BRIDGING OF PLATINACYCLES BY CIS-AZOBENZENES - SYNTHESIS AND PHOTOCHEMICAL STUDY - STRUCTURE OF THE TWISTED INORGANIC Z-1,1,2,2-TETRASUBSTITUTED DIAZAETHENE RO(((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-azoben zenes 2 (ZC6H4N=NC6H4Z; Z = H, p-Me, m-Me, p-Cl) photoisomerized in si tu from the trans-azobenzene precursors. The reaction is carried out i n benzene or chloroform at room temperature to afford the unique inten sively colored adducts 3 in which bridging cis-azobenzenes bind two pl atinum [Pt(C6H3RCH2NMe2)Cl] fragments. The nitrogen atoms of the azobe nzene 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-azoben zene moiety, as confirmed by X-ray crystallography. Compound 3a: C30H3 4N4Cl2Pt2.1/2C6H14, triclinic, space group P1BAR, a = 10.644(3) angstr om, b = 11.932(6) angstrom, c = 13.874(4) angstrom, alpha = 72.22(3)-d egrees, beta = 76.06(2)-degrees, gamma = 83.44(3)-degrees, Z = 2. Addu cts 3 possess the metal-to-ligand charge-transfer (MLCT) transitions i nvolving the azobenzene pi(N=N) orbital, as suggested by a resonance Raman spectroscopy study. The energy of the MLCT transitions depends o n the substituents R and Z conforming to the Hammett law, when the lat ter are varied. Solutions of 3 show a thermochroic behavior due to rev ersible dissociation of the cycloplatinated moieties from cis-azobenze nes. 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, bi nds only one such a fragment to form 4. The latter reaction needs no i rradiation, and the products are not as intensively colored as in the case of the azobenzenes.