Ferromagnetic exchange coupling of vanadium(IV) dpi spins across pyrimidine rings: dinuclear complexes of oxovanadium(IV) bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionate) bridged by pyrimidine derivatives

Charge and Solvent Effects on the Redox Behavior of Vanadyl Salen-Crown Complexes

The incorporation of charged groups proximal to a redox active transition metal center can impact the local electric field, altering redox behavior and enhancing catalysis. Vanadyl salen (salen = N,N'-ethylenebis(salicylideneaminato)) complexes functionalized with a crown ether containing a nonredox active metal cation (V-Na, V-K, V-Ba, V-La, V-Ce, and V-Nd) were synthesized. The electrochemical behavior of this series of complexes was investigated by cyclic voltammetry in solvents with varying polarity and dielectric constant (ε) (acetonitrile, ε = 37.5; N,N-dimethylformamide, ε = 36.7; and dichloromethane, ε = 8.93). The vanadium(V/IV) reduction potential shifted anodically with increasing cation charge compared to a complex lacking a proximal cation (ΔE_1/2 > 900 mV in acetonitrile and >700 mV in dichloromethane). In contrast, the reduction potential for all vanadyl salen-crown complexes measured in N,N-dimethylformamide was insensitive to the magnitude of the cationic charge, regardless of the electrolyte or counteranion used. Titration studies of N,N-dimethylformamide into acetonitrile resulted in cathodic shifting of the vanadium(V/IV) reduction potential with increasing concentration of N,N-dimethylformamide. Binding constants of N,N-dimethylformamide (log(K_DMF)) for the series of crown complexes show increased binding affinity in the order of V-La > V-Ba > V-K > (salen)V(O), indicating an enhancement of Lewis acid/base interaction with increasing cationic charge. The redox behavior of (salen)V(O) and (salen-OMe)V(O) (salen-OMe = N,N'-ethylenebis(3-methoxysalicylideneamine) was also investigated and compared to the crown-containing complexes. For (salen-OMe)V(O), a weak association of triflate salt at the vanadium(IV) oxidation state was observed through cyclic voltammetry titration experiments, and cation dissociation upon oxidation to vanadium(V) was identified. These studies demonstrate the noninnocent role of solvent coordination and cation/anion effects on redox behavior and, by extension, the local electric field.

Vanadyl as a Stable Structural Mimic of Reactive Ferryl Intermediates in Mononuclear Nonheme-Iron Enzymes

The iron(II)- and 2-(oxo)glutarate-dependent (Fe/2OG) oxygenases catalyze an array of challenging transformations via a common iron(IV)-oxo (ferryl) intermediate, which in most cases abstracts hydrogen (H•) from an aliphatic carbon of the substrate. Although it has been shown that the relative disposition of the Fe-O and C-H bonds can control the rate of H• abstraction and fate of the resultant substrate radical, there remains a paucity of structural information on the actual ferryl states, owing to their high reactivity. We demonstrate here that the stable vanadyl ion [(V^IV-oxo)²⁺] binds along with 2OG or its decarboxylation product, succinate, in the active site of two different Fe/2OG enzymes to faithfully mimic their transient ferryl states. Both ferryl and vanadyl complexes of the Fe/2OG halogenase, SyrB2, remain stably bound to its carrier protein substrate (l-aminoacyl-S-SyrB1), whereas the corresponding complexes harboring transition metals (Fe, Mn) in lower oxidation states dissociate. In the well-studied taurine:2OG dioxygenase (TauD), the disposition of the substrate C-H bond relative to the vanadyl ion defined by pulse electron paramagnetic resonance methods is consistent with the crystal structure of the reactant complex and computational models of the ferryl state. Vanadyl substitution may thus afford access to structural details of the key ferryl intermediates in this important enzyme class.

Enhanced ferromagnetic interaction in metallacyclic complexes incorporating m-phenylenediamidato bridges

The double stranded Cu(II)2-metallacyclic complex of formula [Cu2(mbpb)2].2H2O (1) and the triple stranded Ni(II)2-metallacyclic complexes of formula [Ni2(Hmbpb)3]PF6.21H2O (2), [Co(H2O)6][Ni2(mbpb)3)]THF.10H2O (3) and {Ag2(H2O)[Ni2(mbpb)3]}.11H2O (4) (where H2mbpb is the bisbidentate dinucleating bridging ligand 1,3-bis(pyridine-2-carboxamide) benzene) have been synthesised and characterised by single-crystal X-ray diffraction. Within the dinuclear molecules, metal ions are bridged by either fully or semideprotonated bisbidentate ligands, which are coordinated through the pyridine and amidato nitrogen donor atoms. In complex 4 the triple stranded dinuclear Ni2 units are connected to the Ag+ cations through O-amidato bridges and Ag-pi(benzene) interactions to afford a 1D bimetallic chain. Cu2 (1) and Ni2 (2-4) complexes exhibit ferromagnetic coupling between the metal ions through the bridging ligand with J(Cu-Cu) = 21.1 cm(-1) and J(Ni-Ni) in the range of 2.9-3.6 cm(-1), respectively. Amongst copper(II) dinuclear complexes bearing m-phenylenediamidato bridges, complex 1 exhibits the stronger ferromagnetic exchange coupling reported so far. DFT calculations firstly confirm that the spin polarisation mechanism is responsible for the ferromagnetic coupling, and secondly allows us to predict stronger ferromagnetic couplings in Cu(II)(2) complexes with larger tetrahedral distortions of the CuN4 coordination environment.

A magneto-structural study of a hexanuclear (V(IV)=O)6-complex and a tetranuclear mixed-valent [V(III)2V(IV)2] species

Syntheses, crystal structures, and magnetic properties are reported for a hexametallic and a mixed-valent tetrametallic vanadium cluster, namely [Na(L(1))(6)(V(IV)=O)(6)]ClO(4) (1) and [(L(2))(2)V(III)(2)V(IV)(2)(mu-Cl)(6)Cl(4)] (2), where H(2)L(1) represents N-methyldiethanolamine and H(2)L(2) N'N'-di(3,5-di-tert-butyl-salicylidene)-1,3-diaminobenzene. The structure of the cation in 1 is comprised of a hexagon of six (V(IV)=O) units, in which a sodium ion is encapsulated in the center of the hexagon. Thus, the hexametalate ring contains six d(1) ions, each of which is in an octahedral environment of O(5)N atoms. Magnetic studies reveal the cluster to exhibit ferromagnetic exchange interactions (J = +16.7 +/- 0.3 cm(-1)), which is rationalized by the orthogonal neighboring xy planes of d(1) V(IV)=O ions. The structure of complex 2 can be considered as two mixed-valent face-sharing bioctahedral units of [V(III)(mu-Cl)(3)V(IV)] bridged by two m-phenylene linkers. The V(1)(III)...V(2)(IV) distances of av. 3.10 A preclude metal-metal bonding. Variable-temperature magnetic susceptibility data are fitted to obtain the parameters J(1) = -47.6 cm(-1), J(2) = +1.5 cm(-1), g(V(III)) = 1.95, g(V(IV)) = 1.85 (H = -2J x S(i).S(j)). The ferromagnetic coupling J(2) operates at a distance of approximately 7.09 A between two vanadium centers through the spin polarization mechanism due probably to the topology (1,3-substitution) of the m-phenylene bridges. The X-band electron paramagnetic resonance (EPR) spectrum analyzed with S(t) = 1 shows that the mixed-valent V(III)V(IV) species has indeed been formed and the tetranuclear core remains intact in solution. The hyperfine coupling constants indicate the localized mixed-valence nature of 2.

Rational design of 1-D metal–organic frameworks based on the novel pyrimidine-4,6-dicarboxylate ligand. New insights into pyrimidine through magnetic interaction

Single crystal X-ray analysis of compounds H2pmdc.2H2O (1), KHpmdc (2), and K2pmdc (3) shows that the pyrimidine-4,6-dicarboxylate (pmdc) dianion presents an almost planar geometry which confers a potential capability to act as a bis-bidentate bridging ligand, and therefore, to construct 1-D metal complexes. Based on this assumption, we have designed the first six transition metal complexes based on this ligand of formula {[M(micro-pmdc)(H2O)2].H2O}n [M(II) = Fe (4), Co (5), Ni (6), Zn (7), Cu (8)] and {[Cu(micro-pmdc)(dpa)].4H2O}n (9) (dpa = 2,2'-dipyridylamine). The crystal structure of all of these complexes has been determined by single crystal X-ray measurements, except for compound whose X-ray powder diffraction pattern reveals that it is isostructural to compounds 4-7. The bis-chelating pmdc ligand bridges sequentially octahedrally coordinated M(II) centres leading to polymeric chains. The hexacoordination of the metal centres is completed by two water molecules in compounds 4-8 and by the two endocyclic-N atoms of a terminal dpa ligand in compound . Cryomagnetic susceptibility measurements show the occurrence of antiferromagnetic intrachain interactions for compounds and (J = -2.5 (4), -5.2 (6), -32.7 (8), and -0.9 (9) cm(-1)). Model calculations and analyses of the available experimental data have been used to examine the influence of several factors on the nature and magnitude of the magnetic coupling constants in pyrimidine bridged complexes, showing that metal deviation from the pyrimidine mean plane could lead to ferromagnetic behaviour.

Strong Ferromagnetic Exchange Couplings in Copper(II) and Nickel(II) Complexes with a Paramagnetic Tridentate Chelate Ligand, 2,2‘-Bipyridin-6-yl tert-Butyl Nitroxide

A paramagnetic ligand 6bpyNO (2,2'-bipyridin-6-yl tert-butyl nitroxide) is a newcomer in the field of metal-radical hybrid magnetic materials. Complexes [CuII(6bpyNO)Cl2] and [NiII(6bpyNO)2](PF6)2 having highly planar chelate rings showed considerably strong ferromagnetic exchange couplings J/kB = 202 and 192 K, respectively, across the direct radical oxygen coordination bonds (the spin Hamiltonian is defined as -2JSi.Sj).

Substituted m-phenylene bridges as strong ferromagnetic couplers for Cu(II)-bridge-Cu(II) magnetic interactions: new perspectives

Based on a combined theoretical-experimental study, we propose that substituted m-phenylene ligands (m-N-Phi-N) can act as tuneable strong ferromagnetic couplers connecting Cu(II) ions; a new complex presenting that bridge with J close to +15 cm(-1) has been suggested and synthesized.

Targeted Ferromagnetic Coupling in a Trinuclear Copper(II) Complex: Analysis of the St = 3/2 Spin Ground State

The trinuclear Cu(II) complex [(talen)Cu(II)(3)] (1) using the new triplesalen ligand H(6)talen has been synthesized and structurally characterized. The three Cu(II) ions are bridged in a m-phenylene linkage by the phloroglucinol backbone of the ligand. This m-phenylene bridging mode results in ferromagnetic couplings with an S(t) = (3)/(2) spin ground state, which has been analyzed by means of EPR spectroscopy and DFT calculations. The EPR spectrum exhibits an unprecedented pattern of 10 hyperfine lines due to the coupling of three Cu(II) ions (I = (3)/(2)). Resonances around g = 4 in both perpendicular and parallel mode EPR spectra demonstrate a zero-field splitting of D approximately 74 x 10(-4) cm(-1) arising from anisotropic/antisymmetric exchange interactions. The DFT calculations show an alteration in the sign of the spin densities of the central benzene ring corroborating the spin-polarization mechanism as origin for the ferromagnetic coupling.