HEPTACOORDINATE RHENIUM(III)-BIS(TERPYRIDINE) COMPLEXES - SYNTHESES, CHARACTERIZATIONS, AND CRYSTAL-STRUCTURES OF [RE(TERPYRIDINE)2X]2- SUBSTITUTION KINETICS OF [RE(TERPYRIDINE)2OH]2+( (X=OH, CL, NCS) )

The 7-coordinated Re(III) complex [Re(terpy)2OH]2+, where terpy = 2,2' :6',6''-terpyridine, has been synthesized and characterized. Preparati on of this complex from [Re(V)O2(pyridine)4]+ occurs via a reduction/s ubstitution route utilizing 2 molar equiv of terpyridine. The concomit ant production of ReO4- implies the presence of a disproportionation c omponent in the preparative reaction. Replacement of the coordinated h ydroxo ligand by Cl- or NCS- to generate respectively the [Re(terpy)2C l]2+ or [Re(terpy)2NCS]2+ complex proceeds much more rapidly at low pH . The X-ray crystal structure analyses of all three novel complexes ar e reported: [Re(terpy)2OH](PF6)2.H2O crystallizes in the triclinic spa ce group P1BAR with a = 9.099(l) angstrom, b = 10.781(2) angstrom, c = 17.438(3) angstrom, a = 82.87(1)-degrees, beta = 88.85(1)-degrees, ga mma = 86.83(1)-degrees, V = 1694.6(4) angstrom3, and Z = 2. [Re(terpy) 2Cl](PF6)2 crystallizes in the monoclinic space group P2(1)/c with a = 9.257(2) angstrom, b = 19.597(5) angstrom, c = 17.899(5) angstrom, be ta = 94.85(2)-degrees, V = 3235(1) angstrom3, and Z = 4. [Re(terpy)2NC S](SCN)2.1/2H2O crystallizes in the triclinic space group P1BAR with a = 10.067(4) angstrom, b = 11.121(2) angstrom, c = 15.133 (7) angstrom , alpha = 89.63(3)-degrees, beta = 80.04(4)-degrees, gamma = 75.46(3)- degrees, V = 1614(1) angstrom3, and Z = 2. The solution structure is f ully determined by proton NMR and is shown to be similar to that in th e solid state with two equivalent, but asymmetric, terpy ligands. In a ddition, the [Re(terpy)2X]2+ complexes are characterized by elemental analysis, UV-vis, IR, and mass spectroscopy, and/or thin-layer chromat ography. A preliminary kinetic study of the substitution reaction of [ Re(terpy)2OH]2+ to yield [Re(terpy)2NCS]2+ indicates that the hydroxy complex is involved in a rapid protonation equilibrium to yield a reac tive species which then reacts with SCN- in a rate determining step. D erived equilibrium and rate constants at 25-degrees-C and mu = 1.00 M are pK(a) = 1.4(8) and k = 45(4) M-1 s-1. The reverse reaction rate is insignificant under the conditions utilized.