Selective Iron-Catalyzed <i>N</i>‑Formylation of Amines using Dihydrogen and Carbon Dioxide

A family of iron­(II) carbonyl hydride species supported by PNP pincer ligands was identified as highly productive catalysts for the <i>N-</i>formylation of amines via CO₂ hydrogenation. Specifically, iron complexes supported by two different types of PNP ligands were examined for formamide production. Complexes containing a PNP ligand with a tertiary amine afforded superior turnover numbers in comparison to complexes containing a bifunctional PNP ligand with a secondary amine, indicating that bifunctional motifs are not required for catalysis. Systems incorporating a tertiary amine containing a PNP ligand were active for the <i>N-</i>formylation of a variety of amine substrates, achieving TONs up to 8900 and conversions as high as 92%. Mechanistic experiments suggest that <i>N-</i>formylation occurs via an initial, reversible reduction of CO₂ to ammonium formate followed by dehydration to produce formamide. Several intermediates relevant to this reaction pathway, as well as iron-containing deactivation species, were isolated and characterized

Small Molecule Activation Chemistry of Cu–Fe Heterobimetallic Complexes Toward CS₂ and N₂O

In this contribution, we report the reactivity of polar, unsupported Cu–Fe bonds toward small-molecule heteroallenes. Insertion of CS₂ into the polar Cu–Fe bond of (IMes)­Cu-FeCp­(CO)₂ proceeds at mild conditions and results in the simultaneous presence of two unprecedented CS₂ binding modes (μ₃:η⁴ and μ₃:η³) in the same product. Reactivity between N₂O and (NHC)­Cu-FeCp­(CO)₂ complexes also is observed at mild conditions, resulting in migration of the cyclopentadienyl groups from Fe to Cu. Similar reactivity is observed for new (NHC)­Cu-FeCp*­(CO)₂ analogues, whose structural characterization is reported here and reveals two semibridging Cu···CO interactions per molecule. Stoichiometric oxygen atom transfer from N₂O to PPh₃ was mediated by (IMes)­Cu-FeCp­(CO)₂, indicating the presence of an N₂O-activated intermediate that can be intercepted by exogenous reagents

Selective Iron-Catalyzed Deaminative Hydrogenation of Amides

The five-coordinate iron­(II) hydride complex (^<i>i</i>PrPNP)­Fe­(H)­CO (^<i>i</i>PrPNP = N­[CH₂CH₂(P^<i>i</i>Pr₂)]₂) was found to selectively catalyze deaminative hydrogenation of amides to the corresponding amines and primary alcohols. It is one of the most active amide hydrogenation catalysts reported to date, with turnover numbers (TONs) in excess of 1000 observed for multiple substrates and TONs greater than 4000 obtained for activated formanilides. The amide C–N cleavage reactions occur with a preference for electron-withdrawing substituents and with greater activity for formamides compared with acetamides and benzamides. Stoichiometric reactions between (^<i>i</i>PrPNP)­Fe­(H)­CO and formanilide afforded the new iron­(II) complex (^<i>i</i>PrPN^HP)­Fe­(H)­CO­(N­(Ph)­HCO) resulting from N–H addition across the Fe–N bond. Complexes of this type were identified as the resting state during catalytic hydrogenation reactions containing secondary amides. Addition of a Lewis acid cocatalyst provided further enhancement of the productivity of catalytic amide hydrogenations

Heterobimetallic Complexes with Polar, Unsupported Cu–Fe and Zn–Fe Bonds Stabilized by N‑Heterocyclic Carbenes

Heterobimetallic complexes of the formulations (NHC)­Cu–FeCp­(CO)₂ (NHC = IPr, IMes, SIMes), (IPr)­Cu–MoCp­(CO)₃, and (IPr)­(Cl)­Zn–FeCp­(CO)₂ were synthesized in high yield from readily available starting materials and characterized crystallographically. The solid-state structures of the Cu–Fe systems reveal close, secondary interactions between Cu and one CO ligand from the [FeCp­(CO)₂] unit that are absent in the Zn–Fe analogue. The heterobimetallic complexes feature short yet polar Cu–Fe, Cu–Mo, and Zn–Fe bonds in which the electrophilic metal (Cu, Zn) is later in the transition series than the nucleophilic metal (Fe, Mo), thereby subverting the more common early–late heterobimetallic paradigm. DFT analyses were used to assess M–M′ bond polarity and examine effects on M–M′ bonding of systematic modifications to both the nucleophilic and electrophilic fragments. Experimental confirmation of Cu–Fe bond polarity was obtained by analysis of product mixtures resulting from the reactions between (NHC)­Cu–FeCp­(CO)₂ complexes and MeI, which produced (NHC)­Cu–I and Me–FeCp­(CO)₂ products

Heterobimetallic Complexes with Polar, Unsupported Cu–Fe and Zn–Fe Bonds Stabilized by N‑Heterocyclic Carbenes

Heterobimetallic complexes of the formulations (NHC)­Cu–FeCp­(CO)₂ (NHC = IPr, IMes, SIMes), (IPr)­Cu–MoCp­(CO)₃, and (IPr)­(Cl)­Zn–FeCp­(CO)₂ were synthesized in high yield from readily available starting materials and characterized crystallographically. The solid-state structures of the Cu–Fe systems reveal close, secondary interactions between Cu and one CO ligand from the [FeCp­(CO)₂] unit that are absent in the Zn–Fe analogue. The heterobimetallic complexes feature short yet polar Cu–Fe, Cu–Mo, and Zn–Fe bonds in which the electrophilic metal (Cu, Zn) is later in the transition series than the nucleophilic metal (Fe, Mo), thereby subverting the more common early–late heterobimetallic paradigm. DFT analyses were used to assess M–M′ bond polarity and examine effects on M–M′ bonding of systematic modifications to both the nucleophilic and electrophilic fragments. Experimental confirmation of Cu–Fe bond polarity was obtained by analysis of product mixtures resulting from the reactions between (NHC)­Cu–FeCp­(CO)₂ complexes and MeI, which produced (NHC)­Cu–I and Me–FeCp­(CO)₂ products

Heterobimetallic Complexes with Polar, Unsupported Cu–Fe and Zn–Fe Bonds Stabilized by N‑Heterocyclic Carbenes

Heterobimetallic complexes of the formulations (NHC)­Cu–FeCp­(CO)₂ (NHC = IPr, IMes, SIMes), (IPr)­Cu–MoCp­(CO)₃, and (IPr)­(Cl)­Zn–FeCp­(CO)₂ were synthesized in high yield from readily available starting materials and characterized crystallographically. The solid-state structures of the Cu–Fe systems reveal close, secondary interactions between Cu and one CO ligand from the [FeCp­(CO)₂] unit that are absent in the Zn–Fe analogue. The heterobimetallic complexes feature short yet polar Cu–Fe, Cu–Mo, and Zn–Fe bonds in which the electrophilic metal (Cu, Zn) is later in the transition series than the nucleophilic metal (Fe, Mo), thereby subverting the more common early–late heterobimetallic paradigm. DFT analyses were used to assess M–M′ bond polarity and examine effects on M–M′ bonding of systematic modifications to both the nucleophilic and electrophilic fragments. Experimental confirmation of Cu–Fe bond polarity was obtained by analysis of product mixtures resulting from the reactions between (NHC)­Cu–FeCp­(CO)₂ complexes and MeI, which produced (NHC)­Cu–I and Me–FeCp­(CO)₂ products

Heterobimetallic Complexes with Polar, Unsupported Cu–Fe and Zn–Fe Bonds Stabilized by N‑Heterocyclic Carbenes

Heterobimetallic complexes of the formulations (NHC)­Cu–FeCp­(CO)₂ (NHC = IPr, IMes, SIMes), (IPr)­Cu–MoCp­(CO)₃, and (IPr)­(Cl)­Zn–FeCp­(CO)₂ were synthesized in high yield from readily available starting materials and characterized crystallographically. The solid-state structures of the Cu–Fe systems reveal close, secondary interactions between Cu and one CO ligand from the [FeCp­(CO)₂] unit that are absent in the Zn–Fe analogue. The heterobimetallic complexes feature short yet polar Cu–Fe, Cu–Mo, and Zn–Fe bonds in which the electrophilic metal (Cu, Zn) is later in the transition series than the nucleophilic metal (Fe, Mo), thereby subverting the more common early–late heterobimetallic paradigm. DFT analyses were used to assess M–M′ bond polarity and examine effects on M–M′ bonding of systematic modifications to both the nucleophilic and electrophilic fragments. Experimental confirmation of Cu–Fe bond polarity was obtained by analysis of product mixtures resulting from the reactions between (NHC)­Cu–FeCp­(CO)₂ complexes and MeI, which produced (NHC)­Cu–I and Me–FeCp­(CO)₂ products

Synthesis and Characterization of Heterobimetallic Complexes with Direct Cu–M Bonds (M = Cr, Mn, Co, Mo, Ru, W) Supported by <i>N</i>‑Heterocyclic Carbene Ligands: A Toolkit for Catalytic Reaction Discovery

Building upon the precedent of catalytically active (NHC)­Cu–FeCp­(CO)₂ complexes, a series of (NHC)­Cu–[M] complexes were synthesized via the addition of Na⁺[M]⁻ reagents to (NHC)­CuCl synthons. The different [M]⁻ anions used span a range of 7 × 10⁷ relative nucleophilicity units, allowing for controlled variation of nucleophile/electrophile pairing in the heterobimetallic species. Direct Cu–M bonds (M = Cr, Mn, Co, Mo, Ru, W) formed readily when the bulky IPr carbene was used as a support. Crystallographic characterization and computational examination of these complexes was conducted. For the smaller IMes carbene, structural isomerism was observed when using the weakest [M]⁻ nucleophiles, with (IMes)­Cu–[M] and {(IMes)₂Cu}­{Cu­[M]₂} isomers being observed in equilibrium. Collectively, the series of complexes provides a toolbox for catalytic reaction discovery with precise control of structure–function relationships

Ancillary Ligand Effects upon Dithiolene Redox Noninnocence in Tungsten Bis(dithiolene) Complexes

An expanded set of compounds of the type [W­(S₂C₂Me₂)₂L₁L₂]^<i>n</i> (<i>n</i> = 0: L₁ = L₂ = CO, <b>1</b>; L₁ = L₂ = CN^<i>t</i>Bu, <b>2</b>; L₁ = CO, L₂ = carbene, <b>3</b>; L₁ = CO, L₂ = phosphine, <b>4</b>; L₁ = L₂ = phosphine, <b>5</b>. <i>n</i> = 2–: L₁ = L₂ = CN^–, [<b>6</b>]^2–) has been synthesized and characterized. Despite isoelectronic formulations, the compound set reveals gradations in the dithiolene ligand redox level as revealed by intraligand bond lengths, υ_CCchelate, and rising edge energies in the sulfur K-edge X-ray absorption spectra (XAS). Differences among the terminal series members, <b>1</b> and [<b>6</b>]^2–, are comparable to differences seen in homoleptic dithiolene complexes related by full electron transfer to/from a dithiolene-based MO. The key feature governing these differences is the favorable energy of the CO π* orbitals, which are suitably positioned to overlap with tungsten d orbitals and exert an oxidizing effect on both metal and dithiolene ligand via π-backbonding. The CN^– π* orbitals are too high in energy to mix effectively with tungsten and thus leave the filled dithiolene π* orbitals unperturbed. This work shows how, and the degree to which, the redox level of a noninnocent ligand can be modulated by the choice of ancillary ligands(s)