Synthesis and Luminescence of Cyclometalated Compounds with Nitrile and Isocyanide Ligands

Abstract

Mononuclear cationic cyclometalated palladium complexes [Pd(C ∧ N)(NCMe) 2 ]ClO 4 [C ∧ N = benzoquinolinate (bzq) 1, 2-phenylpyridinate (ppy) 2 ], analogous to the previously described platinum complexes [Pt(C ∧ N)(NCMe) 2 ]ClO 4 [C ∧ N = bzq 3, ppy 4 ], and the isocyanide platinum benzoquinolinate [Pt(bzq)(CNR) 2 ]X (R = tert -butyl ( t -Bu, 5 ), 2,6-dimethylphenyl (Xyl, 6 ), 2-naphthyl (2-Np, 7 ); X = ClO 4 − a, PF 6 − b ) have been prepared and characterized. The solid-state structures of the cation [Pt(C ∧ N)(CN-Xyl) 2 ] + with different counteranions ( 6a and 6b ) were found to be different in terms of packing, although in both cases they were dominated by π−π intermolecular interactions. The influence of the counteranion in the UV−vis spectra, both in solution and in the solid state of 5 − 7, is negligible. Time-dependent density-functional theory calculations on cation [Pt(C ∧ N)(CN-Xyl) 2 ] + ( 6 + ) have been performed, suggesting that the lowest absorption is 1 IL in nature mixed with some 1 MLCT character. Acetonitrile platinum complexes ( 3, 4 ) are photoluminescent at low temperature (77 K) and at room temperature, whereas analogous palladium complexes ( 1, 2 ) are emissive only at 77 K (solid-state and glassy acetonitrile). Isocyanide derivatives 5 − 7 are intensely luminescent in all media. The emissions are assigned to ligand-centered fluorescence, to mixed 3 LC/ 3 MLCT phosphorescence, or to excimeric (or ground-state) 3 ππ* or 3 MMLCT [σ*(M)→π*(C ∧ N)] transitions depending on the medium and the excitation wavelength. The effect of the counteranion in governing the degree of aggregation and the extent of the interactions seem to be relatively important, especially in a rigid medium, with the smaller ClO 4 − inducing a more excimeric character. The tendency to form π−π excimers and/or Pt···Pt oligomerization follows the order CN-2-Np > CN- t -Bu > CN-Xyl and ClO 4 − > PF 6 − .