T. Keith Hollis

Air- and Water-Stable Catalysts for Hydroamination/Cyclization. Synthesis and Application of CCC−NHC Pincer Complexes of Rh and Ir

The scope of CCC-NHC pincer complex synthetic methodology by metalation/transmetalation has been extended to Ir. Structural characterization revealed that it is isomorphous with the Rh complex. Both Rh and Ir complexes are efficient catalysts for the hydroamination/cyclization of secondary amines in the presence of air and/or water.

Palladium catalyzed enantioselective rearrangement of allylic imidates to allylic amides

Abstract Recent progress in developing asymmetric Pd(II) catalysts for the [3,3]-sigmatropic rearrangement of allylic imidates to form enantioenriched allylic amides is reviewed. The best catalysts developed to date generate allylic amides in >90% enantiomeric excess (ee) from some allylic imidate substrates. The most recent catalysts developed in our laboratories are based on cyclopalladated ferrocenes having elements of planar chirality. A mechanistic proposal that enantioselection in Pd(II)-catalyzed allylic imidate rearrangements might derive from axial approach of the alkene to the chiral Pd(II) catalyst has guided our design of improved ligands.

Cyclopalladated ferrocenyl amines as enantioselective catalysts for the rearrangement of allylic imidates to allylic amides

Abstract The first examples of the use of cyclopalladated complexes in enantioselective catalysis are reported. Cyclopalladated ferrocenyl amine 6f promotes the rearrangement of some 2-alkenyl imidates to allylically transposed amides in excellent yield and moderate enantioselectivity.

An Efficient Synthesis of Bis-1,3-(3′-aryl-N-heterocycl-1′-yl)arenes as CCC-NHC Pincer Ligand Precursors

A report that demonstrated an efficient methodology for the arylation of imidazoles has been extended to bis(N-heterocyclic) compounds. Using bis(aryl) iodonium salts provides high-yielding access to CCC-NHC ligand precursors in a single step. Examples of arylation using various iodonium salts are reported herein with an investigation into the factors governing their relative rate of reactivity. The metalation of one of these compounds using Zr(NMe2)4 and its subsequent treatment with [Pt(COD)Cl2] to yield a transmetalated product are reported.

The first structural characterization and determination of the isomerization activation parameters of a chiral phosphatitanoceneElectronic supplementary information (ESI) available: X-ray structure determination. See http://www.rsc.org/suppdata/cc/b2/b208945a/

The activation parameters (delta G++298 = 11.5 (+/- 1.0) kcal mol-1, delta H++ = 16.3 (+/- 3.0) kcal mol-1, delta S++ = 16 (+/- 11) cal mol-1 K-1) have been determined for the rac to meso isomerization of a phosphametallocene, bis(3,4-dimethyl-2-phenylphospholyl)titanium dichloride, 2, which has been structurally characterized.

Direct Observation of Ion Pairing at the Liquid/Solid Interfaces by Surface Enhanced Raman Spectroscopy

Ion-pairing, the association of oppositely charged ionic species in solution and at liquid/solid interfaces has been proposed as a key factor for a wide range of physicochemical phenomena. However, experimental observations of ion pairing at the ligand/solid interfaces are challenging due to difficulties in differentiating ion species in the electrical double layer from that adsorbed on the solid surfaces. Using surface enhanced Raman spectroscopy in combination with electrolyte washing, we presented herein the first direct experimental evidence of ion pairing, the coadsorption of oppositely charged ionic species onto gold nanoparticles (AuNPs). Ion pairing reduces the electrolyte concentration threshold in inducing AuNP aggregation and enhances the competitiveness of electrolyte over neutral molecules for binding to AuNP surfaces. The methodology and insights provided in this work should be important for understanding electrolyte interfacial interactions with nanoparticles.

Experimental and Computational Studies of the Mechanisms of Hydroamination/Cyclisation of Unactivated α,ω-Amino-alkenes with CCC-NHC Pincer Zr Complexes*

Four new CCC-NHC pincer Zr complexes have been synthesised, characterised, and used in mechanistic studies in the hydroamination/cyclisation of unactivated amino-alkenes. These Zr pre-catalysts will cyclise a primary amino-alkene, but no reaction was observed for a secondary amino-alkene even in the presence of a primary amine. The empirical rate law, experimentally determined activation parameters, and kinetic isotope effects (KIEs) are reported. Several possible mechanisms, including amido- versus imido-insertion and concerted-insertion versus [2 + 2] cycloaddition mechanisms, were modelled computationally at the PBEPBE level of theory with double-zeta quality basis sets. The formation of a catalytically relevant imido complex via the monoamido complexes was accompanied by in situ formation of ammonium salts of the substrates. The experimental and computational data are consistent with an imido-[2 + 2] cycloaddition mechanism for the CCC-NHC pincer diamido Zr complexes that follow saturation kinetics under catalytically relevant concentrations.

CCC-NHC Pincer Zr Diamido Complexes: Synthesis, Characterisation, and Catalytic Activity in Hydroamination/Cyclisation of Unactivated Amino-Alkenes, -Alkynes, and Allenes*

2-(1,3-Bis-3′-butylimidazol-1′-yl-2′-ylidene)phenylene)bis(dimethylamido) iodo zirconium(iv) (3) and 2-(1,3-bis-3′-butylimidazol-1′-yl-2′-ylidene)phenylene)bis (dimethylamido) bromo zirconium(iv) (4), have been prepared via a modification of the solvent and stoichiometry from the previously reported methodology. The reactivity of 3 and 4 in hydroamination/cyclisation is reported. Both diamido complexes have been found to improve catalytic activity as compared with the previously reported mono-amido analogues. Complexes 3 and 4 were observed to be selective for primary amines over secondary amines in hydroamination/cyclisation. The lack of reactivity with secondary amines is consistent with a mechanism involving requisite formation of a Zr-imido intermediate.

Ferroelectric bimorph cantilever with self-assembled silane layer

Bimorph ferroelectric microcantilevers are fabricated at the locations of the interdomain walls of inversely poled lithium niobate. The cantilevers are excited via the piezoelectric effect using a rf voltage and the natural vibrations are observed via optical detection. The self-assembling layers of two different silanes are chemically deposited on the microcantilevers. The presence of the silane layers is detected by two independent methods: optical detection of a low frequency shift of cantilever natural vibrations and the analysis of Raman spectral lines from the Silane layers. Possible applications include smart biosensors in real time domains.

CCC-NHC Pincer Complexes

Abstract The synthesis of organometallic CCC-NHC pincer complexes is discussed along with their applications. Such complexes span the transition metals of the periodic table with examples including group 4, 5, 8, 9, and 10 metals. Lanthanide, group 3, Ir, and Pd complexes of C^C^C-NHC pincer ligands are also reported. Such a variety highlights the broadly applicable nature of the CCC-NHC ligand set. Organometallic CCC-NHC pincer complexes are employed as efficient catalysts and emitters. Such complexes are achieved via three major routes: metalation with amido regents, transmetalation from early to late metals, or by direct metalation of a suitable proligand. Yields of these syntheses vary greatly, depending upon the metal and the reaction conditions. Researchers have used the modular nature of the ligand precursors to engineer around unwanted side products.