Phosphaalkene Ligand Research Articles

P-C, P-N, and M-N Bond Formation Processes in Reactions of Heterometallic Phosphinidene-Bridged MoMn and MoRe Complexes with Diazoalkanes and Organic Azides to Build Three- to Five-Membered Phosphametallacycles.

Reactions of the heterometallic MoRe complex [MoReCp(μ-PR*)(CO)6] and its MoMn analogue with some small molecules having N-N multiple bonds, such as diazoalkanes and organic azides, were investigated (R* = 2,4,6-C6H2tBu3). Reactions with excess ethyl diazoacetate proceeded slowly and with concomitant denitrogenation to give complexes [MoMCp(μ-η2P,C:κ2P,O-PR*CHCO2Et)(CO)5], which display a bridging phosphaalkene ligand in a novel μ-η2:κ2 coordination mode, while reactions with other diazoalkanes resulted only in the decomposition of the organic reagent. The MoRe complex reacted with benzyl- or p-tolyl azide at room temperature to give the green complexes [MoReCp(μ-η2P,N:κP,N'2-PR*N3R)(CO)6] [R = Bn, p-tol], which display bridging phosphatriazadiene ligands in a novel 6-electron donor coordination mode as a result of a formal [2 + 1] cycloaddition of the terminal N atom of the azide to the Mo-P double bond of the parent complex, followed by coordination of the distal NR nitrogen to the rhenium center. Denitrogenation was only observed for the p-tolyl azide derivative, which upon heating at 333 K yielded [MoReCp{μ-κP:κN-PR*N(p-tol)}(CO)6], a molecule displaying a bridging phosphaimine ligand in a rare κP:κN coordination mode. Analogous reactions of the MoMn phosphinidene complex proceeded similarly at 273 K to give the phosphatriazadiene-bridged derivatives [MoMnCp(μ-η2P,N:κ2P,N'-PR*N3R)(CO)6], but these were thermally unstable and degraded at room temperature to give the mononuclear triazenylphosphanyl complexes [Mn2(κP,N-PR*NHNNR)(CO)3] as major products, along with small amounts of the phosphaimine-bridged complex [MoMnCp{μ-κP:κN-PR*N(p-tol)}(CO)6] in the case of the p-tolyl azide derivative. The structure of the new complexes was analyzed in light of spectroscopic data and single-crystal diffraction studies on selected examples of each type of complex.

Cu(i) complex bearing a PNP-pincer-type phosphaalkene ligand with a bulky fused-ring Eind group: properties and applications to FLP-type bond activation and catalytic CO2 reduction.

Herein, we report the synthesis of [Cu(Eind2-BPEP)][PF6] (2) (Eind2-BPEP = 2,6-bis(2-Eind-2-phosphaethenyl)pyridine, Eind = 1,1,3,3,5,5,7,7-octaethyl-1,2,3,5,6,7-hexahydro-s-indacen-4-yl), a three-coordinated Cu(i) complex bearing a PNP-pincer-type phosphaalkene ligand with bulky fused-ring Eind groups. The Gutmann-Beckett test revealed that complex 2 is highly Lewis acidic and comparable in strength to B(C6F5)3, which is a relatively strong Lewis acid. In addition, 2 is more Lewis acidic than [Cu(Mes*2-BPEP)][PF6] (3), the analogous complex with less-bulky Mes* instead of Eind groups. DFT calculations using model compounds revealed that the higher Lewis acidity of 2 compared to 3 is not due to the electronic effects of the ligand, but due to a reduction in the LUMO energy caused by the steric effect of the bulky Eind groups. When combined with a tertiary amine, the highly Lewis acidic and bulky 2 exhibits the reactivity of a frustrated Lewis pair (FLP) and can activate hydrogen and phenylacetylene. Complexes 2 and 3 were found to catalyze the hydrogenation and hydrosilylation of CO2 in the presence of DBU under relatively mild conditions.

On the Geometrical Stability of Square-Planar Platinum(0) Complexes That Bear a PNP-Pincer-Type Phosphaalkene Ligand (Eind2 -BPEP).

The four-coordinate Pt0 complex [Pt(PPh3 )(Eind2 -BPEP)] (Eind=1,1,3,3,5,5,7,7-octaethyl-1,2,3,5,6,7-hexahydro-s-indacen-4-yl; BPEP=2,6-bis(1-phenyl-2-phosphaethenyl)pyridine), which bears a PNP-pincer-type phosphaalkene ligand (Eind2 -BPEP; PNP=N,N-bis(diphenylphosphine)-2,6-diaminopyridine), were found to adopt a square-planar configuration around the Pt center (τ4 =0.11). This coordination geometry is very uncommon for formal d10 complexes. In this study, a series of ligands with different electronic properties (i.e., DMAP, 2,6-lutidine, PMe3 , tBuNC, and CO) were introduced in place of PPh3 , and their effects on the coordination geometry were examined. X-ray diffraction analysis revealed that all complexes adopted a square-planar configuration (τ4 =0.20-0.27). In contrast, DFT calculations indicated that the geometrical stability towards distortion around Pt varied with the ligand. The complexes with pyridine-based ligands had rigid planar structures, whereas those with π-accepting ligands, such as CO, were relatively flexible towards distortion. The electronic effects of the ligands were reflected in the spectroscopic properties of the complexes, which showed a large color change in the near-infrared region.

A Square-Planar Complex of Platinum(0).

The Pt0 complex [Pt(PPh3 )(Eind2 -BPEP)] with a pyridine-based PNP-pincer-type phosphaalkene ligand (Eind2 -BPEP) has a highly planar geometry around Pt with ∑(Pt)=358.6°. This coordination geometry is very uncommon for formal d10 complexes, and the Pd and Ni homologues with the same ligands adopt distorted tetrahedral geometries. DFT calculations reveal that both the Pt and Pd complexes are M0 species with nearly ten valence electrons on the metals whereas their atomic orbital occupancies are evidently different from one another. The Pt complex has a higher occupancy of the atomic 6s orbital because of strong s-d hybridization due to relativistic effects, thereby adopting a highly planar geometry reflecting the shape and orientation of the partially unoccupied dx2-y2 orbital.

A Square‐Planar Complex of Platinum(0)

AbstractThe Pt0 complex [Pt(PPh3)(Eind2‐BPEP)] with a pyridine‐based PNP‐pincer‐type phosphaalkene ligand (Eind2‐BPEP) has a highly planar geometry around Pt with ∑(Pt)=358.6°. This coordination geometry is very uncommon for formal d10 complexes, and the Pd and Ni homologues with the same ligands adopt distorted tetrahedral geometries. DFT calculations reveal that both the Pt and Pd complexes are M0 species with nearly ten valence electrons on the metals whereas their atomic orbital occupancies are evidently different from one another. The Pt complex has a higher occupancy of the atomic 6s orbital because of strong s–d hybridization due to relativistic effects, thereby adopting a highly planar geometry reflecting the shape and orientation of the partially unoccupied orbital.

Unsymmetrical PNP-Pincer Type Phosphaalkene Ligands Protected by a Fused-Ring Bulky Eind Group: Synthesis and Applications to Rh(I) and Ir(I) Complexes

We recently reported that 2-(phospholanylmethyl)-6-(2-phosphaethenyl)pyridine (PPEP) with a 2,4,6-tri-tert-butylphenyl group (Mes*) as steric protection of the P═C bond serves as a noninnocent ligand on Ir(I), leading to extremely high reactivity toward metal–ligand cooperative activation of ammonia and acetonitrile. The high reactivity is largely due to the strong π-accepting properties of the P═C bond. However, PPEP had a stability problem that provokes the loss of the P═C bond on other transition metals, including Rh(I), and restricts its utilization. This paper describes the synthesis of Eind-PPEP protected by an octaethyl-s-hydrindacen-4-yl group (Eind) instead of Mes*. The fused-ring bulky Eind group successfully prevents the loss of the P═C bond and enables us to compare the reactivity of Rh(I) and Ir(I) complexes toward ammonia. The complex K[RhCl(Eind-PPEP*)], bearing a dearomatized Eind-PPEP* ligand, undergoes simple ligand displacement to give [Rh(NH3)(Eind-PPEP*)], whereas the iridium analogue...

Streamlined Preparation and Coordination Chemistry of Hybrid Phosphine–Phosphaalkene Ligands

A rationally designed and selective synthesis of hybrid phosphine–phosphaalkene ligands E-1a (Cy2PCH2CH═PMes*, Mes* = 2,4,6-tri-tert-butylphenyl) and E-1b (Ph2PCH2CH═PMes*) was developed using phospha-Wittig methodology. The new hybrid ligands E-1a and E-1b were used to prepare the Pd and Pt dichloride complexes Pd(Cy2PCH2CH═PMes*)Cl2 (2a), Pd(Ph2PCH2CH═PMes*)Cl2 (2b), Pt(Cy2PCH2CH═PMes*)Cl2 (3a), and Pt(Ph2PCH2CH═PMes*)Cl2 (3b). The crystal structures of E-1a, E-1b, 2a·1.33CHCl3, 3a·CH3CN, and 3b were determined. DFT calculations (M06/LACV3P**) on 2a revealed that the π* orbital located on the P═C unit is low-lying and accessible. An NBO analysis concluded that the phosphaalkene ligand is a significantly poorer σ donor and a slightly better π acceptor than its tertiary phosphine counterpart, due to the presence of the P═C double bond.

Reactions of [Cu(X)(BPEP-Ph)] (X = PF6, SbF6) with silyl compounds. Cooperative bond activation involving non-coordinating anions

Bond activation of silyl compounds, assisted by the cooperative action of non-coordinating anions, is achieved using Cu(I) complexes coordinated with a PNP-pincer type phosphaalkene ligand, [Cu(X)(BPEP-Ph)] (X = PF6 (1a), SbF6 (1b); BPEP-Ph = 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine). Complexes 1a and 1b react with Me3SiCN to form Me3SiF and Cu(i) cyanide complexes of the formula [Cu(CN-EF5)(BPEP-Ph)] (E = P (2a), Sb (2b)), in which the CN ligand is associated with the EF5 group arising from EF6(-). Formation of the intermediary isonitrile complex [Cu(CNSiMe3)(BPEP-Ph)](+)SbF6(-) (3b) is confirmed by its isolation. Thus, a two-step reaction process involving coordination of Me3SiCN, followed by nucleophilic attack of SbF6(-) on the silicon atom of 3b is established for the conversion of 1b to 2b. Complex 1b cleaves the H-Si bond of PhMe2SiH as well. The isolation and structural identification of [Cu(BPEP-Ph)](+)BAr(F)4(-) (1c) (BAr(F)4 = B{3,5-(CF3)2C6H3}4) as a rare example of a T-shaped, three-coordinated Cu(i) complex is reported.

Iridium(I) Complexes Bearing a Noninnocent PNP‐Pincer‐Type Phosphaalkene ­Ligand: Catalytic Application in the Base‐Free N‐Alkylation of Amines with Alcohols

AbstractA series of IrI complexes [Ir(L)(PPEP*)] [L = Cl– (3), CO (4), tBuNC (5), PMe3 (6), PPh3 (7)], coordinated with a PNP‐pincer‐type phosphaalkene ligand bearing a dearomatized pyridine ring (PPEP*), have been prepared and their catalytic properties for the dehydration/condensation of amines with alcohols has been examined. The catalytic reactions successfully proceed under base‐free conditions to give N‐alkylated amines and their dehydrogenation derivatives (imines). The product selectivity is dependent on L coordinated with Ir(PPEP*). Complexes 4 and 5 that contain π‐accepting ligands (CO, tBuNC) form N‐alkylated amines as the major products in a closed system using a nitrogen‐gas‐filled Schlenk tube. In contrast, complex 7 that contain PPh3 as L produces imines as the major products under a nitrogen‐gas flow. The reason for the selectivity change depending on L is discussed based on stoichiometric reactions using model compounds of presumed catalytic intermediates.

C–H Bond Cleavage of Acetonitrile by Iridium Complexes Bearing PNP-Pincer-Type Phosphaalkene Ligands

A novel parent amido complex of iridium(I), K[Ir(NH2)(PPEP*)] (3), coordinated with a dearomatized PNP-pincer-type phosphaalkene ligand (PPEP*) has been prepared by deprotonation with KHMDS from [Ir(NH2)(PPEP)] (2) having benzophospholanylmethyl and phosphaethenyl groups at the 2,6-positions of pyridine. Complex 3 has two base points at PPEP* and NH2 ligands and, thus, successively reacts with two molecules of CH3CN via heterolytic cleavage of the C–H bond. X-ray structural analysis of the product complex K[Ir(CH2CN)2(PPEP)] (5) reveals remarkable elongation of the P═C bond indicative of the occurrence of strong π-back-donation from iridium to PPEP.

Catalytic Synthesis of an Unsymmetrical PNP-Pincer-Type Phosphaalkene Ligand

An unsymmetrical PNP-pincer-type phosphaalkene ligand, 2-(phospholanylmethyl)-6-(2-phosphaethenyl)pyridine (PPEP), has been prepared from 2,6-bis(2-phosphaethenyl)pyridine (BPEP) by intramolecular C–H addition/cyclization of the 2-phosphaethenyl group with a 2,4,6-tri-tert-butylphenyl substituent (CH═PMes*). The reaction proceeds in hexane in the presence of a catalytic amount of [Pt(PCy3)2] (20 mol %) at 80 °C in a sealed tube, giving PPEP in 32% isolated yield, along with byproduction of 2,6-bis(phospholanylmethyl)pyridine (BPMP) and a Pt(II) phosphanido complex (5). The PPEP ligand reacts with [Rh(μ-Cl)(C2H4)2]2 and [RuCl2(PPh3)3] to afford [RhCl(PPEP)] (6) and [RuCl2(PPh3)(PPEP)] (8), respectively. Complex 6 easily undergoes C–H addition/cyclization at the other CH═PMes* group to afford the 2,6-bis(phospholanylmethyl)pyridine complex [RhCl(BPMP)] (7), whereas 8 is stable against C–H addition/cyclization. Treatment of 8 with tBuOK forms [RuCl(PPh3)(PPEP*)] (9), coordinated with an unsymmetrical PNP-pincer-type phosphaalkene ligand containing a dearomatized pyridine unit (PPEP*). The X-ray structures of 5 and 9 are reported. The reaction processes from BPEP to PPEP and to 5 are discussed based on NMR observations.

Disproportionation of Bis(phosphaethenyl)pyridine Iron(I) Bromide Induced by tBuNC

Redox behavior of Fe(I) complexes bearing a PNP-pincer-type phosphaalkene ligand, 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine (BPEP-Ph), is reported. The four-coordinate Fe(I) complex [FeBr(BPEP-Ph)] (1) readily reacts with 1 equiv of tBuNC in toluene at −35 °C to give [FeBr(tBuNC)(BPEP-Ph)] (2) with a 17e configuration. Complex 2 is fairly stable in neat benzene at room temperature, but smoothly undergoes disproportionation in the presence of added tBuNC to afford the Fe(0) complex [Fe(tBuNC)2(BPEP-Ph)] (3) and the Fe(II) complex [FeBr2(tBuNC)4] (4) along with free BPEP-Ph. The single-crystal X-ray diffraction studies of 3 and 4 reveal distorted trigonal bipyramidal and square pyramidal arrangements around Fe, respectively. The yield of 3 increases up to 50%/2 when 2 equiv of tBuNC is added to the system. A disproportionation process involving a 19e intermediate is proposed.

Synthesis and Structures of Nickel Complexes with a PN-Chelate Phosphaalkene Ligand

Phosphaalkenes with a P═C bond possess extremely strong π-accepting ability, often providing transition metal complexes with interesting structures and properties. This paper describes unique structures of nickel complexes coordinated with a PN-chelate phosphaalkene ligand (PEP = 2-[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine). The PEP ligand combines with [NiBr2(dme)] (dme = 1,2-dimethoxyethane) in toluene to afford [Ni(Br)(μ-Br)(PEP)]2 (1), which reacts with R2Mg(thf)2 in Et2O or THF to form three types of nickel complexes depending on the R groups and reaction conditions. The reaction with Ph2Mg(thf)2 produces a Ni(I) dimer bridged with two μ-Br ligands, [Ni(μ-Br)(PEP)]2 (2). Treatment of 1 with R2Mg(thf)2 (R = Me, Me3SiCH2) at −35 °C leads to dialkyl complexes [NiR2(PEP)] (3 and 4), with a significantly distorted square planar configuration. DFT calculations support the occurrence of effective π-back-bonding between Ni to PEP to cause the structural distortion. On the other hand, the reaction of 1 with R2Mg(thf)2 (R = Me3SiCH2) conducted at a low temperature of −78 °C forms an aryl bromide complex of the formula [Ni(Mes*)(Br)(PEP*)] (5; Mes* = 2,4,6-tBu3C6H2), in which the Mes* group originally bonded to the phosphorus atom of PEP is shifted to nickel; instead, the phosphorus atom is substituted with the R group to form the PEP* ligand. The temperature-dependent formation of 4 or 5 is rationalized by considering a common five-coordinate intermediate.

Mechanism of N–H Bond Cleavage of Aniline by a Dearomatized PNP-Pincer Type Phosphaalkene Complex of Iridium(I)

Detailed mechanistic investigations using kinetic and theoretical methods have been conducted for deprotonative N–H bond cleavage of p-YC6H4NH2 (Y = H, MeO, Me, Cl, Br, NO2) by [K(18-crown-6)][Ir(Cl)(PPEP*)] (1a) bearing a dearomatized PNP-pincer type phosphaalkene ligand (PPEP*) to afford [Ir(NHC6H4Y)(PPEP)] (2) with an aromatized ligand (PPEP). While 1a is in equilibrium with [K(18-crown-6)]Cl (3) and [Ir(PPEP*)] (4) in solution, the N–H bond cleavage proceeds via association of 1a with aniline, where the coordination of aniline to iridium is insignificant; instead, aniline is associated with PPEP* by hydrogen bonding. In contrast, the N–H bond cleavage of ammonia proceeds via the pentacoordinate intermediate [Ir(Cl)(NH3)(PPEP*)]. The difference between the N–H bond cleavage processes of aniline and ammonia is examined by DFT calculations.

Facile N–H Bond Cleavage of Ammonia by an Iridium Complex Bearing a Noninnocent PNP-Pincer Type Phosphaalkene Ligand

A novel PNP-pincer type phosphaalkene complex of iridium bearing a dearomatized pyridine unit (3) has been prepared. Complex 3 rapidly reacts with ammonia at room temperature to afford a parent amido complex in high yield. DFT calculations indicate that the phosphaalkene unit with a strong π-accepting property effectively facilitates the N-H bond cleavage of ammonia via metal-ligand cooperation.

Synthesis, Structures, and Reactivity of Ruthenium Complexes with PNP-pincer Type Phosphaalkene Ligands

2,6-Bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine (BPEP-Ph) reacts with [Ru3(CO)12] or [RuCl(η3-allyl)(CO)3] in toluene at elevated temperatures to afford the three types of PNP-pincer complexes 1–3. The reaction of BPEP-Ph with [Ru3(CO)12] under vacuum forms a mixture of [Ru(CO)2(BPEP-Ph)] (1) and a dicarbonylruthenium(0) complex (2); the latter complex has an unsymmetrical PNP-pincer ligand with 1-phenyl-2-phosphaethenyl and phenyl(benzo[b]phospholan-1-yl)methyl groups at the 2,6-positions of the pyridine core, which is formed by intramolecular C–H addition of a t-Bu group to the P═C bond. On the other hand, the reaction of BPEP-Ph with [RuCl(η3-allyl)(CO)3] affords a dicarbonylruthenium(II) chloride (3) with an unsymmetrical PNP-pincer ligand bearing a dearomatized pyridyl group with 1-phenyl-2-phosphaethenyl and α-(benzo[b]phospholan-1-yl)benzylidene substituents at the 2,6-positions. Treatment of 3 with i-PrOK in THF results in selective formation of 2. Complexes 1–3 have been...

A Bis(phosphaethenyl)pyridine Complex of Iridium(I): Synthesis and Catalytic Application to N-Alkylation of Amines with Alcohols

The iridium(I) complex [IrCl(BPEP-H)] (1), coordinated with 2,6-bis[2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine (BPEP-H) as a PNP-pincer-type phosphaalkene ligand, has been synthesized and fully characterized by elemental analysis, NMR spectroscopy, and X-ray diffraction analysis. Complex 1 (1 mol %) catalyzes N-alkylation of primary and secondary amines with alcohols, leading to the selective formation of secondary and tertiary amines, respectively. Primary amines are smoothly alkylated with a variety of benzylic and aliphatic alcohols (1 or 3 equiv) at 100 °C under basic conditions (CsOH, 10 mol %) to give the corresponding secondary amines in good to high yields. On the other hand, N-alkylation of secondary amines with benzyl alcohol (3 equiv) proceeds in the presence of KH2PO4 (5 mol %) at 140 °C to afford tertiary amines in high yields.

Nitrogen-bridged bidentate phosphaalkene ligands

Nitrogen-bridged bidentate phosphaalkenes reveal several exceptional properties as a new class of P=C-unsaturated diphosphinoamine (PNP) ligands: N–Si bond cleavage of the first N-silylimino-bridged bis-phosphaalkene (R2C=P)2NSiMe3 (R = iPrMe2Si) with chlorides of AgI, AuI, and RhI leads to complexes of the 5c-6π-heteropentadienide imidobisphosphaalkene anion; double Cl– transfer from PtCl2 and PdCl2 to coordinated phosphaalkene phosphorus atoms of P-diphenylphosphanylamino phosphaalkenes R2C=PN(R')PPh2 (R' = Me3Si or iPrMe2Si; R = tBu, 1-Ada) furnishes unprecedented bis-chelate P-chloroylid complexes M[R2CP(Cl)N(R')PPh2]2 (M = Pd, Pt); CO- and RhI-induced coupling of two P-diphenylphosphanylamino phosphaalkenes under disilylketene elimination provides trigonal–bipyramidal RhIII complexes with novel tetradentate dianionic “PNP–C–PNP” ligands R2C[P(–)N(R')PPh2]2.

Gold highlights at the 10th International Conference on Heteroatom Chemistry (ICHAC-10), in Uji, Kyoto, Japan, 20–25 May 2012

There were over 370 participants attending this meeting, coming from diverse fields of heteroatom chemistry. Most presentations were from Japan; however, around 90 participants were from other countries (including China, Korea, Taiwan, India, Iraq, Kuwait, Saudi Arabia, Bulgaria, Poland, Italy, Germany, Netherlands, Sweden, France, UK, Spain, Portugal, USA and Canada), being the majority from universities and research institutes. Gold-related subjects had an importance presence, showing that research on gold keeps increasing all over the world. The synthesis of different gold complexes, the use of gold compounds as catalysts for heterocycle formation and gold nanoparticles or gold electrodes for several applications were referred. A brief description is indicated below. The plenary lecture (PL3) of Wolf du Mont (Technical University of Braunschweig, Germany) dealt with “Nitrogen and oxygen-bridged bifunctional phosphaalkene ligands”. N–Si bond cleavage of the N-silylimino-bridged bis-phosphaalkene [(iPrMe2Si)2C0P]2NSiMe3 with chlorides of Ag , Au and Rh leads to complexes of the 5c-6π-heteropentadiene imidobisphosphanaalkene anion [1]. Figure 1 shows an imidobisphosphaalkene gold complex. In his plenary lecture (PL5), dealing with “Transition metal-catalysed synthesis of heterocycles proceeding with migration of heteroatom groups”, Vladimir Gevorgyan (University of Illinois at Chicago, USA) spoke about novel efficient transition metal-catalysed methodologies for synthesis of multi-substituted carboand heterocycles by incorporation of migrating step(s) in the cyclisation cascade. It was found that in the presence of Cu, Ag and Au catalysts, a number of groups, such as X (X0I, Br, Cl), RS−, SiR3, etc., could undergo 1,2or 1,3-migration. An example referred in a recent publication of the speaker shows an interesting example [2], as a double migratory cascade reaction of αhalogen-substituted propargylic phosphates to produce highly functionalized 1,3-dienes has been developed. This transformation features 1,3-phosphatyloxy group migration followed by 1,3-shifts of bromine and chlorine as well as the

Redox Chemistry of Bis(phosphaethenyl)pyridine Iron Complexes

Redox reactions of iron complexes bearing a PNP-pincer-type phosphaalkene ligand, 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine (BPEP), are reported. The Fe(II) dibromide [FeBr2(BPEP)] (1) is readily reduced by [Cp2Co] to afford the four-coordinate Fe(I) monobromide [FeBr(BPEP)] (2), while 2 reacts with PhCH2Br to reproduce 1. Treatment of 1 with MesMgBr or Mes2Mg(THF)2 (Mes = 2,4,6-Me3C6H2) results in one-electron reduction of 1, followed by transmetalation of the resulting 2 with mesitylmagnesium compounds to give the Fe(I) mesityl complex [FeMes(BPEP)] (3). The single-crystal diffraction study of 3 has revealed a distorted trigonal monopyramidal arrangement around the iron center. SQUID magnetometry has established a low-spin ground state (S = 1/2) of 3. Complex 2 reacts with Me2Mg(THF)2 to afford Fe(0) and Fe(II) complexes (4 and 5, respectively) coordinated with novel multidentate ligand systems containing a phosphonium ylide structure. The formation processes of 4 and 5 v...