Research

Stimuli-responsive materials containing coordination compounds.

We are interested in creating new stimuli-responsive and, in particular, mechanoresponsive materials that utilize coordination compounds to induce desired changes in properties (such as color, luminescence, shape, conductivity) in response to external stimulus such as mechanical activation, light, pH changes or oxidation/reduction.

For this purpose, we utilize conformationally flexible ligands that are known to change their coordination mode to a metal in response to pH changes, redox reactions, temperature or subtle changes in ligand steric properties.

For example, using our expertise in coordination chemistry of transition metal complexes with pyridinophane ligand, we recently developed a new mechanophore based on the pyridinophane copper(I) iodide complex. Our previous and current studies confirm that these complexes show significant changes in their photoluminescent properties in response to subtle changes in steric properties of the alkyl substituent attached to the axial amines, which is due to various degrees of conformational lability of these complexes [1,2]. By incorporating this complex into a polyurethane chain, we obtained a new mechanoresponsive material showing very fast and reversible response to tensile stress [3]. Thus, when the film is elongated, photoluminescence intensity increases, and then quickly returns to the original value after film relaxation. The unique feature of this system is that no bond breaking or formation is involved in this stress sensing mechanism. This gives rise to a very fast and fully reversible response. These materials can be useful as sensitive and reversible stress sensors for evaluating materials properties.

Supported by JSPS Kakenhi Grant # JP18K05247

References:

[1] Filonenko, G. A.; Fayzullin, R. R.; Khusnutdinova, J. R. "Intramolecular Non-Covalent Interactions as a Strategy Towards Controlled Photoluminescence in Copper(I) Complexes" J. Mater. Chem. C. 2017, 5, 1638-1645

[2] Filonenko, G. A.; Khusnutdinova, J. R. "Dynamic Phosphorescent Probe for Facile and Reversible Stress Sensing" Adv. Mater. 2017, 29, 1700563

[3] Patil, P. H.; Filonenko, G. A.; Lapointe, S.; Fayzullin, R. R.; Khusnutdinova, J. R. "Interplay between conformational flexibility and photoluminescent properties of mononuclear pyridinophane copper(I) complexes" Inorg. Chem. 2018, 57, 10009-10027

 

Hydrogenation and hydrogen borrowing reactivity by manganese complexes.

Manganese is the third most abundant transition metal in Earth’s crust, however, until recently, it has rarely been used in reduction catalysis. Recently, there has been a significant interest towards using manganese as an inexpensive and readily available catalyst for hydrogenation and hydrogen transfer reactions. However, at the moment, the vast majority of known manganese catalysts for hydrogenation are based on air-sensitive polydentate phosphine ligands. Although manganese complexes with simple N-donor ligands such as 2,2'-bipyridine have been recently utilized in electrocatalytic CO2 reduction, they have not been reported to be active in homogeneous hydrogenation reactions. 

Our second project aims at developing transition metal-based catalysts that utilize inexpensive first row transition metals (Mn, Fe) in combination simple and robust ligands, which are able to activate strong chemical bonds via cooperation between the metal and the ligand:

Taking inspiration from [Fe]-hydrogenase which activates hydrogen under mild conditions via assistance of the surrounding ligands, we developed a new efficient Mn catalyst for CO2 hydrogenation that utilizes a very simple and robust 6,6’-dihydroxybipyridine ligand. To the best of our knowledge, this is the first example of a homogeneous Mn catalyst for CO2 hydrogenation.

The presence of the two hydroxyl groups in the ortho-positions was a key to high catalytic reactivity. For example, analogous complexes with 2,2'-bipyridine and 6,6'-dimethoxybipyridine showed very low activity, while the complex with two OH groups in the para-positions was significantly less efficient. We were able to obtain over 6000 turnovers for CO2 hydrogenation to formate under mild conditions (65 oC in MeCN). The same complex also catalyzes formation of N,N-diethylformamide from diethylamine, CO2 and H2. This is a rare example of the first row transition metal complex that can be used for CO2 hydrogenation for formamides.

Currently, we are also developing other types of hydrogen borrowing catalytic reactivity by Mn complexes. In addition, we are also studying small molecule activation at manganese center via metal-ligand cooperation using non-phosphine based ligands.

References:

[1] Dubey, A.; Nencini, L.; Fayzullin, R. R.; Nervi, C.; Khusnutdinova, J. R. "Bio-Inspired Mn(I) Complexes for Hydrogenation of CO2 to Formate and Formamide" ACS Catal. 2017, 7, 3864-3868.

[2] Dubey, A.; Rahaman, S. M. W.; Fayzullin, R. R.; Khusnutdinova, J. R. "Transfer hydrogenation of carbonyl groups, imines and N‐heterocycles catalyzed by simple, bipyridine‐based MnI complexes" ChemCatChem 2019, 11, 3844-3852.

 

Polynuclear linear chain complexes and second coordination sphere interaction: new ligand scaffolds for new reactivity patterns

Our group is interested in designing new unsymmetrical ligand scaffolds that can support homo- and heterobimetallic polynuclear “linear chain” complexes and/or feature secondary coordination sphere interactions. The presence of multiple redox-active metal centers with close metal-metal interactions may enable new patterns for small molecule activation and catalysis.

For example, ligands L1 and L2 are two new unsymmetrical ligand scaffolds that feature naphthyridinone backbone, which acts as a bridging ligand of variable denticity, and a chelating mono- or bis-picolylamine capping fragment.

This ligands support copper(I) complexes containing one, two, three or four copper centers with remarkable selectivity, depending on reaction conditions [1]. Using this ligand, we demonstrated stepwise controlled growth of copper(I) chains from mononuclear to tetranuclear species, as well as their selective reversible deconstruction controlled by stoichiometry, solvent or redox reactions. During the process of chain growth, the naphthyridinone fragment “slides” across the metal chain as schematically shown below, and hemilability of the capping fragment enables further chain growth [1]:

Selective formation of heterobimetallic polynuclear species and redox reactions of linear chain complexes are currently investigated using this and similar ligand platforms.

 

Supported by JSPS KAKENHI Grant Number 16F16038; ORW is supported by JSPS Fellowship.

References:

[1] Rivada-Wheelaghan, O; Aristizábal, S.L.; López-Serrano, J.; Fayzullin, R. R; Khusnutdinova, J. R. "Controlled and reversible stepwise growth of copper(I) linear chains enabled via dynamic ligand scaffolds" Angew. Chem. Int. Ed. 2017, 56, 15267 Highlight for general audience.