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New cis-(1,2-azole)-aquo bis(2,2′-bipyridyl)ruthenium(II) (1,2-azole (az*H) = pzH (pyrazole), dmpzH (3,5-dimethylpyrazole), and indzH (indazole)) complexes are synthesized via chlorido abstraction from cis-[Ru(bipy)2Cl(az*H)]OTf. The latter are obtained from cis-[Ru(bipy)2Cl2] after the subsequent coordination of the 1,2-azole. All the compounds are characterized by 1H, 13C, 15N NMR spectroscopy as well as IR spectroscopy. Two chlorido complexes (pzH and indzH) and two aquo complexes (indzH and dmpzH) are also characterized by X-ray diffraction. Photophysical and electrochemical studies were carried out on all the complexes. The photophysical data support the phosphorescence of the complexes. The electrochemical behavior of all the complexes in an Ar atmosphere indicate that the oxidation processes assigned to Ru(II) → Ru(III) occurs at higher potentials in the aquo complexes. The reduction processes under Ar lead to several waves, indicating that the complexes undergo successive electron-transfer reductions that are centered in the bipy ligands. The first electron reduction is reversible. The electrochemical behavior in CO2media is consistent with CO2electrocatalyzed reduction, where the values of the catalytic activity [icat(CO2)/ip(Ar)] ranged from 2.9 to 10.8. Controlled potential electrolysis of the chlorido and aquo complexes affords CO and formic acid, with the latter as the major product after 2 h. Photocatalytic experiments in MeCN with [Ru(bipy)3]Cl2as the photosensitizer and TEOA as the electron donor, which were irradiated with >300 nm light for 24 h, led to CO and HCOOH as the main reduction products, achieving a combined turnover number (TONCO+HCOO–) as high as 107 for 2cafter 24 h of irradiation.

Cationic fac-[Re(CO)3(pz*H)(pypzH)]OTf (pz*H = pyrazole, pzH; 3,5-dimethylpyrazole, dmpzH; indazole, indzH; 3-(2-pyridyl)pyrazole, pypzH) were obtained from fac-[ReBr(CO)3(pypzH)] by halide abstraction with AgOTf and subsequent addition of the corresponding pyrazole. Successive deprotonation with Na2CO3and NaOH gave neutral fac-[Re(CO)3(pz*H)(pypz)] and anionic Na{fac-[Re(CO)3(pz*)(pypz)]} complexes, respectively. Cationic fac-[Re(CO)3(pz*H)(pypzH)]OTf, neutral complexes fac-[Re(CO)3(pz*H)(pypz)], and fac-[Re(CO)3(pypz)2Na] were subjected to photophysical and electrochemical studies. They exhibit phosphorescent decays from a prevalently 3MLCT excited state with quantum yields (Φ) in the range between 0.03 and 0.58 and long lifetimes (τ from 220 to 869 ns). The electrochemical behavior in Ar atmosphere of cationic and neutral complexes indicates that the oxidation processes assigned to ReI→ ReIIoccurs at lower potentials for the neutral complex compared to cationic complex. The reduction processes occur at the ligands and do not depend on the charge of the complexes. The electrochemical behavior in CO2saturated media is consistent with CO2electrocatalyzed reduction, where the values of the catalytic activity [icat(CO2)/icat(Ar)] ranged from 2.7 to 11.5 (compared to 8.1 for fac-[Re(CO)3Cl(bipy)] studied as a reference). Controlled potential electrolysis for the pyrazole cationic (3a) and neutral (4a) complexes after 1 h affords CO in faraday yields of 61 and 89%, respectively. These values are higher for indazole complexes and may be related to the acidity of the coordinated pyrazole.

Considering the interest in processes related to hydrogen storage such as CO2hydrogenation and formic acid (FA) decomposition, we have synthesized a set of Ir, Rh, or Ru complexes to be tested as versatile precatalysts in these reactions. In relation with the formation of H2from FA, the possible applicability of these complexes in the transfer hydrogenation (TH) of challenging substrates as quinoline derivatives using FA/formate as hydrogen donor has also been addressed. Bearing in mind the importance of secondary coordination sphere interactions, N,N′ ligands containing NH2groups, coordinated or not to the metal center, were used. The general formula of the new complexes are [(p-cymene)RuCl(N,N′)]X, X = Cl–, BF4–and [Cp*MCl(N,N′)]Cl, M = Rh, Ir, where the N,N′ ligands are 8-aminoquinoline (HL1), 6-pyridyl-2,4-diamine-1,3,5-triazine (L2) and 5-amino-1,10-phenanthroline (L3). Some complexes are not active or catalyze only one process. However, the complexes [Cp*MCl(HL1)]Cl with M = Rh, Ir are versatile catalysts that are active in hydrogenation of quinolines, FA decomposition, and also in CO2hydrogenation with the iridium derivative being more active and robust. The CO2hydrogenation takes place in mild conditions using only 5 bar of pressure of each gas (CO2and H2). The behavior of some precatalysts in D2O and after the addition of 9 equiv of HCO2Na (pseudocatalytic conditions) has been studied in detail and mechanisms for the FA decomposition and the hydrogenation of CO2have been proposed. For the Ru, Ir, or Rh complexes with ligand HL1, the amido species with the deprotonated ligand are observed. In the case of ruthenium, the formate complex is also detected. For the iridium derivative, both the amido intermediate and the hydrido species have been observed. This hydrido complex undergoes a process of umpolung D+↔ Ir–D. All in all, the results of this work reflect the active role of −NH2in the transfer of H+.

The reactivity of complexes [PdCl2(κ2‐NN′)] [NN′ = 2‐C5H4N‐CH=N‐Ar; Ar = 2‐MeC6H4, a; 4‐MeC6H4, b; 2‐MeOC6H4, c; 4‐MeOC6H4, d; 4‐CF3C6H4, e; 2,6‐Me2C6H3, f; 2‐iPrC6H4, g] and [Pd(κ2‐acac)(κ2‐NN′)][BF4] toward the nucleophilic reagent acetylacetonate (acac) has been explored. This reaction starts with nucleophilic attack on the cabonylic atom of the coordinated imine generating a new C–C bond and an amido complex. The next step in the reaction is proton migration leading to the protonation of the amido nitrogen, and finally replacement of this coordinated nitrogen atom with an anionic carbon atom. The final consequence of the nucleophilic attack on the coordinated imines is the formation of new complexes and the generation of new C–C and C–Pd bonds.

The reaction of the complex [Pd(κ3-NN′O)CH3] [NN′O = 2-Py-CH=N-(o-C6H4)O–] (1a) with nitromethane affords a mixture of 2a and 2b. In this reaction methylation of the ligand takes place. These complexes display exchange equilibrium in solution. The methylation reaction is proposed to occur through nucleophilic attack of the nitronate anion on the iminic carbon atom. DFT calculations are presented in order to support a part of the mechanism and these calculations show that the participation of water in the migration of a proton is important. The crystal structure of the derivative productafter spontaneous release of methane (3) is presented as well. Attempts to extend the reaction to other nitroalkanes resulted in a mixture of complexes that was not easy to characterize. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Palladium complexes of formula [PdLCl]X (X = Cl 1, BF₄ 2) containing the pincer terdentate ligands L [L = 2,6-(ArNHCH₂)₂C₅H₃N (Ar = 4-MeC₆H₄ 1a and 2a, 4-MeOC₆H₄ 1b and 2b); L = 2,6-(4-MeC₆H₄NMeCH₂)₂C₅H₃N 1c and 2c] have been synthesised as cis–trans diastereomeric mixtures. The chiral nature of both amine arms of the pincer ligands accounts for the mixtures of isomers. All the complexes show dynamic behavior assigned to conversion between diastereomers. A ΔG‡₂₇₃ of 68.9 kJ mol−1 for 2a and a ΔG‡₂₇₃ of 65.8 kJ mol−1 for 2c were found through complete DNMR line-shape analysis of the –CH₂– signals in the 1H spectra. The cis–trans conversion requires the inversion of the pyramidal nitrogen of one of the coordinated amine arms. Two putative pathways have been proposed for such inversion: a) dissociation of one of the amine nitrogen atoms followed by inversion of the nitrogen atom and reformation of the nitrogen–palladium bond and b) deprotonation of one of the N–H bond leading to an amido intermediate which can reprotonate on the other side. Experimental evidence is given for both mechanisms. The structural characterization of 1a is described.

The synthesis and structure of a unique asymmetric folded binuclear anionic complex of palladium containing two different hydrazonato ligands and showing a Pd−N(amido) bond cis to a Pd−C(aryl) bond is described.