Luminescent platinum(II) dimers with a cyclometallating aryldiamine ligand

Triflate salts of three (Pt(pip(2)NCN))(2) (mu-L)(2+) (pip(2)NCNH = 1,3-bis(piperidylmethyl)benzene) dimers bridged by a series of nitrogen-donor ligands (L = pyrazine (pyz), 1,2-bis(4-pyridyl)ethane (bpa), trans- 1, 2-bis(4-pyridyl)ethylene (bpe)) are reported. These complexes have been fully characterized by H-1 NMR spectroscopy and elemental analysis. The X-ray crystal structures of [(Pt(pip(2)NCN))(2)(mu-pyz)](CF3SO3)(2) and [(Pt(pip(2)NCN))(2)(mu-bpe)](CF3SO3)(2).2CH(2)-Cl-2 are reported. [(Pt(pip(2)NCN))(2)(mu-pyz)](CF3SO3)(2): triclinic, P (1) over bar, a = 12.5240(5) Angstrom, b = 14.1570(6) Angstrom, c = 14.2928(6) Angstrom: alpha = 106.458(1)degrees, beta = 92.527(1)degrees, gamma = 106.880(1)degrees, V = 2303.46(17) Angstrom(3), Z = 2. [(Pt(pip(2)NCN))(2)(mu-bpe)](CF3SO3)(2).2CH(2)Cl(2): monoclinic, P2(1)/c, a = 10.1288(6) Angstrom, b = 16.3346(g) Angstrom, c = 17.4764(10) Angstrom, beta = 90.882(2)degrees, V = 2891.1(3) Angstrom(3), Z = 2. These structures and solution measurements provide evidence for the strong trans-directing properties of the pip(2)NCN(-) ligand. The electronic structures of these complexes and those of the 4,4'-bipyridine (bpy) dimer, (Pt(pip(2)NCN))(2)(mu-bpy)(2+), also have been investigated by UV-visible absorption and emission spectroscopies, as well as cyclic voltammetry. The accumulated data indicate that variations in the bridging ligands provide remarkable control over the electronic structures and photophysics of these complexes. Notably, the bpa dimer exhibits a broad, low-energy emission from a metal-centered (LF)-L-3 excited state, whereas the bpe and bpy dimers exhibit structured emission from a lowest pyridyl-centered (3)(pi-pi(*)) excited state, In contrast, the pyz dimer exhibits remarkably intense yellow emission tentatively assigned to a triplet metal-to-ligand charge-transfer excited state.