Is Single-4-Ring the Most Basic but Elusive Secondary Building Unit That Transforms to Larger Structures in Zinc Phosphate Chemistry?

Haloaryl
phosphates (X-dippH2, X = Cl, Br, I) react
with zinc acetate in the presence of collidine or 2-aminopyridine
(2-apy) to yield zinc phosphate clusters [Zn­(X-dipp)­(collidine)]4 (X = Cl (1), Br (2), I (3)) and [Zn­(X-dipp)­(2-apy)]4·2MeOH (X = Cl (4), Br (5), I (6)), respectively.
Single-crystal X-ray diffraction studies reveal that collidine and
2-apy capped zinc phosphates 1–6 exist
as discrete tetrameric zinc phosphate molecules, exhibiting a cubane-shaped
D4R core. In contrast, when the same reaction has been carried out
in the presence of 4-cyanopyridine (4-CNpy), polymeric zinc phosphates
{[Zn4(X-dipp)4(4-CNpy)2(MeOH)2]·2H2O}n (X =
Cl (7), Br (8), I (9)) have
been isolated. Compounds 7–9 are
square-wave-shaped, one-dimensional polymers composed of fused S4R
repeating units. The common structural motif found both in D4R cubanes 1–6 and polymers 7–9 is the S4R building block, which presumably undergoes further
fusion because of the coordinative unsaturation at zinc and the simultaneous
presence of free PO. The closed shell cubanes 1–6 are obviously formed by a face-to-face dimerization involving two S4R units in which the two PO
groups are in cis-configuration. On the other hand, the one-dimensional
(1-D) square-wave polymers 7–9 are
formed from a face-to-face association of S4R building units in which
the two PO groups are in a trans-configuration. In order to
stabilize these elusive S4R zinc phosphates, the reaction between
Cl-dippH2 and zinc acetate was carried out in the presence
of excess imidazole as an ancillary ligand (1:1:4), although only
an imidazole decorated cubane cluster [Zn­(Cl-dipp)­(imz)]4.2MeOH (10) was isolated. The chelating N,N′-donor 1,10-phenanthroline ligand was
used to eventually isolate cyclic S4R phosphate [Zn­(μ2-Cl-dipp)­(1,10-phen)­(OH2)]2·MeOH·H2O (11). The change of Zn2+ source
to zinc nitrate and the phosphate source to 2,6-dimethylphenyl phosphate
(dmppH2) led to the isolation of another polymeric phosphate
[Zn­(dmpp)­(MeOH)]n (12), with
a zigzag backbone, formed through an edge-to-edge to polymerization of S4R building units with PO groups in
trans-configuration. The isolation of four different structural types
of zinc phosphates A–D in the present
study can be rationalized in terms of fusion of S4R rings in a variety
of ways to either produce discrete clusters or 1-D polymers.