Carbene Platinum Research Articles

Cyclopropane Intermediates from Insertion Reactions of Platinum–Carbenes: A Route to Heterospiranes

Heteroaromatic-anchored enynals with a pendent alkene group were successfully cyclized through a Huisgen-type [3+2] cycloaddition to give a tetracyclic Pt–carbene complex that underwent insertion into the C–H bond in the β-position to give fused cyclopropanes that are otherwise inaccessible. On heating, the cyclopropanes smoothly rearranged to form the corresponding heterospiranes with excellent levels of stereoselectivity and high yields.

N-heterocyclic carbene platinum complexes functionalized with a polyether chain and silyl group: Synthesis and application as a catalyst for hydrosilylation

GRAPHICAL ABSTRACT

Synthesis of N-Heterocyclic Carbene Platinum Complexes and Application in the Organic Reaction

Synthesis of N-Heterocyclic Carbene Platinum Complexes and Application in the Organic Reaction

Photophysical Enhancement of Triplet Emitters by Coordination-Driven Self-Assembly.

The quantum yields of organic fluorophores used as donors in coordination-driven self-assembly often suffer from the heavy atom effect of nearby metal sites. Here, the role of intersystem crossing from a deactivating process to one that delivers emissive triplet states was reversed. A phosphorescent trans bis-N-heterocyclic carbene platinum(II) compound, Pt(dhim)2 (C≡C-4-py)2 (D1; dhim=1,3-dihexyl-2-H-imidazol-2-ylidene), was used along with other linear donors 4,4'-bipyridine (D2) and 1,4-bis(4-pyridyl ethynyl)benzene (D3) in self-assembly reactions with Pt(dtbpy)X2 acceptors (dtbpy=4,4'-di-tert-butyl-2,2'-bipyridine) to afford three metallacycles. Photophysical investigations revealed that, although the building blocks used to construct M1 have relatively low quantum yields (Φ=1.2 and <1 % for D1 and 2, respectively), the metallacycle has a quantum yield of 14 %. This increase reflects a change in radiative rate constant from 3.6×104 s-1 for D1 to 2.1×105 s-1 for M1.

Cyclometalated N‐Heterocyclic Carbene Platinum(II) Complexes with Bridging Pyrazolates: Enhanced Photophysical Properties of Binuclear Blue Emitters

AbstractThe synthesis of the first examples of phosphorescent binuclear cyclometalated N‐heterocyclic carbene C^C* μ‐pyrazolate platinum(II) complexes are reported and the photophysical characterization of the single isomers is described. The complexes exhibit a strong phosphorescent emission in the blue region of the visible spectrum with very high quantum yields of up to 86 % at room temperature (PMMA film) and 98 % at 77 K (2‐MeTHF).

Highly Site Selective Formal [5+2] and [4+2] Annulations of Isoxazoles with Heterosubstituted Alkynes by Platinum Catalysis: Rapid Access to Functionalized 1,3‐Oxazepines and 2,5‐Dihydropyridines

AbstractPlatinum‐catalyzed formal [5+2] and [4+2] annulations of isoxazoles with heterosubstituted alkynes enabled the atom‐economical synthesis of valuable 1,3‐oxazepines and 2,5‐dihydropyridines, respectively. Importantly, this Pt catalysis not only led to unique reactivity dramatically divergent from that observed under Au catalysis, but also proceeded via unprecedented α‐imino platinum carbene intermediates.

Highly Site Selective Formal [5+2] and [4+2] Annulations of Isoxazoles with Heterosubstituted Alkynes by Platinum Catalysis: Rapid Access to Functionalized 1,3-Oxazepines and 2,5-Dihydropyridines.

Platinum-catalyzed formal [5+2] and [4+2] annulations of isoxazoles with heterosubstituted alkynes enabled the atom-economical synthesis of valuable 1,3-oxazepines and 2,5-dihydropyridines, respectively. Importantly, this Pt catalysis not only led to unique reactivity dramatically divergent from that observed under Au catalysis, but also proceeded via unprecedented α-imino platinum carbene intermediates.

ChemInform Abstract: Lewis Acid Mediated Vinylogous Additions of Enol Nucleophiles into an α,β‐Unsaturated Platinum Carbene.

AbstractThe title reaction provides access to benzofuran and a broad range of indole derivatives.

Decoding catalytic activity of platinum carbene hydrosilylation catalysts

A series of complexes of the formula [Pt(dvtms)(ImPy–R)] (dvtms=1,1,3,3-tetramethyl-1,3-divinyldisiloxane, ImPy–R=2-R-imidazo[1,5-a]pyridine-3-ylidene; R=4-cyanophenyl (4a), 4-trifluoromethylphenyl (4b), phenyl (4c), 4-methoxyphenyl (4d), mesityl (4e), pentafluorophenyl (4f), tert-butyl (4g)) are presented. These compounds are synthesized from Karstedt’s catalyst [Pt2(dvtms)3] and the respective imidazo[1,5-a]pyridinium salts using tBuOK as base. 4a–g were characterized (1H, 13C, 19F, 195Pt NMR, EA, IR, UV–Vis) and investigated by cyclic voltammetry. The compounds are efficient and selective catalysts in the model hydrosilylation reaction of oct-1-ene with HSi3O2Me7. The combination of spectroscopic and theoretical studies on the B3LYP/6-31G∗∗ level of theory reveals a distinct correlation of HOMO energy levels EHOMO, oxidation potential EOx and catalytic activity (TOF). The impact of the substitution pattern introduced at the ImPy–R ligand can be interpreted based on electronics (Hammet correlation) and sterics of the residues R.

Synthesis, spectroscopic characterization and catalytic activity of platinum(II) carbene complexes

A novel platinum complex with 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidenyl ligand has been synthesized and characterized on the basis of elemental analysis, MS, 1H and 13C NMR spectroscopy, X-ray Absorption Spectroscopy and single crystal X-ray diffraction studies. The XRD determination of the complex (monoclinic, C2/c) revealed a structure in which the platinum (II) centre coordinates two chlorides, a carbon atom of the N-heterocyclic carbene (NHC) and is stabilized by coordinating the nitrogen atom of a 3-chloropyridine molecule, forming an over-all square planar geometry. By prolonging the time of the reaction, it was possible to obtain the trans-dichloridobis(3-chloropyridine) platinum(II) complex under the same reaction conditions. The electronic and molecular properties of both complexes were investigated and compared by means of Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS), supported by numerical simulations. The platinum carbene complex obtained was tested in a series of C–H activation and hydrosilylation reactions.

Post-functionalization of platinum–NHC complexes by oxime ligation for ligand targeted therapy

Carbonyl condensation reactions have been used for accessing N-heterocyclic carbene (NHC) platinum bioconjugate complexes.

Lewis Acid Mediated Vinylogous Additions of Enol Nucleophiles into an α,β-Unsaturated Platinum Carbene.

A variety of substituted indoles and benzofurans are accessed via a platinum catalyzed annulation and vinylogous addition of enol nucleophiles. Several β-dicarbonyl compounds participate in the reaction, as do α-nitro and α-cyano carbonyl species. Subjecting the indole products to acidic conditions results in the formation of fused heterocycles.

User-Friendly Platinum Catalysts for the Highly Stereoselective Hydrosilylation of Alkynes and Alkenes.

With a view to addressing the shortcomings of traditional catalysts, a new generation of outstanding N-heterocyclic carbene platinum(0) complexes for the hydrosilylation of unsaturated carbon-carbon bonds is reported. Their discovery and application to the stereoselective addition of various silanes to silylated alkynes, terminal acetylenes, and olefins is presented. Insights into the catalytic cycle and the origin of the stereoselectivity are also discussed.

DFT/TDDFT investigation on the electronic structures and photophysical properties of a series of substituted N-heterocyclic carbene (NHC) platinum(II) complexes

The electronic and photophysical properties of a series of substituted N-heterocyclic carbene (NHC) platinum(II) complexes have been investigated by using the density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The lowest-lying singlet absorptions for complexes 2–5 are attributed to the configurations of HOMO (highest occupied molecular orbital) to LUMO (lowest unoccupied molecular orbital). The emission energies for these complexes are obviously red-shifted in contrast to that of 1. For 3 and 4, the emission energies are nearly the same, consistent with their same energy gaps between of LUMO and HOMO. In addition, ionization potential (IP), electron affinities (EA) and reorganization energy (λ) also were obtained to evaluate the charge transfer and balance properties between hole and electron. From the calculated results, it can be seen that the complex 3 possesses possibly the largest kr value among these complexes due to the lowest ΔES1−T1, larger 3MLCT contribution, and higher μS1⁡ value. It is anticipated that this study could provide useful information for the synthesis of efficient platinum(II) phosphors in OLEDs.

Synthesis, structural characterization and in vitro anti-cancer activity of functionalized N-heterocyclic carbene platinum and palladium complexes

A series of platinum(II) and palladium(II) complexes bearing N-heterocyclic carbene ligands functionalized by specific functionalities such as alcohol, nitrile or ester were synthesized. The novel complexes were fully characterized and X-ray diffraction was used for the structure determination of eight derivatives. The reactivity of the functional groups has been studied; for example, Pt and Pd NHC complexes containing tBu ester functions were easily hydrolyzed giving water-soluble NHC complexes. The palladium complex 21 containing dangling COOH group was evaluated in copper-free Sonogashira coupling reaction. In addition, platinum NHC complexes are attractive metal-based systems for the development of anticancer drugs. Preliminary results of the biological effects on various human cancer and non-cancer cells are also reported.

Reactivity of cationic agostic and carbene structures derived from platinum(II) metallacycles.

This paper describes the formation of new platinacyclic complexes derived from the phosphine ligands PiPr2 Xyl, PMeXyl2 , and PMe2 Ar Xyl 2 (Xyl=2,6-Me2 C6 H3 and Ar Xyl 2=2,6-(2,6-Me2 C6 H3 )2 -C6 H3 ) as well as reactivity studies of the trans-[Pt(C^P)2 ] bis-metallacyclic complex 1 a derived from PiPr2 Xyl. Protonation of compound 1 a with [H(OEt2 )2 ][BArF ] (BArF =B[3,5-(CF3 )2 C6 H3 ]4 ) forms a cationic δ-agostic structure 4 a, whereas α-hydride abstraction employing [Ph3 C][PF6 ] produces a cationic platinum carbene trans-[Pt{PiPr2 (2,6-CH(Me)C6 H3 }{PiPr2 (2,6-CH2 (Me)C6 H3 }][PF6 ] (8). Compounds 4 a and 8 react with H2 to yield the same 1:3 equilibrium mixture of 4 a and trans-[PtH(PiPr2 Xyl)2 ][BArF ] (6), in which one of the phosphine ligands participates in a δ-agostic interaction. DFT calculations reveal that H2 activation by 8 occurs at the highly electrophilic alkylidene terminus with no participation of the metal. The two compounds 4 a and 8 experience C-C coupling reactions of a different nature. Thus, 4 a gives rise to complex trans-[PtH{(E)-1,2-bis(2-(PiPr2 )-3-MeC6 H3 )CHCH}] (7) that contains a tridentate diphosphine-alkene ligand, through agostic CH oxidative cleavage and C-C reductive coupling steps, whereas the C-C coupling reaction in 8 involves classical migratory insertion of its [PtCH] and [PtCH2 ] bonds promoted by platinum coordination of CO or CNXyl. The mechanisms of the CC bond-forming reactions have also been investigated by computational methods.

Chelated bis-N-heterocyclic carbene platinum and palladium units as tunable components of multinuclear complexes.

Heterometallic trinuclear M2M' complexes (M = Rh, Ir; M' = Pt, Pd) containing a platinum or palladium moiety with chelated bis-N-heterocyclic carbene ligands, [(MCp*)2{M'(bisNHC-Cn-R)}(μ3-S)2](BPh4)2 (M = Rh, Ir; M' = Pt, Pd; bisNHC-Cn-R = methylene-, ethylene-, or propylene-bridged bis(N-alkyl-imidazolylidene)), were synthesized by reacting bis(hydrosulfido)platinum(II) or palladium(II) complexs with bisNHC-Cn-R and hydroxo-bridged dinuclear complexes [(MCp*)2(μ-OH)3](BPh4), whose dinuclear structures remained intact during the formation of the trinuclear complexes, which was confirmed by using electrospray mass spectrometry and NMR spectroscopy. The heterometallic trinuclear M2M' complexes with a variety of alkylene bridges in bisNHC-Cn-R showed two reversible reduction waves in the cyclic voltammogram, and the second reduction potentials were affected by the alkylene chain lengths, which caused different dihedral angles between the imidazolylidene rings and the coordination plane of the platinum or palladium center. The M2M' complexes, except for those containing the platinum unit with the ethylene-bridged bisNHC ligands, showed dynamic behavior in solution due to the flapping wing motion of the NHC ligand moieties. Although activation parameters obtained from line-shape analyses on variable-temperature NMR spectra of the complexes suggested that the flapping wing motion occurred without bond cleavage, large negative ΔS(‡) values were obtained for the complexes with the palladium unit with the ethylene-bridged ligand, suggesting that the Pd-Ccarbene bond cleavage, accompanied by coordination of solvent molecules, occurred.

Synthesis and behavior of novel sulfonated water-soluble N-heterocyclic carbene (η(4)-diene) platinum(0) complexes.

A series of water-soluble (NHC)Pt(0)(dvtms) and (NHC)Pt(0)(AE) complexes containing different sulfonated NHC ligands (dvtms = divinyltetramethyldisiloxane and AE = diallyl ether) are reported. The dvtms compounds have been found to be quite robust and to display some conformational rigidity, whereas their AE counterparts are less stable and more flexible. The catalytic evaluation of these complexes in the hydrosilylation of alkynes in water revealed no benefits in favor of the complexes containing the more labile spectator diene (AE), and a fairly regular catalytic behavior for all complexes that restricts the location of the sulfonate group to the proximity of the metal site.

Water-Soluble Mono- and Dimethyl N-Heterocyclic Carbene Platinum(II) Complexes: Synthesis and Reactivity

A family of water-soluble dimethyl complexes of formula cis-[PtMe2(dmso)(NHC·Na)] (2), in which NHC is an anionic N-heterocyclic carbene bearing a sulfonatopropyl chain on one of the nitrogen atoms and a sulfonatopropyl (a), methyl (b), mesityl (c), or 2,6-diisopropylphenyl group (d) on the other, have been prepared. The hydrolytic stability of the Pt–C bonds in these complexes under different neutral, alkaline, and acidic aqueous conditions has also been studied. Complexes 2 were found to be quite stable at room temperature in water under neutral or alkaline conditions. Degradation occurred at higher temperatures but involved C sp3–H activation and C–C reductive elimination processes in addition to Pt–Me bond hydrolysis. Hydrolytic cleavage of the platinum–methyl bonds was favored by good nucleophiles. Thus, the addition of KCN to an aqueous solution of 2 resulted in formation of the monomethyl complexes K[PtMe(CN)2(NHC·Na)] (9), whereas the dimethyl complexes K[PtMe2(CNR)(NHC·Na)] (10) were formed with the isocyanide CNCH2COOK. The addition of stoichiometric amounts of protic acids to aqueous solutions of 2 resulted in the clean cleavage of one or both platinum(II)–methyl bonds. Thus, the reaction of 2 with HCl afforded the complexes [PtClMe(dmso)(NHC·Na)] (3) and [PtCl2(dmso)(NHC·Na)] (4), whereas [PtMe(OH2)(dmso)(NHC)] (5) and [Pt(OH2)2(dmso)(NHC)][BF4] (7) were obtained upon treatment with HBF4. The crystal structure of 9a is remarkable in light of the longitudinal channels around 6 Å in diameter internally decorated with Pt–Me bonds.

Correction to “Vicinal Bisheterocyclizations of Alkynes via Nucleophilic Interception of a Catalytic Platinum Carbene”

Correction to “Vicinal Bisheterocyclizations of Alkynes via Nucleophilic Interception of a Catalytic Platinum Carbene”