Sterically encumbered iridium Bis(N-heterocyclic carbene) Systems: Multiple C-H activation processes and isomeric normal/abnormal carbene complexes

Christina Y. Tang, William Smith, Dragoslav Vidovic, Amber L. Thompson, Adrian B. Chaplin, Simon Aldridge

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The reaction of [Ir(coe)2Cl]2 (coe = cyclooctene) with the N-heterocyclic carbene N,N′-bis(2,6diisopropylphenyl)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]+[BAr 4f]- (2) is formed on reaction with Na[BAr 4f] 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. Hydrogénation 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.

Original languageEnglish
Pages (from-to)3059-3066
Number of pages8
Issue number10
Publication statusPublished - 25 May 2009
Externally publishedYes

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