Filling a void: Isolation and characterization of tetracarboxylato dimolybdenum cations

Article abstract of DOI:10.1021/ic011079r

Dimolybdenum tetracarboxylato cations have been prepared and structurally characterized for the first time. The reactions of the new, quadruply bonded compound, Mo₂(TiPB)₄, where TiPB = 2,4,6-triisopropylphenyl carboxylate, with NOPF₆ and NOBF₄ give the ionic compounds [Mo₂(TiPB)₄]PF₆ and [Mo₂(TiPB)₄]BF₄, respectively. Each product crystallizes in space group P2₁/n and displays an elongation of the Mo-Mo bond of 0.060 and 0.068 A, respectively, over that of the parent compound (2.076(1) A). Each complex displays a characteristic EPR signal, showing hyperfine coupling to the spin active isotopes ⁹⁵Mo and ⁹⁷Mo, with g_∥ = g⊥ = 1.936, that is consistent with the presence of an un paired electron. Electronic spectroscopy indicates the expected red shift in the δ → δ* transition for the cations, due to the loss of exchange energy in going from the two-electron to one-electron system. We have also obtained a small amount of crystalline [Mo₂(O₂CC₄H₉)₄]PF ₆ from the reaction of Mo₂(O₂CC₄H₉)₄ with AgPF₆. This product crystallizes in the space group C2/c, and the Mo-Mo bond is elongated by 0.063 A over that of the neutral parent compound. These ionic compounds have the first isolated and characterized [Mo₂(O₂CR)₄]⁺ cationic species.

Full text of DOI:10.1021/ic011079r

Inorg. Chem. 2002, 41, 1639−1644
Filling a Void: Isolation and Characterization of Tetracarboxylato
Dimolybdenum Cations
,†
†
†
,†,‡
F. Albert Cotton,* Lee M. Daniels, Elizabeth A. Hillard, and Carlos A. Murillo*
Department of Chemistry and Laboratory for Molecular Structure and Bonding, P.O. Box 30012,
Texas A&M UniVersity, College Station, Texas 77842-3012, and Department of Chemistry,
UniVersity of Costa Rica, Ciudad UniVersitaria, Costa Rica
Received October 18, 2001
Dimolybdenum tetracarboxylato cations have been prepared and structurally characterized for the first time. The
reactions of the new, quadruply bonded compound, Mo
with NOPF and NOBF give the ionic compounds [Mo (TiPB)
crystallizes in space group P2 /n and displays an elongation of the Mo−Mo bond of 0.060 and 0.068 Å, respectively,
over that of the parent compound (2.076(1) Å). Each complex displays a characteristic EPR signal, showing hyperfine
2
(TiPB)
4
, where TiPB ) 2,4,6-triisopropylphenyl carboxylate,
6
4
2
4
]PF
6
and [Mo (TiPB) ]BF , respectively. Each product
2
4
4
1
95
97
coupling to the spin active isotopes Mo and Mo, with g
|
) g ) 1.936, that is consistent with the presence of
⊥
an unpaired electron. Electronic spectroscopy indicates the expected red shift in the δ f δ* transition for the
cations, due to the loss of exchange energy in going from the two-electron to one-electron system. We have also
obtained a small amount of crystalline [Mo
product crystallizes in the space group C2/c, and the Mo−Mo bond is elongated by 0.063 Å over that of the neutral
parent compound. These ionic compounds have the first isolated and characterized [Mo (O CR)
]+ cationic species.
2 2 4 9 4 6 2 2 4 9 4 6
(O CC H ) ]PF from the reaction of Mo (O CC H ) with AgPF . This
2
2
4
Introduction
Even more surprising is the fact that although the limited
electrochemical data show that reversible (or at least quasi-
n+
1
Among the thousands of M
2
compounds the most
+
reversible) oxidation to Mo
tials well below +1.0 V, no such species has ever been
2
(O
2
CR)
4
ions occurs at poten-
4+
4+
numerous are those of Mo
2
. The earliest of the Mo
2
compounds to have been reported and then structurally
characterized were those of the paddlewheel tetracarboxylato
2 2 4 2
type, Mo (O CR) . These were also among the earliest M
compounds to be subjected to rigorous molecular orbital
calculations, detailed electronic spectroscopic study, and
photoelectron spectroscopic study. It is, therefore, surprising
to note how little is known about their redox chemistry.
8
2
structurally characterized over the approximately 40 year
3
period since the Mo
We now report that the gap has been filled by the
preparation of [Mo (TiPB) ]PF ‚2CH Cl , 1‚2CH Cl , and
Mo (TiPB) ]BF ‚2CH Cl , 2‚2CH Cl , where TiPB is 2,4,6-
2 2 4
(O CR) compounds were discovered.
n+
2
4
6
2
2
2
2
4
5
[
2
4
4
2
2
2
2
6
triisopropylphenyl carboxylate, and their detailed structural
and physical characterization. For comparison, we also report
7
on the parent compound Mo
of the pivalato cation in [Mo
2
(TiPB)
(O CC
4
, 3, and the structure
]PF , 4.
*
Authors to whom correspondence should be addressed. E-mail:
2
2
4
H
9
)
4
6
cotton@tamu.edu (F.A.C.); murillo@tamu.edu (C.A.M.).
‡
University of Costa Rica.
Texas A&M University.
†
Experimental Section
(
1) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms,
nd ed.; Oxford University Press: Oxford, 1993.
2) (a) Abel, E. W.; Singh, A.; Wilkinson, G. J. Am. Chem. Soc. 1959,
2
General Procedures. All manipulations were carried out in an
inert atmosphere utilizing standard Schlenk and drybox techniques.
All reagents and solvents were obtained commercially. Anhydrous
(
8
1, 3097. (b) Bannister, E.; Wilkinson, G. Chem. Ind. (London) 1960,
319. (c) Stephenson, T. A.; Bannister, E.; Wilkinson, G. J. Am. Chem.
Soc. 1963, 85, 6012.
(
(
(
3) (a) Lawton, D.; Mason, R. J. Am. Chem. Soc. 1965, 87, 921. (b) Cotton,
F. A.; Mester, Z. C.; Webb, T. R. Acta Crystallogr. 1974, B30, 2768.
4) Norman, J. G., Jr.; Kolari, H. J. J. Chem. Soc., Chem. Commun. 1975,
(6) Lichtenberger, D. L.; Johnston, R. L. In Metal-Metal Bonds and
Clusters in Chemistry and Catalysis; J. P. Fackler, Jr., Ed.; Plenum
Press: New York, 1990.
(7) Reference 1, pp 161-162.
(8) For a recent reference, see: Liwporncharoenvong, T.; Luck, R. L. J.
Am. Chem. Soc. 2001, 123, 3615.
649.
5) Cotton, F. A.; Zhong, B. J. Am. Chem. Soc. 1990, 112, 2256 and
references therein.
1
0.1021/ic011079r CCC: $22.00 © 2002 American Chemical Society
Inorganic Chemistry, Vol. 41, No. 6, 2002 1639
Published on Web 02/21/2002

Cotton et al.
Table 1. Crystallographic Data for [Mo2(O2CR)4]0/+ Complexes
‚2CH2Cl2
1
2‚2CH2Cl2
3
4
empirical formula
fw
space group
a, Å
b, Å
c, Å
C66H96Cl4F6Mo2O8P
1496.08
P21/n (No. 14)
9.4884(6)
18.757(1)
20.428(1)
90
98.319(2)
90
3597.4(4)
2
C66H96Cl4F4Mo2O8B
1437.92
P21/n (No. 14)
9.2621(7)
18.755(2)
20.510(2)
90
98.210(2)
90
3526.3(4)
2
C64H92Mo2O8
1181.26
P 1h (No. 2)
9.709(6)
12.035(8)
14.574(6)
100.01(4)
107.95(3)
103.36(6)
1520(2)
1
C20H36F6Mo2O8P
741.34
C2/c (No. 15)
16.764(4)
10.454(2)
19.246(4)
90
115.351(3)
90
3048(1)
4
R, deg
â, deg
γ, deg
3
V, Å
Z
T, °C
λ, Å
-60
-60
-60
-60
0.71073
1.381
0.71073
1.354
0.71073
1.290
0.71073
1.615
3
dcalcd, g/cm
-
1
µ, mm
0.584
0.1354 (0.1423)
0.567
0.0709 (0.0808)
0.464
0.0689 (0.1713)
0.949
0.0330 (0.0765)
a
b
R1 (wR2 )
a
R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) [∑[w(Fo2 - Fc ) ]/∑[w(Fo ) ]] , w ) 1/[σ (Fo ) + (aP) + bP], where P ) [max(Fo or 0) + 2(Fc )]/3.
b
2 2
2 2 1/2
2
2
2
2
2
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
[
dichlorobenzene was purchased from Aldrich Chemical Company
Mo2(O2CR)4]0 Complexes
/+
in Sure-Seal bottles. Dichloromethane was dried over P
and toluene over Na/K alloy, and these solvents were freshly
distilled under N prior to use.
2 5
O , hexanes
1‚2CH2Cl2 2‚2CH2Cl2
3
4
Mo(1)-Mo(1a)a
2.1364(8)
2.074(3)
2.066(3)
2.063(3)
2.062(3)
2.1441(5)
2.071(2)
2.070(1)
2.065(2)
2.065(1)
2.076(1) 2.1512(5)
2.084(4) 2.079(2)
2.113(4) 2.080(2)
2.084(4) 2.073(2)
2.088(4) 2.077(2)
2
Mo(1)-O(1)
Mo(1)-O(2)
Mo(1)-O(3)
Mo(1)-O(4)
Physical Measurements. Elemental analyses were performed
by Canadian Microanalytical Service, Ltd., Delta, British Columbia,
1
Canada. H NMR spectra were recorded on a Unity Plus 300 NMR
spectrometer, with chemical shifts (δ) referenced to CH
cyclic voltammograms were recorded on a BAS 100 electrochemical
analyzer in 0.1 M Bun
NPF solutions with Pt working and auxiliary
electrodes and a Ag/AgCl reference electrode; scan rates were 100
2 2
Cl . The
Mo(1a)-Mo(1)-O(1) 91.70(9)
Mo(1a)-Mo(1)-O(2) 91.67(8)
Mo(1a)-Mo(1)-O(3) 90.6(1)
Mo(1a)-Mo(1)-O(4) 90.57(9)
91.54(5)
91.64(4)
90.71(5)
90.61(5)
89.98[6]
176.96[6]
92.8(1)
90.9(1)
91.4(1)
92.0(1)
89.9[2]
91.31(5)
91.04(5)
91.25(5)
91.04(5)
89.98[8]
4
6
-
1
cis-O-Mo(1)-O
90.0[1]
mV s in all cases. The EPR spectra were recorded on a Bruker
ESP300 spectrometer. The magnetic susceptibility measurements
were recorded on a Quantum Design SQUID MPMS-XL magneto-
meter. UV/vis spectra were recorded on a Cary 17D spectro-
photometer.
trans-O-Mo(1)-O
176.7[1]
176.1[1] 177.67[6]
a
Mo(1) and Mo(1a) are related by an inversion center in all compounds.
refinement results for all compounds are summarized in Table 1.
X-ray Crystallography. Single crystals of 1‚2CH
2
Cl
2
, 2‚2CH
2
-
Selected bond distances and angles are given in Table 2.
-
Cl
2
, and 4 were attached to glass fibers with a small amount of
In 2‚2CH
2
Cl
2
, the BF
4
group resides on the inversion center
silicon grease and mounted on the Bruker SMART system for data
collection using Mo KR radiation at 213(2) K. Single-crystal X-ray
work on 3 was performed on a Nonius FAST diffractometer
utilizing the program MADNES with Mo KR radiation at 213(2)
K. Cell parameters were obtained from an autoindexing routine.
halfway between the dinuclear Mo centers, located directly in line
with the axial positions. The anion was modeled as a rigid
tetrahedron, and further refinement followed by examination of
difference Fourier maps revealed at least two other orientations in
addition to the disorder imposed by the site symmetry. Therefore
-
For 1‚2CH
within a 2θ range of 4.324-51.07°. For 2‚2CH
were refined with 5034 reflections within a 2θ range of 4.554-
4.975°. For 3, cell parameters were refined with 250 reflections
2
Cl
2
, cell parameters were refined with 3118 reflections
three BF
4
units were included at the site as rigid tetrahedra in
Cl , cell parameters
2
which the B-F bonds were allowed to shrink or expand. The B
atoms were not constrained to remain exactly on the inversion
center. The sum of the occupancies was constrained to equal full
occupancy for the site, and one common isotropic displacement
factor was refined for all F atoms and another for all B atoms. The
final occupancies for the three orientations converged to 36.57-
(2)%, 31.06(2)%, and 32.37(2)%.
2
5
within a 2θ range of 18.2-41.6°. For 4, cell parameters were refined
with 6400 reflections within a 2θ range of 4.684-54.98°.
For all compounds, the coordinates of some or all of the non-
hydrogen atoms were found via direct methods using the structure
9
solution program SHELXS. The positions of the remaining non-
Preparation of [Mo
(TiPB) (300 mg, 0.254 mmol) and NOPF
were each combined with 20 mL of CH Cl
was transferred to a flask containing the NOPF
affording a purple solution, which was stirred for 2 h. The mixture
was filtered over Celite to remove any unreacted NOPF and
concentrated to about 10 mL. Red needles of 1‚2CH Cl suitable
2
(TiPB)
4
]PF
6
‚2CH
2
Cl
(45.0 mg, 0.257 mmol)
. The yellow solution
suspension, quickly
2
, 1‚2CH
2 2 2
Cl . Mo -
hydrogen atoms were located by use of a combination of least-
squares refinement and difference Fourier maps in the SHELXL-
4
6
2
2
9
3 program.10 Non-hydrogen atoms were refined with anisotropic
6
displacement parameters, except for disordered portions of the
structures (isopropyl groups in 1‚2CH Cl , fluorine atoms in 2‚
CH Cl , phenyl rings and isopropyl groups in 3, and tert-butyl
2
2
6
2
2
2
2
2
groups in 4). The hydrogen atoms were included in the structure
factor calculations at idealized positions. Cell parameters and
for X-ray structural analysis were grown after 24 h from the slow
diffusion of hexanes into the filtrate. The yield was 0.060 g (52%).
After elimination of interstitial solvent molecules under vacuum:
(
9) Sheldrick, G. M. SHELXS, program for crystal structure solution. Acta
Anal. for C64
6
H
92Mo
O
PF
.99 (6.84). IR (KBr): 2964, 2931, 2870, 1700, 1687, 1655, 1638,
605, 1561, 1543, 1460, 1403, 1320, 1284, 1262, 1156, 1107, 1087
6
. Calcd (found): C, 57.97 (57.51); H,
2
8
Crystallogr. 1990, A46, 467.
(
10) Sheldrick, G. M. In Crystallographic Computing 6; Flack, H. D.,
1
Parkanyi, L., Simon, K., Eds.; Oxford University Press: Oxford, U.K.,
-
1
1993; p 111.
cm .
1640 Inorganic Chemistry, Vol. 41, No. 6, 2002

Tetracarboxylato Dimolybdenum Cations
[
Mo
salt of Mo
After elimination of interstitial solvent molecules: Anal. for C64
Mo BF . Calcd (found): C, 60.62 (60.02); H, 7.31 (7.25). IR
KBr): 2963, 2930, 2870, 1701, 1686, 1655, 1637, 1605, 1562,
2
(TiPB)
4
]BF
4
‚2CH
2
Cl
2
, 2‚2CH
2 2 4
Cl . The corresponding BF
2
(TiPB)
4
was prepared similarly, in comparable yield.
92
H -
O
2 8
4
(
1
cm
544, 1460, 1403, 1320, 1292, 1261, 1194, 1156, 1108, 1089, 1052
-
1
.
Mo
and HTiPB (4.977 g, 20.04 mmol) in 25 mL of o-dichlorobenzene
was refluxed under N for 4 days. Upon cooling, copious yellow
2 4 6
(TiPB) , 3. A mixture of Mo(CO) (2.640 g, 10.00 mmol)
2
solid was afforded, which was filtered and recrystallized by slow
diffusion of hexanes into a hot, saturated toluene solution. The yield,
before recrystallization, was essentially quantitative. Crystals suit-
able for X-ray crystallography were prepared by diffusion of
hexanes into a saturated toluene solution of the product. Anal. for
1
C
64
H92Mo
2 8
O
. Calcd (found): C, 65.08 (64.62); H, 7.85 (7.94). H
NMR δ (ppm, in CD
2
Cl
2
): 7.129 (s, 8 H, aromatic), 3.354 (septet,
8
2
2
1
H, o-isopropyl), 2.932 (septet, 4 H, p-isopropyl), 1.272 (doublet,
4 H, methyl), 1.175 (doublet, 48 H, methyl). IR (KBr): 2960,
933, 2870, 1698, 1605, 1564, 1484, 1464, 1411, 1387, 1360, 1318,
Figure 1. Thermal ellipsoid plot of tetrakis(2,4,6-triisopropylbenzoato)-
dimolybdenum(II), 3. Probability ellipsoids are shown at the 50% level.
Hydrogen atoms and disordered phenyl and isopropyl groups of minor
occupancy have been omitted for clarity.
297, 1259, 1242, 1195, 1158, 1105, 1071, 1054 cm-1
.
[
Mo
prepared from a literature procedure, and AgPF
mmol) were dissolved in 20 and 5 mL of CH Cl
Upon addition of the molybdenum solution to the flask containing
the AgPF solution, a green solution and black precipitate (Ag)
2
(O
2
CC
4
H
9
)
4
]PF
6
, 4. Mo
2
(O
2
CC
4
H
9
)
4
(200 mg, 0.335 mmol),
(85.0 mg, 0.335
, respectively.
1
1
6
2
2
The paramagnetic cationic species, [Mo
2 4 6
(TiPB) ]PF , is
deep red, while the BF salt is somewhat more orange in
4
6
color. These complexes are quite soluble and stable in
dichloromethane, but completely insoluble in hexanes, thus
contributing to the ease of preparation and crystallization.
These solids are moderately air stable and decompose after
about an hour in air, when they turn from red to brown.
However, solutions of these complexes are extremely air-
sensitive and lose all color within a couple of minutes in
air. This process has also been observed via UV/vis
spectroscopy, where all transitions in the visible range
disappear after air exposure.
quickly formed. The mixture was stirred for 1 h and then filtered
over Celite. A few green blocks of 4 suitable for X-ray structural
analysis were grown after 24 h from the slow diffusion of hexanes
into the filtrate. Besides the few crystals, copious yellow and brown
solids precipitated out of solution.
Results and Discussion
Compounds 1, 2, and 3 were synthesized by the following
reactions:
CH Cl
2
2
In 3, the four carboxylato groups bridge the quadruply
Mo (TiPB) + NOPF
8 [Mo (TiPB) ]PF + NO (1)
2 4 6
2
4
6
4+
bonded Mo
2
unit, giving the typical paddlewheel arrange-
ment shown in Figure 1. The Mo-Mo distance of 2.076(1)
Å is marginally shorter by ca. 0.02 Å than that of most of
CH Cl
2
2
Mo (TiPB) + NOBF
8 [Mo (TiPB) ]BF + NO (2)
2
4
4
2
4
4
the previously reported quadruply bonded Mo
2
(O
2 4
CR)
∆
, o-Cl C H
2 6 4
1
compounds, and the structure is similar to that of the
2
Mo(CO) + 4TiPBH
8
6
1
2
chromium analogue.
[
Mo (TiPB) ] + 12CO + 2H (3)
2
4
2
For the oxidized species, shown in Figures 2 and 3, the
structure of the cation is generally similar, but the Mo-Mo
separation, 2.1364(8) Å in 1 and 2.1441(5) Å in 2, is
significantly greater than that of the unoxidized starting
material (2.076(1) Å). Thus, removal of one electron, which
reduces the Mo-Mo bond order to 3.5, increases the distance
by about 0.06-0.07 Å relative to that in the parent
compound. The magnitude of the change in going from the
The neutral, quadruply bonded Mo
2
(TiPB)
4
was obtained
in excellent yield by the classical route. It displays a brilliant
canary yellow color typical of other Mo (O CR) compounds.
3 4
However, it is slightly more air sensitive than Mo (O CCH ) ,
and the crystals turn greenish brown after several hours of
exposure to the atmosphere. It is far more soluble in hexanes,
2
2
4
2
2
toluene, ether, and dichloromethane than Mo
Mo (O CC . This increased solubility makes recrystal-
2 2 3 4
(O CCH ) or
2
4
2
2 4
σ π δ to the σ π δ configuration is typical for such a change,
2
2
4 9 4
H )
as may be seen by comparison with those for the pairs,
lization difficult, and crystals can be obtained only when a
highly saturated solution of hot toluene is layered with
hexanes. The solubility of the compound even in hexanes
reduces the yields for the crystalline material; the extreme
solubility of the compound in dichloromethane precludes
crystallization from this solvent.
4
- 13
3- 14
Mo
2
(SO
4
)
4
and Mo
2
(SO
4
)
4
, with Mo-Mo distances
15
of 2.110(2) and 2.164(2) Å, respectively, and for Mo
2
(hpp)
(where hpp is the anion of 1,3,4,6,7,8-
hexahydro-2H-pyrimido[1,2-a]pyrimidine), where the bond
4
+
16
2 4
and [Mo (hpp) ]
(12) Cotton, F. A.; Hillard, E. A.; Murillo, C. A.; Zhou, H.-C. J. Am. Chem.
Soc. 2000, 122, 416.
(13) Angell, C. L.; Cotton, F. A.; Frenz, B. A.; Webb, T. R. J. Chem.
Soc., Chem. Commun. 1973, 399.
(
11) Stephenson, T. A.; Bannister, E.; Wilkinson, G. J. Chem. Soc. 1964,
2538.
Inorganic Chemistry, Vol. 41, No. 6, 2002 1641

Cotton et al.
Figure 4. Cyclic voltammogram of tetrakis(2,4,6-triisopropylbenzoato)-
dimolybdenum, 3, in dichloromethane.
Figure 2. Thermal ellipsoid plot of the tetrakis(2,4,6-triisopropylbenzoato)-
dimolybdenum(II,III) cation and PF6 anion in 1‚2CH2Cl2. Probability
ellipsoids are shown at the 50% level. Hydrogen atoms, solvent of
crystallization, disordered isopropyl groups, and disordered fluorine atoms
of minor occupancy have been omitted for clarity.
higher for 3 in all cases, when the difference in referencing
procedures is taken into account. The cyclic voltammogram
of Mo
, with E1/2 ) +0.24 V vs Ag/AgCl, while measurements
on solutions of Mo (pivalate) in acetonitrile and THF have
1/2 values of +0.38 V vs SCE and +0.86 V vs Ag wire,
respectively. These results contrast with those reported for
2 2 3 4
(O CCH ) in methanol is similar to that of compound
2
0
3
2
4
2
1
22
E
20
2 4
Mo (aspirinate) , where an irreversible one-electron oxida-
tion wave was observed. It is surprising that all of the noted
compounds are more easily oxidized than 3, yet no structures
of the cations have been reported. To determine the reason
for this gap in the literature, we sought to chemically oxidize
another dimolybdenum carboxylate. We chose the pivalato
derivative, Mo
in dichloromethane and the presence of an electrochemically
reversible oxidation wave (∆E°(CH Cl ) ) 0.133 V), from
which we determined E1/2(CH Cl ) ) 0.522 V (vs Ag/AgCl).
Although crystalline samples of 1‚2CH Cl and 2‚2CH Cl
were obtained in moderate yield, according to eqs 1 and 2,
we found that a similar oxidation of Mo (pivalate) provides
2 2 4 9 4
(O CC H ) , due to its favorable solubility
2
2
2
2
Figure 3. Thermal ellipsoid plot of the tetrakis(2,4,6-triisopropylbenzoato)-
dimolybdenum(II,III) cation in 2‚2CH2Cl2. Probability ellipsoids are shown
at the 50% level. Hydrogen atoms, solvent of crystallization, and disordered
tetrafluoroborate anion have been omitted for clarity.
2
2
2
2
2
4
green crystals of 4 only as a very minor product. Copious
yellow and brown solids settle out of the dichloromethane
solution when carefully layered with hexanes. Furthermore,
these crystals are significantly less stable than those of 1‚
distances are 2.067(1) and 2.127 Å, respectively. However,
the change in the Mo-Mo distances is significantly smaller
upon oxidation of Mo
.0839(9)17 to 2.1194(12) (0.0355 Å). Furthermore, the
increase in charge on the Mo core from the loss of one
2
[µ-η-(NPh)
2 4
CNHPh] , being only
18
2
2CH Cl
2 2
2 2
and 2‚2CH Cl and lose crystallinity within a couple
2
of days. After a great deal of effort, a crystal structure was
obtained, Figure 5, with a Mo-Mo distance of 2.1512(5)
Å, 0.063 Å longer than that in the parent compound.23
Unfortunately, the low yield made bulk measurements
impractical. It is also worth mentioning that the tetrabutyrato
electron causes the Mo-O bonds in the cations to contract
by ca. 0.025 Å relative to the neutral parent compound.
2 2
/Mo
5+)
4
+
The one-electron electrochemical oxidation (Mo
of 3 in dichloromethane, ethanol, and acetonitrile exhibits
values for E1/2 of +0.621, +0.448, and +0.462 V (vs Ag/
AgCl), respectively. A representative cyclic voltammogram
is depicted in Figure 4. While the trend is the same as that
1
9
dimolybdenum cation has also been isolated. However,
electrochemical experiments indicated that the cation was
unstable, with a lifetime on the order of 1 min. Very likely
19
5+
reported for solutions of Mo
2
(butyrate)
4
(E1/2 values of
the reason for the increased stability of the Mo
2
unit, when
+
0.45, +0.30, and +0.39 V vs SCE), the E1/2 values are
surrounded by the bulkier 2,4,6-triisopropylbenzoate anions,
(
14) Cotton, F. A.; Frenz, B. A.; Webb, T. R. J. Am. Chem. Soc. 1973, 95,
(20) Carvill, A.; Higgins, P.; McCann, G. M.; Ryan, H.; Shiels, A. J. Chem.
Soc., Dalton Trans. 1989, 2435.
4431.
(
(
15) Cotton, F. A.; Timmons, D. J. Polyhedron 1998, 119, 7889.
16) Cotton, F. A.; Daniels, L. M.; Liu, C. Y.; Murillo, C. A.; Wilkinson,
C. Angew. Chem., Int. Ed., submitted.
(21) Santure, D. J.; Huffman, J. C.; Sattelberger, A. P. Inorg. Chem. 1985,
24, 371.
(22) (a) Cayton, R. H.; Chisholm, M. H.; Darrington, F. D. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 1481. (b) Cayton, R. H.; Chisholm, M. H. J.
Am. Chem. Soc. 1989, 111, 8921. (c) Cayton, R. H.; Chisholm, M.
H.; Huffman, J. C.; Lobkovsky, E. B. Angew. Chem., Int. Ed. Engl.
1991, 30, 862.
(
(
(
17) Bailey, P. J.; Bone, S. F.; Mitchell, L. A.; Parsons, S.; Taylor, K. J.;
Yellowlees, L. J. Inorg. Chem. 1997, 36, 867.
18) Bailey, P. J.; Bone, S. F.; Mitchell, L. A.; Parsons, S.; Taylor, K. J.;
Yellowlees, L. J. Inorg. Chem. 1997, 36, 5420.
19) Cotton, F. A.; Pedersen, E. Inorg. Chem. 1975, 14, 399.
(23) Cotton, F. A.; Extine, M.; Gage, L. D. Inorg. Chem. 1978, 17, 172.
1642 Inorganic Chemistry, Vol. 41, No. 6, 2002

Tetracarboxylato Dimolybdenum Cations
Figure 6. Electron paramagnetic resonance spectrum of 1 (a) and 2 (b) in
Figure 5. Thermal ellipsoid plot of the tetrakis(pivalato)dimolybdenum-
-
4
-1
frozen dichloromethane; g| ) g⊥ ) 1.936, A| ) 35.60 × 10 cm , and
(II,III) cation and PF6 anion in 4. Probability ellipsoids are shown at the
-
4
-1
A⊥ ) 18.20 × 10 cm
.
5
0% level. Hydrogen atoms and disordered tert-butyl groups of minor
occupancy have been omitted for clarity.
susceptibility measurements were carried out on crystalline
samples of 2 using a SQUID magnetometer at 1000 G in
the temperature range 300-2 K. The data for 2 were
corrected for diamagnetic contribution by measuring the
magnetic susceptibility of the neutral parent compound
as compared to the less bulky pivalate or butyrate groups, is
the capacity of the former to isolate the radical dimetal unit
more effectively from the surrounding environment. This
ability to tune the region of stability might be potentially
useful for the exploitation of such units as one-electron
oxidants for organic substrates. This is an area of great
-
1
(-0.0010 emu mol ). Compound 2 displays a linear 1/ø
plot, with the x-intercept very near zero (1.286 K), and a
slope of 0.3474, as expected for a Curie paramagnet with S
2
4
current interest and where further work might prove
rewarding.
1
2
) / . From eq 5, g was calculated to be 1.93, in rough
The X-band (microwave frequency 9.42 GHz) EPR spectra
of 1 and 2 in frozen dichloromethane at 70 K are consistent
agreement with the value extracted from the EPR data.
1
8
2
with a doublet ground state with both g
|
⊥
and g having the
C ) g (S(S + 1))
(5)
same value of 1.936. The spectra are similar to that reported
+
19
2 2
for [Mo (O CC
3 7
H )
4
] , with g
|
) g ) 1.941, and each
⊥
The room temperature electronic spectrum for 1 shows
may be interpreted in terms of the spin Hamiltonian (eq 4),
-
1
-1
-1
three peaks: 550 (ꢀ ) 4700 M cm ), 365 (ꢀ ) 8400 M
1
2
with S ) / and including species with nuclear spin states
-
1
-1
-1
cm ), and 290 nm (ꢀ ) 10900 M cm ). The spectrum
for 2 is quite similar, with peaks at 530 (ꢀ ) 4400 M cm ),
65 (ꢀ ) 8880 M cm ), and 290 nm (ꢀ ) 11100 M
cm ). Compound 3 exhibits a poorly resolved shoulder at
ca. 390 nm (ꢀ ) 13800 M cm ), a peak at 350 nm (ꢀ )
2000 M cm ), and a shoulder at 290 nm (ꢀ ) 11400
1
2
5
I + I ) J ) 0, /
2
, and 5 with a natural abundance of 56%,
-1
-1
25
37.7%, and 6.3%, respectively. We were not able to directly
-
1
-1
-1
3
observe the 11-line pattern arising from the J ) 5 isotopomer,
likely due to poor instrument resolution. It should be pointed
out that this small component is also not discernible in the
simulation, although it has been added to our model at the
appropriate natural abundance. The spin Hamiltonian pa-
rameters for 1 and 2 are listed in the caption to Figure 6 and
-
1
-
1
-1
-
1
-1
2
M
-
1
-1
cm ). While the δ f δ* transition in Mo
2 2
(O CCR)
4
1
complexes typically occurs at ca. 440 nm, this region of
the spectrum is obscured by what is a large, broad, presum-
ably charge transfer band at 350 nm. It is tempting, albeit
speculative, to assign the shoulder at ca. 390 nm to the δ f
δ* transition for two reasons. First, in light of the observation
that axial ligands tend to lengthen the Mo-Mo bond, one
expects the δ f δ* transition to be at higher energy for the
non-axially ligated compound 3. Second, if we assign the
lowest energy peak in 1 and 2 to the δ f δ* transition, the
difference in transition energy between the one-electron and
1
9
are very similar to those reported for Mo
2
2 3 7 4
(O CC H ) .
1
z
2
z
H ) m[g H S + g (H S + H S )] + A S (I + I ) +
|
z
z
⊥
x
x
y
y
1
|
z
2
1
2
A [S (I + I ) + S (I + I )] (4)
⊥
x
x
x
y
y
y
-
It may be noted that for the [Mo
2 4
(HPO )
4
]3 ion the g
|
and
2
6
g
⊥
were distinguishable (1.894, 1.866), but only barely.
The clear evidence of an unpaired electron from EPR
spectroscopy provoked us to further investigate the magnetic
-
1
two-electron systems is ca. 7300 cm , a fair approximation
of the exchange energy of quadruply bonded molybdenum
+
properties of the cation, [Mo
2
(TiPB)
4
] . Molar magnetic
27
24) See for example the review on the use of Fe(CN)63- for the oxidation
compounds. The large molar absorptivity coefficient of the
putative δ f δ* transition may be explained by significant
mixing with the proximal, intense CT transition.
(
of organic substrates: Leal, J. M.; Garcia, B.; Domingo, P. L. Coord.
Chem. ReV. 1998, 173, 79.
25) Cotton, F. A.; Frenz, B. A.; Pedersen, E.; Webb, T. R. Inorg. Chem.
(
1
975, 14, 391.
(
26) Chang, I.-J.; Nocera, D. G. J. Am. Chem. Soc. 1987, 109, 4901.
(27) Cotton, F. A.; Nocera, D. G. Acc. Chem. Res. 2000, 33, 483.
Inorganic Chemistry, Vol. 41, No. 6, 2002 1643

Cotton et al.
The blue shift of the CT band in 3 from the 428 nm peak28
exhibited by Mo (O CC has been attributed to the
twisting of the phenyl rings out of the CO plane in the case
. Likewise, in the centrosym-
metric 3, one phenyl blade is twisted 68°, while the other is
twisted 34° from the carboxylate plane, thus disrupting the
ligand conjugation and destabilizing the acceptor molecular
orbital for a MLCT transition.
reversible oxidation waves in solution, the 2,4,6-triisopro-
pylphenyl carboxylato complexes exhibit far superior stabil-
ity, and the three compounds whose structures are described
here are the first and only ones that have been isolated and
characterized fully.
2
2
6 5 4
H )
2
2
9
2 2 3 6 2 4
of Mo (O C(2,4,6-Me C H ))
Acknowledgment. We thank Dr. Huay-Keng Loke and
Mr. Matthew Vogt for assistance with the EPR spectra, and
Mr. Bradley W. Smucker and Mr. John F. Berry for
assistance with magnetic measurements. We are grateful to
the National Science Foundation for financial support. E.A.H.
also thanks the NSF for a predoctoral fellowship.
Conclusions
The first examples of dimolybdenum tetracarboxylato
cations have been structurally characterized in [Mo
2 4
(TiPB) ]-
PF , [Mo (TiPB) ]BF , and [Mo (O CC ]PF . The crystal
6
2
4
4
2
2
H )
4 9 4
6
structures, EPR spectra, and electronic spectra all indicate
Supporting Information Available: An X-ray crystallographic
file in CIF format for compounds 1‚2CH
2
Cl
2 2 2
, 2‚2CH Cl , 3, and
that the lone electron resides in a metal-based δ orbital. While
+
4, and IR, NMR, and electronic spectra, magnetic susceptibility
traces, cyclic voltammograms, and EPR simulations. This material
is available free of charge via the Internet at http://pubs.acs.org.
other [Mo
2
(O
2
CR)
4
]
complexes have been accessed via
(
28) Chisholm, M. H.; Huffman, J. C.; Iyer, S. S.; Lynn, M. A. Inorg.
Chim. Acta 1996, 243, 283.
(29) San Filippo, J., Jr.; Sniadoch, H. J. Inorg. Chem. 1976, 15, 2209.
IC011079R
1644 Inorganic Chemistry, Vol. 41, No. 6, 2002