(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.

研究表明，从三配位路易斯酸中自发消除卤化物，可为制备含线性多键合硼给体配体的阳离子金属配合物提供一条极具价值的全新路径。在通式为LnMB(hal)NR₂的硼基体系中，借助富电子[CpM(PR₃)₂]片段，可在极性溶剂中自发生成化学稳定性优异的硼宾配合物（borylene complexes）[LnM(BNR₂)]⁺，该类配合物不仅亲电性极低，且M–B键长极短。这一特征体现在两处：其一为M–B键距（铁硼体系约为1.80 Å）与烷基/芳基取代的硼宾配合物更为相近；其二则最为引人注目——吡啶与该两配位硼中心结合时的放热焓极低（ΔH = −7.4 kcal mol⁻¹，对比三氯化硼的ΔH = −40.7 kcal mol⁻¹）。尽管这种受抑制的反应活性以及极短的M–B键，均暗示金属片段存在强π电子释出效应，但在非对称变体[CpFe(dmpe)(BN{C₆H₄OMe-4}Me)]⁺中，Fe=B键的旋转能垒却极低（约2.9 kcal mol⁻¹）。这种看似矛盾的现象，可通过[CpML₂]⁺片段的最高占据分子轨道（Highest Occupied Molecular Orbital，简称HOMO）与HOMO-2轨道的正交取向得到合理解释：即便在能量最高的构象中，M–B间的π相互作用也不会完全消失。