Reaction of BH₄⁻with {Mo₂Cp₂(μ-SMe)_n} species to give tetrahydroborato, hydrido or dimetallaborane compounds: control of product by ancillary ligands

Abstract

The reaction of mono- or dichloro-dimolybdenum(III) complexes [Mo2Cp2(mu-SMe)2(mu-Cl)(mu-Y)] (Cp=eta5-C5H5; 1, Y=SMe; 2, Y=PPh2; 3, Y=Cl) with NaBH4 at room temperature gave in high yields tetrahydroborato (8), hydrido (9) or metallaborane (12) complexes depending on the ancillary ligands. The correct formulation of derivatives and has been unambigously determined by X-ray diffraction methods. That of the hydrido compound 9 has been established in solution by NMR analysis and confirmed by an X-ray study of the mu-azavinylidene derivative [Mo2Cp2(mu-SMe)2(mu-PPh2)(mu-N=CHMe)] (10) obtained from the insertion of acetonitrile into the Mo-H bond of 9. Reaction of NaBH4 with nitrile derivatives, [Mo2Cp2(mu-SMe)4-n(CH3CN)2n]n+(5, n=1; 6 n=2), afforded the tetrahydroborato compound 8, together with a mu-azavinylidene species [Mo2Cp2(mu-SMe)3(mu-N=CHMe)](14), when n=1, and the metallaborane complex 12, together with a mixed borohydrato-azavinylidene derivative [Mo2Cp2(mu-SMe)2(mu-BH4)(mu-N=CHMe)] (13), when n=2. The molecular structures of these complexes have been confirmed by X-ray analysis. Preparations of some of the starting complexes (3 and 4) are also described, as are the molecular structures of the precursors [Mo2Cp2(mu-SMe)2(mu-X)(mu-Y)] (1, X/Y=Cl/SMe; 2, X/Y=Cl/PPh2; 4, X/Y=SMe/PPh2).

Mixed μ-phosphido or μ-thiolato μ-halo-dimolybdenum(III) compounds [Mo2Cp2(μ-SMe)2(μ-X)(μ-Y)] (X=PPh2, Y=Cl; X=SCH3, Y=Br, I): Electrochemical and structural comparisons – The X-ray structure of [{Mo2Cp2Br(μ-O)(μ-SMe)2}2(μ-MoO4)]

“…The phosphido complexes thus appear ''hotter'' than their tris(thiolato)-bridged analogues and able to react under milder conditions. This is consistent with other results we recently reported [6].…”

“…As is usual in this group of compounds, the bridges in 1-4 are all nearly symmetrical. In 3 (and also in 1 and 2 [6]) the SMe groups cis to the l-Y bridge are syn, with the methyl groups adjacent to Y; in 4 the alternative anti arrangement of these methyl groups is found. The Mo-Mo bond lengths in Table 1 are nearly constant and evidently are insensitive to the nature of the bridging groups.…”

“…The close analogy between the structural characteristics of the triply-bridged {Mo 2 Cp 2 Br(l-SMe) 2 (l-O)} core Table 1 Selected bond lengths (Å ) and angles (°) for the [Mo 2 Cp 2 (l-SMe) 2 (l-X)(l-Y)] complexes 1 (X = PPh 2 ) [6], 2 (X = SMe) [6], 3 (X= SMe) and 4 (X = SMe) a (1) [1][2][3][4]. Though normal electron counting rules require the presence of a Mo-Mo double bond in 5, it is known [8,9] (1) ðl-SMeÞðl-BrÞðl-OÞ is not surprising -see [8,9] (2) and 1.723(2) Å ).…”

“…The crystal structures of [Mo 2 Cp 2 (l-SMe) 3 (l-Y)] (Y = Br (3), I (4)) Single crystals of 3 and 4 were obtained from Et 2 O solution. The resulting molecular structures are compared with those of 1 and 2 [6] in Table 1. In the asymmetric unit of 3 there are three independent but structurally equivalent molecules (A, B and C).…”

“…In this context we have shown by single-crystal X-ray analysis of 1 that the PPh 2 group is trans to chloride; this implies that the conversion of 2 to 1 exclusively involves replacement of the thiolate bridge trans to l-Cl [6]. In addition, investigation of the reactions of 1 and 2 with borohydride revealed that the different activity of the two compounds could be rationalised in term of the differing trans-effect of l-PPh 2 and l-SMe groups (Scheme 1) [6].…”