Reports: DNI353542-DNI3: Imparting Precious Metal Properties to First-Row Metals for C-H Activation: Ligation to Inexpensive, Heavy Main Group Donors
printer friendlyImparting Precious Metal Properties to First-Row Metals for C-H Activation: Ligation to Inexpensive, Heavy Main Group Donors
This research project is aimed towards the ligation of first-row metals with heavy atom (As, Sb, Bi) ligands in an effort to simulate some of the magnetic and reactivity properties of the heavier, precious metals. Work up to this point has focused in the preparation of several antimony (Sb) containing ligands to explore this hypothesis. The ligand preparation is segmented into three portions below.
(A) Tripodal Sb3 Ligand: This ligand served as the primary focus of the PRF proposal. The ligand presents an Sb3 tripod, with each antimony donor bearing two phenyl units (R-SbPh2). We have prepared this ligand in small, yet reproducible quantities following some initial challenges in understanding the thermodynamic nature of isolating Sb-containing ligands and intermediates. Metalation attempts with the Sb3 ligand so far have been hampered due to the high lability of the Sb-Ph bond, and difficulties in metalating first-row metals. Presently, we are preparing an alternate form of the ligand to avoid this problem, namely the di-isopropyl congener. The higher bond strength of the Sb-alkyl unit should promote isolation of the ligand in higher yields, as well as more stable complexations in the presence of first-row metal salts. We are also improving our metalation conditions (solvent, counterion, temperature, ancillary ligand) to more reliably afford the desired complexes. In preparing the ligand and corresponding antimony starting materials, we have gained valuable experience in understanding Sb-based comproportionation reactions, as well as the general requirements to maintain Sb-C bonding motifs during reaction sequences and hands-on manipulations of compounds.
(B) Bidentate Sb2 Ligand: This naphthalene-based ligand provides an alternate pathway towards the ligation of the first row metals with Sb ligands. While the bidentate nature of the ligand is inferior to the tripod described above for reactivity studies, the Sb2 format will allow us to prepare metal complexes for magnetic and spectroscopic measurements.
(C) Tripodal N-Sb3 Ligand: The ligand of type N-Sb3 is derived from a central amine moiety, which is trisubstituted with benzyl(ortho)di(R-group)stibines. We have prepared the NSb3-Ph ligand wherein the two substitutents on each Sb donor are phenyl groups. In preliminary metalation studies with this ligand, the lability of the Sb-phenyl bond, and difficulties with choice of metalation conditions (again, solvent, temperature, counterions) has thus far limited its utility. As also stated in Part A, we are working to improve the ligand stability (Ph-iPr substituents) and reaction conditions.
Auxiliary Projects:
(A) Cobalt Peroxo Alkane Functionalization. We have discovered the spontaneous reaction of a simple N4 Schiff base ligand with Co(II) salts in presence of air that leads to a 'criss-crossed' μ-hydroxo,μ-peroxo species. This dicobalt(III) species can be oxidized to afford the non-oxo Co(IV) species that we postulate remains in the same m-hydroxo,m-peroxo coordination sphere. The Co(IV) character has been confirmed by low temperature EPR in MeCN solvent. Interestingly, addition of glassing agents such as toluene and CH2Cl2 abolish the EPR signal. We have hypothesized that this is due to a C-H activation event at the high-valent cobalt site. For our second contribution in this area, we are now attempting to characterize the organic oxidation products and catalytic capability of the system in presence of O2. We will also test the substrate scope and selectivity of this unusual non-oxo Co(IV) system.
(B) Hydrogen Utilizaton by Iron and Manganese Dicarbonyls related to the Mono-[Fe] Hydrogenase active site. We have also utilized the naturally occurring enzyme mono-[Fe] hydrogenase as inspiration to develop inexpensive hydride transfer catalysts (ideally, H2+CO2→CH3OH, methanol). Manganese and iron carbonyls can be useful in such reactions because their low-spin d6 electronic configurations make them analogous to the precious metals in some respects. We are preparing chelating ligands that replicate two key features of the mono-[Fe] hydrogenase active site, namely i) a unique metal-acyl bonding motif and ii) the presence of an N-ligated pyridone. It is hypothesized that the combination of these unique features in biology will provide a guiding framework toward the development of hydride transfer catalysts (from H2) derived from earth abundant metals.
