Host-guest complexes of C-propyl-2-bromoresorcinarene with aromatic N-oxides\u3csup\u3e*\u3c/sup\u3e

The host-guest complexes of C-propyl-2-bromoresorcinarene with pyridine N-oxide, 3-methylpyridine N-oxide, quinoline N-oxide and isoquinoline N-oxide are studied using single crystal X-ray crystallography and 1H NMR spectroscopy. The C-propyl-2-bromoresorcinarene forms endo-complexes with the aromatic N-oxides in the solid-state when crystallised from either methanol or acetone. In solution, the endo-complexes were observed only in methanol-d4. In DMSO the solvent itself is a good guest, and crystallisation provides only solvate endo-complexes. The C-propyl-2-bromoresorcinarene shows remarkable flexibility when crystallised from either methanol or acetone, and packs into one-dimensional self-included chains. Of special note, crystallising C-propyl-2-bromoresorcinarene with 3-methylpyridine N-oxide from acetone results in a 2:2 dimeric capsular assembly organised through both C−H···πhost and N−O···(H−O)host interactions

Halogen-bonded shape memory polymers

Halogen bonding (XB), a non-covalent interaction between an electron-deficient halogen atom and a Lewis base, is widely adopted in organic synthesis and supramolecular crystal engineering. However, the roadmap towards materials applications is hindered by the challenges in harnessing this relatively weak intermolecular interaction to devise human-commanded stimuli-responsive soft materials. Here, we report a liquid crystalline network comprising permanent covalent crosslinks and dynamic halogen bond crosslinks, which possess reversible thermo-responsive shape memory behaviour. Our findings suggest that I···N halogen bond, a paradigmatic motif in crystal engineering studies, enables temporary shape fixation at room temperature and subsequent shape recovery in response to human body temperature. We demonstrate versatile shape programming of the halogen-bonded polymer networks through human-hand operation and propose a micro-robotic injection model for complex 1D to 3D shape morphing in aqueous media at 37 °C. Through systematic structure-property-performance studies, we show the necessity of the I···N crosslinks in driving the shape memory effect. The halogen-bonded shape memory polymers expand the toolbox for the preparation of smart supramolecular constructs with tailored mechanical properties and thermoresponsive behaviour, for the needs of, e.g., future medical devices.publishedVersionPeer reviewe

A coumarin based gold(I)-alkynyl complex: a new Q1 class of supramolecular hydrogelators

A phosphine-gold(I)-alkynyl-coumarin complex, [Au{7-(prop-2-ine-1-yloxy)-1-benzopyran-2-one}-(DAPTA)] (1), was synthesized and the formation of long luminescent fibers in solution was characterized via fluorescence microscopy and dynamic light scattering. The fibers presented strong blue and green luminescence, suggesting that the gold(I) in the complex increased intersystem crossing due to the heavy atom effect, resulting in a significant increase in triplet emission. The X-ray structure of the fibers indicates that both aurophilic, π-π interactions and hydrogen bonding contribute to their formation in aqueous solvents

Candida antarctica Lipase A-Based Enantiorecognition of a Highly Strained 4-Dibenzocyclooctynol (DIBO) Used for PET Imaging

The enantiomers of aromatic 4-dibenzocyclooctynol (DIBO), used for radiolabeling and subsequent conjugation of biomolecules to form radioligands for positron emission tomography (PET), were separated by kinetic resolution using lipase A from Candida antarctica (CAL-A). In optimized conditions, (R)-DIBO [(R)-1, ee 95%] and its acetylated (S)-ester [(S)-2, ee 96%] were isolated. In silico docking results explained the ability of CAL-A to differentiate the enantiomers of DIBO and to accommodate various acyl donors. Anhydrous MgCl2 was used for binding water from the reaction medium and, thus, for obtaining higher conversion by preventing hydrolysis of the product (S)-2 into the starting material. Since the presence of hydrated MgCl2·6H2O also allowed high conversion or effect on enantioselectivity, Mg2+ ion was suspected to interact with the enzyme. Binding site predictions indicated at least two sites of interest; one in the lid domain at the bottom of the acyl binding pocket and another at the interface of the hydrolase and flap domains, just above the active site.</p

Pyridine N-oxides as Versatile Ligands for Silver(I)

Chapter 1 gives a brief introduction to metallosupramolecular chemistry, silver(I) chemistry and the synthesis and coordination chemistry of pyridine mono-, di- and tri-N-oxides. Chapter 2 describes the syntheses of 17 [16 known and 1 new] pyridine N-oxides using either mCPBA or 50% H₂O₂ as the oxidising agent. It also describes 33 new X-ray crystal structures, of which 23 are silver(I) complexes and the remaining are molecular species, protonated ligands and unreported ligand crystal structures. The N-O group is shown to coordinate with up to three silver atoms with dimensionalities ranging from 0 to 2. Chapter 3 describes the syntheses of 21 [16 known and 5 new] bipyridine N,N′-dioxides. This chapter also contains 27 new crystal structures, with 23 being silver(I) complexes, 1 molecular species and three unreported ligand crystal structures. The N,N′-dioxide ligands adopted upto µ₆-O,O,O,O′,O′,O′ denticity with polymer dimensionality varying from 0 to 2. Chapter 4 involves the syntheses of 4 N,N′,N′′-trioxides and 7 new silver(I) complexes with 2,6′:2′,6′′-terpyridine N,N′,N′′-trioxide and 2 ligand crystal structures. This ligand adopted up to µ₆-O,O,O′,O′,O′′,O′′ with network dimensionality ranging from 0 to 3. Chapter 5 summarises the results with final conclusions and future prospects. Chapter 6 details the synthetic procedures for the ligands and their silver(I) complexes. All the N-oxide ligands were characterised by melting point, ¹H NMR, ¹³C NMR, IR, ESI-MS and elemental analysis, while the silver(I) complexes were characterised by melting point, elemental analysis, IR and ESI-MS

Inclusion complexes of Cethyl-2-methylresorcinarene and pyridine N-oxides: breaking the C–I⋯−O–N+ halogen bond by host–guest complexation

C ethyl-2-Methylresorcinarene forms host–guest complexes with aromatic N-oxides through multiple intra- and intermolecular hydrogen bonds and C–H⋯π interactions. The host shows conformational flexibility to accommodate 3-methylpyridine N-oxide, while retaining a crown conformation for 2-methyl- and 4-methoxypyridine N-oxides highlighting the substituent effect of the guest. N-Methylmorpholine N-oxide, a 6-membered ring aliphatic N-oxide with a methyl at the N-oxide nitrogen, is bound by the equatorial −N–CH3 group located deep in the cavity. 2-Iodopyridine N-oxide is the only guest that manifests intermolecular N–O⋯I–C halogen bond interactions, which are broken down by the host resulting in a 2 : 2 pseudocapsular complex stabilized by additional C–I⋯π interactions between the two 2-iodopyridine N-oxides located in two adjacent hosts. These host–guest complexes were analyzed in the solid state by single crystal X-ray crystallography and in solution by 1H NMR spectroscopy.peerReviewe

Synthesis and structural characterization of new transition metal complexes of a highly luminescent amino-terpyridine ligand

The synthesis, NMR and UV-Vis spectroscopy measurements and X-ray diffraction analysis of four new metal complexes of the amino terpyridine ligand 4́-[4-(4-aminophenyl)phenyl]-2,2́:6́,2́́-terpyridine L, namely [FeL2](ClO4)2 (1), [ZnL2](ClO4)2 (2), [CdL2](ClO4)2 (3) and [PtMe3IL] (4), are reported. The X-ray crystal structures of complexes 1-3 are 1:2 metal:ligand structures with tridentate ligands decorated around the octahedral metal centers. In complex 4, with L in a bidentate coordination mode, the Pt(IV) coordinated methyl and iodine groups form a fac-arrangement. The 1H NMR spectrum of 4 shows three 195Pt-1H resonances for the methyl groups incorporating the fac-arrangement, which confirms the bidentate coordination mode of L in solution. The X-ray crystal structure of L shows a transoid conformation of the pyridine ring nitrogen atoms along the interannular bond in the terpyridine core. Ligand L exhibits a bright blue emission in dichloromethane [fluorescence quantum yield (ΦF) = 68%] and its emission maxima shows significant solvatochromism (461 nm in dichloromethane to 533 nm in N,N-dimethylformamide), while its complexes 2 and 3 are only very weakly emissive and 1 and 4 are found to be non-emissive.peerReviewe

Halogen Bonds in Square Planar 2,5-Dihalopyridine-Copper(II) Bromide Complexes

Halogen bonding in self-complementary 1:2 metal–ligand complexes obtained from copper(II) bromide (CuBr2) and seven 2,5-dihalopyridines were analyzed using single-crystal X-ray diffraction. All presented discrete complexes form 1D polymeric chains connected with C–X···Br–Cu halogen bonds (XB). In (2-chloro-5-X-pyridine)2·CuBr2 (X = Cl, Br, and I) only the C5-halogen and in (2-bromo-5-X-pyridine)2·CuBr2 (X = Cl, Br, and I) both C2- and C5-halogens form C–X···Br–Cu halogen bonds with the X acting as the XB donor and copper-coordinated bromide as the XB acceptor. The electron-withdrawing C2-chloride in (2-chloro-5-X-pyridine)2·CuBr2 complexes has only a minor effect on the C5–X5···Br–Cu XBs, and the X5···Br distances follow the expected order I5 < Br5 < Cl5 with RXB values of 0.91, 0.94, and 0.99, respectively. In (2-bromo-5-X-pyridine)2·CuBr2 complexes, due to the polarization of both halogens, the C2–X2···Br–Cu and C5–X5···Br–Cu RXB values are very similar [0.92–0.99] as a result of the competition between C2- and C5-halogens for XB formation. In addition to the classical halogen bonds, the square-planar CuII complexes exhibit C2–X2···Cu (X = Cl and Br) contacts perpendicular to the Br–Cu–Br plane with shorter C2–Br2···Cu than C2–Cl2···Cu contacts. These interactions induce a pseudo-octahedral geometry for CuII ions. Notably, C2–X2···Br–Cu halogen bonds and the additional C2–X2···Cu contacts are slightly enhanced by the C5-halogen electronegativity.peerReviewe

Conformational changes in Cmethyl-resorcinarene pyridine N-oxide inclusion complexes in the solid state

Aromatic N-oxides interact with Cmethyl-resorcinarene resulting in marked changes in the conformation of the host resorcinarene. In the solid state, 2- and 3-methylpyridine N-oxides form pseudo-capsular 2 : 2 endo host–guest complexes with Cmethyl-resorcinarene stabilized by C–H⋯π interactions. The Cmethylresorcinarene·2-methylpyridine N-oxide complex has a C4v crown conformation, while the Cmethylresorcinarene·3-methylpyridine N-oxide complex has a slightly open C2v boat conformation. On the contrary, other para-substituted and benzo-fused pyridine N-oxides form only exo complexes with Cmethyl-resorcinarene. In the exo complexes, the asymmetry of the guest, conformational flexibility and preferred inter-host hydrogen bonding of Cmethyl-resorcinarenes exclude endo complexation. All the exo complexes form robust 1-D hydrogen bonded chains between the host hydroxyl groups assisted by the guest N–O groups, resulting in a C2v boat conformation.peerReviewe

Metal-bound Nitrate Anion as an Acceptor for Halogen Bonds in mono-Halopyridine-Copper(II) nitrate Complexes

Fifteen n-halopyridine-Cu(NO3)2 complexes (n = 2, 3, 4) obtained from two different solvents, acetonitrile and ethanol, are investigated for C–X···O–N halogen bonds (XBs) in the solid state by single and powder X-ray diffraction. The nitrate anions bind copper(II) via anisobidentate modes and one of three oxygens act as an XB acceptor to halogens on the core pyridine rings. The N-metal coordination activates the electron-deficient π-system and triggers even C2- and C4-chlorines in the corresponding [Cu(2-chloropyridine)2(NO3)2] and [Cu(4-chloropyridine)2(NO3)2(ACN)] complexes to form short C–Cl2/Cl4···O–N halogen bonds. Notably, the C2–Cl2···O–N XBs with a normalized XB distance parameter (RXB) of 0.89 is close to C2–I2···O–N XBs [RXB = 0.88] in the [Cu(2-iodopyridine)2(NO3)2] complex. In overall, the C–X···O–N halogen bonds in the studied complexes range from moderately short to roughly the van der Waals contact distance of the respective X···O atoms (RXB = 0.88–0.99) and have a varying significance in governing the molecular packing respective of the complex. Among the studied complexes, two main coordination modes were observed—distorted octahedral and distorted pentagonal bipyramid—of which the latter results from the coordination of acetonitrile to the Cu(II) ion. The crystal structures showed that the steric bulk of C2-halogens in 2-halopyridines prevent this, while in similar conditions the 3- and 4-halopyridine ligands yield acetonitrile bound Cu(II)-complexes.peerReviewe