Design, Synthesis, and Study of Novel Platforms for Iron-N2 Chemistry and Photoinduced, Copper-mediated C-N Bond Formation

Several new ligand platforms designed to support iron dinitrogen chemistry have been developed. First, we report Fe complexes of a tris(phosphino)alkyl (CPiPr3) ligand featuring an axial carbon donor intended to conceptually model the interstitial carbide atom of the nitrogenase iron-molybdenum cofactor (FeMoco). It is established that in this scaffold, the iron center binds dinitrogen trans to the Calkyl anchor in three structurally characterized oxidation states. Fe-Calkyl lengthening is observed upon reduction, reflective of significant ionic character in the Fe-Calkyl interaction. The anionic (CPiPr3)FeN2- species can be functionalized by a silyl electrophile to generate (CPiPr3)Fe-N2SiR3. This species also functions as a modest catalyst for the reduction of N2 to NH3. Next, we introduce a new binucleating ligand scaffold that supports an Fe(μ-SAr)Fe diiron subunit that coordinates dinitrogen (N2-Fe(μ-SAr)Fe-N2) across at least three oxidation states (FeIIFeII, FeIIFeI, and FeIFeI). Despite the sulfur-rich coordination environment of iron in FeMoco, synthetic examples of transition metal model complexes that bind N2 and also feature sulfur donor ligands remain scarce; these complexes thus represent an unusual series of low-valent diiron complexes featuring thiolate and dinitrogen ligands. The (N2-Fe(μ-SAr)Fe-N2) system undergoes reduction of the bound N2 to produce NH3 (~50% yield) and can efficiently catalyze the disproportionation of N2H4 to NH3 and N2. The present scaffold also supports dinitrogen binding concomitant with hydride as a co-ligand. Next, inspired by the importance of secondary-sphere interactions in many metalloenzymes, we present complexes of iron in two new ligand scaffolds ([SiPNMe3] and [SiPiPr2PNMe]) that incorporate hydrogen-bond acceptors (tertiary amines) which engage in interactions with nitrogenous substrates bound to the iron center (NH3 and N2H4). Cation binding is also facilitated in anionic Fe(0)-N2 complexes. While Fe-N2 complexes of a related ligand ([SiPiPr3]) lacking hydrogen-bond acceptors produce a substantial amount of ammonia when treated with acid and reductant, the presence of the pendant amines instead facilitates the formation of metal hydride species. Additionally, we present the development and mechanistic study of copper-mediated and copper-catalyzed photoinduced C-N bond forming reactions. Irradiation of a copper-amido complex, ((m-tol)3P)2Cu(carbazolide), in the presence of aryl halides furnishes N-phenylcarbazole under mild conditions. The mechanism likely proceeds via single-electron transfer from an excited state of the copper complex to the aryl halide, generating an aryl radical. An array of experimental data are consistent with a radical intermediate, including a cyclization/stereochemical investigation and a reactivity study, providing the first substantial experimental support for the viability of a radical pathway for Ullmann C-N bond formation. The copper complex can also be used as a precatalyst for Ullmann C-N couplings. We also disclose further study of catalytic Calkyl-N couplings using a CuI precatalyst, and discuss the likely role of [Cu(carbazolide)2]- and [Cu(carbazolide)3]- species as intermediates in these reactions. Finally, we report a series of four-coordinate, pseudotetrahedral P3FeII-X complexes supported by tris(phosphine)borate ([PhBP3FeR]-) and phosphiniminato X-type ligands (-N=PR'3) that in combination tune the spin-crossover behavior of the system. Low-coordinate transition metal complexes such as these that undergo reversible spin-crossover remain rare, and the spin equilibria of these systems have been studied in detail by a suite of spectroscopic techniques.