Synthesis and structural characterization of a dimolybdenum complex bridged by the hydroxymethylphenylphosphinate ligand

Article abstract of DOI:10.1007/s10870-011-0096-5

Hydroxymethylphenylphosphinic acid, 1 was synthesized and structurally characterized by single crystal X-ray diffraction. Ligand 1 crystallized in the monoclinic P2₁ space group with two molecules comprising the unit cell and cell dimensions a

Full text of DOI:10.1007/s10870-011-0096-5

J Chem Crystallogr (2011) 41:1317–1322
DOI 10.1007/s10870-011-0096-5
ORIGINAL PAPER
Synthesis and Structural Characterization of a Dimolybdenum
Complex Bridged by the Hydroxymethylphenylphosphinate
Ligand
Linsheng Feng Rudy L. Luck
•
Received: 21 December 2010 / Accepted: 15 April 2011 / Published online: 27 April 2011
Ó Springer Science+Business Media, LLC 2011
Abstract Hydroxymethylphenylphosphinic acid, 1 was
synthesized and structurally characterized by single crystal
X-ray diffraction. Ligand 1 crystallized in the monoclinic
Introduction
Various organic phosphinates have been studied for their
capacity to stabilize a variety of transition metals and to
display a variety of coordination modes (dimers or poly-
P2 space group with two molecules comprising the unit cell
1
˚
and cell dimensions a = 7.5587(13) A, b = 5.9019(8) A,
˚
-
˚
c = 8.808(3) A and b = 90.90(2)°. Molecules were linked
meric chains) including R PO , R = Me [1–3], Ph [2,
2
2
-
up by hydrogen bonding involving all three oxygen atoms
of 1 resulting in 2-dimensional sheets. Reacting 1 with
MoO Cl in a mixture of ethanol and dichloromethane
4–15], C H [16], p-CH OC H [17–19] and R R PO ,
6 11 3 6 4 1 2 2
?
R = Me, R = CH NH [20, 21]. Phosphinic acids have
1
2
2
3
also been investigated as flame retardant materials previ-
ously [22–25]. Hydroxymethylphenylphosphinic acid, 1, is
an asymmetric organic phosphinic acid which has three
oxygen atoms capable of ligation and we anticipated that a
2-dimensional polymeric sheet may be formed through
hydrogen bonding. For example, the oxygen atoms in the
hydroxymethylphenylphosphinate ligand could exhibit
monodentate, bidentate or tridentate bonding modes. In our
lab, we have utilized phosphinates, such as diphenylphos-
phinic acid and dimethylphosphinic acid to prepare
molybdenum tetramers with distorted Mo O cubic cores [2,
2
2
afforded the dinuclear molybdenum complex Mo O Cl
2
2
4
[
(HOCH )PhPOO] , 2 where the two molybdenum atoms
2 2
were bridged by the phosphinate ligand. Complex 2 crys-
tallized in the P21/C space group as a dietherate with
˚
a = 9.208(3) A,
˚
b = 14.353(4) A,
˚
c = 12.805(4) A
and b = 98.64(3)° and contained terminal O and Cl atom
disorder. The two crystallographically identical molybde-
num centers are in distorted octahedral environments.
3
1
Complexes were analyzed by IR, P NMR and X-ray
crystallography.
4
4
26]. We have also previously detailed the structures and
Keywords Hydroxymethylphenylphosphinic acid ꢀ
Dioxomolybdenum ꢀ Molybdenum dimers ꢀ Hydrogen
bonding
catalytic activities of monodentate complexes of the form
MO Cl L (M = Mo, W and L = various monodentate
2 2 2
ligands) [26–30]. Better solubility and catalytic properties
specifically the utilization of H O as a source of oxygen) of
(
2
2
the complexes have been observed in polar and non-chlo-
rinated solvents, such as ethanol, when polar hydroxyl
groups are introduced in the ligand [27]. Therefore, we are
interested in making complexes containing polar ligands,
such as ligand 1, as this may afford better solubility and
catalytic properties. In this work, we describe the synthesis
and characterization of dinuclear dioxo molybdenum com-
plexes bridged by 1 displaying a bridging tridentate ligand
mode. The structure of the hydroxymethylphenylphosphinic
acid was also characterized by single crystal X-ray diffrac-
tion and we became aware of a recent crystallographic report
L. Feng ꢀ R. L. Luck (&)
Department of Chemistry, Michigan Technological University,
1400 Townsend Drive, Houghton, MI 49931, USA
e-mail: rluck@mtu.edu
123

1
318
J Chem Crystallogr (2011) 41:1317–1322
O
on 1 during the preparation of this manuscript (R = 5.67%)
31].
CH Cl
2
2
PbMoO4
+
H
3
C
C
MoO Cl
2
[
25
o
C, 30 mins
2
Cl
Experimental
O
O
O
H
2
C
OH
P
O
CH
2
Cl
2
/EtOH
Mo
Cl
General Method
MoO
2
Cl
2
+
P
O
CH
2
5 oC, 24h
O
P
2
O
H C
O
2
HO
Cl
Mo
O
O
O
Infrared spectra were obtained on a Perkin Elmer Spectrum
3
1
one FT-IR spectrometer. P NMR data were recorded on a
Varian XL-400 spectrometer. A Fisher-Johns melting point
apparatus (Fisher Scientific Company) was used for the
melting point determinations. Solvents were used as
received from commercial suppliers. Chemicals were pur-
chased from Aldrich and all were used as received.
2 4 2 2 2
Scheme 1 Synthesis of Mo dimer Mo O Cl [(HOCH )PhPOO] (2)
was filtered and dried under vacuum at room temper-
ature overnight affording 0.45 g of the molybdenum dimer
Synthesis and Characterization
Mo
2
O
4
Cl
2
[(HOCH
2
)PhPOO]
2
.
Yield
:
72%. M.p.:
-
1
of Hydroxymethylphenylphosphinic Acid
166–168 °C. IR (neat, cm ) 3303(m), 1593(m), 1439(s),
1131 (s, P=O) 1036(w), 1011(w), 993(w), 952(s, Mo = O,
sym), 913(s, Mo = O, asym), 864(w), 771(w), 736(s),
(
HOCH )PhPOOH (1)
2
3
1
Hydroxymethylphenylphosphinic acid was prepared by
modifications to the literature method for bis(hydroxy-
methyl)phosphinic acid [32]. A mixture of phenylphos-
phinic acid (7.1 g, 50 mmol), concentrated hydrochloric
acid (10 mL) and formaldehyde (1.60 g) was charged in a
two neck flask and warmed up to 40–45 °C until a clear
solution was obtained. The solution was then refluxed at
689(s). P NMR(EtOH) d = 42.1 ppm(s, 1P) relative to
PO
H
.
4
3
X-Ray Crystallography
Suitable crystals of compound 1 were obtained by allowing
slow growth from a solution of 1 in isopropyl alcohol.
Those of 2 were obtained by layering an ethanol solution
with diethyl ether for several days. Suitable crystals of 1
and 2 were coated with epoxy resin and mounted on the
tips of thin glass fibers. All measurements were obtained by
using an Enraf–Nonius Turbo CAD4 X-ray diffractometer.
1
20 °C for 30 h. The product was obtained by removing all
the volatiles. 4.6 grams of 1 was afforded after re-crystal-
1
lization in isopropyl alcohol. 1 was characterized by IR, H
3
1
NMR and P NMR. Yield: 54%. M.p.: 124.5–126 °C. IR
-
neat, cm ) 3458(s), 2903(w), 1438(s) 1128(m, P=O)
1
(
3
1
052(m) 967(s) 858(s) 769(m), 742(s), 689(s). P NMR
˚
1
MoKa radiation (k = 0.71073 A) was used for data col-
(
EtOH) d = 35.9 ppm (s, 1P) relative to H PO . The
lection. The collection of the data, solution and refinements
of the structures were accomplished using established
procedures [2, 36–38].
3
4
ligand was also structurally characterized by single X-ray
diffraction.
Synthesis and Characterization of Molybdenum Dimer
Mo O Cl [(HOCH )PhPOO] (2)
2
Results and Discussion
2
4
2
2
The MoO Cl unit was prepared as shown in Scheme 1
2
Synthesis
2
following established procedures [28, 33–35]. A mixture of
lead molybdate (0.693 g, 1.885 mmol), acetyl chloride and
dichloromethane was stirred for 30 min at room tempera-
ture resulting in a brown solution which was obtained by
filtration. The solution was then added in a dropwise
manner to a solution of hydroxymethylphenylphosphinic
acid (0.332 g, 1.885 mmol) in CH Cl /EtOH resulting first
We have previously reported on the synthesis, character-
ization and catalytic properties of dioxomolybdenum and
dioxotungsten complexes supported by the monodentate
hydrophilic ligand hydroxymethyldiphenyl phosphine
oxide [27]. We discovered that by introducing a polar
hydroxymethyl group in the ligand, these complexes
showed better solubility in non-chlorinated solvents, such
as ethanol and may be more applicable for catalytic
2
2
in an orange solution which then changed to a light yel-
lowish-green solution after one hour of stirring. This
solution was then stirred at room temperature for 24 h. The
solution was concentrated under vacuum and hexane added
to precipitate the yellowish-green product. The precipitate
epoxidation reactions utilizing H O as the source of
2 2
oxygen. It was with these results in mind that we decided to
explore the syntheses of molybdenum tetramers, consisting
1
23

J Chem Crystallogr (2011) 41:1317–1322
1319
of an ‘‘Mo O ’’ [2, 26] core, which may be stabilized with
4
Table 1 Crystal data and structure refinement details of complexes 1
and 2
8
the conjugate base of ligand 1. We prepared ligand 1 by
refluxing a mixture of phenylphosphinic acid, concentrated
HCl and formaldehyde for 30 h. The IR spectrum con-
Identification code
Molecular formula
1
2
-
1
C H O P
7
2 10
C H Cl Mo O
9
3
14 16
2
tained an absorption at 1128 cm which may be ascribed
to a P=O bond vibration in agreement with the literature
values [25].
P
2
ꢀ2C
817.23
0.40 9 0.15 9 0.15 0.20 9 0.10 9 0.10
4 10
H O
Formula weight
Crystal size (mm)
Crystal color
172.11
The procedure used to prepare the complex 2 is shown
in Scheme 1. The ‘‘MoO Cl ’’ unit was generated by
Colorless
290 (2)
Light blue
290 (2)
Monoclinic
P21/C
2
2
Temperature (K)
Crystal system
Space group
mixing lead molybdate with acetyl chloride in dichloro-
methane [28, 33–35]. Treatment of equal moles of
Monoclinic
P2
1
‘‘MoO Cl ’’ with 1 in a mixture of dichloromethane and
2 2
˚
a (A)
7.5587(13)
5.9019(8)
8.808(3)
90.90(2)
392.88(16)
2
9.208(3)
14.353(4)
12.805(4)
98.64(3)
1673.1(9)
4
ethanol at room temperature produces the molybdenum
˚
b (A)
dimer 2 as a moisture sensitive, yellowish-green solid in
˚
c (A)
7
9
2% yield. The IR spectrum contains diagnostic bands at
-
1
52 and 913 cm which are due to the symmetric and
b (°)
3
˚
V (A)
asymmetric stretching modes for the cis-dioxomolybdenum
Z
unit (MoO ) [39]. A strong-intensity absorption band at
2
3
Dc (Mg/m )
-
1
131 cm can be assigned to a P=O stretch which is blue
1.455
1.622
1
Absorption
coefficient (mm
0.302
1.057
shifted compared to the equivalent mode in free ligand
-
1
)
-
1128 cm ). The P NMR spectrum of 2 in ethanol dis-
1
31
(
F (000)
180
824
plays a single resonance at 42.1 ppm for the P atom of the
coordinated ligand which is shifted downfield of the reso-
nance for the P atom on the free ligand 1 (35.9 ppm). The
magnitude of this shift, 6.2 ppm, is comparable to that
observed with other related phosphonate ligands [27, 29].
Scan range h (°)
No. of reflections
measured
2.31–24.91
971
2.15–24.99
2933
No. of observed data,
I [ 2r (I)
No. of parameters
902
1968
109
191
a,b
c
d
R1, wR2 [I [ 2r (I)]
R1, wR2 (all data)
Goodness of fit on F
0.0217, 0.0573
0.0235, 0.0606
1.117
0.0506, 0.1137
0.1034, 0.1333
1.144
X-Ray Crystallographic Data
2
Crystal data and details of structural refinement are listed in
Table 1. Selected bond lengths and angles of compound 1
and 2 are given in Tables 2 and 3 respectively.
P
P
) ? (0.0310P) ? 0.1117P] where P = (F
a
R1 = ||F
o
| - |Fc||/ |F
2
o
|
P
P
b
c
d
2 2
c
2 2 1/2
o
) ]]
wR2 = [ [w(F
2
o
- F
) ]/ [w(F
2
2
o
2
o
2
c
w = 1/[r (F
? 2F
2
)/3
A Mercury [40] drawing of ligand 1 is shown in Fig. 1.
One hydroxymethylphenylphosphinic acid molecule com-
prises the asymmetric unit and all H atoms were refined
with riding parameters. The geometry around the phos-
phorus atom is pseudo tetrahedral with the O–P–O and
C–P–C bond angles equal to 116.30(11)° and 108.92(12)°
respectively. The bond angles for O–P–C range from
2
2
2
w = 1/[r (Fo ) ? (0.0.0445P) ? 3.3894P] where P = (F
o
?
2
2
c
F )/3
the a axis, constituting a polar middle section with the
H-bonds consisting of pentatomic rings with a bifurcated H
2
1
donor ðR ð5ÞÞ and a hydrophobic exterior with the pro-
104.03(9)–110.79(12)° probably due to minimization of
steric hindrance with the phenyl ring. There are two sig-
truding phenyl groups. In compound 1, the crystal is built
up from infinite 2-dimensional polymeric sheets held
together by O–HꢀꢀꢀO hydrogen bonds, parallel to the
a–b plane, as shown in Fig. 3. Finally, although complex 1
has crystallized in a chiral and polar space group, the
crystal structure is not since the dipole vectors of adjacent
molecules are approximately perpendicular to the polar
axis but are antiparallel [45].
nificantly different P–O bond distances; a short P=O(2) and
˚
longer P–O(1)H at 1.497(2) and 1.5488(17) A respectively.
These P–O bond distances are within the range of expected
values for P=O and P–OH bond lengths [41, 42]. All of the
O atoms in complex 1 are involved in H-bonding as dis-
played in Fig. 2 with each molecule H-bonding to four
adjacent molecules. Two of these interactions are related
by symmetry and the three unique interactions are listed in
Table 2. The different P–O bond distances results in an
asymmetric hydrogen bridge [43, 44]. The net result of the
A Mercury [40] drawing of complex 2 is shown in
Fig. 4. The structure contains disorder (89/11%) between
terminal Cl and O atoms and these were refined with the
Mo–Cl and Mo–O distances for all Cl and O atoms
2
2
˚
involved constrained to 2.38 and 1.67 A respectively.
H-bonds are polymeric chains ðC ð10ÞÞ, located parallel to
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J Chem Crystallogr (2011) 41:1317–1322
˚
Table 2 Selected bond distances (A), bond angles (°) and H-bonds
for compound 1
Bond distances
P(1)–O(2)
1.497(2)
1.5488(17)
1.803(3)
P(1)–C(1)
O(1)–C(7)
1.799(2)
1.412(3)
P(1)–O(3)
P(1)–C(7)
Bond angles
O(2)–P(1)–O(3)
O(2)–P(1)–C(7)
O(3)–P(1)–C(7)
O(2)–P(1)–C(1)
116.30(11)
109.83(12)
106.61(13)
110.79(12)
O(3)–P(1)–C(1)
C(7)–P(1)–C(1)
C(7)–O(1)–H(1)
P(1)–O(3)–H(31)
104.03(9)
108.92(12)
109.5
109.5
Fig. 1 Mercury [40] representation of 1 with selected atom number-
ing. Thermal ellipsoids are drawn at the 50% probability level
D–HꢀꢀꢀA
d(D–H)
d(HꢀꢀꢀA)
d(DꢀꢀꢀA)
\(DHA)
a
O(1)–H(1)ꢀꢀꢀO1a
0.82
0.82
0.82
2.44
2.29
1.74
3.0949(12)
3.002(3)
2.512(3)
137.5
145.2
157.1
O(1)–H(1)ꢀꢀꢀO2
b
O(3)–H(31)ꢀꢀꢀO2
˚
Hydrogen bonds for HꢀꢀꢀA \ (r(A) ? 2.000 A) and \ DHA [ 110°
Symmetry transformations used to generate equivalent atoms:
a
b
x ? 1, y ? 1/2, -z ? 2; - x, y ? 1/2, -z ? 2
-
˚
Table 3 Selected bond distances (A) and bond angles (°) for
compound 2
Bond distances
Mo(1)–O(2)
1.669(5)
1.754(7)
1.77(4)
Mo(1)–Cl(1)
a
O(1)–P(1)
2.338(2)
1.519(5)
1.503(5)
1.519(5)
1.786(6)
1.815(7)
Mo(1)–O(3)
Mo(1)–O(31)
P(1)–O(11)
a
P(1)–O(1)
Mo(1)–O(11)
2.033(4)
2.113(4)
2.15(3)
Mo(1)–O(1)
P(1)–C(1)
a
Mo(1)–Cl(11)
P(1)–C(7)
Mo(1)–O(71)
2.286(5)
Bond angles
O(2)–Mo(1)–O(3)
O(2)–Mo(1)–O(31)
O(3)–Mo(1)–O(31)
O(2)–Mo(1)–O(11)
O(3)–Mo(1)–O(11)
O(31)–Mo(1)–O(11)
O(2)–Mo(1)–O(1)
O(3)–Mo(1)–O(1)
O(31)–Mo(1)–O(1)
O(11)–Mo(1)–O(1)
O(2)–Mo(1)–Cl(11)
O(31)–Mo(1)–Cl(11)
O(11)–Mo(1)–Cl(11)
O(1)–Mo(1)–Cl(11)
O(2)–Mo(1)–O(71)
O(3)–Mo(1)–O(71)
O(31)–Mo(1)–O(71)
O(11)–Mo(1)–O(71)
102.8(4)
89(2)
O(1)–Mo(1)–O(71)
Cl(11)–Mo(1)–O(71)
O(2)–Mo(1)–Cl(1)
O(3)–Mo(1)–Cl(1)
O(11)–Mo(1)–Cl(1)
O(1)–Mo(1)–Cl(1)
Cl(11)–Mo(1)–Cl(1)
O(71)–Mo(1)–Cl(1)
72.33(17)
87.1(8)
Fig. 2 The hydrogen bonding interactions (dashed lines) of 1
104.4(17)
95.3(2)
96.93(19)
95.4(3)
The dinuclear molybdenum complex is positioned around
an inversion point in the P21/C space group and is H-bon-
ded to two diethyl ether molecules via the H-atoms on the
hydroxyl moiety. The two crystallographically identical
molybdenum centers are bridged together by two tridentate
hydroxymethylphenylphosphinate ligands. The molybde-
90.0(3)
165.22(16)
87.87(13)
91.6(8)
163.5(16)
97.1(2)
159.3(3)
81.4(16)
82.37(17)
103.4(10)
100.6(18)
93.7(8)
83.36(14)
129.3(3)
141(6)
a
P(1) –O(1)–Mo(1)
Cl(1)–O(31)–Mo(1)
a
O(11)–P(1)–O(1)
˚
num atoms, separated by 5.465(2) A are in distorted
115.6(3)
106.1(3)
109.6(3)
110.4(3)
103.4(3)
111.9(3)
152.4(3)
octahedral environments in which the two terminal oxygen
atoms are bonded cis to each other. The molybdenum
centers are also each coordinated to one chloride ligand
and three oxygen atoms from the two bridging
hydroxymethylphenylphosphinate ligands. Each hydro-
xymethylphenylphosphinate ligand forms a five-membered
O(11)–P(1)–C(1)
a
O(1) –P(1)–C(1)
a
159.4(10)
169.4(2)
87.7(3)
O(11)–P(1)–C(7)
a
O(1) –P(1)–C(7)
a
a
C(1)–P(1)–C(7)
89.3(18)
83.10(18)
P(1)–O(11)–Mo(1)
chelating ring through the bidentate –O=PCH OH-linkage
2
to the Mo atoms. These two oxygen atoms (O(71), O(1))
from the hydroxymethylphenylphosphinate ligand are cis
to each other with an O(71)–Mo(1)–O(1) angle of
D–HꢀꢀꢀA
d(D–H)
0.82
d(HꢀꢀꢀA)
d(DꢀꢀꢀA)
2.560(7)
\(DHA)
b
O(71)–H(71)ꢀꢀꢀO(103)
1.75
˚
169.6
72.33(17)° and are located trans to the terminal oxo-
Hydrogen bond for HꢀꢀꢀA \ (r(A) ? 2.000 A) and \DHA [ 110°
a
Symmetry transformations used to generate equivalent atoms: - x ? 2,
ligands, The two P–O distances at P(1)–O(1) = 1.519(5)
˚
and P(1)–O(11) = 1.503(5) A are slightly different
b
y ? 1, -z ? 1; - x ? 2, y ? 1/2, -z ? 1/2
-
1
23

J Chem Crystallogr (2011) 41:1317–1322
1321
Fig. 5 Mercury [40] drawing of the crystal packing of 2 viewed down
the b axis. The C atoms in the ether molecule are lighter in appearance
Fig. 3 The crystal packing of 1 showing the arrangement of the 2D
polymer formed via hydrogen bonds (dashed lines)
The bond distance between Mo(1) and O(2) (ordered) is
˚
.669(5) A whichiswithintheexpectedrangeof1.66–1.76 A
˚
1
[46]. The bond distances for the disordered Mo = Ooxo are
˚
Mo(1)–O(3) = 1.754(7) and Mo(1)–O(31) = 1.77(4) A
which are all significantly longer. The Mo–Cl bond lengths at
˚
Mo(1)–Cl(1) = 2.338(2) and Mo(1)–Cl(11) = 2.15(3) A are
in the range found for Mo–Cl distances in complexes fea-
t
turing disorder [47]. Irregularities in these distances can be
ascribed to this disorder. Interestingly enough this complex
packs so that the polar bonds are located on the bc plane with
the nonpolar phenyl ring and H-bonded diethyl ether mole-
cules in center of the unit cell as illustrated in Fig. 5.
Conclusion
A functionalized organic phosphinic acid, 1, was synthe-
sized and structurally characterized. Structural examination
of the ligand 1 revealed infinite 1-dimentional polymers
formed via intermolecular hydrogen bonds. The dinuclear
dioxomolybdenum complex 2 bridged by tridentate ligand
Fig. 4 Mercury [40] drawing of 2 with selected atom numbering for
the major orientation. Thermal ellipsoids are drawn at the 30%
probability level and the H-bonds to the ether molecules are
illustrated with dashed lines
3
1
1 was also prepared and characterized by IR, P NMR and
single X-ray diffraction. Complex 2 contained disorder
between terminal Cl and O atoms.
probably reflective of the fact that the distances between
the Mo and O atoms are also significantly different with
˚
Mo(1)–O(1) = 2.113(4) and Mo(1)–O(11) = 2.033(4) A.
Supporting Information Available
The fact that there is a wide variance in the P–O–Mo bond
angles with Mo(1)–O(1)–P(1) = 129.3(3) and Mo(1)
O(11)–P(1) = 152.4(3)° illustrated the flexibility of this
kind of coordination mode given the slight differences in
distances noted previously.
Crystallographic data for the structural analysis of the
compounds have been deposited at the Cambridge Crys-
tallographic Data Center (CCDC). The CCDC numbers are
804207 for 1 and 804208 for 2. Copies of the data can be
123

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J Chem Crystallogr (2011) 41:1317–1322
obtained free of charge on application to CCDC, 12 Union
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Acknowledgement We thank Michigan Technological University
for financial support.
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