Single-Step Synthesis of a Heterometallic [Cu2PdL4]2+ Hybrid Metal-Organic Coordination Cage

Abstract

Traditional methods of assembling low-symmetry heterometallic cage architectures are limited to stepwise construction and combinations of inert and labile metal ions, affording complex, anisotropic cage structures by sacrificing synthetic ease. Herein, a heterometallic [Cu₂PdL₄]²⁺ lantern-type cage has been assembled in a single self-assembly step through the use of a heteroditopic ligand with two different metal-binding groups. The resultant cage complex is a fusion of two common lantern-type cage motifs-carboxylate-based metal-organic Cu₄L₄ cages and pyridyl-based Pd₂L₄ coordination cages. Evidence for heterometallic cage formation in solution was provided by ¹H and diffusion-ordered NMR spectroscopy and electrospray ionization mass spectrometry (ESIMS) data, whereas circular dichroism (CD) spectra confirmed the helical nature of the assembly. The heterometallic cage was then exploited in binding heterotopic guests. It is envisioned that the simple design strategy presented herein will ease the assembly of other structurally complex, low-symmetry cage architectures.

Keywords: Cages; Copper(II); Heterometallic complexes; Low‐symmetry; Palladium(II).

Scheme 1

Cartoon illustration of the strategies employed in the self‐assembly of heterometallic cages including a) sequential complexation, b) the metalloligand approach, c) subcomponent self‐assembly, d) postsynthetic metal exchange or insertion, and e) the single‐step synthetic strategy presented in the current work. Graphics inspired by Lewis et al.^{[ 30 ]} and Crowley et al.^{[ 35 ]}

Figure 1

a) Single‐step synthesis of heterometallic cage [Cu₂Pd(L1)₄](BF₄)₂ (1) with a heterotopic ligand design. b) Stacked ¹H NMR spectra (400 MHz, DMSO‐d ₆, 298 K) of HL1 and cage 1 (asterisks (*) designate impurities in the [Pd(CH₃CN)₄](BF₄)₂ starting reagent). c) ESIMS (CH₃CN/DMA) of 1 (isotope pattern shown as inset). d) CD spectra (CH₃CN/DMA) of HL1 (dashed line) and 1 (solid line). The molecular model of 1 was generated via DFT optimization. Colors: carbon (grey), oxygen (red), nitrogen (blue), sulfur (yellow), palladium (purple), and copper (light blue). Hydrogen atoms omitted for clarity.

Figure 2

Stacked partial ¹H NMR spectra (400 MHz, DMSO‐d ₆, 298 K) depicting the titrations of a) Cu(OAc)₂ (2.0 equiv) into a mixture of HL1 (4.0 equiv) and [Pd(CH₃CN)₄](BF₄)₂ (1.0 equiv) and b) [Pd(CH₃CN)₄](BF₄)₂ (1.0 equiv) into a mixture of HL1 (4.0 equiv) and Cu(OAc)₂ (2.0 equiv). The complete heterometallic cage 1 is formed in both cases.