Construction of Larger Molecular Aluminophosphate Cages from the Cyclic Four-Ring Building Unit

New molecular aluminophosphates of
different nuclearity are synthesized
by a stepwise process and structurally characterized. The alkane elimination
reaction of bis­(trimethylsiloxy)­phosphoric acid, OP­(OH)­(OSiMe3)2, with trialkylalanes, AlR3 (R = Me,
Et, iBu), provides the cyclic dimeric aluminophosphates,
[(AlR2{μ2-O2P­(OSiMe3)2})2] (R = Me (1), Et (2), iBu (3)). Unsymmetrically substituted
cyclic aluminophosphonate [(AlMe2{μ2-O2P­(OSiMe3)­(cHex)})2] (cis/trans-4) is prepared by dealkylsilylation
reaction of cHexP­(O)­(OSiMe3)2 with
AlMe3. Molecules 1–4 containing
the [Al2(μ2-O2P)2] inorganic core are structural and spectroscopic models for the
single four-ring (S4R) secondary building units (SBU) of zeolite frameworks.
Compound 1 serves as a starting point in construction
of larger molecular units by reactions with OP­(OH)­(OSiMe3)2 as a cage-extending reagent and with diketones, such
as Hhfacac (1,1,1,5,5,5-hexafluoropentan-2,4-dione) and Hacac (pentan-2,4-dione),
as capping reagents. Reaction of 1 with 4 equiv of Hhfacac
leads to new cyclic aluminophosphate [(Al­(hfacac)2{μ2-O2P­(OSiMe3)2})2] (5), existing in two isomeric (D2 and C2h) forms.
Reaction of 1 with 2 equiv of OP­(OH)­(OSiMe3)2 and 1 equiv of Hhfacac provides a molecular aluminophosphate
[AlMe­{Al­(hfacac)}2{μ3-O3P­(OSiMe3)}2{μ2-O2P­(OSiMe3)2}2{OP­(OSiMe3)3}] (6), while by adding first the Hhfacac and using
3 equiv of OP­(OH)­(OSiMe3)2 we isolate [Al­{Al­(hfacac)}2{μ3-O3P­(OSiMe3)}2{μ2-O2P­(OSiMe3)2}2H­{OP­(O)­(OSiMe3)2}2] (7). These molecules contain units in their cores
that imitate 4=1 SBU of zeolite frameworks. Reaction with the order
of component mixing 1, Hhfacac, OP­(OH)­(OSiMe3)2 at a 1:2:2 molar ratio lead to formation of a larger
cluster [(Al­(AlMe)­{Al­(hfacac)}­{μ3-O3P­(OSiMe3)}2{μ2-O2P­(OSiMe3)2}3)2] (8)
containing both S4R and 4=1 structural units. Similarly, Hacac (pentan-2,4-dione)
provides an isostructural [(Al­(AlMe)­{Al­(acac)}­{μ3-O3P­(OSiMe3)}2{μ2-O2P­(OSiMe3)2}3)2] (9). Both molecules display Al centers in three
different coordination environments.