Five-coordinate platinum (II) alkyne complexes: synthesis, ab initio calculations and crystal and molecular structure of [PtI 2(Me 2phen) η2PhC CPh]·CHCl 3

Abstract

[PtX 2(Me 2phen)] complexes ( 1)Me 2phen = 2,9-dimethyl-1,10-phenanthroline; X = Br, Iy; I, Iz), having a highly distorted square-planar geometry, undergo alkynes uptake in chlorinated solvents to give the five-coordinate trigonal-bipyramidal species [PtX 2(Me 2phen)-(alkyne)] ( 2) (alkyne = ethyne, a; propyne, b; 2-butyne, c; 1-pentyne, d; phenylacetylene, e; methylphenylacetylene, f; diphenylacetylene, g). The equilibrium constants for formation ( K 1) decrease in the order (1>Br) already observed for the analogous reaction with alkenes to give the [PtX 2(Me 2phen)(alkene)] complexes ( 3) (1>Br>Cl) although the differences in K 1 values are much greater in the case of alkynes. In contrast, the number and bulk of substituents at the metal-coordinated carbons have practically no effect on the K 1 of the alkyne complexes, while they greatly reduce the stability of the five-coordinate olefin compounds. Finally, the Δ G of activation for rotation around the Ptalkyne bond calculated by variable temperature 1H NMR spectroscopy (71.5 ± 0.8 kJ mol −1 for 2zb) is considerably smaller than the value found for the corresponding five-coordinate propene complex (86.2 ± 0.8 kJ mol −1). The crystal and molecular structure of the chloroform solvate of [PtI 2(Me 2phen( η 3-PhCCPh)] ( 2zg·CHCl 3) has been determined by X-ray diffraction methods. In the crystals two crystallographically independent complexes and chloroform molecules are present: space group Cmc2, a = 14.367(3), b = 15.529(3), c = 26.959(5)Å, Z = 8, R = 0.0444. The average bending back of the phenyl groups with respect to the CC triple bond axis is 25(1)°. Spectroscopic and crystal data show that the conformation of the coordinated alkynes in these five-coordinate complexes is intermediate between those found for the trigonal Pt(0) and the square-planar Pt(II) alkyne complexes. Ab initio Hartfree-Fock calculations and constrained space orbital variation analyses have shown that in the five-coordinate species ethylene exhibits better σ-donating and π-accepting properties than acetylene.