UNUSUAL ALKYNE-ALKYLIDENE COUPLING REACTIONS AT A RHODIUM-RHODIUM BOND - CRYSTAL AND MOLECULAR-STRUCTURE OF THE BRIDGING BUTADIENE COMPLEX )2RH2(MU-CO)(MU-ETA-2-ETA-2-C(CF3)H=C(CF3)CME=CH2)

Two products isolated from the reaction between (eta-C5H5)2Rh2(mu-CO){ mu-eta2-CF3C2CF3) (1) and N2CMe2 have been characterized spectroscopic ally as (eta-C5H5)2Rh2{mu-CF3C2CF3.CO.N2CMe2} (4) and (eta-C5H5)2Rh2(m u-CO)(mu-CF3C2CF3-CMe2) (5). The structure of 4 is analogous to that p reviously established for the related N2CMePh derivative. It is propos ed that the structure of 5 incorporates a mu-eta1-alkylidene with CF3 and C(CF3)=CMe2 substituents and a bridging carbonyl. When 5 is left i n solution, there is a 1,4-hydrogen shift that converts the bridging a lkylidene to a bridging diene ligand. The resultant complex (eta-C5H5) 2Rh2(mu-CO) {mu-eta2:eta2-C(CF3)H=C(CF3)CMe=CH2}(6) has been character ized by determination of the X-ray crystal structure. Crystal data: C1 8H16F6ORh2; M = 568.1, P1BAR, a = 12.313(2) angstrom, b = 9.365(2) ang strom, c = 8.758(2) angstrom, alpha = 112.16(1)-degrees, beta = 91.97( l)-degrees, gamma = 105.40(1)-degrees; V = 891.4(3) angstrom3, D(calcd ) = 2.12 g cm-3 (Z = 2), D(measd) = 2.11(l) g cm-3, mu = 18.9 cm-1 for Mo Kalpha radiation (lambda = 0.7107 angstrom), final R = 0.040, R(w) = 0.044, from 4252 observed reflections (5201 measured). The reaction of 1 with diazocyclopentadiene, N2C5H4, follows a different course. T here is elimination of nitrogen and a 1,2-hydrogen shift within the C5 H4 unit to form a cyclic 'ene-yne''. A product of formula (eta-C5H5)2R h2(mu-CF3C2CF3.CO)(C=CCH2CH=CH) is obtained, but it has not been possi ble to determine if the C5 ring is attached directly to the Rh-Rh bond (7a) or has condensed with CF3C2CF3.CO to form a bridging pentadienon e (7b). Nitrogen elimination is also the primary step in the reaction of the diazomalonates N2C(CO2R)2, R = Me or t-Bu, with 1. The bridging alkylidenecomplexes H5)2Rh2(mu-CO)(mu-eta1:eta1-CF3C2CF3){mu-C(CO2R)2 } are formed, and these complexes undergo a rearrangement when chromat ographed or irradiated with sunlight to form the bridging divinyl ethe r complexes 1:eta2-C(CF3)=C(CF3)OC(OR)=C-(CO2R)}(8;R=Me,t-Bu). With th e t-Bu system, a second rearrangement occurs as a consequence of hydro lysis of the ester functions. This gives a product formulated as (eta- C5H5)2Rh2(CO){mu-eta2 :eta1-CH(CF3)C(CF3)OC(O)C(CO2H)} (9).