Substituent effects on iron boryl and borylene systems: Unusual reactivity and spectroscopic properties

Synthetic studies illustrate the crucial roles played by the steric and electronic properties of both the metal-bound ancillary ligands, L, and the boron-bound substituents, X, in the structural and reaction chemistry of boryl (L_nMBX₂) and borylene systems (L_nMBX). Thus, the strong σ donor properties of trialkyl phosphine ligands are reflected in the fact that [CpFe(dmpe)(BOMes)]⁺ is shown to be a tractable, room-temperature stable species, whereas [CpFe(CO)₂(BOMes)] ⁺ is not. Moreover the enhanced steric profile of dmpe (compared to a dicarbonyl ancillary ligand set) is shown to influence the propensity for electrophilic alkylation at boryl amino substituents, for example in CpFe(dmpe){B(NMe₂)Cl} vs. CpFe(CO)₂{B(NMe₂)Cl}. Steric factors within the X substituent itself are also of key importance, and provide, for example, a rationale for the differing lability of NMe₂ and NCy₂ substituents in the presence of [Me₃O][BF ₄]. In addition, relatively subtle electronic factors within the B-X linkage have been shown to have a profound effect on spectroscopic properties, with the unusual chemical shift measured for [CpFe(dmpe)(BOMes)]⁺ being influenced by the presence of a heteroatom substituent featuring two lone pairs.