Interpenetrated metal-organic frameworks of self-catenated four-connected mok nets

Article abstract of DOI:10.1039/c1cc10411j

We describe two cobalt metal-organic frameworks built by amide derivative and organodicarboxyl co-ligands, displaying 3-fold interpenetration of 6 ⁵.8-mok nets which are 4-connected self-catenated nets described theoretically in the early nineties.

Full text of DOI:10.1039/c1cc10411j

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Interpenetrated metal–organic frameworks of self-catenated
four-connected mok netsw
Yun Gong,ab Yu-Chao Zhou,b Tian-Fu Liu,a Jian Lu
¨
,*a Davide M. Proserpioc and
Rong Cao*a
Received 21st January 2011, Accepted 29th March 2011
DOI: 10.1039/c1cc10411j
We describe two cobalt metal–organic frameworks built by
amide derivative and organodicarboxyl co-ligands, displaying
3-fold interpenetration of 65.8-mok nets which are 4-connected
self-catenated nets described theoretically in the early nineties.
observed in a high-pressure polymorph of silica, coesite (coe)
where the shortest 8-rings are catenated with other 8-rings
(Fig. 1, left). In 2000, the only known example so far of
coordination polymer with coe topology was isolated.8
O’Keeffe7 also created a new uninodal net by merging together
pairs of the vertices of coe net, that still maintains the self-
catenation property (Fig. 1, right). This new net was believed
‘‘not very likely to be found as a net in a crystal structure’’
because there was one distance between two nodes that was
shorter than the edges.7 The self-catenated net has a vertex
symbol9 of 6.6.6.6.62.82 and named later as mok net
(point symbol 65.8). It is as-yet an unrealized network topology
in crystal structures of CPs/MOFs.10
The contemporary explosion of research interest on coordination
polymers (CPs) and metal–organic frameworks (MOFs) stems
from not only from their promising practical applications1 in
several cutting-edge realms such as catalysis, magnetism,
nonlinear optics, drug delivery, gas storage and separation,
but also their intriguing variety of architectures and
topologies.2 In particular, topology focuses on the network
connectivity generated via the reduction of periodic nets into
node-and-linker/vertex-and-edge representations, which helps
greatly the understanding of structural complexity, variety and
entanglements of the coordination architectures.3 Crystal
engineering of CPs/MOFs has thus been greatly accelerated
along with the cumulative knowledge of the delicate correla-
tions between structures and topology of nets.4 Nets such as
dia, pcu, srs, and structures of known materials and minerals,
have been frequently encountered in CPs/MOFs whereas some
other nets have never been found.5 Molecular mimics of
theoretically available nets are of interest in crystal chemistry
and in mathematics, arousing open puzzles as well.
Self-catenated framework structures represent one subclass
of the most fascinating topological nets in the extraordinary
family of entanglements, featuring one of the ‘shortest rings’
being catenated by other ‘shortest rings’ of the same net.3c,11
Though once only comes out serendipitously, deliberate
synthesis of self-catenated CPs/MOFs has now been proven
being rationally engineered by the combined use of long and
bent organoamine and organocarboxylate ligands.12 Herein
we report two examples of MOF architectures featuring 3-fold
interpenetration of the self-catenated 4-connected uninodal mok
net, formulated as [Co(4,40-BPIPA)(2,6-NDC)]ꢀ2DMF (1)z
and [Co(4,40-BPIPA)(4,40-BPDC)]ꢀ2DMF (2) (4,40-BPIPA =
N,N0-bis-4-pyridyl-isophthalamide, 2,6-NDC = 2,6-naphthalene-
dicarboxylic acid, 4,40-BPDC = 4,40-biphenyldicarboxylic
acid, Scheme 1).
Four-connected (4-c) 3-periodic nets3d captured special
research attention both for their intrinsic interest (all zeolites
are 4-c nets) and from a theoretical point of view: many 4-c
nets have been deduced and summarized firstly by Wells3a
followed by several comprehensive discussions.6 O’Keffee
collected some ‘‘dense and rare’’ 4-c nets in 1991,7 among
which there is the first example of self-catenated 4-c net
Complex 1 crystallizes in monoclinic, space group C2/cy. In
the structure of 1, the crystallographically independent cobalt
center is bonded by two pyridyl N atoms from 4,40-BPIPA and
a State Key Laboratory of Structural Chemistry, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Sciences,
Fujian, Fuzhou 350002, P. R. China. E-mail: rcao@fjirsm.ac.cn,
lujian05@fjirsm.ac.cn; Fax: +8659183714946;
Tel: +8659183796710
b College of Chemistry and Chemical Engineering,
ChongQing University, ChongQing 400044, P. R. China
c
`
Universita degli Studi di Milano, DCSSI, via Venezian 21,
20133 Milano, Italy
w Electronic supplementary information (ESI) available: Synthesis
and general characterizations and additional structural figures. CCDC
764789, 764788 and 772375. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c1cc10411j
Fig. 1 View of the ideal binodal coe (left) and uninodal mok (right)
nets with catenated 8-ring (for coe) and 6-ring (for mok) highlighted.
c
5982 Chem. Commun., 2011, 47, 5982–5984
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subunits is integrated into mok making it self-catenated
(Fig. 3 top-left and top-right). Further complexity arises from
the fact that there are three interpenetrated mok nets (Fig. 3
bottom and S3w). Uncoordinated DMF molecules fill in the
voids of the network and are weakly coordinated via hydrogen
bonds with the amide groups (N–Hꢀ ꢀ ꢀO, 2.98 and 3.07 A).
Numbers of DMF molecules were determined mainly by
elemental analysis and TGA analysis (Fig. S7w).
Complex 2 displays a similar network to complex 1
exhibiting 3-fold interpenetrated framework of mok nets. In
fact, replacement of 2,6-NDC in 1 with longer 4,40-BPDC
result in 2. Thus, complexes 1 and 2 are topologically equi-
valent (Fig. S3w). However, complex 2 is less stable and loses
solvent DMF faster than 1, possibly due to the larger solvent
accessible voids induced by the longer 4,40-BPDC linker in 2.
Calculations using PLATON15 indicate that complex 1 and
complex 2 have solvent accessible volumes of 2688.7 A3
(36.8%) and 3910.2 A3 (44.1%), respectively, after the removal
of guest solvent molecules. The permanent porosity was
demonstrated by the N2 adsorption isotherm of the activated
samples (Fig. S4w). The BET surface area of 1 is calculated to
be 116 m2 gꢁ1 (langmuir surface area 169 m2 gꢁ1) and the total
pore volume is 0.07 cm3 gꢁ1. The former is smaller than the
solvent-accessible surface area estimated from crystal structure
which is probably caused by the incomplete removal of guest
molecules or structural deformation during the thermal
activation of 1 (indeed the heated samples lose crystallinity
as seen from PXRD).16 The pore size obtained by applying the
Horvath-Kawazoe17 (HK) equation to the N2 sorption data is
5.03 A which is consistent with the structure.
Scheme 1 Schematic view of the organic linkers.
four O atoms from two chelating carboxyl groups of 2,6-NDC
(Fig. 2, top-left). The Co–N distances are 2.082(7) A and
2.086(9) A and the Co–O bonds are 2.091(7)/2.209(8) A and
2.131(7)/2.155(7) A. Each Co atom connects to four neigh-
boring ones through two 4,40-BPIPA and two 2,6-NDC, both
as linear linkers, to form an infinite three-dimensional three-
periodic network (3-D). There are three interpenetrated nets
related by a single translational vector (Class Ia),13 with the
shortest interpenetration vector [1/2,1/2,0] (and its symmetrically
equivalent [1/2,–1/2,0]) of 15.11 A. Topological analysis using
TOPOS software14 identifies the net as mok. As mentioned
earlier, another interesting aspect of the resulting mok net is
that it is self-catenated as coe, but now the catenation happens
among 6-rings (Fig. 1). Basic 6-membered cycles are observed
in 1 formed by six Co, two 2,6-NDC and four 4,40-BPIPA. The
cycles have ‘chair’ configurations and build into a wavy
hexagonal (hcb) sub-net (Fig. 2, top-right). A pair of the hcb
sub-nets interpenetrates to each other by sharing the same
average plane (Fig. 2, bottom) to give a 2-fold interpenetrated
subunit. For the sake of clarity, we mark in a random sequence
the six Co centers in a 6-membered cycle as Co1-Co6,
although they are crystallographically identical, and the four
hcb nets of the two neighbouring 2-fold layers (above and
below a central one) as L1-L4. The interconnection among
three 2-fold layers is achieved by Co1-L1, Co2-L2, Co3-L3,
Co4-L4, Co5-L3, and Co6-L4. Thus an overall 3-D mok net is
generated and the feature of interpenetration of the hcb
Fig. 3 Topological presentation the 3-D mok net derived from
complex 1 by reducing the organic linkers as bold lines (top-left);
The 4-connected mok net with highlighted a pair of self-catenated
6-rings. The piercing edges are the long, rigid dicarboxylic molecules
(top-right); A view of the 3-fold interpenetrated mok nets (bottom).
Fig. 2 View of the basic building blocks (top-left), individual hcb net
(top-right), and the doubly interpenetrated hcb nets (top-right and
bottom) in complex 1. Color codes: C, black; N, blue; O, red; Co,
green.
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Curiously, however, when the geometrically and chemically
comparable (but shorter) terephthalic acid (TP) was used as a
substitution of 2,6-NDC or 4,40-BPDC, a new complex,
[Co(4,40-BPIPA)(TP)]ꢀ2DMF (3), was isolated. The Co centers
in complex 3 are bonded by two pyridyl groups of 4,40-BPIPA
(Co–N 2.113(3) A) and four oxygens of the two chelating
carboxylate groups of TP (Co–O 2.070(2) and 2.254(2) A). The
metal centers are cross-linked by the two organic ligands
(spacers) to give a 2-periodic sql layer. The layers are
undulated and give rise to a 2-fold interpenetration sharing
the same average plane (Fig. S5, ESIw). The 2-fold sql stacks
along (010) in an ABAB sequence. The aromatic rings of the
V-shaped 4,40-BPIPA molecules point up- and downwards
from the layers to form a multi-armed bilayer unit, which
interdigitates with adjacent bilayers in a ‘cross finger’ way to
give a 3-D supramolecular network (Fig. S6, ESIw). Guest
DMF molecules reside into the interlayer regions through
hydrogen bonds with the amide groups (N–Hꢀ ꢀ ꢀO, 2.889 A).
The reason for the unexpected structure of complex 3 may
be double: First, TP is considerably shorter than 2,6-NDC and
4,40-BPDC that may not meet the requirement to realize the
self-catenation. In fact, an intrinsic doubly interpenetrated hcb
bilayer subunit is needed to realize mok topology. In the case
of complex 3, instead we have doubly interpenetrated sql
bilayer with 4-rings (with sides of 10.68, 14.31 A), rather than
the needed 6-rings hcb (with sides 12.92–16.95 for 1 and
15.02–16.98 for 2), The second difference is in the conforma-
tion of 4,40-BPIPA observed for 3 (cis, defined by the two
rotatable amido on a benzene ring of the 4,40-BPIPA) versus 1
and 2 (trans), which may be responsible for the variety of the
subunits (see Scheme S1w). Generally the Npyridylꢀ ꢀ ꢀNpyridyl
distance of cis-4,40-BPIPA (11.2 A) is shorter than the
Npyridylꢀ ꢀ ꢀNpyridyl distance of trans-4,40-BPIPA (13.3 A), thus
the overall Coꢀ ꢀ ꢀCo distances are much shorter in 3: the one
bridged by 4,40-BPIPA are 14.31 A versus ca. 16.9 A for 1 and
2. Moreover, the cis-/trans-configuration of 4,40-BPIPA is
somewhat controllable through the fine tuning of reaction
temperature: the low temperature helps the formation of
cis-configuration ligands (potentially result in 4-rings) whereas
high temperature favors trans- configuration ligands (potentially
result in 6-rings) which is consistent with our previous
observations with some geometrically flexible ligands.18
In conclusion, two metal–organic frameworks with
65.8-mok topology have been synthesized and topologically
characterized. The two MOFs reported in this work give
beautiful examples showing a new type of topological nets
and add first members in the inventory of 4-connected
uninodal mok CPs/MOFs. A key to the successful isolation
of the nets reported herein is the combined use of long and
rigid dicarboxylate linkers with a flexible V-shaped pyridyl-
amide derivative. It demonstrates again the theoretically
anticipated topological nets can be accomplished in the area
of CPs/MOFs.
Notes and references
z Syntheses. Red block crystals of complex 1 were originally obtained
by solvothermal reaction of N,N0-bis-4-pyridinyl-isophthalamide,
2,6-naphthalenedicarboxylic acid, and Co(NO3)2ꢀ6H2O (1 : 1 : 1) in
8 mL DMF at 120 1C for 3 days. Red purple block crystals of complex
2 were synthesized at a higher temperature (150 1C) comparing
with
1
and 4,40-biphenyldicarboxylic acid was used instead of
2,6-naphthalenedicarboxylic acid. Red block crystals of complex 3
were synthesized with starting materials of N,N0-bis-4-pyridinyl-
isophthalamide, terephthalic acid, and Co(NO3)2ꢀ6H2O (1 : 1 : 1) at a
lower temperature of 105 1C. For details, see ESIw
y Crystal data: see ESI for detailsw.
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Financial support from the 973 Program (2011CB932504,
2007CB815303), NSFC (20731005, 20821061, and 91022007),
Fujian Key Laboratory of Nanomaterials (2006L2005), and
the Key Project from CAS are gratefully acknowledged.
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F.-X. Xiao, S. R. Batten and R. Cao, Chem.–Asian J., 2008, 3,
542; J. Lu, W.-H. Bi and R. Cao, CrystEngComm, 2009, 11, 2248.
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5984 Chem. Commun., 2011, 47, 5982–5984
This journal is The Royal Society of Chemistry 2011