PR0378
Vibrational contribution to entropy in spin-crossover complexes with
Jahn-Teller effect(IMCE Kyushu Univ.) ○Gergely Juhasz, Kazunari Yoshizawa
Introduction. Spin-crossover (SCO) effect intransition metal complexes offer an opportunity tofabricate bistable magnetic switching systems inmolecular scale, therefore great attention has
been paid to understand the detailed mechanism

NN
N2+Co N
NNof SCO. The SCO is generally considered to be
entropy driven: the driving force of the spintransition is the vibrational entropy associatedFig. 1. Structure of Co(terpy)2]2+.

with the population of anti-bonding metal-ligand orbitals in the high-spin (HS) state.
Recently characterized mononuclear cobalt(II) compounds with long alkyl chains, [Co(Cnterpy)2](BF4)2(n=0-22)(1, Fig.1. ), put the question of driving force in a new perspective.
Compounds 1 exhibit a unique reverse spin-transition behaviour [1], which cannot beexplained on the basis of vibrational entropies. We considered the active Jahn-Teller (JT) modes and distortions in 1 to find a possible mechanism of the cooperativeinteractions and to explain the observed unique thermal behaviour.

Method. DFT calculations were carried out at BP86/6-311G(d) and B3LYP*/6-311G(d)level on the [Co(terpy)2]2+ moiety in low-spin (LS) and high-spin (HS) states as a modelfor 1, using Gaussian03 and ORCA programs. Vibrational levels of the calculated
anharmonic potential surfaces were numerically estimated based on the Fourier Grid
Hamiltonian method.

Results and Conclusion. The molecule in both spin states shows a distortion from anideal D2d geometry due to the Jahn-Teller effect. In the high spin (HS) state, theoperative J-T effect is of first order, however the geometric distortion is relatively small.
In the low-spin (LS) state, the J-T effect is of second order, and it causes an elongated C2vgeometry. The geometry distortion of metal-ligand bonding distances (ѝ0.2 Å) is large

even compared to the geometry distortion
associated with the spin-crossover (ѝ 0.1 Å).
Potential energy scans show that there are800

T effect, and the energy barrier between

E / cm-1

two potential minima associated with the J600

them is around 0.7-0.8 kcal/mol in free400

molecules in LS state (Fig. 2). In solid state,200

this barrier can be considerably higher due
0-3-2-10Q / A*amu

123-1/2

to intermolecular interactions. The highbarrier between the two J-T minima and theassociated large geometry distortion suggest

Fig. 2. Potential energy surface andthat compound 1 can show an abrupt phasevibrational levels along the Jahn-Teller modetransition between static and dynamic J-T

in the low-spin (LS) state.

phases.
The EPR spectral parameters of 1 wassimulated assuming statically distorted molecules, and also for molecules participatingin the dynamic JT process. The results of the calculations compared with previous EPR
experiments strongly support the presence of a static-dynamic J-T phase transition
around 100-200 K in related cobalt (II) compounds.
Based on these findings, we present a simple thermodynamic model that can explain thereverse spin-transition in 1 on single
molecular basis. According to the model, thedriving force of the reverse spin-transition isthe vibrational entropy gain from the dynamicJ-T mode. This entropy gain considerably
shifts the transition temperature (T1/2), hence
the systems shows both regular and reverse
spin-transition.

Reference:[1] Hayami et al., Angew. Chem. Int. Ed. 2005,(44) 4899.

Fig. 3. g values (blue) and PES (gray) forthe cobalt(II) along the Jahn-Teller mode.