59th Annual Report on Research 2014

Progress Report Year 3: 08/01/2013 – 08/31/2014

Previously we reported that computational chemistry studies (DFT/BP86 level) indicate that cyanoalkenes preferentially bind to M(CO)₅, M = Cr, Mo or W, via the nitrogen atom over the C=C bond.

We have carried out competitive kinetic determinations of the activation enthalpy of the thermal decomposition of fumaronitrile (FN) complexes of the chromium and tungsten pentacarbonyl complexes. The complexes were prepared and purified according to literature procedures. In these experiments, we set up a competitive scheme in which a dilute solution of the complex in toluene was exposed to different concentrations of pyridine and a fixed amount of fumaronitrile. Pyridine is a stronger sigma donor than fumaronitrile. Temperature was varied in the 55-70 ^oC for tungsten and 45-60 ^oC for chromium. The kinetic behavior suggested an associative mechanism as the observed rate for decomposition was linear with pyridine concentration in the experimental range of concentrations. The activation energy (E_a) of the dissociation step (rate limiting) was inferred from the Arrhenius equation, assuming that association of ligand is barrierless, as demonstrated for many other systems.  The E_a values obtained were 23.4 ± 1.4 kcal/mol and 26.7 ± 3.3 kcal/mol for (FN)Cr(CO)₅ and (FN)W(CO)₅, respectively. These values agree well with the computed values for bond dissociation energies: 27.8 and 31.6 kcal/mol. The difference is attributed to the lack of correction for PV, solvent effects and the fact that ligand association may not be barrierless, but rather slightly activated by a few kcal/mol. We attempted the quantification of the activation energy for the thermal decomposition of the tetracyanoethylene complexes, but the presence of pyridine induced the formation of a precipitate byproduct at elevated temperatures that prevented the Arrhenius analysis.

We have also investigated, using DFT and bond energy decomposition analysis (BEDA), the effects of electron withdrawing and electron donating substituents in the para- position of the phenyl ring of styrene derivatives on the metal-olefin bond, and along different metals. It is hypothesized that the metal-styrene bond energy could be tuned by the presence of either electron withdrawing or electron donating in the phenyl ring. In these set of complexes the bonding interaction would be not largely influenced by steric effects as the substituent would be at a position distant from the bonding site. We studied the following systems: (CO)₅M-L, M = Cr, Mo, W, (CO)₄Fe-L (L in equatorial position), and (CO)₃Ni-L, with L = styrene, 4-nitrostyrene, 4-vinyl benzaldehyde, 4-fluorostyrene, 4-chlorostyrene, 4-bromostyrene, 4-iodostyrene, 4-hydroxystyrene, 4-methoxystyrene, 4-tert-butoxysyrene, 4-aminostyrene, 4-acetoxystyrene, 4-tert-burylstyrene, and 4-trifluoromethyl styrene. In terms of the Dewar-Chatt-Duncanson (DCD) model, the most electronwitdrawing alkene (styrene) would bind the strongest with the metal. Therefore the complexes with para-nitrostyrene should be the ones with the strongest metal-olefin bond and the complexes with para-aminostyrene should be the ones with the weakest metal-olefin bond. DFT calculations actually indicate a trend opposite to the expected one. The BEDA indicates that the attractive covalent interaction between metal and olefin increases with the increase in electron withdrawing ability of the para-substitutent in agreement with the DCD model. However, the extent of this interaction seems to affect the repulsive effects. This counteracting effect results from the decrease in the metal-alkene bond distance that results from the strong bonding interaction.

As expected the iron complexes are the most stable in the metal series, with bond dissociation energies (BDE) around 63 kcal/mol. Tungsten complexes are more stable (~ 24 kcal/mol) than chromium (~19 kcal/mol) than molybdenum (~18 kcal/mol) but much less stable than the iron complexes. The nickel complexes BDEs (~23 kcal/mol) are comparable to the tungsten complexes.