Sterically encumbered iridium bis(N-heterocyclic carbene) systems : multiple C−H activation processes and isomeric normal/abnormal carbene complexes

The reaction of [Ir(coe)(2)Cl](2) (coe = cyclooctene) with the N-heterocyclic carbene N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) in tetrahydrofuran under anaerobic conditions leads to the formation of two main products, the planar four-coordinate Ir(I) complex Ir(IPr)(IPr")Cl (1) formed by dehydrogenation of one of the IPr isopropyl substituents (to give the mixed NHC/alkene donor IPr") and the trigonal bipyramidal Ir(III) system Ir(IPr)(aIPr)(H)(2)Cl (6), which features both "normal" and "abnormal" C-bound isomers of the NHC ligand. Formation of I presumably proceeds via initial C-H activation at iridium(l); moreover, subsequent reactivity for I initiated by chloride abstraction suggests that C-H oxidative addition chemistry is facile for the methyl C-H bonds of the carbene isopropyl substituent. Thus, the square pyramidal Ir(M) alkene alkyl hydride [Ir(IPr)(IPr")H](+)[BAr(4)(f)](-) (2) is formed on reaction with Na[BAr(4)(f)] in fluorobenzene. In contrast to the Ir(I) system 1, the alignment of the alkene ligand in solid 2 is such that it lies coplanar with the IrC(4) basal plane. Quantum chemical investigations imply that the energetic difference between this alkene orientation and an alternative perpendicular conformation is small (ca. 5 kcal mol(-1)), with steric factors (notably at the alkene 2-position) being important. Hydrogenation of 1 proceeds via an intermediate identified as Ir(IPr)(IPr")(H)(2)Cl to give the Ir(III) dihydride Ir(IPr)(2)(H)(2)Cl (4), the structure of which can be compared with those of the tautomeric isomer 6 and the mixed IPr/IMes carbene complex Ir(IPr)(IMes)(H)(2)CI [IMes = N,N'-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]. Crystallographically determined bond lengths for the Ir-C linkages trans to the respective carbenes imply relatively similar sigma-donor properties for the IPr, aIPr, and IMes ligands