Dissecting out the effect of Ru-OAr bonding in a five-coordinate complex of ruthenium (II)

Blue Ru(o-cat)(PPh3)3 (5; o-cat = ortho-catecholate), obtained in 94% yield by reaction of RuCl2(PPh3)3 (4) with dithallium catecholate, was characterized by NMR, MALDI-MS, IR, and single crystal X-ray analysis, and by a combined electronic spectroscopy and time-dependent density functional theory (TD-DFT) study. The frontier orbitals in 5 participate in a low-energy charge transfer excitation involving donation from the Ru-catecholate π bond (largely localized on catecholate) to a low-lying σ* orbital on Ru-PPh3(apical). The energy of this transition increases on binding a pyridine ligand in the sixth site.Key words: ruthenium aryloxide, catecholate, phosphine, electronic spectroscopy, time-dependent DFT.

Heteroleptic tris-chelates of ruthenium(II): synthesis, spectral characterisation and electrochemical properties

A facile reaction of cis-trans-cis-RuCl(2)(RaaiR')(2) [RaaiR'=1-alkyl-2-(arylazo)imidazole, m-R-C(6)H(4)-NN-C(3)H(2)-NN-1-R', where R=H (a), OMe (b), NO(2) (c) and R'=Me (1), Et (2) and CH(2)Ph (3)] either with 2,2'-bipyridine (bpy) and AgNO(3) followed by NaClO(4) or [Ag(bpy)(2)](ClO(4)) in boiling acetone has isolated red-brown [Ru(bpy)(RaaiR')(2)](ClO(4))(2) (1a-c, 2a-c, 3a-c). The maximum molecular peak of [Ru(bpy)(OMeaaiMe)(2)](ClO(4))(2) (1b) is observed at m/z 888.01 (100%) in the FAB mass spectrum. IR spectra of the complexes show CN and NN stretching at 1590 and 1370cm(-1) which is red shifted by 40 and 90cm(-1) from the free ligand value supports Ru-azo nitrogen pi bonding interaction. The emission spectra in frozen glass (77K) are sharper and considerably more intense than the room temperature spectra. The (1)H NMR spectral measurements suggest methylene, -CH(2)-, in RaaiEt gives a complex AB type multiplet while in RaaiCH(2)Ph it shows AB type quartets. Considering two arylazoimidazole moieties there are 20 different carbon atoms in the molecule which gives a total of 20 different peaks in the (13)C NMR spectrum. In the (1)H-(1)H COSY spectrum of the present complexes, absence of any off-diagonal peaks extending from delta=14.12 and 9.55ppm confirm their assignment of no proton on N(1) and N(3), respectively. Contour peaks in the (1)H-(13)C HMQC spectrum in the present complexes, the absence of any contours at delta=147.12, 160.76, 155.67 and 157.68 ppm assign them to the C(2), C(6), C(8) and C(e and e'') carbon atoms, respectively. Cyclic voltammogram shows Ru(III)/Ru(II) redox couple along with three successive ligand reductions. The plot of difference in potential of first oxidation and reduction versus energy of main MLCT band (nu(CT)) is linear. Electrochemical parametrisation of Ru(III)/Ru(II) redox couple determines ligand potential E(L)(L).

Ruthenium Nitrosyls Derived from Polypyridine Ligands with Carboxamide or Imine Nitrogen Donor(s): Isoelectronic Complexes with Different NO Photolability

As part of our search for photoactive ruthenium nitrosyls, a set of {RuNO}6 nitrosyls has been synthesized and structurally characterized. In this set, the first nitrosyl [(SBPy3)Ru(NO)](BF4)3 (1) is derived from a polypyridine Schiff base ligand SBPy3, while the remaining three nitrosyls are derived from analogous polypyridine ligands containing either one ([(PaPy3)Ru(NO)](BF4)2 (2)) or two ([(Py3P)Ru(NO)]BF4 (3) and [(Py3P)Ru(NO)(Cl)] (4)) carboxamide group(s). The coordination structures of 1 and 2 are very similar except that in 2, a carboxamido nitrogen is coordinated to the ruthenium center in place of an imine nitrogen in case of 1. In 3 and 4, the ruthenium center is coordinated to two carboxamido nitrogens in the equatorial plane and the bound NO is trans to a pyridine nitrogen (in 3) and chloride (in 4), respectively. Complexes 1-3 contain N6 donor set, and the NO stretching frequencies (nuNO) correlate well with the N-O bond distances. All four diamagnetic {RuNO}(6) nitrosyls are photoactive and release NO rapidly upon illumination with low-intensity (5-10 mW) UV light. Interestingly, photolysis of 1 generates the diamagnetic Ru(II) photoproduct [(SBPy3)Ru(MeCN)](2+) while 2-4 afford paramagnetic Ru(III) species in MeCN solution. The quantum yield values of NO release under UV illumination (lambda(max) = 302 nm) lie in the range 0.06-0.17. Complexes 3 and 4 also exhibit considerable photoactivity under visible light. The efficiency of NO release increases in the order 2 < 3 < 4, indicating that photorelease of NO is facilitated by (a) the increase in the number of coordinated carboxamido nitrogen(s) and (b) the presence of negatively charged ligands (like chloride) trans to the bound NO.

Complex series [Ru(tpy)(dpk)(X)]n+ (tpy = 2,2':6',2''-terpyridine; dpk = 2,2'-dipyridyl ketone; X = Cl-, CH3CN, NO2(-), NO+, NO*, NO-): substitution and electron transfer, structure, and spectroscopy

The complex framework [Ru(tpy)(dpk)]2+ has been used to study the generation and reactivity of the nitrosyl complex [Ru(tpy)(dpk)(NO)]3+ ([4]3+). Stepwise conversion of the chloro complex [Ru(tpy)(dpk)(Cl)]+ ([1]+) via [Ru(tpy)(dpk)(CH3CN)]2+ ([2]2+) and the nitro compound [Ru(tpy)(dpk)(NO2)]+ ([3]+) yielded [4]3+; all four complexes were structurally characterized as perchlorates. Electrochemical oxidation and reduction was investigated as a function of the monodentate ligand as was the IR and UV-vis spectroscopic response (absorption/emission). The kinetics of the conversion [4]3+/[3]+ in aqueous environment were also studied. Two-step reduction of [4]3+ was monitored via EPR, UV-vis, and IR (nu(NO), nu(CO)) spectroelectrochemistry to confirm the {RuNO}7 configuration of [4]2+ and to exhibit a relatively intense band at 505 nm for [4]+, attributed to a ligand-to-ligand transition originating from bound NO-.

Photochromism of 2-(Phenylazo)imidazoles

The isomerization behaviors of 2-(phenylazo)imidazole (Pai-H) and 1-N-methyl-2-(phenylazo)imidazole (Pai-Me) have been investigated. The crystal structure of trans-Pai-Me was determined, revealing that key structures around the azo group are nearly identical among azobenzene, Pai-H, and Pai-Me. Pai-Me undergoes reversible cis/trans photoisomerization, whereas Pai-H responds poorly to irradiation. The quantum yields of trans-to-cis isomerization of Pai-Me on 454 and 355 nm excitation are 0.35 +/- 0.03 and 0.25 +/- 0.03, respectively, in toluene. The wavelength-dependent isomerization quantum yield is well-known for azobenzene, but these values are substantially higher than those of azobenzene. The activation energy of thermal cis-to-trans isomerization of Pai-Me in toluene is 79.0 +/- 3.5 kJ mol(-1), which is lower than that of azobenzene by 15 kJ mol(-1). The thermal cis-to-trans isomerization of Pai-H is even faster. Density functional theory calculations were performed, revealing that the energy gaps between the azo n-orbital and the highest pi-orbital of azoimidazoles are much narrower than that of azobenzene. Finally, a preliminary study suggested that metal ions can modulate the absorption spectrum of Pai-Me without a loss of the gross photochromic behavior.