Reversible Ligand Protonation of a Mn(I) PCP Pincer Complex To Afford a Complex with an η2-Caryl–H Agostic Bond

Is Mn(I) More Promising Than Fe(II)-A Comparison of Mn vs Fe Complexes for Olefin Metathesis

Olefin metathesis is one of the most significant transformations in organic chemistry and is an excellent example for efficient homogeneous catalysis. Although most currently used catalysts are primarily based on 4d and 5d metals, cycloaddition and cycloreversion reactions can also be attributed to first-row transition metals, such as Fe. Surprisingly, the potential of Mn(I)-based catalysts for olefin metathesis has been unexplored despite their prominence in homogeneous catalysis and their diagonal relationship to Ru(II). In the present study, we have investigated the prospective capabilities of Mn complexes for cycloaddition and reversion reactions using density functional theory. Therefore, we have initially compared the literature known iron model systems and their isoelectronic Mn counterparts regarding their reactivity and electronic structure. Next, we constructed potential Mn complexes derived from synthetically accessible species, including carbonyl ligands and obeying octahedral geometry. Based on thermodynamic parameters and the calculation of electronic descriptors, we were able to validate the isodiagonal relationship. Our study serves as guidance for the experimental chemist.

Base metal complexes featuring a new pyrazole-derived PCP pincer ligand

A new pyrazole-derived PCP pincer ligand featuring a 1-methylpyrazole backbone tethered to two di(isopropyl)phosphine moieties via phenylene spacers (P(CH)P-iPr) was prepared. When reacting the ligand with group six carbonyl complexes [M(CO)6] (M = Cr, Mo, W) at 130 °C, complexes of the type [M(κ2PN-PCP-iPr)(CO)4] were obtained featuring a κ2P,N-bound ligand with a pendant phosphine arm. Upon an increase of the reaction temperature to 150 °C, in the case of molybdenum, the formation of the complex [Mo(κ3PCP-PCP-iPr)(CO)3] was observed featuring a weak Mo-C bond. DFT calculations reveal that there is no agostic η2-C-H interaction. Treatment of [Mn2(CO)10], [Fe2(CO)9], [Co2(CO)8] and [Ni(COD)2] afforded complexes [Mn(κ3PCP-PCP-iPr)(CO)3], [Fe(κ3PCP-PCP-iPr)(H)(CO)2], [Co(κ3PCP-PCP-iPr)(CO)2] and [Ni(κ3PCP-PCP-iPr)(H)], respectively, where the PCP ligand is coordinated in the typical meridional κ3-fashion. Postfunctionalization of the anionic PCP pincer ligand was possible via N-methylation of the second nitrogen atom of the pyrazole unit with the oxonium salt [Me3O]BF4. Treatment of [Mn(κ3PCP-PCP-iPr)(CO)3] and [Fe(κ3PCP-PCP-iPr)(H)(CO)2] with [Me3O]BF4 resulted in the formation of the cationic complexes [Mn(κ3PCP-PCPMe-iPr)(CO)3]+ and [Fe(κ3PCP-PCPMe-iPr)(Cl)(CO)2]+. In the case of the latter, the chloride ligand seems to originate from the solvent CH2Cl2 undergoing a hydride chloride exchange. All complexes were characterized by means of 1H, 13C{1H}, and 31P{1H} NMR spectroscopy, IR spectroscopy and HR-MS. In addition, the structures of representative complexes were determined by X-ray crystallography.

Insights into the Fluxional Processes of Monomethylcyclohexenyl Manganese Tricarbonyl

Multiple fluxional processes of 6-monomethylcyclohexenylmanganese tricarbonyl [(6-MeC₆H₈)Mn(CO)₃, complex 1] and 5-monomethylcyclohexenylmanganese tricarbonyl [(5-MeC₆H₈)Mn(CO)₃, complex 2] have been explored using density functional theory (DFT) computations. The contributions of four agostomers-1, 2, 3, and 4-to the (MeC₆H₈)Mn(CO)₃ exchange processes were revealed. The computational results demonstrated that the 1, 2-agostic isomerization only occurred via the η⁴-diene hydride transition state (TS-1-2, 14.0 kcal/mol), which is consistent with the experimentally proposed high-energy exchange process (16.0 kcal/mol). Excellent agreement is observed (R² = 0.9862) when comparing the computed and experimentally observed variable temperature ¹H NMR chemical shifts. With these results, important insights into the role of agostic interaction in the homogeneous catalysis process could be made, especially with regard to transition metal catalyzed C-H activation.

Manganese and iron PCP pincer complexes - the influence of sterics on structure and reactivity

The syntheses of various manganese and iron PCP pincer complexes via a solvothermal oxidative addition methodology is described. Upon reacting [Mn₂(CO)₁₀] with the ligands (P(C-Br)P^CH₂-iPr) (1a) and (P(C-Br)P^O-iPr) (1b), Mn(I) PCP pincer complexes [Mn(PCP^CH₂-iPr)(CO)₃] (2a) and [Mn(-PCP^O-iPr)(CO)₃] (2b) were obtained. Protonation of 2a with HBF₄·Et₂O led to the formation of [Mn(κ³P,CH,P-P(CH)P^CH₂-iPr)(CO)₃]BF₄ (3) featuring an η²-C_aryl-H agostic bond. The solvothermal reaction of 1a with [Fe₂(CO)₉] afforded the Fe(II) PCP pincer complex [Fe(PCP^CH₂-iPr)(CO)₂Br] (4). Treatment of 4 with Li[HBEt₃] afforded the Fe(I) complex [Fe(PCP^CH₂-iPr)(CO)₂] (5a). When using the sterically more demanding ligands (P(C-Br)P^CH₂-tBu) (1c) and (P(C-Br)P^O-tBu)(1d) striking differences in reactivity were observed. While neither 1c nor 1d showed any reactivity towards [Mn₂(CO)₁₀], the reaction with [Fe₂(CO)₉] and [Fe(CO)₅] led to the formation of the Fe(I) complexes [Fe(PCP^CH₂-tBu)(CO)₂] (5b) and [Fe(PCP^O-tBu)(CO)₂] (5c). X-ray structures of representative complexes are provided.

Synthesis, Characterization, and Catalytic Reactivity of {CoNO}8 PCP Pincer Complexes

The reaction of coordinatively unsaturated Co(II) PCP pincer complexes with nitric oxide leads to the formation of new, air-stable, diamagnetic mono nitrosyl compounds. The synthesis and characterization of five- and four-coordinate Co(III) and Co(I) nitrosyl pincer complexes based on three different ligand scaffolds is described. Passing NO through a solution of [Co(PCP^NMe-iPr)Cl], [Co(PCP^O-iPr)Cl] or [Co(PCP^CH2-iPr)Br] led to the formation of the low-spin complex [Co(PCP-iPr)(NO)X] with a strongly bent NO ligand. Treatment of the latter species with (X = Cl, Br) AgBF₄ led to chloride abstraction and formation of cationic square-planar Co(I) complexes of the type [Co(PCP-iPr)(NO)]⁺ featuring a linear NO group. This reaction could be viewed as a formal two electron reduction of the metal center by the NO radical from Co(III) to Co(I), if NO is counted as NO⁺. Hence, these systems can be described as {CoNO}⁸ according to the Enemark–Feltham convention. X-ray structures, spectroscopic and electrochemical data of all nitrosyl complexes are presented. Preliminary studies show that [Co(PCP^NMe-iPr)(NO)]⁺ catalyzes efficiently the reductive hydroboration of nitriles with pinacolborane (HBpin) forming an intermediate {CoNO}⁸ hydride species.

Bisphosphine phenol and phenolate complexes of Mn(i): manganese(i) catalyzed Tishchenko reaction

We synthesized new organomanganese complexes using the phenolic “pincer” type ligand H-POP.