Interaction of H2 and Prototypical Solvent Molecules with Cr(CO)5 in the Gas Phase

Calibrating Reaction Enthalpies: Use of Density Functional Theory and the Correlation Consistent Composite Approach in the Design of Photochromic Materials

Acquisition of highly accurate energetic data for chromium-containing molecules and various chromium carbonyl complexes is a major step toward calibrating bond energies and thermal isomerization energies from mechanisms for Cr-centered photochromic materials being developed in our laboratories. The performance of six density functionals in conjunction with seven basis sets, utilizing Gaussian-type orbitals, has been evaluated for the calculation of gas-phase enthalpies of formation and enthalpies of reaction at 298.15 K on various chromium-containing systems. Nineteen molecules were examined: Cr(CO)₆, Cr(CO)₅, Cr(CO)₅(C₂H₄), Cr(CO)₅(C₂ClH₃), Cr(CO)₅(cis-(C₂Cl₂H₂)), Cr(CO)₅(gem-(C₂Cl₂H₂)), Cr(CO)₅(trans-(C₂Cl₂H₂)), Cr(CO)₅(C₂Cl₃H), Cr(CO)₅(C₂Cl₄), CrO₂, CrF₂, CrCl₂, CrCl₄, CrBr₂, CrBr₄, CrOCl₂, CrO₂Cl₂, CrOF₂, and CrO₂F₂. The performance of 69 density functionals in conjunction with a single basis set utilizing Slater-type orbitals (STO) and a zeroth-order relativistic approximation was also evaluated for the same test set. Values derived from density functional theory were compared to experimental values where available, or values derived from the correlation consistent composite approach (ccCA). When all reactions were considered, the functionals that exhibited the smallest mean absolute deviations (MADs, in kcal mol^-1) from ccCA-derived values were B97-1 (6.9), VS98 (9.0), and KCIS (9.4) in conjunction with quadruple-ζ STO basis sets and B97-1 (9.3) in conjunction with cc-pVTZ basis sets. When considering only the set of gas-phase reaction enthalpies (Δ_rH°_gas), the functional that exhibited the smallest MADs from ccCA-derived values were B97-1 in conjunction with cc-pVTZ basis sets (9.1) and PBEPBE in conjunction with polarized valence triple-ζ basis set/effective core potential combination for Cr and augmented and multiple polarized triple-ζ Pople style basis sets (9.5). Also of interest, certainly because of known cancellation of errors, PBEPBE with the least-computationally expensive basis set combination considered in the present study (valence double-ζ basis set/effective core potential combination for Cr and singly-polarized double-ζ Pople style basis sets) also provided reasonable accuracy (11.1). An increase in basis set size was found to have an improvement in accuracy for the best performing functional (B97-1).

Time-resolved infrared (TRIR) study on the formation and reactivity of organometallic methane and ethane complexes in room temperature solution

We have used fast time-resolved infrared spectroscopy to characterize a series of organometallic methane and ethane complexes in solution at room temperature: W(CO)5(CH4) and M(eta5-C5R5)(CO)2(L) [where M = Mn or Re, R = H or CH3 (Re only); and L = CH4 or C2H6]. In all cases, the methane complexes are found to be short-lived and significantly more reactive than the analogous n-heptane complexes. Re(Cp)(CO)2(CH4) and Re(Cp*)(CO)2(L) [Cp* = eta5-C5(CH3)(5) and L = CH4, C2H6] were found to be in rapid equilibrium with the alkyl hydride complexes. In the presence of CO, both alkane and alkyl hydride complexes decay at the same rate. We have used picosecond time-resolved infrared spectroscopy to directly monitor the photolysis of Re(Cp*)(CO)3 in scCH4 and demonstrated that the initially generated Re(Cp*)(CO)2(CH4) forms an equilibrium mixture of Re(Cp*)(CO)2(CH4)/Re(Cp*)(CO)2(CH3)H within the first few nanoseconds (tau = 2 ns). The ratio of alkane to alkyl hydride complexes varies in the order Re(Cp)(CO)2(C2H6):Re(Cp)(CO)2(C2H5)H > Re(Cp*)(CO)2(C2H6):Re(Cp*)(CO)2(C2H5)H approximately equal to Re(Cp)(CO)2(CH4):Re(Cp)(CO)2(CH3)H > Re(Cp*)(CO)2(CH4):Re(Cp*)(CO)2(CH3)H. Activation parameters for the reactions of the organometallic methane and ethane complexes with CO have been measured, and the DeltaH++ values represent lower limits for the CH4 binding enthalpies to the metal center of W-CH4 (30 kJ.mol(-1)), Mn-CH4 (39 kJ.mol(-1)), and Re-CH4 (51 kJ.mol(-1)) bonds in W(CO)5(CH4), Mn(Cp)(CO)2(CH4), and Re(Cp)(CO)2(CH4), respectively.

Electrochemistry and [60]fullerene displacement reactions of (dihapto-[60]fullerene) pentacarbonyl metal(0) (M = Cr, Mo, W)

Cyclic voltammetry (CV) measurements on (eta(2)-C(60))M(CO)(5) complexes (M = Cr, Mo, W) in dichloromethane show three [60]fullerene-centered and reversible reduction/oxidation waves. The E(1/2) values of these waves are shifted to positive values relative to the corresponding values of the uncoordinated [60]fullerene in the same solvent. A Jahn-Teller type distortion of the spherical surface of [60]fullerene promoted by [60]fullerene-metal pi-backbonding may explain the observed positive shifts. Lewis bases (L = piperidine and triphenyl phosphine) displace [60]fullerene from (eta(2)-C(60))M(CO)(5) complexes. Analysis of the activation parameters for the metal-[60]fullerene dissociation, the metal-[60]fullerene bond enthalpies (from DFT computations), and metal-solvent (benzene) bond enthalpies (from DFT computations) suggests appreciable solvent contribution to the transition state leading to formation of the intermediate species solvent-M(CO)(5). Appreciable transition state stabilization due to solvation of the intermediate species is inferred for M = Mo and W. For M = Cr, stabilization of the intermediate species due to solvation is not accompanied by the corresponding transition state stabilization.

Experimental determination of the Cr-C(2)Cl(4) bond dissociation enthalpy in Cr(CO)(5)(C(2)Cl(4)): quantifying metal-olefin bonding interactions

The bond dissociation enthalpy for the Cr-C(2)Cl(4) bond in gas-phase Cr(CO)(5)(C(2)Cl(4)) has been determined to be 12.8 +/- 1.6 kcal/mol using transient infrared spectroscopy. The results of a density functional theory-based energy decomposition analysis are used to quantify the metal-olefin bonding interactions in terms of the bonding description provided by the Dewar-Chatt-Duncanson model (sigma donation and back-bonding). The bond energy decomposition analysis reveals that metal-olefin bond strengths can be strongly influenced by the Pauli repulsion energy and by the energy necessary to deform the olefin and the metal-centered moiety from their equilibrium geometries to their geometry in the final complex. Further, a comparison between the metal-olefin bond strengths and the magnitude of the electronic interactions demonstrates that the energy associated with these deformations is the determining factor in the trends in bond enthalpies in the series of complexes Cr(CO)(5)(C(2)X(4)) (X = H, F, Cl). Though deformation of the Cr(CO)(5) moiety contributes to the overall deformation energy, the major contribution involves deformation of the olefin. This occurs as a consequence of rehybridization of the olefin as a result of metal-olefin back-bonding. The results are discussed in terms of the Dewar-Chatt-Duncanson model, which provides the accepted qualitative description of bonding in organometallic olefin complexes.