One Dianionic Luminophore with Three Coordination Modes Binding Four Different Metals: Toward Unexpectedly Phosphorescent Transition Metal Complexes

This work reports on a battery of coordination compounds featuring a versatile dianionic luminophore adopting three different coordination modes (mono, bi, and tridentate) while chelating Pd(II), Pt(II), Au(III), and Hg(II) centers. An in-depth structural characterization of the ligand precursor (H2 L) and six transition metal complexes ([HLPdCNtBu], [LPtCl], [LPtCNtBu], [LPtCNPhen], [HLHgCl], and [LAuCl]) is presented. The influence of the cations and coordination modes of the luminophore and co-ligands on the photophysical properties (including photoluminescence quantum yields (ΦL ), excited state lifetimes (τ), and average (non-)radiative rate constants) are evaluated at various temperatures in different phases. Five complexes show interesting photophysical properties at room temperature (RT) in solution. Embedment in frozen glassy matrices at 77 K significantly boosts their luminescence by suppressing radiationless deactivation paths. Thus, the Pt(II)-based compounds provide the highest efficiencies, with slight variations upon exchange of the ancillary ligand. In the case of [HLPdCNtBu], both ΦL and τ increase over 30-fold as compared to RT. Furthermore, the Hg(II) complex achieves, for the first time in its class, a ΦL exceeding 60% and millisecond-range lifetimes. This demonstrates that a judicious ligand design can pave the way toward versatile coordination compounds with tunable excited state properties.

Silver(I)-Catalyzed C4-H Amination of 1-Naphthylamine Derivatives with Azodicarboxylates at Room Temperature

A highly facile and efficient protocol for silver(I)-catalyzed C4–H amination of 1-naphthylamine derivatives with readily available azodicarboxylates utilizing picolinamide as a bidentate directing group is reported, providing an alternative strategy for the synthesis of 4-aminated 1-naphthylamine derivatives. The reaction proceeded smoothly in acetone at room temperature undergoing a self-redox process under the base and external oxidant-free conditions, affording the desired products with good to excellent yields.

Overcoming peri- and ortho-selectivity in C-H methylation of 1-naphthaldehydes by a tunable transient ligand strategy

Methyl groups widely exist in bioactive molecules, and site-specific methylation has become a valuable strategy for their structural functionalization. Aiming to introduce this smallest alkyl handle, a highly regioselective peri- and ortho-C-H methylation of 1-naphthaldehyde by using a transient ligand strategy has been developed. A series of methyl-substituted naphthalene frameworks have been prepared in moderate to excellent yields. Mechanistic studies demonstrate that peri-methylation is controlled by the higher electronic density of the peri-position of 1-naphthaldehyde as well as the formation of intermediary 5,6-fused bicyclic palladacycles, whereas experimental studies and theoretical calculations inferred that a 5-membered iridacycle at the ortho-position of 1-naphthaldehyde leads to energetically favorable ortho-methylation via an interconversion between the peri-iridacycle and ortho-iridacycle. Importantly, to demonstrate the synthetic utility of this method, we show that this strategy can serve as a platform for the synthesis of multi-substituted naphthalene-based bioactive molecules and natural products.

Merging Photoredox Catalysis with Transition Metal Catalysis: Direct C4-H Sulfamidation of 1-Naphthylamine Derivatives

A mild and efficient protocol for the copper(I)-catalyzed C4-H sulfamidation of 1-naphthylamine derivatives with diphenylsulfonimide (NHSI) was explored at room temperature, affording the desire produces in moderate to good yields. The control experiments indicated that this visible-light-promoted reaction might proceed via a single-electron-transfer process. In addition, preliminary DFT studies for the intermediates in the catalytic cycle were also explored, indicating that the C4 site in the naphthyl ring is the most likely electrophilic reactive site and providing some exact basis for the plausible mechanism.

Catalytic Gold Chemistry: From Simple Salts to Complexes for Regioselective C-H Bond Functionalization

Gold coordinated to neutral phosphines (R₃ P), N-heterocyclic carbenes (NHCs) or anionic ligands is catalytically active in functionalizing various C-H bonds with high selectivity. The sterics/electronic nature of the studied C-H bond, oxidation state of gold and stereoelectronic capacity of the coordinated auxiliary ligand are some of the associated selectivity factors in gold-catalyzed C-H bond functionalization reactions. Hence, in this review a comprehensive update about the action of different types of gold catalysts, from simple to sophisticated ones, on C-H bond reactions and their regiochemical outcome is disclosed. This review also highlights the catalytic applications of Au(I)- and Au(III)-species in creating new opportunities for the regio- and site-selective activation of challenging C-H bonds. Finally, it also intends to stress the potential applications in selective C-H bond activation associated with a variety of heterocycles recently described in the literature.

Directed C-H Halogenation Reactions Catalysed by PdII Supported on Polymers under Batch and Continuous Flow Conditions

A new generation of N-heterocyclic carbene palladium(II) complexes containing vinyl groups in different positions in the backbone of the N-heterocycle have been developed. The fully characterised monomers were copolymerised with divinylbenzene to fabricate robust polymer supported NHC-Pd^II complexes and these polymers were applied as heterogeneous catalysts in directed C-H halogenation of arenes with a pyridine-type directing group. The catalysts demonstrated medium-high catalytic activity with up to 90 % conversion and 100 % selectivity in chlorination. They are heterogeneous and recyclable (at least six times) with no significant leaching of palladium in batch mode catalysis. The best catalyst was also applied under continuous flow conditions where it disclosed an exceptional activity (90 % conversion) and 100 % selectivity for the mono-halogenated product for at least six days, with no leaching of palladium, no loss of activity and an ability to maintain the original oxidation state of Pd^II .

Iron-Catalyzed Regioselective Remote C(sp2)-H Carboxylation of Naphthyl and Quinoline Amides

Iron(III)-catalyzed regioselective direct remote C-H carboxylation of naphthyl and quinoline amides was developed using CBr₄ and alcohol. The reaction involves a radical pathway using a coordination activation strategy and single electron transfer process. The use of sustainable iron catalysis, selectivity, and the substrate scope are the important practical features.

Ligand and solvent control of selectivity in the C-H activation of a pyridylimine-substituted 1-naphthalene; a combined synthetic and computational study

The pyridylimine-substituted 1-naphthalenes, 2-(1-C10H7)-6-{CR[double bond, length as m-dash]N(2,6-i-Pr2C6H3)}C5H3N (R = Me HLMe, H HLH), react with Na2[PdCl4] in acetic acid at elevated temperature to afford either ortho-C-Hnaphthyl activated (LMe)PdCl (2ortho) or the unactivated adduct (HLH)PdCl2 (1b). Alternatively, 1b and its ketimine analogue (HLMe)PdCl2 (1a), can be prepared by treating (MeCN)2PdCl2 with either HLMe or HLH in chloroform at room temperature. Regio-selective ortho-C-H activation to form 2ortho can also be initiated by the thermolysis of 1a in acetic acid, while no reaction occurs under similar conditions with 1b. Interestingly, the C-H activation of HLMe to give 2ortho is found to be reversible with 100% deuteration of the peri-site occurring on reacting Na2[PdCl4] with HLMe in acetic acid-d4. By contrast, heating 1a in toluene gives a 55 : 45 mixture of 2ortho and its peri-activated isomer 2peri. Pure 2peri can, however, be obtained either from (LMe)PdOAc (3peri) by OAc/Cl exchange or by the sequential reactions of 1a with firstly silver acetate then with aqueous sodium chloride. Intriguingly, a peri to ortho interconversion occurs on heating 2peri in acetic acid to give 2ortho. DFT calculations have been used to investigate the C-H activation steps and it is found that in acetic acid ortho-C-H activation is kinetically and thermodynamically favoured but peri-CH activation is kinetically accessible (ΔΔG‡ = 2.4 kcal mol-1). By contrast in toluene, the reaction appears to be irreversible with the difference in barrier height for ortho- and peri-C-H activation being very small within the error of the method (ΔΔG‡ = 0.7 kcal mol-1), findings that are in agreement with the empirically observed product distribution for 2ortho and 2peri. Single crystal X-ray structures are reported for 1a, 1b, 2ortho and 2peri.

Copper-catalyzed peri-selective direct sulfenylation of 1-naphthylamines with disulfides

A copper-catalyzed peri-selective direct C–H sulfenylation of 1-naphthylamines with disulfides was developed.

Csp3 -H Activation without Chelation Assistance in an Iridium Pincer Complex Forming Cyclometallated Products

Cyclometallation of 8-methylquinoline and 2-(dimethylamino)-pyridine in an iridium-based pincer complex is described. The C-H activation of 2-(dimethylamino)pyridine is not chelation assisted, which has not been described before for Csp³ -H bonds in cyclometallation reactions. The mechanism of the cyclometallation of 2-(dimethylamino)pyridine was studied by DFT calculations and kinetic measurements.

Aromatic C-H Activation in the Triplet Excited State of Cyclometalated Platinum(II) Complexes Using Visible Light

The visible-light driven cyclometalation of arene substrates containing an N-donor heteroaromatic moiety as directing group by monocyclometalated Pt(II) complexes is reported. Precursors of the type [PtMe(C^N)(N^CH)], where N^CH is 2-phenylpyridine (ppyH) or related compunds with diverse electronic properties and C^N is the corresponding cyclometalated ligand, afford homoleptic cis-[Pt(C^N)2] complexes upon irradiation with blue LEDs at room temperature with evolution of methane. Heteroleptic derivatives cis-[Pt(ppy)(C'^N')] are obtained analogously from [PtMe(ppy)(N'^C'H)], where N'^C'H represents an extended set of heteroaromatic compounds. Experimental and computational studies demonstrate an unprecedented C-H oxidative addition, which is initiated by a triplet excited state of metal-to-ligand charge-transfer (MLCT) character and leads to a detectable Pt(IV) methyl hydride intermediate.

Regioselectivity in C-H activation: reagent control in cyclometallation of 2-(1-naphthyl)-pyridine

2-(1-Naphthyl)-pyridine () possesses sp(2) C-H bonds in both the γ- and δ-positions and is therefore a suitable substrate for studying the cyclometallation selectivity with different reagents and conditions. Such selectivity studies are reported. Based on deuterium-exchange experiments it is concluded that cycloruthenation with RuCl2(p-cymene) dimer is reversible with kinetic and thermodynamic preference for γ-substitution. Electrophilic cycloborylation, on the other hand, shows unusual δ-substitution. The previously published cyclopalladation and cycloauration of the substrate was studied in detail and was shown to be irreversible; they proceed under kinetic control and give γ- and δ-substitution for palladium and gold, respectively.

Cyclometallated gold(iii) aryl-pyridine complexes as efficient catalysts for three-component synthesis of substituted oxazoles

Cyclometallated gold(iii) complexes efficiently catalyze the multicomponent reaction between imines, alkynes, and acyl chlorides to form trisubstituted oxazoles.