How Do Ring Size and π-Donating Thiolate Ligands Affect Redox-Active, α-Imino-N-heterocycle Ligand Activation?

Article

pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX −XXX

How Do Ring Size and π‑Donating Thiolate Ligands Aﬀect Redox-

Active, α‑Imino‑N‑heterocycle Ligand Activation?

Benjamin K. Leipzig,^†Julian A. Rees,^†,∥Joanna K. Kowalska,^‡Roslyn M. Theisen,^†,△Matjaz Kavcic,^§

̌

Penny Chaau Yan Poon, Werner Kaminsky,^†,⊥Serena DeBeer,^‡Eckhard Bill,^‡and Julie A. Kovacs*^,†

^†The Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States

^‡Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34−36, D−45470 Mulheim an der Ruhr, Germany

̈

^§Jozef Stefan Institute, 1000 Ljubljana, Slovenia

̌

S

* Supporting Information

ABSTRACT: Considerable eﬀort has been devoted to the

development of ﬁrst-row transition-metal catalysts containing

redox-active imino-pyridine ligands that are capable of storing

multiple reducing equivalents. This property allows abundant

and inexpensive ﬁrst-row transition metals, which favor

sequential one-electron redox processes, to function as

competent catalysts in the concerted two-electron reduction

of substrates. Herein we report the syntheses and character-

ization of a series of iron complexes that contain both π-

donating thiolate and π-accepting (α-imino)-N-heterocycle

redox-active ligands, with progressively larger N-heterocycle

rings (imidazole, pyridine, and quinoline). A cooperative

interaction between these complementary redox-active ligands

is shown to dictate the properties of these complexes. Unusually intense charge-transfer (CT) bands, and intraligand metrical

parameters, reminiscent of a reduced (α-imino)-N-heterocycle ligand (L^•−), initially suggested that the electron-donating thiolate

had reduced the N-heterocycle. Sulfur K-edge X-ray absorption spectroscopic (XAS) data, however, provides evidence for direct

communication, via backbonding, between the thiolate sulfur and the formally orthogonal (α-imino)-N-heterocycle ligand π*-

orbitals. DFT calculations provide evidence for extensive delocalization of bonds over the sulfur, iron, and (α-imino)-N-

heterocycle, and TD-DFT shows that the intense optical CT bands involve transitions between a mixed Fe/S donor, and (α-

imino)-N-heterocycle π*-acceptor orbital. The energies and intensities of the optical and S K-edge pre-edge XAS transitions are

shown to correlate with N-heterocycle ring size, as do the redox potentials. When the thiolate is replaced with a thioether, or

when the low-spin S = 0 Fe(II) is replaced with a high-spin S = 3/2 Co(II), the N-heterocycle ligand metrical parameters and

electronic structure do not change relative to the neutral L⁰ligand. With respect to the development of future catalysts containing

redox-active ligands, the energy cost of storing reducing equivalents is shown to be lowest when a quinoline, as opposed to

imidazole or pyridine, is incorporated into the ligand backbone of the corresponding Fe complex.

both generate high-valent iron oxidants through a concerted

2e⁻OO bond-cleaving step, involving O₂as the terminal

oxidant.^17,23With P450, proton-induced heterolytic cleavage of

the Fe(III)-OOH (Cmpd 0) OO bond,²⁴generates a highly

reactive Fe(IV)O (P450-I).²⁵⁻²⁸The two electrons required

for this step originate from two diﬀerent sources: one from the

Fe and the other from the conjugated porphyrin and axial

cysteinate.²⁹Density functional theory (DFT) calculations

indicate that a signiﬁcant amount of unpaired spin density

resides on the cysteinate sulfur,²⁹indicating that P450-I is best

described as a (por/^cysS)^•+Fe(IV)O moiety.^11,30The role of

the axial cysteinate is not limited to its redox capabilities,

however, as the sulfur has also been shown to increase the

INTRODUCTION

■

Base-metal catalysis is an important area of research aimed at

the design of abundant and inexpensive ﬁrst-row transition-

metal catalysts that are capable of operating under mild

conditions.¹⁻⁷One limitation of ﬁrst-row transition metals,

however, is that they favor sequential one-electron, as opposed

to two-electron (2e⁻), redox changes. Many important

chemical transformations involve concerted 2e⁻chemistry,

and Nature has developed methods of circumventing this

limitation, by using redox-active ligands,^1,8−13such as

porphyrins or thiolates, or cooperative interactions between

metal ions incorporated into clusters or bimetallic systems.¹⁴⁻¹⁸

This is illustrated by the numerous examples of ﬁrst-row

transition metal-containing enzymes that promote complex,

multielectron processes.^{14,15,17−22}For example, soluble meth-

ane monooxygenase (sMMO) and cytochrome P450 (Cmpd I)

Received: November 9, 2017

A

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basicity of the oxo moiety.²³In addition, the cysteinate is

proposed to facilitate protonation of the peroxo, and

heterolytic, as opposed to homolytic, cleavage of the OO

bond, via what is referred to as the “push-eﬀect” (Figure

1).^24,31−33The low-energy v_OOstretch (at 799 cm⁻¹)^34,35of

RESULTS

■

In order to determine how to optimize orbital overlap in

synthetic transition-metal complexes containing synergistic π-

donor and π-acceptor redox-active ligands, and favor

cooperative redox events, we have synthesized, and spectro-

scopically, crystallographically, and electrochemically charac-

terized, a family of novel (α-imino)-N-heterocycle-containing

transition-metal complexes (Figure 2) containing a thiolate in

Figure 1. Thiolate-induced push mechanism of P450 O−O bond

cleavage.

Cmpd 0 has been attributed to the inﬂuence of the axial

cysteinate and low S = 1/2 spin-state of the Fe(III). The

thiolate also inﬂuences the redox-active frontier orbitals via a

highly covalent interaction with the appropriate symmetry

metal d-orbitals. This cooperativity between cysteinate, Fe, and

porphyrin π-orbitals is essential both for the 2e⁻OO bond-

cleaving step, and for the organic redox reservoir to delocalize

the radical cation of Cmpd I.^13,23,36

Figure 2. Complexes studied herein.

The involvement of a redox-active organic cofactor in

promoting P450 chemistry has inspired considerable eﬀorts

toward the development of ﬁrst-row transition-metal catalysts

containing redox-active ligands.^{1,5,6,10,37−39}One such ligand

moiety is the iminopyridine, which is capable of storing

multiple reducing equivalents,^{1,5,6,8−10,37−40}thereby allowing

complexes of ﬁrst-row transition metals to function as

competent catalysts in reactions involving two electrons. For

example, using iminopyridine ligands, Chirik and Ritter have

shown that base-metal catalyzed transformations involving

alkene hydrogenation, C−C bond activation, and diene

hydrosilylation proceed through concerted 2e⁻oxidative-

addition, and reductive-elimination steps,^4,41−43reactivity that

is typically associated with more costly second- and third-row

transition metal catalysts.¹Distributing charge over one or

more ligands has been shown to alleviate the electronic

demands at the metal, and avoid high- and low-valent oxidation

states, thus increasing catalytic turnover numbers, by

minimizing catalyst degradation.⁴³⁻⁴⁶

Appreciable overlap between metal and ligand orbitals

establishes the communication that is necessary for cooperative

redox events.^5,36,47,48Determining how one can optimize

orbital overlap in synthetic systems is imperative to achieving

catalytic function attainable by Nature. To that end, we

describe herein the synthesis and detailed spectroscopic,

crystallographic, and electrochemical characterization of a

family of novel (α-imino)-N-heterocycle complexes containing

a thiolate in the coordination sphere. A synergistic interaction

between the π-donating thiolate, the metal ion, and the π-

accepting (α-imino)-N-heterocycle is shown to govern the

properties of these complexes, and activate the (α-imino)-N-

heterocycle ligand π*-framework via a cis-type push-eﬀect,

reminiscent of P450.³¹⁻³³

the coordination sphere, [Fe^II(tame-N₂S(py)₂)]₂(1),

2+

[Fe^II(tame-N₂S(quino)₂)]₂(2), [Fe^II(tame-N₂S(^MeIm)₂)]₂

2+

(3), [Co^II(tame-N₂S(^MeIm)₂)]₂(4), as well as [Fe^II(tame-

N₂S^Bz(^MeIm)₂(MeCN))]²⁺(5), which contains a thioether in

the coordination sphere.

2+

Synthesis and Characterization of [Fe^II(tame-N₂S-

2+

(py)₂)]₂(1). The synthesis of a ligand framework that

incorporates both a thiolate and an (α-imino)-pyridine moiety

was carried out according to the method outlined in Scheme 1.

Scheme 1

The tripodal thiolate/amine ligand precursor, tame-N₂S, was

synthesized via a previously reported method,⁴⁹with slight

modiﬁcations involving the use of mesylate, as opposed to

tosylate, leaving groups, and Boc-protection of the amines prior

to deprotection of the benzyl-protected thiolate (Scheme S1).

A Schiﬀ-base condensation between the tripodal thiolate/amine

(tame-N₂S) and 2 equiv of 2-pyridine carboxaldehyde in the

presence of FeCl₂was found to aﬀord [Fe^II(tame-N₂S-

2+

(py)₂)]₂(1) containing intact (α-imino)-pyridine moieties

(Scheme 1). As shown in the ORTEP diagram of Figure 3, the

expected ﬁve-coordinate thiolate [Fe^II(tame-N₂S(py)₂)]⁺com-

plex was found to form a dimeric structure, with overall C₂

symmetry, and an Fe(μ-SR)₂Fe core. The thiolates are

symmetrically bridging, and derived from the chelating

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Figure 3. ORTEP of dicationic [Fe^II(tame-N₂S(py)₂)]₂²⁺(1) and [Fe^II(tame-N₂S(^MeIm)₂)]₂²⁺(3) with counterions, solvents of crystallization, and

hydrogen atoms omitted for clarity.

Table 1. Selected M−L Bond Distances (Å) for 1−5

1

2

3

4

5

M(1)−S(1)

M(1)−S(2)

M(2)−S(1)

M(2)−S(2)

2.284(1)

2.290(1)

N/A

2.2734(3)

2.2789(4)

N/A

2.2883(8)

2.3109(8)

2.2965(8)

2.2825(8)

1.917(2)

1.923(2)

1.955(2)

1.966(2

2.3930(5)

2.4653(5)

2.4616(5)

2.3912(5)

2.162(2)

2.145(2)

2.141(2)

2.120(2)

2.123(2)

2.2078(16)

2.121(2)

2.156(2)

2.271(1)

N/A

a

N/A

M(1)−N(1)^imine

M(1)−N(2)^imine

M(1)−N(3)^NHet

M(1)−N(4)^NHet

M(2)−N(7)^imine

M(2)−N(8)^imine

M(2)−N(9)^NHet

M(2)−N(10)^NHet

1.908(4)

1.916(4)

1.997(4)

1.984(4)

N/A

1.949(1)

1.996(1)

1.987(1)

N/A

1.948(4)

1.941(4)

1.996(4)

1.981(4)

N/A

1.933(2)

1.925(2)

1.970(2)

1.973(2)

N/A

a

For this structure, S(2) and Fe(2) are related to S(1) and Fe(1) via a crystallographic inversion center, and are thus labeled S(1)′ or Fe(1′) in the

supplemental crystallographic tables (Supporting Information), where S(1) and S(1)′ are derived from two diﬀerent pentadentate ligands.

pentadentate ligand, as well as from that of the adjacent

chelated metal ion. Two imine and two pyridine nitrogens

complete the pseudo-octahedral coordination sphere. The

planar bidentate (α-imino)-pyridine moieties of 1 are roughly

orthogonal to one another, with a dihedral angle of 70.0°

between N(2)C(12)C(13)N(3) and N(1)C(6)C(5)N(4)

planes. The Fe−N bond lengths (Table 1) are in the range

typically observed with low-spin (S = 0) ferrous complexes,^50,51

and the Fe−S bond lengths are signiﬁcantly shorter than that of

a high-spin (S = 2) Fe(II) ion,^52,53but comparable to that of a

low-spin (S = 1/2) Fe(III) ion.⁵⁴⁻⁵⁷

Scheme 2. Comparison of the Metrical Parameters within

the Structurally Distinct (α-Imino)-pyridine Fragments of 1,

with Those Reported for the Three Diﬀerent Redox States of

This Ligand Fragment⁸

It is well-established that redox-active (α-imino)-pyridine

ligands have characteristic intraligand bond lengths depending

on their formal oxidation state.¹⁰Three redox states are

accessible, L⁰, L^•−, and L²⁻(Scheme 2). Comparison of the

intraligand bond lengths of 1 with those previously established

for the neutral and reduced ligands indicates that the two (α-

imino)-pyridine arms of 1 (Py⁽¹⁾and Py⁽²⁾of Scheme 2) have

C^im−Ci^psodistances close to that of a monoreduced open-shell

L^•−oxidation state (Table 2), and are thus decidedly

“activated”.^9,10Strong reducing agents are typically required

to access the L^•−oxidation state.^6,9,10,45,58

a closed-shell L⁰.^6,7In the absence of an exogenous reductant,

ligand activation of 1 (Table 2) could be due to an unusually

large degree of π-backbonding. Given the redox-active nature of

(α-imino)-pyridines, it is also possible that electronic rearrange-

ment to an Fe(III)-(L)(L^•−) conﬁguration has occurred in 1 as

a consequence of having a thiolate in the coordination sphere.

Both the metal−ligand^{53,54,57,59−63}and intraligand metrical

parameters would be consistent with this. In this scenario, the

However, π-backbonding has also been shown to induce,

albeit only slight (vide infra), activation of the (α-imino)-

pyridine moiety in low-spin Fe(II) complexes, previous

examples of which have bond lengths much closer to that of

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Table 2. Selected Intraligand Bond Distances (Å) for 1−5 versus Typical Neutral and Monoreduced Radical Anion (α-Imino-N-

heterocycle) Ligands¹⁰

1

2

3

4

5

typical L⁰

typical L^•−

a

c

N^im(1)−C^im

1.294(6)

1.300(7)

1.417(7)

1.411(8)

1.294(6)

1.300(7)

1.417(7)

1.297(4)

1.297(3)

1.427(4)

1.301(3)

1.294(3)

1.430(4)

1.423(4)

1.281(5)

1.287(5)

1.432(6)

1.436(6)

N/A

1.275(2)

1.274(2)

1.447(3)

1.449(3)

1.283(2)

1.272(2)

1.442(3)

1.281(5)

1.287(5)

1.436(6)

1.432(6)

N/A

1.28

1.47

1.28

1.47

1.34

1.41

1.34

1.41

N^im(2)−C^im

a

c

C^im(1)−C^ipso

C^im(2)−C^ipso

b

N^im(1)−C^im

N^im(2)−C^im

C^im(1)−C^ipso

C^im(2)−C^ipso

d

N/A

b

d

N/A

1.411(8)

b

N/A

1.451(3)

N/A

a

c

d

Imine(1) bound to Fe(1). Imine(1) bound to Fe(2). Imine(2) bound to Fe(1). Imine(2) bound to Fe(2).

1

2+

Figure 4. H NMR of [Fe^II(tame-N₂S(py)₂)]₂(1, top) versus [Fe^II(tame-N₂S(quino)₂)]₂(2, bottom) in MeCN-d₃at 298 K. Only one-half of

each dimer is shown. The other half is related by symmetry.

radical could be either localized, or delocalized, across both (α-

imino)-pyridine moieties as previously described by Lu and

Wieghardt.¹⁰The H NMR spectrum of 1 (Figure 4) shows

1

sharp peaks in the diamagnetic region, appearing to rule out the

presence of a paramagnetic species in solution, and favor a

closed-shell low-spin Fe(II)L⁰electronic description. Ligand

radicals can, however, be strongly coupled (J = −330 cm⁻¹to

−1370 cm⁻¹) antiferromagnetically to a paramagnetic metal

ion,^9,10meaning that the observed diamagnetic properties of 1

do not necessarily preclude an Fe(III)−(L)(L^•−) electronic

description.

Synthesis and Characterization of [Fe^II(tame-N₂S-

2+

(quino)₂)]₂(2) and [Fe^II(tame-N₂S(^MeIm)₂)]₂(3). In

order to determine how the relative energies of the redox-

active ligand π*- and Fe d-orbitals inﬂuence the properties of

the π-donor/π-acceptor complexes, we systematically varied the

N-heterocycle ring size, from a pyridine ring in 1, to a quinoline

in 2, and an imidazole in 3 (Figure 2). The corresponding iron

complexes, [Fe^II(tame-N₂S(quino)₂)]₂(2) and [Fe^II(tame-

2+

Figure 5. ORTEP of dicationic [Fe^II(tame-N₂S(quino)₂)]₂(2) with

N₂S(^MeIm)₂)]₂(3), were synthesized in the same manner as

2+

counterions, solvents of crystallization, and hydrogen atoms omitted

for clarity.

the pyridine derivative (Scheme 1), using 2-quinolinecarbox-

aldehyde and 1-methyl-2-imidazolecarboxaldehyde in place of

2-pyridinecarboxaldehyde, respectively. These two derivatives

form bimetallic iron complexes in the solid state containing two

bridging thiolates, analogous to the pyridine derivative (Figures

3 and 5). The FeL bond lengths of 1−3 are similar (Table

1), with some minor diﬀerences, suggesting that all three

complexes contain Fe in the same oxidation state and spin state.

The bridging FeS distances are more inequivalent in the

imidazole structure 3, relative to 1, suggesting that the bridge

would more readily cleave (vide infra). In addition, all three

complexes have similar intraligand imine CN and C_imine



C_ipsometrical parameters (Table 2 and Figures S1 −S2), which

are indicative of partially populated redox-active orbitals, as

discussed previously. The tripodal tame-N₂S backbone of C_2h

symmetric 2 has a unique bridging mode, where it spans two

metal centers, as opposed to binding to a single metal ion as

D

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seen in structures 1 and 3. Reﬂecting these diﬀerences, the Fe−

S bonds of 2 are noticeably shorter than those in the pyridine

and imidazole complexes 1 and 3. The distinct bridging motif

of the quinoline complex 2 persists in solution as indicated by

moment of 2.9 μ_Bat 298 K would thus indicate that 35% of the

iron is a paramagnetic monomer (1.76 μmol) in solution, and

65% a diamagnetic dimer (3.26 μmol of Fe, or 1.63 μmol of

dimer). The eﬀective magnetic moment squared (μ_eff²) was

used for this calculation because the measurable macroscopic

quantity χ_pis dependent on μ_eff². One can then use this

1

the H NMR spectrum of Figure 4. While C_2hsymmetric

2+

[Fe^II(tame-N₂S(quino)₂)]₂(2) displays a singlet correspond-

ing to equivalent CH₂protons (H_a) adjacent to the sulfur, and

a singlet corresponding to the imine proton (H_d), the less

information to calculate the dimer/monomer equilibrium

constant at 298 K, K_eq

= (3.20 × 10⁻³)²/(2.96 × 10⁻³) =

298 K

symmetric C₂[Fe^II(tame-N₂S(py)₂)]₂(1) displays two H_a

3.46 × 10⁻³M, as described in the Supporting Information.

Assuming that both dimeric 3 and its monomeric derivative

contribute to the absorbance at λ_max= 619 nm, we can use

Beer’s law (path length l = 1 cm) and the equilibrium

concentrations of dimer and monomer at 298 K to determine

the extinction coeﬃcients for these two species. Using

electronic absorption spectroscopy, equilibrium monomer and

dimer concentrations, [M]_eqand [D]_eq, were determined at 298

K at two diﬀerent initial [Fe]_initconcentrations, using two

simultaneous equations as described in the Supporting

Information. The absorbance response to concentration

changes was nonlinear, providing evidence that more than

one species contributes to the absorbance, the relative ratios of

2+

doublets, and two H_dsinglets indicating that the imine and

methylene protons adjacent to the sulfur are inequivalent, the

latter of which are diastereotopic.

Dissociation of Dicationic 3 into Monocationic

Monomers in Solution. Despite their analogous bridging

modes, in solution, the pyridine and imidazole complexes 1 and

3 display subtle spectroscopic diﬀerences. For example, in

1

contrast to the diamagnetic H NMR spectrum of 1, that of 3

consists of mainly paramagnetic signals (Figure S3), suggesting

that more than one species is present in solution. Given the

more asymmetric Fe₂S₂core (Table 1), and longer, weaker

Fe(1)−S(2) bond in 3, it is likely that the thiolate bridge is

readily cleaved in solution, leading to an equilibrium mixture of

monomeric and dimeric species. Such a change in speciation

could induce a change in spin-state, leading to the observed

which are concentration-dependent, at the wavelength (λ_max

=

619 nm) monitored. The nonlinear Beer’s law behavior would

be consistent with a dimer/monomer equilibrium, and is

inconsistent with population of a higher-spin-state being

responsible for the increase in magnetic moment (Figure S8).

1

paramagnetic features in the H NMR. In order to distinguish

between a paramagnetic monomer ⇋ diamagnetic dimer

equilibrium, and the possibility that that 3 possesses a thermally

accessible higher-spin-state, we collected high-resolution LTQ

ion trap mass spectral data for 3 in MeCN solution. This data

shows isotopomers diﬀering by single, as opposed to half, mass

units (Figure S4), consistent with a monocationic monomer. In

contrast, using identical mass spectrometer settings, the LTQ

ion trap mass spectroscopy data of 2 only contains isotopomers

diﬀering by half-mass units (Figure S5), consistent with a

dicationic dimer. Complex 1 displays a mixture of single and

half mass unit isotopomers (Figures S6 and S7), consistent with

a mixture of monocationic and dicationic species. In order to

determine the spin-state of the monocationic species, we

measured the solution-state μ_efffor 3 over the temperature

range 238−335 K in MeCN-d₃using the Evans method (Figure

S8). The magnetic moment was shown to increase as a function

of temperature (from μ_eff= 2.9 μ_Bat 298 K, to μ_eff= 3.7 μ_Bat

330 K). A magnetic moment of 3.7 μ_Bsuggests that the

monomer is high-spin S = 2, as opposed to intermediate-spin S

= 1, since the latter would have a maximum spin-only μ_effof

2.83 μ_B, and 3.7 μ_Bexceeds that number. Since a monomeric

species would be entropically favored at higher temperatures,

this T-dependent magnetic data supports the possibility that, in

solution, a paramagnetic monomeric species exists in

equilibrium with the crystallographically characterized diamag-

netic dimer 3. Athough one cannot rule out the possibility that

dimeric 3 possesses a thermally accessible higher-spin-state, the

observation of both a dimer and monomer in high-resolution

mass spectral data provides evidence for an equilibrium. Since a

monomeric species would be entropically favored at higher

temperatures, the increase in magnetic moment as the

temperature is increased from 298 to 330 K would be

consistent with a paramagnetic monomer. One can calculate

the fraction of paramagnetic monomer species at 298 K under

the conditions used for the Evans method measurements (5.02

μmol of Fe in 550 μL of MeCN-d₃) using the ratio μ_eff²(expt)/

μ_eff²(theoretical). If 100% of the solution consisted of an S = 2

monomer, then μ_eff(theoretical) would be 4.9 μ_B.A magnetic

Using this method, we determined that solutions of 3 ([3]_init

0.120 and 0.240 mM) consist mainly of monomers at 298 K

([M]_eq= 0.113 and 0.213 mM, respectively).

=

Finally, in order to probe the possibility of an open

coordination site in the monomeric form of 3, preliminary

reactivity studies were conducted with small molecules. In

contrast to 1 and 2, which show no signs of reaction with

Bu₄NN₃, the paramagnetic peaks shift in the ¹H NMR

spectrum of 3 upon the addition of Bu₄NN₃(Figure S3).

Complexes 1 and 2 take hours, or months, respectively, to react

with O₂, whereas 3 reacts with O₂within ∼2 min (k_obs= 4.9 ×

10⁻³s⁻¹), as determined using electronic absorption spectros-

copy (Figure S9) and ¹H NMR. In addition, preliminary results

indicate that 3 reacts with CO. The increased reactivity of 3

would be consistent with the presence of a coordinatively

unsaturated, or solvent-bound, monomeric Fe(II) species in

solution. A detailed investigation of the reactivity of 3 with a

variety of small molecules will be the subject of a subsequent

paper.

Electronic Absorption Spectroscopy. The electronic

absorption spectra of 1−3 are richly featured, with multiple

transitions in the visible region. Solutions of 1 and 2 are

intensely turquoise in color, while 3 is an intense royal blue.

Complexes 1 and 2 have three charge-transfer (CT) bands in

the range 400−720 nm (Figure 6), whereas 3 has only a single

resolved absorption band at λ_max(ϵ, M⁻¹cm⁻¹) = 619 (11 050)

nm (Figure 7). The fact that solutions of 3 consist mainly of a

paramagnetic (S = 2) monomeric species, whereas those of 1

and 2 consist mainly, or solely, of the crystallographically

characterized dimer, explains why the solution spectrum of 3 is

distinctly diﬀerent from that of 1 or 2. The energy of the

lowest-energy band correlates with the N-heterocycle ring size,

shifting to lower energies as the ring size increases. This implies

that transitions associated with these bands involve N-

heterocycle π*-acceptor orbitals (vide infra). Intense (ε ≥ 10⁴

M⁻¹cm⁻¹), low-energy π-to-π* CT transitions (λ_max≤ 700

nm) are a hallmark of iron complexes bearing reduced, L^•−, (α-

E

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Figure 8. Cyclic voltamagram of 1 and 2 in MeCN at 298 K showing

Figure 6. Electronic absorption spectra of 1 and 2 in MeCN at 298 K.

reversible and irreversible oxidation waves. Potentials shown are in mV

vs Fc^0/+

.

Table 3. Redox Potentials for 1−3 in MeCN versus Fc/Fc⁺

1

2

3

(1)

E_1/2

−0.116 V

N/A

+0.112 V

N/A

−0.489 V

+0.152 V

+0.722 V

−1.89 V

(2)

E_pa

E_pc

+0.466 V

−1.74 V

+0.826 V

−1.39 V

proximity of two metal ions in these bimetallic structures, and

the expectation that electrochemical events would not be

independent of one another. The irreversibility of the more

anodic wave suggests that the bimetallic species dissociate upon

oxidation. Reduction waves (Figure S11) are also observed for

all three complexes in the range −1.39 V to −1.89 V (vs Fc^0/+

,

Figure 7. Comparison of the electronic absorption spectra of 3, which

contains a thiolate in the coordination sphere, versus 5, which contains

a thioether in place of the thiolate, illustrating the inﬂuence of the π-

donating thiolate on spectral intensity.

Table 3), with potentials that correlate with N-heterocycle ring

size (E_pc(2) < E_pc(1) < E_pc(3)). Although these potentials are

∼700 mV more negative than most Fe(II) complexes bearing

redox-active iminopyridine ligands, these redox events can be

attributed to N-heterocycle ligand-centered reduction (vide

infra),¹⁰and likely reﬂect the inﬂuence of the electron-donating

thiolate. The relative energies of these reduction potentials

would be consistent with the expected drop in energy of the

lowest unoccupied molecular orbital (LUMO) as the extent of

conjugation within the π-system increases. Consistent with the

¹H NMR, magnetic, and electronic absorption data, the

imidazole complex 3 reproducibly displays an additional

oxidation wave (E_pa⁽²⁾) at +152 mV versus Fc^0/+(Table 3

and Figure S10), attributable to the monomer that is proposed

to exist in equilibrium with the crystallized dimer.

imino)-pyridine ligands.^{10,40,64,6510,40,64,65}In contrast, closed-

shell L⁰(α-imino)-pyridine Fe complexes have Fe → π*

MLCT bands with extinction coeﬃcients in the range 700−

6000 M⁻¹cm⁻¹and λ_max-values in the range 408−600 nm.^6,66,67

This suggests that the N-heterocycle ligands of 1−3 are

reduced. Thiolate ligands have been shown to signiﬁcantly

inﬂuence the intensity of M → π* MLCT bands,^68,69however,

making it possible that the rather large extinction coeﬃcients of

1−3 (Figure 6) reﬂect enhancement of the putative Fe → π*

MLCT transitions via thiolate π-donation (vide infra).

Electrochemistry. All three of the complexes described

herein (1, 2, and 3) display multiple redox events within the

MeCN solvent window. This includes at least two oxidation

waves (Figure 8 and Figure S10), the lowest of which is

reversible (or quasireversible) (Table 3). The potentials of

these oxidation events are in the range typically observed for

metal-centered oxidation of RS−Fe(II) complexes,^55,70,71but

also directly correlate with N-heterocycle ring size (E_1/2(2) >

E_1/2(1) > E_1/2(3)). The latter reﬂects the metal orbital

stabilization that occurs as a result of overlap with π-acceptor

N-heterocycle orbitals, the extent of which increases as the

energy match optimizes with increasing N-heterocycle ring size.

The observation of two oxidation waves (tentatively assigned as

Fe(II)Fe(II) → Fe(II)Fe(III) → Fe(III)Fe(III)) with a more

than 500 mV separation would be consistent with the close

Sulfur K-Edge X-ray Absorption Spectroscopy. Sulfur

K-edge X-ray absorption spectroscopy (XAS) is well-

established as a sensitive measure of metal−sulfur covalency,

as well as ligand radical character.⁷²⁻⁷⁸Excitation of a sulfur 1s

electron to low-lying unoccupied orbitals probes both the

energy of the acceptor orbital, and the extent of mixing between

this orbital and the thiolate sulfur, as reﬂected in peak intensity.

The acceptor orbital may be either a metal 3d (in the case of

ﬁrst-row transition metals), or an unoccupied ligand-based

orbital with S 3p character. The S K-edge XAS spectra of solid-

state samples of 1−3 are shown in Figure 9 (top), and Figure

S12 (full range). These spectra exhibit two intense pre-edge

peaks: a prominent lower-energy pre-edge feature at ∼2471 eV,

and a higher-energy feature at ∼2473 eV (Table S1). The

lowest-energy peak systematically shifts in energy and intensity

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Fe(II) ions, provided the closest reproduction of metrical

parameters. In particular, the unusual intraligand distances

associated with the coordinated (α-imino)-N-heterocycles are

calculated to within 0.005 Å (Table S2), suggesting that the

electronic eﬀects responsible for these structural features are

successfully captured by the calculations. A qualitative MO

diagram for the crystallized dimeric form of 3 is shown in

Figure 10, and for 1 in Figure S16; a quantitative comparison is

Figure 9. Experimental (top) and TD-DFT-calculated (bottom) S K-

edge XAS pre-edge features associated with 1−3. Representative

transition diﬀerence densities for quinoline compound peaks (denoted

2a and 2b) are shown at an isovalue of 0.0015a₀, with regions of

increased electron density (acceptors) shown in purple. The calculated

3

Figure 10. Qualitative molecular orbital diagram for imidazole-3. The

the t_2gorbitals are the highest energy doubly-occupied orbitals, the

ligand π*-orbitals are the LUMOs, and the e_g* orbitals lie to higher

spectrum was shifted by 40.2 eV to align theory with experiment.

−3/2

energy. Molecular orbitals are shown at an isovalue of 0.05 a₀

.

in a manner that correlates with the N-heterocycle ring size

(vide infra), analogous to the experimentally-determined

reduction potentials (Table 3).

included in the Discussion section. Consistent with the

diamagnetic H NMR of 1 (Figure 4), all of the t_2g-type

1

The relative energies of the higher-energy pre-edge feature

(Figure 9, top right) do not display the same trends seen with

the lower-energy pre-edge peak. For example, the peak for the

quinoline complex 2 is highest, as opposed to lowest, in energy

(Table S1). With a low-spin d⁶Fe(II) complex, the metal-

centered t_2gd-orbitals are completely ﬁlled, and thus, transitions

would have to involve the unoccupied Fe e_g* d-orbitals as the

acceptor orbitals. The highest-energy pre-edge feature of 2

(Table S1) reﬂects the shorter Fe−S bond lengths relative to 1

and 3 (Table 1), which should serve to increase both the

covalency of this bond, and the ionization potential of the S 1s

electrons. We note that there should also be S(1s) → C−S σ*

and S(1s) → S 4p transitions to higher energies, possibly near,

or mixed in with, the higher-energy pre-edge feature of Figure

9. These interpretations are explored in more detail using the

density functional theory (DFT) calculations described below.

Density Functional Theory Calculations. Density func-

tional theory (DFT) calculations were performed in order to

generate a molecular orbital picture of complexes 1−3, and

provide a clear understanding of the electronic structure of

each. Geometry optimizations were initiated from crystallo-

graphic coordinates, using both closed-shell and broken-

symmetry singlet conﬁgurations. In all cases the lowest-energy

closed-shell solution, with neutral ligands and low-spin S = 0

orbitals in the DFT-optimized structures are doubly occupied

(Figure 10 and Figure S16). The sulfur character of both the d_xy

and d_xz(or d_yz) orbitals (Table S3) and d-orbital character in

the bonding S p-orbital (Figure 10) indicate that the Fe d-

orbitals have substantial covalent mixing with the bridging

thiolate sulfur orbitals, as is typical for Fe-thiolate complexes.⁵⁴

Consistent with the conjugated portion of the ligand scaﬀold

being π-acidic in nature, the LUMO orbitals are found to be the

(α-imino)-N-heterocycle π*, with the Fe e_g* orbitals at higher

energy (Figure 10). Analogous electronic structures were

determined for 1 (Figure S16) and 2 (Table S3). Inspection

of the calculated orbital compositions in Table S3 reveals that

the quinoline complex 2 has the most substantial covalent

mixing between the ligand π* and thiolate orbitals, and the

most Fe character in the ligand π*-orbitals. The extent of

covalent S(3p)/Lπ* mixing, and % Fe character in Lπ*, is less

with 1, relative to 2, and smallest with 3 (Table S3).

Interestingly, while the sulfur contribution to the t_2gorbitals

is largest for 2, sulfur character in the e_g* orbitals is largest for

3. This highlights the π-donating nature of the thiolate ligands,

which dominates when M/π-acceptor interactions are attenu-

ated with the smaller ring system.

While the relative energies of the molecular orbitals [M(t_2g)

< L(π*) < M(e_g*)] are not surprising in light of the abundant

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literature precedent,^3,4,6,41,51the apparent mixing of the thiolate

sulfur orbitals into the redox-active (α-imino)-N-heterocycle

ligand orbitals (Table S3) is highly unusual. The intensities of

the optical (Figures 6 and 7) and S K-edge pre-edge XAS

(Figure 9) transitions further suggest that there is substantial

communication between the thiolate sulfur 3p and (α-imino)-

N-heterocycle ligand π*-orbitals. On the basis of the orbital

compositions in Table S3, there is extensive delocalization of

the d_{x −y}over both the sulfur (17.8−21.1%) and Lπ* (12.4−

22.7%) orbitals, and this appears to be the most involved of the

Fe 3d orbitals in facilitating the orbital mixing that engenders

this spectral intensity. The nature of the underlying electronic

transitions involved in both types of absorption spectroscopy

was investigated using time-dependent DFT (TD-DFT)

calculations.

As discussed previously, S K-edge XAS provides^13,73a direct

probe of the thiolate bonding in these complexes. The TD-

DFT-calculated pre-edge features of the S K-edge XAS spectra

of 1−3 are shown at the bottom of Figure 9, and employed the

CAM-B3LYP functional. Comparison with the experimental S

K-edge pre-edge XAS spectra (Figure 9, top) indicates that the

energetic trends observed for both features are accurately

reproduced in the calculated spectra. Examination of the

transition diﬀerence densities (Figures S13 −S15) establishes

that the lower-energy S K-edge pre-edge XAS feature indeed

corresponds to a sulfur 1s → (α-imino)-N-heterocycle π*

LUMO transition, as proposed above. Inspection of the

transition diﬀerence densities for the transitions underlying

the higher-energy pre-edge feature, however, reveals a

combination of Fe e_g*-type and C−S σ*-acceptor orbitals,

with signiﬁcant sulfur np character (Figure 9, bottom, Figures

S11 −S13). The experimental intensities of the ﬁrst pre-edge

feature (Figure 9, top) provide evidence for an increase in

sulfur 3p/ligand π* LUMO mixing as the N-heterocycle ring

size increases: imidazole (3) < pyridine (1) < quinoline (2).

Thiolate π-donation into the (α-imino)-N-heterocycle π*

LUMO is likely mediated via backbonding from the ﬁlled Fe

t_2g-type orbitals. It is noted that the TD-DFT-calculated

intensities of the S K-edge XAS pre-edge feature do not

capture the experimental trend, although this shortcoming can

be rationalized by the diﬃculty associated with calculating both

CT transitions and the onset of a continuum, using TD-DFT.⁷⁹

Both of these factors could alter the relative intensities of the

lower- and higher-energy pre-edge features. The agreement

between the TD-DFT-calculated electronic absorption inten-

sities and the experimental intensities, as well as the trend in

energies as a function of ring size, however, suggests that the

electronic eﬀects inﬂuencing the CT intensity may in fact be

successfully captured by the calculations, and that the

discrepancies in the S K-edge XAS spectra could simply be

2

The putative charge-transfer nature of the transitions

involved in the electronic absorption spectra prompted the

use of the range-separated hybrid density functional CAM-

B3LYP for these calculations. As shown in Figure 11, the TD-

due to problems modeling the onset of the continuum.

57

̈

Fe Mossbauer Spectroscopy. The electronic environ-

ment of the iron center of 1−3 was also investigated using ⁵⁷Fe

Mossbauer spectroscopy. The zero-ﬁeld ⁵⁷Fe Mossbauer

̈

spectra of 2 and 3 are shown in Figure 12, and that of 1 is

Figure 11. TD-DFT-calculated electronic absorption spectra of 1 and

2. Transition diﬀerence densities for the most intense transitions are

shown at an isovalue of 0.0015 a₀⁻³, with areas of decreased electron

density (donors) shown in teal and increased electron density

(acceptors) shown in purple.

DFT-calculated electronic absorption spectra for 1 and 2 both

have intense transitions in the visible region. The relative

energies and intensities are in good agreement, although the

absolute transition energies are overestimated by approximately

200 nm when compared to the experimental data (Figure 6).

This is a well-known shortcoming of TD-DFT.⁷⁹The nature of

these transitions can be examined with reasonable ﬁdelity,

however, via the transition diﬀerence densities (Figure 11). For

both complexes, the donors and acceptors associated with the

intense visible region CT transitions are shown to be mixed

Fe−S, and (α-imino)-N-heterocycle π* in nature, respectively

(Figure 11). The iron 3d-orbitals thus appear to mediate charge

transfer from the thiolate 3p to the formally orthogonal (α-

imino)-N-heterocycle π* LUMO.

Figure 12. Zero-ﬁeld ⁵⁷Fe Mossbauer spectra of 2 and 3.

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shown in Figure S15. The compounds exhibit well-resolved

quadrupole doublets with isomer shift parameters, δ, in the

range 0.36−0.41 mm/s, and relatively small quadrupole

splitting parameters, ΔE_Q, in the range 0.38−0.87 mm/s

(Table 4). The isomer shift parameter, δ, depends on the

While δ parameters are sensitive to electron density at the

⁵⁷Fe nucleus, the quadrupole splitting parameter, ΔE_Q, provides

a measure of the asymmetry of the efg at the nucleus. In the

limit of crystal ﬁeld theory, an O_hS = 0 d⁶Fe(II) ion has a

vanishing valence contribution to the efg that should result in

the coalescence of the two quadrupole doublet peaks. However,

the lowered symmetry, π-donating thiolate, and π-accepting (α-

imino)-N-heterocycle of 1−3 should contribute ﬁnite covalence

and a lattice contribution to the efg, in agreement with the

experimental data. As with the isomer shift parameter, the

quadrupole splitting parameter, ΔE_Q, of 3 is smaller than that of

1 and 2 (Table 4). This suggests that there is less asymmetry in

the covalent bonding of 3, and, being isostructural to 1, implies

that the bonding in 3 may be less covalent overall. Finally, the

Table 4. Comparison of the Experimental and Calculated

Mossbauer Parameters for 1−3

|ΔE_Q|

BP86 calcd

δ (mm/s)

BP86 calcd

ΔE_Q(mm/s)

N-heterocycle δ (mm/s) (mm/s)

1

2

3

pyridine

0.36

0.41

0.87

0.85

0.38

0.22

0.39

1.07

−1.09

0.82

quinoline

imidazole

DFT-calculated Mossbauer parameters shown in Table 4 are in

̈

excellent agreement with the experimentally observed trends,

reinforcing the accuracy of the calculated electronic structures.

charge density at the Mossbauer nucleus, reﬂects the oxidation

̈

state and spin-state, and is well below the range established for

ﬁve- and six-coordinate high-spin Fe(II) sites (>1 mm/s).⁸⁰

Unfortunately, these values are consistent not only with low-

spin Fe(II), but also with Fe(III), low-spin or high-spin.

Fortunately, the latter cases, i.e., the presence of oxidized

Fe(III), can be ruled out for the following reasons: (i) The

compounds exhibit a diamagnetic ground state which could not

be explained by any kind of antiferromagnetic spin coupling

between high-spin S = 5/2 Fe(III) and a ligand radical with S′ =

1/2. (ii) Low-spin Fe(III) can be excluded because the

In summary, the ⁵⁷Fe Mossbauer data and accompanying DFT-

calculated Mossbauer parameters suggest that the extent of π-

back-donation into the π-accepting ligands correlates with the

size of the N-heterocycle, as proposed above. These data also

provide evidence for a compensatory contraction of the Fe−S

bonds in 2 to enable stronger covalent π-bonding to the (α-

imino)quinoline ligand.

̈

Examining the Inﬂuence of the Thiolate Ligand and

Metal Ion. The electronic structures of 1−3 developed thus far

highlight the importance of the thiolate and Fe valence orbitals

in tuning back-donation to the (α-imino)-N-heterocycle ligand

π* LUMO and activating the ligand. To systematically explore

these eﬀects, derivatives of imidazole 3 were synthesized in

which either the metal ion or the thiolate was varied. The cobalt

5

corresponding 3d t_2gconﬁguration would exhibit substantial

charge asymmetry in the 3d shell, which would give rise to

substantial valence contribution to the electric ﬁeld gradient

(efg), and a corresponding large quadrupole splitting

parameter, |ΔE_Q| > 1−3 mm/s,⁸⁰which is not experimentally

imidazole derivative, [Co^II(tame-N₂S(^MeIm)₂)]₂(4), was

2+

observed for compounds 1−3. Hence, the Mossbauer spectra

̈

synthesized using a method analogous to that used to

synthesize 3. As shown in Figure 13, 4 has a similar C₂

corroborate the presence of low-spin Fe(II) for compounds 1−

3. This is also supported by the reasonable match between

experimental and calculated Mossbauer parameters obtained

̈

from the orbital analysis of the DFT results given above.

Isomer shifts theoretically correlate with the average Fe−L

bond length.⁸¹Iron−ligand (Fe−L) bond lengths in 1−3 are

modulated by the π-acceptor (α-imino)-N-heterocycle moi-

eties, such that a decrease in Fe → L π-back-donation should

lead to a more positive δ parameter. The isomer shift δ

parameters of Table 4 therefore indicate that Fe → L π-back-

donation is smallest with the imidazole complex 3 (δ = 0.41

mm/s), and comparable in the pyridine and quinoline

complexes 1 and 2 (both δ = 0.36 mm/s). While the former

conclusion is in good agreement with the S K-edge XAS spectra

and TD-DFT calculations (Figure 9, Table S3), one might have

anticipated that the Mossbauer parameters of 2 would be

̈

indicative of more π-backbonding than 1. However, just as π-

accepting ligands can depopulate 3d orbitals, π-donating ligands

such as thiolates can increase the eﬀective 3d orbital population.

Comparison of the metrical parameters of 1 and 2 indeed

reveals that the Fe−S bonds in 2 are shorter by 0.01 Å,

indicative of increased S → Fe π-donation in 2. Thus, the π-

donating thiolate ligands compensate for the increased π-

backbonding with the π-acceptor imino-N-heterocycle π*-

orbitals in 2 by forming more covalent Fe−SR bonds with more

generous S → Fe π-donation, in agreement with the DFT-

calculated d_yz-, d_xz-, and L π*-orbital compositions (Table S3).

This coincidently results in identical isomer shift parameters for

2 and 1. This phenomenon of oﬀsetting perturbations to δ has

been previously investigated theoretically in some detail for

other complexes bearing covalent π-bonding ligands.^82,83

Figure 13. ORTEP of dicationic [Co^II(tame-N₂S(^MeIm)₂)]₂²⁺(4) with

counterions, solvents of crystallization, and hydrogen atoms omitted

for clarity.

symmetric structure in the solid state, with Co−L bond lengths

consistent with high-spin Co(II) (Table 1). The solution-state

magnetic moment of 4 (μ_eff= 3.98 μ_B) is consistent with an S =

3/2 spin-state.

A notable diﬀerence between structures 4 and 3 is that the

Co₂S₂core of 4 is signiﬁcantly more puckered (Figure 13), with

a separation between the MM and SS centroids of 0.72 Å,

compared to 0.34 Å for the Fe₂S₂core of 3. This is likely a

result of the change in d-electron conﬁguration (d⁷Co(II) vs d⁶

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Fe(II)). In addition, the MS bond lengths in 4 are

signiﬁcantly elongated compared to those of 3 (Table 1), by

an average of 0.13 Å. This likely reﬂects the high- versus low-

spin-states of 4 and 3, respectively. The more puckered core

and longer MS bonds of 4 decrease RS → M(t_2g) π-

donation, which should decrease the extent of π-back-donation

into the π-acidic (α-imino)-N-heterocycle π* LUMO. Distinct

diﬀerences in intraligand metrical parameters for cobalt-4

versus iron-3 (Table 2) provide evidence to support this. While

the intra-ligand bond lengths of the Fe complex, 3, resemble a

diﬀerences in intraligand metrical parameters, and electronic

absorption spectral properties, suggest that the π-donating

thiolate ligand plays a key role in activating the π-accepting (α-

imino)-N-heterocycle ligand.

DISCUSSION

■

The interplay between metal and ligand redox orbitals has been

clearly established as an integral component in the function of

both metalloenzymes and synthetic transition-metal catalysts

containing redox-active ligands.^{6,11,23,25,26,84,85}As detailed

herein, the synthesis and characterization of complexes 1−5

provide insight into the cooperativity between the frontier

orbitals of a π-donating thiolate, Fe, and π-accepting (α-imino)-

N-heterocycle ligands with extended π-systems. Cytochrome

P450 is the classic biological example that implements

cooperativity between redox-active orbitals in order to

eﬃciently promote catalysis.^{11,23,25,26,84,85}While many synthetic

catalysts featuring redox-active ligands^1,6,41,86,87derive inspira-

tion from P450,³¹⁻³³only a limited number incorporate

thiolate ligands.^38,39Thiolate ligands have well-established

functional roles in biology, which include tuning redox

potentials,^55,57,88stabilizing low-spin-states,^54,56,57,63or using a

compensatory eﬀect to maintain a constant metal ion Lewis

acidity.⁵⁹They can also inﬂuence the properties of other ligands

in the coordination sphere, by labilizing trans ligands,⁸⁹⁻⁹¹or

altering basicity and O−H bond strengths.^{11,84,92−96}With

P450, the cysteine thiolate directly participates in the redox

chemistry, functioning as part of the reservoir of oxidizing

equivalents necessary to form Cmpd I.^11,84,97,98Despite this

diverse biological utility, an in-depth understanding of the

impact that thiolate ligands have on the electronic structure of

synthetic complexes containing redox-active ligands has yet to

be fully elucidated.

reduced L^•−ligand (C^im−N^im= 1.297(3) Å, C^im−C^ipso

=

1.427(3) Å), intra-ligand bond lengths in the Co complex 4

(C^im−N^im= 1.276(5) Å, C^im−C^ipso= 1.447(4) Å) more closely

resemble those of a closed-shell L⁰ligand (Table 2). Consistent

with this picture, involving less eﬀective RS/M/(α-imino)-

imidazole LUMO π-orbital overlap, the electronic absorption

spectrum of 4 (Figure S18) notably lacks the intense low-

energy CT transitions observed in those of 1−3 (Figures 6 and

7).

The inﬂuence of the thiolate ligand in activating the π-acidic

(α-imino-N-heterocycle) ligand was also investigated, by

synthesizing a thioether derivative of 3. In contrast to thiolate

s t r u c t u r e s 1 − 4 , t h i o e t h e r [ F e ^{I I}( t a m e -

N₂S^Bz(^MeIm)₂(MeCN))]²⁺(5) is monomeric in the solid

state (Figure 14), with a pseudo-octahedral coordination

The series of complexes, 1−5, described herein illustrates the

ability of π-donating thiolate ligands to tune intraligand metrical

parameters and the valence electronic structure of π-acceptor

ligands. For thiolate complexes 1−3, unusual activation of the

(α-imino)-N-heterocycle ligand is observed (Table 2), wherein

imine C^imN bond lengths are elongated, and C^imC^ipso

bonds are shortened, relative to that predicted for a closed-shell

ligand, L⁰, indicating that electron density has shifted into the

LUMO orbital of the (α-imino)-N-heterocycle ligand frame-

work.¹⁰Cobalt(II)-containing 4 and the thioether complex 5

do not exhibit the same ligand activation. As discussed above,

the origin of ligand activation in 1−3 could either be attributed

to a formal reduction of the ligand to L^•−(with strong

antiferromagnetic coupling to the resultant low-spin S = 1/2

Fe(III) ion), or to extraordinarily large amounts of π-

backbonding from low-spin Fe(II) to the π-acidic ligand,⁶

Figure 14. ORTEP of dicationic 5 with counterions, solvents of

crystallization, and hydrogen atoms omitted for clarity.

sphere that is completed by an axial MeCN trans to the

thioether. Although the short Fe−N and Fe−S bond lengths

are most consistent with a low-spin S = 0 ferrous ion at 100 K

(the temperature at which X-ray crystallographic data was

1

collected), the H NMR spectrum of 5 at 298 K displays

both of which would give rise to the observed diamagnetic H

paramagnetically shifted peaks (Figure S19), and the ambient

temperature solution magnetic moment of 5, μ_eff= 5.04 μ_B, is

consistent with an S = 2 spin-state, indicating that there is a

thermally accessible higher-spin-state. Although the Fe−N

bond lengths of the thiolate complex 3 and thioether complex 5

are comparable, the intraligand C^im−N^imand C^im−C^ipso

metrical parameters in 5 are closer to that of a closed-shell

(α-imino)-N-heterocycle (Table 2). Furthermore, although 5

retains an Fe → (α-imino)-pyridine π* CT transition in the

visible region of the electronic absorption spectrum (Figure 7),

it is signiﬁcantly lower in intensity, and to higher energy (λ_max

(ϵ, M⁻¹cm⁻¹) = 512 (2850) nm), relative to that of the thiolate

complex 3 (λ_max(ϵ, M⁻¹cm⁻¹) = 619 (11 500) nm). These

NMR spectra of 1 and 2. The Mossbauer parameters of 1−3

̈

and the metal−ligand bond lengths of 1−3 and 5 are most

consistent with low-spin Fe(II), however. The comparable

FeN bond lengths in thioether-containing 5, and thiolate-

containing 3, as well as the absence of perturbed intraligand

metrical parameters in 5, support the latter description as well,

in agreement with spectroscopic data and DFT calculations

(vide supra).

The extent of metal-to-ligand π-backbonding largely depends

on the population and energy of the metal π-symmetry orbitals,

which can be modulated by metal oxidation state and spin-state,

as well as the nature of supporting ligands. Low-spin ferrous

ions, such as those in 1−3 and 5, have a (t_2g)⁶conﬁguration

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and are optimally suited for π-back-donation to a π-acceptor

ligand. In contrast, the high-spin Co(II) ions in 4 have a

(t_2g)⁵(e_g*)²conﬁguration, with one fewer electron in the π-

symmetry d-orbitals. Additionally, population of the e_g*-orbitals

serves to elongate the metal−ligand bonds, which would

weaken π-backbonding to Co(II) (Table 1). Given that

thiolates are known to be eﬀective π-donors, two of the three

π-symmetry t_2g-orbitals of 1−3 should be relatively electron-

rich. The metal-centered π-symmetry t_2g-orbitals are also of the

appropriate symmetry to back-bond into the (α-imino)-N-

heterocycle π*-orbitals. However, eﬀective communication

between all of these these orbitals requires eﬃcient π-overlap,

which, of course, is dependent on metal−ligand bond lengths

and bond angles. This is evident in the metrical parameters of 4

compared to 3, where longer metal−ligand bonds and a more

puckered diamond core in the former result in negligible (α-

imino)-N-heterocycle ligand activation. The comparable Fe−N

bond lengths, but contrasting intraligand distances in thioether

5 versus thiolate 3, illustrate the inﬂuence of the thiolate on π-

backbonding to the (α-imino)imidazole ligand, in the solid

state. In the solution state, this is supported by the signiﬁcantly

less intense CT bands for Co(II)-4 (ε = 654 M⁻¹cm⁻¹, Figure

S18) and the thioether complex 5 (ε = 2850 M⁻¹cm⁻¹, Figure

7) versus the unusually intense Fe → (α-imino)pyridine π* CT

bands (ε > 10 000 M⁻¹cm⁻¹) for the thiolate complexes 1−3

(Figures 6 and 7). The intensities of the latter are indicative of

optimal orbital overlap.⁵⁴This is supported by the TD-DFT-

calculated transition diﬀerence densities (Figure 11), which

indicate that the charge-donating thiolate contributes to the

intense transitions. Additionally, the Fe → (α-imino)pyridine

π* CT band associated with 5 is shifted to higher energy

relative to 1−3. The latter reﬂects the higher energy of the d-

orbitals in 1−3 caused by π-donation from the thiolate, and the

well-established ability of thiolate ligands to tune the energies

of redox-active metal orbitals.

Evidence for the direct involvement of the π-donating

thiolate ligands in activating the (α-imino)-N-heterocycle

ligand π*-orbitals is provided by the S K-edge XAS spectra,

which show electronic transitions from the thiolate sulfur 1s-

orbital into the (α-imino)-N-heterocycle LUMO π* (Figure 9).

Assignment of this S K-edge XAS pre-edge feature is supported

by the strong correlation between its transition energy and

intensity, and the N-heterocycle ring size (Figure 15). The

intensity of the S K-edge XAS pre-edge transitions increases as

the thiolate character of the LUMO π*-orbitals increases

(Table S3). A quantitative comparison of reduction potentials

(Table 3), S K-edge XAS pre-edge energies (Table S1), and the

DFT-calculated LUMO (α-imino)-N-heterocycle π*-orbital

energies for 1−3 shows (Figure 15) that these properties are

highly correlated.

Figure 15. Correlation between reduction potentials, S K-edge XAS

pre-edge energies, and DFT-calculated LUMO energies (referenced to

their respective HOMO energies), illustrating the eﬀect of variations in

N-heterocycle ring size.

t_2g-type 3d-orbitals. This electronic structure results in

unusually intense RS−Fe → (α-imino)-N-heterocycle π* CT

bands in the electronic absorption spectra, the intensities of

which are reminiscent of complexes containing reduced L^•−

ligands. Sulfur K-edge X-ray absorption spectroscopy (XAS)

and TD-DFT provide evidence for an interaction between the

π-donating thiolate and π-accepting (α-imino)-N-heterocycle

ligand π*-orbitals, via the metal ion d-orbitals. The energies and

intensities of the lowest-energy S K-edge XAS pre-edge feature

are shown to correlate with reduction potentials, DFT-

calculated (α-imino)-N-heterocycle LUMO energies, and N-

heterocycle ring size (Figure 15). Communication between the

π-donor and π-acceptor orbitals is shown to generate an

activated α-imino-N-heterocycle ligand with intraligand metri-

cal parameters close to those of a monoreduced L^•−. This is

unprecedented, and distinctly diﬀerent from most complexes

containing (α-imino)pyridine-ligands, where ligand activation is

only observed upon the addition of a strong reducing agent.

Four components of the geometric and electronic structures of

1−3 were shown to be responsible for the observed ligand

activation: (1) an approximate energy match between the metal

ion and (α-imino)-N-heterocycle ligand π*-orbitals, (2)

eﬀective orbital spatial overlap, (3) a low-spin-state, and most

importantly, (4) the inclusion of a π-donating thiolate in the

coordination sphere. The Fe 3d-orbitals were shown to mediate

charge-transfer from the thiolate sulfur 3p-orbital to the

formally orthogonal (α-imino)-N-heterocycle π* LUMO. We

liken this interaction to the proposed “push-eﬀect” of the P450

ferric hydroperoxo Cmpd 0, wherein electron donation from

the cysteinate ligand into the Fe 3d-orbitals results in the

population of the peroxo O−O σ*-orbital.³¹⁻³³

CONCLUSIONS

■

Herein we have examined the geometric and electronic

structure of a family of metal complexes containing both a π-

donating thiolate ligand, and π-acceptor (α-imino)-N-hetero-

cycle ligands. A combined analysis of X-ray crystal structures

and spectroscopic parameters, supported by theoretical

calculations, indicates that the ground state electronic structure

of 1−3 is heavily delocalized over the thiolate ligand, the metal

center, and the (α-imino)-N-heterocycle. Each of these

complexes is shown to contain low-lying LUMOs that are α-

imino-N-heterocycle ligand π* in character, and HOMOs

which consist of a highly covalent mix of sulfur 3p and metal

The study reported herein also highlights key properties

likely to be beneﬁcial for the development of redox-active (α-

imino)-N-heterocycle base-metal catalysts capable of promoting

concerted 2e⁻reactions.¹⁻³By incorporating redox-active

ligands that are capable of both donating and accepting

electron density to and from the metal ion, charge distribution

could be modulated in a facile manner, thereby potentially

improving catalyst performance. We demonstrated that we can

modulate the extent of metal−ligand redox-active orbital

coupling by altering the size of the ligand π-system. As the

energies of the ligand π*-orbitals decrease in response to

K

DOI: 10.1021/acs.inorgchem.7b02748

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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quinoline is incorporated into the ligand framework.

ASSOCIATED CONTENT

* Supporting Information

■

S

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.inorg-

chem.7b02748.

Experimental details regarding the synthesis and

characterization of the tame-N₂SH ligand, and metal

1

complexes derived therefrom; H NMR spectrum of 3

and 5; magnetic moment vs T plot of 3; cyclic

voltammograms for 1−3; DFT generated MO diagram

and Mossbauer spectrum of 1; electronic absorption

̈

spectrum of 4; crystallographic tables and metrical

parameters for DFT optimized structures of 1−5; and

experimental details for the S K-edge XAS (PDF)

Accession Codes

CCDC 1584846−1584850 contain the supplementary crystal-

lographic data for this paper. These data can be obtained free of

charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-

ing 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.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: kovacs@chem.washington.edu.

ORCID

Joanna K. Kowalska: 0000-0003-1314-2924

Serena DeBeer: 0000-0002-5196-3400

Julie A. Kovacs: 0000-0003-2358-1269

■

Present Addresses

^△Department of Chemistry, University of Wisconsin − Eau

Claire, Eau Claire, Wisconsin 54702, United States

^∥Chemical Sciences Division, Lawrence Berkeley National

Laboratory, Berkeley, CA 94720, United States.

Notes

The authors declare no competing ﬁnancial interest.

^⊥Crystallographer, UW Department of Chemistry.

ACKNOWLEDGMENTS

■

J.A.K. acknowledges the NIH (GM45881) for funding, and

J.K.K., E.B., and S.D. acknowledge the Max Planck Society for

funding. J.A.R. was funded, in part, by a graduate study

scholarship from the German Academic Exchange Service

(DAAD). Aleksandr Forov is thanked for assistance during

beam time, the ESRF is acknowledged for providing beamtime,

and Pieter Glatzel is thanked for technical assistance with data

collection at beamline ID26.

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Coenzyme A Synthase. Acc. Chem. Res. 2004, 37, 775−783.

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Nature 2008, 455, 333−340.

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DOI: 10.1021/acs.inorgchem.7b02748

Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02748

Inorg. Chem. XXXX, XXX, XXX−XXX

Article Doi

How Do Ring Size and π-Donating Thiolate Ligands Affect Redox-Active, α-Imino-N-heterocycle Ligand Activation?

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