Three paramagnetic reduction stages of phenyl-substituted 1,2:9,10-dibenzo[2.2]paracyclophane-1,9-dienes. Radical anions, triplet dianions, and radical trianions as studied by ESR and ENDOR spectroscopy

Abstract

arm protonated. The relatively long time scale for the onset of dissociation of one arm of the tpen ligand in acidic media as compared to the NMR experiment rules out the possibility of a dissociative enantiomerization process.Conclusions. [ Fe(tpen)] (C104)2-2/3H20 undergoes a spincrossover transformation with T, = 365 K in the solid state and T, = 363 K in a DMF solution. This is the first Fe" spin-crossover complex to have a spin-state interconversion rate in the solid state which is faster than can be sensed by 57Fe Mossbauer spectroscopy.The absence of intermolecular interactions, Le., a simple spinequilibrium for each [ Fe(tpen)12' complex, has been demonstrated by, inter alia, dilution studies with the isostructural Zn2+ complex. The results of X-ray structure determinations at 298 and 358 K show that the [Fe(tpen)12' complex has an appreciable trigonal twist up and beyond what is expected for an octahedral complex. Furthermore, this trigonal twist increases as the amount of high-spin content increases. It is suggested that the trigonal twist in [Fe(tpen)12+ leads to an increased spin-orbit interaction between 'Al and 5T2 states as a result of decreasing the energy of intermediate triplet states. This then results in an increased rate of spin-state interconversion. In solution [Fe(tpen)12' exhibits a relatively fast rate of enantiomerization which supports the idea that a trigonal twisting motion is coupled to the spin-state interconversion process.Acknowledgment. We are grateful for funding from National Institutes of Health Grant HLI 3652 (D.N.H.).Supplementary Material Available: Tables of anisotropic thermal parameters for [Fe(tpen)](C104)2 at 298 K and [Fe-(tpen)](C104)2.2/3H20 at 298 and 358 K, as well as chemical analysis, S7Fe Mossbauer, and magnetic susceptibility data and figures showing Mossbauer spectra, magnetic moment vs temperature plots, and temperature dependence of the 60-MHz 'H NMR spectrum of [Fe(tpen)]12 in 1:l MeOH/H,O (31 pages); listing of observed and calculated structure factors (36 pages).Ordering information is given on any current masthead page.

Spin delocalization in radical anions containing two phosphaalkene groups: an EPR study

“…It is worth noting that although the molecules 1 , 2 and 3 contain two groups likely to undergo a one-electron reduction, they give rise only to a radical monoanion whose unpaired electron is delocalized on the whole molecule. Neither localized structures 19 nor diradical species 20 were observed.…”

Redox and magnetic switching in 1,3,5-acceptor-substituted benzenes: reversible formation of radical anions, dianions and trianions in doublet, triplet, and quartet spin states

“…We were unable to detect the half-field signal due to the greatly reduced intensity of this transition in comparison with the ∆m s = 1 transition because of a small zero-field splitting (see below). 1, 28 As indicated in Scheme 1, only a singular combination of redox stages can lead to the experimental EPR signal: The triplet state can only be based on the dianion, 1 2Ϫ -2 K ϩ whereas the quartet state has to originate from the trianion 1 3ؒϪ -3 K ϩ . The observed DЈ = |D|/hc and EЈ = |E|/hc values for the triplet state of 1 2Ϫ -2 K ϩ are 0.0102 and 0.0014 cm Ϫ1 , respectively (Fig.…”

Organic Radical Ions as Studied by Epr Spectroscopy

“…[2.2]Paracyclophanes fused to biphenyl and o-terphenyl could be consecutively reduced to radical anions, triplet dianions, and radical trianions in which the unpaired electron is localized in a different region of the molecule (45). Apart from some 7i-systems of axial symmetry (44), triplet dianions were obtained from compounds containing two naphthalene moieties held by a rigid spacer at a distance of ca.…”

“…Prolonged reduction of many aromatic hydrocarbons by an alkali metal yields diamons which, in some cases, are paramagnetic triplets (44,45). Further contact with the metallic mirror can lead to radical trianions (45,46).…”

“…Further contact with the metallic mirror can lead to radical trianions (45,46). In this way, radical dianions of hydrocarbons with an odd number of n-centers were also produced (47,48).…”