(Dimethylamino)borylene and Related Complexes of Electron-Rich Metal Fragments: Generation of Nucleophile-Resistant Cations by Spontaneous Halide Ejection

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.