Organometallic Trinuclear Niobium Cluster Complex in Aqueous Solution: Synthesis and Characterization of Niobium Complexes Containing Nb-3(mu-eta(2):eta(2) (perpendicular to)-NCCH3)(mu(2)-O)(3)(6+) Cluster Core

The synthesis and characterization of an organometallic trinuclear oxo-bridged niobium cluster complex with perpendicularly coordinated mu(3)eta(2):eta(2)(perpendicular to)-acetonitrile ligand in aqueous solution is reported. Reaction of NbCl5 in acetonitrile with aluminum under argon followed by reaction with aqueous hydrochloric acid affords, after suitable workup, the isolation of the organometallic [Nb-3(mu-eta(2):eta(2)-NCCH3)O-3(H2O)(9)](6+) aqua ion by cation-exchange chromatography. The purple niobium aqua ion in 2 M HCl shows a small peak at 365 nm (epsilon similar to 511 M-1 cm(-1) per Nb) and a broad peak at 565 nm (epsilon similar to 335 M-1 cm(-1)) in the UV-visible region. It is electron paramagnetic resonance (EPR)-active (g = 1.98), but no hyperfine interaction with the Nb-93 nuclear spin (I = 9/2) was observed. The cyclic voltammogram of [Nb-3(mu-eta(2):eta(2)-NCCH3)O-3(H2O)(9)](6+) in 4 M HCl on edge-plane pyrolytic graphite electrode at 50 mV s(-1) in the potential range -1.2 V to +1.1 V (vs SCE) exhibits three anodic peaks at -0.12, +0.53, and +0.85 V and a large cathodic peak at -0.91 V with a slight shoulder at about -0.8 V. The purple aqua ion reacted with potassium thiocyanate to give the green thiocyanate derivative, which was crystallized as ((CH3)(3)NH)(3)[Nb-3(mu-eta(2):eta(2)-NCCH3)O-3(NCS)(9)]center dot 2.5H(2)O (1) and subjected to X-ray structure analysis. Density-functional theory (DFT) calculations fully supported the structure of the cluster.