Influence of Lewis acid charge and proximity in MoMo?M linear chain compounds with M = Na+, Ca2+, Sr2+, and Y3+

Article abstract of DOI:10.1016/j.poly.2015.09.028

The syntheses, X-ray structural characterizations, and electrochemical properties of four new paddlewheel [MMo₂(SNO5)₄Cl]^(n-1)+ (Mⁿ⁺ = Na⁺, Ca²⁺, Sr²⁺, Y³⁺; HSNO5 = monothiosuccinimide) compounds are described here. By changing the Mⁿ⁺ cation charge and size, we demonstrate a method for easily tuning the Lewis acidity of the MoMo quadruple bond and the [Mo₂]^4+/5+ redox potential. As the charge of the Mⁿ⁺ cation is increased, the distal Mo atom becomes more Lewis acidic, leading to a shorter Mo₂-Cl bond distance, and the [Mo₂]^4+/5+ redox couple becomes less accessible. As the Mⁿ⁺ Mo₂ distance is increased, the distal Mo atom becomes less Lewis acidic, leading to a longer Mo₂-Cl bond distance.

Full text of DOI:10.1016/j.poly.2015.09.028

Polyhedron 103 (2016) 71–78
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
@
Influence of Lewis acid charge and proximity in Mo MoÁ Á ÁM linear chain
@
+
2+
2+
3+
compounds with M = Na , Ca , Sr , and Y
⇑
Brian S. Dolinar, John F. Berry
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 July 2015
Accepted 12 September 2015
Available online 16 September 2015
The syntheses, X-ray structural characterizations, and electrochemical properties of four new paddle-
(nÀ1)+
n+
+
2+
2+
3+
wheel [MMo
2
(SNO5)
4
Cl]
(M = Na , Ca , Sr , Y ; HSNO5 = monothiosuccinimide) compounds
n+
are described here. By changing the M cation charge and size, we demonstrate a method for easily tun-
@
4+/5+
ing the Lewis acidity of the Mo Mo quadruple bond and the [Mo
2
]
redox potential. As the charge of
@
n+
the M cation is increased, the distal Mo atom becomes more Lewis acidic, leading to a shorter Mo
2
–Cl
Keywords:
4+/5+
n+
bond distance, and the [Mo
increased, the distal Mo atom becomes less Lewis acidic, leading to a longer Mo –Cl bond distance.
2
2
]
redox couple becomes less accessible. As the M Á Á ÁMo
2
distance is
Dimolybdenum
Heterometallic
Alkali Metal
Ó 2015 Elsevier Ltd. All rights reserved.
Alkaline Earth Metal
Quadruple bond
1
. Introduction
A topic of recent interest in coordination chemistry has been the
effect of hard, redox-inactive, Lewis acidic main group metal ions
on the chemical properties of transition metal complexes [10].
We hypothesized that a closely bound redox-inactive metal ion
The chemistry of quadruply bonded [Mo
2
]4+ compounds has
been studied for the past 50 years, and now it is a mature field
1]. Much of the current work in the field has focused on electronic
and photophysical properties [2], reactivity [3], redox chemistry
4], and the use of Mo units as building blocks in the synthesis
[
might increase the Lewis acidity of a quadruply-bonded Mo
With this idea in mind, we designed the heterotrimetallic [Mo
compound pyLiMo (SNO5) Cl (1, SNO5 = monothiosuccinimidate,
2
unit.
4+
2
]
[
2
2
4
+
of novel heterometallic compounds and supramolecular architec-
see Scheme 1), in which Lewis acidic Li is held in the axial position
tures [5]. The stability of the Mo Mo quadruple bond makes these
of a [Mo
2
]4+ unit [5c]. The Li ion serves to polarize the Mo Mo
+
@
@
@
@
compounds particularly robust and versatile.
bond, increasing the Lewis acidity of the distal Mo atom and
Despite a wealth of chemistry based on equatorial ligand substi-
strengthening the Mo
facilitated by the ambidentate nature of the SNO5 ligand. The
hard O donor atom preferentially binds to the hard acid Li and
2
–Cl axial bond. The synthesis of 1 is
À
tutions, Mo
2
complexes are not nearly as keen to bind ligands in
tetracarboxylates bind terminal
+
the axial sites. For example, Mo
2
chloride ions at distances ranging from 2.84 to 2.89 Å [6], signifi-
the soft S donor atom preferentially binds to the soft acid Mo,
cantly longer (by P0.3 Å!) than the equatorial terminal Mo–Cl dis-
2 4
allowing 1 to self-assemble from a mixture of Mo (OAc) ,
2
À
tances in the [Mo
of Mo complexes contrasts meaningfully with the strong Lewis
acidity of the analogous Rh compounds. The unparalleled catalytic
behavior of Rh compounds may be attributed to this Lewis acidity
8], which is instrumental in the stabilization of key intermediates
Cl
2 8
]
ion [7]. This attenuated axial Lewis acidity
HSNO5, and LiCl (Scheme 1) [5c]. The ease with which 1 is
À
2
synthesized suggests that the ambidentate nature of SNO5 can
2
be harnessed to synthesize a variety of MÁ Á ÁMo
2
heterotrimetallic
2
chain compounds.
[
Herein, we describe the syntheses, X-ray structural characteri-
zation, and electrochemistry of four new MÁ Á ÁMo compounds
(M = Na , Ca , Sr , Y ). The syntheses of these new compounds
in catalysis [8c, 9]. A major goal in the chemistry of metal–metal
multiple bonds would be to endow complexes of the cheap,
Earth abundant metal Mo with some of the chemical behavior
more characteristic of Rh.
2
+
2+
2+
3+
greatly expand the scope of MÁ Á ÁMo
2
heterotrimetallic compounds
À
known, and we show that the [Mo
2
4
(SNO5) Cl] system can be gen-
eralized to interact with Lewis acids over a range of sizes and
charges. Through the structural and electrochemical characteriza-
tion of these compounds, the effect that cation charge and cation
size have on the Lewis acidity and electronic properties of the
⇑
Corresponding author.
E-mail address: berry@chem.wisc.edu (J.F. Berry).
4
+
[
Mo
2
]
unit is probed.
http://dx.doi.org/10.1016/j.poly.2015.09.028
277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
0

7
2
B.S. Dolinar, J.F. Berry / Polyhedron 103 (2016) 71–78
Scheme 1. Synthetic routes to compounds 1–5.
2
. Experimental
with 15 mL pyridine and layered with hexanes, yielding small crys-
tals of 2. The crystals were collected, washed with 3 Â 30 mL hex-
2.1. General
anes, and dried under vacuum. Yield: 56 mg (27%). Anal. Calc. for
C
18.5
H
18.5
N
4.5
O
4
S
4
Mo
2
NaCl (2Á0.5 C
5 5
H N): C, 29.77; H, 2.50; N,
1
All synthetic work was carried out under an inert atmosphere
8.44. Found: C, 29.81; H, 2.64; N, 7.81%. H NMR (500 MHz,
using standard Schlenk and glovebox techniques. All solvents were
rigorously dried prior to use. THF, MeCN, hexanes, and Et O were
DMSO-d6) d 3.53 (br, 8H, S@CCH
2
CH
2
C@O), 2.77 (m, 8H, S@CCH
2
-
À1
CH
2
C@O). IR (ATR, cm ) 1728 (s), 1437 (m), 1457 (m), 1396 (s),
2
dried over molecular sieves and subsequently dried using a
Vacuum Atmospheres solvent purification system and degassed
prior to use. Pyridine was dried over molecular sieves, distilled
1240 (w), 1193 (vs), 1115 (w), 1050 (w), 911 (m), 806 (m), 667 (s).
2
.2.2. Tris-methanol-calciumdimolybdenumchloro-tetrakis(monoth-
iosuccinimidato) chloride [(MeOH) CaMo (SNO5) Cl][Cl] (3)
A 50 mL flask was charged with 164 mg 1Ápy (0.193 mmol),
00 mg CaCl (6.31 mmol), and 20 mL MeOH. The reaction mixture
3
2
4
from barium oxide under N
glovebox prior to use. MeOH was dried sequentially over molecular
sieves and Mg/Mg(OMe) . It was then distilled under N immedi-
2
, and stored in an inert atmosphere
7
2
2
2
was heated with stirring at 70 °C for 5 h. The mixture was filtered,
and the filtrate was collected. After standing overnight, crystals of
ately prior to use. Propylene carbonate was purchased in anhy-
drous form from Sigma Aldrich and further dried by refluxing
(3) formed. These crystals were filtered and washed with
over CaH
glovebox prior to use. Monothiosuccinimide (HSNO5) was synthe-
sized from succinimide and P 10 [11]. Molybdenum acetate
Mo (OAc) ) was synthesized from Mo(CO) , acetic acid, and acetic
(OAc) , HSNO5, and
2 2
for 72 h. It was then distilled under N and stored in a
2
 10 mL MeOH. The solid was then dried under vacuum and
collected. Yield: 65 mg (39%). Anal. Calc. for C19
CaCl
2
.12%. H NMR (300 MHz, DMSO-d ) d 3.49 (m, 8H, S@CCH CH
H
28
N S
4 4
O
7
2
Mo -
4
S
2
: C, 26.67; H, 3.30; N, 6.55. Found: C, 27.16; H, 2.60; N,
(
2
4
6
1
6
7
2
-
anhydride [12]. 1Ápy was synthesized from Mo
2
4
C@O), 3.15 (s, CH
3
OH), 2.73 (m, 8H, S@CCH
2
CH
2
C@O) ppm. IR
LiCl [5c]. All other reagents were purchased from Sigma–Aldrich
and used without further purification. Elemental analyses were
carried out by Midwest Microlabs in Indianapolis, IN, USA. NMR
À1
(
ATR, cm ) 1723 (vs) 1654 (vw), 1388 (m) 1270 (vs), 1248 (vs)
1
230 (vs) 1026 (w), 679 (w).
spectroscopy was performed on
spectrometer or Bruker Avance III 500 MHz Spectrometer. FTIR
ATR) data were obtained using a Bruker TENSOR 27 spectrometer.
a
Bruker AC 300 MHz
2
.2.3. Tris-methanol-strontiumdimolybdenumchloro-tetrakis
(
monothiosuccinimidato) chloride [(MeOH)
3
SrMo (SNO5) Cl][Cl] (4)
Analogously to the synthesis of 3, A 100 mL Schlenk flask was
(12 mmol).
2
4
(
charged with 173 mg 1Ápy (0.204 mmol) and 1.9 g SrCl
2
2
2
.2. Syntheses
These were dissolved in MeOH at room temperature and stirred
overnight resulting in the production of an orange precipitate.
The solid was collected by filtration and washed with 2 Â 25 mL
.2.1. Sodiumdimolybdenum-chlorotetrakis(monothiosuccinimidato)
(
2 4
NaMo (SNO5) Cl) (2)
2
Et O. Yield: 46 mg (25%). X-ray quality crystals could be obtained
A 100 mL Schlenk flask was charged with 138 mg HSNO5
1.20 mmol), 440 mg NaCl (7.5 mmol), and 60 mL MeOH. To the
1
from concentration of the red–orange filtrate to 5 mL. H NMR
(
6
(
(
(
(
DMSO-d ) d 3.51 (m, 8H, S@CCH
s, CH OH), 2.74 (m, 8H, S@CCH
s), 1611 (vw), 1429 (w), 1385 (m), 1260 (vs), 1238 (vs), 1222
vs), 1101 (m, br), 804 (w), 681 (m).
2
CH
CH
2
C@O), 3.17 (s, CH
3
OH), 3.15
resulting suspension was added 190
l
L NEt
3
(1.36 mmol), and
À1
3
2
2
C@O), IR(ATR, cm ) 1716
the mixture was stirred at 60 °C until the NaCl had fully dissolved.
This solution was then transferred via cannula to a flask containing
1
2 4
27 mg Mo (OAc) (0.297 mmol). The reaction mixture became an
orange color, and there was a slow precipitation of orange-brown
solid over a period of 5 min. The reaction mixture was heated at
2.2.4. Tetrakis-methanol-yttriumdimolybdenum-chloro-tetrakis-
monothiosuccinimidato bis-triflate [(MeOH) YMo (SNO5) Cl][OTf]
A flask was charged with 1.32 g Y(OTf) (2.46 mmol) and
4
2
4
2
(5)
7
0 °C overnight. Then, the reaction mixture was cooled to room
3
temperature, filtered, and the solid was washed with 2 Â 25 mL
199 mg 1Ápy (0.234 mmol). These were dissolved in 10 mL of
MeOH and 2 Â 25 mL DCM. The remaining solid was extracted
MeOH. The reaction mixture was stirred at room temperature

B.S. Dolinar, J.F. Berry / Polyhedron 103 (2016) 71–78
73
+
overnight. Then the reaction mixture was filtered and layered with
0 mL Et O. Within a week, X-ray quality crystals of (5) grew. The
crystals were collected by filtration, washed with 3 x 50 mL Et O,
and dried under vacuum. Yield: 140 mg (50%). Anal. Calc. for
Cl0.85 (0.85Á5 + 0.15Á[(MeOH) YMo
), C, 21.88; H, 2.65; N, 4.61. Found: C, 21.43; H,
the Na -ligand solution is added to Mo
2
(OAc)
4
and heated to reflux
8
2
to give the desired compound. Without NaCl in the reaction, cis-
2,2-Mo (SNO5) is predominantly produced [2b]. Indeed, this
reaction also generates some cis-2,2-Mo (SNO5) as a byproduct,
but allowing Na and HSNO5 to precomplex before adding the
Mo (OAc) limits the amount of this undesired byproduct that is
2
2
4
2
4
+
C
22.15
H
32
N
4
S
6.15
O
14.45
F
6.45YMo
2
4
2
(
SNO5)
4
][OTf]
3
2
4
1
+
2
8
8
.65; N, 4.45%. H NMR (500 MHz, DMSO-d6) d 3.49 (m,
formed. The presence of Na thus appears to template the
desired 4,0 arrangement of the ligands, similar to the role of Li+
in the synthesis of 1 [5c].
H, S@CCH
H, S@CCH
2
CH
CH
2
C@O), 3.17 (s, CH
3
OH), 3.15 (s, CH
3
OH), 2.74 (m,
À1
2
2
C@O). IR (ATR, cm ) 1696 (m), 1657 (w), 1405
+
(
m), 1394 (m), 1382 (m), 1290 (s), 1258 (vs), 1225 (s), 1166 (w),
In solution, the Li ion of 1 only has a modest thermodynamic
1
066 (s), 1028 (vs), 880 (w), 802 (w), 761 (vw), 698 (m), 637 (s).
preference for binding (Keq = 95 at 298 K) [5c]. Thus, 1 is
potentially a good starting material for the preparation of a
variety of MÁ Á ÁMo
2
compounds by cation exchange. Compounds
2
.3. X-ray crystallography
+
3
–5 were synthesized in this manner, by exchanging Li for
2
+
2+
3+
either Ca , Sr , or Y (Scheme 1). In these reactions, either
CaCl , SrCl , or Y(OTf) is reacted with 1 in MeOH. Compound 1
Single crystals of 2Á4py, 3Á2MeOH, 3-dimÁ5.5 MeCN, 4Á2MeOH
2
2
3
and 5Á1.6MeOH were selected under paratone oil and attached to
is only sparingly soluble in MeOH, but when the incoming metal
is present in solution, 1 easily dissolves and reacts to form the
desired compound as a precipitate. These reactions can be either
done at reflux (compound 3) or room temperature (compounds 4
and 5). While the former does yield a successful synthesis of the
a MiTeGen MicroMount. They were mounted in a stream of cold
2
N at 100(1) K using an Oxford Cryostat and centered in the X-
ray beam using a video monitoring system. The crystal evaluation
and data collection were performed on a Bruker Quazar APEX-II
diffractometer with Mo K
a radiation (k = 0.71073 Å). The data
compound, it also forms
a significant amount of cis-2,2-
were collected using a routine to survey an entire sphere of recip-
rocal space. The data were integrated using the SAINT routine in
APEX-II and corrected for absorption using SADABS [13]. The struc-
tures were solved via direct methods and refined by iterative cycles
of least-squares refinement on F2 followed by difference Fourier
synthesis using SHELX2013 [14] (see Table 1). All non-hydrogen
atoms were refined anisotropically except where noted below.
The alcohol hydrogen atoms on MeOH components of structures
Mo (SNO5) that must be removed, and reactions at room
2
4
temperature avoid this complication.
3.2. X-ray crystal structures
The X-ray crystal structures of 2–5 are shown in Figs. 1–4, and
important bond distances of 2–5 are compiled in Table 2 and com-
3
Á2MeOH, 4Á2MeOH, and 5Á1.6MeOH were located from the Fourier
pared with those of 1Ápy and its dimer [LiMo
2
(SNO5)
4 2
Cl] (1-dim).
difference map and refined independently. All other hydrogen
atoms were included in the final structure factor calculation at ide-
alized positions and were allowed to ride on the neighboring atoms
with relative isotropic displacement coefficients. The positional
and compositional disorder present in these structures is detailed
in the Supporting Information.
@
Each compound has a very similar Mo Mo bond distance between
@
2
.12 Å and 2.14 Å. These distances are longer than the average
]4+ paddlewheel compounds found
@
Mo Mo bond distance of [Mo
2
@
⁄
in the CSD (ꢀ2.10 Å), likely due to electron donation into
r
and
⁄
p
2
orbitals of the Mo unit by the axial ligands [5c,15].
Compounds 2–5 each exhibit the same 4,0 arrangement of SNO5
ligands found in 1Ápy and 1-dim. The differences between these
structures lie in the coordination environment, charge, and ionic
radius of their Lewis acidic main group cations.
2.4. Electrochemistry
Compounds 1, 2, 3, and 5 were dissolved in propylene carbonate
+
Structure 2Á4py is polymeric, with the Na ion coordinated to a
6
and chloride was removed by addition of a slight excess of TlPF ,
bridging chloride of an adjacent molecule as well as the four
giving dehalogenated compounds 1a, 2a, 3a, and 5a. The gray TlCl
that precipitated was removed by filtration. Cyclic voltammograms
for compounds 1a, 2a, 3a, and 5a were taken in propylene carbon-
ate at room temperature with 2 mM analyte and 100 mM elec-
trolyte (NEt PF ) using a standard glassy carbon electrode for the
4 6
working electrode, a platinum wire for the auxiliary electrode,
and an Ag/Ag electrode as the reference electrode. The solutions
were titrated with the appropriate M(OTf)
no longer changed. All electrochemical potentials were internally
referenced to the ferrocene/ferrocenium couple. The voltammetry
was performed in the range of 1000 mV to 0 mV at a scan rate of
À
SNO5 ligands, giving a square pyramidal coordination geometry
+
around the Na ion. The NaÁ Á ÁMo
2
distance is 3.505(2) Å, which is
distances found in 1Ápy
and 1-dim (3.075(5) Å and 3.049(6) Å, respectively). The Mo –Cl
substantially longer than the LiÁ Á ÁMo
2
2
bond distance increases from 2.6533(6) Å in 1Ápy to 2.776(1) Å in
+
4+
+
2
Á4py. Since Na is more remote from the [Mo
2
]
core than Li
+
4+
is, it has less of an influence on the acidity of the [Mo
2
]
. The lower
n
until the redox wave
À
Lewis acidity as well as the bridging nature of the axial Cl con-
2
tribute to the increased Mo –Cl bond distance in this compound
as compared to the Li complex.
2+
2+
The Ca and Sr ions of 3Á2MeOH and 4Á2MeOH are coordi-
nated by three MeOH ligands as well as the four SNO5 ligands, giv-
ing a seven-coordinate mono-capped trigonal prismatic geometry
À1
1
00 mV s
.
À
3
. Results and discussion
with one SNO5 O atom in the capping position. These structures
also contain two solvent MeOH molecules. One of these forms a
À
3.1. Synthesis
hydrogen bond with the free Cl , and one of these forms a hydro-
À
gen bond with the axially bound Cl , which serves to lengthen the
Each compound reported here was synthesized by one of two
methods: self-assembly or cation exchange of Li for M . Com-
pound 2 was synthesized by the self-assembly method shown in
Mo
2
–Cl bond. The complexes have MÁ Á ÁMo
2
distances of 3.699(1) Å
+
n+
and 3.8138(5) Å, respectively, which are significantly larger than
2+
that of 2Á4py. For 4Á2MeOH, this is expected, since Sr has a much
+
Scheme 1. In this method, the acetate ligands of Mo
2
(OAc)
4
are
larger ionic radius than Na , but for 3Á2MeOH, this is unexpected.
+
+
2+
substituted for HSNO5 while simultaneously installing the Na
The ionic radii of 5-coordinate Na and 7-coordinate Ca are
1.14 Å and 1.20 Å, respectively. However, the average M–OSNO5
bond distance is essentially identical for the two compounds,
+
ion into the compound. In this reaction, the Na is first allowed
À
to bind to the deprotonated SNO5 ligand in MeOH at 60° C. Then,

Table 1
X-ray experimental data for structures 2–5.
Compound
Formula
2Á4py
16ClMo
N)
3Á2MeOH
[C19
(CH
3-dimÁ5.5CH
3
CN
4Á2MeOH
5Á1.6MeOH
C
16
H
N
2 4
NaO S
4 4
Á4
H
28
N
4
O
7 4
S ClCaMo
2
]ClÁ2
C
32
H32Ca
Cl
2 4
Mo
4
N
8
O
8 8
S Á5.5
[C19H
28Cl
2
Mo
2
4
N O
7
S
4
Sr]ClÁ2
C
20
H32Cl0.85Mo
2
4
N O
8
S
4
Y[CF
3
SO
3
]2.15Á1.6
(
C
5
H
5
3
OH)
(CH
3
CN)
(CH
3
OH)
(CH OH)
3
Formula weight
T (K)
1023.29
100.0
919.64
100.0
1773.08
100.0
967.18
100.01
1260.11
100.0
Crystal system
Space group
a (Å)
b (Å)
c (Å)
tetragonal
P4/ncc
13.402(4)
13.402(4)
22.289(6)
90
90
90
4003(3)
4
1.698
orthorhombic
Pbca
19.139(7)
14.502(5)
24.573(8)
90
90
90
6821(4)
8
1.791
orthorhombic
Pnma
26.379(9)
16.334(6)
15.656(6)
90
90
90
6746(4)
4
1.746
orthorhombic
Pbca
19.1231(9)
14.5719(7)
24.850(1)
90
90
90
6924.7(6)
8
1.855
monoclinic
P2 /c
1
12.156(4)
14.561(5)
25.45(1)
90
102.61(1)
90
4396(3)
4
1.904
a
(°)
b (°)
(°)
c
3
V (Å )
Z
q
l
calc mg/mm3
(mm
À1)
0.964
1.338
1.342
2.694
2.308
F(000)
2064.0
3712.0
3534.0
3856.0
2509.0
Crystal size (mm)
Radiation
0.476 Â 0.232 Â 0.078
0.04 Â 0.03 Â 0.03
0.098 Â 0.065 Â 0.063
0.164 Â 0.025 Â 0.023
0.247 Â 0.086 Â 0.080
Mo K (k = 0.71073)
a
Mo K (k = 0.71073)
a
Mo K (k = 0.71073)
a
Mo K (k = 0.71073)
a
Mo K (k = 0.71073)
a
2
h range for data collection
3.654 to 64.276°
3527
3.314 to 55.23
7917
3.024 to 54.798
7924
3.278 to 52.804
7089
3.242 to 52.878
8995
Independent reflections
R
int
0.0314
0.0786
0.0848
0.1088
0.0547
Data/restraints/parameters
Goodness-of-fit (GOF) on F
3527/168/185
1.121
7917/38/433
1.061
7924/930/566
1.134
7089/39/430
1.021
8995/192/632
1.418
2
a,b
Final R indexes [I P 2
r(I)]
R
1
= 0.0257
wR = 0.0709
= 0.0287
wR = 0.0744
R
1
= 0.0243
wR = 0.0482
= 0.0387
wR = 0.0531
R
1
= 0.0415
wR = 0.0945
= 0.0568
wR = 0.1010
R
1
= 0.0346
wR = 0.0625
= 0.0608
wR = 0.0699
R
1
= 0.0699
wR = 0.1430
= 0.0808
wR = 0.1460
2
2
2
2
2
Final R indexes [all data]
R
1
R
1
R
1
R
1
R
1
2
2
2
2
2
a
R
wR
1
= [
R
||F
o
| À |F
c
||]/[
R
|F
o
|].
o o o c
(w(F r )]/3.
)2]]1/2, w = 1/[ 2(F ) + (aP)2 + bP], where P = [max(0 or F2) + 2(F2
b
2
2
2
2
2
2
= [
R
[w(F
o
À F
c
) ]]/[
R

B.S. Dolinar, J.F. Berry / Polyhedron 103 (2016) 71–78
75
Fig. 1. X-ray crystal structure of polymeric 2Á4py. Atoms are drawn as 50% thermal probability ellipsoids. All hydrogen atoms and solvent molecules are omitted for clarity.
Fig. 2. X-ray crystal structures of (a) monomeric 3 and (b) dimeric 3-dim. All atoms are drawn as 50% thermal probability ellipsoids. All hydrogen atoms, except the alcohol
hydrogens of methanol, all counter ions, minor components of disorder, and free solvent molecules are omitted for clarity.
+
2+
Ca2+ is responsible for the increased MÁÁÁMo
indicating that the difference in ionic radii between Na and Ca
has a negligible impact on the structure and does not adequately
explain the ꢀ0.19 Å increase in MÁ Á ÁMo separation. Instead, we
believe this change to result from Coulombic repulsion between
2
distance, the
4+
2
increased charge also exerts a greater influence on [Mo ] , result-
2
ing in a Mo
2
–Cl bond distance closer to that in the corresponding
Li compound. The Mo
–Cl bond distance of 4Á2MeOH is slightly
longer than that of 3Á2MeOH. This slight increase is likely caused
+
2
4
+
n+
2+
[
Mo
2
]
and M . The Mo ions of the dimolybdenum unit more
strongly repel the Ca ion than the Na ion because of the higher
2
separation. In spite
distance as well as the hydrogen bonding
interaction between solvent MeOH and the axial Cl , both
2
+
+
2+
by the slightly longer MÁÁÁMo
2
distance of the Sr compound.
2
+
charge on Ca , resulting in the longer MÁ Á ÁMo
When crystallized from MeCN, 3 dimerizes to form the struc-
2
+
of the longer MÁ Á ÁMo
2
ture 3-dimÁ5.5 MeCN. In this structure, both Ca ions are coordi-
nated by four SNO5 ligands, two chlorides, which bridge the two
halves of the dimer, and one MeCN ligand. This structure has a
slightly longer MÁÁÁMo distance than does 3Á2MeOH, but it has a
À
3
Á2MeOH and 4Á2MeOH have Mo
and 2.707(1) Å, respectively) that are significantly shorter than
the Mo
–Cl bond distance in 2Á4py. While the increased charge of
2
–Cl bond distances (2.7023(9) Å
2
2 2
much shorter Mo –Cl bond distance of 2.630[1] Å. This Mo –Cl

7
6
B.S. Dolinar, J.F. Berry / Polyhedron 103 (2016) 71–78
3+
Structure 5Á1.6MeOH contains an eight-coordinate Y ion that
is coordinated by the four SNO5 ligands and four MeOH ligands
in a square anti-prismatic arrangement. Like 3Á2MeOH and
4
Á2MeOH, 5Á1.6MeOH has a hydrogen bond between one of the
À
partially occupied MeOH solvent molecules and the axial Cl . The
axial Cl attached to the Mo
with a triflate anion (Cl component: 85.5(5)%). This compositional
disorder is representative of the bulk sample as evidenced by the
À
2
unit is compositionally disordered
À
elemental analysis data (vide supra). This structure has a YÁ Á ÁMo
2
distance of 3.745(2) Å, which is slightly longer than the CaÁ Á ÁMo
2
3
+
2+
distance in 3Á2MeOH as the Y ion is repelled by the Mo ions
2
+
more strongly than Ca is. The Mo
2
–Cl bond distance is slightly
shorter than that of 3Á2MeOH and 4Á2MeOH, an effect that can be
3
+
attributed to the increased charge of Y as compared with either
2
+
2+
Ca or Sr
.
In general, the increased charge of the Lewis acidic cation influ-
ences the dimolybdenum center, making it more Lewis acidic and
Fig. 3. X-ray crystal structure of the cation of 4Á2CH
3
OH. All non-hydrogen atoms
are drawn as 50% thermal probability ellipsoids. All hydrogen atoms except the
alcohol hydrogens of methanols are omitted for clarity.
shortening the Mo
2
–Cl bond distance. However, this effect is miti-
4+
gated by the increased repulsion between the [Mo
2
]
moiety and
the cation resulting from the cation’s increased charge as well as by
the formation of hydrogen bonds with solvent methanol.
3.3. Electrochemistry
Cyclic voltammograms (CV) of compounds [LiMo
2
(SNO5)
(3a), and
(5a) were taken in propylene carbonate with
PF as the electrolyte. Compounds
a, 2a, 3a, and 5a were generated in situ by reacting 1, 2, 3, and 5
in propylene carbonate in order to strip away the axial
4 6
][PF ]
(
[
2
1
2 4 6 2 4 6 2
1a), [NaMo (SNO5) ][PF ] (2a), [CaMo (SNO5) ][PF ]
YMo (SNO5) ][PF ]
2 4 6 3
mM analyte and 100 mM NEt
4
6
with TlPF
6
chloride. This was a necessary step for obtaining high quality CVs
4
+/5+
of the compounds because the Mo
2
redox waves of some com-
À
Å
pounds overlapped with the Cl /Cl redox waves. Solid TlCl was
removed by filtration, and CVs were taken on the filtered solutions
4
of 1a, 2a, 3a, and 5a. Each of these exhibits a quasi-reversible Mo
2
Fig. 4. X-ray crystal structure of the cation of 5Á1.6CH
3
OH. All non-hydrogen atoms
are drawn as 50% thermal probability ellipsoids. All hydrogen atoms, except those
on the alcohol groups, and all disordered components are omitted for clarity.
+
/5+
n+
redox wave that moves to a higher potential as M is added to
the solution. These CVs are consistent with metal ion-coupled elec-
tron transfer [17], and they indicate that each of these compounds
n+
reversibly binds M , as we previously reported for the electro-
bond distance is also shorter than those of 1Ápy and 1-dim. Another
chemistry of 1. Further, these CVs are substantially different than
set of heterotrimetallic linear chain compounds recently reported
those of the previously reported trans-2,2-Mo (SNO5)4 and cis-
2
by our group (Mo
dpa) Cl ]OTf, Mo
distances caused by covalent bonding between Ru and Mo
5a,16]. The Mo
–Cl bond distance of 3-dimÁ5.5 MeCN is the
shortest known for a MÁ Á ÁMo heterotrimetallic compound in
which M interacts with the Mo unit in a purely Coulombic
2
Ru(dpa)
4
Cl
2
, Mo
2
–Cl = 2.5262(6) Å, and [Mo
2
Ru
2,2-Mo (SNO5) , ruling out the possibility that 1a, 2a, 3a, and 5a
2
4
(
4
2
2
–Cl = 2.4637(5) Å) have shorter Mo
2
–Cl bond
undergo ligand rearrangement upon halide abstraction. Thus, the
conclusions we draw from this electrochemical study are based
on the reasonable assumption that the solution structures of 1a,
2a, 3a, and 5a, are similar to those of their halogenated
counterparts.
2
[
2
2
2
fashion. Unlike the MeOH solvent molecules of 3Á2MeOH, The
MeCN solvent molecules of 3-dimÁ5.5 MeCN are unable to form
hydrogen bonds with the axial chlorides. Without the hydrogen
Each solution was titrated with a solution of M(OTf) until the
redox wave was no longer affected by additional aliquots of M
n
(OTf)n solution. The final CVs for each compound are shown in
4+
n+
bonding interaction competing with the [Mo
axial Cl , structure 3-dimÁ5.5 MeCN can support a shorter Mo–Cl
2
]
core for the
Fig. 5 and the redox potentials as well as the amount M added
À
to solution are reported in Table 3. The charge of the Lewis acid,
n+
bond distance.
and thus the charge of the [MMo (SNO5) ]
cation, greatly
2
4
Table 2
Selected bond distances for compounds 1–5.
Compound
Mo–Mo (Å)
MÁ Á ÁMo (Å)
2
Mo –Cl (Å)
Mn+–OSNO5 (Å)
Mn+ ionic radius (Å)a
Refs.
1
1
2
3
3
4
5
Ápy
2.1357(3)
2.1354(8)
2.1377(7)
2.1306(6)
2.1361[8]
2.1333(5)
2.120(1)
3.075(5)
3.049(6)
3.505(2)
3.699(1)
3.711[1]
3.8138(5)
3.745(2)
2.6533(6)
2.644(1)
2.776(1)
2.7023(9)
2.630[1]
2.707(1)
2.699(3)
2.182[6]
2.185[7]
2.434(2)
2.4422[9]
2.439[2]
2.554[2]
2.374[3]
0.90
0.90
1.14
1.20
1.20
1.35
1.16
[5c]
[5c]
-dim
Á4py
This work
This work
This work
This work
This work
Á2MeOH
-dimÁ5.5 MeCN
Á2MeOH
Á1.6MeOH
a
Values taken from Ref. [18].

B.S. Dolinar, J.F. Berry / Polyhedron 103 (2016) 71–78
77
]+ (1a, gray) [NaMo
2
4
2 4 2 4 2 4
(SNO5) (SNO5) (SNO5)
]+ (2a, orange), [CaMo ]2+ (3a, green), and [YMo ]3+ (5a, blue). The CVs were taken
Fig. 5. Cyclic voltammograms of [LiMo
(SNO5)
in PC with 2 mM analyte and 100 mM NEt
4
PF6. The currents are normalized. (Color online.)
n+
Table 3
the Mo
2
–Cl bond distance. The charge of M has a profound
4
+/5+
Electrochemical potentials of 1a, 2a, 3a, and 5a.
impact on the redox potential of the [Mo
2
]
couple. Increasing
n+
4+/5+
+
Equivalents Mn+ added
the charge of M from n = 1 to n = 3 allows the [Mo
2
]
redox
Compound
c c
E1/2 vs. F /F (mV)
couple to be tuned over a 100 mV range.
1
2
3
5
a
a
a
a
446
480
567
579
20
19
25
8
Acknowledgements
The authors thank Tristan R. Brown for his help in obtaining ele-
mental analysis data on these compounds. Financial support was
provided under NSF Grant CHE-1300464 and a generous gift from
Paul J. Bender (X-ray diffraction and NMR spectroscopy instrumen-
tation). Finally, the authors wish Malcolm Chisholm a very happy
birthday.
4
+/5+
+
2+
influences the Mo
2
redox potential. The ionic radii of Na , Ca ,
3
+
and Y have a narrow range of 0.06 Å. Thus, the differences in the
CVs of 2a, 3a, and 5a are based primarily on the charge of M ,
rather than differences in the ionic radius. As the charge of M
in these compounds increases from +1 to +3, this potential shifts
from 480 mV to 579 mV. Given the ease of swapping one cation
n+
n+
Appendix A. Supplementary data
for another in these systems, these results demonstrate a simple
method to reliably tune the redox potential of the Mo4
+/5+
couple
2
CCDC 1413783-1413787 contains the supplementary crystallo-
over a range of 100 mV.
graphic data for 2Á4py, 3Á2MeOH, 3-dimÁ5.5MeCN, 4Á2MeOH,
5
Á1.6MeOH. These data can be obtained free of charge via http://
4
. Conclusions
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2015.09.028.
We describe here the synthesis of four new MÁÁÁMo
2
compounds
in which M is a Group I, II, or III cation. Three of these compounds
(
2, 3, and 5) follow a diagonal trend and have very similar ionic
radii, allowing the effect of cation charge on the Lewis acidity of
4
+
4+/5+
[
Mo
2
]
and [Mo
2
]
redox couple to be probed. Compound 4
References
has the same cation charge as 3, but it has a larger ionic radius,
4
+
allowing the effect of increasing the MÁ Á ÁMo
2
distance on [Mo
2
]
[1] (a) F.A. Cotton, C.A. Murillo, R.A. Walton, Multiple Bonds Between Metal
Atoms, 3rd ed., Springer Science and Business Media Inc., New York, 2005;
n+
Lewis acidity to be examined. Increasing the charge of the M does
(
b) L.R. Falvello, B.M. Foxman, C.A. Murillo, Inorg. Chem. 53 (2014) 9441.
4+
increase the Lewis acidity of [Mo
distance, however the effect is attenuated by an increase in the
MÁ Á ÁMo distance resulting from increased repulsion between
2 2
] , shortening the Mo –Cl bond
[
2] (a) G. Li Manni, A.L. Dzubak, A. Mulla, D.W. Brogden, J.F. Berry, L. Gagliardi,
Chem. Eur. J. 18 (2012) 1737;
(
(
(
b) B.S. Dolinar, J.F. Berry, Dalton Trans. 43 (2014) 6165;
c) J. Hicks, S.P. Ring, N.J. Patmore, Dalton Trans. 41 (2012) 6641;
d) M.H. Chisholm, N.J. Patmore, Acc. Chem. Res. 40 (2007) 19;
2
n+
4+
M
and [Mo
2
]
. Increasing the MÁ Á ÁMo
2
distance has a modest
Lewis acidity and slightly increases
4+
effect that decreases [Mo
2
]
(e) L.A. Wilkinson, L. McNeill, A.J. Meijer, N.J. Patmore, J. Am. Chem. Soc. 135

7
8
B.S. Dolinar, J.F. Berry / Polyhedron 103 (2016) 71–78
(
(
(
(
2013) 1723;
[6] (a) K. Jansen, K. Dehnicke, D. Fenske, Z. Naturforsch., B 40 (1985) 13;
(b) F. Apfelbaum, A. Bino, Inorg. Chim. Acta 155 (1989) 191;
(c) F.A. Cotton, L.R. Falvello, A.H. Reid Jr., W.J. Roth, Acta Crystallogr. C 46
(1990) 1815;
f) L.A. Wilkinson, L. McNeill, P.A. Scattergood, N.J. Patmore, Inorg. Chem. 52
2013) 9683;
g) B.G. Alberding, S.E. Brown-Xu, M.H. Chisholm, T.L. Gustafson, C.R. Reed, V.
Naseri, Dalton Trans. 41 (2012) 13097;
(d) B. Udovic, I. Leban, P. Šegedin, Croat. Chem. Acta 72 (1999) 477.
[7] (a) J.V. Brencic, F.A. Cotton, Inorg. Chem. 8 (1969) 7;
(b) J.V. Brencic, F.A. Cotton, Inorg. Chem. 8 (1969) 2698;
(c) I. Leban, P. Šegedin, Inorg. Chim. Acta 85 (1984) 181;
(d) F.A. Cotton, J.H. Matonic, D.de O. Silva, Inorg. Chim. Acta 234 (1995) 115.
[8] (a) F.G. Adly, A. Ghanem, Chirality 26 (2014) 692;
(
(
h) M.H. Chisholm, T.L. Gustafson, C. Turro, Acc. Chem. Res. 46 (2013) 529;
i) B.G. Alberding, M.H. Chisholm, C.B. Durr, J.C. Gallucci, Y. Ghosh, T.F. Spilker,
Dalton Trans. 43 (2014) 11397;
j) B.G. Alberding, M.H. Chisholm, Y.-H. Chou, J.C. Gallucci, Y. Ghosh, T.L.
Gustafson, N.J. Patmore, C.R. Reed, C. Turro, Inorg. Chem. 48 (2009) 4394;
k) B.G. Alberding, M.H. Chisholm, Y.H. Chou, Y. Ghosh, T.L. Gustafson, Y. Liu, C.
Turro, Inorg. Chem. 48 (2009) 11187;
l) B.G. Alberding, M.H. Chisholm, J.A. Gallucci, Y. Ghosh, T.L. Gustafson, Proc.
Natl. Acad. Sci. U.S.A. 108 (2011) 8152;
m) B.G. Alberding, M.H. Chisholm, T.L. Gustafson, C.R. Reed, N. Singh, C. Turro,
J. Cluster Sci. 20 (2009) 307;
n) B.G. Alberding, M.H. Chisholm, B.J. Lear, V. Naseri, C.R. Reed, Dalton Trans.
0 (2011) 10658;
(
(
(b) J. Hansen, H.M.L. Davies, Coord. Chem. Rev. 252 (2008) 545;
(c) A.F. Trindade, J.A.S. Coelho, C.A.M. Afonso, L.F. Veiros, P.M.P. Gois, ACS
Catal. 2 (2012) 370;
(
(d) K.P. Kornecki, J.F. Berry, D.C. Powers, T. Ritter, Metal-Metal Bond-
Containing Complexes as Catalysts for C-H Functionalization, in: K.D. Karlin
(Ed.) Prog. Inorg. Chem., John Wiley & Sons, Inc., Hoboken, 2014, pp. 225–302.;
(e) K.P. Kornecki, J.F. Berry, Chem. Eur. J. 17 (2011) 5827;
(f) K.P. Kornecki, J.F. Berry, Eur. J. Inorg. Chem. (2012) 562;
(g) K.P. Kornecki, J.F. Berry, Chem. Commun. 48 (2012) 12097.
[9] K.P. Kornecki, J.F. Briones, V. Boyarskikh, F. Fullilove, J. Autschbach, K.E.
Schrote, K.M. Lancaster, H.M.L. Davies, J.F. Berry, Science 342 (2013) 351.
[10] (a) E.Y. Tsui, R. Tran, J. Yano, T. Agapie, Nature Chem. 5 (2013) 293;
(b) Y.J. Park, J.W. Ziller, A.S. Borovik, J. Am. Chem. Soc. 133 (2011) 9258;
(c) I.S. Ke, J.S. Jones, F.P. Gabbai, Angew. Chem., Int. Ed. Engl. 53 (2014) 2633;
(d) T.P. Lin, F.P. Gabbaï, J. Am. Chem. Soc. 134 (2012) 12230;
(e) H. Yang, F.P. Gabbaï, J. Am. Chem. Soc. 136 (2014) 10866.
[11] U. Berg, J. Sandström, Acta Chem. Scand. 20 (1966) 689.
[12] G. Holste, H. Schäfer, Z. Anorg. Allg. Chem. 391 (1972) 263.
[13] (a) Bruker-AXS, in, Bruker AXS, Madison, WI, 2014.;
(b) L. Kraus, R. Herbst-Irmer, G.M. Sheldrick, D. Stalke, J. Appl. Crystallogr. 48
(2015) 3.
(
(
4
(
(
o) M.H. Chisholm, J.S. D’Acchioli, B.D. Pate, N.J. Patmore, Inorg. Chem. 44
2005) 1061.
[
[
3] C.A. Murillo, Aust. J. Chem. 67 (2014) 972.
4] (a) M. Nippe, E. Victor, J.F. Berry, Inorg. Chem. 48 (2009) 11889;
(
b) D.W. Brogden, Y. Turov, M. Nippe, G. Li Manni, E.A. Hillard, R. Clérac, L.
Gagliardi, J.F. Berry, Inorg. Chem. 53 (2014) 4777.
5] (a) D.W. Brogden, J.F. Berry, Inorg. Chem. 53 (2014) 11354;
[
(
(
(
(
(
(
(
(
b) D.W. Brogden, J.H. Christian, N.S. Dalal, J.F. Berry, Inorg. Chim. Acta 424
2015) 241;
c) B.S. Dolinar, J.F. Berry, Inorg. Chem. 52 (2013) 4658;
d) M. Nippe, J.F. Berry, J. Am. Chem. Soc. 129 (2007) 12684;
e) M. Nippe, E. Bill, J.F. Berry, Inorg. Chem. 50 (2011) 7650;
f) M. Nippe, Y. Turov, J.F. Berry, Inorg. Chem. 50 (2011) 10592;
g) M. Nippe, E. Victor, J.F. Berry, Eur. J. Inorg. Chem. 2008 (2008) 5569;
h) M. Nippe, J. Wang, E. Bill, H. Hope, N.S. Dalal, J.F. Berry, J. Am. Chem. Soc.
[14] (a) G.M. Sheldrick, XS, Georg-August-Universität Göttingen, Göttingen,
Germany, 2013;
1
(
32 (2010) 14261;
i) K. Mashima, Y. Shimoyama, Y. Kusumi, A. Fukumoto, T. Yamagata, M.
Ohashi, Eur. J. Inorg. Chem. 2007 (2007) 235;
j) M. Ohashi, A. Shima, T. Rüffer, H. Mizomoto, Y. Kaneda, K. Mashima, Inorg.
Chem. 46 (2007) 6702;
(b) G.M. Sheldrick, Acta Crystallogr. C 71 (2015) 3;
(c) O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, H. Puschmann, J.
Appl. Crystallogr. 42 (2009) 339.
(
[15] F.H. Allen, Acta Crystallogr. B 58 (2002) 380.
[16] D.W. Brogden, J.F. Berry, Inorg. Chem. 54 (2015) 7660.
[17] (a) S. Fukuzumi, K. Ohkubo, Coord. Chem. Rev. 254 (2010) 372;
(b) S. Fukuzumi, K. Ohkubo, Y. Morimoto, Phys. Chem. Chem. Phys. 14 (2012)
8472.
(
(
(
k) K. Pal, K. Nakao, K. Mashima, Eur. J. Inorg. Chem. 2010 (2010) 5668;
l) M.H. Chisholm, A.M. Macintosh, Chem. Rev. 105 (2005) 2949;
m) B. Li, H. Zhang, L. Huynh, M. Shatruk, E.V. Dikarev, Inorg. Chem. 46 (2007)
9
(
6
155;
n) M. Koeberl, M. Cokoja, W.A. Herrmann, F.E. Kuehn, Dalton Trans. 40 (2011)
834.
[18] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751.