Nitrous oxide activation by picoline-derived Ni-CNP hydrides.

Oxygen atom transfer (OAT) from N2O to the Ni-H bond of proton-responsive picoline-derived CNP nickel complexes has been investigated both experimentally and theoretically. These Ni-CNP complexes efficiently catalyse the reduction of N2O with pinacolborane (HBpin) under mild conditions.

Synthesis and characterization of enantiopure chiral NH2/SO palladium complexes.

A series of enantiopure chiral NH2/SO palladium complexes have been synthesised with high yields by treating the corresponding tert-butylsulfinamide/sulfoxide derivatives with Pd(CH3CN)2Cl2. The enantiopure chiral ligands were prepared by stereoselective addition of tert-butyl or phenyl methylsulfinyl carbanions to different tert-butylsulfinylimines. In all cases, coordination occurs with concomitant desulfinylation. X-ray studies of the Pd complexes showed a higher trans influence of the phenylsulfinyl group in comparison to that of the tert-butylsulfinyl group. Furthermore, we have obtained and characterised two possible palladium amine/sulfonyl complexes, epimers at sulfur, resulting from N-desulfinylation and coordination of palladium with both oxygens of the prochiral sulfonyl group. The catalytic activity and enantioselectivity of the new Pd(II) complexes of acetylated amine/tert-butyl- and phenylsulfoxides in the carboxylated cyclopropanes arylation reaction has been studied, obtaining the best results with the phenylsulfoxide ligand 25(SC,SS) that produced the final arylated product in a 93 :7 enantiomeric ratio.

Catalytic Nitrous Oxide Reduction with H2 Mediated by Pincer Ir Complexes.

Reduction of nitrous oxide (N2O) with H2 to N2 and water is an attractive process for the decomposition of this greenhouse gas to environmentally benign species. Herein, a series of iridium complexes based on proton-responsive pincer ligands (1-4) are shown to catalyze the hydrogenation of N2O under mild conditions (2 bar H2/N2O (1:1), 30 °C). Among the tested catalysts, the Ir complex 4, based on a lutidine-derived CNP pincer ligand having nonequivalent phosphine and N-heterocyclic carbene (NHC) side donors, gave rise to the highest catalytic activity (turnover frequency (TOF) = 11.9 h-1 at 30 °C, and 16.4 h-1 at 55 °C). Insights into the reaction mechanism with 4 have been obtained through NMR spectroscopy. Thus, reaction of 4 with N2O in tetrahydrofuran-d8 (THF-d8) initially produces deprotonated (at the NHC arm) species 5NHC, which readily reacts with H2 to regenerate the trihydride complex 4. However, prolonged exposure of 4 to N2O for 6 h yields the dinitrogen Ir(I) complex 7P, having a deprotonated (at the P-arm) pincer ligand. Complex 7P is a poor catalytic precursor in the N2O hydrogenation, pointing out to the formation of 7P as a catalyst deactivation pathway. Moreover, when the reaction of 4 with N2O is carried out in wet THF-d8, formation of a new species, which has been assigned to the hydroxo species 8, is observed. Finally, taking into account the experimental results, density functional theory (DFT) calculations were performed to get information on the catalytic cycle steps. Calculations are in agreement with 4 as the TOF-determining intermediate (TDI) and the transfer of an apical hydrido ligand to the terminal nitrogen atom of N2O as the TOF-determining transition state (TDTS), with very similar reaction rates for the mechanisms involving either the NHC- or the P-CH2 pincer methylene linkers.

Enantioselective synthesis of 4-amino-3,4-dihydrocoumarins and their non-cyclic hydroxyester precursors: Biological evaluation for the treatment of glioblastoma multiforme.

The stereoselective addition of ethyl acetate enolate to the C═N bond of N-tert-butylsulfinylimines has been investigated in depth. A significant effect of the LHMDS amount and the N-sulfinylimine nature on the stereoselectivity of the process was observed. Conditions were found where sulfinylimines of differently substituted salicylaldehydes derivatives, ethyl acetate, and LHMDS afforded the corresponding addition products as a single diastereomer in good yields. The developed protocol was successfully applied to the first stereoselective synthesis of differently substituted 4-amino-3,4-dihydrocoumarin derivatives. Computational models confirmed the prominent role of the ortho aryl substituent in the stereoselectivity of the process. A significant and selective cytotoxic activity against Glioblastoma Multiforme (GBM) cancer line has been determined for the noncyclic hydroxy ester derivative.

Chirality influence on the cytotoxic properties of anionic chiral bis(N-heterocyclic carbene)silver complexes.

Complexes Na3[Ag(NHCR)2], 2a-e and 2b'-c', where NHCR is a N-heterocyclic carbene of the 2,2'-(1H-2λ3,3λ4-imidazole-1,3-diyl)dicarboxylate type, were prepared by treatment of compounds HLR, 1a-e and 1b'-c' (2-(1-(carboxyalkyl)-1H-imidazol-3-ium-3-yl)carboxylate), with silver oxide in the presence of aqueous sodium hydroxide. They were characterized by analytical, spectroscopic (infrared, IR, 1H and 13C nuclear magnetic resonance, NMR, and circular dichroism) and X-ray methods (2a). In the solid state, the anionic part of complex 2a, [Ag(NHCH)2]3-, shows a linear disposition of Ccarbene-Ag-Ccarbene atoms and an eclipsed conformation of the two NHC ligands. The proposed bis(NHC) nature of the silver complexes was maintained in solution according to NMR and density functional theory (DFT) calculations. The cytotoxic activity of compounds 2 was evaluated against four cancer cell lines and one non-cancerous cell line and several structure-activity correlations were found for these complexes. For instance, the activity decreased when the bulkiness of the R alkyl group in Na3[Ag(NHCR)2] increased. More interesting is the detected chirality-anticancer relationship, where complexes Na3[Ag{(S,S)-NHCR}2] (R = Me, 2b; iPr, 2c) showed better anticancer activity than those of their enantiomeric derivatives Na3[Ag{(R,R)-NHCR}2] (R = Me, 2b'; iPr, 2c').

Antimicrobial Properties of Amino-Acid-Derived N-Heterocyclic Carbene Silver Complexes.

Complexes {Ag[NHCMes,R]}n (R = H, 2a; Me, 2b and 2b'; iPr, 2c; iBu, 2d), were prepared by treatment of imidazolium precursor compounds [ImMes,R] (2-(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1a, (S)-2-alkyl(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1b-d, and (R)-2-methyl(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1b', with Ag2O under appropriate conditions. They were characterised by analytical, spectroscopic (IR, 1H, and 13C NMR and polarimetry), and X-ray methods (2a). In the solid state, 2a is a one-dimensional coordination polymer, in which the silver(I) cation is bonded to the carbene ligand and to the carboxylate group of a symmetry-related Ag[NHCMes,H] moiety. The coordination environment of the silver centre is well described by the DFT study of the dimeric model {Ag[NHCMes,H]}2. The antimicrobial properties of these complexes were evaluated versus Gram-negative bacteria E. coli and P. aeruginosa. From the observed MIC and MBC values (minimal inhibitory concentration and minimal bactericidal concentration, respectively), complex 2b' showed the best antimicrobial properties (eutomer), which were significantly better than those of its enantiomeric derivative 2b (distomer). Additionally, analysis of MIC and MBC values of 2a-d reveal a clear structure-antimicrobial effect relationship. Antimicrobial activity decreases when the steric properties of the R alkyl group in {Ag[NHCMes,R]}n increase.

Electrophilic activation of alkynes promoted by a cationic alkylidene complex of Pt(II).

Pt(II) alkylidene 1a has been reacted with terminal alkynes to afford ylide complexes 3a-d, resulting from electrophilic activation of the CC bond and its insertion into the platinacyclic fragment of 1a that contains the carbene functionality. DFT calculations indicate that the observed regioselectivity is determined by the nucleophilic attack of the alkyne to the alkylidene carbon.

Homochiral imidazolium-based dicarboxylate silver(I) compounds: synthesis, characterisation and antimicrobial properties.

Complexes [Ag(LR)], 2 (LR = 2,2'-(imidazolium-1,3-diyl)di(2-alkylacetate)), were prepared by treatment of compounds HLR, 1, with Ag2O. They were characterised by analytical, spectroscopic (IR, 1H and 13C NMR and polarimetry) and X-ray methods (2c, 2c' and 2e). In the solid state, these compounds are novel one-dimensional or two-dimensional coordination polymers in which silver(I) cations are connected via the chiral [LR]- anion with unprecedented coordination modes. The antimicrobial properties of these complexes were evaluated. 2a and 2b' showed the best antimicrobial properties (minimal inhibitory concentrations and minimal bactericidal concentration) for Pseudomonas aeruginosa and Escherichia coli pathogens. Eutomers 2b' and 2c' showed slightly better antimicrobial properties than their respective enantiomers 2b and 2c.

Ammonia-Borane Dehydrogenation Catalyzed by Dual-Mode Proton-Responsive Ir-CNNH Complexes.

Metal complexes incorporating proton-responsive ligands have been proved to be superior catalysts in reactions involving the H2 molecule. In this contribution, a series of IrIII complexes based on lutidine-derived CNNH pincers containing N-heterocyclic carbene and secondary amino NHR [R = Ph (4a), tBu (4b), benzyl (4c)] donors as flanking groups have been synthesized and tested in the dehydrogenation of ammonia-borane (NH3BH3, AB) in the presence of substoichiometric amounts (2.5 equiv) of tBuOK. These preactivated derivatives are efficient catalysts in AB dehydrogenation in THF at room temperature, albeit significantly different reaction rates were observed. Thus, by using 0.4 mol % of 4a, 1.0 equiv of H2 per mole of AB was released in 8.5 min (turnover frequency (TOF50%) = 1875 h-1), while complexes 4b and 4c (0.8 mol %) exhibited lower catalytic activities (TOF50% = 55-60 h-1). 4a is currently the best performing IrIII homogeneous catalyst for AB dehydrogenation. Kinetic rate measurements show a zero-order dependence with respect to AB, and first order with the catalyst in the dehydrogenation with 4a (-d[AB]/dt = k[4a]). Conversely, the reaction with 4b is second order in AB and first order in the catalyst (-d[AB]/dt = k[4b][AB]2). Moreover, the reactions of the derivatives 4a and 4b with an excess of tBuOK (2.5 equiv) have been analyzed through NMR spectroscopy. For the former precursor, formation of the iridate 5 was observed as a result of a double deprotonation at the amine and the NHC pincer arm. In marked contrast, in the case of 4b, a monodeprotonated (at the pincer NHC-arm) species 6 is observed upon reaction with tBuOK. Complex 6 is capable of activating H2 reversibly to yield the trihydride derivative 7. Finally, DFT calculations of the first AB dehydrogenation step catalyzed by 5 has been performed at the DFT//MN15 level of theory in order to get information on the predominant metal-ligand cooperation mode.

Zero-valent ML2 complexes of group 10 metals supported by terphenyl phosphanes.

Bulky terphenyl phosphane ligands PMe2Ar' (Ar' = terphenyl group) facilitate the isolation of zero-valent bis-phosphane complexes of nickel, palladium and platinum. The former show coordination numbers greater than two in the solid state due to the existence of Ni-Carene interactions with the terphenyl fragment.

Aerobic intramolecular carbon-hydrogen bond oxidation promoted by Cu(I) complexes.

The oxidation of C-H bonds by copper centres in enzymes with molecular oxygen takes place in nature under ambient conditions. Herein we report a similar transformation in which under ambient pressure and temperature (1 atm, 25 °C) the complex TpMsCu(THF) (TpMs = hydrotris(3-mesityl-pyrazol-1-yl)borate) undergoes the intramolecular oxidation of an alkylic C-H bond with O2, leading to the formation of a trinuclear compound where alkoxy and hydroxyl ligands are bonded to the copper centres, as inferred from X-ray studies. The presence of adventitious Cu(0) derived from the partial decomposition of initial TpMsCu(THF) facilitates the formation of such a trinuclear compound. DFT studies support the reaction taking place through a Cu(iii) alkoxy-hydroxyl copper intermediate.

Hydrogenation/dehydrogenation of N-heterocycles catalyzed by ruthenium complexes based on multimodal proton-responsive CNN(H) pincer ligands.

Ru complexes based on lutidine-derived pincer CNN(H) ligands having secondary amine side donors are efficient precatalysts in the hydrogenation and dehydrogenation of N-heterocycles. Reaction of a Ru-CNN(H) complex with an excess of base produces the formation of a Ru(0) derivative, which is observed under catalytic conditions.

Synthesis, Structure, Reactivity and Catalytic Implications of a Cationic, Acetylide-Bridged Trigold-JohnPhos Species.

The cationic complex [(JohnPhos-Au)3 (acetylide)][SbF6 ] (JohnPhos=(2-biphenyl)di-tert-butylphosphine, L1) has been characterised structurally and features an acetylide-trigold(I)-JohnPhos system; the trinuclear-acetylide unit, coordinated to the monodentate bulk phosphines, adopts an unprecedented µ,η1 ,η2 ,η1 coordination mode with an additional interaction between distal phenyl rings and gold centres. Other cationic σ,π-[(gold(I)L1)2 ] complexes have also been isolated. The reaction of trimethylsilylacetylene with various alcohols (iPrOH, nBuOH, n-HexOH) catalysed by cationic [AuI L1][SbF6 ] complexes in CH2 Cl2 at 50 °C led to the formation of acetaldehyde acetals with a high degree of chemo- and regioselectivity. The reaction mechanism was studied, and several organic and inorganic intermediates have been characterised. A comparative study with the analogous cationic [CuI L1][PF6 ] complex revealed different behaviour; the copper metal is lost from the coordination sphere leading to the formation of cationic vinylphosphonium and copper nanoparticles. Additionally, a new catalytic approach for the formation of this high-value cationic vinylphosphonium has been established.

Ultrasound-assisted one-pot green synthesis of new N- substituted-5-arylidene-thiazolidine-2,4-dione-isoxazoline derivatives using NaCl/Oxone/Na3PO4 in aqueous media.

A rapid and green method for the synthesis of novel N-thiazolidine-2,4-dione isoxazoline derivatives 5 from N-allyl-5-arylidenethiazolidine-2,4-diones 3 as dipolarophiles with arylnitrile oxides via 1,3-dipolar cycloaddition reaction. The corresponding N-allyl substituted dipolarophiles were prepared by one-pot method from thiazolidine-2,4-dione with aldehydes using Knoevenagel condensation followed by N-allylation of thiazolidine-2,4-dione in NaOH aqueous solution under sonication. In addition, the isoxazoline derivatives 5 were synthesized by regioselective and chemoselective 1,3-dipolar cycloaddition using inexpensive and mild NaCl/Oxone/Na3PO4 as a Cl source, oxidant and/or catalyst under ultrasonic irradiation in EtOH/H2O (v/v, 2:1) as green solvent. All synthesized products are furnished in good yields in the short reaction time, and then their structures were confirmed by NMR, mass spectrometry and X-ray crystallography analysis.

A Versatile Approach to Access Trimetallic Complexes Based on Trisphosphinite Ligands.

A straightforward method for the preparation of trisphosphinite ligands in one step, using only commercially available reagents (1,1,1-tris(4-hydroxyphenyl)ethane and chlorophosphines) is described. We have made use of this approach to prepare a small family of four trisphosphinite ligands of formula [CH3C{(C6H4OR2)3], where R stands for Ph (1a), Xyl (1b, Xyl = 2,6-Me2-C6H3), iPr (1c), and Cy (1d). These polyfunctional phosphinites allowed us to investigate their coordination chemistry towards a range of late transition metal precursors. As such, we report here the isolation and full characterization of a number of Au(I), Ag(I), Cu(I), Ir(III), Rh(III) and Ru(II) homotrimetallic complexes, including the structural characterization by X-ray diffraction studies of six of these compounds. We have observed that the flexibility of these trisphosphinites enables a variety of conformations for the different trimetallic species.

Neutral, cationic and anionic organonickel and -palladium complexes supported by iminophosphine/phosphinoenaminato ligands.

We report a series of organometallic nickel and palladium complexes containing iminophosphine ligands R2PCH2C(Ph) = N-Dipp (Dipp = 2,6-diisopropylphenyl; R = iPr, La; R = Ph, Lb; and R = o-C6H4OMe, Lc), synthesized by ligand exchange or oxidative addition reactions, and we investigate the capacity of such ligands to undergo reversible deprotonation to the corresponding phosphinoenaminato species. In the attempted ligand exchange reaction of the nickel bis(trimethylsilyl)methyl precursor [Ni(CH2SiMe3)2Py2] with Lb, the iminophosphine acts as a weak acid rather than a neutral ligand, cleaving one of the Ni-C bonds, to afford the phosphinoenaminato complex [Ni(CH2SiMe3)(L'b)(Py)] (L'b = conjugate base of Lb). We disclose a general method for the syntheses of complexes [Ni(CH2SiMe3)(L)(Py)]+ (L = La, Lb or Lc), and demonstrate that iminophosphine deprotonation is a general feature and occurs reversibly in the coordination sphere of the metal. By studying proton exchange reactions of the cation [Ni(CH2SiMe3)(Lb)(Py)]+ with bases of different strength we show that the conjugate phosphinoenaminato ligand in [Ni(CH2SiMe3)(L'b)(Py)] is a base with strength comparable to DBU in THF. The acyl group in the functionalized aryl complex [Ni(p-C6H4COCH3)(Br)(La)] does not interfere in the iminophosphine deprotonation with NaH. The latter reaction affords the unusual anionic hydroxide species [Ni(p-C6H4COCH3)(OH)(L'a)]-Na+, which was isolated and fully characterized.

Steric Tuning of Sulfinamide/Sulfoxides as Chiral Ligands with C1, Pseudo-meso, and Pseudo-C2 Symmetries: Application in Rhodium(I)-Mediated Arylation.

A new family of sulfinamide/sulfoxide derivatives was synthesized as chiral bidentate ligands by stereoselective additions of methylsulfinyl carbanions to N-tert-butylsulfinylimines. The new ligands, with C1, pseudo-meso, and pseudo-C2 symmetries, were successfully assayed in Rh-catalyzed additions of arylboronic acids to activated ketones. The sterically dissymmetric C1 ligand (RS,SC,RS)-N-[1-(phenylsulfinyl)-3-methylbut-2-yl] tert-butylsulfinamide turned out to be the optimal one, allowing the 1,4-additions of diverse arylboronic acids, on different α,ß-unsaturated cyclic ketones with high chemical yields and enantioselectivities up to >99% ee.

Aluminium(iii) dialkyl 2,6-bisimino-4R-dihydropyridinates(-1): selective synthesis, structure and controlled dimerization.

A family of stable and otherwise selectively unachievable 2,6-bisimino-4-R-1,4-dihydropyridinate aluminium (III) dialkyl complexes [AlR'2(4-R-iPrBIPH)] (R = Bn, Allyl; R' = Me, Et, iBu) have been synthesized, taking advantage of a method for the preparation of the corresponding 4-R-1,4-dihydropiridine precursors developed in our group. All the dihydropyrdinate(-1) dialkyl aluminium complexes have been fully characterized by 1H- 13C-NMR, elemental analysis and in the case 2'a, also by X-ray diffraction studies. Upon heating in toluene solution at 110 °C, the dimethyl derivatives 2a and 2'a dimerize selectively through a double cycloaddition. This reaction leads to the formation of two new C-C bonds that involve the both meta positions of the two 4-R-1,4-dihydropyridinate fragments, resulting the binuclear aluminium species [Me2Al(4-R-iPrHBIP)]2 (R = Bn (3a); allyl (3'a)). Experimental kinetics showed that the dimerization of 2'a obeys second order rate with negative activation entropy, which is consistent with a bimolecular rate-determining step. Controlled methanolysis of both 3a and 3'a release the metal-free dimeric bases, (4-Bn-iPrHBIPH)2 and (4-allyl-iPrHBIPH)2, providing a convenient route to these potentially useful ditopic ligands. When the R' groups are bulkier than Me (2b, 2'b and 2'c), the dimerization is hindered or fully disabled, favoring the formation of paramagnetic NMR-silent species, which have been identified on the basis of a controlled methanolysis of the final organometallic products. Thus, when a toluene solution of [AlEt2(4-Bn-iPrBIPH)] (2b) was heated at 110 °C, followed by the addition of methanol in excess, it yields a mixture of the dimer (4-Bn-iPrHBIPH)2 and the aromatized base 4-Bn-iPrBIP, in ca. 1 : 2 ratio, indicating that the dimerization of 2b competes with its spontaneous dehydrogenation, yielding a paramagnetic complex containing a AlEt2 unit and a non-innocent (4-Bn-iPrBIP)˙- radical-anion ligand. Similar NMR monitoring experiments on the thermal behavior of [AlEt2(4-allyl-iPrBIPH)] (2'b) and [AliBu2(4-allyl-iPrBIPH)] (2'c) showed that these complexes do not dimerize, but afford exclusively NMR silent products. When such thermally treated samples were subjected to methanolysis, they resulted in mixtures of the alkylated 4-allyl-iPrBIP and non-alkylated iPrBIP ligand, suggesting that dehydrogenation and deallylation reactions take place competitively.

Synthesis and structural characterization of homochiral coordination polymers with imidazole-based monocarboxylate ligands.

Chiral Na[(S)-LR] (R = Me, 1a; iPr, 1b; CH2iPr, 1c, and (S)-secBu, 1d) and Na[(R)-LR] (R = Me, 1a') compounds were synthesised following standard procedures. New compounds 1d and 1a' were analytically and spectroscopically characterised. 1a and 1c were structurally identified by single-crystal X-ray diffraction methods as homochiral 2D coordination polymers, {Na(H2O)[(S)-LMe]}n and {Na[(S)-LCH2iPr]}n, respectively. Both (S)-2alkyl,2-(1H-imidazol-1-yl)acetate anions displayed unprecedented coordination modes in these coordination polymers: µ3κ2OκO' for 1a and µ4κ2Oκ2O' for 1c. Enantiomeric species 1a', {Na(H2O)[(R)-LMe]}n, showed the same X-ray powder diffractogram (XRPD) as 1a, in agreement with a similar crystal structure. DFT calculations on the [LR]- anions confirmed their coordination capabilities as ditopic linkers. In fact, the reaction of Na[LR] with several metal salts yielded the following coordination polymers: {Ag[(S)-LMe]}n, 2a, {Ag[(R)-LMe]}n, 2a', {Cu[(S)-LR]2}n (R = Me, 3a; iPr, 3b), {Cu[(R)-LMe]}n, 3a', {Zn[(S)-LR]2}n (R = Me, 4a; iPr, 4b; (S)-secBu, 4d) and {Zn[(R)-LMe]2}n, 4a'. For the known compounds 3a and 4a, this procedure is a new synthetic route that avoided high temperature reaction conditions. New complexes 2, 3a', b, and 4a', b, d were characterised by elemental analysis, infrared and XRPD methods and complex 2a by single-crystal X-ray diffraction. This complex is also a two-dimensional coordination polymer in which the [(S)-LMe]- anion acts as a µ4κN,κ2O,κ2O' bridging ligand. Compounds 1-4a' are the first examples of homochiral coordination polymers with imidazole-monocarboxylate ligands based on non-natural amino acids. Preliminary studies on the metal-catalysed preparation of chiral α-aminophosphonates were carried out but, unfortunately, no enantioselectivity was observed.

Synthesis, Structure and Nickel Carbonyl Complexes of Dialkylterphenyl Phosphines.

The experimental and computational characterization of a series of dialkylterphenyl phosphines, PR2 Ar' is described. The new P-donors comprise five compounds of general formula PR2 Ar Dtbp 2 (R=Me, Et, iPr, c-C5 H9 and c-C6 H11 ); Ar Dtbp 2 = 2,6-C6 H3 -(3,5-C6 H3 -(CMe3 )2 )2 ), and another five PR2 Ar' phosphines containing the bulky alkyl groups iPr, c-C5 H9 or c-C6 H11 , in combination with Ar'=Ar Xyl 2 , Ar Xyl ' 2 , or Ar Ph 2 (L1-L10). Steric and electronic parameters have been determined computationally and from IR and X-ray data obtained for the phosphines and for some derivatives, including tricarbonyl and dicarbonyl nickel complexes, Ni(CO)3 (PR2 Ar') and Ni(CO)2 (PR2 Ar'). In the solid state, the free phosphines PR2 Ar' adopt one of the three possible structures formally related by rotation around the Cipso -P bond. Details on their relative energies and on the influence of the free phosphine structure on its coordination chemistry towards Ni(CO)n (n = 2, 3) fragments has been obtained by experimental and computational methods.