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Reaction of Ti(OBu'), with about 1 equivalent of water in mt-butyl alcohol for 5 d at 100 "C yielded the polytitanate complex [Ti, 8028H][OB~1]17. A single-crystal X-ray diffraction study of its Bu'OH sohate [a = 15.789(4), h = 29.544(5), c = 27.496(5) A, = 91.85(5)"; 2 = 4; space group P2,/c-C:,,] revealed a Ti,8045 metal-oxygen framework having a pentacapped Keggin structure, i.e. a Ti ]. 3040 Keggin unit capped by five T i 0 moieties which lie above five of the six approximately square faces of the T,-distorted cuboctahedron defined by the metals in the Keggin unit. All seventeen of the terminal oxygen atoms are bonded to tert-butyl groups. This complex is the most highly condensed titanium alkoxide hydrolysis product isolated to date and showed good stability in solution, retaining its structure and showing no decomposition after 24 h at ambient temperature in toluene-tert-butyl alcohol solution, according to NMR spectroscopy. Reactions of the complex with water. 2-methylbutan-2-01 and triethylamine are also reported.In 1933 Keggin determined the crystal structure of 12tungstophosphoric acid, revealing the [PW, 2040]3 -anion structure shown in a where a tetrahedral oxoanion is encapsulated by a metal-oxygen cage (W 2036) whose twelve metal centres lie at the vertices of a T,-distorted cuboctahedron.This Keggin structure, the first structure ever definitively assigned to a large early-transition-metal polyoxoanion, proved to be of central importance in heteropolyanion chemistry in that numerous stable polyoxo-tungstate and -molybdate complexes have been shown to adopt this or closely related structures.' The Keggin structure was not, however, observed in Group 5 polyoxoanion chemistry until 1980, when the 14vanadophosphate structure was r e p ~r t e d . ~ Several isostructural polyvanadates have since been r e p ~r t e d . ~ As shown in b. the [PV , 4042]4 -anion has a bicapped Keggin structure in which two V 0 3 + units lie above frans 'square' faces of the distorted metal cuboctahedron in a [PV12040]15 -Keggin anion. Alternativcly. the [PV14042]9-anion can be derived from the 'superkeggin' structure c in which all six square faces of the Keggin metal cuboctahedron have been capped with metal oxide moicties. Indeed, this superkeggin structure was subsequently observed for the 18-vanadosulfate ion and an isostructural is~polyvanadate.~ In this paper we report a new polytitanate complex, [Ti, ,O,,H)[OBu'], 7, which is the first example ot'a Group 4 Keggin complex. As shown in d. this complex tias a [Til 8045]'*-metal-oxygen core structure that can be viewed either as a [Ti1304,,]28-Keggin complex capped by five Ti0' + units or as a defective superkeggin complex, i.e. a [Ti, 9040] ' -superkeggin lacking one Ti02 + moiety. a 9 Experimental Reagents, solvents and general proceduresThe following reagents were purchased from commercial sources and used without further purification: titanium@) isopropoxide (Aldrich), anhydrous tert-butyl acetate (Aldrich)f Basis of the presentation given at Dalton Discussion No. 1, 3...

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The dinucleoside phosphate A2'P5'U, C19H24N7012P, forms monoclinic crystals from aqueous solution, with a0=9-783, b0= 16.013, co= 8.718 A, fl=90"79 ° and two molecules in the unit cell, together with eight molecules of water. The space group is P2~. The phosphate group was located by a rigidbody search and least-squares fit to the sharpened Patterson distribution, and the remainder of the structure was then determined by Fourier methods, phasing first with the phosphate and then with successively increasing numbers of atoms. After least-squares refinement, all of the hydrogen atoms whose positions are fixed by the molecular framework were found in difference maps; some of the others were also indicated. The final R index was 0.060 for 1781 observed reflections. The molecular topography shows no unexpected features, and is compared with related molecules; a detailed summary of the conformations of phosphate diesters is included. Each A2'P5'U molecule is hydrogen-bonded to four adjacent nucleotide molecules and nine water molecules; no hydrogen bonds link base moeities directly, but the bases are stacked so that they overlap slightly, with an average interplanar separation of about 3.4/~. Close contacts of furanose-ring oxygen atoms with nearby bases are also present, as in some other structures of nucleic acid fragments.