Monocarbonyl Complexes of the Type Re2X4(μ-dppm)2(CO) (X = Cl, Br; dppm = Ph2PCH2PPh2) Revisited. Reactions of the Bromo Complex Re2Br4(μ-dppm)2(CO) with tert-Butyl and Xylyl Isocyanides

Article abstract of DOI:10.1021/ic00127a019

One of the two isomeric forms of the complex Re2Br4(μ-dppm)2(CO) (1b), both of which are formed by the reaction of the triply bonded complexRe2Br4(μ-dppm)2 with CO, is shown to have the same A-frame-like structure as that characterized previously for Re2Cl4(μ-dppm)2(CO) (1a) by X-ray crystallography. Crystal data for Re2Br4(μ-dppm)2(CO).CH2Cl2 (1b) at 293 K: monoclinic space group Cc (No. 9) with a = 19.584(5) ?, b = 17.109(2) ?, c = 17.649(6) ?, β = 105.92(2)°,V = 5686(5) ?**3, Z = 4. The structure was refined to R = 0.055 (Rw= 0.073) for 4891 data with I > 3.0σ(I). The CO ligand is terminally bound, and the Re-Re distance of 2.336(1) ? accords with the retention of a Re.tplbond.Re bond. The second isomeric form of Re2X4(μ-dppm)2(CO) has been identified as having an open unsymmetrical structure X3Re(μ-dppm)2ReX(CO). The reaction of 1b with xylyl and tert-butyl isocyanides produces bioctahedral complexes of the type (RNC)Br2Re(μ-dppm)2ReBr2(CO) (R = xylyl (3a), t-Bu (3b)), in which two Re-Br bonds are collinear with the Re.tplbond.Re bond, and the terminally bound RNC and CO ligands are anti to one another on separate Re centers. This structure has been established by a single-crystal X-ray structure determinationof 3b. Crystal data for Re2Br4(μ-dppm)2(CO)(CN-t-Bu).CH2Cl2.2.5C6H6 (3b) at 203 K: monoclinic space group P21/n (No. 14) with a = 12.892(2) ?, b = 19.478(4) ?, c = 27.231(4) ?, β = 93.623(13)°, V = 6824(4) ?**3, Z = 4. The structure was refined to R = 0.066 (Rw = 0.084) for 5166 data with I > 3.0σ(I). The Re-Re distance is 2.381(1) ?. The mixed carbonyl-isocyanide complexes 3a and 3b react with TlO3SCF3 in dichloromethane to give the triply-bonded compounds [Re2Br3(μ-dppm)2(CO)(CNR)]O3SCF3 (R = xylyl (4a), t-Bu (4b)), which have an unsymmetrical structure with the two metal centers possessing different coordination numbers and the CO and RNC ligands trans to one another. This conclusion is based upon a single-crystal X-ray structure determination of 4a. Crystal data for [Re2Br3(μ-dppm)2(CO)(CNxyl)]O3SCF3 (4a) at 293 K: monoclinic space group P21/c (No. 14) with a = 14.182(3) ?,, b = 15.749(4) ?, c = 31.580(5) ?, β = 98.58(2)°, V = 6974(5) ?**3, Z = 4. The structure was refined to R = 0.066(Rw = 0.088) for 5333 data with I > 3.0σ(I). A comparison is madebetween these results and those obtained previously for the related dirhenium chloro complexes which contain CO and RNC ligands.