A bis(azo-imine)palladium(II) system with 10 ligand π 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)<SUB>2</SUB> (Pd(RA)<SUB>2</SUB>), has been isolated in the form of diamagnetic solids by the 6e<SUP>-</SUP>-6H<SUP>+</SUP> reduction of bis(phenylazooximato)palladium(II), Pd(N(O)C(R)NNPh)<SUB>2</SUB> (abbreviated Pd(RB)<SUB>2</SUB>), with ascorbic acid in a mixed solvent (R = Ph, &#945;-naphthyl). Selected spectral features are described. The X-ray structures of Pd(PhA)<SUB>2</SUB> and Pd(PhB)<SUB>2</SUB> have revealed trans-planar geometry consistent with metal oxidation state of +2. Bond length trends within the chelate rings are rationalized in terms of steric and electronic factors. In Pd(PhA)<SUB>2</SUB> a total of 10 ligand &#960; electrons are present, each formally monoanionic ligand contributing five. Model EHMO studies have revealed that the filled HOMO (a<SUB>u</SUB>) in Pd(RA)<SUB>2</SUB> is a bonding combination of two ligand &#960;<SUP>&#8727; </SUP>orbitals with large azo contributions. The LUMO (b<SUB>g</SUB>) is roughly the corresponding antibonding combination. The outer p-electron configuration of Pd(RA)<SUB>2</SUB> is (a<SUB>u</SUB>)<SUP>2</SUP>(b<SUB>g</SUB>)<SUP>0</SUP>. Four successive voltammetric responses, two oxidative and two reductive, are observed. The E<SUB>1/2</SUB> range is -1.3 to +0.8 V vs SCE for Pd(PhA)<SUB>2</SUB> in a 1:9 MeCN-CH<SUB>2</SUB>Cl<SUB>2</SUB> mixture (Pt electrode). EPR and electronic spectra of the electrogenerated one-electron-oxidized complex Pd(PhA)<SUB>2</SUB><SUP>+</SUP> are described. The azo-imine system is compared with imine-imine and azo-azo systems. Crystal data for the complexes are as follows. Pd(PhA)<SUB>2</SUB>: crystal system monoclinic; space group C2/c; a = 18.167(5) &#197;, b = 7.420(3)&#197;, c = 16.527(6) &#197;; &#946; = 92.70(3)&#176;; V = 2225(1) &#197;<SUP>3</SUP>; Z = 4; R = 2.61%, R<SUB>w</SUB> = 3.58%. Pd(PhB)<SUB>2</SUB>: crystal system monoclinic; space group P2<SUB>1</SUB>/n; a = 5.735(5) &#197;, b = 10.797(6) &#197;, c = 18.022(11) &#197;; &#946; = 97.73(6) &#197;; V = 1105(1) &#197;<SUP>3</SUP>; Z = 2; R = 3.37%; R<SUB>w</SUB> = 3.40%