57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

Progress Report Year 1: 01/01/2011 – 08/31/2012

During the first year, efforts concentrated on two projects in which computational and experimental progress were made. The first project involves the determination of the metal-olefin bond energies in the series of complexes M(CO)₅(C₂H_4-nX_n), with M = Cr, Mo or W; X = CN or F, and n = 0 to 4. A full computational study of the determination of the bond formation energies for the reaction: M(CO)₅ + C₂H_4-nX_n → M(CO)₅(C₂H_4-nX_n) have been completed.¹ As expected the computational modeling using density functional theory (DFT) predicts that metal-olefin bond energies decrease as the number of electron withdrawing CN or F substituents increase, which is counter to the qualitative prediction of the traditional Dewar-Chatt-Duncanson model. A bond energy decomposition analysis (BEDA) reveals that even though the covalent interaction of the metal and the olefin is enhanced by the increase of the electron withdrawing substituents, the rehybridization of the olefin that is concurrent with bond formation induces conformational changes (reorganization) that are energetically costly. Both the cyanoethylenes and fluoroethylenes behaved in similar fashion, although the metal-olefin interactions within the cyanoethylene series experience larger repulsive energies than the corresponding fluoroethylenes. Also the fluoroethylenes interact with the metals stronger than the corresponding cyanoethylenes, but they also suffered from larger reorganizational energies.

An experimental determination of the bond energies for the M(CO)₅(C₂H_4-n(CN)_n), M = Cr, Mo, W was also undertaken. We used laser photoacoustic calorimetry (LPAC)² to measure the bond energy between tungsten or chromium and 1-cyanoethylene in hexane solution. The formation of the complex was verified using FTIR spectroscopy. It was found that the metal-1-cyanoethylene bond enthalpy is 32 kcal/mol for chromium and 34 kcal/mol for tungsten, which were larger than the values obtained from the DFT calculations (20 and 26 kcal/mol respectively). Unfortunately LPAC experiments could not be carried out for trans-1,2-dicyanoethylene and tetracyanoethylene because of their poor solubilities in alkanes and absorption interferences with the photolysis laser. This forced us to search for an alternative way of measuring bond strengths. The approach consisted on synthesizing the complexes M(CO)₅(C₂H_4-n(CN)_n), M = Cr, Mo, W, with n = 1, 2 (trans isomer) and 4 according to previously published procedures.^3-5 Then thermal decomposition of the complexes in toluene solution was studied over a temperature range. Kinetic analyses of the decomposition rates with temperature were used to obtain the enthalpy of activation for the process. If a dissociative mechanism for decomposition is assumed, the activation energies will reflect the bond strength.⁶ It was found that the activation enthalpies for the decomposition of Cr(CO)₅(trans-C₂H₂(CN)₂) and Cr(CO)₅(C₂(CN)₄) are similar (27 kcal/mol) and those of Mo(CO)₅(trans-C₂H₂(CN)₂) and Mo(CO)₅(C₂(CN)₄) do not differ much either (27 and 30 kcal/mol respectively). These results indicate that the amount of CN substituents does not influence the bond strength strongly which is counter to the computational DFT results. Previous reports on the M(CO)₅(trans-C₂H₂(CN)₂) complexes (M = Cr, Mo, W)^4,5 and the W(CO)₅(C₂H₃(CN)) complex⁷ suggest that the olefin is not bonded via the double bond (h² mode) but rather via the nitrogen atom in the cyano (N-bonded mode). We have carried out DFT calculations of the metal-olefin bond strength using an N-bonded mode and have obtained results that are in agreement with the experimental kinetic-based results.

The other project is in a computational modeling stage and consists of quantifying the dependence of the metal-olefin bond energy on steric effects. For this purpose we have carried out DFT calculations of the bond formation energy for the reaction: Cr(CO)₅ + C₂H_4-nPh_n → Cr(CO)₅(C₂H_4-nPh_n), Ph = phenyl. The calculations indicate that there is a decrease in the magnitude of the bond formation energy of about 5 kcal/mol per every phenyl group that substitutes a hydrogen in going from ethylene to tetraphenylethylene. Indeed, the calculations indicate that the complex Cr(CO)₅(C₂Ph₄) via an h² bond to the ethylene double bond cannot be formed. The bulky nature of the phenyl substituents hinders the approach of the double bond by the metal.

1.  S. L. Johnson, D. L. Cedeño, "A computational study of the bonding interaction between chromium, molybdenum, or tungsten carbonyl complexes and cyanoethylenes or fluoroethylenes" Abstracts of papers, Joint 46th Midwest & 36th Great Lakes ACS Regional Meeting, St. Louis, MO, October 21, 2011, 486.

2.  D. N. Schlappi, D. L. Cedeño, "Metal-olefin bond energies in M(CO)₅(C₂H_4-nCl_n) M = Cr, Mo, W; n = 0-4: Electron-withdrawing olefins do not increase the bond strength", J. Phys. Chem. A, 2009, 113, 9692-9699.

3.  M. Herberhold, "New metal complexes of tetracyanoethylene" Angew. Chem. Internat. Ed., 1968, 4, 305-306.

4.  I. A. Mour, S. özkar, C. G. Kreiter, "Synthesis and spectroscopic studies of pentacarbonylfumaronitrile-chromium(0), -molybdenum(0), and –tungsten(0)" Z. Naturforsch., 1994, 49b, 1059-1062.

5.  I. A. Mour, S. özkar, "Synthesis and spectroscopic study of pentacarbonyl(h²-tetracyanoethylene) metal(0) complexes of the group 6B elements" Z. Naturforsch., 1994, 49b, 717-720.

6.  F. Kozanoglu, S. Saldamli, S. özkar, "Substitution kinetics of Cr(CO)₅(h²-Z-cyclooctene) with tetracyanoethylene" J. Organomet. Chem., 2002, 658, 274-280.

7.  B. L. Ross, J. G. Grasselli, W. M. Ritchey, H. D. Kaesz, "Spectroscopic studies of the complexes of acrylonitrile and acetonitrile with the carbonyls of chromium, molybdenum, and tungsten" Inorg. Chem. 1963, 2, 1023-1030.