Search

A series of complexes of metal halides with unreduced quinone-type ligands have been synthesized and characterized in detail. The 3,6-di-tert-butyl-o-benzoquinone (1) and 4,6-di-tert-butyl-N-aryl-substituted o-iminobenzoquinones (2-5) (aryl is 2,6-dimethylphenyl in 2, 2-methyl-6-ethylphenyl in 3, 2,6-diethylphenyl in 4, and 2,6-diisopropylphenyl in 5) were used to obtain the molecular complexes with metal 12 group halides as well as with indium(III) iodide. The molecular structures of five complexes, bearing an unreduced form of redox-active ligand, have been established by single-crystal X-ray analysis. The spectral data, electrochemical measurements, and DFT calculations indicate the significant transformations of the molecular orbitals of 1-5 upon complexation with Lewis acids. The reduction potentials of o-(imino)quinones in complexes with metal halides shift into the anodic region versus uncoordinated ones. The choice of metal halide allows varying the shift magnitude up to 1.7 V in 2·CdI2. The change of the oxidizing ability of the 1-5 upon coordination with Lewis acids enables the oxidation of mercury and ferrocene, infeasible for free ligands.

Abstract Poorly soluble in the most part of organic solvents dimeric complexes $${\text{M}}{{{\text{g}}}_{{\text{2}}}}{\text{L}}_{2}^{2}$$·4DMF (I) and $${\text{N}}{{{\text{i}}}_{{\text{2}}}}{\text{L}}_{2}^{2}$$·4DMF (II) (L is 4,4'-(1,4-phenylenebis(azanylylidene))bis(3,6-di-tert-butyl-2-hydroxycyclohexa-2,5-dien-1-one dianion)) are synthesized by the reactions of magnesium and nickel acetates with the ditopic redox-active ligand of the hydroxy-para-iminoquinone type in a DMF solution. The molecular and crystal structures of the synthesized compounds are determined by X-ray diffraction analysis (CIF files CCDC nos. 2045665 (I) and 2045666 (II·3DMF)). The thermal stability is studied by thermogravimetry. The redox-active character of the organic bridging ligand in the dimeric complexes $${\text{M}}{{{\text{g}}}_{{\text{2}}}}{\text{L}}_{2}^{2}$$·4DMF and $${\text{N}}{{{\text{i}}}_{{\text{2}}}}{\text{L}}_{2}^{2}$$·4DMF is confirmed by the data of solid-phase electrochemistry.

Digermylene oxide was prepared by hydrolysis of O,N-heterocyclic germylene, and its reactivity was examined in situ. Its treatment with N-heterocyclic carbene produces dianionic digermylene oxide while its reaction with vanadocene gives the dinuclear derivative, a rare example of a compound containing the vanadium–germylene coordination bond. The product structures were confirmed by X-ray diffraction analysis.

In the present work, the combustion energies of 2,5-dihydroxy-3,6-di-tert-butyl-p-benzoquinone and 4-methoxy-3,6-di-tert-butyl-o-benzoquinone were determined by static-bomb combustion calorimetry i...

Two synthetic approaches were developed for the preparation of 3,6-di-tert-butyl-2-hydroxy-4-pyridinylphenolate (LH), a new zwitter-ionic redox-active diolate type ligand. Two heteroligand five-coordinate tin(IV) derivatives were prepared with this ligand: 3,6-di-tert-butyl-2-oxy-4-pyridinylphenolato(triphenyl)tin(IV) (I) and 3,6-di-tert-butyl-2-oxy-4-pyridinylphenolato(diphenyl)chlorotin(IV) (II). The molecular structures of the ligand LH ∙ 0.5Py and complex I ∙ CH3CN were determined by X-ray diffraction (CIF files CCDC nos. 1974166 (LH), 1974165 (I)). It was shown that the ligand LH and the tin(IV) compound exhibit solvatochromism, which consists in a considerable blue shift with increasing solvent polarity.

New stable O,N-heterocyclic stannylene 1 bearing tetradentate 4,6-di-tert-butyl-N-(PhC(CH3)N(CH2)3NMe2)-o-amidophenolate ligand (RAP) has been synthesized and characterized. Complex 1 is monomeric in solid state and in solution due to intramolecular N → Sn donor–acceptor interactions. Compound 1 was found to be inert towards one-electron oxidants, air and moisture. Oxidative addition of the 3,6-di-tert-butyl-o-benzoquinone (3,6-Q) to stannylene 1 has yielded the new octahedral catecholate Sn(IV) compound RAPSn(3,6-di-tert-butylcatecholate) 2. Reaction of 1 with Lewis acid such as Fe2(CO)9 results in the formation of stable adduct 3 – RAPSn-Fe(CO)4. The molecular structures of obtained compounds were determined by single crystal X-ray diffraction.

Cobalt(iii) complexes with different 4,6-di-tert-butyl-N-(aryl)-o-iminobenzosemiquinone ligands were synthesized. Four-, five-, or six-coordinate cobalt(iii) derivatives can be prepared using various starting metal compounds (CoCl2•6H2O, (acac)2Co•2H2O) and varying substituents in the N-aryl moiety of the organic ligand. The structures of five synthesized compounds were determined by X-ray diffraction. The electronic and spin states of the five-coordinate cobalt(iii) complexes were studied in detail by DFT quantum chemical calculations.

Two heteroleptic NiII complexes combined the redox-active catecholate and 2,2′- bipyridine ligand platforms were synthesized to observe a photoinduced intramolecular ligand-to-ligand charge transfer (LL’CT, HOMOcatecholate → LUMOα-diimine). A molecular design of compound [NiII(3,6-Cat)(bipy)]∙CH3CN (1) on the base of bulky 3,6-di-tert-butyl-o-benzoquinone (3,6-DTBQ) was an annelation of the ligand with an electron donor glycol fragment, producing derivative [NiII(3,6-Catgly)(bipy)]∙CH2Cl2 (2), in order to influence the energy of LL’CT transition. A substantial longwave shift of the absorption peak was observed in the UV-Vis-NIR spectra of 2 compared with those in 1. In addition, the studied NiII derivatives demonstrated a pronounced negative solvatochromism, which was established using a broad set of solvents. The molecular geometry of both compounds can be ascribed as an insignificantly distorted square-planar type, and the π–π intermolecular stacking of the neighboring α-diimines is realized in a crystal packing. There is a lamellar crystal structure for complex 1, whereas the perpendicular T-motifs with the inter-stacks attractive π–π interactions form the packing of complex 2. The redox-active nature of ligand systems was clearly shown through the electrochemical study: a quasi-reversible one-electron reduction of 2,2′-bipyridine and two reversible successive one-electron oxidative conversations (“catecholate dianion—o-benzosemiquinonato radical anion—neutral o-benzoquinone”) were detected.

New monomeric O,N-heterocyclic germylene AdAPGe (1) based on 4,6-di-tert-butyl-N-adamantyl-o-aminophenol is synthesized and structurally characterized. Germylene 1 in the crystal demonstrates weak Ge⋯Ge interactions between adjacent molecules. The redox activity of compound 1 and its known analogs PhAPGe (2), t-BuAPGe (3) was studied using cyclic voltammetry and chemical oxidation by the phenoxy radical. Germylene 1, as opposed to 2 and 3, was found to form a relatively stable o-iminosemiquinonate moiety during oxidation. The last one was detected by EPR spectroscopy in toluene solution at low temperatures. The catalytic activity of germylenes 1–3 and dippAPGe (4) (dipp – 2,6-di-isopropylphenyl) towards hydroboration and cyanosilylation of benzaldehyde was examined. Germylene 1 demonstrates the most potent promoter properties for such reactions, which allowed cyanosilylation and hydroboration of aldehyde under mild conditions with excellent conversion quickly. Theoretical studies were performed to establish the mechanism, and different reaction routes were examined. According to the DFT (B3LYP/def2-SVP) calculation, cyanosilylation and hydroboration processes are initiated by coordinating TMSCN or HBpin with the catalyst. The followed reaction with aldehyde leads to the resulting products.

The new pentacoordinated chloro-bis-o-iminosemiquinonato complexes (imSQOMe)2MnIIICl (1) and (imSQOMe)2FeIIICl (2) based on 4,6-di-tert-butyl-N-(2-methoxyphenyl)-o-aminophenol were synthesized. As established by X-ray diffraction analysis the compounds 1 and 2 are structural analogues. No additional coordination of methoxy groups to metal center was found in the case of both investigated complexes. Electrochemical oxidation leads to the bonding of functional –OMe group with iron(III) ion in compound 2. Triplet ground spin state (S = 1) for 1 is realized due to a prevailing strong antiferromagnetic interaction between unpaired electrons of high-spin manganese(III) (d4, SMn = 2) and two o-iminosemiquinonato radicals (SR = 1/2). The temperature dependence of effective magnetic moment (μeff) indicates the doublet ground spin state (S = 1/2) and thermally occupied quartet spin state (S = 3/2) for complex 2. The performed DFT calculations demonstrate that the magnetic properties of 2 are conditioned by the strong antiferromagnetic exchange of Fe-imSQ type as well as the transformation of iron(III) ion from high-spin configuration to intermediate-spin state (SFe = 5/2 → S = 3/2) with a decrease in temperature as a result of spin-crossover. The latter phenomenon was proved by Mossbauer spectroscopy. At the same time the intermediate-spin manganese(III) isomer (i.s.MnIII, SMn = 1) is destabilized by more than 20 kcal/mol, and therefore spin-crossover in compound 1 is impossible.

New bis-o-iminobenzosemiquinonato derivatives of nickel (imSQC(O)Ph)2Ni (1) and (imSQPh)2Ni (2) that are ligated by functionalized o-aminophenols H2LC(O)Ph and H2LPh, respectively, were synthesized. Complex 1 is distinguished by a square-planar geometry of N2O2 coordination core while the bis-o-iminobenzosemiquinonate 2 demonstrates a slight deviation from planarity. Both complexes 1 and 2 show the strong antiferromagnetic interactions between radical anion ligands as established by variable-temperature magnetic susceptibility measurements. Electrochemical behavior of 1 and 2 is represented by ligand-centered redox transitions: two quasi-reversible one-electron reduction processes and two-electron oxidation one. The salient peculiarity of cyclic voltammogram for 1 is an increase in number of redox waves owing to the quasi-reversible reduction of benzophenone fragments in ligands. Six-coordinated NiII derivative [(imQC(O)Ph)2Ni(H2O)2](OSO2CF3)2 (3) with two o-iminobenzoquinones as well as four-coordinated NiII species [(imSQC(O)Ph)Ni(APC(O)Ph)]CoCp2∙2.1toluene (4∙2.1toluene) including radical-anion and o-amidophenolate forms of redox-active ligand were prepared as products of oxidation and reduction of the initial 1, respectively, in accordance with the CV data. The value of effective magnetic moment (μeff) unambiguously indicates the high-spin state for nodal NiII ion in 3. Low-temperature EPR signal demonstrates rather the ligand-centered nature of unpaired electron in 4 that corresponds to the total doublet ground spin state (S = 1/2) for compound and diamagnetic low-spin (SNi = 0) configuration of the metal center.

The tetracoordinated square-planar CoIII complex (imSQC(O)Ph)CoIII(APC(O)Ph) (1) bearing a radical anion and the closed-shell o-amidophenolate forms of the functionalized o-aminophenol H2LC(O)Ph were synthesized. The intermediate spin state (SCo = 1) CoIII center was found for compound 1. The cyclic voltammogram of derivative 1 contains two oxidative processes and one reductive redox process as well as an additional multi-electron wave at high negative potentials above −2 V, which can involve both the ligand and metal center. One-electron oxidation of 1 by silver triflate produces the [(imSQC(O)Ph)CoII(imQC(O)Ph)]OTf·2toluene (2) derivative with the trigonal prismatic coordination environment of the metal arising from the additional coordination of –C(O)Ph hemilabile groups. This is a first example of a trigonal prismatic coordination polyhedron in cobalt-based complexes featuring o-iminobenzoquinone ligands. The trigonal prismatic geometry achieved by the unique flexibility of the ligand allows metal-to-ligand redox-induced electron transfer (RIET). Chemical oxidation of complex 1 promotes the reduction of CoIII to CoII in compound 2 due to the redox-active nature of o-iminobenzoquinonate ligands. Remarkably, this is the first example of RIET in cobalt-based derivatives with this type of ligand. The oxidative states of the ligands and cobalt ion in both complexes were unequivocally established according to the X-ray data collection by using the utility of “metric oxidation state” (MOS). The spin states of the metal centers were unambiguously determined by density functional theory. The strong antiferromagnetic exchange via metal–ligand interactions is dominant in compounds 1 and 2, giving the doublet (S = 1/2) and triplet (S = 1) ground spin state, respectively.

Optimal conditions for investigations of thermodynamic properties have been determined experimentally by the method of combustion calorimetry for compounds of the o-benzoquinone series. In the present work, the energies of combustion in oxygen were measured at T = 298.15 K by static bomb combustion calorimetry for 3,5-di-tert-butyl-o-benzoquinone and 3,6-di-tert-butyl-o-benzoquinone. The experimental values have been used to calculate the standard (р° = 0.1 MPa) molar enthalpy of combustion ΔcHom and formation ΔfHom for the examined compounds in the crystalline phase.

Nickel(II) and copper(II) bis-o-iminosemiquinone complexes were synthesized based on 4,6-di-tert-butyl-N-(2-methoxyphenyl)-o-aminophenol (Ni(imSQOMe)2 (1) and Cu(imSQOMe)2 (2)), and methyl 2-(3,5-di-tert-butyl-2-hydroxyphenylamino)benzoate (Ni(imSQC(O)OMe)2 (3) and Cu(imSQC(O)OMe)2 (4)). The molecular structures of these complexes were established by X-ray diffraction. The metal atoms in complexes 1, 3, and 4 are in square-planar coordination, whereas the square-planar coordination of the copper atom in complex 2 is substantially distorted, as evidenced by the angle between the planes of the o-iminosemiquinone ligands (22.7°). Complexes 1 and 3 are diamagnetic. The absence of line broadening in the 1H NMR spectra of these compounds at 293 K attests to the singlet spin state. The temperature dependences of the effective magnetic moment for complexes 2 and 4 are indicative of their doublet ground spin state (S = 1/2) and the thermally populated quartet state (S = 3/2). The presence of an unpaired electron on the metal center in the ground spin state for compounds 2 and 4 was confirmed by EPR spectroscopy.