Thermoanalytical study of linkage isomerism in coordination compounds

A Differential Scanning Calorimetry and Theoretical Study on the Isomerization of trans‐[Co(cyclam)(ONO)2]X (X = PF6–, ClO4–)

“…The DSC curves associated with this isomerization reaction at several heating rates are shown in Figure 1. In alignment with previous studies, the appearance of exothermic peaks in these DSC thermograms can be attributed to the thermal conversion of the dinitrito isomers to the dinitro isomers 18–20. However, the thermal stability of these linkage isomers is different from those of recently studied related complexes.…”

“…However, the thermal stability of these linkage isomers is different from those of recently studied related complexes. Our pervious study showed that both chemically prepared pure nitrito and nitro linkage isomers of trans ‐bis(ethylenediamine)cobalt(III) hexafluorophosphate20 and pentaaminecobalt(III) complexes18,19 are metastable states at ambient temperatures and reach an stable equilibrium state at elevated temperatures. In these instances, the stable state is a mixture of the nitro and nitrito isomers as major and minor components, respectively.…”

“…The first observation of the photoinduced linkage isomerism of NO 2 ‐containing transition metal complexes was reported in 1955 by Adell, who found that sunlight irradiation of the yellow crystals of [Co(NH 3 ) 5 (NO 2 )]XY complexes resulted in the corresponding red nitrito isomers of [Co(NH 3 ) 5 ONO]XY 12. To date, thermal nitrito–nitro isomerization has been observed in several complexes, including [Co(MeNH 2 ) 5 ONO] 2+ , cis ‐ and trans ‐[Co(MeNH 2 )(NH 3 ) 4 ONO] 2+ , trans ‐[Co(MeNH 2 )(en) 2 ONO] (en = ethylenediamine), cis ‐ and trans ‐[CoL 14 (ONO)] 2+ (L 14 = 6‐methyl‐1,4,8,11‐tetraazacyclotetradecane‐6‐amine),13 cis ‐ and trans ‐[Co(en) 2 (ONO) 2 ] + ,14 cis ‐[Co(en) 2 (ONO)(NO 2 )] + ,15 trans ‐[Co(cyclam)(NH 3 )(ONO)] 2+ (cyclam = 1,4,8,11‐tetraazacyclotetradecane), trans ‐[Co(en)(dien)ONO] 2+ (dien = diethylenetriamine), [Co(en) 2 (NH 3 )ONO] 2+ ,16 cis ‐[Co(NH 3 ) 4 (ONO) 2 ] + and cis ‐[Co(NH 3 ) 4 (ONO)H 2 O] 2+[17] in solution, and also [Co(NH 3 ) 5 ONO]X 2 , [Co(NH 3 ) 5 NO 2 ]X 2 (X = Cl – , Br – , I – , PF 6 – ),18,19 trans ‐[Co(en) 2 (ONO) 2 ]PF 6 , trans ‐[Co(en) 2 (NO 2 ) 2 ]PF 6 ,20 trans ‐[Ni(en) 2 (NO 2 ) 2 ],11 and M(porphyrin)(NO x )21,22 in the solid state. In addition, the photoisomerization of trans ‐[Cr(cyclam) 2 (ONO) 2 ]BF 4 ,23 M(porphyrin)(NO x ),6,24–26 and [Ni(dppe)(η 1 ‐ONO)Cl] [dppe = 1,2‐bis(diphenylphosphanyl)ethane]27 have been studied in the solid state.…”

“…In aqueous solutions, UV/Vis spectroscopy has been the most common method for the determination of the kinetic and thermodynamic parameters of the nitrito–nitro isomerization 8,13,14,36. However, the most‐used techniques for the investigation of nitrito–nitro linkage isomerization in the solid state have been X‐ray crystallography, [27–30,37–42] NMR43 and IR31,44 spectroscopy, and differential scanning calorimetry (DSC) 18–20. Until recently, IR spectroscopy was the sole method for the kinetic and thermodynamic evaluation of linkage isomerism in the solid state.…”

“…Our previous DSC studies19 showed that the kinetic and thermodynamic parameters of the isomerization depend on the nature of the complex counterions. Also, recent DSC studies on the solid‐state thermal interconversion of Co III nitropentaamine and nitritopentaamine complexes18,19 as well as trans ‐bis(ethylenediamine)20 complexes revealed that the pure nitrito and nitro isomers are both metastable states, which convert to an equilibrium state upon heating. Both nitro and nitrito isomers are present in the stable equilibrium state as major and minor components, respectively.…”