58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

This grant supports research in the Berry lab on bimetallic nitrido complexes having a linear M–M≡N structure featuring a direct metal-metal bond and their potential for performing intra- or inter-molecular N-atom transfer reactions. Since our first report of intramolecular aryl C–H amination via Ru–Ru≡N species, we have been exploring the possibility of C–H amination of benzylic C–H bonds and attempting to assess selectivity for aryl vs benzylic C–H amination. Compounds 1 and 2, depicted in Scheme 1, were designed for this purpose.

Scheme 1

The xylyl substituents on 1 are poised to be attacked by the nitride formed by photolysis or thermolysis of the Ru₂-azide group, which can result in the formation of a six-membered chelate ring in the amido product compound. We have monitored this reaction by EPR spectroscopy, which supports the proposed reaction sequence. We hope to be able to confirm the C-H amination product by obtaining a crystal structure.

Compound 2 utilizes o-tolyl instead of xylyl substituents. Thus, once the Ru₂-nitride is formed post-photolysis or –thermolysis, the N atom may choose to insert into an aryl or benzylic C–H bond, forming either of the two products shown.

Our next goal was to explore the possibility of intermolecular N atom transfer from Ru2-nitrides. To this end, we have synthesized the Ru₂(chp)₄N₃ molecule (chp = 2-chloro-6-hydroxypyridinate), as shown below in Scheme 2, which features no organic ligand fragments surrounding the azide group due to the arrangement of ligands around the metal.  Thus, upon photolytic or thermolytic conversion of the azide to a nitride, no intramolecular C–H functionalization should take place and, instead, it should be possible to observe intermolecular reactivity.

Scheme 2

To date, we have focused on the reaction of the Ru₂(chp)₄N nitrido intermediate with triphenylphosphine.  After photolysis of a solution of Ru₂(chp)₄N₃ in the presence of ten equivalents of PPh₃, and following subsequent workup with HCl gas, we have established by ³¹P NMR that Ph₃PNH₂Cl is formed in > 60 % yield. This product is proposed to result from reaction of the nitrido intermediate with PPh₃ to form a Ru₂–N=PPh₃ complex, which then reacts with HCl to yield the Ru₂(chp)₄Cl starting complex (recovered in quantitative yield) and Ph₃PNH₂Cl. Thus, this reaction represents the first time we have been able to observe intermolecular reactivity with Ru₂ nitrido complexes.  There are two other noteworthy aspects of this reaction.  First, since the Ru₂(chp)₄Cl starting material is recovered in quantitative yield from this process, we can close a synthetic cycle for converting PPh₃ to phosphinimide using stoichiometric amounts of sodium azide as the N atom source (Scheme 3).  Moreover, the phosphinimide product, Ph₃PNH₂Cl, slowly hydrolyzes in water to produce phosphine oxide, Ph₃PO, and ammonium ion.  Thus, this synthetic cycle may be used to produce ammonia.

Scheme 3

                Another question we are currently addressing is whether or not it would be possible to generate the Ru₂ nitride complexes from N₂.  To this end, we are exploring one electron reduction of Ru₂(chp)₄Cl in attempts to synthesize an N₂ complex such as Ru₂(chp)₄(N₂), which would represent the first step towards the goal of converting N₂ into a reactive Ru₂ nitride. Thus far, we have obtained a number of axial ligand adducts of the reduced Ru₂(chp)₄ fragment, and have also found that Ru₂(chp)₄ dimerizes when strongly binding ligands are not present. We have determined equilibrium constants (K_eq) for the reaction shown below for a number of different potential axial ligands, L, including N₂.

                [Ru₂(chp)₄]₂   +   2 L   ==   2 Ru₂(chp)4(L)       K_eq

                We also have ongoing synthetic efforts to prepare the Os₂ and Re₂ analogs of the Ru₂ nitride complexes described here. We recently reported a preliminary communication of Os₂-azide compounds bearing either formamidinate or chp ligands. On the Re₂ front, we have prepared an azido complex (Scheme 4) and explored its photolytic and thermolytic properties. The Re₂ compounds have the advantage that they are diamagnetic, and thus we can follow the reactions by NMR spectroscopy. It appears that the Re₂ compound can undergo conversion to a nitride and intramolecular C–H amination just as in the Ru₂ case, but the conversion is at present not good. We are currently working on ways to improve this.

Scheme 4