M. Victoria Jiménez

Oxidation and β-Alkylation of Alcohols Catalysed by Iridium(I) Complexes with Functionalised N-Heterocyclic Carbene Ligands

The borrowing hydrogen methodology allows for the use of alcohols as alkylating agents for CC bond forming processes offering significant environmental benefits over traditional approaches. Iridium(I)-cyclooctadiene complexes having a NHC ligand with a O- or N-functionalised wingtip efficiently catalysed the oxidation and β-alkylation of secondary alcohols with primary alcohols in the presence of a base. The cationic complex [Ir(NCCH3 )(cod)(MeIm(2- methoxybenzyl))][BF4 ] (cod=1,5-cyclooctadiene, MeIm=1-methylimidazolyl) having a rigid O-functionalised wingtip, shows the best catalyst performance in the dehydrogenation of benzyl alcohol in acetone, with an initial turnover frequency (TOF0 ) of 1283 h(-1) , and also in the β-alkylation of 2-propanol with butan-1-ol, which gives a conversion of 94 % in 10 h with a selectivity of 99 % for heptan-2-ol. We have investigated the full reaction mechanism including the dehydrogenation, the cross-aldol condensation and the hydrogenation step by DFT calculations. Interestingly, these studies revealed the participation of the iridium catalyst in the key step leading to the formation of the new CC bond that involves the reaction of an O-bound enolate generated in the basic medium with the electrophilic aldehyde.

CH bond activation of thiophenes at iridium: a lower energy process than CS bond scission

Abstract The 16-electron fragment [(triphos)IrH], generated in situ by thermolysis of (triphos)Ir(H)2(C2H5) in THF, reacts with thiophene (T) or benzo[b]thiophene (BT) to give, in the temperature range from 67 to 100°C, mixtures of CH and CS insertion product as a result of parallel reactions [triphos = MeC(CH2PPh2)3]. Above 100°C, the (thienyl)dihydride complexes convert to the thermodynamically more stable CS insertion products (butadienethiolate and 2-vinylthiophenolate complexes) through an intramolecular mechanism which does not involve T or BT dissociation. A comparison is made to analogous reactions with dibenzothiophene.

Alkene C-H activations at dinuclear complexes promoted by oxidation.

We are grateful to the Plan Nacional de Investigacion, Ministerio de Ciencia y Tecnologia, for the support of this research (Project No. BQU2000-1170).

Mechanistic Studies on the Catalytic Oxidative Amination of Alkenes by Rhodium(I) Complexes with Hemilabile Phosphines

Cationic rhodium(I) complexes with P,O‐functionalised arylphosphine ligands are efficient catalysts for the regioselective anti‐Markovnikov oxidative amination of styrene with piperidine. The mechanism of the catalytic reaction has been investigated by spectroscopic means under stoichiometric and catalytic conditions. In the presence of piperidine, the catalyst precursor [Rh{κ2‐P,O‐Ph2P(CH2)3OEt}2]+ (5) gave the piperidine complex [Rh{κ1‐P‐Ph2P(CH2)3OEt}2(HNC5H10)2]+ (8) that was transformed into the neutral amido–piperidine species [Rh{κ1‐P‐Ph2P(CH2)3OEt}2(NC5H10)(HNC5H10)] (9) under thermal conditions. NMR studies performed in the presence of styrene under catalytic conditions showed that 9 is a key species in the catalytic oxidative amination of styrene. Related cyclooctadiene‐containing catalyst precursors [Rh(cod){κ1‐P‐Ph2P(CH2)3OEt}n]+ (n=1, 2) also gave 9 under the same conditions. The proposed catalytic cycle has been established by a series of DFT calculations including the transition states of the key steps that have been identified and characterised. These studies have shown that, after elimination of the enamine, regeneration of catalytic active species takes place by direct transfer of the proton of a piperidine ligand to the alkyl group resulting from the insertion of styrene into the RhH bond and formation of ethylbenzene. Against the expectations, the formation of a dihydride intermediate by NH oxidative addition is a highly energy‐demanding process. Catalyst 5 has also been applied for the oxidative amination of substituted vinylarenes with several secondary cyclic and acyclic amines.

Hydride Mobility in Trinuclear Sulfido Clusters with the Core [Rh3(μ-H)(μ3-S)2]: Molecular Models for Hydrogen Migration on Metal Sulfide Hydrotreating Catalysts

The treatment of [{Rh(μ-SH){P(OPh)(3)}(2)}(2)] with [{M(μ-Cl)(diolef)}(2)] (diolef=diolefin) in the presence of NEt(3) affords the hydrido-sulfido clusters [Rh(3)(μ-H)(μ(3)-S)(2)(diolef){P(OPh)(3)}(4)] (diolef=1,5-cyclooctadiene (cod) for 1, 2,5-norbornadiene (nbd) for 2, and tetrafluorobenzo[5,6]bicyclo[2.2.2]octa-2,5,7-triene (tfb) for 3) and [Rh(2)Ir(μ-H)(μ(3)-S)(2)(cod){P(OPh)(3)}(4)] (4). Cluster 1 can be also obtained by treating [{Rh(μ-SH){P(OPh)(3)}(2)}(2)] with [{Rh(μ-OMe)(cod)}(2)], although the main product of the reaction with [{Ir(μ-OMe)(cod)}(2)] was [RhIr(2)(μ-H)(μ(3)-S)(2)(cod)(2){P(OPh)(3)}(2)] (5). The molecular structures of clusters 1 and 4 have been determined by X-ray diffraction methods. The deprotonation of a hydrosulfido ligand in [{Rh(μ-SH)(CO)(PPh(3))}(2)] by [M(acac)(diolef)] (acac=acetylacetonate) results in the formation of hydrido-sulfido clusters [Rh(3)(μ-H)(μ(3)-S)(2)(CO)(2) (diolef)(PPh(3))(2)] (diolef=cod for 6, nbd for 7) and [Rh(2)Ir(μ-H)(μ(3)-S)(2)(CO)(2)(cod)(PPh(3))(2)] (8). Clusters 1-3 and 5 exist in solution as two interconverting isomers with the bridging hydride ligand at different edges. Cluster 8 exists as three isomers that arise from the disposition of the PPh(3) ligands in the cluster (cis and trans) and the location of the hydride ligand. The dynamic behaviour of clusters with bulky triphenylphosphite ligands, which involves hydrogen migration from rhodium to sulfur with a switch from hydride to proton character, is significant to understand hydrogen diffusion on the surface of metal sulfide hydrotreating catalysts.

Rhodium(I) Complexes with Hemilabile Phosphines: Rational Design for Efficient Oxidative Amination Catalysts

A series of cationic square‐planar rhodium(I) complexes of type [Rh(cod){Ar2P(CH2)nZ}]+, [Rh(cod){Ar2P(CH2)nZ}2]+, and [Rh{Ar2P(CH2)nZ}2]+, which contained diverse functionalized hemilabile phosphine ligands of type Ar2P(CH2)nZ (n=1–3; Z=OMe, OEt, OnBu, NMe2, SMe), were synthesized and spectroscopically characterized. The crystal structures of representative compounds were determined by X‐ray diffraction. Most complexes were active catalysts for the anti‐Markovnikov oxidative amination of styrene with piperidine to produce (E)‐1‐styrylpiperidine. Catalyst screening showed a remarkable relationship between the hemilabile ligand, the precatalyst structure, and the catalytic activity. The more‐efficient catalysts were those that had arylphosphine ligands with a 2‐alkoxyethyl‐ or 3‐alkoxypropyl hemilabile moiety, Ar2P(CH2)nOR (n=2, 3; R=Me, Et, nBu). This study has revealed the outstanding catalytic activity of bis‐phosphine complexes [Rh{(4‐R‐C6H4)2P(CH2)3OEt}2][PF6] (R=H, Me, OMe), with unprecedented turnover frequencies of up to 80 h−1 (R=Me) and excellent enamine selectivity (96 %).

On the Synthesis and Chemical Behaviour of the Elusive Bis(hydrosulfido)‐Bridged Dinuclear Rhodium(I) Complexes [{Rh(μ‐SH)(CO)(PR3)}2]

Several bis(hydrosulfido)-bridged dinuclear rhodium(I) compounds, [{Rh(mu-SH)(L)(2)}(2)], have been prepared from rhodium(I) acetylacetonato complexes, [Rh(acac)(L)(2)], and H(2)S(g). Reaction of [Rh(acac){P(OPh)(3)}(2)] with H(2)S(g) affords the dinuclear bis(hydrosulfido)-bridged compound [{Rh(mu-SH){P(OPh)(3)}(2)}(2)] (1). However, reaction of complexes [Rh(acac)(CO)(PR(3))] with H(2)S(g) gives the dinuclear compound [{Rh(mu-SH)(CO)(PR(3))}(2)] (R=Cy, 2; R=Ph, 4) and the trinuclear cluster [Rh(3)(mu-H)(mu(3)-S)(2)(CO)(3)(PR(3))(3)] (R=Cy, 3; R=Ph, 5). The selective synthesis of both type of compounds has been carried out by control of the H(2)S(g) concentration in the reaction media. The trinuclear hydrido-sulfido cluster [Rh(3)(mu-H)(mu(3)-S)(2)(CO)(3)(PPh(3))(3)] (5) has been also obtained by reaction of [{Rh(mu-SH)(CO)(PPh(3))}(2)] (4) with [Rh(acac)(CO)(PPh(3))], proceeding through the trinuclear hydrosulfido-sulfido intermediate [Rh(3)(mu(3)-SH)(mu(3)-S)(CO)(3)(PPh(3))(3)]. The molecular structures of complexes 1 and 3 have been determined by X-ray diffraction methods. Compound 1 is stable in solution, but complexes 2 and 4 slowly transform in solution into the trinuclear hydrido-sulfido clusters 3 and 5, respectively, with the release of H(2)S(g) in a reversible way. (1)H NMR kinetic experiments for the transformation of 4 into 5 have revealed that this transformation follows second-order-type kinetic. The following activation parameters, DeltaH( not equal)=24+/-3 kJ mol(-1) and of DeltaS( not equal)=-223+/-8 J K(-1) mol(-1), have been calculated from the determination of the second-order rate constants in the temperature range 30-45 degrees C. The large negative value of the activation entropy is consistent with an associative character of the rate-determining step. A plausible multistep mechanism based on the chemical behaviour of hydrosulfido-metal complexes and compatible with the kinetic behaviour has been proposed.

Enhancing the hydrogen transfer catalytic activity of hybrid carbon nanotube-based NHC–iridium catalysts by increasing the oxidation degree of the nanosupport

CVD-grown multiwalled carbon nanotubes were purified by applying four different treatments with increasing oxidation severity. The growing severity of the treatment results in progressive oxygen functionalization of the surface along with introduction of an increasing quantity of defects on the carbon nanotube walls. Iridium–N-heterocyclic carbene complexes were covalently anchored to those oxidized surfaces through their surface carboxylic acids via acetyl linkers. The carbon nanotube-based iridium–NHC hybrid materials developed are active in the hydrogen-transfer reduction of cyclohexanone to cyclohexanol with 2-propanol/KOH as hydrogen source but with rather different activity. The catalytic activity of the hybrid catalysts is strongly influenced by the type and amount of oxygenated functionalization resulting from the treatment applied to the support, being the most active and the most oxidized material.

Reactivity of rhodium(III) complexes containing chelating pentachlorophenyl ligands. Molecular structures of fac-[Rh(C6Cl5)3(py)] and mer-[Rh(C6Cl5)3(tBuNC)3]

Abstract The reactions of the octahedral rhodium(III) complexes [Rh(C6Cl5)3] and [Rh(C6Cl5)3Cl]− with neutral monodentate ligands have been studied. With pyridine, triethylphosphine or pyrazole, one of the rhodiumo-chloro bonds of the starting materials is broken and complexes of formula [Rh(C6Cl5)3L] and [Rh(C6Cl5)3ClL]− are obtained. Carbonylation of [Rh(C6Cl5)3] produces the insertion of CO molecules in two of the RhCaryl bonds giving the diacyl complex [Rh{C(O)C6Cl5}2(C6Cl5)(CO)], while the reaction of [Rh(C6Cl5)3] or [Rh(C6Cl5)3Cl]− with the strong σ-donor ligand tBuNC breaks all the rhodiumo-chloro interactions producing the compound mer-[Rh(C6Cl5)3(tBuNC)3]. All the complexes are stable to the air and moisture in the solid state and moderately stable in deoxygenated solutions. The compounds have been studied and characterised by IR, NMR and MS. The crystal structures of fac-[Rh(C6Cl5)3(py)] and mer-[Rh(C6Cl5)3(tBuNC)3] have been determined by X-ray diffraction methods. They display distorted octahedral metal environments but, while the pyridine complex shows two C6Cl5 groups acting as chelate ligands bonded through the ipso-C atom and one of the o-Cl atoms, the isocyanide complex exhibits all the coordinated ligands acting as monodentate groups.

Homo- and hetero-bridged mixed-valence dinuclear complexes which contain the fragment ‘Rh(C6F5)3’

Abstract The reaction of the anionic mononuclear rhodium complex [Rh(C 6 F 5 ) 3 Cl(Hpz)] t- (Hpz = pyrazole, C 3 H 4 N 2 ) with methoxo or acetylacetonate complexes of Rh or Ir led to the heterodinuclear anionic compounds [(C 6 F 5 ) 3 Rh(μ-Cl)(μ-pz)M(L 2 )] [M = Rh, L 2 = cyclo-octa-1,5-diene, COD ( 1 ), tetrafluorobenzobarrelene, TFB ( 2 ) or (CO) 2 ( 4 ); M = Ir, L 2 = COD ( 3 )]. The complex [Rh(C 6 F 5 ) 3 (Hbim)] − ( 5 ) has been prepared by treating [Rh(C 6 F 5 ) 3 (acac)] − with H 2 bim (acac = acetylacetonate; H 2 bim = 2,2′-biimidazole). Complex 5 also reacts with Rh or Ir methoxo, or with Pd acetylacetonate, complexes affording the heterodinuclear complexes [(C 6 F 5 ) 3 Rh(μ-bim)M(L 2 )] − [M = Rh, L 2 = COD ( 6 ) or TFB ( 7 ); M = Ir, L 2 = COD ( 8 ); M = Pd, L 2 = η 3 -C 3 H 5 ( 9 )]. With [Rh(acac)(CO) 2 ], complex 5 yields the tetranuclear complex [{(C 6 F 5 ) 3 Rh(μ-bim)Rh(CO) 2 } 2 ] 2− . Homodinuclear Rh III derivatives [{Rh(C 6 F 5 ) 3 } 2 (μ-L) 2 ] ·- [L 2 = OH, pz ( 11 ); OH, S t Bu ( 12 ); OH, SPh ( 13 ); bim ( 14 )] have been obtained by substitution of one or both hydroxo groups of the dianion [{Rh(C 6 F 5 ) 3 (μ-OH)} 2 ] 2− by the corresponding ligands. The reaction of [Rh(C 6 F 5 ) 3 (Et 2 O) x ] with [PdX 2 (COD)] produces neutral heterodinuclear compounds [(C 6 F 5 ) 3 Rh(μ-X) 2 Pd(COD)] [X = Cl ( 15 ); Br ( 16 )]. The anionic complexes 1–14 have been isolated as the benzyltriphenylphosphonium (PBzPh 3 + ) salts.