Syntheses, electronic absorption, emission, and ion-binding studies of platinum(II) C^N^C and terpyridyl complexes containing crown ether pendants

Abstract

A series of platinum (II) C^N^C complexes, [Pt(C^N^C) (L)] (HC^N^CH = 2,6-diphenylpyridine (dppy); L=Ph2PB15C5 (1, B15C5= benzo[15]crown-5), Ph2PDMP (2, DMP =3,4-dimethoxyphenyl), pyCOA15C5 (3, A15C5=aza[15]crown-5), pyCON-(CH2CH2OCH3)2 (4), pyC≡CB15C5 (5), pyC≡CDMP (6)) and terpyridyl complexes, [Pt(trpy)(L)](X)2 (trpy= 2,2′:6′,2″-terpyridine; L=Ph2PB15C5, X=OTf (7a), PF6 (7b); X=PF6, L= Ph2PDMP (8), pyC≡CB15C5 (9), and pyC≡CDMP (10)) have been successfully synthesized and characterized. The structures of 1, 3, and 7a have been determined by X-ray crystallography. Excitation of complexes 1-6 in EtOH/ MeOH (4:1 v/v) glass gave high-energy structured emission bands, assigned as derived from states of metal-perturbed intraligand (IL) origin. At higher concentrations, complexes 3-6 each displayed an additional, structureless emission band at 600-615 nm, with complexes 5 and 6 showing an obvious increase in the intensity of this emission band when the concentration was increased further. In dichloromethane at room temperature, complexes 3-6 showed, in addition to the high-energy emission at 490-505 nm, an extra, broad emission band at 620-625 nm when the concentration was increased. The emission origins of the low-energy band in glass and in fluid solutions are suggested to be derived from the ground-state oligomerization or aggregation process of the complexes. In the solid state at room temperature, complexes 1-6 each showed a broad, unstructured emission band at 560-600 nm, which was shifted to lower energy upon cooling to 77 K. On the other hand, the terpyridyl analogues 7-10 displayed intense vibronic-structured intraligand (IL) emissions at 460-472 nm in butyronitrile glass at 77 K. Solid-state samples of 9 and 10 displayed strong phosphorescence upon photoexcitation at 298 K and 77 K, tentatively assigned as derived from states of Pt(dπ)→π*(trpy) 3MLCT origin (MLCT = metal-to-ligand charge transfer). The ion-binding properties of complexes 5 and 9 for Na+, Ba2+, and K+ ions have been studied by UVNis spectrophotometric methods, and confirmed by ESI mass spectrometric studies. The ion-binding properties for Na+ ions have also been probed by 1H NMR experiments. For the same crown ether-containing ligand and the same metal ions, the neutral cyclometalated complexes gave larger binding constants than the positively charged terpyridyl analogues.