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, pip2NCN- (pip2NCNH = 1,3-bis(piperidylmethyl)benzene), are reported:  Pt(pip2NCN)(L)+ (2, L = pyridine; 3, L = 4-phenylpyridine;
5, L = 2,6-pyridinedimethanol) and [(Pt(pip2NCN))2(μ-4,4‘-bipyridine)]2+ (4). The complexes have been fully
characterized by 1H NMR spectroscopy, elemental analysis, and X-ray crystallography. Compound 2(CF3SO3-): 
triclinic, P1̄, a = 9.7518(6) Å, b = 12.0132(8) Å, c = 12.6718(9) Å, α = 114.190(2)°, β = 100.745(3)°, γ =
103.545(2)°, V = 1247.95(14) Å3, Z = 2. Compound 3(CF3SO3-):  monoclinic, P21/c, a = 15.550(2) Å, b =
9.7386(11) Å, c = 18.965(3) Å, β = 92.559(7)°, V = 2869.1(6) Å3, Z = 4. Compound 4(CF3SO3-)2·1/2(CH3)2CO: 
monoclinic, I2/a, a = 21.3316(5) Å, b = 9.6526(2) Å, c = 26.1800(6) Å, β = 96.4930(10)°, V = 5356.0(2) Å3,
Z = 4. Compound 5(CF3SO3-)·3/2CHCl3:  monoclinic, P21/n, a = 17.1236(10) Å, b = 9.3591(5) Å, c =
21.3189(11) Å, β = 96.11(3)°, V = 3397.2(3) Å3, Z = 4. The accumulated data indicate that the phenyl group of
pip2NCN- 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.