Skip to main content
NCBI home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2021 Jan 13.
Published in final edited form as: Organometallics. 2019 Dec 23;39(1):201–205. doi: 10.1021/acs.organomet.9b00733

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

Paul O Peterson 1, Stephan M Rummelt 1,‡, Bradley M Wile 1,†, S Chantal E Stieber 1,#, Hongyu Zhong 1, Paul J Chirik 1,*
PMCID: PMC7388646  NIHMSID: NIHMS1062977  PMID: 32728308

Abstract

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 (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 transmetallation (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.

Graphical Abstract

graphic file with name nihms-1062977-f0001.jpg

The success of Suzuki-Miyaura cross-coupling (SMC) with palladium catalysts has motivated the search for similar reactivity with more Earth-abundant first row transition metals.1 Iron is particularly attractive given its high terrestrial abundance and low toxicity. While iron-catalyzed Kumada-type coupling has been known since the 1940s2 and has been performed on an industrial scale,3 iron-catalyzed Suzuki-Miyaura cross coupling is by comparison less developed. Alkyl lithium-activated arylborate nucleophiles (activated with n-BuLi or t-BuLi) are often necessary for transmetallation to iron,4 but these anionic nucleophiles likely function more like organomagnesium reagents applied in Kumada coupling, limiting functional group tolerance and the ease of handling and operation associated with palladium-catalyzed reactions. Achieving transmetallation to iron and using comparatively mild bases (e.g. hydroxide, carbonate, alkoxide) applied routinely in palladium catalysis remains an unmet need and attractive target toward the further development of iron-catalyzed Suzuki-Miyaura cross couplings.

Nakamura and co-workers first reported an iron-catalyzed cross-coupling with a boron nucleophile, where lithium arylborates were coupled with primary and secondary alkyl halides to achieve C(sp2)–C(sp3) coupling.4a Lithium aryl borate species have also been shown by Bedford and co-workers to be nucleophiles for directed C(sp2)–C(sp2) coupling.4c In both cases, formation of the borate nucleophile was essential for a successful catalytic reaction and is likely required for transmetallation. Byers and co-workers have recently reported that amide bases enabled C(sp2)–C(sp3) cross coupling promoted by a cyano bis(oxazoline)-supported iron(II) precatalyst (Scheme 1).5 A key finding in this study was that no coupling was observed when the amide-borate complex was preformed prior to the reaction, suggesting transmetallation occurred from a neutral, rather than anionic, boron nucleophile. Alkoxide bases were ineffective due to aggregation of the resulting iron complexes. Such a pathway is analogous to that proposed for palladium SMC, whereby transmetallation from a neutral boron nucleophile to a hydroxy palladium intermediate is kinetically preferred over the alternative involving a hydroxide-coordinated borate.6

Scheme 1.

Scheme 1.

Open in a new tab

Transmetallation of boron nucleophiles to iron.

Examples of boron-based stoichiometric transmetallation to generate an iron aryl complex are primarily limited to lithium aryl borate nucleophiles.7 Neidig and co-workers demonstrated that the reaction of (SciOPP)FeCl2 (SciOPP = spin-control-intended ophenylene bisphosphine) with lithium phenylborate resulted in two successive transmetallation events to form (SciOPP)Fe(Ph)2, which undergoes reductive elimination to form (SciOPP)Fe(η6-biphenyl).8 Other examples include the reaction of triarylboranes with iron alkyl complexes, forming the corresponding iron aryl complex.9 In these examples, it is proposed that initial alkyl abstraction by the borane forms an activated borate, which becomes the nucleophile for transmetallation.

To explore the possibility of transmetallation from a neutral boron reagent to iron, pyridine(diimine) iron complexes were selected for study using an approach previously applied by our group for cobalt.10 This class of iron compounds exhibits a rich and diverse catalytic chemistry as well as the potential for electronic metal-ligand cooperativity, giving rise to a high density of states.11, 12, 13 More germane to iron-catalyzed cross coupling, both iron aryl, (iPrPDI)Fe(Ar) (Ar = phenyl; iPrPDI = 2,6-(2,6-iPr2-C6H3N=CMe)2C5H3N)),14 and alkoxide (iPrPDI)Fe(OR) (R = Et, allyl, etc.) derivatives have been synthesized with the latter class of compounds isolated following C–O bond cleavage in esters and ethers.15 Notably, the pyridine(diimine) iron aryl complexes possess different magnetic ground states from the iron alkoxides,16 suggesting that a spin-state change may occur during transmetallation, similar to observations previously reported by our group in cobalt-catalyzed SMC.

Suzuki-type transmetallation of a neutral aryl boronic acid or ester nucleophile to an iron amide or alkoxide complex has yet to be demonstrated in stoichiometric reaction and could provide key mechanistic insights on optimal bases, iron precursors and spin states for promoting the essential fundamental organometallic transformation. Here we describe the observation of stoichiometric transmetallation of furanyl-substituted, neutral boron reagents with iron alkoxide derivatives (Scheme 1). The electronic structure implications associated with conversion of an iron alkoxide to the corresponding iron heteroaryl complexes are elucidated and the potential for iron to undergo palladium-like SMC transmetallation reactivity is demonstrated.

Because conversion of a metal-halide to the corresponding alkoxide derivative is a key step in palladium-catalyzed SMC, the salt metathesis of (iPrPDI)FeBr17 with various alkoxide sources was initially studied. Addition of NaOR (R = SiMe3, Me, Et) to a diethyl ether solution of the iron bromide complex generated the corresponding (iPrPDI)FeOR derivatives (Scheme 2a). Pure (iPrPDI)FeOSiMe3 was isolated in 57% yield following recrystallization from pentane. However, using NaOCH3, the salt metathesis protocol yielded paramagnetic iron-containing side products in addition to the targeted (iPrPDI)FeOCH3 complex. Because protonolysis of iron alkyls by free alcohols is known method for the preparation of iron alkoxide complexes,18, 19 pentane solutions of (iPrPDI)Fe(CH2SiMe3) were independently treated with methanol and ethanol, and, following filtration and removal of volatiles, (iPrPDI)FeOCH3 and (iPrPDI)FeOEt were obtained in 71% and 66% yields, respectively, as green solids (Scheme 2b).

Scheme 2.

Scheme 2.

Open in a new tab

Synthesis of (iPrPDI)FeOR complexes by (a) salt metathesis and (b) solvolysis of iron alkyls

These alkoxide compounds exhibit the number of 1H NMR resonances consistent with overall C2v symmetry. As has been previously reported for (iPrPDI)FeX (X = Cl, Br) compounds,22 1H NMR resonances for (iPrPDI)FeOEt were observed over a wide chemical shift range (–300 to 300 ppm) at 23 °C. Specifically, the pyridine 4-CH resonance (371.51 ppm for (iPrPDI)FeOEt) and imine methyl CH3 resonances (−209.74 ppm for (iPrPDI)FeOEt) are diagnostic for these types of complexes. For (iPrPDI)FeOSiMe3, a benzene-d6 magnetic moment of 4.0 μB was measured (Evans method)20, consistent with the spin-only value of 3.87 μB for three unpaired electrons and an overall S = 3/2 iron complex.

The solid-state structures of (iPrPDI)FeOSiMe3, (iPrPDI)FeOEt, and (iPrPDI)FeOMe were determined by single crystal X-ray diffraction and representations of the molecular structures are presented in Figure 1. For all three compounds, a distorted square planar geometry was observed around the iron center with the sum of the bond angles approximating 360° in each case (R = SiMe3: 360.43(17)°, R = Et: 360.20(13)°, R = Me: 359.96(4)°). Near linear N(2)-Fe(1)-O(1) bond angles of 174.39(11)° ((iPrPDI)FeOSiMe3), 177.30(6)° (iPrPDI)FeOEt) and 175.46(2)° ((iPrPDI)FeOCH3) were observed in all cases, suggesting some degree of π-donation from the alkoxide oxygen to iron.

Figure 1.

Figure 1.

Open in a new tab

Solid-state structures of (a) (iPrPDI)FeOEt, (b) (iPrPDI)FeOSiMe3 and (c) (iPrPDI)FeOCH3 at 30% probability ellipsoids, with hydrogen atoms omitted for clarity. Zero-field 57Fe Mössbauer Spectra of (a) (iPrPDI)FeOEt and (b) (iPrPDI)FeOSiMe3 at 80 K in the solid state. Gray circles represent the experimental spectrum, while the blue line indicates the best fit of the data.

Two representative pyridine(diimine) iron alkoxide compounds, (iPrPDI)FeOSiMe3 and (iPrPDI)FeOEt, were also characterized by solid-state 57Fe Mössbauer spectroscopy. (Figure 1). Isomer shifts of 0.75 and 0.74 mm/s were obtained for (iPrPDI)FeOSiMe3 and (iPrPDI)FeOEt, respectively, with quadrupole splittings, ΔEQ, of 1.20 and 1.21 mm/s. Both isomer shifts are comparable with the previously reported value of δ = 0.74 mm/s for (iPrPDI)FeCl, although there is a slight increase in quadrupole splitting when the X-type ligand is changed from Cl to OR. (ΔEQ = 0.73 mm/s for (iPrPDI)FeCl).22 Both the magnetic moment and Mössbauer isomer shift support high spin iron(II) complexes, in analogy to (iPrPDI)FeCl and related complexes assigned previously.21, 22

To provide additional support for the electronic structure assignment of the (iPrPDI)FeOR compounds (R = Et, SiMe3), full-molecule density functional theory studies were conducted at the B3LYP level of theory. The optimized electronic structures are best described as high spin Fe(II), S = 3/2 (broken symmetry (4,1)), with an iron-centered radical antiferromagnetically coupled to a radical anion delocalized on the pyridine diimine ligand (see SI for discussion). A qualitative d-orbital splitting diagram is shown in Figure S6. This electronic structure is comparable to the electronic structure of (iPrPDI)FeX (X = Cl, Br) compounds previously described, which were also computed to exhibit an S = 3/2 broken symmetry (4,1) ground state.22 The computed Mössbauer parameters were δ = 0.65 mm/s and | ΔEQ | = 1.23 mm/s for (iPrPDI)FeOEt and δ = 0.64 mm/s and | ΔEQ | = 1.28 mm/s for (iPrPDI)Fe(OSiMe3), in good agreement with experimentally determined data.

With pyridine(diimine)iron alkoxide complexes in hand, their transmetallation reactivity with neutral aryl boronic esters was studied. Addition of PhBPin to a benzene-d6 solution of (iPrPDI)FeOEt produced no reaction at ambient temperature or upon heating to 80 °C for 24 hours. Taking inspiration from previous work on transmetallation to cobalt,10 the transmetallation of furan-type boron nucleophiles was investigated. Use of either (2-benzofuranyl)BPin or (2-benzofuranyl)BNeo (Pin = pinacolato, Neo = neopentylglycolato) resulted in rapid (<15 min) transmetallation at 23 °C in C6D6 as judged by an immediate color change from light to dark green and observation of (iPrPDI)Fe(benzofuranyl) by 1H NMR spectroscopy. In each case, a second, paramagnetic iron compound, tentatively assigned as the iron-boryl, (iPrPDI)Fe(BPin) or (iPrPDI)Fe(BNeo), accompanied formation of the benzofuranyl transmetallation product. Recrystallization of the product of the reaction with (2-benzofuranyl)BNeo from diethyl ether resulted in isolation of (iPrPDI)Fe(benzofuranyl) as dark green crystals in 90% yield. Attempted transmetallation with (2-benzofuranyl)B(OH)2 and (iPrPDI)FeOEt generated an intractable mixture of products with just trace quantities of (iPrPDI)Fe(2-benzofuranyl) (See SI).

Single crystals of (iPrPDI)Fe(2-benzofuranyl) were obtained from a concentrated solution of (iPrPDI)Fe(2-benzofuranyl) in diethyl ether at −35°, and the solid-state structure confirmed the generation of the iron aryl complex (Figure 2). The bond distances observed in (iPrPDI)Fe(2-benzofuranyl) are consistent with one-electron reduction of the chelate,23 where Nimine−Cimine distances of 1.319 (6) Å and 1.315(6) Å, and Cimine–Cipso bond distances of 1.445(7) Å and 1.437(7) Å are within the range expected for (RPDI)1-. The benzofuranyl moiety is rotated by 51.5(4)° relative to the plane of the pyridine(diimine) chelate. It is possible that this orientation for the benzofuranyl group is stabilized by π-stacking with the aryl groups or simply sterically preferred.

Figure 2.

Figure 2.

Open in a new tab

Top: A spin state change accompanies transmetallation. Solid-state structure (left) of (iPrPDI)Fe(benzofuranyl) at 30% probability ellipsoids, with hydrogen atoms omitted for clarity. Mulliken spin density plot (right) of (iPrPDI)Fe(2-benzofuranyl).

Experimental and full-molecule DFT studies (B3LYP) on (iPrPDI)Fe(2-benzofuranyl) support an intermediate spin iron(II) engaged in antiferromagnetic coupling with a chelate radical anion, resulting in an overall S = 1/2 complex (Figure 2; see SI for discussion). With the electronic structure of (iPrPDI)Fe(2-benzofuranyl) established, the flow of electrons during transmetallation reaction can be discussed. The starting iron alkoxides, (iPrPDI1-)FeIIOR, are high spin ferrous compounds with a monoreduced pyridine(diimine) chelate. Exchange of the weak field alkoxide ligand for a stronger field heteroaryl induced a spin state change at iron from S = 2 to S = 1 and the radical anion nature of the chelate is maintained throughout the reaction (Scheme 3). This behavior is comparable to the analogous transmetallation reaction between (iPrPNP)CoOR (PNP = bis(phosphino)pyridine, R = iPr or CH(Ph)Me), where a spin-state change from S = 1 (tetrahedral) to S = 0 (planar) accompanies the transmetallation step.10 Likewise, the propensity of 2-benzofuranyl boronate esters to undergo transmetallation as compared to the phenyl variant suggests that initial p-coordination of the furan ring facilitates formation of the oxygen boron bond, similar to the model proposed in cobalt-catalyzed SMC.10

Scheme 3.

Scheme 3.

Open in a new tab

Reaction of (iPrPDI)Fe(2-benzofuranyl) with electrophiles.

The potential of (iPrPDI)Fe(2-benzofuranyl) to participate in carbon-carbon bond-forming chemistry was evaluated with both aryl and alkyl halide electrophiles. Addition of one equivalent of cycloheptyl bromide, iodobenzene, bromobenzene, or phenyl triflate produced a blue solid identified as the pyridine(diimine) iron dihalide, (iPrPDI)FeX2 (X = Br, I, OTf) (Scheme 3). Low yields (<15%) of the cross-coupled product were obtained, and the remainder of the organic material was identified as hydrodehalogenated products (R–H) and benzofuran (Scheme 3; see Table S1 for organic product distribution).

In summary, the transmetallation of a neutral boron nucleophile to an iron alkoxide complex was demonstrated. In contrast to previous examples of transmetallation to iron, boronic acid ester nucleophiles commonly employed for palladium SMC are capable of promoting transmetallation. A net spin state change from S = 3/2 to S = 1/2 occurs during transmetallation, which was previously observed for cobalt and may assist in transmetallation. These results suggest that Suzuki-type transmetallation from boron to iron may be a viable pathway for iron-catalyzed cross coupling.

Supplementary Material

Supporting Information

ACKNOWLEDGMENT

We thank the National Institutes of Health (R01 GM121441) for financial support.

Footnotes

The Supporting Information is available free of charge on the ACS Publications website.

NMR Spectra (PDF)

CCDC 1953963, 1953965, 1954249, 1956370 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 441223 336033.

The authors declare no competing financial interest.

REFERENCES

  • 1.(a) Campeau L-C; Hazari N Cross-Coupling and Related Reactions: Connecting Past Success to the Development of New Reactions for the Future. Organometallics 2019, 38, 3–35. [DOI] [PMC free article] [PubMed] [Google Scholar]; (b) Neidig ML; Carpenter SH; Curran DJ; DeMuth JC; Fleischauer VE; Iannuzzi TE; Neate PGN; Sears JD; Wolford NJ Development and Evolution of Mechanistic Understanding in Iron-Catalyzed Cross-Coupling. Acc. Chem. Res 2019, 52, 140–150. [DOI] [PMC free article] [PubMed] [Google Scholar]; (c) Hayler JD; Leahy DK; Simmons EM A Pharmaceutical Industry Perspective on Sustainable Metal Catalysis. Organometallics 2019, 38, 36–46. [Google Scholar]
  • 2.(a) Kharasch MS; Fields EK Factors Determining the Course and Mechanisms of Grignard Reactions. IV. The Effect of Metallic Halides on the Reaction of Aryl Grignard Reagents and Organic Halides. J. Am. Chem. Soc 1941, 63, 2316–2320. [Google Scholar]; (b) Tamura M; Kochi JK Vinylation of Grignard Reagents. Catalysis by Iron. J. Am. Chem. Soc 1971, 93, 1487–1489. [Google Scholar]
  • 3.Piontek A; Bisz E; Szostak M Iron-Catalyzed Cross-Couplings in the Synthesis of Pharmaceuticals: In Pursuit of Sustainability. Angew. Chem. Int. Ed 2018, 57, 11116–11128. [DOI] [PubMed] [Google Scholar]
  • 4.(a) Hatakeyama T; Hashimoto T; Kondo Y; Fujiwara Y; Seike H; Takaya H; Tamada Y; Ono T; Nakamura M Iron-Catalyzed Suzuki–Miyaura Coupling of Alkyl Halides. J. Am. Chem. Soc 2010, 132, 10674–10676. [DOI] [PubMed] [Google Scholar]; (b) Bedford RB; Brenner PB; Carter E; Carvell TW; Cogswell PM; Gallagher T; Harvey JN; Murphy DM; Neeve EC; Nunn J; Pye D Expedient Iron-Catalyzed Coupling of Alkyl, Benzyl and Allyl Halides with Arylboronic Esters. Chem. - Eur. J 2014, 20, 7935–7938. [DOI] [PubMed] [Google Scholar]; (c) O’Brien HM; Manzotti M; Abrams RD; Elorriaga D; Sparkes HA; Davis SA; Bedford RB Iron-Catalysed Substrate-Directed Suzuki Biaryl Cross-Coupling. Nat. Catal 2018, 1, 429–437. [Google Scholar]
  • 5.Crockett MP; Tyrol CC; Wong AS; Li B; Byers JA Iron-Catalyzed Suzuki–Miyaura Cross-Coupling Reactions between Alkyl Halides and Unactivated Arylboronic Esters. Org. Lett 2018, 20, 5233–5237. [DOI] [PubMed] [Google Scholar]
  • 6.(a) Carrow BP; Hartwig JF Distinguishing Between Pathways for Transmetalation in Suzuki–Miyaura Reactions. J. Am. Chem. Soc 2011, 133, 2116–2119. [DOI] [PMC free article] [PubMed] [Google Scholar]; (b) Lennox AJJ; Lloyd-Jones GC Transmetalation in the Suzuki–Miyaura Coupling: The Fork in the Trail. Angew. Chem., Int. Ed 2013, 52, 7362–7370. [DOI] [PubMed] [Google Scholar]; (c) Thomas AA; Denmark SE Pre-Transmetalation Intermediates in the Suzuki-Miyaura Reaction Revealed: The Missing Link. Science 2016, 352, 329–332. [DOI] [PubMed] [Google Scholar]
  • 7.Dunsford JJ; Clark ER; Ingleson MJ Highly Nucleophilic Dipropanolamine Chelated Boron Reagents for Aryl-Transmetallation to Iron Complexes. Dalton Trans. 2015, 44, 20577–20583. [DOI] [PubMed] [Google Scholar]
  • 8.Daifuku SL; Kneebone JL; Snyder BER; Neidig ML Iron(II) Active Species in Iron–Bisphosphine Catalyzed Kumada and Suzuki–Miyaura Cross-Couplings of Phenyl Nucleophiles and Secondary Alkyl Halides. J. Am. Chem. Soc 2015, 137, 11432–44. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 9.Partyka DV Transmetalation of Unsaturated Carbon Nucleophiles from Boron-Containing Species to the Mid to Late d-Block Metals of Relevance to Catalytic C–X Coupling Reactions (X = C, F, N, O, Pb, S, Se, Te). Chem. Rev 2011, 111, 1529–1595. [DOI] [PubMed] [Google Scholar]
  • 10.Neely JM; Bezdek MJ; Chirik PJ Insight into Transmetalation Enables Cobalt-Catalyzed Suzuki–Miyaura Cross Coupling. ACS Cent. Sci 2016, 2, 935–942. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 11.Chirik PJ Carbon-Carbon Bond Formation in a Weak Ligand Field: Leveraging Open-Shell First-Row Transition-Metal Catalysts. Angew. Chem. Int. Ed 2017, 56, 5170–5181. [DOI] [PubMed] [Google Scholar]
  • 12.Hu L; Chen H Substrate-Dependent Two-State Reactivity in Iron-Catalyzed Alkene [2+2] Cycloaddition Reactions. J. Am. Chem. Soc 2017, 139, 15564–15567. [DOI] [PubMed] [Google Scholar]
  • 13.For examples of (PDI)Fe-catalyzed reactions, see:; (a) Small BM; Brookhart M Iron-Based Catalysts with Exceptionally High Activities and Selectivities for Oligomerization of Ethylene to Linear α-Olefins. J. Am. Chem. Soc 1998, 120, 7143–7144. [Google Scholar]; (b) Small BL; Brookhart M; Bennett AMA Highly Active Iron and Cobalt Catalysts for the Polymerization of Ethylene. J. Am. Chem. Soc 1998, 120, 4049–4050. [Google Scholar]; (c) Bart SC; Lobkovsky E Preparation and Molecular and Electronic Structures of Iron(0) Dinitrogen and Silane Complexes and Their Application to Catalytic Hydrogenation and Hydrosilation. J. Am. Chem. Soc, 2004, 126, 13794–13807. [DOI] [PubMed] [Google Scholar]; (d) Bouwkamp MW; Bowman AC; Lobkovsky E; Chirik PJ Iron-Catalyzed [2π + 2π] Cycloaddition of α,ω-Dienes: The Importance of Redox-Active Supporting Ligands. J. Am. Chem. Soc, 2006, 128, 13340–13341. [DOI] [PubMed] [Google Scholar]; (e) Archer AM; Bouwkamp MW; Cortez M-P; Lobkovsky E; Chirik PJ Arene Coordination in Bis(imino)pyridine Iron Complexes: Identification of Catalyst Deactivation Pathways in Iron-Catalyzed Hydrogenation and Hydrosilation. Organometallics, 2006, 25, 4269–4278. [Google Scholar]; (f) Trovitch RJ; Lobovsky E; Bill E; Chirik PJ Functional Group Tolerance and Substrate Scope in Bis(imino)pyridine Iron Catalyzed Alkene Hydrogenation. Organometallics, 2008, 27, 1470–1479. [Google Scholar]; (g) Sylvester KT; Chirik PJ Iron-Catalyzed, Hydrogen-Mediated Reductive Cyclization of 1,6-Enynes and Diynes: Evidence for Bis(imino)pyridine Ligand Participation. J. Am. Chem. Soc, 2009, 131, 8772–8774. [DOI] [PubMed] [Google Scholar]; (h) Russell SK; Darmon JM; Lobkovsky E; Chirik PJ Synthesis of Aryl-Substituted Bis(imino)pyridine Iron Dinitrogen Complexes. Inorg. Chem, 2010, 49, 2782–2792. [DOI] [PubMed] [Google Scholar]; (i) Russell SK; Lobkovsky E; Chirik PJ Iron-Catalyzed Intermolecular [2π + 2π] Cycloaddition. J. Am. Chem. Soc 2011, 133, 8858–8861. [DOI] [PubMed] [Google Scholar]; (j) Yu RP; Darmon JM; Hoyt JM; Margulieux GW; Turner ZR; Chirik PJ High-Activity Iron Catalysts for the Hydrogenation of Hindered, Unfunctionalized Alkenes. ACS Catal., 2012, 2, 1760–1764. [DOI] [PMC free article] [PubMed] [Google Scholar]; (k) Atienza CCH; Tondreau AM; Weller KJ; Lewis KM; Cruse RW; Nye SA; Boyer JL; Delis JGP; Chirik PJ High-Selectivity Bis(imino)pyridine Iron Catalysts for the Hydrosilylation of 1,2,4-Trivinylcyclohexane. ACS Catal., 2012, 2, 2169–2172. [Google Scholar]; (l) Tondreau AM; Atienza CCH; Weller KJ; Nye SA; Lewis KM; Delis JGP; Chirik PJ Iron Catalysts for Selective Anti-Markovnikov Alkene Hydrosilylation Using Tertiary Silanes. Science, 2012, 335, 567–570. [DOI] [PubMed] [Google Scholar]; (m) Hoyt JM; Sylvester KT; Semproni SP; Chirik PJ Synthesis and Electronic Structure of Bis(imino)pyridine Iron Metallacyclic Intermediates in Iron-Catalyzed Cyclization Reactions. J. Am. Chem. Soc, 2013, 135, 4862–4877. [DOI] [PubMed] [Google Scholar]; (n) Schmidt VA; Hoyt JM; Margulieux GW; Chirik PJ Cobalt-Catalyzed [2π + 2π] Cycloadditions of Alkenes: Scope, Mechanism, and Elucidation of Electronic Structure of Catalytic Intermediates. J. Am. Chem. Soc, 2015, 137, 7903–7914. [DOI] [PMC free article] [PubMed] [Google Scholar]; (o) Hoyt JM; Schmidt VA; Tondreau AM; Chirik PJ Iron-Catalyzed Intermolecular [2+2] Cycloadditions of Unactivated Alkenes. Science 2015, 349, 960–963. [DOI] [PubMed] [Google Scholar]; (p) Agahi R; Challinor AJ; Dunne J; Docherty JH; Carter NB; Thomas SP Regiodivergent Hydrosilylation, Hydrogenation, [2π + 2π]-Cycloaddition and C–H Borylation Using Counterion Activated Earth-Abundant Metal Catalysis. Chem. Sci 2019, 10, 5079–5084. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 14.Fernández I; Trovitch RJ; Lobkovsky E; Chirik PJ Synthesis of Bis(imino)pyridine Iron Di- and Monoalkyl Complexes: Stability Differences between FeCH2SiMe3 and FeCH2CMe3 Derivatives. Organometallics 2008, 27, 109–118. [Google Scholar]
  • 15.Trovitch RJ; Lobkovsky E; Bouwkamp MW; Chirik PJ Carbon–Oxygen Bond Cleavage by Bis(imino)pyridine Iron Compounds: Catalyst Deactivation Pathways and Observation of Acyl C–O Bond Cleavage in Esters. Organometallics 2008, 27, 6264–6278. [Google Scholar]
  • 16.Stieber SCE Methods For Determining Electronic Structures and Mechanisms of Bis(Imino)Pyridine Iron Pre-Catalysts Ph.D. Dissertation, Princeton University, Princeton, NJ, 2013. [Google Scholar]
  • 17.Bouwkamp MW; Bart SC; Hawrelak EJ; Trovitch RJ; Lobkovsky E; Chirik PJ Square Planar Bis(imino)pyridine Iron Halide and Alkyl Complexes. Chem. Commun 2005, 27, 3406–3408. [DOI] [PubMed] [Google Scholar]
  • 18.Delle Chiaie KR; Biernesser AB; Ortuño MA; Dereli B; Iovan DA; Wilding MJT; Li B; Cramer CJ; Byers JA The Role of Ligand Redox Non-Innocence in Ring-Opening Polymerization Reactions Catalysed by Bis(imino)pyridine Iron Alkoxide Complexes. Dalton Trans. 2017, 46, 12971–12980. [DOI] [PubMed] [Google Scholar]
  • 19.Biernesser AB; Li B; Byers JA Redox-Controlled Polymerization of Lactide Catalyzed by Bis(imino)pyridine Iron Bis(alkoxide) Complexes. J. Am. Chem. Soc 2013, 135, 16553–16560. [DOI] [PubMed] [Google Scholar]
  • 20.Evans DF The Determination of the Paramagnetic Susceptibility of Substances in Solution by Nuclear Magnetic Resonance. J. Chem. Soc 1959, 2003. [Google Scholar]
  • 21.(a) Cantalupo SA; Fiedler SR; Shores MP; Rheingold AL; Doerrer LH High-Spin Square-Planar CoII and FeII Complexes and Reasons for Their Electronic Structure. Angew. Chem. Int. Ed 2012, 51, 1000–1005. [DOI] [PubMed] [Google Scholar]; (b) Holland PL All Square for High-Spin Iron(II). Nat. Chem 2011, 3, 507–508. [DOI] [PubMed] [Google Scholar]; (c) Römelt C; Weyhermüller T; Wieghardt K Structural Characteristics of Redox-Active Ryridine-1,6-diimine Complexes: Electronic Structures and Ligand Oxidation Levels. Coord. Chem. Rev 2019, 380, 287–317. [Google Scholar]
  • 22.Bart SC; Chłopek K; Bill E; Bouwkamp MW; Lobkovsky E; Neese F; Wieghardt K; Chirik PJ Electronic Structure of Bis(imino)pyridine Iron Dichloride, Monochloride, and Neutral Ligand Complexes: Monochloride, and Neutral Ligand Complexes: A Combined Structural, Spectroscopic, and Computational Study. J. Am. Chem. Soc 2006, 128, 13901–13912. [DOI] [PubMed] [Google Scholar]
  • 23.Knijnenburg Q; Gambarotta S; Budzelaar PH M. Ligand-Centred Reactivity in Diiminepyridine Complexes. Dalton Trans. 2006, 5442–5448. [DOI] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supporting Information
NIHMS1062977-supplement-Supporting_Information.pdf (3.6MB, pdf)

RESOURCES