Carbon dioxide capture from air leading to bis[N-(5-methyl-1H-pyrazol-3-yl-κN 2)carbamato-κO]copper(II) tetrahydrate

A mononuclear square-planar CuII complex was synthesized by reacting 5-methyl-3-pyrazolamine and copper(II) acetate in water under ambient conditions. Diethanolamine was added to facilitate carbon dioxide adsorption, creating an alkaline environment. Structural analysis revealed that the complex crystallizes in the P21/c space group of the monoclinic crystal system, with the central copper(II) atom in a square-planar coordination environment N2O2. Co-crystallized water molecules are present, forming O—H⋯O hydrogen bonds with the CuII mononuclear complex. Hirshfeld surface analysis highlighted the importance of various interactions, including H⋯O/O⋯H, H⋯C/C⋯H, and H⋯N/N⋯H, in providing crystal structure packing.


Chemical context
Currently, global warming stands out as the most significant environmental concern, leading to climate change and giving rise to a range of effects, including elevated sea levels, prolonged droughts, intensified hurricanes, and a surge in extreme weather occurrences (Ochedi et al., 2021).The primary cause of global warming in recent decades can be attributed to the heightened levels of greenhouse gases in the atmosphere, with particular emphasis on the concentration of CO 2 (Aghaie et al., 2018).Power plants, comprising more than 40% of CO 2 emissions, with coal-fired facilities accounting for 73% of fossil fuel-based power plant emissions (Cannone et al., 2021;Mikkelsen et al., 2010), are a significant contributor to the carbon footprint.Given the widespread use of fossil fuels, particularly coal, there is a strong need to develop effective methods for capturing and mitigating CO 2 emissions from power plant flue gases, to help stabilize the atmospheric CO 2 level (Wang et al., 2017).
Various technologies, including adsorption (Milner et al., 2017), absorption (Conway et al., 2013), membrane separations (Sreedhar et al., 2017), cryogenic distillation (Song et al., 2019), and chemical looping (Kronberger et al., 2004), are currently under research and development for capturing CO 2 from flue-gas streams.One potential strategy for reducing carbon emissions in the future involves the utilization of carbon capture and sequestration (CCS) materials.
The process of CCS entails the specific separation and subsequent storage of CO 2 taken from exhaust gas mixtures, which predominantly consist of N 2 , CO 2 , H 2 O, and O 2 , preventing their release into the atmosphere.Following this, the collected CO 2 is transported for either utilization or longterm storage.Amine scrubbing-based chemical capture methods have garnered significant focus and interest (Tang et al., 2005;Mani et al., 2006).
One of the methods for reducing carbon dioxide levels in the environment involves capturing it through the formation of carbamates (Conway et al., 2011;McCann et al., 2009;Zhang et al., 2017).Besides, carbamates can be used as catalysts or useful intermediates in the synthesis of other, morevaluable chemicals (Dell'Amico et al., 2003).Given the necessity of capturing CO 2 to address broader societal needs, in this article we report the synthesis, crystal structure and Hirshfeld surface analysis of a new mononuclear copper(II) complex with (5-methyl-1H-pyrazol-3-yl)carbamic acid -[Cu(5-MeHpzCarb) 2 ]•4H 2 O.

Structural commentary
The title compound crystallizes in the monoclinic space group P2 1 /c, and has a crystal structure built upon neutral mononuclear [Cu(5-MeHpzCarb) 2 ] units (Fig. 1).Co-crystallized water molecules are present in a 1:4 ratio to the complex as interstitial molecules.The asymmetric unit includes one copper site (SOF is 0.5, Wyckoff position 2a), one (5-methyl-1H-pyrazol-3-yl)carbamate ligand and two co-crystallized water molecules.
The Cu II ion displays a square-planar coordination environment (N 2 O 2 ) formed by two nitrogen atoms of pyrazole rings and two oxygen atoms of carboxylate group of (5methyl-1H-pyrazol-3-yl)carbamate ligands.The Cu1-N1 distances are 1.931 (2) A ˚while the Cu1-O1 distances are shorter and account to 1.9140 (17) A ˚.The O1-Cu1-O1 i and N1-Cu1-N1 i bond angles are 180 � , which is typical for a square-planar arrangement (Fig. 1).At the same time, the N1-Cu1-O1 i and N1-Cu1-O1 bond angles slightly deviate from the ideal value of 90 � , which is the result of the formation of the six-membered chelate rings.Selected bond lengths and bond angles are given in Table 1.The Cu1 atom lies within the plane defined by N1-O1-N1 i -O1 i .Additionally, the Cu atom lies within the planes of the aromatic rings, whereas O1 and O1 i are slightly above the plane, with an O1(O1 i )-to-plane distance of 0.182 (3) A ˚.In the crystal structure, monomeric [Cu(5-MeHpzCarb) 2 ] units form layers with Cu1 centres lying in the ab plane.The plane-normal-to-plane-normal angle between the horizontal N1-O1-N1 i -O1 i planes of two adjacent layers is 74.762 (2) � .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.The H atoms of N (p, c) -H, C p -H and O w -H groups (p = pyrazole, c = carbamide, w = water) were positioned geometrically and refined as riding atoms, with C-H = 0.95 A ˚and U iso (H) = 1.2U eq (C) for C p -H groups, N-H = 0.88 A ˚and U iso (H) = 1.2U eq (N) for N (p, c) -H groups and O-H = 0.87 A ˚and U iso (H) = 1.5U eq (O) for O w -H groups.Methyl H atoms were positioned geometrically and were allowed to ride on C atoms and rotate around the C-C bond, with C-H = 0.98 A ˚and U iso (H) = 1.5U eq (C) for the CH 3 groups.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.
(a) Hirshfeld surface representations with the function d norm plotted onto the surface for the different interactions; (b) two-dimensional fingerprint plots, showing the contributions of different types of interactions.

Table 1
Selected bond lengths and bond angles (A ˚, � ).

Table 3
Experimental details.