Synthesis and crystal structure of tetracoordinated molybdenum (II) complexes containing rigid α-diimine ligands

Article abstract of DOI:10.1016/j.poly.2005.08.027

The complex of the general formula (dad)MoCl₂ [3ai, 3bi, 3c] (dad = R′NC(R)-C(R)NR′) is obtained in one step by reduction of [MoCl₄(thf)₂] (1) in the presence of dad ligands (dab: ai, Ar-BIAN: bi, Ph-BIC: c) (2). All new complexes have been characterized by IR and NMR spectroscopy. The X-ray single-crystal diffraction of 3a2 reveals a relatively tetrahedral coordination geometry.

Full text of DOI:10.1016/j.poly.2005.08.027

Polyhedron 25 (2006) 1142–1146
www.elsevier.com/locate/poly
Synthesis and crystal structure of tetracoordinated molybdenum
(II) complexes containing rigid a-diimine ligands
a
b
a,
*
Thouraya Turki , Taha Guerfel , Faouzi Bouachir
a
Laboratoire de Chimie de Coordination, Faculte´ des Sciences de Monastir, 5000 Monastir, Tunisia
b
Laboratoire de Chimie de Solide, Faculte´ des Sciences de Monastir, 5000 Monastir, Tunisia
Received 27 June 2005; accepted 19 August 2005
Available online 10 October 2005
Abstract
The complex of the general formula (dad)MoCl2 [3ai, 3bi, 3c] (dad = R0N@C(R)–C(R)@NR0) is obtained in one step by reduction of
[MoCl4(thf)2] (1) in the presence of dad ligands (dab: ai, Ar-BIAN: bi, Ph-BIC: c) (2). All new complexes have been characterized by IR
and NMR spectroscopy. The X-ray single-crystal diffraction of 3a2 reveals a relatively tetrahedral coordination geometry.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Molybdenum; Chemical reduction; Molybdenum (II) diazadiene complexes; a-Diimine
1. Introduction
properties also offered isolable and stable compounds. It was
therefore of interest to probe the coordination of diazadiene
ligands to these ions. It has been recently shown [15] that a
variety of dad ligands easily add to [CpMoCl2] to yield
[CpMoCl2(dad)] complexes. The metal center, therefore, is
most appropriately described as formally MoIII.
Poli et al. [16] have recently reported about the synthesis
of MoCl2(i-Pr-dad)2 where the dad ligand is coordinated to
the molybdenum center as form B.
Diazadiene complexes of the type (dad)MoCl2 have not
been studied extensively, only one example has been re-
ported recently [(dad)CrCl2] [17].
Similar complexes with phosphine ligands have been
studied by Hawkins set al. [18]
Organometallic compounds with diazadiene (dad) li-
gands have been extensively studied in recent years. While
a number of different binding modes have been discovered
[1] (Fig. 1), the most commonly observed mode is chelation
of two imine nitrogens by one metal. This conjugated dii-
mine structure often leads to stable compounds especially
in lower oxidation states where the strongly back-bonding
characteristics of this ligand group can be utilized. The re-
cent discovery by Brookhart and co-workers [2–9] of Ni(II)
and Pd(II) a-olefin polymerisation catalysts containing
bulky diimine ligands has stimulated renewed interest in
the chemistry of 1,4-diazadiene ligands and their complexes
[1,10–14]. For some time we have been investigating the
coordination, chemistry and the organometallic chemistry
of molybdenum in intermediate oxidation states, mostly II,
III and IV. Ligands with mild p-acidic properties, such as
phosphane, are quite compatible with these systems, but a
wide range of ligands with extreme r- and p-donor/acceptor
In this paper, we report the preparation and character-
ization of novel molybdenum (II) [3ai, 3bi, 3c] derivatives
that contains diazadiene ligands.
2. Results and discussion
The complex (dad)MoCl2 was prepared with good yields
in one step from [MoCl4(thf)2] [19] by reduction with zinc
in the presence of the diazadiene ligand. Other derivative
of MoCl2 have been reported by Pombeiro and Richards
[20] [MoCl2(dad)(CNMe)2] but the main difference being
*
Corresponding author. Present address: INSAT, Chemistry, Zone
Urbaine Nord BP676, Tunis 1080, Tunisia. Tel.: +21698503997; fax:
+21671704329.
E-mail address: bouachirf@yahoo.de (F. Bouachir).
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.08.027

T. Turki et al. / Polyhedron 25 (2006) 1142–1146
1143
R
R
The new complexes (dad)MoCl2, exhibit spectroscopic
data in accord with the proposed structure.
R
R
R
R
The NMR properties unambiguously establish the dia-
magnetic nature of these compounds. In fact the substitu-
ent on the aryl groups of the dad ligand are all
equivalent as expected for the C2v-symmetric isomer.
Chemical shifts of the R0N@C(R)–C(R)@NR0 subunit in
the complexes [3ai, 3bi, 3c] are slightly shielded in compar-
ison to those of the free ligands similar to other complexes
[21]. The C@N carbon appears between 163 and 179 ppm.
In the IR spectra of the complexes (as KBr pellets), the
NR'
NR'
R'N
R'N
NR'
R'N
M
B
M
C
M
A
Fig. 1. Coordination modes of diazadiene ligands.
the use of sodium as the reducing agent and their octahe-
dral geometry. These other derivatives of MoCl2, however,
were not structurally characterized and no details of their
stereochemistry were evident from the spectroscopic stud-
ies. Therefore, an alternative synthetic methods of new
diazadiene molybdenum complexes were explored.
Molybdenum complexes [3ai, 3bi, 3c] (Scheme 1) crystal-
lize as green crystals for 3ai, 3c, and orange brown for 3bi
from CH2Cl2/n-hexane. All these compounds are soluble in
methylene chlorid, dimethylsulfoxid, acetone and acetoni-
trile. They are air-sensitive and non-soluble in unpolar sol-
C@N stretching vibration is observed at 1601–1640 cmꢀ1
,
i.e., shifted by 10–20 cmꢀ1 to lower frequency as compared
to the free ligands.
Single crystal of 3a2 was grown from a mixture of
CH2Cl2and hexane solution at room temperature.
The Ortep diagram of 3a2 is shown in Fig. 2, with
details of the structure determination, including unit-cell
data, summarized in Table 1 and relevant bond distances
and bond angles in Table 2.
1
vents. These compounds have been characterised by H,
The structural analysis confirms the identity of the
molecule. The coordination sphere of the molybdenum
center is composed of the two nitrogen atoms of the chelat-
ing diazadiene ligand and the two dichloride atoms. The
coordination geometry of the molybdenum atom can be
described as slightly distorted tetrahedral. This distortion
is explained by the 78.17(19)ꢁ bite angle measured for the
bidentate diazadiene ligand [N2–Mo–N1]. This angle are
similar to the value of 72.6(3)ꢁ measured for [W(CO)3(dad:
R = H; R0 = i-Pr)(g2-eco)] {eco = (E)-cyclooctene} [22].
The Mo–N distances are considerably shorter than in
the structurally related complex [MoO2Cl2(dad: R = H;
13C NMR and IR spectroscopy.
R
R
13
11
Ph
12
10
R'
R'
5
N
N
N
N
N
9
11
MoCl2
10
MoCl2
MoCl2
7
6
5
9
6
8
N
8
7
Ph
0
˚
R = t-Bu)] [2.399(2) and 2.388(2) A] [23].
R
R
The ligand and metal in this structure are essentially co-
planar consistent with the p-backbonding of this ligand.
The N1–C19, N2–C20 and C19–C20 bond distances of
3ai
3bi
3c
a1 : R = H , Ar = 2.4,6-Me3-C6H2
b1: Ar = 2,6-iPr2 - C6H3
a2 : R = CH3, Ar = 2,4,6- Me3-C6H2 b2: Ar = o-Me-C6H4
a3 : R = CH3 , Ar = 2,6-iPr2-C6H3
˚
1.267(8), 1.258(8) and 1.523(9) A, respectively, confirm
Scheme 1.
the conjugated diimine nature of this ligand and with those
Fig. 2. ORTEP diagram of 3a2 Æ CH2Cl2. Thermal ellipsoids are at 50% probability. Hydrogen are omitted for clarity.

1144
T. Turki et al. / Polyhedron 25 (2006) 1142–1146
Table 1
respectively, which are very closer for other platinum (II)
complexes [25].
Summary of crystallographic data and X-ray experimental details for 3a2
Empirical formula
Formula weight
Crystal system
Space group
Z
C23H30Cl4N2Mo
572.23
monoclinic
P21/c
The o-methylphenyl groups makes an angle of approxi-
mately 90ꢁ to the plane of the C@N bonds, due to the
presence of the o-methyl substituents as appears from the
torsion angles of 93.64 (0.79)ꢁ, ꢀ88.83 (0.76)ꢁ, 85.51
(0.84)ꢁ and ꢀ92.75 (0.79)ꢁ for C20–N2–C7–C8, C20–N2–
C7–C12, C19–N1–C1–C6 and C19–N1–C1–C2, respecti-
vely.
In conclusion, we have described a convenient synthetic
procedure for the preparation of the new diazadiene
molybdenum (II) complexes. The application of these
complexes in catalytic reactions such as oligomerization
and polymerisation of alkenes (ethylene, propylene) and
functional alkenes (methylacrylate) is in progress.
4
˚
a (A)
12.871(8)
14.659(9)
14.404(9)
90
90.62 (7)
90
2717.8(3)
1.399
0.11 · 0.31 · 0.64
293(2)
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
qcalc (g cmꢀ3
)
Crystal dimensions (mm)
T (K)
Radiation (Mo Ka) k (A)
F(000)
˚
0.71073
1168
3. Experimental
2h Range (ꢁ)
Limiting indices
2.1–26.99
ꢀ16 6 h 6 16,
ꢀ18 6 k 6 0,
ꢀ9 6 l 6 18
10325
5860
2700
268
empirical (DIFABS)
XCAD4
SHELX-97
8.08, 22.65
0.996
All manipulations are carried out under an atmosphere
of dry and oxygen-free argon with standard schlenck tech-
niques. Diethylether was purified by reflux over sodium
benzophenone ketyl and distilled under argon prior to
use. Methylene chloride and n-hexane were purified from
P2O5 and distilled under argon.
The diazadiene ligands: dab [26], Ar-BIAN, Ph-BIC
[27]; MoCl4(thf)2 [19] were obtained according to the liter-
ature procedures.
Number of scanned reflections
Number of independent reflections
Number of observed reflections
Number of parameters varied
Absorption correction
Data reductions programs
Programs used
R1 (Fo), wR2 ðF 2oÞ (I > 2r(I)) (%)
Goodness-of-fit on F2
All routine NMR experiments were carried out on a
Bruker AM 300 and infrared spectra on a Bio-Rad FTS-
6000 spectrophotometer.
of complexes to with the bonding mode A can be clearly
˚
˚
assigned (N–C: 1.26–1.30 A; C–C: 1.40–1.55 A) [1]. These
distances are similar to those observed in Mo(CO)4(dad)
complex [24].
3.1. General procedure for the preparation of [dad]MoCl2
The metal-chelate ring (Mo–N2–C20–C19–N1) in com-
plex (3a2) is almost flat as indicated by the torsion angles of
ꢀ1.96 (0.87)ꢁ, 5.21 (0.71)ꢁ and ꢀ2.40(0.71)ꢁ for N2–C20–
C19–N1, Mo–N1–C19–C20 and Mo–N2–C20–C19,
In an inert atmosphere, MoCl4(thf)2 (1 equiv.) was
added to diazadiene ligand (3 equiv.) in 20 ml of anhy-
drous CH2Cl2. Zinc was added (5 equiv.) as a reducing
agent. The solution was stirred at room temperature for
two days. The supernatant was separated by filtration,
and the solvent was removed under vacuum until 3 ml.
Addition of hexane precipitate the product. Pure com-
pound was isolated as a solid by crystallization from meth-
ylene chloride/hexane solution at ꢀ30 ꢁC. Yields ranged
from 52% to 83%.
Table 2
Selected interatomic distances and angles for [2,4,6-Me3-dad]MoCl2 (3a2)
˚
Bond distances (A)
Bond angles (ꢁ)
Mo–N1
Mo–Cl1
N1–C19
N1–C1
C6–C13
C2–C14
C4–C15
C19–C20
Mo–N2
Mo–Cl2
N2–C20
N2–C7
2.089(5)
2.204(2)
1.267(8)
1.464(8)
1.513(11)
1.480(10)
1.491(11)
1.523(9)
2.087(5)
2.220(2)
1.258(8)
1.446(8)
1.494(10)
1.497(10)
1.539(11)
N1–Mo–N2
N2–Mo–Cl1
N2–Mo–Cl2
C20–N2–Mo
C20–N2–C7
C7–N2–Mo
C8–C7–N2
C12–C7–N2
N2–C20–C19
Cl1–Mo–Cl2
N1–Mo–Cl1
N1–Mo–Cl2
C19–N1–Mo
C19–N1–C1
C1–N1–Mo
C6–C1–N1
78.17(19)
115.88(16)
114.53(15)
115.2(4)
121.8(6)
122.9(4)
118.9(6)
117.9(6)
115.7(6)
115.96(9)
113.14(16)
113.58(16)
114.2(4)
122.2(5)
123.5(4)
117.9(6)
118.5(6)
116.5(5)
3.2. [2,4,6-(CH3)3-C6H2NC(H)C(H)NC6H2-2,4,6-
(CH3)3]MoCl2 (3a1)
Following general procedure, from MoCl4(thf)2
(0.57 mmol,
0.22 g),
[2,4,6-(CH3)3-C6H2NC(H)C(H)-
NC6H2-2,4,6-(CH3)3] (1.7 mmol, 0.505 g). Zinc (2.85
mmol, 0.186 g) in 20 ml of methylene chloride was
obtained 0.178g (yield 68%) of 3a1 as a green solid after
crystallization from a mixture n-hexane/CH2Cl2 (8/2).
Decomposition = 310 ꢁC; IR (mC@N = 1615 cmꢀ1).
1H NMR (300 MHz, CDCl3, 25 ꢁC, d [ppm]): d = 2.30
(s, 12H, ortho,ortho0-CH3), 2.49 (6H; para-CH3); 6.936 (s,
4H, Har); 8.76 (1, 2H, H–C@N).
C12–C16
C8–C17
C10-C18
C2–C1–N1
N1–C19–C20

T. Turki et al. / Polyhedron 25 (2006) 1142–1146
1145
13C NMR (75.5 MHz, CDCl3, 25 ꢁC, d [ppm]): d = 19.5
(ortho-CH3); 21.5 (para-CH3); 130.75 (Cortho); 132 (Cmeta);
137, 75 (Cpara); 144 (Cipso); 176.5 (C@N).
133.12 (C10); 139.87 (Cortho); 140.70 (C11); 145.06 (Cipso);
165.63 (C@N).
3.6. [o-Me-C6H4-acenaphtene]MoCl2 (3b2) {Ar-BIAN,
Ar = o-Me-C6H4}
3.3. [2,4,6-(CH3)3-C6H2NC(Me)C(Me)NC6H2-2,4,6-
(CH3)3]MoCl2 (3a2)
Following general procedure, from MoCl4(thf)2 (0.39
mmol, 0.15 g), [o-Me-acenaphtene] (1.1 mmol, 0.424 g).
Zinc (1.95 mmol, 0.127 g) in 20 ml of methylene chloride
was obtained 0.178g (yield 86%) of 3b2 as orange solid after
crystallization from a mixture n-hexane/CH2Cl2 (9/1).
Decomposition = 336 ꢁC; IR (mC@N = 1631, 1591 cmꢀ1).
1H NMR (300 MHz, DMSO-d, 25 ꢁC, d [ppm]): d =
Following general procedure, from MoCl4(thf)2
(0.44 mmol, 0.17 g), [2,4,6-(CH3)3-C6H2NC(Me)C(Me)-
NC6H2-2,4,6-(CH3)3] (1.3 mmol, 0.427 g). Zinc (2.2 mmol,
0.143 g) in 20 ml of methylene chloride was obtained
0.178g (yield 83%) of 3a2 as a green solid after crystalliza-
tion from a mixture n-hexane/CH2Cl2 (8/2). Decomposi-
tion = 367 ꢁC; IR (mC@N = 1650; 1607 cmꢀ1).
2.18 (s, 6H, CH3); 6.83 (d, 2H, H7); 6.92 (d, 2H, Har meta
7.92–7.05 (m, 10H, 6Har 9,10,11 and 4H8,9).
)
1H NMR (300 MHz, CD3CN, 25 ꢁC,
d [ppm]):
d = 2.26–2.36 (m, 24H, ortho,ortho0,para-CH3 + CH3–
C@N); 7.09 (s, 4H, Har).
13C NMR (75.5 MHz, DMSO-d, 25 ꢁC, d [ppm]):
d = 17.15 (MeC@N); 119.75 (Cortho); 120.01 (C7); 124.55
(Cpara); 125.43 (Cortho); 126.89 (Cmeta), 127.36 (C8), 128,37
13C NMR (75.5 MHz, CD3CN, 25 ꢁC, d [ppm]):
d = 17.38 (CH3–C@N); 18.20 (ortho,ortho0-CH3); 19.35
(para-CH3); 128.43 (Cortho); 129.29 (Cmeta); 136.91 (Cpara);
139.79(Cipso); 172.16 (C@N).
0
(C9); 130.27 (C10); 130.96 (Cmeta ): 131.98 (C6); 142.69(C11);
144.30 (Cipso); 163.23 (C@N).
3.7. [(Ph-Bic)]MoCl2 (3c)
3.4. [2,6-i-Pr2-C6H3NC(Me)C(Me)NC6H3-2,6-i-
Pr2]MoCl2 (3a3)
Following general procedure, from MoCl4(thf)2
(0.7 mmol, 0.28 g), [(Ph-BIC)] (2.1 mmol, 0.694 g). Zinc
(3.5 mmol, 0.228 g) in 20 ml of methylene chloride was
obtained 0.165g (yield 52%) of 3c as green solid after crys-
tallization from a mixture n-hexane/CH2Cl2 (9/1). Decom-
position = 318ꢁC; IR (mC@N = 1675; 1631; 1595 cmꢀ1).
1H NMR (300 MHz, acetone-d, 25 ꢁC, d [ppm]):
d = 0.68; 0.86; 0.98 (s, 3H chacun, H12,13,14); 1.55–1.179
(m, 4H, H8,9); 3.15 (d, 1H, H10); 7.18–7.40 (m, 10H, Har).
13C NMR (75.5 MHz, acetone-d, 25 ꢁC, d [ppm]):
d = 11.37; 17.02; 21.20 (C12,13,14); 23, 86, 32, 69 (C8,9); 49,
11 (C7); 49.97 (C10); 56.04 (C11); 121.54, 123.14 (Cortho);
127.76 (Cpara); 129.06, 129.78 (Cmeta); 174.13; 176.10
(C@N).
Following general procedure, from MoCl4(thf)2
(0.6 mmol, 0.23 g), [2,6-i-Pr2-C6H3NC(Me)C(Me)NC6H3-
2,6-i-Pr2] (1.8 mmol, 0.731 g). Zinc (3 mmol, 0.196 g) in
20 ml of methylene chloride was obtained 0.260g (yield
67%) of 3a3 as a brown solid after crystallization from a
mixture n-hexane/CH2Cl2 (9/1). Decomposition = 355 ꢁC;
IR (mC@N = 1654; 1607 cmꢀ1).
1H NMR (300 MHz, DMSO-d, 25 ꢁC, d [ppm]):
d = 1.16–1.08 (m, 24H, Me); 1.98 (s, 6H, CH3–C@N);
2.50 (sept, 4H, CH–i-Pr); 7.18–6.85 (m, 6H, Har).
13C NMR (75.5 MHz, DMSO-d, 25 ꢁC, d [ppm]):
d = 16.63 (CH3–C@N); 23, 18, 23, 56 (CH3–i-Pr); 27.99
(CH–i-Pr); 122.50 (Cmeta); 124.15 (Cpara); 134.76 (Cortho
,
0
C
ortho ); 145.99 (Cipso); 168.23 (C@N).
4. X-ray structure determination
3.5. [2,6-i-Pr2-C6H3-acenaphtene]MoCl2: 3b1{Ar-BIAN,
Ar = 2,6-i-Pr2-C6H3}
A crystal (0.11 · 0.31 · 0.64 mm) suitable for X-ray
intensity data collection was selected. X-ray intensity data
were collected on a MACH3 Enraf Nonius diffractometer
˚
Following general procedure, from MoCl4(thf)2
(0.54 mmol, 0.21 g), [2,6-i-Pr2-acenaphtene] (1.6 mmol,
0.785 g). Zinc (2.7 mmol, 0.176 g) in 20 ml of methylene
chloride was obtained 0.262g (yield 75%) of 3b1 as orange
brown solid after crystallization from a mixture n-hexane/
CH2Cl2 (9/1). Decomposition = 375 ꢁC; IR (mC@N = 1640;
1601 cmꢀ1).
using monochromated Mo Ka radiation (k = 0.71073 A)
at 293 K. Accurate unit cell parameters and orientation
matrix were determined from 25 reflections in the range
13–15ꢁ. Intensity data were collected by using the x–2h
scan mode with a range of 2 < h < 27ꢁ. All the intensity
data were corrected for Lorentz and polarization effects.
The crystal structure solution carried out with direct
methods from the SHELXS-97 permitted the location of all
non-hydrogen atoms. After anisotropic least-squares
refinement, hydrogen atoms were placed at their geometri-
cally calculated positions and refined riding on the corres-
ponding atoms with isotropic thermal parameters. Final
refinement based on the reflections [I > 2r(I)] converged
at R1 = 0.0808, wR2 = 0.2265 and goodness-of-fit = 0.996.
1H NMR (300 MHz, CDCl3, 25 ꢁC, d [ppm]): d = 0.82 (d,
12H, CH3–i-Pr); 1.32 (d, 12H, CH3–i-Pr); 3.26 (sept, 4H,
0
CH–i-Pr); 6.65 (d, 2H; H7); 7.55–7.3 (m, 6H, Hmeta,meta ,para);
8.101 (d, 2H, H9)
13C NMR (75.5 MHz, CDCl3, 25 ꢁC, d [ppm]): d =
24.802 (Me-i-Pr); 29.34 (CH–i-Pr); 125.43 (C7); 126.02
(Cmeta); 127.39 (Cpara); 128.69 (C8); 129.27(C9); 131.44 (C6);

1146
T. Turki et al. / Polyhedron 25 (2006) 1142–1146
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The experimental conditions of data collection, strategy
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Crystallographic data for the structural analysis have
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may be obtained free of charge from the Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336 033 or deposit@ccdc.cam.ac.uk.
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