Chirality: from molecular complexes to coordination assemblies and material networks

H. Amouri & M. Gruselle Chirality in Transion Metal Chemistry : Molecules, supramolecular assemblies and materials Chichester Wiley 2008.

Chirality is always a fascinating subject and is encountered in various fields of modern chemistry. Our group has international expertise in this area, with chirality in coordination chemistry being the cornerstone on which our research activities are based. We have for several years prepared a variety of chiral structures from mononuclear and coordination assemblies, including chiral networks. These compounds exhibit interesting properties:

I- Chiral ortho-methylenequinones

Ortho-methylene quinones act as important intermediates in organic synthesis as well as in chemical and biological processes, but examples of isolated species are rare due to their high reactivity. We have described the synthesis, stabilisation and reactivity of the first iridium and rhodium complexes of o-methylenequinone. These compounds undergo interesting C-C bond formation reactions with a variety of alkenes and alkynes, they also exhibit planar chirality, and their differentiation and splitting are challenging goals, especially in asymmetric C-C coupling reactions.

Figure. o-méthylène quinones chiraux. Acc. Chem. Res.2002, 35, 501. Organometallics  2005, 24, 4240.Synlett 2011, 1357. Chirality 2013, 25, 449. Inorg. Chimi. Acta.  2021, 517, 120208.

II- Tetrahedral complexes of rhodium and iridium enantiopurs with NHC-naphthalimide (NHC = N-heterocyclic carbenes)

On the other hand, we have developed a new approach to obtain a family of iodinated tetrahedral rhodium and iridium complexes with N-heterocyclic-naphthalimide carbenes. Remarkably, these complexes are obtained as two regioisomers containing five and six membered metallacycles. These complexes are chiral and exhibit very high configurational stability.

Figure. Tetrahedral cyclometallated complexes of enantiopure iridium obtained as two regioisomers. Inorg. Chem. 2019, 58, 2930.

We have extended our synthesis method to rhodium analogue complexes in order to study the influence of the metal on their configurational stability in solution. It should be noted that only the 6-membered isomer complex 4 was obtained (see Figure below). In collaboration with Dr. N. Vanthuyne (U. Marseille), we split these complexes and obtained the corresponding enantiopure complexes. 

Figure. Enantiopure complexes of rhodium and iridium prepared and their absolute configuration.

Determination of enantiomerisation barriers of configurational stability

To assess the stability of the rhodium enantiopure complex (R)-4 was dissolved in acetonitrile, heated to 60°C and then analysed by chiral HPLC. Complex 4 remained stable in solution but underwent racemisation. Monitoring of the enantiomeric excess over time revealed a clean first order reaction and allowed the determination of an enantiomerisation barrier of ΔG# = 115.9 kJ/mol. The half-life was determined at 25°C considering zero enantiomerisation entropy, t1/2 = 130 days for 4 at 25°C in acetonitrile. Thus, the enantiopur 4 complex can be handled and evaporated without any risk of loss of enantiopurity.  Similar racemisation kinetics studies carried out on (S)-3 and (R)-2, gave enantiomerisation barriers in acetonitrile, respectively ΔG# = 124.4 kJ/mol and ΔG# = 122.3 kJ/mol. These values confirm the chemical robustness of the iridium complexes and allowed us to estimate their half-life at 25°C, namely t1/2 = 11 years and t1/2 = 5 years for 3 and 4, respectively. This result highlights the role of the size of the 5-membered versus 6-membered metallacycle.

Figure. Configurational stability of tetrahedral complexes with centred chirality. Chemistry 2022, 4, 156.

III- Chiral luminescent compounds with organometallic ligands

In this work, we have designed a new class of luminescent platinum complexes featuring a chiral thioquinonoid ligand. Remarkably, the presence of the chiral organometallic ligand controls the aggregation of these square planar luminophores and imposes a homo- or heterochiral arrangement at the supramolecular level via non-covalent Pt—Pt and π−π interactions. Interestingly, these complexes are red emitters in the crystalline state, and their photophysical properties can be attributed to their aggregation in the solid state (Collaboration Prof. V.W.W. Yam).

Figure. CD curves for (pR, R) and (pS, S) enantiomers of luminescent platinum complexes. Chem. Eur. J. 2016, 22, 8032. (Hot paper – couverture). Posted on the CNRS website in October 2016.

IV-Chiral octahedral luminescent compounds with NHC-naphthalimide carbene ligands (NHC = N-heterocyclic carbenes).

We obtained enantiopure versions of the compounds Δ3anBu, Λ3anBu and Δ4anBu, Λ4anBu in which an n-butyl group is now attached to the nitrogen of the carbene unit to increase the solubility of these complexes (Figure). At room temperature, they act as emitters in the far-red and near-infrared regions. Their optical and chiroptical properties have been studied. Remarkably, the vibrational circular dichroism (VCD) technique and TD-DFT calculations allowed us to determine their stereochemistry.

Figure. Circular dichroism spectra of iridium enantiopure complexes in CH2Cl2 à CM = 10-4 M. Daltons Trans., 2022, 51, 2750.
Figure. Vibrational circular dichroism (VCD) spectra of the two iridium enantiomers Δ4anBu et Λ4anBu. Daltons Trans., 2022, 51, 2750.

These interesting results show that it is possible to obtain complexes with cross properties, namely luminescence and chirality.