Site-selective dihalogenated heteroarene cross-coupling with organometallic reagents usually occurs at the halogen proximal to the heteroatom, enabled by intrinsic relative electrophilicity, particularly in strongly polarized systems. An archetypical example is the Suzuki-Miyaura cross-coupling (SMCC) of 2,4-dibromopyridine with organoboron species, which typically exhibit C2-arylation site-selectivity using mononuclear Pd (pre)catalysts. Given that Pd speciation, particularly aggregation, is known to lead to the formation of catalytically competent multinuclear Pdn species, the influence of these species on cross-coupling site-selectivity remains largely unknown. Herein, we disclose that multinuclear Pd species, in the form of Pd3-type clusters and nanoparticles, switch arylation site-selectivity from C2 to C4, in 2,4-dibromopyridine cross-couplings with both organoboronic acids (SMCC reactions) and Grignard reagents (Kumada-type reactions). The Pd/ligand ratio and the presence of suitable stabilizing salts were found to be critically important in switching the site-selectivity. More generally, this study provides experimental evidence that aggregated Pd catalyst species not only are catalytically competent but also alter reaction outcomes through changes in product selectivity.

A Dichotomy in Cross-Coupling Site Selectivity in a Dihalogenated Heteroarene: Influence of Mononuclear Pd, Pd Clusters, and Pd Nanoparticles-the Case for Exploiting Pd Catalyst Speciation.

Abstract A photoswitchable ligand based on azobenzene is self‐assembled with palladium(II) ions to form a [Pd2(E‐L)4]4+ cage. Irradiation with 470 nm light results in the near‐quantitative switching to a monomeric species [Pd(Z‐L)2]2+, which can be reversed by irradiation with 405 nm light, or heat. The photoswitching selectivity towards the metastable isomer is significantly improved upon self‐assembly, and the thermal half‐life is extended from 40 days to 850 days, a promising approach for tuning photoswitching properties.

Visible‐Light‐Responsive Self‐Assembled Complexes: Improved Photoswitching Properties by Metal Ion Coordination

An azobenzene-functionalized bis-bis(urea) ligand ( L azo ) and phosphate anion assembled to the first photoactive "aniono" constructs, tetrahedron (A 4 L 6 ) and helicate (A 2 L 3 ), which readily undergo interconversion through cis - trans isomerization of the azo group under irradiation/heating. Most strikingly, the tetrahedral cage can accommodate a [18]crown-6 molecule, which can capture two tetramethylammonium (TMA + ) ions with an unusually high affinity, even capable of replacing K + in [K([18]crown-6)] + to form a {(TMA) 2 ⸦([18]crown-6)}⸦cage "Russian doll" structure. Thus, the current work may provide a model for the study on light-driven binding and exchange of biologically important K + ion and TMA+.

Light-Triggered High-Affinity Binding of Tetramethylammonium over Potassium Ions by [18]crown-6 in a Tetrahedral Anion Cage.

Metal complexes associated with photochromic molecules are attractive platforms to achieve smart light-switching materials with innovative and exciting properties due to specific optical, electronic, magnetic or catalytic features of metal complexes and by perturbing the excited-state properties of both components to generate new reactivity and photochemical properties. In this overview, we focus on selected achievements in key domains dealing with optical, redox, magnetic properties, as well as application in catalysis or supramolecular chemistry. We also try to point out scientific challenges that are still faced for future developments and applications.

Metal complexes bearing photochromic ligands: photocontrol of functions and processes.

Different Oxidative Addition Mechanisms for 12- and 14-Electron Palladium(0) Explain Ligand-Controlled Divergent Site Selectivity

New photoswitchable pyridyl-azo-phenyl-decorated tripodal host ligands (Laz) that belong to the cyclotriveratrylene family have been synthesized, and their photoswitching behavior and crystal structures determined. The latter includes a remarkable 7-fold Borromean-weave entanglement of π-π stacked layers. Trigonal bipyramidal {[Pd(en)]3(Laz)2}6+ metallo-cryptophanes (en = ethylenediamine) were formed from these and a previously known pyridyl-azo-phenyl-decorated tripodal host ligand. These coordination cages dissociate at low concentrations and are less robust to photoswitching of the Laz ligands than were previously reported Ir(III)-linked metallo-cryptophanes with similar ligands, reflecting the greater lability of the Pd-N bonds. The {[Pd(en)]3(Laz)2}6+ cages all act as hosts, binding octyl sulfate anions, or N-[2-(dimethylamino)ethyl]-1,8-naphthalimide in a dimethyl sulfoxide solution.

Self-Assembly and Host-Guest Interactions of Pd3L2 Metallo-cryptophanes with Photoisomerizable Ligands.

The trinuclear complexes [{PdI2(pyCl)}3(L1)] C1 and [{PdI2(pyCl)}3(L2)] C2, where pyCl = 3-chloropyridine, L1 = methyl(cyclotriguaiacylenyl)methylbenzimidazol-2-ylidene and L2 = benzyl(cyclotriguaiacylenyl)methylbenzimidazol-2-ylidene, each feature three palladium N-heterocyclic carbene (NHC) centres tethered onto a host-type cyclotriguaiacylene scaffold. Crystal structures of different solvates of complex C1 reveal different host–guest motifs including intra-cavity binding of dioxane guests concomitant with intramolecular halogen bonding interactions of C1. Mononuclear NHC analogues of C1 and C2, namely [PdI2(pyCl)(L3)] C3 and [PdI2(pyCl)(L4)] C4, where L3 = (3-chloropyridyl)-1-(2-methoxyphenyoxy)methyl-3-methylbenzimidazol-2-ylidene and L4 = (3-chloropyridyl)-1-(2-methoxyphenyoxy)methyl-3-benzylbenzimidazol-2-ylidene, were also synthesised and their crystal structures determined. Complexes C1–C4 are competent catalysts for Suzuki Miyaura cross-coupling, and interestingly exhibit a switch in the normal regioselectivity observed for reactions of 2,4-dibromopyridine with aryl boronic acids, usually C2-selective, yielding C4-arylated product preferentially over C2-arylated product.

Edward Britton

Papers

Self-Assembly and Host-Guest Interactions of Pd3L2 Metallo-cryptophanes with Photoisomerizable Ligands.

Cyclotriveratrylene-tethered trinuclear palladium(ii)–NHC complexes; reversal of site selectivity in Suzuki–Miyaura reactions