Relative stabilities of M/NHC complexes (M = Ni, Pd, Pt) against R–NHC, X–NHC and X–X couplings in M(0)/M(ii) and M(ii)/M(iv) catalytic cycles: a theoretical study

Abstract

DFT calculations reveal relative stability of MII/NHC and MIV/NHC complexes of nickel, palladium and platinum against the R–NHC coupling and various reductive elimination reactions that influence catalyst stability/decomposition.

The key role of R–NHC coupling (R = C, H, heteroatom) and M–NHC bond cleavage in the evolution of M/NHC complexes and formation of catalytically active species

“…The effect of metal and its oxidation state on the C–NHC coupling efficiency has been evaluated by DFT calculations for M II /NHC and M IV /NHC complexes of Ni, Pd and Pt. 60 The results indicate that thermodynamic and kinetic stabilities of both M II and M IV complexes 1 against C–NHC coupling decrease as Pt > Pd > Ni. Besides, complexes 1 with a higher oxidation state of the metal are thermodynamically and kinetically less stable than corresponding complexes with metals in lower oxidation states.…”

“…The process results in formation of new C–C bond between NHC and R ( Scheme 1 ). 22 , 23 , 25 , 58 , 60 Complexes 1 are typical catalytic intermediates found in the vast majority of M/NHC catalysed reactions. Decomposition of M/NHC catalysts via C–NHC coupling was detected in Mizoroki–Heck 27 , 62 , 65 and Suzuki–Miyaura 66 couplings, in the oligomerizations of alkenes and alkynes, 67 – 70 in CH-functionalizations of heterocycles, 71 and in many other reactions.…”

“…The poor thermodynamic and kinetic stabilities of the regular Pd IV and Ni IV complexes 1 against R–NHC coupling indicate high probability of the M IV –NHC bond cleavage and implementation of the NHC-disconnected catalytic scenario in the reactions comprising M IV intermediates. 60 …”

“…Ni IV and Pd IV , are susceptible to the reductive elimination of NHC, and the use of such complexes in catalytic systems with assumed participation of M IV species may be inefficient due to their low stability. 60 It should also be noted that activated M 0 /NHC complexes are usually susceptible to M–NHC bond dissociation and prone to ligand displacement and elimination, which may trigger formation of metal clusters and nanoparticles. 25 …”

“…This rate strongly depends on the rate of the M–NHC bond cleavage which, in turn, depends on the structure of the NHC ligand and co-ligands. 27 , 54 , 58 – 60 , 62 , 63 , 97 M/NHC complexes with bulky NHC ligands (or bis-NHC complexes, 59 especially chelated 250 ) are usually more resistant to R–NHC coupling and therefore decompose slower than NHC complexes with non-bulky NHC ligands. For example, the rate of Mizoroki–Heck reactions shows inverse relationship with the stability of Pd/NHC precatalyst.…”