REACTIONS OF THE POLYHYDRIDE COMPLEX REH7(PPH3)2 WITH HYDROXYPYRIDINE AND MERCAPTOPYRIDINE LIGANDS - FORMATION OF HYDRIDO COMPLEXES OF RHENIUM(III), RHENIUM(IV), AND RHENIUM(V) AND THE CHARACTERIZATION OF 8-COORDINATE ISOMERS IN THE SOLID-STATE AND IN SOLUTION

The reactions of the heptahydride complex ReH7(PPh3)2 with 2-hydroxypyridine (Hhp), 2-mercaptopyridine (Hmp) and 2-hydroxypyridine (Hhp), 2-mercaptopyridine (Hmp) and 2-hydroxy-6-methylpyridine (Hmhp) in acetone afford the diamagnetic seven-coordinate complexes ReH(L)2(PPh3)2 (L = hp, mp, mhp), which can be oxidized by one electron when treated with [(eta(5)-C5H5)2Fe]PF6 in dichloromethane to give paramagnetic [ReH(L)2(PPh3)2]PF6. These are rare examples of mononuclear rhenium(IV) hydrides. While the 17-electron mp derivative is stable in solution, the related hp and mhp complexes decompose (probably by a disproportionation mechanism) to give the dihydrides [ReH2(L)2(PPh3)2]PF6 as the major products. In these latter reactions small quantities of the seven-coordinate oxorhenium(V) complexes [ReO(L)2(PPh3)2]PF6 can also be isolated. The crystal structure of [ReO(mhp)2(PPh3)2]PF6.C2H4Cl2 (4) shows that it is based upon a distorted pentagonal bipyramid with a P-Re-P angle of ca. 169-degrees and with the oxo ligand occupying a position within the pentagonal plane. The diamagnetic dihydrido species [ReH2(L)2(PPh3)2]PF6 are also formed in all cases upon treatment of ReH(L)2(PPh3)2 with HPF6 in dichloromethane. The mp derivative is unstable and converts to [ReH(mp)2(PPh3)2]PF6 with loss of H-2. The hp complex [ReH2(hp)2(PPh3)2]PF6 is stable both in the solid state and in solution, and is identical in all respects with the form of this complex that is isolated from the decomposition of [ReH(hp)2(PPh3)2]PF6. On the other hand, the complex of composition [ReH2(mhp)2(PPh3)2]PF6 exists in two geometric isomeric forms. The product that is obtained upon decomposition of the 17-electron complex [ReH(mhp)2(PPh3)2]PF6 is denoted as the cis isomer and is the more thermodynamically stable form of the two. The trans isomer is the form that is obtained by protonation of neutral ReH(mhp)2(PPh3)2. Both isomers have structures that are based upon distorted dodecahedral geometries with the hydrogen and oxygen atoms at the A sites and the nitrogen and phosphorus atoms at the B sites of an MA4B4 dodecahedron. Crystal structures on one form of the trans isomer and two different crystalline forms of the cis isomer have been determined, viz., trans-[ReH2(mhp)2(PPh3)2]PF6.C2H4Cl2 (1), cis-[ReH2(mhp)2(PPh3)2]PF6.(CH3)2CO (2), and cis-[ReH2(mhp)2(PPh3)2]PF6.0.5C2H4Cl2 (3). Each isomer appears to be a classical dihydride. They both have an independent existence in solution although the trans isomer converts very slowly to the cis isomer. This is the first time that eight-coordinate geometric isomers which are stable both in the solid state and in solution have been structurally characterized. These two isomers, as well as [ReH2(hp)2(PPh3)2]PF6, are easily deprotonated by NEt3 and/or DBU to re-form the parent neutral monohydrides ReH(L)2(PPh3)2. Crystal data for 1 (-62-degrees-C): triclinic space group P1BAR (No. 2), a = 13.382 (2) angstrom, b = 13.723 (2) angstrom, c = 15.593 (2) angstrom, alpha = 107.06 (1)-degrees, beta = 96.54 (1)-degrees, gamma = 114.05 (1)-degrees, V = 2408 (2) angstrom 3, Z = 2. Crystal data for 2 (-96-degrees-C): orthorhombic space group Pbca (No. 61), a = 19.292 (2) angstrom, b = 22.399 (2) angstrom, c = 22.314 (2) angstrom, alpha = beta = gamma = 90-degrees, V = 9642 (3) angstrom 3, Z = 8. Crystal data for 3 (+20-degrees-C): triclinic space group P1BAR (No. 2), a = 15.075 (4) angstrom, b = 18.640 (4) angstrom, c = 20.662 (6) angstrom, alpha = 114.53 (1)-degrees, beta = 100.44 (2)-degrees, gamma = 98.40 (2)-degrees, V = 5033 (5) angstrom 3, Z = 4. Crystal data for 4 (+20-degrees-C): monoclinic space group P2(1)/c (No. 14), a = 11.794 (2) angstrom, b = 14.168 (1) angstrom, c = 31.359 (5) angstrom, beta = 110.776 (6)-degrees, V = 4899 (2) angstrom 3, Z = 4. All structures were refined by full-matrix least-squares methods to the following values of R (R(w) given in parentheses) for the stated number of data with I > 3.0-sigma(I): 1, 0.023 (0.030), 5706 data; 2, 0.036 (0.049), 4536 data; 3, 0.059 (0.081), 8473 data; 4, 0.041 (0.049), 4525 data.