Cobalt(ii) acyl intermediates in carbon–carbon bond formation and oxygenation

Abstract

The organocobalt scorpionate compounds To^MCoR (To^M = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; R = Bn, 1; CH₂SiMe₃, 2; Ph, 3; Et, 4; ⁿBu, 5; Me, 6) react in carbonylation, oxidation, and carboxylation reactions via pathways that are distinctly influenced by the nature of the organometallic moiety. The compounds are prepared by reaction of To^MCoCl with the corresponding organolithium or organopotassium reagents. Compounds 1–6 were characterized by 8-line hyperfine coupling to cobalt in EPR spectra and solution phase magnetic measurements (μ_eff = 4–5μ_B) as containing a high-spin cobalt(II) center. The UV-Vis spectra revealed an intense diagnostic band at ca. 700 nm (ε > 1000 M⁻¹ cm⁻¹) associated with the tetrahedral organocobalt(II) center that was assigned to a d ← d transition on the basis of configuration interaction (CI) calculations. Complexes 1–6 react rapidly with CO to form equilibrating mixtures of the low spin organocobalt carbonyl To^MCo(R)CO, acyl To^MCoC(O)R, and acyl carbonyl To^MCo{C(O)R}CO. The ¹H and ¹¹B NMR spectra contained only one set of signals for the CO-treated solutions, whereas the solution-phase IR spectra contained up to two ν_CO and three ν_C(O)R signals with intensities varying depending on the R group (R = Bn, 7; CH₂SiMe₃, 8; Ph, 9; Et, 10; ⁿBu, 11; Me, 12). Single crystal X-ray diffraction of To^MCo{C(O)Et}CO (10) supports its assignment as a square pyramidal cobalt(II) acyl carbonyl complex. Upon evaporation of volatiles, solutions of 8–12 revert to the CO-free organocobalt starting materials 2–6, whereas attempts to isolate benzyl-derived 7 provide an unusual α-alkoxyketone species, characterized by single crystal X-ray diffraction. Despite the differences observed in the carbonylation of 1–6 as a result of varying the R group, compounds 7–12 all react rapidly with O₂ through an oxygenation pathway to afford the corresponding carboxylate compounds To^MCoO₂CR (R = Bn, 13; CH₂SiMe₃, 14; Ph, 15; Et, 16; ⁿBu, 17; Me, 18). In contrast, the insertion of CO₂ into the Co–C bond in 1–6 requires several days to weeks.