Reactivity and Catalysis with Two-Coordinate Metal Complexes

On the basis of our observation that the simple iron amido complex Fe[N(SiMe3)2]2 catalyzes carbonyl hydrosilations,1 a more comprehensive program was initiated to explore the fundamental chemical and catalytic properties of two-coordinate first-row metal complexes. Although this relatively new class of complexes has attracted attention for their exceedingly low-coordinate structures, and more recently for their magnetic properties, very little is known about their chemical reactivity. The 2-coordinate bis(amido) Ni[N(SiMe3)DIPP]2 (DIPP = 2,6-iPr2C6H3), with an unusual, planar (eclipsed) (C)(Si)N–Ni–N(Si)(C) core, was found to be an efficient, selective catalyst for the anti-Markovnikov hydrosilation of olefins.2 Mechanistic studies on this catalysis were complicated by a lack of information concerning fundamental reaction steps that are possible for such 2-coordinate, high-spin metal centers. Thus, investigations focus on defining the fundamental transformations (e.g., redox events, bond activations) that are possible in such low-coordinate systems.

Recent Projects

One-electron reductions of Ni[N(SiMe3)DIPP]2 provide Ni(I) anionic {Ni[N(SiMe3)DIPP]2} and {Ni[N(SiMe3)DIPP]2}– complexes, which was alkylated by MeI to give only the second example of an isolated Ni(III) alkyl complex, Me–Ni[N(SiMe3)DIPP]2. Interestingly, the planar core of 15 is maintained in these transformations, and the T-shaped structure of the latter complex may be understood in terms of DFT-calculated frontier orbitals.3 The discovery of a Ni(III) alkyl complex allowed investigations into the possible role of such species in nickel-catalyzed C–C coupling reactions, as they have often been postulated as important intermediates. However, well-characterized examples had not been shown to participate in a competent catalytic cycle. Complex Ni[N(SiMe3)DIPP]2 was one of the first examples of a first-row metal, two-coordinate catalyst for an organic transformation, and mechanistic studies revealed compelling experimental evidence for the role played by a Ni(III)–alkyl species, as well as those of other isolated, key Ni(I) and Ni(II) intermediates.4 

This study reveals a surprising stability for the Ni[N(SiMe3)DIPP]2 fragment, and suggests the possibility of a new class of catalysts based on the unique ability of first-row transition metals to accommodate very low coordination numbers. For this reason, attempts are being made to expand the types of two-coordinate complexes available for reactivity and catalytic studies. The anionic Ni(I) complexes mentioned above are useful precursors to a number of neutral, two- and three-coordinate complexes of Ni(I).5 In addition, a rare, neutral two-coordinate Fe(I) complex with NHC and amido ligands was synthesized and characterized, and was found to function as a catalyst for alkyne trimerizations to arenes.6 Entry into low-coordinate Cr complexes required use of the bulkier –N(SiiPr3)DIPP amido ligand, and this system has provided analogous, two-coordinate Cr(I), Cr(II) and Cr(III) complexes, as well as the methyl derivative Me–Cr[N(SiiPr3)DIPP]2.7 

References

1. Yang, J.; Tilley, T. D. " Efficient Hydrosilylation of Carbonyl Compounds with the Simple Amide Catalyst Fe[N(SiMe3)2]2" Angew. Chem. Int. Ed. 2010, 49, 10186-10188. http://onlinelibrary.wiley.com/doi/10.1002/anie.201005055/full

2. Lipschutz, M.; Tilley, T. D. "Synthesis and Reactivity of a Conveniently Prepared Two-Coordinate Bis(amido) Nickel(II) Complex" Chem. Comm. 2012, 48, 7146-7148. http://dx.doi.org/10.1039/C2CC32974C

3. Lipschutz, M. I.; Yang, X.; Tilley, T. D. "A Structurally Rigid Bis(amido) Ligand Framework in Low-Coordinate Ni(I), Ni(II) and Ni(III) Analogues Provides Access to a Ni(III) Methyl Complex via Oxidative Addition" J. Am. Chem. Soc. 2013, 135, 15298–15301. http://dx.doi.org/10.1021/ja408151h

4. Lipschutz, M. I.; Tilley, T. D. "Carbon-carbon cross-coupling reactions catalyzed a two-coordinate nickel(II) bis(amido) complex via observable NiI, NiII and NiIII intermediates" Angew. Chem. Int. Ed., 2014, 53, 7290-7294. http://dx.doi.org/10.1002/anie.201404577

5. Lipschutz, M. I.; Tilley, T. D. "A Useful Method for the Preparation of Low-coordinate Ni(I) Complexes via Transformations of the Ni(I) Bis(amido) Complex K{Ni[N(SiMe3)(2,6-iPr2-C6H3)]2}" Organometallics 2014, 33, 5566-5570. http://dx.doi.org/10.1021/om500849u

6. Lipschutz, M. I.; Chantarojsiri, T.; Dong, Y.; Tilley, T. D. "Synthesis, Characterization and Alkyne Trimerization Catalysis of a Heteroleptic Two-Coordinate FeI Complex" J. Am. Chem. Soc. 2015, 137, 6366-6372. http://dx.doi.org/10.1021/jacs.5b02504

7. Cai, I. C.; Lipschutz, M. I.; Tilley, T. D. "A Bis(amido) Ligand Set That Supports Two-Coordinate Chromium in the +1, +2 and +3 Oxidation States" Chem. Comm., 2014, 50, 13062-13065. http://dx.doi.org/10.1039/C4CC06615D