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\DOI{10.5802/crchim.422}
\datereceived{2025-07-07}
\daterevised{2025-08-21}
\dateaccepted{2025-09-10}
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\dateposted{2025-10-24}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{Effect of chloride substitution on the performance of a
benzhydryl-substituted bis(arylimino)pyridyl--cobalt ethylene
polymerization catalyst}

\alttitle{Effet d'une substitution chlorure sur les performances d'un
catalyseur de polym\'{e}risation de l'\'{e}thyl\`{e}ne de type
bis(arylimino)pyridyl-cobalt substitu\'{e} par un groupe benzhydryle}

\author{\firstname{Tian} \lastname{Liu}\CDRorcid{0000-0002-7563-0838}}
\address{Department of Polyethylene, SINOPEC (Beijing) Research
Institute of Chemical Industry Co., Ltd., Beijing, 100013, China}
\address{Key Laboratory of Engineering Plastics and Beijing National
Laboratory for Molecular Science, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, China}
%\email[T. Liu]{}

\author{\firstname{Yanping} \lastname{Ma}\CDRorcid{0000-0003-3106-9724}\IsCorresp}
\addressSameAs{2}{Key Laboratory of Engineering Plastics and Beijing
National Laboratory for Molecular Science, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China}
\email[Y. Ma]{myanping@iccas.ac.cn}

\author{\firstname{Gregory A.} \lastname{Solan}\CDRorcid{0000-0002-9833-6666}\IsCorresp}
\addressSameAs{2}{Key Laboratory of Engineering Plastics and Beijing
National Laboratory for Molecular Science, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China}
\address{Department of Chemistry, University of Leicester, University
Road, Leicester LE1 7RH, UK}
\email[G. A. Solan]{gas8@leicester.ac.uk}

\author{\firstname{Jiahao} \lastname{Gao}\CDRorcid{0009-0005-2602-6596}}
\addressSameAs{2}{Key Laboratory of Engineering Plastics and Beijing
National Laboratory for Molecular Science, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China}
%\email[J. Gao]{}

\author{\firstname{Qiuyue} \lastname{Zhang}\CDRorcid{0000-0003-0368-8253}}
\addressSameAs{2}{Key Laboratory of Engineering Plastics and Beijing
National Laboratory for Molecular Science, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China}
%\email[Q. Zhang]{}

\author{\firstname{Tongling} \lastname{Liang}\CDRorcid{0000-0002-0669-9751}}
\addressSameAs{2}{Key Laboratory of Engineering Plastics and Beijing
National Laboratory for Molecular Science, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China}
%\email[T. Liang]{}

\author{\firstname{Wen-Hua} \lastname{Sun}\CDRorcid{0000-0002-6614-9284}\IsCorresp}
\addressSameAs{2}{Key Laboratory of Engineering Plastics and Beijing
National Laboratory for Molecular Science, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China}
\email[W.-H. Sun]{whsun@iccas.ac.cn}

\shortrunauthors

\thanks{Chinese Academy of Sciences (grant no. 2025PVB0034)}

\keywords{\kwd{2,6-Bis(arylimino)pyridine--cobalt
complex}\kwd{\textit{Ortho}-(4,4\tralicstex{\textit{\textquotesingle}}{$'$}-dichlorobenzhydryl) 
substitution}\kwd{Ethylene polymerization}\kwd{High catalytic 
activity}\kwd{Narrow dispersity}\kwd{Linear polyethylene wax}}

\altkeywords{\kwd{Complexe 2,6-bis(arylimino)pyridine-cobalt}
\kwd{Substitution
ortho-(4,4\tralicstex{\textit{\textquotesingle}}{$'$}-dichlorobenzhydryle)}\kwd{Polym\'{e}risation de
l'\'{e}thyl\`{e}ne}\kwd{Activit\'{e} catalytique
\'{e}lev\'{e}e}\kwd{Dispersit\'{e} \'{e}troite}\kwd{Cire de
poly\'{e}thyl\`{e}ne lin\'{e}aire}}

\begin{abstract}
To address the impact of chloride substitution on the performance of a
benzhydryl-substituted bis(arylimino)pyridyl--cobalt ethylene
polymerization catalyst, this work reports a new family of
unsymmetrical \tralicstex{\textit{N\textquotesingle},\textit{N\textquotesingle},\textit{N\textquotesingle\textquotesingle}}{$N',N,N''$}-cobalt(II) chloride complexes,
\textbf{Co1}--\textbf{Co5}, incorporating one fixed
\textit{N}-2-Me-4,6-bis(4,4\tralicstex{\textit{\textquotesingle}}{$'$}-dichlorodibenzhydryl)phenyl group while the
other \textit{N}-aryl group is sterically and electronically variable (aryl
=\ 2,6-dimethylphenyl; 2,6-diethylphenyl; 2,6-diisopropylphenyl;
2,4,6-trimethylphenyl; 2,6-diethyl-4-methylphenyl). Full
characterization of all five complexes is supported by representative
X-ray structures of \textbf{Co1} and \textbf{Co2} that display
distorted square pyramidal geometries. All cobalt complexes exhibited
high activity, under activation with either methylaluminoxane (MAO) or
modified MAO (MMAO), with levels reaching up to 14.74 ×
10\tsup{6} (g of PE)\tralicstex{\textperiodcentered}{${\cdot}$}(mol of Co)\tralicstex{\tsup{--1}\textperiodcentered{}h\tsup{--1}}{$^{-1}{\cdot}$h$^{-1}$} at 60
\textdegree C producing polyethylene (PE) wax (\tralicstex{\textit{M}\tsub{w}}{$M_{\mathrm{w}}$} range:
10.3--34.3 kg\tralicstex{\textperiodcentered{}mol\tsup{--1}}{${\cdot}$mol$^{-1}$}) with high linearity and narrow
dispersity (\tralicstex{\textit{M}\tsub{w}/\textit{M}\tsub{n}}{$M_{\mathrm{w}}/M_{\mathrm{n}}$} range: 2.1--2.5),
highlighting their good control. With both aluminum activators, the
most sterically demanding cobalt catalyst, \textbf{Co3}, afforded the
highest molecular weight polyethylene but the lowest catalytic
activity.
{\vspace*{-2pt}}
\end{abstract}

\begin{altabstract}
Afin d'\'{e}tudier l'impact de la substitution chlorure sur les
performances d'un catalyseur de polym\'{e}risation de
l'\'{e}thyl\`{e}ne \`{a} base de bis(arylimino)pyridyl-cobalt
substitu\'{e} par du benzhydryle, ce travail pr\'{e}sente une nouvelle
famille de complexes asym\'{e}triques de chlorure de
\tralicstex{\textit{N\textquotesingle},\textit{N\textquotesingle},\textit{N\textquotesingle\textquotesingle}}{$N',N,N''$}-cobalt(II), \textbf{Co1}-\textbf{Co5}, incorporant un
groupe \textit{N}-2-Me-4,6-bis(4,4\tralicstex{\textit{\textquotesingle}}{$'$}-dichlorodibenzhydryl)ph\'{e}nyle fixe
tandis que l'autre groupe \textit{N}-aryle est st\'{e}riquement et
\'{e}lectroniquement variable (aryle =\ 2,6-dim\'{e}thylph\'{e}nyle;
2,6-di\'{e}thylph\'{e}nyle; 2,6-diisopropylph\'{e}nyle;
2,4,6-trim\'{e}thylph\'{e}nyle;
2,6-di\'{e}thyl-4-m\'{e}thylph\'{e}nyle). La caract\'{e}risation des
cinq complexes est \'{e}tay\'{e}e par des structures
repr\'{e}sentatives aux rayons X de \textbf{Co1} et \textbf{Co2} qui
pr\'{e}sentent des g\'{e}om\'{e}tries pyramidales carr\'{e}es
d\'{e}form\'{e}es. Tous les complexes de cobalt ont montr\'{e} une
activit\'{e} \'{e}lev\'{e}e, apr\`{e}s activation par du
m\'{e}thylaluminoxane (MAO) ou du MAO modifi\'{e} (MMAO), avec des
niveaux atteignant jusqu'\`{a} 14.74 × 10\tsup{6} (g de PE)\tralicstex{\textperiodcentered}{${\cdot}$}(mol de Co)\tralicstex{\tsup{--1}\textperiodcentered{}h\tsup{--1}}{$^{-1}{\cdot}$h$^{-1}$} \`{a} 60 \textdegree C, 
produisant une cire de poly\'{e}thyl\`{e}ne {(PE) (plage}
\tralicstex{\textit{M}\tsub{w}}{$M_{\mathrm{w}}$}: 10,3--34,3 kg\tralicstex{\textperiodcentered{}mol\tsup{--1}}{${\cdot}$mol$^{-1}$}) avec une
lin\'{e}arit\'{e} \'{e}lev\'{e}e et une dispersit\'{e} \'{e}troite
(plage \tralicstex{\textit{M}\tsub{w}/\textit{M}\tsub{n}}{$M_{\mathrm{w}}/M_{\mathrm{n}}$}: 2,1-2,5), soulignant leur
bonne  performance. Avec les deux activateurs d'aluminium, le
catalyseur au cobalt le plus exigeant sur le plan st\'{e}rique,
\textbf{Co3}, a permis d'obtenir le poly\'{e}thyl\`{e}ne de masse
molaire plus \'{e}lev\'{e}e, mais l'activit\'{e} catalytique la plus
faible. 
\end{altabstract}

\editornote{Article submitted by invitation}
%\alteditornote{Article soumis sur invitation}

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}

{\vspace*{-2pt}}

\def\figurename{Chart}
\setcounter{figure}{0}
\begin{figure*}
\figType{chart}
{\vspace*{-2pt}}
\includegraphics{ch01}
{\vspace*{-2pt}}
\caption{\label{ch1}Parent 2,6-bis(arylimino)pyridine ({A}) and its
unsymmetrical derivatives ({B}--{E}) incorporating various types of
benzhydryl substitution including the subject of this work ({F}).}
{\vspace*{-2pt}}
\end{figure*}

Catalysts for ethylene polymerization based on late transition metals
have been widely documented since their inception in the mid to late
1990s on account of their exceptional catalytic
performance~\cite{1a,1b,1c,1d,1e,1f,1g,2a,2b,2c,2d,2e,2f,2g,2h,3a,3b,3c,3d,3e,3f,4a,4b,4c,4d,4e,4f,4g,5a,5b,5c,5d,6a,6b,6c,6d,6e,6f,6g}. 
Since these pioneering disclosures,
there has been rapid progress in the use of the 2,6-bis(imino)pyridine
ligand ({A}, Chart~\ref{ch1}) as a chelating support for both iron and
cobalt catalysts, driven by the desire for ever higher activity as well
as improvements in thermal stability; recent progress has been
reviewed~\cite{7,8}. Besides their excellent performance
characteristics, such late transition metal catalysts also produce
linear polyethylene with a broad range of structural properties that
can be modulated by the specifics of the ligand structure~\cite{9a,9b,9c,10a,10b}.
Indeed, the resulting polymers offer promise for applications in a
variety of fields, such as automobiles, pipes, and so on.

In recent years, our group has been interested in designing new
bis(arylimino)pyridyl--iron and \mbox{--cobalt} catalysts~by systematically
varying the types of {$N$}-aryl groups~\cite{11a,11b,11c,11d}. Of note,
unsymmetrical examples incorporating bulky benzhydryl substitution have
emerged including 2,4-dibenzhydryl-6-methylphenyl ({B},
Chart~\ref{ch1}) \cite{11a}, 2-benzhydryl-4,6-dimethylphenyl ({C},
Chart~\ref{ch1}) \cite{11b},
2,4-bis(bis(4-fluorophenyl)methyl)-6-methylphenyl ({D},
Chart~\ref{ch1}) \cite{11c} and
2,4-bis(bis(4-methoxyphenyl)methyl)-6-methylphenyl ({E},
Chart~\ref{ch1}) \cite{11d}. A notable outcome of these studies is that
the \textit{ortho}-benzhydryl substituent can provide protection to the
active center and improve thermal stability of the catalyst.
Furthermore, the electronic variations made to the periphery of the
benzhydryl group ({viz}., \textit{para}-X ${=}$\ H, F, OMe) can
further influence this stability and also affect catalytic activity and
various polymer properties.

With the intention of further probing the impact of the benzhydryl's
\textit{para}-X group on catalyst activity and polymer properties, we
herein focus on the \textit{para}-Cl derivative {F} (Chart~\ref{ch1})
as a support for a cobalt polymerization catalyst. Accordingly, the
synthetic details for a family of {F} are given that encompass a range
of steric and electronic variations made to one of the {$N$}-aryl
groups. This set of unsymmetrical $N',N,N''$-ligands are then used to
prepare their cobalt(II) chloride complexes, which are then applied in
a comprehensive ethylene polymerization evaluation. The findings of
this evaluation are then compared to those reported for cobalt
catalysts bearing {B}--{E} (Chart~\ref{ch1}), with the aim of
highlighting any effects of the \textit{para}-Cl substitution on
catalytic activity, thermal stability, polymer molecular weight, and
dispersity. Full synthetic and characterization data for the complexes,
ligands, and polyethylenes are reported.

\section{Experimental}\label{sec2}
\subsection{General considerations}\label{ssec21} 

Standard Schlenk techniques were used in the handling of air- and
moisture-sensitive compounds and were conducted under an inert nitrogen
atmosphere. Immediately before the polymerization evaluations were
conducted, the reaction solvent, toluene, was heated to reflux over
sodium and then distilled under nitrogen. High-purity ethylene gas was
purchased from Beijing Yansan Petrochemical Co. and used as received.
The activators methylaluminoxane (MAO, 1.30 M solution in toluene) and
modified methylaluminoxane (MMAO, 1.93 M in $n$-heptane) were purchased
from Anhui Botai Electronic Materials Co. Other reagents were purchased
from Aldrich, Acros, or local suppliers (Beijing, China). The \tsup{1}H
and \tsup{13}C NMR spectra of the organic compounds were recorded on a
Bruker DMX 400~MHz instrument at room temperature with TMS as internal
standard; chemical shifts are measured in ppm. Elemental analysis was
recorded on a Flash EA 1112 microanalyzer and FT-IR spectra on a System
2000 FT-IR spectrometer. An Agilent PL-GPC 220 GPC instrument was used
to determine the molecular weight ($M_{\mathrm{w}}$) and molecular
weight distribution ($M_{\mathrm{w}}/M_{\mathrm{n}}$) of the resulting
polyethylene at  150~\textdegree C by using 1,2,4-trichlorobenzene as
the eluting solvent. The melting temperatures ($T_{\mathrm{m}}$) of the
polyethylenes were recorded on a PerkinElmer TA-Q2000 differential
scanning calorimeter (DSC) under a nitrogen atmosphere. Typically, a
sample of about 3.0--5.0 mg was heated up to 160~\textdegree C at a
rate of  20~\textdegree C${\cdot}$min$^{-1}$, kept for 2 min at 
160~\textdegree C to delete the thermal history and then cooled at a
rate of  20~\textdegree C${\cdot}$min$^{-1}$ to $-$40~\textdegree C.
The \tsup{13}C NMR spectra of the polyethylenes were recorded on a
Bruker DMX 500~MHz instrument at 100~\textdegree C; sample preparation
involved a weighed amount of polyethylene (30--50 mg) being dissolved
in 1,1,2,2-tetrachloroethane-$d$\tsub{2} with TMS as an internal
standard. The imine-ketones,
1-(6-(1-(arylimino)ethyl)pyridin-2-yl)ethan-1-ones (aryl ${=}$\ 
2,6-dimethylphenyl; 2,6-diethylphenyl; 2,6-diisopropylphenyl;
2,4,6-trimethylphenyl; 2,6-diethyl-4-methylphenyl) and
2,4-bis(bis(4-chlorophenyl)methyl)-6-methylaniline were prepared as
previously described~\cite{14,15}.

\vspace*{-2pt}

\subsection{Syntheses of
2-\{\{2,4-(({$p$}-ClPh)\tsub{2}CH)\tsub{2}-6-MeC\tsub{6}H\tsub{2}\}N${=}$CMe\}-6-(ArN${=}$CMe)C\tsub{5}H\tsub{3}N
(\textbf{L1}--\textbf{L5})}\label{ssec22}

\vspace*{-2pt}

\subsubsection{Ar: 2,6-Me\tsub{2}C\tsub{6}H\tsub{3} (\textbf{L1})}

\vspace*{-2pt}

A catalytic amount of {$p$}-toluenesulfonic acid was added to a
solution of 1-(6-(1-((2,6-dimethylphenyl)imino)ethyl)
pyridin-2-yl)ethan-1-one (2.54~g, 9.53~mmol) and
2,4-bis(bis(4-chlorophenyl)methyl)-6-methylaniline (5.00~g, 8.66 mmol)
in toluene (30~mL). The resulting mixture was stirred and heated under
reflux for 9~h. Once cooled to room temperature, the reaction mixture
was filtered and all volatile components removed on a rotary
evaporator. The remaining solid was loaded onto a basic alumina column
and eluted with petroleum ether/ethyl acetate (v/v ${=}$\ 500/1) to
afford \textbf{L1} as a pale-yellow solid (2.53~g, 35\%). \tsup{1}{H} NMR
(400~MHz, CDCl\tsub{3}, TMS): ${\delta}$ 
8.48 (d, $J=8.0$~Hz, 1H, Py-H), 
8.30 (d, $J=8.0$~Hz, 1H, Py-H),
7.90 (t, $J=8.0$~Hz, 1H, Py-H),
7.24 (d, $J=8.0$~Hz, 4H, Ph-H),
7.16 (d, $J=8.0$~Hz, 2H, Ph-H),
7.11--7.08 (m, 4H, Ph-H), 
6.96 (d, $J=8.0$~Hz, 4H, Ph-H), 
6.94 (s, 1H, Ph-H),
6.86 (d, $J=8.0$~Hz, 2H, Ph-H),
6.81 (d, $J=8.0$~Hz, 3H, Ph-H),
6.38 (s, 1H, Ph-H), 
5.35 (s, 1H, --CH--), 
5.33 (s, 1H, --CH--), 
2.18 (s, 3H, --CH\tsub{3}), 
2.09 (s, 3H, --CH\tsub{3}), 
2.04 (s, 3H, --CH\tsub{3}), 
1.96 (s, 3H, --CH\tsub{3}), 
1.69 (s, 3H, --CH\tsub{3}).
\tsup{13}C NMR (100~MHz, CDCl\tsub{3}, TMS): ${\delta}$ 168.9, 167.1,
155.3, 154.8, 148.7, 146.8, 142.3, 142.2, 141.3, 140.7, 137.4, 136.8,
132.5, 132.4, 132.4, 132.2, 130.9, 130.6, 130.5, 130.5, 130.5, 129.6,
128.5, 128.3, 128.3, 128.3, 128.0, 127.9, 125.7, 125.4, 123.1, 122.3,
122.0, 55.0, 51.1, 18.0, 17.9, 16.8, 16.4. FT-IR (cm$^{-1}$): 2917
(w), 1900 (w), 1642 ($\nu_{\mathrm{C}=\mathrm{N}}$, m), 1594 (w), 1570
(w), 1489 (s), 1466 (m), 1404 (m), 1363 (m), 1297 (w), 1238 (m), 1209
(m), 1121 (m), 1090 (s), 1015 (m), 825 (m), 795 (m), 761 (m), 685 (m).
HRMS (MALDI-TOF) $m/z$: [M]$^{-}$ Calcd for
C\tsub{50}H\tsub{41}Cl\tsub{4}N\tsub{3} 823.2049, Found 823.2043. Anal.\ 
Calc.\ for C\tsub{50}H\tsub{41}Cl\tsub{4}N\tsub{3} (825.70): C, 72.73;
H, 5.01; N, 5.09\%, Found: C, 72.23; H, 4.94; N, 4.96\%.

\subsubsection{Ar: 2,6-Et\tsub{2}C\tsub{6}H\tsub{3} (\textbf{L2})} 
Using a method and molar ratios of reagents similar to those described
for \textbf{L1}, \textbf{L2} was isolated as a pale-yellow powder (0.36 g,
10\%). \tsup{1}H NMR (400~MHz, CDCl\tsub{3}, TMS): ${\delta}$ 
8.46 (d, $J=8.0$~Hz, 1H, Py-H),
8.30 (d, $J=8.0$~Hz, 1H, Py-H),
7.90 (t, $J=8.0$~Hz, 1H, Py-H),
7.24 (d, $J=8.0$~Hz, 3H, Ph-H),
7.17 (d, $J=8.0$~Hz, 4H, Ph-H),
7.12 (d, $J=8.0$~Hz, 4H, Ph-H),
7.05 (s, 1H, Ph-H), 
6.97 (d, $J=8.0$~Hz, 5H, Ph-H),
6.88 (d, $J=8.0$~Hz, 2H, Ph-H),
6.23 (d, $J=8.0$~Hz, 3H, Ph-H),
6.40 (s, 1H, Ph-H), 
5.36 (s, 1H, --CH--),
5.35 (s, 1H, --CH--), 
2.47--2.35 (m, 4H, --CH\tsub{2}--), 
2.21 (s, 3H, \mbox{--CH\tsub{3}),}
1.97 (s, 3H, --CH\tsub{3}), 
1.71 (s, 3H, --CH\tsub{3}), 
1.20 (t, $J=8.0$~Hz, 3H, --CH\tsub{3}),
1.15 (t, $J=8.0$~Hz, 3H, \mbox{--CH\tsub{3}).}
\tsup{13}C NMR (100~MHz, CDCl\tsub{3}, TMS): ${\delta}$ 
169.0, 166.9, 155.3, 154.8, 147.8, 146.8, 142.3, 142.2, 141.3, 140.7,
137.4, 136.8, 132.5, 132.3, 132.2, 131.2, 130.9, 130.5, 130.5, 129.6,
128.5, 128.5, 128.3, 128.3, 126.0, 125.7, 123.4, 122.3, 122.0, 55.0,
51.1, 24.6, 17.9, 16.8, 13.7. FT-IR (cm$^{-1}$): 2964 (w), 1899 (w),
1640 ($\nu_{\mathrm{C}=\mathrm{N}}$, m), 1568 (w), 1489 (s), 1452 (m), 
1404 (m), 1362 (m), 1296 (w), 1235 (m), 1205 (m), 1120 (m), 1090 (s),
1016 (m), 961 (w), 879 (w), 823 (s), 795 (s), 762 (m), 703 (m), 681
(m). HRMS (MALDI-TOF) $m/z$: [M]$^{-}$ Calcd for
C\tsub{52}H\tsub{45}Cl\tsub{4}N\tsub{3} 851.2362, Found 851.2363. Anal.\ 
Calc.\ for C\tsub{52}H\tsub{45}Cl\tsub{4}N\tsub{3} (853.75): C, 73.16;
H, 5.31; N, 4.92\%, Found: C, 72.85; H, 5.33; N, 4.71\%.

\subsubsection{Ar: 2,6-$^{i}$Pr\tsub{2}C\tsub{6}H\tsub{3} (\textbf{L3})} 
Using a method and molar ratios of reagents similar to those described
for \textbf{L1}, \textbf{L3} was isolated as a pale-yellow powder (0.45 g,
15\%). \tsup{1}H NMR (400~MHz, CDCl\tsub{3}, TMS): ${\delta}$ 
8.46 (d, $J=8.0$~Hz, 1H, Py-H),
8.30 (d, $J=8.0$~Hz, 1H, Py-H),
7.90 (t, $J=8.0$~Hz, 1H, Py-H),
7.25--7.09 (m, 13H, Ph-H), 
7.04 (d, $J=8.0$~Hz, 2H, Ph-H), 
7.00--6.95 (m, 3H, Ph-H), 
6.88 (d, $J=8.0$~Hz, 2H, Ph-H), 
6.83 (d, $J=8.0$~Hz, 2H, Ph-H), 
6.43 (s, 1H, Ph-H), 
5.39 (s, 1H, --CH--), 
5.34 (s, 1H, \mbox{--CH--),} 
2.81--2.69 (m, 2H, --CH--), 
2.20 (s, 3H, --CH\tsub{3}), 
1.97 (s, 3H, --CH\tsub{3}), 
1.71 (s, 3H, --CH\tsub{3}),
1.21 (t, $J=8.0$~Hz, 6H, --CH\tsub{3}),
1.17 (t, $J=8.0$~Hz, 6H, --CH\tsub{3}).
\tsup{13}C NMR (100~MHz, CDCl\tsub{3}, TMS): ${\delta}$ 
170.1, 167.0, 146.4, 145.9, 141.5, 140.7, 136.8, 135.8, 132.3, 132.2,
131.6, 131.0, 130.6, 128.7, 128.6, 128.4, 123.7, 123.0, 122.3, 122.0,
50.9, 28.3, 23.3, 22.9, 21.3, 17.3, 17.1. FT-IR (cm$^{-1}$): 2960
(w), 1904 (w), 1637 ($\nu_{\mathrm{C}=\mathrm{N}}$, m), 1569 (w), 
1489 (s), 1458 (m), 1405 (w), 1364 (m), 1322 (w), 1305 (w), 1237 (m),
1210 (w), 1124 (m), 1090 (s), 1014 (s), 965 (w), 898 (w), 871 (m), 827
(s), 796 (m), 776 (m), 710 (w), 683 (w). HRMS (MALDI-TOF) $m/z$:
[M]$^{-}$ Calcd for C\tsub{54}H\tsub{49}Cl\tsub{4}N\tsub{3} 879.2675,
Found 879.2670. Anal.\ Calc.\ for C\tsub{54}H\tsub{49}Cl\tsub{4}N\tsub{3}
(881.81): C, 73.55; H, 5.60; N, 4.77\%, Found: C, 73.75; H, 5.64; N,
4.73\%.

\subsubsection{Ar: 2,4,6-Me\tsub{3}C\tsub{6}H\tsub{2} (\textbf{L4})}
Using a method and molar ratios of reagents similar to those described
for \textbf{L1}, \textbf{L4} was isolated as a pale-yellow powder (1.06 g,
15\%). \tsup{1}H NMR (400~MHz, CDCl\tsub{3}, TMS): ${\delta}$ 
8.45 (d, $J=8.0$~Hz, 1H, Py-H),
8.28 (d, $J=8.0$~Hz, 1H, Py-H),
7.89 (t, $J=8.0$~Hz, 1H, Py-H),
7.24 (d, $J=8.0$~Hz, 4H, Ph-H),
7.18--7.15 (m, 2H, Ph-H), 
7.11 (d, $J=8.0$~Hz, 2H, Ph-H),
6.96 (d, $J=8.0$~Hz, 4H, Ph-H),
6.90 (d, $J=8.0$~Hz, 2H, Ph-H),
6.87 (d, $J=8.0$~Hz, 2H, Ph-H),
6.81 (d, $J=8.0$~Hz, 3H, Ph-H),
6.39 (s, 1H, Ph-H), 
5.35 (s, 1H, --CH--), 
5.34 (s, 1H, \mbox{--CH--),} 
2.31 (s, 3H, --CH\tsub{3}), 
2.18 (s, 3H, --CH\tsub{3}), 
2.06 (s, 3H, \mbox{--CH\tsub{3}),}
2.01 (s, 3H, --CH\tsub{3}), 
1.96 (s, 3H, --CH\tsub{3}), 
1.69 (s, 3H, --CH\tsub{3}). 
\tsup{13}C NMR (100~MHz, CDCl\tsub{3}, TMS): ${\delta}$
169.0, 167.3, 155.4, 154.7, 146.8, 146.2, 142.3, 142.2, 141.3, 140.7,
137.4, 136.8, 132.5, 132.3, 132.3, 132.3, 132.2, 130.9, 130.5, 130.5,
130.5, 129.6, 128.6, 128.6, 128.5, 128.5, 128.3, 128.2, 125.7, 125.2,
122.3, 121.9, 55.0, 51.1, 20.7, 17.9, 17.9, 17.8, 16.8, 16.4. FT-IR
(cm$^{-1}$): 2913 (w), 1897 (w), 1640 ($\nu_{\mathrm{C}=\mathrm{N}}$, m), 
1569 (m), 1489 (s), 1403 (m), 1362 (m), 1321 (w), 1239 (w), 1213 (m),
1116 (m), 1088 (s), 1013 (s), 962 (w), 823 (s), 794 (s), 761 (w), 745
(w), 703 (m), 682 (w). HRMS (MALDI-TOF) $m/z$: [M]$^{-}$ Calcd
for C\tsub{51}H\tsub{43}Cl\tsub{4}N\tsub{3} 837.2206, Found 837.2201.
Anal.\ Calc.\ for C\tsub{51}H\tsub{43}Cl\tsub{4}N\tsub{3} (839.73): C,
72.95; H, 5.16; N, 5.00\%, Found: C, 73.05; H, 5.17; N, 4.90\%.
\looseness=-1

\subsubsection{Ar: 2,6-Et\tsub{2}-4-MeC\tsub{6}H\tsub{2} (\textbf{L5})}
Using a method and molar ratios of reagents similar to those described
for \textbf{L1}, \textbf{L5} was isolated as a pale-yellow powder (0.76 g,
17\%). \tsup{1}H NMR (400~MHz, CDCl\tsub{3}, TMS): ${\delta}$ 
8.45 (d, $J=8.0$~Hz, 1H, Py-H),
8.28 (d, $J=8.0$~Hz, 1H, Py-H),
7.89 (t, $J=8.0$~Hz, 1H, Py-H),
7.25 (d, $J=8.0$~Hz, 6H, Ph-H),
7.17 (d, $J=8.0$~Hz, 2H, Ph-H),
7.11 (d, $J=8.0$~Hz, 2H, Ph-H),
6.98--6.94 (m, 6H, Ph-H), 
6.87 (d, $J=8.0$~Hz, 2H, Ph-H),
6.82 (d, $J=8.0$~Hz, 3H, Ph-H),
6.39 (s, 1H, Ph-H), 
5.36 (s, 1H, --CH--), 
5.34 (s, 1H, --CH--), 
2.46--2.28 (m, 7H, --CH\tsub{2}-- and --CH\tsub{3}), 
2.20 (s, 3H, --CH\tsub{3}), 
1.97 (s, 3H, --CH\tsub{3}), 
1.71 (s, 3H, --CH\tsub{3}),
1.18 (t, $J=8.0$~Hz, 3H, --CH\tsub{3}),
1.13 (t, $J=8.0$~Hz, 3H, --CH\tsub{3}).
\tsup{13}C NMR (100~MHz, CDCl\tsub{3}, TMS): ${\delta}$ 
169.0, 167.0, 155.4, 154.7, 146.8, 145.2, 142.3, 142.2, 141.3, 140.7,
137.4, 136.8, 132.5, 132.3, 132.3, 132.2, 131.1, 130.9, 130.5, 130.5,
130.5, 129.6, 128.4, 128.3, 128.2, 126.7, 125.7, 122.3, 121.9, 55.0,
51.1, 24.6, 21.0, 17.9, 16.8, 16.7, 13.9, 13.8. FT-IR (cm$^{-1}$):
2965 (w), 1899 (w), 1642 ($\nu_{\mathrm{C}=\mathrm{N}}$, m), 1598 (w), 
1569 (m), 1489 (s), 1454 (m), 1403 (m), 1365 (m), 1326 (w), 1298 (w),
1257 (m), 1208 (m), 1086 (s), 1013 (s), 865 (m), 823 (m), 795 (s), 745
(m), 709 (w), 681 (w). HRMS (MALDI-TOF) $m/z$: [M]$^{-}$ Calcd for
C\tsub{53}H\tsub{47}Cl\tsub{4}N\tsub{3} 865.2519, Found 865.2513. Anal.\ 
Calc.\ for C\tsub{53}H\tsub{47}Cl\tsub{4}N\tsub{3} (867.78): C, 73.36;
H, 5.46; N, 4.84\%, Found: C, 73.07; H, 5.51; N, 4.76\%.

\vspace*{-2pt}

\subsection{Syntheses of
[2-\{\{2,4-(($p$-ClPh)\tsub{2}CH)\tsub{2}-6-MeC\tsub{6}H\tsub{2}\}N${=}$CMe\}-6-(ArN${=}$CMe)C\tsub{5}H\tsub{3}N]CoCl\tsub{2}
(\textbf{Co1}--\textbf{Co5})}

\vspace*{-2pt}

\subsubsection{Ar: 2,6-Me\tsub{2}C\tsub{6}H\tsub{3} (\textbf{Co1})}

\vspace*{-2pt}

Under a nitrogen atmosphere, a Schlenk vessel was loaded with \textbf{L1}
(0.18 g, 0.22~mmol) and CoCl\tsub{2}${\cdot}$6H\tsub{2}O (0.047 g,
0.20~mmol) and the contents dissolved in a mixture of freshly distilled
dichloromethane (5~mL) and ethanol (10~mL). The reaction mixture was
stirred for 9 h at room \mbox{temperature} after which all volatile components
were evaporated under reduced pressure. The remaining residue was
dissolved in dichloromethane, and diethyl ether was added to induce
precipitation. The precipitate was collected by filtration, washed with
diethyl ether, and dried to afford \textbf{Co1} as a brown powder (0.17 g,
91\%). FT-IR (cm$^{-1}$): 2915 (w), 2111 (w), 1623
($\nu_{\mathrm{C}=\mathrm{N}}$, m), 1589 (m), 1489 (s), 1470 (m),  1439
(w), 1403 (w), 1371 (m), 1261 (m), 1212 (m), 1091(s), 1014 (s), 825
(m), 801 (m), 762 (m), 738 (w), 655 (m). HRMS (ESI) $m/z$:
[M--Cl]$^{+}$ Calcd for C\tsub{50}H\tsub{41}Cl\tsub{5}CoN\tsub{3}
917.1070, Found 917.1070. Anal.\ Calc.\ for
C\tsub{50}H\tsub{41}Cl\tsub{6}CoN\tsub{3} (955.53): C, 62.85; H, 4.33;
N, 4.40\%, Found: C, 62.52; H, 4.30; N, 4.26\%.

\subsubsection{Ar: 2,6-Et\tsub{2}C\tsub{6}H\tsub{3} (\textbf{Co2})}
Using a procedure and molar ratios of reagents similar to those
outlined for \textbf{Co1}, \textbf{Co2} was isolated as a brown powder (0.080 g,
97\%). FT-IR (cm$^{-1}$): 2968 (w), 1903 (w), 1623
($\nu_{\mathrm{C}=\mathrm{N}}$, m), 1584 (m), 1489 (s), 1468 (m), 1446
(m), 1404 (w), 1374 (m), 1319 (w), 1263 (m), 1210 (m), 1134 (w), 1088
(s), 1014 (s), 978 (w), 895 (w), 867 (m), 838 (m), 813 (s), 795 (m),
760 (m), 698 (w), 656 (w). HRMS (ESI) $m/z$: [M--Cl]$^{+}$ Calcd for
C\tsub{52}H\tsub{45}Cl\tsub{5}CoN\tsub{3} 945.1383, Found 945.1387.
Anal.\ Calc.\ for C\tsub{52}H\tsub{45}Cl\tsub{6}CoN\tsub{3} (983.59): C,
63.50; H, 4.61; N, 4.27\%, Found: C, 63.13; H, 4.59; N, 4.13\%.

\subsubsection{Ar: 2,6-$^{i}$Pr\tsub{2}C\tsub{6}H\tsub{3} (\textbf{Co3})}
Using a procedure and molar ratios of reagents similar to those
outlined for \textbf{Co1}, \textbf{Co3} was isolated as a brown powder (0.070 g,
84\%). FT-IR (cm$^{-1}$): 2964 (w), 1619
($\nu_{\mathrm{C}=\mathrm{N}}$, m), 1584 (m), 1489 (s), 1469 (m), 1442
(m), 1404 (w), 1373 (m), 1322 (w), 1263 (m), 1214 (m), 1182 (w), 1090
(s), 1014 (s), 940 (w), 895 (w), 834 (m), 796 (s), 765 (m), 699 (m),
656 (w). HRMS (ESI) $m/z$: [M--Cl]$^{+}$ Calcd for
C\tsub{54}H\tsub{49}Cl\tsub{5}CoN\tsub{3} 973.1696, Found 973.1693.
Anal.\ Calc.\ for C\tsub{54}H\tsub{49}Cl\tsub{6}CoN\tsub{3} (1011.64): C,
64.11; H, 4.88; N, 4.15\%, Found: C, 64.03; H, 4.89; N, 4.07\%.

\subsubsection{Ar: 2,4,6-Me\tsub{3}C\tsub{6}H\tsub{2} (\textbf{Co4})}
Using a procedure and molar ratios of reagents similar to those
outlined for \textbf{Co1}, \textbf{Co4} was isolated as a brown powder (0.090 g,
87\%). FT-IR (cm$^{-1}$): 2912 (w), 1980 (w), 1618
($\nu_{\mathrm{C}=\mathrm{N}}$, m), 1585 (m), 1489 (s), 1403 (m), 1372
(m), 1320 (w), 1264 (m), 1225 (m), 1182 (w), 1090 (s), 1014 (s), 893
(w), 839 (m), 806 (m), 763 (m), 738 (m), 654 (w). HRMS (ESI) $m/z$:
[M--Cl]$^{+}$ Calcd for C\tsub{51}H\tsub{43}Cl\tsub{5}CoN\tsub{3}
931.1226, Found 931.1229. Anal.\ Calc.\ for
C\tsub{51}H\tsub{43}Cl\tsub{6}CoN\tsub{3} (969.56): C, 63.18; H, 4.47;
N, 4.33\%, Found: C, 62.78; H, 4.48; N, 4.18\%.

\subsubsection{Ar: 2,6-Et\tsub{2}-4-MeC\tsub{6}H\tsub{2} (\textbf{Co5})}
Using a procedure and molar ratios of reagents similar to those
outlined for \textbf{Co1}, \textbf{Co5} was isolated as a brown powder (0.090 g,
87\%). FT-IR (cm$^{-1}$): 2966 (w), 1908 (w), 1622
($\nu_{\mathrm{C}=\mathrm{N}}$, m), 1587 (m), 1490 (s), 1465 (m), 1404
(m), 1372 (m), 1322 (w), 1265 (m), 1216 (m), 1182 (w), 1090 (s), 1014
(s), 865 (m), 835 (m), 806 (s), 799 (m), 762 (m), 739 (m). HRMS (ESI)
$m/z$:  [M--Cl]$^{+}$ Calcd for
C\tsub{53}H\tsub{47}Cl\tsub{5}CoN\tsub{3} 959.1539, Found 959.1540.
Anal.\ Calc.\ for C\tsub{53}H\tsub{47}Cl\tsub{6}CoN\tsub{3} (997.61): C,
63.81; H, 4.75; N, 4.21\%, Found: C, 63.30; H, 4.72; N, 4.08\%.{\vspace*{-6pt}}

\subsection{Polymerization studies}\label{ssec24}

A 250~mL autoclave, equipped with pressure/temperature control system,
mechanical stirrer, and an ethylene cylinder, was employed to conduct
the ethylene polymerizations. This autoclave was evacuated and
backfilled with nitrogen four times, and then with ethylene gas. The
pre-determined amount of cobalt complex (2~${\upmu}$mol) was then added
to a dry Schlenk tube (100~mL), which had been evacuated and
back-filled with nitrogen three times. Freshly distilled toluene
(25~mL) was injected to dissolve the cobalt complex, and the resulting
solution transferred quickly to the autoclave. More toluene (25~mL) was
injected into the Schlenk tube to dissolve any remaining cobalt complex
and added to the previous solution; this was repeated one more time.
Once the temperature had reached the required value, the pre-determined
amount of activator (MAO or MMAO) was injected into the autoclave with
a syringe along with another 25~mL of toluene to take the total volume
of solvent to 100~mL. With the ethylene pressure set at 10 atm and the
temperature at the pre-identified value (and controlled by circulating
water or using a water/ice bath), the reaction was started by stirring
at 400 rpm. Once the run time was completed, the reactor was cooled to
room temperature, and ethylene pressure vented. The reaction mixture
was then quenched using a 5\% hydrochloric acid in ethanol solution
(100~mL), forming the polyethylene as a white powder. After stirring
and washing for 2 h, the polymer was collected using suction filtration
and dried in a vacuum oven to a constant weight.{\vspace*{-6pt}} 

\subsection{X-ray crystallographic studies}\label{ssec25} 

Single-crystal XRD studies on \textbf{Co1} and \textbf{Co2} were carried out using a
XtaLAB Synergy-R diffractometer with mirror-monochromatic
Cu-K${\alpha}$ radiation ($\lambda=1.54184$~\AA{})  at 170.00~K; the
cell parameters were obtained by global refinement of the positions of
all attained reflections. Direct methods were used to solve the
structures, and these were refined by full-matrix least-squares on
$F^{2}$. All hydrogen atoms were placed in calculated positions.
Structural solution and refinement were performed using the Olex2 1.2
package and SHELXTL~\cite{12}. The application of PLATON software was
used during the structural refinement to squeeze the solvent in the
lattice~\cite{13}. Details of the crystal data and processing
parameters are summarized in Table~S1.

{\vspace*{-6pt}}

\section{Results and discussion}\label{sec3}
\subsection{Synthesis and characterization}\label{ssec31}

Five different examples of
2-[1-(2,4-bis(di(4-chlorophenyl)methyl)-6-methylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridine 
(aryl ${=}$\ 2,6-dimethylphenyl (\textbf{L1}); 2,6-diethylphenyl (\textbf{L2});
2,6-diisopropylphenyl (\textbf{L3}); 2,4,6-trimethylphenyl (\textbf{L4});
2,6-diethyl-4-methylphenyl (\textbf{L5})) were synthesized by the
acid-catalyzed condensation reaction of the corresponding imine-ketone
with 2,4-bis(di(4-chlorophenyl)methyl)-6-methylaniline in toluene under
reflux; related procedures have been reported elsewhere
(Scheme~\ref{sch1})~\cite{14}. Compounds \textbf{L1}--\textbf{L5} were isolated in
moderate yield and were characterized by \tsup{1}H/\tsup{13}C NMR and
FT-IR spectroscopy, and purity further confirmed by elemental analysis
(see experimental).

\begin{scheme*}
\includegraphics{sc01}
\caption{\label{sch1}Synthesis of \textbf{L1}--\textbf{L5} and their use in
forming cobalt(II) chloride complexes \textbf{Co1}--\textbf{Co5}.} 
\end{scheme*}

Interaction of \textbf{L1}--\textbf{L5} with CoCl\tsub{2}${\cdot}$6H\tsub{2}O in a
mixture of ethanol and dichloromethane at room temperature afforded, on
work-up, \textbf{Co1}--\textbf{Co5} in excellent yields (Scheme~\ref{sch1}). FT-IR
spectroscopy proved an effective means of confirming coordination of
the $N',N,N''$-ligands as evidenced by the 
$\nu_{\mathrm{C}=\mathrm{N}}$ stretching vibrations of \textbf{Co1}--\textbf{Co5}
shifting to lower wavenumbers (range: 1618--1623 cm$^{-1}$) when
compared with the free ligands (range: 1637--1642 cm$^{-1}$). In
their ESI mass spectra, fragmentation peaks corresponding to
[M--Cl]$^{+}$ ions were seen for all five complexes. Additionally, the
elemental analysis data were consistent with elemental compositions
based on the general formula LCoCl\tsub{2}. Further confirmation of
their composition was provided by the X-ray structures of
representative \textbf{Co1} and \textbf{Co2} (see below).

Single crystals of \textbf{Co1} and \textbf{Co2} suitable for XRD were
obtained by layering a dichloromethane solution of each complex with
diethyl ether and leaving the mixture to slowly diffuse at room
temperature. Views of \textbf{Co1} and \textbf{Co2} are shown in
Figures \ref{fig1} and \ref{fig2}, respectively; selected bond lengths
and angles for both are collected in Table~\ref{tab1}. Both complexes
\textbf{Co1} and \textbf{Co2} are similar and will be discussed
together. 

\def\figurename{Figure}
\setcounter{figure}{0}
\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}ORTEP representation of \textbf{Co1} with the thermal
ellipsoids set at the 30\% probability level; all hydrogen atoms have
been removed for clarity.}
\end{figure*}

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}ORTEP representation of \textbf{Co2} with the thermal
ellipsoids set at the 30\% probability level; all hydrogen atoms have
been removed for clarity.}
\end{figure*}

%tab1
\begin{table}
\caption{\label{tab1}Selected bond lengths (\AA{}) and angles
(\textdegree) for \textbf{Co1} and \textbf{Co2}{\vspace*{-3pt}}}
\begin{tabular}{ccc}
\thead
& \textbf{Co1} & \textbf{Co2} \\
\endthead
\multicolumn{3}{l}{Bond lengths (\AA{})} \\ 
Co(1)--N(1)  & 2.265(9) & 2.211(3) \\ 
Co(1)--N(2)  & 2.051(8) & 2.050(3) \\ 
Co(1)--N(3)  & 2.195(9) & 2.247(3) \\ 
Co(1)--Cl(1) & 2.304(3) & \02.3070(10) \\ 
Co(1)--Cl(2) & 2.249(3) & 2.2464(9) \vspace*{5pt} \\ 
\multicolumn{3}{l}{Bond angles (\textdegree)} \\ 
N(1)--Co(1)--N(2)  & \074.4(3)  & \074.79(10) \\ 
N(1)--Co(1)--N(3)  & 147.3(3) & 144.68(10) \\ 
N(2)--Co(1)--N(3)  & \075.7(3)  & \074.37(11) \\ 
N(1)--Co(1)--Cl(2) & \098.1(3)  & \099.21(7) \\ 
N(2)--Co(1)--Cl(2) & 141.7(3) & 152.57(9) \\ 
N(3)--Co(1)--Cl(2) & \097.1(2)  & 100.22(8) \\ 
N(1)--Co(1)--Cl(1) & \096.7(2)  & \097.29(7) \\ 
N(2)--Co(1)--Cl(1) & 103.8(3) & \091.86(8) \\ 
N(3)--Co(1)--Cl(1) & 103.0(3) & 100.48(8) \\ 
Cl(2)--Co(1)--Cl(1)& \0\0114.44(11)& 115.54(4)
\botline
\end{tabular}
{\vspace*{-8pt}}
\end{table}

In each case, the coordination geometry of the cobalt center can be
best described as distorted square pyramidal, with the basal plane
defined by N1, N2, N3 and Cl2. The Co1 center sits above the basal
plane by 0.571~\AA{} in \textbf{Co1} and 0.486~\AA{} in \textbf{Co2},
while axial Cl1 further protrudes by 2.853~\AA{} in \textbf{Co1} and
2.735~\AA{} in \textbf{Co2}. With regard to the Co--N distances, the
central Co--N$_{\mathrm{pyridine}}$ bond length [2.051(8)
(\textbf{Co1}), 2.050(3) (\textbf{Co2})~\AA{}] is markedly shorter than
the exterior Co--N$_{\mathrm{imine}}$ ones [2.195(9)--2.265(9) \AA{}],
which likely derives from the good donor properties of the central
pyridine and the constraints of this $N',N,N''$-ligand class; similar
observations have been \mbox{previously} noted~\cite{14}. The presence of
inequivalent {$N$}-aryl groups causes some more modest variations in
the exterior Co--N$_{\mathrm{imine}}$ distances, which is also affected
by the steric properties exerted by the {$N$}-2,6-dimethylphenyl
(\textbf{Co1}) and {$N$}-2,6-diethylphenyl (\textbf{Co2}) groups. In
terms of the Co--Cl bond lengths, some variation is also seen with that
involving the axial chloride (Co--Cl1: 2.304(3)~\AA{} (\textbf{Co1}),
2.3070(10)~\AA{} (\textbf{Co2})) longer than its basal counterpart
(Co1--Cl2: 2.249(3)~\AA{} (\textbf{Co1}), 2.2464(9)~\AA{}
(\textbf{Co2})). The {$N$}-aryl rings are positioned almost
perpendicular with respect to the N1--N2--N3--Co1 coordination plane
with dihedral angles of 81.61\textdegree\ and 77.03\textdegree\ for
\textbf{Co1} and 88.74\textdegree\ and 82.63\textdegree\ for
\textbf{Co2}~\cite{15}. There are no intermolecular contacts of note in
either structure.

\subsection{Catalytic evaluation for ethylene polymerization}
\label{ssec32}

To explore the performance of \textbf{Co1}--\textbf{Co5} as
precatalysts for ethylene polymerization, side-by-side investigations
were conducted using MAO (Table~\ref{tab2}) and MMAO (Table~\ref{tab3})
as activators. All polymerizations were conducted in toluene with the
ethylene pressure initially fixed at 10 atm and the run time at 30~min.
Reaction parameters including run temperature, molar ratio of aluminum
to cobalt, and reaction time were subject to a systematic
investigation. The resulting polyethylenes were characterized by
differential scanning calorimetry (DSC) and gel permeation
chromatography (GPC). Furthermore, high-temperature \tsup{13}C NMR
spectroscopy was undertaken on selected polyethylene samples in order
to provide insight on their microstructural properties. As a matter of
course, gas chromatography was performed on post-reaction mixtures
which, in all cases, gave no evidence of any oligomeric fractions.

\subsubsection{Evaluation of \textbf{Co1}--\textbf{Co5}/MAO as ethylene
polymerization catalysts}\label{sssec321}

To pinpoint an effective set of reaction conditions to screen all five
complexes with MAO, \textbf{Co1} was firstly employed as the test precatalyst
to ascertain the optimal temperature, Al:Co molar ratio, and run time.
Firstly, with the Al:Co ratio set at 2500, the reaction temperature was
changed from 40 to 80~\textdegree C (entries~1--5, Table~\ref{tab2}),
revealing the peak activity of $11.05\times 
10^{6}$~(g~of~PE)${\cdot}$(mol of Co)$^{-1}{\cdot}$h$^{-1}$ to {occur} at
60~\textdegree C (entry 3, Table~\ref{tab2}). On further \mbox{increasing} the
temperature, the level of activity dropped to $2.74\times 10^{6}$  (g
of PE)${\cdot}$(mol of Co)$^{-1}{\cdot}$h$^{-1}$ at 80~\textdegree C 
(entries 5, Table~\ref{tab2}). This finding can likely be accredited to
both decomposition of the active species and the lower concentration of
ethylene in toluene at higher temperature~\cite{16}. Meanwhile, as the
reaction temperature increased, the molecular weight of the resulting
polyethylene decreased from 16.5 down to 5.3~kg${\cdot}$mol$^{-1}$,
which can be accounted for by a higher rate of chain termination as the
temperature was raised. Nevertheless, all polymers generated under
these conditions {exhibited} molecular weights ($M_{\mathrm{w}}$ range:
16.5--5.3 kg mol$^{-1}$) that can classify them best as polyethylene
waxes. The effects of reaction temperature on activity and polymer
molecular weight using \textbf{Co1}/MAO are further displayed in
Figure~\ref{fig3}a, while the corresponding GPC traces are collected
in Figure~\ref{fig3}b.

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}For \textbf{Co1}/MAO: (a) dual plots of polymer
molecular weight and catalytic activity as a function of run
temperature and (b) GPC traces showing variations of log
$M_{\mathrm{w}}$ of the polymer as the run temperature is varied
(entries 1--5, Table~\ref{tab2}).}
\end{figure*}

%tab2
\begin{table*}
\caption{\label{tab2}Ethylene polymerization results using
\textbf{Co1}--\textbf{Co5}/MAO at $P_{\mathrm{C}_{2}\mathrm{H}_{4}}=10$
atm$^{\mathrm{a}}$}
\tabcolsep=4pt
\begin{tabular}{cccccccccc}
\thead
Entry & Pre-cat. & $T$ (\textdegree C) & Al:Co & $t$ (min) & Mass of PE
(g) &  Activity$^{\mathrm{b}}$ & $M_{\mathrm{w}}\,{}^{\mathrm{c}}$
(kg${\cdot}$mol$^{-1}$) &
$M_{\mathrm{w}}/M_{\mathrm{n}}\,{}^{\mathrm{c}}$ &
$T_{\mathrm{m}}\,{}^{\mathrm{d}}$ (\textdegree C) \\ 
\endthead
1 & \textbf{Co1} & 40 & 2500 & 30 & \04.25 & \04.25 & 16.5 & 2.1 & 135.4 \\ 
2 & \textbf{Co1} & 50 & 2500 & 30 & 10.01  & 10.01 & 11.8 & 2.2 & 130.1 \\ 
3 & \textbf{Co1} & 60 & 2500 & 30 & 11.05  & 11.05 & 10.2 & 2.2 & 129.7 \\ 
4 & \textbf{Co1} & 70 & 2500 & 30 & \04.30 & \04.30 & \07.6 & 2.2 & 129.1 \\ 
5 & \textbf{Co1} & 80 & 2500 & 30 & \02.74 & \02.74 & \05.3 & 2.2 & 127.9 \\ 
6 & \textbf{Co1} & 60 & 1500 & 30 & \08.22 & \08.22 & 11.4 & 2.3 & 130.5 \\ 
7 & \textbf{Co1} & 60 & 2000 & 30 & 14.74  & 14.74 & 10.4 & 2.2 & 130.4 \\ 
8 & \textbf{Co1} & 60 & 3000 & 30 & 10.51  & 10.51 & 10.3 & 2.2 & 129.8 \\ 
9 & \textbf{Co1} & 60 & 3500 & 30 & \07.50 & \07.50 & 10.7 & 2.2 & 130.1 \\ 
10 & \textbf{Co1} & 60 & 2000 & \05 & \05.36 & 32.16 & \09.5 & 2.3 & 129.5 \\ 
11 & \textbf{Co1} & 60 & 2000 & 15 & \09.96  & 19.92 & 10.8 & 2.1 & 130.6 \\ 
12 & \textbf{Co1} & 60 & 2000 & 45 & 16.96   & 11.31 & 12.8 & 2.2 & 130.1 \\ 
13 & \textbf{Co1} & 60 & 2000 & 60 & 18.47   & \09.24 & 13.7 & 2.2 & 130.8 \\ 
14$^{\mathrm{e}}$ & \textbf{Co1} & 60 & 2000 & 30 & 10.21 & 10.21 & \09.9 & 2.3 & 129.5 \\ 
15$^{\mathrm{f}}$ & \textbf{Co1} & 60 & 2000 & 30 & \00.84 & \00.84 & \06.4 & 2.4 & 127.8 \\ 
16 & \textbf{Co2} & 60 & 2000 & 30 & 12.57 & 12.57 & 18.6 & 2.3 & 131.4 \\ 
17 & \textbf{Co3} & 60 & 2000 & 30 & \08.55 & \08.55 & 34.2 & 2.1 & 131.6 \\ 
18 & \textbf{Co4} & 60 & 2000 & 30 & 14.60 & 14.60 & 13.0 & 2.2 & 130.3 \\ 
19 & \textbf{Co5} & 60 & 2000 & 30 & \09.34 & \09.34 & 20.4 & 2.2 & 131.5
\botline
\end{tabular}
\tabnote{$^{\mathrm{a}}$Reaction conditions: 2.0~${\upmu}$mol of cobalt precatalyst, 100~mL toluene, 10 atm ethylene; 
$^{\mathrm{b}}$10\tsup{6} (g of PE)${\cdot}$(mol of Co)$^{-1}{\cdot}$h$^{-1}$;
$^{\mathrm{c}}$measured using GPC; 
$^{\mathrm{d}}$measured using DSC;
$^{\mathrm{e}}$ethylene pressure ${=}$\ 5 atm; 
$^{\mathrm{f}}$ethylene pressure ${=}$\ 1 atm.}
\end{table*}

With the run temperature maintained at the optimal 60~\textdegree C,
the Al:Co molar ratio using \textbf{Co1}/MAO was increased from 1500 to 3500;
the results are collected in Table~\ref{tab2} and illustrated in
Figure~\ref{fig4}. On increasing the ratio, the highest activity of
$14.74\times 10^{6}$~(g~of~PE)${\cdot}$(mol of
Co)$^{-1}{\cdot}$h$^{-1}$ was achieved with 2000~molar equivalents
of MAO (entry 7, Table~\ref{tab2}). However, on further raising the
Al:Co molar ratio to 3500, the catalytic activity dropped by nearly
half to $7.50\times 10^{6}$ (g of PE)${\cdot}$(mol of
Co)$^{-1}{\cdot}$h$^{-1}$.  In terms of the polymer molecular
weight, this remained within a narrow range as the molar ratio was
increased, although the slight drop from 11.4 kg${\cdot}$mol$^{-1}$ at
1500 to 10.7~kg${\cdot}$mol$^{-1}$ at 3500 may suggest the onset of
some chain transfer from the cobalt to aluminum~\cite{17a,17b,17c,17d}.

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}For \textbf{Co1}/MAO: (a) dual plots of polymer
molecular weight and catalytic activity as a function of Al:Co molar
ratio and (b) GPC traces showing the modest effect of Al:Co molar ratio
on log $M_{\mathrm{w}}$ (entries 3 and 6--9, Table~\ref{tab2}).} 
\end{figure*}

%tab3
\begin{table*}
\caption{\label{tab3}Ethylene polymerization results using
\textbf{Co1}--\textbf{Co5}/MMAO at $P_{\mathrm{C}_{2}\mathrm{H}_{4}}=10$
atm$^{\mathrm{a}}$}
\tabcolsep=2.5pt
\begin{tabular}{cccccccccc}
\thead
Entry & Precat & $T$ (\textdegree C) & Al:Co & $t$ (min) & Mass of PE
(g) & Activity$^{\mathrm{b}}$ & $M_{\mathrm{w}}\,{}^{\mathrm{c}}$
(kg${\cdot}$mol$^{-1}$) &
$M_{\mathrm{w}}/M_{\mathrm{n}}\,{}^{\mathrm{c}}$ &
$T_{\mathrm{m}}\,{}^{\mathrm{d}}$ (\textdegree C) \\ 
\endthead
1 & \textbf{Co1} & 20 & 2500 & 30 & 3.50 & 3.50 & 38.7 & 2.2 & 132.8 \\ 
2 & \textbf{Co1} & 30 & 2500 & 30 & 3.93 & 3.93 & 27.8 & 2.3 & 132.2 \\ 
3 & \textbf{Co1} & 40 & 2500 & 30 & 2.68 & 2.68 & 17.3 & 2.3 & 130.7 \\ 
4 & \textbf{Co1} & 50 & 2500 & 30 & 1.82 & 1.82 & 14.2 & 2.4 & 130.3 \\ 
5 & \textbf{Co1} & 60 & 2500 & 30 & 1.59 & 1.59 & 13.5 & 2.4 & 130.0 \\ 
6 & \textbf{Co1} & 30 & 2000 & 30 & 3.25 & 3.25 & 29.3 & 2.4 & 131.7 \\ 
7 & \textbf{Co1} & 30 & 3000 & 30 & 4.55 & 4.55  & 31.5 & 2.3 & 132.0 \\ 
8 & \textbf{Co1} & 30 & 3500 & 30 & 3.58 & 3.58 & 29.7 & 2.4 & 132.2 \\ 
9 & \textbf{Co1} & 30 & 4000 & 30 & 3.24 & 3.24 & 29.0 & 2.2 & 132.0 \\ 
10 & \textbf{Co1} & 30 & 3000 & \05 & 2.29 & 13.74\0 & 29.4 & 2.5 & 132.0 \\ 
11 & \textbf{Co1} & 30 & 3000 & 15 & 2.88 & 4.32 & 29.7 & 2.3 & 131.9 \\ 
12 & \textbf{Co1} & 30 & 3000 & 45 & 5.39 & 3.59 & 31.9 & 2.2 & 132.3 \\ 
13 & \textbf{Co1} & 30 & 3000 & 60 & 5.87 & 2.94 & 33.0 & 2.2 & 132.0 \\ 
14$^{\mathrm{e}}$ & \textbf{Co1} & 30 & 3000 & 30 & 2.91 & 2.91 & 29.6 & 2.3 & 132.4 \\ 
15$^{\mathrm{f}}$ & \textbf{Co1} & 30 & 3000 & 30 & 1.05 & 1.05 & 29.4 & 2.4 & 131.8 \\ 
16 & \textbf{Co2} & 30 & 3000 & 30 & 3.55 & 3.55  & 29.6 & 2.3 & 133.3 \\ 
17 & \textbf{Co3} & 30 & 3000 & 30 & 3.32 & 3.32  & 32.0 & 2.3 & 133.9 \\ 
18 & \textbf{Co4} & 30 & 3000 & 30 & 4.07 & 4.07  & 30.6 & 2.1 & 132.4 \\ 
19 & \textbf{Co5} & 30 & 3000 & 30 & 3.35 & 3.35  & 30.1 & 2.1 & 133.3
\botline
\end{tabular}
\tabnote{
$^{\mathrm{a}}$Reaction conditions: 2.0~${\upmu}$mol of cobalt precatalyst, 100~mL toluene, 10 atm ethylene; 
$^{\mathrm{b}}$10\tsup{6} (g of PE)${\cdot}$(mol of Co)$^{-1}{\cdot}$h$^{-1}$;
$^{\mathrm{c}}$measured using GPC; 
$^{\mathrm{d}}$measured using DSC; 
$^{\mathrm{e}}$ethylene pressure ${=}$\ 5 atm; 
$^{\mathrm{f}}$ethylene pressure ${=}$\ 1 atm.}
\end{table*}

\begin{figure*}
\includegraphics{fig05}
{\vspace*{-2pt}}
\caption{\label{fig5}For \textbf{Co1}/MAO: (a) dual plots of polymer molecular
weight and catalytic activity as a function of run time and (b) GPC
traces showing the effect of run time on log $M_{\mathrm{w}}$ of the
polymer (entries 7 and 10--13, Table~\ref{tab2}).} 
{\vspace*{-2pt}}
\end{figure*}

The influence of reaction time on the polymerization behavior of
\textbf{Co1}/MAO and the lifetime of the active species was then investigated
with the reaction temperature fixed at 60~\textdegree C and the Al:Co
molar ratio at 2000. Specifically, the tests were run over 5, 15, 30,
45, and 60 min (entries 7 and 10--13, Table~\ref{tab2}) and revealed
the maximum activity of $32.16\times 10^{6}$~(g~of~PE)${\cdot}$(mol of
Co)$^{-1}{\cdot}$h$^{-1}$ to be detected within 5 min (entries 10,
Table~\ref{tab2}). After this initial spike in activity, the level
slowly decreased to only $9.24\times 10^{6}$~(g~of~PE)${\cdot}$(mol of
Co)$^{-1}{\cdot}$h$^{-1}$ after 1 h (entry 13, Table~\ref{tab2}),
which suggests that \textbf{Co1}/MAO displayed good stability and appreciable
lifetime. \mbox{Evidently,} the active species was quickly generated upon
addition of MAO and then underwent a steady deactivation over
time~\cite{18a,18b}. As for the polymer molecular weight, this was found to
slowly increase over reaction time (Figure~\ref{fig5}), which
demonstrated that the active species remained potent
throughout~\cite{19a,19b}.

With the optimum polymerization conditions established for
\textbf{Co1}/MAO as 60~\textdegree C (run temperature), 2000 (Al:Co
molar ratio), and 30 min (run time), \textbf{Co2}--\textbf{Co5} were
then investigated similarly (entries 16--19, Table~\ref{tab2}). All
precatalysts exhibited high activity and thermal stability with the
activity falling in the order: \textbf{Co1} (2,6-di(Me)) ${>}$
\textbf{Co4} (2,4,6-tri(Me)) ${>}$ \textbf{Co2} (2,6-di(Et)) ${>}$
\textbf{Co5} (2,6-di(Et)-4-Me) ${>}$ \textbf{Co3} (2,6-di($^{i}$Pr)).
Evidently, the steric properties of the precatalyst affect the
performance with more sterically bulky \textit{ortho}-groups impeding
coordination and insertion of ethylene, resulting in lower
activity~\cite{17a}. By contrast, the bulkiest precatalyst \textbf{Co3}
afforded the polymer exhibiting the highest molecular weight (34.2
kg${\cdot}$mol$^{-1}$, entry 17, Table~\ref{tab2}) of the series, where
the steric properties have the additional role of hindering chain
termination pathways leading to polyethylene of relatively high
molecular weight~\cite{17b}. The effects of precatalyst type on
activity and polymer molecular weight are further displayed in
Figure~\ref{fig6}a, while the corresponding GPC traces are collected
in Figure~\ref{fig6}b.

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig06}
{\vspace*{-2pt}}
\caption{\label{fig6}(a) Bar chart displaying catalytic activity and
polymer molecular weight with respect to the cobalt precatalyst and (b)
GPC traces showing log $M_{\mathrm{w}}$ as a function of the
precatalyst (entries 7 and 16--19, Table~\ref{tab2}); all runs
conducted with MAO as activator at
$P_{\mathrm{C}_{2}\mathrm{H}_{4}}=10$ atm.}
\end{figure*}

\subsubsection{Evaluation of \textbf{Co1}--\textbf{Co5}/MMAO as
ethylene polymerization catalysts}\label{sssec322} 

With MMAO now employed as the activator, \textbf{Co1} was again employed to
optimize the conditions of the polymerization. As with MAO, this
initial study focused on the effects of run temperature, Al:Co molar
ratio, and reaction time; the complete set of results are gathered in
Table~\ref{tab3}. With the Al:Co molar ratio firstly set at 2500, the
polymerization temperature was increased from 20 to 60~\textdegree C
(entries 1--5, Table~\ref{tab3}). In this case, the highest activity of
$3.93\times 10^{6}$ (g of PE)${\cdot}$(mol of
Co)$^{-1}{\cdot}$h$^{-1}$  was observed at 30~\textdegree C rather
than 60~\textdegree C as with MAO, highlighting the importance of the
activator on the active catalyst's temperature stability. With regard
to the polyethylene, molecular weights gradually lowered as the
reaction temperature was increased (Figure~\ref{fig7}), which can be
accounted for by the faster chain termination at higher 
temperature~\cite{15}. In comparison with the results obtained using
MAO, the polyethylenes produced using \textbf{Co1}/MMAO generally showed
slightly higher molecular weights but possessed similarly narrow
dispersities ($M_{\mathrm{w}}/M_{\mathrm{n}} \leq 2.4$). Additionally,
\textbf{Co1}/MMAO showed in general lower catalytic activities, which could
plausibly derive from the differences between MMAO and MAO, and their
effects on the active species. \looseness=-1

\begin{figure*}
\includegraphics{fig07}
{\vspace*{-2pt}}
\caption{\label{fig7}For \textbf{Co1}/MMAO: (a) dual plots of polymer
molecular weight and catalytic activity as a function of run
temperature and (b) GPC traces showing the effect of run temperature on
the log $M_{\mathrm{w}}$ of the polymer (entries 1--5,
Table~\ref{tab3}).} 
{\vspace*{-2pt}}
\end{figure*}

Next, the Al:Co molar ratio using \textbf{Co1}/MMAO was altered with the run
temperature maintained at 30~\textdegree C (entries 2 and 6--9,
Table~\ref{tab3}) leading to the highest activity of  $4.55\times
10^{6}$ (g of PE)${\cdot}$(mol of Co)$^{-1}{\cdot}$h$^{-1}$ being
achieved with a ratio of 3000 (entry 7, Table~\ref{tab3}). As with
\textbf{Co1}/MAO, the catalytic activity gradually declined at higher ratios,
while polymer molecular weight remained relatively constant
($M_{\mathrm{w}}$ range: 27.8--31.5 kg mol$^{-1}$) but with a
perceptible decrease especially at higher ratios.

On modifying the reaction time from 5 to 60~min (entries 7 and 10--13,
Table~\ref{tab3}), the activity of \textbf{Co1}/MMAO reached a maximum of
$13.74\times 10^{6}$ (g of PE)${\cdot}$(mol of
Co)$^{-1}{\cdot}$h$^{-1}$ after 5 min and then revealed a gradual
decrease to $2.94\times 10^{6}$ (g of PE)${\cdot}$(mol of
Co)$^{-1}{\cdot}$h$^{-1}$ after 60 min, a finding suggesting that
the active species was formed quickly but then underwent deactivation
as time elapsed~\cite{17c}. The effect of ethylene pressure was also
explored, with the activity of \textbf{Co1}/MMAO found to fall as the pressure
was lowered from 10 atm ($4.55\times 10^{6}$
(g of PE)${\cdot}$(mol of Co)$^{-1}{\cdot}$h$^{-1}$; entry 7) to 5 atm 
($2.91\times 10^{6}$ (g of PE)${\cdot}$(mol of Co)$^{-1}{\cdot}$h$^{-1}$;
entry 14) to 1 atm ($1.05\times 10^{6}$ (g of PE)${\cdot}$(mol of Co)$^{-1}{\cdot}$h$^{-1}$; entry 15); similar
effects were seen with MAO and reflect the importance of ethylene
pressure to chain propagation in these systems.

\begin{figure*}
\includegraphics{fig08}
{\vspace*{1pt}}
\caption{\label{fig8}(a) Bar chart displaying catalytic activity and
molecular weight of the polymer with respect to the cobalt precatalyst
and (b) GPC traces showing log $M_{\mathrm{w}}$ as a function of the
precatalyst (entries 7 and 16--20, Table~\ref{tab3}); all runs
performed using MMAO as activator and 
$P_{\mathrm{C}_{2}\mathrm{H}_{4}}=10$ atm.}
\end{figure*}

\begin{figure*}
\includegraphics{fig09}
\caption{\label{fig9}\tsup{13}C NMR spectrum of the PE sample produced
using \textbf{Co1}/MAO as catalyst at 60~\textdegree C (entry 7,
Table~\ref{tab2}); recorded at 100~\textdegree C in
1,1,2,2-tetrachloroethane-$d_{2}$.}
\end{figure*}

With the optimal conditions for \textbf{Co1}/MMAO now in place (run
temperature ${=}$\ 30~\textdegree C, Al:Co molar ratio ${=}$\ 3000),
the remaining precatalysts \textbf{Co2}--\textbf{Co5} were evaluated
accordingly. In terms of catalytic activity, the trend essentially
mimics that seen with MAO: \textbf{Co1} (2,6-di(Me)) ${>}$ \textbf{Co4}
(2,4,6-tri(Me)) ${>}$ \textbf{Co2} (2,6-di(Et)) ${>}$ \textbf{Co5}
(2,6-di(Et)-4-Me) ${\sim}$ \textbf{Co3} (2,6-di($^{i}$Pr)). Again, the
\textit{ortho}-methyl precatalysts \textbf{Co1} and \textbf{Co4}
\mbox{exhibit} higher catalytic activity than their bulkier
\textit{ortho}-ethyl or \textit{ortho}-isopropyl counterparts
(\textbf{Co2}, \textbf{Co3}, \textbf{Co5}); this observation is
consistent with that observed with MAO. In all cases, the {dispersity}
of the polyethylene is narrow ($M_{\mathrm{w}}/M_{\mathrm{n}}$ range:
2.4--2.1), which is also borne out in the GPC traces
(Figure~\ref{fig8}), which supports the single-site-like nature of the
active species.

\begin{figure*}
{\vspace*{3pt}}
\includegraphics{fig10}
{\vspace*{3pt}}
\caption{\label{fig10}Comparison of thermal stability, catalytic
activity, polyethylene molecular weight, and dispersity for
(\textbf{B}$_{\mathrm{Mes}}$)CoCl\tsub{2},
(\textbf{C}$_{\mathrm{Mes}}$)CoCl\tsub{2},
(\textbf{D}$_{\mathrm{Mes}}$)CoCl\tsub{2}, and
(\textbf{E}$_{\mathrm{Mes}}$)CoCl\tsub{2} (Chart~\ref{ch1}) with the
current precatalyst  (\textbf{F}$_{\mathrm{Mes}}$)CoCl\tsub{2}
(\textbf{Co4}); all polymerization runs were performed at 
$P_{\mathrm{C}_{2}\mathrm{H}_{4}}=10$ atm under their optimal
conditions.}
{\vspace*{3pt}}
\end{figure*}

\subsection{Microstructural analysis of the polyethylenes}
\label{ssec33}

As can be gathered from Tables~\ref{tab2} and~\ref{tab3}, all
polyethylenes display melting temperatures ($T_{\mathrm{m}}$) in excess
of 127~\textdegree C, values that are typical of \mbox{linear} high-density
polyethylene. To corroborate this assertion, two representative samples
obtained using \textbf{Co1}/MAO at 60~\textdegree C (entry 7,
Table~\ref{tab2}; $M_{\mathrm{w}}=10.4$~kg${\cdot}$mol$^{-1}$) and
\textbf{Co1}/MMAO at 30~\textdegree C (entry 7, Table~\ref{tab3};
$M_{\mathrm{w}}=31.5$ kg${\cdot}$mol$^{-1}$) were subjected to
high-temperature \tsup{13}C NMR spectroscopy (recorded at
100~\textdegree C in 1,1,2,2-tetrachloroethane-$d_{2}$, see
Figure~\ref{fig9} and Figure~S3). For the sample obtained using
\textbf{Co1}/MAO at 60~\textdegree C, a characteristic singlet resonance at
${\delta}$ 30.00 corresponds to the methylene repeat unit
(--CH\tsub{2}--) for a linear polyethylene~\cite{15}; the absence of
additional peaks corresponding to chain ends is presumably attributable
to the relatively high molecular weight of the sample.

\subsection{Comparison of the current cobalt catalysts with previously
reported examples}\label{ssec34}

To help contextualize the findings in this work, we have extracted key
performance data for mesityl-containing \textbf{Co4}/MAO
[(\textbf{F}$_{\mathrm{Mes}}$)CoCl\tsub{2}] obtained herein (entry 18,
Table~\ref{tab2}) and assembled this alongside those previously
reported for (\textbf{B}$_{\mathrm{Mes}}$)CoCl\tsub{2},
(\textbf{C}$_{\mathrm{Mes}}$)CoCl\tsub{2},
(\textbf{D}$_{\mathrm{Mes}}$)CoCl\tsub{2}, and
(\textbf{E}$_{\mathrm{Mes}}$)CoCl\tsub{2} (Figure~\ref{fig10}); 
all tests were obtained
under optimized conditions at $P_{\mathrm{C}_{2}\mathrm{H}_{4}}=10$ 
atm using MAO~\cite{11a,11b,11c,11d}. All five systems are structurally related with
(\textbf{B}$_{\mathrm{Mes}}$)CoCl\tsub{2},
(\textbf{D}$_{\mathrm{Mes}}$)CoCl\tsub{2},
(\textbf{E}$_{\mathrm{Mes}}$)CoCl\tsub{2}, and
(\textbf{F}$_{\mathrm{Mes}}$)CoCl\tsub{2} all incorporating
{$N$}-2-Me-4,6-bis(4,4$'$-X\tsub{2}dibenzhydryl)phenyl (X ${=}$\ H, F,
OMe, Cl) substitution while (\textbf{C}$_{\mathrm{Mes}}$)CoCl\tsub{2},
{$N$}-2,4-diMe-6-dibenzhydrylphenyl substitution. For the most closely
related systems (\textbf{B}$_{\mathrm{Mes}}$)CoCl\tsub{2},
(\textbf{D}$_{\mathrm{Mes}}$)CoCl\tsub{2},
(\textbf{E}$_{\mathrm{Mes}}$)CoCl\tsub{2}, and
(\textbf{F}$_{\mathrm{Mes}}$)CoCl\tsub{2}, it is evident that
chloro-containing (\textbf{F}$_{\mathrm{Mes}}$)CoCl\tsub{2} is the most
active ($14.6\times 10^{6}$
(g of PE)${\cdot}$(mol of Co)$^{-1}{\cdot}$h$^{-1}$;  entry 18,
Table~\ref{tab2}) but forms the lowest molecular weight polymer
($M_{\mathrm{w}}=13.0$ kg${\cdot}$mol$^{-1}$; entry 18,
Table~\ref{tab2}). Conversely,
(\textbf{E}$_{\mathrm{Mes}}$)CoCl\tsub{2} bearing an electron donating
methoxy-substituent showed moderate activity, but the \mbox{resulting}
polyethylene afforded the highest molecular weight
($M_{\mathrm{w}}=274.0$  kg${\cdot}$mol$^{-1}$) and broad
dispersity~\cite{11d}. Evidently, these results underline the
electronic influence played by the \textit{para}-groups in the
benzhydryl group on catalytic performance. By comparison,
(\textbf{C}$_{\mathrm{Mes}}$)CoCl\tsub{2} showed the lowest activity
($2.1\times 10^{6}$ (g of PE)${\cdot}$(mol of Co)$^{-1}{\cdot}$h$^{-1}$)
\cite{11b}, which could be the result of a combination of both steric
and electronic effects. Nevertheless, all these classes of
benzhydryl-substituted bis(arylimino)pyridyl--cobalt catalyst impart
excellent control as evidenced by the single-site-like behavior
($M_{\mathrm{w}}/M_{\mathrm{n}}$ range: 2.2--2.6).

\section{Conclusions}\label{sec4}

In summary, five examples of unsymmetrical
2,6-bis(arylimino)pyridine-cobalt complexes (\textbf{Co1}--\textbf{Co5}),
incorporating one
{$N$}-2,4-bis(di(4-chlorophenyl)methyl)-6-methylphenyl group and one
variable {$N$}-aryl group, have been successfully synthesized from
their corresponding free $N',N,N''$-ligands. All complexes were formed
in high yield and fully characterized including by single crystal XRD
for \textbf{Co1} and \textbf{Co2}. Under activation with MAO and MMAO, \textbf{Co1}--\textbf{Co5}
all demonstrated their ability to promote ethylene polymerization with
high activities reaching up to  $14.74\times 10^{6}$
(g of PE)${\cdot}$(mol of Co)$^{-1}{\cdot}$h$^{-1}$ for \textbf{Co1}/MAO. Moreover,
they could deliver this at an appreciable operating temperature of
60~\textdegree C, which is notably higher than that seen in previously
reported related examples. Furthermore, the polymerizations are well
controlled as is evidenced by narrow molecular-weight distributions.
Comparison with a series of structurally related benzhydryl-containing
cobalt catalysts highlights the important role of the
\textit{para}-chloride group. Overall, this is a rare example of a
cobalt-based catalyst system that possesses the combined properties of
high thermal stability and high activity producing highly linear
polyethylene. This also represents an excellent demonstration of the
capacity of rational ligand design to improve the performance of
ethylene polymerization catalysts.

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Funding}

GAS is grateful to the Chinese Academy of Sciences for a President's
International Fellowship for Visiting Scientists (grant no.
2025PVB0034).

\CDRGrant[CAS]{2025PVB0034}

\section*{Underlying data}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.  

The data underlying the article can be obtained from the corresponding
author.

CCDC-2464596 for \textbf{Co1} and 2464597 for \textbf{Co2} contain the
supplementary crystallographic data for this article; these data can be
obtained free of charge via
\url{http://www.ccdc.cam.ac.uk/conts/retrieving.html}.


\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-422-suppl.pdf}}

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