Role of N-Heterocyclic Carbenes as Ligands in Iridium Carbonyl Clusters

Abstract

The low-energy isomers of Ir(CO)(NHC) (x = 1, 2, 4) are investigated with density functional theory (DFT) and correlated molecular orbital theory at the coupled cluster CCSD(T) level. The structures, relative energies, ligand dissociation energies, and natural charges are calculated. The energies of tetrairidium cluster are predicted at the CAM-B3LYP level that best fit the CCSD(T) results compared with the other four functionals in the benchmark calculations. The NHC's behave as stronger σ donors compared with CO's and have higher ligand dissociation energies (LDEs). For smaller isomers, the increase in the LDEs of the CO's and the decrease in the LDEs of the NHC's as more NHC's are substituted for CO's are due to π-back-bonding and electron repulsion, whereas the trend of how the LDEs change for larger isomers is not obvious. We demonstrate a μ-CO resulting from the high electron density of the metal centers in these complexes, as the bridging CO's and the μ-CO's can carry more negative charge and stabilize the isomers. Comparison of calculations for a mixed tetrairidum cluster consisting of two calixarene-phosphine ligands and a single calixarene-NHC ligand in the basal plane demonstrated good agreement in terms of both the ligand substitution symmetry (C derived), as well as the infrared spectra. Similar comparisons were also performed between calculations and experiment for novel monosubstituted calixarene-NHC tetrairidium clusters.

Next-Generation Water-Soluble Homogeneous Catalysts for Conversion of Glycerol to Lactic Acid

“…As proposed by Xiao et al, Ir­(III) compounds likely undergo ligand substitution by an alcohol, deprotonation to form Ir-alkoxide, β-hydride elimination, and finally hydride abstraction using H + to release H 2 . Given that CO substitution in 9 is not facile, we propose the catalytic cycle in Figure . First, the 16-electron Ir­(I) species undergoes O–H oxidative addition with glycerol to form an alkoxy hydride complex; this step has precedents for Ir­(I) complexes .…”

“…The difference in activity observed for [(NHC-ph-SO 3 – ) 2 Ir­(cod)] − ( 7 ) versus [(NHC-ph-SO 3 – ) 2 Ir­(CO) 2 ] − ( 9 ) could be due to (i) more favorable H 2 elimination or O–H reductive elimination from Ir center in 9 with stronger π-acceptor (CO) ligands or (ii) the fact that 7 undergoes an entirely different mechanism than 9 (cycooctadiene ligand in 7 could easily be displaced, opening coordination sites, which is not the case for 9 ) . Although the presence of carbonyl ligands also opens the possibility for cluster formation, , in the case of 9 cluster formation is not likely at the low catalyst loadings used, compared to previous reports. , …”

N-Heterocyclic Carbenes in Materials Chemistry

“…Work has also been carried out on Ir carbonyl clusters, with recent studies including computational work describing the structures, relative energies, natural charges, and ligand dissociation energies of serval [Ir x (CO) z (NHC) y ] clusters. 272 The effects of the NHC on the cluster and the CO ligands were investigated.…”

NHC Carbene–Metal Complex Ligand Binding Energies

“…We categorize the carbenes discussed in the current work as singlet nucleophilic carbenes (SNCs) as exemplified by the NHCs. In the current work, we expand our studies of carbene energetics by predicting carbene binding energies to a variety of metals building on our prior work on carbene binding in models of Grubbs metathesis catalysts and to Ir clusters …”

“…In the current work, we expand our studies of carbene energetics by predicting carbene binding energies to a variety of metals building on our prior work on carbene binding in models of Grubbs metathesis catalysts 19 and to Ir clusters. 20…”