Copper is a bioessential element in biology with truly unique chemical characteristics in its two relevant oxidation states +I and +II. Significant progress has been made in recent years in the elucidation of the frequently surprising biochemistry of this trace element. Those advances were especially furthered through mutual stimulation involving results from biochemistry, molecular biology, and medicine on one hand and the synthesis as well as the structural and spectroscopic characterization of low molecular weight model complexes on the other. The most notable features of protein‐bound active copper are its almost exclusive function in the metabolism of O2 or N/O compounds (NO 2−, N2O) and its frequent association with oxidizing organic and inorganic radicals such as tyrosyl, semiquinones, superoxide, or nitrosyl. This unique biological role of copper can be rationalized given its chemical and assumed evolutionary background.
Valence tautomerism involving coupled Cu+/2+ and quinone−\./2− redox systems has been observed for the first time outside an enzymatic situation (amine oxidases) with chelates L as CuI and CuII affine coligands. Even slight variations of the quinone substitution pattern suffice to shift the valence tautomer equilibrium completely. [(L)CuII(Q−II)] ⇌ [(L)CuI(Q−I)]
Azophenine (7,8-diphenyl-2,5-bis(phenylamino)-p-quinonediimine, L(p)) reacts with [Cu(PPh3)4](BF4) or [Re(CO)(5)Cl] to yield the (Ph3P)(2)Cu(+) or [(OC)(3)ClRe] complex of the tautomeric form 7,8-diphenyl-4,5-bis(phenylamino)-o-quinonediimine, L(o), as evident from structure determinations and from very intense metal-to-ligand charge transfer (MLCT) transitions in the visible region. Time-dependent DFT (TD-DFT) calculations on model complexes [(N intersection N)Re(CO)(3)Cl] confirm the spectroscopic results, showing considerably higher oscillator strengths of the MLCT transition for the o-quinonediimine complexes in comparison to compounds with N intersection N=1,4-dialkyl-1,4-diazabutadiene. The complexes are additionally stabilized through hydrogen bonding between two now ortho-positioned NHPh substituents and one fluoride of the BF(4) (-) anion (Cu complex) or the chloride ligand (Re complex). DFT Calculations on the model ligands p-quinonediimine or 2,5-diamino-p-quinonediimine and their ortho-quinonoid forms with and without Li(+) or Cu(+) are presented to discuss the relevance for metal-dependent quinoproteins.
Paramagnetic complexes between chelating o-quinone derivatives Q"-(Q = 3,5di-tert-butyl~-benzoquinone) and coordinatively unsaturated copper compounds (L)Cu have been obtained and studied by EPR spectroscopy. With nitrogen donor co-ligands, L, e.g. saturated polyamines or a bis(imidazo1e) chelate ligand, the Cu"(Q2-) form was observed. In contrast, tetrahydrothiophene and bi-, tri-or tetra-dentate chelating thioethers favour the Cu'(Q'-) form as ground state as evident from small 63%u coupling (< 1.5 mT), from semiquinone hyperfine splitting and from g factors (<2.010) which are relatively close to the free-electron value. However, the thioether co-ligands generally effect higher metal-coupling constants and g factors in (L)CU'(Q'-) systems than triorganophosphine, triorganoarsine, carbonyl, n-coordinated alkyne or alkene ligands.
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