Synthetic Control of Pt···Pt Separation and Photophysics of Binuclear Platinum Complexes

A new series of luminescent μ-pyrazolate-bridged cyclometalated platinum binuclear complexes having the formula C∧NPt(μ-pz‘)2PtC∧N (C∧N = 2-(2,4-difluorophenyl)pyridyl, pz‘ = pyrazolate for 1, 3,5-dimethylpyrazolate for 2, 3-methyl-5-tert-butylpyrazolate for 3, and 3,5-bis(tert-butyl)pyrazolate for 4) have been synthesized and characterized. The two Pt(C∧N) moieties are bridged by two μ-pyrazolate ligands in an exo-bidentate fashion. A mononuclear complex with an isolated Pt center, C∧NPt(pz)2BEt2, 5, is also described. The X-ray crystal structures of 1−4 show the following Pt−Pt spacings:  1 = 3.3763(7) Å, 2 = 3.1914(9) Å, 3 = 3.0457(7) Å, and 4 = 2.8343(6) Å. At 77 K, the emission energy of the complexes varies from blue (for 1, 2, and 5) to green (for 3) to red (for 4). The changes in the photophysical properties of the binuclear complexes can be correlated with the decreasing Pt−Pt distance; the emissive state changes from a mixed ligand center triplet/metal-to-ligand charge transfer excited state (for 1 and 2) to a lower-energy, Pt−Pt metal−metal-to-ligand charge transfer state (for 3 and 4).