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 \u3e 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(O)R, and acyl carbonyl ToMCo{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 νCO and three νC(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(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

Comparison of S-adsorption on (111) and (100) facets of Cu nanoclusters

In order to gain insight into the nature of chemical bonding of sulfur atoms on coinage metal surfaces, we compare the adsorption energy and structural parameters for sulfur at four-fold hollow (4fh) sites on (100) facets and at three-fold hollow (3fh) sites on (111) facets of Cu nanoclusters. Consistent results are obtained from localized atomic orbital and plane-wave based density functional theory using the same functionals. PBE and its hybrid counterpart (PBE0 or HSE06) also give similar results. 4fh sites are preferred over 3fh sites with stronger bonding by ∼0.6 eV for nanocluster sizes above ∼280 atoms. However, for smaller sizes there are strong variations in the binding strength and the extent of the binding site preference. We show that suitable averaging over clusters of different sizes, or smearing the occupancy of orbitals, provide useful strategies to aid assessment of the behavior in extended surface systems. From site-projected density of states analysis using the smearing technique, we show that S adsorbed on a 4fh site has similar bonding interactions with the substrate as that on a 3fh site, but with much weaker antibonding interactions