Rapid and ordered carbonylation and oxygenation of a cobalt(ii) methyl

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

The organocobalt scorpionate compounds ToMCoR (ToM = 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 ToMCoCl 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 (μeff = 4-5μB) 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 ToMCo(R)CO, acyl ToMCoC([double bond, length as m-dash]O)R, and acyl carbonyl ToMCo{C([double bond, length as m-dash]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 νCO and three νC([double bond, length as m-dash]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 ToMCo{C([double bond, length as m-dash]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 ToMCoO2CR (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.

Cobalt(II) Complexes with N,N,N-Scorpionates and Bidentate Ligands: Comparison of Hydrotris(3,5-dimethylpyrazol-1-yl)borate Tp* vs. Phenyltris(4,4-dimethyloxazolin-2-yl)borate ToM to Control the Structural Properties and Reactivities of Cobalt Centers

Scorpionate ligands Tp* (hydrotris(3,5-dimethylpyrazol-1-yl)borate) and ToM (tris(4,4-dimethyloxazolin-2-yl)phenylborate) complexes of cobalt(II) with bidentate ligands were synthesized. Both Tp* and ToM coordinate to cobalt(II) in a tridentate fashion when the bidentate ligand is the less hindered acetylacetonate. In crystal structures, the geometry of cobalt(II) supported by the N₃O₂ donor set in the Tp* complex is a square-pyramid, whereas that in the ToM complex is close to a trigonal-bipyramid. Both Tp*- and ToM-acac complexes exhibit solvatochromic behavior, although the changing structural equilibria of these complexes in MeCN are quite different. In the bis(1-methylimidazol-2-yl)methylphenylborate (LPh) complexes, Tp* retains the tridentate (к³) mode, whereas ToM functions as the bidentate (к²) ligand, giving the tetrahedral cobalt(II) complex. The bowl-shaped cavity derived from the six methyl groups on ToM lead to susceptibility to the bulkiness of the opposite bidentate ligand. The entitled scorpionate compounds mediate hydrocarbon oxidation with organic peroxides. Allylic oxidation of cyclohexene occurs mainly on the reaction with tert-butyl hydroperoxide (TBHP), although the catalytic efficiency of the scorpionate ligand complexes is lower than that of Co(OAc)₂ and Co(acac)₂. On cyclohexane oxidation with meta-chloroperbenzoic acid (mCPBA), both ToM and Tp* complexes function as catalysts for hydroxylation. The higher electron-donating ToM complexes show faster initial reaction rates compared to the corresponding Tp* complexes.