Tuning spin-crossover transition temperatures in non-symmetrical homoleptic meridional/facial [Fe(didentate)3]2+complexes: what for and who cares about it?

Volume 50

Number 4

28 January 2021

Pages 1129-1516

Dalton

Transactions

An international journal of inorganic chemistry

rsc.li/dalton

ISSN 1477-9226

PAPER

Claude Piguet et al.

Tuning spin-crossover transition temperatures in non-

symmetrical homoleptic meridional/facial [Fe(didentate)₃]²⁺

complexes: what for and who cares about it?

Anniversary

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PAPER

Tuning spin-crossover transition temperatures in

non-symmetrical homoleptic meridional/facial

[Fe(didentate)₃]²⁺complexes: what for and who

cares about it?†

Cite this: Dalton Trans., 2021, 50,

1206

a

Neel Deorukhkar,^aCéline Besnard, ^bLaure Guénée^band Claude Piguet

*

The [FeN₆] chromophores found in [Fe(didentate)₃]²⁺complexes, where didentate is a non-symmetrical

2-(6-membered-heterocyclic ring)-benzimidazole ligand (Lk), exist as mixtures of two geometrical mer

(C₁-symmetry) and fac (C₃-symmetry) isomers. Speciﬁc alkyl-substituted six-membered heterocyclic

rings connected to the benzimidazole unit (pyridines in ligands L1–L3, pyrazines in L4–L5 and pyrimi-

dines in L6–L7) control the ligand ﬁeld strength and the electron delocalization so that [Fe^II(Lk)₃]²⁺display

tunable thermally-induced spin transitions in solution. Thermodynamic, spectroscopic (UV-Vis, NMR) and

magnetic studies in solution demonstrate that [Fe(L6)₃]²⁺(L6 = 1-methyl-2-(pyrimidin-2-yl)-1H-benzo[d]

imidazole) exhibits a close to room temperature spin transition (T_1/2= 273(3) K) combined with a high

Fe;L6

1;3

stability formation constant (logðβ

Þ ¼ 21:8ð9Þ in acetonitrile), which makes this complex suitable for

the potential modulation of lanthanide-based luminescence in polymetallic helicates. A novel method is

proposed for assigning speciﬁc thermodynamic spin crossover parameters to fac-[Fe(L6)₃]²⁺and mer-[Fe

(L6)₃]²⁺isomers in solution. The observed diﬀerence relies mainly on the entropic content ΔS^mer− ΔS^fac

SCO

= 11(1) J mol⁻¹K⁻¹, which favors the spin transition for the meridional isomer. Intermolecular interactions

occurring in the crystalline state largely overcome minor thermodynamic trends operating in diluted solu-

tions and a single conﬁgurational isomer is usually observed in the solid state. Among the thirteen solved

crystal structures 1–13 containing the [M(Lk)₃]²⁺cations (M = Fe, Ni, Zn, Lk = L6–L7), pure meridional

isomers are observed six times, pure facial isomers also six times and a mixture (44% mer and 56% fac) is

detected only once. Solid-state magnetic data recorded for the Fe^IIcomplexes show the operation of

slightly cooperative spin transitions in 7 ( fac-[Fe(L6)₃]²⁺) and 12 (mer-[Fe(L7)₃]²⁺). For the meridional

isomer in 6, a two-step spin state transition curve, associated with two phase transitions, is detected.

Received 6th November 2020,

Accepted 15th December 2020

DOI: 10.1039/d0dt03828h

rsc.li/dalton

Cambi et al. of Fe^IIIcomplexes with dithiocarbamate ligands

displaying thermal spin-state equilibria are still considered as

Introduction

Faced with the duality of paramagnetism measured for the first experimental demonstration of what is known as the

[Fe(H₂O)₆]²⁺and diamagnetism for [Fe(CN)₆]⁴⁻Pauling spin crossover (SCO) phenomenon.^5–7The formulation of the

,

exploited valence bond theory in 1931 to develop the concept of crystal-field theory,⁸further extended and improved by the

‘magnetic criterion of bond type’ (Scheme 1a).^1,2He also recog- ligand field theory,^9–11provided a better description of chemical

nized that two molecular systems with diﬀerent spin states could bonding for open-shell transition metal complexes (Scheme 1b).

be present simultaneously at a given temperature, according that The SCO phenomenon exhibited by d⁶Fe^IIin an octahedral

the energy diﬀerence between them is comparable with thermal environment could be then quantified as twice the diﬀerence

energy (mRT with 1 ≤ m ≤ 10).^3,4The concomitant isolation by between the ligand field strength Δ_octand the spin pairing

energy P, the latter being modeled with the help of the Racah

parameters B and C to give P = 2B + 4C ≈ 19B (Scheme 1b).¹²

A

^aDepartment of Inorganic and Analytical Chemistry, University of Geneva, 30 quai

E. Ansermet, CH-1211 Geneva 4, Switzerland. E-mail: Claude.Piguet@unige.ch

^bLaboratory of Crystallography, University of Geneva, 24 quai E. Ansermet, CH-1211

Geneva 4, Switzerland

complete ligand-field picture is provided by the relevant

Tanabe–Sugano diagram, which displays the relative energies of

all the Russell–Saunders multiplet terms arising from the d⁶

configuration as a function of the Δ_oct/B ratio (Scheme 1c).¹³

The strict equality Δ_oct= P, E_H^°_Sꢀ E_L^°_S¼ 2ðΔ_octꢀ PÞ ¼ 0,

leads to Δ_oct/B ≈ 19 and an equal population of ¹A₁and ⁵T₂

†Electronic supplementary information (ESI) available. CCDC 2041614–2041630.

For ESI and crystallographic data in CIF or other electronic format see DOI:

10.1039/d0dt03828h

1206 | Dalton Trans., 2021, 50, 1206–1223

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dard Racah parameters.^12,16Consequently, an adequate model-

ing of SCO behaviors requires looking beyond ligand-field

theory with the help of quantum chemistry for mapping quan-

titatively the energy surfaces at equilibrium geometries associ-

ated with the ¹A₁and ⁵T₂terms.²¹Although significant pro-

gress is being made along this line,^18–23reliable models for

the prediction of molecular SCO systems are still limited

because they involve open-shell species and small energy

diﬀerences.²⁴The systematic exploration of classes of ligands

is thus the rule for designing molecular SCO complexes in

coordination chemistry. Myriads of six-coordinate SCO Fe^II

complexes have been thus actively synthesized, characterized

and a posteriori rationalized. This global eﬀort has been regu-

larly and extensively reviewed during the last two decades in

specialized journals^14,25–35and in two comprehensive

monographies.^36,37There is no doubt that the pseudo-octa-

hedral [FeN₆] chromophore, where N stands for a heterocyclic

nitrogen donor atom, is one of the most studied and versatile

unit for implementing Fe^IISCO properties in molecular com-

plexes. Some ‘rules of thumb’ could be extracted from empiri-

cal analyses of structure–properties relationships. For instance,

Shatruk and coworkers used the interatomic separation

between the N-donor atoms in multidentate chelate ligands as

a metric guide for programming SCO properties in Fe^IIcom-

plexes,³⁸whereas Brooker, Garden and coworkers reported on

more sophisticated density functional theory (DFT) calcu-

lations for predicting the ¹⁵N NMR shifts in 1,2,4-triazole

ligands, which can be then used as a criterion for rationalizing

SCO behaviors.²⁴

Short-range and long-range orders produced by packing

interactions in solid state samples drastically complicate the

situation because intermolecular energies usually dominate

the minor energy changes accompanying the spin state tran-

sition at the molecular level,^30,39–42a phenomenon often at the

origin of specific phase transitions accompanying the spin

crossover process in the solid state.^39,43Whereas spin-state

Scheme 1 a) Illustration of early Pauling’s magnetic criterion of bond

type; (b) crystal ﬁeld/ligand ﬁeld approach to spin crossover phenom-

enon (P is the electron spin pairing energy); (c) Tanabe–Sugano diagram

for a d⁶metal ion completed with a central red domain, which corres-

ponds to non-accessible ligand-ﬁeld strengths and green domains transitions implying solvated complexes in solution can be

where low-spin and high-spin complexes co-exist at accessible temp-

satisfyingly modeled with the standard entropy of mixing

ꢂ

ꢃ

eratures (0 ꢁ E_h^°_sꢀ E_l^°_sꢁ 2000 cm^ꢀ1). Adapted from ref. 16 and 17.

according to the simple chemical equilibrium (1) and associ-

44,45

ated stability constant K_SCO

,

intermolecular communi-

cations produce some cooperative eﬀects in the solid state,

levels represented by a vertical dashed line (Scheme 1c). which can be implemented either by adding an interaction

However, this approach is misleading and physically unsound term Γ(x_hs, T, P) = γ·x_hs·(1 − x_hs) to give eqn (2), where γ is an

since the vertical dashed line in the Tanabe–Sugano diagram adjustable interaction parameter,^43,46or by considering non-

(Scheme 1c) refers to a non-equilibrium geometry. Indeed, the randomly distributed domains containing n molecules of like

population of the antibonding orbitals in high-spin Fe^IIis spin in eqn (3) and (4), where C_pare the heat capacities at con-

accompanied by a 10% elongation of the Fe–N bond lengths¹⁴stant pressure.^43,47

together with minor geometrical distortions,¹⁵which aﬀect

K_SCO

Δ_octand P during the spin transition. Taking this into account,

a domain in the Tanabe–Sugano diagram is no more accessi-

ble to coordination complexes (red surface in Scheme 1c),

whereas two restricted domains (green surfaces in Scheme 1c)

II

ꢀꢀꢀ*

Fe

)ꢀꢀꢀ Fe_high‐spin

low‐spin

ꢀ

ꢁ

ð1Þ

ð2Þ

1 ꢀ x_hs

ΔG_SCOΔH_SCOΔS_SCO

ꢀ lnðK_SCOÞ ¼ ln

¼

ꢀ

x_hs

RT

R

HS

correspond to the limits 11 000 ≤ Δ

≤ 12 500 cm⁻¹and

oct

19 000 ≤ Δ ≤ 22 000 cm⁻¹for which the phenomenon of a

LS

ꢀ

ꢁ

oct

1 ꢀ x_hs

ΔH_SCOþ γð1 ꢀ 2x_hsÞ ΔS_SCO

thermal spin transition (ΔE ¼ E_H^°_Sꢀ E_L^°_S¼ mRT with 1 ≤ m ≤

ln

¼

ꢀ

x_hs

RT

R

10) can be expected in Fe^IIcoordination complexes with stan-

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ꢀ

ꢁ

1 ꢀ x_hs

nΔG_SCOnΔH_SCOnΔS_SCO

chance to propose some reliable design in solids, and major

interest in the SCO community moved toward the systematic

structural exploration of classes of ligands and complexes for

designing addressable (via temperature, pressure, light)

switches implemented in various materials (solids, films,

nanoparticles, liquid crystals, etc.).^37,48–53The deliberate com-

bination of SCO properties with other physical or chemical

ln

¼

ꢀ

ð3Þ

ð4Þ

x_hs

RT

R

with

ꢄ

ꢅ

ꢄ

ꢅ

ꢆ

ꢇ

ꢄ

ꢅ

4RT₁₌₂

ΔH_S²_CO

C_p‐LST₁₌₂þ C_p‐HST₁₌₂

n ¼

C_pT₁₌₂

ꢀ

2

Since precise chemical programming of spin state tran- properties in a synergetic fashion in molecular entities, sum-

sitions are not accessible in solution, there is much less marized under the term of multifunctionality,^54–56remains in

its infancy despite remarkable eﬀorts focused on the modu-

lation of magnetic communication^57,58and luminescence.⁵⁹

For instance, the modulation of an emissive lanthanide by an

adjacent SCO Fe^IImetallic site in an isolated dimetallic

complex has been recently attempted (Fig. 1), but it proved to

depend crucially on some ultra-precise tuning of the spin state

equilibrium, which is currently beyond rational chemical

design.⁶⁰

Starting from the didentate 2-(α-methyl-substituted pyri-

dine)-benzimidazole ligands L1–L3 (Scheme 2), which exactly

model the didentate units bound to Fe^IIin the [EuFeL₃]⁵⁺heli-

cate, some systematic exploration of structurally related

2-(α-methyl-substituted pyrazine)-benzimidazole L4–L5 allowed

some shifts of T_1/2in the mononuclear [Fe(didentate)₃]²⁺

model complexes, but none of them improved the situation in

Fig. 1 Molecular structure of the [EuFeL₃]⁵⁺helicate in which red

luminescence (620 nm) is modulated via energy transfer (ET) to the

adjacent SCO Fe^IIunit.⁶⁰

Scheme 2 Chemical structures of the didentate ligands L1–L7, together with the electronic properties of the associated pseudo-octahedral

[Ni(Lk)₃]²⁺complexes and thermodynamic SCO properties of the pseudo-octahedral [Fe(Lk)₃]²⁺complexes in CD₃CN solutions.

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the helicates.^16,61Moreover, simple mononuclear [Fe

(didentate)₃]²⁺complexes exist as mixtures of meridional and

facial isomers, for which only the latter one can be taken as a

valuable model for the triple-stranded [EuFeL₃]⁵⁺helicates.⁶¹

We report here the connection of a 2-pyrimidine ring (L6)

or a 4-pyrimidine ring (L7) to the benzimidazole ring, which

eventually provides [Fe(Lk)₃]²⁺complexes possessing lower

transition temperatures compatible with luminescence modu-

lation in triple-stranded helicates (Scheme 2).

A novel

approach for the estimation of specific thermodynamic SCO

parameters for meridional and facial isomers in solution is

described.

Results and discussion

Design, synthesis and characterization of the didentate

pyrimidine–benzimidazole ligands L6 and L7

In an elegant contribution,²⁴Brooker, Garden and co-workers

suggested that the ¹⁵N NMR chemical shifts recorded for the

binding nitrogen atom of the azine ring in a series of didentate

triazole–azine ligands (L^Azine) can be taken as a valuable indi-

cator for both the electronic density borne by the N-donor

atoms and their capacity to control the critical temperature

T_1/2of spin transitions for [Fe(L^Azine)₂(NCBH₃)₂] in solution.

The decreasing order found for T_1/2(pyrazine > pyridine > pyri-

midine) matches that reported in Scheme 2 for the two first

related complexes [Fe(L4)₃]²⁺(pyrazine) > [Fe(L1)₃]²⁺(pyridine)

and encourages us to prepare 2-pyrimidine (L6) and 4-pyrimi-

dine (L7) analogues for (i) shifting T_1/2toward lower tempera-

tures in [Fe(Lk)₃]²⁺and (ii) further exploring the eﬀect of meri-

dional ↔ facial isomerization on the SCO properties.

Scheme 3 Multistep synthesis of the didentate ligands L6 (2-pyrimi-

dine) and L7 (4-pyrimidine).

The ligands L6–L7 were thus synthesized (Scheme 3) using

a well-established strategy based on modified Philips conden-

sation reactions for the formation of benzimidazoles as key

steps.⁶²Pyrimidine-carboxylic acids, contrary to pyrazine-2-car-

boxylic acid,^16,62could be activated in situ to pyrimidine-2-carbo-

nyl chloride through standard reactions with either thionyl chlor-

ide or oxalyl chloride. The ¹H NMR and ¹³C NMR spectra

(Scheme 4a and Fig. S1–S12†) point to fast rotations around the

inter-annular C–C bond leading to average C_s-symmetry on the

NMR time scale. The lack of NOE eﬀect between the protons of

the methyl group of the benzimidazole and the pyrimidine

proton at the 5-position of the pyrimidine in L7 suggests a pre-

ferred average planar conformation in solution with an anti

location of the N-donor atoms in the free didentate ligand (shown

in Schemes 3 and 4). These structural characteristics are sup-

ported by gas-phase energies computed as a function of the

interplanar angle between the two aromatic rings (Fig. S13†).

Finally, indirect ¹H–¹⁵N detection (Fig. S6 and S12†)⁶³shows

that the N-donor atom of 2-pyrimidine in L6 is less shielded

Scheme 4 a) ¹H NMR chemical shifts (with respect to tetramethylsilane

at 0 ppm in CD₃CN) and (b) ¹⁵N NMR chemical shifts (with respect to

nitromethane at 381.7 ppm in CD₂Cl₂) observed for ligands L6 and L7.

than that in 4-pyridimidine in L7 (Scheme 4b), an experi- diﬀraction analysis (Tables S1–S6 and Fig. S15–S20†). Each

mental result which is confirmed by the larger negative charge unit cell contains two slightly diﬀerent molecules of the

computed for the azine donor group in L7 (Fig. S14†).

ligands, both confirming the formation of almost planar

Slow diﬀusion of n-hexane into concentrate solutions of L6 didentate aromatic units adopting anti arrangements of the

and L7 in dichloromethane yielded crystals suitable for X-ray N-donor atoms (Fig. 2).

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ꢈ

ꢉ

2þ

M;Lk

1;3

M

^2þþ 3Lk Ð MðLkÞ₃

β

ð7Þ

Taking the experimental ¹⁵N HMR chemical shifts

(Scheme 4b) as a body of evidence for optimizing M-σL

binding and stability upon coordination to spherical and ionic

Zn²⁺, one can reasonably predict β₁^Z_;ⁿ_n^;L7> β₁^Z_;ⁿ_n^;L6. However, the

reverse situation holds with a cumulative constant log(β^Z_1;ⁿ₃^;L7) =

14.07(4), found for [Zn(L7)₃]²⁺, being almost 8 orders of magni-

Fig. 2 Molecular structures of the two diﬀerent molecules in the unit

cells of the crystal structures of (a) L6 (interplanar angles 5.93(5)° and

9.71(5)°) and (b) L7 (interplanar angles 7.65(8)° and 12.73(8)°). The

N-donor atoms are highlighted as blue spheres.

tude smaller than log(β^Z_1;ⁿ₃^;L6) = 21.8(1) (Table 1). Translated

into ligand speciation,⁷³these thermodynamic constants

imply that the target saturated complex [Zn(L6)₃]²⁺is quantitat-

ively formed (>98%) in solution for a stoichiometric ratio

Zn : L6 = 1 : 3 and a total ligand concentration of 10 mM

(Fig. 3a). For [Zn(L7)₃]²⁺, a maximum of 82% of this complex

can be detected in acetonitrile for a total ligand concentration

of 10 mM (Fig. 3b). Its quantitative formation (≥97%) requires

a ten times larger total concentration of ligand L7 (100 mM,

Fig. 3c).

Complexation of the didentate pyrimidine–benzimidazole

ligands L6 and L7 with Zn^II, Ni^IIand Fe^IIin solution

Reactions of L6 or L7 with M(CF₃SO₃)₂(M = Zn, Fe) or Ni(BF₄)₂

in acetonitrile is characterized by a red-shift of the ligand-cen-

tered π* ← n,π electronic transitions (Fig. S21a–S24a†). Similar

shifts were originally reported for 2,2′-bipyridine,⁶⁴and could be

theoretically assigned to the anti → syn conformational change of

the α,α′-diimine binding unit accompanying its chelate complexa-

tion to the metal.^65,66The spectrophotometric titrations of sub-

millimolar concentrations of the ligands L6 and L7 in dry aceto-

nitrile with Zn(CF₃SO₃)₂were therefore exploited for the quanti-

tative analysis of the successive formation of three complexes,

characterized by smooth endpoints at Zn/Lk = 1 : 3, Zn/Lk = 1 : 2,

Zn/Lk = 1 : 1 (Fig. S21b–S22b†). This trend was further confirmed

by factor analysis (Fig. S21c–S22c†), which suggested the exist-

ence of four individual absorbing eigenvectors Lk, ZnLk, Zn(Lk)₂

and Zn(Lk)₃with satisfying re-constructed absorption spectra

(Fig. S21d–S22d†).^67–70Non-linear least-squares fit to equilibria

We conclude that σ-donation from the bound ligand to

Zn²⁺is not the only important thermodynamic driving force.

Comparison with the previous azine found in [Zn(L4)_n]²⁺

(Table S7†)⁷⁴shows that L4 (2-pyrazine, pK_a= 0.65), which is a

much weaker σ-donor but a stronger π-acceptor than L7,

indeed provides stability constants up to three orders of mag-

nitude larger than those observed for [Zn(L7)_n]²⁺(4-pyrimi-

dine, pK_a= 1.23). Surprisingly, the stability constants of

[Zn(L6)_n]²⁺(2-pyrimidine, pK_a= 1.23) largely overcome those of

its azine analogues with L4 and L7, and can be only compared

with that produced by the more basic 2-pyridine ring (pK_a

=

5.2) in [Zn(L1)_n]²⁺(Table S7†). In fact, the complexation of L1,

L4 and L7 with the entering cation is limited by some con-

siderable energetic penalties accompanying the anti → syn

conformational change of the chelate α,α′-diimine unit. On the

contrary, L6 is ‘symmetrically preorganized’ and no ligand

ð5Þ reorganization is required. This beneficial eﬀect is reminiscent

to the gain of four orders of magnitude in stability observed in

(5)–(7) provided satisfying estimates for the formation macrocon-

M;Lk

1;n

stants β

gathered in Table 1 (column 1, entries 1 and 2).^71–73

2þ M;Lk

M

^2þþ Lk Ð ½MðLkÞꢂ

β

1;1

going from [Zn(2,2′-bipyridine)₃]²⁺(log(β_1,3

[Zn(1,10-phenanthroline)₃]²⁺(log(β_1,3) = 17.1(1)) in water.⁷⁵

) = 13.2(2)) to

ꢈ

ꢉ

2þ

M;Lk

1;2

M

^2þþ 2Lk Ð MðLkÞ₂

β

ð6Þ

−

Table 1 Stability constants for the successive formation of [M(Lk)_n]²⁺(M = Zn, Ni, Fe, Lk = L6, L7, n = 1, 2, 3, non-coordinating anions: ClO₄

,

CF₃SO₃or BF₄⁻) in acetonitrile (293 K), metal–ligand interaction parameter ( f^M,Lk) and inter-ligand interaction parameter (u^Lk,Lk), and associated

−

energies ΔG_aﬃnity= −RTln( f^M,Lk) and ΔE_interaction= −RTln(u^Lk,Lk

)

log( f^M,L

)

ΔG_aﬃnity/kJ mol⁻¹

−34.4(2.4)

log(u^L,L

)

ΔE_interaction/kJ mol⁻¹

−3.6(2.8)

[Zn(L6)_n]²⁺

log(β_1,1) = 7.02(4)

log(β_1,2) = 15.2(1)

6.0(4)

0.7(5)

log(β_1,3) = 21.8(1)

[Zn(L7)_n]²⁺

[Fe(L6)_n]²⁺

[Ni(L7)_n]²⁺

log(β_1,1) = 5.10(1)

log(β_1,2) = 10.22(2)

log(β_1,3) = 14.07(4)

log(β_1,1) = 5.0(1)

log(β_1,2) = 10.95(1)

log(β_1,3) = 15.70(1)

log(β_1,1) = 4.11(1)

log(β_1,2) = 9.22(1)

log(β_1,3) = 13.65(1)

3.9(2)

4.1(5)

3.0(2)

−22.1(9)

0.2(2)

0.3(6)

1.0(3)

−1.3(1.1)

−1.6(3.2)

−5.8(1.5)

−23.2(2.7)

−16.8(1.3)

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ꢊ

ꢋ

ꢊ

ꢋ

Lk;Lk

=RT

k;j

ꢀ

ΔE

M;Lk

i;affinity

Lk;Lk

k;l

(ΔG

¼ ꢀRT ln f_i^M;Lk

)

and

u

¼ e

accounts for the free energy of interligand interactions

ꢊ

ꢋ

Lk;Lk

k;l

Lk;Lk

k;l

(ΔE

¼ ꢀRT ln u

), which operate when two ligands

are bound to the same metallic centre.

^M;Lk=RT

stat

chem

M;Lk

1;n

¼ e^ꢀΔG

¼ K

ꢃ K

1;n

β

1;n

!

n

Y

ð8Þ

ꢃ

f_i^M;Lk

ꢃ

Lk;Lk

u

k;l

stat

1;n

i¼1

k,l

Application of eqn (8) to all possible geometric isomers of

pseudo-octahedral [Zn(Lk)(S)₄]²⁺(one isomer, S = solvent),

[Zn(Lk)₂(S)₂]²⁺(five stereoisomers) and [Zn(Lk)₃]²⁺(two stereoi-

somers) has been previously detailed (Fig. S25†)⁷⁴and pro-

vides eight micro-constants which can be combined to give a

global model (eqn (9)–(11)) for the three macro-constants char-

acterizing equilibria (5)–(7).

Zn;Lk

1;1

β

¼ 24f ^Zn;Lk

ð9Þ

ꢊ

ꢄ

ꢅ

2

Zn;Lk

1;2

Lk;Lk

trans;fac

Lk;Lk

β

¼ 12 f ^Zn;Lk

u

þ u

trans;mer

ꢋ

ð10Þ

Lk;Lk

cis;fac

Lk;Lk

cis;mer

þ4u

þ 4u

ꢆ

ꢇ

ꢊ

ꢋ

ꢊ

ꢋ

2

ꢄ

ꢅ

3

Zn;Lk

1;3

Lk;Lk

cis;fac

Lk;Lk

cis;fac

Lk;Lk

cis;mer

β

¼ 16 f ^Zn;Lk

u

þ3 u

ð11Þ

Assuming the operation of a single average intermetallic

Lk;Lk

trans;fac

Lk;Lk

trans;mer

Lk;Lk

cis;fac

Lk;Lk

interaction

u

’ u

; u_Lk;Lk

cis;mer

reduces the number microscopic descriptors to f^Zn,Lkand

u^Lk,Lkfor modelling the three macroscopic constants obtained

by spectrophotometry (eqn (9), (12) and (13)).

ꢄ

ꢅ

2

Zn;Lk

1;2

β

¼ 120 f ^Zn;Lku^Lk;Lk

ð12Þ

ð13Þ

ꢄ

ꢅ ꢄ

ꢅ

3

Zn;Lk

1;3

β

¼ 64 f ^Zn;Lku^Lk;Lk

Linear least-squares fits of the logarithmic forms provide

ꢄ

ꢅ

Zn;Lk

affinity

the aﬃnity parameters ΔG

¼ ꢀRTln f ^Zn;Lkand interli-

Fig. 3 Concentration proﬁles obtained using HySS2009 simulation⁷³

and the formation constants collected in Table 1 for the complex

species (a) [Zn(L6)_n]²⁺at |L6|_tot= 10 mM, and (b and c) [Zn(L7)_n]²⁺at

(b) |L7|_tot= 10 mM and (c) |L7|_tot= 100 mM (acetonitrile, 293 K).

ꢄ

ꢅ

Lk;Lk

gand interactions ΔE

¼ ꢀRTln u^Lk;Lkgathered in

interaction

Table 1 (columns 3–6), which satisfyingly reproduce the experi-

mental data. Altogether, this rough thermodynamic model

establishes that (i) the aﬃnity of the selected didentate ligands

for spherical Zn²⁺(ꢀΔG^M;Lk

_1;i;affinity) follows the trend L1 (2-pyri-

A deeper and quantitative analysis relies on the site binding

dine) ≈ L6 (2-pyrimidine) > L4 (2-pyrazine) ≫ L7 (4-pyrimidine)

model (eqn (8)),^76–78which combines a statistical factor K

stat

and (ii) the successive binding of ligands leading to [Zn(Lk)₃]²⁺

1;n

(related to the contribution produced by the change in

rotational entropy between the reactants and products and to

Lk;Lk

interaction

follows a weakly cooperative procedure (ΔE

ꢁ 0).

Repeating the spectrophotometric titrations for submilli-

molar concentrations of the ligands L6 and L7 in dry aceto-

nitrile with Fe(CF₃SO₃)₂lead to smoother endpoints for L6

(Fig. S23†) and to only poor drifts for L7. Satisfying factor ana-

lysis and non-linear least-squares fit using equilibria (5)–(7)

the entropy of mixing of enantiomers),^79–81with a chemical

chem

contribution K

made up of two microscopic chemical para-

1;n

ꢊ

ꢋ

ΔG^M_1;i^;_;^L_a^k_ffinity=RT

meters f_i^M;Lkand u_k^L_;^k_l^;Lk. The first term f_i^M;Lk¼ e^ꢀ

stands for the intermolecular microscopic aﬃnities (including could be obtained for L6. The associated macro-constants

Fe;Lk

desolvation) characterizing the free energy of inner sphere con-

β

are however drastically reduced by two to five orders of

1;n

nection of the didentate binding site in Lk to M^IImagnitude in going from Zn²⁺/L6 (Table 1, entry 1) to Fe²⁺/L6

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Fig. 4 Meridional to facial isomerization process operating in [M(Lk)₃]²⁺complexes (eqn (14)) together with its modeling with the help of the site

binding model. The molecular structures are those found in the crystal structures of [Zn(L6)₃](ClO₄)₂(CH₃CN)_2.5(1).

(Table 1, entry 3). Though less pronounced, the same trend (>95%) formed and display dynamically average C₃-symmetri-

was previously reported for 2-pyrazine-benzimidazole L4 cal symmetries on the NMR time scale (Fig. S28 and S29†).

ligands in going from Zn²⁺to Fe²⁺(Table S7†).⁶¹The replace- Upon stepwise decrease of the temperature, the dynamic

exchange process slows down so that the ¹H NMR spectra

Fe;L7

ment of ligand L6 with L7 leads to β

ꢄ β₁^F_;^e_n^;L6, a trend

1;n

which strictly mirrors that previously observed for Zn²⁺.

display two series of resolved signals, which can be assigned

to a mixture of mer-[Zn(Lk)₃]²⁺(C₁symmetry, three signals for

a given proton) and fac-[Zn(Lk)₃]²⁺(C₃symmetry, one signal

for a given proton, Fig. S28 and S29†). Integration of low temp-

erature ¹H NMR data provides the experimental ratio of the

Finally, titrations of L7 with analogous Ni²⁺cations (Fig. S24†)

Fe;L7

1;n

Ni;L7

in line with the well-known

1;n

additionally fixes β

ꢁ β

Irving–Williams series (Table 1, entry 4).^16,82,83Consequently,

the speciation profiles computed for Fe²⁺/L6 (Fig. S26†) mirror

those highlighted for Zn²⁺/L7 (Fig. 4b and c), which possess

similar stability constants, but the Fe²⁺/L7 system is expected

to produce much smaller quantities of [Fe(L7)₃]²⁺, which are

overcome by those computed for [Ni(L7)₃]²⁺(Fig. S27†).

concentrations mer-[Zn(Lk)₃]²⁺/fac-[Zn(Lk)₃]²⁺, from which

Zn;Lk

mer!fac

K

can be estimated at diﬀerent temperatures. Van’t Hoﬀ

plots display linear correlations (Fig. S30†) and lead to enthal-

pic and entropic contributions collected in Table 2 together

with speciations highlighted in Fig. 5. As previously reported

for mer-[Zn(Lk)₃]²⁺→ fac-[Zn(Lk)₃]²⁺equilibria with L1 (2-pyri-

dine) and L4 (2 pyrazine, Table S8†),⁷⁴the latter mer → fac iso-

Meridional to facial isomerization process operating for [Zn

(Lk)₃]²⁺(Lk = L6 and L7) in solution

merization process for L6 and L7 is solvent dependent and

A focus on the target [M(Lk)₃]²⁺complexes requires the vari-

able-temperature analysis of the thermodynamic equilibrium

(14) linking the meridional and facial isomers (Fig. 4). This

challenge can be addressed for diamagnetic closed-shell

[Zn(Lk)₃]²⁺cations with the help of variable-temperature

¹H NMR spectra.

Zn;Lk

mer!fac

deviates from a pure statistical behavior (ΔH

¼ 0 and

Zn;Lk

mer!fac

ꢀTΔS

= −RT·ln(1/3) = 2.7 kJ mol⁻¹at 298 K).

The enthalpic preference for the facial isomer (−7.3 ≤

Zn;Lk

mer!fac

ΔH

≤ −2.8 kJ mol⁻¹, Table 2), in which each benzimida-

zole N-donor atom is bound trans to the N-atom of the pyridi-

midine ring, is reminiscent of the thermodynamic trans

influence.^74,84The opposite entropic preference for the meri-

At room temperature in CD₃CN and CD₃OD, both

[Zn(L6)₃]²⁺(10 mM) and [Zn(L7)₃]²⁺(0.1 M) are quantitatively

Zn;Lk

mer!fac

Zn;Lk

mer!fac

Table 2 Thermodynamic enthalpic (ΔH

) and entropic (ΔS

) contributions to equilibrium (14) in solution

Zn;Lk

mer!fac

Zn;Lk

mer!fac

Zn;Lk;°

mer!fac

/kJ mol⁻¹

ΔS

/J mol⁻¹K⁻¹

ΔG

^b/kJ mol⁻¹

T_1/2/K

a

Complex

Solvent

ε_r

ΔH

[Zn(L6)₃]²⁺

[Zn(L7)₃]²⁺

CD₃CN

CD₃OD

37.5

33.0

−2.8(1)

−6.9(5)

−7.3(9)

−16.2(4)

−30(2)

−38(4)

2.1(2)

2.2(5)

3.8(1.5)

171(8)

227(26)

195(32)

^aRelative dielectric constants. ^bΔG_m^Zn_e^;_r^L_!^k;_f^°_acare calculated at T = 298 K.

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Fig. 6 Plot of molar paramagnetic magnetic susceptibility (χ^p_M^araT)

versus temperature (T) for a 10.2 mM solution of the complex [Fe(L6)₃]²⁺

in CD₃CN (green), and a 21.3 mM solution of the complex [Fe(L7)₃]²⁺in

CD₃CN (orange). The black dashed curves represent the best ﬁt obtained

by using eqn (20) and the thermodynamic and magnetic parameters col-

lected in Table 3.

¹H NMR signals lie below the freezing point of CD₃CN (228 K).

In deuterated methanol, some resolved ¹H NMR signals can be

obtained for T < 228 K (Fig. S29†) for both [Zn(L6)₃]²⁺(Fig. 5b)

and [Zn(L7)₃]²⁺(Fig. 5c).

Electronic and spin crossover properties of [Fe(L6)₃]²⁺and

[Fe(L7)₃]²⁺in solution

According to Busch and co-workers,⁸⁵the easily accessible

ligand field strengths measured for the pseudo-octahedral

[Ni(Lk)₃]²⁺complexes can be used as a reliable benchmark

(11 200 ≤ Δ_oct(Ni^II) ≤ 12 400 cm⁻¹) for predicting and rationa-

lizing spin-crossover operative in the related Fe^IIcomplexes.

The absorption spectra recorded for 0.1 M acetonitrile solution

of [Ni(Lk)₃]²⁺(Lk = L6, L7, Fig. S31 and S32†) display Ni(³T₂←

³A₂) transitions at 11 079 cm⁻¹for L6 and 11 237 cm⁻¹for L7,

which can be used as estimates for Δ_oct(Table S9†).^61,86–88

Altogether, the four [Ni(Lk)₃]²⁺complexes (Lk = L1, L4, L6 and

L7) possess very similar electronic properties with an average

value of Δ_oct/B = 13.2(2) compatible with SCO behavior around

room temperature for the related [Fe(Lk)₃]²⁺complexes

(Scheme 2).^61,85Variable temperature ¹H NMR spectra

recorded for 0.1 mM solution of [Fe(L6)₃]²⁺in acetonitrile

Fig. 5 Reconstruction of the mole fractions of the meridional and facial

isomers for the complex [Zn(L6)₃]²⁺in (a) CD₃CN and (b) CD₃OD, and (c)

for the complex [Zn(L7)₃]²⁺in CD₃OD as a function of temperature. The

circles represent the accessible experimental data for which the

dynamic of mer ↔ fac isomerization is slow on the ¹H NMR time scale.

Zn;Lk

mer!fac

dional isomer (4.8 ≤ ꢀTΔS

≤ 11.3 kJ mol⁻¹at 298 K, (233–298 K) indeed unambiguously demonstrate the existence

Table 2), which is significantly larger than the pure statistical of temperature-dependent paramagnetic shifts which are diag-

contribution of 2.7 kJ mol⁻¹at 298 K, has been previously nostic for the formation of increasing amounts of fast-relaxing

attributed to the larger relaxation of the solvent organization high-spin Fe^II(S = 2) at high temperature (Fig. S33†). However,

around the meridional isomer which possesses a smaller the broadening of the signals due to the combination of low-

dipole moment.⁷⁴

spin to high-spin transformation with thermally-activated mer

Due to the remarkable stability found for [Zn(L6)₃]²⁺in ↔ fac isomerization prevents the change in chemical shifts to

solution, the kinetic rate constant of the mer ↔ fac isomeriza- be exploited for extracting reliable speciation in solution and

tion process (eqn (14)) is small enough to give resolved ¹H associated spin state equilibria.^89,90

NMR spectra for each isomer on the NMR time scale in aceto-

Alternatively, the total magnetic susceptibility of a paramag-

nitrile prior to reach its freezing point (233 ≤ T ≤ 258 K, netic solute χ^p_M^ara(cm³mol⁻¹) can be obtained with the help of

Fig. 6a and S28†). The situation changes with the less stable Evans’ method,^45,91–93adapted for the temperature depen-

[Zn(L7)₃]²⁺complex, and the coalescence temperature for the dence of solvent density⁹⁴and for an adequate treatment of

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the various diamagnetic corrections (eqn (15); S_f= 4π/3 is the The consideration of spin-state eqn (1) eventually transforms

shape factor of the superconducting magnet, ν₀is the Larmor eqn (16) into eqn (18) (see ESI Appendix 1†).⁶¹

frequency of the spectrometer in Hz, Δv = v_ref− v_intis

χ^p_M^araT ꢀ ðC_lsþ T ꢃ TIP_lsÞ

the diﬀerence in Hz between the frequency of the standard

x_hs¼ 1 ꢀ x_ls

¼

ð17Þ

ðC_hsþ T ꢃ TIP_hsÞ ꢀ ðC_lsþ T ꢃ TIP_lsÞ

ðC_lsꢀ C_hsþ T ꢃ ðTIP_lsꢀ TIP_hsÞÞ

(tert-butanol) in contact with the complex of interest (v_ref

=

δ_ref·v_o) and that of the standard (v_int= δ_int·v_o) placed in the

coaxial tube in absence of complex, M is the molecular

weight in g·mol⁻¹of the paramagnetic complex or of its dia-

magnetic analogue when Fe^IIis replaced with Zn^II, and m is

χ^p_M^araT ¼

ꢊ

ꢋꢋ

ΔH_sco

1

1 þ exp _Rꢃ ΔS_sco

ꢀ

ð18Þ

T

þ C_hsþ T ꢃ TIP_hs

the concentration of the pertinent complex in g cm⁻³

,

The non-linear least-square fits of the experimental para-

magnetic data depicted in Fig. 6 for [Fe(L6)₃]²⁺and [Fe(L7)₃]²⁺

by using eqn (18) provides Curie constants, TIP and thermo-

dynamic parameters ΔH_SCOand ΔS_SCOfor the spin crossover

equilibrium gathered in Table 3 (see ESI Appendix 1 for details†).

The Curie constants C and TIP values found for [Fe(L6)₃]²⁺

and [Fe(L7)₃]²⁺(Table 3, entries 5–6) are typical for the [FeN₆]

chromophores previously investigated in [Fe(didentate)₃]

complexes.^{16,61,97–102}One notes that C_HS> 3.00 cm³K mol⁻¹

(spin only value) can be taken as the signature of some partially

unquenched orbital momentum in this pseudo-octahedral

Fig. S34†).^95,96

ꢀ

ꢁ

1

v_oS_f

Δv^paraM^paraΔv^diaM^dia

χ^p_M^ara

¼

ꢀ

ð15Þ

m^para

m^dia

Application of eqn (15) for the variable-temperature shifts

1

of the H-NMR signals recorded for tert-butanol in acetonitrile

solutions of [Fe(L6)₃]²⁺(10.2 mM, Fig. S34†) and [Fe(L7)₃]²⁺

(21.3 mM), while using [Zn(L6)₃]²⁺(13 mM) as the diamagnetic

reference, eventually gave plots of χ^p_M^araT versus T, which are

diagnostic for the operation of spin crossover in solution

(Fig. 6).

environment. The thermodynamic parameters 19.5 ≤ ΔH_SCO

≤

23 kJ mol⁻¹and 71 ≤ ΔS_SCO≤ 83 J mol⁻¹K⁻¹measured for

[Fe(L6)₃]²⁺and [Fe(L7)₃]²⁺also match the usual range found for

similar complexes in solution.^45,97,98Interestingly, the pyrimidine

rings in ligands L6 and L7 produce the weakest enthalpic change

accompanying the spin crossover process in solution compared

with pyridine in L1 and pyrazine in L4. Due to partial H/S

compensation^101–105established for SCO processes,¹⁰⁵a satisfy-

ing ΔH_SCO= α·ΔS_SCO+ β linear correlation is observed for the

series of [Fe(Lk)₃]²⁺complexes with α = 601(103) K and β =

−24 936(9020) J mol⁻¹(Fig. S35†). Consequently, the transition

temperature T_1/2= ΔH_SCO/ΔS_SCO= α + (β/ΔS_SCO) in CD₃CN

decreases with ΔS_SCO(since β < 0) and reaches its lowest value

for [Fe(L6)₃]²⁺(273 K) followed by [Fe(L7)₃]²⁺(279 K), [Fe(L1)₃]²⁺

(313 K) and [Fe(L4)₃]²⁺(351 K, Table 3, column 9).

The general expression of the Curie law, corrected for dia-

magnetism, for a mixture containing high-spin (mole fraction

x_hs) and low-spin (mole fraction x_ls) complexes is given in eqn

(16), where C_hsand C_lsare the Curie constants for the high-

spin and low-spin forms (C = (g²/8)S(S + 1) within the frame of

the spin-only approximation) and TIP_hsand TIP_lsare the temp-

erature-independent paramagnetic contributions.^39,45

χ^p_M^araT ¼ x_hsꢃðC_hsþ TꢃTIP_hsÞ þ x_lsꢃðC_lsþ TꢃTIP_lsÞ

ð16Þ

The introduction of mass balance (x_ls+ x_hs= 1) gives a

straightforward access to the speciation (eqn (17) and Fig. 7).

However, the unavoidable mer ↔ fac equilibrium (14)

depicted in Fig. 4 implies that the above rough analysis of the

SCO processes operating in [Fe(didentate)₃]²⁺complexes in

solution is of limited pertinence when the didentate ligand is

non-symmetrical.^33,61A pioneering attempt to obtain specific

SCO thermodynamic contributions for fac-[Fe(Lk)₃]²⁺and mer-

[Fe(Lk)₃]²⁺isomers used non-covalent lanthanide tripods to

force facial arrangement around Fe^IIas illustrated in the dime-

tallic triple-stranded helicate (Fig. 1). The associated thermo-

dynamic data recorded in solution for the SCO processes

occurring in LaFe-helicates, where the didentate units bound

to Fe^IImirrors ligands L1 and L4, were taken as valuable

models for fac-[Fe(Lk)₃]²⁺complexes. The thermodynamic

characteristics of mer-[Fe(Lk)₃]²⁺could be then deduced from

the χ^p_M^araT versus T plots recorded for standard solution mix-

Fe;Lk

mer!fac

tures according that K

are at hand at each temperature.⁶¹

Fig. 7 Plot of high-spin mole fraction (estimated from χ^p_M^araT by using

eqn (17)) versus temperature (T) for a 10.2 mM solution of the complex

[Fe(L6)₃]²⁺in CD₃CN (green circles) and its analysis using eqn (20) as

originating from fac-[Fe(L6)₃]²⁺(blue trace) and mer-[Fe(L6)₃]²⁺(red

We propose here a parallel approach which takes reasonably

for granted that the Curie constants C and the TIP contri-

butions are identical for the fac and mer isomers of the same

complex. Moreover, one also assumes (still reasonably) that

Fe;Lk

Zn;Lk

trace) controlled by the isomerization constant K

’ K

.

mer!fac

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Table 3 Magnetic and thermodynamic parameters obtained from the non-linear least square ﬁts of χ^p_M^araT vs. T plots recorded for [Fe(Lk)₃]²⁺com-

plexes in solution (CD₃CN)

C_hs/cm³

TIP_hs× 10⁻⁶

/

C_ls/cm³

TIP_ls× 10⁻⁶

/

ΔH_sco

/

ΔS_sco/

Complex

Anion

K mol⁻¹

cm³mol⁻¹

K mol⁻¹

cm³mol⁻¹

kJ mol⁻¹

J mol⁻¹K⁻¹

T_1/2/K

Ref.

[Fe(bipy)₃]^{2+ a}

[Fe(L2)₃]^{2+ b}

[Fe(L1)₃]²⁺

[Fe(L4)₃]²⁺

[Fe(L6)₃]²⁺

[Fe(L7)₃]²⁺

CF₃₋SO₃

PF₆₋

—

0.00(8)

—

351(55)

—

28.6(3)

35.7(3)

19.5(2)

23.01(2)

—

91.5(8)

101(1)

71.3(6)

82.5(1)

—

313(4)

351(5)

273(3)

279(1)

61

−

3.44(2)

358(57)

PF₆

3.44^c

358^c

0.00^c

0.00

351^c

−

CF₃₋SO₃

3.44^c

358^c

351^c

342(3)

346.2(3)

BF₄₋

4.10(3)

3.595(3)

369(3)

368.8(2)

This work

PF₆

^a100% low-spin at all studied temperatures. ^b100% high-spin at all studied temperatures. ^cC_lsand TIP_lsfound for [Fe(bipy)₃]²⁺and C_hsand TIP_hs

found for [Fe(L2)₃]²⁺have been used in ref. 65 for fitting the SCO behaviors.

the thermodynamic data extracted from VT-NMR analysis for

The diﬀerent enthalpic contributions are reminiscent of the

the isomerization process operating in [Zn(Lk)₃]²⁺holds true concept of trans influence which is thought to strengthen Fe–N

Zn;Lk

for [Fe(Lk)₃]²⁺(in other words K^Fe;Lk

_mer!facðTÞ ’ K

ðTÞ). With

mer!fac

bonds in the facial isomer. The entropic preference exhibited

by the meridional isomer might be relevant to solvation

eﬀects. Indeed, the primary change in solvation energy accom-

panying the low-spin to high-spin transition is produced by

this in mind, eqn (16) can re-written as eqn (19) and the intro-

duction of the thermodynamic constants K_mer→facand K_SCO

finally yields eqn (20) (see ESI Appendix 2 for details†).⁶¹

the expansion of [Fe(Lk)₃]²⁺considered as

a

charged

change

monopole.^106,107

The

associated

endergonic

χ^p_M^araT ¼ ðx^facþ x^merÞ ꢃ ðC_hsþ TꢃTIP_hsÞ

hs

ΔΔ_solvG_monopole∝ −(z²_hs/R_hs− z_ls²/R_ls) modeled with Born

equation¹⁰⁶is expected to be very similar for both meridional

and facial isomers since their cationic charges are identical

ð19Þ

fac

mer

þ ðx þ x Þ ꢃ ðC_lsþ TꢃTIP_lsÞ

ls

0

@

1

fac‐FeðLkÞ₃

Fe;Lk

mer‐FeðLkÞ₃

K

ꢃ K

mer!fac

K

Fe;Lk

SCO

χ^p_M^araT ¼

þ

ꢃ ðC_hsþ T ꢃ TIP_hsÞþ

ꢃ ðC_lsþ T ꢃ TIP_lsÞ

SCO

A

ꢊ

ꢋꢊ

ꢋ

ꢊ

ꢋꢊ

ꢋ

fac‐FeðLkÞ₃

mer‐FeðLkÞ₃

Fe;Lk

mer!fac

1 þ K

SCO

mer!fac

0

@

1

ð20Þ

Fe;Lk

mer!fac

K

1

A

ꢋꢊ

ꢋ

þ

ꢊ

ꢋꢊ

ꢋ

fac‐FeðLkÞ₃

mer‐FeðLkÞ₃

Fe;Lk

1 þ K

mer!fac

1 þ K

SCO

mer!fac

fac

mer

hs

mer

ls

The systematic application of the van’t Hoﬀ relationship (z

= z_hs= z

= z = 2) and their ionic radii are very close

ls

ΔG° = –RTln(K) = ΔH° − TΔS° for estimating the various stabi- (R_fac≈ R_mersince V_fac≈ V_mer). The minor, but non-negligible

Zn;Lk

mer!fac

additional dipolar contribution to the solvation energy can be

lity constants together with (i) the use of ΔH

and

estimated with the help of Onsager equation to give

ΔΔ_solvG_dipole∝ −(μ_hs²/R_i,hs³− μ_ls²/R_i,ls³).^108–111Again, the radius

of the spherical cavity R_icut from the dielectric when the

solute, taken as spherical, is immersed into the solvent, is

similar for both isomers (R_i,fac≈ R_i,mer), but the magnitude of

Zn;Lk

ΔS

gathered in Table 2 for L6 in CD₃CN and (ii)

mer!fac

Fe;Lk

Zn;Lk

mer!fac

K

’ K

, allows the satisfying non-linear least-

mer!fac

square fits with eqn (20) of χ^p_M^araT versus T plots for fac-[Fe

(L6)₃]²⁺and mer-[Fe(L6)₃]²⁺(Fig. 7). In complete agreement

with the previous analysis using a loosely constrained triple-

stranded LaFe helicate for modeling fac-[Fe(Lk)₂]²⁺(Lk = L1,

L4),⁶¹this alternative approach avoids the synthesis of the

related helicates and confirms that the SCO enthalpies are

only marginally larger for the facial isomer (≈1 kJ mol⁻¹).

2

2 112

the dipole moments are diﬀerent with μ_fac> μ_mer

.

Since

R_i,hs> R_i,ls, then ΔΔ_solvG_dipole> 0 and the contribution of

dipole solvation disfavors the low-spin to high-spin spin tran-

sition more strongly for the facial isomer than for the meridio-

fac

mer

SCO

nal isomer as measured by ΔS

> ΔS

.

SCO

However, the SCO entropy is systematically more favorable for

fac

the meridional isomer mer-[Fe(L6)₃]²⁺by ΔS − ΔS_SCO= 11(1)

Isolation, structures and magnetic properties of [M(L6)₃]X₂

mer

SCO

J mol⁻¹K⁻¹, a value mirroring those previously reported for

and [M(L7)₃]X₂in the solid state (M = Fe, Ni, Zn and X =

−

CF₃SO₃⁻, BF₄⁻, PF₆⁻, ClO₄

)

mer-[Fe(L1)₃]²⁺(14(2) J mol⁻¹K⁻¹) and mer-[Fe(L4)₃]²⁺, (11(1) J

mol⁻¹K⁻¹).⁶¹In going from facial to meridional isomer in Stoichiometric M : Lk = 1 : 3 mixtures of L6 or L7 (10 mM) with

[Fe(Lk)₃]²⁺, H/S anti-compensation seems to be the rule since Ni(BF₄)₂·6H₂O or Zn(CF₃SO₃)₂or Fe(ClO₄)₂·xH₂O or Fe

the spin transitions, which occur in mer-[Fe(Lk)₃]²⁺are conco- (BF₄)₂·6H₂O in acetonitrile were evaporated to dryness. The

mitantly favored over fac-[Fe(Lk)₃]²⁺by (i) a lower enthalpy cost resulting microcrystalline powders were re-dissolved in aceto-

and a (ii) larger entropic gain (see Fig. 7).

nitrile or methanol and allowed to crystallize by evaporation,

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or by diﬀusion of tert-butyl methyl ether after metathesis in (imidazolylimine)₃](Cl)(PF₆) salts with diﬀerent solvent

presence of

large excess (10 eq.) of (ⁿBu)₄NClO₄or contents,^113,114the thirteen crystal structures 1–13, which

a

(ⁿBu)₄NBF₄or (ⁿBu)₄NPF₆to give primary salts [Ni(L6)₃] incorporate similar [M(Lk)₃]²⁺cations with various poorly coor-

(BF₄)₂·1.55CH₃CN·0.4H₂O, [Zn(L6)₃](CF₃SO₃)₂·0.6H₂O, [Fe(L6)₃] dinating anions and solvent molecules, are statistically parti-

(BF₄)₂·0.15CH₃CN·1.2H₂O, [Ni(L7)₃](BF₄)₂·0.85H₂O, [Zn(L7)₃] tioned between six pure facial isomers (2, 4, 5, 7, 8, 9), six pure

(PF₆)₂·1.25H₂O, and [Fe(L7)₃](PF₆)_1.76(BF₄)_0.24·0.6H₂O com- meridional isomers (3, 6, 10, 11, 12, 13) and one mixture of

plexes in 46–72% yield (Table S10†). Single crystals of meridional/facial (44/56%) isomers (1, Fig. 8 and Tables S41

suﬃcient quality for X-ray diﬀraction studies could be and S42†). All complexes display poorly distorted octahedral

obtained for complexes 1–13 gathered in Fig. 8 (Tables S11– [MN₆] chromophores¹¹⁵with bond lengths within the expected

S40 and Fig. S36–S48†). Reminiscent to the empirical isolation range.^116,117The Fe–N bond lengths measured for [Fe(L6)₃]

of either pure facial or meridional isomers reported for [Fe (ClO₄)₂(6), and [Fe(L6)₃](BF₄)₂·(C₅H₁₂O)_0.5·(C₂H₅N)_0.5(7) at

180 K (2.03 to 2.15 Å, Table S41†) are longer than standard

Fe_ls–N bonds^26,38found in [Fe(L7)₃](PF₆)(BF₄)·CH₃OH (12) and

[Fe(L7)₃](PF₆)_1.72(ClO₄)_0.28·CH₃OH (13) at 150 K (1.98–1.99 Å.

Table S42†). This suggests that the iron complexes with

ligands L6 exists as mixtures of low-spin and high-spin in 6

and 7 at 180 K, while those with L7 in 12 and 13 are mainly

low-spin at 150 K.

A second collection of X-ray diﬀraction data at 100 K for

complexes [Fe(L6)₃](ClO₄)₂(6, meridional isomer) and [Fe(L6)₃]

(BF₄)₂·(C₅H₁₂O)_0.5(7, facial isomer) indeed showed a signifi-

cant decrease of the Fe–N bond length, which converged to

1.99 Å at low temperature as expected for pure low-spin [FeN₆]

units (Table S43†). Interestingly, complex 6 at 100 K adopts the

monoclinic crystal system with one meridional isomer per

asymmetric unit, and transforms into a triclinic system with

two diﬀerent meridional complexes (noted as Fe–A and Fe–B)

per unit cell at 180 K. A careful look at the bond lengths

measured in the crystal structure at 180 K (Table S25†)

indeed shows diﬀerent average bond lengths with 2.01(1) Å for

Fe–A and 2.08(3) Å for Fe–B units. A detailed variable-

temperature X-ray diﬀraction analysis of mer-[Fe(L6)₃]²⁺in

6

revealed

two

successive

phase

transitions,

monoclinic ¹⁶⁰ꢀ^ꢀ!^{170 K}triclinic ²⁴⁰ꢀ^ꢀ!^{250 K}monoclinic, accompanied

by a two-steps spin crossover transition (Table S44†).

Complexes Fe–B (50% of total iron content) undergo the

low-spin to high-spin transition within the 165–210 K range

(Fig. 9a). The remaining Fe–A sites see their coordination

spheres to be slightly shrunk when low-spin Fe–B is trans-

formed into high-spin Fe–B. Then the Fe–A sites undergo their

own spin transition in the 220–250 K range (Fig. 9a). As

expected, this extension of the Fe–N bond lengths is

accompanied by a rather regular increase of the volume of the

unit cell with small bumps at 170 and 240 K (Fig. 9b), which

are confirmed by diﬀerential scanning calorimetry (Fig. S49†).

The distinction between two complex molecules with the

same chemical formula, but exhibiting specific transitions in

the same solid is not unprecedented and was theoretically

rationalized by Sasaki and Kambara.¹¹⁸Related experimental

observations of this phenomenon within the SCO topics have

Fig. 8 Molecular structures of the [M(Lk)₃]²⁺cations in the crystal been reported for both Fe^IIIcomplexes¹¹⁹and Fe^IIcom-

plexes,¹²⁰which displayed an ‘intermediate-phase’ (IP) order-

ing phenomenon leading to a two-step spin-crossover behav-

structures of [Zn(L6)₃](ClO₄)₂(C₂H₃N)_2.5(1), [Zn(L6)₃](BF₄)₂·(CH₃CN)₂(2),

[Ni(L6)₃](ClO₄)₂(3), [Ni(L6)₃](ClO₄)₂·C₂H₃N (4), [Ni(L6)₃](BF₄)₂·C₂H₃N (5),

[Fe(L6)₃](ClO₄)₂(6), [Fe(L6)₃](BF₄)₂·(C₅H₁₂O)_0.5·(C₂H₅N)_0.5(7), [Zn(L7)₃]

(ClO₄)₂(8) [Zn(L7)₃](PF₆)₂(9) [Ni(L7)₃](ClO₄)_1.48(PF₆)_0.52·CH₃CN (10),

ior. The IP is defined by the limiting temperatures of the two-

phase transitions and is usually formed of approximately

equal proportions of LS and HS states. It is therefore not so

[Ni(L7)₃](BF₄)₂·CH₃OH·(CH₃CN)_0.5(11), [Fe(L7)₃](PF₆)(BF₄)·CH₃OH (12)

and [Fe(L7)₃](PF₆)_1.72(ClO₄)_0.28·CH₃OH (13).

1216 | Dalton Trans., 2021, 50, 1206–1223

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Fig. 9 Evolution of (a) Fe−N bond lengths in the monoclinic phases

(green) and in the triclinic phase with Fe−A (red) and Fe−B (blue) and (b)

cell volumes of [Fe(L6)₃](ClO₄)₂(6) in its crystalline state as a function of

temperature during heating.

surprising that paramagnetic χ^p_M^araT versus T plot obtained by

SQUID measurements for [Fe(L6)₃](ClO₄)₂(6, meridional

isomers) in the solid state display two-successive spin tran-

sitions in the 100–300 K range (Fig. 10a). On the other side,

[Fe(L6)₃](BF₄)₂·(C₅H₁₂O)_0.5(7, facial isomers) and [Fe(L7)₃](PF₆)

(BF₄)·CH₃OH (12, meridional isomers) exhibit regular one-step

spin transitions (Fig. 10b and c). Interestingly, [Fe(L6)₃]

Fig. 10 Plots of molar paramagnetic susceptibility (χ_MT) versus temp-

erature (T) collected with increments of 1 K min⁻¹in a constant mag-

netic ﬁeld of 5000 Oe between 5–300 K for (a) [Fe(L6)₃](ClO₄)₂(6, meri-

dional isomers) and 4–400 K for (b) [Fe(L6)₃](BF₄)₂·(C₅H₁₂O)_0.5(7, facial

isomers) and (c) [Fe(L7)₃](PF₆)(BF₄)·CH₃OH (12, meridional isomers). The

red points represents the experimental data while the dashed black

traces are built by using the ﬁtted values of ΔH_sco, ΔS_sco, and γ (Table 4,

see text).

(BF₄)₂·(C₅H₁₂O)_0.5·(C₂H₅N)_0.5(7) exhibits

a complete and

abrupt spin transition, which allows the reliable estimation of

a Curie constant C_ls= 0.0080(3) cm³K mol⁻¹and temperature-

independent paramagnetism TIP_ls= 387(14) × 10⁻⁶cm³mol⁻¹

in the pure low-spin state and C_hs= 3.53(13) cm³K mol⁻¹

TIP_hs= 494(18) × 10⁻⁶cm³mol⁻¹in the pure high-spin state.

Taking these values as reasonable approximations for all low-

spin, respectively high-spin Fe^IIcomplexes with ligands L6 and

L7, it is possible to apply eqn (17) for transforming the plots of

the molar paramagnetic susceptibility (χ_MT) versus tempera-

ture (T) collected in Fig. 10 into plots of high-spin mole frac-

tions (x_hs) as a function of temperature (Fig. S50†).

analytically as a function of T with eqn (2), but the temperature

can be expressed as a function of x_hsin eqn (21).

ΔH_SCOþ γð1 ꢀ 2x_hsÞ

T ¼

ð21Þ

R ꢃ ln½ð1 ꢀ x_hsÞ=x_hsꢂ þ ΔS_SCO

According to the various shapes observed for x_hsversus T

plots (Fig. S50†), the simple van’t Hoﬀ relationship (eqn (1)) is

not adequate and should be replaced with eqn (2) where the mental

Non-linear least squares fits using eqn (21) for the experi-

〈T; x_hs〉 couples measured for [Fe(L6)₃]

adjustable interaction parameter γ takes into account inter- (BF₄)₂·(C₅H₁₂O)_0.5(7, facial) and [Fe(L7)₃](PF₆)(BF₄)·CH₃OH

molecular interactions.^43,46However, x_hscannot be expressed (12, meridional) yield the thermodynamic constants ΔH_sco

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Table 4 Thermodynamic parameters (ΔH_sco

,

ΔS_sco

,

and γ) associated with the SCO process in [Fe(L6)₃](ClO₄)₂(6), and [Fe(L6)₃]

(BF₄)₂·(C₅H₁₂O)_0.5·(C₂H₅N)_0.5(7), and [Fe(L7)₃](PF₆)(BF₄)·CH₃OH (12)

[Fe(L6)₃]²⁺(6)

Fe–A

Complex

Fe–B

[Fe(L6)₃]²⁺(7)

[Fe(L7)₃]²⁺(12)

Isomer

Meridional

10.6(4)

42(2)

Meridional

3.58(6)

20.1(3)

178(4)

2.12(3)

Facial

15.5(9)

75(4)

Meridional

13.45(1)

39.5(4)

ΔH_SCO/kJ mol⁻¹

ΔS_SCO/J mol⁻¹K⁻¹

T_1/2/K

251(12)

2.54(9)

207(16)

3.7(2)

341(3)

γ/kJ mol⁻¹

AF^a

1.71(2)

4.61 × 10⁻³

5.01 × 10⁻⁴

8.51 × 10⁻³

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

ꢄ

ꢅ

ꢄ

ꢅ

P

2

exp

hs

exp

hs

^aAgreement factor AF ¼

x

ꢀ x

=

x

.

calcd

hs

and ΔS_scoand the cooperativity factor γ collected in Table 4 spin fraction is solely assigned to x^B_hsin the 140–200 K range

(columns 4 and 5) and which satisfyingly reproduce the experi- and to x^A_hsin the 225–300 K domain allows two separated non-

mental data (Fig. 10b, c and S50b, c†).

linear least-square fits of the 〈T; x_hs〉 couples with eqn (21) for

Since the energies of intermolecular interactions overpass the two successive transitions detected in mer-[Fe(L6)₃](ClO₄)₂

those induced by the molecular SCO processes, ΔH_sco, ΔS_sco(6, Fig. 10a). The associated approximated thermodynamic

observed in the solid state for [Fe(L6)₃](BF₄)₂·(C₅H₁₂O)_0.5

·

parameters are collected in Table 4 (columns 2 and 3) and

(C₂H₅N)_0.5(7), and [Fe(L7)₃](PF₆)(BF₄)·CH₃OH (12, Table 4) sig- show transition temperatures T_1/2(Fe–B) = 178(4) K and

nificantly deviate from those recorded in solution (Table 3). T_1/2(Fe–A) = 251(12) K in line with the phase transition

Consequently, the transition temperature found for fac-[Fe observed by DSC (Fig. S48,† T_1/2= 167 K and 241 K). Some

(L6)]²⁺in acetonitrile (T_1/2= 309(23) K in Fig. 7) does not match slight positive cooperativity 2.1 ≤ γ ≤ 2.5 kJ mol⁻¹operates

that found in the crystal structure of 7 (T_1/2= 207(16) K). The within each spin state transition in mer-[Fe(L6)₃](ClO₄)₂(6)

systematic operation of positive cooperativity observed for [Fe and, as expected from the very limited cooperativity factors,

(L6)₃](BF₄)₂·(C₅H₁₂O)_0.5(7, γ = 3.7(2) kJ mol⁻¹), and [Fe(L7)₃] the χ^p_M^araT versus T plots recorded for 6 and 12 for heating–

(PF₆)(BF₄)·CH₃OH (12, γ = 1.71(2) kJ mol⁻¹) in the solid state is cooling cycles show no hysteresis.

more instructive and suggests that these crystal lattices are well-

adapted for accommodating the larger high-spin form. The

value of γ found for 7 in the solid state is slightly larger than

Conclusion

2RT_1/2(γ > 2RT_1/2= 3.4 kJ mol⁻¹) and we must consider the

possibility that the latter complex might undergo a hysteretic

transition between the low-spin and the high-spin states.^39,40

However, the record of magnetic susceptibility along a complete

heating and cooling cycle showed no interpretable hysteresis,

but some drift resulting from the evaporation of volatile intersti-

tial solvent molecules (Fig. S51†).

Even if it may appear tedious, there is currently no more

eﬃcient method to tune SCO properties of a [FeN₆] unit than

the use of some semi-empirical trial/analysis/optimization pro-

cedures within a family of closely related complexes as pro-

posed for the L1–L7 series. In this context, [Fe(L6)₃]²⁺emerges

to be the best candidate in term of both stability and spin tran-

sition temperature in solution, to be connected to luminescent

lanthanide stains in metallosupramolecular architectures.

These systematic studies (i) confirmed the unpredictability of

SCO behaviors and associated thermodynamic parameters

observed in solid state materials, and (ii) allowed us to focus

our attention on the eﬀect of mer ↔ facial isomerization on

the spin state transitions operating for [Fe(didentate)₃]²⁺in

solution (didentate stands for a non-symmetrical diimine

ligand). As expected from the minor thermodynamic trans

influence reported for the formation of pseudo-octahedral

Finally, molar magnetic susceptibilities (χ^p_M^ara) recorded for

mer-[Fe(L6)₃](ClO₄)₂(6) at variable temperatures between

5–300 K indeed confirmed the crystallographic studies and the

operation of two successive SCO transitions, each aﬀecting

50% of the total iron complexes (Fig. 10a). The magnetic data

could be rationalized with eqn (22), which considers an equi-

molar mixture of two complexes Fe–A (T_1/2at high tempera-

ture) and Fe–B (T_1/2at low temperature) in the solid state.

X

ꢈ

ꢄ

ꢅ

2χ_MT ¼

xⁱ_hsꢃ C_hⁱ_sþ T ꢃ TIPⁱ_hs

complexes along the 3d series,^74,84ΔH_SCOare slightly more

i¼A;B

ð22Þ

ꢄ

ꢅ ꢄ

ꢅꢉ

fac

SCO

þ 1 ꢀ xⁱ_hsꢃ C_lⁱ_sþ T ꢃ TIP_lⁱ_s

positive, and thus unfavourable, for the facial isomers (ΔH

mer

SCO

− ΔH

≈ 1 kJ mol⁻¹). This trend is amplified by a larger drift

Introducing the single set of magnetic parameter C_ls

=

originating from an entropic preference for SCO operating in

fac

SCO

0.008 cm³K mol⁻¹, C_hs= 3.53 cm³K mol⁻¹, TIP_ls= 387 × 10⁻⁶the meridional isomers in solution ΔS

− ΔS

≈ 11 J

mer

SCO

cm³mol⁻¹and TIP_hs= 494 × 10⁻⁶cm³mol⁻¹into eqn (22) mol⁻¹K⁻¹. Since the same entropic gap has been previously

transforms the experimental χ_MT values recorded for mer-[Fe reported in triple-stranded FeLn helicates, where the facial

(L6)₃](ClO₄)₂(6, Fig. 10a) into total high-spin mole fractions isomer is prevented to transform into meridional isomer by

= x^A_hs+ x_h^B_s(Fig. S50a†). Assuming that the growth of high- the non-covalent lanthanide tripod,⁶¹we conclude that, in

tot

hs

x

1218 | Dalton Trans., 2021, 50, 1206–1223

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solution, mer-[Fe_hs(didentate)₃]²⁺is systematically entropically Preparation of N-methyl-N-(2-nitrophenyl)pyrimidine-2-

favored over fac-[Fe_hs(didentate)₃]²⁺by dipolar solvation carboxamide (3)

eﬀects. With the [Fe(2-pyrimidine-benzimidazole)₃] units as

Thionyl chloride (7.41 mL, 12.15 g, 102.14 mmol, 9.26 eq.) was

added dropwise to the mixture of pyrimidine-2-carboxylic acid

(1, 1.37 g, 11.03 mmol, 1 eq.) dissolved dry 1,2-dichloroethane

(35 mL). Dimethylformamide (300 μL) was added to the reac-

programmed in [Fe(L6)₃]²⁺, we are now equipped for the

exploration of dinuclear Fe^II–Ln^IIIcomplexes, the lanthanide-

based luminescence of which could be modulated by Fe^IIspin

crossover at the molecular level.

tion mixture. The solution was refluxed for 5 h under an inert

atmosphere of nitrogen, then cooled and evaporated to

dryness. The residue was dissolved in 40 mL of dry 1,2-dichloro-

ethane. N-Methyl-2-nitroaniline (2, 4.19 g, 27.57 mmol, 2.5 eq.)

dissolved in 10 mL 1,2-dichloroethane was added dropwise to

the solution. The solution was refluxed 15 h during which two

portions of (iPr)₂NEt (Hünig’s base, 3.5 mL, 20.09 mmol,

Experimental section

General

Chemicals were purchased from Sigma-Aldrich and Acros and

used without further purification unless otherwise stated.

1.82 eq.) were added after 30 min and 12 h. The solution was

Dichloromethane, 1,2-dichloroethane, tert-butyl methyl ether

concentrated under vacuum to aﬀord a solid residue which

and N,N-dimethylformamide were dried through an alumina

cartridge. Silica-gel plates (Merck, 60 F₂₅₄) were used for thin-

was partitioned between CH₂Cl₂(400 mL) and half saturated

aqueous NH₄Cl (4 × 120 mL). The aqueous phase was further

layer

chromatography,

SiliaFlash®

silica

gel

P60

extracted with CH₂Cl₂(3 × 50 mL). The combined organic

phase was dried over anhydrous Na₂SO₄, concentrated under

vacuum and purified by column chromatography (silica,

CH₂Cl₂/MeOH 99.2 : 0.8) to give 2.62 g of N-methyl-N-(2-nitro-

phenyl)pyrimidine-2-carboxamide (3, 10.14 mmol, yield 92%)

as a beige solid. ¹H NMR (CDCl₃, 400 MHz, 298 K) δ/ppm:

mixture of rotamers A (86.5%) and B (13.5%): 3.41 (3H, s, B),

(0.04–0.063 mm), and Acros silica gel 60 (0.035–0.07 mm) were

used for preparative column chromatography.

Preparation of pyrimidine-2-carboxylic acid (1)

Pyrimidine-2-carboxylic acid was synthesized according to a

slightly modified reported procedure.¹²¹Pyrimidine-2-carboni-

trile (5.02 g, 47.76 mmol, 1 eq.) was stirred with 5 M NaOH

(35 mL) at 55 °C for 3 h. The solution was neutralized (pH 2.0)

with diluted aqueous HCl and evaporated to dryness. The

resulting solid was suspended in acetonitrile, refluxed for 1 h,

and filtered when hot to remove the insoluble salts. The fil-

trate was evaporated under vacuum to dryness. The resulting

solid was re-dissolved in methanol and crystallized to give

3.21 g of pyrimidine-2-carboxylic acid (2, 25.8 mmol, yield

54%) as colourless thin needles. ¹H NMR (CDCl₃, 400 MHz,

298 K) δ/ppm: 7.61 (1H, t, ³J = 4.9 Hz), 9.00 (2H, d, ³J = 4.9 Hz),

10.62 (br).

3

3.55 (3H, s, A), 7.08–7.75 (4H, m, A and B), 7.95 (1H, dd, J =

8.4 Hz, A), 8.10 (1H, dd, ³J = 8.6 Hz, B), 8.53 (2H, d, ³J = 4.8 Hz,

A), 8.91 (2H, d, ³J = 4.9 Hz, B). ESI-MS (soft positive mode –

MeOH + CHCl₃+ HCOOH)- [3 + H]⁺= 259.3 (exp.) 259.3 (calc.),

[3 + Na]⁺= 281.4 (exp.) 281.2 (calc.), 213.4, 136.4, 119.3, 107.3

(fragments).

Preparation of 1-methyl-2-(pyrimidin-2-yl)-1H-benzo[d]

imidazole (L6)

N-Methyl-N-(2-nitrophenyl)pyrimidine-2-carboxamide

(3,

2.60 g, 10.06 mmol, 1 eq.) was dissolved in a mixture of

EtOH : DMF (20 mL : 25 mL). Sodium dithionite (7 g,

40.20 mmol, 3.97 eq.) was added to the solution and the temp-

Preparation of N-methyl-2-nitroaniline (2)

1-Chloro-2-nitrobenzene (30 g, 190.4 mmol, 1 eq.) and methyl- erature of the mixture was raised to 60 °C when 20 mL of dis-

amine (197.7 mL 40% wt. in water, 2.28 mol, 12 eq.) were tilled water was added. After refluxing for 24 h, the mixture

introduced into a Carius tube equipped with a magnetic stirrer was neutralized using aqueous ammonia and the solvents

and heated at 120 °C for 48 h. The excess of methylamine was were removed in vacuo. The concentrate was dissolved in

rotary evaporated, and the residual brown oil was partitioned CH₂Cl₂(50 mL) and washed with water (3 × 200 mL). The

between CH₂Cl₂(300 mL) and half-saturated aqueous NH₄Cl aqueous layers were collectively re-extracted with CH₂Cl₂(3 ×

(300 mL). The organic layer was separated and the aqueous 50 mL). The combined organic fractions were then concen-

phase was further extracted with CH₂Cl₂(3 × 150 mL). The trated under vacuum and purified by column chromatography

combined organic extracts were dried over anhydrous Na₂SO₄, (silica, CH₂Cl₂/MeOH 98 : 2) to yield L6 (5.86 mmol, yield

filtered and the solvent evaporated to dryness. The resulting 58%). The compound was crystallized as needles by slow

red oil was purified by column chromatography (silica, diﬀusion of n-hexane into CH₂Cl₂. ¹H-NMR (CD₃CN, 400 MHz,

CH₂Cl₂) to give N-methyl-2-nitroaniline (1, 178.57 mmol, yield 298 K) δ/ppm; 4.175 (3H, s), 7.29–7.33 (1H, ddd, ³J = 5.4–8.1

4

3

4

94%) as a deep red/orange oil, which was crystallized upon tri- Hz, J = 1.2 Hz), 7.37–7.41 (1H, ddd, J = 6.8–8.2 Hz, J = 1.2

1

3

4

turation. H NMR (CDCl₃, 400 MHz, 298 K) δ/ppm = 8.13 (1 H, Hz), 7.47 (1 H, t, J = 4.8 Hz), 7.57 (1 H, ddd, J = 8.1 Hz, J =

dd, J = 8.8 Hz, J = 1.6 Hz), 8.00 (1 H, bs), 7.43 (1 H, ddd, J = 0.9 Hz), 7.78 (1 H, ddd, ³J = 8.0 Hz, ⁴J = 0.8 Hz), 8.95 (2 H, d, ³J

3

4

3

8.8 Hz, J = 7.2 Hz, J = 1.6 Hz), 6.81 (1 H, dd, J = 8.6 Hz, J = = 4.8 Hz). ¹³C-NMR (CD₃CN, 101 MHz, 298 K) δ/ppm; 159.59

3

4

3

4

3

4

1.0 Hz), 6.62 (1 H, ddd, J = 8.4 Hz, J = 7.2 Hz, J = 1.2 Hz), (C_q), 158.48 (CH), 150.33 (C_q), 143.59 (C_q), 138.12 (C_q), 124.72

2.99 (3 H, s).

(CH), 123.46 (CH), 121.81 (CH), 121.27 (CH), 111.67 (CH),

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4

3

4

3

33.28 (CH₃). ESI-MS (soft positive mode – MeOH + CHCl₃

HCOOH): [L6 + H]⁺= 211.3 (exp.) 211.09 (calc.), [L6 + Na]⁺

+

=

7.59 (1H, dt, J = 1 Hz, J = 8.2 Hz), 7.76 (1H, dt, J = 1 Hz, J =

4

3

8.1 Hz), 8.32 (1H, dd, J = 1.6 Hz, J = 5.2 Hz), 8.89 (1H, d, J =

233.1 (exp.) 233.08 (calc.), 170.3 (fragments). Elemental ana- 5.2 Hz), 9.28 (1H, d, J = 1.6 Hz). ¹³C-NMR (CD₃CN, 101 MHz,

4

lysis: calculated (%): C 68.56, H 4.79, N 26.65; found (%): C 298 K) δ/ppm; 158.99 (CH), 158.38 (C_q), 148.79 (C_q), 143.51

68.39, H 4.69, N 26.82.

(C_q), 138.73 (C_q), 125.13 (CH), 123.82 (CH), 121.38 (CH),

121.03 (CH), 111.78 (CH), 33.54 (CH₃). ESI-MS (soft positive

mode – MeOH + CHCl₃+ HCOOH) [L7 + H]⁺= 210.6 (exp.)

211.09 (calc.), [L7 + Na]⁺= 233.1 (exp.) 233.08 (calc.), 82.4 (frag-

Preparation of N-methyl-N-(2-nitrophenyl)pyrimidine-4-

carboxamide (4)

Oxalyl chloride (55.33 mL, 14.0 g, 110.3 mmol, 10 eq.) was ments). Elemental analysis: calculated (%): C 68.56, H 4.79, N

added dropwise to a mixture of pyrimidine-2-carboxylic acid 26.65; found (%): C 68.32, H 4.72, N 27.00.

(1.37 g, 11.03 mmol, 1 eq.) dissolved in dry dichloromethane

Preparation [M(L6)₃]X₂and [M(L7)₃]X₂complexes (M = Fe, Ni,

(35 mL). Dimethylformamide (300 μL) was added to the

mixture to catalyze the reaction. The solution was refluxed for

−

Zn and X = CF₃SO₃⁻, BF₄⁻, PF₆⁻, ClO₄

)

2 h under an inert atmosphere of nitrogen, then cooled and In a typical synthesis, 0.3 mmol (3 eq.) of the ligand L6 or

evaporated to dryness. The residue was dissolved in 40 mL of ligand L7 dissolved in 2 mL of acetonitrile was added to

dry dichloromethane. N-Methyl-2-nitroaniline (2, 4.19 g, 0.1 mmol (1 eq.) of Ni(BF₄)₂·6H₂O or Zn(CF₃SO₃)₂or Fe

27.57 mmol, 2.5 eq.) dissolved in 10 mL dichloromethane was (ClO₄)₂·xH₂O or Fe(BF₄)₂·6H₂O in 2 mL acetonitrile. The result-

added dropwise to the solution. The solution was refluxed for ing mixtures were stirred under an inert atmosphere for 3 h

15 h during which two portions of (iPr)₂NEt (Hünig’s base, for ensuring complete dissolution of all components and then

3.5 mL, 20.09 mmol, 1.82 eq.) were added after 30 min and evaporated to dryness to yield microcrystalline powders of the

12 h. The solution was concentrated under vacuum to aﬀord a respective complexes. The powders were re-dissolved in aceto-

solid residue which was partitioned between CH₂Cl₂(400 mL) nitrile or methanol and allowed to crystallize by evaporation,

and half saturated aqueous NH₄Cl (4 × 120 mL). The aqueous or diﬀusion of tert-butyl methyl ether into acetonitrile or

phase was further extracted with CH₂Cl₂(3 × 50 mL). The com- methanol after metathesis in presence of a large excess (10

bined organic phase was dried over anhydrous Na₂SO₄, con- eq.) of (ⁿBu)₄NClO_4,(ⁿBu)₄NBF₄or (ⁿBu)₄NPF₆, respectively, to

centrated under vacuum and purified by column chromato- give primary salts [Ni(L6)₃](BF₄)₂·1.55CH₃CN·0.4H₂O, [Zn(L6)₃]

graphy (silica, CH₂Cl₂/MeOH 99 : 1) to give 2.62 g of N-methyl- (CF₃SO₃)₂·0.6H₂O, [Fe(L6)₃](BF₄)₂·0.15CH₃CN·1.2H₂O, [Ni(L7)₃]

N-(2-nitrophenyl)pyrimidine-4-carboxamide (4, 8.34 mmol, (BF₄)₂·0.85H₂O,

[Zn(L7)₃](PF₆)₂·1.25H₂O,

[Fe(L7)₃]

yield 76%) as a brown viscous liquid. ¹H NMR (CDCl₃, (PF₆)_1.76(BF₄)_0.24·0.6H₂O complexes in 46–72% yield. Only non-

400 MHz, 298 K) δ/ppm: mixture of rotamers A (15.1%) and B perchlorate complexes were characterized by elemental ana-

(84.9%): 3.49 (3H, s, B), 3.52 (3H, s, A), 7.34–7.77 (4H, m, A lyses (Table S10†). Single crystals of suﬃcient quality for X-ray

3

4

and B), 7.95 (1H, dd, J = 8.2 Hz, J = 1.4 Hz, B), 8.11 (1H, dd, diﬀraction studies could be obtained and X-ray crystal struc-

³J = 8.1 Hz, ⁴J = 1.1 Hz, A), 8.75 (1H, d, ³J = 5.1 Hz, B), 8.95 (1H, tures could be solved for [Zn(L6)₃](ClO₄)₂(C₂H₃N)_2.5(1), [Zn

d, ³J = 5.0 Hz, A), 8.78 (1H, d, ³J = 1.2 Hz, B), 9.33 (1H, A). (L6)₃](BF₄)₂·(CH₃CN)₂(2), [Ni(L6)₃](ClO₄)₂(3), [Ni(L6)₃]

ESI-MS (soft positive mode – MeOH + CHCl₃+ HCOOH): [4 + (ClO₄)₂·C₂H₃N (4), [Ni(L6)₃](BF₄)₂·C₂H₃N (5), [Fe(L6)₃](ClO₄)₂

H]⁺= 259.3 (exp.) 259.3 (calc.), [4 + Na]⁺= 281.3 (exp.) 281.2 (6), [Fe(L6)₃](BF₄)₂·(C₅H₁₂O)_0.5·(C₂H₅N)_0.5(7), [Zn(L7)₃](ClO₄)₂

(calc.), 213.4, 184.3, 136.3, 119.3, 108.3 (fragments).

(8) [Zn(L7)₃](PF₆)₂(9) [Ni(L7)₃](ClO₄)_1.48(PF₆)_0.52·CH₃CN (10),

[Ni(L7)₃](BF₄)₂·CH₃OH·(CH₃CN)_0.5(11), [Fe(L7)₃](PF₆)

(BF₄)·CH₃OH (12) and [Fe(L7)₃](PF₆)_1.72(ClO₄)_0.28·CH₃OH (13).

Caution! Dry perchlorates may explode and should be handled

(4, in small quantities and with the necessary precautions.^122,123

Preparation of 1-methyl-2-(pyrimidin-4-yl)-1H-benzo[d]

imidazole (L7)

N-Methyl-N-(2-nitrophenyl)pyrimidine-4-carboxamide

2.60 g, 10.06 mmol, 1 eq.) was dissolved in EtOH : DMF

(20 mL : 25 mL). Sodium dithionite (7 g, 40.20 mmol, 3.97 eq.)

Spectroscopic and analytical measurements

was added to the solution and the temperature of the mixture ¹H, ¹³C NMR and ¹H–¹⁵N HMBC spectra were recorded at

was raised to 60 °C when 20 mL of distilled water was added. 298 K on a Bruker Avance 400 MHz spectrometer. Chemical

After refluxing for 24 h, the mixture was neutralized using shifts are given in ppm with respect to TMS for ¹H and ¹³C

aqueous ammonia and the solvents were removed in vacuo. and nitromethane for ¹⁵N. Spectrophotometric titrations were

The concentrate was dissolved in CH₂Cl₂(50 mL) and washed performed with a J&M diode array spectrometer (Tidas series)

with water (3 × 200 mL). The combined aqueous layers were connected to an external computer. In a typical experiment,

extracted with CH₂Cl₂(3 × 50 mL). The organic fraction was 25 cm³of ligand L6 or ligand L7 in acetonitrile (∼2 × 10⁻⁴M)

then concentrated under vacuum and purified by column was titrated at 298 K with a solution of Fe(CF₃SO₃)₂and Ni

chromatography (silica, CH₂Cl₂/MeOH 98.5 : 1.5) to yield L7 (BF₄)₂·6H₂O or Zn(CF₃SO₃)₂(∼2 × 10⁻³M) in acetonitrile under

(5.21 mmol, yield 52%). The compound was crystallized as an inert atmosphere. After each addition of 33 μL, the absor-

needles by slow diﬀusion of n-hexane into CH₂Cl₂. ¹H-NMR bance was recorded using Hellma optrodes (optical path

(CD₃CN, 400 MHz, 298 K) δ/ppm; 4.31 (3H, s), 7.33 (1H, ddd, ⁴J length 0.1 cm) immersed in the thermostated titration vessel

3

4

3

= 1.2 Hz, J = 7.9 Hz), 7.41 (1H, ddd, J = 1.2 Hz, J = 8.0 Hz), and connected to the spectrometer. Mathematical treatment of

1220 | Dalton Trans., 2021, 50, 1206–1223

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Dalton Transactions

Paper

the spectrophotometric titrations was performed with factor

analysis^67–69and with ReactLab™ Equilibria (previously Specfit/

32).^69,71,72Pneumatically-assisted electrospray (ESI-MS) mass

spectra were recorded from ∼1 × 10⁻⁴M (ligands) and ∼1 × 10⁻³

M (complexes) solutions on an Applied Biosystems API 150EX

LC/MS System equipped with a Turbo Ionspray source. Elemental

analyses were performed by K. L. Paglia from the Microchemical

Laboratory of the University of Geneva. Electronic spectra in the

UV-Vis region were recorded at 293 K from solutions in CH₃CN

with a PerkinElmer Lambda 1050 using quartz cells of 0.1 or

1.0 mm path length. Solid-state absorption spectra were recorded

with a PerkinElmer Lambda 900 using capillaries. Solid-state

magnetic data were recorded on MPMS 3 QUANTUM DESIGN

magnetometers using magnetic fields of 1000–5000 Oe and at

1 K min⁻¹rates within the 5–400 K range (5–300 K range for per-

chlorate containing complexes). The magnetic susceptibilities

were corrected for the magnetic response of the sample holder

and the diamagnetism of the compounds by using the approxi-

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Tuning spin-crossover transition temperatures in non-symmetrical homoleptic meridional/facial [Fe(didentate)3]2+complexes: what for and who cares about it?

DOI: 10.1039/d0dt03828h

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