Diverse reactivity of stereoisomers containing quadruply bonded dimolybdenum with oxygen: Formation of [{Mo(η-N,N′- diisopropylbenzamidinato)oxo}2(μ-acetato)2(μ-oxo)]

Article abstract of DOI:10.1016/j.ica.2005.01.002

The reaction of quadruply bonded dimolybdenum complex, [Mo ₂(μ-OAc)₄] (1), with lithiated amidinato, Li[(N ⁱPr)₂CR] (R = ^tBu; 2a, Me; 2b, Ph; 2c), was investigated. The reaction of 1 with 2a afforded the dark-red solid, whereas the product was so highly unstable that the product was not able to be characterized. In the case of acetamidinato 2b, lantern-type mixed-ligand quadruply bonded dimolybdenum complex, [Mo₂(μ-OAc){μ-(N ⁱPr)₂CMe}₃] (3), was obtained as a yellow solid. In the reaction with benzamidinato 2c, symmetrical lantern-type dimolybdenum complex, [Mo₂(μ-OAc)₂ {μ-(N ⁱPr)₂CPh}₂] (4), was isolated as a yellow solid. In the latter reaction, intermediary red compound (5), which is considered to be stereoisomer of 4 possessing non-lantern-type skeleton, was formed. However, isolation of 5 as a single component was not successful due to isomerization to 4. Complex 5 readily reacted with dry oxygen to give dimolybdenum(V) complex, [{Mo(η-(NⁱPr)₂CPh)oxo} ₂ (μ-OAc)₂(μ-oxo)] (6), as a red solid. These complexes were characterized spectroscopically as well as, in some cases, by X-ray analyses.

Full text of DOI:10.1016/j.ica.2005.01.002

Inorganica Chimica Acta 358 (2005) 2363–2370
www.elsevier.com/locate/ica
Diverse reactivity of stereoisomers containing quadruply bonded
dimolybdenum with oxygen: formation of
[{Mo(g-N,N0-diisopropylbenzamidinato)oxo}2(l-acetato)2(l-oxo)]
a,
a
a
Yoshitaka Yamaguchi *, Sou Ozaki , Hideto Hinago ,
b
Kimiko Kobayashi , Takashi Ito
a
a
Department of Materials Chemistry,Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama
240-8501, Japan
b
The Institute of Physical and Chemical Research (RIKEN), Hirosawa, Wako 351-0198, Japan
Received 8 December 2004; accepted 11 January 2005
Abstract
The reaction of quadruply bonded dimolybdenum complex, [Mo2(l-OAc)4] (1), with lithiated amidinato, Li[(NiPr)2CR]
(R = tBu; 2a, Me; 2b, Ph; 2c), was investigated. The reaction of 1 with 2a afforded the dark-red solid, whereas the product was
so highly unstable that the product was not able to be characterized. In the case of acetamidinato 2b, lantern-type mixed-ligand
quadruply bonded dimolybdenum complex, [Mo2(l-OAc){l-(NiPr)2CMe}3] (3), was obtained as a yellow solid. In the reaction with
benzamidinato 2c, symmetrical lantern-type dimolybdenum complex, [Mo2(l-OAc)2 {l-(NiPr)2CPh}2] (4), was isolated as a yellow
solid. In the latter reaction, intermediary red compound (5), which is considered to be stereoisomer of 4 possessing non-lantern-type
skeleton, was formed. However, isolation of 5 as a single component was not successful due to isomerization to 4. Complex 5 readily
reacted with dry oxygen to give dimolybdenum(V) complex, [{Mo(g-(NiPr)2CPh)oxo}2 (l-OAc)2(l-oxo)] (6), as a red solid. These
complexes were characterized spectroscopically as well as, in some cases, by X-ray analyses.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Molybdenum complexes; Amidinato ligands; Dinuclear complexes; Mo–Mo bond; Oxo complexes
1. Introduction
There have been a lot of excellent studies on the quadru-
ply bonded dimolybdenum complexes, which are often
named as lantern-type complexes from a viewpoint of
their shapes. Recently, extensive attention has been fo-
cused on the formation of the linearly ordered multinu-
clear transition-metal complexes by assembling the
metals along the lines with the metal–metal multiple
bonds [3]. However, there are few examples of the inves-
tigation on the direct reactivity toward the quadruply
bonded Mo2 core in perpendicular directions [4].
Amidinato is one of the most important ancillary li-
gands in the coordination chemistry because it acts not
only as a chelating mode but also as a bridging one
[1]. The remarkable ability of an amidinato ligand, in
particular of a formamidinato ligand formulated as
[(NR0)2CH]ꢀ, to construct the bridging mode of coordi-
nation may be useful to build up the dinuclear com-
plexes containing the metal–metal multiple bonds [2].
The recent study on amidinato complexes has re-
vealed that the steric repulsion between substituents on
the amidinato ligand governs the coordination mode
of the ligand: an amidinato ligand bearing sterically
bulky substituents is likely to act as a chelating ligand
*
Corresponding author. Tel./fax: +814 5339 3932.
E-mail address: yyama@ynu.ac.jp (Y. Yamaguchi).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.01.002

2364
Y. Yamaguchi et al. / Inorganica Chimica Acta 358 (2005) 2363–2370
(Chart 1) [5]. Thus we are interested in the introduction
of sterically hindered amidinato ligands onto the Mo2
core for the purpose of producing new non-lantern-type
Mo2 complexes. Herein, we report that the reaction of
[Mo2(l-OAc)4] (1) with a series of Li[(NiPr)2CR]
(R = tBu; 2a, Me; 2b, Ph; 2c) resulted in the formation
of new mixed-ligand lantern-type dimolybdenum com-
plexes. In the course of this study, we found the forma-
tion of non-lantern-type complex of dimolybdenum, the
reactivity of which is also described.
was then stirred overnight to give a brown solution.
After removing the volatiles under reduced pressure,
the residual solid was extracted with pentane. The sol-
vent was evaporated off under high vacuum and spectro-
scopically pure 3 was obtained as a greenish yellow solid
(111 mg, 0.16 mmol, 89%). Anal. Calc. for C26H54Mo2-
N6O2: C, 46.29; H, 8.07; N, 12.46. Found: C, 46.18; H,
1
8.08; N, 12.39. H NMR (d, in C6D6): 0.87 (d, J = 6.4
Hz, 12H, Pr–CH3), 1.04 (d, J = 6.4 Hz, 12H, Pr–
i
i
i
CH3), 1.42 (d, J = 6.9 Hz, 12H, Pr–CH3), 2.35 (s, 6H,
amidinato–CCH3), 2.42 (s, 3H, amidinato- or acetate–
CCH3), 2.49 (s, 3H, amidinato- or acetate–CCH3),
i
3.80 (sept, J = 6.9 Hz, 2H, Pr–CH), 4.23 (sept, J = 6.4
2. Experimental
i
Hz, 4H, Pr–CH). 13C{1H} NMR (d, in C6D6): 16.0
2.1. General procedures
(amidinato-CCH3), 21.3 (amidinato- or acetate–
CCH3), 23.8 (iPr–CH3), 24.2 (amidinato- or acetate–
CCH3), 25.1 (iPr–CH3), 25.2 (iPr–CH3), 50.4 (iPr–CH),
51.6 (iPr–CH), 161.7, 162.9, 175.6 (amidinato and ace-
tate central carbons).
All manipulations involving air- and moisture-sensi-
tive compounds were carried out under an atmosphere
of dry argon, which was purified by SICAPENT (Merck
Co., Inc.), by using a standard Schlenk tube or high vac-
uum techniques. All solvents were distilled over appro-
priate drying agents prior to use. [Mo2(l-OAc)4] (1) [6]
and Li(amidinato) (2a, 2b, and 2c) [5b,7] were prepared
according to the literature methods. Other reagents em-
ployed in this research were used without further
purification.
1H, 13C{1H}, and 31P{1H} NMR spectra were re-
corded on a JEOL EX-270 spectrometer at ambient tem-
perature unless otherwise mentioned. H and 13C{1H}
NMR chemical shifts were reported in ppm relative to
internal Me4Si. 31P{1H} NMR chemical shifts were re-
corded in ppm relative to external 85% H3PO4. All cou-
pling constants were recorded in Hz. Splitting patterns
are indicated as s, singlet; d, doublet; t, triplet; sept, sep-
tet; m, multiplet; and br, broad signal. Elemental analy-
ses were performed by a Perkin–Elmer 240C.
2.3. Preparation of [Mo2(l-OAc)2{l-(NiPr)2CPh}2]
(4)
[Mo2(l-OAc)4] (1) (289 mg, 0.68 mmol) and Li[(-
NiPr)2CPh] (2c) (301 mg, 1.43 mmol) were put into a
Schlenk tube, which was attached to a high-vacuum line.
THF (ca. 15 mL) was added by a trap-to-trap-transfer
technique at ꢀ78 ꢁC and the reaction mixture was al-
lowed to be warmed to room temperature. The mixture
was then refluxed overnight to give a yellow solution.
After removing the volatiles under reduced pressure,
the residual solid was extracted with pentane. The sol-
vent was evaporated off under high vacuum, and spec-
troscopically pure 4 was obtained as a yellow solid
(452 mg, 0.63 mmol, 93%). Anal. Calc. for C30H44Mo2-
N4O4: C, 50.28; H, 6.19; N, 7.82. Found: C, 50.10; H,
1
1
6.20; N, 7.81. H NMR (d, in C6D6): 0.93 (d, J = 6.6
Hz, 24H, Pr–CH3), 2.59 (s, 6H, acetate–CH3), 4.02
2.2. Preparation of [Mo2(l-OAc){l-(NiPr)2CMe}3]
(3)
i
i
(sept, J = 6.6 Hz, 4H, Pr–CH), 7.12–7.39 (m, 10H,
i
Ph). (d, in CDCl3): 0.68(d, J = 6.6 Hz, 24H, Pr–CH3),
[Mo2(l-OAc)4] (1) (76 mg, 0.18 mmol) and Li-
[(NiPr)2CMe] (2b) (87 mg, 0.59 mmol) were put into a
Schlenk tube, which was attached to a high-vacuum line.
THF (ca. 10 mL) was added by a trap-to-trap-transfer
technique at ꢀ78 ꢁC and the reaction mixture was al-
lowed to be warmed to room temperature. The mixture
2.56 (s, 6H, acetate–CH3), 3.77 (sept, J = 6.6 Hz, 4H,
iPr–CH), 7.36–7.52 (m, 10H, Ph). 13C{1H} NMR (d,
in C6D6): 23.7 (acetate–CCH3), 26.3 (iPr–CH3), 51.7
(iPr–CH), 127.8, 128.4, 129.4, 138.5 (Ph), 166.9, 177.1
(amidinato and acetate central carbons).
2.4. Formation of [Mo2(l-OAc)2{g-(NiPr)2CPh}2] (5)
[Mo2(l-OAc)4] (1) (268 mg, 0.63 mmol) and Li-
[(NiPr)2CPh] (2c) (271 mg, 1.29 mmol) were put into a
Schlenk tube, which was attached to a high-vacuum line.
THF (ca. 15 mL) was added by a trap-to-trap-transfer
technique at ꢀ78 ꢁC and the reaction mixture was al-
lowed to be warmed to room temperature. After several
hours, the volatiles were removed under reduced pres-
1
Chart 1. Coordination modes of amidinato ligand.
sure. The residual red solid was subjected to H NMR

Y. Yamaguchi et al. / Inorganica Chimica Acta 358 (2005) 2363–2370
2365
1
measurement. H NMR (d, in C6D6): 0.97 (d, J = 6.3
tion corrections and an empirical absorption
correction (W scan) were made.
A diffraction measurement of 6 was made on an En-
i
i
Hz, 12H, Pr–CH3), 1.57 (d, J = 6.3 Hz, 12H, Pr–
CH3), 2.63 (s, 6H, acetate–CH3), 3.75 (sept, J = 6.3
i
Hz, 4H, Pr–CH), 6.77–7.05 (m, 10H, Ph). Complex 5
raf-Nonius CAD4 diffractometer by using graphite-
˚
was not able to be isolated due to its tendency to isom-
erize to 4.
monochromated Mo Ka radiation (k = 0.71069 A).
The data collections were carried out at 23 1 ꢁC using
the x–2h scan technique. Cell constants and an orienta-
tion matrix for data collection were determined from 25
reflections with 2h angles in the range 20.84–36.06ꢁ. In
the reduction of data, Lorentz and polarization correc-
tions and an empirical absorption correction (W scan)
were made.
Crystallographic data and the results of measure-
ments are summarized in Table 1. The structures were
solved by heavy-atom Patterson methods (DIRDIF94
PATTY) [8] for 3 or by direct methods (SIR 92) [9] for 4
and 6, and expanded using Fourier techniques [10].
Least-squares refinements were carried out using
SHELXL-97 [11] linked to TEXSAN [12]. All of the non-
hydrogen atoms were refined anisotropically. All hydro-
gen atoms were introduced at the ideal positions with
2.5. Preparation of [{Mo(g-(NiPr)2CPh)(oxo)}2(l-
OAc)2(l-oxo)] (6)
[Mo2(l-OAc)4] (1) (111 mg, 0.26 mmol) and Li-
[(NiPr)2CPh] (2c) (134 mg, 0.64 mmol) were put into a
Schlenk tube, which was attached to a high-vacuum line.
THF (ca. 10 mL) was added by a trap-to-trap-transfer
technique at ꢀ78 ꢁC and the reaction mixture was al-
lowed to be warmed to ꢀ30 ꢁC. The reaction mixture
was again cooled to ꢀ78 ꢁC, and then oxygen (1 atm),
which had been purified by SICAPENT, was intro-
duced. After being allowed to be warmed to room tem-
perature, the volatiles were removed under reduced
pressure. The residual liver solid was extracted with hex-
ane. The fractional recrystallization gave the title com-
plex 6 (65 mg, 0.085 mmol, 33%) as a red crystal and
4 (60 mg, 0.084 mmol, 32%) as a yellow crystal. Anal.
Calc. for C30H44Mo2N4O7: C, 47.13; H, 5.80; N, 7.33.
Found: C, 47.25; H, 5.71; N, 7.30. 1H NMR (d, in
˚
the C–H bond distance of 0.96 A. The methyl hydrogen
atoms were refined by using riding models and the rest
of hydrogen atoms were fixed at the calculated
positions.
i
C6D6): 1.38 (d, J = 6.6 Hz, 6H, Pr–CH3), 1.41 (d,
i
i
J = 6.6 Hz, 6H, Pr–CH3), 1.69 (d, J = 6.6 Hz, 6H, Pr–
CH3), 1.71 (s, 3H, acetato-CH3), 1.71 (d, J = 6.6 Hz,
3. Results and discussion
i
6H, Pr–CH3), 1.73 (s, 3H, acetate–CH3), 3.85 (sept,
3.1. Reaction of [Mo2(l-OAc)4] (1) with Li(amidinato)
(2). Formation of mixed-ligand lantern-type complexes
i
J = 6.6 Hz, 2H, Pr–CH), 3.98 (sept, J = 6.6 Hz, 2H,
iPr–CH), 7.03-7.28 (m, 10H, Ph). (d, in CDCl3): 1.15
(d, J = 6.6 Hz, 6H, Pr–CH3), 1.16 (d, J = 6.6 Hz, 6H,
i
In order to systematically investigate on the reaction
of dimolybdenum complex 1 with Li(amidinato), we se-
lected three amidinato ligands containing a series of sub-
stituents on the central carbon, Li[(iPrN)2CR] (R = tBu;
2a, Me; 2b, Ph; 2c), which are easily prepared by the
reaction of 1,3-diisopropylcarbodiimide with the corre-
sponding RLi (Chart 2).
Firstly, the reaction of dimolybdenum complex 1
with Li(amidinato) 2a in THF was examined (Scheme
1). The reaction mixture was changed from a yellow
solution to a homogeneous red solution from which a
dark-red solid was obtained, which was, however, too
unstable to be characterized. Cotton has reported that
the fully chelated red crystalline dimolybdenum com-
plex, [Mo2(g-o-Me2NCH2C6H4)4], is very air-sensitive
[13]. Taking into account that most of lantern-type com-
plexes, most of which are yellow, are relatively stable to-
ward air and moisture, it is feasible to consider that
instability of the present product might come from the
formation of non-lantern-type complex.
iPr–CH3), 1.40 (d, J = 6.6 Hz, 6H, Pr–CH3), 1.49 (d,
i
i
J = 6.6 Hz, 6H, Pr–CH3), 1.69 (s, 3H, acetato-CH3),
2.36 (s, 3H, acetate–CH3), 3.69 (sept, J = 6.6 Hz, 2H,
i
iPr–CH), 3.88 (sept, J = 6.6 Hz, 2H, Pr–CH), 7.26-
7.49 (m, 10H, Ph). 13C{1H} NMR (d, in C6D6): 23.1,
23.4, 23.5, 24.0, 24.5 (iPr–CH3 or acetato–CH3), 50.9,
51.2 (iPr–CH), 126.7, 128.6, 128.9, 129.6, 131.7 (Ph),
180.9, 181.0 (OCO), 199.9 (s, NCN).
2.6. Experimental procedure for X-ray crystallography
Suitable single crystals were obtained by recrystalliza-
tion from toluene (3), pentane (4), or from hexane (6), at
ꢀ30 ꢁC and were mounted on glass fibers.
Diffraction measurements of 3 and 4 were made on a
Rigaku AFC-7R automated four-circle diffractometer
with graphite-monochromated Mo Ka radiation
˚
(k = 0.71069 A). The data collections were carried out
at 23 2 ꢁC using the x–2h scan technique. Cell con-
stants and an orientation matrix for data collection were
determined from 25 reflections with 2h angles in the
range 29.77–29.98ꢁ for 3 and 29.67–29.98ꢁ for 4, respec-
tively. In the reduction of data, Lorentz and polariza-
In the reaction of 1 with acetamidinato 2b, tris(amid-
inato)acetato complex 3 was obtained as a greenish yel-
low crystal in good yield (Scheme 2). X-ray diffraction
study on the product revealed that the complex 3 in a

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Y. Yamaguchi et al. / Inorganica Chimica Acta 358 (2005) 2363–2370
Table 1
Summary of crystal data for 3, 4, and 6
3
4
6
Empirical formula
Formula weight
Crystal color, habit
Crystal size (mm)
Crystal system
C26H54Mo2N6O2
674.63
C30H44Mo2N4O4
716.58
C30H44Mo2N4O7
764.58
red, plate
yellow, plate
0.25 · 0.18 · 0.08
triclinic
yellow, prismatic
0.25 · 0.15 · 0.15
monoclinic
0.45 · 0.38 · 0.15
triclinic
ꢀ
ꢀ
Space group
Lattice parameters
P1ð#2Þ
P21/c (#14)
P1ð#2Þ
˚
a (A)
8.860(3)
10.396(2)
18.121(3)
89.99(1)
80.64(2)
74.57(2)
1586.0(7)
2
9.735(2)
9.876(1)
17.741(1)
90
12.774(1)
13.743(2)
10.8262(9)
95.405(9)
107.914(8)
96.15(1)
1781.7(4)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
91.13(1)
90
3
˚
V (A )
1705.4(4)
2
1.395
Z
Dc (g cmꢀ3
F(000)
l (Mo Ka) (cmꢀ1
2hmax (ꢁ)
Reflections measured
)
1.413
1.425
784.00
704.00
8.20
60.0
736.00
7.70
60.0
)
7.48
54.9
9818
5511
8529
Independent reflections (Rint
)
9265 (0.026)
6558
341
4969 (0.022)
3151
186
8162 (0.013)
6630
398
Number of observed (I > 2.0r(I))
Number of variables
Residuals: Ra; Rw
0.030; 0.090
1.03d
0.033; 0.101e
1.02e
0.025; 0.070f
1.05f
b
d
c
GOF
ꢀ3
˚
dqmax,min (e A
)
0.47, ꢀ0.65
0.34, ꢀ0.35
0.29, ꢀ0.40
a
R = RiFo| ꢀ |Fci/R|Fo| for observed data.
2
2 1=2
b
c
d
e
f
Rw ¼ ½RwðF 2o ꢀ F 2c Þ =RwðF 2oÞ ꢁ for all data.
GOF = [Rw(|Fo| ꢀ |Fc|)2/(No ꢀ Nv)]1/2
.
w ¼ ½r2ðF 2oÞ þ ð0:0404P2 þ 0:0560Pꢁꢀ1, where P ¼ ðF o2 þ 2F 2c Þ=3.
w ¼ ½r2ðF o2Þ þ ð0:0421PÞ þ 0:3913Pꢁꢀ1, where P ¼ ðF o2 þ 2F 2c Þ=3.
2
w ¼ ½r2ðF o2Þ þ ð0:0328PÞ þ 0:5551Pꢁꢀ1, where P ¼ ðF o2 þ 2F 2c Þ=3.
2
Me
iPr
iPr
iPr
N
iPr
N
iPr
N
iPr
N
N
N
Me
i
Pr
i
N
Pr
tBu
Mo N
N
[Mo2(µ-OAc)4]
+
3 eqs.
Li
Me
Me
Ph
Mo
N
N
iPr
O
N
N
N
O
iPr
iPr
iPr
iPr
iPr
1
2b
Me
2a
2b
2c
3
Chart 2. Entry of amidinato ligands employed in this research.
Scheme 2.
of doublets with the same integration (12 protons each),
i
assignable to Pr-methyl groups on the amidinato li-
iPr
N
tBu
[Mo2(µ-OAc)4]
+
4eqs.
Li
dark-red solid
(highly unstable)
gands, were observed. Methine protons on the amidi-
nato ligands displayed two sets of septets and the
integration ratio was 4H/2H. The pathway for the for-
mation of non-lantern-type complex in this reaction is
not clear at the present stage.
N
iPr
1
2a
Scheme 1.
solid state possesses a lantern-type skeleton bearing
three amidinato ligands and one acetato ligand. One
of the three amidinato ligands is located in the coplanar
with the acetato ligand and the rest two amidinato li-
gands are located in mutually trans positions (vide in-
fra). This structure was found to be maintained in the
solution state, too. In the 1H NMR spectrum, three sets
Treatment of 1 with two equivalents of 2c in THF un-
der refluxing conditions yielded a homogeneous yellow
solution from which yellow crystals were obtained in
1
93% yield. Elemental analysis, H and 13C NMR spec-
tra, and X-ray structure analysis confirmed the forma-
tion of the symmetrical lantern-type complex bearing
two amidinato and two acetato ligands (4) (Scheme 3).

Y. Yamaguchi et al. / Inorganica Chimica Acta 358 (2005) 2363–2370
2367
Ph
iPr
iPr
iPr
N
N
N
O
Mo O
N
[Mo2(µ-OAc)4]
+
2 eqs. Li
Ph
Mo
N
N
iPr
O
O
iPr
iPr
1
2c
Ph
4
Scheme 3.
1
In H NMR spectrum, isopropyl protons on the amidi-
nato ligands were seen as a doublet at 0.92 ppm and as a
septet at 4.02 ppm with a coupling constant of 6.6 Hz.
Methyl protons on the acetato ligands were observed
at 2.59 ppm as a singlet.
The ORTEP drawings of 3 and 4 are displayed in
Figs. 1 and 2, respectively. The crystal data and selected
bond distances and angles are listed in Tables 1–3,
respectively. The molecular structure of 3 is almost iden-
tical to those of [Mo2(l-OAc){l-(NPh)2CMe}3] [14].
˚
The Mo–Mo distance, 2.0702(3) A, is consistent with a
Mo–Mo quadruple bond. The Mo–N bond distances
˚
that are trans to the acetato oxygen are 2.125(2) A for
˚
Mo(1)–N(3) and 2.134(2) A for Mo(2)–N(4). These dis-
tances are slightly shorter than the remaining Mo–N
˚
bond distances (2.153(2)–2.158(2) A). These structural
Fig. 2. ORTEP drawing of complex 4 with thermal ellipsoid drawn at
the 30% probability level. All hydrogen atoms are omitted for clarity.
features are also similar to those in [Mo2(l-OAc){l-
(NPh)2CMe}3] [14]. The structure of complex 4 is almost
similar to those of [Mo(l-OAc)2{l-(NSiMe3)2CPh}2]
3.2. Reinvestigation on the reaction of 1 with 2c.
Formation of a non-lantern-type dinuclear complex,
[Mo2(l-OAc)2{g-(NiPr)2CPh}2] (5)
[15]
nyl)formamidinato)2}] [16]. The quadruply bonded
and
[Mo(l-OAc)2{l-(N,N0-di(2-methoxyphe-
˚
Mo–Mo distance in 4 is 2.0677(4) A.
The reaction of 1 with 2c in THF was investigated in
1
detail by H NMR (Scheme 4). The reaction mixture of
1 with 2c showed a homogeneous red solution at the
Table 2
˚
Selected bond distances (A) and angles (ꢁ) for complex 3
Mo(1)–Mo(2)
Mo(1)–O(1)
Mo(1)–N(1)
Mo(1)–N(3)
Mo(1)–N(5)
Mo(2)–O(2)
Mo(2)–N(2)
Mo(2)–N(4)
Mo(2)–N(6)
2.0702(3)
2.175(2)
2.158(2)
2.125(2)
2.156(2)
2.174(2)
2.153(2)
2.134(2)
2.156(2)
O(1)–Mo(1)–N(3)
N(1)–Mo(1)–N(5)
O(1)–Mo(1)–N(1)
O(1)–Mo(1)–N(5)
N(1)–Mo(1)–N(3)
N(3)–Mo(1)–N(5)
O(2)–Mo(2)–N(4)
N(2)–Mo(2)–N(6)
O(2)–Mo(2)–N(2)
O(2)–Mo(2)–N(6)
N(2)–Mo(2)–N(4)
N(4)–Mo(2)–N(6)
173.94(7)
171.73(7)
88.23(8)
85.56(7)
94.83(8)
90.87(8)
174.83(7)
171.24(9)
88.21(8)
85.47(8)
94.05(8)
91.78(8)
Fig. 1. ORTEP drawing of complex 3 with thermal ellipsoid drawn at
the 30% probability level. All hydrogen atoms are omitted for clarity.

2368
Y. Yamaguchi et al. / Inorganica Chimica Acta 358 (2005) 2363–2370
Table 3
and two amidinato ligands are located in mutually cis
position, respectively. Structures B and C exhibit non-
lantern-type skeleton. In the case of B, two amidinato li-
gands act as a bridging ligand and each acetato ligand
coordinates to the molybdenum center in a chelating
fashion. The structure C is a reverse to the structure
B. Recently, Zou and Ren [15] have reported that dinu-
clear complex [Mo2(OAc)2{(NSiMe3)2CPh}2] possesses
a set of stereoisomers; one is a lantern-type complex
which is an isostructure to 4 and the other is a non-lan-
tern-type complex corresponding to the structure C in
Chart 3. The tentative assignment of complex 5 to the
structure C was further supported by its reaction with
O2 (vide infra).
˚
Selected bond distances (A) and angles (ꢁ) for complex 4
Mo(1)–Mo(1*)
Mo(1)–O(1)
Mo(1)–O(2*)
Mo(1)–N(1)
Mo(1)–N(2*)
2.0677(4)
2.123(2)
2.122(2)
2.135(2)
2.132(3)
O(1)–Mo(1)–O(2*)
N(1)–Mo(1)–N(2*)
O(1)–Mo(1)–N(1)
O(1)–Mo(1)–N(2*)
O(2*)–Mo(1)–N(1)
O(2*)–Mo(1)–N(2*)
N(1)–Mo(1)–N(2*)
175.52(8)
174.02(9)
89.16(9)
90.26(9)
90.59(9)
89.53(9)
174.02(9)
4
iPr
N
3.3. Reaction of [Mo2(l-OAc)2{g-(NiPr)2CPh}2] (5)
with O2
THF
reflux
[Mo2(µ-OAc)4]
+
2 eqs.
heat
Li
Ph
N
iPr
5
We are interested in the reactivity of the dinuclear
complex 5 with various molecules, because it has the
structure of ‘‘open’’ quadruply bonded Mo2 framework
allowing the direct reaction toward the Mo–Mo quadru-
ple bond. To examine the reactivity of 5, we used the
mixture of complexes 5 and 4 because the former was
not able to be isolated as single product as mentioned
above. The reactions of 5 with some unsaturated mole-
cules such as olefin and acetylene resulted in isomeriza-
tion to 4.
THF
-78˚C
2c
1
r.t.
Scheme 4.
initial stage. After treating 1 with 2c in THF at ꢀ78 ꢁC,
the reaction mixture was gradually allowed to be
warmed to room temperature. After removing the vola-
1
tiles, the obtained red solid was subjected to H NMR
1
measurement. The H NMR spectrum showed that the
formation of 4 was accompanied with another complex
(5). The latter complex showed the two doublets
(d = 0.97 and 1.57) and one septet (d = 3.75) assignable
On treatment of the mixture of 4 and 5 with dry O2 (1
atm) in THF at ꢀ78 ꢁC, oxo complex of dimolybdenum
was obtained as a red crystal (Scheme 5). In this reac-
tion, complex 4 remained almost intact. X-ray diffrac-
tion study revealed that the product has two terminal
oxo ligands attached to each molybdenum center and
one bridging oxo ligand between Mo2 cores and that
two amidinato and two acetato ligands act as chelating
and bridging modes, respectively, as shown by the OR-
TEP drawing in Fig. 3. Each molybdenum center shows
the pseudo-pentagonal bipyramidal geometry position-
ing terminal oxo and acetato oxygen in axial sites. From
the skeleton of complex 6, it may be plausible to assign
the structure C depicted in Chart 1 as a possible geom-
etry for complex 5.
i
to Pr–methyl and –methine protons, respectively, with
a coupling constant of 6.3 Hz. Acetato methyl protons
were observed at 2.63 ppm as a singlet signal. Although
we tried to isolate this complex (5), a solid isolated from
the solution was consisted of only 4. On monitoring the
1
time-course of the mixture of complexes 4 and 5 by H
NMR, the intensity of the signals due to complex 5
was found to be reduced gradually and the signals for
complex 4 were observed finally. From this observation
and the results of thermal synthetic conditions men-
tioned above, it is plausible to consider that the complex
5 is a stereoisomer of 4 and that the complex 5 is a ki-
netic product and 4 is a thermodynamic one.
On the basis of spectral evidence, we proposed three
plausible structures for 5 as depicted in Chart 3. The
structure A has a lantern-type skeleton and two acetate
The crystal data and selected bond distances and an-
gles are listed in Tables 1 and 4, respectively. The Mo–
˚
Mo distance in 6, 2.8088(3) A, is somewhat longer than
the range of the single bond distances in Mo(V)-Mo(V)
O
O
O2
(1 atm)
O
O
O
O
O
iPr
N
O
N
O
O
N
N
N
O
O
O
iPr
Ph
iPr
Ph
N
iPr
O
N
N
Mo
N
Mo
Mo
N
Mo
N
Mo
Mo
Mo
Mo
Mo
N
Mo
N
O
O
N
O
Ph
Ph
O
O
N
N
N
N
O
O
O
N
iPr
O
N
iPr
iPr
iPr
A
B
C
5
6
Chart 3. Plausible structures for complex 5.
Scheme 5.

Y. Yamaguchi et al. / Inorganica Chimica Acta 358 (2005) 2363–2370
2369
Fig. 3. ORTEP drawing of complex 6 with thermal ellipsoid drawn at the 30% probability level. All hydrogen atoms are omitted for clarity.
formation of O = PPh3 was observed by 31P NMR spec-
troscopy (d = 27.9). The 1H NMR spectrum showed the
consumption of complex 6, whereas the product formed
was not able to be identified because of the complexity
of the products.
Table 4
˚
Selected bond distances (A) and angles (ꢁ) for complex 6
Mo(1)–Mo(2)
Mo(1)–O(1)
Mo(1)–O(3)
Mo(1)–O(5)
Mo(1)–O(7)
Mo(1)–N(1)
Mo(1)–N(2)
Mo(2)–O(2)
Mo(2)–O(4)
Mo(2)–O(6)
Mo(2)–O(7)
Mo(2)–N(3)
Mo(2)–N(4)
2.8088(3)
2.210(2)
2.127(2)
1.671(2)
1.921(1)
2.105(2)
2.135(2)
2.253(2)
2.098(1)
1.669(2)
1.914(2)
2.116(2)
2.142(2)
4. Concluding remarks
We investigated the systematic reaction of dimolyb-
denum complex 1 with Li(amidinato) in order to pro-
duce non-lantern-type complexes by introduction of
sterically hindered amidinato ligands onto Mo2 core.
These investigations resulted in the formation and isola-
tion of lantern-type mixed-ligand dimolybdenum com-
plexes (3 and 4). Although non-lantern-type complex
was not isolated, the evidence for the formation of such
complex (5) was obtained. Complex 5 showed high reac-
tivity toward O2 to give dinuclear oxo complex (6). High
potential reactivity seems to be accumulated on the qua-
druple bond in Mo2 core and thus it is hoped that utili-
zation of this potential for the activation of various
substrates will contribute to the development of the no-
vel catalytic processes. Further investigations on the
syntheses and reactions of non-lantern-type complexes
of Mo2 are now in progress.
O(1)–Mo(1)–O(5)
N(1)–Mo(1)–N(2)
O(2)–Mo(2)–O(6)
N(3)–Mo(2)–N(4)
Mo(1)–O(7)–Mo(2)
168.71(8)
61.91(7)
171.05(7)
61.36(7)
94.15(6)
˚
(2.73–2.80 A) [17]. Complex 6 showed the diamagnetic
nature [17a] indicated by NMR spectroscopy (see Sec-
tion 2). Therefore, the Mo–Mo bond in complex 6 is
considered to be a single bond. The terminal oxo–Mo
bond distances are 1.671(2) and 1.669(2) A, distances
of which are consistent with the reported Mo(V) = O
distances [18]. The bridging oxo–Mo bond distances
˚
˚
(1.921(1) and 1.914(2) A) are longer than terminal
oxo–Mo distances.
Recently, Cotton et al. [4] reported [Mo2(g-OAc)2{l-
(NXyl)2CH}2(l-oxo)2] complex, which has bridging
amidinato and chelating acetato ligands. This complex
has a coordination mode reverse to 6. It is interesting
to note that the CottonÕs complex was obtained by the
reaction of the lantern-type complex formulated as
[Mo2(l-OAc)2{l-(NXyl)2CH}2] with O2 and that it has
the formal oxidation state of Mo(IV). Although the
reaction of the lantern-type complex 4 with O2 did not
take place, complex 6 was formed by the reaction of 5
with O2 in our case.
Acknowledgment
The authors are grateful to Ms. Keiko Yamada
(Chemical Analysis Division, RIKEN) for her kind help
in the elemental analyses.
Appendix A. Supplementary data
ORTEP drawings of complexes 3, 4, and 6 with the
atom-numbering scheme have been deposited at the edi-
torial office. Atomic coordinates, thermal parameters,
and bond lengths and distances in CIF format have been
deposited at the Cambridge Crystallographic Data Cen-
ter, 12 Union Road, Cambridge CB2 1EZ, UK and
For the purpose of transforming the complex 6 into
dimolybdenum (IV) complex similar to that reported
by Cotton, we examined the reduction of 6 with triphe-
nylphosphine as a reducing agent. In this reaction, the

2370
Y. Yamaguchi et al. / Inorganica Chimica Acta 358 (2005) 2363–2370
[6] A.B. Brignole, F.A. Cotton, Inorg. Synth. 13 (1972) 81.
[7] Y. Yamaguchi, H. Nagashima, Organometallics 19 (2000) 725.
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W.P. Bosman, R. de Gelder, R. Israel, J.M.M. Smits, Technical
Report of the Crystallography Laboratory, University on Nijme-
gen, The Netherlands, 1994.
copies can be obtained on request, free of charge, by
quoting the publication citation and the deposition
numbers 260458–260460. Supplementary data associ-
ated with this article can be found, in the online version,
at doi:10.1016/j.ica.2005.01.002.
[9] SIR 92: A. Altomare, M.C. Burla, M. Camalli, M. Cascarano, C.
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(1994) 435.
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