INTERMETAL OXYGEN, SULFUR, SELENIUM, AND NITROGEN ATOM TRANSFER-REACTIONS

Although atom transfer reactions have been known for over 40 years, an understanding of intermetal oxygen atom transfer processes still remains underdeveloped. Furthermore, atom transfer reactions involving other multiply bonded elements, M=S, M=Se, or M≡N have been reported only recently. Nonetheless, important aspects of these fundamental reactions are emerging. The oxygen atom transfer processes shown in reactions 32–45 allow metal complexes to be ranked in order of their reactivity toward atom transfer. It is apparent that these reactions are driven in part by M=O bond strengths. The metalloporphyrin-based reactions also indicate that electronic factors such as oxidation state and d-electron configurations have a significant role in the type of atom transfer reactions that take place. A more systematic approach in terms of developing a thermodynamic scale for oxygen atom transfer reactions has been summarized by Holm.11b In this approach, a thermodynamic oxygen atom transfer “half-reaction” is used (eq 62). Representative enthalpies [formula omitted] for selected half-reactions are listed in Table V. The ΔH limits shown in this table were established by the reaction or lack of reaction of metal complexes with oxygen atom donor-acceptor reagents with known enthalpies for half-reaction 62. Unfortunately, only a limited thermodynamic data set has been developed and much more work remains to be done. A similar thermodynamic scale for nitrogen atom transfer has not been developed yet. At this point, a severe limitation is the lack of complete nitrogen atom transfer examples. Furthermore, the only known cases involve transfer between two metal complexes. Reactions 36–45 illustrate another significant consideration. In these examples, the chloro and oxo functional groups, which are present in the metal complexes, are both capable of serving as reaction sites and in fact do result in competitive chlorine atom and oxygen atom transfer. This demonstrates the complications in transition metal chemistry arising from relatively polar, weak metal-ligand bonds. However, when a less reactive supporting ligand system is used, such as porphyrins or other macrocycles, regioselective reactions of functional groups can be achieved in transition metal complexes. Furthermore, these types of ligands have allowed the extension of inner-sphere redox processes to intermetal sulfur and selenium atom transfer reactions. © 1993, American Chemical Society. All rights reserved.