this strategy can push magnetic ordering to reasonable temperatures when threedimensional compounds are prepared.
18.6.2
Hybrid Metal Oxides
It is reasonable to expect that both the thermal stability and magnetic ordering
temperatures in hybrid systems may be improved by synthesizing materials with
extended M–O–M arrays. In other words, as the connectivity between transition
metals through oxygen increases, hybrid metal oxides should assume properties
closer to transition metal oxides and further from purely organic polymers. In this
section, we focus exclusively on transition metal systems, since their parent oxides
have the most exciting properties [139].
The earliest examples of such materials are the pillared layered structures, consisting of metal–oxygen sheets connected via diphosphonate bridges [140]. Porous
materials may be synthesized by combining both a rigid diphosphonate (such as
biphenylenebis(phosphonate)) and phosphite, where the average pore size may be
adjusted by varying the ratio of acids used in the synthesis. One important advantage of this approach is that materials with pores between the ranges covered by
crystalline zeolites and mesopores may be prepared. The relatively small crystallite
sizes and their tendency to grow as flat plates allows the synthesis of materials
with both nano- and mesoporosity. Because of their tunable pore sizes, aptitude for
functionalization, and moderately high thermal stability, these materials may find
similar applications to those of zeolites.
Polyoxometalate chemistry has been used to prepare a number of new metal
oxygen arrays. For example, many new molybdenum oxygen structures [141] have
been generated with diamines playing a similar charge balancing and templating
role as in aluminosilicate chemistry. Jacobson and coworkers have prepared several
transition metal-vanadate hybrid materials [142] with open architectures and reversible hydration/dehydration [143]. However, for the later transition metals, such
architectures are less likely due to the difficulty in maintaining high oxidation
states in hydro/solvothermal reactions.
More recently, several groups, particularly those of Fe ´rey and Cheetham, have
focused on metal carboxylates as a promising family for synthesizing novel hybrid
metal oxides. Fe ´rey and coworkers have used hydrothermal synthesis to prepare
the first vanadium carboxylate, V(OH)(1,4-benzenedicarboxylate) [144]. The structure consists of a 1D chain of octahedral V
3þ bridged in four directions by the dicarboxylate to create large channels. Despite having only 1D M–O–M connectivity,
long-range antiferromagnetic ordering is observed at 95 K. Along with an isostructural Cr
3þ analog [145], these materials may be dehydrated to show zeolitic
porosity with BET surface areas as high as 1500 m
2 g
À1 .
Livage and Fe ´rey used the flexible, a,o-dicarboxylate succinic acid to prepare
several structures, MIL-9 [146] and MIL-16 [147], with two dimensional CoaOaCo
sheets. Nickel succinate [148] (Figure 18.9) provides a striking example of this
new class of hybrid materials with a honeycomb framework based on a three18 Nanoporous Materials
612
18.6.2
Hybrid Metal Oxides
It is reasonable to expect that both the thermal stability and magnetic ordering
temperatures in hybrid systems may be improved by synthesizing materials with
extended M–O–M arrays. In other words, as the connectivity between transition
metals through oxygen increases, hybrid metal oxides should assume properties
closer to transition metal oxides and further from purely organic polymers. In this
section, we focus exclusively on transition metal systems, since their parent oxides
have the most exciting properties [139].
The earliest examples of such materials are the pillared layered structures, consisting of metal–oxygen sheets connected via diphosphonate bridges [140]. Porous
materials may be synthesized by combining both a rigid diphosphonate (such as
biphenylenebis(phosphonate)) and phosphite, where the average pore size may be
adjusted by varying the ratio of acids used in the synthesis. One important advantage of this approach is that materials with pores between the ranges covered by
crystalline zeolites and mesopores may be prepared. The relatively small crystallite
sizes and their tendency to grow as flat plates allows the synthesis of materials
with both nano- and mesoporosity. Because of their tunable pore sizes, aptitude for
functionalization, and moderately high thermal stability, these materials may find
similar applications to those of zeolites.
Polyoxometalate chemistry has been used to prepare a number of new metal
oxygen arrays. For example, many new molybdenum oxygen structures [141] have
been generated with diamines playing a similar charge balancing and templating
role as in aluminosilicate chemistry. Jacobson and coworkers have prepared several
transition metal-vanadate hybrid materials [142] with open architectures and reversible hydration/dehydration [143]. However, for the later transition metals, such
architectures are less likely due to the difficulty in maintaining high oxidation
states in hydro/solvothermal reactions.
More recently, several groups, particularly those of Fe ´rey and Cheetham, have
focused on metal carboxylates as a promising family for synthesizing novel hybrid
metal oxides. Fe ´rey and coworkers have used hydrothermal synthesis to prepare
the first vanadium carboxylate, V(OH)(1,4-benzenedicarboxylate) [144]. The structure consists of a 1D chain of octahedral V
3þ bridged in four directions by the dicarboxylate to create large channels. Despite having only 1D M–O–M connectivity,
long-range antiferromagnetic ordering is observed at 95 K. Along with an isostructural Cr
3þ analog [145], these materials may be dehydrated to show zeolitic
porosity with BET surface areas as high as 1500 m
2 g
À1 .
Livage and Fe ´rey used the flexible, a,o-dicarboxylate succinic acid to prepare
several structures, MIL-9 [146] and MIL-16 [147], with two dimensional CoaOaCo
sheets. Nickel succinate [148] (Figure 18.9) provides a striking example of this
new class of hybrid materials with a honeycomb framework based on a three18 Nanoporous Materials
612