Complexes involving the W(II) (acac)2 fragment have been synthesized from a convenient pentacarbonyl iodide tungsten anionic starting material. Oxidation with elemental iodine and subsequent addition of the acac anion via deprotonation of 2,4-pentanedione, results in a reactive eighteen electron tricarbonyl complex, W(CO)3(acac)2. This complex readily reacts at ambient temperature with alkynes, phosphines, and nitriles to liberate carbon monoxide and coordinate the incoming ligand.
This chemistry mirrors similar work done previously with two dialkyldithiocarbamate ancillary ligands, however, the oxygen donor acac ligands avoid side reactions such as oxidation and cleavage which hampered the reactivity of the W(II)(dtc)2 system. Further studies will include attempts at sidebound nitriles, and the reactivity of those complexes, as well as using b-diketiminate ligands (NacNac) to mimic the oxygen and sulfur donor system.
Aryloxycarbyne complexes of the general formula Tp'(CO)2MºC(p-OC6H4R) (M = Mo, W and R = NO2, H, CH3, OMe) were synthesized1 via chloride displacement from the chlorobyne complex 1 by the corresponding aryloxide (Scheme 1).
This synthetic methodology also provides a route to cationic phosphonium carbyne complexes [Tp'(CO)2MºCPR3][PF6] (M = Mo, W and R = PMe2Ph, PPh3, PCy3) (3).1 Since treatment of 1 with KO(p-C6H4NO2) gave poor yields of the aryloxycarbyne 2a, the cationic phosphonium carbyne complexes were used as reagents. However, treatment of 2a with KO(p-C6H4NO2) produced the neutral h2-ketenyl coupled product 4 by nucleophilic addition to the metal (Scheme 2). Attempts to make the p-methoxyphenoxide
coupled product by the analogous reaction generates exclusively the aryloxycarbyne 2d.2 The only difference between the two reactions is the electronic nature of the para substituents on the phenoxide nucleophile so variation of the electronic properties of the nucleophile should expose the details of the different reactivities.
1. Jamison, G. M.; White, P. S.; Templeton, J. L. Organometallics 1991, 10, 1954-59.
2. G.M. Jamison, P. S. White, and J. L. Templeton; unpublished results.
Reduction of imines can occur stereoselectively, but the ratio of stereoisomers often correlates directly to the E/Z ratio of the starting imine reagent. There are several examples of metal-coordinated imines that add nucleophiles stereoselectively. However, nucleophile addition is complicated with coordinated, N-protio substituted imines as the substrate, since deprotonation of the imine nitrogen becomes a competing reaction. In order to address this issue, we seek to utilize secondary amine precursors, and our initial target was a cyclic imine complex based on pyrrolidine. Cyclic imine complexes have two potential advantages as substrates: (1) they are not subject to E/Z isomer options, (2) they have an N-alkyl substituent that eliminates deprotonation at nitrogen as an option.
Studies of the diastereoselectivity of nucleophile addition to the imine complex are accessible with this system. Future studies will include exploration of the reactivity of piperidine and azetidine ligated complexes.
I - Water Gas Shift Chemistry and H2 Elimination
We are currently pursuing new chemistry of d6 Tp'Pt(IV) complexes. We reported the formation of a stable methyl hydride complex, Tp'PtMe2H, in 1996, and have recently completed the syntheses of Tp'PtMe(H)2 and Tp'PtH3 via WGS chemistry. Interconversions of Pt(IV) and Pt(II) complexes via acid induced reductive elimination of methane from Tp'PtMe(R)H (R = Me, H) have generated an array of protonated [(HTp')Pt(II)(R)L] + products that can be deprotonated to form neutral products, Tp'Pt(II)MeL. Currently we are attempting to study the reductive elimination of hydrogen from Tp'Pt(R)H2 (R=Me, H). We have found that protonation of these species at low temperature in the presence of a strong π-acid that elimination will occur. Low temperature NMR experiments have shown that the elimination occurs directly from a 6-coordinate π-acid trapped Pt(IV) cation. Future work will attempt to modify the system in order to make it active for WGS catalysis.
II - C-H Bond Activation Processes
Lewis acid assisted methane elimination are also under investigation. Thermolysis of Tp'PtMeH2 in the presence of B(C6F5 )3 in aromatic solvents (Ar-H) leads to the formation of C-H bond activation products, Tp'PtAr(H) 2. Mechanistic studies on methane elimination from Tp'PtMe2H complement the synthetic and structural investigations outlined above.
Gentle heating of Tp'PtMe2H with B(C6F5 )3 in selected alkane solvents leads to formation of Tp'Pt(alkyl)(Me)(H) complexes. When the alkyl has an available b-hydrogen, these complexes convert to Tp'Pt(η2-olefin)H upon further heating. Protonation of Tp'Pt(R)(H)2 [R= Me or Ph] at -78°C and addition of alkenes followed by deprotonation also yields Tp'Pt(η>2-olefin)H complexes. These hydride complexes and analogous Tp'Pt(η2-olefin)(R) complexes [R= Ph, Me, Et] are being investigated. Alkane activation competition experiments, coordinated olefin isomerization reactions, and insertion of ethylene into Pt-R bonds (R =H or Ph) are being studied.
The coordination of transient species to transition metals enables the thorough study of otherwise unstable reaction intermediates. Vinylidene, HC=C :, a tautomer of ethylene, has been implicated as an intermediate in a variety of organic transformations, including α-eliminations from vinyl halides and related compounds.
Recent efforts in our lab, in collaboration with Professor Fergus J. Lalor at University College in Cork, Ireland have led to the synthesis of the first examples of air and water stable anionic vinylidene complexes, specifically a series of mononuclear Group VI vinylidene complexes of the type [NEt4][Tp′(CO)2Mo=C=C(CN)(p-X-C6H4)].
Variation of the substituent in the para position of the aryl group provides a handle for altering charge delocalization and thus probing the role of electronic factors in the behavior of these anions. Variable temperature 1H NMR studies have revealed dynamic behavior in these complexes. In addition, reactivity with electrophiles at the β-carbon and spontaneous oxidative conversion to carbyne complexes Tp′(CO)2Mo ºC-C(O)(p-X-C6H4) in the presence of O2 are being investigated.