An electrochemical and DFT study on selected β-diketiminato metal complexes

Abstract

Selected homoleptic metal β-diketiminates M^IL and M^IIL₂ [M^I = Li or K, M^II = Mg, Ca or Yb; L: L^Ph = {N(SiMe₃)C(Ph)}₂CH, L^But = N(SiMe₃)C(Ph)C(H)C(Bu^t)N(SiMe₃), L* = {N(C₆H₃Prⁱ₂-2,6)C(Me)}₂CH] have been studied by cyclic voltammetry (CV). The primary reduction (E_p^red, the peak reduction potential measured vs. SCE in thf containing 0.2 M [NBu₄][PF₆] with a scan rate 100 mV s⁻¹ at a vitreous carbon electrode at ambient temperature) is essentially ligand-centred: E_p^red being ca. −2.2 V (LiL^Ph and KL^Ph) and −2.4 V [Mg(L^Ph)₂, LiL^But and Ca(L^Ph)₂], while LiL* is significantly more resistant to reduction (E_p^red = −3.1 V). These observations are consistent with the view that the two (L^Ph) or single (L^But) C-phenyl substituent(s), respectively, are available for π-electron-delocalisation of the reduced species, whereas the N-aryl substituents of L* are unable to participate in such conjugation for steric reasons. The primary reduction process was reversible on the CV-time scale only for LiL^But, Ca(L^Ph)₂ and Yb(L^Ph)₂. For the latter this occurs at a potential ca. 500 mV positive of Ca(L^Ph)₂, consistent with the notion that the LUMO of Yb(L^Ph)₂ has substantial metal character. The successive reversible steps, each separated by ca. 500 mV, indicate that there is strong electronic communication between the two ligands of Yb(L^Ph)₂. The overall three-electron transfer sequence shows that the final reduction level corresponds to [Yb^II(L^Ph)²⁻(L^Ph)³⁻]. DFT calculations on complexes Li(L^Ph)(OMe₂)₂ and Li₂(L^Ph)(OMe₂)₃ showed that both HOMO and LUMO orbitals are only based on the ligand with a HOMO–LUMO gap of 4.21 eV. Similar calculations on a doubly reduced complex Yb{(µ-L^Ph)Li(OMe₂)}₂ demonstrated that there is a considerable Yb atomic orbital contribution to the HOMO and LUMO of the complex.