Cerocene Revisited: The Electronic Structure of and Interconversion Between Ce2(C8H8)3and Ce(C8H8)2

Abstract

New synthetic procedures for the preparation of Ce(cot)2, cerocene, from [Li(thf)4][Ce(cot)2], and Ce2(cot)3 in high yield and purity are reported. Heating solid Ce(cot)2 yields Ce2(cot)3 and COT while heating Ce2(cot)3 with an excess of COT in C6D6 to 65 °C over four months yields Ce(cot)2. The solid state magnetic susceptibility data of these three organocerium compounds show that Ce(cot)2 behaves as a TIP (temperature independent paramagnet) over the temperature range of 5−300 K, while that of Ce2(cot)3 shows that the spin carriers are antiferromagnetically coupled below 10 K; above 10 K, the individual spins are uncorrelated, and [Ce(cot)2]− behaves as an isolated f1 paramagnet. The EPR spectra recorded at 1.5 K confirm that Ce2(cot)3 and [Ce(cot)2]− have a ground-state of MJ= ± 1/2. The LIII edge XANES of Ce(cot)2 (Booth, C.H.; Walter, M.D.; Daniel, M.; Lukens, W.W., Andersen, R.A., Phys. Rev. Lett. 2005, 95, 267202) and Ce2(cot)3 over 30−500 K are reported; the Ce(cot)2 XANES spectra show Ce(III) and Ce(IV) signatures up to a temperature of approximately 500 K, whereupon the Ce(IV) signature disappears, consistent with the thermal behavior observed in the melting experiment. The EXAFS of Ce(cot)2 and Ce2(cot)3 are reported at 30 K; the agreement between the molecular parameters for Ce(cot)2 derived from EXAFS and single crystal X-ray diffraction data are excellent. In the case of Ce2(cot)3 no X-ray diffraction data are known to exist, but the EXAFS are consistent with a “triple-decker” sandwich structure. A molecular rationalization is presented for the electronic structure of cerocene having a multiconfiguration ground-state that is an admixture of the two configurations Ce(III, 4f1)(cot1.5-)2 and Ce(IV, 4f0)(cot2-)2; the multiconfigurational ground-state has profound effects on the magnetic properties and on the nature of the chemical bond in cerocene and, perhaps, other molecules.