A Promising Approach for Controlling the Second Coordination Sphere of Biomimetic Metal Complexes: Encapsulation in a Dynamic Hydrogen‐Bonded Capsule

Abstract

The design of biomimetic models of metalloenzymes needs to take into account many factorsa nd is therefore ac hallenging task. We propose in this work an original strategy to control the second coordinations phere of a metal centre and its distal environment. Ab iomimetic complex, reproducing the first coordination sphere, is encapsulated in as elf-assembled hydrogen-bondedc apsule. The cationic complex is co-encapsulated with its counter-anion or with solventm olecules. The capsule is dynamic, allowing a fast in/out exchange of the co-encapsulated species. It also provides both ah ydrogen-bonding site in the second coordination spherea nd as ource of protona si tc an be deprotonatedi nt he presence of the complex,providing ag lobally neutralh ost-guest assembly.T hiss imple and broad scope strategyi su nprecedented in biomimetic studies. The approach appearstobeavery promising method for the stabilisation of reactive species and for the studyo ft heir reactivity.

Inspiration from Nature: Influence of Engineered Ligand Scaffolds and Auxiliary Factors on the Reactivity of Biomimetic Oxidants

“…In enzymes, substrate binding and positioning is an essential feature to guide a reaction to high selectivity. Although most biomimetic studies have focused on the first-coordination sphere of the oxidant as compared to enzymatic systems, over the years a number of studies have been reported on how the substrate approach can affect reactivity patterns. ,− In particular, in biomimetic chemistry surrounding an oxidant with bulky groups, placing the short-lived intermediate in a cavity is useful for trapping and characterizing these species. This has been proved particularly useful for CO 2 reduction reactions on metal-porphyrin systems where the encapsulated substrate was tightly bound. − …”

Encapsulated Neutral Ruthenium Catalyst for Substrate‐Selective Oxidation of Alcohols

“…It is important to note that the same reactions performed in the absence of resorcinarene, and thus of a capsular entity, led to poor enantioselectivity [10] . Although encapsulation of cationic metal complexes such as cobalt, [11] copper, [12] gold, [13] iridium, [14] ruthenium [15] or zinc [12] have been reported, only few of the corresponding capsular systems were employed in catalysis. Among these are N ‐heterocyclic carbene gold complexes, which were used in competitive hydration of alkynes, [13c] as well as ruthenium complexes suitable for ring closing metathesis [15b] .…”

A caged tris(2-pyridylmethyl)amine ligand equipped with a Ctriazole–H hydrogen bonding cavity

“…These UV-vis data are consistent with the formation of the azido complex Cu II (Hm-TriA-TPA)(N 3 )(OTf ). 2,7,11 In TPAcomplexes bearing intramolecular H-bonding donors, the potential presence of stabilizing interactions with the azido moiety has been associated with a blue-shift of the LMCT band, as well as a blue-shift of the ν(N-N) stretching frequency. 2,17 The value of the LMCT band observed in the case of Hm-TriA-TPA was therefore compared with that of its To further support the stabilization offered by Hm-TriA-TPA, the two azido complexes have been isolated (see the ESI †) and their antisymmetric N 3 − IR stretch was compared.…”

“…8 In particular, the archetypal TPA ligand has been equipped with well-defined cavities by means of its covalent substitution, 9,10 or host–guest encapsulation into a H-bonded capsule. 11 In this context, TPA -based hemicryptophanes are organic cages built from a bowl-shaped cyclotriveratrylene (CTV) cap, connected to the tripodal ligand via three linkers. We have recently demonstrated that TPA -hemicryptophanes with methylene or phenyl linkers could, respectively, control the helical arrangement of the ligand 12 and lead to enhanced oxidation catalysts.…”

“…7 Azido complexes [(Cu II - TPA X )(N 3 − )] + have been extensively studied by electronic and vibrational spectroscopies. 2,11 They display typical azido → Cu II LMCT transitions and ν (N–N) stretching frequency. On this basis, we explored the possibility of binding N 3 − to our caged Cu( ii ) complex.…”