Controlled synthesis of 3d-4d heterobimetallic complexes of a symmetrical tetraiminodiphenolate macrocycle - structural, spectroscopic, and redox properties

Abstract

The mononuclear complexes [Rh(LH₂)Cl₂](ClO₄)·3H₂O (1) and [Pd(LH₂)](ClO₄)₂ (2) of the tetraiminodiphenolate macrocycle L^2- have been synthesized by the transmetallationreaction between [Pb(LH₂)](ClO₄)₂, and RhCl₃·xH₂O and Na₂[PdCl₄], respectively. In these compounds, the uncoordinated imino nitrogen atoms are protonated and are hydrogen bonded to the phenolate oxygen atoms to stabilize them against hydrolytic cleavage. Using these mononuclear complexes, the following heterodinuclear complexes: [Rh^IIICl₂(L)Pd^II](X) [X = ClO₄ (3), PF₆ (4)], [Pd^II(L)M^II(H₂O)₂](ClO₄)₂ [M = Mn (5), Fe (6), Co (7), Ni (8)], [Pd^II(L)M^II](ClO₄)₂ [M = Cu (9), Zn (10)]; and the dipalladium(II) complex [Pd₂L](ClO₄)₂ (11) have been synthesized. The crystal structure determined for [RhCl₂(L)Pd](PF₆) (4) shows an axially elongated octahedral geometry for Rh^III and a square-planar geometry for Pd^II. In [Pd^II(L)Cu^II](ClO₄)₂ (9), both the metal ions lie in the flat plane of the macrocyclic ligand and they are positionally disordered with equal occupancies. The Cu^II center has an apically elongated square-pyramidal geometry due to the weak perturbation by an oxygen atom of a perchlorate anion. Complexes 2, 10, and 11 on excitation at 400 nm exhibit luminescence at room temperature at 505, 437, and 443 nm, respectively. Spectrofluorimetric titrations of [Pd^II(LH₂)]2+ with the acetate salts of zinc(II) and palladium(II) have shown that the formation of [Pd(L)Zn]²⁺ and [Pd₂L]²⁺ complex species are accompanied by blue shift of luminescence with increased and reduced emission intensities, respectively. The hyperfine-shifted ¹H NMR spectral features of the paramagnetic compounds 7-9 have been analyzed by determining their longitudinal (T₁) and transverse (T₂) relaxation times. Cyclic and square-wave voltammetric measurements have been carried out for complexes 7-9 in acetonitrile. For complexes 8 and 9, reversible one-electron reduction occurs with E_½ = -0.10 V for Pd^IICu^II/Pd^IICu^I and -0.80 V for Pd^IINi^II/Pd^IINi^I versus Ag/AgCl. Complex 7 undergoes irreversible reduction for cobalt(II) at -0.78 V.