A unique pair of stereoisomers: Synthesis and structures of cis-Mo2(μ-acetate)2[η2-N,N'-bis(trimethylsilyl)benzamidinate]2 and trans-Mo2(μ-acetate)2[μ-N,N'-bis(trimethylsilyl)benzamidinate]2

Article abstract of DOI:10.1016/S0020-1693(00)00096-7

The reaction between lithium bis(trimethylsilyl)benzamidinate and dimolybdenum tetraacetate yields a mixture of two stereoisomers: trans- (2a) and cis-Mo₂(O₂CCH₃)₂[PhC(NSi(CH₃)₃)₂]₂ (2b). While the acetates coordinate to the Mo₂ core via bridging mode in both compounds, the benzamidinates are bridging in 2a and chelating in 2b. The Mo-Mo bond lengths are 2.0692(4) and 2.1241(3) ? for 2a and 2b, respectively. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(00)00096-7

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Inorganica Chimica Acta 304 (2000) 305–308
Note
A unique pair of stereoisomers: synthesis and structures of
cis-Mo2(m-acetate)2[h2-N,N%-bis(trimethylsilyl)benzamidinate]2 and
trans-Mo2(m-acetate)2[m-N,N%-bis(trimethylsilyl)benzamidinate]2
Gang Zou, Tong Ren *
Department of Chemistry, Uni6ersity of Miami, Coral Gables, FL 33124, USA
Received 19 November 1999; accepted 28 February 2000
Abstract
The reaction between lithium bis(trimethylsilyl)benzamidinate and dimolybdenum tetraacetate yields a mixture of two
stereoisomers: trans- (2a) and cis-Mo2(O2CCH3)2[PhC(NSi(CH3)3)2]2 (2b). While the acetates coordinate to the Mo2 core via
bridging mode in both compounds, the benzamidinates are bridging in 2a and chelating in 2b. The MoꢀMo bond lengths are
,
2.0692(4) and 2.1241(3) A for 2a and 2b, respectively. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Molybdenum complexes; Benzamidinate complexes; Acetate complexes; Dinuclear complexes
ligand exhibits both bridging and chelating modes when
binding to a dimolybdenum(II) core.
1. Introduction
More than half of metalꢀmetal bonded dimolybden-
um(II) compounds are supported by bidentate ligands
(LꢀL) with an overwhelming majority having the biden-
tate ligand in a bridging mode [1]. In contrast, few
bidentate ligands coordinate to Mo2(II) core in a
chelating mode. Best known examples are the
Mo2X4(PꢀP)2 compounds with X as halide and PꢀP as
a bidentate phosphine R2P(CH2)mPR2 (m\1), where
isomers with PꢀP in both chelating (a) and bridging (b)
modes are abundant [1]. Occurrence of Mo2(II) com-
pounds containing two nitrogen donor chelating lig-
ands (NꢀN) is rare and previous examples include
cis-Mo2(OAc)2[(pz)2BEt2]2 and cis-Mo2(OAc)2[(pz)2-
BH]2 [2], Mo2(Et2Bpz2)2[Et2B(OH)pz]2 [3], [Mo2(m-O2-
CCF3)2(bpy)2]2+ and Mo2(h1-O2CCF3)4(bpy)2 [4]. We
report herein a unique example where a bidentate NꢀN
2. Results and discussion
Previous work demonstrated that reaction between
Mo2(O2CPh)4 and two equivalents of PhC(NSiMe3)-
[N(SiMe3)2]
afforded
trans-Mo2(O2CPh)2[m-N,N%-
PhC(NSiMe3)2]2 (1) as the sole product [5]. However,
the reaction between Mo2(O2CCH3)4 and excess
LiPhC(NSiMe3)2 (1:7 mole ratio) results in two prod-
ucts, 2a and 2b, with a low combined yield (36% based
on Mo). Compounds 2a and 2b differ drastically in
polarity: the former can be extracted with hexanes,
while the latter is insoluble even in CH2Cl2. Thus, it
was quite a surprise when X-ray diffraction studies
clearly revealed that they are stereoisomers, as shown
by the ORTEP plots in Figs. 1 and 2.
The molecular structure of 2a, trans-Mo2(m-
O2CCH3)2[m-N,N%-PhC(NSiMe3)2]2, is almost identical
to that of trans-Mo2(O2CPh)2[m-N,N%-PhC(NSiMe3)2]2
* Corresponding author. Tel.: +1-305-284 6617; fax: +1-305-284
1880.
E-mail address: tren@miami.edu (T. Ren)
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(00)00096-7

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G. Zou, T. Ren / Inorganica Chimica Acta 304 (2000) 305–308
[5] in both the overall ligand arrangement and the
metric parameters around the dimolybdenum(II) core.
bis(trimethylsilyl)benzamidinates.
Reflecting
the
strength of repulsion, 2b exhibits the largest NꢀMoꢀMo
angles (114.75(4) and 109.49(5)°) among the comparable
compounds, namely cis-Mo2(OAc)2[(pz)2BEt2]2 (108.9°),
cis-Mo2(OAc)2[(pz)2BH]2 (106.7°) [2], and [Mo2(m-
O2CCF3)2(bpy)2]2+ (100.6°) [4]. As a result of the steric
,
The MoꢀMo distance in 2a, 2.0692(4) A, is consistent
with a MoꢀMo quadruple bond and slightly decreased
,
,
from that of 1 (2.083 A). The average MoꢀO (2.123 A)
,
and MoꢀN (2.161 A) bond lengths in 2a are slightly
constraint of
a
four member chelating ring
longer than the corresponding distances observed in 1
(see Table 1). All bridging ligands conform to an
eclipsed geometry around an Mo2 core in 2a with the
largest torsional angle about 4° (NꢀMoꢀMo%ꢀN%).
The molecular structure of 2b, cis-Mo2(m-
O2CCH3)2[h2-N,N%-PhC(NSiMe3)2]2, contrasts sharply
with that of both 1 and 2a. Both bis(trimethylsi-
lyl)benzamidinate ligands coordinate to the Mo2 core in
a chelating mode, which enforces a cis arrangement
upon two bridging acetates. The MoꢀMo bond length,
(NꢀMoꢀN%ꢀC), the N1ꢀMoꢀN2 angle is only 61.92°.
Besides the unexpected chelating geometry of the
amidinate ligands, low solubility of 2b in common
organic solvents is also puzzling, especially considering
the presence of four trimethylsilyl groups. Careful exam-
ination of packing diagrams of 2b failed to reveal any
substantial intermolecular interaction, such as the for-
mation of a dating bond from a carboxy oxygen center
to an adjacent Mo(II) center or p–p stacking between
two phenyl rings. Similarly, no obvious intermolecular
interaction was revealed for the ‘virtually insoluble’
cis-Mo2(OAc)2[(pz)3BH]2 [2]. A subtle dipole–dipole
attraction is perhaps responsible for the limited solubil-
ity of both 2b and cis-Mo2(OAc)2[(pz)3BH]2. The solu-
bility difference between 2a and 2b explains the higher
synthetic yield of the latter: immediate crystallization of
2b during the reaction prevents subsequent conversion
to the less strained isomer 2a. Attempts to monitor the
possible equilibrium between 2a and 2b in solution by
1H NMR failed due to the extreme air-sensitivity of
both compounds.
,
2.1241(3) A, is about the same as the MoꢀMo bond
,
distance of cis-Mo2(OAc)2[(pz)2BEt2]2 (2.129 A) and
,
cis-Mo2(OAc)2[(pz)2BH]2 (2.147 A) [2], and slightly
shorter than [Mo2(m-O2CCF3)2(bpy)2]2+ (2.181 A) [4].
,
Compared with 1 and 2a, the MoꢀMo bond length in
2b is significantly elongated, which is largely attributed
to the strong repulsion between two chelating
Table 1
,
Selected bond lengths (A) and angles (°) for molecules 2a and 2b
2a
2b
MoꢀMoA
MoꢀO(1)
MoꢀO(2)
MoꢀN(1)
MoꢀN(2)
O(1)ꢀC(1)
O(2)ꢀC(3)
N(1)ꢀC(5)
N(2)ꢀC(5)
Si(1)ꢀN(1)
Si(2)ꢀN(2)
2.0692(4)
2.1163(16)
2.1297(17)
2.1641(19)
2.159(2)
1.265(2)
1.265(2)
1.335(3)
1.331(3)
1.758(2)
1.763(2)
MoꢀMoA
MoꢀO(1)
MoꢀO(2)
MoꢀN(1)
MoꢀN(2)
O(1)ꢀC(1)
O(2)ꢀC(1A)
N(1)ꢀC(3)
N(2)ꢀC(3)
Si(1)ꢀN(1)
Si(2)ꢀN(2)
2.1241(3)
2.1031(14)
2.1135(13)
2.2005(16)
2.1859(15)
1.267(3)
1.264(3)
1.335(2)
1.324(2)
1.7531(17)
1.7484(16)
Fig. 1. ORTEP of molecule 2a at 20% probability level.
MoAꢀMoꢀO(1)
MoAꢀMoꢀO(2)
MoAꢀMoꢀN(1)
MoAꢀMoꢀN(2)
O(1)ꢀMoꢀN(1)
O(2)ꢀMoꢀN(1)
O(1)ꢀMoꢀN(2)
O(2)ꢀMoꢀN(2)
91.97(5)
92.10(5)
92.96(5)
93.62(6)
92.22(7)
87.90(7)
92.70(7)
86.71(7)
O(1)ꢀMoꢀMoA
O(2)ꢀMoꢀMoA
MoAꢀMoꢀN(1)
MoAꢀMoꢀN(2)
N(2)ꢀMoꢀN(1)
O(1)ꢀMoꢀN(2)
O(2)ꢀMoꢀN(1)
N(2)ꢀC(3)ꢀN(1)
91.76(4)
90.58(4)
114.75(4)
109.49(5)
61.92(6)
101.91(6)
99.42(6)
116.12(17)
N(2)ꢀC(5)ꢀN(1) 121.1(2)
N(2)ꢀMoꢀMoAꢀ 3.96(11)
N(1A)
O(1)ꢀMoꢀMoAꢀ 3.29(6)
O(1A)
N(2A)ꢀMoAꢀ
MoꢀN(1)
O(2A)ꢀMoAꢀ
MoꢀO(1)
5.04(6)
2.40(6)
O(2)ꢀMoꢀMoAꢀ 1.98(6)
O(2A)
Fig. 2. ORTEP of molecule 2b at 20% probability level. Methyl groups
of SiMe3 were removed for clarity.

G. Zou, T. Ren / Inorganica Chimica Acta 304 (2000) 305–308
307
Table 2
3. Experimental
Crystal data for compounds 2a and 2b
Crystal 2a
C30H26N4O4Si4Mo2 C30H26N4O4Si4Mo2
3.1. General
2b
Empirical formula
Formula weight
Temperature (K)
Wavelength (Mo
LiPhC(NSiMe3)2 [6] and Mo2(OAc)4 [7] were
prepared and purified as previously described. THF
and hexanes were distilled over Na/benzophenone
under N2 before use. CH2Cl2 was distilled over CaH2
under N2.
837.0
837.0
300(2)
0.71073
300(2)
0.71073
,
Ka) (A)
Crystal system
orthorhombic
Pbcn (no. 60)
18.3083(8)
11.9440(5)
18.3818(7)
90
4019.6(3)
4
1.383
monoclinic
C2/c (no. 15)
21.8386(9)
10.8279(4)
18.5368(7)
117.0500(10)
3903.8(3)
4
Space group
,
a (A)
,
b (A)
3.2. Synthesis of compounds 2a and 2b
,
c (A)
i (°)
3
To a suspension of 0.89 g Mo2(OAc)4 (2.07 mmol) in
20 ml THF was added dropwise a solution of
LiPhC(NSiMe3)2 (14 mmol in 20 ml THF). The
reaction mixture turned red immediately and was
stirred overnight at room temperature. The reaction
mixture was filtered to yield a dark-brown filtrate and
an orange crystalline solid. Removal of THF from the
filtrate resulted in a brown residue, which was extracted
with hot hexanes. Yellow microcrystals were obtained
upon the distillation of hexanes, and further
recrystallization from ether–hexanes yields 2a as canary
needles (0.18 g, 10% based on Mo). The orange solid
was recrystallized from hot THF to afford 2b as orange
blocks (0.45 g, 26% based on Mo). Solutions of
compounds 2a and 2b turn brown instantaneously
when exposed to air, while their solids degrade within a
few minutes upon exposure.
,
V (A )
Z
Dcalc (Mg m−3
)
1.424
Absorption coeffi-
cient (mm−1
F(000)
0.779
0.802
)
1728
1728
Max and min
transmission
0.890 and 0.832
0.930 and 0.797
Crystal size (mm3)
q Range for data
collection (°)
0.23×0.20×0.15
2.04–28.00
0.26×0.11×0.09
2.09–28.00
Index ranges
−245h520,
−155k515,
−165l524
−285h519,
−135k514,
−245l524
14 239
Reflections collected 28 481
Independent
reflections
4852 [Rint=0.0469] 4695 [Rint=0.0276]
Completeness (%)
100.0 (to q=28.00°) 99.7 (to q=28.00°)
Refinement method full-matrix
full-matrix
least-squares on F2 least-squares on F2
Data/restraints/
parameters
4852/0/210
4695/0/207
Goodness-of-fit on
1.004
0.944
F2
3.3. Crystallographic analysis
Final R indices
[I\2|(I)]
R indices (all data)
R1=0.0283,
wR2=0.0657
R1=0.0535,
wR2=0.0750
0.407 and −0.368
R1=0.0243,
wR2=0.0547
R1=0.0359,
wR2=0.0570
0.360 and −0.244
Single crystals were obtained by slow cooling of
either a hexane solution (2a) or a THF solution (2b).
For both 2a and 2b, crystals were wedged into capil-
laries filled with deoxygenated mineral oil. Data collec-
tions were performed at 300 K on a Bruker Smart 1000
CCD-based X-ray diffractometer equipped with a Mo-
Largest difference
peak and hole
−3
,
(e A
)
,
target X-ray tube (u=0.71073 A) operated at 2400 W
power. Data were measured using ꢀ scans of 0.3° per
frame for 10 s, such that a hemisphere was collected. A
total of 1271 frames were collected with a final resolu-
crystallographic twofold axis that links four carbon
atoms of two bridging acetates. The asymmetric unit of
2b also consists of one half of the molecule and is
related to the other half via a crystallographic twofold
axis that goes through the midpoint of MoꢀMoA vec-
tor and bisects the planes of bridging acetates. With all
non-hydrogen atoms being anisotropic and all hydro-
gen atoms in calculated positions and riding mode,
both structures were refined to convergence by least-
squares method on F2, SHELXL-93, incorporated in
,
tion of 0.757 A in each set. No decay was indicated by
the recollection of the first 50 frames at the end of data
collection. The frames were integrated with the Bruker
SAINT software package using a narrow-frame integra-
tion algorithm [8], which also corrects for the Lorentz,
polarization, and absorption effects.
The structures of 2a and 2b were solved and refined
in the space groups Pbcn and C2/c, respectively. Posi-
tions of all non-hydrogen atoms were located by direct
methods. The asymmetric unit of 2a consists of one half
of the molecule, which is related to the other half via a
SHELXTL-PC V 5.03 [9–11]. Crystal data and details of
data collection and structure solution are summarized
in Table 2.

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G. Zou, T. Ren / Inorganica Chimica Acta 304 (2000) 305–308
4. Supplementary material
References
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(1991) 106.
Crystallographic data have been deposited with the
Cambridge Crystallographic Data Centre with the fol-
lowing deposition numbers: CCDC 140510 (2b) and
CCDC 140511 (2a). Copies of this information may be
obtained free of charge from The Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: +44-
1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
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[8] SAINT V 6.035, Software for the CCD Detector System, Bruker-
AXS Inc., 1999.
[9] SHELXTL 5.03 (Window-NT Version), Program Library for
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Acknowledgements
Starting fund and CCD diffractometer fund (to T.R.)
from the University of Miami are gratefully acknowl-
edged. Partial support from the University of Miami
Research Council is also acknowledged.
.