Highly sterically encumbered silanols with diamidoheterocycle substituents as models for isolated Si–OH groups. stabilization of terminal O3Si-O-M(NR2)3 (M= Ti, Zr, Hf; R= Me, Et) metallosilicates

• In this manuscript, we describe two synthetic pathways for very bulky silanol ligands with the general formula (LO)( t BuO) 2 Si(OH) based on diamidocarbene skeletons. They are highly stable even against condensation or hydrolysis. Furthermore, we demonstrate that by selecting the size of the diamido skeleton, we can modify the steric protection of the Si–OH group until it is comparable to that of isolated silanol groups on the silica surface. • This was confirmed by the successful isolation of the first examples of molecular metallosilicates with terminal O 3 Si–O–M(NMe 2 ) 3 (M = Zr, Hf) groups, which are currently known only from single-site species immobilized on the silica surface. • The compounds were characterised by several analytical techniques, and in most cases also by single crystal X-Ray diffraction, to determine their molecular structures. Furthermore, Hirshfeld atom refinement, DFT calculations and QTAIM analysis of the resulting wavefunctions were used to describe the electron density distribution in the O 3 Si–O–H and O 3 Si–O–M(NR 2 ) 3 inorganic core in some of the compounds. Molecular models of silica-immobilized molecular species play an important role in their characterization and study. Herein, we present two bulky silanols designed specifically to stabilize terminal metallosilicates O 3 Si(µ-O)M IV (NR 2 ) 3 . Their synthesis involves a) the oxidative addition of ( t BuO) 2 Si(OH) 2 to the diamidocarbene (CH 3 ) 2 C{(O=C){N(Mes)} 2 C: (Mes= 2,4,6-Me 3 C 6 H 2 ) yielding ( L 1 O)( t BuO) 2 Si(OH) ( 1; L 1 = (CH 3 ) 2 C{(O=C)N(Mes)} 2 CH), and b) three-step route to ( L 2 O)( t BuO) 2 Si(OH) ( 5; L 2 = {(O=C)N(Mes)} 2 CH) that involves a reaction of Si(OAc) 4 with the {(O=C)N(Mes)} 2 C(H)OH alcohol giving ( L 2 O)Si(OAc) 3 ( 3 ), followed by a substitution of two OAc groups by O t Bu moieties giving ( L 2 O)( t BuO) 2 Si(OAc) ( 4 ) that is hydrolyzed to 5 . Unlike the previously reported silanols L β M III (H)(μ-O)( t BuO) 2 Si(OH) (M III = Al( A ), Ga( B )) based on the β-diketiminate ligand L β = HC{C(Me)N(2,6- i Pr 2 C 6 H 3 )} 2 – , silanols 1 and 5 are air-stable facilitating their handling. The synthesis and structural characterization of seven metallosilicate complexes ( L x O)( t BuO) 2 Si(µ-O)M IV (NR 2 ) 3 ( x = 1 or 2; M IV = Ti, Zr, Hf; R= Me, Et) ( 6 – 12 ) with a 1:1 ligand:M IV ratio confirmed their suitability as molecular models for isolated Si–OH groups on the silica surface. Quantum Crystallography and DFT calculations were used to evaluate the electronic structures of all species, revealing the importance of the steric hindrance provided by bulky organic residues on the stability of the metallosilicates.