Sterically Encumbered Iridium Bis(N-heterocyclic carbene) Systems: Multiple C−H Activation Processes and Isomeric Normal/Abnormal Carbene Complexes

Abstract

The reaction of [Ir(coe)2Cl]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)2Cl (6), which features both “normal” and “abnormal” C-bound isomers of the NHC ligand. Formation of 1 presumably proceeds via initial C−H activation at iridium(I); moreover, subsequent reactivity for 1 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(III) alkene alkyl hydride [Ir(IPr′)(IPr”)H]+[BArf4]− (2) is formed on reaction with Na[BArf4] 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 IrC4 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)2Cl to give the Ir(III) dihydride Ir(IPr)2(H)2Cl (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)2Cl [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 σ-donor properties for the IPr, aIPr, and IMes ligands.