The rational design of 3d-metal-based single-molecule
magnets (SMM)
requires a fundamental understanding of their intrinsic electronic
and structural properties and how they translate into experimentally
observable features. Here, we determined the magnetic properties of
the linear iron(I) silylamides K{crypt}[FeL2] and [KFeL2] (L = −N(Dipp)SiMe3; crypt = 4,7,13,16,21,24-Hexaoxa-1,10-diazabicyclo[8.8.8]hexacosan).
For the former, slow-relaxation of the magnetization with a spin reversal
barrier of U
eff = 152 cm–1 as well as a closed-waist magnetic hysteresis and magnetic blocking
below 2.5 K are observed. For the more linear [KFeL2],
in which the potassium cation is encapsulated by the aryl substituents
of the amide ligands, the relaxation barrier and the blocking temperature
increase to U
eff = 184 cm–1 and T
B = 4.5 K, respectively. The increase
is rationalized by a more pronounced axial anisotropy in [KFeL2] determined by dc-SQUID magnetometry. The effective relaxation
barrier of [KFeL2] is in agreement with the energy spacing
between the ground and first-excited magnetic states, as obtained
by field-dependent IR-spectroscopy (178 cm–1), magnetic
measurements (208 cm–1), as well as theoretical
analysis (212 cm–1). In comparison with the literature,
the results show that magnetic coercivity in linear iron(I) silylamides
is driven by the degree of linearity in conjunction with steric encumbrance,
whereas the ligand symmetry is a marginal factor.
We describe the synthesis of the linear N-heterocyclic carbene (NHC)-based chromium(I) silylamide [(IMes)Cr(N{Dipp}-Si i Pr 3 )] (IMes = 1,3-dimesityl-imidazol-2-ylidene, Dipp = 2,6-di-isopropylphenyl) via protolytic ligand displacement of the homoleptic chromium(I) silylamide [KCr(N{Dipp}Si i Pr 3 )] with the imidazolium salt IMes•HCl. This could also be extended to the bulkier NHC IDipp (IDipp = 1,3-bis(2,6-di-iso-propylphenyl)imidazol-2-ylidene). Analysis of the magnetic properties of [(IMes)Cr(N{Dipp}Si i Pr 3 )] reveals an isotropic high-spin S = 5/2 state. First reaction studies with this linear chromium(I) complex yields with CO the four-coordinate chromium dicarbonyl complex [(IMes)Cr(CO) 2 (N{Dipp}Si i Pr 3 )], with benzyl bromide oxidation to the trigonal chromium(II) bromide [(IMes)-Cr(Br)(N{Dipp}Si i Pr 3 )], and with DippN 3 the first three-coordinate chromium(III) imide ([(IMes)Cr(NDipp)(N{Dipp}Si i Pr 3 )]).
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