Tripodal Silanolate Ligands Expand [MoX] Chemistry Beyond Its Traditional Borders.

Homodimeric complexes [XMo≡MoX] are commonplace, but in no case is the corresponding monomeric [MoX] species known; conversely, none of the very rare monomeric complexes [MoX] has the respective homodimeric analogue. This mutual exclusivity ends with the present study; on top, an entirely unprecedented class of heterodimers of type [XMo≡MoY] is reported. Key to success was the use of tripodal silanolates as ancillary ligands; the fence formed by properly chosen peripheral substituents shields the sensitive Mo(+3) center; homodimerization of the resulting [MoX] complexes is then kinetically strongly disfavored, though possible. The monomers are able to cleave NO and convert -dihalides into metal alkylidynes; they exist in different binding modes, in which the basal phenyl ring of the ligand backbone is either completely unengaged with the central metal or tightly bound to it, depending on whether the ligand sphere is complemented by solvent molecules or not. If the latter are sufficiently labile, a surprisingly facile heterodimerization of the d electron fragments will ensue; the resulting products [XMo≡MoY] incorporate the intact Cummins complex [(Bu)(Ar)N]Mo (Ar = 3,5-dimethylphenyl) as one of their constituents, which is famous for not engaging in metal-metal triple bonding otherwise. Heterodimerization was also observed with simple -butoxide ligands. The new type of heterodimers features unusually long yet robust Mo≡Mo bonds, which are notably polarized according to DFT. However, there is no direct correlation between the extreme Mo≡Mo bond lengths and the strikingly deshielded Mo NMR signals, since ligand-based orbitals can also markedly affect the shielding tensor.