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Organometallics 2005, 24, 778-784

Reviews
Single Electron Transfer Reactions in the Synthetic Organometallic Chemistry of First-Row Transition Metals
Kevin M. Smith†
Department of Chemistry, University of Prince Edward Island, 550 University Avenue, Charlottetown, Prince Edward Island, Canada C1A 4P3 Received October 17, 2004

The discovery of new applications in small-moleculeactivation, olefin polymerization, and organic synthesis has stimulated interest in well-defined, paramagnetic, first-row transitionmetal organometallic compounds. Single-electron-transfer reactions have proved to be a useful tool in the targeted synthesis of such complexes. In this review, several examples from the recent literature are used to illustrate this strategy. The case studies include-diketiminato complexes of Cr(II) and Cr(III), three-coordinate Ni(II) species, and -diketiminato complexes of V(III), V(IV), and V(V).
Introduction The preparation of well-defined yet catalytically active complexes provides an exciting challenge to synthetic organometallic chemists. Once the fundamental structural requirements for catalytic activity have been identified, the steric and electronicproperties of the ancillary ligands can be systematically altered to enhance the reactivity profile of the resulting catalyst. Over the past 25 years, organometallic success stories such as stereoselective olefin polymerization and olefin metathesis have relied on this interplay between inorganic synthesis and catalysis.1,2 Perhaps the most inspiring example of this relationship has been the ongoingdevelopment of d0 catalysts of molybdenum.3 By comparison, the synthetic methodology for paramagnetic organometallic complexes of the first-row transition metals is not nearly as advanced. While this may be due in part to the perception that second-row metals are more important catalytically, there are also significant chemical challenges to be overcome. The stability of several adjacentoxidation states effectively precludes reactions based on two-electron oxidative addition and reductive elimination processes, as are seen in the familiar catalytic cycles based on d10 and d8 complexes of palladium4 or d6 and d4 ruthenium species.5 Instead, disproportionation and/or radical reactivity is often observed, and single electron transfer (SET)
E-mail: kmsmith@upei.ca. (1) Coates, G. W. Chem.Rev. 2000, 100, 1223-1252. (2) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18-29. (3) Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42, 4592-4633. (4) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 41764211. (5) Trost, B. M.; Toste, F. D.; Pinkerton, A. B. Chem. Rev. 2001, 101, 2067-2096.
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reactivity is common.6 Even within a single oxidation state, multiplespin states are sometimes accessible.7 Perhaps most significantly, the presence of unpaired electrons renders NMR spectroscopy less useful in characterizing paramagnetic complexes.8 As a result of these factors, multistep inorganic syntheses involving paramagnetic first-row metals remain relatively uncommon.9 Much of the published work concerns the preparation of homoleptic species or the use ofredox reactions to generate transient 19e or 17e metalloradical species from diamagnetic 18e organometallic precursors.10 The potential for paramagnetic organometallic synthesis has brightened in recent years, in part due to technological advancements. The use of CCD detectors has dramatically decreased the minimum crystal size required for single-crystal X-ray diffraction, to the point where X-raystructural determination has become almost mandatory for confirming the identity of all new paramagnetic products. While it remains difficult to definitively characterize paramagnetic complexes by NMR spectroscopy,8 more researchers are using NMR as a screening technique. Improvements in computer speed and in commercially available computational packages have led to the increased utility of...