Chemistry of the Rhenium−Azopyridine Family: An Oxo Parent and Derivatives Thereof Including a Novel Oxo−Imido Dimer

Abstract

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 Re^VOCl₃(ClL), 2, which furnishes Re^III(OPPh₃)Cl₃(ClL), 3, upon treatment with PPh₃. Aromatic amines, ArNH₂, convert 2 to the imido complex Re^V(NAr)Cl₃(ClL), 5, and the unusual oxo−imido dimer (ClL)Cl₂(O)Re^VORe^V(NAr)Cl₂(ClL), 7. The complex Re^III(OPPh₃)Cl₃(HL), 4, has been generated from Re^VOCl₃(PPh₃)₂ and HL. Reaction of 4 with HL has yielded Re^V(NPh)Cl₃(HL), 6, via azo splitting. The complexes have been characterized with the help spectral, magnetic, and X-ray structural data (2, 3, 5c (Ar = pClC₆H₄) and 7·CH₂Cl₂ (Ar = pMeC₆H₄)). 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) Å, in 2 and Re−N(imido) bond, 1.719(5) Å, 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)°) and unsymmetrical, the Re(1)−O(2) bond, 1.849(7) Å, having a large double-bond character (Re(2)−O(2), 1.954(7) Å). In effect, cis-Re^VO₂ acts as a monodentate oxygen ligand toward Re^VNAr in 7. In all cases the pyridine nitrogen binds trans to the oxo, OPPh₃, or NAr donor. Bond length data are consistent with the presence of substantial d(Re)−π*(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 ∼ 1.4 V) better than the oxo ligand in 2 (∼1.9 V). Parallely it is more difficult to reduce the azo group in 5 (∼−0.4 V) than in 2 (∼0.0 V). In 7 the metal (∼1.0 V) and azo (∼−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 π-acidity and electron-withdrawing character of the azo function relative to the aldimine function. This also makes the transfer of the Re^VO 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 P1̄, a = 7.118(4) Å, b = 8.537(4) Å, c = 13.231(9) Å, α = 79.16(5)°, β = 78.03(5)°, γ = 70.96(4)°, V = 737.2(7) ų, Z = 2; 3, empirical formula C₂₉H₂₃Cl₄N₃OPRe, crystal system monoclinic, space group P2₁/n, a = 11.264(2) Å, b = 15.221(3) Å, c = 17.628(4) Å, β = 94.21(3)°, V = 3014(1) ų, Z = 4; 5c, empirical formula C₁₇H₁₂Cl₅N₄Re, crystal system triclinic, space group P1̄, a = 9.683(3) Å, b = 10.898(3) Å, c = 11.522(3) Å, α = 63.67(2)°, β = 71.24(2)°, γ = 86.79(2)°, V = 1026(1) ų, Z = 2; 7·CH₂Cl₂, empirical formula C₃₀H₂₅Cl₈N₇O₂Re₂, crystal system triclinic, space group P1̄, a = 12.522(6) Å, b = 12.857(8) Å, c = 13.182(7) Å, α = 67.75(4)°, β = 88.30(4)°, γ = 82.09(4)°, V = 1945(2) ų, Z = 2.