REACTIONS OF THE DIRHENIUM(II) COMPLEXES RE2X4(DPPM)2(X = CL OR BR DPPM = PH2PCH2PPH2) WITH ISOCYANIDES .4. ISOMERISM IN MIXED CARBONYL-ISOCYANIDE COMPLEXES OF STOICHIOMETRY [RE2CL3(DPPM)2(CO)2(CNR)]N+(N = 0 OR 1) AND [RE2CL3(DPPM)2(CO)(CNR)2]+(R = T-BU OR XYLYL), WHICH POSESS EDGE-SHARED BIOCTAHEDRAL STRUCTURESN
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The multiply bonded dirhenium isocyanide complexes Re2Cl4(dppm)2(CNR) (R = t-Bu or xylyl) react with gaseous CO at room temperature and 1 atm pressure, in the presence of T1PF6, to yield the mixed isocyanide-carbonyl species [Re2Cl3(dppm)2-(CO)2(CNR)]PF6(la, R = t-Bu; lb, R = xylyl). The reaction of Re2Cl4(dppm)2(CO)(CN-t-Bu) with CO in the presence of T1PF6also provides a high yield route to la. In the case of both la and lb, thermal isomerization to the isomers (2a and 2b), which contain an all-cis arrangement of CO and CNR ligands, occurs. Complexes 2a and 2b and others of this kind can be prepared directly by the reaction of Re2Cl4(dppm)2(CO)2with RNC in the presence of T1PF6; the xylylNC derivative has also been prepared from the reaction of Re2Cl4(dppm)2(CO)(CNxylyl) with CO and T1PF6. The spectroscopic (IR, NMR, and electronic absorption) and electrochemical properties of 1 and 2, while generally similar to one another, nonetheless show some important differences. Complexes 1 and 2 can be reduced (by one electron) to their neutral, structurally related analogues 3 and 4, respectively, by using acetone solutions of cobaltocene. Representative X-ray crystal structures of la and 4 (R = i-Pr) have helped aid in the characterization of these systems. Compound la, [Re2Cl3(dppm)2(CO)2(CN-t-Bu)]PF6, crystallizes along with two formula units of dichloromethane in the triclinic system, space group PI, with a = 10.950 (4) A, b = 12,353 (4) A, c = 13.358 (5) A, a = 108.55 (3), 96.56 (3), y = 104.77 (3), V = 1618 (1) A3, and Z = 1. Difference-structure direct methods was found to be the only available technique leading to successful development of the structure. Least-squares refinement of 745 variables to 5478 data gave final residuals of R = 0.025 and Rw= 0,035 and quality of fit 0.896. This compound has an edge-sharing bioctahedral structure and a Re-Re bond distance of 2.605 (1) A. The equatorial plane, which comprises the [Re2Cl3(CO)2(CN-t-Bu)] unit, has a cis arrangement of two CO ligands on one side, while the other side has the t-BuNC ligand, viz. Cl(CO)Re(-Cl)(-CO)ReCl(CN-t-Bu). The molecule Cl(CO)Re(Cl)(-CO)(-dppm)2ReCl(CN-i-Pr) (4) crystallizes in the tetragonal system, space group P43212, with a = 15.033 (7) A, c = 26.366 (14) A, V = 5959 (7) A3, and Z = 4. The structure was refined to residuals of R = 0.054 and Rw= 0.074 and quality of fit 1.558. The Re-Re bond distance is 2.718 (1) A. The terminal sites occupied by the carbonyl and isocyanide ligands are disordered due to the presence of a crystallographic twofold axis. An infrared spectrum of the crystalline sample used in this study is in accord with the structure found in the solid state. The molecule possesses one bridging and one terminal carbonyl ligand as well as a terminal CN-i-Pr ligand. Structural isomerism has also been encountered in the case of the mixed carbonyl-isocyanide complexes of stoichiometry [Re2Cl3(dppm)2(CO)(CNR)2]PF6(5, R = t-Bu; 6 and 7, R = xylyl). These complexes can be prepared from the monocarbonyl Re2Cl4(dppm)2(CO) or from Re2Cl4(dppm)2(CO)(CNR) by using 2 or 1 equiv of the RNC ligand, respectively, in the presence of T1PF6. Whereas 5 is believed to contain dppm bridges and terminally bound CO and t-BuNC, ligands, the isomers 6 and 7 of the xylyl isocyanide derivative most likely possess structures that are closely akin to those of 1 and 2, respectively. Thermal isomerization of 6 to the all-cis isomer 7 occurs in 1,2-dichloroethane. 1987, American Chemical Society. All rights reserved.
