Structural and dynamic stereochemistry of .alpha.-polyoxomolybdate (.alpha.-Mo8O264-)

Abstract

20) In many epoxide cyclization experiments, significant amounts of acyclic ketone resulting from simple rearrangement are formed, despite the greatly enhanced reaction rate due to the neighboring ir bond.6 As an explanation for this and other cyclization results, there may be generated with participation some type of partially cyclized intermediate cation common to all processes, reacting with external nucleophiles (or bases) to give acyclic and monocyclic material as well as with further double bonds to give polycyclics. As a particularly attractive feature, this hypothesis (proposed by Professor J. Brauman) accounts both for the rate acceleration in production of noncyclized material as well as the lack of extra acceleration in the rate of polycycle formation.

Hydrothermal Synthesis and Crystal Structure of [{Cu(2,2′-bpy)2}2Mo8O26]: a β-octamolybdate Cluster Covalently Bonded to Two {Cu(2,2′-bpy)2}2+ Coordination Complexes via Bridging oxo Groups

“…The present structure highlights an unusual mode of bonding between the molybdenum}oxo clusters and the copper coordination complexes; that is, the -(Mo O )\ cluster bonds to the (Cu(2,2-bpy) )> cations via bridging oxo groups that connect two adjacent molybdenum sites. The (Mo O )\ moiety is composed of eight edge-sharing (MoO ) octahedra and thus displays the characteristic -octamolybdate arrangement with two Mo O subunits stacking together (27). All molybdenum sites exhibit #6 oxidation state and are crystallographically independent, possessing octahedral coordination geometry with di!erent distortion extents.…”

A bond model for polyoxometalate ions composed of MO6 octahedra (MOk polyhedra with k>4)

“…Long M-O distances are considered by many authors to characterize ionic bonds [23][24][25][26][27][28]141] that can undergo facile cleavage [26][27][28]. Thus a number of polyoxometalate ions are described to be composed of an 02-or other anion surrounded by or otherwise combined with an (MO3)( cage, ring, or similar structure; for example (02 )(M6018) (z M60~9) ' (02 )2(M04012)2 (= (fi-) M08046), or (MoO2-)2(M06018) (= a-Mo804~) [3,[26][27][28]142].…”

“…Thus a number of polyoxometalate ions are described to be composed of an 02-or other anion surrounded by or otherwise combined with an (MO3)( cage, ring, or similar structure; for example (02 )(M6018) (z M60~9) ' (02 )2(M04012)2 (= (fi-) M08046), or (MoO2-)2(M06018) (= a-Mo804~) [3,[26][27][28]142]. Such structures are in direct contrast to the view presented in this article: the structurestabilizing effect of the inner bond valence is reduced and the large distance between the charges of the cations and polyanions that would result for salts of most of such species do not allow the necessary charge compensation according to the relevant bond length-bond valence functions.…”

“…Thus a number of polyoxometalate ions are described to be composed of an O z-or another anion surrounded by or otherwise combined with an (MO3)~ cage, ring, or something similar, for example, (O2-)(M6Ol8) (--M6029) or (po3-)(M12036) (=PM12030) [26][27][28][29][30][31]. For the case of the XM120~0-heteropolyoxo anions, where an XO~ n-tetrahedron is assumed to be surrounded by an M12036 cage [26,[29][30][31], this view has been questioned [32, 33a, 34a].…”

Solid state coordination chemistry of polyoxomolybdate clusters: Hydrothermal synthesis and structures of and (Hpt=5(4′-pyridyl)tetrazole)

“…Of this class of materials, composites constructed from embedded octamolybdate clusters, {Mo 8 O 26 } 4À , are quite common [52]. A curious feature of the structural chemistry is the observation of eight isomeric forms of octamolybdates, a through H [53][54][55][56][57][58][59][60][61][62].…”