4E01
Metal!Ligand Cooperation in H2 Production and H2O Decomposition on a Ru(II)PNN Complex: the Role of Ligand Dearomatization!Aromatization(бᎲ໻ᄺ‫ܜ‬ᇢ⠽䊾࣪ᄺⷨお᠔) "Jun Li, Yoshihito Shiota, Kazunari Yoshizawa

The sunlight-driven splitting of water into H2 and O2 is an attractive and challenging issue
in science today. More recently, Milstein’s group achieved a distinct scheme in which H2 andO2 were generated in consecutive thermal- and light-driven steps.1 Heating an aromatic Ru(II)hydrido-hydroxo complex [C5NH3(CH2PtBu2)(CH2NEt2)Ru(H)(CO)(OH)] in refluxing water
for three days resulted in the evolution of H2 with formation of a cis dihydroxo complex.
Exposure of this complex to light released O2 and regenerated the starting hydrido-hydroxocomplex. This scheme based on such a simple homogeneous system presents a new idea forthe splitting of water. This active complex bears a tridentate PNN pincer ligand, which candonate or accept a proton via facile dearomatization/aromatization process, cooperated withthe metal center. This kind of metal PNN (or PNP) complexes have been found powerful formany novel reactions than involves O!H or C!H bond activation. To our knowledge, there islittle theoretical work to study this kind of metal complexes and these interesting reactions sofar. Thus, in this paper, we performed detailed DFT calculations to investigate the energetics
and mechanism of the thermal H2 production from water splitting mediated by this Ru(II)PNN complex.
All calculations on a model system (with PMe2 and NMe2 groups replacing PtBu2 and NEt2
groups) using the B3LYP hybrid functional were carried out with the Gaussian 03 program.
The structure of each stationary point was optimized by taking into account bulk solvent(water) effects with polarizable continuum model (IEF-PCM).
The most likely pathway for H2 formation and H2O decomposition on this Ru(II) PNN
complex is shown in Scheme 1. In the aromatic complex 1, a proton can transfer from thePNN ligand to the hydride and also to the hydroxyl group. But the latter is reversible. On theother hand, once H2 is formed, it will be liberated but to be cleaved again. After H2 liberation,
the succeeding H2O decomposition occurs readily, which favorably leads to the transdihydroxo complex under thermokinetics control. However, the cis dihydroxo complex is still
the final main product, as a result of thermodynamic control. Although the overall reaction isslightly endergonic, H2 escapes from the reaction system into gas phase at elevatedtemperature, driving the reaction to the product side. The rate-limiting step is the formation ofH2, which has a relatively high activation barrier of 37.6 kcal/mol (Figure 1). But water canfacilitate this step as a bridging molecule (Figure 2). All of these may be the reason why theyield of the final product 8 is 45%, and 25% of 1 remains after the reaction proceeds for threedays at 100°C.
Scheme 1

Figure 1. Free energy (298 K, in kcal/mol) profile forH2 production and H2O decomposition on a Ru(II)PNN complex. The energies in gas phase are also
given in the parentheses.

Figure 2. Free energy (298 K, in kcal/mol)
profile for addition of a H2O bridge as partof the coupling of the Ru-bound hydridewith a captive proton.

References[1] Kohl, S. W.; Weiner, L.; Schwartsburd, L.; Konstantinovski, L.; Shimon, L. J. W.; BenDavid, Y.; Iron, M. A.; Milstein, D. Science 2009, 324, 74.