Synthesis, structure, spectroscopic properties, and electrochemistry of rare earth sandwich compounds with mixed 2,3-naphthalocyaninato and octaethylporphyrinato ligands

A series of 14 heteroleptic rare earth sandwich complexes [M^III-(nc)(oep)] (M=Y, La-Lu except Ce and Pm; nc=2,3-naphthalocyaninate; oep = octaethylporphyrinate) have been prepared by a one-pot procedure from the corresponding [M(acac)₃] · nH₂O (acac = acetylacetonate), metal-free porphyrin H₂(oep), and naphthalonitrile in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in n-octanol. The molecular structures of four of these complexes (M= Sm, Gd, Y, Lu) are isostructural, exhibiting a slightly distorted square antiprismatic geometry with two domed ligands. The interplanar distance decreases from 2.823 to 2.646 Å along the series as a result of lanthanide contraction. The whole series of complexes have also been characterized spectroscopically. All the electronic absorptions, except the two B-bands due to nc and the oep rings, are metal-dependent, indicating that there are substantial π-πinteractions. The hole or the unpaired electron in these double-deckers is delocalized over both macrocyclic ligands, as evidenced by the co-appearance of the IR marker bands for the nc^·- (1315-1325cm^-1) and oep^·- (1510-1531 cm^-1) π radical ions. Three one-electron oxidation couples and up to three one-electron reduction couples have been revealed by electrochemical methods. All the potentials are linearly dependent on the size of the metal center. The changes in absorption spectra during the first electro-oxidation and reduction of the La^III, Eu^III, and Y^III double-deckers have also been studied spectroelectrochemically. The spectral data recorded for [M^III(nc)(oep)]^- (M=Y, La-Lu except Ce and Pm) reduced chemically with hydrazine hydrate are in accord with those obtained by spectroelectrochemical methods. The first heteroleptic naphthalocyaninate-containing triple-deckers [M^III₂(nc)-(oep)₂] (M = Nd, Eu) have also been prepared by a raise-by-one-story method by using [M^III(nc)(oep)] (M=Nd, Eu), [M(acac)₃] · nH₂O (M = Nd, Eu), and H₂(oep) as starting materials. The compounds adopt a symmetrical triple-decker structure with two outer oep rings and one inner nc ring, which has been confirmed by ¹H NMR spectroscopy and X-ray structural determination of the Nd complex. Both compounds give a near-IR absorption at 1021 nm (for M = Nd) or 1101 nm (for M=Eu), which has rarely been observed for neutral (or hole-free) triple-decker complexes and can be ascribed to the lowest-energy Q'(0, 0) transition. Similarly to the double-decker analogues, these triple-decker complexes undergo a series of one-electron transfer processes with a relatively small potenial gap (1.1-1.2 V) between the first oxidation and the first reduction.