The combined use of P(2-MeOC6H4)3 (L1) and TMEDA as ligands effectively prevents defect formation in palladium-catalyzed direct arylation polymerization (DArP) to give donor–acceptor type alternating copolymers (DA polymers) with diketopyrrolopyrrole (DPP) units. The reaction of 3,6-bis(5-bromo-2-thienyl)diketopyrrolopyrrole (1-Br) and 3,4-dicyanothiophene (2-H) in toluene at 100 °C for 6 h in the presence of only L1 afforded a notable amount of insoluble materials via branching and cross-linking; in addition, the soluble part (Mn = 3100) included a large quantity of homocoupling defects (12.5%). In contrast, in the presence of L1 and TMEDA, the formation of insoluble materials was completely suppressed, and homocoupling defects decreased to 1.6%. Furthermore, the molecular weight of poly(1-alt-2) remarkably increased (Mn = 24 500, 97% yield). The mixed ligand catalyst using L1 and TMEDA was also effective against DArP of 1-Br with thienopyrroledione (3-H) or 3,4-propylenedioxythiophene (4-H) to afford DA polymers with DPP units in high yields (poly(1-alt-3): Mn = 36 800, 100% yield; poly(1-alt-4): Mn = 19 000, 80% yield). Thermal and optical properties of the resulting polymers are reported.

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)]+BArF4− (1c) (BArF4 = B{3,5-(CF3)2C6H3}4) as a rare example of a T-shaped, three-coordinated Cu(I) complex is reported.

This paper reports a novel mixed ligand catalyst for palladium-catalyzed direct arylation polymerization (DArP) of 2,6-diiododithienosilole (DTS-I2) and thienopyrroledione (TPD-H2) to give poly(DTS-alt-TPD). It has been documented that this monomer combination has a marked tendency to form homocoupling and branching defects in polymer chains, and the latter defects eventually lead to the formation of insoluble materials. This paper demonstrates that the combined use of P(o-MeOC6H4)3 and TMEDA ligands effectively prevents the defect formation. The side reactions that afford structural defects constitute a sequential process triggered by the reduction of DTS-I units. TMEDA as a simple diamine effectively inhibits the reduction of DTS-I units, giving poly(DTS-alt-TPD) (MnGPC = 20 000) in high cross-coupling selectivity (99%).

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 K[IrCl(Eind-PPEP*)] causes N–H bond cleavage to form [Ir(NH2)(Eind-PPEP)]. DFT calculations indicate a thermodynamic cause of the metal-dependent product change.

Palladium-catalyzed direct arylation polymerization (DArP) has recently attracted much attention as a simple and easy means of synthesizing π-conjugated polymers. We have reported a novel catalytic system for DArP. While the already known catalytic systems for direct arylation commonly require the use of a highly polar solvent such as DMA, our system is sufficiently reactive in less polar THF and toluene. A key to this unique catalytic property is the use of P(2-MeOC6H4)3 (L1), having o-MeO substituents as the supporting ligand. In this paper, we examine the role of L1 through the isolation of presumed catalytic intermediates. The arylpalladium acetate complexes [PdAr(OAc)(L1)]n (Ar = Ph (1a), 2,6-Me2C6H3 (5b)) are prepared by oxidative addition of ArBr to [Pd{P(o-tolyl)3}2] in the presence of L1, followed by anionic ligand exchange with AgOAc. X-ray diffraction analysis reveals that 1a is a dimeric compound (n = 2) bridged by two types of μ-OAc ligands, whereas 5b is a monomeric species (n = 1) coordinated with PO-chelating L1 (L1-κ2PO). Although the solid-state structures are different, both complexes form an equilibrium mixture of two kinds of monomeric compounds in solution, [PdAr(OAc-κO)(L1-κ2PO)] (5) and [PdAr(OAc-κ2O)(L1-κP)] (2). The equilibrium ratio of 5 to 2 is nearly independent of Ar ligands, and the reaction rate with 2-methylthiophene (3) is little affected by Ar ligands and solvents (toluene, THF, and DMA). These tendencies make a sharp contrast to those observed for the PPh3 analogues [PdAr(OAc)(PPh3)]n (Ar = Ph, 2,6-Me2C6H3), whose reactivity changes with Ar ligands and solvents remarkably. A mechanistic understanding of the unique ligand effect of L1 is presented.

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.

This paper reports the synthesis of alternating donor–acceptor copolymers (DA polymers) containing thiazole units via palladium-catalyzed direct arylation polymerization (DArP), which has recently emerged as a viable alternative to conventional cross-coupling polycondensations. While thiazole-based C–H monomers such as benzodithiazole and thiazolothiazole are poorly reactive toward DArP, thiazolothiazoles (TzTz) capped with 5-bromothienyl and thienyl groups at the 2,5-positions, 2,5-bis[5-bromo-3-(2-octyldodecyl)thiophene-2-yl]thiazolo[5,4-d]thiazole (1-Br) and 2,5-bis[3-(2-octyldodecyl)thiophene-2-yl]thiazolo[5,4-d]thiazole (1-H), successfully react with C–H and C–Br monomers, respectively. The reactions proceed in THF at 100 °C in the presence of Pd2(dba)3 and P(2-MeOC6H4)3 (L1) as catalyst precursors and pivalic acid and Cs2CO3 as additives to afford TzTz-containing DA polymers with high molecular weight (Mn up to 88 100) and good solubility in almost quantitative yields. A high-temperature NMR analysis of the resulting polymers reveals well-controlled structures containing a small amount of homocoupling defects.

This paper reports the synthesis of an alternating copolymer consisting of dithienosilole (DTS) and thienopyrroledione (TPD) units via palladium-catalyzed direct arylation polymerization (DArP). Although DArP has recently attracted much attention as an easy synthetic method of π-conjugated polymers without the need for prepreparation of organometallic monomers, there are two major problems that must be eliminated. One is homocoupling producing structural defects in the polymer chain, and the other is byproduction of insoluble materials. In this study, we have found that the combined use of P(2-MeOC6H4)3 and P(2-Me2NC6H4)3 ligands effectively prevents these side reactions, resulting in poly(DTS-alt-TPD) (MnGPC = 15000, Mw/Mn = 2.57) with good solubility in high yield (84%).

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.

Treatment of the Fe(I) mesityl complex [Fe(Mes)(BPEP-Ph)] (BPEP-Ph = 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine) with π-acid ligands (L = CO, RNC) leads to one-electron reduction via Mes group migration from Fe to P, followed by homolytic elimination of the 2,4,6-tBu3C6H2 group, to afford Fe(0) complexes of the formula [Fe(L)2(BPEP-Ph*)] (BPEP-Ph* = 2-[1-phenyl-2-mesityl-2-phosphaethenyl]-6-[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine). This reduction process is supported by radical trapping experiments and theoretical studies. The 2,4,6-tBu3C6H2˙ radical is captured by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in high yield. DFT calculations reveal the mechanism of Mes group migration with a reasonable energy profile.

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.

The palladium-catalyzed direct arylation of heteroarenes with aryl halides has attracted considerable attention as a simple cross-coupling process that does not need organometallic reagents. It is generally accepted that this catalysis proceeds via an arylpalladium carboxylate intermediate, which produces direct arylation products via the sequence of three elementary processes: (a) substrate coordination, (b) C–H bond cleavage, and (c) C–C reductive elimination. This paper describes kinetic investigations into the effects of four kinds of PAr3 ligands on direct arylation using the isolated complexes [Pd(2,6-Me2C6H3)(μ-O2CMe)(PAr3)]4 (1: Ar = Ph (a), 4-MeOC6H4 (b), 4-FC6H4 (c), 4-F3CC6H4 (d)). While 1a–d have a tetrameric structure in the solid state, they are in rapid equilibrium with the monomeric species [Pd(2,6-Me2C6H3)(O2CMe-κ2O)(PAr3)] in solution. Complexes 1a–d react with 2-methylthiophene (3) and benzothiazole (4) in THF at 65 °C to give the corresponding direct arylation products in high yields. The reactivity order of 1a–d is reversed according to the heteroarene substrates; the reaction with 3 is accelerated by electron-deficient PAr3 (1b < 1a < 1c < 1d), whereas that with 4 is facilitated by electron-donating PAr3 (1d < 1c < 1a < 1b). The reasons for the opposite ligand effects are examined by DFT calculations using the model compounds [PdPh(O2CMe-κ2O)(PAr3)]. Unlike the general assumption, the C–H bond cleavage process is relatively insensitive to electronic properties of PAr3. Instead, the reaction of 3 invokes the C–C reductive elimination process as the rate-determining step, and the activation energy is significantly reduced by electron-deficient PAr3. On the other hand, although the rate-determining step for 4 is assigned to the C–H bond cleavage process, the transition state is little affected by PAr3 ligands. In this case, electron-donating PAr3 destabilizes the precursor complex for C–H bond cleavage, thereby reducing the activation barrier for the rate-determining step.

π-Conjugated polymers with a donor–acceptor (DA) combination of repeating units possess a narrow HOMO–LUMO gap, thus resulting in a high device performance in solar cells. This paper reports an improved catalytic system for the synthesis of DA polymers containing 5-(2-hexyldecyl)-5H-thieno[3,4-c]pyrrole-4,6-dione-1,3-diyl (TPD) group as the acceptor unit via palladium-catalyzed direct arylation polymerization. Although a related study has been reported ( Angew. Chem. Int. Ed. 2012, 51, 2068), we attempted to reduce the catalyst loading because the palladium residue in π-conjugated polymers has been known to produce a detrimental effect on device performance. As a result, the amount of palladium could be reduced to 1/8 by using PdCl2(MeCN)2 and P(C6H4-o-OMe)3 (L1) as catalyst precursors. The polymerization smoothly proceeds at 100 °C in THF in the presence of pivalic acid and Cs2CO3 to afford TPD-based DA polymers 3a–3d containing the following donor units in almost quantitative yields: 4,4′-dioctyl-2,2′-bithiophene-5,5′-diyl (3a, Mn = 36800, Mw/Mn = 2.20), 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl (3b, Mn = 31100, Mw/Mn = 2.44), 3,4-(2,2′-dioctylpropylenedioxy)thiophene-2,5-diyl (3c, Mn = 68200, Mw/Mn = 3.04), and 2,5-bis(2-ethylhexyloxy)-1,4-phenylene (3d, Mn = 65500, Mw/Mn = 2.21). A detailed analysis of the structure of 3a is reported.

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.

Direct arylation polymerization has attracted a great deal of attention as an efficient method for synthesizing π-conjugated polymers without the use of organometallic reagents. This paper describes that Pd2(dba)3·CHCl3 (0.5 mol %) and P(C6H4-o-OMe)3 (L1, 2.0 mol %), in conjunction with a carboxylic acid (AcOH or PivOH) and a base (Cs2CO3), exhibit high catalyst performance in polycondensation of 2,7-dibromo-9,9-dioctylfluorene and 1,2,4,5-tetrafluorobenzene. Unlike conventional catalytic direct arylation systems that have need to use highly polar solvents such as DMA and DMF, the present catalytic system is sufficiently reactive in THF and toluene to afford poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(2,3,5,6-tetrafluoro-1,4-phenylene)] (PDOF-TP) with extremely high molecular weight (Mn up to 347 700) in high yield.

Reaction of [(η5-C5H4Me)4Fe4(HCCH)(HCCBr)](PF6) ([2](PF6)) with AgPF6 in acetonitrile gave [(η5-C5H4Me)4Fe4(HCCH)(HCCNCMe)](PF6)2 ([3](PF6)2). The X-ray diffraction study revealed that the cationic [CCH] subunit is stabilized through coordination of the acetonitrile molecule to the cationic carbon atom. As a synthon for a donor-free [(η5-C5H4Me)4Fe4(HCCH)(HCC)]2+, [(η5-C5H4Me)4Fe4(HCCH)(HCCL)](PF6)2 ([6](PF6)2, L = pyrazine) was synthesized by the reaction of [2](PF6) with AgPF6 in the presence of pyrazine. Treatment of [6](PF6)2 with tertiary amines in acetonitrile led to deprotonation of acetonitrile to form [(η5-C5H4Me)4Fe4(HCCH)(HCCCH2CN)](PF6) ([11](PF6)). Treatment of [6](PF6)2 with maleimide in the presence of tertiary amine in acetonitrile allowed the functionalization of the coordinated acetonitrile through the nucleophilic attack of maleimide at the nitrile carbon atom.

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.

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 isolated as crystalline compounds and examined in detail by IR and NMR spectroscopy, X-ray diffraction studies, and DFT calculations. Complex 1 exhibits catalytic activity toward N-alkylation of amines (RNH2) with alcohols (R′CH2OH) to give imines (RN═CHR′) as major products.

The palladium-catalyzed direct arylation of aromatic compounds with aryl halides has been proposed to involve an arylpalladium carboxylate intermediate. However, isolated arylpalladium complexes, which undergo C–H bond cleavage of aromatic substrates without the aid of additional activators or promoters, have been scarcely documented. This paper reports that [PdAr(μ-O2CR)(PPh3)]n complexes (1: Ar = Ph, 2-MeC6H4, 2,6-Me2C6H3; R = Me, tBu) successfully react with 2-methylthiophene (2) in the absence of additives to afford 5-aryl-2-methylthiophenes (3) in high yields. The reactivity increases with increasing bulkiness of the Ar group, whereas the bulky pivalate ligand (R = tBu) reduces the reactivity as compared with the acetate ligand (R = Me). Complex 1 is in equilibrium with the monomeric species [PdAr(O2CR-κ2O)(PPh3)] (5) in solution, as confirmed by IR spectroscopy. Kinetic examinations have suggested that the direct arylation proceeds via 5, which undergoes C–H bond cleavage of 2. Complex 1 serves as a good catalyst for direct arylation of 2 with aryl bromides.

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 via an [FeMe(BPEP)] intermediate are discussed on the basis of their X-ray structures.

A 3:1 molar ratio mixture of [(η5-C5H4Me)4Fe4(HCCBr)2](PF6) (1a) and [(η5-C5H4Me)4Fe4(HCCH)(BrCCBr)](PF6) (1b) was converted to [(η5-C5H4Me)4Fe4(μ3-CH)2(μ3-CNPh)2](PF6)2 (2) upon treatment with aniline, followed by NiPr2Et and finally [Cp2Fe](PF6). The X-ray diffraction analysis revealed that 2 can be described as a cubane-type tetrairon cluster possessing two μ3-CH and two μ3-isonitrile ligands. Treatment of 2 with 2.5 equiv of [Cp2Co] gave the neutral form 3, formulated as [(η5-C5H4Me)4Fe4(HCCH)2(μ3-CNPh)2]. The redox reactions were chemically reversible; treatment of 3 with [Cp2Fe](PF6) reproduced 2 quantitatively. The structure of 3 was determined by X-ray diffraction analysis. The molecule exhibits a butterfly geometry resulting from the scission of one of the iron–iron bonds of the tetrahedron in 2. In accordance with the conversion of the core structure from tetrahedron to butterfly, the coupling of two μ3-CH ligands occurs to form an acetylene ligand. Further treatment of 3 with LiAlH4 followed by air-oxidation resulted in reductive coupling of two isonitrile ligands to give a bis(acetylene) cluster, [(η5-C5H4Me)4Fe4(HCCH)2](PF6) (6).

CuI complexes bearing BPEP as a PNP-pincer type phosphaalkene ligand undergo effective bonding interactions with SbF6− and PF6− as non-coordinating anions to give [Cu(SbF6)(BPEP)] and [Cu2(BPEP)2(μ-PF6)]+, respectively [BPEP = 2,6-bis(1-phenyl-2-phosphaethenyl)pyridine]. NMR and theoretical studies indicate a reduced anionic charge of the μ-PF6 ligand, which is induced by the strong π-accepting ability of BPEP.

A 15-electron iron complex with a formal Fe(I) center, [FeBr(BPEP)] (BPEP = 2,6-bis(1-phenyl-2-phosphaethenyl)pyridine), was prepared by one-electron reduction of the dibromide precursor [FeBr2(BPEP)]. The single-crystal diffraction analysis revealed a distorted trigonal monopyramidal arrangement around the iron center, and SQUID magnetometry established the S = 3/2 ground state. The Mössbauer isomer shift value (δ = 0.59 mm/s) was consistent with a high-spin Fe(I) center of [FeBr(BPEP)]. DFT calculations for a model complex revealed two highly delocalized molecular orbitals formed by bonding and antibonding interactions between the dz2 (Fe) and π* (BPEP) orbitals. Orbital occupancy analysis demonstrated the electronic structure with a high-spin Fe(I) center. The effective dπ−pπ interaction between iron and BPEP was concluded to be responsible for the highly distorted structure of [FeBr(BPEP)], with its rather uncommon trigonal monopyramidal configuration.

Dehydrohalogenative polycondensation of 2-bromo-3-hexylthiophene was successful with Herrmann’s catalyst and tris(2-dimethylaminophenyl)phosphine as catalyst precursors, giving head-to-tail poly(3-hexylthiophene) (HT-P3HT) with high molecular weight (Mn = 30 600, Mw/Mn = 1.60) and high regioregularity (98%) in almost quantitative yield (99%).

A series of platinum complexes of the type [Pt(D)(A)] having two kinds of π-conjugated ligand systems with donor and acceptor properties are prepared, using p-substituted diphenylacetylenes (tolan-X; X = OMe, H, CF3), 1,2-benzenedithiolato (bdt) and 1,3-dithia-2-thione-4,5-dithiolato (dmit) as π-donor ligands (D), and 1,2-diaryl-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutenes (DPCB-Y; diaryl = 2C6H4-p-Y (Y = OMe, H, CF3), biphenyl-2,2′-diyl) as π-acceptor ligands (A). The electronic structures of the resulting complexes are examined in detail to investigate π-orbital interaction occurring in DPCB-Y complexes with a low-coordinate phosphorus ligand. The 31P NMR chemical shifts are highly sensitive to the Pt–P distances of the complexes, and linearly correlated with the bond lengths. The UV-vis absorption spectra of [Pt(tolan-X)(DPCB-Y)] display π–π* transitions in the visible region, which are shifted to longer wavelengths as the electron-donating ability of X and electron-withdrawing ability of Y increase, respectively. The complex [Pt(tolan-OMe)(DPCB-CF3)] with a donor–acceptor combination of substituents exhibits particularly low-energy absorption. The absorption is further red-shifted for [Pt(bdt)(DPCB-phen)] and [Pt(dmit)(DPCB-phen)] having dithiolates as strong π-donor ligands.

All-cis and all-trans isomers of poly[(arylenevinylene)-alt-(5-octyloxy-1,3-phenylenevinylene)]s having three kinds of arylene groups (PmPVs: arylene = m-phenylene (1a), p-phenylene (1b), 4,4′-biphenylene (1c)) have been synthesized in almost perfect stereoregularity by two types of palladium-catalyzed polycondensation reactions, respectively. Suzuki−Miyaura-type polycondensation of (Z,Z)-bis(2-bromoethenyl)arenes with 5-octyloxy-1,3-benzenediboronic acid pinacolate affords all-cis 1a−c, whereas Hiyama-type polycondensation of (E,E)-bis(2-silylethenyl)arenes with 5-octyloxy-1,3-diiodobenzene forms all-trans 1a−c. The resulting polymers with relatively short effective π-conjugation lengths interrupted by m-phenylene units undergo two-way photoisomerization to give PmPVs containing cis- and trans-vinylene groups in nearly 1:1 ratios irrespective of the arylene groups of staring polymers.

A series of styrylpalladium phosphine complexes, trans-[Pd(CH═CHAr)Br(PMeAr′2)2] (1: Ar = Ph, 4-MeOC6H4, 4-MeC6H4, 4-F3CC6H4, 4-O2NC6H4; Ar′ = Ph, 4-MeC6H4, 4-FC6H4), have been prepared, and their P−C reductive elimination in the presence of added PMeAr′2 (1−4 equiv) in CD2Cl2 has been examined by kinetic experiments. All complexes are converted to [Pd(η2-ArCH═CHPMeAr′2)(PMeAr′2)2]Br (3) at 40 °C in high selectivity. The kinetic data are consistent with the reaction process involving prior association of 1 with PMeAr′2 to form a five-coordinate intermediate, trans-[Pd(CH═CHAr)Br(PMeAr′2)3] (A), which subsequently undergoes P−C reductive elimination to give 3. The pseudo-first-order rate constant (kobsd) increases as the amount of added PMeAr′2 ([PMeAr′2]) increases, according to the equation 1/kobsd = 1/kK[PMeAr′2] + 1/k, where K = [A]/[1][PMeAr′2] and k stands for the rate constant for the conversion of A to 3. The association constant K and the rate constant k exhibit a good Hammett correlation with the σp values of para substituents on the Ar and Ar′ groups, respectively: log(KY/KH) = [+0.84(3)]σp + 0.02(1) (for Ar groups); log(KY/KH) = [+4.2(4)]∑σp + 0.09(7) (for Ar′ groups); log(kY/kH) = [−2.43(9)]σp + 0.01(3) (for Ar groups); [−4.8(4)]∑σp − 0.11(8) (for Ar′ groups). Thus, the K value increases as the electron-withdrawing ability of para substituents increases, while the k value increases as the electron-donating ability of para substituents increases. The P−C reductive elimination mechanism from A to 3 is discussed.

A di-μ-hydroxo-dirhodium complex bearing a low-coordinate phosphorus ligand, [Rh2(μ-OH)2(DPCB)2] (2; DPCB = 1,2-diphenyl-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene), is prepared from [Rh2(μ-Cl)2(DPCB)2] (1) and KOH. Complex 2 serves as a good precursor for hydrido complexes, affording [Rh2(μ-H)(μ-OH)(DPCB)2] (3) and [Rh2(μ-H)2(DPCB)2] (7) by the reactions with HSiMe2Ph and HCO2H, respectively. Although complex 7 is too unstable to be isolated, its formation is evidenced by a trapping experiment using 1,3-cyclohexadiene to give [Rh(η3-C6H9)(DPCB)] (8). Complex 2 reacts with CO to afford [Rh2(μ-κP,P;κP,C-DPCB)(CO)4] (9) and [Rh2(μ-CO)2(DPCB)2] (10), which are interconvertible in solution. Complex 9 is the stable form under a CO atmosphere even in the presence of free DPCB, but readily converted to 10 by purging the CO gas from the solution. The X-ray structures of 2, 3, 9, 10, and related [Rh2(μ-H)(μ-OMe)(DPCB)2] (4) are reported. Two Rh(DPCB) moieties in 10 are oriented orthogonal to each other in the crystal; namely, one of the Rh centers has a square-planar configuration, whereas the other is in a tetrahedral arrangement with the μ-CO and DPCB ligands, showing flexible electronic properties of DPCB.

The (E)- and (Z)-styryl isomers of trans-[Pd(CH═CHPh)Br(PMePh2)2] (1a) and [Pd(η2-PhCH═CHPMePh2)Br(PMePh2)] (2a) were prepared, and their C−P reductive elimination (1a → 2a) and C−P oxidative addition (2a → 1a) behaviors examined. Kinetics and thermodynamics of the reactions are strongly affected by E/Z configurations of the styryl group and solvent polarity. Complex (E)-1a readily undergoes C−P reductive elimination in CD2Cl2 as a polar solvent in high selectivity. On the other hand, while the (Z)-isomer of 1a is unreactive toward reductive elimination, (Z)-2a undergoes C−P oxidative addition favorably in nonpolar benzene. X-ray diffraction analysis and DFT calculations for 1a and 2a provided reasonable accounts for these reaction features. Kinetic examinations revealed two types of C−P reductive elimination processes, which involve predissociation and association of the PMePh2 ligand, respectively.

Reaction of [(η5-C5H4Me)4Fe4(HCCH)(HCCBr)](PF6) (1) with HPPh2 in the presence of NEt3, followed by treatment of [Cp2Co], afforded [(η5-C5H4Me)4Fe4(HCCH)(HCC−PPh2)] (2). The electron-rich [4Fe−4C] core substituent leads to the extremely electron-releasing character of the phosphine part, estimated by the JPSe coupling constant of the corresponding selenide [(η5-C5H4Me)4Fe4(HCCH)(HCC−P(Se)Ph2)] (3). Reaction of 2 with [AuCl(SMe2)] gave [(η5-C5H4Me)4Fe4(HCCH)(HCC−P(AuCl)Ph2)] (4). The cyclic voltammogram of 4 shows two reversible one-electron oxidation waves, indicating the existence of one- and two-electron oxidized forms.

Reaction of (Z)-styryl bromide (1) with PhB(OH)2 in toluene at 80 °C for 1 h in the presence of Pd(PPh3)4 (1.5 mol %) and an aqueous solution of K2CO3 (3 equiv) affords (Z)-stilbene as the cross-coupling product in 99% yield. On the other hand, the same reaction of the (E)-isomer of 1 forms considerable amounts of homocoupling products (1,4-diphenylbutadiene (2, 22%) and biphenyl (27%)) in addition to the cross-coupling product ((E)-stilbene, 73%). The formation of 2 was examined by kinetic experiments using trans-Pd(CH═CHPh)Br(PMePh2)2 (3) as a model of the presumed intermediate. Complex 3 reacts with 1 at 50 °C for 5 h, giving 2 (91%) and trans-PdBr2(PMePh2)2 (4, 92%), together with a small amount of Pd(η2-PhCH═CHPMePh2)Br(PMePh2) (5, 8%). The reaction rate shows first-order dependence on the concentration of 3 and 5, respectively, but is independent of the concentration of 1. A novel homocoupling process induced by P−C reductive elimination from 3 giving 5 is proposed.

1,2-Bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobuta[l]phenanthrene (DPCB-phen) as a low-coordinated phosphorus ligand forms the platinum(0) alkyne complexes [Pt(alkyne)(DPCB-phen)] with extended π-conjugated systems. The colors of the complexes are highly dependent on alkyne ligands, showing a marked variation from reddish orange to teal.

The complex [RuCl(μ-Cl)(CO)(DPCB-OMe)]2 (1a), bearing a low-coordinated phosphorus ligand (DPCB-OMe = 1,2-bis(4-methoxyphenyl)-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene), is readily reduced to [RuH(μ-Cl)(CO)(DPCB-OMe)]2 (2a) by the reaction with water and HSiMe2Ph. The reaction proceeds via a [RuCl2(CO)(H2O)(DPCB-OMe)] intermediate, which is characterized by X-ray diffraction analysis. Complexes 1a and 2a serve as highly efficient catalysts for Z-selective hydrosilylation of phenylacetylene. The reason for the high catalyst efficiency of DPCB-OMe complexes has been investigated by reaction and structure analysis of the presumed intermediate [Ru(CH═CHPh)Cl(CO)(DPCB-OMe)] (3a). It has been found that 3a has ample space to associate with hydrosilane and, therefore, readily undergoes metathesis between Ru−C and H−Si bonds. This structural feature in conjunction with the strong π-accepting ability of the DPCB-OMe ligand leads to highly efficient catalysis for Z-selective hydrosilylation of terminal alkynes.

(S)-1-Phosphaethenyl-2-diarylphosphanylferrocenes with planar chirality (Fc(CH═PMes*)PAr2: PAr2 = PPh2 (3a), P(1-naphthyl)Ph (3b)) are prepared in high yields from optically active 2-phosphanylferrocenecarboxaldehydes by the phospha-Peterson reactions with Mes*P(Li)SiMe3 in THF. The stereochemistry of 3b is determined by X-ray diffraction analysis. Compounds 3a and 3b readily react with [PtMe2(μ-SMe2)]2 in Et2O to afford dimethyl complexes with bidentate coordination of these ligands (PtMe2(L): L = 3a (4a), 3b (4b)). The X-ray structure of 4b reveals almost identical trans-influence of the phosphaethenyl and phosphanyl groups, showing comparable σ-donating abilities of those components. Treatment of 3a and 3b with [Pd(η3-allyl)(μ-Cl)]2 in CH2Cl2 in the presence of AgOTf forms the corresponding π-allyl complexes [Pd(η3-allyl)(L)]OTf (L = 3a (5a), 3b (5b)), respectively, which are mixtures of diastereomers with endo- and exo-oriented π-allyl ligands. Complex 5a catalyzes hydroamination of 1,3-cyclohexadiene with aniline in toluene in the presence of 5 Å molecular sieves at room temperature, giving N-cyclohexen-3-ylaniline in 84% yield.

The vinylideneruthenium(II) complexes bearing bulky and basic tertiary phosphine ligands, RuCl2(CCHPh)L2 (L = PPri3, PCy3), serve as good catalyst precursors for (Z)-selective cross-dimerization between arylacetylenes and silylacetylenes in the presence of N-methylpyrrolidine.

Treatment of the Fe(I) mesityl complex [Fe(Mes)(BPEP-Ph)] (BPEP-Ph = 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine) with π-acid ligands (L = CO, RNC) leads to one-electron reduction via Mes group migration from Fe to P, followed by homolytic elimination of the 2,4,6-tBu3C6H2 group, to afford Fe(0) complexes of the formula [Fe(L)2(BPEP-Ph*)] (BPEP-Ph* = 2-[1-phenyl-2-mesityl-2-phosphaethenyl]-6-[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine). This reduction process is supported by radical trapping experiments and theoretical studies. The 2,4,6-tBu3C6H2˙ radical is captured by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in high yield. DFT calculations reveal the mechanism of Mes group migration with a reasonable energy profile.

The palladium-catalyzed direct arylation of heteroarenes with aryl halides has emerged as a viable alternative to conventional cross-coupling reactions. This paper reports a detailed mechanistic study on factors controlling the reactivity of heteroarenes in direct arylation with well-defined models of the presumed intermediate [PdAr(O2CMe-κ2O)L] (1). Although recent theoretical studies have provided a reasonable description of the mechanism of C–H bond cleavage by 1, its model compounds so far tested have been evidently less reactive than that expected. We found that [PdPh(O2CMe-κ2O)(PPh3)] (1a) and [Pd(2,6-Me2C6H3)(O2CMe-κ2O)(PPh3)] (1c), generated in situ from isolated [PdPh(μ-O2CMe)(PPh3)]2 (4a) and [Pd(2,6-Me2C6H3)(μ-O2CMe)(PPh3)]4 (4c), respectively, react with a variety of heteroarenes in almost quantitative yields. The reactivity order of heteroarenes was evaluated by competitive reactions, showing that benzothiazole (8) is significantly less reactive than 2-methylthiophene (6), despite the acidity of 8 (pKa = 27) being much higher than that of 6 (pKa = 42). This reason was examined by kinetic experiments using 1c as well as DFT calculations using the model compound [PdPh(O2CMe-κ2O)(PH3)] (1d). Both heteroarenes reacted with 1 via a sequence of three elementary processes (i.e., substrate coordination, C–H bond cleavage, and C–C reductive elimination), but their energy profiles were significantly different from each other. The reaction of 6 obeyed simple second-order kinetics, and the deuterium-labeling experiments and DFT calculations indicated the occurrence of rate-determining reductive elimination. On the other hand, the reaction of 8 displayed saturation kinetics due to the occurrence of relatively stable coordination of 8 prior to C–H bond cleavage. This coordination stability enhances the activation barrier for C–H bond cleavage, thereby causing the modest reactivity of 8. Thus, although the previous mechanistic studies on direct arylation have been focused largely on the C–H bond cleavage process, not only the C–H bond cleavage but also the substrate coordination and C–C reductive elimination must be considered.

CuI complexes bearing BPEP as a PNP-pincer type phosphaalkene ligand undergo effective bonding interactions with SbF6− and PF6− as non-coordinating anions to give [Cu(SbF6)(BPEP)] and [Cu2(BPEP)2(μ-PF6)]+, respectively [BPEP = 2,6-bis(1-phenyl-2-phosphaethenyl)pyridine]. NMR and theoretical studies indicate a reduced anionic charge of the μ-PF6 ligand, which is induced by the strong π-accepting ability of BPEP.

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)]+BArF4− (1c) (BArF4 = B{3,5-(CF3)2C6H3}4) as a rare example of a T-shaped, three-coordinated Cu(I) complex is reported.