Naphthalene and Anthracene Cobaltates(1−): Useful Storable Sources of an Atomic Cobalt Anion

Abstract

Reductions of CoBr(2) or cobaltocene by 3 equiv of potassium anthracene radical anion in tetrahydrofuran (THF) afford 60-80% yields of bis(anthracene)cobaltate(1-) (1), of interest as a readily accessible and quite labile source of spin-paired atomic Co(-). Although the unsolvated potassium salt of 1 is thermally unstable at 20 °C, the [K(18-crown-6)(THF)(2)](+) salt of 1 functions as a useful storable crystalline reagent for Co(-) in several reactions. Previously known classic cobaltates, [CoL(4)](-), for L = 1/2 (1,3-butadiene) (2), PF(3) (3), and P(OiPr)(3) (5), were obtained directly from 1 and structurally characterized for the first time. Anion 3 is noteworthy because it appears to possess the shortest known Co-P distance, av = 2.012(4) Å. Although the naphthalene analogue of 1 is not yet available as a pure substance, low-temperature reductions of CoBr(2) or cobaltocene by naphthalene radical anion in the presence of 1,5-cyclooctadiene (COD) afford variable yields (80-90% from CoCp(2)) of (naphthalene)(COD)cobaltate(1-) (10). Ready displacement of naphthalene in 10 by L = 1,3-butadiene, 2,2'-bipyridine, and COD occurs to give good yields of the respective [Co(L)(COD)](-), all of which have been structurally characterized. Both ligands in 10 are displaced by tert-butylisocyanide to afford [Co(CNtBu)(4)](-) (16), the first isolable and structurally characterized homoleptic alkylisocyanometalate. The molecular structure of 16 shows unprecedented bending of the isocyanides, av C-N-C = 137(2)°, for homoleptic isocyanide complexes.

Heteroatom‐Free Arene‐Cobalt and Arene‐Iron Catalysts for Hydrogenations

“…[10][11][12] Unambiguous distinction between homogeneous and heterogeneous catalysts is intricate, [21] yet our observations are consistent with a homogeneous mechanism. The loss of suitable p acceptors after complete hydrogenation of 1-dodecene (and 2 % anthracene from the precatalyst) resulted in catalyst precipitation.…”

“…This is consistent with the assumption that p acceptors other than anthracene are also capable of stabilizing reactive low-valent homogeneous metal species. [10] Reaction progress analyses (GC-FID) fully support the notion of an initial anthracene substitution which is hydrogenated only after the stoichiometric substrate (styrene) is entirely consumed (Figure 1). Hg poisoning (addition of 150 equiv Hg per [Co] at the start of the reaction and at 25 % conversion, respectively) showed no loss of activity (Table 1, entry 1, and see Figure S4 in the Supporting Information).…”

“…The reaction of styrene under 2 bar D 2 gave a,b-[D 2 ]-ethylbenzene. [10] Bis(anthracene)cobaltate 1 was highly active in the hydrogenation of alkenes, ketones, and imines (1-5 mol % cat., 1-10 bar H 2 , 20-60 8C). [24] Transfer hydrogenation between 1,3-diphenylpropan-2-ol (4 equiv) and 4-methylstyrene in the presence of 1 indeed afforded 1-ethyl-4-methylbenzene with 100 % selectivity (18 % yield) in the absence of an H 2 atmosphere (Scheme 7).…”

“…[6b,c] A PNP pincer ligand was recently used by Hanson and coworkers to stabilize a cobalt catalyst with 15 valence electrons that allowed hydrogenation of alkenes, ketones, and imines after activation by Brookharts acid. [10,11] Unlike many catalytic reactions where this scenario would entail catalyst aggregation and deactivation, the presence of a large excess of p-acidic ligands in hydrogenations (of unsaturated molecules such as alkenes, arenes, and carbonyl compounds) can effect rapid ligand exchange between p ligands and thus assure prolonged catalyst activity in the homogeneous phase. [8] The same research group reported high activities of bis(imino)pyridine-and bis(arylimidazol-2-ylidene)pyridine-iron and -cobalt catalysts bearing two labile N 2 ligands (Scheme 1).…”

“…Polymerization was observed only to a minor extent (< 5 %). Exchange of the cation by using [K(18-crown-6){Co(C 14 H 10 ) 2 }] [10] instead of 1 afforded identical yields in reactions of various styrenes. [13,17] A comparative study of initial reaction rates revealed no significant electronic effect of the para substituent of different styrene derivatives (F, H, Me, OMe).…”