In this study, we synthesized a new Co(II) complex, NMe42Co(bpyO2)2 (1), using deprotonated 2,2′-bipyridine-6,6′-diol ligands (bpyO2 2–). This compound exhibits a significant zero-field splitting ...(D) value. The far-infrared magneto spectroscopy and high-frequency and field electron paramagnetic resonance (HFEPR) measurements indicated that compound 1 possesses D = −54.8 cm–1 and E ∼ 0 cm–1. These findings were subsequently confirmed by other experimental data, including DC magnetic susceptibilities and variable temperature and variable magnetic field reduced magnetizations. Additionally, we conducted a series of AC magnetic susceptibility measurements to investigate the kinetics of magnetization relaxation. Below 6.6 K and under zero external magnetic field, fast quantum tunneling of magnetization (QTM) dominates (∼570 Hz), and temperature-independent out-of-phase signals are observed. Above 8.1 K, temperature-dependent behavior is observed. Furthermore, we examined the AC magnetic susceptibility behavior under external magnetic fields ranging from 300 to 4000 G. The effect of QTM is significantly reduced in the presence of an external magnetic field. Temperature-dependent behavior is primarily governed by Raman relaxation. Through structural analysis of compound 1 and a series of pure nitrogen-coordinated single-ion magnets (SIMs), we propose that the oxo substituents from the double-deprotonated form of the 2,2′-bipyridine-6,6′-diol ligands donate their negative charge to the pyridine ring, forming amido anion sites. This triggers a more pronounced out-of-phase signal than that observed in pure pyridine-coordinated compounds. Moreover, we observed intermolecular interactions, including intermolecular hydrogen bonding, which, to some extent, influenced the slow relaxation of molecules. Therefore, we speculate that the slow relaxation phenomenon of compound 1 may be attributed to the combination of oxo back-donating effects and intermolecular interactions.
The synthesis and properties of an unusual, neutrally charged and volatile N,N,N′,N′-tetramethylethylenediamine trimethyl manganese(III) complex, (TMEDA)MnMe3, are described, along with its facile ...disproportionation to the corresponding Mn(II) and Mn(IV) complexes. Characterization by single-crystal XRD, UV–vis spectroscopy, high-frequency and -field EPR (HFEPR), magnetic susceptibility, and density functional theory (DFT) computations indicate that the (TMEDA)MnMe3 electronic structure can be described as largely square pyramidal Mn(III) centered. The paucity of manganese(III) polyalkyls and the simplicity and reactivity of this compound implicate it as a potentially useful synthetic building block.
The nitrido‐ate complex (PN)2Ti(N){μ2‐K(OEt2)}2 (1) (PN−=(N‐(2‐PiPr2‐4‐methylphenyl)‐2,4,6‐Me3C6H2) reductively couples CO and isocyanides in the presence of DME or cryptand (Kryptofix222), to form ...rare, five‐coordinate TiII complexes having a linear cumulene motif, K(L)(PN)2Ti(NCE) (E=O, L=Kryptofix222, (2); E=NAd, L=3 DME, (3); E=NtBu, L=3 DME, (4); E=NAd, L=Kryptofix222, (5)). Oxidation of 2–5 with FcOTf afforded an isostructural TiIII center containing a neutral cumulene, (PN)2Ti(NCE) (E=O, (6); E=NAd (7), NtBu (8)) and characterization by CW X‐band EPR spectroscopy, revealed unpaired electron to be metal centric. Moreover, 1e− reduction of 6 and 7 in the presence of Kryptofix222cleanly reformed corresponding discrete TiII complexes 2 and 5, which were further characterized by solution magnetization measurements and high‐frequency and ‐field EPR (HFEPR) spectroscopy. Furthermore, oxidation of 7 with Fc*B(C6F5)4 resulted in a ligand disproportionated TiIV complex having transoid carbodiimides, (PN)2Ti(NCNAd)2 (9). Comparison of spectroscopic, structural, and computational data for the divalent, trivalent, and tetravalent systems, including their 15N enriched isotopomers demonstrate these cumulenes to decrease in order of backbonding as TiII→TiIII→TiIV and increasing order of π‐donation as TiII→TiIII→TiIV, thus displaying more covalency in TiIII species. Lastly, we show a synthetic cycle whereby complex 1 can deliver an N‐atom to CO and CNAd.
A titanium nitride undergoes N−C bond coupling with carbon monoxide and isocyanides to form TiII cumulene complexes (PN)2Ti(NCE)− (E=O, NAd, NtBu). Oxidation of these affords the TiIII cumulene (PN)2Ti(NCE). Further oxidation affords a TiIV bis‐cumulene trans‐(PN)2Ti(NCNAd)2. Structural, electrochemical, and spectroscopic studies (HFEPR, CW X‐band, NMR and IR spectroscopies), including 15N isotopic labelling studies are presented and discussed to understand the bonding and topology.
A new linear trinuclear Co(II)3 complex with a formula of {Co(μ-L)}2Co has been prepared by self-assembly of Co(II) ions and the N3O3-tripodal Schiff base ligand H3L, which is obtained from the ...condensation of 1,1,1-tris(aminomethyl)ethane and salicylaldehyde. Single X-ray diffraction shows that this compound is centrosymmetric with triple-phenolate bridging groups connecting neighboring Co(II) ions, leading to a paddle-wheel-like structure with a pseudo-C 3 axis lying in the Co–Co–Co direction. The Co(II) ions at both ends of the Co(II)3 molecule exhibit distorted trigonal prismatic CoN3O3 geometry, whereas the Co(II) at the middle presents an elongated trigonal antiprismatic CoO6 geometry. The combined analysis of the magnetic data and theoretical calculations reveal strong easy-axis magnetic anisotropy for both types of Co(II) ions (|D| values higher than 115 cm–1) with the local anisotropic axes lying on the pseudo-C 3 axis of the molecule. The magnetic exchange interaction between the middle and ends Co(II) ions, extracted by using either a Hamiltonian accounting for the isotropic magnetic coupling and ZFS or the Lines’ model, was found to be medium to strong and antiferromagnetic in nature, whereas the interaction between the external Co(II) ions is weak antiferromagnetic. Interestingly, the compound exhibits slow relaxation of magnetization and open hysteresis at zero field and therefore SMM behavior. The significant magnetic exchange coupling found for {Co(μ-L)}2Co is mainly responsible for the quenching of QTM, which combined with the easy-axis local anisotropy of the CoII ions and the collinearity of their local anisotropy axes with the pseudo-C 3 axis favors the observation of SMM behavior at zero field.
A new linear trinuclear Co(II)3 complex with a formula of {Co(μ-L)}2Co has been prepared by self-assembly of Co(II) ions and the N3O3-tripodal Schiff base ligand H3L, which is obtained from the ...condensation of 1,1,1-tris(aminomethyl)ethane and salicylaldehyde. Single X-ray diffraction shows that this compound is centrosymmetric with triple-phenolate bridging groups connecting neighboring Co(II) ions, leading to a paddle-wheel-like structure with a pseudo-C3 axis lying in the Co-Co-Co direction. The Co(II) ions at both ends of the Co(II)3 molecule exhibit distorted trigonal prismatic CoN3O3 geometry, whereas the Co(II) at the middle presents an elongated trigonal antiprismatic CoO6 geometry. The combined analysis of the magnetic data and theoretical calculations reveal strong easy-axis magnetic anisotropy for both types of Co(II) ions (|D| values higher than 115 cm-1) with the local anisotropic axes lying on the pseudo-C3 axis of the molecule. The magnetic exchange interaction between the middle and ends Co(II) ions, extracted by using either a Hamiltonian accounting for the isotropic magnetic coupling and ZFS or the Lines' model, was found to be medium to strong and antiferromagnetic in nature, whereas the interaction between the external Co(II) ions is weak antiferromagnetic. Interestingly, the compound exhibits slow relaxation of magnetization and open hysteresis at zero field and therefore SMM behavior. The significant magnetic exchange coupling found for {Co(μ-L)}2Co is mainly responsible for the quenching of QTM, which combined with the easy-axis local anisotropy of the CoII ions and the collinearity of their local anisotropy axes with the pseudo-C3 axis favors the observation of SMM behavior at zero field.
A new linear trinuclear Co(II)
complex with a formula of {Co(μ-L)}
Co has been prepared by self-assembly of Co(II) ions and the N
O
-tripodal Schiff base ligand H
L, which is obtained from the ...condensation of 1,1,1-tris(aminomethyl)ethane and salicylaldehyde. Single X-ray diffraction shows that this compound is centrosymmetric with triple-phenolate bridging groups connecting neighboring Co(II) ions, leading to a paddle-wheel-like structure with a pseudo-
axis lying in the Co-Co-Co direction. The Co(II) ions at both ends of the Co(II)
molecule exhibit distorted trigonal prismatic CoN
O
geometry, whereas the Co(II) at the middle presents an elongated trigonal antiprismatic CoO
geometry. The combined analysis of the magnetic data and theoretical calculations reveal strong easy-axis magnetic anisotropy for both types of Co(II) ions (|
| values higher than 115 cm
) with the local anisotropic axes lying on the pseudo-
axis of the molecule. The magnetic exchange interaction between the middle and ends Co(II) ions, extracted by using either a Hamiltonian accounting for the isotropic magnetic coupling and ZFS or the Lines' model, was found to be medium to strong and antiferromagnetic in nature, whereas the interaction between the external Co(II) ions is weak antiferromagnetic. Interestingly, the compound exhibits slow relaxation of magnetization and open hysteresis at zero field and therefore SMM behavior. The significant magnetic exchange coupling found for {Co(μ-L)}
Co is mainly responsible for the quenching of QTM, which combined with the easy-axis local anisotropy of the Co
ions and the collinearity of their local anisotropy axes with the pseudo-
axis favors the observation of SMM behavior at zero field.
A mononuclear iron(III) porphyrin compound exhibiting unexpectedly slow magnetic relaxation, which is a characteristic of single‐ion magnet behaviour, is reported. This behaviour originates from the ...close proximity (≈550 cm−1) of the intermediate‐spin S=3/2 excited states to the high‐spin S=5/2 ground state. More quantitatively, although the ground state is mostly S=5/2, a spin‐admixture model evidences a sizable contribution (≈15 %) of S=3/2 to the ground state, which as a consequence experiences large and positive axial anisotropy (D=+19.2 cm−1). Frequency‐domain EPR spectroscopy allowed the mS= |±1/2⟩→|±3/2⟩ transitions to be directly accessed, and thus the very large zero‐field splitting in this 3d5 system to be unambiguously measured. Other experimental results including magnetisation, Mössbauer, and field‐domain EPR studies are consistent with this model, which is also supported by theoretical calculations.
Admix it up: The first mononuclear high‐spin d5 (S=5/2) iron(III) compound that shows both in‐plane magnetic anisotropy and single‐ion magnet behaviour with applied external magnetic field is reported. Despite the expected isotropic spin ground state, it has large and positive magnetic anisotropy (D=+19.2 cm−1), which can be attributed to the contribution of quartet excited states (i.e., spin‐admixed character of the ground state).
Tryptophan-based free radicals have been implicated in a myriad of catalytic and electron transfer reactions in biology. However, very few of them have been trapped so that biophysical ...characterizations can be performed in a high precision context. In this work, tryptophan derivative-based radicals were studied by high-frequency/high-field electron paramagnetic resonance (HFEPR) and quantum chemical calculations. Radicals were generated at liquid nitrogen temperature with a photocatalyst, sacrificial oxidant, and violet laser. The precise g-anisotropies of L- and D-tryptophan, 5-hydroxytryptophan, 5-methoxytryptophan, 5-fluorotryptophan, and 7-hydroxytryptophan were measured directly by HFEPR. Quantum chemical calculations were conducted to predict both neutral and cationic radical spectra for comparison with the experimental data. The results indicate that under the experimental conditions, all radicals formed were cationic. Spin densities of the radicals were also calculated. The various line patterns and g-anisotropies observed by HFEPR can be understood in terms of spin-density populations and the positioning of oxygen atom substitution on the tryptophan ring. The results are considered in the light of the tryptophan and 7-hydroxytryptophan diradical found in the biosynthesis of the tryptophan tryptophylquinone cofactor of methylamine dehydrogenase.
The synthesis and properties of an unusual, neutrally charged and volatile N,N,N',N'-tetramethylethylenediamine trimethyl manganese(III) complex, (TMEDA)MnMe3, are described, along with its facile ...disproportionation to the corresponding Mn(II) and Mn(IV) complexes. Characterization by single-crystal XRD, UV–vis spectroscopy, high-frequency and -field EPR (HFEPR), magnetic susceptibility, and density functional theory (DFT) computations indicate that the (TMEDA)MnMe3 electronic structure can be described as largely square pyramidal Mn(III) centered. Finally, the paucity of manganese(III) polyalkyls and the simplicity and reactivity of this compound implicate it as a potentially useful synthetic building block.
Three new closely related Co
Y
complexes of general formula Co(μ-L)(μ-X)Y(NO
)
(X
=NO
1, benzoate 2, or 9-anthracenecarboxylato 3) have been prepared with the compartmental ligand ...N,N',N''-trimethyl-N,N''-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H
L). In these complexes, Co
and Y
are triply bridged by two phenoxide groups belonging to the di-deprotonated ligand (L
) and one ancillary anion X
. The change of the ancillary bridging group connecting Co
and Y
ions induces small differences in the trigonally distorted CoN
O
coordination sphere with a concomitant tuning of the magnetic anisotropy and intermolecular interactions. Direct current magnetic, high-frequency and -field EPR (HFEPR), frequency domain Fourier transform THz electron paramagnetic resonance (FD-FT THz-EPR) measurements, and ab initio theoretical calculations demonstrate that Co
ions in compounds 1-3 have large and positive D values (≈50 cm
), which decrease with increasing the distortion of the pseudo-octahedral Co
coordination sphere. Dynamic ac magnetic susceptibility measurements indicate that compound 1 exhibits field-induced single-molecule magnet (SMM) behavior, whereas compounds 2 and 3 only display this behavior when they are magnetically diluted with diamagnetic Zn
(Zn/Co=10:1). In view of this, it is always advisable to use magnetically diluted complexes, in which intermolecular interactions and quantum tunneling of magnetism (QTM) would be at least partly suppressed, so that "hidden single-ion magnet (SIM)" behavior could emerge. Field- and temperature-dependence of the relaxation times indicate the prevalence of the Raman process in all these complexes above approximately 3 K.
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