COMMUNICATIONS spectroscopy. The low g value (2.00374) and the coupling constants (u,,,, = 0.338 mT, uHm = 0.133 mT, aN = 0.802 mT), which are typical of aminyl radicals, enabled the presence of a nitroxyl radical to be ruled out.[''] The diradical 17 ( g = 2.00374) prepared by dehydrogenation of the model compound 11 could be confirmed by ESR spectroscopy. Zero field splitting at 135 K (toluene, glass) was determined to be 1.76 mT. The distance of 1.1 7 nm found from this by point dipole estimation is in good agreement with the calculated distance between the aminyl centers of I .O2 nm. [191 17 The new polymer PPSA has a well-defined structure and a high molecular weight. It is colorless, stable at temperatures up to 380 'C, very soluble in many common organic solvents, and dopable. Continuing investigations into its optical and electronic properties and into the conversion of 1 into the polyradical 15 are in progress.

The role played by vanadium in biochemistry has become widely appreciated in recent years.1-3 Its involvement ranges from being the active-site metal in a number of enzymes (bromoperoxidase and nitrogenase) to being a potent inhibitor of others.4-7 Among this latter group are the phosphate-metabolizing enzymes such as RNase, where bound vanadate has been proposed to function as a transition state analog of the pentavalent trigonal bipyramidal intermediate of phosphate hydrolysis.8 This supposition, first presented by Lindquist et al., was based on the known coordination

Plasmid DNA is cleaved by dimeric 0x0-bridged vanadium(II1) complexes without exogenous oxidants or reductants.

Tetranuclear V(III) complexes, [HB(pz)(3)](4)V(4)(&mgr;-C(6)H(5)OPO(3))(4) (I), its acetonitrile solvate (I.4CH(3)CN), and [HB(pz)(3)](4)V(4)(&mgr;-O(2)NC(6)H(4)OPO(3))(4).4C(7)H(8).H(2)O (II), and tetranuclear vanadyl complexes, (t-Bupz)(4)V(4)O(4)(&mgr;-C(6)H(5)PO(3))(4).2H(2)O (III) and (t-Bupz)(5)V(4)O(4)(&mgr;-C(6)H(5)PO(3))(4).4CH(3)CN.0.6 H(2)O (IV), have been prepared and characterized by spectroscopic, magnetic, and electrochemical methods (pz = pyrazole, t-Bupz = tert-butylpyrazole). The use of organic solvents and bulky organic groups as ancillary ligands leads to formation of neutral species instead of the anionic clusters commonly found in the hydrothermal synthesis of vanadium organophosphate/phosphonate systems. Complexes I.4CH(3)CN and IV have also been characterized by single-crystal X-ray diffraction. Crystal data: I.4CH(3)CN, triclinic, P&onemacr;, a = 15.495(3) Å, b = 17.000(3) Å, c = 17.949(4) Å, alpha = 89.17(3) degrees, beta = 86.00(3) degrees, gamma = 78.60(3) degrees, Z = 2; IV, triclinic, P&onemacr;, a = 15.541(3) Å, b = 16.340(2) Å, c = 19.069(5) Å, alpha = 83.58(2) degrees, beta = 79.67(2) degrees, gamma = 63.68(1) degrees, Z = 2. Both are closed clusters, the core structure of the first consisting of a cubane-like arrangement of metal octahedra and phosphate tetrahedra and the core structure of the second consisting of a distorted, collapsed variant of the first. Unlike other vanadium phosphate clusters, these compounds form in the absence of a central, templating agent. As such they represent the simplest form of a closed cluster in which steric forces and cluster connectivity requirements play the primary role in organizing the cluster framework.

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