N. Gabriel Lemcoff

Predicting the Cis−Trans Dichloro Configuration of Group 15−16 Chelated Ruthenium Olefin Metathesis Complexes: A DFT and Experimental Study

Gradient-corrected (BP86) and hybrid (M06-L) density functional calculations were used to study the relative stability of cis and trans-dichloro X-chelated benzylidene ruthenium complexes (X = O, S, Se, N, P). Calculations in the gas phase differed from experimental results, predicting the trans-dichloro configuration as being more stable in every case. The addition of Poisson-Boltzmann (PBF) continuum approximation (dichloromethane) corrected the disagreement and afforded energies consistent with experimental results. Novel N, Se, and P chelated ruthenium olefin metathesis complexes were synthesized to evaluate calculation predictions. These findings reinforce the importance of including solvent corrections in DFT calculations of ruthenium metathesis catalysts and predict that stronger sigma donors as chelating atoms tend to electronically promote the unusual and less active cis-dichloro configuration.

Studies on Electronic Effects in O‐, N‐ and S‐Chelated Ruthenium Olefin‐Metathesis Catalysts

A short overview on the structural design of the Hoveyda-Grubbs-type ruthenium initiators chelated through oxygen, nitrogen or sulfur atoms is presented. Our aim was to compare and contrast O-, N- and S-chelated ruthenium complexes to better understand the impact of electron-withdrawing and -donating substituents on the geometry and activity of the ruthenium complexes and to gain further insight into the trans-cis isomerisation process of the S-chelated complexes. To evaluate the different effects of chelating heteroatoms and to probe electronic effects on sulfur- and nitrogen-chelated latent catalysts, we synthesised a series of novel complexes. These catalysts were compared against two well-known oxygen-chelated initiators and a sulfoxide-chelated complex. The structures of the new complexes have been determined by single-crystal X-ray diffraction and analysed to search for correlations between the structural features and activity. The replacement of the oxygen-chelating atom by a sulfur or nitrogen atom resulted in catalysts that were inert at room temperature for typical ring-closing metathesis (RCM) and cross-metathesis reactions and showed catalytic activity only at higher temperatures. Furthermore, one nitrogen-chelated initiator demonstrated thermo-switchable behaviour in RCM reactions, similar to its sulfur-chelated counterparts.

Polycyclooctadiene Complexes of Rhodium(I): Direct Access to Organometallic Nanoparticles

The search for new materials with novel properties continues to be a main driving force in materials science. For example, organic nanoparticles have recently emerged as valuable compounds for various applications. Some of the proposed uses for organic nanoparticles are as vehicles for controlled release of small molecules, the encapsulation of fluorescent markers, and the change of physical properties of materials. One of the methods to generate organic nanoparticles is the intramolecular cross-linking of dendrimers or polymers, which is usually achieved by the formation of a covalent bond between pendant functional groups on the polymer chain. Alternatively, coordination chemistry can be used for the well-defined intramolecular cross-linking of polymers, but this method is not widespread. This latter method has the advantage to intrinsically introduce a metal to the organic nanoparticle lattice. In this context, metal-containing polymers have become quite popular during the past few decades. The appeal of these hybrid compounds is attributed to their ability to impart some of the physical and functional properties of both the polymer and the incorporated metal to the material that is obtained. In this way, the advantage of facile fabrication of organic polymers can be combined with the magnetic, electronic, optical, and catalytic potential of metals. Thus, novel compounds such as conductive and semiconductive materials for electroluminescence, photovoltaics, electrocatalysis, and nanotechnological applications may be devised. The most common synthetic methodologies to incorporate a metal to a polymer are either the use of metalcontaining monomers or the coordination of the metal to a pre-existing polymer chain that contains suitable ligands. As an example of the first approach, Gladysz and co-workers recently reported the synthesis of p-bound iridium complexes of polyacetylene through the ring-opening metathesis polymerization of [(h-C5H5)Ir(h -C6H6)] by using Schrockand Grubbs-type initiators. The metallopolymer thus obtained consisted of cis-butadiene moieties bound to cyclopentadienyl–iridium complexes. Examples based on the latter strategy were reported by the groups of Wang and Manners. In the first case, stable metal-containing micelles were prepared through coordination of metal ions to a terdendate-ligand-grafted PVP block of poly(4-vinylpyridine)-bpoly(ethyleneoxide). In the second case, reversible crosslinking by Pt–olefin coordination of polyisoprene-poly(ferrocenyldimethylsilane) micelles was shown. Herein, we report a very convenient and straightforward synthetic route to well-defined metal-containing polymeric nanoparticles of ROMP-derived poly(1,5-cyclooctadiene) (poly(COD)) that is intramolecularly cross-linked by p-bound rhodium(I) fragments, and study their physical properties. The traditional procedure for the preparation of Rh/ diene complexes such as 2 involves treatment of RhCl3·H2O with 1,5-cyclooctadiene in hot, deoxygenated aqueous ethanol in the presence of sodium carbonate. However, this procedure would not be suitable for the preparation of organometallic complexes of poly(COD), chiefly due to the poor solubility of poly(COD) in polar solvents. Thus, an alternative route starting from the commercially available chlorobis(ethylene)rhodium(I) dimer (1), which possesses two labile ethylene molecules that are coordinated to the rhodium metal, was developed. In order to obtain a mechanistic understanding of the exchange process, we first attempted a simple ligandexchange reaction of complex 1 with 1,5-cyclooctadiene (COD) and 1,5-hexadiene (Scheme 1). Mixing of COD and 1 in toluene afforded the expected chloro(1,5-cyclooctadiene)rhodium(I) dimer (2) through the smooth exchange of ethylene with COD moieties in just one hour at room temperature. An upfield shift of the proton signal from d = 5.53 ppm to d = 4.23 ppm in the H NMR spectrum could be readily observed, confirming the formation of 2 (see the

Light-induced olefin metathesis

Summary Light activation is a most desirable property for catalysis control. Among the many catalytic processes that may be activated by light, olefin metathesis stands out as both academically motivating and practically useful. Starting from early tungsten heterogeneous photoinitiated metathesis, up to modern ruthenium methods based on complex photoisomerisation or indirect photoactivation, this survey of the relevant literature summarises past and present developments in the use of light to expedite olefin ring-closing, ring-opening polymerisation and cross-metathesis reactions.

A general approach to mono- and bimetallic organometallic nanoparticles

A comprehensive methodology to prepare nanometric size organometallic particles (ONPs) containing rhodium(I), iridium(I) and nickel(0) with ROMP-derived polycycloocta-1,5-diene (pCOD) by a controlled single chain collapse mechanism was developed. The polymeric complexes could be produced via direct exchange of the respective labile ligands of metal complexes by the 1,5-hexadiene elements in pCOD, or via in situ reduction of metal ions in the presence of the polymer. These well-defined π-bound polymeric complexes were characterized by UV-Vis spectroscopy, dynamic light scattering (DLS) and size exclusion chromatography (SEC) measurements and the resulting polymer sizes were found to be inversely proportional to the amount of metal added due to concomitant single chain collapse. Moreover, these procedures were readily extended to the synthesis of organobimetallic nanoparticles containing two metals; which could be added in commutative order and specific metal ratios. The embedded metal elements were found to be readily accessible for applications in catalysis, where the close proximity of the catalytic centers led to distinctive reactivity compared to the isolated complexes.

Practical synthesis of water-soluble organic nanoparticles with a single reactive group and a functional carrier scaffold

A new approach to prepare functional organic nanoparticles (ONPs) from linear polymers is described. The nanoparticles are obtained by ring-opening metathesis polymerisation (ROMP) of functionalised norbornene dicarboximides, side-chain amidation with tri-O-allyl-TRIS, and ring-closing metathesis (RCM). The synthesis is quite flexible and mild, allowing preparation of organic- and aqueous-soluble particles with narrow molecular weight (MW) distributions that are tunable, ranging from MW ≈ 10 to >100 kD with diameters between ca. 5 and 50 nm. The use of functional monomer(s) and/or chain-transfer agents (CTAs) leads to the fully controlled synthesis of nanoparticles containing single or multiple reactive functional groups. This non-toxic ONP scaffold can be readily used as a protective carrier for a broad range of groups using the appropriate monomers, such as a fluorescein-functionalised monomer that affords fluorescent ONPs with significantly enhanced photostability.

Facile acetal dynamic combinatorial library

We herein report our first results on the use of simple acetalation chemistry in the service of dynamic combinatorial libraries (DCLs); the reaction between triethylene glycol and 4-nitrobenzaldehyde afforded a DCL of more than 15 cyclic and acyclic species; all of which were separated and characterized; the smaller macrocyclic compounds were successfully amplified by the use of ammonium ions.

New supramolecular host systems. 41. novel diacetal podands, diazacrowns and cryptands

Abstract New and important key podands bearing the cis -1,3,5,7-tetraoxadecalin (TOD) core are reported, viz., 2,6-di(hydroxymethyl)- ( 2 ) and 2,6-di(aminomethyl)- cis -1,3,5,7-TOD ( 6 ), which lead to the first diazacrown-TOD compounds ( 9a ) and ( 9b ) and a corresponding cryptand ( 11 ) in the series. 9a&b exhibit very good alkali and earth-alkali ion inclusion ability with selectivity for K + and Ba +2 . Molecular dynamics (Insight/Discover/AMBER) on the crowns and cryptand were performed, including reparametrized MM3-GE calculations. A convincing case for the TOD based hosts was thus established.

A Light-Activated Olefin Metathesis Catalyst Equipped with a Chromatic Orthogonal Self-Destruct Function.

A sulfur-chelated photolatent ruthenium olefin metathesis catalyst has been equipped with supersilyl protecting groups on the N-heterocyclic carbene ligand. The silyl groups function as an irreversible chromatic kill switch, thus decomposing the catalyst when it is irradiated with 254 nm UV light. Therefore, different types of olefin metathesis reactions may be started by irradiation with 350 nm UV light and prevented by irradiation with shorter wavelengths. The possibility to induce and impede catalysis just by using light of different frequencies opens the pathway for stereolithographic applications and novel light-guided chemical sequences.

Regioselective Chromatic Orthogonality with Light‐Activated Metathesis Catalysts

The ability to selectively guide consecutive chemical processes towards a preferred pathway by using light of different frequencies is an appealing concept. Herein we describe the coupling of two photochemical reactions, one the photoisomerization and consequent activation of a sulfur-chelated latent olefin-metathesis catalyst at 350 nm, and the other the photocleavage of a silyl protecting group at 254 nm. Depending on the steric stress exerted by a photoremovable neighboring chemical substituent, we demonstrate the selective formation of either five- or six-membered-ring frameworks by light-triggered ring-closing metathesis. The orthogonality of these light-induced reactions allows the initiation of these processes independently and in interchangeable order, according to the wavelength of light used to promote them.