Crystal structure, spectroscopy and magnetism of binuclear complexes of mercury and lanthanides; the Ln(NCS)(HMPA)4(μ-SCN)2HgCl(SCN) type

Abstract

A series of lanthanide compounds of type Ln(NCS)(HMPA)4,(μ-SCN)2HgCl(SCN) (Ln = Pr, Nd, Eu) were synthesized and grown in the form of single crystals. The crystal structure of the neodymium complex was determined by X-ray diffraction. Its space group is Cc, with the following unit cell parameters; a = 17.338(3) Å, b = 15.795(3) Å, c = 21.828(4) Å, β = 107.65(3)°. The structure has an unexpected architecture in which one Cl− ion, four SCN− ions, and four oxygen atoms of HMPA groups are engaged in the metal ion coordination. The binuclear complex is composed of two types of subunits; seven coordinated Nd (III) and four coordinated Hg (II). The results obtained were compared with the earlier published data on the crystal structures of polynuclear complexes with ions of the IIa group (Zn or Cd). Luminescence, excitation of luminescence and absorption spectra of lanthanide (Pr, Nd, Eu) single crystals, as well as vibrational IR and Raman spectra at 293, 77 and 4K, were recorded. Non-trivial results of reabsorption of the d-level of Pr(III) emission by 3H4 → 3 PJ, 1D2 transitions were observed with simultaneous detection of emission from the 3P0 level after excitation in the UV region. The experimental oscillator strengths of the transitions were determined from the absorption spectra and parametrized in terms of the Judd-Ofelt intensity parameters Ωλ (λ = 2, 4, 6). Satisfactory results for the calculation with low errors of estimation of the parameters were obtained for a crystal of the Nd-Hg compound, which reproduced the intensities of the electronic transitions well. Positive values of Ωλ were evaluated for Pr(III) after including the 3H4 → 3F2 hypersensitive transition (obeying selection rules δJ = 2, δL = 2) in the calculations. Based on the above results, the radiative rate constant can be determined. Strong vibronic components were found in the low temperature spectra for both types of ligands involved in metal ion coordination. The vibronic transitions are mainly associated with modes of groups directly coordinated to the metal ions. Electron-phonon coupling including the resonant vibronic effect was analysed based on IR and Raman data. Magnetic susceptibility measurements were carried out down to 1.7 K. Correlation of the spectra and magnetic properties with details of the structure of the title compound was studied.