Polyoxometalates (POMs) are discrete, molecular metal oxides with dimensions ranging from about one to tens of ngstroms, a wide variety of topologies and compositions, and an extensive range of chemical and electronic properties which, together with their thermal and oxidative stabilities, are leading to applications in catalysis, electrooptics, magnetics, medicine, and biology. [1][2][3] Consequently, POMs are attractive as functional components of active materials. In recent years, systematic methods for POM synthesis and derivatization have been developed, providing an expanding range of robust "designer" components for "bottom-up" materials synthesis, and a major challenge now facing those engaged in POM research is to devise generic methods for constructing functional nanoscale architectures from these versatile building blocks.The self-assembly of organic monolayers on surfaces has developed over the last two decades into a powerful strategy for the construction of hierarchical structures from molecular components and, although self-assembled inorganic monolayers have received much less attention, several groups have investigated the incorporation of POMs into surface-confined structures. Ordered monolayers of POMs have been obtained on silver and gold by adsorption from solution, [4,5] and evaporative solution deposition has been used to produce catalytically active POM layers on highly oriented pyrolytic graphite (HOPG). [6][7][8] Hybrid organic-POM multilayered magnetic structures have been produced by LangmuirBlodgett techniques, [9][10][11] while electrostatic layer-by-layer assembly with cationic polyelectrolytes has produced more robust hybrid structures.[12] These approaches rely upon either electrostatic interactions or ill-defined chemisorption for the assembly of POM monolayers or multilayers and, to our knowledge, the only example of well defined covalent attachment to a surface is that of a thiol-derivatized POM on gold nanoparticles.[13] Our previous work has shown that monofunctional alkoxide POM derivatives [(RO)MW 5 O 18 ] nÀ (M = Ti, Zr, Nb) are accessible through hydrolytic aggregation reactions involving metal alkoxides.

The methoxo-bridged, dimeric, ZrIV-substituted Lindqvist-type polyoxometalate (POM) (nBu4N)6[{(mu-MeO)ZrW5O18}2], (TBA)61, has been synthesized by stoichiometric hydrolysis of Zr(OnPr)4, [{Zr(OiPr)3(mu-OnPr)(iPrOH)}2], or [{Zr(OiPr)4(iPrOH)}2] and [{WO(OMe)4}2] in the presence of (nBu4N)2WO4, providing access to the systematic nonaqueous chemistry of ZrW5 POMs for the first time and an efficient route to 17O-enriched samples for 17O NMR studies. 1H NMR provided no evidence for dissociation of 1 in solution, although exchange with MeOH was shown to be slow by an EXSY study. Reactions with HX at elevated temperatures gave a range of anions [{XZrW5O18}n]3n- (X = OH, 3; OPh, 4; OC6H4Me-4, 5; OC6H4(CHO)-2, 6; acac, 7; OAc, 8), where n = 2 for 3 and n = 1 for 4-8, while 1H and 17O NMR studies of hydrolysis of 1 revealed the formation of an intermediate [(mu-MeO)(mu-HO)(ZrW5O18)2]6-. Electrospray ionization mass spectrometry of 1 and 3 illustrated the robust nature of the ZrW5O18 framework, and X-ray crystal structure determinations showed that steric interactions between ligands X and the ZrW5O18 surface are important. The coordination number of Zr is restricted to six in aryloxides 4 and 5, while seven-coordination is achieved in the chelate complexes 6-8. Given the inert nature of the methoxo bridges in 1, protonation of ZrOW sites is proposed as a possible step in reactions with HX. The diphenylphosphinate ligand in [(Ph2PO2)ZrW5O18]3- was found to be labile and upon attempted recrystallization the aggregate [(mu3-HO)2(ZrW5O18)3H]7- 9 was formed, which was found to be protonated at ZrOZr and ZrOW sites. This work demonstrates the flexibility of the {ZrW5O18}2- core as a molecular platform for modeling catalysis by tungstated zirconia surfaces.

The tetra-n-butylammonium (TBA) salt of [(MeO)TiW(5)O(18)](3-) 1 was reacted with alcohols ROH to give primary, secondary and tertiary alkoxide derivatives [(RO)TiW(5)O(18)](3-) (R = Et 2, (i)Pr 3 and (t)Bu 4), whilst hydrolysis afforded [(mu-O)(TiW(5)O(18))(2)](6-) 5 rather than the hydroxo derivative (R = H). In reactions with (i)PrOH and (t)BuOH, impurity peaks observed at 1015 and 1020 ppm in the (17)O NMR spectra indicate alkoxide degradation and Ti=O bond formation via reactions analogous to those occurring at the surfaces of solid heteropolyacids. Aryloxides [(ArO)TiW(5)O(18)](3-) were prepared by reacting 1 with phenols ArOH (Ar = C(6)H(5) 6, C(6)H(4)Me-4 7, C(6)H(4)(t)Bu-4 8, C(6)H(4)OH-4 9, C(6)H(4)OH-3 10, C(6)H(3)(OH)(2)-3,5 11 and C(6)H(4)CHO-2 13). TiW(5)O(18) units were linked by reacting 1 with 9 to give [(mu-1,4-OC(6)H(4)O)(TiW(5)O(18))(2)](6-) 12. (17)O and (183)W NMR spectra are reported and X-ray crystal structures were obtained for TBA salts of anions 3-10 and 13, which showed that the titanium is six-coordinate in all cases. Reactions were monitored by (1)H NMR, including a 2D-EXSY study of methoxo exchange, and the slow rates observed are probably associated with the reluctance of titanium in these anions to achieve seven-coordination.

Polyoxometalates (POMs) are discrete, molecular metal oxides with dimensions ranging from about one to tens of ngstroms, a wide variety of topologies and compositions, and an extensive range of chemical and electronic properties which, together with their thermal and oxidative stabilities, are leading to applications in catalysis, electrooptics, magnetics, medicine, and biology. [1][2][3] Consequently, POMs are attractive as functional components of active materials. In recent years, systematic methods for POM synthesis and derivatization have been developed, providing an expanding range of robust "designer" components for "bottom-up" materials synthesis, and a major challenge now facing those engaged in POM research is to devise generic methods for constructing functional nanoscale architectures from these versatile building blocks.The self-assembly of organic monolayers on surfaces has developed over the last two decades into a powerful strategy for the construction of hierarchical structures from molecular components and, although self-assembled inorganic monolayers have received much less attention, several groups have investigated the incorporation of POMs into surface-confined structures. Ordered monolayers of POMs have been obtained on silver and gold by adsorption from solution, [4,5] and evaporative solution deposition has been used to produce catalytically active POM layers on highly oriented pyrolytic graphite (HOPG). [6][7][8] Hybrid organic-POM multilayered magnetic structures have been produced by LangmuirBlodgett techniques, [9][10][11] while electrostatic layer-by-layer assembly with cationic polyelectrolytes has produced more robust hybrid structures.[12] These approaches rely upon either electrostatic interactions or ill-defined chemisorption for the assembly of POM monolayers or multilayers and, to our knowledge, the only example of well defined covalent attachment to a surface is that of a thiol-derivatized POM on gold nanoparticles.[13] Our previous work has shown that monofunctional alkoxide POM derivatives [(RO)MW 5 O 18 ] nÀ (M = Ti, Zr, Nb) are accessible through hydrolytic aggregation reactions involving metal alkoxides.