SYNTHESIS, STRUCTURE, AND SOLUTION EQUILIBRIA OF DIAMAGNETIC AND PARAMAGNETIC AZOOXIMATES OF BIVALENT PLATINUM - GEOMETRICAL CHANGE ASSOCIATED WITH LIGAND-BASED ELECTROPROTIC REACTIONS

(Phenylazo)benzaldoxime, PhN=NC(Ph)=NOH (Hpbo), reacts with K2PtCl4 in alkaline media affording trans-Pt(pbo)(2). One-electron-one-proton el ectroprotic transformation converts it to a cis complex isolated as th e dimer [cis-Pt(pbo)(Hpbo)](2). Both complexes are diamagnetic in the solid state. In trans-Pt(pbo)(2), the PtN4 coordination sphere is plan ar, the metal atom being a center of inversion. In the cis dimer one m onomer is stacked on the other such that the Pt...Pt (3.235(1) Angstro m) midpoint is a center of inversion. Each metal atom is displaced by 0.088 Angstrom from its N-4 plane toward its neighbor. Unsymmetrical a nd nearly linear O...H-O bridging is present within each monomer. In d ichloromethane solution, the equilibrium dimer (s = 0) reversible arro w 2 monomer (s = 1/2) exists. The paramagnetic monomer is EPR-active ( g = 1.987). The intensity of the EPR-signal as well as the paramagneti c moment progressively diminishes with decreasing temperature due to t he shift of the equilibrium to the left. The Delta H and Delta S value s associated with dissociation are 8.3(+/- 2) kcal mol(-1) and 23.5(5) cal mol(-1) K-1, respectively. EHMO studies have revealed that the un paired electron in the cis monomer (idealized point group C-s) resides in an azoimine orbital (3A'') with a large azo-pi() contribution. Th e EPR spectrum is consistent with this. The cis monomer is essentially a stable free radical. In dimer (C-i) formation 3A''-3A'' and d(z)2-d (z)2 interactions contribute significantly to binding. The doubly occu pied 6A(g) HOMO of the dimer corresponds to the bonding 3A''-3A'' comb ination. Accordingly the average N=N length in the cis dimer (1.31 Ang strom) is longer than that in the trans complex (1.28 Angstrom) in whi ch the azo-pi() orbitals are empty and constitute the LUMO. The cis m onomer displays quasireversible one-electron oxidation (E(1/2), 0.52 V vs SCE) and in presence of NEt(3) the oxidized complex is deprotonate d affording the sterically favorable trans-Pt(pbo)(2). The latter is r econverted to the O...H-O stabilized cis complex via one-electron redu ction (E(1/2), -0.28 V) and proton addition. In this remarkable family , metal geometry is controlled by electroprotic transfer at ligand sit es: azo-pi() (electron transfer) and oximato oxygen (proton transfer) . Crystal data for the complexes are as follows. trans-Pt(pbo)(2): cry stal system monoclinic; space group P2(1)/n; a = 5.646(4) Angstrom, b = 10.784(7) Angstrom, c = 18.367(14) Angstrom; beta =,98.11(5)0; V = 1 107(1) Angstrom(3); Z = 2; R = 2.74%; R(W) = 3.20%. [cis-Pt(pbo)(Hpbo) ](2): crystal system monoclinic; space group C2/c; a = 25.183(8) Angst rom, b = 8.849(4) Angstrom, c = 20.839(8) Angstrom; beta = 90.09(3)deg rees; V = 4644(3) Angstrom(3); Z = 4; R = 2.42%; R(W) = 2.90%.