Paramagnetic Hexacyanometalates. The Diversity of Spin Distribution Studied by ¹³C and ¹⁵N MAS NMR Spectroscopy

Abstract

With the aim of probing the spin density distribution in the open-shell cyanometallates Cs2K[M(CN)6] (M = Cr, Mn, Fe), K3[M(CN)6] (M = Mn, Fe), K4[M(CN)6] (M = Cr, Mn), and K4[V(CN)7] have been studied by solid-state (13)C and (15)N NMR spectroscopy. The signals appear in strongly shifted and broad ranges ((13)C, -2100 to -8900 ppm; (15)N, -1900 to 2400 ppm) except K4[V(CN)7], which is NMR-silent. Analysis of the isotropic signal shifts yields negative spin density in all carbon 2s orbitals (up to 12.2% at the six ligands of [Mn(CN)6](3-)) and positive spin density in all nitrogen 2s orbitals (up to 1.1% at the six ligands of [Mn(CN)6](4-) and [Fe(CN)6](3-)). This is in accord with the induction of alternating spin at the CN ligands by successive polarization of σ bonds triggered by the spin center M(n+). The signal shift anisotropies are related to spin in the carbon and nitrogen 2pπ and 2pσ orbitals. In the case of Cs2K[Cr(CN)6] and K4[Cr(CN)6] much positive spin is found in the nitrogen 2pπ orbitals, which corresponds to direct M → N spin transfer. On Cs2K[M(CN)6] (M = Mn, Fe), the 2pπ spin density at nitrogen is negative. The results are in accord with and extend the data of polarized neutron diffraction and EPR spectroscopy. Owing to high signal resolution, small deviations of the [M(CN)6](n-) ions from octahedral symmetry and disorder of crystal layers have been detected. This corresponds to the crystal symmetry and to Jahn-Teller distortion. The disorder entails a scatter of spin densities. In the case of K4[Mn(CN)6] it reaches 19% for the C 2s orbitals and 80% for the N 2s orbitals with regard to the respective smallest spin population.

Probing the Local Magnetic Structure of the [FeIII(Tp)(CN)3]− Building Block Via Solid‐State NMR Spectroscopy, Polarized Neutron Diffraction, and First‐Principle Calculations

“…À3 )a nd the unpaired electron fractions are reported in Table 2. [31,100] On one hand, the polarizationm echanism of the M-C link (z axis) is consistentw itht he retained axial symmetry of the 13 CMAS NMR spectra. The involvement of 1s orbitals through cross terms with 2s orbitals is not excluded, but that should not affect the general conclusions.…”

“…The broad sideband features do not allow nonequivalent cyanido ligands to be distinguished, likely because the coupling with the 14 N quadrupolar nuclei and the dipolar interaction with paramagnetic neighbor ions lead to a significant line broadening, as observed previously for other reported cyanido complexes . The line broadening may also arise from anisotropic bulk magnetic susceptibility and the existence of a small gradient temperature in the rotor.…”

“…[31,100] The line broadening may also arise from anisotropicb ulk magnetic susceptibility and the existence of a small gradientt emperature in the rotor.H owever,asignificantly better agreement between the experimental and simulated spectra was obtained when assuming two different types of 13 CN ligand are present in the ratio 2:1( Figure 3). [31,100] The line broadening may also arise from anisotropicb ulk magnetic susceptibility and the existence of a small gradientt emperature in the rotor.H owever,asignificantly better agreement between the experimental and simulated spectra was obtained when assuming two different types of 13 CN ligand are present in the ratio 2:1( Figure 3).…”

“…However,the analysis of the EPR data relies on effective Hamiltonian (g-tensors and/ or zero-field splitting parameters). [31][32][33] However,n one of these previousw orks were subjected to ab-initio calculations. We have shown that PND analyses on as ingle crystal (at moderate fields and in three orthogonal directions) give access to the magnetic susceptibility tensor, and we obtained ad irect pictureo ft he magnetic anisotropy on the iron ion, revealing as trong axial anisotropy along the C 3 pseudo-axis.…”

“…From an experimental point of view, the challenge consists in finding a reliable way to evaluate the pseudo‐contact contribution in order to extract the Fermi‐contact term from the paramagnetic chemical shift. Some of us previously used simple models based on the point‐dipole approximation to carry out such studies on cyanide‐based materials such as hexacyanometallate and Prussian blue analogues . However, none of these previous works were subjected to ab‐initio calculations.…”