Pyridylazole Chelation of Oxorhenium(V) and Imidorhenium(V). Rates and Trends of Oxygen Atom Transfer from ReVO to Tertiary Phosphines

Abstract

The concerned azoles are 2-(2-pyridyl)benzoxazole (pbo) and 2-(2-pyridyl)benzthiazole (pbt). These react with ReOCl₃(PPh₃)₂ in benzene, affording Re^VOCl₃(pbo) and Re^VOCl₃(pbt), which undergo facile oxygen atom transfer to PPh₂R (R = Ph, Me) in dichloromethane solution, furnishing Re^III(OPPh₂R)Cl₃(pbo) and Re^III(OPPh₂R)Cl₃(pbt). The oxo species react with aniline in toluene solution, yielding the imido complexes Re^V(NPh)Cl₃(pbo) and Re^V(NPh)Cl₃(pbt). The X-ray structures of pbt, ReOCl₃(pbt), Re(OPPh₃)Cl₃(pbt), and Re(NPh)Cl₃(pbo) are reported. The lattice of pbt consists of stacked dimers. In all the complexes the azole ligand is N,N-chelated and the ReCl₃ moiety is meridionally disposed. In ReOCl₃(pbt) the metal−oxo bond length is 1.607(9) Å. The second-order rates and the associated activation parameters of the oxygen atom transfer reactions of the Re^VO chelates with PPh₂R are reported. The large and negative entropy of activation (∼ −24 eu) is consistent with an associative pathway involving nucleophilic phosphine attack. The rate increases with phosphine basicity (PPh₂Me > PPh₃) and azole heteroatom electronegativity (O(pbo) > S(pbt)). Logarithmic rate constants for ReOCl₃(pbo), ReOCl₃(pbt), and ReOCl₃(pal) are found to correlate linearly with Re^VIO/Re^VO reduction potentials (pal is pyridine-2-(N-p-tolyl)aldimine). The relatively low rate constant of ReOCl₃(pbt) compared to that of ReOCl₃(pal) is consistent with the observed shortness of the metal−oxo bond in the former. Crystal data are as follows: (pbt) empirical formula C₁₂H₈N₂S, crystal system orthorhombic, space group Pca2₁, a = 13.762(9) Å, b = 12.952(8) Å, c = 11.077(4) Å, V = 1974(2) ų, Z = 8; (ReOCl₃(pbt)) empirical formula C₁₂H₈Cl₃N₂OSRe, crystal system monoclinic, space group P2₁/c, a = 11.174(7) Å, b = 16.403(10) Å, c = 7.751(2) Å, β = 99.35(4)°, V = 1401.8(13) ų, Z = 4; (Re(NPh)Cl₃(pbo)) empirical formula C₁₈H₁₃Cl₃N₃ORe, crystal system monoclinic, space group P2₁/c, a = 9.566(6) Å, b = 16.082(8) Å, c = 11.841(5) Å, β = 94.03(4)°, V = 1817(2) ų, Z = 4.