This award by the Inorganic, Bioinorganic, and Organometallic Chemistry Program supports the work of Professor Jonas Peters at the Massachusetts Institute of Technology to investigate (phosphinoborate)iron(I) compounds that are structurally unusual by virtue of their relatively low coordination numbers and geometry. These complexes feature iron-nitrogen multiple bonds. These complexes have been found to mediate C-O cleavage pathways that effect partial or complete CO extrusion or reductive C-C coupling to produce oxalate. The synthesis of monometallic and bimetallic iron(oxo) species will also be explored particularly in terms of dihydrogen activation. Finally, the ability to activate carbon dioxide and release iron carbonyl products opens the door to using carbon dioxide as a synthon for C-C coupling reactions that exploit CO as a carbene or carbyne precursor. The synthesis and reactivities of iron(carbenes) and (carbynes) with hydrogen will also be investigated. In addition to the reductive degradation of carbon dioxide which may have important ramifications for the reduction of greenhouse gases and the development of new methods of carbon management, this research will encourage undergraduate and graduate students to become sensitive to energy conversion issues as they enter their chemical careers.
Award No.: 1062716 Research Activities: Our major research activities focus on the synthesis of new iron and other transition metal coordination targets due to their affinity for reacting with CO/CO2 in reactions that may lead to more valuable chemicals. We are also exploring nickel, palladium, and platinum complexes of unusual geometries, as they might reveal new C-H bond activation reactions. Our program will enable us to explore chemistry relevant to Fe-mediated C-O cleavage and C-C bond formation from CO2 and CO. We have discovered a host of multi-electron group transfer processes. We have found that certain tris(phosphino)borate supported iron(I) species, [PhBPR3]Fe(I), react with CO2 to mediate (i) C-O cleavage pathways for partial or complete CO extrusion, or (ii) reductive C-C coupling to produce oxalate. Such reactivity is highly unusual for later first row metals and warrants detailed exploration. Due to its vast supply and proposed role in global warming, CO2 would be an ideal source for fine chemicals and fuels. Transfer of an oxygen atom from CO2 is thus desirable but difficult, due to the stability of CO2. Nature employs late transition metals in a [NiFe] bimetallic core that facilitates this transformation. In contrast to this, most examples of coordination complexes that deoxygenate CO2 feature early transition, lanthanide and actinide metals. Examples of later metals displaying similar reactivity are comparatively rare. An alternative transformation involves direct coupling of two CO2 molecules to form oxalate, but well-defined, homogeneous metal complexes that mediate this reaction remain uncommon. To our knowledge, there is only one such reported system. Our group is interested in the small molecule chemistry triggered by unusual oxidation states and geometries of iron. Recently, we have become interested in the tris(phosphine)borane ligand (o-iPr2PC6H4)3B, or iPrTPB, developed by Borissou and coworkers. We hypothesized that we could exploit the coordinative flexibility of the boron center and stabilize geometries conducive to both high and low oxidation state intermediates. We have found that iPrTPB does coordinate iron in the desired manner, with all three phosphines bound. Additionally, we have been able to isolate and structurally characterize a number of complexes with varying apical ligands and found that the boron-iron interaction strength tracks with the iron oxidation state, varying from 2.608 Å for the pseudo-tetrahedral (iPrTPB)FeN(p-OMePh) to 2.293 Å for the trigonal bipyramidal [(iPrTPB)Fe(N2)][Na]. We are also investigating two routes to C-O bond cleavage: O-atom abstraction and O-atom functionalization. In the first method, we will add early transition metal complexes to a metal carbonyl species to encourage M-O bond formation and C-O bond cleavage. The second method will attempt to functionalize the oxygen atom, thereby weakening the C-O bond and allowing for easier cleavage. This method was used recently in our group to generate an iron-siloxycarbyne species where the M-O bond was formed, though C-O bond cleavage was ultimately not realized. Furthermore, we have synthesized and studied a series of novel, Group 10 metal complexes of the tris(phosophino)silyl ligand [SiPR3]. The unique steric and electronic properties of this ligand provide access to a series of trigonal bipyramidal cations {[SiPR3]M(L)}+ that feature weakly-coordinated ligands in the apical position. In the platinum system, the phenyl-substituted ligand enables coordination of weak donor ligands such as dichloromethane, diethyl ether, and benzene. The solid-state structure of the toluene adduct shows a close contact between the cationic metal center and an aryl C-H bond, which could be an unusual example of a directly observed intermolecular agostic interaction; these types of interactions are of interest as they are hypothesized to precede C-H bond activation. With the more isopropyl-substituted ligand, we are able to exclude the fifth, axial ligand to afford the four-coordinate, trigonal pyramidal complex {[SiPiPr3]Pt}+, as well as the palladium analogue. The geometries of these complexes stand in sharp contrast to prototypical d8 square-planar Pt and Pd geometries. The corresponding cationic nickel complexes coordinate dinitrogen, providing one of the first examples of N2 bound to a Ni(II) center. The N2 ligands can be displaced by dihydrogen, generating the first examples of H2 adducts of Ni. Additionally, the bound dihydrogen ligands can be deprotonated by a base to generate the neutral hydride species. These compounds may shed light on the mechanism of H2 activation in nature by nickel-iron hydrogenases. We are currently also exploring the chemistry of diiron complexes of an asymmetric ligand, H[NOP], with both hard and soft binding sites. The design of this ligand scaffold allows an electron-rich metal center to be coupled to a more acidic metal center, a set-up which may facilitate the activation of CO2 in a manner similar to that of the [NiFe] bimetallic core found in nature.
