A BIS(AZO-IMINE)PALLADIUM(II) SYSTEM WITH 10 LIGAND PI-ELECTRONS - SYNTHESIS, STRUCTURE, SERIAL REDOX, AND RELATIONSHIP TO BIS(AZOOXIMATES)AND OTHER SPECIES

The first azo-imine chelate system, Pd(N(H)C(R)NNPh)(2) (Pd(RA)(2)), h as been isolated in the form of diamagnetic solids by the 6e(-) -6H(+) reduction of bis(phenylazooximato)palladium(II), Pd(N(O)C(R)NNPh)(2) (abbreviated Pd(RB)(2)), with ascorbic acid in a mixed solvent (R = Ph , alpha-naphthyl). Selected spectral features are described. The X-ray structures of Pd(PhA)(2) and Pd(PhB)(2) have revealed trans-planar ge ometry consistent with metal oxidation state of +2. Bond length trends within the chelate rings are rationalized in terms of steric and elec tronic factors. In Pd(PhA)(2) a total of 10 ligand pi electrons are pr esent, each formally monoanionic ligand contributing five. Model EHMO studies have revealed that the filled HOMO (a(u)) in Pd(RA)(2) is a bo nding combination of two ligand pi orbitals with large azo contributi ons. The LUMO (b(g)) is roughly the corresponding antibonding combinat ion. The outer pi-electron configuration of Pd(RA)(2) is (a(u))(2)(b(g ))(0). Four successive voltammetric responses, two oxidative and two r eductive, are observed. The E(1/2) range is -1.3 to +0.8 V vs SCE for Pd(PhA)(2) in a 1:9 MeCN-CH2Cl2 mixture (Pt electrode). EPR and electr onic spectra of the electrogenerated one-electron-oxidized complex Pd( PhA)(2)(+) are described. The azo-imine system is compared with imine- imine and azo-azo systems. Crystal data for the complexes are as follo ws. Pd(PhA)2: crystal system monoclinic; space group C2/c; a = 18.167( 5) Angstrom, b = 7.420(3) Angstrom, c = 16.527(6) Angstrom; beta = 92. 70(3)degrees; V = 2225(1) Angstrom(3); Z = 4; R = 2.61%, R(w) = 3.58%. Pd(PhB)(2): crystal system monoclinic; space group P2(1)/n; a = 5.735 (5) Angstrom, b = 10.797(6) Angstrom, c = 18.022(11) Angstrom; beta = 97.73(6) Angstrom; V = 1105(1) Angstrom(3); Z = 2; R = 3.37%; R(w) = 3 .40%.