Better Together: Functional Heterobimetallic Macrocyclic and Cage-like Assemblies

Abstract

Metallosupramolecular chemistry has attracted the interest of generations of researches due to the versatile properties and functionalities of oligonuclear coordination complexes. Quite a number of different discrete cages were investigated, mostly consisting of only one type of ligand and one type of metal cation. Looking for ever more complex structures, heterobimetallic complexes became more and more attractive, as they give access to new structural motifs and functions. In the last years substantial success has been made in the design and synthesis of cages consisting of more than one type of metal cations, and a rapidly growing number of functional materials has appeared in the literature. This Minireview describes recent developments in the field of discrete heterometallic macrocycles and cages focusing on functional materials that have been used as host-systems or as magnetic, photo-active, redox-active, and even catalytically active materials.

Keywords: functional materials; heterobimetallic complexes; metallosupramolecular cages; supramolecular catalysts; supramolecular chemistry.

Figure 1

Reaction of 1 with Mo₂(OAc)₄ at high temperatures yielded the core–shell assembly 2 that could be further transformed to heterobimetallic 3 via metathesis of metal ions.34

Scheme 1

Cage‐to‐cage transformations starting from tetrahedral cage 4 to heterobimetallic 7 upon addition of 5. Compound 7 could then be transformed to the homometallic tetrahedron 9 upon addition of subcomponent 8.36

Scheme 2

Transformation of cubic cages 10 and 11 to trigonal bipyramidal assemblies 14 and 15 upon addition of the bisdentate ligand dppp.37

Figure 2

X‐ray crystal structures of a) Zr₆Pd₃ trigonal bipyramid 16;41 b) host–guest complex of pyrene⊂17;42 c) octahedral host–guest complex of 18 with three encapsulated BF₄ ⁻ ions and one BPh₄ ⁻ anion;47 (guest molecules are shown as space‐fill models; hydrogen atoms, non‐coordinated solvent molecules and non‐encapsulated counter anions are omitted for clarity; color code: light blue—zirconium, petrol—palladium, yellow—rhodium, orange—copper, red—iron, blue—nitrogen, bright red—oxygen, green—chlorine, green‐yellow—fluorine, beige—boron, grey—carbon).

Figure 3

Ligand 19 can be used to form cage 20 in a stepwise assembly. Cage 20 was found to encapsulate an additional [Na(BF₄)₄]³⁻ complex (color code: purple—cobalt, light pink—ruthenium, pink—sodium, light green—fluorine, beige—boron, blue—nitrogen, grey—carbon).48

Scheme 3

[Rh₄]‐fragment 21 could be used as a metalloligand for the assembly of heterobimetallic 22, which again could be used to separate p‐xylene from its aromatic constitution isomers. Guest release was achieved either by extraction with diethyl ether or by exposure to sunlight. Color code: yellow—rhenium, silver—silver, green—chlorine, bright red—oxygen, blue—nitrogen, grey—carbon.49

Scheme 4

Allosteric regulation of guest binding properties of heterobimetallic cube 24 (color code: red—iron, light green—molybdenum, bright red—oxygen, blue—nitrogen, grey—carbon).^[53, 54]

Figure 4

X‐ray crystal structures of enantiomerically pure 25 and 26 (top) (color code: orange—copper, greenish—nickel, purple—cobalt, blue—nitrogen, grey—carbon). Plot of χ_M T vs. T (left axis), χ_M ⁻¹ vs. T (right axis) and corresponding linear fit of (a) 25 and (b) 26 (adapted with permission from ref. [57], copyright 2018, American Chemical Society).

Figure 5

Solid state structures of 27, 28, and 29 (top) (color code: red—iron, orange—copper, greenish—nickel, purple—cobalt, bright red—oxygen, blue—nitrogen, yellow—sulfur, grey—carbon). a) Plot of χT vs. T for 27–29 (B=0.1 T) (solid lines are a fit of the experimental data, dashed lines are the Curie constants). b) Magnetization of 27–29 in field ranges of T=2–7 K and B=0–7 T (dashed lines indicate the saturation values expected for the field‐induced alignment of all isotropic spin centers) (reprinted with permission from ref. [59], copyright 2018, American Chemical Society).

Figure 6

Solid state structure of [Dy₈Cu₆] 30 (color code: dark green—dysprosium, orange—copper, red—oxygen, blue—nitrogen, grey—carbon). (a) Magnetic susceptibility χ and χT vs. T. (b) Temperature dependence of the out‐of‐phase components of the AC susceptibility for frequencies of 100–5000 kHz (reproduced with permission from ref. [61], copyright 2016, Royal Society of Chemistry)

Figure 7

Chemical shifts of a selected proton in temperature‐dependent ¹H‐NMR experiments in [D₄]MeOH. Empty squares 31, filled squares C₇₀@31, and calculated molar susceptibility χ_M based on the ideal solution model (black lines, scaled) (adapted from ref. [72], copyright 2017, Wiley‐VCH).

Figure 8

Solid state structure of nanoball 32 (top) (color code: red—iron, orange—copper, yellow—sulfur, blue—nitrogen, beige—boron, grey—carbon). (a) Plot of χ_MT vs. T per Fe^II ion (empty circles) and the light‐induced excited metastable high‐spin state with subsequent thermal relaxation (filled squares). (b) Cycling of green‐ (filled squares) and red‐ (empty squares) light‐induced switching between high spin (‘on’) and low‐spin (‘off) states (adapted from ref. [73], copyright 2009, Wiley‐VCH).

Figure 9

Palladium(II)‐based 33 shows photosensitized ¹O₂‐production upon irradiation with sun light (ON‐state), whereas copper‐based 34 is a dormant photosensitizer (OFF‐state), because of the electron transfer (ET) from the zinc(II) porphyrin to copper(II) cations and can be switched ‘on’ by protonation.77

Figure 10

(a) Stepwise assembly and structure of 38 as determined by XRD analysis (hydrogen atoms, solvate and counter anions are omitted in the crystal structure; color code: petrol—palladium, green—europium, red—oxygen, blue—nitrogen, grey—carbon). (b) Excitation (dashed lines) and emission (solid lines) spectra of 37 (black, 3.33×10⁻⁶  m) and 38 (red, 1.67×10⁻⁶  m) (excitation and emission intensity of 37 is 10 times amplified for clarity). (c) Luminescence emission spectra (λ_ex=345 nm) of 38 (20 μm) with the addition of 0–120 μm of PCL‐Na. Inset shows photographs before and after the addition of PCL‐Na (adapted with permission from ref. [78], copyright 2018, American Chemical Society).

Scheme 5

Stepwise assembly of the [Ru₈Pd₆] truncated octahedron 41, capable of guest encapsulation.79

Scheme 6

Oxidation of 42 to 43 with CCl₄, coupled with a hinge motion. Reverse reaction proceeded via treatment with activated Mg.82

Scheme 7

Chemical and electrochemical switching between folded 44 and unfolded 45 upon addition or removal of Cu^I moieties.91

Scheme 8

Heterobimetallic cage 47 encapsulated guest 48 with a central Co^II cation, which could then be used to perform radical‐type reactions.94

Scheme 9

Enantiomerically pure 41 can transform 53 into 54 in a stereo‐ and regioselective manner upon irradiation with a blue LED.100