Modications of Tanabe-Sugano d6diagram induced by radical ligand eld ab initio inspection of a FeII-verdazyl molecular complex

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Modiﬁcations of Tanabe-Sugano d6diagram

induced by radical ligand ﬁeld: ab initio

inspection of a Fe(II)-verdazyl molecular complex

Pablo Roseiro,†Saad Yalouz,∗,†David J. R. Brook,‡Nadia Ben Amor,¶and

Vincent Robert†

†Laboratoire de Chimie Quantique, Institut de Chimie, CNRS/Universit´e de Strasbourg, 4

rue Blaise Pascal, 67000 Strasbourg, France

‡Department of Chemistry, San Jos´e State University, One Washington Square, San Jos´e,

CA 95192, USA

¶Laboratoire de Physique et Chimie Quantiques, UMR 5626 Universit´e Paul Sabatier, 118

route de Narbonne, 31062 Toulouse, France

E-mail: yalouzsaad@gmail.com

Abstract

Quantum entanglement between the spin states of a metal centre and radical lig-

ands is suggested in an iron(II) [Fe(dipyvd)2]2+ compound (dipyvd = 1-isopropyl-3,5-

dipyridil-6-oxoverdazyl). Wavefunction ab initio (Diﬀerence Dedicated Conﬁguration

Interaction, DDCI) inspections were carried out to stress the versatility of local spin

states. We named this phenonmenon excited state spinmerism, in reference to our

previous work (see Roseiro et. al., ChemPhysChem 2022, e202200478) where we in-

troduced the concept of spinmerism as an extension of mesomerism to spin degrees of

freedom. The construction of localized molecular orbitals allows for a reading of the

arXiv:2210.02325v2 [quant-ph] 7 Feb 2023

wavefunctions and projections onto the local spin states. The low-energy spectrum is

well-depicted by a Heisenberg picture. A 60 cm−1ferromagnetic interaction is calcu-

lated between the radical ligands with the Stotal = 0 and 1 states largely dominated

by a local low-spin SF e = 0. In contrast, the higher-lying Stotal = 2 states are super-

positions of the local SF e = 1 (17%, 62%) and SF e = 2 (72%, 21%) spin states. Such

mixing extends the traditional picture of a high-ﬁeld d6Tanabe-Sugano diagram. Even

in the absence of spin-orbit coupling, the avoided crossing between diﬀerent local spin

states is triggered by the ﬁeld generated by radical ligands. This puzzling scenario

emerges from versatile local spin states in compounds which extend the traditional

views in molecular magnetism.

Introduction

The synthesis and characterization of molecule-based magnetic systems has been an intense

research area for decades, prompted by the need for information storage devices and advances

in quantum technologies (see for example Refs.1–4). The motivations for targeting such

complexes stem from their physical-chemical properties ranging from long coherence time5–9

to manipulation possibilities.10–15 In the presence of spin-switchable units, the local spin state

of archetypical iron(II) or cobalt(II) (3d6and 3d7, respectively) ions can be controlled using

external ﬁelds such as light or temperature. Important structural modiﬁcations are then

observed, with changes in bond distances of up to 0.2 ˚

A (i.e. 10 %)16 and deep changes in

charge distribution (up to 0.5 electrons).17 The latter are responsible for hysteretic behavior

in materials,18 and have long been underestimated whereas they are the main characteristics

of valence tautomers.19 More recently, transition metal ions combined with organic ligands

have been considered as possible targets in the development of molecular-based quantum

units of information, e.g. qubits or qudits (see for example Refs.1–4). Theoretical studies

have also revealed the potential interest of radical ligands for the manipulation of quantum

information via the entanglement of local spin degrees of freedom.20,21 A prerequisite is

the binding of radical ligands to paramagnetic metal centers without losing their open-shell

character. In this respect, oxoverdazyl-based ligands have proven to fulﬁll such requirement,

giving rise to a wealth of magnetic coupling schemes.22–24 Furthermore, the ﬂexibility and

well-established redox activity of organic-based compounds make such materials particularly

interesting. Not only can inter-unit interactions be modulated with speculated spin-crossover

behaviour25, but the ﬁeld generated by several open-shell ligands may give rise to unusual

and puzzling scenarios.

Recently, it has been suggested that the energy spectrum of a cobalt(II) ion coordinated

to open-shell radicals may not be readily interpreted. At the crossroads of exchange coupled

and spin-crossover systems, this compound has questioned the traditional pictures emerg-

ing from a metal ion, either high-spin or low-spin, in the electrostatic ﬁeld of neighbouring

ligands.20,26 Wavefunction-based calculations supported that the spin states are character-

ized by combinations of various local spin states on the cobalt(II) center. In particular, the

ground state displays a structure-sensitive admixture of low-spin SCo = 1/2 in a dominant

high-spin SCo = 3/2 , a feature of entanglement named spinmerism. The mixing was fur-

ther interpreted by inspecting a d7Tanabe-Sugano diagram that exhibits a doublet-quartet

crossing in the intermediate ligand ﬁeld regime. The spinmerism phenomenon can be seen

Fe2+

Figure 1: [Fe(dipyvd)2]2+ compound (1) from Ref27.

as an extension of mesomerism to spin degrees of freedom with entanglement in between two

local sub-parts of a molecule. Since the ligand ﬁeld includes Coulomb and direct exchange

contributions in a complex built on spin-coupled partners, one may wonder whether diﬀer-

ent local spin states may coexist on the metal ion. The introduction of radical ligands may

indeed disrupt the assumption of a given spin state on the metal centre in the description of

ground and excited states.

Inspired by these observations, the pseudo-octahedral iron(II)-oxoverdazyl compound

[Fe(dipyvd)2]2+, with dipyvd = 1- isopropyl-3,5-dipyridil-6-oxoverdazyl (1) was considered

as another prototype to be examined (see Figure 1).27 Magnetic and spectroscopic proper-

ties were reported in the literature and complemented by density functional theory-based

calculations. Iron(II) being a spin-crossover metal ion, any intermediate ligand ﬁeld should

be appropriate to observe the coexistence of a SF e = 0 (low spin) and SF e = 2 (high spin).

From the Fe-N bond distances values (ca. 1.9 ˚

A), a low-spin iron center is expected and was

indeed conﬁrmed.27 As seen in Figure 2, the d6Tanabe-Sugano diagram exhibits a crossing

between the excited triplet and excited quintet states. Therefore, a similar manifestation of

spin states mixing observed in the ground state of cobalt(II) complex might appear in the ex-

cited states of this iron(II) analogue. Guided by this observation in the high-ﬁeld regime, we

thought that excited state spinmerism might be anticipated, with total spin states resulting

from combinations of local SF e = 1 and SF e = 2. For all these reasons, ab initio calculations

were carried out to inspect the energy spectrum of 1(shown in Fig. 1), dominated by local

spins modiﬁcations. The examined energy window consists of spin states characterized by

similar charge distributions, leaving out ligand-to-metal and metal-to-ligand charge transfer

states. The eigen-states were constructed using localized molecular orbitals (LMOs) to allow

for projections onto the local spin states of the metal and ligands, SF e and SL, respectively.

The numerical results presented in this work highlight the presence of a strong quantum

entanglement between the local spin states, i.e. aspinmerism eﬀect, of the metal ion and

the radical ligand environment. This evidence reshuﬄes the standard views in molecular

magnetism and simultaneously opens up new perspectives for technological development

at the interface of physics and chemistry. In Light-Induced Excited Spin-State Trapping

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ModicationsofTanabe-Suganod6diagraminducedbyradicalligandeld:abinitioinspectionofaFe(II)-verdazylmolecularcomplexPabloRoseiro,ySaadYalouz,,yDavidJ.R.Brook,zNadiaBenAmor,{andVincentRobertyyLaboratoiredeChimieQuantique,InstitutdeChimie,CNRS/UniversitedeStrasbourg,4rueBlaisePascal,67000Strasbourg,F...

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Modications of Tanabe-Sugano d6diagram induced by radical ligand eld ab initio inspection of a FeII-verdazyl molecular complex

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