Asymmetric phase diagram and dimensional crossover in a system of spin-1/2 dimers under applied hydrostatic pressure

We present the magnetic and structural properties of [Cu(pyrazine)$_{0.5}$(glycine)]ClO$_4$ under applied pressure. As previously reported, at ambient pressure this material consists of quasi-two-dimensional layers of weakly coupled antiferromagnetic dimers which undergo Bose-Einstein condensation of triplet excitations between two magnetic field-induced quantum critical points (QCPs). The molecular building blocks from which the compound is constructed give rise to exchange strengths that are considerably lower than those found in other $S = 1/2$ dimer materials, which allows us to determine the pressure evolution of the entire field-temperature magnetic phase diagram using radio-frequency magnetometry. We find that a distinct phase emerges above the upper field-induced transition at elevated pressures and also show that an additional QCP is induced at zero-field at a critical pressure of $p_{\rm c} = 15.7(5)$ kbar. Pressure-dependent single-crystal X-ray diffraction and density functional theory calculations indicate that this QCP arises primarily from a dimensional crossover driven by an increase in the interdimer interactions between the planes. While the effect of quantum fluctuations on the lower field-induced transition is enhanced with applied pressure, quantum Monte Carlo calculations suggest that this alone cannot explain an unconventional asymmetry that develops in the phase diagram.Comment: Submitted to Phys. Rev.

Asymmetric phase diagram and dimensional crossover in a system of spin-spin- 1/2 dimers under applied hydrostatic pressure

We present the magnetic and structural properties of [Cu(pyrazine)0.5 (glycine)]ClO4 under applied pressure. As previously reported, at ambient pressure this material consists of quasi-two-dimensional layers of weakly coupled antiferromagnetic dimers which undergo Bose-Einstein condensation of triplet excitations between two magnetic field-induced quantum critical points (QCPs). The molecular building blocks from which the compound is constructed give rise to exchange strengths that are considerably lower than those found in other S=1/2 dimer materials, which allows us to determine the pressure evolution of the entire field-temperature magnetic phase diagram using radio-frequency magnetometry. We find that a distinct phase emerges above the upper field-induced transition at elevated pressures and also show that an additional QCP is induced at zero-field at a critical pressure of pc=15.7(5),kbar. Pressure-dependent single-crystal X-ray diffraction and density functional theory calculations indicate that this QCP arises primarily from a dimensional crossover driven by an increase in the interdimer interactions between the planes. While the effect of quantum fluctuations on the lower field-induced transition is enhanced with applied pressure, quantum Monte Carlo calculations suggest that this alone cannot explain an unconventional asymmetry that develops in the phase diagram

Heterobimetallic [NiFe] complexes containing mixed CO/CN− ligands: analogs of the active site of the [NiFe] hydrogenases

The development of synthetic analogs of the active sites of [NiFe] hydrogenases remains challenging and, in spite of the number of complexes featuring a [NiFe] center, those featuring CO and CN− ligands at the Fe center are under-represented. We report herein the synthesis of three bimetallic [NiFe] complexes [Ni(N2S2)Fe(CO)2(CN)2], [Ni(S4)Fe(CO)2(CN)2] and [Ni(N2S3)Fe(CO)2(CN)2] that each contain a Ni center that bridges through two thiolato S donors to a {Fe(CO)2(CN)2} unit. X-ray crystallographic studies on [Ni(N2S3)Fe(CO)2(CN)2], supported by DFT calculations, are consistent with a solid state structure containing distinct molecules in the singlet (S = 0) and triplet (S = 1) states. Each cluster exhibits irreversible reduction processes between −1.45 to −1.67 V vs Fc+/Fc and [Ni(N2S3)Fe(CO)2(CN)2] possesses a reversible oxidation process at 0.17 V vs Fc+/Fc. Spectroelectrochemical infrared (IR) and electron paramagnetic resonance (EPR) studies, supported by density functional theory (DFT) calculations, are consistent with a NiIIIFeII formulation for [Ni(N2S3)Fe(CO)2(CN)2]+. The SOMO in [Ni(N2S3)Fe(CO)2(CN)2]+ is based on Ni 3dz² and 3p S with the S contributions deriving principally from the apical S-donor. The nature of the SOMO corresponds to that proposed for the Ni-C state of the [NiFe] hydrogenases for which a NiIIIFeII formulation has also been proposed. A comparison of the experimental structures, and the electrochemical and spectroscopic properties of [Ni(N2S3)Fe(CO)2(CN)2] and its [Ni(N2S3)] precursor, together with calculations on the oxidized [Ni(N2S3)Fe(CO)2(CN)2]+ and [Ni(N2S3)]+ forms suggests that the binding of the {Fe(CO)(CN)2} unit to the {Ni(CysS)4} center at the active site of the [NiFe] hydrogenases suppresses thiolate-based oxidative chemistry involving the bridging thiolate S donors. This is in addition to the role of the Fe center in modulating the redox potential and geometry, and supporting a bridging hydride species between the Ni and Fe centers in the Ni-C state.

Halogen-substituted ureas for anion binding: solid state and solution studies

Herein, we report the synthesis and the anion binding properties of a family of N,N′-diphenylureas L1-L15, bearing on the aromatic ring(s) halogens (chlorine and iodine) and/or nitro or trifluoromethyl electron-withdrawing groups. The analysis of the crystal structures obtained from single crystal X-ray diffraction experiments shows that self-assembled chains or tapes connected via N–H···O hydrogen bonds are the most commonly adopted arrangements for this type of molecules in the crystal lattice. In the presence of anion guests or solvent molecules with competing hydrogen bond donors and acceptors, other supramolecular arrangements can be observed. Solution studies conducted in DMSOd 6/0.5% H2O by means of 1H-NMR titrations show the formation of 1:1 adducts with all receptors. The different observed affinities of the receptors for the anion guests were rationalised in terms of steric hindrance of the substituents on the phenyl rings and their electron-withdrawing properties

Heterobimetallic [NiFe] complexes containing mixedCO/CN− ligands: analogs of the active site of the [NiFe]hydrogenases

The development of synthetic analogs of the active sites of [NiFe] hydrogenases remains challenging and, in spite of the number of complexes featuring a [NiFe] center, those featuring CO and CN− ligands at the Fe center are under-represented. We report herein the synthesis of three bimetallic [NiFe] complexes [Ni(N2S2)Fe(CO)2(CN)2], [Ni(S4)Fe(CO)2(CN)2] and [Ni(N2S3)Fe(CO)2(CN)2] that each contain a Ni center that bridges through two thiolato S donors to a {Fe(CO)2(CN)2} unit. X-ray crystallographic studies on [Ni(N2S3)Fe(CO)2(CN)2], supported by DFT calculations, are consistent with a solid state structure containing distinct molecules in the singlet (S = 0) and triplet (S = 1) states. Each cluster exhibits irreversible reduction processes between −1.45 to −1.67 V vs Fc+/Fc and [Ni(N2S3)Fe(CO)2(CN)2] possesses a reversible oxidation process at 0.17 V vs Fc+/Fc. Spectroelectrochemical infrared (IR) and electron paramagnetic resonance (EPR) studies, supported by density functional theory (DFT) calculations, are consistent with a NiIIIFeII formulation for [Ni(N2S3)Fe(CO)2(CN)2]+. The SOMO in [Ni(N2S3)Fe(CO)2(CN)2]+ is based on Ni 3dz² and 3p S with the S contributions deriving principally from the apical S-donor. The nature of the SOMO corresponds to that proposed for the Ni-C state of the [NiFe] hydrogenases for which a NiIIIFeII formulation has also been proposed. A comparison of the experimental structures, and the electrochemical and spectroscopic properties of [Ni(N2S3)Fe(CO)2(CN)2] and its [Ni(N2S3)] precursor, together with calculations on the oxidized [Ni(N2S3)Fe(CO)2(CN)2]+ and [Ni(N2S3)]+ forms suggests that the binding of the {Fe(CO)(CN)2} unit to the {Ni(CysS)4} center at the active site of the [NiFe] hydrogenases suppresses thiolate-based oxidative chemistry involving the bridging thiolate S donors. This is in addition to the role of the Fe center in modulating the redox potential and geometry, and supporting a bridging hydride species between the Ni and Fe centers in the Ni-C state.

Comparative study of the structural and magnetic properties of Mn1/3NbS2 and Cr1/3NbS2

We compare the magnetism of single crystals of the intercalated transition metal dichalcogenides Cr1/3NbS2 and Mn1/3NbS2 using techniques such as dc and ac susceptibility and Lorentz transmission electron microscopy (LTEM). We present a detailed structural investigation of these materials using electron and single-crystal x-ray diffraction measurements to show how substitutional disorder and stacking faults can manifest in Cr1/3NbS2 and Mn1/3NbS2, and give rise to additional superlattice reflections in diffraction patterns acquired from Mn1/3NbS2. Magnetic susceptibility and LTEM measurements show Cr1/3NbS2 displays chiral helimagnetism below its magnetic ordering temperature (TC) of 111 K, while there is no evidence that Mn1/3NbS2 exhibits helimagnetic ordering below its transition temperature TC = 45 K

Halogen-substituted ureas for anion binding: solid state and solution studies

Herein, we report the synthesis and the anion binding properties of a family of N,N′-diphenylureas L1-L15, bearing on the aromatic ring(s) halogens (chlorine and iodine) and/or nitro or trifluoromethyl electron-withdrawing groups. The analysis of the crystal structures obtained from single crystal X-ray diffraction experiments shows that self-assembled chains or tapes connected via N–H···O hydrogen bonds are the most commonly adopted arrangements for this type of molecules in the crystal lattice. In the presence of anion guests or solvent molecules with competing hydrogen bond donors and acceptors, other supramolecular arrangements can be observed. Solution studies conducted in DMSOd 6/0.5% H2O by means of 1H-NMR titrations show the formation of 1:1 adducts with all receptors. The different observed affinities of the receptors for the anion guests were rationalised in terms of steric hindrance of the substituents on the phenyl rings and their electron-withdrawing properties

Diagnostic value of exome and whole genome sequencing in craniosynostosis

Background. Craniosynostosis, the premature fusion of one or more cranial sutures, occurs in ~1 in 2250 births, either in isolation or as part of a syndrome. Mutations in at least 57 genes have been associated with craniosynostosis, but only a minority of these are included in routine laboratory genetic testing. Methods. We utilised exome or whole genome sequencing to seek a genetic cause in a cohort of 40 subjects with craniosynostosis, selected by clinical or molecular geneticists as being high priority cases, and in whom prior clinically-driven genetic testing had been negative. Results. We identified likely associated mutations in 15 patients (37.5%), involving 14 different genes. All genes were mutated in single families, except for IL11RA (2 families). We classified the other positive diagnoses as follows: commonly mutated craniosynostosis genes with atypical presentation (EFNB1, TWIST1); other core craniosynostosis genes (CDC45, MSX2, ZIC1); genes for which mutations are only rarely associated with craniosynostosis (FBN1, HUWE1, KRAS, STAT3); and known disease genes for which a causal relationship with craniosynostosis is currently unknown (AHDC1, NTRK2). In two further families, likely novel disease genes are currently undergoing functional validation. In 5 of the 15 positive cases, the (previously unanticipated) molecular diagnosis had immediate, actionable consequences for either genetic or medical management (mutations in EFNB1, FBN1, KRAS, NTRK2, STAT3). Conclusions. This substantial genetic heterogeneity, and the multiple actionable mutations identified, emphasises the benefits of exome/whole genome sequencing to identify causal mutations in craniosynostosis cases for which routine clinical testing has yielded negative results

Diagnostic value of exome and whole genome sequencing in craniosynostosis

Background Craniosynostosis, the premature fusion of one or more cranial sutures, occurs in ~1 in 2250 births, either in isolation or as part of a syndrome. Mutations in at least 57 genes have been associated with craniosynostosis, but only a minority of these are included in routine laboratory genetic testing. Methods We used exome or whole genome sequencing to seek a genetic cause in a cohort of 40 subjects with craniosynostosis, selected by clinical or molecular geneticists as being high-priority cases, and in whom prior clinically driven genetic testing had been negative. Results We identified likely associated mutations in 15 patients (37.5%), involving 14 different genes. All genes were mutated in single families, except for IL11RA (two families). We classified the other positive diagnoses as follows: commonly mutated craniosynostosis genes with atypical presentation (EFNB1, TWIST1); other core craniosynostosis genes (CDC45, MSX2, ZIC1); genes for which mutations are only rarely associated with craniosynostosis (FBN1, HUWE1, KRAS, STAT3); and known disease genes for which a causal relationship with craniosynostosis is currently unknown (AHDC1, NTRK2). In two further families, likely novel disease genes are currently undergoing functional validation. In 5 of the 15 positive cases, the (previously unanticipated) molecular diagnosis had immediate, actionable consequences for either genetic or medical management (mutations in EFNB1, FBN1, KRAS, NTRK2, STAT3). Conclusions This substantial genetic heterogeneity, and the multiple actionable mutations identified, emphasises the benefits of exome/whole genome sequencing to identify causal mutations in craniosynostosis cases for which routine clinical testing has yielded negative results