Photolysis and oxidation of azidophenyl-substituted radicals: delocalization in heteroatom-based radicals

2-(4-Azidophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (14), 2-(4-azidophenyl)benzimidazole-1-oxide-3-oxyl (16), 2-(4-azidophenyl)-1,2,6-triphenylverdazyl (19), 2-(3-azidophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (21), and (3-azidophenyl)-N-tert-butyl-N-aminoxyl (25) were photolyzed in frozen solution to give S = 3/2 state ESR spectra of the corresponding nitrenophenyl radicals with the following zero-field splitting parameters: |D/hc| = 0.277 cm(-1), |E/hc| < or = 0.002 cm(-1) (7 from 14); |D/hc| = 0.256 cm(-1), |E/hc| < or = 0.002 cm(-1) (8 from 16); |D/hc| = 0.288 cm(-1), |E/hc| < or = 0.002 cm(-1) (9 from 19); |D/hc| = 0.352 cm(-1), |E/hc| = 0.006 cm(-1) (10 from 21); |D/hc| = 0.336 cm(-1), |E/hc| = 0.004 cm(-1) (11 from 25). UB3LYP/6-31G computations and ESR spectroscopic analyses suggest that these are nitreno radicals, even para-linked systems with possible quinonoidal resonance forms. Neat samples of azidophenyl radicals 14 and 21 showed bulk paramagnetic behavior, consistent with the lack of close contacts in their crystal structures. Efforts to make photolabile coordination complexes of 14 and 21 with paramagnetic transition metal ions were unsuccessful: Cu(ClO4)2 x 6H2O instead oxidized them to the corresponding diamagnetic nitrosonium perchlorate salts.     

Limits of delocalization in through-conjugated dinitrenes: aromatization or bond formation?

[reaction: see text] Dinitrenes 4 and 5 both can form quinonoidal structures by conjugative bond formation. However, ESR spectroscopy detects a thermally populated, excited-state, triplet quinonoidal structure only for 4, with a zero-field splitting of |D/hc| = 0.0822 cm(-)(1), |E/hc| congruent with 0.0 cm(-)(1). The tendency to maintain aromaticity in the additional ring of 5 favors a dinitrene structure (with one less formal pi-bond) over a quinonoidal structure. The thermally populated quintet state of 5 has a zero-field splitting of |D/hc| = 0.287 cm(-)(1), |E/hc| 相似文献   

Photochemistry of furyl- and thienyldiazomethanes: spectroscopic characterization of triplet 3-thienylcarbene

Photolysis (λ > 543 nm) of 3-thienyldiazomethane (1), matrix isolated in Ar or N(2) at 10 K, yields triplet 3-thienylcarbene (13) and α-thial-methylenecyclopropene (9). Carbene 13 was characterized by IR, UV/vis, and EPR spectroscopy. The conformational isomers of 3-thienylcarbene (s-E and s-Z) exhibit an unusually large difference in zero-field splitting parameters in the triplet EPR spectrum (|D/hc| = 0.508 cm(-1), |E/hc| = 0.0554 cm(-1); |D/hc| = 0.579 cm(-1), |E/hc| = 0.0315 cm(-1)). Natural Bond Orbital (NBO) calculations reveal substantially differing spin densities in the 3-thienyl ring at the positions adjacent to the carbene center, which is one factor contributing to the large difference in D values. NBO calculations also reveal a stabilizing interaction between the sp orbital of the carbene carbon in the s-Z rotamer of 13 and the antibonding σ orbital between sulfur and the neighboring carbon-an interaction that is not observed in the s-E rotamer of 13. In contrast to the EPR spectra, the electronic absorption spectra of the rotamers of triplet 3-thienylcarbene (13) are indistinguishable under our experimental conditions. The carbene exhibits a weak electronic absorption in the visible spectrum (λ(max) = 467 nm) that is characteristic of triplet arylcarbenes. Although studies of 2-thienyldiazomethane (2), 3-furyldiazomethane (3), or 2-furyldiazomethane (4) provided further insight into the photochemical interconversions among C(5)H(4)S or C(5)H(4)O isomers, these studies did not lead to the spectroscopic detection of the corresponding triplet carbenes (2-thienylcarbene (11), 3-furylcarbene (23), or 2-furylcarbene (22), respectively).     

A perimidine-derived non-Kekulé triplet diradical

6,9-Di(tert-butyl)-1-methyltetrazolo[1,5-a]perimidine (1) has been synthesized from naphthalene in seven steps. The EPR spectra, recorded after irradiation of 1 in a butyronitrile matrix at 77 K (lambda = 351 nm) and in Ar and Xe matrixes at 4.6 K (lambda > or = 345 nm), showed a six-line, high-field signal (Delta m(S) = +/- 1), centered at 3350 G in butyronitrile, along with a half-field signal (Delta m(S) = +/- 2), which is characteristic for triplets. Simulation of the observed EPR spectra gave values for the zero-field splitting parameters of |D/hc|/cm(-1) = 0.0105, |E/hc|/cm(-1) = 0.0014 in butyronitrile and |D/hc|/cm(-1) = 0.0107, |E/hc|/cm(-1) = 0.0016 in Ar. These EPR parameters are consistent with the diradical 5,8-di(tert-butyl)-2-(N-methylimino)perimidine-1,3-diyl ((3)2) as source of the EPR spectra. Linearity of the Curie-Weiss plot and UB3LYP and (14/14)CASPT2 calculations of the singlet-triplet energy difference (DeltaE(ST) approximately 8-10 kcal/mol) indicate that the triplet is the ground state of 2, as predicted for such a nondisjoint diradical.     

2,3,5,6-Tetrafluorophenylnitren-4-yl: electron paramagnetic resonance spectroscopic characterization of a quartet-ground-state nitreno radical

2,3,5,6-Tetrafluorophenylnitren-4-yl (5) was synthesized in argon at 4 K via the photolysis of 2,3,5,6-tetrafluoro-4-iodo-phenyl azide (6). Electron paramagnetic resonance (EPR) spectroscopy allows us to observe triradical 5 in its quartet state with the zero-field splitting (ZFS) parameters |D/hc| = 0.285 and |E/hc| = 0.043 cm-1. The quartet ground state of 5 is in accordance with our previous infrared (IR) spectroscopic investigation, in which the high-spin quartet state, but no low-spin doublet state, of 5 was observed in solid argon at 4 K [Wenk, H. H.; Sander, W. Angew. Chem., Int. Ed. 2002, 41, 2742-2745]. Because annealing of the matrix at temperatures of >10 K results in the rapid recombination of the highly reactive species 5 with I atoms produced during the photolysis of 6, the Curie-Weiss behavior could not be investigated. However, the absence of low-spin states in the IR investigations, as well as the results of ab initio and density functional theory (DFT) calculations, strongly suggest that 5 has a robust quartet ground state that is best-described as an unprecedented sigma,sigma,pi-triradical. The ZFS of 5 has been successfully reproduced by DFT calculations, which furthermore provide qualitative insight into the origin of the observed EPR parameters.     

Syntheses, optical and intramolecular magnetic properties of mono- and di-radicals based on nitronyl-nitroxide and oxoverdazyl groups appended to 2,6-bispyrazolylpyridine cores

This paper presents the synthesis of a series of nitronyl-nitroxide (NN), oxoverdazyl (OVZ) based mono-, and bi-radicals attached to 4-phenyl-2,6-bispyrazolylpyridine coupling unit, their optical, electron spin resonance (ESR) spectroscopic studies and computational analysis. The ESR studies revealed that the axial zero-field splitting (zfs) parameter of the NN biradical (|D/hc| = 0.00719 cm(-1)) is larger than the OVZ biradical (|D/hc| = 0.00601 cm(-1)). Additionally both biradicals displayed forbidden half-field transitions (ΔM(s) = ±2; g(av) ~ 4.01) at 170 K demonstrating their triplet nature. The cryogenic ESR measurements of the two biradicals showed a Curie magnetic behaviour of the ΔM(s) = ±2 signal intensities (χ(EPR)) down to 4.2 K. A detailed comparative analysis of the strength of hyperfine coupling, spin density distribution, zfs and the spin-spin exchange coupling (J) of both NN and OVZ based biradicals showed that the ground state spin multiplicity of both biradicals is probably triplet (S = 1) or it is nearly degenerate singlet-triplet states with J(NN)?J(OVZ).     

Donor stabilized borylnitrene: a highly reactive BN analogue of vinylidene

A donor atom stabilized borylnitrene, 2-nitreno-1,3,2-benzodioxaborole 4c, is characterized by matrix isolation IR, UV, and ESR spectroscopy as well as multiconfiguration SCF and CI computations. UV irradiation (lambda = 254 nm) of the corresponding azide 6c, isolated in solid argon at 10 K, produces 4c in high yield. The oxygen donor atoms in 4c result in a triplet ground state (|D/hc| = 1.492 cm(-)(1), |E/hc| = 0.004 cm(-)(1)) for the borylnitrene. The lowest energy singlet state ((1)A(1)) is 33 kcal mol(-)(1) higher in energy and closely related to the ground state of vinylidene. Under the conditions of matrix isolation, triplet 4c is photochemically and thermally stable toward rearrangement to the corresponding cyclic iminoborane. Photochemical irradiation (lambda > 550 nm) of 4c rather causes an efficient reaction with molecular nitrogen, lying in matrix sites nearby, to give 6c. Similarly, photochemical, but not thermal, trapping of 4c with CO is possible and results in the corresponding isocycanate 9c. Thermal reaction of 4c with O(2) in doped argon matrixes at 35 K could be observed by IR spectroscopy to result in borylnitroso-O-oxide 17c as shown by (18)O(2) labeling experiments and DFT computations. The diradical 17c is very photolabile and quickly rearranges to the nitritoborane 16c upon irradiation.     

Incorporation of stable organic radicals of the tris(2,4, 6-trichlorophenyl)methyl radical series to pyrrole units as models for semiconducting polymers with high spin multiplicity. 1

The condensation reactions between (4-amino-2,6-dichlorophenyl)bis(2, 4,6-trichlorophenyl)methyl radical and acetylacetone or 1, 4-bis(5-methyl-2-thienyl)-1,4-butanedione yield [2,6-dichloro-4-(2, 5-dimethyl-1-pyrrolyl)phenyl]bis(2,4,6-trichlorophenyl)methyl radical (3(*)()) and [2,6-dichloro-4-[2, 5-bis(5-methyl-2-thienyl)-1-pyrrolyl]phenyl]bis(2,4, 6-trichlorophenyl)methyl radical (4(*)()), respectively. EPR studies of both radicals 3(*)() and 4(*)() in CH(2)Cl(2) solution suggest a weak electron delocalization with coupling constant values of 1.25 and 1.30 G, respectively, with the six aromatic hydrogens. Their electrochemical behavior was analyzed by cyclic voltammetry. Both radicals show reversible reduction processes at E degrees = -0.69 V and -0.61 V versus SSCE, respectively, and anodic peak potentials at E(p)(a) = 1.10 and 0.72 V, respectively, versus SSCE at a scan rate (nu) of 200 mV s(-)(1), being reversible for radical 4(*)(). X-ray analysis of radical 3(*)() shows a high value (65 degrees ) of the dihedral angle between the 2,5-dimethylpyrrolidyl moiety and the phenyl ring. Smooth oxidation of radical 4(*)() in CH(2)Cl(2) containing trifluoroacetic acid gives an ionic diradical species with a weak electron interaction (|D/hc| = 0.0047 cm(-)(1)). A Curie plot of the Deltam(s)() = +/-2 signal intensity versus the inverse of the absolute temperature in the range between 4 and 70 K suggests a triplet or a nearly degenerate singlet-triplet ground state.     

Selective formation of triplet alkyl nitrenes from photolysis of beta-azido-propiophenone and their reactivity

Photolysis of beta-azido propiophenone derivatives, 1, with built-in sensitizer units, leads to selective formation of triplet alkyl nitrenes 2 that were detected directly with laser flash photolysis (lambdamax = 325 nm, tau = 27 ms) and ESR spectroscopy (|D/hc| = 1.64 cm-1, |E/hc| = 0.004 cm-1). Nitrenes 2 were further characterized with argon matrix isolation, isotope labeling, and molecular modeling. The triplet alkyl nitrenes are persistent intermediates that do not abstract H-atoms from the solvent but do decay by dimerizing with another triplet nitrene to form azo products, rather than reacting with an azide precursor. The azo dimer tautomerizes and rearranges to form heterocyclic compound 3. Nitrene 2a, with an n,pi* configuration as the lowest triplet excited state of the its ketone sensitizer moiety, undergoes intramolecular 1,4-H-atom abstraction to form biradical 6, which was identified by argon matrix isolation, isotope labeling, and molecular modeling. beta-Azido-p-methoxy-propiophenone, with a pi,pi* lowest excited state of its triplet sensitizer moiety, does not undergo any secondary photoreactions but selectively yields only triplet alkyl nitrene intermediates that dimerize to form 3b.     

Low-valent low-coordinated manganese(I) ion dimer: a temperature dependent W-band EPR study

W-Band EPR spectra of [[HC(CMeNAr)(2)]Mn](2) (Ar = 2,6-(i)Pr(2)C(6)H(3)) have been measured at different temperatures. The spectra show a behavior which is typical for an antiferromagnetically coupled dimer with excited states populating upon increasing temperature. By following the intensity variation of the different features of the spectra with temperature, we attributed different groups of resonances to the S = 1, 2, and 3 states of the dimer. Their corresponding spin Hamiltonian parameters were derived from simulations. The zero-field-splitting parameters measured in this way were D(S=1) = 1.57 cm(-1) and E(S=1) = 0.064 cm(-1), D(S=2) = 0.266 cm(-1) and E(S=2) = 0.0045 cm(-1), and D(S=3) = 0.075 cm(-1) and E(S=3) = 0. On the basis of the molecular structure of the system, we could estimate that zero-field splitting (ZFS) is the result of anisotropic exchange and single-ion anisotropic contributions of similar magnitude (|D| approximately 0.2 cm(-1)). These results allow a deeper insight into the electronic structure of the Mn(I) centers in low-coordination environments, further supporting the electronic structure of Mn(I) to be 4s(1)3d(5), as previously indicated by DFT calculations.     

Extraction and Crystal Structure of Karounidiol

1 INTRODUCTION Karounidiol is a natural pentacyclic triterpene compound, which was identified as a novel compound initially from the Japanese commercial seeds of Trichosanthes kirilowii Maxim. (Fam. Cucurbitaceae)[1], and then isolated from the Chinese commercial seeds of T. kirilowii Maxim. and T. rosthornii Harms[2, 3] and the Japanese wild plant seeds of T. cucumeroides (Ser.) Maxim.[4]. It has not been isolated from other plants up to now, so we are thinking about how to use th…     

Spectroscopic and magnetic properties of an iodo Co(I) tripodal phosphine complex

Reaction of the tripodal phosphine ligand 1,1,1-tris((diphenylphosphino)phenyl)ethane (PhP3) with CoI(2) spontaneously generates a one-electron reduced complex, [(PhP3)Co(I)(I)] (1). The crystal structure of 1 reveals a distorted tetrahedral environment, with an apical Co-I bond distance of ～2.52 ?. Co(II/I) redox occurs at an unusually high potential (+0.38 V vs. SCE). The electronic absorption spectrum of 1 exhibits an MLCT peak at 320 nm (ε = 8790 M(-1) cm(-1)) and a d-d feature at 850 nm (ε = 840 M(-1) cm(-1)). Two more d-d bands are observed in the NIR region, 8650 (ε = 450) and 7950 cm(-1) (ε = 430 M(-1) cm(-1)). Temperature dependent magnetic measurements (SQUID) on 1 (solid state, 20-300 K) give μ(eff) = 2.99(6) μ(B), consistent with an S = 1 ground state. Magnetic susceptibilities below 20 K are consistent with a zero field splitting (zfs) |D| = 8 cm(-1). DFT calculations also support a spin-triplet ground state for 1, as optimized (6-31G*/PW91) geometries (S = 1) closely match the X-ray structure. EPR measurements performed in parallel mode (X-band; 0-15?000 G, 15 K) on polycrystalline 1 or frozen solutions of 1 (THF/toluene) exhibit a feature at g≈ 4 that arises from a (Δm = 2) transition within the M(S) = <+1,-1> manifold. Below 10 K, the EPR signal decreases significantly, consistent with a solution zfs parameter (|D|≈ 8 cm(-1)) similar to that obtained from SQUID measurements. Our work provides an EPR signature for high-spin Co(I) in trigonal ligation.     

STRUCTURE OF 1-CHLOHOMETHYI-3,4,6,7,10,11-TRIBENZO-2,8,9-TRIOXA-5-AZA- 1-SILATRICYCLO-(3.3.3. O1,5)-UNDECANE C_(19)H_(14)ClNO_3Si

<正> Mr=367.88, Monoclinic, C2/c, a=17.758(7), b=ll.838(3), c= 16.793(7)A, β=103.79(3)°,V=3428(2)A3, Z=8,μ(MoKα)=3.10cm-1, F(000)=1520, Dx=1.43g/cm3, room temperature. The final R=0.074, Rw=0.073 with wt=[c2 (|Fo|) +0.0006|Fo| 2]-1, s=1.26 for 1465 independent reflections with |Fo|> 2.5(|Fo)). The length of dative N-Si bond is 2.256A, the Cl atom in -CH2Cl group is disordered.     

Reactive carbon-chain molecules: synthesis of 1-diazo-2,4-pentadiyne and spectroscopic characterization of triplet pentadiynylidene (H-C[triple bond]C-:C-C[triple bond]C-H)

1-Diazo-2,4-pentadiyne (6a), along with both monodeuterio isotopomers 6b and 6c, has been synthesized via a route that proceeds through diacetylene, 2,4-pentadiynal, and 2,4-pentadiynal tosylhydrazone. Photolysis of diazo compounds 6a-c (lambda > 444 nm; Ar or N2, 10 K) generates triplet carbenes HC5H (1) and HC5D (1-d), which have been characterized by IR, EPR, and UV/vis spectroscopy. Although many resonance structures contribute to the resonance hybrid for this highly unsaturated carbon-chain molecule, experiment and theory reveal that the structure is best depicted in terms of the dominant resonance contributor of penta-1,4-diyn-3-ylidene (diethynylcarbene, H-C[triple bond]C-:C-C[triple bond]C-H). Theory predicts an axially symmetric (D(infinity h)) structure and a triplet electronic ground state for 1 (CCSD(T)/ANO). Experimental IR frequencies and isotope shifts are in good agreement with computed values. The triplet EPR spectrum of 1 (absolute value(D/hc) = 0.6157 cm(-1), absolute value(E/hc) = 0.0006 cm(-1)) is consistent with an axially symmetric structure, and the Curie law behavior confirms that the triplet state is the ground state. The electronic absorption spectrum of 1 exhibits a weak transition near 400 nm with extensive vibronic coupling. Chemical trapping of triplet HC5H (1) in an O2-doped matrix affords the carbonyl oxide 16 derived exclusively from attack at the central carbon.     

pi-topology and spin alignment utilizing the excited molecular field: observation of the excited high-spin quartet (S = 3/2) and quintet (S = 2) states on purely organic pi-conjugated spin systems.

As a model system for the photoinduced/photoswitched spin alignment in a purely organic pi-conjugated spin system, 9-[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (1a), 9-[3-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (1b), 9,10-bis[4-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (2a), and 9,10-bis[3-(4,4,5,5-tetramethyl-1-yloxyimidazolin-2-yl)phenyl]anthracene (2b) were designed and synthesized. In these spin systems, 9-phenylanthracene and 9,10-diphenylanthracene were chosen as photo spin couplers and iminonitroxide was chosen as a dangling stable radical. Time-resolved electron spin resonance (TRESR) spectra of the first excited states with resolved fine-structure splittings were observed for 1a and 2a in an EPA or a 2-MTHF rigid glass matrix. Using the spectral simulation based on the eigenfield method, the observed TRESR spectra for 1a and 2a were unambiguously assigned as an excited quartet (S = 3/2) spin state (Q) and an excited quintet (S = 2) spin state (Qu), respectively. The g value and fine-structure splitting for the quartet state of 1a were determined to be g(Q) = 2.0043, D(Q) = 0.0235 cm(-1), and E(Q) = 0.0 cm(-1). The relative populations (polarization) of each M(S)() sublevel in Q were determined to be P(+1/2') = P(-1/2') = 0.5 and P(+3/2') = P(-3/2') = 0.0 with an increasing order of energy in zero magnetic field. The spin Hamiltonian parameters for Qu are g = 2.0043, D = 0.0130 cm(-1), and E = 0.0 cm(-1), and the relative populations in Qu were determined to be P(0') = 0.30, P(-1') = P(+1') = 0.35 and P(-2') = P(+2') = 0.0. These are the first observations of a photoexcited quartet and a quintet high-spin state in pi-conjugated triplet-radical pair systems. In contrast high-spin excited states were not observed for 1b and 2b, the pi-topological isomers of 1a and 2a, showing the role of pi-topology in the spin alignment of the excited states. Since a weak antiferromagnetic exchange interaction was observed in the ground state of 2a, the clear detection of the excited quintet high-spin state shows that the effective exchange coupling between the two dangling radicals through the diphenylanthracene spin coupler has been changed from antiferromagnetic to ferromagnetic upon photoexcitation. Thus, a photoinduced spin alignment utilizing the excited triplet molecular field was realized for the first time in the purely organic pi-conjugated spin system. Furthermore, the mechanism for the generation of dynamic electron spin polarization was investigated for the observed quartet and quintet states, and a plausible mechanism of the enhanced selective intersystem crossing was proposed. Ab initio molecular orbital calculations based on density functional theory were carried out to determine the electronic structures of the excited high-spin states and to understand the mechanism of the spin alignment utilizing the excited molecular field. The role of the spin delocalization and the spin polarization mechanisms were revealed on the photoexcited state.     

Fully state-selected VMI study of the near-threshold photodissociation of NO(2): variation of the angular anisotropy parameter

Velocity-map ion imaging (VMI) has been used to study the angular distribution of the NO fragment generated in the photodissociation of NO(2) at a variety of photolysis wavelengths. Images were recorded for the channels NO (2)Pi(1/2) (v = 0, J= 3/2, 11/2 and 21/2) + O ((3)P(2,1)), for excitation energies ranging from the onset (E(avl)/hc = 0 cm(-1)) to E(avl)/hc approximately 900 cm(-1). The angular anisotropy parameter beta was obtained as a function of available energy. Photofragment multiphoton ionization (PHOMPI) spectra were also recorded in the energy range E(avl)/hc = 0-300 cm(-1) for each of these channels. Large fluctuations of beta as a function of E(avl) were found in all observed dissociation channels. These variations are discussed in terms of the lifetimes of the originally photoexcited overlapping resonances in the A(2)B(2) state of NO(2), the dynamics of which are strongly influenced by nonadiabatic coupling with the X[combining tilde](2)A(1) state. The potential use of this photolysis process for production of cold oxygen atoms is discussed.     

N-(2-methoxycarbonyl-2-yloethyl) (3-nitrenophenyl)aminyl: EPR observation of a quintet hetero-spin molecule

[reaction: see text]. The EPR observation of a ground-state quintet hetero-spin molecule, N-(2-methoxycarbonyl-2-yloethyl) (3-nitreno-phenyl)aminyl 3, in a MTHF matrix at cryogenic temperatures is reported. The molecule has two different kinds of triplet sites, and the ZFS parameters of the quintet are /D/hc/ = 0.170 cm(-1) and /E/hc/ = 0.004 cm(-1). As a result of the nature of the 1,3-localized biradical, the quintet 3 was thermally labile over 5 K.     

Magneto-structural correlations in dinuclear Mn(III) compounds with formula [{Mn(L)(NN)}(μ-O)(μ-2-RC6H4COO)2{Mn(L')(NN)}]n+

Three dinuclear Mn(III) compounds with oxo and carboxylato bridges have been synthesized and characterized by X-ray diffraction: [{Mn(L)(NN)}(μ-2-ClC(6)H(4)COO)(2)(μ-O){Mn(L')(NN)}](n+) with NN = 2,2'-bipyridine (1 and 2) or 1,10-phenanthroline (3). The counteranion is either NO(3)(-) (1 and 3) or ClO(4)(-) (2) and the monodentate positions (L, L') could be occupied by molecules of water or the counteranion. For compound 1, L = H(2)O and L' = NO(3)(-); compound 2 shows two different dinuclear units and L and L' could be H(2)O or ClO(4)(-), and for compound 3 both monodentate positions are occupied by nitrate anions. The magnetic properties of the three compounds have been analyzed using the Hamiltonian H = -JS(1)·S(2). Compound 1 exhibits a dominant ferromagnetic behavior, with J = 3.0 cm(-1), |D(Mn)| = 1.79 cm(-1), |E(Mn)| = 0.60 cm(-1) with intermolecular interactions zJ' = -0.18 cm(-1). Due to the anisotropy of the Mn(III) ions, the ground state S = 4 shows ZFS with |D(4)| = 0.58 cm(-1). Compounds 2 and 3 show antiferromagnetic couplings, with J = -10.9 and -0.3 cm(-1), respectively. The magnetic interaction in this kind of compound depends on several structural factors. In the present work, the distortion around manganese ions, the torsion angle between the phenyl ring and the carboxylate group and the relative disposition of the coordination octahedra have been analyzed.     

NC-(CF2)4-CNSSN radical containing 1,2,3,5-dithiadiazolyl radical dimer exhibiting triplet excited states at low temperature and thermal hysteresis on melting-solidification: structural, spectroscopic, and magnetic characterization

A high yield, one-pot synthesis of the 1,2,3,5-dithiadiazolyl radical NC-(CF2)4-CNSSN radical by reduction of the corresponding 1,3,2,4-dithiadiazolium salt is reported. In the solid state, the title compound is dimerized in trans-cofacial fashion with intra-dimeric Sdelta+...N(delta-) interactions of ca. 3.2 angstroms, and the dimeric units are linked by electrostatic -C triple bond N(delta-)...Sdelta+ interactions forming an infinite chain. Magnetic susceptibility measurements performed on the solid state sample indicate a magnetic moment of 1.8 microB per dimer (1.3 microB per monomer) at 300 K and a good fit to the Bleaney-Bowers model in the temperature range 2-300 K with 2J = -1500 +/- 50 cm(-1), g = 2.02(5), rho = 0.90(3)%, and TIP = 1.25(4) x 10(-3) emu mol(-1). The [NC-(CF2)4-CNSSN radical]2 dimer is the second example of a 1,2,3,5-dithiadiazolyl radical dimer with an experimentally detected triplet excited state as probed by solid-state EPR [2J = -1730 +/- 100 cm(-1), |D| = 0.0278(5) cm(-1), |E| = 0.0047(5) cm(-1)]. The value of the singlet-triplet gap has enabled us to estimate the "in situ" dimerization energy of the radical dimer as ca. -10 kJ mol(-1). The diradical character of the dimer was calculated [CASSCF(6,6)/6-31G*] as 35%. The title radical shows magnetic bistability in the temperature range of 305-335 K as probed by the solid-state EPR presumably arising from the presence of a metastable paramagnetic supercooled phase. Bistability is accompanied by thermochromic behavior with a color change from dark green (dimeric solid) to dark brown (paramagnetic liquid).     

Magneto-structural studies of two new cobalt(II)-N,N-diisobutylisonicotinamide compounds: [CoLCl2]n and [Co(L)2(H2O)4][CoLBr3]2·2H2O

Two similar synthetic pathways using the ligand N,N-diisobutylisonicotinamide (L) with anhydrous CoX(2) salts (being X = Cl(-), Br(-)) led to different species: a one-dimensional system, [CoLCl(2)](n), 1, and an ionic product [Co(L)(2)(H(2)O)(4)][CoLBr(3)](2)·2H(2)O, 2, respectively. Compound 1 is a polymer in which ligand L coordinates to tetrahedral Co(II) ions in a bidentate bridging fashion using the pyridine nitrogen and carbonyl oxygen atoms. Compound 2 consists of one octahedral cationic [Co(L)(2)(H(2)O)(4)](2+) entity and two tetrahedral anionic [CoLBr(3)](-) units. In this system, the ligand molecules coordinate only through the pyridine nitrogen atoms. The magnetic properties of 1 and 2 were investigated in the temperature range of 2.0 to 300.0 K and correlations between both (due to the existence of similar features) examined. The study of the magnetic properties of 1 was carried out by considering each Co(II) ion as a perfectly isolated system, hence, J = 0, but taking into account a significant zero-field splitting contribution due to distortions on the tetrahedral environment of the cobalt atoms. The fit of the magnetic susceptibility data together with reduced magnetization vs H/T measurements provided similar parameters (|D| = 10.8 cm(-1), g(⊥) = 1.92, g(‖) = 2.92 for the former and |D| = 11.04 cm(-1)and g = 2.05 for the latter, respectively). On the other hand, the magnetic response of compound 2 has been analyzed using a model which considers the presence of two tetrahedral and one octahedral Co(ii) ions (Co(Td) and Co(Oh)). The study was carried out in two separated blocks, above and below 80 K, where only the most significant effects at each interval of temperature were considered. As a result, the analysis of the magnetic data shows weak antiferromagnetic interactions between the Co(Oh)and the two Co(Td) ions (J = -0.41 cm(-1)) in 2. The best fit parameters were g(Co(Td)) = 2.89, g(Co(Oh)) = 3.50, |D(Co(Td))| = 10.62 cm(-1), |E(Co(Td))| = 2.95 cm(-1), Δ = 240.9 cm(-1) and J(L-S) = -107.1 cm(-1), from where λ was calculated with a final value of -144.8 cm(-1) (J(L-S) = Aκλ). The approximations performed to obtain these values provide reasonable results in agreement with compound 1 and also to other systems in the literature.