Competitive Transmetalation of First-Row Transition-Metal Ions between Trinuclear Triple-Stranded Side-by-Side Helicates.

A hybrid tris-bidentate neutral ligand (L) composed of a central 2,2'-bipyridine and two terminal triazolyl-pyridine chelating units connected by methylene spacers is employed to synthesize trinuclear triple-stranded side-by-side helicates of first-row transition-metal(II) ions. Three such new homometallic helicates L3M3(OTf)6 [ M = Cu2+ (4); Ni2+ (5); Co2+ (6)], along with our recently reported helicates L3Fe3(OTf)6 (1), L3Zn3(OTf)6 (2), and L3Fe2Zn(OTf)6 (3) are taken into consideration for competitive formation and transmetalation studies. Single-crystal X-ray structures of L3Cu3(OTf)6 (4) and L3Ni3(OTf)6 (5) show the formation of trinuclear triple-stranded side-by-side helicates with alternating Λ and Δ chiralities at the metal ions as earlier observed in cases of L3Fe3(OTf)6 (1), L3Zn3(OTf)6 (2), and L3Fe2Zn(OTf)6 (3). ESI-FTICR mass spectrometry and UV-vis spectroscopy studies show that helicates L3Fe3(OTf)6 (1), L3Zn3(OTf)6 (2), L3Fe2Zn(OTf)6 (3), and L3Co3(OTf)6 (6) can easily be transmetalated to helicate L3Cu3(OTf)6 (4) in the presence of Cu(OTf)2. On the other hand, only a trace amount of heterometallic helicate L3Ni2Cu(OTf)6 forms even after several days, when Cu(OTf)2 is added to a the solution of homometallic helicate L3Ni3(OTf)6 (5). Further, we have demonstrated the formation of a heterometallic helicate L3Ni2Co(OTf)6 (7) from a 1:1:1 reaction mixture of L, Ni(OTf)2, and Co(OTf)2, which can also be prepared from homometallic helicate L3Co3(OTf)6 (6) by transmetalation with Ni(OTf)2.

Polyamide-Polyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase.

Polyamide-polyamine hybrid macrobicycle L is explored with respect to its ability to bind α,ω-dicarboxylate anions. Potentiometric studies of protonated L with the series of dianions from succinate (suc2-) through glutarate (glu2-), α-ketoglutarate (kglu2-), adipate (adi2-), pimelate (pim2-), suberate (sub2-), to azelate (aze2-) have shown adipate preference with association constant value of K = 4900 M-1 in a H2O/DMSO (50:50 v/v) binary solvent mixture. The binding constant increases from glu2- to adi2- and then continuously decreases with the length of the anion chain. Further, potentiometric studies suggest that hydrogen bonding between the guest anions and the amide/ammonium protons of the receptor also contributes to the stability of the associations along with electrostatic interactions. Negative-mode electrospray ionization of aqueous solutions of host-guest complexes shows clear evidence for the selective formation of 1:1 complexes. Single-crystal X-ray structures of complexes of the receptor with glutaric acid, α-ketoglutaric acid, adipic acid, pimelic acid, suberic acid, and azelaic acid assist to understand the observed binding preferences. The solid-state structures reveal a size/shape complementarity between the host and the dicarboxylate anions, which is nicely reflected in the solution state binding studies.

Threading of various 'U' shaped bidentate axles into a heteroditopic macrocyclic wheel via NiII/CuII templation.

The threading of 'U' shaped bent axles having diverse functionalities (Axle1-Axle10) is investigated by using a heteroditopic amido-amine macrocyclic (MC) wheel via NiII or CuII metal ion templation. These bent shaped axles are the derivatives of 4,4'-substituted 2,2'-bipyridine, which are composed of various terminal groups like alkene, alkyne, bromide, hydroxyl and azide. Such metallo [2]pseudorotaxanes are well characterised by ESI-MS, EPR and FT-IR spectroscopic studies, UV-Vis absorption studies, elemental analysis and single-crystal X-ray diffraction studies wherever possible. Experimental evidence supports 1 : 1 : 1 ternary complexation between MC, the metal ion and axle. The single crystal X-ray structures of three CuII templated ternary complexes (PR1', PR3' and PR7') show the penta-coordination arrangement around the templating metal ion. Interestingly, judicious selection of chemical functionalities in the complementary wheel and axle components enables to show the existence of various covalent and non-covalent interactions.

Selective recognition and extraction of KBr via cooperative interactions with a urea functionalized crown ether dual-host.

Selective solid-liquid extraction of KBr is demonstrated for the first time with a crown ether based pentafluorophenyl urea functionalized dual-host receptor. (1)H-NMR and ITC studies have been carried out to illustrate the effect of cooperativity towards the recognition of alkali metal salts.

Formation and Transmetalation Mechanisms of Homo- and Heterometallic (Fe/Zn) Trinuclear Triple-Stranded Side-by-Side Helicates.

A novel linear hybrid tris-bidentate neutral ligand having 2,2'-bipyridine and two terminal triazolylpyridine coordination sites (L) was efficiently synthesized and explored in the synthesis of trinuclear triple-stranded homometallic side-by-side helicates L3Fe3(OTf)6 (1) and L3Zn3(OTf)6 (2), in which the three metal centers display alternating Λ and Δ configurations. Selective formation of the analogous heterometallic side-by-side helicate L3Fe2Zn(OTf)6 (3) was achieved from a mixture of L, Fe(CH3CN)2(OTf)2, and Zn(OTf)2 (1:1:1) in acetonitrile at room temperature. Various analytical techniques, i.e., single-crystal X-ray diffraction and NMR and UV/vis spectroscopy, were used to elucidate the sequence of the metal atoms within the heterometallic helicate, with the Zn(2+) at the central position. The formation of 3 was also achieved starting from either L3Zn3(OTf)6 or L3Fe3(OTf)6 by adding Fe(CH3CN)2(OTf)2 or Zn(OTf)2, respectively. ESI-MS and (1)H NMR studies elucidated different transmetalation mechanisms for the two cases: While a Zn(2+)-to-Fe(2+) transmetalation occurs by the stepwise exchange of single ions on the helicate L3Zn3(OTf)6 at room temperature, this mechanism is almost inoperative for the Fe(2+)-to-Zn(2+) transmetalation in L3Fe3(OTf)6, which is kinetically trapped at room temperature. In contrast, dissociation of L3Fe3(OTf)6 at higher temperature is required, followed by reassembly to give L3Fe2Zn(OTf)6. The reassembly follows an interesting mechanistic pathway when an excess of Zn(OTf)2 is present in solution: First, L3Zn3(OTf)6 forms as the high-temperature thermodynamic product, which is then slowly converted into the thermodynamic heterometallic L3Fe2Zn(OTf)6 product at room temperature. The temperature-dependent equilibrium shift is traced back to significant entropy differences resulting from an enhancement of the thermal motion of the ligands at high temperature, which destabilize the octahedral iron terminal complex and select zinc in a more stable tetrahedral geometry.

Encapsulation of [(SO4)4(H2O)112]8⁻ clusters in a metal organic framework of pyridyl functionalized cyanuric acid based tris-urea.

Encapsulation of hydrated sulfate in a bowl-shaped metal organic coordination polymer formed by Zn(2+) assisted self-assembly of a 3-pyridyl terminated cyanuric acid platform based urea receptor is reported in aqueous medium. Trapping of an unusual [(SO4)4(H2O)12](8-) cluster in a [Zn(H2O)6](2+) capped self-assembled structure is characterized by single crystal X-ray crystallography. Furthermore, selective binding of SO4(2-) is established from the (1)H-NMR titration study.

[2]Rotaxane with multiple functional groups.

High-yield syntheses of Cu(II)- and Ni(II)-templated [2]pseudorotaxane precursors (CuPRT and NiPRT, respectively) were achieved by threading bis(azide)bis(amide)-2,2'-bipyridine axle into a bis(amide)tris(amine) macrocycle. Single-crystal X-ray structural analysis of CuPRT revealed complete threading of the axle fragment into the wheel cavity, where strong aromatic π-π stacking interactions between two parallel arene moieties of the wheel and the pyridyl unit of axle are operative in addition to metal ion templation. Attachment of a newly developed bulky stopper molecule with a terminal alkyne to CuPRT via a Cu(I)-catalyzed azide-alkyne cycloaddition reaction failed as a result of dethreading of the azide-terminated axle under the reaction conditions. However, the synthesis of a metal-free [2]rotaxane containing triazole with other functionalities in the axle was achieved in ∼45% yield upon coupling between azide-terminated NiPRT and the alkyne-terminated stopper. The [2]rotaxane was characterized by mass spectrometry, 1D and 2D NMR (COSY, DOSY, and ROESY) experiments. Comparative solution-state NMR studies of the [2]rotaxane in its unprotonated and protonated states were carried out to locate the position of the wheel on the axle of the metal-free [2]rotaxane. Furthermore, a variable-temperature (1)H NMR study in DMSO-d6 of [2]rotaxane supported the kinetic inertness of the interlocked structure, where the newly developed stopper prevents dethreading of the 30-membered wheel from the axle.

Synthesis of a preorganized hybrid macrobicycle with distinct amide and amine clefts: tetrahedral versus spherical anions binding studies.

A new C3v symmetric amido-amine hybrid macrobicycle, L is synthesized toward anion recognition in its protonated states. L contains tri-amide and tetra-amine clefts separated by p-phenylene spacers. The solid-state structure of methanol-encapsulated L exhibits an overall cavity length of ~12.0 Å where the amide and amine -NH protons are converged toward the center of the respective cavities. Conformational analysis of L in solution is established by NOESY NMR. Anion binding of [H3L](3+) with spherical (Cl(-), Br(-), I(-)) and tetrahedral (ClO4(-), SO4(2-)) anions are carried out by isothermal titration calorimeter in dimethylsulfoxide. The association of halides with [H3L](3+) is endothermic and entropy driven. However, association of tetrahedral anions is exothermic in nature and both entropy- and enthalpy-driven. The overall association constants show the following order: HSO4(-) > Br(-)> Cl(-) ≈ ClO4(-). Single crystal X-ray structures of ClO4(-) and Br(-) complexes of protonated L show encapsulation of ClO4(-) in the amide cleft of [H2L](2+) (complex 1) and encapsulation of Br(-) in the ammonium cleft of [H3L](3+) (complex 2). Further, preorganization of L toward encapsulation of spherical and tetrahedral anions is established by comparing its amide, amine, and overall cavity dimensions with 1 and 2.

Selective recognition of sulphate in a Cu(II) assisted 1D polymer of a simple pentafluorophenyl substituted pyridyl-urea via second sphere coordination.

A pentafluorophenyl (­C6F5) substituted 3-pyridyl urea, L(1), is explored extensively to demonstrate SO4(2-) binding exclusively via second sphere coordination in the cavity of a 1D polymeric self-assembly of L(1), selectively assisted by Cu2+. A single crystal X-ray diffraction study depicts SO4(2-) encapsulation in the C2 symmetric cleft via nine hydrogen bonding interactions contributed by eight urea protons of four L(1) moieties in [CuL(1)4(DMF)2]SO4 (1). To revalidate the importance of Cu2+ selective anion coordination via exclusive second sphere coordination, a complex of L(1) and Cu(NO3)2, i.e. [CuL(1)4(H2O)2](NO3)2 (2), is also isolated and characterized by a single crystal X-ray diffraction study. When SO4(2-) salts of different metal ions such as Co2+/Ni2+ are employed, the first sphere coordination of SO4(2-) is observed in cases of complexes [CoL(1)3(DMF)2SO4] (3) and [NiL(1)3(DMF)2SO4] (4) respectively. These results clearly suggest the importance of Cu2+ towards anion recognition via purely second sphere coordination in the case of complexes 1 and 2. To understand the importance of (­C6F5) substitution in the design of L(1) towards such recognition of SO4(2-) in 1, the phenyl (­C6H5) analogue of L(1), i.e.L(2), is allowed to complex with the SO4(2-) salt of Cu2+. Interestingly, L(2) shows first sphere SO4(2-) coordination in the complex [CuL(2)2(DMF)(H2O)2SO4] (5). Solution state UV-Vis experiments of L(1) with various copper salts such as Cu(ClO4)2, CuSO4, Cu(NO3)2, CuCl2 and CuBr2 in DMF show the formation of a binary complex corresponding to 1. Further, Cu(2+) selective second sphere coordination of SO4(2-) in solution is also demonstrated by UV-Vis studies of complexes isolated from the mixtures of various Cu(2+) salts and/or SO4(2-) salts of different metal ions.

A series of amino acid functionalized tripodal hexaamide anion receptors: ion-pair-assisted capped-cleft formation by a pentafluorophenyl-functionalized amide.

A new series of tris(2-aminoethyl)amine (tren)-based L-alanine amino acid backboned tripodal hexaamide receptors (L1-L5) with various attached moieties based on electron-withdrawing fluoro groups and lipophilicity have been synthesized and characterized. Detailed binding studies of L1-L5 with different anions, such as halides (F(-), Cl(-), Br(-), and I(-)) and oxyanions (AcO(-), BzO(-) (Bz=benzoyl), NO(3)(-), H(2)PO(4)(-), and HSO(4)(-)), have been carried out by isothermal titration calorimetric (ITC) experiments in acetonitrile/dimethylsulfoxide (99.5:0.5 v/v) at 298 K. ITC titration experiments have clearly shown that receptors L1-L4 invariably form 1:1 complexes with Cl(-), AcO(-), BzO(-), and HSO(4)(-), whereas L5 forms a 1:1 complex only with AcO(-). In the case of Br(-), I(-), and NO(3)(-), no appreciable heat change is observed owing to weak interactions between these anions and receptors; this is further confirmed by (1)H NMR spectroscopy. The ITC binding studies of F(-) and H(2) PO(4)(-) do not fit well for a 1:1 binding model. Furthermore, ITC binding studies also revealed slightly higher selectivity of this series of receptors towards AcO(-) over Cl(-), BzO(-), and HSO(4)(-). Solid-state structural evidence for the recognition of Cl(-) by this new category of receptor was confirmed by single-crystal X-ray structural analysis of the complex of tetrabutylammonium chloride (TBACl) and L1. Single-crystal X-ray diffraction clearly showed that the pentafluorophenyl-functionalized amide receptor (L1) encapsulated Cl(-) in its cavity by hydrogen bonds from amides, and the cavity of L1 was capped with a TBA cation through hydrogen bonding and ion-pair interactions to form a capped-cleft orientation. To understand the role of the cationic counterpart in solution-state Cl(-) binding processes with this series of receptors (L1-L4), a detailed Cl(-) binding study was carried out with three different tetraalkylammonium (Me(4) N(+), Et(4) N(+), and Bu(4) N(+)) salts of Cl(-). The binding affinities of these receptors with different tetralkylammonium salts of Cl(-) gave binding constants with the TBA cation in the following order: butyl>ethyl>methyl. This study further supports the role of the TBA countercation in ion-pair recognition by this series of receptors.