Low-valence anionic α-diimine iron complexes: Synthesis, characterization, and catalytic hydroboration studies

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Low-Valence Anionic α‑Diimine Iron Complexes: Synthesis,

Characterization, and Catalytic Hydroboration Studies

Thomas M. Maier, Martin Gawron, Peter Coburger, Michael Bodensteiner, Robert Wolf,*

Nicolaas P. van Leest, Bas de Bruin, Serhiy Demeshko, and Franc Meyer

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ABSTRACT: The synthesis of rare anionic heteroleptic and

homoleptic α-diimine iron complexes is described. Heteroleptic

BIAN (bis(aryl)iminoacenaphthene) complexes 1-[K([18]c-6)-

(thf)_0.5] and 2-[K([18]c-6)(thf)₂] were synthesized by reduction

of the [(BIAN)FeBr₂] precursor complex using stoichiometric

amounts of potassium graphite in the presence of the

corresponding oleﬁn. The electronic structure of these para-

magnetic species was investigated by numerous spectroscopic

57

̈

analyses (NMR, EPR, Fe Mossbauer, UV−vis), magnetic

measurements (Evans NMR method, SQUID), and theoretical

techniques (DFT, CASSCF). Whereas anion 1 is a low-spin

complex, anion 2 consists of an intermediate-spin Fe(III) center.

Both complexes are eﬃcient precatalysts for the hydroboration of

carbonyl compounds under mild reaction conditions. The reaction of bis(anthracene) ferrate(1−) gave the homoleptic BIAN

complex 3-[K([18]c-6)(thf)], which is less catalytically active. The electronic structure was elucidated with the same techniques as

described for complexes 1-[K([18]c-6)(thf)_0.5] and 2-[K([18]c-6)(thf)₂] and revealed an Fe(II) species in a quartet ground state.

row transition metal catalysis to gain control over the often

very reactive intermediates and to mimic the behavior of

precious metals.⁴A redox-active ligand is capable of storing

and releasing electrons and can therefore be directly involved

in catalytic reactions by facilitating net 2e⁻processes that

would otherwise be disfavored.⁵A prominent class of redox-

active ligands is the α-diimine family, members of which are

almost always good π-acceptors and so well able to stabilize

electron-rich metal centers. There are many examples of such

transition-metal α-diimine complexes known, in particular 3d

metals such as nickel⁶or cobalt,⁷which exhibit interesting

catalytic properties. From a sustainibility point of view,

especially low-valence iron complexes with redox-active α-

diimine ligands provide an attractive motif for applications in

sustainable catalysis.

INTRODUCTION

■

Organometallic and coordination chemistry can contribute to a

sustainable future by supporting the development of eﬃcient

catalysts based on earth-abundant metals.¹One key objective

in these areas is the development of ﬁrst-row transition metal

compounds that replace precious metals in existing reactions

or potentially even mediate entirely new reactions, which are

not yet accessible. Iron is the most earth-abundant transition

metal, and it is a pivotal element for catalysis because it is

cheap and shows a high synthetic versatility due to its formal

oxidation states ranging from −2 to +6. Meanwhile, some of

the most important classes of large-scale chemical reactions are

catalytic reductions (hydrogenations, hydroborations, and

hydrosilylations) which play crucial roles in the synthesis of

numerous ﬁne chemicals, agrochemicals, fragrances, and food

additives.²In this context, low-valence highly reducing iron

species unsurprisingly became compounds of special interest,

which can be used as eﬃcient catalysts.³

Metal-mediated catalytic cycles typically consist of elemen-

tary two-electron processes such as oxidative addition or

reductive elimination. Precious metals such as rhodium

[Rh(I)/Rh(III)] or palladium [Pd(0)/Pd(II)] easily undergo

such redox reactions and hence are typically favored from this

point of view. In contrast, for ﬁrst-row transition metal

complexes, one-electron reactions are often observed. The

concept of using redox-active ligands was introduced for ﬁrst-

As a result, the synthesis of low-valence α-diimine iron

complexes bearing an unsaturated hydrocarbon as second

ligand has attracted signiﬁcant interest over the last two

decades (Figure 1).⁸⁻²⁰In these complexes, the hydrocarbon is

Received: September 3, 2020

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Figure 1. Previously reported low-valence α-diimine iron complexes with hydrocarbon ligands, and the new complexes presented in this work. R =

H, Me; R¹= H, (A−C) Me, (D, E) BIAN; R²= aryl, alkyl; Dipp = 2,6-diisopropylphenyl; solv = solvent molecule.

supposed to act as a “placeholder”, which is easily replaced by

other substrates to initiate the catalytic cycle. These hydro-

carbons are typically simple arenes as used for example in net-

neutral complexes A−E shown in Figure 1 synthesized by the

groups of Zenneck (A),⁸Chirik (B),⁹Song (C),¹⁰and

Findlater (D and E, all Figure 1).^11,12In these complexes, the

C−C bond of the α-diimine unit is signiﬁcantly shortened

relative to the free ligand, whereas the C−N bonds were

stretched, which can be explained by an electron transfer from

the metal center to the now formally anionic ligand. Of

particular interest are the complexes D and E by Findlater and

co-workers, who chose a bis(aryl)iminoacenaphthene (BIAN)

ligand with a polyarene backbone instead of a simple α-diimine

ligand with a saturated backbone.^11,12BIAN ligands have been

known since the 1960s and were established in coordination

chemistry by Elsevier and co-workers.^13,14This ligand class

beneﬁts from a rigid backbone that ensures that the donor

nitrogen atoms are ﬁxed in the cis orientation that is required

for metal coordination. Fedushkin and co-workers intensively

studied the redox behavior of the ^DippBIAN (Dipp = 2,6-

diisopropylphenyl) ligand. Remarkably, it is possible to reduce

^DippBIAN to its tetraanionic state using sodium metal.¹⁵

However, such an extreme formal charge is rather unusual in

transition metal complexes and usually neutral, monoanionic,

or dianionic BIAN ligands are observed. Findlater and co-

workers synthesized the [(^DippBIAN)Fe(arene)] (arene =

benzene, toluene) complexes D and E by reduction of

[(^DippBIAN)FeCl₂] with Na/Hg in the presence of the

corresponding arene. The benzene complex acts as active

precatalyst for the polymerization of ^L-lactide, whereas the

toluene adduct can be used as a precatalyst in the

hydrosilylation of ketones and aldehydes under solvent-free

conditions. Metrical data taken from single-crystal X-ray

crystallography support the hypothesis of a monoanionic α-

diimine ligand in those complexes. Independently, the group of

Jacobi von Wangelin studied the hydrogenation of oleﬁns

mediated by an in situ catalyst formed from [(^DippBIAN)FeCl₂]

and n-butyllithium in toluene.¹⁶

Chelating oleﬁns such as 1,5-cyclooctadiene (cod) can also

be used as labile hydrocarbon ligands. Such complexes have

been reported by the groups of Chirik, Ritter, and

Findlater.^9,17,18Chirik and co-workers synthesized the neutral

tetrahedral iron complex F, which contains cod and an α-

diimine unit based on biacetyl.⁹This complex is an eﬃcient

precatalyst for the hydrogenation of simple oleﬁns under mild

conditions (0.3 mol % [Fe], 4 bar H₂, r.t. (r.t. = room

temperature)), although the formation of the catalytically

inactive benzene complex B was observed for some styrenic

substrates. Ritter and co-workers observed the catalytic

dimerization of 1,3-butadiene to cod on a >100 g scale using

the similar complex H.¹⁷In the same report, the electronic

structure of this complex was investigated with various

techniques (SQUID magnetization measurements, ⁵⁷Fe

̈

Mossbauer spectroscopy, and DFT calculations), suggesting

the presence of a high-spin Fe(I) (S = 3/2) center, which is

antiferromagnetically coupled to an α-diimine radical anion.

Findlater and co-workers introduced the BIAN ligand in this

class of complexes, resulting in the formation of complex

[(^DippBIAN)Fe(η⁴-cod)] (I).¹⁸The group of Chirik further

contributed the neutral heteroleptic iron α-diimine complexes

J and K.¹⁹Both complexes are coordinated by an α-diimine

ligand based on biacetyl with a ligand-sphere completed either

by two −CH₂SiMe₃ligands (J) or by isoprene (K).

In contrast to the research on neutral heteroleptic iron α-

diimine complexes, there are only three reports about anionic

B

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Figure 2. Previously reported homoleptic low-valence α-diimine iron complexes and new anionic complexes presented in this work. R¹, R², R³, R⁴=

alkyl or aryl.

complexes, coming from the groups of Lichtenberg, de Bruin,

and Grutzmacher (L−N). In these complexes, the oleﬁn and

diimines investigated were all 1,4-diaza-1,3-dienes derived from

biacetyl and the resulting complexes were only analyzed by

elemental analysis. Later on, Walther and co-workers

investigated the chemistry of benzil dianil and electron-rich

metal centers P.²²The iron complex was investigated by

magnetic methods (μ_eff= 3.23 μ_Bat r.t.), as well as

spectroscopic techniques (IR, UV−vis). Chirik and co-workers

synthesized the neutral bis(α-diimine) iron complex Q by

reduction of the corresponding α-diimine iron(II) chloride

precursor with Na/Hg.⁹This was the ﬁrst compound of this

type that was structurally characterized by single-crystal X-ray

crystallography, revealing a tetrahedral geometry at iron. The

metrical data, in particular the relatively long C−C bond and

short C−N bond, in combination with magnetic susceptibility

measurements, suggested a signiﬁcant degree of α-diimine

ligand reduction. Wieghardt and co-workers subsequently

̈

α-diimine ligands were combined in one framework using a

trop₂dad ligand (trop = 5H-dibenzo[a,d]cycloheptene-5-yl,

dad = diazadiene).²⁰The central iron atom of complex L

adopts an almost square-planar geometry. The electronic

structures of these complexes were studied by various

experimental techniques and DFT calculations using the

broken-symmetry approach. Single-crystal X-ray crystallogra-

phy on the [Na(thf)₃]⁺salt L suggested the presence of a

dianionic diazadiene bound to Fe(I). The low-spin d⁷

electronic conﬁguration was further conﬁrmed by SQUID

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̈

analysis, Fe Mossbauer spectroscopy, and EPR studies.

Analogous complexes M and N featuring a saturated backbone

were also prepared and fully characterized. These complexes

were used for the eﬃcient dehydrogenation of dimethylamine-

borane and the dehydrogenative coupling of silanes and 1,4-

dibenzenemethanol.

provided a detailed analysis of the electronic structure of the

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̈

species R by a combination of Fe Mossbauer spectroscopy,

Although there are several reports on heteroleptic α-diimine

complexes, the corresponding homoleptic counterparts remain

scarce (Figure 2).^9,21−24These complexes have limited

potential for applications in catalysis because of their limited

capability to generate vacant coordination sites. Thus, such

species have mainly served as subjects for electronic structure

analysis. The group of tom Dieck synthesized the ﬁrst low-

valent bis(α-diimine) iron species O by the reduction of bis(α-

diimine) iron(II) chloride salts with sodium metal.²¹The α-

magnetic susceptibility measurements, single-crystal X-ray

crystallography, and density-functional theory using the

broken-symmetry approach.²³It was concluded that the

commonly used description of the electron structure with a

central Fe(0) atom is in fact not correct. Instead, the

complexes each contain two monoanionic α-diimine ligands

(S = 1/2) with a high-spin Fe(II) (S = 2) center. Because of

the antiferromagnetic coupling between the α-diimine ligands

and the iron atom, an overall spin of S = 1 is observed for the

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Scheme 1. Synthesis of the Anion [{(^DippBIAN)Fe(η⁴-cod)}]⁻(1) by Reduction of an Fe(II) Precursor (Left) or Ligand

Exchange Starting from an Fe(-II) Complex (Right)

Scheme 2. Synthesis of 2-[K([18]c-6)(thf)₂] by Reduction of [(^DippBIAN)FeBr₂] Using KC₈in the Presence of 2,5-

Norbornadiene

ground state. This work established the fact that α-diimines are

redox-active ligands. Later, a similar outcome was reported by

Fedushkin and co-workers, who synthesized neutral

[(^DippBIAN)₂Fe] (S) by the reaction of K[^DippBIAN] with

FeI₂.²⁴The C−C and C−N bond lengths of the BIAN ligand

have been very well investigated as a function of the oxidation

state of the ligand. In the case of [(^DippBIAN)₂Fe], the

structural data suggest the formation of a monoanionic ligand

and consequently an Fe(II) center. Magnetic measurements

and theoretical calculations also support the presence of

antiferromagnetic coupling.

Recently, we reported amine-borane dehydrogenation and

transfer hydrogenation catalyzed by the α-diimine cobaltates

[K(thf)_x{(^ArBIAN)Co(η⁴-cod)}] [Ar = Mes (2,4,6-trimethyl-

phenyl), Dipp].^7c,25This work showed that such highly

reduced α-diimine metal anions can be eﬀectively used in

reductive catalysis. This suggested that an analogous Fe

complex should be a promising precatalyst for similar

reactions. Herein, we report the synthesis, characterization,

and catalytic applications of highly reduced anionic hetero-

leptic and homoleptic α-diimine iron complexes.

monoanionic complex [Li(thf){(^DippBIAN)Fe(η⁴-cod)}] (1-

[Li(thf)]) and not the expected dianion. A plausible

explanation for this observation is that LiBIAN is formed as

a side product by an outer-sphere redox process, although this

has not been conﬁrmed experimentally. A ligand to metal

[

^DippBIAN:Fe(2−)] ratio of 1.5:1 seemed to be ideal after

optimization of the reaction conditions, although it is formally

a 2:1 reaction (see the Supporting Information for details).

Complex 1-[Li(thf)] was isolated as dark crystals after workup

and crystallization from n-hexane in 55% yield (41 mg) (see

Scheme 1). Unfortunately, scale up of this reaction proved to

be diﬃcult. Therefore, a second approach involving reduction

of the Fe(II) precursor [(^DippBIAN)FeBr₂] using strong

reductants such as alkali metals was investigated. [(^DippBIAN)-

FeBr₂] can be prepared in large scale (>20 g) in a single step

from ^DippBIAN and FeBr₂. Reduction of this complex using

potassium graphite (KC₈, 3 equiv.) in the presence of excess

1,5-cyclooctadiene was found to be more eﬃcient than with

lithium or potassium metal. Thus, [{(^DippBIAN)Fe(η⁴-cod)}]⁻

(1) was isolated as 1-[K([18]c-6)(thf)_0.5] in good yield (78%)

and at multigram scale (>7 g) after crystallization from THF/

n-hexane.

It is of further interest to investigate the inﬂuence of the

bis(oleﬁn) ligand with respect to the structure and reactivity.

The replacement of the 1,5-cyclooctadiene (cod) ligand in our

proposed ferrate precatalyst by 2,5-norbornadiene (nbd) was

attempted. Therefore, the same reductive approach was

pursued: reaction of [(^DippBIAN)FeBr₂] with potassium

graphite and an excess of 2,5-norbornadiene followed by the

addition of 18-crown-6 ([18]c-6) to facilitate crystallization

RESULTS AND DISCUSSION

■

Synthesis of an Anionic α-Diimine Fe Complex. As

[K(thf)_x{(^ArBIAN)Co(η⁴-cod)}] is synthesized from the

cobalt(1−) source [Co(η⁴-cod)₂]⁻by simple ligand exchange,

it was sought out to synthesize the analogous iron complex in

the same manner starting from dianionic bis(cylooctadiene)

ferrate(2−) [Li₂(dme)₄{Fe(η⁴-cod)₂}].²⁶Surprisingly, the

reaction of this ferrate with ^DippBIAN gave exclusively the

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Scheme 3. Synthesis of 3-[K([18]c-6)(thf)]

Figure 3. Solid-state structures of 1-[Li(thf)] (left) and 2-[K([2.2.2]cryptand)] (right) with thermal ellipsoids drawn at the 40% probability level.

Selected bond lengths (Å) and angles (deg) are as follows. For 1-[Li(thf)]: N1−C1 1.385(4), C1−C1′ 1.388(5), C21−C22 1.409(5), Fe1−N1

1.971(2), Fe1−C21 2.045(3), and N1−Fe1−N1′ 82.1(1). For 2-[K([2.2.2]cryptand)]: C1−C2 1.385(6) Å, C1−N1 1.382(6) Å, C2−N2 1.371(6)

Å, Fe1−N1 1.961(4) Å, Fe1−N2 2.001(3) Å, Fe1−C3 2.023(4) Å, Fe1−C4 2.042(5) Å, C3−C44 1.562(6) Å, C4−C44 1.585(6) Å, N1−Fe1−N2

84.8(2), and C3−Fe−C4 86.7(2). Hydrogen atoms, solvent molecules (three THF molecules in the case of 2-[K([2.2.2]cryptand)]) and cation 2-

[K([2.2.2]cryptand)]⁺are omitted for clarity.

from THF/n-hexane. Unexpectedly, a salt of the complex

[{(^DippBIAN)Fe(η⁴-nbd)}]⁻was not obtained under the

reaction conditions. Instead, single-crystal X-ray crystallog-

raphy revealed that two norbornadiene ligands underwent C−

C coupling to form a bis(norbornyl) ligand, resulting in a new

complex 2-[K([18]c-6)(thf)₂], which was isolated in high yield

(83%) as deep-green crystals (Scheme 2).

match the calculated value for 3-[K([18]c-6)(thf)] bearing

one THF molecule (calcd.: 73.77; found: 72.03), whereas the

values for hydrogen (calcd., 7.88; found, 7.57) and nitrogen

(calcd.: 3.91; found: 3.88) are close to the calculated values.

For sample 3₂-[K([18]c-6)(thf)], the CHN calculated values

are in reasonable agreement with the experimentally

determined values (C 73.30, H 7.76, N 3.81).

For the synthesis of homoleptic anionic bis(α-diimine) iron

complexes, the bis(anthracene) ferrate(1−) anion served as an

available source for an iron anion.²⁷First, the reactivity of

^DippBIAN toward this anion was investigated. Regardless if 1 or

2 equiv. of the α-diimine were added, only homoleptic and no

heteroleptic complexes were isolated (Scheme 3). Thus, the

replacement of both anthracene ligands furnished the

homoleptic bis α-diimine ferrate complex 3-[K([18]c-6)(thf)].

This complex is a rare example of an anionic bis(α-diimine)

complex.²⁸Complex 3-[K([18]c-6)(thf)], was isolated in 46%

crude yield and was recrystallized from THF/n-hexane (sample

3₁-[K([18]c-6)(thf)]), resulting in 15% yield of isolated

product. Another recrystallization of sample 3₁-[K([18]c-

6)(thf)] gave sample 3₂-[K([18]c-6)(thf)] in 7% yield. Sample

3₁-[K([18]c-6)(thf)] contains an unknown impurity, which

can be removed by the second recrystallization. The character-

ization for sample 3₁-[K([18]c-6)(thf)] is described in the

Supporting Information.

Single-Crystal X-ray Crystallography. Complexes 1-

[K([18]c-6)(thf)_0.5] and 1-[Li(thf)] were structurally charac-

terized by single-crystal X-ray crystallography. Unfortunately,

the structure of 1-[K([18]c-6)(thf)_0.5] was highly disordered

and it was impossible to determine reliable bond distances.

However, the connectivity and overall structure were

conﬁrmed. In contrast, the crystal structure of 1-[Li(thf)]

gave good values (R₁= 0.0383; wR₂= 0.0929) and is shown in

Figure 3. The complex crystallizes in the orthorhombic space

group Pmn2₁with two molecules per formula unit. The central

iron atom Fe1 is coordinated in an almost perfect square-

planar environment (twist angle of 0.5°) by two nitrogen

atoms and two CC double bonds. The elongated C1−N1

(1.385(4) Å) and shortened C1−C1′ (1.388(5) Å) bond

lengths are in the range of dianionic BIAN ligands (cf. 1.40 Å

(C−C bond) and 1.39 Å (C−N bond) for Na₂[^DippBIAN]).¹⁵

Consequently, an oxidation state of +1 with a d⁷conﬁguration

for iron can be assigned. The lithium cation is η⁴-coordinated

to the dianionic moiety of the BIAN framework as well as by

one THF molecule. Compound 1-[Li(thf)] is therefore a

According to CHN analysis, sample 3₁-[K([18]c-6)(thf)]

shows slight impurities as the carbon value does not perfectly

E

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contact-ion pair. It is worth noting that the recently published

cobaltate [Li(thf){(^DippBIAN)Co(η⁴-cod)}] is isostructural to

this compound.^7c

iron atom Fe1 is coordinated by the two nitrogen atoms of

BIAN (N1, N2) and two carbon atoms (C3, C4) of the

bis(norbornyl) ligand in a distorted tetrahedral structure with a

twist angle of 53.4°. Analysis of the C−C (C1−C2 1.385(6)

Å) and C−N (C1−N1 1.382(6) Å, C2−N2 1.371(6) Å)

bonds indicated a dianionic BIAN ligand.¹⁵In combination

with the dianionic bis(norbornyl) ligand an oxidation state of

+3 for iron can be assumed. Two molecules of norbornadiene

have clearly dimerized through formation of a C−C bond

(oleﬁn coupling) to give a bis(norbornyl) ligand and a ﬁve-

membered, perfectly planar ferracycle. Stereochemically, the

ligand exists in an exo-trans−exo conformation. Such a

norbornadiene dimerization at the metal center is rare and

only known for three other systems, two based on iridium and

one on zirconium. For the latter, Rosenthal and co-workers

used a zirconocene alkyne complex as starting material and

observed a twisted and not perfectly planar ﬁve-membered

zirconacycle.²⁹In 1980, Osborn, Fraser, and co-workers

reported on an octahedral iridium complex bearing a dimerized

norbornadiene ligand, one acetylacetonate ligand, and an

additional single η⁴-norbornadiene ligand.³⁰In there, the

iridacycle is also nonplanar, being folded with one carbon atom

displaced 0.24 Å out of the plane of the ring atoms. Recently,

Oro and co-workers also synthesized iridium complexes

containing this structural motif.³¹All are octahedral iridium

complexes with ﬁve-membered, nonplanar iridacycles.

Although crystallographic analysis of 2-[K([18]c-6)(thf)₂]

allowed determination of the connectivity of the ferrate anion,

signiﬁcant disorder prevented a more detailed structural

analysis. Fortunately, the [2.2.2]cryptand complex 2-[K-

([2.2.2]cryptand)] prepared in an analogous manner provided

a structure that could be reﬁned to a much more satisfactory

degree of accuracy. The complex crystallizes in the non-

centrosymmetric space group P2₁with one molecule per

asymmetric unit, which is shown in Figure 4. Here the central

Crystals of 3-[K([18]c-6)(thf)] suitable for single-crystal X-

ray diﬀraction were obtained by slow diﬀusion of n-hexane into

a THF solution. More than 10 crystals were examined. It is

noteworthy that all of these had the same space group and cell

parameters. A full single-crystal X-ray structure analysis on one

of the crystals revealed that complex 3-[K([18]c-6)(thf)]

crystallizes in the monoclinic space group P21/n with four

molecules of 3-[K([18]c-6)(thf)] per formula unit. The

molecular structure is shown in Figure 4. There is no close

interaction between the cation [K([18]c-6)(thf)₂]⁺and the

anion [(^DippBIAN)₂Fe]⁻. The iron atom is coordinated via four

nitrogen atoms. The dihedral angle between the two BIAN

Figure 4. Solid-state structure of 3-[K([18]c-6)(thf)] with thermal

ellipsoids drawn at the 40% probability level. Selected bond lengths

(Å) and angles (deg): N1−C1 1.351(3), N2−C2 1.359(2), N3−C3

1.362(3), N4−C4 1.364(2), C1−C2 1.411(3), C3−C4 1.406(3),

Fe1−N1 2.090(1), Fe1−N2 1.976(1), Fe1−N3 2.048(1), Fe1−N4

2.072(1), N1−Fe1−N2 81.58(1), and N3−Fe1−N4 82.56(6).

Hydrogen atoms and [K([18]c-6)(thf)]⁺cation are omitted for

clarity.

Figure 5. Experimental (Exp.) and simulated (Sim.) X-band EPR spectra of 1-[K([18]c-6)(thf)_0.5] (left) and 2-[K([18]c-6)(thf)₂] (right)

recorded in a ^MeTHF glass at 20 K. Experimental parameters: microwave frequency, 9.37466 GHz (1-[K([18]c-6)(thf)_0.5]) and 9.376835 GHz (2-

[K([18]c-6)(thf)₂]); modulation amplitude, 4.000 G; power, 0.6325 mW. The experimental spectrum of 1-[K([18]c-6)(thf)_0.5] could be ﬁtted

with the following parameters for an S = 1/2 system on a nucleus with a nuclear spin of 0, which we attribute to iron. g₁₁= 2.20, g₂₂= 2.10, g₃₃

=

2.01, W₁₁= 10, W₂₂= 11, and W₃₃= 13. The experimental spectrum of 2-[K([18]c-6)(thf)₂] can be ﬁtted using the following parameters for an S =

3/2 system. g₁₁= 2.25, g₂₂= 2.25, g₃₃= 1.90, D = 15 cm⁻¹, E = 0.8 cm⁻¹, E/D = 0.05.

F

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Figure 6. VT-¹H NMR spectra (400 MHz, THF-d₈) of complex 3-[K([18]c-6)(thf)] (sample 3₂-[K([18]c-6)(thf)]) in the range from 298 to 328

K.

ligands is 42.2° and the coordination geometry can therefore

be described as neither square planar nor tetrahedral, but

rather as a distorted structure that lies somewhere between

these two extremes. The C−C bond lengths of the α-diimine

fragment (C1−C2 1.411(3) Å, C3−C4 1.406(3) Å) are similar

to those observed in Na₂[^DippBIAN] (C1−C2 1.402(4) Å),

suggesting a dianionic BIAN ligand. It has to be mentioned

though that the experimentally observed C−N bond lengths

(C1−N1 1.351(3) Å, C2−N2 1.359(2) Å, C3−N3 1.362(3)

Å, C4−N4 1.364(2) Å) are in between the values for the

dianionic BIAN ligand in Na₂[^DippBIAN] (C1−N1 1.387(4) Å,

C2−N2 1.386(4) Å), and the monoanionic BIAN ligand (C1−

N1 1.336(3) Å, C1−N1 1.325(3) Å) in [(^DippBIAN)Ni(η⁴-

cod)].^15,32Consequently, a clear assignment from metric data

is not feasible.

NMR spectroscopy. 1-[K([18]c-6)(thf)_0.5] showed broad

resonances from 9 to −16 ppm in THF-d₈, whereas 2-

[K([18]c-6)(thf)₂] exhibits sharper signals in a range of 70 to

−55 ppm (see Figures S1 and S2). Both complexes were also

investigated by EPR spectroscopy in a 2-methyltetrahydrofuran

(^MeTHF) glass at 20 K (Figure 5). 1-[K([18]c-6)(thf)_0.5]

shows a rhombic signal with g₁₁= 2.22, g₂₂= 2.10, and g₃₃

=

2.01. No hyperﬁne interaction (HFI) to nitrogen is observed,

which in combination with the simulated g-values is consistent

with the proposed Fe(I) structure from single-crystal X-ray

crystallography. Calculation of the EPR parameters at the

B3LYP³³/def2-TZVP³⁴level of theory gave g values of 2.13,

2.08, and 2.01, which are close to the experimentally observed

g values. Hyperﬁne coupling constants were predicted to be

small for all protons (A^H≤ 1 MHz) and nitrogen atoms (A^N

11

¹H NMR and EPR Spectroscopy. 1-[K([18]c-6)(thf)_0.5]

= −4.73, A^N= −5.69, and A^N= −7.61). In all cases, these

22

33

1

and 2-[K([18]c-6)(thf)₂] are paramagnetic according to H

coupling constants were too small to be observed in the

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obtained experimental spectrum. Moreover, the small A^N

conﬁrms the proposed spin density on the iron center and

agrees with the presence of a dianionic closed-shell BIAN

ligand. 2-[K([18]c-6)(thf)₂] shows a completely diﬀerent EPR

spectrum. Only a broad signal at half ﬁeld was observed, which

indicated the presence of an intermediate or high-spin iron

complex. The experimental spectrum of 2-[K([18]c-6)(thf)₂]

as approximated by simulation using the same approach as for

complex 1-[K([18]c-6)(thf)_0.5] with an S = 3/2 system and g₁₁

= 2.25, g₂₂= 2.25, g₃₃= 1.90, D = 15 cm⁻¹, E = 0.8 cm⁻¹(E/D

= 0.05). Herein, D and E describe the axial and rhombic

component of the zero ﬁeld splitting, respectively. Notably, we

were not able to satisfactorily simulate the experimental

spectrum by consideration of an S = 5/2 system.

Mossbauer spectroscopy at 80 K. A polycrystalline sample of 1-

[K([18]c-6)(thf)_0.5] showed an asymmetric doublet spectrum

with an isomer shift of δ = 0.23 mm s⁻¹and a quadrupole

splitting of |ΔE_Q| = 2.26 mm s⁻¹(Figure 8, left). The

asymmetry of the doublet spectrum is typical for an iron

species with an odd number of unpaired electrons featuring

slow paramagnetic relaxation, and it is consequently in line

with the result of EPR spectroscopy. 2-[K([18]c-6)(thf)₂] also

gave a doublet with a similar isomer shift of δ = 0.17 mm s⁻¹,

but signiﬁcantly smaller quadrupole splitting of |ΔE_Q| = 1.32

mm s⁻¹(Figure 8, right). The ﬁve coordinated pyridine-

diimine iron tetrazene complex [(^iPrPDI)Fe{(NR)NN(NR)}]

(^iPrPDI = 2,6-(ArN = CMe)₂C₅H₃N; Ar = Dipp, R = 3,5-

Me₂C₆H₃) prepared by Chirik and co-workers was proposed to

̈

have an intermediate-spin Fe(III) center based on Mossbauer

Complex 3-[K([18]c-6)(thf)] is also paramagnetic in

parameters δ = 0.43 mm s⁻¹and |ΔE_Q| = 1.37 mm s⁻¹;³⁵

whereas the isomer shift in 2-[K([18]c-6)(thf)₂] is signiﬁ-

cantly lower, SQUID magnetometry (see below) indicates 2-

[K([18]c-6)(thf)₂] to also feature intermediate-spin Fe(III),

Complex 3-[K([18]c-6)(thf)] was also investigated by ⁵⁷Fe

1

solution according to H NMR spectroscopy in THF-d₈and

shows 14 resonances between +50 and −50 ppm (see Figure

S3). This is almost in accordance with the expected number of

resonances (15) for two chemically nonequivalent ^DippBIAN

ligands and one 18-crown-6 molecule. Another possibility is

the presence of two isomeric complexes. Variable-temperature

̈

Mossbauer spectroscopy at 80 K. As shown in Figure 9, this

1

polycrystalline sample of 3₂-[K([18]c-6)(thf)] gave rise to a

symmetric quadrupole doublet with an apparent isomer shift δ

= 0.78 mm s⁻¹and |ΔE_Q| = 2.10 mm s⁻¹. The values observed

for 3₂-[K([18]c-6)(thf)] are consistent with those of related

(VT) H NMR spectra of sample 3₂-[K([18]c-6)(thf)] in the

range from 298 to 328 K showed that no resonances appear or

disappear upon heating and only minor shifts of the signals are

observed (Figure 6).

The complex 3-[K([18]c-6)(thf)] (sample 3₂-[K([18]c-

6)(thf)]) was analyzed by EPR spectroscopy in a ^MeTHF glass

at 20 K. The resulting spectrum of sample 3₂-[K([18]c-

high-spin Fe(II) complexes. For example, Holland and co-

workers reported Fe Mossbauer data for a neutral tetra-

57

̈

coordinated iron complex with a tetrazene and a β-

diketiminato ligand.³⁶This compound shows a similar ⁵⁷Fe

6)(thf)] shows a rhombic signal with simulated g-values (g₁₁

=

−1

̈

Mossbauer isomer shift (δ = 0.69(2) mm s ) and a smaller

2.13, g₂₂= 2.10, g₃₃= 2.06) consistent with an S = 3/2 ground

state with large zero-ﬁeld splitting and rhombicity (D = 20

cm⁻¹, E = 4.1 cm⁻¹, E/D = 0.20) (see Figure 7). Notably, no

quadrupole splitting (|ΔE_Q| = 1.32(4) mm s⁻¹) as observed for

57

̈

3₂-[K([18]c-6)(thf)]. Comparable Fe Mossbauer data have

also been reported for the anionic reduction product of

hyperﬁne interaction to nitrogen nuclei is observed.

Holland’s complex (δ = 0.81(2) mm s⁻¹, |ΔE_Q| = 1.40(2) mm

57

̈

Fe Mossbauer Spectroscopy. 1-[K([18]c-6)(thf)_0.5]

36

57

s⁻¹). Nonetheless, the observed Fe Mossbauer data are

and 2-[K([18]c-6)(thf)₂] were further investigated by ⁵⁷Fe

̈

clearly in line with the presence of a high-spin Fe(II) species.

57

̈

Magnetic Measurements. The Fe Mossbauer data for

complex 1-[K([18]c-6)(thf)_0.5] are also in agreement with the

magnetic moment μ_eff= 2.0(1) μ_Bdetermined in solution

(THF-d₈) at ambient temperature by the Evans NMR

method,³⁷which corresponds to one unpaired electron

(spin-only value: 1.73 μ_B). Unfortunately, SQUID magnet-

ization measurements on 1-[K([18]c-6)(thf)_0.5] were un-

successful presumably due to the presence of minor amounts

of ferromagnetic particles in the sample. 2-[K([18]c-6)(thf)₂]

is also paramagnetic. The magnetic moment determined in

solution (THF-d₈) by Evans NMR method is μ_eff= 3.9(1) μ_B

at ambient temperature, which is almost identical to the spin-

only value for three unpaired electrons (3.87 μ_B). A SQUID

magnetization measurement in the temperature range 2−295

K gave χ_MT = 2.03 cm³mol⁻¹K at 295 K, which corresponds

to μ_eff= 4.0 μ_B(Figure 10). The measurement can be well

simulated with a spin S = 3/2 (g = 2.08, D = 17.0 cm⁻¹), which

is in line with the EPR results. Thus, these experiments further

support the assignment of an Fe(III) center with an

intermediate spin of S = 3/2.

The magnetic moment was determined for sample 3₂-

[K([18]c-6)(thf)] by the Evans NMR method in THF-d₈

solution.³⁷The observed magnetic moment was μ_eff= 4.7(1)

μ_B(or χ_MT = 2.76 cm³mol⁻¹K) at ambient temperature. This

value is much closer to the calculated spin-only values for three

unpaired electrons (1.875 cm³mol⁻¹K; quartet state) than for

ﬁve unpaired electrons (4.375 cm³mol⁻¹K; sextet state).

Figure 7. Experimental (Exp.) and simulated (Sim.) X-band EPR

spectra of 3-[K([18]c-6)(thf)] (sample 3₂-[K([18]c-6)(thf)])

recorded in ^MeTHF glass at 20 K. Experimental parameters:

microwave frequency, 9.375682 GHz; modulation amplitude, 4.000

G; power, 0.6325 mW. Sample 3₂-[K([18]c-6)(thf)] can be simulated

as an S = 3/2 species: g₁₁= 2.13, g₂₂= 2.10, g₃₃= 2.06, D = 20 cm⁻¹, E

= 4.1 cm⁻¹, E/D = 0.20.

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57

̈

Figure 8. Fe Mossbauer spectra of 1-[K([18]c-6)(thf)_0.5] (left) and 2-[K([18]c-6)(thf)₂] (right); exp = experimental, sim = simulated.

antiferromagnetic coupling of a HS-Fe(II) ion (S = 2) with the

ligand based radical (S = 1/2) with ﬁt parameters g = 2.38 and

D = 22.5 cm⁻¹. The simulation of the χ_MT measurement with

these parameters reveals a deviation from this model at

temperatures higher than 50 K. This deviation may be

explained by the presence of a small amount of magnetic

nanoparticles, which are even not detectable by elemental

57

̈

analysis and Fe Mossbauer, but are known to be readily

formed under highly reducing conditions as used for the

synthesis of the present iron complexes. The magnetization of

these particles is saturated and consequently their magnetic

susceptibility vanishes at high magnetic ﬁelds in VTVH

measurements. Attempts to analyze the χ_MT curve using a

model with two spin carriers and moderate antiferromagnetic

exchange were unsuccessful.

Quantum-Chemical Calculations. Theoretical investiga-

tions using DFT and CASSCF methods were used to analyze

the electronic structures of the anions of 1-[K([18]c-

6)(thf)_0.5] and 2-[K([18]c-6)(thf)₂] in comparison with

experimental results. Geometries were optimized at the

OPBE-D3BJ/def2-TVZP^34,38,39level of theory in the gas

phase. The optimized geometry for the anion of 1-[K([18]c-

6)(thf)_0.5] is in good agreement with the structure determined

by single-crystal X-ray crystallography. DFT calculations

conﬁrmed that spin density is localized at the metal center,

as shown in Figure 12 (left). Analysis of the orbitals showed

that the highest occupied orbital is a singly occupied molecular

orbital (SOMO) with high d character (see the Supporting

Information). Thus, a d⁷electronic conﬁguration with a Fe(I)

species is plausible and consistent with experimental and

theoretical data. State-averaged CASSCF-DLPNO-NEVPT2/

def2-mTZVP def2-TZVP/C calculations were conducted

including three diﬀerent ground state multiplicities (M_s= 2S

+1 = 2, 4, 6) to get an insight into the electronic structure of

the anion of 2-[K([18]c-6)(thf)₂]. The quartet state was

energetically most favored while the sextet (ΔE = +39.9 kcal

mol⁻¹) and doublet state (ΔE = +26.6 kcal mol⁻¹) are

signiﬁcantly higher in energy. The quartet state corresponds to

an Fe(III) intermediate spin system (S = 3/2) with d⁵

conﬁguration at iron. Moreover, the spin-density obtained

from a single-point DFT calculation is mainly localized on the

metal center (Figure 12, right). Thus, the theoretical data are

consistent with the experimental data.

57

̈

Figure 9. Fe Mossbauer spectrum of sample 3₂-[K([18]c-6)(thf)];

exp = experimental, sim = simulated.

Figure 10. χ_MT versus T plot of 2-[K([18]c-6)(thf)₂]; exp =

experimental, sim = simulated.

For more detailed analysis, the SQUID experiments on solid

3₂-[K([18]c-6)(thf)] in form of variable temperature−variable

ﬁeld (VTVH) magnetization measurement as well as temper-

ature dependent susceptibility measurement were performed

(Figure 11). The VTVH measurement can be successfully

ﬁtted assuming an S = 3/2 model as a result of strong

The anion of 3-[K([18]c-6)(thf)] was analyzed using the

same computational method (CASSCF) as mentioned for

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Figure 11. Variable temperature−variable ﬁeld (VTVH) magnetization measurements as M_molversus μ_BB/k_BT (left) and χ_MT versus T plot at 0.5

T (right) for 3-[K([18]c-6)(thf)] (sample 3₂-[K([18]c-6)(thf)]). Solid lines represent the calculated curve ﬁts (see text).

for a detailed discussion of the quantum chemical calcu-

lations).

Cyclic Voltammetry. Complexes 1-[K([18]c-6)(thf)_0.5]

and 2-[K([18]c-6)(thf)₂] both possess two potentially redox-

active sites: the central iron atom and the redox-active BIAN

ligand. Both complexes are similar in their redox properties

according to their cyclic voltammograms (Figures 13 and 14).

One reversible and at least one irreversible oxidation is

observed. In the case of complex 1, the chemically reversible

process at E_1/2= −1.98 V vs Fc/Fc⁺is consistent with previous

results by Findlater and co-workers, who prepared the neutral

complex [(^DippBIAN)Fe(η⁴-cod)] by Na/Hg reduction of

Figure 12. Spin-density plots according to Mulliken population

analysis of 1-[K([18]c-6)(thf)_0.5] (left) and 2-[K([18]c-6)(thf)₂]

[(^DippBIAN)FeCl₂] and cod.¹⁸On the basis of the structural

data obtained for the anionic complex 1-[K([18]c-6)(thf)_0.5]

(vide supra) and the neutral compound [(^DippBIAN)Fe(η⁴-

cod)] reported by Findlater and co-workers, both species

presumably contain an Fe(I) center. The structural parameters

(in particular C−C and C−N bond lengths) suggest the

presence of a BIAN²⁻dianion in 1-[K([18]c-6)(thf)_0.5] and a

BIAN⁻monoanion in [(^DippBIAN)Fe(η⁴-cod)]. Accordingly,

the reversible process observed for 1-[K([18]c-6)(thf)_0.5] at

(right); isosurface value = 0.05.

complex 2-[K([18]c-6)(thf)₂]. For both complexes, the

doublet states are signiﬁcantly higher in energy (+21 kcal

mol⁻¹) compared to the most stable high-spin conﬁguration.

The quartet and sextet conﬁgurations are very similar in

energy, the former being preferred by 1.7 kcal mol⁻¹for

complex 3-[K([18]c-6)(thf)]. On the basis of an analysis of

the orbital occupations resulting from the CASSCF-DLPNO-

NEVPT2 calculations, a high-spin Fe(II) center with one

monoanionic and one dianionic BIAN ligand is conceivable for

complex 3-[K([18]c-6)(thf)] (see the Supporting Information

E

_1/2= −1.98 V vs Fc/Fc⁺may be assigned as a ligand-centered

redox event.

Complex 2-[K([18]c-6)(thf)₂] can also be reversibly

oxidized by one electron at a similar, but slightly less negative

potential (E_1/2= −1.71 V vs Fc/Fc⁺). Based on the similar

Figure 13. Cyclic voltammograms of 1-[K([18]c-6)(thf)_0.5] in THF/ⁿBu₄NPF₆, scan rate: 100 mV s⁻¹(left) and 50 mV s⁻¹(right).

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Figure 14. Cyclic voltammograms of 2-[K([18]c-6)(thf)₂] in THF/ⁿBu₄NPF₆, scan rate: 100 mV s⁻¹(left) and 50 mV s⁻¹(right).

Figure 15. Cyclic voltammogram of 3₂-[K([18]c-6)(thf)] in THF/ⁿBu₄NPF₆in the range −2750 to 500 mV (left, scan rate: 100 mV s⁻¹) and

−2500 to −1400 mV (right, 50 mV s⁻¹).

structure of 1-[K([18]c-6)(thf)_0.5] and 2-[K([18]c-6)(thf)₂],

we speculate that this also a ligand-centered process. An

additional irreversible process observed at −0.85 V for 1-

[K([18]c-6)(thf)_0.5] and −0.8 V for 2-[K([18]c-6)(thf)₂] is

assigned to the oxidation of the complexes.

Introduction, it is a key challenge to develop earth-abundant

transition metal catalysts.⁴⁰The catalytic hydroboration of

carbonyl compounds using pinacolborane (HBpin) was chosen

as a proof of principle, which is an important organic reaction

and aﬀords widely used borate esters and the corresponding

alcohols by facile hydrolysis.^41,42This catalytic reaction has

been intensively studied using precious metal catalysts, but

there has also been recent progress in using 3d metal

complexes based on Mn,⁴³Co,⁴⁴Ni,⁴⁵and Fe.⁴⁶⁻⁵¹Although

there is a number of reports on iron-catalyzed alkene

hydroboration, only a few examples for the hydroboration of

carbonyl compounds are described in literature. Findlater and

co-workers reported the hydroboration of ketones and

aldehydes with HBpin (1.5 equiv.) using catalytic amounts of

Fe(acac)₃(10 mol %) and NaBEt₃H (10 mol %) in THF. The

reaction proceeds to completion at ambient temperature

within 1 day.⁴⁶Subsequently, Bai and co-workers described a

more eﬃcient system suitable for various ketones and

aldehydes using a lower catalyst loading (2.5 mol %) based

on a low-coordinate tritert-butylphosphoranimido-iron(II)

dimer.⁴⁷An iron(II) hydride complex was also developed by

Tong, Wang, and co-workers and used for the exclusive

hydroboration of aldehydes at low catalyst loading (0.1 mol

%).⁴⁸One of the most active systems for aldehydes so far is an

imine coupled [Fe−N₂S₂]₂complex (0.1 mol %) reported by

Baker and co-workers, which is capable of reducing aldehydes

Complex 3-[K([18]c-6)(thf)] shows three potentially redox-

active sites (one iron atom and two α-diimine ligands). The

cyclic voltammograms (CVs) of sample 3₂-[K([18]c-6)(thf)]

were recorded in THF/ⁿBu₄NPF₆. The CV of 3₂-[K([18]c-

6)(thf)] shows two quasi-reversible processes with waves at

−1.6 and −2.2 V vs Fc/Fc⁺and waves assigned to quasi-

reversible or irreversible processes at −0.4 V and −0.2 V

(Figure 15). As mentioned in the Introduction, Fedushkin and

co-workers synthesized the neutral [(^DippBIAN)₂Fe] complex

and proposed an Fe(II) center with two monoanionic BIAN

ligands.²⁴On the basis of this assignment, it seems likely that

the ﬁrst oxidation event is a ligand-centered process. There is

an additional wave at −1.8 V, which might arise from a

secondary redox process or an impurity. Moreover, the CV of

sample 3₂-[K([18]c-6)(thf)] shows an irreversible reduction at

−2.7 V.

Catalytic Hydroboration of Carbonyl Compounds.

Because of their high reduction potential, it was anticipated

that complexes 1−3 should be excellent candidates as catalysts

for reductive transformations such as hydrogenation, hydro-

boration, and hydrosilylation. As already mentioned in the

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to primary borate esters with HBpin within 30 min at ambient

temperature.⁴⁹Two systems based on the cooperation of iron

and silicon were reported by Fenske and co-workers and So

and co-workers. Both systems were applied in the hydro-

boration of ketones at ambient temperature with HBpin and

catalyst loadings of 1 mol % (Fenske) or 10 mol % (So).⁵⁰

Recently, Gade and co-workers showed that a chiral bis-

(oxazolinyl-methylidene)isoindoline iron alkyl complex is able

to catalyze the hydroboration of various functionalized

ketones, which led to chiral halohydrines, oxaheterocycles,

and amino-alcohols after workup. These reactions were

performed at −30 °C and achieved remarkable turnover

frequencies of more than 40 000 h⁻¹.⁵¹

and a small excess of HBpin (1.03 equiv.) at 25 °C in THF.

Under these optimized conditions, the complexes 2-[K([18]c-

6)(thf)₂] and 3-[K([18]c-6)(thf)] were tested as well.

Although the homoleptic complex 3-[K([18]c-6)(thf)] is

signiﬁcantly less active than 1-[K([18]c-6)(thf)_0.5], the

heteroleptic complex 2-[K([18]c-6)(thf)₂] displayed similar

activity. A plausible reason for the diﬀerent reactivities of 1-

[K([18]c-6)(thf)_0.5]/2-[K([18]c-6)(thf)₂] and 3-[K([18]c-6)-

(thf)] is the absence of a vacant coordination site. However,

the activation processes for 1-[K([18]c-6)(thf)_0.5] and 2-

[K([18]c-6)(thf)₂] must be diﬀerent because of the fact that 2-

[K([18]c-6)(thf)₂] is coordinated by two alkyl ligands.

Complex 1-[K([18]c-6)(thf)_0.5] was selected for further

investigations. The optimized reaction conditions described

above using 0.1 mol % of catalyst were applied to various para-

substituted acetophenone derivatives, in order to investigate

functional group tolerance. Gratifyingly, the system was found

to be compatible with electron-donating (OMe, Me), as well as

electron-withdrawing groups (CF₃). Furthermore, halogenated

(F, Cl, Br, I) acetophenones were cleanly hydroborated with

no observable dehydrohalogenation, a problem sometimes

observed for other highly reduced metalates.²⁵Pendant amine

(NMe₂) and carboxylic ester (COOMe) were also tolerated.

Furthermore, eﬃcient hydroboration was observed for 3-

methylacetophenone, propiophenone, the aliphatic substrate 2-

octanone, and 2-acetyl pyridine (Scheme 4). More sterically

hindered substrates such as 2-methylacetophenone or

benzophenone were also viable substrates, although slightly

extended reaction times were required to achieve full

conversion. Prolonged reaction times were also necessary for

substrates such as 1-cylohexylethan-1-one, benzaldehyde, and

α,β-unsaturated ketones. The hydroboration of 4-nitroaceto-

We decided to investigate the hydroboration of acetophe-

none using 1−3 and HBpin as a model reaction (Table 1).

Table 1. Hydroboration of Acetophenone with HBPin

Using Fe Precatalysts 1−3

a

HBpin

(equiv.)

solvent

(mL)

time

yield

a

entry

catalyst(mol %)

(min) (conversion)

1

1-[K([18]c-6)

1.03

THF

(0.1)

THF

(0.1)

THF

(0.1)

10

>99

(thf)_0.5] (0.1)

2

3

2-[K([18]c-6)

(thf)₂] (0.1)

3-[K([18]c-6)

(thf)] (0.1)

>99

67 (67)

a

Yields and conversions were determined by quantitative GC vs

internal n-pentadecane.

Optimization of the reaction conditions was performed using

α-diimine ferrate 1-[K([18]c-6)(thf)_0.5]. Best activity and full

conversion were ﬁrst obtained using 0.1 mol % catalyst loading

a

Scheme 4. Substrate Scope for the Hydroboration of Carbonyls Catalyzed by 1-[K([18]c-6)(thf)_0.5]

a

Yields correspond to isolated alcohols. Reaction conditions: Substrate (1 mmol), HBpin (1.03 mmol), 1-[K([18]c-6)(thf)_0.5] (0.001 mmol), THF

(0.5 mL), 25 °C. [a] = yield determined by quantitative GC-FID vs. internal n-pentadecane; [b] 30 min; [c] 1 h.

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Figure 16. Reaction proﬁle for of the hydroboration of acetophenone by HBpin catalyzed by 1-[K([18]c-6)(thf)_0.5]. Reaction conditions:

Acetophenone (2 mmol), HBpin (2 mmol), 1-[K([18]c-6)(thf)_0.5] (0.05 mol % (left), 0.036 mol % (right)), THF (4 mL), 25 °C. Data points

were obtained from time-resolved FT-IR spectra (see the Supporting Information for details).

phenone was not selective as nitro group reduction was

observed as well. 4-Acetylphenol was not hydroborated at all.

Notably, no CC bond reduction was observed for α,β-

unsaturated ketone substrates. Moreover, the aldehyde group

in 4-acetylbenzaldehyde was hydroborated more rapidly than

the ketone moiety as proven by GC-MS, despite benzaldehyde

itself requiring a longer reaction time for conversion than

acetophenone.

In order to gain more insight into the reaction mechanism,

the model hydroboration reaction of acetophenone, the

decrease in the CO stretching band (ν_(CO)= 1689

cm⁻¹) was monitored by in situ IR spectroscopy (Figure 16).

Full consumption of acetophenone was achieved within 30 s at

25 °C with a catalyst loading of 0.05 mol % (1-[K([18]c-

6)(thf)_0.5]); this corresponds to a turnover frequency of

220000 h⁻¹. Catalyst 1-[K([18]c-6)(thf)_0.5] therefore repre-

sents one of the most active iron catalysts for the hydro-

boration of ketones reported to date. For comparison, a

turnover frequency of 43 500 h⁻¹at −30 °C was reported by

Gade and co-workers for 4′-ﬂuoroacetophenone hydroboration

of HBPin using the current state-of-the-art bis(oxazolinyl-

methylidene)isoindoline iron catalyst.⁵¹

Unfortunately, attempts to further characterize or isolate the

resulting species have so far been unsuccessful.

CONCLUSION

■

This work shows that bis(aryl)iminoacenaphthene ligand

BIAN is able to stabilize highly reduced iron complexes. 1-

[K([18]c-6)(thf)_0.5], 2-[K([18]c-6)(thf)₂], and 3-[K([18]c-

6)(thf)] are accessible by reduction of α-diimine iron(II)

dibromide salts with potassium graphite or by ligand exchange

using suitable low-valence ferrate precursors. The stability of

these compounds is attributed to the redox-active nature of the

BIAN ligand, which is present in the mono- or dianionic state

in the molecular structures of these complexes. An additional

cod ligand is present in 1-[K([18]c-6)(thf)_0.5] and a

dimerization product of norbornadiene is observed for 2-

[K([18]c-6)(thf)₂], whereas compound 3-[K([18]c-6)(thf)] is

a homoleptic complex of iron with two BIAN ligands. The

electronic structures of the ferrate anions in these complexes

were analyzed in detail by various experimental techniques (¹H

NMR and EPR spectroscopy, single-crystal X-ray crystallog-

57

̈

raphy, Fe Mossbauer spectroscopy, SQUID magnetization

measurement) as well as theoretical calculations at the DFT

and CASSCF level. These data revealed an Fe(I) center with S

= 1/2 for 1-[K([18]c-6)(thf)_0.5] and an Fe(III) center with

rare intermediate spin S = 3/2 for 2-[K([18]c-6)(thf)₂]. For

complex 3-[K([18]c-6)(thf)], the S = 3/2 and S = 5/2 states

are close in energy with a small energy diﬀerence of 1.7 kcal

mol⁻¹in favor of the quartet state revealed by the CASSCF

calculations.

In addition to this monitoring study, we also conducted a

poisoning experiment using mercury, which is known to

selectively inhibit the catalytic activity of heterogeneous

catalysts. The hydroboration of acetophenone by 1-[K([18]-

c-6)(thf)_0.5] was not negatively inﬂuenced by the presence of

mercury (one drop).⁵²Several illustrative stoichiometric

reactions were also carried out. The reaction of a molar 1:1

ratio of 1-[K([18]c-6)(thf)_0.5] and HBPin did not lead to any

new signal in the ¹¹B NMR spectrum, suggesting a lack of any

direct, productive reaction between these two components.

Conversely, a brown solution of 1-[K([18]c-6)(thf)_0.5] turned

immediately green upon addition of 1 equiv. of acetophenone.

Moreover, the ν_(CO)stretching band of acetophenone

disappeared from the IR spectrum of the resulting mixture,

indicating a direct reaction between 1-[K([18]c-6)(thf)_0.5] and

As an initial test of their potential in homogeneous catalysis,

the complexes were successfully applied in the catalytic

hydroboration of carbonyl compounds with HBpin at ambient

temperature. Gratifyingly, 1-[K([18]c-6)(thf)_0.5] and 2-[K-

([18]c-6)(thf)₂] were highly active. In particular complex 1-

[K([18]c-6)(thf)_0.5] showed a high TOF = 220 000 h⁻¹for the

reduction of acetophenone and a broad functional group

tolerance. These initial results are promising for the future

application of highly reactive ferrate complexes as catalysts for

a wider range of catalytic transformations.

1

the substrate. A H NMR spectrum of the 1:2 reaction of 1-

[K([18]c-6)(thf)_0.5] and acetophenone in THF-d₈led to

disappearance of all signals of 1-[K([18]c-6)(thf)_0.5] in the

spectrum. New signals appeared in the range of 81 to −15 ppm

(see Figure S48). We conclude that complex 1-[K([18]c-

6)(thf)_0.5] seems to be the precatalyst of the reaction.

EXPERIMENTAL SECTION

Materials. All experiments were performed under an atmosphere

of dry argon (argon 4.6, Linde) using standard Schlenk techniques or

■

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an MBraun UniLab Glovebox. Solvents were dried and degassed with

an MBraun SPS800 solvent-puriﬁcation system. THF and diethyl

ether were stored over molecular sieves (3 Å). n-Hexane was stored

over a potassium mirror. 1,2-Dimethoxyethane was stirred over K/

benzophenone, distilled, and stored over molecular sieves (3 Å). All

chemicals were purchased from commercial suppliers and used as

received, if not stated otherwise. [Li₂(dme)₂{Fe(η⁴-cod)₂}],⁵³KC₈,⁵⁴

^DippBIAN,⁵⁵[(^DippBIAN)FeBr₂],⁵⁶and [K([18]c-6)(thf)₂][Fe(η⁴-

C₁₄H₁₀)₂]²⁷were prepared according to literature procedures.

Instruments. CHN analyses were recorded by the analytical

department of the University of Regensburg with a Micro Vario Cube

(Elementar). ESI-MS spectra carried out by the analytical department

of the University of Regensburg, Agilent Q-TOF 6549 UHD. Melting

points were measured on samples in sealed capillaries on a Stuart

SMP10 melting point apparatus. UV−vis spectra were recorded on an

Ocean Optics Flame spectrometer (Varian Cary 50 Spectropho-

tometer) in a Quartz cuvette with a layer thickness of 1 cm at room

temperature with a concentration of 1 × 10⁻⁴to 1 × 10⁻⁶M. ¹H, ¹¹B,

¹³C{¹H}, and ¹⁹F NMR spectra in solutions were recorded on Bruker

Avance 300 and BrukerAvance 400 if not stated otherwise. Chemical

shifts are given relative to solvents resonances in the tetramethylsilane

(¹H, ¹³C), BF₃·OEt₂(¹¹B), CFCl₃(¹⁹F) scale. The following

abbreviations have been used for multiplicities: s = singlet, d =

doublet, t = triplet, q = quartet, sept = septet, m = multiplet, dd =

doublet of doublet, dt = doublet of triplet.

Single-Crystal X-ray Crystallography. The single-crystal X-ray

diﬀraction (XRD) data were recorded on an Agilent Super Nova

diﬀractometer with an Atlas CCD detector. Microfocus Cu K_α

radiation (λ = 1.54184 Å) was used in each measurement. Empirical

multiscan and analytical absorption corrections⁶²were applied to the

data. The structures were solved with SHELXT⁶³and least-squares

reﬁnements on F²were carried out with SHELXL.⁶⁴The program

package Olex2⁶⁵was used. Crystal data for 1-[Li(thf)], 2-[K([2.2.2]-

cryptand)(thf)₃], and 3-[K([18]c-6)(thf)] are given in section 4

below.

Synthesis of [K([18]c-6)(thf)_0.5][(^DippBIAN)Fe(η⁴-cod)] (1-[K([18]c-

6)(thf)_0.5]). 1,5-Cyclooctadiene (1.0 mL, 8.1 mmol, 5.8 equiv.) was

added to a solution of [(^DippBIAN)FeBr₂] (1.0 g, 1.4 mmol, 1.0

equiv.) in 100 mL of THF. The solution was cooled to −60 °C and

KC₈(0.6 g, 4.4 mmol, 3.15 equiv.) was added in portions over 5 min

and then slowly warmed to ambient temperature. Meanwhile, the

solution turned yellow-brownish. After stirring overnight at ambient

temperature, the resulting suspension was ﬁltered and [18]c-6 (0.37 g,

1.4 mmol, 1.0 equiv.) in 15 mL of THF was added. The solution was

concentrated to 80 mL of THF, layered with 80 mL of n-hexane and

stored at −10 °C. Brown-green block shaped crystals were formed

within a few days. [K([18]c-6)(thf)_0.5][(^DippBIAN)Fe(η⁴-cod)] (1-

[K([18]c-6)(thf)_0.5]) was isolated in 78% yield by decanting the

solvent and drying the remaining crystalline solid in vacuo. Crystals

suitable for single-crystal X-ray crystallography were obtained by

recrystallization from THF/n-hexane. Yield: 1.09 g (1.086 mmol,

Cyclic Voltammetry. Cyclic voltammetry experiments were

performed in a single-compartment cell inside a nitrogen-ﬁlled

glovebox using a CH Instruments CH1600E potentiostat. The cell

was equipped with a platinum disc working electrode (2 mm

diameter) polished with 0.05 μm alumina paste, a platinum wire

counter electrode, and an Ag/AgNO₃reference electrode. The

supporting electrolyte, tetra-n-butylammonium hexaﬂuorophosphate,

was dried in vacuo at 110 °C for 3 days. All redox potentials are

reported vs the ferrocenium/ferrocene (Fc⁺/Fc) couple.

1

78%). Melting point: T > 70 °C: decomposition to a black oil. H

NMR (400.13 MHz, 300 K, THF-d₈): δ [ppm]: 8.63 (m, Int 1), 4.77

(br s, Int 0.7), 4.35 (br s, Int 1), 3.51 (br s, overlap with THF-d₈, Int

12.5), 2.18 (br s, Int 3.3), 1.29 (br s, Int 1.7), 0.89 (br s, Int 1), 0.43

(br s, Int 2.5), − 16.2 (br s). EPR: Measured in ^MeTHF glass at 20 K.

Microwave frequency, 9.37466 GHz; modulation amplitude, 4.000 G;

power, 0.6325 mV. The experimental spectrum could be ﬁtted with

the following parameters for an S = 1/2 system on a nucleus with a

nuclear spin of 0. g₁= 2.217, g₂= 2.095, g₃= 2.005, W₁= 10, W₂= 11,

and W₃= 13. UV−vis (THF): λ_max/nm (ε/L mol⁻¹cm⁻¹) = 294

(21 000), 408 (12 000), 697 (5000). CHN analysis found (calcd.) for

C₅₆H₇₆N₂KFeO₆·(C₄H₈O)_0.5: C, 69.78 (69.37); H, 7.82 (8.03); N,

2.47 (2.79). Magnetic moment (Evans NMR method in THF-d₈₋a₁t

EPR Spectroscopy. The experimental X-band EPR spectra was

recorded on a Bruker EMX spectrometer (Bruker BioSpin

Rheinstetten) equipped with a He temperature-control cryostat

system (Oxford Instruments). The g values were calculated with the

ORCA software package⁵⁷at the B3LYP⁵⁸/def2-TZVP⁵⁹level of

theory. The spectra of 1−3 were analyzed and simulated using the

W95EPR program of Prof. Frank Neese or EasySpin (easyspin.org).⁶⁰

298 K): μ_eff= 2.0(1) μ_B. ⁵⁷Fe Moßbauer spectrum: δ = 0.23 mm s ;

̈

|ΔE_Q| = 2.26 mm s⁻¹.

57

Fe Mossbauer Spectroscopy. Fe Mossbauer spectra were

Synthesis of [Li(thf){(^DippBIAN)Fe(η⁴-cod)}] (1-[Li(thf)]). ^DippBIAN

(75.2 mg, 0.15 mmol, 1.5 equiv.) was dissolved in 2 mL of THF and

added dropwise as cooled (−30 °C) solution to a −30 °C cold

solution of [Li₂(dme)₂{Fe(η⁴-cod)₂] (46.7 mg, 0.10 mmol, 1.0

equiv.) in 2 mL of THF (Note: the dissolved ferrate contains some

black particles). The olive-green solution turned dark brown and was

warmed up to room temperature and stirred for 24 h. The solvent was

evaporated and residue extracted with 20 mL n-hexane and ﬁltered.

Dark green-brown crystals were obtained by slow evaporation at room

temperature and isolated by decanting the mother liquor and dried in

̈

recorded with a ⁵⁷Co source in a Rh matrix using an alternating

̈

constant acceleration Wissel Mossbauer spectrometer operated in the

transmission mode and equipped with a Janis closed cycle helium

cryostat. Isomer shifts are given relative to Fe metal at ambient

temperature. Simulation of the experimental data was performed with

the Mfit program using Lorentzian line doublets.⁶¹

Magnetic Susceptibility Measurements. Temperature-depend-

ent magnetic susceptibility measurements were carried out with a

Quantum-Design MPMS-XL-5 or with a MPMS3 SQUID magneto-

meter in the range from 295 (or 300) to 2.0 K at a magnetic ﬁeld of

0.5 T. The powdered samples were contained in a polypropylene or

polycarbonate capsule and ﬁxed in a nonmagnetic straw. Each raw

data ﬁle for the measured magnetic moment was corrected for the

diamagnetic contribution of the sample holder. The molar

susceptibility data were corrected for the diamagnetic contribution.

Experimental data for 2-[K([18]c-6)(thf)₂] and 3₂-[K([18]c-6)(thf)]

were modeled with the julX program using a ﬁtting procedure to the

appropriate spin Hamiltonian for Zeeman splitting and zero-ﬁeld

splitting, eq 1.

1

vacuo. Yield: 40.8 mg (0.055 mmol, 55%). H NMR (400.13 MHz,

300 K, THF-d₈): δ [ppm]: 8.73 (br s), 7.20 (br s), 6.92 (br s), 6.07

(br s), 5.82 (br s), 5.50 (br s), 5.17−4.04 (m), 3.04−2.02 (m), 1.39−

0.82 (m), 0.41 (br s), − 0.98 (br s). CHN analysis found (calcd.) for

C₄₈H₆₀N₂LiFeO: C, 76.57 (77.51); H, 7.88 (8.13); N, 3.55 (3.77).

Synthesis of [K([18]c-6)(thf)₂][(^DippBIAN)Fe(C₁₄H₁₆)] (2-[K([18]c-

6)(thf)₂]). [(^DippBIAN)FeBr₂] (506 mg, 0.71 mmol, 1.0 equiv.) and

2,5-norbornadiene (1.4 mL, 13.8 mmol, 19.4 equiv.) were dissolved in

10 mL THF and cooled to −70 °C. Potassium graphite (KC₈, 304

mg, 2.25 mmol, 3.2 equiv.) was added in small portions. The color

changed from orange-brown to deep green. The reaction mixture was

warmed to room temperature and stirred overnight. The suspension

was ﬁltered and [18]c-6 (187 mg, 0.71 mmol, 1.0 equiv.) in 5 mL of

THF was added to the ﬁltrate. The solution was reduced to 10 mL

and layered with 25 mL of n-hexane. Dark-green crystals of [K([18]c-

6)(thf)₂][(^DippBIAN)Fe(C₁₄H₁₆)] (2-[K([18]c-6)(thf)₂]) were

formed within 1 day, isolated, and dried in vacuo. Recrystallization

from THF/n-hexane aﬀorded crystals that were suitable for single-

crystal X-ray crystallography. The same reaction was conducted with

Ä

É

Å

Ñ

2

z

1

3

̂

⃗

H = μ BS + D S − S(S + 1)

Å

B

Å

(1)

Å

Ç

Ñ

Ö

For 2-[K([18]c-6)(thf)₂], the temperature-independent paramagnet-

ism (TIP = 650 × 10⁻⁶cm³mol⁻¹) was included.

Quantum Chemical Calculations. DFT and CASSCF calcu-

lations were performed using the ORCA program package.⁵⁷See the

Supporting Information for further details.

N

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̈

[2.2.2]cryptand instead of [18]c-6 to obtain crystals of the

[2.2.2]cryptand complex 2-[K([2.2.2]cryptand)] suitable for single-

crystal X-ray crystallography, which was not further characterized.

Characterization data for 2-[K([18]c-6)(thf)₂]: Yield: 0.7 g (0.59

mmol, 83%). Melting point: T > 200 °C: decomposition to a black

oil. ¹H NMR (400.13 MHz, 300 K, THF-d₈): δ [ppm, Int = integral]:

69.2 (s, Int 1.5), 44.0 (s, Int 1), 30.5 (s, Int 1.2), 23.4 (s, Int 2), 14.6

(br s, Int 1.7), 14.4 (s, Int 2), 11.6 (s, Int 1.8), 4.9 (br s, Int 6), 4.8 (s,

Int 3.5), 3.4 (s, Int 24, 18[c]-6), − 7.9 (m, Int 7.4), − 10.2 (s, Int

1,5), − 15.6 (s, Int 4.7), − 55.4 (br s, Int 0.06). EPR, measured in

^MeTHF glass at 20 K; microwave frequency, 9.376835 GHz;

modulation amplitude, 4.000 G; power, 0.6325 mV. The experimental

spectrum of 2-[K([18]c-6)(thf)₂] can be ﬁtted using the following

parameters for a S = 3/2 system. g₁₁= 2.25, g₂₂= 2.25, g₃₃= 1.90, D =

15 cm⁻¹, E = 0.8 cm⁻¹, E/D = 0.05. UV−vis (THF): λ_max/nm (ε/L

mol⁻¹cm⁻¹) = 298 (22 000), 366 (13 500), 418 (8000), 740 (5000).

CHN analysis found (calcd) for C₆₂H₇₆N₂KFeO₆·(C₄H₈O)₂: C, 70.57

(70.74); H, 7.84 (8.14); N, 2.13 (2.36). Magnetic moment (Evans

[K([18]c-6)(thf)], μ_eff= 4.7(1) μ_B. ⁵⁷Fe Moßbauer spectrum: sample

3₂-[K([18]c-6)(thf)], δ = 0.78 mm s⁻¹, |ΔE_Q| = 2.10 mm s⁻¹; sample

3₁-[K([18]c-6)(thf)], δ = 0.76 mm s⁻¹, |ΔE_Q| = 1.63 mm s⁻¹. A ﬁtting

procedure revealed that the simulated spectrum can be divided into

two subunits (sub1 and sub2) with isomer shifts of δ = 0.75 mm s⁻¹

(share of 75%, sub1) and δ = 0.78 mm s⁻¹(share of 25%, sub2). The

corresponding quadrupole splitting values are |ΔE_Q| = 1.60 mm s⁻¹

(sub1) and |ΔE_Q| = 2.17 mm s⁻¹(sub2).

ASSOCIATED CONTENT

* Supporting Information

■

sı

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02606.

Single-crystal X-ray crystallography; ¹H NMR, EPR, ⁵⁷Fe

Moßbauer, and UV−vis spectra; cyclic voltammetry;

computational details; catalytic hydroboration (PDF)

̈

NMR method in THF-d₈at 298 K), μ_eff= 3.9(1) μ_B; ⁵⁷Fe Moßbauer

Accession Codes

̈

spectrum, δ = 0.17 mm s⁻¹; |ΔE_Q| = 1.32 mm s⁻¹.

CCDC 2026690−2026692 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.

Synthesis of [K([18]c-6)(thf)_0.5][(^DippBIAN)₂Fe] (3-[K([18]c-6)(thf)]).

[K([18]c-6)(thf)₂][Fe(η⁴-C₁₄H₁₀)₂] (653 mg, 0.76 mmol, 1.0 equiv.)

was dissolved in 60 mL of THF and ^DippBIAN (761 mg, 1.52 mmol,

2.0 equiv.) was added as solid to the solution. The color immediately

changed from green to orange and the reaction mixture was stirred for

48 h. The solvent was removed and the residue washed with 20 mL of

n-hexane. The residue was extracted with 40 mL of THF and ﬁltered.

The solution was layered with 50 mL of n-hexane and stored at room

temperature. After a few days, black crystals of 3-[K([18]c-6)(thf)]

were isolated (490 mg, 0.35 mmol, 46%) and dried in vacuo. Several

samples were used for elemental analysis and spectroscopic

characterization. These samples were obtained by two diﬀerent

methods: Samples 3₁-[K([18]c-6)(thf)]: Samples labeled 3₁-[K([18]-

c-6)(thf)] were obtained by recrystallizing the crude product from

THF/n-hexane. Samples 3₂-[K([18]c-6)(thf)]: This sample was

obtained by recrystallization of a sample 3₁-[K([18]c-6)(thf)] from

THF/n-hexane. For both types of samples, crystals suitable for single-

crystal X-ray crystallography were obtained by diﬀusion of n-hexane

into a concentrated THF solution. Several crystalline samples were

examined by X-ray crystallography. All examined crystals had the

same unit-cell parameters as reported in the Supporting Information

(section 6.5.4). Yield of crude product, 490 mg (0.35 mmol, 46%);

sample 3₁-[K([18]c-6)(thf)], 163 mg (0.11 mmol, 15%); sample 3₂-

[K([18]c-6)(thf)], 82 mg (0.06 mmol, 7%); melting point (sample

AUTHOR INFORMATION

Corresponding Author

■

Robert Wolf − Institute of Inorganic Chemistry, University of

Regensburg, 93040 Regensburg, Germany; orcid.org/0000-

0003-4066-6483; Email: robert.wolf@ur.de

Authors

Thomas M. Maier − Institute of Inorganic Chemistry, University

of Regensburg, 93040 Regensburg, Germany

Martin Gawron − Institute of Inorganic Chemistry, University of

Regensburg, 93040 Regensburg, Germany

Peter Coburger − Institute of Inorganic Chemistry, University of

Regensburg, 93040 Regensburg, Germany

Michael Bodensteiner − Institute of Inorganic Chemistry,

University of Regensburg, 93040 Regensburg, Germany

Nicolaas P. van Leest − van ’t Hoﬀ Institute for Molecular

Sciences (HIMS), University of Amsterdam, 1098 XH

Amsterdam, The Netherlands

Bas de Bruin − van ’t Hoﬀ Institute for Molecular Sciences

(HIMS), University of Amsterdam, 1098 XH Amsterdam, The

Netherlands; orcid.org/0000-0002-3482-7669

Serhiy Demeshko − Institute of Inorganic Chemistry, University

of Gottingen, 37077 Gottingen, Germany

Franc Meyer − Institute of Inorganic Chemistry and

International Center for Advanced Studies of Energy Conversion

1

3₁-[K([18]c-6)(thf)]), T > 200 °C; decomposition to a black oil. H

NMR (400.13 MHz, 300 K, THF-d₈): of sample 3₂-[K([18]c-

6)(thf)]: δ [ppm]: 47.3 (s, Int: 1.0), 46.8 (br s, Int: 0.8), 7.1 (br s,

Int: 3.5), 5.2 (s, Int: 1.5), 3.7 (br s, Int: 6.5), 3.58 (THF_coordinated), 2.8

(br s, Int: 1), 2.3 (br s, Int: 1), 1.74 (THF_coordinated), 1.29 (n-hexane),

1.2 (br s, Int: 0.4), 1.0 (br s, Int: 0.5), 0.89 (n-hexane), −3.4 (br s,

Int: 2.6), −3.4 (s, Int: 2.8), −11.0 (br s, Int: 1.0), −17.3 (br s, Int:

2.8), −48.3 (br s, Int: 0.4). EPR: Samples 3₁-[K([18]c-6)(thf)] and

3₂-[K([18]c-6)(thf)] were measured in a frozen ^MeTHF glass at 20 K.

Sample 3₁-[K([18]c-6)(thf)]: microwave frequency, 9.375682 GHz;

modulation amplitude, 4.000 G; power, 0.6325 mW. Sample 3₁-

[K([18]c-6)(thf)] can be simulated as an S = 3/2 species: g₁₁= 2.20,

̈

(ICASEC), University of Gottingen, 37077 Gottingen,

Germany; orcid.org/0000-0002-8613-7862

g

₂₂= 2.12, g₃₃= 2.06, D = 20 cm⁻¹, E = 4.1 cm⁻¹, E/D = 0.20. Sample

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c02606

3₂-[K([18]c-6)(thf)]: microwave frequency, 9.375682 GHz; modu-

lation amplitude, 4.000 G; power, 0.6325 mV. Sample 3₂-[K([18]c-

6)(thf)] can be simulated as a S = 3/2 species: g₁₁= 2.13, g₂₂= 2.10,

g₃₃= 2.06, D = 20 cm⁻¹, E = 4.1 cm⁻¹, E/D = 0.20. UV−vis (THF):

λ_max/nm (ε/L mol⁻¹cm⁻¹). Sample 3₁-[K([18]c-6)(thf)]: 360

(20000, shoulder), 435 (7000), 505 (6500), 588 (3500). Sample

3₂-[K([18]c-6)(thf)]: 360 (18000, shoulder), 435 (8500), 505

(8000), 588 (4000). CHN analysis found (calcd.) for

C₈₄H₁₀₄N₄KFeO₆·(C₄H₈O). Results for sample 3₁-[K([18]c-6)(thf)]:

C, 72.03 (73.77); H, 7.57 (7.88); N, 3.88 (3.91). Results for sample

3₂-[K([18]c-6)(thf)]: C, 73.30 (73.77); H, 7.76 (7.88); N, 3.81

(3.91). Magnetic moment (Evans NMR method in THF-d₈at 298

K): sample 3₁-[K([18]c-6)(thf)], μ_eff= 4.7(1) μ_B; sample 3₂-

Author Contributions

T.M.M. performed the experimental work and wrote the

manuscript. M.G. assisted with the synthetic work. P.C.

assisted with the quantum-chemical calculations. N.P.v.L. and

B.d.B. recorded and analyzed EPR data. M.B. helped with the

reﬁnement of single-crystal X-ray crystallography data. S.D.

57

̈

and F.M. conducted and interpreted Fe Mossbauer spectra

and SQUID magnetization measurements. R.W. supervised

and directed the project.

O

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Article

(10) Janes, T.; Rawson, J. M.; Song, D. Syntheses and structures of

Li, Fe, and Mo derivatives of N,N ′-bis(2,6-diisopropylphenyl)-o-

phenylenediamine. Dalton Trans. 2013, 42, 10640−10648.

(11) Wekesa, F. S.; Arias-Ugarte, R.; Kong, L.; Sumner, Z.;

McGovern, G. P.; Findlater, M. Iron-Catalyzed Hydrosilylation of

Aldehydes and Ketones under Solvent-Free Conditions. Organo-

metallics 2015, 34, 5051−5056.

Funding

European Reseach Council (ERC GoG 772299) Fonds der

Chemischen Industrie (fellowship to T.M.M.)

Notes

The authors declare no competing ﬁnancial interest.

(12) Brown, L. A.; Wekesa, F. S.; Unruh, D. K.; Findlater, M.; Long,

B. K. BIAN-Fe(η⁶-C₆H₆): Synthesis, characterization, and l-lactide

polymerization. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2824−

2830.

ACKNOWLEDGMENTS

■

Financial support by the European Research Council (ERC

CoG 772299) and the Fonds der Chemischen Industrie

(fellowship to T.M.M.) is gratefully acknowledged. We thank

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(b) Dvolaitzky, M. C. R. Acad. Sci. Prais, Ser. C 1969, 268, 1811.

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Julia Marsch (University of Regensburg) and Andreas Rosch

(University of Regensburg) for experimental assistance, and

̈

Dr. Daniel J. Scott (Universitat Regensburg) for assistance

with manuscript preparation. Prof. Robert Kretschmer

(University of Jena) is thanked for providing access to a

ReactIR device.

Asselt, R.; Gielens, E. E. C. G.; Rulke, R. E.; Elsevier, C. J. Isolation of

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alkyl- and acyl-palladium complexes containing rigid bidentate

nitrogen ligands by stepwise successive insertion of CO and alkenes.

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