Phenanthridine-Containing Pincer-like Amido Complexes of Nickel, Palladium, and Platinum

Proligands based on bis­(8-quinolinyl)­amine (<b>L1</b>) were prepared containing one (<b>L2</b>) and two (<b>L3</b>) benzo-fused N-heterocyclic phenanthridinyl (3,4-benzoquinolinyl) units. Taken as a series, <b>L1</b>–<b>L3</b> provides a ligand template for exploring systematic π-extension in the context of tridentate pincer-like amido complexes of group 10 metals (<b>1-M</b>, <b>2-M</b>, and <b>3-M</b>; <b>M</b> = Ni, Pd, Pt). Inclusion of phenanthridinyl units was enabled by development of a cross-coupling/condensation route to 6-unsubstituted, 4-substituted phenanthridines (<b>4-Br</b>, <b>4-NO</b>_<b>2</b>, <b>4-NH</b>_<b>2</b>) suitable for elaboration into the target ligand frameworks. Complexes <b>1-M</b>, <b>2-M</b>, and <b>3-M</b> are redox-active; electrochemistry and UV–vis absorption spectroscopy were used to investigate the impact of π-extension on the electronic properties of the metal complexes. Unlike what is typically observed for benzannulated ligand–metal complexes, extending the π-system in metal complexes <b>1-M</b> to <b>2-M</b> to <b>3-M</b> led to only a moderate red shift in the relative highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap as estimated by electrochemistry and similarly subtle changes to the onset of the lowest-energy absorption observed by UV–vis spectroscopy. Time-dependent density functional theory calculations revealed that benzannulation significantly impacts the atomic contributions to the LUMO and LUMO+1 orbitals, altering the orbital contributions to the lowest-energy transition but leaving the energy of this transition essentially unchanged

Site-Selective Benzannulation of <i>N</i>‑Heterocycles in Bidentate Ligands Leads to Blue-Shifted Emission from [(<i>P^N</i>)Cu]₂(μ-X)₂ Dimers

Benzannulated bidentate pyridine/phosphine (<i>P^N</i>) ligands bearing quinoline or phenanthridine (3,4-benzoquinoline) units have been prepared, along with their halide-bridged, dimeric Cu­(I) complexes of the form [(<i>P^N</i>)­Cu]₂(μ-X)₂. The copper complexes are phosphorescent in the orange-red region of the spectrum in the solid-state under ambient conditions. Structural characterization in solution and the solid-state reveals a flexible conformational landscape, with both diamond-like and butterfly motifs available to the Cu₂X₂ cores. Comparing the photophysical properties of complexes of (quinolinyl)­phosphine ligands with those of π-extended (phenanthridinyl)­phosphines has revealed a counterintuitive impact of site-selective benzannulation. Contrary to conventional assumptions regarding π-extension and a bathochromic shift in the lowest energy absorption maxima, a blue shift of nearly 40 nm in the emission wavelength is observed for the complexes with larger ligand π-systems, which is assigned as phosphorescence on the basis of emission energies and lifetimes. Comparison of the ground-state and triplet excited state structures optimized from DFT and TD-DFT calculations allows attribution of this effect to a greater rigidity for the benzannulated complexes resulting in a higher energy emissive triplet state, rather than significant perturbation of orbital energies. This study reveals that ligand structure can impact photophysical properties for emissive molecules by influencing their structural rigidity, in addition to their electronic structure

Cross-trienamines in Asymmetric Organocatalysis

Cross-conjugated trienamines are introduced as a new concept in asymmetric organocatalysis. These intermediates are applied in highly enantioselective Diels–Alder and addition reactions, providing functionalized bicyclo[2.2.2]­octane compounds and γ′-addition products, respectively. The nature of the transformations and the intermediates involved are investigated by computational calculations and NMR analysis

Cross-trienamines in Asymmetric Organocatalysis

Cross-conjugated trienamines are introduced as a new concept in asymmetric organocatalysis. These intermediates are applied in highly enantioselective Diels–Alder and addition reactions, providing functionalized bicyclo[2.2.2]­octane compounds and γ′-addition products, respectively. The nature of the transformations and the intermediates involved are investigated by computational calculations and NMR analysis

Asymmetric Organocatalytic Formal [2 + 2]-Cycloadditions via Bifunctional H-Bond Directing Dienamine Catalysis

A new concept in organocatalysis allowing for the construction of cyclobutanes with four contiguous stereocenters with complete diastereo- and enantiomeric control by a formal [2 + 2]-cycloaddition is presented. The concept is based on simultaneous dual activation of α,β-unsaturated aldehydes and nitroolefins by amino- and hydrogen-bonding catalysis, respectively. A new bifunctional squaramide-based aminocatalyst has been designed and synthesized in order to enable such an activation strategy. The potential and scope of the reaction are demonstrated, and computational studies which account for the stereochemical outcome are presented

Carbon–Carbon Bond-Forming Reactions of α-Thioaryl Carbonyl Compounds for the Synthesis of Complex Heterocyclic Molecules

Strategies for the formation of carbon–carbon bonds from the α-thioaryl carbonyl products of substituted lactams are described. Although direct functionalization is possible, a two step process of oxidation and magnesium-sulfoxide exchange has proven optimal. The oxidation step results in the formation of two diastereomers that exhibit markedly different levels of stability toward elimination, which is rationalized on the basis of quantum mechanical calculations and X-ray crystallography. Treatment of the sulfoxide with <i>i</i>-PrMgCl results in the formation of a magnesium enolate that will undergo an intramolecular Michael addition reaction to form two new stereogenic centers. The relationship between the substitution patterns of the sulfoxide substrate and the efficiency of the magnesium exchange reaction are also described

Carbon–Carbon Bond-Forming Reactions of α-Thioaryl Carbonyl Compounds for the Synthesis of Complex Heterocyclic Molecules

Strategies for the formation of carbon–carbon bonds from the α-thioaryl carbonyl products of substituted lactams are described. Although direct functionalization is possible, a two step process of oxidation and magnesium-sulfoxide exchange has proven optimal. The oxidation step results in the formation of two diastereomers that exhibit markedly different levels of stability toward elimination, which is rationalized on the basis of quantum mechanical calculations and X-ray crystallography. Treatment of the sulfoxide with <i>i</i>-PrMgCl results in the formation of a magnesium enolate that will undergo an intramolecular Michael addition reaction to form two new stereogenic centers. The relationship between the substitution patterns of the sulfoxide substrate and the efficiency of the magnesium exchange reaction are also described