Polyoxotitanates join the Keggin family: synthesis, structure and reactivity of [Ti18O28H][OBut]17

Abstract

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...

Synthesis, Characterization, Cytotoxicity, and Hydrolytic Behavior of C2‐ and C1‐Symmetrical TiIV Complexes of Tetradentate Diamine Bis(Phenolato) Ligands: A New Class of Antitumor Agents

“…It is also quite surprising that in all processes in which a bis(phenolato) ligand is released, its hydrolysis rate is similar to that of the isopropoxo groups, which are hydrolyzed quite slowly for [Ti(L n )A C H T U N G T R E N N U N G (OiPr) 2 ] (n = 1-4). [42][43][44][45][46] Overall, these observations are consistent with those obtained by the UV/Vis measurements regarding formation of a new species in substantial amounts for the biologically active complexes [Ti(L 2 ] in THF and stirring at room temperature for three days gave a significant amount of single crystals that, somewhat surprisingly, [42][43][44][45][46] turned out to be the starting (Figure 1, top). However, whereas increasing the amount of water to 100 equivalents gave similar results when the reaction was mixed for only 1.5 h, different single crystals suitable for X-ray crystallography were obtained in 95% yield when the reaction was allowed to stir with 50 equivalents of water for three days.…”

“…Therefore, if steric crowding allows, a new polynuclear species may form upon replacement of the isopropoxo groups; however, this process is not rapid for this family of complexes. [42][43][44][45][46] Effect of ligand lability in [Ti(L , which is able to form a trinuclear complex, it is plausible that the strong binding of the catecholato ligand and perhaps also the different symmetry of the original complex prohibit this interaction, as the complex symmetry was previously observed to have a strong influence on activity. [33] Interestingly, this more inert complex also exhibits reduced cytotoxicity against both HT- (Figure 17).…”

Polynuclear Bismuth‐Oxo Clusters: Insight into the Formation Process of a Metal Oxide

“…The most extensively studied compounds are those of Group IV. Several titanium-oxo alkoxides have been fully characterised, [54] in which the predominant coordination number of the titanium atoms within the metal-oxo clusters is six. The large oxygen atoms play a structure-directing role.…”

Heterobimetallic sol–gel precursors and intermediates

“…W. G. Klemperer compiled a series of molecular clusters Ti a O b (OH/OR) c , with nuclearities (index a) ranging from 3 to 18 (a few more were isolated later) from partially hydrolyzed titanium alkoxides [29,30] and showed how the structures depend on the Ti/O ratio (a:b), i.e., the degree of condensation. Modification of Ti(OR) 4 by bidentate ligands (BL) adds a second dimension, because the structures of the clusters Ti a O b (OR/OH) c (BL) d not only depend on the Ti/O (a/b) but also on the Ti/BL (a/d) ratio [21].…”