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, (PDI)FeOEt (PDI = 2,6-(2,6-Pr2-CHN=CMe)CHN; R = Me, Et, SiMe), to yield the corresponding iron benzofuranyl derivative was studied. Synthesis of the requisite iron alkoxide complexes was accomplished either by salt metathesis between (PDI)FeCl and NaOR (R = Me, Et, SiMe) or by protonation of the iron alkyl, (PDI)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 (PDI)FeOR compound as an essentially planar, high-spin, = 3/2 compound engaged in antiferromagnetic coupling with a radical anion on the chelate ( = 3/2; = 2, = 1/2). The resulting iron benzofuranyl product, (PDI)Fe(2-benzofuranyl) was characterized by X-ray diffraction and in combination with magnetic measurements, spectroscopic and computational data, was identified as an overall = 1/2 compound, demonstrating that a net spin-state change accompanies transmetallation ( = 1, = 1/2). These findings may be relevant to further development of iron-catalyzed Suzuki-Miyaura cross-coupling with neutral boronate esters and alkoxide bases.