Tang, Christina Y., Smith, W. (William), Vidovic, Dragoslav, Thompson, Amber L., Chaplin, Adrian B. and Aldridge, S. (Simon). (2009) Sterically encumbered iridium bis(N-heterocyclic carbene) systems : multiple C−H activation processes and isomeric normal/abnormal carbene complexes. Organometallics, Vol.28 (No.10). pp. 3059-3066. ISSN 0276-7333

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Abstract

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.

Item Type:	Journal Article
Subjects:	Q Science > QD Chemistry
Divisions:	Faculty of Science > Chemistry
Library of Congress Subject Headings (LCSH):	Iridium, Activation reactions, Heterocyclic compounds, Carbenes (Methylene compounds), Transition metal catalysts
Journal or Publication Title:	Organometallics
Publisher:	American Chemical Society
ISSN:	0276-7333
Date:	25 May 2009
Volume:	Vol.28
Number:	No.10
Page Range:	pp. 3059-3066
Identification Number:	10.1021/om9000082
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access
Funder:	Engineering and Physical Sciences Research Council (EPSRC)
Grant number:	EP/F0160OX/1 (EPSRC)
URI:	http://wrap.warwick.ac.uk/id/eprint/40531

Data sourced from Thomson Reuters' Web of Knowledge

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