Iron Olefin Metathesis: Unlocking Reactivity and Mechanistic Insights.

Olefin metathesis catalyzed by iron complexes has garnered substantial interest due to iron's abundance and nontoxicity relative to ruthenium, yet its full potential remains untapped, largely because of the propensity of iron carbenes to undergo cyclopropanation instead of cycloreversion from a metallacycle intermediate. In this report, we elucidate the reactions of [{PC(sp)P}Fe(L)(N)], ([PC(sp)P] = bis[2-(diisopropylphosphino)phenyl]methylene) with strained olefins, unveiling their capability to yield metathesis-related products. Our investigations led to the isolation of a structurally characterized metallacyclobutane during the reaction with norbornadiene derivatives, ultimately leading to a ring-opened iron alkylidene.

[2+2] Cycloadditions with an Iron Carbene: A Critical Step in Enyne Metathesis.

Ruthenium carbenes, famously used in olefin metathesis, have impacted numerous research areas, ranging from synthesis to materials and biology. Although in the same group as ruthenium, iron carbenes showing similar reaction patterns have not been reported. Such targets are of high interest because the use of a sustainable metal would lead to a decreased cost, toxicity, and environmental impact of the corresponding compounds.

Oxidation reactions of a nucleophilic palladium carbene: mono and bi-radical carbenes.

Palladium(ii) cationic carbene radical and neutral bi-radical complexes were synthesized from a previously reported Pd(ii) carbene in the presence of one and two electron oxidants. When [{PC(sp2)P}tBuPd(PMe3)] (1, [PC(sp2)P]tBu = (bis[2-(di-iso-propylphosphino)-4-tertbutylphenyl]methylene)) was treated with [Cp2Fe][X] (X = BArF4, ArF = 3,5-(CF3)2C6H3, or PF6), the corresponding radical cationic complexes [{PC˙(sp2)P}tBuPd(PMe3)][X] (2: X = BArF4, 3: PF6) were isolated and characterized. Magnetic moment measurements and EPR spectroscopy indicated the presence of a ligand centered unpaired electron.

NI(ii) phosphine and phosphide complexes supported by a PNP-pyrrole pincer ligand.

The reaction between [(PNP)NiCl] (1, PNP = 2,5-bis((di-iso-propylphosphino)-methyl)-1H-pyrrolide) and TlPF in the presence of a monodentate phosphine ligand led to cationic nickel phosphine and phosphite complexes, [(PNP)Ni(PHPh)][PF] (2), [(PNP)Ni(PMe)][PF] (3), and [(PNP)Ni{P(OMe)}][PF] (4). Compound 2 can be deprotonated resulting in the generation of a terminal phosphido complex, [(PNP)Ni(PPh)] (5). When 3 is subjected to a base, a methyl proton of PMe is abstracted to afford [(PNP)Ni(CHPMe)] (6), containing a methylene bridge between Ni and the external phosphine.

Formation of Palladium η -Bound Chalcogenoketones across a Pd -C Bond.

A series of chalcogen analogues encompassing a ketone and chalcogenoketones [{PC(=E)P}Pd(PMe )] (E=O, S, Se, Te) was generated from a nucleophilic palladium carbene compound, [{PC(sp )P}Pd(PMe )] ([PC(sp )HP]=bis[2-(diisopropylphosphino)-phenyl]methyl, iPr P-C H -CH-C H -PiPr ). The thio-, seleno-, and telluroketone were all synthesized by means of an atom transfer from the respective chalcogens. The ketone analogue, however, required the use of nitrobenzene or nitrosobenzene as the oxygen-atom transfer agent.

Iron pyrrole-based PNP pincer ligand complexes as catalyst precursors.

The structure of a pincer ligand consists of a backbone and two `arms' which typically contain a P or N atom. They are tridentate ligands that coordinate to a metal center in a meridional configuration. A series of three iron complexes containing the pyrrole-based PNP pincer ligand 2,5-bis[(diisopropylphosphanyl)methyl]pyrrolide (PNP) has been synthesized.

Heterobimetallic Pd-K carbene complexes one-electron reductions of palladium radical carbenes.

Heterobimetallic Pd-K carbenes featuring Pd-C-K moieties were synthesized an unprecedented sequential substitution/reduction reaction from a radical precursor, [{PC˙(sp)P} PdI] ([PC(sp)P] = bis[2-(di-iso-propylphosphino)-4--butylphenyl]methylene). Polymeric structures were observed in the solid state for the heterobimetallic compounds that can be interrupted in the presence of a donor solvent.

C-H activation of ethers by pyridine tethered PCsp3P-type iridium complexes.

Iridium PCsp3P complexes featuring a novel bis(2-diphenylphosphinophenyl)-2-pyridylmethane ligand (PC(Py)HP) are reported. C-H activation reactions between the dihydride complex [(PC(Py)P)Ir(H)2] and tetrahydrofuran or methyl tert-butyl ether in the presence of a hydrogen acceptor, norbornene (NBE), at ambient temperature led exclusively to the hydrido oxyalkyl complexes, [(PC(Py)P)IrH(C4H7O)] and [(PC(Py)P)IrH(CH2O(t)Bu)], respectively. The internal pyridine donor is important and stabilizes these species by coordination to the iridium center.

E-H (E = B, Si, Ge) bond activation of pinacolborane, silanes, and germanes by nucleophilic palladium carbene complexes.

The reactivity of two nucleophilic palladium carbenes, [PC(sp(2))P]Pd(PMe3) and [PC(sp(2))P]Pd(PPh3), where [PC(sp(2))P] = bis[2-(di-iso-propylphosphino)phenyl]methylene, toward the E-H bond activation of Ph4-nEHn (E = Si, Ge; n = 1-3) and pinacolborane (HBpin) is discussed. Unlike previous reports, both types of isomer species, hydride [PC(EHn-1Ph4-n)P]PdH or [PC(Bpin)P]PdH and silyl/germyl [PC(H)P]Pd(EHn-1Ph4-n), were observed depending on the substrate and the phosphine ligand, showing that the polarity of the Pd-C bond can be tuned by the phosphine substituents.

Redox-induced umpolung of transition metal carbenes.

Metal carbene complexes have been at the forefront of organic and organometallic synthesis and are instrumental in guiding future sustainable chemistry efforts. While classical Fischer and Schrock type carbenes have been intensely studied, compounds that do not fall within one of these categories have attracted attention only recently. In addition, applications of carbene complexes rarely take advantage of redox processes, which could open up a new dimension for their use in practical processes.

Frustrated Lewis pair-like reactions of nucleophilic palladium carbenes with B(C6F5)3.

The reactions of two nucleophilic palladium carbene complexes with the strong Lewis acid B(C6F5)3 afforded two zwitterionic products. One of them features a remote nucleophilic attack at the para-carbon of the supporting ligand, while the other indicates C-F activation of B(C6F5)3. Both behaviours are reminiscent of the reactivity of frustrated Lewis pairs due to the steric inaccessibility of the nucleophilic carbon center, but are unprecedented for transition metal carbene complexes.

Carbon-hydrogen bond activation, C-N bond coupling, and cycloaddition reactivity of a three-coordinate nickel complex featuring a terminal imido ligand.

The three-coordinate imidos (dtbpe)Ni═NR (dtbpe = (t)Bu2PCH2CH2P(t)Bu2, R = 2,6-(i)Pr2C6H3, 2,4,6-Me3C6H2 (Mes), and 1-adamantyl (Ad)), which contain a legitimate Ni-N double bond as well as basic imido nitrogen based on theoretical analysis, readily deprotonate HC≡CPh to form the amide acetylide species (dtbpe)Ni{NH(Ar)}(C≡CPh). In the case of R = 2,6-(i)Pr2C6H3, reductive carbonylation results in formation of the (dtbpe)Ni(CO)2 along with the N-C coupled product keteneimine PhCH═C═N(2,6- (i)Pr2C6H3). Given the ability of the Ni═N bond to have biradical character as suggested by theoretical analysis, H atom abstraction can also occur in (dtbpe)Ni═N{2,6-(i)Pr2C6H3} when this species is treated with HSn((n)Bu)3.

Ag(I) and Tl(I) precursors as transfer agents of a pyrrole-based pincer ligand to late transition metals.

A PNP ligand, PN(pyr)P ((PN(pyr)P)H = 2,5-bis((di-iso-propylphosphino)methyl)pyrrole), which employs a pyrrole unit as a central anionic nitrogen donor, was designed. The corresponding group 10 metal chlorides as well as iridium and ruthenium compounds were isolated. In order to conduct this work, [(PN(pyr)P)Tl] and [(PN(pyr)P)Ag]2 were synthesized and characterized.

Flexible coordination of diphosphine ligands leading to cis and trans Pd(0), Pd(II), and Rh(I) complexes.

A series of diphosphine ligands (i)Pr2P-C6H4-X-C6H4-P(i)Pr2 (for ligand L(1), X = CH2; for ligand L(2), X = CH2CH2) was investigated to determine the preference for cis/trans coordination to palladium(0), palladium(II), and rhodium(I). Increasing the length of the bridging alkyl backbone from one to two carbons changes the geometry of the resulting palladium(II) complexes, with L(1) coordinating preferentially cis, while L(2) coordinates in a trans fashion. Coordination to Pd(0) leads to L(1)Pd(dba) and L(2)Pd(dba), in which both ligands accommodate a P-M-P angle close to 120°.

Coordination of a hemilabile pincer ligand with an olefinic backbone to mid-to-late transition metals.

The coordination chemistry of a neutral tPCH═CHP pincer (tPCH═CHP = 2,2'-bis(di-iso-propylphosphino)-trans-stilbene) with metals that form stable complexes in the +1 oxidation state was studied and (tPCH═CHP)CoCl, (tPCH═CHP)CoCl(CO), (tPCH═CHP)RhCl, (tPCH═CHP)Cu(OTf), [(tPCH═CHP)Cu][PF6], and [(tPCH═CHP)Ag][PF6] were synthesized and characterized. In order to determine whether the coordination mode is dependent on the oxidation state of the metal, some +2 metal complexes, (tPCH═CHP)CoCl2 and (tPCH═CHP)FeBr2, were also investigated. The coordination of the olefinic backbone is not observed in (tPCH═CHP)FeBr2, (tPCH═CHP)CoCl2, (tPCH═CHP)Cu(OTf), or [(tPCH═CHP)Ag][PF6], but η(2)-coordination is present in [(tPCH═CHP)CoCl][BAr(F)4], [(tPCH═CHP)FeBr][BAr(F)4], (tPCH═CHP)CoCl, (tPCH═CHP)CoCl(CO), (tPCH═CHP)RhCl, and [(tPCH═CHP)Cu][PF6].

Three-coordinate nickel carbene complexes and their one-electron oxidation products.

The synthesis and characterization of two new carbene complexes, (dtbpe)Ni═CH(dmp) (1; dtbpe = 1,2-bis(di-tert-butylphosphino)ethane; dmp = 2,6-dimesitylphenyl) and (dippn)Ni═CH(dmp) (2; dippn = 1,8-bis(di-iso-propylphosphino)naphthalene), are described. Complexes 1 and 2 were isolated by photolysis of the corresponding side-bound diazoalkane complexes, exemplified by (dtbpe)Ni{η(2)-N2CH(dmp)} (3). The carbene complexes feature Ni-C distances that are short and Ni-C-C angles at the carbene carbon that are intermediate between 120° and 180° (155.

Synthesis and reactivity of two-coordinate Ni(I) alkyl and aryl complexes.

Reaction of [(IPr)Ni(μ-Cl)]2 (1-Cl; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with ClMg{CH(SiMe3)2}·Et2O affords (IPr)Ni{CH(SiMe3)2} (2), a two-coordinate Ni(I) alkyl complex. An analogous two-coordinate aryl derivative, (IPr)Ni(dmp) (dmp = 2,6-dimesitylphenyl), can be similarly prepared from Li(dmp) and 1-Cl. Reaction of 2 with alkyl bromides gives the three-coordinate Ni(II) alkyl halide complex (IPr)Ni{CH(SiMe3)2}Br.

Synthesis and characterization of three-coordinate Ni(III)-imide complexes.

A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. Oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III)-imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl.

Hydrogen-atom abstraction from Ni(I) phosphido and amido complexes gives phosphinidene and imide ligands.

Hydrogen-atom abstraction from M-E(H) to generate M═E-containing complexes (E = PR, NR) is not well studied because only a few complexes are known to undergo such reactions. Hydrogen-atom abstraction from nickel(I) phosphide and amide complexes led to the corresponding phosphinidene and imide compounds. These reactions are unparalleled in the organometallic chemistry of nickel and feature an unusual example of a transition-metal phosphinidene synthesized by hydrogen-atom abstraction.

Reactions of CO(2) and CS(2) with 1,2-bis(di-tert-butylphosphino)ethane complexes of nickel(0) and nickel(I).

Reaction of CS(2) with [(dtbpe)Ni](2)(η(2),μ-C(6)H(6)) (1; dtbpe =1,2-bis(di-tert-butylphosphino)ethane) in toluene gives the carbon disulfide complex (dtbpe)Ni(η(2)-CS(2)) (2), characterized by standard spectroscopic methods and X-ray crystallography. Reaction of CS(2) with the Ni(I) complex (dtbpe)Ni(OSO(2)CF(3)) gives the diamagnetic, trimetallic cluster [{(dtbpe)Ni(κ(1),η(2)-CS(2))}(2)(dtbpe)Ni][SO(3)CF(3)](2) (3-OTf). The solid-state structure of 3-OTf reveals that the two CS(2) ligands bind η(2) to two (dtbpe)Ni centers and κ(1) to the third, unique (dtbpe)Ni in the complex dication, and NMR spectroscopic data indicate that this structure is maintained in solution.

Arrested 1,2-hydrogen migration from silicon to nickel upon oxidation of a three-coordinate Ni(I) silyl complex.

Reaction of the dimeric Ni(I) chloride complex [(dtbpe)NiCl](2) (1) with dimesitylsilyl potassium affords the three-coordinate Ni(I) silyl complex (dtbpe)Ni(SiHMes(2)) (2). Alternatively, 2 can be prepared by an oxidative-addition reaction of Mes(2)Si(H)OTf (Tf = CF(3)SO(3)) with the nickel(0) complex [(dtbpe)Ni](2)(mu-C(6)H(6)) (3), with (dtbpe)Ni(OTf) (4) formed as an easily separable byproduct. The one-electron oxidation of 2 by ferrocenium affords diamagnetic [(dtbpe)Ni(mu-H)SiMes(2)][BAr(F)(4)] (5), a Ni(II) complex formed by partial 1,2-H migration from silicon to nickel and featuring an unusual 3-center, 2-electron bonding motif between Ni, Si, and the bridging H.

Monomeric and dimeric disulfide complexes of nickel(II).

Elemental sulfur reacts with a bulky bis(phosphine)nickel(0) complex to give a monomeric nickel(II) eta(2)-disulfido complex, oxidation of which results in the elimination of sulfur with dimerization to give an eta(2),eta(2)-disulfidodinickel(II) derivative in which the S-S bond can be reductively cleaved in a redox-reversible fashion.

Synthesis and characterization of side-bound aryldiazo and end-bound nitrosyl complexes of nickel.

Structural characterization of a nickel aryldiazo complex supported by the bulky 1,2-bis(di-tert-butylphosphino)ethane ligand reveals square-planar Ni geometry and an unusual side-on coordination mode for the N2R moiety, while the related nitrosyl complex displays trigonal-planar geometry at Ni and end-on coordination of the NO ligand.