科研之友
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摘要
Selected homoleptic metal beta-diketiminates (ML)-L-I and (ML2)-L-II [M-I = Li or K, M-II = Mg, Ca or Yb; L: L-Ph = {N(SiMe3) C(Ph)}(2)CH, L-But = N(SiMe3) C( Ph) C( H) C(Bu-t) N(SiMe3), L* = {N(C6H3Pri (2)-2,6) C(Me)}(2)CH] 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 (LiLPh and KLPh) and - 2.4 V [Mg(L-Ph)(2), LiLBut 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 pi-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 LiLBut, 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 Li-2(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{(mu-L-Ph) Li(OMe2)}(2) demonstrated that there is a considerable Yb atomic orbital contribution to the HOMO and LUMO of the complex.
- 出版日期2006
- 单位中国地震局
