Hydrogenation of Terminal Alkenes Catalyzed by Air-Stable Mn(I) Complexes Bearing an N-Heterocyclic Carbene-Based PCP Pincer Ligand.

Efficient hydrogenations of terminal alkenes with molecular hydrogen catalyzed by well-defined bench stable Mn(I) complexes containing an N-heterocyclic carbene-based PCP pincer ligand are described. These reactions are environmentally benign and atom economic, implementing an inexpensive, earth abundant non-precious metal catalyst. A range of aromatic and aliphatic alkenes were efficiently converted into alkanes in good to excellent yields. The hydrogenation proceeds at 100 °C with catalyst loadings of 0.25-0.5 mol %, 2.5-5 mol % base (KOt Bu) and a hydrogen pressure of 20 bar. Mechanistic insight into the catalytic reaction is provided by means of DFT calculations.

Activated Mn-MACHO Complexes Form Stable CO2 Adducts.

Manganese(I) carbonyl complexes bearing a MACHO-type ligand (HN(CH2 CH2 PR2 )2 ) readily react in their amido form with CO2 to generate 4-membered {Mn-N-C-O} metallacycles. The stability of the adducts decreases with the steric demand of the R groups at phosphorous (R=isopropyl>adamantyl). The CO2 -adducts display generally a lower reactivity as compared to the parent amido complexes. These adducts can thus be interpretated as masked forms of the active amido catalysts and potentially play important roles as off-loop species or branching points in catalytic transformations of carbon dioxide.

Well-Defined Highly-Coordinated Copper(III) Iodide and Pincer Tris(trifluoromethyl)copper Complexes.

Copper(III) iodide and bromide complexes representing a unique combination of highly-coordinated metal and soft polarizable anions were synthesized and fully characterized, including X-ray crystallography. Ligand substitution in well-defined highly-coordinated copper complex PyCu(CF3)3 with pincer ligands was achieved to give formally octahedral copper(III) complexes.

Photocatalytic Reduction of CO2 into CO with Cyclometalated Pt(II) Complexes of N

In this work, density functional theory (DFT) calculations were employed to study the photocatalytic reduction of CO2 into CO using a series of Pt(II) square planar complexes with the general formula [Pt(5-R-dpb)Cl] (dpb = 1,3-di(2-pyridyl)benzene anion, R = H, N,N-dimethylaniline,T thiophene, diazaborinine). The CO2-into-CO conversion process is thought to proceed via two main steps, namely the photocatalytic/reduction step and the main catalytic step. The simulated absorption spectra exhibit strong bands in the range 280-460 nm of the UV-Vis region. Reductive quenching of the T1 state of the complexes under study is expected to be favorable since the calculated excited state redox potentials for the reaction with sacrificial electron donors are highly positive. The redox potentials reveal that the reductive quenching of the T1 state, important to the overall process, could be modulated by suitable changes in the N^C^N pincer ligands. The CO2 fixation and activation by the three coordinated Pt(II) catalytically active species are predicted to be favorable, with the Pt-CO2 bond dissociation energies D0 in the range of -36.9--10.3 kcal/mol. The nature of the Pt-CO2 bond of the Pt(II) square planar intermediates is complex, with covalent, hyperconjugative and H-bonding interactions prevailing over the repulsive electrostatic interactions. The main catalytic cycle is estimated to be a favorable exergonic process.

Carbon Dioxide Hydrogenation to Formate Catalyzed by a Neutral, Coordinatively Saturated Tris-Carbonyl Mn(I)-PNP Pincer-Type Complex.

CO2 catalytic hydrogenation to formate was achieved (TONmax =ca. 3800) in the presence of the neutral, halide-free, coordinatively saturated tris(carbonyl) manganese pincer-type complex [Mn(PNP)(CO)3 ], bearing a diarylamido pincer-type PNP ligand, using DBU as base and LiOTf as Lewis acid additive, under mild reaction conditions (60 bar, 80 °C). DFT calculations suggest that the precatalyst activation key step occurs by intermolecular, base assisted dihydrogen heterolytic splitting rather than by the expected ligand-assisted intramolecular MLC-type mechanism.

Hydrogenation of CO2 by a Bifunctional PC(sp3 )P Iridium(III) Pincer Complex Equipped with Tertiary Amine as a Functional Group.

Reversible hydrogen storage in the form of stable and mostly harmless chemical substances such as formic acid (FA) is a cornerstone of a fossil fuels-free economy. In the past, we have reported a primary amine-functionalized bifunctional iridium(III)-PC(sp3 )P pincer complex as a mild and chemoselective catalyst for the additive-free decomposition of neat formic acid. In this manuscript, we report on the successful application of a redesigned complex bearing tertiary amine functionality as a catalyst for mild hydrogenation of CO2 to formic acid. The catalyst demonstrates TON up to 6×104 and TOF up to 1.7×104  h-1 . In addition to the practical value of the catalyst, experimental and computational mechanistic studies provide the rationale for the design of improved next-generation catalysts.

Unsymmetrical Pd(II) Pincer Complexes with Benzothiazole and Thiocarbamate Flanking Units: Expedient Solvent-Free Synthesis and Anticancer Potential.

Driven by the growing threat of cancer, many research efforts are directed at developing new chemotherapeutic agents, where the central role is played by transition metal complexes. The proper ligand design serves as a key factor to unlock the anticancer potential of a particular metal center. Following a recent trend, we have prepared unsymmetrical pincer ligands that combine benzothiazole and thiocarbamate donor groups. These compounds are shown to readily undergo direct cyclopalladation, affording the target S,C,N-type Pd(II) pincer complexes both in solution and in the absence of a solvent. The solid-phase strategy provided the complexes in an efficient and ecologically friendly manner. The resulting palladacycles are fully characterized using nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy and, in one case, by single-crystal X-ray diffraction (XRD). The solvent-free reactions are additionally analyzed by powder XRD. The pincer complexes exhibit remarkable cytotoxicity against several solid and blood cancer cell lines, including human colorectal carcinoma (HCT116), breast cancer (MCF7), prostate adenocarcinoma (PC3), chronic myelogenous leukemia (K562), multiple plasmacytoma (AMO1), and acute lymphoblastic leukemia (H9), with the dimethylamino-substituted derivative being particularly effective. The latter also induced an appreciable level of apoptosis in both parental and doxorubicin-resistant cells K562 and K562/iS9, vindicating the high anticancer potential of this type of palladacycles.

Dipyrromethane-Based PGeP Pincer Germyl Rhodium Complexes.

A family of germyl rhodium complexes derived from the PGeP germylene 2,2'-bis(di-isopropylphosphanylmethyl)-5,5'-dimethyldipyrromethane-1,1'-diylgermanium(II), Ge(pyrmPi Pr2 )2 CMe2 (1), has been prepared. Germylene 1 reacted readily with [RhCl(PPh3 )3 ] and [RhCl(cod)(PPh3 )] (cod=1,5-cyclooctadiene) to give, in both cases, the PGeP-pincer chloridogermyl rhodium(I) derivative [Rh{κ3 P,Ge,P-GeCl(pyrmPi Pr2 )2 CMe2 }(PPh3 )] (2). Similarly, the reaction of 1 with [RhCl(cod)(MeCN)] afforded [Rh{κ3 P,Ge,P-GeCl(pyrmPi Pr2 )2 CMe2 }(MeCN)] (3). The methoxidogermyl and methylgermyl rhodium(I) complexes [Rh{κ3 P,Ge,P-GeR(pyrmPi Pr2 )2 CMe2 }(PPh3 )] (R=OMe, 4; Me, 5) were prepared by treating complex 2 with LiOMe and LiMe, respectively. Complex 5 readily reacted with CO to give the carbonyl rhodium(I) derivative [Rh{κ3 P,Ge,P-GeR(pyrmPi Pr2 )2 CMe2 }(CO)] (6), with HCl, HSnPh3 and Ph2 S2 rendering the pentacoordinate methylgermyl rhodium(III) complexes [RhHX{κ3 P,Ge,P-GeMe(pyrmPi Pr2 )2 CMe2 }] (X=Cl, 7; SnPh3 , 8) and [Rh(SPh)2 {κ3 P,Ge,P-GeMe(pyrmPi Pr2 )2 CMe2 }] (9), respectively, and with H2 to give the hexacoordinate derivative [RhH2 {κ3 P,Ge,P-GeMe(pyrmPi Pr2 )2 CMe2 }(PPh3 )] (10). Complexes 3 and 5 are catalyst precursors for the hydroboration of styrene, 4-vinyltoluene and 4-vinylfluorobenzene with catecholborane under mild conditions.

A High-Valent Ru-PCP Pincer Catalyst for Hydrogenation of Carbonyl and Carboxyl Compounds under Molecular Hydrogen.

Low-valent metals traditionally dominate the domain of catalytic hydrogenation. However, metal-ligand cooperating (MLC) catalytic systems, operating through heterolytic H-H bond splitting by a Lewis acidic metal and a basic ligand site, do not require an electron-rich metal. On the contrary, high-valent metals that induce weaker back donation facilitate heterolytic bond activation. Here we report, for the first time, the efficient hydrogenation of carbonyl and carboxyl compounds under molecular hydrogen catalyzed by a structurally well-defined RuIV catalyst bearing a bifunctional PCP pincer ligand. The catalyst exhibits reactivity toward molecular hydrogen superior to that of the low-valent analog and allows hydrogen activation even at room temperature.

Atom Swapping on Aromatic Rings: Conversion from Phosphinine Pincer Metal Complexes to Metallabenzenes Triggered by O2 Oxidation.

Herein, we report a novel method for the synthesis of metallabenzenes by swapping the phosphorus atom in an aromatic phosphinine ring with transition metal fragments. The oxidation of a phosphine-phosphinine-phosphine pincer iridium complex by O2 triggered the replacement of the phosphorus atom of the phosphinine ring by an iridium fragment to afford iridabenzene. Dianionic rhodabenzene was also synthesized from a phosphinine rhodium complex by oxidation of the phosphorus atom, followed by subsequent reduction using metallic potassium. The aromaticity of the newly synthesized irida- and rhoda-benzenes was evaluated both experimentally and theoretically.

Reactions of Late First-Row Transition Metal (Fe-Zn) Dichlorides with a PGeP Pincer Germylene.

The reactivity of the PGeP germylene 2,2'-bis(di-isopropylphosphanylmethyl)-5,5'-dimethyldipyrromethane-1,1'-diylgermanium(II), Ge(pyrmPiPr2 )2 CMe2 , with late first-row transition metal (Fe-Zn) dichlorides has been investigated. All reactions led to PGeP pincer chloridogermyl complexes. The reactions with FeCl2 and CoCl2 afforded paramagnetic square planar complexes of formula [MCl{κ3 P,Ge,P-GeCl(pyrmPiPr2 )2 CMe2 }] (M=Fe, Co). While the iron complex maintained an intermediate spin state (S1 ; µeff =3.0 µB ) over the temperature range 50-380 K, the effective magnetic moment of the cobalt complex varied linearly with temperature from 1.9 µB at 10 K to 3.6 µB at 380 K, indicating a spin crossover behavior that involves S1/2 (predominant at T<180 K) and S3/2 (predominant at T>200 K) species. Both cobalt(II) species were detected by electron paramagnetic resonance at T<20 K. The reaction of Ge(pyrmPiPr2 )2 CMe2 with [NiCl2 (dme)] (dme=dimethoxyethane) gave a square planar nickel(II) complex, [NiCl{κ3 P,Ge,P-GeCl(pyrmPiPr2 )2 CMe2 }], whereas the reaction with CuCl2 involved a redox process that rendered a mixture of the germanium(IV) compound GeCl2 (pyrmPiPr2 )2 CMe2 and a binuclear copper(I) complex, [Cu2 {µ-κ3 P,Ge,P-GeCl(pyrmPiPr2 )2 CMe2 }2 ], whose metal atoms are in tetrahedral environments. The reaction of the germylene with ZnCl2 led to the tetrahedral derivative [ZnCl{κ3 P,Ge,P-GeCl(pyrmPiPr2 )2 CMe2 }].

Computational Study of Bridge Splitting, Aryl Halide Oxidative Addition to PtII , and Reductive Elimination from PtIV : Route to Pincer-PtII Reagents with Chemical and Biological Applications.

Density functional theory computation indicates that bridge splitting of [PtII R2 (µ-SEt2 )]2 proceeds by partial dissociation to form R2 Pta (µ-SEt2 )Ptb R2 (SEt2 ), followed by coordination of N-donor bromoarenes (L-Br) at Pta leading to release of Ptb R2 (SEt2 ), which reacts with a second molecule of L-Br, providing two molecules of PtR2 (SEt2 )(L-Br-N). For R=4-tolyl (Tol), L-Br=2,6-(pzCH2 )2 C6 H3 Br (pz=pyrazol-1-yl) and 2,6-(Me2 NCH2 )2 C6 H3 Br, subsequent oxidative addition assisted by intramolecular N-donor coordination via PtII Tol2 (L-N,Br) and reductive elimination from PtIV intermediates gives mer-PtII (L-N,C,N)Br and Tol2 . The strong σ-donor influence of Tol groups results in subtle differences in oxidative addition mechanisms when compared with related aryl halide oxidative addition to palladium(II) centres. For R=Me and L-Br=2,6-(pzCH2 )2 C6 H3 Br, a stable PtIV product, fac-PtIV Me2 {2,6-(pzCH2 )2 C6 H3 -N,C,N)Br is predicted, as reported experimentally, acting as a model for undetected and unstable PtIV Tol2 {L-N,C,N}Br undergoing facile Tol2 reductive elimination. The mechanisms reported herein enable the synthesis of PtII pincer reagents with applications in materials and bio-organometallic chemistry.

Synthesis and Characterization of Cobalt NCN Pincer Complexes.

A series of cobalt complexes, stabilized by a monoanionic tridentate NCN pincer ligand, was synthetized and characterized. Preparation of the paramagnetic 15 VE complex [Co(NCNCH2-Et)Br] (1) was accomplished by transmetalation of Li[2,6-(Et2NCH2)2C6H3] with CoBr2 in THF. Treatment of this air-sensitive compound with NO gas resulted in the formation of the diamagnetic Co(III) species [Co(NCNCH2-Et)(NO)Br] (2) as confirmed by X-ray diffraction. This complex features a strongly bent NO ligand (Co-N-O∠135.0°). The νNO is observed at 1609 cm-1 which is typical for a bent metal-N-O arrangement. Coordinatively unsaturated 1 could further be treated with pyridine, isocyanides, phosphines and CO to form five-coordinate 17 VE complexes. Oxidation of 1 with CuBr2 led to the formation of the Co(III) complex [Co(NCNCH2-Et)Br2]. Treatment of [Co(NCNCH2-Et)Br2] with TlBF4 as halide scavenger in acetonitrile led to the formation of the cationic octahedral complex [Co(NCNCH2-Et)(MeCN)3](BF4)2. A combination of X-ray crystallography, IR-, NMR- and EPR-spectroscopy as well as DFT/CAS-SCF calculations were used to characterize all compounds.

Direct Arylation in the Presence of Palladium Pincer Complexes.

Direct arylation is an atom-economical alternative to more established procedures such as Stille, Suzuki or Negishi arylation reactions. In comparison with other palladium sources and ligands, the use of palladium pincer complexes as catalysts or pre-catalysts for direct arylation has resulted in improved efficiency, higher reaction yields, and advantageous reaction conditions. In addition to a revision of the literature concerning intra- and intermolecular direct arylation reactions performed in the presence of palladium pincer complexes, the role of these remarkably active catalysts will also be discussed.

A Comparative Study on the Thermodynamics of Halogen Bonding of Group†10 Pincer Fluoride Complexes.

The thermodynamics of halogen bonding of a series of isostructural Group 10 metal pincer fluoride complexes of the type [(3,5-R2 -tBu POCOPtBu )MF] (3,5-R2 -tBu POCOPtBu =κ3 -C6 HR2 -2,6-(OPtBu2 )2 with R=H, tBu, COOMe; M=Ni, Pd, Pt) and iodopentafluorobenzene was investigated. Based on NMR experiments at different temperatures, all complexes 1-tBu (R=tBu, M=Ni), 2-H (R=H, M=Pd), 2-tBu (R=tBu, M=Pd), 2-COOMe (R=COOMe, M=Pd) and 3-tBu (R=tBu, M=Pt) form strong halogen bonds with Pd complexes showing significantly stronger binding to iodopentafluorobenzene. Structural and computational analysis of a model adduct of complex 2-tBu with 1,4-diiodotetrafluorobenzene as well as of structures of iodopentafluorobenzene in toluene solution shows that formation of a type I contact occurs.

A Reversible Liquid-to-Liquid Organic Hydrogen Carrier System Based on Ethylene Glycol and Ethanol.

Liquid organic hydrogen carriers (LOHCs) are powerful systems for the efficient unloading and loading molecular hydrogen. Herein, a liquid-to-liquid organic hydrogen carrier system based on reversible dehydrogenative coupling of ethylene glycol (EG) with ethanol catalysed by ruthenium pincer complexes is reported. Noticeable advantages of the current LOHC system is that both reactants (hydrogen-rich components) and the produced esters (hydrogen-lean components) are liquids at room temperature, and the dehydrogenation process can be performed under solvent and base-free conditions. Moreover, the hydrogenation reaction proceeds under low hydrogen pressure (5 bar), and the LOHC system has a relatively high theoretical gravimetric hydrogen storage capacity (HSC>5.0 wt %), presenting an attractive hydrogen storage system.

Cyclometalated Ruthenium Pincer Complexes as Catalysts for the α-Alkylation of Ketones with Alcohols.

Ruthenium PNP pincer complexes bearing supplementary cyclometalated C,N-bound ligands have been prepared and fully characterized for the first time. By replacing CO and H- as ancillary ligands in such complexes, additional electronic and steric modifications of this topical class of catalysts are possible. The advantages of the new catalysts are demonstrated in the general α-alkylation of ketones with alcohols following a hydrogen autotransfer protocol. Herein, various aliphatic and benzylic alcohols were applied as green alkylating agents for ketones bearing aromatic, heteroaromatic or aliphatic substituents as well as cyclic ones. Mechanistic investigations revealed that during catalysis, Ru carboxylate complexes are predominantly formed whereas neither the PNP nor the CN ligand are released from the catalyst in significant amounts.

Terminal Alkyne Coupling Reactions Through a Ring: Effect of Ring Size on Rate and Regioselectivity.

Terminal alkyne coupling reactions promoted by rhodium(I) complexes of macrocyclic NHC-based pincer ligands-which feature dodecamethylene, tetradecamethylene or hexadecamethylene wingtip linkers viz. [Rh(CNC-n)(C2 H4 )][BArF 4 ] (n=12, 14, 16; ArF =3,5-(CF3 )2 C6 H3 )-have been investigated, using the bulky alkynes HC≡CtBu and HC≡CAr' (Ar'=3,5-tBu2 C6 H3 ) as substrates. These stoichiometric reactions proceed with formation of rhodium(III) alkynyl alkenyl derivatives and produce rhodium(I) complexes of conjugated 1,3-enynes by C-C bond reductive elimination through the annulus of the ancillary ligand. The intermediates are formed with orthogonal regioselectivity, with E-alkenyl complexes derived from HC≡CtBu and gem-alkenyl complexes derived from HC≡CAr', and the reductive elimination step is appreciably affected by the ring size of the macrocycle. For the homocoupling of HC≡CtBu, E-tBuC≡CCH=CHtBu is produced via direct reductive elimination from the corresponding rhodium(III) alkynyl E-alkenyl derivatives with increasing efficacy as the ring is expanded. In contrast, direct reductive elimination of Ar'C≡CC(=CH2 )Ar' is encumbered relative to head-to-head coupling of HC≡CAr' and it is only with the largest macrocyclic ligand studied that the two processes are competitive. These results showcase how macrocyclic ligands can be used to interrogate the mechanism and tune the outcome of terminal alkyne coupling reactions, and are discussed with reference to catalytic reactions mediated by the acyclic homologue [Rh(CNC-Me)(C2 H4 )][BArF 4 ] and solvent effects.

Synthesis and Photophysical Properties of T-Shaped Coinage-Metal Complexes.

The photophysical properties of a series of T-shaped coinage d10 metal complexes, supported by a bis(mesoionic carbene)carbazolide (CNC) pincer ligand, are explored. The series includes a rare new example of a tridentate T-shaped AgI complex. Post-complexation modification of the AuI complex provides access to a linear cationic AuI complex following ligand alkylation, or the first example of a cationic square planar AuIII -F complex from electrophilic attack on the metal centre. Emissions ranging from blue (CuI ) to orange (AgI ) are obtained, with variable contributions of thermally-dependent fluorescence and phosphorescence to the observed photoluminescence. Green emissions are observed for all three gold complexes (neutral T-shaped AuI , cationic linear AuI and square planar cationic AuIII ). The higher quantum yield and longer decay lifetime of the linear gold(I) complex are indicative of increased phosphorescence contribution.

(Electro-)chemical Splitting of Dinitrogen with a Rhenium Pincer Complex.

The splitting of N2 into well-defined terminal nitride complexes is a key reaction for nitrogen fixation at ambient conditions. In continuation of our previous work on rhenium pincer mediated N2 splitting, nitrogen activation and cleavage upon (electro)chemical reduction of [ReCl2(L2)] {L2 = N(CHCHPtBu2)2 -} is reported. The electrochemical characterization of [ReCl2(L2)] and comparison with our previously reported platform [ReCl2(L1)] {L1 = N(CH2CH2PtBu2)2 -} provides mechanistic insight to rationalize the dependence of nitride yield on the reductant. Furthermore, the reactivity of N2 derived nitride complex [Re(N)Cl(L2)] with electrophiles is presented.