Binuclear Rhodium(I) Carbonyl Carboxylate Complexes: DFT Study of Structural and Spectral Properties

Abstract

During the last decades, rhodium complexes have attracted the attention of researchers as they are efficient catalysts of diverse, including practically significant, reactions in organic synthesis (methanol carbonylation, alkene and alkyne hydrogenation, and alkene hydroformylation). Among rhodium compounds that deserve attention, special emphasis should be put on rhodium(I) carbonyl complexes, which are often used as precursors of catalytic systems and function as active intermediates in catalytic cycles. The sensitivity of the carbonyl ligand to the electronic state of the central atom and particularly characteristic stretching vibrations of the carbonyl ligand make it possible to use the ν ( CO ) stretching frequency as a source of useful information on the donor-acceptor interactions in the coordination sphere and on the mutual ligand effects.In a previous publication [1], the spectral (IR and NMR) characteristics of the carbonyl groups in rhodium(I) carbonyl carboxylate complexes have been determined and some correlations between these characteristics and the electronegativity of the substituent R in the carboxylate ligand were found. It was also found that transition from dicarbonyl (per one rhodium atom) com-, is accompanied by substantial changes in the spectroscopic parameters of the carbonyl groups. These observations were interpreted [1] using the intuitive views on ligand effect transfer through the central atom. The authors of [1] also paid attention to the fact that the IR spectra of the binuclear carbonyl carboxylate complexes differ appreciably in the carbonyl stretching region (the intensity ratio of the observed absorption bands) from the spectrum of the chloride analog [ Rh ( µ -Cl )( CO ) 2 ] 2 . This difference was tentatively attributed to the change in the dihedral angle ( q ) formed by the coordination planes of the rhodium atoms on passing from the carboxylate complexes to the complex with monoatomic chloride bridges. In view of a successful experience of DFT description of the structure and vibrational spectra of the mononuclear rhodium(I) carbonyl complexes [2-4], we performed such quantum-chemical calculations for interpreting the experimental data concerning binuclear rhodium(I) complexes, [ Rh ( µ -Cl )( CO ) 2 ] 2 and [ Rh ( µ -RCOO )( CO ) 2 ] 2 (R = H, CH 3 , CF 3 ). The study aimed at answering the following questions:-to what extent the results of quantum-chemical calculations can reproduce the set of experimental structural and spectral parameters of each compound separately;-whether the calculation results reported previously [1] reflect the correlations between these parameters and what characteristics of the electronic structure of the complex (atomic charges, orbital populations, bond orders) may by considered responsible for these correlations;-whether the calculations confirm the assumption made previously [1] that the intensity ratio of the ν ( CO ) stretches in the IR spectra of binuclear complexes correlates with the value of the dihedral angle between the Rh ( CO ) 2 ...

NMR (13C, 17O), IR and Raman Studies of Poly-nuclear Carbonyls Transition Metal Carbonyls : A DFT Application

“…All the metals in the above named first three carbonyls were spatially equivalent while in the latter two carbonyls,all the metals were not equivalent.But no where, the two metals or any two COs were found to be magnetically equivalent.Excepting Ir 4 (CO) 12 where all the12 CO groups were spatially equivalent,in other four carbonyls, CO groups were found to be two or more types spatially.The first metal and the responding spatially equivalent other metal/s possessed same k and j values. For CO groups attached to one metal and spatially equivalent CO groups attached to other spatially equivalent metal/s, k and j values of 13 C nuclei possessed the same values.…”

“…Depending upon the geometry, even the COs attached to the same metal might differ spatially to give more than one value of s z C, d 12 300 511 total values of their s parameters respectively (Tables: 3-6). (3) The same inference was drawn from the charges on the metal ions where the spatially equivalent metals possessed the same charges while the spatially different metals possessed different charges (Table: 8 .The d orbitals involved in this hybridization were: n d x z, n d y z {n=4,5; M=Ru, Os} with major lobes pointing towards the vertices though not as directly as in the case of an octahedron.…”

“…But unlike the vast variety of NMR parameters of 20 mono-and bi-nuclear [8][9][10][11][12][13][14] carbonyls studied by our group of workers 1,4 ,only a little had been reported on NMR studies by computational methods for poly-nuclear [18][19][20][21][22] carbonyls as follows:…”

“…Variable temperature 13 C NMR spectra of the carbonyls M 3 (CO) 12 (M = Fe, Ru, and Os) and other related compounds 18 were reported. At least three CO scrambling processes had been shown to operate in these system.…”

“…The present study includes 5 poly-nuclear carbonyls of transition metals in their zero oxidation states as: [M 3 (CO) 12 O of CO(g) and slight increase in positive charge on the metals of the five carbonyls.…”