Coordination Chemistry

Fine-Tuning Redox Properties of Heteroleptic Molybdenum
Complexes through Ligand-Ligand-Cooperativity

Benedict J. Elvers,* Sebastian Pätsch, Siva S. M. Bandaru, Vera Krewald, Carola Schulzke,
and Christian Fischer*

Dedicated to Professor Joachim Heinicke valuing his lifetime achievement in phosphorous and nitrogen-based chemistry.

Abstract: Heteroleptic molybdenum complexes bearing
1,5-diaza-3,7-diphosphacyclooctane (P2N2) and non-in-
nocent dithiolene ligands were synthesized and electro-
chemically characterized. The reduction potentials of
the complexes were found to be fine-tuned by a
synergistic effect identified by DFT calculations as
ligand-ligand cooperativity via non-covalent interac-
tions. This finding is supported by electrochemical
studies combined with UV/Vis spectroscopy and tem-
perature-dependent NMR spectroscopy. The observed
behavior is reminiscent of enzymatic redox modulation
using second ligand sphere effects.

The active sites of many metalloenzymes benefit from
controlled reaction environments, which often positively
influence the overall reaction by interactions of or with the
second ligand sphere.[1] Attempts to chemically model these
effects have resulted in the field of metal-ligand coopera-
tivity (MLC)[2] and in stabilization strategies for
supramolecular complex assembly via ligand-ligand interac-
tion (LLI) utilizing non-covalent interactions (NCI).[3] Vari-
ous types of MLC have been reported, from internal pH
modulation,[4] pendant reactive sites,[2d,5] and structural
stabilization of distinct coordination geometries,[6] to redox

non-innocence.[7] Modeling the intricate influence of the
second ligand sphere on the redox properties in a small
metal ion bearing compound is challenging. Subtle redox
potential changes of transition metal complexes are achieved
most often by substituent variation in the ligand backbones
and by distortion of the coordination geometry through a
rigid ligand backbone, thereby altering the electronic
structures of the complexes.[6,8]

A much less common and decidedly more difficult
approach towards redox tuning exploits intramolecular inter-
actions in the second ligand sphere. For instance, hydrogen
bonds from urea-derived tripodal ligands were used to stabilize
an oxo ligand and tune its reactivity in high-valent complexes.[9]

As a side effect, the hydrogen bonds indirectly modulate the
electron density at the transition metal and thus its redox
properties. In a more directed approach, hydrogen bonding
was used to obtain “pseudo” chelating ligands, changing their
bite angle and, consequently, the overall electronic
structures.[3d,e,g,10] In nickel complexes with one multidentate
ligand, a varied chirality in the ligand backbone altered the
exhibited π–π interactions, resulting in a slightly different
geometry and thus modulated redox potential.[11] This observa-
tion could be considered a rare case of cooperativity in the
outer coordination sphere between the substituents of one
ligand. With regard to metal-ligand cooperativity, two types of
ligands known for their MLC ability are particularly note-
worthy: 1,5-diaza-3,7-diphosphacyclooctanes (short: P2N2) and
ene-dithiolates (or: dithiolenes), see Figure 1. The P2N2 ligand
is often associated with proton reduction/hydrogen oxidation
reactions in which its pendant amines and internal proton

[*] Dr. B. J. Elvers, S. Pätsch, Dr. S. S. M. Bandaru,
Prof. Dr. C. Schulzke, Dr. C. Fischer
Bioinorganic Chemistry, Institute for Biochemistry, University of
Greifswald
17489 Greifswald (Germany)
E-mail: benedict.elvers@uni-greifswald.de

christian.fischer@uni-greifswald.de

Dr. B. J. Elvers
Biophysical Chemistry, Institute for Biochemistry, University of
Greifswald
17489 Greifswald (Germany)

Prof. Dr. V. Krewald
Theoretical Chemistry, Institute for Chemistry, TU Darmstadt
64287 Darmstadt (Germany)

© 2023 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution Non-Commercial
NoDerivs License, which permits use and distribution in any med-
ium, provided the original work is properly cited, the use is non-
commercial and no modifications or adaptations are made.

Figure 1. Chemical structures of the investigated molybdenum mono-
dithiolene complexes and their respective ligands (p-tol=p-tolyl,
cy=cyclohexyl). For molecular structures of the four complexes, see
Figure S1.

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How to cite: Angew. Chem. Int. Ed. 2023, 62, e202303151
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relays are utilized.[12] The dithiolene ligand is a notorious non-
innocent ligand known for its ability to influence the redox
properties of the coordinated metal directly.[13]

While optimizing the properties of molybdenum com-
plexes with varying combinations of these two types of
ligands (Figure 1) with the aim to balance their electronic
properties, stabilities, reactivities, and catalytic abilities, an
unexpected yet rather remarkable synergistic effect of the
two ligands was noticed.

Complexes with monodithiolene ligands are known to
exhibit rather close-lying potentials upon reduction.[13e, 14] In
molybdenum complexes 2 and 3 with a ligand sphere that
combines two distinct dithiolene ligands with the novel P2

p-

tolN2
tBu ligand (1p-tol), a highly unusual modulation of the redox

behavior was observed. In the voltammograms of 2 and 3
(Figure 2), the two half-step potentials (E1/2) at �� 2 V are

almost collapsed, essentially beginning to merge into one 2e�

event. Differential pulse voltammetry (DPV) verified that
indeed two 1e� reduction events take place (Figure S2). To
assess the contribution of the P2N2 ligand to the observed
phenomenon, a second set of monodithiolene complexes, 4
and 5, was synthesized using the known P2

cyN2
tBu (1cy).[15] For

these complexes, no such collapse was observed (Figure S3
and Table 1). In the oxidative regions, the typical 2e�

oxidation is found for each of the four complexes. As
described earlier,[13e] this is followed by a chemical reaction
(EC-mechanism), releasing CO and yielding a solvent-coordi-
nated complex. Further proof for the latter is provided by the
XRD (X-ray diffraction) molecular structure of [Mo-
(CH3CN)3(cydt)(P2

p� tolN2
tBu)]2+ (6) (Figure S4).[16]

Density functional theory (DFT) calculations were per-
formed to better understand the distinct electrochemical

Figure 2. Close-up of the scan rate dependent cyclic voltammetry of [Mo(CO)2(dt)(P2
p-tolN2

tBu)] (dt= xdt (2), left; dt=cydt (3), right). Measurements
were performed in CH3CN using 0.1 M (n-Bu4N)(PF6) as supporting electrolyte. Potentials are summarized in Table 1. Full cyclic voltammograms
are shown in Figure S3.

Table 1: Half step potentials (E1/2), cathodic peak potential (Epc), and anodic peak potential (Epa) of [Mo(CO)2(dt)(P2
p-tolN2

tBu)], dt= xdt (2), dt=cydt
(3) and [Mo(CO)2(dt)(P2

cyN2
tBu)], dt= xdt (4), dt=cydt (5). Measurements were performed in CH3CN at room temperature using 0.1 M (n-

Bu4N)(PF6) as a supporting electrolyte at 100 mVs� 1.

(2) (3) (4) (5)

E1/2
1 � 1.971 V[a] � 2.097 V[a] � 2.124 V � 2.246 V

Epc
1 � 1.993 V[a] � 2.130 V[a] � 2.163 V � 2.278 V

Epa
1 � 1.949 V[a] � 2.064 V[a] � 2.085 V � 2.215 V

ΔU 1 44 mV[a] 66 mV[a] 78 mV 63 mV
E1/2

1(DPV) � 1.971 V � 2.120 V – –
E1/2

2 � 1.871 V[a] � 1.965 V[a] � 1.882 V � 1.966 V
Epc

2 � 1.902 V[a] � 1.998 V[a] � 1.922 V � 2.002 V
Epa

2 � 1.839 V[a] � 1.932 V[a] � 1.841 V � 1.929 V
ΔU 2 63 mV[a] 66 mV[a] 81 mV 73 mV
E1/2

2(DPV) � 1.889 V � 1.995 V – –
additional. Epc – � 0.521 V � 0.423 V � 0.584 V
E1/2

3 – 0.021 V 0.098 V � 0.047 V
Epc

3 – � 0.011 V 0.073 V � 0.090 V
Epa

3 0.232 V 0.052 V 0.122 V � 0.003 V
ΔU 3 – 63 mV 49 mV 88 mV
Δ (E1/2

1� E1/2
2) 100 mV 132 mV 242 mV 280 mV

Δ (E1/2
1(DPV)� E1/2

2(DPV)) 82 mV 123 mV – –
Δ (E1/2

1� E1/2
2)calc

[b] 159 mV 201 mV 227 mV 265 mV

[a] Half-step potentials and peak potentials roughly estimated by turning points in the cyclic voltammograms. [b] Calculated as final single point
energy differences from DFT calculations with the TPSSh functional.

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behavior of the P2N2 coordinated complexes starting with
coordinates obtained from XRD measurements.[17] In the
optimized structures of singly and doubly reduced forms of 2
and 3, the dithiolene ligands xdt and cydt, respectively, are
bent at the two sulfur atoms, leading to a stacked arrangement
with the adjacent p-tolyl substituents of the P2N2 ligand
(Figure 3 and S5). At first deemed a case of an optimization
slip-up based on using the BP86 functional, this type of
structure is, in fact, consistently found also with other density
functionals in which the NCIs in the second ligand sphere are
captured by the respective dispersion correction according to
Grimme (D3zero, see Computational Details). For the singly
reduced complex 21� the density functionals BP86, TPSSh,[18]

and PBE0[19] give comparable results. The BP86 and TPSSh
functionals produce almost identical, acute bending angles
(151° and 153°), whereas with PBE0, a slightly less bent
structure results (159°, Figure 3). The TPSSh density functional
produced the best agreement with experimental IR and UV/
Vis spectra of the neutral species 2 (Figures S6–S8 and
Table S2). In the following, hence, only the results based on
TPSSh calculations are discussed.

The bending of the dithiolene (Mo-SS-dtbackbone) was
observed to be most pronounced with an aromatic ring system
on both P2N2 and dithiolene ligand (21� : 151°). It decreases in
the order 31� , 41� , and 51� (163°, 170°, 172°) and is hence the
smallest when both ligands are non-aromatic cyclohexyl
derivatives. Most notably, the bending positively correlates
with the degree of signal merging in the reductive regions of
the voltammograms (Figure S10). A similar but decidedly
distinct effect was observed for high-valent dithiolene metal
complexes and defined as the “folding angle”.[20] In that case
the dithiolene bending compensates for the electron deficiency
by improved S-π-donation. In the present case, the ligand
motion (Figure S13 and Table S3) must have a different origin
since the d-orbitals are nearly or completely filled.

In the case of the xdt coordinated complex 21� , the NCI
can be best described as parallel offset π-stacking[21] with a
distance of 3.9 Å between both arene rings in the computed
structures. Replacing the p-tolyl group with hydrogen in
silico leads to a relaxed structure with no dithiolene bend
(177°, Figure S11). We can therefore conclude that ligand
bending results from a NCI of the dithiolene backbone with
the p-tolyl substituent. Switching off the dispersion correc-
tion during the geometry relaxation of 21� resulted in a
structure without NCI in the second coordination sphere
(Figure S12). The difference in energy for 21� was calculated
to be 2.4 kcalmol� 1 lower for the “stacked” structure, which
is in accordance with similar intramolecular London dis-
persion forces.[11b] The most significant degree of stabiliza-
tion of the “stacked” structure is seen when both ligands are
aromatic (21� ) and decreases with the gradual replacement
of the arene groups by cyclohexyl-derived moieties (31� :
1.7 kcalmol� 1; 41� : 1.1 kcalmol� 1; 51� : 1.2 kcalmol� 1) just as
the degree of dithiolene bending does. We note that these
minor energy differences should be taken as an indication of
the more stable species, not as a quantification of the
stabilization energy due to dispersion.

Regarding the experimentally observed near-collapse of
the reduction events, the theoretical differences between the
potentials for the two reduction steps (Δ=(E1/2

1� E1/2
2)calc) can

be calculated from final single point energy differences.[22] We
find that more closely spaced potentials are correlated with
more stabilized dithiolene bending (2 Δ: 159 mV, 3 Δ: 201 mV,
4 Δ: 227 mV, and 5 Δ: 265 mV) in accordance with the
experimental observations (Table 1 and S4).

The character of the HOMO of 21� is dominated by a
Mo-carbonyl π-bonding interaction with some sulfur non-
bonding contribution, while the SOMO (LUMO of 2) is best
described as a Mo-dithiolene π-anti-bonding orbital. Despite
the bending of the dithiolene ligand, the delocalized π-
system consisting of Mo d-orbitals and sulfur and dithiolene
carbon p-orbitals remains as intact (see Figure S14–S16) as
in comparable complexes.[13e,14] The geometry change and
ligand bending is facilitated by molybdenum by decreasing
its SOMO contribution via the dx2 � y2 orbital from 12.6%
(non-stacked) to 6.9% (stacked) while increasing its dxz
contribution from 8.2% (non-stacked) to 14.3% (stacked)
(Table S5). In consequence, the molybdenum orbital share
of the SOMO is geometrically linked to the degree of the
dithiolene tilt. The dxy orbital contribution to the SOMO
(6.5 vs. 6.6%) remains essentially the same in the distinct
geometries. However, the axial donors (CO and P) almost
halve their contributions to the SOMO in the “stacked”
species (stacked: CO 8.5%; P 4.2%; non-stacked: CO
14.1%, P 6.8%; Table S7).

The first reduction step is also associated with a change
in coordination geometry from trigonal prismatic to octahe-
dral, as was shown before by DFT calculations and reported
for related monodithiolene complexes.[13e, 14,23] Such a torsion
is evident also here from the Bailar-Twist angles[24] of the
“stacked” (52.2°) and “non-stacked” (45.2°) structures of 21�

compared to the parent complex (2) (3.1°; Table S1). All
four complexes investigated here exhibit the same computa-
tional geometry change upon reduction. However, only the

Figure 3. Superposition of singly reduced [Mo(CO)2(P2
p-tolN2

tBu)(xdt)]1�

(21� ) calculated with different density functionals (BP86: light blue;
TPSSh: blue; PBE0: orange) in which dispersion is parameterized
differently; visualization with Mercury. See Figure S9 for additional
perspectives.

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P2
p-tolN2

tBu species additionally engage in a significant degree
of ligand stacking due to NCIs. Since DFT calculations
suggest the stacking interaction to be energetically relatively
weak (21� : 2.4 kcal, 31� : 1.7 kcal; Tables S4), it should be
rather thermo-responsive. While cooling is expected to
stabilize the stacking and support the collapse of the two
redox events in the voltammograms, heating should do the
reverse. This hypothesis was investigated by temperature-
dependent cyclic voltammetry (Figure 4).

Commonly, temperature-dependent cyclic voltammetric
studies result in a congruent shift of all signals either up- or
down-potential.[25] Here, in contrast, with lower temper-
atures, the two reductive potentials of 2 and 3 moved
towards each other, resulting in a more substantial overlap
or even full merge (2) of these events and a greater
separation at higher temperatures. This observation agrees
with the weak NCI between adjacent arenes indicated by
DFT calculations as the underlying cause.

With a recently published method for spectral decomposi-
tion of spectro-electrochemical UV/Vis data, the time-depend-
ent concentration (Figure S17) and the pure component
spectra of all four investigated monodithiolene complexes
were extracted including those of their corresponding reduced
states that were generated only in situ (Figure S18–S21).[13e,26]

These spectra were then used to validate the ligand stacking
further. For [Mo(CO)2(P2

p-tolN2
tBu)(xdt)]1� (21� ) (Figure S18),

the two bands observed experimentally at 567 nm and 470 nm
are better reproduced with the computational transitions as
calculated for the “stacked” than for the “non-stacked”
species.

For further experimental evidence of the unusual behavior
of the compounds’ coordination spheres, the complexes were
reduced in situ and monitored by 31P NMR spectroscopy. In
the case of 3, [Co(Cp*)2] was used as the reducing agent
resulting merely in the release of the phosphane ligand. In
contrast, when 2 was reacted with KC8 at varying temper-
atures, two new doublets appeared (in addition to the starting
material and free ligand, Figure S22). These exhibit a 2J31P-31P

coupling constant of 40.2 Hz that likely corresponds to an
octahedral complex with two chemically inequivalent 31P
atoms. NOESY studies were then performed to directly
characterize the aromatic rings’ interaction. These show a
correlation signal (Figure S23 and S24) for the protons of the

tolyl residue (�7.01 ppm) and the xdt ligand (�7.84 ppm)
that is absent in the starting material.

Considering all experimental and DFT data, the ob-
served intramolecular interaction between the dithiolene
and the P2N2 ligand that results in fine-tuning the complexes’
redox properties can be described as ligand-ligand coopera-
tivity (LLC) arising from non-covalent interactions. The
geometric change in the coordination sphere upon reduction
results in spatial proximity of the backbone of the non-
innocent dithiolene ligand and the substituent on the
phosphane ligand. This proximity allows both ligands to
interact electronically after a folding motion of the dithio-
lene. The dithiolene bending and moving away from the
equatorial plane likely results in a reduced orbital overlap
between sulfur p and molybdenum d orbitals and, hence, a
reduced supply of electron density from S to Mo. This would
be in accordance with a facilitated reduction of the metal
center and a less negative redox potential for the second
reduction, as was observed electrochemically. The induced
“conversation” between the ligands modulates the electronic
structure of the complex, thereby directly influencing the
redox properties, as evidenced by the cyclic voltammograms.
This type of second ligand sphere interaction of two distinct
ligands in a single complex is unprecedented for molecular
compounds and may be considered a model system for
second coordination sphere effects in metalloenzymes. The
detailed understanding of the observed LLC and subsequent
redox modulation, as provided herein, should help facilitate
the development of new molecular redox catalysts exploiting
such second-ligand sphere effects in the future.

Acknowledgements

C.S. gratefully acknowledges general financial support from
the DFG (SCHU 1480/4-2). B.J.E. thanks the Deutsche
Bundesstiftung Umwelt (DBU, AZ 20018/562) for financial
support during his Ph.D. thesis and Prof. Mihaela Delcea for
the freedom to pursue independent research. Open Access
funding enabled and organized by Projekt DEAL.

Conflict of Interest

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available
from the corresponding author upon reasonable request.

Keywords: Ligand-Ligand Cooperativity · Non-Innocence
Ligands · Noncovalent Interactions · Redox Chemistry

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Manuscript received: March 6, 2023
Accepted manuscript online: April 14, 2023
Version of record online: May 9, 2023

Angewandte
ChemieCommunications

Angew. Chem. Int. Ed. 2023, 62, e202303151 (5 of 5) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH

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