Luminescent Platinum(II) Complexes with Terdentate N∧C∧C Ligands.

The synthesis, structure, and luminescence of Pt(II) complexes of the type [Pt(N∧C∧C)(L)] are reported, where N∧C∧C is a terdentate ligand resulting from the cycloplatination of 2-(3,5-diphenoxyphenyl)pyridine or 2-(4,4″-dimethyl-[1,1':3',1″-terphenyl]-5'-yl)pyridine, and L represents a monodentate ancillary ligand, which can be γ-picoline, 4-pyridinecarboxaldehyde, PPh3, n-butyl or 2,6-dimethylphenyl isocyanide, CO, or the N-heterocyclic carbenes 1-butyl-3-methylimidazol-2-ylidene or 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene. Derivatives bearing CO, isocyanides, or carbenes showed the highest stabilities in solution, whereas the pyridine and PPh3 derivatives establish ligand-exchange equilibria in acetonitrile. Different supramolecular structures are observed in the solid state, which largely depend on the nature of the ancillary ligand. Isocyanides and CO favor π interactions between the aromatic rings, metallophilic Pt···Pt contacts, or a combination of both. In contrast, pyridine ligands may lead to bimolecular assemblies driven by C-H···O, C-H···Pt, or C-H/π hydrogen bonds. Luminescence was examined in fluid solution, poly(methyl methacrylate) matrices, and the solid state at 298 K, and in 2-methyltetrahydrofuran glasses at 77 K. The majority of derivatives show highly efficient emissions from 3ILCT/MLCT or 3ILCT/MLCT/LLCT excited states of monomeric species. The formation of excimers and different types of emissive aggregates are demonstrated, which lead to red-shifted emissions of different origins and characteristics depending on the involved noncovalent interactions.

Photochemically Induced Cyclometalations at Simple Platinum(II) Precursors.

Photochemical cycloplatinations of 2-arylpyridines and related C∧N ligands, as well as terdentate heteroaromatic N∧N∧C, N∧C∧N, and N∧C∧C compounds, are demonstrated using (Bu4N)2[Pt2Cl6] or [PtCl2(NCPh)2] as precursors at room temperature. Mono- or bis-cyclometalated Pt(II) complexes with C∧N ligands are obtained depending on excitation wavelength and precursor. Monitoring experiments show that photoexcitation enables both the N-coordination and the subsequent C-H metalation. Photochemical synthetic protocols have been developed, which are advantageous with respect to the established thermal procedures and have allowed the synthesis of the first Pt(II) complexes with N∧C∧C ligands.

Phosphorescent Tris-cyclometalated Pt(IV) Complexes with Mesoionic N-Heterocyclic Carbene and 2-Arylpyridine Ligands.

The synthesis, structure, photophysical properties, and electrochemistry of the first series of Pt(IV) tris-chelates bearing cyclometalated aryl-NHC ligands are reported. The complexes have the general formula [Pt(trz)2(C∧N)]+, combining two units of the cyclometalated, mesoionic aryl-NHC ligand 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene (trz) with a cyclometalated 2-arylpyridine [C∧N = 2-(2,4-difluorophenyl)pyridine (dfppy), 2-phenylpyridine (ppy), 2-(p-tolyl)pyridine (tpy), 2-(2-thienyl)pyridine (thpy), 2-(9,9-dimethylfluoren-2-yl)pyridine (flpy)], and presenting a mer arrangement or metalated aryls. They exhibit a significant photostability under UV irradiation and long-lived phosphorescence in the blue to yellow color range, arising from 3LC excited states involving the C∧N ligands, with quantum yields of up to 0.34 in fluid solution and 0.77 in the rigid matrix at 298 K. The time-dependent density functional theory (TD-DFT) calculations reveal that nonemissive, deactivating excited states of ligand-to-metal charge-transfer (LMCT) character are pushed to high energies as a consequence of the strong σ-donating ability of the carbenic moieties, making the Pt(trz)2 subunit an essential structural component that enables efficient emissions from the chromophoric C∧N ligands, with potential application for the development of different Pt(IV) emitters with tunable properties.

Strongly Luminescent Pt(IV) Complexes with a Mesoionic N-Heterocyclic Carbene Ligand: Tuning Their Photophysical Properties.

The synthesis, electrochemistry, and photophysical properties of a series of bis-cyclometalated Pt(IV) complexes that combine the mesoionic aryl-NHC ligand 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene (trz) with either 1-phenylpyrazole or 2-arylpyridine (C∧N) are reported. The complexes (OC-6-54)-[PtCl2(C∧N)(trz)] bearing cyclometalating 2-arylpyridines present phosphorescent emissions in the blue to yellow color range, which essentially arise from 3LC(C∧N) states, and reach quantum yields of ca. 0.3 in fluid solutions and almost unity in poly(methyl methacrylate) (PMMA) matrices at 298 K, thus representing a class of strong emitters with tunable properties. A systematic comparison with the homologous C2-symmetrical species (OC-6-33)-[PtCl2(C∧N)2], which contains two equal 2-arylpyridine ligands, shows that the introduction of a trz ligand leads to significantly lower nonradiative decay rates and higher quantum efficiencies. Computational calculations substantiate the effect of the carbene ligand, which raises the energy of dσ* orbitals in these derivatives and results in the higher energies of nonemissive deactivating 3LMCT states. In contrast, the isomers (OC-6-42)-[PtCl2(C∧N)(trz)] are not luminescent because they present a 3LMCT state as the lowest triplet.

Visible light driven generation and alkyne insertion reactions of stable bis-cyclometalated Pt(iv) hydrides.

Hydride complexes resulting from the oxidative addition of C-H bonds are intermediates in hydrocarbon activation and functionalization reactions. The discovery of metal systems that enable their direct formation through photoexcitation with visible light could lead to advantageous synthetic methodologies. In this study, easily accessible dimers [Pt2(µ-Cl)2(C^N)2] (C^N = cyclometalated 2-arylpyridine) are demonstrated as a very convenient source of Pt(C^N) subunits, which promote photooxidative C-H addition reactions with different 2-arylpyridines (N'^C'H) upon irradiation with blue light. The resulting [PtH(Cl)(C^N)(C'^N')] complexes are the first isolable Pt(iv) hydrides arising from a cyclometalation reaction. A transcyclometalation process involving three photochemical steps is elucidated, which occurs when the C^N ligand is a monocyclometalated 2,6-diarylpyridine, and a detailed analysis of the photoreactivity associated with the Pt(C^N) moiety is provided. Alkyne insertions into the Pt-H bond of a photogenerated Pt(iv) hydride are also reported as a demonstration of the ability of this class of compounds to undergo subsequent organometallic reactions.

Selective synthesis, reactivity and luminescence of unsymmetrical bis-cyclometalated Pt(iv) complexes.

Pt(ii) complexes cis-N,N-[PtCl(C^N)(N'^C'H)], where C^N represents a monocyclometalated 2,6-diaryl- or 2-arylpyridine ligand and N'^C'H is an N-coordinated 2-arylpyridine, are selectively obtained from bridge-cleavage reactions of dimers [Pt2(µ-Cl)2(C^N)2] with excess N'^C'H at room temperature; isolation and characterization of derivatives of this kind is reported for the first time. Oxidation with PhICl2 affords Pt(iv) complexes [PtCl2(C^N)(C'^N')], bearing two cyclometalated ligands in an unsymmetrical arrangement. The abstraction of the two chlorides using AgOTf at 120 °C in the presence of an additional 2-arylpyridine ligand leads to mer isomers of tris-cyclometalated Pt(iv) complexes if C^N derives from a 2-arylpyridine, whereas it results in a reductive C-C coupling if C^N is a monocyclometalated 2,6-diarylpyridine. Complexes [PtCl2(C^N)(C'^N')] show phosphorescence in frozen PrCN glasses arising from essentially 3LC excited states localized on the cyclometalated ligand with the lowest π-π* transition energy. The combined photophysical data and computational results substantiate a variable degree of MLCT admixture into the emitting state depending on the atom trans to the metalated carbon of the chromophoric ligand (Cl or N), which has an appreciable effect on the characteristics of the observed luminescence.

Luminescent Platinum(IV) Complexes Bearing Cyclometalated 1,2,3-Triazolylidene and Bi- or Terdentate 2,6-Diarylpyridine Ligands.

The synthesis, structure, and photophysical properties of luminescent PtIV complexes that combine cyclometalated 1,2,3-triazolylidene and bi- or terdentate 2,6-diarylpyridine ligands are reported. The targeted complexes represent the first examples of PtIV species with a cyclometalated mesoionic aryl-NHC ligand. They exhibit moderate or weak emissions in fluid solution at 298 K arising from 3 LC states, which become very intense in poly(methyl methacrylate) (PMMA) matrices at 298 K. DFT and TD-DFT calculations confirm that the chromophoric ligand is the cyclometalated 2,6-diarylpyridine and show that the aryl-NHC ligand exerts a beneficial effect on the emission efficiencies of these derivatives by increasing the energy of deactivating LMCT excited states with respect to comparable PtIV complexes with cyclometalated 2-arylpyridine ligands.

Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications.

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

Unveiling the role of ancillary ligands in acceptorless benzyl alcohol dehydrogenation and etherification mediated by mesoionic carbene iridium complexes.

We synthesized a set of triazolylidene iridium(iii) complexes [IrCp*(C^N)L]n+ (Cp* = pentamethylcyclopentadienyl, C^N = C,N-bidentate coordinating pyridyl-triazolylidene) containing different neutral or anionic ancillary ligands L and evaluated their impact on the catalytic activity in alcohol conversion. We demonstrate that these ancillary ligands have a strong influence on the catalytic selectivity and direct whether the iridium center preferentially catalyzes either the dehydrogenation or the dehydration of benzyl alcohol. Ligand exchange experiments provide a direct correlation of ligand lability with catalytic activity and selectivity. These results underline the relevance of ancillary ligands and provide a rational approach to tailor the catalytic activity of the iridium center towards aldehyde formation (loss of H2) or etherification (elimination of H2O).

A Diels-Alder reaction triggered by a [4 + 3] metallacycloaddition.

The Tp(Me2)Ir(III) complex 1-OH2 (Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate), which contains a labile molecule of water and an iridium-bonded alkenyl moiety (-C(R)═C(R)-(R=CO2Me)) as part of a benzo-annulated five-membered iridacycle, reacts readily with the conjugated dienes butadiene and 2,3-dimethylbutadiene to afford the corresponding Diels-Alder products. Experimental and DFT studies are in accordance with an initial [4 + 3] cyclometalation reaction between the diene and the five-coordinated 16-electron organometallic fragment 1 (generated from 1-OH2 by facile water dissociation). The reaction can be extended to a related TpIr(III) complex (Tp = hydrotris(pyrazolyl)borate) that also features a labile ligand (i.e., 2-THF).

Building a parent iridabenzene structure from acetylene and dichloromethane on an iridium center.

Parenthood: The reaction of [TpIr(C2H4)2] (1) (Tp=hydrotris(pyrazolyl)borate) with acetylene in CH2 Cl2 affords a 1:1 mixture of the "parent" metallabenzene 2 (that is, all the ring carbon centers are CH units) and the ß-Cl substituted vinyl species 3. Generation of 2 is by the coupling of an iridacyclopentadiene (formed from two acetylene molecules at the Ir center) with the dichloromethane-derived chlorocarbene ":C(H)Cl" and a subsequent α-Cl elimination event.

Experimental evidences in favour of the hydroxylamine→nitrene-water tautomerization on the coordination sphere of Ir(III) centres.

And, to round off … A series of Ir(III) 5-membered metallacycles with an Ir-CH2 bond, react with aq. NH2OH with formation of hydride 6-membered iridacyclic complexes, which contain an Ir-NH=CH- imine functionality (see scheme).