Novel reactivity at iron centers supported by poly(phosphino)borate ligands

Abstract

The reactivity of the iron(II) alkyl species [PhBPiPr3]FeMe ([PhBPiPr3] = PhB(CH2PiPr2)3-) towards Si-H bonds is presented. Reaction of [PhBPiPr3]FeMe with primary aryl silanes results in the unusual η3 silane adducts [PhBPiPr3]Fe(H)(η3-H2SiMeR). X-ray crystallography, Mossbauer spectroscopy, and theoretical calculations confirm this structural assignment; however, solution NMR experiments suggest a degree of fluxionality in solution. Low valent, tris(phosphino)borate iron platforms have been shown to facilitate the activation of white phosphorus, P4. The iron(I) precursors {[PhBPiPr3]Fe}2(μ-N2) and [PhBPPh3]Fe(PPh3) react with P4 to quantitatively generate {[PhBPiPr3]Fe}2(μ-P4) and {[PhBPPh3]Fe}2(μ-P4), respectively. These unique iron(II) dimers bridged by square P42- units have been characterized structurally and spectroscopically, and their reactivity has been examined. A simplified electronic structure calculation is presented to aid in discussion of bonding within these complexes. Motivated by the versatility of the tris(phosphino)borate ligands, a new family of tripodal hybrid bis(phosphino)pyrazolylborate ligands, [PhBPtBu2(pz')]- ([PhBPtBu2(pz')]- = PhB(CH2PtBu2)2(pz')-), has been prepared and characterized. The synthesis, spectroscopy, and solid-state structures of four-coordinate, pseudo-tetrahedral iron(II) and cobalt(II) halide complexes supported by these ligands is presented. To compare the electron-releasing ability of these ligands with their [PhBPR3] analogues, the cyclic voltammetry of these complexes is introduced. Potential routes to a terminal cobalt or iron nitride complex via extrusion of N2 from coordinated azide and metathesis with the N-atom transfer reagent Li(dbabh) are investigated. Reduction of the [PhBPtBu2(pz')]MX halide complexes in the presence of excess phosphine generates low valent [PhBPtBu2(pz')]MI(PMe3) precursors. These precursors react with organic azides to generate cobalt(III) and iron(III) imides. Initial reactivity studies indicate that these imides are more moderately more reactive than the corresponding tris(phosphino)borate complexes. The electrochemistry of the [PhBPtBu2(pz')]FeIII(NR) imides features a quasi-reversible to fully reversible oxidation event, dependent on choice of pyrazolyl substituents and scan rate. This oxidation can be achieved chemically to generate the isolable cationic iron(IV) imides, [PhBPtBu2(pz')]FeIV(NR)+. The structural and spectroscopic characterization of these highly unusual complexes is discussed.