Communication www.rsc.org/chemcomm CHEMCOMM Mimicking oxide surfaces: different types of defects and ligand coordination at well defined positions of a molybdenum oxide based nanocluster† Achim Müller,* Rabindranath Maiti, Marc Schmidtmann, Hartmut Bögge, Samar K. Das and Wenjian Zhang Fakultät für Chemie der Universität, D-33501, Bielefeld, Germany. E-mail: a.mueller@uni-bielefeld.de Received (in Cambridge, UK) 10th July 2001, Accepted 17th August 2001 First published as an Advance Article on the web 18th September 2001 The mixed valence cluster anion of the compound (NH 4 ) 32 [Mo VI 110 Mo V 28 O 416 H 6 (H 2 O) 58 (CH 3 CO 2 ) 6 ]·xH 2 O 1 (x ~ 250), synthesized under one-pot conditions, contains well-defined different types of defects—missing groups compared to the complete parent {Mo 154 } type cluster with full D 7d symmetry—and acetate ligand coordination; this proves that the giant-wheel type anion can be considered as an object with a variety of nanoscale structural features (“nanostructured landscape”) allowing reactions at a variety of well defined centers. It is still a challenge to understand details of the interaction mechanisms of substrates with the surfaces of heterogeneous catalysts, 1,2 as for instance in the case of transition metal oxides which play an important role in industrial processes, e.g. in selective oxidations. 3 Of particular interest is MoO 3 , which shows an enormous versatility of catalytical properties. 3a A tremendous step to this end would be to consider the well defined discrete giant metal–oxide based cluster species, i.e., nanoreactors (which show the same or a similar structure including defects on their large surfaces), as relevant catalyt- ically active bulk materials, the surface of which is difficult to investigate. Remarkably, very little is known as yet about the influence of such defects present on the surface of an oxide, and their role in determining the catalytic properties. 3b Here we report on the synthesis of (NH 4 ) 32 [Mo VI 110 Mo V 28 O 416 H 6 - (H 2 O) 58 (CH 3 CO 2 ) 6 ]·xH 2 O 1 (x ~ 250) which shows different types of well defined defects, substrate interactions/activation, and even other important structural features such as Mo VI/V type redox as well as acid sites, important for the catalytic action of molybdenum–oxide based catalysts. In particular the pentago- nal bipyramidal MoO 7 polyhedra seem to be of interest for understanding selective oxidations. 3c When an aqueous solution of ammonium heptamolybdate is reduced with hydrazinium sulfate in the presence of a high acetic acid concentration, blue crystals of 1 precipitate within three weeks.‡ Compound 1 was characterized by elemental analysis, thermogravimetric analysis (to determine the crystal water content), cerimetric titrations [for the determination of the (formal) number of Mo V centers], spectroscopic methods (IR, resonance Raman, VIS–NIR)§ and single crystal X-ray struc- ture analysis, including bond valence sum (BVS) calculations (to determine the number and positions of H 2 O and OH groups as well as the formal number of Mo V centers).¶ The relatively low solubility of 1 in water is due to the abundance of acetate ligands and ammonium cations. The crystal structure of 1 shows in the lattice the presence of a derivative of the “classical” tetradecameric type of molybde- num–oxide based anionic giant wheel, formulated with its characteristic building blocks as [{Mo 2 } 14 {Mo 8 } 14 - {Mo 1 } 14 ] 142 = [{(O) 2 NMo VI (H 2 O)(m-O)(H 2 O)Mo VI N (O) 2 } 2+ 14 {Mo VI/V 8 O 26 (m 3 -O) 2 H(H 2 O) 3 Mo VI/V } 32 14 ] 142 · [Mo VI 126 Mo V 28 O 462 H 14 (H 2 O) 70 ] 142 2a. 4 In 1a (Fig. 1) there are six {Mo 2 } units missing while two bidentate acetate ligands are coordinated to other abundant {Mo 2 } units thereby replacing four H 2 O ligands with the formation of {Mo 2 Ac}* units. In addition, four {Mo 1 } units are—compared to 2a— “released”. Describing the “release” of these {Mo 1 } units is somewhat difficult (see Fig. 2): formally, 4 {{MoO 2 } 2+ + H 2 O + H + } collectives of 2a are removed and 4 {CH 3 CO 2 2 + 3H + } added, thus leading to the schematic description of 1a as [{Mo 2 } 6 {Mo 2 Ac}* 2 {Mo 8 } 10 {Mo 8 Ac}* 4 {Mo 1 } 10 ] 322 or to the formula (NH 4 ) 32 [Mo VI 110 Mo V 28 O 416 H 6 (H 2 O) 58 (CH 3 - CO 2 ) 6 ]·xH 2 O (x ~ 250) for the resulting compound 1, as stated † Dedicated to Prof. J. Strähle on the occasion of his 65th birthday. Fig. 1 Side view of the structure of 1a in crystals of 1 in ball and stick representation with enlarged C atoms of the acetate ligands. Fig. 2 Schematic representation (view from the inside of the rings) of the structural change from 2a to 1a at one representative {Mo 5 O 6 } type compartment built up by four {Mo 8 } and one {Mo 1 } type atoms: formally, one MoO 2 2+ group, i.e. the {Mo 1 } type atom with the terminal oxygen atom bonded to it and the m-O atom bridging the Mo atom and the pentagonal Mo center of the {Mo 8 } group, is removed from 2a. The vacant position due to the loss of m-O at the pentagonally coordinated Mo atom is filled by an O atom of an acetate ligand while the other O atom of the carboxylate group substitutes a H 2 O ligand of a neighbouring Mo center. The two m 3 -O type atoms (one of which is protonated in 2a) of the complete {Mo 5 O 6 } compartment of 2a situated on the equator are doubly protonated in 1a. Consequently, a protonation at the m 3 -O type atom of a neighbouring intact compartment is not possible due to space limitations. This journal is © The Royal Society of Chemistry 2001 2126 Chem. Commun., 2001, 2126–2127 DOI: 10.1039/b106092a Downloaded by Georgetown University Library on 06 March 2013 Published on 18 September 2001 on http://pubs.rsc.org | doi:10.1039/B106092A View Article Online / Journal Homepage / Table of Contents for this issue