Multinuclear NMR Spectroscopy Research Articles

Effect of Terminal Hydroxy Groups on the Structural Properties of Acyl Thioureas.

The 1-acyl thiourea family [R1C(O)NHC(S)NR2R3] exhibits the flexibility to incorporate a wide variety of substituents into their structure. The structural attributes of these compounds are intricately tied to the type and extent of substitution. In the case of 3-mono-substituted thioureas (R2 = H), the conformational behavior is predominantly shaped by the presence of an intramolecular N-H···O=C hydrogen bond. This study delves into the structural consequences stemming from the inclusion of substituents possessing hydrogen-donor capabilities within four novel 1-acyl-3-mono-substituted thiourea derivatives. A comprehensive suite of analytical techniques, encompassing FTIR, Raman spectroscopy, multinuclear (1H and 13C) NMR spectroscopy, single-crystal X-ray diffraction, and supported by computational methods, notably NBO (Natural Bond Orbital) population analysis, Hirshfeld analysis, and QTAIM (Quantum Theory of Atoms in Molecules), was harnessed to scrutinize and characterize these compounds. In the crystalline state, these compounds exhibit an intricate interplay of intermolecular interactions, prominently featuring an expansive network of hydrogen bonds between the hydroxy (-OH) groups and the carbonyl and thiocarbonyl bonds within the 1-acyl thiourea fragment. Notably, the topological analysis underscores significant distinctions in the properties of the acyl thiourea fragment and the intramolecular >C=O···H-N bond when transitioning from the isolated molecule to the crystalline environment.

Anticancer potential of NSAID-derived tris(pyrazolyl)methane ligands in iron(II) sandwich complexes.

Tris(pyrazolyl)methane (tpm), 2,2,2-tris(pyrazolyl)ethanol (tpmOH) and its esterification derivatives with ibuprofen and flurbiprofen (tpmIBU and tpmFLU) were used as ligands to obtain complexes of the type [Fe(tpmX)2]Cl2 (1-4). The tpmIBU and tpmFLU ligands and corresponding complexes 3 and 4 were characterized by IR and multinuclear NMR spectroscopy, and the structure of tpmIBU was elucidated by single crystal X-ray diffraction. Complexes 1-4 were also assessed for their behaviour in aqueous media (solubility in D2O, octanol/water partition coefficient, stability in physiological-like conditions). The antiproliferative activity of ligands and complexes was determined on A2780, A2780cis and A549 cancer cell lines and the non-cancerous HEK 293T and BJ cell lines. The ligands and complexes were investigated for their ability to inhibit COX-2 (cyclooxygenase) and HNE (4-hydroxynonenal) enzymes. Complexes 3 and 4 exhibited cytotoxicity that may be attributed predominantly to their bioactive fragments, while DNA binding and enhancement of ROS production do not appear to play any significant role.

Homoleptic nickel(II)-dithiocarbamates as potent homogeneous electrocatalysts for hydrogen evolution reaction: Effect of positional isomeric –OH group on electrocatalytic properties

Three positional isomeric homoleptic Ni(II) dithiocarbamate complexes having general formula [Ni(L)2] (L=N-benzyl-1-(2-hydroxyphenyl) dithiocarbamate (Ni-1); N-benzyl-1-(3-hydroxyphenyl) dithiocarbamate (Ni-2) and N-benzyl-1-(4-hydroxyphenyl) dithiocarbamate (Ni-3)) have been synthesized and characterized by elemental analysis, FT-IR, UV–Vis., multinuclear NMR spectroscopy and single crystal X-ray diffraction. The single crystal X-ray analysis for Ni-2 reveals distorted square planar geometry around Ni(II), that is satisfied by four sulfur centers of two N-benzyl-1-(3-hydroxyphenyl) dithiocarbamate ligands. The nature of weak interactions in Ni-2 have been explored using Hirshfeld surface analysis, which suggest that the percentage contribution of O···H interactions is more than S···H interactions. These complexes have been employed as homogeneous catalysts for electrochemical hydrogen evolution reaction suggesting turnover frequency (TOF) of 566.2, 457.2 and 867.9 s−1 for Ni-1, Ni-2 and Ni-3, respectively on adding 30 mM trifluoroacetic acid (TFA). Also, the overpotential (η) values for Ni-1, Ni-2 and Ni-3 are 0.862, 0.872 and 0.723 V, respectively. The variation in the electrocatalytic properties was explained on the basis of theoretical calculations which suggested that the para-OH isomer complex possess relatively larger HOMO-LUMO energy gap, compared to ortho-OH and meta-OH isomeric complexes and indicated the relative less stable nature of para-OH isomer complex and its better susceptibility towards electrons at a lower negative potential.

Rare earth mixed sandwich complexes with tetraalkylphospholide and cyclooctatetraenide ligands

A series of rare earth (mono)phospholide mixed sandwich complexes of the general form [M(PC4R4)(COT)(THF)n] (M = Sc, Y, La, Lu; R = Me, Et; COT = cyclooctatetraenide, {C8H8}2–; n = 0 to 2) have been isolated by the treatment of iodide precursors, [M(COT)(I)(THF)n], (1M, M = Sc, Y, Lu, n = 2; M = La, n = 3) with potassium phospholide salts, [K(PC4R4)]n (R = Me, Et). The solid-state molecular structures and speciation of these sandwich complexes depends upon both the ionic radius of the rare earth metal, along with small steric and solubility differences which arise between the two per-alkylated phospholide ligands. The smaller {PC4Me4} ligand gave monomeric Lewis-base free [M(C8H8)(PC4Me4)] (2M, M = Sc, Lu) with smaller rare earths Sc(III) and Lu(III), but moving to larger ions Y(III) and La(III), the products were poorly soluble and could only be isolated as THF-adducts, [Y(C8H8)(PC4Me4)(THF)] (3) and [La(C8H8)(PC4Me4)(THF)2] (4). The slightly increased steric demands of {PC4Et4} gave monomeric Lewis-base free complexes for Sc(III), Lu(III), and Y(III) in [M(C8H8)(PC4Et4)] (5M, M = Sc, Y, Lu), whereas for La(III) a dimeric complex was isolated, [La(C8H8)(µ-PC4Et4)]2 (6). We also report the synthesis and molecular structures of 1Sc and 1Lu, as well as [LuI3(THF)3] (7) for the first time. All complexes were characterised by single crystal X-ray diffraction, multi-nuclear NMR, UV-Vis-NIR and ATR-IR spectroscopies in addition to elemental analysis.

New (O-Si) chelate with SiC2O2 framework: N-[acetimidate(dimethylsilyl)methyl]acetamide hydrochloride

The interaction of N-(trimethylsilyl)acetamide with ClCH2SiMe2F led to the formation of previously unknown (O-Si) chelate compound N-[acetimidate(dimethylsilyl)methyl]acetamide hydrochloride, containing imidate group as axial subsituent. This compound is new example of the little-known (O-Si) chelates with C = O→SiC2O coordination unit. Its structure was confirmed by the multinuclear NMR spectroscopy and X-ray analysis. The O1 and O2 atoms occupy the axial positions, and both bonds Si-O have the same length 1.931 Å.

Bridging Titanium Nitrido Complexes Containing A Linear Ti-N-Ti Core with A Two-Coordinate Nitrido Ligand.

A series of titanium m2-nitrido complexes supported by the triamidoamine ligand Xy-N3N (Xy-N3N = {(3,5-Me2C6H3)NCH2CH2}3N3-) is reported. The titanium azido complex [(Xy-N3N)TiN3] (1-N3), prepared by salt metathesis of the chloride complex [(Xy-N3N)TiCl] (1-Cl) with NaN3, reacted with lithium metal or with alkali metal naphthalenides (alkali metal M = Na, K, and Rb) in THF to give the corresponding dinuclear m2-nitrido complexes M[(Xy-N3N)Ti=N-Ti(Xy-N3N)] (2-M; M = Li, Na, K, Rb). Single crystal X-ray diffraction studies of 2-Li, 2-Na, and 2-K revealed alkali metal dependent structures in the solid state. While 2-Li and 2-K contain a m2-nitrido ligand with a linear Ti-N-Ti core, 2-Na includes a m3-nitrido ligand as part of a T-shape Ti2NaN fragment with the sodium cation weekly coordinated to the nitrido nitrogen atom. When the synthesis of the nitrido complexes was carried out in the presence of excess alkali metals, selective decomposition of the nitrido complexes was observed affording some intractable titanium species along with the trialkali metal salts [M3(Xy-N3N)] (3-M) (M = Li, Na, K, and Rb). These salts were also prepared by deprotonation of (Xy-N3N)H3 with the corresponding alkali metal hexamethyldisilazide and characterized by multinuclear NMR spectroscopy as well as single crystal X-ray diffraction.

Synthesis and Ligand Properties of Silsesquioxane‐Caged Phosphite T7Ph7P

AbstractThe synthesis of the phosphorus‐capped heptaphenylsilsesquioxane T7Ph7P is reported. We show that, unlike previous examples, it readily forms Lewis acid‐base adducts with boranes of different steric demand and the FeCO4 fragment. All complexes were characterized by multinuclear NMR and IR spectroscopy in solution and the solid state. The molecular structures of the adducts with BPh3 and B(C6F5)3 were determined from XRD suitable single crystals. The relative conformation of the Ph and C6F5 rings in T7Ph7P−B(C6F5)3 suggests favorable π‐interactions, stabilizing the adducts to such a degree that attempts to activate H2 or CO2 were unsuccessful.

C[sbnd]H activation of furanyl and thiofuranyl substrates catalyzed by trans-dichloro[1-cinnamyl-3-arylmethyl-benzimidazol-2-yliden]pyridine palladium(II) complexes

The present study explores the potential of six NHC-palladium(II) complexes derived from N-cinnamyl-N’-alkylbenzimidazolium salts, namely trans-dichloro[1,3-dicinnamyl-benzimidazol-2-yliden]pyridine palladium(II) (3a), trans-dichloro[1-cinnamyl-3-benzyl-benzimidazol-2-yliden]pyridine palladium(II) (3b), trans-dichloro[1-cinnamyl-(fluorobenzyl)-benzimidazol-2-yliden]pyridine palladium(II) (2-fluorobenzyl 3c, 3-fluorobenzyl 3d and 4-fluorobenzyl 3e,) and trans-dichloro[1-cinnamyl-(4-nitrobenzyl)-benzimidazol-2-yliden]pyridine palladium(II) (3f), as precatalyst for the cross-coupling reactions between furanyl and thiofuranyl derivatives and arylbromides via CH activation of the heterocycles. The structure of the six palladium(II) complexes has been elucidated through the use of multinuclear NMR, FT-IR and mass spectroscopy. Furthermore, the square-planar geometry of the organometallic ion was confirmed by single crystal X-ray diffraction study carried out on complex 3c. The latter complex proved to be the most effective precatalyst agent, with observed conversions higher than 78% when 4-bromoacetophenone and 1-bromonaphthalene were employed.

Design of efficient benzimidazole-derived N-heterocyclic carbene Ag(I) catalysts for aldehyde–amine–alkyne coupling

A mild catalytic protocol for aldehyde, amine, and alkyne coupling (A3-coupling) allows for the selective synthesis of propargyl amines using 5,6-dimethylbenzimidazole-derived N-heterocyclic carbene (BNHC) silver(I) catalysts. A series of BNHC Ag(Ⅰ) halide complexes were synthesized and their structures have been elucidated through the use of multinuclear NMR and FT-IR spectroscopy. Furthermore, the pseudo-linear geometry of two different compounds was substantiated by single-crystal X-ray diffraction. Silver-based A3-coupling was achieved for both acyclic and cyclic secondary amines using a diverse set of alkynes to afford propargyl amines with yields of up to 95 %. The present approach is environmentally benign and water is generated as the sole byproduct.

Beyond transition block metals: exploring the reactivity of phosphine PTA and its oxide [PTA(O)] towards gallium(iii).

The water-soluble cage-like phosphine PTA (1,3,5-triaza-7-phosphaadamantane) and its phosphine oxide derivative [PTA(O)] (1,3,5-triaza-7-phosphaadamantane-7-oxide) were used to explore their reactivity towards two gallium(iii)-halide precursors, namely GaCl3 and GaI3, for the first time. By using various reaction conditions, a series of N-mono-protonated phosphine salts with [GaCl4]- or [I]- as counterions were obtained in all cases, while the formation of coordinated Ga-PTA and Ga-[PTA(O)] complexes was not observed. All compounds were characterized in solution using multinuclear NMR spectroscopy (1H, 13C{1H}, 31P{1H} and 71Ga) and in the solid state using FT-IR spectroscopy and X-ray crystal diffraction. The new Ga-phosphine salts resulted stable and highly soluble in aqueous solution at room temperature. Density functional theory (DFT) calculations were also performed to further rationalize the coordination features of PTA with Ga3+ metal ion, highlighting that the phosphorus-gallium bond is about twice weaker than the phosphorus-metal bond commonly established by PTA with transition metals such as gold. Furthermore, the mono-protonation of PTA (or [PTA(O)]) makes the formation of ionic gallium-PTA coordination complexes thermodynamically unstable, as confirmed experimentally by the formation of Ga-phosphine salts reported herein.

Synthesis, structural characterization, Hirshfeld surface analysis and thermal properties of dibutyltin(IV)-based complexes containing thiocyanate and pyridyl ligands

Dibutyltin(IV) complex [SnBu2(NCS)2] (1) has been prepared from the reaction of SnBu2Cl2 and potassium thiocyanate in a molar ratio of Sn:KNCS/1:2 in ethanol. The reaction of [SnBu2(NCS)2] (1) with 4,4ꞌ-di-tert-butyl-2,2ꞌ-bipyridine (bu2bpy) in a molar ratio of Sn:bu2bpy/1:1 resulted in the formation of [SnBu2(NCS)2(bu2bpy)] (2). The products were characterized using elemental analysis, IR, and multinuclear NMR (1H, 13C, 119Sn) spectroscopy. The X-ray crystal structure determination of the complex [SnBu2(NCS)2(bu2bpy)] (2) revealed that the geometry around the Sn(IV) atom is highly distorted octahedral with trans-[SnBu2] configuration. On the other hand, the new single crystals of bu2bpy (3) were characterized by X-ray crystallography from the crystallization of 2 in diethyl ether. The various types of weak interactions in their networks have been assessed using the 3D Hirshfeld surface analysis and 2D fingerprint maps for 2, and 3 which show the major contribution of intermolecular interactions is related to H…H interactions. The thermal properties analysis showed that the complex [SnBu2(NCS)2] (1) is stable up to 153 °C while the complex [SnBu2(NCS)2(bu2bpy)] (2) is stable up to 190 °C.

Aminocarbyne-Alkyne Coupling in Diruthenium Complexes: Exploring the Anticancer Potential of the Resulting Vinyliminium Complexes and Comparison with Diiron Homologues.

New diruthenium complexes based on the scaffold Ru2Cp2(CO)2 (Cp = η5-C5H5) and containing a bridging vinyliminium ligand, [2a-d]CF3SO3, were synthesized through regioselective coupling of alkynes with an aminocarbyne precursor (85-90% yields). The reaction involving phenylacetylene proceeded with the formation of a diruthenacyclobutene byproduct, [4]CF3SO3 (10% yield). Complexes [2a-d]+ undergo partial alkyne extrusion in contact with alumina or CDCl3. All products were characterized by elemental analysis, infrared and multinuclear NMR spectroscopy, and single crystal X-ray diffraction in two cases. Complexes [2a-d]+ revealed an outstanding stability in DMEM cell culture medium at 37 °C (<1% degradation over 72 h). These complexes exhibited cytotoxicity in human colon colorectal adenocarcinoma HT-29 cells in the low micromolar range, with lower IC50 values than those obtained with the homologous diiron complexes previously reported. Evaluation of ROS (reactive oxygen species) production and O2 consumption rate (OCR) highlighted the higher potential of Ru2 complexes, compared to the Fe2 counterparts, to impact mitochondrial activity, with the heterometallic Ru2-ferrocenyl complex [2d]+ showing the best performance.

The Chlorido-Bismuth Dication: A Potent Lewis Acid Captured in a Hepta-Coordinate Species with a Stereochemically Active Lone Pair.

The stabilization of simple, highly reactive cationic species in molecular complexes represents an important strategy to isolate and characterize compounds with uncommon or even unprecedented structural motifs and properties. Here we report the synthesis, isolation, and full characterization of chlorido-bismuth dications, stabilized only by monodentate dimethylsulfoxide (dmso) ligands: [BiCl(dmso)6][BF4]2 (1) and [BiCl(μ2-dmso)(dmso)4]2[BF4]4 (2). These compounds show unusual distorted pentagonal bipyramidal coordination geometries along with high Lewis acidities and have been analyzed by multinuclear NMR spectroscopy, elemental analysis, IR spectroscopy, single-crystal X-ray diffraction, and density functional theory calculations. Attempts to generate the bromido- and iodido-analogs gave dmso-stabilized tricationic bismuth species.

Palladium-Catalyzed Cross-Coupling Reaction via C–H Activation of Furanyl and Thiofuranyl Substrates

The present study explores the potential of four NHC-palladium(II) complexes derived from (Z)- or (E)-styryl-N-alkylbenzimidazolium salts, namely trans-dichloro-[(Z)-1-styryl- 3-benzyl-benzimidazol-2-yliden]pyridine palladium(II) (6), trans-dichloro-[(E)-1-styryl-3-benzyl- benzimidazol-2-yliden]pyridine palladium(II) (7), trans-dichloro-[(Z)-1-styryl-3-(3-fluorobenzyl)- benzimidazol-2-yliden]pyridine palladium(II) (8) and trans-dichloro-[(E)-1-styryl-3- (3-fluorobenzyl)-benzimidazol-2-yliden]pyridine palladium(II) (9), to be use as pre-catalysts for the cross-coupling reactions between furanyl or thiofuranyl derivatives and arylbromides via the C–H activation of the heterocycles. The structures of the four Pd(II) complexes have been elucidated through the use of multinuclear NMR, FT-IR and mass spectroscopy. Furthermore, the cis or trans conformation of the styryl substituents and the geometry of two different compounds was substantiated by single-crystal X-ray diffraction, which was carried out on organometallic species 6, 8 and 9. After the optimization of catalytic conditions, which was carried out with 1 mol% of pre-catalyst with KOAc as a base in dimethylacetamide at 120 °C for 3 h, complex 6 proved to be the most effective pre-catalyst agent, with full or quasi full conversions being observed in the cross-coupling of 4-bromoacetophenone with 2-butylfuran, 1-(2-furanyl)-ethanone, furfuryl acetate, furfural, 1-(2-thienyl)-ethanone, thenaldehyde and 2-methylthiophene.

Direct Synthesis of C-Substituted [RC(O)CH2-CB11H11]− Carborate Anions

A new synthetic method for the synthesis of C-substituted [RC(O)CH2-CB11H11]− carborate anions has been developed. The reaction of [closo-B11H11]2− with terminal alkynes in the presence of a copper catalyst leads to insertion into the boron cluster, and C-substituted [RC(O)CH2-CB11H11]− carborate anions are formed. These reactions are strongly dependent on the reaction conditions, the solvents, and the alkynes used. The alkynes HCCCO2Et, HCCCO2Me, and HCCCONH2 lead to the formation of [NH2C(O)CH2-CB11H11]− as the final product in aqueous ammonia solution. In contrast, the reaction using the alkyne HCCCOMe yields [MeC(O)CH2-CB11H11]−. The products have been fully characterized by multinuclear NMR and IR spectroscopy as well as mass spectrometry. The crystal structures of K[NH2C(O)CH2-CB11H11] and [NEt3CH2Cl][NH2C(O)CH2-CB11H11] have been determined.

DPPM-Bridged Binuclear Pt(II) Pincer Complexes: Chemistry, Structure, and Photophysics in Solution Revisited.

A series of luminescent binuclear ([dppm{Pt(NNC)}2]2+) and mononuclear ([PPh3Pt(NNC)]+) complexes containing pincer ligands were synthesized and characterized. Photophysical characteristics of both types of complexes were studied in dichloromethane solution. In the solid phase, the binuclear compounds adopt a syn configuration where the {Pt(NNC)} fragments are held together due to intramolecular Pt-Pt bonding and π-stacking of the pincer ligand aromatic systems. Analysis of the complexes' molecular structure in solution by multinuclear NMR spectroscopy showed that the stacked intramolecular configuration is retained in fluid media, which is in complete agreement with a considerable red shift of the emission wavelength due to formation of the intramolecular Pt-Pt bond, leading to the transformation of an emissive excited state to 3MMLCT. It was also found that triethylamine quenches the emission of both types of complexes; the mechanism of quenching is a combination of dynamic and static channels of excited-state deactivation. In the case of binuclear complexes, deprotonation of the dppm methylene bridge by triethylamine also contributes to the chromophore quenching. To explain the observed chemistry of binuclear complex interactions with Et3N, a chemical equilibrium scheme was suggested, which was confirmed by quantitative monitoring of the 31P signal variations as a function of triethylamine concentration.

Complexes of Alkali Metal and the Ammonium Ion with the Cyclodimethylsiloxane Ligands D7 and D8 (D=Me2SiO)

AbstractWe present complexes of alkali metal ions and the ammonium ion with the large cyclodimethylsiloxanes D7 and D8 (D=Me2SiO). The formation of such complexes is accompanied by rearrangements of the ligands to form complexes with a suitable ring size for the metal used. These rearrangements could be suppressed by suitable reaction conditions and rapid work‐up of the reaction mixtures. This enabled the isolation of [Li(D7)][GaI4] (1), [Li(D8)][GaI4] (2), [Na(D8)][GaI4] (3), [K(D8)][GaI4] (4) and [NH4(D8)][GaI4] (5). All of the prepared compounds were analysed using single‐crystal X‐ray diffraction as well as multinuclear NMR spectroscopy. This reveals very different coordination modes in the compounds obtained.

Regioselective Cobalt(II) Catalyzed C-H Functionalization and [3 + 2] Annulation of N-Arylguanidines with 1,3-Diynes.

8-Quinolinyl directed N-arylguanidines were subjected to cobalt(II) catalyzed C-H functionalization/1,3-diyne annulation under mild conditions to afford 40 alkynylated indole guanidines in 34% to 92% yields. The scope of the substrates was explored. The reported procedure is mostly regioselective and tolerates various functional groups in the substrates. The regioselectivity of the reactions was assessed with the aid of multinuclear NMR spectroscopy and X-ray crystallography. Competition and labeling experiments were carried out to support the proposed mechanism.

A lithium–aluminium heterobimetallic dimetallocene

Homobimetallic dimetallocenes exhibiting two identical metal atoms sandwiched between two η5 bonded cyclopentadienyl rings is a narrow class of compounds, with representative examples being dizincocene and diberyllocene. Here we report the synthesis and structural characterization of a heterobimetallic dimetallocene, accessible through heterocoupling of lithium and aluminylene fragments with pentaisopropylcyclopentadienyl ligands. The Al–Li bond features a high ionic character and profits from attractive dispersion interactions between the isopropyl groups of the cyclopentadienyl ligands. A key synthetic step is the isolation of a cyclopentadienylaluminylene monomer, which also enables the structural characterization of this species. In addition to their structural authentication by single-crystal X-ray diffraction analysis, both compounds were characterized by multinuclear NMR spectroscopy in solution and in the solid state. Furthermore, reactivity studies of the lithium–aluminium heterobimetallic dimetallocene with an N-heterocyclic carbene and different heteroallenes were performed and show that the Al–Li bond is easily cleaved.

Synthesis, Spectroscopic Characterization, and Photophysical Studies of Heteroleptic Silver Complexes Bearing 2,9-Bis(styryl)-1,10-phenanthroline Ligands and Bis[(2-diphenylphosphino)phenyl] Ether

Three new heteroleptic Ag(I) complexes, labeled as [AgL(POP)]BF4 (1–3), were successfully synthesized and comprehensively characterized. Here, L represents 2,9-bis((E)-4-methoxystyryl)-1,10-phenanthroline (L1), 2,9-bis((E)-4-methylthiostyryl) -1,10-phenanthroline (L2), and 2,9-bis((E)-4-diethylaminostyryl)-1,10-phenanthroline (L3), while POP stands for Bis[(2-diphenylphosphino)phenyl] ether. The stability of these compounds in solution was confirmed through multinuclear 1D (1H, 13C, 31P) and 2D NMR (COSY, NOESY, HMBC, HSQC) spectroscopies. Additionally, their molecular structure was elucidated via X-ray crystallography. The photophysical properties of the complexes were assessed both in the solid state and in solution (dichloromethane). Compounds 1–3 demonstrated moderate emissions in solution, with quantum yields ranging from 11–23%. Interestingly, their solid-state luminescent behavior differed. Large bathochromic shifts (42–75 nm) of the emission maxima and a decrease in quantum yields (2.5–9.5%) were evident, possibly due to the presence of excimers. Compound 3 stands out as a rare example of an Ag(I) red-color emitter.