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Reports: G3 48624-G3: Chelating Pyridyl Disulfide Ligands as Redox Modulators for Aerobic Oxidation
As noted in the first progress report, funds from the Grant #48624-G3 were directed to two projects that differed from the original proposal. While different in topic, these two projects have become the main focus for our young group and the PRF-G funds were instrumental in establishing the work. Seed funding from the PRF-G program enabled us to successfully obtain an NSF CAREER Award to continue our research. We are extremely grateful for the support and are proud to summarize our results below. Thank you.
I. Research Program in Chemistry of Low-Coordinate Metal Isocyanides:
Out largest and highest-profile project focuses on the construction and reactivity of low-coordinate transition metal isocyanide complexes. Isocyanides are isolobal surrogates to carbon monoxide (CO) and the goal of this project is to generate and study condensed-phase analogues of the unsaturated binary transition metal carbonyls. These carbonyls (e.g. Mo(CO)3, Fe(CO)4, Co(CO)4 and Ni(CO)3) have been exclusively studied in the gas-phase. Accordingly, limited information is known about their electronic/geometric structure or reactivity patterns, despite the fact that these compounds have been paramount to the development of organometallic chemistry. To generate stable mimics of the unsaturated carbonyls, my group has introduced m-terphenyl isocyanides as ligands in transition-metal chemistry. This project has resulted in several high-quality publications, which are summarized below.
Publication#1: In this important communication we disclosed the two-coordinate, unsaturated metal isocyanide complex Pd(CNArDipp2)2. Two-coordinate Pd(CNArDipp2)2 features our second m-terphenyl isocyanide ligand, CNArDipp2 (Dipp = 2,6-(i-Pr)2C6H3; see publication #4, below). Palladium(0) bis-isocyanide complexes were long sought in the literature and Pd(CNArDipp2)2 represents the first definitive example. In addition, Pd(CNArDipp2)2 shows remarkable reactivity toward small molecule substrates. Highlights include i) the ability of Pd(CNArDipp2)2 to effect Suzuki-Miyaura C-C bond formation under remarkably mild conditions and ii) the ability of Pd(CNArDipp2)2 to effect an unprecedented one-electron reduction of the N-O unit in nitrosobenzene (PhNO). In fact, this latter reaction uncovered the redox non-innocence of nitrosobenzene.
Publication#2: In this article we disclose the synthesis of the encumbering m-terphenyl isocyanide CNArDipp2, which is a more encumbering version of CNArMes2. The most important finding is that CNArDipp2 only furnishes bis-isocyanide complexes in cases where CNArMes2 forms tris-derivatives. Another highlight is a sterically promoted fac-to-mer isomerization in a Group 6-M(CO)3L3 complex, as this is traditionally thermodynamically disfavored.
Publication#3: In this communication, we report the synthesis and characterization of a series of stable tetra-isocyanide cobalt complexes that mimic the binary carbonyls [Co(CO)4]-, Co(CO)4 and [Co(CO)4]+. These carbonyls have been proposed as the active species in industrial hydroformylation processes, but longstanding controversy has remained concerning the precise electronic and geometric structure of these simple complexes. This is especially true for the neutral species Co(CO)4. Using a combination of variable-temperature crystallographic, EPR spectroscopic, magnetic and Density Functional Theory studies, we elucidated, in atomic-level detail, the subtle structural and spectroscopic features of the d9 complex Co(CNArMes2)4 and showed how these characteristics are shared with the binary carbonyl Co(CO)4. In addition, we showed how the 1+, 0 and 1- charge states of the [Co(CNArMes2)4] can be modulated by discrete one-electron events. This publication has also set the stage for several additional studies on the reactivity of the unsaturated cobalt isocyanide complexes in various charge states.
Publication#4: In this article, we disclose synthetic and structural details for tris-CNArMes2 cobalt halide complexes which may serve as valuable precursors to anionic, neutral and cationic complexes of the formulation [Co(CNArMes2)3]n (n = 1+, 0, 1-). Major findings include an interesting valence-state series wherein three CNArMes2 ligands are accommodated by Co(I), Co(II) and Co(III) ions with only iodide completing the primary coordination sphere. Spectroscopic and magnetic studies characterize these and other cobalt tris-isocyanide halide complexes featured in the paper.
Publication#5: In this communication, we detail the synthesis, structural properties and reactivity of the mixed isocyanide/carbonyl manganese monoanions [Mn(CO)2(CNArMes2)3]- and [Mn(CO)3(CNArDipp2)2]-. These new manganese monoanions bridge the stability gap between the unstable penta-isocyanide anion [Mn(CNXyl)5]- reported by Cooper and the robust anion [Mn(CO)5]-. However, they incorporate the steric protection offered by the m-terphenyl framework. This feature is demonstrated by the addition of SnCl2 to the sodium salt of [Mn(CO)3(CNArDipp2)2]- to afford the chloride substituted metallostannylene ClSnMn(CO)3(CNArDipp2)2. The latter is the first example of an unprotected metallostannylene. This publication was submitted as an invited contribution to the Emerging Investigators themed issue of Chemical Communications.
II. Project in the Chemistry of Low-Coordinate Iridium Hydrides and Energy Processes: A second group project focuses on low-coordinate iridium hydride complexes and their ability to activate H2O and H2 in transformations relevant alternative-energy processes. The major impetus for this project arose from literature reports of H2O oxidative addition at low-valent iridium centers. However, many of the precursors or products of H2O oxidative addition are reported to be unstable with respect to H2O loss or ligand degradation. We felt we could make an impact in this area though improved ligand design strategies. Thus, we set out to develop a new class of encumbering phosphine ligands that are resistant to oxidative addition, but do not feature fluorinated substituents. This strategy has worked and has produced significant new results as detailed in the publication below.
Publication#6: In this communication, we describe the isolation and characterization of two four-coordinate iridium(I) monohydrides. These compounds have been proposed as reactive intermediates in a host of important transformations including C-H activation, hydroformylation and hydrogenation reactions since the 1970’s. However, definitive evidence for their existence has never been achieved. Using our C-H activation resistant methyleneadamantyl phosphine, we showed that such complexes could indeed be isolated. In this work, we also demonstrated a unique and reversible N2 binding/C-H reductive elimination scheme, H2O activation to yield the first transition metal dihydride hydroxide and a hydride deprotonation reaction yielding an anionic four-coordinate iridium(I) monohydride of unique construction. With respect to alternative-energy processes, one of the more important aspects of this work is the demonstration that dihydrogen rapidly expels water from the dihydride hydroxide complex at room temperature. This is notable since several researchers are pursing similar iridium complexes as catalysts for both electrochemical and photolytic water splitting.
