The synthesis, structure and nuclear magnetic resonance properties of some titanium relatives of Amavadin: [Δ-Ti(R,R-hidpa)2]2–, [Δ,Λ-Ti(R,R-hidpa)2]2– and [Δ,Λ-Ti(hida)2]2– [H3hidpa = 2,2′-(hydroxyimino)dipropionic acid, H3hida = N-hydroxyiminodiacetic acid]

Abstract

The compound [TiO(acac)₂] (acac = acetylacetonate) reacts with N-hydroxyiminodiacetic acid (H₃hida) or (R,R)-2,2′-(hydroxyimino)dipropionic acid (R,R-H₃hidpa) to yield [Ti(hida)₂]^2– or [Ti(R,R-hidpa)₂]^2–, respectively. The anion [Ti(R,R-hidpa)₂]^2– has been obtained from H₂O in the presence of Ca²⁺ as [Ca(H₂O)₄][Ca(H₂O)₅]₃[Δ-Ti(R,R-hidpa)₂]₄·2H₂O 3 which crystallises in the space group P2₁ (no. 4) with a = 14.258(2), b = 16.516(3), c = 19.169(3) Å, β = 110.907(9)°; least-squares refinement of 6082 reflections and 1126 variables converged with agreement factors of R = 0.069 and R′ = 0.054. The anions possess the co-ordination geometry identified for other Amavadin-style complexes, viz. eight-co-ordination achieved by the ligation of two mutually trans and staggered η²-NO groups and four unidentate carboxylate oxygen atoms; Ti–N(NO) 2.04(1)–2.10(1), Ti–O(NO) 1.962(9)–2.001(9), Ti–O(CO₂) 2.024(9)–2.100(9), N–O 1.38(1)–1.44(1) Å. The lattice of compound 3 comprises of parallel zigzag strands of alternating [Δ-Ti(R,R-hidpa)₂]^2– and [Ca(H₂O)_x]²⁺ (x = 4 or 5) linked interstrand by Ca^II binding to carboxylato-oxygen atoms and intra- and inter-strand by hydrogen bonding from the H₂O molecules to the oxygen atoms of the anions. The anions are obtained exclusively in the Δ-helical form and this has enabled ¹H and ¹³C NMR studies to follow epimerisation at the Ti^IV centre to produce a mixture of the Δ- and Λ-helical forms. Carbon-13 NMR spectroscopy has been used to investigate the interaction of the carboxylato groups of [Ti(hida)₂]^2– with solvents.