Studies of dirhodium(II) tetrakis (trifluoroacetate). 6. The first structural characterization of axial alkyne complexes, Rh-2(O2CCF3)(4)(Ph2C2)(n) (n=1, 2): Diphenylacetylene as a bifunctional ligand

The solid-state reactions of Rh-2(O2CCF3)(4) with diphenylacetylene (Ph2C2) have produced the first two dirhodium(II,II) carboxylate complexes with co ordinated alkynes having overall compositions of 1:2 (1) and 1:1 (2), Compo unds 1 and 2 have been characterized by elemental analysis and IR and mass spectroscopy, and their structures have been determined by X-ray diffractio n, The bisadduct 1 has two alkyne groups coordinating to axial positions of the dirhodium tetrakis(trifluoroacetate) molecule. In the crystal structur e of 2 the diphenylacetylene is acting as a bifunctional ligand, binding on e dirhodium unit through the carbon-carbon triple bond while coordinating t o another one via its phenyl group, thus forming an extended polymeric stru cture. The alkyne coordination in both complexes is an eta(2)-type without disruption of the triple-bond system (the C=C distances are 1.204(6) Angstr om (1) and 1.206(8) Angstrom (2); the average C=C-C angles are 168.3(4)degr ees (1) and 169.2(6)degrees (2)), The axial Rh-C-alkyne contacts are averag ed to 2.530(4) Angstrom in 1 and to 2.494(5) Angstrom in 2. The coordinatio n of Rh to one of the phenyl groups of Ph2C2 in (2) is off-centered (3,4), with the two closest Rh-C-arene distances being 2.696(5) and 2.750(6) Angst rom.