The synthesis and characterization of phenoxy(imine) iron(II) alkyl precatalysts for C(sp 2 )−C(sp 3 ) Suzuki−Miyaura crosscoupling of aryl boronic esters and alkyl bromides is described. Addition of phenoxyimines (FI) to (py) 2 Fe(CH 2 SiMe 3 ) 2 (py = pyridine) afforded the high-spin iron(II) alkyl derivatives, (FI)Fe-(CH 2 SiMe 3 )(py) with varying N-imine substituents. With both neopentyl glycol-protected (BNeo) and pinacol-protected boronic ester (BPin) aryl nucleophiles, an iron-catalyzed cross-coupling method was realized that utilizes mild alkoxide bases. Optimal performance was observed in nonpolar solvents with anisole and fluorobenzene identified as more benign alternatives to benzene. The scope of this transformation includes high efficiency C(sp 2 )−C(sp 3 ) bond formation with both primary and secondary alkyl bromides with electron-deficient aryl and heteroaryl nucleophiles. Substrates with base-sensitive functionality including ester and nitrile groups were tolerated, highlighting the broader compatibility with an alkoxide base. Radical clock experiments support the formation of electrophile-derived radicals during catalysis, and experiments with preformed potassium aryl boronates demonstrate the role of boronate intermediates in transmetalation.

Pincer-type [P 2 Si]Rh complexes featuring a rhodium−silicon bond are shown to facilitate well-defined stoichiometric reductions of CO 2 with Si−O bond formation by two different pathways: (a) hydride transfer to CO 2 followed by formate migration to silicon, or (b) complete scission of the CO bond at the Rh−Si unit to afford a product with siloxide and carbonyl ligands. A combined experimental and computational study shows that the latter process occurs by anomalous insertion of CO 2 into the polarized Rh δ− −Si δ+ bond, a finding that is confirmed by extending the reactivity to an unchelated system. The siloxide carbonyl product can be further elaborated by reaction with water or pinacolborane to give structurally distinct CO 2 reduction products. Taken together, these results demonstrate how metal/main-group bonds can be tuned to direct migratory insertion reactivity.

The two-electron oxidative addition of aryl and alkyl halides to a reduced iron dinitrogen complex with a strong-field tridentate pincer ligand has been demonstrated. Addition of iodobenzene or bromobenzene to (3,5-Me2 MesCNC)­Fe­(N2)2 (3,5-Me2 MesCNC = 2,6-(2,4,6-Me-C6H2-imidazol-2-ylidene)2-3,5-Me2-pyridine) resulted in rapid oxidative addition and formation of the diamagnetic, octahedral Fe­(II) products (3,5-Me2 MesCNC)­Fe­(Ph)­(N2)­(X), where X = I or Br. Competition experiments established the relative rate of oxidative addition of aryl halides as I > Br > Cl. A linear free energy of relative reaction rates of electronically differentiated aryl bromides (ρ = 1.5) was consistent with a concerted-type pathway. The oxidative addition of alkyl halides such as methyl-, isobutyl-, or neopentyl halides was also rapid at room temperature, but substrates with more accessible β-hydrogen positions (e.g., 1-bromobutane) underwent subsequent β-hydride elimination. Cyclization of an alkyl halide containing a radical clock and epimerization of neohexyl iodide-d 2 upon oxidative addition to (3,5-Me2 MesCNC)­Fe­(N2)2 are consistent with radical intermediates during C­(sp3)–X bond cleavage. Importantly, while C­(sp2)–X and C­(sp3)–X oxidative addition produces net two-electron chemistry, the preferred pathway for obtaining the products is concerted and stepwise, respectively.

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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, ( iPr PDI)FeOEt ( iPr PDI = 2,6-(2,6-i Pr2-C 6 H 3 N=CMe) 2 C 5 H 3 N; R = Me, Et, SiMe 3 ), to yield the corresponding iron benzofuranyl derivative was studied. Synthesis of the requisite iron alkoxide complexes was accomplished either by salt metathesis between ( iPr PDI)FeCl and NaOR (R = Me, Et, SiMe 3 ) or by protonation of the iron alkyl, ( iPr 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 ( iPr PDI)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, ( iPr 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 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.