The Groves–Spiro Dioxomanganese(V) Story

Abstract

bonding · manganese · porphyrin · reactivity · stability Theintroductionintheseminal1962Ballhausen-Graypaperthat provided the first molecular orbital description of metaloxygen multiple bonds ( Figure 1) concluded with the following statement: "It also might be hoped that an understanding of the principal features of the bonding in VO 2+ will be a helpful guide in attempts to develop a general theory of the electronic structures of MO n+ and MO 2 n+ complexes." [1] The last sentences in that paper (published in the first issue of Inorg. Chem.) read as follows: "Indeed, it is clear that any complete discussion of the electronic structures of the oxycations of the transition and actinium series must allow for substantial oxygen to metal p-bonding. satisfy the p-bonding capacities of the two 3d p orbitals of a first transition series metal ion, but it takes at least two oxygens to satisfy the combined p-bonding capacities offered by the 5f and 6d orbitals of the metal ions in the actinium series."The enormous importance of metal-oxygen multiple bonds in reaction chemistry was recognized many years later, when the central role of key oxo-metal intermediates in biological processes was uncovered. The research group of J. T. Groves pioneered the isolation of synthetic complexes with terminal oxo-metal bonds, including porphyrin-chelated, and {Ru(O) 2 } 2+ complexes; and T. G. Spiros group played a leading role in the characterization of both natural and synthetic oxo-metal porphyrins by vibrational spectroscopy.[2] The work of Groves and Spiro not only had an impact on heme and porphyrin chemistry, but also influenced both related (corrole-chelated {Cr(O)} 3+ and {Mn(O)} 3+ complexes, for example) [3] and quite different (non-heme complexes of {Fe(O)} 2+ and {Fe(O)} 3+ , for example) [4] systems. The Ballhausen-Gray predictions were right on the mark until most recently, as dioxo metal complexes were limited to the second and third transition-metal series, and, although the analysis did not rule out the possible existence of MO 2 n+ species for 3d metal ions, there was only indirect evidence for their involvement in certain cases. This situation has now taken a dramatic new turn, with the most recent publication by Groves, Spiro, and co-workers. [5] The isolation of (oxo)manganese(V) porphyrins appears to have been much more difficult than that of (oxo)chromium(V) and (oxo)iron(V) porphyrins, the famous Compound I analogue that is nowadays accepted to have an (oxo)iron(IV) porphyrin radical structure. [6] Recall that it took 20 years from the time (oxo)manganese(V) was first proposed as a key intermediate in catalysis until such a complex was unambiguously characterized by Jin and Groves.[7] The diamagnetism of that (and related) species is entirely consistent with the original Ballhausen-Gray MO analysis, as the d 2 low-spin state (paired electrons in the d xy orbital, with the manganese-oxo bonds along the z axis) is obtained only because the d xz and d yz metal orbitals are at relatively high energy, as these orbi...

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

“…These complexes were synthesized by treating Mn(III) porphyrins with H 2 O 2 in the presence of excess n Bu 4 OH ( Figure 26 b). 233 , 234 These compounds exhibited Raman ν sym (O=Mn V =O) values of 741–744 cm –1 , which were decreased by 21–26 cm –1 and 39–44 cm –1 , respectively, upon single and double 18 O substitution on O=Mn V =O moiety. Significantly, ν assym (O=Mn V =O) values detected by IR were predictably different, confirming the linear, triatomic structure of the dioxoMn(V) unit ( Figure 26 c).…”

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

“…However, the structure of this complex was not assigned correctly and it was subsequently reformulated as a trans-dioxo Mn V porphyrin complex, based on reinterpretation of the spectroscopic data and independent generation of another example of a trans-dioxo Mn V porphyrin complex. 137 …”

C–H oxidation by hydroxo manganese(v) porphyrins: a DFT study

“…All these prototropic species are involved in the oxo-hydroxo tautomerism. 17,18 Both [Mn(OH) 2 (por)] + , at more acidic pH, and MnO(por)(OH), at less acidic pH, are Mn V (OH) species.…”