Cis-protected Pd Research Articles

Rational design of metal–organic cages to increase the number of components via dihedral angle control

The general principles of discrete, large self-assemblies composed of numerous components are not unveiled and the artificial formation of such entities is a challenging topic. In metal–organic cages, design strategies for tuning the coordination directions in multitopic ligands by the bend and twist angles were previously developed to solve this problem. In this study, the importance of remote geometric communications between components is emphasized, realizing several types of metal–organic assemblies based on dihedral angle control in multitopic ligands although they have the same coordination directions. Self-assembly of a tritopic ligand with dihedral angles θ = 36.5° and a cis-protected Pd(II) ion affords M9L6 and M12L8 cages as kinetic and thermodynamic products, respectively, whereas an M12L8 sheet is formed when θ = 90°. Geometric analyses of strains in the subcomponent rings reveals that remote geometric communications among neighboring multitopic ligands through coordination bonds are key for large assemblies.

Formation of a low-symmetry Pd8 molecular barrel employing a hetero donor tetradentate ligand, and its use in the binding and extraction of C70.

The majority of reported metallo-supramolecules are highly symmetric homoleptic assemblies of M x L y type, with a few reports on assemblies that are obtained using multicomponent self-assembly or using ambidentate ligands. Herein, we report the use of an unsymmetrical tetratopic ligand (Lun) containing pyridyl and imidazole donor sites in combination with a cis-protected Pd(ii) acceptor for the formation of a low-symmetry M8Lun 4 molecular barrel (UNMB). Four potential orientational isomeric (HHHH, HHHT, HHTT, and HTHT) molecular barrels can be anticipated for the M8Lun 4 type metallo-assemblies. However, the formation of an orientational isomer (HHTT) of the barrel was suggested from single-crystal X-ray diffraction and 1H NMR analysis of UNMB. Two large open apertures at terminals and the hydrophobic confined space surrounded by four aromatic panels of Lun make UNMB a potential host for bigger guests. UNMB encapsulates fullerenes C70 and C60 favoured by non-covalent interactions between the fullerenes and aromatic panels of the ligand molecules. Experimental and theoretical studies revealed that UNMB has the ability to bind C70 more strongly than its lower analogue C60. The stronger affinity of UNMB towards C70 was exploited to separate C70 from an equimolar mixture of C70 and C60. Moreover, C70 can be extracted from the C70⊂UNMB complex by toluene, and therefore, UNMB can be reused as a recyclable separating agent for C70 extraction.

Diastereoselectively self-sorted low-symmetry binuclear metallomacrocycle and trinuclear metallocage.

A pyridine/aniline appended unsymmetrical bis-monodentate ligand N-(3-aminophenyl)nicotinamide, Lun is synthesized via condensation of nicotinic acid with excess m-phenylene diamine. A low-symmetry binuclear complex of the Pd2L'2Lun2 type and an extremely rare trinuclear complex of the Pd3Lun6 type are produced by self-assembly of the ligand Lun with cis-protected palladium(II) (i.e., PdL') and palladium(II), respectively. Two isomers (i.e. [(2,0), (2,0)] and [(1,1), (1,1)]-forms) are theoretically possible for the Pd2L'2Lun2-type complex whereas nine isomers can be envisaged in the case of the Pd3Lun6-type arrangement. However, one of the isomers of the Pd2L'2Lun2-type complex as well as the one for the Pd3Lun6-type complex are experimentally obtained. The exclusive formation of specific isomers could be predicted from the 1D/2D NMR study in the solution state and the DFT calculations in the gas phase/implicit solvent media. The formation of the predicted all-(1,1)-[Pd2(en)2Lun2](NO3)4 has been confirmed by a single-crystal XRD study. DFT calculations for the isomers of the Pd3Lun6-type arrangement show that a [cis(2,2), cis(2,2), cis(2,2)] isomer is energetically favourable than the alternatively predicted [trans(2,2), trans(2,2), trans(2,2)] isomer. Conformational changes within the build of the exclusively formed isomers are proposed on the basis of NMR study.

Conformational Solvatomorphism in a [2]Catenane

The combination of a flexible pyridine-appended ligand (L1) with 4,4′-bipyridine (L2) and cis-protected Pd(II) units, Pd(L′)(NO3)2, in water yielded a concentration-dependent equilibrium mixture of the corresponding [2]catenanes [Pd2(L′)2(L1)(L2)]2(NO3)8 and their constituent macrocycles [Pd2(L′)2(L1)(L2)](NO3)4. The cis-protecting units (L′) used were ethylenediamine (en), tetramethylethylenediamine (tmeda), 2,2′-bipyridine (bpy), and 1,10-phenanthroline (phen). Crystallization of the [2]catenanes via slow evaporation of their CH3CN/H2O (1:1) solutions yielded a single crystalline form in the case of [Pd2(2,2′-bpy)2(L1)(L2)]2(NO3)8 and two concomitant crystalline forms (solvates) in the case of [Pd2(phen)2(L1)(L2)]2(NO3)8. The formation of the [2]catenane is facilitated by noncovalent interactions such as N–H···O, C–H···O, and C–H···π contacts and π···π stacking. The solvates of [Pd2(phen)2(L1)(L2)]2(NO3)8 revealed differences in the orientation of the amide carbonyl group and conformational differences in the ligands and phen units in the macrocycles and may thus be termed conformational solvatomorphs. The molecular packing in all the structures was found to be driven by intermolecular π···π stacking interactions between the aromatic rings in their cis-protected Pd(II) units. However, because of the awkward shape of the molecules, the closest 2,2′-bpy and phen units in all the structures overlap differently rather than the commonly observed “head-to-tail” or “head-to-head” modes.

Hierarchical Assembly toward Nanoparticles of a Chiral Palladium Supramolecular Complex Based on Bicyclo[3.3.1]nonane Framework

The cyclic supramolecular coordination constructs were synthesized via metal-directed self-assembly from cis-protected Pd(II) precursor and chiral and racemic C2 symmetric V-shaped bicyclo[3.3.1]no...

Self-Assembly Processes of Octahedron-Shaped Pd6L4 Cages.

The self-assembly processes of two kinds of octahedron-shaped M6L4 cages consisting of cis-protected Pd(II) complexes and organic tritopic ligands were investigated. Whether kinetically trapped species larger than the cages are produced or the M6L4 cage is assembled without the formation of such kinetic traps is determined by a balance between the rates of oligomerization and intramolecular cyclization, which is affected by slight changes in the chemical structure of the tritopic ligand. A numerical analysis of the experimental data based on a reaction network model where 249 reactions between the possible 56 species were considered revealed the self-assembly pathways of one of the two M6L4 cages.

Self-Assembled Molecular Squares as Supramolecular Tectons

Concentration-dependent equilibria of molecular squares [Pd4(L′)4(L)4](NO3)8 and triangles [Pd3(L′)3(L)3](NO3)6 were obtained when cis-protected Pd(II) units [PdL′(NO3)2] (L′ = tmeda, 2,2′-bpy, and phen) were combined independently with 4,4′-bipyridine (L) in water. However, complexation of [PdL′(OTs)2] with L resulted in exclusive formation of the corresponding molecular squares. The addition of AgOTs to each mixture of square and triangle led to a shift in the equilibrium, resulting in the disappearance of the triangles and exclusive formation of the corresponding squares. The crystal structures of the molecular squares [Pd4(L′)4(L)4](OTs)8 revealed a pair of tosylate anions encapsulated in the hydrophobic cavity of the square. Further, [Pd4(2,2′-bpy)4(L)4](OTs)8 and [Pd4(phen)4(L)4](OTs)8 exhibited solvatomorphism, yielding two crystalline forms each, respectively. The cationic units in these crystals associate through intermolecular π···π stacking interactions wherein the cis-protecting units (i.e., 2...

Self-assembled molecular squares as supramolecular tectons

Concentration-dependent equilibria of molecular squares [Pd4(L′)4(L)4](NO3)8 and triangles [Pd3(L′)3(L)3](NO3)6 were obtained when cis-protected Pd(II) units [PdL′(NO3)2] (L′ = tmeda, 2,2′-bpy, and phen) were combined independently with 4,4′-bipyridine (L) in water. However, complexation of [PdL′(OTs)2] with L resulted in exclusive formation of the corresponding molecular squares. The addition of AgOTs to each mixture of square and triangle led to a shift in the equilibrium, resulting in the disappearance of the triangles and exclusive formation of the corresponding squares. The crystal structures of the molecular squares [Pd4(L′)4(L)4](OTs)8 revealed a pair of tosylate anions encapsulated in the hydrophobic cavity of the square. Further, [Pd4(2,2′-bpy)4(L)4](OTs)8 and [Pd4(phen)4(L)4](OTs)8 exhibited solvatomorphism, yielding two crystalline forms each, respectively. The cationic units in these crystals associate through intermolecular π···π stacking interactions wherein the cis-protecting units (i.e., 2...

Cis-Protected palladium(ii) based binuclear complexes as tectons in crystal engineering and the imperative role of the cis-protecting agent

The shape and molecular packing of cis-protected Pd(ii) based binuclear complexes depends upon the ligand and cis-protecting agents.

Reversible Mechanical Interlocking of D-Shaped Molecular Karabiners bearing Coordination-Bond Loaded Gates: Route to Self-Assembled [2]Catenanes.

Complexation of 1,4-phenylenebis(methylene) diisonicotinate, L1, with cis-protected Pd(II) components, [Pd(L')(NO3 )2 ], in an equimolar ratio yielded binuclear complexes, 1 a-d of [Pd2 (L')2 (L1)2 ](NO3 )4 formulation where L' stands for ethylenediamine (en), tetramethylethylenediamine (tmeda), 2,2'-bipyridine (bpy), and phenanthroline (phen). The combination of 4,4'-bipyridine, L2, with the cis-protected Pd(II) units is known to yield molecular squares, 2 a-d. However, 2 b-d coexist with the corresponding molecular triangles, 3 b-d. Combination of an equivalent each of the ligands L1 and L2 with two equivalents of cis-protected Pd(II) components in DMSO resulted in the D-shaped heteroligated complexes [Pd2 (L')2 (L1)(L2)](NO3 )4 , 4 a-d. Two units of the D-shaped complexes interlock, in a concentration dependent fashion, to form the corresponding [2]catenanes [Pd2 (L')2 (L1)(L2)]2 (NO3 )8 , 5 a-d under aqueous conditions. Crystal structures of the macrocycle [Pd2 (tmeda)2 (L1)(L2)](PF6 )4 , 4 b'', and the catenane [Pd2 (bpy)2 (L1)(L2)]2 (NO3 )8 , 5 c, provide unequivocal support for the proposed molecular architectures.

Dynamic [2]Catenation of Pd(II) Self-assembled Macrocycles in Water

Abstract A catenated structure is formed through dynamic self-assembly of 1,3-bis(4-pyridyloxy)propane and cis-protected Pd(II) metal with ethylenediamine in water. 1H NMR spectra have revealed that the formation of [2]catenane is dependent on the concentration of macrocyclic ring. We have performed the structural analysis of macrocyclic ring and [2]catenane using H–H COSY and ESI MS. The dynamic process of the catenation has been elucidated by van’t Hoff plot and the formation of [2]catenane is enthalpically favored in D2O, strongly suggesting the hydrophobic interactions for the formation of [2]catenane. Diffusion-ordered NMR spectroscopy (DOSY) distinguishes between dinuclear self-assembled macrocyclic ring and [2]catenane, indicating that [2]catenanes diffuse slower than macrocyclic rings in D2O.

Self‐Discriminating Self‐Assembly of Dinuclear Heterochiral Rhombs from Tröger’s Base Derived Bis(pyridyl) Ligands

Five racemic dissymmetric bis(pyridyl) ligands based on 2,8- or 3,9-difunctionalised Tröger's base derivatives have been synthesised. Only those derived from a 2,8-difunctionalised scaffold were found to undergo selective self-assembly to discrete self-assembled dinuclear metallosupramolecular aggregates of rhomboid shape upon coordination to cis-protected Pd(2+) or Pt(2+) ions, as evidenced by ESI mass spectrometry, NMR spectroscopy and single-crystal X-ray diffraction. Interestingly, these processes were found to be highly diastereoselective leading to the formation of C(2v)-symmetric heterochiral assemblies in a self-discriminating manner.

Dynamic Self‐Assembly of an M3L6 Molecular Triangle and an M4L8 Tetrahedron from Naked PdII Ions and Bis(3‐pyridyl)‐Substituted Arenes

The pyridine-appended nonchelating bidentate ligands 1,4-bis(3-pyridyl)benzene (1) and 4,4'-bis(3-pyridyl) biphenyl (2) were complexed with a naked Pd(II) ion for the construction of molecular cage compounds. Prior to these experiments, the complexation of the ligands with cis-[Pd(en)(NO3)2] was also examined, because self-assemblies from the cis-protected Pd(II) ion were expected to be simple motifs that constitute the assemblies from naked Pd(II) ion. The structures of the self-assembled compounds resulting from 1 and [Pd(en)(NO3)2] depended on the solvent employed. In aqueous solution, an M2L2 trenchlike compound was obtained. In dimethyl sulfoxide, however, a mixture of the M2L2 trench and an M3L3 macrocycle was found in equilibrium, the dynamic nature of which was confirmed by the concentration-dependent nature of the species. At higher concentration, an M4L4 macrocycle was mostly observed. The complexation of 1 with naked Pd(II) ions was expected to produce novel structures that are combinations of the M(n)L(n) type frameworks. A peculiar tetrahedral M4L8 assembly was obtained quantitatively from 1 and Pd(NO3)2, rather than the smallest possible M3L6 double-walled triangle. Interestingly, the use of Pd(CF3SO3)2 resulted in the sole formation of the latter structure. Thus, the anion is important as a template in the formation of these assemblies. Ligand 2, which contains an extra p-phenylene unit compared to 1, behaved in a similar manner when treated with [Pd(en)(NO3)2], but showed subtle differences with naked Pd(II) ions. With Pd(NO3)2, 2 gave mostly a tetrahedron along with a double-walled triangle. With Pd(CF3SO3)2, this longer ligand formed a double-walled triangle with a negligible amount of tetrahedra. A single discrete assembly of a perfect tetrahedron was obtained from 2 and Pd(II) ions by choosing p-tosylate as a counterion.

Side chain-directed complementary cis-coordination of two pyridines on Pd(II): Selective multicomponent assembly of square-, rectangular-, and trigonal prism-shaped molecules

This paper describes a new strategy to regulate multicomponent assembly from two kinds of pyridine-based ligands and cis-protected Pd(II) ions. The introduction of sterically hindered substituents to only one of the two ligands directs the complementary cis-coordination of the two lingads on the Pd(II) center, leading to the selective multicomponent assembly of two- and three-dimensional polynuclear Pd(II) complexes (e.g., square-, rectangular-, and trigonal prism-shaped molecules).

Single‐Walled Carbon Nanotubes Under the Influence of Dynamic Coordination and Supramolecular Chemistry

A dynamic coordinative-directed solubilization of single-walled carbon nanotubes (SWNTs) in aqueous solutions has been achieved through a combination of a Zn(II) metalloporphyrin complex and a cis-protected Pd(II) complex, which are believed to form charged acyclic and/or cyclic adducts on or around the side walls of SWNTs. The solubilization of SWNTs in aqueous solution only occurs when these acyclic and/or cyclic complexes are allowed to enter simultaneously into a self-assembly process with SWNTs under mild conditions. The aqueous solubility properties that these dynamic complexes confer upon SWNTs are believed to involve noncovalent bonding interactions between the two entities. They have been probed in solution using ultraviolet and visible absorption spectroscopy and in thin films using high-resolution transmission electron microscopy. The supramolecular electronic effects that the individual components of their acyclic and/or cyclic complexes impart upon a single semiconducting SWNT have been probed within a nanotube field-effect transistor device.

Metal driven self-assembly of pyridine appended ligands with cis-protected/naked Pd(ii) ion: a comparative study

Discrete self-assemblies prepared by designed pyridine appended ligands with cis-protected Pd(II) ion (5) are described. If ligands 2, 3 and 4 are separately treated with (5), the formation of (i) dynamic equilibrium of macrocyclic M2L2 (6) and M4L4 assemblies (7); (ii) macrocyclic M2L2 product (9); and (iii) macrobicyclic M3L2 (11) occurred, respectively. The assembly so obtained from the combination of a ligand and (5) may be considered as a module or reminiscent of the assembly that is expected from the complexation of same ligand with naked Pd(II). In fact, complexation of the above mentioned ligands with naked Pd(II) resulted in discrete structures. This is in contrast to the possible polymeric structure usually obtained from such types of ligands in combination with naked Cd(II), Ni(II) etc. The result that the ligands 2, 3, and 4 with naked Pd(II) give macrotricyclic M2L4 (8); macropentacyclic M4L8 (10); and M6L8 spherical assembly (12), respectively, establishes interesting strategies. Thus we have shown rational comparison of a particular ligand for either cis-protected or naked Pd(II) ion for first time.

Metal-directed self-assembly of two- and three-dimensional synthetic receptors

This article reviews recent progress in the study of the transition-metal mediated self-assembly of two- and three-dimensional synthetic receptors. Whereas macrocyclization under kinetic control is undoubtedly an unfavorable process, the self-assembly strategy offers quite efficient methods for constructing macrocycles under thermodynamic control. In particular, cis-protected Pd(II) and Pt(II) blocks are quite effective in obtaining the cyclic framework from simple molecules. Examples disclosed in this article are spontaneously assembled in quantitative yields by just mixing component molecules. This approach is successfully applied to the construction of cage compounds. The self-assembly of nanosized macrocycles and cages is also discussed.

Self-assembling [2]catenanes: molecular magic rings

This article reviews the recent progress of catenane synthesis. A focus is self-assembled catenanes containing transition metals in their backbones. Treatment of a cis-protected Pd(II) complex with a pyridine-based simple ligand gave rise to the self-assembly of an equilibrium mixture of a Pd(II)-linked catenane and its component ring. In this equilibrium, a catenane is formed via rapid slippage of two preformed molecular rings, reminiscent of magic rings well known to magicians. The catenane became an overwhelmingly dominant species at high concentrations in polar media. In a counterpart platinum system, an inert Pt(II)-pyridine bond became labile only at a high temperature in polar media due to the inertia of a Pt(II)-pyridine bond. Thus a Pt(II)-linked catenane, which self-assembled from two component rings under the forced conditions, never dissociated into the separate rings. The mechanism of the rapid slippage of molecular rings will be also discussed.