Crystal Engineering of 6-Carboxy-4-aryl-2,2′-bipyridine Complexes: Potent Chelators with Intrinsic Intermolecular Affinity

Article abstract of DOI:10.1021/acs.cgd.7b00683

Herein we report the coordination chemistry and structural properties of two multifunctional terdentate 2,2′-bipyridine derivatives 4- and 3-(4-nitrophenyl)-2,2′-bipyridine-6-carboxylate, L1/L2, respectively. We report eight new coordination complexes wit

Full text of DOI:10.1021/acs.cgd.7b00683

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Article
Crystal engineering of 6-carboxy-4-aryl-2,2#-bipyridine
complexes: potent chelators with intrinsic intermolecular affinity
Hannah L. Dalton, Chris S Hawes, and Thorfinnur Gunnlaugsson
Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 16 Jun 2017
Downloaded from http://pubs.acs.org on June 16, 2017
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Crystal engineering of 6ꢀcarboxyꢀ4ꢀarylꢀ2,2′ꢀbipyridine complexes: potent
chelators with intrinsic intermolecular affinity
Hannah L. Dalton, Chris S. Hawes and Thorfinnur Gunnlaugsson*
School of Chemistry & Trinity Biomedical Sciences Institute, Trinity College Dublin, The University of Dublin,
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Dublin 2, Ireland.
Email: gunnlaut@tcd.ie
Herein we report the coordination chemistry and structural properties of two multifunctional terdentate
2
,2′ꢀbipyridine derivatives 4ꢀ and 3ꢀ(4ꢀnitrophenyl)ꢀ2,2´ꢀbipyridineꢀ6ꢀcarboxylate, L1/L2 respectively.
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II
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We report 8 new coordination complexes with Cd , Co and Co , and study their behaviour in the
crystalline phase with the view of establishing L1 and L2 as robust and functional chelators for use in
extended metallosupramolecular systems. The divalent complexes [Cd(L1) ]·H O 1, [Cd(L2) ] 2,
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2
[
Co(L1) ] 5 and [Co(L2) ] 7 are mononuclear complexes with distorted octahedral coordination
2 2
geometries dictated by the two meridionallyꢀcoordinated L1/L2 ligands. Also reported are two trivalent
byproducts [Co(L1) ]NO 6 and [Co(L2) ]NO 8 which were formed in trace quantities during the initial
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screening of complexes 5 and 7, respectively. The extended structures of these complexes are dominated
by substantial intermolecular interactions, including anion···π hole interactions. Modification of the
reaction conditions from
1
and
2
gives two closelyꢀrelated trinuclear cluster species
[Cd (L1) (NO ) (DMF) ]NO 3 and [Cd (L2) (NO ) (DMF) ]NO ·DMF 4, in which three cadmium ions,
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with pentagonal bipyramidal coordination geometries, form discꢀshaped assemblies with L1/L2, which
undergo further weak interactions between neighbouring complexes in the crystalline state. With these
outcomes L1 and L2 are established as exciting new building blocks for metallosupramolecular
assemblies involving dꢀblock metal ions.
Contact Author: Prof. Thorfinnur Gunnlaugsson
School of Chemistry and Trinity Biomedical Sciences Institute (TBSI)
Trinity College Dublin
Dublin 2, Ireland
Phone: +353ꢀ1ꢀ896 3459
Emails: gunnlaut@tcd.ie
https://thorrigunnlaugsson.wordpress.com/
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Crystal engineering of 6ꢀcarboxyꢀ4ꢀarylꢀ2,2′ꢀ
bipyridine complexes: potent chelators with intrinsic
intermolecular affinity
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Hannah L. Dalton, Chris S. Hawes and Thorfinnur Gunnlaugsson*
School of Chemistry and Trinity Biomedical Sciences Institute, Trinity College Dublin, The
University of Dublin, Dublin 2, Ireland. Email: gunnlaut@tcd.ie
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Abstract
Herein we report the coordination chemistry and structural properties of two multifunctional
terdentate 2,2′ꢀbipyridine derivatives 4ꢀ and 3ꢀ(4ꢀnitrophenyl)ꢀ2,2´ꢀbipyridineꢀ6ꢀcarboxylate,
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III
L1/L2 respectively. We report 8 new coordination complexes with Cd , Co and Co , and study
their behaviour in the crystalline phase with the view of establishing L1 and L2 as robust and
functional chelators for use in extended metallosupramolecular systems. The divalent complexes
[
Cd(L1) ]·H O 1, [Cd(L2) ] 2, [Co(L1) ] 5 and [Co(L2) ] 7 are mononuclear complexes with
2 2 2 2 2
distorted octahedral coordination geometries dictated by the two meridionallyꢀcoordinated
L1/L2 ligands. Also reported are two trivalent byproducts [Co(L1) ]NO 6 and [Co(L2) ]NO 8
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which were formed in trace quantities during the initial screening of complexes 5 and 7,
respectively. The extended structures of these complexes are dominated by substantial
intermolecular interactions, including anion···π hole interactions. Modification of the reaction
conditions from
1
and
2
gives two closelyꢀrelated trinuclear cluster species
[
Cd (L1) (NO ) (DMF) ]NO 3 and [Cd (L2) (NO ) (DMF) ]NO ·DMF 4, in which three
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cadmium ions, with pentagonal bipyramidal coordination geometries form discꢀshaped
assemblies with L1/L2, which undergo further weak interactions between neighbouring
complexes in the crystalline state. With these outcomes L1 and L2 are established as exciting
new building blocks for metallosupramolecular assemblies involving dꢀblock metal ions.
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Introduction
In the onꢀgoing drive towards new functional supramolecular systems, a constant need exists
for the discovery and development of new ligand systems, binding motifs and molecular
scaffolds, in order to diversify the existing chemical toolbox and rationally pursue specific
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applications. To this end, significant progress has been made recently in establishing modular
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,5
ligand sets for further development into supramolecular systems. Doing so requires a thorough
basis in the fundamental coordination chemistry and structural trends of archetypal systems, to
establish the necessary understanding of the subtle geometric and electronic effects which are
6ꢀ9
well known to strongly influence the bulk properties of such systems. The polypyridine family
of ligands are wellꢀknown metallosupramolecular ligand scaffolds, widely exploited in transition
metal coordination chemistry, due to the predictable coordination behaviour and favourable
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electronic and magnetic properties of bipyridine and terpyridine dꢀmetal ion complexes.
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These structures have found myriad applications, including as dyeꢀsensitized solar cells, redox
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photocatalysts, molecular electronics and spin crossover devices.
As wellꢀestablished
starting points, functional 2,2ʹꢀbipyridine derivatives are ideally suited for the installation of new
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substitution patterns and functional groups. Although direct derivatisation of the deactivated
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pyridine ring itself can be synthetically cumbersome compared to other nitrogen heterocycles,
new synthetic methods are continually emerging for the synthesis of functional bipyridine
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ligands.
The scope of possible modification to the 2,2ʹꢀbipyridine backbone includes the incorporation
of additional chelating groups at the 6 and/or 6ʹ positions, with the view of increasing the metal
binding affinity, while functionalisation at the 4 and/or 4ʹ positions allows for the grafting of
additional chemical functionality, or structural handles, for engineering intermolecular
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interactions.
Work by Kruger et al. has demonstrated the utility of a carboxylate moiety in
the 6 position for the generation of polymeric assemblies involving strong chelation of the
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terdentate N,N,Oꢀbinding pocket to Cu ions with bridging through additional carboxylate
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functionality. In that case, the unsymmetric substitution of the bipyridine skeleton was
achieved by partial decarboxylation of the tetracarboxylic acid precursor. Some examples are
known of simple bipyridine ligands unsymmetrically substituted with a carboxylate group at a
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single 6 position,
although the corresponding 1,10ꢀphenanthroline analogue is more
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common.
Examples of structurally characterised 6ꢀcarboxyꢀ4ꢀarylꢀ2,2ʹꢀbipyridine derivatives are
relatively uncommon, although the naturally occurring antitumor antibiotic Streptonigrin is one
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such fascinating example, Scheme 1.
Interestingly, the coordination of metal ions to
Streptonigrin has been shown to promote DNA binding and lead to an enhanced degree of
toxicity. However, to date, only three reports have emerged of metal complexes of 6ꢀcarboxyꢀ4ꢀ
arylꢀ2,2ʹꢀbipyridine ligands, which can be considered simplified tridentate analogues of
Streptonigrin in metal binding affinity without the added complication of the redoxꢀactive
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3ꢀ35.
quinone ring.
In most of these cases, lanthanide ions were employed in order to access the
photophysical antenna properties of the extended aromatic ligand backbone, and to provide a
better geometric match with the steric demands of the ligand being better suited to high
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6
coordination numbers. As such, the coordination chemistry of these systems with dꢀblock metal
ions remains a fertile area for further exploration. Here we describe the coordination chemistry
of 4ꢀ(4/3ꢀnitrophenyl)ꢀ2,2´ꢀbipyridineꢀ6ꢀcarboxylate (L1/L2, Scheme 1) with cobalt and
cadmium ions, elucidating two unique crystalline phases for each combination of metal and
ligand, and contrast the structural properties of the resulting 8 compounds.
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Scheme 1 Structures of NH L1, EtL1 and NH L2, with numbering scheme for NMR assignment
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and Streptonigrin
Experimental
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The chalcone precursors, Scheme 2, were synthesised according to literature procedures.
All other reagents and solvents were purchased from SigmaꢀAldrich, Merck or Fisher Scientific,
were of reagent grade or better, and were used as received. All NMR spectra were recorded using
either a 400 MHz Bruker Avance III or 600 MHz Bruker Avance II spectrometer, operating at
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400.1 or 600.1 MHz for H NMR and 100.2 or 150.2 MHz for C NMR. Chemical shifts are
reported in ppm referenced relative to the internal residual solvent signals. Electrospray mass
spectra were recorded on a Micromass LCT spectrometer or a MALDI QToF Premier, running
Mass Lynx NT V 3.4 on a Waters 600 controller connected to a 996 photodiode array detector
with HPLCꢀgrade methanol or acetonitrile. High resolution mass spectra were determined by a
peak matching method, using leucine enkephaline (TyrꢀGlyꢀGlyꢀPheꢀLeu) as the standard
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reference (m/z = 556.2771). All accurate masses were reported within ±5 ppm. Melting points
were determined using an IA9000 digital melting point apparatus in air and are uncorrected.
Infrared spectra were recorded on a Perkin Elmer Spectrum One FTꢀIR spectrometer fitted with a
Universal ATR Sampling Accessory. Elemental analyses were carried out at the Microanalytical
Laboratory, School of Chemistry and Chemical Biology, University College Dublin.
Thermogravimetric analysis was performed using a PerkinꢀElmer Pyris 1 TGA instrument, with
samples (1 – 5 mg) mounted in alumina pans and heated in the range 25 – 500 °C at a rate of 5
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°C/min under a constant N flow of 20 mL/min. Xꢀray powder diffraction patterns were
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measured on a Bruker D2 Phaser instrument operating using Cu Kα (λ = 1.54178 Å) radiation
and a Lynxeye detector at room temperature, with samples mounted on a zeroꢀbackground
silicon single crystal sample stage. The patterns collected at room temperature were compared
with the patterns simulated from the single crystal data (collected at 100K) to establish phase
purity of each crystalline material (Supporting Information).
Synthesis of 4/3-(nitrophenyl)-2,2´ bipyridine-6-carboxylate (L1 and L2, respectively)
The appropriate chalcone (2.05 g, 1 eq, 9.04 mmol), 2ꢀpyridacyl pyridinium iodide (1.79 g, 1
eq, 9.04 mmol) and ammonium acetate (5.74 g, 8 eq, 72.39 mmol) were added to H O (80 mL)
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and heated under reflux for 5 hrs. The resulting precipitate was isolated by filtration and washed
with H O (2 × 50 mL) and acetone (2 × 50 mL) and airꢀdried, to yield L1/L2 as offꢀwhite flaky
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solids. In both cases, the solids obtained analysed for ammonium salts or mixed phases of the
type [(HL) (NH L) ]; L1 was obtained as the ammonium salt hemipentahydrate
x
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1ꢀx
NH L1·2.5H O (x = 0), while L2 exhibited the bulk formula [(HL2) (NH L2) ]·2.33H O.
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0.2
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0.8
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Both materials were characterised and reacted onwards using these formulae.
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L1: Yield 1.83g (53%). m.p. 238ꢀ240 °C (decomp.); Found C, 53.54; H, 4.51; N, 14.37;
Calculated for C H N O ·2.5H O [NH L1·2.5H O] C, 53.26; H, 4.99; N, 14.62%; m/z (HRꢀ
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ꢀ
ꢀ
ESI ) 320.0671 ([MꢀH] , calculated for C H N O 320.0667); δ (DMSOꢀd , 400 MHz) 8.66
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H
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(H ,d, J = 1.8 Hz, 1H), 8.51 (H , d, J = 8.0 Hz, 1H), 8.38 (H , d, J = 8.9 Hz, 2H), 8.24 (H , d, J
5
4
9+8
6
=
=
1.8 Hz, 1H), 8.16 (H10+7, d, J = 8.8 Hz, 2H), 8.00 (H , td, J = 7.7, 1.8 Hz, 1H), 7.50 (H , ddd, J
3 2
7.4, 4.7, 1.0 Hz, 1H); δ (DMSOꢀd ) 167.6 (COO), 157.6 (C ) 155.2 (C ), 155.1 (C ) 149.2
C
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(C ), 147.7 (C ꢀNO ), 145.9 (C ), 144.2 (C ), 137.4 (C ), 128.3 (C ), 124.7(C ), 124.3
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ꢀ1
(
C9+10), 121.3 (C ), 121.1 (C ), 117.8 (C ); ν
(ATR, cm ) 2981, 1588, 1510, 1415, 1330,
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4
5
max
1256, 1104, 1003, 855, 788, 764, 752.
L2: Yield 1.62 g (48%). m.p. 219ꢀ221 °C (decomp.); Found C, 54.37; H, 4.17; N, 14.36;
+
Calculated for [(HL2) (NH L2) ]·2.33H O C, 54.17; H, 4.83; N, 14.12%; m/z (HRꢀESI )
0.2
4
0.8
2
+
3
22.0828 ([M+H] , calc. for C H N O 322.0832); δ (600 MHz, DMSOꢀd ) 8.70 (H , d, J =
17 12 3 4 H 6 1
4
.0 Hz, 1H), 8.64 (H , d, J = 1.7 Hz, 1H), 8.56 (H , t, J = 1.8 Hz, 1H), 8.49 (H , d, J = 7.9 Hz,
5
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1H), 8.32 (H , dd, J = 8.0, 2.0 Hz, 2H), 8.23 (H , d, J = 1.8 Hz, 1H), 7.97 (H , td, J = 7.8, 1.7
7
+8
6
3
Hz, 1H), 7.82 (H , t, J = 8.0 Hz, 1H), 7.46 (H , ddd, J = 7.4, 4.8, 1.0 Hz, 1H); δ (DMSOꢀd , 600
9
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C
6
MHz) 168.3 (COO), 158.8 (C ), 155.8 (C ), 155.7 (C ), 149.6 (C ), 149.0 (C ꢀ NO ), 146.5
1
6
12
11
1
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(C ), 139.9 (C ) 137.9 (C ), 133.9 (C ), 131.4 (C ) 124.7 (C ), 124.2 (C ), 121.9 (C ), 121.6
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9
2
8
10
ꢀ1
(
C ), 121.6 (C ), 118.0 (C ); ν
(cm ) 2981,1579, 1516, 1413, 1316, 1105, 1001, 885, 854,
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6
5
max
788,751, 685.
Synthesis of Ethylꢀ4ꢀ(nitrophenyl)ꢀ2,2´ꢀbipyridineꢀ6ꢀcarboxylate (EtL1)
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L1 (1.0 g, 1 eq, 2.6 mmol) was suspended in EtOH (80 mL) and SOCl (0.7 mL, 3.7 eq, 9.6
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mmol) and heated to 80 °C for 24 hrs. The solvent was removed by vacuum distillation, and the
resulting hydrochloride salt was taken up in H O (50 mL) and neutralised with sodium
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bicarbonate solution (3 x 15 mL). The precipitated EtL1 was obtained by filtration as a beige
solid (0.97 g, quant.). m.p 182ꢀ184 °C (decomp.); Found C, 62.16; H, 4.09; N, 11.21; Calculated
+
+
for C H N O ·H O C, 62.12; H, 4.66; N, 11.44%; m/z (HRꢀESI ) 350.1156 ([M+H] calc. for
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C H N O 350.1141); δ (400 MHz, DMSO) 8.91 (H ,s, 1H), 8.77 (H , d, J = 4.2 Hz, 1H), 8.48
19
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H
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(H , d, J = 7.9 Hz, 1H), 8.43 (H7+10, d, J = 8.5 Hz, 2H), 8.40 (H , s, 1H), 8.22 (H d, J = 8.6 Hz,
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6
8+9,
2
H), 8.06 (H , t, J = 7.6 Hz, 1H), 7.56 (H , m, 1H), 4.47 (CH q, J = 7.1 Hz, 2H), 1.42 (CH , t, J
3 2 2, 3
=
7.1 Hz, 3H). δ (DMSOꢀd ) 164.4 (COO), 156.4(C ), 153.9 (C ), 149.5 (C ), 148.7 (C ),
C 6 12 11 1 15
148.1 (C ꢀNO ) 147.5 (C ), 142.8 (C ), 137.7 (C ) 128.7 (C ) 125.0 (C ) 124.4 (C8+10) 122.6
1
5
2
13
14
3
7+9
2
ꢀ1
(
C ) 121.2 (C ) 121.0 (C ), 61.6 (CH ), 14.2 (CH ); ν (cm ) 3116, 3088, 2983, 2937, 1752,
6
5
4
2
3
max
1584, 1513, 1426, 1350, 1235, 1167, 1081, 1024, 904, 850, 783, 752, 695, 620.
Synthesis of [Cd(L1) ]·H O 1
2
2
Cadmium nitrate tetrahydrate (8 mg, 26 ꢁmol) was dissolved in DMF:H O (2:1, 3 mL) and
2
EtL1 (10 mg, 27 ꢁmol) was added. The reaction mixture was sealed in a Teflonꢀcapped vial and
heated at 80 °C for 8 hrs. The resulting colourless crystals were isolated by filtration to give 1.
Yield 7 mg, 35 %. m.p (decomp.) >280 °C; Found: C, 52.49; H, 2.72; N, 10.7%; calculated for
ꢀ1
C H CdN O C, 52.97%; H, 2.87; N, 10.9%; ν (ATR, cm ) 3398, 3072, 1632, 1599, 1556,
34
22
6
9
max
1
513, 1482, 1430, 1375, 1344, 1312, 1246, 1164, 105, 1009, 891, 860, 824, 796, 756, 719, 683,
64, 637, 600, 554. Phase purity was established by Xꢀray powder diffraction (Supporting
6
Information)
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2
2
2
2
2
2
2
2
2
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3
3
3
3
3
3
4
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4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Synthesis of [Cd(L2) ] 2
2
Cadmium nitrate tetrahydrate (8 mg, 26 ꢁmol) was dissolved in DMF:H O (2:1, 3 mL) and L2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(10 mg, 26 ꢁmol) was added. The reaction mixture was sealed in a Teflonꢀcapped vial and
heated at 100 °C for 14 hrs. The resulting colourless crystals were isolated by filtration to give 2
which, as discussed in the text, unavoidably contained a trace quantity of inorganic impurities.
Yield 3mg, 15%. m.p (decomp.) >280 °C; Found C, 53.33; H, 2.57 N, 10.95% calculated for
ꢀ
1
C H CdN O C, 54.23; H, 2.67; N, 11.16%; ν (ATR, cm ) 3059, 1635, 1602, 1577, 1522,
max
34
20
6
8
1
485, 1422, 1415, 1644, 1305, 1282, 1245, 1164, 1078, 1020, 879, 797, 762, 750, 741, 723, 695,
67, 631, 597, 569. Phase purity was established by Xꢀray powder diffraction (Supporting
6
Information)
Synthesis
of
polynuclear
complexes
[Cd (L1) (NO ) (DMF) ]NO
3
3
and
3
3
3 2
4
[
Cd (L2) (NO ) (DMF) ]NO ·DMF 4
3 3 3 2 4 3
Cadmium nitrate tetrahydrate (18 mg, 58 ꢁmol) was dissolved in DMF (0.5 mL) and EtL1/L2
(20 mg, 54/52 ꢁmol) was added. The reaction mixture was sealed in a Teflonꢀcapped glass vial
and heated at 100 °C for 18 hrs. On cooling to room temperature, the resulting colourless crystals
were isolated by filtration. The solvent content was found to be variable depending on the age
and drying conditions of the isolated sample, as discussed in the text.
[
Cd (L1) (NO ) (DMF) ]NO 3: Yield 18 mg, 56 %. m.p (decomp.) >280 °C; Found C 40.57;
3 3 3 2 4 3
H, 2.81; N, 11.33; calculated for C H N O Cd ·3DMF·1.5H O C, 40.45; H, 2.84; N, 11.47%
5
1
30 12 21
3
2
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8
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ꢀ1
νmax (ATR, cm ) 3389, 2935, 1641, 1585, 1555, 1516, 1447, 1410, 1383, 1342, 1299, 1248,
1
107, 1075, 1035, 1009, 908, 857, 799, 776, 754, 688, 665, 633, 618,598, 586, 576. Phase purity
was established by Xꢀray powder diffraction (Supporting Information)
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14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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45
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47
48
49
50
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53
54
55
56
57
58
59
60
[Cd (L2) (NO ) (DMF) ]NO ·DMF 4: Yield 12 mg, 38 %. m.p (decomp.) >280 °C; Found C
3
3
3 2
4
3
4
0.54; H, 2.65; N, 11.37; calculated for C H N O Cd ·3DMF·1.5H O C, 40.45; H, 2.84; N,
51 30 12 21 3 2
ꢀ1
1
1.47%; νmax (ATR, cm ) 3387, 3066, 1638, 1599, 1580, 1553, 1525, 1482, 1424, 1380, 1347,
1308, 1246, 1163, 1109, 1075, 1050, 1019, 888, 797, 781, 741, 719, 692, 670, 625, 557. Phase
purity was established by Xꢀray powder diffraction (Supporting Information)
Synthesis of [Co(L1/L2) ] 5 and 7
2
Cobalt(II) nitrate hexahydrate (3 mg, 10 ꢁmol) was dissolved in DMF:H O (1:1, 3 mL) and
2
EtL1/L2 (10 mg, 27/26 ꢁmol) was added. The reaction mixture was sealed and heated at 100 °C
for 18 hrs. On cooling to room temperature, the resulting orange rod shaped crystalline material
was isolated by filtration.
[
Co(L1) ] 5: Yield 3 mg, 43 %. m.p (decomp.) >280 °C; Found: C, 57.89; H, 2.76; N, 11.97;
2
ꢀ1
calculated for C H CoN O C, 58.37; H, 2.88; N, 12.01%; ν (cm ) 3070, 2284,2163, 1981,
3
4
20
6
8
max
1931, 1638, 1604, 1525,1482, 1442, 1421, 1347, 1310, 1280, 1247, 1186, 1162, 1124, 1084,
1
070, 1024, 972, 938, 898, 890, 823, 880, 793, 764, 754, 741, 730, 724, 694, 669, 628, 608, 577,
62 . Phase purity was established by Xꢀray powder diffraction (Supporting Information)
5
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
[
Co(L2) ] 7 Yield 6 mg, 86%. m.p (decomp.) > 280 °C; Found C, 57.86; H, 2.76; N, 11.72;
2
ꢀ1
calculated for C H CoN O C, 58.37; H, 2.88, N. 12.01%; ν (ATR, cm ) 3068, 1638, 1604,
34 20
6
8
max
1
526, 1482, 1421, 1347, 1280, 1161, 1123, 1071, 1005, 898, 880, 823, 793, 764, 754, 741, 684.
28, 607, 559. Phase purity was established by Xꢀray powder diffraction (Supporting
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
6
Information).
II
III
Synthesis of mixed Co /Co phases containing [Co(L1/L2) ]NO impurity 6 and 8
2
3
Cobalt(II) nitrate hexahydrate (8 mg, 27 ꢁmol) was added to DMF (1 mL) and EtL1/L2 (10
mg, 27/26 ꢁmol) was added. The reaction mixture was sealed and heated at 100 °C for 24 hrs,
II
III
giving a red crystalline solid which contained both Co /Co phases. Individual crystals of 6 and
8 were manually isolated from this mixture for single crystal Xꢀray diffraction analysis.
Scheme 2. Synthetic pathway for NH L1, EtL1 and NH L2 from benzaldehyde precursors; (i)
4
4
sodium pyruvate, H O/EtOH, 2M NaOH/HCl (ii) Pyridacyl pyridinium iodide, ammonium
2
37,38
acetate, H O
(iii) SOCl , EtOH.
2
2
Xꢀray Crystallography
Structural and refinement parameters are presented in Table 1. All diffraction data were
collected using a Bruker APEXꢀII Duo dualꢀsource instrument using graphiteꢀmonochromated
Mo Kα (λ = 0.71073 Å) or microfocus Cu Kα (λ = 1.54178 Å) radiation as specified. Datasets
were collected using ω and φ scans with the samples immersed in oil and maintained at a
constant temperature of 100 K using a Cobra cryostream. The data were reduced and processed
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using the Bruker APEX suite of programs. Multiꢀscan absorption corrections were applied
4
0
using SADABS. The diffraction data were solved using SHELXT and refined by fullꢀmatrix
4
1ꢀ43
least squares procedures using SHELXLꢀ2015 within the OLEXꢀ2 GUI.
The functions
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2
2
2
2
2
ꢀ1
2
2
minimized were Σw(F ꢀF ), with w=[σ (F )+aP +bP] , where P=[max(F ) +2F ]/3. All nonꢀ
o
c
o
o
c
hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms
were placed in calculated positions and refined with a riding model, with isotropic displacement
parameters equal to either 1.2 or 1.5 times the isotropic equivalent of their carrier atoms. In cases
where U or position restraints were necessary, these were employed sparingly and only for the
ij
purpose of maintaining chemically sensible geometries and ADPs. Specific refinement strategies
are outlined in the combined crystallographic information file. CCDC 1544479ꢀ1544485.
Results and Discussion
Ligand Synthesis
3
7ꢀ38
The syntheses of L1 and L2 were based on modified literature procedures.
The appropriate
nitrobenzaldehyde was reacted with sodium pyruvate to generate the precursor chalcone, which
was subjected to a Michael addition with 2ꢀpyridacyl pyridinium iodide to yield the dipyridyls
L1 and L2, respectively (Scheme 1). The bulk formulations of L1 and L2 in the solid form were
3
4,35
ascertained by elemental analysis as ammonium salts
or mixtures of protonation states (see
experimental), and all solutionꢀphase characterisation was consistent with the expected species in
solution. Largely due to the poor solubility of these compounds and the resulting complexes, the
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
shortꢀwavelength absorption maxima of the electronꢀdeficient aromatic chromophores (λmax
=
280 nm or below, in both cases) and lack of any measurable fluorescence, no meaningful
information could be derived as to their solution phase coordination behaviour with divalent
cadmium or cobalt ions through UVꢀVisible absorption studies. As such, the discussion herein is
focused on the coordination chemistry of L1 and L2 in the crystalline phase.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
While the formulation of L2 obtained from the reaction mixture proved sufficiently soluble for
further reaction under solvothermal conditions, we encountered difficulties with the very low
solubility of [NH L1] in common solvents, which hindered its reactivity with metal ions. Instead,
4
relying on the wellꢀknown tendency for ester groups to undergo metalꢀmediated in situ
4
4
hydrolysis at high temperatures, the ethyl ester EtL1 was instead employed as a feedstock for
these reactions. The ester was prepared using a thionyl chlorideꢀmediated esterification of L1
and was used in all subsequent preparations, which provided an efficient route to the desired
complexes.
Synthesis and structure of mononuclear Cd complexes of L1 and L2
Structure of [CdL1 ]·H O 1
2
2
The reaction of EtL1 in a 2:1 DMF:H O mixture with cadmium nitrate tetrahydrate at 100 °C
2
yielded pale yellow crystalline material after 18 hrs. The single crystal Xꢀray diffraction data for
[
Cd(L1) ]·H O 1 were solved and refined in the triclinic space group P-1. The asymmetric unit
2 2
contains two L1 molecules deprotonated and coordinating to a cadmium(II) ion, and a nonꢀ
coordinating water molecule (Figure 1). Two nonꢀequivalent ligands coordinate to the cadmium
ion in a distorted octahedral geometry providing narrow cis bite angles of 71.27(7)° and
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6
9.87(7)° for N2ꢀCd1ꢀO1 and N5ꢀCd1ꢀO5, respectively, and 69.46(7)° and 69.69(7)° for N2ꢀ
Cd1ꢀN1 and N4ꢀCd1ꢀN5, respectively. These angles give a large octahedral distortion parameter
45
Σ of 195°, consistent with a size mismatch between the cadmium ion and the ligand binding
pocket when coordinating in a bisꢀtridentate mode. The CdꢀN bonds to the outer pyridine
nitrogen atoms N1 and N4 (2.369(2) and 2.389(2) Å) are longer than the bonds to the central
pyridine nitrogen atoms N2 and N5 (2.283(2) and 2.285(2) Å) and the carboxylate oxygen atoms
O1 and O5 (2.306(2) and 2.329(2) Å, respectively). Both ligands exhibit some distortion from
complete planarity; the nitrophenyl rings are each inclined from their attached pyridine rings
with torsion angles 24.0(3)° (C13ꢀC12ꢀC8ꢀC9) and 12.7(3)° (C34ꢀC29ꢀC25ꢀC26). A very large
dihedral angle is also observed between two coordinating pyridine rings on one molecule of L1,
10
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54
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56
57
58
59
60
th
with the torsion angle (N5ꢀC23ꢀC22ꢀN4) of 18.9(3) ° falling outside the 98 percentile of known
torsions for chelating 2, 2´ꢀbipyridine groups (Supporting Information).
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3
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7
8
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Figure 1 Structure of complex 1, with labelling scheme for coordinating atoms (top) and
showing hydrogen bonding interactions between adjacent complexes via lattice water molecules
(bottom). Selected hydrogen atoms are omitted for clarity.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Adjacent complexes in the structure of 1 are linked into pairs by hydrogen bonding through
two equivalent lattice water molecules. Nonꢀcoordinating carboxylate oxygen atom O2 and
coordinating oxygen atom O5 act as acceptors in these interactions, at O···O bond distances
2.753(3) and 2.776(3) Å and OꢀH···O angles of 179.02(13) and 167.55(12)°, respectively. The
extended structure of 1 is defined by a series of faceꢀtoꢀface and edgeꢀtoꢀface πꢀπ interactions,
and CꢀH···O hydrogen bonding interactions, forming a densely packed overall structure. Both
unique L1 molecules contain a ‘bay’ of three inwardsꢀdirected CꢀH groups; with each of the
groups electron deficient by virtue of attachment to pyridine or nitrophenyl rings, these would be
expected to participate in nonꢀclassical hydrogen bonding interactions with Lewis basic
46ꢀ48
groups.
In the case of 1, the two L1 groups interact with the lattice water molecule O9 and
nonꢀcoordinating carboxylate oxygen atom O6 through these groups. Each of the C···O distances
fall in the range 3.363(3) ꢀ 3.598(3) Å, and all six CꢀH···O angles lie between 139.19(15) ꢀ
177.87(15) °, indicative of genuine CꢀH···O hydrogen bonding interactions, and visible on the
dnorm Hirshfeld surface plot (ESI). The large bipyridyl torsion angle described above may be
related to these interactions, particularly those involving the CꢀH groups at the pyridine 3ꢀ and 6ꢀ
positions.
Structure of [Cd(L2) ] 2
2
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Reacting L2 with cadmium nitrate under similar conditions to 1 gave colourless crystalline
material after 14 hours. Despite screening a wide variety of reaction conditions for the formation
of 2, we consistently obtained small quantities of inorganic impurities alongside the main
crystalline product, which were evident both as unexpected Bragg peaks in the PXRD analysis at
2θ = 10°, 25° and 50° (Supporting Information), and consistently low values for carbon and
nitrogen in microanalysis. These impurities became more prevalent at longer reaction times or
with higher temperatures, and most likely correspond to concurrent formation of inorganic
cadmium or cadmium formateꢀbased products alongside the major product; such phases are very
frequently observed in metalꢀorganic framework syntheses as a result of solvent hydrolysis and
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60
4
9,50
redox sideꢀprocesses.
The diffraction data for [CdL2 ]·H O 2 were solved and refined in the orthorhombic space
2
2
group Pna2 , where the asymmetric unit consists of four unique L2 ligands deprotonated and
1
4
coordinated to two unique cadmium(II) ions, each adopting a distorted octahedral N O2
coordination sphere (See Figure 2). The four unique NꢀCdꢀO cis bite angles lie in the range
70.2(2) ꢀ 70.8(2) °, and similarly the NꢀCdꢀN bite angles are between 70.2(3) and 71.4(3) °,
comparing well to complex 1. Although the octahedral sum of distortion, Σ = 195 and 193 ° for
Cd1 and Cd2 respectively, resemble that of 1, the coordination geometries of both cadmium ions
display a greater tendency towards trigonal prismatic character (Supporting Information), with
trigonal faces defined by the two chelating nitrogen atoms of one L2 molecule and the
coordinating oxygen atom of the other. This assignment is supported by the particularly large cis
angles between terminal pyridine nitrogen atoms N1, N4, N7 and N10, and the central pyridine
nitrogen atoms of opposing ligands N5, N2, N11 and N8, respectively. These angles, which lie in
the range 120.6(3) – 132.2(3) °, help to define flat square faces about the pseudoꢀtrigonal axis.
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5
5
5
5
5
6
The related angles in 1, of 103.66(7) and 118.95(7)°, are much more consistent with the distorted
octahedral geometry. All of the coordination bond lengths, falling in the range 2.250(7) ꢀ
2.373(7) Å, are consistent with those observed in complex 1, with bonds to the central pyridine
0
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8
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3
4
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0
nitrogen atoms displaying the shortest distances. Unlike 1, all of the pyridineꢀpyridine torsions in
2 are near planarity (in the range 5.3(9) – 7.8(9) °), and the phenylꢀpyridyl torsion angles display
comparable values, in the range 15.7(11) – 49.1(10)°.
Figure 2 Structure of complex 2, with labelling scheme for coordinating atoms (top) and
showing polydentate CꢀH···O hydrogen bonding interactions between adjacent complexes in the
extended structure (bottom). Selected hydrogen atoms are omitted for clarity.
Two predominant intermolecular interaction modes are present between complexes within the
structure of 2. Nonꢀequivalent pairs of complexes associate with faceꢀtoꢀface πꢀπ interactions
between the two dipyridyl groups, with (dipyridylꢀdipyidyl) mean interplanar angle of 2.2° and
minimum interatomic distance of 3.250(12) Å (C4ꢀC40). These interactions are supplemented by
reciprocated chelating CꢀH···O hydrogen bonding interactions between the inwardsꢀdirected Cꢀ
H groups of each ring on one complex and the coordinating carboxylate oxygen atom (O5/O9) of
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the adjacent species. For the tightly bound dimers, the six CꢀH···O interactions display C···O
distances in the range 3.247(10) ꢀ 3.576(11) Å, and CꢀH···O angles 148.8(6) ꢀ 171.0(5)°. These
5
1,52
interactions are also a dominant feature on the calculated Hirshfeld surface (ESI).
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Additionally, the dimeric association of neighbouring complexes leads to unusually close
contacts between the nitro groups and the chelating pyridine nitrogen atoms of the adjacent
ligand, with N···O distances 2.793(10) and 2.932(11) for O3···N11 and O15···N5, respectively.
These dimeric assemblies are further linked to neighbouring complexes by weaker CꢀH···O
hydrogen bonding interactions to nonꢀcoordinating carboxylate oxygen atoms, although the
influence of this interaction on the extended structure is lessened by the lack of any substantial πꢀ
π overlap between the adjacent complexes. Additionally, various CꢀH···O interactions involving
the nitro groups, and CꢀH···π interactions are present within the structure, contributing to a
densely packed extended structure containing no void volume or encapsulated solvent molecules.
Synthesis of polynuclear Cd complexes
·
[
Cd (L1) (NO ) (DMF) ]NO 3 and [Cd (L2) (NO ) (DMF) ]NO DMF 4
3
3
3 2
4
3
3
3
3 2
4
3
Interestingly, during the screening process for the optimised syntheses of 1 and 2, two
additional phases were discovered. Reacting either EtL1 or HL2 with cadmium nitrate at higher
concentrations in pure DMF and 1:1 M:L stoichiometries gave rise to a different crystalline
material (in both cases) compared to those observed in 1 and 2. The single crystal Xꢀray
diffraction data of [Cd (L1) (NO ) (DMF) ]NO 3 were solved and refined in the triclinic space
3
3
3 2
4
3
group P-1, with the asymmetric unit containing three unique L1 ligands, deprotonated and
II
coordinating to three unique Cd ions, forming a trinuclear complex. Each cadmium ion adopts a
sevenꢀcoordinate pentagonal bipyramidal geometry. The two axial positions of each metal ion
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are occupied by either nitrate anions or DMF molecules, while the equatorial positions are
occupied by a single tridentate chelate from one molecule of L1 and a chelating carboxylate
group from another L1 molecule; each ligand coordinates in a ꢁ ꢀκN,N′,O:κO,Oʹ bridging mode
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(Figure 3). The overall assembly resembles an approximately threefoldꢀsymmetric disc with a
central sixꢀmembered Cd O ring, with the three Cd···Cd distances 4.819(1), 4.686(1) and
3
3
4.6745(9) Å for Cd3ꢀCd2, Cd2ꢀCd1 and Cd1ꢀCd3, respectively. This approximate symmetry is
disrupted by the unsymmetric distribution of the axial ligands, where cadmium ion Cd3 has both
axial positions occupied by DMF molecules, while Cd1 and Cd2 are coordinated by one nitrato
and one DMF ligand each, in an anti arrangement to one another across the central plane. For
charge balance, the asymmetric unit also contains one disordered nonꢀcoordinating nitrate anion,
while no nonꢀcoordinating solvent molecules were detected crystallographically. All of the
metalꢀligand bond distances are greater in 3 than their equivalents in the related mononuclear
complex 1, falling in the range 2.299(7) – 2.353(6) Å for CdꢀN bonds and 2.289(7) – 2.507(6) Å,
consistent with the higher coordination number of the metal ions in 3.
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Figure 3 Structure of complex 3, showing the central cluster core and coordination
environment with labels for coordinating atoms (top) and the complete structure of the complex
(bottom). Nonꢀcoordinating nitrate anion and all hydrogen atoms are omitted for clarity.
The largely flat trinuclear clusters in 3 associate through various faceꢀtoꢀface πꢀπ interactions,
supported by CꢀH···O interactions between the L1 backbone and the nitrato and DMF ligands of
adjacent complexes. These interactions are generally reminiscent of those observed in the
structures of the two mononuclear complexes. In addition, the bridging coordination geometry
provides additional opportunities for chelated CꢀH···O interactions on the outer faces of each L1
molecule in concert with the outer pyridine CꢀH groups of adjacent ligands. The extended
structure of 3 can be considered in terms of columns aligned parallel to the crystallographic a
axis consisting of the central metal cores and axial ligands, with the discꢀshaped periphery of L1
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groups extending into the b and c directions and interdigitated with those of adjacent columns,
supported by πꢀπ and CꢀH···O interactions. These columns are arranged in a honeycomb rod
packing motif, with the interstitial spaces occupied by the disordered nitrate anions.
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The reaction of HL2 with cadmium nitrate under similar conditions to 3 gave colourless
crystals of [Cd (L2) (NO ) (DMF) ]NO ·DMF 4, the diffraction data for which were solved and
3
3
3 2
4
3
refined in the triclinic space group Pꢀ1. The asymmetric unit of 4 is closely related to that of 3,
II
containing a trinuclear complex consisting of three sevenꢀcoordinate pentagonal bipyramidal Cd
ions, three deprotonated L2 ligands, four DMF and two nitrato ligands, and a nonꢀcoordinating
nitrate anion and lattice DMF molecule. The three cadmium ions and ligand molecules in 4 adopt
equivalent coordination geometries to those in 3, and the CdꢀCd distances in the range 4.5999(7)
ꢀ
4.8394(6) Å are in close agreement with those observed in 3. The arrangement of nitrato and
DMF ligands in 4 is slightly different, with both nitrato ligands occupying the axial positions of
cadmium ion Cd2, while the coordination spheres of the other two cadmium ions are completed
by DMF ligands only. No meaningful differences in metalꢀligand bond lengths are observed
between the three cadmium ions, or with the equivalent distances in the structure of 3.
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Figure 4 Structure of complex 4, showing core connectivity with labelling scheme for
coordinating atoms (top) and complete structure of the complex (bottom). Nonꢀcoordinating
DMF molecule and nitrate anion, positional disorder in coordinating DMF ligand, and all
hydrogen atoms are omitted for clarity.
Adopting a similar discꢀlike shape, the intermolecular interactions in the structure of 4 are
also related to those observed in 3. The packing motifs are largely dominated by πꢀπ interactions,
augmented by CꢀH···O interactions between the electron deficient aromatic CꢀH groups and the
oxygen atoms of axial ligands from adjacent molecules. Numerous such interactions can be
expressed in the Hirshfeld surface with mapped dnorm plot for the complex (Supporting
Information), and features on the fingerprint plot reminiscent of 3 and the mononuclear
complexes 1 and 2 are present (See discussion section). As was the case in 3, adjacent complexes
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overlap along the a axis into columns which interdigitate with one another via these interactions.
However, rather than the regular honeycomb packing motif of these columns observed in 3, in 4
these columns are arranged into layers oriented in the ac plane by intermolecular πꢀπ and Cꢀ
H···O interactions, while the interactions in the b direction separate the metal sites by a greater
degree.
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Figure 5 Comparison in the extended packing motif for complexes 3 (left) and 4 (right); while
a single stack of 3 interacts with three others in a pseudoꢀtrigonal packing arrangement, stacks of
4
associate into dimers at the anti-anti edge before further associating (c axis horizontal).
Interestingly, both complexes 3 and 4 exhibited sensitivity to atmospheric water on extended
drying in ambient air. Thermogravimetric analysis was conducted on two batches of 3 and 4, one
of which was freshly synthesised whereas the other had been exposed to air for 3 weeks. The
freshly isolated samples both exhibit small initial mass losses (ca. 4%) corresponding to surface
and loosely bound solvent, plus mass losses in the temperature range 100ꢀ230 °C corresponding
to loss of the coordinating and nonꢀcoordinating DMF molecules (measured 14% and 16%,
calculated 16% and 19% for 3 and 4, respectively). On standing in air for three weeks, the higher
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temperature steps (100 ꢀ 230 °C) are reduced to 4% and 8% for 3 and 4, respectively, while both
compounds exhibit an increased 8% mass loss between room temperature and 100 °C. The loss
of lattice DMF molecules and incorporation of atmospheric water in both cases is corroborated
by microanalysis on the airꢀdried samples, which showed both compounds can be described by
the formulation [Cd (L) (NO ) ]·1.5DMF·3H O (calculated 6.5% wt. DMF, 3 % wt. H O), but
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3
3
3 3
2
2
not suitably approximated by the original loading of 4ꢀ5 DMF molecules per cluster. Adsorption
of atmospheric water is often observed in both polymeric frameworks and porous molecular
materials, and can lead to structural changes particularly where the guest molecules are loosely
53ꢀ55
bound or arranged in contiguous solvent channels.
Exposure to air for extended periods led to
cracking and consequent loss of single crystallinity in both 3 and 4, preventing the monitoring of
this process by Xꢀray diffraction. Analysis by PXRD of the dried sample shows degradation of
the original Bragg peaks but the emergence of no new peaks in the pattern, suggesting a loss of
crystallinity rather than a transition to a new crystalline phase (Supporting Information)
II
III
Synthesis and structure of Co and Co complexes of L1 and L2
After the examination of the coordination chemistry of L1 and L2 with the labile coordination
II
sphere of Cd , and the emergence of both mononuclear and trinuclear species for each ligand, we
II
turned our attention to Co , with the expectation of forming more wellꢀdefined mononuclear
complexes with regular octahedral geometry. Surprisingly, however, the reaction between either
EtL1, HL1 or HL2 in DMF at 100 °C gave mixtures of products after 24 hours. The
predominant products were the divalent species [Co(L1) ] 5 and [Co(L2) ] 7, however a
2
2
crystalline trivalent impurity was identified in both cases, with the formulation [Co(L1) ]NO 6
2
3
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or [Co(L2) ]NO 8. Although such in-situ metal ion oxidations under solvothermal conditions are
2
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5
6
not especially uncommon, we were unable to generate either trivalent species on larger scales
or as a pure phase. Nonetheless, modifying the reaction solvent to 1:1 H O/DMF was adequate to
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remove the trivalent impurity from the isolated material and generate each of the divalent
materials as crystalline phases in sufficient quantity to conduct the necessary analysis to both
asses the purity and fully characterise the complexes.
Structure of [Co(L1) ] 5 and [Co(L1) ]NO 6
2
2
3
The diffraction data for 5 were solved and refined in the monoclinic space group C2/c, with the
II
asymmetric unit containing one molecule of L1, deprotonated and coordinating to a Co ion
occupying a crystallographic special position. Two equivalent ligand molecules coordinate to the
cobalt ion in a distorted octahedral geometry, with each ligand providing narrow cis bite angles
of 76.95(13)° and 75.42(14)° for O1ꢀCo1ꢀN2 and N1ꢀCo1ꢀN2, respectively. These angles are
comparable to those observed in the mononuclear complexes 1 and 2, although the octahedral
distortion parameter Σ of 132° is considerably smaller, consistent with the greater tendency
7
II
10
II
towards regular octahedral geometry for the d Co ion compared to the d Cd ion. The metalꢀ
ligand bond lengths also show some level of distortion, with the CoꢀN bond to the central
pyridine nitrogen atom N1 of 2.041(2) Å considerably shorter than the bonds to N2 and O1, at
2.142(5) and 2.114(4) Å, respectively. The ligand adopts a relatively planar conformation, with a
slight twist in the nitrophenyl ring relative to the central pyridine ring of 29.9(7) ° (C9ꢀC8ꢀC12ꢀ
C17 dihedral angle).
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Figure 6 Structure of complex 5, with labelling scheme for coordinating atoms (top) and
reciprocated polydentate CꢀH···O interactions between adjacent molecules in the extended
structure (bottom). Symmetry codes used to generate equivalent atoms: (i) 1ꢀx, +y, ½ꢀz.
Adjacent molecules of 5 associate through a πꢀπ stacking interaction coupled with a series of Cꢀ
H···O interactions. The dipyridyl moieties from two molecules associate in an offset faceꢀtoꢀface
fashion, with mean interplanar distance 3.40 Å between parallel dipyridyl units. This interaction
places the coordinating carboxylate oxygen atom O1 in the vicinity of several electron deficient
CꢀH groups from the adjacent molecule, with C···O distances 3.422(5), 3.270(5) and 3.670(5) Å
and CꢀH···O angles 152.5(3), 161.0(3) and 156.0(3)° for C7, C4 and C13, respectively. These
interactions, similar to those observed for complex 2, and which are a dominant feature on the
calculated Hirschfeld surface for the molecule (ESI), are reciprocated between pairs of
molecules, and align complexes into columns parallel to the c axis. Adjacent columns associate
through weaker πꢀπ and CꢀH···π interactions involving the terminal nitrophenyl group.
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The diffraction data for the trivalent species [Co(L1) ]NO 6 was refined in the triclinic
2
3
+
space group Pꢀ1, with the asymmetric unit containing one [Co(L1) ] cation and an associated
2
nonꢀcoordinating nitrate anion. The geometry of the complex is closely related to that observed
in 5, with the expected contraction in MꢀL bond lengths, each of which is shortened by ca. 0.1 Å
compared to the divalent complex. Again, the bonds to the central pyridine rings (1.855(2) and
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1
.857(2) Å for N2 and N5, respectively) are shorter than those to the outer pyridine groups
(
1.943(2) and 1.959(2) Å for N1 and N2, respectively) and the carboxylate oxygen atoms
(1.931(2) and 1.913(2) Å for O1 and O5, respectively). The octahedral distortion parameter is
III
also significantly reduced to 63°, consistent with a better fit of the smaller Co ion within the
binding pocket.
Figure 7 Structure of complex 6 with partial atom labelling scheme (Top), and overlaid
structures of complex 5, shown in blue, and complex 6, shown in red (bottom). Hydrogen atoms
are omitted for clarity. The distance N3···O11 corresponding to the anion···πꢀhole contact is
2
.873(3) Å.
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The intermolecular interactions of 6 are related to those observed in 5, with variations based on
slight differences in ligand geometry and the presence of the nitrate anion within the lattice. One
unique L1 group engages in a similar πꢀπ stacking/CꢀH···O hydrogen bonding mode as observed
in 5, albeit with considerably weaker interꢀcomplex interactions. The mean interplanar distance
of 3.52 Å between the equivalent dipyridyl mean planes and C···O distances in the range
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3.430(3) – 4.306(3) Å for this interaction are both indicative of a significantly weaker attractive
force between the two molecules. The other L1 molecule interacts with the nitrate anion through
an unusually short electrostatic interaction between nitrate oxygen atom O11 and nitrogen atom
N3 from the aromatic nitro group, of NꢀO distance 2.873(3) Å. In similar systems this anionꢀπ
5
7ꢀ59
interaction is accounted for by the presence of a nitroaromatic πꢀhole.
This interaction is
accompanied by a substantial bending of the phenyl ring of the L1 molecule out of the
meridional plane of the metal centre in the opposite direction from the nearby anion. The
differences in geometry can be conveniently visualised by overlaying the two molecular
structures (Figure 7). The interplanar angle of 12.0° between the nitrate mean plane and the mean
plane of the aromatic nitro group CꢀNO shows the two group are nearꢀparallel to each other.
2
Besides these interactions, the extended structure is densely packed, with no encapsulated
solvent molecules or void volume, and is largely defined by a series of CꢀH···O interactions
involving the nitrate anion, and additional CꢀH···π interactions of the terminal phenyl ring.
Structure of [Co(L2) ] 7 and [Co(L2) ]NO 8
2
2
3
L2 was reacted with cobalt nitrate hexahydrate in DMF at 100 °C to yield light brown
crystalline material after 24 hrs, which contained a mixture of the divalent [Co(L2) ] 7 and the
2
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