Photoreversible Multiple Additions of Hydrogen to a Highly Unsaturated Platinum−Rhenium Cluster Complex

The compound Pt₃Re₂(CO)₆(PBu^t₃)₃, 1, was obtained from the reaction of Re₂(CO)₁₀ with Pt(PBu^t₃)₂ in octane solvent at reflux. Compound 1 consists of a trigonal bipyramidal cluster of five metal atoms with three platinum atoms in the trigonal plane and the two rhenium atoms in the apical positions. The metal cluster is formally unsaturated by 10 electrons. Compound 1 sequentially adds 3 equiv of hydrogen at room temperature/1 atm to form the series of compounds Pt₃Re₂(CO)₆(PBu^t₃)₃(μ-H)₂, 2, Pt₃Re₂(CO)₆(PBu^t₃)₃(μ-H)₄, 3, and Pt₃Re₂(CO)₆(PBu^t₃)₃(μ-H)₆, 4. A small but significant kinetic isotope effect was observed, k_H/k_D = 1.3. The rate of addition of hydrogen is unaffected by the presence of a 20-fold excess of free PBu^t₃ in solutions of 1. Compounds 2−4 each consist of a trigonal bipyramidal cluster of three platinum and two rhenium atoms similar to that of 1. The hydrido ligands in 2−4 bridge the platinum−rhenium bonds and are arranged to give structures having overall C_2v symmetry for 2 and 3 and approximate D_3h symmetry for 4. Some of the hydrido ligands were expelled from 4 in the form of hydrogen upon exposure of solutions to UV−vis irradiation to yield compound 3 and then 2 in reasonable yields, but the elimination of all hydrido ligands to yield 1 was achieved only under the most forcing UV irradiation and then only with a major loss of the complex due to decomposition. The electronic structures of 1−4 were investigated by DFT calculations. Additional DFT calculations have suggested some mechanisms for the activation of hydrogen at multicenter metal sites without ligand eliminations prior to the hydrogen additions.