Anton W. Gal

Rhodium and Iridium β‐Diiminate Complexes – Olefin Hydrogenation Step by Step

The bulky β-diiminate ligands [(2,6-C6H3X2)NC(Me)CHC(Me)N(2,6-C6H3X2)]– (X = Me, LMe; X = Cl, LCl) have been found to be effective in stabilizing low coordination numbers (CN) in Rh and Ir complexes. The 14- complex LMeRh(COE) (COE = cyclooctene) has a three-coordinate T-shaped Rh environment and is nonagostic. Coordinative unsaturation is avoided by incorporation of a small ligand (e.g. N2, MeCN, olefins), by the intramolecular coordination of a chlorine atom in LClRh(COE), or by an agostic interaction in LMeRh(norbornene). In solution at room temperature, LMeRh(COE) undergoes rapid isomerization according to the allyl hydride mechanism; the corresponding 2,3-dimethylbutene complex actually prefers the allyl hydride structure. Rhodium(I) complexes of LMe and LCl catalyze olefin hydrogenation; hydrogenation of 2,3-dimethylbutene has been shown to be preceded by isomerization. The shielding properties of the bulky β-diiminate ligands allow direct observation of a number of reactive intermediates or their iridium analogues, including an olefin–dihydrogen complex (with Rh) and an olefin dihydride (with Ir). These observations, together with calculations on simple model systems, provide us with snapshots of a plausible hydrogenation cycle. Remarkably, hydrogenation according to this cycle appears to follow a 14-e/16-e path, in contrast to the more usual 16-e/18-e paths.

Selective Oxidation of [RhI(cod)]+ by H2O2 and O2

2-Rhodaoxetanes have thus far not been invoked as intermediates in rhodium-catalysed oxidation of olefins. Oxygenation of one COD double bond in cationic complexes [′N3′RhI(cod)]+ by H2O2 and O2 is now found to result in 2-rhodaoxetanes that subsequently rearrange (see figure). Insight into their modes of formation and rearrangement might contribute to a better understanding of late transition metal catalysed oxidation of olefins.

2-Rhodaoxetanes: Their Formation of Oxidation of [RhI(ethene)]+ and Their Reactivity upon Protonation

New cationic, pentacoordinate complexes [(TPA)Rh1(ethene)]+, [1a]+, and [(MeTPA)Rh1(ethene)]+, [1b]+, have been prepared (TPA = N,N,N-tri(2-pyridylmethyl)amine, MeTPA = N-[(6-methyl-2-pyridyl)-methyl]-N,N-di(2-pyridylmethyl)amine). Complex [1a]+ is selectively converted by aqueous HCl to [(TPA)RhIII-(ethyl)Cl]+, [2a]+. The same reaction with [1b]+ results in the [(MeTPA)RhIII-(ethyl)Cl]+ isomers [2b]+ and [2c]+. Treatment of [1a]+ and [1b]+ with aqueous H2O2 results in a selective oxygenation to the unsubstituted 2-rho-da(III)oxetanes (1-oxa-2-rhoda(III)cyclo-butanes) [(TPA)RhIII(kappa2-C,O-2-oxyethyl)]+, [3a]+, and [(MeTPA)RhIII(kappa2-C,O-2-oxyethyl)]+, [3b]+. The reactivity of 2-rhodaoxetanes [3a]+ and [3b]+ is dominated by the nucleophilic character of their 2-oxyethyl oxygen. Reaction of [3a]+ and [3b]+ with the non-coordinating acid HBAr(f)4 results in the dicationic protonated 2-rhodaoxetanes [(TPA)RhIII(kappa2-2-hydroxyethyl)]2+, [4a]2+, and [(MeTPA)RhIII(kappa2-2-hydroxyethyl)]2+, [4b]2+. These eliminate acetaldehyde at room temperature, probably via a coordinatively unsaturated kappa1-2-hydroxyethyl complex. In acetonitrile, complex [4a]2+ is stabilised as [(TPA)-RhIII(kappa1-2-hydroxyethyl)(MeCN)]2+, [5a]2+, whereas the MeTPA analogue [4b]2+ continues to eliminate acetaldehyde. Reaction of [3a]+ with NH4Cl and Mel results in the coordinatively saturated complexes [(TPA)RhIII(kappa1-2-hydroxyethyl)(Cl)]+, [6a]+, and [(TPA)-RhIII(kappa1-2-methoxyethyl)(I)+, [7a]+, respectively. Reaction of [3a]+ with NH4+ in MeCN results in formation of the dicationic metallacyclic amide [(TPA)-RhIII [kappa2-O,C-2-(acetylamino)ethyl]]2+, [9]2+, via the intermediates [4a]2+, [5a]2+ and the metallacyclic iminoester [(TPA)RhIII[kappa2-N,C-2-(acetimidoyloxy)ethyl]]2+, [8]2+. The observed overall conversion of the [Rh(I)(ethene)] complex [1a]+ to the metallacyclic amide [9]2+ via 2-rhodaoxetane [3a]+, provides a new route for the amidation of a [RhI(ethene)] fragment.

Dioxygen Activation by a Mononuclear IrII–Ethene Complex

In an attempt to gain a mechanistic insight into the rhodiumand iridium-catalyzed oxygenation of olefins, we have recently investigated stoichiometric oxygenation of N ligand RhI± and IrI ± olefin complexes by O2 (olefin ethene, propene, 1,5-cyclooctadiene).[1, 2] The reactivity of RhI± and IrI ± ethene fragments towards dioxygen varied between ethene displacement (Figure 1a), formation of mixed O2 ± ethene complexes (Figure 1b), C O bond making (giving a 3-metalla( )-1,2-dioxolane; Figure 1c), and combined C O bond making and O O bond breaking (giving a 2-metalla( )oxetane; Figure 1d) The outcome of the oxygenation reaction varies with the N ligand and the central metal.

Coordination and oxidative addition at a low-coordinate rhodium(I) β-diiminate centre

The reaction of 14e [L(Me)Rh(coe)] (1; L(Me)[double bond]ArNC(Me)CHC(Me)NAr, Ar[double bond]2,6-Me(2)C(6)H(3); coe[double bond]cis-cyclooctene) with phenyl halides and thiophenes was studied to assess the competition between sigma coordination, arene pi coordination and oxidative addition of a C-X bond. Whereas oxidative addition of the C-Cl and C-Br bonds of chlorobenzene and bromobenzene to L(Me)Rh results in the dinuclear species [[L(Me)Rh(Ph)(micro-X)](2)] (X=Cl, Br), fluorobenzene yields the dinuclear inverse sandwich complex [[L(Me)Rh](2)(anti-micro-eta(4):eta(4)-PhF)]. Thiophene undergoes oxidative addition of the C-S bond to give a dinuclear product. The reaction of 1 with dibenzo[b,d]thiophene (dbt) in the ratio 1:2 resulted in the formation of the sigma complex [L(Me)Rh(eta(1)-(S)-dbt)(2)], which in solution dissociates into free dbt and a mixture of the mononuclear complex [L(Me)Rh(eta(4)-(1,2,3,4)-dbt)] and the dinuclear complex [[L(Me)Rh](2)(micro-eta(4)-(1,2,3,4):eta(4)-(6,7,8,9)-dbt)]. The latter could be obtained selectively by the 2:1 reaction of 1 and dbt. Reaction of 1 with diethyl sulfide produces [L(Me)Rh(Et(2)S)(2)], which in the presence of hydrogen loses a diethyl sulfide ligand to give [L(Me)Rh(Et(2)S)(H(2))] and catalyses the hydrogenation of cyclooctene.

Enhanced Reactivity of 2‐Rhodaoxetanes through a Labile Acetonitrile Ligand

New cationic, square-planar, ethene complexes [(Rbpa)RhI(C2H4)]+ [2a]--[2c]+ (Rbpa = N-alkyl-N,N-di(2-pyridylmethyl)amine; [2a]+: alkyl =R=Me; [2b]+: R = Bu; [2c]+: R = Bz) have been selectively oxygenated in acetonitrile by aqueous hydrogen peroxide to 2-rhoda(III)oxetanes with a labile acetonitrile ligand, [(Rbpa)RhIII(kappa2-C,O-CH2CH2O-)(MeCN)]+, [3a]+-[3c]+. The rate of elimination of acetaldehyde from [(Rbpa)RhIII(kappa2-C,O-CH2CH2O-)(MeCN)]+ increases in the order R = Me< R = Bu< R = Bz. Elimination of acetaldehyde from [(Bzbpa)RhIII(kappa2-C,O-CH2CH2O)(MeCN)]+ [3c]+, in the presence of ethene results in regeneration of ethene complex [(Bzbpa)RhI(C2H4)]+ [2c]+, and closes a catalytic cycle. In the presence of Z,Z-1,5-cyclooctadiene (cod) the corresponding cod complex [(Bzbpa)RhI(cod)]+ [6c]+ is formed. Further oxidation of [3c]+ by H2O2 results in the transient formylmethyl-hydroxy complex [(Bzbpa)RhIII(OH)[kappa1-C-CH2C(O)H]]+ [5c]+.

Steric Control over Arene Coordination toβ-Diiminate Rhodium(I) Fragments

The bulky ligands Lx− (Lx=(2, 6-C6H3X2)NC(Me)CHC(Me)N(2, 6-C6H3X2), X=Cl, Me) can be used to generate fluxional mononuclear arene complexes [LXRh(η4-arene)] (arene=benzene, toluene, m-xylene, mesitylene), which for X=Me disproportionate to fluxional dinuclear complexes [{LMeRh}2(anti-μ-arene)]. For both mononuclear and dinuclear complexes, steric interactions do not stop the fluxionality but govern the preferred orientation of the methyl-substituted arenes, thus allowing indirect determination of the static NMR parameters. For the μ-arene complexes, two distinct types of fluxionality are proposed on the basis of calculations: ring rotation and metal shift. In the solid state, the toluene complex has an η4(1,2,3,4):η4(3,4,5,6)-bridged structure; the NMR analysis indicates that the benzene and m-xylene complexes have similar structures. The mesitylene complex, however, has an unprecedented η3(1,2,3):η3(3,4,5)-bridged structure, which is proposed to correspond to the transition state for arene rotation in the other cases. Steric factors are thought to be responsible for this reversal of stabilities.

3‐Rhoda‐1,2‐dioxolanes through Dioxygenation of a Rhodium–Ethene Complex by Air

Proposed as intermediates in the catalytic oxidation of olefins to ketones, 3-rhoda-1,2-dioxolanes (κ2 C1 ,O2 -2-peroxyethyl rhodium complexes) have now been prepared by oxygenation of solid [(N4 -ligand)RhI (ethene)]PF6 with air. This process leads to stable isomeric 3-rhoda-1,2-dioxolanes A and B. Upon substitution of PF6- by BPh4- only isomer B is obtained. The X-ray structure of isomer B is presented.

Dioxygenation of Sterically Hindered (Ethene)RhI and -IrI Complexes to (Peroxo)RhIII and (Ethene)(peroxo)IrIII Complexes

New cationic, five-coordinate bis(ethene)iridium(I) complexes [(κ3-Me3-tpa)IrI(ethene)2]+ (12+) and [(κ3-Me2-dpa-Me)IrI(ethene)2]+ (13+) have been prepared {Me3-tpa = N,N,N-tris[(6-methyl-2-pyridyl)methyl]amine, Me2-dpa-Me = N-methyl-N,N-bis[(6-methyl-2-pyridyl)methyl]amine}. Complexes 12+ and 13+ lose one ethene fragment in solution, yielding the five-coordinate mono(ethene) complex [(κ4-Me3-tpa)IrI(ethene)]+ (14+) and the four-coordinate mono(ethene) complex [(κ3-Me2-dpa-Me)IrI(ethene)]+ (15+), respectively. [(κ4-Me3-tpa)RhI(ethene)]+ (11+), the rhodium analogue of 14+, was also prepared. Whereas rhodium complex 11+ is stable in acetonitrile at room temperature, the iridium analogue 14+ converts to the cyclometallated (acetonitrile)(hydrido) complex 16+ within 72 h by dissociation of the unique 6-methylpyridyl fragment and oxidative addition of the 6-methylpyridyl C3−H bond. The four-coordinate mono(ethene) complex 15+ is even less stable in solution; it converts to a mixture of compounds within 18 h. Reaction of the mono(ethene)RhI complex 11+ with O2 yields the peroxo complex 17+ by ethene displacement. In contrast, the mono(ethene)IrI complexes 14+ and 15+ bind O2 without the loss of ethene, leading to unprecedented (ethene)(peroxo)IrIII complexes 18+ and 19+. At room temperature, peroxo complex 17+ does not react with ethene and, quite remarkably, C−O bond formation does not occur in the (ethene)(peroxo) complexes 18+ and 19+. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Ethylene coordination, insertion, and chain transfer at a cationic aluminum center: A comparative study with Ab Initio correlated level and density functional methods

The performance of correlated ab initio methods and DFT methods was compared for the propagation and chain transfer steps of ethylene polymerization by a model aluminum–amidinate system, [{HC(NH)2}AlCH2CH3]+. All methods agree that the main chain transfer mechanism is β‐hydrogen transfer to the monomer (BHT), and that this is substantially easier than propagation; implications for the real Jordan system are discussed briefly. Counterpoise corrections are necessary to obtain reasonable olefin complexation energies. Activation energies are consistently lower at DFT (BP86, B3LYP) than at ab initio levels [MP2, MP3, MP4, CI, CCSD(T)]; the differences are particularly large (16 kcal/mol) for the BHT reaction. This is suggested to be related to the known problem of DFT in describing hydrogen bridged systems.