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Synthesis and reactivity of the methoxozirconium pentatungstate ( nBu4N)6[{(μ-MeO)ZrW5O 18}2]: Insights into proton-transfer reactions, solution dynamics, and assembly of {ZrW5O18}2- building blocks

Lookup NU author(s): Dr John Errington, Dr Sagar Petkar, Paul Middleton, Emeritus Professor William McFarlane, Emeritus Professor Bill Clegg, Dr Ross Harrington

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Abstract

The methoxo-bridged, dimeric, ZrIV-substituted Lindqvist-type polyoxometalate (POM) (nBu4N)6[{(μ-MeO) ZrW5O18}2], (TBA)61, has been synthesized by stoichiometric hydrolysis of Zr(OnPr)4, [{Zr(OiPr)3(μ-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 [{XZrW 5O18}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 [(μ-MeO)(μ-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 [(Ph 2PO2)ZrW5O18]3- was found to be labile and upon attempted recrystallization the aggregate [(μ3-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 {ZrW5O 18}2- core as a molecular platform for modeling catalysis by tungstated zirconia surfaces. © 2007 American Chemical Society.


Publication metadata

Author(s): Errington RJ, Petkar SS, Middleton PS, McFarlane W, Clegg W, Coxall RA, Harrington RW

Publication type: Article

Publication status: Published

Journal: Journal of the American Chemical Society

Year: 2007

Volume: 129

Issue: 40

Pages: 12181-12196

ISSN (print): 0002-7863

ISSN (electronic): 1520-5126

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/ja0725495

DOI: 10.1021/ja0725495


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