Cyclometalated Platinum(II) Complexes of N∧C∧N-Coordinating, Terdentate Ligands: the Contrasting Influence of Pyrazolyl and Pyridyl Rings on Luminescence

1,3-Bis(1-pyrazolyl)-5-methyl-benzene, HL2, undergoes cyclometalation at the C-2 position upon reaction with K2PtCl4, to generate an N boolean AND C boolean AND N-coordinated complex, (PtLCl)-Cl-2. This compound is luminescent in degassed solution at 298 K, emitting in the blue region of the spectrum on the microsecond time scale (lambda(max) = 453 nm, tau = 4.0 mu, Phi(IUM) = 0.02, in CH2Cl2). Compared to the analogous complex Pt(dpyb)Cl that incorporates pyridyl rather than pyrazole rings {dpybH = 1,3-di(2-pyridyl)-benzene}, the excited state is displaced to higher energy by 1700 cm(-1). This effect is rationalized in terms of the poorer pi-acceptor nature of pyrazolyl compared to pyridyl rings, leading to destabilization of the lowest unoccupied molecular orbital, which is largely localized on the heteroaromatic rings in both cases. Cyclic voltammetry and density functional theory calculations reinforce this interpretation, and suggest that the lowest-energy excited state is probably best described as heavily mixed pi(L)/d(pt)/p(Cl) -> pi*(L) (IL/MLCT/LLCT) in character. 5-Aryl-substituted analogues of HL2 are accessible in three steps from 1,3,5-tribromobenzene by Pd-catalyzed cross-coupling with aryl boronic acids, followed by copper-catalyzed bromo-iodo exchange, and subsequent amination with pyrazole under relatively mild conditions also catalyzed by copper. The corresponding Pt(II) complexes display red-shifted and more intense luminescence compared to (PtLCl)-Cl-2. Ligands incorporating one pyrazole and one pyridyl ring are also accessible; for example, 1-(1-pyrazolyl)-3-(2-pyridyl) benzene, HL6. Their complexes are highly luminescent in solution; for example, for (PtLCl)-Cl-6, lambda(max) = 487 nm, T = 6.9 mu S, Phi(Ium) = 0.55, in dilute solution in CH2Cl2. At elevated concentrations, (PtLCl)-Cl-6 displays an additional excimeric emission band that is substantially blue-shifted compared to that displayed by Pt(dpyb)Cl (bands centered at 645 and 695 nm, respectively), indicating that the presence of the pyrazole ring destabilizes the excimer. The introduction of a methyl substituent into the central aryl ring of such complexes is sufficient to eliminate the excimer emission.