Dedicated to Professor Christoph Elschenbroich on the occasion of his 70th birthday Spin 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 Me 2 )(dbc)](BPh 4 )·0.8 MeCN·0.2 Et 2 O (1) containing the dimethyl derivative of the tetraazamacrocyclic ligand 2,11diaza[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-Scheme 1. Preparation of compounds 1 and 2. Figure 1. Perspective view of the complex cation in 1 showing 50 % thermal ellipsoids; selected bond lengths []:

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-

A comprehensive spectroscopic and structural investigation of [Co (l-N tBu )(dbsq)][B(p-C H Cl) ] (1, l-N tBu =N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane, dbsq =3,5-di-tert-butylsemiquinonate), the first known octahedral complex with a low-spin (ls) Co semiquinonate ground state, is reported. Above 200 K, solids as well as solutions of 1 exhibit thermally induced spin-crossover (SCO) from the ls to the high-spin (hs) Co semiquinonate state instead of the frequently observed valence tautomerism from ls Co catecholate to hs Co semiquinonate. DFT calculations demonstrate that the (closed shell) Co catecholate suffers from a triplet instability leading to the ls Co semiquinonate ground state. The thorough temperature-dependent spectroscopic study of the SCO enables a photophysical investigation. Thus, by selective photoexcitation of the ls fraction of 1 in solution at room temperature, ultrafast conversion to the hs state is observed using femtosecond electronic and IR-vibrational (infrared) transient absorption spectroscopy. The kinetics of the photocycle is described by a stretched exponential with τ=3.3-3.6 ps and β=0.52-0.54, representing an upper limit for the hs-ls relaxation time. This is, to our knowledge, the fastest interconversion ever determined for a SCO complex, and is attributed to the special situation that in 1 a Co complex is coordinated to a π-radical ligand allowing very efficient coupling between the ls and hs spin states.

The dinuclear complex Me 2 )) 2 (BiBzIm)](ClO 4 ) 2 · 2EtCN (1) has been investigated by Mössbauer spectroscopy carried out in the temperature range from 5 to 150 K with externally applied magnetic fields of up to B = 5 T. By means of a consistent simulation of all experimental data sets within the Spin Hamiltonian formalism, the zerofield splitting D and the rhombicity parameter E/D of the ferrous high-spin (HS) site in this complex was determined to be D = −15.0 ± 1.0 cm −1 and E/D = 0.33 respectively. The sign of the quadrupole splitting of the HS site is positive which indicates that this iron site of the dinuclear complex 1 has an electronic ground state with the d xy orbital being twofold occupied.

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