Transition metal–carbohydrate chemistry. Part 2. Homoleptic diacetoneglucose complexes of aluminium and Group 4 metals

Abstract

The synthesis of homoleptic 1,2:5,6-di-O-isopropylidene-α-D-glucofuranose (HL) complexes of aluminium and Group 4 metals (Ti, Zr or Hf) was achieved by ligand displacement from AlEt₃ and [M(PhCH₂)₄](M = Ti, Zr or Hf) with HL. The products were characterized by ¹H and ¹³C NMR, which showed the sugar ligands to be equivalent and free to rotate about the M–O bond, even at 193 K. Pyridine adducts of all the complexes were obtained and the crystal structures of two of them, [AlL₃(py)] and [ZrL₄(Py)₂], determined. The reaction of [AlL₃] with two equivalents of 4,4′-bipyridine (4,4′-bipy) led to the formation of the one-to-one adduct [AlL₃(4,4′-bipy)]. Complexation of [TiL₄] by 1,10-phenanthroline (phen) gave the Lewis-base adduct [TiL₄(phen)] which shows a strong steric effect of the base and a clear differentiation of the sugar ligands cis and anti to the N donor atoms. Optical rotatory power values [α]_D were measured for all the complexes. Crystallographic details: [AlL₃(py)], trigonal, space group R3, a=b=c= 10.639(2)Å, α=β=γ= 93.18(2)°, Z= 1 and R= 0.049 for 2154 independent observed reflections; [ZrL₄(py)₂], triclinic, space group P1, a= 13.843(1), b= 12.682(1), c= 10.942(1)Å, α= 87.73(1), β= 73.32(1), γ= 66.96(1)°, Z= 1 and R= 0.057 for 3336 independent observed reflections.