A tale of two complexes, PtMen(RN=CHCH=NR) (n = 2 and n = 4, R = cyclohexyl): Why do PtII and PtIV complexes exhibit virtually identical redox behavior and colors?

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Chemistry - A European Journal, 1 (1): 95--99 (1995)
DOI: 10.1002/chem.19950010114


In spite of their similar cyclic voltammograms, absorption spectra, and solvatochromic behavior, the two 1,4-diazabutadiene title complexes exhibit markedly different photoreactivities and underlying electronic structures, as evident from absorption and EPR spectra of the persistent anion radical forms. The lowest excited state of the nonphotoreactive PtII system (CyN=CHCH=NCy)PtMe2 has MLCT (metal-to-ligand charge-transfer, 5d $\rightarrow$ p*) character, and the EPR spectrum of the corresponding anion radical at 〈g〉 = 2.016 exhibits sizable metal/ligand orbital mixing. However, the structurally characterized PtIV complex (CyN=CHCH=NCy)PtMe4 (C2/c; a 2021.6(2), b 805.3(1), c 1254.2(1) pm; b 111.05(1)°; V = 1905.7(4) $\times$ 106 pm3; Z = 4) has a low-lying photoreactive LLCT (ligand-to-ligand and charge-transfer, sPt-C $\rightarrow$ p*) excited state in which the axial Pt-C bonds are activated, as already suggested by the longer Pt-C(ax) bonds (214.0(8) pm) relative to Pt-C(eq) in the ground state (204.5(5) pm). The anion radical of the PtIV complex has lost the long-wavelength absorption band in the visible; it shows a well-resolved EPR spectrum at 〈g〉 = 1.9945 with p-ligand and 195Pt hyperfine structure and a small g anisotropy. A qual. MO scheme is presented to account for the similar frontier-orbital energy differences despite dissimilar underlying electronic structures. on SciFinder(R)



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