Insensitivity of Magnetic Coupling to Ligand Substitution in a Series of Tetraoxolene Radical-Bridged Fe2 Complexes

of Tetraoxolene Radical-Bridged Fe Complexes

Article

Insensitivity of Magnetic Coupling to Ligand Substitution in a Series

Agnes E. Thorarinsdottir, Ragnar Bjornsson, and T. David Harris*

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.9b03736

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sı Supporting Information

ABSTRACT: The elucidation of magnetostructural correlations

between bridging ligand substitution and strength of magnetic

coupling is essential to the development of high-temperature

molecule-based magnetic materials. Toward this end, we report the

I I

series of tetraoxolene-bridged Fe

complexes

[

(Me TPyA) Fe ( L)]

(Me TPyA = tris(6-methyl-2-

LH₂= 3,6-dimethoxy-2,5-

dihydroxo-1,4-benzoquinone, LH = 3,6-dichloro-2,5-dihydroxo-

OMe

pyridylmethyl)amine; n = 2:

,4-benzoquinone, Na₂[ L] = sodium 3,6-dinitro-2,5-dihydroxo-

SMe

,4-benzoquinone; n = 4:

L = 3,6-bis(dimethylsulfonium)-2,5-

dihydroxo-1,4-benzoquinone diylide) and their one-electron-

reduced analogues. Variable-temperature dc magnetic susceptibility

data reveal the presence of weak ferromagnetic superexchange between Fe centers in the oxidized species, with exchange constants

−

of J = +1.2(2) (R = OMe, Cl) and +0.3(1) (R = NO , SMe ) cm . In contrast, X-ray diﬀraction, cyclic voltammetry, and Mo

ssbauer

spectroscopy establish a ligand-centered radical in the reduced complexes. Magnetic measurements for the radical-bridged species

−

reveal the presence of strong antiferromagnetic metal−radical coupling, with J = −57(10), −60(7), −58(6), and −65(8) cm for R

x−•

OMe, Cl, NO , and SMe , respectively. The minimal eﬀects of substituents in the 3- and 6-positions of L on the magnetic

2 2

coupling strength is understood through electronic structure calculations, which show negligible spin density on the substituents and

associated C atoms of the ring. Finally, the radical-bridged complexes are single-molecule magnets, with relaxation barriers of U =

eff

0(1), 41(1), 38(1), and 33(1) cm⁻for R = OMe, Cl, NO , and SMe , respectively. Taken together, these results provide the ﬁrst

examination of how bridging ligand substitution inﬂuences magnetic coupling in semiquinoid-bridged compounds, and they establish

design criteria for the synthesis of semiquinoid-based molecules and materials.

INTRODUCTION

molecule-based magnets must be increased to near or even

above room temperature.

■

Molecule-based magnetic materials, ranging from discrete

In targeting molecule-based magnets with high operating

temperatures, the strength of magnetic exchange interactions

between spin centers, quantiﬁed by the exchange constant J,

represents a vitally important parameter. Speciﬁcally, the value

of J between paramagnetic centers is directly correlated to the

mono- and multinuclear metal complexes to extended

−9

networks,

have garnered tremendous interest over the

past few decades owing predominately to their potential

advantages over established solid-state inorganic materials,

such as adjustable chemical compositions, access through low-

temperature synthetic routes, and chemical processabili-

isolation of the spin ground state of single-molecule magnets,

d,6g,8−10

the relaxation barrier of single-chain magnets, and the

ty.

Moreover, the employment of bridging ligands

magnetic ordering temperature of permanent magnets.

with predictable binding modes, such as cyanide and a wide

range of organic linkers, grants exquisite synthetic control over

both the bulk crystal structure and the local metal coordination

environment and, therefore, enables the directed assembly of

Despite this critical relationship between operating temper-

ature and J, the vast majority of molecule-based magnets

feature paramagnetic metal centers that are coupled through

diamagnetic bridging ligands via a superexchange mechanism.

,6,8,11−15

compounds with targeted structures and properties.

Indeed, molecule-based materials may provide a new

generation of designer magnets, with the potential to transform

our current energy landscape through their utilization in

technologies such as spin-based data processing, magneto-

resistive sensing, magnetic gas separations, and permanent

Received: December 24, 2019

,16−18

magnet generators.

Nevertheless, for many of these

applications to be realized, the operating temperatures of

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Nevertheless, despite the tremendous potential of quinoid-

Article

Scheme 1. Redox Series

based ligands in the development of magnetic materials,

systematic studies that probe the eﬀects of ligand substitution

on the magnetic coupling strength in semiquinoid-bridged

systems are conspicuously lacking. The elucidation of these

magnetostructural correlations is essential to derive a clear and

complete understanding of the factors that govern the strength

and sign of magnetic coupling in metal-semiquinoid com-

pounds, and such eﬀorts may ultimately inform the design of

molecule-based magnets with high operating temperatures.

The strength of magnetic exchange through diamagnetic

benzoquinoid ligands has previously been found to vary with

the electronic properties of the ring-bound substituents in

Redox series of deprotonated tetraoxolenes, which can act as

bis(bidentate) bridging ligands.

nets.

complexes and two-dimensional (2D) ferrimag-

While superexchange interactions through short oxo and cyano

ligands can be suﬃciently strong to, for instance, aﬀord

magnets that order above room temperature,

magnitude of J decreases dramatically as the number of atoms

4g,33

While the substituent eﬀects are substantial for 2D

II III

c,d,6b,11b

Mn Cr frameworks, leading to a twofold increase in magnetic

ordering temperature when the electronegativity of the

substituents is decreased, these eﬀects are subtle for Cu

the

b,c

in the bridging ligand increases,

thereby hindering the

complexes due to a weak contribution from the substituents to

development of materials based on strong superexchange

coupling through chemically tunable organic bridging ligands.

As an alternative to superexchange coupling, one key

strategy to promote strong magnetic coupling involves the

incorporation of a radical bridging ligand between para-

magnetic metal ions, so as to engender strong direct magnetic

exchange by virtue of direct overlap of the metal- and radical-

the magnetic orbitals. Furthermore, remote ligand sub-

stitution can vary the coupling strength by nearly threefold in

mononuclear Mn and Cu semiquinoid complexes. Here,

electron-withdrawing substituents were found to provide

stronger ferromagnetic interactions than electron-donating

substituents for the Cu complexes. In contrast, no clear

trend was observed for the antiferromagnetically coupled Mn

u,13,22

based molecular orbitals.

Indeed, this approach has

found signiﬁcant success, as exempliﬁed by extensive studies

analogues, which was postulated to stem from the presence of

u,2a,e,p,23

4,14a−c,24

on systems featuring nitroxide,

organonitrile,

multiple convoluting exchange pathways. These studies

notwithstanding, a systematic investigation of the electronic

eﬀects of ligand substituents on magnetic coupling in

perchlorotriphenylmethyl, triplet carbene, and pyrazine

radical ligands. Nevertheless, the low charges and monodentate

binding mode of the coordinating functional groups in these

ligands often limit the strength of metal−ligand interactions

and hamper the directed assembly of compounds with well-

deﬁned structures.

semiquinoid-bridged compounds remains elusive.

Herein, we report the series of tetraoxolene-bridged Fe

x− n+

complexes [(Me TPyA) Fe ( L )] (x = 2, n = 2: R = OMe,

Cl, NO ; x = 0, n = 4: R = SMe ; Me TPyA = tris(6-methyl-2-

Bis(bidentate) benzoquinoid ligands oﬀer an appealing

platform for the construction of radical-bridged compounds

with strong magnetic coupling, as these ligands (i) undergo

facile redox chemistry to generate the semiquinoid radical (see

Scheme 1), (ii) form strong metal−ligand bonds and

compounds with predictable structures owing to their

bis(bidentate) binding mode and large negative charge, and

iii) display a high capacity for chemical modiﬁcation, where a

wide range of donor atoms and ring-bound substituents can be

readily installed. Indeed, a number of dinuclear com-

chain compounds,

featuring semiquinoid bridging ligands have already

been shown to exhibit strong metal−radical interactions.

pyridylmethyl)amine) and the radical-bridged congeners

x−• n+

[

(Me TPyA) Fe ( L )] (x = 3, n = 1: R = OMe, Cl,

3 2 2

NO ; x = 1, n = 3: R = SMe ). While the identity of R is shown

to substantially inﬂuence the strength of superexchange

interactions through the diamagnetic tetraoxolene, it has

remarkably little impact on the strength of the metal−radical

interaction in the radical-bridged analogues despite the

presence of substituents with immensely diﬀerent electronic

character across the series. To our knowledge, this study

provides the ﬁrst systematic examination of how ligand

substitution inﬂuences J in semiquinoid-bridged compounds,

and it establishes synthetic design criteria to be considered in

the construction of future semiquinoid-based materials.

(

β,28,29

2w,30

plexes,

works

and extended net-

,31

Figure 1. Synthesis of complexes [(Me TPyA) Fe ( L)] (R = OMe, Cl, NO , SMe ), as observed in 1-OMe (n = 2), 1-Cl (n = 2), 1-NO (n =

), and 1-SMe (n = 4). X = H for R = OMe, Cl; X = Na for R = NO ; X = nothing for R = SMe .

2 2 2

Inorg. Chem. XXXX, XXX, XXX−XXX

4+/2+

3+/1+

Figure 2. Crystal structures of the cationic complexes [(Me TPyA) Fe ( L)]

(left) and [(Me TPyA) Fe ( L)]

(right), as observed in 1-

R·solvent and 2-R·solvent (R = OMe, Cl, NO , SMe ), respectively. Orange, green, yellow, red, blue, and gray spheres represent Fe, Cl, S, O, N,

and C atoms, respectively; H atoms are omitted for clarity.

isostructural Fe^I^Icomplexes bridged by tetraoxolene deriva-

tives with an array of substituents in the 3- and 6-positions,

RESULTS AND DISCUSSION

■

Syntheses, Structures, and Electrochemistry. With the

aim to better understand the eﬀects of ligand substitution on

electron delocalization and magnetic coupling in semiquinoid

radical-bridged magnets, we employed dinuclear complexes as

model systems owing to their structural simplicity and ease of

magnetic characterization. Speciﬁcally, we targeted a series of

ranging from electron-donating OMe groups to strongly

electron-withdrawing SMe groups.

Reaction of LH (R = OMe, Cl), Na [ L], or ^S^M^eL·

2CH COOH with 2 equiv each of Fe(BF ) ·6H O and

Me TPyA in MeCN, followed by puriﬁcation and subsequent

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Table 1. Selected Mean Interatomic Distances (Å) and Octahedral Distortion Parameter (∑_s_u_m) for the Cationic Complexes in

a b

-R·Solvent and 2-R·Solvent

-OMe·

2-OMe·

2.0MeCN

1-Cl·

2-Cl·

1-NO ·

1-SMe ·

2-SMe ·0.9MeCN·

.0MeCN

0.7H O

0.5Et O

4.0MeCN

2-NO₂

4.0MeCN

0.5Et O

Fe−N

2.210(2)

2.003(3)

2.203(3)

2.103(3)

1.294(5)

1.245(5)

1.270(4)

1.516(6)

1.429(6)

1.363(6)

1.436(4)

7.996(3)

106.3(5)

2.2466(6)

1.992(1)

2.112(1)

2.052(1)

1.313(2)

1.294(2)

1.304(2)

1.470(2)

1.407(2)

1.389(2)

1.422(1)

7.8652(9)

125.8(2)

2.195(3)

2.051(5)

2.187(3)

2.119(3)

1.269(6)

1.238(8)

1.254(5)

1.532(9)

1.409(7)

1.389(9)

1.443(5)

8.018(2)

110.2(7)

2.234(2)

1.991(3)

2.130(3)

2.060(2)

1.304(5)

1.283(5)

1.293(4)

1.468(5)

1.403(5)

1.395(6)

1.422(3)

7.823(2)

112.3(4)

2.1998(5)

2.0372(8)

2.1782(9)

2.1077(6)

1.258(2)

1.240(2)

1.249(1)

1.532(2)

1.411(2)

1.386(2)

1.443(1)

8.0219(8)

109.8(2)

2.2223(7)

2.010(2)

2.143(1)

2.076(1)

1.293(2)

1.278(2)

1.285(2)

1.465(2)

1.404(2)

1.396(2)

1.422(2)

7.9330(9)

111.7(2)

2.196(2)

2.039(2)

2.210(3)

2.125(2)

1.254(4)

1.240(3)

1.247(3)

1.526(4)

1.404(4)

1.388(4)

1.439(3)

8.017(3)

2.227(1)

2.038(2)

2.140(2)

2.084(1)

1.292(3)

1.280(3)

1.286(2)

1.465(3)

1.416(3)

1.408(3)

1.430(2)

7.9972(6)

110.1(3)

Fe−O1

Fe−O2

Fe−O

C1−O1

C2−O2

C−O

C1−C2

C2−C3

C3−C1A

C−C

Fe···Fe

∑_s_u_m

111.2(3)

See Figure 2 for the atomic numbering scheme. Average distances for speciﬁc types of bonds are shown in bold. Intramolecular Fe···Fe distance.

Octahedral distortion parameter (∑_s_u_m) = sum of the absolute deviation from 90° for the 12 cis angles in [FeN O ].

crystallization, aﬀorded the compounds [(Me TPyA) -

compounds as the diamagnetic ^RL^x⁻(x = 2: R = OMe, Cl,

OMe

Fe₂( L)](BF₄)₂(1-OMe), [(Me TPyA) Fe ( L)](BF ) ·

NO ; x = 0: R = SMe ). Finally, the Fe complexes in 1-OMe·

4 2

.3H O (1-Cl), [(Me TPyA) Fe ( L)](BF₄)₂(1-NO ),

and [(Me TPyA) Fe ( L)](BF ) (1-SMe ) as dark green

4.0MeCN, 1-Cl·0.7H O, 1-NO ·4.0MeCN, and 1-SMe ·

2 2 2

SMe

4.0MeCN feature a mean intramolecular Fe···Fe distance

4 4

crystalline solids (see Figure 1). Slow diﬀusion of Et O vapor

ranging from 7.996(3) to 8.0219(8) Å, in accord with values

into concentrated MeCN solutions of 1-R (R = OMe, Cl, NO₂,

reported for other high-spin benzoquinoid-bridged Fe

compounds.

28d,29a,36

SMe ) gave dark orange plate-shaped crystals of 1-OMe·

Notably, the similar structural metrics

.0MeCN, 1-Cl·0.7H O, 1-NO ·4.0MeCN, and 1-SMe ·

for 1-OMe·4.0MeCN, 1-Cl·0.7H O, 1-NO ·4.0MeCN, and 1-

2 2

.0MeCN suitable for single-crystal X-ray diﬀraction analysis.

, with

SMe ·4.0MeCN indicate that the solid-state structure of this

All compounds crystallized in the triclinic space group P1

the exception of 1-SMe ·4.0MeCN, which crystallized in the

family of Fe complexes is minimally aﬀected by the nature of

the benzoquinoid substituents. Furthermore, comparison of

monoclinic space group P2 /c (see Tables S1 −S4). In general,

the average Fe−O distances and the octahedral distortion

the structures of [(Me TPyA) Fe ( L)] (n = 2: R = OMe,

parameters (∑

)

in 1-R·solvent to values reported for

sum

Cl, NO ; n = 4: R = SMe ) consist of two crystallographically

analogous tetraoxolene-bridged Fe complexes bearing TPyA

equivalent [(Me TPyA)Fe] moieties connected by a

deprotonated L (x = 2: R = OMe, Cl, NO ; x = 0: R =

capping ligands (Fe−O = 2.089(2)−2.138(2) Å; ∑

sum

x−

28,37

121.6(5)−185.4(3)°)

illustrates the lack of signiﬁcant

SMe ) bridging ligand, with a crystallographic site of inversion

steric eﬀects imposed by the Me groups of Me TPyA on the

x−

located at the center of L (see Figure 2). Each Fe center

resides in a distorted octahedral coordination environment

comprised of two cis-oriented O atoms from L (x = 2: R =

OMe, Cl, NO ; x = 0: R = SMe ) and four N atoms from the

Fe coordination environment in 1-R·solvent.

To probe the eﬀects of tetraoxolene substitution on the

electronic structure of 1-R and to assess the feasibility of

isolating the radical-bridged congeners, cyclic voltammetry

experiments were performed for MeCN solutions of these

compounds at 298 K. The cyclic voltammograms of 1-R are

depicted in Figure 3. Each voltammogram exhibits two

reversible processes at E₁_/₂= +0.22 and −1.11 V, +0.33 and

x−

Me TPyA capping ligand.

The mean Fe−N and Fe−O bond distances across the series

fall in the ranges of 2.195(3)−2.210(2) Å and 2.103(3)−

.125(2) Å, respectively, consistent with reported distances for

high-spin S = 2 Fe centers in similar coordination environ-

ments (see Table 1).

−0.86 V, +0.41 and −0.58 V, and +0.45 and −0.47 V versus

28d,35−37

0/1+

Within the bridging ligand, the

[Cp Fe]

for 1-OMe, 1-Cl, 1-NO , and 1-SMe , respec-

2 2

average C−O distance ranges from 1.247(3) to 1.270(4) Å

tively. On the basis of precedent in other benzoquinoid-

bridged Fe₂complexes, we assign the wave at negative

across the series, which falls between the value expected for a

R x−/(x+1)−•

single and a double bond. Moreover, the mean C1−C2 bond

distance of 1.516(6)−1.532(9) Å, typical for a single bond, is

substantially longer than the average C2−C3 and C3−C1A

bond distances, which range from 1.404(4) to 1.429(6) Å and

from 1.363(6) to 1.389(9) Å, respectively, across the series.

These collective distances strongly suggest that the bridging

ligand in 1-OMe·4.0MeCN, 1-Cl·0.7H O, 1-NO ·4.0MeCN,

potential to the ligand-centered redox process L

= 2: R = OMe, Cl, NO ; x = 0: R = SMe ) and the wave at

III

positive potential to the metal-based Fe Fe /Fe Fe

28d,29a,b,40

couple.

The variation of E₁_/₂with the identity of

the substituents for both ligand- and metal-based processes

correlates linearly with the Hammett substituent constant (σ_p),

which quantiﬁes the electronic properties of substituents by

considering both inductive and resonance eﬀects (see Figures

and 1-SMe ·4.0MeCN is best described as two localized 6-π-

II II

II III

electron fragments connected by two C−C single bonds. The

observation of two drastically diﬀerent Fe−O bond distances,

speciﬁcally shorter Fe−O1 distances of 2.003(3)−2.051(5) Å

and longer Fe−O2 distances of 2.1782(9)−2.210(3) Å, further

supports the formulation of the bridging ligand in these

S1 and S2). In particular, E₁_/₂for the Fe Fe /Fe Fe couple

is anodically shifted by 0.23 V as the substituents are varied

from electron-donating OMe groups to strongly electron-

withdrawing SMe groups, while the concurrent anodic shift

in E₁_/₂for the ligand-centered process is 0.64 V. The more

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

OMe

compounds [(Me TPyA) Fe ( L)](BF )·2.0MeCN (2-

OMe·2.0MeCN), [(Me TPyA) Fe ( L)](BF )·0.5Et O (2-

Cl·0.5Et O), [(Me TPyA) Fe ( L)](BF ) (2-NO ), and

SMe

[

(Me TPyA) Fe ( L)](BF ) ·0.9MeCN·0.5Et O (2-SMe ·

3 2 2 4 3 2 2

.9MeCN·0.5Et O). Subsequent drying of these crystals

under reduced pressure gave the desolvated forms 2-R (R =

OMe, Cl, NO , SMe ) in moderate yields of 57−88%. IR

spectroscopy for solid samples reveals changes in peak

−

frequencies and intensities in the 1200−1600 cm spectral

range when moving from 1-R to 2-R (R = OMe, Cl, NO ,

SMe ) (see Figures S17 −S22), suggesting diﬀerent net C−C

and/or C−O bond order for the tetraoxolene bridging ligand

in these two series of compounds.

OMe

L)]⁺,

The cationic complexes [(Me TPyA) Fe (

[

(Me TPyA) Fe ( L)] , [(Me TPyA) Fe ( L)] , and

SMe

(Me TPyA) Fe ( L)] in 2-R·solvent feature very similar

structures to those in 1-R·solvent. The two Fe sites in each

molecule are related through a crystallographic inversion

center, with the exception of slightly inequivalent Fe centers

in [(Me TPyA) Fe ( L)] stemming from crystal packing of

−

sum

the (BF ) counterion. The nearly identical values of ∑

for 1-R·solvent and 2-R·solvent illustrate that the coordination

geometry at Fe is not signiﬁcantly aﬀected by the substituents

and redox state of the bridging ligand. In contrast, close

comparison of the bond distances between the two series of

compounds reveals several key diﬀerences. First, the mean C−

C bond distance decreases slightly by 0.6−1.5%, from

.436(4)−1.443(5) to 1.422(1)−1.430(2) Å, in moving from

-R·solvent to 2-R·solvent. Moreover, the mean C−O bond

distance for 2-R·solvent varies from 1.285(2) to 1.304(2) Å

across the series, which represents a 2.7−3.1% increase as

compared to the values obtained for the oxidized analogues.

These structural changes upon reduction reﬂect a net increase

in C−C bond order and a net decrease in C−O bond order, in

Figure 3. Cyclic voltammograms for solutions of 1-R in MeCN

containing 100 mM (Bu N)(PF ) supporting electrolyte, collected at

x−

98 K; R = OMe (red), Cl (green), NO (blue), SMe (gold). Scan

2 2

agreement with a ligand-centered reduction from

−

−1

rate = 100 mV s for R = OMe, Cl, NO ; 25 mV s^f^o^r^R⁼^S^M^e2^.

R (x+1)−•

(x = 2: R = OMe, Cl, NO ; x = 0: R = SMe ), as has

Black vertical lines and arrows denote the open-circuit potentials and

scan directions, respectively.

been observed for similar benzoquinoid-bridged metal

8d,29a,b

complexes.

Furthermore, the mean Fe−O bond

distance of 2.052(1)−2.084(1) Å in the 2-R·solvent series is

1.5−2.8% shorter than corresponding distances in 1-R·solvent,

and the mean intramolecular Fe···Fe distance decreases to a

similar degree in moving from 1-R·solvent to 2-R·solvent (R =

pronounced change in E₁_/₂for ^RL^x⁻^/⁽^x⁺¹⁾⁻^•(x = 2: R = OMe,

Cl, NO ; x = 0: R = SMe ) is consistent with the substituents

primarily aﬀecting the energy levels of the ligand; however, the

clear variation in the metal-based redox potential indicates that

the electronic properties of the substituents signiﬁcantly

modulate the metal−ligand interactions as well. The remaining

OMe, Cl, NO

, SMe ). Together, these observations highlight

the stronger Fe−O interactions in the radical-bridged

x−• n+

complexes [(Me

OMe, Cl, NO ; x = 1, n = 3: R = SMe

d i a m a g n e t i c l i g a n d - b r i d g e d a n a l o g u e s

TPyA) Fe

( L )] (x = 3, n = 1: R =

2 2

/1+

oxidation event at +0.56 V versus [Cp Fe]

for 1-OMe is

) than in the

II III

III III

assigned to the metal-based Fe Fe /Fe Fe oxidation,

x− n+

whereas the additional reversible redox event at E = −1.17

[(Me TPyA) Fe ( L )] (x = 2, n = 2: R = OMe, Cl,

3 2 2

/1+

V versus [Cp Fe]

for 1-SMe is assigned to the ligand-

NO ; x = 0, n = 4: R = SMe ) owing to the increase in anionic

2 2

SMe

−•/2−

based

couple.

charge. As a result of stronger interactions between the Fe

Together, the cyclic voltammetry measurements suggest that

centers and the bridging ligand in 2-R·solvent, the average Fe−

N bond distance increases slightly by 1.0−1.8% in moving

from 1-R·solvent to 2-R·solvent (R = OMe, Cl, NO , SMe ).

I I

t h e r a d i c a l - b r i d g e d F e

c o m p l e x e s

x−• n+

[

(Me TPyA) Fe ( L )] (x = 3, n = 1: R = OMe, Cl,

NO ; x = 1, n = 3: R = SMe ) should be chemically accessible.

Mossbauer Spectroscopy. To conﬁrm the presence of a

Toward this end, dark green MeCN solutions of 1-R were

bridging ligand-centered reduction and further probe the

treated with stoichiometric quantities of the reductant Cp Co

eﬀects of tetraoxolene substitution on the electronic structures

to give red-brown (R = OMe, Cl, NO ) or green-brown (R =

of 1-R and 2-R, zero-ﬁeld Fe Mo

collected for polycrystalline samples at 80 K. The Mo

ssbauer spectra were

ssbauer

SMe ) solutions. H NMR analysis revealed the formation of

new paramagnetic species (see Figures S3 −S16 ) with generally

more broad peaks relative to the oxidized analogues.

spectra for 1-R exhibit a single sharp doublet (see Figure 4,

top). Lorentzian ﬁts to the data give an isomer shift of δ =

1.076(3)−1.111(3) mm s and a quadrupole splitting of ΔE

= 2.36(2)−2.61(2) mm s across the series (see Table 2).

−

Subsequent diﬀusion of Et O vapor into these solutions

−1

aﬀorded red-orange (R = OMe, Cl, NO ) or orange (R =

SMe ) plate-shaped crystals of the one-electron reduced

These parameters are consistent with high-spin Fe centers in

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 4. Zero-ﬁeld ⁵⁷Fe Mo

blue), SMe (gold). Black crosses represent experimental data, and colored lines correspond to Lorentzian ﬁts to the data. Each vertical scale bar

ssbauer spectra for polycrystalline samples of 1-R (top) and 2-R (bottom) at 80 K; R = OMe (red), Cl (green), NO₂

(

represents an absorption of 1%.

Table 2. Summary of Parameters Obtained from Fits to Zero-Field ⁵⁷Fe Mo

ssbauer Spectra for 1-R and 2-R at 80 K

-OMe

2-OMe

1-Cl

2-Cl

1-NO₂

2-NO₂

1-SMe₂

2-SMe₂

δ (mm s⁻¹)

1.081(3)

2.61(2)

0.263(3)

0.306(3)

1.108(3)

2.28(2)

0.330(3)

0.256(3)

1.076(3)

2.57(2)

0.249(2)

0.240(2)

1.118(3)

2.31(2)

0.286(3)

0.243(3)

1.087(3)

2.36(2)

0.371(4)

0.362(4)

1.108(3)

2.33(2)

0.274(3)

0.253(3)

1.111(3)

2.38(2)

0.302(4)

0.315(4)

1.121(3)

2.30(2)

0.319(3)

0.291(3)

ΔE (mm s⁻¹)

Γ_L(mm s⁻¹)

Γ_R(mm s⁻¹)

See Figure 4 for the experimental data and corresponding ﬁts using Lorentzian doublets. The uncertainties in the parameter values were estimated

from a combination of experimental and statistical ﬁtting errors as described in the Experimental Section. Γ denotes the width of the left line of a

quadrupole doublet. Γ denotes the width of the right line of a quadrupole doublet.

a pseudo-octahedral geometry and agree well with values

origin of this asymmetry requires detailed variable-temperature

analysis that is beyond the scope of this work. Overall, the

29,36,37,40,42

reported for dinuclear complexes

and extended

solids featuring Fe ions in similar coordination environ-

ments. The spectra for 2-R display a similar quadrupole

doublet (see Figure 4, bottom), with an isomer shift of δ =

foregoing Mo

ssbauer spectroscopic analysis corroborates the

assignment of the Fe complexes in 1-R as [(Me TPyA) -

x− n+

Fe ( L )] (x = 2, n = 2: R = OMe, Cl, NO ; x = 0, n = 4: R

−

x−• n+

.108(3)−1.121(3) mm s and a quadrupole splitting of ΔE_Q

= SMe ) and 2-R as [(Me TPyA) Fe ( L )] (x = 3, n = 1:

2 3 2 2

−

2.28(2)−2.33(2) mm s for the series (see Table 2). The

R = OMe, Cl, NO ; x = 1, n = 3: R = SMe ), in accord with

nearly identical isomer shifts in 1-R and 2-R conﬁrm the one-

electron reduction from 1-R to 2-R to be centered on the

bridging ligand. Notably, compounds 2-R exhibit a smaller

quadrupole splitting than their corresponding oxidized

analogues 1-R. This diﬀerence is most pronounced for the

OMe- and Cl-substituted derivatives (see Table 2) and likely

single-crystal X-ray diﬀraction analysis.

UV−Vis−NIR Spectroscopy. To gain further insight into

the electronic structures of 1-R and 2-R, UV−vis−NIR (NIR =

near-infrared) absorption spectra were collected for MeCN

solutions at 298 K. The spectra for all compounds show an

intense absorption band centered at 261−263 nm (ε

max

−1

stems from the change in ligand ﬁeld at the Fe centers

28 700−37 700 M

cm ), as depicted in Figure 5.

associated with the reduction of the bridging ligand. The slight

asymmetry of the quadrupole doublet for 1-OMe and 2-R may

be attributed to several eﬀects, including slow spin relaxation of

Considering the identical feature in the spectrum for Me TPyA

(see Figure S23 ) and the invariance of λ_m_a_xand ε_m_a_xon the

oxidation state of the bridging ligand, we assign this absorption

44a,45

S = 2 Fe ions, sample texture eﬀects,

and lattice

to a π−π* transition occurring within the Me TPyA capping

44a,45a

vibrational anisotropy.

A complete understanding of the

ligand. The spectra for 1-R display an additional strong band

Inorg. Chem. XXXX, XXX, XXX−XXX

Figure 6. Variable-temperature dc magnetic susceptibility data for 1-

R, collected under an applied ﬁeld of 1 T. Colored circles represent

experimental data, and black lines correspond to ﬁts to the data.

signiﬁcantly aﬀected by the nature of the substituents, although

the establishment of a clear trend is complicated by broad

features and diﬀerences in overall charges. Along these lines,

the diﬀuse reﬂectance spectra collected for microcrystalline

samples of 1-R and 2-R (see Figures S34 −S41) show similar

features as the solution spectra, suggesting that the substituents

also play an important role in determining the electronic

properties of these compounds in the solid state.

Static Magnetic Properties. To probe and compare

magnetic interactions in 1-R and 2-R, variable-temperature dc

magnetic susceptibility data were collected for microcrystalline

samples under an applied ﬁeld of 1 T. The resulting plots of

Figure 5. UV−Vis spectra for solutions of 1-R (top) and 2-R

(

bottom) in MeCN at 298 K.

in the near-UV region that exhibits a progressive bathochromic

−

−1

shift (λ_m_a_x= 298−350 nm; ε = 25 800−34 700 M cm )

max

as the electron-donating ability of the substituents increases

(

see Figure 5, top). On the basis of the similarity with the

spectra for the free ligands (see Figures S24 −S27 ) and

χ T versus T for 1-R are shown in Figure 6. At 300 K, the

−1

literature precedent for other complexes bearing quinoid

values of χ T = 6.90, 7.25, 7.32, and 7.47 cm K mol for 1-

ligands, this band is ascribed to a π−π* transition within the

OMe, 1-Cl, 1-NO , and 1-SMe , respectively, correspond to

bridging ligand. Notably, the spectra for 2-OMe, 2-Cl, and 2-

two magnetically noninteracting S = 2 Fe centers with g =

2.14, 2.20, 2.21, and 2.23, respectively. As the temperature is

decreased, the data for 1-OMe and 1-Cl undergo a gradual

then rapid increase, reaching maximum values of 9.28 and 9.82

NO feature two broad tetraoxolene-centered π−π* bands

−

−1

at λ = 313−334 nm (ε = 12 000−16 700 M cm ) and

max

−

−1

λ_m_a_x= 406−478 nm (ε = 10 200−13 800 M cm ) for the

max

−1

series, which are signiﬁcantly weaker than those for the

cm K mol at 10 K, respectively. This increase in χ T with

oxidized analogues (see Figure 5, bottom). In contrast, the

decreasing temperature is indicative of weak ferromagnetic

coupling between the Fe centers via a superexchange

spectrum for 2-SMe exhibits a markedly diﬀerent proﬁle than

those for 2-OMe, 2-Cl, and 2-NO . Speciﬁcally, multiple

mechanism through the diamagnetic tetraoxolene ligand to

overlapping tetraoxolene-centered π−π* bands are present in

the 300−500 nm range, with λ = 369 nm (ε = 21 000

give an S = 4 ground state. Below 10 K, χ T decreases sharply

−1

to minimum values of 4.54 and 4.76 cm K mol at 2.0 K for

1-OMe and 1-Cl, respectively, likely the result of Zeeman

splitting, zero-ﬁeld splitting, and/or weak intermolecular

interactions. In contrast, the temperature dependence of χ_MT

for 1-NO and 1-SMe is not as pronounced as observed for 1-

max

−

−1

cm ). This discrepancy most likely arises from the

diﬀerent charges of the Fe ₂complexes in these compounds.

Close comparison of the vis−NIR region of the spectra

obtained for 1-R and 2-R reveals that compounds 1-R

generally feature stronger absorption in the NIR region than

do 2-R, while additional bands are observed between 525 and

OMe and 1-Cl. Rather, the χ T data for 1-NO show a gradual

−1

increase to a maximum value of χ T = 7.52 cm K mol at 15

50 nm in the spectra for 2-R (see Figures S28 −S33). We

tentatively assign these new bands to charge-transfer

transitions based on the molar absorptivity values (ε

K and then undergo a sharp decline to a minimum value of

−1

3.79 cm K mol at 2.0 K. Similarly, the value of χ T for 1-

SMe is relatively constant above 60 K but then decreases

max

−

−1

770−3960 M cm ). Taken together, the spectral changes

gradually as the temperature is decreased from 60 K, and more

−1

observed upon reduction of the bridging ligand are in good

agreement with the associated color change from dark green

sharply below 10 K, to a minimum value of 3.93 cm K mol

at 2.0 K. The diﬀerent magnetic behavior observed for 1-NO₂

for 1-R to red-brown (R = OMe, Cl, NO ) or green-brown (R

and 1-SMe than that observed for 1-OMe and 1-Cl likely

stems from weaker magnetic exchange interactions through

diamagnetic bridging ligands bearing electron-withdrawing

SMe ) for 2-R. Furthermore, these studies demonstrate that

the solution electronic structures of 1-R and 2-R are

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Table 3. Summary of Parameters Obtained from Fits to Magnetic Data for 1-R and 2-R

-OMe

1-Cl

1-NO₂

1-SMe₂

2-OMe

2-Cl

2-NO₂

2-SMe₂

D (cm⁻¹)

−4.8(4)

2.11(3)

+1.2(2)

−8.0(6)

2.16(3)

+1.2(2)

−7.0(4)

2.20(2)

+0.3(1)

−15(2)

2.22(3)

−16.9(2)

2.11

−57(10)

50(1)

3.3(6)

−12.4(1)

−20.7(3)

−19.9(3)

2.14

−60(7)

2.23

−58(6)

2.36

−65(8)

J (cm⁻¹)

+0.3(1)

(cm⁻¹

)

41(1)

10(2)

38(1)

18(3)

33(1)

110(30)

eff

τ₀(ns)

These values were obtained from a simultaneous ﬁt to low-temperature magnetization and dc magnetic susceptibility data, as described in the

Experimental Section . This value of D was obtained from an individual ﬁt to low-temperature magnetization data using ﬁxed values of g and J, as

described in the Experimental Section and Table S5 . These values were obtained from ﬁts to low-temperature magnetization data, as described in

the Experimental Section. These values of J were obtained from ﬁ ts to dc magnetic susceptibility data in the temperature range 60−300 K using

the spin Hamiltonian provided in eq 3 in the Experimental Section . These values were obtained from ac magnetic susceptibility measurements

collected under zero applied dc ﬁeld, as described in the Experimental Section .

32,33

substituents, as has been previously observed,

and/or

larger zero-ﬁeld splitting.

To assess the presence of magnetic anisotropy and conﬁrm

the spin ground states in 1-R, low-temperature magnetization

data were collected at selected dc ﬁelds (see Figures S42 −

S45 ). The saturation magnetization values of the resulting

−

isoﬁeld curves fall in the range of M = 5.26−6.37 μ mol

across the series, in accord with the presence of an S = 4

ground state and signiﬁcant magnetic anisotropy for all

compounds.

To quantify the magnetic exchange interactions and

magnetic anisotropy in 1-R, the variable-temperature dc

magnetic susceptibility data and low-temperature magnet-

ization data were simultaneously ﬁt to the Van Vleck equation

according to the spin Hamiltonian provided in eq 1 (see

Experimental Section ). Fits to the data give exchange constants

of J = +1.2(2), +1.2(2), +0.3(1), and +0.3(1) cm⁻for 1-OMe,

-Cl, 1-NO , and 1-SMe , respectively, along with values of the

2 2

Figure 7. Variable-temperature dc magnetic susceptibility data for 2-

R, collected under an applied ﬁeld of 1 T. Colored circles represent

experimental data, and black lines correspond to ﬁts to the data.

axial zero-ﬁeld splitting parameter D ranging from −4.8(4) to

−

15(2) cm and g = 2.11(3)−2.22(3) across the series (see

Table 3). Note that the values of g obtained from these ﬁts are

in excellent agreement with those estimated from the χ_MT

values at 300 K. Furthermore, the magnitude and sign of J for

decreasing the temperature to reach maximum values of

−1

.04, 8.44, 8.60, and 9.41 cm K mol at 30, 35, 35, and 35 K

-R is consistent with other examples of benzoquinoid-bridged

for 2-OMe, 2-Cl, 2-NO , and 2-SMe , respectively. This

28d,40

Fe complexes.

Interestingly, the value of J is identical

upturn in χ T with decreasing temperature suggests signiﬁcant

for 1-OMe and 1-Cl, and this value is 4 times larger than the

magnetic coupling between the two Fe centers and semi-

value of J = +0.3(1) cm⁻obtained for both 1-NO and 1-

quinoid radical ligand via a direct exchange mechanism. To

quantify this interaction and to determine whether it is

ferromagnetic or antiferromagnetic in nature, the data were ﬁt

in the temperature range of 60−300 K to the Van Vleck

equation according to the spin Hamiltonian provided in eq 3

SMe , demonstrating that strongly electron-withdrawing ligand

substituents decrease the magnetic coupling strength through a

diamagnetic tetraoxolene bridge. The lack of a linear

correlation between J and the Hammett substituent constant

^σp ^f^o^r^t^h^e¹^-^R^s^e^rⁱ^e^s^m^a^y^b^e^a^t^t^rⁱ^b^u^t^e^d^t^o^t^h^e^a^bⁱ^lⁱ^t^y^o^f^h^a^l^o^g^eⁿ

substituents to donate a lone pair of electrons. Speciﬁcally, the

electron-donating resonance eﬀects may outweigh the

electron-withdrawing inductive eﬀects for the Cl substituents

and thus render the electronic properties of L close to that

(see Experimental Section) to give exchange constants of J =

−1

−

57(10), −60(7), −58(6), and −65(8) cm for 2-OMe, 2-

Cl, 2-NO , and 2-SMe , respectively (see Table 3), with g =

.09(4), 2.14(3), 2.17(3), 2.23(4), respectively. Note that the

Cl 2−

introduction of an Fe ···Fe superexchange coupling term in

the spin Hamiltonian does not signiﬁcantly a ﬀ ect the metal−

radical exchange coupling constants (see Table S6 ). Interest-

ingly, the antiferromagnetic coupling in 2-Cl is in contrast to

the ferromagnetic coupling (J = +19 cm ) for the related

complex [(TPyA) Fe ( L )] . This drastically diﬀerent

OMe 2−

when coordinated to metal ions. Indeed, the similar

ssbauer parameters

UV−vis absorption spectra and Mo

obtained for 1-OMe and 1-Cl support this hypothesis.

Furthermore, similar magnetic properties have been observed

for paramagnetic metal complexes featuring aromatic ligands

−

II Cl 3−•

28d

with OMe and Cl substituents.

magnetic behavior may stem from structural diﬀerences

The plots of χ T versus T for 2-R exhibit a markedly

imposed by the bulkier Me TPyA capping ligand in 2-R;

diﬀerent proﬁle than those for 1-R (see Figure 7). The values

of χ T at 300 K are 6.10, 6.41, 6.59, and 7.00 cm K mol for

however, the lack of a crystal structure for

−1

II Cl 3−•

[(TPyA) Fe ( L )] precludes further insight.

2 2

-OMe, 2-Cl, 2-NO , and 2-SMe , respectively. As the

The rapid decline in χ T below 30 K can be attributed to

temperature is decreased to 150 K, χ T undergoes a gradual

Zeeman splitting, zero-ﬁeld splitting, and/or weak intermo-

increase and then increases nearly linearly upon further

lecular interactions. Indeed, low-temperature magnetization

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

data for 2-R reveal the presence of substantial zero-ﬁeld

splitting, with ﬁts to the data giving values of D = −16.9(2),

−

12.4(1), −20.7(3), and −19.9(3) cm for 2-OMe, 2-Cl, 2-

NO , and 2-SMe , respectively, and g = 2.11, 2.14, 2.23, 2.36,

respectively (see Figures S46 −S49 and Tables 3 and S7 −S10).

Note that the values of g obtained from the low-temperature

magnetization data agree well with those obtained from the

variable-temperature dc susceptibility data. Interestingly, for

both the 1-R and 2-R series of compounds, the value of g

follows the trend R = OMe < Cl < NO < SMe . This increase

in g across the series may stem from increasing electron-

withdrawing ability of the substituents, although we note that

other eﬀects, such as those arising from crystal packing, cannot

be ruled out.

The values of J for 2-R represent 48−217-fold increases

from those observed for the 1-R series (see Table 3),

demonstrating the much stronger Fe −radical direct exchange

coupling than Fe ···Fe superexchange coupling, similar to

previous observations for dinuclear benzoquinoid-bridged

8d,29a,b

complexes.

However, the substituents bear a remark-

ably insigniﬁcant eﬀect on the magnitude of metal−radical

coupling in 2-R. This is in contrast with the clearly distinct

χ_MT versus T proﬁles and values of J for R = OMe, Cl and R =

NO , SMe , observed for the 1-R series. These results suggest

that the eﬀects of ligand substituents on magnetic coupling

strength in tetraoxolene-bridged complexes are highly depend-

ent on the oxidation state of the bridging ligand. Accordingly,

the contributions from substituents to ligand-based orbitals

that mediate magnetic coupling between paramagnetic metal

centers through a diamagnetic bridge are likely signiﬁcantly

greater than such contributions to the ligand-based magnetic

orbital of radical states. This discrepancy may be attributed to

the more favorable donation of electron density into the

diamagnetic ring than into the radical ring, owing to the greater

negative charge and electron delocalization in the latter.

Density Functional Theory Calculations. To provide

greater insight into the eﬀects of ligand substitution on J in 1-R

and 2-R, broken-symmetry density functional theory (DFT)

calculations were performed. The possible spin couplings were

explored by calculating M = 4 and M = 0 determinants for

Figure 8. Calculated spin densities for 1-Cl (top) and 2-Cl (bottom)

shown at 0.01 isovalue with α-spin in magenta and β-spin in cyan.

Green, red, blue, and gray spheres represent Cl, O, N, and C atoms,

−

respectively; H atoms and (BF

)

anions are omitted for clarity.

Calculations were performed on the high-spin M = 4 and the lowest-

energy M_S

= /₂broken-symmetry solutions for 1-Cl and 2-Cl,

respectively, using the experimental single-crystal X-ray structure

geometries. See Supporting Information for analogous spin densities

for other complexes.

than in 1-OMe and 1-Cl. The presence of large β-spin

densities on the coordinating O atoms and the four C atoms

on the tetraoxolene ring to which they are attached in 2-R are

consistent with stronger magnetic coupling in the radical-

bridged Fe complexes that is antiferromagnetic in nature (see

the 1-R series and M = / , / , and / determinants for the

Figures 8, bottom, and S53 −S55 and Tables S17 −S20). The β-

spin density is distributed over all four O donor atoms in 2-

NO and 2-SMe , whereas the spin density is largely localized

-R series. The DFT energies reveal a small energy diﬀerence

between the M = 4 and M = 0 spin states and weak magnetic

coupling for 1-R (see Table S11) that are consistent with the

small values of J obtained from magnetic measurements, albeit

the sign is not correctly predicted for all compounds, likely

stemming from the weak interactions. In contrast, computa-

on one set of O atoms in 2-OMe and 2-Cl, suggesting diﬀerent

dominant exchange pathways within the series. Nevertheless,

the computed spin densities for 2-R are in accord with the

minimal impact of bridging ligand substitution on the magnetic

coupling strength, as negligible spin density is located on the

substituents and the C atoms to which they are bonded.

Inspection of the molecular orbitals further supports the

insigniﬁcant eﬀects of ligand substitution on the value of J for

2-R . The molecular orbital energy-level diagrams (see Figures

S56 −S63) for 1-R and 2-R reveal that the unpaired electron on

the radical bridging ligand in 2-R occupies a bonding β-spin π

orbital that has negligible contribution from the substituents

and the associated C atoms of the ring (see Figures S64 −S67).

tions reveal that the M = / spin state is lowest in energy for

all members of the 2-R series, and the calculated exchange

−

constants of J = −58 to −85 cm are in good agreement with

experimental values (see Table S12 ).

The computed spin density distributions (M = 4 solution

for 1-R and M = / solution for 2-R) show that, for both

series of Fe complexes, the substituents at the 3- and 6-

positions on the bridging ligand do not contribute signi ﬁcantly

to the magnetic orbitals (see Figures 8 and S50 −S55 ). The

analogous α-spin densities localized on the coordinating O

atoms and Fe atoms in 1-OMe and 1-Cl are consistent with

identical values of J for these compounds (see Figures 8, top,

and S50 and Tables S13 and S14 ). Furthermore, the smaller α-

spin densities on the four O donors in 1-NO and 1-SMe (see

These results are in accord with those obtained for free

Cl 3−• 28e

Because the overlap between this ligand-based π

orbital and Fe-based t orbitals predominantly determines the

magnitude of the exchange coupling in these compounds, the

nature of the bridging ligand substituents should have minimal

eﬀect. Moreover, analysis of spin density distributions in

related mono- and dinuclear transition-metal complexes

Figures S51 and S52 and Tables S15 and S16 ) are in accord

with weaker ferromagnetic coupling in 1-NO and 1-SMe₂

Inorg. Chem. XXXX, XXX, XXX−XXX

The Supporting Information is available free of charge at

Article

S98). Note, however, that this ostensible substituent depend-

ence of U_e_f_fis complicated by a nonconstant τ across the

series, which inﬂuences U . Alternatively, inspection of the

eff

relaxation time τ across the series at 2.00, 4.00, and 6.00 K

reveals the absence of a discernible trend (see Figures S99 −

S101 ). As such, the data reported here do not conclusively

show the relaxation dynamics of 2-R to be dependent on

tetraoxolene substitution.

At lower temperatures, the data begin to deviate from

linearity and ﬁnally reach a plateau below 3.00, 2.75, and 4.00

K for 2-Cl, 2-NO , and 2-SMe , respectively, suggesting the

presence of additional fast relaxation processes, such as

quantum tunneling and/or spin−spin relaxation, that shortcut

the energy barrier. These additional relaxation processes are

most prominent for 2-SMe , as the temperature-dependent

Figure 9. Variable-frequency out-of-phase ac susceptibility data for 2-

OMe, collected under zero applied dc ﬁeld in the temperature range

of 2.00−6.75 K (left), and corresponding Arrhenius plot of relaxation

time (right). Colored lines are a guide to the eye, and the black line

corresponds to a linear ﬁt to the data.

features in the plot of χ ″ versus ν are observed at much

higher frequencies than those for 2-OMe, 2-Cl, and 2-NO .

This discrepancy may arise from the diﬀerent charges of the

Fe complexes in these compounds and/or the steric bulk of

the SMe groups.

CONCLUSIONS AND OUTLOOK

featuring terminal or bridging semiquinone, iminosemiqui-

■

III 49

none, and diiminosemiquinone radical ligands and Fe ,

The foregoing results demonstrate the insigniﬁcant eﬀects of

III 50

II 51

II 52

II53

Co , Ni , Ru , and Pt

metal centers of diﬀerent

ligand substitution on the metal−radical exchange coupling in

geometries reveals similar insigniﬁcant contribution from the

tetraoxolene radical-bridged Fe complexes. Moving from R =

substituents at 3- and 6-positions on the quinoid ring to the

OMe or Cl to R = NO or SMe in the Fe complexes featuring

total spin density. This suggests that the minimal inﬂuence of

the diamagnetic form of the bridging ligand leads to a decrease

−

the ring-bound substituents on magnetic coupling strength

in coupling strength from J = +1.2(2) cm to J = +0.3(1)

−

^m^a^yⁿ^o^t^b^e^lⁱ^mⁱ^t^e^d^t^o^t^e^t^r^a^o^x^o^l^eⁿ^e^-^b^rⁱ^d^g^e^d^F^e2 complexes, as

cm . In stark contrast, the one-electron-reduced, radical-

observed in 2-R, but could generally apply to metal-

semiquinoid compounds.

bridged complexes exhibit exchange coupling constants of J =

−

57(10), −60(7), −58(6), and −65(8) cm for R = OMe,

Dynamic Magnetic Properties. Finally, the presence of

large negative values of D for 1-R and 2-R prompted us to

probe potential slow magnetic relaxation in these compounds.

Accordingly, variable-frequency ac magnetic susceptibility data

were collected under zero applied dc ﬁeld in the temperature

range of 2.00−8.00 K. For 1-R, only onsets of peaks in the out-

Cl, NO , and SMe , respectively. This insensitivity of J on

ligand substitution is rationalized through electronic structure

calculations, which show minimal spin density on R and the

associated C atoms of the tetraoxolene ring. This systematic

investigation suggests that substitution of the 3- and 6-

positions of quinoid bridging ligands is not an eﬀective strategy

to increase metal−radical coupling. Rather, selection of ligand

substituents should perhaps be based on considerations such as

synthetic accessibility of the ligand itself or of the resulting

metal compound, or the potential of the substituent to impart

secondary function such as conductivity or photophysical

properties.

of-phase component (χ ″) of the ac susceptibility were

observed above 2.00 K and below 1488 Hz, indicating too

fast magnetic relaxation (see Figures S68 −S75). In stark

contrast, compounds 2-R exhibit pronounced temperature-

and frequency-dependent peaks in both the in-phase (χ ′) and

out-of-phase component (χ ″) of the ac susceptibility (see

Figures 9, left, and S76 −S90), which demonstrates that the

radical-bridged complexes are indeed single-molecule magnets.

These data were employed to construct Cole−Cole plots (see

Figures S91, S92, S94, and S96), which were ﬁt using the

ASSOCIATED CONTENT

sı Supporting Information

https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03736.

■

generalized Debye model to estimate relaxation times (τ).

The corresponding Arrhenius plots of relaxation time exhibit

linear regions at higher temperatures between 4.75 and 6.75 K,

between 4.75 and 6.50 K, between 4.25 and 6.25 K, and

Additional experimental details, characterization data,

and computational data for 1-R and 2-R (R = OMe, Cl,

NO , SMe ), including crystallographic data for 1-OMe·

between 5.50 and 7.00 K for 2-OMe, 2-Cl , 2-NO , and 2-

SMe , respectively (see Figures 9, right, S93, S95, and S97 ),

4.0MeCN, 1-Cl·0.7H O, 1-NO ·4.0MeCN, 1-SMe ·

indicating a thermally activated relaxation process for all

complexes. Fits to the data in these temperature ranges aﬀord

.0MeCN, 2-OMe·2.0MeCN, 2-Cl ·0.5Et O, 2-NO ,

2 2

and 2-SMe ·0.9MeCN·0.5Et O (PDF)

spin relaxation barriers of U = 50(1), 41(1), 38(1), and

eff

−

3(1) cm

for 2-OMe, 2-Cl, 2-NO , and 2-SMe ,

Accession Codes

respectively, with corresponding pre-exponential factors of τ

CCDC 1926459−1926466 contain the supplementary crys-

tallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by

emailing data_request@ccdc.cam.ac.uk, or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

−

−8

3.3(6) × 10 , 1.0(2) × 10 , 1.8(3) × 10 , and 1.1(3) ×

−

s (see Table 3). These values are similar to those obtained

29a,b

for related tetraazalene-bridged Fe ₂complexes.

Interest-

ingly, the plot of U versus the Hammett substituent constant

eff

σ_pfor the series 2-R reveals a linear relationship (see Figure

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

i) Lin, P.-H.; Burchell, T. J.; Clerac, R.; Murugesu, M. Dinuclear

Article

Springer-Verlag: Berlin, Germany, 2006; Vol. 122, pp 1 −161.

Dysprosium(III) Single-Molecule Magnets with a Large Anisotropic

Barrier. Angew. Chem., Int. Ed. 2008 , 47 , 8848 −8851. (j) Bagai, R.;

Christou, G. The Drosophila of Single-Molecule Magnetism:

AUTHOR INFORMATION

Corresponding Author

T. David Harris − Department of Chemistry, Northwestern

University, Evanston 60208, Illinois, United States; Department

of Chemistry, University of California, Berkeley 94720,

California, United States; orcid.org/0000-0003-4144-

■

Chang, C. J.; Long, J. R. Slow Magnetic Relaxation in a High-Spin

(

Mn O (O CR) (H O) ]. Chem. Soc. Rev. 2009, 38, 1011−1026.

k) Freedman, D. E.; Harman, W. H.; Harris, T. D.; Long, G. J.;

00X; Email: dharris@berkeley.edu

Iron(II) Complex. J. Am. Chem. Soc. 2010, 132, 1224 −1225.

Authors

l) Rinehart, J. D.; Fang, M.; Evans, W. J.; Long, J. R. Strong

Exchange and Magnetic Blocking in N ₂ -Radical-Bridged Lanthanide

Agnes E. Thorarinsdottir − Department of Chemistry,

Northwestern University, Evanston 60208, Illinois, United

States; orcid.org/0000-0001-9378-4454

Complexes. Nat. Chem. 2011, 3, 538 −542. (m) Rinehart, J. D.; Fang,

M.; Evans, W. J.; Long, J. R. A N

Radical-Bridged Terbium

Ragnar Bjornsson − Department of Inorganic Spectroscopy,

Complex Exhibiting Magnetic Hysteresis at 14 K. J. Am. Chem. Soc.

011, 133, 14236−14239. (n) Mougel, V.; Chatelain, L.; Pecaut, J.;

Max-Planck-Institut fur Chemische Energiekonversion, Mulheim

̈ ̈

Caciuffo, R.; Colineau, E.; Griveau, J.-C.; Mazzanti, M. Uranium and

Manganese Assembled in a Wheel-Shaped Nanoscale Single-Molecule

Magnet with High Spin-Reversal Barrier. Nat. Chem. 2012, 4, 1011−

1017. (o) Ganivet, C. R.; Ballesteros, B.; de la Torre, G.; Clemente-

Juan, J. M.; Coronado, E.; Torres, T. Influence of Peripheral

Substitution on the Magnetic Behavior of Single-Ion Magnets Based

an der Ruhr 45470, Germany; orcid.org/0000-0003-2167-

374

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.9b03736

13 , 5110 −5148. (q) Zhang, P.; Guo, Y.-N.; Tang, J. Recent

Notes

on Homo- and Heteroleptic Tb Bis(phthalocyaninate). Chem. - Eur.

J. 2013, 19, 1457 −1465. (p) Woodruff, D. N.; Winpenny, R. E. P.;

Layfield, R. A. Lanthanide Single-Molecule Magnets. Chem. Rev. 2013 ,

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

Advances in Dysprosium-Based Single Molecule Magnets: Structural

Overview and Synthetic Strategies. Coord. Chem. Rev. 2013 , 257 ,

This work was supported by the Department of Energy (DE-

SC0019356). X-ray crystallography, NMR spectroscopy, and

IR spectroscopy were performed at the Integrated Molecular

Structure Education and Research Center at Northwestern

Univ., which has received support from the Soft and Hybrid

Nanotechnology Experimental Resource (NSF NNCI-

1728 −1763. (r) Pedersen, K. S.; Bendix, J.; Clerac, R. Single-Molecule

Magnet Engineering: Building-Block Approaches. Chem. Commun.

2014 , 50 , 4396 −4415. (s) Meihaus, K. R.; Long, J. R. Actinide-Based

Single-Molecule Magnets. Dalton Trans. 2015 , 44, 2517−2528.

(t) Craig, G. A.; Murrie, M. 3d Single-Ion Magnets. Chem. Soc. Rev.

2015, 44 , 2135 −2147. (u) Demir, S.; Jeon, I.-R.; Long, J. R.; Harris,

T. D. Radical Ligand-Containing Single-Molecule Magnets. Coord.

Chem. Rev. 2015, 289−290, 149−176. (v) Chen, Y.-C.; Liu, J.-L.;

Ungur, L.; Liu, J.; Li, Q.-W.; Wang, L.-F.; Ni, Z.-P.; Chibotaru, L. F.;

Chen, X.-M.; Tong, M.-L. Symmetry-Supported Magnetic Blocking at

542205), the State of Illinois, and the International Institute

for Nanotechnology. Quantitative analysis of Fe was performed

at the Northwestern Univ. Quantitative Bio-element Imaging

Center. Quantum chemical calculations were performed at a

computing cluster at the Max Planck Institute for Chemical

Energy Conversion. We thank Mr. B. D. Coleman, Dr. D. Z.

Zee, Dr. J. P. S. Walsh, and Ms. Y. Wang for experimental

assistance and helpful discussions. R. B. thanks the Max Planck

Society for support.

20 K in Pentagonal Bipyramidal Dy(III) Single-Ion Magnets. J. Am.

Chem. Soc. 2016, 138, 2829−2837. (w) Liu, J.; Chen, Y.-C.; Liu, J.-L.;

Vieru, V.; Ungur, L.; Jia, J.-H.; Chibotaru, L. F.; Lan, Y.; Wernsdorfer,

W.; Gao, S.; Chen, X.-M.; Tong, M.-L. A Stable Pentagonal

Bipyramidal Dy(III) Single-Ion Magnet with a Record Magnetization

Reversal Barrier over 1000 K. J. Am. Chem. Soc. 2016, 138, 5441−

5450. (x) Ding, Y.-S.; Chilton, N. F.; Winpenny, R. E. P.; Zheng, Y.-Z.

On Approaching the Limit of Molecular Magnetic Anisotropy: A

Near-Perfect Pentagonal Bipyramidal Dysprosium(III) Single-Mole-

cule Magnet. Angew. Chem., Int. Ed. 2016, 55, 16071−16074. (y) Guo,

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