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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= [ABSTRACT FROM AUTHOR]

A series of dichlorophenyl dithiadiazolyls (DCP-DTDA) have been prepared and structurally characterized by X-ray diffraction (1 = 2,3-DCP-DTDA; 2 = 2,4-DCP-DTDA; 3 = 2,5-DCP-DTDA, 4 = 3,4-DCP-DTDA and 5 = 3,5-DCP-DTDA). All the radicals adopt -cofacial π*-π* dimers with intra-dimer S...S contacts in the range 3.119(8)--3.300(5) Å. The propensity for chloro-aromatics to adopt a β-sheet structure assists formation of lamellar structures in 1-5 with intra-stack Cl⋯Cl contacts in the range 3.44(1)-3.82(1) Å. Perpendicular to the stacking direction, the packing is dominated by formation of intermolecular S⋯Cl contacts. Variable-temperature magnetic susceptibility measurements on 1-5 in the range 5-300 K reveal the onset of paramagnetism around 200 K. Solid-state EPR studies reveal that this paramagnetism is associated with thermal population of a triplet excited state. A good fit to the Bleaney-Bowers model determined the exchange interactions (2 ) between the radicals in the dimers to be in the range -904 to -1529 cm-1. Computational studies of the dimerisation process indicate that at intra-dimer S⋯S distances in excess of 3.2 Å the closed-shell singlet becomes destabilized with respect to both the open-shell singlet and triplet state configurations. [ABSTRACT FROM AUTHOR]

Abstract: The perovskite-like salt [p-ClC6H4NH3]2CuCl4 1 exhibits a wealth of magnetic and structural phase transitions which have been probed by variable temperature single crystal X-ray diffraction, SQUID magnetometry, resonant ultrasound spectroscopy (RUS), EPR spectroscopy, DSC measurements and DFT calculations. Single crystal X-ray diffraction studies between room temperature and 3 K reveal a rich tapestry of structural changes; at 298 K the structure conforms to a monoclinic setting but undergoes a first order phase transition upon cooling below ∼275 K to a higher symmetry orthorhombic cell. This is facilitated by a transition to an intermediate phase at ∼277 K. Whilst the intermediate phase has a limited stability window (∼2 K) and has not been structurally determined, the two discrete phase transitions at 275.5 K and 277 K have been clearly detected by differential scanning calorimetry, EPR spectroscopy and RUS studies. On further cooling a dynamic relaxation process is observed in RUS measurements, evidenced by a Debye-like peak in the dissipation at ∼140 K. Residual electron density maps from single crystal X-ray diffraction studies reveal that this may be associated with a freezing out of the NH3⋯Cl hydrogen-bonding between cation and anion frameworks upon cooling. The activation barrier for this order/disorder process was estimated to be at least 27 kJ mol−1 from the RUS data. Variable temperature dc SQUID data reveal that 1 is a 2D ferromagnet with antiferromagnetic interactions between layers below 9 K. Analysis of the temperature dependence of the magnetic susceptibility for T > 40 K reveals that 1 exhibits Curie–Weiss behaviour with θ = +22.8 K indicative of dominant ferromagnetic interactions. Good agreement is observed between the strength of the ferromagnetic interaction extracted from the Weiss constant (J/k = +22.8 K) and that calculated by DFT (J/k = +25 to +28 K) and from EPR studies (J/k = +17 K). The presence of short range ferromagnetic interactions is reflected in a marked temperature dependence of the g-factors determined from EPR spectroscopy below 20 K and possibly a small elastic anomaly in the RUS data. RUS studies indicate a very small elastic anomaly associated with the transition to long range order, implying weak or no magnetoelastic effect. At fields above ∼15 G a spin flip transition is induced and 1 displays metamagnetic behaviour, with a saturation magnetization of 0.96 μ B. [Copyright &y& Elsevier]

7-(4-Fluorophenyl) and 7-phenyl-substituted 1,3-diphenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl radicals were characterized by X-ray diffraction analysis and variable-temperature magnetic susceptibility studies. The radicals pack in 1D π stacks of equally spaced slipped radicals with interplanar distances of 3.59 and 3.67 Å and longitudinal angles of 40.97 and 43.47°, respectively. Magnetic-susceptibility studies showed that both radicals exhibit antiferromagnetic interactions. Fitting the magnetic data revealed that the behavior is consistent with 1D regular linear antiferromagnetic chain with J=−12.9 cm−1, zJ′=−0.4 cm−1, g=2.0069 and J=−11.8 cm−1, zJ′=−6.5 cm−1, g=2.0071, respectively. Magnetic-exchange interactions in benzotriazinyl radicals are sensitive to the degree of slippage, and inter-radical separation and subtle changes in structure alter the fine balance between ferro- and antiferromagnetic interactions. [ABSTRACT FROM AUTHOR]

The structure of the canted antiferromagnet β- p-NCC6F4CNSSN ( 1) was determined from synchrotron powder-diffraction studies in the pressure range 0-21.6 kbar. Radical 1 crystallizes in the orthorhombic space group Fdd2, but undergoes an asymmetric contraction of the unit-cell size with increasing pressure. At the molecular level, this contraction of the unit cell is simultaneously accommodated by: 1) an increase in twist angle between aryl and heterocyclic rings; and 2) a shortening of the intermolecular S ⋅⋅⋅N contacts, which propagate the magnetic-exchange pathway. DFT calculations based on the structures in this pressure range revealed an increase in the magnetic-exchange interaction ( J) with increasing pressure, and an excellent correlation was observed between J and the magnetic-ordering temperature, which increased from 36 K at ambient pressure up to 70 K at 16 kbar. [ABSTRACT FROM AUTHOR]

X-ray studies show that 1,3-diphenyl-7-(thien-2-yl)-1,4-dihydro-1,2,4-benzotriazin-4-yl ( 6) adopts a distorted, slipped π-stacked structure of centrosymmetric dimers with alternate short and long interplanar distances (3.48 and 3.52 Å). Cyclic voltammograms of 7-(thien-2-yl)benzotriazin-4-yl 6 show two fully reversible waves that correspond to the −1/0 and 0/+1 processes. EPR and DFT studies on radical 6 indicate that the spin density is mainly delocalized over the triazinyl fragment. Magnetic susceptibility measurements show that radical 6 obeys Curie-Weiss behavior in the 5-300 K region with C=0.378 emu K mol−1 and θ=+4.72 K, which is consistent with ferromagnetic interactions between S=1/2 radicals. Fitting the magnetic susceptibility revealed the behavior is consistent with an alternating ferromagnetic chain ( g=2.0071, J1=+7.12 cm−1, J2=+1.28 cm−1). [ABSTRACT FROM AUTHOR]

Abstract: EPR, electrochemical studies and neutron diffraction studies on a large range of 1,2,3,5-dithiadiazolyl radicals have shown that the spin density distribution is essentially unaffected by substituent effects. The variation of magnetic properties previously reported for a series of perfluorophenyl-dithiadiazolyl radicals p-XC6F4CNSSN (X=CN, Br, NO2 and NCC6F4) is therefore due to differences in molecular packing. The nearest-neighbour magnetic exchange interactions in these radicals are probed through DFT studies (B3LYP/6-311G**) and are shown to exhibit a variety of both ferromagnetic and antiferromagnetic interactions. All exchange interactions with | J |>1cm−1 coincide with close heterocyclic contacts between dithiadiazolyl rings (3.18–4.00Å) and produce | J | values up to 40K. An analysis of these interactions indicates that the dominant exchange pathway would appear to be a direct exchange mechanism between radical centres, with orthogonality of the singly-occupied molecular orbitals (SOMOs) favouring ferromagnetic interactions. [Copyright &y& Elsevier]

The cover picture shows a 2:1 charge transfer complex formed from a tetrathiafulvalene (TTF)–pyridylverdazyldiradical cation and tetracyanoquinonedimethane (TCNQ) which crystallizes in a mixed‐stack arrangement. These building blocks show promise for the development of spintronic devices where the spin of the electrons as well as their charge can be used to process information. More details can be found in the Full Paper by Melanie Pilkington et al. on page 1624 in Issue 11, 2015 (DOI: 10.1002/cplu.201500309). [ABSTRACT FROM AUTHOR]

The syntheses, structures, and magnetic properties of three new coordination complexes, tetranuclear [Zn2L³(OAc)(OMe)]2⋅3MeOH⋅H2O (3), trinuclear [Ni3(L³)3]⋅6H2O (4), and a 1-D chain {[Cu2L³(OAc)2]2⋅H2O}n (6), of a polydentate, doubly deprotonated, 3,3'-disubstituted bipyridine ligand [L³]2- are reported. The X-ray crystal structures demonstrate that the ditopic ligand provides a flexible N3 donor set for transition metal ions where each binding pocket shifts from fac to intermediate fac/mer to the mer isomer affording a Ni3 triangle, a Zn4 tetramer, and a 1-D Cu(II) polymer, respectively. This variation in coordination preference is rationalized with the aim of designing future ligands with controlled coordination modes. Magnetic susceptibility studies on 4 reveal it belongs to the rare family of ferromagnetically coupled [Ni3] clusters. In contrast, magnetic studies of the 1-D chain 6 reveal weak antiferromagnetic interactions due to the poor orbital overlap of the singly occupied Cu(II) ... orbitals with the one-atom bridge that connects them along the Jahn-Teller distortion axis. [ABSTRACT FROM AUTHOR]

1-Phenyl-3-trifluoromethyl-l,4-dihydrobenzo[e][1,2,4]triazin-4-yl is the first example of a hydrazyl radical that shows a reversible sharp spin transition fully completed within 5(1) K. The nominally first-order transition takes place at ca. 58(2) K and proceeds via subtle changes of intra- and interstack interactions between two similar structural phases. The low-temperature phase (5—60 K) is diamagnetic and has a singlet ground state (2Jexp= -166.8 cm-1, gsolid = 2.0042, ρ = 0.2%) stemming from a multicenter two-electron interaction. The high-temperature phase (60—300 K) is paramagnetic as a result of noninteracting S = 1/2 spins arising from weakly bound dimers. [ABSTRACT FROM AUTHOR]

Titanium dioxide (TiO2) doped with transition-metal ions (M) has potentially broad applications in photocatalysis, photovoltaics, and photosensors. One approach to these materials is through controlled hydrolysis of well-defined transition-metal titanium oxo cage compounds. However, to date very few such cages have been unequivocally characterized, a situation which we have sought to address here with the development of a simple synthetic approach which allows the incorporation of a range of metal ions into titanium oxo cage arrangements. The solvothermal reactions of Ti(OEt)4 with transition-metal dichlorides (MIICl2, M = Co, Zn, Fe, Cu) give the heterometallic transition-metal titanium oxo cages [Ti4O(OEt)15(MCl)] [M = Co (2), Zn (3), Fe (4), Cu (5)], having similar MTi4(μ4-O) structural arrangements involving ion pairing of [Ti4O(OEt)15]- anion units with MCl+ fragments. In the case of the reaction of MnCl2, however, two MnII ions are incorporated into this framework, giving the hexanuclear Mn2Ti4(μ4-O) cage [Ti4O(OEt)15(Mn2Cl3)] (6) in which the MCl+ fragments in 2-5 are replaced by a [ClMn(μ-Cl)MnCl]+ unit. Emphasizing that the nature of the heterometallic cage is dependent on the metal ion (M) present, the reaction of Ti(OEt)4 with NiCl2 gives [Ti2(OEt)9(NiCl)]2 (7), which has a dimeric Ni(μ-Cl)2Ni bridged arrangement arising from the association of [Ti2(OEt)9]- ions with NiCl+ units. The syntheses, solid-state structures, spectroscopic and magnetic properties of 2-7 are presented, a first step toward their applications as precursor materials. [ABSTRACT FROM AUTHOR]

Knowledge of the spin-density distribution in the dithiadiazolyl radical ring (DTDA) constitutes a major step towards the understanding of the magnetic and electronic properties of the rich magnetism of DTDA derivatives. The O2N·C6F4CNSSN radical was chosen as the most favourable CNSSN derivative to study the spin distribution in this kind of free radicals by polarised-neutron diffraction. Spin-density maps obtained for the O2N C6F4CNSSN⋅ radical show that almost all the spin density is localised on the sulphur and nitrogen atoms of the CNSSN⋅ ring. A small negative spin density on the carbon atom of the CNSSN⋅ ring and a negligible spin density over the rest of the radical are observed, in good agreement with ab initio calculations. [Copyright &y& Elsevier]

The incorporation of the electronegative cyano group into the structure of the title dithiazolyl radical 1 induces a lamellar packing motif that favors spin-transition behavior. Variable-temperature magnetic studies on 1 reveal a reversible phase transition at 250 K between a diamagnetic low-temperature phase and a paramagnetic high-temperature phase. X-ray crystal structures at 180(2) and 300(2) K reveal that both phases crystallize in the space group P21/c, with the phase transition associated with a doubling of the crystallographic a axis. The concomitant change in magnetic properties is associated with a change from a paramagnetic regular stack of radicals and a diamagnetic distorted π stack in which an energy gap opens up at the Fermi level corresponding to a Peierls-like transition. [ABSTRACT FROM AUTHOR]