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

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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 ReVOCl3(ClL), 2, which furnishes ReIII(OPPh3)Cl3(ClL), 3, upon treatment with PPh3. Aromatic amines, ArNH2, convert 2 to the imido complex ReV(NAr)Cl3(ClL), 5, and the unusual oxo−imido dimer (ClL)Cl2(O)ReVOReV(NAr)Cl2(ClL), 7. The complex ReIII(OPPh3)Cl3(HL), 4, has been generated from ReVOCl3(PPh3)2 and HL. Reaction of 4 with HL has yielded ReV(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-ReVO2 acts as a monodentate oxygen ligand toward ReVNAr 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 (ReVI/ReV or ReIV/ReIII) and azo redox. The imido ligand in 5 stabilizes the ReVI state (E1/2 ∼ 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 ReVO 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) Å3, 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) Å3, 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) Å3, 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) Å3, Z = 2.

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