60th Annual Report on Research 2015

Guidelines for the Project: This project represents the original proposal content from the PI, regarding the use of novel, heavy-atom containing ligands (As, Sb, Bi) to chelate catalytically active first-row metals. Essentially, the goal of the project is to impart some of the precious metal properties of expensive metals (like Pt, Rh, Ir, etc) to the base metals by surrounding the base metal with ‘soft’ heavy-atom donors. Like the precious metals, heavy-atom donors possess significant spin-orbit coupling constants (in quantum: J value)

Research Achieved: In this project we have made significant progress in assembling antimony (Sb) based ligands and resultant metal complexes. The novel ligand tri-isopropylantimony(III) has been prepared, and its complexes of Cu(I), Ni(II) and Co(II) have been isolated. The copper complexation reaction yields cuboids of general formula [Cu4(X)4(SbiPr3)4], which exhibit Cu–Cu bonding motifs and luminescent behavior at low temperatures. The cobalt and nickel complexes [mu-(I)2-(Co(SbiPr3))2] and [Ni(SbiPr3)2(CO)(I)2] have been isolated and structurally characterized. We are now investigating the magnetic properties of these complexes for comparison with their ‘light-atom’ congeners (where L = PiPr3 or AsiPr3) and their heavier atom congeners (where L = BiiPr3). Indeed, we have already prepared the novel ligand BiiPr3 in gram amounts and high yields, and are pursuing similar complexation reactions of CuI, NiI2, CoI2, FeI2 and MnI2.

Research in Progress: To address the poor ligation ability of antimony-based ligands (Sb is a weak sigma-donor), we are continuing to assemble both tripodal (Sb3) and tetrapodal (Sb4) ligands. Our original plan was to utilize the ‘phenylated’ antimony analog of the well-known ligand Triphos, and thus we successfully synthesized [CH3(CH2Sb(Ph)2)3] (Sb3-Ph). However, one of the peculiarities of antimony chemistry is the lability of the Sb–Ph bond in the presence of HCl, metal halides or (in our hand) open shell transition metals. Thus we found Sb3-Ph to be not applicable to the goals of this project. [As a result, we pursued isolation of complexes derived from tri-alkyl antimony ligands, as detailed in the previous paragraph.] Presently, we have isolated the corresponding ‘all-alkyl’ Sb3 ligand, namely [CH3(CH2Sb(CH3)2)3] (Sb3-Me). We are presently scaling up the synthesis of this ligand, and also working to isolate the more strongly chelating congener Sb3-iPr. Regarding the tetrapodal Sb4 approach, we have now isolated the key intermediate Sb(CHCH2)3, and are working to optimize conditions for the addition of Na–SbR2 or Li–SbR2 (where R = CH3, iPr) to generate the chelate Sb4-R.

Ancillary Projects

(A) Using Earth-Abundant Iron for Dihydrogen (H2) Activation and Hydride Transfer

Another main thrust that has been partially supported by the PRF funds is the development of iron-based catalyst for H2 activation and hydride transfer. Our primary inspiration for this project is the enzyme mono-iron hydrogenase, which utilizes a single Fe center in an unusual ligation environment to activate H2 and perform a hydride transfer in the biological reduction of CO2. In the last year, we have replicated the active site iron environment (fac CNS ligation) in small molecule format, in several ways. First, we have utilized an equatorial CNS ligand to isolate the complex [(CNS)Fe(CO)2(Br)]. Interestingly while this coordination environment replicates the identity of the donors in the active site, it does not replicate the proper arrangement of donors (mer vs fac). Consequently, the complex is not active for either H2 activation or hydride abstraction from organic substrates. In contrast, we have also assembled an anthracene-based scaffold that allows for the facial arrangement of the necessary donors. The complex [(Anth-CNS)Fe(Br)] does activate H2 (NH/ND scrambling) and serves as a potent catalyst for hydride abstraction from organic catalysts. We are now expanding the scope of hydrocarbon substrates for selective hydride abstraction, as well as pursuing high-pressure H2 NMR experiments to drive hydrogenation of suitable substrates.

(B) Iron-Based Carbonyl Carbido Clusters for Ethylene/Acetylene Reduction and Dinitrogen (N2) Reductive Aminations of Hydrocarbons

Lastly, the postdoc sponsored by the PRF grant worked on developing a novel set of iron-carbido-carbonyl clusters for the reduction of N2. This work is inspired by the biological active site of nitrogenase, which contains a unique [Fe6•C] core, where C = carbide C4–. Close inspection of the literature revealed that the [Fe6•C] motif has been investigated in the context of metal carbonyl clusters, especially those involving iron. Therefore, we have synthesized several existing and new iron-carbido-carbonyl clusters, such as [Fe6•C•(CO)16]2–, [Fe6•C•(CO)18], [Fe5•C•(CO)14]2–, [Fe4•C•(CO)12]2–, and the heterometallic [MoFe5•C•(CO)16]2–. Upon treatment with KC8 as reductant under N2 atmosphere, the clusters exhibit varying extents of N2 reduction, captured by a silane and detected as N(Si(CH3)3)3. While some clusters also exhibit a parasitic THF-ring opening reaction, other clusters are selective for the N2 reduction process. Presently, we are finalizing the synthetic, structural and reactivity details for the publication of several projects in this area.