Exceptionally strong electronic coupling between [Mo2] units linked by substituted dianionic quinones

Article abstract of DOI:10.1039/b710804d

Ligands derived from N-CH₃ substituted benzoquinonemonoimines are exceedingly good facilitators of electronic communication between two quadruply bonded dimolybdenum units and provide record values for comproportionation constants. The Royal So

Full text of DOI:10.1039/b710804d

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Exceptionally strong electronic coupling between [Mo2] units linked by
substituted dianionic quinones{
F. Albert Cotton,{ Jia-Yi Jin, Zhong Li, Carlos A. Murillo* and Joseph H. Reibenspies
Received (in Cambridge, UK) 16th July 2007, Accepted 28th September 2007
First published as an Advance Article on the web 19th October 2007
DOI: 10.1039/b710804d
Ligands derived from N–CH3 substituted benzoquinonemo-
noimines are exceedingly good facilitators of electronic
communication between two quadruply bonded dimolybde-
num units and provide record values for comproportionation
constants
The study of electronic interactions between redox centers having
structurally identical single metal sites mediated by various
Scheme 1
bridging groups has been a topic of great interest since the
synthesis of the Creutz–Taube ion in the 1960s.1 Conjugated
group (the anion of 2,5-dihydroxy-1,4-benzoquinone (dhbq22), III
in Scheme 2, and its derivatives (IV), X = Cl and NO2.8b
electron carriers, such as polyenes, polypyrroles and polycon-
Here we report that analogues of [Mo2]dioxolene[Mo2] ([Mo2]
range electronic coupling.2 These complexes not only provide an
represents an Mo2(formamidinate)3 unit) having two O atoms in
densed aromatics have been utilized as linkers to promote long-
the dioxolene linker replaced by isoelectronic N–CH3 groups have
elegant model for the electron transfer processes in biochemistry,
been made and one of these compounds give the largest KC yet
4+
known for species containing quadruply bonded Mo2 units.
but also have potential for use as molecular wires and single-
molecule transistors in a new computer generation.3
These N-substituted benzoquinonemonoimines first made by
Braunstein and co-workers10 have attracted attention because
Recently our laboratory and others have been interested in
4+
analogues containing two dimetal species, usually Mo2 units,
linked by a variety of bridging ligands,4 in which electronic
they are antiaromatic having 6p + 6p electrons (Scheme 2). For the
linker precursors, two isomers that differ in the position of the N
communication between dimetal units can be conveniently
atoms within the ring have been isolated, 1,5-dihydroxy-2,4-
evaluated. Because structural and spectroscopic properties of
di(methylamino)benzene (V, cis) and 1,4-dihydroxy-2,5-dimethyl-
dimetal units are generally well understood such units provide
several advantages for probing electronic coupling.5
aminebenzene (VI, trans).
The respective pairs of pairs having Mo2 units, isomers 1 and 2,
Electrochemistry has been extensively used to evaluate how linkers
were synthesized as shown in eqn (1) by substitution of the labile
acetate ligand in Mo2(DAniF)3(O2CCH3)11 by the more basic
the well-known relationship that relates comproportionation
facilitate electronic communication between metal centers by using
1/2/25.69 6
constants with DE1/2 values, KC = eDE
. By selectively
N-substituted benzoquinonemonoimines linkers.§ Here DAniF
represents the anion N,N9-di-p-anisylformamidinate.
changing the linkers, KC’s ranging from the statistical value of 4 to
as much as 6.2 6 1013 have been observed.7
2Mo2(DAniF)3(O2CCH3)zH2C8H8N2O2
It should be mentioned that the most frequently used type of
ligands, namely dianions of dicarboxylic acids (I in Scheme 1), led
to low KC values.8 However, other work has shown that amidate
ligands (II) possess attractive properties since such groups are
much stronger Lewis bases than the carboxylate groups and
substituents on the N atoms can be synthetically adjusted both
sterically and electronically.9 Electronic coupling in these amidate
bridged compounds is usually much stronger than in those linked
by the corresponding carboxylates. Other ligands that allow strong
electronic communication between Mo2 units are the dioxolene
(1)
2NaOCH3=THF
DCDCDCCA ½Mo2(DAniF)3ꢀ2(C8H8N2O2)
r:t:
Department of Chemistry and Laboratory for Molecular Structure and
Bonding, P. O. Box 3012, Texas A&M University, College Station,
Texas, 77842-30012, USA. E-mail: murillo@tamu.edu;
Tel: +1 9798453646
{ Electronic supplementary information (ESI) available: Tables of
crystallography data and selected bond distances and angles. The UV-
vis spectra of 1 and 2 and EPR spectrum of 1+. See DOI: 10.1039/
b710804d
Scheme 2
{ Deceased on February 20, 2007.
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Chem. Commun., 2008, 211–213 | 211

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suggest that the electron conjugation within the ring likely follows
a 6p + 6p pattern as shown in Scheme 2.
The cyclic voltammogram (CV) of 1 in CH2Cl2 shows two well-
separated, reversible waves at 310 and 1160 mV (vs. Ag/AgCl)
(Fig. 2). These two redox waves correspond to two one-electron
oxidation processes that take place at the [Mo2] centers (vide infra).
The DE1/2 of 850 mV for 1, represents the largest potential
separation in compounds having two [Mo2] units joined by any of
a large variety of linkers. The corresponding comproportionation
constant for 1 is 2.34 6 1014. For 2 the two E1/2 values are 345 and
1125 mV. The slightly smaller DE1/2 of 780 mV corresponds to a
KC of 1.54 6 1013. The large electronic communication and large
KC values are in accord with what has been observed for
compounds containing these linkers and single metal units,
M(acac), where M = Ni or Pd.12
Fig. 1 The cores for compounds 1 and 2. Thermal ellipsoids are drawn
at the 40% probability level, and all p-anisyl groups and hydrogen atoms
are omitted for clarity.
The electrochemical data for 1, 2 and selected [Mo2(DAniF)3]-
(L)[Mo2(DAniF)3] compounds are shown in Table
1 for
comparison purposes. These data clearly shows that the family
of compounds having benzoquinones and benzoquinonemonoi-
mines have an exceptionally large ability to delocalize electrons
with the cis-substituted N-methyl benzoquinonemonoimine being
the best. It should be noted that the oxalate linker, which
represents the best dicarboxylate linker for electron delocalization
has a KC many orders of magnitude smaller than 1. Because C–C
distances in each linker C6 ring differ significantly (vide
supra), electron delocalization in both isomers 1 and 2 is expected
to extend through a 14-atom ring that includes the linker
(Scheme 2) and the two dimolybdenum units, similarly to the
dhbq22 analogue.8b
After routine treatment of the reaction mixtures, crystals of 1
formed by diffusion of ethanol into the solution of 1 dissolved in
THF while crystals of 2 were obtained by diffusion of hexanes into
solutions of 2 dissolved in CH2Cl2–acetone (1 : 1)." The core
structures are shown in Fig. 1. Compound 1 crystallized in the
space group P21/c with one molecule in a general position and five
disordered THF molecules. The linker and the two dimolybdenum
units are essentially planar with the two Mo2 axes essentially
parallel to each other. The Mo–Mo distances are 2.0966(7) and
˚
2.0945(7) A and the separation between Mo2 units is 8.816 A.
˚
¯
Compound 2 crystallized in the triclinic space group P1 with
It should also be noted that the electronic spectra for 1 and 2
show intense absorption bands at 750 and 779 nm, respectively (see
ESI{). Because the existence of low energy metal-to-ligand charge
transfer (MLCT) is critical for the electronic coupling between the
dimetal units by facilitating an ‘electron hopping’ pathway,13 these
MLCT transitions account not only for the green color of the
compounds but are also consistent with the exceptional strong
electronic coupling in these species.
Z = 1. There are also four interstitial CH2Cl2 molecules per unit
cell. The two Mo2 units are related by an inversion center located
at the center of the linker C6 ring. The two sets of quadruply
bonded Mo2 bonds and the linker are essentially flat, providing
what appears to be a well conjugated platform for electron
transport between the metal redox sites. This is different from the
analogue linked by dioxolene (dhbq22) which has a chair
conformation.8b The Mo–Mo distance of 2.0902(7) A is within
˚
Definite confirmation of the delocalized nature of the odd
electron in the singly oxidized species 1+ and 2+ is provided by the
EPR spectra taken on samples of the corresponding precursors
that had been oxidized in situ by adding slightly less than one
equivalent of FeCp2PF6. The EPR spectrum for 2+ is given in
Fig. 3 and that for 1+ in the ESI.{
the typical range for Mo–Mo quadruple bonds.6 The separation
˚
between the two Mo2 units of 8.832 A is similar to that in 1 and
about 0.3 A longer than that of the dhbq22 analogue (see Table 1).
˚
The C–C bond lengths within the linker C6 ring show variations of
˚
˚
0.06–0.08 A for both 1 and 2, i.e., C(1)–C(2) is long (1.457(5) A)
˚
and C(2)–C(3) is short (1.389(5) A) for 2. The corresponding
In both cases the g values are less than ca. 2.0 which are typical
for organic radicals. The values of 1.944 for 1+ and 1.947 for 2+,
distances in 1 are similar. These large differences in bond distances
Table 1 Electrochemical data for 1, 2 and selected [Mo2(DAniF)3]-
(L)[Mo2(DAniF)3] compounds (L = linker)
a
d /A
1 b
E1/2
2 b
E1/2
˚
L
DE1/2 KC
Ref.
1
2
8.816
8.832
310
345
1160
1125
563
780
861
850
780
763
795
816
212
2.34 6 1014 This work
1.54 6 1013 This work
7.92 6 1012 8b
dhbq22 8.536 2200
ca22 c
na22 c
oxalate 6.953
8.718
—
215
45
294
2.75 6 1013 8b
6.23 6 1013 8b
506
3.8 6 103
7
a
The distance d between the two [Mo2] units measured by the
centers of two Mo2 axis. Potentials are in mV referenced to Ag/
AgCl and E1/2 = (Epa + Epc)/2 obtained from the CV for isomers 1
b
Fig. 2 CVs and the differential pulse voltammograms (DPVs) for 1 (left)
and 2 (right) recorded in CH2Cl2 solution with potentials referenced to
Ag/AgCl.
and 2. ca22 = Chloranilate, na22 = nitranilate.
c
212 | Chem. Commun., 2008, 211–213
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diffusion of ethanol into a solution of the product in a mixture of CH2Cl2
and acetone (1 : 1). Yield: 210 mg (50%). UV-vis, lmax/nm (e/M21 cm21):
383 (2.2 6 103), 511 (5.8 6 102), 779 (1.4 6 104). Anal. Calc. for
C98H98Mo4N14O14 (2): C, 56.37, H, 4.73 N, 9.39. Found: C, 56.35; H, 4.70;
N, 9.35%.
" Crystal data for [Mo2(DAniF)3]2{cis-C6H2(ONCH3)2}?5THF, 1?5THF,
monoclinic, space group P21/c, a = 16.5233(18), b = 24.371(3), c =
3
˚
˚
˚
29.569(3) A, b = 106.110(2)u, V = 11440(2) A , T = 213(2) K, Z = 4, m(l =
0.71073 A) = 0.501 mm21, Dc = 1.417 Mg m23, 85184 reflections
measured, 20356 unique, and 14709 (I . 2s(I)) used in the calculations;
R1 = 0.1030, wR2 = 0.1835. Interstitial solvent: C4H4O.
Crystal data for [Mo2(DAniF)3]2{trans-C6H2(ONCH3)2}?4CH2Cl2,
¯
2?4CH2Cl2, triclinic, space group P1, a = 11.345(4), b = 16.008(6), c =
˚
Fig. 3 X-Band EPR spectrum for 2+ in CH2Cl2 solution at room
temperature. The simulated spectrum is in very good agreement with the
experimental data as shown in the ESI.{
3
˚
16.851(6) A, a = 70.111(7), b = 73.723(6), c = 70.823(6)u, V = 2668.7(17) A ,
T = 213(2) K, Z = 1, m(l = 0.71073 A) = 0.727 mm21, Dc = 1.505 Mg m23
,
˚
13608 reflections measured, 8836 unique, and 6169 (I . 2s(I)) used in the
calculations; R1 = 0.0907, wR2 = 0.1809. Interstitial solvent: CH2Cl2.
CCDC 652613 and 652614 for 1 and 2, respectively. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b710804d
are consistent with a metal-based oxidation. More importantly the
small hyperfine values of 11.8 6 1024 cm21 for 1+ and 10.8 6
1024 for 2+ are characteristic of delocalized systems containing
pairs of pairs with dimolybdenum units.8 These A values are ca.
1 (a) C. Creutz, Prog. Inorg. Chem., 1983, 30, 1; (b) D. E. Richardson and
H. Taube, Coord. Chem. Rev., 1984, 60, 107; (c) P. Chen and T. Meyer,
Chem. Rev., 1998, 98, 1439; (d) A. Ferretti, A. Lami, L. F. Murga,
I. A. Shehadi, M. J. Ondrechen and G. Villani, J. Am. Chem. Soc., 1999,
121, 2594; (e) W. Kaim, A. Klein and M. Gloeckle, Acc. Chem. Res.,
2000, 33, 755; (f) K. D. Demadis, C. M. Hartshorn and T. J. Meyer,
Chem. Rev., 2001, 101, 2655; (g) B. S. Brunschwig, C. Creutz and
N. Sutin, Chem. Soc. Rev., 2002, 31, 168; (h) V. C. Lau, L. A. Berben
and J. R. Long, J. Am. Chem. Soc., 2002, 124, 9042.
50% of those in analogous compounds having localized oxidations
+ 14
and that in the parent ion Mo2(DAniF)4
.
In conclusion, two isomers having [Mo2] units linked by N–CH3
substituted benzoquinonemonoimines give pairs of pairs with an
exceptionally large electronic coupling.
We gratefully acknowledge the Robert A. Welch Foundation,
the National Science Foundation (IR/D program) and Texas
A&M University for financial support. We also thank Dr J. H.
Reibenspies and Dr Qinliang Zhao for help with X-ray crystal-
lography and EPR spectroscopy, respectively, and Prof. P.
Braunstein for helpful suggestions.
2 (a) L. M. Tolbert, X. Zhao, Y. Ding and L. A. Bottomley, J. Am. Chem.
Soc., 1995, 117, 12891; (b) J. P. Sutter, D. M. Grove, M. Beley,
J. P. Collin, N. Veldman, A. L. Spek, J. P. Sauvage and G. V. Koten,
Angew. Chem., Int. Ed. Engl., 1994, 33, 1282.
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C. W. Spangler, Inorg. Chem., 1996, 35, 3735.
4 (a) F. A. Cotton, C. Lin and C. A. Murillo, Acc. Chem. Res., 2001, 34,
759; (b) F. A. Cotton, C. Lin and C. A. Murillo, Proc. Natl. Acad. Sci.
U. S. A., 2002, 99, 4810; (c) M. H. Chisholm and A. M. Macintosh,
Chem. Rev., 2005, 105, 2949; (d) M. H. Chisholm and N. J. Patmore,
Acc. Chem. Res., 2007, 40, 19.
5 D. E. Richardson and H. Taube, Inorg. Chem., 1981, 20, 1278.
6 F. A. Cotton, Multiple Bonds Between Metal Atoms, ed. F. A. Cotton,
C. A. Murillo and R. A. Walton, Springer Science and Business Media,
Inc., New York, 3rd edn, 2005.
7 F. A. Cotton, J. P. Donahue and C. A. Murillo, J. Am. Chem. Soc.,
2003, 125, 5436.
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F. A. Cotton, C. A. Murillo, D. Villagra´n and R. Yu, J. Am. Chem.
Soc., 2006, 128, 3281.
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2006, 45, 9765; (b) F. A. Cotton, L. M. Daniels, J. P. Donahue, C. Y. Liu
and C. A. Murillo, Inorg. Chem., 2002, 41, 1354.
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Notes and references
§ Experimental details: All reactions and manipulations were performed
under a nitrogen atmosphere, using either a drybox or standard Schlenk
line techniques. Solvents were purified under argon using a Glass Contour
solvent purification system or distilled over the corresponding drying agent
under nitrogen. The starting materials, Mo2(DAniF)3(O2CCH3)11 and the
linker precursors for IV (1,5-dihydroxy-2,4-di(methylamino)benzene) and
V (1,4-dihydroxy-2,5-di(methylamino)benzene)10 were prepared according
to reported procedures.
For the preparation of [Mo2(DAniF)3]2{cis-C6H2(ONCH3)2}, 1, 0.80 mL
of a 0.50 M sodium methoxide solution in methanol was added slowly,
with stirring, to a suspension of Mo2(DAniF)3(O2CCH3)2 (406 mg,
0.400 mmol) and 1,5-dihydroxy-2,4-di(methylamino)benzene (33 mg,
0.20 mmol) in 30 mL of THF. The reaction mixture was stirred overnight
and during this time the colour changed from yellow-brown to dark green.
The solvent was removed under reduced pressure, and dichloromethane
(20 mL) was added to the residue. The resulting mixture was filtered. The
volume of the filtrate was reduced to a volume of about 5 mL under
vacuum and 40 mL hexanes were added to precipitate a green solid. The
solvent was then decanted and the solid was washed with ethanol (2 6
15 mL) and hexanes (2 6 15 mL), and dried under vacuum. The solid was
dissolved in 15 mL of THF and the solution was then layered with ethanol.
Dark green crystals formed within 2 days. Yield: 250 mg (60%). UV-vis,
11 F. A. Cotton, C. Y. Liu, C. A. Murillo, D. Villagra´n and X. Wang,
J. Am. Chem. Soc., 2003, 125, 13564.
12 (a) O. Siri, J. Taquet, J. Collin, M.-M. Rohmer, M. Be´nard and
P. Braunstein, Chem.–Eur. J., 2005, 11, 7247; (b) O. Siri, P. Braunstein,
J. Taquet, J.-P. Collin and R. Welter, Dalton Trans., 2007, 1481.
13 (a) M. H. Chisholm, R. J. H. Clark, J. Gallucci, C. M. Hadad and
N. J. Patmore, J. Am. Chem. Soc., 2004, 126, 8303; (b) F. A. Cotton,
Z. Li, C. Y. Liu and C. A. Murillo, Inorg. Chem., 2007, 46, 7840.
14 (a) F. A. Cotton, C. Y. Liu, C. A. Murillo and Q. Zhao, Inorg. Chem.,
2007, 46, 2604; (b) F. A. Cotton, J. P. Donahue, P. L. Huang,
C. A. Murillo and D. Villagra´n, Z. Anorg. Allg. Chem., 2005, 631, 2606.
l
max/nm (e/M21 cm21): 390 (8.0 6 102), 500 (4.0 6 102), 750 (6.6 6 103).
Anal. Calc. for C98H98Mo4N14O14 (1): C, 49.36 H, 4.23 N, 8.06. Found: C,
49.59; H, 4.10; N, 8.24%.
For the preparation of [Mo2(DAniF)3]2{trans-C6H2(ONCH3)2}, 2, a
similar procedure as that for 1 was used starting from 1,4-dihydroxy-2,5-
di(methylamino)benzene (33 mg, 0.20 mmol) and Mo2(DAniF)3-
(O2CCH3)2 (406 mg, 0.400 mmol). Green crystals were obtained by
This journal is ß The Royal Society of Chemistry 2008
Chem. Commun., 2008, 211–213 | 213