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).

The combination of CoCl and 1,3-dienes is known to catalyze challenging alkyl-alkyl cross-coupling reactions between Grignard reagents and alkyl halides, but the mechanism of these valuable transformations remains speculative. Herein, electrospray-ionization mass spectrometry is used to identify and characterize the elusive intermediates of these and related reactions. The vast majority of detected species contain low-valent cobalt(I) centers and diene molecules. Charge tagging, deuterium labeling, and gas-phase fragmentation experiments elucidate the likely origin of these species and show that the diene not only binds to Co as a π ligand, but also undergoes migratory insertion reactions into Co-H and Co-R bonds. The resulting species have a strong tendency to form anionic cobalt(I) ate complexes, the superior nucleophilicity of which should render them highly reactive toward electrophilic substrates and, thus, presumably is the key to the high catalytic efficiency of the system under investigation. Upon the reaction of the in situ formed cobalt(I) ate complexes with organyl halides, only the final cross-coupling product could be detected, but no cobalt(III) species. This finding implies that this reaction step proceeds in a direct manner without any intermediate or, alternatively, that it involves an intermediate with a very short lifetime.

>The combination of CoCl2 with bidentate phosphines is known to catalyze challenging cross‐coupling and Heck‐type reactions, but the mechanisms of these valuable transformations have not been established. Here, we use electrospray‐ionization mass spectrometry to intercept the species formed in these reactions. Our results indicate that a sequence of transmetalation, reductive elimination, and redox disproportionation convert the cobalt(II) precatalyst into low‐valent cobalt complexes. These species readily transfer single electrons to alkyl bromides, which thereupon dissociate into alkyl radicals and Br−. In cross‐coupling reactions, the alkyl radicals add to the cobalt catalyst to form observable heteroleptic complexes, which release the coupling products through reductive eliminations. In the Heck‐type reactions, the low abundance of newly formed ionic species renders the analysis more difficult. Nonetheless, our results also point to the occurrence of single‐electron transfer processes and the involvement of radicals in these transformations.

Anionic coordination polymerizations proceed via highly reactive intermediates, whose in situ analysis has remained difficult. Here, we show that electrosprayionization mass spectrometry is a promising method to obtain detailed information on the polymerization process. Focusing on polymerization reactions of 1,3dienes initiated by CoCl 2 /RLi (R = Me, nBu, tBu, Ph), we directly observe the growing polymer chains and characterize the active anionic cobalt centers by gasphase fragmentation experiments. On the basis of these results, we suggest a plausible mechanism for the polymerization reaction. Moreover, the ESI mass spectra permit the determination of molecular weight distributions, which are in good agreement with those derived from NMR-spectroscopic as well as MALDI mass-spectrometric measurements, and afford a wealth of kinetic data.

Anionische Koordinationspolymerisationen verlaufen über hochreaktive Zwischenstufen, deren in situ‐Analyse schwierig bleibt. Hier zeigen wir, dass Elektrosprayionisations‐Massenspektrometrie eine vielversprechende Methode ist, um detaillierte Informationen über den Polymerisationsprozess zu erhalten. Für die durch CoCl2/RLi (R=Me, nBu, tBu, Ph) initiierte Polymerisation von 1,3‐Dienen initiiert beobachten wir die wachsenden Ketten direkt und charakterisieren die aktiven Cobalt‐Zentren durch Fragmentierungsexperimente in der Gasphase. Darauf basierend schlagen wir einen möglichen Mechanismus für die Polymerisation vor. Die ESI‐Massenspektren erlauben zudem die Bestimmung der Molmassenverteilung, welche mit denen aus NMR‐Messungen und MALDI‐Massenspektrometrie gut übereinstimmen, und liefern eine Vielzahl an kinetischen Daten.