New chemistry of the triply bonded divanadium (V24+) unit and reduction to an unprecedented V23+ core

Article abstract of DOI:10.1021/ic034646e

We report here the synthesis and characterization of seven new divanadium compounds with the paddlewheel geometry bridged by nitrogen-donating ligands. Five of these contain the diamagnetic V₂⁴⁺ core with short V-V distances of less

Full text of DOI:10.1021/ic034646e

Inorg. Chem. 2003, 42, 6063−6070
New Chemistry of the Triply Bonded Divanadium (V 4+) Unit and
2
Reduction to an Unprecedented V 3+ Core
2
F. Albert Cotton,* Elizabeth A. Hillard, Carlos A. Murillo,* and Xiaoping Wang
Department of Chemistry and Laboratory for Molecular Structure and Bonding, P.O. Box 30012,
Texas A&M UniVersity, College Station, Texas 77842-3012
Received June 10, 2003
We report here the synthesis and characterization of seven new divanadium compounds with the paddlewheel
geometry bridged by nitrogen-donating ligands. Five of these contain the diamagnetic V 4+ core with short V−V
2
distances of less than 2.0 Å, consistent with a formal triple bond. The V−V distances vary with the basicity of the
bridging ligands; more basic ligands such as those of the guanidinate type have the shorter metal−metal separations,
and those with the less basic formamidinate groups have longer separations. One compound, V (DPhF)4 (DPhF )
2
the anion of N,N′-diphenylformamidine), has been reduced by one electron, and two structures, [K(THF)3]V (DPhF)4
2
and [K(18-crown-6)(THF)2]V (DPhF)4, have been obtained. These have an unprecedented V 3+ core where the
2
2
formal oxidation state of each vanadium atom is +1.5. The decrease in V−V bond distance and the multiline EPR
spectrum in the reduced species provide evidence that these two molecules contain a bond of order 3.5.
Introduction
Although it was in 1964 that the quadruply bonded Re2
by the ability of two Cr(II) atoms to come together to form
a high-multiplicity, quadruple metal-metal bond. In much
the same way, triply bonded divanadium compounds might
be expected to be thermodynamically stable. Of course, this
would also be true for the heavier group 5 metals, niobium
and tantalum. However, the +2 ions of these metals have
been shown to cleave C-N bonds in formamidinato ligands,9
6+
unit (in Re2Cl82-) was recognized1 and in 1965 that the exist-
5+
4+
ence of Tc2 (in Tc2Cl83-) and Mo2 (in Mo2(O2CCH3)4)
was established,2,3 it was not until 1992 that the first
compound of the triply bonded V2 core, V2(DTolF)4
4+
(DTolF ) the anion of N,N′-di-p-tolylformamidine, which
is shown in Figure 1a), was reported.4 In the meantime
thousands of compounds containing metal-metal bonds have
been synthesized and characterized, and have found applica-
tions in catalysis,5 medicine,6 and supramolecular chemistry.7
These molecules are, for the most part, composed of second-
and third-row transition metals8 as first-row metals are less
prone to support metal-metal bonds. The exception is
chromium, and its uniqueness in this regard may be explained
(6) (a) Asara, J. M.; Hess, J. S.; Lozada, E.; Dunbar, K. R.; Allison, J. J.
Am. Chem. Soc. 2000, 122, 8. (b) Catalan, K. V.; Hess J. S.; Maloney,
M. M.; Mindiola, D. J.; Ward, D. L.; Dunbar, K. R. Inorg. Chem.
1999, 38, 3904. (c) Dale, L. D.; Dyson, T. M.; Tocher, D. A.; Tocher,
J. H.; Edwards, D. I. Anti-Cancer Drug Des. 1989, 4, 295. (d) Aoki,
K.; Yamazaki, H. J. Chem. Soc., Chem. Commun. 1980, 186. (e)
Howard, R. A.; Kimball, A. P.; Bear, J. L. Cancer Res. 1979, 39,
2568. (f) Fu, P.; Bradley, P. M.; Turro, C. Inorg. Chem. 2001, 40,
2476. (g) Aoki, K.; Salam, M. A. Inorg. Chim. Acta 2001, 316, 50.
(7) (a) Cotton, F. A.; Daniels, L. M.; Lin, C.; Murillo, C. A. J. Am. Chem.
Soc. 1999, 121, 4538. (b) Cotton, F. A.; Lin, C.; Murillo, C. A. Chem.
Commun. 2001, 11. (c) Cotton, F. A.; Lin, C.; Murillo, C. A. J. Chem.
Soc., Dalton Trans. 2001, 499. (d) Cotton, F. A.; Daniels, L. M.; Lin,
C.; Murillo, C. A.; Yu, S.-Y. J. Chem. Soc., Dalton Trans. 2001, 502.
(e) Bonar-Law, R. P.; McGrath, T. D.; Singh, N.; Bickley, J. F.;
Steiner, A. Chem. Commun. 1999, 2457. (f) Bickley, J. F.; Bonar-
Law, R. P.; Femoni, C.; MacLean, E. J.; Steiner, A.; Teat, S. J. J.
Chem. Soc., Dalton Trans. 2000, 4025. (g) Bonar-Law, R. P.; Bickley,
J. F.; Femoni, C.; Steiner, A. J. Chem. Soc., Dalton Trans. 2000, 4244.
(h) Bonar-Law, R. P.; McGrath, T. D.; Singh, N.; Bickley, J. F.;
Femoni, C.; Steiner, A. J. Chem. Soc., Dalton Trans. 2000, 4343. (i)
Bonar-Law, R. P.; McGrath, T. D.; Bickley, J. F.; Femoni, C.; Steiner,
A. Inorg. Chem. Commun. 2001, 4, 16.
* To whom correspondence should be addressed: cotton@tamu.edu
(F.A.C.); murillo@tamu.edu (C.A.M.).
(1) Cotton, F. A.; Curtis, N. F.; Harris, C. B.; Johnson, B. F. G.; Lippard,
S. J.; Mague, J. T.; Robinson, W. T.; Wood, J. S. Science 1964, 145,
1305.
(2) Cotton, F. A.; Bratton, W. K. J. Am. Chem. Soc. 1965, 87, 921.
(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) Cotton, F. A.; Daniels, L. M.; Murillo, C. A. Angew. Chem., Int. Ed.
Engl. 1992, 31, 737.
(5) See for example: (a) Doyle, M. P.; Ren, T. Prog. Inorg. Chem. 2001,
49, 113. (b) Estevan, F.; Herbst, K.; Lahuerta, P.; Barberis, M.; Pe´rez-
Prieto, J. Organometallics 2001, 20, 950. (c) Catalysis by Di- and
Polynuclear Metal Cluster Complexes; Adams, R. D., Cotton, F. A.,
Eds.; Wiley-VCH: New York, 1998.
(8) Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms,
2nd ed.; Oxford University Press: Oxford, 1993.
(9) See for example: Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Wang,
X. Inorg. Chem. 1997, 36, 896.
10.1021/ic034646e CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/21/2003
Inorganic Chemistry, Vol. 42, No. 19, 2003 6063

Cotton et al.
the chemistry of multiply bonded divanadium compounds
is quite limited. Presented here is a report of work which
shows that this impression is incorrect. We have found that
many other V24+-containing compounds can be made, and
include in this report the characterization of V2(DPhF)4, 1,
V2(DAniF)4, 2, V2(DClPhF)4, 3, V2(TPG)4, 4, and V2(ap)4,
5, the ligands of which are depicted in Figure 1.
We have investigated the electrochemistry of these mol-
ecules and have been rewarded with the observation of
reversible (or at least quasi-reversible) reduction waves in
all cases. This has led to the isolation of the first stable
3+
tetragonal paddlewheel complex with a rare M2 core in
[K(THF)3]V2(DPhF)4, 6, in which the potassium ions lie
between the formamidinate anions as reported in our
preliminary communication.17 Here we also report the
structure of a more stable analogue in which the K+ ions
are trapped by a crown ether, namely, [K(18-crown-6)(THF)2]-
n+
V2(DPhF)4, 7. Oxidation states for isolated M2 units had
been previously restricted to only three values, namely n )
4, 5, and 6, although the oxidation number 7 was recently
discovered for M ) Os18 and Re19 in complexes of the type
[M2(hpp)4Cl2]PF6. While very high oxidation numbers are
not uncommon in transition metal chemistry, it has been
thought that these would not favor metal-metal bond
formation due to the known contraction experienced by the
d orbitals as the positive charge increases. On the other hand,
oxidation numbers of less than 2 are uncommon for transition
metals in general, except in the case of the coinage metals
or when noninnocent ligands, i.e., those with π-acceptors,
are present.20 Therefore the average oxidation state of +1.5
Figure 1. Precursors of the ligands that have been used to make
divanadium compounds: (a) N,N′-di-p-tolylformamidine, HDTolF; (b)
1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, Hhpp; (c) N,N′-di-
cyclohexylformamidine, HDCyF; (d) N,N′-diphenylformamidine, HDPhF;
(e) N,N′-di-p-anisylformamidine, HDAniF; (f) N,N′-di-p-chloroformamidine,
HDClPhF; (g) 1,2,3-triphenylguanidine, HTPG; (h) anilinopyridine, Hap.
and compounds with a metal-metal bond have only been
obtained in the case of ligands with fused-ring systems such
as hpp10,11 (the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido-
[1,2-a]pyrimidine, which is shown in Figure 1b) and 7-aza-
indole11,12 which resist cleavage. The +2 oxidation state is
more accessible for vanadium than for the heavier congeners
niobium or tantalum and, therefore, might be expected to
yield more compounds containing triple metal-metal bonds.
Indeed, theoretical calculations as early as 1985 suggested
that V2(carboxylato)4 compounds should be stable,13 but thus
far these molecules have eluded all synthethic efforts, and
only compounds containing the oxo-centered V3O(carbox-
3+
in these unique V2 cores is highly unusual.
Experimental Section
General Considerations. All syntheses and sample manipula-
tions were carried out under an atmosphere of dry and deoxygenated
argon with standard Schlenk and drybox techniques. Solvents were
distilled under nitrogen from Na/K-benzophenone. VCl3‚3THF was
prepared according to the literature method21 and stored at -35 °C
prior to use to prevent loss of THF. Di-p-anisylformamidine and
di-p-chlorophenylformamidine were prepared by a reported method.22
Methyllithium (1.6 M in diethyl ether) was purchased from Acros
Organics. Sodium triethylborohydride (1 M in THF), diphenyl-
formamidine, triethyl orthoformate, p-anisidine, p-chloroaniline,
2-anilinopyridine, and 18-crown-6 were purchased from Aldrich
Chemical Co.; 1,2,3-triphenylguanidine was purchased from TCI
America. Potassium graphite was prepared by combining an 8:1
molar ratio of graphite and K, and stirring under argon at 130 °C
until the solid turned bronze in color. Then it was stored at -35
°C in a drybox until it was used.
n+
ylate)6 species, n ) 0, 1, are known.14
Nonetheless, in the 10 years since the report of V2(DTolF)4,
the chemistry of the V24+ core has languished. Only two other
4+
15
compounds of V2 have been described, V2(DCyF)4
(DCyF ) N,N′-dicyclohexylformamidinate, which is shown
in Figure 1c) and V2(hpp)4.16 Both of these compounds are
similar to the first one, but very little has been learned about
the properties of these compounds (e.g., spectra, electro-
chemistry), and thus the general impression has arisen that
(10) Cotton, F. A.; Matonic, J. H.; Murillo, C. A. J. Am. Chem. Soc. 1997,
119, 7889.
(11) Cotton, F. A.; Matonic, J. H.; Murillo, C. A. J. Am. Chem. Soc. 1998,
120, 6047.
Physical Measurements. Elemental analyses were performed
by Canadian Microanalytical Service, Ltd., Delta, British Columbia,
(12) (a) Tayebani, M.; Feghali, K.; Gambarotta, S.; Yap, G. P. A. Inorg.
Chem. 2001, 40, 1399. (b) Tayebani, M.; Feghali, K.; Gambarotta,
S.; Yap, G. P. A.; Thompson, L. K. Angew. Chem., Int. Ed. 1999, 38,
3659.
(13) Cotton, F. A.; Diebold, M. P.; Shim, I. Inorg. Chem. 1985, 24, 1510.
(14) See for example: Cotton, F. A.; Extine, M. W.; Falvello, L. R.; Lewis,
D. B.; Lewis, G. E.; Murillo, C. A.; Schwotzer, W.; Toma´s, M. Inorg.
Chem. 1986, 25, 3505.
(17) Cotton, F. A.; Hillard, E. A.; Murillo, C. A. J. Am. Chem. Soc. 2003,
125, 2026.
(18) Cotton, F. A.; Dalal, N. S.; Huang, P.; Murillo, C. A.; Stowe, A. C.;
Wang, X. Inorg. Chem. 2003, 42, 670.
(19) Berry, J. F.; Cotton, F. A.; Huang, P.; Murillo, C. A., J. Chem. Soc.,
Dalton Trans. 2003, 1218.
(20) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; John Wiley & Sons: New York, 1999.
(21) Manzer, L. Inorg. Synth. 1982, 21, 135.
(15) Berno, P.; Shoukang, H.; Minhas, R.; Gambarotta, S. J. Am. Chem.
Soc. 1994, 116, 7417.
(16) Cotton, F. A.; Timmons, D. J. Polyhedron 1998, 17, 179.
(22) Bradley, W.; Wright, I. J. Chem. Soc. 1956, 640.
6064 Inorganic Chemistry, Vol. 42, No. 19, 2003

New Chemistry of the Triply Bonded DiWanadium (V24+) Unit
1
Canada. H NMR spectra were recorded on a Mercury 300 NMR
Electronic Spectroscopy. λmax (nm) (ꢀ/M-1 cm-1) in THF for
1: 464 (90, sh), 398 (1700). For 2: 488 (300), 414 (540). For 3:
455 (50, sh), 405 (1600). For 4: 588 (730). For 5: 533 (950), 411
(2500). For the formamidinato compounds there was also a very
weak shoulder around 560 nm, which was not discernible in the
spectrum of V2(ap)4 or V2(TPG)4.
spectrometer. The cyclic voltammograms were recorded on a BAS
100 electrochemical analyzer in THF solutions containing 0.1 M
Bun NPF6 with Pt working and auxiliary electrodes and a Ag/AgCl
reference electrode; scan rates were 100 mV s-1 in all cases. The
EPR spectra were recorded on a Bruker ESP300 9.458 GHz
spectrometer. UV/vis spectra were recorded on a Shimadzu 2501-
PC spectrophotometer.
4
Crystallographic Studies. Single crystals of 1-7 were obtained
as described above. Each crystal was mounted on a glass fiber with
silicone grease and transferred to a goniometer. The crystals were
cooled to -60 °C under a stream of nitrogen, except for 2, which
was cooled to -80 °C. Geometric and intensity data were collected
on a Bruker SMART 1000 CCD area detector system using 0.3°
ω-scans at 0°, 90°, and 180° in φ, with Mo KR radiation (λ )
0.71073 Å). Cell parameters for 1-6 were determined using the
program SMART.25 Data reduction and integration were performed
with the software package SAINTPLUS.26 Absorption corrections
were applied using the program SADABS.27 Crystal and space
group symmetries for all compounds were determined using the
XPREP program.28 For all compounds, the positions of some or
all of the non-hydrogen atoms were found by direct methods or
the Patterson method using the program SHELXS.29 The positions
of the remaining non-hydrogen atoms were located by use of a
combination of least-squares refinement and difference Fourier maps
in the SHELXL-9730 program. Non-hydrogen atoms were refined
with anisotropic displacement parameters. The hydrogen atoms were
included in the structure factor calculations at idealized positions.
Crystal data and refinement results for all compounds are listed in
Table 1. Selected bond distances and angles are listed in Table 2.
Crystals of 7 exhibited nonmerohedral twinning. The twin
indexing program GEMINI31 gave two similar, but statistically
different unit cells. The cell parameters were further optimized with
the nonlinear least squares lattice parameter routine of SMART.
Two sets of integrated intensities were generated with each of the
two contributing orientation matrixes. The intensity data set from
the major component (a ) 12.479(2) Å, b ) 12.592(2) Å, c )
13.363(2) Å, R ) 65.337(2)°, â ) 65.078(2)°, γ ) 88.078(2)°, V
) 1704.6(4) Å3) was used successfully for structure solution and
refinement (Table 1).
Abbreviations Used. DTolF ) the anion of N,N′-di-p-tolylform-
amidine; DCyF ) the anion of N,N′-dicyclohexylformamidine; hpp
) the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimi-
dine; DPhF ) the anion of N,N′-diphenylformamidine; DAniF )
the anion of N,N′-di-p-anisylformamidine; DClPhF ) the anion of
N,N′-di-p-chloroformamidine; TPG ) the anion of 1,2,3-triphenyl-
guanidine; ap ) the anion of anilinopyridine. The word “hexanes”
describes a mixture of C6H14 isomers.
Preparation of V2(NXN)4 Compounds. Compounds 1-5 were
prepared as previously reported.4 In a typical reaction, 0.40 g (1.07
mmol) of VCl3‚3THF was dissolved in 10 mL of THF and reduced
by dropwise addition of 1 equiv of NaEt3BH at -78 °C. In a
separate flask, 2 equiv of the corresponding ligand were deproto-
nated with MeLi at -78 °C. Each solution was brought to room
temperature, and then cooled again to -78 °C. The ligand solution
was then transferred by cannula into the V(II) solution. After stirring
for 15-30 min at room temperature, the THF was removed by
vacuum evaporation and the residue was extracted with ca. 40 mL
of warm toluene (benzene in the case of 4 and 5) and filtered
through Celite. X-ray quality crystals were grown by the slow
diffusion of hexanes into the corresponding toluene or benzene
solution. Crystalline yields of compounds 1-5 were approximately
40%.
Preparation of 6. In a typical reaction, a solution of 60 mg
(0.068 mmol) of V2(DPhF)4 in 15 mL of THF was added by cannula
to a suspension of 25 mg (0.19 mmol) of KC8 in 10 mL of THF at
-78 °C. The mixture was stirred at -78 °C for 1 h, after which
time the color of the solution turned from red to green. The mixture
was filtered through Celite, and a mixture of hexanes (10 mL) was
added. The solution was placed in a freezer at -10 °C. Very dark
green blocklike crystals of 6 were obtained after 3 days, with a
yield of approximately 10%.
Preparation of 7. The same procedure was followed for the
synthesis of 6, except, after reduction, the green filtrate was layered
with an ether solution of 1.5 equiv of 18-crown-6. Slow diffusion
yielded very dark green crystals after 3 weeks. Yields for this
reaction were quantitative.
The potassium cation in 7 was found on a crystallographic
inversion center and was coordinated by one 18-crown-6 and two
THF solvent molecules. The 18-crown-6 moiety was disordered in
two positions with occupancies of 0.546(3) and 0.454(3), respec-
tively. The THF molecules were also disordered and were refined
with restraints on the distances.
NMR Spectroscopy. 1H NMR for 1 (C6D6, ppm): 5.953 (d, 16
H), 6.700-6.790 (m, 24 H), 10.007 (s, 4H). For 2 (CDCl3, ppm):
3.607 (s, 24 H), 5.803 (d, 16 H), 6.341 (d, 16 H), 10.017 (s, 4 H).
For 3 (C6D6, ppm): 5.517 (d, 16 H), 6.667 (d, 16 H), 9.680 (s, 4
H). For 4 (CDCl3, ppm): 6.322 (s, 2H), 6.624-6.706 (m, 6H),
6.839 (t, 4H), 6.943 (d, 4H). For 5 (C6D6, ppm): complex signals
ranging from 5.792 to 6.983 ppm.23
Elemental Analyses. Anal. Calcd (found) for 1: C, 70.74
(70.92); H, 5.02 (5.18); N, 12.69 (12.64). For 2, C60H60V2N8O8:
C, 64.17 (63.84); H, 5.39 (5.76); N, 9.98 (8.99). For 3, C52H36V2-
N8Cl8: C, 53.92 (54.05); H, 3.13 (3.10); N, 9.67 (9.59). For 4,
C79H67V2N12: C, 73.76 (73.23); H, 5.25 (5.27); N, 13.07 (12.20).
For 5, C47H39V2N8: C, 69.03 (68.49); H, 4.81 (4.91); N, 13.70
(13.79). The instability of compound 6 precluded satisfactory
analysis.24
Results and Discussion
The five new V24+ compounds 1-5, along with the three
previously known, are listed in Table 3, where some
additional information about each one is also presented. It
will be noted that these are all similar in three respects. (1)
Each one is a neutral paddlewheel molecule with four NXN-
bridging ligands. (2) All compounds have been accurately
(24) Elemental analysis for 7 was satisfactory for H and N, but low for C.
(25) SMART for Windows NT; Bruker AXS Inc.: Madison, WI, 1997-
2001.
(26) SAINTPLUS for NT; Bruker AXS Inc.: Madison, WI, 1997-2001.
(27) SADABS, V2.03, Bruker/Siemens area detector absorption and other
corrections; Bruker AXS Inc.: Madison, WI, 2000.
(28) XPREP; Bruker AXS Inc.: Madison, WI, 1997-2001.
(29) Sheldrick, G. M. SHELXS-97: Program for Crystal Structure Solution;
University of Go¨ttingen: Go¨ttingen, Germany, 1986-1997.
(30) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(31) GEMINI V1.02; Bruker AXS Inc.: Madison, WI, 1999.
(23) The complexity of the NMR spectra for anilinopyridinate compounds
has been previously alluded to by Tocher and Tocher: Tocher, D.
A.; Tocher, J. H. Inorg. Chim. Acta 1985, 104, L15.
Inorganic Chemistry, Vol. 42, No. 19, 2003 6065

Cotton et al.
Table 1. Crystal Data and Structure Refinement for 1-7
compound
1
2
3
4‚4benzene
5‚2benzene
6
7
empirical formula
fw
space group
a (Å)
b (Å)
c (Å)
C52H44N8V2
882.83
P2/n
17.874(1)
10.3011(8)
24.416(2)
90
102.156(2)
90
C60H60N8O8V2
1123.04
P1h
C52H36Cl8N8V2
1158.37
Fddd
26.596(2)
27.238(2)
29.798(2)
90
90
90
21586(2)
16
1.426
C
100H88N12V2
C
56 H48N8V2
C64H68KN8O3V2
1138.24
Pna21
20.249(1)
12.4492(6)
22.957(1)
90
90
90
5786.9(5)
4
1.306
C72H84KN8O8V2
1330.45
P1h
1559.70
P1h
934.90
P1h
10.2646(6)
10.3332(6)
13.9625(9)
80.265(1)
75.071(1)
81.160(1)
1400.9(2)
1
14.405(1)
15.082(1)
21.210(2)
72.170(2)
79.445(2)
69.961(2)
4105.3(6)
2
9.908(1)
10.163(1)
12.761(1)
97.989(2)
109.990(2)
101.325(2)
1154.1(2)
1
12.479(2)
12.592(2)
13.363(2)
65.337(2)
65.078(2)
88.078(2)
1704.6(4)
1
R (deg)
â (deg)
γ (deg)
vol (Å3)
Z
4394.7(6)
4
1.334
d (calc, g/cm3)
1.331
0.396
1.262
0.285
1.345
0.453
1.296
0.396
µ (mm-1
)
0.472
0.786
0.448
final R indices
[I > 2σ(I)]a,b
R indices
R1 ) 0.057,
wR2 ) 0.136
R1 ) 0.089,
wR2 ) 0.153
R1 ) 0.040,
wR2 ) 0.104
R1 ) 0.048,
wR2 ) 0.110
R1 ) 0.037,
wR2 ) 0.086
R1 ) 0.060,
wR2 ) 0.100
R1 ) 0.055,
wR2 ) 0.106
R1 ) 0.111,
wR2 ) 0.128
R1 ) 0.030,
wR2 ) 0.085
R1 ) 0.032,
wR2 ) 0.087
R1 ) 0.046,
wR2 ) 0.081
R1 ) 0.081,
wR2 ) 0.095
R1 ) 0.089,
wR2 ) 0.186
R1 ) 0.108,
wR2 ) 0.199
(all data)
2
2
2
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo - Fc )2]/∑[w(Fo )2]]1/2, w ) 1/[σ2(Fo ) + (aP)2 + bP], where P ) [max(Fo or 0) + 2(Fc )]/3.
Table 2. Selected Bond Lengths and Anglesa
1
2
3
4
5
6
7
1.979(1)
1.978(1)
1.9876(5)
1.974(1)
1.982(1)
1.9521(7)
1.9425(4)
1.9295(8)
1.924(2)
V-V (Å)
2.101[3]
2.102[3]
2.106[2]
94.33[4]
89.68[6]
171.12[6]
2.093[2]
2.113[2]
2.094[3]
93.82[7]
89.8[1]
2.149[1] (py)
2.046[1] (im)
2.142[3]
95.03[9]
89.6[1]
2.155[4]
95.0[1]
89.6[2]
170.0[2]
V-N (Å)
94.46[7]
94.47[7]
94.21[6]
94.42[6]
94.49[3]
89.66[5]
168.37(4)
V-V-N (deg)
cis-N-V-N (deg)
trans-N-V-N (deg)
89.7[1]
89.7[1]
89.72[9]
89.66[9]
171.0[1]
171.0[1]
171.6[1]
171.2[1]
172.3[1]
169.7[1]
a Values in square brackets represent errors in average measurements.
4+
Table 3. Compounds Containing the V2 Core
It should be noted that special attention must be given to
the careful preparation and purification of the starting
materials and to control of stoichiometry to ensure success
of the reaction. Purple solutions of VCl2 in THF are made
by reducing VCl3(THF)3 with an equivalent of NaEt3BH.
Since VCl3(THF)3 loses coordinated solvent, it is necessary
to use freshly prepared crystalline material or crystals that
have been stored at low temperature in a drybox. Whenever
excess of V(III) is present, it is not unusual to find
tris-chelating compounds such as V(formamidinate)3 as
contaminants.32 Alternatively, an excess of ligand can
generate species such as [V(formamidinate)3]-. All com-
pounds are air-sensitive; therefore all reactions are carried
out under Ar, and solvents must be rigorously dried by fresh
distillation over K/Na alloy. Contact with oxygen has been
shown to produce compounds such as the dinuclear dioxo-
bridged V2(µ-O)2(µ-formamidinate)2(η2-formamidinate)2 com-
pounds or the mononuclear VO(formamidinate)2 species.32
Even when precautions are taken to avoid contaminants, it
is often found that, upon removal of the reaction solvent
(THF) in the preparation of compounds such as 1-5, oily
red residues are obtained generally commingled with some
red solid. We found that, in the case of V2(DClPhF)4, limiting
the reaction time to 15 min at room temperature decreased
the oiliness of the crude residue. Generally, the source of
V-V bond
torsion
reduction
potential, V
ligand
DTolF
distance (Å)
angles (deg)
ref
4, 32
1.978(2)
5.4(2)a
9.6(6)b
c
DCyF
hpp
1.968(2)
1.932(1)
c
c
15
16
0
c
1.979(1)
1.978(1)
2.1
2.8
-1.46
DPhFd
DAniF
DClPhFd
this work
this work
this work
1.9876(5)
0
-1.77
-1.23
1.974(1)
1.982(1)
1.0
7.7
TPG
ap
1.9521(7)
1.9425(4)
14.0
0
-1.99
-1.82
this work
this work
a Tetragonal polymorph. b Orthorhombic polymorph. c Not reported.
d Two crystallographically independent molecules in the unit cell.
characterized as to structure by single-crystal X-ray crystal-
lography, and the V-V distances are all within the relatively
narrow range 1.93-1.99 Å, while torsion angles are close
to 0°. (3) Each of the compounds dissolved in THF displays
a (quasi)reversible reduction wave.
Syntheses. The paddlewheel compounds 1-5 were syn-
thesized by the following metathesis reaction, where NXN
represents the anion of the bidentate ligand:
2VCl2‚nTHF + 4Li(NXN) THF, rt8 V2(NXN)4 + 4LiCl
6066 Inorganic Chemistry, Vol. 42, No. 19, 2003
(32) Cotton, F. A.; Daniels, L. M.; Murillo, C. A. Inorg. Chem. 1993, 32,
2881.

New Chemistry of the Triply Bonded DiWanadium (V24+) Unit
the oiliness can be removed by washing with small amounts
of benzene or toluene, leaving the products as red solids.
After removal of THF, the crude residues are best allowed
to remain under reduced pressure for only a short time, from
1
about 15 min to /2 h. If, for any reason, there is accidental
exposure of the solid to a small amount of atmospheric
oxygen, darkening of the surface occurs. Should this happen,
the discolored surface layer can be removed by washing the
solid with small amounts of toluene or THF, although it is
best to avoid having to wash the residue at all, because the
compounds are moderately soluble in these solvents. The
products can then be separated from the side product, LiCl,
by extraction with warm toluene or benzene, filtration
through Celite, and diffusion of hexanes to produce X-ray
quality crystals. Interestingly, although the reaction mixtures
and benzene (or toluene) extracts are always dark red in
color, crystals of V2(TPG)4 are green. This feature, the
oiliness of the residue in all cases, and the fact that the
crystallization solutions remain highly colored even after
complete diffusion suggest that a dark red uncharacterized
and highly soluble side product is uniformly obtained. It
should be noted that reactions with less basic ligands such
as N,N′-di(3,5-dichlorophenyl)formamidinate do not produce
dinuclear compounds and only the corresponding tris-
chelated mononuclear complex Li(THF)4[V(formamidinate)3]
can be isolated.33 Attempts to allow the reaction to proceed
at higher temperatures lead to cleavage of the formamidinate
groups as indicated by crystallographic studies of the
vanadium products. Cleavage of formamidinate groups has
been documented for reactions with the heavier congeners.9
Reaction of VCl2 solutions with N,N′-di-p-trifluoromethyl-
phenylformamidinate does not produce isolable dinuclear
complexes either.
Figure 2. Thermal ellipsoid plot of V2(DPhF)4, 1. Thermal ellipsoids are
shown at the 30% probability level; hydrogen atoms have been omitted for
clarity. Only one of the two independent molecules in the asymmetric unit
is shown.
The reduction of 1 is accomplished by treatment of the
precursor with KC8 yielding a brilliant green solution (within
which graphite is suspended) after stirring a few minutes at
-78 °C. Rapid crystallization of 6 must be carried out by
filtering the THF reaction mixture and adding just enough
hexanes to evolve a precipitate. By placing the solution in a
freezer at -10 °C, crystals form within 48 h. Rapid
crystallization at low temperature is very important as slower
crystallization rates by diffusion of hexanes into the THF
solution yields only red 1. Some of compound 1 is generally
present even when short crystallization times are used. On
the other hand, when crown ether is introduced, the
compound becomes insoluble in THF and 7 is quite stable
in the solid form, resisting decomposition at room temper-
ature under an inert atmosphere indefinitely.
Figure 3. Thermal ellipsoid plot of V2(DAniF)4, 2. Thermal ellipsoids
are shown at the 30% probability level; hydrogen atoms have been omitted
for clarity.
(33) Preparation and crystal data for Li(THF)4[V(formamidinate)3]: This
compound was made by following the same procedure as for
compounds 1-5. When the toluene filtrate was layered with hexanes,
only a red oil was obtained. The oil was redissolved by heating the
hexanes/toluene supernatant, and the flask was stored at 10 °C. After
2 weeks, copious oil and a few red crystals were obtained. Empirical
formula: C55H53Cl12LiN6O4V, P1h, a ) 11.0014(7) Å, b ) 15.975(1)
Å, c ) 18.905(1) Å, R ) 98.468(1)°, â ) 96.081(1)°, γ ) 105.737(1)°,
V ) 3125.7(3) Å3, Z ) 2, T ) 213(2) K.; cell parameters were refined
with 4405 reflections within a 2θ range of 4.408-44.578°. A total of
8151 unique reflections (2θ e 45.04°) were measured. Full-matrix
least-squares refinement of F2 (712 parameters) converged to R1 )
0.090 and wR2 ) 0.153 for all data.
Figure 4. Thermal ellipsoid plot of one of the two crystallographically
independent molecules in V2(DClPhF)4, 3. Thermal ellipsoids are shown at
the 30% probability level; hydrogen atoms have been omitted for clarity.
Structural Considerations. The thermal ellipsoid plots
of the five new V24+ molecules listed in Table 3 are shown
in Figures 2-6. Each compound has the typical paddlewheel
geometry with V-V bond distances ranging from 1.943 Å
Inorganic Chemistry, Vol. 42, No. 19, 2003 6067

Cotton et al.
Figure 7. Thermal ellipsoid plot of [K(THF)3]V2(DPhF)4, 6. Thermal
ellipsoids are shown at the 30% probability level; hydrogen atoms and THF
carbon atoms have been omitted for clarity.
in 5 to 0.812 in 2. For Cr2(DTolF)4, the FSR is 0.814,4 while
the shortest Cr-Cr bond in Cr2(2-MeO-5-MeC6H3)4 has a
FSR of 0.770.8
Figure 5. Thermal ellipsoid plot of V2(TPG)4, in 4‚4benzene. Thermal
ellipsoids are shown at the 30% probability level; hydrogen atoms and
interstitial benzene molecules have been omitted for clarity.
The crystal structure of [K(THF)3]V2(DPhF)4, 6, is shown
in Figure 7. The anion has a paddlewheel structure, somewhat
similar to that of 1, but there is a decrease in the V-V
distance from 1.979(1) to 1.9295(8) Å and a small increase
in the average V-N distance. The shortening of the V-V
bond of 0.05 Å is consistent with the addition of one electron
into a V-V bonding orbital and the increase in the formal
bond order from 3 to 3.5. The magnitude of this change
suggests that the additional electron resides in the δ-orbital,
generating a σ2π4δ configuration in the reduced species. The
influence of a δ electron on the bond length may be
compared to the case of the dimolybdenum carboxylates,
whose electronic structure is well understood. The difference
n+
in bond length between Mo2(TiPB)4 (where TiPB is the
anion of 2,4,6-triisopropylbenzoic acid and n ) 0 and 1)
with a σ2π4δ2 quadruple bond and the σ2π4δ oxidized species
is +0.06 Å.34 This is similar to the change in bond length of
-0.05 Å when going from a σ2π4 to a σ2π4δ configuration
in the divanadium compounds. The change in V-N distances
is also indicative of the lowering of the overall charge on
the dimetal core; the average distance increases from 2.101[3]
to 2.142[3] Å upon reduction. Similar variations are observed
in the series of compounds M2(hpp)4n+ (n ) 0, 1, 2), for M
) Mo35 and W.36 Finally, the decrease in the average torsion
angle from about 2.4° in the neutral species to about 1.6° in
the reduced species points toward improved δ orbital overlap.
There is an additional feature in the structure of 6, namely,
the presence of the K+ ion, coordinated by only three THF
molecules and associated also with the DPhF ligands. As
shown in Figure 7, the K+ cation is found in the cleft created
by two of the formamidinate groups. This type of associa-
tion of an alkali metal cation with some of the ligands of an
Figure 6. Thermal ellipsoid plot of V2(ap)4, 5‚2benzene. Thermal ellipsoids
are shown at the 30% probability level; hydrogen atoms have been omitted
for clarity.
in 5 to 1.988 Å in 2. The longer bond distances correspond
to the formamidinates, while the shorter ones belong to
compounds with the more basic guanidinate and amino-
pyridinate ligands. The diamagnetism exhibited by 1-5 and
the short V-V bond distances are consistent with the
assignment of a σ2π4 triple bond to each. It is notable that
the V-V bonds are the only known metal-metal bonds,
other than the fully developed Cr-Cr quadruple bonds, that
are shorter than 2.00 Å. The V-V bond is so short, in fact,
that it is shorter than the natural bite of the formamidinate
ligands. This is manifested in the distortion of the ligand
bridge shown by the V-V-N angles, which are all greater
than 90°, and in the trans N-V-N angles, which are all
less than 180°. For comparison, the “formal shortness ratio”
(FSR),8 a measure of bond shortness normalized to atomic
size, of the V-V bond in these molecules ranges from 0.794
(34) Cotton, F. A.; Daniels, L. M.; Hillard, E. A.; Murillo, C. A. Inorg.
Chem. 2002, 41, 1639.
(35) (a) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Timmons, D. J.;
Wilkinson, C. C. J. Am. Chem. Soc. 2002, 124, 9249. (b) Cotton, F.
A.; Gruhn, N. E.; Gu, J.; Huang, P.; Lichtenberger, D. L.; Murillo, C.
A.; Van Dorn, L. O.; Wilkinson, C. C. Science 2002, 298, 1971.
(36) (a) Cotton, F. A.; Huang, P.; Murillo, C. A. Inorg. Chem. Commun.
2003, 6, 121. (b) Cotton, F. A.; Huang, P.; Murillo, C. A.; Wang, X.
Inorg. Chem. Commun. 2003, 6, 121.
6068 Inorganic Chemistry, Vol. 42, No. 19, 2003

New Chemistry of the Triply Bonded DiWanadium (V24+) Unit
Figure 9. Cyclic voltammogram of V2(DPhF)4, 1, in THF with 0.1 M
Bu4NPF6 supporting electrolyte. E1/2 ) -1.46 vs Ag/AgCl. Scan rate )
0.1 V/s.
Figure 8. Thermal ellipsoid plot of [K(18-crown-6)(THF)2]V2(DPhF)4,
7. Thermal ellipsoids are shown at the 30% probability level. Only one
orientation of the disordered phenyl rings and crown ether is shown. THF
molecules coordinated apically to the K+ cation and hydrogen atoms have
been omitted for clarity.
torsion angle in 7 is 0°, as required by crystallographic
symmetry.
Electrochemistry. Because of our recent interest in the
7+
oxidation of M24+ complexes to M25+, M26+, and even M2
,
M2 paddlewheel molecule is not without precedent and has
been observed in Nb2(hpp)411 and W2(hpp)4.36 Although these
compounds were first isolated in crystals of M2(hpp)4‚
2NaEt3BH,11,37 having Na+ ions occupying pockets between
the paddles, the metal-metal bond distances were essentially
identical whether with or without the associated alkali
cations. In 6, there are long K‚‚‚N separations averaging
3.124[3] Å and three THF molecules at a relatively long
average K‚‚‚O distance of 2.729[4] Å, similar to those found
in other K(THF) containing compounds.38 Even though the
M-M distances were unaffected by the presence or absence
of the alkali metal in such dinuclear Nb and W complexes,
this is not a trend that is necessarily general especially when
small changes in distances, such as the difference of 0.05 Å
between V-V bond distances in 1 and 6, are being
considered.
In order to establish that the K+ cation does not seriously
perturb the paddlewheel geometry and that there is not a
significant effect in the metal-metal separation, the potas-
sium cation in 6 was sequestered by reaction with 18-crown-
6. The result is 7, the structure of which is shown in Figure
8, where the K+ cation is not associated with the divanadium
species in any way. The V-V bond distance of 1.924(2) Å
is the same, within 3σ, as that found in 6 (1.9295(8) Å).
This is also true for the V-N distances, which are only
slightly longer, averaging 2.155[4] Å as compared to 2.142[3]
Å in 6. However, small changes are observed in the cis
N-V-N bond angles which are more square and uniform
in the absence of the associated K+ cation, 88.9(2)°, 89.5(2)°,
89.8(2)°, and 90.1(2)°, while the presence of the cation in 6
gives rise to a slight opening of the angle between the two
blades embracing the cation to 93.7[1]° as compared to
90.5[1]° on the other side of the molecule. The average
we investigated the electrochemistry of V24+ compounds, and
this has led to the important advance in the chemistry of
metal-metal bonds discussed above, namely, the preparation
and structural characterization of the first tetragonal paddle-
3+
wheel complex having an M2 core.39 While no reversible
oxidation occurs, there is a reversible reduction. In all the
thousands of tetragonal paddlewheel compounds having
3+
metal-metal bonded dinuclear units, reduction to an M2
core is a very unusual observation.39,40 The reduction wave
in the cyclic voltammogram for 1 is shown in Figure 9. The
reduction product can be formally considered to provide an
example of the rare oxidation state V+ known only in
relatively exotic species such as [V(mesitylene)2]+.41 How-
ever, we consider it more likely that the additional electron
is located in the δ bonding orbital, where it is delocalized in
3+
a V2 core which would be expected to have an overall
electron configuration of σ2π4δ and a metal-metal bond
order of 3.5, consistent with the shortening of the V-V in
going from 1 to 6 and 7.
Each of the compounds, 1-5, exhibits a similar reduction
wave, the potentials of which are listed in Table 3. There is
a notable trend in reduction potential: as the bridging ligand
becomes more basic, the reduction potentials become more
negative; it becomes more difficult to add an additional
electron to the valence, presumably δ-type, orbital. A similar,
but opposite, correlation between oxidation potentials and
the ligand basicity of various formamidinate ligands has been
shown in dinickel,42 diruthenium,43 and dimolybdenum44
(39) A report of the possible existence of a tetragonal paddlewheel complex
3+
having a Co2 core has appeared, but structural characterization is
lacking: He, L.-P.; Yao, C.-L.; Naris, M.; Lee, J. C.; Korp, J. D.;
Bear, J. L. Inorg. Chem. 1992, 31, 620.
(40) There are only two sets of structurally characterized compounds having
M33+ units, M ) Fe and Co, but these are neutral, highly paramagnetic
molecules having the formula M2[RC(NPh)2]3, R ) H, C6H5, and only
three formamidinate or benzamidinate bridging ligands instead of four
ligands in 6 and 7. See: (a) Cotton, F. A.; Daniels, L. M.; Falvello,
L. R.; Matonic, J. H.; Murillo, C. A. Inorg. Chim. Acta 1997, 256,
269. (b) Cotton, F. A.; Daniels, L. M.; Maloney, D. J.; Matonic, J.
H.; Murillo, C. A. Inorg. Chim. Acta 1997, 256, 283.
(41) See for example: (a) Calderazzo, F.; Ferri, I.; Pampaloni, G.; Englert,
U. Organometallics 1999, 18, 2452. (b) Calderazzo, F.; Pampaloni.
G.; Rocchi, L.; Marchetti, F. J. Organomet. Chem. 1991, 417, C16.
(c) Reference 20, pp 733-735.
(37) Cotton, F. A.; Huang, P.; Murillo, C. A.; Timmons, D. J. Inorg. Chem.
Commun. 2002, 5, 501.
(38) For comparison see Evans et al. (Evans, W. J.; Brady, J. C.; Ziller, J.
W. Inorg. Chem. 2002, 41, 3340), Karsch et al. (Karsch, H. H.; Volker,
G. W.; Reisky, M. Chem. Commun. 1999, 1695), and Ye´lamos et al.
(Ye´lamos, C.; Heeg, M. J.; Winter, C. H. Inorg. Chem. 1998, 37,
3892), where the K-O distances are 2.638(2) Å in [K(DMI)(THF]n
(DMI ) 2,3-dimethylindolide), 2.708 Å in [(THF)K(C5H4CH3CH2-
PPh2)]n, and 2.716(5) Å in [(Ph2C3HN2)K(THF)]6, respectively.
Inorganic Chemistry, Vol. 42, No. 19, 2003 6069

Cotton et al.
glass of 6 at 6 K gave a multiline spectrum, shown in Figure
10, which indicates that the electron is coupling with each
51V (I ) 7/2, ∼100%) atom equally. A simulation of the main
feature gives a g value of 1.9999. Although this is close to
the free-electron value, the complicated hyperfine splitting
pattern indicates that much of the unpaired electron density
is localized on the dimetal core. A similiar spectrum was
obtained for 7 in an acetonitrile glass.
Concluding Remarks
With the preparation of a series of paddlewheel compounds
having ligands of varied basicity, we have shown that
vanadium offers a rich chemistry. A careful analysis of the
synthetic methodology shows the importance of stoichio-
metric control, purity of the starting materials, and control
of the reaction conditions for the successful preparation of
tetragonal paddlewheel complexes with short vanadium-
vanadium triple bonds of less than 2.0 Å. Our data indicate
that the length of the V-V bond depends on the basicity of
the bridging ligands, with those having the more basic
guanidinate core being shorter than those having the less
basic formamidinate groups. Furthermore, use of highly
electron withdrawing substituents such in N,N′-di(3,5-di-
chlorophenyl)formamidinate favors the formation of tris-
chelating mononuclear complexes or even cleavage of the
formamidinate groups. An important effect is observed in
the reduction potential also. More importantly, divanadium
complexes with the less basic formamidinate ligands can be
chemically reduced with KC8 to generate the first tetragonal
paddlewheel complexes with a V23+ core and a bond between
vanadium atoms of order 3.5, with a σ2π4δ electronic
configuration having one unpaired electron delocalized in
the V2 unit. The formal oxidation state of 1.5 for each
Figure 10. Electron paramagnetic resonance spectrum (9.5 GHz) of 6 in
a THF glass at 6 K.
paddlewheel-type compounds. Furthermore, the V-V dis-
tances correlate fairly well with the reduction potentials. The
more negative reduction potentials belong to the molecules
with the shorter bonds, while the more accessible potentials
correspond to those with the longer bonds.
Finally, the reduction potentials correlate with the low-
4+
energy electronic absorptions. Each of the V2 molecules
with formamidine ligands display two weak transitions, the
first ranging from 455 to 488 nm for the DClPhF and DAniF
compounds, respectively, and the second one appearing as
a very weak shoulder at around 560 nm. For V2(ap)4 the
first transition is shifted to 533 nm. In the case of V2(TPG)4,
only one transition is observed at 588 nm. Although these
transitions cannot at present be assigned with certainty, it is
clear that, as the transition becomes more energetic, the
reduction potential becomes less negative. This trend allows
the following interpretation: (1) the LUMO is destabilized
as the ligands become more basic, blue-shifting the transi-
tions and facilitating reduction, and (2) the low-energy
transition has the same character in each compound. A
similar correlation was found in the reduction of diruthenium
formamidinates.45 We are pursuing DFT and other types of
calculations to learn more about the electronic structure of
these molecules.
3+
vanadium atom in these V2 units is unprecedented in
coordination chemistry without π-donor ligands but more
importantly has allowed extension of the attainable range of
oxidation states in metal-metal bonded systems which a year
n+
ago was restricted to the range of +4 to +6 for n in M2
units and that has now been extended to the range of n of
+3 to +7.
Acknowledgment. We thank Mr. Chris Fewox and Dr.
Murali Ayaluru for help with EPR spectroscopy, which was
supported by NSF CHE-0092010. We are grateful to the
National Science Foundation and Robert Welch Foundation
for support, and E.A.H. thanks the NSF for a predoctoral
fellowship.
EPR Spectroscopy. For the reduced species in 6 and 7,
our view that the additional electron is introduced into the δ
metal bonding orbital, where it is delocalized in a V23+ core,
is further supported by EPR spectroscopy. A frozen THF
Supporting Information Available: A displacement ellipsoid
plot in pdf and CIF of Li(THF)3[V(di(3,5-dichlorophenyl)form-
amidinate)3] and additional crystallographic data in CIF format for
1-7. This material is available free of charge via the Internet at
http://pubs.acs.org.
(42) Lin, C.; Protasiewicz, J. D.; Ren, T. Inorg. Chem. 1996, 35, 7455.
(43) (a) Lin, C.; Ren, T.; Valente, E. J.; Zubkowski, J. D. J. Organomet.
Chem. 1999, 579, 114. (b) Lin, C.; Ren, T.; Valente, E. J.; Zubkowski,
J. D J. Chem. Soc., Dalton Trans. 1998, 571.
(44) Lin, C.; Protasiewicz, J. D.; Smith, E. T.; Ren, T. Inorg. Chem. 1996,
35, 6422.
(45) Lin, C.; Ren, T.; Valente, E. J.; Zubkowski, J. D.; Smith, E. T. Chem.
Lett. 1997, 753.
IC034646E
6070 Inorganic Chemistry, Vol. 42, No. 19, 2003