Cortlandt G. Pierpont

Studies on charge distribution and valence tautomerism in transition metal complexes of catecholate and semiquinonate ligands

Transition metal complexes containing first row metal ions chelated by catecholate (Cat) and semiquinonate (SQ) ligands have been found to have localized electronic structures with quinone ligands bonded in both SQ and Cat electronic forms. In exceptional cases the balance of metal and quinone orbital energies is sufficiently close as to permit magnetic and spectroscopic observations on isomers differing in charge distribution together under equilibrium conditions. Equilibria occurring between Mn(SQ−) and Mn+1(Cat2−) redox isomers have been observed in solution and in the solid state for complexes of Co, Mn, and Cu. The temperature range over which equilibria may be observed, is defined by enthalpy and entropy changes associated, mainly, with changes in charge and spin-state of the metal ion. This effect has been considered as an example of valence tautomerism (VT). In this review we present an overview of recent research on complexes of quinone ligands that exhibit reversible shifts in charge distribution under equilibrium conditions.

Unique properties of transition metal quinone complexes of the MQ3 series

Abstract Transition metal complexes of ligands derived from o -benzoquinones have been shown to exhibit unique versatility in their electronic, structural and magnetic properties. Examples of these properties are described for complexes of the MQ 3 series with ligands coordinated in the form of semiquinonate (SQ) radical anions, catecholate dianions (Cat), and unreduced benzoquinones (BQ). Electrochemical activity may occur at either the metal or ligand; SQ complexes exhibit temperature dependent magnetic properties resulting from M–SQ, SQ–SQ, and intermolecular exchange coupling; equilibria between M I (SQ)/M II (Cat) redox isomers has been observed to occur in solution and in the solid state; charge localization has resulted in the formation of complexes containing mixed-valence ligands; strong π donation from Cat oxygens has been found to stabilize metal ions in unusually high oxidation states. Examples of these properties from among compounds of the MQ 3 series are described.

Nickel complexes containing catecholate, benzoquinone and semiquinone radical ligands

Abstract Quinone complexes of nickel have been prepared by treating Ni(CO)4 with 3,6-di-tert-butyl-1,2-benzoquinone (3,6-DBBQ). Reactions carried out with stoichiometric quantities of 3,6-DBBQ lead to the formation of monomeric, square planar Ni(3,6-DBSQ)2. Spins of the radical semiquinone ligands are coupled antiferromagnetically to result in near diamagnetism. Electrochemical characterization has shown that the complex undergoes reduction in two one-electron steps. Chemical reduction with CoCp2 results in the formation of (CoCp2)[Ni(3,6-DBCat)2]. EPR spectra recorded on this product show anisotropy that is consistent with the Ni(III) formulation. Reactions carried out with Ni(CO)4 and an excess of 3,6-DBBQ lead to the formation of Ni)3,6-DBBQ)(3,6-DBCQ)2. Crystalligraphic characterization Ni(3,6-DBBQ)(3,6-DBSQ)2. Crystallographic characterization (Ni(3,6-DBBQ) (3,6-DBSQ)2: orthorhombic, Ccca, a = 20.092(6), b = 24.335(8), c = 18.340(4) A , V = 8967(4) A 3 , Z = 4 ) has shown that the metal is octahedral and that the benzoquinone and semiquinone ligands are differentiable by characteristic CO, CC and Ni bond lengths. Magnetic properties are slightly temperature dependent due to either weak antiferromagnetic NiSQ exchange or offsetting NiSQ (ferromagnetic) and SQSQ (antiferromagnetic) exchange interactions.

Dopamine complexes of iron in the etiology and pathogenesis of Parkinson’s disease

Parkinsons disease (PD) is the second most common neurodegenerative disease after Alzheimers. The main pathological hallmark of Parkinsons is the deterioration and death of neurons that produce the neurotransmitter dopamine. Much of the neuronal damage takes place in the substantia nigra, a small region of the midbrain that contains the cell bodies of neurons that produce dopamine. The deterioration and death of dopaminergic neurons are directly associated with misfolding and aggregation of proteins, principally alpha-synuclein, that are natively unfolded. Present also in the substantia nigra is an unusually high concentration of vestigial iron. Protein misfolding in non-genetic (sporadic) cases of PD has been associated with reactive oxygen species formed as products of O(2) reduction by the combination of dopamine and iron. Combinations of Fe(3+), dopamine hydrochloride (DA(H+)Cl), and various ancillary ligands have been studied as a function of pH in aqueous solution to determine the optimum pH for complex formation. With ancillary ligands (L(4)) derived from nitrilotriacetic acid and ethylenediamine diacetic acid spectral changes are consistent with the formation of L(4)Fe(DA(H+)) species that reach a maximum concentration at pH 7.2. With edta as the ancillary ligand, spectral features at pH 7 resemble those of Fe(3+)-catecholate complexes that contain catecholate ligands bonded through a single oxygen. This demonstrates the ability of the dopamine catechol functionality to penetrate the coordination sphere of even exceptionally stable iron chelates.

Valence tautomerism for catechol/semiquinone complexes of the trans-M(Bupy)2(3,6-DBQ)2 (MMn, Fe, Co) series

Studies directed at the investigation of metal-quinone electron transfer have been carried out for members of the M(Bupy)2(3,6- DBQ)2 series where Bupy is 4-tert-butylpyridine, 3,6-DBQ is the 3,6-di-tert-butylbenzoquinone ligand in either its reduced semiquinonate or catecholate form, and M is Mn, Fe or Co. Structural characterization on Mn(Bupy)2(3,6-DBCat)2 and Fe(Bupy)2(3,6-DBSQ)(3,6-DBCat) (Mn(Bupy)2(3,6-DBCat)2: triclinic, P1, a=10.294(15), b=10.632(8), c=11.085(5) A, α=93.00(3), β=90.78(3), γ=112.12(3)°, V=1122(1) A3 and Z=1; Fe(Bupy)2(3,6-DBSQ)(3, 6-DBCat): triclinic, P1, a=10.326(2), b=10.646(2), c=11.192(2) A, α=92.40(2), β=92.17(1), γ=111.87(1)°, V=1138.9(4) A3 and Z=1) has shown that the molecules are in the trans isomeric form. Features of the inner coordination spheres about both metal ions are in accord with the charge distributions indicated. Crystallographically imposed inversion symmetry disorders the Cat and SQ ligands of the iron complex. Both Co(Bupy)2(3,6-DBSQ)(3,6-DBCat) and Mn(Bupy)2(3,6-DBSQ)(3,6-DBCat) show low-energy charme transfer transitions in the region between 3000 and 5000 cm−1, and exhibit valence tautomeric equilibria in solution and in the solid state. Equilibria for the cobalt complex involve shifts between CoIII(Bupy)2(3,6-DBSQ)(3,6-DBCat) and CoII(Bupy)2(3,6-DBSQ)2 charge distributions in separate electron transfer and metal spin transition steps. Solid state equilibria for the manganese complex appear to occur between MnIII(Bupy)2(3,6-DBSQ)(3,6-DBCat) and MnIV(Bupy)2(3,6-DBCat)2 charge distributions, with an additional shift to the MnII(Bupy)2(3,6-DBSQ)2 form in toluene solution. Fe(Bupy)2(3,6-DBSQ)(3,6-DBCat) shows a low intensity transition at 2150 nm, but fails to show evidence for a tautomeric equilibrium.

Synthesis and characterization of V(V)(3,6-DBSQ)(3,6-DBCat)2, a d(0) metal complex with dioxygenase catalytic activity.

Transition-metal complexes containing redox-active quinoid ligands are of interest because of their catalytic capabilities in multielectron, substrate-activation reactions such as dioxygenase catalysis using O(2). The new catecholate complex V(V)(3,6-DBSQ)(3,6-DBCat)(2) (where 3,6-DBSQ = 3,6-di-tert-butylsemiquinone and 3,6-DBCat = 3,6-di-tert-butylcatecholate) was synthesized by combining VO(acac)(2) with 1 equiv of 3,6-DBBQ (where 3,6-DBBQ = 3,6-di-tert-butylbenzoquinone) and 2 equiv of H(2)(3,6-DBCat) in dry methanol under an inert atmosphere. The resultant complex was characterized by single-crystal X-ray diffraction, elemental analysis, near-IR, UV/vis, and electron paramagnetic resonance (EPR) spectroscopy. The crystallography as well as the near-IR and EPR studies suggest that the radical spin is localized on the 3,6-DBSQ ligand at room temperature, making V(V)(3,6-DBSQ)(3,6-DBCat)(2) a type 1 mixed-valence complex. Initial dioxygenase catalysis studies reveal that V(V)(3,6-DBSQ)(3,6-DBCat)(2) is a good dioxygenase precatalyst for the substrate H(2)(3,6-DBCat) with O(2) in ca. 600 total turnovers to >93% intra- and extradiol products with only 1-2% of the undesired benzoquinone autoxidation product.

Methoxide Coordination at the Pocket of [CuIITpCum,Me] and a Simple Model for the Cu Center of Galactose Oxidase

An unusually negative oxidation potential is found for the tyrosine residue in the center of fungal galactose oxidase. The complex [Cu(TpCum,Me ){O(MeS)C6 H4 }] (see picture on the right; TpCum,Me =hydrotris(pyrazolyl)borate) offers insight into the mode of (cysteinyl-tyrosine) coordination to the copper center, and the reason for the low oxidation potential.

Electron distribution within bis(quinone)ruthenium complexes. Structural characterization on delocalized (bipyridine)bis(quinone)ruthenium and on the localized trans-bis(3-chloropyridine)bis(3,5-di-tert-butyl-1,2-semiquinone)ruthenium(III) cation

Abstract Structural information obtained on complexes containing semiquinone and catecholate ligands has provided insights on charge distribution. Ligand CO lengths found for trans-Ru(4-t-Bupy)2(DBQ)2 averaged 1.321(5) A, midway between values of 1.34 and 1.29 A expected for localized catecholate and semiquinone ligands. This value may arise from crystallographic disorder of a localized RuIII(SQ)(Cat) species or it may result from charge delocalization over both quinone ligands. Crystallographic characterization of Ru(bpy)(Q)2, where Q is the unsubstituted o-quinone ligand, has shown that the complex molecule crystallizes without imposed symmetry on the molecule, in a crystal structure that places both quinone ligands in different solid-state environments. The four independent CO lengths average 1.321(5) A, without significant variation, strongly suggesting charge delocalization over both quinone ligands. Characterization on the trans-Ru(3-Clpy)2(DBQ)2+ cation, formed by one-electron oxidation of a neutral species that would be expected to show ligand charge delocalization, has shown ligand CO lengths of 1.293(4) A, exactly the value expected for semiquinone ligands. Oxidation of the neutral complex appears to occur at a ligand delocalized electronic level to give a ligand localized bis(semiquinone) product, RuIII(SQ)2+.

Bending crystals. Solid state photomechanical properties of transition metal complexes containing semiquinonate ligands

The properties of transition metal complexes containing catecholate and radical semiquinonate ligands have often been found to be unusual and unexpected. Crystals of Rh(CO)2(3,6-DBSQ), containing the 3,6-di-tert-butyl-1,2-semiquinonate ligand, form as long thin needles that are observed to bend reversibly upon irradiation with NIR light. Crystallographic characterization reveals a stacked solid state lattice with planar molecules aligned with metal atoms atop one another. Electronic spectra recorded in the solid state and in solution show an intense band at 1600 nm that maps the energy dependence of crystal bend angle. The transition is a property of the stacked assembly, rather than of an individual complex molecule, and appears associated with an MLCT process that transfers charge from an antibonding band formed by interacting Rhdz2 orbitals to the vacant quinone π* orbital. Related observations have been made on the [Co(μ-pyz)(3,6-DBSQ)(3,6-DBCat)]npolymer. Photomechanical properties appear associated with electronic transitions that lead to a physical change in axial length of a linear polymer, coupled with a soft solid state lattice that permits axial contraction/expansion without crystal fracture.

Iminoquinone coordination to copper(I) in the [Cu(PhenoxBQ)(μ-Cl)]2 dimer

The addition of 2,4,6,8-tetra-tert-butylphenoxazin-l-one (PhenoxBQ) to an acetonitrile solution of CuCl leads to the formation of the [Cu(PhenoxBQ)(μ-Cl)]2 dimer. Structural characterization on crystals of the dimer obtained by recrystallization from dichloromethane (triclinic, P1, a = 10.559(1), b = 11.914(3), c = 13.530(4) A, α = 85.55(2), β = 74.57(2), γ = 75.52(2)°, V = 1588.5(6) A3, Z = 1, R = 0.079) has shown that the coordination geometry about the metals is tetrahedral. The CuCu separation of 2.826(1) A and the CuCl lengths of 2.215(3) and 2.289(2) A are unusually short among chloro-bridged Cu(I) dimers. Addition of PhenoxBQ to CuCl2 results in the formation of [Cu(PhenoxBQ)Cl2]2, also formulated as a dimer on the basis of its magnetic and electrochemical properties. Both dimers show the four-membered redox series that appears characteristically for complexes containing two PhenoxBQ or PhenoxBQ ligands. In the case of [CuI(PhenoxBQ)Cl]2 an oxidation at +1.043 V (versus NHE) has been assigned as the Cu(I)Cu(II) couple associated with oxidation of one metal ion of the dimer.