High-field EPR study and crystal and molecular structure of trans-RSSR-[CrCl₂(cyclam)]_nX (X = ZnCl₄²⁻, Cl⁻and Cl⁻·4H₂O·0.5HCl)

Abstract

For the first time, HF-EPR (94.5 GHz) spectroscopy has been used to determine crystal field parameters in chromium(III) coordination compounds. The large zero-field splitting parameters of the dark-green photochromic trans-RSSR-[CrCl(2)(cyclam)](2)ZnCl(4), 1, the red-purple trans-RSSR-[CrCl(2)(cyclam)]Cl, 2, and the red-purple trans-RSSR-[CrCl(2)(cyclam)]Cl.4H(2)O.0.5HCl, 3, where cyclam = 1,4,8,11-tetraazacyclotetradecane, have been obtained. A full analysis of EPR spectra at 94.5 GHz of diluted complexes 1, 2 and 3 at 300 K revealed that they are extremely sensitive to D and E values. The rhombic distortion was precisely determined for each compound. For 1, g= 2.01, D=-0.305 cm(-1), E= 0.041 cm(-1) and lambda=|E/D|= 0.1396; for 2, g= 2.01; D=-0.348 cm(-1), E= 0.042 cm(-1) and lambda=|E/D|= 0.1206 and for 3, g= 1.99, D=-0.320 cm(-1), E= 0.041 cm(-1) and ambda=|E/D|= 0.1281. The EPR study at 94.5 GHz at 10 K allowed us to confirm the sign of the D value for all compounds. These data indicate that at room temperature the crystal field is mainly rhombic and as the temperature decreases, the rhombicity of the D tensor increases slightly. These found differences between 1, 2 and 3 allowed us to establish the importance of the intermolecular interactions in the solid state due to different hydrogen bonding networks in their crystalline arrangement.

Terminological confusions and problems at the interface between the crystal field Hamiltonians and the zero-field splitting Hamiltonians—Survey of the CF=ZFS confusion in recent literature

“…The two notions CFPs and ZFSPs have been evidently considered as synonymous by Solano-Peralta et al [142]. The ZFSPs appearing in their SH as well as several times in text were correctly named as, e.g.…”

“…'zero-field splitting: axial (D) and rhombic (E) components'. However, in Abstract it is stated, quote [142]: 'For the first time, HF-EPR (94.5 GHz) spectroscopy has been used to determine crystal field parameters in chromium(III) coordination compounds. The large zero-field splitting parameters of… have been obtained' and 'the crystal field is mainly rhombic'.…”

EMR-related problems at the interface between the crystal field Hamiltonians and the zero-field splitting Hamiltonians

“…(1) Confusion between the CF (LF) and SH (ZFS) quantities, mainly the CF=ZFS confusion exemplifi ed by consideration of the two notions: the CF parameters (CFPs) and the ZFS parameters evidently as synonymous, occurs in recent EMR--related papers, e.g., [22][23][24][25][26][27][28][29][30][31][32]; for critical comments, see [14]. Apart from the problems and confusions identifi ed in the review on molecular magnets [16], serious forms of the CF=ZFS confusion occur also in the EMR and SMM/MNM related textbook [13].…”

Crystal structure and energy transfer of trans-RSSR-[CrCl2(cyclam)]3[Cr(CN)6]·14H2O

“…Very recently, it has been found that the differences in the magnetic and optical properties of the trans-RSSR-[CrCl 2 (cyclam)] n X n , when X = Cl À , ZnCl 2À 4 compounds, have been attributed to supramolecular interactions between the chromium cations and the ZnCl 2À 4 [1][2][3]. On the other hand, from luminescence studies of the double salt [Cr(en) 3 ][Cr(CN) 6 ] AE XH 2 O, it has been reported that an energy transfer occurs because lifetime of the 2 E g state of donor [Cr(en) 3 ] 3+ (6.0 ns) is in agreement with rise time of the 2 E g state of acceptor [Cr(CN) 6 ] 3À (6.2 ns) [4].…”

“…Within a project about the characterisation of the counterion role in the explanation of the photochromism exhibited by trans-RSSR-[CrCl 2 (cyclam)] 2 ZnCl 4 [1,2] and in order to understand which are the factors that are affecting this property, we synthesised and characterised by X-ray diffraction the trans-[CrCl 2 (cyclam)] 3 [Cr(CN) 6 ] AE 14H 2 O. We also carried out a photoluminescence study on this compound.…”