Synthesis, molecular structure and fluxional behavior of the elusive [HRu 4 (CO) 12 ] 3- carbonyl anion.

The elusive mono-hydride tri-anion [HRu 4 (CO) 12 ] 3- (4) has been isolated and fully characterized for the first time. Cluster 4 can be obtained by the deprotonation of [H 3 Ru 4 (CO) 12 ] - (2) with NaOH in DMSO. A more convenient synthesis is represented by the reaction of [HRu 3 (CO) 11 ] - (6) with an excess of NaOH in DMSO. The molecular structure of 4 has been determined by single-crystal X-ray diffraction (SC-XRD) as the [NEt 4 ] 3 [4] salt. It displays a tetrahedral structure of pseudo C 3v symmetry with the unique hydride ligand capping a triangular Ru 3 face. Variable temperature (VT) 1 H and 13 C{ 1 H} NMR experiments indicate that 4 is fluxional in solution and reveal an equilibrium between the C 3v isomer found in the solid state and a second isomer with C s symmetry. Protonation-deprotonation reactions inter-converting H 4 Ru 4 (CO) 12 (1), [H 3 Ru 4 (CO) 12 ] - (2), [H 2 Ru 4 (CO) 12 ] 2- (3), [HRu 4 (CO) 12 ] 3- (4) and the purported [Ru 4 (CO) 12 ] 4- (5) have been monitored by IR and 1 H NMR spectroscopy. Whilst attempting the optimization of the synthesis of 4, crystals of [NEt 4 ] 2 [Ru 3 (CO) 9 (CO 3 )] ([NEt 4 ] 2 [7]) were obtained. Anion 7 contains an unprecedented CO 3 2- ion bonded to a zero-valent Ru 3 (CO) 9 fragment. Finally, the reaction of 6 as the [N(PPh 3 ) 2 ] + ([PPN] + ) salt with NaOH in DMSO affords [Ru 3 (CO) 9 (NPPh 3 )] - (9) instead of 4. Computational DFT studies have been carried out in order to support experimental evidence and the location of the hydride ligands as well as to shed light on possible isomers.