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The spectroelectrochemically generated infrared (IR) and near-infrared spectra of the homo-bimetallic, hexa-1,3,5-triyn-1,6-diyl-bridged complex cations [{Cp*(dppe)M}(μ-C≡CC≡CC≡C){M(dppe)Cp*}]+ (M ...

A convenient modification to the established synthetic routes for the preparation of complexes [M(CCR)(dppe)Cp′] (M = Fe, Ru; Cp′ = η5-C5H5 (Cp), η5-C5Me5 (Cp*); R = C6H5, C6H4-p-OMe, C6H4-p-NO2, tBu) and [Ru(CCR)(PPh3)2Cp] from the corresponding [MCl(PP)Cp′] complex and the alkyne HCCR via an intermediate vinylidene is described. The complexes are generally obtained as high quality crystalline samples directly from the reaction mixture. In agreement with previous studies, the iron complexes undergo a predominantly metal-centred oxidation, whilst there is greater involvement of the acetylide ligand in the oxidation processes associated with the ruthenium analogues. Analysis of the redox properties and spectroelectrochemical investigations reveal limited differences in the gross electronic structures of the Cp vs. Cp* derivatives in each of the Fe and Ru series.

The electronic structure and associated spectroscopic properties of ligand-bridged, bimetallic 'mixed-valence' complexes of the general form {M}(μ-B){M

Single-molecule conductance experiments using the STM-based I(s) method and samples of N,N’-di(4-(trimethylsilylethynyl)benzyl)-4,4’-bipyridinium bis(tetrafluoroborate) ([1](BF4)2) prepared on gold substrates with low-surface coverage of [1](BF4)2 (Γ = 1.25·10⁻¹¹ mol·cm⁻²) give rise to molecular junctions with two distinct conductance values. From the associated break-off distances and comparison experiments with related compounds the higher conductance junctions are attributed to molecular contacts between the molecule and the electrodes via the N,N’-dibenzyl-4,4´-bipyridinium (viologen) moiety and one trimethylsilylethynyl (TMSE) group (G = (5.4 ± 0.95)×10⁻⁵ G0, break-off distance (1.56 ± 0.09) nm). The second, lower conductance junction (G = (0.84 ± 0.09)×10⁻⁵ G0) is consistent with an extended molecular conformation between the substrate and tip contacted through the two TMSE groups giving rise to a break-off distance (1.95 ± 0.12) nm that compares well with the Si...Si distance (2.0 nm) in the extended molecule. Langmuir monolayers of [1](BF4)2 formed at the air-water interface can be transferred onto a gold-on-glass substrate by the Langmuir-Blodgett (LB) technique to give well-ordered, compact films with surface coverage Γ = 2.0·10⁻¹⁰ mol·cm⁻². Single-molecule conductance experiments using the STM-based I(s) method reveal only the higher conductance junctions (G = (5.4 ± 0.95)×10⁻⁵ G0, break-off distance (1.56 ± 0.09) nm) due to the restricted range of molecular conformations in the tightly packed, well-ordered LB films., S.M. and P.C. are grateful for financial assistance from Ministerio de Economía y Competitividad from Spain and fondos FEDER in the framework of projects MAT2016-78257-R. S.M. and P.C. also acknowledge DGA/fondos FEDER (construyendo Europa desde Aragón) for funding the research group Platón (E-54). S.M. acknowledges funding from the University of Zaragoza (grant number JIUZ02016-CIE-04). R.J.N, S.J.G and D.C.M are grateful for financial assistance from the EPSRC (grant EP/M029522/1). P.J.L. and J.B.G.G. gratefully acknowledge support from the Australian Research Council (FT120100073; DP140100855).

The reaction of [MoBr(dppe)(η-C7H7)] (dppe = Ph2PCH2CH2PPh2) with HC CC CSiMe3 and Na[BPh4] in 1:1 NHEt2/THF as solvent yields the aminoallenylidene complex [Mo{C C C(Me)NEt2}(dppe)(η-C7H7)][BPh4], [1][BPh4]. The reaction likely proceeds via nucleophilic addition of NHEt2 at Cγ of a butatrienylidene intermediate. Structural and spectroscopic characterisation of [1][BPh4] indicate a significant contribution of an iminium alkynyl resonance form to the overall structure of the heteroatom stabilised allenylidene ligand. The X-ray structural study of [1][BPh4] determines a Mo Cα bond length of 2.077(3) A, intermediate between that of the cumulenic diphenylallenylidene analogue [Mo(C C CPh2)(dppe)(η-C7H7)][PF6] (1.994(3) A) and the alkynyl compound [Mo(C CPh)(dppe)(η-C7H7)] (2.138(5) A). Complex [1][BPh4] undergoes a reversible one-electron oxidation with E½ = −0.19 V with respect to the FeCp2/FeCp2+ couple and the stable 17-electron radical dication [1]2+ is readily observed by spectroelectrochemical methods. IR spectroelectrochemistry in CH2Cl2 demonstrates that the ν(CCC) stretch, characteristic of the allenylidene ligand, shifts to higher wavenumber (from 1959 to 2032 cm−1) as a result of oxidation of [1]+ to [1]2+, consistent with a strongly metal-centred redox process and an enhancement in the alkynyl character of the allenylidene ligand following one-electron oxidation.

Climate warming is supposed to enlarge the area climatically suitable to the naturalization of alien garden plants in temperate regions. However, the effects of a changing climate on the spread of naturalized ornamentals have not been evaluated by spatially and temporarily explicit range modelling at larger scales so far. Here, we assess how climate change and the frequency of cultivation interactively determine the spread of 15 ornamental plants over the 21st century in Europe. We coupled species distribution modelling with simulations of demography and dispersal to predict range dynamics of these species in annual steps across a 250 x 250 m raster of the study area. Models were run under four scenarios of climate warming and six levels of cultivation intensity. Cultivation frequency was implemented as size of the area used for planting a species. Although the area climatically suitable to the 15 species increases, on average, the area predicted to be occupied by them in 2090 shrinks under two of the three climate change scenarios. This contradiction obviously arises from dispersal limitations that were pronounced although we assumed that cultivation is spatially adapting to the changing climate. Cultivation frequency had a much stronger effect on species spread than climate change, and this effect was non‐linear. The area occupied increased sharply from low to moderate levels of cultivation intensity, but levelled off afterwards. Our simulations suggest that climate warming will not necessarily foster the spread of alien garden plants in Europe over the next decades. However, climatically suitable areas do increase and hence an invasion debt is likely accumulating. Restricting cultivation of species can be effective in preventing species spread, irrespective of how the climate develops. However, for being successful, they depend on high levels of compliance to keep propagule pressure at a low level.

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.

Tetrakis(ferrocenylethynyl)ethene (1) has been prepared in four steps from ethynyl ferrocene (2). In a dichloromethane solution containing 10−1 M NBu4[PF6], only a single oxidation process is observed by cyclic voltammetry, corresponding to the independent oxidation of the four ferrocenyl moieties. However, in dichloromethane containing 10−1 M NBu4[BArF4] electrolyte, where [BArF4]– is the weakly associating anion [B{C6H3(CF3)2-3,5}4]–, four distinct oxidation processes are resolved, although further spectroelectrochemical investigation revealed essentially no through bond interaction between the individual ferrocenyl moieties. Quantum chemical treatment of 1 identified several energetic minima corresponding to different relative orientations of the ferrocenyl moieties and the plane of the all-carbon bridging fragment. Further computational investigation of the corresponding monocation [1]+ supported the notion of charge localisation with no evidence for significant through bond electronic interactions.

International audience; The electronic structures of the prototypical bimetallic buta-1,3-diyn-1,4-diyl-bridged radical cation complexes [{M(dppe)Cp'}(2)(mu-C CC C)](+) (M= Fe, Cp' = Cp* (1a), Cp (1b); M = Ru, Cp' = Cp* (2a), Cp (2b)) have been (re)investigated using a combination of UV-vis-NIR and IR spectroelectrochemistry, and quantum chemical calculations based on both dispersion-corrected global (BLYP35-D3) and local (Lh-SsirPW92-D3) hybrid functionals. Following analysis of new and existing data, including the IR-active v(C C) bands, the iron compounds [1](+) are reclassified as valence-trapped (Robin and Day Class II) mixed-valence complexes, in contrast to the ruthenium complexes [2](+), which are delocalized (Robin and Day Class III) systems. All members of the series exist as a thermally populated distribution of conformers in solution, and the overlapping spectroscopic profiles make the accurate extraction of the parameters necessary for the analysis of [1](+) and [2](+) within the framework of the Marcus-Hush model extremely challenging. Analysis of the spin-density distributions from a range of conformational minima provides an alternative representation of the degree of charge localization, and a comparison between members of the series is presented.

International audience; The complexes FcCH=C{1,4-C≡C-C6H4-C≡CM(dppe)Cp∗}2 (Fc = ferrocenyl (FeCp(η-C5H4-); M = Fe (1), Ru (2)) were prepared from FcCH=C{1,4-C≡C-C6H4-C≡CSiMe3}2 (3) via a desilylation/metalation protocol in good (2; 65%) to excellent (1; 97%) yield. The iron compound 1 could also be prepared in a stepwise fashion by desilylation of 3 to give FcCH=C{1,4-C≡C-C6H4-C≡CH}2 (4), reaction with FeCl(dppe)Cp∗ to give the vinylidene complex FcCH=C{1,4-C≡C-C6H4-CH=C=Fe(dppe)Cp∗}2](PF6)2 (5(PF6)2; 65%), and deprotonation. The cyclic voltammograms of 1 and 2 are characterized by an initial oxidation wave resulting from the overlap of two closely spaced oxidation processes, the potentials of which are sensitive to the identity of M, and a subsequent, one-electron-oxidation wave. Thus, while the dications 12+ and 22+ could be prepared by oxidation with 2 equiv of ferrocenium hexafluorophosphate and isolated as the PF6 - salts 1(PF6)2 and 2(PF6)2 at low temperature, the monocations 1+ and 2+ could only be detected and studied as comproportionated mixtures of 1, 1(PF6), 1(PF6)2 and 2, 2(PF6), 2(PF6)2. A combination of EPR spectroscopy, IR and NIR spectroelectrochemistry, and DFT quantum chemical calculations reveal subtle distinctions in the electronic structures of 1(PF6)n and 2(PF6)n (n = 0-2). The HOMOs of 1 and 2 are more heavily distributed over the metal-diethynylbenzene arm trans to the ferrocenyl moiety. While one-electron oxidation of 1 gives 1(PF6), in which the spin density is similarly distributed along the branch of the molecule trans to the ferrocenyl group, the spin density in 2(PF6) is more extensively, but not fully, delocalized. Further analysis of the ESR, NIR, and IR spectra reveals that charges are essentially localized in 1(PF6) and 1(PF6)2 on the IR time scale, but ground-state exchange between the Fe(dppe)Cp∗ moieties can take place via the ferrocenyl moiety on the slower ESR time scale. For 2(PF6) and 2(PF6)2, optical charge transfer processes between the ferrocenyl moiety and the organometallic branches can also be observed, consistent with the increased coupling between the Ru(dppe)Cp∗ and Fc moieties that are linked by a linear conjugation pathway through the bridging-ligand backbone. © 2018 American Chemical Society.

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.

Broad-band near-infrared (NIR) transient absorption (TA) spectroscopy has been used for the first time to probe an all-carbon-bridged organometallic radical cation complex. The compound [{Ru(PPh3)2Cp}2(μ-C≡CC≡C)]+ ([1]+) was investigated in dichloromethane and acetonitrile solutions, using laser excitation at 700, 800, 900, and 1000 nm; these wavelengths span the NIR absorption band envelope. The resulting TA spectra were found to be independent of excitation wavelength and consist of an excited state absorption feature with a peak at ca. 1150 nm and the corresponding bleach signal of the ground-state NIR absorption band, which both decay to 0 over the 50 ps time window investigated. Data were analyzed globally and fit collectively for each of the four different excitation wavelengths, with the resulting best fit to a biexponential decay function indicating two processes with slightly different time scales of ca. 1.5 and ca. 9.0 ps involved in the relaxation to the ground state.

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.

Phosphinoalkynes P(C≡CC6H4Me-4)Ph2 (1) and P(C≡CC6H4C≡CC6H4Me-4)Ph2 (2) have been prepared from CuI catalysed reactions of the corresponding 1-alkyne and PClPh2. The trimethylamine-N-oxide promoted reaction of PPh3, 1 or 2 with [Fe2(μ-pdt)(CO)6] (pdt = propanedithiolate) affords derivatives [Fe2(μ-pdt)(CO)5{PRPh2}] [R=Ph (3), C≡CC6H4Me (5), C≡CC6H4C≡CC6H4Me (6)] or, at elevated temperatures, [Fe2(μ-pdt)(CO)4(PPh3)2] (4). The cyclic voltammograms of compounds 3 and 4 feature almost fully reversible one-electron oxidation processes and an irreversible reduction, whilst the electrochemical response of the alkynyl phosphine substituted complexes 5 and 6 is irreversible for both oxidation and reduction. IR spectroelectrochemical studies of 4 are consistent with an oxidation processes leading to a delocalized or (Class III) mixed valence [FeFe]3+ core in which the iron centers have an average oxidation state of 1.5. The molecular structures of the alkynyl phosphine substituted clusters 5 and 6 are also reported.

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.

In an extension of earlier studies concerning drugs and odorants based on disila-substituted tetrahydronaphthalene and indane skeletons, such as the musk odorant disila-phantolide (1), 1-(1,1,2,2,3,3,6-heptamethyl-1,2,3-trisilaindan-5-yl)ethanone (methyltrisila-phantolide, 2) and a series of related 5,6-disubstituted 1,2,3-trisilaindanes (3−5) were prepared in multistep syntheses. Compounds 2−5 were characterized by 1H, 11B (5 only), 13C, and 29Si NMR studies. In addition, compounds 3−5 were studied by single-crystal X-ray diffraction. The 1,2,3-trisilaindane skeleton proved to have limited stability toward oxidation agents. The oxidation product 6, a novel 1,3,4-trisilaisochroman system, was isolated and characterized. The trisila-phantolide derivatives 2 and 3 do not display a typical musk odor, but instead possess a slightly creamy-lactonic odor with coumarinic aspects of very weak intensity (odor thresholds, >500 ng L−1 air).

et al., 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., R.J.N. and S.J.H. thank EPSRC for funding (grant EP/H035184/1 and EP/K007785/1). S.M. and P.C. are grateful for financial assistance from Ministerio de Economía y Competitividad from Spain in the framework of projects CTQ2012-33198 and CTQ2013-50187-EXP and support from DGA and Fondos Feder for funding the Platon research group. P.J.L. thanks EPSRC for funding and also gratefully acknowledges support from the Australian Research Council (DP 140100855) and the award of a Future Fellowship (FT120100073). H.M.O. is grateful for financial assistance from Secretaria Nacional de Educación Superior, Ciencia, Tecnología e Innovación from Ministerio de Educación (Ecuador). F.H. thanks EPSRC for funding (EP/K00753X/1)

Thanks to their synthetic versatility, the half-sandwich metal chlorides MCl(dppe)(η5-C5R5) [M = Fe, Ru; dppe = 1,2-bis(diphenylphosphino)ethane, R = H (cyclopentadiene, Cp), CH3 (pentamethylcyclopentadiene, Cp*)] are staple starting materials in many organometallic laboratories. Here we present an overview of the synthetic methods currently available for FeCl(dppe)Cp*, FeCl(dppe)Cp, RuCl(dppe)Cp*, and RuCl(dppe)Cp, and describe in detail updated and optimised multigram syntheses of all four compounds.

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.

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.

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 SiCH2Si 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 RARβ,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 RARβ,γ-selective retinoid.