The Use of Bridging Ligand Substituents to Bias the Population of Localized and Delocalized Mixed-Valence Conformers in Solution.

The electronic structure and associated spectroscopic properties of ligand-bridged, bimetallic 'mixed-valence' complexes of the general form {M}(µ-B){M+ } are dictated by the electronic couplings, and hence orbital overlaps, between the metal centers mediated by the bridge. In the case of complexes such as [{Cp*(dppe)Ru}(µ-C≡CC6 H4 C≡C){Ru(dppe)Cp*}]+ , the low barrier to rotation of the half-sandwich metal fragments and the arylene bridge around the acetylene moieties results in population of many energy minima across the conformational energy landscape. Since orbital overlap is also sensitive to the particular mutual orientations of the metal fragment(s) and arylene bridge through a Karplus-like relationship, the different members of the population range exemplify electronic structures ranging from strongly localized (weakly coupled Robin-Day Class II) to completely delocalized (Robin-Day Class III). Here, we use electronic structure calculations with the hybrid density functional BLYP35-D3 and a continuum solvent model in combination with UV-vis-NIR and IR spectroelectrochemical studies to show that the conformational population in complexes [{Cp*(dppe)Ru}(µ-C≡CArC≡C){Ru(dppe)Cp*]+ , and hence the dominant electronic structure, can be biased through the steric and electronic properties of the diethynylarylene (Ar) moiety (Ar=1,4-C6 H4 , 1,4-C6 F4 , 1,4-C6 H2 -2,5-Me2 , 1,4-C6 H2 -2,5-(CF3 )2 , 1,4-C6 H2 -2,5-i Pr2 ).

A Spectroscopic and Computationally Minimal Approach to the Analysis of Charge-Transfer Processes in Conformationally Fluxional Mixed-Valence and Heterobimetallic Complexes.

Class II mixed-valence bimetallic complexes {[Cp'(PP)M]C≡C-C≡N[M'(PP)'Cp']}2+ (M, M'=Ru, Fe; PP=dppe, (PPh3 )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.

Rational Control of Conformational Distributions and Mixed-Valence Characteristics in Diruthenium Complexes.

The electronic characteristics of mixed-valence complexes are often inferred from the shape of the inter-valence charge transfer (IVCT) band, which usually falls in the near infrared (NIR) region, and relationships derived from Marcus-Hush theory. These analyses typically assume one single, dominant molecular conformation. The NIR spectra of the prototypical delocalised (Class III Robin-Day mixed-valence) complexes [{Ru(pp)Cp'}2 (µ-C≡C-C≡C)]+ ([1]+ : Cp'=Cp, pp=(PPh3 )2 ; [2]+ : Cp'=Cp, pp=dppe; [3]+ : Cp'=Cp*, pp=dppe) feature a 'two-band' pattern, which complicates band-shape analysis using these traditional methods. In the past, the appearance of sub-bands within or near the IVCT transition has been attributed to vibronic effects or localised d-d transitions. Quantum-chemical modelling of a series of rotational conformers of [1]+ -[3]+ reveals the two components that contribute to the NIR absorption band envelope to be a π-π* transition and an MLCT transition. The MLCT components only gain appreciable intensity when the orientation of the half-sandwich ruthenium ligand spheres deviates from idealised cis (Ω P-Ru-Ru-P=0°) or trans (Ω P-Ru-Ru-P=180°) conformations. The increased steric demand of the supporting ligands, together with some underlying inter-phosphine ligand T-shaped CH⋅⋅⋅π stacking interactions across the series [1]+ to [2]+ to [3]+ results in local minima biased towards such non-idealised conformations of the metal-ligand fragments (Ω P-Ru-Ru-P=33-153°). Experimentally, this is indicated by appearance of multiple bands within the IR ν˜ (C≡C) band envelopes and increasing intensity of the higher-energy MLCT transition(s) relative to the π-π* transition across the series, and the appearance of a pronounced 'two-band' pattern in the experimental NIR absorption envelopes. These conformational effects and the methods of analysis presented here, which combine analysis of IR and NIR spectra with quantum-chemical calculations on a range of energetically similar conformational minima, are expected to be quite general for mixed-valence systems.

Electrochemical Single-Molecule Transistors with Optimized Gate Coupling.

Electrochemical gating at the single molecule level of viologen molecular bridges in ionic liquids is examined. Contrary to previous data recorded in aqueous electrolytes, a clear and sharp peak in the single molecule conductance versus electrochemical potential data is obtained in ionic liquids. These data are rationalized in terms of a two-step electrochemical model for charge transport across the redox bridge. In this model the gate coupling in the ionic liquid is found to be fully effective with a modeled gate coupling parameter, ξ, of unity. This compares to a much lower gate coupling parameter of 0.2 for the equivalent aqueous gating system. This study shows that ionic liquids are far more effective media for gating the conductance of single molecules than either solid-state three-terminal platforms created using nanolithography, or aqueous media.

Cross-Conjugated Systems Based On An (E)-Hexa-3-en-1,5-diyne-3,4-diyl Skeleton: Spectroscopic and Spectroelectrochemical Investigations.

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.

Mixed-valence ruthenium complexes rotating through a conformational Robin-Day continuum.

The conformational energy landscape and the associated electronic structure and spectroscopic properties (UV/Vis/near-infrared (NIR) and IR) of three formally d(5)/d(6) mixed-valence diruthenium complex cations, [{Ru(dppe)Cp*}2(µ-C≡CC6H4C≡C)](+), [1](+), [trans-{RuCl(dppe)2}2(µ-C≡CC6H4C≡C)](+), [2](+), and the Creutz-Taube ion, [{Ru(NH3)5}2(µ-pz)](5+), [3](5+) (Cp = cyclopentadienyl; dppe = 1,2-bis(diphenylphosphino)ethane; pz = pyrazine), have been studied using a nonstandard hybrid density functional BLYP35 with 35 % exact exchange and continuum solvent models. For the closely related monocations [1](+) and [2](+), the calculations indicated that the lowest-energy conformers exhibited delocalized electronic structures (or class III mixed-valence character). However, these minima alone explained neither the presence of shoulder(s) in the NIR absorption envelope nor the presence of features in the observed vibrational spectra characteristic of both delocalized and valence-trapped electronic structures. A series of computational models have been used to demonstrate that the mutual conformation of the metal fragments--and even more importantly the orientation of the bridging ligand relative to those metal centers--influences the electronic coupling sufficiently to afford valence-trapped conformations, which are of sufficiently low energy to be thermally populated. Areas in the conformational phase space with variable degrees of symmetry breaking of structures and spin-density distributions are shown to be responsible for the characteristic spectroscopic features of these two complexes. The Creutz-Taube ion [3](5+) also exhibits low-lying valence-trapped conformational areas, but the electronic transitions that characterize these conformations with valence-localized electronic structures have low intensities and do not influence the observed spectroscopic characteristics to any notable extent.

Refining the interpretation of near-infrared band shapes in a polyynediyl molecular wire.

Spinning to improve (band) shape: A blend of theoretical and experimental work demonstrates that the rotational conformation of mixed-valence complexes influences the low-energy (NIR) transitions in such molecules. Interpretations of the NIR band shapes are presented.

Disila-analogues of the synthetic retinoids EC23 and TTNN: synthesis, structure and biological evaluation.

Silicon chemistry offers the potential to tune the effects of biologically active organic molecules. Subtle changes in the molecular backbone caused by the exchange of a carbon atom for a silicon atom (sila-substitution) can significantly alter the biological properties. In this study, the biological effects of a two-fold sila-substitution in the synthetic retinoids EC23 (4-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalen-2-ylethynyl)benzoic acid (4a)) and TTNN (6-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydronaphthalen-2-yl)-2-naphthoic acid (7a)) as well as their corresponding analogues with an indane instead of a 1,2,3,4-tetrahydronaphthalene skeleton (compounds 5a and 8a) were investigated. Two-fold C/Si exchange in 4a, 5a, 7a and 8a leads to the silicon-analogues disila-EC23 (4b), 5b, disila-TTNN (7b) and 8b, which contain a 1,2,3,4-tetrahydro-1,4-disilanaphthalene (4b, 7b) or 1,3-disilaindane skeleton (5b, 8b). Exchange of the SiCH(2)Si moiety of 5b for an SiOSi fragment leads to the disiloxane 6 (2-oxa-1,3-disilaindane skeleton). The EC23 derivative 5a, the TTNN derivative 8a and the silicon-containing analogues 4b, 5b, 6, 7b and 8b were synthesised, and the biological properties of the C/Si pairs 4a/4b, 5a/5b, 7a/7b and 8a/8b and compound 6 were evaluated in vivo using RAR isotype-selective reporter cells. EC23 (4a) and its derivatives disila-EC23 (4b), 5a, 5b and 6 are very potent RAR agonists, which are even more potent than the powerful reference compound TTNPB. Disila-substitution of EC23 (4a) and 5a leads to a moderate decrease in RARα activation, whereas the RARß,γ activation is almost not affected. In contrast, two-fold C/Si exchange in the weak retinoid agonist TTNN (7a) and 8a resulted in considerably different effects: a significant increase (7a→7b) and almost no change (8a→8b) in transcription activation potential for all three RAR isotypes. Disila-TTNN (7b) can be regarded as a powerful RARß,γ-selective retinoid.

Sandwich and half-sandwich metal complexes derived from cross-conjugated 3-methylene-penta-1,4-diynes.

The cross-conjugated ethynyl-vinylidene [Ph2C[double bond, length as m-dash]C(C[triple bond, length as m-dash]CH){C(H)[double bond, length as m-dash]CRu(PPh3)2Cp}]PF6 ([4a]PF6), 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(PPh3)2Cp}]PF6 ([4b]PF6), and ethynyl-alkynyl Ph2C[double bond, length as m-dash]C(C[triple bond, length as m-dash]CH){C[triple bond, length as m-dash]CRu(PPh3)2Cp} (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(PPh3)2Cp} (5b) compounds (Cp = η5-cyclopentadienyl) have been prepared from reactions of the known 3-methylene-penta-1,4-diynes Ph2C[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(PPh3)2Cp]. 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]CAuPPh3)2 (7) with RuCl(PPh3)2Cp were unsuccessful. The related bis- and tris(ferrocenyl) derivatives Ph2C[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]CBr2 (1a) and FcC(H)[double bond, length as m-dash]CBr2 (1b). Cyclic voltammetry of 6a and 6b using n-Bu4N[PF6] 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-Bu4N[B{C6H3(CF3)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.

Long range charge transfer in trimetallic mixed-valence iron complexes mediated by redox non-innocent cyanoacetylide ligands.

The reaction of Fe(C≡CC≡N)(dppe)Cp (1) with one-half equivalent of [trans-Fe(N≡CMe)2(dppx)2][BF4]2 (dppx = dppe ([2][BF4]2) or dppm ([3][BF4]2)) affords trimetallic [trans-Fe{N≡CC≡CFe(dppe)Cp}2(dppx)2][BF4]2 (dppx = dppe [4][BF4]2; dppx = dppm [5][BF4]2). Both [4][BF4]2 and [5][BF4]2 undergo three, one-electron oxidation processes, arising from sequential oxidation of the two terminal Fe(C≡CR)(dppe)Cp moieties and finally the central Fe(N≡CR)2(dppx)2 fragment. The redox products [4](n+) and [5](n+) (n = 3, 4) have been characterised by UV-vis-NIR and IR spectroelectrochemistry. The shifts in the characteristic ν(C≡CC≡N) bands upon oxidation demonstrate not only the localised electronic structure of the trications, but also the redox non-innocence of the cyanoacetylide ligands. The trimetallic [formally Fe(II/II/III) mixed-valence] complexes [4](3+) and [5](3+) feature two distinct IVCT transitions, one associated with charge transfer from the central 18-electron {Fe(N≡CR)2(dppx)2}(2+) to terminal {Fe(C≡CR)(dppe)Cp}(+) moiety, and a lower energy transition involving charge transfer between the terminal Fe fragments which are separated by the redox active 9-atom, 10-bond -C≡C-C≡N{Fe(dppx)2}N≡C-C≡C- bridge. The tetracationic complexes [4](4+) and [5](4+) generated by a further stepwise oxidation exhibit a single {Fe(N≡CR)2(dppx)2}(2+)→{Fe(C≡CR)(dppe)Cp}(+) IVCT transition.