Role of Coordination Geometry on the Magnetic Relaxation Dynamics of Isomeric Five-Coordinate Low-Spin Co(II) Complexes

Abstract

To investigate the influence of the coordination geometry on the magnetization relaxation dynamics, two geometric isomers of a fivecoordinate low-spin Co(II) complex with the general molecular formula [Co(DPPE) 2 Cl]SnCl 3 (DPPE = diphenylphosphinoethane) were synthesized and structurally characterized. While one isomer has a square pyramidal geometry (Co-SP (1)), the other isomer figures a trigonal bipyramidal geometry (Co-TBP (2)). Both complexes were already reported elsewhere. The spin state of these complexes is unambiguously determined by detailed direct current (dc) magnetic data, X-band, and high-frequency EPR measurements. Slow relaxation of magnetization is commonly observed for systems with S > 1/2. However, both 1 and 2 show field-induced slow relaxation of magnetization. Especially 1 shows relaxation times up to τ = 35 ms at T = 1.8 K, which is much longer than the reported values for undiluted Co(II) low-spin monomers. In 2, the maximal field-induced relaxation time is suppressed to τ = 5 ms. We attribute this to the change in g-anisotropy, which is, in turn, correlated to the spatial arrangement of ligands (i.e., coordination geometry) around the Co(II) ions. Besides the detailed electronic structure of these complexes, the experimental observations are further corroborated by theoretical calculations.

Cobalt-Catalyzed Hydrogenation Reactions Enabled by Ligand-Based Storage of Dihydrogen

“…EPR spectra of organic radicals are typically more isotropic and with all g -values near g = 2.0, suggesting that a Co­(II) electronic structure may be appropriate . Indeed, literature examples where a five- or six-coordinate Co­(III) center is bound to a ligand radical as well as previous examples with DHP radical ligands, namely, [ t Bu,Tol DHP 2–• ]Ni and [ Ph,Tol DHP 2–• ]­Ni, all have smaller g -anisotropy than that of 1 , further supporting a [DHP – ]­Co­(II) resonance structure. ,, While the greater g -anisotropy of 1 is different from low spin, square-planar cobalt complexes, it is distinctly similar to related tetrahedral or see-saw complexes . The best simulation we have obtained uses hyperfine coupling (MHz) to both 59 Co ( A xx = 57.6, A yy = 62.4, A zz = 58.8) and 14 N ( A xx = 35.4, A yy = 44.7, A zz = 10.7), although we note that the complicated pattern means that alternative spin systems, for instance, those with coupling to more than one 14 N nucleus, may also provide satisfactory fits.…”

“…9a , 21 , 9c While the greater g -anisotropy of 1 is different from low spin, square-planar cobalt complexes, 22 it is distinctly similar to related tetrahedral or see-saw complexes. 23 The best simulation we have obtained uses hyperfine coupling (MHz) to both 59 Co ( A xx = 57.6, A yy = 62.4, A zz = 58.8) and 14 N ( A xx = 35.4, A yy = 44.7, A zz = 10.7), although we note that the complicated pattern means that alternative spin systems, for instance, those with coupling to more than one 14 N nucleus, may also provide satisfactory fits. While the g -anisotropy and 59 Co hyperfine constants support a Co(II) oxidation state, the large 14 N hyperfine suggests that there is still significant spin on the DHP ligand.…”

Slow magnetic relaxation of a S = 1/2 copper(ii)-substituted Keggin-type silicotungstate

“…To date, slow magnetic relaxation in S = 1/2 systems has been reported in mononuclear complexes of V IV , 2 d , e , h , 8 Mn IV , 9 Fe III , 10 Co II , 11 Ni I , 12 Ni III , 13 and Cu II . 2 h ,14 These complexes carry organic ligands and reported Raman exponents, n , of around 7–3.…”

Magnetic properties of two coordination isomeric cobalt(ii) single-ion magnets

“…In particular, research on the magnetic properties of isomeric Co(II) complexes provides a unique platform for exploring the magneto-structural correlation. 20 To the best of our knowledge, few examples of six-and seven-coordinate isomeric Co (II)-SIMs have been investigated, except for the Co(II) complexes bearing nitrite ligands reported by Sato, which exhibit coordination numbers switching between six and seven. 21 With the design of isomeric molecular magnets in mind, we have developed a new strategy to construct mononuclear Co(II) complexes based on the bidentate ligand 1,8-naphthyridine (napy) and nitrate ion.…”