X. Feng et al. / Journal of Solid State Chemistry 206 (2013) 277–285
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compared with those of homodinuclear lanthanide xanthene-9-
carboxylate complexes [14].
The binuclear unit is between outer benzene rings, giving an
arborization-like array, however, the pyridine and phenyl ring are
not coplanar. The dihedral angles between pyridine and phenyl ring
within the same Hdppꢀ ligand is 48.831, while the dihedral angles
between two phenyl rings sharing the common Pr(III) ion are is
71.261, and the carboxyl group is nearly in the plane of benzene
ring. The second category of ligand is fully deprotonated, acting as a
penta-dentate ligand. This entangled ligand displays μ4-kO, O′: kO:
k O: kO′ fashion to link four adjacent Pr(III) ions. Both two
carboxylic groups of dpp2ꢀ adopt the bis-(bridging) bidentate as
well as monodentate modes (see Scheme 1(b)). While, each Hdppꢀ
ligand employs the 3-carboxylate group oxygen to chelate another
Pr(1) ion and using 4-hydroxyl oxygen connect the third Pr(1) ion.
The two neighboring Pr(1) ions are double linked via 3-carboxylate
group to form a Pr2 binuclear unit with the Prꢁ ꢁ ꢁ Pr separation of
5.522 Å. These dinuclear segments are grafted on to the infinite 1D
double-stranded chain arrays along the crystallographic c-axis
through the carboxylate oxygen atoms from phthalate moiety and
oxygen atoms derived from the hydroxyl-pyridine entity, as shown
in Fig. 1b, and projection view along ab plane is shown in Fig. S3,
Supporting materials. The nearest Prꢁ ꢁ ꢁ Pr separation within this
chain is 6.586 Å. Meantime, Hdppꢀ ligand acts as a linker using the
3-carboxylate oxygen, consequently interconnects these 1-D chains
into a 2D corrugated grid along a-axis, as displayed in Fig. 2a, in
which the adjacent binuclear units are linked together by the μ2-O
bridging from 4-carboxylate group of the Hdppꢀ ligand to generate
a parallelogram aggregate composed of four neighboring Pr(1) ions,
and the diagonal-to-diagonal distances are 12.41, 10.34 Å, respec-
tively (see Fig. 1b). The individual nets are lattice in nature, and the
rhombus 4, 4 order topology layers arises [15]. Moreover the
puckered chain connected by Hdppꢀ, covers and reinforces this
2D sheet from upside, as shown in Fig. 2b, but this does not result in
interpenetrating formation. The coordination modes of the ligand
are essentially different from that of divalent zinc and cadmium
coordination polymers containing analogs flexible ligand [16], in
which, two carboxylate groups have a dihedral angle of 14.81 and
86.51 toward the plane of the corresponding phenyl rings. The
combination of the pyridine ring and phenyl ring are twisted across
the CꢀN bond and these twisted carboxylate groups result in the
formation of (CdꢀL)n and (CdꢀPA)n helical chains.
Fig. 1. (a) The coordination environment of Pr(III) ion in the symmetry unit of 1
viewed along ac plane. (b) Polyhedral diagram of the 1D double-stranded chains of
Pr(1) ions linked by the full depronated dpp2ꢀ ligand viewed approximately down
b-axis in 1.
on 1D double-stranded lanthanide-carboxylate motifs. Therefore,
structure of 1 was selected and described in details to represent
their frameworks. As illustrated in Fig. 1a, the symmetric unit of 1
contains two crystallographically independent Pr(III) cations and
four H2dpp ligands, whereas the fromate anion does not present in
the final product. Interestingly, there are no solvent or crystal-
lization water molecules, even without coordinated terminal aqu-
eous ligand in the structure, which proves that it is crystallized from
an ethanol–water aqueous medium. Presumably the abundance of
coordination sites and chelating ability of the mixed donor ligand
preclude the possibility of water binding to central Ln(III) ion, and
this restriction of O–H vibrational resonance always enhances the
luminescence of the given lanthanoid [12]. The Pr(III) ion is eight
coordinated bonding six H2dpp anion ligands. The extremely
distorted dodecahedronal geometry Pr{O8} around the central ion
is completed by eight O atoms, of which six carboxylate O atoms are
from two distinct carboxylic groups and two O atoms are from
hydroxyl pyridine moieties of two independent Hdppꢀ ligands.
The Pr1–O bond lengths fall in the range of 2.358(2)–2.730(3) Å,
with an average value of ca. 2.439 Å for octa-coordinated Pr(1) ions
(see Table S1). This is closely related to those observed for other
Pr(III) complexes with similar amino acid-like ligands [13]. Among
the six multi-carboxylic ligands, four are completed deprotonated,
while the other two just are partially deprotonated, being denoted
as dpp2ꢀ and Hdppꢀ respectively. It is interesting that both two
kinds of ligand adopt the chelating and bi-monodentate bridging
fashions, linking neighboring Pr(III) ion (see Scheme 1(a)).
Two adjacent Pr(III) ions are doubly bridged by two carboxylic
groups from Hdppꢀ, exhibiting a cyclooctane-like configuration, as
indicated in Fig. S1, Supporting materials. The two Pr(III) metal
units are exactly identical, making the molecule symmetry with
an inversion symmetry center, each. This binuclear array can be
These 2D layers are further interconnected and extended
through terminal oxygen atoms from carboxylate and hydroxyl
pyridine, into a 3D convent complicated coordination networks
consequently, as demonstrated in Fig. 3a. The distance between
the adjacent layers is 13.309 Å. This array can be comparable to the
series of lanthanide coordination polymers containing the 2,2′-
bipyridyl-4,4′-dicarboxylic acid with the similar crystal system
[17]. Packing view of the 3D structure along b c plane projection is
shown in Fig. S3, in Supporting materials. The hydrogen bonding
(see Fig. S4 and Tab S2) and
observed.
π
–
π
stacking interactions are also
A better insight into the nature of this framework can be
achieved by the application of a topological approach, i.e. reducing
the multidimensional structures to simple node and connecting
nets. The dpp2ꢀ bridged dinuclear Pr(III) unit was treated as one
node, producing a 4, 4-connected net square lattice topology
(Fig. 2a). Then the ligand further interconnected these 2D sheet
via carboxylic O and phenolic-hydroxyl O into a (4, 6)-connected
3D framework. The total Schläfli symbol is (43526) (4355667).[18]
as illustrated in Fig. 3b. It should be pointed out that in present
work from the light lanthanide praseodymium complex of 1 to
the heavy lanthanide erbium complex 5, they all exhibit the same
3D lattice array. This case is different from those lanthanide
coordination polymers based on the multi-N, O-donor ligands
Feng, Xun
