ESR and Magnetic Susceptibility Studies on High-Spin Tetrahedral Cobalt(II)–Thiolate Complexes: An Approach to Rubredoxin-Type Active Sites

Abstract

Magnetic susceptibility and ESR studies on high-spin (S=3⁄2) tetrahedral cobalt(II)–thiolate complexes were performed for A2[Co(SPh)4]; A=Me4N (1a), Et4N (1b), Ph4P (1c) and (Et4N)2[Co(SR)4]; R=C6H4-p-Me (2), C6H4-p-Cl (3), C6F5 (4). It was found that the magnitude of zero-field splitting and the anisotropy of g-factor are exceptionally large for 1c as D≈−100 cm−1, E≈0, gx≈gy<gz. This parameter set shows that Co(SPh)42− in 1c is strongly affected by an axial distortion. On the contrary, a rhombic distortion with moderate magnitude was found for 1a (D*=6.5 cm−1, E⁄D=−0.21) and 1b (D*=5.0 cm−1, E⁄D=0.19). The results for the other compounds are D*=7.0 cm−1, E⁄D=0.14 for 2, D*=12 cm−1, E⁄D=0.10 for 3, and D*=2.0 cm−1, E⁄D=0.02 for 4. Attempts were made to correlate the magnetic properties and the coordination geometry for 1a–c by use of available X-ray data. As a result, the importance of the orientation of the thiolate ligands was confirmed.

High-spin Co(II) in monomeric and exchange coupled oligomeric structures: Magnetic and magnetic circular dichroism investigations

“…3). The D-parameter obtained, equal to −30 cm −1 looks rather large for the tetrahedrally coordinated [5].…”

“…But, there is a remarkable overlap between particular intervals corresponding to different coordinations. Some rare cases are reported with a high value of ZFS which lies far outside the typical intervals, namely D ≈ −100 cm −1 for one distorted tetrahedrally coordinated Co(II) ion [5]. The value of ZFS D = −30 cm −1 found in this work for the tetrahedrally coordinated Co(tu) 4 (NO 3 ) 2 ·H 2 O is on the one hand much lower, but, if compared to the typical D values, still high.…”

“…The only exception is the binuclear complex [Co 2 ( -OAc) 3 (urea)(tmen) 2 ][OTf] (5) for which a positive exchange constant J ex = 18 cm −1 was obtained. The major difference between the antiferromagnetically coupled complex (3) and ferromagnetically coupled complex (5) is the substitution of the hydroxamate bridge in (3) by an acetato bridge in (5).…”

“…Compared to compound (3), the bridging hydroxamate group is substituted by an acetate group in (5). In order to compare, in com- pound (4) the bridging water substitutes the bridging oxygen of the hydroxamate group in compound (3) and the acetate group in compound (5).…”

“…Whereas for octahedral coordination, D could be up to 100 cm −1 (or even more), for tetrahedral coordination D values are moderate, up to around 10 cm −1 [5]. Surprisingly, D values of 100 cm −1 were reported even for a high-spin tetrahedral Co(II)-thiolate complex [5]. Fivefold coordinated Co(II) complexes have D values about 70 cm −1 [6].…”