Syntheses and electronic spectroscopy of [PtL(L')][ClO4] complexes (HL = 6-phenyl-2,'2'-bipyridine; L' = pyridine, 4-aminopyridine, 2-aminopyridine, and 2,6-diaminopyridine)

Abstract

Four cyclometalated Pt(II) complexes, [PtL(L')][ClO4] [HL = 6-phenyl-2,2'-bipyridine; L' = pyridine (1), 4-aminopyridine (2), 2-aminopyridine (3), 2,6-diaminopyridine (4)], were designed and synthesized to probe intramolecular N···Pt interactions. The crystal structures of the compounds show that the pyridine ligands are almost perpendicular to the planes of the molecules. In addition, the pendant NH2 groups of the 2-aminopyridine and 2,6-diaminopyridine ligands are close to the metal centers in complexes 3 and 4, with the Pt-N(H2) distances (3.065(3)-3.107(3) Å) significantly shorter than the sum of the van der Waals radii of Pt and N. These compounds were also studied by electronic spectroscopy. All the complexes display intense intraligand π → π* transitions at 200-340 nm (ε = 104-103 M-1 cm-1) and moderately intense (ε ~ 103 M-1 cm-1) metal (Pt)-to-ligand (π*) charge-transfer (MLCT) transitions. For 1 and 2, the MLCT transitions occur at ~390 nm, but the MLCT transition of 4 is exceptionally low in energy (492 nm). The low-temperature emission spectra of the complexes in frozen EMD glass indicate that 3ππ* is the emissive excited state for 1 and 2 but the emission of 3 is from a 3MLCT excited state. On the basis of the spectroscopic results, the order of energy of the MLCT excited states is established as 1 ~ 2 > 3 > 4. It is proposed that the red shifts of the MLCT transitions in 3 and 4 are due to increased electron-donating abilities of the ancillary pyridine ligands and intramolecular interactions between the orbitals of amine nitrogen lone pairs. Crystal data for the complexes are as follows. 1: triclinic P1̄, Z = 2, a = 8.7917(2) Å b = 10.6398(3) Å, c = 11.9592(3) Å, α = 107.130(1)°, β = 92.522(1)°, γ = 111.509(1)°. 2·CH3CN: triclinic P1̄, Z = 2, a = 7.0122(4) Å, b = 12.9653(8) Å, c = 14.0283(9) Å, α = 107.3100(10)°, β = 102.7640(10)°, γ = 91.6320(10)°. 3·CH3CN: triclinic P̄, Z = 2, a = 7.6459(1) Å, b = 10.8433(1) Å, c = 14.8722(2) Å, α = 99.383(1)°, β = 93.494(1)°, γ = 101.385(1)°. 4·CH3CN: triclinic P̄, Z = 2, a = 7.862(2) Å, b = 10.977(3) Å, c = 14.816(5) Å, α = 99.34(2)°, β = 92.64(2)°, γ = 104.11(2)°.