Chemistry of transition-metal complexes containing functionalized phosphines: synthesis and structural analysis of rhodium(I) complexes containing allyl and cyanoalkylphosphines.

A series of related acetylacetonate-carbonyl-rhodium compounds substituted by functionalized phosphines has been prepared in good to excellent yields by the reaction of [Rh(acac)(CO)] (acac is acetylacetonate) with the corresponding allyl-, cyanomethyl- or cyanoethyl-substituted phosphines. All compounds were fully characterized by P, H, C NMR and IR spectroscopy. The X-ray structures of (acetylacetonato-κO,O')(tert-butylphosphanedicarbonitrile-κP)carbonylrhodium(I), [Rh(CHO)(CO)(CHN)] or [Rh(acac)(CO)(BuP(CHCN)}] (2b), (acetylacetonato-κO,O')carbonyl[3-(diphenylphosphanyl)propanenitrile-κP]rhodium(I), [Rh(CHO)(CHN)(CO)] or [Rh(acac)(CO){PhP(CHCHCN)}] (2h), and (acetylacetonato-κO,O')carbonyl[3-(di-tert-butylphosphanyl)propanenitrile-κP]rhodium(I), [Rh(CHO)(CHN)(CO)] or [Rh(acac)(CO){BuP(CHCHCN)}] (2i), showed a square-planar geometry around the Rh atom with a significant trans influence over the acetylacetonate moiety, evidenced by long Rh-O bond lengths as expected for poor π-acceptor phosphines. The Rh-P distances displayed an inverse linear dependence with the coupling constants J and the IR ν(C[triple-bond]O) bands, which accounts for the Rh-P electronic bonding feature (poor π-acceptors) of these complexes. A combined study from density functional theory (DFT) calculations and an evaluation of the intramolecular H...Rh contacts from X-ray diffraction data allowed a comparison of the conformational preferences of these complexes in the solid state versus the isolated compounds in the gas phase. For 2b, 2h and 2i, an energy-framework study evidenced that the crystal structures are mainly governed by dispersive energy. In fact, strong pairwise molecular dispersive interactions are responsible for the columnar arrangement observed in these complexes. A Hirshfeld surface analysis employing three-dimensional molecular surface contours and two-dimensional fingerprint plots indicated that the structures are stabilized by H...H, C...H, H...O, H...N and H...Rh intermolecular interactions.