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.

Synthesis and Physical Properties of Polyfluorinated Cycloparaphenylenes.

Fluorinated [6]- and [9]cycloparaphenylene (CPP) derivatives, 8F-[6]CPP and 12F-[9]CPP, were synthesized based on the previous synthesis of the parent CPPs. While the reductive aromatization conditions used in the final step of the synthesis of the parent CPPs did not work for the fluorinated compounds, the use of PBr3 and SnCl2 in acetonitrile successfully accomplished the desired transformation. The structures of F-CPPs were determined by single-crystal X-ray analysis. Photo- and electrochemical analyses and host-guest chemistry revealed the effects of the introduction of fluorine atoms.

Strain-Induced Double Carbon-Carbon Bond Activations of Cycloparaphenylenes by a Platinum Complex: Application to the Synthesis of Cyclic Diketones.

The carbon-carbon (C-C) bond activation of [n]cycloparaphenylenes ([n]CPPs) by a transition-metal complex is herein reported. The Pt0 complex Pt(PPh3 )4 regioselectively cleaves two C-C σ bonds of [5] CPP and [6]CPP to give cyclic dinuclear platinum complexes in high yields. Theoretical calculations reveal that the relief of ring strain drives the reaction. The cyclic complex was further transformed into a cyclic diketone by using a CO insertion reaction.

Synthesis of a 1,2-Dithienylethene-Containing Donor-Acceptor Polymer via Palladium-Catalyzed Direct Arylation Polymerization (DArP).

This paper reports the synthesis of D-A polymers containing 1,2-dithienylethene (DTE) units via palladium-catalyzed direct arylation polymerization (DArP). The reaction of dibromoisoindigo (1-Br) and DTE (2-H), in the presence of Pd2(dba)3·CHCl3 (0.5 mol%), P(2-MeOC6H4)3 (L1) (2 mol%), pivalic acid (1 equiv) as catalyst precursors, and Cs2CO3 (3 equiv) as a base affords poly(1-alt-2) with a high molecular weight (Mn up to 44,900). Although, it has been known that monomers, with plural C⁻H bonds, tend to form insoluble materials via direct arylation at undesirable C⁻H positions; the reaction of 1-Br and 2-H cleanly proceeds without insolubilization. The resulting polymer has a well-controlled structure and exhibits good charge transfer characteristics in an organic field-effect transistor (OFET), compared to the polymer produced by Migita⁻Kosugi⁻Stille cross-coupling polymerization. The DArP product displays an ideal linear relationship in the current⁻voltage curve, whereas the Migita⁻Kosugi⁻Stille product shows a VG-dependent change in the charge mobility.

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.

PNP-Pincer-Type Phosphaalkene Complexes of Late Transition Metals.

This account summarizes our recent studies on PNP-pincer-type phosphaalkene complexes. Phosphaalkenes with a P=C bond possess an extremely low-lying π* orbital and have a marked tendency to engage in strong π back-bonding with transition metals. This particular ligand property provides PNP-pincer complexes with unique structures and reactivities. 2,6-Bis(phosphaethenyl)pyridine leads to the isolation of coordinatively unsaturated complexes of Fe(I) and Cu(I); the former adopts a trigonal monopyramidal configuration, whereas the latter has a strong affinity for PF6- and SbF6- as non-coordinating anions. Unsymmetrical PNP-pincer-type phosphaalkene complexes of Ir(I) bearing a dearomatized pyridine unit instantly cleave the N-H bond of NH3 and the C-H bond of MeCN at room temperature. The dearomatized iridium complexes catalyze the dehydrative coupling of amines with alcohols to afford N-alkylated amines and imines in high yields.

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.

Reduction of an Fe(I) mesityl complex induced by π-acid ligands.

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.

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 of end-capped regioregular poly(3-hexylthiophene)s via direct arylation.

The synthesis of regioregular head-to-tail poly(3-hexylthiophene)s capped with aryl groups (Ar-HT-P3HTs) has been accomplished by palladium-catalyzed polycondensation of 2-bromo-3-hexylthiophene (1) via direct arylation. A variety of aryl groups are installed at the initiated end in 86%-98% selectivity using aryl bromides and iodides as capping agents. The polymerization proceeds via a two-stage process. Before monomer 1 is consumed, the competitive formation of end-capped and non-capped HT-P3HTs is operative, where the molecular weight increases linearly with monomer conversion. After 1 is consumed, the resulting polymers are coupled with each other to afford highly end-capped HT-P3HTs.

Synthesis and coordination behavior of Cu(I) bis(phosphaethenyl)pyridine complexes.

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

Palladium-catalyzed dehydrohalogenative polycondensation of 2-bromo-3-hexylthiophene: an efficient approach to head-to-tail poly(3-hexylthiophene).

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 (M(n) = 30,600, M(w)/M(n) = 1.60) and high regioregularity (98%) in almost quantitative yield (99%).

Electronic structure of four-coordinate iron(I) complex supported by a bis(phosphaethenyl)pyridine ligand.

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 [FeBr(2)(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 Mossbauer isomer shift value (delta = 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 d(z(2)) (Fe) and pi* (BPEP) orbitals. Orbital occupancy analysis demonstrated the electronic structure with a high-spin Fe(I) center. The effective dpi-ppi 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.

Redox-responsive recombination of carbon-carbon bonds on flexible tetrairon cores.

When a brown powder of 2a was dissolved in acetonitrile, 2a was converted to 2b. Equilibrium was reached at a 74:26 molar ratio within 1 week at 303 K. The isomerization proceeds through a cubane-like transition state, in which recombination of a carbon-carbon bond occurs.

Catalytic applications of transition-metal complexes bearing diphosphinidenecyclobutenes (DPCB).

The preparation and characterization of sterically protected diphosphinidenecyclobutenes bearing two two-coordinate phosphorus atoms are described from the viewpoint of the development of a novel type of bidentate ligand for transition-metal complexes. They form complexes with group 6 metals such as chromium, molybdenum and tungsten, group 8 metals such as ruthenium, group 10 metals such as palladium and platinum, and group 11 metals such as copper and gold. Some of them can be used as catalysts for synthetic reactions such as cross-coupling of the Sonogashira type, Suzuki-Miyaura coupling, Ullmann coupling, Kosugi-Migita-Stille coupling, cyanation, polymerization of ethylene, dehydrogenative hydrosilylation of ketones, hydroamination of 1,3-butadienes, and direct alkylation or amination of allylic alcohols of Tsuji-Trost reactions.

Stereoselective synthesis of cis- and trans-oligo(phenylenevinylene)s via palladium-catalyzed cross-coupling reactions.

cis-Oligo(phenylenevinylene)s (OPVs) are synthesized by Suzuki-Miyaura coupling of arylboronic acids with (Z)-bromoalkenes in over 96% geometrical purity. On the other hand, trans-oligo(phenylenevinylene)s can be synthesized by Hiyama coupling of aryl iodide with (E)-alkenylsilanes in almost perfect purities. Effect of pi-conjugation chain length on photoisomerization behavior of OPVs is described.

Highly (Z)-selective hydrosilylation of terminal alkynes catalyzed by a diphosphinidenecyclobutene-coordinated ruthenium complex: application to the synthesis of (Z,Z)-bis(2-bromoethenyl)arenes.

[reaction: see text] Complex 1 bearing a diphosphinidenecyclobutene ligand (DPCB-OMe) catalyzes highly stereoselective hydrosilylation of diethynylarenes with HSiMe2Ph to afford (Z,Z)-bis(2-silylethenyl)arenes. Treatment of the hydrosilylation products with N-bromosuccinimide causes bromodesilylation in a stereospecific manner, giving (Z,Z)-bis(2-bromoethenyl)arenes in high geometrical purity (>98%).

(Z)-Selective cross-dimerization of arylacetylenes with silylacetylenes catalyzed by vinylideneruthenium complexes.

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

Stereocontrolled synthesis and optical properties of all-cis poly(phenylene vinylenes) (PPVs): a method for direct patterning of PPVs.

Geometrically pure, all-cis poly(phenylene vinylenes) (PPVs) are synthesized by Suzuki-Miyaura-type polycondensation of 2,5-dioctyloxy-1,4-benzenediboronic acid with (Z,Z)-bis(2-bromoethenyl)benzenes, which are prepared by ruthenium-catalyzed (Z)-selective double hydrosilylation of diethynylbenzenes, followed by bromodesilylation of the resulting (Z,Z)-bis(2-silylethenyl)benzenes with N-bromosuccinimide. The all-cis PPVs thus obtained undergo one-way photoisomerization to the corresponding trans-PPVs both in solution and in the solid. This phenomenon is applied to direct microscale patterning of PPVs onto a quartz substrate.