2, Tufts University, Medford, Massachusetts, United States
3, Universidad Nacional Autónoma de México, Mexico-City, Cd. Mexico, Mexico
Despite of evident, in particular morphological differences, in terms of surface chemistry nanodiamond (ND) has much in common with graphene oxide (GO). Both of them have oxygenated functional groups on their surfaces. Among the most important and abundant ones are carboxylic groups COOH, which form the basis of functionalization chemistry for both ND and GO. Recently we tested a novel approach to GO functionalization, which allows for facile generation of a paramagnetic material by combining two diamagnetic components: GO and square-planar tetraazamacrocyclic cations [Ni(cyclam)]2+ and [Ni(tet b)]2+.1 The underlying chemistry is the conversion of the square-planar diamagnetic complexes to pseudooctahedral paramagnetic ones when coordinated to COOH group under basic conditions, which implies the change from low-spin to high-spin state of nickel(II) ions. Based on the similarity between ND and GO surface chemistry, we attempted coordination functionalization of ND with the same tetraazamacrocyclic cations, under the same conditions as in the case of GO.1 Nevertheless, the results we obtained were negative. DFT calculations (PBE functional with the empirical correction by Grimme) were employed to explain why our attempts to coordinatively functionalize ND failed. Our explanation is based on the comparison of calculated binding energies for low-spin (singlet) and high-spin (triplet) complexes of model carboxylates GO− and ND− with the two tetraazamacrocyclic cations. The calculated energies of complex formation were interpreted in terms of ΔΔE3-1 values, which quantify the difference in stability for the triplet and singlet complexes: a negative ΔΔE3-1 value means that triplet complex is more stable, and vice versa. The results obtained do not completely exclude the possibility of formation of high-spin [Ni(cyclam)]2+ carboxylate derivatives on ND. However, the comparison of ΔΔE3-1 values in the case of [Ni(tet b)]2+ explicitly demonstrated that the formation of high-spin complex is highly unfavorable with ND− contrary to GO− model, for which ΔΔE3-1 values obtained are 13.22 and -4.64 kcal/mol, respectively. In addition to binding energies, we analyzed the optimized complex geometries (in particular Ni−O distances), HOMO−LUMO parameters and spin density plots.
This work was supported by the projects CONACYT-250655 and UNAM DGAPA-IN200516.
(1) V. A. Basiuk, et al., Appl. Surf. Sci., 371, 16 (2016).