Magnetic Studies on Hexahalorhenate(IV) Salts of Ferrocenium Cations [Fe(C₅R₅)₂]₂[ReX₆] (R = H, CH₃; X = Cl, Br, I)

Abstract

The hexahalorhenate(IV) salts of formula [Fe(C5H5)2]2[ReX6], with X = Cl (1), Br (2), and I (3), and [Fe(C5Me5)2]2[ReX6], with X = Cl (4), Br (5), and I (6) ([Fe(C5Me5)2]+ = decamethylferrocenium cation), have been synthesized and the structures of 1, 2, and 4 determined by single-crystal X-ray diffraction. 1, 2, and 4 crystallize in the orthorhombic system, space groups Pbca (1 and 2) and Ibam (4), with a = 14.099(2) A, b = 16.125(2) A, and c = 22.133(15) A, for 1, a = 14.317(3) A, b = 16.848(3) A, and c = 22.099(2) A for 2, and a = 15.8583(5) A, b = 15.9368(5) A, and c = 16.9816(6) A for 4. The three structures are made up of discrete [ReX6]2- anions and ferrocenium cations held together by electrostatic forces. There are anion-anion contacts in 1 and 2 but only through one direction. The [ReX6]2- octahedra are arranged along the y axis forming chains of Re and X atoms, -Re-X...X-Re-X...X-Re-, where the intermolecular X...X distances are shorter than the van der Waals distances. A somewhat greater separation between the anions occurs in 4. The magnetic properties of 1-6 were investigated in the temperature range 2.0-300 K. 1, 2, 4, and 5 exhibit an antiferromagnetic coupling between the anions, whereas a ferromagnetic coupling between anions and cations is the dominant interaction in 3. 6 behaves as a magnetically isolated compound, its susceptibility being the simple addition of the independent contributions of the uncoupled paramagnetic cations and anions.

Magnetic anisotropy in two- to eight-coordinated transition–metal complexes: Recent developments in molecular magnetism

“…The high sensitivity of the Re IV ion to the crystal field strength with this coordination geometry was confirmed by magnetic investigations of heavier halide analogues 78-80 (Fig. 27) [139]. The counter-cation in these complexes is a paramagnetic Fe III center, but its bulkiness prevents the propagation of intermolecular magnetic superexchange interactions.…”

“…However, the most interesting cases involved Re IV metal ions with octahedral geometry in a series of complexes (Fig. 27), which can be categorized as: (i) hexahalorhenates(IV) [119,[139][140][141][142][143][144][145][146] (79), and I (80)) [139], (ii) pentahalorhenates(IV) with one labile coordination site [147,148] such as (NH 2 Me 2 )[Re IV X 5 (DMF)] (where X = Cl (81) and Br (82)) [147], (iii) tetrahalorhenates(IV) with cis-blocking chelating organic ligands [110,[149][150][151][152][153][154][155][156] such as (NBu 4 ) 2 [Re(ox)X 4 ] (where ox = oxalate, X = Cl (83) and Br (84) For a hexa-coordinated 3d 3 ion with ideal O h geometry, significantly high first-order SOC and thus large magnetic anisotropy will be obtained from the triply degenerate ground state (involving d xy , d xz , and d yz orbitals) if Jahn-Teller distortion, which inevitably breaks the degeneracy and minimizes the SOC, is not present. In octahedral complexes, Jahn-Teller distortion splits the lowest lying triply degenerate d orbitals, thereby leading to two sets of orbitals, i.e., d xy and (d xz , d yz ), irrespective of whether the nature of the distortion is compression (where the energy order is d xy < (d xz , d yz )) or elongation (where the energy order is d xy > (d xz , d yz )).…”

“…Hence, quantifying the magnetic anisotropy of these complexes in magnetic studies appears to be quite tricky because of intermolecular magnetic superexchange interactions, even when the Re IV centers are separated by diamagnetic species in a crystal lattice [140,144,[146][147][148]157]. However, the isolation of mononuclear rhenate(IV) complexes with bulkier cations (even though they are paramagnetic [139]) cancels these intermolecular interactions, thereby allowing the estimation of magnetic anisotropy [119, 139-142, 146, 149-151, 153, 156, 158, 159, 168-171].…”

“…The counter-cation in these complexes is a paramagnetic Fe III center, but its bulkiness prevents the propagation of intermolecular magnetic superexchange interactions. Detailed magnetic investigations obtained g factors = 1.82, 1.85, and 1.83, and the absolute axial ZFS parameters were |D| = 16.0, 14.5, and 12.0 cm -1 for the Cl (78), Br (79), and I (80) analogues, respectively [139]. The D parameters for (AsPh 4 ) 2 [Re IV Cl 6 ] 2-and (NBu 4 ) 2 [Re IV Cl 6 ] 2-were positive with magnitudes of 13 and 9 cm -1 , respectively [156].…”

A Different 'Spin' on Rhenium Chemistry. Synthetic Approaches and Perspectives of 17-Electron Rhenium Complexes

“…The main difficulty, which remains a great challenge for the physical chemists, is that it is hard to predict and control the sign and magnitude of D. To achieve the desired anisotropy one idea is to move down the periodic table and utilize complexes of second-and thirdrow transition-metal ions, for which the greater spin-orbit coupling increases the magnitude of the zero-field splitting. The rhenium ion is particularly suited for this purpose due to: i) a large degree of spin delocalization of its complexes onto the ligands [32] and ii) its remarkable magnetic anisotropy due to the high value of the spin-orbit coupling parameter. [33] The strategy of utilizing second-and third-row transition-metal ions in the construction of SMM has been recently and successfully applied for the synthesis of Mo(iii)-based cyanide clusters [34,35] and in one case, for the preparation of a family of mixed-metal cyanide cubes containing octahedral Re(ii) vertices.…”