Diastereoselective generation of triple-stranded helicates induced by gem-dimethyl groups on a linker

The diastereoselectivity of the self-assembly of bis(dipyrromethene) ligands with trivalent metals was improved by introducing gem-dimethyl groups to the linker.     

Long-range stereocontrol in the self-assembly of two-nanometer-dimensioned triple-stranded dinuclear helicates

A series of bisimine-bridged dicatechol ligands 2-H(4)-5-H(4) were synthesized and were used to prepare triple-stranded dinuclear helicate-type complexes with a length of up to more than 2 nm. X-ray structural analyses of Na(4)[(2)(3)V(2)], Na(4)[(3)(3)Ti(2)], Na(4)[(4)(3)Ti(2)], and Na(4)[(5)(3)Ti(2)], as well as temperature-dependent NMR investigations of Na(4)[(4)(3)Ti(2)] and Na(4)[(5)(3)Ti(2)] show that, in the case of the rigid linear ligands 2 and 3, and of the ligand 5, which possesses C(2h) symmetry in its idealized structure, homochiral helicates are diastereoselectively formed. Ligand 4, on the other hand, with idealized C(2v) symmetry, leads with surprisingly high selectivity to the formation of the heterochiral meso-helicate. This is attributed to the ability of ligand 4 to adopt a less-restricted conformation in the meso compound than in the helical complex. NMR investigations indicate that both complex units of Na(4)[(4)(3)Ti(2)] invert (LambdaDelta-->DeltaLambda) simultaneously, while in the case of Na(4)[(5)(3)Ti(2)] a stepwise racemization proceeds.     

Discriminating between lanthanide ions: self-assembly of heterodimetallic triple-stranded helicates

A bis-terdentate segmental ligand has been designed which self-assembles with lanthanide ions of different size to yield heterodimetallic triple-stranded helicates.     

Dicatechol-diimines: easily accessible ligands for the self-assembly of dinuclear triple-stranded helicates

Dicatechol ligands 3b-g-H4 are simply prepared by imine formation of 2,3-dihydroxybenzaldehyde 2 with a series of different diamines 1b-g . An X-ray structural analysis was obtained for the butyl-bridged compound 3e-H4, showing an intramolecular proton transfer and the formation of a chinoidic "keto-amine" structure. The dicatechol derivatives 3b-g-H4 form dinuclear triple-stranded helicates M4[(3)3Ti2] with titanium(IV) ions in the presence of alkali-metal carbonate. For the phenyl- and the trans-1,4-cyclohexyl-bridged complexes, K4[(3b)3Ti2] and Na4[(3f)3Ti2], X-ray structures were obtained.     

Ruthenium(II) as a novel "labile" partner in thermodynamic self-assembly of heterobimetallic d-f triple-stranded helicates

Unsymmetrical substituted bidentate benzimidazol-2-ylpyridine ligands L2 and L3 react with [Ru(dmso)(4)Cl(2)] in ethanol to give statistical 1:3 mixtures of fac-[Ru(Li)(3)](2+) and mer-[Ru(Li)(3)](2+) (i=2, 3; DeltaGtheta(isomerisation)=-2.7 kJ mol(-1)). In more polar solvents (acetonitrile, methanol), the free energy of the facial<=>meridional isomerisation process favours mer-[Ru(Li)(3)](2+), which is the only isomer observed in solution at the equilibrium (DeltaGtheta(isomerisation)< or = -11.4 kJ mol(-1)). Since the latter process takes several days for [Ru(L2)(3)](2+), fac-[Ru(L2)(3)](2+) and mer-[Ru(L2)(3)](2+) have been separated by chromatography, but the 28-fold increase in velocity observed for [Ru(L3)(3)](2+) provides only mer-[Ru(L3)3](ClO(4))2 after chromatography (RuC(60)H(51)N(9)O(8)Cl(2), monoclinic, P2(1)/n, Z=4). The facial isomer can be stabilised when an appended tridentate binding unit, connected at the 5-position of the benzimidazol-2-ylpyridine unit in ligand L1, interacts with nine-coordinate lanthanides(III). The free energy of the facial<=>meridional isomerisation is reversed (DeltaGtheta(isomerisation)> or =11.4 kJ mol(-1)), and the Ru--N bonds are labile enough to allow the quantitative thermodynamic self-assembly of HHH-[RuLu(L1)(3)]5+ within hours ([RuLu(L1)3](CF(3)SO(3))(4.5)Cl(0.5)(CH(3)OH)(2.5): RuLuC(106)H(109)Cl(0.5)N(21)O(19)S(4.5)F(13.5), triclinic, P(-)1, Z=2). Electrochemical and photophysical studies show that the benzimidazol-2-ylpyridine units in L1-L3 display similar pi-acceptor properties to, but stronger pi-donor properties than, those found in 2,2'-bipyridine. This shifts the intraligand pi-->pi* and the MLCT transitions toward lower energies in the pseudo-octahedral [Ru(Li)(3)](2+) (i=2, 3) chromophores. The concomitant short lifetime of the (3)MLCT excited state points to efficient, thermally activated quenching via low-energy Ru-centred d-d states, a limitation which is partially overcome by mechanical coupling in HHH-[RuLu(L1)(3)]5+.     

Diastereoselective assembly of pentanuclear circular helicates

Reaction of a ligand which contains two N-donor and O-donor tridentate domains separated by a 1,3-phenylene spacer unit with Zn(2+) ions results in a pentanuclear circular helicate [Zn(5)(L)(5)](10+) and this structure persists in both the solid and solution state. The formation of this high nuclearity species is governed by unfavourable steric interactions between the phenyl units which destabilize the simple linear helicate. Incorporation of enantiopure units within the ligand strand controls the diastereoselectivity with up to 80% d.e.     

Observation of slow magnetic relaxation in triple-stranded lanthanide helicates

Two dinuclear triple-stranded helicates [Ln(2)L(3)](3+) (Ln = Dy and Tb) obtained via self-assembly from the ligand HL (2,6-diformyl-4-methylphenol di(benzoylhydrazone)) and lanthanide perchlorate have been synthesized and characterized. The crystal structural analysis demonstrates that three ligand strands wrap around a pseudo-threefold axis defined by the two metal ions, leading to a 'meso'-relation between the right- (Δ) and left-hand (Λ) configurations of [Ln(2)L(3)](3+) in the crystal. Each Ln(III) ion is coordinated by nine donor atoms in a distorted tricapped trigonal-prismatic arrangement. Alternating current (ac) susceptibility measurements of [Dy(2)L(3)](3+) reveal a frequency-dependent out-of-phase signal under a 700 Oe dc field, indicating the onset of the slow relaxation of magnetization with a roughly estimated activation energy of ～5 K and τ(0) of 10(-7) s.     

Lanthanide triple-stranded helicates: controlling the yield of the heterobimetallic species

Two unsymmetrical ditopic hexadentate ligands designed for the simultaneous recognition of two different trivalent lanthanide ions have been synthesized, L(AB2) and L(AB3), where A represents a tridentate benzimidazole-pyridine-benzimidazole coordination unit, B2 a diethylamine-substituted benzimidazole-pyridine-carboxamide one, and B3 a chlorine-substituted benzimidazole-pyridine-carboxamide moiety. Under stoichiometric 2:3 (Ln/L) conditions, these ligands self-assemble with lanthanide ions to yield triple-stranded bimetallic helicates. The crystal structures of four helicates with L(AB3) of composition [LnLn'(L(AB3))3](ClO4)6.solv (CeCe, PrPr, PrLu, NdLu) show the metal ions embedded into a helical structure with a pitch of about 13.2-13.4 A. The metal ions lie at a distance of 9.1-9.2 A and are nine-coordinated by the three ligand strands, which are oriented in a HHH (head-head-head) fashion, where all ligand strands are oriented in the same direction. In the presence of a pair of different lanthanide ions in acetonitrile solution, the ligand L(AB3) shows selectivity and gives high yields of heterobimetallic complexes. L(AB2) displays less selectivity, and this is shown to be directly related to the tendency of this ligand to form high yields of HHT (head-head-tail) isomer. A fine-tuning of the HHH left arrow over right arrow HHT equilibrium and of the selectivity for heteropairs of Ln(III) ions is therefore at hand.     

TD-DFT investigation of triple-stranded helicates with bis(benzene-o-dithiolato) ligands

The electronic spectra of triple-stranded complexes of Ti(IV) with bis(benzene-o-dithiolato) ligands (H(4)-L(1)=1,2-bis(2,3-dimercaptobenzamido)ethane; H(4)-L(2)=1,2-bis(2,3-dimercaptobenzamido)benzene) were investigated at the TD-DFT level of theory employing B3LYP/LANL2DZ. The influence of the nature (aliphatic or aromatic) of the spacer bridging both {Ti(S(2)C(6)H(3))(3)} units on the absorptive features of the dinuclear complexes was also studied. B3LYP/LANL2DZ leads to spectra accounting for four absorption bands. At the lowest-energy region, the most important transitions are due to ligand-to-metal charge transfer (LMCT), in which out-of-plane ligand-centered orbitals and titanium-based MOs are involved. In going to the blue-region, a third band was detected with excitations showing an important contribution from ligand-to-ligand charge transfer (LLCT) and indeed, a combined LMCT+LLCT character has been considered. This observation seems to arise from a decrease in the metallic character of the LUMO-derivatives involved in the excitations. The origin of the absorption band at the highest-energy part of the spectrum is assigned to a LLCT. The influence of the nature of the spacer on the molar absorption coefficients (?) for this band has been clearly observed. The complex bearing aliphatic spacer shows ? of about 5 E+4 M(-1) cm(-1), while the one containing an aromatic spacer present a value of ? of about 2 E+5 M(-1) cm(-1).     

Three double- or triple-stranded chains constructed from different cobalt(II) salts and a flexible bis-triazole ligand

Three coordination compounds, {[Co(btrp)₂(H₂O)₂]?·?NO₃?·?H₂O} _n (1), {[Co(btrp)₂(H₂O)₂]?·?H₂O?·?2H₂btc} _n (2), and {[Co(btrp)₃]?·?2ClO₄} _n (3) (btrp?=?1,3-bis(1,2,4-triazol-1-yl)propane; H₃btc?=?benzene-1,3,5-tricarboxylic acid), have been prepared via solvothermal method and characterized by single-crystal X-ray diffraction and elemental analyses. Compound 1 possesses a 1-D double-stranded chain composed of ribbons of 20-membered cycles. Binuclear water clusters link adjacent nitrate anions to form a 1-D supramolecular helix in the structure. Compound 2 has a 1-D double-stranded chain wherein free H₂btc ligands constitute 1-D negative chains through classical hydrogen-bonding interactions (O–H?···?O). Compound 3 exhibits a triple-stranded 1-D chain. For 1–3, 3-D supramolecular structures are consolidated by interchain weak hydrogen-bonding interactions as well as electrostatic interactions.     

Cerium-based triple-stranded helicates as luminescent chemosensors for the selective sensing of magnesium ions

Cerium-based triple-strand helicates Ce-CL(1) and Ce-CL(2) were achieved via self-assembly from malonohydrazone derived bis-tridentate ligands H(2)CL(1) and H(2)CL(2), respectively. Structure analysis of Ce-CL(1) shows that six oxygen atoms of the β-diketone groups on the ligands form a lantern-like cavity inside the helicate. Thus the helicates exhibit crown ether recognition behaviors and could work as luminescent magnesium chemosensors. The restricted geometry constraints of the internal cavities provide high selectivities of the helical probes towards Mg(2+) ion over other alkaline and alkaline-earth ions such as Li(+), Na(+), K(+), Ca(2+) and Ba(2+).     

Thermodynamic parameters governing the self-assembly of head-head-head lanthanide bimetallic helicates

The heterobitopic ligands L ABX (X=1, 2, 3, 4 or 5), differing only by a Cl or NEt(2) substituent, have been designed to complex with a pair of lanthanide ions to form triple-stranded bimetallic helicates of overall composition [Ln2(L ABX)3]6+. The percentage of HHH (head-head-head) isomer, in which each of the three ligand strands coordinates to the same lanthanide ion with the same coordination unit, is deciding the ability of the ligands to selectively form heterobimetallic complexes containing one luminescent and one magnetic or two different luminescent ions. It deviates significantly from the statistical value of 25 % and ranges from 6-20 % for L AB2 complexes to 93-96 % for L AB4 complexes. The equilibrium between HHT (head-head-tail) and HHH isomers has been investigated in detail for homobimetallic helicates (Ln=Y, La, Ce, Pr, Nd, Sm, Eu, Lu) by means of variable temperature NMR and thermodynamic parameters have been determined. The equilibrium is characterized by small values of DeltaH and DeltaS, which vary in opposite direction along the lanthanide series for complexes with the same ligand in a way that keeps the value of DeltaG almost constant. The results are interpreted in terms of differences in interstrand stacking, ion-dipole interactions and metal-metal repulsion.     

Rational design of a ternary supramolecular system: self-assembly of pentanuclear lanthanide helicates

The self-assembly of the first pentanuclear helicate was predicted on the structural basis obtained for linear and tetranuclear parent supramolecular compounds. Accordingly, the designed ternary supramolecular system requires appropriate polytopic organic receptors, which were successfully synthesized. Indeed, the formation of pentanuclear complexes was experimentally evidenced with NMR and ESMS spectra that perfectly reflect the expected pattern. The structural features in the europium pentanuclear complex are highlighted with semiempirical molecular modeling. The present work validates the combinatorial approach leading to the thermodynamically driven formation of tower-like pentanuclear edifices.     

Pentanuclear and heptanuclear helicates by self-assembly of d10 metal ions and a rigid aromatic bis-bidentate chelator

The self-assembly of Zn(II) and Cd(II) ions with a bis-bidentate ligand 3,5-bis(benzimidazol-2-yl)pyrazole (H 3L) was studied by Electrospray ionization mass spectrometry, (1)H NMR measurements, and single-crystal X-ray diffraction analyses. Reaction of Zn(ClO 4) 2.6H 2O and Cd(ClO 4) 2.6H 2O with H 3L in DMF gave two pentanuclear complexes [(Zn 5(mu 3-O)(H 2L) 6)(ClO 4) 2.DMF.9.5H 2O ( 1) and [Cd 5(mu 3-O)(H 2L) 6](ClO 4)(OH).4.75DMF.0.25EtOH.10.5H 2O ( 2), in which the trigonal-bipyramidal core structures are bridged by mu 3-oxo and pyrazolate rings of the monodeprotonated H 2L. When Na 3PO 4.12H 2O was used in the reaction system of CdBr 2.4H 2O and H 3L, [Cd 5(mu 3-O)(H 2L) 6]Br 2.4.5DMF.6.5H 2O ( 3) and [Cd 7(mu 6-PO 4)(mu-Br) 3(H 2L) 6](HPO 4).DMF.10H 2O ( 4) were isolated. 3 displays the same core structure as that of 2, whereas 4 exhibits a turbinate, heptanuclear core which is bridged by a mu 6-PO 4, three mu-Br, and three pyrazolate rings. All of the pentanuclear and heptanuclear cores are surrounded by three pairs of bis-bidentate H 2L (-) ligands with offset pi-pi stacking, showing propeller-like molecular structures and triple-stand helicates. Electrospray ionization mass spectrometry studies and (1)H NMR measurements demonstrate that the pentanuclear complexes have different stability in the solution, depending on the metal ions and the counteranions. Furthermore, both 1 and 2 emit blue fluorescence with nanosecond luminescent lifetimes in DMF at room temperature.     

Tuning the self-assembly of lanthanide triple stranded heterobimetallic helicates by ligand design

The heterobitopic ligands L(AB4) and L(AB5) have been designed and synthesised with the ultimate aim of self-assembling dual-function lanthanide complexes containing either a magnetic and a luminescent probe or two luminescent probes emitting at different wavelengths. They react with lanthanide ions to form complexes of composition [Ln(2)(L(ABX))(3)](6+) of which three (X = 4; Ln = Pr, Nd, Sm) have been isolated and characterised by means of X-ray diffraction. The unit cells contain triple-stranded helicates in which the three ligand strands are wrapped tightly around the two lanthanide ions. In acetonitrile solution the ligands form not only homobimetallic, but also heterobimetallic complexes of composition [Ln(1)Ln(2)(L(ABX))(3)](6+) when reacted with a pair of different lanthanide ions. The yield of heterobimetallic complexes is analyzed in terms of both the difference in ionic radii of the lanthanide ions and of the inherent tendency of the ligands to form high percentages of head-head-head (HHH) helicates in which all three ligand strands are oriented in the same direction with respect to the Ln-Ln vector. The latter is very sensitive to slight modifications of the tridentate coordinating units.     

Diastereoselective radical mediated alkylation of a chiral glycolic acid derivative

Radical alkylation of 2-(tert-butyl)-2-methyldioxolan-4-one, a chiral equivalent of glycolic acid, occurs with good to high diastereoselectivity that compares favorably with the corresponding enolate alkylation. The importance of the position of the transition state position, early or late, is highlighted.     

A simple thermodynamic model for rationalizing the formation of self-assembled multimetallic edifices: application to triple-stranded helicates

Reaction of the bis-tridentate ligand bis[1-ethyl-2-[6'-(N,N-diethylcarbamoyl)pyridin-2'-yl]benzimidazol-5-yl]methane (L2) with Ln(CF(3)SO(3))(3).xH(2)O in acetonitrile (Ln = La-Lu) demonstrates the successive formation of three stable complexes [Ln(L2)(3)](3+), [Ln(2)(L2)(3)](6+), and [Ln(2)(L2)(2)](6+). Crystal-field independent NMR methods establish that the crystal structure of [Tb(2)(L2)(3)](6+) is a satisfying model for the helical structure observed in solution. This allows the qualitative and quantitative beta23 (bi,Ln1,Ln2)characterization of the heterobimetallic helicates [(Ln(1))(Ln(2))(L2)(3)](6+). A simple free energy thermodynamic model based on (i) an absolute affinity for each nine-coordinate lanthanide occupying a terminal N(6)O(3) site and (ii) a single intermetallic interaction between two adjacent metal ions in the complexes (DeltaE) successfully models the experimental macroscopic constants and allows the rational molecular programming of the extended trimetallic homologues [Ln(3)(L5)(3)](9+).     

Diastereoselective self-assembly of chiral diamine-chelated aryllithiums to dimeric aggregates

Aryllithium compounds [LiC6H4(CH2N(Et)CH2CH2NEt2)-2]2 (2b), [LiC6H4(CH(Me)N(Me)CH2CH2NMe2-(R))-2]2 ((R)-3b), and [LiC6H4(CH(Me)N(Me)CH2CH2NMe2-(rac))-2]2 ((rac)-3b) were synthesized and characterized in the solid state and in solution. X-ray crystallographic studies of 2b and (R)-3b and molecular weight determinations of 2b, (R)-3b, and (rac)-3b by cryoscopy in benzene showed that, both in the solid state and in apolar, noncoordinating solvents such as benzene, these compounds exist as discrete dimeric aggregates. For (R)-3b and (rac)-3b the aggregation process of two monomeric aryllithium units to one dimer is highly diastereoselective.