Structures, relative energies, and ligand dissociation energies of iridium carbonyl phosphine clusters

Abstract

a b s t r a c tThere is significant interest in the catalytic properties of substituted iridium carbonyl clusters but little thermodynamic information available characterizing them. The low-energy isomers of Ir x (PH 3 ) y (CO) z (x = 1, 2, 4) were investigated with density functional theory and correlated molecular orbital theory at the coupled cluster CCSD(T) level. The relative energies and ligand dissociation energies were calculated. Differences in relative energies are consequences of both electronic and steric effects of the phosphines and carbonyls. The calculations predict three fundamental structural types for Ir 2 (PH 3 ) y (CO) z : C 2v , C 2 , and D 3d . Ten exchange-correlation functionals were used for the ligand dissociation energy calculations in addition to CCSD(T) for the smaller clusters. The xB97X-D functional gave the most consistent ligand dissociation energies as compared with the CCSD(T) benchmark calculations, and, so it was used to predict the dissociation energies for larger clusters when CCSD(T) calculations were infeasible. The dissociation energies characteristic of Ir 4 (PH 3 ) y (CO) z were in the range of $30 to $60 kcal/mol. Dissociation of a bridging ligand often involved a hydrogen atom transfer from a phosphine to a coordinatively unsaturated iridium atom and a phosphine converting from a bridging site to an equatorial site. The products of such reactions are predicted to have lower relative energies than other isomers.Phosphines act as r-electron donors, and there is a trend of an increase in carbonyl ligand dissociation energies as more phosphines are substituted in the small clusters.

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

“…For a single phosphine ligand, we predict that the energy of the monosubstituted tetrairidium cluster with a ax PH 3 in a C 3vderived structure is higher in energy than the T d -derived structure by 1.1 kcal/mol at the ω-B97X-D level. 39 The C 3vderived structure for Ir 4 (CO) 11 PH 3 with an ax PH 3 is predicted to be lower in energy by 2.8 kcal/mol at the M06 level. A single crystal X-ray crystallography study showed a C 3v -derived structure with a phosphine-calixarene ligand in the axial position, 81 as is generally found.…”

“…A transformation of a bri NHC into an eq NHC (reactions (4) and (35), Figure S7 in the Supporting Information) can occur during a dissociation of CO, analogous to the transformation of a bri PH 3 into an eq PH 3 considered previously. 39 The NHC ligand that shares the Ir atom with an eq CO ligand moves to this eq position after dissociation of the CO. In reactions ( 82), ( 83), (108), and (113), carbonyls binding to 3 Ir atoms (μ 3 -CO) are observed in the products 9o, 9p, 11f, and 11h.…”

“…For the parent Ir 4 (CO) 12 clusters, the isomer with the T d structure has the lowest energy. We previously predicted that the T d -based structure is of lower energy when one CO ligand is substituted by a PH 3 , but that the C 3 v -based structure is lower in energy after a second CO is replaced by a PH 3 . The lowest-energy isomer of Ir 4 (CO) 11 (NHC) is derived from the C 3v structure ( 4c ), with the isomer derived from the T d structure ( 4e ) being 2.5 kcal/mol higher in energy.…”

“…As more NHC ligands are substituted for the CO ligands in the tetramer, the LDEs of the NHC ligands in the isomers with the lowest energies decrease for most isomers, consistent with the trend for the PH 3 ligands predicted previously. 39 The LDEs of the NHC ligands decrease by a larger amount as compared to PH 3 because of stronger electron repulsion and steric effects. There is, however, no discernible trend for how the LDEs of the CO ligands change in the Ir 4 (CO) y (NHC) z isomers, consistent with what we have found for the Ir 4 (PH 3 ) y (CO) z clusters.…”

“…14 Mononuclear iridium complexes have also been widely used as catalysts in oxidation (e.g., of alcohols, 15−17 phenols, 18−20 and amines 21,22 ), C−H bond activation, 23−25 hydrogenation, 26,27 cycloaddition (e.g., [2 + 2 + 2], 28−30 [2 + 2 + 1], 31,32 and [4 + 2] 33,34 ), cycloisomerization, 12,13 and ringopening reactions. 35 Ir(CO) n (n = 1−4) and Ir 2 (CO) 8 clusters have been reported to be the products of the reaction of Ir with CO. 36−38 Ir 2 (CO) 8 clusters generated in matrices at 10−50 K dimerized into Ir 4 (CO) 12 at ∼200 K. 36,37 Recently, we reported a theoretical investigation of iridium carbonyl phosphine complexes Ir x (PH 3 ) y (CO) z (x = 1, 2, 4) 39 after earlier work showed that the tetrairidium clusters could have more than one kind of active site, in which the reactivity at the apical Ir site was controlled by three calixarene-phosphine ligands bonded to the basal plane. 40 We have now extended this work to include N-heterocyclic carbenes (NHCs or stable singlet nucleophilic carbenes) instead of the phosphines as ligand because of the importance of NHC's in catalysis.…”