A family of 13 tetranuclear zinc(ii)-lanthanide(iii) complexes of a [3 + 3] Schiff-base macrocycle derived from 1,4-diformyl-2,3-dihydroxybenzene

Abstract

A family of thirteen tetranuclear heterometallic zinc(II)-lanthanide(III) complexes of the hexa-imine macrocycle (L^Pr)⁶⁻, with general formula Zn^II₃Ln^III(L^Pr)(NO₃)₃·xsolvents (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm or Yb), were prepared in a one-pot synthesis using a 3 : 1 : 3 : 3 reaction of zinc(II) acetate, the appropriate lanthanide(III) nitrate, the dialdehyde 1,4-diformyl-2,3-dihydroxybenzene (H₂L¹) and 1,3-diaminopropane. A hexanuclear homometallic zinc(II) macrocyclic complex [Zn₆(L^Pr)(OAc)₅(OH)(H₂O)]·3H₂O was obtained using a 2 : 0 : 1 : 1 ratio of the same reagents. A control experiment using a 1 : 0 : 1 : 1 ratio failed to generate the lanthanide-free [Zn₃(L^Pr)] macrocyclic complex. The reaction of H₂L¹ and zinc(II) acetate in a 1 : 1 ratio yielded the pentanuclear homometallic complex of the dialdehyde H₂L¹, [Zn₅(L¹)₅(H₂O)₆]·3H₂O. An X-ray crystal structure determination revealed [Zn₃^IIPr^III(L^Pr)(NO₃)₂(DMF)₃](NO₃)·0.9DMF has the large ten-coordinate lanthanide(III) ion bound in the central O₆ site with two bidentate nitrate anions completing the O₁₀ coordination sphere. The three square pyramidal zinc(II) ions are in the outer N₂O₂ sites with a fifth donor from DMF. Measurement of the magnetic properties of [Zn^II₃Dy^III(L^Pr)(NO₃)₃(MeOH)₃]·4H₂O with a weak external dc field showed that it has a frequency-dependent out-of-phase component of ac susceptibility, indicative of slow relaxation of the magnetization (SMM behaviour). Likewise, the Er and Yb analogues are field-induced SMMs; the latter is only the second example of a Yb-based SMM. The neodymium, ytterbium and erbium complexes are luminescent in the solid phase, but only the ytterbium and neodymium complexes show strong lanthanide-centred luminescence in DMF solution.