Formic Acid Modified Ti(OCHMe2)4. Syntheses, Characterization, and X-ray Structures of Ti4(μ4-O)(μ-O)(OFc)2(μ-OR)4(OR)6 and Ti6(μ3-O)6(OFc)6(OR)6 (OFc = O2CH; OR = OCHMe2)

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Abstract
Novel structural arrangements for carboxylic acid modified titanium alkoxides were obtained from stoichiometric reactions between titanium isopropoxide (Ti(OPri)4) and formic acid (HOFc). The 1:1 reaction in toluene forms the tetranuclear species Ti4O2(OFc)2(OPri)10, 1. The structure of 1 was solved in the triclinic space group P1̄ with a = 13.034(3) Å, b = 13.467(2) Å, c = 13.996(2) Å, α = 98.79(1)°, β = 104.68(2)°, and γ = 97.82(2)° for Z = 2. The general structure of 1 resembles two face-shared [Ti−O]4 cubes with one set of mirror-related titanium atoms removed. The oxygen atoms are represented by one μ4-O, one μ-O, and four μ-OR ligands, with the remaining sites filled by two OFc ligands and six terminal alkoxides. Increasing the stoichiometry to 1:2 (Ti/HOFc) leads to the isolation of Ti6O6(OFc)6(OR)6, 2. The structure of 2 was solved in the monoclinic space group P21/c with a = 8.968(2) Å, b = 26.520(4) Å, c = 20.046(2) Å, and β = 93.19(1)° for Z = 4. The structure of 2 consists of two offset six-membered [Ti−(μ3-O)]3 rings joined through Ti−O bonds. The OFc ligands are arranged externally around the central hexagon-prism, oscillating between the top and bottom rings. Compound 1 adopts a very symmetrical arrangement in solution due to the labile OFc ligands and was found to undergo “aging” by a trans-esterification mechanism, the rate of which is enhanced by heating. Compound 2 maintains its solid-state structure in solution.