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

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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 ReVOCl3(pbo) and ReVOCl3(pbt), which undergo facile oxygen atom transfer to PPh2R (R = Ph, Me) in dichloromethane solution, furnishing ReIII(OPPh2R)Cl3(pbo) and ReIII(OPPh2R)Cl3(pbt). The oxo species react with aniline in toluene solution, yielding the imido complexes ReV(NPh)Cl3(pbo) and ReV(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 ReVO 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 ReVIO/ReVO 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) Å3, 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) Å3, 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) Å3, Z = 4.

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