Theoretical approaches to the description of the electronic structure and chemical reactivity of transition metal compounds

Electronic control of the regiochemistry in palladium-phosphine catalyzed intermolecular Heck reactions

Density functional theory calculations of the transition-state structures and reaction barriers for the C-C coupling between monosubstituted eta(2)-olefins and eta(1)-vinyl for neutral [PdI(PH(3))(vinyl)(RCHCH(2))] and cationic [Pd(H(2)PCH(2)PH(2))(vinyl)(RCHCH(2))](+) (R = OMe, Me, and CN) depend mostly on the regiochemistry and not on the starting position of the olefin substituent. The regiochemistry is thus implicit in the electronic structure of the precursor complex. A selectivity index, Omega, based on electrostatic and frontier orbital interactions gives a good correlation with experiment for vinylations or arylations. The model correctly predicts that the regiochemistry for R = OMe, Me, and CN is the same for both neutral and cationic Pd complexes while for R = CH(2)OH the regiochemistry reverses. The latter is confirmed by explicit calculations of the transition-state energies. Selectivity indices are computed for 13 substituents: CO(2)Me, CN, CF(3), Ph, H, Me, CH(2)OH, CH(2)NMe(2), 2-pyrolidone, CH(2)SiMe(3), OAc, OMe, and F. Cationic conditions systematically give larger Omega values and thus tend to favor coupling at the alpha carbon on the olefin. The Omega values are approximately additive and can be used to predict the regiochemistry for disubstituted olefins.

Are Gas-Phase Reactions of Five-Coordinate Divalent Metal Ion Complexes Affected by Coordination Geometry?

Five-coordinate metal complex ions of the type [ML](2+) [where M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and L= 1,9-bis(2-pyridyl)-2,5,8-triazanonane (DIEN-(pyr)(2)) and 1,9-bis(2-imidazolyl)-2,5,8-triazanonane (DIEN-(imi)(2)] have been reacted with acetonitrile in the gas phase using a modified quadrupole ion trap mass spectrometer. The kinetics and thermodynamics of these reactions show that the reactivity of these complexes is affected by metal electronic structure and falls into three groups: Mn(II) and Ni(II) complexes are the most reactive, Fe(II) and Co(II) complexes exhibit intermediate reactivity, and Cu(II) and Zn(II) complexes are the least reactive. To help explain the experimental trends in reactivity, theoretical calculations have been used. Due to the relatively large size of the metal complexes involved, we have utilized a two-layered ONIOM method to perform geometry optimizations and single point energy calculations for the [ML](2+) and [ML + CH(3)CN](2+) systems. The calculations show that the reactant five-coordinate complexes ([ML](2+)) exhibit structures that are slightly distorted trigonal bipyramidal geometries, while the six-coordinate complexes ([ML + CH(3)CN](2+)) have geometries that are close to octahedral. The Delta G values obtained from the ONIOM calculations roughly agree with the experimental data, but the calculations fail to completely explain the trends for the different metal complexes. The failure to consider all possible isomers as well as adequately represent pi-d interactions for the metal complexes is the likely cause of this discrepancy. Using the angular overlap model (AOM) to obtain molecular orbital stabilization energies (MOSE) also fails to reproduce the experimental trends when only sigma interactions are considered but succeeds in explaining the trends when pi interactions are taken into account. These results indicate that the pi-donor character of the CH(3)CN plays a subtle, yet important, role in controlling the reactivity of these five-coordinate complexes. Also, the AOM calculations are consistent with the experimental data when the [ML](2+) complexes have high-spin trigonal bipyramidal configurations. Generally, these results suggest that ion-molecule reactions can be very sensitive to metal complex coordination geometry and thus may have some promise for providing gas-phase coordination structure.