Direct Observation of Transmetalation from a Neutral Boronate Ester to a Pyridine(diimine) Iron Alkoxide

Transmetallation of the neutral boronate esters, (2-benzofuranyl)BPin and (2-benzofuranyl)BNeo (Pin = pinacolato, Neo = neopentylglycolato) to a representative pyridine(diimine) iron alkoxide complex, (^iPrPDI)FeOEt (^iPrPDI = 2,6-(2,6-ⁱPr2-C₆H₃N=CMe)₂C₅H₃N; R = Me, Et, SiMe₃), to yield the corresponding iron benzofuranyl derivative was studied. Synthesis of the requisite iron alkoxide complexes was accomplished either by salt metathesis between (^iPrPDI)FeCl and NaOR (R = Me, Et, SiMe₃) or by protonation of the iron alkyl, (^iPrPDI)FeCH2SiMe3, by the free alcohol R'OH (R' = Me, Et). A combination of magnetic measurements, X-ray diffraction, NMR and Mössbauer spectroscopies and DFT calculations identified each (^iPrPDI)FeOR compound as an essentially planar, high-spin, S = 3/2 compound engaged in antiferromagnetic coupling with a radical anion on the chelate (S _Total = 3/2; S _Fe = 2, S _PDI = 1/2). The resulting iron benzofuranyl product, (^iPrPDI)Fe(2-benzofuranyl) was characterized by X-ray diffraction and in combination with magnetic measurements, spectroscopic and computational data, was identified as an overall S = 1/2 compound, demonstrating that a net spin-state change accompanies transmetallation (S _Fe = 1, S _PDI = 1/2). These findings may be relevant to further development of iron-catalyzed Suzuki-Miyaura cross-coupling with neutral boronate esters and alkoxide bases.