Alkene Metalates as Hydrogenation Catalysts

Abstract

First‐row transition‐metal complexes hold great potential as catalysts for hydrogenations and related reductive reactions. Homo‐ and heteroleptic arene/alkene metalates(1−) (M=Co, Fe) are a structurally distinct catalyst class with good activities in hydrogenations of alkenes and alkynes. The first syntheses of the heteroleptic cobaltates [K([18]crown‐6)][Co(η4‐cod)(η2‐styrene)2] (5) and [K([18]crown‐6)][Co(η4‐dct)(η4‐cod)] (6), and the homoleptic complex [K(thf)2][Co(η4‐dct)2] (7; dct=dibenzo[a,e]cyclooctatetraene, cod=1,5‐cyclooctadiene), are reported. For comparison, two cyclopentadienylferrates(1−) were synthesized according to literature procedures. The isolated and fully characterized monoanionic complexes were competent precatalysts in alkene hydrogenations under mild conditions (2 bar H2, r.t., THF). Mechanistic studies by NMR spectroscopy, ESI mass spectrometry, and poisoning experiments documented the operation of a homogeneous mechanism, which was initiated by facile redox‐neutral π‐ligand exchange with the substrates followed by H2 activation. The substrate scope of the investigated precatalysts was also extended to polar substrates (ketones and imines).

Mechanisms of Cobalt/Phosphine‐Catalyzed Cross‐Coupling Reactions

“…Presumably, the apparent stability of L 2 Co − and Li 2 L 2 Co + arises to a good deal from their 18 valence‐electron count. Numerous examples of isoelectronic cobalt(−I) species are known in the literature, of which the biphosphinine cobalt(−I) anion appears to be most closely related to the cobalt(−I) complexes found in the present study . L 2 Co − and Li 2 L 2 Co + could originate from a reductive elimination of a cobalt(I) species or from the redox disproportionation of a neutral cobalt(0) precursor.…”

From Ruthenium to Iron and Manganese—A Mechanistic View on Challenges and Design Principles of Base‐Metal Hydrogenation Catalysts

“…This illustrates that well‐defined catalysts are not necessarily required for olefin hydrogenation. Very recently, the utilization of alkene metalates ( 4 ) as catalysts for the hydrogenation of olefins was reported . Simple π‐ligand exchange (arene by olefin) generates the catalytically active species, which is able to activate H 2 .…”

Iron‐Gallium and Cobalt‐Gallium Tetraphosphido Complexes

“…The 1 H NMR spectrum of the isolated crystals of 2 dissolved in [D 8 ]THF displays five multiplets for coordinated anthracene with an integral ratio of 2:2:2:2:2. The chemical shifts of these resonances are in the range δ = 6.90 ppm to 2.69 ppm, which is common for anthracene coordinated to a transition metal center , . The observed high‐field shift of the anthracene signals is explained by π‐donation from the metal atom to the π‐acceptor ligand, which reduces the aromaticity of anthracene .…”

“…The chemical shifts of these resonances are in the range δ = 6.90 ppm to 2.69 ppm, which is common for anthracene coordinated to a transition metal center. [17,24] The observed high-field shift of the anthracene signals is explained by π-donation from the metal atom to the π-acceptor ligand, which reduces the aromaticity of anthracene. [17] Two chemically distinct 2,6-diisoproplyphenyl groups are observed in accordance with the single-crystal Xray structure, which are also discernible in the 13 C{ 1 H} NMR spectrum.…”