Syntheses, Characterizations, and Single-Crystal X-ray Structures of Soluble Titanium Alkoxide Phosphonates

Reactions of Ti(OiPr)4 with different phosphonic acids RPO3H2 (R = Ph, 4-CNPh, Me, tBu) in organic solvents
have been investigated. In the presence of small amounts of water, the new molecular titanium oxide alkoxide
phosphonates [Ti4(μ3-O)(OiPr)5(μ-OiPr)3(RPO3)3]·DMSO [R = Ph (1), Me (2), tBu (3), 4-CNPh (4)] were isolated.
The single-crystal X-ray structure analyses of 1 and 2 revealed hexacoordinated titanium atoms and a connectivity
of (111) for each phosphonate. Under rigorous exclusion of water, the reaction of Ti(OiPr)4 with tert-butylphosphonic
acid in toluene gave the titanium phosphonate tetramer [Ti(OiPr)2(tBuPO3)]4 (5). A single-crystal X-ray structure
analysis of 5 revealed a 5 + 1 coordination of the titanium atoms as a result of the (112) connectivity of each
phosphonate; such a coordination mode has never been reported for a titanium phosphate, phosphonate, or
phosphinate. Compounds 1−5 were characterized by FT-IR, 31P MAS NMR, and solution multinuclear NMR
(1H, 13C{1H}, 31P{1H}) spectroscopies. 13C CP MAS NMR experiments were carried out on arylphosphonates 1
and 4. Solution NMR experiments were also used to investigate the exchange reaction between 1 and 2 and the
conversion of 5 to [Ti4(μ3-O)(OiPr)5(μ-OiPr)3(tBuPO3)3]·iPrOH by partial hydrolysis in the presence of Ti(OiPr)4.
The phosphonate clusters 1−5 are soluble in organic solvents and are likely intermediates in the sol−gel processing
of inorganic−organic hybrids based on titanium oxide and phosphonate groups that we are currently developing.