Synthesis of a double-activated switchable molecule via ruthenium¡©acetylide barbituric derivatives Jean-Luc Fillaut,* Matthew Price Andrew L. Johnson and Johann Perruchon Institut de Chimie de Rennes Laboratoire ¡®Organom¢¥etalliques et Catalyse¡� UMR 6509 CNRS - Universit¢¥e de Rennes 1 Campus de Beaulieu 35042 Rennes France. E-mail jean-luc.fillaut@univ-rennes1.fr Received (in Cambridge UK) 19th October 2000 Accepted 8th March 2001 First published as an Advance Article on the web 30th March 2001 Ruthenium¡©s-acetylide derivatives connected to barbituric acceptors through a p-conjugated bridge define a series of molecular systems presenting highly tunable properties. Scheme 2 Several molecular systems capable of performing elementary tasks on the molecular level have been synthesized in recent years.1 In most cases the molecular device obeys a binary rule; i.e.an input provides a positive or negative output response.2 This ability to switch on/off a function through an external parameter (light electron transfer chemical reaction) defines the concept of a molecular switch. Another objective is the building of molecules which exhibit useful electronic/photonic functions that can be externally controlled.3 With this in mind organometallic and coordination compounds are currently the subject of considerable investigations.4 Transition metal complexes with h1-alkynyl ligands (LnMC¡�CR) attract particular interest mostly as precursors of molecules containing a linear array of delocalized p-systems between two different functionalities.5 In this context we set out to investigate metal¡©alkynyl donor¡©acceptor systems as prototypes for molecular materials possessing switchable properties through chemical or redox modifications. We now describe the design and synthesis of ruthenium(II)¡©alkynyl connected to a barbituric acceptor selected for its ability in stabilizing charge delocalized canonical structures through an heteroaromatic limit form as a first example of this concept. corresponding vinylidene is a well-known process.7,8 Addition of an excess of a strong acid (HCl CF3CO2H) to 2a was therefore monitored by 31P NMR a significant shift of the signal for the 4 equivalent phosphorus nuclei from 48.6 to 39.6 ppm was observed. This latter value is consistent with the formation of a trans-chlororuthenium¡©vinylidene cationic complex 3 (Scheme 2).9 The presence of a quintuplet (4J[P,H] = 3 Hz) at 3.96 ppm in the 1H NMR spectrum of 3 is also in agreement with such a RuNCNCH cationic fragment.Knoevenagel condensation of 16 (Scheme 1) with barbituric acid yields the target compound 2a as an air-stable blue powder. Its structure was confirmed by spectrometric methods.¢Ó The UV¡©VIS spectra of complexes 1 and 2a characterized by intense (e 1.2¡©2 3 104 M21 cm21) MLCTs at lmax = 410 and 547 nm respectively in THF [Fig. 1(a)] proved the donor¡© acceptor nature of these systems. Complex 2a possesses three independant sites for successive protonation¡©deprotonation sequences (Scheme 2) which permit us to modulate the donor¡©acceptor coupling in 2a alternatively at the donor or at the acceptor head as depicted through dramatic colour changes.The protonation on the bcarbon of transition metal acetylide complexes leading to their Simultaneously a rapid colour change from purple to red (lmax = 495 nm e = 1.7 3 104 M21 cm21) [Fig. 1(b)] was observed. This reveals the weakening of the donor¡©acceptor electronic coupling arising from an alteration of the metal¡© acetylide moiety. This hypothesis was confirmed by electrochemical studies on the Ru2+/Ru3+ redox couple. Complex 2a presents a reversible oxidation wave in its cyclic voltammogram [Fig. 2(a)] at E1/2 = 560 mV vs. SCE in CH2Cl2,¢Ô assigned to the Ru(II)/(III) oxidation. The addition of a slight excess of trifluoroacetic acid to 2a resulted in the total disappearance of the initial anodic wave as expected for a vinylidene derivative [see Fig.2(a,b)]. As the CV for 2a reappears upon addition of K2CO3 to 3 complete reversibility and high speed switching were observed for both the protonation (acetylide to vinylidene) and deprotonation (vinylidene to acetylide) reactions. Scheme 1 Synthesis of the barbituric derivative 2a. Fig. 1 Absorption spectra of 2 in THF (a) pure 2a; (b) after addition of 1 equiv. of CF3CO2H to 2a; (c) after addition of 2 equiv. of DBU to 2a. DOI 10.1039/b100384b This journal is �Ï The Royal Society of Chemistry 2001 739 Chem. Commun. 2001 739¡©740 Fig. 2 Cyclic voltammograms‡ of 2a in CH2Cl2 at a scan rate of 200 mV s21 (a) pure 2a; (b) after addition of 1 equiv.of CF3CO2H on 2a; (c) after addition of 1 equiv. of DBU on 2a (see text). On the other hand the addition of a strong base (KOH DBU (DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene)) to 2a induces an immediate colour change from purple to pale yellow (lmax = 380 nm e = 1.5 3 104 M21 cm21) [Fig. 1(c)]. A minor variation of the 31P NMR shift for the phosphorus ligands from 48.6 to 49.5 ppm is observed. This process is perfectly reversible upon addition of a weak acid (CH3CO2H). Both these observations are consistent with the weakening of the acceptor strength of the barbituric moiety as a result of its deprotonation. This hypothesis was ascertained by monitoring the modifications of the Ru(II)/(III) oxidation wave of 2a upon sequential addition of DBU.This resulted in the growth of a new anodic wave at a less positive potential (E1/2 = 460 mV vs. SCE in CH2Cl2) in accord with the formation of a more electron rich species whereas the initial wave (E1/2 = 560 mV vs. SCE) was progressively disappearing [Fig. 2(c) for instance these concomitant reversible waves were observed upon addition of 1 equiv. DBU]. Two equivalents of base were necessary to afford complete conversion from the purple compound 2a to its yellow analogue. This process was therefore monitored by 1H NMR spectroscopy. As expected complete disappearance of both the NH (at 8.2 and 7.9 ppm) and a significant shift of the ethylenic CH signal (from 8.5 to 8.1 ppm) at the barbituric head resulted from the addition of 2 equiv.of DBU. Addition of 1 equiv. of base gave an unclear outcome (partial disappearance and broadening of the most significant signals of 2a). At this stage two 31P NMR signals were observed at 48.6 and 49.5 ppm which indicated that the deprotonation process was not complete. This process was monitored by 1H NMR upon addition of a large excess of the less basic N-ethyldiisopropylamine. The initial signals for the NH groups disappeared but Chem. Commun. 2001 739–740 740 only slight variations were observed for the ethylenic proton at 8.5 ppm and the aromatic system at 8.2 and 6.4 ppm. Neither colour change (UV–VIS spectra) nor electrochemical potential shift for the oxidation of 2 were observed.Finally complete conversion from 2a into its yellow analogue were obtained when 1 equiv. of DBU was added to the solution containing the weaker base. We assume then that a double NH deprotonation at the barbituric head9 is necessary to induce considerable electronic changes (Scheme 2 2b represents one of the ketoenolic forms of the monodeprotonated species whereas 2c is a conceivable mesomeric form of the bi-deprotonated derivative). This second mode of switching is different from the first one which acts more specifically on the metal–acetylide moiety. The synthesis of complex 2a has therefore proven an effective strategy for designing highly sensitive derivatives whose properties are adjustable alternatively at the donor or at the acceptor head.On the other hand such a switching mode could permit us to control the hydrogen binding ability of the barbituric head either by deprotonation on this residue or by protonation on the ruthenium moiety. This research was supported by the CNRS the MENR the Socrates-Erasmus Program (financial support to M. P. from the University of Edinburgh). Notes and references † Selected values for 2a 1H NMR (CDCl3 ppm) d 8.46 (s 1 H NCH) 8.20 and 6.47 (dd 4 H J 8.5 Hz aromatic) 8.03 and 7.88 (brs 2 3 1 H NH) 7.4–6.9 (m 40 H aromatic of the dppe) 2.65 (m 8 H CH2); 31P NMR (CDCl3 ppm) d 48.7 (s RuPPh2); 13C{1H} NMR (CDCl3 75.47 MHz ppm) d 163.9 161.5 and 148.9 (CNO); 159.1 (CH) 158.6 (Ru–C·C) 138.8–123.2 (C aromatics) 126.9 (Ru–C·C); 110.3 (Cq.barbituric) 30.5 (CH2 dppe); IR (CH2Cl2 cm21) n 2038 (C·C); MS (FAB) m/z 1172.1895 [M]+ calcd. 1172.1901. ‡ The electrochemistry of 2a was carried out at 298 K in a standard threeelectrode system (platinum working/auxiliary electrode and SCE reference electrode) using a 0.1 M dm23 [Bun 4N][PF6]–CH2Cl2 solution as electrolyte. 1 V. Balzani M. Gomez-Lopez and J. F. Stoddart Acc. Chem. Res. 1998 31 405; A. Credi M. Montaldi V. Balzani S. J. Langford F. M. Raymo and J. F. Stoddart New J. Chem. 1998 1061; J.-P. Sauvage Acc. Chem. Res. 1998 31 611. For examples of multimode switching see L. Gobbi P. 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