CpIr(dab) (dab = 1,4-Bis(2,6-dimethylphenyl)-1,4-diazabutadiene):  A Coordinatively Unsaturated Six-π-Electron Metallaheteroaromatic Compound?

1,4-Bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene (dab) forms the structurally characterized iridium(III) complex [Cp*IrCl(dab)](PF6): C28H35ClF6IrN2P, orthorhombic, space group Pnma, a = 16.187(2) Å, b = 15.823(2) Å, c = 11.677(1) Å, V = 2990.8(6) Å3, Z = 4, and R = 0.0588. On reaction with NaBH3CN this compound does not form an iridium(III) hydride but the coordinatively unsaturated reduced product Cp*Ir(dab): C28H35IrN2, monoclinic, space group P21/n, a = 8.484(2) Å, b = 14.535(3) Å, c = 20.956(4) Å, β = 98.88(3)°, V = 2553.2(9) Å3, Z = 4, and R = 0.0586. The inverted relation d CC (=1.334(15) Å) < d CN (=1.379(13) and 1.366(14) Å) in the dab ligand of Cp*Ir(dab) suggests that the reduction has occurred primarily at that ligand to form an ene-1,2-diamido/iridium(III) moiety or, alternatively, a six-π-electron metallaheteroaromatic system. Ab initio pseudopotential calculations of model complexes [CpIr(HNCHCHNH)]0/2+ support this description of the bonding.