Reactions of acetylenes with noble-metal carbonyl halides. Part 7. Synthesis and chemical characterization of cationic and neutral tetrasubstituted cyclobutadiene complexes of platinum(II). X-Ray structure of dichloro(η4-1,2-dimethyl-3,4-diphenylcyclobutadiene)-(triphenylphosphine)platinum(II)

Abstract

cis-[Pt(CO)₂Cl₂] reacts at 20 °C with 1-phenylpropyne, in non-polar solvents, to give ionic species of the type [Pt₂(C₄Me₂Ph₂)₂Cl₃][Pt(CO)Cl₃](1) with the methyl groups of the η⁴-cyclobutadiene ring in a cis configuration. Analytical and spectroscopic data of complex (1) show that the cation is a dimeric platinum(II) complex with three bridging chlorine atoms; each platinum is also co-ordinated to a tetrasubstituted cyclobutadiene ring. Complex (1) is easily transformed into the corresponding neutral tetrasubstituted-cyclobutadiene derivative [Pt₂(C₄Me₂Ph₂)₂Cl₄](2). Complex (2) reacts with triphenylphosphine to give the monomeric derivative [Pt(C₄Me₂Ph₂)(PPh₃)Cl₂](5a). Evidence for the formation of the isomer with trans methyl groups for reactions under different conditions is reported; organic by-products, obtained by dimerisation of acetylenes with CO insertion, and hexasubstituted benzene have been isolated. The crystal structure of complex (5a) has been solved by Patterson and Fourier methods from counter data and refined by diagonal least squares to a final conventional R of 0.046 for 4 358 independent observed reflections. Crystals of (5a) are monoclinic, space group P2₁/c, with unit-cell dimensions a= 10.900(4), b= 23.437(6), c= 13.289(4)Å, β= 113.5(5)°, and Z= 4. The crystals contain discrete molecules in which the co-ordination around the Pt atom is distorted tetrahedral. Selected bond lengths (Å) are: Pt–Cl(1) 2.425(3), Pt–Cl(2) 2.430(2), Pt–P 2.353(2). The cyclobutadiene ring is asymmetrically co-ordinated to platinum [Pt–C distances in the range 2.116(9)–2.214(8)Å]; the C–C bond distances in the ring, which is planar, are equal to within the estimated standard deviations [av. 1.47(1)Å].