A crystallographic and EPR study of the fluxional Cu(ii) ion in [CuL2][BF4]2 (L = 2,6-dipyrazol-1-ylpyridine)

Abstract

This paper reports a crystallographic and EPR study of pseudo-Jahn-Teller fluxionality in [Cu(L 1 ) 2 ][BF 4 ] 2 (1; L 1 = 2,6-dipyrazol-1-ylpyridine). For 50 ≤ T ≤ 350 K, the Cu() ion in crystalline 1 is fluxional, with its axis of pseudo-Jahn-Teller elongation being disordered about the two N{pyrazole}-Cu-N{pyrazole} axes. The crystallographic data for 1 at these temperatures are well reproduced by a two-state model that neglects intermolecular interactions, but which yields an unusually small pseudo-Jahn-Teller radius (S pJT ) for the compound. This was confirmed by measuring S pJT independently from the mean-square displacement amplitudes (MSDAs) in 1 and [Zn(L 1 ) 2 ][BF 4 ] 2 (2). At 41 K, 1 undergoes a phase transformation to a new polymorph containing three molecules per asymmetric unit, which have static structures and exhibit more normal S pJT values. Q-band EPR data show that the proportion of static spins in a powdered sample of 1 grows in relatively slowly as the temperature is lowered below 40 K.

A [2 × 2] nickel(ii) grid and a copper(ii) square result from differing binding modes of a pyrazine-based diamide ligand

“…Indeed, EPR spectra with a superficial similarity to that of 4 have been attributed to copper(II) ions subjected to fluxional Jahn-Teller distortions 24 or in a d z 2 ground state. 25 In the present case, the essentially temperature independent appearance of both spectra from room temperature down to below 3 K contra-indicates these interpretations. More importantly, interpretations in terms of total spin S = 1/2 systems are clearly untenable given the tetranuclear configuration of these complexes and the EPR spectral features must arise from Table 3 Spin Hamiltonian parameters: g values, spin state multiplicities (S) and zero-field splitting parameters (D) and line widths for the simulated spectra of 4 shown in Fig.…”

CuII Complexes and Coordination Polymers with Pyridine or Pyrazine Amides and Amino Benzamides—Structures and EPR Patterns

“…The ∆g values for this group of complexes lie markedly below 0.2 and their g av values range from 2.1 to 2.17. A detailed inspection of the metrics around Cu II showed very similar geometries for [Cu(4-Aba) 2 (EtOH) 2 ](ClO 4 ) 2 (15) at 150 K and [Cu(4-Aba) 2 (EtOH) 2 ](BF 4 ) 2 ( 16) at 298 K. We therefore concluded that in this group either very tiny changes in geometry and with this a change of the character of the electronic ground state from the predominant {dx 2 -y 2 } 1 (axial) to a {dz 2 } 1 electronic ground state (rhombic or inverse axial) [42,47,48,54,55] were decisive and axial or rhombic spectra were observed. Importantly, this group is defined by a rather low g anisotropy ∆g.…”

Intramolecular Spin State Locking in Iron(II) 2,6-Di(pyrazol-3-yl)pyridine Complexes by Phenyl Groups: An Experimental Study

“…In addition, we synthesized a zinc(II) complex Zn(L1) 2 (ClO 4 ) 2 to check how an introduction of a non-spin transition metal ion (that shows no Jahn-Teller effect) would affect the distortion of the complex [M(L1) 2 ] 2+ and to identify if it is caused by the ligand design. Diamagnetic analogues of spin transition compounds are not usually structurally analysed [30,31] (e.g., one zinc(II) complex out of a hundred of Fe(II) complexes with 1-or 3-bpp ligands available in CSD, as of 2018), even though they are often used for metal dilution in magnetic measurements and are known to influence the spin state behaviour [32]. They can also be useful as a diamagnetic analogues of the paramagnetic compounds, such as cobalt(II) or HS iron(II) complexes, in an analysis of the NMR data by the recently proposed approach [29].…”

“…This behaviour indicates a D2d double axial distortion common for the iron(II) complexes of terpy [43] and both families of bpp ligands [12], with a geometry of the HS state deformed toward an edge-bicapped tetrahedron (ebcT-6) [10]. Such a distortion of the zinc(II) complex, however, cannot be explained by the Zn-N bond lengths being close to those of the HS iron(II) ion, as the zinc(II) ion has a more octahedral-like coordination environment in other complexes of bpp available in Cambridge structural database, CSD (see, e.g., refcodes JIZCUS [44], YODNOW [32], YODNUC [32], XEGFEU [30] and XIMSIV [31]). (Table 1) and NMR-based approach, respectively.…”