The role of pi-pi, stacking interactions in square planar palladium complexes. Combined quantum mechanics/molecular mechanics QM/MM studie

Density functional (DFT) studies and hybrid QM/MM-DFT calculations demonstrate the importance of pi-pi stacking interactions in determining the structural features of two exemplary d(8) palladium complexes, PdBr(p-NCC6H4)({S}-MeO-Biphep), 1, and PdBr(C6F5)-({S}-MeO-Biphep), 2. Despite the superficial similarity of the two compounds, the former shows marked distortions from square planar geometry, while the latter exhibits an almost ideal structure. Attractive pi-pi stacking interactions between two pairs of P-phenyl rings and the arene backbone of the MeO-Biphep are the main origin of the distortion in complex 1. The planar structure of complex 2 is preferred as a consequence of an additional stacking interaction between one P-phenyl ring and the pentafluorophenyl a-ligand. The artificial introduction of an analogous stacking interaction in complex 1 reestablishes an ideal square planar geometry, thus demonstrating that switching on/off specific pi-pi interactions distinctly alters the coordination geometry. These results reveal a previously unrecognized role for pi-pi stacking interactions in the stabilization of structural features in transition metal compounds. This suggests pi-pi stacking interactions as a potential new design principle in tailoring coordination compounds

PGSE diffusion studies on chelating phosphine complexes of ruthenium(II). Solvent dependence and ion pairing

POSE 1H and 19F diffusion data, D values, and 1H 19F HOESY NMR spectra for a series of [RuCl(p-cymene)(BINAP)]X salts, 2a-f, have been recorded in CD 2Cl 2 (X = BF 4- (2a), CF 3 SO 3- (2b), BArF - (2c), PF 6-(2d), SbF 6- (2e), Cl - (2f)). The solvent dependence of the D values for 2a - c in methanol, chloroform, dichloromethane, and acetone and the resulting ion-pairing effects are discussed. On the basis of HOESY data, it is suggested that small anions in chloroform solution approach the ruthenium atom from the side of the molecule remote from the chloride and slightly below the complexed arene. PGSE diffusion data for the Ru-aqua complexes [Ru(H 2O) 3(CO)(dppe)]X 2 (X = BF 4- (3a), CF 3SO 3- (3b), SbF 6 (3c), N(O 2SCF 3) 2- (3d)) are reported. The observed D values for 3a-d partially reflect hydrogen bonding from complexed water to the anions. © 2003 American Chemical Society

Dialkyl effect on enantioselectivity: pi-stacking as a structural feature in P,N complexes of palladium(II)

A phosphino,oxazoline P,N-bidentate ligand, 4, containing 3,5-di-tert-butylphenyl groups has been prepared. In the Heck arylation of dihydrofuran, 4 is shown to afford higher ee's than either 2 or 3, the unsubstituted and m-dimethylphenyl analogues, respectively. Several Pd(0) complexes of 4 are reported. The exchange dynamics of Pd(4)(dba) are shown to involve an interconversion of diastereomers via an intramolecular process. The X-ray structure for PdCl2(4), 8, was determined by X-ray diffraction methods. Comparison of data with PdCl2(2), 9, and PdCl2(3), 10, suggests that differing amounts of stacking influence the structures of these relatively simple Pd complexes, with 9 and 10 revealing the strongest, pi-pi interactions. An estimation of the van der Waals energies involved in the interaction supports a ca. 4 kcal/mol stabilization via pi-pi stacking

NMR, PGSE diffusion, and X-ray diffraction studies of lithium and potassium salts derived from diphenylphosphino(o-cyanophenyl)aniline and their crown ether complexes

H-1, P-31, and Li-7 pulsed-gradient spin-echo (PGSE) diffusion and variable-temperature NMR results for THF solutions of the lithium and potassium salts derived from diphenylphosphino(o-cyanophenyl)aniline are reported and compared to the solid-state results obtained via X-ray diffraction studies. The solution results favor mononuclear salts, sometimes strongly ion paired, whereas the solid-state data reveal dinuclear species. The structures of the products from reactions of these salts with crown ethers are determined via PGSE and H-1 Overhauser NMR methods

Mononuclear and dinuclear complexes with a [Ru(tBu2PCH2CH2PtBu2)(CO)] core.

Thermolysis of solid [Ru(d(t)bpe)(CO)2Cl2](2, d(t)bpe =(t)Bu2PCH2CH2P(t)Bu2) under vacuum affords the five-coordinate complex [Ru(d(t)bpe)(CO)Cl2] (4), which was shown by X-ray crystallography to contain a weak remote agostic interaction. In solution, 4 can be readily trapped by CO, CH3CN or water to give [Ru(d(t)bpe)(CO)(L)Cl2](L = CO, 2; L = CH3CN, 6; L = H2O, 7). Reaction of 4 with AgOTf/H2O yields the tris-aqua complex [Ru(d(t)bpe)(CO)(H2O)3](OTf)2 (8), which has been structurally characterised and probed in solution by pulsed-gradient spin echo (PGSE) NMR spectroscopy. The water ligands in 8 are labile and easily substituted to give [Ru(d(t)bpe)(CO)(NCCH3)3](OTf)2 (10) and [Ru(d(t)bpe)(CO)(DMSO)3](OTf)2 (11). In the presence of CO, the tris-aqua complex undergoes water-gas shift chemistry with formation of the cationic hydride species [Ru(d(t)bpe)(CO)3H](OTf) (12) and CO2. X-Ray crystal structures of complexes 2, 4, 6, 8 and 11-12 are reported along with those for [{Ru(d(t)bpe)(CO)}2(mu-Cl)2(mu-OTf)](OTf) (3), [{Ru(d(t)bpe)(CO)}2(mu-Cl)3][Ru(d(t)bpe)(CO)Cl3](5) and [Ru(d(t)bpe)(CO)(H2O)2(OTf)](OTf)(9)

1H, 19F-HOESY and PGSE diffusion studies on ionic liquids: The effect of co-solvent on structure

1H, 19F-HOESY studies on the ionic liq. based on 1-butyl-3-methylimidazolium tetrafluoroborate, [C4C1i.m.]BF4, [C4C1i.m.][N(OTf)2], and, partially, [C4C1i.m.]PF6, are reported. The addn. of methanol separates the ions; however, in dichloromethane soln. the anions and cations show strong HOESY contacts even after diln. and taken together with the PGSE diffusion measurements, the data suggest strong ion pairing in this solvent