Synthesis and Electrochemical Characterization of Bimetallic Ruthenium Complexes with the Bridging η2(σ, σ)-1,3-Butadiyne-1,4-Diyl Ligand

O complexo [ cis-{RuCl(bpy)2(μ-C≡C-)}]2 (1) foi obtido pelo tratamento de 1 equiv de 1,4bis(trimetilsilil)-1,3-butadiino ou bis(trimetilsilil)acetileno com 2 equiv do complexo cis[RuCl2(bpy)2].2H2O, NaF e NaBF 4 na mistura de solventes metanol/CH 2 l2 (10/1) com rendimentos de 52% e 35%, respectivamente. Análises de RMN de 1H, 13C{H} e principalmente eletroquímica, confirmaram que o mesmo produto foi obtido dos dois métodos. Análise de ( 1) através de voltametria cíclica, no i tervalo de potencial de 0 a 1,20 V mostrou dois picos de oxidação quasi-reversíveis referentes ao par redox Ru(II)/Ru(III). Os dois processos redox são separados por 520 V, indicando comunicação eletrônica significante entre os dois centros metálicos


Introduction
Organometallic polymers whose metal centers are joined by organic ligands with delocalized π-systems have been investigated extensively over the last several years 1 .Such species are of interest due to their potential usefulness in the areas of electronics and materials science 2 .Organometallic polymers, with transition metals linked by a polyynediyl ligand, M-(C≡C) n -M, have attracted increasing attention from various viewpoints 3 .The π-conjugated polycarbon system is extended to the two terminal metal units and such systems are expected to display attractive properties resulting from i) π-conjugation along the rodlike linkage, ii) stabilization of odd-electron (mixed-valent) species formed by oxidation and reduction, and iii) hyperpolarizability.One particular type of organometallic polymer, with transition metals linked by a 1,3-butadiyne-1,4-diyl ligand, is known to have a rigid, rodlike structure and exhibits unusual properties both in solution and in the solid state 4 .Here we report the synthesis and characterization of a complex containing the η 2 (σ,σ)-1,3-butadiyne-1,4-diyl ligand, synthesized from the reactions of cis-[RuCl 2 (bpy) 2 ] .2H 2 O (bpy = bipyridine) with 1,4-bis(trimethylsilyl)-1,3-butadiyne or bis(trimethylsilyl)acetylene.
Electrochemical measurements were performed on an EG&G Princeton Applied Research (PAR) M273A electrochemical analyzer interfaced to an IBM computer employing PAR 270 electrochemical software.A standard three-electrode cell was designed to allow the tip of the reference electrode to closely approach the working electrode.Positive feedback IR compensation was applied routinely.All measurements were carried out under dry argon, in anhydrous deoxygenated acetonitrile; solution were ca.1x10 -3 mol dm -3 with respect to the compounds under study and ca.1x10 -1 mol dm -3
As described, the BF 4 -anion acts as a halide abstractor 7 (eq 2) to promote the complexation of the terminal alkyne at the ruthenium center, giving the vinylidene complex isolated in many cases as the final product of the reaction (eq 3).
However, under these conditions, the cleavage of the trimethylsilyl group is associated with the formation of a stoichiometric amount of a strong base.This base in situ deprotonates the vinylidene intermediate, allowing the formation of the bis-(ruthenium alkynyl) complex 1 as the final product of the reaction (eq 4). {cis-[RuCl(bpy To our knowledge, this is the first example reported so far of a diynediyl complex bearing the RuCl(bpy) 2 fragment.Because of the complexity of the 1 H and 13 C spectra it is not possible to use it to characterize the complex.The complex was characterized by elemental analysis and, mainly, by electrochemical studies.The absence of the MeOH absorption of the C≡C stretching in the IR spectrum and the presence of the two quasi-reversible oxidation peaks in the cyclic voltammogram revealed by the electrochemical studies, attributed to the couple Ru(II)/Ru(III), suggest that compounds 1 is a dimer (see discussion below).
Complex 2 was prepared by reaction of the free ligand, H-C≡C-SiMe 3 with cis-[RuCl (bpy) 2 (CH 3 COCH 3 )] + (eq 5, 6) The monoacetone complex 8 is known to be a valuable synthetic intermediate.The preparation of the monoacetone complex must be carefully timed 5 .After short reaction times (<1.5 h) the complex is not completely formed, the limiting factor being the rate of dissolution of cis-[RuCl 2 (bpy) 2 ] .2H 2 O in acetone.After long reaction times (>1.5 h) a dark, red-brown precipitate begins to form.Isolation of this complex showed it to be identical with dimer cis-[RuCl(bpy) 2 ] 2 [BF 4 ] 2 by cyclic voltametry 9 .Isolation of complex 2 was achieved by precipitation from an acetone solution by adding diethyl ether, after filtration through an alumina plug to remove salts.

H and 13 C{ H} NMR spectra.
The of 1 H NMR spectrum of [RuCl 2 (bipy) 2 ], in (DMSO-d 6 ) has been discussed in the literature 10 and shown to be more complex than expected for the cis or trans configurations.The reason for the complexity of the spectrum is that a mixture of cis and trans compounds were present in solution, with possible solvent interation 10 .In the cis configuration, the molecule has no symmetry so that the 16 bipyridine hydrogens are expected to be unique.A firstorder coupling scheme predicts eight doublets and eight triplets for the 1 H NMR spectrum assigned to the aromatic hydrogens of bipyridine 5 .In fact, complexes 1 -2 showed a more complicated pattern of 1 H and 13 C{ 1 H}NMR spectra in the aromatic region, which would be expected for a cis or trans compounds.The 13 C{ 1 H}NMR spectra of the complexes studied are similar in the aromatic region and display five sets of resonances, concentrated at δ 160.4 -158.2, 155.2 -149.5, 139.0 -136.9, 127.6 -126.7 and 124.7 -123.7.Each set of resonance exhibits more peaks than would be expected for the cis configuration.This is in agreement with the fact that the pyridine groups of each ligand are not magnetically equivalent and both cis and trans configurations probably would be present.Moreover, in these complexes, the 13 C NMR signals of the Ru-C≡C-carbons are masked by the signals of pyridyl carbons.
This compound exhibited two quasi-reversible oxidation peaks at E 1/2 +2.07 and +0.410 V vs Ag/AgCl and two quasi-reversible reduction peaks at E 1/2 -1.48 and -1.61 V vs Ag/AgCl.The peaks at +2.07 and +0.410 V were attributed to the Ru(III)/Ru(IV) and Ru(II)/Ru(III) couples, respectively.The two reduction peaks at -1.48 and -1.61V were ascribed to the bpy ligand.These results are consistent with the previous study on this complex 9 .The same electrochemical behavior was shown by compound 2 (see Figure 2).This compound shows two quasi-reversible oxidation peaks at E 1/2 +1.53 and +0.410 V vs Ag/AgCl, owing to the Ru(III)/Ru(IV) and Ru(II)/Ru(III) couples, respectively, and a irreversible reduction process at -1.25 V and two quasi-reversible peaks at -1.39 and -1.56 V vs Ag/AgCl, due to the bpy ligand.Table 1 shows that the potential of the Ru(II)/Ru(III) couple in complex cis-[RuCl 2 (bpy) 2 ] and 2 is the same as in compound 1, which implies that the Ru(II) orbital energy is unchanged in the complex 2 when of the substitution of the trimethyl(silyl) acetylide by one chloride on cis-[RuCl 2 (bpy) 2 ] complex.The cyclic voltammetric response for dimeric complex 1 is shown in Figure 3.This complex displays one irreversible oxidation peak at +2.05 V. vs Ag/AgCl, ascribed to the Ru(III)/Ru(IV) couple and two quasi-reversible reduction peaks at -1.42 and -1.61 V vs Ag/AgCl, due to the reduction of the bpy ligand.Two oxidation peaks at +0.351 and +0.871 V vs Ag/AgCl with the i pc /i pa current ratio of unity were attributed to the Ru(II)/Ru(III) couple.This means that the neutral dimer undergoes two successive one-electron oxidations to yield the mono-and the dications, respectively (eq 7).The ∆E p value for the two redox processes is 520 mV, clearly indicates a strong communication between the two ruthenium centers propagated throughout the orbitals of the -C≡C-C≡C-bridge.This reflects the extent of the delocalization between metal centers in the ground state.The important stabilization of the mixed-valence Ru(II)/Ru(III) state is shown by the large value of the comproportionation constant, K C = 6.13 x 10 8 .The one-dimensional-C 4 -bridge acts as a molecular wire to convey the odd electron from one metal center to the other.The high value of ∆E p = 720 V, corresponding to a K C = 1.60 x 10 12 , reported for complex ([FeCp*(dppe)]-C≡C-C≡C-[FeCp*(dppe)]) 11 , shows that the delocalization, greatly favored by the -C x -bridge, strongly depends on the electronic structure of the metal unit.
Figure 4 shows the cyclic voltammetric response for complex 1, synthesized using bis(trimethylsilyl) acetylene as the ligand.The one irreversible oxidation peak observed at +2.02 V vs Ag/AgCl was ascribed to the Ru(III)/ Ru(IV) couple, the two quasi-reversible peaks at -1.42 and -1.62 V vs Ag/AgCl were assigned to the reduction of the bpy ligand and the two reversible peaks (i pc /i pa = 1) at +0.354 and +0.874 V vs Ag/AgCl were ascribed to the dimer Ru(II)/Ru(III) couple that undergoes two successive one-electron oxidations (DE p = 520 mV) to yield the monoand the dications, respectively.
The data on Table 1 and the electrochemical behavior presented in Figures 3 and 4 for compound 1 obtained from 1,4-bis(trimethylsilyl)1,3-butadiyne or bis(trimethylsilyl) acetylene are very similar.We therefore, suggest that these compounds are the same.As stated above, dimerization of the ligand may occur, owing of the mechanism of the reaction and the temperature used, since the reaction does not occur at room temperature.Moreover, it was expected that the communication between the two metallic centers of dimer complexes having a bridged acetylenic linkage would be greater than that between metallic centers linked by a bridging butadiyne.Indeed, biferrocenyl derivatives bridged by acetylenic linkages Fc-C≡C-Fc and Fc-C≡C-C≡C-Fc have already been described and showed similar cyclic voltammograms with ∆E p 130 and 100 mV, respectively 12,13 .Since ∆E values for the two compounds reported herein are the same, 520mV, these compounds should be the same.