Synthesis and properties of organolanthanide complexes with chelate ligands

Abstract

Dicyclopentadienyl lanthanide chlorides, (C 5H 5) 2LnCl, are key intermediates for the synthesis of the compounds containing LNC σ bonds, LnN and LnO bonds. However, probably due to lanthanide contraction effect, the early lanthanocene chloride complexes, particularly for La, Ce, Pr and Nd, have not yet been synthesized [1-3]. Recently, some organometallic chemists have made their efforts to study how to stabilize the dicyclopentadienyl early lanthanide chloride. Using the di(trimethylsilyl)-lanthanide chlorides. By using a bulky pentamethylcyclopentadienyl as a ligand, Evans [4] and Andersen [5] had succeeded in stabilizing dicyclopentadienyl cyclopentadienyl as a ligand, Lappert [6] succeeded in stabilizing early (f 0–f 3) lanthanocene chlorides by generating an anionic complex. We now wish to report here two methods for the stabilization of early lanthanocene chlorides by using a chelate ligand. 1. By using ring-bridged dicyclopentadiene instead of two separated cyclopentadienes as a ligand we have succeeded in synthesizing the stabilized early lanthanocene chlorides tetrahydrofuran coordinated neutral complexes [7]. Because our results are different from Tsutsui's [8], we further synthesized six 1,1′-trimethylenedicyclopentadienyl lanthanide chlorides. All the products so far obtained were found to be tetrahydrofuran coordinated neutral complexes without exception. Anhydrous lanthanide chlorides reacted with disodium salt 1,3-dicyclopentadienyl-propane in THF at −50 °C to give tetrahydrofuran coordinated 1,1′-trimethylenedicyclopentadienyl lanthanide chlorides (eqn. 1). ▪ Furthermore, we found that the coordinated tetrahydrofuran could be replaced by 2,2′-bipyridyl to afford 2,2′-bipyridyl complexes which exhibit better stability to air and moisture (eqn. 2). ▪ It should be noted that even when the molar ratios of chlorides to 2,2′-bipyridyl in starting material were changed from 1:1 to 1:2, the atomic ratios of lanthanide metal to nitrogen in products remained 1:1. The infrared spectra of 2,2′-bipyridyl complexes indicate that four characteristics absorption peaks of pyridine ring at 408, 621, 994 and 1580 cm −1 all shift to higher frequencies as compared with that in [C 5H 4(CH 2) 3C 5H 4]LnCl·2,2′-bpy) 0.5. It implies that a pair of electrons on nitrogen is coordinated to the metal to form a coordinated covalent bond. However, the dicyclopentadienyl lanthanide chlorides reacted with 2,2′-bipyridyl giving 2,2′-bipyridyl complexes, the atomic ratios of metal to nitrogen in products always were 1:2 and the coordination number is nine (eqn. 3). The infrared spectra of these complexes indicate that four characteristics absorption peaks of pyridine ring all shift to higher frequencies too, in agreement ▪ with that of [C 5H 4(CH 2) 3C 5H 4]LnCl·(2,2′-bpy) 0.5. The X-ray photoelectron spectra of five dicyclopentadienyl lanthanide chlorides and their corresponding bipyridyl complexes (Cp 2LnCl·2,2′-bpy) were measured. The binding energies of both Ln 4d and Cl 2p decrease as Cp 2LnCl forms Cp 2LnCl·2,2′-bpy complexes. This fact also demonstrated that the lone pair of electrons on nitrogen is coordinated to the metal to form a coordinated covalent bond. It was unexpectedly found that the binding energies of Cl 2p also decrease. It seems that the electron also shifts toward chlorine. The binding energies of N 1s increase at the same time. 2. The second method of stabilizing early (f 0–f 3) lanthanocene chlorides we adopted is to use other types of chelate ligand, 2,2′-bipyridyl or o-phenanthroline to satisfy the coordination saturation, and thus improve the stabilization. We succeeded for the first time in synthesizing the dicyclopentadienyl early lanthanide chloride 2,2′-bipyridyl or o-phenanthroline coordination compounds (eqns. 4,5). They were allowed to react with sodium trifluoroacetate to afford the corresponding derivatives Cp 2LnOCOCF 3·nB (eqn. 6). The 1H NMR spectrum of Cp 2LaOCOCF 3·2phen was determined. It indicates that the δ values of coordinated o-phenanthroline shift to lower field as compared with that in o-phenanthroline. ▪ B = 2,2′-bpy; Ln = La, Nd B = o-phen; Ln = La, Ce; N = 2; Ln = Pr, Nd; N = 1 or 2 By using ring-bridged dicyclopentadiene, 2,2′-bipyridyl or o-phenanthroline as a ligand the disproportionation of early lanthanocene chloride is thus prevented, the stabilization of early lanthanocene chlorides achieved, and the early lanthanocene chlorides can be isolated.