Alkyne Ligands Research Articles

Mechanistic Study of the Rhodium‐Catalyzed [3+2+2] Carbocyclization of Alkenylidenecyclopropanes with Alkynes

A systematic theoretical study has been performed on the recently reported Rh(I)-catalyzed [3+2+2] carbocyclization reactions between alkenylidenecyclopropanes (ACPs) and alkynes. With the aid of theoretical calculations, two possible mechanisms, that is, alkene-carbometalation-first and alkyne-carbometalation-first mechanisms, are examined in this study. In the oxidative addition step, the possibility of reaction on either the distal or proximal C-C bond of the cyclopropane group has been evaluated. The calculations indicate that the alkene-activation-first mechanism is more favored for the overall catalytic cycle. This mechanism involves four steps, that is, oxidative addition of the distal (rather than the proximal) C-C bond of cyclopropane group, alkene carbometalation, alkyne carbometalation, and reductive elimination. The rate-determining step in the overall catalytic cycle is the carbometalation of the alkyne (i.e., the alkyne-insertion step) and this step also determines the regioselectivity. Finally, the origin of the regioselectivity is determined by the steric effect (i.e., the steric crowding between the electron-withdrawing group on alkyne and other ligands on the rhodium center) in the alkyne-insertion step.

Alkyne-functionalized palladium nanoparticles: Synthesis, characterization, and electrocatalytic activity in ethylene glycol oxidation

1-Octyne-stabilized palladium nanoparticles (PdHC8, core dia. 2.50±0.28nm) were prepared by a simple reduction procedure. FTIR studies showed that the self-assembly of 1-octyne onto the Pd nanoparticle surfaces involved the effective breaking of the terminal HC bonds and the formation of Pd–vinylidene interfacial linkages by a tautomeric rearrangement process. With this conjugated metal–ligand interfacial bond, the nanoparticles exhibited unique photoluminescence properties that were analogous to those of diacetylene derivatives. Furthermore, thermogravimetric analysis showed that there were about 150 ligands per nanoparticle, with an average footprint of ca. 13Å2 per capping ligand on the nanoparticle surface, consistent with a head-on configuration of the alkyne ligands on the Pd surface. Interestingly, the PdHC8 nanoparticles exhibited apparent electrocatalytic activity in the oxidation of ethylene glycol in alkaline media that was about twice that of commercial Pt/C, as manifested in cyclic voltammetric and chronoamperometric studies. Such an apparent improvement of the electrocatalytic activity was most probably ascribed to the partial removal of the alkyne capping ligands whereby the remaining molecules provided a relatively hydrophobic chemical environment and hence facilitated the removal of water generated from the oxidative dehydration reaction of ethylene glycol. Additionally, the enhanced electron density within the Pd metal cores as a result of the intraparticle charge delocalization between the particle-bound acetylene moieties might also facilitate the formation of PdC bonds in Pd-[C2H3(OH)2]ads, which was presumed to be the rate-determining step in ethylene glycol oxidation on Pd surfaces.

Transformations of Alkynes at a Cyclotriphosphato Ruthenium Complex

Photolysis of the cyclotriphosphato complex (PPN)[Ru(P3O9)(η6-C6H6)] (2) with bpy (2,2′-bipyridyl) in alcohols generated a violet solution of (PPN)[Ru(P3O9)(bpy)(ROH)] (3) with a labile alcohol ligand. Complexes 3 were readily oxidized during recrystallization to give the Ru(III) alkoxo complexes (PPN)[Ru(OR)(P3O9)(bpy)] (4). Treatment of 3 with internal alkynes led to selective formation of η2-alkyne complexes (PPN)[Ru(P3O9)(bpy)(η2-RC≡CR′)] (7), which did not undergo vinylidene rearrangement under heating and photoirradiation conditions in contrast to its dppe analogues. On the other hand, diphenylacetylene complex 7c (R = R′ = Ph) gave the α-ketocarbene complex (PPN)[Ru(P3O9)(bpy)(═CPhCOPh)] (8) on reaction with m-CPBA. This observation provides a very rare example for the direct and controlled oxidation of a coordinated alkyne ligand. Reactions of 3 with the terminal alkynes in alcohols and in DMF afforded the alkoxycarbene complexes (PPN)[Ru(P3O9)(bpy){═C(OR)CH2CR′}] (9) and the carbonyl complex (PPN...

Synthesis and Dynamic Properties of a Triruthenium Complex Containing μ3-η2(∥)-Ethyne and μ3-Methylidyne Ligands: Equilibrium of an Ethyne–Hydrido Complex with a Nonclassical μ3-Vinyl Complex

A triruthenium complex containing μ3-η2(∥)-ethyne and μ3-methylidyne ligands, (CpRu)3{μ3-η2(∥)-HC≡CH}(μ3-CH)(μ-H)2 (2a), was synthesized via the treatment of {Cp*Ru(μ-H)}3(μ3-H)2 (3) (Cp* = η5-C5Me5) with propene. In the low-temperature region, VT 1H NMR spectra showed that complex 2a is in equilibrium with a nonclassical μ3-vinyl complex, (CpRu)3(μ3-η2:η2-HC═CH2)(μ3-CH)(μ-H) (5a), which features an interaction between one of the β-vinyl protons and a Ru center. The presence of the unusual 3c-2e interaction of the β-C–H bond of the vinyl group is strongly supported by the small JC–H value (98 Hz) found for the β-carbon as well as the small JH–H value for the α-vinyl proton. Treatment of the equilibrated mixture of phenyl-substituted complexes (CpRu)3{μ3-η2(∥)-HC≡CPh}(μ3-CH)(μ-H)2 (2b) and (CpRu)3(μ3-η2:η2-HC═CHPh)(μ3-CH)(μ-H) (5b) with tBuNC afforded a μ-styryl complex, (CpRu)3(μ-η2-HC═CHPh)(μ3-CH)(μ-tBuNC)(μ-H) (6), while analogous treatment of 2a and 5a resulted in the formation of μ3-dimetalloallyl complex (CpRu)3{μ3-η3-CHCHCN(H)tBu}(μ3-CH)(μ-H) (7). These results imply that the coordinatively saturated μ3-η2(∥)-alkyne−μ3-alkylidyne complex can generate a vacant site via the migration of a hydrido ligand onto an alkyne ligand to yield a μ-vinyl intermediate.

Synthesis of a heterometallic trinuclear cluster of ruthenium and iridium containing a perpendicularly coordinated alkyne ligand and its dynamic behavior

Heterometallic clusters containing a perpendicularly coordinated alkyne ligand, (Cp*Ru)2(Cp*Ir)(μ3–η2:η2(⊥)-PhCCR)(μ-H)2 (6a, R = Me; 6b, R = Ph), were prepared by the reaction of a trinuclear heterobimetallic tetrahydrido complex of ruthenium and iridium, (Cp*Ru)2(Cp*Ir)(μ-H)3(μ3-H) (2), with internal alkyne. X-ray diffraction studies clearly showed that the alkyne ligand was preferentially attached to the Ru–Ru bond in a perpendicular manner rather than to the Ru–Ir bond. Although no detectable changes were observed in a VT-NMR study on 6a, VT-NMR studies on 6b implied that the alkyne ligand underwent a switchback motion at the heterometallic site. The presence of the minor isomer 8b, in which the alkyne ligand was attached to the Ru–Ir bond, was also proposed by the density functional theory (DFT) calculations. The energy difference between 6b and 8b is estimated at 4.1 kcal mol−1. These results suggest that the coordination site of the alkyne ligand was determined thermodynamically.

New Linear π-Conjugated Diruthenium Compounds Containing Axial Tetrathiafulvalene-acetylide Ligands

Under the weak base conditions, diruthenium(III) tetrakis-N,N′-dimethylbenzamidinate (DMBA) nitrate (Ru2(DMBA)4(NO3)2) reacted with electroactive tetrathiafulvalene acetylene ligands, HC≡C–TTF1 (5-ethynyl-2-(4,5-dimethyl-1,3-dithiol-2-ylidene)benzo[d][1,3]dithiole) and HC≡C–TTF2 (2-(5-ethynylbenzo[d][1,3]dithiol-2-ylidene)benzo[d][1,3]dithiole), to afford new compounds trans-Ru2(DMBA)4(C≡C–TTF1)2 (1) and trans-Ru2(DMBA)4(C≡C–TTF2)2 (2), respectively. The trans orientation and the planar nature of the ethynyltetrathiafulvalene ligands around the Ru2(III,III) core were supported by the single-crystal X-ray diffraction study of compound 1. Both compounds 1 and 2 and their TTF ligand precursors were characterized with the spectroscopic and voltammetric techniques, which revealed a minimal electronic interaction between the two TTF moieties within the same compound. The electronic structure of trans-Ru2(DMBA)4(C≡C–TTF)2 was analyzed based on a DFT calculation of a model compound, and the resultant distribution of valence MOs is consistent with the voltammetric results.

Photophysical and DFT Characterization of Novel Pt(II)-Coupled 2,5-Diaryloxazoles for Nonlinear Optical Absorption

Several new bis-phosphine platinum(II) complexes with 2,5-diaryl-substituted oxazole-containing alkyne ligands have been synthesized and optically characterized in solution. Measurements of nonlinear absorption showed strong attenuation of laser light at 532 and 600 nm. The light absorption of the Pt complexes was shifted from the near-UV region for the ground state to the red region for the excited triplet state, and was associated with large extinction coefficients. The optical limiting effect can be explained by triplet-triplet excited state absorption in conjunction with fast excited singlet-to-triplet intersystem crossing and slow triplet-to-ground-state decay, in comparison with the pulse length of the laser. DFT calculations show good predictability of the S(0)-S(1) and S(0)-T(1) energy gaps and offer insight into the interaction strength between Pt and the alkyne ligands. The use of this type of ligand, with weak absorption for the Pt(II) complexes in the visual wavelength range as a key feature, enables the possibility to further improve these molecular systems for nonlinear absorption applications.

Dinuclear Olefin and Alkyne Complexes of Platinum(II)

AbstractBis(olefin) complexes of Zeise's dimer type – [{PtCl2(cis‐MeHC=CHMe)}2] (2) or [{PtCl2(c‐Hex)}2] (3, c‐Hex = cyclohexene) – were prepared by treatment of K2[PtCl4] in the presence of SnCl2 or K[PtCl3(cis‐MeHC=CHMe)] with the corresponding olefin. Zeise's dimer, [{PtCl2(H2C=CH2)}2] (1), was found to react in chloroform or dichloromethane with the internal alkynes RC≡CtBu (R = Me, tBu), provided that the cleaved‐off ethene was removed, to yield bis(alkyne) complexes of Zeise's dimer type [{PtCl2(RC≡CtBu)}2] (R = Me, 11; tBu, 12). Without removal of the ethene, mononuclear mixed ethene/alkyne complexes cis‐[PtCl2(H2C=CH2)(RC≡CtBu)] (R = Me, 8; tBu, 9) and cis‐[PtCl2(H2C=CH2)2] (10) were formed. Analogous reactions with alkynes RC≡CR′ bearing sterically less demanding substituents (R, R′ = Me, Et, nPr, Ph) led to (cyclobutadiene)platinum(II) complexes [PtCl2(C4R2R′2)] (R/R′ = Me/Me 4; Et/Et 5; Me/nPr 6) and [PtCl2(C4Me2Ph2)] (7). Furthermore, use of acetone (instead of CHCl3/CH2Cl2) as solvent in the reactions between 1 and sterically undemanding substituted alkynes resulted in bridge cleavage of Zeise's dimer (1) and in aldol addition and condensation reactions, thereby confirming why Chatt et al. in his analogous classical experiments (1961) observed only decomposition. Complexes 2–7 and 11/12 were isolated in pure states and fully characterized by elemental analysis, NMR spectroscopy, and X‐ray diffraction measurements (2, 11, 12). For the dinuclear bis(olefin) (1–3) and bis(alkyne) complexes (11, 12) solvent‐dependent equilibria between the transoid and cisoid isomers were observed, and these could even be ascertained crystallographically for complex [{PtCl2(tBuC≡CtBu)}2] (12), which crystallized both as the transoid isomer 12a and as the cisoid isomer 12b·CHCl3. Furthermore, consistent with DFT calculations, NMR measurements provided evidence of fast rotation and of hindered rotation of olefin and alkyne ligands, respectively, resulting in conformational isomers in 8 and 11.

DFT Study of Internal Alkyne-to-Disubstituted Vinylidene Isomerization in [CpRu(PhC≡CAr)(dppe)]+

Internal alkyne-to-vinylidene isomerization in the Ru complexes ([CpRu(η(2)-PhC≡CC(6)H(4)R-p)(dppe)](+) (Cp = η(5)-C(5)H(5); dppe = Ph(2)PCH(2)CH(2)PPh(2); R = OMe, Cl, CO(2)Et)) has been investigated using a combination of quantum mechanics and molecular mechanics methods (QM/MM), such as ONIOM(B3PW91:UFF), and density functional theory (DFT) calculations. Three kinds of model systems (I-III), each having a different QM region for the ONIOM method, revealed that considering both the quantum effect of the substituent of the aryl group in the η(2)-alkyne ligand and that of the phenyl groups in the dppe ligand is essential for a correct understanding of this reaction. Several plausible mechanisms have been analyzed by using DFT calculations with the B3PW91 functional. It was found that the isomerization of three complexes (R = OMe, CO(2)Et, and Cl) proceeds via a direct 1,2-shift in all cases. The most favorable process in energy was path 3, which involves the orientation change of the alkyne ligand in the transition state. The activation energies were calculated to be 13.7, 15.0, and 16.4 kcal/mol, respectively, for the three complexes. Donor-acceptor analysis demonstrated that the aryl 1,2-shift is a nucleophilic reaction. Furthermore, our calculation results indicated that an electron-donating substituent on the aryl group stabilizes the positive charge on the accepting carbon rather than that on the migrating aryl group itself at the transition state. Therefore, unlike the general nucleophilic reaction, the less-electron-donating aryl group has an advantage in the migration.

Syntheses and characterizations of α-iminopyridine compounds (alkylN[dbnd]CHpy)2Fe(L/Xn), and an assessment of redox non-innocence

Treatment of cis-(Me3P)4FeMe2 with two equiv neoPeNCH(2-py) afforded {κ-N,N′-neoPeNCH(2-pyridyl)}2FeMe2 (2-P). Reductions of FeCl2 with Na/Hg in the presence of neoPeNCH(2-pyridyl) or neoHexNCH(2-pyridyl) and L (H2CCHCO2Me, PhCCPh, PMe3) gave tentative evidence of {κ-N,N′-neoPeNCH(2-pyridyl)}2Fe(H2CCHCO2Me) (2-P), but {κ-N,N′-neoPeNCH(2-pyridyl)}2Fe(PhCCPh) (5-P), {κ-N,N′-neoHexNCH(2-pyridyl)}2Fe(PhCCPh) (5-H), and {κ-N,N′-neoHexNCH(2-pyridyl)}2Fe(PMe3) (6-H) were all isolated in good yield. Phosphine complex 6-H could be alkylated with CH3I or CH3OTf to afford [{κ-N,N′-neoHexNCH(2-pyridyl)}2Fe(PMe3)Me]+I− (7-H-I) or [{κ-N,N′-neoHexNCH(2-pyridyl)}2Fe(PMe3)Me]+ OTf− (7-H-OTf). [{κ-N,N′-RNCH(2-pyridyl)}3Fe][OTf]2 (R=neoPe, 8-P-OTf; neoHex, 8-H-OTf) complexes were also prepared from Fe(OTf)2 in order to identify potential byproducts in certain reactions. Compounds 2-P and 7-H-OTf were tested for ethylene oligomerization activity, to no avail. Complexes 2-P, 5-H, 6-H, and 7-H-OTf were structurally characterized and evaluated by Mössbauer spectroscopy, and 5-H was shown to possess a redox non-innocent alkyne ligand, while the α-alkyliminopyridine ligands of 6-H were classified as radical anions, rendering all compounds describable as Fe(II).

Synthesis of a Heterometallic Trinuclear Cluster Containing Ruthenium and Cobalt and Its Reactivity with Internal Alkynes

A heterometallic tetrahydrido cluster composed by ruthenium and cobalt, (Cp*Ru)2(Cp*Co)(μ-H)3(μ3-H) (1) (Cp* = η5-C5Me5), was exclusively synthesized by the reaction of Cp*Ru(μ-H)4RuCp* with (Cp*CoCl)2 in the presence of LiBEt3H. X-ray diffraction and VT-NMR studies showed that 1 has a similar structure to the Ir and Rh analogues in terms of the positions of the hydrido ligands, but the size of the trinuclear site is considerably smaller than that of the iridium analogue because of the smaller covalent radius of cobalt. Like the iridium analogue, complex 1 reacted with internal alkynes to yield an equilibrated mixture of perpendicularly coordinated alkyne complexes. However, the structure of the major isomer was different from that of the iridium analogue: the alkyne ligand on the Ru2Co core was preferentially coordinated to one of the Ru–Co bonds.

Steric Effects in the Oxidative Addition of MeI to a Sulfido-Bridged ZrRh2Early–Late Heterobimetallic Compound

Reaction of the early–late heterobimetallic compound [Cptt2Zr(μ3-S)2{Rh(CO)}2(μ-dppm)] (1) with MeI affords the unusual oxidative addition product [Cptt2Zr(μ3-S)2Rh2(μ-CO)(μ-dppm)(I)(COCH3)] (4), showing the presence of a bridging carbonyl and a terminal acetyl ligand. The optimized structure of 4 by DFT calculations is further substantiated by the spectroscopic data of the 13CO-labeled complex 4*. The same reaction carried out on the structurally related gem-dithiolate dinuclear complexes [Rh2(μ-S2CR2)(CO)2(PPh3)2] (2, 3) gives the one-center oxidative addition acetyl products [Rh2(μ-S2CR2)(COCH3)(I)(CO)(PPh3)2] (R2 = −(CH2)5–, 5; R = Bn (benzyl), 6). Reaction of 3 with molecular iodine affords the dinuclear compounds [Rh2(μ-S2CBn2)(I)2(μ-CO)(PPh3)2] (7) and [Rh2(μ-S2CBn2)(I)2(CO)2(PPh3)2] (8), the transannular oxidative addition product, in a 40:60 ratio. Compound 7 was also formed in the reaction of 3 with MeI after a prolonged reaction time. The molecular structures of compounds 5 and 7 have been dete...

Vanadocene complexes with asymmetric alkyne ligands, [Cp2V(1-2η-R1C=CR2)

A number of vanadocene complexes with asymmetrically substituted acetylenes, [Cp2V-(1-2η-R1C=CR2)] (R1 = Ph and SiMe2Ph; R2 = 4-biphenyl, 1-naphthyl, and Ge(CF3)3), were obtained and examined by ESR spectroscopy. The isotropic and anisotropic ESR spectra of the complexes containing the phenyl, biphenyl, naphthyl, or dimethyl(phenyl)silyl substituent in the alkyne ligand are typical of d1 complexes of vanadocene with alkynes. This is not the case of the complex containing the electron-withdrawing tris(trifluoromethyl)germyl group, whose isotropic and anisotropic ESR spectra show anomalous values of g factor and hyperfine coupling constant.

Platinum nanoparticles functionalized with acetylene derivatives: Electronic conductivity and electrocatalytic activity in oxygen reduction

Stable platinum nanoparticles were prepared by the self assembly of acetylene derivatives (1-alkynes, 4-ethylphenylacetylene, and 4-tert-butylphenylacetylene) onto bare Pt colloid surfaces. Transmission electron microscopic measurements showed that the nanoparticles exhibited an average core size of 2.85±0.62nm. FTIR study showed that with the acetylene ligands adsorbed onto the Pt nanoparticle surface, the CH vibrational stretches disappeared completely, along with a substantial redshift of the CC vibrational stretch, as compared to those of the monomeric ligands. These were ascribed to the breaking of the CH bond and the formation of PtsurfaceH and PtsurfaceC at the metal–ligand interface. The conjugated bonding interactions between the triple bond and Pt were found to lead to extensive intraparticle charge delocalization between the acetylene moieties, and hence unique photoluminescence properties of the nanoparticles. For nanoparticles functionalized with 4-ethylphenylacetylene or 4-tert-butylphenylacetylene, the excitation and emission peak positions showed an apparent redshift as compared to those of 1-alkyne-capped platinum nanoparticles. Electronic conductivity measurements of the nanoparticle solids showed that for the nanoparticles capped with 1-alkynes or 4-ethylphenylacetylene, the temperature dependence of the ensemble conductivity was consistent with that of semiconducting materials, whereas for the 4-tert-butylphenylacetylene-capped nanoparticles, metallic behaviors were observed instead. An apparent discrepancy was also observed in the electrocatalytic reduction of oxygen in alkaline media, where the specific activity was all markedly better than that of commercial Pt/C catalysts, with the best performance by the 4-ethyphenylacetylene-capped Pt nanoparticles. These observations were accounted for by the deliberate manipulation of the electronic structure of the Pt nanoparticles by the organic ligands.

Formation and Reactivity of a Co4-μ-Alkyne Cluster from a Co(I)-Alkene Complex

Highly reactive Co(I) complex [CpCo(H2C═CHSiMe3)2] (1) easily forms a tetranuclear Co4 cluster in hydrocarbon solvents under mild conditions, possessing a bridging alkyne ligand stemming from an unusual C–H activation of the H2C═CHSiMe3 ligand. The cluster was structurally characterized, and the catalytic reactivity in [2+2+2] cycloaddition, hydroformylation, and hydrogenation reactions investigated. Interesting differences were found and compared to the mononuclear complex 1, which could be relevant for the real catalytically active species.

Synthesis and characterization of diacetyl platinum(II) complexes with two primary and secondary amine ligands

[Pt(COMe)2(bpy)] (2; bpy = 2,2′-bipyridine) and [Pt(COMe)2{H(Me)dmg}] (5; H(Me)dmg = MeO–NC(Me)–C(Me)N–OH) were found to react with primary and secondary amines yielding diacetyl platinum(II) complexes with two monodendate amine ligands [Pt(COMe)2(NH2R)2] (R = Bn, 3a; CH2CH2Ph, 3b; Et, 3c; i-Pr, 3d; CH2CHCH2, 3e; Cy, 3f; Bn = benzyl, Cy = cyclohexyl) and [Pt(COMe)2(NHR2)2] (R = Me, 6a; Et, 6b), respectively. The equilibrium of these ligand exchange reactions was investigated by NMR experiments and DFT calculations showing that complex 5 is the more preferable starting complex and a large excess of the amine has to be used. The sterically demanding diisopropylamine was found to react with 5 yielding a thermally highly unstable dinuclear bis(acetyl) bridged complex [{Pt(COMe){NH(i-Pr)2}}2(μ-COMe)2] (7). Analogous reactions with ethylenediamine derivatives resulted in the formation of [Pt(COMe)2(N^N)] (N^N = ethylenediamine, en, 8a; N,N′-dimethylethylenediamine, 8b; N,N-dimethylethylenediamine, 8c; N,N,N′,N′-tetramethylethylenediamine, TMEDA, 8d). All complexes were fully characterized by microanalysis/high-resolution ESI mass spectrometry, by NMR (1H, 13C, 195Pt) and IR spectroscopies as well as by single-crystal X-ray diffraction measurements (3a/3d). Due to the high trans influence of the acetyl ligands, the Pt–N bonds were found to be relatively long (2.164(2)–2.182(3)Å). The resulting weak coordination of the amines gave rise to a decomposition of complexes 3 under CO extrusion yielding carbonyl–methyl complexes.

Synthesis and Structure of Cubane-Type Tetrairon Clusters Possessing μ3-Isonitrile Ligands. Reductive Coupling of Two Isonitriles on Redox-Responsive Tetrairon Reaction Sites

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).

Electronic conductivity of alkyne-capped ruthenium nanoparticles

Ruthenium nanoparticles (2.12 ± 0.72 nm in diameter) were stabilized by the self-assembly of alkyne molecules (from 1-hexyne to 1-hexadecyne) onto the Ru surface by virtue of the formation of Ru-vinylidene interfacial linkages. Infrared measurements depicted three vibrational bands at 2050 cm(-1), 1980 cm(-1) and 1950 cm(-1), which were ascribed to the vibrational stretches of the terminal triple bonds that were bound onto the nanoparticle surface. Thermogravimetric analysis showed that there were about 65 to 96 alkyne ligands per nanoparticle (depending on the ligand chainlength), corresponding to a molecular footprint of 20 to 15 Å(2). This suggests that the ligands likely adopted a head-on configuration on the nanoparticle surface, consistent with a vinylidene bonding linkage due to interfacial tautomeric rearrangements. With this conjugated interfacial bonding interaction, electronic conductivity measurements of the corresponding nanoparticle solid films showed that the nanoparticles all exhibited linear current-potential curves within the potential range of -0.8 V to +0.8 V at varied temperatures (200 to 300 K). The ohmic characters were partly ascribed to the spilling of core electrons into the organic capping layer that facilitated interparticle charge transfer. Furthermore, based on the temperature dependence of the nanoparticle electronic conductivity, the activation energy for interparticle charge transfer was estimated to be in the range of 70 to 90 meV and significantly, the coupling coefficient (β) was found to be 0.31 Å(-1) for nanoparticles stabilized by short-chain alkynes (1-hexyne, 1-octyne, and 1-decyne), and 1.44 Å(-1) for those with long alkynes such as 1-dodecyne, 1-tetradecyne, and 1-hexadecyne. This may be accounted for by the relative contributions of the conjugated metal-ligand interfacial bonding interactions versus the saturated aliphatic backbones of the alkyne ligands to the control of interparticle charge transfer.

A comparative study of the topology of the experimental electron density within 2 and 4e− donor alkyne complexes

A comparison of the topology of the experimental electron density, as revealed by high resolution X-ray diffraction, is provided for two prototypal transition metal alkyne complexes where the alkyne formally behaves as a 2 or 4e(-) donor. A higher value of the electron density ρ(r)(bcp) at the M(T)···C bond critical point (bcp), a lower value of ρ(r)(bcp) at the coordinated C≡C bcp, outwardly bent MC bond paths and a close to zero ellipticity for the C[triple bond, length as m-dash]C bond constitute the topological signature of a 4e(-) donor alkyne ligand.

Mechanistic Study of Rhodium/xantphos-Catalyzed Methanol Carbonylation

Rhodium/iodide catalysts modified with the xantphos ligand are active for the homogeneous carbonylation of methanol to acetic acid using either pure CO or CO/H2. Residues from catalytic reactions contain a Rh(III) acetyl complex, [Rh(xantphos)(COMe)I2] (1), which was isolated and crystallographically characterized. The xantphos ligand in 1 adopts a “pincer” κ3-P,O,P coordination mode with the xanthene oxygen donor trans to the acetyl ligand. The same product was also synthesized under mild conditions from [Rh(CO)2I]2. Iodide abstraction from 1 in the presence of donor ligands (L = MeCN, CO) gives the cationic acetyl species [Rh(xantphos)(COMe)I(L)]+, whereas in CH2Cl2 migratory CO deinsertion gives [Rh(xantphos)(Me)I(CO)]+ (4), which reacts with H2 to liberate methane, as observed in catalytic reactions using syngas. A number of Rh(I) xantphos complexes have been synthesized and characterized. Oxidative addition of methyl iodide to the cation [Rh(xantphos)(CO)]+ is very slow but can be catalyzed by addition of an iodide salt, via a mechanism involving neutral [Rh(xantphos)(CO)I] (6). IR spectroscopic data and DFT calculations for 6 suggest the existence in solution of conformers with different Rh–O distances. Kinetic data and activation parameters are reported for the reaction of 6 with MeI, which proceeds by methylation of the Rh center and subsequent migratory insertion to give 1. The enhancement of nucleophilicity arising from a Rh- - -O interaction is supported by DFT calculations for the SN2 transition state. A mechanism for catalytic methanol carbonylation based on the observed stoichiometric reaction steps is proposed. A survey of ligand conformations in xantphos complexes reveals a correlation between P–M–P bite angle and M–O distance and division into two broad categories with bite angle <120° (cis) or >143° (trans).