Cyclometalated phosphine-based pincer complexes: Mechanistic insight in catalysis, coordination, and bond activation

One of the initial steps in the development of therapeutic agents is the identification of lead compounds that bind to the receptor or enzyme target of interest. Many analogs of these lead compounds are then synthesized to define the key recognition elements for maximal activity. In general, many compounds must be evaluated in both the lead identification and optimization steps. Increasing burdens have been placed on these efforts due to the large number of new therapeutic targets that continue to be identified thorough modern molecular biology methods.1 To address this demand, very powerful chemical and biological methods have been developed for the generation of large combinatorial libraries of peptides2 and oligonucleotides3 that are then screened against a receptor or enzyme to identify high-affinity ligands or potent inhibitors, respectively. While these studies have clearly demonstrated the power of library synthesis and screening strategies, peptides and oligonucleotides generally have poor oral activities and rapid in vivo clearance;4 therefore their utility as bioavailable therapeutic agents is often limited. Due to the favorable pharmacokinetic properties of many small organic molecules (<600-700 molecular weight),5 the design, synthesis, and evaluation of libraries of these compounds6 has rapidly become a major frontier in organic chemistry. Lorin A. Thompson was born in Lexington, KY, in 1970. He received the Bachelor of Science degree from the University of North Carolina, Chapel Hill, in 1992 where he worked under the guidance of Joseph Desimone. He is currently pursuing his doctorate in the laboratory of Jonathan Ellman at UC Berkeley where he is the 1994 Glaxo-Wellcome fellow. His research interests include the development of synthetic methodology for organic library construction.

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Synthesis and Applications of Small Molecule Libraries.

http://alpha.chem.umb.edu/chemistry/ch611/documents/C-Htransitionmetalbonds.pdf

Carbonhydrogen-transition metal bonds

C-H oxidation has a long history and an ongoing presence in research at the forefront of chemistry and interrelated fields. As such, numerous highly useful articles and reviews have been written on this subject. Logically, these are generally written from the perspective of the scope and limitations of the reagents employed. This Minireview instead attempts to emphasize chemoselectivity imposed by the nature of the substrate. Consequently, many landmark discoveries in the field of C-H oxidation are not discussed, but hopefully the perspective taken herein will allow C-H oxidation reactions to be more readily incorporated into synthetic planning.

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If C-H bonds could talk: selective C-H bond oxidation.

Owing to their high uptake capacity at low temperature and excellent reversibility kinetics, metal–organic frameworks have attracted considerable attention as potential solid-state hydrogen storage materials. In the last few years, researchers have also identified several strategies for increasing the affinity of these materials towards hydrogen, among which the binding of H2 to unsaturated metal centers is one of the most promising. Herein, we review the synthetic approaches employed thus far for producing frameworks with exposed metal sites, and summarize the hydrogen uptake capacities and binding energies in these materials. In addition, results from experiments that were used to probe independently the metal–hydrogen interaction in selected materials will be discussed.

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Hydrogen Storage in Microporous Metal-Organic Frameworks with Exposed Metal Sites

Reported were the first examples of isolable transition-metal complexes containing a coordinated dihydrogen molecule, characterized by a variety of spectroscopic and structural methods to possess n/sup 2/-bonded hydrogen. The dihydrogen ligand is symmetrically coordinated in an n/sup 2/ mode with average tungsten-hydrogen distances of 1.95 (23) angstroms (x-ray) and 1.75 angstroms (neutron, ..delta..F). The H-H separation is 0.75 (16) angstrom (x-ray) and 0.84 angstroms (neutron, ..delta..F), slightly larger than that obtained from free H/sub 2/ (0.74 angstroms). The H/sub 2/ ligand axis is approximately parallel to the trans phosphorous-phosphorus direction. Vibrational spectra of solid samples of the H/sub 2/, D/sub 2/, and HD forms (M = W) are consistent with coordination of molecular H/sub 2/. The H/sub 2/ complexes are significant in that they may represent an arrested form of oxidative addition of H/sub 2/ to metals. 1 table (DP)

Characterization of the first examples of isolable molecular hydrogen complexes, M(CO)3(PR3)2(H2) (M = molybdenum or tungsten; R = Cy or isopropyl). Evidence for a side-on bonded dihydrogen ligand

Redetermination of the crystal structure of dimanganese decacarbonyl and determination of the crystal structure of dirhenium decacarbonyl. Revised values for the manganese-manganese and rhenium-rhenium bond lengths in dimanganese decacarbonyl and dirhenium decacarbonyl

The synthesis, reactivity, and molecular structures of coordinatively and electronically unsaturated complexes M(CO)/sub 3/(PR/sub 3/)/sub 2/ (M = Mo, W; R = Cy, i-Pr) are described. Ligands that bind reversibly to W(CO)/sub 3/(PCy/sub 3/)/sub 2/ include H/sub 2/, N/sub 2/, C/sub 2/H/sub 4/, H/sub 2/O, ROH, and thiophene; irreversibly bound ligands are MeCN, pyridine, NH/sub 3/, CyNH/sub 2/, and PR/sub 3/. The structures of W(CO)/sub 3/(PCy/sub 3/)/sub 2/ and W(CO)/sub 3/(P(i-Pr)/sub 3/)/sub 2/ involve incipient intramolecular oxidative addition of a distal phosphine C-H bond to the metal. The three-center M...H-C interaction in W(CO)/sub 3/(PCy/sub 3/)/sub 2/ has W-H(11a)-C(11) = 127.6/sup 0/, W-C(11) = 2.945 (6) A, and W-H(11a) approx. 2.27 A. Crystal data for W(CO)/sub 3/(PCy/sub 3/)/sub 2/ are the following: space group P1, a = 10.300 (1) A, b = 12.675 (2) A, c = 15.473 (1) A, ..cap alpha.. = 91.44 (1)/sup 0/, ..beta.. = 90.37 (1)/sup 0/, ..gamma.. = 103.99 (1)/sup 0/, Z = 2, rho(calcd) = 1.34 g cm/sup -3/, R = 0.025, 6211 reflections. Crystal data for W(CO)/sub 3/(P-i-Pr/sub 3/)/sub 2/ are the following: P2/sub 1//m, a = 8.425 (1) A, b = 13.375 (2) A, c = 12.039 (2) A, ..beta.. = 109.58 (1)/supmore » 0/, Z = 2, rho(calcd) = 1.53 g cm/sup -3/, R = 0.034, 2212 reflections. Since these molecules add dihydrogen to form stable eta/sup 2/-H/sub 2/ complexes, they afford an unprecedented opportunity to observe both H-H and C-H bond activation at a single metal center.« less

Molecular hydrogen complexes of the transition metals. 2. Preparation, structure, and reactivity of W(CO)3(PCy3)2 and W(CO)3(P-iso-Pr3)2, .eta.2-H2 complex precursors exhibiting metal.cntdot..cntdot..cntdot.hydrogen-carbon interaction.

Das Tetrachloroalkylidinwolframat (I) reagiert mit Alkinen zu Produkten, fur die aufgrund einer Rontgenstrukturanalyse des Methyl-Derivats (RG P21/c, Z=4) die Cyclopentadienylalkin-Struktur (III) bestatigt wird.

Reaction of tungsten(VI) alkylidyne complexes with acetylenes to give tungstenacyclobutadiene and cyclopentadienyl complexes

Multiple metal-carbon bonds. 25. Preparation and structure of tungsten neopentylidene hydride, neopentylidene carbonyl, and neopentylidene ethylene complexes

Harvey J. Wasserman

Papers

Characterization of the first examples of isolable molecular hydrogen complexes, M(CO)3(PR3)2(H2) (M = molybdenum or tungsten; R = Cy or isopropyl). Evidence for a side-on bonded dihydrogen ligand

Redetermination of the crystal structure of dimanganese decacarbonyl and determination of the crystal structure of dirhenium decacarbonyl. Revised values for the manganese-manganese and rhenium-rhenium bond lengths in dimanganese decacarbonyl and dirhenium decacarbonyl

Molecular hydrogen complexes of the transition metals. 2. Preparation, structure, and reactivity of W(CO)3(PCy3)2 and W(CO)3(P-iso-Pr3)2, .eta.2-H2 complex precursors exhibiting metal.cntdot..cntdot..cntdot.hydrogen-carbon interaction.

Reaction of tungsten(VI) alkylidyne complexes with acetylenes to give tungstenacyclobutadiene and cyclopentadienyl complexes

Multiple metal-carbon bonds. 25. Preparation and structure of tungsten neopentylidene hydride, neopentylidene carbonyl, and neopentylidene ethylene complexes