Chemistry of the rhenium-azopyridine family: An oxo parent and derivatives thereof including a novel oxo-imido dimer

The concerned azo ligands are 2-(phenylazo)pyridine (HL) and 2-((p-chlorophenyl)azo)pyridine (ClL). The reaction of KReO4 with HL in hot concentrated HCl is attended with metal reduction and ligand chlorination affording the oxo complex (ReOCl3)-O-V(ClL), 2, which furnishes Re-III(OPPh3)Cl-3(ClL), 3, upon treatment with PPh3. Aromatic amines, ArNH2, convert 2 to the imido complex Re-V(NAr)Cl-3(ClL), 5, and the unusual oxo-imido dimer (ClL)-Cl-2(O)(ReOReV)-O-V(NAr)Cl-2(ClL), 7. The complex Re-III(OPPh3)Cl-3(HL), 4, has been generated from (ReOCl3)-O-V(PPh3)(2) and HL. Reaction of 4 with HL has yielded Re-V(NPh)Cl-3(HL), 6, via azo splitting. The complexes have been characterized with the help spectral, magnetic, and X-ray structural data (2, 3, 5c (Ar = pClC(6)H(4)) and 7.CH2Cl2 (Ar = pMeC(6)H(4))). In 2, 3, and 5c the ReCl3 fragment is meridionally disposed, and in 7 the ReCl2 fragments have a trans configuration. The Re-O(oxo) bond, 1.663(6) Angstrom, in 2 and Re-N(imido) bond, 1.719(5) Angstrom, in 5c are triple bonds. The corresponding bonds are slightly longer in 7 wherein the (O)Re(1)-O(2) -Re(2)(NAr) bridge is angular(151.0(5)degrees) and unsymmetrical, the Re(1)-O(2) bond, 1.849(7) Angstrom, having a large double-bond character (Re(2)-O(2), 1.954(7) Angstrom). In effect, cis-(ReO2)-O-V acts as a monodentate oxygen ligand toward (ReNAr)-N-V in 7. In all cases the pyridine nitrogen binds trans to the ore, OPPh3, or NAr donor. Bond length data are consistent with the presence of substantial d(Re)-pi*(azo) back-bonding. In acetonitrile solution the complexes display electrochemical one-electron metal (Re-VI/Re-V or Re-IV/Re-III) and azo redox. The imido ligand in 5 stabilizes the Re-VI state (E-1/2 similar to 1.4 V) better than the oxo ligand in 2 (similar to 1.9 V). Parallely it is more difficult to reduce the azo group in 5 (similar to-0.4 V) than in 2 (similar to 0.0 V). In 7 the metal (similar to 1.0 V) and azo (similar to-0.4 V) couples correspond to the imido and oxo halves, respectively. The significantly higher (by 0.2-0.6 V) metal reduction potentials of the azopyridine compared to pyridine-2-aldimine complexes is ascribed to the superior pi-acidity and electron-withdrawing character of the azo function relative to the aldimine function. This also makes the transfer of the (ReO)-O-V oxygen function much more facile under azopyridine chelation as in 2. For the same reason, ReOCl3(PPh3)(2) reacts with HL affording only 4 while it reacts with pyridine-2-aldimines furnishing oxo species. Crystal data for the complexes are as follows: 2, empirical formula C11H8Cl4N3ORe, crystal system triclinic, space group P1, a = 7.118(4) Angstrom, b = 8.537(4) Angstrom, c = 13.231(9) Angstrom, alpha = 79.16(5)degrees, beta = 78.03(5)degrees, gamma = 70.96(4)degrees, V= 737.2(7) Angstrom(3), Z= 2; 3, empirical formula C29H23Cl4N3OPRe, crystal system monoclinic, space group P2(1)/n, a = 11.264(2) Angstrom, b = 15.221(3) Angstrom, c = 17.628(4) Angstrom, beta = 94.21(3)degrees, V = 3014(1) Angstrom(3), Z = 4; 59, empirical formula C(17)Hi(2)Cl(5)N(4)Re, crystal system triclinic, space group P1, a = 9.683(3) Angstrom, b = 10.898(3) A, c = 11.522(3) Angstrom, alpha 63.67(2)degrees, beta 71.24(2)degrees, gamma 86.79(2)degrees, V = 1026(1) Angstrom(3), Z = 2; 7 CH2Cl2, empirical formula C30H25Cl8N7O2Re2, crystal system triclinic, space group P1, a = 12.522(6) Angstrom, b = 12.857(8) Angstrom, c = 13.182(7) Angstrom, alpha = 67.75(4)degrees, beta = 88.30(4)degrees, gamma = 82.09(4)degrees, V = 1945(2) Angstrom(3), Z = 2.