Higher spin states in some low-energy bis(tetramethyl-1,2-diaza-3,5-diborolyl) sandwich compounds of the first row transition metals: boraza analogues of the metallocenes

Abstract

The known sandwich compound [η⁵-(CH₂)₃N₂(BPh)₂CMe]₂Fe in which adjacent C₂ units are replaced by isoelectronic BN units can be considered as a boraza analogues of ferrocene similar to borazine, B₃N₃H₆, considered as a boraza analogue of benzene. In this connection, the related bis(1,2,3,5-tetramethyl-1,2-diaza-3,5-diborolyl) derivatives (Me₄B₂N₂CH)₂M (M = Ti, V, Cr, Mn, Fe, Co, Ni) for all of the first row transition metals have been optimized using density functional theory for comparison with the isoelectronic tetramethylcyclopentadienyl derivatives (Me₄C₅H)₂M. Low-energy sandwich structures having parallel B₂N₂C rings in a trans orientation are found for all seven metals. The 1,2-diaza-3,5-diborolyl ligand appears to be a weaker field ligand than the isoelectronic cyclopentadienyl ligand as indicated by higher spin ground states for some (η⁵-Me₄B₂N₂CH)₂M sandwich compounds relative to the corresponding metallocenes (η⁵-Me₄C₅H)₂M. Thus (η⁵-Me₄B₂N₂CH)₂Cr has a quintet ground state in contrast to the triplet ground state of (η⁵-Me₄C₅H)₂Cr. Similarly, the sextet ground state of (η⁵-Me₄B₂N₂CH)₂Mn lies ∼18 kcal mol⁻¹ below the quartet state in contrast to the doublet ground state of the isoelectronic (Me₄C₅H)₂Mn. These sandwich compounds are potentially accessible by reaction of 1,2-diaza-3,5-diborolide anions with metal halides analogous to the synthesis of [η⁵-(CH₂)₃N₂(BPh)₂CMe]₂Fe.