Mössbauer, Electron Paramagnetic Resonance, and Magnetic Susceptibility Studies on Members of a New Family of Cyano-Bridged 3d-4f Complexes. Demonstration of Anisotropic Exchange in a Fe−Gd Complex

Content Provider	Scilit
Author	Stoian, Sebastian Paraschiv, Carmen Kiritsakas, Nathalie Lloret, Francesc Münck, Eckard Bominaar, Emile L. Andruh, Marius
Copyright Year	2010
Abstract	The synthesis and crystallographic characterization of a new family of M(μ-CN)Ln complexes are reported. Two structural series have been prepared by reacting in water rare earth nitrates $(Ln^{III}$ = La, Pr, Nd, Sm, Eu, Gd, Dy, Ho) with $K_{3}[M(CN)_{6}$] $(M^{III}$ = Fe, Co) in the presence of hexamethylenetetramine (hmt). The first series consists of six isomorphous heterobinuclear complexes, $[(CN)_{5}M-CN-Ln(H_{2}O)_{8}$]·2hmt ([FeLa] 1, [FePr] 2, [FeNd] 3, [FeSm] 4, [FeEu] 5, [FeGd] 6), while the second series consists of four isostructural ionic complexes, $[M(CN)_{6}][Ln(H_{2}O)_{8}$]·hmt ([FeDy] 7, [FeHo] 8, [CoEu] 9, [CoGd] 10). The hexamethylenetetramine molecules contribute to the stabilization of the crystals by participating in an extended network of hydrogen bond interactions. In both series the aqua ligands are hydrogen bonded to the nitrogen atoms from both the terminal $CN^{−}$ groups and the hmt molecules. The [FeGd] complex has been analyzed with$ ^{57}$Fe Mössbauer spectroscopy and magnetic susceptibility measurements. We have also analyzed the [FeLa] complex, in which the paramagnetic $Gd^{III}$ is replaced by diamagnetic $La^{III}$, with$ ^{57}$Fe Mössbauer spectroscopy, electron paramagnetic resonance (EPR), and magnetic susceptibility measurements, to obtain information about the low-spin $Fe^{III}$ site that is not accessible in the presence of a paramagnetic ion at the complementary site. For the same reason, the [CoGd] complex, containing diamagnetic $Co^{III}$, was studied with EPR and magnetic susceptibility measurements, which confirmed the S = 7/2 spin of $Gd^{III}$. Prior knowledge about the paramagnetic sites in [FeGd] allows a detailed analysis of the exchange interactions between them. In particular, the question of whether the exchange interaction in [FeGd] is isotropic or anisotropic has been addressed. Standard variable-temperature magnetic susceptibility measurements provide only the value for a linear combination of $J_{x}$, $J_{y}$, and $J_{z}$ but contain no information about the values of the individual exchange parameters $J_{x}$, $J_{y}$, and $J_{z}$. In contrast, the spin-Hamiltonian analysis of the variable-field, variable-temperature Mössbauer spectra reveals an exquisite sensitivity on the anisotropic exchange parameters. Analysis of these dependencies in conjunction with adopting the g-values obtained for [FeLa], yielded the values $J_{x}$ = +0.11 $cm^{−1}$, $J_{y}$ = +0.33 $cm^{−1}$, and $J_{z}$ = +1.20 $cm^{−1}$ $(Ŝ_{1}·J·Ŝ_{2}$ convention). The consistency of these results with magnetic susceptibility data is analyzed. The exchange anisotropy is rooted in the spatial anisotropy of the low-spin $Fe^{III}$ ion. The condition for anisotropic exchange is the presence of low-lying orbital excited states at the ferric site that (i) effectively interact through spin−orbit coupling with the orbital ground state and (ii) have an exchange parameter with the Gd site with a value different from that for the ground state. Density functional theory (DFT) calculations, without spin−orbit coupling, reveal that the unpaired electron of the t_{2g}$^{5}$ ground configuration of the $Fe^{III}$ ion occupies the xy orbital, that is, the orbital along the plane perpendicular to the Fe···Gd vector. The exchange-coupling constants for this orbital, $j_{xy}$, and for the other $t_{2g}$ orbitals, $j_{yz}$ and $j_{xz}$, have been determined using a theoretical model that relates them to the anisotropic exchange parameters and the g-values of $Fe^{III}$. The resulting values, $j_{yz}$ = −5.7 $cm^{−1}$, $j_{xz}$ = −4.9 $cm^{−1}$, and $j_{xy}$ = +0.3 $cm^{−1}$ are quite different. The origin of the difference is briefly discussed.
Related Links	http://europepmc.org/articles/pmc2856468?pdf=render https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2856468/pdf
Ending Page	3401
Page Count	15
Starting Page	3387
ISSN	00201669
e-ISSN	1520510X
DOI	10.1021/ic902516r
Journal	Inorganic Chemistry
Issue Number	7
Volume Number	49
Language	English
Publisher	American Chemical Society (ACS)
Publisher Date	2010-03-12
Access Restriction	Open
Subject Keyword	Journal: Inorganic Chemistry Inorganic Chemistry 3d-4f Interaction Magnetic Susceptibility Anisotropic Exchange Coupling
Content Type	Text
Resource Type	Article
Subject	Physical and Theoretical Chemistry Inorganic Chemistry