René de Gelder

Report on the sixth blind test of organic crystal structure prediction methods

The results of the sixth blind test of organic crystal structure prediction methods are presented and discussed, highlighting progress for salts, hydrates and bulky flexible molecules, as well as on-going challenges.

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.

Azetidine based ligands in boron catalyzed asymmetric Diels-Alder reactions

Abstract The preparation of a new class of azetidine-based auxiliaries and their selectivity in the BBr3 catalyzed Diels-Alder reaction is described. The results are compared with a similar proline-derived ligand and a known prolinol auxiliary. Results show that selectivities are highly dependent on the dienophile and the substituent of the chiral auxiliary.

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.

Self-association and self-assembly of molecular clips in solution and in the solid state

Clip molecules based on diphenylglycoluril form well-defined dimeric structures in chloroform solution and in the solid state. In solution the dimerization process is based on favourable π-π interactions and cavity filling effects. A combination of favourable π-π interactions and crystal packing forces determine the self-assembly of clips in the solid state. The geometry that the clip molecules adopt in solution and in a series of X-ray crystal structures is compared with favourable geometries predicted by molecular modelling calculations.

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.

Synthesis, characterisation and chiroptical properties of ‘click’able polyisocyanopeptides

Rigid rod polyisocyanopeptides have been synthesised containing acetylene groups on the side arms as scaffolds for multifunctional derivatisation by the copper-catalysed click reaction between an acetylene and an azide. All materials were characterised in detail by spectroscopic procedures and for the processable polymers, atomic force microscopy was used to determine the molecular weight parameters. The solubility properties of the synthesised macromolecules are very dependent on the stereochemistry and/or the presence of solubilising trimethylsilyl groups on the acetylene function. The potential for derivatisation of the acetylene-containing materials using click chemistry was successfully demonstrated by the reaction of these polymers with aliphatic tails functionalised with azide moieties.

Squaring cooperative binding circles

The cooperative binding effects of viologens and pyridines to a synthetic bivalent porphyrin receptor are used as a model system to study how the magnitudes of these effects relate to the experimentally obtained values. The full thermodynamic and kinetic circles concerning both activation and inhibition of the cage of the receptor for the binding of viologens were measured and evaluated. The results strongly emphasize the apparent character of measured binding and rate constants, in which the fractional saturation of receptors with other guests is linearly expressed in these constants. The presented method can be used as a simple tool to better analyze and comprehend the experimentally observed kinetics and thermodynamics of natural and artificial cooperative systems.