Cyclic polyamidato dianions as bridges between Mo24+ units: Synthesis, crystal structures, electrochemistry, absorption spectra, and electronic structures

Article abstract of DOI:10.1021/ja0358044

Compounds in which quadruply bonded Mo₂⁴⁺ units, Mo₂(DAniF)₃ (DAniF = N,N′-di-p-anisylformamidinate), are linked by cyclic diamidate anions have been synthesized and characterized by X-ray crystallography and spectroscopic methods. As identified by the diamidate linker, these compounds are 4,6-dioxypyrimidinate (2), 2,3-dioxypyrazinate (3), 2,3-dioxyquinoxalinate (4), 2,3-dioxy-5,6-dicyanopyrazinate (5), and cyanurate (6). With uracilate, a dinuclear unlinked 1:1 adduct is formed, Mo₂(DAniF)₃-(uracilate) (1). The cyclic voltammograms of 3-5 reveal significantly larger AE_1/2 values (258 mV-308 mV) than that of the oxalate linked analogue (212 mV), which is indicative of greater charge delocalization in the mixed valent Mo₂⁴⁺/Mo₂⁵⁺ species and hence greater communication between the two Mo₂ units. ΔE_1/2 for 2 is substantially lower than those for 3-5. This difference is attributed to the meta disposition of the two amidate groups in 4,6-dioxypyrimidinate as compared to their ortho arrangement in the pyrazinate-type linkers. The absorption spectra of the linked compounds 3-5 are more complex than those of the analogous polyunsaturated dicarboxylate linked compounds and reveal at least two significant absorption bands within the region 420-550 nm. Compound 2 also has two bands but with significantly lower intensity. Time dependent DFT calculations upon 2 and 3 indicate rather different electronic structures for these two structural isomers. The two bands for 3 have δ → π* character, and the π* type orbitals have substantial contributions from the Mo₂ units as well as from the diamidate linker. The excitations observed in 2 are mainly metal based. The differences between the electronic spectra of 2 and 3 are consistent with the electrochemistry in underscoring the profound physical effect of changing the symmetry of the diamidate linker.

Full text of DOI:10.1021/ja0358044

Published on Web 06/25/2003
4+
Cyclic Polyamidato Dianions as Bridges between Mo2
Units: Synthesis, Crystal Structures, Electrochemistry,
Absorption Spectra, and Electronic Structures
F. A. Cotton,*,† James P. Donahue,† Carlos A. Murillo,*,† Lisa M. Pe´rez,‡ and
Rongmin Yu†
Contribution from the Laboratory for Molecular Structure and Bonding,
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77842-3012 and
Laboratory for Molecular Simulation, Department of Chemistry, Texas A&M UniVersity,
College Station, Texas 77842-3012
Received April 25, 2003; E-mail: cotton@tamu.edu; murillo@tamu.edu
4+
Abstract: Compounds in which quadruply bonded Mo2
units, Mo2(DAniF)3 (DAniF ) N,N′-di-p-
anisylformamidinate), are linked by cyclic diamidate anions have been synthesized and characterized by
X-ray crystallography and spectroscopic methods. As identified by the diamidate linker, these compounds
are 4,6-dioxypyrimidinate (2), 2,3-dioxypyrazinate (3), 2,3-dioxyquinoxalinate (4), 2,3-dioxy-5,6-dicyanopy-
razinate (5), and cyanurate (6). With uracilate, a dinuclear unlinked 1:1 adduct is formed, Mo2(DAniF)3-
(uracilate) (1). The cyclic voltammograms of 3-5 reveal significantly larger ∆E1/2 values (258 mV-308
mV) than that of the oxalate linked analogue (212 mV), which is indicative of greater charge delocalization
5+
in the mixed valent Mo24+/Mo2 species and hence greater communication between the two Mo2 units.
∆E1/2 for 2 is substantially lower than those for 3-5. This difference is attributed to the meta disposition of
the two amidate groups in 4,6-dioxypyrimidinate as compared to their ortho arrangement in the pyrazinate-
type linkers. The absorption spectra of the linked compounds 3-5 are more complex than those of the
analogous polyunsaturated dicarboxylate linked compounds and reveal at least two significant absorption
bands within the region 420-550 nm. Compound 2 also has two bands but with significantly lower intensity.
Time dependent DFT calculations upon 2 and 3 indicate rather different electronic structures for these two
structural isomers. The two bands for 3 have δ f π* character, and the π* type orbitals have substantial
contributions from the Mo2 units as well as from the diamidate linker. The excitations observed in 2 are
mainly metal based. The differences between the electronic spectra of 2 and 3 are consistent with the
electrochemistry in underscoring the profound physical effect of changing the symmetry of the diamidate
linker.
Introduction
properties of molecules of type Ia (Scheme 1). Unfortunately,
to this day these molecules have, apparently, defied all efforts
The relative effectiveness of various groups that may link
structurally two metal atoms, or groups of metal atoms, in
communicating electronic changes from one metal atom(s) to
the other(s) is clearly a fundamental question in chemistry. With
respect to molecules with a single metal atom at each end,
enormous effort and ingenuity have already gone into answering
it, and a great deal of progress has been made.1 The very next
larger entity after a single metal atom is, obviously, a pair of
metal atoms. In this realm, there is still much that we do not
know. The earliest serious effort to link pairs of dimetal units
was published in 1991 by Chisholm et al.,2 who studied solution
to isolate them in a form suitable for standard X-ray crystal-
lographic characterization, although Chisholm has carried out
other pertinent studies.3
In 1998, the isolation and structural characterization of
molecules of type I, namely those in Ib, were reported.4 More
detailed, structurally complete studies of many Ib molecules
with a highly varied array of X units have appeared more
recently,5 and many more complex but related structures (e.g.,
squares, triangles, and loops) have also been prepared, crystal-
lographically defined, and studied in other ways.6 In nearly all
(2) Cayton, R. H.; Chisholm, M. H.; Huffman, J. C.; Lobkovsky, E. B. J. Am.
Chem. Soc. 1991, 113, 8709.
† Laboratory for Molecular Structure and Bonding.
‡ Laboratory for Molecular Simulation.
(3) (a) Bursten, B. E.; Chisholm, M. H.; Hadad, C. M.; Li, J.; Wilson, P. J.
Chem. Commun. 2001, 2382. (b) Bursten, B. E.; Chisholm, M. H.; Clark,
R. J. H.; Firth, S.; Hadad, C. M.; MacIntosh, A. M.; Wilson, P. J.;
Woodward, P. M.; Zaleski, J. M. J. Am. Chem. Soc. 2002, 124, 3050. (c)
Bursten, B. E.; Chisholm, M. H.; Clark, R. J. H.; Firth, S.; Hadad, C. M.;
Wilson, P. J.; Woodward, P. M.; Zaleski, J. M. J. Am. Chem. Soc. 2002,
124, 12244.
(1) For example, see: (a) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1. (b) Chen,
P.; Meyer, T. J. Chem. ReV. 1998, 98, 1439. (c) Ferretti, A.; Lami, A.;
Murga, L. F.; Shehadi, I. A.; Ondrechen, M. J.; Villani, G. J. Am. Chem.
Soc. 1999, 121, 2594. (d) Kaim, W.; Klein, A.; Gloeckle, M. Acc. Chem.
Res. 2000, 33, 755. (e) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J.
Chem. ReV. 2001, 101, 2655. (f) Brunschwig, B. S.; Creutz, C.; Sutin, N.
Chem. Soc. ReV. 2002, 31, 168. (g) Lau, V. C.; Berben, L. A.; Long, J. R.
J. Am. Chem. Soc. 2002, 124, 9042.
(4) Cotton, F. A.; Lin, C.; Murillo, C. A. J. Chem. Soc., Dalton Trans. 1998,
3151.
9
8900
J. AM. CHEM. SOC. 2003, 125, 8900-8910
10.1021/ja0358044 CCC: $25.00 © 2003 American Chemical Society

Cyclic Polyamidato Dianions as Bridges between Mo24+ Units
A R T I C L E S
+
+
Scheme 1. Dicarboxylate Linked M2(O2CR)3 (Ia) and M2(ArNC(H)NAr)3 (Ib) Units
Scheme 2. Structures of Diamidate (IIa) and Diamidinate
(IIb) Linkers
all six, we report the crystal structures and their electrochemistry.
From the pair of oxidation potentials in each of the five linked
molecules, we infer the abilities of the linkers to delocalize
charge over both dimetal units when only one has been oxidized.
In all the compounds mentioned above, the communication
between the dimetal centers has been studied by electrochem-
istry. The effectiveness of communication has been assessed in
terms of ∆E1/2, the difference between the first and second
oxidation potentials.9 These have ranged from <100 mV to a
high of 225 mV for dicarboxylate-linked molecules of type Ib.
of the compounds alluded to so far, the linkers have been
dicarboxylic acids. The question naturally arose as to whether
diamidates (IIa) and diamidinates (IIb) (Scheme 2) would be
more efficient than carboxylates in effecting electronic coupling
between dimetal units. One report has appeared describing the
deliberate synthesis, structures, and other properties of two such
compounds.7 In this work, two straightforward examples of type
IIa diamidates were used.
2-
Still higher values were found when the O2CXCO2 linkers
2-
were replaced by EO4 ions, E ) S, Mo, and W.10
In the work reported here, we have sought the answers to
two main questions: (1) Can molecules of type IIa be made in
which the diamidate bridge is a cyclic polyamidate (or a
tautomer thereof)? (2) How effective will cyclic polyamidate
linkers be in mediating communication between the two dimetal
units? In planning this research, we considered all the precursors
to the dianionic linkers shown in Scheme 3 (IIIa). Practical
considerations (e.g., commercial availability, accessibility via
synthesis, probability of adding significantly to our understand-
ing in return for the additional work required) limited our choice
to those reported here.
It should be noted that some of the ligand precursors have
important roles in biological processes, and an enormous amount
of literature has been dedicated to a few of them. For example,
cyanuric acid and uracil, which have a close structural relation-
ship, have been often studied for their ability to form hydrogen
bonds in structures that vary from simple pairs to extended
supramolecular networks.11 Much of this work has been based
on the concept of base pair recognition that mimics the pairing
of adenine (A) with thymine (T) and guanine (G) with cytosine
(C) by hydrogen bond formation that is so important in the
formation of the DNA double helix.12 However, the amount of
literature on the interaction of this type of compound with
transition metal atoms (which are abundant in enzymes) is very
limited.
We have recognized that there is a special class of diamidates
that have no dicarboxylate analogues and that these could give
rise to coupling between dimetal units that might exceed that
which can be achieved in other ways. The precursors of the
entire set of five isomeric cyclic diamidate ligands (or potential
ligands) are depicted in Scheme 3(IIIa); for simplicity all are
shown in the same extreme tautomeric form. Our primary goal
in this work was to see how much synthetic chemistry could
be accomplished by employing these as linkers between two
+
(DAniF)3Mo2 units and measuring electrochemically how
effective they are in mediating electronic communication.
Scheme 3 (IIIb), analogously, shows two similar compounds
that have also been used as linkers. Among those in Scheme 3
(IIIa), only two of the five, B and C, have been incorporated
into the type of product we were seeking. Uracil, A, could not
be incorporated into this type of structure for steric reasons,
but a 1:1 adduct with (DAniF)3Mo2+ has been made. There can
be little doubt that E would also be incapable, for steric reasons,
of serving as a linker (a point to be discussed later). The
precursor of the potential linker D is poorly characterized and
difficult to synthesize without sterically unencumbering sub-
stituents,8 and we have not undertaken to use it.
Experimental Section
Materials and Methods. All manipulations and procedures were
conducted under N2 using either an N2 drybox or standard Schlenk
line techniques. Solvents were distilled and/or degassed immediately
Thus, we report a total of six compounds here in which linkers
B, C, F, G, and H as well as the ligand A have been used. For
(9) Richardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278.
(10) Cotton, F. A.; Donahue, J. P.; Murillo, C. A. Inorg. Chem. 2001, 40, 2229.
(11) For example, see: (a) MacDonald, J. C.; Whitesides, G. M. Chem. ReV.
1994, 94, 2383. (b) Pedireddi, V. R.; Ranganathan, A.; Ganesh, K. N. Org.
Lett. 2001, 3, 99. (c) Prins, L. J.; Hulst, R.; Timmerman, P.; Reinhoudt, D.
N. Chem.sEur. J. 2002, 8, 2288. (d) Falvello, L. R.; Garde, R.; Toma´s,
M. Inorg. Chem. 2002, 41, 4599.
(12) (a)Watson, J. D.; Crick, F. H. C. Nature 1953, 171, 737. (b) Saenger, W.
Principles of Nucleic Acid Structure; Springer-Verlag: New York, 1984.
(c) Neidle, S. Nucleic Acid Structure and Recognition; Oxford University
Press: Oxford, 2002.
(5) (a) Cotton, F. A.; Donahue, J. P.; Lin, C.; Murillo, C. A. Inorg. Chem.
2001, 40, 1234. (b) Cotton, F. A.; Donahue, J. P.; Murillo, C. A. Inorg.
Chem. Commun. 2002, 5, 59. (c) Cotton, F. A.; Donahue, J. P.; Murillo,
C. A. J. Am. Chem. Soc. 2003, 125, 5436.
(6) (a) Cotton, F. A.; Lin, C.; Murillo, C. A. Acc. Chem. Res. 2001, 34, 759.
(b) Cotton, F. A.; Lin, C.; Murillo, C. A. Proc. Nat. Acad. Sci., U.S.A.
2002, 99, 4810.
(7) Cotton, F. A.; Daniels, L. M.; Donahue, J. P.; Liu, C. Y.; Murillo, C. A.
Inorg. Chem. 2002, 41, 1354.
(8) Adachi, J.; Sato, N. J. Heterocyclic Chem. 1986, 23, 871.
9
J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003 8901

A R T I C L E S
Cotton et al.
Scheme 3. Five Monocyclic Diamide Ligands (IIIa)a
a For convenience, these linking ligands are all written in the hydroxyamine tautomeric form. Other available cyclic diamide ligands (IIIb) are also shown
b
as well as the potentially trischelating ligand cyanuric acid (IIIc). These distances are between the Mo atoms and the N or O atoms of the linker.
prior to use; MeCN was twice distilled under N2, first from activated
molecular sieves and then from CaH2, CH2Cl2 was dried and distilled
from P2O5, and THF, Et2O, and hexanes were distilled from Na/K-
benzophenone. Chlorobenzene was not distilled but dried by filtration
through anhydrous Na2SO4 and degassed with vigorous N2 bubbling
confirmed with axial photographs. All data were corrected for Lorentz
and polarization effects. Data were processed using an ellipsoid-mask
algorithm (the program PROCOR16), and the program SORTAV17 was
used to correct for absorption. Data for [Mo2(DAniF)3(uracilate)]‚CH2-
Cl2‚C6H14 (1‚CH2Cl2‚C6H14), [Mo2(DAniF)3]2(µ-2,3-dioxypyrazinate)‚
4CH2Cl2 (3‚4CH2Cl2), [Mo2(DAniF)3]2(µ-2,3-dioxyquinoxalinate)‚
3CH2Cl2 (4‚3CH2Cl2), [Mo2(DAniF)3]2(µ-2,3-dicyano-5,6-dioxypyrazin-
ate)‚2CH2Cl2‚Et2O (5‚2CH2Cl2‚Et2O), and [Mo2(DAniF)3]2(µ-cyanurate)‚
C6H5Cl‚Et2O (6‚C6H5Cl‚Et2O) were collected using a Bruker SMART
1000 CCD area detector system using ω scans. The first 50 frames
were recollected at the end of the data collection to monitor for crystal
decay, but no significant decomposition was observed. Cell parameters
were determined using the program SMART.18 Data reduction and
integration were performed with the software package SAINT,19 which
corrects for Lorentz and polarization effects, while absorption correc-
tions were applied by using the program SADABS.19
13
immediately prior to use. Mo2(DAniF)3Cl2 and 2,3-dihydroxypyra-
zine14 were prepared by literature methods, and all tetraethylammonium
salts of the polyamidate compounds were prepared and isolated as solids
by neutralizing the corresponding diamide compound with 2 equiv of
Et4NOH followed by carefully drying under vacuum.
Physical Methods. Elemental analyses were performed by Canadian
Microanalytical Service, Delta, British Columbia, upon crystalline
samples that were dried overnight under vacuum. Where the presence
of residual solvent was indicated by 1H NMR spectroscopy, it was
1
factored into the expected elemental analysis. H NMR spectra were
recorded on a Varian XL-200AA NMR spectrometer with chemical
shifts referenced to the protonated solvent residual. Absorption spectra
were measured at room temperature under N2 using a Shimadzu UV-
2501 PC spectrophotometer. The cyclic voltammograms and differential
pulse voltmmograms were taken with a CH Instruments model-
CH1620A electrochemical analyzer in 0.1 M Bu4NPF6 solution in CH2-
Cl2 with Pt working and auxiliary electrodes, an Ag/AgCl reference
electrode, and a scan rate of 100 mV/s. All the potential values are
referenced to the Ag/AgCl electrode, and under the present experimental
conditions, the E1/2 (Fc+/Fc) consistently occurred at +440 mV in CH2-
Cl2 and at +574 mV in THF.
X-ray Structure Determinations. The six molybdenum compounds
prepared from ligands A-C and F-H are indicated with numbers in
bold-faced type as 1-3 and 4-6, respectively. Single crystals suitable
for X-ray diffraction analysis were grown by diffusion of Et2O (1, 3-5)
or hexanes (2) into a CH2Cl2 solution of the corresponding product.
Crystals of 6‚C6H5Cl‚Et2O were obtained by slow diffusion of a 1:10
Et2O:hexanes mixture into a saturated C6H5Cl solution. Single-crystal
X-ray work on [Mo2(DAniF)3]2(µ-4,6-dioxypyrimidinate) (2) was
performed on a Nonius Fast diffractometer utilizing the program
MADNES.15 A suitable crystal was mounted on the tip of a quartz
fiber with a small amount of silicone grease and transferred to a
goniometer head. Cell parameters were obtained from an autoindexing
routine and were refined with 250 reflections within a 2θ range 18.1-
41.6°. Cell dimensions and Laue symmetry for the crystals were
In all structures, the positions of the Mo atoms were found via direct
methods using the SHELXL software.20 The positions of the remaining
non-hydrogen atoms were revealed by subsequent cycles of least-
squares refinements followed by difference Fourier syntheses. All
hydrogen atoms, except H(8A) in 1 and H(15) in 6, were added in
calculated positions and refined isotropically as riding atoms with
displacement parameter values equal to 1.2 times those of the carbon
atoms to which they are attached. Cell parameters and refinement results
for all compounds are summarized in Table 1, while key metric
parameters are collected in Tables 2 and 3.
Computational Details. All calculations were performed with the
Gaussian 98 (G98) suite of programs21 using a double-ú basis set (D95)
on the C, N, O, and H atoms.22 A small (1s2s3s2p3p3d) effective core
potential (ECP) was used for the Mo atoms with a double-ú quality
basis set.23 To investigate the electronic structure of compounds 2 and
(16) (a) Kabsch, W. J. Appl. Crystallogr. 1988, 21, 67. (b) Kabsch, W. J. Appl.
Crystallogr. 1988, 21, 916.
(17) Blessing, R. H. Acta Crystallogr. 1995, A51, 33.
(18) SMART for Windows NT, version 5.618; Bruker Analytical X-ray Sys-
tems: 2000.
(19) SAINT+ for NT, Version 6.28A.; Bruker Analytical X-ray Systems: 2001.
(20) Sheldrick, G. M. SHELX-97 Programs for Crystal Structure Analysis.
Institu¨t fu¨rAnorganische Chemie der Universita¨t: Tammanstrasse 4,
D-3400, Go¨ttingen, Germany, 1998.
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
(13) Cotton, F. A.; Daniels, L. M.; Jordan, G. T., IV; Lin, C.; Murillo, C. A. J.
Am. Chem. Soc. 1998, 120, 3398.
(14) Adachi, J.; Sato, N. J. Org. Chem. 1972, 37, 221.
(15) Pflugrath, J. W.; Messerschmidt, A. MADNES, Munich Area Detector (New
ECC) System, version EEC 11/1/89, with enhancements by Nonius
Corporation, Delft, The Netherlands. A description of MADNES appears
in the following: Messerschmidt, A.; Pflugrath, J. W. J. Appl. Crystallogr.
1987, 20, 306.
9
8902 J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003

Cyclic Polyamidato Dianions as Bridges between Mo24+ Units
A R T I C L E S
Table 1. Crystallographic Data for the Linked Compounds [Mo2(DAniF)3]2(OCN-X-NCO) and for 1‚CH2Cl2‚C6H14
compound
formula
1‚CH Cl2‚C H
2
3‚4CH Cl2
4‚3CH Cl2
5‚2CH Cl2‚Et2O
6‚C H Cl‚Et2O
2
6
14
2
2
2
6 5
C56H64Cl2Mo2-
N8O8
1239.93
triclinic
P1h
11.0140(8)
15.354(1)
17.810(1)
64.940(1)
87.207(1)
86.798(1)
2723.1(3)
2
C94H92Mo4-
C98H100Cl8Mo4-
C101H100Cl6Mo4-
C102H104Cl4Mo4-
C103H106ClMo4-
15O16
N
14O14
N14O14
N14O14
N16O15
N
fw, g mol-1
crystal system
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
2025.58
monoclinic
P21/n
10.613(2)
24.806(4)
33.840(2)
90
90.72(1)
90
8909(2)
4
2365.28
triclinic
P1h
2330.41
triclinic
P1h
2319.57
monoclinic
P2/c
16.581(3)
17.679(3)
18.365(3)
90
110.131(3)
90
5055(2)
2
2229.24
monoclinic
C2/c
30.021(2)
11.8606(9)
56.867(4)
90
94.857(1)
90
20176(3)
8
12.467(1)
14.620(2)
15.275(2)
71.936(2)
83.479(2)
71.188(2)
2505.3(5)
1
12.5325(8)
14.3280(9)
15.380(1)
72.668(1)
84.678(1)
72.656(1)
2516.4(3)
1
Z
dcalc
1.512
1.510
1.568
1.538
1.524
1.468
(g cm-3
)
µ (mm-1
2θ range
(deg)
)
0.621
4.4-55.0
0.623
4.0-45.1
0.722
3.5-45.1
0.716
4.2-50.1
0.663
3.3-45.1
0.585
3.2-45.1
λ, Å
0.710 73
-60
0.710 73
-60
0.710 73
-60
0.710 73
-60
0.710 73
-60
0.710 73
-60
T, °C
goodness-of-fit
1.041
1.102
1.041
1.079
1.142
1.088
R1a, wR2b (I > 2σ(I))
0.046, 0.120
0.052, 0.118
0.054, 0.144
0.054, 0.139
0.067, 0.155
0.073, 0.154
2
2
2
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo - Fc )2]/w(F0 )2]1/2, w ) 1/[σ2(Fo ) + (aP)2 + bP], where P ) [max(Fo or 0) + 2(Fc )]/3.
Table 2. Selected Interatomic Distances for Compounds 1-6
1
2
3
4
5
6
d,a Å
Mo(1)-Mo(2)
Mo(3)-Mo(4)
Mo-Ob
7.256
7.081
7.089
2.0904(6)
7.128
7.318
2.0938(4)
2.128(3)
2.200(3)
2.0893(8)
2.0844(8)
2.099(4)
2.121(4)
2.229(5)
2.212(5)
2.131(5)
2.151(5)
2.137(5)
2.174(5)
2.131(5)
2.149(5)
2.137(6)
2.101(5)
2.142(5)
2.155(6)
2.132(5)
2.120(5)
2.0917(7)
2.153(4)
2.148(4)
2.095(1)
2.145(5)
2.192(5)
2.097(1)
2.096(1)
2.136(5)
2.128(5)
2.245(6)
2.247(6)
2.147(7)
2.126(6)
2.144(8)
2.145(7)
2.124(7)
2.136(8)
2.163(8)
2.120(7)
2.133(8)
2.153(7)
2.131(7)
2.158(8)
2.21(1)
Mo-Nb
2.16(1)
Mo(1)-N(1)
Mo(1)-N(3)
Mo(1)-N(5)
Mo(2)-N(2)
Mo(2)-N(4)
Mo(2)-N(6)
Mo(3)-N(7)
Mo(3)-N(9)
Mo(3)-N(11)
Mo(4)-N(8)
Mo(4)-N(10)
Mo(4)-N(12)
2.169(3)
2.120(3)
2.182(3)
2.144(3)
2.127(3)
2.126(3)
2.161(5)
2.131(5)
2.159(5)
2.122(5)
2.147(5)
2.142(5)
2.171(4)
2.126(4)
2.166(4)
2.136(4)
2.144(4)
2.148(4)
2.178(6)
2.138(6)
2.170(6)
2.120(7)
2.133(6)
2.130(7)
selected bond distances
for linkers
O(7)-C(48)
1.277(4)
O(8)-C(49)
1.249(5)
O(1)-C(1)
1.292(7)
O(2)-C(2)
1.288(7)
O(7)-C(47)
1.326(7)
N(7)-C(47)
1.318(7)
O(7)-C(1)
1.296(6)
N(7)-C(1)
1.368(6)
O(7)-C(46)
1.278(8)
N(7)-C(46)
1.346(9)
O(15)-C(3)
1.211(9)
O(13)-C(1)
1.280(9)
N(7)-C(48)
1.365(5)
N(7)-C(49)
1.366(5)
C(1)-N(13)
1.395(8)
C(2)-N(14)
1.395(9)
N(7)-C(48)
1.38(1)
C(47)-C(47A)
1.45(1)
C(1)-C(1A)
1.459(8)
N(7)-C(7A)
1.45(2)
N(7)-C(47)
1.387(9)
N(8)-C(48)
1.15(1)
O(14)-C(2)
1.277(9)
N(13)-C(1)
1.32(1)
a d ) distance between centroids of the two Mo2 units. b Distances to linker N and O atoms.
3, single point energy calculations24 were performed using density
functional theory (DFT)25 with the Becke three parameter hybrid
exchange functional26 and the Lee-Yang-Parr correlation functional
(B3LYP).27 For feasibility purposes, the calculations were simplified
by replacing each p-anisyl substituent with a hydrogen atom. To gain
insight into the electronic transitions responsible for the observed UV-
vis spectra of 2 and 3, time-dependent density functional theory28
(TDDFT) calculations were performed on Origin 2000 and Origin 3800
computers at the Texas A&M University Supercomputing facility and
on an SGI Power Challenge in the Department of Chemistry, Texas
A&M University.
Preparation of Compounds. The procedure used was that of method
B in ref 5a.
Mo2(DAniF)3(uracilate) (1). Tetraethylammonium uracilate was
used in the stoichiometric ratio of 1:1 with Mo2(DAniF)3Cl2. Yield:
yellow prisms, 40%. H NMR (ppm in CD2Cl2): 10.23 (br, s, 1H,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.6, A.7, and A.9;
Gaussian, Inc.: Pittsburgh, PA 1998.
1
N-H), 8.52 (s, 2H, -NCHN-), 8.42 (s, 1H, aromatic C-H), 6.98
(m, 2H, aromatic C-H), 6.38-6.74 (m, 20H, aromatic C-H), 6.10-
6.24 (m, 4H, aromatic C-H), 3.71 (s, 6H, -OCH3), 3.65 (s, 6H,
-OCH3), 3.64 (s, 18H, -OCH3). Absorption spectrum (CH2Cl2) λmax
(ꢀM): 257 (48 700), 297 (50 200), 405 (7230), 460 (3880). Anal. Calcd
for C49H48Mo2N8O8: C, 55.06; H, 4.53; N, 10.48. Found: C, 54.75;
H, 4.45; N, 10.38%.
(22) Dunning, T. H.; Hay, P. J. In Methods of Electronic Structure Theory, Vol.
3; Schaefer, H. F., III, Ed.; Plenum Press: New York, 1977.
(23) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(24) The convergence criterion for the SCF was increased from a default value
of 10-4 to 10-8
.
(25) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules;
Oxford University Press: Oxford, 1989.
(26) (a) Becke, A. D. Phys. ReV. A. 1998, 38, 3098. (b) Becke, A. D. J. Chem.
Phys. 1993, 98, 1372. (c) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(27) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B 1998, 37, 785.
(28) Casida, M. E.; Jaorski, C.; Casida, K. C.; Salahub, D. R. J. Chem. Phys.
1998, 108, 4439.
[Mo2(DAniF)3]2(µ-4,6-dioxypyrimidinate) (2). Yield: orange plates,
1
40%. H NMR (δ in CD2Cl2): 8.58 (s, 4H, -NCHN-), 8.30 (s, 1H,
pyrimidinate C-H), 8.25 (s, 2H, -NCHN-), 6.59-6.70 (m, 16H,
9
J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003 8903

A R T I C L E S
Cotton et al.
Table 3. Selected Angles for Compounds 1-6
1
2
3
4
5
6
Φ,a deg
35.9
0
0
5.1
3.2
83.8(2)
X(1)-Mo(1)-N(1)b
X(1)-Mo(1)-N(3)b
X(1)-Mo(1)-N(5)b
X(2)-Mo(2)-N(2)b
X(2)-Mo(2)-N(4)b
X(2)-Mo(2)-N(6)b
X(3)-Mo(3)-N(7)
X(3)-Mo(3)-N(9)
X(3)-Mo(3)-N(11)
X(4)-Mo(4)-N(8)
X(4)-Mo(4)-N(10)
X(4)-Mo(4)-N(12)
N(1)-Mo(1)-N(5)
N(2)-Mo(2)-N(6)
N(7)-Mo(3)-N(11)
N(8)-Mo(4)-N(12)
90.9(1)
177.7(1)
90.4(1)
87.3(1)
171.6(1)
89.4(1)
85.9(2)
170.2(2)
88.9(2)
92.5(2)
176.8(2)
85.5(2)
85.8(2)
171.6(2)
90.1(2)
91.3(2)
176.1(2)
84.7(2)
172.9(2)
174.0(2)
174.1(2)
172.9(2)
87.7(2)
174.5(2)
87.4(2)
86.1(2)
174.5(2)
89.6(2)
92.1(3)
172.7(2)
84.4(3)
84.4(3)
173.7(3)
92.2(3)
91.9(2)
176.6(2)
88.1(2)
89.1(2)
173.0(2)
90.3(2)
171.3(3)
88.3(2)
88.4(2)
177.7(3)
92.1(3)
85.5(3)
170.8(2)
88.1(2)
88.8(2)
177.5(3)
92.6(2)
170.9(3)
174.8(3)
172.5(3)
173.9(3)
175.4(1)
173.1(1)
173.4(2)
172.9(2)
174.3(1)
173.1(1)
175.2(2)
173.6(3)
selected bond angles
for linker
C(48)-N(7)-C(49) O(1)-C(1)-N(13)
O(7)-C(47)-N(7)
119.3(5)
O(7)-C(1)-N(7)
122.5(8)
O(7)-C(46)-N(7)
119.2(6)
O(1)-C(1)-N(13)
122.9(7)
120.1(3)
116.6(5)
O(7)-C(48)-N(7)
118.2(3)
O(2)-C(2)-N(14)
117.6(6)
C(47)-N(7)-C(48)
116.9(7)
C(1)-N(7)-C(7A) C(46)-N(7)-C(47)
124(1) 116.3(6)
O(2)-C(2)-N(14)
122.6(7)
O(8)-C(49)-N(7)
120.8(3)
N(13)-C(3)-N(14) N(7)-C(7)-C(47A) O(7)-C(1)-C(1A) N(7)-C(46)-C(46A) N(13)-C(3)-O(3)
127.9(6)
121.0(7)
118.4(7)
120.7(4)
120.2(7)
O(8)-C(49)-N(8)
120.2(3)
C(1)-C4)-C(2)
120.4(7)
a Φ is the twist angle between the Mo2 axes. b For 2 and 6, X(1) ) O(1), X(2) ) N(13), X(3) ) O(2), X(4) ) N(14); for 1, 3 - 5, X(1) ) N(7), X(2)
) O(7).
aromatic C-H), 6.50 (s, 1H, pyrimidinate C-H), 6.45 (d, 4H, aromatic,
C-H), 6.48 (d, 8H, aromatic C-H), 6.40 (d, 4H, aromatic C-H), 6.24
(d, 4H, aromatic C-H), 6.10 (d, 4H, aromatic C-H), 5.84 (d, 4H,
aromatic C-H), 3.66 (s, 12H, -OCH3), 3.59 (s, 12H, -OCH3), 3.58
(s, 6H, -OCH3), 3.51 (s, 6H, -OCH3). The resonance for the N-H
hydrogen of the cyanurate linker was obscured in this spectrum. MS
ESI+: 1021 (MH22+/2), 1086 (MH+ - Mo2(DAniF)3).
C-H), 6.17-6.32 (m, 16H, aromatic C-H), 6.09 (d, 8H, aromatic
C-H), 5.96 (d, 4H, aromatic C-H), 3.71 (s, 12H, -OCH3), 3.64 (s,
6H, -OCH3), 3.59 (s, 18H, -OCH3). Absorption spectrum (CH2Cl2)
λmax (ꢀM): 269 (106 000), 295 (103 000), 337 (sh, 62 800), 405 (sh,
10 600), 469 (4270). Anal. Calcd for [Mo2(DAniF)3]2(µ-4,6-dioxy-
pyrimidinate)‚0.5CH2Cl2, C99.5H93Cl1Mo4N14O14: C, 54.88; H, 4.53; N,
9.48. Found: C, 54.80; H, 4.58; N, 9.54%.
Results and Discussion
[Mo2(DAniF)3]2(µ-2,3-dioxy-1,4-pyrazinate) (3). Yield: yellow-
orange needles, 42%. 1H NMR (ppm in CD2Cl2): 8.53 (s, 2H,
-NCHN-), 8.40 (s, 4H, -NCHN-), 7.87 (dd, 1H, aromatic C-H),
6.18-6.67 (m, 48H, aromatic C-H), 5.99(dd, 1H, aromatic, C-H),
3.73 (s, 6H, -OCH3), 3.68 (s, 12H, -OCH3), 3.67 (s, 12H, -OCH3),
3.64 (s, 6H, -OCH3). Absorption spectrum (CH2Cl2) λmax (ꢀM): 267
(104 000), 294 (106 000), 420 (16 100), 477 (14 000). Anal. Calcd for
C94H92Mo2N14O14: C, 55.74; H, 4.58; N, 9.68. Found: C, 54.53; H,
4.59; N, 9.46%.
Syntheses. The method of synthesis involves reduction of
the mixed valent Mo2(DAniF)3Cl2 compound with Zn metal in
acetonitrile (eq 1) followed by an in situ reaction with the
corresponding dianion of the linker, L, as its Et4N+ salt, as
shown in eq 2:
2Mo2(DAniF)3Cl2 + Zn + 4MeCN f
[Mo2(DAniF)3(MeCN)2]2ZnCl4 (1)
[Mo2(DAniF)3]2(µ-2,3-dioxyquinoxalinate) (4). Yield: orange blocks,
43%. H NMR (ppm in CD2Cl2): 8.64 (s, 2H, -NCHN-), 8.35 (s,
2[Mo2(DAniF)3(MeCN)2]+ + L2-
f
1
4H, -NCHN-), 6.54-6.70 (m, 20H, aromatic C-H), 6.41-6.48 (m,
20H, aromatic C-H), 6.33(d, 4H, aromatic C-H), 6.24 (d, 4H, aromatic
C-H), 6.19 (d, 2H, aromatic C-H), 4.53 (m, 2H, aromatic C-H),
3.74 (s, 6H, -OCH3), 3.67 (s, 12H, -OCH3), 3.65 (s, 62H, -OCH3),
3.64 (s, 6H, -OCH3). Absorption spectrum (CH2Cl2) λmax (ꢀM): 267
(sh, 106 000), 278 (107 000), 306 (100 000), 447 (sh, 12 100), 479
(15 200), 511 (sh, 11800). Anal. Calcd for C98H94Mo4N14O14: C, 56.71;
H, 4.56; N, 9.45. Found: C, 56.87; H, 4.59; N, 9.39%.
(DAniF)3Mo2-L-Mo2(DAniF)3 (2)
The neutral linked complexes generally precipitate directly from
acetonitrile solution as microcrystalline solids that are conve-
niently isolated by filtration. Although air-sensitive in solution,
these compounds are moderately stable to air as solids but are
best stored for prolonged periods of time under N2.
In the particular case of uracilate, it was found that the 1:1
adduct, 1, was the only species isolated regardless of whether
the mono- or dianion was used and despite variation in the
stoichiometry of the uracilate dianion that was employed. This
result appears to be due to the prohibitive steric congestion that
[Mo2(DAniF)3]2(µ-2,3-dicyano-5,6-dioxypyrazinate) (5). Yield:
violet blocks, 35%. 1H NMR (ppm in CD2Cl2): 8.65 (s, 2H,
-NCHN-), 8.42 (s, 4H, -NCHN-), 6.42-6.68 (m, 36H, aromatic
C-H), 6.44 (d, 4H, aromatic C-H), 6.30 (d, 4H, aromatic C-H), 6.20
(d, 4H, aromatic C-H), 3.72 (s, 6H, -OCH3), 3.70 (s, 12H, -OCH3),
3.68 (s, 12H, -OCH3), 3.64 (s, 6H, -OCH3). Absorption spectrum
(CH2Cl2) λmax (ꢀM): 258 (89 600), 280 (89 500), 300 (88 100), 344 (sh,
48 000), 484 (sh, 11 600), 513 (14 200), 550 (12 300). Anal. Calcd for
C96H90Mo4N64O14: C, 55.55; H, 4.37; N, 10.79. Found: C, 55.50; H,
4.45; N, 10.58%.
+
would arise from the juxtaposition of the two Mo2(DAniF)3
units about the uracilate core, a point which is illustrated
schematically in Figure 1a. In this context, the futility of
attempting to employ 3,6-dihydroxypyradizine (E in Scheme 3
(IIIa)) as a linking ligand is patently obvious. As shown in
Figure 1a, 3,6-dihydroxypyradizine would produce even more
1
[Mo2(DAniF)3]2(µ-cyanurate) (6). Yield: yellow plates, 53%. H
NMR (ppm in TDF): 8.66 (s, 4H, -NCHN-), 8.20 (s, 2H,
-NCHN-), 6.76 (d, 8H, aromatic C-H), 6.58-6.63 (m, 16H, aromatic
+
steric crowding between two Mo2(DAniF)3 units than does
9
8904 J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003

Cyclic Polyamidato Dianions as Bridges between Mo24+ Units
A R T I C L E S
complex with a uracilate ligand32 as well as the first in which
uracilate serves as a bridging ligand between two multiply
bonded metal atoms. The closely related 1-methyl uracilate
ligand, however, has been reported in a similar N-C-O binding
mode in several diplatinum(III,III) compounds in which at least
a formal Pt-Pt single bond exists.33
Compound 2 is the first structurally characterized compound
in which the anion of 4,6-hydroxypyrimidine is a ligand to any
sort of transition metal. The only prior study of the coordination
chemistry of this particular ligand deals with measurements of
the stability constants of the complexes formed with CoII, MnII,
NiII, and ZnII without isolation and physical characterization of
the species formed.34 The significant twist angle of 35.9° (Table
3) between the two Mo2 axes is readily apparent in the thermal
ellipsoid drawing in Figure 2. The eight non-hydrogen atoms
of the dioxypyrimidinate linker have a mean deviation of 0.0674
Å from the mean plane which they define, with the greatest
deviation being 0.164 Å for N(14). The appreciable distortion
observed in the crystal structure of 2 may be plausibly attributed
to crystal packing effects since it is the only compound in the
set which crystallizes without solvent in the lattice and since it
has virtually the same size and shape as 6, which has an
essentially planar core.
Compounds 3-5 are related inasmuch as the linking ligands
all contain the 2,3-dioxypyrazine core. All three molecules have
essentially planar core structures with little (5.1° for 5) or no
(for 3 and 4) twist between the Mo2 axes. Molecules 3 and 4
crystallize upon inversion centers in the space group P1h with
the result that the linking ligand is seen as a 1:1 overlay of two
orientations in which the oxygen and nitrogen atoms are
interchanged. For convenience, this disorder may be envisioned
by imagining 3 and 4 to be fixed and then rotating the 2,3-
dioxypyrazinate or 2,3-dioxyquinoxalinate ligand 180° around
the C47-C47A or C1-C1A axis, respectively, to generate its
other positional variant. This imposed disorder introduces large
uncertainties into the bond parameters of these linking ligands
(Tables 2 and 3). In the case of 4, this disorder necessitated
restraining the N(7)-C(1) and O(7)-C(1) bond distances to
chemically reasonable values. In compound 5, there is a 2-fold
axis bisecting the C(46)-C(46A) bond and extending parallel
to the Mo(1)-Mo(2) axis, thereby generating the right half of
the molecule from the left. This symmetry element, however,
does not generate the type of disorder observed in 3 and 4.
Compound 3 is the only structurally characterized transition
metal complex with 2,3-dioxypyrazine in any coordination mode
and, furthermore, appears to be the first structurally characterized
molecule, organic or inorganic, which contains the 2,3-dioxy-
pyrazine fragment. The closest related structures which occur
in the Cambridge Structural Database are those with the 2,3-
piperazinedione core,35 where the C-C bond of the aryl ring
(C46-C48 in 3) has been reduced to a single bond. A molecule
closely related to 4, again with the pivalate anion as supporting
Figure 1. (a) Relative disposition of the Mo2 units with diamidate ligands
A and E. (b) Relative steric requirements of the pivalate and DAniF ligands.
uracilate, so its use as a linker was not attempted here. We note
that Chisholm et al. have succeeded in bridging two Mo24+ units
with the dianion of 3,6-dihydroxypyradizine.29 In this system,
however, the sterically less demanding pivalate ion was
employed as supporting ligand on the Mo2 units; the comparative
steric requirements of the two ligands are made clear in Figure
1b.
Structures. The core structures of molecules 1-6 are
presented in Figure 2 with 50% probability ellipsoids and all
p-anisyl substituents and hydrogen atom omitted for clarity
except for the N-H hydrogen atoms in 1 and 6. Cell parameters
and refinement statistics for all the structures are presented in
Table 1, while selected bond distances and angles are collected
in Tables 2 and 3, respectively. The separation between the
4+
centroids of the Mo2 units, d, for 2-6 falls within the
relatively narrow range 7.08-7.32 Å, thus permitting an
assessment of the effects of the linker in mediating electronic
communication at approximately constant d (Vide infra).
Compound 1 was the species isolated and crystallized in
attempts to prepare a linked molecule, but it is merely one
isomer of the two conceivable ones which might arise in the
formation of a 1:1 adduct between Mo2(DAniF)3+ and uracilate.
1
Furthermore, as revealed by H NMR, 1 is stable in CH2Cl2
solution and does not exist in equilibrium with the isomer in
which N1 of uracil is coordinated to molybdenum (see Figure
1a for numbering scheme). The N3 proton which has been
removed is the less acidic of the two (pKa ) 9.43,30 although
the difference in relative acidity between the two sites is only
slight), suggesting that coordination at N3 is preferred because
it is the more basic site. The origin of the proton on N1 of the
uracilate ligand (N8 in Figure 2) may be due to either incomplete
deprotonation by 2 equiv of Et4NOH (pKa ) 13.5 for the second
deprotonation) or perhaps incomplete drying of the (Et4N)2-
(uracilate) salt once it is formed. The position of H(8A) in 1
(Figure 2) was visible in the final difference maps and was
permitted to refine.
The transition metal coordination chemistry of uracilate has
been rather unevenly explored but is perhaps furthest developed
for PtII, where, among the more notable achievements that have
been reported, uracilate has served as a linker in the formation
of cationic squares with (en)Pt2+ corner endpieces.31 Compound
1 appears to be the only structurally characterized molybdenum
(31) (a) Rauter, H.; Hillgeris, E. C.; Lippert, B. Chem. Commun. 1992, 1385.
(b) Rauter, H.; Hillgeris, E. C.; Erxleben, A.; Lippert, B. J. Am. Chem.
Soc. 1994, 116, 616. (c) Navarro, J. A. R.; Freisinger, E.; Lippert, B. Eur.
J. Inorg. Chem. 2000, 147.
(32) The only other molybdenum complex with uracilate as a ligand which can
be found in the literature was not structurally characterized: Dias, A. R.;
Duarte, M. T.; Galva˜o, A. M.; Garcia, M. H.; Marques, M. M.; Salema,
M. S. Polyhedron 1995, 14, 675.
(33) (a) Lippert, B.; Scho¨llhorn, H.; Thewalt, U. Inorg. Chem. 1986, 25, 407.
(b) Scho¨llhorn, H.; Eisenmann, P.; Thewalt, U.; Lippert, B. Inorg. Chem.
1986, 25, 3384.
(29) Cayton, R. H.; Chisholm, M. H.; Putilina, E. F.; Folting, K. Polyhedron
1993, 12, 2627.
(30) Jona´sˇ, J.; Gut, J. Collect. Czech. Chem. Commun. 1962, 27, 716.
(34) Sengupta, S.; Bannerjee, N. R. J. Ind. Chem. Soc. 1990, 67, 313.
9
J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003 8905

A R T I C L E S
Cotton et al.
Figure 2. The core structures of the six polyamido molybdenum compounds. Displacement ellipsoids are drawn at the 50% probability level. The p-anisyl
groups attached to the N atoms and all hydrogen atoms are omitted for clarity, except for H(8A) on 1 and H(15) in 6. To the right of each structure, the
number of each compound, the cyclic voltammogram (above) and the differential pulse voltammogram (below) are shown. These electrochemical measurements
were recorded in CH2Cl2 solutions.
ligand, was previously reported by Chisholm et al.2,29 although
it was not structurally characterized by X-ray diffraction.
Compound 4 is the first crystallographically characterized
example of a transition metal complex in which 2,3-dioxyqui-
noxalinate coordinates in the N-C-O amidate mode rather than
as an ene-1,2-diolate ligand. The only other crystallographically
characterized transition metal complexes with this ligand, [FeIII-
(2,3-dioxyquinoxalinate)3]3- 36 and Cp*Ru(NO)(2,3-dioxyqui-
noxalinate),37 display the latter binding mode. Molecule 5, like
4, is the sole instance of a structurally characterized transition
9
8906 J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003

Cyclic Polyamidato Dianions as Bridges between Mo24+ Units
A R T I C L E S
Table 4. Electrochemical Dataa for Compounds 1 - 6
metal compound in which its ligand coordinates in the amidate
motif rather than as an ene-1,2-diolate ligand. The transition
metal chemistry of 2,3-dioxy-5,6-dicyanopyrazine is quite scant
and limited to a few reports describing its utility in the formation
of extended two-dimensional structures with CuII.38
peak positionsb (mV)
1/2 (1+/0)
∆E1/2(mV)
E
E1/2 (2+/1+)
K c
c
1
2
3
4
5
6
172
99
131
156
339
272
286
389
464
602
424
187
258
308
263
152
1.5 × 103
2.3 × 104
1.6 × 105
2.8 × 104
3.7 × 102
The coordination chemistry of cyanuric acid (H in Scheme
3a) is confined to simple monodentate O- or N-bound complexes
with middle and late first row transition metal ions, principally
CoIII,39 MnII,39 CuII,40 and NiII.41 Compound 6 is thus the only
crystallographically characterized complex of this ligand with
molybdenum, the sole example in which cyanurate is coordi-
nated to any sort of M2 unit and, furthermore, the only case in
which the N-C-O amidate coordination mode is observed for
this ligand. The core structure of 6 is essentially planar, in
contrast to that of 2, with only a slight twist angle of 3.2°
between the Mo2 units. The short C(3)-O(3) bond distance of
1.211(9) Å in the cyanurate ligand is indicative of a double
bond. The diffraction data for this compound was of a quality
sufficient to reveal the presence of the hydrogen atom bound
to N(15), and its position was refined. It is clear that the same
steric considerations that preclude formation of a linked species
with the uracilate dianion (Figure 1a) also render impossible
the formation of the trigonally symmetric molecule [Mo2-
(DAniF)3]3(µ3-cyanurate).
The structures of three of the molecular precursors of the six
linking ligands, namely uracil,42 1,4-dihydro-2,3-quinoxalinedi-
one,43 and cyanuric acid,44 have been previously described and
provide useful comparisons for assessing the tautomeric form
of the ligand anions as coordinated to the Mo2 unit. These three
neutral ligands all exist in the keto amine tautomeric form in
the crystalline state, a formulation born out by the short carbon-
oxygen bond distances and by the visibility of the hydrogen
atoms themselves on the nitrogen atoms in the electron density
maps. In the corresponding Mo2 compounds, the carbon oxygen
bond lengths are substantially lengthened, while the carbon
nitrogen bond lengths are relatively unaffected. In compounds
3 and 4, it should be noted that the previously described
crystallographic disorder renders these bond distances consider-
ably less reliable then those in the other compounds. Taken as
a whole, however, the structural results support a description
of the ligand anions as enolate type anions rather than as keto
aminate anions.
a All potentials are referenced to Ag/AgCl. Data for 1-5 are from CH2Cl2
solutions, while data for 6 are from a THF solution; E1/2 for Fc+/Fc is +440
b
mV in CH2Cl2 and +574 mV in THF. E1/2 ) (Epa + Epc)/2 from the CV.
1/2/25.69
c Kc is calculated from the formula Kc ) e∆E
.
Table 5. Electrochemical Data for Selected Linked Pairs of
4+
Mo2 Units
peak positions (mV)
E
E
∆E
1/2
1/2
1/2
(1+/0) (2+/1+) (mV)
K c
reference
c
[Mo2(DAniF)3]2(µ-O2CCO2)
[Mo2(DAniF)3]2(µ-SO4)
[Mo2(DAniF)3]2(µ-MoO4)
[Mo2(DAniF)3]2(µ-WO4)
[Mo2(DAniF)3]2(µ-Cl)4
+294 +506 212 3.8 × 103
+93 +321 228 7.2 × 103
+81 +392 311 1.8 × 105
+102 +387 285 6.6 × 104
+271 +798 527 8.1 × 108
5a,c
10
10
10
a
b
b
a
[Mo2(DAniF)3]2(µ-Zn(OMe)4)
[Mo2(DAniF)3]2(µ-Co(OMe)4)
-208
-205
30 238 1.1 × 104
60 265 3.0 × 104
[Mo2(DAniF)3]2(µ-fluoflavinate) +140 +1020 880 7.5 × 1014
a Unpublished data. b Cotton, F. A.; Liu, C. Y.; Murillo, C. A. Inorg.
Chem. 2003, 42, in press.
∆E1/2 values sufficiently high as to permit their determination
as E(2)1/2-E(1) in a straightforward manner directly from the
1/2
cyclic voltammograms. The pyrazinate linkers in 3-5 are all
more effective than oxalate (See Table 5) in delocalizing charge
in the mixed valent species and thereby mediating electronic
communication between the two Mo2 units. These ∆E1/2 values
are ∼45, ∼95, and ∼50 mV greater, respectively, than that of
the oxalate linked molecule despite having slightly greater values
of d (d ) 6.953 Å for [Mo2(DAniF)3]2(µ-O2CCO2) (see Table
5)). Comparing 4 and 5, one observes that benzannulation of
the pyrazine ring has a significantly greater effect upon ∆E1/2
than substitution with cyano groups. Compound 4 therefore
represents the best candidate in this series for the successful
isolation of a one-electron oxidized species and is the object of
current research in this laboratory. We note that Chisholm’s
analogue to 4,29 which has Mo2(O2CCMe3) units in lieu of Mo2-
(DAniF)3 units, has a ∆E1/2 of 366 mV rather than 308 mV.
This difference corresponds to an order of magnitude increase
in Kc (1.5 × 106) over our compound 4 (1.6 × 105). For
molecule 5, the E(1)1/2 and E(2)1/2 values are shifted approximately
200 mV to more strongly oxidizing potentials relative to 3 due
to the highly electron withdrawing nature of the cyano groups.
The potentials for 4 are also shifted toward higher values relative
to 3, but as one might anticipate, the effect is much less
pronounced since 4 has only the effect of increased delocal-
ization but not the inductive influence of the highly electron
withdrawing cyano substituents.
Electrochemistry. The electrochemical results are presented
graphically in Figure 2 and summarized numerically in Table
4. The compounds with pyrazine-type linkers, 3-5, all have
(35) For instance, see: (a) Lenstra, A. T. H.; Bracke, B.; van Dijk, B.; Maes,
S.; Vanhulle, C.; Desseyn, H. O. Acta Crystallogr. 1998, B54, 851. (b)
Bellouard, F.; Chuburu, F.; Kervarec, N.; Toupet, L.; Triki, S.; Le Mest,
Y.; Handel, H. J. Chem. Soc., Perkin Trans. 1 1999, 3499.
(36) Mun˜oz, M. C.; Ruiz, R.; Traianidis, M.; Aukauloo, A.; Cano, J.; Journaux,
Y.; Ferna´ndez, I.; Pedro, J. R. Angew. Chem., Int. Ed. Engl. 1998, 37, 1834.
(37) Yang, K.; Bott, S. G.; Richmond, M. G. J. Chem. Cryst. 1995, 25, 283.
(38) Adachi, K.; Sugiyama, Y.; Kumagai, H.; Inoue, K.; Kitagawa, S.; Kawata,
S. Polyhedron 2001, 20, 1411.
(39) Falvello, L. R.; Pascual, I.; Toma´s, M. Inorg. Chim. Acta 1995, 229, 135.
(40) (a) Falvello, L. R.; Pascual, I.; Toma´s, M.; Urriolabeitia, E. P. J. Am. Chem.
Soc. 1997, 119, 11894. (b) Hart, R. D.; Skelton, B. W.; White, A. H. Aust.
J. Chem. 1992, 45, 1927. (c) Server-Carrio, J.; Folgado, J.-V. Polyhedron
1998, 17, 1495.
(41) (a) Palade, T.; Nutiu, M. ReV. Chim. 1986, 37, 80. (b) Falvello, L. R.;
Pascual, I.; Toma´s, M. Inorg. Chim. Acta 1995, 229, 135, (c) Falvello, L.
R.; Hitchman, M. A.; Palacio, F.; Pascual, I.; Schultz, A. J.; Stratemeier,
H.; Toma´s, M.; Urriolabeitia, E. P.; Young, D. M. J. Am. Chem. Soc. 1999,
121, 2808.
(42) Stewart, R. F.; Jensen, L. H. Acta Crystallogr. 1967, 23, 1102.
(43) Svensson, C. Acta Chem. Scand. 1976, B30, 581.
(44) Coppens, P.; Vos, A. Acta Crystallogr. 1971, B27, 146.
Compound 2 is a significantly poorer mediator of the effect
of oxidation of one Mo2 unit upon the other, with a ∆E1/2 of
187 mV. This point is further underscored by considering that,
since 2 is a relatively rigid molecule and its structure in the
crystalline state and in solution ought to be essentially the same,
a more meaningful value for d in the context of understanding
the electrochemistry is the distance between Mo(2) and Mo(4)
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A R T I C L E S
Cotton et al.
Figure 3. Comparison of the cyclic voltammetry of 6 in THF (blue trace)
vs CH2Cl2 (red trace).
(6.357 Å in Figure 2) since that value represents the closest
approach of the Mo2 units. Despite having the ends of the Mo2
units substantially closer to one another than 3-5 and arguably
contributing more to ∆E1/2 through electrostatic or through-space
effects, 2 still has a substantially lower ∆E1/2. Within the present
set of compounds, no comparisons can be made which would
separate the relative contributions of electrostatic and orbital
coupling effects.
The electrochemical traces for compound 6 are atypically
broad, most notably in the differential pulse voltammogram of
Figure 2, and suggest that the interpretation of its electrochem-
istry is not as straightforward as it is with the other compounds.
Figure 4. UV-vis spectra in CH2Cl2 of selected compounds as identified
by their cyclic polyamidate linker. (Upper panel) Spectra of the three
compounds with pyrazine-type linkers. (Lower panel) Comparison of the
two structural isomers with 2,3-dioxypyrazinate (red) and 4,6-dioxypyri-
midinate (blue) as linkers.
1
The H NMR spectroscopy of 6 indicates a complex solution
behavior for this molecule which is not observed for 1-5. At
least three distinct species appear to be present in significant
amounts in dichloromethane solution even when highly crystal-
line samples of 6 are used. The origin and identities of these
species are unclear, but possible explanations include the
existence of an equilibrium between N-H and O-H tautomers
of the µ-cyanurate linker, partial dissociation of 6 into unlinked
species, and/or the formation of N-chloro cyanurate species in
feature in individual Mo2 quadruply bonded compounds with a
ground state electronic configuration of σ2π4δ2. This δ f δ*
1
1
transition, also known as A1g f A2u, is due to the excitation
of one electron to the σπδδ* state and is intrinsically weak
due to the small overlap integral between δ orbitals, even though
such transitions are symmetry allowed.47 For example, when
the ligands are carboxylate, sulfate, or water, the intensities of
the δ f δ* transition have ꢀmax values of a few hundreds up to
∼1000. The intensities can increase but usually by not much
more than a factor of 2 when mixing with MLCT transitions
occurs.
1
the presence of chloroalkanes.45 The H NMR spectrum in
deuterated THF indicates that formation of other species is
largely suppressed although not completely eliminated in this
solvent. This point is corroborated by the observation of
significantly better resolved electrochemical traces for 6 when
these measurements are repeated in THF rather than CH2Cl2
(Figure 3). The values for 6 in Table 4 are those obtained from
a THF solution.
The absorption spectra of the cyclic polyamidate linked
molecules 2-5 were initially expected to be very similar to the
polyunsaturated dicarboxylate linked compounds of type Ib but
instead were observed to display significant differences (Figure
4). The electronic spectra of 3, 4, and 5 (Figure 4, upper panel)
reveal at least two low energy features of comparable intensity.
These features are well resolved in 3 and less so in 4 and 5. In
4, it appears that three distinct absorptions exist, two of which
merely appear as shoulders. The transitions responsible for these
colors must involve orbitals of the pyrazinate linker since that
is the only variable being changed in this set of compounds.
The pronounced shift to lower energy in moving from 3 to 5
accords with the intuitive expectation that the energy of the π
Electronic Spectra and Structures. In our preceding study
of the spectra and electronic structures of compounds of type
Ib with polyunsaturated dicarboxylate linkers,46 time-dependent
DFT calculations revealed the lowest energy absorptions to be
HOMO f LUMO excitations which were essentially Mo2 δ
f dicarboxylate π* metal-to-ligand charge transfers. These
features in the electronic spectra were noteworthy because they
were significantly lower in energy and higher in intensity than
the δ f δ* transition typically observed as the lowest energy
(45) Elemental analyses upon samples of 6 which have been dissolved in CH2-
Cl2 and evaporated to dryness indicate the presence of substantial amounts
of residual chlorine.
(46) Cotton, F. A.; Donahue, J. P.; Murillo, C. A.; Pe´rez, L. M. J. Am. Chem.
Soc. 2003, 125, 5486.
(47) Cotton. F. A.; Walton, R. A. Multiple Bonds between Metal Atoms;
Clarendon: Oxford, 1993.
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8908 J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003

Cyclic Polyamidato Dianions as Bridges between Mo24+ Units
A R T I C L E S
Figure 5. Illustration of the 0.04 contour surface diagram of the time-dependent DFT calculated molecular orbital responsible for the two lowest energy
transitions in molecules 2 and 3. In the case of 2, the MOs involved are primarily metal based. For 3, substantial mixing occurs between the molybdenum
atoms and the diamidate linking ligand. The lowest energy transition in 3 is a HOMO f LUMO transition.
system of the pyrazine core would be lowered by benzannulation
to form the quinoxaline system and by substitution with highly
electron withdrawing cyano groups in 5. A second interesting
comparison to note is that between molecules 2 and 3 (Figure
4, lower panel). The significant differences in their absorption
maxima and band intensities suggest the involvement of
molecular orbitals of substantially different composition.
Time-dependent DFT calculations upon molecules 2 and 3
were undertaken to obtain at least a qualitative insight into the
nature of the absorption bands shown in Figure 4. The results
indicate that not only are 2 and 3 quite different from the
polyunsaturated dicarboxylate linked molecules such as [Mo2-
(DAniF)3]2(µ-trans-O2CCHdCHCO2) but also that they are
qualitatively quite different from one another despite being
structural isomers. The absorption band observed at 477 nm
for 3, which is the HOMO f LUMO excitation, is a δ f π*
charge transfer with a substantial metal contribution to the
composition of the π* orbital (Figure 5, right side). The higher
energy transition in 3 is an excitation from the HOMO
(essentially the Mo2 δorbital) to an MO which is largely δ* in
character but which also has an appreciable amount of ligand
π* character mixed into it from the diamidate linker.
In contrast, the two lowest energy bands in 2 are both metal
based and have relatively little involvement from the dioxypy-
rimidinate linker. This fact explains the significant decrease in
intensity of these two shoulders at 469 and 405 nm relative to
the absorptions in 3 since metal-to-ligand charge transfer bands
generally have much higher extinction coefficients due to more
efficient electron density shifts between the orbitals involved.
The lowest energy band in 2 involves a δ f π* transition, but
the π* orbital is essentially metal based and can be clearly seen
as the out of phase interaction of the dxz orbitals of the two
Mo2 units in 2 (Figure 5, left side). DFT results indicate that
the shoulder at 405 nm in 2 may be assigned as the Mo2 δ f
δ* transition. This dramatic difference between the electronic
absorption spectra of 2 and 3, which are structural isomers, is
a result which in hindsight finds an intuitive grasp in considering
the difference between the aromatic systems of the 2,3-
dioxypyrazine and 4,6-dioxypyrimidine rings. For the latter ring
system, the meta disposition of the ring nitrogen atoms and the
oxy substituents limits the resonance forms that can be drawn
for this ligand as compared to the 2,3-dioxypyrazine ligand.
Apparently this affects the energetics or the symmetry or both
of the π system, possibly because of a discontinuity introduced
by the presence of a node(s), such that the δ orbitals of the
Mo2 units can no longer effectively interact with it.
Summary and Conclusions
The present work extends the family of linked pairs of Mo2-
(DAniF)3 units, which until now has been comprised of
molecules joined by dicarboxylates (-O2CXCO2-), tetrahedral
oxo anions (EO42-, E ) S, Mo, and W), and N,N′-diaryltere-
phthaloyldiamidate linkers, to include a distinctly new set of
compounds with cyclic diamidate linkers. With uracilate as
potential linker, only the 1:1 adduct Mo2(DAniF)3(uracilate),
1, is formed for steric reasons. Similarly, cyanurate is capable
of coordinating only two Mo2(DAniF)3 units rather than three
in a trigonally symmetric fashion. The uncoordinated nitrogen
of the cyanurate ligand is found to be the site of the remaining
proton.
In the set of compounds 1-6, there is a surprising number
of firsts in the transition metal chemistry of these diamidate
ligands. In addition to being a rare instance of a structurally
characterized uracilate complex of molybdenum, 1 is also a
unique example in which uracilate spans two multiply bonded
metal atoms. Compounds 2 and 3, with 4,6-dioxypyrimidinate
and 2,3-dioxypyrazinate, are the only well-characterized transi-
tion metal complexes of any kind with these ligands. No
molecule, organic or inorganic, has been previously character-
ized with the 2,3-dioxypyrazinate core. Compound 5 represents
the first structurally characterized complex of 2,3-dioxy-5,6-
dicyanopyrazinate binding as an amidate type ligand. An
analogue of 4 has been previously described by Chisholm, but
its crystal structure was not reported. To the best of our
9
J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003 8909

A R T I C L E S
Cotton et al.
knowledge, compound 6 represents a new coordination mode
for cyanurate, all prior examples being limited to simple
monodentate N-bound or O-bound complexes with this ligand.
The cyclic voltammograms of 3-5 (∆E1/2 ) 258, 308, 263
mV, respectively) reveal that they are substantially better
mediators of electronic communication than even the oxalate-
linked molecule. By the Robin-Day classification scheme,48
the one electron oxidized form of 4 would represent a type III
fully delocalized system. It is undoubtedly the case that the π
system of the pyrazinate ring is intimately involved in facilitating
this charge delocalization. In contrast, the electrochemistry of
2, although it is a structural isomer of 3, is significantly less
effective in mediating communication, an effect which may be
attributable to the different symmetry of the 4,6-dioxypyrim-
idinate ring. Compound 6 has been found to have a rather
complex and unexpected solution behavior that displays a strong
solvent dependence. In THF, however, a single species pre-
ponderates.
calculations on 3 identify its lowest energy band as a δ f π*
transition, where the π* orbital has appreciable metal as well
as diamidate ligand character. This excitation is the HOMO f
LUMO transition. The higher energy absorption in 3 involves
a transition from the δ orbital to an MO which has mixed δ*
and π* character to it. Unlike 3, the two lowest energy features
in 2 both appear to be metal based, the lower energy one being
a δ f π* type transition while the other is essentially a δ f
δ* excitation. The electronic spectra and DFT calculations
reinforce the observation from the electrochemistry results that
2 and 3 are in fact very different electronically and that the
latter, probably for symmetry reasons, is somewhat better than
the former in mediating electronic communication. These cyclic
polyamidate linker pairs bear little resemblance to their N,N′-
diarylterephthaloyldiamidate linked analogues and must be
regarded as a distinctly different type of compound.
Acknowledgment. This research was supported by the
National Science Foundation and the Robert A. Welch Founda-
tion. J.P.D. gratefully acknowledges the support of an NIH
postdoctoral fellowship.
The electronic absorption spectra of 3-5 are in many respects
consistent with the electrochemistry in supporting the idea that
these compounds are in fact quite different than their dicar-
boxylate linked congeners. In particular, they have two or three,
rather than one, features in the 420-550 nm region. Compound
2 has two unresolved shoulders of appreciably lower intensity
than the absorption bands in 3-5. Time-dependent DFT
Supporting Information Available: Thermal ellipsoid draw-
ings at the 50% probability level with complete atomic labeling
in PDF in addition to X-ray crystallographic data for compounds
1-6 in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
(48) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247.
JA0358044
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8910 J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003