Double Rotors with Fluxional Axles: Domino Rotation and Azide-Alkyne Huisgen Cycloaddition Catalysis

The simple preparation of the multicomponent devices [Cu4 (A)2 ]4+ and [Cu2 (A)(B)]2+ , both rotors with fluxional axles undergoing domino rotation, highlights the potential of self-sorting. The concept of domino rotation requires the interconversion of axle and rotator, allowing the spatiotemporal decoupling of two degenerate exchange processes in [Cu4 (A)2 ]4+ occurring at 142 kHz. Addition of two equiv of B to rotor [Cu4 (A)2 ]4+ afforded the heteromeric two-axle rotor [Cu2 (A)(B)]2+ with two distinct exchange processes (64.0 kHz and 0.55 Hz). The motion requiring a pyridine→zinc porphyrin bond cleavage is 1.2×105 times faster than that operating via a terpyridine→[Cu(phenAr2 )]+ rupture. Finally, both rotors are catalysts due to their copper(I) content. The fast domino rotor (142 kHz) was shown to suppress product inhibition in the catalysis of the azide-alkyne Huisgen cycloaddition.

Substrate-dependent allosteric regulation by switchable catalytic molecular tweezers

Allosteric regulation is exploited by biological systems to regulate the activity and/or selectivity of enzymatic reactions but remains a challenge for artificial catalysts. Here we report switchable terpy(Zn-salphen)2 molecular tweezers and their metal-dependent allosteric regulation of the acetylation of pyridinemethanol isomers. Zinc-salphen moieties can both act as a Lewis acid to activate the anhydride reagents and provide a binding site for pyridinemethanol substrates. The tweezers’ conformation can be reversibly switched between an open and a closed form by a metal ion stimulus. Both states offer distinct catalytic profiles, with closed tweezers showing superior catalytic activity towards ortho substrates, while open tweezers presenting higher rate for the acetylation of meta and para substrates. This notable substrate dependent allosteric response is rationalized by a combination of experimental results and calculations supporting a bimetallic reaction in the closed form for ortho substrate and an inhibition of the cavity for meta and para substrates.

Molecular brakes based on the Zn(ii) porphyrin dimer

The rotational movement of molecular brakes based on two Zn–porphyrin units interconnected by a spacer may be reversibly locked by addition of an external auxiliary ligand.

A pyridyl-benzimidazole based molecular luminescent turnstile

A molecular turnstile based on a luminescent pyridyl-benzimidazole stator and a rotor containing a pyridyl coordinating site may be reversibily switched between its open and closed states upon binding/unbinding of silver cations.

Six States Switching of Redox-Active Molecular Tweezers by Three Orthogonal Stimuli

A six level molecular switch based on terpyridine(Ni-salphen)₂ tweezers and addressable by three orthogonal stimuli (metal coordination, redox reaction, and guest binding) is reported. By a metal coordination stimulus, the tweezers can be mechanically switched from an open "W"-shaped conformation to a closed "U"-shaped form. Theses two states can each be reversibly oxidized by the redox stimulus and bind to a pyrazine guest resulting in four additional states. All six states are stable and accessible by the right combination of stimuli and were studied by NMR, XRD, EPR spectroscopy, and DFT calculations. The combination of the supramolecular concepts of mechanical motion and guest binding with the redox noninnocent and valence tautomerism properties of Ni-salphen complexes added two new dimensions to a mechanical switch.

Switchable platinum-based tweezers with Pt-Pt bonding and selective luminescence quenching

Molecular tweezers incorporating peripheral platinum salphen complexes and a central chelating terpyridine group have been synthesized. The terpyridine can be switched upon metal binding between a free 'W' shaped form and a coordinated 'U' form. The crystallographic structure of the zinc-closed molecular tweezers was obtained and presented a strong π-stacking between the Pt-salphen units associated with a Pt-Pt bond. The luminescence properties, notably in response to selected guest ions (Zn(2+), Pb(2+), Hg(2+)) and the resulting mechanical motion, have been investigated by UV-Vis and emission spectroscopy. While ion coordination to the terpy resulted in no significant changes in the luminescence, a selective intercalation of a second Hg(2+) associated with a large differential quenching was observed.

Mechanical switching of magnetic interaction by tweezers-type complex

An original approach using a mechanical motion to control the magnetic interaction between the two spin centers of terpy(Cu–salphen)₂ complexes is presented.

A bi-stable Pt(ii) based molecular turnstile

Playing with the competition between H- and coordination-bonds, a bi-stable unsymmetrical organometallic turnstile may be switched between two closed states.

Terpy(Pt-salphen)2 switchable luminescent molecular tweezers

The design and synthesis of switchable molecular tweezers based on a luminescent terpy(Pt-salphen)2 (1; terpy=terpyridine) complex is reported. Upon metal coordination, the tweezers can switch from an open "W"-shaped conformation to a closed "U"-shaped form that is adapted for selective recognition of cations. Closing of the tweezers by metal coordination (M=Zn(2+), Cu(2+), Pb(2+), Fe(2+), Hg(2+)) was monitored by (1)H NMR and/or UV/Vis titrations. During the titration, exclusive formation of the 1:1 complex [M(1)] was observed, without appearance of an intermediate 1:2 complex [M(1)2]. The crystallographic structure of the 1:1 complex was obtained with Pb(2+) and showed a distorted helical structure. Selective intercalation of Hg(2+) cations by the closed "U" form was observed. The tweezers were reopened by selective metal decoordination of the terpyridine ligand by using tris(2-aminoethyl)amine (tren) as a competitive ligand without modification of the Pt-salphen complex. Detailed photophysical studies were performed on the open and closed tweezers. Structured emission was observed in the open form from the Pt-salphen moieties, with a high quantum yield and a long lifetime. The emission is slightly modified upon closing with 1 equivalent of Zn(2+) or Hg(2+), whereas a dramatic quenching was obtained upon intercalation of additional Hg(2+).

A luminescent molecular turnstile

Optical reading of the open and closed states of a molecular turnstile.

Optical reading of the open and closed states of a molecular turnstile

The reversible switching between the open and closed states of a molecular turnstile can be addressed by emission spectroscopy.

A platinum turnstile with a palladium lock

An organometallic molecular turnstile composed of a stator based on an α,ω-diphosphine bearing, in a symmetric fashion, the 2,6-pyridyl diamide moiety as a central tridentate chelating unit and a rotor composed of Pt(II) equipped with two pyridyl groups in trans configuration was designed. The switching between its open and closed states using Pd(II) was investigated both in solution and in the solid state.