Massimo Boiocchi

A two-channel molecular dosimeter for the optical detection of copper(II)

A cyclam-like macrocycle with an integrated push-pull chromophore selectively detects Cu2+ inclusion through both orange-to-yellow colour change and quenching of the green fluorescence.

The Interaction of Fluoride with Fluorogenic Ureas: An ON1–OFF–ON2 Response

The anion binding tendencies of the two fluorogenic ureas L(1)H and L(2)H, containing the 2-anthracenyl and 1-pyrenyl moieties as signaling units, respectively, have been investigated in MeCN and DMSO by absorption, emission, and (1)H NMR spectroscopies. The formation of stable 1:1 receptor:anion H-bond complexes has been confirmed by structural studies on the crystalline [Bu4N][L(1)···Cl] and [Bu4N][L(2)H···CH3COO] salts. Complexation induces significant variations of the emission properties of L(1)H and L(2)H according to a multifaceted behavior, which depends upon the fluorogenic substituent, the solvent, and the basicity of the anion. Poorly basic anions (Cl(-), Br(-)) cause a red shift of the emission band(s). Carboxylates (CH3COO(-), C6H5COO(-)) induce fluorescence quenching due to the occurrence of an electron-transfer process taking place in the locally excited complex [*L-H···X](-). However, this excited complex may undergo an intracomplex proton transfer from one urea N-H fragment to the anion, to give the tautomeric excited complex [L···H-X](-)*, which emits at higher wavelength. F(-) displays a unique behavior: It forms with L(1)H a stable [L-H···F](-) complex which in the excited state undergoes intracomplex proton transfer, to give the poorly emissive excited tautomer [L···H-F](-)*. With L(2)H, on moderate addition of F(-), the 1:1 H-bond complex forms, and the blue fluorescence of pyrene is quenched. Large excess addition of F(-) promotes deprotonation of the ground-state complex, according to the equilibrium [L(2)H···F](-) + F(-) ⇆ [L(2)](-) + HF2(-). The deprotonated receptor [L(2)](-) is distinctly emissive (yellow fluorescence), which generates the fluorimetric response ON(1)-OFF-ON(2) of receptor L(2)H with respect to F(-).

Chiral receptors for phosphate ions

The binding tendencies of the enantiomeric forms, R,R and S,S, of the neutral receptor 1 towards anions were investigated through UV-vis and 1H NMR titration experiments in DMSO. Both enantiomers form stable H-bond complexes with carboxylates and phosphates. In particular, receptor 1 strongly binds two H2PO4- ions according two stepwise equilibria, in which logK2 is higher than logK1. Such an unusual cooperativity effect is to be ascribed to the formation of strong H-bond interactions between the two H2PO4- anions, when bound to the two urea subunits of the receptor, as demonstrated by the crystal and molecular structures of the 1 : 2 complex salt: [Bu4N]2[R,R-1...(H2PO4)2]. The S,S enantiomer forms an H-bond complex with the biologically relevant D-2,3-diphosphoglycerate anion, whose association constant is twice that of the R,R complex. Such an effect is ascribed to the different structural features of the two diastereomeric complexes in solution, as shown by 31P NMR studies.

Moderate and Advanced Intramolecular Proton Transfer in Urea–Anion Hydrogen-Bonded Complexes

The study of the interactions of the three urea-based receptors AH, BH(+) and CH(2+) with a variety of anions, in MeCN, has made it possible to verify the current view that hydrogen bonding is frozen proton transfer from the donor (the urea N-H fragment in this case) to the acceptor (the anion X(-)). The poorly acidic, neutral receptor AH establishes two equivalent hydrogen bonds N-H···X(-), with all anions, including CH(3)COO(-) and F(-), in which moderate proton transfer from N-H to the anion takes place. The strongly acidic, dicationic receptor CH(2+) forms, with most anions, complexes in which two inequivalent hydrogen bonds are present: one involving moderate proton transfer (N-H···X(-)) and one in which advanced proton transfer has taken place, described as N(-)···H-X. The degree of proton advancement is directly related to the basic tendencies of the anion. The cationic receptor BH(+) of intermediate acidic properties only forms complexes with two inequivalent hydrogen bonds (moderate+advanced proton transfer) with CH(3)COO(-) and F(-), and complexes with two equivalent hydrogen bonds (moderate proton transfer) with all the other anions. Moreover, [B···HF] and [C···HF](+), on addition of a second F(-) ion, lose the bound HF molecule to give HF(2)(-). Release of CH(3)COOH, with the formation of [CH(3)COOH···CH(3)COO](-), also takes place with the [B···CH(3)COOH] complex in the presence of a large excess of anion.

Redox Active Cage for the Electrochemical Sensing of Anions

The tripodal system [1]3+ forms a 1:1 complex with CoII in which the metal is octahedrally coordinated by three bpy fragments. The [CoII(1)]5+ complex provides a cavity suitable for solvent or anion inclusion. X-ray diffraction studies on the crystalline complex salt of formula [CoII(1)...H2O]Cl(PF6)(4).2MeCN have shown that a water molecule is included in the cavity and the water oxygen atom receives six H-bonds from the C-H fragments of the three imidazolium subunits and of the three proximate pyridine rings, according to a slightly distorted trigonal prismatic geometry. Anion inclusion in an aqueous MeCN solution induces a distinct cathodic shift of the potential of the CoIII/CoII couple, whose magnitude decreases along the series: Cl->Br- approximately NCO->I- approximately NCS-, which reflects anion tendencies to receive H-bonds from the receptor. The variation of the water content in the MeCN solution (from 0 to 20%) induces a gradual change of the voltammetric response to anion titration: from two well distinguished peaks at a fixed potential to a single peak progressively shifted to a more cathodic potential. Such a behavior parallels the gradual decrease of the equilibrium constant for anion inclusion into the [CoII(1)]5+ receptor.

Structurally-variable, rigid and optically-active D2 and D3 macrocycles possessing recognition properties towards C60.

The reactivity of aromatic dicarboxylic acids, in combination with an axially-chiral, suitable dibenzylic alcohol, derived from BINOL, has been exploited in one-pot esterification reactions for the direct formation of several rigid, homochiral macrocycles. Cyclic adducts possessing, respectively, average molecular D(2) and D(3) symmetries, have been characterized in pure forms, with isolated yields and selectivities which are substantially different from those rising from purely statistical arguments. NMR and CD spectroscopies detect the structural and shape variability in the scaffolds, reflected both in terms of large changes in chemical shifts of selected proton resonances, and in terms of the variation of the CD signature related to the dihedral angle defined by the binaphthyl units embedded in the rigid cyclic skeleton. The crystal structure of two homochiral D(2) macrocycles show the formation of ordered nanostructures in the solid state, with the naphthyl rings of the binaphthyl units packing intermolecularly in an eclipsed-like conformation to yield nonhelical tubular arrangements. The larger D(3) cyclic adducts are able to afford stable complexes with C(60) in toluene solution, with comparable binding strengths, yet whose stoichiometries are dependent on small variations in the spacing units and therefore in the shapes of the internal cavities of the cyclic structures.

Octahedral Copper(II) and Tetrahedral Copper(I) Double-Strand Helicates: Chiral Self-Recognition and Redox Behavior

The racemic form of 5 ((RR)5 + (SS)5) gives dinuclear complexes of 2:2 stoichiometry both with Cu(II), acting as a bis-terdentate ligand, and with Cu(I), acting as a bis-bidentate ligand. Single crystal X-ray diffraction studies have shown that the Cu(II) complex exists as double-strand homochiral helicate molecules: P,P-[Cu(2)(II)((RR)5)(2)](4+) and M,M-[Cu(2)(II)((SS)5)(2)](4+); in which the two trans-1,2-cyclohexanediamine subunits have the same chirality for of the two strands. Each Cu(II) metal center is six-coordinated according to a cis-octahedral geometry and interacts with a NNO donor subunit of each strand. The Cu(I) complex, when crystallized from THF in the presence of (rac)5, gives a double-strand homochiral helicate complex and in the solid state forms a racemic mixture of the homochiral metal complexes M,M-[Cu(2)(I)((RR)5)(2)](2+) and P,P-[Cu(2)(I)((SS)5)(2)](2+). When crystallizing from a MeCN solution, Cu(I) and (rac)5 give rise to the heterochiral nonhelicate dimeric complex [Cu(2)(I)((RR)5)((SS)5)](2+), in which the two strands of the dimer have inverse configuration of the trans-1,2-cyclohexanediamine subunits and are assembled side-by-side. In both structural architectures, the Cu(I) centers are four-coordinated by two nitrogen atoms from each strand, according to a distorted tetrahedral geometry. In MeCN solution, the dinuclear Cu(II) complex disassembles to give the mononuclear species, which, on reduction at a platinum electrode in a cyclic voltammetry experiment, gives two Cu(I) mononuclear complexes that quickly assemble to give the dinuclear Cu(I) complex. This complex undergoes two consecutive one-electron oxidation processes, but the dinuclear Cu(II) species that forms decomposes in less than 1 s. On the contrary, the [Cu(2)(I)((rac)5)(2)](2+) complex is stable in MeCN solution and undergoes two one-electron oxidation processes to give a form of dinuclear Cu(II) complex that lasts in solution for more than 20 s.

Enantioselective Cycloadditions of 2-Alkenoylpyridine-N-oxides Catalysed by a Bis(oxazoline)/CuII Complex: Structure of the Reactive Intermediate

Diels-Alder and 1,3-dipolar cycloadditions involving (E)-3-aryl-1-(pyridin-2-yl-N-oxide)prop-2-en-1-ones as the 2π components are efficiently catalysed by bis(oxazoline)-Cu(II) complexes. The cycloadducts are obtained in quantitative yields with up to 98 % ee; absolute configurations were determined by X-ray analysis. The structure of the reactive complex, determined by X-ray analysis, is fully consistent with the stereochemical outcome of the catalysed process (approach of the diene or nitrone to the less hindered face of the coordinated pyridine-N-oxide derivative).

Further insights on the high–low spin interconversion in nickel(II) tetramine complexes. Solvent and temperature effects

The spin interconversion equilibrium involving the [Ni(II)(cyclam)]2+ complex has been investigated in a variety of polar solvents, at varying temperatures. The greater the donor tendencies of the solvent, the higher the endothermicity of the high-to-low-spin conversion. In particular, a positive linear relationship exists between DeltaHdegrees and Gutmanns Donor Number (DN). In the same way, higher donor tendencies of the solvent favour the occurrence of the Ni(II)-to-Ni(III) oxidation process and negative linear relationship has been found between the E1/2(Ni(III)/Ni(II)) and DN. General behaviour is related to the intensity of the metal-solvent axial bonds in the octahedrally elongated cyclam complexes (of both Ni(II) and Ni(III)).

Site preference and local geometry of Sc in garnets: Part II. The crystal-chemistry of octahedral Sc in the andradite-Ca3Sc2Si3O12 join

Abstract Investigation of scandium incorporation in garnets along the synthetic Ca3Fe23+Si3O12-Ca3Sc2Si3O12 (adr.CaSc) join, based on the same multi-technique approach used in the companion paper (Oberti et al. 2006a), shows that (1) Sc is incorporated exclusively at the Y octahedron; (2) the local coordination of Sc is slightly different in Sc-poor than in Sc-rich compositions (Sc-O = 2.06 Å in CaSc10 vs. 2.10 Å in CaSc30-90); (3) the local coordination of Ca is also slightly different in Sc-poor than in Sc-rich compositions [Ca1,2-O are 2.34(2) and 2.48(2) Å in CaSc10 and 2.36(2) and 2.50(2) Å in CaSc90, with Δ fixed at 0.14 Å in all the samples]; (4) the linear increase of the unit-cell edge along the join derives from multiple changes in the geometry of the different polyhedra and from the rotation of the tetrahedron around the 4̄ axis (α rotation), and cannot be modeled from extrapolation of the behavior observed along the Ca3Al2Si3O12-Ca3Fe23+Si3O12 (grs.adr) join. CaSc-rich garnets, where a large X dodecahedron coexists with a large Y octahedron and a Z tetrahedron occupied by Si, similar to pyrope-grossular garnets, have the highest α values observed to date in calcium silicate garnets. Slightly lower α values are observed in pyrope and almandine, but correspond to a different structural arrangement, where a small X dodecahedron coexists with a small Y octahedron. These results further confirm the efficiency of a combined short- and long-range approach for understanding the properties of garnet solid solutions.