N-Heterocyclic carbenes (NHCs) and their palladium complexes have been developed to facilitate the formation of carbon-carbon and carbon-heteroatom bonds.[1] NHC complexes exhibit unique chemical properties such as strong Pd-NHC s bonding, which enhances the stabilities of active organometallic compounds relative to conventional phosphane complexes.[2] Moreover, chiral NHC ligands have been synthesized to promote asymmetric catalysis. [3] Most of the chiral NHC ligands that have been utilized for asymmetric Pd II catalysis are monodentate, as Lee and Hartwig demonstrated moderate enantioselectivities (71-76 % ee) in a-arylation.[4] However, monodentate ligands caused practical difficulties including concomitant formation of inactive palladium-ligand complexes, such as those with a trans conformation. Bidentate NHC ligands exhibited better stabilities and selectivities: Douthwaite reported better enantioselectivities (up to 92 % ee) for asymmetric allylic alkylation [5a, b] than reactions employing the corresponding monodentate ligand.[5c]We envisioned tridentate NHCs would enhance the stabilities of Pd II complexes and enantioselectivities of various asymmetric reactions. As depicted in Figure 1, we sought a "chiral {Pd(OAc) 2 } complex" and designed and synthesized novel chiral tridentate NHC-Pd II complexes (II). Notably, ligand systems with NHCs, amidates, and oxygen functionalities (a C,N,O triad) could exert high electron densities and strong coordination on the Pd II complexes to increase stabilities even in nucleophilic solvents such as water and alcohols. Therefore, labile ligands such as water, alcohols, and acetonitrile are likely to be removed easily and thus enhance the reactivities and efficiencies of NHC-Pd catalysts.We report herein the synthesis of chiral tridentate NHC-Pd II complexes and their applications in an asymmetric oxidative Heck-type reaction as a proof of concept.The preparation of chiral ligands 4 is shown in Scheme 1. Hydroxyamide compounds 2 were prepared by reduction of amino acids 1 and subsequent N-alkylation with bromoacetyl bromide. Treatment of 2 with benzimidazole in the presence of KOH in DMF provided compounds 3, which were subjected to methylation to yield the amido alcohol substituted benzimidazolium salts 4. The structure was confirmed by 1 H NMR spectroscopic analysis; new peaks assigned to the NCH 3 appeared at d = 4.18 (4 a) and 4.15 ppm (4 b). Also, the imidazole H resonances shifted significantly as expected for iodine salts, appearing at d = 9.55 (4 a) and 9.50 ppm (4 b).Because direct coordination of ligands 4 to palladium was not efficient under numerous conditions, the ligands were transferred to palladium via silver NHC complexes.[6] As described in Scheme 2, compounds 4 a and 4 b were treated with Ag 2 O in CH 2 Cl 2 to give silver NHC complexes.

A discrete, air, protic, and thermally stable (NNC)Ir(III) pincer complex was synthesized that catalytically activates the CH bond of methane in trifluoroacetic acid; functionalization using NaIO 4 and KIO 3 gives the oxy-ester.The most efficient methane hydroxylation catalysts utilize Pd II , Pt II , Hg II , or Au I/III cations that operate by a sequence of electrophilic CH bond activation (CHA) 1 followed by reductive oxy-functionalization (ROF). 2 Because of their electrophilic character, these catalysts are highly susceptible to poisoning by water or methanol, requiring strong acid solvents to minimize this inhibition. To minimize this inhibition we are modifying the highly effective Pt(bipyrimidine)Cl 2 motif 1b,3 to reduce its electrophilicity by increasing the electron density at the metal center.In 2003 Goddard and co-workers 4a used the mechanism that they had established 4b for the (bpym)PtCl 2 system to set up a quantum mechanical rapid prototyping (QMRP) strategy enabling 1000s of ligand-metal oxidation state and solvent conditions to be sampled rapidly. This study showed that Ir III complexed to an NNC ligand motif (a simplified version of 1-TFA) should react with methane by CHA with an activation barrier o30 kcal mol À1 , should not be poisoned by H 2 O, and could undergo reductive functionalization through an Ir V intermediate to generate a functionalized product.We report here the experimental realization of this predicted system. This study provides a lesson in using QM theory to examine a large number of systems to quickly narrow the candidates down to a select few, and shows the experimental processes required to develop a stable functional catalyst.The first step was to extend the functional groups in the simplified NNC ligand to form a stable framework using donating C-ligands. 3d However, the key challenges are ensuring catalyst stability, maintaining efficient CHA, 5-7 and developing compatible oxy-functionalization reactions. 3a,8,9 Because Ir(I) to Ir(III) conversion is likely with electrondonating ligands under protic and oxidizing conditions (e.g. Scheme 1A), 8 we focused our efforts on catalytic systems based on CHA with Ir(III) followed by either direct O-atom insertion 10 from YO and then hydrolysis (Scheme 1B) or oxidation by YO to an Ir V -CH 3 species, followed by ROF (Scheme 1C). 9 We observed that treatment of (NN)(NC)Ir(III)(Me)OTf (NN = k 2 -4,4 0 -di-tert-butyl-2,2 0 -bipyridine, NC = Z 2 -(N,C-3)-6-phenyl-2,2 0 -bipyridine) with PhI(TFA) 2 (PITFA) 8c leads to efficient ROF to generate methyl oxy-esters, presumably via an Ir V -CH 3 intermediate, we designed a catalyst that would activate the methane CH bond with an Ir(III) motif and then utilize a similar ROF route.We report an (NNC)Ir III pincer complex (NNC = Z 3 -6-phenyl-2,2 0 -bipyridine) that catalytically activates and functionalizes methane. Scheme 2 shows the synthetic route to the trans(Et,TFA)-(NNC tBu )Ir(TFA)(C 2 H 4 )Et, 1-TFA (TFA = trifluoroacetate). Treatment of 6-phenyl-4,4 0 -di-tert-butyl-2,2 0 -bip...

Heteromultimetallic hydride clusters containing both rare-earth and d-transition metals are of interest in terms of both their structure and reactivity. However, such heterometallic complexes have not yet been investigated to a great extent because of difficulties in their synthesis and structural characterization. Here, we report the synthesis, X-ray and neutron diffraction studies, and hydrogen addition and release properties of a family of rare-earth/d-transition-metal heteromultimetallic polyhydride complexes of the core structure type 'Ln(4)MH(n)' (Ln = Y, Dy, Ho; M = Mo, W; n = 9, 11, 13). Monitoring of hydrogen addition to a hydride cluster such as [{(C(5)Me(4)SiMe(3))Y}(4)(μ-H)(9)Mo(C(5)Me(5))] in a single-crystal to single-crystal process by X-ray diffraction has been achieved for the first time. Density functional theory studies reveal that the hydrogen addition process is cooperatively assisted by the Y/Mo heteromultimetallic sites, thus offering unprecedented insight into the hydrogen addition and release process of a metal hydride cluster.

Starkes Fluor: In Einklang mit theoretischen Studien zu α‐Fluorcarbanionen ergab eine Röntgenstrukturanalyse für das α‐Fluorbis(phenylsulfonyl)methid‐Anion eine pyramidale Konfiguration (siehe Bild). Anspruchsvolle Rechnungen und NMR‐spektroskopische Studien zeigten, dass elektronenziehende Substituenten die Eigenschaften von Bis(phenylsulfonyl)methid‐Anionen entscheidend beeinflussen.

Two chiral tetraphenylenes, 2,15-dideuteriotetraphenylene (7) and 2,7-dimethyltetraphenylene (15) were synthesized and resolved to address the tetraphenylene inversion barrier problem. Neutron diffraction investigation of enantiopure 7 showed that the molecule retained its chirality integrity during its synthesis from enantiopure precursors and therefore rules out the possibility of the tetraphenylene framework possessing a low-energy barrier to inversion. Thermal study on 15 and tetraphenylene 1 further revealed that their inversion barriers were not overcome up to 600 degrees C, at which temperature these compounds underwent skeletal contraction into triphenylene with activation energies of 62.8 and 58.2 kcal/mol, respectively. This result is supported by computational studies which yielded an inversion barrier of 135 kcal/mol for tetraphenylene as a consequence of the peri-hydrogen repulsions at its planar conformation.