Systematic Preparation of Mo24+ Building Blocks for Supramolecular Assemblies

Article abstract of DOI:10.1021/ic035433s

The preparation of additional and useful building blocks for the construction of supramolecular entities with quadruply bonded Mo ₂⁴⁺ units has been explored, and five new mixed-ligand complexes with three types of ligands and various basicities are reported. The ligands used were the DAniF (N,N′-di-p-anisylformamidinate) anion, the acetate anion, and neutral acetonitrile molecules. The formamidinate ligands are the least labile, and the acetonitrile molecules are the most labile. This difference as well as a relatively strong trans directing influence by the formamidinate anions in ligand substitution reactions allows designed synthesis of various mixed-ligand building blocks, including rare pairs of cis and trans isomers, The new compounds are cis-Mo₂(DAniF)₂(O ₂CCH₃)₂ (1), trans-Mo₂(DAniF) ₂(O₂CCH₃)₂ (2), trans-[Mo ₂(DAniF)₂(O₂CCH₃)(CH ₃CN_eq)₂]BF₄ (3), trans-[Mo ₂(DAniF)₂(CH₃CN_eq)4](BF ₄)₂ (4), and [Mo₂(O₂CH ₃)(CH₃CN_eq)₆(CH₃CN _ax)](BF₄)₃ (5), where eq and ax designate equatorial and axial ligands, respectively. A comparison with some previously synthesized complexes is given along with a discussion of the overall reactivity of all compounds.

Full text of DOI:10.1021/ic035433s

Inorg. Chem. 2004, 43, 2267−2276
Systematic Preparation of Mo24+ Building Blocks for Supramolecular
Assemblies
F. Albert Cotton,* Chun Y. Liu, and Carlos A. Murillo*
Department of Chemistry and Laboratory for Molecular Structure and Bonding, PO Box 30012,
Texas A&M UniVersity, College Station, Texas 77842-3012
Received December 13, 2003
The preparation of additional and useful building blocks for the construction of supramolecular entities with quadruply
bonded Mo24+ units has been explored, and five new mixed-ligand complexes with three types of ligands and
various basicities are reported. The ligands used were the DAniF (N,N′-di-p-anisylformamidinate) anion, the acetate
anion, and neutral acetonitrile molecules. The formamidinate ligands are the least labile, and the acetonitrile molecules
are the most labile. This difference as well as a relatively strong trans directing influence by the formamidinate
anions in ligand substitution reactions allows designed synthesis of various mixed-ligand building blocks, including
rare pairs of cis and trans isomers. The new compounds are cis-Mo2(DAniF)2(O CCH ) (1), trans-Mo2(DAniF)2-
2
3 2
(O CCH ) (2), trans-[Mo2(DAniF)2(O CCH )(CH CN ) ]BF4 (3), trans-[Mo2(DAniF)2(CH CN ) ](BF4)2 (4), and
2
3 2
2
3
3
eq 2
3
eq 4
[Mo2(O CH )(CH CN ) (CH CN )](BF ) (5), where eq and ax designate equatorial and axial ligands, respectively.
2
3
3
eq 6
3
ax
4 3
A comparison with some previously synthesized complexes is given along with a discussion of the overall reactivity
of all compounds.
Introduction
significant number of papers have appeared in which Zn2+
and other ions have been employed too.2b
In the past few years there has been great interest in
supramolecular entities,1 in particular those that contain
building blocks with metal species joined by appropriate
polydentate linkers.2 The development of designed, conver-
gent strategies for their synthesis has a high priority. De-
pending on the substituents, the coordination number, and
the geometry of such metal containing species, it is possible
to produce corner pieces that will favor triangular, square,
or other polygonal arrangements.3 Much work to date has
been done using M(diamine)2+ or M(diphosphine)2+, M )
Pd and Pt, corner pieces and neutral bidentate linkers which
form supramolecular ions with high positive charges. A
In our laboratory, we have introduced and developed the
idea of combining metal-metal bonded species containing
two nonlabile bridging formamidinate ligands with polycar-
boxylate linkers. This has resulted in a variety of M2-con-
taining architectures, including loops, squares, triangles,
complex polygons, and extended three-dimensional materials
for systems where M ) Mo,4 Rh,4 and Ru.5 Most of the
2+
work has been done with cis-M2(formamidinate)2 corner
piece precursors such as I which favor square and sometimes
triangular assemblies. Previous efforts to produce the iso-
* Authors to whom correspondence should be addressed. E-mail:
Cotton@tamu.edu (F.A.C.); Murillo@tamu.edu (C.A.M.).
(1) (a) Fujita, M. Acc. Chem. Res. 1999, 32, 53. (b) Lehn, J.-M.
Supramolecular ChemistrysConcepts and PerspectiVes; VCH: Wein-
heim, Germany, 1995. (c) Kim, J.; Chen, B.; Reineke, T. M.; Li, H.;
Eddaoudi, M.; Moler, D. B.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem.
Soc. 2001, 123, 8239.
(2) See for example: (a) Fujita, M. Chem. Soc. ReV. 1998, 27, 417. (b)
Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853.
(c) Holliday, G. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40,
2022. (d) Swieger, G. F.; Malefetse, T. J. Chem. ReV. 2000, 100, 3483.
(e) Navarrro, J. A. R.; Lippert, B. Coord. Chem. ReV. 1999, 915.
(3) See for example: (a) Jiang, H.; Lin, W. J. Am. Chem. Soc. 2003,
125, 8084. (b) Yamamoto, T.; Arif, A. M.; Stang, P. J. J. Am. Chem.
Soc. 2003, 125, 12309 and references therein. (c) Dinolfo, P. H.; Hupp,
J. T. Chem. Mater. 2001, 13, 3113.
2+
meric trans-M2(formamidinate)2 species, which might be
used to assemble other geometrical entities such as ladders
10.1021/ic035433s CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/02/2004
Inorganic Chemistry, Vol. 43, No. 7, 2004 2267

Cotton et al.
of the type II, have also been successful, but so far only
tetraalkylammonium salts of dicarboxylic acids.4 The use of
+
carboxylate analogues, e.g., cis-Mo2(O2CBut)3(CH3CNeq)2 ,
as building blocks has turned out to be problematical.10,11
The isolation of products in crystalline form has proven very
difficult because of the higher lability of the carboxylate
anions, compared to that of the formamidinate ligands, during
substitution reactions with dicarboxylate linkers.
The strategy for success in consistent, designed, convergent
synthesis of specific structures as compounds that can be
isolated as pure, well-formed crystals is to employ starting
materials with a hierarchy of ligand labilities. The series, in
order of increasing lability, that has served our purposes well
with methyl groups present in the 2 and 6 positions of the
aryl groups of the formamidinate anions, as in trans-
Cr2(DXylF)2(O2CCH3)2(THF)2, DXylF ) N,N′-dixylylfor-
mamidinate.6 Other isolated examples of dimolybdenum
compounds having mixed amidinate and carboxylate ligands,
some with trans arrangements, have also been prepared.7
The first systematic approach to the production of various
M2 building blocks was done using dimolybdenum species
with a mixture of formamidinate and acetonitrile ligands.8
These generally have from four to zero N,N′-diarylforma-
midinate groups (DArF) occupying the equatorial positions
-
is DAniF- < RCO2 < CH3CN.
Recently, we expanded our efforts in the construction of
molecules with pairs of Mo2 units, IV, by using the more
basic diamidate linkers which enable better electronic com-
munication between Mo2 units.12 However, in early synthetic
4+
(eq) of the paddlewheel Mo2 species, with the rest of the
equatorial positions occupied by acetonitrile molecules. Axial
(ax) acetonitrile molecules might also be present. These
compounds, of general formula Mo2(DArF)4-n(CH3CNeq)2n-
(CH3CNax)m, m ) 0-2, are useful in substitution reactions
because the formamidinate ligands are not displaced in
reactions with dicarboxylate linkers while the acetonitrile
molecules are. These useful starting materials are made by
reacting quadruply bonded Mo2(DArF)4, IIIa, where Ar is
often Ani ) anisyl, in acetonitrile as solvent with the nec-
essary quantities of acids such as OEt3BF4 or HBF4 to give
efforts reactions of Mo2(DArF)4-n(CH3CNeq)2nn+ species with
diamidate anions did not provide the target molecules
because acetonitrile molecules are very reactive toward the
more basic ligands. Nucleophilic attacks by bases upon
coordinated acetonitrile are known, for example in reactions
producing acetamidate groups, and such reactions are
catalyzed by transition metal ions.13 To obviate this problem
we employed Mo2(formamidinate)3(acetate) as a useful and
reactive starting material from which the labile acetate groups
are preferentially replaced by the diamidate linkers.14 The
acetate displacement approach has also been used success-
+
species such as Mo2(DArF)3(CH3CNeq)2 , cis-Mo2(DArF)2-
3+ 8
(CH3CNeq)42+, and Mo2(DArF)(CH3CNeq)6
. The fully
5+
fully for the synthesis of Ru2 compounds.15
(6) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Schooler, P. Dalton
2000, 2001.
(7) See for example: (a) Wu, Y.-Y.; Chen, J.-D.; Liou, L.-S.; Wang, J.-
C. Inorg. Chim. Acta 2002, 336, 71. (b) Weng, H.-Y.; Lee, C.-W.;
Chuang, Y.-Y.; Suen, M.-C.; Chen, J.-D.; Hung, C.-H. Inorg. Chim.
Acta 2003, 351, 89. (c) Zou, G.; Ren, T. Inorg. Chim. Acta 2000,
304, 305. (d) Cotton, F. A.; Ilsley, W. H.; Kaim, W. Inorg. Chem.
1981, 20, 930. (e) Zinn, A.; Weller, F.; Dehnicke, K. Z. Anorg. Allg.
Chem. 1991, 594, 106.
(8) Chisholm, M. H.; Cotton, F. A.; Daniels, L. M.; Folting, K.; Huffman,
J. C.; Iyer, S. S.; Lin, C.; MacIntosh, A. M.; Murillo, C. A. J. Chem.
Soc., Dalton Trans. 1999, 1387.
(9) Cotton, F. A.; Wiesinger, K. J. Inorg. Synth. 1992, 29, 134.
(10) Cayton, R. H.; Chisholm, M. H.; Huffman, J. C.; Lobkovsky, E. B. J.
Am. Chem. Soc. 1991, 113, 8709.
(11) However, if the two carboxylate groups are connected forming cis
dicarboxylates, they can be used as building blocks. See for example:
(a) Bonar-Law, R. P.; McGrath, T. D.; Singh, N.; Bickley, J. F.;
Steiner, A. Chem. Commun. 1999, 2457. (b) Bonar-Law, R. P.;
McGrath, T. D.; Bickley, J. F.; Femoni, C.; Steiner, A. Inorg. Chem.
Commun. 2001, 4, 16.
solvated species, Mo2(CH3CNeq)84+, is obtained similarly
from Mo2(O2CCH3)4, IIIb.9 These mixed formamidinate-
acetonitrile species can be successfully used in the construc-
tion of supramolecular entities by reacting them with
(12) Cotton, F. A.; Liu, C. Y.; Murillo, C. A.; Villagra´n, D.; Wang, X. J.
Am. Chem. Soc. 2003, 125, 13564.
(4) (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. Natl. Acad. Sci.
U.S.A. 2002, 99, 4810.
(5) Angaridis, P.; Berry, J. F.; Cotton, F. A.; Murillo, C. A.; Wang, X. J.
Am. Chem. Soc. 2003, 125, 10327.
(13) (a) Concolino, T. E.; Eglin, J. L.; Staples, R. J. Polyhedron 1999, 18,
915. (b) Eglin, J. L. Comments Inorg. Chem. 2002, 23, 23. (c)
Dequeant, M.; Eglin, J. L.; Graves-Brook, M. K.; Smith, L. T. Inorg.
Chim. Acta 2003, 351, 141. (d) Cotton, F. A.; Daniels, L. M.; Murillo,
C. A.; Wang, X. Polyhedron 1998, 17, 2781.
2268 Inorganic Chemistry, Vol. 43, No. 7, 2004

4+
Mo2 Building Blocks for Supramolecular Assemblies
formamidine (2.56 g, 10.0 mmol) in 100 mL of THF was added
slowly, and with stirring, 20 mL of a 0.5 M solution of NaOCH3
in methanol. The color turned red, then brown. The reaction mixture
was stirred for 5 h at room temperature, after which some colorless
sodium acetate was visible. Then the solvent was removed under
reduced pressure, and dichloromethane (ca. 40 mL) was added to
the residue; then the mixture was filtered. The volume of the filtrate
was then reduced to about 15 mL under vacuum. While the mixture
was stirred vigorously, ethanol (ca. 40 mL) was added, producing
a light yellow solid and a dark brown solution. The solid was
separated and then washed with ethanol (2 × 20 mL), followed by
20 mL of hexanes, and dried under vacuum. Yield: 3.50 g (86%).
1H NMR (δ, ppm in CDCl3): 8.45 (s, 2H, -NCHN-), 6.72 (d,
8H, aromatic C-H), 6.71 (d, 8H, aromatic C-H), 3.72 (s, 12H,
-OCH3), 2.61 (s, 6H, -CH3). Anal. Calcd for C34H36Mo2N4O8:
C, 49.56; H, 4.40; N, 6.80. Found: C, 49.69; H, 4.33; N, 6.69.
In the present work we examine in more detail the prep-
aration of mixed-ligand complexes having an Mo2 nu-
4+
cleus where the ligands are formamidinates, acetate, and
acetonitrile, and report five new compounds. These are cis-
Mo2(DAniF)2(O2CCH3)2 (1), trans-Mo2(DAniF)2(O2CCH3)2
(2), trans-[Mo2(DAniF)2(O2CCH3)(CH3CNeq)2]BF4 (3), trans-
[Mo2(DAniF)2(CH3CNeq)4](BF4)2 (4), and [Mo2(O2CCH3)(CH3-
CNeq)6(CH3CNax)](BF4)3 (5). Compounds 1 and 2, and also
4 and the previously reported cis-[Mo2(DAniF)2(CH3CNeq)4]-
(BF4)2,8 are both pairs of cis and trans stereoisomers, which
have so far been uncommon in dimetal chemistry.16 Each of
the isomers has been synthesized selectively, and by a
different synthetic route. This provides a unique opportunity
to study the factors that influence ligand substitution and
reactivity of dimolybdenum compounds. A general discussion
of the syntheses of mixed-ligand compounds is provided.
Preparation of trans-[Mo2(DAniF)2(O2CCH3)(CH3CNeq)2]BF4,
3. trans-Mo2(DAniF)2(O2CCH3)2 (0.410 g, 0.500 mmol) was
dissolved in a mixture of 15 mL of dichloromethane and 5 mL of
acetonitrile. To the yellow solution was added 1.00 mL of
triethyloxonium tetrafluoroborate dropwise, with stirring. The
mixture was stirred for 30 min, generating a golden yellow solution.
The volume was reduced to ca. 10 mL; then, with vigorous stirring,
30 mL of ether was added, precipitating a sticky brown solid. After
the solvent was decanted, the residue was dissolved in 5 mL of
acetonitrile, and ether (20 mL) was added with stirring. This
produced a golden yellow solid, which was washed with an
additional 10 mL of ether after filtration. The product was dried
under vacuum. Yield: 0.258 g (55%). A golden yellow crystalline
product was obtained by diffusion of ether into a dichloromethane-
acetonitrile solution (5:5 mL) of the product. 1H NMR (δ, ppm in
CD3CN): 8.92 (s, 2H, -NCHN-), 6.912 (d, 8H, aromatic C-H),
6.88 (d, 8H, aromatic C-H), 3.78 (s, 12H, -OCH3), 2.61 (s, 3H,
-CH3 from acetate). 2.02 (s, 6H, -CH3 from acetonitrile). Anal.
Calcd for C36H40BF4Mo2N6O7 (3‚H2O): C, 45.42; H, 4.23; N, 8.83.
Found: C, 45.06; H, 4.17; N, 8.47.
Experimental Section
Materials and Methods. Solvents used were freshly distilled
under N2 by employing standard procedures or dried and degassed
using a Glass Contour solvent purification system. All synthetic
operations were conducted under N2 using Schlenk line techniques.
The materials used for the study, Mo2(O2CCH3)4,17 IIIb or B, Mo2-
(DAniF)4,18 IIIa or D, cis-Mo2(O2CCH3)2(CH3CNeq)4](BF4)2,19 A,
cis-[Mo2(DAniF)2(CH3CNeq)4](BF4)2, E, (DAniF ) N,N′-di-p-
anisylformamidinate), [Mo2(DAniF)(CH3CNeq)6](BF4)3, C,8 were
prepared by following a literature procedure. Other commercially
available chemicals were used as received.
Physical Measurements. Elemental analyses were performed
by Canadian Microanalytical Service, Delta, British Columbia,
1
Canada. H NMR spectra were recorded on a Mercury-300 NMR
spectrometer with chemical shifts (δ ppm) referenced to CDCl3
for 1 and 2, CD3CN for 3 and 4, and DMSO-d6 for 5.
Preparation of cis-Mo2(DAniF)2(O2CCH3)2, 1. To cis-
[Mo2(DAniF)2(CH3CNeq)4](BF4)2 (1.04 g, 1.00 mmol) was added
an excess amount of anhydrous NaO2CCH3 (0.25 g, 3.05 mmol)
in 20 mL of acetonitrile. A yellow precipitate formed in ca. 10
min. The mixture was stirred at room temperature for 2 h. After
filtration, the solid was extracted with ca. 15 mL of dichlo-
romethane; addition of 40 mL of hexanes gave a light yellow
precipitate, which was collected by filtration and dried under
vacuum. Yield: 0.680 g (83%). 1H NMR (δ, ppm in CDCl3): 8.41
(s, 2H, -NCHN-), 6.61 (d, 8H, aromatic C-H), 6.55 (d, 8H,
aromatic C-H), 3.71 (s, 12H, -OCH3), 2.65(s, 6H, -CH3). Anal.
Calcd for C34H36Mo2N4O8: C, 49.56; H, 4.40; N, 6.80. Found: C,
49.50; H, 4.34; N, 6.79.
Preparation of trans-[Mo2(DAniF)2(CH3CNeq)4](BF4)2, 4. A
purple solution containing [Mo
2(DAniF)(CH3CN)6](BF ) (0.479
4 3
g, 0.500 mmol) in 10 mL of acetonitrile was mixed with a solution
of HDAniF (0.128 g, 0.500 mmol) in 10 mL of acetonitrile. With
stirring, the color of the mixture turned to orange red. After 1 h,
40 mL of ether was added to produce an orange solid, which was
washed first with ether (2 × 20 mL) and then with dichloromethane
(2 × 10 mL). The solid was then collected by filtration and dried
under vacuum. Yield: 0.330 g (64%). Single crystals for X-ray
analysis were obtained by diffusion of ether into an acetonitrile
1
solution. H NMR (δ, ppm in CD3CN): 9.18 (s, 2H, -NCHN-),
Preparation of trans-Mo2(DAniF)2(O2CCH3)2, 2. To a suspen-
sion of Mo2(OCCH3)4 (2.14 g, 5.00 mmol) and N,N′-di-p-anisyl-
7.04 (d, 8H, aromatic C-H), 6.89 (d, 8H, aromatic C-H), 3.77 (s,
12H, -OCH3), 1.99 (s, 12H, -CH3). Anal. Calcd for C38H42B2F8-
Mo2N8O4: C, 43.72; H, 4.06; N, 10.73. Found: C, 43.58; H, 3.90;
N, 10.38.
(14) (a) Cotton, F. A.; Liu, C. Y.; Murillo, C. A.; Wang, X. Inorg. Chem.
2003, 42, 4619. (b) Cotton, F. A.; Liu, C. Y.; Murillo, C. A.; Villagra´n,
D. J. Am. Chem. Soc. 2003, 125, 13564.
Preparation of [Mo2(O2CCH3)(CH3CN)6(CH3CNax)](BF4)3, 5.
To a suspension of Mo2(O2CCH3)4, (0.535 g, 1.25 mmol) in 20
mL of acetonitrile was added slowly 8 mL of triethyloxonium
tetrafluoroborate (1.0 M in CH2Cl2), with stirring. A red solution
which formed immediately was stirred for 1 h while the color turned
magenta. Solvent removal under reduced pressure left a magenta
solid, which was washed thoroughly with diethyl ether and dried
under vacuum. Yield: 0.660 g (70%). The crystalline product was
obtained by layering an acetonitrile solution (15 mL) with hexanes
(15) Angaridis, P.; Berry, J. F.; Cotton, F. A.; Lei, P.; Lin, C.; Murillo, C.
A.; Villagra´n, D. Inorg. Chem. Commun. 2004, 7, 9.
(16) There is a pair of isomers which have the composition Mo2(µ-acetate)2-
(N,N′-bis(trimethylsilyl)benzamidinate)2 which were isolated from the
same reaction mixture. Only one of them has a paddlewheel structure
with two trans acetate and two bridging benzamidinate groups, while
the other has two cisoid acetate bridges but each of the benzamidinate
groups is chelated to one Mo atom. See ref 7c.
(17) Brignole, A. B.; Cotton, F. A. Inorg. Synth. 1972, 13, 87.
(18) Lin, C.; Protasiewicz, J. D.; Smith, E. T.; Ren, T. Inorg. Chem. 1996,
35, 6422.
(19) Cotton, F. A.; Reid, A. H.; Schwotzer, W. Inorg. Chem. 1985, 24,
3965.
1
(0.5 mL) followed by diethyl ether (40 mL). H NMR (δ, ppm in
DMSO-d6): 2.82 (s, 3H, -CH3 from acetate), 2.05 (s, 18H, -CH3
Inorganic Chemistry, Vol. 43, No. 7, 2004 2269

Cotton et al.
Table 1. X-ray Crystallographic Data for 1-5
1
2
3‚1.5C4H10O
4
5‚0.5CH3CN
empirical formula
fw
space group
a, Å
b, Å
C34H36Mo2N4O8
820.55
C34H36Mo2N4O8
820.55
C42H54BF4Mo2N6O7.5
1041.60
P21/c (No. 14)
17.860(1)
21.679(1)
12.3258(8)
90
94.761(1)
90
4755.7(5)
4
C38H42B2F8Mo2N8O4
1040.3
C17H15.5B3F12Mo2N7.5O2
819.26
P21 (No. 4)
8.087(2)
22.374(4)
19.358(4)
90
101.747(4)
90
3461.0(1)
4
P212121 (No. 19)
10.7893(5)
17.5542(9)
18.2354(9)
90
90
90
3453.7(3)
4
P21/n (No. 14)
12.9175(5)
9.6959(5)
18.294(1)
90
104.471(1)
90
2218.5(2)
2
P1h (No. 2)
11.1461(7)
16.220(1)
19.106(1)
101.377(1)
95.957(1)
106.340(1)
3202.5(4)
4
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
T, K
213
213
213
213
213
λ, Å
0.71073
1.575
0.780
0.71073
1.578
0.782
0.71073
1.455
0.596
0.71073
1.557
0.647
0.71073
1.699
0.882
0.066(0.128)
d
calcd, g/cm3
µ, mm-1
R1a (wR2b)
0.078(0.164)
0.024(0.059)
0.048(0.106)
0.031(0.073)
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo - Fc )2]/∑[w(Fo )2]]1/2
.
2
2
2
Table 2. Selected Bond Lengths (Å) for 1-5
Double arrows with an X signify an interconversion of cis/
trans isomer pairs that was not observed. Letters O and N
represent oxygen atoms of acetate groups or nitrogen atoms
of formamidinate groups, respectively, and L represents an
equatorial CH3CN molecule. The identity of each previously
known compound was confirmed by comparing the 1H NMR
spectrum of the prepared sample with that of an authentic
sample made as described in the literature, and/or by com-
paring values of the crystal unit cell dimensions. The absence
of an arrow indicates that the reaction was not studied.
Reaction conditions for each process are described briefly
in the scheme footnote. In all cases, crystallographic charac-
terization and 1H NMR spectra are available (Vide infra). It
should be noted that Scheme 1 shows 10 species of a total
of 21 possible combinations for the three ligands (forma-
midinate, acetate, and acetonitrile) under study. Of those not
3‚
5‚
1a
2
1.5C4H10
O
4
0.5CH3CNa
Mo(1)-Mo(2)
Mo(1)-Mo(1A)
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(1)-O(1)
Mo(1)-O(3)
Mo(2)-O(2)
Mo(2)-O(4)
2.092(2) 2.0831(3) 2.114(1)
2.1558(5)
2.1056(4)
2.155(12) 2.140(2)
2.121(14) 2.136(2)
2.152(5) 2.137(2)
2.142(5) 2.114(2)
2.149(4)
2.154(4)
2.126(4)
2.136(4)
2.153(4)
2.140(4)
2.127(4)
2.089(3)
2.162(14) 2.152(2)
2.121(14) 2.132(2)
2.170(8) 2.146(2)
2.126(5) 2.123(2)
2.121(4)
2.129(10) 2.125(2)) 2.170(4)
2.156(11) 2.122(2)
2.125(11) 2.113(2)
2.150(11) 2.118(2)
2.071(5)
2.066(3)
a Data for one of the two crystallographically independent but chemically
similar molecules in 1 and 5.
from acetonitrile). Anal. Calcd for C14H21B3F12Mo2N6O2: C, 22.07;
H, 2.78; N, 11.03. Found: C, 22.30; H, 3.95; N 10.80.
shown, three have been studied before and are described
X-ray Structure Determinations. Each of the single crystals
for 1-5 was mounted on the tip of a quartz fiber attached to a
goniometer head. Data were collected on a Bruker SMART 1000
CCD area detector system. The structures were solved by direct
methods using the SHELXS-97 program20 and refined using the
SHELXL-97 program.21 Crystals of 1 were racemic twins, and the
structure was refined to a BASF coefficient of 0.64. Non-hydrogen
atoms were refined with anisotropic displacement parameters.
Hydrogen atoms were added in calculated positions. Crystal-
lographic data for 1, 2, 3‚1.5(C2H5)2O, 4, and 5‚0.5CH3CN are in
Table 1, and selected bond distances in Table 2.
4+ 9
elsewhere, namely, Mo2(CH3CNeq)8
,
Mo2(DAniF)3-
(O2CCH3),12 and Mo2(DAniF)3(CH3CNeq)2+.22 Others which
remain unreported, e.g., trans-Mo2(O2CCH3)2(CH3CNeq)4
2+
,
were not studied because we did not deem them likely to be
useful synthetic building blocks, nor did we see them as
essential for understanding the patterns of chemical reactivity.
The scheme permits several useful generalizations and a
few specific observations:
(1) Replacement of formamidinate groups by acetonitrile
molecules requires the strongest conditions, such as the use
of Et3OBF4 with a trace amount of H2O or even HBF4.
(2) Acetate ligands are replaced also by acetonitrile in the
presence of CF3SO3H, Et3OBF4, or HBF4‚Et2O, or other acids
with weakly coordinating anions.
(3) Acetonitrile molecules can be readily substituted by
carboxylate and formamidinate anions.
(4) Carboxylates can be replaced by formamidinates, but
the opposite is not possible under any of the conditions
studied. Thus, ligand lability decreases from acetonitrile to
acetate to formamidinate ligands.
Results and Discussion
Synthetic Overview. Reactions pertinent to the syntheses
of all five new compounds are summarized in Scheme 1,
where compounds 1-5 are in red boxes. Other species with
an Mo24+ unit that play a role in this study are labeled A-E,
and are in blue boxes. All reactions are coded by arrows
with lower case italic letters (a-o). Solid arrows indicate
reactions that lead exclusively to the species to which the
arrows are pointing, while an X over the arrow indicates an
attempted reaction that did not lead to the target product.
(5) Whenever cis and trans isomers exist, each can be
made in pure form, but it is necessary to use different
(20) Sheldrick, G. M. SHELXS-97. Program for crystal structure solution.
Acta Crystallogr. 1990, A46, 467.
(21) Sheldrick, G. M. SHELXL-97. Program for Crystal Structure Analysis;
University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(22) (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. J.
Am. Chem. Soc. 2003, 125, 5436.
2270 Inorganic Chemistry, Vol. 43, No. 7, 2004

4+
Mo2 Building Blocks for Supramolecular Assemblies
a
Scheme 1
a
Reaction conditions for some processes are as follows: (a) mixing of 5 with 1 equiv of NaO2CCH3 in CH3CN; (b) reaction of B with excess (Et3O)BF4
in CH3CN; (c) reaction of B with 4 equiv of (Et3O)BF4 in CH2Cl2/CH3CN solution with a trace of H2O, see ref 18; (d) reaction of B with 2 equiv of
HDAniF/NaOCH3 in THF; (e and f) reaction of A with 2 equiv of HDAniF/NaOCH3 in CH3CN; (g) reaction of 2 with 2 equiv of (Et3O)BF4 in CH2Cl2/
CH3CN solution in the presence of a trace of H2O; (h) refluxing of 1 or 2 in toluene for 2 h; (i) refluxing of 4 or E in CD3CN for 0.5 h; (j) mixing of C
with 1 equiv of HDAniF in CH3CN; (k) mixing of 2 with 2 equiv of HDAniF/NaOMe in THF; (l) reaction of 2 with 4 equiv of (Et3O)BF4 in CH3CN; (m)
mixing of E with an excess of NaOCH3 in CH3CN; (n) addition of an excess of (Et3O)BF4 plus a trace of H2O to D in CH3CN; (o) reaction of D with 4
equiv of (Et3O)BF4 in CH3CN with a trace of H2O, see ref 8. Unless otherwise noted, reactions were carried out at room temperature.
synthetic methods.16 For example, 2 which has a trans
disposition of the formamidinate and acetate groups is made
by reacting Mo2(O2CCH3)4 with 2 equiv each of neutral
HDAniF and NaOMe (reaction d). Control of the stoichi-
ometry is essential for the success of this reaction, because
any excess of formamidinate will react further to produce
Mo2(DAniF)3(O2CCH3), a compound that has been used for
the synthesis of species having pairs of Mo2 units,12,14b e.g.,
IV. On the contrary, the corresponding cis isomer 1 can be
made only by an indirect route in which Mo2(DAniF)4 is
(reaction o), in a reaction that yields exclusively the cis
species. The acetonitrile ligands are then replaced by acetate
ions with retention of the cis conformation (reaction m).
Likewise, the syntheses of trans and cis isomers 4 and E,
respectively, require different procedures. The known cis
isomer E has been made in excellent yield by reaction of D
with 4 equiv of trialkyloxonium tetrafluoroborate in aceto-
nitrile, but the trans isomer 4 has to be prepared by an
indirect route in which three of the four formamidinate
groups in D are replaced by six acetonitrile molecules to
give C, which then reacts with 1 equiv of HDAniF to give
2+
transformed to the cis-Mo2(DAniF)2(CH3CNeq)4 cation
Inorganic Chemistry, Vol. 43, No. 7, 2004 2271

Cotton et al.
Scheme 2
the trans isomer 4. It should be noted that a random
substitution would be expected to yield a mixture of cis to
trans products in a 2:1 ratio but only 4 is observed. In this
reaction, strong base or deprotonated DAniF anion should
be avoided to prevent possible nucleophilic attack upon the
coordinated acetonitrile molecules.23 The addition of diethyl
ether, a Lewis base, shifts the equilibrium toward the
products by removing the acid that is produced as HBF4-
(OEt2).
Scheme 3
(6) In some reactions such as that of E to give 1, the
stereochemistry is retained and both reactants and product
have a cis conformation. However, in process l (reaction of
2 to produce E) the trans disposition of the formamidinate
anions changes and a cis species is formed. Likewise, cis A
gives only trans 2 but no cis 1.
or intermediate associative mechanism (Ia).25 For such
mechanisms, the axial sites are the most likely pathway for
ligands to enter or leave the M2 unit.
General Stability of Isomers 1 and 2, and 4 and E. Once
isolated, the four compounds are indefinitely stable in the
solid state under an inert atmosphere. They are also stable
in solution at room temperature. For each pair of cis and
Let us first consider the replacement of formamidinate
groups in Mo2(DAniF)4, D, by CH3CN molecules in the
presence of acid (reaction o). Presumably, substitution
reactions occur in a stepwise manner; protonation forms an
intermediate with one monodentate, neutral HDAniF group,
and the open coordination site is occupied by an acetonitrile
molecule. Then the neutral formamidine is displaced by
another acetonitrile molecule to give a species that has been
reported earlier (but made by a different synthethic route),
namely, the [Mo2(DAniF)3(CH3CN)2]+ cation.22 This process
is shown in Scheme 2. Because of the stronger trans directing
influence of the bridging formamidinate group relative to
that of that of the monodentate acetonitrile ligands, replace-
ment of a second bridging group is expected to occur giving
a cis geometry, as confirmed by the transformation of D into
E. Similarly B yields A in the acetate analogues, although
1
trans isomers the H NMR spectra show small but distinct
differences that allow unambiguous identification of each
compound. To evaluate the possible interconversion of
isomers, yellow solutions of 1 and 2 were taken indepen-
dently to reflux in toluene for 2 h (reaction h). Under these
conditions, no isomer interconversion was observed.24
Similarly, orange-red solutions of 4 and E, prepared in
CD3CN in an NMR tube, were heated slowly under nitrogen
from 50 to 85 °C over a period of 1.5 h and then refluxed
gently at this temperature for an additional 0.5 h (reaction
i). The 1H NMR spectra did not change. Thus, heating under
these conditions does not cause interchange between the two
isomers.
Reactivity and Evidence of a Trans Effect. The isolation
and lack of interconversion of cis and trans isomers of these
mixed-ligand species indicate that their formation is not
thermodynamically regulated but kinetically controlled in
much the same way as kinetic control determines the
substitution reactions in the common textbook example of
formation of the square planar complexes cis-Pt(NH3)2Cl2
and trans-Pt(NH3)2Cl2. Indeed the similarities go beyond the
selective formation of isomers. The basicity of the three
+
no intermediate Mo2(O2CCH3)3(CH3CN)2 has been ob-
served.
For the replacement of acetate groups in Mo2(O2CCH3)4,
B, by formamidinate ligands, the substitution leading to the
trans 2 is also easily understood in terms of a stepwise, trans
directing effect as shown in Scheme 3.26 In the intermediate
compound Mo2(DAniF)(O2CCH3)3 (which has not been
observed), the strong trans directing DAniF makes replace-
ment of the acetate trans to the DAniF ligand, resulting in
formation of the trans isomer, the preferred choice.
The methodology to synthesize trans-[Mo2(DAniF)2-
(CH3CNeq)4](BF4)2, 4, was explicitly designed with the
concept of a trans effect in mind. This compound is poten-
tially useful for the development of ladder-type molecular
chains, II, but this building block precursor has not been
accessible by direct synthesis.8 This preparation also provided
an opportunity to test the proposed stepwise mechanism and
the trans-directing influence. Indeed, addition of 1 equiv of
DAniF (as HDAniF) to [Mo2(DAniF)(CH3CNeq)6]3+, C,
ligands studied here decreases in the order DAniF-
>
-
O2CCH3 > CH3CN. Accordingly the observed lability is
the opposite, with formamidinates being more difficult to
displace while the acetonitrile molecules are the most easily
displaced. Furthermore, it appears that each of these ligands
exerts a trans effect that is proportional to its basicity. This
trans effect seems to be the major factor influencing the
geometry of the product in any given substitution reaction,
and it is also manifested in the bond distances (Vide infra).
Earlier work supports the idea that, in substitution reactions
of quadruply bonded dimolybdenum compounds, bond
formation and breaking takes place via an associative (A)
(23) Cotton, F. A.; Daniels, L. M.; Donahue, J. P.; Liu, C. Y.; Murillo, C.
A. Inorg. Chem. 2002, 41, 1354.
(25) Casas, J. M.; Cayton, R. H.; Chisholm, M. H. Inorg. Chem. 1991, 30,
360.
(24) The only changes observed in the 1H NMR spectra were due a
small amount of disproportionation with apparent formation of
Mo2(DAniF)3(O2CCH3).
(26) Here and in all other schemes, the species in brackets have not been
isolated; furthermore, the charges on all ions have been removed for
simplicity.
2272 Inorganic Chemistry, Vol. 43, No. 7, 2004

4+
Mo2 Building Blocks for Supramolecular Assemblies
Scheme 4
Scheme 6
Scheme 5
for the substitution of each acetate group. In the first step,
replacement of a labile acetate anion by two acetonitrile
molecules does not change the geometry. Therefore, a
geometry conversion must take place during the replacement
of the second acetate group by two additional acetonitrile
molecules. However, direct replacement of the second labile
acetate group would produce a trans, and not the observed
cis species. Because interconversion of the isomers E and 4
does not occur, the geometry change must take place during
the second substitution process and it is probably effected
displaces two CH3CN molecules, and gives trans 4 as the
only product. Scheme 4 shows a cartoon of the reaction.
However, an attempt to prepare trans-[Mo2(O2CCH3)2-
(CH3CNeq)4]2+ using a similar approach, viz., by reaction of
[Mo2(O2CCH3)(CH3CNeq)6]3+, 5, with 1 equiv of NaO2CCH3,
led exclusively to cis-[Mo2(O2CCH3)2(CH3CNeq)4]2+, A. This
indicates that the fairly labile and less basic acetate groups
are not capable inducing the trans substitution. Thus, it is
likely that in this case the kinetic factors do not offset the
higher thermodynamic stability of the cis isomer.
+
by the presence of a trace of water in the OEt3 precursor
which can react with the formamidinate groups.
These two geometry transformation reactions are not
straightforward and show that other factors can be important
too. It appears that the geometry of the final product is
determined by the basicity of the ligands and the geometric
arrangement of the ligands in the precursor at the last step.
Contrary to those reactions in Schemes 2-4, where the
intermediates have the most labile ligands opposite to the
trans directing ligands, those in Schemes 5 and 6 have
intermediates with the most labile ligands in positions other
than trans to the formamidinate groups. Therefore in the
latter, the lability of the ligands and the trans directing effect
do not work synergistically. It is only under these circum-
stances that a change of geometry has been observed.
Although it is not immediately obvious how the change takes
place, it is reasonable to suppose that some degree of axial
coordination is involved in the transition states. For example,
in Scheme 5, the acetate group trans to the formamidinate
ligand could be activated with concomitant dissociation of
one of the oxygen atoms.27 With support from the associative
mechanism mentioned earlier,25 a possible sequence for the
substitution of DAniF for acetonitrile can be described in
this way: the acetate group is displaced from the trans
position by the incoming DAniF, and then, through axial
coordination, this acetate anion displaces the labile CH3CN
molecules, completing the substitution and geometry conver-
sion. Because these effects are not expected to be exclusive
to those reactions in Schemes 5 and 6, they must be slower
than those in which there is a trans inducing effect.
The reaction of cis-[Mo2(DAniF)2(CH3CNeq)4](BF4)2, E,
with NaO2CCH3 leading to cis-Mo2(DAniF)2(O2CMe)2, 1,
is also understandable in terms of the proposed kinetic
considerations. In the cation cis-[Mo2(DAniF)2(CH3CNeq)4]2+,
the labile CH3CN molecules are trans to the DAniF ligands.
The addition of one acetate group produces cis-[Mo2(DAniF)2-
(O2CCH3)(CH3CNeq)2]+ as the intermediate product. Because
of the strong trans directing influence of the formamidinate
ligands, the second incoming acetate ligand displaces the
remaining acetonitrile molecules, which are also trans to a
DAniF ligand, maintaining the cis geometry of the precursor.
Cis/Trans Geometry Transformations. The initial cis or
trans configuration of a given reactant does not guarantee
that the geometry will be retained in the product. An example
is provided by reaction e, in which A yields 2. Here a
rearrangement of ligands and conversion of geometry from
cis to trans is involved, as shown in Scheme 5. In the
intermediate cation [Mo2(DAniF)(O2CCH3)2(CH3CN)2]+,
which has not been observed but appears to be the logical
intermediate in the stepwise addition of a formamidinate
ligand to A, the labile acetonitrile molecules are expected
to be the leaving ligands. If a second step were to proceed
similarly, this would give 1, which cannot be formed in this
manner, as shown in reaction f. It should be noted that, in
the intermediate monocation, the species that is opposite to
the trans directing DAniF group is an acetate anion while
the very labile acetonitrile molecules are cis to the DAniF
ligands.
It should also be noted that a geometric conversion from
cis to trans occurs when cis-[Mo2(DPhF)2(CH3CN)4]2+ reacts
with pyridine, DPhF ) N,N′-diphenylformamidinate, giving
trans-[Mo2(DPhF)2(py)4]2+.8 In the former, weaker ligands
(CH3CN) displace a stronger acetate ligand, but in the latter,
stronger (more basic) pyridine ligands substitute the weaker
Another example of change in geometry is observed in
the reaction of 2 to E, which is expected to proceed via 3
(Scheme 6). Here, strict control of the acidity is necessary
and only 2 equiv of triethyloxonium tetrafluoroborate is used
(27) A related pathway has been invoked for dirhodium complexes. See
for example: Crawford, C. A.; Matonic, J.; Streib, W. E.; Huffman,
J. C.; Dunbar, K. R.; Christou, G. Inorg. Chem. 1993, 32, 3125.
Inorganic Chemistry, Vol. 43, No. 7, 2004 2273

Cotton et al.
CH3CN ligands. In this case, the bulky pyridine molecules
adopt a transoid configuration.
Other Observations. Compound E is the most extensively
used building block precursor in the construction of su-
pramolecular species with dicarboxylate anions. It is made
by reaction of D with trialkyloxonium tetrafluoroborate as a
source of acid. However, E is not suitable for making an
assembly with more basic ligands, such as diamidates.23
Because 1 has the same geometry setting as E, we believe
that it will be useful for the synthesis of square or triangular
supramolecular entities with strongly basic ligands.28 Simi-
larly, 3 and 4 should provide the basis for the construction
of ladder type arrangements, II.
Because of the variety of ligands in 3, this compound
offers some interesting possibilities that will be explored in
due course. The compound can be prepared using a method
similar to the procedures for other dimetal acetonitrile
derivatives using triethyloxonium tetrafluoroborate in the
presence of a trace of water.8 However, the reaction must
be handled very carefully. To remove one of the acetate
ligands from 2, trans-Mo2(DAniF)2(O2CMe)2, 2 equiv of the
acidic reagent is required and the stoichiometry must be
strictly controlled because of the lability of the acetate ligand;
a mixture of dichloromethane and acetonitrile is used to
create a homogeneous reaction system. It is also important
to wash the crude product thoroughly.
Figure 1. Molecular structure of 1 showing the paddlewheel structure
with cisoid formamidinate and acetate groups. Displacement ellipsoids have
been drawn at the 40% probability level, and hydrogen atoms have been
omitted for clarity.
Colors of the Compounds. It is well-known that the
electronic spectra of compounds of the type Mo2(carboxylate)4
and Mo2(formamidinate)4 is dominated by the δ f δ*,
transition which gives them a yellow color.29 Replacement
of some of these groups by acetonitrile molecules causes a
significant change in color from yellow to blue as the num-
ber of CH3CN groups increases. Thus, Mo2(CH3CNeq)8-
4+
(CH3CNax)2 is blue-purple,9 while 5 is pink30 and C is
purple.8 Compounds E and 4 are orange-red, and A is red.
All others shown in Scheme 1 are yellow.
Structures. As shown in Figures 1 and 2, compounds 1
and 2 are stereoisomers having a paddlewheel structure in
which the four paddles are two formamidinate and two
acetate groups. In the former the formamidinate ligands are
cisoid to each other but transoid in the latter. The isomer
cis-Mo2(DAniF)2(O2CCH3)2 crystallizes in the monoclinic
space group P21 while the corresponding trans complex is
found in the orthorhombic space group P212121. The
Mo-Mo bond distances for 1, 2.089(2) and 2.092(2) Å for
the two crystallographically independent molecules in the
unit cell, are essentially the same as 2.0831(3) Å for 2, which
is consistent with the presence of quadruple bonds in the
Mo24+ units.18,31 However, there are some notable differences
in Mo-N and Mo-O bond distances as seen in Tables 2
Figure 2. A drawing of the molecular structure of 2 with 40% probability
ellipsoids showing the transoid configuration of the acetate and formamidi-
nate groups. All hydrogen atoms have been omitted for clarity.
Table 3. Cisoid and Transoid Mo-Ligand Distances (Å) Relative to a
Formamidinatea Group
1 and 2
E and 4
5
C
trans 2.140[3] (2)
Mo-NDAniF
Mo-OAcetate
Mo-Nacetonitrile
cis
2.13[1] (1)
trans 2.14[1] (1)
cis
2.120[2] (2)
trans
cis
2.181[4] (E) 2.152[2] 2.16[1]
2.141[2] (4) 2.132[5] 2.14[2]
a Acetate in 5.
and 3. The Mo-N bond lengths in 1, with an average of
2.13[1] Å, are slightly shorter than those in 2 (average of
2.140[3] Å), but the Mo-O bond distances in 1 are longer
than those in 2 (averages of 2.14[1] Å vs 2.120[ 2] Å,
respectively). Thus, all bonds trans to formamidinate nitrogen
atoms are slightly elongated. These data are collected in
Table 3 to facilitate the comparison between Mo-to-ligand
distances in these cis/trans isomers and other similar species.
(28) Indeed, we have isolated a molecular triangle from a reaction with
N,N′-diphenyltetraphthaloyldiamidate: Cotton, F. A.; Liu, C. Y.;
Murillo, C. A. Unpublished results.
(29) Cotton, F. A.; Nocera, D. G. Acc. Chem. Res. 2000, 33, 483.
(30) A pink compound that has not been structurally characterized but
reported to be [Mo2(O2CCH3)2(CH3CN)5](BF3OH)2 is likely to be
analogous to 5, because of the color. See: Telser, J.; Drago, R. L.
Inorg. Chem. 1984, 23, 1798.
(31) Cotton, F. A.; Mester, Z. C.; Webb, T. R. Acta Crystallogr. 1974,
B30, 2768.
2274 Inorganic Chemistry, Vol. 43, No. 7, 2004

4+
Mo2 Building Blocks for Supramolecular Assemblies
Figure 4. A view of the cation in 4 drawn with 40% probability ellipsoids.
All hydrogen atoms have been omitted for clarity.
Figure 3. A view of the cation in 3‚1.5Et2O showing the three different
types of ligands and the transoid configuration of the formamidinate groups.
Displacement ellipsoids have been drawn at the 40% probability level, and
all hydrogen atoms have been omitted for clarity.
This lengthening of bond distances of the groups that are
trans to the formamidinate ligands is consistent with the
kinetic trans influence of the formamidinate groups relative
to those of the acetate groups.32
The cation in 3‚1.5(C2H5)2O, shown in Figure 3, resides
on a general position in the monoclinic space group P21/c
and has three different types of ligands: two transoid,
bridging formamidinates, one bridging acetate, and two
acetonitrile molecules that occupy equatorial positions. The
4+
interstitial molecules of diethyl ether are far from the Mo2
unit and do not interact with it. The Mo-Mo bond distance
of 2.114(1) Å is longer than those in the paddlewheel
compounds 1 and 2, which is in accord with the presence of
only three bridging groups. All the Mo-ligand distances,
2.135[5] Å for Mo-NDAniF, 2.161[6] Å for Mo-Nacetonitrile
,
and 2.121[5] Å for Mo-O, are in the expected range.8
Figure 5. A view of one of the two crystallographically independent
cations in 5‚0.5CH3CN showing the weak interaction with an axial
acetonitrile molecule. Displacement ellipsoids are drawn at the 40%
probability level. Hydrogen atoms have been omitted for clarity.
Compound 4 crystallizes in a monoclinic space group P21/
n, with the midpoint of each Mo-Mo bond on an inversion
center. Figure 4 shows the trans geometry of the dication
which is accompanied by two BF4- counteranions. In contrast
to the cis analogue and cis-[Mo2(O2CCH3)2(CH3CNeq)4(CH3-
CNax)2](BF4)2, in which there are two axial CH3CN mol-
ecules,8,19,33 the axial sites in 4 are empty. Also, there are no
interstitial solvent molecules in the unit cell. The Mo-Mo
bond distance of 2.1056(4) Å is noticeably shorter than that
of 2.1439(6) Å in the cis isomer E and that of 2.134(2) Å
for the cis acetate compound, A. This may be due, at least
in part, to the lack of axial ligation which generally shortens
the metal-metal bond lengths.34 An interesting issue is why
there are no CH3CN molecules in the axial positions in the
trans compound 4, despite the fact that it is a cation. It is
known that electronic, structural, or crystallographic factors
may affect the axial interactions on dimetal units. Thus, the
structural parameters might render some hints. The average
Mo-NDAniF bond distance of 2.119[2] Å is slightly longer
than that of the cis analogue, 2.101[3] Å; but, as shown in
Table 3, the average Mo-Nacetonitrile bond distance of 2.141-
[2] Å is significantly shorter than that of 2.181[4] Å in the
cis isomer.8 Here, again, we see the impact of the nature of
the coordination of ligands that elongate the bonds that are
trans to the nonlabile DAniF group. For this reason, the
equatorial coordination of the CH3CN molecules to the Mo24+
unit is stronger in the trans isomer than in the cis isomer. In
(32) Similar elongations, which have been attributed to a trans influence,
have been observed recently in singly bonded dirhodium compounds.
See for example: Chifotides, H. T.; Catalan, K. V.; Dunbar, K. R.
Inorg. Chem. 2003, 42, 8739-8747.
(34) Cotton, F. A.; Hillard, E. A.; Murillo, C. A.; Zhou, H.-C. J. Am. Chem.
Soc. 2000, 122, 416.
(33) Pimblett, G.; Garner, C. D. J. Chem. Soc., Dalton Trans. 1986, 1257.
Inorganic Chemistry, Vol. 43, No. 7, 2004 2275

Cotton et al.
other words, the electron density in the Mo24+ unit is larger
in the trans isomer than in the cis one, which reduces the
attraction of molybdenum atoms for the axial ligands in 4.
Complex 5 crystallizes in space group P1h, with Z ) 4.
There are two crystallographically independent, but similar
cations (one is shown in Figure 5) which reside on general
positions. Each has only one bridging acetate group. The
remaining equatorial positions are occupied by acetonitrile
molecules. Furthermore, one of the axial sites is occupied
by a loosely bound acetonitrile molecule with an Mo-N
distance of 2.605 or 2.716 Å; the other axial position is
empty. The Mo-Mo bond distances of 2.1558(5) and
2.1539(5) Å are very similar to that of 2.152(1) Å found in
the formamidinate analogue, C. For 5 and C, the CH3CN
molecules trans to the acetate or DAniF ligands, respectively,
have Mo-Nacetonitrile distances longer than those correspond-
ing to the cis Mo-Nacetonitrile groups (see Table 3).
formamidinate, acetate, and acetonitrile ligands, all of which
show different substitution rates, which permits the selective
replacement of ligands. Furthermore, a significant trans effect
of the formamidinate ligands permits significant control in
the syntheses of cis and trans species. This is somewhat
similar to that found in square platinum(II) compounds. The
possibility that this methodology can be extended to other
M2 species will be investigated soon.
Acknowledgment. We thank Mr. J. F. Berry and Dr.
Peng Lei for supplying the crystal data for 5, and the National
Science Foundation, the Robert A. Welch Foundation, and
Texas A&M University (through the Laboratory for Molec-
ular Structure and Bonding) for financial support. We also
thank Dr. X. Wang for crystallographic advice.
Supporting Information Available: X-ray crystallographic data
in CIF format for 1, 2, 3‚1.5(C2H5)2O, 4, and 5‚0.5CH3CN. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Concluding Remarks. The systematic preparation of
various building block precursors usable for the construction
of supramolecular species has been accomplished using
IC035433S
2276 Inorganic Chemistry, Vol. 43, No. 7, 2004