NCN Trianionic Pincer Ligand Precursors: Synthesis of Bimetallic, Chelating Diamide, and Pincer Group IV Complexes

This report details the synthesis of new NCN trianionic pincer ligand precursors and metalation reactions to form group (IV) complexes. N,N′-[1,3-phenylenebis(methylene)]bis-2,6-diisopropylaniline [2,6-iPrNCN]H3 (8) was converted to the N,N′-substituted Si(IV), Sn(IV), Mg(II), and Zn(II) derivatives. [2,6-iPrNCHN](SiMe3)2 (9-Si) and [2,6-iPrNCHN](SnMe3)2 (9-Sn) form by first treating 8 with MeLi followed by Me3MCl, where M = Si or Sn. Single crystal X-ray experiments indicate 8, 9-Si, and 9-Sn have similar structural features in the solid state. [2,6-iPrNCHN](μ-MgCl·THF)2 (12) forms by treating 8 with MeMgCl, and its solid state structure revealed a bis-μ-MgCl bridging unit. The 1H NMR spectrum of 12 reveals a dynamic process occurs in solution. A variable temperature 1H NMR experiment failed to quench the dynamic process. {[2,6-iPrNCHN]Zn}2 (13) forms upon treating {[2,6-iPrNCHN]Li2}2 (10) with anhydrous ZnCl2 and is a dimer in the solid state. Again, dynamic 1H NMR behavior is observed, and a mechanism is provided to explain the apparent low symmetry of 13 in solution. Extension of the aliphatic arm of the NCN ligand provides the new NCCCN pincer ligand precursors N,N′-(2,2′-(1,3-phenylene)bis(ethane-2,1-diyl))bis(3,5-bis(trifluoromethyl)aniline) [3,5-CF3NCCCN]H3 (16) and [3,5-CF3NCCHCN](SiMe3)2 (17). A more rigid ligand architecture was accessed by synthesis of the anthracene derived pincer ligand anthracene-1,8-diylbis(N-3,5-bistrifluormethylaniline) [3,5-CF3NCCanthCN]H3 (18). Treating {Zr(NMe2)4}2 with 2 equiv of 16 provides the dimer {(μ-3,5-CF3NCCHCN)Zr(NMe2)3NHMe2}2 (19). Treating Hf(NMe2)4 with 18 provides the bimetallic complex (μ-3,5-CF3NCCHanthCN){Hf(NMe2)3NHMe2}2 (20) in which one ligand bridges two Hf(IV) ions. Salt metathesis between 10 and ZrCl2(NMe2)2(THF)2 provides the mononuclear complex [2,6-iPrNCHN]Zr(NMe2)2 (21) in which the NCN ligand is bound as a chelating diamide. Thermoysis of 21 does not lead to formation of a trianionic pincer complex. Instead, treating HfCl4 with {[2,6-iPrNCN]Li3}2 (11) followed by MeLi provides the trianionic pincerate complex [2,6-iPrNCNHfMe2][Li(DME)2] (23). In the solid state the Hf ion has distorted trigonal bipyramidal geometry.