2.1 Synthesis and characterization of [Fe₂(CO)₄(κ²-dmpe)(μ-pdt)] (1)

Treatment of [Fe₂(CO)₆(μ-pdt)] with dmpe in refluxing THF for 1 h resulted in the formation of the unsymmetrically disubstituted complex [Fe₂(CO)₄(κ²-dmpe)(μ-pdt)] (1) with moderate yields (47%) (Scheme 3).

Reaction conditions to yield 1 are milder than those used for synthesizing its dppe analogue [6a]. The replacement of two carbonyl ligands by dmpe does not require either the presence of Me₃NO nor warming in refluxing toluene overnight. This reflects the expected behaviour of the dmpe ligand that is a better electron-donor than dppe. Compound 1 was characterized by elemental analyses, IR and NMR spectroscopies. Typical ν(CO) bands are observed at 2008(s), 1936(s), 1893(w) cm⁻¹ in the infrared spectrum of a CH₂Cl₂ solution of 1. These absorptions are as expected shifted to lower frequencies relative to those of the dppe analogue [ν(CO) = 2019(s), 1949(s), 1906(w) cm⁻¹] [6a], and they are very close to those of [Fe₂(CO)₄(κ²-phen)(μ-pdt)] (2008(s), 1939(s) and 1897(s) cm⁻¹) [8]. The ¹H NMR spectrum of 1 displays a set of poorly resolved resonances for the pdt and dmpe groups. The ¹³C–{¹H} NMR spectrum at 298 K of 1 shows a carbonyl pattern characteristic of an unsymmetrically disubstituted complex, i.e. a singlet and a triplet (J = 12.0 Hz) at 218.4 and 221.2 ppm. These signals are assigned unambiguously to the {Fe(CO)₃} and {Fe(CO)P₂} moieties, respectively. The ³¹P–{¹H} NMR spectrum of 1 in CD₂Cl₂ exhibits two singlets at 64.2 and 62.7 ppm in an approximate 1:6 ratio. Upon cooling, the more deshielded resonance signal splits into two sets of signals appearing as ill-resolved AB quartets, and the peak at 62.7 ppm splits into two singlets (Fig. 1 and extended Fig. a in Supplementary data). The presence in solution of four isomers is hence observed at low temperature. These data agree with previous studies of analogous compounds, which indicate that fluxional processes related to the diphosphine group and to the dithiolate bridge are operative in solution [4a,6]. Accordingly, the former peak was assigned to a basal–apical isomer and the latter to a basal–basal form, with two possible orientations of the pdt bridge for each isomer (Fig. 1). It should be noted that unlike the dppe compound [Fe₂(CO)₄(κ²-dppe)(μ-pdt)], the basal–basal isomer of 1 is here the major species. In our previous work, we found that the ratios between the basal–basal and basal–apical isomers of [Fe₂(CO)₄(κ²-dppe)(μ-pdt)] depended on the solvent [6a], suggesting a slow equilibrium between the two forms. This was supported by the following experiment: when [Fe₂(CO)₄(κ²-dppe)(μ-pdt)] was completely dissolved in CD₂Cl₂ or toluene-d₈ at 190 K, the ³¹P–{¹H} NMR spectrum at 190 K of the resulting solution showed only the presence of the basal–apical isomer. The basal–basal form of the dppe complex was detected when the temperature reached 223 K, indicating that the equilibrium becomes operative.

X-ray analysis of single crystals of the complex 1 obtained from a diethyl ether solution established without any ambiguity the binding mode of the bidendate diphosphine to a single iron atom in a basal–basal position (Fig. 2). One of the more striking features of the structure of 1 is the Fe–Fe distance (2.6038(5) Å), which is longer than that observed in [Fe₂(CO)₄(κ²-dppe)(μ-pdt)] (2.547(7) Å) [6a] and is very close to that determined in other related compounds containing a basal–basal diphosphine, such as [Fe₂(CO)₄{κ²-(PPh₂)₂NR}(μ-pdt)] (R = ⁱPr, 2.6236(4) Å; allyl, 2.6042(4) Å) and [Fe₂(CO)₄(κ²-dppm)(μ-pdt)] (2.5879(7) Å) [3]. In addition, the basal–basal chelation of the diphosphine induces a noticeable dissymmetry of the Fe₂S₂ core, which is revealed by a shortening of the Fe–S bonds trans to the Fe–P bonds [S(1)–Fe(1), 2.2182(7); S(1)–Fe(2), 2.2658(7); S(2)–Fe(1), 2.2369(6); S(2)–Fe(2), 2.2755(7) Å]. A similar effect has been also observed in [Fe₂(CO)₄(κ²-phen)(μ-pdt)] [8]. As expected, complexes [Fe₂(CO)₄(κ²-dppe)(μ-pdt)] and [Fe₂(CO)₄(κ²-dmpe)(μ-pdt)] possess very near bite angles (P–Fe–P), 87.7(4)° and 86.05(3)°, respectively.

2.2 Protonation study of 2

Protonation of 1 with HBF₄·Et₂O was carried out in CH₂Cl₂ at 298 K, giving immediately a red solution of the μ-hydride derivative [Fe₂(CO)₄(κ²-dmpe)(μ-H)(μ-pdt)](BF₄) (2) (Scheme 4).

IR carbonyl absorptions of 2 are as expected shifted to higher energy (2093(s), 2036(s), 1972(s) cm⁻¹) relative to those of 1. The ¹H NMR spectrum of 2 in CD₂Cl₂ confirms that protonation occurs at the Fe–Fe site. Accordingly, a characteristic high-field signal assigned to an Fe₂(μ-H) group appears as a triplet at −13.92 ppm with a coupling constant, ²J_PH, of 22.0 Hz, in agreement with a basal–basal position of the diphosphine in 2. The ³¹P–{¹H} NMR spectrum displays a singlet at 68.0 ppm. Addition of Et₃N (6 equiv) to a solution of 2 in dichloromethane resulted in the slow deprotonation of 2 in 1 within 2 h.

Protonation experiments of 1 at low temperature confirmed previously reported results concerning unsymmetrically disubstituted derivatives [Fe₂(CO)₄(κ²-L₂)(μ-pdt)] [L₂ = dppe, phen, bis(N-heterocyclic carbene)] [6–8].

Treatment of 1 with excess of HBF₄·OEt₂ in CD₂Cl₂ at 188 K resulted in the immediate appearance, in the ¹H NMR spectrum, of a signal at −4.68 ppm, corresponding to the formation of a species 3 featuring a terminal hydride at the unsubstituted iron atom (Fig. 3). The ³¹P–{¹H} NMR spectrum displayed a singlet at 65.5 ppm. The stability of this species up to 243 K allowed us to record a ¹³C–{¹H} NMR spectrum, which shows in the carbonyl region three ill-resolved signals at, 236.2, 212.0 and 203.0 ppm, with relative intensities of 1, 1 and 2 (Fig. b in the Supplementary data). The more deshielded chemical shift at 236.2 ppm suggests either the presence of a semi-bridging carbonyl or a strong electron-donor effect of the hydride in a trans position relative to this CO group. An ¹H–¹³C HMBC 2D NMR experiment shows a correlation between the hydride signal and the carbonyl peaks at 236.2 and 203.0 ppm (Fig. c in the Supplementary data). This strongly suggests, as it has been proposed in previous works [6–8], the presence in 3 of an {FeH(CO)₃} moiety with a terminal hydride, two equivalent carbonyl groups (basal position) and a third semi-bridging carbonyl trans to the hydride. Until now, neither reliable low-temperature IR data nor suitable single crystals of 3 can be obtained. The transformation of intermediate 3 upon increasing the temperature was monitored by ¹H NMR and ³¹P–{¹H} NMR. No noticeable change was observed up to 243 K. Above this temperature, a triplet at −14.46 ppm (J_PH = 20 Hz) and a doublet at −14.34 ppm (J_PH = 25 Hz) appeared in the ¹H NMR spectrum, while the ³¹P–{¹H} NMR spectrum displayed a singlet at 68.0 ppm and an AB pattern at 67.2 (d, J_PP = 31.2 Hz) and 65.9 (d, J_PP = 31.2 Hz) (Fig. 3). These observations accord with the formation of μ-hydride species with basal–basal (2_ba–ba) and basal–apical (2_ba–ap) coordination of the diphosphine ligand. Compound 2_ba–ap appears to be an intermediate and its complete transformation into 2_ba–ba was observed when the temperature raised to 298 K. Surprisingly, unlike the related dppe compound, the ¹H NMR spectrum of the dmpe derivative 2, in acidic solution at low temperature, did not exhibit any signal that could be assigned to a terminal hydride attached to the iron atom supporting the dmpe ligand.

In order to check the influence of the dissymmetry on the protonation of the disubstituted diiron centre, we decided to study the protonation of the symmetrical complex [Fe₂(CO)₄(PMe₃)₂(μ-pdt)] [12] under conditions similar to those above. Its treatment with excess (3 equiv) of HBF₄·OEt₂ in CD₂Cl₂ solution at 198 K resulted in the immediate appearance of two signals at −14.73 ppm (singlet) and −14.86 ppm (doublet, J = 22 Hz). These resonances are assigned to two species having a bridging hydride and the phosphine groups in apical–apical and basal–apical position. Couplings between bridging hydride and phosphorus atom lying in apical position are not observed. This result suggests that both the basicity of the diiron site and the unsymmetrical substitution are required to get a terminal hydride in disubstituted tetracarbonyl dithiolato systems [Fe₂(CO)₄L₂(μ-pdt)]. Finally, on warming the solution to 298 K, the two previous hydride signals disappeared, while a triplet appeared at −15.2 ppm, which was assigned to the already known basal–basal product [Fe₂(CO)₄(PMe₃)₂(μ-pdt)(μ-H)]⁺ reported by Darensbourg et al. [12]. Such an assignment was corroborated by recording the ³¹P–{¹H} NMR spectrum. Moreover, similar investigation realized with the symmetrically substituted derivative [Fe₂(CO)₄{P(OMe)₃}₂(μ-pdt)] [13] gave results close to those reported above. The ¹H NMR spectrum in CD₂Cl₂ at 178 K indicated the formation of three bridging hydride species upon protonation by HBF₄·OEt₂ at this temperature (see Fig. d in the Supplementary data). When the temperature reached 298 K, the conversion of two of these isomers into the third one was observed and was achieved after four days.

2.3 Reaction of 1 with arphos (arphos = Ph₂AsPCH₂CH₂PPh₂)

Heating [Fe₂(CO)₆(μ-pdt)] with the mixed phosphine–arsine Ph₂PCH₂CH₂AsPh₂ (arphos) and Me₃NO·2H₂O in refluxing THF for 4 h gave the disubstituted complex [Fe₂(CO)₄(arphos)₂(μ-pdt)] (4b). The formation of the monosubstituted derivative [Fe₂(CO)₅(arphos)(μ-pdt)] (4a) was observed in some experiments, but only small amounts of this species are isolated after purification by chromatography (Scheme 5).

The coordination of the phosphine–arsine ligand was revealed by the deshielding of chemical shift of the phosphorus atom in the ³¹P–{¹H} spectra (CD₂Cl₂) of 4a and 4b. The related signal is observed at −12.1 ppm in the free molecule Ph₂PCH₂CH₂AsPh₂ and at 60.4 and 58.3 ppm in 4a and 4b, respectively. IR spectra in the carbonyl region of 4a and 4b in CH₂Cl₂ solution were typical of monosusbtituted (ν(CO): 2044(s), 1981(vs), 1959(sh) and 1928(w) cm⁻¹) and disubstituted (ν(CO): 1989(s), 1951(s), 1919(s) cm⁻¹) phosphine derivatives [13]. ¹H NMR spectra of 4a and 4b show the expected signals for arphos and pdt groups. The structure of complex 4b was confirmed by X-ray analysis of single crystals obtained from dichloromethane–ether solutions (Fig. 4). The main feature of this structure is the presence of two dangling monodentate arphos ligands in apical positions. The Fe₂S₂ core has a typical butterfly structure. Distances and angles are unexceptional and very close to values determined for diiron dithiolate analogues [14].

Given these results and in the absence of a diiron chelate compound, we decided not to pursue the work with the phosphine–arsine ligand.

4.1 Methods and materials

All the experiments were carried out under an inert atmosphere, using Schlenk techniques for the syntheses. Solvents were deoxygenated and dried according to standard procedures. [Fe₂(CO)₆(μ-pdt)] was prepared according to the reported procedure [15]. The NMR spectra (¹H, ³¹P) were recorded at room temperature in CD₂Cl₂ solution with a Bruker AMX 400, AC300 and DRX 500 spectrometers and were referenced to SiMe₄ (¹H), H₃PO₄ (³¹P). ¹H–¹³C experiments were carried out on a Bruker DRX 500 spectrometer. The infrared spectra were recorded on a Nicolet Nexus Fourier transform spectrometer. Chemical analyses were made by the “Service de microanalyse I.C.S.N.” Gif-sur-Yvette (France), or the “Centre de microanalyse du CNRS”, Vernaison (France). Crystal data for compounds 1a–c and 2a were collected with an Oxford Diffraction X-Calibur-2 CCD diffractometer, equipped with a jet cooler device and graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The structures were solved and refined by standard procedures [16].

4.2 X-ray structure analysis

Crystal data: 1, C₁₃H₂₂Fe₂O₄P₂S₂, F.Wt = 480.07, T = 295(2) K, monoclinic, space group P2₁/c, a = 12.9610(9), b = 10.5514(7), c = 14.4917(9) Å, β = 93.133(5)°, V = 1978.9(2) ų, Z = 4. Refinement of 212 parameters gave R1 = 0.0326, wR2 = 0.0824 and |Δρ| < 0.827 e Å⁻³ for all 6164 unique reflections. Compound 4b, C₅₉H₅₄As₂Fe₂O₄P₂S₂, F.Wt = 1214.62, T = 170(2) K, monoclinic, space group P2₁/n, a = 12.2473(6), b = 17.3095(12), c = 26.5034(14) Å, β = 91.856(5)°, V = 5615.6(6) ų, Z = 4. Refinement of 510 parameters gave R1 = 0.0564, wR2 = 0.1316 and |Δρ| < 0.638 e Å⁻³ for all 6988 unique reflections. Further crystallographic data for 1 and 4b are provided as supporting information.

4.3 Synthesis of 1

4.4 Protonation reaction of 1

Protonation of 1. A solution of 1 (0.1 g, 0.208 mmol) in dichloromethane (10 mL) was treated with 2 equiv of HBF₄·Et₂O. The mixture was stirred for 15 min. The volume was then reduced under vacuum and diethyl ether was added to precipitate a red powder of 2 (0.95 g, 80% yield). IR (CH₂Cl₂, cm⁻¹): ν(CO) 2093(s), 2036(s), 1972(s). ¹H NMR (CD₂Cl₂, 25 °C) δ: 2.75 (m, 2H, pdt), 2.58 (m, 2H, dmpe), 2.54 (m, 1H, pdt), 2.42 (m, 2H, pdt), 2.18 (m, 2H, dmpe), 1.98 (m, 1H, pdt), 1.62 (d, J_PH = 9.5 Hz, 6H, dmpe), 1.46 (d, J_PH = 10,0 Hz, 6H, dmpe), −13.92 (t, J_PH = 22.0 Hz, 1H, Fe₂(μ-H)). ³¹P–{¹H} NMR (25 °C) δ: 68.0(s).

4.5 Reaction of [Fe₂(CO)₆(μ-pdt)] with arphos (arphos = Ph₂AsPCH₂CH₂PPh₂)

A solution of [Fe₂(CO)₆(μ-pdt)] (0.20 g, 0.52 mmol) and arphos (0.44 g, 1.04 mmol) in THF (100 mL) was heated under reflux in the presence of Me₃NO·2H₂O (0.185 g, 1.10 mmol) for 4 h. The orange solution became red-brown. After evaporation of the solvent, the residue was chromatographed on a silica gel column. Elution with hexane–dichloromethane mixtures afforded two red–orange bands. The first gave the monosubstituted complex 4a and the second the disubstituted derivative 4b. Compound 4b was isolated as a red powder (0.16 g, 25% yield).

Compound 4a: IR (CH₂Cl₂, cm⁻¹): ν(CO) 2044(s), 1981(vs), 1959(sh) and 1928(w). ¹H NMR (CD₂Cl₂, 25 °C) δ: 7.6–7.3 (m, 20H, arphos), 2.51 (m, 2H, arphos), 2.14 (m, 2H, arphos), 1.77 (m, 2H, pdt), 1.41 (m, 4H, pdt). ³¹P–{¹H} NMR (25 °C) δ: 60.4(s).

Compound 4b: IR (CH₂Cl₂, cm⁻¹): ν(CO) 1989(s), 1951(s), 1919(s). ¹H NMR (CDCl₃, 25 °C) δ: 7.6–7.3 (m, 40H, arphos), 2.53 (m, 2H, arphos), 2.23 (m, 2H, arphos), 1.59 (m, 4H, pdt), 1.08 (m, 2H, pdt). ³¹P–{¹H} NMR (25 °C) δ: 58.3(s). Anal. Calcd (%) for C₅₉H₅₄As₂Fe₂O₄P₂S₂. CH₂Cl₂: C, 55.4; H, 4.3. Found: C, 55.1; H, 4.2.