Luminescent Benzoquinolate‐Isocyanide Platinum(II) Complexes: Effect of Pt⋅⋅⋅Pt and π⋅⋅⋅π Interactions on their Photophysical Properties

The neutral compounds [Pt(bzq)(CN)(CNR)] (R=tBu ( 1 ), Xyl ( 2 ), 2‐Np ( 3 ); bzq= benzoquinolate, Xyl=2,6‐dimethylphenyl, 2‐Np=2‐napthyl) were isolated as the pure isomers with a trans‐C_bzq,CNR configuration, as confirmed by ¹³C{¹H} NMR spectroscopy in the isotopically marked [Pt(bzq)(¹³CN)(CNR)] (R=tBu ( 1′ ), Xyl ( 2′ ), 2‐Np ( 3′ )) derivatives (δ¹³C_CN≈110 ppm; ¹J(Pt,¹³C)≈1425 Hz]. By contrast, complex [Pt(bzq)(C≡CPh)(CNXyl)] ( 4 ) with a trans‐N_bzq,CNR configuration, has been selectively isolated from [Pt(bzq)Cl(CNXyl)] (trans‐N_bzq,CNR) using Sonogashira conditions. X‐ray diffraction studies reveal that while 1 adopts a columnar‐stacked chain structure with Pt–Pt distances of 3.371(1) Å and significant π???π interactions (3.262 Å), complex 2 forms dimers supported only by short Pt???Pt (3.370(1) Å) interactions. In complex 4 the packing is directed by weak bzq???Xyl and bzq???C≡E (C, N) interactions. In solid state at room temperature, compounds 1 and 2 both show a bright red emission (?=42.1 % 1 , 57.6 % 2 ). Luminescence properties in the solid state at 77 K and concentration‐dependent emission studies in CH₂Cl₂ at 298 K and at 77 K are also reported for 1 , 1·CHCl3 , 2 , 2' , 2·CHCl3 , 3 , 4 .     

Influence of the Li⋅⋅⋅π Interaction on the H/X⋅⋅⋅π Interactions in HOLi⋅⋅⋅C6H6⋅⋅⋅HOX/XOH (X=F,Cl, Br,I) Complexes

The influences of the Li???π interaction of C₆H₆???LiOH on the H???π interaction of C₆H₆???HOX (X=F, Cl, Br, I) and the X???π interaction of C₆H₆???XOH (X=Cl, Br, I) are investigated by means of full electronic second‐order Møller–Plesset perturbation theory calculations and “quantum theory of atoms in molecules” (QTAIM) studies. The binding energies, binding distances, infrared vibrational frequencies, and electron densities at the bond critical points (BCPs) of the hydrogen bonds and halogen bonds prove that the addition of the Li???π interaction to benzene weakens the H???π and X???π interactions. The influences of the Li???π interaction on H???π interactions are greater than those on X???π interactions; the influences of the H???π interactions on the Li???π interaction are greater than X???π interactions on Li???π interaction. The greater the influence of Li???π interaction on H/X???π interactions, the greater the influences of H/X???π interactions on Li???π interaction. QTAIM studies show that the intermolecular interactions of C₆H₆???HOX and C₆H₆???XOH are mainly of the π type. The electron densities at the BCPs of hydrogen bonds and halogen bonds decrease on going from bimolecular complexes to termolecular complexes, and the π‐electron densities at the BCPs show the same pattern. Natural bond orbital analyses show that the Li???π interaction reduces electron transfer from C₆H₆ to HOX and XOH.     

Observation of CH⋅⋅⋅π Interactions between Methyl and Carbonyl Groups in Proteins

Protein structure and function is dependent on myriad noncovalent interactions. Direct detection and characterization of these weak interactions in large biomolecules, such as proteins, is experimentally challenging. Herein, we report the first observation and measurement of long‐range “through‐space” scalar couplings between methyl and backbone carbonyl groups in proteins. These J couplings are indicative of the presence of noncovalent C−H⋅⋅⋅π hydrogen‐bond‐like interactions involving the amide π network. Experimentally detected scalar couplings were corroborated by a natural bond orbital analysis, which revealed the orbital nature of the interaction and the origins of the through‐space J couplings. The experimental observation of this type of CH⋅⋅⋅π interaction adds a new dimension to the study of protein structure, function, and dynamics by NMR spectroscopy.     

Construction of Hetero‐Four‐Layered Tripalladium(II) Cyclophanes by Transannular π⋅⋅⋅π Interactions

A synthetic strategy for the generation of new molecular species utilizing a provision of nature is presented. Nano‐dimensional (23(2)×21(1)×16(1) ų) hetero‐four‐layered trimetallacyclophanes were constructed by proof‐of‐concept experiments that utilize a suitable combination of π???π interactions between the central aromatic rings, tailor‐made short/long spacer tridentate donors, and the combined helicity. The behavior of the unprecedented four‐layered metallacyclophane system offers a landmark in the development of new molecular systems.     

Intramolecular Metal⋅⋅⋅π‐Arene Interactions in Neutral and Cationic Main Group Compounds

The role of intramolecular metal???π‐arene interactions has been investigated in the solid‐state structures of a series of main group compounds supported by the bulky amide ligands, [N(^tBuAr^≠)(SiR₃)]^? (^tBuAr^≠=2,6‐(CHPh₂)₂‐4‐tBuC₆H₂, R=Me, Ph). The lithium and potassium amide salts showed different patterns of solvation and demonstrated that the SiPh₃ substituent is able to be involved in stabilizing the electrophilic metal. These group 1 metal compounds served as ligand transfer reagents to access a series of bismuth(III) halides. Chloride extraction from Bi(N{^tBuAr^≠}{SiPh₃})Cl₂ using AlCl₃ afforded the 1:1 salt [Bi(N{^tBuAr^≠}{SiPh₃})Cl][AlCl₄]. This was accompanied by a significant rearrangement of the stabilizing π‐arene contacts in the solid‐state. Attempted preparation of the corresponding tetraphenylborate salt resulted in phenyl‐transfer and generation of the neutral Bi(N{^tBuAr^≠}{SiPh₃})(Ph)Cl.     

Thiophenylazobenzene: An Alternative Photoisomerization Controlled by Lone‐Pair⋅⋅⋅π Interaction

Azoheteroarene photoswitches have attracted attention due to their unique properties. We present the stationary photochromism and ultrafast photoisomerization mechanism of thiophenylazobenzene (TphAB). It demonstrates impressive fatigue resistance and photoisomerization efficiency, and shows favorably separated (E)‐ and (Z)‐isomer absorption bands, allowing for highly selective photoconversion. The (Z)‐isomer of TphAB adopts an unusual orthogonal geometry where the thiophenyl group is perfectly perpendicular to the phenyl group. This geometry is stabilized by a rare lone‐pair???π interaction between the S atom and the phenyl group. The photoisomerization of TphAB occurs on the sub‐ps to ps timescale and is governed by this interaction. Therefore, the adoption and disruption of the orthogonal geometry requires significant movement along the inversion reaction coordinates (CNN and NNC angles). Our results establish TphAB as an excellent photoswitch with versatile properties that expand the application possibilities of AB derivatives.     

Quantification of CH⋅⋅⋅π Interactions: Implications on How Substituent Effects Influence Aromatic Interactions

Attractive interactions between a substituted benzene ring and an α‐substituted acetate group were determined experimentally by using the triptycene model system. The attractive interaction correlates well with the Hammett constants σ_m (R²=0.90), but correlates much better with the acidity of the α‐protons (R²=0.98).     

Guest Binding via N−H⋅⋅⋅π Bonding and Kinetic Entrapment by an Inorganic Macrocycle

Modern supramolecular chemistry is overwhelmingly based on non‐covalent interactions involving organic architectures. However, the question of what happens when you depart from this area to the supramolecular chemistry of structures based on non‐carbon frameworks remains largely unanswered, and is an area that potentially provides new directions in molecular activation, host–guest chemistry, and biomimetic chemistry. In this work, we explore the unusual host–guest chemistry of the pentameric macrocycle [{P(μ‐N^tBu}₂NH]₅ with a range of anionic and neutral guests. The polar coordination site of this host promotes new modes of guest encapsulation via hydrogen bonding with the π systems of the unsaturated C≡C and C≡N bonds of acetylenes and nitriles as well as with the PCO^? anion. Halide guests can be kinetically locked within the structure by oxidation of the phosphorus periphery by oxidation to P^V. Our study underscores the future promise of p‐block macrocyclic chemistry.     

Highly Selective Synthesis of Iridium(III) Metalla[2]catenanes through Component Pre‐Orientation by π⋅⋅⋅π Stacking

A series of molecular metalla[2]catenanes featuring Cp*Ir vertices have been prepared by the template‐free, coordination‐driven self‐assembly of dinuclear iridium acceptors and 1,5‐bis[2‐(4‐pyridyl)ethynyl]anthracene donors. The metalla[2]catenanes were formed by using a strategically selected linker type that is capable of participating in sandwich‐type π–π stacking interactions. In the solid state, the [2]catenanes adopt two different configurations depending on the halogen atoms at the dinuclear metal complex bridge. Altering the solvent or the concentration, as well as the addition of guest molecules, enabled controlled transformations between metalla[2]catenanes and tetranuclear metallarectangles.     

Self‐Assembly and (Hydro)gelation Triggered by Cooperative π–π and Unconventional CH⋅⋅⋅X Hydrogen Bonding Interactions

Weak C? H???X hydrogen bonds are important stabilizing forces in crystal engineering and anion recognition in solution. In contrast, their quantitative influence on the stabilization of supramolecular polymers or gels has thus far remained unexplored. Herein, we report an oligophenyleneethynylene (OPE)‐based amphiphilic Pt^II complex that forms supramolecular polymeric structures in aqueous and polar media driven by π–π and different weak C‐H???X (X=Cl, O) interactions involving chlorine atoms attached to the Pt^II centers as well as oxygen atoms and polarized methylene groups belonging to the peripheral glycol chains. A collection of experimental techniques (UV/Vis, 1D and 2D NMR, DLS, AFM, SEM, and X‐Ray diffraction) demonstrate that the interplay between different weak noncovalent interactions leads to the cooperative formation of self‐assembled structures of high aspect ratio and gels in which the molecular arrangement is maintained in the crystalline state.