EPR Spectra Of Solutions Research Articles

Experimental confirmation of the formation of collective modes of the magnetization motion of paramagnetic particles in dilute solutions due to spin exchange

Experimental confirmation of the manifestations of new spin exchange paradigm in EPR spectra of 14N nitroxide radical solutions is presented. It was shown that in the region of relatively low concentrations of radicals, the two side components of the spectrum have a mixed shape (the sum of the absorptive line and dispersive line). The dispersion contributions in these two lines have opposite signs. As the concentration of radicals increases, the contribution of dispersion passes through an extremum and in the region of maximum contribution of dispersion, the contribution of absorption to these two lines changes sign. In the region of high concentrations of radicals, when one homogeneously broadened line is practically observed, it turns out that these side components have resonant frequencies that do not coincide with the frequency of the center of gravity of the spectrum.

Vanadium-Containing Keggin-Type Polyoxometalates, [VM12O40]3- and [VVM11O40]4- (M = Mo, W): Structural Characterization and Voltammetric, NMR, and EPR Studies Related to Electrochemical Reduction at Framework and Central Vanadium Sites.

Vanadium is accommodated in both the framework (VoutV) and central positions (VinV) in the Keggin-type polyoxometalates (POMs) [VinVVoutVM11O40]4- (M = Mo, W; VinVVoutVM11) and in the central position in [VinVM12O40]3- (VinVM12). The structures of the VinVVoutVM11 class have been determined by X-ray crystallography and compared to those of VinVM12 reported previously. A major feature of interest with POMs is their capacity for very extensive reduction, particularly when protonation accompanies the electron transfer step. With VinVVoutVM11 and VinVM12 POMs, knowledge as to whether reduction occurs at V or M sites and the concomitant dependence on acidity has been obtained. Frozen solution EPR spectra obtained following bulk electrolysis showed that the one-electron reduction of VinVMo12 occurs at the molybdenum framework site to give VinVMoVMo11. In contrast, EPR spectra of one-electron reduced VinVW12 at <30 K are consistent with the electron being accommodated on the central V atom in a tetrahedral environment to give VinIVW12. In the case of VinVVoutVM11, the initial reduction occurs at the framework VoutV site to give VinVVoutIVM11. The second electron is delocalized over the Mo framework in two-electron reduced VinVVoutIVMoVMo10, whereas it is accommodated on the central V site in VinIVVoutIVW11. The distance between VinIV and VoutIV in VinIVVoutIVW11 estimated as 3.5 ± 0.2 Å from analysis of the EPR spectrum is consistent with that obtained in VinVVoutVW11 from crystallographic data. Simulations of the cyclic voltammograms as a function of CF3SO3H acid concentration for the initial reduction processes provide excellent agreement with experimental data obtained in acetonitrile (0.10 M [n-Bu4N][PF6]) and allowed acid association constants to be estimated and compared with the literature values available for [XVoutVM11O40]n- (X = S (n = 3), P and As (n= 4); M = Mo, W). The interpretation of the voltammetric data is supported by 51V NMR measurements on the oxidized VV forms of the POMs.

Resolving a Half-Century-Long Controversy between (Magneto)optical and EPR Spectra of Single-Electron-Reduced [PcFe]-, [PcFeL]-, and [PcFeX]2- Complexes: Story of a Double Flip.

The reduction of iron(II) phthalocyanine (Pc(2-)FeII) or its bisaxially coordinated complexes results in the formation of the purple/red [PcFe]-, [PcFeL]-, and [PcFeX]2- (L is neutral and X is anionic ligand) species. The X-ray structure of the [K(DME)4][PcFe] complex exhibits a square-planar [PcFe]- anion. 1H NMR spectra of the reduced species have one or two phthalocyanine broad peaks between 15 and 17 ppm. Solution magnetic moments are consistent with the presence of a single unpaired electron. A solid-state Mössbauer spectrum of [K(DME)4][PcFe] is consistent with an early report [Taube, R. Pure Appl. Chem.1974, 38, 427-438]. The solid-state EPR spectrum of the [PcFe]- anion is close to that recorded by Konarev et al. [ Dalton Trans.2012, 41, 13841-13847]. Solution EPR spectra of reduced species have axial symmetry (g⊥ ∼ 2.08-2.17 and g|| ∼ 1.95-1.96) and correlate well with spectra reported by Lever and Wilshire in 1978 [ Inorg. Chem.1978, 17, 1145-1151]. The UV-vis spectra of pentacoordinated [PcFeL]- and [PcFeX]2- anions consist of the characteristic bands around 810, 690, and 515 nm. These bands correlate well with the set of MCD pseudo A-terms and resemble transitions in the [Pc(3-)M]- and [Pc(3-)ML]- compounds. The UV-vis and MCD spectra of [PcFeL]- and [PcFeX]2- complexes are in stark contrast to the crystallographically characterized reference [Pc(2-)CoI]- anion, which is EPR silent, has a regular diamagnetic 1H NMR spectrum, and has an intense Q-band at 699 nm, which correlates well with the strong MCD A-term. The DFT and TDDFT calculations are suggestive of the iron(II) center in a (dxy)2(dxz,yz)3(dz2)1 (s = 1) electronic configuration that is antiferromagnetically coupled with the one-electron-reduced Pc(3-) ligand (i.e., [Pc(3-)FeII]-, [Pc(3-)FeIIL]-, and [Pc(3-)FeIIX]2-). The calculated EPR, Mössbauer, and UV-vis spectra of [PcFe]-, [PcFeL]-, and [PcFeX]2- complexes are in excellent agreement with the experimental data, thus resolving the controversy between axial s = 1/2 like EPR and Pc(3-)-like UV-vis spectra of these compounds.

Meso-Carbon Atom Nucleophilic Attack Susceptibility in the Sterically Strained Antiaromatic Bis-BODIPY Macrocycle and Extended Electron-Deficient BODIPY Precursor.

A sterically strained 32π-electron antiaromatic bis-BODIPY macrocycle in which two BODIPY fragments are linked by p-divinylbenzene groups was prepared and characterized. Unlike regular BODIPYs, the fluorescence in this macrocycle is quenched. The broad signals in the NMR spectra of the macrocycle were explained by the vibronic freedom of the p-divinylbenzene fragments. The possible diradicaloid nature of the macrocycle was excluded on the basis of variable-temperature EPR spectra in solution and in solid state, which is indicative of its closed-shell quinoidal structure. The meso-C-H bond in the macrocycle and its precursor BODIPY dialdehyde 3 forms a weak hydrogen bond with THF and is susceptible for the nucleophilic attack by organic amines and cyanide anion. The reaction products of such a nucleophilic attack have meso-sp3 carbon atoms and were characterized by NMR, mass spectrometry and, in one case, X-ray crystallography. Unlike the initial bis-BODIPY macrocycle, the adducts have strong fluorescence in the 400 nm region. The electronic structure and spectroscopic properties of new chromophores were probed by density functional theory (DFT) and time-dependent DFT (TDDFT) calculations and correlate well with the experimental data.

Template Synthesis, Spectroscopic Characterization and Antimicrobial Studies of Oxovanadium(IV) Tetraazamacrocyclic Complexes Derived from 1, 8-Diaminonaphthalene and Substituted β-Diketones

A new series of oxovanadium (IV) tetraazamacrocyclic complexes of type [VO(mac)]SO4, have been synthesized via in situ reaction of 1,8-diaminonaphthalene and substituted β-diketones (1-phenyl-3-(phenyl/4-chlorophenyl/4-hydroxyphenyl/4- methoxyphenyl/4-nitrophenyl)-diketone) in the presence of oxovanadium(IV) sulphate in methanol. Attempts to synthesize the corresponding metal free macrocyclic ligands did not prove successful. Complexes were characterized on the basis of elemental analyses, conductance measurements, magnetic properties and spectral (electronic, IR, and EPR) data. The magnetic moments of the complexes lie in the range 1.70-1.78BM suitable for complexes with one unpaired electron and paramagnetic nature. The infrared spectra indicates that the ligands coordinate through four aza nitrogen atoms to vanadium(IV). The fluid solution EPR spectra show an eight-line pattern typical for a mononuclear VO(IV) complexes. The spectral studies support a square pyramidal geometry for the oxovanadium(IV) complexes. The oxovanadium complexes were tested for their antibacterial activity against bacteria Bacillus subtilis, Staphylococcus aureus, Escherichia coli and antifungal activity against fungi Aspergillus niger and Candida albicans. The complexes show moderate to good activity as compared to standard antibiotic tetracycline and the antifungal drug fluconazole.

Mono Versus Dinuclear Vanadium(V) Complexes: Solvent Dependent Structural Versatility and Electro Syntheses of Mixed-Valence Oxovanadium(IV/V) Entities in Solution.

Two mononuclear oxidovanadium(V) complexes type of [VVO(L1)(OMe)(MeOH)] (1), [VVO(L2)(OMe)(MeOH)] (2) and two [V2O3]4+ core of μ-oxidodioxidodivanadium(V) complexes (L1)(O)VV–O–VV(O)(L1) (3) and (L2)(O)VV–O–VV(O)(L2) (4) and two complexes [VVO(L1)(8-Hq)] (5) and [VVO(L2)(8-Hq)] (6) incorporating 8-hydroxyquinoline (8-hq) as co-ligand have been reported where L1 [(E)-N′-(2-hydroxybenzylidene)cinnamohydrazide] and L2 [(2E,N′E)-N′-(2-hydroxybenzylidene)-3-(naphthalen-1-yl)acrylohydrazide] are the dianionic forms of the conjugated keto-imine functionalized substituted hydrazone ligands. The μ-oxidodioxidodivanadium complexes are generated upon switching the solvent from methanol to acetonitrile. The X-ray analysis showed octahedral geometry for the mononuclear complexes 1, 2 and 5 but oxido-bridged dinuclear complexes 3 and 4 formed penta-coordinated square-pyramidal geometry about metal atoms. Two mixed-valence species of type II, 3a and 4a, of general formulae (L)(O)VIV–O–VV(O)(L), are being generated upon constant potential electrolysis (CPE) of 3 and 4 respectively. Frozen solution EPR spectra have 13 hyperfine lines, revealing the unpaired electron is majorly localized on one of the two vanadium centres. All these complexes have been well characterized by physio-chemical techniques and the density functional theory (DFT) calculations were applied to obtain further insight into the electronic structure of this type of molecule. The oxidomethoxido complexes 1 and 2 were taken to investigate the catechol oxidase mimicking activity following the oxidation of 3,5-di-tert-butyl catechol (3,5-DTBC) to 3,5-di-tert-butyl benzoquinone (3,5-DTBQ).

A Sterically Hindered Derivative of 2,1,3-Benzotelluradiazole: A Way to the First Structurally Characterised Monomeric Tellurium-Nitrogen Radical Anion.

Interaction of the tetradentate redox-active 6,6'-[1,2-phenylenebis(azanediyl)]bis(2,4-di-tert-butylphenol) (H4 L) with TeCl4 leads to neutral diamagnetic compound TeL (1) in high yield. The molecule of 1 has a nearly planar TeN2 O2 fragment, which suggests the formulation of 1 as TeII L2- , in agreement with the results of DFT calculations and QTAIM and NBO analyses. Reduction of 1 with one equivalent of [CoCp2 ] leads to quantitative formation of the paramagnetic salt [CoCp2 ]+ [1].- , which was characterised by single-crystal XRD. The solution EPR spectrum of [CoCp2 ]+ [1].- at room temperature features a quintet due to splitting on two equivalent 14 N nuclei. Below 150 K it turns into a broad singlet line with two weak satellites due to the splitting on the 125 Te nucleus. Two-component relativistic DFT calculations perfectly reproduce the a(14 N) HFI constants and A∥ (125 Te) value responsible for the low-temperature satellite splitting. Calculations predict that the additional electron in 1.- is localised mainly on L, while the spin density is delocalised over the whole molecule with significant localisation on the Te atom (≥30 %). All these data suggest that 1.- can be regarded as the first example of a structurally characterised monomeric tellurium-nitrogen radical anion.

Interpretation of the Nature of the Mixed Form of Resonance Lines of the Electron Paramagnetic Resonance Spectrum in a New Paradigm of Spin Exchange: Abnormal "Resonance" of Non-Resonant Spins.

It is shown that the asymmetric shape of lines in the electron paramagnetic resonance spectra of paramagnetic particle solutions caused by spin exchange in bimolecular particle collisions is due to the fact that, in the experiment, under the conditions of slow spin exchange, an absorption signal of nominally resonant spins and a "resonance" dispersion signal of nominally nonresonant spins with an abnormal phase of their quantum coherence are registered simultaneously. The mixing of these two signals is different for different resonance lines in the observed spectrum. A phenomenological model is proposed that can be used to find the phase of the "resonance" signal of non-resonant spins. The obtained results can be transferred to other systems to study the mechanisms of spectral diffusion, for example, chemical exchange by magnetic resonance methods.

Synthesis, structural and DFT interpretation of a Schiff base assisted Mn(III) derivative

A mononuclear MnIII derivative, [Mn(L)(SCN)(H2O)] (1) [where H2L = N,N′-bis(salicyaldehydene)-1,3-diaminopropan-2-ol] has been synthesized and systematically characterized. In 1, the central MnIII ion is linked to the NNOO donor atoms of the potentially binucleating precursor L2− and additionally coordinates one water molecule and the pseudohalide SCN−, thus yielding a distorted octahedral geometry. The UV–Vis spectra of 1 shows an intense band corresponding to a 5T2g − 5Eg transition at 545 nm; thus presuming the octahedral geometry around the Mn ion. The solid and solution EPR spectra of 1 have been simulated with an HP 53150A microwave frequency counter which has confirmed the +3 oxidation state of the metal ion. We have also conducted a DFT computational study which has confirmed that compound 1 self assembles into a two-dimensional network via O‒H⋯O, π–π, and unconventional C‒H … π(SCN) interactions involving the π-system of the thiocyanate. The room temperature magnetic susceptibility of compound 1 has yielded an effective magnetic moment (μeff) value of 4.96 B.M.

Potentiometric and spectroscopic studies of the complex formation between copper(II) and Gly-Leu-Phe or Sar-Leu-Phe tripeptides

Metal complexes involving a chelation between copper(II) and the tripeptides, sarcosyl-l-leucyl-phenylalanine (Sar-Leu-Phe) and glycyl-l-leucyl-phenylalanine (Gly-Leu-Phe), have been identified as potential therapeutic drugs for rheumatoid arthritis. However, in solution the speciation and exact coordination of these tripeptides to copper(II) is unknown and as a result this was investigated using glass electrode potentiometry, ultraviolet–visible spectrophotometry, infrared spectroscopy and 1H NMR and CW EPR spectroscopies. The same basicity was found for the terminal amine of both tripeptides, but the methyl group on Sar decreased the copper(II) complex stability constants by 0.38–1.67 log units. Four species: ML, MLH-1, ML2H-1 and MLH-2 were found for both tripeptides in the pH range 2–11. The dissociated protons are from the amide groups. Room temperature and frozen solution EPR spectra support the existence of MLH-1, MLH-2 and ML2H-1 (a large excess was required for the ML2H-1 species). The two species, MLH-2 and ML2H-1, have a CuN3O equatorial chromophore, which therefore produce similar room temperature EPR spectra, with similar magnetic parameters. Frozen solution EPR spectra revealed different anisotropic magnetic parameters in axial symmetry, thus suggesting the presence of two different geometries. The lower g|| and the higher A|| values indicate that the geometry of the MLH-2 complexes tends more towards square planar, instead of the usual expected tetragonally elongated octahedral geometry. Copper(II) could coordinate to each tripeptide via an amine-N, two amide-Ns, two carbonyl-Os and a carboxyl-O. The proposed coordination modes were validated using quantum mechanical calculations.

Crystal structure and EPR spectra of (Bu4N)2[V(dmit)3

The tris-chelate complex of vanadium(IV) (Bu4N)2[V(dmit)3] is prepared from VCl3 and (Bu4N)2[Zn(dmit)2] (dmit = isotrithionedithiolate C3S5 2–) and characterized by single crystal XRD and mass spectrometry. The complex crystallizes in the space group Pna21 and has a distorted octahedral environment of vanadium. The complex is paramagnetic and gives a characteristic EPR spectrum in both solution and solid phase. The g factors and hyperfine interaction constants are determined.

A polynuclear and two dinuclear copper(II) Schiff base complexes: Synthesis, characterization, self-assembly, magnetic property and DFT study

Three elongated (4+1) square pyramidal copper(II) complexes, [Cu2(L)2(μ1,1-N3)2]·H2O (1), [Cu2(L)2(μ1,1-NCO)2] (2) and [Cu(L)(μ1,5-dca)]n (3) [HL=1((2-(methylamino)ethylimino)methyl)naphthalen-2-ol and dca=dicyanamide], have been synthesized using a tridentate N2O donor Schiff base ligand (HL) and characterized. Complexes 1 and 2 are centrosymmetric dimers in which copper(II) centres are connected by asymmetric double end on pseudohalide bridges. Complex 3 features a 1D zigzag chain in which copper(II) centres are connected by end to end dca bridges. Variable temperature (2–300K) magnetic susceptibility measurements indicate the presence of antiferromagnetic exchange coupling between copper(II) centres in complexes 1 (J=−2.313cm−1) and 3 (J=−0.344cm−1), whereas complex 2 shows ferromagnetic exchange coupling between copper(II) centres (J=0.513cm−1). DFT calculations also corroborate the data. The fluid solution EPR spectra recorded at 293K are typical of copper(II) species. Significant supramolecular interactions are explored using high level DFT calculations (BP86-D3/def2-TZVP) and characterized by Bader’s theory of “atoms-in-molecules”.

Binuclear copper(II) complexes based on acyldihydrazones of imino-, oxo-, and thiodiacetic acids and 2-hydroxyacetophenone

The results of studying the binuclear copper(II) complexes with acyldihydrazones of amino-, oxo-, and thiodiacetic acids and 2-hydroxyacetophenone were described. The temperature variations of the EPR spectra of solutions with a seven-line hyperfine structure indicating an exchange character of interaction between unpaired electrons and the two equivalent nuclei of copper atoms were studied.

An EPR and voltammetric study of simple and mixed copper(II) complexes with l- or d-glutamate and l-arginate in aqueous solution

EPR CW measurements at room and low temperature have been carried out on the systems containing 63copper(II) nitrate, l-Arg and l-or d-Glu or dl- racemic mixtures of both amino acids in aqueous solution or in mixed solvents. For these copper(II) bis and ternary complexes, RT EPR spectra revealed that mixtures of cis and trans conformers are really present in aqueous solution. In most cases, for these systems, the cis conformer is more abundant at the lowest temperature (275K). Increasing gradually the temperature from 275K to 305K the cis/trans ratio tends towards the trans conformer. This latter is less abundant in the case of [Cu(l-Glu)2], because of the possible interaction of negative side chain carboxylate anions with the water molecules axially bound, and for [Cu(l-Arg)(d-Glu)], probably owing to a possible electrostatic interaction between the two opposite charged side chains which are on the same side of the CuN2O2 coordination plane. Low temperature frozen solution EPR spectra run on these systems show the presence of only one absorbing specie thus giving reason to the stabilization of one conformer over the other, in particular, the cis conformer. There are no evident differences in the anisotropic Spin Hamiltonian parameters derived from the frozen EPR solution spectra of these systems, which are characterized by a tetragonally elongated octahedral stereochemistry. SWV measurements confirmed the EPR spectroscopic data, actually giving rise to the observation of slightly asymmetrical peaks, which revealed two near formal redox potentials assigned to the two conformers.

Structural, Magnetic and DFT studies on a Charge-Transfer Salt of a Tetrathiafulvalenepyridyl-(1,5-diisopropyl) verdazyl Diradical Cation.

A new tetrathiafulvalene (TTF) donor covalently appended with a 1,5-diisopropylverdazyl radical through a cross-conjugated pyridyl linker (3) has been prepared and characterised. Reaction of 3 with tetracyanoquinonedimethane (TCNQ) afforded the 2:1 charge-transfer complex (3)2 ⋅TCNQ (4), in which the IR and structural data are consistent with 0.25 e- charge transfer from the TTF donor (D) to the TCNQ acceptor (A). The TTF and TCNQ molecules adopt a mixed-stack D⋅⋅⋅D⋅⋅⋅A arrangement that does not facilitate conduction. A solution EPR spectrum of 4 comprises a broad featureless singlet, which is consistent with the presence of a TCNQ radical anion. Theoretical studies were performed to probe the exchange interactions within selected fragments of 4 with and without charge transfer. In the absence of charge transfer, DFT calculations reveal weak antiferromagnetic exchange between verdazyl radicals within the (3)2 monoradical unit. However, partial oxidation of the dimer (3)2 to the diradical cation leads to an S= ground state, in which the verdazyl radical spins are now aligned co-parallel as a consequence of antiferromagnetic exchange to the additional delocalised TTF-based spin containing unit. The magnetic properties of 4 are consistent with a net S= spin state per formula unit with dominant antiferromagnetic interactions between spin-bearing building blocks.

Conformational Properties of Seven Toac-Labeled Angiotensin I Analogues Correlate with Their Muscle Contraction Activity and Their Ability to Act as ACE Substrates.

Conformational properties of the angiotensin II precursor, angiotensin I (AngI) and analogues containing the paramagnetic amino acid TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) at positions 0, 1, 3, 5, 8, 9, and 10, were examined by EPR, CD, and fluorescence. The conformational data were correlated to their activity in muscle contraction experiments and to their properties as substrates of the angiotensin I-converting enzyme (ACE). Biological activity studies indicated that TOAC0-AngI and TOAC1-AngI maintained partial potency in guinea pig ileum and rat uterus. Kinetic parameters revealed that only derivatives labeled closer to the N-terminus (positions 0, 1, 3, and 5) were hydrolyzed by ACE, indicating that peptides bearing the TOAC moiety far from the ACE cleavage site (Phe8-His9 peptide bond) were susceptible to hydrolysis, albeit less effectively than the parent compound. CD spectra indicated that AngI exhibited a flexible structure resulting from equilibrium between different conformers. While the conformation of N-terminally-labeled derivatives was similar to that of the native peptide, a greater propensity to acquire folded structures was observed for internally-labeled, as well as C-terminally labeled, analogues. These structures were stabilized in secondary structure-inducing agent, TFE. Different analogues gave rise to different β-turns. EPR spectra in aqueous solution also distinguished between N-terminally, internally-, and C-terminally labeled peptides, yielding narrower lines, indicative of greater mobility for the former. Interestingly, the spectra of peptides labeled at, or close, to the C-terminus, showed that the motion in this part of the peptides was intermediate between that of N-terminally and internally-labeled peptides, in agreement with the suggestion of turn formation provided by the CD spectra. Quenching of the Tyr4 fluorescence by the differently positioned TOAC residues corroborated the data obtained by the other spectroscopic techniques. Lastly, we demonstrated the feasibility of monitoring the progress of ACE-catalyzed hydrolysis of TOAC-labeled peptides by following time-dependent changes in their EPR spectra.

Synthesis, structure, and characterizations of a new antiferromagnetic manganese(II) dichloro-bridged 1-D polymer decorated by 5-amino-1-H-tetrazole

[Mn(5-ATZ)2Cl2]n (1) (5-ATZ – 5-amino-1-H-tetrazole) was synthesized from the reaction of 5-ATZ and manganese(II) chloride and isolated by solution evaporation at room temperature. 1 was characterized by elemental analysis, X-ray crystallography, infrared, and EPR spectroscopy as well as magnetic measurements. In the crystal structure, [Mn(5-ATZ)2Cl2] units are linked by double μ2-bridging chlorides to form 1-D chains parallel to the a-axis. The Mn sphere approximates to octahedral with the metal coordinated by four chlorides in the equatorial plane and two 5-ATZ molecules, bound through their ring nitrogens, in axial positions. The intramolecular N–H···Cl hydrogen bond between the 5-ATZ amino group and the adjacent coordinated Cl− stabilizes the chain. N–H···N hydrogen bonds between adjacent chains form a 3-D supramolecular framework. No hyperfine coupling to the Mn nuclei (I = 5/2) is observed in the powdered EPR spectrum of 1 at 77 K. The frozen solution EPR spectrum provides evidence of the mononuclearity of 1 in methanol. The magnetic properties have been analyzed using the Hamiltonian H = –J∑Si · Si+1 with J = −1.38(3) cm−1 and g = 2.00(1). A small value of the exchange parameter is typical for 1-D six-coordinate bis(μ2-chloro) Mn(II) polymers.

Synthesis and characterization of new chiral Cu(ii)-N4 complexes and their application in the asymmetric aza-Henry reaction.

Cu(ii) Schiff base complexes Cu(ii)- and Cu(ii)- based on 2-acetyl pyridine with both (1R,2R)-1,2-diaminocyclohexane and (1S,2S)-1,2-diaminocyclohexane were synthesized in a single step. Subsequent reduction of ligands and with NaBH4 followed by complexation with Cu(OTf)2 resulted in generation of two more additional chiral centers in complexes Cu(ii)- and Cu(ii)-. The ligands and their corresponding complexes were well characterized by several spectral techniques like (1)H-NMR, (13)C-NMR, LC-MS, CD, UV-Vis spectroscopy and microanalysis. The respective Cu(ii) complexes derived from ligands and were investigated using both the solution and solid state EPR spectra. The particular orientation of the reduced complex with Cu(OTf)2 was confirmed by the X-ray crystal structure of the corresponding complex. All the catalytic protocols were applied in the asymmetric aza-Henry reaction to evaluate the catalytic properties of the Cu(ii) complexes in the present study.

Steric Effects in Reactions of Decamethyltitanocene Hydride with Internal Alkynes, Conjugated Diynes, and Conjugated Dienes

Titanocene hydride [Cp*2TiH] (Cp* = η5-C5Me5) (1) readily inserts simple internal alkynes R1C≡CR2 into its Ti–H bond, yielding titanocene alkenyl Ti(III) compounds of two structural types. The less sterically congested products [Cp*2Ti(R1C═CHR2)] (2a–e) contain a σ1-bonded alkenyl group, whereas the products bearing at least one trimethylsilyl substituent and other bulky substituents (R1 = SiMe3; R2 = SiMe3, 4a; CMe3, 4b; and Ph, 4c) possess a remarkable Ti–H agostic bond of the σ1-bonded alkenyl group. This feature is consistent with solution EPR spectra of 4a–4c showing a doublet due to coupling of the hydrogen nucleus with the Ti(III) d1 electron. Compound 1 reacts with one molar equivalent of conjugated buta-1,3-diynes (RC≡C)2 to give η3-butenyne complexes (R = SiMe3, 5a; CMe3, 5b). The Ti(III) complexes 2a–2e and 5a and 5b were oxidatively chlorinated with PbCl2 to give Ti(IV) chloro-alkenyl complexes [Cp*2TiCl(R1C═CHR2)] 3a–3e and chloro-alkenynes 6a and 6b, respectively. 1H and 13C NMR spectra of 3a–3e and 6a and 6b revealed that these compounds form equilibria of two atropisomers differing by the anti- and syn-position of the chlorine and the alkenyl hydrogen atoms. Such atropisomers are denoted by appended (a) and (b), respectively. Compound 1 reacted with 1,3-butadiene to give a thermally stable π-bonded 1-methylallyl complex (7) and with penta-1,3-diene to give a thermally labile 1,3-dimethylallyl complex (8). In toluene-d8 solutions 7 dissociated at 80 °C and 8 at room temperature to give [Cp*Ti(C5Me4CH2)] and corresponding alkenes. Other methyl-substituted dienes, isoprene, 4-methylpenta-1,3-diene, and 2,3-dimethylbuta-1,3-diene, did not yield observable π-bonded allyl products; the dienes were, however, hydrogenated to olefins with concomitant formation of [Cp*Ti(C5Me4CH2)]. Compound 1 was shown to catalyze the hydrogenation of the alkynes and dienes to olefins and ultimately to alkanes under lower than atmospheric hydrogen pressure at room temperature. Single-crystal structures were determined for 3d(a), 3e(a), 4a–4c, 5a, 6b, and 7.

Charge Transfer States in Stable Neutral and Oxidized Radical Adducts from Carbazole Derivatives

In this paper we report the spectral properties of the stable radical adducts 1(•)-3(•), which are formed by an electron donor moiety, the carbazole ring, and an electron acceptor moiety, the polychlorotriphenylmethyl radical. The molecular structure of radical adduct 1(•) in the crystalline state shows a torsion angle of approximately 90° between the phenyl and the carbazole rings due to steric interactions. They exhibit a charge transfer band in the visible range of the electronic spectrum. All of them are chemically oxidized with copper(II) perchlorate to the respective cation species, which show a strong charge transfer band into the near-infrared region of the spectrum. Radical adducts 1(•)-3(•) and the corresponding stable oxidized species 1(+)-3(+) are real organic mixed-valence compounds due to the open-shell nature of their electronic structure. Charge transfer bands of the cation species are stronger and are bathochromically shifted with respect to those of the neutral species due to the greater acceptor ability of the positively charged central carbon atom of the triphenylmethyl moiety. The cationic species 1(+)-3(+) are diamagnetic, as shown by the absence of a signal in the EPR spectrum in acetonitrile solution at room temperature, but they show an intense and unique band in frozen solutions (183 K).