Enhancing the Value of Free Metals in the Synthesis of Lanthanoid Formamidinates: Is a Co-oxidant Needed?

Treatment of N,N'-bis(aryl) formamidines (Ar-FormH), N, N'-bis(2,6-difluorophenyl)formamidine (DFFormH) or N, N'-bis(2,6-diisopropylphenyl) formamidine (DippFormH), with europium metal in CH3CN is an efficient synthesis of the divalent complexes: [{Eu(DFForm)(2)(CH3CN)(2)}(2)] (Eu1) or [Eu(DippForm)(2)(CH3CN)(4)] (Eu2). The synthetic method was extended to ytterbium, but the metal required activation by addition of Hg-0. With DFFormH in CH3CN, [{Yb(DFForm)(2)(CH3CN)}(2)] (Yb1) was obtained in good yield, and [Yb(DFForm)(2)(thf)(3)] (Yb3) was obtained from a synthesis in CH3CN/THF. Thus, this synthetic method completely circumvents the use of either salt metathesis, or redox transmetallation/protolysis (RTP) protocols to prepare divalent rare-earth formamidinates. Heating Yb1 in PhMe/C6D6 resulted in decomposition to trivalent products, including one from a CH3CN activation process. For a synthetic comparison, divalent ytterbium DFForm and DippForm complexes were synthesised by RTP reactions between Yb-0, Hg(R)(2) (R=Ph, C6F5), and ArFormH in THF, leading to the isolation of either [Yb(DFForm)(2)(thf)(3)] (Yb3), or the first five coordinate rare-earth formamidinate complex [Yb(DippForm)(2)(thf)] (Yb4b), and, from adjustment of the stoichiometry, trivalent [Yb(DFForm) 3(thf)] (Yb6). Oxidation of Yb3 with benzophenone (bp), or halogenating agents (TiCl4(thf)(2), Ph3CCl, C2Cl6) gave [Yb(DFForm)(3)(bp)] or [Yb(DFForm)(2)Cl(thf)(2)], respectively. Furthermore, the structural chemistry of divalent ArForm complexes has been substantially broadened. Not only have the highest and lowest coordination numbers for divalent rareearth ArForm complexes been achieved in Eu2 and Yb4b, respectively, but also dimeric Eu1 and Yb1 have highly unusual ArForm bridging coordination modes, either perpendicular mu-1 kappa(N:N'):2 kappa(N:N') in Eu1, or the twisted mu-1 kappa(N:N'):2 kappa(N':F') DFForm coordination in Yb1, both unprecedented in divalent rare-earth ArForm chemistry and in the wider divalent rare-earth amidinate field.