Heterometal Cubane-Type MFe<sub>3</sub>S<sub>4</sub> Clusters (M = Mo, V) Trigonally\nSymmetrized with Hydrotris(pyrazolyl)borate(1−) and\nTris(pyrazolyl)methanesulfonate(1−) Capping Ligands

Field: Chemistry (Physical Sciences)

A series of heterometal cubane-type clusters containing [VFe<sub>3</sub>S<sub>4</sub>]<sup>2+</sup> and [MoFe<sub>3</sub>S<sub>4</sub>]<sup>3+,2+</sup> cores has been prepared.\nLigand substitution of [(DMF)<sub>3</sub>VFe<sub>3</sub>S<sub>4</sub>Cl<sub>3</sub>]<sup>-</sup> affords [(Tpms)VFe<sub>3</sub>S<sub>4</sub>L<sub>3</sub>]<sup>2-</sup> (L = Cl<sup>-</sup> (<b>8</b>), EtS<sup>-</sup> (<b>9</b>), <i>p</i>-MeC<sub>6</sub>H<sub>4</sub>S<sup>-</sup>,\n<i>p</i>-MeC<sub>6</sub>H<sub>4</sub>O<sup>-</sup>). A new procedure for the preparation of molybdenum single cubanes is introduced by the reaction\nof recently reported [(Tp)MoS(S<sub>4</sub>)]<sup>-</sup> with FeCl<sub>2</sub>/NaSEt to afford [(Tp)MoFe<sub>3</sub>S<sub>4</sub>Cl<sub>3</sub>]<sup>-</sup> (<b>1</b>, 75% yield). This procedure\nis more efficient that the existing multistep synthesis of single cubanes, which generally affords clusters of mirror\nsymmetry. Also prepared were [(Tp)MoFe<sub>3</sub>S<sub>4</sub>L<sub>3</sub>]<sup>-</sup> (L = EtS<sup>-</sup> (<b>2</b>), <i>p</i>-MeC<sub>6</sub>H<sub>4</sub>S<sup>-</sup>). Reduction of <b>1</b> with borohydride\ngives [(Tp)MoFe<sub>3</sub>S<sub>4</sub>Cl<sub>3</sub>]<sup>2-</sup> (<b>5</b>, 67%). Owing to the nature of the heterometal ligand, all clusters have idealized\ntrigonal symmetry, reflected in their <sup>1</sup>H NMR spectra. Trigonal structures are demonstrated by crystallography of\n(Bu<sub>4</sub>N)[<b>1</b>,<b>2</b>], (Bu<sub>4</sub>N)<sub>2</sub>[<b>5</b>]·MeCN, and (Me<sub>4</sub>N)<sub>2</sub>[<b>8</b>,<b>9</b>]. The availability of <b>1</b> and <b>5</b> allows the first comparison of structures\nand <sup>57</sup>Fe isomer shifts of [MoFe<sub>3</sub>S<sub>4</sub>]<sup>3+,2+</sup> in a constant ligand environment. Small increases in most bond distances\nindicate that an antibonding electron is added in the reduction of <b>1</b>. Collective synthetic and electrochemical results\nfrom this and other studies demonstrate the existence of the series of oxidation states [VFe<sub>3</sub>S<sub>4</sub>]<sup>3+,2+,1+</sup> and\n[MoFe<sub>3</sub>S<sub>4</sub>]<sup>4+,3+,2+</sup> whose relative stabilities within a given series are strongly ligand dependent. Isomer shifts indicate\nthat the reduction of <b>1</b> largely affects the Fe<sub>3</sub> subcluster and are consistent with the formal descriptions [MoFe<sup>3+</sup><sub>2</sub>Fe<sup>2+</sup>S<sub>4</sub>]<sup>3+</sup> (<b>1</b>) and [MoFe<sup>3+</sup>Fe<sup>2+</sup><sub>2</sub>S<sub>4</sub>]<sup>2+</sup> (<b>5</b>). Reaction of <b>1</b> with excess Li<sub>2</sub>S in acetonitrile affords the double cubane\n{[(Tp)MoFe<sub>3</sub>S<sub>4</sub>Cl<sub>2</sub>]<sub>2</sub>(μ<sub>2</sub><sub>-</sub>S)}<sup>2-</sup>, whose sulfide-bridged structure is supported by two sequential reductions separated\nby 290 mV, in analogy with previously reported double cubanes of higher charge. Trigonally symmetric single\ncubanes eliminate isomers in the formation of double cubanes and other cluster structures, and may be of considerable\nvalue in the preparation of new types of M−Fe−S clusters. (Tpms = tris(pyrazolyl)methanesulfonate(1−); Tp =\nhydrotris(pyrazolyl)borate(1−).)