Back in 2008, a report surfaced from the laboratory of Prof. Karol Grela describing the behavior of a series of naphthalene-based Hoveyda-Grubbs type catalysts (1a-c).1 Somewhat surprisingly, at least in my eyes upon initial assessment, the complexes displayed very drastic differences in terms of catalytic activity. Complex 1b behaved in a similar fashion to the parent 2nd generation Grubbs-Hoveyda complex, readily initiating at 0 °C, whereas complexes 1a and 1c required temperatures in excess of 100 °C to show similar activity. After considerable thought and analysis, this phenomenon was explained by the Clar rule, which describes the electronic character of polycyclic aromatic hydrocarbons (PAHs). Essentially, this rule states that PAHs will maximize the number of completed aromatic sextets and the location of these sextets will display more aromatic character adding increased stability. Looking at the case of anthracene and phenanthrene, the linear anthracene has only one sextet delocalized over all three rings, whereas, phenanthrene has two sextets localized on the external rings. By analogy, when considering the Ru-benzylidene-ether chelate as an extension of the naphthalene framework, complexes 1a and 1c can be compared to phenanthrene, supplying increased stabilization within the 5-membered chelate, whereas, complex 1b corresponds to an anthracene mimic in which this stabilization is much less.
Building on this work, a recent independent report from the Barbasiewicz et al. described the synthesis and activity of a missing member of the naphthalene family based on pleiadiene, in which the two coordinating sites of the chelate (Ru=C and O-iPr ether) are located on different rings.2 The electronics of this π-system suggest that this analog should possess the lowest level of stabilization within the chelate ring, providing a fast initiating catalyst. Indeed, when complex 2 was synthesized and tested in some standard ring closing metathesis (RCM) reactions, it displayed extremely fast initiation, albeit at the expense of stability. This added some further evidence to this idea of chelate stability; however, the authors do point out that this effect could also be the result of a distorted geometry about the Ru-center present in this complex that could weaken the Ru-O coordination.
In yet another report, Barbasiewicz et al. describe some forays into bimetallic systems based on this idea of chelate stabilization.3 Again, with phenanthrene and anthracene as the model, bimetallic complexes like 3a-b were envisioned in hopes that some sort of cooperative behavior may be realized. Numerous attempts to synthesize and isolate a variety of phenanthrene-like complexes proved unsuccessful. Isolated solids were comprised of complex mixtures which could not be further purified by traditional means. The lone success proved to be the isolation of the bimetallic anthracene-like complex 4. This result goes against that observed for the naphthalene-based complexes 1a-c in which the angular phenanthrene-like complexes exhibit increased stability compared to the linear anthracene-like complex. The authors propose, that unlike the naphthalene system, the presence of two chelates off of the central benzene core are not enough to overcome the aromatic stabilization of the benzene core itself, thus, the angular bimetallic complexes don’t gain any added stability. Complex 4 was tested in some standard RCM reactions and its activity was very similar to a related mono-metallic complex, suggesting the lack of any cooperation between the two Ru-centers. This result isn’t overly surprising since linear anthracene-like complexes display behavior similar to traditional Hoveyda-based systems. Even though the outcome of the bimetallic study wasn’t as anticipated and the results were met with limited success, I found the idea to be very interesting. The idea of cooperative mechanisms with respect to bimetallic complexes is largely an unexplored area in olefin metathesis and with future creative ideas and designs, as described by Barbasiewicz, new reactivity profiles may be observed that will open the door to unique chemistries.
1 Barbasiewicz, M; Szadkowska, A.; Makal, A.; Jarzembska, K; Woźniak, K; Grela, K. “Is the Hoveyda-Grubbs Complex a Vinylogous Fisher-Type Carbene? Aromaticity-Controlled Activity of Ruthenium Metathesis Catalysts.” Chem. Eur. J. 2008, 14, 9330-9337.
2 Barbasiewicz, M; Grudzień, K.; Malinska, M. “A Missing Relative: A Hoveyda-Grubbs Catalyst Bearing a Peri-Substituted Naphthalene Framework.” Organometallics 2012, 31, 3171-3177.
3 Grudzień, K.; Malinska, M.; Barbasiewicz, M. “Synthesis and Properties of Bimetallic Hoveyda-Grubbs Metathesis Catalysts.” Organometallics 2012, 31, ASAP.