Synthesis and characterization of heptacoordinated molybdenum(II) complexes supported with 2,6-bis(pyrazol-3-yl)pyridine (bpp) ligands.

Functional pincer ligands that engage in metal-ligand cooperativity and/or are capable of redox non-innocence have found a great deal of success in catalysis. These two properties may be found in metal complexes of the 2,6-bis(pyrazol-3-yl)pyridine (bpp) ligands. With this goal in mind, we have attempted the coordination of 2,6-bis(5-trifluoromethylpyrazol-3-yl)pyridine (LCF3) and its Bu analogue 2,6-bis(5--butylpyrazol-3-yl)pyridine (LtBu) to Mo(0) by reactions with mixed phosphine/carbonyl complexes [Mo(CO)(MeCN)(PMePh)] 1-3 (1 ≤ ≤ 3).

Photoluminescent ruthenium(II) bipyridyl complexes containing phosphonium ylide ligands.

A P-ylide Ru(II) bipyridyl complex was readily synthetized and fully characterized, constituing one of the rare examples of photoluminescent metal ylide complexes. Its photophysical and redox properties have been compared with those of related NHC and cyclometalated Ru complexes and exploited in visible-light photocatalyzed SET and EnT processes.

Impact of the Methylene Bridge Substitution in Chelating NHC-Phosphine Mn(I) Catalyst for Ketone Hydrogenation.

Systematic modification of the chelating NHC-phosphine ligand (NHC = N-heterocyclic carbene) in highly efficient ketone hydrogenation Mn(I) catalyst fac-[(PhPCHNHC)Mn(CO)Br] has been performed and the catalytic activity of the resulting complexes was evaluated using acetophenone as a benchmark substrate. While the variation of phosphine and NHC moieties led to inferior results than for a parent system, the incorporation of a phenyl substituent into the ligand methylene bridge improved catalytic performance by ca. 3 times providing maximal TON values in the range of 15000-20000.

Iridium(I) complexes with bidentate NHC ligands as catalysts for dehydrogenative directed C-H silylation.

A series of (NHC)(cod)Ir(I) complexes bearing NHC-carboxylate ligands were efficiently synthesized and fully characterized. Their solid-state structures confirmed the bidentate coordination mode of these LX-type NHC ligands. These unprecedented iridium(I) complexes demonstrated efficient catalytic activities in dehydrogenative directed C-H silylation of arenes, and allowed for excellent -selectivity control with aromatic silylating agents.

Hydrosilylation Reactions Catalyzed by Rhenium.

Hydrosilylation is an important process, not only in the silicon industry to produce silicon polymers, but also in fine chemistry. In this review, the development of rhenium-based catalysts for the hydrosilylation of unsaturated bonds in carbonyl-, cyano-, nitro-, carboxylic acid derivatives and alkenes is summarized. Mechanisms of rhenium-catalyzed hydrosilylation are discussed.

Experimental and Theoretical Insights into the Electronic Properties of Anionic N-Heterocyclic Dicarbenes through the Rational Synthesis of Their Transition Metal Complexes.

The lithiation of the NHC ligand backbone in Cp(CO)Mn(IMes) followed by transmetalation on the C4 carbenic position with Cp(CO)FeI led to the heterobimetallic complex Cp(CO)Mn(IMes)Fe(CO)Cp bearing the anionic ditopic imidazol-2,4-diylidene IMes ligand. Subsequent treatment of the later with TfOH induced a selective decoordination of the [Cp(CO)Mn] fragment to form the cationic abnormal NHC complex [Cp(CO)Fe(IMes)](OTf), which was further derivatized to the bis(iron) IMes complex [Cp(CO)Fe(IMes)Fe(CO)Cp](OTf) by reaction with AmOK and Cp(CO)FeI. The effect of the metalation at the C4 or C2 position on the imidazole ring on the electronic donation properties of the associated C2 and C4 carbenic centers in the IMes ligand was quantified through systematic experimental and theoretical studies of IMes, IMes, and IMes complexes.

Manganese and rhenium-catalyzed selective reduction of esters to aldehydes with hydrosilanes.

The selective reduction of esters to aldehydes, via the formation of stable alkyl silyl acetals, was, for the first time, achieved with both manganese, -Mn(CO)- and rhenium -Re(CO)- catalysts in the presence of triethylsilane as reductant. These two methods provide a direct access to a large variety of aliphatic and aromatic alkyl silyl acetals (30 examples) and to the corresponding aldehydes (13 examples) upon hydrolysis. The reactions proceeded in excellent yields and high selectivity at room temperature under photo-irradiation conditions (LED, 365 nm, 40 W, 9 h).

Bis[diphenylphosphino]methane and its bridge-substituted analogues as chemically non-innocent ligands for H activation.

Deprotonation of fac-[(κ2P,P-Ph2PCH(R)PPh2)Mn(CO)3Br] (R = H, Me, Ph) produces the corresponding diphosphinomethanide derivatives fac-[(κ3P,C,P-Ph2PC(R)PPh2)Mn(CO)3], which are prone to activate H2 to form the hydride complexes fac-[(κ2P,P-Ph2PCH(R)PPh2)Mn(CO)3H]. The substitution of the dppm bridge improves dramatically the reaction efficiency and this was rationalized by DFT calculations.

Rhenium-Catalyzed Reduction of Carboxylic Acids with Hydrosilanes.

Re(CO) efficiently catalyzes the direct reduction of various carboxylic acids under mild conditions (rt, irradiation 350 or 395 nm). While aliphatic carboxylic acids were readily converted to the corresponding disilylacetals with low catalyst loading (0.5 mol %) in the presence of EtSiH (2.

Phosphine-NHC Manganese Hydrogenation Catalyst Exhibiting a Non-Classical Metal-Ligand Cooperative H Activation Mode.

Deprotonation of the Mn NHC-phosphine complex fac-[MnBr(CO) (κ P,C-Ph PCH NHC)] (2) under a H atmosphere readily gives the hydride fac-[MnH(CO) (κ P,C-Ph PCH NHC)] (3) via the intermediacy of the highly reactive 18-e NHC-phosphinomethanide complex fac-[Mn(CO) (κ P,C,C-Ph PCHNHC)] (6 a). DFT calculations revealed that the preferred reaction mechanism involves the unsaturated 16-e mangana-substituted phosphonium ylide complex fac-[Mn(CO) (κ P,C-Ph P=CHNHC)] (6 b) as key intermediate able to activate H via a non-classical mode of metal-ligand cooperation implying a formal λ -P-λ -P phosphorus valence change. Complex 2 is shown to be one of the most efficient pre-catalysts for ketone hydrogenation in the Mn series reported to date (TON up to 6200).

Synthesis of Quinolines Through Acceptorless Dehydrogenative Coupling Catalyzed by Rhenium PN(H)P Complexes.

A practical and sustainable synthesis of substituted quinolines was achieved through the annulation of 2-aminobenzyl alcohol with various secondary alcohols, ketones, aldehydes, or nitriles, under hydrogen-borrowing conditions. Under the catalysis of well-defined rhenium complexes bearing tridentate diphosphinoamino ligands, the reaction proceeded efficiently (31 examples were isolated with yields up to 96 %) affording a variety of quinoline derivatives.

Manganese catalyzed α-methylation of ketones with methanol as a C1 source.

The direct α-methylation of ketones with methanol under hydrogen borrowing conditions using a well-defined manganese PN3P complex as a pre-catalyst was, for the first time, achieved. The reactions typically proceed at 120 °C for 20 h with 3 mol% pre-catalyst loading and in the presence of NaOtBu (50 mol%) as base. The scope of the reaction was extended to the α-methylation of esters.

Manganese catalyzed reductive amination of aldehydes using hydrogen as a reductant.

A one-pot two-step procedure was developed for the alkylation of amines via reductive amination of aldehydes using molecular dihydrogen as a reductant in the presence of a manganese pyridinyl-phosphine complex as a pre-catalyst. After the initial condensation step, the reduction of imines formed in situ is performed under mild conditions (50-100 °C) with 2 mol% of catalyst and 5 mol% of tBuOK under 50 bar of hydrogen. Excellent yields (>90%) were obtained for a large combination of aldehydes and amines (40 examples), including aliphatic aldehydes and amino-alcohols.

Transfer Hydrogenation of Carbonyl Derivatives Catalyzed by an Inexpensive Phosphine-Free Manganese Precatalyst.

A very simple and inexpensive catalytic system based on abundant manganese as transition metal and on an inexpensive phosphine-free bidendate ligand, 2-(aminomethyl)pyridine, has been developed for the reduction of a large variety of carbonyl derivatives with 2-propanol as hydrogen donor. Remarkably, the reaction proceeds at room temperature with low catalyst loading (down to 0.1 mol %) and exhibits a good tolerance toward functional groups.

Ising-type Magnetic Anisotropy and Slow Relaxation of the Magnetization in Four-Coordinate Amido-Pyridine Fe Complexes.

A family of four-coordinate Fe complexes formed with N,N'-chelating amido-pyridine ligands was synthesized, and their magnetic properties were investigated. These distorted tetrahedral complexes exhibit significant magnetic anisotropy with zero-field splitting parameter D ranging between -17 and -12 cm. Ab initio calculations enabled identification of the structural factors that control the nature of the magnetic anisotropy and the rationalization of the variation of D in these complexes.

Efficient and selective N-alkylation of amines with alcohols catalysed by manganese pincer complexes.

Borrowing hydrogen (or hydrogen autotransfer) reactions represent straightforward and sustainable C-N bond-forming processes. In general, precious metal-based catalysts are employed for this effective transformation. In recent years, the use of earth abundant and cheap non-noble metal catalysts for this process attracted considerable attention in the scientific community.

Direct synthesis of dicarbonyl PCP-iron hydride complexes and catalytic dehydrogenative borylation of styrene.

A new and efficient method based on the simple metalating reagent Fe(CO)5 has been developed for the straightforward synthesis of well defined cyclometalled PCP iron carbonyl pincer complexes. The reaction proceeds cleanly under mild conditions at 30 °C and UV irradiation. Four hydride pincer complexes are synthesized and fully characterized as well as an intermediate dinuclear species.

Iron-Catalyzed α-Alkylation of Ketones with Alcohols.

A general and benign iron-catalyzed α-alkylation reaction of ketones with primary alcohols has been developed. The key to success of the reaction is the use of a Knölker-type complex as catalyst (2 mol %) in the presence of Cs2 CO3 as base (10 mol %) under hydrogen-borrowing conditions. Using 2-aminobenzyl alcohol as alkylation reagent allows for the "green" synthesis of quinoline derivatives.

Iron-catalyzed C-H borylation of arenes.

Well-defined iron bis(diphosphine) complexes are active catalysts for the dehydrogenative C-H borylation of aromatic and heteroaromatic derivatives with pinacolborane. The corresponding borylated compounds were isolated in moderate to good yields (25-73%) with a 5 mol% catalyst loading under UV irradiation (350 nm) at room temperature. Stoichiometric reactivity studies and isolation of an original trans-hydrido(boryl)iron complex, Fe(H)(Bpin)(dmpe)2, allowed us to propose a mechanism showing the role of some key catalytic species.

Methylation of secondary amines with dialkyl carbonates and hydrosilanes catalysed by iron complexes.

Methylation of secondary amines was achieved using dimethyl carbonate or diethyl carbonate as the C1 source under the catalysis of well-defined half-sandwich iron complexes bearing an N-heterocyclic carbene ligand. The reaction proceeded under mild conditions in the presence of hydrosilanes as the reductants, and the amines were obtained with good to excellent isolated yields.

Selective reduction of carboxylic acids to aldehydes through manganese catalysed hydrosilylation.

The direct reduction of carboxylic acids to disilylacetals was achieved through a manganese catalyzed hydrosilylation reaction in the presence of triethylsilane under mild conditions, at r.t. and under UV irradiation (350 nm).

Unexpected selectivity in ruthenium-catalyzed hydrosilylation of primary amides: synthesis of secondary amines.

Selective ruthenium-catalyzed reductive coupling of primary amides under hydrosilylation conditions is achieved using an one pot procedure. Using 3 equiv. of phenylsilane and [RuCl2(mesitylene)]2 (1-2 mol%) as the catalyst at 100 °C under neat conditions, secondary symmetric amines were obtained in good yields and with high chemoselectivities.

Selective switchable iron-catalyzed hydrosilylation of carboxylic acids.

Selective reduction of carboxylic acids either to aldehydes or alcohols is achieved using a one pot procedure based on iron-catalyzed hydrosilylations. Using phenylsilane and (COD)Fe(CO)(3) catalyst under UV-irradiation at rt, alcohols were obtained specifically in good yields, whereas aldehydes were selectively obtained using TMDS and (t-PBO)Fe(CO)(3) catalyst under thermal activation.