NOVEL BINUCLEAR BIS(DIMETHYLPHOSPHINO)METHANE-BRIDGED COMPLEXES OF RHODIUM CONTAINING 2 AND 3 ALKYNE MOLECULES

The bis(alkyne) complexes [Rh2(CO)2(mu-RC=CR)(mu-R'C=CR')(dmpm)2] (R = R'= CO2Me (1); R = CO2Me, R' = CF3 9.833 dmpm = Me2PCH2PMe2) are prepared by addition of dimethyl acetylenedicarboxylate to [Rh2(CO)2(mu-RC=CR) (dmpm)2] (R = CO2Me, CF3). Addition of excess hexafluorobutyne (HFB) to [Rh2(CO)2(mu-HFB)(dmpm)2] yields [Rh2(HFB)2(mu-CO)(mu-HFB)(dmpm)2] (4), in which one alkyne is eta-2-bound to each rhodium while the third is bridging. Reaction of 4 with carbon monoxide yields [Rh2(CO)2(eta-2-HFB)(mu-HFB)(dmpm)2] (5), containing one terminal and one bridging HFB group, and this species rearranges to [Rh2(CO)2(mu-HFB)2(dmpm)2] (3), analogous to species 1 and 2. All bis(alkyne) complexes 1-3 are unreactive and do not react with CO OT With additional alkyne. The structures of 2 and 3 have been determined. Compound 2 crystallizes in the triclinic space group P1BAR with a = 9.7825 (8) angstrom, b = 9.833 (1) angstrom, c = 9.3791 (9) angstrom, alpha = 92.390 (8)-degrees, beta = 99.893 (7)-degrees, gamma = 118.180 (8)-degrees, V = 775.5 angstrom3, and Z = 1, while 3 crystallizes in the monoclinic space group P2(1)/n with a = 9.474 (1) angstrom, b = 15.813 (2) angstrom, c = 9.917 (1) angstrom, beta = 95.34 (1)-degrees, V = 1479.3 angstrom3, and Z = 2. For compound 2 the disorder present affects only the substituents on the alkynes (CO2Me, CF3); all other atoms are well behaved. In spite of this disorder in 2 all atoms refined well, converging to R = 0.030 and R(w) = 0.049 based on 2486 observations. Compound 3 refined to R = 0.040 and R(w) = 0.043 based on 1588 observations. Both compounds have a trans arrangement of diphosphines with the alkynes bound on opposite faces of the complex as cis-dimetalated olefins. The Rh-Rh separations of 2.6989 (4) angstrom (2) and 2.696 (1) angstrom (3) are quite short; however, all parameters associated with the alkyne ligands are as expected for this binding mode.