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.

Potassium C–F Interactions and the Structural Consequences in N,N′-Bis(2,6-difluorophenyl)formamidinate Complexes

“…Initial extension of the coordination of the DFForm ligand in 1 shows that the nitrogen atoms are further bridging to an adjacent potassium ion in a μ-(N,N′):(N,N′) manner ( Figure 1C, also Figure 2A). Such formamidinate bridging is known for other s-and f-block complexes [16,22,31]. This bridging is mirrored by a symmetry equivalent DFForm ligand, generating a potassium nitrogen based cube of volume: 6.78 Å 3 ( Figure 1C).…”

“…This is likely due to a combination of steric pressure, which starves the metal centre from coordination of additional donors, and increased electron donation from the aromatic ring caused by the alkyl substituents. Another means to engage the aromatic component in coordination is through the addition of donor functionalities (e.g., OMe, F), especially in the ortho-positions, thereby transforming the formamidinate ligand into a tri- [15][16][17], or tetra-dentate (e.g., N,N ,X or N,N ,X,X ) [17], chelate, with examples across a variety of different metal classes [18]. For s-block chemistry however, the use of such ligands has been restricted to very few examples, namely the use of N,N -bis(2-fluorophenyl)formamidine (FForm) [19].…”

“…One of the fluorinated formamidinate ligands used was N,N -bis(2,6-difluorophenyl)formamidinate (DFForm) in both trivalent [16,17], and divalent [16] rare-earth complexes. Despite the presence of the additional fluorine atoms, the observation of any M-F interaction was rare, typically only occurred in unsolvated species, and interactions were displaced on coordination of donor solvents [16,17]. It is likely that the smaller ionic radii of the trivalent rare-earths, compared with the larger potassium ion [29], create a significant strain in the NCHN bite of the DFForm ligand when it coordinates the fluorine atoms, and therefore donor solvent coordination is preferred.…”

“…This contrasts the group one complexes of the non-fluorinated N,N′-di(aryl)formamidinate ligands [21][22][23], which readily retain coordinating solvent. Since then, we have expanded the use of fluorinated formamidinate ligands to f-block chemistry [9,16,17,[24][25][26][27][28], in a variety of different contexts [5]. One of the fluorinated formamidinate ligands used was N,N′-bis (2,6-difluorophenyl)formamidinate (DFForm) in both trivalent [16,17], and divalent [16] rare-earth complexes.…”

“…Since then, we have expanded the use of fluorinated formamidinate ligands to f-block chemistry [9,16,17,[24][25][26][27][28], in a variety of different contexts [5]. One of the fluorinated formamidinate ligands used was N,N′-bis (2,6-difluorophenyl)formamidinate (DFForm) in both trivalent [16,17], and divalent [16] rare-earth complexes. Despite the presence of the additional fluorine atoms, the observation of any M-F interaction was rare, typically only occurred in unsolvated species, and interactions were displaced on coordination of donor solvents [16,17].…”