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Start date: 2000 × End date: 2019 × Subject: chemistry × Author: Alley, Kerwyn Graham ×

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    Towards valence tautomerism in polynuclear complexes
    Alley, Kerwyn Graham ( 2012)
    The synthesis, structural and physical characterisation of families of dinuclear cobalt complexes with the bridging bis-o-dioxolene ligands 3,3,3',3'-tetramethyl-1,1'-spirobis(indane-5,5',6,6'-tetrol) (spiroH4) and 3,3'-bis-2-hydroxy-1,4-naphthoquinone (bhnqH2) as well as dinuclear nickel complexes with N1,N2-bis(2-((E)-(3,5-di-tert-butyl-2-hydroxyphenylimino)methyl)phenyl)oxalamide (H4Ltert) and N1,N2-bis(2-((E)-(2-hydroxy-3,5-dimethylphenylimino)methyl)phenyl)oxalamide (H4Lmethyl) were undertaken, with an emphasis on investigating potential valence tautomeric (VT) behaviour. A new family of dinuclear cobalt complexes incorporating deprotonated spiroH4 and ancillary ligands derived from tris(2-pyridylmethyl)amine (TPA) was investigated. Systematically increasing the number of methyl groups at the 6-position of the pyridine arms of the ancillary ligand was shown to fine tune the charge distribution within each complex. Pure samples of [Co2(spiro)(TPA)2]2+ (12+), [Co2(spiro)(MeTPA)2]2+ (22+), [Co2(spiro)(Me2TPA)2]2+ (32+), [Co2(spiro)(Me3TPA)2]2+ (42+), [Co2(spiro)(TPA)2]3+ (53+), [Co2(spiro)(TPA)2]4+ (64+) have been isolated. In the solid state, crystallographic analysis, variable temperature magnetic susceptibility and X-ray absorption spectroscopy (XAS) data revealed 12+, 42+ and 64+ exist in the LS-CoIII-(spirocat-cat)4--LS-CoIII, HS-CoII-(spiroSQ-SQ)2--HS-CoII and LS-CoIII-(spiroSQ-SQ)4--LS-CoIII redox states, respectively, invariant of temperature (2-360 K). In solution, electrochemistry, electronic absorption and EPR spectroscopy analysis are consistent with the above assigned redox states, and also confirmed the presence of another redox state of LS-CoIII-(spiroSQ-cat)3--LS-CoIII in complex 53+. Due to spiroconjugation, electronic communication is observed across the spirocyclic carbon atom in the spiro ligand, which gives rise to weak antiferromagnetic coupling between the two halves of (spiroSQ-SQ)2- in 64+ and electron delocalisation across the ligand for (spiroSQ-cat)3- in 53+. The only complex to exhibit a temperature dependent charge distribution is 32+. Variable temperature magnetic susceptibility and XAS data are consistent with two transitions in the solid state occurring at around 100 and 300 K, whereas UV-visible absorption spectroscopy indicated that two transitions occur at 200 and 250 K in butyronitrile. Therefore, complex 32+ undergoes two thermally-induced VT transitions between LS-CoIII-(spirocat-cat)4--LS-CoIII and HS-CoII-(spiroSQ-cat)3--LS-CoIII and then to HS-CoII-(spiroSQ-SQ)2--HS-CoII. Evidence of a photo-induced VT transition was observed in the solid state at 10 K. Analogous to the spiro complexes, three new dinuclear complexes [Co2(bhnq)(MenTPA)2]2+ (n = 1 for 72+; 2 for 82+; 3 for 92+) with the deprotonated form of bhnqH2 were isolated. In the solid and solution states all three of these complexes existed in the HS-CoII-(bhnqQ-Q)2--HS-CoII redox state independent of temperature (2-300 K). Increasing the number of methyl substituents on the TPA ligand favours the stabilisation of HS-Co(II). An increase in distortion from octahedral coordination around each cobalt centre was correlated with the decrease in room temperature magnetic susceptibility due to quenching of the orbital contribution. A search for potential dinuclear nickel VT systems afforded two square-planar and two octahedral complexes: [Ni2(Ltert)] (10), [Ni2(Lmethyl)] (11), [Ni2(Ltert)(py)4] (12) and [Ni2(Lmethyl)(py)4] (13), where the metal is Ni(II). None of these complexes displayed evidence of a VT transition. Chemical (with (NH4)2Ce(NO3)6) and electrochemical oxidation of 10 led to the decomposition of the complex, giving N1,N2-bis(2(5,7-di-tert-butylbenzo[d]oxazol-2-yl)phenyl)oxalamide (14).