Metal-ligand bond dissociation energies in the Ni, Pd, and Pt complexes with N-heterocyclic carbenes: effect of the oxid
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Metal—ligand bond dissociation energies in the Ni, Pd, and Pt complexes with N-heterocyclic carbenes: effect of the oxidation state of the metal (0, +2) A. V. Astakhov, S. B. Soliev, and V. M. Chernyshev M. I. Platov South-Russian State Polytechnic University, 132 ul. Prosveshcheniya, 346428 Novocherkassk, Russian Federation. E-mail: [email protected] A DFT study was carried out of how the nature of metal, the oxidation state of the metal (0 and +2), as well as the structures of N-heterocyclic carbene (NHC) and other ligands influence the heterolytic dissociation energies of the metal—ligand bond in the complexes M-NHC (M = Ni, Pd, Pt). It was shown that a change in the oxidation state of the metal can be followed by a considerable change in the M—NHC bond dissociation energy (up to nearly 21 kcal mol–1), which is also strongly influenced by the ligand in the trans-position to NHC. Key words: N-heterocyclic carbenes, coordination compounds, nickel, palladium, platinum, bond dissociation energies, catalysis.
Nickel, palladium, and platinum complexes with N-heterocyclic carbenes (NHC) are widely used as homogeneous catalysts.1—6 They are also intensively studied as luminescent materials7—9 and cancer drugs.10—12 Wide use of metal complexes with NHC (M-NHC) is underlain by higher stability of such systems compared to other types of homogeneous catalysts since the M—NHC bond is very strong.1,2,13 However, M-NHC complexes can decompose in solutions due to dissociation of the М—NHC bond, which often occurs in rather mild conditions.14—20 Dissociation of the metal—ligand bond is of great importance for catalysis and other practical applications because it usually causes deactivation of catalysts6,21—23 or changes in the nature of active centers and in the mechanism of catalysis.24—27 Contrary to this, elimination of auxiliary ligands is often necessary to activate the catalyst, M-NHC.16 Thus, the catalytic properties of complexes M-NHC depend strongly on their ability to break down with heterolytic dissociation of the metal—ligand bond. Therefore, the heterolytic bond dissociation energy (BDE),28 which can be calculated by quantum chemistry methods, is often used to predict the catalytic properties of complexes.29—31 To date, both experimental and theoretical BDE values have been determined for various complexes M-NHC.32—40 For instance, the influence of (i) the structure of the NHC ligand on the M—NHC bond dissociation energies (BDEM—NHC) in complexes with Ni32,34,37 and Pd36,37,39,40 and (ii) halide ligands on the BDE in complexes PdII-NHC39 were studied. Steric factors, especially, bulky substituents at nitrogen atoms of the NHC ligands affect strongly the BDEM—NHC values.29,30 However, it should be noted that the oxidation state of the metal usually
changes in the course of catalysis and these changes can influence the stability of the metal—ligand bonds. For instance, the catalytic cycles of the cross-coupling, hydrogenation, and СН-activation reactions very often involve interconversions of the intermediates M 0-N
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