Nickel(II) complexes based on l-amino-acid-derived ligands: synthesis, characterization and study of the role of the supramolecular structure in carbon dioxide capture

Two l-amino-acid-based NiII complexes are reported; while the l-tyrosine derivative is a symmetrical μ3-carbonate-bridged self-assembled trinuclear NiII complex whose formation involves CO2 uptake, the l-phenylalanine analog is a mononuclear system which does not exhibit the same behaviour.


Analysis of carbonate-metal complexes using the Crystallographic Data Base (CSD)
References S1-S15 show all L-Tyr-based-ligands Ni(II) coordination complexes included at the CSD, respectivelly.S16 References S6-S15 correspond to unsubstituted L-Tyr (10 hits) and S17-S20 to L-Phe (8 hits) Ni(II)-systems.     For the structure with ID code OHARUL, since the H atoms of the coordinated water molecules were not refined, it was not analyzed.

Figure S6
H-bond sets between the coordinated water molecules and the carbonate bridging ligand for structures found at the CSD related to 1Ni-SC. S21-S23 Synthesis of Ligand 3. It was prepared following the procedure described for 1 except that 200mg

Synthesis and characterization
(1.1mmol) of L-Tyr and 88mg de NaOH (2.2mmol) were used, and after the addition of NaBH4 the reaction mixture was stirred at RT and not under reflux. Yield: 150mg (50.2%). 1 H NMR (D2O/NaOD): δ 2.71 (d, 2H, J = 6Hz), 3.22 (t, 1H, J = 6Hz), 3.51 (d, 1H, J = 12Hz), 3.70 (d, 1H, J = 12Hz), 6.50 (d, 2H, J = 8Hz), 6.90 (d, 2H, J = 8Hz), 7.25 -7.35 (m, 5H). 13 C NMR (D2O/NaOD): δ 38.11, 50.98, 64.76, 118.52, 123.51, 127.27, 128.58, 128.61, 130.49, 138.96, 164.50, 181.69 Synthesis of complex 3Ni starting form Ni(NO3)2·6H2O. In a 5mL vial, 6mg (0.02mmol) of Ni(NO3)2·6H2O were dissolved in 1mL of methanol and then, a solution of the sodium salt of ligand 3 was added. The solution of the ligand was prepared using 5mg (0.018mmol) of 3, 1.6mg (0.04mmol) of NaOH and 1mL of methanol. The resulting blue-green solution of the complex was stirred overnight and then after removing the cap of the vial and covering with parafilm foil with small holes, it was open to the air and left for slow evaporation of the solvent at room temperature. After c.a. 3 days, light-blue crystalline aggregates were obtained. A lot of different crystallization protocols were explored but unfortunately, so far any of them were successful, and thus we were not able to obtain suitable crystalline material to perform single crystal x-ray experiments. Selected          After that, a smaller endothermic peak is observed, which it is associated with structural-solvent loss (coordinated water molecules). Finally, the decomposition of the complexes is observed.

Figure S22
TGA obtained for 2Ni-CA. A moderate mass loss at the beginning, associated with a residual superficial solvent lost, could be appreciated. After that, a strong mass loss due to the decomposition of the complex is observed.

Figure S23
TGA and DSC analysis superposed for 2Ni-CA. An intense exothermic peak is seen in the region where the solvent molecules are lost. After that, another highly exothermic peak is observed, which it is associated with structural-solvent loss (coordinated water molecules). Finally, the decomposition of the complex appears as a moderate peak at the end.

XRD studies Single Crystal XRD tables
Crystals of 1Ni-SC suitable for XRD studies were obtained after slow evaporation of the reaction solvent. After removing the crystals from their mother liquor, they were immediately covered by mounting oil and after a fast inspection, a suitable crystal was mounted in a loop and placed in the diffractometer at low temperature. Several crystals were tested using an Oxford Diffraction Gemini E lab diffractometer with Mo Kα (λ = 0.71 Å) radiation, but due to the partial loss of solvent molecules during the experiment and the decrease of the crystal quality, the resulting data were not good enough to give place to acceptable resolutions. The full data collection was planned using the CrysAlis Pro strategy tool S29 and data were reduced using CrysAlis Pro programme. A Gaussian method implemented in WinGX S30 or a numerical model S31 was used for the absorption correction. Crystals of complex 1Ni obtained from different Ni(II) salts were studied by single crystal XRD using an Oxford Diffraction Gemini E lab diffractometer with Mo Kα (λ = 0.71 Å) radiation as well. Full data collection for 1Ni-SC and 2Ni-SC were performed at a Synchrotron beamline. 1-y,1+x-y,+z; 2 +y-x,1-x,+z Table S5. Hydrogen Bonds features developed in 1Ni-SC, bond lengths in Å and angles in (⁰).

D H A d(D-H)/Å d(H-A)/Å d(D-A)/Å D-H-A/°
Nevertheless, all three diffractograms are in good agreement.
─ Simulated from single crystal SRD experiments of 1Ni-SC ─ Experimental using a sample of 1Ni-SC ─ Experimental using a sample of 1Ni-PM (1Ni-SC dried at RT)

Figure S25
X-ray diffraction patterns for 2Ni-CA using different Nickel (II) salts for the synthesis.

Additional structural analysis
The molecular structure of 1Ni-SC was evaluated along with the data deposited at the CSD and it was confirmed that the structural parameters of all fragments are in agreement with average values of the most relevant bond lengths and angles.

Figure S26
Statistical evaluation for carbonate angles in 1Ni complex.   The ligands in the structure of 2Ni, one of them showed a disordered carboxylate moiety and besides, the relative location of the aromatic rings respect to each other is also different in each ligand. In one of the ligands the rings are displayed in a parallel fashion as it was observed for the structure of complex 1Ni, giving place to a long distance π-π interaction (distance between centroids 3.825 Å), but the other has the piperonal moiety a bit away from the expected location. The angle between the planes described by the rings shows a torsion of 43.26°. A possible explanation for that could be associated with the effect of the intermolecular interactions developed by the DMF located in the proximity of the second ligand. It is probable that the H-bonds established by this molecule and the carboxylate and amino groups would balance the distortion generated in the rings and the absence of any π-interaction ( Figure   S31).

Figure S31
Conformation of the ligands in 2Ni-SC. Supramolecular structure of 2Ni-SC is constructed by an infinite linear chain described by consecutive units of the complex along crystallographic axis c, which interacts with a zig zag arrangement extended along crystallographic axis a and also, by C-H···O contacts involving the piperonal moieties which are displayed along crystallographic axis b.

Figure S33
Details of the H-bonds exhibited in 2Ni-SC.