A porous Mn(v) coordination framework with PtS topology: Assessment of the influence of a terminal nitride on CO2 sorption

Article abstract of DOI:10.1039/c3dt51721g

A new coordination framework material, [Zn{MnN(CN)₄(H ₂O)}]·2H₂O·MeOH, has been characterised crystallographically and the effect of a terminal nitride on the N₂, H₂ and CO₂ sorption capacities of the material assessed through porosimetery measurements and DRIFTS.

Full text of DOI:10.1039/c3dt51721g

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A porous Mn(V) coordination framework with PtS
topology: assessment of the influence of a terminal
nitride on CO2 sorption†
Cite this: Dalton Trans., 2013, 42, 13308
Received 27th June 2013,
Accepted 31st July 2013
DOI: 10.1039/c3dt51721g
Michael J. Murphy, Deanna M. D’Alessandro and Cameron J. Kepert*
www.rsc.org/dalton
A new coordination framework material, [Zn{MnN(CN)4(H2O)}]· and have been of interest for heterogeneous catalysis due to
2H2O·MeOH, has been characterised crystallographically and the presence of coordinatively unsaturated metal sites within
the effect of a terminal nitride on the N2, H2 and CO2 sorption their structures, examples of which have incorporated metallo-
capacities of the material assessed through porosimetery measure- porphyrins and post-synthetically modified ligands.14–16 On
ments and DRIFTS.
this basis, a preliminary assessment of the catalytic perform-
ance of [Zn{MnN(CN)4(H2O)}]·2H2O·MeOH for the epoxidation
of cyclopentene was performed.
The crystal structure of the solvated framework was solved
and refined in the tetragonal space group P42/mmc with unit
cell dimensions a = 7.51110(10) and c = 13.4593(5) Å at 100 K
(Fig. 1–3).‡ The asymmetric unit consists of Mn(V) and Zn(II)
centres bridged by a cyanido ligand (Fig. 1). The Mn(V) site
Coordination frameworks have increasingly been investigated
as materials for CO2 capture and storage due to their high
internal surface areas and large pore volumes and to the tune-
able nature of their structures.1 Current research has identified
coordinatively unsaturated metal sites and pendant functional
groups as features that lead to enhanced uptake or selectivity
of CO2 over other guest molecules, due in part to their electro-
static interactions with the quadrupole moment of CO2.2–5 The
inclusion of nitrogen-containing groups within coordination
frameworks, for example via the grafting of amine groups onto
unsaturated metal sites, has primarily been aimed at mimick-
ing the acid–base interactions of the alkanolamines that are
currently used to chemically absorb CO2.6–9 The incorporation
of these functionalities can result in significant increases in
CO2 adsorption compared with non-grafted materials at the
low partial pressures relevant to flue gas capture.
In the present study, the influence of a terminal nitride
ligand on N2, H2 and CO2 sorption in a new cyanido-based
coordination framework, [Zn{MnN(CN)4(H2O)}]·2H2O·MeOH,
with PtS topology, has been assessed. The secondary building
unit (SBU) [MnN(CN)4]2− possesses interesting catalytic10 and
electrochemical11 properties in the solution state by virtue of
the active Mn(V) centre, and is known to give rise to unique
thermal expansion12 and gas sorption properties13 when incor-
porated into framework materials. Frameworks with the PtS
topology contain both square planar and tetrahedral nodes,
forms
a distorted octahedron, with four bridging C-co-
ordinated cyanides in the equatorial plane and with terminal
nitride (MnuN distance = 1.64(3) Å) and water (Mn–OH2 dis-
tance = 2.20(2) Å) bound axially. The entire MnN(CN)4(H2O)
unit is 50 : 50 disordered about the ac-mirror plane, with there
being no evidence for long-range ordering of its orientation
despite an expectation that the distorted tetrahedral coordi-
nation geometry of the Zn(II) centre might favour this (see
ESI†). The crystalline framework contains a 3D PtS-type pore
network that occupies 56% of the crystal volume; the narrowest
pore windows in the ab plane have dimensions 3.7 × 4.4 Å2
and in the c-direction 5.2 × 5.2 Å2 (Fig. 2 and 3). The axial
Mn(V) sites project directly into this pore space.
School of Chemistry, The University of Sydney, NSW, 2006, Australia.
E-mail: cameron.kepert@sydney.edu.au; Fax: +61 (2) 9351-3329;
Tel: +61 (0)2 9351-7482
†Electronic supplementary information (ESI) available: Experimental details,
single crystal structure parameters, PXRD, TGA, IR and gas sorption data. CCDC
933557. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3dt51721g
Fig. 1 The coordination spheres of the Mn(V) and Zn(II) centres of [Zn{MnN(CN)4-
(H2O)}]·2H2O·MeOH.
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Fig. 2 Extended crystal structure of [Zn{MnN(CN)4(H2O)}]·2H2O·MeOH viewed
parallel to the a-axis. Framework disorder and pore solvent have been removed
for clarity.
Fig. 4 Adsorption and desorption isotherms for N2 (filled and open circles)
and H2 (filled and open squares) at 77 K for [Zn{MnN(CN)4(MeOH)}].
Fig. 3 Extended crystal structure of [Zn{MnN(CN)4(H2O)}]·2H2O·MeOH viewed
parallel to the c-axis. Framework disorder and pore solvent have been removed
for clarity.
Fig. 5 Adsorption and desorption isotherms for N2 (filled and open triangles)
and CO2 (filled and open circles) at 298 K for [Zn{MnN(CN)4(MeOH)}].
A N2 sorption isotherm was measured at 77 K on a metha-
nol-washed sample of the framework that had been desolvated
pore volumes, such as Prussian blues (8–12 wt% CO2 uptake at
at 90 °C for 24 h under vacuum to produce [Zn{MnN(CN)4- 1 bar).21,22 The hydrogen sorption capacity of [Zn{MnN(CN)4-
(MeOH)}] (see Fig. 4). Interpretation of the reversible type
I isotherm using the Brunauer–Emmett–Teller (BET) equation
yielded an estimated surface area of 537.3 0.6 m2 g−1. This is
comparable to other cyanide-based frameworks such as Prus-
sian blues, [M3{Co(CN)6}2] (M = Mn, Fe, Co, Ni and Cu), and
(MeOH)}] at 77 K (Fig. 4) is 0.84 wt% at 1 bar, which is also
in the range observed for Prussian blues.17,18
The isosteric heat of adsorption (−Qst) for CO2 was calcu-
lated from the Clausius–Clapeyron equation using adsorption
isotherms collected at 298, 308 and 318 K (see ESI†). At low
tetracyanido-based frameworks, which have surface areas in the coverage, −Qst was 34 kJ mol−1, which is indicative of a physi-
ranges of 560–870 m2 g−1 17,18
,
and 127–398 m2 g−1, respectively.19
sorptive interaction between CO2 and the framework, possibly
In comparison, the closest structurally related framework,
to a small proportion of coordinately unsaturated Mn(V) sites.
Zn[Ni(CN)4], which is reported to contain two interpenetrated With increasing coverage, −Qst declined rapidly to 21 kJ mol−1
,
then more gradually to 15 kJ mol−1 at higher loadings, which
is consistent with the physisorption of CO2 onto itself.
networks, is effectively non-porous, with a surface area of
8.32 m2 g−1 20 The saturation uptake of 6.5 mmol g−1 corres-
.
ponds to 1.8 molecules of N2 per formula unit.
The 298 K CO2 isotherm (Fig. 5) shows a reversible uptake
of 5.0 wt% (1.14 mmol g−1) at 298 K and 1 bar, corresponding
Diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) was employed to probe the interaction of CO2 with
adsorption sites in the framework (see ESI†). A methanol-
to 0.3 molecules of CO2 per formula unit. This uptake is washed sample of [Zn{MnN(CN)4(H2O)}]·2H2O·MeOH was acti-
broadly comparable with those of frameworks with similar
vated in situ at 90 °C for 16 h whereupon the broad peak
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centred at 3000 cm−1 disappeared, indicating the removal of
the occluded water solvent; however, the peaks at 2856 and
2926 cm−1, which are indicative of coordinated MeOH,23,24
were retained. Activation of the framework under vacuum at
temperatures above 110 °C resulted in an amorphous powder
with reduced surface area (ca. 230 m2 g−1), indicative of frame-
work collapse. Consequently, it was postulated that the
6-coordination of the Mn(V) site is important to the structural
integrity of the framework upon guest desorption.
To investigate whether the terminal nitride ligands interacts
strongly with the guest CO2 molecules, the ν(MnN) stretch at
1090 cm−1 in the DRIFTS spectrum of [Zn{MnN(CN)4(MeOH)}]
was monitored at various loadings of CO2 (see ESI†). As the
concentration of CO2 increased, no change in ν(MnN) was
observed, indicating that the nitride ligand is not sufficiently
nucleophilic to interact strongly with CO2.
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This work was supported by the Australian Research 19 J. T. Culp, S. Natesakhawat, M. R. Smith, E. Bittner,
Council and the Science and Industry Endowment Fund. We
gratefully acknowledge Dr Mohammad Choucair for collecting
the DRIFTS data.
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Notes and references
‡Crystal data for C5H10MnN5O4Zn (M = 324.49): tetragonal, space group P42/mmc
(no. 131), a = 7.51110(10), c = 13.4593(5) Å, V = 759.33(3) Å3, Z = 2, T = 100(2) K,
μ(Mo-Kα) = 2.416 mm−1, ρcalc = 1.419 g mm−3, 11 832 reflections measured
(9.78 ≤ 2θ ≤ 52.68), 469 unique (Rint = 0.0413). R1 = 0.0429 (I ≥ 2σ(I)) and wR2
=
24 L. J. Burcham, L. E. Briand and I. E. Wachs, Langmuir,
2001, 17, 6164–6174.
0.1233 (all data). CCDC 933557 contains the supplementary crystallographic data
for this paper.
13310 | Dalton Trans., 2013, 42, 13308–13310
This journal is © The Royal Society of Chemistry 2013