(Dimethylamino) borylene and related complexes of electron-rich metal fragments : generation of nucleophile-resistant cations by spontaneous halide ejection

Abstract

Spontaneous halide ejection from a three-coordinate Lewis acid has been shown to offer a remarkable new route to cationic metal complexes featuring a linear, multiply bonded boron-donor ligand. The exploitation of electron-rich [CpM(PR3)2] fragments within boryl systems of the type LnMB(hal)NR2 leads to the spontaneous formation in polar solvents of chemically robust borylene complexes, [LnM(BNR2)]+, with exceptionally low electrophilicity and short M–B bonds. This is reflected by M–B distances (ca. 1.80 Å for FeB systems) which are more akin to alkyl-/aryl-substituted borylene complexes and, perhaps most strikingly, by the very low exothermicity associated with the binding of pyridine to the two-coordinate boron center (ΔH = −7.4 kcal mol–1, cf. −40.7 kcal mol–1 for BCl3). Despite the strong π electron release from the metal fragment implied by this suppressed reactivity and by such short M–B bonds, the barrier to rotation about the Fe═B bond in the unsymmetrical variant [CpFe(dmpe)(BN{C6H4OMe-4}Me)]+ is found to be very small (ca. 2.9 kcal mol–1). This apparent contradiction is rationalized by the orthogonal orientations of the HOMO and HOMO-2 orbitals of the [CpML2]+ fragment, which mean that the M–B π interaction does not fall to zero even in the highest energy conformation.