Graphene oxide and nanodiamond: same carboxylic groups, different complexation properties

Abstract

DFT calculations (PBE functional with the empirical correction by Grimme) were employed to explain why our attempts to coordinatively functionalize nanodiamond (ND) with tetraazamacrocyclic cations [Ni(cyclam)]²⁺ and [Ni(tet b)]²⁺, and to generate paramagnetic hybrid materials in this way, failed, contrary to the successful functionalization of graphene oxide (GO) reported previously (Appl. Surf. Sci., 2016, 371, 16–27). The explanation offered is based on the comparison of binding energies for low-spin (singlet) and high-spin (triplet) complexes of model carboxylate ions GO⁻ and ND⁻ with the two tetraazamacrocycles. The formation energies were interpreted in terms of ΔΔE₃₋₁ values, which characterize the difference in stability for the triplet and singlet complexes (negative values mean that triplet state is more stable, and positive, that singlet state is more stable). While the results obtained do not rule out completely the possibility of forming high-spin [Ni(cyclam)]²⁺ carboxylate derivatives on ND, in the case of [Ni(tet b)]²⁺ comparison of the ΔΔE₃₋₁ values explicitly demonstrated that the formation of high-spin complex is highly unfavorable with ND⁻ contrary to GO⁻ model: ΔΔE₃₋₁ values obtained are 13.22 and −4.64 kcal mol⁻¹, respectively. For comparison, similar data are presented for a series of simpler carboxylates. In addition to binding energies and ΔΔE₃₋₁ values, for all the systems studied we analyzed Ni–O distances, spin density plots and HOMO−LUMO parameters.