Rational synthesis of high nuclearity Mo/Fe/S clusters:: The reductive coupling approach in the convenient synthesis of (Cl4-cat)2Mo2Fe6S8(PR3)6 [R = Et, nPr, nBu] and the new [(Cl4-cat)2Mo2Fe2S3O(PEt3)3Cl]•1/2(Fe(PEt3)2(MeCN)4) and (Cl4-cat)2Mo2Fe3S5(PEt3)5 clusters

A general method for the synthesis of high nuclearity Mo/Fe/S clusters is presented and involves the reductive coupling of the (Et4N)(2)[(Cl-4-cat)MoOFeS2Cl2] (1) and (Et4N)(2)[Fe2S2Cl4] (H) clusters. The reaction of I and II with Fe(PR3)(2)Cl-2 or sodium salts of noncoordinating anions such as NaPF6 or NaBPh4 in the presence of PR3 (R = Et, Pr-n, or Bu-n) affords (Cl-4-cat)(2)Mo2Fe6S8(PR3)(6) [R = Et (IIIa), Pr-n (IIIb), Bu-n (IIIc)], Fe6S6(PEt3)(4)Cl-2 (IV) and (PF6)[Fe6S8((PPr3)-Pr-n)(6)] (V) as byproducts. The isolation of (Et4N)[Fe(PEt3)Cl-3] (VI), NaCl, and SPEt3 supports a reductive coupling mechanism. Cluster IV and V also have been synthesized by the reductive self-coupling of compound II. The reductive coupling reaction between I and II by PEt3 and NaPF6 in a 1:1 ratio produces the (Et4N)(2)[(Cl-4-cat)Mo(L)Fe3S4Cl3] clusters [L = MeCN (VIIa), TBF (VIIb)]. The hitherto unknown [(Cl-4-cat)(2)Mo2Fe2S3O(PEt3)(3)Cl-3] cluster (VIII) has been isolated as the 2:1 salt of the (Fe(PEt3)(2)(MeCN)(4))(2+) cation after the reductive self-coupling reaction of 1 in the presence of Fe(PEt3)(2)Cl-2. Cluster VIII crystallizes in the monoclinic space group P2(1)/c with a = 11.098(3) Angstrom, b = 22.827(6) Angstrom, c =- 25.855(6) Angstrom, beta = 91.680(4)degrees, and Z = 4. The formal oxidation states of metal atoms in VIII have been assigned as Mo-III, Mo-IV, Fe-II, and Fe-III on the basis of zero-field Mossbauer spectra. The Fe(PEt3)2(MeCN)4 cation of VIII is also synthesized independently, isolated as the BPh4- salt (IX), and has been structurally characterized. The reductive coupling of compound I also affords in low yield the new (Cl-4-cat)(2)Mo2Fe3S5(PEt3)(5) cluster (X) as a byproduct. Cluster X crystallizes in the monoclinic space group P2(1)/n with a = 14.811(3) Angstrom, b = 22.188(4) Angstrom, c = 21.864(4) Angstrom,beta = 100. 124(3)degrees, and Z = 4 and the structure shows very short Mo-Fe, Fe-Fe, Mo-S, Fe-S bonds. Them oxidation states of the metal atoms in this neutral cluster (X) have been assigned as (MoMoFeFeFeII)-Mo-IV-Fe-III-Fe-II-Fe-II based on zero-field Mossbauer and magnetic measurement. All Fe atoms are high spin and two of the three Fe-Fe distances are found at 2.4683(9) Angstrom and 2.4721(9) Angstrom.