Synthesis and Catalytic Reactivity of Cobalt Pincer Nitrosyl Hydride Complexes

Ligand-free MnBr2-Catalyzed Chemo- and Stereoselective Hydroboration of Terminal Alkynes

Developing simple and benign protocols for synthesizing alkenylboronates is crucial as they are synthetically valuable compounds in various organic transformations. In this work, we report a straightforward ligand-free protocol for synthesizing alkenylboronates via atom-economical hydroboration of alkynes with HBpin catalyzed by a manganese salt. The reaction shows a high level of chemo and regioselectivity for the terminal alkynes and exclusively produces E-selective alkenylboronates. The hydroboration scope is vast, with the resilience of a range of synthetically beneficial functionalities, such as halides, ether, alkenyl, silyl and thiophenyl groups. This reaction proceeds through the involvement of a metal-hydride intermediate. The developed alkenylboronate can be smoothly converted to useful C-C, C-N and C-I bond-forming reactions.

Exploring the Effect of Pincer Rigidity on Oxidative Addition Reactions with Cobalt(I) Complexes

Cobalt complexes containing the 2,6-diaminopyridine-substituted PNP pincer (iPrPNMeNP = 2,6-(iPr2PNMe)2(C5H3N)) were synthesized. A combination of solid-state structures and investigation of the cobalt(I)/(II) redox potential established a relatively rigid and electron-donating chelating ligand as compared to iPrPNP (iPrPNP = 2,6-(iPr2PCH2)2(C5H3N)). Based on a buried volume analysis, the two pincer ligands are sterically indistinguishable. Nearly planar, diamagnetic, four-coordinate complexes were observed independent of the field strength (chloride, alkyl, aryl) of the fourth ligand completing the coordination sphere of the metal. Computational studies supported a higher barrier for C-H oxidative addition, largely a result of the increased rigidity of the pincer. The increased oxidative addition barrier resulted in stabilization of (iPrPNMeNP)Co(I) complexes, enabling the characterization of the cobalt boryl and the cobalt hydride dimer by X-ray crystallography. Moreover, (iPrPNMeNP)CoMe served as an efficient precatalyst for alkene hydroboration likely because of the reduced propensity to undergo oxidative addition, demonstrating that reactivity and catalytic performance can be tuned by rigidity of pincer ligands.

Solvent-free hydroboration of alkenes and alkynes catalyzed by rhodium-ruthenium nanoparticles on carbon nanotubes

A heterogeneous catalyst consisting of bimetallic rhodium-ruthenium particles immobilized on carbon nanotubes was used in the hydroboration reaction and proved highly effective for a variety of alkenes and alkynes. The reactions were carried out with low catalytic loadings (0.04 mol%), under solvent-free conditions, and at room temperature. In addition, to demonstrate its recyclability, the catalyst was recovered by a simple centrifugation process and reused over 5 consecutive cycles without losing any activity.

Control over Selectivity in Alkene Hydrosilylation Catalyzed by Cobalt(III) Hydride Complexes

Two new bisphosphine [PCP] pincer cobalt(III) hydrides, [(L¹)Co(PMe₃)(H)(Cl)] (^L11, L¹ = 2,6-((Ph₂P)(Et)N)₂C₆H₃) and [(L²)Co(PMe₃)(H)(Cl)] (^L21, L² = 2,6-((ⁱPr₂P)(Et)N)₂C₆H₃), as well as one new bissilylene [SiCSi] pincer cobalt(III) hydride, [(L³)Co(PMe₃)(H)(Cl)] (^L31, L³ = 1,3-((PhC(^tBuN)₂Si)(Et)N)₂C₆H₃), were synthesized by reaction of the corresponding protic [PCP] or [SiCSi] pincer ligands L¹H, L²H, and L³H with CoCl(PMe₃)₃. Despite the similarities in the ligand scaffolds, the three cobalt(III) hydrides show remarkably different performance as catalysts in alkene hydrosilylation. Among the PCP pincer complexes, ^L11 has higher catalytic activity than complex ^L21, and both catalysts afford anti-Markovnikov selectivity for both aliphatic and aromatic alkenes. In contrast, the catalytic activity for alkene hydrosilylation of silylene complex ^L31 is comparable to phosphine complex ^L11, but a dependence of regioselectivity on the substrates was observed: While aliphatic alkenes are converted in an anti-Markovnikov fashion, the hydrosilylation of aromatic alkenes affords Markovnikov products. The substrate scope was explored with 28 examples. Additional experiments were conducted to elucidate these mechanisms of hydrosilylation. The synthesis of cobalt(I) complex (L¹)Co(PMe₃)₂ (^L17) and its catalytic properties for alkene hydrosilylation allowed for the proposal of the mechanistic variations that occur in dependence of reaction conditions and substrates.

Hydroboration of Terminal Alkenes and trans-1,2-Diboration of Terminal Alkynes Catalyzed by a Manganese(I) Alkyl Complex

A Mn^I‐catalyzed hydroboration of terminal alkenes and a 1,2‐diboration of terminal alkynes with pinacolborane (HBPin) is described. For alkenes, anti‐Markovnikov hydroboration takes place; for alkynes the reaction proceeds with excellent trans‐1,2‐selectivity. The most active pre‐catalyst is bench‐stable alkyl bisphosphine Mn^I complex fac‐[Mn(dippe)(CO)₃(CH₂CH₂CH₃)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn–alkyl bond to yield an acyl intermediate, which undergoes B−H bond cleavage of HBPin (for alkenes) and rapid C−H bond cleavage (for alkynes), forming the active Mn^I boryl and acetylide catalysts [Mn(dippe)(CO)₂(BPin)] and [Mn(dippe)(CO)₂(C≡CR)], respectively. A broad variety of aromatic and aliphatic alkenes and alkynes was efficiently and selectively borylated. Mechanistic insights are provided based on experimental data and DFT calculations revealing that an acceptorless reaction is operating involving dihydrogen release.

Hydroboration of Terminal Alkenes and trans-1,2-Diboration of Terminal Alkynes Catalyzed by a Manganese(I) Alkyl Complex

A MnI-catalyzed hydroboration of terminal alkenes and a 1,2-diboration of terminal alkynes with pinacolborane (HBPin) is described. For alkenes, anti-Markovnikov hydroboration takes place; for alkynes the reaction proceeds with excellent trans-1,2-selectivity. The most active pre-catalyst is bench-stable alkyl bisphosphine MnI complex fac-[Mn(dippe)(CO)3(CH2CH2CH3)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate, which undergoes B-H bond cleavage of HBPin (for alkenes) and rapid C-H bond cleavage (for alkynes), forming the active MnI boryl and acetylide catalysts [Mn(dippe)(CO)2(BPin)] and [Mn(dippe)(CO)2(C≡CR)], respectively. A broad variety of aromatic and aliphatic alkenes and alkynes was efficiently and selectively borylated. Mechanistic insights are provided based on experimental data and DFT calculations revealing that an acceptorless reaction is operating involving dihydrogen release.

Bent and Linear {CoNO}8 Entities: Structure and Bonding in a Prototypic Class of Nitrosyls

Among the isoelectronic ligands CN^-, CO, and NO⁺, an oblique bonding to the metal is well-established for the nitrosyl ligand, with M-N-O angles down to ≈120°. In the last decades, the nitrosyl community got into the habit of addressing a bent-bonded nitrosyl ligand as ¹NO^-. Thus, because various redox forms of a nitrosyl ligand seem to exist, the ligand is considered to be "noninnocent" because of the obvious ambiguity of an oxidation state (OS) assignment of the ligand and metal. Among the bent-bonded species, the low-spin {CoNO}⁸ class is prototypic. From this class, some 20 new nitrosyl compounds, the X-ray structure determinations of which comply with strict quality criteria, were analyzed with respect to the OS issue. As a result, the effective OS method shows a low-spin d⁸ Co^I-NO⁺ couple instead of a negative OS of the ligand at the BP86/def2-TZVP (+D3, +CPCM with infinite permittivity) level of theory. The same holds for some new members of the linear subclass of {CoNO}⁸ compounds. For all compounds, a largely invariable "real" charge of ≈ -0.3 e was obtained from population analyses. All of these electron-rich d⁸ species strive to manage Pauli repulsion between the metal electrons and the lone pair at the nitrosyl's nitrogen atom, with the bending of the CoNO unit as the most frequent escape.