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Reports: DNI354178-DNI3: Preparation of Low-Coordinate Bis(Alkoxide) Metal Complexes and Their Reactivity in Bond Formation and Bond Activation Reactions Involving Azide Precursors

Stanislav Groysman, Wayne State University

The goal of this project is to develop a new family of transition-metal complexes catalyzing C-N and N-N bond formation, with particular emphasis on creating highly reactive nitrene functionality in an exceptionally weak-field ancillary ligand environment. The design of the system relies on a newly developed bulky alkoxide ligand that leads selectively to low-coordinate bis(alkoxide) 3d transition metal complexes. During the last funding period (year 1 of this project), our endeavors focused primarily on two research directions: (1) synthesis and characterization of the full series of the 3d bis(alkoxide) complexes for Cr(II) – Cu(II); (2) investigation of the reactivity of the iron bis(alkoxide) complex Fe(OR)2(THF)2 with alkyl/aryl azides. The reaction of MCl2 with two equivalents of LiOR (where OR = OCtBu2Ph and M = Cr, Mn, Fe, Co) initially forms the seesaw geometry complexes of [M2Li2Cl2(OR)4] composition (1-4, Figure 1 below). In contrast, the reaction of NiCl2(dme) (or NiBr2(dme)) with two equivalents of LiOR forms the distorted trigonal planar complexes [NiLiX(OR)2(THF)2] 5 (X = Cl) and 6 (X = Br). An intriguing reactivity was observed for Cu(II): treatment of CuCl2 or CuBr2 with two equivalents of LiOR affords the Cu(I) product Cu4(OR)4. This unusual reduction (in which alkoxide serves as a reductant) was further investigated using UV-vis and NMR spectroscopy. UV-vis spectroscopy established the existence of a short-lived intermediate, whereas in-situ NMR of the reaction mixture demonstrated presence of the products consistent with the β-scission of the alkoxide radical. Based on these data, a mechanism for the formation of Cu4(OR)4 was proposed (Figure 2), in which the initially formed [CuLiX(OR)2(THF)2] (analogous to the isolated Ni complex) undergoes reduction by the alkoxide ligand to form isolated [CuI4(OR)4], while the resulting alkoxide radical undergoes a β-scission to form a variety of organic products.      Figure 1. Reactions of LiOR with 3d metals M = Cr – Cu.   Figure 2. Mechanism explaining formation of the CuI4(OR)4 from the reaction of CuII precursors with the alkoxide ligand.   Following the synthesis of the seesaw clusters, we sought to transform them into mononuclear bis(alkoxide) complexes. This transformation was achieved using thallium hexafluorophosphate due to the insolubility of thallium chloride. Complexes Mn(OR)2(THF)2, Fe(OR)2(THF)2 and Co(OR)2(THF)2 were obtained in good yields as colorless (Mn and Fe) and violet (Co) crystals by recrystallization from hexane. The complexes M(OR)2(THF)2 are all high-spin species exhibiting distorted tetrahedral geometry with larger angles between the alkoxides and smaller angles between THF ligands (for the example of Fe(OR)2(THF)2, see Figure 3). No formation of M(OR)2(THF)2 complexes was observed for M = Cr and Ni.  
       

 Figure 3. The structure of Fe(OR)2(THF)2, 50% probability ellipsoids.   We have also investigated the reactivity of Fe(OR)2(THF)2 with alkyl and aryl azides. Several different reactivity modes were observed. The reaction of Fe(OR)2(THF)2 with an alkyl (adamantyl) azide led unexpectedly to the reductive coupling of azides, forming an hexazene moiety. In contrast, treatment of Fe(OR)2(THF)2 with aryl azides leads invariably to the formation of highly reactive transient iron-nitrene complexes. DFT calculations indicate that this formally iron(IV)-imido species [Fe(OR)2(NAr)] is in fact iron(III) antiferromagnetically coupled to an imido radical. Two different reactivity patterns were observed for the transient [Fe(OR)2(NAr)] species. For the relatively bulky Ar groups (mesityl, 2,6-diethylphenyl), coupling of the nitrene functionality is observed to form the resulting diazene. This reaction is catalytic: using as little as 1 mol% of Fe(OR)2(THF)2 leads to the fast and quantitative formation of azomesitylene. Smaller Ar groups (Ar = 4-methylphenyl, 4-trifluoromethylphenyl, 3,5-dimethylphenyl) do not form diazenes, yielding the dinuclear iron(III)-imido complexes instead (Figure 4).        Figure 4. Reactivity of Fe(OR)2(THF)2 with alkyl and aryl azides.   To summarize, our group has developed a new family of bis(alkoxide) complexes of the middle and late first-row transition metals and is currently investigating their reactivity. Fe(OR)2(THF)2 has demonstrated very promising initial results, displaying rich chemistry with various azides via both reductive coupling and reductive splitting. Catalytic formation of azo compounds (diazenes) was also observed. Our future endeavors in this project (year 2) will focus on (1) the investigation of the C-N bond formation reactivity involving metal-nitrene functionality; (2) attempts to form metal-carbene species analogous to the observed metal-nitrene complexes.