Cobalt(II) complex formation in [C4mim][Tf2N]

Room temperature ionic liquids (RTILs) have been used as "green" substitutes of organic solvents in a number of applications, due to several advantages over the latter and numerous useful physicochemical properties [1] As far as the separation and recycling of “critical” metals is concerned, the use of hydrophobic RTILs as extracting phases in hydrometallurgical processes has been proposed in the last decade [2]. In particular, the recycling of cobalt assumed growing importance due to the increased demand related to his use in expanding markets, such as Li-ion batteries or electric motors [3]. Therefore the knowledge of speciation and structure of the Co(II) ion in RTILs can provide key information for developing efficient recylcing processes. Among the available RTILs, those based on the N,N’-alkylimidazolium (CnCmim+) cation and bis(trifluoromethylsulfonyl)imide (Tf2N-) anion have been extensively studied for metal extractions and electrochemical depositions. However, only few works[4]are focused on the nature of the dissolved metals and their speciation in RTILs. With the aim to fill this gap, in this communication, we report the results on Co(II) complex formation with nitrate and chloride anions in [C4mim][Tf2N] (C4mim = 1-methyl-3 butylimidazolium). The nature of the species formed in [C4mim][Tf2N] is determined experimentally by means of spectrophotometry and calorimetry. Density Functional Theory (DFT) and molecular dynamics (MD) calculations are employed to get information on the structure and solvation of the complexes. Our results show that stable CoXj (j = 1-4 and 1-3 for Cland NO3- , respectively) are formed. In the case of Cl- , a change of coordination (octahedral → tetrahedral) occurs (Figure 1) in the 1:2 species. MD simulations provided the Co(II) coordination number and information on the arrangement of the [Tf2N]- anions in solution. Also, it is observed that the second solvation shell, mostly composed by [C4mim]+ cations, contracts with the increase of the number of bound ligands (j). [1] Armand, M., Endres, F., MacFarlane, D. R. , Ohno, H., Scrosati, B. Nat. Mater. 2009, 8, 621-629. [2] Abbott, A. P., Frisch, G., Hartley, J., Ryder, K. S. Green Chem. 2011, 13 (3), 471-481. [3] Piatek., J. S. Afyon, T. M. Budnyak, S. Budnyk, M. H Sipponen, A. Slabon, Adv. Energy Mater. 2020, 2003456. [4] M. Busato, A. Lapi, P. D’Angelo, A. Melchior, J. Phys. Chem. B 2021, 125 (24), 6639.