By Karlee Bamford, Treasurer for the GCI
What is ecocatalysis? I had never heard this term before until reading a recent publication from Grison and coworkers in the RSC journal Green Chemistry entitled “Ecocatalyzed Suzuki cross coupling of heteroaryl compounds”.1 In this work, the authors perform the familiar Suzuki cross-coupling of arylboronic acids (Figure 1) with heteroaryl halides. However, they use a thoroughly unfamiliar palladium catalyst: the common water hyacinth (Figure 2).In broad terms, ecocatalysis is the use of plant-derived, metal-based catalysts in chemical reactions. If it were not for the author’s graphical abstract illustrating the plant body performing catalysis, I would have assumed that this was a more standard bioinorganic paper featuring a plant extract as catalyst. While the propensity of certain plants and microbiota to uptake (“phytoextract”) particular contaminants has long been used in waste water purification, for example in the uptake of inorganic phosphates (e.g. [PO4]3-),3 ecocatalysis represents a clever progression from plants being used in chemical sequestration to chemical transformation.
Plants currently used in phytoremediation, that is, the removal of contaminants such as heavy metals from anthropogenically polluted environments, could clearly be used in the production of ecocatalysts.4 One current use for such metal-laden plants is in phytomining as so-called bio-ore.5 This metal extraction process ultimately results in the majority of the plant bio-mass being wasted through the energy-consumptive process of incineration, whereas an ecocatalyst such as EcoPd requires that same bio-mass as a kind of ligand support.
Grison and colleagues report reaction times, conditions, and yields (typically >90 %) for their “EcoPd” catalyst that are competitive with typical Suzuki cross coupling experiments and catalysts, both homogenous and heterogeneous. Remarkably, the primarily root-based EcoPd catalyst can be reclaimed and reused without loss of activity, as the authors demonstrated in a control study that involved recycling the catalyst four times over. Finally, the palladium content of the used catalyst can be quantitatively recovered by rhizofiltration, that is, by returning the elemental palladium obtained in post-synthesis work-up to a new plant specimen for metal uptake. In practical terms, this involves filtering the post-synthesis solution, dissolving the isolated solids with aqua regia, and diluting the resultant palladium-containing solution with water before reintroducing it to the roots of E. crassipes.
Ecocatalysis is an entirely new and emerging field of chemistry (circa 2013) being pioneered by Grison and coworkers at The Laboratory of Bio-inspired Chemistry and Ecological Innovations (University of Monpellier) in France. Their research has furnished several other noteworthy ecocatalysts (EcoM’s) featuring nickel (EcoNi),6 zinc (EcoZn),7 manganese (EcoMn),8 copper (EcoCu),9 which have proven effective in Biginelli, Diels-Alder, reductive amination, and Ullmann reactions, respectively.
This new approach to catalysis is not only charmingly novel – at least to a non-bioinorganically-minded chemist such as myself – but it also offers a real solution to the problematic dependence of catalysis on pure precious metals. The plants themselves provide a means for both harvesting and using low-abundance metals in a format that does not require complicated ligand design and is consistent with homogenous catalysis. Clearly, EcoPd and other such EcoM may not be suitable replacements in every metal-catalyzed transformation, but they nonetheless provide a new avenue for recycling precious metals and realising catalyst sustainability.
The range of possible ecocatalysts is, in my mind, astounding. Plant species that are known to preferentially accumulate heavy metals, known as accumulators and hyperaccumulators, are greater than 500 in number and sequester metals from across the p- and d-block of the periodic table, each to varying extents.10 As the tolerance and preference for certain transition metals is in part gene-regulated,11 it is conceivable that genetic modification and controlled environmental conditions could in the future yield heavy metal-specific plant species for sequestration and, perhaps, subsequent ecocatalysis.
- G. Clavé, F. Pelissier, S. Campidelli and C. Grison, Green Chemistry, 2017, DOI: 10.1039/c7gc01672g.
- Used with permission from Larry Allain, hosted by the USDA-NRCS PLANTS Database.
- J. Lv, J. Feng, Q. Liu and S. Xie, Int. J. Mol. Sci., 2017, 18.
- C. Grison, Environmental Science and Pollution Research, 2015, 22, 5589-5591.
- R. R. Brooks, M. F. Chambers, L. J. Nicks and B. H. Robinson, Trends in Plant Science, 1998, 3, 359-362.
- C. Grison, V. Escande, E. Petit, L. Garoux, C. Boulanger and C. Grison, RSC Adv., 2013, 3, 22340.
- V. Escande, T. K. Olszewski and C. Grison, Comptes Rendus Chimie, 2014, 17, 731-737.
- V. Escande, A. Velati, C. Garel, B.-L. Renard, E. Petit and C. Grison, Green Chemistry, 2015, 17, 2188-2199.
- G. Clavé, C. Garel, C. Poullain, B.-L. Renard, T. K. Olszewski, B. Lange, M. Shutcha, M.-P. Faucon and C. Grison, RSC Adv., 2016, 6, 59550-59564.
- H. Sarma, Journal of Environmental Science and Technology, 2011, 4 118-138.
- S. Jan and J. A. Parray, Approaches to Heavy Metal Tolerance in Plants, Springer Singapore, Singapore, 2016.