Synthesis, characterization and spectrochemical studies on a few binuclear μ-oxo molybdenum(V) complexes of pyrimidine derived Schiff base ligands

Article abstract of DOI:10.1007/s12039-010-0028-5

Ten new binuclear singly oxo-bridged molybdenum complexes (complexes 1-10) were prepared using five pyrimidine derived Schiff base ligands and two Mo(V) precursors (NH₄)₂MoOCl₅ and (NH ₄)₂MoOBr₅. The ligands are prepared by the condensation of 4,6-dimethyl 2-hydrazino pyrimidine with salicylaldehyde (for HL₁), o-hydroxy acetophenone (for HL₂) and substituted salicylaldehydes (for HL₃, HL₄ and HL₅) respectively. These ligands are already reported as good donors for Mo(VI) state. The μ-oxo Mo(V) complexes reported here bears a distorted octahedral geometry around each Mo atom with either N₂O₂Cl or N ₂O₂Br chromophores. Fine variations in the spectroscopic behaviour of the complexes are observed in accordance with the varying electron donating properties of the ligands. All the complexes are unstable in solution and X-ray quality crystal of complex 1 could be isolated. All the complexes are characterized by IR and UV-Vis spectra.

Full text of DOI:10.1007/s12039-010-0028-5

J. Chem. Sci., Vol. 122, No. 2, March 2010, pp. 239–245. © Indian Academy of Sciences.
Synthesis, characterization and spectrochemical studies on a few
binuclear µ-oxo molybdenum(V) complexes of pyrimidine derived
Schiff base ligands
a
SAMIK GUPTA , SOMNATH ROY , TARAK NATH MANDAL , KINSUK DAS ,
a
a
b
a
SANGITA RAY , RAY J BUTCHER and SUSANTA KUMAR KAR *
c
a,
a
Department of Chemistry, University College of Science, 92 A.P.C. Road, Kolkata 700 009
b
Department of Chemistry, Haldia Government College, Debhog, Purba, Midnapur 721 657
c
Department of Chemistry, Howard University, 2400 Sixth Street, N.W., Washington, DC 20059, USA
e-mail: skkar_cu@yahoo.co.in
MS received 28 April 2009; revised 2 July 2009; accepted 29 July 2009
Abstract. Ten new binuclear singly oxo-bridged molybdenum complexes (complexes 1–10) were pre-
pared using five pyrimidine derived Schiff base ligands and two Mo(V) precursors (NH4)2MoOCl5 and
(NH4)2MoOBr5. The ligands are prepared by the condensation of 4,6-dimethyl 2-hydrazino pyrimidine
with salicylaldehyde (for HL1), o-hydroxy acetophenone (for HL2) and substituted salicylaldehydes (for
HL3, HL4 and HL5) respectively. These ligands are already reported as good donors for Mo(VI) state. The
μ-oxo Mo(V) complexes reported here bears a distorted octahedral geometry around each Mo atom with
either N2O2Cl or N2O2Br chromophores. Fine variations in the spectroscopic behaviour of the complexes
are observed in accordance with the varying electron donating properties of the ligands. All the
complexes are unstable in solution and X-ray quality crystal of complex 1 could be isolated. All the com-
plexes are characterized by IR and UV-Vis spectra.
Keywords. Molybdenum(V); Schiff base; crystal structure; binuclear complex; spectral study.
1. Introduction
been determined crystallographically the under-
standing of the transient Mo(V) state is still heavily
6
dependant on spectroscopy. With the knowledge of
The chemistry of molybdenum is quite interesting
because of its large number of accessible oxidation
states as well as its ability to form stable complexes
with oxygen, nitrogen and sulphur containing ligands
with diverse coordination numbers and stereo-
approximate coordination sphere of Mo in the metal-
loenzymes at hand, several attempts have been made
to model the active sites of these enzymes and study
their enzyme mimetic activity. From the last quarter
of twentieth century the literature has been flooded
with reports of model complexes mostly containing
1
chemistries. Perhaps this versatility is what qualifies
it for use by nature in various essential life processes
2
as constituent part of several metalloenzymes. In
7
the Mo(VI) state and Schiff base ligands. The Mo(V)
fact, Mo is the only essential trace element of the
second transition series and is an integral part of
complexes are less described and compared to the
Mo(VI) species. In fact, up to the first half of this
decade only a few crystal structures of Mo(V)
3
oxotransferase and nitrogenase enzymes.
4
8
complexes with Schiff bases could be found at the
Several molybdoenzymes consist of two Mo
atoms per molecule suggesting binuclear Mo sites
and joined by one or two oxo or sulphido bridges.
The Mo atom alternates +VI and +IV states with the
5
involvement of transient Mo(V) state also. Though
6+
the active site of the fully oxidized (Mo ) and
Cambridge structural Database (version 5⋅26, No-
vember 2004). The chemistry of Mo(V) is domi-
4+
9
nated by spin paired oxo-bridged dimers with Mo2O3
2+
or Mo2O4 cores. Mononuclear paramagnetic Mo(V)
complexes are not so common in literature. Because
of a tendency of Mo(V) species to form oxo-bridged
binuclear complexes preparation of mononuclear
complexes requires careful choice of the ligands
4+
reduced form (Mo ) of metalloenzymes has already
*For correspondence
239

240
Samik Gupta et al
HL4: m.p. 165°C Yield: 1⋅70 g (60%). IR (KBr
which stereochemically hinders formation of binu-
10
clear Mo core. In nature, however, the pterin bound pellet) ν/cm : 3275 (N–H), 3191 (O–H), 1528 (C=N),
enzymes such as DMSO reductase are common in 1610 (C=C). H NMR (300 MHz, CDCl3, 25°C, TMS)
microbial systems and are mononuclear in nature.
–1
1
11
δ/ppm: (6⋅63) (s, 1H, –C5–H pyrimidine), (6⋅9–7⋅8)
Apart from their utility as model compounds for (m, 4H, –C6H4), (8⋅2) (s, 1H, –CH=N–), (12⋅8)
enzymes the Mo(V) complexes have also found use (s, 1H, aromatic-OH), (2⋅28) (s, 1H, N–H of hydra-
as catalysts for oxygen transfer. Mo(V) complexes zone), (2⋅46–2⋅53 ) (m, 6H, (CH3).
are known to ‘speed up’ the transfer of oxygen
HL5: m.p.: above 200°C Yield: 2⋅28 g (80%). 3321
1
from DMSO or pyridine-N-oxide to triphenyl (N–H), 3187 (O–H), 1535⋅2 (C–N), 1612 (C=C). H
12
phosphine.
NMR (300 MHz, CDCl3, 25°C, TMS) δ/ppm: (6⋅62)
In the present work, we have continued our en- (s, 1H, –C5–H pyrimidine), (6⋅9–7⋅6) m, 4H, –C6H4),
deavour to explore the chemistry of Mo complexes (8⋅2) (s, 1H, –CH=N–), (12⋅8) (s, 1H, aromatic-OH),
with heterocycle based ligands and synthesized ten (2⋅28) (s, 1H, N–H of hydrazone), (2⋅46–2⋅53)
new μ-oxo binuclear Mo complexes of pyrimidine (m, 6H, CH3).
derived Schiff base ligands. These complexes are
prepared from two standard Mo(V) precursors
2.1 Synthesis of the complexes 1–10
(NH4)2MoOCl5 and (NH4)2MoOBr5. The complexes
bear octahedral arrangement about each Mo center
The complexes [Mo2O3(L1)2Cl2] (1), [Mo2O3
(L2)2Cl2] (2), [Mo2O3(L3)2Cl2] (3), [Mo2O3(L4)2Cl2]
using the tridentate NNO donor ligand, two oxo
groups (one terminal and the other bridging) and a
(4) and [Mo2O3(L5)2Cl2] (5) were synthesized by
adding an ethanolic solution (10 mL) of (NH4)2
shown that difference in electron donating abilities
loosely bound halide moiety. Previously we have
MoOCl5 (0⋅5 mmol, 0⋅160 g) as the common reagent
of Schiff base ligands affect the spectroscopic prop-
with ethanolic solutions of HL1 (0⋅5 mmol, 0⋅122 g),
HL2 (0⋅5 mmol, 0⋅136 g), HL3 (0⋅5 mmol, 0⋅201 g)
justifiable correlations. Here also, we have found
erties in the corresponding Mo(VI) complexes with
13
and HL4 (0⋅5 mmol, 0⋅158 g) and HL5 (0⋅5 mmol,
0⋅167 g) respectively. A deep magenta colouration
of the solution developed immediately on mixing the
that similar effects are observed for a series of Mo(V)
complexes.
two components. The respective complex separated as
2. Experimental
a violet microcrystalline compound on refluxing the
(NH4)2MoOCl5 and (NH4)2MoOBr5 were prepared as
reaction mixture for 15 min at water bath tempera-
14
described in the literature. Reagent grade solvents
were used as received. 4,6-dimethyl 2-hydrazino
pyrimidine was prepared as described in the litera-
ture. It was filtered, washed with ethanol and dried
over fused CaCl2.
X-ray quality crystals of 1 were grown by slow
diffusion of a dilute ethanolic solution of (NH4)2
MoOCl5 into a dilute ethanolic solution of ligand
HL1 in a sealed glass tube. When the solution of the
two precursors diffused into one another crystals of
6 formed and deposited on the inner wall of the
glass tube.
15
ture. Salicylaldehyde, 5-bromo salicylaldehyde,
5-chlorosalicylaldehyde, 5-nitrosalicylaldehyde and
o-hydroxyacetophenone were purchased from Al-
drich Chemical Company, USA and used as received.
The ligands HL1, HL2 were prepared as described in
16
our previous publication. HL3, HL4 and HL5 were
prepared using a similar technique by condensation
of 10 mmol of 5-bromo salicylaldehyde, 5-chloro-
salicylaldehyde or 5-nitrosalicylaldehyde with
10 mmol of 4,6-dimethyl 2-hydrazino pyrimidine in
each case.
Preparation of complexes 6–10 were affected
by taking equimolar proportion of (NH4)2MoOBr5
and the respective ligands and applying the same
method.
[Mo2O3(L1)2Cl2] (1): Yield 0⋅130 g (33%). Ele-
mental analysis: Anal. Calc. (%) for C26N8O5H26
Mo2Cl2: C: 39⋅39 H: 3⋅28 N: 14⋅14; Found C:
38⋅91 H: 3⋅25N: 14⋅05.
HL3: m.p. 167°C. Yield: 2⋅68 g (80%). IR (KBr
–1
pellet) ν/cm : 3321 (N–H), 3187 (O–H), 1535⋅2
(C=N), 1612 (C=C). H NMR (300 MHz, CDCl3,
1
25°C, TMS) δ/ppm: (6⋅62) (s, 1H, –C5–H Pyrimi-
dine), (6⋅9–7⋅6) (m, 4H, –C6H4), (8⋅2) (s, 1H,
–CH=N–), (12⋅8) (s, 1H, aromatic-OH), (2⋅28) (s,
1H, (N–H of hydrazone), (2⋅46–2⋅53) (m, 6H, CH3).
[Mo2O3(L2)2Cl2] (2): Yield 0⋅143 g (35%). Ele-
mental analysis: Anal. Calc. (%) for C28N8O5
H30Mo2Cl2: C: 40⋅97 H: 3⋅65 N: 13⋅65; Found C:
40⋅61H: 3.52N: 13⋅30.

Studies on a few binuclear μ-oxo molybdenum(V) complexes of pyrimidine derived Schiff base ligands
241
[Mo2O3(L3)2Cl2] (3): Yield 0⋅166 g (35%). Ele- samples, at the Servei de Magnetoquímica of the
mental analysis: Anal. Calc. (%) for C26N8O5H24 Universitat de Barcelona, with a Quantum Design
Mo2Cl2Br2: C: 32⋅84H: 2⋅52N: 11⋅78; Found C: SQUID MPMS-XL susceptometer apparatus work-
32⋅45H: 2⋅38N: 11⋅50.
ing in the range 2–300 K under magnetic field of
[Mo2O3(L4)2Cl2] (4): Yield 0⋅129 g (35%). Ele- approximately 500 G (2–30 K) and 1000 G (35–
mental analysis: Anal. Calc. (%) for C26N8O5H24 300 K). Diamagnetic corrections were estimated
Mo2Cl4: C: 36⋅23H: 2⋅78N: 13⋅00; Found C: 35⋅91H: from Pascal tables. The EPR spectra on 1–10 have
2⋅65N: 12⋅70.
been recorded on X-band Bruker Spectrometer (ESR
[Mo2O3(L5)2Cl2] (5): Yield 0⋅132 g (30%). Ele- 300E), working with an oxford helium liquid
mental analysis: Anal. Calc. (%) for C26N10O9H24 cryostat for variable temperature.
Mo2Cl2: C: 35⋅37H: 2⋅72N: 15⋅87; Found C: 35⋅33H:
2⋅61N: 15⋅66.
[Mo2O3(L1)2Br2] (6): Yield 0⋅132 g (30%). Ele-
2.3 Crystallographic measurement
mental analysis: Anal. Calc. (%) for C26N8O5H26
The crystal of complex 1 was measured on an Enraf-
Mo2Br2: C: 35⋅45H: 3⋅40N: 12⋅72; Found C:
Nonius CAD-4 diffractometer using MoKα radia-
35⋅51H: 3⋅27N: 12⋅50.
[Mo2O3(L2)2Br2] (7): Yield 0⋅159 g (35%). Ele-
tion (λ = 0⋅71073 Å) and a graphite monochromator
at –80°C. The unit cell dimensions were obtained
from a least squares fit to the setting angles of 25,
mental analysis: Anal. Calc. (%) for C28N8O5H30
Mo2Br2: C: 37⋅00H: 3⋅30N: 12⋅33; Found C:
36⋅90H: 3⋅22N: 12⋅30.
[Mo2O3(L3)2Br2] (8): Yield 0⋅182 g (35%). Ele-
mental analysis: Anal. Calc. (%) for C26N8O5H24
accurately determined reflections. The data were
collected using a ω – 2θ scan technique with a 2θ
range of 4–55° and were corrected by an empirical
method for absorption and Lorentz and polarization
Mo2Br4: C: 30⋅05H: 2⋅31N: 10⋅78; Found C:
29⋅61H: 2⋅29N: 10⋅30.
effect. Calculations were carried out using a XCAD-
49 (data reduction), SHELXTL (absorption correc-
[Mo2O3(L4)2Br2] (9): Yield 0⋅166 g (35%). Ele-
tions, structure solution refinements and molecular
mental analysis: Anal. Calc. (%) for C26N8O5H24
graphics). The structures were solved using direct
methods and refined on F2 using full matrix least
Mo2Cl2Br2: C: 32⋅80H: 2⋅78N: 11⋅77; Found C:
32⋅51H: 2⋅61N: 11⋅41.
[Mo2O3(L5)2Br2] (10): Yield 0⋅145 g (30%).
squares techniques with anisotropic displacement
factors for all non-hydrogen atoms. Positions of the
Elemental analysis: Anal. Calc. (%) for C26N10O9H24
Hydrogen atom were calculated from the geometry
Mo2Br2: C: 32⋅16H: 2⋅47N: 14⋅43; Found C:
32⋅04H: 2⋅30N: 14⋅52.
of the molecular skeleton and their thermal dis-
placement parameters were refined isotropically on
a group-wise basis. Crystal parameters of complex 1
are provided in table 1. Selected bond distances and
2.2 Physical measurements
bond angles are provided in table 2.
Elemental analyses (C, H and N), IR spectra (KBr
–1
discs, 4000–200 cm ), of the ligands and the com-
3. Results and discussion
plexes and UV-Vis spectra (DMF)of the complexes
were done with a Perkin-Elmer Model 240°C CHN 3.1 Synthesis and spectroscopic analysis
analyzer, a Jasco FTIR model 420 spectrophoto-
The ligands HLn (n = 1–5) have been used to prepare
the μ-oxo molybdenum(V) complexes 1–5 by react-
ing them with the Mo(V) precursor (NH4)2MoOCl5.
were carried out using Sycopel Model 77 AEW2
meter and a Hitachi U-3501 spectrophotometer, re-
spectively. Cyclic voltammetry (CV) experiments
Similarly, using (NH4)2MoOBr5 with the same series
1820F/S instrument. The measurements were per-
of ligands complexes 6–10 were prepared (scheme
formed at 300 K in a DMF solution containing
–3
–4
1). All the ligands despite of their varying e donat-
ing ability, are found to be good donors towards
both Mo(V) species. The reaction between the metal
precursors and ligand solutions take place with im-
mediate colour change at room temperature and preci-
pitation of the respective complexes within an hour.
0⋅2 M TEAP and 10 –10 M Mo(VI) complexes 1–
5 and 11 deoxygenated by bubbling with nitrogen. A
platinum wire, a platinum coil and a SCE were used
as a working, a counter and reference electrodes, re-
spectively. Magnetic susceptibility measurements
for complex 1 was carried out on polycrystalline

242
Samik Gupta et al
The complexes are spin paired oxo-bridged binu- stretch. The peaks for ν (N–H) of hydrazone are ob-
–1 16
clear units. The binuclear nature of the complexes is served at 3275–3321 cm . In complexes 1–10 the
a result of an inherent property of the Mo(V) that it ν (O–H) bands are found absent as a result of depro-
is inclined to produce oxo-bridged complexes with tonation of the ligands to form these complexes. All
4+
Mo2O3 or Mo2O4 cores, often regardless of the the ligands and the complexes contain the peaks for
2+
18
ligands used. Mononuclear complexes of Mo(V) are ν (C=N) and ν (C=C) stretch. However, when
comparatively less described. Changes in the reac- these peaks from the ligands are compared to those
tion conditions such as carrying out the reactions at in the complexes a slight blue shift (by a maximum
–1
room temperature did not alter the composition of of 35 cm ) may be observed. In the ligands the
–1
the products. Characterization of these complexes ν(C=N) appears at 1528–1546 cm . In the com-
was however hampered considerably by their low plexes the C=N stretch values are found to appear at
–1
solubility in common organic solvents and their low lower energy zones at around 1523–1539 cm . In
stability in oxygenated solvents as DMF and DMSO. the oxo-bridged Mo(V) complexes 1–5 the ν(Mo=O)
–1 19
IR spectra of the ligands (HLn, n = 1–5) show broad (terminal) is observed at 935–959 cm . Here the
–1
bands at 3170–3191 cm for H-bonded ν (O–H) effects of the donating capacity of the ligands are
very obvious. It is found that the lowest energy
stretching band is observed in complex 2 where the
Table 1. Crystal parameters for complex 1.
ligand is HL2, the most electron donating one. With
Identification code
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Complex 6
C26H26Cl2Mo2N8O5
793⋅33
the reduction of the electron donation capacity of the
ligands this stretching frequency value gradually
undergoes blue shift in 1, 3, 4 and finally has the
highest value in 5 (same trend of values are
observed for complexes 6–10). The antisymmetric
and symmetric stretching values for bridging Mo–O
203(2)
0⋅71073
Orthorhombic
Space group
Pnca
a = 15⋅3153(5) Å
α = 90°
b = 13⋅7183(6) Å
β = 90°
c = 14⋅1143(5) Å
γ = 90°
2965⋅41(19)
8
Unit cell dimensions
–1
–1 20
are found at 799–832 cm and 757–772 cm .
Here also the same correlation may be drawn. How-
ever there is not much difference in these peaks when
the chloro complexes are compared to the bromo
complexes of the same ligand showing that both the
chloro and bromo donors are weakly bound and
contributes little to the stability of the complexes.
Electronic spectra of the complexes 1–10 in DMF
display two strong absorption bands in the region
293–312 nm and 329–355 nm. These peaks may be
assigned as charge transfer transitions of the type
N(pπ)–Mo(dπ) LMCT and O(pπ)–Mo(dπ) LMCT
3
)
Volume (Å
Z
Density (calc) (gm/cm )
3
1⋅777
Absorption coefficient (mm ) 1584
–1
F(000)
Crystal size (mm )
1584
3
0⋅55 × 0⋅43 × 0⋅31
Thetarange for data collection 4⋅77 to 30⋅63°
Index ranges
–21 ≤ h ≤ 21,
–19 ≤ k ≤ 15,
–20 ≤ l ≤ 18
21
respectively, as the ligand based orbitals are either
N or O donor types. The slight change in λ values
within each set of peaks going from 1 to 5 (and from
6 to 10 as well) may be due to the difference in elec-
tron donating capacity of the ligands. When HL1 is
compared to HL3 and HL4 and HL5, i.e. salicylalde-
hyde derived ligand compared to its 5-bromo-
salicylaldehyde, 5-chlorosalicylaldehyde and 5-nitro-
salicylaldehyde counterparts, the ligands HL3, HL4
and HL5 are gradually less electron-rich due to the
electron withdrawing nature of the substituents. This
makes HOMO of HL3, HL4 and HL5 higher in en-
ergy compared to that of HL1. As a result, the elec-
tronic transitions in the respective complex (3, 4 and
5) experience a red shift compared to those in 1. On
Reflections collected
Independent reflections
Completeness to theta
Absorption correction
17220
4195 [R(int) = 0⋅0481]
98⋅9%
Semi-empirical from
equivalents
Max. and min. transmission
Refinement method
1⋅00000 and 0⋅70959
Full-matrix least-
squares on F2
4195/0/197
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices
0⋅998
R1 = 0.0404,
wR2 = 0⋅0905
R1 = 0.0904
wR2 = 0⋅1075
0⋅708 and –0⋅868
[I > 2 sigma(I)]R indices
(all data)
Largest diff. peak
–3
and hole (eÅ )

Studies on a few binuclear μ-oxo molybdenum(V) complexes of pyrimidine derived Schiff base ligands
Table 2. Selected bond angles and bond distances of complex 1.
243
Selected bonds
Values (Å)
Selected angles
(°)
Mo –Cl
Mo –O1
Mo –O2
Mo –O3
Mo –N1
Mo –N3
N1 –N2
2⋅5098(10)
1⋅969(3)
1⋅8792(3)
1⋅676(3)
2⋅296(3)
2⋅188(3)
1⋅380(4)
Cl
Cl
–Mo
–Mo
–Mo
–Mo
–Mo
–Mo
–Mo
–Mo
–Mo
–Mo
–Mo
–Mo
–Mo
–Mo
–Mo
–O1
89⋅81(8)
165⋅63(9)
91⋅11(10)
79⋅16(7)
82⋅98(7)
93⋅36(8)
101⋅80(11)
80⋅02(10)
151⋅93(10)
101⋅93(13)
87⋅59(12)
87⋅69(8)
170⋅13(12)
105⋅41(11)
72⋅00(9)
–O2
–O3
–N1
–N3
–O2
–O3
–N1
–N3
–O3
–N1
–N3
–N1
–N3
–N3
Cl
Cl
Cl
O1
O1
O1
O1
O2
O2
O2
O3
O3
N1
Mo
–O2 –Mo_a 177⋅78(19)
Scheme 1. Scheme showing method of preparation of complexes 1–10.
the other hand, HL2 being more e donating than plexes were found to be EPR silent. Study of the
HL1, the CT bands for 2 are more blue shifted com- electrochemical behaviour of the complexes was
pared to 1. Electronic spectra of all the complexes attempted using cyclic voltammetry but the results
1–10 in DMF solution shows another CT band at were unsatisfactory due to the instability of the
22
518–556 nm which is characteristic of binuclear complexes in DMF and DMSO.
μ-oxo Mo(V) complexes. None of these peaks are
stable in DMF solution and disappears over a period
3.2 Structure of complex 1
of 24 h indicating solvolysis in DMF solvent. The
The neutral μ-oxomolybdenum(V) complex 1 (fig-
diffuse reflectance spectra (DRS) of these com-
ure 1) bears a distorted octahedral geometry around
each Mo atom. The ligand HL1, in its deprotonated
form occupies three of the six coordination sites at
the metal centre. One terminal oxo atom, one bridging
oxo atom and a chloride ion complete the coordina-
tion spheres. The tridentate monoanionic donor
plexes show a broad band implying several promi-
nent transitions with maxima between 522 and
565 nm. Spectroscopic characters of individual
complexes are given in table S2 in the (Supplemen-
tary Information).
All the complexes were diamagnetic at room tem-
perature and their spin paired nature did not alter
with variation of temperature either. All the com-
–
ligand (L1 ) binds the molybdenum(V) centre
through a pyrimidine nitrogen (N3), the azomethine

244
Samik Gupta et al
nitrogen (N1) and the enolate oxygen (O1). The the coordination sphere N1–Mo–O3 bears the high-
ligand forms a five-membered and a six-membered est value of 170⋅13°. Values of all the other angles
chelate ring at the metal centre in the equatorial XY support in favour of distorted octahedral structure.
23
plane. One oxo group (O3) is present in the same VSEPR theory can explain the order O3–Mo–
plane trans to the azomathine nitrogen. The bridging O1~O3–Mo–N3 > N1–Mo–N3 ~ N3–Mo–O1
and
oxo group (O2) is present in the axial plane with the also O3–Mo–Cl > Cl–Mo–N1. The Mo–O2–Mo
chloride donor. The molybdenum atoms are found to bridge angle may have values ranging from 148° to
be deviated from the mean equatorial plane towards 180°. The low value of this angle is often associated
the bridging oxo group by 0⋅119 Å at one end of the with the presence of Mo–Mo bonding. However,
bridge and by 0⋅098 Å at the other end. A dihedral higher the value, the lower is the probability of Mo–
angle of 16⋅77°–17⋅10° between the pyrimidine and Mo bonding. Here the bridge angle value is 177⋅78°,
the phenyl ring in the bound ligand reveals that con- almost linear arrangement as in many other singly
27
siderable deviation from planarity has taken place in oxo-bridged Mo(V) complexes. This rule out any
the ligand to facilitate chelation. However, it may be Mo–Mo bonds. The distance between the two Mo
noted that all the ligand atoms which form the two atoms (3⋅759 Å) supports non bonded arrangement.
chelate rings maintain remarkable planarity to
A two-fold intermolecular H-bonding interaction
ensure adequate electron delocalization (maximum is observed in the complex involving the hydrogen
deviation 0⋅150 Å from their mean plane). This is bound to the azomethine nitrogen and the chloride
unlike our previously reported dioxomolybde- donors, i.e. N2–H2A⋅⋅⋅Cl in the complex (figure S1).
16
num(VI) complex of the same ligand where the This creates an infinite one-dimensional array of
phenolate oxygen was found to be deviated by H-bonded molecules organized in a zigzag fashion.
28
(0⋅29 Å) from the mean plane. The Mo=O bond Three other unconventional H-bonding interactions
distances are unexceptional. As expected, the axial of even lesser strength helps to build the three
24
Mo–O bond which is also bridging in nature is dimensional packing in the crystal. These are C10–
longer than the equatorial one by ca. 0⋅203 Å. The H10C⋅⋅⋅O3 (intramolecular), C11---H11A⋅⋅⋅Cl and
Mo–O1 bond (1⋅96 Å) is longer than both Mo–O2 C13---H13B⋅⋅⋅O3 (both intermolecular) (see table SI
and Mo–O3. Between the two bonds Mo–N1 and supplementary information). Among these three
Mo–N3 the former is a little longer due to the trans nonconventional weak H bonding interactions the
25
effect of the oxo group. The Mo–Cl bond length of latter two are noteworthy as the distance of interac-
26
2⋅51 Å shows that the chloride is loosely bound.
tion and the angle falls within the range required for
The bond lengths in the coordination sphere are not significant contribution to the 3D structure (distance
appreciably different from those in the previous between 3⋅0–4⋅0 (Å) and angle 150°–180°) (table SI
dioxomolybdenum(VI) complex despite of the in supplementary information). These two interac-
change in Mo oxidation state. Among the angles in tions create the binding force between the infinite
one-dimensional H-bonded molecular strand. An
interesting feature is the contribution made by one
–
of the coordinated Cl ions which gets involved in
two types of weak bond interactions. A favourable
configuration of the ligands bound to the Mo centers
enables a two-fold intramolecular π–π interaction
between the pyrimidine and phenyl rings of the
ligands with centroid to centroid distances of 3⋅52 Å.
4. Conclusion
Ten new Mo(V) complexes are prepared which are
useful additions in the literature of binuclear oxo-
bridged Mo(V) chemistry as they are based on Schiff
bases derived from the biologically important hetero-
cycle, pyrimidine. Only one of these complexes
could be fully characterized as preparation of their
Figure 1. Structural representation of complex 1.
single crystals are heavily hampered by their poor

Studies on a few binuclear μ-oxo molybdenum(V) complexes of pyrimidine derived Schiff base ligands
245
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The complexes exhibit slight but perceivable
changes in their spectroscopic characteristics depen-
ding on the variations of electron donating abilities
of the used ligands.
Supplementary information
CCDC-678941 contains the supplementary crystal-
lographic data for complex 1. These data can be
obtained free of charge from The Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif. A figure showing one-dimen-
sional H bonding array of complex 1, table showing
H-bonding parameters and spectroscopic characteri-
zation table have been presented as supplementary
materials as figure S1, tables S1 and 2 (see
www.ias.ac.in/chemsci).
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