(Electrochemical) Properties and Computational Investigations of Ferrocenyl-substituted Fe3(μ3-PFc)2(CO)9 and Co4(μ4-PFc)2(CO)9 Clusters and Their Reduced Species

Clusters and Their Reduced Species

Article

(

Electrochemical) Properties and Computational Investigations of

Ferrocenyl-substituted Fe (μ ‑PFc) (CO) and Co (μ ‑PFc) (CO)

Marcus Korb,* Xianming Liu, Sebastian Walz, Marco Rosenkranz, Evgenia Dmitrieva, Alexey A. Popov,

and Heinrich Lang *

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c00276

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sı Supporting Information

ABSTRACT: The formation of ferrocenyl-functionalized iron and

cobalt carbonyl clusters is reported, based on a reaction of FcPCl

(

3) (Fc = Fe(η -C H )(η -C H )) with Fe (CO) and Co (CO) ,

5 5 5 4 2 9 2 8

respectively. Therein, nido-Fe (CO) (μ -PFc) (4) and nido-

Co (CO) (μ -PFc) (5) clusters were obtained as the ﬁrst

diferrocenyl-substituted carbonyl clusters with a symmetrical cluster

core. Cluster 4 shows two reversible one-electron processes within

the anodic region, based on Fc/Fc redox events, as well as two

processes in the cathodic region. In situ IR and electron

paramagnetic resonance (EPR) measurements of all electronic states

conﬁrmed an Fc-based oxidation and a core-based reduction. On the basis of the results of a single-crystal X-ray analysis of structures

of 4 and 5, computational studies of the highest occupied molecular orbital−lowest unoccupied molecular orbital energies, the spin

density, quantum theory of atom-in-molecule delocalization indices, and the atomic charges were performed to explain the

experimental results. The latter revealed a reorganization of the cluster core upon reduction and the existence of weak P···P

interactions in 4 and 5. Ferrocenyl-related redox processes, occurring reversibly in case of 4, were absent for 5, due to a diﬀerent

distribution of the HOMO energies. EPR measurements furthermore conﬁrmed the core-based radical anion and the formation of a

2−

decomposition product at potentials lower than [M] (M = Fe, Co).

INTRODUCTION

clusters.

Furthermore, their interactions with unpaired

■

electrons gives further information within electron para-

magnetic resonance (EPR) measurements. Introduction of

ferrocenyls has yet been achieved by replacement of carbonyls

Compounds bearing redox-active ferrocenyl groups are of

broad interest in organic and organometallic chemistry, since

they allow for the study of several features, for example,

electronic communication between the redox centers (donor−

with various ferrocenylphosphines, for example, FcPPh or

,1′-bis(diphenylphosphino)ferrocene (dppf), in a subsequent

acceptor) through a conjugated backbone. Several six- and

ﬁve-membered (hetero)aromatics were investigated with

regard to their ability to conduct electrons through oligomers

based on their 2,5-connection pattern. The ferrocenyl termini

has been chosen, due to its stability in both the neutral and the

oxidized forms, which allows study of the electronic properties

in situ and of the respective isolated species. Furthermore,

well-separated redox events can be observed for polyferroceny-

lated systems, which allows study of even more complex

reaction step after the synthesis of the cluster itself or within

catalytic reactions using metal carbonyls such as Fe (CO) .

2 5

Although numerous cluster geometries and substituents were

investigated, the direct attachment of a ferrocenyl group at a

cluster P atom constituent is still pending, due to the elaborate

synthesis of FcPCl₂. Therefore, attempts to synthesize

carbonyl clusters solely resulted in a small yield of

polyferrocenyl cluster A (Chart 1), whereby FcP and Fc₂P

groups were incorporated, due to the inaccessibility of pure

−4

molecules.

Although well-studied in the 1970s, iron- and cobalt-based

carbonyl clusters have yet not been investigated toward their

ability to conduct electrons between neutral and oxidized

redox-active groups. Examples, where the Fc moiety is attached

via a carbon atom, have already been reported, by, for example,

protecting ethynylferrocenes within a Nicolas reactions or by

the reaction of FcHgCl and HCCo (CO) . Of special interest

FcPCl₂. Further examples are the single Fc-substituted

Received: January 28, 2020

is the incorporation of phosphorus atoms, since their variable

binding modes allow for the stabilization of diﬀerent types of

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Chart 1. Clusters

obtained phosphine 2 with a 1.9 M phosgene solution,

based on a recently published procedure, gave 3 in an excellent

overall yield of 76%.

For the synthesis of P-containing metal carbonyl clusters

various synthetic methodologies were established, that is, the

reaction of PRCl (R = single-bonded organic group) with

Na [Fe(CO) ] or Na[Co(CO) ], or with neutral metal

carbonyls such as Fe (CO) , of which the latter acts as metal

carbonyl source and as reducing agent, respectively. As

depicted in Scheme 2, treatment of 3 with Fe (CO) (ratio

Scheme 2. Synthesis

Selected ferrocenyl-functionalized Co (top) and Fe (bottom)

13,14,16

carbonyl-based clusters

and “non-innocent” κ -coordination of

a dppf ligand towards an Fe(CO) fragment in the mono-oxidized

species.

cluster B, in which a sulfur atom acts as a further cluster

constituent and dimer C consisting of a central FcPS unit,

bridging two Co S moieties. Regarding iron carbonyls,

ferrocenyl-containing cluster D has been reported, containing

more than one iron center.

Recently, investigations at [Fe(CO) (κ -dppf)] as a

Synthesis of clusters 4 and 5. (i) Toluene, 2 h, 50 °C; (ii) THF, 2 h,

40 °C.

dimetallic model compound revealed a direct Fe···Fe inverse

charge transfer between both metals (E, Chart 1). The

exchange of spin density from Fe to Fe indicates that

multimetallic cluster cores could show a similar behavior and

enable communication through the cluster core in the oxidized

:7) or Co (CO) (ratio 1:1.34) in either toluene or

2 8

tetrahydrofuran (THF) at 50 or 40 °C produced the Fe- and

Co-based carbonyl phosphinidene clusters 4 and 5 (Scheme

state.

). The reactions were run until the evolvement of CO ﬁnished

Herein, we present the synthesis of the diferrocenyl iron and

(

Experimental Section). While trimetallic cluster 4 was

cobalt carbonyl clusters Fe (CO) (μ -PFc) and Co (CO) -

obtained in 42% isolated yield, 5 was only formed in ∼1%.

As the main component within the latter reaction, cluster

Co (CO) was isolated. Increasing the 3-to-Co (CO) ratio

(

μ -PFc) . Their physical properties were investigated by

3 2

electrochemical, spectroscopic, and computational methods, as

well as by single-crystal X-ray diﬀraction analysis. In addition,

the single- and double-reduced states were investigated by

computational methods to explain the appropriate EPR

behavior.

to 1:6 did not improve the yield of 5.

The P{ H} NMR spectrum of 4 shows a resonance signal

at 316 ppm, which is characteristic for such type of clusters.

The signal is located between the respective isostructural

phenyl (320 ppm) and p-CF -C H derivative (312 ppm).

RESULTS AND DISCUSSION

■

However, for cluster 5 no P{ H} NMR signal could be

Dichlorophosphines are one of the most common starting

materials for the synthesis of metal carbonyl clusters, and

hence dichloroferrocenylphosphine 3 was synthesized as

detected, due to exchange phenomena of the bridging

31 59

9,23−26

carbonyls and J( P, Co) couplings.

H and C{ H}

NMR studies revealed sharp resonance signals for 4 and 5 for

shown in Scheme 1. Reduction of ferrocenylphosphonate 1

the C H and C H protons and carbons (Experimental

with Li[AlH ] and subsequent chlorination of the thus-

Section). The carbonyl ligands in 4 give rise to a single

resonance at 213.3 ppm in the C{ H} NMR spectrum at

Scheme 1. Synthesis

ambient temperature, conﬁrming a rapid exchange of the axial

and equatorial carbonyls on the NMR time scale.

Solid-State Structures. The molecular structures of

clusters 4 and 5 in the solid state were determined by single-

crystal X-ray diﬀraction analysis. Suitable crystals were

obtained by evaporation of dichloromethane solutions

containing either 4 or 5. ORTEP diagrams are shown in

Figures 1 (4) and 2 (5), and selected bond distances and

angles are summarized in Tables 1 and 2. Both clusters

crystallize in centrosymmetric space groups (4, orthorhombic

Cmc2 ; 5, monoclinic P2 /c) with half a molecule in the

12,19,20

Synthesis of dichloroferrocenylphosphine 3.

(i) Li[AlH₄],

ClSiMe , THF, 45 °C, 18 h; (ii) COCl (1.9 M in toluene), CH Cl ,

5 °C.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

equivalent atoms: (A) −x , y , z .

Article

Table 1. Selected Bond Distances and Angles of 4

bond distances, Å

Fe1−Fe2

bond angles, deg

2.6634(8)

2.6657(8)

2.2094(8)

2.2643(9)

2.2282(9)

3.5864(8)

2.6027(14)

1.641(6)

1.653(6)

1.801(3)

1.783(5)

1.800(3)

1.772(5)

1.809(3)

1.775(5)

P1−Fe1−P1

P1−Fe2−P1

P1−Fe3−P1

72.17(4)

70.16(4)

71.47(4)

Fe2−Fe3

Fe1−P1

Fe2−P1

Fe3−P1

Fe1···Fe3

P1···P1^A

Fe4−Ct1

Fe4−Ct2

Fe1−C1

Fe1−C2

Fe2−C3

Fe2−C4

Fe3−C5

Fe3−C6

Fe1−P1−Fe3

107.84(3)

C1−Fe1−C1

C1−Fe1−C2

C3−Fe2−C3

C3−Fe2−C4

C5−Fe3−C5

C5−Fe3−C6

87.39(19)

104.91(14)

99.9(2)

97.10(13)

92.3(2)

98.38(15)

Ct1−Fe4−Ct2

176.17(10)

Figure 1. ORTEP (50% probability level) illustration of the molecular

structure of 4 with the atom-numbering scheme. All hydrogen atoms

were omitted for clarity. Symmetry operation for generating

−

CO

1.139(4) to 1.152(6)

Ct represents the centroids of the respective C

Ct2) planes. Symmetry operation: (A) −x, y, z.

(Ct1) and C

(

Table 2. Selected Bond Distances and Angles of 5

bond distances, Å

Co1−Co2 2.5225(9)

bond angles, deg

P1−Co1−P1

P1−Co2−P1

70.28(4)

70.29(3)

Co1−Co2

Co1−P1

Co1 −P1

2.6890(10)

2.2552(11)

2.2541(9)

2.2398(9)

2.2686(13)

3.6875(18)

3.6865(17)

2.5955(13)

1.6416(7)

1.6563(7)

1.791(3)

Co1−P1−Co1

Co2−P1−Co2

109.72(4)

109.71(3)

90.02(4)

89.98(4)

96.68(12)

95.11(12)

80.81(10)

6.48(17)

6.48(15)

2.57(13)

0.78(12)

176.15(3)

Co2−P1

Co1−Co2−Co1

Co2−Co1−Co2

C3−Co1−C4

C1−Co2−C2

Co1−C5−Co2

C1−Co2−Co1−C3

C2−Co2−Co1−C4

C1−Co2−Co1 −C4

Co2 −P1

Co1···Co1^A

Co2···Co2

P1···P1^A

Fe4−Ct1

Fe4−Ct2

Co1−C3

Co1−C4

Co1−C5

Co2−C5

Co2−C1

Co2−C2

1.801(3)

1.947(3)

1.945(3)

1.795(3)

C2−Co2−Co1 −C3

Ct1−Fe4−Ct2

μ CO

1.128(3) to 1.132(3)

1.155(3)

1.795(3)

μ CO

Figure 2. ORTEP (30% probability level) illustration of the molecular

structure of 5 with the atom-numbering scheme. All hydrogen atoms

were omitted for clarity. Symmetry operation for generating

equivalent atoms: (A) −x + 1, −y, −z + 1.

Ct represents the centroids of the respective C H (Ct1) and C H

(

Ct2) planes. Symmetry operation: (A) −x + 1, −y, −z + 1.

(

comparable with the average of literature values) and giving

asymmetric unit. Symmetry-equivalent atoms are generated by

a mirror plane in the case of 4, which is located within the

rise to a very weak binding interaction, which strengthens upon

reduction (for more details see below and Table 6). The

Fe(CO)₃moiety is positioned with 1.9119(7) Å in the

midpoint above this plane with equal distances to Fe1

(2.6634(8) Å), Fe3 (2.6657(8) Å), and P1/P1 (2.2643(9)

Å) (Figure 1).

central Fe unit, and an inversion center in the midpoint of the

octahedron of 5. On the basis of these symmetry operations,

the ferrocenyl ligands are oriented syn-periplanar in 4 and anti-

periplanar in 5. Both clusters exhibit similar Ct−Fe−Ct tilt

angles (4, 176.17(10); 5, 176.15(3) °; Ct = centroid) and a

rather eclipsed rotation of the cyclopentadienyls (<8.7(2) °).

nido-Cluster 4 possesses a distorted tetragonal-pyramidal

structure with Fe P as tetragon and an Fe(CO) moiety in

The Fe P plane is with 11.89(15)° almost coplanar with the

C H planes of the ferrocenyl ligands (rmsd = 0.0033). The

Fe ···Fe P units intersect almost perpendicularly (84.82(4)°).

The structure of 4 indicates a rather small steric impact of the

ferrocenyls on the core, whereas the syn-orientation of both

ferrocenyls is contrary to literature, where a trigonal-

2 2

the apical position, similar to Fc(CO) -capped diferrocenyl

cycobutadienes.

The four-membered Fe P plane (root-mean-square devia-

bipyramidal Fe P arrangement is dominant. To prove that

3 2

tion (rmsd) = 0.0589) is signiﬁcantly distorted with a

nonbinding Fe1···Fe3 distance of 3.5864(8) Å and a short

P···P interaction of 2.6027(14) Å. The latter is a common

the shape of the cluster core is not temperature-dependent, an

additional measurement at ambient temperature was per-

formed, revealing equal bond geometries and cell parameters

(for more details see the Supporting Information ). The

feature, lying within the sum of the van der Waals radii

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

and square-wave voltammetry (SWV).

Article

carbonyls at the Fe₃building block reveal an eclipsed

Table 3. Cyclic Voltammetry Data of 4

conformation for Fe1 and Fe2 (C1−Fe1−Fe2−C3,

redox process

1st reduction

E°′ (ΔE_p)

ΔE°′

.75(14)°), whereas the Fe2−Fe3 bond intersects the C5−

−1605 (68)

−1755 (64)

Fe3−C5 angle with 48.20(10)°. The interaction via an

150

nd reduction

eclipsed conformation is most probably the reason for the

Fe1Fe3P1/Fe1Fe3P1 plane intersection of 10.35(3)°.

st oxidation

nd oxidation

295 (62)

465 (63)

^C^o^-^B^a^s^e^d^c^l^u^s^t^e^r⁵^e^x^hⁱ^bⁱ^t^s^a^dⁱ^s^t^o^r^t^e^d^o^c^t^a^h^e^d^r^a^l^C^o4^P

170

core with a planar Co tetragon (rmsd = 0.000) and the P

−1

atoms in apical positions. In contrast to 4, cluster 5 exhibits

bridging carbonyls, resulting in a shortening of the involved

Co−Co bonds from 2.6890(10) to 2.5225(9) Å (Table 2).

The bridging carbonyls exhibit increased Co−C bond lengths

of 1.945(3) Å compared to 1.791(3)−1.801(3) Å for a single

Potentials (in mV) vs FcH/FcH , scan rate 100 mV s at a glassy-

carbon electrode of 1.0 mmol L solutions in anhydrous dichloro-

methane containing 0.1 mol L of [N Bu ][B(C F ) ] as supporting

−

6 5 4

electrolyte at 25 °C. Formal potential. Diﬀerence between the

cathodic and anodic peak potential. Potential diﬀerence between the

ﬁrst and the second redox processes.

Co−C bonding, resulting in an increase of the CO bond

length from 1.132(3) to 1.155(3) Å. Cluster 5 exhibits with

.5955(13) Å a short P···P distance. Both ferrocenyls are

instead are delocalized over the Co rectangle in 5 (Figure 11).

positioned above the short edges of the central Co unit,

Hence, oxidation processes in 5 are core-based, which most

probably explains the irreversible redox behavior. Together

with a low overall yield and potential impurities of Co (CO)

directing toward the bridging carbonyls. On the basis of the

symmetry operation, both ferrocenyls are coplanar toward each

other (0.00(19)°). They are slightly bent away from the core

by 8.09(8)°, most probably to avoid a steric interaction with

the bridging carbonyls.

Electrochemistry. The electrochemical behavior of

clusters 4 and 5 (Figures 3 and S2, see the Supporting

Information ) were investigated by cyclic voltammetry (CV)

this species was not investigated further.

Cluster 4 was studied under anodic and cathodic conditions,

respectively, and over the whole range of the electrochemical

window (Figure 3). Therein, no noticeable decomposition or

change of the potential of the redox events occurred,

evidencing a high redox stability under both oxidative and

reductive conditions.

The ferrocenyl-related redox events at 295 and 465 mV are

less anodically shifted by the electron-withdrawing eﬀect of the

Fe (CO) P core compared to an Fe (CO) motif, as

evidenced for studies of FcPPh -substituted Fe and Ru clusters

with a maximum of 410 mV. A weak withdrawing eﬀect is

caused by a monometallic Fe(CO) fragment, as shown for

Fe(CO) (dppf), where the iron carbonyl-based oxidation

(

−150 mV) occurred prior to the dppf ligand (30 mV).

To the contrary, cluster 4 did not show an oxidation of the

Fe P -carbonyl core. The Fc-related processes reveal a redox

separation of 170 mV, although both groups are positioned on

opposite sides of the cluster. Spectro-electrochemical (SEC)

IR studies of 4 showed a lowering of the electron density

within the cluster core upon oxidation of the ﬁrst ferrocenyl

substituent aﬀecting the redox potential of the second unit.

Hence, the redox separation is based on electrostatic

interactions (see below).

Oxidation processes at the clusters are predominantly based

on redox-active substituents. The absence of such groups

Figure 3. Cyclic voltammograms (black and purple: scan rates 100

mV s ) and square-wave voltammogram (gray: step height 25 mV,

results in the formation of an Fe(CO) -based radical, which

−1

can solely be observed in measurements performed at −30

pulse width 5 s, amplitude 5 mV) of 4 in a dichloromethane solution

−

°C. To the contrary, a reduction of the Fe (CO) P cluster

(

1.0 mmol L ) at 25 °C measured with a glassy carbon working

3 9 2

−

core is well-studied and occurs for several cluster geometries,

electrode. Supporting electrolyte 0.1 mol L of [N Bu ][B(C F ) ]).

Fc* = Fe(η -C Me ) ).

6 5 4

(

whereby an irreversible behavior may be observed.

5 2

The two one-electron reductions of cluster 4 occur as

reversible processes at −1605 and −1755 mV. These values are

comparable to the respective isostructural Ph-substituted

The electrochemical measurements were performed in

anhydrous dichloromethane solutions under inert conditions

cluster, bearing p-OMe (−1530 and −1760 mV) or p-NMe

−1

containing [N Bu ][B(C F ) ] (0.1 mol L ) as supporting

groups (−1630 and −1790 mV), conﬁrming the electron-

6 5 4

electrolyte, which stabilizes highly charged species and

donating character of the ferrocenyls.

32−34

minimizes ion pairing eﬀects.

For further details see the

Experimental Section. The CV data are summarized in Table

. Iron cluster 4 shows four well-separated reversible one-

In contrast to the ferrocenyl-based redox events that aﬀect

the Fe P core by electrostatic interactions, the reduction

occurs within the Fe P core, where the negative charge is

3 2

electron redox events, whereas for 5 several irreversible events

were observed, indicating a rapid decomposition (Figure S3 ).

This is due to a diﬀerent localization of the HOMO orbitals,

which are based on the ferrocenyls in case of 4 (Figure 9), and

equally distributed, as veriﬁed by IR and EPR measurements.

Because of the observed redox separation of the ferrocenyl-

and cluster-based processes in the anodic and cathodic region

of 4, in situ UV−vis/near-infrared (NIR) SEC measurements

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

All values are given in inverse centimeters.

Article

were additionally performed to study the charge transfer

behavior. However, no intervalence charge transfer (IVCT)

absorptions could be observed (Figure S1). This shows that no

can aﬀect IR bands of adjacent carbonyl ligands could be

shown for 1,1′-dithiolatoferrocene-hexacarbonyldiiron. Fur-

ther increase of the potential results in the formation of [4] ,

−1

direct electronic communication between the Fc and Fc units

accompanied by an equal hypsochromic shift of ∼10 cm for

occurs, excluding an interaction through the Fe P cluster core,

the most intense bands. The IR spectra of [4] reveal a

distribution of ﬁve ν(CO) stretching frequencies similar to the

neutral species 4 (Figure 4). The sole diﬀerence is the change

of the relative intensity of bands toward each other.

although a weak P···P interaction was found within the

computational studies (see below).

IR Spectro-electrochemistry. The IR spectro-electro-

chemistry was applied to investigate the charge distribution

upon oxidation and reduction of cluster 4, while monitoring

the shift of the ν_C_Ostretching frequency.

That the electron density of a cluster core and hence the

value of the respective CO stretching frequencies can be

aﬀected even by an electrostatic interactions of a distant redox-

active group was recently shown for an Fe (CO) (PR)

The neutral species exhibits the typical pattern of an

Fe (CO) P core, consisting of ﬁve CO bands, whereby the

butterﬂy-type cluster bearing R = CH Fc. Although the sp

lowest-energy frequency occurs as a shoulder of band 4 (Table

carbon of R prevented a direct communication of the Fc’s with

the Fe P , a hypsochromic shift of the ν(CO) bands of ∼8

−

8a,37,38

) at 1994 cm (Figure 4).

The electron-donating

−

cm was observed upon double oxidation from the neutral

−

Table 4. IR Data of the Carbonyl Region of 4 upon

species. The smaller band shift of 4 cm per Fc unit,

−1

Oxidation

compared to 10 cm herein, follows an increased spatial

distance of the redox-active groups toward the cluster core.

SEC IR measurements of 4 were also performed under

v (CO)

[4]⁺

[4]²⁺

Δv

2065 (w)

2035 (vs)

2075

2046

2085

2056

2036

2013

10/10

reductive conditions to investigate the impact onto the cluster

_c_o_r_e₍_F_i_g_u_r_e₅₎_._S_u_c_c_e_s_s_i_v_e_r_e_d_u_c_t_i_o_n_o_f₄_t_o_[₄_]− is

035

2027

013

1998

accompanied by a bathochromic shift of the carbonyl

2013 (s)

−1

stretching frequencies by ∼50 cm (Table 5). Compared to

−1

a diﬀerence of 10 cm caused by the electrostatic interaction

1994 (m)

of the Fc’s, the increased shift is in accordance with a reduction

of the cluster core. Such a behavior is well-known and has been

reported for isostructural Ph-based derivatives. Higher Δv

−

values of ∼100 cm are solely observed for Fe(CO) (dppf),

where an intramolecular Fe →Fe charge transfer occurs, and

the proximity of 3.70 Å between the two Fe atoms causes an

increased inﬂuence on the carbonyls.

−

Further reduction toward [4] reveals an ambiguous

behavior, since no further bathochromic shifts are observed.

Instead, bands at higher energy were raising, which rapidly

results in a further change of the spectra (Figure 5), indicating

9,42

a decomposition/reorganization of the cluster core.

This is

a common phenomenon for Fe (CO) (PR) clusters and is

38,40

caused by a μ /μ ﬂip of one of the P groups

and is also

known for S-functionalized hexacarbonyldiiron fragments.

Comparison of the potential diﬀerences for the formation of

2−

[

4] and [4] from the respective single charged species [4]

−

and [4] within the in situ measurements showed a potential

range of 560 mV for the oxidation process, whereas solely 125

−

mV is required to convert [4] into a further species, before

Figure 4. IR spectra (1925−2100 cm ) of 4 at 25 °C in

−

the IR pattern further changes. This is contrary to the CV

dichloromethane (2 mmol L ) at rising potentials: (left) −200 to

40 mV; (right) 840 to 1400 mV, purple: [4], green: [4] ; blue:

4] . All potentials vs Ag/AgCl; supporting electrolyte 0.1 mol L of

data, where a similar redox separation (170 mV [4] → [4] ;

50 mV [4] → [4] ) was observed, forecasting a similar

−

2−

−1

[

N Bu ][B(C F ) ]; Pt working electrode.

behavior within SEC measurements. The diﬀerences within

both methods might be due to the use of the Pt working

electrode within the optically transparent thin-layer electro-

chemical (OTTLE) cell for the in situ investigations and the

glassy carbon for the recording of CV data. Slight diﬀerences

between both electrodes were observed within EPR measure-

ments (see below, Figures 6 and S2 ). This phenomenon is

well-known and is caused by a diﬀerent interaction with the

6 5 4

ferrocenyls causes a bathochromic shift for each band,

compared to the respective phenyl derivative.

of the Fc groups onto the IR frequencies is therefore

comparable with isostructural p-OMe- and p-NMe -substituted

8a,38

The impact

phenyl clusters, similar to the inﬂuence onto the redox

potential (vide supra).

Stepwise oxidation of 4 to [4] results in a hypsochromic

surface of the electrode as described recently. The

−

shift of all bands by ∼10 cm . The initial bands do not

completely disappear, indicating that a less symmetrical species

is formed, based on diﬀerent oxidation states of the ferrocenyls.

This in turn supports absence of a communication through the

core and the presence of a noncoupled class I system according

reorganization of the cluster core, which occurs slowly within

the progress of the electrochemical measurements, also

explains the formation of a further species during the

spectro-electrochemical IR measurements.

EPR Spectroelectrochemistry. To get further insights

into the localization of the charges in the diﬀerent oxidation

to Robin and Day. That oxidation of ferrocenyl substituents

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

All values are given in inverse centimeters.

Article

−

−1

Figure 5. IR spectra (1825−2100 cm ) of 4 at 25 °C in dichloromethane (2 mmol L ) at decreasing potentials: −300 to −1925 mV (left);

−

2−

−

1925 to −2000 mV (middle); −2000 to −2100 mV (right); purple: [4], blue: [4] ; green: [4] ; orange: decomposition. All potentials vs Ag/

AgCl; supporting electrolyte [N Bu ][B(C F ) ]; Pt working electrode.

6 5 4

Table 5. IR Data of the Carbonyl Region of 4 upon

Reduction

The EPR signal is a triplet with a 1:2:1 intensity ratio

ascribed to two equivalent P nuclei with a( P) of 13.3 G.

The time/potential proﬁle of the EPR intensity allows its

assignment to the ﬁrst reduction step. The EPR parameters of

ν (CO)

[4]⁻

Δν

2065

2035

2013

1994

the radical anion are very similar to those of a series of

1986

1971

1935

−

[

Fe (CO) (PR) ] radical anions produced by chemical

reduction of the corresponding Fe (CO) (PR) clusters by

cobaltocene, reported by Kochi et al., or by Na/Hg.

Kochi et al. also found that the formed [Fe (CO) (PR) ]

−

species with two equivalent a( P) constants slowly (within

hours) converted to another type of a radical anion with a g-

factor near 2.04 and nonequivalent a( P) constants of 25 and

G. The authors suggested that the second type of species

results from a μ - to μ -binding transition of one of the Fe−P

bonds, causing a loss of equivalence between both P atoms.

Herein, the formation of such radicals occurred, when the

measurement was performed in a broad potential window

going well beyond the second reduction potential of 4. In such

case, already the second and subsequent electrochemical cycles

−

showed the existence of the new radical anion [6] with a g-

factor of 2.0392 (Figure 7). Its EPR signal is a doublet of

Figure 6. Left: Cyclic voltammogram, measured in an SEC cell

doublets with a( P) values of 25 G and ca. 2 G.

−

(

sweep rate 3 mV s ) and potential dependence of the EPR intensity

purple and blue dots). Right: EPR spectrum of the radical anion [4]

The rapid change of the cluster geometry under cathodic

conditions is in accordance with the in situ IR measurements

−

measured in situ during electrochemical reduction of Fe (CO) (PFc)

−

(

4) in a dichloromethane solution (0.5 mmol L ) at 25 °C with a Pt

−

1 n

working electrode. Supporting electrolyte 0.5 mol L of [N Bu ][B-

(

C F ) ]. All potentials vs the AgCl-coated Ag wire.

6 5 4

states of 4, in situ EPR measurements were performed. Within

the anodic region no EPR signal could be detected (Figure

S2 ), which conﬁrms Fc-located oxidation processes. During the

electrochemical reduction of 4 the formation of a radical anion

with a g-factor of 2.0192 could be observed (Figure 6). The

EPR signal appears together with the ﬁrst reduction event in

the CV corresponding to the formation of a radical anion. The

intensity of the signal reaches a maximum and decreases during

the second reduction process, caused by the formation of the

2−

diamagnetic species [4] . In the backward potential scan, the

EPR intensity increases again during the ﬁrst oxidation event in

the CV and completely vanishes at the ﬁnal potential

Figure 7. EPR spectrum of species [6]⁻(left) formed within the

beginning of the second cycle of a CV measurement of cluster [4] and

its proposed structure (right).

(

reversible redox behavior of the system).

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

radical anion [4 ] (Figure 8 ). The latter species was generated

Article

performed herein, where the necessity of a complete reduction

of the whole cell volume also resulted in the rapid formation of

a less symmetric species (Figure 5). This phenomenon has

already been observed, whereby the reaction of the

isostructural Fe (CO) (PR) clusters with sodium amalgam

but not suﬃciently resolved with regard to the standard

deviation (Table 1). The highest occupied molecular orbital

(HOMO) of 4 is predominantly localized on the ferrocenyls

(Figure 9 ), which are therefore oxidized at the two ﬁrst

required 8 h at 25 °C to form 50% and 24 h for a complete

conversion.

To get more information on the anisotropy of the hyperﬁne

parameters, we also studied the EPR spectrum of the ﬂash-

frozen CH Cl solution of the electrochemically produced

−

Figure 9. HOMO and LUMO isosurfaces in 4 (top) and spin density

−

isosurface in [4] (red: positive; green: negative).

oxidation steps observed in cyclic voltammetry. The lowest

unoccupied molecular orbital (LUMO) of 4 is delocalized over

the Fe P cluster core with similar contributions of all ﬁve

atoms (Figure 9).

Structure optimization of the radical anion revealed

considerable distortion of the Fe P moiety upon reduction.

Figure 8. Experimental EPR spectrum of the frozen solution of [4]⁻

black) at 100 K compared to a simulated spectrum (red). The radical

anion was formed while keeping a CH Cl solution of 4 at the

potential of the ﬁrst reduction process for 3 min, followed by ﬂash-

freezing at 100 K.

The Fe2···Fe3 distance is increased strongly in comparison to

the pristine molecule (from 2.644 to 3.173 Å), whereas the

Fe1···Fe2 distance is shortened from 2.632 to 2.589 Å. The

ferrocenyl fragments in the anion are twisted considerably with

regard to their virtually parallel arrangement in the neutral

molecule. For the dianion, the singlet state is predicted to be

more stable than the triplet state by 0.94 eV. Structurally, the

dianion is similar to the monoanion, but the asymmetry of the

(

while the solution at the potential of the ﬁrst reduction process

was kept at ambient temperature, whereby no further species

was formed. The experimental spectrum was ﬁtted using

density functional theory (DFT) calculations, which predicted

g- and a-tensors as starting parameters. The best match to the

experimental spectrum was obtained for a rhombic g-tensor

Fe fragment is enhanced further, with Fe1···Fe2 and Fe2···Fe3

distances changing to 2.547 and 3.639 Å, respectively.

Charging of the cluster substantially aﬀects the shape of the

square-planar Fe1 < P > Fe3 fragment. In the neutral form the

(

1.9998, 2.0194, 2.0474). Conservative estimations of the

Fe1 < P ···Fe3 < P plane intersection of 8° is close to

−

a( P) tensors from the ﬁt are (−8.9, −11.6, −18.5) and

planarity. In monoanion [4] the angle increases to 23° and

2−

(

−8.0, −16.1, −20.2) G, but large variations in the a-tensors

reaches 38° in [4] .

−

lead to marginal changes in the spectrum, indicating that the

resolution of the experimental spectrum is not suﬃcient for

reliable determination of hyperﬁne tensors.

The spin density in the radical anion [4] is predominantly

localized on the iron atoms (Figure 9), which is diﬀerent from

what one might expect based on the LUMO shape of the

pristine cluster. Fe3 has the highest positive Mulliken spin

population of +0.91; the spin population of Fe2 is +0.39,

whereas the spin density on Fe1 is negative with a net

population of −0.19. The spin population of each phosphorus

atom is also negative with −0.04. The negative spin density

DFT Calculations. Full-electron DFT calculations at the

PBE0-D3BJ/TZVP level of theory with scalar-relativistic DKH

correction were performed with Orca suite to get further

insights into the structures and spin density distribution in 4 in

the pristine and reduced forms. For charged species, solvation

by dichloromethane was taken into account with the

COSMO model. Optimized geometry parameters for the

neutral molecule agree well with those obtained by the single-

crystal X-ray diﬀraction measurement performed at 125 K. The

Fe P moiety is close to C symmetry. Deviations are caused

consequently leads to negative P hyperﬁne coupling

constants. For the two slightly nonequivalent P atoms DFT-

predicted hyperﬁne tensors (−14.4, −15.8, −20.5) and

(−15.0, −17.0, −20.1) G, lead to isotropic a ( P) hyperﬁne

iso

constants of −16.9 and −17.4 G, respectively. These values are

by the eclipsed and staggered conformations of the CO ligands

of the capping Fe atom (Fe2) with respect to those of Fe1- and

Fe3-labeled Fe(CO) fragments of the crystal structure (Figure

in reasonable agreement with the experimental a ( P)

iso

constant of 13.3 G. Computations predicted a rhombic g-

tensor (1.9987, 2.0095, 2.0259), giving the isotropic factor g_i_s_o

= 2.0114, which is considerably larger than the free-electron g-

). According to DFT calculations, the Fe1···Fe2 distance for

the eclipsed conformation is shorter by 0.012 Å, which is also

factor of 2.0023 but still smaller than the experimental g_i_s_o=

indicated by the single-crystal X-ray diﬀraction measurement,

2.0192.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Bonding Interactions in Fe₃P₂upon Reduction.

Bonding interactions in the Fe P core of 4 were analyzed

with the help of Bader’s quantum theory of atom-in-molecule

(

QTAIM). The labeling of the atoms corresponds to those of

the solid-state structures (Figures 1 and 2). From the multiple

QTAIM parameter, the delocalization index δ(A, B) was

focused, which shows the number of electron pairs shared

between atoms A and B, and can be interpreted as the bond

order. Table 6 lists delocalization indices in the Fe P fragment

Table 6. QTAIM Delocalization Indices (δ) and Atomic

Charges (q) in Diﬀerent Charged States of [4]

δ(A, B)

δ(Fe2, Fe3)

Q = 0

0.31

Q = −1

Q = −2

0.11

0.07

δ(Fe1, Fe2)

0.36

0.39

δ(Fe2, P1)/δ(Fe2, P2)

δ(Fe3, P1)/δ(Fe3, P2)

δ(Fe1, P1)/δ(Fe1, P2)

δ(P1, P2)

0.71/0.72

0.81/0.81

0.81/0.80

0.23

0.72/0.72

0.74/0.74

0.76/0.76

0.31

0.73/0.72

0.68/0.69

0.74/0.73

0.37

q(Fe2)

0.47

0.51

0.50

q(Fe3)

0.56

0.62

0.53

q(Fe1)

0.52

0.51

0.49

q(P1)/q(P2)

0.50/0.50

0.41/0.39

0.43/0.41

Figure 10. Visualization of the ELF in Fe (CO) (PFc) (left) and

3 9 2

−

Fe (CO) (PFc) (right) and Co (CO) (PFc) (bottom). Valence

δ(P, C) is ∼0.9 in all charge states.

V (Fe,P) and V (Co,P) basins are shown in blue (isosurfaces are

iso

plotted at η = 0.76), core C(Fe) and C(Co) basins are orange and

purple (η = 0.76), and valence V (Fe,Fe) and V (Co,Co) basins are

green (η = 0.30). Other basins are not shown for clarity.

in diﬀerent charge states of 4. In the pristine molecule,

QTAIM analysis reveals considerable Fe1···Fe2 and Fe2···Fe3

bonding interactions with δ(Fe, Fe) values of 0.31 and 0.36,

respectively. Fe···P interactions are essentially single bonds

with δ(Fe2, P) of 0.71−0.72 and δ(Fe1, P)/δ(Fe3, P) indices

near 0.80. There is also a small but non-negligible bonding

interaction between both phosphorus atoms with δ(P1, P2) =

iso

whereas the V (Fe1,Fe2) basin with an attractor value η = 0.40

has a basin population of 0.39. Thus, both QTAIM and ELF

methods independently reveal a weak Fe1−Fe2 and Fe2−Fe3

bonding, the former being somewhat stronger.

.23. Upon reduction of 4, the Fe2···Fe3 interaction is

Bonding Interactions in Co P . Computational studies

weakened dramatically, which is in line with the increase of

the corresponding interatomic distance. The Fe1···Fe2

bonding is slightly increased to the δ(Fe1, Fe3) index of

4 2

were also performed for 5 and its charged states in order to

understand the bonding interactions in the molecule and

explain its nonreversible electrochemical behavior. In pristine

.39. P1···P2 bonding is also strengthened to δ(P1, P2) = 0.37

, the QTAIM analysis revealed non-negligible Co−Co

bonding interactions with delocalization indices of 0.30 and

.34 (Table 7). The smaller value corresponds to the μ-CO

in the dianion. The Fe2−P delocalization indices are not

aﬀected by the number of charges, whereas Fe1−P and Fe3−P

bonds are somewhat weakened.

bridged Co−Co interaction. Therein, 0.36 is a value for a

nonsupported interaction. The delocalization indices are

opposed with Co−Co bond lengths derived from the single-

crystal X-ray diﬀraction measurement (Table 2). The P−Co

Visualization of the bonding interactions is convenient with

the use of the electron localization function (ELF), which is

isomorphous to the laplacian of the electron density and helps

in localization of the regions with enhanced localization of

electrons. Topological analysis of ELF divides the space into

basins of diﬀerent synapticity, such as monosynaptic core C(A)

basins or disynatpic valence V(A,B) basins, corresponding to

core electron shells and covalent bonds, respectively. Figure 10

Table 7. Delocalization Indices (δ) and Atomic Charges (q)

−

in 5 and [5a]

_[₅_a_]−

δ(A, B)

2−

shows selected ELF basins in 4 and [4] . The Fe−P bonding

is described by disynaptic V (Fe,P) basins. Positions of their

attractors (maxima) deviate from the Fe−P lines pointing to

the certain strain in the structure. Attractor values for V (Fe,P)

δ(Co1, Co2) bridged

δ(Co1, Co2 ) direct

δ(Co1, P)

δ(Co2, P)

0.30

0.34

0.69

0.68

0.24

0.29

0.65

basins are in the range from η = 0.82 for V (Fe2,P) to η = 0.91

for V (Fe1,P) and V (Fe3,P) in the neutral molecule. If plotted

δ(P1, P2)

0.25

0.32

at the isosurface of 0.76 as in Figure 10, three V (Fe,P) basins

are merged into one superbasin.

q(Co1)

q(Co2)

0.35

0.36

0.39

In the neutral molecule, ELF analysis revealed the presence

of two disynaptic Fe−Fe basins with small attractor values of η

0.44 in V (Fe1,Fe2) and η = 0.26 in V (Fe2,Fe3), which

q(P)

0.60

0.50

have both substantially diﬀerent spatial extension (Figure 10)

and diﬀerent basin populations of 0.35 and 0.12, respectively.

δ(P, C) is 0.87; δ(Co1, C5) is 0.76. The symmetry operation: (A)

In the dianion, the V (Fe2,Fe3) basin is not found any more,

−x + 1, −y, −z + 1 refers to the solid state structure (Figure 2).

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

indices are in the range of 0.68−0.69. In similarity to cluster 4,

a non-negligible P−P interaction with the delocalization index

of 0.25 is found as well. However, bond paths for neither Co−

Co nor P−P pairs were found.

that the Co−Co indices in the monoanion are reduced by ca

20% when compared to the neutral molecule, whereas the P−P

index is increased from 0.25 to 0.32 (Table 7). Overall, there

are no dramatic diﬀerences in the bonding situation in 5 and

−

Selected ELF basins of 5 are shown in Figure 10. The Co−P

[5] . But further charging resulted in a dramatic distortion of

−

bonding is described by four disynaptic V (Co,P) basins for

the molecule in [5] . Namely, DFT optimization resulted in

each P vertex. Attractor values for V (Co,P) basins are in the

the collapse of the Co rectangle and changing of the P−Co

range of η = 0.82−0.83. If plotted at the isosurface of 0.76 as in

bonding from μ to μ mode (Figure S4).

Figure 11 , four V (Co,P) basins are merged into one torus-

CONCLUSION

■

The formation of ferrocenyl-functionalized iron and cobalt

carbonyl clusters is reported, based on a reaction of

ferrocenylphosphine FcPCl (Fc = Fe(η -C H )(η -C H ))

and Fe (CO) and Co (CO) , respectively. Therein, trans-

substituted nido-Fe (CO) (μ -PFc) (4) and nido-

Co (CO) (μ -PFc) (5) were obtained as the ﬁrst trans-

diferrocenyl-substituted carbonyl clusters. Single-crystal X-ray

diﬀraction analysis revealed C (4) and C (5) symmetrical

structures with distorted square-planar pyramidal and a

compressed octahedral geometry, respectively. The iron-

based cluster revealed two reversible one-electron processes

within the anodic region, based on Fc/Fc redox events, as well

as two processes in the cathodic region. Spectro-electro-

chemical IR measurements revealed a hypsochromic shift of

−

the ν_C_Ostretching of 10 cm upon oxidation and a

−

Figure 11. HOMO and LUMO isosurfaces in 5 (top) and spin

density isosurface in [5a] (red: positive; green: negative).

bathochromic shift of 50 cm upon reduction. The former

−

is based on the electrostatic interaction of Fc toward the core,

which is supported by the absence of an IVCT band in the

NIR region. Computational studies of the HOMO−LUMO

energies, the spin density, QTAIM delocalization indices, and

the atomic charges, based on the single-crystal X-ray structures

of both clusters, revealed that the latter is caused by the

shaped superbasin. For the nonsupported Co−Co interactions,

the ELF analysis also revealed the presence of two disynaptic

Co−Co basins, V (Co1,Co2′), with a small attractor value of η

0.36 and basin population of 0.26. The small population

^dⁱ^s^t^rⁱ^b^u^tⁱ^oⁿ^o^f^t^h^e^e^l^e^c^t^r^oⁿ^t^h^r^o^u^g^h^t^h^e^F^e3^P core

agrees with the small but non-negligible delocalization index of

accompanied by a dramatic weakening of the Fe2···Fe3 bond

.36. Substantial P−P interactions are not seen in the ELF

2−

from 0.36 (4) to 0.07 in [4] , and the weak P···P interaction

strengthens from 0.23 to 0.37. This in turn facilitates the

reorganization of the cluster core upon reduction and explains

the formation of further species within the electrochemical

measurements. The calculated atomic charges (q) of the iron

description.

The HOMO and LUMO of 5 are delocalized over the Co₄P₂

moiety (Figure 11), indicating that it should be redox active in

both cathodic and anodic ranges, and that redox processes may

aﬀect the structure of the Co P core. However, in contrast to

atoms within the Fe P core do not show signiﬁcant changes

, where the HOMO is predominantly located on the

3 2

upon reduction, due to a compensation of the electron-

withdrawing carbonyls.

ferrocenyls (Figure 10), cluster 5 shows it delocalized over

the Co P moiety (Figure 11). This explains why (reversible)

EPR measurements did not detect a signal within the anodic

region, which conﬁrms an Fc-located process. Within the

Fc/Fc -based processes are not observed in the CV of the

latter species, indicating that especially processes in the anodic

region may aﬀect the structure of the Co P core. For the

−

cathodic region, the single reduced species [4] reveals a signal

−

at g = 2.0192 including a coupling with two chemically equal

phosphorus atoms of 13.3 G, which supports the assumption of

a symmetrical electron delocalization within the core. The EPR

intensity reduces upon the formation of the diamagnetic

monoanion [5] , DFT calculations revealed two almost

isoenergetic conformations denoted hereafter as [5a] and

5b] (Figure S4). In [5a] , the symmetry of the structure is

preserved, but unsupported Co−Co distances are elongated to

.779 Å (compare to 2.632 Å calculated for 5), whereas CO-

−

[

2−

species [4] . The rise of a second EPR signal (g = 2.0392;

bridged Co−Co distances remain essentially unaltered. P−P

a( P) = 25 and 2 G) furthermore conﬁrmed reorganization of

the initial symmetrical core-based radical anion into a less-

symmetrical decomposition product at potentials, lower than

−

distance is reduced from 2.482 Å in 5 to 2.458 Å in [5a] . In

−

[

5b] , the structure is distorted so that one unsupported

2−

Co−Co distance is 2.713 Å, whereas another one is increased

for [4] at Pt working electrodes.

to 2.851 Å. CO-bridged Co−Co distances as well as P−P

Cluster 5, revealed a similar behavior upon reduction,

accompanied with a strengthening of the P···P interaction from

−

distance in [5b] are the same as in [5a] . It is reasonable to

−

2−

suggest that [5a] and [5b] are interconverting easily, and

thus calculations show the structure of the [5] anion is very

0.25 to 0.32. However, calculation of [5] revealed a collapse

−

of the Co₄rectangle. The core-based distribution of the

labile with considerable elongation of unsupported Co−Co

HOMO energies in 5, compared to ferrocenyl-located orbitals

−

distances. Spin density in [5] is distributed equally between

in 4, explain the absence of reversible Fc/Fc -related redox

all Co atoms with an additional small negative polarization of

the bridging CO ligands (Figure 11). QTAIM analysis showed

events in case of 5, resulting in an irreversible core-based

process upon oxidation.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

FcH/FcH , ΔE = 60 mV, while the FcH/FcH couple itself was at

EXPERIMENTAL SECTION

■

20 mV versus Ag/Ag , ΔE = 61 mV.

General. All reactions were performed under an atmosphere of

argon using standard Schlenk techniques. Reaction ﬂasks were heated

at reduced pressure with a heat gun and ﬂushed with argon. This

procedure was repeated thrice. If necessary, solvents were

deoxygenated by standard procedures. For column chromatography

silica with a particle size of 40−60 μm (230−400 mesh (ASTM)) was

used.

UV−Vis−NIR and IR Spectro-electrochemistry. Spectro-

electrochemical UV−vis−NIR and IR measurements of 2.0 mmol·

−1

solutions of 4 in anhydrous dichloromethane containing 0.1 mol·

−1

of [N Bu ][B(C F ) ] as the supporting electrolyte were

6 5 4

performed in an OTTLE (quartz windows) cell with a Varian

Cary 5000 spectrophotometer at 25 °C. The values obtained by

−1

deconvolution could be reproduced within ε : 100 L·mol ·cm ;

max

Reagents. Tetrahydrofuran was puriﬁed by distillation from

sodium/benzophenone ketyl. Dichloromethane, hexane, and toluene

were dried and puriﬁed with an MBraun SPS−800 puriﬁcation system

−1

ν_m_a_x50 cm ; Δν 50 cm . Spectro-electrochemical IR measure-

1/2

−1

ments of solutions of 4 (2 mmol·L ) in anhydrous dichloromethane

−1

containing [N Bu ][B(C F ) ] (0.1 mol·L ) as supporting electro-

6 5 4

and stored over molecular sieves (4 Å). Compounds Fe (CO) ,

lyte were performed in an OTTLE cell with CaF windows.

FcP(O)(OEt) (1), FcPH (2), FcPCl (3), and [N( Bu) ][B-

EPR Spectro-electrochemistry. The in situ EPR spectro-

electrochemistry was performed in an optical cavity (ER 4104OR,

Bruker). The EPR spectra were recorded using EMXplus X-Band CW

spectrometer (Bruker BioSpin) at 100 kHz modulation and a

microwave power of 5 mW. The EPR spectrometer was linked to a

HEKA potentiostat/galvanostat PG 390. The triggering was

performed by software package PotMaster v2 × 90.2 (HEKA

Elektronik Dr. Schulze GmbH). A spectro-electrochemical ﬂat cell

with a three-electrode arrangement consisting of a laminated platinum

mesh (Goodfellow) as a working electrode, a platinum wire as a

counter electrode, and a silver chloride-coated silver wire as a pseudo

reference electrode was used. The potentials are denoted against the

pseudo reference electrode. For the spectro-electrochemical measure-

ments, the dichloromethane (puriss, absolute, over molecular sieve

(H O ≤ 0.005%), ≥99.5%, Sigma-Aldrich) solution containing 0.5

mmol·L of 4 and 0.5 mmol·L of [N Bu ][B(C F ) ] was used.

The solutions were purged with nitrogen for 1 h. The cell was

assembled under nitrogen atmosphere.

Synthesis of Fe (CO) (μ -PFc) (4). Compound 3 (580 mg, 2.02

mmol) was placed in a Schlenk tube under inert conditions, followed

by the addition of Fe (CO) (5.11g, 14.04 mmol) and 140 mL of

toluene. The mixture was placed in a cold oil bath, slowly heated to 50

°C, and stirred for 1 h, until gas evolution was ﬁnished. The mixture

was ﬁltered through a plug of silica by using CH Cl , followed by

removal of all volatiles with a rotary evaporator. The residue was

puriﬁed by column chromatography (silica, 4 × 14 cm column size).

Cluster Fe (CO) was removed using hexane. The eluent was

changed to a 4:1 hexane/toluene mixture (v/v), which gave 4 as a

sustained red band. Removal of all volatiles gave 4 as a red solid.

Crystals suitable for single-crystal X-ray diﬀraction analysis were

obtained by evaporation of a CH Cl solution of 4 at ambient

33,52

(

C F ) ]

were synthesized according to published procedures.

5 4

Ferrocene, phosgene (1.9 M in toluene), Li[AlH ] (600 mg tablets),

BuLi (1.9 M in hexane), ClP(O)(OEt) , and Fe(CO) were

purchased from commercial suppliers and were used without further

puriﬁcation.

Instruments. Fourier transform infrared (FT-IR) spectra were

recorded as KBr pellets in transition mode. NMR spectra were

recorded with a Bruker Avance III 500 spectrometer (500.3 MHz for

H, 125.8 MHz for C, and 202.5 MHz for P) are reported with

chemical shifts in δ (ppm) units downﬁeld from tetramethylsilane

with the solvent as the reference signal (chloroform-d : H at 7.26

ppm and ¹³C{ H} at 77.16 ppm), by external standards ( P{ H}

relative to 85% H PO , 0.0 ppm and P(OMe) , 139.0 ppm), or by the

H solvent lock signal. The melting or decomposition points were

−

−1

determined by using a Gallenkamp MFB 595 010 M melting point

apparatus. Elemental analyses were performed with a Thermo

FlashAE 1112 instrument. High-resolution mass spectra were

recorded with a Bruker Daltonik micrOTOF-QII spectrometer.

Single-Crystal X-ray Diﬀraction Analysis. Data were collected

with an Oxford Gemini S diﬀractometer (4) and a Venture D8

diﬀractometer (4, 5) with Cu Kα radiation (λ = 1.541 84 Å; 4, 5) and

Mo Kα radiation (λ = 0.710 73 Å, 4) at 125 K (4) and ambient

conditions (4, 5). Measurements with Cu Kα radiation at the D8

Venture device were performed with a ﬁne-focus source. The

molecular structures were solved by direct methods using SHELXS-

4 6 5 4

and reﬁned by full-matrix least-squares procedures on F using

55,56

SHELXL-13.

All non-hydrogen atoms were reﬁned anisotropi-

cally, and a riding model was employed in the treatment of the

hydrogen atom positions, except as otherwise noted. Graphics of the

molecular structures were created by using ORTEP.

Electrochemistry. Electrochemical measurements of 4 and 5 (1.0

conditions.

−1

mmol·L ) using 0.1 mol·L [N Bu ][B(C F ) ] as the supporting

Yield: 360 mg (0.43 mmol, 43% based on FcPCl ). Anal. Calcd for

electrolyte in anhydrous, oxygen-free dichloromethane were per-

formed in an argon-purged cell at 25 °C with a Radiometer Voltalab

PGZ 100 electrochemical workstation interfaced with a personal

C H Fe O P (851.62 g/mol): C, 40.90; H, 2.13. Found: C, 40.75;

9 2

H, 2.52%. mp 230 °C (decomp). H NMR (CDCl , δ): 4.39 (s, 10H,

C H ), 4.64−4.66 (m, 4H, C H ), 4.69−4.71 (m, 4H, C H ) ppm.

2,54,58

computer.

electrode cell containing a Pt auxiliary electrode, a glassy carbon

For the voltammetric measurements a three-

C{ H} NMR (CDCl , δ): 70.8 (s, 10C, C H ), 72.8 (br, s, 4C,

3 5 5

C H ), 73.9 (br, 4C, C H ), 213.3 (s, 9C, CO) ppm. P{ H} NMR

(CDCl , δ): 316.2 ppm. IR data (KBr, v): 2965, 2925, 2854, 2065,

2035, 2013, 1994, 1157, 1031, 833, cm .

working electrode (surface area 0.031 cm ), and a Ag/Ag (0.01

mmol L⁻[AgNO ]) reference electrode ﬁxed on a Luggin capillary

−1

was used. The working electrode was pretreated by polishing on a

Buehler microcloth ﬁrst with 1 μm and then with a 1/4 μm diamond

paste. The reference electrode was constructed from a silver wire

Synthesis of Co (CO) (μ -PFc) (5). Compound 3 (1.00 g, 3.5

mmol) was placed in a Schlenk tube under inert conditions, followed

by the addition of Co (CO) (1.61g, 4.7 mmol) and 50 mL of

−

−1

inserted into a 0.01 mmol L solution of [AgNO ] and 0.1 mol L

tetrahydrofuran. The mixture was placed in an oil bath, slowly heated

to 40 °C, and stirred for 3 h, until gas evolution was ﬁnished. All

volatiles were removed in vacuo. The residue was taken up in a

minimum amount of CH Cl and mixed with silica. Puriﬁcation was

of an [N Bu ][B(C F ) ] acetonitrile solution in a Luggin capillary

6 5 4

with a CoralPor tip. This Luggin capillary was inserted in a second

Luggin capillary containing a 0.1 mol L⁻[N Bu ][B(C F ) ]

dichloromethane solution and a CoralPor tip. Experiments under

the same conditions showed that all reduction and oxidation

potentials were reproducible within ±5 mV. Experimental potentials

were referenced against an Ag/Ag⁺reference electrode, but the

presented results are referenced against ferrocene as an internal

realized by column chromatography (silica, 2.5 × 30 cm column size)

with a 1:8 hexane/toluene mixture (v/v). After the elution of

Co (CO) as a sustained purple band, cluster 5 was obtained.

Removal of all volatiles gave 5 as a dark red solid. Crystals suitable for

single-crystal X-ray diﬀraction analysis were obtained by evaporation

of a CH Cl solution of 5.

standard as required by IUPAC. To achieve this, each experiment

was repeated in the presence of 1.0 mmol L⁻of decamethylferrocene

Yield: 20 mg (0.02 mmol, 1% based on FcPCl ). mp 102 °C. H

NMR (CDCl , δ): 4.74 (m, 4H, CH an C H ), 4.57 (m, 4H, CH an

(

Fc*). Data were processed on a Microsoft Excel worksheet to set the

formal reduction potentials of the FcH/FcH couple to 0.0 V.

Under our conditions the Fc*/Fc* couple was at −614 mV versus

C H ), 4.30 (s, 10H, CH an C H ). C{ H} NMR (CDCl , δ): 71.0

5 4 5 5 3

(C H ), no further signals detectable. P{ H} NMR (CDCl , δ): no

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

(b) Hildebrandt, A.; Ruffer, T.; Erasmus, E.; Swarts, J. C.; Lang, H.

Article

signal detectable. IR data (KBr, v): 2965, 2925, 2854, 2065, 2035,

A. Hexaferrocenylbenzene. Chem. Commun. 2006, 2572−2574.

A Star-Shaped Supercrowded 2,3,4,5-Tetraferrocenylthiophene: Syn-

−

013, 1994, 1157, 1031, 833, cm .

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00276.

thesis, Solid-State Structure, and Electrochemistry. Organometallics

■

2010, 29, 4900−4905. (c) Hildebrandt, A.; Schaarschmidt, D.; van

As, L.; Swarts, J. C.; Lang, H. Diferrocenes containing thiadiazole

connectivities. Inorg. Chim. Acta 2011, 374 , 112 −118. (d) Hilde-

brandt, A.; Schaarschmidt, D.; Lang, H. Electronically Intercommu-

nicating Iron Centers in Di- and Tetraferrocenyl Pyrroles. Organo-

metallics 2011, 30, 556−563. (e) Hildebrandt, A.; Schaarschmidt, D.;

Claus, R.; Lang, H. Influence of Electron Delocalization in

Heterocyclic Core Systems on the Electrochemical Communication

in 2,5-Di- and 2,3,4,5-Tetraferrocenyl Thiophenes, Furans, and

Pyrroles. Inorg. Chem. 2011, 50, 10623−10632. (f) Yu, Y.; Bond, A.

D.; Leonard, P. W.; Vollhardt, K. P. C.; Whitener, G. D. Syntheses,

structures, and reactivity of radial oligocyclo-pentadienyl metal

complexes: penta(ferrocenyl)cyclopentadienyl and congeners.

Angew. Chem. 2006, 118, 1826−1831; Angew. Chem., Int. Ed. 2006,

The synthesis of all starting materials, NMR spectra of

all new compounds. Note: CCDC 1935472 (4 at 125

K), 1935473 (4 at 295 K), and 1935474 (5) (PDF)

Accession Codes

CCDC 1935472 −1935474 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.

5, 1794−1799. (g) Jiao, J.; Long, G. J.; Rebbouh, L.; Grandjean, F.;

Beatty, A. M.; Fehlner, T. P. Properties of a Mixed-Valence

III

(Fe ) (Fe ) Square Cell for Utilization in the Quantum Cellular

2 2

AUTHOR INFORMATION

Corresponding Authors

Marcus Korb − Faculty of Sciences, School of Molecular Sciences,

The University of Western Australia, 6009 Crawley, Perth,

Western Australia, Australia; orcid.org/0000-0001-5453-

■

Automata Paradigm for Molecular Electronics. J. Am. Chem. Soc.

2005, 127, 17819−17831. (h) Kaleta, K.; Hildebrandt, A.; Strehler,

F.; Arndt, P.; Jiao, H.; Spannenberg, A.; Lang, H.; Rosenthal, U.

Ferrocenyl-substituted metallacycles of titanocenes. Angew. Chem.

011, 123, 11444−11448; Angew. Chem., Int. Ed. 2011 , 50 , 11248 −

1252. (i) Hildebrandt, A.; Pfaff, U.; Lang, H. 5-Membered

137 ; Email: marcus.korb@uwa.edu.au

Heinrich Lang − Faculty of Natural Sciences, Institute of

Chemistry, Inorganic Chemistry, Technische Universitat

heterocycles with directly-bonded sandwich and half-sandwich

termini as multi-redox systems: synthesis, reactivity, electrochemistry,

structure and bonding. Rev. Inorg. Chem. 2011, 31, 111 −141.

Chemnitz, D-09107 Chemnitz, Germany; Center for Materials,

Architectures and Integration of Nanomembranes, D-09126

Chemnitz, Germany; orcid.org/0000-0001-9744-7906 ;

Phone: +49 (0)371-531-21210; Email: heinrich.lang@

chemie.tu-chemnitz.de ; Fax: +49 (0)371-531-21219

(2) (a) Pfaff, U.; Hildebrandt, A.; Korb, M.; Lang, H. The influence

of an ethynyl spacer on the electronic properties in 2,5-ferrocenyl-

substituted heterocycles. Polyhedron 2015, 86, 2−9. (b) Pfaff, U.;

Hildebrandt, A.; Schaarschmidt, D.; Hahn, T.; Liebing, S.; Kortus, J.;

Lang, H. Di- and Triferrocenyl (Hetero)Aromatics: Synthesis,

Characterization, (Spectro-)Electrochemistry, and Calculations. Orga-

nometallics 2012 , 31 , 6761 −6771. (c) Miesel, D.; Hildebrandt, A.;

Lang, H. Molecular electrochemistry of multi-redox functionalized 5-

membered heterocycles. Curr. Opn. Electrochem. 2018, 8, 39−44.

(d) Miesel, D.; Hildebrandt, A.; Korb, M.; Low, P. J.; Lang, H.

Synthesis and (Spectro)electrochemical Behavior of 2,5-Diferrocenyl-

Authors

Xianming Liu − Faculty of Natural Sciences, Institute of

Chemistry, Inorganic Chemistry, Technische Universitat

Chemnitz, D-09107 Chemnitz, Germany

Sebastian Walz − Faculty of Natural Sciences, Institute of

Chemistry, Inorganic Chemistry, Technische Universitat

e) Korb, M.; Pfaff, U.; Hildebrandt, A.; Ruffer, T.; Lang, H. 3,4-

-phenyl-1 H -phosphole. Organometallics 2013, 32, 2993−3002.

Ferrocenyl-Functionalized Pyrroles: Synthesis, Structure, and

Spectro)Electrochemical Studies. Eur. J. Inorg. Chem. 2014, 2014,

051−1061. (f) Pfaff, U.; Filipczyk, G.; Hildebrandt, A.; Korb, M.;

Chemnitz, D-09107 Chemnitz, Germany

Marco Rosenkranz − Leibniz Institute for Solid State and

Materials Research (IFW Dresden), 01069 Dresden, Germany

Evgenia Dmitrieva − Leibniz Institute for Solid State and

Materials Research (IFW Dresden), 01069 Dresden, Germany;

orcid.org/0000-0001-7490-617X

Alexey A. Popov − Leibniz Institute for Solid State and

Materials Research (IFW Dresden), 01069 Dresden, Germany;

orcid.org/0000-0002-7596-0378

Lang, H. 1,3,5-Triferrocenyl-2,4,6-tris(ethynylferrocenyl)-benzene − a

new member of the family of multiferrocenyl-functionalized cyclic

systems. Dalton Trans. 2014, 43, 16310−16321. (g) Pfaff, U.;

Hildebrandt, A.; Korb, M.; Schaarschmidt, D.; Rosenkranz, M.;

Popov, A.; Lang, H. Five-Membered Heterocycles as Linking Units in

Strongly Coupled Homobimetallic Group 8 Metal Half-Sandwich

Complexes. Organometallics 2015, 34, 2826−2840. (h) Miesel, D.;

Hildebrandt, A.; Korb, M.; Wild, D. A.; Low, P. J.; Lang, H. Influence

of P-Bonded Bulky Substituents on Electronic Interactions in

Ferrocenyl-Substituted Phospholes. Chem. - Eur. J. 2015, 21,

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c00276

Notes

11545−11559. (i) Speck, J. M.; Korb, M.; Schade, A.; Spange, S.;

The authors declare no competing ﬁnancial interest.

Lang, H. Ferrocenes Bridged by Ethylenediamino Thiophene: Varying

Charge Transfer Properties in a Series of 3,4-Di-N -substituted 2,5-

Diferrocenyl Thiophenes. Organometallics 2015, 34, 3788−3798.

ACKNOWLEDGMENTS

■

(j) Miesel, D.; Hildebrandt, A.; Korb, M.; Schaarschmidt, D.; Lang,

M.K. thanks the Forrest Research Foundation for a PostDoc

Fellowship and the Univ. of Western Australia for ﬁnancial

Support. We appreciate the technical support with computa-

tional resources in IFW Dresden by Ulrike Nitzsche.

H. Transition-Metal Carbonyl Complexes of 2,5-Diferrocenyl-1-

phenyl-1 H -phosphole. Organometallics 2015, 34, 4293−4304.

(3) (a) Hildebrandt, A.; Al Khalyfeh, K.; Nawroth, J. F.; Jordan, R.

Electron Transfer Studies on Conjugated Ferrocenyl-Containing

Oligomers. Organometallics 2016, 35, 3713−3719. (b) Pfaff, U.;

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