Chelate-Ring-Dependent Shifts in Redox Isomerism for the Co(Me₂N(CH₂)_nNMe₂)(3,6-DBQ)₂(n= 1−3) Series, Where 3,6-DBQ Is the Semiquinonate or Catecholate Ligand Derived from 3,6-Di-tert-butyl-1,2-benzoquinone

Abstract

Intramolecular electron transfer between CoII(SQ) and CoIII(Cat) redox isomers has been investigated for the series of complexes Co(N-N)(3,6-DBQ)2, where 3,6-DBQ is the semiquinonate or catecholate form of 3,6-di-tert-butyl-1,2-benzoquinone and N-N is tetramethylmethylenediamine (tmmda), tetramethylethylenediamine (tmeda), or tetramethylpropylenediamine (tmpda). Measurements of magnetic properties and changes in electronic spectra indicate that the transition temperature (T 1/2) for Co(III)/Co(II) redox isomerism in the solid state and in toluene solution is the lowest for Co(tmpda)(3,6-DBQ)2. This is attributed to the flexibility of the six-membered chelate ring of Co(tmpda) and a positive contribution to ΔS for the equilibrium. Transition temperatures for the tmmda and tmeda analogues are more than 150 deg higher than that for the tmpda species. Structural characterization of CoIII(tmmda)(3,6-DBSQ)(3,6-DBCat), CoIII(tmeda)(3,6-DBSQ)(3,6-DBCat), and CoII(tmpda)(3,6-DBSQ)2 has shown that the Co(tmmda) and Co(tmeda) chelate rings are conformationally ordered, while there is considerable disorder in the Co(tmpda) chelate ring. In toluene solution, T 1/2 is approximately 15 deg higher for Co(tmeda)(3,6-DBSQ)2 than for the tmmda analogue due to the more contracted chelate ring of CoIII(tmmda)(3,6-DBSQ)(3,6-DBCat). For the Co(N-N)(3,6-DBQ)2 series, T 1/2 varies as tmpda ≪ tmmda < tmeda.

Novel Co complex with high transformation temperature of valence tautomerism

“…The abrupt increase in µ eff indicates the occurrence of phase transition at about 380 K. To the best of our knowledge, the T c value of complex d is the highest among the species exhibiting thermal valence tautomerism. The change in the magnetization behavior is consistent with the valence tautomeric behavior, which has been reported by Jung et al [10,11].…”

Novel Dinuclear Redox‐isomeric Complexes with a Tetrapodal Pyridine‐based Ligand

“…A complex containing N 1 , N 1 , N 3 , N 3 ‐tetramethylpropane‐1,3‐diamine as a neutral N‐donor ligand and two 3,6‐di‐ tert ‐butyl‐ o ‐semiquinones, [(Me 2 N(CH 2 ) 3 NMe 2 )Co(3,6‐DBQ) 2 ], was already described in the literature 9. The coordination cores of this complex and complexes 1 – 3 are very similar, and this complex is the closest structural analogue of 1 – 3 .…”

“…The coordination cores of this complex and complexes 1 – 3 are very similar, and this complex is the closest structural analogue of 1 – 3 . Thermodynamic parameters of transition for [(Me 2 N(CH 2 ) 3 NMe 2 )Co(3,6‐DBQ) 2 ] (Δ H ° = 14.2 kJ · mol –1 ; Δ S ° = 80 J · mol –1 · K –1 ; T = 178 K) were calculated from the temperature dependence of the magnetic susceptibility 9. It should be noted that these parameters are close to those calculated for 1 and 3 from the temperature dependence of the heat capacities.…”

“…Higher sigma donor abilities stabilize the Co III (SQ)(Cat) isomer, whereas π‐electron withdrawal shifts the equilibrium towards the Co II (SQ) 2 isomer. Enlarging of structural lability of the N‐donor ligand favors the Co II (SQ) 2 isomer due to the entropy contribution to the equilibrium constant 9. The ability of the N‐donor ligand to exist in different conformations causes a hysteresis loop in the temperature dependence of the magnetic moment 10.…”

Switching metal complexesviaintramolecular electron transfer: connections with solvatochromism

“…The observation of VT behavior in both solution and crystalline phases, as observed for 5, is not always guaranteedin fact, there are numerous examples of VT observed in solution but not in the solid state, and vice versa. 42,[73][74][75] The temperature range over which a solid state VT transition is observed may be inaccessible in liquid solution, and additionally, the solvent-solute interactions may be very different to the intermolecular interactions in the crystalline phase. For example, despite the lack of VT in solution samples of 4, the species displays thermally induced VT in crystalline samples, as demonstrated by Mössbauer spectroscopy.…”