Reversible Transformation between a [PdL2]2+ “Figure-of-Eight” Complex and a [Pd2L2]4+ Dimer: Switching On and Off Self-Recognition

Structural changes to metallosupramolecular assemblies resulting in the release or uptake of guests are currently well established, whereas transformations turning on and off specific self-recognition are far less developed. We report a novel ligand (2,6-bis(1-(3-pyridin-4-yl)phenyl-1H-1,2,3-triazol-4-yl)pyridine) possessing a tridentate central metal-binding site flanked by two pendant pyridyl arms. In a 2:1 ratio with Pd^II metal ions, a spiro-type [PdL₂]²⁺ “Figure-of-eight” complex forms with the central tridentate binding pocket unoccupied. The introduction of an additional one equivalent of Pd^II metal ion results in the conversion to a dimeric [Pd₂L₂]⁴⁺ molecule with the tridentate pocket occupied. There is site-specific self-recognition between dimers in solution with strong NOE peaks between adjacent molecules. The self-recognition between dimers can be turned off in two ways: firstly, adding another equivalent of Pd^II metal ion brings about binding to the previously uncoordinated pyridyl arms that are key to the self-recognition event, and; secondly, addition of sufficient ligand to return the stoichiometry to 2:1 regenerates the [PdL₂]²⁺ complex. Hence, the self-recognition event can be turned on or off through simple variation of L:Pd^II stoichiometry.     

Complexes of Cu(I) and Pd(II) with (+)-camphor and (–)-cavrone thiosemicarbazones: Synthesis,structure, and cytotoxicity of the Pd(II) complex

CuLCl, CuL¹Cl, PdLCl₂, and PdL¹Cl₂ complexes [L and L¹ being (+)-camphor and (–)-carvone thiosemicarbazones, respectively] have been synthesized. The structure of binuclear [Pd₂L²²Cl₄] complex has been determined by means of X-ray diffraction. The L² ligand (dehydrogenated (–)-carvone thiosemicarbazone) is coordinated via the bridging S atom to two Pd atoms. The complexes of Cu(I) and Pd(II) presumably have polynuclear and binuclear structure, respectively. These facts are in good agreement with IR and NMR spectroscopy as well as mass spectrometry data which indicate the coordination of L and L¹ ligands via the S atom. The influence of L¹ and PdL¹Cl₂ on viability of the Hep2 cell line has been studied. The PdL¹Cl₂ complex is more cytotoxic than L¹ ligand.     

Dynamic Open Coordination Cage from Nonsymmetrical Imidazole–Pyridine Ditopic Ligands for Turn‐On/Off Anion Binding

This work demonstrates a new nonconventional ligand design, imidazole/pyridine‐based nonsymmetrical ditopic ligands ( 1 and 1 ^S ), to construct a dynamic open coordination cage from nonsymmetrical building blocks. Upon complex formation with Pd²⁺ at a 1:4 molar ratio, 1 and 1 ^S initially form mononuclear PdL₄ complexes (Pd²⁺( 1 )₄ and Pd²⁺( 1 ^S )₄) without formation of a cage. The PdL₄ complexes undergo a stoichiometrically controlled structural transition to Pd₂L₄ open cages ((Pd²⁺)₂( 1 )₄ and (Pd²⁺)₂( 1 ^S )₄) capable of anion binding, leading to turn‐on anion binding. The structural transitions between the Pd₂L₄ open cage and the PdL₄ complex are reversible. Thus, stoichiometric addition (2 equiv) of free 1 ^S to the (Pd²⁺)₂( 1 ^S )₄ open cage holding a guest anion ((Pd²⁺)₂( 1 ^S )₄?G^?) enables the structural transition to the Pd²⁺( 1 ^S )₄ complex, which does not have a cage and thus causes the release of the guest anion (Pd²⁺( 1 ^S )₄+G^?).     

Self‐assembly and Cycling of a Three‐state PdxLy Metallosupramolecular System

The use of stimuli to induce reversible structural transformations in metallosupramolecular systems is of keen interest to chemists seeking to mimic the way that Nature effects conformational changes in biological machinery. While a wide array of stimuli have been deployed towards this end, stoichiometric changes have only been explored in a handful of examples. Furthermore, switching has generally been between only two distinct states. Here we use a simple 2‐(1‐(pyridine‐4‐methyl)‐1H‐1,2,3‐triazol‐4‐yl)pyridine “click” ligand in combination with Pd^II in various stoichiometries and concentrations to quantitatively access and cycle between three distinct species: a [PdL₂]²⁺ monomer, a [Pd₂L₂]⁴⁺ dimer, and a [Pd₉L₁₂]¹⁸⁺ cage.     

Chiral complexes of PdCl<Subscript>2</Subscript> with hydroxypyrazolylquinoline and pyrazolylquinoline based on the monoterpenoid (+)-3-carene: Synthesis and structures

The chiral complexes [PdL¹Cl₂] (I) and [PdL²Cl₂] (II) (where L¹ and L² are hydroxypyrazolylquinoline and pyrazolylquinoline, respectively, based on the monoterpenoid (+)-3-carene) were obtained and examined using X-ray diffraction. The crystal structures of complexes I and II are built from mononuclear acentric molecules. The Pd²⁺ ions coordinate two N atoms of the chelating bidentate ligand L¹ or L² and two Cl atoms. The coordination polyhedron Cl₂N₂ is a square distorted in a tetrahedral manner. In structure I, adjacent molecules are linked by intermolecular contacts and hydrogen bonds Cl···H-O, which gives rise to chains aligned with the axis x. In structure II, contacts that are substantially shorter than the van der Waals interactions were not detected.     

Untersuchungen zur reaktivität von metall-π-komplexen : XXIX. Synthesewege und reaktivität von (PdPd)- und (PtPt)-zweikernkomplexen des typs (μ-C5H5)(μ-allyl)M2L2

The binuclear complexes (Cp)(2-RC₃H₄)M₂L₂ are formed either on reaction of equimolar amounts of CpM(2-RC₃H₄) and L (where L is a tertiary phosphine, phosphite or arsine) or by a “1 + 1” addition of CpM(2-RC₃H₄) and ML₂. The NMR data suggest that in all complexes the cyclopentadienyl and allyl ligands are analogously coordinated to both metal atoms and thus sandwich the LMML unit. CpPd(2-ClC₃H₄) reacts with L to give CpPd(L)Cl and allene. The reaction of CpPd(2-ClC₃H₄) and PdL₂ (L = P(i-Pr)₃) leads, probably via the intermediate (Cp)(Cl)Pd₂L₂, to the unsymmetrical binuclear complex Cp(L)PdPd(L)(2-ClC₃H₄) which isomerizes on heating to give (2-CpC₃H₄)(Cl)Pd₂L₂. The reactions of the (PdPd)-complexes (Cp)(2-RC₃H₄)Pd₂L₂ with electrophilic and nucleophilic reagents proceed predominantly by cleavage of the metal-to-metal bond. With I₂, HCl and MeI a mixture of mononuclear cyclopentadienylpalladium and allylpalladium complexes is always formed. In the reaction of (Cp)(2-MeC₃H₄)Pd₂L₂ with HBr, however, the formation of binuclear complexes with bromide as bridging ligand occurs. An exchange of L is only observed in the reaction of (Cp)(2-MeC₃H₄)Pd₂L₂ with trimethylphosphine.     

Synthesis of binuclear complexes of PdCl2 with chiral ethylenediamine dioxime (H2L1), piperazine dioxime (H2L2), and propylenediamine dioxime (H2L3), the derivatives of the natural monoterpenoid (R)-(+)-limonene. The crystal structures of [Pd2(H2L1)Cl4] and [Pd2(H2L2)Cl4]

The diamagnetic complexes [Pd₂(H₂L¹)Cl₄] (I), [Pd₂(H₂L²)Cl₄] (II), and Pd₂(H₂L³)Cl₄(III) with chiral ligands derived from the natural monoterpenoid (R)-(+)-limonene are obtained (H₂ L¹ is ethylenediamine dioxime, H₂L² is piperazine dioxime, and H₂L³ is propylenediamine dioxime). According to X-ray diffraction data, the crystal structures of complexes I and II are composed of binuclear acentric molecules. The coordination polyhedra PdN₂Cl₂ are trapeziums (squares distorted in a tetrahedral manner) made up of two N atoms of the tetradentate bridging cyclic ligands H₂L¹ and H₂L² and two Cl atoms. The fragments PdCl₂ are trans in the complexes. The ¹³C and ¹H NMR spectra of complexes I and II in CDCl₃ also suggest their binuclear structures.     

Synthesis of binuclear complexes of PdCl2 with chiral α,α′-diamino-meta-xylene dioximes H2L1, H2L2, and H2L3, the derivatives of the terpenes (+)-3-carene, (R)-(+)-limonene, and (S)-(−)-α-pinene. Crystal structure of [Pd2(H2L1)Cl4]

Chiral α,α′-diamino-meta-xylene dioximes H₂L¹, H₂L², and H₂L³ were obtained from the naturally occurring terpenoids (+)-3-carene, (R)-(+)-limonene, and (S)-(?)-α-pinene, respectively. Reactions of these ligands with PdCl₂ gave the diamagnetic complexes Pd₂(H₂L¹)Cl₄ (I), Pd₂(H₂L²)Cl₄ (II), and Pd₂(H₂L³)Cl₄ (III). According to X-ray diffraction data, the crystal structure of complex I consists of acentric binuclear molecules [Pd₂(H₂L¹)Cl₄]. The coordination polyhedron PdN₂Cl₂ is a square distorted in a tetrahedral manner (trapezium) made up of two N atoms of the tetradentate bridging cyclic ligand H₂L¹ and two Cl atoms. The fragments PdCl₂ in the complex are cis to each other. According to the ¹H NMR spectra of complexes I–III in CDCl₃, the organic ligands are coordinated through the N atoms; in solution, the complexes exist in several forms.     

Synthesis and Crystal Structures of Dinuclear Palladium and Mononuclear Nickel Complexes of Macrocyclic N8S4 Ligands

The N₈S₄ donor ligand L¹ has been investigated regarding its capability to support the formation of coordinatively unsaturated Pd₂ complexes and its use as a starting material for functionalized N₈S₄ systems. L¹ represents a macrotricyclic ligand comprising four 4‐tert‐butyl‐2,6‐bis(aminomethyl)thiophenolate units, whose N and S atoms are linked by ethylene units. Treatment of L₁ with [Pd(NCMe)₂Cl₂] produced the dinuclear complex [Pd₂Cl₂(H₂L¹)]⁴⁺, which was isolated as its pale‐yellow perchlorate salt [Pd₂Cl₂(H₂L¹)](ClO₄)₄ (H₂ 1 ) and characterized by elemental analysis, IR, NMR and MS spectroscopy and X‐ray crystallography. The structure shows two planar PdN₂SCl units which are located in the central 24‐membered ring of L¹. Reaction of L¹ with CH₂O/HCO₂H under Eschweiler‐Clarke conditions followed by deprotection with sodium in liquid ammonia furnished the permethylated octaamine‐tetrathiophenol H₄L⁴. The identity of H₄L⁴ was ascertained by an X‐ray crystal structure determination of one of its metal complexes.     

Further studies on reactions of organic halides with disilanes catalysed by transition metal complexes

The interaction of a range of organic halides with (Cl₃Si)₂ or (Me₃Si)₂ in the presence of a variety of transition metal catalysts (very predominantly Pd⁰ or Pd^II complexes) have been examined. PhSiMe₃ was formed from PhCl[m.y., 15%] (m.y. - maximum yield), PhBr (m.y., 92%, with [PdL₂Br₂] as catalyst (L - PPh₃)), and (contrary to earlier reports) PhI (m.y. 51%, with [PdL₂I₂]). MeSiCl₃ was formed from MeBr (m.y., 78% with [PdL₄]) and MeI (m.y., 91% with [PdL₄]), and EtSiCl₃ from EtBr (m.y., 49%, with [PdL₂“Br₂]; L” - P(C₆H₄OMe-p)₃) and EtI (m.y. 45%, with [PdL₄]). Me₄Si was satisfactorily formed from MeBr (m.y. 42%, with [PdL₄]). Evidence was obtained for the formation of Me₃SiCF₃ from CF₃I. Very poor yields of XC₆H₄CH₂SiMe₃ were obtained from XC₆H₄CH2Br (X - H orp-Me) (with X - H some PhSiMe₃ was formed), butp-O₂NC₆H₄CH₂SiMe₃ was formed in 48% yield fromp-O₂NC₆H₄CH₂Cl with [PdL“₄] as catalyst. PhCOSiMe₃ was formed from PhCOCl (m.y. 52% with [PdL₂I₂]. The nickel complex [NiL₄] was moderately effective as a catalyst for reactions between (Cl₃Si)₂ and MeBr, EtBr, or PhCH₂Br. The new complex [PdL₂(SiCl₃)₂] was prepared by treatment of [PdL₄] with (Cl₃Si)₂ or Cl₃SiH, and shown to catalyse the reaction between MeBr and (Cl₃Si)₂.     

Synthesis,structure, and characterization of picolyl- and benzyl-linked biphenyl palladium N-heterocyclic carbene complexes and their catalytic activity in acylative cross-coupling reactions

N-heterocyclic carbene ligands with picolyl (L¹H₂Br₂, L³H₂Br₂) and benzyl (L²H₂Br₂, L⁴H₂Br₂) linked biphenyl backbone were synthesized and characterized. Their palladium(II) complexes [PdL¹]Br₂ ( 1 ), [PdL²Br₂] ( 2 ), [PdL³]Br₂ ( 3 ), and [PdL⁴Br₂] ( 4 ) were synthesized by direct method using Pd(OAc)₂. All complexes ( 1 – 4 ) were characterized by CHN analysis, electrospray ionization-MS, nuclear magnetic resonance, and single-crystal X-ray diffraction. Molecular structures confirm the distorted square planar geometry around the Pd(II) center. All of them showed good catalytic activity in acylative Suzuki cross coupling of phenyl boronic acid with benzoyl chloride to afford benzophenone in good yields.     

η-Phosphole tricarbonylmanganates: a new type of chelating ligands for transition metals

3,4-Dimethylphosphacymantrene (1) reacts with tert-butyllithium to give the corresponding (η⁴-3,4-dimethyl-1-tert-butylphosphole)tricarbonylmanganate (2) which can act as a chelating ligand (L) toward a Pd₂Cl₂ dimeric core. The X-ray crystal structure of L₂Pd₂Cl₂ (3) shows a bite angle of 60.5°.     

Chiral Self‐Discrimination and Guest Recognition in Helicene‐Based Coordination Cages

Chiral nanosized confinements play a major role for enantioselective recognition and reaction control in biological systems. Supramolecular self‐assembly gives access to artificial mimics with tunable sizes and properties. Herein, a new family of [Pd₂L₄] coordination cages based on a chiral [6]helicene backbone is introduced. A racemic mixture of the bis‐monodentate pyridyl ligand L¹ selectively assembles with Pd^II cations under chiral self‐discrimination to an achiral meso cage, cis‐[Pd₂ L^1P ₂ L^1M ₂]. Enantiopure L¹ forms homochiral cages [Pd₂ L^1P/M ₄]. A longer derivative L² forms chiral cages [Pd₂ L^2P/M ₄] with larger cavities, which bind optical isomers of chiral guests with different affinities. Owing to its distinct chiroptical properties, this cage can distinguish non‐chiral guests of different lengths, as they were found to squeeze or elongate the cavity under modulation of the helical pitch of the helicenes. The CD spectroscopic results were supported by ion mobility mass spectrometry.     

Visible-Light Switching of Metallosupramolecular Assemblies**

A photoswitchable ligand and palladium(II) ions form a dynamic mixture of self-assembled metallosupramolecular structures. The photoswitching ligand is an ortho-fluoroazobenzene with appended pyridyl groups. Combining the E-isomer with palladium(II) salts affords a double-walled triangle with composition [Pd₃L₆]⁶⁺ and a distorted tetrahedron [Pd₄L₈]⁸⁺ (1 : 2 ratio at 298 K). Irradiation with 410 nm light generates a photostationary state with approximately 80 % of the E-isomer of the ligand and results in the selective disassembly of the tetrahedron, the more thermodynamically stable structure, and the formation of the triangle, the more kinetically inert product. The triangle is then slowly transformed back into the tetrahedron over 2 days at 333 K. The Z-isomer of the ligand does not form any well-defined structures and has a thermal half-life of 25 days at 298 K. This approach shows how a thermodynamically preferred self-assembled structure can be reversibly pumped to a kinetic trap by small perturbations of the isomer distribution using non-destructive visible light.     

Metallkomplexe von Farbstoffen. VI. Phosphan-Nickel-, Palladium- und Platin-Komplexe sowie Pentamethyl-cyclopentadienyl-Rhodium- und Iridium-Komplexe von Bis(2-hydroxyaryl)azo-Farbstoffen

Metal Complexes of Dyes. Phosphine-Nickel, Palladium, Platinum Complexes and Pentamethylcyclopentadienyl Rhodium and Iridium Complexes of 2,2′-Dihydroxyazoarenes The terdentate dianions of 2,2′-dihydroxyazobenzene (L¹H), 1-(2-hydroxy-4-nitrophenylazo)-2-naphthol (L²H), 1-(2-hydroxy-5-nitrophenylazo)-2-naphthol (L³H) and 1-phenyl-3-methyl-4-(2-hydroxy-5-nitrophenylazo)-5-pyrazolone (L⁴H) form with chloro bridged complexes [(R₃P)MCl₂]₂ (M = Pd, Pt; R = Ph, nBu), [(n⁵-C₅Me₅)MCl₂]₂ (M = Rh, Ir) and with (nBu₃P)₂NiCl₂ the metal dye complexes (R₃P)ML (M = Ni, Pd, Pt) and (C₅Me₅)ML (M = Rh, Ir). The structures of (Ph₃P)PtL¹ and (nBu₃P)PdL³ have been determined by X-ray diffraction. For the complexes (n⁵-C₅Me₅)ML (M = Rh, Ir) with asymmetric metal centers two diastereoisomers are detected by nmr spectroscopy which points to the ?hydrazone”? form of the azo ligand with a pyramidalized N-atom.     

Stoichiometrically Controlled Revocable Self‐Assembled “Spiro” versus Quadruple‐Stranded “Double‐Decker” Type Coordination Cages

The simple combination of Pd^II with the tris‐monodentate ligand bis(pyridin‐3‐ylmethyl) pyridine‐3,5‐dicarboxylate, L , at ratios of 1:2 and 3:4 demonstrated the stoichiometrically controlled exclusive formation of the “spiro‐type” Pd₁L₂ macrocycle, 1 , and the quadruple‐stranded Pd₃L₄ cage, 2 , respectively. The architecture of 2 is elaborated with two compartments that can accommodate two units of fluoride, chloride, or bromide ions, one in each of the enclosures. However, the entry of iodide is altogether restricted. Complexes 1 and 2 are interconvertible under suitable conditions.     

Complexes of nickel(II) and palladium(II) with thiobenzamide

Summary The electronic and vibrational spectra of Ni^II and Pd^II complexes with thiobenzamide, L, are discussed. L acts as a sulphur donor ligand. The Pd^II compounds and (NiL₄)(ClO₄)₂ are square planar. PdL₂Cl₂ has acis-structure, while PdL₂X₂ (X=Br or I) istrans; NiL₄Cl₂ istrans-octahedral. The i.r. bands due to(M.S) and(MX) have been assigned. The influence of the anions on the properties of the complexes, both in solution and in the solid state, is discussed.     

Catenation and Aggregation of Multi‐Cavity Coordination Cages

A series of metal‐mediated cages, having multiple cavities, was synthesized from Pd^II cations and tris‐ or tetrakis‐monodentate bridging ligands and characterized by NMR spectroscopy, mass spectrometry, and X‐ray methods. The peanut‐shaped [Pd₃L¹₄] cage deriving from the tris‐monodentate ligand L¹ could be quantitatively converted into its interpenetrated [5Cl@Pd₆L¹₈] dimer featuring a linear {[Pd‐Cl‐]₅Pd} stack as an unprecedented structural motif upon addition of chloride anions. Small‐angle neutron scattering (SANS) experiments showed that the cigar‐shaped assembly with a length of 3.7 nm aggregates into mono‐layered discs of 14 nm diameter via solvophobic interactions between the hexyl sidechains. The hepta‐cationic [5Cl@Pd₆L¹₈] cage was found to interact with polyanionic oligonucleotide double‐strands under dissolution of the aggregates in water, rendering the compound class interesting for applications based on non‐covalent DNA binding.     

Solvent‐Dependent Self‐Assembly Behaviour and Speciation Control of Pd6L8 Metallo‐supramolecular Cages

The C₃‐symmetric chiral propylated host‐type ligands (±)‐tris(isonicotinoyl)‐tris(propyl)‐cyclotricatechylene ( L1 ) and (±)‐tris(4‐pyridyl‐4‐benzoxy)‐tris(propyl)‐cyclotricatechylene ( L2 ) self‐assemble with Pd^II into [Pd₆L₈]¹²⁺ metallo‐cages that resemble a stella octangula. The self‐assembly of the [Pd₆( L1 )₈]¹²⁺ cage is solvent‐dependent; broad NMR resonances and a disordered crystal structure indicate no chiral self‐sorting of the ligand enantiomers in DMSO solution, but sharp NMR resonances occur in MeCN or MeNO₂. The [Pd₆( L1 )₈]¹²⁺ cage is observed to be less favourable in the presence of additional ligand, than is its counterpart, where L=(±)‐tris(isonicotinoyl)cyclotriguaiacylene ( L1 a ). The stoichiometry of reactant mixtures and chemical triggers can be used to control formation of mixtures of homoleptic or heteroleptic [Pd₆L₈]¹²⁺ metallo‐cages where L= L1 and L1 a .     

Complexation of palladium cyanide with imidazolidine-2-thione and its derivatives

Summary Pd(CN)₂ reacts with imidazolidine-2-thione (Imt), 1,3-diazinane-2-thione(Diaz), 1,3-diazipnane-2-thione (Diap) and their derivatives to yield complexes of stoichiometry [PdL₂(CN)₂] or [PdL(CN)₂] (L = Imt, Diaz or Diap and L = Imt having N-Me, Et or Pr substituents), which were characterized by elemental analysis, i.r., ¹H and ¹³C n.m.r. spectroscopy. Both mono- and bis ligand complexes are thought to be square planar with the monoligand binding to metal via sulphur (bridging) and the bis ligand via the monodentate thione group. The ¹³C enriched Pd(¹³CN)₂ complex was prepared and the ¹³C n.m.r. recorded. The C-2 resonance of ¹³C n.m.r. of Imt, Diaz or Diap complexes of the copper(I), silver(I), gold(I) and palladium(II) were compared.