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43410-G3
Developing Late Metal Oxo Complexes of Strongly Donating Ligands

Jeremy M. Smith, New Mexico State University

Insights into C-H activation by high valent iron imido complexes

A number of iron(IV) imido complexes [PhB(MesIm)₃FeNR]OTf (R = ^tBu, Ad) are readily prepared by oxidation of the iron(III) precursors (Scheme 1). Despite their high oxidation state, these thermally stable complexes do not participate in nitrene transfer reactions with substrates such as alkenes and phosphines. However, they are able to activate reasonably strong C-H bonds, such as those in 1,4-cyclohexadiene (BDE = 77(1) kcal/mol).

Scheme 1. Synthesis of [PhB(MesIm)₃FeNR]OTf. Inset: X-ray crystal structure of [PhB(MesIm)₃FeNAd]BPh₄

A thermodynamic investigation into hydrogen atom abstraction by these complexes was undertaken to provide insight into the experimental results (Scheme 2). Cyclic voltammetry of [PhB(MesIm)₃FeNAd]OTf reveals a Fe^III/IV couple at E_1/2 = -0.98 V (vs Cp₂Fe⁺/Cp₂Fe), showing that despite the high oxidation state, these complexes are poor oxidants. The pK_a of the product arising from hydrogen atom transfer, [PhB(MesIm)₃Fe-N(H)Ad]⁺, was determined by a combination of experimental and computational methods, pK_a(MeCN) = 37(3). These data allow the N-H BDFE (free energy) of [PhB(MesIm)₃FeNAd]OTf to be calculated, N-H BDFE = 88(5) kcal/mol.

Scheme 2.

These results show that C-H activation reaction is driven by the high basicity of the iron(III) imido intermediate. The lack of reactivity towards group transfer may be due to the poor oxidizing ability of the iron(IV) imido complex, although steric effects cannot be eliminated at this stage. Therefore, these results provide insights into the driving forces for C-H activation as well as the factors that affect reaction selectively.

To provide insights into the effect of the metal oxidation state on C-H activation reaction, we have also begun to investigate the thermodynamics of hydrogen atom transfer to the iron(III) imido PhB(MesIm)₃FeN^tBu. It is notable that this complex can prepared by reaction of the stable 2,4,6-tri(tert-butyl)phenoxy radical (O-H BDE = 81 kcal/mol) with the iron(II) amido complex PhB(MesIm)₃Fe-N(H)^tBu. Thus, it is likely that the N-H BDFE of PhB(MesIm)₃Fe-N(H)^tBu is lower than of its iron(III) amido congener [PhB(MesIm)₃Fe-N(H)^tBu]⁺.

Electrophilic iron nitrido complexes

An exciting finding is that terminal iron nitrido complexes can be isolated and crystallized (Scheme 3). Photolysis of iron(II) azido complexes leads to the thermally stable iron nitrido complexes PhB(RIm)₃FeN (R = ^tBu, Mes) in high yield. These are the first terminal iron nitrido complexes to be structurally characterized.

Initial reactivity studies show that the nitrido ligand has electrophilic character. Thus, PhB(^tBuIm)₃FeN reacts with PPh₃ to form the imidophosphine complex PhB(^tBuIm)₃Fe-N=PPh₃. Reaction of PhB(^tBuIm)₃FeN with electrophiles such as MeOTf, Me₃SiOTf and Ph₃COTf result in decomposition of the complex. A kinetic investigation of the reaction between PhB(^tBuIm)₃FeN and PPh₃ gives the rate law rate = k[Fe][PPh₃], with k = 3.65(6) × 10^-4 M^-1s^-1 at 320 K. Preliminary results of a Hammett study shows that electron-donating substituents increase the rate of reaction (r =-0.66), consistent with nucleophilic attack of the phosphine at the nitrido ligand.

Scheme 3. Synthesis of iron nitridos, R = ^tBu, Mes. Inset A: X-ray structure of PhB(^tBuIm)₃FeN. Inset B: ¹⁵N NMR of PhB(^tBuIm)₃FeN.

We have  begun investigations into N-H bond formation from the nitrido complexes. While acids have been found to decompose PhB(^tBuIm)₃FeN, the non-acidic hydride Co(dppe)₂H, which possesses a very weak Co-H bond (Co-H BDE = 59 kcal/mol) reacts to form PhB(^tBuIm)₃Fe-NH₂, with a selectivity of ca. 50 % (unoptimized). The parent amido complex PhB(^tBuIm)₃Fe-NH₂ can be independently prepared from PhB(^tBuIm)₃FeCl and LiNH₂. This suggests that hydrogen atom transfer may be an alternative method for creating N-H bonds.

While PhB(^tBuIm)₃Fe≡N does not react with H₂, the more open complex PhB(MesIm)₃Fe≡N reacts with 1 atm H₂ at room temperature to form new diamagnetic product(s). While complete characterization of the product(s) from this reaction is still required, at this stage it is important to note that free ammonia is not detected.

4. Career Impact

The funding provided by the ACS-PRF has allowed the PI to conduct and disseminate research. The PI presented some of the results in this report as a poster at the Inorganic Reaction Mechanisms Gordon Conference in February 2007. This poster was one of eight selected for a 15 minute poster presentation. Results were also presented at the Organometallic Gordon Conference and the International Conference on Biological Inorganic Chemistry (ICBIC), both of which were held in July 2007. The PI was also selected to be a participant in the NSF-sponsored Inorganic Chemistry Workshop in June 2007. Three papers have been published from results obtained during the course of the award, two have been submitted, and another two are in preparation. The results in this report have been used as the basis for an NSF-CAREER proposal and an NIH-SCORE proposal. Finally, new spectroscopic collaborations have been established with Joshua Telser (Roosevelt College) and Brian M. Hoffman (Northwestern University)

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