FULL PAPER DOI: 10.1002/ejic.200700792 (Pentamethylcyclopentadienyl)iridium-PTA (PTA = 1,3,5-Triaza-7-phospha- adamantane) Complexes and Their Application in Catalytic Water Phase Carbon Dioxide Hydrogenation Mikael Erlandsson, [a,b] Vanessa R. Landaeta, [a][‡] Luca Gonsalvi,* [a] Maurizio Peruzzini, [a] Andrew D. Phillips, [a,b] Paul J. Dyson, [b] and Gábor Laurenczy* [b] Keywords: Reduction / Iridium(III) complexes / Water-soluble ligands / Hydrogenation / Catalysis The water-soluble iridium(III) complexes [Cp*Ir(PTA)Cl 2 ](1) and [Cp*Ir(PTA) 2 Cl]Cl (2) (PTA = 1,3,5-triaza-7-phosphaada- mantane) have been synthesised and characterised by spec- troscopy and X-ray crystallography. The complexes were evaluated as catalyst precursors for the hydrogenation of CO 2 and hydrogen carbonate in aqueous solutions, in the ab- sence of amines or other additives, under relatively mild con- Introduction In recent years, there has been increased concern over the emission of green house gases, and the possible conse- quences of these emissions. [1] Carbon dioxide is the major green house gas emission and is becoming increasingly abundant in the atmosphere. To solve the twin problems of depleting fossil fuel sources, and the associated climate change, the possibility of fixing CO 2 through catalytic hy- drogenation, thereby transforming this feed-stock into an economical C 1 building block is being investigated. [2] The reduction of CO 2 through both homogeneous [3] and hetero- geneous [4] catalysis has been investigated under different conditions, [5] and the use of enzymatic catalysis has also been reported. [6] However, there has been an increased interest in the development of CO 2 reduction in water [7] since CO 2 is highly soluble in water, [8] and importantly water is environmentally benign. Furthermore, it is rela- tively easy to recycle water-soluble catalysts from biphasic systems comprising water/organic solvents. [9] The possible primary products from the reduction of CO 2 in water in- [a] Istituto di Chimica del Composti Organometallici, Consiglio Nazionale delle Ricerche (ICCOM-CNR), Via Madonna del Piano 10, 50019 Sesto Fiorentino, Firenze, Italy E-mail: l.gonsalvi@iccom.cnr.it [b] Institut des Sciences et Ingénierie Chimiques, Ecole Polytech- nique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland E-mail: gabor.laurenczy@epfl.ch [‡] Current address: Departamento de Química, Universidad Simón Bolívar, Apartado 89000, Caracas 1080A, Venezuela Supporting information for this article is available on the WWW under http://www.eurjic.org or from the author. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2008, 620–627 620 ditions. Complex 1 performed poorly while 2 catalyses the hydrogenation with moderate activity. The catalytically active monohydride [Cp*Ir(PTA) 2 H] + was identified by multi- nuclear NMR spectroscopy and its nature confirmed by inde- pendent synthesis. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) clude formic acid, formaldehyde, methanol and methane, but generally only the formation of formic acid is observed [Equation (1)]. CO 2 +H 2 h HCOOH (1) The catalytic hydrogenation of CO 2 in water solutions also has the advantage that the free energy for the reaction between completely dissolved reagents is slightly negative (ΔG° = –4 kJ/mol), while the same gas-phase reaction be- tween CO 2 and H 2 , which produces liquid formic acid, has a positive difference in free energy (ΔG° = +33 kJ/mol). [10] Moreover, when hydrogenation of CO 2 is performed in water, the CO 2 /hydrogen carbonate/carbonate equilibrium has to be taken into consideration [Equation (2)]. CO 2 +H 2 O h H 2 CO 3 h HCO 3 – +H + [rlhar ] CO 3 2– +2H + (2) Because of this equilibrium, the active species of the re- action may vary depending on pH, temperature and CO 2 pressure, and information concerning the identity of the active species may be provided by multinuclear NMR spec- troscopy. [11] Extensive studies have been performed on the hydrogenation of CO 2 which show that the reduction is very slow at low pH, with high rates achieved around pH 8, sug- gesting that the actual substrate is hydrogen carbonate. [11,12] For the catalytic hydrogenation of CO 2 in aqueous solu- tions, complexes based on the transition metals Rh and Ru dominate, both under conventional and supercritical condi- tions, [13] and only a few reports have been published on the use of cyclopentadienyl iridium catalysts. [14] Recently, sev- eral papers have described water-soluble Ir and Rh cyclo- pentadienyl complexes and Ru arene complexes that cata- lyse the reduction of CO 2 and hydrogen carbonate in aque-