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 (μ-4,4'-bipyridine)] 2 + (4). The complexes have been fully characterized by 1 H NMR spectroscopy, elemental analysis, and X-ray crystallography. Compound 2(CF 3 SO 3 - ): triclinic, P1, a = 9.7518(6) A, b = 12.0132(8) A, c = 12.6718(9) A, α = 114.190(2)°, β = 100.745(3)°, y = 103.545(2)°, V = 1247.95(14) A 3 , Z = 2. Compound 3(CF 3 SO 3 -): monoclinic, P2 1 /c, a = 15.550(2) A, b = 9.7386(11) A, c = 18.965(3) A, β = 92.559(7)°, V= 2869.1(6) A 3 , Z= 4. Compound 4(CF 3 SO 3 -) 2 .½(CH 3 ) 2 CO: monoclinic, /2/a, a = 21.3316(5) A, b = 9.6526(2) A, c = 26.1800(6) A, β = 96.4930(10)°, V = 5356.0(2) A 3 , Z = 4. Compound 5(CF 3 SO 3 -).3/2CHCl 3 : monoclinic, P2 1 /n, a = 17.1236(10) A, b = 9.3591(5) A, c = 21.3189(11) A, β = 96.11(3)°, V= 3397.2(3) A 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 π-π* state, indicating that substitution of the pyridyl ligand results in a dramatic change in the orbital character of the emissive state.