Christian R. Goldsmith, PhD , Auburn University
Work during this period focused on the synthesis and reactivity of transition metal complexes with a number of polydentate ligands. These were used primarily for the oxidation of hydrocarbon substrates, although one of the complexes was explored as a contrast agent for magnetic resonance imaging (MRI).
I. Metal Complexes with Methylated Derivatives of Bispicen
In the last progress report, Mn(II) and Fe(II) complexes with methylated derivatives of N,N'- bis(2-pyridylmethyl)-1,2-ethanediamine (bispicen) were reported.1 The iron complex with one of the dimethylated derivatives was oxidized to generate a short-lived species that was identified as [Fe(LMe2)(O)Cl2].2 This species resembles the active oxidant in the mechanism proposed for non-heme iron halogenases.3 The [M(LMen)Cl2] complexes promote the chlorination of weak C-H bonds using meta-chloroperbenzoic acid as the terminal oxidant.2 The metal-based oxidant does not activate C-H bonds on tertiary carbons, suggesting that the methyl groups on the LMe2 ligand are successfully tuning the regioselectivity of the oxidation. The observed chlorination is mild compared to systems previously reported by Que and Comba, and the LMen systems cannot chlorinate alkanes.4-6 Que and Comba's oxidants employ relatively rigid ligands, which led us to hypothesize that the flexibility of the bispicen framework is hastening intramolecular decomposition of the catalyst.1
II. N,N'-bis(2-pyridylmethyl)-bis(ethylacetate)-1,2-ethanediamine (debpn)
The debpn ligand was explored for its potential to stabilize Fe(IV) species and to direct oxidative catalysis towards the less sterically congested portions of hydrocarbon substrates. A series of metal complexes were prepared with the ligand.7 The Mn(II) and Fe(II) species are heptacoordinate; whereas, the Co(II), Ni(II), and Zn(II) compounds are hexacoordinate. The [Mn(debpn)(H2O)]2+ compound was found to be water-stable and was investigated as a contrast agent for MRI.7 The abilities of the Mn(II) and Fe(II) complexes to oxidize alkane and alkene substrates were investigated. [M(depbn)(H2O)]2+-mediated alkene epoxidation and alkane hydroxylation/chlorination chemistry occurs more slowly than with hexacoordinate catalysts. We are currently trying to understand these differences using computational methods.
III. N,N'-bis(phenylmethyl)-N,N'-bis(2-pyridinylmethyl)-1,2-cyclohexanediamine (bbpc)
Unlike the previous ligands, bbpc contains a 1,2-cyclohexanediamine linkage, which has been shown to limit conformational dynamics.8 The compound [Fe(bbpc)(MeCN)2](SbF6)2 was synthesized and used as a catalyst for alkane oxygenation and chlorination.9 When H2O2 is used as a terminal oxidant, the catalyst has a strong preference for hydroxylation; consequent oxidation of the organic alcohols to carbonyl compounds is hindered. Furthermore, the benzyl groups on the ligand tune the regioselectivity, once again limiting the ability of the oxidant to activate thermodynamically weaker but more sterically congested C-H bonds.9 With respect to the chlorination, H2O2 serves as a competent terminal oxidant. Thus far, we have gotten the chlorination to turn over 2.2 times, albeit with a substantial amount of oxygenated side-products.
IV. Phenanthroline (phen) Derivatives
In our prior report, we noted that metal complexes with phen derivatives were unable to promote hydrocarbon chlorination or bromination. We subsequently assessed the impact of electronic perturbations on the epoxidation chemistry of the manganese complexes.10 Installing an electron-withdrawing group on the 5-position of the phen noticeably decreases the speed of alkene epoxidation by commercially available peracetic acid. We subsequently prepared and investigated [Ga(phen)2Cl2]Cl.11 Unexpectedly, this redox-inactive compound also catalyzes alkene epoxidation by peracetic acid.10 Unlike other olefin epoxidation reactions catalyzed by Group XIII metals,12 the chemistry occurs cleanly, with no allylic oxidation or ring-opening under our optimum conditions.
(1) Coates, C. M.; Hagan, K.; Mitchell, C. A.; Gorden, J. D.; Goldsmith, C. R. Dalton Trans. 2011, 40, 4048-4058.
(2) Goldsmith, C. R.; Coates, C. M.; Hagan, K.; Mitchell, C. A. J. Mol. Catal. A 2011, 335, 24-30.
(3) Blasiak, L. C.; Vaillancourt, F. H.; Walsh, C. T.; Drennan, C. L. Nature 2006, 440, 368-371.
(4) Comba, P.; Wunderlich, S. Chem. Eur. J. 2010, 16, 7293-7299.
(5) Kojima, T.; Leising, R. A.; Yan, S.; Que, L., Jr. J. Am. Chem. Soc. 1993, 115, 11328-11335.
(6) Leising, R. A.; Zang, Y.; Que, L., Jr. J. Am. Chem. Soc. 1991, 113, 8555-8557.
(7) Zhang, Q.; Gorden, J. D.; Beyers, R. J.; Goldsmith, C. R. Inorg. Chem. 2011, 50, in press.
(8) Costas, M.; Que, L., Jr. Angew. Chem. Int. Ed. 2002, 41, 2179-2181.
(9) He, Y.; Gorden, J. D.; Goldsmith, C. R. 2011, under revision.
(10) Jiang, W.; Goldsmith, C. R., 2011, manuscripts in preparation.
(11) Carty, A. J.; Dymock, K. R.; Boorman, P. M. Can. J. Chem. 1970, 70, 3524-3529.
(12) Stoica, G.; Santiago, M.; Jacobs, P. A.; Perez-Ramirez, J.; Pescarmona, P. P. Appl. Catal. A 2009, 371, 43-53.