Temperature dependence of the spin state and geometry in tricobalt paddlewheel complexes with halide axial ligands

Trinuclear cobalt paddlewheel complexes, [Co3(dpa)4X2] (dpa = the anion of 2,2'-dipyridylamine, X = Cl-, Br-, -NCS-, -CN-, (NC)2N-), are known to demonstrate a thermally-induced spin-crossover (SCO). Despite a wealth of structural and magnetic information about such complexes, the role of the axial ligand on the characteristic SCO temperature (T1/2) remains ambiguous. The situation is complicated by the observation that the solid state geometry of the complexes, symmetric or unsymmetric, with respect to the central cobalt ion, also appears to influence the SCO behavior. In order to seek trends in the relationship between the nature of the axial ligand, geometry and magnetic properties, we have prepared the first examples of tricobalt paddlewheel complexes with axial fluorido and iodido ligands, as well as two new chlorido and bromido solvates. Their SCO properties are discussed in the context of an examination of previously reported chlorido and bromido adducts. The main conclusions are: (1) T1/2 values follow the trend I- < Br- ≈ Cl- < F-; (2) while the molecular geometry is predominantly guided by crystal packing for the Cl-, Br- and I- derivatives, the presence of an axial fluoride may favor a more symmetric core; (3) the magnetic characterization of a second example of an unsymmetric complex supports the observation that they display dramatically lower T1/2 values than their symmetric analogues; and (4) SCO in crystallographically symmetric compounds apparently occurs without loss of molecular or crystallographic symmetry, while a gradual geometric transformation linking the temperature dependence of quasi-symmetric to unsymmetric in crystallographically unconstrained compounds was found

Proton cascade in a molecular solid: H/D exchange on mobile and immobile water

Working without pores: In non-porous, channel-free crystals of a manganese citrate coordination polymer, neutron diffraction reveals that the water molecules, both metal-bound and co-crystallized, undergo full hydrogen/deuterium exchange. Neutron diffraction analyses show a pattern of hydrogen disorder that can be interpreted in terms of the Grotthuss proton-cascade mechanism. © 1999-2020 John Wiley & Sons, Inc

Ice Regelation: Hydrogen-bond extraordinary recoverability and water quasisolid-phase-boundary dispersivity

Regelation, i.e., ice melts under compression and freezes again when the pressure is relieved, remains puzzling since its discovery in 1850’s by Faraday. Here we show that hydrogen bond (O:H-O) cooperativity and its extraordinary recoverability resolve this anomaly. The H-O bond and the O:H nonbond possesses each a specific heat η(x)(T/Θ(Dx)) whose Debye temperature Θ(Dx) is proportional to its characteristic phonon frequency ω(x) according to Einstein’s relationship. A superposition of the η(x)(T/Θ(Dx)) curves for the H-O bond (x = H, ω(H) ~ 3200 cm(−1)) and the O:H nonbond (x = L, ω(L) ~ 200 cm(−1), Θ(DL) = 198 K) yields two intersecting temperatures that define the liquid/quasisolid/solid phase boundaries. Compression shortens the O:H nonbond and stiffens its phonon but does the opposite to the H-O bond through O-O Coulomb repulsion, which closes up the intersection temperatures and hence depress the melting temperature of quasisolid ice. Reproduction of the T(m)(P) profile clarifies that the H-O bond energy E(H) determines the T(m) with derivative of E(H) = 3.97 eV for bulk water and ice. Oxygen atom always finds bonding partners to retain its sp(3)-orbital hybridization once the O:H breaks, which ensures O:H-O bond recoverability to its original state once the pressure is relieved