Redox Potential Tuning of Dimolybdenum Systems through Systematic Substitution by Guanidinate Ligands

Article abstract of DOI:10.1021/acs.inorgchem.9b03394

We report the synthesis and characterization of a series of dimolybdenum paddlewheel complexes of the type Mo₂(DAniF)_4-n(hpp)_n (n = 1-3), where DAniF is the anion of N,N′-di-p-anisyl-formamidine and hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine. The effect on the electronic structure of these tetragonal paddlewheel dimolybdenum compounds was studied upon systematic substitution of formamidinate ligands by the more basic guanidinates. Mo-Mo distances in the paddlewheel structures decreased upon guanidinate ligand substitution, and were found to be 2.0844(6) and 2.0784(6), for Mo₂(DAniF)₃(hpp) (1) and trans-Mo₂(DAniF)₂(hpp)₂ (2), respectively. Electrochemical studies show that the half-wave potential of the Mo₂⁵⁺/Mo₂⁴⁺ couple shifts cathodically upon ancillary ligand substitution ranging from -0.286 V for the tetraformamidinate complex to -1.795 V for the tetraguanidinate analogue and with redox potentials of -0.75, -1.07, and -1.14 V for 1, 2, and 3 (Mo₂(DAniF)(hpp)₃), respectively. The presence of a second redox event assigned to the Mo₂⁶⁺/Mo₂⁵⁺ couple was not observed until two guanidinate ligands were introduced. Raman spectroscopy shows that the v(M-M) stretch gets systematically strengthened upon formamidinate ligand substitution by the guanidinate ligand hpp. The induced delta bond destabilization by the basic hpp ligand was measured using DFT calculations by tracking the energy of the frontier orbitals. The decrease in the HOMO-LUMO energy gap was supported by the red shift in the UV-vis spectra of the compounds: 412, 442, and 450 nm for 1, 2, and 3, respectively.

Full text of DOI:10.1021/acs.inorgchem.9b03394

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Redox Potential Tuning of Dimolybdenum Systems through
Systematic Substitution by Guanidinate Ligands
́
́
Nancy Rodríguez-Lopez, Nathalie Metta, Alejandro J. Metta-Magana, and Dino Villagran*
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ABSTRACT: We report the synthesis and characterization of a
series of dimolybdenum paddlewheel complexes of the type
Mo2(DAniF)4−n(hpp)n (n = 1−3), where DAniF is the anion of
N,N′-di-p-anisyl-formamidine and hpp is the anion of 1,3,4,6,7,8-
hexahydro-2H-pyrimido[1,2-a]pyrimidine. The effect on the
electronic structure of these tetragonal paddlewheel dimolybde-
num compounds was studied upon systematic substitution of
formamidinate ligands by the more basic guanidinates. Mo−Mo
distances in the paddlewheel structures decreased upon guanidi-
nate ligand substitution, and were found to be 2.0844(6) and
2.0784(6), for Mo2 (DAniF)3 (hpp) (1) and trans-
Mo2(DAniF)2(hpp)2 (2), respectively. Electrochemical studies
4+
show that the half-wave potential of the Mo25+/Mo2 couple
shifts cathodically upon ancillary ligand substitution ranging from −0.286 V for the tetraformamidinate complex to −1.795 V for the
tetraguanidinate analogue and with redox potentials of −0.75, −1.07, and −1.14 V for 1, 2, and 3 (Mo2(DAniF)(hpp)3),
5+
respectively. The presence of a second redox event assigned to the Mo26+/Mo2 couple was not observed until two guanidinate
ligands were introduced. Raman spectroscopy shows that the v(M−M) stretch gets systematically strengthened upon formamidinate
ligand substitution by the guanidinate ligand hpp. The induced delta bond destabilization by the basic hpp ligand was measured
using DFT calculations by tracking the energy of the frontier orbitals. The decrease in the HOMO−LUMO energy gap was
supported by the red shift in the UV−vis spectra of the compounds: 412, 442, and 450 nm for 1, 2, and 3, respectively.
Scheme 1. Structural Ligands Used in Multiple Bonded
Bimetallic Complexes in This Study: Carboxylate,
Formamidinate, and Guanidinate Ligands
INTRODUCTION
■
The chemistry of metal−metal-bonded bimetallic complexes
has been enhanced by the use of bidentate ligands such as
carboxylates, amidinates, and guanidinates.1 The nature of
such supporting ligands has a strong effect on the oxidation
states, redox potentials, and electronic properties of these
compounds.2,3 The increased attention to guanidinate ligands
is a result of their capability to stabilize higher oxidation states
in bimetallic complexes.4−10 This ability is a result of the
delocalization of the charge across the guanidinate back-
bone.11,12 Of special interest is the bicyclic hpp ligand (hpp is
the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyr-
imidine) due to its high Lewis basicity (Scheme 1),13,14
which allows the stabilization of the most easily ionizable
family of complexes. W2(hpp)4, W2(TMhpp)4, and
W2(TEhpp)4 present onset photoelectron ionization energies
(3.51, 3.45, and 3.40 eV respectively) lower than the
ionization energy for the cesium atom (3.89 eV).1,15,16 The
low ionization energies for compounds containing hpp have
been associated with the strong antibonding interaction
between the π orbitals of the ligand and the δ orbitals of
bimetallic compounds of the M2L4 type are observed when
the hpp ligand is introduced into the system.1
Research on bicyclic guanidinate ligands has centered on
enhancing the solubility of bimetallic complexes in a variety of
organic solvents16,18 as well as fine-tuning their electronic and
Received: November 25, 2019
the M2 unit,15 therefore destabilizing the M2 core and
4+
4+
favoring the oxidation to M2 and M2 species.13,17,18
Consequently, large cathodic shifts in the redox potentials of
5+
6+
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electrochemical properties.19,20 The individual effect of the
ligands on the electronic properties of the system has been
explored in similar bimetallic complexes by systematic
replacement of the ancillary ligands.21,22 Prior work with
carboxylate and formamidinate ligands in dimolybdenum
complexes has revealed that when carboxylates are systemati-
cally replaced by formamidinate ligands there is an increase in
the stability of the oxidized species as well as a shift in the
reduction potentials.23 Previous results show a remarkable
X-ray Structure Determination. Suitable crystals for X-ray
diffraction were prepared by the diffusion of hexanes into THF
solutions of 1 and 2. Each crystal was mounted with a small amount
of silicone grease and centered in the goniometer of a Bruker
SMART APEX CCD system equipped with a graphite mono-
chromator and a Mo Kα fine-focus tube (λ = 0.71073 Å). Data for all
crystals was collected at 100 K with no crystal decay observed during
the collection. The frames were integrated with the Bruker SAINT
Software package using a narrow-frame algorithm. Data was
corrected for absorption effects via the multiscan method (SADABS).
All structures were solved and refined by direct methods using the
Bruker SHELXTL software package. The Mo−Mo distances for 1
and 2 are given in Table 1, while their crystallographic data are listed
in Table 2.
4+
capacity of hpp to fine-tune the redox potential of the Mo2
unit in M2L4 complexes. When bicyclic guanidinates are
introduced into dimolybdenum paddlewheel complexes, the
most stable species is the triply bonded Mo26+.8 Nonetheless,
a study where formamidinates are systematically replaced by
guanidinate ligands has not been reported. Thus, the effect of
the substitution on the electronic structure and reduction
potentials of the dimolybdenum center has not been explored.
Herein we report the synthesis, characterization, and study
of a series of compounds of the form Mo2(DAniF)4−n(hpp)n,
where n = 1, 2, 3. We have synthesized and characterized
dimolybdenum paddlewheel units with different proportions
of formamidinate and guanidinate ligands in an attempt to
fine-tune the electronic structure of these compounds and to
gain a better understanding of the electron-donor properties
of basic ligands such as acetates, formamidinates, and
guanidinates when attached to a bimetallic system. The
degree of tuning was quantified by exploring their electro-
chemical properties and comparing their prospective redox
potentials, Raman shifts, and electronic absorption. Density
functional theory (DFT) studies were performed to compare
the respective energies of the compounds and to understand
their electronic structure. The effect of ligand substitution on
the electronic structures of the dimetal core was investigated
using X-ray diffraction, electrochemistry, Raman, and UV−vis
spectroscopies and were correlated with the results from DFT
calculations.
Table 1. Experimental and Calculated Mo−Mo Bond
Lengths (Å) for Compounds I−IV and 1−3
experimental
calculated
I
2.0934(8)40
2.0892(8)12
2.0964(5)13
2.0831(4)
II
III
1
2.12328
2.11451
2.10730
a
2
2.0784(6)
3
IV
2.067(1)1
a
5+
Isolation of the singly oxidized Mo2 compound, 2·PF6, rendered a
Mo−Mo distance of 2.114(2) Å, corresponding to the decrease in
bond order from 4 to 3.5. See the SI for complete crystallographic
information.
Table 2. X-ray Crystallographic Data for 1 and 2
Mo2(DAniF)3(hpp), 1 trans-Mo2(DAniF)2(hpp)2, 2
formula
FW, g·mol−1
crystal system
space group
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
C52H57Mo2N9O6
1095.94
monoclinic
P21/c
36.359(3)
12.0642(8)
23.0343(16)
90
106.0600(10)
90
9709.5(11)
8
C44H52Mo2N10O4
976.83
triclinic
P1
̅
9.7483(7)
10.6331(7)
11.4180(8)
65.7350(10)
86.1570(10)
74.3210(10)
1037.51(12)
1
EXPERIMENTAL SECTION
■
General Procedures. All synthesis and material handling
procedures were conducted under an N2 atmosphere using a drybox
or standard Schlenk techniques, unless otherwise noted. All of the
used solvents were dried and degassed using a Pure Process
Technology solvent purification system. The materials used for this
study: HDAniF,24 Mo2(O2CCH3)4 (I),25 Mo2(DAniF)3(O2CCH3)
(II),26 Mo2(DAniF)4 (III),27 trans-Mo2(DAniF)2(O2CCH3)2,21 and
Z
dcalc (g·cm−3
)
1.499
1.563
μ (mm−1
2θ (deg)
λ, Å
)
0.577
0.662
1.96−27.43
0.71073
22
[Mo2(DAniF)(CH3CN)6](BF4)3 were prepared following literature
1.17−31.00
0.71073
100(2)
procedures. 1,3,4,6,7,8-Hexahydro-2H-pyrimido[1,2-a]pyrimidine
(Hhpp) and other commercially available chemicals were used as
received.
T, K
100(2)
GOF
R1,
wR2(I > 2σ(I))
0.951
0.0535, 0.1210
1.036
0.0483, 0.1072
Physical Measurements. Elemental analyses were performed by
Midwestern Microlab, LLC (Indianapolis, IN) and Galbraith
Laboratories, Inc. (Knoxville, TN). UV−vis spectra were collected
on a SEC2000 Spectra System with Visual Spectra 2.1 software from
350 to 800 nm. Infrared spectra were recorded on an Agilent Cary
630 FTIR spectrometer. Raman spectra were obtained on a Thermo
a
b
a
b
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑[w(Fo − Fc )2]/
2
2
2
∑w(F0 )2]1/2, w = 1/[σ2(Fo ) + (aP)2 + bP], where P = [max(Fo
or 0) + 2(Fc2)]/3.
1
Scientific DXR SmartRaman spectrometer using a 532 nm laser. H
NMR spectra were recorded on Bruker 300 and 400 MHz NMR
spectrometers with chemical shifts referenced to the residual protic
signal of C6D6. Electrochemical analyses by cyclic voltammetry (CV)
were performed in THF using a CHI760D potentiostat with a Pt
working electrode, Pt mesh auxiliary electrode, Ag/Ag+ reference
electrode, a scan rate of 100 mV/s, and 0.1 M TBAPF6 as an
electrolyte. Ferrocene was added at the end of the run as an internal
standard.
M o 2 ( D A n i F ) 3 ( h p p ) ( 1 ) .
A
m i x t u r e o f y e l l o w
Mo2(DAniF)3(O2CCH3) (1.02 g, 1.00 mmol) and Hhpp (0.139 g,
1.00 mmol) was dissolved in 40 mL of THF. While the mixture was
stirred, 2 mL of a 0.5 M solution of NaOCH3 in methanol were
slowly added. The color first turned red and then brown. The
reaction was stirred for 5 h at room temperature, producing white
crystalline sodium acetate. After removal of the solvent under
reduced pressure, the residue was extracted with 20 mL of
B
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a
Scheme 2. Reaction Conditions for the Procurement of Compounds 1−3
a
(a) Mixing I with 3 equiv of HDAniF and NaOCH3 in THF; (b) reaction of II with 1 equiv of Hhpp and NaOCH3 in THF; (c) addition of 2
equiv of HDAniF and NaOCH3 to I in THF; (d) addition of 2 equiv of Hhpp and NaOCH3 in THF to trans-Mo2(DAniF)2(O2CCH3)2; (e)
reaction of II with 1 equiv of HDAniF and NaOCH3 in THF; (f) reaction of trans-Mo2(DAniF)2(O2CCH3)2 with 2 equiv of HDAniF and
NaOCH3 in THF; (g) mixing III with 3 equiv of HBF4 in CH3CN:DCM; (h) addition of 3 equiv of Hhpp and NaOCH3 to
[Mo2(DAniF)(CH3CN)6](BF4)3 in THF. The tetraguanidinate (IV) is obtained by the reaction of Mo2(O2CF3)4 with 4 equiv of Hhpp in THF.
Compounds 1−3 are allocated in bold line boxes, while other compared compounds (I−IV) can be found in dashed line boxes. Letters O and N
are used to represent the oxygen and nitrogen atoms in the acetates, formamidinates, and guanidinates ligands, while L refers to an acetonitrile
molecule. All reactions were carried out under an inert atmosphere at room temperature unless otherwise noted.
dichloromethane; filtration removed NaO2CCH3. The volume of the
filtrate was reduced to about 5 mL with vacuum evaporation. Ethanol
(10 mL) was added to the residue with vigorous stirring. A bright-
yellow solid and a dark-brown supernatant solution were obtained.
After decantation, the solid was washed with ethanol (2 × 10 mL),
followed by 10 mL of hexanes, and dried under vacuum. Yield: 901
Mo2(DAniF)(hpp)3 (3). To a mixture of purple [Mo2(DAniF)-
(CH3CN)6](BF4)3 (0.600 g, 0.650 mmol) and Hhpp (0.270 g, 1.95
mmol) in THF, NaOCH3 (3.90 mL) 0.5 M in methanol was added
dropwise. The color turned bright orange upon addition of the
sodium salt. The reaction was stirred for 5 h. After 5 h, the stirring
was stopped and the solution was allowed to settle. An orange solid
could be seen at the bottom, and the supernatant was decanted. The
solid was washed with diethyl ether (2 × 10 mL) and dried under
1
mg, 82%. H NMR (δ in C6D6): 1.75 (q, 4H, −2CH2), 2.89 (t, 4H,
−2CH2), 3.21 (s, 6H, −OCH3) 3.22 (s, 12H, −OCH3), 3.45 (t, 4H,
2CH2), 6.58 (d, 8H, aromatic C−H) 6.67 (d, 16H, aromatic C−H),
8.56 (s, 2H, −NCHN−), 8.58 (s, 1H, −NCHN−). UV−vis λmax: 412
nm. Raman (cm−1): ν = 487. ESI-MS (m/z): calcd 1097.3 [M + H+],
found 1097.5 [M + H+]. Anal. Calcd for Mo2C52N9O6H57: C, 56.99;
N, 11.5; H, 5.24. Found: C, 56.79; N, 11.27; H, 5.35.
1
vacuum. Yield: 395 mg, 73%. H NMR (δ in C6D6): 1.53 (q, 8H,
−2CH2), 2.58 (t, 8H, −2CH2), 2.89 (t, 4H, −2CH2), 2.92 (q, 4H,
−2CH2), 3.10 (t, 8H, −2CH2), 3.33(s, 6H, −OCH3), 3.46, (t, 4H,
−2CH2), 6.62 (d, 4H, aromatic C−H), 6.77 (d, 4H, aromatic C−H),
8.33 (s, 1H, −NCHN−). UV−vis λmax: 450 nm. Raman (cm−1): ν =
505. ESI-MS (m/z): calcd 822.2 [M − 2OCH3 + Na+], found 821.9
[M − 2OCH3 + Na+].
trans-Mo2(DAniF)2(hpp)2, 2. A mixture of yellow trans-
Mo2(DAniF)2(O2CCH3)2 (0.500 g, 0.610 mmol) and Hhpp (0.160
g, 1.15 mmol) was dissolved in 40 mL of THF. While the solution
was stirred, 2.30 mL of a 0.5 M solution of NaOCH3 in methanol
were slowly added. The color turned orange, and with further stirring
the solution turned a brighter shade of orange. The reaction was
stirred for 5 h. After 5 h, the stirring was stopped and the solution
was allowed to settle. An orange solid and a dark brown supernatant
solution were obtained. After decantation, the solid was washed with
diethyl ether (2 × 10 mL) and dried under vacuum. Yield: 495 mg,
COMPUTATIONAL DETAILS
■
Density functional theory (DFT) calculations were performed
with the hybrid Becke-3 parameter exchange functional and
the Lee−Yang−Parr nonlocal correlation functional
(B3LYP)28,29 as implemented by the Gaussian 0930 (Revision
C.01) program suit. A double-ζ-quality basis set (cc-
pvdz)31−35 was used on nonmetal atoms (carbon, nitrogen,
oxygen, and hydrogen). An effective core potential (ECP)
representing the 1s2s2p3s3p3d4p core was employed for the
molybdenum atoms, along with the associated double-ζ basis
set (LANL2DZ).36−39 The absence of imaginary vibrations in
the frequency calculations for all compounds indicated that
the gas-phase geometry optimizations were minima in the
1
83%. H NMR (δ in C6D6): 1.79 (q, 8H, −2CH2), 2.92 (t, 8H,
−2CH2), 3.29 (s, 12H, −OCH3), 3.59 (t, 8H, −2CH2), 6.62 (d, 8H,
aromatic C−H), 6.74 (d, 8H, aromatic C−H), 8.41 (s, 2H,
−NCHN−). UV−vis λmax: 442 nm. Raman (cm−1): ν = 494. ESI-
MS (m/z): calcd 980.24 [M + H+], found 980.5 [M + H+]. Anal.
Calcd for Mo2C44N10O4H54: C, 53.99; N, 14.31; H, 5.56. Found: C,
53.81; N, 14.07; H, 5.47. ESI-MS: m/z: 982.40.
C
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potential energy surface. Polarizability derivatives (Raman
intensities) were computed using the keyword Freq = Raman
in the Gaussian 09 package. Time-dependent density
functional theory (TD-DFT) was used to calculate the
electronic transition energies for the 15 lowest singlet excited
states of all neutral compounds.40−46 All calculations were
performed in a 44-processor PowerWolf PSSC supercomputer
cluster running Linux Red Hat 4.1.2-54 located at the
University of Texas at El Paso. Isosurface plots of frontier
molecular orbitals were generated using Avogadro software
with isodensity values of 0.04.
[Mo2(DAniF)(CH3CN)6]3+ was first obtained by reacting
tetrafluoroboric acid, HBF4, and Mo2(DAniF)4 (III) (Scheme
2, reaction conditions in g).27 The removal of formamidinates
is more difficult compared to the displacement of acetate
groups in I due to their increased basicity. Their substitution
requires stronger reactants (HBF4) than NaOCH3 used in the
displacement of acetates in I. Compounds with the formula
Mo2(DAniF)4−n(CH3CNeq)2n, are useful since acetonitrile
molecules can be easily displaced in substitution reactions.21
−
In the substitution of DAniF by CH3CN, HBF4 protonates
the formamidinate ligand to a monodentate intermediate,
while a neutral acetonitrile occupies the open coordination
site.22 Finally, the MeCN ligands are substituted by hpp to
obtain Mo2(hpp)3(DAniF) (3) as an orange powder in 73%
yield (Scheme 2, reaction h). This compound is partially
soluble in most donor and aromatic solvents and is highly air-
sensitive.
Structural Determinations. Single crystals suitable for
crystallography were obtained by the diffusion of hexanes into
a THF solution of the compounds. The structures of 1 and 2
provided in Figure 1 show a paddlewheel structure with
RESULTS AND DISCUSSION
■
Molecular Design and Syntheses. Redox tuning can be
studied through ligand substitution by taking advantage of the
well-defined redox properties of quadruply bonded Mo2
complexes. An overview of the synthetic routes followed for
the synthesis of all involved compounds is shown in Scheme
2, where 1−3 are placed in bold line boxes. Other species
compared in the study are written off as I−IV and can be
found in dashed line boxes. The depicted reactions are listed
with italicized lowercase letters a−h. Letters O and N are used
to represent the oxygen and nitrogen atoms found in the
ancillary ligands (acetates, formamidinates, and guanidinates),
while L refers to an acetonitrile molecule in an equatorial
position. A brief description of the reactions for the synthesis
of each species can be found in the footnote associated with
Scheme 2. The use of carboxylates and formamidinate ligands
allows compounds bearing different patterns of ancillary
ligands: one carboxylate and three formamidinates as well as
two carboxylates and two formamidinates in either the cis or
trans configuration. The remaining carboxylates in such
precursors could be replaced by the more basic guanidinates
to afford the anticipated compounds.
As reported,25 starting material Mo2(O2CCH3)4 (I) can be
used to synthesize Mo2(DAniF)3(O2CCH3) (II). The
stoichiometric addition of the more basic formamidinate
ligand (HDAniF) compared to acetate allows their sub-
stitution to afford II (Scheme 2, route a). Compound 1 was
synthesized by replacing the acetate ligand in II with the
bicyclic guanidinate hpp (Scheme 2, following conditions in
b). Mo2(DAniF)3(hpp) (1) was obtained in 82% yield as a
yellow powder. Compound 1 is considerably stable as a solid
under an inert atmosphere. It does not react with chlorinated
solvents, and it is soluble in donor solvents such as THF as
well as aromatic solvents such as benzene.
The synthesis of trans isomer Mo2(DAniF)2(hpp)2 (2) was
performed similarly to 1. Precise control of the stoichiometry
for the synthesis of precursor trans-Mo2(DAniF)2(O2CCH3)2
was crucial (Scheme 2, conditions in c). Excess of
formamidinate would otherwise react to yield III. Once the
precursor was successfully obtained, the replacement of the
acetate ligands by hpp was completed (Scheme 2, following
reaction conditions in d). trans-Mo2(DAniF)2(O2CCH3)2 (2)
was obtained as an orange powder in 83% yield. Compound 2
is stable in the solid state under an inert atmosphere. It reacts
with chlorinated solvents and is only partially soluble in other
solvents. While we were not able to obtain the cis isomer, no
interconversion from the trans to cis configuration was
detected.
Figure 1. Crystal structures for 1 and 2 with ellipsoids drawn at the
50% probability level. All hydrogen atoms have been omitted for
clarity.
formamidinate and guanidinate structural ligands. Figure 1a
shows the molecular structure of Mo2(DAniF)3(hpp) (1) with
selected bond distances and angles given in Table S1.
Compound 1 crystallizes in monoclinic space group P21/c
with Z = 8. The M−M dimolybdenum bond distance
2.0831(4) Å, falls in the range of 2.06−2.17 Å, which
corresponds to that of a typical Mo2 quadruple bonded
In contrast to the direct synthetic pathway of 1 and 2,
Mo2(DAniF)(hpp)3 (3) was prepared through an indirect
route (Scheme 2). In order to afford 3, precursor
D
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compound.47,48 The replacement of the acetate ligand in
starting material Mo2(DAniF)3(O2CCH3) (II) by the
guanidinate ligand to form 1 was shown to affect the
metal−metal bond distance. Compared to the reported Mo−
Mo bond in II, of 2.0892(8) Å, the metal−metal distance was
shortened by 0.0061 Å. The Mo−Nhpp bond distance in 1 is
on average 2.1315 Å, while the average bond lengths for
formamidinate nitrogen atoms Mo−Ntrans and Mo−Ncis are
2.1605 and 2.1532 Å, respectively. The bond distances found
trans to guanidinate nitrogen atoms are slightly longer than
those cis to them. This lengthening trend of trans groups is
similar to the trans influence of both guanidinate and
formamidinate groups relative to carboxylates.21,49,50 Mean-
−
while, the average N−C bond lengths were 1.357 Å for hpp
and 1.331 Å for −DAniF, which are consistent with the
delocalized charge in these bonds.
Compound 2, where two guanidinates in the trans
configuration have been introduced into the system,
Figure 2. Cyclic voltammograms for 1, 2, and 3 (1 mM) in THF
with potentials referenced to Fc/Fc+. The electrochemical analyses
were performed with Pt working and auxiliary electrodes and an Ag/
AgCl reference electrode, with 0.1 M TBAPF6 as the electrolyte and
a scan rate of 100 mV/s. The poor solubility of 2 and 3 is reflected in
the decreased current response.
̅
crystallizes in the triclinic space group P1. The molecule
occupies a special position (Z = 1) having the center of the
M−M bond sit on an inversion point. Therefore, the two
Mo(DAniF)(hpp) half-units are crystallographically equiva-
lent. Figure 1b shows the molecular structure for trans-
Mo2(DAniF)2(hpp)2 (2) with selected bond distances and
angles given in Table S1. The Mo−Mo distance of 2.0784(6)
Å is shorter than that in 1 by 0.006 Å as expected from the
addition of a second hpp unit. The Mo−Nhpp bond distance in
2 is on average 2.138(3) Å, whereas that for Mo−NDAniF is
2.158(3) Å. We were unable to obtain single crystals of 3
suitable for adequate data collection.
Table 3. Redox Potentials in Volts (vs Fc/Fc+) for Mo2L4
Compounds I−IV and 1−3
2
5+
2
4+
E1/2 (Mo26+/Mo2
)
E1/2 (Mo25+/Mo2
)
I
−0.120
−0.286
−0.38124
−0.754
−0.988
−1.136
−1.7957
II
III
1
2
3
−0.693
−0.763
−0.9687
The C−N and C−C bond distances observed in the
guanidinate ligands for complexes 1 and 2 are comparable. In
addition, distances of 1.45 and 1.51 Å were observed for the
C−N and C−C bonds in the formamidinate ligands,
respectively. On the other hand, the two equivalent C−N
bonds of the CN3 unit are shorter, with a bond length of 1.34
Å. This observation is in agreement with the atoms in the hpp
core allowing charge delocalization and causing the guanidine
core to demonstrate planarity (178.9°). In addition, the
shorter Mo−Nhpp distance for the hpp ligand when compared
to the Mo−N distance for the formamidinate ligand is a
reflection of the stronger binding of hpp due to the higher
ligand basicity.
IV
vs Fc/Fc+) when compared to II and III. The lower oxidation
potential of 1 is expected due to the increase in the electron
density on the metal center as a result of the increased Lewis
basicity of hpp.14
The bicyclic guanidinate, hpp, has shown the ability to
stabilize higher oxidation states in bimetallic complexes due to
its π-donor nature and increased basicity.16,52 The CV for
Mo2(hpp)4 (IV) shows two reversible one-electron redox
events at E1/2 (1) = −1.795 V and E1/2 (2) = −0.968 V
4+
5+
corresponding to the oxidation of Mo2 to Mo25+and Mo2
to Mo26+, respectively.8 The charge stabilization ability of hpp
can be observed when a second guanidinate is introduced in
2. Two one-electron reversible redox processes are obtained at
E1/2 (1) = −0.988 V and E1/2 (2) = −0.693 V, analogous to
those for IV. The half-wave potentials, E1/2 (1) and E1/2 (2),
Electrochemical Studies. Dimolybdenum paddlewheel
structures show one-electron oxidations that leads to the
51
5+
formation of a Mo2 unit. However, when hpp is used as a
ligand in Mo2(hpp)4 (IV), two one-electron oxidations are
observed.8 Thus, to understand the ability of hpp to stabilize
higher oxidation states the electrochemical properties of I, II,
and III and 1, 2, and 3 were studied. Cyclic voltammograms
(CV) for the studied compounds and their precursors are
depicted in Figure 2 and Figure S8, respectively. Electro-
chemical data for the compounds presented in this work as
well as precursors I, II, III, and IV are given in Table 3, and
all redox potentials have been referenced against the
ferrocene/ferrocenium couple (Fc/Fc+). Compound I shows
a one-electron reversible event with a half-wave potential
(E1/2) at −0.120 V, which corresponds to the reversible
4+
5+
5+
were assigned to the oxidation of Mo2 to Mo2 and Mo2
to Mo26+, respectively. The redox potential at −0.693 V
(Figure 2) indicates that the oxidation of 2+ is easier than that
of I and III. The measured half-wave potential for the
[Mo2]4+/5+ pair in 2 is significantly more negative than that of
1 by 0.234 V. To note, the decreased solubility of 2 compared
to that of I, II, and 1 in THF causes a considerable decrease
in the current in the cyclic voltammogram.
The replacement of a third formamidinate by hpp in 3
shows a similar voltammogram to that of 2, with two one-
electron reversible redox events at E1/2 (1) = −1.136 V and
E1/2 (2) = −0.763 V vs Fc/Fc+. Analogous to 2, these
4+
oxidation of Mo2 to Mo25+. Similarly, II and III display a
one-electron oxidation with E1/2 = −0.286 and −0.381 V,
4+
5+
respectively. The replacement of one formamidinate by a
processes correspond to the oxidation of Mo2 to Mo2 and
4+
5+
5+
guanidinate ligand in 1 shifts the Mo2 to Mo2 oxidation
potential by 0.5 V in the negative direction (E1/2 = −0.754 V
Mo2 to Mo26+, respectively. The redox potential for the
[Mo2]5+/4+ pair in 3 is 0.148 V more negative than in 2. This
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trend continues when the final formamidinate is replaced to
excitation of 532 nm and are found in Figure 4. For nonaxially
substituted carboxylate dimolybdenum complexes such as I,
4+
5+
get IV where the E1/2 for the oxidation of Mo2 to Mo2
occurs at −1.795 V vs Fc/Fc+. In addition, the low solubility
of 3, as observed for 2, is reflected in the decreased redox
current observed in Figure 2.
The systematic substitution of formamidinates by guanidi-
nates allows to probe the ability of hpp to destabilize the
4+
5+
quadruple bond. The measured E1/2 for the Mo2 to Mo2
oxidation shifts cathodically as guanidinates are added to the
4+
Mo2 unit. It is evident that the guanidinate ligand, hpp,
tunes the redox potentials of these bimetallic molybdenum
systems due to the high basicity of the anion along with its
n+
ability to interact strongly with the Mo2 core via its
delocalized π electrons in the central guanidinate core.52
Therefore, the δ bonding orbital is destabilized upon
increasing the electron-donating ability of the ancillary ligand.
The redox properties of these series of compounds show the
increase in basicity for −OAc < −DAniF < −hpp, with
guanidinate being the most basic. Consequently, the
guanidinate ligands are the most difficult to substitute while
acetates are the easiest ones due to their increased lability.
Moreover, a trans influence is noticed proportionally to the
basicity of the ligand, and such influence is also manifested in
the Mo−N bond distance (Table S1).
Figure 4. Raman spectra of compounds 1 (ν = 487 cm−1, bottom
trace), 2 (ν = 494 cm−1, middle trace), and 3 (ν = 505 cm−1, top
trace) at room temperature.
Spectroscopic Properties. The UV−vis spectra of
compounds 1−3 show two absorption peaks: one near the
UV region (∼300 nm) and one in the visible region (∼450
nm). The low-energy transition in the region of 400−600 nm,
assigned to Mo2 δ → Mo2 δ*, has been well established in the
literature for quadruply bonded paddlewheel dimolybdenum
compounds.22,53,54 The energy of this transition relies on the
nature of the ancillary ligands.55 The absorption spectra of 1−
3 are shown in Figure 3. Compound 1 shows a band at 412
the band associated with the symmetric ν(Mo−Mo) vibration
appears at 404 cm−l (Figure S9), while for the halide species it
is 342 8 cm−l.56 The strongest Raman signal was assigned
to the Mo−Mo stretching mode as supported by DFT
calculations. The ν(Mo−Mo) of 1 shows a Raman shift at 487
cm−1. The trans isomer, 2, has a Mo−Mo shift at 494 cm−1.
Finally, compound 3 has a Mo−Mo vibration at 505 cm−1.
This complex shows strong fluorescence, which accounts for
the high intensity detected. The sequential addition of
guanidinate ligands shifts the ν(Mo−Mo) Raman signal, this
observation is consistent with the destabilization of the
Mo−Mo bond due to the high basicity and electron-donating
abilities of guanidinate ligands.57
DFT Calculations. Density functional theory (DFT)
calculations were performed on precursors III and IV, and
all synthesized compounds, 1−3, to gain insight into the
electronic structure of their Mo2 cores, their ground-state
geometries, and the nature of their frontier orbitals. The gas-
phase geometry optimizations were performed using the
crystal structure parameters as the starting point for the
calculations. The calculated gas-phase Mo−Mo bond distance
for 1 is 2.12328 Å, which is comparable to the experimentally
obtained 2.0831(4) Å. For 2, the DFT obtained metal−metal
bond is 2.11451 Å while the experimental one is 2.0784(6) Å.
Meanwhile, the calculated Mo−Mo bond for 3 is 2.10730 Å.
There is a consistent overestimation of the Mo−Mo bond by
about 0.04 Å, which has been well established in the literature
when B3LYP and other functionals are employed.16,58 The
shortening of the M−M bond distance is consistent with the
greater overlap of the δ bond due to the addition of
guanidinates as ancillary ligands. This effect is described by
the linear relationship of the M−M bond distance with the
number of guanidinate ligands surrounding the bimetallic
center (Figure 5). The fitting of the data gives a correlation
coefficient of (R2) 0.921. Another correlation is also observed
between the energy of the δ → δ* transition and the metal−
metal bond distance. As the metal−metal bond length
increases, the energy of the δ → δ* transition increases as
well.
Figure 3. Electronic spectra of compounds 1 (λmax = 412 nm; red
spectrum), 2 (λmax = 442 nm; purple spectrum), and 3 (λmax = 450
nm; blue spectrum) in THF solution at room temperature.
nm, arising from the δ → δ* transition. Similarly, compounds
2 and 3 presented a band at 442 and 450 nm, respectively. A
detailed electronic transition analysis was performed using
time-dependent density functional theory (TD-DFT) calcu-
lations (vide supra). Raman spectroscopy was used to further
probe the electronic structure and the strength of the metal−
metal bond in 1−3. The spectra were obtained with laser
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The electronic structure of 1 shows that the HOMO
(−3.929 eV) is metal-based (66%) with δ character. Contrary
to typical paddlewheel complexes, the σ orbital is found to be
less stable than the π orbitals, at −5.932 eV (HOMO−8). The
HOMO−9 and HOMO−10 (at −5.997 eV and −6.156 eV,
respectively) are the π orbitals showing the strong ligand
interaction between the guanidinate N atoms and the metal
center. The LUMO of 1 is the metal-based δ* found at
−0.790 eV. Compound 2 has an onset LUMO energy of
−0.505 eV, which is 0.285 eV higher in energy than that for 1.
The Mo2 unit makes the largest contribution to the HOMO
with 65% at −3.520 eV. Similar to 1, the σ orbital (HOMO−
4) is less stable than the π orbitals at −5.515 eV. Finally,
HOMO−9 and HOMO−11 are found at −5.921 and −6.027
eV, respectively. On the other hand, 3 has a HOMO energy of
−3.198 eV. Interestingly, the metal character of the
compound decreases again with respect to that of 1 and 2
to 64%. Once again, the σ orbital is found before the π
orbitals at −5.290 eV (HOMO−4), while HOMO−5 and
HOMO−8 are found at −5.516 and −5.655 eV, respectively.
The reduction potentials for this series are linearly correlated
to the number of guanidinates in the complexes and the onset
energy of the HOMOs with R2 values of 0.9228 (Figure 8a)
and 0.9258 (Figure 8b), respectively. A comparison between
the energy levels of the metal-based frontier orbitals for III, 1,
2, 3, and IV shows that the relative energy of such orbitals
increases from that of the tetraformamidinate paddlewheel
compound, III, to that of the tetraguanidinate one, IV. The
large energy range of the HOMO (∼1.5 eV) shows that
Figure 5. Plot of the dependence of the Mo−Mo bond distance on
the number of guanidinates in III, 1, 2, and IV. The squares are the
measured values, and the solid line is the least-squares fit of the data.
Figure 6 depicts the HOMO and LUMO orbitals for
compounds 1−3. The highest occupied molecular orbital
−
−
substitution of DAniF for hpp results in a large change in
the electronic structure of the paddlewheel compounds. This
trend correlates with the ligand donor ability that increases
gradually as formamidinates get replaced by the guanidinate
hpp. Furthermore, the systematic addition of electron-
donating hpp ligands results in a smaller HOMO−LUMO
energy gap, thus decreasing the metal character of the δ
orbitals (HOMO) due to a greater mixing of metal- and
ligand-based orbitals.55 The magnitude of the half-wave
potential for the synthesized compounds follows the order 1
< 2 < 3 based on their HOMO energies as the number of
guanidinates are introduced into the system. This observation
is in agreement with the obtained electrochemical data, where
4+
the destabilization of the Mo2 core is reflected in the
negative shift of the redox potentials that favors the formation
5+
6+
of the M2 and M2 species.
Figure 6. Illustration of the HOMO and LUMO for compounds 1−3
Density functional theory was used to help assign the
ν(Mo−Mo) vibrational modes. Table 4 presents the
experimental and calculated Raman shifts of ν(Mo−Mo).
We can observe that the frequencies for the dimolybdenum
stretch in compounds 1−3 are between 487 and 505 cm−1.
Compared to the vibration of the tetraacetate moiety, the
higher frequency of the vibration for 1−3 can be attributed to
a smaller Mo−Mo bond and the increase in the metal−metal
bond strength.60 The calculated Raman shifts obtained by
DFT show an increase in the frequency for the dimolybdenum
stretch as a function of the number of guanidinates being
substituted with a correlation coefficient (R2) of 0.9676
(Figure 8c), consistent with the reduction of the HOMO−
LUMO gap. Nonetheless, the difference in the calculated
frequencies between compounds 1 and 3 is 21 cm−1 (0.06
kcal/mol).
with 0.04 contour surfaces calculated with DFT.
(HOMO) for all of the compounds was established as a
metal-based δ orbital with some degree of ligand mixing. The
composition of the HOMO attributes the lower redox
potential wave in the cyclic voltammograms for 1−3 to the
5+/4+
Mo2
process. On the other hand, the lowest unoccupied
molecular orbital (LUMO) is a metal-based δ* orbital, except
in Mo2(hpp)4 where the LUMO is a metal-based σ* while
LUMO + 1 is the δ*.59 A molecular orbital diagram for the
frontier molecular orbitals of the Mo2(DAniF)4−n(hpp)n (n =
0, 1, 2, 3, 4) molecules is given in Figure 7, which allows a
comparison of relative energies for III, 1, 2, 3, and IV. The
highest HOMO energy observed was at −2.813 eV for
Mo2(hpp)4 (IV), with 64% metal character, while the lowest
and most stable orbital is that for Mo2(DAniF)4 (III) with an
onset HOMO δ energy of −4.32 eV and 75% metal character.
Time-dependent density functional theory calculations were
performed on geometry-optimized compounds 1−3 to help
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Figure 7. Diagram showing the respective relative energies of the molecular orbitals of III, 1, 2, 3, and IV. The metal-based character of the δ
HOMO orbital is displayed. The blue lines indicate the filled orbitals for all complexes, while the red lines represent their empty orbitals.
Figure 8. Plot of the dependence of (a) half-wave potentials (E1/2) on the number of guanidinates around the Mo2 core, with zero being the
tetraformamidinate (III); (b) potential separation (E1/2) on the calculated energy of the highest occupied molecular orbitals for III, 1, 2, 3, and
IV; (c) the Raman shifts for the ν(Mo−Mo) stretch on the guanidinate ligands for 1, 2, and 3; and (d) the lowest-energy absorption band
associated with the δ → δ* electronic transition on the calculated HOMO−LUMO energy gap for 1, 2, 3, and IV. The squares are the measured
values, and the solid lines are the least-square fits of the data.
assign the lowest-energy transition observed in the UV−vis
spectra for each compound. Absorption spectra of 1 shows a
band at 412 nm arising from the δ → δ* transition. Similarly,
2 presented a band at 442 nm while the same signal appeared
at 450 nm for 3. These bands correspond to the HOMO →
LUMO transition calculated to be 520, 542, and 572 nm for
1, 2, and 3, respectively. A bathochromic shift is present in the
electronic absorption spectra for these molecules as
guanidinates are substituted in the Mo−Mo center, from the
tetraformamidinate (III) (430 nm) to the tetraguanidinate
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Table 4. Experimental and Calculated Raman Shifts for
ν(Mo−Mo)
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03394.
experimental (cm−1
)
calculated (cm−1
)
I
III
1
2
3
404
404
487
494
505
407
489
492
510
501
Selected bond lengths and angles for X-ray-character-
ized compounds 1 and 2; H NMR for 1−3; calculated
1
and experimentally obtained mass spectra of 1−3;
synthesis, X-ray crystallographic data, and XYZ coor-
dinates of [Mo2(DAniF)2(hpp)2](PF6); cyclic voltam-
mograms of I, II, and III; Raman spectra of I and III;
infrared spectra of 2; and Cartesian coordinates of the
DFT-optimized geometries in the gas phase (PDF)
IV
complex (IV) (460 nm) as depicted in Figure 8d with a
correlation coefficient (R2) of 0.8993. This feature is
attributed to the delocalization of the uncoordinated nitrogen
lone pair into the ligand π system, which narrows the
HOMO−LUMO energy gap as observed in the calculated
relative energies in Figure 7.61 Meanwhile, the higher-energy
transitions observed at 290, 295, and 330 nm for 1, 2, and 3,
respectively, were calculated at 375, 362, and 358 nm. These
high-energy transitions correspond to the δ → πM* transition
for 1, πM → δ* in the case of 2, and δ → πL for 3.
Experimental and calculated wavelengths for the δ to δ*
transition of I, III, 1, 2, 3, and IV are displayed in Table 5.
Accession Codes
CCDC 1970116−1970117 and 1979462 contain the supple-
mentary crystallographic 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.
AUTHOR INFORMATION
■
Table 5. Experimentally Obtained and Calculated λmax in
the Electronic Spectra for I, III, 1, 2, 3, and IV
Corresponding Author
́
Dino Villagran − Department of Chemistry and Biochemistry,
experimental (nm)
calculated (nm)
The University of Texas at El Paso, El Paso, Texas 79968,
United States; orcid.org/0000-0002-5798-3584;
Email: dino@utep.edu
I
III
1
30360
43024
412
520
542
572
Authors
2
3
IV
442
450
46061
́
Nancy Rodríguez-Lopez − Department of Chemistry and
Biochemistry, The University of Texas at El Paso, El Paso,
Texas 79968, United States
Nathalie Metta − Department of Chemistry and Biochemistry,
The University of Texas at El Paso, El Paso, Texas 79968,
United States
Alejandro J. Metta-Magana − Department of Chemistry and
Biochemistry, The University of Texas at El Paso, El Paso,
Texas 79968, United States
CONCLUSIONS
■
A series of quadruply bonded dimolybdenum compounds
were successfully synthesized and characterized. Compounds
1 and 2 were structurally characterized by single-crystal X-ray
crystallography presenting a Mo−Mo distance of 2.0844(6)
and 2.0784(6) Å, respectively. Data obtained from Raman
spectroscopy shows an increase in the Raman shift with the
addition of guanidinate ligands, 487, 494, and 505 cm−1 for 1,
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03394
Notes
The authors declare no competing financial interest.
2, and 3, respectively. Electrochemical studies showed that the
ACKNOWLEDGMENTS
4+/5+
■
Mo2
redox process can be tuned over a range of nearly
We thank Prof. Luis Echegoyen for access to the Raman
spectrometer. N.R.-L. acknowledges financial support from
CONACYT (Consejo Nacional de Ciencia y Tecnologia)
Mexico. We acknowledge the NSF-MRI program (CHE-
0.4 V due to the increased ligand basicity. In addition, we
have shown the ability of hpp to stabilize higher oxidation
states for the Mo2 unit. The addition of two and three
guanidinate ligands shows the ability of these complexes to
perform multielectron redox chemistry. DFT calculations
depict a strong influence on the guanidinate ligand in the
difference in energy levels between the compounds. These
observations show greater metal−ligand mixing as formami-
dinates were replaced by guanidinates, thus reducing the metal
character of the HOMO. Moreover, the decrease in energy for
the δ → δ* transition (as guanidinates are introduced into the
Mo2 core) is in agreement with an increase in the d orbital
overlap and bond strength, hence shortening the Mo−Mo
bond distances.
́
́
1827875) for the funding to purchase an X-ray diffractometer.
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