From Induced-Fit Assemblies to Ternary Inclusion Complexes with Fullerenes in Corannulene-Based Molecular Tweezers.

The participation of the tether moiety in fullerene recognition of corannulene-based molecular tweezers is known to be an important factor. In the present work, we describe the synthesis of a set of fullerene receptors bearing two corannulene units located at a suitable distance to effectively interact with C60 and C70. The tether comprises a fluorene-like scaffold where an assortment of different groups with variable electronic properties has been grafted. The photophysical and electrochemical properties of all final compounds have been unveiled and correlated to the donor/acceptor (DA) nature of the tether. Despite these strong variations, their affinity toward fullerenes cannot be correlated in any way to simple DA behavior as the main contribution to the interaction correspond to London dispersion forces. We found, however, that the sulfur-derived subfamily is able to adapt better to the fullerene outer surface slightly increasing the charge transfer and electrostatic attractive interactions being the most outstanding example the case of thiophene 4-S with C70 as it is capable of forming a ternary 2:1 inclusion complex in solution with an electronic binding energy that offsets entropy and desolvation penalties typically associated with higher-order inclusion complexes.

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.

Octapodal Corannulene Porphyrin-Based Assemblies: Allosteric Behavior in Fullerene Hosting.

An octapodal corannulene-based supramolecular system has been prepared by introducing eight corannulene moieties in a porphyrin scaffold. Despite the potential of this double picket fence porphyrin for double-tweezer behavior, NMR titrations show exclusive formation of 1:1 adducts. The system exhibits very strong affinity for C60 and C70 (K1 = (2.71 ± 0.08) × 104 and (2.13 ± 0.1) × 105 M-1, respectively), presenting selectivity for the latter. Density functional theory (DFT) calculations indicate that, in addition to the four corannulene units, the relatively flexible porphyrin tether actively participates in the recognition process, resulting in a strong synergistic effect. This leads to a very strong interaction with C60, which in turn also induces a large structural change on the other face (second potential binding site), leading to a negative allosteric effect. We also introduced Zn2+ in the porphyrin core in an attempt to modulate its flexibility. The resulting metalloporphyrin also displayed single-tweezer behavior, albeit with slightly smaller binding constants for C60 and C70, suggesting that the effect of the coordination of fullerene to one face of our supramolecular platform was still transmitted to the other face, leading to the deactivation of the second potential binding site.

Tris(2-pyridyl) Bismuthines: Coordination Chemistry, Reactivity, and Anion-Triggered Pyridyl Coupling.

A series of new tris(2-pyridyl) bismuthine ligands of the type [Bi(2-py')3] have been prepared, containing a range of substituents at various positions within their pyridyl rings (py'). They can act as intact ligands or, as a result of the low C-Bi bond energy, exhibit noninnocent reactivity in the presence of metal ions. Structural studies of Li+ and Ag+ complexes show that the coordination to metal ions using their pyridyl-N atoms and to anions using the Lewis acidity of their Bi(III) centers can be modified by the presence of substituents within the 2-pyridyl rings, especially at the 6- or 3-positions, which can block the donor-N or Lewis acid Bi sites. Electron withdrawing groups (like CF3 or Br) can also severely reduce their ability to act as ligands to metal ions by reducing the electron donating ability of the pyridyl-N atoms. Noninnocent character is found in the reactions with Cu+ and Cu2+, resulting in the coupling of pyridyl groups to form bipyridines, with the rate of this reaction being dependent on the anion present in the metal salts. This leads to the formation of Bi(III)/Cu(I) complexes containing hypervalent [X2Bi(2-R-py)]- (X = Cl, Br) anions. Alternatively, the tris(2-pyridyl) bismuthine ligands can act as 2-pyridyl transfer reagents, transferring 2-py groups to Au(I) and Fe(II).

Identification by NMR of key compounds present in beer distillates and residual phases after dealcoholization by vacuum distillation.

BACKGROUND: Nowadays, low alcohol and non-alcohol beer intake has increased due to expanding concerns about healthy diets. However, there are still appreciable differences between non-alcoholic beer and conventional beer, particularly regarding flavor. Vacuum distillation is commonly used to remove ethanol from the beer in industrial processes and it is used here. RESULTS: The presence of n-propanol, isobutanol, 3-methylbutanol, 2-methylbutanol, 2-phenylethanol, ethyl acetate, isoamyl acetate, and acetaldehyde, which are key compounds responsible for aroma and flavor of beer, have been analyzed using nuclear magnetic resonance (NMR) spectroscopy in commercial beers and also in the corresponding distillates and residual phases after dealcoholization. CONCLUSION: The compounds present in each phase were identified by monodimensional and bidimensional NMR spectra. The compounds that are completely removed or that remain in the residue of the conventional beers studied are described in detail. The presence of these compounds in dealcoholized beer would be beneficial in keeping the aroma and flavor in dealcoholized beer. © 2020 Society of Chemical Industry.

A Tris(3-pyridyl)stannane as a Building Block for Heterobimetallic Coordination Polymers and Supramolecular Cages.

The systematic assembly of supramolecular arrangements is a persistent challenge in modern coordination chemistry, especially where further aspects of complexity are concerned, as in the case of large molecular mixed-metal arrangements. One targeted approach to such heterometallic complexes is to engineer metal-based donor ligands of the correct geometry to build 3D arrangements upon coordination to other metals. This simple idea has, however, only rarely been applied to main group metal-based ligand systems. Here, we show that the new, bench-stable tris(3-pyridyl)stannane ligand PhSn(3-Py)3 (3-Py=3-pyridyl) provides simple access to a range of heterometallic SnIV /transition metal complexes, and that the presence of weakly coordinating counter anions can be used to build discrete molecular arrangements involving anion encapsulation. This work therefore provides a building strategy in this area, which parallels that of supramolecular transition metal chemistry.

Dual-Tweezer Behavior of an Octapodal Pyrene Porphyrin-Based System as a Host for Fullerenes.

The incorporation of eight pyrene units in a single porphyrin core exhibits a great synergistic effect, resulting in high affinity toward C60 and C70. This octapyrene porphyrin is easily accessible by a straightforward two-step synthetic approach that involves an octuple Suzuki reaction. The new supramolecular platform can present single- or double-tweezer fullerene hosting behavior. The switch from double- to single-tweezer behavior is triggered by the simple coordination of Zn2+ to the porphyrin. Both the octapyrene porphyrin 2HPOP and its zinc metalloporphyrin analogue ZnPOP show very high affinity for C60 and C70, while simultaneously allowing the discrimination of C70 over C60 in a C60/C70 mixture. The use of 2HPOP and ZnPOP for the enrichment of real fullerene mixtures is also demonstrated.

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.

Synergistic Effect of Tetraaryl Porphyrins Containing Corannulene and Other Polycyclic Aromatic Fragments as Hosts for Fullerenes. Impact of C60 in a Statistically Distributed Mixture of Atropisomers.

Symmetric meso-tetraarylporphyrins bearing phenanthrene, pyrene, and corannulene moieties in meta positions have been synthesized in a straightforward procedure under microwave irradiation by quadruple Suzuki-Miyaura reactions. Their (1)H NMR spectra showed the typical pattern of four atropisomers distributed according to their statistical ratio not properly separable due to their fast isomerization. Their ability to bind buckminsterfullerene has been tested with the whole mixture, and different behaviors have been found, α4 isomer corannulene-substituted porphyrins being the best hosts in the family.

Stereoselective aldol addition to rhenium(I) complexes and reversible dimerization with epimerization of the metal center.

Herein, we report several examples of stereoselective aldol additions of aldehydes or ketones to ReI tricarbonyl complexes to form monomeric derivatives in good yields. The metal-centered chirality defines the final stereochemistry of the carbon atom of the monomeric ReI complex after the addition. However, it cannot control the resulting stereochemistry of the enolate part, and thus, if the α-carbon atom of the reagent is prochiral, a mixture of diastereoisomers is obtained. On the other hand, all of the monomeric complexes can be reversibly dimerized in basic media to form cis dimers, for which an epimerization of the metal-centered chirality is required in order to avoid steric congestion. All of these results are supported by exhaustive crystallographic analysis.

Structural and dimensional control of porphyrin capsules using Group 15 tris(3-pyridyl) linkers.

While supramolecular chemistry involving organic and metallo-organic host assemblies is a well-established and important field with applications in gas-storage, drug-delivery and the regio- and stereo-control of organic reactions, the use of main group elements in this setting (beyond the second row of the p-block) has been little explored. In this paper we show how periodic trends in the p-block can provide the means for systematic size and structural control in an important class of supramolecular porphyrin-based capsules. The formation of molecular and extended 2D capsule arrangements between the heavier Group 15 tris(3-pyridyl) linkers Sb(3-py)3 and Bi(3-py)3 and the metallo-porphyrins MTPP (M = Zn, Mg; TPP = tetraphenylporphyrin, 3-py = 3-pyridyl) is the first study involving heavier Group 15 pyridyl linkers. The increase in C-E bond length in the E(3-py)3 linkers moving down Group 15 (from E = P, to Sb, to Bi) can be used to alter the dimensions and structural preference of the capsules, as can oxidation of the Group 15 bridgehead atoms themselves. The subtle changes in the dimensions and Lewis acidity of the encapsulates have a dramatic effect on the rate and selectivity of the catalytic oxidative cleavage of organic diols and catalytic oxidation of α-hydroxyketones. By providing simple tools for modulating the chemical and steric properties of the capsules this work should have direct applications for the tuning of the activity and specificity of a range of catalytic systems based on main-group-based capsules of this type.

Iminopyridine complexes of manganese, rhenium, and molybdenum derived from amino ester methylserine and peptides Gly-Gly, Gly-Val, and Gly-Gly-Gly: self-assembly of the peptide chains.

Complexes containing pyridine-2-carboxaldehyde (pyca) ligand acting as κ(2)-(N,O) chelates in [MX(CO)(3)(pyca)] (M = Mn, Re; X = Cl, Br), or [MoX(methallyl)(CO)(2)(pyca)] (X = Cl, Br), are good precursors for iminopyridine complexes derived from amino esters and peptides of formula [MX(CO)(3)(py-2-C(H)═NCHX-COOY)] or [MoX(methallyl)(CO)(2)(py-2-C(H)═NCHX-COOY)], via Schiff condensation of the aldehyde function of pyca with the terminal NH(2) group of the amino ester or peptide. X-ray determinations confirm the structures and show that in solid phase the peptide chains assemble through H-bonds adopting different patterns which depend on the geometry of the metal-ligand fragments. The H-bonding patterns have been analyzed in detail and described by using graph set methods. In most cases, Mo complexes show intramolecular arrangement involving the halogen (Cl or Br) and an NH group of the side chain. For the Mn and Re complexes, the peptide side arms form infinite chains, helices, and rings. In many cases, the terminal carboxylic O-H function is engaged in a "terminal" H-bond with a polar molecule of solvent (THF or acetone), instead of forming the usual head-to-head arrangement found in simple carboxylic acids. For the longer tripeptide Gly-Gly-Gly, a discrete, dimeric association is observed, in which the peptide chains show antiparallel arrangement with a complementary disposition of the internal N-H and C═O functions. DOSY experiments in solution show significant changes in the diffusion rates upon addition of OPBu(3), which indicate H-bonding interaction of OPBu(3) with the peptide hydrogens.

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.

Coordination versus coupling of dicyanamide in molybdenum and manganese pyrazole complexes.

The reactions of cis-[MoCl(η(3)-methallyl)(CO)(2)(NCMe)(2)] (methallyl = CH(2)C(CH(3))CH(2)) with Na(NCNCN) and pz*H (pzH, pyrazole, or dmpzH, 3,5-dimethylpyrazole) lead to cis-[Mo(η(3)-methallyl)(CO)(2)(pz*H)(µ-NCNCN-κ(2)N,N)](2) (pzH, 1a; dmpzH, 1b), where dicyanamide is coordinated as bridging ligand. Similar reactions with fac-[MnBr(CO)(3)(NCMe)(2)] lead to the pyrazolylamidino complexes fac-[Mn(pz*H)(CO)(3)(NH═C(pz*)NCN-κ(2)N,N)] (pzH, 2a; dmpzH, 2b), resulting from the coupling of pyrazol with one of the CN bonds of dicyanamide. The second CN bond of dicyanamide in 2a undergoes a second coupling with pyrazole after addition of 1 equiv of fac-[MnBr(CO)(3)(pzH)(2)], yielding the dinuclear doubly coupled complex [{fac-Mn(pzH)(CO)(3)}(2)(µ-NH═C(pz)NC(pz)=NH-κ(4)N,N,N,N)]Br (3). The crystal structure of 3 reveals the presence of two isomers, cis or trans, depending on whether the terminal pyrazoles are coordinated at the same or at different sides of the approximate plane defined by the bridging bis-amidine ligand. Only the cis isomer is detected in the crystal structure of the perchlorate salt of the same bimetallic cation (4), obtained by metathesis with AgClO(4). All the N-bound hydrogen atoms of the cations in 3 or 4 are involved in hydrogen bonds. Some of the C-N bonds of the pyrazolylamidino ligand have a character intermediate between single and double, and theoretical studies were carried out on 2a and 3 to confirm its electronic origin and discard packing effects. Calculations also show the essential role of bromide in the planarity of the tetradentate ligand in the bimetallic complex 3.

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.

Self-Resetting Bistable Redox Molecular Machines for Fullerene Recognition.

Addressing control over molecular machines resulting in variable output modulation by mimicking nature mechanisms is a current hot topic. The exploitation of reversibility in thiol/disulfide motifs in chemical systems flanked by nonplanar corannulene moieties capable to recognize fullerenes is presented herein. Two redox-based machines have been conceived for this purpose: an ON/OFF switch that activates its binding properties upon dimerization and a self-resetting (i.e., with an automated backward process) host that substantially modulates its affinity.

Synthesis and decarbonylation reactions of the triiron phosphinidene complex [Fe3Cp3(µ-H)(µ3-PPh)(CO)4]: easy cleavage and formation of P-H and Fe-Fe bonds.

The binuclear phosphine complex [Fe(2)Cp(2)(µ-CO)(2)(CO)(PH(2)Ph)] (Cp = η(5)-C(5)H(5)) reacted with the acetonitrile adduct [Fe(2)Cp(2)(µ-CO)(2)(CO)(NCMe)] in dichloromethane at 293 K to give the trinuclear hydride-phosphinidene derivative [Fe(3)Cp(3)(µ-H)(µ(3)-PPh)(CO)(4)] as a mixture of cis,anti and trans isomers (Fe-Fe = 2.7217(6) Å for the cis,anti isomer). In contrast, photochemical treatment of the phosphine complex with [Fe(2)Cp(2)(CO)(4)] gave the phosphide-bridged complex trans-[Fe(3)Cp(3)(µ-PHPh)(µ-CO)(2)(CO)(3)] as the major product, along with small amounts of the binuclear hydride-phosphide complexes [Fe(2)Cp(2)(µ-H)(µ-PHPh)(CO)(2)] (cis and trans isomers), which are more selectively prepared from [Fe(2)Cp(2)(CO)(4)] and PH(2)Ph at 388 K. The photochemical decarbonylation of either of the mentioned triiron compounds led reversibly to three different products depending on the reaction conditions: (a) the 48-electron phosphinidene cluster [Fe(3)Cp(3)(µ-H)(µ(3)-PPh)(µ-CO)(2)] (Fe-Fe = 2.592(2)-2.718(2) Å); (b) the 50-electron complex [Fe(3)Cp(3)(µ-H)(µ(3)-PPh)(µ-CO)(CO)(2)], also having carbonyl- and hydride-bridged metal-metal bonds (Fe-Fe = 2.6177(3) and 2.7611(4) Å, respectively); and (c) the 48-electron phosphide cluster [Fe(3)Cp(3)(µ-PHPh)(µ(3)-CO)(µ-CO)(2)], an isomer of the latter phosphinidene complex now having three intermetallic bonds (Fe-Fe = 2.5332(8)-2.6158(8) Å).

ON/OFF metal-triggered molecular tweezers for fullerene recognition.

Herein, we report molecular tweezers for fullerene recognition based on 2,2'-bipyridine-bearing corannulene motifs. The syn or anti confirmation can be selected simply by Cu(I) coordination/decoordination, thus controlling the fullerene recognition capability of the system on demand and leading to the formation of effective metal-triggered ON/OFF molecular tweezers.

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.

Porphyrin-based systems containing polyaromatic fragments: decoupling the synergistic effects in aromatic-porphyrin-fullerene systems.

In this work, we report a two-step synthesis that allows the introduction of four pyrene or corannulene fragments at the para position of meso-tetraarylporphyrins using a microwave-assisted quadruple Suzuki-Miyaura reaction. Placing the PAHs at this position, further from the porphyrin core, avoids the participation of the porphyrin core in binding with fullerenes. The fullerene hosting ability of the four new molecular receptors was investigated by NMR titrations and DFT studies. Despite having two potential binding sites, the pyrene derivatives did not associate with C60 or C70. In contrast, the tetracorannulene derivatives bound C60 and C70, although with modest binding constants. In these novel para-substituted systems, the porphyrin core acts as a simple linker that does not participate in the binding process, which allows the system to be considered as two independent molecular tweezers; i.e., the first binding event is not transmitted to the second binding site. This behavior can be considered a direct consequence of the decoupling of the porphyrin core from the binding event.