Azo-dimethylaminopyridine-functionalized Ni(II)-porphyrin as a photoswitchable nucleophilic catalyst

Abstract

We present the synthesis and the photochemical and catalytic switching properties of an azopyridine as a photoswitchable ligand, covalently attached to a Ni(II)-porphyrin. Upon irradiation with 530 nm (green light), the azopyridine switches to the cis configuration and coordinates with the Ni2+ ion. Light of 435 nm (violet) isomerizes the ligand back to the trans configuration, which decoordinates for steric reasons. This so-called record player design has been used previously to switch the spin state of Ni2+ between singlet and triplet. We now use the coordination/decoordination process to switch the catalytic activity of the dimethylaminopyridine (DMAP) unit. DMAP is a known catalyst in the nitroaldol (Henry) reaction. Upon coordination to the Ni2+ ion, the basicity of the pyridine lone pair is attenuated and hence the catalytic activity is reduced. Decoordination restores the catalytic activity. The rate constants in the two switching states differ by a factor of 2.2, and the catalytic switching is reversible.

Solid-state photochromic arylazopyrazole-based transition metal complexes

“…On the other hand, specific designs are desired in metal-based molecular systems, which give additional advantages in modulating inherent physicochemical properties such as the electrocatalytic, magnetic, redox and biological behaviour of these complexes by light. Such complexes find applications in light-induced ligand-driven spin crossover complexes, 5–7 redox switches, 8,9 photoswitchable catalysts, 10–12 nonlinear optics, 13,14 logics and memories, 15 supramolecular chemistry, 16–18 etc . Indeed, phototunability can be introduced into transition metal complexes by incorporating different organic photochromes within the ligand frameworks.…”

Traffic Lights for Catalysis: Stimuli‐Responsive Molecular and Extended Catalytic Systems

“…A fundamentally different approach toward achieving switchable catalysis is coupling of organic photoswitches with catalytically active organometallic complexes. [55][56][57][58][59][60][97][98][99][100][101][102][103][104][105][106] Similar to the purely-organic systems described above, photochromic organometallic complexes can take advantage of significant changes in geometric parameters or physicochemical properties of the complex upon isomerization of a photochromic ligand. Moreover, access to d-orbitals from transition metals expands the scope of geometries available for photoswitch integration in comparison with many organic systems.…”

“…Rather than geometric control of reaction rate or yield, a different approach to switchable catalysis can be achieved through modulation of the electronic properties of the metal center for catalytically active organometallic complexes. [57][58][59][60]103] For example, change in a metal center's oxidation state could significantly alter the catalytic activity. Subtle shifts in ligand field strength, as well as LMCT, MLCT, or PET could be utilized to affect the metal oxidation states.…”

Ni-Centered Coordination-Induced Spin-State Switching Triggered by Electrical Stimulation

“…The few examples reported to date in the literature involve a quite limited number of transition metals, namely, Fe(II), 27−29 Mn(V), 30 or Ni-(II), 24,25,31−38 selected for their ability to undergo HS ↔ LS transitions following the addition or removal of ligand(s) from the metal center (Figure 1a). 31,39 All of these achievements involve the use of chemicals (pH) 27,40,41 or light 24,[31][32][33][34][35]38,42,43 as triggers for molecular motions, leading to an increase or decrease in the coordination number of the metal. In contrast, far less progress has been made in these directions with electron-responsive systems, even though electricity stands as a particularly attractive, controllable, and clean trigger in the perspective of applications in solid-state devices.…”

“…, from a high-spin (HS) to a low-spin (LS) state, are triggered by rational/drastic modifications of the first coordination sphere of a given metal center upon stimulation. The few examples reported to date in the literature involve a quite limited number of transition metals, namely, Fe­(II), − Mn­(V), or Ni­(II), ,,− selected for their ability to undergo HS ↔ LS transitions following the addition or removal of ligand(s) from the metal center (Figure a). , All of these achievements involve the use of chemicals (pH) ,, or light ,− ,,, as triggers for molecular motions, leading to an increase or decrease in the coordination number of the metal. In contrast, far less progress has been made in these directions with electron-responsive systems, even though electricity stands as a particularly attractive, controllable, and clean trigger in the perspective of applications in solid-state devices. − The few examples of redox-triggered magnetic switching reported so far in literature are based on Prussian blue analogues or transition-metal complexes exhibiting valence tautomerism. − …”