Isolation of a Terminal Co(III)-Oxo Complex

Abstract

Late transition metal oxo complexes with high d-electron counts have been implicated as intermediates in a wide variety of important catalytic reactions; however, their reactive nature has often significantly limited their study. While some examples of these species have been isolated and characterized, complexes with d-electron counts >4 are exceedingly rare. Here we report that use of a strongly donating tris(imidazol-2-ylidene)borate scaffold enables the isolation of two highly unusual Co-oxo complexes which have been thoroughly characterized by a suite of physical techniques including single crystal X-ray diffraction. These complexes display O atom and H atom transfer reactivity and demonstrate that terminal metal oxo complexes with six d-electrons can display strong metal-oxygen bonding and sufficient stability to enable their characterization. The unambiguous assignment of these complexes supports the viability of related species that are frequently invoked, but rarely observed, in the types of catalytic reactions mentioned above. The studies described here change our understanding of the reactivity and bonding in late transition metal oxo complexes and open the door to further study of the properties of this class of elusive and important intermediates.

M−O Bonding Beyond the Oxo Wall: Spectroscopy and Reactivity of Cobalt(III)‐Oxyl and Cobalt(III)‐Oxo Complexes

“…The IRPD spectrum of d 6 cobalt(III)‐oxo complex 1 in Figure a shows a ν(Co−O) band at 752 cm −1 , which shifts to 718 cm −1 upon 18 O labeling, consistent with the ν(Co−O) assignment. The position of this band is similar to that of analogous sextet iron(III)‐oxo complexes, 63 cm −1 lower than the ν(Co−O) band in the trigonal singlet cobalt(III)‐oxo complex reported by Goetz et al . and 18 cm −1 lower than the ν(Co−O) band in the quartet cobalt(IV)‐oxo complex reported by Wang et al .…”

M−O Bonding Beyond the Oxo Wall: Spectroscopy and Reactivity of Cobalt(III)‐Oxyl and Cobalt(III)‐Oxo Complexes

“…TheI RPD spectrum of d 6 cobalt(III)-oxo complex 1 in Figure 2a shows a n(CoÀO) band at 752 cm À1 ,which shifts to 718 cm À1 upon 18 Ol abeling,c onsistent with the n(CoÀO) assignment. Thep osition of this band is similar to that of analogous sextet iron(III)-oxo complexes, [32] 63 cm À1 lower than the n(Co À O) band in the trigonal singlet cobalt(III)-oxo complex reported by Goetz et al [12] and 18 cm À1 lower than the n(CoÀO) band in the quartet cobalt(IV)-oxo complex reported by Wang et al [11] The n(CoÀO) frequencyt hus indicates that the Co À Ob ond order is less than two. Furthermore,t he d(CH) Py bands (highlighted in yellow in Figure 2a-e) around 770 cm À1 are sensitive to the occupancy of the d x 2 Ày 2 2 orbital, as evident from the comparison of ironoxo complexes 3, 3·H 2 O, and 4 with different d x 2 Ày 2 orbital occupancies (Figure 2c-e).…”

“…[1,2] When progressing to late transition metals,t he occupancyo fp-antibonding metal do rbitals increases,a nd metal-oxo bonds become weaker and more reactive.T his culminates in the rich chemistry of iron-oxo compounds,s uch as heme (P450) [3][4][5] and non-heme iron enzymes [6,7] that inspired the development of important C À H oxidation catalysts. [8][9][10] In contrast, metal-oxo complexes beyond Group 8a re rare and exist only in square-pyramidal, [11] trigonal-pyramidal, [12,13] and, importantly,s quare-planar [14] coordination geometries in which electrons occupy MÀ On on-bonding (or weakly p * -antibonding) do rbitals.S tabilization of metal-oxo complexes beyond Group 8h as also been achieved by coordination of oxygen to Lewis acids. [15] Earlier reports of pseudo-octahedral platinum, gold, and palladium complexes were subsequently disproved for incorrect structural assignment.…”

A bis‐Pyrazolate Pincer on Reduced Cr Deoxygenates CO2: Selective Capture of the Derived Oxide by CrII

“…[14] The oxide product of carbon oxygen bond cleavage here is readily captured by carbon dioxide, meaning that any Cr(O 2À )i ntermediate is nucleophilic. [15] The structureo ft he [LCr] 2 unit hostinga na dditional ligand,O 2À ,i nt he dianion [L 2 Cr 2 (m 2 -O)] 2À is shown in the Supporting Information. This is a potentialm echanistic participant, following reaction with CO 2 .…”