Reactivity of a Stable and Highly Electron-Rich (η⁶-Diborabenzene)nickel(0) Synthon

The reaction of the diborabenzene (DBB) nickel(0) pogo-stick complex [(η⁶-DBB)Ni(CO)] (II) with a large excess of [Ni(CO)₄] yields the dark green, unstable dinickel(0) complex [(η⁶-DBB)Ni(μ-CO)Ni(CO)₃] (1), which loses one CO ligand to yield the purple, bimetallic Ni⁰₂ half-sandwich complex [(η⁶-DBB)Ni₂(μ-CO)(η¹-CO)₂] (2). The addition of the chromium aminoborylene complex [(OC)₅Cr{BN(TMS)₂}] (TMS = trimethylsilyl) to II does not result in the expected borylene transfer but in the formation of the black Ni⁰-Cr⁰ complex [(η⁶-DBB)Ni(μ-CO)Cr(CO)₅] (3), alongside the dimeric iminoborane [(TMS)BN(TMS)]₂ (4), which results from the rearrangement of the released BN(TMS)₂ aminoborylene moiety. Furthermore, the oxidative addition of methyl triflate (MeOTf) to II leaves the (η⁶-DBB)Ni moiety intact and provides the ionic Ni^II half-sandwich complex [(η⁶-DBB)NiMe(CO)]OTf (5), while reaction with pentaphenylborole (PPB) yields the unique, dark-blue, unsymmetrical sandwich complex [(η⁶-DBB)Ni₂(μ-CO)₂(η⁵-PPB)] (6). DFT calculations point toward 6 being a dinickel(0) complex with a neutral aromatic DBB and a neutral antiaromatic PPB ligand.