Tuning the electronic structures of platinum(II) complexes with a cyclometalating aryldiamine ligand

Triflate salts of four platinum(II) pyridyl complexes with a mer-coordinating tridentate pincer ligand, pip(2)NCN(-) (pip(2)-NCNH = 1,3-bis(piperidylmethyl)benzene), are reported: Pt(pip(2)NCN)(L)(+) (2, L = pyridine; 3, L = 4-phenylpyridine; 5, L = 2,6-pyridinedimethanol) and [(Pt(pip(2)NCN))(2)(mu-4,4'-bipyridine)](2+) (4). The complexes have been fully characterized by H-1 NMR spectroscopy, elemental analysis, and X-ray crystallography. Compound 2(CF3SO3-): triclinic, P1, a = 9.7518(6) Angstrom, b = 12.0132(8) Angstrom, c = 12.6718(9) Angstrom, alpha = 114.190(2)degrees, beta = 100.745(3)degrees, gamma = 103.545(2)degrees, V = 1247.95(14) Angstrom(3), Z = 2. Compound 3(CF3SO3-): monoclinic, P2(1)/c, a = 15.550(2) Angstrom, b = 9.7386(11) Angstrom, c = 18.965(3) Angstrom, beta = 92.559(7)degrees, V = 2869.1(6) Angstrom(3), Z = 4. Compound 4(CF3SO3-)(2).1/2(CH3)(2)CO: monoclinic, /2/a, a = 21.3316(5) Angstrom, b = 9.6526(2) Angstrom, c = 26.1800(6) Angstrom, beta = 96.4930(10)degrees, V = 5356.0(2) Angstrom(3), Z = 4. Compound 5(CF3SO3-).3/2CHCl(3): monoclinic, P2(1)/n, a = 17.1236(10) Angstrom, b = 9.3591(5) Angstrom, c = 21.3189(11) Angstrom, beta = 96.11(3)degrees, V = 3397.2(3) Angstrom(3), Z = 4. The accumulated data indicate that the phenyl group of pip(2)NCN(-) labilizes the trans pyridyl ligand. The electronic structures were investigated using cyclic voltammetry, as well as UV-visible absorption and emission spectroscopies. Red emission from 2 in rigid media originates from a lowest triplet ligand field excited state, whereas yellow-green emissions from 3 and 4 originate from a lowest pyridyl ligand-centered triplet pi-pi* state, indicating that substitution of the pyridyl ligand results in a dramatic change in the orbital character of the emissive state.