Combined Experimental and Computational Studies on Carbon−Carbon Reductive Elimination from Bis(hydrocarbyl) Complexes of (PCP)Ir

Abstract

The reductive elimination of carbon-carbon bonds is one of the most fundamentally and synthetically important reaction steps in organometallic chemistry, yet relatively little is understood about the factors that govern the kinetics of this reaction. C-C elimination from complexes with the common d (6) six-coordinate configuration generally proceeds via prior ligand loss, which greatly complicates any attempt to directly measure the rates of the specific elimination step. We report the synthesis of a series of five-coordinate d (6) iridium complexes, ( (tBu)PCP)Ir(R)(R'), where R and R' are Me, Ph, and (phenyl-substituted) vinyl and alkynyl groups. For several of these complexes (R/R' = Ph/Vi, Me/Me, Me/Vi, Me/CCPh, and Vi/CCPh, where Vi = trans-CHCHPh) we have measured the absolute rate of C-C elimination. For R/R' = Ph/Ph, Ph/Me, and Ph/CCPh, we obtain upper limits to the elimination rate; and for R/R' = CCPh/CCPh, a lower limit. In general, the rates decrease (activation barriers increase) according to the following order: acetylide < vinyl approximately Me < Ph. Density functional theory (DFT) calculations offer significant insight into the factors behind this order, in particular the slow rates for elimination of the vinyl and, especially, phenyl complexes. The transition states are calculated to involve rotation of the aryl or vinyl group around the Ir-C bond, prior to C-C elimination, such that the group to which it couples can add to the face of the aryl or vinyl group. This rotation is severely hindered by the presence of the phosphino -t-butyl groups that lie above and below the plane of the aryl/vinyl group in the ground state. Accordingly, calculations predict dramatically different relative rates of elimination from the much less sterically hindered complexes ( (H)PCP)Ir(R)(R'). For example, the barrier to elimination from ( (H)PCP)Ir(Me) 2 is 20 kcal/mol, which is 2 kcal/mol greater than from the ( (tBu)PCP)Ir analogue. In contrast, the activation enthalpies calculated for vinyl-vinyl and phenyl-phenyl elimination from ( (H)PCP)Ir are remarkably low, only 2 and 9 kcal/mol, respectively; these values are 16 and 22 kcal/mol less than those of the corresponding ( (tBu)PCP)Ir complexes. Moreover, since these eliminations are very nearly thermoneutral, the barriers are calculated to be equally low for the reverse reactions [C-C oxidative addition to ( (H)PCP)Ir]. The absence of differences in intraligand CC bond lengths in the transition states relative to the ground states, combined with a comparison of calculated "face-on" and "planar" transition states for C-C coupling, suggests that the critical importance of the aryl/vinyl rotation is based on geometric or steric factors rather than electronic ones. Thus there is no evidence for participation of the pi or pi* orbitals of the aryl or vinyl groups in the formation of the C-C bond, although a small pi effect cannot be rigorously excluded. Likewise, the results do not support the hypothesis that the degree of directionality of the carbon-based orbital u...

Density Functional Theoretical Study on the C-H Coupling Reaction from Ir(III) Complexes

“…The Ir-CMe bond length in 1 + was 2.10 Å, which was comparable to the value that was found in the X-ray structure 2 and the related pincer Ir(III)-methyl complexes. 5 The Ir-CPh bond length in 2 + was 2.07 Å, which was also comparable to the value that was found in the related pincer Ir(III)-phenyl complexes. 5 The phenyl ring in 2 + was vertical to the plane that was defined by Ir and its pincer ligand, and therefore, the hydrogen atom that was attached to Ir was also in the same plane.…”

“…5 The Ir-CPh bond length in 2 + was 2.07 Å, which was also comparable to the value that was found in the related pincer Ir(III)-phenyl complexes. 5 The phenyl ring in 2 + was vertical to the plane that was defined by Ir and its pincer ligand, and therefore, the hydrogen atom that was attached to Ir was also in the same plane. The Ir-H bond lengths were 1.54 Å in In 1-σ + , the Ir-H bond length was elongated to 1.83 Å, and the Ir-CMe bond was lengthened to 1.48 Å.…”

Formation of 5-membered metallacycles at iPrPCPIr by C–H, O–H, and C–CO bond cleavage

“…2.26 Å versus 3.85 Å) and, accordingly, the calculated C-C activation pathway involves a fully perpendicular approach. As evidenced by the separate rotamers formed from C-H activation of biphenylene with tBu PCPIr, as well as its reluctance to reductively eliminate C (Ph) -C bonds [42], rotation of phenyl groups bound to tBu PCPIr is severely hindered. By extension, in the double C-H activation of biphenyl it is likely that the initial C-H activation occurs with the un-activated Ph ring syn to the hydride since rotation from the anti-addition product to form 1 would be restricted (Scheme 4).…”

A Well-Defined (POCOP)Rh Catalyst for the Coupling of Aryl Halides with Thiols

“…Goldman, Krogh-Jespersen, et al previously showed that, for Ar−X (X = Me, Ph, CHCH 2 , CCH) RE from five-coordinate (PCP)Ir(Ar)(X), the aryl group must be oriented "face-on" toward X in the transition state. 29 The steric influence of the phosphines in the ground state of (PCP)Ir(Ar)(X) favors the "side-on" orientation of the aryl group with respect to X, and the necessary rotation of the aryl can be a considerable component of the activation barrier. This situation can be contrasted with the typical (R 3 P) n Pd(Ar)(X) (n = 1, 2) intermediates, where the steric bulk of the phosphines favors "face-on" orientation of the aryl with respect to X in the ground state and encourages faster RE.…”