Florencia Fagalde

Synthesis and properties of [Ru(tpy)(4,4′-X2bpy)H]+ (tpy=2,2′:6′,2″-terpyridine, bpy=2,2′-bipyridine, X=H and MeO), and their reactions with CO2

Abstract A novel type of hydrido complex [Ru(tpy)(4,4′-X2bpy)H]+ (X=H and MeO) was synthesized. The stronger hydridic character of the complexes compared with [Ru(bpy)2(L)H]+ type complexes (L=CO, PPh3 and AsPh3) was demonstrated by the relatively high chemical shifts of RuH in the 1H NMR spectra and by higher reactivities with CO2. The reactions of [Ru(tpy)(4,4′-X2bpy)H]+ with CO2 occurred at second-order rate constants varying from (4.69±0.02) to (5.51±0.04)×10−3 M−1 s−1 depending on both solvent and X, giving the formato complexes [Ru(tpy)(4,4′-X2bpy)(OCHO)]+ quantitatively. The rate constant was increased with the increase of solvent acceptor number, and the reaction of [Ru(tpy){4,4′-(MeO)2bpy}H]+ with CO2 was found to be 3.6 times faster than that of [Ru(tpy)(bpy)H]+. These results suggest that nucleophilic attack of the hydride ligand to the carbon atom of CO2 is the rate determining step for the formation of the formato complex. The structure of the formato complex [Ru(tpy)(bpy)(OCHO)](PF6) was determined by X-ray crystallographic analysis.

DISTANCE DEPENDENCE OF INTRAMOLECULAR ELECTRON TRANSFER PARAMETERS IN MIXED-VALENCE ASYMMETRIC COMPLEXES OF RUTHENIUM

Abstract New mixed-valence complexes of the type [(terpy)(bipy)Ru II LRu III (NH 3 ) 5 ] 5+ ] (terpy = 2,2′ : 6′,2″-terpyridine, bipy = 2,2′-bipyridine) with L = pz and BPE (pz = pyrazine; BPE = trans -1,2-bis(4-pyridyl)ethylene) exhibit metal-to-metal (Ru b II →Ru a III ; Ru b = Ru bonded to bipyridine, Ru a = Ru bonded to ammine) charge transfer transitions in the visible region, due to the strong asymmetry of the redox sites. Although the electronic coupling element of the pz-bridged complex is higher than that of the BPE analogue, both complexes are considered partially delocalized (Robin and Day class II). From a comparison of these data and those from closely related compounds, the distance dependence of intramolecular electron transfer parameters has been determined over a range of metal-to-metal distances r from 5 to ≊ 14 A , good correlations being obtained, the electronic coupling H AB and the molar absorptivity ϵ max decreasing exponentially with r . The bridging ligands appeat to behave as electronic π mediaors with intermediate conducting properties ( β = 0.40 A −1 ). the reorganization energy λ increases with r , but for r > 9 A , the intramolecular electron transfer back reactions Ru a II → Ru b III fall in the barrierless regime, where the nuclear factor shows small distance dependence. In order to slow charge recombination after photoexcitation, it may be possible to combine more asymmetric redox sites, and by manipulation of the distance between them, generate intermediate values of H AB , thereby causing Marcus “inverted” behaviour.

New mono- and di-nuclear mixed complexes of cyanides and 2,3-bis (2′-pyridyl) pyrazine with ruthenium

Abstract The synthesis and spectroscopic, electrochemical and emissive properties of a new mononuclear mixed complex, of formula [Ru (dpp) (CN) 4] 2− (dpp = 2,3-bis (2′-pyridyl) pyrazine are reported. The spectroscopic (IR, UV-visible, and emission) results demonstrate that dpp has a π-acceptor capacity intermediate between bpy and bpz (bpy = 2,2′-bipyridine, bpz = 2,2′-bipyrazine) , when coordinated to ruthenium cyanides. The redox potential of the RuII\RuIII couple and the observed luminiscence at room temperature imply its possible use as a photosensitizer. Since dpp has also the extra ability, when compared to bpy, to form supramolecular complexes, new dinuclear complexes with dpp bridging Ru (CN) 4 n− (n = 1,2) moieties can also be obtained. The corresponding mixed-valent complex, [ (CN) 4RuII (dpp) RuIII (CN) 4] 3−, has the characteristics of a Class (II) species, as disclosed by the weak interaction between both metallic centres.

Base hydrolysis of acetonitrile coordinated to a ruthenium(II)_polypyridine complex

Abstract On coordination to Ru(terpy)(bipy)2+ (terpy = 2,2′ : 6′,2″-terpyridine, bipy = 2,2′-bipyridine), acetonitrile is converted to acetamide in aqueous basic solution, through hydroxide attack with a catalytic factor of ca 3 × 103 (kOH = 4.6 × 10−3 M−1 s−1). This is a remarkable effect for a d6 transition metal in the (II) oxidation state, and can be ascribed to π backbonding from the metal to the polypyridyl ligands, which makes the ruthenium(II) centre more electropositive than ruthenium(II) in the Ru(NH3)52+ moiety. The final hydrolysis product is the [Ru(terpy)(bipy)(OH)]+ complex, since amides, being poor π-acceptor ligands, are rapidly released from the coordination sphere of ruthenium(II).

Preparation and spectroscopic, electrochemical and photophysical properties of mono-, di-nuclear and mixed-valence species derived from the photosensitizing group (2,2′-bipyridine)(2,2′:6′,2″-terpyridine)ruthenium(II)

New mono-, di-nuclear, and mixed-valence complexes derived from the photosensitizing moiety Ru(terpy)(bipy)2+(bipy = 2,2′-bipyridine, terpy = 2,2′:6′,2″-terpyridine) have been prepared and characterized by spectroscopic, electrochemical and luminescence techniques. The mononuclear species [Ru(terpy)(bipy)(4,4′-bipy)]2+(4,4′-bipy = 4,4′-bipyridine) exhibits λmax at 466 and 420 nm in MeCN, E½(RuIII–RuII)= 1.23 V (vs. saturated calomel electrode, SCE) in MeCN and λem= 625 nm in an EtOH–MeOH (4:1 v/v) glass at 77 K. The dinuclear complex [(terpy)(bipy)RuII(4,4′-bipy)RuII(NH3)5]4+ has a new absorption maximum at 538 nm and a new voltammetric wave at E½= 0.39 V in MeCN ascribable to a metal-to-ligand charge transfer and a RuIII–RuII redox process respectively involving the capping ammineruthenium group. On oxidation of this species with bromine vapour the mixed-valence complex [(terpy)(bipy)RuII(4,4′-bipy)RuIII(NH3)5]5+ is obtained, which seems to exhibit a metal-to-metal charge-transfer band, at λmax≈640 nm. Both dinuclear species emit at λem≈620 nm at 77 K, but with reduced intensity with respect to the parent mononuclear complex, thus pointing to the operation of reductive and oxidative quenching processes for the II,II and the II,III complexes, respectively. The weak metal–metal interaction detected in the mixed-valence ion implies the possibility of using these systems as ‘molecular switches’ and/or models for the obtention of charge-separated states.

Kinetics and mechanism of the redox reactions of binuclear complexes [XnRuII–L–RuII,III (NH3)5] (n+1)+ (X=polypyridines; L=cyanide and N-heterocyclic derivatives) with peroxydisulfate

Abstract The fully reduced (R) and mixed-valence (M) complexes [XnRuII–L–RuII,III (NH3)5]n, (n+1)+, with Xn=2,2′-bipyridine (bpy), 2,2′:6′,2″-terpyridine (terpy) and L=cyanide, pyrazine (pz), 4,4′-bipyridine (4,4′-bpy), 4-cyanopyridine (4-CNpy) and trans-1,2 (4-pyridyl)ethylene (bpe), were prepared in aqueous solution. The R complexes reacted with peroxydisulfate yielding the M complexes, with oxidation at the RuII (NH3)5 fragment. Both R and M were characterized through UV–VIS spectroscopy and electrochemistry. The one-electron oxidation reactions were measured under stopped-flow conditions for L=pz, 4,4′-bpy and 4-CNpy (T=25.0°C, I=0.1 mol dm−3, pH=4.8); the rate law was −d[R]/dt=k2 [R] [S2O fn2 8−]. A plot of ln ket against E0Ru (the redox potential at the pentaammine fragments) showed a good linear behavior, consistent with a Marcus LFE relationship for intramolecular electron-transfer in the ion pairs {R//S2O8}. The results fitted into a broader correlation including other binuclear and mononuclear [RuII (NH3)5L]n complexes, after due account of electrostatic corrections. The mixed-valence complexes decayed slowly in excess of peroxydisulfate, suggesting a complete oxidation process through direct attack at the remaining RuII-polypyridine center. These oxidation rates could also be included in the Marcus plot. A mechanistic analysis of both one-electron processes was made, considering evidences favoring direct attack of S2O 28− on each metal center; alternative paths with intramolecular electron-transfer assistance for oxidation of the mixed-valence complexes were discarded.

New asymmetric N-heterocyclic-bridged mixed-valent dinuclear complexes of rhenium and ruthenium

Abstract Two new mixed-metal mixed-valent complexes, of formulae: [(CO)3(bpy)ReI(bpe)RuIII(NH3)5]4+ and [(CO)3(bpy)ReI(4-CNpy)RuIII(NH3)5]4+ (where bpy=2,2′-bipyridine, bpe=trans-1,2-bis(4-pyridil)ethene, 4-CNpy=4-cyanopyridine), were prepared and characterized as solids and in acetonitrile solutions. From their spectroscopic, electrochemical and photophysical properties, the values for the reorganization energies λ and electronic coupling elements HAB for the metal-to-metal intramolecular electron transfers mediated by the N-heterocyclic bridges were calculated, using the Marcus–Hush formalism. For the 4-CNpy derivative, the reverse ruthenium-to-rhenium charge recombination—that should follow light excitation—is expected to be in the normal region, while for the bpe derivative this reaction is predicted to lie in the inverted region. The differences in redox potentials and in λ for both species account for these changes, which are relevant in the design of efficient photoconverters.

Nitrile hydrolysis in a rhenium(I)–ruthenium(III) dinuclear complex

Kinetic data for the hydrolysis reaction: [(CO)3(bpy)ReI(4-CNpy)RuIII(NH3)5]4+ + H2O → [(CO)3(bpy)ReI(4-C(O)NHpy)RuIII(NH3)5]3+ + H+ (bpy = 2,2′-bipyridine, 4-CNpy = 4-cyanopyridine and 4-C(O)NHpy = isonicotinamido), that occurs after oxidation with of the dinuclear species [(CO)3(bpy)ReI(4-CNpy)RuII (NH3)5]3+, have been obtained in aqueous solutions by spectrophotometric techniques. The observed rate constant, k h  = (8.6 ± 0.5) × 10−3 s−1, at 25°C, pH = 3.0 (CF3COOH) and I = 0.1 M (KCl), is ca. three times higher than the corresponding one for the mononuclear complex [(4-CNpy)RuIII(NH3)5]3+, indicating that the catalytic effect of the RuIII(NH3)5 moiety is enhanced by coordination of the free N of 4-CNpy to the ReI(CO)3(bpy) moiety. The value of k h is even higher than that of the dinuclear complex [(trpy)(bpy)RuII (4-CNpy)RuIII(NH3)5]5+ (trpy = 2,2′:6′,2″-terpyridine), reflecting the fact that carbonyls are much stronger π-acceptors than polypyridines.

Nitrile hydrolysis induced by oxidation of a binuclear diruthenium complex

Abstract On oxidation of the dinuclear complex [(terpy)(bipy)Ru II (4-pyCN)Ru II (NH 3 ) 5 ] 4+ (terpy = 2,2′:6′,″-terpyridine, bipy = 2,2′-bipyridine, 4-pyCN = 4-cyanopyridine) by excess S 2 O 8 2− in an aqueous solution a mixed-valent species is formed, which undergoes nitrile hydrolysis according to the reaction: [(terpy)(bipy)Ru II (4-pyCN)Ru III (NH 3 ) 5 ] 5+ +H 2 O → [(terpy)(bipy)Ru II (4-pyC(O)NH)Ru III (NH 3 ) 5 ] 4+ + H + [with 4-pyC(O)NH = isonicotinamido]. The observed rate constant k h = 5.8 × 10 −3 s −1 at 25.0°C, pH = 3.3 (CF 3 COOH), μ = 0.1 M (KCl), is ca twice as high as that of the mononuclear complex [(4-pyCN)Ru III (NH 3 ) 5 ] 3+ , but ca half that of the dinuclear species (CN) 5 Fe II (4-pyCN)Ru III (NH 3 ) 5 and ca four times lower than that of the dinuclear complex [(NH 3 ) 5 Ru III (4-pyCN) Ru III (NH 3 ) 5 ] 6+ , indicating that the catalytic effect of a Ru III (NH 3 ] 5 group on the rate of nitrile hydrolysis is somewhat enhanced by a Ru II (terpy)(bipy) moiety, due to an inductive effect, although to a less extent than ay a Fe II (CN) 5 or a Ru III (NH 3 ) 5 group, probably because of the π-back-bonding ability of Ru II , which makes the nitrile carbon atom of 4-pyCN less susceptible to the nucleophilic attack of a water molecule.

Tuning of the reorganization energies by 'innocent' co-ligands in novel mixed-valent dinuclear ruthenium complexes

Abstract Two novel mixed-valent ruthenium complexes, of formulae [(tpy)(bpy)RuII(pz)RuIII(edta)]+ and [(CN)4RuII(bpz)RuIII(edta)]3− (where tpy=2,2′:6′,2″-terpyridine, bpy=2,2′-bipyridine, pz=pyrazine, bpz=2,2′-bipyrazine, edta=ethylenediaminetetraacetate), were prepared and spectroscopically characterized in aqueous solutions and as solid salts with suitable counterions. For comparison purposes, two new ruthenium complexes: a dinuclear species, of formula [(CN)4RuII(bpz)RuII(NH3)5], and a trinuclear species, of formula {(CN)4RuII(bpz)[RuII(NH3)5]2}2+, were also prepared and characterized in aqueous solutions. From spectral data of metal-to-metal charge transfer (MMCT) absorption bands, a Hush analysis was made, and the reorganization energies for the intramolecular electron transfers were calculated. A dramatic change in these values was disclosed when comparing both mixed-valent species, which can be explained on the basis of the influence exerted by the ‘innocent’ co-ligands (polypyridines or cyanides) bonded to the ruthenium centers. This tuning is an important factor in devising molecular devices for energy conversion.