PHOTOCHEMISTRY OF POLYOXOVANADATES .1. FORMATION OF THE ANION-ENCAPSULATED POLYOXOVANADATE [V15O36(CO3)](7-) AND ELECTRON-SPIN POLARIZATIONOF ALPHA-HYDROXYALKYL RADICALS IN THE PRESENCE OF ALCOHOLS

Prolonged photolysis of aqueous solutions containing [V4O12](4-) and M eOH at pH 9 (pH adjusted by use of K2CO3) led to the formation of K5H2 [V15O36(CO3)].14.5H(2)O. A single-crystal X-ray structural analysis of dark green crystals of K5H2[V15O36(CO3)].14.5H(2)O [triclinic, space group P (1) over bar, a = 12.361(4), b = 18.149(5), c = 11.415(4) Angs trom, alpha = 93.44(3). beta = 107.76(3), gamma = 93.54(3)degrees, Z = 2, R = 0.081 for 4182 independent data with I > 5 sigma(I)] showed th at the pentadecavanadate encapsulates CO32-, with approximate D-3h, sy mmetry and formally contains eight V-IV and seven V-V centres. The co- ordination geometries at the vanadium centres in the anion consist of twelve (V-IV, V-V) VO5 square pyramids and three (VO6)-O-IV octahedra. Each of the VO6 octahedra co-ordinates the CO32- oxygen atoms with V- O bond lengths 2.2616(8)-2.3710(7) Angstrom. The photochemical encapsu lation of other anions such as Cl-, Br-, NO3- or PO33- in the [V15O36] (5-) spherical cluster shell is observed in a similar way, which is ap plicable to the photofixation of CO2. The photoinduced electron transf er to the O --> V ligand-to-metal charge-transfer (l.m.c.t.) excited s tate of [V4O12](4-) from alcohols such as MeOH, EtOH or PrOH was inves tigated with chemically induced dynamic electron polarization (CIDEP) with 100 ns time resolution. The formation and decay of alpha-hydroxya lkyl radicals. following pulsed-laser excitation of the polyoxometalat e was also observed for [Mo7O24](6-) and [W10O32](4-). Observation of an emissive CIDEP ESR pattern for the alpha-hydroxyalkyl radicals reve aled that the reaction precursor is the excited triplet state of the O --> M (M = V, Mo or W) l.m.c.t. which undergoes hydrogen abstraction from alcohols to yield the one-protonated reduced polyoxometalate (for example [V4O12H](4-)) and the alpha-hydroxyalkyl radical. The absence of distortion in the ESR polarization of alpha-hydroxyalkyl radicals and the external magnetic field effect indicated that the interaction between [V4O12H](4-) alpha-hydroxyalkyl radicals is extremely weak in water.