Cyclophane Ligand Research Articles

Ligand Noninnocence in β-Diketiminate and β-Diketimine Copper Complexes.

Metal-ligand cooperative systems have a long precedent in catalysis, with the classification depending on the site of substrate bond cleavage and formation and on redox state changes. Recently, our group reported the participation of a β-diketiminate ligand in chemical bonding to heterocumulenes such as CO2 and CS2 by tricopper complexes, leading to cooperative catalysis. Herein, we report the reactivity of these copper clusters, [Cu3EL]- (E = S, Se; L = tris(β-diketiminate) cyclophane ligand), toward other electrophiles, viz. alkyl halides and Brønsted acids. We identified a family of ligand-functionalized complexes, Cu3EL (R) (R = primary alkyls), and a series of disubstituted products, Cu3EL (R)2, through single-crystal X-ray diffraction, mass spectrometry, and infrared and UV-visible spectroscopy. As part of mechanistic studies on these alkylation reactions, we evaluated the acid-base reactivity of these complexes and the influence of the backbone substitution on the reduction potential. Implications of these findings for ligand noninnocence and the relevance of the metal core as a cofactor for the ligand's reactivity are discussed.

Metal-ligand bonding situation in ruthenophanes containing i,j-xylylene-linked bis(NHC)cyclophane ligands

The electronic structure of a isomeric series of ruthenophanes containing NHC-cyclophanes derived from imidazolium-linked cyclophanes was studied in the light of energy decomposition analysis and natural bond orbitals. The relative stability, bridge tensions and the metal-ligand bonding situation in a set complexes (1–9) formed by isomers of the i,j-xylylene-linked bis(NHC)cyclophane ligand were determined. Isomers containing aromatic moieties in both anti and syn conformations were evaluated. The relative stability of the isomeric series was evaluate by means of electronic structure calculations at DFT level, including the already synthesized and characterized Baker's complex 1, which was identified as the most stable, followed by complex 4. Our calculations reveal that by using as starting material a imidazolium-linked-m,o-cyclophane salt under similar conditions as used by Baker [1], the complex 4 can be likewise obtained. The calculated strain energies imposed on the ligand tether revealed that the strain in NHC-cyclophanes bridges is considerably high. The EDA-NOCV results concerning metal-ligand bonding situation reveal that the stabilization between the cyclophanes and the {RuCl}+ fragments is mainly due to the orbital term, followed by the electrostatic interaction. The orbital stabilization is comprised mainly of σ-donation from carbenes to ruthenium(II) ion, which is the most significant contribution, followed by the metal→ligand π-backdonation.

ChemInform Abstract: Insights into Small Molecule Activation by Multinuclear First‐row Transition Metal Cyclophanates

AbstractReview: 98 refs.

Synthesis of Trinuclear Tin(II), Germanium(II), and Aluminum(III) Cyclophane Complexes

The synthesis and characterization of trinuclear GeII and SnII chlorides and a trialuminum complex supported by a trinucleating tris(β-diketiminate) cyclophane ligand (L3–) are reported. The in situ deprotonation of H3L with benzylpotassium and subsequent reaction with GeCl2·dioxane or SnCl2 afforded (GeCl)3L (1) and (SnCl)3L (2) in 42 and 60% yields, respectively. Each GeII and SnII atom is three-coordinate and exhibit pseudotrigonal pyramidal geometry as anticipated for three-coordinate divalent group 14 cations. UV/visible spectra collected on THF solutions of 1 and 2 display a bathochromic shift in the absorption from 1 to 2 (from 361 to 375 nm). Addition of AlMe3 to toluene solutions of H3L resulted in the formation of (AlMe2)2AlMe3HL (3), which possesses two NCCCN chelated AlMe2 moieties. The third β-diketimine arm remains protonated and adopts an atypical trans conformation with an AlMe3 coordinated to the solvent exposed imine N atom.

Synthesis and characterization of phosphorescent cyclometalated Ir and Pt heteroleptic complexes using cyclophane-based chelates

The synthesis and characterization of a series of neutral cyclometalated Pt(II) and Ir(III) heteroleptic complexes containing cyclophane substituted chelates is reported. The complexes have the general formula of (C^N)Pt(O^O) or (C^N)2Ir(O^O) where C^N is a monoanionic cyclophane ligand 2-([2.2]-paracyclophane-4-yl)pyridyl (pCpy) or 1-([2.2]-paracyclophane-4-yl)pyrazolyl (pCpz) and (O^O) is an ancillary ligand; acetylacetonato (acac) or dipivolylmethanato (dpm). The pCpyH and pCpzH ligand precursors were obtained as racemates due to the planar chirality of the cyclophane ring system. Two diastereomers of (pCpz)2Ir(acac), one with C1 symmetry (ΛRS, ΔSR) and the other with C2 symmetry (ΛRR, ΔSS) were isolated, whereas only one C1 diastereomer was obtained for (pCpy)2Ir(acac). Inherent strain in the cyclophane core and transannular interaction between the bridged phenyl rings leads to a 250–400mV cathodic shift in the oxidation potentials relative to analogous complexes with ppy or ppz ligands. The destabilization of the HOMO leads to a corresponding red shift in the absorption and emission spectra from the complexes with pCpy and pCpz ligands. The Pt and Ir complexes with pCpy ligands are strongly emissive at room temperature, whereas complexes with pCpz ligands are only weakly emissive as they still suffer from temperature-dependent nonradiative deactivation to metal-centered ligand field states.

A three-coordinate Fe(ii) center within a [3Fe-(μ3-S)] cluster that provides an accessible coordination site.

A μ3-sulfide bridged triiron cluster(ii,ii,iii) supported by a cyclophane ligand undergoes metal-based reduction to yield an all-ferrous species. The latter complex incorporates a three-coordinate iron center that provides an accessible coordination site to a solvent molecule.

Insights into small molecule activation by multinuclear first-row transition metal cyclophanates

The rational design of trimetallic transition metal clusters supported by a trinucleating cyclophane ligand, L(3-), and the reactivities of these complexes with dinitrogen and carbon dioxide are discussed. Emphasis is placed on the differences in the observed reactivity between these trimetallic cyclophane complexes and that of the mono- and dinuclear transition metal compounds.

Reactivity of Hydride Bridges in High-Spin [3M-3(μ-H)] Clusters (M = FeII, CoII).

The designed [3M-3(μ-H)] clusters (M = Fe(II), Co(II)) Fe3H3L (1-H) and Co3H3L (2-H) [where L(3-) is a tris(β-diketiminate) cyclophane] were synthesized by treating the corresponding M3Br3L complexes with KBEt3H. From single-crystal X-ray analysis, the hydride ligands are sterically protected by the cyclophane ligand, and these complexes selectively react with CO2 over other unsaturated substrates (e.g., CS2, Me3SiCCH, C2H2, and CH3CN). The reaction of 1-H or 2-H with CO2 at room temperature yielded Fe3(OCHO)(H)2L (1-CO2) or Co3(OCHO)(H)2L (2-CO2), respectively, which evidence the differential reactivity of the hydride ligands within these complexes. The analogous reactions at elevated temperatures revealed a distinct difference in the reactivity pattern for 2-H as compared to 1-H; Fe3(OCHO)3L (1-3CO2) was generated from 1-H, while 2-H afforded only 2-CO2.

Modeling biological copper clusters: synthesis of a tricopper complex, and its chloride- and sulfide-bridged congeners.

The synthesis and characterization of a family of tricopper clusters housed within a tris(β-diketimine) cyclophane ligand (H3L) that bear structural similarities to biological copper clusters are reported. In all complexes, each Cu atom is held within the N2-chelate of a single β-diketiminate arm. Reaction of L(3-) with CuCl affords an anionic complex containing a μ3-chloride donor in the central cavity, whereas there is no evidence for bromide incorporation in the product of the reaction of L(3-) with CuBr (Cu3L). Cu3L reacts with elemental sulfur to generate the corresponding air-stable mixed-valent (μ3-sulfido)tricopper complex, Cu3(μ3-S)L, which represents the first example of a sulfide-bridged copper cluster in which each metal center is both coordinatively unsaturated and held within a N-rich environment. The calculated LUMO is predominantly Cu-S π* in character and delocalized over all three metal centers, which is consistent with the isotropic ten-line absorption (g ∼ 2.095, A ∼ 33 G) observed at room temperature in EPR spectra of the one-electron chemically reduced complex, [Cu3(μ3-S)L](-).

Ruthenium(II) complexes of N-heterocyclic carbenes derived from imidazolium-linked cyclophanes.

The present work seeks to characterize, in the light of electronic structure calculations, an unusual metal-[(η(1)-NHC)2:(η(6)-arene)] bonding situation in a set of ruthenium(ii) complexes containing the ortho-xylylene-linked-bis(NHC)cyclophane ligand (NHC-cyclophane) (), which binds to the ruthenium center through two carbene carbons and one of the arene rings. The nature of ruthenium(ii)-[(η(1)-NHC)2:(η(6)-arene)] bonding was investigated in the light of EDA-NOCV, NBO and QTAIM analyses by adopting as a model compound. The interplay between the ortho-cyclophane scaffold with different families of five-membered carbenes, such as imidazole, , triazole-based NHCs (Enders' carbenes), , and P-heterocyclic carbenes (PHCs), , was investigated. The metal-[(η(1)-NHC)2:(η(6)-arene)] bonding situation was also extended to heavier analogues, such as N-heterocyclic silylenes (NHSi) and N-heterocyclic germylenes (NHGe), in order to address how the basicity of NHC, NHSi and NHGe is affected by the cyclophane framework. The results reveal that ruthenium(ii)-[(η(1)-NHC)2:(η(6)-arene)] is more covalently than electrostatically bonded and that the degree of covalence is larger in PHCs than in NHCs or Enders' carbenes. It is also revealed that the covalent character in the ruthenium(ii)-[(η(1)-NHGe)2:(η(6)-arene)] and ruthenium(ii)-[(η(1)-NHSi)2:(η(6)-arene)] bonds is larger than in ruthenium(ii)-[(η(1)-NHC)2:(η(6)-arene)].

Syntheses and structure of 2,11-dithia[3.3](2,6)pyrazinophane and its coordination complexes of Cu(I) and Co(II)

The cyclophane ligand, 2,11-dithia[3.3](2,6)pyrazinophane (L), was synthesized by a simpler method and characterized by 1H NMR, which was used to synthesize coordination complexes by reactions with Cu+ (I) and Co2+ (II) cations. Single-crystal X-ray analysis revealed that the conformation of the ligand L is syn(boat-chair), while in I and II it adopts syn(boat-chair) and syn(chair-chair), respectively. In the former coordination compound, Cu(I) is coordinated with two nitrogen and one sulfur atom of the ligand. In the latter one, Co(II) is coordinated with two nitrogen and two sulfur atoms of the ligand. In complexes I and II, the formation of three-dimensional structure depends on π-π-stacking and hydrogen bonds.

A new chiral di-N-heterocyclic carbene (NHC) cyclophane ligand and its application in palladium enantioselective catalysis

This report details the synthesis and characterization of the first chiral diNHC cyclophane ligand and its palladium allyl complex. The complex was tested for 1,4-conjugate addition of phenylboronic acid to 2-cyclohexen-1-one and provides R-3-phenylcyclohexanone in 50% enantiomeric excess.

A Mechanistic Investigation on Copolymerization of Ethylene with Polar Monomers Using a Cyclophane-Based Pd(II) α-Diimine Catalyst

A detailed mechanistic investigation of the copolymerization of ethylene and methyl acrylate (MA) by a Pd(II) cyclophane-based alpha-diimine catalyst is reported. Our previous observations of unusually high incorporations of acrylates in copolymerization using this catalyst (J. Am. Chem. Soc. 2007, 129, 10062) prompted us to conduct a full mechanistic study on ethylene/MA copolymerization, which indicates a dramatic departure from normal Curtin-Hammett kinetic behavior as observed in copolymerization using the normal Brookhart type of Pd(II) alpha-diimine catalysts. Further investigation reveals that this contrasting behavior originates from the axial blocking effect of the cyclophane ligand hindering olefin substitution and equilibration. In equilibrium studies of ethylene with nitriles, the cyclophane catalyst was found to more strongly favor the linearly binding nitrile ligands as compared to the standard acyclic Pd(II) alpha-diimine catalysts. Ethylene exchange rates in the complexes [(N--N)PdMe(C(2)H(4))](+) (N--N = diimine) were measured by 2D EXSY NMR spectroscopy and found to be over 100 times slower in the cyclophane case. Measurement of the slow equilibration of ethylene, methyl acrylate, and 4-methoxystyrene in cyclophane-based Pd(II) olefin complexes by (1)H NMR and fitting of the obtained kinetic plots allowed for the estimation of exchange rates and equilibrium constants of the olefins. After extrapolation to typical polymerization temperature, DeltaG(double dagger) = 20.6 and 16.4 kcal/mol for ethylene-methyl acrylate exchange in the forward (ethylene displacement by methyl acrylate) and reverse directions, respectively. These values are of similar magnitude to the previously determined migratory insertion barriers of ethylene (DeltaG(double dagger) = 18.9 kcal/mol) and methyl acrylate (DeltaG(double dagger) = 16.3 kcal/mol) under equivalent conditions, but contrast strongly to the rapid olefin exchange seen in the Brookhart acyclic catalyst. The large barrier to olefin exchange hinders olefin pre-equilibrium, decreasing the cyclophane catalyst's ability to preferentially incorporate one monomer (in this case ethylene) over the other, thus giving rise to high comonomer incorporations.

Nickel(II) and Palladium(II) Polymerization Catalysts Bearing a Fluorinated Cyclophane Ligand: Stabilization of the Reactive Intermediate

The synthesis and characterization of Ni(II) and Pd(II) α-diimine olefin polymerization catalysts bearing a fluorinated cyclophane-based ligand were performed. Fluorine was placed in such a manner as to interact with the metal center from the axial direction. The catalysts were active in the polymerization of ethylene, showing substantial differences in both catalytic behavior and polymer size and structure as compared to their nonfluorinated analogues. Both catalysts afforded polymer of comparatively low branching density and high molecular weight. The Ni(II) catalysts, from precursor [Ni(acetylacetonato)(F-Cyc)]+ salts (F-Cyc = fluorinated cyclophane), exhibited enhanced thermal stability by remaining active after 70 min with little loss in polymerization activity at 105 °C. The Pd(II) catalysts from salts of [Pd(F-Cyc)Me(NCR)]+ (NCR = nitrile) afforded polymer of molecular weights far higher than the nonfluorinated analogue. Additionally, polymerization activity was directly related to ethylene feed pr...

Efficient Incorporation of Polar Comonomers in Copolymerizations with Ethylene Using a Cyclophane-Based Pd(II) α-Diimine Catalyst

Polar olefins were copolymerized with ethylene using a cyclophane-based palladium(II) α-diimine catalyst. The incorporation levels of both methyl acrylate and tert-butyl acrylate are unusually high compared to what is obtained with the corresponding acyclic catalyst. Mechanistic studies by low-temperature NMR spectroscopy revealed that the differences between the insertion barriers for ethylene and MA (ΔΔG⧧) are similar for the acyclic and cyclophane catalysts. The equilibration of comonomers, fast on experimental time scales for the acyclic catalyst, was too slow to be measured for the cyclophane catalyst. These results suggest that ligand substitution is significantly retarded by the bulky cyclophane ligand. The reduction of the rate of monomer exchange with respect to monomer insertion reduces the catalyst's ability to discriminate between comonomers, hence resulting in the high incorporation levels of acrylates in copolymerizations with ethylene.

Luminescence Studies of the Intracellular Distribution of a Dinuclear Gold(I) N‐Heterocyclic Carbene Complex

A bidentate cyclophane N-heterocyclic carbene ligand has been used to synthesize a new dinuclear Au{sup I} complex of the formula [Au{sub 2}L{sub 2}]{sup 2+}. The short Au {hor_ellipsis} Au distance imposed by the rigid cyclophane ligand leads to a red-shifted luminescence profile that enables the complex to be used as a luminescent probe for distribution studies in single living cancer cells.

Role of Anions on the Crystal Structures of Copper(II) and Zinc(II) Complexes of a Tunable Butterfly Cyclophane Macrocycle

Three crystal structures of a ditopic cyclophane ligand (L) in which two 1,5,8,12-tetraamine molecules have been attached through methylene spacers to the ortho positions of a benzene ring are reported. The first one (1) corresponds to the tetraprotonated free macrocycle (H4L4+) having two tetrachlorozincate(II) counteranions (C24H54O2N8Cl8Zn2, a = 9.1890(2) A, b = 14.0120(3) A, c = 15.3180(3) A, alpha = 89.2320(7) degrees , beta = 82.0740(6) degrees , gamma = 83.017(1) degrees , Z = 2.00, triclinic, P); the second one (2) is of a binuclear Cu2+ complex having coordinated chloride anions and perchlorate counteranions (C24H58O14N8Cl4Cu2 a = 9.9380(2) A, b = 30.2470(6) A, c = 53.143(1) A, orthorhombic, F2dd, Z = 18), and the third one (3) corresponds to an analogous Zn2+ complex that has been crystallized using triflate as counteranion (C26H(51.2)O(6.6)N8Cl2F6S2Zn2 a = 8.472(5) A, b = 9.310(5), c = 13.745(5) A, alpha = 84.262(5) degrees , beta = 77.490(5) degrees , gamma = 73.557(5) degrees , triclinic, P, Z = 2). The analysis of the crystallographic data clearly shows that the conformation of the macrocycle and, in consequence, the overall architecture of the crystals are controlled by the anions present in the moiety, pi-pi-stacking associations, and hydrogen bonding interactions. The protonation and stability constants for the formation of the Cu2+ and Zn2+ complexes in aqueous solution have been determined potentiometrically in 0.15 mol dm(-3) NaClO4 at 298.1 K. Intramolecular hydrogen bonding defines the protonation behavior of the compound. Positive cooperativity is observed in the formation of the Cu2+ complexes.

Development of a New Dimeric Cyclophane Ligand: Application to Enhanced Diastereo‐ and Enantioselectivity in the Catalytic Synthesis of β‐Lactams.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Development of a new dimeric cyclophane ligand: application to enhanced diastereo- and enantioselectivity in the catalytic synthesis of beta-lactams.

We detail the synthesis of a new C(2)-symmetric bis(cyclophane) ligand system that can be thought of as electronically analogous to binol, but which possesses the added "third dimension" of cyclophane chirality. The ligand synthesis involves a spontaneous (but unexpected) atropisomerization to the desired product. We have employed this ligand to form a metal complex that is an effective cocatalyst for the highly enantio- and diastereoselective catalytic asymmetric synthesis of a beta-lactam.

Complexation of transition metal ions M+ (M = Fe, Co, Ni, Cu, Zn, Ag) by [2.n]paracyclophane-enes (n = 3, 4, 5, 6) in the gas phase: effect of the molecular cavity on the complexation capability

Complex formation in the gas phase between transition metal ions M+ = Fe+, Co+, Ni+, Cu+, Zn+, and Ag+ and dimethyl[2.n]paracyclophane-enes 1–4(n = 3–6) has been studied by secondary ionization mass spectrometry (SIMS) and tandem mass spectrometry. With the exception of Zn+, complex formation was observed if dry mixtures of salts of a transition metal and a cyclophane were bombarded with a 30 keV primary Cs+ ion beam in a SIMS ion source. Using liquid SIMS, abundant signals of a complex [M(cyclophane)]+ were only obtained for Ag+ and NBA as liquid matrix. Besides a large signal of M+ and a significant signal of [M(cyclophane)]+ all SIMS mass spectra contained a distinct peak of the molecular ion [cyclophane]·+ and a series of small peaks of hydrocarbon fragment ions. Dimeric adduct ions [M(cyclophane)2]+, typical of gas phase complexation of transition metal ions and planar aromatic hydrocarbons, are only detected in the mass spectra of mixtures of 1 and salts of Co or Ag. It is concluded from the experimental results that complex formation occurs in the gas phase of the SIMS ion source by ion/molecule reaction between the sputtered metal ion M+ and the neutral cyclophane evaporating from the heated target holder in competition with charge transfer. By this model the efficiency of complex formation between a certain metal ion and cyclophane can be estimated from the intensity ratio r = [M(cyclophane)]+/[cyclophane]·+. From this ratio and the results of separate experiments, in which two M+ compete in complex formation with cyclophane 3 or in which a mixture of cyclophanes is used for complexation of Ag+, it is shown that complex formation increases in the series Fe+ < Co+ ≈ Ni+ < Cu+ ≪ Ag+ and in the series 1 < 2 < 3 ≈ 4. The selectivity in the series of cyclophanes as well as the lack of formation of dimeric complexes [M(cyclophane)2]+ concurs with the formation of “in” complexes (IC) or “side-on” complexes (SC), in which the metal ion is more or less buried in the cavity of the cyclophane ligand. This is corroborated by collision-induced dissociation experiments, which show—with the exception of Ag+ complexes—intense losses of small hydrocarbon fragments and/or extensive decomposition of the complex ions, but no major dissociation into the components, and by semiempirical AM1 analysis of the structures of the complexes.