Variable-temperature ESR Spectra Research Articles

A mixed-valence Mn6 cluster capped by nitronyl nitroxide units

A mixed-valence Mn6 cluster, [Mn6O2(t-BuCO2)10(H2O)(NNPy)2] (2), has been synthesized by the assembly of [Mn6O2(t-BuCO2)10(THF)4] · THF (1) with a nitronyl nitroxide radical molecule, 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NNPy). Complex 2 consists of [Mn6O2]10+ core and two NNPy radical attachments, where the [Mn6O2]10+ core is composed of two MnIII ions (i.e., the inner Mn ions) bridged by two μ4-O2− and four MnII ions (i.e., the outer Mn ions) surrounding the inner MnIII ions via the two μ4-O2−, forming an edge-shared double-tetrahedral polygon (vertices: Mn ions, centers: μ4-O2−). Bridges by pivalate groups in μ2- and μ3-modes assist the coordination spheres around these Mn ions. Two of the outer MnII ions in trans position are, to the sixth coordination site, occupied by NNPy molecule (pyridine-N coordination). One of the outer MnII ions is occupied by a water molecule to complete octahedral geometry, but the rest of them is vacancy taking a square-pyramidal geometry. The variable-temperature direct current (DC) magnetic susceptibility was collected in the temperature range of 1.81 to 300 K at a 1 kOe appled field. The molar magnetic susceptibility (χM) steadily increases from 0.053 emu mol−1 at 300 K to 0.29 emu mol−1 at 20 K, showing a kink (peak), and then increases rapidly to 0.70 emu mol−1 at 1.81 K. The profile of this magnetic behavior indicates that the intracluster spin cancellation takes place bellow 20 K to derive an SMn6 = 0 by strong antiferromagnetic interaction between Mn ions. This is consequently leading to paramagnetic isolation of only the two nitronyl nitroxide units as if remaining as exchange-uncorrelated spin carriers. The variable-temperature ESR spectra show a sharp signal below 30 K with g = 2.00, proving the presence of free nitronyl nitroxide radical, in which double integral of such signal obeys the Curie–Weiss law that follows the susceptibility result in the low-temperature region.

Synthesis, Crystal Structure and Properties of a 4,4′-Bipyridine Bridged Trigonal-Bipyramidal Copper Homobinuclear Complex with Tris(2-Aminoethyl)amine

The synthesis of the binuclear complex [(tren)Cu(4,4'-bipy)Cu(tren)] (ClO4)4 where tren = tris(2-aminoethyl) amine and 4,4'-bipy = 4,4'-bipyridine, is described. Single-crystal X-ray diffraction study of the homobinuclear complex shows that two copper(II) ions are bridged by 4,4'-bipyridine and each copper(II) ion is trigonal-bipyramidally coordinated, with tren occupying four sites [Cu-N = 2.030(2), 2.047(2), 2.078(2), and 2.119(2) Å respectively] and a bridging 4,4'-bipyridine in the apical position. The Cu-Cu distance is 11.11 Å. In variable-temperature ESR spectra, the presence of hyperfine structure suggest that some interaction exists between the two copper(II) ions. Temperature-dependent susceptibility studies indicate that it is a weak ferromagnetic interaction with 2J = 1.23 cm-1.

SYNTHESIS, CHARACTERIZATION, PHOTOCHROMISM AND CRYSTAL STRUCTURE OF A CHARGE-TRANSFER COMPLEX BETWEEN AN ORGANIC SUBSTRATE AND A POLYOXOMETALATE [C10H10N]4GeMo12O40 · 6DMF (C10H10N = 7-METHYL-QUINOLINE)

A charge-transfer complex, [C10H10N]4GeMo12O40 · 6DMF, between the heteropolymolybdate anion, GeMo12O4− 40 and the organic cation, protonated 7-methyl-quinoline has been synthesized and characterized. Light yellow crystals of the title compound as a DMF solvate of formula [C10H10N]4GeMo12O40 · 6DMF were crystallized from DMF by diffusing acetone vapor into the solution. Spectroscopic data support the presence of a significant electronic interaction between the electron-rich aromatic organic cations and the inorganic anion in the solid state. The complex is strongly photosensitive under irradiation with sunlight, resulting in charge-transfer by oxidation of the 7-methyl-quinoline and reduction of the polyoxometalate. The variable temperature ESR spectra indicate thermal electron delocalization occurs among the Mo atoms in the title compound.

Effect of Substitution on the ESR Spectra and Electronic Ground State of ZnTPP Cation Radicals

Zinc(II) complexes of substituted tetraphenylporphyrins, Zn[T(X–P)P], where X = H, p-F, p-Cl, m-Cl, p-CH3O, and m-CH3O, have been synthesized and characterized. Peripheral substitution shows marked changes in the chemical shifts of the phenyl proton resonances and E1/2 values of the redox couples in the anodic region while the UV/Vis spectra are unaffected. The monocation radicals of these complexes are generated by chemical oxidation with bromine. The ESR spectra reveal the formation of two types of radical species, Zn[T(X–P)P]+·Br– (species I) and Zn[T(X–P)(Brn–P)]+·Br– (species II): Species I at 298 K shows a spectrum, with well-resolved bromine hyperfine features, characteristic of a 2A2u electronic ground state while species II, except for the complex with X = m-CH3O, shows a featureless, isotropic resonance attributable to a 2A1u state. Zn[T(m-CH3O–P)(Brn–P)]+·Br–, on the other hand, exhibits nine resolved nitrogen hyperfine features corresponding to a 2A2u state. Variable temperature ESR spectra (77–298 K) indicate reduction in the bromine and nitrogen hyperfine coupling constants and an increase in the g value of species I from 2.0049 to 2.0060 with lowering temperature and suggest a labile electronic ground state for species I. The p-CH3O substituted complex exhibits an electronic transformation from 2A2u to 2A1u while the remaining complexes, including m-CH3O, show a transformation from 2A2u to an admixed 2A1u/2A2u state. The effect of substitution on the variable temperature ESR spectra are discussed.

The isolation, characterisation, gas phase electron diffraction and crystal structure of the thermally stable radical [CF3CSNSCCF3]˙ †

The first structural characterisation of a heterocyclic free radical in all phases has been achieved. 4,5-Bis(trifluoromethyl)-1,3,2-dithiazol-2-yl, [CF3CSNSCCF3]˙ 4d, a 7π radical, was prepared quantitatively on reduction of [4d][AsF6] with various reducing agents. It is a blue gas over a green paramagnetic liquid [19F NMR δ −59.7], which freezes to a diamagnetic black-green solid. There is a remarkably large volume increase on melting implying that the liquid consists of discrete monomeric radicals, and consistently there is no tendency to dimerise in CCl3F solution and the vibrational spectra attributable to the monomer are essentially identical in all phases. The thermodynamic properties obtained for the vapourisation, sublimation and melting processes are very similar to those of related diamagnetic materials. 4d is remarkably thermochemically robust giving decomposition products identical to those from photochemical decomposition, a quantitative mixture of CF3CSSCCF3 (and its oligomers) and CF3CNSNCCF3. The reaction chemistry is described. The crystal structure shows radical 4d to be a planar heterocyclic ring which associates in the solid as a diamagnetic tetramer. The structure of the gaseous monomer determined by electron diffraction is very similar to that in the solid phase. Comprehensive physical measurements were made including the UV-visible spectra of the bright blue solutions, variable temperature magnetic susceptibility and quantitative variable temperature ESR spectra both of which confirm a phase change from a diamagnetic solid to a paramagnetic liquid on melting, the vibrational spectra, and the photoelectron spectrum of the gaseous free radical. A study of the behavior of RCSNSCR˙ (R = H, CH3 or CF3) given by the reduction of RCSNSCR+ on increase of concentrations of radical is reported, including the isolation of HCSNSCCF3˙ at low temperature. The experimental and calculated [UMPW1PW91/6-31+G*] structures of 4d are in good agreement. These and related calculations also support the interpretation of the vibrational and photoelectron spectra, and the thermodynamic properties of 4d, as well as provide possible explanations for the relative stabilities of RCSNSCR˙, and imply that the weak intramolecular interactions in 4d in the solid state are largely ionic Sδ+⋯Nδ− interactions.

Intramolecular Electron Transfer between Doubly Four σ-Bond-Linked Tetraalkylhydrazine Cationic and Neutral Units

Rate constants kESR for intramolecular electron transfer between the reduced and oxidized hydrazine units of dimeric 2-tert-butyl-3−isopropyl-2,3-diazabicyclo[2.2.2]octylhydrazine radical cations which are doubly linked through the bicyclic units by four σ-bonds (aBIT+, sBIT+, and aBIH+) were determined by simulations of their variable temperature ESR spectra in methylene chloride to be 10.5(7), 9.6(3), and 12.4(4) × 107 s-1 at 298 K, respectively. These cations show solvent sensitive charge transfer absorption bands from which the vertical electron transfer excitation energy, λ, and the electronic coupling, VJ, were determined by simulation of the charge transfer bands using vibronic coupling theory (ref 13). Partitioning between solvent and vibrational components of λ was made assuming that the average energy of the vibrational modes coupled to the electron transfer, hνv, is 2.29 kcal/mol (800 cm-1). The ESR rate constants at 298 K for s- and aBIT+ are factors of 23 and 26, respectively, larger than kcal, calculated from λs, λv, hνv, and VJ estimated using a vibronic coupling theory analysis of the charge transfer bands. The ratio kcal(350)/kcal(250) is 7.4 and 9.4(kESR(350)/kESR(250)) for sBIT+ and aBIT+, respectively. Previously reported data for the doubly-linked four σ-bond dimeric N,N‘-bis-2,3-diazabicyclo[2.2.2]octyl hydrazine (22H+) was also reanalyzed using vibronic coupling theory. kESR/kcal in acetonitrile at 298 K is 20, but kcal(350)/kcal(250) is 25(kESR(350)/kESR(250)). Possible reasons for the rather poor agreement with theory are suggested.

Synthesis, characterization and crystal structure of a novel tris(2-aminoethyl) amine copper(II) complex with an asymmetric coordinated 4,5-diazafluorene-9-hydrazine

The compound, tris(2-aminoethyl)amine-4,5-diazafluorene-9-hydrazine copper(II) perchlorate complex, [Cu(C 6H 18N 4)(C 11H 8N 4](ClO 4) 2 ( 1), has been designed, synthesized and characterized. The X-ray analysis reveals that the copper(II) is coordinated by four nitrogen atoms from tis(2-aminoethyl)amine (tren) and two nitrogen atoms from 4,5-diazafluorene-9-hydrazine (dzfh) to form an unusual (4+1+1′) six-coordinate geometry. This structure is very rare, and to out knowledge, is a unique example of a tripodal copper(II) complex with an asymmetric bidentate phenanthroline derivative. The 110 K ESR and electronic reflectance spectra studies demonstrate that the coordinated geometry of copper(II) is quite similar to square-based pyramidal stereochemistry, instead of the trigonal bipyramidal stereochemistry, which is the common structure for [Cu(tren)L] 2+ (L = monodentate ligand) complexes. The electronic and variable-temperature ESR spectra in absolute DMF suggest that the two nitrogen atoms of the dzfh coordinate to the copper(II) ion in the two-dimensional fluxional model. For comparison, the complex, Cu(tren)(Py)(ClO 4) 2 (Py is pyridine) ( 2, was also

Synthesis, Characterization, and Crystal Structure of a Novel Copper(II) Complex with an Asymmetric Coordinated 2,2‘-Bipyridine Derivative: A Model for the Associative Complex in the Ligand-Substitution Reactions of [Cu(tren)L]2+?

The titled compound, (tris(2-aminoethyl)amine)(4,5-diazafluoren-9-one) copper(II) perchlorate, [Cu(C(6)H(18)N(4))(C(11)H(6)N(2)O)(ClO(4))(2)], 1, has been designed, synthesized, and characterized. The electronic and ESR spectra are very different from those of [Cu(tren)L](2+) complexes where L is monodentate ligand. The X-ray analysis revealed that the complex crystallizes in the monoclinic space group P2(1)/c, with a = 10.726(6) Å, b = 14.921(7) Å, c = 14.649(4) Å, beta = 95.13(3) degrees, and Z = 4. The copper(II) ion is coordinated by four nitrogen atoms from tris(2-aminoethyl)amine (tren) and two nitrogen atoms from 4,5-diazafluoren-9-one (dzf) to form an unusual six-coordinate (4 + 1 + 1') geometry. The structure is very rare, and to our knowledge, it is the first example of an asymmetric bidentate phenanthroline derivative metal complex. The structure could be used as a model of the associative complex in the ligand-exchange and ligand-substitution reactions of [Cu(tren)L](2+) and the catalytic mechanisms of enzymes involving copper sites. From the electronic and variable-temperature ESR spectra in solution, the possible mechanism of these reactions has also been proposed. As a comparison, the complex [Cu(tren)(ImH)(ClO(4))(2)], 2, was also synthesized and characterized, where ImH is imidazole.

Synthesis, characterization and electrochemistry of a binuclear copper(II) complex of schiff base derived from 2-aminomethyl-benzimidazole and salicylaldehyde

A dinuclear Cu II compound (Cu 2L 2 2+) with the phenoxide form of N-salicylidine 2-aminomethylbenzimidazole has been synthesized and characterized on the basis of elemental analysis, UV-visible, IR and variable temperature ESR spectra, magnetic moment and electrochemical studies. Thermal studies indicate that the decomposition takes place in two stages. Lowering of ESR peak intensity with lowering in temperature indicates the dimeric nature of the compound. A stopped flow kinetic study indicated that the acid catalysed decomposition of the compound occurred via two consecutive steps, thus providing further evidence for the dimeric nature of the compound. Evidence of equilibrium NH deprotonation of the benzimidazole moiety of Cu 2L 2 2+ at 0.005 ⩽ [OH −], mol dm −3 ⩽ 0.02 is presented.

Experimental molecular dynamics of an alanine-based helical peptide determined by spin label electron spin resonance.

The alanine-based 3K(I) peptide is reported to be very helical in aqueous solution. We have prepared a series of six nitroxide spin labeled analogs of the 3K(I) sequence and measured the variable-temperature ESR spectra for each in order to reveal the position-dependent peptide dynamics. From analysis of these local dynamics under helix-forming conditions at 1 degree C, we find that the helix termini show greater local dynamics than the peptide cancer. Further, the C-terminus is more mobile than the N-terminus. Even in the helix-promoting solvent trifluoroethanol, the results indicate that there is still substantially greater dynamics at the helix termini than at the peptide center. The unfolded state is also investigated, and we find that the peptide unfolded by guanidine hydrochloride is somewhat different than that found for high-temperature aqueous solution. Recently it was suggested that short 16-mer peptides may adopt a 3(10)-helix structure instead of the expected alpha-helix. The data presented here at 1 degree C show that there is sufficient disorder within the peptide to accommodate the 3(10) structure. Also calculated are the backbone torsional fluctuations, and the results compare well to those from computer molecular dynamics studies. A proposal is outlined that explains how the enhanced dynamics found at the C-terminus results from the exposure of the helix hydrogen bonds to aqueous solvent in this region of the peptide.

Formation in solution, preparation, crystal structure and magnetic characterization of di-µ-hydroxo-bis[diaqua(2,2′-bipyrimidine)copper(II)] diperchlorate dihydrate

A new binuclear complex of formula [Cu2(bipym)2(H2O)4(OH)2][ClO4]2·2H2O (bipym = 2,2′-bi-pyrimidine) has been synthesised and its crystal structure determined by X-ray crystallographic methods. It crystallizes in the triclinic space group P, with a= 7.021(1), b= 10.004(1), c= 10.721(1)A, α= 100.83(1), β= 95.80(1), γ= 99.05(1)° and Z= 1. Least-squares refinement of 2888 reflections with I > 3σ(I) and 256 parameters gave a final R′= 0.0522. The structure consists of discrete centrosymmetric di-µ-hydroxo-copper(II) dimers with 2,2′-bipyrimidine as outer ligand, weakly co-ordinated and crystallization water molecules, and unco-ordinated perchlorate anions. The co-ordination geometry around each copper(II) ion is approximately elongated tetragonal octahedral with the two bridging hydroxo groups and two nitrogen atoms of bipym building the equatorial plane and two water molecules occupying the axial positions. The Cu(1)–OH–Cu(1a) bridging angle and the intramolecular Cu(1)⋯ Cu(1a) separation are 95.0(1)° and 2.870(1)A, respectively. The temperature dependence of the magnetic susceptibility, studied in the range 300–4 K, shows a relatively strong intramolecular ferromagnetic coupling (+147 cm–1 for the singlet–triplet energy gap). Further evidence for the triplet ground state is provided by the variable-temperature ESR spectra. The strength of the exchange interaction is discussed on the basis of the structural features and correlated with reported magneto-structural data on parent double hydroxide-bridged copper(II) complexes. The formation of hydroxo complexes of [Cu(bipym)(H2O)]2+ has been investigated by potentiometry in aqueous solutions [equation (i); log K1=–10.350(1)(25 °C, 0.1 mol dm–3 NaNO3)]. 2[Cu(bipym)(H2O)2]2+ [graphic omitted] [Cu2(bipym)2(OH)2]2++ 2H++ 2H2O (i) The value of this stability constant is compared to those reported for related CuIIL complexes where L are bidentate N-donor ligands.

Alternating linewidth effects in the ESR spectrum of 5-6-dihydro-6-methyluracil

Variable temperature ESR spectra of radicals formed by x-irradiation of single crystals of DH6MU are reported. (AIP)