(301d) The Reaction of Tetramethylallene with Rh2(CO)4Cl2. A Combined in-Situ Vibrational Spectroscopies, Spectral Reconstruction and DFT Study

Highly reactive allenes are being increasingly used as reagents in complex organic syntheses, particularly in metal-mediated syntheses (both stoichiometric as well as homogeneous catalytic). Accordingly, a better understanding of the coordination chemistry of allenes with metal complexes is of interest. In order to understand some of the coordination chemistry of allenes with rhodium, the reaction of tetramethylallene with Rh2(CO)4Cl2 was performed in anhydrous hexane under argon atmosphere with multiple perturbations of Rh2(CO)4Cl2 and tetramethylallene. The reaction was monitored by in-situ FTIR and Raman spectroscopies and the collected spectra were further analyzed with the BTEM family of algorithms [1]. GC-MS analysis and DFT calculations were performed in order to identify the organometallic species present. The geometric optimizations were performed using PBEPBE density function with DGDZVP basis set without any restrictions and the FTIR/Raman vibrational frequencies were calculated afterwards [2]. A mononuclear allene complex Rh(CO)2Cl(ç3-C7H12) and a mononuclear diene complex Rh(CO)Cl(ç4-C7H12) were the primary rhodium complexes observed. Their relative concentrations were also obtained by least-squares fit. No discernible C=C vibrations were observed for Rh(CO)2Cl(ç3-C7H12) in both the FTIR and Raman spectra, thus supporting the suggested coordination mode of the C7H12 moiety. DFT calculations indicate that (1) Rh(CO)2Cl(ç3-C7H12) has a distorted tetrahedral structure and (2) the conformation of the coordinated diene moiety for Rh(CO)Cl(ç4-C7H12) is transoid, due primarily to steric effects. The optimized geometries of the two major species are plotted in Fig. 1. The present contribution shows that with the combination of in-situ spectroscopic measurement, spectral reconstruction and DFT calculations, some new insights into this organometallic reaction were achieved. References 1. (a) E. Widjaja, C.Z. Li and M. Garland, Organometallics 21(9) (2002) 1991. (b) W. Chew, E. Widjaja and M. Garland, Organometallics 21(9) (2002) 1982. (c) H.J. Zhang, M. Garland, Y.Z. Zeng and P. Wu, J. Am. Soc. Mass Spectrom., 14 (2003) 1295. (d) F. Gao, A. D. Allian, H.J. Zhang, S.Y. Cheng and M. Garland, J. Catal. 241 (2006) 189. (e) H.J. Zhang, W. Chew and M. Garland, Appl. Spectrosc. 61 (2007) 1366. (f) F. Gao, H. J. Zhang, L. F. Guo and M. Garland, Chemom. Intell. Lab Syst. 95 (2009) 94. 2. A.D. Allian, M. Tjahjono and M. Garland, Organometallics 25 (2006) 2182;