2.1 Synthesis and spectroscopic studies

We have previously shown that the reaction of sodium with Bu^tPCl₂ and PCl₃ in the ratio 12:4:1 in THF gives a mixture of Na[cyclo-(P₃Bu^t₂)] [5a,6b], Na[cyclo-(P₄Bu^t₃)] [6b], cyclo-(P₄Bu^t₄) [10], Na₂(P₄Bu^t₄) [5a] and sodium tetra-tert-butylcyclopentaphosphanide Na[cyclo-(P₅Bu^t₄)]. The last complex was obtained in reasonable yield after optimization of this reaction [7]. Analogously, K[cyclo-(P₅Bu^t₄)] was prepared, but it was very difficult to separate from K₂(P₄Bu^t₄) and thus always obtained in low yield [2f]. When lithium is employed, a mixture of cyclo-(P₄Bu^t₄) [10], Li₂(P₄Bu^t₄), and lithium tetra-tert-butylcyclopentaphosphanide Li[cyclo-(P₅Bu^t₄)] (1) is formed after 6 hours at 80 °C; keeping the reaction mixture at this temperature increased the amount of 1 (to 90% according to ³¹P{¹H} NMR after 1 d, or 95% after 2 d) (Scheme 1). Isolation and recrystallization from THF/TMEDA gave [Li(tmeda)₂][cyclo-(P₅Bu^t₄)] (1b) in good yield. The ³¹P{¹H} NMR spectrum of 1b shows three sets of signals which correspond to an ABB’CC’ spin system. Chemical shifts and coupling constants were successfully calculated by using the program SPINWORKS [11]. The coupling constants are similar to those observed for the analogous Na and K compounds (Table 1)¹.

According to the spectroscopic data and based on the steric requirements of the tert-butyl groups, only one of the four conformations, the all-trans isomer, is preferred as was previously observed for related compounds [7,8,9] and confirmed by X-ray diffraction studies on 1b (Section 2.2). Additionally, the ¹H NMR and ¹³C{¹H} NMR spectra of 1b showed the expected signals.

2.2 Structural studies

Orange crystals of 1b were obtained from THF/TMEDA solution at –28 °C. Compound 1b crystallizes in the noncentrosymmetric monoclinic space group Cc with four molecules in the unit cell. Selected bond lengths and angles for 1b are given in Table 2.

Compound 1b consists of separated [Li(tmeda)₂]⁺ cations and [cyclo-(P₅Bu^t₄)]⁻ anions (Fig. 1a). The lithium cation is coordinated in an usual distorted tetrahedral fashion by four nitrogen atoms of two TMEDA molecules (Li–N 210.4(8) to 214.4(8) pm, see for example [12]), which prevents interaction of the cation with the anion (Fig. 1b) and thus results in a “naked” [cyclo-(P₅Bu^t₄)]⁻ anion (Li(1)⋯P(1) 494.0(6) pm). This type of anionic two-coordinate phosphorus atom is typical in metal complexes of white phosphorus (see for example [13] and references therein), but rare for linear or cyclic oligophosphanides [2,3,7,9,14]. Only a few examples display an ion-separated structure, such as [K(18-crown-6)(thf)₂][P₄HMes₄] (Mes = 2,4,6-Me₃C₆H₂), in which the potassium cation is coordinated by 18-crown-6 and two THF molecules [3a], and the lithium phosphanides [Li(12-crown-4)₂][PPh₂] and [Li(12-crown-4)₂][PMes(BMes₂)] [14]. In known lithium oligophosphanides the coordination of only one TMEDA molecule and a phosphorus atom was observed [2i], rather than the coordination of two TMEDA molecules as in 1b.

The [cyclo-(P₅Bu^t₄)]⁻ anions show an all-trans arrangement of the tert-butyl groups on the phosphorus atoms with an envelope conformation of the chiral anionic P₅ ring (best plane is defined by P(1)–P(3)–P(4)–P(5), deviation of P(2) from this plane is 87.5 pm (P(1) is the anionic phosphorus atom). This conformation is different from those found in [Na(thf)₄][cyclo-(P₅Bu^t₄)] [7] and [K(pmedta)(thf)][cyclo-(P₅Bu^t₄)] [2g], in which the neutral phosphorus atoms are almost coplanar, while the anionic phosphorus atom lies about 80 pm above this plane. The P(1)–P(5) bond length of 212.5(2) pm in 1b is about 8 pm shorter than the other P–P bonds, which have values ranging from 220.7(2) to 222.6(2) pm (Table 2), validating the larger coupling constant ¹J(P_A−P_B) = ¹J(P_A−P_B’) of −394.9(1) Hz observed in the ³¹P{¹H} NMR spectrum. A comparison of selected structural parameters of alkali and coinage metal complexes containing the cyclo-P₅Bu^t₄⁻ ligand is given in Table 3.

4.1 General methods

All experiments were performed under an atmosphere of dry argon using standard Schlenk techniques. The NMR spectra were recorded at 25 °C on a Bruker AVANCE-DRX-400 spectrometer. ¹H NMR (400.13 MHz) and ¹³C NMR (100.16 MHz): internal standard solvent, external standard TMS; ³¹P NMR (161.9 MHz): external standard 85% H₃PO₄. FAB mass spectra were recorded in a MASPEC II spectrometer with 3-nitrobenzyl alcohol as matrix. IR spectrum: a KBr pellet containing 1b was prepared in a nitrogen-filled glove box and the spectrum was recorded on a Perkin-Elmer System 2000 FTIR spectrometer in the range 350–4000 cm⁻¹. All solvents were purified by distillation, dried, saturated with argon, and stored over potassium mirror. Elemental analyses were performed on a VARIO EL (Heraeus). The melting point was determined in a sealed capillary under argon and is uncorrected. Bu^tPCl₂ was prepared according to the literature procedure [15].

4.2 Data collection and structure refinement of 1b

The data of 1b were collected on a CCD Oxford Xcalibur S (λ(Mo_Kα) = 71.073 pm) using ω and φ scans mode. Semi-empirical from equivalents absorption corrections were carried out with SCALE3 ABSPACK [16]. The structures were solved with direct methods [17]. Structure refinement was carried out with SHELXL-97 [17]. All non-hydrogen atoms were refined anisotropically and hydrogen atoms were placed in calculated positions and refined with calculated isotropic displacement parameters. Table 4 lists crystallographic details. CCDC 768441 (1b) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

4.3 Synthesis of [Li(tmeda)₂][cyclo-(P₅Bu^t₄)] (1b)

A solution of PCl₃ (2.89 g, 0.021 mol) and Bu^tPCl₂ (13.35 g, 0.084 mol) in THF (50 mL) was carefully added at 0 °C to lithium sand prepared by trituration of Li (1.75 g, 0.25 mol) in THF (300 mL). The reaction mixture was then warmed to room temperature and heated under reflux for 2 d. After 1 h, the initially colorless solution had turned yellow-green, and it changed to dark brown after one day. THF was then removed under reduced pressure and the brown residue extracted with n-hexane (450 mL) to give a green-brown solution. The solvent was removed under reduced pressure to give a brown oily solid (Li[cyclo-(P₅Bu^t₄)] (1), 8.20 g, 85%). Recrystallization of this solid from THF/TMEDA (10/1) gave orange needles of [Li(tmeda)₂][cyclo-(P₅Bu^t₄)] (1b; m.p. 139.3–141.6 °C) at −27 °C. Yield of 1b 7.32 g (56%); IR (KBr): 2951 (s), 2889 (s), 2857 (s), 2368 (w), 2287 (w), 1908 (w), 1648 (w), 1458 (s), 1386 (m), 1359 (s), 1261 (s), 1169 (s), 1046 (s), 897 (m), 806 (s), 667 (w), 559 (m), 500 (w), 481 (m), 440 (w), 406 (w); ¹H NMR (400 MHz, d₈-THF, 25 °C): δ = 1.21 (d, ³J(P,H) = 3.0 Hz, 9H; Bu^t), 1.26 (d, ³J(P,H) = 3.0 Hz, 9H; Bu^t), 1.29 (d, ³J(P,H) = 3.0 Hz, 18H; Bu^t), 2.16 (s, 24H, Me of TMEDA), 2.31 (s, 8H, CH₂ of TMEDA); ¹³C{¹H} NMR (100.6 MHz, d₈-THF, 25 °C): δ = 30.4 (m, C(CH₃)₃), 31.1 (m, C(CH₃)₃), 32.6 (br, C(CH₃)₃), 33.4 (br, C(CH₃)₃), 41.8 (s, Me of TMEDA); 56.7 (s, CH₂ of TMEDA). ³¹P{¹H} NMR (Table 1); FAB MS, matrix: 3-NBA; m/z (%): 383.1 (19.7) [M⁺ − Li(tmeda)₂], 327.1 (100.0) [M⁺ − Li(tmeda)₂ − Bu^t + H], 270.1 (18.5) [M⁺ − Li(tmeda)₂ − 2 × Bu^t + H], 239.0 (14.1) [M⁺ − cyclo-(P₅Bu^t₄)]; elemental analysis (%) found (calcd for C₂₈H₆₈LiN₄P₅, M = 622.65 g/mol): C 54.01 (53.59); H 11.01 (10.77); N 9.00 (9.25).