Back to Table of Contents

44703-GB3
Amphiphilic Metal Complexes: A Novel Approach to Aqueous Nanosystems with Catalytic Function

Juan Noveron, University of Texas at El Paso

The development of new metal complexes with alkyl (C_nH_2n+1, n>10) and oligoether (CH₂CH₂O-)₄ components that allow them to self-organize into nanoscale supramolecular phases in water while retaining their active coordination chemistry was the focus of our research. The new lipid metal complexes developed in this project pose a new avenue for further innovation for green catalysis design and the development of functional materials in water that may play a role in the environmental remediation, biomass conversion and in the manufacture of pharmaceuticals.

1. Lipid Metal Complexes. We synthesized and characterized seventeen amphiphilic ligands (1-17), Figure 1. The lipid groups introduced self-assembly properties to these molecules and their corresponding metal complexes in water. They allowed us to investigate the intrinsic chemical and physical properties of their corresponding metal complexes. In this final report, we highlight some of the most interesting results that we obtained during the Grant period.

Figure 1. Lipid ligand systems synthesized (1-17).

a. Novel Copper(I, II) lipid complexes with dioxygen-activation functions. Lipid Cu(I) and (II) complexes of ligands 1, 3, 7, 11 and 15 were synthesized and characterized with EXAFS, electrospray mass spectrometry (ESMS), and UV-vis spectroscopy. The mononuclear Cu(I) complexes of these ligands formed vesicles in water. The lipid ligand 15 generated multi-layered vesicles, Figure 2a, in which the Cu(I) sites are located within a hydrophobic environment that stabilized the copper m-oxo species. The [(15)₂Cu₂(m₂-O)₂] was observed in EXAFS, Figure 2b. The vesicles generated from Cu(I) complexes of 1, 3, 7, and 11  did not generate kinetically stable m-oxo species in water. The oxidative potential of [(15)₂Cu₂(m₂-O)₂] was investigated using triphenylphospine (Ph₃P) as the substrate. We found that [(15)₂Cu₂(m₂-O)₂] mediates the oxidation of Ph₃P (55% yield) to the corresponding phosphine oxide (Ph₃P=O) in water at 4 ^oC, Figure 2c. The control revealed essentially no oxidation of Ph₃P in the absence of the metal under similar conditions. The use of the EXAFS line at the Stanford Synchrotron Radiation Laboratory (SSRL) was used to collect X-ray spectroscopy data to study the coordination chemistry of seventeen metal lipid complexes during the period of the Grant. Time-lapse EXAFS was used to follow the oxo-transfer reaction from dioxygen to phosphine model substrates mediated with Cu(I) lipid complexes, Scheme 1.

Figure 2. (A) AFM scan of [Cu(15)(ACN)]⁺ vesicles in water. (B) Above: Fourier transformed EXAFS  of [Cu(15)(ACN)]⁺ (dotted line) and fitting (solid line). Below: Cu-lipid complex upon reaction with dioxygen at 77 K. (C) ³¹P NMRs of the products of the reactions of [(15)₂Cu₂(m₂-O)₂] with triphenylphosphine in water.

Scheme 1. Concept illustration of this reaction.

Similarly, vesicles prepared in water with [Cu(II)(10)(OTf)₂], where OTf = trifluoromethanesulfonate, exhibit oxidase activity with pyridyl-containing secondary alcohols such as 4,4'-dipyridylmethanol and catalyze the two-electron oxidation to the corresponding ketone, Figure 3.

Figure 3. Oxidase activity of vesicles of [Cu(II)(10)(OTf)₂] in water.

b. Lipid Cu(II) and Zn(II) complexes with hydrolytic catalytic properties in water. We prepared coordinatively unsaturated Cu(II) and Zinc(II) complexes of ligands 3,4,7, 12, and 15 and prepared their corresponding metallo-liposomes in water. These lipid complexes catalyze the hydrolysis of carboxylic esters and phosphate esters at mild conditions (pH 7.1 and 22 ^oC), Figure 4. Hydrolytic function was probed with the model substrates p-nitrophenylacetate and p-nitrophenylphosphate.

Figure 4. Catalytic hydrolysis of p-nitrophenylacetate by lipid Zn(II) complexes A and B in water (pH 7, 22 ^oC). (Time scale is in min.)

c. Lipid-metal Coordination Networks and their Self-assembly Properties in Water. We discovered that reactions of [Cu(II)(7)(OTf)] with 4,4'-trimethylenedipyridyl (4,4-TDP) generate one dimensional coordination networks that self-assemble in water into nanoscopic toroidal structures. Using X-ray crystallography, we crystallized the non-lipid model of this complex, Figure 5.

Figure 5. Crystal structure of the one-dimensional coordination Cu(II) network and close-up of Cu(II) site showing interactions with ligands and the -OTf counter ion.

When the lipid coordination network [Cu(II)(7)( 4,4-TDP)] (OTf)₂ is placed in water (1 mM, 25 ^oC), it folds into toroidal-like structures, Figure 6. Molecular weight determinations with dynamic light scattering (DLS) are currently undergoing, and attempts to control the polymer size with capping coordinating molecules is being explored. These materials could lead to robust catalytic systems that operate in water.

Figure 6. Above: Scheme of lipid coordination polymers. Below: Optical microscope images of toroidal structures generated from the lipid-directed folding of the 1-D coordination network of [Cu(II)(7)( 4,4-TDP)]_n (OTf)_2n in water.

d. Lipid Pt(II) and Pd(II) complexes and their self-assembly properties in water. We also prepared the Platinum(II) and Palladium(II) lipid complexes with ligands 13 and 14.  Two examples are displayed in Figure 7 and 8. Lipid metallacycles of Pt(II) generated micelles in water.

Figure 7. Cryo-TEM image of dinuclear lipid Pt(II) complex with ligand 14.

Figure 8. Tetranuclear Pt(II) macrocyclic lipid complex self-assembled into micelles in water. (a) Computer model (b) surface electronic potential. (c) AFM scans on mica. (d) ³¹P NMR.

Back to top