BRIDGING OF PLATINACYCLES BY CIS-AZOBENZENES - SYNTHESIS AND PHOTOCHEMICAL STUDY - STRUCTURE OF THE TWISTED INORGANIC Z-1,1,2,2-TETRASUBSTITUTED DIAZAETHENE ((Z)-1,2-DIPHENYLDIAZAETHENE)BIS[CHLORO(((DIMETHYLAMINO)METHYL)PHENYL-C,N)PLATINUM(II)]

The cycloplatinated chloro-bridged N,N-dimethylbenzylamine complexes 1, [Pt(C6H3RCH2NMe2)Cl]2 (R = H, 4-MeO), undergo addition to cis-azobenzenes 2 (ZC6H4N=NC6H4Z; Z = H, p-Me, m-Me, p-Cl) photoisomerized in situ from the trans-azobenzene precursors. The reaction is carried out in benzene or chloroform at room temperature to afford the unique intensively colored adducts 3 in which bridging cis-azobenzenes bind two platinum [Pt(C6H3RCH2NMe2)Cl] fragments. The nitrogen atoms of the azobenzene are trans to the Pt-C bonds, and the chloro ligands are located at different sites with respect to the C-N=N-C plane of the cis-azobenzene moiety, as confirmed by X-ray crystallography. Compound 3a: C30H34N4Cl2Pt2.1/2C6H14, triclinic, space group P1BAR, a = 10.644(3) angstrom, b = 11.932(6) angstrom, c = 13.874(4) angstrom, alpha = 72.22(3)-degrees, beta = 76.06(2)-degrees, gamma = 83.44(3)-degrees, Z = 2. Adducts 3 possess the metal-to-ligand charge-transfer (MLCT) transitions involving the azobenzene pi*(N=N) orbital, as suggested by a resonance Raman spectroscopy study. The energy of the MLCT transitions depends on the substituents R and Z conforming to the Hammett law, when the latter are varied. Solutions of 3 show a thermochroic behavior due to reversible dissociation of the cycloplatinated moieties from cis-azobenzenes. The dissociation is a key process to account for the rate law of the thermal dissociation of 3 which is given by k(obsd) = k(o) + k[Pt]t, where [Pt]t is the total concentration of 3. Interestingly, benzo[c]cinnoline, which may be considered as a mimetic of cis-azobenzene, binds only one such a fragment to form 4. The latter reaction needs no irradiation, and the products are not as intensively colored as in the case of the azobenzenes.