Vanadium-Induced Nucleophilic IPSO Substitutions in a Coordinated Tetrachlorosemiquinone Ring: Formation of the Chloranilate Anion as a Bridging Ligand

Modulation of slow magnetic relaxation in Gd(III)‐tetrahalosemiquinonate complexes

Incorporating lanthanoid(III)‐radical magnetic exchange coupling is a possible route to improving the performance of lanthanoid (Ln) single‐molecule magnets (SMMs), molecular materials that exhibit slow relaxation and low temperature quantum tunnelling of the magnetization. Complexes of Gd(III) can conveniently be used as model systems to study the Ln‐radical exchange coupling, thanks to the absence of the orbital angular momentum that is present for many Ln(III) ions. Two new Gd(III)‐radical compounds of formula [Gd(18‐c‐6)X₄SQ(NO₃)].I₃ (18‐c‐6=18‐crown‐6, X₄SQ⋅⁻=tetrahalo‐1,2‐semiquinonate, 1: X=Cl, 2: X=Br) have been synthesized, and the presence of the dioxolene ligand in its semiquinonate form confirmed by X‐ray crystallography, UV‐Visible‐NIR spectroscopy and voltammetry. Static magnetometry and EPR spectroscopy indicate differences in the low temperature magnetic properties of the two compounds, with antiferromagnetic exchange coupling of J_Gd‐SQ∼−2.0 cm⁻¹ (H_ex=−2J_Gd‐SQ(S_GdS_SQ)) determined by data fitting. Interestingly, compound 1 exhibits slow magnetic relaxation in applied magnetic fields while 2 relaxes much faster, pointing to the major role of packing effects in modulating slow relaxation of the magnetization.

Nitrochloranilic acid: a novel asymmetrically substituted quinoid bridging ligand for design of coordination polymers

A series of alkali salts and transition metal complexes of a novel asymmetrically substituted quinoid ligand, 3-nitro-6-chloro-2,5-dihydroxyquinone (nitrochloranilic acid, H₂NCA) was prepared and characterised.

Nitranilic acid as a basis for construction of coordination polymers: from discrete monomers to 3D networks

Six novel transition metal complexes of nitranilic acid, a highly versatile ligand, were studied. Topologies of coordination polymers ranging from 0D (discrete monomers) to 3D were observed.

Aerial Oxidation of a V(IV)-Iminopyridine Hydroquinonate Complex: A Trap for the V(IV)-Semiquinonate Radical Intermediate

The reaction of 2,5-bis[N,N'-bis(2-pyridyl-aminomethyl)aminomethyl]-p-hydroquinone (H2bpymah) with VO(2+) salts in acetonitrile or water at a low pH (2.2-3.5) results in the isolation of [{V(IV)(O)(Cl)}2(μ-bpymah)], the p-semiquinonate complex [{V(IV)(O)(Cl)}2(μ-bpymas)](OH), the cyclic mixed-valent hexanuclear compound [{V(V)(O)(μ-O)V(IV)(O)}(μ-bpymah)]3, and [(V(V)O2)2(μ-bpymah)]. [{V(IV)(O)(Cl)}2(μ-bpymas)](OH) is an intermediate of the radical-mediated oxidation of [{V(IV)(O)(Cl)}2(μ-bpymah)] from O2. At lower pH values (2.2), a reversible intramolecular electron transfer from the metal to the ligand of [{V(IV)(O)(Cl)}2(μ-bpymas)](OH) is induced with the concurrent substitution of chlorine atoms by the oxygen-bridging atoms, resulting in the formation of [{V(V)(O)(μ-O)V(IV)(O)}(μ-bpymah)]3. The metal complexes were fully characterized by X-ray crystallography, infrared (IR) spectroscopy, and magnetic measurements in the solid state, as well as by conductivity measurements, UV-vis spectroscopy, and electrochemical measurements in solution. The oxidation states of the metal ions and ligands were determined by the crystallographic data. The [{V(IV)(O)(Cl)}2(μ-bpymah)]-[{V(IV)(O)(Cl)}2(μ-bpymas)](OH) redox process is electrochemically reversible. The V(IV) ion in the semiquinonate compound exhibits a surprisingly low oxophilicity, resulting in the stabilization of OH(-) counterions at acidic pH values. An investigation of the mechanism of this reaction reveals that these complexes induce the reduction of O2 to H2O2, mimicking the activity of enzymes incorporating two redox-active centers (metal-organic) in the active site.

Vanadium(IV/V)-p-dioxolene temperature induced electron transfer associated with ligation/deligation of solvent molecules

Reaction of an aqueous solution of NaVO3 or a methanol solution of [VO(acac)2] with 2,5-bis((bis(2-hydroxyethyl)amino)methyl)hydroquinone, H6bdeah, results in the formation of two major vanadium species characterized by X-ray crystallography: the [(V(5+)O)2(bdeah)] and the [(V(4.5+)O)2(bdeas)S2] (S = DMSO or MeOH). The vanadium ions in the two species have a trigonal pyramidal and an octahedral coordination sphere respectively. Variable temperature UV-Vis and (51)V NMR spectroscopy as well as EPR and electrochemistry showed a temperature induced electron transfer. The diamagnetic [(V(5+)O)2(bdeah)] is the main species at high temperature. At low temperature one electron is transferred from the bdeah(6-) to the two vanadium centers resulting in the [(V(4.5+)O)2(bdeas)S2] species [H5bdeas = 2,5-bis((bis(2-hydroxyethyl)amino)methyl)-1,4-semiquinone]. The thermodynamic parameters of this intramolecular electron transfer were calculated by UV-Vis (ΔH = -36 ± 2 kJ mol(-1) and ΔS = -129 ± 5 J mol(-1) K(-1)) and (51)V NMR spectroscopy (ΔH = -37 ± 2 kJ mol(-1) and ΔS = -109 ± 5 J mol(-1) K(-1)). The electron transfer is a result of the large change of entropy which is associated with the ligation of the solvent molecules and the geometry change. EPR spectroscopy shows that most of the electron density in [(V(4.5+)O)2(bdeas)S2] is mainly located on the two vanadium ions.

Characterization of noninnocent metal complexes using solid-state NMR spectroscopy: o-dioxolene vanadium complexes

(51)V solid-state NMR (SSNMR) studies of a series of noninnocent vanadium(V) catechol complexes have been conducted to evaluate the possibility that (51)V NMR observables, quadrupolar and chemical shift anisotropies, and electronic structures of such compounds can be used to characterize these compounds. The vanadium(V) catechol complexes described in these studies have relatively small quadrupolar coupling constants, which cover a surprisingly small range from 3.4 to 4.2 MHz. On the other hand, isotropic (51)V NMR chemical shifts cover a wide range from -200 to 400 ppm in solution and from -219 to 530 ppm in the solid state. A linear correlation of (51)V NMR isotropic solution and solid-state chemical shifts of complexes containing noninnocent ligands is observed. These experimental results provide the information needed for the application of (51)V SSNMR spectroscopy in characterizing the electronic properties of a wide variety of vanadium-containing systems and, in particular, those containing noninnocent ligands and that have chemical shifts outside the populated range of -300 to -700 ppm. The studies presented in this report demonstrate that the small quadrupolar couplings covering a narrow range of values reflect the symmetric electronic charge distribution, which is also similar across these complexes. These quadrupolar interaction parameters alone are not sufficient to capture the rich electronic structure of these complexes. In contrast, the chemical shift anisotropy tensor elements accessible from (51)V SSNMR experiments are a highly sensitive probe of subtle differences in electronic distribution and orbital occupancy in these compounds. Quantum chemical (density functional theory) calculations of NMR parameters for [VO(hshed)(Cat)] yield a (51)V chemical shift anisotropy tensor in reasonable agreement with the experimental results, but surprisingly the calculated quadrupolar coupling constant is significantly greater than the experimental value. The studies demonstrate that substitution of the catechol ligand with electron-donating groups results in an increase in the HOMO-LUMO gap and can be directly followed by an upfield shift for the vanadium catechol complex. In contrast, substitution of the catechol ligand with electron-withdrawing groups results in a decrease in the HOMO-LUMO gap and can directly be followed by a downfield shift for the complex. The vanadium catechol complexes were used in this work because (51)V is a half-integer quadrupolar nucleus whose NMR observables are highly sensitive to the local environment. However, the results are general and could be extended to other redox-active complexes that exhibit coordination chemistry similar to that of the vanadium catechol complexes.