4-Phosphino-Substituted N-Heterocyclic Carbenes (NHCs) from the Abnormal Reaction of NHCs with Phosphaalkenes

The activation of the PC bond of phosphaalkenes with N-heterocyclic carbenes (NHCs) offers a convenient means to introduce new functionality at the 4-position of an NHC. Treatment of MesPCRR′ (<b>2a</b>: R = R′ = Ph; <b>2b</b>: R = Ph, R′ = 2-C₅H₄N; <b>2c</b>: R = R′ = 4-C₆H₄F) with 1,3-dimesitylimidazol-2-ylidene [:C­(NMesCH)₂, IMes, Mes = 2,4,6-Me₃C₆H₂] affords 4-phosphino-2-carbenes :C­(NMes)₂CHC­(PMesCHRR′)NMes [<b>1a</b>: R = R′ = Ph; <b>1b</b>: R = Ph, R′ = 2-C₅H₄N; <b>1c</b>: R = R′ = 4-C₆H₄F]. Significantly, these functional NHCs retain the parent carbene functionality and, in addition, contain a phosphine moiety. The preparation and spectroscopic characterization of the complexes [(<b>1a</b>)­M­(cod)­Cl] (<b>4</b>) and <i>cis</i>-[(<b>1a</b>)­M­(CO)₂Cl] (<b>5</b>) (M = Ir, Rh) are reported. The average CO stretching frequencies (ν̅_CO^av) for <b>5_M=Ir</b>, <b>5_M=Rh</b>, and authentic samples of <i>cis</i>-[(IMes)­M­(CO)₂Cl] (M = Ir, Rh) are presented as a means to evaluate the donor properties of 4-phosphino-2-carbene <b>1a</b>. The average CO stretching frequency is lower energy for <b>5_M=Ir</b> than for <i>cis</i>-[(IMes)­Ir­(CO)₂Cl] <b>(</b>Δν̅_CO^av = −2 cm^–1), whereas the opposite is observed between <b>5_M=Rh</b> and <i>cis</i>-[(IMes)­Ir­(CO)₂Cl] <b>(</b>Δν̅_CO^av = 2 cm^–1). The molecular structures are reported for <b>1a</b>, <b>1b</b>, <b>1c</b>, <b>4_M=Ir</b>, and <b>5_M=Ir</b>

Addition-Isomerization Polymerization of Chiral Phosphaalkenes: Observation of Styrene–Phosphaalkene Linkages in a Random Copolymer

These studies provide the first evidence for styrene–phosphaalkene connectivities in a phosphaalkene copolymer. The synthesis and structural characterization of new phosphaalkene–oxazolines, ArPC­(Ph)­(3-C₆H₄Ox) [<b>1a</b>,<b>b</b>, Ar = Mes (<b>1a</b>), Mes* (<b>1b</b>), Ox = CNOCH­(^<i>i</i>Pr)­CH₂], are reported. The radical-initiated homo- and copolymerization of <b>1a</b> with styrene affords <i>P</i>-functional poly­(methylene­phosphine) (<b>4a</b>: <i>M</i>_n = 5300 g mol^–1, PDI = 1.2) and poly­(methylene­phosphine-<i>co</i>-styrene) (<b>5a</b>: <i>M</i>_n = 4000 g mol^–1, PDI = 1.1). Multinuclear NMR spectroscopic analyses of <b>4a</b> and <b>5a</b> provided evidence for the predominance of an addition-isomerization mechanism for the radical polymerization of <b>1a</b>. In addition, signals could be assigned to CHPh–P­(CHPhAr) (i.e., S–<b>1a</b>) and ArCH₂–CH₂ (i.e., <b>1a</b>–S) linkages in copolymer <b>5a</b>. With a monomer feed ratio of <b>1a</b>:S (1:2, 33 mol % <b>1a</b>) the inverse gated ¹³C­{¹H} NMR spectrum suggested an incorporation of 19 mol % <b>1a</b> in copolymer <b>5a</b>. Polymers <b>4a</b> and <b>5a</b> were further functionalized to Au­(I)-containing macromolecules [<b>4a</b>·AuCl: <i>M</i>_n = 13 000, PDI = 1.2; <b>5a</b>·AuCl: <i>M</i>_n = 7500, PDI = 1.1]

Addition-Isomerization Polymerization of Chiral Phosphaalkenes: Observation of Styrene–Phosphaalkene Linkages in a Random Copolymer

These studies provide the first evidence for styrene–phosphaalkene connectivities in a phosphaalkene copolymer. The synthesis and structural characterization of new phosphaalkene–oxazolines, ArPC­(Ph)­(3-C₆H₄Ox) [<b>1a</b>,<b>b</b>, Ar = Mes (<b>1a</b>), Mes* (<b>1b</b>), Ox = CNOCH­(^<i>i</i>Pr)­CH₂], are reported. The radical-initiated homo- and copolymerization of <b>1a</b> with styrene affords <i>P</i>-functional poly­(methylene­phosphine) (<b>4a</b>: <i>M</i>_n = 5300 g mol^–1, PDI = 1.2) and poly­(methylene­phosphine-<i>co</i>-styrene) (<b>5a</b>: <i>M</i>_n = 4000 g mol^–1, PDI = 1.1). Multinuclear NMR spectroscopic analyses of <b>4a</b> and <b>5a</b> provided evidence for the predominance of an addition-isomerization mechanism for the radical polymerization of <b>1a</b>. In addition, signals could be assigned to CHPh–P­(CHPhAr) (i.e., S–<b>1a</b>) and ArCH₂–CH₂ (i.e., <b>1a</b>–S) linkages in copolymer <b>5a</b>. With a monomer feed ratio of <b>1a</b>:S (1:2, 33 mol % <b>1a</b>) the inverse gated ¹³C­{¹H} NMR spectrum suggested an incorporation of 19 mol % <b>1a</b> in copolymer <b>5a</b>. Polymers <b>4a</b> and <b>5a</b> were further functionalized to Au­(I)-containing macromolecules [<b>4a</b>·AuCl: <i>M</i>_n = 13 000, PDI = 1.2; <b>5a</b>·AuCl: <i>M</i>_n = 7500, PDI = 1.1]

Addition-Isomerization Polymerization of Chiral Phosphaalkenes: Observation of Styrene–Phosphaalkene Linkages in a Random Copolymer

These studies provide the first evidence for styrene–phosphaalkene connectivities in a phosphaalkene copolymer. The synthesis and structural characterization of new phosphaalkene–oxazolines, ArPC­(Ph)­(3-C₆H₄Ox) [<b>1a</b>,<b>b</b>, Ar = Mes (<b>1a</b>), Mes* (<b>1b</b>), Ox = CNOCH­(^<i>i</i>Pr)­CH₂], are reported. The radical-initiated homo- and copolymerization of <b>1a</b> with styrene affords <i>P</i>-functional poly­(methylene­phosphine) (<b>4a</b>: <i>M</i>_n = 5300 g mol^–1, PDI = 1.2) and poly­(methylene­phosphine-<i>co</i>-styrene) (<b>5a</b>: <i>M</i>_n = 4000 g mol^–1, PDI = 1.1). Multinuclear NMR spectroscopic analyses of <b>4a</b> and <b>5a</b> provided evidence for the predominance of an addition-isomerization mechanism for the radical polymerization of <b>1a</b>. In addition, signals could be assigned to CHPh–P­(CHPhAr) (i.e., S–<b>1a</b>) and ArCH₂–CH₂ (i.e., <b>1a</b>–S) linkages in copolymer <b>5a</b>. With a monomer feed ratio of <b>1a</b>:S (1:2, 33 mol % <b>1a</b>) the inverse gated ¹³C­{¹H} NMR spectrum suggested an incorporation of 19 mol % <b>1a</b> in copolymer <b>5a</b>. Polymers <b>4a</b> and <b>5a</b> were further functionalized to Au­(I)-containing macromolecules [<b>4a</b>·AuCl: <i>M</i>_n = 13 000, PDI = 1.2; <b>5a</b>·AuCl: <i>M</i>_n = 7500, PDI = 1.1]

Copper(I) Complexes of Pyridine-Bridged Phosphaalkene-Oxazoline Pincer Ligands

The synthesis of enantiomerically pure pyridine-bridged phosphaalkene-oxazolines ArPC­(Ph)­(2,6-C₅H₃NOx) (<b>1</b>, Ar = Mes/Mes*, Ox = CNOCH­(<i>i</i>-Pr)­CH₂/­CNOCH­(CH₂Ph)­CH₂) is reported. This new ligand forms a κ­(P), κ²(NN) dimeric complex with copper­(I) (<b>7</b>) that dissociates into a cationic κ³(PNN) monomeric complex upon addition of a neutral ligand {[<b>1a</b>·CuL]­OTf (<b>8a</b>–<b>e</b>): L = PPh₃ (<b>a</b>), P­(OPh)₃ (<b>b</b>), 2,6-lutidine (<b>c</b>), 4-DMAP (<b>d</b>), 1-methylimidazole (<b>e</b>)}. The P–Cu bond lengths in <b>8</b> are influenced by the π-accepting/σ-donating properties of L, and this can be observed by changes in the δ³¹P_PC NMR shift. The donor–acceptor properties in complexes of type <b>8</b> have also been investigated by UV/vis spectroscopy and density functional theory calculations

Reaction of an Enantiomerically Pure Phosphaalkene-Oxazoline with MeM Nucleophiles (M = Li and MgBr): Stereoselectivity and Noninnocence of the P‑Mesityl Substituent

The addition of alkyl nucleophiles (MeM, M = Li, MgBr) across the PC bond of an enantiomerically pure phosphaalkene-oxazoline followed by protonation of the C anion affords phosphines with three chirality centers. The formation of palladium­(II) complexes of the resultant phosphines permitted structural characterization of the products by X-ray diffraction. The choice of nucleophile has a profound effect on the product distributions. For instance, the Grignard reagent adds in a diastereoselective manner to give one major phosphine product with P- and C-stereocenters. In contrast, addition of methyllithium has proven not only to be less stereoselective but also affords a fascinating cyclic phosphine product. Both the Grignard and RLi reactions involve proton transfer from the <i>o</i>-Me of the P-Mes substituent even though the products are quite different in each case

Copper(I) Complexes of Pyridine-Bridged Phosphaalkene-Oxazoline Pincer Ligands

The synthesis of enantiomerically pure pyridine-bridged phosphaalkene-oxazolines ArPC­(Ph)­(2,6-C₅H₃NOx) (<b>1</b>, Ar = Mes/Mes*, Ox = CNOCH­(<i>i</i>-Pr)­CH₂/­CNOCH­(CH₂Ph)­CH₂) is reported. This new ligand forms a κ­(P), κ²(NN) dimeric complex with copper­(I) (<b>7</b>) that dissociates into a cationic κ³(PNN) monomeric complex upon addition of a neutral ligand {[<b>1a</b>·CuL]­OTf (<b>8a</b>–<b>e</b>): L = PPh₃ (<b>a</b>), P­(OPh)₃ (<b>b</b>), 2,6-lutidine (<b>c</b>), 4-DMAP (<b>d</b>), 1-methylimidazole (<b>e</b>)}. The P–Cu bond lengths in <b>8</b> are influenced by the π-accepting/σ-donating properties of L, and this can be observed by changes in the δ³¹P_PC NMR shift. The donor–acceptor properties in complexes of type <b>8</b> have also been investigated by UV/vis spectroscopy and density functional theory calculations

Homo- and Heteropolynuclear Complexes Containing Bidentate Bridging 4‑Phosphino-N-Heterocyclic Carbene Ligands

The abnormal reaction of phosphaalkenes with N-heterocyclic carbenes (NHC) offers a convenient method to introduce new functionality at the backbone of an NHC. The 4-phosphino-substituted NHC (<b>1a</b>) derived from 1,3-dimesitylimidazol-2-ylidene (IMes) and MesPCPh₂ is shown to be an effective bifunctional ligand for Au­(I) and Pd­(II). Several new complexes are reported: <b>2a</b>: <b>1a</b>·Au_<i>C</i>Cl, <b>3a</b>: <b>1a</b>·(AuCl)₂, <b>4a</b>: [(<b>1a</b>)₂Au_<i>C</i>]­Cl, <b>5a</b>: [(<b>1a</b>·Au_<i>P</i>Cl)₂Au_<i>C</i>]­Cl], and <b>6a</b>: <b>1a</b>·(Pd_<i>C</i>) (Au_<i>P</i>Cl). The reaction of 4-phosphino-NHC <b>1b</b>, derived from 1,3-di­(cyclohexyl)­imidazol-2-ylidene (ICy) and MesPC­(4-C₆H₄F)₂, with (tht)­AuCl (2 equiv, tht = tetrahydrothiophene) affords the fascinating tetranuclear <b>5b</b> [(<b>1b</b>·Au_<i>P</i>Cl)₂Au_<i>C</i>]­[AuCl₂]. The molecular structure of <b>5b</b> features a close Au···Au contact (3.0988(4) Å) between the bis­(carbene)­gold­(I) cation and the dichloroaurate­(I) anion. The buried volumes (%<i>V</i>_bur) and Tolman cone angles for representative 4-phosphino-NHCs calculated from structural data are compared to related carbenes and phosphines. The molecular structures are reported for complexes <b>3a</b>, <b>4a</b>, <b>5b</b>, and <b>6a</b>

Poly(<i>p</i>‑phenylenediethynylene phosphine)s and Related π‑Conjugated Phosphine–Diyne Polymers: Synthesis, Characterization and Photophysical Properties

Despite the considerable synthetic challenge, polymers that incorporate phosphorus atoms within or adjacent to a π-conjugated backbone framework are of interest due to the ability to modulate the photophysical properties of the polymer based on the chemical environment at phosphorus. We report the synthesis and characterization of poly­(<i>p</i>-phenylenediethlynylene phosphine)­s, <b>1a</b>–<b>1e</b>, a new class of σ–π conjugated polymer. Polymers <b>1a</b>–<b>1e</b> were prepared by a Ni-catalyzed coupling of dialkynes with phenyldichlorophosphine in the presence of excess triethylamine and have molecular weights (<i>M</i>_w) up to 12,000 g mol^–1 (vs. polystyrene). While the polymers <b>1b</b>–<b>1e</b> are nonfluorescent, blue fluorescence is observed upon oxidation of the phosphorus centers (Φ_soln = 0.12–0.28). Polymers <b>1b</b>·O–<b>1e</b>·O also exhibit yellow/green fluorescence in the solid state (Φ_solid = 0.04–0.08). These results introduce a fascinating new class of main group-containing macromolecule and lay the groundwork for their utilization in sensory applications and/or their incorporation into optoelectrical devices

An Addition–Isomerization Mechanism for the Anionic Polymerization of MesPCPh₂ and <i>m</i>‑XylPCPh₂

We report that the anionic polymerization of P-mesityl and <i>m</i>-xylyl-substituted phosphaalkenes follows an unusual addition–isomerization mechanism. Specifically, the polymerization of ArPCPh₂ [Ar = Mes (<b>1a</b>), <i>m</i>-Xyl (<b>1b</b>)] involves the hindered nucleophilic anion intermediate, Ⓟ–P­(Ar)–CPh₂^–, which undergoes a proton migration from the <i>o</i>-CH₃ of the Mes/<i>m</i>-Xyl moiety to the −CPh₂ moiety to afford a propagating benzylic anion. This mechanism is supported by the preparation of model compounds MeP­(CHPh₂)-4,6-Me₂C₆H₂–2-CH₂–CPh₃ (<b>2a</b>) or MeP­(CHPh₂)-6-MeC₆H₃–2-CH₂–CPh₃ (<b>2b</b>), which were both crystallographically characterized. Polymerization of <b>1a</b> or <b>1b</b> in THF solution using <i>n</i>-BuLi (2 mol %) revealed ¹H and ¹³C NMR signals assigned to −CH₂– and −CHPh₂ groups consistent with an addition–isomerization polymerization mechanism to afford poly­(methylene­phosphine) <b>3a</b> or <b>3b</b>. A large kinetic isotope effect (≤23) was determined for the <i>n</i>-BuLi-initiated polymerization of <b>1a</b>-<i>d</i>₉ compared to <b>1a</b> in THF at 50 °C, consistent with C–H (or C–D) activation as the rate-determining step. This C–H activation step was modeled using DFT computations which revealed that the intramolecular proton transfer from the <i>o</i>-CH₃ of the Mes moiety to the −CPh₂ moiety has an activation energy (<i>E</i>_a = +18.5 kcal mol^–1). For comparison, this computational value was quite close to the experimentally measured activation energy of propagation ArPCPh₂ in THF [<i>E</i>_a = 14.0 ± 0.9 kcal mol^–1 (<b>1a</b>), 15.6 ± 2.8 kcal mol^–1 (<b>1b</b>)]