Isolation and Crystallographic Characterization of an Octavalent Co2O2 Diamond Core

Abstract

High-valent cobalt oxides play a pivotal role in alternative energy technology as catalysts for water splitting and as cathodes in lithium-ion batteries. Despite this importance, the properties governing the stability of high-valent cobalt oxides and specifically possible oxygen evolution pathways are not clear. One root of this limited understanding is the scarcity of high-valent Co(IV)-containing model complexes; there are no reports of stable, well-defined complexes with multiple Co(IV) centers. Here, an oxidatively robust fluorinated ligand scaffold enables the isolation and crystallographic characterization of a Co(IV)₂-bis-μ-oxo complex. This complex is remarkably stable, in stark contrast with previously reported Co(IV)₂ species that are highly reactive, which demonstrates that oxy-Co(IV)₂ species are not necessarily unstable with respect to oxygen evolution. This example underscores a new design strategy for highly oxidizing transition-metal fragments and provides detailed data on a previously inaccessible chemical unit of relevance to O-O bond formation and oxygen evolution.

Figure 1.

(A) Crystal structure of 2. Hydrogen atoms bound to carbon atoms and co-crystallized solvent removed for clarity. Selected bond (in Å): O1–O2 = 1.335(12), Co–O1 = 1.955(8), Co–O2 = 1.917(8), Co–Cl = 2.285(3); Co–C1 = 1.936(10); Co–C2 = 1.955(10), Co–C3 1.961(10). (B) IR spectra of ¹⁶O-2 (black) and ¹⁸O-2 (red) of 2 with the shift of the O–O stretching frequency at 1102 cm⁻¹ shown. (C) Plot of O–O stretching frequency against O–O bond length for 2 (black star), end-on Co(III)-superoxides (teal), side-on Co(III)-peroxides (red), and side-on metal-dioxygen complexes more generally (green). Data from references^,–,,. The grey curve gives a fit to Badger’s rule, L = 75.78 ν^−2/3 + 0.60. Note that the fit includes O₂, which is not shown as it is not within the plotted values.

Figure 2.

Structures of the diamond core complexes 3 and 4. Hydrogens bound to carbon, co-crystallized solvent, and [BAr^F₄]⁻ counteranions are omitted for clarity. (A) 3 Co1–Co1′ = 2.6801(8) Å, Co1–O1 = 1.832(2) Å, Co1–O1′ = 1.836(2) Å, Co1–C1 = 1.926(3) Å; Co1–C12 = 1.976(3) Å; Co1–C23 = 1.974(3) Å. The Co centers are equivalent by inversion symmetry and C1 is located on the axial carbene. (B) 4 Co1–Co1 = 2.980(1) Å, Co1–O1 = 1.885(3) Å, Co1–O1 = 1.892(3) Å, Co1–C1 = 1.920(4) Å; Co1–C12 = 1.888(4) Å; Co1–C23 = 1.948(4) Å. The Co centers are equivalent by inversion symmetry and C12 is located on the axial carbene.

Figure 3.

(A) Co K-edge spectra of 1, 2, and 3. (B) Fourier transform EXAFS spectrum and fit of 3 along with the spectrum and fit in k-space (inset). The fit indicates a Co-Co bond distance of 2.6 Å.

Scheme 1.

Synthesis of complexes 1 and 2.

Scheme 2.

Dimerization of 1 and 2 to generate the bis-μ-oxo complex 3 and subsequent reduction to the bis-μ-hydroxo complex 4.