Molecular Nickel Phosphide Carbonyl Nanoclusters: Synthesis, Structure, and Electrochemistry of [Ni 11 P(CO) 18 ] 3- and [H 6-n Ni 31 P 4 (CO) 39 ] n- (n = 4 and 5).

The reaction of [NEt4 ]2 [Ni6 (CO)12 ] in thf with 0.5 equiv of PCl3 affords the monophosphide [Ni11 P(CO)18 ]3- that in turn further reacts with PCl3 resulting in the tetra-phosphide carbonyl cluster [HNi31 P4 (CO)39 ]5- . Alternatively, the latter can be obtained from the reaction of [NEt4 ]2 [Ni6 (CO)12 ] in thf with 0.8-0.9 equiv of PCl3 . The [HNi31 P4 (CO)39 ]5- penta-anion is reversibly protonated by strong acids leading to the [H2 Ni31 P4 (CO)39 ]4- tetra-anion, whereas deprotonation affords the [Ni31 P4 (CO)39 ]6- hexa-anion. The latter is reduced with Na/naphthalene yielding the [Ni31 P4 (CO)39 ]7- hepta-anion. In order to shed light on the polyhydride nature and redox behavior of these clusters, electrochemical and spectroelectrochemical studies were carried out on [Ni11 P(CO)18 ]3- , [HNi31 P4 (CO)39 ]5- , and [H2 Ni31 P4 (CO)39 ]4- . The reversible formation of the stable [Ni11 P(CO)18 ]4- tetra-anion is demonstrated through the spectroelectrochemical investigation of [Ni11 P(CO)18 ]3- . The redox changes of [HNi31 P4 (CO)39 ]5- show features of chemical reversibility and the vibrational spectra in the νCO region of the nine redox states of the cluster [HNi31 P4 (CO)39 ]n- (n = 3-11) are reported. The spectroelectrochemical investigation of [H2 Ni31 P4 (CO)39 ]4- revealed the presence of three chemically reversible reduction processes, and the IR spectra of [H2 Ni31 P4 (CO)39 ]n- (n = 4-7) have been recorded. The different spectroelectrochemical behavior of [HNi31 P4 (CO)39 ]5- and [H2 Ni31 P4 (CO)39 ]4- support their formulations as polyhydrides. Unfortunately, all the attempts to directly confirm their poly hydrido nature by 1 H NMR spectroscopy failed, as previously found for related large metal carbonyl clusters. Thus, the presence and number of hydride ligands have been based on the observed protonation/deprotonation reactions and the spectroelectrochemical experiments. The molecular structures of the new clusters have been determined by single-crystal X-ray analysis. These represent the first examples of structurally characterized molecular nickel carbonyl nanoclusters containing interstitial phosphide atoms.