Electron delocalization in vinyl ruthenium substituted cyclophanes: Assessment of the through-space and the through-bond pathways
Abstract
Pseudo-para[2.2]paracyclophane- and [2.1]orthocyclophane-bridged diruthenium complexes 2 and 3 with two interlinked electroactive styryl ruthenium moieties have been prepared and investigated. Both complexes undergo two reversible consecutive one-electron oxidation processes which are separated by 270 or 105 mV. Stepwise electrolysis of the neutral complexes to first the mixed-valent radical cations and then the dioxidized dications under IR monitoring reveal incremental shifts of the charge-sensitive Ru(CO) bands and allow for an assignment of their radical cations as moderately or very weakly coupled mixed-valent systems of class II according to Robin and Day. Ground-state delocalization in the mixed-valent forms of these complexes as based on the CO band shifts is considerably larger for the “closed” paracyclophane as for the “half-open” orthocyclophane. Experimental findings are backed by the calculated IR band patterns and spin density distributions for radical cations of slightly simplified model complexes 2Me
+ and 3Me+ with the PiPr3 ligands replaced by PMe3. Radical cations 2+ and 3+ feature a characteristic NIR band that is neither present in their neutral or fully oxidized forms nor in the radical cation of the monoruthenium [2.2]paracyclophane complex 1 with just one vinyl ruthenium moiety. These bands are thus assigned as intervalence charge-transfer (IVCT) transitions. Our results indicate that, for the radical cations, electronic coupling “through-space” via the stacked styrene decks is significantly more efficient than the “through-bond” pathway.
Graphical abstract
[2.2]Paracyclophane- and half-open [2.1]orthocyclophane-bridged diruthenium complexes are compared with respect to the electronic coupling in their mixed-valent states. Ground-state delocalization as measured by Geiger’s charge distribution parameter is considerably larger for the paracyclophane in spite of nearly identical “through-bond” distances for the two complexes.
Highlights
►Styryl ruthenium complexes derived from [2.2]para- and [2.1]orthocyclophanes. ►electrochemical and spectroelectrochemical studies. ►spectroscopic characterization of the mixed-valent radical cations and the dications. ►quantitative assessment of electron delocalization within the radical cations of dinuclear complexes. ►mutual relevance of the through-bond and the through-space pathways to overall electron delocalization.
Introduction
π-Stacking has long been recognized as an important non-covalent interaction governing the organization of matter [1], [2], [3], [4], [5]. Implications are numerous and include, inter alia, the photophysics of luminophores [6], [7], [8] or of polymers having extended arene substituents or backbones [9], [10], the formation of self-assembled coordination cages [3], [5], [11] and the exchange of magnetic [12], [13], [14], [15], [16], [17], [18] or electronic information [8], [16], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. π-Stacking is also held responsible for rapid charge migration over oligonucleotides and DNA [29], [30], [31], [32] or columnar mesophases made up of suitably functionalized disk-shaped building blocks with extended aromatic cores [32], [33], [34], [35], [36], [37]. Hupp et al. have beautifully demonstrated that rectangular tetrametal box molecules with extended diimine π-ligands as the long sides form, upon partial reduction, mixed-valent radical anions that exhibit electronic coupling through-space [38], [39], [40]. A clear dependency of the strength of the electronic interaction on the stacking distance was observed.
[n.n]Paracyclophanes have a particularly successful history as testbeds for such interactions [41], [42], [43], [44]. The groups of Neugebauer [45], [46] and, more recently, Grampp and Lambert [47] have elegantly utilized the radical cations derived from electron-rich methoxy or bis(triarylamine) substituted [n.n]paracyclophanes as probes for electron delocalization on the EPR timescale and noted that cyclophane bridges behave more like unsaturated and conjugated than as saturated bridges in terms of the electronic coupling conveyed by them [42].
We herein report [2.2]paracyclophanes where one or both arene decks are elaborated into styryl ruthenium moieties RuCl(CH
CHPh’)(CO)(PiPr3)2. The {RuCl(CO)(PiPr3)2} tags render the corresponding styryl substituents electroactive at fairly low potential and stabilize their associated radical cations to about the same extent as dialkyl amino groups, thus making them amenable to spectroscopic investigations. As an added benefit with respect to classical organic electron donating groups they also provide the characteristic Ru(CO) stretch as a reporter of the charge density at the metal center. Its change in position upon oxidation is a convenient spectroscopic probe of the loss of electron density from the metal atom. Moreover, the CO band pattern and the relative CO band shifts are indicative of the degree of intrinsic charge delocalization on the short vibrational timescale of ca. 10−12 s in di- and oligonuclear complexes. Here we apply these Ru(CO) tags to measure electron delocalization in a diruthenium pseudo-para-divinyl[2.2]paracyclophane complex. The issue of the contribution of the saturated alkylene straps to the overall electron delocalization is addressed by comparison with the 3,7-divinyl-10,11-dihydro-5H-dibenzo[a,d]cycloheptene-derived diruthenium complex 3 that maintains the motif of two doubly alkyl linked styryl ruthenium moieties but lacks the π-stacking of the individual styryl ruthenium subunits as it is present in paracyclophanes (see Chart I).
Section snippets
General considerations
All reactions and manipulations were conducted using standard Schlenk techniques. Solvents were dried over appropriate drying agents, distilled under nitrogen and stored in airtight glass bulbs. All NMR solvents were degassed by five “freeze–pump–thaw” cycles and stored in airtight Schlenk tubes over appropriate molecular sieves. 1H, 13C and 31P NMR spectra were recorded on a Bruker Avance 300 (300.13 MHz) or a Bruker Avance 400 (400.13 MHz) NMR spectrometer. Voltammetric measurements were
[2.2]Paracyclophane complexes 1 and 2
The mono- and diruthenium [2.2]paracylophane complexes 1 and 2 of Chart 1 were prepared by hydroruthenation of 4-ethynyl[2.2]paracyclophane or of pseudo-para-diethynyl[2.2]paracyclophane with RuClH(CO)(PiPr3)2 and accordingly characterized by multinuclear NMR and IR spectroscopy. 1H and 13C NMR spectra of the unsymmetrically substituted monoruthenium complex 1 feature separate resonances for every individual carbon and hydrogen atom which could be assigned on the basis of one- and
Discussion
The main issue of this study was to experimentally probe for the strength of the electronic coupling in the mixed-valent radical cations 2+ and 3+ and for the extent to which the π-stacking (or through-space) pathway and the through-bond pathway via the saturated alkylene straps contribute to it. While information as to the extent of electron delocalization in mixed-valent systems can in principle be obtained from several spectroscopic or even computational methods, the oxidation-induced IR
Conclusions
Complexes 2 and 3 feature two interlinked styryl-Ru(CO)Cl(PiPr3)2 subunits, whose phenyl rings are integrated into a [2.2]paracyclophane or a [2.1]orthocyclophane system. These two architectures have virtually identical through-bond distances between the individual styryl ruthenium chromophores and their metal centers but strongly differ with respect to the mutual arrangements of the arene decks. Enforced π-stacking at distances well beyond the sum of the van der Waals radii in the case of the
Acknowledgment
We are grateful to Deutsche Forschungsgemeinschaft (project Wi 1272/7–2) for financial support of this work. S. Z. wishes to thank the Ministry of Education of the Czech Republic (Grant COST LD11086) and the Grant Agency of the Academy of Sciences of the Czech Republic (KAN100400702). We also wish to thank Silvia Domingo-Köhler and Dr. Malte Drescher for the recording of additional EPR spectra of complex 1+. R. F. W. also wishes to thank a referee for insightful comments and Prof. Dr. Stephan
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Present address: Institut Charles Gerhardt – UMR5253, Equipe CMOS (Chimie Moléculaire et Organisation du Solide), Université de Montpellier 2 – CC1701, Place Eugène Bataillon, F-34095 Montpellier Cedex 05, France.