The pH dependent phase transfer of an organometallic complex: Synthesis, characterization, and crystal structure of a W(0) tetracarbonyl with an acidic pyridine-imine ligand

Article abstract of DOI:10.1016/j.inoche.2003.09.015

Reaction of the asymmetrical pyridine-imine ligand L (where L=benzoic acid, 3-[(2-pyridinylmethylene)amino]) with W(CO)₃(EtCN)₃ and CO (g) affords a W(O) tetracarbonyl in which the Schiff-base ligand coordinates in a bidentate fashion. The complex was characterized by IR, NMR, and UV-visible spectroscopy and elemental analysis. The solid-state structure of W(CO)₄(L), which crystallized in the orthorhombic space group Pbca with unit cell parameters a = 22.497(4) ?, b = 14.112(6) ?, and c = 13.675(3) ? was established by X-ray crystallography. Deprotonation of the -CO₂H group on the ligand with base results in the formation of a water-soluble organometallic complex which can be partitioned between organic and aqueous phases in a biphasic system by pH adjustment.

Full text of DOI:10.1016/j.inoche.2003.09.015

Inorganic Chemistry Communications 7 (2004) 14–17
www.elsevier.com/locate/inoche
The pH dependent phase transfer of an organometallic
complex: synthesis, characterization, and crystal structure of a W(0)
tetracarbonyl with an acidic pyridine–imine ligand
1
Brian P. Buffin , Philip J. Squattrito , Abiola O. Ojewole
*
Department of Chemistry, Western Michigan University, Wood Hall, Kalamazoo, MI 49008, USA
Received 2 September 2003; accepted 20 September 2003
Published online: 22 October 2003
Abstract
Reaction of the asymmetrical pyridine–imine ligand L (where L ¼ benzoic acid, 3-[(2-pyridinylmethylene)amino]) with
W(CO)3(EtCN)3 and CO (g) affords a W(0) tetracarbonyl in which the Schiff-base ligand coordinates in a bidentate fashion. The
complex was characterized by IR, NMR, and UV–visible spectroscopy and elemental analysis. The solid-state structure of
ꢀ
ꢀ
W(CO)4(L), which crystallized in the orthorhombic space group Pbca with unit cell parameters a ¼ 22:497ð4Þ A, b ¼ 14:112ð6Þ A,
ꢀ
and c ¼ 13:675ð3Þ A was established by X-ray crystallography. Deprotonation of the –CO2H group on the ligand with base results
in the formation of a water-soluble organometallic complex which can be partitioned between organic and aqueous phases in a
biphasic system by pH adjustment.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Pyridinylimine ligand; Tungsten(0) complex; Water-soluble complex; Phase transfer
Research in the area of aqueous organometallic
chemistry has primarily focused on complexes in which
a hydrophilic phosphine ligand is used for the promo-
tion of water solubility [1]. Recently we communicated
our success at forming stable water-soluble Pt(0) and
Pd(0) complexes with nitrogen-based pyridinylimine and
diazabutadiene ligands [2] as well as our ability to ex-
tend this chemistry into higher valent Group 10 metal
complexes [3,4]. Despite a growing interest in water-
soluble organometallic compounds as recoverable
‘‘green’’ catalysts [5,6], the use of nitrogen ligands in this
area of research has remained relatively unexplored.
One of the major drawbacks to aqueous-phase catalysis
is the diminished activity that often results from the
limited mass transfer of hydrophobic organic substrates
into the catalyst phase [1]. Herein we describe the syn-
thesis and characterization of a stable metal carbonyl
with an acidic pyridine–imine ligand and the ability to
transfer this complex between organic and aqueous
phases via simple pH adjustment, a protocol that may
prove useful for catalyst partitioning in other systems.
The pyridinylimine ligand L (Fig. 1) was prepared via
the condensation of 3-aminobenzoic acid with 2-pyri-
dinecarboxaldehyde in refluxing ethanol, with benzene
added to promote the azeotropic removal of water by-
product, and isolated as an air-stable pale yellow solid in
79% yield [4]. The observation of signals at 8.62 ppm
1
and 162.5 ppm in the H and 13C NMR spectra of L,
respectively, are consistent with formation of the imine.
Addition of water to L results in the immediate hydro-
lysis of the imine bond as evidenced by the appearance
1
of an aldehyde signal in the H NMR data.
A deep blue–green solution was formed upon addition
of Tetrahydrofuran (THF) to a 1:1 mixture of L and
W(CO)3(EtCN)3, suggesting displacement of two labile
propionitrile molecules and coordination of the ligand as
a bidentate nitrogen donor [7]. The resulting air-sensitive
mononitrile compound, W(CO)3(EtCN)(L), steadily
scavenged CO from decomposition products to afford
* Corresponding author. Tel.: +1-2693872420; fax: +1-2693872909.
E-mail address: brian.buffin@wmich.edu (B.P. Buffin).
1
Present address: Department of Chemistry, Central Michigan
University, Mt. Pleasant, MI 48859, USA.
1387-7003/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2003.09.015

B.P. Buffin et al. / Inorganic Chemistry Communications 7 (2004) 14–17
15
Fig. 1. Synthetic scheme for the preparation of W(CO)4(L).
the tetracarbonyl product W(CO)4(L) (Fig. 1). Addition
of CO to a solution of the tricarbonyl mononitrile species
or utilization of W(CO)4(C7H8) (where C7H8 ¼ norbor-
nadiene) as the tungsten starting material provided a
more convenient synthesis of W(CO)4(L) [8]. The deep
purple color of W(CO)4(L), with a lowest energy metal-
to-ligand charge transfer (MLTC) absorption of kmax
(CH2Cl2) ¼ 548 nm in the electronic spectrum, is con-
sistent with literature precedent for the coordination of
conjugated pyridine–imine ligands to low valent metals
[9–11]. W(CO)4(L) was fully characterized by IR and
NMR spectroscopic methods, elemental analysis, and
the molecular structure in the solid state was determined
via X-ray diffraction. Diagnostic of ligand coordination
in the tetracarbonyl compound is an overall downfield
two. When bulkier non-hydrogen atoms are located in
the ortho-position(s), the phenyl ring has been shown to
adopt a position perpendicular to that of the metal-
chelate in related compounds [13,14]. The relatively
ꢀ
short C ¼ N bond length of 1.30(1) A for C(6)–N(2),
when compared to the C(7)–N(2) bond length of 1.42(1)
ꢀ
ꢀ
A and the C(5)–N(1) length of 1.37(1) A in the pyridine
ring, is consistent with a high degree of double bond
character in the imine bond and a lack of electron de-
localization in the metal-chelate of W(CO)4(L). All of
the bond lengths and angles are similar to those in re-
lated compounds [15]. The hydrogen atom attached to
O(2) could not be located, but is inferred from the rel-
ꢀ
atively long C(13)–O(2) bond length of 1.30(1) A.
Analysis of the molecular packing in the unit cell reveals
1
shift of signals in the H NMR data. The imine proton
and the ortho-proton of the pyridine ring are most in-
fluenced by ligand coordination to the W(CO)4 center,
experiencing downfield shifts of 0.75 and 0.58 ppm, re-
spectively. Similar resonance shifts have been observed
in related compounds [7]. The three band pattern in the
carbonyl region of the IR spectrum of W(CO)4(L)
(mCO ¼ 2009 (m), 1901 (s), 1851 (m) cmꢀ1) is consistent
with a pseudo C2v symmetry for the cis-tetracarbonyl
complex.
The molecular structure of W(CO)4(L) with the as-
sociated atom-numbering scheme are depicted in Fig. 2
with selected bond lengths and angles given in the figure
caption [12]. The geometry around the W(0) d6 metal
center in W(CO)4(L) is a distorted octahedron, where
the relatively small N(1)–W(1)–N(2) bond angle of
72.5(3)° is a result of chelating ligand steric constraints.
Consistent with a planar arrangement of the pyridine–
imine ligand, the bond angles around the metal center
which includes the ligand nitrogen atoms and the car-
bonyl groups containing C(15) and C(17) sum to
360.1(9)°. A least-squares plane analysis of the 8 non-
hydrogen atoms comprising the pyridine–imine ligand
and the W atom reveals a planar chelation of the ligand
to the metal, with a mean and maximum deviation from
Fig. 2. ORTEP representation of the molecular structure of
W(CO)4(L) with 50% probability ellipsoids (H atoms and the THF
ꢀ
solvent molecule omitted for clarity). Selected bond distances (A) and
angles (°): W(1)–N(1) ¼ 2.252(9), W(1)–N(2) ¼ 2.238(9), W(1)–C(14) ¼
2.03(1), W(1)–C(15) ¼ 1.95(1), W(1)–C(16) ¼ 2.05(1), W(1)–C(17) ¼
1.95(1), N(1)–C(5) ¼ 1.37(1), N(2)–C(6) ¼ 1.30(1), N(2)–C(7) ¼ 1.42(1),
C(6)–C(5) ¼ 1.43(2), O(1)–C(13) ¼ 1.21(1), O(2)–C(13) ¼ 1.30(1), N(1)–
W(1)–N(2) ¼ 72.5(3), N(1)–W(1)–C(14) ¼ 95.7(4), N(1)–W(1)–C(15) ¼
173.5(5), N(1)–W(1)–C(16) ¼ 91.1(4), N(1)–W(1)–C(17) ¼ 96.1(5),
N(2)–W(1)–C(14) ¼ 90.9(4), N(2)–W(1)–C(15) ¼ 101.4(5), N(2)–W(1)–
C(16) ¼ 92.1(4), N(2)–W(1)–C(17)¼ 168.3(4), O(1)–C(13)–O(2) ¼ 126(1).
ꢀ
planarity of 0.037 and 0.07(1) A for C(4), respectively.
The phenyl ring of the pyridine–imine ligand containing
C(7)–C(12) is tilted relative to the metal-chelate ring
with a calculated dihedral angle of 45.06° between the

16
B.P. Buffin et al. / Inorganic Chemistry Communications 7 (2004) 14–17
Fig. 3. pH-dependent phase transfer of W(CO)4(L).
no tendency for the –COOH groups in adjacent mole-
cules to hydrogen bond. Individual molecules do not
appear to be facing each other, and there are no O–O
plished over 20 times with no evidence of complex de-
composition. Protonation of the W(CO)4(L)ꢀ anion in
the aqueous phase was also accomplished by addition of
acetic acid (pKa ¼ 4.76), bubbling of CO2 (g) (carbonic
acid, pKa ¼ 6:35), and with dihydrogenphosphate ion
(pKa ¼ 7:20). m-Nitrophenol is not a strong enough acid
(pKa ¼ 8:39) to bring about complex protonation and
transfer to the organic phase. The reaction chemistry of
this metal complex and the ability to promote the pH
dependent phase transfer of active catalytic species is
currently under investigation.
ꢀ
contacts involving O(1) and O(2) out to 3.2 A. This is in
marked contrast to Group 6 metal carbonyls with a
related pyridine–imine ligand containing a hydroxyl
group in a position para to the imine nitrogen, where a
range of hydrogen-bonding motifs have been observed
in the solid state [15,16].
W(CO)4(L) is soluble in a variety of organic solvents
including methanol, ethanol, THF, CH2Cl2, DMF and
DMSO but is insoluble in non-polar hydrocarbon sol-
vents. Although the complex is water insoluble, addition
of NaOH base to a suspension of the complex in water
results in its immediate dissolution through deprotona-
tion of the acidic –CO2H group and formation of the
charged species, W(CO)4(L)ꢀNaþ. An independent syn-
thesis of W(CO)4(L)ꢀNaþ from Lꢀ and W(CO)4(C7H8)
confirmed the identity of the water-soluble species. Thus,
the reactive imine group of the ligand has been effectively
stabilized towards hydrolysis through coordination to
W(0). The MLCT band in the electronic spectrum of
W(CO)4(L) undergoes a blue shift of 48 nm (kmax ¼ 500
nm) upon transfer to aqueous solution. This solvato-
chromism is comparable to that observed in related wa-
ter-soluble compounds [17,18]. Protonation of the
carboxylate ion results in a partitioning of the organo-
metallic complex into the organic phase in a biphasic
system. For example, addition of HCl to an aqueous
solution of W(CO)4(L)ꢀNaþ in a CH2Cl2–H2O biphase
results in formation of W(CO)4(L) as a neutral species
and a complete transfer into the CH2Cl2 phase, as
monitored by UV–visible, IR and NMR spectroscopy
(Fig. 3). Regeneration of the anionic complex by the
addition of base results in transfer back into the aqueous
phase. This pH dependent phase transfer was accom-
Supplementary material
Crystallographic data for the structural analysis of
W(CO)4(L) in CIF format has been deposited with the
Cambridge Crystallographic Data Center under CCDC
no. 217828. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).
Acknowledgements
Author acknowledges Donors of The Petroleum
Research Fund, administered by the American Chemi-
cal Society, for partial support of this research (PRF#
32973-GB3) and to Western Michigan University.
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ꢀ
ꢀ
magnetic stir bar was charged with 1.00
g
(2.3 mmol) of
orthorhombic, Pbca (No. 61), a ¼ 22:497ð4Þ A, b ¼ 14:112ð6Þ A,
3
c ¼ 13:675ð3Þ A, V ¼ 4342ð3Þ A , Z ¼ 8, dcalc ¼ 1:82 g cmꢀ3
,
ꢀ
ꢀ
ꢀ
W(CO)3(EtCN)3 and 0.52 g (2.3 mmol) of L and then flushed
with nitrogen. Tetrahydrofuran (THF, 30 ml) was added by
syringe, producing a dark blue–green solution. Carbon monoxide
gas was subsequently bubbled through the solution for 15 min to
afford an intense purple color reminiscent of the aqueous
permanganate ion, and the solution was sealed under an atmo-
sphere of CO (g) for 16 h to complete the transformation to the
tetracarbonyl product. The resulting solution was filtered through
CeliteÒ to remove insoluble tungsten byproducts, and hexane (30
ml) was layered on top of the THF filtrate. After allowing the
solvents to diffuse for approximately 16 h, solvents were partially
removed under reduced pressure, and precipitation of product was
accomplished by cooling to )10 °C. Isolation of the precipitated
solid by filtration and washing with pentane gave 0.95 g (1.8
mmol, 79%) of a dark purple, air-stable solid after vacuum drying.
1H NMR (acetone–d6): d 13.5 (br, 1H), 9.37 (s, 1H), 9.32 (d, 1H),
8.40 (d, 1H), 8.27 (m, 1H), 8.24 (m, 1H), 8.11 (m, 1H), 7.91 (m,
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parameter ratio ¼ 7.1, RðFoÞ ¼ 0:037, RwðFoÞ ¼ 0:034, GOF ¼ 1.88,
ꢀ3
ꢀ
max and min peaks in final DF ¼ 1:27 and )1.18e A
.
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