Binuclear Magnesium, Calcium, and Zinc Complexes Based on Nitrogen–Nitrogen‐Coupled Salicylaldiminate and β‐Diketiminate Ligands

Abstract

The diprotic bis(salicylaldimine) ligand N,N′‐bis(3,5‐di‐tBu‐salicylidene)diimine, (1‐NN)H2, was smoothly deprotonated by reaction with two equivalents of a metal amide M[N(SiMe3)2]2 (M = Mg, Ca or Zn) to form a bimetallic metal amide complex (1‐NN)[MN(SiMe3)2]2. This heteroleptic complex was only stable in the case of Mg and (1‐NN)[MgN(SiMe3)2·thf]2 was structurally characterized by X‐ray diffraction. For Ca and Zn homoleptic products could be obtained: (1‐NN)Ca·(thf)2 and (1‐NN)Zn. The latter crystallized as a trimer with approximate C3‐symmetry. The diprotic N–N coupled bis(β‐diketimine) ligand [RHN–C(Me)=C–C(Me)=N–N=C(Me)–C=C(Me)–NHR, R = 2,6‐di‐iPr‐C6H3], which is abbreviated as (2‐NN)H2, can be deprotonated only under much harsher reaction conditions. Attempted synthesis of (2‐NN)[Mg(nBu)]2 gave after ligand exchange the homoleptic species (2‐NN)Mg and Mg(nBu)2. For Ca, the complexes [(2‐NN)Ca]2 and (2‐NN)[CaN(SiMe3)2·thf]2 both have been prepared and structurally characterized by X‐ray diffraction. The latter heteroleptic complex is in benzene solution in equilibrium with [(2‐NN)Ca]2 and Ca[N(SiMe3)2]2·(thf)2. Reaction of (2‐NN)H2 with excess of Et2Zn gave the heteroleptic complex (2‐NN)[ZnEt]2 and is stable towards ligand exchange reactions. The complex is not active in CO2/cyclohexene oxide copolymerization. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Comparative Study of Lactide Polymerization with Lithium, Sodium, Potassium, Magnesium, Calcium, and Zinc Azo­naphthoxide Complexes

“…The Zn-O bond lengths are similar to those reported previously, [13,14] and the Zn-N bonds are slightly shorter than the Zn-N bonds of imino Table 2. Ring-opening polymerization of l-lactide by 1-12.…”

“…[13,14b,14c] The N-N bond length of the azo group is similar to that in the analogous compound 7·2THF; therefore, we can discount any electronic retrodonation from the Zn centre to the azo group, as suggested for the magnesium complex. [11,13,15] Complexes 10 and 11 share similar structural features (Figures 2 and 3). Two tridentate azonaphthoxide ligands are attached to the metal centre in a distorted octahedral geometry, in which the hexagonal environment is composed of two naphthoxy oxygen atoms, two nitrogen atoms, and two methoxy oxygen atoms.…”

“…15 7.34 (m, 1 H, C 10 H 6 ), 7.02 (m, 2 H, C 6 H 5 ), 6.86 (m, 1 H, C 6 H 5 ), 6.32 (m 1 H, C 10 H 6 ) ppm. 13 C NMR (100.6 MHz, 293 K, C 6 D 6 ): δ = 164. 0, 156.0, 137.3, 134.9, 133.5, 129.7, 128.7, 128.3, 127.6, 127.1, 126.3, 122.3, 122.1, 120.9 ppm.…”

“…13 C NMR (100.6 MHz, 293 K, C 6 D 6 /C 5 D 5 N): δ = 164. 7, 156.0, 137.0, 135.6, 133.8, 128.9, 127.5, 127.1, 122.5, 122.0, 121.4 ppm.…”

“…l-Lactide was purchased from Aldrich and then purified by recrystallization from toluene and sublimation under vacuum. The NMR spectra were recorded with a Bruker 400 spectrometer ( 1 H 400 MHz, 13 (C, H, N) were performed with a Perkin-Elmer 2400 instrument. The molecular weights of polymers in THF at room temperature were determined by gel permeation chromatography (GPC) with a Varian HPL apparatus calibrated to the polystyrene standards and corrected with a 0.58 factor.…”