Cobalt-Promoted Tandem C(sp²)–H Activation and C–C Coupling of Vinyldiphenylphosphine: σ- vs π‑Coordination Modes within Cyclometalation

We have studied the reactivity of a series of <i>terminal</i> alkenyldiphenylphosphines {Ph₂P­(CH₂)_<i>n</i>-CHCH₂), <i>n</i> = 0, 1, 2, 3} under mild conditions (−70 °C) in reaction with univalent Co­(I). Methane and ethane elimination occurs when different olefinic <i>P,C</i>-coordination modes are established. Subsequent C–H activation and C–C coupling take place in the reaction of Co­(CH₃)­(PMe₃)₄ with vinyldiphenylphosphine (1:2 molar ratio) to form a novel anionic <i>P,C,P</i>-type ligand with asymmetric ring sizes attached to the cobalt center of (PMe₃)₂Co­{κ³-(<i>P,C,P</i>)-(Ph₂)­CH₂CHCH-CH­(PPh₂)} (<b>1</b>). X-ray study of <b>1</b> revealed that the complex has a unique cyclometalated structure of a three-membered ring within the newly assembled bis-phosphine ligand. Cyclometalation via selective vinyl-group C­(sp²)–H activation of stoichiometric amounts allyl-diphenylphosphine with Co­(CH₃)­(PMe₃)₄ afforded for the first time σ-allyl coordination incorporated in a five-membered cobaltacycle of (PMe₃)₂Co­{κ²(<i>P,C</i>)-HCCHCH₂PPh₂} (<b>2</b>); subsequent C–C coupling is not observed. By contrast, equimolar mixtures of pentenyl-diphenylphosphine and Co­(CH₃)­(PMe₃)₄ afforded a η³-coordinated π-allyl complex of composition (PMe₃)₂Co­{η³-κ⁴-(<i>P,C,C,C</i>)-H₂CCH-CH­(CH₂)₂PPh₂} (<b>3</b>). All unprecedented complexes were characterized by multinuclear NMR spectroscopy and X-ray diffraction, and their mechanisms of formation are discussed

Synthesis and Catalytic Application in Hydrosilylation of the Complex <i>mer-</i>Hydrido(2-mercaptobenzoyl)tris(trimethylphosphine)cobalt(III)

The sulfur-coordinated acyl­(hydrido)­cobalt­(III) complex <b>1</b> was synthesized by reaction of thiosalicylaldehyde with CoMe­(PMe₃)₄. The crystal structure of <b>1</b> was determined by X-ray diffraction. Complex <b>1</b> is an excellent catalyst for the hydrosilylation of aldehydes and ketones under mild conditions. This might be the first example of hydrosilylation of aldehydes and ketones catalyzed by (hydrido)cobalt complexes

Coordination Chemistry of Cyclic Disilylated Germylenes and Stannylenes with Group 11 Metals

Reactions of Et₃P adducts of bissilylated germylenes and stannylenes with gold, silver, and copper cyanides led to cyanogermyl or -stannyl complexes of the respective metals. In the course of the reaction the phosphine moved to the metal, while the cyanide migrated to the low-coordinate group 14 element. The respective gold complexes were found to be monomeric, whereas the silver and copper complexes exhibited a tendency to dimerize in the solid state. Attempts to abstract the phosphine ligand with B­(C₆F₅)₃ led only to the formation of adducts with the borane coordinating to the cyanide nitrogen atom

The Connection between NHC Ligand Count and Photophysical Properties in Fe(II) Photosensitizers: An Experimental Study

Four homo- and heteroleptic complexes bearing both polypyridyl units and N-heterocyclic carbene (NHC) donor functions are studied as potential noble metal-free photosensitizers. The complexes [Fe^II(L1)­(terpy)]­[PF₆]₂, [Fe^II(L2)₂]­[PF₆]₂, [Fe^II(L1)­(L3)]­[PF₆]₂, and [Fe^II(L3)₂]­[PF₆]₂ (terpy = 2,2′:6′,2″ terpyridine, L1 = 2,6-bis­[3-(2,6-diisopropylphenyl)­imidazol-2-ylidene]­pyridine, L2 = 2,6-bis­[3-isopropylimidazol-2-ylidene]­pyridine, L3 = 1-(2,2′-bipyridyl)-3-methylimidazol-2-ylidene) contain tridentate ligands of the C^N^C and N^N^C type, respectively, resulting in a Fe-NHC number between two and four. Thorough ground state characterization by single crystal diffraction, electrochemistry, valence-to-core X-ray emission spectroscopy (VtC-XES), and high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) in combination with ab initio calculations show a correlation between the geometric and electronic structure of these new compounds and the number of the NHC donor functions. These results serve as a basis for the investigation of the excited states by ultrafast transient absorption spectroscopy, where the lifetime of the ³MLCT states is found to increase with the NHC donor count. The results demonstrate for the first time the close interplay between the number of NHC functionalities in Fe­(II) complexes and their photochemical properties, as revealed in a comparison of the activity as photosensitizers in photocatalytic proton reduction