C(sp)-H, S-H, and Sn-H Bond Activation with a Cobalt(I) Pincer Complex

This study focuses on the stoichiometric reactions of {2,6-(iPr2PO)2C6H3}Co(PMe3)2 with terminal alkynes, thiols, and tin hydrides as part of an effort to develop catalytic, two-electron processes with cobalt. This specific Co(I) pincer complex proves to be effective for cleaving the C(sp)-H, S-H, and Sn-H bonds to give oxidative addition products with the general formula {2,6-(iPr2PO)2C6H3}CoHX(PMe3) (X = alkynyl, thiolate, and stannyl groups) along with the free PMe3. These reactions typically reach completion when the substituents on acetylene, sulfur, and tin are electron-withdrawing groups (e.g., phenyl, pyridyl, and alkenyl groups). In contrast, alkyl-substituted acetylenes, 1-pentanethiol, and tributyltin hydride are partially converted due to the equilibria with the corresponding oxidative addition products. The Co(I) pincer complex is not a hydrothiolation catalyst but capable of catalyzing the hydrostannation of terminal alkynes with Ph3SnH to produce β-(Z)-alkenylstannanes selectively.

Selective Transfer Semihydrogenation of Alkynes Catalyzed by an Iron PCP Pincer Alkyl Complex

Two bench-stable Fe(II) alkyl complexes [Fe(κ3PCP-PCP-iPr)(CO)2(R)] (R = CH2CH2CH3, CH3) were obtained by the treatment of [Fe(κ3PCP-PCP-iPr)(CO)2(H)] with NaNH2 and subsequent addition of CH3CH2CH2Br and CH3I, respectively. The reaction proceeds via the anionic Fe(0) intermediate Na[Fe(κ3PCP-PCP-iPr)(CO)2]. The catalytic performance of both alkyl complexes was investigated for the transfer hydrogenation of terminal and internal alkynes utilizing PhSiH3 and iPrOH as a hydrogen source. Precatalyst activation is initiated by migration of the alkyl ligand to the carbonyl C atom of an adjacent CO ligand. In agreement with previous findings, the rate of alkyl migration follows the order nPr > Me. Accordingly, [Fe(κ3PCP-PCP-iPr)(CO)2(CH2CH2CH3)] is the more active catalyst. The reaction takes place at 25 °C with a catalyst loading of 0.5 mol%. There was no overhydrogenation, and in the case of internal alkynes, exclusively, Z-alkenes are formed. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups including halides, nitriles, unprotected amines, and heterocycles. Mechanistic investigations including deuterium labeling studies and DFT calculations were undertaken to provide a reasonable reaction mechanism.

Solvothermal synthesis of cobalt PCP pincer complexes from [Co2(CO)8]

Treatment of [Co2(CO)8] with the ipso-substituted P(C-X)PY ligands (X = Br, Cl; R = iPr, tBu) bearing Y = NH and CH2 linkers under solvothermal conditions affords the five-coordinate Co(I) and Co(III) complexes [CoI(PCPY-R)(CO)2] and [CoIII(PCPY-R)X2]. The later are paramagnetic exhibiting a solution magnetic moment in the range of 3.0-3.3 μB which is consistent with a d6 intermediate spin system corresponding to two unpaired electrons. In the case of P(C-X)PY ligands (X = Br, Cl; R = tBu; Y = NH) the formation of the square planar Co(II) complex [Co(PCPNH-tBu)X] was favored. This complex gives rise to a magnetic moment of 1.8 μB being consistent with a d7 low spin system corresponding to one unpaired electron. All complexes are characterized by means of spectroscopic techniques (NMR, IR), HR-MS. Representative complexes were also characterized by X-ray crystallography.

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Supplementary Information

The online version contains supplementary material available at 10.1007/s00706-023-03123-x.

Applications of iron pincer complexes in hydrosilylation reactions

Due to its abundance, low cost and low toxicity, the first-row transition metal, iron is widely preferred as a catalyst in organic synthesis. The only drawback of lower selectivity due to high reactivity and low stability of the metal centre is tuned by using pincer ligands of different types. The different iron pincer complexes thus prepared are extensively used in catalyzing different types of organic reactions with great selectivity and functional group tolerance under moderate reaction conditions. In this review, we focus on the applications of iron pincer complexes in hydrosilylation reactions, especially the hydrosilylation of carbonyl derivatives and alkene/alkynes.

Iron pincer complexes are efficient in catalyzing various organic reactions with excellent selectivity and functional group tolerance at moderate reaction conditions. This review focuses on the applications of iron pincer complexes in hydrosilylation reactions.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Emergence and Applications of Base Metals (Fe, Co, and Ni) in Hydroboration and Hydrosilylation

Base metal catalysis offers an alternative to reactions, which were once dominated by precious metals in hydrofunctionalization reactions. This review article details the development of some base metals (Fe, Co, and Ni) in the hydroboration and hydrosilylation reactions concomitant with a brief overview of recent advances in the field. Applications of both commercially available metal salts and well-defined metal complexes in catalysis and opportunities to further advance the field is discussed as well.

POCN Ni(ii) pincer complexes: synthesis, characterization and evaluation of catalytic hydrosilylation and hydroboration activities

(POCN)Ni(ii) complexes were found to mediate a variety of carbonyl hydroboration reactions, including chemoselective hydroboration of benzaldehyde and hydroborative reduction of amides.

Phosphine-assisted C–H bond activation in Schiff bases and formation of novel organo cobalt complexes bearing Schiff base ligands

The sp² C–H bond activation of the HCN moiety in diphenylphosphino benzalimines was realized using CoMe(PMe₃)₄ with the elimination of methane.

Synthesis and catalytic application of [PPP]-pincer iron, nickel and cobalt complexes for the hydrosilylation of aldehydes and ketones

The catalytic performance of [PPP]-pincer hydrido Fe, Ni and Co complexes for the hydrosilylation of aldehydes and ketones was explored.

Synthesis of silyl iron hydride via Si–H activation and its dual catalytic application in the hydrosilylation of carbonyl compounds and dehydration of benzamides

A silyl iron hydride as a dual catalyst was synthesized for the reduction of carbonyl compounds and the dehydration of amides.

Non-precious metal complexes with an anionic PCP pincer architecture

This perspective article provides an overview of the advancements in the field of non-precious metal complexes featuring anionic PCP pincer ligands with the inclusion of aliphatic systems. It covers research from the beginning in 1976 until late 2015 and provides a summary of key developments in this area, which is, to date, limited to the metals nickel, cobalt, iron, and molybdenum. While the research in nickel PCP complexes is already quite extensive, the chemistry of cobalt, iron, and molybdenum PCP complexes is comparatively sparse. With other non-precious metals such as copper, manganese, chromium or vanadium no PCP complexes are known as yet. In the case of nickel PCP complexes already many catalytic applications such as Suzuki-Miyaura coupling, C-S cross coupling, Kharasch and Michael additions, hydrosilylation of aldehydes and ketones, cyanomethylation of aldehydes, and hydroamination of nitriles were reported. While iron PCP complexes were found to be active catalysts for the hydrosilylation of aldehydes and ketones as well as the dehydrogenation of ammonia-borane, cobalt PCP complexes were not applied to any catalytic reactions. Surprisingly, only one molybdenum PCP complex is reported, which was capable of cleaving dinitrogen to give a nitride complex. This perspective underlines that the combination of cheap and abundant metals such as nickel, cobalt, and iron with PCP pincer ligands may result in the development of novel, versatile, and efficient catalysts for atom-efficient catalytic reactions.

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

An efficient method based on the simple metalating reagent Fe(CO)₅ leads to the synthesis of cyclometalled PCP iron carbonyl pincer complexes which are active catalytic precursors for the selective dehydrogenative borylation of styrene.