Unusual Stabilisation of Remarkably Bent Tetra-Cationic Tetra-radical Intermolecular Fe(III) µ-Oxo Tetranuclear Complexes.

A hitherto unknown series of air stable, π-conjugated, remarkably bent tetra-cation tetra-radical intermolecular Fe(III) µ-oxo tetranuclear complex, isolated from the dication diradical diiron(III) porphyrin dimers, has been synthesised and spectroscopically characterised along with single crystal X-ray structure determination of two such molecules. These species facilitate long-range charge/radical delocalisation through the bridge across the entire tetranuclear unit manifesting an unusually intense NIR band. Assorted spin states of Fe(III) centres are stabilised within these unique tetranuclear frameworks: terminal six-coordinate iron centres stabilise the admixed intermediate spin states while the central five-coordinate iron centres stabilise the high-spin states. Variable temperature magnetic susceptibility measurements indicated strong antiferromagnetic coupling for the Fe(III)-O-Fe(III) unit while the exchange interactions between the Fe centres and the porphyrin π-cation radicals are weaker as supported both by magnetic data and DFT calculations. The nature of orbital overlap between the SOMOs of Fe(III) and π* orbital of the porphyrin was found to rationalise the observed exchange coupling, establishing such a complex magnetic exchange in this tetranuclear model with a significant bioinorganic relevance.

Probing Bis-FeIV MauG: Isolation of Highly Reactive Radical Intermediates.

MauG is a diheme enzyme that utilizes two covalently bound c-type heme centers. We report here step-wise oxidations of a synthetic analogue of MauG in which two heme centers are bridged covalently through a flexible linker containing a pyrrole moiety. One- and two-electron oxidations produce monocation radical and dication diradical intermediates, respectively, which, being highly reactive, undergo spontaneous intramolecular rearrangement involving the pyrrole bridge itself to form indolizinium-fused chlorin-porphyrin and spiro-porphyrinato heterodimers. Unlike in MauG, where the two oxidizing equivalents produce the bis-FeIV redox state, the synthetic analogue of the same, however, stabilizes two ferric hemes, each coupled with a porphyrin π-cation radical. The present study highlights the possible role played by the bridge in the electronic communication.

Does Larger Cavity-Size Really Help Bigger Anions to Bind? A Scrutiny on Core-Expanded Calix[4]pyrroles and Their Properties.

Calix[4]pyrroles are an important class of oligopyrrolic macrocycles and have found applications in many diverse fields including anion recognition. To modulate the properties of the calix[4]pyrrole, several structural modifications are realized. The core-expansion has attracted extra attention as it provides larger cavity-size compared to parent calix[4]pyrrole(s). This review highlights the synthetic development of various core-expanded calix[4]pyrroles and their applications in anion-binding properties. Emphasis is given to the changes in the binding properties observed with expanded versions of calix[4]pyrrole(s) in both solution and the solid states. The expanded versions of calix[4]pyrrole do not always show higher binding affinities for larger anions as anticipated. Rather, they display reduced affinities with the anions. The truncated form or asymmetric nature of the expanded versions of calix[4]pyrrole does not probably allow to access all the available binding sites for the anions and hence reduced binding affinities are observed. The receptors which contain a greater number of binding sites and are somehow rigid or preorganized apparently show enhanced binding affinities for anions. The relative binding constants for halide series indicate that the enlarged molecules are more beneficial for largest iodide among others. However, most of the receptors show selectivity towards smallest fluoride over other anions studied.

The syn and anti isomers of the porphyrinogen-like precursor of calix[4]phyrin: isolation, X-ray structure, anion binding and fluoride-ion-mediated proton-deuterium exchange studies.

Porphyrinogen-like precursors of calix[4]phyrins are presumed to be unstable owing to their auto-oxidation. In contrast to this, the syn and the anti isomers of a calix[4]pyrrole molecule containing pyridine moieties at the meso positions were isolated and their structures were determined by single crystal X-ray diffraction studies. Both the isomers gave the same calix[4]phyrin molecule upon oxidation. The anion binding properties of both the isomers were studied in DMSO-d6 by the EQNMR method, which showed that they have a preference of binding with the F(-) ion over the other large sized halide and oxo anions. In addition, the F(-) ion mediated H/D exchange process was monitored by the (19)F NMR method. The solution state structures of the 1 : 1 F(-) ion complexes containing deuterium atoms formed by a random but sequential substitution of NH protons by deuterium atoms were identified from their multiplicity patterns observed in the proton coupled (19)F NMR spectrum, which are supported by the proton decoupled (19)F NMR spectrum showing one singlet for each type of F(-) ion complex in solution for both the syn and anti isomers, correlating with their solid state structures.

A tripyrrolylmethane-based macrobicyclic triazacryptand: X-ray structure, size-selective anion binding, and fluoride-ion-mediated proton-deuterium exchange studies.

A new class of tripyrrolylmethane-based triazacryptand with bridgehead carbons and acyclic molecules were synthesized by the Mannich reaction of tripyrrolylmethane with primary or secondary amine hydrochloride and formaldehyde, respectively. The structure of the triazacryptand was determined by X-ray diffraction (XRD) method. The anion binding properties of both the bicyclic and acyclic receptors were studied by (1)H NMR titration method. The binding studies showed that both receptors exhibit very high affinity and bind strongly with the F(-) ion in DMSO-d(6). However, the binding constant of azacryptand with F(-) is much higher than that of the acyclic receptor. This is attributed to the preorganization of the azacryptand having a specific cavity size, and the strength and the number of hydrogen bonds formed by the F(-) ion. This is supported by the crystal structures of F(-), Cl(-), and Br(-) ion complexes of the bicyclic receptor and by DFT calculations. The X-ray structures showed that the azacryptand receptor forms an inclusion complex with only the F(-) ion; other anions bind in the clefts of the macrobicycle, thus supporting a size-selective anion binding behavior. The high affinity and the selectivity of the macrobicycle as a neutral receptor of the F(-) ion in the presence of other competitive anions in DMSO-d(6) were confirmed by (1)H NMR spectroscopy. Furthermore, the F(-)-ion-mediated hydrogen-deuterium exchanges were monitored by (19)F NMR spectroscopy, showing multiplets based on the formation of all possible deuterium-exchanged fluoride complexes in solution.

Synthesis and X-ray structures of novel macrocycles and macrobicycles containing N,N-di(pyrrolylmethyl)-N-methylamine moiety: preliminary anion binding study.

The [2 + 2] Schiff base condensation reactions between the newly synthesized dialdehyde, N,N-di(α-formylpyrrolyl-α-methyl)-N-methylamine), and ethylenediamine or p-phenylenediamine dihydrochloride readily afforded the 30- and 34-membered large size macrocycles in very high yields. Subsequent reduction reactions of these macrocycles with NaBH(4) gave the corresponding saturated macrocyclic hexaamines in good yields. The analogous reaction of the new dialdehyde with a triamine molecule afforded the [3 + 2] Schiff base macrobicycle in high yield, which was then reduced by reaction with NaBH(4) to give the saturated macrobicycle. All these compounds were characterized by spectroscopic methods. The anion binding properties of the saturated macrocycles having the ethylene and the phenylene linkers in CDCl(3) were studied by NMR titration methods. Although they have similar pyrrolic and amine NH groups their binding properties are different and interesting, owing to the conformational flexibility or rigidness rendered by the ethylene or phenylene groups, respectively. The macrocycle having the ethylene linkers binds anions in a 1:1 fashion, while the other receptor having the phenylene linkers prefers to bind anions in a sequential 1:2 fashion and has a multiple equilibria between a 1:1 and a 1:2 complexes, as shown by their binding constants, curve fittings by EQNMR, and Job plots. The X-ray structures of the 1:2 methanol, the aqua and the benzoate anion complexes of the macrocycles show two cavities in which the guests are bound, correlating with the high affinity found for the formation of stable 1:2 complexes in solution. The X-ray structure showed that the macrobicycle Schiff base adopts an eclipsed paddle-wheel shaped conformation and exhibits an out-out configuration at the bridgehead nitrogen atoms.

Dipyrrolylmethane-based macrobicyclic azacryptand: synthesis, X-ray structures, conformational and anion binding properties.

A new class of macrobicyclic azacryptand containing dipyrrolylmethane subunits with nitrogen bridgeheads was synthesized by the Mannich reaction of the dipyrrolylmethane in the presence of aqueous ammonia. The azacryptand exhibits a staggered conformation in the solid state, but is in a dynamic equilibrium with the eclipsed conformation in solution studied by the variable-temperature (1)H NMR methods. The azacryptand has a specific size suitable only for fluoride ion; large anions such as NO(3)(-) bind in the clefts of the macrobicycle as shown by the X-ray structures of its fluoride ion inclusion and the nitrate anion complexes. The anion binding studies showed that it has high selectivity and affinity for fluoride ion in acetone over other anions studied, which was supported by (1)H and (19)F NMR methods. The azacryptand has fast fluoride ion-mediated proton-deuterium exchanges with acetone-d(6) studied by the (19)F NMR method.

Synthesis, structural characterization, and bonding analysis of two-coordinate copper(I) and silver(I) complexes of pyrrole-based bis(phosphinimine): new metal-pyrrole ring π-interactions.

The reaction between 2,5-bis(diphenylphosphinomethyl)pyrrole and Me3SiN3 gave the new pyrrole-based bis(phosphinimine) L1H in an excellent yield. L1H reacts with [CuCl(COD)]2, AgBF4, or AgOTf to give the corresponding two-coordinate mononuclear ionic complex formulated as [M{(L1H)-κ2N,N}]+[X]- where M = Cu and Ag; X = [CuCl2], BF4 or OTf. Their single crystal X-ray diffraction studies confirmed the two-coordinate geometry formed by the chelate bonding mode of L1H. These 10-membered metalacycles exhibit planar chirality and were also characterized by spectroscopic methods. In addition, in all three structures, there exists a hitherto unknown π-interaction between the pyrrole ring atoms and metal, represented as η2-(Cα-N) in the copper(i) complex, and η3-(Cα-N-Cα') in the silver(i) complexes. These weak interactions were supported by DFT calculations in terms of their electron densities, non-covalent interaction plots and the decrease in the aromaticity of the pyrrole ring.

Versatility of the bis(iminopyrrolylmethyl)amine ligand: tautomerism, protonation, helical chirality, and the secondary coordination sphere with halogen bonds in the formation of copper(II) and nickel(II) complexes.

The reaction of N,N-di(2,6-bis(isopropyl)phenylimino-pyrrolyl-α-methyl)-N-methylamine H2L1 with copper(i) sources such as CuX (X = Cl (1), Br (2), and I (3)) afforded bis(chelated) ionic copper(ii) complexes of the type [CuL1H]X. A similar type of mononuclear structure was obtained with Cu(NO3)2·(H2O)3. Conversely, binuclear copper(ii) complexes [Cu2(µ-L1)(µ-OOCCH3)(µ-OH)](4) and [Cu2(µ-L1H)(µ-OOCPh)(µ-O)] (5) were obtained from the reaction of Cu(O2CR)2·H2O with H2L1. Notably, these reactions in the presence of a base yielded the neutral copper(ii) complex [CuL1] (6). This product was also obtained from the reaction of complex 2 or 4 with NaOH in methanol. All structures feature a dianionic imino-pyrrole motif and a protonated central amine function except 4. The reaction of H2L1 with NiCl2·DME gave the mononuclear complex [NiCl2(L1H2)], 7. In contrast to this, the reaction of the newly synthesized sterically less encumbered ligand N,N-di(phenylimino-pyrrolyl-α-methyl)-N-methylamine H2L2 with NiCl2·DME gave the binuclear complex [NiCl(L2H2)(HOMe)]2[Cl]2 (8). Both 7 and 8 show the amine-azafulvene ligand form and coordination of the central amine. The reaction of complex 7 with NaHBEt3 yielded a neutral complex [NiL1] (8) containing the imino-pyrrole form. In the molecular structures, interesting secondary coordination spheres incorporating guest molecules such as CHCl3 and MeOH in the crystal lattices and the presence of helical enantiomers were observed and analysed. In one case, CHCl3 was found inside an unusual cage-like structure supported by halogen bonds. Preliminary DFT calculations on the geometry of the nickel complex with H2L1 showed that the pentacoordinated tbp geometry is more stable than the square planar geometry.

Hydroxo-bridged diiron(iii) and dimanganese(iii) bisporphyrins: modulation of metal spins by counter anions.

The chemistry of oxo/hydroxo-bridged diheme centers, connected covalently through bridges, has attracted much attention recently. Close approach of the two heme centers in the µ-hydroxo complex results in an unequal core deformation which leads to the unusual stabilization of two different spin states of iron in a single molecular framework. The spin states are also counter-anion specific and are reversibly interconvertable. An increased separation between the heme centers, however, leads to a weaker inter-ring interaction and, hence, renders the iron centers equivalent. The counter anion has been found to perturb the spin state ordering of iron via H-bonding interaction, switching positions between counter anion and axial ligand, ion-dipole interaction, charge polarization etc. A tightly associated counter anion with one of the heme centers generates significant steric effect in both the solid state and solution and induces significant change in the structure and properties, including the iron spin state, without affecting the overall topology of the complex or the metal oxidation state. A brief account of our systematic investigation on this subject is presented in the present Perspective article.

Synthesis and structural characterization of anion complexes with azacalix[2]dipyrrolylmethane: effect of anion charge on the conformation of the macrocycle.

Tetrahomodiazacalix[2]dipyrrolylmethane 1, [-CH2(C4H3N)CR2(C4H3N)CH2N(Me)-]2 an expanded version of the calix[4]pyrrole system, has the tendency to change its ring conformation (1,3-alternate) upon anion binding analogous to calix[4]pyrrole. However, owing to its tertiary amine nitrogen atoms in the ring, it can be protonated and becomes a powerful cationic receptor for anions, besides its inherent hydrogen bonding features. Macrocycle 1 binds with a series of monoanions BF4(-), Cl(-), PhCOO(-) and ClO4(-), and their X-ray structures showed that the ring conformation constitutes the 1,2-alternate form. Upon binding with dianions SO4(2-), CrO4(2-), SiF6(2-) and S2O3(2-), the ring conformation changes to the cone conformation. The intermediate partial cone conformation results for complexes with NO3(-) and Cr2O7(2-) ions. The change in the orientation of the pyrrole NH groups depending on the charge of the anion demonstrates the flexibility of the macrocycle and the dicationic macrocycle stabilizes the anions via both hydrogen bonding and electrostatic interactions.

Synthesis and structural characterization of silver(I), copper(I) coordination polymers and a helicate palladium(II) complex of dipyrrolylmethane-based dipyrazole ligands: the effect of meso substituents on structural formation.

A new class of multidentate dipyrrolylmethane based ditopic tecton, 1,9-bis(3,5-dimethylpyrazolylmethyl)dipyrrolylmethane, containing diethyl (L1) or cyclohexylidene (L2) substituents at the meso carbon atom were readily synthesized in 28-45% yields in two different ways starting from dipyrrolylmethanes. A one dimensional coordination polymer structure ([(L2)Ag][BF4])n was obtained when L2 was treated with AgBF4, whereas the analogous reaction between L1 and AgBF4 afforded the dicationic binuclear metallacycle complex [(L1)2Ag2][BF4]2. In addition, yet another coordination polymeric structure [(L1)CuI]n was obtained from the reaction between L1 and CuI. The analogous reaction of L1 with [Pd(PhCN)2Cl2] afforded the binuclear palladium complex [(L1)2Pd2Cl4] having a double-stranded helicate structure. The observed structural differences are attributed to the effects of the substituents present at the meso carbon atom of the ligand, in addition to the nature of the metal centre, coordination number and the preferred geometry.

Macrocyclic and acyclic molecules synthesized from dipyrrolylmethanes: receptors for anions.

A new class of macrocyclic and acyclic molecules was synthesized by the Mannich reactions of dipyrrolylmethanes to investigate anion recognition. The X-ray structures of the macrocycle and sulfate complexes are reported.