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

Transmetalation 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 where the iron is engaged in antiferromagnetic coupling
with a radical anion on the chelate (STotal = 3/2; SFe = 2, SPDI = −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
transmetalation (SFe = 1, SPDI = −1/2). These findings may be relevant to
further development of iron-catalyzed Suzuki–Miyaura cross-coupling
with neutral boronate esters and alkoxide bases.