62nd Annual Report on Research 2017

Introduction

Previously we found that a Co(II) complex with N,N'-dibenzyl-N,N'-bis(2-pyridylmethyl)-1,2-cyclohexanediamine (bbpc, Scheme 1) was capable of catalyzing the oxidation of weak C-H bonds by iodosobenzene (PhIO) and could be oxidized to a Co(III)-peroxo species capable of aldehyde deformylation. Much of this past year’s work was devoted to finishing its study.¹ The remainder of this report will focus on unpublished work.

In an attempt to improve the C-H activation catalysis observed for the bbpc system, we subsequently explored a new ligand with more chemically robust sulfonyl linkages between the phenyl group and the metal-coordinating portion of the molecule: N-benzenesulfonyl-N,N’-bis(2-pyridylmethyl)-1,2-cyclohexanediamine (bsbpc, Scheme 1). Attempts to install two phenyl groups using the sulfonyl linkages were attempted but have thus far not yielded clean product.

Synthesis and Spectroscopic Characterization

The bsbpc ligand was prepared in 66% yield from a reaction between N,N’-bis(2-pyridylmethyl)-1,2-cyclohexanediamine and benzenesulfonyl chloride. Mixing bsbpc and Co(ClO₄)₂ provides [Co^II(bsbpc)(MeCN)₂](ClO₄)₂ (1) in 63% yield (Figure 1). Based on the short Co-N bond lengths observed in the crystal structure, the metal center is low-spin Co(II). The substantially longer Co-N bonds to the tertiary amine and the MeCN trans to it are consistent with a Jahn-Teller distortion; this is also consistent with a low-spin d⁷ metal center. Although the Co(II) complex with bbpc was not crystallized, a variety of spectroscopic measurements suggest that it is instead high-spin.¹

Reactivity

Mixtures of 1 and PhIO can activate weak C-H bonds. As was found with with [Co(bbpc)(MeCN)₂](ClO₄)₂, an additional Lewis acid beyond the Co(II) is not needed.¹

9,10-Dihydroanthracene is converted exclusively into anthracene (Table 1). The reactions complete in under 30 min, and yield 6 equiv. of anthracene per equiv. of Co(II). Oxygenated products, such as anthrone and anthroquinone, are not observed, and running the reaction under N₂ has no significant impact on the product distribution. The yields are slightly lower under air. Neither cyclohexane nor cyclohexene react with 1 and PhIO to observable degrees, even when 500 mM of these substrates is added. As was found for the bbpc system, meta-chloroperbenzoic acid (MCPBA) can substitute for PhIO as the terminal oxidant, albeit with lower yields of oxidized hydrocarbons.¹ H₂O₂ and O₂ were not competent terminal oxidants for the C-H activation. The bsbpc ligand does not appear to support aldehyde deformylation chemistry, although I should emphasize that only a single substrate, 2-phenylpropionaldehyde, has thus far been investigated and only briefly.

Further work is underway to confirm that the bsbpc ligand remains intact throughout the C-H activation reactions. We currently speculate that the lower activity thus far observed for the bsbpc catalyst may result from electronic differences rather than altered ligand stability.

Impact of Research

The support from the ACS-PRF has allowed me to develop cobalt-catalyzed C-H activation into an active area of research. Prior to my group’s work, cobalt had never been documented to catalyze the oxidation of DHA. All observed cobalt-promoted C-H activation was instead stoichiometric with respect to the metal, and even that relied upon the presence of an additional Lewis acid such as Sc(III). The work is therefore redefining how cobalt can be used in oxidative catalysis and has elevated the profile of my research group.

The grant has also allowed one of my graduate students, Angela Bell-Taylor, to focus exclusively on her research. Her work has advanced more quickly due to her lesser teaching obligation. The unpublished results described here will form the basis of at least one more high-end publication which will improve her competitiveness in upcoming job searches.

Reference

1. Q. Zhang, A. Bell-Taylor, F. M. Bronston, J. D. Gorden, C. R. Goldsmith. Inorg. Chem. 2017, 56, 773.

Table 1. Oxidation of 9,10-Dihydroanthracene (DHA) by PhIO and MCPBA Catalyzed by 1

Standard reaction conditions: All reactions were run at 298 K in 2.5 mL of MeCN. The starting concentrations of the cobalt(II) catalyst (1) and the substrate in all reactivity assays were 1.0 mM and 50 mM, respectively. 25 equiv. of PhIO, relative to the 1, were added at the beginning of the reaction. For each time point, an aliquot of the reaction mixture was diluted with ether, filtered through silica gel, and analyzed via GC. The products were identified by comparing the retention times with those of authentic samples of anthracene. The concentrations of each organic product were calibrated relative to that of an internal standard (dichlorobenzene) with a known concentration. ^aPhIO is the limiting reagent. ^bTurnover number, defined as the number of moles of oxidized organic product generated per mole of 1.