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 (LnMBX2) and borylene systems (LnMBX). 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][BF4]. 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.