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            Work during this period focused primarily on the synthesis and reactivity of metal complexes with bis(2-pyridylmethyl)-1,2-ethanediamine (bispicen) and its derivatives. Other research included the study of manganese and copper complexes with derivatives of 1,10-phenanthroline.

I. Metal Complexes with Electronically Modified Bispicen Derivatives

            Manganese and iron complexes with N-(4-X)benzyl-N,N'-(bis(2-pyridylmethyl)-1,2-ethanediamine (X = H, OMe, Cl, NO₂) were reported just prior to the beginning of the funding period.^{1, 2} Since these compounds featured prominently in the original proposal, they will be described briefly. The 4-benzyl substituent alters the redox properties of the chelated metal ions through inductive effects. The effect is minor, shifting the M(III/II) reduction potential over a range of 50 mV, and does not meaningfully impact the hydrocarbon oxidation chemistry. Upon oxidation with meta-chloroperbenzoic acid (MCPBA), the compounds can chlorinate cyclohexene, albeit in sub-stoichiometric yields.²

II. Metal Complexes with Methylated Bispicen Derivatives

            The bispicen framework was systematically methylated at the 6-positions on the pyridine rings and on the secondary amines (L^Men). These derivatives include N,N'-dimethyl-N,N'-(bis(2-pyridylmethyl))-1,2-ethanediamine (L^Me2) and N,N'-dimethyl-N,N'-(bis(6-methyl-2-pyridylmethyl))-1,2-ethanediamine (L^Me4). The tetradentate N-donor ligands can subsequently react with MnCl₂ or FeCl₂ to yield metal complexes with the general formula [M(L^Men)Cl₂]. The reaction between three equiv. of MnCl₂ and two equiv. of bispicen yields an asymmetric, linear trinuclear Mn(II) complex.³

The structures of the mononuclear Mn(II) compounds demonstrate that the bispicen framework can chelate metal ions in any of three conformations: cis-a, cis-b, and trans.⁴ Prior to our work, only the cis-a conformation had been observed for this class of ligand,^5-9 prompting others to conclude that the ethylene linkage was inflexible and incompatible with conformations other than the cis-a. The lack of a straightforward connection between the extent of methylation and the conformer observed in the crystal state led us to hypothesize that the metal compounds were dynamic, with the conformers interchanging in solution. EPR and ¹H NMR analyses of solution samples confirm that the solid-state structures are not exclusively maintained in solution and are instead consistent with a mixture of conformers.⁴

Upon the oxidation of [Fe(L^Me2)Cl₂] with MCPBA, a transient feature is observed spectrophotometrically.¹⁰ Parallel mass spectrometry analysis suggests that this feature corresponds to [Fe^IV(L^Me2)(O)Cl₂]. This species is reminiscent of the active oxidant in the proposed mechanism for non-heme iron halogenases.¹¹ The [M(L^Men)Cl₂] complexes can promote the chlorination of benzylic and allylic C-H bonds using MCPBA as the terminal oxidant.¹⁰ The iron compounds are approximately twice as efficient at chlorinating hydrocarbons as their manganese analogs. The observed chlorination is mild compared to systems previously reported by Que and Comba.^12-14 Que and Comba's oxidants, which are [Fe^IV(L)(O)Cl] species, can chlorinate cyclohexane; whereas, ours cannot oxidize alkanes. As with these systems, the turnover numbers are low, even with excess MCPBA.

Extensive ligand degradation is observed during the reaction. Que and Comba's oxidants employ relatively rigid ligands, which leads us to hypothesize that the flexibility of the bispicen framework is promoting intramolecular decomposition of the catalyst. Consequently, we are currently exploring catalysts employing ligands with the more rigid 1,2-cyclohexanediamine backbone.

Intriguingly, the oxidants generated from the [Fe(L^Men)Cl₂] precursors can react with toluene and ethylbenzene but not cumene, which has its weakest C-H bond on a tertiary carbon instead of a primary or secondary one. Ethylbenzene is chlorinated to 1-chloroethylbenzene, but no dichlorinated products are observed, even when only 1 equiv. of substrate relative to the oxidant is used. The results suggest that the oxidants are regioselective, preferring to activate C-H bonds on less sterically hindered primary and secondary carbons over weaker bonds on tertiary carbons. The L^Me4 ligand leads to decreased activity with secondary C-H bonds, demonstrating that the ligands' methyl groups can modulate the regioselectivity of the halogenation reactions by restricting the access of potential substrates to the reactive portions of the Fe^IV oxidant.

III. Reactivity of Metal Complexes with Phenanthroline Derivatives

            Copper and manganese complexes with derivatives of 1,10-phenanthroline were prepared with the general formulae [Mn(R-phen)₂Cl₂] and [Cu(R-phen)₂Cl]Cl. Attempts to chlorinate hydrocarbons with these complexes were not successful. Currently, we are assessing the impact that of electronic perturbations on the epoxidation chemistry of the manganese complexes. We have found that installing an electron-withdrawing group on the 5-position of the phenanthroline significantly decreases the epoxidation chemistry. We are currently investigating electron-rich phenanthroline derivatives.

References

  (1)        Coates, C. M.; Nelson, A.-G. D.; Goldsmith, C. R. Inorg. Chim. Acta 2009, 362, 4797-4803.

(2)        Coates, C. M.; Nelson, A.-G. D.; Goldsmith, C. R. Inorg. Chim. Acta 2010, 363, 199-204.

(3)        Coates, C. M.; Fiedler, S. R.; McCullough, T. L.; Albrecht-Schmitt, T. E.; Shores, M. P.; Goldsmith, C. R. Inorg. Chem. 2010, 49, 1481-1486.

(4)        Coates, C. M.; Hagan, K.; Mitchell, C. A.; Gorden, J. D.; Goldsmith, C. R., 2010, submitted.

(5)        Britovsek, G. J. P.; England, J.; White, A. J. P. Dalton Trans. 2006, 1399-1408.

(6)        Collins, M. A.; Hodgson, D. J.; Michelsen, K.; Towle, D. K. J. Chem. Soc., Chem. Commun. 1987, 1659-1660.

(7)        Goodson, P. A.; Hodgson, D. J. Inorg. Chem. 1989, 28, 3606-3608.

(8)        Hureau, C.; Blondin, G.; Charlot, M.-F.; Philouze, C.; Nierlich, M.; Cesario, M.; Anxolabéhère-Mallart, E. Inorg. Chem. 2005, 44, 3669-3683.

(9)        Poussereau, S.; Blondin, G.; Cesario, M.; Guilhem, J.; Chottard, G.; Gonnet, F.; Girerd, J.-J. Inorg. Chem. 1998, 37, 3127-3132.

(10)      Goldsmith, C. R.; Coates, C. M.; Hagan, K.; Mitchell, C. A., 2010, submitted.

(11)      Blasiak, L. C.; Vaillancourt, F. H.; Walsh, C. T.; Drennan, C. L. Nature 2006, 440, 368-371.

(12)      Comba, P.; Wunderlich, S. Chem. Eur. J. 2010, 16, 7293-7299.

(13)      Kojima, T.; Leising, R. A.; Yan, S.; Que, L., Jr. J. Am. Chem. Soc. 1993, 115, 11328-11335.

(14)      Leising, R. A.; Zang, Y.; Que, L., Jr. J. Am. Chem. Soc. 1991, 113, 8555-8557.