Diruthenium Aryls: Structure, Bonding, and Reactivity

2020-07-27T17:31:31Z (GMT) by Adharsh Raghavan
The chemistry of metal–metal (M–M) multiply bonded compounds has fascinated inorganic chemists for a period spanning more than five decades. Since the elucidation of the quadruple bond by Cotton in 1964, thousands of compounds featuring M–M bonds have been isolated and studied. Of these, dinuclear units supported by four bidentate ligands forming a ‘paddlewheel’ motif represent a class of compounds that present unique molecular and electronic structures, and useful electrochemical and magnetic properties.

Over the last two and a half decades, our laboratory has focused on studying diruthenium paddlewheel complexes for their easeof preparation, rich electrochemical properties,and remarkable stability. We have isolated a vast number of diverse diruthenium alkynyls in multiple oxidation states, bearing different paddlewheel (equatorial) ligand systems and studied their molecular and electronic structures. Taking advantage of the extended conjugation that exists between the Ru2core and the poly-alkynyl ligand motif, we have also found applications for them in prototypical flash-memory devices. At this juncture, we sought to expand the organometallic chemistry of Ru2to complexes featuring Ru–aryl linkages.

The ‘aryl anion’ is, based on pKa, twenty orders of magnitude more basic than the corresponding acetylide. Arguably, this difference should result in a more electron-rich dinuclear core with new electronic structures waiting to be explored. Although kinetically more reactive than metal–alkynyls, metal–aryls are still more stable than the corresponding metal–alkyls. However, for second-row transition metals like ruthenium, kinetic instability issues are somewhat more suppressed than for their first-row counterparts.

Armed with the knowledge that it was reasonable to expect somewhat stable metal–aryl complexes, the synthesis and characterization, and analyses of molecular and electronic structures of diruthenium aryls were attempted. By employing relatively simple lithium-halogen exchange reactions, both mono and bis-aryl complexes of diruthenium have been isolated. Additionally, two different oxidation states of diruthenium have beenaccessed, namely Ru2(II,III)and Ru2(III,III),by judiciously modifying the paddlewheel ligands. Following this, preliminary reactivity studies of Ru2(II,III) monoaryls of the form Ru2(ap)4Ar were performed, which yielded surprising results. This work led to the conclusion that the diruthenium–aryl interaction is an example of a metal–metal–ligand interaction that can bring reactivity to the distal metal site. Moreover, it was found that even minor changes in axial ligands can bring about major upheavals in electronic structure.

Computational investigations into the electronic structure of the above-mentioned compounds have faced many a barrier because of the complexity of the system. The deep mixing of the metal–metal and metal–ligand valence manifolds is more easily isolated into its constituent parts in the case of relatively simple structures such as the monoaryls, Ru2II,IIIL4Ar. However, electronic structure calculations are fraught with difficulties in the case of heavily distorted axially disubstituted mono and bis-aryls, (X)Ru2III,IIIL4Ar and Ru2III,IIIL4Ar2, respectively. Ru2III,IIIL4Ar2complexes present an interesting case of second order Jahn-Teller distortion (SOJT), which has been adequately modeled. However, the more heavily distorted case of XRu2(ap)4Ar (X = CCH, CN, CO, etc.) pose greater computational challenges, such as low-lying excited states, spin-admixed ground states and difficulties in isolating metal and ligand contributions to the valence manifold.

Our investigations into diruthenium aryls began as a mere curiosity that arose out of a serendipitous discovery. Two years later, our continued efforts in this direction have yielded rather fruitful results. The unusual structures and associated complex bonding motifs in these systems have taught us about the importance of metal–metal–ligand interactions as more than just a sum of metal–metal and metal–ligand parts.