Cubane-Type Co₄S₄ Clusters: Synthesis, Redox Series, and Magnetic Ground States

Abstract

The recent demonstration that the carbene cluster [Fe₄S₄(Prⁱ₂NHCMe₂)₄] (9) is an accurate structural and electronic analogue of the fully reduced cluster of the iron protein of A. vinelandii nitrogenase, including a common S = 4 ground state, raises the issue of the existence and magnetism of other [M₄S₄L₄]^z clusters, none of which are known with transition metals other than iron. The system CoCl₂/Prⁱ₃P/(Me₃Si)₂S/THF assembles [Co₄S₄(PPrⁱ₃)₄] (3) which is converted to [Co₄S₄(Prⁱ₂NHCMe₂)₄] (5) upon reaction with carbene. The clusters support the redox series [3]^1−/0/1+ and [5]^0/1+/2+; monocations (4, 6) have been isolated by chemical oxidation. Redox potentials and substitution reactions indicate that the carbene is the more effective electron donor to tetrahedral Fe^II and Co^II sites. Clusters 3–6 have the same overall cubane-type geometry as 9. Neutral clusters 3 and 5 have an S = 3 ground state. As with the S = 4 state of 9 with local spins S_Fe = 2, the septet spin state can be described in terms of the coupling of three parallel and one antiparallel spins S_Co = 3/2. The octanuclear clusters [Co₈S₈(PPrⁱ₃)₆]^0,1+ were isolated as minor byproducts of the formation and chemical oxidation of 3. The clusters exhibit a rhomb-bridged noncubane (RBNC) structure, whereas clusters with the Fe₈S₈ core possess the edge-bridged double-cubane (EBDC) stereochemistry. There are two structural solutions for the M₈S₈ core in the form of topological isomers whose stability may depend on valence electron count. A conceptual model for the RBNC ← EBDC interconversion is presented. (Prⁱ₂NHCMe₂ = C₁₁H₂₀N₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene)

Introduction

The cubane-type core unit Fe₄(μ₃-S)₄ is prominently represented within the vast array of structures of iron-sulfur clusters with weak field^1–3 and strong field⁴ terminal ligands. Our interest in weak field clusters derives from their well-known role as accurate structural and electronic analogues of protein-bound clusters {Fe₄S₄(S_Cys)₃L] in which ligand L is cysteinate, another amino acid side chain, or hydroxide/water.² In such clusters the core oxidation states [Fe₄S₄]^3+,2+,1+ are common and all have been isolated in synthetic clusters.² The demonstration of the all-ferrous oxidation state [Fe₄S₄]⁰ in the iron protein of A. vinelandii (Av) nitrogenase,^5–8, and more recently in a dehydratase activator protein from A. fermentans,⁹ upon reduction of the proteins with Ti(III) citrate provided an imperative for the synthesis and full characterization of heretofore unknown synthetic analogues of this state. The first all-ferrous clusters produced were the phosphine species [Fe₄S₄(PR₃)₄] (R = Prⁱ, Bu^t, C₆H₁₁), but all attempts to isolate these compounds in substance resulted in core aggregation accompanying phosphine dissociation to afford the dicubanes [Fe₈S₈(PR₃)₆] and tetracubanes [Fe₁₆S₁₆(PR₃)₈].^10,11 More recently, fully reduced clusters have been isolated in the form of [Fe₄S₄(CN)₄]^{4− 12} and [Fe₄S₄(Prⁱ₂NHCMe₂)₄] with the N-heterocyclic carbene ligand 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene.¹³ The more stable carbene cluster has been shown to be a meaningful representation of the all-ferrous cluster in proteins.^13,14 In particular, a detailed analysis of its Mössbauer and EPR spectra has established an S_t = 4 ground state,¹⁵ which amongst all biological clusters has been found only in those of the Av iron protein⁷ and the activator protein.⁹

The foregoing results support the proposition that the exchange coupling leading to the observed ground state is intrinsic to the [Fe₄S₄]₀ core and is not significantly influenced by the terminal ligands. An associated issue is the ground spin states in other fully reduced cubane-type [M₄S₄]⁰ clusters with variable M. The corresponding core units are unknown with other metals that manifest a tetrahedral stereochemical preference, necessitating synthesis of a new family of clusters. Cobalt(II) commonly exhibits this preference. However, we note that in the brief structural survey of Co-S clusters in Figure 1, which includes nuclearities n = 4,^16,17 6,^18–22 7,^23,24 and 8,²⁵ tetrahedral coordination is found only with n = 7 and 8, the large majority of the clusters are mixed-valence, and cubane stereochemistry is limited to a single example with Co^III ([Cp₄Co₄S₄]¹⁷). If Co-Se clusters are considered, different examples emerge which include the cubane [Co₄Se₄(PPh₃)₄]²⁶ and [Co₈Se₈(PPh₃)₆]^0,1+,²⁷ with a composition analogous to [Fe₈S₈(PR₃)₆]. In this investigation, we have prepared and structurally characterized the cubane-type clusters [Co₄S₄(Prⁱ₂NHCMe₂)₄] and [Co₄S₄(PPrⁱ₃)₄], examined their redox chemistry, and determined magnetic ground states. As will be seen, these clusters exhibit an exchange coupling pattern and a highly paramagnetic ground state consonant with the properties of [Fe₄S₄]⁰ clusters.

Figure 1.

Structures of Co-S clusters with nuclearities of four to eight showing the exclusive or usual terminal ligand atoms associated with each. The five η⁵-C₅Me₅ ligands of Co₆S₆ are omitted for clarity.

Experimental Section

Preparation of Compounds

All reactions and manipulations were performed under a pure dinitrogen atmosphere using either Schlenk techniques or an inert atmosphere box. Solvents were passed through an Innovative Technology or MBraun solvent purification system prior to use. Solvent removal steps were performed in vacuo. Selected compounds were analyzed (H.Kolbe, Mulheim, Germany). All compounds except [Co(PrⁱNHCMe₂)₂Cl₂] were identified by X-ray structural determinations.

[Co(Prⁱ₂NHCMe₂)₂(SBu^t)₂]

To a suspension of CoCl₂ (0.13 g, 1.0 mmol) in THF (10 mL) was added a solution of Prⁱ₂NHCMe₂ (1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene,²⁸ 0.36 g, 2.0 mmol) in THF (5 mL). The reaction mixture was stirred for 4 h. Addition of a solution of Bu^tSNa (0.22 g, 2.0 mmol) in THF (10 mL) resulted in formation of a green suspension. The mixture was stirred for 2 d and filtered. Vapor diffusion of n-hexane into the filtrate afforded the product as a green crystalline solid (0.43 g, 72%). Absorption spectrum (benzene): λ_max (ε_M) 344 (6820), 406 (2490), 456 (1460), 564 (353), 652 (1280), 717 (1144) nm. ¹H NMR (C₆D₆): δ 18.87 (12), 6.80 (9); (THF-d₈): δ 18.04 (12), 7.54 (9).

[Co(Prⁱ₂NHCMe₂)₂Cl₂]

To a suspension of CoCl₂ (0.13 g, 1.0 mmol) in benzene (30 mL) was added a solution of Prⁱ₂NHCMe₂ (0.36 g, 2.0 mmol) in THF (3 mL). The reaction mixture was stirred for 2 d. The blue suspension was filtered and the solid was washed with ether (2 × 5 mL) and dried in vacuo. The product was obtained as a blue powder (0.44 g, 90%). Absorption spectrum (benzene): λ_max (ε_M) 312 (828), 597 (440), 640 (920) nm. ¹H NMR (C₆D₆): δ 14.63 (1), 8.78 (2).

[Co₄S₄(PPrⁱ₃)₄]

Method A of the preparation of [Co₄S₄(Prⁱ₂NHCMe₂)₄] (see below) was followed to the point of the brown oily residue, which was dissolved in THF (5 mL) and the solution was filtered. Layering acetonitrile (20 mL) on the filtrate caused separation of the product as black crystalline blocks (0.34 g, 70%). ¹H NMR (C₆D₆): δ 14.71 (1), 1.80 (6). Anal. Calcd. C, 43.03; H, 8.43; Co, 23.46; P, 12.33; S, 12.76. Found: C, 42.94; H, 8.39; Co, 23.49; P, 12.38; S, 12.77. A very small quantity of platelike black crystals was separated from the bulk product and shown to be [Co₈S₈(PPrⁱ₃)₆] by an X-ray structure determination.

[Co₄S₄(PPrⁱ₃)₄](BF₄)

To a solution of [Co₄S₄(PPrⁱ₃)₄] (0.19 g, 0.20 mmol) in THF (5 mL) was added [Cp₂Fe](BF₄) (0.055 g, 0.20 mmol). The reaction mixture was stirred overnight. Solvent was removed, the deep brown residue was dissolved in acetonitrile (2 mL), and the solution was filtered. Vapor diffusion of ether into the brown filtrate afforded the product as deep brown block-like crystals (0.15 g, 70%). Absorption spectrum (THF): λ_max (ε_M) 370 (sh, 13900) nm. ¹H NMR (CD₃CN): δ 2.65 (6), 2.32 (1). Anal. Calcd. for C₃₆H₈₄BCo₄F₄P₄S₄: C, 39.60; H, 7.76; Co, 21.59; P, 11.35; S, 11.75. Found: C, 39.41; H, 7.49; Co, 22.19; P, 11.44. An accurate sulfur analysis was not obtained. A small amount of brown platelike crystals was also obtained from the bulk sample and shown by an X-ray structure determination to [Co₈S₈(PPrⁱ₃)₆](BF₄).

[Co₄S₄(Prⁱ₂NHCMe₂)₄]

Method A

To a suspension of CoCl₂ (0.26 g, 2.0 mmol) and PPrⁱ₃ (0.64 g, 4.00 mmol)²⁹ in THF (10 mL) was added a solution of (Me₃Si)₂S (0.43 g, 2.4 mmol) in THF (10 mL). The mixture was stirred for 2 d, solvent was removed, and deep brown oily residue was dissolved in THF (10 mL). The solution was filtered. The filtrate was treated with a solution of Prⁱ₂NHCMe₂ (0.72 g, 4.0 mmol) in THF (10 mL), resulting in a brown suspension, which was stirred for 7 d. The reaction mixture was filtered. Vapor diffusion of n–hexane into the brown filtrate afforded the product as a brown crystalline solid (0.28 g, 50%). ¹H NMR (C₆D₆): δ 7.37 (1), 5.38 (2), −11.6 (br). Anal. Calcd. for C₄₄H₈₀Co₄N₈S₄: C, 48.70; H, 7.43; Co, 21.71; N, 10.33; S, 11.82. Found: C, 47.61; H, 7.29; Co, 21.23; N, 10.07; S, 11.53.

Method B

A solution of [Co₄S₄(PPrⁱ₃)₄] (0.20 g, 0.20 mmol) in THF (5 mL) was treated with a solution of Prⁱ₂NHCMe₂ (0.16 g, 0.90 mmol) in THF (5 mL). The reaction mixture was refluxed overnight and filtered. Solvent was removed from the filtrate, the deep brown oily residue was dissolved in benzene (5 mL), and the solution was filtered. Vapor diffusion of hexanes into the brown filtrate yielded the product as deep brown crystals (0.17 g, 80%). The ¹H NMR spectrum of this material was identical with that of the product from Method A.

[Co₄S₄(Prⁱ₂NHCMe₂)₄](PF₆)

To a solution of [Co₄S₄(Prⁱ₂NHCMe₂)₄] (0.11 g, 0.10 mmol) in THF (5 mL) was added (C₇H₇)(PF₆) (0.024 g, 0.10 mmol). The mixture was stirred overnight. Solvent was removed, the deep brown residue was dissolved in THF (2 mL), and the solution was filtered. Vapor diffusion of n-hexane into the brown filtrate yielded the product as brown crystals (0.055 g, 40%). ¹H NMR (CD₃CN): δ 11.75 (1), 9.53 (2).

[Co₄S₄(Prⁱ₂NHCMe₂)₄](BPh₄)

This compound was prepared on the same scale by the previous procedure but with use of NaBPh₄ (0.034 g, 0.10 mmol) in 5 mL of acetonitrile. It was obtained as brown crystals (0.097 g, 70%) with an identical ¹H NMR spectrum of the cation. Absorption spectrum (THF): λ_max (ε_M) 374 (sh, 14100) nm.

In the sections that follow, compounds are referred to by the designations in the Chart.

Chart.

Designation of Compounds

X-ray Structure Determinations

The structures of the seven compounds in supplemental Table S-1 were determined. Diffraction-quality crystals were obtained as follows: 3 and 7, THF/acetonitrile; 5, benzene/hexane; [4](BF₄) and [8](BF₄), acetonitrile/ether; 1, [6](BPh₄), THF/hexane. Crystallizations were performed at room temperature. Crystals were coated with Paratone-N oil and mounted on a Bruker APEX CCD-based diffractometer equipped with an Oxford low-temperature apparatus. Data were collected with scans of 0.3 s/frame for 30 s. Cell parameters were retrieved with SMART software and refined using SAINT software on all reflections. Data integration was performed with SAINT, which corrects for Lorentz polarization and decay. Absorption corrections were applied using SADABS. Space groups were assigned unambiguously by analysis of symmetry and systematic absences determined by XPREP. All structures were solved and refined using SHELXTL. Metal and first coordination sphere atoms were located from direct-methods E-maps; other non-hydrogen atoms were found in alternating difference Fourier synthesis and least-squares refinement cycles and during final cycles were refined anisotropically. Hydrogen atoms were placed in calculated positions employing a riding model. Final crystal parameters and agreement factors are reported in Table S-1.³⁰ Three of the four PPrⁱ₃ ligands of 3 are each disordered over two positions with equal occupancies such that the two Co-P vectors are each site form angles of 19.1(1)°, 23.8(1)°, and 24.4(1)°.

Other Physical Measurements

All measurements were performed under anaerobic conditions. Absorption spectra were recorded with a Varian Cary 50 Bio spectrophotometer. ¹H NMR spectra were obtained with a Varian AM-400 spectrometer. Cyclic voltammetry measurements were made with a BioAnalytical Systems Epsilon potentiostat/galvanostat in THF solutions using a sweep rate of 100 mV/s, a glassy carbon working electrode, 0.1 M (Bu₄N)(PF₆) supporting electrolyte, and an SCE reference electrode. Under these conditions, E_½ = 0.55 V for the [Cp₂Fe]^0,1+ couple.

Magnetic susceptibility data were obtained with powder samples at 2–300 K using a SQUID susceptometer with a field of 1.0 T (MPMS-7, Quantum Design, calibrated with a palladium reference sample, error <2%). Multiple-field variable-temperature measurements were done at 1 T, 4 T, and 7 T also at 2–300 K with the magnetization sampled in equidistant steps on a 1/T temperature scale. The data were corrected for diamagnetic contributions by using Pascal's constants,³¹ as well as for temperature-independent paramagnetism. The susceptibility and magnetization data for the cluster compounds were simulated using the usual spin-Hamiltonian operator 1 for a system of four high spin Co(II) sites with local spin S_i = 3/2. Here J_i,j are the isotropic coupling constants for the exchange interaction between the individual cobalt ions (i = 1, 4), g_i are the average electronic g-values, and D_i and E/D_i are the average zero-field splitting and rhombicity parameters for the ions. Alternatively, the data for the cluster compounds were also simulated using the total spin S_t of the ground state only in conjunction with the usual Hamiltonian operator 2 for a single spin S where S was set to S_t and g = g_t, D = D_t, and E/D = (E/D)_t. For mononuclear complexes, the usual spin-Hamiltonian operator 2 for S = 3/2 was used. Simulations were done with our own package julX for exchange-coupled systems.³² The values were summed over a 16-point Lebedev grid^33,34 to account for the powder distribution with respect to the field. For some samples, intramolecular interactions were taken into account in the simulations by using a Weiss temperature Θ_W as a perturbation of the temperature scale, kT’ = k(T - Θ_W).

$$\hat{H}=-2{\displaystyle {\sum }_{j<i}}{J}_{i,j}{\widehat{\stackrel{\rightharpoonup }{S}}}_i·{\widehat{\stackrel{\rightharpoonup }{S}}}_j+{\displaystyle {\sum }_i}{g}_i\beta {\widehat{\stackrel{\rightharpoonup }{S}}}_i\cdot \stackrel{\rightharpoonup }{\mathrm{B}}+{\displaystyle {\sum }_i}{D}_i[{{\hat{S}}_{i,z}}^2-5/4+E/D_i({{\hat{S}}_{i,x}}^2-{{\hat{S}}_{i,y}}^2)]$$ (1)

$$\hat{H}=g\beta \widehat{\stackrel{\rightharpoonup }{\mathrm{S}}}·\stackrel{\rightharpoonup }{\mathrm{B}}+D[{{\hat{S}}_z}^2-1/3S(S+1)+E/D({{\hat{S}}_x}^2-{{\hat{S}}_y}^2)]$$ (2)

Results and Discussion

Mononuclear Complexes

In seeking clusters with the cubane-type [Co₄S₄] core unit, we note stabilization of [Co₄Se₄] species by tertiary phosphines²⁶ as well as reduced [Fe₄S₄]^0,1+ clusters by phosphine^10,11 and N-heterocyclic carbene¹³ terminal ligation. Anionic ligands such as thiolate increase cluster negative charge and promote oxidative instability. Mononuclear Co(II) phosphine complexes, many of the tetrahedral type [Co(PR₃)₂L₂], abound. While carbene complexes of the divalent ions Fe^II, Ni^II, Pd^II, and Pt^II have been extensively investigated in this decade, ^35–37 recent work on Co^II carbenes has largely utilized tripodal ligands favoring tetrahedral stereochemistry.^38,39 We wished to ascertain whether carbenes would be effective in stabilizing unconstrained high-spin tetrahedral Co^II resembling sites in a cubane-type cluster. The complexes [Co(Prⁱ₂NHCMe₂)₂(SBu^t)₂] and [Co(Prⁱ₂NHCMe₂)₂Cl₂] are readily prepared. The structure of 1, shown in Figure 2, reveals tetrahedral stereochemistry with bond angles in the range 106–117°, normal Co-S bond lengths when compared to other tetrahedral [Co(SR)₂L₂] complexes (2.24–2.27 A),^40,41 and Co-C bond lengths the same as or at most ca. 0.05 A longer than those in other tetrahedral Co^II carbenes.^38,39 In benzene solution, the complexes exhibit visible ligand field bands of tetrahedral (⁴A_2g→⁴T_1g(P)) parentage at energies similar to those observed for [Co(SR)₂L₂] species.⁴¹

Figure 2.

Structure of [Co(Prⁱ₂NHCMe₂)₂(SBu^t)₂] showing 50% probability ellipsoids and the atom labeling scheme. Selected values: Co-C 2.085[1] A, Co-S 2.287[1] A, S1-Co-S2 116.66(6)°, C1-Co-C2 103.1(2)°, C1-Co-S1 112.1(2)°, C1-Co-S2 106.5(2)°, C2-Co-S1 106.0(2)°, C2-Co-S2 111.7(2)°.

The temperature dependencies of the magnetic moments of complexes 1 and 2 were determined at 2–300 K. Above 150 K, both exhibit constant moments of ca. 4.2 μ_B, which is close to the spin-only value of 3.87 μ_B (g = 2) for S = 3/2. Below 150 K, magnetic moments of both compounds increase and reach maxima near 10 K, a behavior typical of intermolecular ferromagnetic spin coupling. The full temperature dependencies could be reasonably well simulated with S = 3/2, g = 2.180 and Θ_W = +0.8 K for 1, and g = 2.186 and Θ_W = +1.2 K for 2.³⁰ Reliable values for zero-field splitting parameters, however, could not be determined because of the effects of intermolecular interactions. A rough upper limit of |D| < 5 cm⁻¹ could be estimated from systematic variations of D and Θ_W in the simulations. These results show that when paired with conventional ligands the carbene, like phosphines, behaves as a normal weak field ligand that stabilizes tetrahedral Co^II in the absence of any ligand-imposed structural preference. Both ligand types were utilized in cluster synthesis.

Cubane-type Clusters

(a) Neutral Clusters

Synthetic methods are summarized in Figure 3. Phosphine cluster 3 is obtained by self-assembly reaction 3. Carbene cluster 5 is also prepared by self- assembly. In that system, 3 is first formed and subjected in situ to ligand displacement reaction 4 by the addition of excess Prⁱ₂NHCMe₂. Alternatively, preisolated 3 is treated with a small excess of carbene to afford 5. Both methods lead to essentially the same yield of isolated product (ca. 50–60%). Analogous iron-sulfur carbene cluster 9 is also prepared by in a similar self-assembly system but with the intermediate formation of edge-bridged double cubane 10¹³ (see below). No intermediate was encountered in the preparation of 5 by self-assembly. However, in the preparation of 3 by reaction 3, a very small amount of a black crystalline byproduct was obtained. Its composition was established as [Co₈S₈(PPrⁱ₃)₆], comparative to 11, by an X-ray structure determination. As will be seen, the structures of clusters 7 and 11 are not the same. Reactions 4 and 5¹³ provide a clear demonstration of the stronger binding affinity of a carbene vs. trialkylphosphine toward a common site, here tetrahedral Fe^II or Co^II

$$4[\text{Co}{({{\text{PPr}}^i}_{\mathit{3}})}_{\mathit{2}}{\text{Cl}}_2]+4{({\text{Me}}_3\text{Si})}_2\mathrm{S}\to [{\text{Co}}_4{\mathrm{S}}_4{({{\text{PPr}}^i}_{\mathit{3}})}_{\mathit{4}}]+8{\text{Me}}_3\text{SiCl}+4{{\text{PPr}}^i}_3$$ (3)

$$[{\text{Co}}_4{\mathrm{S}}_4{({{\text{PPr}}^i}_{\mathit{3}})}_{\mathit{4}}]+4{{\text{Pr}}^i}_2{\text{NHCMe}}_2\to [{\text{Co}}_4{\mathrm{S}}_4{({{\text{Pr}}^i}_2{\text{NHCMe}}_2)}_4]+4{{\text{PPr}}^i}_3$$ (4)

$$[{\text{Fe}}_8{\mathrm{S}}_8{({{\text{PPr}}^i}_3)}_6]+8{{\text{Pr}}^i}_2{\text{NHCMe}}_2\to 2[{\text{Fe}}_4{\mathrm{S}}_4{({{\text{Pr}}^i}_2{\text{NHCMe}}_2)}_4]+6{{\text{PPr}}^i}_3$$ (5)

Figure 3.

Scheme for the preparation of phosphine clusters 3, 4, 7, and 8 and carbene clusters 5 and 6. Clusters 7 and 8 were obtained in low yields as reaction by-products.

The structures of 3 and 5, set out in Figures 4 and 5, respectively, reveal the desired cubane-type stereochemistry with similar core dimensions as evident from the metric information in Table S-2. The Co^II sites have trigonally distorted tetrahedral stereochemistry. In cluster 5, the mean Co-C bond lengths are ca. 0.1 A longer than in mononuclear 1, owing to steric interactions with the bulky carbene and thiolate ligands.

Figure 4.

Structures of [Co₄S₄(PPrⁱ₃)₄]^0,1+ showing 30% probability ellipsoids and the atom numbering schemes. Ligands P(1,3,4)Prⁱ₃ of the neutral cluster are disordered over two positions (not shown). The cation has crystallo-graphically imposed T_d symmetry. Isopropyl groups are omitted for clarity.

Figure 5.

Structures of [Co₄S₄(Prⁱ₂NHCMe₂)₄]^0,1+ showing 30% probability ellipsoids and the atom numbering schemes.

The Co₄S₄ cores approach T_d symmetry; no systematic deviations from that symmetry are evident. The structural similarity between 3 and 5 and their close relationship to iron-sulfur cluster 9 is emphasized by the comparisons of mean distances and volumes calculated from atomic coordinates⁴² in Table 1. The cause of the 0.09 A longer mean Co-Co distance and correspondingly larger Co₄ and Co₄S₄ volumes in 3 compared to 5 is unclear. The longer mean Fe-S bond length and larger S₄ and Fe₄S₄ volumes in 9 vs. the cobalt clusters arises from the difference of 0.05 A in Shannon radii⁴³ between tetrahedral Fe^II and Co^II.

Table 1.

Summary of Core Structural Comparisons of Cubane-type Clusters [M₄S₄L₄]^0,1+(M = Co, Fe; L = PPrⁱ₃, Prⁱ₂NHCMe₂)

cluster	M-S (A)a	M-M (A)a	V(M4) (A3)b	V(S4) (A3)	V(M4S4) (A3)
[Co4S4(PPri3)4]c	2.223[8]	2.603[9]	2.08	5.26	8.50
[Co4S4(Pri2NHCMe2)4]d	2.25[1]	2.69[2]	2.30	5.33	9.12
[Fe4S4(Pri2NHCMe2)4]e,f	2.33[2]	2.68[1]	2.26	6.14	9.47
[Co4S4(PPri3)4]1+ f	2.206(1)	2.607(1)	2.10	5.06	8.44
[Co4S4(Pri2NHCMe2)4]1+ g	2.22[1]	2.66[2]	2.22	5.06	8.77

^aMean values.

^bVolume calculations: ref. 43; Shannon tetrahedral crystal radii (A): Fe^II 0.77, Co^II 0.72.

^c130 K.

^d105 K.

^eRef. 13, 193 K.

^f193 K.

^g100 K.

(b) Redox Series and Oxidized Clusters

Voltammograms of THF solutions prepared from the two neutral clusters are presented in Figure 6. Each supports two quasireversible processes, a reduction and oxidation for 3 and two oxidations for 5. The latter showed no reduction out to −2.0 V. The data are summarized and compared with the behavior of iron cluster 9¹³ and [Fe₄S₄(PPrⁱ₃)]^{1+ 11} in Figure 7. Two trends in the redox couples are defined by decisively large potential differences: E_Fe ^0/1+ < E_Co^0/1+ at constant ligand and E_carbene^0/1+;1+/2+ < E_phosphine^0/1+;1+/2+ at constant metal. The first trend has one precedent with cubane (Fe₄S₄, CoFe₃S₄) clusters.⁴⁴ The second and more significant trend implies that carbene is a more effective σ-donor to the core, resulting in a greater ease of electron loss. This statement is consistent with previous evidence that carbenes are more effective electron donors than the more basic phosphine ligands,^45–47 We are unaware of other quantitative data bearing on the comparative redox potentials of carbene and phosphine ligation.

Figure 6.

Cyclic voltammograms (100 mV/s) of ca. 3 mM THF solutions prepared from [Co₄S₄(PPrⁱ₃)₄] (upper) and [Co₄S₄(Prⁱ₂NHCMe₂)₄] (lower). Peak potentials vs. SCE are indicated.

Figure 7.

Summary of redox reactions of [M₄S₄(PPrⁱ₃)₄] and [M₄S₄(Prⁱ₂NHCMe₂)₄] (M = Fe, Co) in THF solutions showing E_½ values vs. SCE. The potential of the top reaction refers to dichloromethane solution.

Clusters 3 and 5 are readily oxidized to monocations 4 and 6 by ferrocenium and tropylium ions, respectively, and isolated as crystalline salts. Their structures retain the cubane-type geometry (Figures 4 and 5) with dimensions similar to those of the neutral clusters. While these reactions lead to formal mixed-valence (3Co^II + Co^III) cores, the metric parameters in Table S-3³⁰ provide no distinction in metal sites and thus suggest electron delocalization in the [Co₄S₄]¹⁺ core or disorder. The small decreases in S₄ and Co₄S₄ volumes imply (but not proved) removal of an electron from an antibonding orbital with substantial sulfur character.

(c) Magnetic Ground States

The temperature dependence of the effective magnetic moment of carbene cluster 5, depicted in Figure 8, steeply increases from 4.7 μ_B at 2 K to 6.7 μ_B at 10 K, which persists up to about 100 K. The moment then decreases, reaching 5.2 μ_B at 300 K. These large magnetic moments are not necessarily expected because a simple analysis of a symmetic [Co₄S₄]⁰ core with equal exchange couplings between metal sites predicts a diamagnetic ground state (S_t = 0). In contrast, the value observed for 5 at 10–100 K is close to the spin-only value μ_so = 2(3·4)^½ = 6.93 μ_B for S = 3. Accordingly, a preliminary simulation with S_t = 3 and g_t = 1.936 (dashed line) provides a good approximation to the experimental data up to about 100 K, including the decline of μ_eff below 10 K where the effect appears to be due to field saturation. Above 120 K the simple model fails, apparently because manifolds with S_t < 3 become populated. The low average g-value used in the approximate treatment is remarkable because Co^II, with a more than half-filled d-shell (3d⁷), should have g > 2. This matter is discussed below.

Figure 8.

Temperature dependence of the effective magnetic moment of solid [Co₄S₄(Prⁱ₂NHCMe₂)₄] measured at applied field B = 1 T. The dashed line is a preliminary simulation with S = 3, g = 1.936, and D = 0. The solid line represents a generic spin Hamiltonian simulation with four spins: S_i = 3/2, i = 1–4, arranged in pyramidal topology with one unique site (spin 1) and two different exchange coupling constants J = −420 cm⁻¹ for the interactions (1–2), (1–3), and (1–4), and J' = −100 cm⁻¹ for (2–3), (3–4), and (4–2). Other parameters are g₁ = 2.17, g₂₃₄ = 2.0, and D_i = 0. Inset: spin-coupling scheme for the S_t = 3 ground state.

The magnetic data for phosphine cluster 3 in Figure 9 reveals μ_eff ≈6.6 μ_B at 10–100 K, similar to the behavior of 5. The maximum at lower temperatures is indicative of intermolecular interactions, and the values above 100 K decrease much less than those of 5. The latter behavior indicates a different extent of spin coupling for the two clusters and a higher separation of excited spin states for 3. The ground state spin S_t = 3 was corroborated by multifield magnetization measurements and data fits. Nesting of the isofield magnetization curves at B = 1, 4, and 7 T is consistent only with a spin septet with weak zero-field splitting, although it proved difficult to simulate accurately the entire data set because of intermolecular interactions. The data at 4 T and 7 T approach saturation close to M_mol/Ngμ_B = 3, expected forS_t = 3 with g = 2. The deviation suggests a lower g-value for the ground state, as found in simulations.

Figure 9.

Temperature dependence of the effective magnetic moment of solid [Co₄S₄(PPrⁱ₃)₄] measured at applied field B = 1 T. The solid line represents a generic spin Hamiltonian simulation with four spins: S_i = 3/2, i = 1–4, arranged in pyramidal topology with one unique site (spin 1) and two different exchange coupling constants J = −600 cm⁻¹ for the interactions (1–2), (1–3), and (1–4), and J' = −100 cm⁻¹ for (2–3), (3–4), and (4–2). Other parameters are g₁ = 2.27, g₂₃₄ = 2.0, D_i = 0, Θ_W = 0.7 K. Inset: multi-field variable-temperature measurement of magnetization. The solid lines represent spin Hamiltonian simulations obtained with S_t = 3, g_t = 1.90, D_i = −1.95, E/D_i = 0.2, Θ_W = 0.7 K.

The paramagnetism of 3 and 5 is reminiscent of the situation with [Fe₄S₄]⁰ clusters of the iron protein of Av nitrogenase,⁷ the dehydratase activator protein,⁹ and synthetic cluster 9.^13–15 The four Fe^II sites are exchange-coupled via four μ₃-S bridges such that the local spins S_Fe = 2 yield S_t = 3·2 -2 = 4 for the cluster ground state. It has been observed that the all-ferrous clusters must have lower than cubic symmetry and the distortion appears to stabilize the highly reduced system.^14,15 Spectroscopic asymmetry first became evident in the Mössbauer spectrum of the Av iron protein, which showed two resolved quadrupole doublets in the 3:1 intensity ratio with distinct quadrupole and magnetic hyperfine couplings.⁵ The spin of one Fe^II site is aligned opposite to those of the other three sites. The same 3:1 pattern is found with the activator protein and synthetic cluster.

Although we do not have positive indication of a 3:1 site core geometric distortion in 3 and 5, and hyperfine measurements are elusive for these integer spin systems, we assume the same spin topology.⁴⁸ The 3:1 spin-coupling scheme readily rationalizes the ground state of [Co₄S₄]⁰ cores with S_Co = 3/2: S_t = 3(3/2) - 3/2 = 3. Corresponding spin Hamiltonian simulations yield convincing results for the low temperature magnetic data of 5 (Figure 8). Two independent coupling constants have been used to model in the antiferromagnetic interaction of three similar and one different metal site. The observed spin arises from the antiparallel orientation of site (1) relative to sites (2–4) when J-coupling exceeds J’-coupling.

While the magnetic data firmly establish the S_t = 3 ground state for clusters 3 and 5, certain limitations in the analyses presented in Figures 8 and 9 are noted. The specified coupling constants are not unique. Many other J/J’ combinations yield virtually the same result including the decrease in moment of 5 above 120 K. Apparently, we cannot observe or resolve a sufficient number of excited states to arrive at a unique fit. The lower part of the spin energy spectrum computed using the parameters for the spin Hamiltonian simulation for 5 consists of a sequence of septet, quintet, triplet and singlet states (Figure S-3A).³⁰ The corresponding Boltzman population of the lowest manifolds (Figure S-3B) qualitatively explains the decrease in μ_eff above 120 K as the onset of thermal population of the first excited spin quintet. However, we were unable to find satisfactory combinations of coupling constants J_ij that would shift the triplet and singlet states sufficiently close to the ground state to improve the fit. The virtually constant moment of 3 at 10–300 K indicates exclusive population of the spin septet state over than temperature interval. While this behavior constrains the possible J/J’ values, a unique solution is not possible. The values give for J and J’ (Figure 9) exemplify those required for a satisfactory fit.⁴⁹

In the context of the spin coupling scheme, the low g-values of the S_t = 3 ground state derive mainly from the negative contribution of the unique site (1) with respect to the other three if g₁ > 2 supersedes the positive contributions g₂₃₄ to g_t from the other sites. Because S₁ is oriented antiparallel to the total spin, the basic spin projection scheme of the fictitioius intermediate spin S₂₃₄ = 9/2 formed by the parallel arrangement of S₂, S₃, and S₄ and the unique site S₁ yields for the resulting total spin g_t = −3/8 g₁ + 11/8 g₂₃₄. Here g₂₃₄ is the same as the g-value of each Co^II at sites 2, 3, and 4. In the simulation for 5, we obtained the correct (observed) value g_t = 1.936 with g₁ = 2.17 and g₂₃₄ = 2, and for 3 g_t = 1.90 with g₁ = 2.27 and g₂₃₄ = 2. Again, the choice of values is not unique, but those given demonstrate that the experimental data are consistent with the site values g_Co > 2.

Octanuclear Clusters

As noted above, phosphine cluster 7 was separated manually in small amounts in the synthesis of 3 by self-assembly. Its structure, which has imposed centrosymmetry, is provided in Figure 10. The [Co₈(μ₃-S)₆(μ₄-S)₂] core is not built of cubane units. Two Co₃S₃ fragments (Co(1,2,4)S(1,3,4) and its symmetry equivalent) are bridged by a total of four Co-(μ₄-S) and four Co-(μ₃-S) interactions to an interior Co(3,3A)S(2,2A) rhomb with dimensions Co-S 2.250[3] A, Co-Co 2.575(1) A, S-Co-S 69.81(3)°, and Co-S-Co 110.19(3)°. The Co₈ part of the structure may be visualized as two Co₅ square pyramids sharing an edge (Co3-Co3A)with two apical Co atoms (Co4,4A) on opposite sites of the two basal planes which are themselves coplanar. Also identifiable are two Co₄S square pyramids with the same common edge and oppositely placed apical μ₄-S(2,2A) atoms. All Co sites are four-coordinate and approach tetrahedral stereochemistry. Atoms Co(3,3A) show the most pronounced deviation, with one large angle S3-Co3-S4A = 133.87(3)°; other angles at these atoms are the 100.6–110.2° range.

Figure 10.

Structure of [Co₈S₈(PPr ⁱ₃)₆] showing 30% probability ellipsoids and the atom numbering scheme. The cluster has crystallographically imposed centrosymmetry. Isopropyl groups are omitted for clarity. Selected interatomic distances (A) and angles (deg): Co-(μ₃-S) 2.21[4], Co-(μ₄-S) 2.24[1], Co-Co 2.59[7], Co3–Co4 2.952(1), Co3A–Co4 2.913(1), Co-P 2.24[1], S2-Co3-S2A 110.19(3), Co3-S2-Co3A 69.81(3).

Similarly, in the preparation of cation cluster 4, the compound [8](BF₄) was obtained as a byproduct in minor amounts. The structure of the cation (not shown) is nearly indistinguishable from that of 7, and there is no evidence of a discrete Co^III site. Further structural details on 7 and 8 are available.³⁰ These structures are precedented only by the compounds [Co₈Se₈(PPh₃)₆][CoCl₃(PPh₃)] and [Co₈Se₈(PPh₃)₆][Co₆Se₈(PPh₃)₆] originating from the reaction of [CoCl₃(PPh₃)]¹⁻ and (Me₃Si)₂Se.²⁷

Although we have not been able to devise a synthetic method for which 7 is the principal product, we present this cluster here because of our interest in and utilization of Fe₈S₈ double cubane clusters in the preparation of other high-nuclearity clusters. Evidently, for [M₈S₈] cores with tetrahedral sites there are at least two structural solutions, the edge-bridged double cubane (EBDC) and the rhomb-bridged non-cubane (RBNC). The present examples of these structures are topological isomers. When considered in terms of core valence electron count, stable homometallic structures are currently restricted to 119–120 e⁻ for RBNC (7, 8) and 110–112 e⁻ for EBDC (10, 11, [Fe₈S₈(PPrⁱ₃)₄(SSiPh₃)₂]¹¹).⁵⁰ This observation suggests that the two structures might be of competitive energy somewhere in between these limits. We note the conceptual least-motion RBNC ← EBDC interconversion of Figure 11. Proceeding from RBNC, cluster deconstruction affords two M₃S₃ fragments and the interior rhomb. Rhomb rotation by 90° around the S-S axis and recombination with the trinuclear fragments generates the EBDC. The process in effect converts two square pyramidal M₄S entities in the original structure to two trigonal bipyramidal M₄S substructures The cluster core [Fe₄Co₄S₈], if synthetically accessible with tolerably small dimensional differences (Table 1), is one candidate with which to address the issue of relative stability of these two structures.

Figure 11.

Conceptual topological transformation between a rhomb-bridged noncubane (RBNC) and edge-bridged double cubane (EBDC) structures of clusters with [M₈S₈] cores.

Summary

The following are the principal results and conclusions of this investigation.

1. The assembly system CoCl₂/Prⁱ₃P/(Me₃Si)₂S in THF affords the cluster [Co₄S₄(PPrⁱ₃)₄], which upon reaction with the carbene Prⁱ₂NHCMe₂ yields [Co₄S₄(Prⁱ₂NHCMe₂)₄]. These compounds can be oxidized to monocations by reactions with ferrocenium or tropylium. These species are the first examples of [M₄S₄L₄]^z clusters with cubane-type stereochemistry, tetrahedral M sites, and a transition metal other than iron.

2. The clusters in (1) are essentially isostructural with one another and with [Fe₄S₄(Prⁱ₂NHCMe₂)₄]. Small dimensional differences between the cobalt and iron carbene cluster are mainly due to differences in M^II radii. Structures of mixed-valence cation clusters do not reveal electronically localized sites.

3. Ligand substitution of phosphine by carbene in (1) and with iron clusters¹³ and lower redox potentials for the 0/1+ and 1+/2+ couples of carbene vs. phosphine clusters at constant metal are consistent with carbene being the better electron donor, at least to tetrahedral Co^II and Fe^II.

4. [Co₄S₄(PPrⁱ₃)₄] and [Co₄S₄(Prⁱ₂NHCMe₂)₄] have S_t = 3 ground states,⁵¹ which may be rationalized in terms of coupling amongst three parallel and one antiparallel S = 3/2 spins of the Co^II sites. This 3:1 pattern of spin-coupling applies also to [Fe₄S₄(Prⁱ₂NHCMe₂)₄], whose S_t = 4 ground state motivated this investigation. A theoretical model of the origin of the spin-coupling scheme for the cobalt clusters is not yet available. A model based on spontaneous distortions of [Fe₄S₄]⁰ core has been recently described.¹⁵

5. The [Co₈S₈]⁰ core of [Co₈S₈(PPrⁱ₃)₆], obtained as a minor biproduct in the synthesis of [Co₄S₄(PPrⁱ₃)₄], is compositionally analogous to but structurally different from the [Fe₈S₈]⁰ core of known clusters. The clusters are topological isomers. The cobalt cluster is a rhomb-bridged noncubane (RBNC) and the iron cluster an edge-bridged double cubane (EBDC). Core relative stabilility may be related to valence electron count. A conceptual model for the RBNC ← EBDC interconversion is presented. The EBDC structure is populated by homometallic Fe₈S₈ and heterometallic M₂Fe₆S₈ cluster whereas the RBNC structure is currently known only for Co₈Q₈ (Q = S, Se) clusters.