Chemical and redox non-innocence in low-valent molybdenum β diketonate complexes: novel pathways for CO2 and CS2 activation

The investigation of fundamental properties of low-valent molybdenum complexes bearing anionic ligands is crucial for elucidating the molybdenum's role in critical enzymatic systems involved in the transformation of small molecules, including the nitrogenase's iron molybdenum cofactor, FeMoco. The β-diketonate ligands in [Mo(acac)3] (acac = acetylacetonate), one of the earliest low-valent Mo complexes reported, provide a robust anionic platform to stabilize Mo in its +III oxidation state. This complex played a key role in demonstrating the potential of low-valent molybdenum for small molecule activation, serving as the starting material for the preparation of the first reported molybdenum dinitrogen complex. Surprisingly however, given this fact and the widespread use of β-diketonate ligands in coordination chemistry, only a very limited number of low-valent Mo β-diketonate complexes have been reported. To address this gap, we explored the redox behavior of homoleptic molybdenum tris-β-diketonate complexes, employing a tertiary butyl substituted diketonate ligand (dipivaloylmethanate, tBudiket) to isolate and fully characterize the corresponding Mo complexes across three consecutive oxidation states (+IV, +III, +II). We observed marked reactivity of the most reduced congener with heterocumulenes CE2 (E = O, S), yet with very distinct outcomes. Specifically, CO2 stoichiometrically carboxylates one of the β-diketonate ligands, while in the presence of excess CS2, catalytic reductive dimerization to tetrathiooxalate occurs. Through the isolation and characterization of reaction products and intermediates, we demonstrate that the observed reactivity results from the chemical non-innocence of the β-diketonate ligands, which facilitates the formation of a common ligand-bound intermediate, [Mo( tBudiket)2( tBudiket·CE2)]1- (E = O, S). The stability of this proposed intermediate dictates the specific reduction products observed, highlighting the relevance of the chemically non-innocent nature of β-diketonate ligands.

CS2/CO2 Utilization Using Mukaiyama Reagent as a (Thio)carbonylating Promoter: A Proof-of-Concept Study

We report on the reaction of ethylene-terminated heteroatoms (C₂X; X = N, O, and S) with CS₂/CO₂ using Mukaiyama reagent (2-chloro-1-methylpyridinium iodide, CMPI) as a promoter for the preparation of imidazolidin-2-one, oxazolidin-2-one, 1,3-dioxolan-2-one, 1,3-dithiolan-2-one, and their thione counterparts at ambient temperature and pressure. Spectroscopic measurements, viz., ¹H/¹³C nuclear magnetic resonance (NMR) and ex situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy methods verified the reaction of CS₂/CO₂ with the ethylene-based substrates and subsequently the formation of cyclic products. The experimental data indicated the formation of the enol-form of imidazolidin-2-one and oxazolidin-2-one, while the keto-form was obtained for their thione correspondents. Furthermore, density functional theory calculations revealed the stability of the keto- over the enol-form for all reactions and pointed out the solvent effect in stabilizing the latter.

In situ activation of green sorbents for CO2 capture upon end group backbiting

Thermolysis of a urethane end group was observed as a first time phenomenon during activation. This unzipping mechanism revealed a new amine tethering point producing a diamine-terminated oligourea ([10]-OU), acting as a green sorbent for CO₂ capturing. The oligomer backbites its end group to form propylene carbonate (PC), as proved by in situ TGA-MS, which can reflect the polymer performance by maximizing its capturing capacity. Cross polarization magic angle spinning (CP-MAS) NMR spectroscopy verified the formation of the proven ionic carbamate (1:2 mechanism) with a chemical shift at 161.7 ppm due to activation desorption at higher temperatures, viz., 100 °C (in vacuo) accompanied with bicarbonate ions (1:1 mechanism) with a peak centered at 164.9 ppm. Fortunately, the amines formed from in situ thermolysis explain the abnormal behavior (carbamates versus bicarbonates) of the prepared sample. Finally, ex situ ATR-FTIR proved the decomposition of urethanes, which can be confirmed by the disappearance of the pre-assigned peak centered at 1691 cm^-1. DFT calculations supported the thermolysis of the urethane end group at elevated temperatures, and provided structural insights into the formed products.