Heptacoordinate rhenium(III)-bis(terpyridine) complexes: Syntheses, characterizations, and crystal structures of [re(terpyridine)2X]2+ (X = OH, Cl, NCS). Substitution kinetics of [re(terpyridine)2OH]2+

The 7-coordinated Re(III) complex [Re(terpy)2OH]2+, where terpy = 2,2?:6?,6?- terpyridine, has been synthesized and characterized. Preparation of this complex from [RevO2(pyridine)4]+ occurs via a reduction/substitution route utilizing 2 molar equiv of terpyridine. The concomitant production of ReO4 - implies the presence of a disproportionation component in the preparative reaction. Replacement of the coordinated hydroxo ligand by Cl- or NCS- to generate respectively the [Re(terpy)2Cl]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 are reported: [Re(terpy)2OH](PF6)2·H2O crystallizes in the triclinic space group P1? with a = 9.099(1) Å, b = 10.781(2) Å, c = 17.438(3) Å, ? = 82.87(1)°, ? = 88.85(1)°, ? = 86.83(1)°, V = 1694.6(4) Å3, and Z = 2. [Re(terpy)2Cl](PF6)2 crystallizes in the monoclinic space group P21/c with a = 9.257(2) Å, b = 19.597(5) Å, c = 17.899(5) Å, ?= 94.85(2)°, V = 3235(1) Å3, and Z = 4. [Re(terpy)2-NCS](SCN)2*1/2H2O crystallizes in the triclinic space group P1? with a = 10.067(4) Å, b = 11.121(2) Å, c = 15.133(7) Å, ? = 89.63(3)°, ? = 80.04(4)°, ? = 75.46(3)°, V = 1614(1) Å3, and Z = 2. The solution structure is fully determined by proton NMR and is shown to be similar to that in the solid state with two equivalent, but asymmetric, terpy ligands. In addition, the [Re(terpy)2X]2+ complexes are characterized by elemental analysis, UV-vis, IR, and mass spectroscopy, and/or thin-layer chromatography. 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 reactive species which then reacts with SCN- in a rate determining step. Derived equilibrium and rate constants at 25°C and ? = 1.00 M are pKa = 1.4(8) and k = 45(4) M-1 s-1. The reverse reaction rate is insignificant under the conditions utilized.