On the cyclodimerization of acetylenes to cyclobutadiene problem: the synthesis, crystal and molecular structure of (η5-cyclopentadienyl)-(η4-1,2-diphenylcyclobuta[l]phenanthrene)rhodium

Abstract A reaction between 2,2′-bis(phenylethynyl)biphenyl and η5-cyclopentadienylcarbonylrhodium has been found to produce several organometallic products, including (η 5 -cyclopentadienyl)(η 4 -1,2-diphenylcyclobuta[ l ]phenanthrene)rhodium has been determined by single-crystal X-ray diffraction using data acquired by a computer-controlled diffractometer. The substance crystallizes in the triclinic system, space group P 1 , with cell constant of a 10.023(1), b 11.146(3), c 22.314(4) A, α 101.26(2), β 97.53(1) and γ 96.55(1)°. The observed and calculated densities (for two molecules in the asymmetric unit) are 1.45(1) and 1.446 g/cm 3 , respectively. The structure was solved by Patterson, Fourier and least-squares refinement techniques. Final discrepancy indices are R(F) - 3.6% and R w (E) = 4.2%. The (η 5 -C 5 H 5 )Rh fragments have a normal disctances and angles, the Cp rings being planar as expected. The cyclobutadiene rings are also planar (largest deviations from planarity being 0.003 A) and parallel to the Cp rings. The most important change, as a result of the constraints of the fused cyclobuta[ l ]phenanthrene system, is the trapezoidal shape of the (η 4 -C 4 ) ring. The effects of fusion and of strain, caused by the crowding of two hydrogen atoms at the open side of phenanthrene, on the cyclobuta[ l ]-phenanthrene system are discussed.