Oxidation chemistry of axially protected Mo2 and W2 quadruply bonded compounds

Article abstract of DOI:10.1021/ic901965b

Reported is a facile, high-yielding one-pot synthesis of the quadruply bonded ditungsten (II,II) compound W₂(dpa)₄ (1) (dpa=2,2′-dipyridylamide), which was obtained from W(CO)₆ at high temperature in naphthalene. A similar

Full text of DOI:10.1021/ic901965b

Inorg. Chem. 2009, 48, 11889–11895 11889
DOI: 10.1021/ic901965b
Oxidation Chemistry of Axially Protected Mo2 and W2 Quadruply Bonded
Compounds
Michael Nippe, Eric Victor, and John F. Berry*
Department of Chemistry, University of Wisconsin—Madison, 1101 University Ave. Madison, Wisconsin 53706
Received October 5, 2009
Reported is a facile, high-yielding one-pot synthesis of the quadruply bonded ditungsten (II,II) compound W2(dpa)4 (1)
(dpa=2,20-dipyridylamide), which was obtained from W(CO)6 at high temperature in naphthalene. A similar reaction in
1,2-dichlorobenzene furnished a ditungsten (III, III) species as the major product that was crystallized as
[W2(dpa)3Cl2][BPh4] (3). The [W2(dpa)3Cl2]þ cation is better prepared by oxidation of 1 with SO2Cl2. Compound
1 was characterized by X-ray crystallography and cyclic-voltammetry, and is compared with its earlier reported
molybdenum analogue, Mo2(dpa)4 (2). One-electron oxidation products of 1 and 2, [W2(dpa)4][BPh4] (1BPh4) and
[Mo2(dpa)4][BPh4] (2BPh4), respectively, have also been synthesized. The crystallographically determined
˚
˚
metal-metal distances of 2.23 A and 2.14 A in 1BPh4 and 2BPh4, respectively, are in agreement with metal-metal
bond orders of 3.5. Unlike most previously reported Mo25þ and W25þ compounds, the primary coordination spheres
around the M2-units in 1/1BPh4 and 2/2BPh4 remain unchanged upon one-electron oxidation, because the tridentate
dpa ligand hinders axial coordination of exogenous ligands.
4þ
Introduction
compounds, and (2) W2 compounds are extraordinarily
air-sensitive. For example, the generation of binuclear sys-
tems employing (N,N) donor-ligands is significantly easier
for Mo than for W: Mo2(N,N)4 species are immediately
accessible either by a direct reaction of commercially avail-
able Mo(CO)6,7 or by a ligand substitution reaction of the
easily prepared Mo2(OAc)4 and the desired ligand.8 The
preparation of an analogous W2(N,N)4 compound typically
involves a series of steps starting from comproportionation of
W(CO)6 and WCl6 to produce WCl4, which is further
reduced by an electron-donor such as NaHBEt3 and subse-
quently reacted with a ligand salt to produce the quadruply
bonded target W2(N,N)4.9 Even though one-pot reactions
between W(CO)6 and hydroxy-pyridines in high-boiling
solvents (diglyme) at elevated temperatures have been shown
to produce W2(N,O)4 species,10-12 the analogous reactions
with (N,N)-ligands, such as formamidines, yield either in-
complete substituted species (e.g., W2(N,N)4(CO)2) or fur-
nish compounds with only triply bonded W26þ cores.6 These
Combination of eight d-electrons from two divalent group
VI transition metal ions (Cr2þ, Mo2þ, W2þ) (d4) can result in
the formation of metal-metal quadruple bonds, involving
one σ-, two π-, and one δ-bond.1 These electron rich binu-
4
clear M—M units, which are of general interest because of
their unique electronic and optical properties,2 advance our
understanding of bonding since their oxidation removes
electrons from the metal-metal bonding orbitals, resulting
in longer M-M distances and lower formal bond orders. The
types of chelating ligands currently utilized in quadruply
bonded compounds range from O,O donors such as carboxy-
lates3 to N,O donors such as hydroxypyridinates4 to N,N
donors.5 Compounds containing a W24þ unit are less numer-
ous than their Mo24þ counterparts.6 Two factors account for
the relative scarcity of W24þ compounds: (1) a more involved
4þ
synthetic procedure is necessary for the synthesis of W2
*To whom correspondence should be addressed. E-mail: berry@
chem.wisc.edu.
(1) Cotton, F. A. In Multiple Bonds between Metal Atoms, 3rd ed.; Cotton,
F. A., Murillo, C. A., Walton, R. A:, Eds.; Springer Science and Business Media,
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Inc: New York, 2005.
(10) Cotton, F. A.; Ilsley, W. H.; Kaim, W. Inorg. Chem. 1980, 19, 1450–
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(3) Chisholm, M. H. Dalton Trans. 2003, 3821–3828.
(4) Brown, D. J.; Chisholm, M. H.; Gribble, C. W. Dalton Trans. 2007,
1793–1801.
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(6) Cotton, F. A.; Donahue, J. P.; Hall, M. B.; Murillo, C. A.; Villagran,
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r
2009 American Chemical Society
Published on Web 11/18/2009
pubs.acs.org/IC

11890 Inorganic Chemistry, Vol. 48, No. 24, 2009
Nippe et al.
reactions are further complicated by the inferior thermal
stability of formamidines, which prevents the use of high
reaction temperatures that are necessary for the removal of
carbonyl ligands from tungsten.
We report here the first direct one pot synthesis of a binu-
clear quadruply bonded W(II) compound with (N,N)-donor
ligands, W2(dpa)4 (1) (dpa = 2,20-dipyridylamide) starting
from commercially available W(CO)6 and dpaH in refluxing
naphthalene. Compound 1 is of interest to us as a precursor
for new heterotrimetallic complexes.13-15
a VARIAN CARY 50 Scan UV-visible Spectrophotometer
using quartz cells (path length 1.00 cm). 1H NMR spectra were
recorded on a Bruker ACþ 300 NMR spectrometer.
X-ray Structure Determinations. W2(dpa)4 (1). A purple
needle-shaped crystal of 1 with approximate dimensions
0.04 ꢀ 0.02 ꢀ 0.02 mm was selected under oil under ambient
conditions and attached to the tip of a MiTeGen MicroMount.
The crystal was mounted in a stream of cold nitrogen at 100(2) K
and centered in the X-ray beam of a Bruker SMART APEXII
˚
diffractometer with Cu KR (λ=1.54178 A) radiation. The initial
cell constants were determined through an auto-indexing rou-
tine built into the APEXII program, but were also read into
CELLNOW. The crystal was found to be a two-component twin
with domains related to each other by a 180° rotation around the
b axis. A full sphere of data was collected to a resolution of
Here the redox chemistry of 1 and the analogous dimo-
lybdenum compound Mo2(dpa)4 (2)16 is investigated. Inter-
estingly, the axial positions in 1 and 2 are well protected by
coordination of the pendant pyridine groups of the dpa (vide
infra), which prohibits the binding of exogenous ligands. By
˚
0.82 A. Using the TWINABS routine an HKLF4 file was created
using reflections from both domains. The structure was solved
using the Patterson method and refined by least-squares refine-
ment on F2 followed by difference Fourier synthesis. All hydro-
genatomswereincluded inthe final structurefactorcalculation at
idealized positions and were allowed to ride on the neighboring
atoms with relative isotropic displacement coefficients. The metal
atoms occupy positions along a crystallographic 2-fold axis.
Using the HKLF5 file for all reflections and refining the batch
scale factor (final value 0.42) decreased the R1 further (Table 1).
[W2(dpa)4][BPh4] (1BPh4), [Mo2(dpa)4][BPh4] (2BPh4), and
[W2(dpa)3Cl2][BPh4] (3). Crystals were selected under oil under
ambient conditions and block-(1BPh4 and 2BPh4) or needle-(3)
shaped single crystals were attached to the tip of a nylon loop.
The crystals were mounted in a stream of cold nitrogen at 100(2)
K and centered in the X-ray beam of a Bruker CCD-1000
diffractometer (Mo KR) using a video camera. The data were
successfully indexed by an auto-indexing routine built into the
SMART program.18 The structures were solved using direct
methods and refined by least-squares refinement on F2 followed
by difference Fourier synthesis. All hydrogen atoms were in-
cluded in the final structure factor calculation at idealized
positions and were allowed to ride on the neighboring atoms
with relative isotropic displacement coefficients.
4þ
retaining the primary coordination sphere of the M2 and
M25þ units we can accurately analyze the geometric changes
caused by removal of one electron from the δ-manifold of
1 and 2.
Experimental Section
Materials and Methods. All reactions were carried out under a
dry N2 atmosphere using Schlenk techniques and glovebox
methods. Solvents diethyl ether (Et2O), tetrahydrofuran
(THF), and hexanes were purified using a Vacuum Atmospheres
solvent purification system. Dichloromethane was freshly dis-
tilled under an N2 atmosphere over CaH2 prior to use. W(CO)6
(STREM), naphthalene (Sigma-Aldrich), 1,2-dichloroben-
zene (Sigma-Aldrich) tetrabutylammonium hexafluorophos-
phate ([NBu4][PF6]), tetrabutylammonium tetraphenylborate
([NBu4][BPh4], Sigma-Aldrich), SO2Cl2 (1 M in CH2Cl2 from
Sigma-Aldrich), and I2 (Sigma-Aldrich) were purchased and
used as received. The ligand dpaH (2,20-dipyridylamine, Sigma-
Aldrich) was recrystallized from hot hexanes prior to use.
Mo2(dpa)4 (2) was prepared according to a literature proce-
dure.15 Cyclic voltammograms (CVs) were taken on a BAS
Epsilon-EC instrument using CH2Cl2 solutions with 0.1 M
NBu4PF6 and <1 mM substrate. The electrodes were as follows:
glassy carbon (working), Pt wire (auxiliary), and Ag/AgCl in
CH3CN (reference). The potentials were referenced versus
the ferrocene/ferrocenium redox couple, by externally added
ferrocene. Elemental analysis was carried out by Columbia
Analytical Services (formerly Desert Analytics) in Arizona,
U.S.A. Mass spectrometry data were recorded at the Mass
Spectrometry Facility of the Chemistry Instrument Center of
the University of Wisconsin-Madison. Matrix-assisted laser
desorption/ionization (MALDI) mass spectra were obtained
using a Bruker REFLEX II (Billerica, MA) equipped with a
337 nm laser, a reflectron, delayed extraction, and a time-
of-flight (TOF) analyzer. In the positive ion mode, the accelera-
tion voltage was 25 kV. EPR spectra were recorded at room
temperature on CH2Cl2 solutions using a Bruker EleXsys EPR:
E-500-A console with ER 049SX SuperX Bridge and SuperX
Cavity. The parameters were as follows for 1BPh4/2BPh4:
microwave frequency = 9.834548/9.836937 GHz, microwave
power=0.6325 mW, center Field=3772/3599 G, sweep width=
600 G, modulation amplitude=1 G, modulation frequency=
3 kHz, time constant = 1.28 ms. The IR spectra were taken
on a BRUKER TENSOR 27 using KBr techniques. The
UV-vis spectra were recorded under an atmosphere of N2 on
W2(dpa)4 (1). W(CO)6 (1.03 g, 2.92 mmol) and dpaH (1.00 g,
5.84 mmol) were combined at room temperature with naphtha-
lene (5 g). The flask was placed into a preheated sandbath
(250 °C) and stirred for 2 h. Successive color changes from
colorless to yellow, orange, red, purple brown, and finally blue
were observed. The solution was allowed to cool down to room
temperature, and the resulting solid was subsequently washed
with hot hexanes (2 ꢀ 50 mL, 1 ꢀ 30 mL) and CH2Cl2 (20 mL).
The remaining blue crystalline solid was then kept at 90 °C under
vacuum for 1 h. Yield: 1.21 g (79%). Anal. Calcd for C40H32W2N12
(1): C, 45.82%; H, 3.08%; N, 16.03%. Found: C, 45.74%;
H, 3.15%; N, 15.86%. MALDI-Mass spectrum (100% peak):
m/z=1047.8 [1]þ. 1H NMR (CD2Cl2, ppm): 8.16-8.15 (m, 4 H),
7.53-7.49 (m, 8 H), 6.79-6.77 (m, 4 H). IR (KBr, cm-1): 2360 m,
2341 m, 1596 s, 1523 m, 1458 s, 1438 s, 1363 w, 1340 w, 1313 w,
1278 w, 1242 w, 1149 m, 1016 w, 991 w, 952 w, 910 w, 769 m, 734 w.
X-ray qualtity crystals of 1 were obtained by heating 500 mg
of 1 in naphthalene (4 g) to 200 °C for 15 min in a 100 mL
Schlenk flask. The resulting dark blue solution was slowly
cooled and kept at 130 °C for 10 h after which small needle
shaped crystals, suitable for X-ray crystallography, had formed.
[W2(dpa)4][BPh4] (1BPh4). CH2Cl2 (45 mL) was added to a
mixture of blue 1 (300 mg, 0.29 mmol) and I2 (36 mg, 0.14 mmol)
at room temperature, and the dark purple mixture was stirred
for 40 min. The purple mixture was filtered into a flask contain-
ing dried [NBu4][BPh4] (163 mg, 0.29 mmol), and the filtrate was
(13) Nippe, M.; Berry, J. F. J. Am. Chem. Soc. 2007, 129, 12684–12685.
(14) Nippe, M.; Timmer, G. H.; Berry, J. F. Chem. Commun. 2009, 4357–
4359.
(15) Nippe, M.; Victor, E.; Berry, J. F. Eur. J. Inorg. Chem. 2008, 5569–
5572.
(16) Suen, M. C.; Wu, Y.; Chen, J. D.; Keng, T. C.; Wang, J. C. Inorg.
Chim. Acta 1999, 288, 82–89.
(17) Schubert, E. M. J. Chem. Educ. 1992, 69, 62–64.
(18) SADABS V.2.05, SAINT V.6.22, SHELXTL V.6.10 & SMART
5.622; Bruker-AXS: Madison, WI, 2000-2003.

Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11891
Table 1. Crystallographic Data
1BPh4
(CH2Cl2/hexanes)
1BPh4
(CH2Cl2/Et2O)
2 BPh4
(CH2Cl2/hexanes)
2 BPh4
(CH2Cl2/Et2O)
compound
formula
1
3 2CH2Cl2
3
W2(dpa)4
W2(dpa)4
(B(C6H5)4)
monoclinic
C2/c
20.648(4)
18.568(4)
16.291(3)
W2(dpa)4
(B(C6H5)4)
monoclinic
P21
8.9775(7)
27.614(2)
10.7148(8)
Mo2(dpa)4
(B(C6H5)4)
monoclinic
C2/c
20.596(2)
18.554(2)
16.288(2)
Mo2(dpa)4
(B(C6H5)4)
monoclinic
P21
8.9852(6)
27.653(2)
10.6952(8)
W2(dpa)3Cl2
(B(C6H5)4) 2CH2Cl2
triclinic
3
crystal system
space group
monoclinic
C2/c
18.1386(7)
11.5046(4)
16.6777(6)
P1
˚
a, A
12.892(2)
13.561(2)
16.281(2)
95.344(3)
97.282(3)
106.848(3)
2676.9(7)
2
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
101.348(3)
122.096(3)
90.388(2)
122.106(1)
90.401(1)
3
˚
V, A
3412.2(2)
4
2.041
5219.3
2656.2(4)
2
1.710
5272.5(10)
4
1.501
2657.3(3)
2
1.490
Z
4
1.717
0.0244, 0.0608
F, Mg m-3
R1a, wR2b
(I < 2σ(I))
R1a, wR2b
(all data)
1.784
0.0372, 0.0957
0.0362, 0.0829
0.0205, 0.0530
0.0288, 0.0690
0.0219, 0.0580
0.0551, 0.0913
0.0321, 0.0727
0.0213, 0.0562
0.0383, 0.0752
0.0233, 0.0638
0.0549, 0.1136
P
P
P
P
a R1 = ||Fo| - |Fc||/ |Fo|. b wR2 = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2, w = 1/σ2(Fo2) þ (aP)2 þ bP, where P = [max(0 or Fo2) þ 2(Fc2)]/3.
subsequently layered with Et2O. Crystals of 1BPh4 (P21) suita-
ble for X-ray crystallography were obtained after 1 day. Layer-
ing CH2Cl2 solutions of 1BPh4 with hexanes instead of ether
resulted in monoclinic crystals with spacegroup C2/c.
temperature. The mixture was filtered into a flask containing
dried [NBu4][BPh4] (130 mg, 0.23 mmol), and the filtrate was
subsequently layered with Et2O. A few needle shaped crystals
were obtained after 2 days.
Yield: 240 mg (61%). Anal. Calcd for C64H52W2N12B
(1BPh4): C, 56.20%; H, 3.83%; N, 12.29%. Found: C, 55.37%;
H, 3.97%; N, 12.26%. MALDI-Mass spectrum (100% peak): m/z=
1047.7 [1]þ. IR (KBr, cm-1): 3051 w, 2997 w, 1600 m, 1581 m,
1456 s, 1431 s, 1369 w, 1305 w, 1280 w, 1228 w, 1151 w, 1018 w,
960 w, 848 w, 773 w, 763 w, 732 w, 703 w, 613 w.
MALDI-Mass spectrum (100% peak): m/z=949.8 [W2(dpa)3-
Cl2]þ, 1039.7 [W2(dpa)3ClI]þ, and 1131.8 [W2(dpa)3I2]þ.
[W2(dpa)3Cl2][Cl] (5). The dropwise addition of a 1 M solu-
tion of SO2Cl2 in CH2Cl2 (0.36 mL) to a vigorously stirred blue
mixture of W2(dpa)4 (360 mg, 0.34 mmol) and dried LiCl (120
mg, 2.86 mmol) in CH2Cl2 (30 mL) at room temperature caused
an initial color change to purple and then to brown. After
stirring the mixture for 1 h, tetrahydrofuran (THF, 20 mL)
was added, and the mixture was stirred for 10 h. All solvents
were removed under reduced pressure, and the resulting brown
solid was dried under vacuum. Extraction with CH2Cl2 (30 mL)
yielded a brown solution that was layered with Et2O. Crystals of
[Mo2(dpa)4][BPh4] (2BPh4). CH2Cl2 (45 mL) was added to a
mixture of red 2 (300 mg, 0.34 mmol) and I2 (87 mg, 0.34 mmol)
at room temperature, and the resulting dark brown mixture was
stirred for 40 min. The brown mixture was filtered into a flask
containing dried [NBu4][BPh4] (193 mg, 0.34 mmol). This solu-
tion was layered with Et2O, and crystals of 2BPh4 (P21) suitable
for X-ray crystallography were obtained after 1 day. Layering
CH2Cl2 solutions of 2BPh4 with hexanes instead of ether
resulted in monoclinic crystals with spacegroup C2/c.
5 CH2Cl2 were obtained within 3 days. Yield: 140 mg (38%).
Anal. Calcd for C31H26Cl5N9W2 (5 CH2Cl2): C, 34.81%; H,
2.45%; N, 11.79%. Found: C, 34.81%; H, 2.49%; N, 11.19%.
3
3
Yield: 250 mg (61%). Anal. Calcd for C64H52Mo2N12B
(2BPh4): C, 64.49%; H, 4.40%; N, 14.10%. Found: C, 62.37%;
H, 4.29%; N, 13.39%. MALDI-Mass spectrum (100% peak):
m/z=872 [2]þ. IR (KBr, cm-1): 3037 w, 2995 w, 2362 w, 2339 w,
1598 m, 1581 m, 1458 s, 1433 s, 1367 w, 1305 w, 1280 w, 1228 w,
1153 w, 1143 w, 1016 w, 846 w, 775 w, 763 w, 732 w, 702 w.
[W2(dpa)3Cl2][BPh4] (3). 1,2-Dichlorobenzene (dcb) (20 mL)
was added to W(CO)6 (400 mg, 1.14 mmol) and dpaH (290 mg,
1.71 mmol) in a 100 mL Schlenk flask. The flask was equipped
with a water cooled reflux condenser, placed into a preheated
sand bath (250 °C), and stirred under heavy reflux for 12 h. The
resulting brown mixture was filtered, and the filtrate was dried
under reduced pressure. The remaining brown solid was ex-
tracted with CH2Cl2 (50 mL) and filtered into a flask containing
dried [NBu4][BPh4] (320 mg, 0.57 mmol). Layering the dark
brown solution with Et2O resulted in the formation of a brown
precipitate, that was collected by filtration and recrystallized
two times from CH2Cl2/Et2O yielding small amounts of brown
MALDI-Mass spectrum (100% peak): m/z = 982.6
[W2(dpa)3Cl3]þ, 949.6 [W2(dpa)3Cl2]þ, 912.7 [W2(dpa)3Cl]þ.
1H NMR (CD2Cl2, ppm): 8.42 (d, 2 H), 7.98-7.93 (m, 4 H),
7.82 (t, 2 H), 7.63 (d, 2 H), 7.32-7.23 (m, 6 H), 7.14-7.09 (m, 2),
7.02-6.98 (m, 4 H), 6.83 (t, 2 H). IR (KBr, cm-1): 1604 m, 1590
s, 1559 m, 1481 s, 1457 s, 1443 s, 1375 w, 1349 w, 1265 m, 1127 w,
1151 m, 1111 m, 1025 w, 955 w.
Results and Discussion
Synthesis. The synthesis of W2(dpa)4 (1) is efficiently
accomplished in the reaction of commercially available
W(CO)6 with 2 equiv of dpaH in refluxing naphthalene
(reaction a, Scheme 1). A color change to dark blue
indicates that the reaction is complete.
The low solubility of 1 in common solvents made its
crystallization difficult, but it was possible to obtain
crystals suitable for X-ray crystallography by slow cool-
ing of a solution of 1 in naphthalene.
When the reaction of W(CO)6 and dpaH was carried
out in refluxing 1,2-dichlorobenzene (reaction b,
Scheme 1) instead of naphthalene, the resulting brown
mixture contained both 1 and the binuclear cation
[W2(dpa)3Cl2]þ. These could be separated by filtration
to yield a brown solution. Evaporation of the solvent
yielded a brown powder that was dissolved in CH2Cl2.
Addition of [NBu4][BPh4] to this brown solution and slow
needle shaped crystals of 3 2CH2Cl2 and brown precipitate.
3
It was not possible to separate these very fine needles from
the precipitate to obtain pure enough samples for elemental
analysis. MALDI-Mass spectrum (100% peak): m/z = 915.2
[W2(dpa)3Cl]þ, 950.2 [W2(dpa)3Cl2]þ. 1H NMR (CD2Cl2, ppm):
8.41 (d, 2 H), 7.96 (d, 2 H), 7.80 (m, 6 H), 7.45 (d, 4 H), 7.23 (m,
23 H), 7.09-6.88 (m, 30 H), 6.83-6.73 (m 12 H).
[W2(dpa)3X2][BPh]4 (4, X=Cl, I). CH2Cl2 (25 mL) was added
to solid 1 (210 mg, 0.20 mmol) and I2 (190 mg, 0.75 mmol), and
the greenish-brownish mixture was stirred for 12 h at room

11892 Inorganic Chemistry, Vol. 48, No. 24, 2009
Nippe et al.
Scheme 1. (a) T ∼ 220 °C; (b) T ∼ 180 °C; (c) 3 I2, [NBu4][BPh4], 12 h;
(d) 0.5 I2, [NBu4][BPh4], 40 min
Figure 1. Molecular structures of 1 (left) and the [W2(dpa)4]þ cation in
1BPh4 (right) with displacement ellipsoids drawn at the 30% probability
level. Hydrogen atoms have been omitted for clarity.
W
2.755(5) A, while the W Npy(l) distances to the other W
Npy(l) distances on one end of the molecule are both
˚
˚
atom are 3.109(5) A. As a possible result of these weak
3 3 3
3 3 3
solvent diffusion of Et2O yielded brown crystals of
[W2(dpa)3Cl2][BPh4] (3). There are two possible pathways
for the formation of 3 in this reaction: (1) the competitive
direct synthesis of 3 over 1 and (2) the oxidation of
previously formed 1 and concomitant loss of one dpa
ligand to form 3. To distinguish between these pathways,
pure 1 (generated from reaction a) was refluxed in
1,2-dichlorobenzene, and the formation of [W2(dpa)3Cl2]þ
was observed by MALDI-TOF MS. We further investi-
gated the two-electron oxidation of 1 by addition of
excess I2 to 1 in CH2Cl2 (reaction c). From this reaction
mixture a small amount of crystalline material of
[W2(dpa)3X2][BPh4] (4) was obtained, in which X is a
mixture of Cl- and I-. The presence of [W2(dpa)3Cl2]þ,
[W2(dpa)3ClI]þ, and [W2(dpa)3I2]þ species in 4 was also
verified by mass spectrometry, which strongly suggests
the involvement of CH2Cl2 as a chloride source. Having
established that formation of 3 during reaction (b) in-
volves oxidation of 1, the use of 1,2-dichlorobenzene as a
solvent in the preparation of 1 is disfavored and therefore
reaction (a) is the preferred method to cleanly obtain 1.
Our optimized reaction conditions for the preparation of
the [W2(dpa)3Cl2]þ cation are as follows: Stoichiometric
oxidation of 1 with SO2Cl2 in CH2Cl2 in the presence of
LiCl yielded the desired ditungsten (III, III) species as the
chloride salt [W2(dpa)3Cl2]Cl (5) in high purity.
˚
axial interactions, the WtW bond length is about 0.03 A
longer than in W2(hpp)4 (hpp=the anion of 1,3,4,6,7,8-
hexahydro-2H-pyrimido[1,2-a]pyrimidine)20 and in the
closely related W2(PhNpy)4 (PhNpy = 2-anilinopyridi-
nate).21 The M-Na and M-Npy(s) distances are shorter
than in the molybdenum analogue of 1, Mo2(dpa)4 (2), by
˚
∼0.03 and 0.04 A, respectively (Table 2).
The above-mentioned weak coordination of the pen-
dant pyridine rings to the axial regions of the M2 unit
proffers 1 and 2 as ideal compounds to study structural
changes upon oxidation of M24þ to M25þ without altering
the primary coordination sphere around the metals, as
these axial sites are protected against coordination of
exogenous ligands. The M-M distances in 1BPh4 and
2BPh4 (Figure 2) are longer than in 1 and 2 by 0.033 and
˚
0.046 A, respectively, which is consistent with the removal
of one electron from the highest metal-metal bonding
orbital (δ), resulting in a formal bond order of 3.5.
Interestingly, the M-Na bond lengths decrease upon
˚
oxidation by 0.03 and 0.06 A for W and Mo, whereas
˚
the M-Npy(s) bonds increase by 0.025 A for W and do
not significantly change in the case of Mo. The most
dramatic change, however, is observed for the M Npy-
3 3 3
(l) distances: The sum of all M Npy(l) distances (Table
3 3 3
˚
3 and 4) is 0.57 A (P21)/ 0.62 A (C2/c), and 0.60 A (P21)/
˚
˚
Treatment of 1 in CH2Cl2 with a stoichiometric amount
of iodine furnished a one-electron oxidized species which
could be isolated and crystallized as [W2(dpa)4][BPh4]
(1BPh4). The same procedure was also successful when
applied to the molybdenum analogue of 1, Mo2(dpa)4
(2),16 furnishing [Mo2(dpa)4][BPh4] (2BPh4).
˚
0.70 A (C2/c) shorter in 1BPh4 and 2BPh4, respectively,
than in their quadruply bonded parent complexes. The
Npy(l) shortening could clearly indicate enhanced
M
3 3 3
interactions between the Npy(l) atoms and the M2 units.
In detail, the predominant interaction is anticipated to be
increased electron donation from the Npy(l) lone pair into
the metal dxz and dyz orbitals, thereby increasing the
population of metal-metal π* orbitals and elongating
the M-M bond length. Quantitative evaluation of this
effect can be made using the direction angle, defined by
Crystal Structures. The crystal structure of 1 is shown
in Figure 1, and relevant bond distances are given in Table
2. Each of the four dpa ligands is bonded via one amido N
atom (Na) and one pyridine N atom (Npy(s)) with inter-
estingly equal averaged metal-ligand distances of
Cotton as the M-Na-C-Npy(l) torsion angle.19 For
P
˚
2.132[5] A. Additionally the N atoms of the pendant
small values of the direction angle ( < 20°), the orienta-
tion of Npy(l) is optimal to allow for π-type interactions
with M2. The sum of the direction angles in 1 and 2 are
106.8[5]° and 104.2[5]°, respectively, very similar with
pyridine rings (Npy(l)) are weakly coordinated to the
W2-unit in a manner consistent with the structures of
Cr2(dpa)4,19 Mo2(dpa)4,16 and WMo(dpa)4.14 In 1, the
(19) (a) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Pascual, I.; Zhou,
H.-C. J. Am. Chem. Soc. 1999, 121, 6856–6861. (b) Edema, J. J. H.;
Gambarotta, S.; Meetswa, A.; Spek, A. L.; Smeets, W. J. J.; Chiang, M. Y. J.
Chem. Soc., Dalton Trans. 1993, 789.
(20) Cotton, F. A.; Huang, P.; Murillo, C. A.; Timmons, D. J. Inorg.
Chem. Commun. 2002, 5, 501–504.
(21) Chakravarty, A. R.; Cotton, F. A.; Shamshoum, E. S. Inorg. Chem.
1984, 23, 4216–4221.

Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11893
Table 2. Selected Interatomic Distances in 1, 2,16 1BPh4, and 2BPh4
1BPh4
2BPh4
1
C2/c
P21
2
C2/c
P21
˚
M-M, A
2.1934(4)
2.132[5]
2.132[5]
2.775(5)
3.109(5)
2.2275(5)
2.1465[3]
2.102[3]
2.713(3)
2.861(3)
2.2259(2)
2.157[3]
2.101[3]
2.780(3), 2.797(3)
2.619(3), 2.998(3)
2.097(2)
2.178[4]
2.167[4]
2.786(5)
3.155(4)
2.1423(4)
2.167[2]
2.114[2]
2.720(2)
2.868(2)
2.1429(2)
2.176[2]
2.108[2]
2.808(2), 2.815(2)
2.630(2), 3.029(2)
˚
M-Npy(s), A
˚
M-Na(s), A
˚
M1 Npy(l), A
3 3 3
˚
M2 Npy(l), A
3 3 3
6þ
25,26
˚
W2 species (∼2.4 A),
and could suggest a bond
order lower than 3 with strong antiferromagnetic cou-
pling between the two W atoms. This bonding scheme
finds precedence in the diamagnetic compounds (tBuO)4-
W2(μ-PPh2)2 (d(W-W) = 2.59 A) and (Me2N)4W2-
(μ-PCy2)2 (d(W-W)=2.57 A).
27
˚
28
˚
Electrochemistry and Spectroscopy. The significantly
higher solubilities of species 1BPh4 and 2BPh4 as com-
pared to their quadruply bonded parents made it possible
to investigate their redox properties by means of cyclic
Figure 2. Molecular structure of 2BPh4 with displacement ellipsoids
drawn at the 30% probability level. Hydrogen atoms have been omitted
for clarity.
voltammetry (CV) (Figure 4 and 5). In both CVs we
observe a reversible M2
4þ/5þ
redox couple at -1193 mV
for 1BPh4 and at -832 mV for 2BPh4 (potentials are
referencedversus the ferrocene/ferrocenium redox couple).
Interestingly, the MoW4þ/5þ couple in the recently pub-
lished heterodimetallic compound MoW(dpa)414 appears
at a potential that lies between those reported here (-1043
mV). Compounds 1BPh4 and 2BPh4 experience a second
those reported for Cr2(dpa)4 (104[3]°), and indicate rather
weak π* interactions.19 Upon oxidation, even though all
the M Npy(l) distances shorten, the direction angles
3 3 3
do not change significantly. Therefore, no significant
increase in π* interactions is expected, and the strong
decrease in M Npy(l) distances may be solely attributed
oxidation at -578 mV and -44 mV, respectively, which is
3 3 3
5þ/6þ
to electrostatic interactions and the decrease of the metal
d-orbital radii upon oxidation.
We also investigated the molecular structure of 3 by
assigned to the M2
oxidation. This electron transfer
appears to be quasi-reversible for 1BPh4 (but not for
2BPh4) depending on the potential range.
6þ
X-ray crystallography. The W2 unit is surrounded by
To interrogate the electronic structure of 1BPh4 and
2BPh4 further, X-band EPR spectra of CH2Cl2 solutions
of these compounds were measured at room temperature
(Figure 6). The spectra were simulated assuming complete
delocalization of the single electron over both metal atoms
and taking into account the natural abundance of 14.31%
for isotope 183W (I = 1/2) for 1BPh4 and a combined
abundancy of 25.47% for isotopes 95Mo and 97Mo (I =
5/2) in the case of 2BPh4. Compounds 1BPh4 and 2BPh4
display isotropic signals with giso=1.863 (Aiso=95 MHz)
and giso=1.951 (Aiso=56 MHz), repectively. The g-values
are very similar to those observed for [W2(O2CtBu)4)][PF6]
(giso=1.814, Aiso=51 G) and [Mo2(O2CtBu)4)][PF6] (giso=
1.941, Aiso = 27 G),29,30 and the fact that they are both
lower than the free-electron g value agrees with the ex-
pectation that the compounds have a less than half-filled
valence d shell.
two Cl- ions and three ligating dpa ligands. Two of the
dpa moieties adopt a chelating/bridging binding mode in
which one W atom is chelated by one pyridine N atom
and the central amide N atom and the other pyridine N
atom binds to the second W atom (Figure 3, Table 5).
Interestingly, one of the dpa moieties in 3 shows the rare
“doubly chelating/bridging”22 coordination mode23,24
that has so far never been reported for any metal-metal
bonded compound. The molecule has overall C2 symme-
try with the C2 axis passing through the central N atom of
the doubly chelating/bridging ligand and the midpoint
between the two W atoms. The coordination geometry of
the W atoms is best described as capped octahedral. The
overall charge suggests a formal oxidation state of þ3 for
both tungsten ions. Additionally, the W-W distance of
˚
˚
2.5310(7) A is more than 0.30 A longer than in 1BPh4, as a
result of the lower bond order in 3 as compared to 1 and
1BPh4 and the loss of one chelating dpa ligand. The
Upon M24þ/5þ oxidation characteristic changes in the
electronic absorption spectra are expected. The 1(δfδ*)
transition in quadruply bonded compounds is often very
weak and obscured by stronger charge-transfer bands, as
1
diamagnetism of 3 has been established by H-NMR
spectroscopy,17 and would be consistent with a W-W
triple bond. However, the W-W distance in 3 is signifi-
cantly longer than usually observed for triply bonded
2
in the case of 1 and 2 (Figure 7). However, the (δfδ*)
transitions in M2 compounds is commonly encoun-
5þ
(22) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Zhou, H.-C. Inorg.
Chim. Acta 2000, 305, 69–74.
(27) Buhro, W. E.; Chisholm, M. H.; Folting, K.; Eichhorn, B. W.;
Huffman, J. C. J. Chem. Soc, Chem. Commun. 1987, 845–847.
(28) Buhro, W. E.; Chisholm, M. H.; Folting, K.; Huffman, J. C.; Martin,
J. D.; Streib, W. E. J. Am. Chem. Soc. 1988, 110, 6563–6565.
(29) Chisholm, M. H.; D’Acchioli, J. S.; Pate, B. D.; Patmore, N. J.;
Dalal, N. S.; Zipse, D. J. Inorg. Chem. 2005, 44, 1061–1067.
(30) Cotton, F. A.; Daniels, L. M.; Hillard, E. A.; Murillo, C. A. Inorg.
Chem. 2002, 41, 1639–1644.
(23) Berry, J. F.; Cotton, F. A.; Murillo, C. A. Inorg. Chim. Acta 2004,
357, 3847–3853.
€
(24) Muller-Buschbaum, K.; Quitmann, C. C. Inorg. Chem. 2006, 45,
2678–2687.
(25) Chisholm, M. H. Acc. Chem. Res. 1990, 23, 419–425.
(26) Manke, D. R.; Loh, Z.-H.; Nocera, D. G. Inorg. Chem. 2004, 43,
3618–3624.

11894 Inorganic Chemistry, Vol. 48, No. 24, 2009
Nippe et al.
Table 3. Selected Structural Parameters for the Ditungsten Compounds 1 and 1BPh4
1
1BPh4 (C2/c)
1BPH4 (P21)
direction angle, deg
˚
Npy(l), A
˚
Npy(l), A
˚
Npy(l), A
direction angle, deg
W
direction angle, deg
W
W
3 3 3
3 3 3
3 3 3
12.9(5)
12.9(5)
40.5(5)
2.775(5)
2.775(5)
3.109(5)
3.109(5)
11.768[5]
29.0(3)
29.0(3)
9.9(3)
9.9(3)
77.8[3]
2.861(3)
2.861(3)
2.713(3)
2.713(3)
11.148[3]
27.5(3)
29.3(3)
30.0(3)
13.7(3)
100.5[3]
2.797(3)
2.780(3)
2.998(3)
2.619(3)
11.194[3]
40.5(5)
P
106.8[5]
Table 4. Selected Structural Parameters for the Dimolybdenum Compounds 2 and 2BPh4
2
2BPh4 (C2/c)
2BPH4 (P21)
˚
Mo Npy(l), A
3 3 3
˚
Mo Npy(l), A
˚
Mo Npy(l), A
direction angle, deg
direction angle, deg
direction angle, deg
3 3 3
3 3 3
11.6(5)
11.6(5)
40.5(4)
2.786(5)
2.786(5)
3.155(4)
3.155(4)
11.882[5]
28.6(2)
28.6(2)
9.9(2)
9.9(2)
77.0[2]
2.868(2)
2.868(2)
2.720(2)
2.720(2)
11.176[2]
29.9(2)
27.7(2)
12.9(2)
30.3(2)
100.8[2]
2.808(2)
2.815(2)
2.630(2)
3.029(2)
11.282[2]
40.5(4)
P
104.2[5]
Figure 5. CV of 2BPh4 in CH2Cl2 at several potential ranges.
Figure 3. Molecular structure of [W2(dpa)3Cl2]þ from 3 with displace-
ment ellipsoids drawn at the 30% probability level. Hydrogen atoms and
the BPh4- counterion have been omitted for clarity.
Table 5. Selected Interatomic Distances in the [W2(dpa)3Cl2]þ Cation of 3 in
3 2CH2Cl2
3
˚
˚
X
d(W1-X), A
X
d(W2-X), A
W2 2.5348(4)
Cl 2.445(1)
Cl 2.407(2)
N3 2.186(4)
N2 2.128(5)
N9 2.190(5)
N8 2.095(5)
N6 2.152(4)
N1 2.184(5)
N2 2.136(5)
N4 2.211(5)
N5 2.107(4)
N7 2.151(5)
Figure 6. Experimental and simulated EPR spectra of solutions of
1BPh4 (black) and 2BPh4 (red) in CH2Cl2 measured at room temperature.
Figure 7. Electronic absorption spectra of solutions of 1 (black),
1BPh4 (blue), 2 (red), and 2BPh4 (green) in CH2Cl2 measured at room
temperature.
Figure 4. CV of 1BPh4 in CH2Cl2 at several potential ranges.

Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11895
tered at lower energy.31 For CH2Cl2 solutions of 1BPh4
and 2BPh4 we observe bands at 781 and 801 nm, which we
assign to the 2(δfδ*) transitions, and which are slightly
lower in energy than the 2(δfδ*) transition reported for
M2(O2CtBu)4þ species (760 nm).29
2BPh4 the shortest Mo-Mo distance for Mo2 com-
pounds with bridging (N,N) donor ligands. Structural and
5þ
5þ
spectroscopic evidence supports the M2 assignment of
these complexes with an unpaired electron in the M-M
δ orbital and an overall M-M bond order of 3.5.
Summary. In summary, the quadruply bonded com-
pound 1 may be prepared directly from W(CO)6 and dpaH
in refluxingnaphthalene. This isa remarkableobservation,
since preparation of related W2(N,N)4 compounds is
usually quite involved. The choice of solvent for the
reaction is critical, as we found that using 1,2-dichloro-
benzene furnishes the cationic binuclear W(III) species 3.
Acknowledgment. We thank the National Science
Foundation for support under CHE-0745500 and CHE-
0741901 (Bruker EleXsys EPR) and CHE-9208463
(Bruker AC-300). We thank Ilia Guzei for assistance in
the refinement of the crystal structure of 4.
4þ
The W2 unit in 1 is protected against axial coordi-
Note Added after ASAP Publication. This article was
released ASAP on November 18, 2009, with footnote 30
missing. The correct version was posted on November 19,
2009.
nation by the pendant pyridine rings of the dpa ligand.
We therefore prepared the one-electron oxidized species
1BPh4 and its molybdenum analogue 2BPh4, in which the
5þ
M2 units retained their primary coordination sphere.
As a possible result of this encapsulation we found in
Supporting Information Available: Additional information as
noted in the text. This material is available free of charge via the
Internet at http://pubs.acs.org.
(31) Hopkins, M. D.; Gray, H. B. Polyhedron 1987, 6, 705–714.