Research Plans for the Period December 1, 2000 - November 30, 2001:

This coming year, our research will focus on 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 extensive success in C-C cleavage with biphenylene, a molecule with a weaker C-C bond than biphenyl. The success includes not only several new platinum based metal systems of the type [Pt(chelating phosphine)] but also related rhodium systems for C-C cleavage. In addition, cyclobutanones have also been found to under ring-opening decarbonylation involving C-C cleavage. A new system has been investigated using a nickel-bisphosphine complex in which C-CN bonds can be reversibly cleaved. This chemistry appears to be extensive, and will continue to be investigated in the coming year.

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 now completing our detailed studies and kinetic analysis 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. We are currently extending these studies to branched hydrocarbons.

The third area we have been active in is C–F bond activation of fluoroalkanes. We have discovered that Cp*2ZrH2 is capable of cleaving a wide variety of aliphatic C-F bonds, generating Cp*2ZrHF and the reduced hydrocarbon. No other transition metal based system has shown this type of reactivity. For example, 1-fluorohexane is reduced to hexane and perfluoropropene is completely reduced to propane. CFCs are also very reactive, first producing HFCs via C-Cl reduction and eventually HCs via C-F reduction. 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, 1999- November 30, 2000.

1. Tris-pyrazolylborate Studies.

We have made many advances in our studies of rhodium tris-pyrazolylborate complexes for C-H bond activation with regard to alkane complex intermediates. Generation of the 16-electron fragment {[HB(3,5-dimethylpyrazolyl)3]Rh(CNCH2CMe3)} (Tp'RhL) coordinated to an alkane allows the determination of the relative rates of the processes available to the alkane s-complex, such as C-H activation, migration down the alkane chain, or simple dissociation. Several 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.

Both of these reactions occur via an intermediate methane s-complex, and to simulate the kinetics for the interconversion isotope effects on both reductive elimination and oxidative addition were determined. The reductive elimination isotope effect was determined by comparing the rate of disappearance of the secondary isopropyl deuteride complex Tp'Rh(L)(CHMe2)D with the isopropyl hydride complex Tp'Rh(L)(CHMe2)H. The rate determining step in each of these reactions involves formation of the secondary propane s-complex, so that kH/kD for this fundamental step could be determined (kbc in eq 1). The oxidative addition isotope effect for the reverse reaction was determined by examining the kinetic products in the activation of CH2D2 (eq 2). Over time, the kinetic distribution adjusted to give a thermodynamic distibution favoring deuterium on carbon by a factor of ~2. Using these isotope effects, the simulation of the scrambling of Tp'Rh(L)(CH3)D could be successfully modelled as indicated in Scheme 1, and the relative rate contstants for a methane s-complex determined. Remarkably, the methane complex undergoes C-H oxidative addition 11x faster than it undergoes dissociation.

Similar rearrangement studies were carried out using Tp'Rh(L)(R)D where R = ethyl, propyl, butyl, pentyl, and hexyl. As the chains become longer, new rate constants are necessary to include migration up and down the chain and dissociation from primary vs secondary carbons. These simulations proved to be possible, allowing the determination of the relative rates for the processes available to any given alkane complex. However, once the chain length reached 4 carbons (C4), no new rate constants are needed to simulate the behavior of the system so that the pentyl and hexyl hydrides could be simulated using the previously determined rate constants.

The conclusions of this study are summarized in the bar chart shown in Fig.1 below. For methane, C-H activation is strongly preferred over dissociation, whereas for ethane, the rates of these two processes are comparable. End-to-end migration in ethane is slower, still. For propane, terminal C-H activation is favored over dissociation to a lessor extent than propane, but greater than ethane. Migration from the end to the middle of propane is slightly slower than C-H activation. For the secondary propane complex, migration to the end and dissociation occur at about the same rate.

Figure 1.

One interesting point learned in these modelling studies is that one cannot obtain absolute rates for these processes, but only relative rates. Furthermore, one cannot compare the rates of processes for different alkanes or even for different alkane complexes within the same alkane. The reason for this is seen by examination of the free energy picture for the scrambling in propyl deuteride complex (Fig.2). We do not know the absolute energies of the alkane s-complexes, and therefore cannot obtain an absolute rate for any single process involving these complexes. We can, however, learn about the differences in barrier heights for the reactions open to any one of these complexes.

Figure 2.

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) 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), via a Pt(IV) intermediate, which in turn reductively eliminates tetraphenylene at 115 ° C. At 120 ° C the reaction is catalytic, converting 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.

Recently we have extended this chemistry to include the smaller chelate ligand, But2PCH2PBut2 (dtbpm). We had anticipated that this ligand might permit the activation of less sterically accessible C-C bonds. While the platinum dtbpm complex does activate biphenylene more easily than the dippe complex, there is a competing side reaction involving ligand dissociation to create a dimer that renders these compounds unsuccessful for the desired chemistry (eq 3).

We have also been successful in generating the 16-electron rhodium analog of this compound. Now, biphenylene can be activated cleanly and insertion reactions with substrates such as alkynes and CO can be carried out catalytically (eq 4). Furthermore, we have discovered that cyclobutanones undergo C-C cleavage and CO deinsertion to give the rhodium carbonyl complex and cyclopropane (eq 5). The reaction is catalytic at elevated temperatures.

We have also discovered that the nickel complex [Ni(dippe)H]2 reacts with benzonitrile to give first an h2-nitrile complex, which then undergoes C-C cleavage of the carbon-CN bond (eq 6). Furthermore, the reaction does not go to completion but forms and equilibrium mixture of the h2-nitrile and C-CN oxidative addition product. We know of no such example of reversible C-C cleavage in the literature. Other examples of aryl C-CN cleavage are under investigation.

 

3. C-F Bond Cleavage Studies

We have reported that 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. This reaction is difficult to study in that the starting complex, [Cp2ZrH2]2, is insoluble in most solvents. We have now begun studies with the soluble, more reactive Cp*2ZrH2 and found that this molecule cleaves a wide variety of aromatic and aliphatic C-F bonds.

Systematic studies have shown that primary, secondary, and tertiary C-F bonds can all be cleaved with progressively greater difficulty (Scheme 2). In addition, di-fluorosubstituted carbons can be made to react with even more forcing conditions. Tri-fluoromethyl groups scarcely react at all even under extreme conditions.

Scheme 2.

Most remarkable, however, even trifluoromethyl C-F bonds can be easily cleaved if they are adjacent to a double bond. 3,3,3-trifluoropropene is completely defluorinated in 5 min at room temperature to give the zirconium-n-propyl hydride complex (Scheme 3). Perfluoropropene undergoes a similar reaction to give the same product. Details of the mechanism are under further study. Defluorination reactions are also seen with nonafluorohexene, perfluorocyclobutene, perfluorocyclopentene, perfluorobenzene, trifluorotoluene, and related substrates. Chlorofluorocarbons (CFCs) react rapidly to give first fluorocarbons (HFCs), which then are converted to hydrocarbons (HCs) in accord with the above established reactivities (Scheme 4).

Scheme 3.

 

Scheme 4.

Publications appearing during the current grant period,

December 1, 1998 - November 30, 2000, 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. "Carbon–Fluorine 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, 4660-4668.
  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, 10327-10331.
  15. Science, Perspectives, "Conquering the Carbon-Hydrogen Bond," 2000, 287, 1942.
  16. "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 2000, 300-302, 138-150.
  17. "Aliphatic Carbon-Fluorine Bond Activation using (C5Me5)2ZrH2," Bradley M. Kraft, Rene J. Lachicotte, and William D. Jones*, , J. Am. Chem. Soc. 2000, ASAP, August 18.