The reaction of CO 2 with Et 3 SiH catalyzed by the nickel complex [(dippe)Ni(μ-H)] 2 (1) afforded the reduction products Et 3 SiOCH 2 OSiEt 3 (12%), Et 3 SiOCH 3 (3%), and CO, which were characterized by standard spectroscopic methods. Part of the generated CO was found as the complex [(dippe)Ni(CO)] 2 (2), which was characterized by single-crystal Xray diffraction. When the same reaction was carried out in the presence of a Lewis acid, such as Et 3 B, the hydrosilylation of CO 2 efficiently proceeded to give the silyl formate (Et 3 SiOC(O)H) in high yields (85−89%), at 80 °C for 1 h. Further reactivity of the silyl formate to yield formic acid, formamides, and alkyl formates was also investigated.

The selective catalytic hydrogenation and cyclization of levulinic acid (LA) into valuable γ-valerolactone (GVL) catalyzed by different palladium compounds was achieved in water under mild conditions with high yields. Either formic acid (FA) or molecular hydrogen (H 2 ) was used as a hydrogen source. The precatalyst [(dtbpe)PdCl 2 ] (dtbpe = 1,2-(bis-di-tert-butylphosphino)ethane) (1) was highly active in the processes of LA hydrogenation (TON of 2100 and TOF of 2100 h −1 ) and in the dehydrogenation of formic acid to produce H 2 and carbon dioxide. The catalytically active complexes [(dtbpe)Pd(H)Cl)] ( 2) and [(dtbpe) 2 Pd 2 (μ-H) 3 ] + (3) and the catalytically inactive complex [(dtbpe) 2 Pd 2 (μ-H) (μ-CO)] + (4) all formed in situ and were identified as species resulting from FA decomposition.

Copper complexes with N3S donors mimic the CuM site of copper monooxygenases and react with O2 affording side-on cupric-superoxo complexes capable of H-abstraction from dihydroanthracene and THF. Spectroscopic and DFT data of the Cu-superoxos support a spin triplet ground state for the side-on complexes, as well as a hemilabile thioether.

Homogeneous catalytic hydrodefluorination (HDF) of fluoroaromatics under thermal conditions was achieved using nickel(0) compounds of the type [(dippe)Ni(η(2)-C6F6-nHn)] where n = 0-2, as the catalytic precursors. These complexes were prepared in situ by reacting the compound [(dippe)Ni(μ-H)]2 with the respective fluoroaromatic substrate. HDF seems to occur homogeneously, as tested by mercury drop experiments, producing the hydrodefluorinated products. However, despite previous findings by other groups, we found that these HDF reactions were actually the result of direct reaction of the alkylphosphine with the fluoroaromatic substrate. This metal- and silane-free system is the first reported example of a phosphine being able to hydrodefluorinate on its own.

The transfer hydrogenation of diphenylacetylene to yield cis- and trans-stilbenes was achieved using a variety of amines as hydrogen donors and the complex 1 ([(dippe)Ni(μ-H)]2) in catalytic amounts (0.5% mol). The use of nucleophilic amines such as pyrrolidine in neat conditions afforded the hydroamination of diphenylacetylene, in moderate to high yields. Cyclization of 2-ethynylaniline also was carried out under similar conditions, with 1 in catalytic amounts, but in low yield, mainly due to the formation of homocoupling products of the starting material. The hydrogenation of diphenylacetylene by using other nitrogenated compounds such as aromatic N-heterocycles was addressed to give a metal-mediated process, using 1 in stoichiometric amounts.

A method using CO2 and PhSiH3 for the methylation of primary and secondary aliphatic amines catalyzed by Ni (0) complexes was developed, selectively producing the monomethylated products in moderate to good yields. For that purpose, two catalysts were used: [(dippe)Ni(μ-H)]2 and the commercially available Ni(COD)2/dcype, both of which were rather efficient in this process. With a slight experimental modification, the reaction allowed the production of monomethylated ureas in good yields by using low amounts of PhSiH3. On the basis of the experimental results, we propose a possible reaction mechanism for the formation of the new C–N bond.

Complexes of the type [{(dippe)Ni} n (η2-Cα,Cβ-1,4-dien-3-one)] (dippe = 1,2-bis(diisopropylphosphino)ethane); n= 1, 2; enone = aromatic 1,4-pentadien-3-ones) were synthesized. The “[(dippe)Ni]” moiety derived from [(dippe)Ni(μ-H)]2 η2-coordinated to the C,C double bonds of the corresponding α,β-unsaturated enone and was fully characterized using a variety of spectroscopic techniques, for instance, single-crystal X-ray diffraction, nuclear magnetic resonance (NMR), and mass spectrometry. The complexes were assessed in a catalytic transfer hydrogenation process using methanol (CH3OH) as a hydrogen donor. This alcohol turned out to be a very efficient reducing and alkylating agent of 1,4-pentadien-3-ones, under neat conditions. The current methodology allowed the selective reduction of CC bonds in α,β-unsaturated enones to yield enones and saturated ketones by a homogeneous catalytic pathway, whereas by a heterogeneous pathway, the process leads to the formation of mono- and dimethylated ketones. In the latter case, the occurrence of nickel nanoparticles in the reaction media was found to participate in the catalytic alkylation of such dienones.

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