Molecular Alumo- and Gallosilicate Hydrides Functionalized with Terminal M(NR2)3 and Bridging M(NR2)2 (M = Ti, Zr, Hf; R = Me, Et) Moieties

A general synthetic strategy for the systematic synthesis of group 4 MIV heterometallic complexes LMIII(H)­(μ-O)­Si­(μ-O)­(OtBu)2} n MIV(NR2)4–n (L = {[HC­{C­(Me)­N­(2,6- i Pr2C6H3)}2; MIII = Al or Ga; n = 1 or 2; MIV = Ti, Zr, Hf; R = Me, Et), based on alumo- or gallosilicate hydride ligands bearing a Si–OH moiety, is presented. The challenging isolation of these metalloligands involved two strategies. On the one hand, the acid–base reaction of LAlH2 with (HO)2Si­(OtBu)2 yielded LAlH­(μ-O)­Si­(OH)­(OtBu)2 (1), while on the other hand, the oxidative addition of (HO)2Si­(OtBu)2 to LGa produced the gallium analog (2). These metalloligands successfully stabilized two hydrogen atoms with different acid–base properties (MIII–H and SiO–H) in the same molecule. Reactivity studies between 1 and 2 and group 4 amides MIV(NR2)4 (MIV = Ti, Zr, Hf; R = Me, Et) and tuning the reactions conditions and stoichiometry led to isolation and structural characterization of heterometallic complexes 3–11 with a 1:1 or 2:1 metalloligand/MIV ratio. Notably, some of these molecular heterometallic silicate complexes stabilize for the first time terminal (O3Si–O−)­MIV(NR2)3 moieties known from single-site silica-grafted species. Furthermore, the aluminum-containing heterometallic complexes possess Al–H vibrational energies similar to those reported for modified alumina surfaces, which makes them potentially suitable models for the proposed MIV species grafted onto silica/alumina surfaces with hydride and dihydride architectures.