Towards accurate estimates of the spin-state energetics of spin-crossover complexes within density functional theory: a comparative case study of cobalt(ii) complexes

Abstract

We report a detailed DFT study of the energetic and structural properties of the spin-crossover Co(ii) complex [Co(tpy)(2)](2+) (tpy = 2,2':6',2''-terpyridine) in the low-spin (LS) and the high-spin (HS) states, using several generalized gradient approximation and hybrid functionals. In either spin-state, the results obtained with the functionals are consistent with one another and in good agreement with available experimental data. Although the different functionals correctly predict the LS state as the electronic ground state of [Co(tpy)(2)](2+), they give estimates of the HS-LS zero-point energy difference which strongly depend on the functional used. This dependency on the functional was also reported for the DFT estimates of the zero-point energy difference in the HS complex [Co(bpy)(3)](2+) (bpy = 2,2'-bipyridine) [A. Vargas, A. Hauser and L. M. Lawson Daku, J. Chem. Theory Comput., 2009, 5, 97]. The comparison of the and estimates showed that all functionals correctly predict an increase of the zero-point energy difference upon the bpy → tpy ligand substitution, which furthermore weakly depends on the functionals, amounting to . From these results and basic thermodynamic considerations, we establish that, despite their limitations, current DFT methods can be applied to the accurate determination of the spin-state energetics of complexes of a transition metal ion, or of these complexes in different environments, provided that the spin-state energetics is accurately known in one case. Thus, making use of the availability of a highly accurate ab initio estimate of the HS-LS energy difference in the complex [Co(NCH)(6)](2+) [L. M. Lawson Daku, F. Aquilante, T. W. Robinson and A. Hauser, J. Chem. Theory Comput., 2012, 8, 4216], we obtain for [Co(tpy)(2)](2+) and [Co(bpy)(3)](2+) best estimates of and , in good agreement with the known magnetic behaviour of the two complexes.

Guest Modulation of Spin‐Crossover Transition Temperature in a Porous Iron(II) Metal–Organic Framework: Experimental and Periodic DFT Studies

“…Although several studies have assessed the performance of different computational methods for the calculation of absolute T c values, this task still remains challenging even for modern DFT functionals and post-HF methods mainly because insufficient accuracy on the estimation of electronic energy differences. [22][23][24][25][26][27] In the case of the empty clathrate (1), a T c of 425 K can be calculated from numerical frequency calculations; this value is significantly higher than the experimental value (290 K) and indicates that the relative stability of the low-spin state is overestimated, and thus we have to keep in mind that spin-crossover temperatures are highly sensitive to small changes in energy and that such big discrepancies are not strange in this kind of calculations. While accurate absolute transition temperatures are hard to obtain, reasonable estimations of relative T c values can be calculated if the error in HS-LS energy gaps remains systematic for a given set of structurally related systems.…”

Towards quantifying the role of exact exchange in predictions of transition metal complex properties

“…For a balanced treatment of both static correlation and dynamic correlation also in the absence of self-interaction error, multireference wavefunction techniques have been used [43][44][45][46][47][48][49][50] to study spin crossover complexes up to around 45 atoms in size. 48 The predominant wavefunction method employed in the study of spin-crossover complexes is CASPT2, and, while it scales more expensively than density functional theory approaches, recent improvements in scaling 44 have made larger systems tractable.…”

Spin‐State Energetics of FeII Complexes – The Continuing Voyage Through the Density Functional Minefield

“…[31][32][33] In contrast, DFT is relatively cheap, but its performance often depends on whether a particular protocol happens to reproduce the available data. [29,30,34,35] Fortunately, this may not be a serious shortcoming, because the trends are often correct, and thus, as described by Vargas et al, [34] a more accurate absolute picture can be obtained by "anchoring" the calculations with an example where the spin-state energetics are accurately known. [34] We [36] .…”

“…[29,30,34,35] Fortunately, this may not be a serious shortcoming, because the trends are often correct, and thus, as described by Vargas et al, [34] a more accurate absolute picture can be obtained by "anchoring" the calculations with an example where the spin-state energetics are accurately known. [34] We [36] . However, varying the whole ligand set alters the energies a lot and is thus not a very exacting test.…”