Search

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]

Reaction of ArSNᆖC(py)2 [Ar = 2,4-dinitrophenyl, (O2N)2C6H3] with CuCl2·2H2O in DMF yielded the tetranuclear complex [Me2NH2][Cu4Cl6{py2CO2}{py2C(OH)O}] (1) through metal-catalysed hydrolysis of both DMF and the thiazyl chain. Notably direct reaction of dipyridylketone (py2CO) with CuCl2·2H2O in DMF yielded the mononuclear pseudo-octahedral complex [CuCl2(py2C(OH)2)(DMF)]·DMF (2) in which the 1,1-gem diol, py2C(OH)2 acts as a tripodal ligand. Heating of 2 in DMF/MeOH promoted aggregation and formation of the polymer [Cu2Cl3{py2(OMe)O}]∞ (3). The structures of 1–3 were determined by X-ray diffraction. [ABSTRACT FROM AUTHOR]

Two homochiral M–O–M bridged mixed valence complexes [CoIII 2 CoII(mpm) 6 ]·(ClO 4) 2 (1) and [CoII 2 CoIII 2 (μ 3 -OMe) 2 (μ-mpm) 4 (NO 3) 2 (MeO) 2 ] (2) with linear and defect dicubane topologies respectively were prepared and characterized from the reaction of R - and S -mpmH (where mpmH = -methyl-2- pyridinemethanol) with an appropriate Co(II) salt. Two mixed-valence cobalt complexes, [CoIICoIII 2 (mpm) 6 ](ClO 4) 2 (1) and [CoII 2 CoIII 2 (μ 3 -OMe) 2 (μ-mpm) 4 (NO 3) 4 ] (2), where mpm is the deprotonated form of R or S α-methyl-2-pyridinemethanol, have been structurally and magnetically characterized. Both complexes crystallize in chiral space groups, where 1 is a trinuclear compound with a linear Co(III)–Co(II)–Co(III) topology and 2 is a tetranuclear cluster with a defect dicubane-type structure that has two missing vertices. In both cases the mpm− ligand bridges adjacent Co ions in a η1:η2:μ manner. The molecular geometries of the complexes reveal the Co ions are octahedral, where the greatest deviation from O h symmetry is apparent for the HS d7 Co(II) ions. The chirality of 1 and 2 were studied by circular dichroism spectroscopy where opposite Cotton effects are observed for the R- and S -enantiomers of both complexes. Magnetic susceptibility studies reveal the presence of largely quenched first order orbital angular momentum together with weak ion or intermolecular dipole exchange. [ABSTRACT FROM AUTHOR]

A linear Cu 4 complex containing a combination of square pyramidal and octahedrally coordinated Cu(II) ions is reported. The metal centres are linked via diazine and chloride bridges. Magnetic studies reveal the presence of strong antiferromagnetic interactions via the diazine bridge and moderate ferromagnetic interactions via the chloro bridges. The pyrimidinyl hydrazone ligand O C{NHN C(Me)(pm)} 2 (pm = 2-pyrimidinyl) (L 3 H 2) has two potentially protic H atoms and multiple donor atoms. The reaction of L 3 H 2 with CuCl 2 ·2H 2 O yielded the tetranuclear complex [Cu 4 (L 3 H) 2 Cl 6 (CH 3 OH) 2 ] (1). Structural studies on 1 reveal the molecule is located about a crystallographic inversion centre, affording two crystallographically independent Cu(II) centres with the two symmetry equivalent [Cu 2 (L 3 H)Cl 3 CH 3 OH] units connected via µ 2 -Cl bridges to generate a linear tetranuclear copper complex. The ligand is singly deprotonated (L 3 H−) and twisted around the diazine N N′ bond, affording an N 3 donor set for the Cu1 centre and an N 2 O donor set for the Cu2 centre. The Cu1 ion is five coordinate with an [N 3 ] donor set from the L 3 H− ligand and the remaining two sites occupied by Cl− anions. The Cu2 ion is six coordinate with the L 3 H− ligand offering a mer [N 2 O] donor set, with the three remaining sites occupied by two Cl− anions and a MeOH molecule. Magnetic studies reveal the presence of strong antiferromagnetic interactions via the diazine bridge linking the Cu1 and Cu2 ions and moderate ferromagnetic interactions via the chloro bridges located about the inversion centre, connecting the Cu2 and Cu2i ions. [ABSTRACT FROM AUTHOR]

The synthesis of the novel redox active ligand 3-(2′,6′-pyrimidine)-benzo-1,2,4-thiadiazine (pmbtdaH) is reported. The radical pmbtda can be prepared by in situ 1 e − oxidation and its radical character confirmed by EPR spectroscopy, along with DFT calculations of the spin density distribution. Reaction of pmbtdaH with ZnCl 2 ·2H 2 O in a 1:1 mol ratio afforded the dinuclear complex Zn 2 Cl 4 (pmbtdaH) 2 ( 1 ) in which the two trigonal bipyramidal metal centres are linked via μ 2 -bridging Cl − ligands while the pmbtdaH ligand binds in an N,N′-chelate fashion via N(4) and the pyrimidinyl N(1) atom. The outcome of the reaction of pmbtdaH with Zn(hfac) 2 (hfac = hexafluoroacetylacetonate) proved sensitive to the crystallization method; the mononuclear octahedral complex Zn(hfac) 2 (pmbtdaH) ( 2 ) in which the ligand coordinates in a similar N,N′-chelate fashion to that observed in 1 was isolated by layering a concentrated CH 2 Cl 2 solution with hexane. Conversely the same reaction mixture afforded the tetranuclear complex Zn 4 (hfac) 6 (pmbtda ox ) 2 ( 3 ) when left undisturbed in a sealed vial. In 3 the ligand is both deprotonated and oxidized forming an anionic ligand in which there are two potential N,N′-chelate pockets and an additional S-oxide donor. The pmbtda ox − ligand bridges between a Zn(hfac) 2 and a Zn(hfac) unit to form Zn 2 (hfac) 3 (pmbtda ox ). Two such Zn 2 (hfac) 3 (pmbtda ox ) units dimerise through the S-oxide oxygen atom to form a centrosymmetric tetranuclear Zn 4 (hfac) 6 (pmbtda ox ) 2 cluster. The structures of 1 – 3 have been determined by X-ray diffraction, elemental analysis, IR and UV–Vis spectroscopies as well as 1 H NMR spectroscopy for complex 2 . [ABSTRACT FROM AUTHOR]

The oxidative addition reaction of bis (4′,5′-di-methoxybenzo-)-1,2,5,6-tetrathiocin ( 1 ) to zero-valent Pt(PPh 3 ) 4 under microwave irradiation for 30 min afforded a mixture of the title Pt(II) complexes, (dmobdt)Pt(PPh 3 ) 2 ( 2 ) and [(dmobdt)Pt(PPh 3 )] 2 ( 3 ) [dmobdt 2− = 4,5-dimethoxybenzene-1,2-dithiolate]. Under extended reaction times the dimetallic complex 3 was isolated as the exclusive Pt-containing product indicating 2 is an intermediate en route to 3 . The structures of 1 – 3 have been determined by X-ray diffraction. [ABSTRACT FROM AUTHOR]

Abstract: The metal-templated synthesis of the bimetallic macrocyclic complex {CoII(N3O2)Cl(OH2)}2·2Cl·H2O (2) containing two N3O2 macrocyclic binding pockets was achieved from the metal-templated Schiff-base condensation of 2,2′-6,6′-tetracetyl-4,4′-bipyridine together with 3,6-dioxaoctane-1,8-diamine. Slow diffusion of (2) with K4[Fe(CN)6] in a (1:1) mixture of methanol and water afforded the 1-D coordination polymer [{Co(N3O2)(OH2)}2{Fe(CN)6}]⋅5H2O (5) whose structure was determined by X-ray diffraction. The polymer comprises two covalently tethered pentagonal bipyramidal Co(II) macrocycles both axially coordinated to a single [Fe(CN)6]4− unit in a zig-zag arrangement, highlighting the versatility of this family of dimeric building blocks for the self-assembly of higher order structures. Magnetic studies on (2) reveal that the ions exist as isolated S =3/2 spin systems with no first-order spin–orbit coupling, but with significant axial zero field splitting (D =23cm−1, g iso =2.37), indicating that the back-to-back N3O2 macrocyclic ligand does not offer an efficient magnetic exchange pathway. The magnetic response of (5) in the high temperature region (T >50K) is identical to (2) indicating dominant zero field splitting effects. However, on further cooling an upturn in χT is consistent with weak ferromagnetic exchange (J/k =+7.8K) via the [Fe(CN)6]4− bridging unit, affording weakly-coupled CoII dimer units within the polymer chain of (5). [Copyright &y& Elsevier]

The coordination chemistry of a flexible polytopic ligand containing diazine and hydrazone functionalities has been investigated with Mn to afford a coordination polymer. Magnetic studies reveal the complex is an S = 5/2 Mn(II) dimer with weak antiferromagnetic interactions between the dimers, consistent with the isolation of a 1-D Heisenberg chain. [Display omitted] The polydentate ligand L H μ contains bipyridine, hydrazone and oxime functionalities and reacts with Mn(CH 3 COO) 2 ·4H 2 O to afford an alternating one dimensional chain comprising Mn(II) ions, [{Mn 2 (L H)(CH 3 COO) 3 (CH 3 OH) 2 }·CH 3 OH] n (1). The structure of 1 contains two Mn(II) ions in the asymmetric unit; Mn1 adopts an unusual pentagonal bipyramidal coordination geometry comprising an [N 3 O 4 ] donor set, while Mn2 adopts a more conventional octahedral geometry coordinated by an [NO 5 ] donor set. The two manganese ions within the dinuclear unit are connected to each either via O hydrazone and syn-syn O acetate bridges. These dinuclear units are then linked by anti-anti O acetate bridges, affording a magnetic 1-D alternating chain topology. Magnetic studies on 1 reveal the presence of weak antiferromagnetic interactions via O hydrazone and acetate bridges. The magnetism of 1 was modelled using a dimer model [Ĥ = −2JŜ 1 Ŝ 2 ], with inter-dimer exchange modelled using a mean field term (zJ') to afford g = 2.046 ± 0.002, J/k = -1.03 ± 0.02 cm−1 and zJ' = −0.06 ± 0.01 cm−1. [ABSTRACT FROM AUTHOR]

Abstract: Dithiazolyl radicals with π-stacking motifs have attracted particular interest because of their ability to exhibit spin-switching between diamagnetic distorted π-stacks and paramagnetic regular π-stacked structures through a solid state phase transition. Previous studies indicate that inclusion of electronegative heteroatoms into the backbone favours lamellar structures. This methodology has been extended to the synthesis and characterisation of the title compound, 4′-cyanobenzo-1,3,2-dithiazolyl (4-NCBDTA). Its electronic structure is probed through DFT calculations, cyclic voltammetry and EPR spectroscopy and its crystal structure determined by X-ray powder diffraction at room temperature. Variable temperature SQUID magnetometry reveals that 4-NCBDTA undergoes two phase transitions, each exhibiting bistability; a high temperature phase transition occurs at room temperature (T C↓ =291K, T C↑ =304K, ΔT =13K); whilst the low temperature phase transition occurs below liquid nitrogen temperatures (T C↓ =37K, T C↑ =28K;ΔT =9K). [Copyright &y& Elsevier]

Abstract: We have studied, by means of ab initio calculations, the magnetic interaction mechanisms in four radical crystals, X–C6F4–CNSSN (X =O2N, α-NC, β-NC, Br), which has allowed us to explain their different magnetic behaviour (ferromagnetism, antiferromagnetism, paramagnetism, spin frustration, etc.). First, we have identified the magnetic exchange pathways considering those with distances between two atoms of different dithiadiazolyl rings shorter than 7Å and those with an intermolecular distance between an atom of the heterocyclic ring and an atom in a neighbouring radical shorter than 4Å. Second, the calculations have been carried out in the framework of the DFT Broken Symmetry. Following this procedure we have determined the magnitude and the sign of the relevant coupling constants for the X–C6F4–CNSSN (X =O2N, α-NC, β-NC, Br) radicals. In the cases where the radicals order magnetically, ordering temperatures determined with our ab initio calculations agree very well with the experimental ones. Thus, in the case of the O2N derivative ferromagnetic ordering is observed below 1.3K, in very good agreement with an ordering temperature around 1.6K predicted from our calculated exchange constants and using a mean field approximation. [Copyright &y& Elsevier]

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]

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 p-O2N·C6F4·CNSSN⋅ radical was chosen as the most favourable CNSSN⋅ derivative to study the spin distribution in this kind of free radical by polarised neutron diffraction. Spin-density maps obtained for the p-O2N·C6F4·CNSSN⋅ 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 hydrogen bonds and C H⋯π interactions administer the crystal structure. • One of the most efficient methods for producing nanoparticles is ultrasonic irradiation. • The strong intermolecular π⋯π stacking interaction, in addition to hydrogen bonds, creates the system more stable. The zinc coordination complexes [ZnBr 2 (DTDS)]⋯H 2 O (1) [DTDS = 1,2-di(1 H -1,2,4-triazol-3-yl) disulfane] and [ZnI(2,2′-bipy)(chb)] (2) (2,2′-bipy = 2,2′-bipyridine; Hchb = 5-chloro-2-hydroxybenzophenone) were prepared as single crystals by conventional solution chemistry and as crystalline nanoparticles under ultrasonic irradiation. The structures of 1 and 2 were determined by single crystal X-ray diffraction. PXRD confirmed that the nanoparticles have the same structure. They were additionally characterized by scanning electron microscopy. The structures were analyzed by Hirshfeld surface analyses to provide more information on the supramolecular interactions and crystal packing. [ABSTRACT FROM AUTHOR]