Low-Symmetry Self-Assembled Coordination Complexes with Exclusive Diastereoselectivity: Experimental and Computational Studies

Abstract

A pyridine/aniline appended unsymmetrical bidentate ligand N-(4-(4-aminobenzyl)phenyl)nicotinamide, investigated in this work has two well-separated coordination sites. Combination of the ligand with cis-protected palladium(II) (i.e., PdL′) and palladium(II) in separate reactions produced the corresponding Pd 2 L′ 2 L un 2 and extremely rare Pd 2 L un 4 type self-assembled binuclear complexes, respectively. Notably, both varieties of complexes are prepared from a common ligand system. Two diastereomers (i.e., (2,0) and (1,1)-forms) are possible for Pd 2 L′ 2 L un 2 type complex, whereas four diastereomers (i.e., (4,0), (3,1), trans(2,2), and cis(2,2)-forms) can be imagined for the Pd 2 L un 4 type complex. However, exclusive diastereoselectivity was observed, and the complexes formed belong to (1,1)-Pd 2 L′ 2 L un 2 and cis(2,2)-Pd 2 L un 4 forms. The diastereomers are predicted from NMR study in solution and DFT calculations in gas-phase and implicit-solvent media; however, single-crystal structures of both the complexes provided unambiguous support. The rare Pd 2 L un 4 type complex is studied in further detail. Parameters like counteranion, concentration, temperature, and stoichiometry of metal to ligand did not influence the diastereoselectivity in complex formation. DFT calculations show the cis(2,2) form to be the most stable, followed by the (3,1) isomer. The lowest conformational strain in the bound ligand strands in the cis(2,2)-arrangement along with optimal intermolecular interactions makes it the energetically most stable of all the isomers. Molecular dynamics (MD) simulations were carried out to visualize the self-assembly process toward the formation of Pd 2 L un 4 type complex and the free energy difference between the cis(2,2) and (3,1) isomers. Snapshots of MD simulation elucidate the step-by-step progress of complexation leading to the cis(2,2)-isomer.

Tuning the Size and Geometry of Heteroleptic Coordination Cages by Varying the Ligand Bent Angle

“…Several groups have started to investigate such lowsymmetry Pd assemblies. Two synthetic strategies have been explored in this context: a) the use of ligands with reduced symmetry (e. g., lack of C 2 symmetry), [18,[48][49][50][51] and b) the controlled formation of heteroleptic assemblies using two different ligands. [52][53][54][55] The latter strategy has resulted in the discovery of several mixed-ligand assemblies of the general formula [Pd n L x L' y ] 2n + (x + y = 2n).…”

High‐Throughput Computational Evaluation of Low Symmetry Pd2L4Cages to Aid in System Design**

“… [45] Perhaps unsurprisingly, in each of these 12 instances, the cis ‐Pd 2 L 4 isomer was predicted to be the favoured structure, again in agreement with previous work. [ 45 , 47 , 48 ] Indeed, for most of the ligands (51 of 60; 85 %), the cis cage isomer was found to be the most stable (Figure S4a).…”

“… [43] Crowley and co‐workers recently reported a [PdPtL 4 ] cage in which the different labilities of the two metal ions allowed selective sequestration of the Pd II ions to open the cage without complete dissociation of the ligands. [44] We[ 45 , 46 ] and others[ 47 , 48 , 49 , 50 ] have recently begun to explore an alternative approach that uses unsymmetrical ligands to access lower symmetry structures. [51] The lack of bilateral symmetry introduced into the ligand structure means four possible isomers of the resultant dipalladium cage can form (Figure 1 ).…”

A Robust Photocatalytic Hybrid Material Composed of Metal‐Organic Cages and TiO2 for Efficient Visible‐Light‐Driven Hydrogen Evolution

“…B3LYP density functional, with LanL2DZ basis set for Pd, and 6‐31G basis set for C, H, N and S, were employed. Solvent effect (DMSO, ϵ=46.7) was taken into account by using the polarizable continuum model [38] …”