Activation of Carbon-Hydrogen Bonds via 1,2-Addition across M-X (X = OH or NH2) Bonds of d6 Transition Metals as a Potential Key Step in Hydrocarbon Functionalization: A Computational Study

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Published on Web 10/06/2007
Activation of Carbon-Hydrogen Bonds via 1,2-Addition
across M-X (X = OH or NH2) Bonds of d6 Transition Metals
as a Potential Key Step in Hydrocarbon Functionalization:
A Computational Study
Thomas R. Cundari,*,t Thomas V. Grimes,t and T. Brent Gunnoe*,t
Contribution from the Department of Chemistry and Center for Advanced Scientific
Computing and Modeling (CASCaM), University of North Texas, P.O. Box 305070, Denton,
Texas 76203-5070, and Department of Chemistry, North Carolina State University,
Raleigh, North Carolina 27695-8204
Received June 6, 2007; E-mail: tomc@unt.edu
Abstract: Recent reports of 1,2-addition of C-H bonds across Ru-X (X = amido, hydroxo) bonds of TpRu-
(PMe3)X fragments {Tp = hydridotris(pyrazolyl)borate} suggest opportunities for the development of new
catalytic cycles for hydrocarbon functionalization. In order to enhance understanding of these transformations,
computational examinations of the efficacy of model d6 transition metal complexes of the form [(Tab)M-
(PH3)2X]< (Tab = tris-azo-borate; X = OH, NH2; q = -1 to +2; M = Tc', Re', Ru", Co"', Ir"', Ni'V, Pt"v) for
the activation of benzene C-H bonds, as well as the potential for their incorporation into catalytic
functionalization cycles, are presented. For the benzene C-H activation reaction steps, kite-shaped transition
states were located and found to have relatively little metal-hydrogen interaction. The C-H activation
process is best described as a metal-mediated proton transfer in which the metal center and ligand X
function as an activating electrophile and intramolecular base, respectively. While the metal plays a primary
role in controlling the kinetics and thermodynamics of the reaction coordinate for C-H activation/
functionalization, the ligand X also influences the energetics. On the basis of three thermodynamic criteria
characterizing salient energetic aspects of the proposed catalytic cycle and the detailed computational
studies reported herein, late transition metal complexes (e.g., Pt, Co, etc.) in the d6 electron configuration
{especially the TabCo(PH3)2(OH)- complex and related Co(Ill) systems} are predicted to be the most
promising for further catalyst investigation.

1. Introduction
The development of catalysts for the functionalization of
carbon-hydrogen bonds is an important pursuit that could
impact both commodity and fine chemical markets. Transition
metal systems that initiate stoichiometric metal-mediated activa-
tion of carbon-hydrogen bonds are known,' and many of these
systems function at ambient conditions. The most commonly
invoked mechanisms for the C-H bond cleavage step include
oxidative addition (OA), o-bond metathesis (SBM), and elec-
trophilic substitution (ES). Despite the success of metal-mediated
C-H activation, the incorporation of stoichiometric C-H
activations into catalytic cycles remains relatively rare,2 and this
is especially true for the functionalization of aliphatic hydro-
carbons. Noteworthy examples of catalytic conversions include
T University of North Texas.
North Carolina State University.
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10.1021/ja074125g CCC: $37.00 2007 American Chemical Society

Cundari, Thomas R., 1964-; Grimes, Thomas V. & Gunnoe, T. Brent. Activation of Carbon-Hydrogen Bonds via 1,2-Addition across M-X (X = OH or NH2) Bonds of d6 Transition Metals as a Potential Key Step in Hydrocarbon Functionalization: A Computational Study. [Washington, DC]. UNT Digital Library. http://digital.library.unt.edu/ark:/67531/metadc77141/. Accessed August 29, 2014.