Formation of cis-enediyne complexes from rhenium alkynylcarbene complexes

Dimerization of the alkynylcarbene complex Cp(CO)(2)Re=C(Tol)C(triple bond)CCH(3) (8) occurs at 100 degrees C to give a 1.2:1 mixture of enediyne complexes [Cp(CO)(2)Re](2)[eta(2),eta(2)-TolC(triple bond)CC(CH(3))=C(CH(3))C(triple bond)CTol] (10-Eand 10-Z), showing no intrinsic bias toward trans-enediyne complexes. The cyclopropyl-substituted alkynylcarbene complex Cp(CO)(2)Re=C(Tol)C(triple bond)CC(3)H(5) (11) dimerizes at 120 degrees C to give a 5:1 ratio of enediyne complexes [Cp(CO)(2)Re](2)[eta(2),eta(2)-TolC(triple bond)C(C(3)H(5))C=C(C(3)H(5))C(triple bond)CTol] (12-E and 12-Z); no ring expansion product was observed. This suggests that if intermediate A formed by a [1,1.5] Re shift and having carbene character at the remote alkynyl carbon is involved, then interaction of the neighboring Re with the carbene center greatly diminishes the carbene character as compared with that of free cyclopropyl carbenes. The tethered bis-(alkynylcarbene) complex Cp(CO)(2)Re=C(Tol)C(triple bond)CCH(2)CH(2)CH(2)C(triple bond)CC(Tol)= Re(CO)(2)Cp (13) dimerizes rapidly at 12 degrees C to give the cyclic cis-enediyne complex [Cp(CO)(2)Re](2)[eta(2),eta(2)-TolC(triple bond)CC(CH(2)CH(2)CH(2))=CC(triple bond)CTol] (15). Attempted synthesis of the 1,8-disubstituted naphthalene derivative 1,8-[Cp(CO)(2)Re=C(Tol)C(triple bond)C](2)C(10)H(6) (16), in which the alkynylcarbene units are constrained to a parallel geometry, leads to dimerization to [Cp(CO)(2)Re](2)(eta(2),eta(2)-1,2-(tolylethynyl)acenaphthylene] (17). The very rapid dimerizations of both 13 and 16 provide compelling evidence against mechanisms involving cyclopropene intermediates. A mechanism is proposed which involves rate-determining addition of the carbene center of A to the remote alkynyl carbon of a second alkynylcarbene complex to generate vinyl carbene intermediate C, and rearrangement of C to the enediyne complex by a [1,1.5] Re shift.     

C-H and P-C(Ph) activation competitive processes caused by interaction with the solvate [cis-Pt(C6F5)2(thf)2

The study of the reaction between the ethylene [Pt(eta-H2C = CH2)(PPh3)2] or alkyne [Pt(eta2-HC [triple bond] CR)(PPh3)2] (R = SiMe3 1, Bu(t) 2) complexes with [cis-Pt(C6F5)2(thf)2] (thf = tetrahydrofuran) has enabled us to observe the existence of competitive processes between the activation of a P-C(Ph) bond on the PPh3 ligand, to give the binuclear derivative [cis-(C6F5)2Pt(mu-Ph)(mu-PPh2)Pt(PPh3)] 3, and the activation of a C-H bond of the unsaturated group, to give the corresponding (mu-hydride)(mu-vinyl) [cis, cis-(PPh3)2Pt(mu-H)(mu-1kappaC(alpha):eta2-CH = CH2)Pt(C6F5)2] 4 or (mu-hydride)(mu-alkynyl) [cis,cis-(PPh3)2Pt(mu-H)(mu-1kappaC(alpha):eta2-C [triple bond]CR)Pt(C6F5)2] (R = SiMe3 5, Bu(t) 6) compounds, respectively. The monitoring of these reactions by NMR spectroscopy has allowed us to detect several intermediates, and to propose a mechanism for the C-H bond activation. In addition, the structures of the (muo-hydride)(mu-alkynyl) complex 5 and the unprecedented (mu-hydride)(mu-vinyl) derivative 4 have been obtained by X-ray crystallographic analyses.     

First X-ray characterization and theoretical study of pi-alkyne,alkynyl-hydride,and vinylidene isomers for the same transition metal fragment [Cp*Ru(PEt3)2]+

The reaction of the chloro-complex [CpRuCl(PEt(3))(2)] with acetylene gas in methanol gave the pi-alkyne complex [CpRu(eta(2)-HCtbd1;CH)(PEt(3))(2)][BPh(4)] (1), which has been structurally characterized by X-ray analysis. The alkyne complex undergoes spontaneous isomerization even at low temperature, yielding the metastable alkynyl-hydride complex [CpRu(H)(Ctbd1;CH)(PEt(3))(2)][BPh(4)] (2), as the result of the oxidative addition of the alkyne C-H bond. This compound has also been structurally characterized despite it tautomerizes spontaneously into the stable primary vinylidene [CpRu(=C=CH(2))(PEt(3))(2)][BPh(4)] (3). This species has been alternatively prepared by a two-step deprotonation/protonation synthesis from the pi-alkyne complex. Moreover, the reaction of the initial chloro-complex with monosubstituted alkynes HCtbd1;CR (R = SiMe(3), Ph, COOMe, (t)Bu) has been studied without detection of pi-alkyne intermediates. Instead of this, alkynyl-hydride complexes were obtained in good yields. They also rearrange to the corresponding substituted vinylidenes. In the case of R = SiMe(3), the isomerization takes place followed by desilylation, yielding the primary vinylidene complex. X-ray crystal structures of the vinylidene complexes [CpRu(=C=CH(2))(PEt(3))(2)][BPh(4)] (3) and [CpRu(=C=CHCOOMe)(PEt(3))(2)][BPh(4)] (10) have also been determined. Both, full ab initio and quantum mechanics/molecular mechanics (QM/MM) calculations were carried out, respectively, on the model system [CpRu(C(2)H(2))(PH(3))(2)](+) (A) and the real complex [CpRu(C(2)H(2))(PEt(3))(2)](+) (B) to analyze the steric and electronic influence of ligands on the structures and relative energies of the three C(2)H(2) isomers. QM/MM calculations have been employed to evaluate the role of the steric effects of real ligands, whereas full ab initio energy calculations on the optimized QM/MM model have allowed recovering the electronic effects of ligands. Additional pure quantum mechanics calculations on [CpRu(C(2)H(2))(PH(3))(2)](+) (C) and [CpRu(C(2)H(2))(PMe(3))(2)](+) (D) model systems have been performed to analyze in more detail the effects of different ligands. Calculations have shown that the steric effects induced by the presence of bulky substituents in phosphine ligand are responsible for experimentally observed alkyne distortion and for relative destabilization of the alkyne isomer. Moreover, increasing the phosphine basicity and sigma donor capabilities of ligands causes a relative stabilization of an alkynyl-hydride isomer. The combination of both steric and electronic effects, makes alkyne and alkynyl-hydride isomers to be close in energy, leading to the isolation of both complexes.     

Mechanism of the mild functionalization of arenes by diboron reagents catalyzed by iridium complexes. Intermediacy and chemistry of bipyridine-ligated iridium trisboryl complexes

This paper describes mechanistic studies on the functionalization of arenes with the diboron reagent B(2)pin(2) (bis-pinacolato diborane(4)) catalyzed by the combination of 4,4'-di-tert-butylbipyridine (dtbpy) and olefin-ligated iridium halide or olefin-ligated iridium alkoxide complexes. This work identifies the catalyst resting state as [Ir(dtbpy)(COE)(Bpin)(3)] (COE = cyclooctene, Bpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl). [Ir(dtbpy)(COE)(Bpin)(3)] was prepared by independent synthesis in high yield from [Ir(COD)(OMe)](2), dtbpy, COE, and HBpin. This complex is formed in low yield from [Ir(COD)(OMe)](2), dtbpy, COE, and B(2)pin(2). Kinetic studies show that this complex reacts with arenes after reversible dissociation of COE. An alternative mechanism in which the arene reacts with the Ir(I) complex [Ir(dtbpy)Bpin] after dissociation of COE and reductive elimination of B(2)pin(2) does not occur to a measurable extent. The reaction of [Ir(dtbpy)(COE)(Bpin)(3)] with arenes and the catalytic reaction of B(2)pin(2) with arenes catalyzed by [Ir(COD)(OMe)](2) and dtbpy occur faster with electron-poor arenes than with electron-rich arenes. However, both the stoichiometric and catalytic reactions also occur faster with the electron-rich heteroarenes thiophene and furan than with arenes, perhaps because eta(2)-heteroarene complexes are more stable than the eta(2)-arene complexes and the eta(2)-heteroarene or arene complexes are intermediates that precede oxidative addition. Kinetic studies on the catalytic reaction show that [Ir(dtbpy)(COE)(Bpin)(3)] enters the catalytic cycle by dissociation of COE, and a comparison of the kinetic isotope effects of the catalytic and stoichiometric reactions shows that the reactive intermediate [Ir(dtbpy)(Bpin)(3)] cleaves the arene C-H bond. The barriers for ligand exchange and C-H activation allow an experimental assessment of several conclusions drawn from computational work. Most generally, our results corroborate the conclusion that C-H bond cleavage is turnover-limiting, but the experimental barrier for this bond cleavage is much lower than the calculated barrier.     

Thermal activation of hydrocarbon C-H bonds initiated by a tungsten allyl complex

Gentle thermolysis of the allyl complex, CpW(NO)(CH(2)CMe(3))(eta(3)-H(2)CCHCMe(2)) (1), at 50 degrees C in neat hydrocarbon solutions results in the loss of neopentane and the generation of transient intermediates that subsequently activate solvent C-H bonds. Thus, thermal reactions of 1 with tetramethylsilane, mesitylene, and benzene effect single C-H activations and lead to the exclusive formation of CpW(NO)(CH(2)SiMe(3))(eta(3)-H(2)CCHCMe(2)) (2), CpW(NO)(CH(2)C(6)H(3)-3,5-Me(2))(eta(3)-H(2)CCHCMe(2)) (3), and CpW(NO)(C(6)H(5))(eta(3)-H(2)CCHCMe(2)) (4), respectively. The products of reactions of 1 with other methyl-substituted arenes indicate an inherent preference of the system for the activation of stronger arene sp(2) C-H bonds. For example, C-H bond activation of p-xylene leads to the formation of CpW(NO)(CH(2)C(6)H(4)-4-Me)(eta(3)-H(2)CCHCMe(2)) (5) (26%) and CpW(NO)(C(6)H(3)-2,5-Me(2))(eta(3)-H(2)CCHCMe(2)) (6) (74%). Mechanistic and labeling studies indicate that the transient C-H-activating intermediates are the allene complex, CpW(NO)(eta(2)-H(2)C=C=CMe(2)) (A), and the eta(2)-diene complex, CpW(NO)(eta(2)-H(2)C=CHC(Me)=CH(2)) (B). Intermediates A and B react with cyclohexene to form CpW(NO)(eta(3)-CH(2)C(2-cyclohexenyl)CMe(2))(H) (18) and CpW(NO)(eta(3)-CH(2)CHC)(Me)CH(2)C(beta)H(C(4)H(8))C(alpha)H (19), respectively, and intermediate A can be isolated as its PMe(3) adduct, CpW(NO)(PMe(3))(eta(2)-H(2)C=C=CMe(2)) (20). Interestingly, thermal reaction of 1 with 2,3-dimethylbut-2-ene results in the formation of a species that undergoes eta(3) --> eta(1) isomerization of the dimethylallyl ligand following the initial C-H bond-activating step to yield CpW(NO)(eta(3)-CMe(2)CMeCH(2))(eta(1)-CH(2)CHCMe(2)) (21). Thermolyses of 1 in alkane solvents afford allyl hydride complexes resulting from three successive C-H bond-activation reactions. For instance, 1 in cyclohexane converts to CpW(NO)(eta(3)-C(6)H(9))(H) (22) with dimethylpropylcyclohexane being formed as a byproduct, and in methylcyclohexane it forms the two isomeric complexes, CpW(NO)(eta(3)-C(7)H(11))(H) (23a,b). All new complexes have been characterized by conventional spectroscopic methods, and the solid-state molecular structures of 2, 3, 4, 18, 19, 20, and 21 have been established by X-ray crystallographic analyses.     

By What Mechanisms Are Metal Cyclobutadiene Complexes Formed from Alkynes?

The mechanism of formation of an eta4-cyclobutadiene complex from a metallacycle, generated by oxidative coupling of two acetylenes with the fragments CpRuCl, [CpRu(PH3)]+, CpCo, and CpRh, was investigated by means of DFT/B3LYP calculations. Two distinct pathways can be envisaged. 1) A multistep reaction, which can be denoted the Vollhardt mechanism, proceeding via a cyclopropenyl carbene and a tetrahedrane-type intermediate. 2) A one-step transformation involving the formation of a third M-C bond with rearrangement of the metallacyclic ring. Although path 2 is definitely favored over path 1, both pathways are energetically prohibitive unless substituents are present on the acetylene. For the CpRuCl system with HC triple bond CR the barrier varies with R in the series H approximately Ph>Me>SiMe3. On going from H to SiMe3, the barrier for path 2 drops from 41.1 to 26.8 kcal mol(-1). This latter value is already reachable, in agreement with experiment. Whereas the reaction mechanisms involving the fragments CpCo, CpRh, and CpRuCl are very similar (but not identical owing to the additional ligand in CpRuCl), those of [CpRu(PH3)]+ reveal a modification with serious consequences. In both paths 1 and 2, the originally planar metallacycle experiences first a bending distortion induced by the sigma-donor strength of P (in contrast to Cl), which compensates the loss of electrons from the ring brought about by bending. The bent metallacycle is already electronically asymmetric and thus the further course of the reaction is facilitated.     

Acetylene to vinylidene rearrangements on electron rich d6 metal centers: a density functional study

The acetylene to vinylidene isomerization on several Ru(II) d(6) metal fragments with different electron richness of the metal center has been investigated by means of density functional theory calculations. We considered the [(eta(5)-C(5)Me(5))Ru(dippe)](+), [(eta(5)-C(5)Me(5))Ru(dmpe)](+), [(eta(5)-C(5)H(5))Ru(PMe(3))(2)](+), [(eta(6)-C(6)Me(6))(PMe(3))ClRu](+), [(eta(5)-C(5)H(5))Ru(CO)(PPh(3))](+) and [eta(6)-C(6)H(6))(PMe(3))ClRu](+), species which are quite common in the chemistry of cationic Ru(II) complexes and span a wide range of electron-richness. For each of the considered fragments, the minima on the potential energy surfaces for the two possible isomerization mechanisms, i.e. through a direct 1,2-hydrogen shift or through a hydrido-alkynyl intermediate, have been localized. A linear correlation has been found between the C=C stretching frequencies of the vinylidene complexes, as an estimate of the electron richness, and the stability of the corresponding hydrido-alkynyl intermediates. For the most electron-rich among the considered fragments, [(Cp*)(dippe)Ru(HCCH)](+), the hydrido-alkynyl species has been found essentially isoenergetic with the alkyne complex (only 1.9 kcal mol(-1) higher), in agreement with the experimental evidence showing for this system an equilibrium between these two species. For the same [(Cp*)(dippe)Ru](+) fragment, a detailed analysis of the reaction profiles for the two possible acetylene rearrangement pathways has been performed. Our results show that once the eta(2)-C-H coordinated acetylene intermediate is accessed, the system can easily evolve towards a hydrido-alkynyl intermediate, this process being kinetically favored with respect to the direct 1,2-shift leading to the vinylidene product.     

Synthesis and characterization of mono- and binuclear platinum complexes containing terminal and bridging diynyldiphenylphosphine ligands

A series of mononuclear platinum complexes containing diynyldiphenylphosphine ligands [cis-Pt(C(6)F(5))(2)(PPh(2)C[triple bond]CC(6)H(4)C[triple bond]CR)L](n)(n= 0, L = tht, R = Ph 2a, Bu(t)2b; L = PPh(2)C[triple bond]CC(6)H(4)C[triple bond]CR, 4a, 4b; n=-1, L = CN(-), 3a, 3b) has been synthesized and the X-ray crystal structures of 4a and 4b have been determined. In order to compare the eta2-bonding capability of the inner and outer alkyne units, the reactivity of towards [cis-Pt(C(6)F(5))(2)(thf)(2)] or [Pt(eta2)-C(2)H(4))(PPh(3))(2)] has been examined. Complexes coordinate the fragment "cis-Pt(C(6)F(5))(2)" using the inner alkynyl fragment and the sulfur of the tht ligand giving rise the binuclear derivatives [(C(6)F(5))(2)Pt(mu-tht)(mu-1kappaP:2eta2-C(alpha),C(beta)-PPh(2)C[triple bond]CC(6)H(4)C[triple bond]CR)Pt(C(6)F(5))(2)](R = Ph 5a, Bu(t)5b). The phenyldiynylphosphine complexes 2a, 3a and 4a react with [Pt(eta2)-C(2)H(4))(PPh(3))(2)] to give the mixed-valence Pt(II)-Pt(0) complexes [((C(6)F(5))(2)LPt(mu-1kappaP:2eta2)-C(5),C(6)-PPh(2)C[triple bond]CC(6)H(4)C[triple bond]CPh))Pt(PPh(3))(2)](n)(L = tht 6a, CN 8a and PPh(2)C[triple bond]CC(6)H(4)C[triple bond]CPh 9a) in which the Pt(0) fragment is eta2-complexed by the outer fragment. Complex 6a isomerizes in solution to a final complex [((C(6)F(5))(2)(tht)Pt(mu-1kappaP:2eta2)-C(alpha),C(beta)-PPh(2)C[triple bond]CC(6)H(4)C[triple bond]CPh))Pt(PPh(3))(2)]7a having the Pt(0) fragment coordinated to the inner alkyne function. In contrast, the tert-butyldiynylphosphine complexes 2b and 3b coordinate the Pt(0) unit through the phosphorus substituted inner acetylenic entity yielding 7b and 8b. By using 4a and 2 equiv. of [Pt(eta2)-C(2)H(4))(PPh(3))(2)] as precursors, the synthesis of the trinuclear complex [cis-((C(6)F(5))(2)Pt(mu-1kappaP:2eta2)-C(5),C(6)-PPh(2)C[triple bond]CC(6)H(4)C[triple bond]CPh)(2))(Pt(PPh(3))(2))(2)]10a, bearing two Pt(0)(PPh(3))(2)eta2)-coordinated to the outer alkyne functions is achieved. The structure of 7a has been confirmed by single-crystal X-ray diffraction.     

Ruthenium-catalyzed synthesis of alkylidenecyclobutenes via head-to-head dimerization of propargylic alcohols and cyclobutadiene-ruthenium intermediates

The reaction of propargylic alcohols with carboxylic acid, or phenol derivatives, in the presence of the precatalyst [RuCl(cod)(C5Me5)] leads selectively to a variety of alkylidenecyclobutenes through head-to-head dimerization of propargylic alcohol. The first step is the formation of a cyclobutadiene-ruthenium intermediate resulting from the head-to-head coupling of two molecules of propargylic alcohol. On protonation with strong acids (HPF6, HBF4) dehydration of the cyclobutadiene complex leads to formation of an alkylidenecyclobutenyl-ruthenium complex. The X-ray structure of one such complex, [RuCl(C5Me5)(eta4-R'CCH--CH--C=CR2)] (R'=cyclohexen-1-yl, CR2 = cyclohexylidene) has been determined. Carboxylate is added at the less substituted carbon of the cyclic allylic ligand. DFT/B3 LYP calculations confirm that the intermediate arising from head-to-head coupling of alkyne to the RuClCp* species yields the cyclobutadiene-ruthenium complex more easily with propargylic alcohol than with acetylene.     

The nature of the indenyl effect

The eta(5)-to-eta(3) coordination shift of cyclopentadienyl (Cp=C(5)H(5)(-)) and indenyl (Ind=C(9)H(7)(-)) ligands in molybdenocene complexes, [(eta(5)-Cp')(eta(5)-Cp)Mo(CO)(2)](2+) (Cp'=Cp or Ind), driven by a two-electron reduction of those species, was studied and compared by means of molecular orbital calculations (B3LYP HF/DFT hybrid functional, DZP basis sets). The results obtained, in terms of optimized geometries, relative energies, and bond analysis parameters, compare well with the experimental data, and verify the well-known indenyl effect, that is, a significantly more facile eta(5)-to-eta(3) rearrangement for the indenyl ligand when compared to cyclopentadienyl. However, the study of the folding of free Cp and Ind, combined with the (eta(5/3)-Cp')-M bond analysis, shows that the observed difference is not the result of an intrinsic characteristic of the indenyl ligand, such as the traditionally accepted aromaticity gain in the benzene ring formed in eta(3)-Ind complexes. Instead, it is directly related to the Cp'-M bond strength. While the difference in the energy required to fold the two free ligands is negligible (< or =1 kcal mol(-1) for folding angles up to 20 degrees), the (eta(5)-Cp)-M bond is stronger than that of (eta(5)-Ind)-M; however, the opposite situation is found for the eta(3) coordination mode. The net result, for Cp'=Ind, is a destabilization of the eta(5) complexes and a stabilization of the eta(3) intermediates or transition states yielding smaller activation energies and faster reaction rates for processes in which that is the rate-determining step.     

Formation of pyridine from acetylenes and nitriles catalyzed by RuCpCl, CoCp, and RhCp derivatives - A computational mechanistic study

The mechanism of the catalytic formation of pyridines from the coupling of two alkynes and the nitriles NCR (R = H, Me, Cl, COOMe) with the fragments CpRuCl, CpCo, and CpRh has been investigated by means of DFT/B3LYP calculations. According to the proposed mechanism, the key reaction step is the oxidative coupling of two alkyne ligands to give metallacyclopentatriene (Ru, Rh) and metallacyclopentadiene (Co) intermediates. In the case of ruthenium, this process is thermodynamically clearly favored over the oxidative coupling between one alkyne and one nitrile ligand to afford an azametallacycle. This alternative pathway however cannot be dismissed in the case of Co and Rh. The rate determining step of the overall catalytic cycle is the addition of a nitrile molecule to the metallacyclopentatriene and metallacyclopentadiene intermediates, respectively, which has to take place in a side-on fashion. Competitive alkyne addition leads to benzene formation. Thus, also the chemoselectivity of this reaction is determined at this stage of the catalytic cycle. In the case of the RuCpCl fragment, the addition of nitriles R-CN and acetylenes RCCH has been studied in more detail. For R = H, Cl, and COOMe the side-on addition of nitriles is kinetically more favored than alkyne addition and, in accordance with experimental results, pyridine formation takes place. In the case of R = Me nitrile addition could not be achieved and the addition of alkynes to give benzene derivatives seems to be kinetically more favored. Once the nitrile is coordinated facile C-C bond coupling takes place to afford an unusual five- and four-membered bicyclic ring system. This intermediate eventually rearranges to a very unsymmetrical azametallaheptatriene complex which in turn provides CpRuCl(κ¹-pyridine) via a reductive elimination step. Completion of the catalytic cycle is achieved by an exergonic displacement of the respective pyridine product by two acetylene molecules regenerating the bisacetylene complex.     

Chemical and electrochemical reduction of polyarene manganese tricarbonyl cations: hapticity changes and generation of syn- and anti-facial bimetallic eta4,eta6-naphthalene complexes

(Eta6-naphthalene)Mn(CO)(3)(+) is reduced reversibly by two electrons in CH(2)Cl(2) to afford (eta4-naphthalene)Mn(CO)(3)(-). The chemical and electrochemical reductions of this and analogous complexes containing polycyclic aromatic hydrocarbons (PAH) coordinated to Mn(CO)(3)(+) indicate that the second electron addition is thermodynamically easier but kinetically slower than the first addition. Density functional theory calculations suggest that most of the bending or folding of the naphthalene ring that accompanies the eta6 --> eta4 hapticity change occurs when the second electron is added. As an alternative to further reduction, the 19-electron radicals (eta6-PAH)Mn(CO)(3) can undergo catalytic CO substitution when phosphite nucleophiles are present. Chemical reduction of (eta6-naphthalene)Mn(CO)(3)(+) and analogues with one equivalent of cobaltocene affords a syn-facial bimetallic complex (eta4,eta6-naphthalene)Mn(2)(CO)(5), which contains a Mn-Mn bond. Catalytic oxidative activation under CO reversibly converts this complex to the zwitterionic syn-facial bimetallic (eta4,eta6-naphthalene)Mn(2)(CO)(6), in which the Mn-Mn bond is cleaved and the naphthalene ring is bent by 45 degrees . Controlled reduction experiments at variable temperatures indicate that the bimetallic (eta4,eta6-naphthalene)Mn(2)(CO)(5) originates from the reaction of (eta4-naphthalene)Mn(CO)(3)(-) acting as a nucleophile to displace the arene from (eta6-naphthalene)Mn(CO)(3)(+). Heteronuclear syn-facial and anti-facial bimetallics are formed by the reduction of mixtures of (eta6-naphthalene)Mn(CO)(3)(+) and other complexes containing a fused polycyclic ring, e.g., (eta5-indenyl)Fe(CO)(3)(+) and (eta6-naphthalene)FeCp(+). The great ease with which naphthalene-type manganese tricarbonyl complexes undergo an eta6 --> eta4 hapticity change is the basis for the formation of both the homo- and heteronuclear bimetallics, for the observed two-electron reduction, and for the far greater reactivity of (eta6-PAH)Mn(CO)(3)(+) complexes in comparison to monocyclic arene analogues.     

Metal-induced B-H activation: addition of acetylene, propyne, or 3-methoxypropyne to Rh(Cp*), Ir(Cp*), Ru(p-cymene), and Os(p-cymene) half-sandwich complexes containing a chelating 1,2-dicarba-closo-dodecaborane-1,2-dichalcogenolato ligand

The addition reactions of the 16e half-sandwich complexes [M(eta5-Cp*)[E2C2(B10H10)]] (Cp*=pentamethylcyclopentadienyl: 1S: E=S, M=Rh; 2S: E=S; M=Ir; 2Se: E=Se, M=Ir) and [M(eta6-p-cymene)[S2C2(B10H10)]] (p-cymene=4-isopropyltoluene; 3S: M=Ru; 4S: M=Os), with acetylene, propyne, and 3-methoxypropyne lead to the 18e complexes 5-19 with a metal-boron bond in each case. The reactions start with an insertion of the alkyne into one of the metal-chalcogen bonds, followed by B-H activation, transfer of one hydrogen atom from the carborane via the metal to the terminal carbon of the alkyne, and concomitant ortho-metalation of the carborane. The E-eta2-CC and the C(1)B units are arranged either cisoid or transoid at the metal. X-ray structural analyses are reported for one of the starting 16e complexes (4S), the cisoid complex 12S (from 2S and HC[triple bond]C-CH3), and the transoid complexes 9S and 14S (from 1S and HC[triple bond]C-CH2OMe, and from 3S and HC[triple bond]CH, respectively). All new complexes 5-19 were characterized by NMR spectroscopy (1H, 11B, 13C, and 77Se and 103Rh NMR spectroscopy when appropriate).     

Eta6-mesityl,eta1-imidazolinylidene-carbene-ruthenium(II) complexes: catalytic activity of their allenylidene derivatives in alkene metathesis and cycloisomerisation reactions

The reaction of electron-rich carbene-precursor olefins containing two imidazolinylidene moieties [(2,4,6-Me(3)C(6)H(2)CH(2))NCH(2)CH(2)N(R)Cdbond;](2) (2a: R=CH(2)CH(2)OMe, 2 b R=CH(2)Mes), bearing at least one 2,4,6-trimethylbenzyl (R=CH(2)Mes) group on the nitrogen atom, with [RuCl(2)(arene)](2) (arene=p-cymene, hexamethylbenzene) selectively leads to two types of complexes. The cleavage of the chloride bridges occurs first to yield the expected (carbene) (arene)ruthenium(II) complex 3. Then a further arene displacement reaction takes place to give the chelated eta(6)-mesityl,eta(1)-carbene-ruthenium complexes 4 and 5. An analogous eta(6)-arene,eta(1)-carbene complex with a benzimidazole frame 6 was isolated from an in situ reaction between [RuCl(2)(p-cymene)](2), the corresponding benzimidazolium salt and cesium carbonate. On heating, the RuCl(2)(imidazolinylidene) (p-cymene) complex 8, with p-methoxybenzyl pendent groups attached to the N atoms, leads to intramolecular p-cymene displacement and to the chelated eta(6)-arene,eta(1)-carbene complex 9. On reaction with AgOTf and the propargylic alcohol HCtbond;CCPh(2)OH, compounds 4-6 were transformed into the corresponding ruthenium allenylidene intermediates (4-->10, 5-->11, 6-->12). The in situ generated intermediates 10-12 were found to be active and selective catalysts for ring-closing metathesis (RCM) or cycloisomerisation reactions depending on the nature of the 1,6-dienes. Two complexes [RuCl(2)[eta(1)-CN(CH(2)C(6)H(2)Me(3)-2,4,6)CH(2)CH(2)N- (CH(2)CH(2)OMe)](C(6)Me(6))] 3 with a monodentate carbene ligand and [RuCl(2)[eta(1)-CN[CH(2)(eta(6)-C(6)H(2)Me(3)-2,4,6)]CH(2)CH(2)N-(CH(2)C(6)H(2)Me(3)-2,4,6)]] 5 with a chelating carbene-arene ligand were characterised by X-ray crystallography.     

Reactivity of (eta(6)-arene)tricarbonylchromium complexes toward additions of anions, cations, and radicals.

A computational and experimental study of additions of electrophiles, nucleophiles, and radicals to tricarbonylchromium-complexed arenes is reported. Competition between addition to a complexed arene and addition to a noncomplexed arene was tested using 1,1-dideuterio-1-iodo-2-((phenyl)tricarbonylchromium)-2-phenylethane. Reactions under anionic and cationic conditions give exclusive formation of 1,1-dideuterio-1-((phenyl)tricarbonylchromium)-2-phenylethane arising from addition to the complexed arene. Radical conditions (SmI(2)) afford two isomeric products, reflecting a 2:1 preference for radical addition to the noncomplexed arene. In contrast, intermolecular radical addition competition experiments employing ketyl radical addition to benzene and (benzene)tricarbonylchromium show that addition to the complexed aromatic ring is faster than attack on the noncomplexed species by a factor of at least 100,000. Density functional theory calculations using the B3LYP method, employing a LANL2DZ basis set for geometry optimizations and a DZVP2+ basis set for energy calculations, for all three reactive intermediates showed that tricarbonylchromium stabilizes all three types of intermediates. The computational results for anionic addition agree well with established chemistry and provide structural and energetic details as reference points for comparison with the other reactive intermediates. Intermolecular radical addition leads to exclusive reaction on the complexed arene ring as predicted by the computations. The intramolecular radical reaction involves initial addition to the complexed arene ring followed by an equilibrium leading to the observed product distribution due to a high-energy barrier for homolytic cleavage of an exo bond in the intermediate cyclohexadienyl radical complex. Mechanisms are explored for electrophilic addition to complexed arenes. The calculations strongly favor a pathway in which the cation initially adds to the metal center rather than to the arene ring.     

Reduction of an eta2-iminoacyl ligand to eta2-iminium enabled by adjacent carbon monoxide ligand replacement with a variable electron donor alkyne ligand in a cationic Tungsten(II) bis(acetylacetonate) complex

Cationic iminoacyl-carbonyl tungsten complexes of the type [W(CO) (eta (2)-MeNCR)(acac) 2] (+) (acac = acetylacetonate; R = Ph ( 1a), Me ( 1b)) easily undergo thermal substitution of CO with two-electron donors to yield [W(L)(eta (2)-MeNCR)(acac) 2] (+) (L = tert-butylisonitrile [R = Ph ( 2a), Me ( 2b)], 2,6-dimethylphenylisonitrile [R = Me ( 2c)], triphenylphosphine [R = Ph ( 3a), Me ( 3c)], and tricyclohexylphosphine [R = Ph ( 3b)]). Tricyclohexylphosphine complex 3b exhibits rapid, reversible phosphine ligand exchange at room temperature on the NMR time scale. Photolytic replacement of carbon monoxide with either phenylacetylene or 2-butyne occurs efficiently to form [W(eta (2)-alkyne)(eta (2)-MeNCR)(acac) 2] (+) complexes ( 5a- d) with a variable electron donor eta (2)-alkyne paired with the eta (2)-iminoacyl ligand in the W(II) coordination sphere. PMe 3 adds to 1a or 5b to form [W(L)(eta (2)-MeNC(PMe 3)Ph)(acac) 2] (+) [L = CO ( 4), MeCCMe ( 6)] via nucleophilic attack at the iminoacyl carbon. Addition of Na[HB(OMe) 3] to 5b yields W(eta (2)-MeCCMe)(eta (2)-MeNCHPh)(acac) 2, 8, which exhibits alkyne rotation on the NMR time scale. Addition of MeOTf to 8 places a second methyl group on the nitrogen atom to form an unusual cationic eta (2)-iminium complex [W(eta (2)-MeCCMe)(eta (2)-Me 2NCHPh)(acac) 2][OTf] ( 9[OTf], OTf = SO 3CF 3). X-ray structures of 2,6-dimethylphenylisonitrile complex 2c[BAr' 4 ], tricyclohexylphosphine complex 3b[BAr' 4 ], and phenylacetylene complex 5a[BAr' 4 ] confirm replacement of CO by these ligands in the [W(L)(eta (2)-MeNCR)(acac) 2] (+) products. X-ray structures of alkyne-imine complexes 6[BAr' 4 ] and 8 show products resulting from nucleophilic addition at the iminoacyl carbon, and the X-ray structure of 9[BAr' 4 ] reflects methylation at the imine nitrogen to form a rare eta (2)-iminium ligand.     

Synthesis, structure, and reactivity of Group 4 metallacycles incorporating a Me2C-linked cyclopentadienyl-carboranyl ligand

Group 4 metallacycles [eta5:sigma-Me2C(C5H4)(C2B10H10)]Ti[eta2-N(Me)CH2CH2N(Me)] (1a), [eta5:sigma-Me2C(C5H4)(C2B10H10)]Zr[eta2-N(Me)CH2CH2N(Me)](HNMe2) (1b) and [eta5:sigma-Me2C(C5H4)(C2B10H10)]M[eta2-N(Me)CH2CH2CH2N(Me)] (M = Ti (2a), Zr (2b), Hf (2c)) were synthesized by reaction of [eta5:sigma-Me2C(C5H4)(C2B10H10)]M(NMe2)(2) (M = Ti, Zr, Hf) with MeNH(CH2)(n)NHMe (n = 2, 3). These metal complexes reacted with unsaturated molecules such as 2,6-Me2C6H3NC, PhNCO and PhCN to give exclusively M-N bond insertion products. The M-C(cage) bond remained intact. Such a preference of M-N over M-C(cage) insertion is suggested to most likely be governed by steric factors, and the mobility of the migratory groups plays no obvious role in the reactions. This work also shows that the insertion of unsaturated molecules into the metallacycles is a useful and effective method for the construction of very large ring systems.     

Theoretical study of the ruthenium-catalyzed cyclocotrimerization of alkynes with isocyanates and iothiocyanates: cemoselective formation of pyridine-2-one and thiopyrane-2-imine

The cyclocotrimerization of acetylene with isocyanate HNCO and isothiocyanate HNCS, mediated by CpRuCl, is theoretically investigated on the basis of DFT/B3LYP calculations. By these means, the experimental result can be rationalized as to why, with HNCO, a nitrogen-heterocycle is formed, but with HNCS a sulfur-heterocycle is formed. According to the proposed mechanism, the key reaction step is the addition of a double bond to a metallacyclopentatriene formed by oxidative coupling of two acetylene ligands coordinated to CpRuCl, giving a bicyclic carbene intermediate. This double-bond-addition is initiated by eta1 attack at the ruthenium center, and it is just the attacking atom that is going to be incorporated into the cycle. Thus, the chemoselectivity originates from the fact that, for HNCO, N attack is preferred over O, but for HNCS, S attack is preferred over N. The onward reaction is a reductive elimination to give a coordinatively unsaturated metallaheteronorbornene intermediate finally rearranging to a ligated heterocycle. Completion of the cycles is achieved by an exothermic displacement of the respective heterocyclic product by two acetylene molecules which regenerates the bisacetylene complex.     

Reactions of an amphoteric terminal tungsten methylidyne complex.

Treatment of [Tp'(CO)(2)W triple bond C--PPh(3)][PF(6)] (Tp' = hydridotris(3,5-dimethylpyrazolylborate)) with Na[HBEt(3)] in THF forms the methylidyne complex Tp'(CO)(2)W triple bond C--H via formyl and carbene intermediates Tp'(CO)(C(O)H)W triple bond C- PPh(3) and Tp'(CO)(2)W=C(PPh(3))(H), respectively. Spectroscopic features reported for Tp'(CO)(2)W triple bond C--H include the W triple bond C stretch (observed by both IR and Raman spectroscopy) and the (183)W NMR signal (detected by a (1)H, (183)W 2D HMQC experiment). Protonation of the Tp'(CO)(2)W triple bond C--H methylidyne complex with HBF(4).Et(2)O yields the cationic alpha-agostic methylidene complex [Tp'(CO)(2)W=CH(2)][BF(4)]. The methylidyne complex Tp'(CO)(2)W triple bond C-H can be deprotonated with alkyllithium reagents to provide the anionic terminal carbide Tp'(CO)(2)W triple bond C--Li; a downfield resonance at 556 ppm in the (13)C NMR spectrum has been assigned to the carbide carbon. The terminal carbide Tp'(CO)(2)W triple bond C-Li adds electrophiles at the carbide carbon to generate Tp'(CO)(2)W triple bond C--R (R = CH(3), SiMe(3), I, C(OH)Ph(2), CH(OH)Ph, and C(O)Ph) Fischer carbynes. A pK(a) of 28.7 was determined for Tp'(CO)(2)W triple bond C--H in THF by titrating the terminal carbide Tp'(CO)(2)W triple bond C--Li with 2-benzylpyridine and monitoring its conversion to Tp'(CO)(2)W triple bond C--H with in situ IR spectroscopy. Addition of excess Na[HBEt(3)] to neutral Tp'(CO)(2)W triple bond C--H generates the anionic methylidene complex [Na][Tp'(CO)(2)W=CH(2)]. The synthetic methodology for generating an anionic methylidene complex by hydride addition to neutral Tp'(CO)(2)W triple bond C--H contrasts with routes that utilize alpha-hydrogen abstraction or hydride removal from neutral methyl precursors to generate methylidene complexes. Addition of PhSSPh to the anionic methylidene complex in solution generates the saturated tungsten product Tp'(CO)(2)W(eta(2)-CH(2)SPh) by net addition of the SPh(+) moiety.     

Ten-electron coordination and reactivity of an arylphosphinidene ligand

Photochemical decarbonylation of [Mo2Cp2(mu-PR*)(CO)4] (Cp = eta5-C5H5; R* = 2,4,6-C6H2tBu3) gives [Mo2Cp2(mu-kappa1:kappa1,eta6-PR*)(CO)2], which shows the first example of a remarkable 10-electron donor arylphosphinidene ligand which bridges two Mo atoms through its phosphorus atom while being pi-bonded to one Mo center through the six carbon atoms of the aryl ring. This causes a severe pyramidal distortion of the P-bound C atom. The complex adds CO to give [Mo2Cp2(mu-kappa1:kappa1,eta4-PR*)(CO)3], which has an 8-electron donor PR* ligand, and then the parent complex [Mo2Cp2(mu-PR*)(CO)4]. Protonation of [Mo2Cp2(mu-kappa1:kappa1,eta6-PR*)(CO)2] gives the hydride [Mo2Cp2(H)(mu-kappa1:kappa1,eta6-PR*)(CO)2]+, which undergoes P-C bond cleavage and hydride migration, affording the phosphido cation [Mo2Cp2(mu-P)(eta6-R*H)(CO)2]+.