SYNTHESIS, STRUCTURE, AND FLUXIONAL PROPERTIES OF THE CATIONIC CLUSTERS [(ETA-C5H5)2RH3(CO)(CF3C2CF3)(CNR)3]+PF6- (R = I-PR, TERT-BU) - UNEXPECTED ATTACHMENT OF THE ALKYNE IN THE CRYSTAL-STRUCTURE

The cationic cluster complexes [(η-C5H5)2Rh3(CO)(CF3C2CF3)(CNR)3]+PF6- are formed when (η-C5H5)2Rh2(μ-CO)(μ-CF3C2CF3) is treated with [Rh(CNR)4]+PF6- (R = i-Pr, t-Bu). The structure of the complex with R = t-Bu and one solvent molecule (CH2Cl2) has been determined by X-ray crystallography. Crystal data: C31H39Cl2F12N3OPRh3, Mr 1108.25, monoclinic, P21/m, a = 17.509 (2) Å, b = 12.360 (2) Å, c = 10.154 (1) Å, Z = 2, final R = 0.064 for 1635 “observed” reflections. The molecular structure confirms loss of one CNR ligand in the addition reaction and reveals a symmetrical structure with an edge-bridging CO and a face-bridging CF3C2CF3. The CO is attached to the two (η-C5H5)Rh sites, and the alkyne is σ-attached to these two rhodium atoms and π-bonded to the metal of the Rh(CNR)3 unit. This is contrary to expectation; π-bonding is normally to the least electron attracting metal center. All CNR ligands are terminal. Spectroscopic results indicate that two isomers coexist in solution for each of the clusters (R = t-Bu, i-Pr). The symmetrical solid-state structure is preserved for the minor isomer in solution, and the NMR spectra of this isomer are not temperature-dependent. Variable-temperature NMR results show that the major isomer is fluxional at room temperature; the limiting spectra at low temperature are consistent with an unsymmetrical structure in which the alkyne is σ-attached to one (η-C5H5)Rh-Rh(CNR)3 bond and π-bonded to the remaining (η-C5H5)Rh. The CO adopts a semi-face-bridging location with the weakest interaction to Rh(CNR)3. © 1990, American Chemical Society. All rights reserved.