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A systematic study of the influence of ligand field on the slow magnetic dynamics of Co()-diimine compounds
Herein we report heteroleptic Co( ii ) diimine complexes [Co(H 2 bip) 2 Cl 2 ] ( 1 ), [Co(H 2 bip) 2 Br 2 ] ( 2 ), [Co(H 2 bip) 3 ]Br 2 ·1MeOH ( 3 ) and [Co(H 2 bip) 2 (Me 2 bpy)]Br 2 ·(MeCN) 0.5 ·(H 2 O) 0.25 ( 4 ) (H 2 bip = 2,2′-bi-1,4,5,6-tetrahydropyrimidine, bpy = 2,2′-dipyridyl, Me 2 bpy = 4,...
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Published in: | Dalton transactions : an international journal of inorganic chemistry 2021-08, Vol.5 (31), p.1737-1748 |
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Main Authors: | , , |
Format: | Article |
Language: | English |
Subjects: | |
Citations: | Items that this one cites Items that cite this one |
Online Access: | Get full text |
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Summary: | Herein we report heteroleptic Co(
ii
) diimine complexes [Co(H
2
bip)
2
Cl
2
] (
1
), [Co(H
2
bip)
2
Br
2
] (
2
), [Co(H
2
bip)
3
]Br
2
·1MeOH (
3
) and [Co(H
2
bip)
2
(Me
2
bpy)]Br
2
·(MeCN)
0.5
·(H
2
O)
0.25
(
4
) (H
2
bip = 2,2′-bi-1,4,5,6-tetrahydropyrimidine, bpy = 2,2′-dipyridyl, Me
2
bpy = 4,4′-Me-2,2′-dipyridyl), purposefully prepared to enable a systematic study of magnetic property changes arising from the increase of overall ligand field from σ/π-donor chlorido (
1
) to π-acceptor 4,4′Me-2,2′bpy (
4
). The presence of axial and rhombic anisotropy (
D
and
E
) of these compounds is sufficient to allow
1-4
to show field-induced slow relaxation of magnetization. Interestingly, we found as the effective ligand field is increased in the series, rhombicity (
E
/
D
) decreases, and the magnetic relaxation profile changes significantly, where relaxation of magnetization at a specific temperature becomes gradually faster. We performed mechanistic analyses of the temperature dependence of magnetic relaxation times considering Orbach relaxation processes, Raman-like relaxation and quantum tunnelling of magnetization (QTM). The effective energy barrier of the Orbach relaxation process (
U
eff
) is largest in compound
1
(19.2 cm
−1
) and gradually decreases in the order
1
>
2
>
3
>
4
giving a minimum value in compound
4
(8.3 cm
−1
), where the Raman-like mechanism showed the possibility of different types of phonon activity below and above ∼2.5 K. As a precursor of
1
, the tetrahedral complex [Co(H
2
bip)Cl
2
] (
1a
) was also synthesized and structurally and magnetically characterized: this compound exhibits slow relaxation of magnetization under an applied dc field (1800 Oe) with a record slow relaxation time of 3.39 s at 1.8 K.
A systematic study of four structurally-related hexacoordinate Co(
ii
) diimine complexes demonstrates links between ligand donor (σ/π) characteristics and slow magnetic dynamics and spin-lattice relaxation times. |
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ISSN: | 1477-9226 1477-9234 |
DOI: | 10.1039/d0dt01597k |