V. I. Nevodchikov

An EPR investigation of the thermodynamics and kinetics of a reversible intramolecular metal-ligand electron transfer in rhodium complexes

Abstract The temperature dependence of a reversible intramolecular metal-ligand electron transfer for a series of semiquinone-rhodium(I) complexes in equilibrium with their catecholate-rhodium(II) isomers has been investigated by EPR spectroscopy, in a wide range of solvents. Both the thermodynamics and kinetics of this process are reported, along with a discussion of the role of the solvent and of the electronic and molecular structure of the complexes.

ESR investigation of the substitution reactions in rhodium(I) complexes with spin-labeled ligands

Abstract A number of new spin-labelled Rh I complexes containing both the 3,6-ditert-butyl- o -benzosemiquinone (3,6-SQ) fragment and n- and π-donor ligands have been prepared. The tetracoordinate derivatives of the composition L 2 Rh-(3,6-SQ), where L  CO, P(OPh) 3 , L  1/2 1,5-COD and the pentacoordinate complex (PPh 3 ) 2 Rh(3,6-SQ)(CO) were isolated in individual state, the formation of other rhodium compounds was registered by ESR spectroscopy. The presence of an o -benzosemiquinolate ligand in the molecule with the unpaired electron located essentially on this fragment does not significantly influence on the reactivity of the metal ion in most cases; the n- and π-donor ligands exchange reactions studied by ESR confirm this fact. (PPh 3 ) 2 Rh(3,6-SQ) has an abnormal distribution of spin density of the unpaired electron in the molecule, mostly located on the metal atom, this derivative bearing a close analogy to the rhodium(II) ( d 7 ) complexes.

Reaction on the addition of some organometallic compounds to 3,6-di-tert-butyl-o-benzoquinone: new o-quinones

Abstract 3,6-Di- tert -butyl- o -benzosemiquinone is very convenient for the investigation of the interaction between organometallic R 2 M and carbonyl compounds. The reaction of organometallic compounds R 2 M with o-quinone, where R = Ph, Me, Et, Pr, 1 Pr or t Bu and M ≡ Zn, Cd, and Al, proceeds in two ways: (1) the single-electron oxidation of the organometallic compound by o -quinone, resulting in derivatives of alkyl(phenyl)oxyphenols; (2) the polar 1,2 and 1,4 addition of organometallic molecules to o -quinone as to conjugated ketone, which produces hydroxycyclohexadienones after hydrolysis. The relationship between these two is defined by the nature of the organometallic compound. The products of polar addition are rearranged easily in different ways, finally resulting in new o-quinones: 3-(R)-6- tert -butyl- o -benzoquinones and 3,6-di- tert -butyl-4-(R)- o -benzoquinones where R ≡ alkyl.

SYNTHESIS AND ESR-CHARACTERIZATION OF RADICAL ANION COMPLEXES OF LANTHANUM. X-RAY CRYSTAL STRUCTURE OF THE MIXED BIPY, BIPY-1 COMPLEX OF LANTHANUM(III ) LAI2(BIPY)(BIPY)(DME) : EVIDENCE FOR AN INTER-LIGAND CHARGE TRANSFER

Abstract The reaction of [LaI2(THF)3]2[C10H8] with excess of 2,2′-bipyridine (bipy) in DME or THF results in the formation of mixed radical anion ligand/neutral ligand complexes [LaI2(bipy(0))(bipy(−1))(L)n (DME)(1) and (THF)2 (2), whereas a reactant reactant ratio of 1:2 generates the mono-bipy complex [LaI2(bipy)(DME)2] (3). ESR and IR spectroscopic data show a fast shuttle electron transfer (SIET) between the neutral and the radical anion of bipy in1 and2. The X-ray structure shows1 to be monoclinic, space group P2/c, Z = 2, with a = 10.0320(12), b = 9.7510(11), c = 16.296(8)A˚, β = 90.43(2)°. The structure was refined toR = 0.0360 for 2562 observed reflections (1 > 2σ(1)). It shows the lanthanum atom eight-coordinated by two iodine, two oxygen and four nitrogen atoms.

Reduction of vanadium(II) to vanadium(O) by naphthaleneytterbium: synthesis and X-ray crystal structure of the two-dimensional multidecker complex [(η5-C5H5)V(μ-η6:η2-C10H8)Yb(THF)-(η5-C5H5)]n

Abstract The reaction of (C 5 H 5 ) 2 V with an excess of (C 10 H 8 )Yb(THF) 2 in tetrahydrofuran at room temperature generated the polymeric two-dimensional multidecker complex [(η 5 -C 5 H 5 )V(μ-η 6 :η 2 -C 10 H 8 )Yb(THF)(η 5 -C 5 H 5 )] n , ( 1 ), which is also obtained from the reaction of YbI 2 (THF) 2 with an equimolar mixture of KC 5 H 5 and K[(C 5 H 5 )V(C 10 H 8 )] in tetrahydrofuran. The new compound was characterized by elemental analyses, magnetic susceptibility, IR and ESR spectra. The X-ray structural analysis of 1 showed the compound to be monoclinic, space group C2/c , Z =8, with a =19.07(5), b =13.61(3), c =17.40(3) A, β=91.9(2)° and D calc =1.84 g cm −3 . The structure was solved from 2482 observed reflections with F o > 4σ(F o ) and refined to a final R value of 0.047. The molecular structure consists of infinite zig-zag chains formed by (η 5 -C 5 H 5 )Yb moieties and with one (η 5 -C 5 H 5 )V(η 6 -C 10 H 8 ) unit coordinated η 2 via the naphthalene to each Yb atom.

Reduction of 2,5-di-tert-butylcyclopentadienone and pyridine with thulium diiodide. Structures of the complexes TmI2(THF)2[η5-But2C5H2O]TmI2(THF)3 and [TmI2(C5H5N)4]2(μ2-N2C10H10)

Reduction of 2,5-di-tert-butylcyclopentadienone with two equivalents of thulium diiodide in tetrahydrofuran afforded the binuclear thulium(iii) complex with the cyclopentadienyl oxide ligand, viz., TmI2(THF)2[η5-But2C5H2O]TmI2(THF)3 (1). Shielding of the carbonyl carbon atom with two tert-butyl substituents prevents pinacolization of the ketyl radical anions that formed upon one-electron reduction of cyclopentadienone. The reaction of thulium diiodide with an excess of pyridine in tetrahydrofuran gave a product of reductive coupling of two pyridine radical anions, viz., [TmI2(C5H5N)4]2(μ2-N2C10H10) (2). The structures of complexes 1 and 2 were established by single-crystal X-ray diffraction analysis.

New tetrahydro-2,3-naphthoquinones

ConclusionsA preparation is reported for o-benzoquinones and catechols containing tertiary alkyl substituents in the ortho position.

Influence of o-benzoquinone nature on initiation of radical polymerization by the o-benzoquinone—tert-amine system

Abstracto-Benzoquinones initiate radical polymerization of methacrylates under visible light irradiation in the presence of tertiary amines. Spectral sensitivity of the initiating system coincides with absorption bands of o-benzoquinone attributed to the S(π→π*) (λmax ≈ 400 nm) and S(n→π*) (λmax ≈ 600 nm) transitions. The amine radicals (Am·) initiating polymerization are generated by the photoreduction of Q in the presence of AmH from the triplet radical pair 3(QH·, Am·). The yield of Am· depends on the difference between the volumes of substituents in the 3 and 6 positions of the quinoid ring and is maximal for symmetrically substituted o-benzoquinones. For a series of derivatives of symmetrical 3,6-di-tert-butyl-o-benzoquinone, the rate of photopolymerization of α,ω-bis(methacryloyloxyethyleneoxycarbonyloxy)ethyleneoxyethylene (OCM-2) in the presence of N,N-dimethylaniline is determined by the free energy (ΔGe) of electron transfer from the amine to photoexcited o-benzoquinone. The ΔGe value includes the energies of oxidation of the amines and reduction of the o-quinones and the energy of the 0→0 transition of the triplet excited state of o-benzoquinones, which are equal to their redox potentials. The photopolymerization rate is maximal for ΔGe ≈ 0.

C60-fullerenides of ytterbium and lutetium

Abstract Cp#2Yb (Cp#=C5H4(CH2)2NMe2) has been obtained by reaction of YbI2(THF)2 with 2 equiv. of C5H4(CH2CH2NMe2)K in THF. The X-ray structure analysis shows a bent structure with intramolecular coordination of both nitrogen atoms to ytterbium. The reaction of C60-fullerene with Cp#2Yb leads to the formation of the fullerenide derivative [Cp#2Yb]2C60, which shows an ESR signal in the solid state and in THF solution at room temperature (solid: ΔH = 50 G, g = 1.9992; solution: ΔH = 10 G, g = 2.0001) and a magnetic moment of 3.6 BM. The lutetium fullerenides CpLu(C60)(DME) (3) and Cp∗Lu(C60)(DME)(C6H5CH3) (4), ( Cp = η 5 − C 5 H 5 , Cp ∗ = η 5 − C 5 Me 5 ), were obtained by reaction of C60 with CpLu(C10H8) (DME) and Cp∗Lu(C10H8) (DME) in toluene. Both complexes are paramagnetic (μeff = 1.4 and 0.9 BM) and exhibit temperature-dependent ESR signals (293 K: g = 1.992 and 2.0002 respectively).

New sterically hindered di-o-quinones of the biphenyl series

New di-o-quinones of the biphenyl series, namely, 2,2′-dialkyl-5,5′-di-tert-butylbiphenyl-3,4,3′,4′-diquinones, were synthesized. Their structures were established by IR and NMR spectroscopy. The molecular structure of 2,2′-dimethyl-5,5′-di-tert-butylbiphenyl-3,4,3′,4′-diquinone was established by X-ray structural analysis. The structure is characterized by orthogonal (the torsion angle is 82.9°) mutual arrangement ofo-benzoquinone fragments. ESR studies demonstrated that chemical reduction of diquinone proceeds in four oneelectron stages to form paramagnetic mono- and trianions as intermediates. Quinopyrocatechols, which are intermediates in the synthesis of di-o-quinones, were isolated and characterized.