Simultaneous control of the mechanical properties and adhesion of human umbilical vein endothelial cells to suppress platelet adhesion on a supramolecular substrate.

The demand for artificial blood vessels to treat vascular disease will continue to increase in the future. To expand the application of blood-compatible poly(2-methoxyethyl acrylate) (pMEA) to artificial blood vessels, control of the mechanical properties of pMEA is established using supramolecular cross-links based on inclusion complexation of acetylated cyclodextrin. The mechanical properties, such as Young's modulus and toughness, of these pMEA-based elastomers change with the amount of cross-links, maintaining tissue-like behavior (J-shaped stress-strain curve). Regardless of the cross-links, the pMEA-based elastomers exhibit low platelet adhesion properties (approximately 3% platelet adherence) compared with those of poly(ethylene terephthalate), which is one of the commercialized materials for artificial blood vessels. Contact angle measurements imply a shift of supramolecular cross-links in response to the surrounding environment. When immersed in water, hydrophobic supramolecular cross-links are buried within the interior of the materials, thereby exposing pMEA chains to the aqueous environment; this is why supramolecular cross-links do not affect the platelet adhesion properties. In addition, the elastomers exhibit stable adhesion to human umbilical vein endothelial cells. This report shows the potential of combining supramolecular cross-links and pMEA.

A Fluxional Cu8 H6 Cluster Supported by Bis(diphenylphosphino)methane and its Facile Reaction with CO2.

A copper hydride cluster [Cu8 (µ-H)6 (µ-dppm)5 ](PF6 )2 (dppm=bis(diphenylphosphino)methane) was prepared from reaction of [CuH(PPh3 )]6 with dppm in the presence of [Cu(CH3 CN)4 ]PF6 and exhibited fluxional behaviors in solution where the hydrides and the phosphines are scrambling around the trans-bicapped octahedral Cu8 framework. The Cu8 H6 complex showed facile reactivity with CO2 (1 atm, RT) to afford a tricopper complex, [Cu3 (µ-H)(µ-O2 CH)(µ-dppm)3 ]PF6 , which could be developed to unprecedented hydrosilylation of CO2 catalyzed by multinuclear CuH species under mild conditions.

Chiral Self-Recognition between Stereogenic Tetrapalladium Units Affording Pd8 Chains Supported by Homochiral Tetraphosphines.

By using a chiral tetraphosphine, rac-bis[(diphenylphosphinomethyl)phenylphosphino]methane (rac-dpmppm), linear octapalladium chains were synthesized as discrete molecules of [Pd8 (µ-rac-dpmppm)4 L2 ](BF4 )4 (L=CH3 CN, dmf, XylNC), [Pd8 (µ-rac-dpmppm)4 ](BF4 )4 , and [Pd8 (µ-rac-dpmppm)4 (Cl)2 ](BF4 )2 , which are stable in the solution states and characterized by spectroscopic and crystallographic methods to reveal the octapalladium chains supported by homochiral four tetraphosphines. Variable-temperature NMR studies for a 1:1 mixture of [Pd8 (µ-rac-dpmppm)4 (dmf)2 ](BF4 )4 and [Pd8 (µ-meso-dpmppm)4 (dmf)2 ]-(BF4 )4 in [D7 ]DMF revealed that the Pd8 chains were dissociated at higher temperature (T≈140 °C) into the Pd4 units of {Pd4 (µ-rac-dpmppm)2 }2+ and {Pd4 (µ-meso-dpmppm)2 }2+ , and they were thermodynamically self-aligned to restore the Pd8 chains at lower temperature (T<60 °C), through perfect chiral self-recognition between the stereogenic tetrapalladium units.

Electron-rich linear triplatinum complexes stabilized by a spinning tetraphosphine, tris(diphenylphosphinomethyl)phosphine.

Linear triplatinum complexes with 48e(-), [Pt3(µ-tdpmp)2(RNC)2](PF6)2 (R = 2,6-xylyl (3), (t)Bu (4)), were synthesized by using a branched tetraphosphine, tris(diphenylphosphinomethyl)phosphine (tdpmp), and characterized by crystallographic and spectroscopic analyses to show their novel dynamic behaviour in the solution state, in which the linear Pt3 unit was stabilized by two spinning tetraphosphine ligands.

Oxidative addition of an aromatic ortho C-H bond of tetraphosphine to asymmetric diiridium(i) centres.

Reactions of a tetraphosphine, meso-bis{[(diphenylphosphinomethyl)phenyl]phosphino}propane (dpmppp), with [IrCl(cod)]2 and CO (1 atm) or isocyanide (RNC) in the presence of NH4PF6 at 80-100 °C in dichloromethane/acetonitrile/acetone and/or methanol mixed solvents afforded asymmetric diiridium(ii) complexes, [Ir2(H)(Cl)(µ-(dpmppp-H)-κP(4)C)(CO)3]PF6 (1) and [Ir2(H)(µ-(dpmppp-H)-κP(4)C)(RNC)4)]-(PF6)2 (R = 2,6-xylyl (2), 2,4,6-mesityl (3); dpmppp-H = {PPh(o-C6H4)CH2P(Ph)(CH2)3P(Ph)CH2PPh2}(-)). A similar reaction with (t)BuNC resulted in the formation of a mononuclear Ir(III) complex of [Ir(H)(dpmppp-κP(3))((t)BuNC)2](PF6)2 (4). Complexes 1-3 were characterized by ESI mass spectrometry, (1)H and (31)P NMR spectroscopy and X-ray diffraction analyses. They were found to consist of cis/trans-P,P asymmetric Ir(II)-Ir(II) bonded dinuclear structures derived from oxidative addition of an ortho C-H bond of dpmppp (Ir-Ir = 2.8044(2) Å (1), 2.8569(2) Å (2), and 2.8524(5) Å (3)), resulting in a [IrPCCIr] intermetallic cyclometal-bridge and a terminal hydride. DFT calculations indicated the presence of Ir-Ir, Ir-H, and Ir-Cortho covalent bonds. Initial stages of the reactions with CO and XylNC at room temperature were investigated by (31)P{(1)H} NMR spectroscopy and found to contain a symmetrical Ir(I) dinuclear unit with dpmppp that was readily transformed into 1 and 2 upon heating. The Ir intermediate with XylNC, [Ir2(XylNC)4(µ-dpmppp)](PF6)2 (6), was isolated and characterized by X-ray crystallography and DFT calculations as an electron-deficient 32e(-) Ir species involving a Ir(I)→Ir(I) dative bond (2.7989(5) Å). The reaction pathways from 6 to 2 were investigated by DFT calculations. The present study suggested that a novel oxidative addition of an ortho C-H bond proceeded on the cis/trans-P,P asymmetric diiridium(i) scaffold supported by the tetraphosphine, dpmppp, which was assumed to be facilitated by dimetal cooperation with switching Ir→Ir dative interactions.

Gold and Silver Chains Supported by Linear Hexaphosphine Ligands.

A new linear hexaphosphine, rac-cis,cis,trans-bis{[(diphenylphosphinomethyl)phenylphosphinomethyl]phenylphosphino}methane (P6), was synthesized and isolated as a pure isomer, confirmed by transforming to the corresponding phosphine sulfide. The methylene-bridged linear hexaphosphine readily organized flexible gold(I) and silver(I) hexanuclear chains, [M6(µ-P6)2]X6 (X6 = (OTf)6, M = Au (1), Ag (2); X6 = Cl2(PF6)4, M = Au (3)). The hexaphosphine also supported a tetrasilver(I) complex [Ag4(µ-P6)2](OTf)4 (4), which was readily transformed by treatment with AgOTf into 3, revealing a drastic alternation of the two P6 arrangement. The hexagold(I) chains exhibited a considerably red-shifted absorption (∼410 nm) and emission (540-580 nm) to (1) [5dσ*→6pσ] and from (3) [5dσ*→6pσ] excited states of the metal centers, respectively. The new linear hexaphosphine could be a useful tool to construct linear metal clusters as subnano building blocks.

Stepwise Expansion of Pd Chains from Binuclear Palladium(I) Complexes Supported by Tetraphosphine Ligands.

Reaction of [Pd2(XylNC)6]X2 (X = PF6, BF4) with a linear tetraphosphine, meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm), afforded binuclear Pd(I) complexes, [Pd2(µ-dpmppm)2]X2 ([2]X2), through an asymmetric dipalladium complex, [Pd2(µ-dpmppm)(XylNC)3](2+) ([1](2+)). Complex [2](2+) readily reacted with [Pd(0)(dba)2] (2 equiv) and an excess of isocyanide, RNC (R = 2,6-xylyl (Xyl), tert-butyl ((t)Bu)), to generate an equilibrium mixture of [Pd4(µ-dpmppm)2(RNC)2](2+) ([3'](2+)) + RNC ⇄ [Pd4(µ-dpmppm)2(RNC)3](2+) ([3](2+)), from which [Pd4(µ-dpmppm)2(XylNC)3](2+) ([3a](2+)) and [Pd4(µ-dpmppm)2((t)BuNC)2](2+) ([3b'](2+)) were isolated. Variable-temperature UV-vis and (31)P{(1)H} and (1)H NMR spectroscopic studies on the equilibrium mixtures demonstrated that the tetrapalladium complexes are quite fluxional in the solution state: the symmetric Pd4 complex [3b'](2+) predominantly existed at higher temperatures (>0 °C), and the equilibrium shifted to the asymmetric Pd4 complex [3b](2+) at a low temperature (∼-30 °C). The binding constants were determined by UV-vis titration at 20 °C and revealed that XylNC is of higher affinity to the Pd4 core than (t)BuNC. In addition, both isocyanides exhibited higher affinity to the electron deficient [Pd4(µ-dpmppmF2)2(RNC)2](2+) ([3F'](2+)) than to [Pd4(µ-dpmppm)2(RNC)2](2+) ([3'](2+)) (dpmppmF2 = meso-bis[{di(3,5-difluorophenyl)phosphinomethyl}phenylphosphino]methane). When [2]X2 was treated with [Pd(0)(dba)2] (2 equiv) in the absence of RNC in acetonitrile, linearly ordered octapalladium chains, [Pd8(µ-dpmppm)4(CH3CN)2]X4 ([4]X4: X = PF6, BF4), were generated through a coupling of two {Pd4(µ-dpmppm)2}(2+) fragments. Complex [2](2+) was also proven to be a good precursor for Pd2M2 mixed-metal complexes, yielding [Pd2Cl(Cp*MCl) (Cp*MCl2)(µ-dpmppm)2](2+) (M = Rh ([5](2+)), Ir ([6](2+)), and [Au2Pd2Cl2(dpmppm-H)2](2+) ([7](2+)) by treatment with [Cp*MCl2]2 and [AuCl(PPh3)], respectively. Complex [7](2+) contains an unprecedented PC(sp(3))P pincer ligand with a PCPCPCP backbone, dpmppm-H of deprotonated dpmppm. The present results demonstrated that the binuclear Pd(I) complex [2](2+) was a quite useful starting material to extend the palladium chains and to construct Pd-involved heteromultinuclear systems.

Self-alignment of low-valent octanuclear palladium atoms.

A linear tetraphosphine, meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm) was used to synthesize linear octapalladium-extended metal atom chains as discrete molecules of [Pd8(µ-dpmppm)4](BF4)4 (1) and [Pd8(µ-dpmppm)4L2](BF4)4 (L=2,6-xylyl isocyanide (XylNC; 2), acetonitrile (3), and N,N-dimethylformamide (dmf; 4)), which are stable in the solution states and show interesting temperature-dependent photochemical properties in the near IR region. Variable temperature NMR studies demonstrated that at higher temperature T≈140 °C the Pd8 chains were dissociated into Pd4 fragments, which were thermodynamically self-aligned to restore the Pd8 chains at lower temperature T<60 °C. The coldspray ionization mass spectra suggested a possibility for further aggregation of the linear tetrapalladium units.

Facile insertion of carbon dioxide into Cu2(µ-H) dinuclear units supported by tetraphosphine ligands.

Reactions of meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm) with Cu(I) species in the presence of NaBH4 afforded di- and tetranuclear copper hydride complexes, [Cu2(µ-H)(µ-dpmppm)2]X (1) and [Cu4(µ-H)2(µ4-H)(µ-dpmppm)2]X (2) (X=BF4, PF6). Complex 1 undergoes facile insertion of CO2 (1 atm) at room temperature, leading to a formate-bridged dicopper complex [Cu2(µ-HCOO)(dpmppm)2]X (3). The experimental and DFT theoretical studies clearly demonstrate that CO2 insertion into the Cu2(µ-H) unit occurred with the flexible dicopper platform. Complex 2 also undergoes CO2 insertion to give a formate-bridged complex, [Cu4(µ-HCOO)3(dpmppm)2]X, during which the square Cu4 framework opened up to a linear tetranuclear chain.

Strongly luminous tetranuclear gold(I) complexes supported by tetraphosphine ligands, meso- or rac-bis[(diphenylphosphinomethyl)phenylphosphino]methane.

A series of tetragold(I) complexes supported by tetraphosphine ligands, meso- and rac-bis[(diphenylphosphinomethyl)phenylphosphino]methane (meso- and rac-dpmppm) were synthesized and characterized to show that the tetranuclear Au(I) alignment varies depending on syn- and anti-arrangements of the two dpmppm ligands with respect to the metal chain. The structures of syn-[Au4 (meso-dpmppm)2X]X'3 (X = Cl; X' = Cl (4 a), PF6 (4 b), BF4 (4 c)) and syn-[Au4 (meso-dpmppm)2]X4 (X = PF6 (4 d), BF4 (4 e), TfO (4 f); TfO = triflate) involved a bent tetragold(I) core with a counter anion X incorporated into the bent pocket. Complexes anti-[Au4 (meso-dpmppm)2]X4 (X = PF6 (5 d), BF4 (5 e), TfO (5 f)) contain a linearly ordered Au4 string and complexes syn-[Au4 (rac-dpmppm)2X2]X'2 (X = Cl, X' = Cl (6 a), PF6 (6 b), BF4 (6 c)) and syn-[Au4 (rac-dpmppm)2]X4 (X = PF6 (6 d), BF4 (6 e), TfO (6 f)) consist of a zigzag tetragold(I) chain supported by the two syn-arranged rac-dpmppm ligands. Complexes 4 d-f, 5 d-f, and 6 d-f with non-coordinative large anions are strongly luminescent in the solid state (λmax = 475-515 nm, Φ = 0.67-0.85) and in acetonitrile (λmax = 491-520 nm, Φ = 0.33-0.97); the emission was assigned to phosphorescence from (3) [dσ*σ*σ* pσσσ] excited state of the Au4 centers on the basis of DFT calculations as well as the long lifetime (a few µs). The emission energy is predominantly determined by the HOMO and LUMO characters of the Au4 centers, which depend on the bent (4), linear (5), and zigzag (6) alignments. The strong emissions in acetonitrile were quenched by chloride anions through simultaneous dynamic and static quenching processes, in which static binding of chloride ions to the Au4 excited species should be the most effective. The present study demonstrates that the structures of linear tetranuclear gold(I) chains can be modified by utilizing the stereoisomeric tetraphosphines, meso- and rac-dpmppm, which may lead to fine tuning of the strongly luminescent properties intrinsic to the Au(I) 4 cluster centers.

Flexible, linear, tetranuclear palladium complexes supported by tetraphosphine ligands with electron-withdrawing groups.

A linearly ordered tetraphosphine containing electron-withdrawing substituent groups on the outer phosphorus atoms, meso-bis[{di(3,5-difluorophenyl)phosphinomethyl}phenylphosphino]methane (dpmppmF2), was prepared and reacted with [Pd2(RNC)6](PF6)2 and Pd(dba)2 to afford tetranuclear palladium complexes, [Pd4(µ-dpmppmF2)2(RNC)3](PF6)2 (R = 2,6-xylyl (Xyl) (1), 2,4,6-mesityl (2), 2,6-diisopropylphenyl (3) and tert-butyl (4)), which involve an asymmetric {(RNC)Pd4(CNR)2}(2+) core supported by two dpmppmF2 ligands in anti-arrangement. Each terminal of the Pd4 chain was capped by terminal isocyanide and a semi-bridging RNC is introduced into one terminal Pd site. Mechanistic investigation suggested that the dipalladium(I) complex, [Pd2(µ-dpmppmF2)2(RNC)2](PF6)2 (R = Xyl (6)), was a key intermediate to trap Pd(0) species by the uncoordinated outer phosphine pendants with electron-withdrawing groups. Variable-temperature UV-vis and (31)P{(1)H}, (1)H NMR spectroscopic studies demonstrated that the tetrapalladium complexes are quite fluxional in the solution state at high temperature (>20 °C) relating to a symmetric structure of [Pd4(µ-dpmppmF2)2(RNC)2](PF6)2, and the asymmetric solid state structures are retained even in the solution at low temperature (<-60 °C). Theoretical calculations with DFT methods on the asymmetric (R = Xyl (1)) and symmetric (R = Xyl (1')) structures suggested that contribution of Pd(0)→Pd(I)-Pd(0)-Pd(I) with 60 cluster valence electrons (CVEs) would be dominant in 1, while the symmetric structure of 1' can be recognized as Pd(I)-Pd(0)-Pd(0)-Pd(I) with 58 CVEs. The new tetraphosphine dpmppmF2 was proven very effective in organizing dynamically flexible tetrapalladium chains.

Reversible dioxygen binding on asymmetric dinuclear rhodium centres.

Electron-deficient dinuclear rhodium complexes [Rh2Cl2(µ-dpmppp)(RNC)] (1), with the linear tetraphosphine ligand dpmppp, showed reversible binding of molecular oxygen to form asymmetric dirhodium η(2)-peroxo complexes [Rh2Cl2(O2)(µ-dpmppp)(RNC)] (2) stabilized by a Rh→Rh dative bond.

Wheel-shaped icosanuclear homo- and heterometallic complexes of Ni(II), Co(II), and Cu(II) ions supported by unsymmetrical aminoalcohol ligands.

Reactions of M(OAc)2·4H2O (M = Ni, Co) with 3-[benzyl(2-hydroxyethyl)amino]-1-propanol (H2L) in the presence of pyridine or triethylamine afforded novel homometallic icosanuclear wheel-shaped complexes [M20L4(HL)4(OAc)28] (M = Ni (1), Co (2)), which consist of a central M(II)12 single-stranded, nearly planar loop with four peripheral [M2(HL)(OAc)2] fragments attached in an S4 symmetrical fashion. The complexes can alternatively be recognized as saddle-shaped wheel structures, in which four tetranuclear units of [M4L(HL)(OAc)7](2-) are connected by four M(2+) ions (M5). The tetranuclear unit itself can be derived from an ideal C2 symmetrical [M4(HL)2(µ-η(2)-OAc)4(µ-η(1),η(1)-OAc)2(η(1),η(1)-OAc)](-) structure through deprotonation of the HL(-) ligand, and is composed of two plane-shared M3O4 incomplete cubanes in which the M2 and M3 atoms are involved in the central fused plane and the M1 and M4 atoms are disposed at the apex sites. Mixed-metal icosanuclear complexes [NixM20-xL4(HL)4(OAc)28] (3, M = Co, x = 9.5) and [Ni12M8L4(HL)4(OAc)28] (4, M = Cu) were also synthesized by using equimolar amounts of Ni(II) and M(II) ions, and were shown to have similar structures to 1 and 2. X-ray crystallographic and fluorescent analyses revealed that complex 3 contains nonstoichiometric amounts of Ni(2+) and Co(2+) ions in the ratio of 9.5:10.5 and that these are disordered at every metal site. In striking contrast, complex 4 has a stoichiometric formula of Ni12Cu8, which was confirmed by the Jahn-Teller elongation of Cu(2+) ions, and consequently, the M2 and M5 positions are occupied exclusively by the Cu(2+) ions. The temperature-dependent direct current (dc) magnetic susceptibility data showed the presence of ferromagnetic exchange interactions in the Ni homometallic (1) and NiCu bimetallic (4) complexes, while the Co homometallic (2) and NiCo bimetallic (3) complexes exhibited antiferromagnetic interactions due to spin-orbit coupling effects of the octahedral Co(II) ions. The present results demonstrate that the unsymmetrical aminoalcohol ligand H2L is quite effective in organizing the homo- and heterometallic icosanuclear wheel-shaped metal arrangements.

Concerto catalysis--harmonising [NiFe]hydrogenase and NiRu model catalysts.

This communication reports the successful merging of the chemical properties of a natural [NiFe]hydrogenase (Desulfovibrio vulgaris Miyazaki F) and our previously reported [NiRu] hydrogenase-mimic. The catalytic activity of both the natural enzyme and the mimic is almost identical, with the exception of working pH ranges, and this allows us to use them simultaneously in the same reaction flask. In such a manner, isotope exchange between D(2) and H(2)O could be conducted over an extended pH range (about 2-10) in one pot under mild conditions at ambient temperature and pressure.

pH-Dependent isotope exchange and hydrogenation catalysed by water-soluble NiRu complexes as functional models for [NiFe]hydrogenases.

The pH-dependent hydrogen isotope exchange reaction between gaseous isotopes and medium isotopes and hydrogenation of the carbonyl compounds have been investigated with water-soluble bis(mu-thiolate)(mu-hydride)NiRu complexes, Ni(II)(mu-SR)(2)(mu-H)Ru(II) {(mu-SR)(2) = N,N'-dimethyl-N,N'-bis(2-mercaptoethyl)-1,3-propanediamine}, as functional models for [NiFe]hydrogenases. In acidic media (at pH 4-6), the mu-H ligand of the Ni(II)(mu-SR)(2)(mu-H)Ru(II) complexes has H(+) properties, and the complexes catalyse the hydrogen isotope exchange reaction between gaseous isotopes and medium isotopes. A mechanism of the hydrogen isotope exchange reaction between gaseous isotopes and medium isotopes through a low-valent Ni(I)(mu-SR)(2)Ru(I) complex is proposed. In contrast, in neutral-basic media (at pH 7-10), the mu-H ligand of the Ni(II)(mu-SR)(2)(mu-H)Ru(II) complexes acts as H(-), and the complexes catalyse the hydrogenation of carbonyl compounds.

A dinuclear Ni(mu-H)Ru complex derived from H2.

Models of the active site in [NiFe]hydrogenase enzymes have proven challenging to prepare. We isolated a paramagnetic dinuclear nickel-ruthenium complex with a bridging hydrido ligand from the heterolytic cleavage of H2 by a dinuclear NiRu aqua complex in water under ambient conditions (20 degrees C and 1 atmosphere pressure). The structure of the hexacoordinate Ni(mu-H)Ru complex was unequivocally determined by neutron diffraction analysis, and it comes closest to an effective analog for the core structure of the proposed active form of the enzyme.

Synthesis and crystal structure of an open capsule-type octanuclear heterometallic sulfide cluster with a linked incomplete double cubane framework without an intramolecular inversion center.

An open capsule-type octanuclear heterometallic sulfide cluster without an intramolecular inversion center [Ru(eta(6)-C(6)Me(6)){P(OMe)(3)}{MoO(mu(3)-S)(3)}(CuI)(2)](2) (5) has been synthesized for the first time by stepwise connection of three mononuclear building blocks, i.e., (i) [RuCl(2)(eta(6)-C(6)Me(6)){P(OMe)(3)}] (1a) as an octahedral terminal building block to control the direction of cluster expansion, (ii) [MoOS(3)](2)(-) as a tetrahedral polydentate building block owing to the strong coordination ability of the S atoms, and (iii) a CuI building block to form a trigonal planar (mu-S)(2)CuI unit or to form a linkage unit of two incomplete cubane-type octanuclear frameworks. The stepwise connection was made in the following order: [RuCl(2)(eta(6)-C(6)Me(6)){P(OMe)(3)}] (1a, mononuclear) --> [Ru(eta(6)-C(6)Me(6)){P(OMe)(3)}{MoOS(mu(2)-S)(2)}] (2a, dinuclear) --> [Ru(eta(6)-C(6)Me(6)){P(OMe)(3)}{MoO(mu(2)-S)(2)(mu(3)-S)}CuI] (3a, butterfly-type trinuclear) --> [Ru(eta(6)-C(6)Me(6)){P(OMe)(3)}{MoO(mu(3)-S)(3)}(CuI)(2)](2) (5). When P(OMe)(3) was replaced by P(OEt)(3), which is more bulky than P(OMe)(3), in the starting ruthenium building block [RuCl(2)(eta(6)-C(6)Me(6)){P(OEt)(3)}] (1b, mononuclear), only the tetranuclear incomplete single cubane cluster [Ru(eta(6)-C(6)Me(6)){P(OEt)(3)}{MoO(mu(3)-S)(3)}(CuI)(2)] (6) was generated, owing to the steric effect of P(OEt)(3).