Theoretical modeling of valence tautomeric dinuclear cobalt complexes. Adducts of Co^IIdiketonates with cyclic redox-active tetraone ligands

Abstract

The approach to the employment of the mechanism of valence tautomerism (VT) for the design of molecular 2-qubit quantum gates has been extended to adducts 5-8 of Co(II) bis-(malonates), bis-(acetylacetonates), bis-(hexafluoroacetylacetonates) and bis-(trifluoroacetylacetonates) with tetradentate tetraone (cyclic di-o-quinones) redox-active piperazine-2,3,5,6-tetraone L5 (X = NH), 3,3,6,6-tetramethylcyclohexane-1,2,4,5-tetraone L5 (X = C(CH3)2), cyclopenta[fg]acenaphthylene-1,2,5,6-tetraone L6, cyclopenta[fg]acenaphthylene-3,4,7,8-tetraone L7 and pyrene-4,5,9,10-tetraone L8 and computationally studied using the B3LYP*/6-311++G(d,p) method. The calculations reveal ferromagnetic ordering of unpaired electrons in the low-spin electromeric forms of complexes 8 (R1, R2 = H, CH3, CF3) on the basis of L8, which provides for the paramagnetic character of all three interconverting electromers of 8 and makes it possible to realize the two-step mechanism of thermally driven migration of paramagnetic centers between the pyrene-tetraone fragment and metal ions. Through the structural variation of the ancillary diketonate ligands the energy gaps between the electromeric forms of adducts 8 and energy barriers for their interconversion were adjusted to the range of values typical of thermal VT rearrangements. The best energy parameters for the occurrence of thermal two-step VT rearrangements involving all three paramagnetic electromeric forms can be achieved for complex 8 (R1 = CH3, R2 = CF3), which was shown to meet the principal conditions for compounds with a potential role of spin qubit carriers: thermal stability with respect to dissociation into the components, thermally accessible energy barriers for the interconversion of the electromers and weak coupling between their paramagnetic centers.

One-Step versus Two-Step Valence Tautomeric Transitions in Tetraoxolene-Bridged Dinuclear Cobalt Compounds

“…The estimated values of the J parameters (Table S7) indicate ferromagnetic exchange interactions between the hs-Co II and closest (coordinating) dioxolene, while spin coupling within the redox-active pyrene moiety bears strong ferromagnetic nature ( J Sq1–Sq2 = 159 cm –1 ). The latter is in agreement with the earlier theoretical investigation of pyrene-4,5,9,10-tetraone and compounds on its basis …”

“…The latter is in agreement with the earlier theoretical investigation of pyrene-4,5,9,10-tetraone and compounds on its basis. 87 Despite the fact that the low-spin diamagnetic state {ls-Co III (phen) 2 −pyrene Cat−Cat −ls-Co III (phen) 2 } is also thermally achievable (ΔE = 3.5 kcal mol −1 ), no VT transition is expected between the {ls-Co III (phen) 2 −pyrene Cat−Cat −ls-Co III (phen) 2 } and {ls-Co III (phen) 2 −pyrene Cat−Sq −hs-Co II (phen) 2 } charge distributions, since VT behavior, being an entropy-driven process, is proceeding via a transition from the low-spin structure with lower entropy to the high-spin state with longer metal−ligand bonds (and higher entropy) and not vice versa. Thus, the DFT calculations on 1 predict a partial VT transition {ls-Co III (phen) 2 −pyrene Cat−Sq −hs-Co II (phen) 2 } ⇌ {hs-Co II (phen) 2 −pyrene Sq−Sq −hs-Co II (phen) 2 } accompanied by the change of the total spin of the system from S = 2 to S = 4.…”

Understanding the Origin of One- or Two-Step Valence Tautomeric Transitions in Bis(dioxolene)-Bridged Dinuclear Cobalt Complexes

“…Density functional theory calculations were performed for compounds 1a , 2b , 3a , and 4a in the gas phase to examine their magnetic behavior (optimized structures and energies are given in the Supporting Information); 2b was selected over 2a to explore the effect of pp and pd geometric isomers. Valence tautomeric mononuclear complexes have been investigated with DFT in the past, ,,− but to date, DFT calculations of dinuclear cobalt VT systems have been used to predict VT transitions in hypothetical complexes only. − To analyze dinuclear VT transitions, we draw a parallel with DFT investigations of two-step SCO systems. , As such, all three charge distributions in a dinuclear complex can be considered to be in equilibrium. The nature of the transition is then described by the thermochemical parameter ρ, given by eq 1 where state energies, E , are given relative to E {Co III -cat-cat-Co III }. …”

Rational design of potential spin qubits manipulated by the valence tautomerism mechanism: quantum-chemical modeling of the trinuclear transition metal complexes with bischelate linkers

“…To sum up, the DFT B3LYP*/6-311++G(d,p) calculations performed on a broad series of electrically neutral adducts of Co II diketonates with the functionalized redox-active di-o-quinones 2 (M = Co, Ni, Cu, Zn; X = O, NR 0 ; R = H, CH 3 , CF 3 ; R 0 = H, CH 3 , Ph) and model compounds 3-6 allowed uncovering of two groups of complexes 2 (M = Cu and Co; X = O; R = CF 3 ) that may potentially have the properties of molecular spin qubits, magnetic characteristics of which can be additionally tuned through the occurrence of low-energy barrier VT rearrangements. The results of the calculations extend the approach 17,18,52 to the application of the mechanism of valence tautomerism for the design of molecular 2-qubit quantum gates to trinuclear metal complexes with transition metal containing linker groups. ), relative energies (DE) and relative enthalpies at 298 K (DH 298 ) (all energy values are given in kcal mol À1 ) and exchange spin coupling constants (J b , given in cm À1 ) of the adducts 2 (M = HS Co II ; X = O; R = CH 3 , CF 3 ) calculated by the DFT B3LYP*/ Tables S5 and S7).…”

“…In the development of these studies, we have assumed and computationally justified [16][17][18] that the dinuclear complexes designed on the basis of redox-active (non-innocent 19 ) ligands capable of undergoing thermally or photochemically initiated reversible intramolecular rearrangements caused by electron transfer between the metal and ligand centers (valence tautomerism VT [20][21][22][23][24][25] can acquire useful properties of controllable switching of their spin states allowing additional manipulation by spin qubits. The involvement in the spin exchange processes of paramagnetic centers located on the organic moieties is an important distinction of the dinuclear complexes with redox-active ligands from the weakly coupled metal complexes in which spin densities are concentrated at the metal ions.…”