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A series of trans -RuL(PPh 3 ) 2 (nitrile) and {RuL(PPh 3 ) 2 }. 2 -μ-(nitrile)-based complexes [where L = 2,2'-(3,4-diphenyl-pyrrole-2,5-diyl)dipyridine (dpp), di(pyridin-2-yl)isoindoline-1,3-diimine (bpi), or 4-(4-methoxyphenyl)-6-phenyl-2,2'-bipyridine (Pbpy); and nitrile = 1,4-dibenzontirile, 4-ethynylbenzonitrile, or dicyanamide] were synthesized and characterized, and their electrochemical and photochemical behaviors were investigated. Those complexes that contained a significant nitrile contribution to their 3 MLLCT show a release of their nitrile ligand (when L = dpp or Pbpy and the nitrile ligand = 4-dibenzontirile, or 4-ethynylbenzonitrile) with dissociation constants up to 8.09 × 10 -4 s -1 ., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Gelation by small molecules is a topic of enormous importance in catalysis, nanomaterials, drug delivery, and pharmaceutical crystallization. The mechanism by which gelators self-organize into a fibrous gel network is poorly understood. Herein, we describe the crystal structures and gelation properties of a library of bis(urea) compounds and show, via molecular dynamics simulations, how gelator aggregation progresses from a continuous pattern of supramolecular motifs to a homogeneous fiber network. Our model suggests that lamellae with asymmetric surfaces scroll into uniform unbranched fibrils, while sheets with symmetric surfaces undergo stacking to form crystals. The self-assembly of asymmetric lamellae is associated with specific molecular features, such as the presence of narrow and flexible end groups with high packing densities, and likely represents a general mechanism for the formation of small-molecule gels., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Controlling the orientation of complex molecules in molecular junctions is crucial to their development into functional devices. To date, this has been achieved through the use of multipodal compounds (i.e., containing more than two anchoring groups), resulting in the formation of tri/tetrapodal compounds. While such compounds have greatly improved orientation control, this comes at the cost of lower surface coverage. In this study, we examine an alternative approach for generating multimodal compounds by binding multiple independent molecular wires together through metal coordination to form a molecular bundle. This was achieved by coordinating iron(II) and cobalt(II) to 5,5'-bis(methylthio)-2,2'-bipyridine ( L 1 ) and (methylenebis(4,1-phenylene))bis(1-(5-(methylthio)pyridin-2-yl)methanimine) ( L 2 ) to give two monometallic complexes, Fe-1 and Co-1 , and two bimetallic helicates, Fe-2 and Co-2 . Using XPS, all of the complexes were shown to bind to a gold surface in a fac fashion through three thiomethyl groups. Using single-molecule conductance and DFT calculations, each of the ligands was shown to conduct as an independent wire with no impact from the rest of the complex. These results suggest that this is a useful approach for controlling the geometry of junction formation without altering the conductance behavior of the individual molecular wires.

The present work provides an insight into the effect of connectivity isomerization of metal-2,2'-bipyridine complexes. For that purpose, two new 2,2'-bipyridine (bpy) ligand systems, 4,4'-bis(4-(methylthio)phenyl)-2,2'-bipyridine (L meta ) and 5,5'-bis(3,3-dimethyl-2,3-dihydrobenzothiophen-5-yl)-2,2'-bipyridine (L para ) were synthesized and coordinated to rhenium and manganese to obtain the corresponding complexes MnL meta (CO) 3 Br, ReL meta (CO) 3 Br, MnL para (CO) 3 Br, MoL para (CO) 4 and ReL para (CO) 3 Br. The experimental and theoretical results revealed that coordination to the para system, i.e., the metal ion peripheral to the conductance path, gave a slightly increased conductance compared to the free ligand attributed to the reduced highest occupied molecular orbital (HOMO)-least unoccupied molecular orbital (LUMO) gap. The meta-based system formed a destructive quantum interference feature that reduced the conductance of a S···S contacted junction to below 10 -5.5 G o , reinforcing the importance of contact group connectivity for molecular wire conductance., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Pathway complexity results in unique materials from the same components according to the assembly conditions. Here a chiral acyl-semicarbazide gelator forms three different gels of contrasting fibre morphology (termed 'gelmorphs') as well as lyotropic liquid crystalline droplets depending on the assembly pathway. The gels have morphologies that are either hyperhelical (HH-Gel), tape-fibre (TF-Gel) or thin fibril derived from the liquid crystalline phase (LC-Gels) and exhibit very different rheological properties. The gelator exists as three slowly interconverting conformers in solution. All three gels are comprised of an unsymmetrical, intramolecular hydrogen bonded conformer. The kinetics show that formation of the remarkable HH-Gel is cooperative and is postulated to involve association of the growing fibril with a non-gelling conformer. This single molecule dynamic conformational library shows how very different materials with different morphology and hence very contrasting materials properties can arise from pathway complexity as a result of emergent interactions during the assembly process., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

A series of seven new platinum(II) complexes PtL n Cl have been prepared, where L n is an NCN -coordinating ligand comprising a benzene ring 1,3-disubstituted with two different azaheterocycles. In PtL 1-5 Cl, one heterocycle is a simple pyridine ring, while the other is an isoquinoline, a quinoline, a pyrimidine (L 1 , L 2 , L 3 ), or a p -CF 3 - or p -OMe-substituted pyridine (L 4 and L 5 ). PtL 6 Cl incorporates both a p -CF 3 and a p -OMe-substituted pyridine. The synthesis of the requisite proligands HL n is achieved using Pd-catalyzed cross-coupling methodology. The molecular structures of six of the Pt(II) complexes have been determined by X-ray diffraction. All the complexes are brightly luminescent in deoxygenated solution at room temperature. The absorption and emission properties are compared with those of the corresponding symmetrical complexes featuring two identical heterocycles, PtL n sym Cl, and of the parent Pt(dpyb)Cl containing two unsubstituted pyridines [dpybH = 1,3-di(2-pyridyl)benzene]. While the absorption spectra of the nonsymmetrical complexes show features of both PtL n sym Cl and Pt(dpyb)Cl, the emission generally resembles that of whichever of the corresponding symmetrical complexes has the lower-energy emission. PtL 1 Cl differs in that─at room temperature but not at 77 K─it displays emission bands that can be attributed to excited states involving both the pyridine and the isoquinoline rings, despite the latter being unequivocally lower in energy. This unusual behavior is attributed to thermally activated repopulation of the former excited state from the latter, facilitated by the very long-lived nature of the isoquinoline-based excited state. At elevated concentrations, all the complexes show an additional red-shifted emission band attributable to excimers. For PtL 1 Cl, the excimer strikingly dominates the emission spectra at all but the lowest concentrations (<10 -5 M). Trends in the energies of the excimers and their propensity to form are compared with those of the symmetrical analogues.

Stereogenic sp 3 -hybridized carbon centres are fundamental building blocks of chiral molecules. Unlike dynamic stereogenic motifs, such as sp 3 -nitrogen centres or atropisomeric biaryls, sp 3 -carbon centres are usually fixed, requiring intermolecular reactions to undergo configurational changes. Here we report the internal enantiomerization of fluxional carbon cages and the consequences of their adaptive configurations for the transmission of stereochemical information. The sp 3 -carbon stereochemistry of the rigid tricyclic cages is inverted through strain-assisted Cope rearrangements, emulating the low-barrier configurational dynamics typical for sp 3 -nitrogen inversion or conformational isomerism. This dynamic enantiomerization can be stopped, restarted or slowed by external reagents, while the configuration of the cage is controlled by neighbouring, fixed stereogenic centres. As part of a phosphoramidite-olefin ligand, the fluxional cage acts as a conduit to transmit stereochemical information from the ligand while also transferring its dynamic properties to chiral-at-metal coordination environments, influencing catalysis, ion pairing and ligand exchange energetics., (© 2023. The Author(s).)

The balance between strain relief and aromatic stabilization dictates the form and function of non-planar π-aromatics. Overcrowded systems are known to undergo geometric deformations, but the energetically favourable π-electron delocalization of their aromatic ring(s) is typically preserved. In this study we incremented the strain energy of an aromatic system beyond its aromatic stabilization energy, causing it to rearrange and its aromaticity to be ruptured. We noted that increasing the steric bulk around the periphery of π-extended tropylium rings leads them to deviate from planarity to form contorted conformations in which aromatic stabilization and strain are close in energy. Under increasing strain, the aromatic π-electron delocalization of the system is broken, leading to the formation of a non-aromatic, bicyclic analogue referred to as 'Dewar tropylium'. The aromatic and non-aromatic isomers have been found to exist in rapid equilibrium with one another. This investigation demarcates the extent of steric deformation tolerated by an aromatic carbocycle and thus provides direct experimental insights into the fundamental nature of aromaticity., (© 2023. The Author(s).)

The materials property of ferroelectricity is intimately linked with symmetry-changing phase transitions. Characterizing such transitions is therefore essential for understanding molecular ferroelectrics. In this paper, we explore the temperature and thermal history dependence of polymorphic phase transitions in the multiaxial molecular ferroelectric 18-crown-6 oxonium tetrachloro-gallium(III). We have solved the structures of two previously suggested polymorphs (D and Y) ab initio from high-temperature powder diffraction data. We also report the structure of a new polymorph (X) using low-temperature powder diffraction data and identify a fifth (W) that can form on cooling. These polymorphs can be related using two distinct group-subgroup trees. Structure types A-C observed in this and related compounds can be derived from high-temperature polymorph D by group-subgroup relationships. The X and Y polymorphs can be described as child structures of a hypothetical polymorph Z using a molecular rotational distortion mode description. The ferroelectric properties of the various polymorphs can be rationalized based on our structural findings., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

A porous, nonsolvated polymorph of the voltage-gated sodium channel blocker mexiletine hydrochloride absorbs iodine vapor to give a pharmaceutical cocrystal incorporating an I 2 Cl - anion that forms a halogen-π interaction with the mexiletine cations. The most thermodynamically stable form of the compound does not absorb iodine. This example shows that vapor sorption is a potentially useful and underused tool for bringing about changes in pharmaceutical solid form as part of a solid form screening protocol., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Platinum(II) complexes of NNC -cyclometalating ligands based on 6-phenyl-2,2'-bipyridine (HL 1 ) have been widely investigated for their luminescence properties. We describe how PtL 1 Cl and five analogues with differently substituted aryl rings, PtL 2-6 Cl, can be oxidized with chlorine and/or iodobenzene dichloride to generate Pt(IV) compounds of the form Pt( NNC -L n )Cl 3 ( n = 1-6). The molecular structures of several of them have been determined by X-ray diffraction. These PtL n Cl 3 compounds react with 2-arylpyridines to give a new class of Pt(IV) complex of the form [Pt( NNC )( NC )Cl] + . Elevated temperatures are required, and the reaction is accompanied by competitive reduction processes and generation of side-products; however, four examples of such complexes have been isolated and their molecular structures determined. Reaction of PtL 1 Cl 3 with HL 1 similarly generates [Pt( NNC -L 1 ) 2 ] 2+ , which we believe to be the first example of a bis-tridentate Pt(IV) complex. The lowest-energy bands in the UV-vis absorption spectra of all the PtL n Cl 3 compounds are displaced to higher energy relative to the Pt(II) precursors, but they red-shift with the electron richness of the aryl ring, consistent with predominantly 1 [π Ar → π* NN ] character to the pertinent excited state. A similar trend is observed for the [Pt( NNC )( NC )Cl] + complexes. They display phosphorescence in solution at room temperature, centered around 500 nm for [PtL 1 (ppy)Cl] + and [Pt(L 1 ) 2 ] 2+ , and 550 nm for methoxy-substituted derivatives. The lifetimes are in the microsecond range, rising to hundreds of microseconds at 77 K, consistent with triplet excited states of primarily 3 [π Ar → π* NN ] character with relatively little participation of the metal.

Three simple bisamide derivatives ( G1 , G2 and G3 ) with different structural modifications were synthesized with easy synthetic procedures in order to test their gel behaviour. The outcomes showed that hydrogen bonding was essential in gel formation; for this reason, only G1 provided satisfactory gels. The presence of methoxy groups in G2 and the alkyl chains in G3 hindered the hydrogen bonding between N-H and C=O that occurred G1 . In addition, G1 provided thermally and mechanical stable gels, as confirmed with T sol and rheology experiments. The gels of G1 were also responsive under pH stimuli and were employed as a vehicle for drug crystallization, causing a change in polymorphism in the presence of flufenamic acid and therefore providing the most thermodynamically stable form III compared with metastable form IV obtained from solution crystallization.

We present a study of aggregate excited states formed by complexes of the type Pt(N^C^N)X, where N^C^N represents a tridentate cyclometallating ligand, and X = SCN or I. These materials display near-infrared (NIR) photoluminescence in film and electroluminescence in NIR OLEDs with λ max EL = 720-944 nm. We demonstrate that the use of X = SCN or I modulates aggregate formation compared to the parent complexes where X = Cl. While the identity of the monodentate ligand affects the energy of Pt-Pt excimers in solution in only a subtle way, it strongly influences aggregation in film. Detailed calculations on aggregates of different sizes support the experimental conclusions from steady-state and time-resolved luminescence studies at variable temperatures. The use of X = I appears to limit aggregation to the formation of dimers, while X = SCN promotes the formation of larger aggregates, such as tetramers and pentamers, leading in turn to NIR photo- and electroluminescence > 850 nm. A possible explanation for the contrasting influence of the monodentate ligands is the lesser steric hindrance associated with the SCN group compared to the bulkier I ligand. By exploiting the propensity of the SCN complexes to form extended aggregates, we have prepared an NIR-emitting OLED that shows very long wavelength electroluminescence, with λ max EL = 944 nm and a maximum EQE = 0.3 ± 0.1%. Such data appear to be unprecedented for a device relying on a Pt(ii) complex aggregate as the emitter., Competing Interests: The authors have no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We report an approach to obtain drug-mimetic supramolecular gelators, which are capable of stabilizing metastable polymorphs of the pharmaceutical salt mexiletine hydrochloride, a highly polymorphic antiarrhythmic drug. Solution-phase screening led to the discovery of two new solvated solid forms of mexiletine, a type C 1,2,4-trichlorobenzene tetarto-solvate and a type D nitrobenzene solvate. Various metastable forms were crystallized within the gels under conditions which would not have been possible in solution. Despite typically crystallizing concomitantly with form 1, a pure sample of form 3 was crystallized within a gel of ethyl methyl ketone. Various type A channel solvates were crystallized from gels of toluene and ethyl acetate, in which the contents of the channels varied from those of solution-phase forms. Most strikingly, the high-temperature-stable form 2 was crystallized from a gel in 1,2-dibromoethane: the only known route to access this form at room temperature. These results exemplify the powerful stabilizing effect of drug-mimetic supramolecular gels, which can be exploited in pharmaceutical polymorph screens to access highly metastable or difficult-to-nucleate solid forms., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

A hair care mixture formed from a gluconamide derivative and 3-hydroxypropyl ammonium gluconate is known to strengthen hair fibers; however, the mechanism by which the mixture affects hair is unknown. To give insight into the aggregation of the target gluconamide and potential interactions between the gluconate-derived mixture and hair fibers, a range of systems were characterized by X-ray crystallography namely two polymorphic forms of the target gluconamide and three salts of 3-hydroxypropylammonium with sulfuric acid, methane sulfonic acid, and oxalic acid. The gluconamide proves to aggregate and becomes a supramolecular gelator in aniline and benzyl alcohol solution. The resulting gels were characterized by rheology, scanning electron microscopy, proton nuclear magnetic resonance, Fourier transform infrared spectroscopy, and powder X-ray diffraction., Competing Interests: The authors declare the following competing financial interest(s): Ashland LLC is a commercial producer of Fiberhance, (© 2022 The Authors. Published by American Chemical Society.)

Miltefosine is a repurposed anticancer drug and currently the only orally administered drug approved to treat the neglected tropical disease leishmaniasis. Miltefosine is hygroscopic and must be stored at subzero temperatures. In this work, we report the X-ray structures of miltefosine monohydrate and methanol solvate, along with 12- and 14-carbon chain analogue hydrates and a solvate. The three hydrates are all isostructural and are conformational isomorphs with Z ' = 2. Water bridges the gap between phosphocholine head groups caused by the interdigitated bilayer structure. The two methanol solvates are also mutually isostructural with the head groups adopting a more extended conformation. Again, the solvent bridges the gap between head groups in the bilayer. No anhydrous form of miltefosine or its analogues were isolated, with dehydration resulting in significantly reduced crystallinity. This arises as a result of the integral role that hydrogen-bond donors (in the form of water or solvent molecules) play in the stability of the zwitterionic structures., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Herein, we expose how the antagonistic relationship between solid-state luminescence and photocyclization of oligoaryl alkene chromophores is modulated by the conjugation length of their alkenyl backbones. Heptaaryl cycloheptatriene molecular rotors exhibit aggregation-induced emission characteristics. We show that their emission is turned off upon breaking the conjugation of the cycloheptatriene by epoxide formation. While this modification is deleterious to photoluminescence, it enables formation of extended polycyclic frameworks by Mallory reactions. We exploit this dichotomy (i) to manipulate emission properties in a controlled manner and (ii) as a synthetic tool to link together pairs of phenyl rings in a specific sequence. This method to alter the tendency of oligoaryl alkenes to undergo photocyclization can inform the design of solid-state emitters that avoid this quenching mechanism, while also allowing selective cyclization in syntheses of polycyclic aromatic hydrocarbons., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Bicyclic triazolium scaffolds are widely employed in N -heterocyclic carbene (NHC) organocatalysis. While the incorporation of a fused ring was initially for synthetic utility in accessing chiral, modular triazolyl scaffolds, recent results highlight the potential for impact upon reaction outcome with the underpinning origins unclear. The common first step to all triazolium-catalyzed transformations is C(3)-H deprotonation to form the triazolylidene NHC. Herein, we report an analysis of the impact of size of the fused (5-, 6-, and 7-membered, n = 1, 2, and 3, respectively) ring on the C(3) proton transfer reactions of a series of bicyclic triazolium salts. Rate constants for the deuteroxide-catalyzed C(3)-H/D-exchange of triazolium salts, k DO , were significantly influenced by the size of the adjacent fused ring, with the kinetic acidity trend, or protofugalities, following the order k DO ( n = 1) > k DO ( n = 2) ≈ k DO ( n = 3). Detailed analyses of X-ray diffraction (XRD) data for 20 triazolium salts (including 16 new structures) and of computational data for the corresponding triazolylidene NHCs provide insight on structural effects of alteration of fused ring size. In particular, changes in internal triazolyl NCN angle and positioning of the most proximal CH 2 with variation in fused ring size are proposed to influence the experimental protofugality order.

Bis(urea)s based on the 4,4'-methylenebis(2,6-diethylphenylene) (4,4'-MDEP) spacer are highly effective low molecular weight gelators, and the first single crystal structure of a bis(urea) based on this spacer is reported. The structure is a conformational isomorph with eight crystallographically independent molecules ( Z ' = 8) arranged in four tennis-ball type dimers with the 2,6-diethylphenylene units adopting five different conformations in the ratio 4:5:3:2:2. The awkward shape and conformational promiscuity arising from the orientations of the ethyl groups in this system is linked to its gelation behavior. A total of seven 4,4'-MDEP derivatives have been prepared, and six are versatile gelators, confirming the particularly effective nature of the MDEP spacer. Only the nitrophenyl derivative does not form gels, likely because of intramolecular CH···O hydrogen bonding arising from the electron-withdrawing nature of the nitro substituent and hence inhibition of the urea α-tape hydrogen-bonded motif., Competing Interests: The authors declare no competing financial interest., (© 2022 American Chemical Society.)

The structure of the commercially important polyvinylpyrrolidone-hydrogen peroxide complex can be understood by reference to the co-crystal structure of a hydrogen peroxide complex and its mixed hydrates of a two-monomer unit model compound, bisVP·2H 2 O 2 . The mixed hydrates involve selective water substitution into one of the two independent hydrogen peroxide binding sites.

Transfer hydrogenation (TH) is a powerful synthetic tool in the production of secondary alcohols from ketones by using a non-H 2 hydrogen source along with metal catalysts. Among homogeneous catalysts, Ru(II) complexes are the most efficient catalysts. In our research, six novel ruthenium(II) complexes bearing bipyridine-based ligands [Ru(L1)Cl 2 ] (1), [Ru(L1)(PPh 3 )Cl]Cl (2) and [Ru(L2)Cl 2 ] (3) and N-heterocyclic carbene-supported pyridine (NCN) ligands [RuCp(L3)]PF 6 (4), [RuCp*(L3)]PF 6 (5), and [Ru( p -cymene)(L3)Cl]PF 6 (6) (where L1 = 6,6'-bis(aminomethyl)-2,2'-bipyridine, L2 = 6,6'-bis(dimethylaminomethyl)-2,2'-bipyridine and L3 = 1,3-bis(2-methylpyridyl)imidazolium bromide) were synthesised and characterised by NMR spectroscopy, HRMS, and X-ray crystallography. The catalytic transfer hydrogenation of 28 ketones in 2-propanol at 80 °C in the presence of KO t Bu (5 mol%) was demonstrated and the effect of ligands is highlighted. The results show that catalyst 1 exhibits improved TH efficiency compared to the commercially available Milstein catalyst and displays higher catalytic activity than 2 due to the steric effect from PPh 3 . From a combination of kinetic data and Eyring analysis, a zero-order dependence on the acetophenone substrate is observed, implying a rate-limiting hydride transfer step, leading to the proposed inner-sphere hydride transfer mechanism.

A dithienylethene molecule involving carborane clusters shows remarkable fatigue resistance and high contrast visual colour changes when irradiated with alternating ultraviolet and visible light. The fluorescence of this assembly can be switched on and off when irradiated in the solid state but not in the solution state.

Control of molecular chirality is a fundamental challenge in organic synthesis. Whereas methods to construct carbon stereocentres enantioselectively are well established, routes to synthesize enriched heteroatomic stereocentres have garnered less attention 1-5 . Of those atoms commonly present in organic molecules, nitrogen is the most difficult to control stereochemically. Although a limited number of resolution processes have been demonstrated 6-8 , no general methodology exists to enantioselectively prepare a nitrogen stereocentre. Here we show that control of the chirality of ammonium cations is easily achieved through a supramolecular recognition process. By combining enantioselective ammonium recognition mediated by 1,1'-bi-2-naphthol scaffolds with conditions that allow the nitrogen stereocentre to racemize, chiral ammonium cations can be produced in excellent yields and selectivities. Mechanistic investigations demonstrate that, through a combination of solution and solid-phase recognition, a thermodynamically driven adductive crystallization process is responsible for the observed selectivity. Distinct from processes based on dynamic and kinetic resolution, which are under kinetic control, this allows for increased selectivity over time by a self-corrective process. The importance of nitrogen stereocentres can be revealed through a stereoselective supramolecular recognition, which is not possible with naturally occurring pseudoenantiomeric Cinchona alkaloids. With practical access to the enantiomeric forms of ammonium cations, this previously ignored stereocentre is now available to be explored., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)

Wide-dose-range 2D radiochromic films for radiotherapy, such as GAFchromic EBT, are based on the lithium salt of 10,12-pentacosadiynoic acid (Li-PCDA) as the photosensitive component. We show that there are two solid forms of Li-PCDA-a monohydrated form A and an anhydrous form B. The form used in commercial GAFchromic films is form A due to its short needle-shaped crystals, which provide favorable coating properties. Form B provides an enhanced photoresponse compared to that of form A, but adopts a long needle crystal morphology, which is difficult to process. The two forms were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, CP-MAS 13 C solid-state NMR spectroscopy, and thermogravimetric analysis. In sum, these data suggest a chelating bridging bidentate coordination mode for the lithium ions. The sodium salt of PCDA (Na-PCDA) is also reported, which is an ionic cocrystal with a formula of Na + PCDA - ·3PCDA. The PCDA and PCDA - ligands display monodentate and bridging bidentate coordination to the sodium ion in contrast to the coordination sphere of the Li-PCDA forms. In contrast to its lithium analogues, Na-PCDA is photostable., Competing Interests: The authors declare no competing financial interest., (© 2021 American Chemical Society.)

The spin crossover salt [Fe(bpp) 2 ](isonicNO) 2 ⋅ 2.4 H 2 O (1⋅2.4 H 2 O) (bpp=2,6-bis(pyrazol-3-yl)pyridine; isonicNO=isonicotinate N-oxide anion) exhibits a very abrupt spin crossover at T 1/2 =274.4 K. This triggers a supramolecular linkage (H-bond) isomerization that responds reversibly towards light irradiation or temperature change. Isotopic effects in the thermomagnetic behavior reveal the importance of hydrogen bonds in defining the magnetic state. Further, the title compound can be reversibly dehydrated to afford 1, a material that also exhibits spin crossover coupled to H-bond isomerization, leading to strong kinetic effects in the thermomagnetic properties., (© 2020 Wiley-VCH GmbH.)

The iron(ii) salt [Fe(bpp) 2 ](isonicNO) 2 ·HisonicNO·5H 2 O ( 1 ) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; isonicNO = isonicotinate N-oxide anion) undergoes a partial spin crossover (SCO) with symmetry breaking at T 1 = 167 K to a mixed-spin phase (50% high-spin (HS), 50% low-spin (LS)) that is metastable below T 2 = 116 K. Annealing the compound at lower temperatures results in a 100% LS phase that differs from the initial HS phase in the formation of a hydrogen bond (HB) between two water molecules (O4W and O5W) of crystallisation. Neutron crystallography experiments have also evidenced a proton displacement inside a short strong hydrogen bond (SSHB) between two isonicNO anions. Both phenomena can also be detected in the mixed-spin phase. 1 undergoes a light-induced excited-state spin trapping (LIESST) of the 100% HS phase, with breaking of the O4W⋯O5W HB and the onset of proton static disorder in the SSHB, indicating the presence of a light-induced activation energy barrier for proton motion. This excited state shows a stepped relaxation at T 1 (LIESST) = 68 K and T 2 (LIESST) = 76 K. Photocrystallography measurements after the first relaxation step reveal a single Fe site with an intermediate geometry, resulting from the random distribution of the HS and LS sites throughout the lattice., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

The synthesis and photophysical properties of a set of iridium(iii) complexes featuring tridentate N^N^O-coordinating ligands are described, of generic structure [Ir(N^C^N-dpyx)(N^N^O-L n )] + (n = 1 to 4) (dpyx = 1,3-dipyridyl-4,6-dimethylbenzene). The proligands HL n are Schiff bases synthesised by condensation of salicylaldehydes with N-methyl-hydrazinopyridines: they are able to coordinate to the Ir(iii) via lateral pyridine-N and phenolate-O - atoms and a central hydrazone-N atom; the four examples differ in the substitution pattern within the phenolate ring. The bis-tridentate coordination is confirmed by X-ray diffraction. The complexes are phosphorescent in solution at ambient temperature, with higher quantum yields and longer lifetimes than those of structurally related bis-cyclometallated complexes with an N^N^C-coordinating ligand. Related proligands H 2 L 5 and H 2 L 6 have been prepared from 4,6-bis(1-methyl-hydrazino)pyrimidine. They feature a central pyrimidine and two N^N^O units. They are shown to bind as ditopic, bis-tridentate ligands with two iridium(iii) ions, leading to unprecedented dinuclear complexes of the form [{Ir(N^C^N)} 2 (O^N^N-N^N^O-L n )] 2+ (n = 5, 6; N^C^N = dpyx or 1,3-dipyridyl-4,6-difluoro-benzene), with an intramolecular IrIr distance of around 6 Å determined crystallographically. Mononuclear analogues [Ir(N^C^N-dpyx)(N^N^O-HL n )] + have also been isolated. The dinuclear complexes display a well-defined and unusually intense lowest-energy absorption band in the visible region, around 480 nm. They emit much more efficiently than their mononuclear counterparts, even though the emission wavelengths are comparable. Their superior performance appears to be due to an enhancement in the radiative rate constant, affirming conclusions drawn from recent related studies of dinuclear Ir(iii) and Pt(ii) complexes with ditopic, pyrimidine-based cyclometallating ligands.

In this work we develop photoreactive cocrystals/salts of a commercially-important diacetylene, 10,12-pentacosadiynoic acid (PCDA, 1 ) and report the first X-ray crystal structures of PCDA based systems. The topochemical reactivity of the system is modified depending on the coformer used and correlates with the structural parameters. Crystallisation of 1 with 4,4'-azopyridine ( 2 ), 4,4'-bipyridyl ( 3 ), and trans -1,2-bis(4-pyridyl)ethylene ( 4 ) results in unreactive 2 : 1 cocrystals or a salt in the case of 4,4'-bipiperidine ( 5 ). However, salt formation with morpholine ( 6 ), diethylamine ( 7 ), and n -butylamine ( 8 ), results in highly photoreactive salts 12·7 and 1·8 whose reactivity can be explained using topochemical criteria. The salt 1·6 is also highly photoreactive and is compared to a model morpholinium butanoate salt. Resonance Raman spectroscopy reveals structural details of the photopolymer including its conformational disorder in comparison to less photoactive alkali metal salts and the extent of solid state conversion can be monitored by CP-MAS NMR spectroscopy. We also report an unusual catalysis in which amine evaporation from photopolymerised PCDA ammonium salts effectively acts as a catalyst for polymerisation of PCDA itself. The new photoreactive salts exhibit more reactivity but decreased conjugation compared to the commercial lithium salt and are of considerable practical potential in terms of tunable colours and greater range in UV, X-ray, and γ-ray dosimetry applications., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The synthesis and photophysical properties of 22 platinum(ii) complexes featuring N^N^O-coordinating ligands are described. The complexes have the form Pt(N^N^O-Ln)Cl (n = 1 to 20). The tridentate ligands comprise lateral pyridine and phenolate rings, offering the metal N and O coordination respectively, linked via an imine or hydrazone unit that provides a further, central N atom for coordination. The proligands HLn, some of which have previously been reported for the coordination of 1st row transition metal ions in other contexts, are Schiff bases that are readily synthesised by condensation of salicylaldehydes either with 8-aminoquinoline (to generate imine-based ligands HL1-4) or with 2-hydrazinopyridines (to generate hydrazone-based proligands HL5-20). The Pt(ii) complexes are prepared under mild conditions upon treatment of the proligands with simple Pt(ii) salts. Metathesis of the chloride ligand by an acetylide is possible, as exemplified by the preparation of two further complexes of the form Pt(N^N^O-Ln)(-C[triple bond, length as m-dash]C-Ar), where Ar = 3,5-bis(trifluoromethyl)phenyl. Nine of the complexes have been characterised in the solid state by X-ray diffraction. The imine-based complexes have intense low-energy absorption bands around 520 nm attributed to charge-transfer transitions. They display deep red phosphorescence in solution at ambient temperature, with λmax in the range 635-735 nm, quantum yields up to 4.6% and lifetimes in the microsecond range. The hydrazone complexes that feature a py-NH-N[double bond, length as m-dash]C-Ar linker display pH-dependent absorption spectra owing to the acidity of the hydrazone NH: these complexes have poor photostability in solution. In contrast, their N-methylated analogues (i.e., py-NMe-N[double bond, length as m-dash]C-Ar) show no evidence of photodecomposition. They are phosphorescent in solution at room temperature in the 600-640 nm region, the emission maximum being influenced by substituents in the phenolate ring. The results show how simply prepared tridentate Schiff base ligands - which offer the metal a combination of 5- and 6-membered chelate rings - can provide access to phosphorescent Pt(ii) complexes that have superior emissive properties to those of terpyridines, for example.

Class II mixed-valence bimetallic complexes {[Cp'(PP)M]C≡C-C≡N[M'(PP)'Cp']} 2+ (M, M'=Ru, Fe; PP=dppe, (PPh 3 ) 2 ; Cp'=Cp*, Cp) exist as conformational ensembles in fluid solution, with a population of structures ranging from cis- to trans-like geometries. Each conformer gives rise to its own series of low-energy intervalence charge-transfer (IVCT) and local d-d transitions, which overlap in the NIR region, giving complex band envelopes in the NIR absorption spectrum, which prevent any meaningful attempt at analysis of the band shape. However, DFT and time-dependent (TD)DFT calculations with dispersion-corrected global-hybrid (BLYP35-D3) or local hybrid (lh-SsirPW92-D3) functionals on a small number of optimised structures chosen to sample the ground state potential energy hypersurfaces of each of these complexes has proven sufficient to explain the major features of the electronic spectra. Although modest in terms of computational expense, this approach provides a more accurate description of the underlying molecular electronic structure than would be possible through analysis of the IVCT band by using the static point-charge model of Marcus-Hush theory and derivatives, or TDDFT calculations from a single (global) minimum energy geometry., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Knowledge of the crystal structure of a monometallic inorganic molecule is often sufficient to calculate its electronic structure and interpret its magnetic properties. Here we show that for a series of nine-coordinate lanthanide complexes based on the 1,4,7-tris[(6-carboxypyridin-2-yl)methyl]-1,4,7-triazacyclononane ligand, the electronic structure is hypersensitive to geometric structure and to the presence of noncoordinated lattice solvent, which renders the magnetic and spectroscopic properties very difficult to interpret. We explore possible explanations for the peculiar electron paramagnetic resonance (EPR) spectra and conclude that a number of entangled factors are at play across the samples. Hence, great care should be taken in the interpretation of EPR spectra for systems with small magnetic anisotropy, even when the molecular structure is known.

Supramolecular gel phase crystallization offers a new strategy for drug polymorph screening and discovery. In this method, the crystallization outcome depends on the interaction between solute and gel fibre. While supramolecular gels have shown success in producing new polymorphs and crystals with novel morphologies, role of the gel and nature of gel-solute interaction remains largely unexplored. The present study aims to provide a comprehensive picture of the structural evolution of a supramolecular gel produced from a bis(urea) based gelator (G) in the presence of a polymorphic drug carbamazepine (CBZ). The structural aspects of the gel have been assessed by single crystal X-ray analysis, X-ray powder diffraction (XRPD) and solid state NMR spectroscopy. Small Angle Neutron Scattering (SANS) has been used to follow the changes in gel structure in the presence of CBZ. Visual evidence from morphological study and structural evolution observed at a macroscopic level from rheological measurements, shows good agreement with the SANS results. The concentration of the gelator and the relative proportion of G to CBZ were found to be crucial factors in determining the competitive nucleation events involving gelation and crystallization. At a critical G to CBZ ratio the effect of CBZ on gel structure was maximum and fiber bundling in the gel was found to be critically affected. This study offers important information about how the interplay of gelator assembly and gel-solute interactions can fine-tune the nucleation events in a supramolecular gel phase crystallization.

Dynamic covalent rearrangements of fluxional carbon cages, such as bullvalenes and barbaralanes, impart 'shapeshifting' molecular properties. Here, a series of five barbaralanes each interconvert dynamically between two constitutional isomers in solution, but resolve to single isomers upon crystallisation. Unexpectedly, the minor solution-phase isomers are resolved in two instances. Through dynamic NMR, crystallographic and DFT analyses, we show that the isomer observed in the solid state is not a direct consequence of the equilibrium distribution in solution or any specific noncovalent interactions. Rather, the dynamic preferential crystallisation is dictated by differences in molecular size and shape.

Controlling the orientation of molecular conductors on the electrode surfaces is a critical factor in the development of single-molecule conductors. In the current study, we used the scanning tunnelling microscopy-based break junction (STM-BJ) technique to explore 'bare-bones' tripodal molecular wires, employing different anchor groups (AGs) at the 'top' and 'bottom' of the tripod. The triarylphosphine tris(4-(methylthio)phenyl)phosphine and its corresponding phosphine sulfide showed only a single high conductance feature in the resulting 1- and 2-dimensional conductance histograms, whereas analogous molecules with fewer than three thiomethyl AGs did not show clear conductance features. Thus, by systematic molecular modifications and with the aid of supporting DFT calculations, the binding geometry, with respect to the surface, was elucidated., Competing Interests: The authors declare no competing financial interest., (This journal is © The Royal Society of Chemistry.)

Metal complexes are receiving increased attention as molecular wires in fundamental studies of the transport properties of metal|molecule|metal junctions. In this context we report the single-molecule conductance of a systematic series of d 8 square-planar platinum(ii) trans-bis(alkynyl) complexes with terminal trimethylsilylethynyl (C[triple bond, length as m-dash]CSiMe 3 ) contacting groups, e.g. trans-Pt{C[triple bond, length as m-dash]CC 6 H 4 C[triple bond, length as m-dash]CSiMe 3 } 2 (PR 3 ) 2 (R = Ph or Et), using a combination of scanning tunneling microscopy (STM) experiments in solution and theoretical calculations using density functional theory and non-equilibrium Green's function formalism. The measured conductance values of the complexes (ca. 3-5 × 10 -5 G 0 ) are commensurate with similarly structured all-organic oligo(phenylene ethynylene) and oligo(yne) compounds. Based on conductance and break-off distance data, we demonstrate that a PPh 3 supporting ligand in the platinum complexes can provide an alternative contact point for the STM tip in the molecular junctions, orthogonal to the terminal C[triple bond, length as m-dash]CSiMe 3 group. The attachment of hexyloxy side chains to the diethynylbenzene ligands, e.g. trans-Pt{C[triple bond, length as m-dash]CC 6 H 2 (Ohex) 2 C[triple bond, length as m-dash]CSiMe 3 } 2 (PPh 3 ) 2 (Ohex = OC 6 H 13 ), hinders contact of the STM tip to the PPh 3 groups and effectively insulates the molecule, allowing the conductance through the full length of the backbone to be reliably measured. The use of trialkylphosphine (PEt 3 ), rather than triarylphosphine (PPh 3 ), ancillary ligands at platinum also eliminates these orthogonal contacts. These results have significant implications for the future design of organometallic complexes for studies in molecular junctions.

Stable benzotriazinyl radicals (Blatter's radicals) recently attracted considerable interest as building blocks for functional materials. The existing strategies to derivatize Blatter's radicals are limited, however, and synthetic routes are complex. Here, we report that an inexpensive, commercially available, analytical reagent Nitron undergoes a previously unrecognized transformation in wet acetonitrile in the presence of air to yield a new Blatter-type radical with an amide group replacing a phenyl at the C(3)-position. This one-pot reaction of Nitron provides access to a range of previously inaccessible triazinyl radicals with excellent benchtop stabilities. Mechanistic investigation suggests that the reaction starts with a hydrolytic cleavage of the triazole ring followed by oxidative cyclization. Several derivatives of Nitron were prepared and converted into Blatter-type radicals to test the synthetic value of the new reaction. These results significantly expand the scope of using functionalized benzotriazinyls as stable radical building blocks.

The cross-conjugated ethynyl-vinylidene [Ph 2 C[double bond, length as m-dash]C(C[triple bond, length as m-dash]CH){C(H)[double bond, length as m-dash]CRu(PPh 3 ) 2 Cp}]PF 6 ([4a]PF 6 ), and [FcC(H)[double bond, length as m-dash]C(C[triple bond, length as m-dash]CH){C(H)[double bond, length as m-dash]CRu(PPh 3 ) 2 Cp}]PF 6 ([4b]PF 6 ), and ethynyl-alkynyl Ph 2 C[double bond, length as m-dash]C(C[triple bond, length as m-dash]CH){C[triple bond, length as m-dash]CRu(PPh 3 ) 2 Cp} (5a), and FcC(H)[double bond, length as m-dash]C(C[triple bond, length as m-dash]CH){C[triple bond, length as m-dash]CRu(PPh 3 ) 2 Cp} (5b) compounds (Cp = η 5 -cyclopentadienyl) have been prepared from reactions of the known 3-methylene-penta-1,4-diynes Ph 2 C[double bond, length as m-dash]C(C[triple bond, length as m-dash]CH) 2 (3a) and [FcCH[double bond, length as m-dash]C(C[triple bond, length as m-dash]CH) 2 ] (3b) with [RuCl(PPh 3 ) 2 Cp]. The compounds derived from 3b incorporating the more electron-rich alkene proved to be unstable during work-up, and attempts to prepare bis(ruthenium) complexes from 3a and 3b or from transmetallation reactions of the bis(alkynylgold) complex FcCH[double bond, length as m-dash]C(C[triple bond, length as m-dash]CAuPPh 3 ) 2 (7) with RuCl(PPh 3 ) 2 Cp were unsuccessful. The related bis- and tris(ferrocenyl) derivatives Ph 2 C[double bond, length as m-dash]C(C[triple bond, length as m-dash]CFc) 2 (6a) and FcCH[double bond, length as m-dash]C(C[triple bond, length as m-dash]CFc) 2 (6b) were more readily obtained from Pd(ii)/Cu(i) catalysed cross-coupling reactions of FcC[triple bond, length as m-dash]CH with the 1,1-dibromo vinyl complexes PhC[double bond, length as m-dash]CBr 2 (1a) and FcC(H)[double bond, length as m-dash]CBr 2 (1b). Cyclic voltammetry of 6a and 6b using n-Bu 4 N[PF 6 ] as the supporting electrolyte shows broad, overlapping waves arising from the sequential oxidation of the ferrocenyl moieties in electronically and chemically similar environments. Electrostatic effects between the ferrocenyl moieties are enhanced in solutions of the weakly ion-pairing electrolyte n-Bu 4 N[B{C 6 H 3 (CF 3 ) 2 -3,5} 4 ], leading to better resolution of the individual electrochemical processes. The comparative IR spectroelectrochemical response of 6a and 6b suggest the vinyl ferrocene moiety in 6b undergoes oxidation before the ethynyl ferrocene fragments. There is no evidence of electronic coupling between the metallocene moieties and [6a] + , [6b] n+ (n = 1, 2) are best described as Class I mixed-valence compounds.

Ytterbium and yttrium complexes of octadentate ligands based on 1,4,7,10-tetraazacyclododecane with a coordinated pyridyl group and either tricarboxylate (L 1 ) or triphosphinate (L 2 ) donors form twisted-square-antiprismatic structures. The former crystallizes in the centrosymmetric group P2 1 /c, with the two molecules related by an inversion center, whereas the latter was found as an unusual kryptoracemate in the chiral space group P2 1 . Pure shift NMR and EXSY spectroscopy allowed the dynamic exchange between the (RRR)-Δ-(δδδδ) and (RRR)-Λ-(λλλλ) TSAP diastereomers of the [Y.L 2 ] complex to be detected. The rate-limiting step in the exchange between Δ and Λ isomers involves cooperative ligand arm rotation, which is much faster for [Ln.L 1 ] than for [Ln.L 2 ]. Detailed analysis of NOESY, COSY, HSQC, and HMBC spectra confirms that the major conformer in solution is (RRR)-Λ-(λλλλ), consistent with crystal structure analysis and DFT calculations. The magnetic susceptibility tensors for [Yb.L 1 ] and [Yb.L 2 ], obtained from a full pseudocontact chemical shift analysis, are very different, in agreement with a CASSCF calculation. The remarkably different pseudocontact shift behavior is explained by the change in the orientation of the pseudocontact shift field, as defined by the Euler angles of the susceptibility tensor.

For the first time, a series of 25 pseudo-octahedral pyridylphosphinate metal complexes (Ru, Os, Rh, Ir) has been synthesised and assessed in biological systems. Each metal complex incorporates a pyridylphosphinate ligand, a monodentate halide and a capping η(6)-bound aromatic ligand. Solid- and solution-state analyses of two complexes reveal a structural preference for one of a possible two diastereomers. The metal chlorides hydrolyse rapidly in D2O to form a 1 : 1 equilibrium ratio between the aqua and chloride adducts. The pKa of the aqua adduct depends upon the pyridyl substituent and the metal but has little dependence upon the phosphinate R' group. Toxicity was measured in vitro against non-small cell lung carcinoma H460 cells, with the most potent complexes reporting IC50 values around 50 μM. Binding studies with selected amino acids and nucleobases provide a rationale for the variation in toxicity observed within the series. Finally, an investigation into the ability of the chelating amino acid l-His to displace the phosphinate O-metal bond shows the potential for phosphinate complexes to act as prodrugs that can be activated in the intracellular environment.

The phenol-diamide compound, 5-(tert-butyl)-2-hydroxy-N1,N3-bis(2-hydroxyethyl)isophthalamide (), is water-soluble, non-cytotoxic, and capable of both, trapping ROS species and chelating Cu(ii) and Fe(iii) ions; these combined properties confer a protective effect against ROS induced cell death.

Reactions of cis-RuCl2(dppm)2 with various terminal alkynes, of the type HC≡CC6H4-4-R (1 equiv.), in the presence of TlBF4 have resulted in the formation of cationic vinylidene complexes trans-[RuCl(=C=CHC6H4-4-R)(dppm)2]BF4 ([1]BF4). These complexes can be isolated, or treated in situ with a suitable base (Proton Sponge, 1,8-bis(dimethylamino)naphthalene) to yield the mono-alkynyl complexes trans-RuCl(C≡CC6H4-4-R)(dppm)2 (2). Through similar reactions between cis-RuCl2(dppm)2 with 2 equiv. of alkyne, TlBF4 and base, trans-bis(alkynyl) complexes, trans-Ru(C≡CC6H4-4-R)2(dppm)2 (3), can be isolated when R is an electron withdrawing substituent (R = NO2, COOMe, C≡CSiMe3), whereas reactions with alkynes bearing electron donating substituents (R = OMe and Me) form cationic η(3)-butenynyl complexes [Ru(η(3)-{HC(C6H4-4-R)=CC≡CC6H4-4-R})(dppm)2](+) ([4](+)). This work highlights the importance of the electronic character of the alkyne in influencing product outcome.

A series of cross-conjugated compounds based on an (E)-4,4'-(hexa-3-en-1,5-diyne-3,4-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) skeleton (1-6) have been synthesized. The linear optical absorption properties can be tuned by modification of the substituents at the 1 and 5 positions of the hexa-3-en-1,5-diynyl backbone (1: Si(CH(CH3)2)3, 2: C6H4C≡CSi(CH3)3, 3: C6H4COOCH3, 4: C6H4CF3, 5: C6H4C≡N, 6: C6H4C≡CC5H4N), although attempts to introduce electron-donating (C6H4CH3, C6H4OCH3, C6H4Si(CH3)3) substituents at these positions were hampered by the ensuing decreased stability of the compounds. Spectroelectrochemical investigations of selected examples, supported by DFT-based computational studies, have shown that one- and two-electron oxidation of the 1,2-bis(triarylamine)ethene fragment also results in electronic changes to the perpendicular π-system in the hexa-3-en-1,5-diynyl branch of the molecule. These properties suggest that (E)-hexa-3-en-1,5-diynyl-based compounds could have applications in molecular sensing and molecular electronics.

The diimino-diphenolato neutral square-planar Ni(ii) complex, NiL2, is readily oxidised with 2 equiv. of Ag[SbF6], to produce an unprecedented octahedral Ni(ii) tris(phenoxyl) radical complex, [Ni(L˙)3][SbF6]2. This study reveals, for the first time, the X-ray structure of a metal-tri-phenoxyl radical complex.

Reactions of the vinylidene complexes [M([double bond, length as m-dash]C[double bond, length as m-dash]CH2)(PP)Cp']PF6 with NH3 readily afford the acyclic amino carbene complexes [M{C(Me)NH2}(PP)Cp']PF6 [M(PP)Cp' = Fe(dppe)Cp, Ru(dppe)Cp, Ru(PPh3)2Cp, Ru(dppe)Cp*]. Crystallographic, spectroscopic and computational data support a description of these complexes in terms of a M-{C(Me)[double bond, length as m-dash]N(+)H2} mesomeric form, and simple σ-coordination of the singlet acyclic amino carbene.

The ethynyl-phenylene substituted 2,2':6',2''-terpyridine (tpy) derivatives, 4-(phenyl-ethynyl)-2,2':6',2''-terpyridine (L(1)), 4-(methoxyphenyl-ethynyl)-2,2':6',2''-terpyridine (L(2)), 4-(tolyl-ethynyl)-2,2':6',2''-terpyridine (L(3)) and 4-(nitrophenyl-ethynyl)-2,2':6',2''-terpyridine (L(4)) have been used to synthesize four new [RuCl(2,2'-bipyridine)(L(n))]PF6 based complexes. Electronic absorption, resonance Raman, cyclic voltammetry and spectroelectrochemistry aided by DFT calculations were used to explore the influence of the alkynyl substituents on the electronic structures, photochemical and redox properties of the complexes. Furthermore, it is shown that the addition of ethynyl phenyl moieties to the 4-position of the tpy ligand does not have a detrimental effect on these complexes, or the analogous aqua complexes, with respect to their ability to photocatalyse the oxidation of 4-methoxybenzyl alcohol to the corresponding benzaldehyde.

The synthesis and characterization of a series of cobalt compounds, coordinated by the redox-active macrocyclic biquinazoline ligand, Mabiq [2-4:6-8-bis(3,3,4,4-tetramethyldihydropyrrolo)-10-15-(2,2'-biquinazolino)-[15]-1,3,5,8,10,14-hexaene-1,3,7,9,11,14-N6], is presented. The series includes the monometallic Co(Mabiq)Cl2 (1), Co(Mabiq)Cl (2), and Co(Mabiq) (4), with formal metal oxidation states of 3+ → 1+. A binuclear cobaltous compound, Co2(Mabiq)Cl3 (3), also was obtained, providing the first evidence for the ability of the Mabiq ligand to coordinate two metal ions. The electronic structures of the paramagnetic 2 and 3 were examined by electron paramagnetic resonance spectroscopy and magnetic susceptibility studies. The Co(II) ion that resides in the N4-macrocylic cavity of 2 and 3 adopts a low-spin S = (1)/2 configuration. The bypirimidine functionality in 3 additionally coordinates a high-spin S = (3)/2 cobaltous ion in a tetrahedral environment. The two metal ions in 3 are weakly coupled by magnetometry. The square-planar, low-valent 4 offers one of a limited number of examples of structurally characterized N4-macrocyclic Co(I) compounds. Spectroscopic and density functional theory computational data suggest that a Co(II)(Mabiq(•)) description may be a reasonable alternative to the Co(I) formalism for this compound.

Reaction of cis-[RuCl2(dppm)2]BF4 with TlBF4 and 1,4-diethynyl-benzenes results in the formation of the vinylidene cations trans-[Ru([double bond, length as m-dash]C[double bond, length as m-dash]CH-C6H2-2,5-R2-4-C[triple bond, length as m-dash]CH)Cl(dppm)2](+) (R = H, Me). Subsequent reaction with [N(n)Bu4]Cl results in nucleophilic attack at the coordinated organic ligand, but not at the expected metal-bound carbon atom. Instead, trans-[Ru(C[triple bond, length as m-dash]C-C6H2-2,5-R2-4-CCl[double bond, length as m-dash]CH2)Cl(dppm)2] was generated which, when coupled with DFT calculations, provides evidence for an intermediate quinoidal cumulene complex.

The ligands 4'-(4-(methylthio)phenyl)-2,2':6',2″-terpyridine (L(1)), 4'-((4-(methylthio)phenyl)ethynyl)- 2,2':6',2″-terpyridine (L(2)), and bis(tridentate) bridging ligand 2,3,5,6-tetra(pyridine-2-yl)pyrazine (tpp) were used to prepare the complexes [Ru(L(1))2][PF6]2 ([1][PF6]2, [Ru(L(2))2][PF6]2 ([2][PF6]2), [{(L(1))Ru}(μ-tpp){Ru(L(1))}][PF6]4 ([3][PF6]4), and [{(L(2))Ru}(μ-tpp){Ru(L(2))}][PF6]4 ([4][PF6]4). Crystallographically determined structures give S···S distances of up to 32.0 Å in [4](4+). On the basis of electrochemical estimates, the highest occupied molecular orbitals of these complexes fall between -5.55 and -5.85 eV, close to the work function of clean gold (5.1-5.3 eV). The decay of conductance with molecular length across this series of molecules is approximately exponential, giving rise to a decay constant (pseudo β-value) of 1.5 nm(-1), falling between decay factors for oligoynes and oligophenylenes. The results are consistent with a tunnelling mechanism for the single-molecule conductance behavior.