The Chemistry of Tris(phosphino)borate Manganese and Iron Platforms

Abstract

The coordination chemistry of monovalent and divalent manganese complexes supported by the anionic tris(phosphino)borate ligand [PhBPiPr3] is presented. The halide complexes, [PhBPiPr3]MnCl and [PhBPiPr3]MnI, have been characterized by XRD, SQUID magnetometry, and EPR spectroscopy. The halide [PhBPiPr3]MnI serves as a precursor to manganese azide, alkyl, and amide species: [PhBPiPr3]Mn(N3), [PhBPiPr3]Mn(CH2Ph), [PhBPiPr3]Mn(Me), [PhBPiPr3]Mn(NH(2,6-iPr2Ph)), [PhBPiPr3]Mn(dbabh), and [PhBPiPr3]Mn(1-Ph(isoindolate)). Collectively, they represent an uncommon motif of low-coordinate polyphosphine-supported manganese species. Some of our synthetic efforts to generate [PhBPiPr3]Mn?Nx species are described, as are theoretical DFT studies that probe the electronic viability of these multiply bonded target structures. Two tris(phosphino)borate ligands, [PhBPter3] and [PhBPCH2Cy3] are introduced that feature terphenyl and methylcyclohexyl groups on the phosphine arms, respectively. The iron chlorides, [PhBPter3]FeCl and [PhBPCH2Cy3]FeCl, have been prepared as precursors to iron nitrides. Addition of the nitride transfer reagent Li(dbabh) to [PhBPCH2Cy3]FeCl produced the terminal nitride, [PhBPCH2Cy3]Fe(N). The 15N NMR spectrum of the labeled species, [PhBPCH2Cy3]Fe(15N), contains a peak at 929 ppm, consistent with a terminal nitride functionality. Mossbauer spectroscopy of the nitride shows a low isomer shift value of 0.34(1) mm/s and an exceptionally large quadrupole splitting of 6.01(1) mm/s. Reduction of [PhBPCH2Cy3]FeCl generates a masked iron(I) species that is highly reactive. Combustion analysis of this species is consistent with [PhBPCH2Cy3]Fe. Other physical methods including VT NMR, EPR, and IR spectroscopies suggest the presence of a paramagnetic species in equilibrium with a diamagnetic species. The paramagnetic component is postulated to be an Fe(III) hydride, wherein a ligand C-H bond has been cyclometalated at the metal center. The reactivity of [PhBPCH2Cy3]Fe is consistent with iron(I). For example, its reaction with PMe3 and 1-adamantylazide affords the phosphine adduct, [PhBPCH2Cy3]Fe(PMe3), and the iron imide, [PhBPCH2Cy3]Fe(NAd), respectively. Interestingly, [PhBPCH2Cy3]Fe undergoes redox reactions with benzene to give initially a benzene adduct, {[PhBPCH2Cy3]Fe}2(mu-eta3:eta3-C6H6), which decomposes to {[PhBPCH2Cy3]Fe}2(mu-eta5:eta5-6,6'-bicyclohexadienyl) via radical C-C bond coupling. Finally, [PhBPCH2Cy3]Fe readily reduces CO2 at rt to give as the major product {[PhBPCH2Cy3]Fe}2(mu-CO)(mu-O), wherein a C=O bond has been cleaved. The minor product has not been definitively established, but one possibility is the oxalate-bridged dimer {[PhBPCH2Cy3]Fe}2(mu-eta2:eta2-O2CCO2) that results from reductive coupling of two CO2 molecules.