Specific Incorporation of Chalcogenide Bridge Atoms in Molybdenum/Tungsten-Iron-Sulfur Single Cubane Clusters

Abstract

An extensive series of heterometal-iron-sulfur single cubane-type clusters with core oxidation levels [MFe₃S₃Q]^3+,2+ (M = Mo, W; Q = S, Se) has been prepared by means of a new method of cluster self-assembly. The procedure utilizes the assembly system [(^tBu₃tach)M^VIS₃]/FeCl₂/Na₂Q/NaSR in acetonitrile/THF and affords product clusters in 30–50% yield. The trisulfido precursor acts as a template, binding Fe^II under reducing conditions and supplying the MS₃ unit of the product. The system leads to specific incorporation of a μ₃-chalcogenide from an external source (Na₂Q) and affords the products [(^tBu₃tach)MFe₃S₃QL₃]^0/1− (L = Cl⁻, RS⁻), among which are the first MFe₃S₃Se clusters prepared. Some 16 clusters have been prepared, 13 of which have been characterized by X-ray structure determinations including the incomplete cubane [(^tBu₃tach)MoFe₂S₃Cl₂ (μ₂-SPh)], a possible trapped intermediate in the assembly process. Comparisons of structural and electronic features of clusters differing only in atom Q at one cubane vertex are provided. In comparative pairs of complexes differing only in Q, placement of one selenide atom in the core increases core volumes by ca. 2% over the Q = S case, sets the order Q = Se > S in Fe-Q bond lengths and Q = S > Se in Fe-Q-Fe bond angles, causes small positive shifts in redox potentials, and has an essentially nil effect on ⁵⁷Fe isomer shifts. Iron mean oxidation states and charge distributions are assigned to most clusters from isomer shifts. (^tBu₃tach = 1,3,5-tert-butyl-1,3,5-triazacyclohexane)

Introduction

The large majority of homo- and heterometallic iron-sulfur clusters of biomimetic relevance are synthesized under three protocols:¹ (i) self-assembly, consisting of self-organizing reactions between simple mononuclear precursors and ligand reagents, and (ii) fragment condensation in which a preformed di- or polynuclear cluster reacts with a mononuclear species, itself, or another cluster. Certain clusters formed by these methods can undergo (iii) core rearrangement, reorganization of a pre-existing cluster from (i) or (ii) to a different core geometry. Examples include formation of Fe₃S(SR)₃,² Fe₄S₄^3–5, and MFe₃S₄ (M = Mo, W)^1,6,7 clusters by self-assembly, MFe₃S₄ (M = V⁸, Ni^9,10), and M₂Fe₆S₈ (M = Fe,^11,12 Mo,^13–15) clusters by fragment condensation, and M₂Fe₆S₉ clusters (M = Mo,^16,17 V¹⁸) by core rearrangement. Frequently, clusters employed in fragment condensation are prepared by self-assembly, in which the bridging ligand reagent is metal-bound or free sulfide, hydrosulfide, sulfur, RSSR, or RSSSR and a reductant, or (R₃Si)₂S. Cluster products obtained in this way have the invariant feature of bridging sulfide core atoms, always of the μ₃-S mode when the core structure adopts the prevalent cubane-type geometry. Recent advances in synthesis have afforded new types of clusters with internal (M₂Fe₆S₉) and interstitial (Fe₈S₇)^19–21 μ₆-S bridge components.

The foregoing high-nuclearity clusters are directly pertinent to synthetic endeavors directed toward the P^N (Fe₈S₇) and FeMo-cofactor (MoFe₇S₉X) clusters of nitrogenase which contain internal μ₆-S and interstitial μ₆-X (X = C, N, or O) bridging elements, respectively.^22–24 A largely unsolved problem is the deliberate installation of non-sulfur bridge atoms (preferably X) in clusters of nuclearity four and higher. If the problem is approached with less complex cubane-type species, Fe₄(μ₃-Se)₄^25–34 and MFe₃Se₄ (M = Mo, W)³⁵ clusters, prepared by self-assembly, become relevant as a first step in that direction. Lee and coworkers have made significant progress by preparation of two unprecedented types of cubane-type clusters, Fe₄(NR)₄ by self-assembly,^36,37 and the mixed-ligand clusters Fe₄S_n(NR)_4-n (n = 1–3) by remarkable fragment condensation reactions.³⁸ The latter results are noteworthy because of first successful introduction of nitrogen bridges (as imides) and the isolation of clusters with mixed-atom bridges. Early in the development of iron-sulfur cluster chemistry, it was shown that the clusters [Fe₄Q₄(SR)₄]^2−/3− (Q = S, Se) sustain core atom exchange reactions to form [Fe₄S_nSe_4-n(SR)₄]^2−/3− in solution; mixed bridge atom clusters were detected by ¹H NMR but not isolated.²⁶

Recently, we have been experimenting with a new type of cluster assembly reaction based on trisulfido complexes of general formulation [L₃M^VIS₃]^0,1−. These are intended to act as templates in cluster self-assembly by providing in the product cluster a desired metal-trisulfido fragment MS₃ which supports metal binding when reduced and whose other coordination sites are protected by a tridentate ligand L₃. This work utilizes the compounds [(^tBu₃tach)M^VIS₃] (M = Mo, W; see Chart for abbreviations) prepared in this laboratory.³⁹ Assembly reaction systems allow specific incorporation of a single selenide atom leading to cores of the type MS₃Se that may be isolated in stable clusters. We report the synthesis, structures, and selected properties of such clusters.

Chart 1.

Abbreviations and Designation of Compounds

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 solvent purification system prior to use. In the preparations that follow, reactants were nearly always used as suspensions in solvents of specified volume, filtrations were through Celite, solvent removal steps were performed in vacuo, and products were washed with ether and dried. Sodium sulfide, sodium selenide, and ferrous chloride were used in anhydrous form from commercial sources. All new compounds were identified by combinations of ¹H NMR spectroscopy (in Me₂SO-d₆), X-ray structure determinations, and elemental analysis (Midwest Microlab, LLC, Indianapolis, IN 46250). Neutral compounds with three thiolate ligands are soluble in acetonitrile, THF and Me₂SO. Neutral compounds with one thiolate or no thiolate ligand and cluster salts are soluble in acetonitrile and Me₂SO. All compounds are extremely air-sensitive and must be handled accordingly.

[(^tBu₃tach)MoFe₂S₃Cl₂ (μ₂-SPh)]

To a suspension of [(^tBu₃tach)MoS₃]³⁹ (45 mg, 0.10 mmol) in 1 mL of methanol was added a solution of NaSPh (27 mg, 0.20 mmol) in 2 mL of methanol followed by a solution of FeCl₂ (25 mg, 0.20 mmol) in 5 mL of methanol. The reaction mixture was stirred for 12 h. The red-brown solution was evaporated to dryness, the solid was extracted with 5 mL of THF, and the extract was filtered. The volume of the filtrate was reduced to ca. 0.5 mL, affording the product as a black crystalline solid (35 mg, 47%) which sometimes contained an impurity of [(^tBu₃tach)MoFe₃S₄Cl₃]. The compound was identified by an X-ray structure determination.

[(^tBu₃tach)MoFe₃S₄Cl₃]

To [(^tBu₃tach)MoS₃] (45 mg, 0.10 mmol) in 1 mL of acetonitrile was added NaSPh (13.2 mg, 0.10 mmol) in 2 mL of acetonitrile followed by Na₂S (12 mg, 0.15 mmol) in 2 mL of acetonitrile. FeCl₂ (38 mg, 0.30 mmol) in 5 mL THF was immediately added to the solution and the reaction mixture was stirred for 36 h. The red-brown solution was evaporated to dryness, the solid was extracted with 5 mL of acetonitrile, and the extract was filtered. The filtrate was evaporated to dryness. The residue was dissolved in 1 mL of acetonitrile, the solution was filtered, and the filtrate was diffused with ether at −35°C for 2 d. The product was obtained as a black crystalline solid (32 mg, 40%), occasionally with the preceding cluster as an impurity. The compound was identified by an X-ray structure determination.

[(^tBu₃tach)MoFe₃S₄Cl₂(SPh)]

To [(^tBu₃tach)MoS₃] (45 mg, 0.10 mmol) in 1 mL of acetonitrile was added NaSPh (27 mg, 0.20 mmol) in 2 mL acetonitrile followed by Na₂S (12 mg, 0.15 mmol) in 2 mL of acetonitrile. FeCl₂ (38 mg, 0.3 mmol) in 5 mL THF was immediately added and the reaction mixture was stirred for 36 h. The red-brown solution was evaporated to dryness, the residue was extracted with 5 mL of acetonitrile, and the extract was filtered. The filtrate was evaporated to dryness. The residue was dissolved in 1 mL of acetonitrile, filtered, and diffused with ether at −35°C for 2 d to give the product as a black crystalline solid (25 mg, 30%). ¹H NMR: δ 14.60 (m-H, 2), 3.33 (CH₂, 6), 2.48 (CH₃, 27), −2.00 (o-H, 2), −4.18 (p-H, 1).

[(^tBu₃tach)WFe₃S₄Cl₂ (SPh)]

The preceding method on the same scale but with use of [(^tBu₃tach)WS₃]³⁹ was followed. The product was isolated as a black crystalline solid (28 mg, 30%). ¹H NMR: δ 14.23 (m-H, 2), 3.34 (CH₂, 6), 2.73 (CH₃, 27), −3.60 (o-H, 2), −3.99 (p-H, 1)

[(^tBu₃tach)MoFe₃S₃SeCl₂(SPh)]

The procedure on the same scale for [(^tBu₃tach)MoFe₃S₄Cl₂ (SPh)] was followed but with use of Na₂Se (19 mg, 0.15 mmol) and no Na₂S added. The product was obtained as a black crytalline solid (30 mg, 35%). ¹H NMR: δ 14.60 (m-H, 2), 3.33 (CH₂, 6), 2.48 (CH₃, 27), −1.71 (o-H, 2), −4.27 (p-H, 1).

[(^tBu₃tach)WFe₃S₃SeCl₂(SPh)]

The preceding method on the same scale for [(^tBu₃tach)WFe₃S₄Cl₂(SPh)] was employed but with use of Na₂Se (19 mg, 0.15 mmol) and no Na₂S added. The product was obtained as a black crystalline solid (30 mg, 31%). ¹H NMR: δ 14.25 (m-H, 2), 3.34 (CH₂, 6), 2.48 (CH₃, 27), −3.47 (o-H, 2), −4.11 (p-H, 1).

[(^tBu₃tach)MoFe₃S₄(SEt)₃]

To [(^tBu₃tach)MoS₃] (45 mg, 0.10 mmol) in 1 mL of acetonitrile was added NaSEt (34 mg, 0.40 mmol) in 2 mL of acetonitrile followed by Na₂S (12 mg, 0.15 mmol) in 2 mL acetonitrile. FeCl₂ (38 mg, 0.3 mmol) in 5 mL of THF was immediately added and the reaction mixture was stirred for 36 h. The red-brown solution was evaporated to dryness, the solid was extracted with 5 mL of acetonitrile, and the extract was filtered. The filtrate was evaporated to dryness. The residue was dissolved in 1 mL of acetonitrile, filtered, and diffused with ether at −35°C for 2 d, affording the product as a black crystalline solid (45 mg, 54%).¹H NMR: δ 9.09 (SCH₂ CH₃, 9), 3.30 (CH₂, 6), 2.06 (CH₃, 27), −8.44 (SCH₂ CH₃, 6).

[(^tBu₃tach)MoFe₃S₄(SEt)₃](Na)

The preceding procedure on the same scale was used but with an increased amount of NaSEt (42 mg, 0.50 mmol). The compound was isolated as a black crystalline solid (55 mg, 52%). Anal. Calcd. for C₂₁H₄₈Fe₃MoN₃NaS₇: C, 29.55; H, 5.67; N, 4.92. Found: C, 30.03; H, 5.93; N, 4.51. ¹H NMR: δ 9.60 (SCH₂CH₃, 9), 3.30 (CH₂, 6), 2.05 (CH₃, 27), −9.32 (SCH₂CH₃, 6).

[(^tBu₃tach)MoFe₃S₄(SMe)₃][Na(MeCN)]

The preceding method on the same scale was used but with NaSMe (35 mg, 0.50 mmol). The compound was isolated as a black crystalline solid (45 mg, 53%). ¹H NMR: δ 3.32 (CH₂,6), 2.05 (CH₃, 27) −9.67 (SCH₃, 9). Anal. Calcd. for C₁₈H₄₂Fe₃MoN₃NaS₇·C₂H₃N: C, 28.18; H, 5.32; N, 6.57. Found: C, 28.56; H, 5.38; N, 6.34.

[(^tBu₃tach)MoFe₃S₃Se(SEt)₃][Na(CH₃CN)₃]

To [(^tBu₃tach)MoS₃] (45 mg, 0.10 mmol) in 1 mL of acetonitrile was added NaSEt (42mg, 0.5 mmol) in 2 mL of acetonitrile followed by Na₂Se (19 mg, 0.15 mmol) in 2 mL acetonitrile. FeCl₂ (38 mg, 0.3 mmol) in 5 mL THF was immediately added and the reaction mixture was stirred for 36 h. Reaction workup is the same as for the preceding sodium salt, yielding the product as a black crystalline solid (50 mg, 45%). ¹H NMR: δ 10.25 (SCH₂CH₃, 9), 3.30 (CH₂, 6), 1.36 (CH₃, 27), −8.48 (SCH₂CH₃, 6).

[(^tBu₃tach)MoFe₃S₃Se(SMe)₃][Na(Et₂O)]

The preceding method on the same scale was used but with NaSMe (35 mg, 0.50 mmol). The compound was isolated as a black crystalline solid (45 mg, 50%). ¹H NMR: δ 3.35 (CH₂,6), 1.35 (CH₃, 27) −8.74 (SCH₃, 9). Anal. Calcd. for C₁₈H₄₂Fe₃MoN₃NaS₆Se·C₄H₁₀O: C, 26.83; H, 5.29; N, 4.69. Found: C, 26.98; H, 5.09; H, 4.79.

[(^tBu₃tach)WFe₃S₃Se(SEt)₃]

To [(^tBu₃tach)WS₃] (54 mg, 0.10 mmol) in 1 mL of acetonitrile was added NaSEt (42 mg, 0.50 mmol) in 2 mL of acetonitrile followed by Na₂Se (19 mg, 0.15 mmol) in 2 mL of acetonitrile. FeCl₂ (38 mg, 0.30 mmol) in 5 mL of THF was immediately added and the reaction mixture was stirred for 36 h. The workup procedure was the same as for [(^tBu₃tach)MoFe₃S₄(SEt)₃], yielding the product as a black crystalline solid (40 mg, 41%). Anal. Calcd. for C₂₁H₄₈Fe₃N₃S₆SeW: C, 26.13; H, 5.01; N, 4.35. Found: C, 26.02; H, 5.09; N, 4.33. ¹H NMR): δ 8.39 (SCH₂CH₃, 9), 3.29 (CH₂, 6), 1.52 (CH₃, 27), −7.72 (SCH₂CH₃, 6).

[(^tBu₃tach)WFe₃S₄(SEt)₃]

The preceding method was followed on the same scale but with use of Na₂S (12 mg, 0.15 mmol) instead of Na₂Se, affording the product as black crystals (40 mg, 43%). ¹H NMR: δ 8.77 (SCH₂CH₃, 9), 3.35 (CH₂, 6H), 1.91 (CH₃, 27), −9.29 (SCH₂CH₃, 6).

[(^tBu₃tach)MoFe₃S₄(SPh)₃]

To [(^tBu₃tach)MoS₃] (45 mg, 0.1 mmol) in 1 mL of acetonitrile was added NaSPh (52 mg, 0.4 mmol) in 2 mL acetonitrile followed by Na₂S (12 mg, 0.15 mmol) in 2 mL of acetonitrile. FeCl₂ (38 mg, 0.3 mmol) in 5 mL of THF was immediately added to the solution and the reaction mixture was stirred for 36 h. The red-brown solution was evaporated to dryness, the solid was extracted with 5 mL of THF, and the extract was filtered. The filtrate was taken to dryness. The residue was dissolved in 1 mL of acetonitrile; the solution was filtered and diffused with ether at −35°C for 2 d, affording the product as a black crystalline solid (33 mg, 31%). Anal. Calcd. for C₃₃H₄₈Fe₃MoN₃S₇: C, 40.66; H, 4.96; N, 4.31. Found: C, 39.55; H, 4.97; N, 4.45. ¹H NMR: δ 14.81 (m-H, 6), 3.31 (CH₂, 6), 2.48 (CH₃, 27), −2.68 (o-H, 6), −4.24 (p-H, 3).

[(^tBu₃tach)WFe₃S₄(SPh)₃]

The preceding method was followed on the same scale but with use of [(^tBu₃tach)WS₃]. The product was obtained as a black crystalline solid (35 mg, 30%). ¹H NMR: δ 14.24 (m-H, 6), 3.31 (CH₂, 6), 2.71 (CH₃, 27), −3.63 (o-H, 6), −3.99 (p-H, 3).

[(^tBu₃tach)MoFe₃S₃Se(SPh)₃]

To[(^tBu₃tach)MoS₃] (45 mg, 0.10 mmol) in 1 mL of acetonitrile was added NaSPh (52 mg, 0.40 mmol) in 2 mL of acetonitrile followed by Na₂Se (19 mg, 0.15 mmol) in 2 mL of acetonitrile. A suspension of ferrous chloride (38 mg, 0.3 mmol) in 5 mL THF was immediately added to the solution and the reaction mixture was stirred for 36 h. The reaction workup followed that for [(^tBu₃tach)MoFe₃S₄(SPh)₃], leading to the product as a black solid (35 mg, 33%) which analyzed as an acetonitrile hemisolvate (in conformity with the X-ray structure). Anal. Calcd. for C₃₃H₄₈Fe₃MoN₃S₆Se·0.5 C₂H₃N: C, 39.19; H, 4.79; N, 4.70. Found: C, 38.90; H, 4.69; N, 4.43. ¹H NMR: δ 14.75 (m-H, 6), 3.30 (CH₂, 6), 2.22 (CH₃, 27), −1.99 (o-H, 6), −4.28 (p-H, 3).

[(^tBu₃tach)WFe₃S₃Se(SPh)₃]

The preceding procedure was employed but with use of [(^tBu₃tach)WS₃], affording the product as a black solid (35 mg, 31%). Anal. Calcd. for C₃₃H₄₈Fe₃N₃S₆SeW: C, 35.72; H, 4.36; N, 3.79. Found: C, 35.51; H, 4.38; N, 3.77. ¹H NMR : δ 14.34 (m-H, 6), 3.31 (CH₂, 6), 2.48 (CH₃, 27), −3.64 (o-H, 6), −4.15 (p-H, 3).

In the sections following, clusters are numerically designated according to the Chart

X-ray Structure Determinations

The structures of the 13 compounds in Tables 1 and 2 were determined. Diffraction-quality crystals were obtained by ether diffusion into acetonitrile solutions. Crystal mounting and data collections were performed as described⁴⁰ on a Bruker APEX II CCD single-crystal diffractometer. Data for compound 7b were collected at the Advanced Photon Source. None of the crystals showed significant decay during the data collection procedure. Raw data were integrated and corrected for Lorentz and polarization effects using Bruker APEX II program suite.⁴¹ Absorption corrections were performed using SADABS. Space groups were assigned by analysis of symmetry and systematic absences (determined by XPREP) and were further checked by PLATON. Structures were solved by direct methods and refined against all data in the 2θ ranges by full-matrix least-squares on F² with the SHELXL program suite⁴² using the OLEX 2 interface.⁴³ Hydrogen atoms at idealized positions were included in final refinements. The OLEX 2 interface was used for structure visualization and drawing ORTEP figures (shown in Figure 3 and 4). Refinement details and explanations (wherever necessary) are included in the individual CIF files. Crystallographic data and final agreement factors are given in Tables 1 and 2.⁴⁴

Table 1.

Crystallographic Data for Compounds at 100 K^a and 15 K^b (7b).

compounds	2	3	4a	4b	6	7a	8a
formula	C21H38Cl2Fe2 MoN3S4. 1.5(CH4O)	C15H33Cl3Fe3 MoN3S4	C21H38Cl2Fe3Mo N3S5	C21H38Cl2Fe3 MoN3S4Se	C21H48Fe3 MoN3S7	C21H48Fe3N3 S6SeW	C18H42Fe3MoN3 NaS7. 0.5(C4H10O)
formula weight	787.42	753.56	827.28	874.17	830.6	965.39	848.57
crystal system	monoclinic	orthorhombic	monoclinic	monoclinic	monoclinic	orthorhombic	monoclinic
space group	Cc	Pnma	P21	P21/n	Pc	Pna21	P21/n
Z	2	4	2	4	2	4	2
a, Å	10.314(4)	18.3532(18)	14.516(3)	9.8517(8)	9.8714(19)	19.982(3)	9.8665(14)
b, Å	17.255(7)	14.7661(14)	10.013(2)	19.9821(17)	10.0395(19)	16.568(2)	20.871(3)
c, Å	19.572(9)	9.5624(9)	14.518(3)	17.4884(15)	19.241(3)	9.7591(12)	17.322(2)
α, deg	90	90	90	90.00	90	90.00	90
β, deg	93.301(9)	90	112.545(3)	90.024(2)	120.723(7)	90.00	91.349(3)
γ, deg	90	90	90	90.00	90	90.00	90
V, Å3	3477(3)	2591.5(4)	1948.9(7)	3442.7(5)	1639.2(5)	3230.8(7)	3566.0(9)
dcalcd, g/cm3	1.504	1.931	1.410	1.687	1.683	1.985	1.581
µ, mm−1	1.594	2.76	1.828	3.066	2.138	1.491	1.979
max, min peaks, e.Å−3	1.709, −1.416	3.720, −7.290	1.565, −1.106	1.173, −1.388	1.926, −1.138	4.204, −3.746	2.794, −2.199
2θ range, deg	4.16–51.46	4.44–51.48	3.38–51.5	3.1–51.44	4.06–51.4	1.86–32.46	3.06–51.42
R1c (wR2d)	0.0638(0.1040)	0.0687(0.1662)	0.0787(0.2229)	0.0801(0.1446)	0.0973(0.2010)	0.0659(0.1967)	0.0565 (0.1549)
GOF (F2)	1.082	1.062	1.065	1.086	1.035	1.039	1.067

^aMo Kα radiation (λ = 0.71073 Å).

^bSynchrotron source (λ = 0.41328 Å).

^cR₁=Σ| |F₀| − |F_c| | /Σ |F₀|.

^dwR₂ = {Σ[w(F₀² − F_c²)²]/Σ[w(F₀²)²]}^1/2

Table 2.

Crystallographic Data for Compounds at 100 K^a

compounds	8b	9a	9b	10a	10b	11a
formula	C18H42Fe3Mo N3NaS6Se. 0.5(C4H10O)	C27H57Fe3Mo N6NaS7. (C4H10O)	C27H57Fe3Mo N6NaS6Se. (C4H10O)	C33H48Fe3Mo N3S7. 2(C2H3N)	C33H48Fe3Mo N3S6.Se 0.5 (C2H3N)	C37H54Fe3N5S7W
formula weight	895.46	1050.88	1097.77	1056.83	1042.14	1144.67
crystal system	monoclinic	monoclinic	monoclinic	monoclinic	triclinic	monoclinic
space group	P21/n	P21/n	P21/n	P21/n	P-1	P21/n
Z	2	4	4	4	2	4
a, Å	9.9215(10)	13.1400(15)	13.1604(18)	13.137(3)	12.296(3)	13.1609(17)
b, Å	20.895(2)	18.913(2)	19.000(3)	20.952(5)	18.323(4)	20.997(3)
c, Å	17.3048(18)	19.779(2)	19.816(3)	16.063(4)	19.022(4)	16.036(2)
α, deg	90.00	90.00	90.00	90.00	81.471(3)	90
β, deg	91.424(2)	93.348(2)	93.553(2)	96.336(4)	76.904(3)	96.772(2)
γ, deg	90.00	90.00	90.00	90.00	78.096(3)	90
V, Å3	3586.4(6)	4907.1(10)	4945.6(12)	4394.4(19)	4061.2(14)	4400.4(10)
dcalcd, g/cm3	1.658	1.423	1.474	1.597	1.704	1.728
µ, mm−1	2.925	1.456	2.138	1.616	2.587	3.941
max, min peaks, e.Å−3	2.818, −3.046	1.504, −1.406	1.585, −2.076	4.140, −1.396	3.149, −1.924	3.514, −1.718
2θ range, deg	3.06–51.38	3.62–51.36	3.76–51.4	3.20–51.46	3.00–51.62	3.68–51.38
R1b (wR2c)	0.0683(0.1942)	0.0785(0.2070)	0.0778(0.1927)	0.0672(0.1573)	0.0732(0.1930)	0.0299(0.0803)
GOF (F2)	1.077	1.066	1.082	1.047	1.099	1.057

^aMo Kα radiation (λ = 0.71073 Å).

^bR₁=Σ | |F₀| − |F_c| | /Σ |F₀|.

^cwR₂ = {Σ[w(F₀² − F_c²)²]/Σ[w(F₀²)²]}^1/2

Figure 3.

Structures of single cubanes (2, 3, 7b, 9a, 9b and 11a) shown with 50% probability ellipsoids and partial atom labeling schemes. Hydrogen atoms are omitted for clarity.

Figure 4.

Structures of single cubanes (4a, 4b, 8a, 8b, 10a and 10b) shown with 50% probability ellipsoids and partial atom labeling schemes. Hydrogen atoms are omitted for clarity. 8a and 8b exist in dimeric forms involving two sodium atoms each coordinated with diethylether molecule of one half occupancy.

Other Physical Measurements

¹H NMR spectra were obtained with a Varian M400 spectrometer in Me₂SO-d₆ solutions. Absorption spectra were measured on a Varian Cary 50 Bio spectrophotometer. ⁵⁷Fe Mössbauer spectra were measured with a constant acceleration spectrometer. Data were analyzed with Igor Pro 6 software (Wavemetrics, Portland, OR); isomer shifts are referenced to iron metal at room temperature. Cyclic voltammetry measurements were made with a BioAnalytical Systems Epsilon potentiostat/galvanostat in acetonitrile solutions at 100 mV/s using a glassy carbon working electrode, 0.1 M (Bu₄N)(PF₆) supporting electrolyte, and an SCE reference electrode.

Results and Discussion

Synthesis and Scope

In the cluster assembly systems of Figure 1, the trisulfido complexes [(^tBu₃tach)M^VIS₃] (M = Mo, W) are utilized as scaffolds in which an intact MS₃ group is converted to the plausible heterometal unit [MFe₃(μ₃-S)₃] and the tridentate ^tBu₃tach ligand blocks reaction at three M sites. This unit is capable of incorporating a fourth chalcogenide atom to complete the cubane-type core [MFe₃S₃Q]^3+,2+ (Q = S, Se). Preparation of 1a and 1b involves sulfide transfer to [(^tButach)M^VIO₃] by Lawesson's reagent ((p-MeOC₆H₄PS₂)₂).³⁹ During reinvestigation of the isolation of 1a, we obtained a molybdenum derivative of this reagent, [(^tBu₃tach)MoO(μ₂-S)₂(S)P(p-C₆H₄OMe)], whose X-ray structure suggests that it may be an intermediate in the reaction. Because no Mo or W complex with Lawesson's reagent has been obtained previously, we include the structure of this compound.⁴⁴

Figure 1.

Schematic depiction of the synthesis of (Mo/W)-Fe-(S/Se) clusters.

The cluster assembly systems in Figure 1 consist of 1a or 1b, FeCl₂, Na₂Q, and NaSR in THF/acetonitrile. The quantity of thiolate controls the number of iron-bound ligands and core redox state as reflected by cluster charge, leading to isolation of the neutral clusters 3, 4ab, 6, 10ab, and 11ab with 3+ cores and monanionic clusters 8ab and 9ab with 2+ cores. With 1b only the neutral clusters 5ab, 7ab, and 11ab have been isolated. Neutral clusters are formed under the generalized stoichiometry of reaction 1 (n = 1, 2, 4) and monoanionic clusters by reaction 2 (Q = S, Se for both) in which additonal thiolate acts as a reductant. Under the conditions that afford reduced molybdenum clusters 9ab, neutral tungsten clusters 7ab are isolated. This result is apparently another manifestation of the greater difficulty in reducing the tungsten member of an isoligated isostructural Mo/W pair of complexes.⁴⁵

$$2[({}^{\mathrm{t}}{\text{Bu}}_3\text{tach}){\text{MS}}_3]+6{\text{ FeCl}}_2+2\mathrm{n}\text{ NaSPh}+2{\text{ Na}}_2\mathrm{Q}\mathbf{\to }2[({}^{\mathrm{t}}{\text{Bu}}_3\text{tach}){\text{MFe}}_3{\mathrm{S}}_3{\text{QCl}}_{4-\mathrm{n}}{(\text{SPh})}_{\mathrm{n}-1}]+\text{PhSSPh}+2(\mathrm{n}+2)\text{NaCl}$$ (1)

$$[({}^{\mathrm{t}}{\text{Bu}}_3\text{tach}){\text{MS}}_3]+3{\text{ FeCl}}_2+5\text{NaSR}+{\text{Na}}_2\mathrm{Q}\mathbf{\to }\text{Na}[({}^{\mathrm{t}}{\text{Bu}}_3\text{tach}){\text{MFe}}_3{\mathrm{S}}_3\mathrm{Q}{(\text{SR})}_3]+\text{RSSR}+6\text{ NaCl}$$ (2)

The cubane clusters exhibit isotropically shifted ¹H NMR spectra as a consequence of paramagnetism, presumably similar to that of the [MoFe₃S₄]³⁺ (S = 3/2⁴⁶) and [MoFe₃S₄]²⁺ (S = 2^46,47) oxidation states determined for other clusters. These spectra, four of which (7b, 8a, 9a, 11a) encompassing both oxidation states, are included in Figure 2, are useful in cluster identification. The alternating signs of the o-H (+10.9 ppm), m-H (−7.0 ppm) and p-H (+11.1 ppm) isotropic shifts of benzenethiolate cluster 11a indicate that these shifts are dominantly contact in origin.⁴⁸ The same behavior is exhibited by 4ab, 5ab, 10ab, and 11b. The spectrum of methanethiolate cluster 8a (−9.67 ppm) leads the assignment of the CH₂ (−9.32 ppm)) and CH₃ (9.60ppm) signals of ethanethiolate cluster 9a and also 6, 7ab, and 9b. While these spectra could have been assigned on the basis of relative intensities, the oppositely signed isotropic shifts of the SEt group (CH₂, −8.57 ppm; CH₃, +11.52 ppm) are unexpected. Nearly always, contact shifted proton resonances of n-alkyl groups attenuate along the chain with the same sign in the absence of dipolar contributions to the shifts, a behavior observed for other [MoFe₃S₄]³⁺ clusters.^49–51 The first examples of ¹H NMR spectra of [MoFe₃S₄]²⁺ clusters with alkylthiolate ligands are reported here.

Figure 2.

¹H NMR spectra of representative clusters in Me₂SO-d₆. Signal assignments are indicated. Peaks denoted as a and b are CH₃ CN and (CH₃ CH₂)₂O respectively.

Also prepared in this work, by means of reaction 3, is the incomplete cubane 2, so designated because of the vacancy of an iron atom in one vertex of a cubane. The cluster, shown in Figure 3, may be considered as a trapped intermediate in the cluster assembly

$$2[({}^{\mathrm{t}}{\text{Bu}}_3\text{tach}){\text{MoS}}_3]+4{\text{ FeCl}}_2+4\text{ NaSPh}\mathbf{\to }2[({}^{\mathrm{t}}{\text{Bu}}_3\text{tach}){\text{MoFe}}_2{\mathrm{S}}_3{\text{Cl}}_2(\text{SPh})]+\text{PhSSPh}+4\text{NaCl}$$ (3)

process. Once the μ₂-SPh bridge is formed, cubane formation does not proceed and 2 is isolated as a stable solid. On occasion, cubane 3 is found as an impurity in the preparation of 2 and conversely. The X-ray structure of one crystal contained both 2 and 3.⁴⁴ Cluster 3 has also been isolated in low yield by reaction 1 with no externally added Na₂S, indicating that initial complex 1a is susceptible to sulfide extraction. This event may lead to undesirable side reactions and may contribute to the observed moderate yields (ca. 30–50%) of the cubane clusters. However, all compounds were isolated by essentially the same procedure as crystalline solids that meet analytical and/or ¹H NMR standards of purity, and are readily accessible.

Cluster Structures

The cubane formulation of clusters has been verified by X-ray structure determinations of 12 compounds. Structures (except 6) are presented in Figures 3 and 4. Bond distances and angles involving variable atom Q = S, Se are collected in Table 3 together with cluster volumes calculated from atom coordinates. Other bond distances and angles are unexceptional and are available elsewhere.⁴⁴ We make several brief observations. (i) Mo-S and W-S distances in the entire set of clusters occur in the range 2.29–2.35 Å, and are substantially longer that the Mo^VI-S distance of 2.173(6) Å in 1a,³⁹ a behavior consistent with reduction to the M^III/IV level in reactions 1–3. (ii) N-M-N angles are nearly invariant at 59.5–61.3° and correspond closely to those in other M^0,IV,VI complexes (58–61°) with the same 3 ligand.^39,52,53 These angles, dictated by the 6-membered ring of ^tBu₃tach, suggest chelate ring strain larger than that in the less trigonally distorted, generally robust complexes of the type [M^III–VI(Me₃tacn)L₃]^z in which the ligand is a 9-membered ring with N-M-N angles in the range 73–79° (M = Mo, W).^39,54–57 Ring detachment owing to strain may occur during synthesis but the binding is sufficient to allow serviceable yields. (iv) Structurally comparable pairs of clusters 4ab, 8ab, 9ab, and 10ab differ only in atom Q and afford the bond length and angle orders Fe-Se > Fe-S and Fe-S-Fe > Fe-Se-Fe for individual and mean values. Although the standard deviation of mean bond lengths is large, the differences between comparable pairs is 0.10–0.13 Å; likewise, the difference between angles is 1.6–3.4°.

Table 3.

Selected Bond Distances, Angles and Core Volumes.

compoundsa	Fe-Q bond distancesb (Å)		Fe-Q-Fe anglesb (°)
	Q = S	Q = Se	Q = S	Q = Se
3	2.267(3), 2.299(3)c, 2.299(3)c	-----	72.75(9)c, 72.75(9)c, 71.3(1)	-----
4a	2.234(3), 2.255(3), 2.264(4)	-----	71.4(1), 71.4(1), 71.62(10)	-----
4b	-----	2.339(2), 2.382(2), 2.418(3)	-----	68.25(7), 70.60(7), 70.77(7)
6	2.249(7), 2.235(6), 2.322(7)	-----	73.8(2), 71.7(2), 71.0(2)	-----
7b	-----	2.349(3), 2.413(3), 2.424(3)	-----	67.43(8), 69.81 (8), 70.46(8)
8a	2.264(2), 2.303(2), 2.331(2)	-----	69.12(5), 71.70(5), 73.11(6)	-----
8b	-----	2.388(2), 2.389(2), 2.412(2)	-----	66.90(4), 68.88(5), 70.12(5)
9a	2.270(3), 2.328(2), 2.336(3)	-----	69.98(8), 71.99(8), 72.55(8)	-----
9b	-----	2.367(2), 2.425(2), 2.434(2)	-----	67.05(5), 69.40(5), 69.90(5)
10a	2.236(2), 2.287(3), 2.315(3)	-----	70.99(8), 73.21(8), 73.75(8)	-----
10b	-----	2.326(2), 2.400(2), 2.410 (2)	-----	67.64(5), 70.03(5), 70.06 (5)
11a	2.220(1), 2.292(1), 2.306(1)	-----	70.97(3), 73.24(3), 74.11(3)	-----
	core volumes (Å3)
	MFe3	Q4	MFe3Q4
9a	2.34	5.72	9.46
9b	2.36	5.94	9.64
10a	2.33	5.57	9.35
10b	2.33	5.83	9.49

^aM = Mo except 7b and 11a (M = W);

^bSee Figure 1 for position of Q in clusters;

^cEqual by symmetry element.

Oxidation States and Charge Distribution

A property of interest in heterometal iron-sulfur clusters is charge distribution as approximated by metal oxidation states. The procedure in this laboratory is to determine iron (mean) oxidation states by ⁵⁷Fe isomer shifts and infer that of the heterometal by difference. Mössbauer spectra of eight clusters were measured at 100 K; isomer shifts (δ) and quadrupole splittings are listed in Table 4. All spectra consist of a single quadrupole doublet, indicating delocalized electronic structures of these mixed valence clusters. Iron oxidation states s were calculated from the empirical relation δ = 1.43 – 0.40s which applies to tetrahedral FeS₄ sites at or near 77 K.⁴ The values s = 2.33+ and 2.67+ correspond to the formal designations M³⁺Fe²⁺₂Fe³⁺ and M³⁺Fe²⁺Fe³⁺₂, respectively. Reduced clusters 9a (2.32+) and 9b (2.40+) belong to the first designation. Oxidized clusters 8a and 10a (both 2.67+), as practically all [MFe₃S₄]³⁺ clusters examined in this laboratory, belong to the second. The empirical relationship does not strictly apply to 4ab because of chloride ligation. Values for 7b and 10b are intermediate, and no conclusions can be drawn.

Table 4.

Mössbauer Parameters (100 K) for Selected Clusters.

compounds	δ (mm/s)a	ΔEQ (mm/s)
4a	0.42	0.88
4b	0.45	0.84
7b	0.44	0.88
8a	0.36	0.76
9a	0.50	1.12
9b	0.47	0.99
10a	0.36	0.76
10b	0.41	0.83

^aReferenced with Fe metal at room temperature.

Sulfur vs. Selenium

The difference in ionic radii between Se²⁻ and S²⁻ is 1.91 – 1.84 = 0.07 Å and between covalent radii is 1.17 – 1.04 = 0.13 Å.⁵⁸ These differences are reflected in the preceding bond distance and angle orders, which are characteristic of all comparative pairs of S/Se compounds of any type in which only atom Q is varied. A further indication of these effects is seen in the core volumes of two comparative pairs, 9ab and 10ab. While values of the MFe₃ tetrahedra are practically invariant, the entire core MFe₃Q₄ volume increases by 1.9% (9ab) and 1.5% (10ab). The effect is small and expectedly smaller than replacing 4S with 4Se, as in the pair [Fe₄Q₄(SPh)₄]²⁻ where the core volumes are 9.55 ų (Q = S)⁵⁹ and 10.54 ų (Q = Se),²⁵ a 10.4% increase. Other properties are summarized in Figure 5 for the comparative pair 9ab as an example. The energy of the charge transfer band near 380 nm is unchanged and band intensites are only slightly affected. The clusters shown two chemically reversible redox steps in the −0.7 to −2.0 V range with the potentials of the selenium-containg cluster being more positive²⁵ by 10 and 50 mV. Mössbauer spectra are nearly the same and isomer shifts are within experimental uncertainty of ca. ±0.02 mm/s.

Figure 5.

Absorption spectra (in acetonitrile), redox couples (in acetonitrile) and Mössbauer spectra (100 K) of complexes 9a and 9b.

Summary

This report describes the synthesis, characterization, and comparative study of isostructural Mo/W-Fe-S single cubane clusters that differ by only one vertex atom Q = S or Se. The following are the principal results and findings of this investigation.

1. A new self-assembly system has been developed for heterometal-iron-sulfur single cubane clusters that allows selective incorporation of sulfide or selenide at one vertex of the structure. The method utilizes as templates the compounds [(^tBu₃tach)M^VIS₃] (M = Mo, W), which furnish three of the four chalcogenide atoms in the product cubane core [MFe₃S₃Q] as the MS₃ fragment.

2. An extensive series of clusters with the cores [MFe₃S₃Q]^3+,2+ has been prepared by assembly reactions 1 and 2 (30–50% purified yields) and characterized by X-ray structure determinations. The presence of Q = Se retains the cubane structure with small changes in bond distances and angles that are intrinsic to the structural behavior of selenium vs. sulfur.

3. A possible trapped intermediate [(^tBu₃tach)MoFe₂S₃Cl₂(μ₂-SPh)] in the assembly process has been isolated and shown to have an incomplete cubane structure with one vertex iron atom missing.

4. Based on the comparative properties of [(^tBu₃tach)MoFe₃S₃Q(SEt)₃]¹⁻, the Q = Se atom produces an ca. 2% increase in core volume, slightly less negative redox potentials (less easily oxidized), and virtually no change in ⁵⁷Fe isomer shift or UV-visible absorption spectrum. Isomer shifts indicate that both clusters correspond to the M³⁺Fe²⁺₂Fe³⁺ oxidation state formalism. These findings are consistent with the properties of the pair [Fe₄Q₄(SPh)₄]²⁻ for which, however, the effects of selenide become more pronounced because of complete chalcogenide substitution.²⁵

The existence of the cluster [Fe₄S₃(N^tBu)Cl₄]^2–38 makes evident the ability of weak-field cubane sterechemistry to incorporate a much smaller non-sulfur atom (Fe-N 1.95 Å) in addition to sulfide itself (mean Fe-S 2.31(4) in 9a) and the larger selenide (mean Fe-Se 2.41(4) in 9b). This raises the possibility that anions containing oxygen or nitrogen may be incorporated as μ₃ bridges using the methods employed in this work. This investigation is underway. Thereafter, it remains to be learned if non-sulfur bridge atoms can be introduced in larger structures and manipulated into an interstitial position by core rearrangement.