N-Heterocyclic Carbene Copper (I) Complexes Incorporating Pyrene Chromophore: Synthesis, Crystal Structure, and Luminescent Properties.

Luminescent N-heterocyclic carbene chloride copper (I) complexes incorporating pyrene chromophore (1-Pyrenyl-NHC-R)-Cu-Cl, (3, 4) have been prepared and fully characterized. Two complexes were prepared with R = methyl (3) and R = naphthyl groups (4) at the nitrogen center of the carbene unit to tune their electronic properties. The molecular structures of 3 and 4 have been elucidated by X-ray diffraction and confirm the formation of the target compounds. Preliminary results reveal that all compounds including the imidazole-pyrenyl ligand 1 are emissive in the blue region at room temperature in solution and in solid-state. All complexes display quantum yields comparable or higher when compared to the parent pyrene molecule. Interestingly replacement of the methyl by naphthyl group increases the quantum yield by almost two-folds. These compounds might show promise for applications as optical displays.

Luminescent Complexes of Platinum, Iridium, and Coinage Metals Containing N-Heterocyclic Carbene Ligands: Design, Structural Diversity, and Photophysical Properties.

The employment of N-heterocyclic carbenes (NHCs) to design luminescent metal compounds has been the focus of recent intense investigations because of the strong σ-donor properties, which bring stability to the whole system and tend to push the d-d dark states so high in energy that they are rendered thermally inaccessible, thereby generating highly emissive complexes for useful applications such as organic light-emitting diodes (OLEDs), or featuring chiroptical properties, a field that is still in its infancy. Among the NHC complexes, those containing organic chromophores such as naphthalimide, pyrene, and carbazole exhibit rich emission behavior and thus have attracted extensive interest in the past five years, especially carbene coinage metal complexes with carbazolate ligands. In this review, the design strategies of NHC-based luminescent platinum and iridium complexes with large spin-orbit-coupling (SOC) are described first. Subsequent paragraphs illustrate the recent advances of luminescent coinage metal complexes with nucleophilic- and electrophilic-based carbenes based on silver, gold, and copper metal complexes that have the ability to display rich excited state emissions in particular via thermally activated delayed fluorescence (TADF). The luminescence mechanism and excited state dynamics are also described. We then summarize the advance of NHC-metal complexes in the aforementioned fields in recent years. Finally, we propose the development trend of this fast-growing field of luminescent NHC-metal complexes.

Rational Design of Mono- and Bi-Nuclear Cyclometalated Ir(III) Complexes Containing Di-Pyridylamine Motifs: Synthesis, Structure, and Luminescent Properties.

Heteroleptic cyclometalated iridium (III) complexes (1-3) containing di-pyridylamine motifs were prepared in a stepwise fashion. The presence of the di-pyridylamine ligands tunes their electronic and optical properties, generating blue phosphorescent emitters at room temperature. Herein we describe the synthesis of the mononuclear iridium complexes [Ir(ppy)2(DPA)][OTf] (1), (ppy = phenylpyridine; DPA = Dipyridylamine) and [Ir(ppy)2(DPA-PhI)][OTf] (2), (DPA-PhI = Dipyridylamino-phenyliodide). Moreover, the dinuclear iridium complex [Ir(ppy)2(L)Ir(ppy)2][OTf]2 (3) containing a rigid angular ligand "L = 3,5-bis[4-(2,2'-dipyridylamino)phenylacetylenyl]toluene" and displaying two di-pyridylamino groups was also prepared. For comparison purposes, the related dinuclear rhodium complex [Rh (ppy)2(L)Rh(ppy)2][OTf]2 (4) was also synthesized. The x-ray molecular structure of complex 2 was reported and confirmed the formation of the target molecule. The rhodium complex 4 was found to be emissive only at low temperature; in contrast, all iridium complexes 1-3 were found to be phosphorescent in solution at 77 K and room temperature, displaying blue emissions in the range of 478-481 nm.

Cyclometalated Rhodium and Iridium Complexes Containing Masked Catecholates: Synthesis, Structure, Electrochemistry, and Luminescence Properties.

Two neutral cyclometalated rhodium and iridium coordination assemblies [(F2ppy)2M(η-Cat)], M = Rh, (2) and M = Ir, (3) (F2ppy: 2,4-difluorophenylpyridine), displaying a masked catecholate (η-Cat = η-O∧O) are described. The catecholate ligand is π-bonded to an organometallic Cp*Ru(II) moiety. The latter brings stability to the whole system in solution and suppresses the formation of the related paramagnetic semiquinone complex. The determination of the molecular structure of the iridium complex [(F2ppy)2Ir(η-Cat)] (3) corroborates the formation of the target compound and reveals the generation of a rare two-dimensional (2D) honeycomb supramolecular architecture in the solid state, in which the Δ-enantiomer self-assembles with the Λ-enantiomer through encoded π-π interactions among individual units. The electrochemistry of complexes 2 and 3 was investigated and showed that reduction occurs at very negative potentials (∼-2.2 V versus saturated calomel electrode (SCE)), while oxidation of the cyclometalated Rh and Ir centers occurs at 0.8 and 0.86 V. In contrast to complexes with 1,2-dioxolene chelates, which are nonemissive, the heterodinuclear diamagnetic complexes 2 and 3 were found to be emissive at room temperature both in solution and in the solid state. Moreover, at 77 K in a solid state, both compounds display opposite emission behavior, for instance, complex 3 displays a blue-shifted emission, while rhodium compound 2 exhibits red-shifted emission to lower energy.

Enantiopure, luminescent, cyclometalated Ir(III) complexes with N-heterocyclic carbene-naphthalimide chromophore: design, vibrational circular dichroism and TD-DFT calculations.

A series of chiral cyclometalated iridium complexes of the type [Ir(C^N)2(C^C:)], {(C^N) = ppy (2); dfppy (3)} featuring a naphthalimide N-heterocyclic carbene ligand (C^C:) = (Naphthalimide-NHC) are described and fully characterized. The racemic complexes rac-2 and rac-3 were resolved via chiral column chromatography techniques into their corresponding enantiopure species Δ-2, Λ-2, Δ-3, Λ-3 as confirmed by their CD curves. This unique class of molecules containing organic and inorganic chromophores might be used as a platform to probe the stereochemical effect on the photophysical properties. Vibrational circular dichroism (VCD) was used as an important tool to assign successfully the stereochemistry of the enantiomers. TD-DFT calculations are also advanced to support the experimental studies and to rationalize the observed results.

Extra hydrogen bonding interactions by peripheral indole groups stabilize benzene-1,3,5-tricarboxamide helical assemblies.

Benzene-1,3,5-tricarboxamide monomers derived from alkyl esters of tryptophan (BTA Trp) self-assemble into helices with an inner threefold hydrogen bond network surrounded by a second network involving the indole N-H groups. As a consequence of this extra stabilization of its helical assemblies, BTA Trp forms more viscous solutions than a range of ester and alkyl BTAs.

Capturing a Square Planar Gold(III) Complex Inside a Platinum Nanocage: A Combined Experimental and Theoretical Study.

A novel synthetic procedure was set up to gain access to platinum coordination cages Pt2L4, which are less investigated compared to their palladium counterparts. This Pt2L4 nanocage exhibits an adequate cavity for guest encapsulation. Indeed, the Au(III) metal complex [Au(bdt)2]- (bdt = benzene-1,2-dithiolate) was successfully captured inside the cavity, in contrast to the analogous palladium cage which failed to host the gold complex. This result represents a rare example where a metal complex with thio-ligands can be encapsulated in a coordination cage. Moreover, it highlights the role of the metal center and the robustness of the platinum cage for host-guest chemistry. This discovery will inspire researchers in this area to pay more attention to Pt-cages. The host-guest system was fully characterized by NMR techniques and X-ray crystallographic analysis. Moreover, the nature of the host-guest interaction in this unique example was investigated and rationalized by DFT computational studies.

Unique Class of Enantiopure N-Heterocyclic Carbene Half-Sandwich Iridium(III) Complexes with Stable Configurations: Probing Five-Membered versus Six-Membered Iridacycles.

A unique class of enantiopure N-heterocyclic carbene half-sandwich iridium complexes is reported. These compounds display stable configurations at the metal center, as demonstrated by their chiroptical properties. Remarkably, because of the nature of the naphthalimide molecule, two regioisomers containing five-membered [( R)-2a and ( S)-2a] and six-membered [( R)-2b and ( S)-2b] iridacycles were obtained. Density functional theory calculations are advanced to rationalize their relative stability.

Cyclometalated N-heterocyclic carbene iridium(iii) complexes with naphthalimide chromophores: a novel class of phosphorescent heteroleptic compounds.

A series of cyclometalated N-heterocyclic carbene complexes of the general formula [Ir(C^N)2(C^C:)] has been prepared. Two sets of compounds were designed, those where (C^C:) represents a bidentate naphthalimide-substituted imidazolylidene ligand and (C^N) = ppy (3a), F2ppy (4a), bzq (5a) and those where (C^C:) represents a naphthalimide-substituted benzimidazolylidene ligand and (C^N) = ppy (3b), F2ppy (4b), bzq (5b). The naphthalimide-imidazole and naphthalimide-benzimidazole ligands 1a,b and the related imidazolium and benzimidazolium salts 2a,b were also prepared and fully characterized. The N-heterocyclic carbene Ir(iii) complexes have been characterized by NMR spectroscopy, cyclic voltammetry and elemental analysis. Moreover, the molecular structures of one imidazolium salt and four Ir(iii) complexes were determined by single-crystal X-ray diffraction. The structures provide us with valuable information, most notably the orientation of the naphthalimide chromophore with respect to the N-heterocyclic carbene moiety. All compounds are luminescent at room temperature and in a frozen solvent at 77 K, exhibiting a broad emission band that extends beyond 700 nm. The presence of the naphthalimide moiety changes the character of the lowest excited state from 3MLCT to 3LC, as corroborated by DFT and TD-DFT calculations. Remarkably, replacing imidazole with a benzimidazole unit improves the quantum yields of these compounds by decreasing the knr values which is an important feature for optimized emission performance. These studies provide valuable insights about a novel class of N-heterocyclic carbene-based luminescent complexes containing organic chromophores and affording metal complexes emitting across the red-NIR range.

Chiral two bladed ML2 metallamacrocycles: design, structures and solution behavior.

Chiral two bladed complexes of [Co(L)2][BF4]2 (1) and [Zn(L)2][BF4]2 (2) containing an atropoisomeric semi-rigid bidentate ligand L were synthesized and characterized. They are obtained as homochiral species but in a racemic mixture via a ligand self-sorting mechanism. These enantiomers can be differentiated in solution using optically active anions.

Luminescent Cyclometalated Platinum Complexes with π-Bonded Catecholate Organometallic Ligands.

A series of cyclometalated platinum(II) complexes of the type [(ppy)Pt(LM)]n+ (n = 0, 1) with π-bonded catecholates acting as organometallic ligands (LM) have been prepared and characterized by analytical techniques. In addition, the structures of two complexes of the series were determined by single-crystal X-ray diffraction. The packing shows the formation of a 1D supramolecular assembly generated by dPt-πCp* interactions among individual units. All complexes are luminescent in the solid state and in solution media. The results of photophysics have been rationalized by means of density functional theory (DFT) and time-dependent DFT investigations.

New N

We describe Pd(ii) and Pt(ii) complexes of an N^C^N-coordinating pincer-like ligand featuring two lateral pyridine rings and a 6-membered carbene core. Their crystal structures display 1-dimensional chains with short π-π and M(ii)M(ii) interactions. Such interactions also impact on the photophysical properties, with the Pt(ii) complex being luminescent in the solid state at room temperature.

Highly Phosphorescent Crystals of Square-Planar Platinum Complexes with Chiral Organometallic Linkers: Homochiral versus Heterochiral Arrangements, Induced Circular Dichroism, and TD-DFT Calculations.

A novel class of chiral luminescent square-planar platinum complexes with a π-bonded chiral thioquinonoid ligand is described. Remarkably the presence of this chiral organometallic ligand controls the aggregation of this square planar luminophor and imposes a homo- or hetero-chiral arrangement at the supramolecular level, displaying non-covalent Pt-Pt and π-π interactions. Interestingly these complexes are highly luminescent in the crystalline state and their photophysical properties can be traced to their aggregation in the solid state. A TD-DFT calculation is obtained to rationalize this unique behavior.

Induced Circular Dichroism in Phosphine Gold(I) Aryl Acetylide Urea Complexes through Hydrogen-Bonded Chiral Co-Assemblies.

Phosphine gold(I) aryl acetylide complexes equipped with a central bis(urea) moiety form 1D hydrogen-bonded polymeric assemblies in solution that do not display any optical activity. Chiral co-assemblies are formed by simple addition of an enantiopure (metal-free) complementary monomer. Although exhibiting an intrinsically achiral linear geometry, the gold(I) aryl acetylide fragment is located in the chiral environment displayed by the hydrogen-bonded co-assemblies, as demonstrated by induced circular dichroism (ICD).

Induced phosphorescence from Pt → Ag and Ag(i)...Ag(i) metallophilic interactions in benzenedithiolatodiimine-Pt2/Ag2 clusters: a combined experimental and theoretical investigation.

We report the synthesis and luminescence properties of a novel platinum(ii)-silver(i) cluster exhibiting supramolecular donor-acceptor dative Pt → Ag bonds as well as d(10)...d(10) argentophilic interactions. This compound was obtained by a self-assembly process upon mixing silver triflate with the [Pt(bdt)(bpy)] (1) building block. The new compound was characterized by infrared, NMR ((1)H, (13)C) and UV-vis spectroscopy. In addition, the molecular structure was unequivocally ascertained by a single-crystal X-ray diffraction study and shows that the assembly is identified as [(Pt2Ag2(o-bdt)2(bpy)2](CF3SO3)2 (2). Remarkably, the structure shows that complex 2 exists as two isomers I and II displaying different types of argentophilic interactions Ag...Ag as well as Pt → Ag dative bonds. These interactions are highly important and turn on phosphorescent emissions at low temperature depending on the nature of the isomer. The photophysical properties of both isomers were investigated and rationalized through TD-DFT calculations. This work paves the way to the synthesis of novel supramolecular assemblies displaying peculiar properties imparted from metallophilic as well as donor-acceptor metal-metal interactions.

A unique class of neutral cyclometalated platinum(II) complexes with π-bonded benzenedithiolate: synthesis, molecular structures and tuning of luminescence properties.

A unique class of neutral cyclometalated platinum(ii) complexes with π-bonded benzenedithiolate are reported including two X-ray molecular structures. To the best of our knowledge these are the first structures to be reported for cyclometalated platinum complexes with a benzenedithiolate ligand. All of the complexes are luminescent in fluid solution at room temperature and in frozen solvent glasses at 77 K and their emission properties can be tuned through ligand variation.

Tuning excited states of bipyridyl platinum(II) chromophores with π-bonded catecholate organometallic ligands: synthesis, structures, TD-DFT calculations, and photophysical properties.

A series of bipyridyl (bpy) Pt(II) complexes with π-bonded catecholate (cat) [(bpy)Pt(LM)][BF4]n (2-5) (LM = Cp*Rh(cat), n = 2; Cp*Ir(cat), n = 2; Cp*Ru(cat), n = 1; and (C6H6)Ru(cat), n = 2) were prepared and fully characterized. The molecular structures of the four compounds were determined and showed that the solid-state packing is different and dependent on the π-bonded catecholate unit. For instance, while the (bpy)Pt(II) complexes 2 and 3 with rhodium and iridium catecholates did not show any Pt···Pt interactions those with the ruthenium catecholates 4 and 5 showed the presence of Pt···Pt and π-π interactions among individual units and generated one- and two-dimensional supramolecular chains. The photophysical properties of these compounds 2-5 were investigated and showed that all compounds are luminescent at low temperature, in contrast to the well-known parent compound [(C6H4O2)Pt(bpy)] (1), which is weakly luminescent at 77 K. Time-dependent density functional theory studies are advanced to explain this difference in behavior and to highlight the role of the π-bonded catecholate system.

Encapsulation of a metal complex within a self-assembled nanocage: synergy effects, molecular structures, and density functional theory calculations.

A novel palladium-based metallacage was self-assembled. This nanocage displayed two complementary effects that operate in synergy for guest encapsulation. Indeed, a metal complex, [Pt(NO2)4](2-), was hosted inside the cavity, as demonstrated by solution NMR studies. Single-crystal X-ray diffraction shows that the guest adopts two different orientations, depending on the nature of the host-guest interactions involved. A density functional theory computational study is included to rationalize this type of host-guest interaction. These studies pave the way to a better comprehension of chemical interaction and transformation within confined nanospaces.

An unprecedented cyclometallated platinum(II) complex incorporating a phosphinine co-ligand: synthesis and photoluminescence behaviour.

The first luminescent cyclometallated platinum(II) complex bearing phosphinine co-ligands is reported. The long-lived luminescence, which it displays at 77 K, is assigned to a phosphinine-localised triplet excited state on the basis of electrochemical data and TD-DFT calculations.

Selenoquinones stabilized by ruthenium(II) arene complexes: synthesis, structure, and cytotoxicity.

A new series of monoselenoquinone and diselenoquinone π complexes, [(η(6) -p-cymene)Ru(η(4) -C6 R4 SeE)] (R=H, E=Se (6); R=CH3 , E=Se (7); R=H, E=O (8)), as well as selenolate π complexes [(η(6) -p-cymene)Ru(η(5) -C6 H3 R2 Se)][SbF6 ] (R=H (9); R=CH3 (10)), stabilized by arene ruthenium moieties were prepared in good yields through nucleophilic substitution reactions from dichlorinated-arene and hydroxymonochlorinated-arene ruthenium complexes [(η(6) -p-cymene)Ru(C6 R4 XCl)][SbF6 ]2 (R=H, X=Cl (1); R=CH3 , X=Cl (2); R=H, X=OH (3)) as well as the monochlorinated π complexes [(η(6) -p-cymene)Ru(η(5) -C6 H3 R2 Cl)][SbF6 ]2 (R=H (4); R=CH3 (5)). The X-ray crystallographic structures of two of the compounds, [(η(6) -p-cymene)Ru(η(4) -C6 Me4 Se2 )] (7) and [(η(6) -p-cymene)Ru(η(4) -C6 H4 SeO)] (8), were determined. The structures confirm the identity of the target compounds and ascertain the coordination mode of these unprecedented ruthenium π complexes of selenoquinones. Furthermore, these new compounds display relevant cytotoxic properties towards human ovarian cancer cells.