Photochemical generation of a reactive rhenium(III) oxo complex and its curious mode of cleavage of dioxygen

Abstract

The rhenium(V) oxo oxalate complex (HBpz3)ReO(C204) (HBpz3 = hydridotris(l-pyrazolyl)borato) has been synthesized in three steps from potassium perrhenate. It has been characterized spectroscopically and its molecular structure determined by X-ray crystallography. When irradiated with UV light, the oxo oxalate complex undergoes an internal redox reaction, predominantly losing carbon dioxide and generating a reactive rhenium complex. The characterization of the transient photoproduct as the rhenium(III) oxo complex (HBpz3)Re( 0) is inferred from its reactions with trapping reagents: for example, photolysis in the presence of phenanthrenequinone gives a rhenium-(V) oxo catecholate complex in good yield. (HBpZ3)Re(0) also reacts with 62, yielding the rhenium(VII) complex (HBpz3)Re03, formally the result of four-electron oxidation. Labeling studies show that only one oxygen atom in the product comes from O2, with the second deriving from an oxalate ligand. That unimolecular four-electron reduction of oxygen does not occur readily in this system, despite its great exothermicity, may be due to a general symmetry-imposed barrier to cleavage of 02 at a single metal center. Crystal data for (HBpz3)ReO(C2O4)-0.5C6H6: a = 8.082 (2) A, b = 9.125 (3) A, c = 13.219 (3) A, a = 84.03 (2)°, 0 = 74.26 (2)°, y = 72.47 (2)°, triclinic, P\, Z = 2.

Reactions of vanadium, molybdenum and rhenium tris(pyrazolyl)borate-stabilised oxometal complexes with B(C6F5)3: crystal structures of [Mo{OB(C6F5)3}{HB(dmpz)3}(S2CNMe2)] and [Mo{OB(C6F5)3}{HB(dmpz)3}(OCH2CH2O)] (dmpz = 3,5-dimethylpyrazolyl)

“…In addition IR spectroscopy revealed absorptions assigned to the C 2 O 4 2Ϫ unit at 1759 and 1618 cm Ϫ1 which are very similar to those observed for the starting metal complex at 1753, 1732 and 1676 cm Ϫ1 . 19 These findings indicate that little perturbation of the oxalate ligand has occurred and suggest that the Lewis acid has preferentially attacked the Re᎐ ᎐ O unit (Fig. 3).…”

“…The complex [ReO{HB(pz) 3 }(C 2 O 4 )] offers three types of nucleophilic oxygen atoms; two derived from the oxalate ligand, M᎐O᎐C and C᎐ ᎐ O groups, and a third formed by a terminal Re᎐ ᎐ O site. 19 Addition of 1 equivalent of B(C 6 F 5 ) 3 produces a Lewis acid-metal complex adduct, [Re{OB(C 6 F 5 ) 3 }{HB(pz) 3 }-(C 2 O 4 )] 6. As observed for compounds 4 and 5, 1 H NMR spectroscopy clearly indicates that the pattern of signals due to the HB(pz) 3 ligand in 6 is consistent with an approximately octahedral fac ReN 3 L 2 LЈ geometry indicating that the Lewis acid has reacted with the Re᎐ ᎐ O unit (Table 1).…”

Oxorhenium(V) Complexes of Carbohydrate Ligands

“…The new compounds were prepared according to a modification of Brown and Mayer's method [7] by reaction of dichloro[hydridotris(pyrazolyl)borato]oxorhenium() [(tpb)ReOCl 2 ], triethylamine, and the carbohydrate in refluxing methanol (Scheme 1). To prevent formation of oligomeric rhenium complexes -obviously the main side reaction -a fourfold excess of carbohydrate over rhenium was used.…”

“…[7] Spectroscopic Experiments: 1 H and 13 C NMR: Jeol EX-400. 1 H-13 C HMQC and NOE difference spectra: Jeol eclipse ϩ500; mass spectra: Jeol JMS-700 (ionisation method: DEI ϩ ).…”

“…The molar ratio of the isomers depends on the solvent used. In acetonitrile, the solvent used by Brown and Mayer, [7] the syn/anti ratio increases to 4:1, whereas with water as the medium in a heterogeneous reaction, followed by dissolution of the product in dimethyl sulfoxide, a 5:1 syn/anti ratio is obtained. It should be noted, however, that due to the kinetic inertness of the complexes, these values obviously do not represent the equilibrium mixture.…”