Electronic Structure of Ytterbium Bis-indenyl and -cyclopentadienyl α-Diimine Complexes: A DFT and MS-CASPT2 Investigation

Abstract

The electronic structure of Yb(Cp′)2(N–N) complexes with Cp′ = η5-C5R5 (Cp*) or η5-C9H7 (Ind) and N–N = DAB (N,N′-tert-butyl-1,4-diazabutadiene), bpy (2,2′-bipyridine), and pyca ((E)-N-(pyridine-2-ylmethylene)aniline) was investigated by means of DFT and ab initio (CASSCF/CASPT2) calculations. Whereas the agreement between experimental features and theory is fair for the Yb(Ind)2bpy molecule, the description of the electronic ground state of Yb(Ind)2DAB and of the Cp* complexes is more problematic. The relative energies of the closed-shell singlet, lowest open-shell singlet, and triplet were calculated for Yb(Ind)2DAB with various functionals at the DFT level, which overstabilize the closed-shell singlet. All functionals place the open-shell singlet energetically close to the triplet state. The best functionals (B3LYP, M06, TPSSh) estimate the singlet–triplet energy gap in the range 17–28 kJ·mol–1, in disagreement with the experimental data. The electronic structure of the smaller and more symmetric system Yb(η5-C5H5)2(N–N) (N–N = DAB, bpy), for which DFT fails at describing the ground state, has been investigated by CASSCF/CASPT2 calculations. The lowest energy electronic ground state corresponds to a (4f(Yb))2(π*DAB)0-(4f(Yb))0(π*DAB)2 (1A1) state, nearly degenerate to the triplet 4f(Yb)1(π*DAB)1 configuration according to a diradical picture.