Synthesis and Reactivity of Highly Electron-Rich Zerovalent Group 10 Diborabenzene Pogo-Stick Complexes.

A cyclic alkyl(amino)carbene (CAAC)-stabilized 1,4-diborabenzene (DBB, 1 ) reacts with the group 10 precursor [Ni(CO) 4 ] to yield the DBB pogo-stick complex [(η 6 -DBB)Ni(CO)] ( 2 ) as a dark-green crystalline solid. The IR-spectroscopic and X-ray crystallographic data of 2 highlight the strong π-donor properties of the DBB ligand. The reaction of 1 with [M(nbe) 2 ] (M = Pd, Pt; nbe = norbornene) yields the unique zerovalent heavier group 10 arene pogo-stick complexes [(η 6 -DBB)M(nbe)] ( 3-M ), isolated as dark-purple and black crystalline solids, respectively. 3-Pd and 3-Pt show strong near-IR (NIR) absorptions at 835 and 904 nm, respectively. Time-dependent density functional theory (TD-DFT) calculations show that these result from the S 0 →S 1 excitation, which corresponds to a transfer of electron density from a metal d orbital aligned with the z direction (d xz or d yz ) to a d orbital located in the xy plane (d xy or d x 2 -y 2 ), with the redshift for 3-Pt resulting from the higher spin-orbit coupling (SOC). In complex 2 , electron donation from the nickel center into the carbonyl 2π* orbital destabilizes the DBB···Ni interaction, resulting in an absorption at a higher energy. Complexes 2 and 3-M react with [Fe(CO) 5 ] to yield the doubly CO-bridged M(0)→Fe(0) (M = Ni, Pd, Pt) metal-only Lewis pairs (MOLPs) 4-M as black (M = Ni, Pt) and dark-turquoise (M = Pd) crystalline solids. Furthermore, 3-Pt undergoes oxidative Sn-H addition with Ph 3 SnH to yield the corresponding Pt(II) stannyl hydride, [(η 6 -DBB)PtH(SnPh 3 )] ( 5 ).