Ligand Effects on Reactivity of Cobalt Acyl Complexes

Abstract

Hydrogenolysis reactions of cobalt acyl complexes, [Co(CO)3(L)(COR)] (L = phosphine, R = Me, n Pr) have been monitored using in situ IR spectroscopy at moderate temperatures (<75 °C) and pressures (<25 bar). The reactions provide a model for the product-formation step in phosphine-modified, cobalt-catalyzed hydroformylation. The reaction kinetics are dependent on L, with the fastest rate being observed for the complex containing n-pentyl-9-phosphabicyclo[4.2.1]nonane (a 5-PhobPC5). The observed dependence of rate on H2 and CO pressure is consistent with a mechanism involving initial CO dissociation, followed by reaction of [Co(CO)2(L)(COR)] with H2. Isotopic exchange experiments, monitored by IR spectroscopy, demonstrate that both terminal and acetyl carbonyls of [Co(CO)3(L)(COR)] exchange with free 13CO. Kinetic data are also reported for reactions of [Co(CO)3(L)(COR)] with triphenyltinhydride. A zero-order dependence on [Ph3SnH] (at large excess) and positive values of ΔS ‡ demonstrate rate-determining CO dissociation. For a series of less bulky, symmetrical phosphines, the rates follow the sequence PEt3 < PMe2Ph ∼ PEtPh2 < PPh3 < P(4-ClC6H4)3, in accord with their electron-donating strength. Higher rates are found for more bulky phosphines, and the fastest rate is again found for L = a 5-PhobPC5. Calculations using density functional theory indicate that the CO dissociation energy for [Co(CO)3(L)(COMe)] is influenced by the stereoelectronic properties of L, with steric bulk having a substantial effect. X-ray crystal structures are reported for [Co(CO)3(PEtPh2)(COMe)], [Co(CO)3(s-PhobPC5)]2, and [Co(CO)3(a 5-PhobPC5)]2.

Synthesis and Reactivity of Organometallic Intermediates Relevant to Cobalt‐Catalyzed Hydroformylation

“…Addition of excess 13 CO gas (1.2–6 equiv) to a [D 6 ]benzene solution of the compound resulted in exchange of the CO ligands with the gaseous CO as judged by 13 C NMR spectroscopy (see Figure S7 in the Supporting Information). These findings are consistent with the reported observations that CO exchange between HCo(CO) 4 and 13 CO is too fast to monitor at −30 °C . Analogous rhodium complexes, (P‐P)Rh(CO) 2 H, also undergo rapid carbonyl–CO exchange with a half‐life of 4 seconds determined for the Xantphos example at 40 °C (Xantphos=4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene)…”

“…These findings are consistent with the reported observations that CO exchange between HCo(CO) 4 and 13 CO is too fast to monitor at À30 8C. [18] Analogous rhodium complexes, (P-P)Rh(CO) 2 H, also undergo rapid carbonyl-CO exchange [19b] with a half-life of 4 seconds determined for the Xantphos example at 40 8C (Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene). [21] The presence of p-donors increases the substitutional lability of cis ligands in octahedral complexes such as (CO) 5 MnBr.…”

“…Studies by Clarke and co‐workers on (bisphosphine)rhodium hydroformylation established that the nature of the alkyl ligand (branched versus linear) affects the thermodynamics of CO migratory insertion by as much as 1.8 kcal mol −1 . The influence of a bidentate phosphine, however, is unknown …”

Synthesis and Reactivity of Organometallic Intermediates Relevant to Cobalt‐Catalyzed Hydroformylation

“…Communications bis(diazaphospholane) (BDP) rhodium and styrene. [19] In contrast to rhodium where linear and branched acyls are observed, [18] the rate of b-hydrogen elimination for cobalt is faster than CO insertion, prohibiting observation of acyl intermediates in the absence of added CO.…”

Nanotubular TiO2-supported amorphous Co–B catalysts and their catalytic performances for hydroformylation of cyclohexene

“…Several million tons of aldehydes are produced annually by the reaction [1]. For decades, the Co-based [2,3], Ir-based [4,5] and Rh-based [6] catalysts for hydroformylation are widely investigated. Indeed, the Co-based catalysts are much less expensive than Rh-based and Ir-based catalysts, and are able to hydroformylate olefins selectively to aldehydes effectively.…”