DFT Prediction and Experimental Investigation of Valence Tautomerism in Cobalt-Dioxolene Complexes

Abstract

The family of complexes of general formula [Co(Me n tpa)(Xdiox)] + (tpa = tris(2-pyridylmethyl)amine, n = 0−3 corresponds to successive methylation of the 6-position of the pyridine rings; X = Br 4 , Cl 4 , H 4 , 3,5-Me 2 , 3,5-tBu 2 ; diox = dioxolene) was investigated by density functional theory (DFT) calculations to predict the likelihood of valence tautomerism (VT). The OPBE functional with relativistic and solvent corrections allowed accurate reproduction of trends in spin-state energetics, affording the prediction of VT in complex [Co(Me 3 tpa)(Br 4 diox)] + (1 + ). One-electron oxidation of neutral precursor [Co II (Me 3 tpa)-(Br 4 cat)] (1) enabled isolation of target compounds 1(PF 6 ) and 1(BPh 4 ). Solution variable-temperature UV−vis absorption and Evans method magnetic susceptibility data confirm DFT predictions that 1 + exists in a temperature-dependent valence tautomeric equilibrium between low-spin Co(III)-catecholate and high-spin Co(II)-semiquinonate forms. The solution VT transition temperature of 1 + is solvent-tunable with critical temperatures in the range of 291−359 K for the solvents measured. Solid-state magnetic susceptibility measurements of 1(PF 6 ) and 1(BPh 4 ) reveal the onset of VT transitions above room temperature.

Single-ion anisotropy and exchange coupling in cobalt(ii)-radical complexes: insights from magnetic and ab initio studies

“…As 1-Zn is isomorphous with 1, the diluted analog with 5% cobalt doped in zinc, 1-Co 0.05 , was synthesized using the procedure from 1 with appropriate amounts of zinc(II) chloride and cobalt(II) chloride. 75 The mole % of Co was determined to be 4.68(4)% by ICP-OES, a value that is consistent with the structural analysis. While PXRD indicates the bulk sample of 2-Zn(PF 6 ) is predominantly isomorphous with 2(PF 6 ), the bulk sample with 5% dilution of cobalt in zinc (2-Co 0.05 (PF 6 )) resulted in a mixture of phases ( Fig.…”

“…The crystal structures of 1 and 1-Zn have been reported by some of us previously. 75 The crystal structure of 3(PF 6 )$tol has been reported previously, at 150 K and from a crystal whose quality was less than ideal (nal R 1 : 0.084). 42 As this structure was to be the focus of ab initio calculations, we have recollected the data using a better quality crystal at 100 K using a Rigaku XtaLAB Synergy-S Dual Microfocus X-ray diffractometer using Cu-Ka radiation (Table 1).…”

“…(3(PF 6 )) have been previously reported, 42,75 and the syntheses of new compounds [Co(Me 3 tpa)(trop)](PF 6 ) (2(PF 6 )), [Zn(Me 3tpa)(trop)](PF 6 ) (2-Zn(PF 6 )), [Zn(Me 3 tpa)(dbsq)](PF 6 ) (3-Zn(PF 6 )) and 1-Co 0.05 (5% 1 in 1-Zn) have been adapted from the syntheses of 1, 3(PF 6 ) and [Co(Me 4 cyclam)(trop)](PF 6 ). 45…”

“…Infrared spectra conrm that 1-Zn, 2-Zn(PF 6 ) and 3-Zn(PF 6 )$tol are isostructural with their cobalt analogs (see ref. 75 and Fig. S3 and S4 †).…”

“…Compound 1 precipitates directly from the reaction solution as a red solid and was recrystallized from DMF/diethyl ether. 75 Compound 2(PF 6 ) precipitates as a pale orange solid. A series of small-scale recrystallizations of 2(PF 6 ) with toluene were performed to obtain single crystals of diffraction quality.…”