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

Abstract

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-

Spectroscopic, Structural, and Kinetic Investigation of the Ultrafast Spin Crossover in an Unusual Cobalt(II) Semiquinonate Radical Complex

“…Characterization of the SCO properties of solid 1 a) Temperature-dependent structural investigation:A s referredt oi nt he Introduction, because the previously reported magnetic measurements [30] by themselves cannotp rovide any information about the temperature at which the SCO starts to occur in the solid state, structure determinations were performed on the same selected single crystal at different temperatures between 100 and 300 Ka t5 0Kintervals. Al ist of selected bond lengths is given in Ta bleS1( Supporting Information).…”

“…The bond lengths resultingf rom the structure determination at 400 Ka swell as the magnetic data were taken and adapted from ref. [30].…”

“…The occurrence of this unique SCO was unambiguously established in the solid state on the basis of the characteristic temperature-dependent changes of the equatorial Co II ÀN py and Co II ÀOb ond lengths observed in two structure determinations carriedo ut at 100 and 400 K, respectively. [30] In addition, the increasei nm agnetic susceptibility above 200 K demonstrates that higher spin levels of the hs Co II semiquinonate complexs tart to become populated.A t400 K(the highest availablet emperature), the SCO is not complete. The magnetic data indicatec omplete reversibility of the SCO process and furthermore, togetherw ith the structural data, that complex 1 is stable at 400 Kint he solid state.…”

“…Thus we succeeded in demonstratingt he occurrence of SCO in an octahedral Fe II bis(imino)acenaphthalene radicalc omplex [29] as well as in an octahedral Co II semiquinonate radicalcomplex. [30] The latter complex is unique in two aspects. First, in general, octahedral mononuclear Co II semiquinonate complexes are only known to contain hs Co II ions.…”

“…Secondly, in any of the reported octahedral hs Co II semiquinonate complexes that are also capable of changing their electronic states upon lowering the temperature, the underlying process has only been linked to VT.I ns olid 1,o nt he other hand, at emperature-induced SCO transition from an ls to ah sC o II semiquinonate complex has been observed (Scheme 1). [30] Note that in both processes, VT and SCO, the exciteds tate is ah sC o II semiquinonate state; only the corresponding ground states differ.…”