Molecular Nickel Phosphide Carbonyl Nanoclusters: Synthesis, Structure, and Electrochemistry of [Ni11P(CO)18]3- and [H6-nNi31P4(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 [Ni11P(CO)18]3- that in turn further reacts with PCl3 resulting in the tetra-phosphide carbonyl cluster [HNi31P4(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 [HNi31P4(CO)39]5- penta-anion is reversibly protonated by strong acids leading to the [H2Ni31P4(CO)39]4- tetra-anion, whereas deprotonation affords the [Ni31P4(CO)39]6- hexa-anion. The latter is reduced with Na/naphthalene yielding the [Ni31P4(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 [Ni11P(CO)18]3-, [HNi31P4(CO)39]5-, and [H2Ni31P4(CO)39]4-. The reversible formation of the stable [Ni11P(CO)18]4- tetra-anion is demonstrated through the spectroelectrochemical investigation of [Ni11P(CO)18]3-. The redox changes of [HNi31P4(CO)39]5- show features of chemical reversibility and the vibrational spectra in the νCO region of the nine redox states of the cluster [HNi31P4(CO)39]n- (n = 3-11) are reported. The spectroelectrochemical investigation of [H2Ni31P4(CO)39]4- revealed the presence of three chemically reversible reduction processes, and the IR spectra of [H2Ni31P4(CO)39]n- (n = 4-7) have been recorded. The different spectroelectrochemical behavior of [HNi31P4(CO)39]5- and [H2Ni31P4(CO)39]4- support their formulations as polyhydrides. Unfortunately, all the attempts to directly confirm their poly hydrido nature by 1H 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.