A dodecamethoxy[6]cycloparaphenylene consisting entirely of hydroquinone ethers: unveiling in-plane aromaticity through a rotaxane structure.

[n]Cycloparaphenylenes ([n]CPPs, where n is the number of phenylene groups), consisting of 1,4-linked phenylene unit, have attracted much attention due to their cyclic π-conjugated structures and physical properties. However, functionalizing of the benzene rings of smaller [n]CPPs (n < 7) has been a challenge due to ring strain and steric hindrance of the substituents that hampers their synthesis. Here we show successful synthesis of a new [6]CPP derivative with twelve methoxy groups at the 2,5-positions of all benzene rings by utilizing our developed CPP synthesis method via a macrocyclic gold complex. This molecule exhibited a significantly higher oxidation potential caused by the electron-donating ability of the methoxy groups and the tubular molecular conformation, allowing facile oxidation to give dicationic species with in-plane aromaticity. Furthermore, this molecule successfully included with the guest molecules with a flexible alkyl chain in the cavity, enabling the creation of a CPP-based rotaxane, which exploited its mechanically interlocked molecular structure to the first experimental observation that the in-plane aromaticity in the center of the macrocycle.

Synthesis of organic carbamates as polyurethane raw materials from CO2: the quest for metal alkoxides as regenerable reagents.

It is well known that the utilization of carbon dioxide (CO2) for chemical materials is attracting research attention from the viewpoint of the carbon cycle. To contribute to the reduction of CO2 emission through such CO2 utilization reactions and counteract global climate change, the target compounds should be core chemical products that are distributed in large quantities and used for a long time. One such synthetic target is isocyanates that are used as raw materials for the production of polyurethanes, which are versatile polymeric materials with a service life of approximately 10 years. However, since direct synthesis of isocyanate from CO2 is quite difficult due to equilibrium constraints, a method via the use of its alcohol adduct, organic carbamate, as a precursor has been proposed. In this Perspective, we present regenerative metal alkoxide reactants, such as tin alkoxide, titanium alkoxide, and alkoxysilane, as environmentally benign reactants for the synthesis of organic carbamates from CO2. We also present a practical and environmentally friendly method for the highly efficient synthesis of various organic carbamates, including industrially important diisocyanate precursors, from 1 atm CO2 using alkoxysilanes. Furthermore, prospects for the practical application of these carbamate synthesis reactions are also discussed.

Dynamic Au-C σ-Bonds Leading to an Efficient Synthesis of [n]Cycloparaphenylenes (n = 9-15) by Self-Assembly.

The transmetalation of the digold(I) complex [Au2Cl2(dcpm)] (1) (dcpm = bis(dicyclohexylphosphino)methane) with oligophenylene diboronic acids gave the triangular macrocyclic complexes [Au2(C6H4) x (dcpm)]3 (x = 3, 4, 5) with yields of over 70%. On the other hand, when the other digold(I) complex [Au2Cl2(dppm)] (1') (dppm = bis(diphenylphosphino)methane) was used, only a negligible amount of the triangular complex was obtained. The control experiments revealed that the dcpm ligand accelerated an intermolecular Au(I)-C σ-bond-exchange reaction and that this high reversibility is the origin of the selective formation of the triangular complexes. Structural analyses and theoretical calculations indicate that the dcpm ligand increases the electrophilicity of the Au atom in the complex, thus facilitating the exchange reaction, although the cyclohexyl group is an electron-donating group. Furthermore, the oxidative chlorination of the macrocyclic gold complexes afforded a series of [n]cycloparaphenylenes (n = 9, 12, 15) in 78-88% isolated yields. The reorganization of two different macrocyclic Au complexes gave a mixture of macrocyclic complexes incorporating different oligophenylene linkers, from which a mixture of [n]cycloparaphenylenes with various numbers of phenylene units was obtained in good yields.

Digold(I) Thianthrenyl Complexes. Effect of Diphosphine Ligands on Molecular Structures in the Solid State and in Solution.

A series of digold complexes possessing two thianthrenyl ligands, Au2(Thi)2(Ph2P(CH2) n PPh2) (Thi: 1-thianthrenyl; 1: n = 1, 2: n = 2, 3: n = 3, 4: n = 4), were prepared and characterized by crystallographic and spectroscopic measurements. X-ray crystallography of complexes 1 and 3 revealed U-shaped structures with short Au-Au distances [3.2171(3) Å and 3.0735(2) Å]. Complex 2 and three of the four structure-determined molecules of complex 4 showed structures without Au-Au contacts. UV-vis spectroscopic measurements of 1-4 and TD-DFT calculations of the two conformers of 1 revealed that complexes 1 and 3 in the solution phase contained conformers with Au(I)-Au(I) interactions in a much higher proportion than complexes 2 and 4. As a result, complexes with diphosphine ligands containing an odd number of methylene groups preferred structures with Au-Au interactions in the solid state and in solution. Oxidation of 1 with 2 equiv of PhICl2 yielded a mixture of monomeric and dimeric thianthrenes and its dimer via ligand elimination and C-C coupling, respectively.

Ferrocene-Containing Pseudorotaxanes in Crystals: Aromatic Interactions with Hammett Correlation.

Single crystals of pseudorotaxanes, [(FcCH2NH2CH2Ar)(DB24C8)][PF6] (DB24C8 = dibenzo[24]crown-8, Fc = Fe(C5H4)(C5H5), Ar = -C6H3-3,4-Cl2, -C6H3-3,4-F2, -C6H4-4-F, -C6H4-4-Cl, -C6H4-4-Br, -C6H3-3-F-4-Me, -C6H4-4-I) and [(FcCH2NH2CH2C6H4-4-Me)(DB24C8)][Ni(dmit)2] (dmit = 1,3-dithiole-2,4,5-dithiolate), were obtained from solutions containing DB24C8 and ferrocenylmethyl(arylmethyl)ammonium. X-ray crystallographic analyses of the pseudorotaxanes revealed that the aryl ring of the axle moiety and the catechol ring of the macrocyclic component were at close centroid distances and parallel or tilted orientation. The structures with parallel aromatic rings showed correlation of the distances between the centroids to Hammett substituent constants of the aryl groups.

Transmetalation of boronic acids and their derivatives: mechanistic elucidation and relevance to catalysis.

The Suzuki-Miyaura reaction (the cross-coupling reaction of boronic acids with organic halides catalysed by Pd complexes) has been recognised as a useful synthetic organic reaction that forms a C(sp2)-C(sp2) bond. The catalytic cycle of the reaction involves the transmetalation of aryl- and alkenylboronic acids with Pd(II) complexes. It migrates the aryl and alkenyl groups of boronic acid to Pd and produces a Pd-C bond. Many studies have investigated the mechanism of transmetalation. They elucidated the mechanism of the organometallic reaction and its role as a fundamental step in catalytic reactions. This perspective reviews studies on the transmetalation of aryl- and alkenylboronic acids with Pd(II) complexes. Emphasis was laid on the structures and chemical properties of the intermediate Pd complexes and the effects of OH- on the pathways of the catalytic Suzuki-Miyaura reaction. The reactions of arylboronic acids with Rh(I)-OH complexes were investigated, which are relevant to the mechanism of Rh-catalysed addition of aryl boronic acids to enones and aldehydes. Recent studies on the transmetalation of boronic acids with other late transition metals such as Fe(II), Co(I), Pt(II), Au(III), and Au(I) are presented with the related catalytic reactions and their utilisation in the synthesis of aromatic molecules and π-conjugated materials.

Synthesis of a Ni Complex Chelated by a [2.2]Paracyclophane-Functionalized Diimine Ligand and Its Catalytic Activity for Olefin Oligomerization.

A diimine ligand having two [2.2]paracyclophanyl substituents at the N atoms (L1) was prepared from the reaction of amino[2.2]paracyclophane with acenaphtenequinone. The ligand reacts with NiBr2(dme) (dme: 1,2-dimethoxyethane) to form the dibromonickel complex with (R,R) and (S,S) configuration, NiBr2(L1). The structure of the complex was confirmed by X-ray crystallography. NiBr2(L1) catalyzes oligomerization of ethylene in the presence of methylaluminoxane (MAO) co-catalyst at 10-50 °C to form a mixture of 1- and 2-butenes after 3 h. The reactions for 6 h and 8 h at 25 °C causes further increase of 2-butene formed via isomerization of 1-butene and formation of hexenes. Reaction of 1-hexene catalyzed by NiBr2(L1)-MAO produces 2-hexene via isomerization and C12 and C18 hydrocarbons via oligomerization. Consumption of 1-hexene of the reaction obeys first-order kinetics. The kinetic parameters were obtained to be ΔG‡ = 93.6 kJ mol-1, ΔH‡ = 63.0 kJ mol-1, and ΔS‡ = -112 J mol-1deg-1. NiBr2(L1) catalyzes co-dimerization of ethylene and 1-hexene to form C8 hydrocarbons with higher rate and selectivity than the tetramerization of ethylene.

Synthesis and reactivity of boryloxorhodium complexes. Relevance to intermolecular transmetalation from boron to rhodium in Rh-catalyzed reactions.

The synthesis of a dimeric boryloxorhodium complex having the Rh-O-Bpin scaffold from the reaction of [(cod)Rh(OMe)]2 or [(cod)Rh(OH)]2 with an arylboronate has been achieved. The obtained dirhodium complex is converted into mononuclear complex [(cod)Rh(OBpin)(PPh3)], which reacts with arylboronic acid to afford the complex with an Rh-aryl bond via transmetalation from boron to rhodium. The dimeric boryloxorhodium complex catalyzes the 1,4-addition of arylboronic acid to cyclohexene-2-one.

Cyclodextrin Rotaxanes of Pt Complexes and Their Conversion to Pt Nanoparticles.

The cationic Pt complex (Pt(NC6H4-C6H4N-(CH2)10-O(C6H3-3,5-(OMe)2)(MeN-(CH2CH2NMe2)2))+ was prepared by the reaction of alkylbipyridinium ligand with a nitrateplatinum(II) complex. Mixing the complex and α- and ß-cyclodextrins in aqueous media produced the corresponding [2]rotaxanes with 1:1 stoichiometry. γ-Cyclodextrin and the Pt complex formed a rotaxane having components in a 1:1 or 2:1 molar ratio. The results of mass and nuclear magnetic resonance (NMR) measurements confirmed the rotaxane structures of the Pt complexes. Transmission electron microscopy (TEM) and atomic force microscope (AFM) analyses revealed the formation of micelles or vesicles. The addition of NaBH4 to the rotaxanes in aqueous media formed Pt nanoparticles with diameters of 1.3-2.8 nm, as characterized by TEM. The aggregated size of the nanoparticles formed from the rotaxane did not change even at 70 °C, and they showed higher thermal stability than those obtained from the reduction of the cyclodextrin-free Pt complex.

A Macrocyclic Gold(I)-Biphenylene Complex: Triangular Molecular Structure with Twisted Au2 (diphosphine) Corners and Reductive Elimination of [6]Cycloparaphenylene.

The digold(I) complex [Au2 Cl2 (Cy2 PCH2 PCy2 )] reacts with 4,4'-diphenylene diboronic acid to form a triangular macrocyclic complex with twisted Au-P-C-P-Au groups at the three corners. The synthesis of the complex and its chemical oxidation produced [6]cycloparaphenylene ([6]CPP) in 59 % overall yield.

Bimolecular fusion of [Pd3(µ-CN-C6H3Me2-2,6)3(CN-C6H3Me2-2,6)3] induced by Ph2GeH2: formation of the redox-active Pd6Ge2 complex.

The reaction of Ph2GeH2 with a planar tripalladium(0) complex, [Pd3(µ-CN-C6H3Me2-2,6)3(CN-C6H3Me2-2,6)3], selectively afforded a hexagonal bipyramidal Pd6Ge2 complex, [Pd6(µ-GePh2)2(CN-C6H3Me2-2,6)8(µ-CN-C6H3Me2-2,6)2]. The molecule is stabilized by bridging coordination of isonitrile and GePh2 ligands, although all the Pd-Pd bonds are weak, as revealed by DFT calculations. The resulting complex undergoes two successive redox processes at E1/2 = -1.35 and -1.03 V (vs. Fc+/Fc), which correspond to the stepwise oxidation of the Pd(0)6 complex.

Reversible Laser-Induced Bending of Pseudorotaxane Crystals.

This study investigated the dynamic photoresponse of pseudorotaxane crystals with azobenzene and ferrocenyl groups in the axle component. X-ray crystallography showed pseudorotaxanes with a methylazobenzene group and a dibromophenylene ring in the cyclic component to exhibit twisting of the trans-azobenzene groups at torsion angles of 17° and 38°, respectively. Repeated alternating laser irradiation of the crystals at 360 and 445 nm produced bending of 20-30° in opposite directions, with no evidence of decay. Under 445 nm irradiation, bending took place within 0.3 s. A crystal of nonsubstituted pseudorotaxane showed bending of only 2° under 360 nm irradiation due to multiple π-π interactions between the planar trans-azobenzene groups. The pseudorotaxane crystals have two chromophores, bent rapidly and reversibly on irradiation at rates depending on the molecular structure.

Cationic and Neutral Rotaxanes Having Different Functional Groups in the Axle Molecule and Their Coordination to PtII.

Dibenzo[24]crown-8 (DB24C8) forms rotaxanes with a linear molecule having a dialkylammonium group and a triazole group as well as with the acetylation product of a cationic axle molecule. The former cationic rotaxane is stabilized by multiple intermolecular hydrogen bonds between the NH2+ and oxyethylene groups. The neutral rotaxane contains the macrocycle in the vicinity of the terminal aryl group. The co-conformation of both the cationic and neutral rotaxanes can be fixed by coordination of the triazole group of the axle molecule to PtCl2 (dmso)2 . A 1 H NMR spectroscopic study on the thermodynamics of the Pt coordination revealed a larger association constant for the rotaxanes than for the corresponding axle molecules and a larger value for the neutral rotaxane than for the cationic rotaxane.

Planar PtPd3 Complexes Stabilized by Three Bridging Silylene Ligands.

A heterobimetallic PtPd3 complex supported by three bridging diphenylsilylene ligands, [Pt{Pd(dmpe)}3 (µ3 -SiPh2 )3 ] (1, dmpe=1,2-bis(dimethylphosphino)ethane), has been synthesized from mononuclear Pd and Pt complexes. The hexagonal core composed of Pt, Pd, and Si atoms is slightly larger than that of the tetrapalladium complex, [Pd{Pd(dmpe)}3 (µ3 -SiPh2 )3 ] (2). Reaction of PhSiH3 with complex 1 in the presence and absence of Ph2 SiH2 results in the formation of a tetranuclear complex with silyl and hydride ligands at the Pt center, [PtH(SiPh2 H){Pd(dmpe)}3 (µ3 -SiHPh)3 ] (3), and an octanuclear complex, [{Pt{Pd(dmpe)}3 (µ3 -SiHPh)3 }2 (κ2 -dmpe)] (5), respectively. Both M-Si (M=Pt, Pd) bond lengths and the 29 Si NMR chemical shifts of 1 and 2 are located between those of mononuclear late transition-metal complexes with a silylene ligand and complexes with donor-stabilized silylene ligands. CuI and AgI adducts of 1 and 2, formulated as [M(µ-M'I){Pd(dmpe)}3 (µ3 -SiPh2 )3 ] (M=Pt, Pd; M'=Cu, Ag), undergo elimination of CuI (AgI) and regenerate the tetrametallic complexes upon heating or addition of a chelating diphosphine. Elimination of AgI from 2-AgI occurs more rapidly than elimination of CuI from 2-CuI, as determined from the results of kinetics experiments.

Rapid and reversible photoinduced switching of a rotaxane crystal.

Crystalline phase transitions caused by external stimuli have been used to detect physical changes in the solid-state properties. This study presents the mechanical switching of crystals of ferrocene-containing rotaxane controlled by focused laser light. The expansion and contraction of the crystals can be driven by turning on and off laser light at 445 nm. The irradiation-induced expansion of the crystal involves elongation along the a, b and c axes at 30 °C, whereas heating of the crystal at 105 °C causes the shortening of c axis. The expansions reversibly occur and have the advantage of a rapid relaxation (reverse) process. Single-crystal X-ray crystallography reveals the detailed structural changes of the molecules, corresponding to a change in the size of the crystals on laser irradiation. This molecular crystal behaviour induced by laser irradiation, is demonstrated for the remote control of objects, namely, microparticle transport and microswitching in an electric circuit.

Synthesis of Optically Active Polystyrene Catalyzed by Monophosphine Pd Complexes.

Cationic Pd(II) monophosphine complexes derived from α- and ß-cyclodextrins (CDs) promote the homopolymerization of styrene under carbon monoxide pressure. Although reversible CO coordination takes place under catalytic conditions according to (13) C NMR studies with (13) C-enriched CO, both complexes catalyze the formation of CO-free styrene polymers. These macromolecules display optical activity as a result of the presence of stereoregular sequences within the overall atactic polymer.

Olefin Polymerization Catalyzed by Double-Decker Dipalladium Complexes: Low Branched Poly(α-Olefin)s by Selective Insertion of the Monomer Molecule.

Dipalladium complexes of a cyclic bis(diimine) ligand with a double-decker structure catalyze polymerization of ethylene and α-olefins and copolymerization of ethylene with 1-hexene. The polymerization of 1-hexene yields a polymer that is mainly composed of the hexamethylene unit formed by 2,1-insertion of the monomer into the palladium-carbon bond, followed by chain-walking (6,1-insertion). The polymerization of 4-methyl-1-pentene proceeds by 2,1-insertion with a selectivity of 92-97 %, and affords the polymer with methyl and 2-methylhexyl branches. 2,1-Insertion occurs selectively in all of the polymerization reactions of α-olefins catalyzed by the dipalladium complexes. Ethylene polymerization with the catalyst at 100 °C lasts over 24 h, whereas the monopalladium-diimine catalyst loses its activity within 8 h at 60 °C. Polyethylene obtained by the dipalladium catalyst is less-branched and has a higher molecular weight compared to that of the monopalladium catalyst under the same conditions. Copolymerization of ethylene with 1-hexene affords solid products with melting points and molecular weights that vary depending on the polymerization time, suggesting formation of a block and/or gradient copolymer.

Bond formation and coupling between germyl and bridging germylene ligands in dinuclear palladium(I) complexes.

The dinuclear palladium(I) complexes [L(Ar2 HGe)Pd(µ-GeAr2 )2 Pd(GeHAr2 )L] (Ar=Ph, p-Tol; L=PMe3 , tBuNC) contain terminal germyl and bridging germylene ligands with the experimentally observed Ge⋅⋅⋅Ge bond lengths of 2.8263(4) Š(L=PMe3 ) and 2.928(1) Š(L=tBuNC), which are close to the longest Ge-Ge bond reported to date [2.714(1) Å]. Significant Ge⋅⋅⋅Ge interactions between the germylene and germyl ligands (PMe3 complexes > tBuNC complexes) are supported by DFT calculations, Wiberg bond indices (WBI), and natural bond orbital (NBO) analyses. Exchanging tBuNC for PMe3 ligands increases the Ge⋅⋅⋅Ge interaction, and simultaneously activates two Pd-Ge bonds. Adding the chelating diphosphine 1,2-bis(diethylphosphino)ethane (depe) to the PMe3 complexes results in the intramolecular coupling of germyl and germylene ligands followed by extrusion of a digermane.

Dipalladium catalyst for olefin polymerization: introduction of acrylate units into the main chain of branched polyethylene.

A dipalladium complex with a double-decker structure catalyzes ethylene-acrylate copolymerization to produce the branched polymer containing the acrylate units in the polymer chain, not at the branch terminus. The cooperation of the two palladium centers, which are fixed in a rigid framework of the macrocyclic ligand, is proposed to have a significant dinuclear effect on the copolymerization.

Dynamic properties of molecular tweezers with a bis(2-hydroxyphenyl)pyrimidine backbone.

4,6-Bis(2-hydroxyphenyl)-2-alkylpyrimidines with two anthryl or 9-ethylnylanthryl substituents at the positions para to the OH groups prefer a U-shaped conformation supported by two intramolecular OH⋅⋅⋅N hydrogen bonds in the solid state and in CDCl3 solution. The compound with a hexyl substituent on the pyrimidine group and two 9-ethynylanthryl arms at the hydroxyphenyl groups forms a 1:1 complex with 2,4,7-trinitrofluorenone. Its association constant K(a) was estimated to be 2100 M(-1) at 298 K, which is larger than those of other molecular tweezers (K(a) < 1000 M(-1)). DFT calculations suggested that the complex adopts a stable conformation supported by intramolecular hydrogen bonds among the OH groups and the pyrimidine ring as well as by intermolecular π-π interaction between the anthryl groups and 2,4,7-trinitrofluorenone. Addition of nBu4NF to a solution of the molecular tweezers or their complexes causes the cleavage of one or two OH⋅⋅⋅N hydrogen bonds, formation of new O⋅⋅⋅HF hydrogen bonds, and changes in the molecular conformation. The resulting structure of the molecular tweezers contains nonparallel anthryl groups, which do not bind the guest molecule. Photochemical measurements on 4,6-bis(2-hydroxyphenyl)-2-methylpyrimidine with two anthryl substituents showed negligible luminescence (quantum yield ϕ<0.01), owing to photoinduced electron transfer of the molecule with a U-shaped structure. However, the O-hexylated compound exhibits emission from the anthryl groups with ϕ=0.39.