Double Helicate Research Articles

Synthesis and Properties of Silver(I) and Copper(I) Helicates with Imine-Bridged Oligobipyridine Ligands

Oligobipyridine ligands containing one or two imine bridges were found to form double helicates by treatment with copper(I) or silver(I). The properties of the complexes are similar to those of oxapropylene-bridged oligobipyridines. Titration of a mixture of the bis(bipyridine) and the tris(bipyridine) ligands with silver(I) hexafluorophosphate showed that helicates formed with strict self-recognition.

Synthesis and Properties of Silver(I) and Copper(I) Helicates with Imine-Bridged Oligobipyridine Ligands

Oligobipyridine ligands containing one or two imine bridges were found to form double helicates by treatment with copper(I) or silver(I). The properties of the complexes are similar to those of oxapropylene-bridged oligobipyridines. Titration of a mixture of the bis(bipyridine) and the tris(bipyridine) ligands with silver(I) hexafluorophosphate showed that helicates formed with strict self-recognition.

Diastereoselective formation of chiral helicates

Chiral 2,2′:6′,2″:6″,2‴-quaterpyridines bearing fused chiral groups in the 5,6- and 5‴,6‴-positions preferentially form structurally characterised P or M dinuclear double helicates in ≈99% diastereomeric excess upon coordination to copper(I).

Preparation of the new podand ligand SP(pzpy) 3 [pzpy = 3-(2-pyridyl)-pyrazol-1-yl], and the syntheses and crystal structures of copper(II) and copper(I) complexes of its hydrolysis product [OSP(pzpy) 2] −, and a double helical copper(I) complex of [O 2P(pzpy) 2] −

The new podand ligand SP(pzpy) 3 [L 1; pzypy = 3-(2-pyridyl)-pyrazol-1-yl], containing three bidentate pyrazolyl-pyridine arms linked to an apical PS group via the pyrazolyl N 1 atoms, was prepared and crystallographically characterised. Reaction with Cu(II) or Cu(I) resulted in complexes of the partially-hydrolysed ligand [OSP(pzpy) 2] − (L 2), containing two bidentate chelating arms linked by a thiophosphinate fragment. [Cu 2(L 2) 2(μ-pzpy) 2] contains a planar dinuclear copper(II) core with two pyrazolate bridges linking the metal centres, and one L 2 ligand attached to each metal by one bidentate arm with the other arm pendant. In [Cu 2(L 2) 2] each ligand L 2 acts as an N, N-donor to one metal ion and an N, S-donor to the other with a non-coordinated pyridyl group, such that each metal ion has an N 3 S donor set; this complex is achiral with a “side-by-side” disposition of bridging ligands. For comparison purposes we also crystallographically characterised [Cu 2(L 3) 2], where L 3 is [O 2P(pzpy) 2] −, an analogue of L 2 but with a phosphinate rather than thiophosphinate group. This complex is a dinuclear double helicate in which each ligand acts as an N, N-bidentate chelate to each metal ion. Comparison of this with the structure of [Cu 2(L 2) 2] shows that it is the involvement of the thiophosphinate S atom of L 2 in coordination that prevents helication from occurring.

Enantioselective Formation of Double and Triple Helicates of Silver(I): The Role of Stacking Interactions

Despite having ligands with almost identical conformations, the structures of the AgI complexes of 1 and 2 differ significantly: ligand 1 forms a double-helical complex of P chirality, and 2 gives a triple-helical complex (depicted on the right). The reason for the difference appears to be the existence of strong interligand stacking in the latter case.

Steric Control of Directional Isomerism in Dicopper(I) Helicates of Asymmetrically Substituted 2,2‘:6‘,2‘‘:2‘‘,6‘‘‘-Quaterpyridine Derivatives

Derivatives of 2,2‘:6‘,2‘‘:2‘‘,6‘‘‘-quaterpyridine have been prepared which are asymmetrically substituted with alkyl groups in the 4- or 6-position and with various substituents in the 4‘-position. These ligands form dicopper(I) double helicates which have been investigated by 1H and 13C NMR spectroscopic techniques. The formation of helical isomers is shown to depend on the intramolecular interactions between the constituent helicands of the double helicate; 4‘-methyl substituents undergo steric interactions with the 4-substituent of the partner helicand, leading to a modest selectivity, although bulky 4-substituents decrease selectivity. In the absence of 4‘-substituents, the smaller pitch permits steric interactions between like 4-substituents of the component helicands. In each case, formation of the head-to-head helicate isomer is preferred.

Diastereoselective formation of P and M dicopper(i) double helicates with chiral 2,2′:6′,2″-terpyridines

Chiral 2,2′:6′,2″-terpyridines bearing bornyl groups in the 6-position are prepared; these ligands preferentially form P or M dinuclear double helicates upon coordination to copper(I) which have been structurally characterised.

Diastereoselective formation of disilver(I) double helicates with chiral 2,2′:6′,2″:6″,2′′′-quaterpyridines

Chiral 2,2′:6′,2″:6″,2′′′-quaterpyridines bearing fused chiral groups in the 5,6- and 5′′′,6′′′-positions are prepared; these ligands preferentially form P or M dinuclear double helicates upon coordination to silver(I), one of which has been structurally characterised.

Synthesis and Structure of a Bis(Double Helicate) and its Cryptatoclathrate

AbstractThe structures of the hitherto unknown bis(double helicate) 10 and its cryptatoclathrate (10)2·2THF were unequivocally determined by X‐ray diffraction. Bis(double helicate) 10 is formed in a one‐pot synthesis starting from CH‐acidic bis (tetrazolylmethyl ketone) 9 and Zn(OAc)2. The formation of racemic, homochiral 10 from [Zn2L32] fragments, which are formed in a self‐assembly process, is governed by chiral self‐recognition. According to NMR studies only 10 is present in solution. 13C CP/MAS NMR spectroscopy and X‐ray analysis confirm aggregation of 10 with two molecules of THF to yield the inclusion compound (10)2·2 THF in the solid state.

Investigation of the Self‐Assembly Pathway of Pentanuclear Helicates by Electrospray Mass Spectrometry

AbstractThe self‐assembly of the pentanuclear double helicates Hh, Ha and He from the corresponding oligobipyridine strands Lh, La and Le and Cu1 ions has been investigated by NMR and electrospray mass spectrometry (ESMS). Where as Hh is assembled rapidly (in less than 20 min), He (about 20 h) and especially Ha (about 60 h) form much more slowly. The rate decreases strongly with increasing steric bulk of the substituents in the 4,4′‐positions on the bipyridine units; this indicates that the search processes (wrapping, unwrapping) that lead to the final helicate are strongly hindered by the size of the substituents. The ESMS data give information about the species present in solution under different conditions and allow the formulation of possible formation pathways, which may involve, in particular, helicates of hairpin type.

Trinuclear Double Helicates of Iron(II) and Nickel(II): Self‐assembly and resolution into helical enantiomers

AbstractThe synthesis of the linear tris[terpyridine] 1 with three tridentate binding sites is described. The reaction with metal ions of octahedral coordination geometry, such as FeII or NiII, leads to the self‐assembly of trinuclear complexes [M3(1)2]6+, which display properties in agreement with a double helical structure. The trinuclear iron (II) helicate has been resolved into its enantiomers.

Self‐Assembly of a Circular Double Helicate

Self‐Assembly of a Circular Double Helicate

Heterodimetallic double helicates from redistribution reactions

Dinickel(II) and dicopper(I) double helicates of 4′,4‴-bis(methylsulfanyl)-2,2′:6′,2″:6″,2‴ :6‴, 2â�—-quinquepyridine (ms2qpy) ligands react to form heterodimetallic double helicates [CuNi(ms2qpy)2]3+; the complex [CuNi(ms2-qpy)2][PF6]3·4H2O is structurally characterised.

Self-recognition in helicate self-assembly: spontaneous formation of helical metal complexes from mixtures of ligands and metal ions.

On treatment with copper(I) ions, mixtures of the [oligo(2,2')-bipyridine] strands 2-5 yield spontaneously the double helicates 2H(2)-2H(5) without significant crossover. Similarly, when a mixture of the two tris-bypyridine ligands 1 and 3a (containing different substitutions) is allowed to react with copper(I) and nickel(II) ions, only the double helicate 2H(3a) and the triple helicate 3H(1) are formed. Thus, these systems undergo self-assembly of helicates with self-recognition. The process represents a self/nonself-discrimination involving the preferential binding of like metal ions by like ligand strands in a mixture to selectively assemble into the corresponding helicates. In a broader perspective, these results point to a change in paradigm from pure compounds to instructed mixtures--that is from seeking chemical purity to designing programmed systems composed of mixtures of instructed components capable of spontaneously forming well--defined superstructures through self-processes.

Dinuclear double helicates incorporating a 1,3-phenylene spacer; the crystal and molecular structure of diacetato-1x2O-,2x2O-bis[µ-1,3-bis(4-methylthio-2,2′-bipyridin-6-yl)benzene-1x2N,N′:2x2N″,N‴]dinickel bis(hexafluorophosphate)

Linking two bipyridine units by a 1,3-phenylene spacer has provided a novel class of ligand which promotes the spontaneous self-assembly of double helicates upon reaction with transition-metal ions. Interaction with copper(I) or silver(I) resulted in dinuclear double-helical complexes with the metal ions occupying pseudo-tetrahedral co-ordination sites. Reaction with cobalt(II) or nickel(II) acetates gave similar dinuclear double-helical structures with a didentate acetate ligand completing the co-ordination sphere of each metal to give a pseudo-octahedral geometry. The crystal structure of [Ni2L22(O2CMe)2][PF6]2[L2= 1,3-bis(4-methylthio-2,2′-bipyridin-6-yl)benzene] has been determined. The two metal centres are separated by 5.875 A. In contrast to the helicates formed by the linear oligopyridines, no significant π-stacking interactions are observed between pyridine moieties on the helical strands.

Spontaneous Self‐Assembly of a Dinickel(II) Double Helicate Containing a 1, 3‐Benzenediyl Spacer Group

A new type of potentially helical ligand is represented by 1. The twist of the molecule's chain required to form double helical complexes by self-organization occurs at the CC bonds between the arene rings. Ligand 1 forms double helical complexes with Cu+, Ag+, and Ni2+ salts. A crystal structure analysis of the complex [Ni2(OAc)2(lb)2](PF6)2 revealed neither intermetallic interactions nor stacking interactions between almost coplanar rings.

Helicity Induction in Helicate Self‐Organisation from Chiral Tris(bipyridine) Ligand Strands

AbstractThe chiral tris, and pentakis(bipyridine) ligand strands 3a and 4 were synthesised, each in optically pure form with (S,S)‐configuration. Ligand 3a yielded substituted double‐helical metal complexes, derived from the parent trihelicate structure 2, with CuI and AgI ions. The spectral data, in particular the 1H‐NMR spectra and the large positive Cotton effect, indicate that helicate formation occurs with very high induction of helicity. Together with consideration of the steric effects in the two possible helical diastereoisomers (P)‐1 and (M)‐1 that may be formed, the data favour the preferential generation of the right‐handed double helicate (P)‐1 from the tris(bipyridine) strand 3a of(S,S)‐configuration.