Neutron diffraction study of [Pt(en)₂][Pt(en)₂I₂](ClO₄)₄ at 20 K: structure and evidence of a new phase transition

Abstract

The structure at 20K of fully deuterated [Pt(en)2]-[Pt(en)212](C104) 4, where en=ethylenediamine, has been determined by neutron diffraction on a single crystal. It is shown that the symmetry is monoclinic with cell parameters a= 16.650(7), b = 5.760(2), c= 14.751(2)A and fl=99.19(1) '~, that is, with respect to the previously reported room-temperature structures, a doubling of the c parameter is observed. The space group is C2/c and the refinements lead to an R value of 0.0374. All the bond lengths (C--C, C--N, C--D, N--D and CI--O) and angles are consistent with the expected values and the main difference with respect to the previously reported room-temperature structures results in an ordering of the stacking of the ethylenediamines. The iodines are distributed over two sites, which gives two PtmI distances, 2.712 (3),~, attributed to the ptIV--I bond, and 3.048(3),~, attributed to PtnmI. Actually, very weak diffraction peaks that provide evidence for an additional doubling of the a and b parameters are also observed and a space group C2 is therefore proposed. This could result from a very slight ordering of the chains with respect to one another, but, due to the weakness of such signals and the large number of atoms to be considered, no reliable refinement has been obtained in C2. On the other hand, on heating the crystal the diffraction peaks with I odd greatly decrease in intensity between 150 and 160K, which unambiguously argues for the existence of a phase transition presumably connected to an order-disorder of the ethylenediamines.

Molecular Motions in Halogen-Bridged One-Dimensional Pt Complexes, [PtII(en)2][PtIVX2(en)2](ClO4)4 (X = Cl, Br) Studied by 2H and 1H NMR

“…The ∆ values obtained for chloro-complexes in the present study are larger than 2 -5 kJ mol 1 , determined for the iodo-complex [6]. This difference can be explained by the loose crystal packing of the iodocomplex [5,15], which is favorable for the puckering motion. In fact, it is concluded from X-ray and neutron diffraction measurements [5,7,15] that the steric hindrance for the motion around en-ligands in intrachain for iodo-complex [5,15] is smaller than that for the chloro-complex [7].…”

“…This difference can be explained by the loose crystal packing of the iodocomplex [5,15], which is favorable for the puckering motion. In fact, it is concluded from X-ray and neutron diffraction measurements [5,7,15] that the steric hindrance for the motion around en-ligands in intrachain for iodo-complex [5,15] is smaller than that for the chloro-complex [7]. From these reports and the present studies of 2 H NMR spectra and 1D measure-ments, most of the en-ligands in the chloro-complex have stable -or -and -or -conformations in the low and high-temperature phases, respectively, and the puckering motion of the en-skeleton takes place in the large asymmetric double minimum potential, but the lifetime, i.e., the population of the excited state is quite low because of the high ∆ value.…”

“…Characteristic electronic excitations in mixed valence states [4], and charge and spin migrations along the one-dimensional chains have been revealed. [Pt(en) 2 ][PtI 2 (en) 2 ](ClO 4 ) 4 was shown to have a structural phase transition with an order-disorder type as for the conformation of "en" chelate rings at 150 -160 K [5]. We showed from studies of 1 H and 2 H NMR spectra and spin-lattice relaxation times 1 that the disorder of the en conformation in the high-temperature phase is not static but dynamic, where the puckering motion of Pt(en) chelate rings between and conformations was well explained by the model of two-site jumps between two asymmetric potential wells with an energy difference of 2 -5 kJ mol 1 [6].…”

EXAFS Spectroscopy of Fractional Mixed-Valence Charge Density Wave Systems

“…These results are then used in Artemis to fit the data by varying parameters in eq . The structures of the PtX complexes have been determined previously by X-ray diffraction. ,− These crystallographic results were used in the Atoms program (version 0.9.25) to generate the atomic positions that were input into FEFF9, where calculations of the scattering potentials were done using a self-consistent field (SCF) approach with Hedin–Lundqvist self-energy correction. By default, each element in a material is given a unique potential.…”

Ultra-low temperature structure determination of a Mn12 single-molecule magnet and the interplay between lattice solvent and structural disorder

“…High pressure crystallography reveals a structural change commensurate with the change in relaxation rates, specifically related to the switching of a misaligned Jahn-Teller axis into alignment with the magnetic easy axis. 27 Many molecular crystals undergo structural transformations upon cooling as well as under pressure, [28][29][30] and it is possible that this could happen to 1 in a way that would influence the ultra-low temperature magnetic properties and quantum behaviour. …”