18.4.4.3 Cobalt(II) and Manganese Phosphates
Cobalt(II) has a special place in the open framework family because it is one of the
few transition metal cations that easily exhibits tetrahedral coordination, in addition to five and sixfold ones. Moreover, magnetic couplings can give rise to interesting magnetic properties, similar to those described above for iron. Furthermore,
it is well known that Co doping enhances the catalytic performance of certain zeolites and related compounds. For instance, CoAPO-5 and CoAPO-11 have been
used for the autoxidation of cyclohexane [77] and p-cresol [78].
Recently, a substantial effort was made to synthesize pure cobalt phosphates
with an open framework, using either inorganic or organic templates. The first
success [79] concerned the pseudo-tetragonal three-dimensional CoPO4Á0.5C 2 Á
H 10 N 2 . The framework consists of three systems of channels with eight-membered
windows limited by Co and P tetrahedra in strict alternation. The dimensions are
0:39 Â 0:47 nm
2 , and the compound becomes antiferromagnetic at 2 K. The threedimensional character exists only with ethylenediamine. As soon as the length of
the carbon chain increases (e.g. 1,3-diaminopropane and 1,4-diaminobutane), the
solids become two dimensional [80] with planes built up from corner sharing trans
chains of Co
2þ tetrahedra linked by phosphate groups.
Subsequently, Stucky’s group [81], being aware that the tremendous success
with the aluminophosphate system was in part due to the initial recognition of
structural analogies between AlPO 4 and SiO 2 , sought to determine if the same
analogies also exist between cobalt phosphates and aluminosilicate minerals. This
design approach led them to characterize, using alkali (M ¼ Na, K, Rb) and NH 4
þ
ions, chiral tetrahedral MCoPO 4 cobalt phosphates. They exhibit either the zeolite
Li-ABW structure type with NH 4
þ and Rb, or a hexagonal structure, intermediate
between the ABW and tridymite structures, with Na, K, and NH 4
þ . The latter
shows 6-ring channels. This paper clearly shows the difficulty of the corresponding
chemistry and underlines the drastic influence of pH and the nature of the solvent
on the synthesis of the desired products. Despite the usually antiferromagnetic
behavior of these CoPO 4 materials, the sodium compound is ferromagnetic at 2 K.
Manganese phosphates with an open framework are very scarce, but the
chemistry of new phases in this area is emerging with some recent reports of
organically-templated, layered manganese phosphates [82, 83] and a threedimensional system with narrow channels, NH 4 MnPO 4 , which orders ferromagnetically at @17 K [84].
18.4.4.4 Copper and Nickel Phosphates
While open-framework copper phosphates have proven difficult to prepare using
conventional hydro/solvothermal reactions, Hwu and coworkers have developed a
novel approach using molten salt fluxes to synthesize several microporous copper
phosphates. Examples have been prepared with salt occupying relatively large
channels in several different structure types [85, 86], including homochiral single
crystals [87]. In at least one of these compounds, the salt can be replaced with
water (except for some Cs
þ cations necessary for charge balance), resulting in an
18.4 Open-Framework Metal Phosphates 603
Cobalt(II) has a special place in the open framework family because it is one of the
few transition metal cations that easily exhibits tetrahedral coordination, in addition to five and sixfold ones. Moreover, magnetic couplings can give rise to interesting magnetic properties, similar to those described above for iron. Furthermore,
it is well known that Co doping enhances the catalytic performance of certain zeolites and related compounds. For instance, CoAPO-5 and CoAPO-11 have been
used for the autoxidation of cyclohexane [77] and p-cresol [78].
Recently, a substantial effort was made to synthesize pure cobalt phosphates
with an open framework, using either inorganic or organic templates. The first
success [79] concerned the pseudo-tetragonal three-dimensional CoPO4Á0.5C 2 Á
H 10 N 2 . The framework consists of three systems of channels with eight-membered
windows limited by Co and P tetrahedra in strict alternation. The dimensions are
0:39 Â 0:47 nm
2 , and the compound becomes antiferromagnetic at 2 K. The threedimensional character exists only with ethylenediamine. As soon as the length of
the carbon chain increases (e.g. 1,3-diaminopropane and 1,4-diaminobutane), the
solids become two dimensional [80] with planes built up from corner sharing trans
chains of Co
2þ tetrahedra linked by phosphate groups.
Subsequently, Stucky’s group [81], being aware that the tremendous success
with the aluminophosphate system was in part due to the initial recognition of
structural analogies between AlPO 4 and SiO 2 , sought to determine if the same
analogies also exist between cobalt phosphates and aluminosilicate minerals. This
design approach led them to characterize, using alkali (M ¼ Na, K, Rb) and NH 4
þ
ions, chiral tetrahedral MCoPO 4 cobalt phosphates. They exhibit either the zeolite
Li-ABW structure type with NH 4
þ and Rb, or a hexagonal structure, intermediate
between the ABW and tridymite structures, with Na, K, and NH 4
þ . The latter
shows 6-ring channels. This paper clearly shows the difficulty of the corresponding
chemistry and underlines the drastic influence of pH and the nature of the solvent
on the synthesis of the desired products. Despite the usually antiferromagnetic
behavior of these CoPO 4 materials, the sodium compound is ferromagnetic at 2 K.
Manganese phosphates with an open framework are very scarce, but the
chemistry of new phases in this area is emerging with some recent reports of
organically-templated, layered manganese phosphates [82, 83] and a threedimensional system with narrow channels, NH 4 MnPO 4 , which orders ferromagnetically at @17 K [84].
18.4.4.4 Copper and Nickel Phosphates
While open-framework copper phosphates have proven difficult to prepare using
conventional hydro/solvothermal reactions, Hwu and coworkers have developed a
novel approach using molten salt fluxes to synthesize several microporous copper
phosphates. Examples have been prepared with salt occupying relatively large
channels in several different structure types [85, 86], including homochiral single
crystals [87]. In at least one of these compounds, the salt can be replaced with
water (except for some Cs
þ cations necessary for charge balance), resulting in an
18.4 Open-Framework Metal Phosphates 603