Néstor E. Katz

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.

The mechanism of the reactions of pentacyanonitrosylferrate(II) with ammonia and ethylenediamine

Abstract The reaction of NH3 with the pentacyanonitrosylferrate(II) ion has been studied spectrophotometrically at different pH values and concentrations of base. At low [NH3] and pH 13, suggesting the onset of direct attack of NH3 on NO+. Similar results are obtained when ethylenediamine is used as a nucleophile

Syntheses and characterization of new mononuclear and dinuclear complexes derived from ruthenium polypyridines

Abstract New complexes containing the Ru(bpy)(trpy) 2+ moiety (bpy=2,2′-bipyridine; trpy=2,2′:6′,2″-terpyridine) as a photosensitizer, 4-CNpy (=4-cyanopyridine) as a bridging ligand and Ru(NH 3 ) 5 n+ as an electron donor ( n =2) or acceptor ( n =3) capping group were synthesized and characterized by spectroscopic and electrochemical techniques. In the mononuclear complex [Ru II (bpy)(trpy)(4-CNpy)] 2+ (I) IR, UV-Vis and cyclic voltammetry data ( v CN(nitrile)=2237 cm −1 ; λ max =432 and 464 nm, in CH 3 CN; E 1 2 (Ru III /Ru II )=1.23 V, in CH 3 CN versus SCE) point to a pyridine-bonded isomer of 4-CNpy. In the bridged complex [Ru II (bpy)(trpy)(4-CNpy)Ru II (NH 3 ) 5 ] 4+ ( II ) the nitrile end of 4-CNpy coordinates to a capping Ru II (NH 3 ) 5 group, as disclosed by the shifts in spectral absorptions ( v CN(nitrile)=2174 cm −1 ; λ max =490 nm, in CH 3 CN) and the occurrence of a new voltammetric wave ( E 1/2 =0.74 V, in CH 3 CN versus SCE). A related bridged species, [Ru II (bpy)(trpy)(4-CNpy)Fe II (CN) 5 ] − ( IIl ) was obtained and characterized in aqueous solution (λ max =460 nm, in water). The mixed-valence complex [Ru II (bpy)(trpy)(4-CNpy)Ru III (NH 3 ) 5 ] 5+ ( IV ) can be prepared in CH 3 CN solution (λ max =438 nm) by oxidation of II with Ce(IV); it presents a comproportionation constant K c =2x10 8 , thus indicating a high stability arising from the asymmetric nature of this species. The ‘supramolecular’ systems described in this work can be used as models for systematic studies of intramolecular electron transfers.

Cyano-bridged complexes of bis(2,2'-bipyridine)(pyridine)ruthenium(II)

Abstract New complexes with the Ru(bpy) 2 (py) 2+ moiety (bpy = 2,2′-bipyridine, py = pyridine) connected through a cyano group to Ru(NH 3 3+ 5 and Fe(CN) 2− 5 as electron acceptors have been prepared and their spectroscopic, electrochemical and photophysical properties investigated. Cyano-bridging is disclosed by changes in the shape and position of the cyanide stretching vibration, ν(CN), in the IR spectrum of the dinuclear ruthenium species, as compared with the mononuclear parent complex. Blue shifts in the lowest energy d π → π* (Ru → bpy) metal-to-ligand charge transfer (MLCT) transition occur when going from [Ru(bpy) 2 (py)(CN)] + ( A ) to [(bpy) 2 (py)Ru II CNRu III (NH 3 ) 5 ] 4+ ( B ) and to [(bpy) 2 (py)Ru II CNFe III (CN) 5 ] − ( C ), thus pointing to the existence of nitrile-bound pentaammineruthenium(III) and pentacyanoferrate(II) capping groups in the mixed-valence species B and C . Besides, new intense and broad absorptions at 697 (in HCl 0.01 M) and 700 nm (in H 2 O/Me 2 CO, 1:1 v/v) appear in B and C , respectively, and can be assigned to metal-to-metal charge transfer (MMCT) or “intervalence” transitions. The luminescence of A is completely quenched in B , even at 77 K, a fact which can be explained on the basis of efficient excited-state electron transfer to form the electronic mixed-valence isomer of B . The strong asymmetric nature of B , as deduced from cyclic voltammetry data (the difference in redox potentials between both ruthenium sites amounts to 1.30 V), together with a strong electronic coupling [ H AB = 220 cm −1 , calculated from the “intervalence” absorption data] indicate that the back electron transfer (or charge recombination) from the MMCT excited state of B probably lies in the “inverted” region.

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.

Qualitatively and quantitatively different solvatochromism of the MLCT and MMCT absorption bands of centrosymmetric acceptor-bridged diiron(II, II) and diiron(III, II) cyanide complexes

Abstract The energies of the metal-to-ligand charge transfer (MLCT) and metal-to-metal charge transfer (MMCT) absorption bands of a number of heterocycle-bridged diiron(II,II) homovalent and diiron(III,II) mixed-valent complexes correlate linearly with Gutmanns acceptor number (AN). The compounds are (NEt4)6[(NC)5Fe(μ-tz)Fe(CN)5] and (NEt4)5[(NC)5Fe(μ-tz)Fe(CN)5], tz=1,2,4,5-tetrazine; (NEt4)4[(NC)4Fe(μ-bptz)Fe(CN)4] and (NEt4)3[(NC)4Fe(μ-bptz)Fe(CN)4], bptz=3,6-bis(2-pyridyl)-1,2,4,5-tetrazine; (NEt4)4[(NC)4Fe(μ-bmtz)Fe(CN)4], bmtz=3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine; (NEt4)4[(NC)4Fe(μ-bpym)Fe(CN)4], bpym=2,2′-bipyrimidine. Mononuclear analogues of the tz and bpym ligands were also studied. Various degrees of negative and positive solvatochromism are observed for centrosymmetric dinuclear systems. Unusual observations include the large negative solvatochromism of MLCT bands in Fe(II)Fe(II) species lacking a permanent dipole moment, the positive solvatochromism of MLCT/LMCT bands and the small negative solvatochromism of MMCT bands in the valence-averaged Fe2.5Fe2.5 systems.

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.