Research Plans for the Period December 1, 1999 - November 30, 2000:

This coming year, we will focus our research on three of the items presented in our proposal where we have had success. These include: (1) carbon-carbon bond cleavage reactions, (2) fundamental studies of C-H bond cleavage reactions of trispyrazolylboraterhodium complexes, and (3) carbon-fluorine bond activation. We have made progress in each of these areas over the past year, as described in our report, and will continue our studies in these areas.

Our carbon-carbon bond cleavage study is based upon the notion that metal-phenyl bonds are the strongest metal-carbon bonds. Cleavage of the C-C bonds in biaryl systems will therefore give two very strong metal-aryl bonds, and consequently offers the most thermodynamically preferred situation for observing C-C cleavage.

We have had some success in C-C cleavage with biphenylene, a molecule with a weaker C-C bond than biphenyl. The success has to do with the use of several new platinum based metal systems for C-C cleavage. We have completed studies of the C-C cleavage of biphenylene by Pt(PEt3)3, which catalytically produces tetraphenylene. We have shown that small molecules such as carbon monoxide, isonitriles, and acetylenes can be catalytically inserted into the C-C bond. Over the next two years we will extend the studies to include palladium and nickel complexes for comparison. We will also continue studies of complexes of the type [Pt(chelating phosphine)]. Control of the bite angle of the chelate has been found to be an important factor in tailoring the reactivity of the PtL2 fragment, and we will take advantage of knowledge of these factors. The phosphine But2PCH2PBut2 has already been synthesized for these studies. It will be attached to platinum and a Pt0 precursor used to look for C-C activation of biaryl systems.

Another area that we will be investigating involves a continuation of our studies with the tris-pyrazolylborate fragment [Tp'Rh(CNCH2CMe3)] and the examination of new derivatives of the type [Tp'Rh(L)]. We are currently involved in detailed studies of the selectivities available to the intermediate alkane complexes, specifically, C-H insertion vs. dissociation vs. migration down the alkyl chain. By using deuterium labeling, we have been able to monitor the isomeric species involved and provide for the first time kinetic information about the dynamics of these intermediates. The work requires sophisticated kinetic modeling, and the results have been very well received at recent lectures at the ACS meeting, UC Berkeley, Harvard, and MIT. We expect to extend these studies to branched hydrocarbons in the future.

The third area we have been active in is CF bond activation of fluoroaromatics. We discovered a rhodium hydride that was capable of cleaving these bonds via a mechanism involving nucleophilic aromatic substitution. We have extended these reactions to zirconium and found entirely different pathways for CF bond cleavage involving electrophilic activation and radical pathways. These results have led to evidence for the generation of perfluorobenzyne as an intermediate and have also provided oligomers (n<20) of perfluoro-polyphenylene. We will extend this work to other zirconium, hafnium, and titanium compounds during the coming year to compare and contrast the different reactivities of these reactants.

Progress Report for the Period
December 1, 1998- November 30, 1999.

1. Tris-pyrazolylborate Studies.

We have made many advances in our studies of rhodium tris-pyrazolylborate complexes for C-H bond activation. Generation of the 16-electron fragment {[HB(3,5-dimethylpyrazolyl)3]-Rh(CNCH2CMe3)} (Tp'RhL) in the presence of cyclopropane results in CH activation of the hydrocarbon. The cyclopropyl hydride complex 1 rearranges in benzene solvent to the metallacyclobutane complex, , 2. Thermolysis of the rhodacyclobutane complex produces an h2-propylene complex 4. The related complex Tp'Rh(CN-2,6-xylyl)(C2H4) has been structurally characterized and displays h3-Tp' coordination, both in the solid state and in solution. Thermolysis of the rhodacyclobutane complex in the presence of neopentylisocyanide leads to insertion of isocyanide into both RhC bonds of the metallacycle. Cyclobutane undergoes CH but not CC bond cleavage. We have also established that 11B NMR spectroscopy of these compounds can be used to reliably assess the coordination geometry of the trispyrazolylborate ligand in solution.

Reaction of the complex Tp'Rh(CNneo)(CH=CH2)Cl (neo = CH2CMe3, Tp' = hydrotris(3,5-dimethylpyrazolyl)borate) with Cp2ZrH2 leads to the formation of Tp'Rh(CNneo)(CH=CH2)H, 5. This complex is also formed upon photolysis of a solution of Tp'Rh(CNneo)(PhN=C=Nneo) containing ethylene or by thermal reaction of Tp'Rh(CNneo)(c-hexyl)H with ethylene. The vinyl hydride complex rearranges to the more stable h2-ethylene complex 6 with a half-life of 8 h at 22 C. Photolysis of a solution of Tp'Rh(CNneo)(PhN=C=Nneo) in liquid propylene produces the allylic activation product Tp'Rh(CNneo)(CH2CH=CH2)H, 7, which rearranges (t = 3 days at 22 C) to the h2-propylene complex 8. Allylic activation is also seen with isobutylene to give 9, but loss of olefin is observed at 22 C in benzene solution to generate Tp'Rh(CNneo)(Ph)H (t = 16.6 h). Photolysis of a t-butylethylene solution of Tp'Rh(CNneo)(PhN=C=Nneo) produces the trans-vinyl hydride complex 10, which loses t-butylethylene to generate Tp'Rh(CNneo)(Ph)H (t = 113 days at 22 C).

Two sets of experiments have been performed that provide indirect evidence for the involvement of alkane s-complexes in oxidative addition/reductive elimination reactions of Tp'Rh(L)(R)H complexes (Tp' = tris-3,5-dimethylpyrazolylborate, L = CNCH2CMe3). First, the methyl deuteride complex Tp'Rh(L)(CH3)D was observed to rearrange to Tp'Rh(L)(CH2D)H prior to loss of CH3D. Similarly, Tp'Rh(L)(CD3)H rearranges to Tp'Rh(L)(CD2H)D prior to loss of CD3H. Second, the rate of elimination of methane from Tp'Rh(L)(CH3)H in benzene/perfluorobenzene solvent mixtures was found to be dependent upon the concentration of benzene, indicating an associative component to the reductive elimination of methane. Both of these processes, and their rates, are accommodated by the reversible formation of alkane s-complexes prior to dissociation of alkane.

 

 

 

2. C-C Bond Cleavage Studies

The complexes Pt(PEt3)3 and Pd(PEt3)3 cleave the C-C bond of biphenylene to give (PEt3)2Pt(2,2'-biphenyl), 1 and (PEt3)2Pd(2,2'-biphenyl), respectively. Heating (PEt3)2Pt(2,2'-biphenyl) in the presence of biphenylene leads to C-C cleavage of a second biphenylene to give (PEt3)2Pt(2,2'-tetraphenyl), 2, via a Pt(IV) intermediate. 2 reductively eliminates tetraphenylene at 115 ° C. At 120 ° C the reaction is catalytic, Pt(PEt3)3 or 1 convert biphenylene to tetraphenylene.

The nickel alkyne complexes (dippe)Ni(RC= CR), (R = Ph, Me, CO2Me, or CH2OCH3) were found to be catalysts for the conversion of biphenylene and excess alkyne into the corresponding 9,10-disubstituted phenanthrenes. Fluorenone was catalytically produced by heating (dippe)Ni(CO)2, biphenylene and CO. Catalytic insertion of 2,6-xylylisocyanide into the strained C-C bond of biphenylene was also achieved by heating (dippe)Ni(2,6-xylylisocyanide)2, excess biphenylene and 2,6-xylylisocyanide.

3. C-F Bond Cleavage Studies

The zirconium hydride dimer [Cp2ZrH2]2 reacts with C6F6 at ambient temperature to give Cp2Zr(C6F5)F as the major product along with Cp2ZrF2, C6F5H and H2. Neither the reaction rate nor the product ratio is affected by changes in H2 pressure or the concentration of C6F6. The reaction follows zero-order kinetics. A mechanism is shown in Scheme 1 below.

The thermal decomposition of Cp2Zr(C6F5)2 in THF results in the slow formation of Cp2Zr(C6F5)F and tetrafluorobenzyne. The same reaction performed in the presence of durene or furan also results in the formation of Cp2Zr(C6F5)F and the respective Diels-Alder adducts of tetrafluorobenzyne (Scheme 2). If Cp2Zr(C6F5)2 is heated in the presence of C6F6, linear chains of perfluoroarenes are rapidly generated along with Cp2Zr(C6F5)F (Scheme 3). The disappearance of Cp2Zr(C6F5)2 is observed to slow dramatically after 30-80 % completion, with the extent of reaction being inversely dependent on the concentration of C6F6. Dual mechanisms involving a rapid radical chain and a slower tetrafluorobenzyne producing reaction are proposed to account for these observations.

Scheme 1:

Scheme 2:

Scheme 3:

Publications appearing or in press during the current grant period,
December 1, 1998 - November 30, 1999, resulting from DOE support:

 

  1. "Facile C-N Bond Cleavage Mediated by Electron-Rich Cyclopentadienyl Cobalt(I) Complexes," Helmut Werner, Gerhard Hörlin, and William D. Jones, J. Organomet. Chem. 1998, 562, 45-51.
  2. "Carbon-Hydrogen and Carbon-Carbon Bond Activation of Cyclopropane by a Hydridotrispyrazolylborate Rhodium Complex," Douglas D. Wick, Todd O. Northcutt, Rene J. Lachicotte, and William D. Jones, Organometallics 1998, 17, 4484-4492.
  3. "Catalytic Hydrogenolysis of Biphenylene with Platinum, Palladium, and Nickel Phosphine Complexes," Brian L. Edelbach, David A. Vicic, Rene J. Lachicotte, and William D. Jones, Organometallics 1998, 17, 4784-4794.
  4. "11B NMR: A New Tool for the Determination of Hapticity of Trispyrazolylborate Ligands," Todd O. Northcutt, Rene J. Lachicotte and William D. Jones, Organometallics 1998, 14, 5148-5152.
  5. "Insertion of Elemental Sulfur and SO2 into the Metal-Hydride and Metal-Carbon Bonds of Platinum Compounds," Michael S. Morton, Rene J. Lachicotte, David Vicic, and William D. Jones, Organometallics, 1999, 18, 227-234.
  6. "Energetics of Homogeneous Intermolecular Vinyl and Allyl Carbon-Hydrogen Bond Activation by the 16 Electron Coordinatively Unsaturated Organometallic Fragment [Tp'Rh(CNCH2CMe3)]," William D. Jones and Douglas D. Wick, Organometallics 1999, 18, 495-505.
  7. "Topics in Organometallic Chemistry. Activation of Unreactive Bonds and Organic Synthesis," Chapter 2, Activation of C-H Bonds. Stoichiometric Reactions, William D. Jones, Springer-Verlag, 1999, Berlin.
  8. "Photochemical C-H Activation and Ligand Exchange Reactions of CpReH2(PPh3)2. Phosphine Dissociation is Not Involved," William D. Jones*, Glen P. Rosini, and John A. Maguire, Organometallics, 1999, 18, 1754-1760.
  9. "Evidence for Methane Sigma-Complexes in Reductive Elimination Reactions from Tp'Rh(L)(CH3)H," Douglas D. Wick, Kelly A. Reynolds, and William D. Jones, J. Am. Chem. Soc. 1999, 121, 3974-3983.
  10. "A new synthetic route to ligands of the general composition R2PCH2ER'2 (E = P, As) and some rhodium complexes derived thereof," Justin Wolf, Matthias Manger, Ulrich Schmidt, Guido Fries, Dietmar Barth, Birgit Weberndörfer, David A. Vicic, William D. Jones, Helmut Werner, J. Chem. Soc., Dalton Trans. 1999, 1867-1876.
  11. "CarbonFluorine Bond Cleavage by Zirconium Metal Hydride Complexes," Brian L. Edelbach, A. K. Fazlur Rahman, Rene J. Lachicotte, and William D. Jones, Organometallics 1999, 18, 3170-3177
  12. "Catalytic Carbon-Carbon Bond Activation and Functionalization by Nickel Complexes," Brian L. Edelbach, Rene J. Lachicotte, and William D. Jones, Organometallics 1999, 18, 4040-4049.
  13. "Catalytic Carbon-Carbon and Carbon-Silicon Bond Activation and Functionalization by Nickel Complexes," Brian L. Edelbach, Rene J. Lachicotte, and William D. Jones, Organometallics 1999, 18, in press.
  14. "Generation of Perfluoro-Polyphenylene Oligomers via Intramolecular C-F activation from Cp2Zr(C5F5)2: A Dual Mechanism involving a Radical Chain and Release of Tetrafluorobenzyne," Brian L. Edelbach and William D. Jones, J. Am. Chem. Soc. 1999, 121, in press.
  15. "Insertions of Electrophiles into Metal Hydride Bonds. Reactions of (C5Me5)Rh(PMe3)H2 with Activated Alkynes to Produce h2-Alkene Complexes," Anthony D. Selmeczy and William D. Jones, Inorg. Chim. Acta 1999, in press.