1,2-Azolylamidino ruthenium(II) complexes with DMSO ligands: electro- and photocatalysts for CO2 reduction.

New 1,2-azolylamidino complexes fac-[RuCl(DMSO)3(NHC(R)az*-κ2N,N)]OTf [R = Me (2), Ph (3); az* = pz (pyrazolyl, a), indz (indazolyl, b)] are synthesized via chloride abstraction from their corresponding precursors cis,fac-[RuCl2(DMSO)3(az*H)] (1) after subsequent base-catalyzed coupling of the appropriate nitrile with the 1,2-azole previously coordinated. All the compounds are characterized by 1H NMR, 13C NMR and IR spectroscopy. Those derived from MeCN are also characterized by X-ray diffraction. Electrochemical studies showed several reduction waves in the range of -1.5 to -3 V. The electrochemical behavior in CO2 media is consistent with CO2 electrocatalytic reduction. The catalytic activity expressed as [icat(CO2)/ip(Ar)] ranged from 1.7 to 3.7 for the 1,2-azolylamidino complexes at voltages of ca. -2.7 to -3 V vs. ferrocene/ferrocenium. Controlled potential electrolysis showed rapid decomposition of the Ru catalysts. Photocatalytic CO2 reduction experiments using compounds 1b, 2b and 3b carried out in a CO2-saturated MeCN/TEOA (4 : 1 v/v) solution containing a mixture of the catalyst and [Ru(bipy)3]2+ as the photosensitizer under continuous irradiation (light intensity of 150 mW cm-2 at 25 °C, λ > 300 nm) show that compounds 1b, 2b and 3b allowed CO2 reduction catalysis, producing CO and trace amounts of formate. The combined turnover number for the production of formate and CO is ca. 100 after 8 h and follows the order 1b < 2b ≈ 3b.

Cation- and Anion-Mediated Supramolecular Assembly of Bismuth and Antimony Tris(3-pyridyl) Complexes.

The use of antimony and bismuth in supramolecular chemistry has been largely overlooked in comparison to the lighter elements of Group 15, and the coordination chemistry of the tripodal ligands [Sb(3-py)3] and [Bi(3-py)3] (L) containing the heaviest p-block element bridgehead atoms has been unexplored. We show that these ligands form a common hybrid metal-organic framework (MOF) structure with Cu(I) and Ag(I) (M) salts of weakly coordinating anions (PF6-, SbF6-, and OTf-), composed of a cationic substructure of rhombic cage (M)4(L)4 units linked by Sb/Bi-M bonding. The greater Lewis acidity of Bi compared to Sb can, however, allows anion···Bi interactions to overcome Bi-metal bonding in the case of BF4-, leading to collapse of the MOF structure (which is also seen where harder metals like Li+ are employed). This study therefore provides insight into the way in which the electronic effects of the bridgehead atom in these ligand systems can impact their supramolecular chemistry.

Synthesis of tris(3-pyridyl)aluminate ligand and its unexpected stability against hydrolysis: revealing cooperativity effects in heterobimetallic pyridyl aluminates.

We report the elusive metallic anion [EtAl(3-py)3]- (3-py = 3-pyridyl) (1), the first member of the anionic tris(3-pyridyl) family. Unexpectedly, the lithium complex 1Li shows substantial protic stability against water and alcohols, unlike related tris(2-pyridyl)aluminate analogues. This stability appears to be related to the inability of the [EtAl(3-py)3]- anion to chelate Li+, which precludes a decomposition pathway involving Li/Al cooperativity.

Luminescent cis-Bis(bipyridyl)ruthenium(II) Complexes with 1,2-Azolylamidino Ligands: Photophysical, Electrochemical Studies, and Photocatalytic Oxidation of Thioethers.

New 1,2-azolylamidino complexes cis-[Ru(bipy)2(NH═C(R)az*-κ2N,N)](OTf)2 (R = Me, Ph; az* = pz, indz, dmpz) are synthesized via chloride abstraction after a subsequent base-catalyzed coupling of a nitrile with the previously coordinated 1,2-azole. The synthetic procedure allows the easy obtainment of complexes having different electronic and steric 1,2-azoylamidino ligands. All of the compounds have been characterized by 1H, 13C, and 15N NMR and IR spectroscopy and by monocrystal X-ray diffraction. Photophysical studies support their phosphorescence, whereas their electrochemistry reveals reversible RuII/RuIII oxidations between +1.13 and +1.25 V (vs SCE). The complexes have been successfully used as catalysts in the photooxidation of different thioethers, the complex cis-[Ru(bipy)2(NH═C(Me)dmpz-κ2N,N)]2+ showing better catalytic performance in comparison to that of [Ru(bipy)3]2+. Moreover, the significant catalytic performance of the dimethylpyrazolylamidino complex is applied to the preparation of the drug modafinil, which is obtained using ambient oxygen as an oxidant. Finally, mechanistic assays suggest that the oxidation reaction follows a photoredox route via oxygen radical anion formation.

(1,2-Azole)bis(bipyridyl)ruthenium(II) Complexes: Electrochemistry, Luminescent Properties, And Electro- And Photocatalysts for CO2 Reduction.

New cis-(1,2-azole)-aquo bis(2,2'-bipyridyl)ruthenium(II) (1,2-azole (az*H) = pzH (pyrazole), dmpzH (3,5-dimethylpyrazole), and indzH (indazole)) complexes are synthesized via chlorido abstraction from cis-[Ru(bipy)2Cl(az*H)]OTf. The latter are obtained from cis-[Ru(bipy)2Cl2] after the subsequent coordination of the 1,2-azole. All the compounds are characterized by 1H, 13C, 15N NMR spectroscopy as well as IR spectroscopy. Two chlorido complexes (pzH and indzH) and two aquo complexes (indzH and dmpzH) are also characterized by X-ray diffraction. Photophysical and electrochemical studies were carried out on all the complexes. The photophysical data support the phosphorescence of the complexes. The electrochemical behavior of all the complexes in an Ar atmosphere indicate that the oxidation processes assigned to Ru(II) → Ru(III) occurs at higher potentials in the aquo complexes. The reduction processes under Ar lead to several waves, indicating that the complexes undergo successive electron-transfer reductions that are centered in the bipy ligands. The first electron reduction is reversible. The electrochemical behavior in CO2 media is consistent with CO2 electrocatalyzed reduction, where the values of the catalytic activity [icat(CO2)/ip(Ar)] ranged from 2.9 to 10.8. Controlled potential electrolysis of the chlorido and aquo complexes affords CO and formic acid, with the latter as the major product after 2 h. Photocatalytic experiments in MeCN with [Ru(bipy)3]Cl2 as the photosensitizer and TEOA as the electron donor, which were irradiated with >300 nm light for 24 h, led to CO and HCOOH as the main reduction products, achieving a combined turnover number (TONCO+HCOO-) as high as 107 for 2c after 24 h of irradiation.

Experimental study of speciation and mechanistic implications when using chelating ligands in aryl-alkynyl Stille coupling.

Neutral palladium(ii) complexes [Pd(Rf)X(P-L)] (Rf = 3,5-C6Cl2F3, X = Cl, I, OTf) with P-P (dppe and dppf) and P-N (PPh2(bzN)) ligands have chelated structures in the solid-state, except for P-L = dppf and X = Cl, were chelated and dimeric bridged structures are found. The species present in solution in different solvents (CDCl3, THF, NMP and HMPA) have been characterised by 19F and 31P{1H} NMR and conductivity studies. Some [Pd(Rf)X(P-L)] complexes are involved in equilibria with [Pd(Rf)(solv)(P-L)]X, depending on the solvent and X. The ΔH° and ΔS° values of these equilibria explain the variations of ionic vs. neutral complexes in the range 183-293 K. Overall the order of coordination strength of solvents and anionic ligands is: HMPA ≫ NMP > THF and I-, Cl- > TfO-. This coordination preference is determining the complexes participating in the alkynyl transmetalation from PhC[triple bond, length as m-dash]CSnBu3 to [Pd(Rf)X(P-L)] (X = OTf, I) in THF and subsequent coupling. Very different reaction rates and stability of intermediates are observed for similar complexes, revealing neglected complexities that catalytic cycles have to deal with. Rich information on the evolution of these Stille systems after transmetalation has been obtained that leads to proposal of a common behaviour for complexes with dppe and PPh2(bzN), but a different evolution for the complexes with dppf: this difference leads the latter to produce PhC[triple bond, length as m-dash]CRf and black Pd, whereas the two former yield PhC[triple bond, length as m-dash]CRf and [Pd(C[triple bond, length as m-dash]CPh)(SnBu3)(dppe)] or [Pd(C[triple bond, length as m-dash]CPh)(SnBu3){PPh2(bzN)}].

Luminescent Rhenium(I)tricarbonyl Complexes Containing Different Pyrazoles and Their Successive Deprotonation Products: CO2 Reduction Electrocatalysts.

Cationic fac-[Re(CO)3(pz*H)(pypzH)]OTf (pz*H = pyrazole, pzH; 3,5-dimethylpyrazole, dmpzH; indazole, indzH; 3-(2-pyridyl)pyrazole, pypzH) were obtained from fac-[ReBr(CO)3(pypzH)] by halide abstraction with AgOTf and subsequent addition of the corresponding pyrazole. Successive deprotonation with Na2CO3 and NaOH gave neutral fac-[Re(CO)3(pz*H)(pypz)] and anionic Na{fac-[Re(CO)3(pz*)(pypz)]} complexes, respectively. Cationic fac-[Re(CO)3(pz*H)(pypzH)]OTf, neutral complexes fac-[Re(CO)3(pz*H)(pypz)], and fac-[Re(CO)3(pypz)2Na] were subjected to photophysical and electrochemical studies. They exhibit phosphorescent decays from a prevalently 3MLCT excited state with quantum yields (Φ) in the range between 0.03 and 0.58 and long lifetimes (τ from 220 to 869 ns). The electrochemical behavior in Ar atmosphere of cationic and neutral complexes indicates that the oxidation processes assigned to ReI → ReII occurs at lower potentials for the neutral complex compared to cationic complex. The reduction processes occur at the ligands and do not depend on the charge of the complexes. The electrochemical behavior in CO2 saturated media is consistent with CO2 electrocatalyzed reduction, where the values of the catalytic activity [icat(CO2)/icat(Ar)] ranged from 2.7 to 11.5 (compared to 8.1 for fac-[Re(CO)3Cl(bipy)] studied as a reference). Controlled potential electrolysis for the pyrazole cationic (3a) and neutral (4a) complexes after 1 h affords CO in faraday yields of 61 and 89%, respectively. These values are higher for indazole complexes and may be related to the acidity of the coordinated pyrazole.

Copper complexes for the promotion of iminopyridine ligands derived from ß-alanine and self-aldol additions: relaxivity and cytotoxic properties.

In the study presented herein, we explore the ability of copper complexes with coordinated pyridine-2-carboxaldehyde (pyca) or 2-acetylpyridine (acepy) ligands to promote the addition of amines (Schiff condensation) and other nucleophiles such as alcohols (hemiacetal formation). Distinct reactivity patterns are observed: unlike pyca complexes, acepy copper complexes can promote self-aldol addition. The introduction of a flexible chain via Schiff condensation with ß-alanine allows the possibility of chelate ring ring-opening processes mediated by pH. Further derivatization of the complex [CuCl(py-2-C(H)[double bond, length as m-dash]NCH2CH2COO)] is possible by replacing its chloride ligand with different pseudohalogens (N3-, NCO- and NCS-). In addition to the change in their magnetism, which correlates with their solid-state structures, more unexpected effects in their cytotoxicity and relaxitivities are observed, which determines their possibility to be used as MRI contrast agents. The replacement of a chloride by another pseudohalogen, although a simple strategy, can be used to critically change the cytotoxicity of the Schiff base copper(ii) complex and its selectivity towards specific cell lines.

Copper Complexes in the Promotion of Aldol Addition to Pyridine-2-carboxaldehyde: Synthesis of Homo- and Heteroleptic Complexes and Stereoselective Double Aldol Addition.

CuCl2·2H2O and Cu(ClO4)2·6H2O are able to promote aldol addition of pyridine-2-carboxaldehyde (pyca) with acetone, acetophenone, or cyclohexenone under neutral and mild conditions. The general and simple one-pot procedure for the aldol addition to Cu(II) complexes accesses novel Cu complexes with a large variety of different structural motifs, from which the aldol-addition ligand can be liberated by treatment with NH3. Neutral heteroleptic complexes in which the ligand acts as bidentate, or homoleptic cationic complexes in which the ligand acts as tridentate can be obtained depending on the copper salt used. The key step in these reactions is the coordination of pyca to copper, which increases the electrophilic character of the aldehyde, with Cu(ClO4)2 leading to a higher degree of activation than CuCl2, as predicted by DFT calculations. A regio- and stereoselective double aldol addition of pyca in the reaction of Cu(ClO4)2·6H2O with acetone leads to the formation of a dimer copper complex in which the novel double aldol addition product acts as a pentadentate ligand. A possible mechanism is discussed. The work is supported by extensive crystallographic studies.

Polymer [Pd(CH2SO2C6H4Me)2]n, a precursor to remarkably stable Pd organometallics.

A polymer [Pd(CH2SO2C6H4Me)2]n is obtained by thermolysis of cis-[Pd(CH2SO2C6H4Me)2(NCMe)2] to release the MeCN ligands. The corresponding coordination sites are then occupied by weak Pd-O bonds, easier to break than the previous Pd-N bonds. This allows us to produce from the polymer cis complexes containing ligands weaker than NCMe, such as acetone or water. The complexes cis-[Pd(CH2SO2C6H4Me)2{OC(CD3)2}2], cis-[Pd(CH2SO2C6H4Me)2(OH2)2], and cis-[Pd(CH2SO2C6H4Me)2(OH2){OC(CD3)2}], and cyclic dimers [Pd(CH2SO2C6H4Me)2(OH2)]2 with bridging methylsulphone groups are formed. The Pd : PPh3 : OH2 1 : 1 : 1 reaction of the polymer produces cis-[Pd(CH2SO2C6H4Me)2(OH2)(PPh3)], which isomerizes to trans-[Pd(CH2SO2C6H4Me)2(OH2)(PPh3)], with water O-coordinated to Pd and making hydrogen bonds to the two SO2 groups as seen in its X-ray structure. A similar role is played by RNH2 groups in the structures of trans-[Pd(CH2SO2C6H4Me)2(NH3)(PPh3)] and the dimer µ-(N2H4)(trans-[Pd(CH2SO2C6H4Me)2(PPh3)])2. In addition to these interesting intramolecular hydrogen bonding properties provided by the SO2 groups, the structural and 1H NMR data available suggest that the CH2SO2C6H4Me group is an interesting kind of strong alkyl σ donor, with high trans influence, and forms very stable Pd complexes extraordinarily resistant to reductive elimination and to hydrolysis by water at room temperature.

Dimethylzinc-Mediated Addition of Phenylacetylene to α-Diketones Catalyzed by Chiral Perhydro-1,3-benzoxazines.

An efficient, enantioselective Me2Zn-mediated monoaddition of phenylacetylene to α-diketones in the presence of a chiral perhydro-1,3-benzoxazine ligand is described. At temperatures higher than -20 °C, a kinetic resolution of the resulting α-hydroxy ketone occurs, which greatly improves the enantioselectivity although with moderate chemical yield. The alkynylation of nonsymmetrical aromatic diketones with electronically different substituents on the aromatic rings proceed with high regioselectivity. This procedure allows the preparation of α-hydroxy-α-ynyl ketones as highly enantioenriched materials.

Assembling nonplanar polyaromatic units by click chemistry. Study of multicorannulene systems as host for fullerenes.

Novel compounds with two or three corannulene subunits have been obtained by "click chemistry". These exotic systems were synthesized in high yields using the ethynylcorannulene as common reagent. The synergistic action as receptors for fullerenes of several corannulene blocks has been evaluated. It was found that the three-armed derivatives showed efficient complexation abilities toward C60. Furthermore, a new compound having two corannulene subunits linked to a hexahelicene scaffold has a remarkable affinity constant. Finally, theoretical calculations have been performed to evaluate the formation of their relative adducts containing a C60 molecule.

Enhanced association for C70 over C60 with a metal complex with corannulene derivate ligands.

The geometry imposed by the coordination sphere around the metal, together with the choice of the "arms" can be advantageously used to build corannulene-based molecular tweezers, which show great affinities for C60 and C70, as revealed by NMR titration experiments, mass spectroscopy, DFT calculations and the single crystal X-ray structural analysis of the compound C60 ⊂1.

[Pd(Fmes)2(tmeda)]: a case of intermittent C-H···F-C hydrogen-bond interaction in solution.

The X-ray structure of the title compound [Pd(Fmes)2 (tmeda)] (Fmes=2,4,6-tris(trifluoromethyl)phenyl; tmeda=N,N,N',N'-tetramethylethylenediamine) shows the existence of uncommon CH⋅⋅⋅FC hydrogen-bond interactions between methyl groups of the TMEDA ligand and ortho-CF3 groups of the Fmes ligand. The (19) F NMR spectra in CD2 Cl2 at very low temperature (157 K) detect restricted rotation for the two ortho-CF3 groups involved in hydrogen bonding, which might suggest that the hydrogen bond is responsible for this hindrance to rotation. However, a theoretical study of the hydrogen-bond energy shows that it is too weak (about 7 kJ mol(-1) ) to account for the rotational barrier observed (ΔH(≠) =26.8 kJ mol(-1) ), and it is the steric hindrance associated with the puckering of the TMEDA ligand that should be held responsible for most of the rotational barrier. At higher temperatures the rotation becomes fast, which requires that the hydrogen bond is continuously being split up and restored and exists only intermittently, following the pulse of the conformational changes of TMEDA.

η6-Hexahelicene complexes of iridium and ruthenium: running along the helix.

The first η(6)-complexes of iridium and ruthenium coordinated to helicenes have been obtained. Hexahelicene (1), 2,15-dimethylhexahelicene (2), and 2,15-dibromohexahelicene (3) react with [Cp*IrCl(2)](2) and AgBF(4) in CD(3)NO(2) to afford quantitatively the complexes [Cp*Ir(η(6)-1)][BF(4)](2) (4A), [Cp*Ir(η(6)-2)][BF(4)](2) (5A), and [Cp*Ir(η(6)-3)][BF(4)](2) (6A), respectively. In all cases, the final thermodynamic products are similar, and they exhibit coordination between the 12 e(-) metal fragment [IrCp*](2+) and the terminal ring of the helicene. Monitoring the reaction by NMR shows formation of intermediates, some of which have been fully characterized in solution. These intermediates exhibit the metal fragment coordinated to the internal rings. We have also synthesized the bimetallic complex [(Cp*Ir)(2)(µ(2)-η(6):η(6)-2)][BF(4)](4) (7), achieving coordination between two units [IrCp*](2+) and the helicene 2. Following an analogous methodology, we have prepared the complex [(η(6)-cymene)Ru(η(6)-2)][BF(4)](2) (8), which has been studied by X-ray diffraction, confirming the preferential binding to the terminal aromatic ring.

Macrocycle formation by proton-template-induced dimerization of complexes with (alkoxoimino)pyridine.

The existence of a strong hydrogen bond between two molecules of an (alkoxoimino)pyridinemanganese(I) complex induces their dimerization and the formation of a metallamacrocycle. The expected intramolecular attack of the alkoxo moiety is disfavored.

Unexpected chemoselectivity in the Schiff condensation of amines with eta(2)(C,O)- eta(1)(O)-coordinated aldehyde.

The aldehyde function coordinated as eta(1); eta(2) is able to discriminate between aliphatic and aromatic amines in the Schiff condensation reaction.

Luminescent goldI carbenes from 2-pyridylisocyanide complexes: structural consequences of intramolecular versus intermolecular hydrogen-bonding interactions.

Isocyanide [AuX(CNPy-2)] (X = Cl, C6F5, fluoromesityl, 1/2 octafluorobiphenyl) and carbene [AuX{C(NR1R2)(NHPy-2)}] (R1R2NH = primary or secondary amines or 1/2 primary diamine) gold(I) complexes have been synthesized and characterized. For X = Cl, the carbene complexes show aurophilic interactions. The fragment NHPy-2, formed in the carbenes, can give rise to intra- (for primary amines) or intermolecular (for secondary amines) hydrogen bonds, depending on the amine used. These bonds and contacts have been studied in the solid state and in solution. The intermolecular hydrogen bonds are split in an acetone solution, but the intramolecular ones, which close a six-membered ring, survive in solution. Except for the fluoromesityl derivatives, the carbene complexes display luminescent properties.

Is there any bona fide example of O-H...F-C bond in solution? The cases of HOC(CF3)2(4-X-2,6-C6H2(CF3)2) (X = Si(i-Pr)3, CF3).

The reinvestigation of the title compounds, which are the only examples reported to show experimentally (by NMR) O-H...F-C bonds in solution proves that the NMR data were misinterpreted and the restrictions to rotation of one CF(3) group are due to crowding, not to intramolecular O-H...F-C bond.

Genuine examples of tetrahedral tetradentate sulfide ligand bridging four Pd atoms: controlled formation of [(mu(4)-S){(mu(2)-X)Pd(2)(C(wedge)N)(2)}(2)] (X = OH or Cl; HC(wedge)N = p-C(2)H(5)OC(6)H(4)CH=NC(6)H(4)-p-C(2)H(5)) complexes.

A smooth reaction of [(mu3-S)(mu3-OH)Pd3(CwedgeN)3] (2) with [(mu2-X)2Pd2(CwedgeN)2] (2:1; HCwedgeN = p-C2H5OC6H4CH=NC6H4-p-C2H5, X = OH, Cl) provides [(mu4-S){(mu2-OH)Pd2(CwedgeN)2}2] (3) and [(mu4-S)(mu2-Cl)(mu2-OH)Pd4(CwedgeN)4] (4). Treatment of 3 with HCl (molar ratio 1:2) leads to the corresponding tetranuclear complex [(mu4-S)(mu2-Cl)2Pd4(CwedgeN)4] (5). The three complexes contain a (mu4-S)Pd4 core. A density functional theory study of the bonds in 3 supports that the bonding of the S atom can be described in terms of four two-center two-electron S-Pd bonds, in contrast to most other (mu4-S)M4 systems in the literature, where the presence of M-M bonds prevents a bond-localized description of the molecule. The X-ray structures of 2, 3, and 5 are reported.