Rapid Access to Encapsulated Molecular Rotors via Coordination-Driven Macrocycle Formation.

Macrocycle formation that relies upon trans metal coordination of appropriately placed pyridine ligands within an arylene ethynylene construct provides rapid and reliable access to molecular rotators encapsulated within macrocyclic stators. Showing no significant close contacts to the central rotators, X-ray crystallography of AgI -coordinated macrocycles provides plausibility for unobstructed rotation or wobbling of rotators within the central cavity. Solid-state 13 C NMR of PdII -coordinated macrocycles supports the notion of unobstructed movement of simple arenes in the crystal lattice. Solution 1 H NMR studies indicate complete and immediate macrocycle formation upon the introduction of PdII to the pyridyl-based ligand at room temperature. Moreover, the formed macrocycle is stable in solution; a lack of significant changes in the 1 H NMR spectrum upon cooling to -50 °C is consistent with the absence of dynamic behavior. The synthetic route to these macrocycles is expedient and modular, providing access to rather complex constructs in four simple steps involving Sonogashira coupling and deprotection reactions.

Complementary, Cooperative Ditopic Halogen Bonding and Electron Donor-Acceptor π-π Complexation in the Formation of Cocrystals.

This study expands and combines concepts from two of our earlier studies. One study reported the complementary halogen bonding and π-π charge transfer complexation observed between isomeric electron rich 4-N,N-dimethylaminophenylethynylpyridines and the electron poor halogen bond donor, 1-(3,5-dinitrophenylethynyl)-2,3,5,6-tetrafluoro-4-iodobenzene while the second study elaborated the ditopic halogen bonding of activated pyrimidines. Leveraging our understanding on the combination of these non-covalent interactions, we describe cocrystallization featuring ditopic halogen bonding and π-stacking. Specifically, red cocrystals are formed between the ditopic electron poor halogen bond donor 1-(3,5-dinitrophenylethynyl)-2,4,6-triflouro-3,5-diiodobenzene and each of electron rich pyrimidines 2- and 5-(4-N,N-dimethyl-aminophenylethynyl)pyrimidine. The X-ray single crystal structures of these cocrystals are described in terms of halogen bonding and electron donor-acceptor π-complexation. Computations confirm that the donor-acceptor π-stacking interactions are consistently stronger than the halogen bonding interactions and that there is cooperativity between π-stacking and halogen bonding in the crystals.

Self-Assembly of Complementary Components Using a Tripodal Bismuth Compound: Pnictogen Bonding or Coordination Chemistry?

Triple pnictogen bonding refers to the ability of a pnictogen atom to engage in three simultaneous pnictogen bonds (PnBs) to a complementary partner through a single pnictogen atom. This supramolecular strategy was recently introduced as a unique facet of pnictogen bonding as compared to other named supramolecular interactions. Here, the ability of bismuth to participate in this phenomenon is demonstrated using Bi((NC9H7)3CH3). The study reveals that Bi engages in stronger PnBs than the analogous Sb system. The results have been contrasted with Bi systems that form strong coordination bonds, and analysis of the electron density along the bond path reveals key differences. The solution behavior of these newly synthesized supramolecules were studied by PFGSE NMR spectroscopy and they are found to remain intact in solution. Molecular design strategies that allow for triple pnictogen bonding should find use in the fields of molecular recognition and crystal engineering.

Triple-Pnictogen Bonding as a Tool for Supramolecular Assembly.

Supramolecular assembly utilizing simultaneous formation of three pnictogen bonds around a single antimony vertex was explored via X-ray crystallography, solution NMR, and computational chemistry. An arylethynyl (AE) ligand was designed to complement the three electrophilic regions around the Sb compound. Though solution studies reveal large binding constants for individual pyridyl units with the Sb donor, the rigidity and prearrangement of the AE acceptor proved necessary to achieve simultaneous binding of three acceptors to the Sb-centered pnictogen-bond donor. Calculations and X-ray structures suggest that negative cooperativity upon sequential binding of three acceptors to a Sb center limits the utility of triple-pnictogen bonding pyridyl acceptors. These limitations can be negated, however, when positive cooperativity is designed into a complementary acceptor ligand.

Effects of Halogen and Hydrogen Bonding on the Electronics of a Conjugated Rotor.

The electronic properties of a pyrazine-containing arylene ethynylene unit are influenced by hydrogen bond and halogen bond donors that are held in proximity of the pyrazine rotor. These interactions are evident with iodine- and bromine-centered halogen bonds and O-H- and C-H-based hydrogen bonds. Bathochromic shifts of UV-vis and fluorescence signals are the best indicators of this intramolecular attraction. The effects can be attenuated in solvents that are less favorable for intramolecular halogen or hydrogen bonding, such as 2-propanol, and amplified in solvents that are supportive, such as toluene. Intramolecular attractions promote planarity in the pyrazine ethynylene system, likely increasing the effective conjugation of the unsaturated backbone. Additionally, computations at the B3LYP and M062X levels of theory using 6-311++G(2d,p) and aug-cc-pVTZ basis sets suggest that the Lewis acidity of the halogen and hydrogen atoms influences electronic behavior even in the absence of conformational constraints.

Intramolecular halogen bonding supported by an aryldiyne linker.

Intramolecular halogen bonds between aryl halide donors and suitable acceptors, such as carbonyl or quinolinyl groups, held in proximity by 1,2-aryldiyne linkers, provide triangular structures in the solid state. Aryldiyne linkers provide a nearly ideal template for intramolecular halogen bonding as minor deviations from alkyne linearity can accommodate a variety of halogen bonding interactions, including O···Cl, O···Br, O···I, N···Br, and N···I. Halogen bond lengths for these units, observed by single crystal X-ray crystallography, range from 2.75 to 2.97 Å. Internal bond angles of the semirigid bridge between halogen bond donor and acceptor are responsive to changes in the identity of the halogen, the identity of the acceptor, and the electronic environment around the halogen, with the triangles retaining almost perfect co-planarity in even the most strained systems. Consistency between experimental results and structures predicted by M06-2X/6-31G* calculations demonstrates the efficacy of this computational method for modeling halogen-bonded structures of this type.

Halogen-Bonded Supramolecular Parallelograms: From Self-Complementary Iodoalkyne Halogen-Bonded Dimers to 1:1 and 2:2 Iodoalkyne Halogen-Bonded Cocrystals.

The formation of supramolecular parallelograms utilizing iodoalkyne-pyridine halogen bonding is described. The crystal structures of four iodoalkynyl-substituted (phenylethynyl)pyridines demonstrate the feasibility of discrete self-complementary dimer formation. These compounds 3-(2-iodoethynyl-phenylethynyl) pyridine (1), 2-(3-iodoethynyl-phenylethynyl) pyridine (2), 3-(4,5-difluoro-2-iodoethynyl-phenylethynyl) pyridine (3), and 2-(5-iodoethynyl-2,4-dimethylphenylethynyl) pyridine (4) all form parallelogram-shaped dimers with two self-complementary short N-I halogen bonds. The potential formation of iodoalkynyl halogen-bonded supramolecular macrocycles is demonstrated by the formation of a discrete halogen-bonded parallelogram-shaped complex in the 1:1 cocrystal formed from the bis iodoalkyne, 1-iodoethynyl-2-(3-iodoethynyl-phenylethynyl)-4,5-dimethoxybenzene (6), and the dipyridyl, 5-phenyl-2-(pyridin-3-ylethynyl)pyridine (7). Furthermore, discrete supramolecular parallelograms form within the 2:2 cocrystal formed between 1,2-bis(iodoethynyl)-4,5-difluorobenzene and the dipyridyl 4-(3-pyridylethynyl) pyridine (8).

π-Complexation and C-H hydrogen bonding in the formation of colored cocrystals.

The present study evaluates the potential combination of charge-transfer electron-donor-acceptor π-π complexation and C-H hydrogen bonding to form colored cocrystals. The crystal structures of the red 1:1 cocrystals formed from the isomeric pyridines 4- and 3-{2-[4-(dimethylamino)phenyl]ethynyl}pyridine with 1-[2-(3,5-dinitrophenyl)ethynyl]-2,3,5,6-tetrafluorobenzene, both C14H4F4N2O4·C15H14N2, are reported. Intermolecular interaction energy calculations confirm that π-stacking interactions dominate the intermolecular interactions within each crystal structure. The close contacts revealed by Hirshfeld surface calculations are predominantly C-H interactions with N, O, and F atoms.

Evidence of enhanced conjugation in ortho-arylene ethynylenes with transition metal coordination.

The effective conjugation of ortho and ortho-alt-para-arylene ethynylenes, with appropriately positioned pyridine and pyrazine heterocycles, increases upon binding to Ag(I) and Pd(II) cations. Significant bathochromic shifts in the electronic spectra, witnessed upon introduction of these metal bridges, are consistent with enhanced electron delocalization in the unsaturated backbone. Control studies suggest that this electronic behavior is attributable exclusively (in the case of Ag(I)) or partially (in the case of Pd(II)) to conformational restrictions of the conjugated backbones.

Co-operative halogen bonds and nonconventional sp-C-H...O hydrogen bonds in 1:1 cocrystals formed between diethynylpyridines and N-halosuccinimides.

The rapid evaporation of 1:1 solutions of diethynylpyridines and N-halosuccinimides, that react together to form haloalkynes, led to the isolation of unreacted 1:1 cocrystals of the two components. The 1:1 cocrystal formed between 2,6-diethynylpyridine and N-iodosuccinimide (C4H4INO2·C9H5N) contains an N-iodosuccinimide-pyridine I...N halogen bond and two terminal alkyne-succinimide carbonyl C-H...O hydrogen bonds. The three-dimensional extended structure features interwoven double-stranded supramolecular polymers that are interconnected through halogen bonds. The cocrystal formed between 3,5-diethynylpyridine and N-iodosuccinimide (C4H4INO2·C9H5N) also features an I...N halogen bond and two C-H...O hydrogen bonds. However, the components form essentially planar double-stranded one-dimensional zigzag supramolecular polymers. The cocrystal formed between 3,5-diethynylpyridine and N-bromosuccinimide (C4H4BrNO2·C9H5N) is isomorphous to the cocrystal formed between 3,5-diethynylpyridine and N-iodosuccinimide, with a Br...N halogen bond instead of an I...N halogen bond.

5-{[4-(Di-methyl-amino)-phen-yl]ethyn-yl}pyrimidine-1,2,3,5-tetra-fluoro-4,6-di-iodo-benzene (1/2).

The treatment of 5-{[4-(di-methyl-amino)-phen-yl]ethyn-yl}pyrimidine with a threefold excess of 1,2,3,5-tetra-fluoro-4,6-di-iodo-benzene in di-chloro-methane solution led to the formation of the unexpected 1:2 title co-crystal, C14H13N3·2CF4I2. In the extended structure, two unique C-I⋯N halogen bonds from one of the 1,2,3,5-tetra-fluoro-4,6-di-iodo-benzene mol-ecules to the pyrimidine N atoms of the 5-{[4-(di-methyl-amino)-phen-yl]ethyn-yl}pyrimidine mol-ecule generate [110] chains and layers of these chains are π-stacked along the a-axis direction. The second 1,2,3,5-tetra-fluoro-4,6-di-iodo-benzene mol-ecule resides in channels formed parallel to the a-axis direction between stacks of 5-{[4-(di-methyl-amino)-phen-yl]ethyn-yl}pyrimidine mol-ecules and inter-acts with them via C-I⋯π(alkyne) contacts.

A structural and computational comparison of close contacts and related intermolecular energies of interaction in the structures of 1,3-diiodo-5-nitrobenzene, 1,3-dibromo-5-nitrobenzene, and 1,3-dichloro-5-nitrobenzene.

1,3-Diiodo-5-nitrobenzene, C6H3I2NO2, and 1,3-dibromo-5-nitrobenzene, C6H3Br2NO2, crystallize in the centrosymmetric space group P21/m, and are isostructural with 1,3-dichloro-5-nitrobenzene, C6H3Cl2NO2, that has been redetermined at 100 K for consistency. While the three-dimensional packing in all three structures is similar, the size of the halogen atom affects the nonbonded close contacts observed between molecules. Thus, the structure of 1,3-diiodo-5-nitrobenzene features a close Type 1 I...I contact, the structure of 1,3-dibromo-5-nitrobenzene features a self-complementary nitro-O...Br close contact, while the structure of 1,3-dichloro-5-nitrobenzene also has a self-complementary nitro-O...Cl interaction, as well as a bifurcated C-H...O(nitro) close contact. Notably, the major energetically attractive intermolecular interaction between adjacent molecules in each of the three structures corresponds to a π-stacked interaction. The self-complementary halogen...O(nitro) and C-H...O(nitro) interactions correspond to significant cohesive attraction between molecules in each structure, while the Type 1 halogen-halogen contact is weakly cohesive.

Conformational control through co-operative nonconventional C-H...N hydrogen bonds.

We report the design, synthesis, and crystal structure of a conjugated aryleneethynyl molecule, 2-(2-{4,5-dimethoxy-2-[2-(2,3,4-trifluorophenyl)ethynyl]phenyl}ethynyl)-6-[2-(pyridin-2-yl)ethynyl]pyridine, C30H17F3N2O2, that adopts a planar rhombus conformation in the solid state. The molecule crystallizes in the space group P-1, with Z = 2, and features two intramolecular sp2-C-H...N hydrogen bonds that co-operatively hold the arylethynyl molecule in a rhombus conformation. The H atoms are activated towards hydrogen bonding since they are situated on a trifluorophenyl ring and the H...N distances are 2.470 (16) and 2.646 (16) Å, with C-H...N angles of 161.7 (2) and 164.7 (2)°, respectively. Molecular electrostatic potential calculations support the formation of C-H...N hydrogen bonds to the trifluorophenyl moiety. Hirshfeld surface analysis identifies a self-complementary C-H...O dimeric interaction between adjacent 1,2-dimethoxybenzene segments that is shown to be common in structures containing that moiety.

Arylethynyl Helices Supported by π-Stacking and Halogen Bonding.

Co-crystallization of a pyridyl-containing arylethynyl (AE) moiety with 1,4-diiodotetrafluorobenzene leads to unique, figure-eight shaped helical motifs within the crystal lattice. A slight twist in the AE backbone allows each AE unit to simultaneously interact with haloarene units that are stacked on top of one another. Left-handed (M) and right-handed (P) helices are interspersed in a regular pattern throughout the crystal. The major driving forces for assembly are 1) halogen bonding between the pyridyl nitrogen atoms and the iodine substituents of the haloarene, with N⋅⋅⋅I distances between 2.81 and 2.84 Å, and 2) π-π stacking of the haloarenes, with distances of approximately 3.57 Šbetween centroids. Halogen bonding and π-π stacking not only work in concert, but also seem to mutually enhance one another. Calculations suggest that the presence of π-π stacking modestly intensifies the halogen bonding interaction by <0.2 kcal/mol; likewise, halogen bonding to the haloarene enhances the π-π stacking interaction by 0.59 kcal/mol.

Dialkynyl carbene derivatives: generation and characterization of triplet tert-butylpentadiynylidene (t-Bu-C≡C-C̈-C≡C-H) and dimethylpentadiynylidene (Me-C≡C-C̈-C≡C-Me).

Triplet carbenes t-butylpentadiynylidene (t-BuC(5)H, 1a) and dimethylpentadiynylidene (MeC(5)Me, 1b) have been produced photochemically from their corresponding diazo compound precursors and studied spectroscopically in cryogenic matrices (N(2) or Ar) at 10 K. The infrared, electronic absorption, and electron paramagnetic resonance spectra of these species exhibit numerous similarities to the spectra of pentadiynylidene (HC(5)H) and methylpentadiynylidene (MeC(5)H) recorded previously. EPR spectra yield zero-field splitting parameters that are typical for triplet carbenes with axial symmetry (t-BuC(5)H, 1a: |D/hc| = 0.61 cm(-1), |E/hc| ∼ 0 cm(-1); MeC(5)Me, 1b: |D/hc| = 0.62 cm(-1), |E/hc| ∼ 0 cm(-1)). Electronic spectra are characterized by weak absorptions (T(1) ← T(0)) in the near-UV and visible region (350-430 nm) with extended vibronic progressions. The electronic transitions of several -C(5)- carbenes are compared, and an apparent dependence of the transition wavelength on the level of alkyl substitution of the carbon chain is found. Chemical trapping of triplet 1a in an O(2)-doped matrix affords carbonyl oxides derived predominantly from attack at C-3. Both t-BuC(5)H (1a) and MeC(5)Me (1b) undergo photochemical rearrangement upon UV irradiation.

Synthesis of simple diynals, diynones, their hydrazones, and diazo compounds: precursors to a family of dialkynyl carbenes (R(1)-C≡C-C-C≡C-R(2)).

A variety of substituted pentadiynols, -diynals, and -diynones have been prepared en route to precursors to dialkynyl carbenes (R(1)-C≡C-C-C≡C-R(2)). In light of the marginal stability associated with these simple systems, several strategies were required to assemble the carbon backbones. The requisite five-carbon skeletons were prepared using 4 + 1, 3 + 2, 2 + 2 + 1, and 2 + 1 + 1 + 1 coupling methodologies. The Dess-Martin periodinane serves as an excellent method for the oxidation of pentadiynols to diynals and diynones, although many of the oxidized products are sufficiently reactive that they were not isolated; rather, they were generated in situ and intercepted with nucleophiles such as tosylhydrazide or trisylhydrazide. The hydrazone derivatives are generally reliable precursors to diazo compounds and carbenes, although cyclization of the hydrazone to afford a pyrazole can be a complicating factor in certain instances.

Ditopic halogen bonding with bipyrimidines and activated pyrimidines.

The potential of pyrimidines to serve as ditopic halogen-bond acceptors is explored. The halogen-bonded cocrystals formed from solutions of either 5,5'-bipyrimidine (C8H6N4) or 1,2-bis(pyrimidin-5-yl)ethyne (C10H6N4) and 2 molar equivalents of 1,3-diiodotetrafluorobenzene (C6F4I2) have a 1:1 composition. Each pyrimidine moiety acts as a single halogen-bond acceptor and the bipyrimidines act as ditopic halogen-bond acceptors. In contrast, the activated pyrimidines 2- and 5-{[4-(dimethylamino)phenyl]ethynyl}pyrimidine (C14H13N3) are ditopic halogen-bond acceptors, and 1:1 halogen-bonded cocrystals are formed from 1:1 mixtures of each of the activated pyrimidines and either 1,2- or 1,3-diiodotetrafluorobenzene. A 1:1 cocrystal was also formed between 2-{[4-(dimethylamino)phenyl]ethynyl}pyrimidine and 1,4-diiodotetrafluorobenzene, while a 2:1 cocrystal was formed between 5-{[4-(dimethylamino)phenyl]ethynyl}pyrimidine and 1,4-diiodotetrafluorobenzene.

Spectroscopy and photochemistry of triplet methylpentadiynylidene (Me-C[triple bond]C-:C-C[triple bond]C-H).

Triplet carbene methylpentadiynylidene, MeC(5)H (1), was investigated in cryogenic matrices by IR, UV/vis, and EPR spectroscopy. Broadband irradiation (lambda > 497 nm) of the isomeric diazo compounds, 1-diazo-hexa-2,4-diyne (2) or 2-diazo-hexa-3,5-diyne (3), generates triplet carbene 1. EPR spectra yield zero-field splitting parameters (|D/hc| = 0.62 cm(-1), |E/hc| < 0.0006 cm(-1)), which are typical for a triplet carbene with axial symmetry. The electronic spectrum of triplet 1 is characterized by a weak absorption in the near-UV and visible region (350-430 nm) with vibronic progressions corresponding to excitations of the acetylenic stretching and the terminal C[triple bond]C-H bending modes. Chemical trapping of triplet 1 in an O(2)-doped matrix affords carbonyl oxides derived predominantly from attack at C-3. Upon irradiation at lambda > 399 nm, triplet 1 undergoes photochemical 1,2-hydrogen migration to form hex-1-ene-3,5-diyne (6).

C-I...N and C-I...π halogen bonding in the structures of 1-benzyliodoimidazole derivatives.

Halogen bonding is a well-established and intensively studied intermolecular interaction that has also been used in the preparation of functional materials. While polyfluoroiodo- and polyfluorobromobenzenes have been widely used as aromatic halogen-bond donors, there have been very few studies of iodoimidazoles with regard to halogen bonding. We describe here the X-ray structures of three iodoimidazole derivatives, namely 1-benzyl-2-iodo-1H-imidazole, C10H9IN2, (1), 1-benzyl-4-iodo-1H-imidazole, C10H9IN2, (2), and 1-benzyl-2-iodo-1H-benzimidazole, C14H11IN2, (3), and the halogen bonds that dominate the intermolecular interactions in each of these three structures. The three-dimensional structure of (1) is dominated by a strong C-I...N halogen bond, with an N...I distance of 2.8765 (2) Å, that connects the molecules into one-dimensional zigzag ribbons of molecules. In contrast, the three-dimensional structures of (2) and (3) both feature C-I...π halogen-bonded dimers.

Conjugated metallorganic macrocycles: opportunities for coordination-driven planarization of bidentate, pyridine-based ligands.

Two conjugated systems that can be constrained to planarity via metal coordination have been generated and their metal complexes studied. The potential for these architectures to be incorporated into metal-sensing arylene ethynylene/vinylene oligomers and polymers was probed by verifying that these ligands (1) bind strongly to Ag(I) and Pd(II) cations, and (2) that this event leads to complexes that are planar. Single crystal structures confirm that introduction of Ag(I) or Pd(II) cations enforces planarity in the newly formed macrocycles. Likewise, (1)H-NMR titration studies reveal stoichiometric binding of Pd(II) and strong binding of Ag(I) (K(a (Ligand 1)) = 1.3 × 10(2) M(-1); K(a (Ligand 2)) = 5.4 × 10(2) M(-1)) for each conjugated ligand.