Lewis acid enhanced axial ligation of [Mo2]4+ complexes

Article abstract of DOI:10.1021/ic400275x

We report here the syntheses, X-ray crystal structures, electrochemistry, and density functional theory (DFT) single-point calculations of three new complexes: tetrakis(monothiosuccinimidato)dimolybdenum(II) [Mo ₂(SNO5)₄, 1a], tetrakis(6-thioxo-2-piperidinonato) dimolybdenum(II) [Mo₂(SNO6)₄, 1b], and chlorotetrakis(monothiosuccinimidato)pyridinelithiumdimolybdenum(II) [pyLiMo₂(SNO5)₄Cl, 2-py]. X-ray crystallography shows unusually short axial Mo₂-Cl bond lengths in 2-py, 2.6533(6) A, and dimeric 2-dim, 2.644(1) A, which we propose result from an increased Lewis acidity of the Mo₂ unit in the presence of the proximal Li ⁺ ion. When 2-py is dissolved in MeCN, the lithium reversibly dissociates, forming an equilibrium mixture of (MeCNLiMo₂(SNO5) ₄Cl) (2-MeCN) and [Li(MeCN)₄]⁺[Mo ₂(SNO5)₄Cl]^- (3). Cyclic voltammetry was used to determine the equilibrium lithium binding constant (room temperature, K _eq = 95 ± 1). From analysis of the temperature dependence of the equilibrium constant, thermodynamic parameters for the formation of 2-MeCN from 3 (ΔH = -6.96 ± 0.93 kJ mol^-1 and ΔS = 13.9 ± 3.5 J mol^-1 K^-1) were extracted. DFT calculations indicate that Li⁺ affects the Mo-Cl bond length through polarization of metal-metal bonding/antibonding molecular orbitals when lithium and chloride are added to the dimolybdenum core.

Full text of DOI:10.1021/ic400275x

Article
pubs.acs.org/IC
Lewis Acid Enhanced Axial Ligation of [Mo2]4+ Complexes
Brian S. Dolinar and John F. Berry*
Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States
S
* Supporting Information
ABSTRACT: We report here the syntheses, X-ray crystal structures, electro-
chemistry, and density functional theory (DFT) single-point calculations of three
new complexes: tetrakis(monothiosuccinimidato)dimolybdenum(II)
[Mo2(SNO5)4, 1a], tetrakis(6-thioxo-2-piperidinonato)dimolybdenum(II)
[Mo2(SNO6)4, 1b], and chlorotetrakis(monothiosuccinimidato)-
pyridinelithiumdimolybdenum(II) [pyLiMo2(SNO5)4Cl, 2-py]. X-ray crystallog-
raphy shows unusually short axial Mo2−Cl bond lengths in 2-py, 2.6533(6) Å,
and dimeric 2-dim, 2.644(1) Å, which we propose result from an increased
Lewis acidity of the Mo2 unit in the presence of the proximal Li+ ion. When 2-py
is dissolved in MeCN, the lithium reversibly dissociates, forming an equilibrium
mixture of (MeCNLiMo2(SNO5)4Cl) (2-MeCN) and [Li(MeCN)4]+[Mo2(SNO5)4Cl]− (3). Cyclic voltammetry was used to
determine the equilibrium lithium binding constant (room temperature, Keq = 95
1). From analysis of the temperature
dependence of the equilibrium constant, thermodynamic parameters for the formation of 2-MeCN from 3 (ΔH° = −6.96 0.93
kJ mol−1 and ΔS° = 13.9 3.5 J mol−1 K−1) were extracted. DFT calculations indicate that Li+ affects the Mo−Cl bond length
through polarization of metal−metal bonding/antibonding molecular orbitals when lithium and chloride are added to the
dimolybdenum core.
nitriles, and a pyrazole derivative,7 or with anionic ligands,
INTRODUCTION
Metal−metal-bonded compounds having the paddlewheel-type
structure, shown in Chart 1a, have played a major role in the
■
including Cl−, Br−, I−, and [BF4]−.8 Dimolybdenum carbox-
ylates can also associate through intermolecular axial
interactions with O atoms from an adjacent molecule.9 In all
of these cases, however, these axial ligands are bound
exceptionally weakly, with Mo−Lax bond lengths often up to
0.4−0.6 Å longer than their corresponding Mo−Leq bond
lengths; more often, [Mo2]4+ complexes eschew axial ligands
entirely. Thus, new strategies for the axial functionalization of
[Mo2]4+ complexes are of interest.
Chart 1. (a) Paddlewheel-Type Structure Supported by
a
Bridging Anionic Equatorial Ligands [X−Y−Z]− and (b)
Qualitative MO Diagram of Metal−Metal Interactions for
[Mo2]4+
The indifference of the [Mo2]4+ unit toward axial ligands has
its origin in the electronic structure of the Mo2 unit. Chart 1b
depicts a qualitative molecular orbital (MO) diagram of a
[Mo2]4+ species. The principal empty Mo-centered orbital that
would have the correct symmetry to overlap with a σ-type lone-
pair orbital on a Lewis base is the Mo−Mo σ* orbital. This σ*
orbital is elevated in energy by the strong interaction between
2
the dz orbitals of the two Mo atoms, resulting in poor energetic
matching and, hence, poor overlap with ligand-based σ-type
lone-pair orbitals. Thus, axial ligands bind weakly to Mo2
complexes.
Because of our interest in the reactivity of M2 complexes, we
wanted to see if it were possible to increase the Lewis acidity of
Mo2 species and strengthen Mo2−axial ligand bonding. An
attractive approach to this would be to design complexes that
bring a Lewis acid into close proximity with the Mo2 unit, thus
activating it and making it more susceptible to attack by a Lewis
base. A similar strategy was recently explored by the Gabbaı
a
The metal−metal bond order, n, can range from 0 to 4, and Lax are
axial donor ligands.
development of coordination chemistry1 and continue to be of
interest for their catalytic,2 photophysical,3 electronic,4 and
structural properties5 as well as reactivity.6 The reactivity of
ligands in the axial position of these complexes is very
important and plays a key role in applications such as catalysis.
Only a few [Mo2]4+ compounds are known that are axially
ligated with either neutral ligands, such as tetrahydrofuran,
̈
Received: February 1, 2013
Published: March 29, 2013
© 2013 American Chemical Society
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Article
group, in which inert Hg2+ and Au+ ions were activated by
conjoining them to a proximal Lewis acidic Ar4Sb+ ion,
resulting in further ligation of these metals.10 Also, the Thomas
group has recently demonstrated that Lewis acidic Zr4+ ions can
activate Co+, yielding a complex capable of splitting CO
bonds in CO2.11
Scheme 1. (a) Synthesis Pathway for Forming trans-2,2-
Mo2(SNOX)4 Complexes as well as 2-py and (b) Proposed
Equilibrium Involving Li+ Dissociation from Compound 2-
MeCN
With regards to Lewis acid activation of Mo2 systems, the
closest examples were reported by our group on linear
heterotrinuclear Mo Mo···M complexes using either Co2+ or
Fe2+, in which chloride ligands bind the open Mo2 axial
position, resulting in Mo−Cl bond distances significantly
shorter than those of unactivated Mo2 units.8g,h We
4+
hypothesized, therefore, that stronger Lewis acids, such as
alkali metals, should result in more Lewis acidic Mo2 units and
stronger bonds between Mo and axial ligands.
In order to promote the formation of such complexes, a new
type of ligand capable of forming trimetallic complexes was
designed. As opposed to dipyridylamine ligands used in our
earlier Mo Mo···M work, which have three nitrogen bases, we
have designed a ligand containing a combination of hard and
soft bases so that hard−soft acid−base (HSAB) theory could be
used to design heterometallic compounds with element-specific
structures. Alkoxyalkylphosphines,12 phosphinoamides,11,13 and
thiopyridines14 have been used to synthesize heterometallic
complexes using a similar concept. Monothiosuccinimide
(HSNO5) and 6-thioxo-2-piperidinone (HSNO6), shown in
Chart 2, are two ligands that are ideal for this type of chemistry.
sequentially over molecular sieves and CaH2 and distilled under N2
prior to use. Tetrahydrofuran (THF) was dried using a Vacuum
Atmospheres solvent purification system and degassed with N2 prior to
use. Pyridine was dried sequentially over molecular sieves and barium
oxide. It was then distilled under N2 and stored in a glovebox prior to
use. All other commercial reagents were used as received without
further purification. Lawesson’s reagent, glutarimide, succinimide, P2S5,
lithium hexafluorophosphate, tetrabutylammonium hexafluorophos-
phate, trifluoroacetic acid, acetic acid, and molybdenum carbonyl were
purchased from Sigma Aldrich. Molybdenum acetate [Mo2(OAc)4]
was synthesized from molybdenum carbonyl and acetic acid.15
Molybdenum trifluoroacetate [Mo2(TFA)4] was synthesized from
Mo2(OAc)4 and trifluoroacetic acid.16 Monothiosuccinimide
(HSNO5) was prepared from succinimide and P2S5.17 6-Thioxo-2-
piperidinone (HSNO6) was prepared from glutarimide and Law-
esson’s reagent.18 Elemental analysis was carried out by Midwest
Microlabs in Indianapolis, IN. Mass spectrometry data were recorded
at the Mass Spectrometry Facility of the Chemical Instrument Center
of the University of WisconsinMadison. Matrix-assisted laser
desorption/ionization mass spectrometry (MALDI-MS) spectra were
obtained using a Bruker REFLEX II spectrometer equipped with a 337
nm laser, a reflectron, delayed extraction, and a time-of-flight analyzer.
The acceleration voltage was 25 kV. IR spectra were taken on a Bruker
Chart 2. Ligands Monothiosuccinimide and 6-Thioxo-2-
piperidinone
These two ligands differ only in their ring size, and they
incorporate hard and soft bases that would facilitate the
syntheses of a heterometallic complex containing a hard Lewis
acidic fragment and a [Mo2]4+ unit. The sulfur on this ligand is
a soft base, and it is able to effectively bind molybdenum, while
the oxygen is a hard base, making it ideal to bind alkali ions.
Herein we report the syntheses and characterization of two
unactivated dimolybdenum complexes, Mo2(SNO5)4 (1a) and
Mo2(SNO6)4 (1b), and a dimolybdenum complex that is
activated by the presence of a Lewis acidic lithium cation,
pyLiMo2(SNO5)4Cl (2-py). These complexes (Scheme 1a)
have been structurally characterized by X-ray crystallography.
We also provide electrochemical evidence that 2-py reversibly
dissociates Li+ when dissolved in acetonitrile (MeCN) to form
MeCNLiMo2(SNO5)4Cl (2-MeCN) in equilibrium with [Li-
(MeCN)4]+[Mo2(SNO5)4Cl]− (3; Scheme 1b). To gain more
insight into the electronic structure of this complex, density
functional theory (DFT) geometry optimizations and single-
point calculations were performed on these complexes.
1
TENSOR 27 using ATR techniques. H NMR spectra were recorded
7
on a Bruker AC+ 300 spectrometer. 13C and Li NMR spectra were
recorded on a Bruker Avance-500 spectrometer.
trans-2,2-Tetrakis(monothiosuccinimidato)dimolybdenum(II)
(Mo2(SNO5)4, 1a). A flask was charged with 246 mg of HSNO5 (2.14
mmol), 331 mg of Mo2(TFA)4 (0.521 mmol), and 30 mL of pyridine.
The resulting reaction mixture was heated to reflux with stirring for 3
h. The reaction mixture was then allowed to cool to room
temperature, and the solvent was removed under vacuum. The
resulting residue was washed with 3 × 20 mL of Et2O. This gave an
orange-brown powder, which was extracted with 20 mL of hot
CH2Cl2. Layering this extract with hexanes gave 1a as a brown powder.
The solid was filtered in air and washed with 3 × 20 mL of hexanes
and 1 × 20 mL of Et2O. Yield: 199 mg (59.0%). 1H NMR (300 MHz,
CDCl3): δ 3.563 (m, 8H), 2.885 (m, 8H). 13C NMR (500 MHz,
CDCl3): δ 215.85, 215.57, 186.91, 186.58, 39.55, 32.30. (The doubling
of the signals at 215 and 186 ppm is likely due to the presence of
multiple conformers in solution.) MALDI-MS (1a+): m/z 651. IR
(ATR, cm−1): 1755 w, 1725 m, 1438 m, 1415, m, 1398 m, 1248 m,
1191 vs, 990 vw, 963 w, 917 m, 804 m, 733 m. UV−vis (CH2Cl2, λ
EXPERIMENTAL SECTION
General Procedures. All synthetic manipulations were carried out
under an inert N2 atmosphere using standard Schlenk and glovebox
techniques unless otherwise stated. CH2Cl2 and C2H4Cl2 were dried
■
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Table 1. X-ray Crystallographic Solution Details for Compounds 1 and 2-py
1a
1b
2-py
2-dim
empirical formula
C16H16Mo2N4O4S4
C23H30N4O4S4Cl3Mo2
C26H26LiN6O4S4ClMo2
C34H36Cl6Li2Mo4N8O8S8
fw
648.45
852.98
849.04
1551.53
temperature/K
λ/Å
100(1)
100(1)
100(1)
100(1)
1.54178
0.71073
0.71073
0.71073
cryst syst
space group
a/Å
orthorhombic
monoclinic
orthorhombic
monoclinic
Pccn
P21/n
P212121
P21/n
10.6032(7)
10.6871(3)
8.6895(3)
8.704(3)
b/Å
12.591(1)
19.8411(6)
13.7221(5)
22.530(8)
c/Å
15.842(2)
15.3151(5)
26.1281(9)
13.543(5)
β/deg
volume/Å3
90
106.159(1)
90
108.07(2)
2115.0(3)
3119.2(2)
3115.5(2)
2525(2)
Z
4
4
4
2
ρcalc/mg mm−3
2.036
1.816
1.810
2.041
a
final R indexes [I ≥ 2σ(I)]
final R indexes [all data]
R1 = 0.0271, wR2 = 0.0733
R1 = 0.0281, wR2 = 0.0741
R1 = 0.0290, wR2 = 0.0664
R1 = 0.0389, wR2 = 0.0709
R1 = 0.0186, wR2 = 0.0381
R1 = 0.0204, wR2 = 0.0387
−0.03(2)
R1 = 0.0309, wR2 = 0.0621
R1 = 0.0504, wR2 = 0.0683
Flack Parameter
a
R1 = 3||Fo| − |Fc||/[3|Fo|]. wR2 = {3[w(Fo − Fc2)2]/[3[w(Fo )2]]}1/2, w = 1/σ2(Fo ) + (aP)2 + bP, where P = [max(0 or Fo ) + 2(Fc2)]/3.
2
2
2
2
(nm) [ε (M−1 cm−1)]): 414 [5400], 495 [830]. Elem anal. Calcd for
C16H16S4N4O4Mo2 (1a): C, 29.63; H, 2.49; N, 8.64. Found: C, 29.64;
H, 2.57; N, 7.96.
dimensions 0.370 × 0.190 × 0.090 mm3 was selected. For 2-py, a red
plate-shaped crystal with dimensions 0.210 × 0.184 × 0.029 mm3 was
selected. For 2-dim, an orange rod-shaped crystal with dimensions
0.100 × 0.050 × 0.030 mm3 was selected. Crystals were attached to the
tip of a MiTeGen MicroMount, mounted in a stream of cold nitrogen
at 100(1) K, and centered in the X-ray beam using a video monitoring
system. Crystal evaluation and data collection were performed on a
Bruker Quazar SMART APEX-II diffractometer with Cu Kα (λ =
1.54178 Å; 1a) or Mo Kα (λ = 0.71073 Å; 1b, 2-py, and 2-dim)
radiation. The data were collected using a routine to survey an entire
sphere of reciprocal space and indexed by the SMART program, and
the structures were solved via direct methods and refined by iterative
cycles of least-squares refinement on F2 followed by difference Fourier
synthesis.19 All H atoms were included in the final structure factor
calculation at idealized positions and allowed to ride on the
neighboring atoms with relative isotropic displacement coefficients.
The details concerning X-ray crystallographic solutions and
refinement for compounds 1a, 1b, and 2-py are tabulated in Table
1. For each structure, the model was refined to a low wR2 value (<0.10
for each case). After refinement of the model, compound 2-py had a
featureless final Fourier map. For compounds 1a and 1b, the final
Fourier maps had peaks of 1.34 and 1.36 e Å−3, respectively. These
highest peaks were located closest to the heaviest atoms in the models
(Mo) and were interpreted as noise. For compound 2-py, the absolute
structure was established by anomalous dispersion using the method of
Flack.20
Electrochemistry. Compounds 2-MeCN and 3 were prepared in
situ by dissolving 2-py in a 100 mM solution of NBu4PF6 in MeCN at
room temperature under ambient conditions and subsequently
titrating the solution with either LiPF6 or 12-crown-4 ether,
respectively. Cyclic voltammetry for compounds 1a, 1b, 2-MeCN,
and 3 was performed on solutions of 1 mM analyte and 100 mM
electrolyte (NBu4PF6) at temperatures ranging from −29 to +20 °C
using a standard glassy carbon electrode as the working electrode, a
platinum wire as the auxiliary electrode, and Ag/Ag+ as the reference
electrode. All electrochemical potentials were internally referenced to
the ferrocene/ferrocenium (Fc/Fc+) couple. The voltammetry was
performed in the range of +900 to −500 mV for compounds 2-MeCN
and 3 or to −1700 mV for compounds 1a and 1b at a scan rate of 100
mV s−1. The solvents used were MeCN for compounds 1a, 2-MeCN,
and 3 and CH2Cl2 for compound 1b.
trans-2,2-Tetrakis(6-thioxo-2-piperidinonato)dimolybdenum(II)
(Mo2(SNO6)4, 1b). A flask was charged with 320 mg of HSNO6 (2.48
mmol), 264 mg of Mo2(OAc)4 (0.572 mmol), 148 mg of LiCl, and 15
mL of pyridine, and the resulting reaction mixture was heated to reflux
with stirring for 16 h. The solvent was then removed under vacuum,
leaving a red oily residue. The residue was triturated with 2 × 15 mL of
Et2O, yielding a fine red solid. The solid was extracted with 20 mL of
1,2-dichloroethane, and crystals were grown from the resulting
solution by layering with 80 mL of hexanes. Compound 1b was
isolated by filtration as a red microcrystalline solid. Yield: 201 mg
1
(46.2%). H NMR (300 MHz, CDCl3): δ 3.325 (t, J = 6 Hz, 8H),
2.521 (t, J = 6.6 Hz, 8H), 2.156 (p, J = 6.3 Hz, 8H). 13C (500 MHz,
CDCl3): δ 207.96, 177.53, 39.13, 31.69, 21.86. MALDI-MS (1b+): m/
z 705. IR (ATR, cm−1) 2959 vw, 1675 m, 1604 w, 1521 w, 1485 w,
1441 s, 1407 s, 1333 m, 1323 m, 1261 vs, 1245 vs, 1181 vs, 1115 vs,
1070 w, 1042 w, 1012 w, 975 m, 962 m, 912 m, 846 w, 758 m, 677 s,
651 s. UV−vis (THF, λ (nm) [ε (M−1 cm−1)]): 460 [12000]. Elem
anal. Calcd for C20H24S4N4O4Mo2 (1b): C, 34.09; H, 3.43; N, 7.95.
Found: C, 34.03; H, 3.41; N, 7.84.
4 , 0 - C h l o r o t e t r a k i s ( m o n o t h i o s u c c i n i m i d a t o ) -
pyridinelithiumdimolybdenum(II) (pyLiMo2(SNO5)4Cl, 2-py). A flask
was charged with 326 mg of HSNO5 (2.83 mmol), 303 mg of
Mo2(OAc)4 (0.706 mmol), and 320 mg of LiCl (7.6 mmol) dissolved
in 20 mL of pyridine. The resulting solution was heated to 100 °C
without stirring. Within a few hours, red-orange crystalline 2-py began
to precipitate from the solution. After 16 h, the reaction was cooled to
room temperature. Compound 2-py was collected by filtration, washed
with 3 × 20 mL of THF and 2 × 20 mL of hexanes, and dried under
1
vacuum. Yield: 369 mg (61.6%). H NMR (300 MHz, MeCN-d3): δ
8.59 (m, 2H), 7.745 (tt, J = 7.8 and 1.8 Hz, 1H), 7.34 (m, 2H), 3.58
(m, 8H), 2.78 (m, 8H). 13C NMR (500 MHz, MeCN-d3): δ 192.73,
150.76, 137.04, 124.83, 95.74, 41.29, 33.39. 7Li NMR (500 MHz,
DMF): δ 0.547. IR (ATR, cm−1): 2963 vw, 1724 m, 1569 vw, 1441 w
1429 w, 1390 w, 1259 s, 1236 s, 1217 s, 1086 m, 1034 s, 1018 s, 962 w,
937 w, 863 w, 797 vs, 753 w, 703 m, 684 m, 665 m, 629 w. UV−vis
(MeCN, λ (nm) [ε (M−1 cm−1)]): 433 [5300], 521 [530]. Elem anal.
Calcd for C26H26N6S4O4Mo2Li (2-py·py): C, 36.78; H, 3.09; N, 9.90.
Found: C, 36.64; H, 3.09; N, 9.80.
X-ray-quality crystals of dimeric [LiMo2(SNO5)4Cl]2 (2-dim) were
also obtained by crystallizing 2-py from CH2Cl2/py/hexanes.
X-ray Crystallography. Suitable crystals of 1a, 1b, 2-py, and 2-
dim were selected under oil and ambient conditions. For 1a, an orange
plate-shaped single crystal with dimensions 0.118 × 0.087 × 0.075
mm3 was selected. For 1b, an orange-yellow block-shaped crystal with
DFT Calculations. Restricted Kohn−Sham geometry optimization
and single-point calculations were carried out on 1a and 2-py through
the ORCA calculation package using the BP86 functional.21 The def2-
TZVP (Mo and Cl atoms) and def2-SVP (all other atoms) basis sets
from the Karlsruhe group,22 which are automatically recontracted in
ORCA for use with the scalar relativistic zeroth-order regular
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Table 2. Important Bond Distances and Bond Angles for the X-ray Crystal Structures of Compounds 1a, 1b, 2-py, and 2-dim
Mo2(SNO5)4 (1a)
Mo2(SNO6)4 (1b)
pyLiMo2(SNO5)4Cl (2-py)
[LiMo2(SNO5)4Cl]2 (2-dim)
D(Mo−Mo) (Å)
D(Mo−N) (Å)
D(Mo−S) (Å)
D(Mo−Cl) (Å)
D(Li−O)eq (Å)
D(Li···Mo) (Å)
2.1112(4)
2.145[2]
2.4753[8]
2.1150(2)
2.153[3]
2.481[8]
2.1357(3)
2.119[2]
2.5172[6]
2.6533(6)
2.182[6]
3.075(5)
176.24(2)
178.7(1)
2.1354(8)
2.123[3]
2.514[1]
2.644(1)
2.185[7]
3.049(6)
174.95(2)
177.3(1)
A(Mo−Mo−Cl) (deg)
A(Li−Mo−Mo) (deg)
Mo2(SNO6)4Cl was obtained but only in very poor yield.23
Formation of this compound is likely disfavored because of the
smaller binding pocket available for Li+ in complexes of the
SNO6 ligand compared with complexes of the SNO5 ligand.
X-ray Crystallography. A list of relevant bond lengths and
bond angles for compounds 1a, 1b, 2-py, and 2-dim described
here is included in Table 2.
Compound 1a crystallized in the orthorhombic space group
Pccn from the slow diffusion of hexane into a solution of the
compound in dichloromethane (Figure 1). The complex has
approximation (ZORA), were used. Structures for compounds 1a and
2-py were calculated using initial atomic coordinates taken from the
crystal structures and then optimized until the energy change between
steps was less than 10−6 hartree. All calculations were optimized with a
Grid4 optimization grid and tight self-consistent-field convergence
criteria.
RESULTS AND DISCUSSION
■
Synthesis. Compounds 1a and 1b were synthesized in
useful yields by reacting free HSNO5 and HSNO6 ligands with
Mo2(TFA)4 and Mo2(OAc)4, respectively, under N2 in
refluxing pyridine. For these reactions, pyridine serves two
purposes. First, it is a solvent with a high boiling point (∼130
°C at 1.1 bar), so the reactions can be heated to a high enough
temperature to proceed to completion. Second, the basicity of
pyridine drives this reaction by removing the acetic or
trifluoroacetic acid formed via ligand substitution.
Lithium chloride is an important additive that aids in the
purification of 1b. The pyridinium acetate byproduct formed in
the reaction is not soluble in Et2O or hexanes, but it is soluble
in 1,2-dichloroethane and CH2Cl2, as is 1b. When LiCl is added
to the reaction, lithium acetate and pyridinium chloride are
formed instead of pyridinium acetate, and so the purification of
1b can be accomplished by washing the mixture with Et2O and
subsequent extraction with 1,2-dichloroethane because now
only 1b is soluble in this solvent. When LiCl is not added to the
reaction mixture, only an impure, noncrystalline product can be
obtained from the reaction. Both compounds 1a and 1b are
somewhat air-stable under ambient conditions, but they
degrade slowly after exposure to air and moisture over the
course of several months.
Figure 1. X-ray crystal structure of compound 1a. All atoms are drawn
as 50% thermal probability ellipsoids, and all H atoms are omitted for
clarity. The structure of the compound is the trans-2,2 isomer.
Compound 2-py is the product of the reaction of
Mo2(OAc)4 with HSNO5 and excess LiCl under N2 in hot
pyridine (100 °C). The product precipitates from pyridine,
greatly simplifying workup and leading to good yields (61.6%).
Like 1a and 1b, 2-py is similarly air- and moisture-stable. 2-py
is sparingly soluble in most noncoordinating solvents such as
dichloromethane and toluene, but it is more soluble in strongly
coordinating solvents such as dimethyl sulfoxide, MeCN,
pyridine, and N,N-dimethylformamide (DMF). The lithium-
bound pyridine ligand in 2-py is labile. Simple recrystallization
of 2-py from a mixture of CH2Cl2 and pyridine affords 2-dim,
in which the py ligand is lost and replaced by an intermolecular
Li−O interaction with an O atom from an adjoining
LiMo2(SNO5)4Cl species. When 2-py is dissolved in
coordinating solvents, it is assumed that the donor solvent
replaces the py ligand. 2-py can also be delithiated, as evidenced
by the electrochemistry of 2-py in MeCN (vide infra). In
solution, the lithiated and nonlithiated species are in
equilibrium, as illustrated in Scheme 1b.
idealized D2d symmetry, and a crystallographic 2-fold axis
bisects the Mo−Mo vector. Following the conventional
nomenclature for isomeric paddlewheel-type compounds
bridged by ligands bearing two different donor atoms,1 the
trans-2,2 isomer of 1a is observed here. This means that two of
the SNO5 ligands are aligned one way along the Mo−Mo bond
axis, while the other two ligands are aligned in the opposite
direction. Each Mo atom in complex 1a is, therefore, bound to
two S atoms trans to each other and two N atoms trans to each
other. The Mo−Mo distance of 2.1112(4) Å is typical of a Mo2
quadruple bond.15,16 The Mo−N bond lengths are within the
range of reported values for similar dimolybdenum complexes
containing four equatorial N−S ligands (2.130−2.199 Å), while
the Mo−S bond lengths are slightly longer than the range
reported in the literature (2.444−2.468 Å).24 The sp2
hybridization of the carbonyl and thioxo C atoms of each
ligand in compound 1a results in the entire SNO5 ligand being
planar.
Synthesis of the HSNO6 complex analogous to 2-py
[pyLiMo2(SNO6)4Cl] was attempted as well, and Li-
The X-ray crystal structure for 1b is shown in Figure 2.
Compound 1b crystallizes in the monoclinic space group P21/n
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Figure 3. X-ray crystal structure of monomeric 2-py All atoms are
drawn as 50% thermal probability ellipsoids, and all H atoms are
omitted for clarity. 2-py crystallizes with an additional molecule of
pyridine in the asymmetric unit (not shown).
Figure 2. X-ray crystal structure of 1b. All atoms are drawn as 50%
thermal probability ellipsoids, and all H atoms are omitted for clarity.
1b crystallizes with 1.5 molecules of 1,2-dichloroethane in the
asymmetric unit (not shown).
with 1.5 1,2-dichloroethane solvent molecules in the asym-
metric unit. Like 1a, 1b adopts the trans-2,2 configuration. The
average Mo−N, Mo−S, and Mo−Mo bond distances are all
similar to the corresponding bond distances for 1a. Unlike the
structure of compound 1a, in 1b, the six-membered ring of the
SNO6 ligand is puckered with the C atom in the 4 position out
of the plane of the ligand.
In both compounds 1a and 1b, the Mo−N bond distance is
significantly shorter than the Mo−S bond distance primarily
because of the difference between the atomic radii of the N and
S atoms. The difference in these bond lengths, >0.3 Å, results in
the ligand being canted, pushing the O atoms of opposite
ligands closer together. In 1b, the nonbonded distance between
the O atoms (3.28 Å) is significantly shorter than the distance
between the O atoms in 1a (4.60 Å). In the SNO6 case, steric
crowding by the O atoms limits the available space for Li+ to
bind, whereas in the SNO5 case, the O atoms are not nearly as
close together, providing a much larger binding pocket to
accommodate a Li+ cation. It is likely that the preference for the
trans-2,2 configuration also stems from the fact that the O
atoms could be too close together in the other possible isomers.
Compound 2-py crystallizes from the pyridine reaction
mixture in the noncentrosymmetric space group P212121 as a
monomer, as shown in Figure 3. The compound also can
crystallize from a noncoordinating solvent such as CH2Cl2 as a
pyridine-free dimer (2-dim; Figure 4), which has bond
distances and bond angles very similar to those of 2-py. The
crystal structure shows that 2-py adopts the 4,0 isomer as
opposed to the 2,2 isomer of compounds 1a and 1b, as well as
all previously synthesized [Mo2]4+ complexes with N,S
equatorial ligands.24 Also, unlike 1a and 1b, 2-py contains a
coordinated Li+ ion along the Mo Mo axis at a distance of 3.07
Å from the Mo2 unit. It therefore appears that Li+ acts as a
template for the SNO5 ligands, allowing formation of the 4,0
isomer.
Figure 4. X-ray crystal structure of dimeric 2-dim. All atoms are drawn
as 50% thermal probability ellipsoids, and all H atoms are omitted for
clarity. 2-dim crystallizes with two additional molecules of CH2Cl2 per
dimer (not shown).
in compounds 1a and 1b by 0.025 Å. The differences in the
Mo−Mo bond distance are statistically significant but may not
carry much chemical significance.
A comparison of the axial Mo−Cl bond distances from 2-py
and 2-dim and those of other known Mo2−Cl(ax) compounds
reported in the CSD is listed in Table 3. The previously
synthesized Mo2 complexes can be divided into two categories:
Mo2 complexes with no additional Lewis acid present and Mo2
complexes with a late transition metal acting to increase the
Mo2 Lewis acidity. The Mo−Cl bond distances in unactivated
[Mo2]4+−Cl complexes range from 2.714(1) to 2.864[2] Å
with an average of 2.8242[5] Å. The [Mo2]4+ complexes
activated by a late-transition-metal Lewis acid, in general, have
shorter Mo−Cl bond lengths [2.720(1) and 2.707(1) Å]. The
axial Mo−Cl bond distances in 2-py and 2-dim are much
shorter than those in either of these classes of compounds.
Thus, the increased Lewis acidity of Li+ activates the [Mo2]4+
core far better than even late transition metals.
The Mo−Cl distances in 2-py and 2-dim are 2.6533(6) and
2.644(1) Å, respectively, which are the shortest distances yet
reported for an axial Cl− ion trans to a Mo Mo bond. The
Mo−Mo bond distances are 2.1356(3) and 2.1354(8) Å,
respectively, which are longer than the Mo−Mo bond distances
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Table 3. Mo−Mo and Mo−Cl Distances of Unactivated,
Late-Transition-Metal-Activated, and Alkali-Metal-Activated
a
[Mo2]4+-Cl Complexes
Mo−Mo
Mo−Cl
compound
(Å)
(Å)
Unactivated
Mo2(O2CH)4KCl8a
Mo2Cl2(OAc)2(μ-dppa)2
2.106[2]
2.152(2)
2.154[1]
2.864[2]
2.862(3)
2.877[3]
8b
{[trans-Mo2(O2CCF3)2(μ-dppa)]3(μ6-CO3)(μ-
Cl)3}F8c
8d
Mo2(O2CC6H3(NH3)2)4Cl8
2.107(1)
2.1312(3)
2.1719(8)
2.854(2)
2.7747(5)
2.714(1)
[Pd2Cl2(CNC6H3Me2-2,6)4][Mo2(O2CCF3)4]8e
8f
Mo2Cl2(OAc)2(μ-dppma)2
Late-Transition-Metal Activated
8g
Mo2Fe(dpa)4Cl2
2.168(3)
2.707(1)
2.720(1)
8h
Mo2Co(dpa)4Cl2(CH2Cl2)2
2.1027(5)
Alkali-Metal Activated
pyLiMo2(SNO5)4Cl (2-py)
2.1356(3)
2.1354(8)
2.6533(6)
2.644(1)
[LiMo2(SNO5)4Cl]2 (2-dim)
a
dppa = N,N-bis(diphenylphosphino)amine, dppma = N,N-bis-
Figure 6. Potential of the MeCN solution of 2-py as a function of (a)
the equivalents of Li+ added and (b) the equivalents of 12-crown-4
added. All potentials are referenced to the Fc/Fc+ redox couple.
(diphenylphosphino)methylamine, and dpa = dipyridylamine.
4+
Electrochemistry. Quadruply bonded Mo2 compounds
5+
can often be oxidized to the corresponding Mo2 level in
which the Mo−Mo bond order is 3.5.25,6b The new compounds
reported here all show one quasi-reversible wave in their
respective cyclic voltammograms (Figure 5), consistent with
Scheme 2. Square Scheme for the Electrochemistry of
Complexes 2-MeCN and 3
Figure 5. Cyclic voltammograms of compounds 1a (pink), 1b
(orange), 2-MeCN (navy), and 3 (blue).
4+/5+
On the basis of the experiments described above, the Mo2
potentials of 2-MeCN and 3 are assigned as shown in Table 4.
4+/5+
the Mo2
redox couple. While 1a and 1b show reversible
waves at 388 and 351 mV vs Fc/Fc+, respectively, the redox
potential of 2-py depends strongly on the conditions of the
electrochemistry and, most importantly, on the [Li+]
concentration supplied by the supporting electrolyte (Figure
6). For example, a solution of 2-py in 0.1 M NBu4PF6 in
MeCN shows a reversible wave at 204 mV, which becomes less
accessible as LiPF6 is added to the supporting electrolyte,
ultimately reaching a value of 319 mV. If, instead, the strong Li+
Table 4. Electrochemical Potentials for Oxidation of
Complexes 1−3 as Well as Other [Mo2]4+ Complexes with
a
N,S Equatorial Ligands
potential vs Fc/Fc+
compound
(mV)
solvent
2,2-Mo2(SNO5)4 (1a)
2,2-Mo2(SNO6)4 (1b)
388
351
319
MeCN
CH2Cl2
MeCN
4,0-MeCNLiClMo2(SNO5)4
chelating agent 12-crown-4 ether is added to a solution of 2-py,
(2-MeCN)
4+/5+
4,0-[Mo2(SNO5)4Cl]− (3)
89
180
240
210
92
MeCN
CH2Cl2
CH2Cl2
CH2Cl2
DMF
the Mo2
wave becomes drastically more accessible,
24b
reaching a plateau at 89 mV. These data are consistent with a
Li+ complexation/decomplexation equilibrium in which the Li+
ion of 2-py may be reversibly removed. For the species involved
in this equilibrium, we propose the structures 2-MeCN and 3
(Scheme 2); for the former compound, we make the reasonable
assumption that, upon being dissolved in MeCN, the pyridine
ligand in 2-py is quickly replaced with MeCN. For 3, we
hypothesize that the axial Cl− does not dissociate, which is
consistent with the low oxidation potential of the compound.
Mo2(Ph2PC(S)NMe)4
24b
Mo2(Ph2PC(S)NPh)4
24b
Mo2(Me2NC(S)NMe)4
24e
Mo2({NPh}C(S)CCPh)4
297
520
1320
THF
24c
Mo2(2-mercaptoquinoline)4
CH2Cl2
CH2Cl2
a
All potentials are given referenced to the Fc/Fc+ couple.
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Because the metalated and demetalated complexes have
different oxidation potentials, this system is an example of
metal-coupled electron transfer (MCET).26 In MCET, the
redox potential of a compound behaves according to eqs 1 and
2, which are derived from the Nernst equation using Hess’s law.
complexation with Li+. The ligands of 3 are already rigidly held
by the [Mo2]4+ unit, eliminating this entropy sink.
Interestingly, the value of ΔH is negative for 3 but positive
for 3+. In binding Li+ to 3, the compound is stabilized by the
hard acid−hard base interaction of the four O atoms and Li+.
When Li+ binds to 3+, it is also stabilized by these same
interactions, but it is destabilized to a greater degree by
Coulombic repulsion between the increased charge of the Mo2
core and Li+. This Coulombic repulsion proves to contribute
more to the overall enthalpy of the reaction than stabilization
by the Lewis acid−Lewis base interactions.
RT
E = E2 −
E = E3 −
[ln(Kred)]
[ln(Kox)]
(1)
F
RT
F
(2)
Thus, for the 2-MeCN/3 equilibrium, the thermodynamics
of lithium binding to both 3 and 3+ can be calculated using the
oxidation potentials of pure 2-MeCN and 3 as well as
determining the oxidation potential of an equilibrium mixture
of the two and by using the square scheme shown in Scheme 2.
At 20 °C, the oxidation potential of a solution of 2-py in MeCN
with no additional Li+ or 12-crown-4 ether added is 204 mV,
which corresponds to an equilibrium constant Kred = 95 1.
The equilibrium constants for lithium binding to the oxidized
complex (3+) and nonoxidized complex (3) were determined at
temperatures ranging from −29 to +20 °C. Van’t Hoff plots of
these equilibrium constants are shown in Figure 7. From these
In comparison, the enthalpy of lithium binding to 12-crown-
4 ether is more exothermic than lithium binding to 3 by about
17 kJ mol−1.28 The difference in enthalpy likely stems from the
eight total Li−O bonds formed in complexation between 12-
crown-4 ether and Li+, as opposed to the four Li−O bonds
formed when Li+ binds to 3.
DFT Calculations. The optimized bond distances and bond
angles are shown in Table 5 and compared to the crystal
Table 5. Experimental and Calculated Bond Distances and
Bond Angles for 1a and 2-py
1a exp
1a calcd
2-py exp
2-py calcd
Mo−Mo (Å)
2.1112(4)
2.145[2]
2.4753[8]
2.1077
2.1325
2.4965
2.1356(3)
2.119[2]
2.5172[6]
3.075(5)
2.6533(6)
169.60[5]
166.7[1]
2.1664
2.1234
2.5192
3.0430
2.5066
172.56
163.92
Mo−N avg (Å)
Mo−S avg (Å)
Mo−Li (Å)
Mo−Cl (Å)
S−Mo−S avg (deg)
N−Mo−N avg (deg)
168.30[2]
168.19[6]
165.16
168.40
structure values. For compound 1a, the calculated structure
accurately reproduces the crystal structure geometry. The
calculated Mo−Mo bond distance of 2.1077 Å only differs by
0.0035 Å from the crystal structure value of 2.1112(4) Å. The
Mo−Mo bond distance calculated here corresponds to a Mayer
bond order of 3.3, slightly less than the idealized value of 4. The
average Mo−N and Mo−S bond distances differ minimally
from the crystal structure values. The calculated N−Mo−N
bond angle is very close to the crystal structure value, but there
is a more significant difference between the calculated S−Mo−
S bond angle and the crystal structure value [Δ(N−Mo−N) =
0.21°; Δ(S−Mo−S) = 3.18°].
For compound 2-py, the calculated model adequately
reproduces how the equatorial ligands bind to the Mo2 core,
as shown by how well the calculated Mo−N and Mo−S bond
distances agree with the crystal structure values, but the axial
Mo−Cl bond distance is significantly underestimated by ∼0.15
Å. However, the potential energy surface (PES) for stretching
of the Mo−Cl bond (SI, Figure S1) indicates that elongating
the Mo−Cl bond to the experimental value only increases the
total energy by ∼5 kJ mol−1, which can be caused by crystal
packing effects. It should be noted, though, that increasing the
Mo−Cl distance to a “normal” value, above 2.8 Å, incurs a
greater energetic penalty (>14 kJ mol−1). Thus, the DFT results
clearly indicate stabilization of an unusually short Mo−Cl bond
in 2-py.
Figure 7. Van’t Hoff plots for the equilibrium between (a) 2-MeCN
and 3 and (b) 2-MeCN+ and 3+. Equilibrium constants were measured
over a range of −29 to +20 °C.
plots, the standard enthalpy and entropy of lithium binding to 3
and 3+ were determined (ΔH° = −6.96 0.93 kJ mol−1 and
ΔS° = 13.9 3.5 J mol−1 K−1 for 3 and ΔH° = 15.2 1.3 kJ
mol−1 and ΔS° = 13.9 4.9 J mol−1 K−1 for 3+). The positive
value for the change in the entropy in both of these cases is
sensible. Free Li+ ions in MeCN solution are usually
coordinated to four MeCN molecules in a tetrahedral fashion.27
Binding lithium to 3 will therefore result in the net gain of two
molecules in the system, increasing the entropy.
It is useful to compare the data obtained here with the
complexation of Li+ with 12-crown-4 ether in MeCN, which
yields a 2:1 ligand/Li+ complex. Formation of this complex is
entropically disfavored by ∼10 J mol−1 K−1.28 We speculate that
the negative entropy of the 12-crown-4 system is largely due to
a loss of conformational flexibility of the 12-crown-4 ether upon
Single-point calculations performed on the optimized
structures of 1a and 2-py give insight into their electronic
structures. MO diagrams based on the DFT results for the
metal−metal bonding orbitals of 1a and 2-py are shown in
Figure 8. The metal−metal bonding orbitals are the most
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Figure 8. MO diagrams of compounds 1a and 2-py. Only the orbitals with significant contributions from the Mo atoms are shown. The percent
contributions of the major components of the σ-type orbitals are indicated next to the diagram of those orbitals.
important orbitals because they govern how these complexes
interact with the axial Li+ ion and Cl− ion. For 1a, the MO
diagram shows a deviation from the predicted qualitative MO
diagram shown in Chart 1 in that the Mo−Mo π-bonding
orbitals are lower in energy than the Mo−Mo σ-bonding
orbital, which is raised in energy because of antibonding
interactions with the SNO5 ligand. As expected, the highest
occupied MO (HOMO) is a Mo−Mo δ-bonding orbital, and
the lowest unoccupied MO (LUMO) is a Mo−Mo δ-
antibonding orbital, as predicted by the qualitative MO
diagram.
For compound 2-py, the HOMO is a σ-type orbital (σ2;
Figure 8) rather than a δ-type orbital, which is a major
deviation from the predicted qualitative MO diagram in Chart
1. Elevation of the σ2 orbital to HOMO over the δ-type orbital
results from interaction of the Mo2 core with the axial Cl−, as
shown in Scheme 3. If we take the Mo−Mo−Cl vector to be
while the σ1 orbital is brought down in energy by the
interaction.
The most striking difference between the MO diagrams of
compounds 1a and 2-py is shown in polarization of the σ-type
orbitals in 2-py compared with 1a. In compound 1a, each σ-
type MO has a 50% contribution from each Mo atom.
However, in 2-py, the orbitals are not evenly distributed. In the
σ1 orbital, the MO is polarized 4.96:1 toward the Mo atom
directly bound to the Cl−, and in the σ2 orbital, the MO is
polarized 3.79:1 toward the Mo atom closest to Li+. This
polarization is due to the interaction of Li+ and Cl− with Mo2
and, to a lesser extent, the 4,0 disposition of the SNO ligands,
but the individual contribution of Li+ cannot be determined by
this MO diagram alone.
Geometry optimization and single-point calculations were
also performed on [pyLiMo2(SNO5)4]+, which is 2-py from
which the Cl− ligand has been removed. The σ* and σb orbitals
are the primary orbitals with which a σ-type orbital on Cl− will
interact when forming an axial Mo−Cl bond. In the σ* orbital
shown in Figure 9, the orbital is polarized toward the open axial
position by significant mixing of the Mo 5pz orbital with the
Scheme 3. Qualitative MO Diagram Illustrating the Origin of
the σ-Type Orbitals in 2-py
the z axis, then the Cl pz lone-pair orbital is energetically well
situated to interact with the filled Mo2 σ orbital, with which it
forms a bonding (σ1) and antibonding (σ2) combination. The
Mo2 σ* orbital also mixes with the Cl pz and Mo2 σ, yielding σ3,
which is antibonding with respect to both the Mo−Mo and
Mo−Cl interactions. Thus, the two Mo atoms and the Cl−
ligand form something akin to a 3-center 4-electron σ bond.
The σ2 orbital is elevated in energy to become the HOMO,
Figure 9. σ* and σb orbitals of [pyLiMo2(SNO5)4]+ showing
polarization caused by the presence of Li+.
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2
Georgiev, V. P.; McGrady, J. E. Chem. Sci. 2012, 3, 1319−1329.
(e) Manni, G. L.; Dzubak, A. L.; Mulla, A.; Brogden, D. W.; Berry, J.
F.; Gagliardi, L. Chem.Eur. J. 2012, 18, 1737−1749.
4dz orbital of the exposed axial Mo atom. This polarization
makes the σ* orbital much more available to interact with a Cl−
σ-type orbital. In the σb orbital, the polarization is less
pronounced because it is too low in energy to mix with the 5pz
orbital. As a result of polarization of these orbitals, the Cl−
ligand forms a much stronger bond with the Mo2 core of 2-py
than in unactivated Mo2Cl complexes.
(5) (a) Chisholm, M. H.; Macintosh, A. M. Chem. Rev. 2005, 105,
2949−2976. (b) Cotton, F. A.; Lin, C.; Murillo, C. A. Acc. Chem. Res.
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CONCLUSIONS
(6) (a) Nippe, M.; Timmer, G. H.; Berry, J. F. Chem. Commun. 2009,
4357−4359. (b) Nippe, M.; Victor, E.; Berry, J. F. Inorg. Chem. 2009,
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2856−2859. (e) Long, A. K. M.; Timmer, G. H.; Pap, J. S.; Snyder, J.
L.; Yu, R. P.; Berry, J. F. J. Am. Chem. Soc. 2011, 133, 13138−13150.
(f) Nippe, M.; Turov, Y.; Berry, J. F. Inorg. Chem. 2011, 50, 10592−
10599. (g) Turov, Y.; Berry, J. F. Dalton Trans. 2012, 41, 8153−8161.
(h) Timmer, G. H.; Berry, J. F. Chem. Sci. 2012, 3, 3038−3052.
(7) (a) Cotton, F. A.; Ilsley, W. H.; Kaim, W. Inorg. Chem. 1980, 19,
3586−3589. (b) Cotton, F. A.; Favello, L. R.; Han, S.; Wang, W. Inorg.
Chem. 1983, 22, 4106−4112. (c) Cayton, R. H.; Chisholm, M. H.;
Huffman, J. C.; Lobkovsky, E. B. J. Am. Chem. Soc. 1991, 113, 8709−
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Polyhedron 1986, 5, 31−33. (f) Cotton, F. A.; Reid, A. H., Jr.;
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■
Cationic lithium has been successfully shown by X-ray
crystallography to activate an axial site of a Mo2 complex.
The activated compound was shown to thermodynamically
favor lithium binding, which is reversible in coordinating
solvents. DFT calculations on these compounds gave geo-
metries in close agreement with experiment, and they provided
an electronic explanation for the increased affinity of Mo2 for
the axial Cl− ligand in a Lewis acid activated complex. The
Lewis acid activation results from polarization of the MOs and
subsequent strengthening of the Mo−Cl bond, as evidenced by
the X-ray crystal data and DFT calculations. On the basis of the
data shown here, it is possible that these compounds could also
find utility as Li+ ion sensors.
ASSOCIATED CONTENT
■
S
* Supporting Information
Kuhn, F. E. Inorg. Chim. Acta 1996, 252, 257−264. (h) Cotton, F. A.;
̈
Crystallographic data in CIF format and computed PES for the
Mo−Cl bond in 2-py. This material is available free of charge
via the Internet at http://pubs.acs.org.
Daniels, L. M.; Murillo, C. A.; Wang, X. Polyhedron 1998, 17, 2781−
2793. (i) Day, E. F.; Huffman, J. C.; Folting, K.; Christou, G. J. Chem.
Soc., Dalton Trans. 1997, 2837−2841. (j) Cotton, F. A.; Wiesinger, K.
J. Inorg. Chem. 1991, 30, 871−873. (k) Xue, W.-M.; Kuhn, F. E.;
̈
Zhang, G.; Herdtweck, E.; Raudaschl-Sieber, G. J. Chem. Soc., Dalton
Trans. 1999, 4103−4110. (l) Kuang, S.-M.; Fanwick, P. E.; Walton, R.
A. Inorg. Chim. Acta 2000, 305, 102−105.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: berry@chem.wisc.edu.
Notes
(8) (a) Robbins, G. A.; Martin, D. S. Inorg. Chem. 1984, 23, 2086−
2093. (b) Wu, Y.-Y.; Chen, J.-D.; Liou, L.-S.; Wang, J.-C. Inorg. Chim.
Acta 1997, 258, 193−199. (c) Suen, M.-W.; Tseng, G.-W.; Chen, J.-D.;
Keng, T.-C.; Wang, J.-C. Chem. Commun. 1999, 1185−1186.
(d) Udovic, B.; Leban, I.; Segedin, P. Croat. Chem. Acta 1999, 72,
477−499. (e) Yi, J.; Miyabayashi, T.; Ohashi, M.; Yamagata, T.;
Mashima, K. Inorg. Chem. 2004, 43, 6596−6599. (f) Arnold, D. I.;
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge financial support from the National
Science Foundation under Grants CHE-0745500, CHE-
9208463 (Bruker AC 360), and CHE-0840494 (computational
facilities) and a generous bequest from Paul J. Bender (Bruker
Avance 500).
Cotton, F. A.; Kuhn, F. E. Inorg. Chem. 1996, 35, 4733−4737.
̈
(g) Nippe, M.; Bill, E.; Berry, J. F. Inorg. Chem. 2011, 50, 7650−7661.
(h) Nippe, M.; Victor, E.; Berry, J. F. Eur. J. Inorg. Chem. 2008, 5569−
5572. (i) Cotton, F. A.; Fanwick, P. E. Inorg. Chem. 1983, 22, 1327−
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