Robin G. Pritchard

Hybrid organic–inorganic rotaxanes and molecular shuttles

The tetravalency of carbon and its ability to form covalent bonds with itself and other elements enables large organic molecules with complex structures, functions and dynamics to be constructed. The varied electronic configurations and bonding patterns of inorganic elements, on the other hand, can impart diverse electronic, magnetic, catalytic and other useful properties to molecular-level structures. Some hybrid organic–inorganic materials that combine features of both chemistries have been developed, most notably metal–organic frameworks, dense and extended organic–inorganic frameworks and coordination polymers. Metal ions have also been incorporated into molecules that contain interlocked subunits, such as rotaxanes and catenanes, and structures in which many inorganic clusters encircle polymer chains have been described. Here we report the synthesis of a series of discrete rotaxane molecules in which inorganic and organic structural units are linked together mechanically at the molecular level. Structural units (dialkyammonium groups) in dumb-bell-shaped organic molecules template the assembly of essentially inorganic ‘rings’ about ‘axles’ to form rotaxanes consisting of various numbers of rings and axles. One of the rotaxanes behaves as a ‘molecular shuttle’: the ring moves between two binding sites on the axle in a large-amplitude motion typical of some synthetic molecular machine systems. The architecture of the rotaxanes ensures that the electronic, magnetic and paramagnetic characteristics of the inorganic rings—properties that could make them suitable as qubits for quantum computers—can influence, and potentially be influenced by, the organic portion of the molecule.

A Star of David catenane

We describe the synthesis of a [2]catenane that consists of two triply entwined 114-membered rings, a molecular link. The woven scaffold is a hexameric circular helicate generated by the assembly of six tris(bipyridine) ligands with six iron(II) cations, with the size of the helicate promoted by the use of sulfate counterions. The structure of the ligand extension directs subsequent covalent capture of the catenane by ring-closing olefin metathesis. Confirmation of the Star of David topology (two rings, six crossings) is provided by NMR spectroscopy, mass spectrometry and X-ray crystallography. Extraction of the iron(II) ions with tetrasodium ethylenediaminetetraacetate affords the wholly organic molecular link. The self-assembly of interwoven circular frameworks of controlled size, and their subsequent closure by multiple directed covalent bond-forming reactions, provides a powerful strategy for the synthesis of molecular topologies of ever-increasing complexity.

Electronic and steric effects in manganese Schiff-base complexes as models for the water oxidation complex in photosystem II. The isolation of manganese-(II) and -(III) complexes of 3- and 3,5-substituted N,N′-bis(salicylidene)ethane-1,2-diamine (H2salen) ligands

Manganese-(II) and -(III) complexes of substituted N,N′-bis(salicylidene)ethane-1,2-diamine (H2salen) ligands H2L (substituents are in the 3, 5 or 3,5 positions of the phenyl rings of the salen moiety) have been prepared and thoroughly characterised. The reaction of Mn(ClO4)2·6H2O with H2L in ethanol in air normally leads to manganese(III) complexes ligated by both the N2O2 ligand and water molecule(s). However, by employing electron-withdrawing substituents on the ligand, e.g. 3-Br,5-NO2, a manganese(II) complex can be obtained. A ‘borderline’ ligand is represented by the 5-NO2 derivative (nsalen), which produces a manganese(II) complex contaminated with a small amount of a manganese(III) species. Using a more rigorous oxidising agent in the synthesis, [Fe(η-C5H5)2][FeCl4], drives the reaction totally to a manganese(III) complex [Mn(nsalen)Cl(H2O)]. In addition to magnetic susceptibility studies, cyclic voltammetry has been employed. All the complexes exhibit an oxidation and reduction peak, the reversible character being confirmed by pulse voltammetry. Pulse voltammetry also confirmed the nature of the manganese(II) species [Mn(bnsalen)(H2O)2·2H2O [H2bnsalen =N,N′-bis(3-bromo-5-nitrosalicylidene)ethane-1,2-diamine] and that a slight amount of a manganese(III) species is present in [Mn(nsalen)(H2O)2]·2H2O. Six complexes have been crystallographically characterised. Despite the retention of an octahedral manganese environment in all of them, the supramolecular structures exhibit a wide diversity. The 3,5-dichloro and 5-bromo salen complexes containing co-ordinated water display combined π and hydrogen bonding, as well as dimerisation. The complex [{Mn(µ-dbsalen)(µ-O)}2](dbsalen = 3,5-dibromo derivative) offers an alternative bridging arrangement, and [Mn(bsalen)(MeOH)(OClO3)]·H2O (bsalen = 5-bromo derivative) highlights the versatility of the manganese centre in these systems where, unexpectedly, perchlorate is co-ordinated in place of a lattice water. A more subtle rearrangement of supramolecular structure is obtained in [Mn(nsalen)Cl(H2O)] where the usual combination of π- or hydrogen-bonding interaction is modified by the corresponding ability of the 5-NO2 substituent.

Molecular amino-phosphonate cobalt–lanthanide clusters

The use of 1-amino-1-cyclohexyl phosphonic acid, a functionalised phosphonate, leads to the synthesis of two new structural types for 3d-4f phosphonate cages with unusual structural cores and which show high magnetocaloric effects.

Cytotoxic Michael-type amine adducts of α-methylene lactones alantolactone and isoalantolactone

Two series of cytotoxic (IC50, K562 cell line, 1–24 μM) α-aminomethyl substituted lactones 3 and 4 were prepared by stereoselective Michael-type addition of amines to alantolactone (1) and isoalantolactone (2). The lactones 1 and 2 and their amine adducts induce apoptosis and act as alkylating agents.

Synthesis and characterisation of mercaptoimidazole, mercaptopyrimidine and mercaptopyridine complexes of platinum(II) and platinum(III). The crystal and molecular structures of tetra(2-mercaptobenzimidazole)- and tetra(2- mercaptoimidazole)platinum(II) chloride

Abstract Reaction of K2[PtCl4] with 2-mercaptobenzimidazole (HL2) and 2-mercaptoimidazole (HL3) in aqueous ethanol afforded the new, crystalline, square-planar platinum(II) complexes [Pt(HL2)4]Cl2.EtOH (4) and [Pt(HL3)4]Cl2.2H2O (5), the crystal and molecular structures of which are reported. In both complexes the ligands are present in the thione form with coordination taking place through the sulphur atom only. The Pt–S bond lengths are 2.304(4) and 2.312(3) A in complex (4) and 2.314(12) and 2.317(12) A in complex (5). The S–Pt–S bond angles are 90.33(13) and 89.67(13)° in complex (4) and 94.46(4) and 85.54(4)° in complex (5). Reaction of the ligands 4-hydroxy-2-mercaptopyrimidine (HL4) and 2-mercaptopyridine (HL5) with K2[PtCl4] gave the new, dimeric platinum(III) complex [Pt2(L4)4Cl2] (6) and [Pt2(L5)4Cl2]·EtOH (7) which is similar to a previously reported complex prepared by a different method. In these complexes the organic ligands are bridging N, S donors, the chlorides are terminally coordinated to each platinum and there is a platinum–platinum bond. The reaction of 4-amino-2-mercaptopyrimidine (HL6) with K2[PtCl4] gave the monomeric platinum(II) complex cis-[Pt(HL6)2Cl2]·2H2O (8) in which each HL6 ligand is coordinated to the metal ion via the exocyclic amino group. Reaction of 4,6-dihydroxy-2-methylmercaptopyrimidine (HL7) with K2[PtCl4] gave the dimeric platinum(II) complex of formula [Pt2(L7)2(HL7)2]Cl2 (9), which is suggested to contain bridging L7 and HL7 ligands.

Mercury(II) and gold(III) derivatives of 2-phenyl pyridines and 2-phenyl-4-(methylcarboxylato)quinoline

Abstract ortho -Mercurated derivatives of several substituted 2-phenylpyridines and of 2-phenyl-4-(methylcarboxylato)quinoline have been prepared and characterised. C,N-chelated gold(III) derivatives, [AuCl 2 (C 6 H 4 C 5 H 3 RN)] (R=H, 3-Me, 3,5-Me 2 , 4-Pr n , 4-Bu t ) are prepared more efficiently by trans -metallation reactions than by direct reaction of AuCl 4 − with the phenylpyridines. The new gold complexes were characterised spectroscopically and a variety of substitution reactions have been effected. As is usual with unsymmetrical bidentate ligands, the softer donor atom is found trans to the N-donor of the pyridine unit. Crystal-structure determinations are reported for [Au(OAc)(pmpy)(py)]ClO 4 , [Au(ppy)(dpbt)]BPh 4 , [Au(ppy)(S 2 CNMe 2 )]BPh 4 and AuCl(pqcm) 2 [Hppy=2-phenylpyridine, Hpmpy=2-phenyl-3-methylpyridine, Hpqcm=2-phenyl-4-(methylcarboxylato)quinoline].

A classification of spin frustration in molecular magnets from a physical study of large odd-numbered-metal, odd electron rings

The term “frustration” in the context of magnetism was originally used by P. W. Anderson and quickly adopted for application to the description of spin glasses and later to very special lattice types, such as the kagomé. The original use of the term was to describe systems with competing antiferromagnetic interactions and is important in current condensed matter physics in areas such as the description of emergent magnetic monopoles in spin ice. Within molecular magnetism, at least two very different definitions of frustration are used. Here we report the synthesis and characterization of unusual nine-metal rings, using magnetic measurements and inelastic neutron scattering, supported by density functional theory calculations. These compounds show different electronic/magnetic structures caused by frustration, and the findings lead us to propose a classification for frustration within molecular magnets that encompasses and clarifies all previous definitions.

High performance, acene-based organic thin film transistors

1,4,8,11-Methyl-substituted 6,13-triethylsilylethynylpentacene shows extended pi-pi overlap when deposited from solution, yielding organic thin film transistors with high and reproducible hole mobility with negligible hysteresis.

Tetrameric Cyclic Double Helicates as a Scaffold for a Molecular Solomon Link

A Solomon link, colloquially termed a “Solomon knot” (a link in Alexander–Briggs notation[1]), is a topology of two interwoven rings that cross each other four times in the simplest representation (Figure 1).[2] Such doubly-entwined [2]catenanes are still rare,[3–5] with only two small-molecule examples with wholly organic backbones reported[4,5] to date. The Solomon link is the most complex topology to have been produced[4] using Sauvage’s pioneering route[6] of generating higher order interlocked structures through the connection of the termini of linear double-stranded metal helicates. In principle,[2b,d] cyclic double helicates[7] can provide the crossings required for a range of topologies, while simultaneously positioning connecting sites in close proximity to aid the macrocyclization reactions that can be problematic when employing long linear helicates[8] (Figure 1). A small-molecule pentafoil knot (five crossings) was recently prepared using a pentameric circular helicate scaffold.[9] Here we report on the use of a tetrameric circular helicate as the basis for a Solomon link, illustrating the general utility of this approach for the assembly of complex molecular topologies. Figure 1 Ring-closing cyclic metal double helicates for the formation of topologically complex molecules. A pentameric circular double helicate is the scaffold (five crossings) required for a pentafoil knot,[9] and a tetrameric circular double helicate (four crossings) ... The ligand used in our earlier synthesis of a pentafoil knot[9] was based on a tris(bipyridine) motif employed[7a,b,d] by Lehn to assemble penta- and hexameric cyclic helicates, but with both outer bipyridine units replaced by 2-formylpyridine groups that could condense with amines to form imines and generate tris(bidentate) ligand strands. As well as providing a convenient way of connecting metal binding components, imine bond formation is reversible, imparting an ‘error checking’ mechanism during the assembly process.[10] Incorporating an additional oxygen atom in the ethylene spacer between each bipyridine group of Lehn’s tris(bipyridine) ligand led to cyclic tetrameric helicates.[7b] Accordingly, in an attempt to generate the four crossings required for a Solomon link, we introduced a similar structural change to the ligand used in the pentafoil knot synthesis in the form of 1 (for the synthesis of 1 see the Supporting Information) and investigated its coordination chemistry with primary amines and FeII salts (Scheme 1). Scheme 1 Synthesis of cyclic and linear iron(II) helicates. Reaction conditions: a) FeX2, RCH2NH2, DMSO, 60 °C, 24 h; b) excess KPF6 (aq). DMSO=dimethyl sulfoxide. The reaction of 1 with n-hexylamine and FeCl2 (DMSO, 60 °C, 24 h, Scheme 1)[8] produced an intensely colored purple solution typical of low-spin iron(II) tris(diimine) complexes. After 24 hours, the product was isolated in 47 % yield as the hexafluorophosphate salt 2 by precipitation with aqueous KPF6. Electrospray ionization mass spectrometry (ESI-MS; see the Supporting Information, Figure S1) revealed that 2 was a metal–ligand tetramer with the formula [Fe4L4](PF6)8][11] (L=bis(imine) ligand resulting from the condensation of 1 with two molecules of n-hexylamine). 1H NMR spectroscopy (Figure 2 a) indicated that 2 was highly symmetrical, with the splitting of the diastereotopic CH2-O-CH2 protons consistent with the chiral (racemic) helicate topology shown in Scheme 1. The yield of 2 was increased to 71 % (yield of isolated product) when employing 4.4 equivalents of the iron(II) salt (see the Supporting Information, Figure S9). Figure 2 1H NMR spectra (CD3CN, 500 MHz) for a) cyclic tetramer 2, b) linear triple helicate 3 (green, signals marked * correspond to trace amounts of 2), c) a 1:1 mixture of cyclic tetramer 4 (black) and linear triple helicate ... The formation of the tetrameric cyclic helicate was not limited to the use of FeCl2 as the iron(II) salt (Scheme 1), both Fe(BF4)2 and Fe(ClO4)2 also produced 2, although in significantly lower yields (see the Supporting Information, Figure S13) and contaminated with other polymeric and oligomeric by-products. When FeBr2 was employed as the iron source, a different main product was obtained (Scheme 1), which was identified as the linear trinuclear triple helicate ([Fe3L3]6+) 3 by 1H NMR spectroscopy (Figure 2 b) and ESI-MS (see the Supporting Information, Figure S14). A linear triple helicate with a lifetime of a few minutes was previously observed as an intermediate during the formation of pentameric cyclic helicates using Lehn’s tris(bipyridine) ligand.[7d] While 3 is a much longer-lived species, it is not clear whether this is because the linear triple helicate is particularly stable as the bromide salt, or whether the assembly/disassembly/rearrangement of the various linear and circular helicates and oligomers is markedly slower using FeBr2, perhaps as a result of their limited solubility. Substituting n-hexylamine for 4-methylbenzylamine in the reaction of 1 with FeCl2 gave a mixture of two species (Figure 2 c), identified by ESI-MS (Supporting Information, Figures S3 and S5) as the cyclic tetramer 4 and the linear triple helicate ([Fe3L3]6+) 5 (Scheme 1). Using our standard reaction protocol with an initial concentration of 1 of 2.2 mm, the ratio of 4/5 was approximately 1:1, however the distribution of cyclic-double-helicate/linear-triple-helicate was significantly altered by small variations in concentration: using an initial concentration of 8.8 mm of 1, more than 95 % of the reaction product was the higher order (four ligands, four metal ions) circular helicate 4 after 24 hours, whereas starting with a concentration of 0.55 mm of 1, the reaction produced more than 85 % of the lower nuclearity (three ligands, three metal ions) linear helicate 5 over the same time period (Supporting Information, Figure S15).[12] In contrast, the yield of the analogous n-hexylamine-derived cyclic tetramer 2 was essentially invariant over this concentration range and no linear triple helicate was observed, illustrating the influence that subtle changes in the ligands can have over the outcomes of the self-assembly reactions. In order to link the end groups of the open cyclic helicate to generate a Solomon link, we employed 2,2′-(ethylenedioxy)bis(ethylamine), a diamine that is stereoelectronically predisposed to adopt low-energy turns.[9] The reaction of 1 with the diamine and FeCl2 in DMSO for 24 hours, with subsequent anion exchange with aqueous KPF6, generated the Solomon link 6 in 75 % yield of isolated product (Scheme 2).[13] Scheme 2 Synthesis of molecular Solomon link 6. Reaction conditions: a) FeCl2, 2,2′-(ethylenedioxy)bis(ethylamine), DMSO, 60 °C, 24 h; b) excess KPF6 (aq), 75 % (over two steps). The 1H NMR spectrum (CD3CN, 500 MHz, Figure 2 d) of 6 is very similar to that of the tetrameric cyclic helicate 2 derived from n-hexylamine (Figure 2 a), including the splitting pattern for the diastereotopic CH2-O-CH2 protons. ESI-MS (Supporting Information, Figure S7) confirmed that 6 had a structural formula consistent with a Solomon link. Single crystals of 6 suitable for X-ray crystallography were grown by slow diffusion of diethyl ether into a nitromethane solution of 6, and the structure was confirmed by X-ray crystallography (Figure 3). The solid-state structure shows the two organic macrocycles interlocked by the four crossings that define the topology of a Solomon link. The iron atoms are close-to-coplanar and lie on the vertices of a square with Fe–Fe distances of just over 1 nm. Despite the high yield, as for the related pentafoil knot,[9] the octahedral coordination geometry of the iron(II) centers is amongst the most distorted [Fe(N-ligand)6] structures in the Cambridge Structural Database[14] (see the Supporting Information for details). The -OCH2CH2O- units in the linking group adopt close-to-gauche conformations (59–73°). Two PF6− counter ions are positioned directly above and below the center of the helicate (Figure 3 a) and form bifurcated CH⋅⋅⋅F interactions with the eight Ha protons, which are particularly electron-poor because of the ligand coordination to the iron(II) dications (Supporting Information, Figures S16 and S17). Figure 3 X-Ray crystal structure of Solomon link 6. a) Viewed in the plane of FeII ions (all but two PF6− anions omitted); b) viewed from above the center of the macrocycle cavities (all PF6− anions omitted). The C atoms of one ... The one-pot synthesis of molecular Solomon link 6 assembles four iron(II) cations, four bis(aldehyde) and four bis(amine) building blocks to generate two interwoven 68-membered-ring macrocycles with four crossings in 75 % isolated yield. The assembly process for the tetrameric cyclic double helicate forms the basis for the Solomon link synthesis and is sensitive to structural changes in the amine, the concentration and the anion used (even though the reaction product is not the result of an anion-template mechanism). The synthesis of Solomon link 6 and the earlier pentafoil knot[9] show that cyclic helicates of different sizes can act as highly efficient and effective scaffolds for intricate molecular topologies.