Antiferromagnetic Coupling in a Six-Coordinate High Spin Cobalt(II)−Semiquinonato Complex

Caneschi, Andréa; Dei, Andrea; Gatteschi, Dante; Tangoulis, Vassilis

doi:10.1021/ic020243n

Cited by 83 publications

(95 citation statements)

References 44 publications

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“…However they could be consistent with the existence of a weak antiferromagnetic interaction between the cobalt(ii) ion and the semiquinonate radical ligand, as recently established by some of us to be operative in some related octahedral cobalt(ii) ± semiquinonate complexes. [18] As observed for all the other cobalt systems undergoing valence-tautomeric equilibria, [19] no evidence of the formation of a low-spin Co II ± semiquinonato species was found. This species is expected to be characterized by a triplet electronic ground state, on the basis of the orthogonality of the d s (Co) and p*(SQ) magnetic orbitals.…”

Section: Resultsmentioning

confidence: 76%

Pressure- and Temperature-Induced Valence Tautomeric Interconversion in ao-Dioxolene Adduct of a Cobalt-Tetraazamacrocycle Complex

et al. 2001

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An electronic switch at the molecular level has been realized by using a class of ionic compounds of the formula [Co(L)(diox)]Y (L = tetraazamacrocyclic ligand, Y = mononegative anion). Such compounds undergo temperature- and pressure-induced intramolecular one-electron transfer equilibria. The transition temperature of interconversion varies with the nature of the counterions Y (Y = PF6, BPh4, I). Surprisingly the effect of the anion on the transition temperature is not only governed by its volume but also by its coulombic interaction.

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Section: Resultsmentioning

confidence: 76%

Pressure- and Temperature-Induced Valence Tautomeric Interconversion in ao-Dioxolene Adduct of a Cobalt-Tetraazamacrocycle Complex

et al. 2001

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“…The pattern of absorption bands observed in the spectra of 3 and 4 suggests the existence of six-coordinate cobalt(II)-semiquinonato chromophores. [49] The broad transitions centred at 12 500 cm À1 (800 nm) can be assigned to internal ligand transitions, whereas the pattern of bands at 15 000-19 000 cm À1 (665-525 nm) are attributed to charge-transfer (CT) transitions involving the d orbitals of the metal ion and the singly occupied p* orbital of the ligand.…”

Section: Complex the Complex [Coa C H T U N G T R E N N U N G (Me 2 mentioning

confidence: 99%

Tuning the Charge Distribution and Photoswitchable Properties of Cobalt–Dioxolene Complexes by Using Molecular Techniques

Beni

Dei

Laschi

et al. 2008

Chemistry A European J

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A series of cobalt complexes [Co(Me(n)tpa)(diox)]PF(6)sol (diox=3,5-di-tert-butyl-1,2-dioxolene; sol=ethanol, toluene; tpa=tris(2-pyridylmethyl)amine) were prepared by using tripod-like Me(n)tpa (n=0, 1, 2, 3), derived from tpa by successive introduction of methyl groups into the 6-position of the pyridine moieties, as an ancillary ligand. The steric hindrance induced by this substitution modulates the redox properties of the metal acceptor, thus determining the charge distribution of the metal-dioxolene moiety at room temperature. All of these complexes were characterised by using diffractometric studies, electronic spectroscopic analysis, and magnetic susceptibility measurements. In the solid state, the [Co(Me(n)tpa)(diox)](+) ions (n=0, 1) can be described as diamagnetic cobalt(III)-catecholato derivatives, whereas a cobalt(II)-semiquinonato description seems appropriate for the paramagnetic [Co(Me(3)tpa)(diox)](+) complex. The complex [Co(Me(2)tpa)(diox)]PF(6)C(2)H(5)OH undergoes entropy-driven valence tautomeric interconversion at room temperature. Optically induced valence tautomerism was observed by irradiation of [Co(Me(n)tpa)(diox)]PF(6) complexes (n=0, 1, 2) at cryogenic temperatures. The different relaxation kinetics of the photoinduced metastable phases are related to the respective free-energy changes of the interconversion, as estimated by cyclic voltammetric experiments at room temperature, and to the different lattice interactions, as supported by structural data. These results show the importance of molecular techniques for controlling the relaxation properties of photoinduced metastable species. At the same time, this behaviour strongly suggests that this paradigm exhibits intrinsic limits because of the less controllable factors that affect the process.

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“…This spincrossover process is, however, not complete at 400 K. In reported high-spin cobalt(II) semiquinonate complexes, the exchange coupling between the S = 3/2 spin of the high-spin cobalt(II) ion and the S = 1/2 spin of the semiquinonate radical is generally described as antiferromagnetic, leading to a spin ground state of S = 1 with a moderate coupling constant. [3,14] Any fitting of the magnetic data to a theoretical model is problematic because both the spin transition process and the exchange coupling contribute to the values of the c M T curve above 200 K, no data are available to us above 400 K (the high-temperature limit of the SQUID magnetometer), and the removal of degeneracy of the 4 T 1g state in a high-spin octahedral cobalt(II) ion as a result of distortion of the ligand field and spin-orbit coupling can complicate a magnetic analysis even without the occurrence of spin crossover. [14] We felt that any attempt to simulate the powder susceptibility data including all those effects would suffer from severe overparametrization.…”

Section: Dedicated To Professor Christoph Elschenbroich On the Occasimentioning

confidence: 99%

“…[3,14] Any fitting of the magnetic data to a theoretical model is problematic because both the spin transition process and the exchange coupling contribute to the values of the c M T curve above 200 K, no data are available to us above 400 K (the high-temperature limit of the SQUID magnetometer), and the removal of degeneracy of the 4 T 1g state in a high-spin octahedral cobalt(II) ion as a result of distortion of the ligand field and spin-orbit coupling can complicate a magnetic analysis even without the occurrence of spin crossover. [14] We felt that any attempt to simulate the powder susceptibility data including all those effects would suffer from severe overparametrization. Hence, no unambiguous and reliable values for the exchange coupling constant and the critical spin-crossover temperature can be derived at this time.…”

Section: Dedicated To Professor Christoph Elschenbroich On the Occasimentioning

confidence: 99%

Temperature‐Induced Spin‐Transition in a Low‐Spin Cobalt(II) Semiquinonate Complex

et al. 2010

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Dedicated to Professor Christoph Elschenbroich on the occasion of his 70th birthdaySpin crossover and valence tautomerism are examples of processes that can be utilized as a basis for achieving molecular switches.[1] Whereas the spin-crossover process is characterized by a temperature-, pressure-, or light-induced change of the electronic state of the metal ion to one with a different spin multiplicity, [2] valence tautomerism entails an intramolecular redox reaction between a metal ion and a coordinated ligand, which, in a few instances, is accompanied by a change in the spin state of the metal ion.[3] Various reported low-spin cobalt(III) catecholate complexes, which can be transformed into high-spin cobalt(II) semiquinonate complexes by raising the temperature, provide excellent examples of the latter process. In contrast, spin-crossover chemistry is dominated by octahedral iron(II) complexes with a FeN 6 coordination sphere; [2] however, there are only very few known octahedral cobalt(II)-containing spin-crossover complexes.[4] Herein we describe the first cobalt(II) semiquinonate complex that displays spin-crossover properties rather than valence tautomerism.The starting point of our investigation was the olive-green cobalt(III) 3,5-di-tert-butylcatecholate (dbc 2À ) complex [Co-(L-N 4 Me 2 )(dbc)](BPh 4 )·0.8 MeCN·0.2 Et 2 O (1) containing the dimethyl derivative of the tetraazamacrocyclic ligand 2,11-diaza[3.3](2,6)pyridinophane (L-N 4 Me 2 ) as coligand. This complex was obtained in 42 % yield by oxidation of the red cobalt(II) catecholate complex Me 2 )(dbc)] (prepared in situ from equimolar solutions of cobalt(II) perchlorate, L-N 4 Me 2 , and 3,5-di-tert-butylcatecholate) with ferrocenium tetrafluoroborate ([Fe(Cp) 2 ](BF 4 ); Cp = cyclopentadienyl), followed by a metathesis reaction with sodium tetraphenylborate (Scheme 1). In accordance with the description of 1 as a cobalt(III) catecholate complex, solutions and solids of this substance are diamagnetic. X-ray structure analysis of 1 also supports this assignment.[6] Figure 1 shows a perspective view of the complex cation in 1. Because of the small size of the macrocyclic ring, the coordinated ligand L-N 4 Me 2 is folded along the N amine -N amine axis, thereby rendering a distorted cis-octahedral coordina-

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Antiferromagnetic Coupling in a Six-Coordinate High Spin Cobalt(II)−Semiquinonato Complex

Cited by 83 publications

References 44 publications

Pressure- and Temperature-Induced Valence Tautomeric Interconversion in ao-Dioxolene Adduct of a Cobalt-Tetraazamacrocycle Complex

Pressure- and Temperature-Induced Valence Tautomeric Interconversion in ao-Dioxolene Adduct of a Cobalt-Tetraazamacrocycle Complex

Tuning the Charge Distribution and Photoswitchable Properties of Cobalt–Dioxolene Complexes by Using Molecular Techniques

Temperature‐Induced Spin‐Transition in a Low‐Spin Cobalt(II) Semiquinonate Complex

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