Molybdenum-carbon functionalities supported by a quadridentate Schiff-base ligand: The reactivity of trans-dichloro[N,N′-bis-(salicylidene)-o-phenylenediaminato]molybdenum(IV)

Article abstract of DOI:10.1039/a802148a

The parent compounds trans-[Mo(salophen)Cl₂] 1 [salophen = N,N′-bis(salicylidene)-o-phenylenediamine dianion] and trans-[Mo(tbsalophen)Cl₂] 2 [tbsalophen = N,N′-bis(3,5-di-tert-butylsalicylidene)-o-phenylenediamine dianion] were obta

Full text of DOI:10.1039/a802148a

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Molybdenum–carbon functionalities supported by a quadridentate
Schiff-base ligand: the reactivity of trans-dichloro[N,NЈ-bis-
(salicylidene)-o-phenylenediaminato]molybdenum(IV)‡
Euro Solari,a Cristiano Maltese,a Mario Latronico,b Carlo Floriani,*,†,a Angiola Chiesi-Villac and
Corrado Rizzolic
a Institut de Chimie Minérale et Analytique, BCH, Université de Lausanne, CH-1015 Lausanne,
Switzerland
b Dipartimento di Ingegneria e Fisica dell’Ambiente, Università della Basilicata, I-85100 Potenza,
Italy
c Dipartimento di Chimica, Università di Parma, I-43100 Parma, Italy
The parent compounds trans-[Mo(salophen)Cl2] 1 [salophen = N,NЈ-bis(salicylidene)-o-phenylenediamine
dianion] and trans-[Mo(tbsalophen)Cl2] 2 [tbsalophen = N,NЈ-bis(3,5-di-tert-butylsalicylidene)-o-phenylene-
diamine dianion] were obtained from the corresponding Schiff bases and [MoCl4(MeCN)2]. The alkylation of 1
proceeded very differently depending on the alkylating agent. The reaction with (PhCH2)2Mg led to the formation
of trans-[Mo(salophen)(PhCH2)2] 3, containing two alkyl groups at the metal, while with Mg(mes)Br (mes =
2,4,6-Me3C6H2) arylation occurred both at the imino carbons and at the metal, via a rather complex mechanism.
Reduction of 1 led to an intermediate Mo(salophen), which was intercepted by PhCCPh. The plausible inter-
mediate metallocyclopropene rearranged with the migration of one of the carbons to an imino group and
dimerization of the monomeric unit. The salophen ligand displayed a bridging bonding mode in the dimer,
containing a Mo᎐Mo fragment and two bridging vinyl functionalities. The proposed structures are supported
᎐
by three X-ray analyses.
Over the last two decades the M(η-C5H5)2 moiety has become
representative of an organometallic fragment which can be
used in the stabilization of M᎐C bonds and in a wide variety of
Experimental
General
metal-assisted reactions. In the meantime very few attempts
have been made to move away from this convenient frag-
ment and to improve upon it. One strategy is the use of alkoxo
groups as ancillary ligands,1 a second consists of the replace-
ment of at least one C5H5 ring with an amido group,2 while a
third makes use of macrocyclic ligands, which until recently
have been confined to co-ordination chemistry in its strictest
sense. The only popular use of these macrocycles has been in
the formation of organometallic derivatives of the pairs Fe,
Ru, and Co, Rh in porphyrin chemistry,3 with other cases
being spread over a range of metals. In the case of early
transition metals five major approaches can be mentioned, in
chronological order: the use of (i) tetradentate Schiff bases, i.e.
salen;4 (ii) dibenzotetramethyltetraaza[14]annulenes;5 (iii) the
porphyrin skeleton;6 (iv) meso-octaalkylporphyrinogen;7 (v)
calix[4]arenes.8
Whilst using tetradentate Schiff bases as ancillary ligands in
the organometallic chemistry of titanium() and zirconium()
it was discovered that, though very reactive metal–carbon
bonds could be formed, the alkyl or aryl groups migrated to the
highly electrophilic imino carbons of the ligand.4a,d The use of
lower oxidation states, namely TiIII, rendered the imino groups
less electrophilic and enabled the isolation and study of M᎐C
bond functionalities.4c
All reactions were carried out under an atmosphere of purified
nitrogen. Solvents were dried and distilled before use by stand-
ard methods. Proton NMR and IR spectra were recorded on
AC-200, DPX-400 Bruker, and Perkin-Elmer FT 1600 instru-
ments, respectively. Magnetic susceptibility measurements were
made on a MPMS5 SQUID susceptometer (Quantum Design
Inc.) operating at a magnetic field strength of 3 kG.
Syntheses
Compound 1. Tri-n-butylamine (19 cm3, 79.2 mmol) was
added to an orange solution of H2salophen (11.39 g, 36.0 cm3)
in thf (350 cm3). The complex [MoCl4(MeCN)2] (11.5 g, 36
mmol) was then added and the resulting black suspension
refluxed for 1 d. The black microcrystalline solid was filtered off
and dried in vacuo (16.6 g, 83%) (Found: C, 52.15; H, 4.03; N,
5.15. C24H22Cl2MoN2O3 requires C, 52.10; H, 4.01; N, 5.06%).
IR (Nujol): ν max/cmϪ1 1600s and 1567s. µeff = 2.75 µB at 300 K.
Compound 2. Tri-n-butylamine (19 cm3, 80 mmol) was added
to a light yellow solution of H2tbsalophen [N,NЈ-bis(3,5-di-tert-
butylsalicylidene)-o-phenylenediamine] (19.6 g, 36 mmol) in thf
(300 cm3). The complex [MoCl4(MeCN)2] (11.6 g, 36 mmol)
was then added, and the resulting black suspension refluxed for
1 d. The black microcrystalline solid was filtered off and dried
in vacuo (17.0 g, 66%). Crystals suitable for X-ray analysis were
grown from thf solution (Found: C, 61.32; H, 6.63; N, 3.88.
C36H46Cl2MoN2O2 requires C, 61.28; H, 6.57; N, 3.97%). IR
(Nujol): ν max/cmϪ1 1600m and 1572m. µeff = 2.65 µB at 300 K.
The present study describes how a molybdenum() bonded
to the tetradentate salophen ligand [N,NЈ-bis(salicylidene)-o-
phenylenediamine dianion] behaves in the alkylation reaction
and the formation of Mo᎐C functionalities. This is quite a rare
co-ordination environment for molybdenum engaged in organo-
metallic derivatization, and it can be usefully extended to other
macrocyclic ligands.
Compound 3. Dibenzylmagnesium (28.5 cm3, 0.3  in thf, 8.5
mmol) was added to a frozen brown suspension of compound 1
(4.7 g, 8.5 mmol) in benzene (200 cm3)–1,4-dioxane (5 cm3). The
solution slowly changed from dark brown to dark red. It was
stirred overnight. The resulting dark red suspension was taken
to dryness and benzene (200 cm3) added. The suspension was
† E-Mail: carlo.floriani@icma.unil.ch
‡ Non-SI units employed: G = 10Ϫ4 T, µB = 9.27 × 10Ϫ24 J TϪ1
.
J. Chem. Soc., Dalton Trans., 1998, Pages 2395–2400
2395

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then filtered, taken to dryness and n-hexane (60 cm3) added.
The reddish brown solid was then collected and dried in vacuo
(3.6 g, 71%) (Found: C, 68.5; H, 4.80; N, 4.45. C34H28MoN2O2
requires C, 68.92; H, 4.76; N, 4.73%). IR (Nujol): ν max/cmϪ1
1599m and 1576m. µeff = 2.53 µB at 300 K. The hydrolysis of 3
by aqueous HCl gave PhCH3 (GC-MS) in the expected 1:2
Mo:PhCH3 ratio.
R
R
O
Cl
O
H2salophen or
H2tbsalophen
Mo
Cl
[MoCl4(MeCN)2]
NBun
CH
N
R
N
CH
R
3
R = H, trans-[Mo(salophen)Cl2]•thf
R = But, trans-[Mo(tbsalophen)Cl2]
1
2
Compound 4. Mesitylmagnesium bromide (22.8 cm3, 0.83 
in thf, 19 mmol), was added dropwise to a frozen brown suspen-
sion of compound 1 (5.3 g, 9.5 mmol) in benzene (200 cm3)–1,4-
dioxane (4 cm3). The mixture slowly changed from dark brown
to dark green and was stirred overnight. The resulting dark
green suspension was taken to dryness and benzene (200 cm3)
added. The green mixture was then filtered, taken to dryness
and n-hexane (50 cm3) added. The greenish brown precipitate
was collected and dried in vacuo (4.1 g, 67%). Crystals suitable
for X-ray analysis were grown in an n-hexane solution (Found:
C, 73.16; H, 6.23; N, 3.90. C47H47MoN2O2 requires C, 73.52; H,
6.17; N, 3.65%). IR (Nujol): ν max/cmϪ1 1569m. µeff = 1.83 µB at
300 K.
(PhCH2)2Mg
Mg(mes)Br
CH2Ph
Mo
O
O
CH
N
mes
mes
N
CH
CH2Ph
O
N
CH
Mo
CH N
mes
O
[Mo(salophen)(PhCH2)2] 3
4
Scheme 1
to the metal atom along the Mo(1)᎐C(41) and Mo(1)᎐N(1)
bonds (general background 0.43 e ÅϪ3).
Compound 5. Compound 1 (7.2 g, 13.0 mmol), Na (0.6 g, 26.0
mmol) and PhCCPh (2.9 g, 16.0 mmol) were added to thf (200
cm3) and the mixture was stirred for 5 d. It slowly changed from
violet to dark green. The resulting solution was then filtered (to
remove NaCl), concentrated (50 cm3) and diethyl ether (50 cm3)
added. The dark green precipitate was collected and dried in
vacuo (6.8 g, 80%). Crystals suitable for X-ray analysis were
grown in a thf–n-hexane solution (Found: C, 68.79; H, 5.24; N,
3.48. C76H64Mo2N4O6 requires C, 68.85; H, 5.50; N, 3.82%). IR
(Nujol): ν max/cmϪ1 1589s and 1533m. µeff = 0.9 µB at 300 K.
The refinement of complex 4 was straightforward. In 2 the
C(26), C(27), C(28) methyl carbon atoms of a tert-butyl group
showed high thermal parameters indicating the presence of dis-
order. The atoms were then split over two positions (A and B)
isotropically refined with site occupation factors of 0.5. In
complex 5 the thf solvent molecule of crystallization was found
to be heavily affected by disorder which was solved by splitting
the atoms over three positions, A, B and C, isotropically refined
with site occupation factors of 0.5, 0.25 and 0.25, respectively.
During refinement the C᎐O and C᎐C bond distances within the
disordered thf molecules were constrained to 1.42(1) and
1.54(1) Å respectively. The isotropic model applied to the dis-
ordered atoms in 2 and 5 was the most satisfactory obtained
after unsuccessful attempts to allow the ‘partial’ atoms to vary
anisotropically.
X-Ray crystallography
Suitable crystals were mounted in glass capillaries and sealed
under nitrogen. The reduced cells were obtained with use of
TRACER.9 Crystal data and details associated with data col-
lection are given in Table 1. Data were obtained using a single-
crystal diffractometer (Rigaku AFC6S) at 133 K. For inten-
sities and background the individual reflection profiles were
analysed.10 The structure amplitudes were obtained after the
usual Lorentz-polarization corrections, and the absolute scale
was established by the Wilson method.11 The crystal quality was
tested by ψ scans showing that crystal absorption effects could
not be neglected. Data for all complexes were then corrected
for absorption using a semiempirical method.12 The function
minimized during the least-squares refinements was Σw(∆F 2)2.
Anomalous scattering corrections were included in all struc-
ture factor calculations.13b Scattering factors for neutral atoms
were taken from ref. 13(a) for non-hydrogen atoms and from
ref. 14 for H. Structure solutions were based on the observed
reflections [I > 2σ(I)] while the refinements were based on the
unique reflections having I > 0. The structures were solved by
the heavy-atom method starting from a three-dimensional
Patterson map.15 Refinements were done by full-matrix least
squares first isotropically and then anisotropically for all non-H
atoms except for the disordered atoms in 2 and 5. The hydrogen
atoms were located in a Fourier-difference map and introduced
in the refinements as fixed atom contributions (Uiso = 0.05 Å2).
The H atoms associated with the disordered carbon atoms were
ignored. In the last stage of refinement the weighting scheme
CCDC reference number 186/1001.
See http://www.rsc.org/suppdata/dt/1998/2395/ for crystallo-
graphic files in .cif format.
Results and Discussion
The synthesis of the parent compounds 1 and 2 was carried out
by treating [MoCl4(MeCN)2] with H2salophen in the presence
of a base, i.e. Bun N (Scheme 1). Both compounds were
3
obtained as microcrystalline black solids. They have the
expected d2 high-spin configuration with a magnetic moment of
2.75–2.65 µB at 300 K. The structures of 1 and 2 were clarified
by X-ray analysis carried out on the more soluble derivative 2.
A SCHAKAL17 view of complex 2 is given in Fig. 1, with
selected bond distances and angles in Table 2. Complex 2 crys-
tallizes with a thf molecule. The co-ordination around molyb-
denum is pseudo-octahedral, two chlorine atoms being bonded
to a Mo(tbsalophen) moiety in a trans arrangement [Cl(1)᎐
Mo᎐Cl(2) 179.2(1)Њ] (Fig. 1). The Mo᎐Cl bond distances are not
significantly different from each other and are in good agree-
ment with those found for instance in [Mo(acacen)Cl2] [H2-
acacen = 4,4Ј-ethylenedinitrilobis(pentan-2-one)],18 while the
Mo᎐O and Mo᎐N bond distances are significantly shorter and
longer respectively than those in [Mo(acacen)Cl2]. However,
they fall in the range of values found for molybdenum Schiff
base complexes [1.920(2)–2.112(2) and 2.070(6)–2.304(3) Å for
Mo᎐O and Mo᎐N respectively].18–20 The Mo atom lies on the
N2O2 core which shows considerable tetrahedral distortion
(Table 3). The normal to the mean co-ordination plane forms a
dihedral angle of 1.7(1)Њ with the Cl᎐Mo᎐Cl line. The two six-
membered chelation rings are slightly folded with respect to the
N ؒ ؒ ؒ O lines (Table 3). The metal lies nearly on the o-phenylene
moiety, the maximum displacement from the mean plane
2
w = 1/[σ2(Fo ) ϩ (aP)2] [with P = (Fo2 ϩ 2Fc2)/3 was applied with
a resulting in the value of 0.0634, 0.1052, 0.0513 for 2, 4 and 5
respectively]. All calculations were performed using SHELXL
93.16 The final difference map for 5 showed no unusual features,
with no significant peaks above the general background. For 2
two residual peaks of 0.98 and 0.90 e ÅϪ3 were found in proxim-
ity to the metal atom in the direction of the Mo(1)᎐N(2) and
Mo(1)᎐O(1) bonds (general background 0.40 e ÅϪ3). For 4 two
residual peaks of 1.32 and 1.28 e ÅϪ3 were found in proximity
2396
J. Chem. Soc., Dalton Trans., 1998, Pages 2395–2400

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Table 1 Experimental data for the X-ray diffraction studies on crystalline complexes 2, 4 and 5
2
4
5
Formula
M
C36H46Cl2MoN2O2ؒC4H8O
777.7
C47H47MoN2O2
767.8
C68H48Mo2N4O4ؒ2C4H8O
1321.2
Crystal system
Space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
Triclinic
P1 (no. 2)
Monoclinic
P21/n (no. 14)
9.188(2)
30.228(4)
13.615(2)
Orthorhombic
Pbcn (no. 60)
24.185(4)
14.435(2)
17.044(2)
¯
13.648(3)
15.105(5)
11.149(3)
108.48(3)
105.37(2)
63.91(2)
1936.3(10)
2
1.334
44.08
0.669–1.000
6516
103.99(2)
90
γ/Њ
U/Å3
3669.2(11)
4
1.390
32.83
0.643–1.000
5959
5950.2(15)
4
1.475
39.85
0.822–1.000
4919
Z
Dc/g cmϪ3
µ/cmϪ1
Transmission coefficients
Unique data used in refinement (I > 0)
Unique observed data [I > 2σ(I)]
No. parameters refined
4910
430
4104
469
3303
412
R
wR2
0.051
0.143
0.068
0.196
0.042
0.121
¹
2 2
λ = 1.541 78 Å; R = Σ|∆F|/Σ|Fo| calculated on the unique observed data [I > 2σ(I)]; wR2 = (Σw|∆F2|2/Σw|Fo | )² calculated on the unique data with
I > 0.
Table 2 Selected bond distances (Å) and angles (Њ) for complexes 2
and 4
2
4
Mo᎐X*
Mo᎐Cl(2)
Mo᎐O(1)
2.400(1)
2.397(1)
1.928(5)
1.930(3)
2.129(4)
2.131(5)
1.341(5)
1.346(6)
1.309(7)
1.432(9)
1.431(7)
1.306(6)
1.424(10)
2.120(6)
1.944(6)
1.938(6)
1.958(5)
1.994(7)
1.361(10)
1.365(10)
1.510(10)
1.400(10)
1.391(9)
1.494(10)
1.522(10)
1.550(9)
1.495(9)
1.543(10)
Mo᎐O(2)
Mo᎐N(1)
Mo᎐N(2)
O(1)᎐C(1)
O(2)᎐C(20)
N(1)᎐C(7)
N(1)᎐C(8)
N(2)᎐C(13)
N(2)᎐C(14)
C(6)᎐C(7)
C(7)᎐C(21)
C(14)᎐C(15)
C(14)᎐C(31)
Fig. 1 A SCHAKAL drawing of complex 2. Disorder affecting the
C(25)–C(28) tert-butyl group has been omitted for clarity
1.427(8)
through all the atoms including Mo being 0.042(5) for N(2).
The tetrasubstituted Schiff base assumes a stacked conform-
ation, the two peripheral aromatic rings being parallel [dihedral
angle between them 1.0(2)Њ, stacking distance 0.97(2) Å] and
tilted up and down with respect to the N2O2 core by 21.2(4)Њ
(mean).
N(1)᎐Mo᎐N(2)
O(2)᎐Mo᎐N(2)
O(2)᎐Mo᎐N(1)
O(1)᎐Mo᎐N(2)
O(1)᎐Mo᎐N(1)
O(1)᎐Mo᎐O(2)
Cl(1)᎐Mo᎐Cl(2)
N(1)᎐Mo᎐C(41)
O(2)᎐Mo᎐C(41)
N(1)᎐C(7)᎐C(6)
C(6)᎐C(7)᎐C(21)
N(1)᎐C(7)᎐C(21)
N(2)᎐C(14)᎐C(15)
N(2)᎐C(14)᎐C(30)
C(15)᎐C(14)᎐C(31)
76.4(2)
85.9(2)
160.0(2)
160.4(2)
86.1(2)
112.7(2)
179.2(1)
79.6(3)
84.7(2)
127.9(2)
166.2(2)
92.7(2)
91.1(2)
The alkylation of compound 1 with Mg(CH2Ph)2 leads to the
expected trans-dibenzyl derivative 3. The proposed structure is
109.2(3)
122.2(2)
115.4(5)
111.4(5)
112.9(6)
106.2(5)
110.2(6)
119.4(6)
supported by the presence in the IR spectrum of the C᎐N band
᎐
at 1599 cmϪ1, and hydrolysis gave the expected amount of
PhCH3 confirming the presence of both benzyl groups at the
metal without any migration to the imino carbons.
127.2(5)
126.9(5)
The reaction of compound 1 with Mg(mes)Br (mes =
mesityl) led to 4 the formation of which requires the arylation
of both the imino carbons and the metal, along with the oxid-
ation of MoIV to MoV. The oxidation remains quite obscure
since we cannot invoke a disproportionation reaction MoV to
MoIII because 4 is formed in a yield of ca. 70%. The arylation
at the imino groups is the consequence of a preliminary oxid-
ation of the metal to MoV which enhances the electrophilicity
of the imino groups. The structure of 4 is shown in Fig. 2, with
selected bond distances and angles in Table 2.
Complex 4 contains two mesityl groups bonded to the two
C(7) and C(14) imino carbon atoms of a salophen ligand
[C(7)᎐C(21) 1.550(9), C(14)᎐C(31) 1.543(10) Å]. A third mesityl
group is σ bonded to molybdenum() which achieves five-co-
ordination involving the N2O2 set of donor atoms at the base
(Fig. 2). Molybdenum is 0.088(1) Å out of the plane through
* X = Cl(1) for complex 2 and C(41) for complex 4.
the σ-bonded aromatic ring at a Mo᎐C(41) distance of 2.120(6)
Å falling in the range of values observed for Mo᎐Cσ-aryl bonds
[mean 2.151(10) Å].21 The C(41)–C(46) ring is nearly perpen-
dicular to the N2O2 core, the dihedral angle between them being
83.0(1)Њ. Molybdenum is out of the N2O2 core toward the apical
aromatic carbon by 0.536(1) Å. The N2O2 core shows remark-
able tetrahedral distortions (Table 3). The co-ordination poly-
hedron should be described as a distorted trigonal bipyramid
involving the N(1), O(1), C(41) atoms in the equatorial plane
and the O(1) and N(2) atoms at the apices. Molybdenum is out
of the equatorial plane by 0.099(1) Å toward O(1). The double
J. Chem. Soc., Dalton Trans., 1998, Pages 2395–2400
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Table 3 Comparison of relevant structural parameters (distances in Å, angles in Њ) within the Mo(salophen) units for complexes 2, 4 and 5
2
4
5
Distances of atoms from the N2O2 cores
O(1)
O(2)
N(1)
N(2)
Mo
Ϫ0.159(5)
0.159(5)
0.118(5)
Ϫ0.095(4)
0.010(2)
Ϫ0.264(6)
0.281(6)
0.320(6)
Ϫ0.338(6)
0.536(1)
0.019(4)
Ϫ0.031(4)
Ϫ0.002(3)
0.009(3)
0.987(1)
Distances of atoms from the O(1)C(3)N(1) mean planes
O(1)
N(1)
C(1)
C(6)
C(7)
Mo
Ϫ0.041(4)
0.049(5)
0.085(5)
Ϫ0.019(6)
Ϫ0.077(6)
0.524(1)
Ϫ0.076(6)
0.103(6)
0.066(7)
0.080(7)
Ϫ0.166(7)
0.361(1)
Ϫ0.007(3)
0.029(4)
0.006(5)
0.035(5)
Ϫ0.060(5)
0.722(1)
Distances of atoms from the O2C3N2 cores
O(2)
N(2)
C(14)
C(15)
C(20)
Mo
0.032(4)
Ϫ0.044(5)
0.077(6)
0.004(6)
Ϫ0.080(6)
Ϫ0.516(2)
0.179(5)
Ϫ0.229(6)
0.424(7)
Ϫ0.126(7)
Ϫ0.247(7)
0.132(1)
0.104(3)
Ϫ0.142(3)
0.455(4)
Ϫ0.193(4)
Ϫ0.313(6)
1.118(1)
Folding* along the N(1) ؒ ؒ ؒ O(1) line
Folding along the N(2) ؒ ؒ ؒ O(2) line
20.9(2)
20.4(2)
15.4(2)
10.7(2)
26.7(2)
45.3(2)
Angle between Mo᎐N(1)᎐O(1) and Mo᎐N(2)᎐O(2) planes
Angle between the mean O(1)C(3)N(1) and O(2)C(3)N(2) planes
11.0(2)
11.2(2)
53.0(2)
45.6(2)
54.6(1)
79.8(1)
Torsion angle N(1)᎐C(8)᎐C(13)᎐N(2)
4.6(8)
Ϫ5.1(9)
Ϫ4.0(6)
* The folding is defined as the dihedral angle between the Mo,N,O and OC3N planes of a six-membered chelation ring.
N(2) ؒ ؒ ؒ O(2) six-membered ring (Table 3). The deformation of
the Schiff base is clearly indicated by the dihedral angle of
48.1(2)Њ between the C(15)–C(20) ring and the N2O2 core. The
N(1) ؒ ؒ ؒ O(1) six-membered chelation ring undergoes a minor
distortion upon arylation, mainly as a result of the sp3 char-
acter of the C(7) carbon atom (Table 3), the dihedral angle
between the C(1)–C(6) ring and N2O2 core being 18.7(2)Њ. The
metal lies approximately on the o-phenylene moiety, the max-
imum displacement from the mean plane through all the atoms
including Mo being 0.086(6) Å for N(2). The orientation of the
C(21)–C(26) ring which is nearly perpendicular to the N2O2
core [dihedral angle 86.5(2)Њ] means that the C(27) o-methyl
group approaches the six-co-ordination site of molybdenum
[Mo ؒ ؒ ؒ H(271) 2.56 Å, C(41)᎐Mo ؒ ؒ ؒ H(271) 152.3Њ]. Rotation
of the mesityl groups C(31)–C(36) and C(41)–C(46) around the
C᎐C and Ti᎐C bonds respectively is inhibited by mutual steric
interactions, and rotation of the third mesityl group around the
C(7)᎐C(21) bond is forbidden by steric interaction with the
phenylene bridge.
With the purpose of accessing other kinds of M᎐C bonds, we
attempted the reduction of compound 1 in the presence of
diphenylacetylene. This reaction led to the formation of the
dimeric complex 5, via a complex pathway. The reduction of 1
in the absence of any reactive substrate would probably produce
a metal–metal bonded dimer, as in the case of [Mo(acacen)Cl2],
leading to [(acacen)MoєMo(acacen)].20 In the presence of
diphenylacetylene the intermediate MoII is intercepted by
PhCCPh (see species A in Scheme 2) and finally one of the M᎐C
bonds of the metallacyclopropene migrates to one of the imino
carbons. This event leads to loss of the square-planar rigid
arrangement of the salophen ligand and allows dimerization of
the monomeric unit, as shown for 5 in Scheme 2.
The structure of complex 5 is shown in Fig. 3, with selected
bond distances and angles in Table 4. Complex 5 is a dimeric
unit which contains two tetraanionic ligands L (Fig. 3) derived
from the alkylation of the C(14) imino carbon atom [C(14)᎐
C(21) 1.502(6) Å] by PhCCPh. The complex has a crystallo-
graphically imposed C2 symmetry (Fig. 4). In a dimer each lig-
and exhibits a tris-chelating behaviour toward the Mo atom
through the O(1), N(1) and N(2) atoms while it binds the MoЈ
Fig. 2 A SCHAKAL drawing of complex 4
arylation of salophen leads to a tetraanionic ligand, which
would account for the marked shortening of the Mo᎐N dis-
tances compared with those of complex 2 (see Table 2). The
Mo᎐O distances are not significantly different from those in
complex 2. The conformation of the Mo(salophen) moiety is
strongly affected by arylation which moves the two mesityl
ligands to the opposite sides of the co-ordination plane. As a
result the two C(41)–C(46) and C(31)–C(36) rings become
nearly parallel to each other to avoid steric interactions
[dihedral angle 23.1(2)Њ]. As a consequence the peripheral
C(15)–C(20) aromatic ring is pushed down to produce a
half-umbrella conformation and a strong puckering of the
2398
J. Chem. Soc., Dalton Trans., 1998, Pages 2395–2400

View Article Online
Ph
Ph
N
Table 4 Selected bond distances (Å) and angles (Њ) for complex 5
O
O
C
C
Ph2C2
–NaCl
1 + 2Na
Mo
Mo(1)᎐Mo(1Ј)
Mo(1)᎐O(1)
Mo(1)᎐N(1)
Mo(1)᎐N(2)
Mo(1)᎐N(2Ј)
Mo(1)᎐C(22)
Mo(1)᎐C(22Ј)
Mo(1Ј)᎐O(2)
O(1)᎐C(1)
2.544(1)
2.007(3)
2.220(4)
2.114(4)
2.176(3)
2.528(5)
2.226(5)
2.002(2)
1.320(6)
1.336(6)
N(1)᎐C(7)
N(1)᎐C(8)
1.295(6)
1.407(7)
1.441(5)
1.494(6)
1.434(8)
1.503(7)
1.502(6)
1.354(7)
1.483(7)
1.503(6)
CH
N
CH
N(2)᎐C(13)
N(2)᎐C(14)
C(6)᎐C(7)
C(14)᎐C(15)
C(14)᎐C(21)
C(21)᎐C(22)
C(21)᎐C(31)
C(22)᎐C(41)
A
Ph
Ph
O(2)᎐C(20)
C
C
O
O
N(2Ј)᎐Mo(1)᎐C(22Ј)
N(2)᎐Mo(1)᎐N(2Ј)
N(2)᎐Mo(1)᎐C(22)
N(1)᎐Mo(1)᎐C(22Ј)
N(1)᎐Mo(1)᎐N(2Ј)
N(1)᎐Mo(1)᎐N(2)
O(1)᎐Mo(1)᎐C(22Ј)
O(1)᎐Mo(1)᎐N(2Ј)
O(1)᎐Mo(1)᎐N(1)
O(2Ј)᎐Mo(1)᎐C(22)
72.0(2)
73.0(1)
67.1(1)
165.6(2)
104.9(1)
72.8(2)
97.9(1)
165.3(1)
82.2(1)
156.9(1)
N(1)᎐C(7)᎐C(6)
124.0(5)
106.6(4)
110.2(4)
109.9(4)
116.1(4)
117.2(4)
126.5(4)
Mo
N(2)᎐C(14)᎐C(21)
N(2)᎐C(14)᎐C(15)
C(15)᎐C(14)᎐C(21)
C(14)᎐C(21)᎐C(31)
C(14)᎐C(21)᎐C(22)
C(22)᎐C(21)᎐C(31)
Mo(1Ј)᎐C(22)᎐C(21) 113.1(3)
C(21)᎐C(22)᎐C(41) 114.8(4)
Mo(1Ј)᎐C(22)᎐C(41) 125.8(4)
CH
N
N CH
B
dimerization
Ph Ph
Ph
Ph
Primes denote the transformation Ϫx, y, 1.5 Ϫ z.
C
C
O
O
CH
C
C
O
O
Mo
N
Mo
N
CH
CH
CH
N
N
5
Scheme 2
Fig. 4 A SCHAKAL drawing of complex 5. Primes denote a trans-
formation of Ϫx, y, 1.5 Ϫ z
[3.023(5) Å] and Mo(1Ј)᎐C(21) [3.025(5) Å] distances rule out
any possible η2-C᎐C interaction with the metal. Considering
᎐
the Mo(1)᎐C(22) interaction the metal atoms exhibit seven-co-
ordination which could be described as a monocapped dis-
torted octahedron (Fig. 5) with the N(1), O(1), N(2Ј), C(22Ј)
atoms defining the equatorial plane and the O(2Ј) and C(22)
atoms at the apices. The N(2) atom is capping the N(1), N(2Ј),
C(22) face. In a dimer the two octahedra share the C(22)᎐N(2Ј)
edge. The two Mo᎐N(2) distances are significantly different
[Mo᎐N(2) 2.114(4), Mo᎐N(2Ј) 2.176(3) Å]. In spite of the
bridging role of N(2) these values are significantly shorter than
the Mo᎐N(1) distance [2.220(4) Å] as a consequence of the
alkylation of the C(14) imino carbon. The Mo᎐O bond dis-
tances are significantly longer than those observed in complexes
2 and 4. The Mo᎐MoЈ distance [2.544(1) Å]22 suggests a double
metal–metal bond. The alkylation by PhCCPh strongly affects
the conformation of the original Schiff base, the N2O2 inner core
being removed by the twist of the O(2)᎐C(20)–C(15) phenoxide
moiety which forms a dihedral angle of 57.4(2)Њ with the o-
phenylene ring. Nevertheless, the four N and O donor atoms
still define a plane (Table 3). The dihedral angle between the
o-phenylene ring and the O(1)᎐C(1)–C(6) phenoxide moiety is
Fig. 3 A SCHAKAL drawing of the ligand in complex 5 with the
numbering scheme adopted
atom (Ј Ϫx, y, 1.5 Ϫ z) through the O(1), N(2) and C(22) atoms
(Table 4). This results in the formation of a five-membered
metallacycle [N(2), C(14), C(21), C(22), Mo(1Ј)]. The Mo(1)᎐
C(22Ј) [2.226(5) Å] and C(21)᎐C(22) [1.354(7) Å] bond distances
are in the range found for Mo᎐C(R)᎐CR functionalities
᎐
2
[Mo᎐Cav 2.193(8), C᎐C 1.346(4) Å].21 The group of N(2),
᎐
av
C(14), C(21), C(22) atoms gives rise to another metallacycle
with Mo(1) if the C(22)᎐ Mo(1) contact distance of 2.528(5) Å
is taken into account. The hypothesis that this contact distance
can be explained by a weak π interaction rather than mere geo-
metrical effects is supported by the pyramidality of the C(22)
carbon atom, which is out of the plane passing through the
three bonded atoms [C(21), C(41), Mo(1Ј)] by 0.239(5) Å, but not
by the C(21)–C(22) distance [1.354(7) Å] which is not signific-
antly longer than a localized double bond. The Mo(1) ؒ ؒ ؒ C(21)
J. Chem. Soc., Dalton Trans., 1998, Pages 2395–2400
2399

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Fig. 5 A SCHAKAL drawing of the co-ordination polyhedra in com-
plex 5. Primes denote a transformation of Ϫx, y, 1.5 Ϫ z
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nearly perpendicular forming a dihedral angle of 79.8(1)Њ.
Conclusion
This report deals with unprecedented Mo᎐C bond function-
alities supported by a tetradentate Schiff-base ligand. The
organometallic functionalization of the parent compound
[Mo(salophen)Cl2] 1 led to the formation of a bis(benzyl)-
molybdenum() derivative, containing two alkyl groups trans
to each other in trans-[Mo(salophen)(PhCH2)2]. However, the
arylation of 1 occurred both at the metal and at the imino
groups of the salophen ligand with the formation of an unusual
molybdenum() aryl derivative. The metal and the electrophilic
carbon of the imino groups were also involved when the
Mo(salophen) intermediate, produced from the reduction of 1,
was intercepted by PhCCPh, leading to a dimeric Mo᎐MoЈ
᎐
compound containing two bridging vinyl functionalities.
Acknowledgements
We thank the ‘Fonds National Suisse de la Recherche
Scientifique’ (Bern, Switzerland, Grant No. 20-46’590.96) and
Ciba Speciality Chemicals (Basle, Switzerland) for financial
support.
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