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

1,4-Bis(2,6-dimethylphenyl)-1,4-diaza-1,3-butadiene (dab) forms the structurally characterized iridium(III) complex [CpIrCl(dab)](PF(6)): C(28)H(35)ClF(6)IrN(2)P, orthorhombic, space group Pnma, a = 16.187(2) Å, b = 15.823(2) Å, c = 11.677(1) Å, V = 2990.8(6) Å(3), Z = 4, and R = 0.0588. On reaction with NaBH(3)CN this compound does not form an iridium(III) hydride but the coordinatively unsaturated reduced product CpIr(dab): C(28)H(35)IrN(2), monoclinic, space group P2(1)/n, a = 8.484(2) Å, b = 14.535(3) Å, c = 20.956(4) Å, beta = 98.88(3) degrees, V = 2553.2(9) Å(3), 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 CpIr(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-pi-electron metallaheteroaromatic system. Ab initio pseudopotential calculations of model complexes [CpIr(HNCHCHNH)](0/2+) support this description of the bonding.