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

Abstract

Treatment of N,N'-bis(aryl)formamidines (ArFormH), N,N'-bis(2,6-difluorophenyl)formamidine (DFFormH) or N,N'-bis(2,6-diisopropylphenyl)formamidine (DippFormH), with europium metal in CH3 CN is an efficient synthesis of the divalent complexes: [{Eu(DFForm)2 (CH3 CN)2 }2 ] (Eu1) or [Eu(DippForm)2 (CH3 CN)4 ] (Eu2). The synthetic method was extended to ytterbium, but the metal required activation by addition of Hg(0) . With DFFormH in CH3 CN, [{Yb(DFForm)2 (CH3 CN)}2 ] (Yb1) was obtained in good yield, and [Yb(DFForm)2 (thf)3 ] (Yb3) was obtained from a synthesis in CH3 CN/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/C6 D6 resulted in decomposition to trivalent products, including one from a CH3 CN 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, C6 F5 ), 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)] (Yb4 b), and, from adjustment of the stoichiometry, trivalent [Yb(DFForm)3 (thf)] (Yb6). Oxidation of Yb3 with benzophenone (bp), or halogenating agents (TiCl4 (thf)2 , Ph3 CCl, C2 Cl6 ) 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 rare-earth ArForm complexes been achieved in Eu2 and Yb4 b, respectively, but also dimeric Eu1 and Yb1 have highly unusual ArForm bridging coordination modes, either perpendicular μ-1κ(N:N'):2κ(N:N') in Eu1, or the twisted μ-1κ(N:N'):2κ(N':F') DFForm coordination in Yb1, both unprecedented in divalent rare-earth ArForm chemistry and in the wider divalent rare-earth amidinate field.

Keywords: X-ray structures; lanthanides; reactivity; redox transmetallation; synthetic methods.