CYCLOBUTYNE LIGANDS .5. C-H BOND-CLEAVAGE VERSUS RING-OPENING OF ALPHA,PI-COORDINATED CYCLOBUTENYL LIGANDS

The reaction of 1-bromocyclobutene with Os3(CO)10(NCMe)2 at 25-degrees-C yielded two new complexes: Os3(CO)10(mu-Br)(mu-CCHCH2CH2) (1; 44%) and OS3(CO)9(mu-Br)(mu3-CCHCH2CH2) (2; 16%). Compound 1 contains a bridging sigma,pi-coordinated cyclobutenyl and a bridging bromo ligand across the open edge of a triosmium cluster. Compound 2 contains a metalated sigma,pi-coordinated bridging alkenyl ligand. Compound 2 was obtained from 1 in good yield both by thermal- and irradiation-induced ring-opening cleavage of a carbon-carbon bond of the cyclobutenyl ligand. Compound 1 was decarbonylated by treatment with Me3NO at 0-degrees-C and was converted into a mixture of 2 (37%) and the new compound Os3(CO)9(mu-Br)(mu3-eta2-C2CH2CH2)-(mu-H) (3; 8%). Compound 3 contains a new example of a triply bridging cyclobutyne ligand formed by the transfer of the alkenyl hydrogen atom of the cyclobutenyl ligand to the metal atoms. Interestingly, compound 3 could not be transformed to 2 by either thermal or photochemical methods. Crystal data for 1: space group P1BAR, a = 15.577(1) angstrom, b = 20.220(3) angstrom, c = 9.555(2) angstrom, alpha = 91.39(2)-degrees, beta = 94.30(2)-degrees, gamma = 93.54(l)-degrees, Z = 6, 5295 reflections, R = 0.027. Crystal data for 2: space group P2(1)/n, a = 13.348(3) angstrom, b = 9.588(3) angstrom, c = 15.036(3) angstrom, beta = 108.93(2)-degrees, Z = 4, 1753 reflections, R = 0.034. Crystal data for 3: space group P1BAR, a = 9.237(2) angstrom, b = 13.922(5) angstrom, c = 7.779(2) angstrom, alpha = 92.57(2)-degrees, beta = 111.32(2)-degrees, gamma = 88.31(2)-degrees, Z = 2, 1727 reflections, R = 0.029.