Group 13 and lanthanide complexes with mixed O,S anionic ligands derived from maltol

Article abstract of DOI:10.1021/ic048693y

Four mixed O,S binding ligand precursors derived from maltol (3-hydroxy-2-methyl-4-pyrone) have been chelated to gallium(III), indium(III); and lanthanide(III) ions to yield a series of metal complexes. The four ligand precursors include two pyranthiones, 3-hydroxy-2-methyl-4-pyranthione, commonly known as thiomaltol (Htma), and 2-ethyl-3-hydroxy-4-pyranthione, commonly known as ethylthiomaltol (Hetma), and two pyridinethiones, 3-hydroxy-2-methyl-4(H)- pyridinethione (Hmppt) and 3-hydroxy-1,2-dimethyl-4-pyridinethione (Hdppt). Dimeric forms of the pyridinethiones, Hmppt dimer and Hdppt dimer, were also isolated and characterized. Complete characterization of the monomeric organic compounds is reported including acidity constants and crystal structures of Htma, Hetma, and Hdppt dimer. Reacting the four monomeric ligand precursors with Ga³⁺ and In³⁺ ions yielded new tris(bidentate ligand) complexes. X-ray-quality crystals of the fac isomer of Ga(tma)₃ were also obtained. New complexes with a range of lanthanides (Ln³⁺) were also synthesized with the two pyranthiones, Htma and Hetma. The synthesis reactions yielded complexes of the type LnL₃·xH₂O and LnL₂(OH)·xH₂O, as indicated by elemental analysis and spectroscopic evidence such as mass spectral data and IR and NMR spectra.

Full text of DOI:10.1021/ic048693y

Inorg. Chem. 2005, 44, 2666−2677
Group 13 and Lanthanide Complexes with Mixed O,S Anionic Ligands
Derived from Maltol
Vishakha Monga, Brian O. Patrick, and Chris Orvig*
Medicinal Inorganic Chemistry Group, Department of Chemistry, UniVersity of British Columbia,
2036 Main Mall, VancouVer, British Columbia V6T 1Z1, Canada
Received September 17, 2004
Four mixed O,S binding ligand precursors derived from maltol (3-hydroxy-2-methyl-4-pyrone) have been chelated
to gallium(III), indium(III), and lanthanide(III) ions to yield a series of metal complexes. The four ligand precursors
include two pyranthiones, 3-hydroxy-2-methyl-4-pyranthione, commonly known as thiomaltol (Htma), and 2-ethyl-
3-hydroxy-4-pyranthione, commonly known as ethylthiomaltol (Hetma), and two pyridinethiones, 3-hydroxy-2-methyl-
4(H)-pyridinethione (Hmppt) and 3-hydroxy-1,2-dimethyl-4-pyridinethione (Hdppt). Dimeric forms of the pyridinethiones,
Hmppt dimer and Hdppt dimer, were also isolated and characterized. Complete characterization of the monomeric
organic compounds is reported including acidity constants and crystal structures of Htma, Hetma, and Hdppt dimer.
Reacting the four monomeric ligand precursors with Ga3+ and In3+ ions yielded new tris(bidentate ligand) complexes.
X-ray-quality crystals of the fac isomer of Ga(tma)3 were also obtained. New complexes with a range of lanthanides
(Ln3+) were also synthesized with the two pyranthiones, Htma and Hetma. The synthesis reactions yielded complexes
of the type LnL3‚xH2O and LnL2(OH)‚xH2O, as indicated by elemental analysis and spectroscopic evidence such
as mass spectral data and IR and NMR spectra.
Introduction
a primary driving force for the development of the coordina-
tion chemistry of these elements. Group 13 and lanthanide
complexes have shown numerous biochemical responses in
vivo (vide infra). It has been postulated that biological effects
are seen because these metal ions have chemical and physical
properties (e.g., ionic radii) similar to those of metal ions
found naturally in biological systems, elements such as Fe3+,
Ca2+, and Na+. A major requirement for coordination
complexes to be used as radiopharmaceuticals is in vivo
thermodynamic stability: the metal-ligand binding should
be stable enough to resist hydrolysis of the metal ion and
competition from endogenous biomolecules that can behave
as ligands, such as the plasma protein transferrin. Because
of their myriad of potential biological applications, our group
has studied the coordination chemistry of group 13 and
lanthanide metal ions extensively over the years.7-11 Two
classes of bidentate O,O (pro)ligands that have been studied
Many gallium(III), indium(III), and lanthanide(III) com-
plexes are widely used as pharmaceutical agents for their
diagnostic or therapeutic (chemotherapeutic and radiothera-
peutic) properties.1-4 Whereas the distinctive magnetic
properties of lanthanide(III) ions (Gd3+, Ho3+, Eu3+) are used
for MRI (magnetic resonance imaging),3,5 emissions from
the radioactive isotopes of Ga, In, and Ln can be utilized
for SPECT (single photon emission computed tomography)1
and PET (positron emission tomography).6 These imaging
techniques have been successfully used to diagnose cancer,
infection, thrombosis, and kidney and liver abnormalities,
as well as many other cardiac and neurological disorders.1
The radioactive isotopes of group 13 metal ions are
becoming indispensable in nuclear medicine and are currently
* To whom correspondence should be addressed. Tel.: 604-822-4449.
Fax: 604-822-2847. E-mail: orvig@chem.ubc.ca.
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1999, 99, 2293. (b) Lauffer, R. B. Chem. ReV. 1987, 87, 901.
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2666 Inorganic Chemistry, Vol. 44, No. 8, 2005
10.1021/ic048693y CCC: $30.25
© 2005 American Chemical Society
Published on Web 03/24/2005

Complexes with Mixed O,S Ligands DeriWed from Maltol
extensively in our group are 3-hydroxy-4-pyrones and
3-hydroxy-4-pyridinones.7,12-15 The synthesis, solution stud-
ies, and biodistribution of group 13 complexes with 3-hy-
droxy-4-pyrone and 3-hydroxy-2-methyl-4-pyridinone de-
rivatives (Hma,12 Hmpp,16,17 and Hdpp16,17) were reported
previously by our group. For the pyridinone ligands, it was
found that the resulting complexes were stable to hydrolysis
and the overall formation constants were much higher than
those of Ga-transferrin, a commonly used radiopharmaceu-
tical for SPECT (log â3 ≈ 3617 vs log K1 ≈ 2018 for Ga-
transferrin). The efficacies of the 67Ga-labeled complexes
were evaluated in vitro and in vivo and were compared to
that for 67Ga-citrate.19 It was found that these complexes were
stable in vivo (in rabbits and dogs) and accumulated in the
heart.19 At present, Ga(ma)3 is being revisited and evaluated
as an orally active agent for the treatment of several cancers.20
Lanthanide complexes with Hma and Hdpp have also been
reported by a few research groups.21-23
lanthanide complexes with sulfur-containing ligands have
been evaluated for potential in radiotherapy. 166Holmium
(which undergoes â- decay to 166Er) was chelated to N,N′-
ethylenedi-L-cysteine (EC, a N2S2 donor set); the 166Ho-
labeled complex showed the desired rapid renal clearance
when used for post-angioplasty intravascular radiotherapy
(IVRT),26 thus promoting the application of other such
complexes in the future.
Herein, four compounds, two pyranthiones, Htma and
Hetma, and two pyridinethiones, Hmppt and Hdppt, are
reported as potential monoanionic bidentate mixed hard-
soft donors with an O,S binding moiety. For all four thio
compounds, the ligands bind through a deprotonated hy-
droxyl oxygen, O- and a neutral thiocarbonyl group, CdS.
Because O- is considered to be a hard donor, it was expected
to bind strongly to the harder metal ions (e.g., Ga3+, Lu3+),
whereas binding to the softer S (from CdS) should be
favored by the softer metal ions (e.g., In3+, La3+). Thiomaltol,
one of the ligands studied in this work, is the thio analogue
of maltol, an approved food additive in the U.K., U.S., and
Canada. The coordination chemistry of the maltolato ligand
has been explored thoroughly with vanadium,8 group 13,12
lanthanide, and many other metal ions.27-29 Comparing the
well-documented work on maltolato and other similar (pro)-
ligands (ethylmaltol and 3-hydroxy-4-pyridinones) with that
on the thio analogues should provide insight concerning the
differences in chemical and biological properties of the
resultant metal complexes. The thiomaltolato anion has been
chelated previously to various metal ions (Cu, Hg, Ni, Fe,
Mo, In, V, Pb, Sn, Cd, Co) to explore the coordination and
extraction ability of this ligand.30-33 Recently, there seems
to be renewed interest in this compound as a ligand because
of its trans-influencing ability.31 A series of pyridinethiones
was also patented recently for therapeutic antioxidant proper-
ties34 and effectiveness against carcinoma.35 When coordi-
nated to M3+ ions in a 1:3 ratio (M3+/L-), the resulting
complexes with these ligands are expected to be low in
molecular weight (∼500-640), neutral and six coordinate
with an O3S3 coordination sphere, all properties important
The chemical and physical properties of elements, how-
ever, vary down a group and across a period. Therefore, for
this study, ligands for Ga, In, and Ln metal ions were
designed keeping in mind the general trend in the chemical
properties of these metal ions (vide infra). For the group 13
metal ions, hardness (i.e., affinity for hard donors such as O
and N) decreases down the group (e.g., Ga3+ is harder than
In3+), whereas hardness increases with decreasing ionic size
across the series for the lanthanides (e.g., Lu3+ is harder than
La3+). Considering similar conditions, several researchers
have synthesized numerous Ga(III) and In(III) complexes
with bidentate and multidentate sulfur-containing ligands and
tested the radio-labeled complexes for in vivo stability and
biodistribution as imaging agents.24,25 Despite the fact that
lanthanides are considered to be hard metal ions, some
(12) Finnegan, M. M.; Lutz, T. G.; Nelson, W. O.; Smith, A.; Orvig, C.
Inorg. Chem. 1987, 26, 2171.
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S.; Yuen, V. G.; Orvig, C. J. Am. Chem. Soc. 1995, 117, 12759.
(14) Zhou, Y. M.Sc. Thesis, University of British Columbia, Vancouver,
BC, Canada, 1993; p 8.
(15) Nelson, W. O.; Karpishin, T. B.; Rettig, S. J.; Orvig, C. Can. J. Chem.
1988, 66, 123.
(16) Nelson, W. O.; Karpishin, T. B.; Rettig, S. J.; Orvig, C. Inorg. Chem.
1988, 27, 1045.
(17) Clevette, D. J.; Lyster, D. M.; Nelson, W. O.; Rihela, T.; Webb, G.
A.; Orvig, C. Inorg. Chem. 1990, 29, 667.
(18) Harris, W. R.; Pecoraro, V. L. Biochemistry 1983, 22, 292.
(19) Zhang, Z.; Lyster, D. M.; Webb, G. A.; Orvig, C. Nucl. Med. Biol.
1992, 19, 327.
(26) Chakraborty, S.; Unni, P. R.; Banerjee, S.; Samuel, G.; Das, T.; Sarma,
H. D.; Ramamoorthy, N.; Pillai, M. R. A. Nucl. Med. Biol. 2001, 28,
309.
(27) Thompson, K. H.; Chiles, J.; Yuen, V. G.; Tse, J.; McNeill, J. H.;
Orvig, C. J. Inorg. Biochem. 2004, 98, 683.
(28) Wu, A.; Kennedy, D. C.; Patrick, B. O.; James, B. R. Inorg. Chem.
2003, 42, 7579.
(29) Fryzuk, M. D.; Jonker, M. J.; Rettig, S. J. Chem. Commun. 1997, 4,
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(20) http://www.titanpharm.com/products/gm_product.html (accessed Jan
2004).
(21) Dutt, N. K.; Sharma, U. U. M. J. Inorg. Nucl. Chem. 1975, 37, 1801.
(22) Ahmed, S.; Burgess, J.; Hurman, B.; Parsons, S. A. Polyhedron 1994,
13, 23.
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(31) Lewis, J. A.; Puerta, D. T.; Cohen, S. M. Inorg. Chem. 2003, 42,
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(32) Uhlemann, E.; Petzold, W.; Raab, M. Z. Anorg. Allg. Chem. 1984,
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(23) Fregona, D.; Faraglia, G.; Graziani, R.; Casellato, U.; Sitran, S. Gazz.
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Inorganic Chemistry, Vol. 44, No. 8, 2005 2667

Monga et al.
under Ar. Yields are reported for the analytically pure compounds
and were calculated on the basis of the respective metal starting
material for the metal complexes.
for demonstrating higher thermodynamic stability in vivo.
The five-membered chelate ring that forms as a result of the
coordination of O- and S to the metal ion should also add
to the stability of the complex.
This work describes in detail the synthesis and character-
ization (including the acidity constants) of thiomaltol; its
close analogue, ethylthiomaltol (thio analogue of another
approved food additive, ethylmaltol); and two 3-hydroxy-
4-pyridinethiones, Hmppt and Hdppt.
Instrumentation. Infrared spectra were recorded as KBr disks
in the range of 4000-500 cm-1 on a Galaxy Series 5000 FTIR
spectrometer and were standardized to polystyrene. Mass spectra
were obtained with a Kratos MS 50 (electron-impact ionization mass
spectrometry, EIMS), a Kratos Concept II H32Q (Cs+ liquid
secondary ion mass spectrometry, LSIMS), or a Micromass LCT
or Bruker Esquire Ion Trap (electrospray ionization mass spec-
trometry, ESIMS) spectrometer. Mr. Peter Borda or Mr. Minaz
Lakha in the UBC chemistry department performed the elemental
analyses for C, H, N, and S. 1H and 13C{1H} NMR spectra,
respectively, were recorded on Bruker AC-200E (200, 50.2 MHz)
or Bruker AV300 (300, 75.5 MHz) NMR spectrometer with δ
referenced to the deuterated solvent. For the 2D HMQC and HMBC
NMR experiments, a Bruker AV400 spectrometer at 400 MHz was
used. Melting points were measured using a Gallenkamp melting
point apparatus and are corrected. UV-visible spectra were
recorded using a Hewlett-Packard 8543 diode array spectropho-
tometer.
Synthesis of the Ligand Precursors. Please refer to the
Supporting Information for detailed characterization of the organic
compounds.
The coordination chemistry of these ligand precursors with
group 13 metal ions gallium and indium has also been
explored in detail and compared to that of their oxygen
analogues. Some new lanthanide (Ln ) La, Pr, Nd, Sm, Gd,
Ho, Yb, and Lu) complexes with the two pyranthiones, Htma
and Hetma, are also reported. It was expected that the
bidentate mixed O,S donor ligands would favor both types
of metal ions (hard and soft) and might be an ideal choice
for synthesizing strong metal complexes down the group
(group 13) or across the (lanthanide) series. Furthermore,
the influence of O to S substitution on the binding strength
of these diverse trivalent metal ions can be examined.
Another unique feature of these ligands is the covalent
coordinate bond to the thiocarbonyl group (CdS); most of
the other sulfur-containing ligands tested as imaging agents
were chelated through a deprotonated thiol, S-. It is
noteworthy that the sulfur atom in a thiolate group has a
tendency to bridge metal centers and form polymeric
species.36 With a thiocarbonyl binding moiety (instead of a
thiolate group), isolation of the desirable monomeric species
is more probable. Furthermore, it was of interest to examine
the difference in binding properties when chelating a neutral
CdS moiety instead of the anionic S-.
(i) Thiomaltol, 3-Hydroxy-2-methyl-4-pyranthione, Htma.
Phosphorus pentasulfide (P2S5, 8.94 g, 0.04 mol) was added slowly
to a solution of maltol (25.38 g, 0.202 mol) in 1,4-dioxane (200
mL) at 45 °C. The resulting solution was heated with stirring for
3 h, cooled to room temperature, and clarified by filtration. The
filtrant was washed with 10 mL of 1,4-dioxane. The filtrate was
added to water (500 mL), and a yellow crystalline product instantly
precipitated. The water/dioxane mixture (including the precipitate)
was placed in the refrigerator (4 °C, 2 h) to maximize precipitation
of the product, which was filtered out to yield the pure yellow
crystalline product (13.30 g, 0.094 mol, 46% yield) that had a
pungent smell. Slow evaporation of a hexane solution of Htma gave
X-ray-quality crystals. Anal. Calcd (found) for C6H6O2S: C, 50.69
(50.58); H, 4.25 (4.27); S, 22.55 (22.74).
(ii) Ethylthiomaltol, 2-Ethyl-3-hydroxy-4-pyranthione,
Hetma. Ethylthiomaltol was prepared in a manner similar to that
used for thiomaltol. P2S5 (3.69 g, 0.017 mol) was added slowly to
a solution of ethylmaltol (11.63 g, 0.083 mol) in 1,4-dioxane (25
mL) at 45 °C. The resulting solution was heated with stirring
overnight, cooled to room temperature, and clarified by filtration.
The filtrate was washed with 10 mL of 1,4-dioxane and the filtrant
(solvent) was evaporated to yield a brown viscous oily layer.
Column purification (silica gel, 230-400 mesh, 7:3 ethyl acetate/
hexane solvent) of the reaction mixture yielded a viscous brown
oil (7.23 g, 0.046 mol, 56% yield) that had a distinct smell. Placing
the brown oil in a flask on dry ice and slowly warming the flask to
room temperature gave yellow X-ray-quality crystals of Hetma. It
was noted that the compound decomposes in air (on melting) after
a few days; hence, it was always stored under Ar or vacuum. Anal.
Calcd (found) for C7H8O2S: C, 53.83 (53.78); H, 5.16 (5.24); S,
20.53 (20.60).
Experimental Section
Materials and Methods. All chemicals [P2S5, NaCl, NaOH, 40%
aqueous MeNH2, NH4OH, HCl, DTT (dithiothreitol), Ga(NO3)3‚
9H2O, Ga(ClO4)3‚6H2O, In(NO3)3‚3H2O, In(ClO4)3‚6H2O, Ln-
(ClO4)3‚6H2O, NaOH, NEt3] were obtained from Sigma-Aldrich
or Fisher Scientific and were used as received without further
purification. Maltol and ethylmaltol were obtained from Pfizer or
Cultor Food Science, potassium biphthalate (KHP) from Anachemia,
and silica gel 230-400 mesh from Silicycle. All solvents were
reagent-grade and were obtained from Fisher Scientific. Water was
deionized (Barnstead D8902 and D8904 Cartridges) and distilled
(Hytrex II GX50-9-7/8 and GX100-9-7/8 filter cartridges) before
use. All synthetic reactions were carried out in degassed solvents
(iii) 3-Hydroxy-2-methyl-4(H)-pyridinethione, Hmppt. Thio-
maltol (3.54 g, 0.025 mol) was dissolved in degassed water (330
mL) at 78 °C. As the solution was slowly cooled to 34 °C, degassed
ethanol (150 mL) was added to redissolve the thiomaltol that
precipitated as the temperature was decreased. At this point,
concentrated NH4OH (8.4 mL, 0.126 mol) was added dropwise to
the solution. After the solution (pH ≈ 9) had been stirred for 36 h
at 34 °C, it was concentrated in vacuo and cooled at 4 °C overnight.
(36) Rodr´ıguez, A.; Romero, J.; Garc´ıa-Va´zquez, J. A.; Sousa, A.; Zubieta,
J.; Rose, D. J.; Maresca, K. Inorg. Chim. Acta 1998, 281, 70.
2668 Inorganic Chemistry, Vol. 44, No. 8, 2005

Complexes with Mixed O,S Ligands DeriWed from Maltol
Pale brown needles of the pure product were collected by filtering
the solution (1.51 g, 43%). Anal. Calcd (found) for C6H7NOS: C,
51.04 (50.85); H, 5.00 (4.95); N, 9.92 (9.72); S, 22.71 (22.66).
(+ESIMS): m/z 532 ([KGaL3]+), 494 ([HGaL3]+), 352 ([GaL2]+).
IR (cm-1, (4 cm-1): 1578, 1499 (ring vibrations), 820 (νC-O-C),
664 (νGa-O).
(ii) Tris(thiomaltolato)indium(III), In(tma)3. In(tma)3 was
synthesized according to the same method as employed for
Ga(tma)3, except that the molar ratio of In3+ to L- was 1:3 instead
of 1:6, using In(NO3)3‚3H2O (0.66 g, 1.86 mmol) and Htma (0.7925
g, 5.58 mmol) to yield 0.69 g, 69%. Anal. Calcd (found) for
C18H15InO6S3: C, 40.16 (40.36); H, 2.81 (2.79); S, 17.87 (17.90).
(iv) 3-Hydroxy-1,2-dimethyl-4-pyridinethione, Hdppt. To a
solution of thiomaltol (1.97 g, 0.014 mol) in degassed water (150
mL) at 78 °C was added 40% methylamine (11.9 mL, 0.14 mol),
dropwise, over a period of 2 h. The resulting deep red solution
was heated for 12 h. Concentration in vacuo and cooling at 4 °C
overnight gave 1.49 g of a mixture of mostly Hdppt and some Hdppt
dimer. Column purification (silica gel, 230-400 mesh, 3:1 ethyl
acetate/methanol solvent) of this mixture yielded pale yellowish-
brown fine needles of the pure compound (1.00 g, 47%). Anal.
Calcd (found) for C7H9NOS: C, 54.17 (54.25); H, 5.84 (5.70); N,
9.02 (9.02); S, 20.66 (20.60).
2
1H NMR (300 MHz, DMSO-d6): δ 8.18 (1H, d, J ) 4.26 Hz,
OCH), 7.56 [1H, d, 2J ) 4.23 Hz, HCC(S)], 2.48 (3H, s, CH3). 13
C
NMR (75.5 MHz, DMSO-d6): δ 173.70 (CdS), 156.84 (C-O-
In), 153.66 (C-CH3), 146.60 (OCH), 122.01 [HCC(S)], 16.33
(CH3). MS (+ESIMS): m/z 538 ([InL3]+), 397 ([InL2]+). IR (cm-1
,
(4 cm-1): 1570, 1482 (ring vibrations), 822 (νC-O-C), 658 (νIn-O).
(iii) Tris(ethylthiomaltolato)gallium(III), Ga(etma)3.
Ga(NO3)3‚9H2O (0.0441 g, 0.106 mmol) was added to a solution
of Hetma (0.0988 g, 0.63 mmol) in 10 mL of degassed water and
5 mL of MeOH under Ar at 70 °C. As the pH of the solution was
raised slowly (over 10 min) with 1 M NaOH (0.2 mL, 0.2 mmol)
to ∼7.5, a yellow solid precipitated from the solution. The resulting
reaction mixture was refluxed for 3 h and then cooled to room
temperature. A yellow precipitate was collected by vacuum
filtration, washed twice with 5 mL of cold water, and dried
overnight in vacuo to yield 0.05 g, 90%. Anal. Calcd (found) for
(v) 4,4′-Dithiobis(2-methylpyridinium-3-oxide), Hmppt Dimer.
The synthesis of Hmppt dimer was done in a fashion analogous to
that for Hmppt except that the temperature was maintained at 78
°C for the duration of the reaction (36 h) and no ethanol was added
to the reaction mixture. To verify the disulfide bond in the dimer,
an NMR experiment was done with a reducing agent, dithiothreitol
(DTT). DTT (0.011 g, 0.071 mmol) was added to a solution of
Hmppt dimer (0.0047 g, 0.017 mmol) in DMSO-d6. The 1H NMR
spectrum of the solution was taken before and after addition of
DTT to monitor the reduction. Hmppt dimer was immediately
reduced to its monomeric form, Hmppt. Anal. Calcd (found) for
C12H12N2O2S2‚2H2O: C, 45.56 (45.68); H, 5.10 (5.10); N, 8.85
(8.97).
1
C21H21GaO6S3: C, 47.12 (47.31); H, 3.95 (3.94). H NMR (300
2
MHz, DMSO-d6): δ 8.17 (1H, d, J ) 4.63 Hz, OCH), 7.46 [1H,
2
2
d, J ) 4.62 Hz, HCC(S)], 2.79 (2H, q, J ) 7.51 Hz, CH2), 1.11
(3H, t, J ) 7.51 Hz, CH3). 13C NMR (75.5 MHz, DMSO-d6): δ
2
176.26 (CdS), 154.97 (C-O-Ga), 154.80 (C-C2H5), 147.54
(OCH), 120.43 [HCC(S)], 22.67 (CH2), 10.35 (CH3). MS
(+ESIMS): m/z 558 ([NaGaL3]+), 380 ([GaL2]+). IR (cm-1, (4
cm-1): 1572, 1497 (ring vibrations), 822 (νC-O-C), 662 (νGa-O).
(iv) Tris(ethylthiomaltolato)indium(III), In(etma)3. In(etma)3
was synthesized according to the same method as employed for
Ga(etma)3, except that the molar ratio of In3+ to L- was 1:3 instead
of 1:6, using In(NO3)3‚3H2O (0.167 g, 0.47 mmol) and Hetma
(0.2208 g, 1.42 mmol) to yield 0.17 g, 61%. Anal. Calcd (found)
for C21H21InO6S3‚0.5H2O: C, 42.79 (42.53); H, 3.76 (3.47); S, 16.32
(16.69). 1H NMR (300 MHz, DMSO-d6): δ 8.21 (1H, d, 2J ) 4.62
Hz, OCH), 7.57 [1H, d, 2J ) 4.62 Hz, HCC(S)], 2.86 (2H, q, 2J )
4,4′-Dithiobis(1,2-dimethylpyridinium-3-oxide), Hdppt Dimer.
A small amount of Hdppt dimer always formed with Hdppt and
was isolated by column purification as described for Hdppt. To
verify the disulfide bond in the dimer, an NMR experiment was
done with dithiothreitol (DTT). DTT (0.01 g, 0.065 mmol) was
added to a solution of Hdppt dimer (0.0049 g, 0.016 mmol) in
DMSO-d6. The 1H NMR spectrum of the solution was taken before
and after addition of DTT to monitor the reduction. Hdppt dimer
was immediately reduced to its monomeric form, Hdppt. Anal.
Calcd (found) for C14H16N2O2S2‚2H2O: C, 48.82 (49.11); H, 5.85
(5.94); N, 8.13 (8.23).
2
7.50 Hz, CH2), 1.13 (3H, t, J ) 7.50 Hz, CH3). 13C NMR (75.5
MHz, DMSO-d6): δ 174.15 (CdS), 157.43 (C-C2H5), 156.40 (C-
O-In), 146.64 (OCH), 122.01 [HCC(S)], 22.93 (CH2), 10.22 (CH3).
MS (+ESIMS): m/z 1005 ([In2L5]+), 603 ([NaInL3]+), 425
([InL2]+), 270 ([InL]+). IR (cm-1, (4 cm-1): 1573, 1487 (ring
vibrations), 824 (νC-O-C), 657 (νIn-O).
Note: Metal perchlorate salts should be handled with care.
Synthesis of the Gallium(III) and Indium(III) Complexes. (i)
Tris(thiomaltolato)gallium(III), Ga(tma)3. Ga(NO3)3‚9H2O (0.0564
g, 0.135 mmol) was added to a solution of Htma (0.116 g, 0.82
mmol) in 10 mL of degassed water under Ar at 75 °C. As the pH
of the solution was raised slowly (over 10 min) with 1 M NaOH
(0.4 mL, 0.4 mmol) to ∼7.5, a yellow solid precipitated from the
solution. The resulting reaction mixture was refluxed for 3 h and
then cooled to room temperature. A yellow precipitate was collected
by vacuum filtration, washed twice with 5 mL of cold water, and
dried overnight in vacuo to yield 0.06 g, 90%. Anal. Calcd (found)
for C18H15GaO6S3: C, 43.83 (43.50); H, 3.07 (2.91). 1H NMR (300
(v)
Tris(2-methyl-3-oxy-4-pyridinethionato)gallium(III),
Ga(mppt)3. Ga(ClO4)3‚6H2O (0.0755 g, 0.16 mmol) was added to
a solution of Hmppt (0.0693 g, 0.49 mmol) in 10 mL of dry,
degassed MeCN under Ar at room temperature. As the pH of the
solution was raised slowly (over 10 min) with NEt3 (0.075 mL,
0.54 mmol), a beige solid precipitated from the solution. The
resulting reaction mixture was stirred for 3 h and then vacuum
filtered. The beige product was washed twice with 5 mL of cold
MeCN and dried overnight in vacuo to yield 0.054 g, 68%. Anal.
Calcd (found) for C18H18GaN3O3S3‚2H2O: C, 41.08 (41.22); H,
1
4.21 (4.30); N, 7.98 (7.56). H NMR (300 MHz, DMSO-d6): δ
2
MHz, DMSO-d6): δ 8.15 (1H, d, J ) 4.64 Hz, OCH), 7.45 [1H,
7.51 (1H, br, OCH), 7.39 [1H, br, HCC(S)], 2.38 (3H, s, CH3). 13
C
d, 2J ) 4.65 Hz, HCC(S)], 2.41 (3H, s, CH3). 13C NMR (75.5 MHz,
NMR (75.5 MHz, DMSO-d6): δ 121.92 [HCC(S)], 15.83 (CH3)
(see Results and Discussion for explanation of the other 13C signals).
MS (+ESIMS): m/z 529 ([KGaL3]+), 350 ([GaL2]+), 210 ([GaL]+).
DMSO-d6):
δ 175.73 (CdS), 155.41 (C-O-Ga), 150.88
(C-CH3), 147.52 (OCH), 120.45 [HCC(S)], 15.94 (CH3). MS
Inorganic Chemistry, Vol. 44, No. 8, 2005 2669

Monga et al.
IR (cm-1, (4 cm-1): 2919 (νN-H), 1590, 1458, 1316, 1228 (ring
and C-N-C vibrations), 607 (νGa-O).
of cold EtOH and dried overnight in vacuo to yield 0.060 g, 53%.
IR (cm-1, (4 cm-1): 1580, 1497, 824 (ring vibrations and νC-O-C),
662 (νPr-O).
(vi)
Tris(2-methyl-3-oxy-4-pyridinethionato)indium(III),
Potentiometric Equilibrium Measurements. Potentiometric
measurements were carried out with an automatic titration system
consisting of a Metrohm 713 pH meter equipped with a Metrohm
6.0123.100 pH glass electrode, a Metrohm 6.0726.100 silver
chloride reference electrode, a model 665 Metrohm Dosimat
autoburet, and water-jacketed 50-mL titration vessels maintained
at 25.0 ( 0.1 °C with a Julabo UC circulating bath. Both the pH
meter and the autoburet were controlled by an IBM-compatible PC
so that the titrations were automated. The titrations were controlled
by a locally written Qbasic program. Ar, passed through water and
2 M NaOH (to exclude CO2), was bubbled through the solutions
during the titrations.
All solutions were prepared with freshly boiled, deionized,
distilled water that was purged with Ar while being boiled and
cooled to ensure removal of CO2. The electrodes were calibrated
before each titration by titrating a known amount of aqueous HCl
with a known amount of NaOH. A plot of current (mV, calculated)
vs pH gave a working slope and intercept so that the pH could be
read as -log [H+] directly. The ionic strength (I) of the titration
solution was kept constant with 0.16 M NaCl. The value of pKw
used at I ) 0.16 M NaCl and T ) 25 °C was 13.76.37 The NaOH
solution was standardized potentiometrically with potassium
biphthalate.
Acidity constants for the compounds were determined by titrating
50 mL of aqueous 3.0 mM HCl (I ) 0.16 M NaCl, T ) 25 °C) in
the presence of 0.5-0.7 mM solution of the desired compound (HL)
under Ar with ∼1-1.5 mL of ∼0.1 M NaOH. The constants were
calculated using the data within the range ∼2.5 e pH e 10.5 on
an IBM-compatible computer containing a Pentium II processor
using a curve-fit procedure (a Newton-Gauss nonlinear least-
squares program). The final values for the acidity constants for all
compounds were obtained from the average of 10 independent
titrations.
X-ray Crystallographic Analyses. The X-ray crystal structures
were determined by mounting the crystals on a glass fiber. The
measurements were made on a Rigaku/ADSC CCD area detector
with graphite-monochromated Mo KR radiation. All data were
processed and corrected for Lorentz and polarization effects and
absorption using the d*TREK program38 or the multiscan technique
(SADABS).39 The structures were solved by direct methods40 and
expanded using Fourier techniques.41 Final refinements were carried
out using teXsan.42
In(mppt)3. In(mppt)3 was synthesized according to the same method
as employed for Ga(mppt)3 using In(ClO4)3‚6H2O (0.077 g, 0.14
mmol) and Hmppt (0.0586 g, 0.42 mmol) to yield 0.036 g, 50%.
Anal. Calcd (found) for C21H21InN3O3S3‚3H2O: C, 36.68 (36.63);
H, 4.10 (3.92); N, 7.13 (6.99). 1H NMR (300 MHz, DMSO-d6): δ
7.35 (1H, br, OCH), 7.23 (1H, br, HCC(S)), 2.25 (3H, s, CH3). 13
C
NMR (75.5 MHz, DMSO-d6): δ 159.75 (C-O-In), 158.99 (Cd
S), 130.75 (C-CH3), 123.50 (N-CH), 121.23 [HCC(S)], 15.27
(CH3). MS (+ESIMS): m/z 558 ([NaInL3]+), 536 ([HInL3]+), 395
([InL2]+). IR (cm-1, (4 cm-1): 2911 (νN-H), 1581, 1454, 1314,
1225 (ring and C-N-C vibrations), 594 (νIn-O).
(vii) Tris(1,2-dimethyl-3-oxy-4-pyridinethionato)gallium(III),
Ga(dppt)3. Ga(dppt)3 was synthesized according to the same
method as employed for Ga(mppt)3 using Ga(ClO4)3‚6H2O (0.0484
g, 0.102 mmol) and Hdppt (0.0493 g, 0.32 mmol) to yield 0.032 g,
60%. Anal. Calcd (found) for C21H24GaN3O3S3: C, 47.38 (47.57);
H, 4.54 (4.32); N, 7.89 (8.29). 1H NMR (300 MHz, DMSO-d6): δ
7.36 (1H, d, 2J ) 6.54 Hz, OCH), 7.09 [1H, d, 2J ) 6.54 Hz, HCC-
(S)], 3.81 (3H, s, N-CH3), 2.36 (3H, s, CH3). 13C NMR (75.5 MHz,
DMSO-d6): δ 159.92 (CdS), 158.94 (C-O-Ga), 131.08 (C-
CH3), 127.43 (N-CH), 120.98 [HCC(S)], 43.28 (N-CH3), 13.20
(CH3). MS (+ESIMS): m/z 378 ([GaL2]+), 224 ([GaL]+). IR (cm-1
,
(4 cm-1): 1584, 1294 (ring and C-N-C vibrations), 529 (νGa-O).
(viii) Tris(1,2-dimethyl-3-oxy-4-pyridinethionato)indium(III),
In(dppt)3. In(dppt)3 was synthesized according to the same method
as employed for Ga(mppt)3 using In(ClO4)3‚6H2O (0.0454 g, 0.082
mmol) and Hdppt (0.0383 g, 0.25 mmol) to yield 0.030 g, 64%.
Anal. Calcd (found) for C21H24InN3O3S3: C, 43.68 (43.28); H, 4.19
1
(4.16); N, 7.28 (7.39). H NMR (300 MHz, DMSO-d6): δ 7.37
(1H, d, 2J ) 6.47 Hz, OCH), 7.21 [1H, d, 2J ) 6.32 Hz, HCC(S)],
3.84 (3H, s, N-CH3), 2.41 (3H, s, CH3). 13C NMR (75.5 MHz,
DMSO-d6): δ 160.15 (C-O-In), 157.13 (CdS), 133.42 (C-CH3),
126.47 (N-CH), 122.76 [HCC(S)], 43.59 (N-CH3), 13.36 (CH3).
MS (+ESIMS): m/z 1000 ([In2L5]+), 600 ([NaInL3]+), 423
([InL2]+), 270 ([HInL]+). IR (cm-1, (4 cm-1): 1584, 1310 (ring
and C-N-C vibrations), 521 (νIn-O).
General Procedure for the Synthesis of the Ln(III)-
Pyranthione Complexes. (See Results and Discussion and Sup-
porting Information for the detailed characterization of all of the
Ln(III) complexes.)
(i) Tris(thiomaltolato)lanthanum(III), La(tma)3. La(ClO4)3‚
6H2O (0.0655 g, 0.13 mmol) was added to a solution of Htma
(0.112 g, 0.79 mmol) in 10 mL of EtOH under Ar at room
temperature. As the pH of the solution was raised slowly (over 20
min) with 6 M NaOH (0.06 mL, 0.36 mmol), a yellow solid
precipitated from the solution. The resulting reaction mixture was
stirred for 3 h and then vacuum filtered to collect the product. The
yellow precipitate was washed with 5 mL of cold EtOH and dried
overnight in vacuo to yield 0.065 g, 83%. IR (cm-1, (4 cm-1):
1586, 1503, 824 (ring vibrations and νC-O-C), 658 (νLa-O).
ORTEP diagrams from the X-ray structures of Htma and Hetma,
plus Hdppt dimer are shown in Figures S1 and 1, respectively. For
Ga(tma)3, the material crystallized as the fac isomer. For complete
details of the X-ray crystallographic analyses, please refer to the
Supporting Information.
(37) Martell, A. E.; Motekaitis, R. J. Determination and Use of Stability
Constants, VCH: New York, 1988; p 4.
(38) d*TREK: Area Detector Software, version 7.11; Molecular Structure
Corporation: The Woodlands, TX, 2001.
(39) SADABS: Bruker Nonius Area Detector Scaling and Absorption
Correction, version 2.05; Bruker AXS Inc.: Madison, WI.
(40) SIR97: Altomare, A.; Burla, M. C.; Cammali, G.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, A. J. Appl. Crystallogr. 1999, 32, 115.
(41) DIRDIF94: Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman,
W. P.; de Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF-94
Program System; Technical Report of the Crystallography Laboratory;
University of Nijmegen: Nijmegen, The Netherlands, 1994.
(42) TeXsan: Crystal Structure Analysis Package; Molecular Structure
Corporation: The Woodlands, TX, 1985 and 1992.
(ii)Hydroxobis(thiomaltolato)praseodymium(III),Pr(tma)2(OH).
Pr(ClO4)3‚6H2O (0.135 mL of 50% w/w solution, 0.12 mmol) was
added to a solution of Htma (0.1017 g, 0.72 mmol) in 10 mL of
EtOH under Ar at room temperature. As the pH of the solution
was raised slowly (over 20 min) with 6 M NaOH (0.1 mL, 0.6
mmol), a yellow solid precipitated from the solution. The resulting
reaction mixture was stirred for 3 h and then vacuum filtered to
collect the product. The yellow precipitate was washed with 5 mL
2670 Inorganic Chemistry, Vol. 44, No. 8, 2005

Complexes with Mixed O,S Ligands DeriWed from Maltol
The pyridinethiones Hdppt and Hmppt were prepared by
doing a ring-conversion reaction with Htma (Scheme 1), in
a fashion analogous to the synthesis of their oxygen
analogues (pyridinones) synthesized in our laboratory previ-
ously.45 The ring oxygen in Htma was replaced with N-H
or N-CH3 by reacting Htma with aqueous NH4OH or
MeNH2 to obtain Hmppt or Hdppt, respectively. For the
N-H pyridinethione, if the reaction mixture was left
refluxing at the higher temperature (80 °C), the major product
obtained was Hmppt dimer. For Hdppt, a small amount of
dimer always formed with the monomer despite varying
temperature and solvents. A different route to synthesize
Hdppt by reacting the pyridinone with Lawesson’s reagent
to convert the carbonyl group to the thiocarbonyl group was
reported recently.34 This synthesis, however, includes many
byproducts, requires multiple purification steps, and gives a
low yield (20%). The procedure reported herein is much
simpler. The reaction is a one-step process, followed by
simple column purification, and gives the product in a higher
yield (> 47%).
Figure 1. ORTEP diagram of Hdppt dimer (50% thermal ellipsoids).
Scheme 1. Synthesis of the Compoundsa
All of the organic compounds were characterized com-
pletely by EA, EIMS, mp, and IR and NMR spectroscopies
(see Supporting Information for complete details). The most
significant (expected) change in the IR spectrum is the
appearance of νCdS and disappearance of νCdO. Because of
substitution with a heavier atom (sulfur), νCdS appears at a
lower value (∼1176-1196 cm-1) as compared to νCdO
(∼1650 cm-1). This is in agreement with various other
reports for νCdS in pyranthiones and pyridinethiones.46 The
ν
CdO/νCdS ratio for maltol to thiomaltol was found to be
a Conditions: (a) excess P2S5, dioxane, 45 °C; (b) excess NH4OH, H2O/
EtOH, 34 °C, 36 h; (c) excess 40% MeNH2, H2O, 75 °C, 12 h.
∼1.40, in agreement with other similar studies for a
conjugated system.47,48 For the pyridinethiones, νCdS was the
most intense peak in the spectrum and was also confirmed
by its absence in the IR spectra of the pyridinethione dimers.
Results and Discussion
1
The most diagnostic features of the H NMR spectra in
The pyranthiones thiomaltol (Htma) and ethylthiomaltol
(Hetma) were prepared by reacting maltol/ethylmaltol with
P2S5 in a 1:2 ratio followed by recrystallization with hexanes
and column purification to obtain pure Htma and Hetma,
respectively. The syntheses were based on the work of
Blazevic et al.,43 but some of the conditions were modified
significantly (temperature; reaction time; volume of the
solvents; ratio of reagents, i.e., maltol/ethylmaltol to P2S5;
and recrystallization solvent) to improve the yields (Scheme
1). While this work was ongoing, Cohen and co-workers31
described the synthesis of Htma using P2S5 in combination
with hexamethyldisiloxane (HMDO) based on the work by
all four monomeric thio compounds were the expected AB
split doublets for the ring hydrogens (Ja,b ≈ 4-5 Hz for
pyranthiones, Ja,b ≈ 6.4-6.6 Hz for pyridinethiones), a
singlet for the methyl hydrogens on C-1 (or ethyl hydrogens
for Hetma), and a sharp singlet for the hydroxyl hydrogen.
As compared to those in their oxygen analogues, the ring
hydrogen peaks in pyranthiones and pyridinethiones shift
downfield (Table S1). The ring hydrogen next to CdS, Hb,
shows the most significant downfield shift (∆δ ≈ 1-1.2
ppm). The lack of intramolecular hydrogen bonding (due to
the replacement of CdS for CdO) is evident from the upfield
shift of the hydroxyl peak with respect to that in their oxygen
analogues (∆δ ≈ 0.6 ppm). With respect to their synthetic
precursor, Htma, the ring hydrogens in the pyridinethiones
shift upfield. This was expected because of the positive
Curphey.44 Although Cohen et al.’s procedure reports31
a
higher overall yield (70% vs 46% in this work) for Htma,
the synthesis described here is simple, faster (one-step), and
less expensive. There is a CAS registry number (84642-
58-0) for ethylthiomaltol (Hetma); however, this is the first
reported synthesis for this compound, to our knowledge.
(45) Nelson, W. O. Ph.D. Thesis, University of British Columbia, Van-
couver, BC, Canada, 1988; p 17.
(46) Spinner, E. J. Org. Chem. 1958, 23, 2037.
(43) Blazevic, K.; Jakopcic, K.; Hahn, V. Bull. Sci. Cons. Acad. Sci. (RSF
Yugosl.) 1969, 14, 1.
(44) Curphey, T. J. Tet. Lett. 2000, 41, 9963. (b) Curphey, T. J. J. Org.
Chem. 2002, 67, 6461. (c) Curphey, T. J. Tet. Lett. 2002, 43, 371.
(47) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 2nd ed.;
Chapman and Hall: London, 1980; Vol. 2, p 215.
(48) Luttringhaus, A.; Mecke, R.; Grohmann, J. Chem. Ges. Dtsch.
Demokrat Rep. 1956, 152.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2671

Monga et al.
Table 1. Acidity Constants for the Thio Compounds and Their Oxygen
Analogues (I ) 0.16 M NaCl, T ) 25 °C, [HL]a) 0.5-0.7 mM)
Table 2. Selected Bond Lengths (Å) and Angles (deg) in Hdppt Dimer
Bond Lengths (Å)
pKa1 of corresponding
oxygen analogues
S(1)-S(1*)
S(1)-C(1)
O(1)-C(2)
N(1)-C(3)
N(1)-C(4)
N(1)-C(7)
2.0472(7)
1.763(1)
1.278(2)
1.362(2)
1.351(2)
1.482(2)
C(1)-C(2)
C(1)-C(5)
C(2)-C(3)
C(4)-C(5)
C(3)-C(6)
1.424(2)
1.383(2)
1.427(2)
1.377(2)
1.489(2)
compd
pKa1
Htma
8.12 ( 0.02
8.16 ( 0.03
8.93 ( 0.02
9.44 ( 0.01
8.67 ( 0.03b
8.72 ( 0.03b
9.80 ( 0.01c
9.86 ( 0.03c
Hetma
Hmppt
Hdppt
Bond Angles (deg)
104.22(5)
a HL: general abbreviation for protonated ligand precursor; b Reference
S(1)-S(1*)-C(1)
S(1)-C(1)-C(2)
S(1)-C(1)-C(5)
C(3)-N(1)-C(4)
C(3)-N(1)-C(7)
C(4)-N(1)-C(7)
N(1)-C(3)-C(2)
N(1)-C(3)-C(6)
N(1)-C(4)-C(5)
O(1)-C(2)-C(1)
O(1)-C(2)-C(3)
C(2)-C(1)-C(5)
C(1)-C(2)-C(3)
C(1)-C(5)-C(4)
C(2)-C(3)-C(6)
119.9(1)
120.8(1)
123.4(1)
121.9(1)
115.8(1)
119.4(1)
120.0(1)
49. c Reference 50 (I ) 0.15 M NaCl).
112.1(1)
126.0(1)
123.0(1)
119.4(1)
117.5(1)
119.9(1)
120.1(1)
inductive effect resulting from the substitution of the ring
oxygen atom with the less electronegative nitrogen (Table
S1).
The molecular structures of the dimeric form of pyridi-
1
nethiones were verified by H NMR spectroscopy. Dithio-
For Hmppt, tautomerism must be considered in determin-
ing its pKa1 value. Deprotonated Hmppt (mppt-) can have
two tautomeric forms. An attempt was made to determine
two separate deprotonation constants but the results showed
that the OH and NH hydrogens are exchangeable and have
the same pKa value (i.e., pKa1).
threitol (DTT), a well-known reagent for reducing disulfide
bonds, was used to confirm the presence of a disulfide bond.
1H NMR spectra were taken before and after addition of
DTT. As expected, the representative peaks for the dimer
disappeared from the 1H NMR spectrum, and peaks for the
monomer appeared almost immediately, indicating complete
reduction of the disulfide bond (and hence verifying its
existence).
13C NMR spectra for all of the compounds were also
recorded, and peaks were assigned by running 2D-HMQC
and/or HMBC NMR experiments (Figure S2).
In addition to the usual methods of characterization, acidity
constants (OH hydrogen) were also determined for all of the
monomeric organic compounds. All of the organic mono-
meric compounds (and their oxygen analogues) synthesized
in this project are amphoteric in nature. Potentiometric
titrations were done to determine the acidity constants (Ka1)
for the hydroxyl hydrogen of the compounds. Table 1 lists
the pKa1 values for all four monomeric organic compounds
and their oxygen analogues.
Comparing pyranthiones with pyridinethiones, it was found
that the pKa1 values for the pyranthiones are smaller. The
pyranthiones are more acidic because of the higher stabilities
of their conjugate bases. Because of its higher electronega-
tivity, the ring oxygen delocalizes (and hence, stabilizes) the
negative charge of the deprotonated hydroxyl oxygen better
than does the N-R group. Consequently, the pyridinethiones
are more basic.
X-ray-quality crystals for Htma were obtained by slow
evaporation from its hexane solution (Figure S1). While this
work was ongoing, the structure of Htma was published by
Cohen and co-workers.31 Although the synthesis and recrys-
tallization methods are different, the crystallographic data
for Htma found in this work are identical to those reported
by Cohen and co-workers (Tables S2-S4). X-ray-quality
crystals for Hetma were also obtained by cooling the brown
oil form of the compound in dry ice and then warming it
slowly to room temperature (Figure S1). As compared to
maltol (CdO, bond length ≈ 1.25 Å),51 the CdS bond
lengths in Htma and Hetma were found to be longer (∼1.675
and 1.681 Å, Table S3), as expected. The C(1)-C(6) distance
in Htma (1.481 Å) is almost exactly the same as that in maltol
(1.482 Å), indicating that the methyl group is the least
affected by the thio substitution.
In comparison to their oxygen analogues, the pyranthiones
and pyridinethiones were found to be more acidic (Table
1).49,50 The slightly lower (more acidic) pKa values were
expected because of the less electronegative sulfur. Further-
more, the polarizable nature of sulfur increases the delocal-
ization of the negative charge of the deprotonated hydroxyl
oxygen. This makes the conjugate bases of the thio com-
pounds more stable. Another contributing factor to the higher
acidity is the decrease in hydrogen bonding between the
thiocarbonyl sulfur and OH hydrogen as compared to that
between the carbonyl oxygen and OH hydrogen.
X-ray-quality crystals for Hdppt dimer were obtained by
slow evaporation of its solution in ethyl acetate (Figure 1
and Table S5). The C-S bond length (1.763 Å, see Table
2) in Hdppt dimer is longer than that in Htma (1.675 Å)
(49) Song, B.; Aebischer, N.; Orvig, C. Inorg. Chem. 2002, 41, 1357.
(50) Clevette, D. J.; Nelson, W. O.; Nordin, A.; Orvig, C.; Sjo¨berg, S.
Inorg. Chem. 1989, 28, 2079.
(51) Burgess, J.; Fawcett, J. Russell, D. R.; Hider, R. C. Hossain, M. B.
Stoner, C. R.; Van Der Helm, D. Acta Crystallogr. C: Cryst. Struct.
Commun. 1996, 52, 2917.
2672 Inorganic Chemistry, Vol. 44, No. 8, 2005

Complexes with Mixed O,S Ligands DeriWed from Maltol
Scheme 2. Syntheses of Tris(pyranthionato)Ga(III) and In(III)
Complexes
pKa values of the ligand precursors because the conjugate
bases of Hmppt and Hdppt are more stable than are those of
Htma and Hetma. This also contributed to the reasonable
yields of these complexes (∼50-68%) even though a weaker
base (NEt3) was used.
All eight complexes precipitated from the respective
reaction mixtures and were collected by vacuum filtration,
washed with cold water (or cold acetonitrile), and dried
overnight in vacuo. All of the Ga(III) and In(III) complexes
with the pyranthiones were yellow solids, whereas the
Ga(III)/In(III)-pyridinethione complexes were beige. At-
tempts at recrystallizing the complexes (in hot solvents)
resulted in loss of product because of decomposition.
Washing the precipitated solid thoroughly with cold solvents
to remove excess (or unreacted) ligand or other side products
and drying the solid overnight in vacuo yielded pure products
with expected analytical results.
Scheme 3. Syntheses of Tris(pyridinethionato)Ga(III) and In(III)
Complexes
All of the group 13 metal complexes were found to be
air-stable. All of the complexes were characterized by
elemental analysis; positive ion electrospray ionization (ESI)
or liquid secondary ion (LSI) mass spectrometry; and IR and
1H, 13C and 2D (HMBC) NMR spectroscopies (where
possible), all of which gave the expected patterns. Poten-
tiometric titrations were attempted to determine the stepwise
formation constants for the pyranthionato group 13 com-
plexes. Because of precipitation at very low pH for the In-
(III) complexes and no complexation of the Ga(III) ions with
the ligands at the low concentrations used, no data were
obtained.
Elemental analyses of all of the complexes correlated well
with the calculated values. With the exception of In(etma)3,
Ga(mppt)3, and In(mppt)3, which are hydrates, all other
Ga(III) and In(III) complexes were found to be ML3 species.
For the NH pyridinethione complexes, the formation of
hydrates is similar to that also found for the vanadium
complexes with this ligand and probably portrays the
increased hydrogen-bonding ability of the N-H hydrogen
in this ligand.53
Positive ion detection mode mass spectrometry was used
to analyze all of the complexes, and diagnostic mass spectra
were obtained with the expected isotopic patterns. As
expected and reported12 for other ML3 species, the most
intense peak for all of the group 13 complexes corresponded
to [ML2]+, generated by the gas-phase loss of one ligand.
Other major peaks in the spectra of the Ga(III) and In(III)
complexes were the protonated parent ion peak [HML3]+ or
its sodium or potassium adduct ([NaML3]+ or [KML3]+). For
most of the ML3 complexes (group 13 and Ln), a peak
corresponding to [M2L5]+ was also detected, as is usually
seen for tris(bidentate ligand) trivalent metal ion, based on
the corresponding oxygen analogues.12,16
indicating a C-S single bond (Table S6 and S7). The S-S
distance (2.047 Å) is typical of disulfide bonds and is
representative of an S-S bond with electron-withdrawing
groups attached.52 The C-C distances in the ring, ranging
from 1.377 to 1.427 Å, are also longer, indicating more
delocalization, but the C-CH3 distance is still similar to those
in Htma and Hma [C(3)-C(6) ) 1.489 Å].
Four neutral gallium(III) complexes [Ga(tma)3, Ga(etma)3,
Ga(mppt)3, and Ga(dppt)3] and indium(III) complexes
[In(tma)3, In(etma)3, In(mppt)3, and In(dppt)3] were synthe-
sized by adding the metal salt to a solution of the ligand
precursor and adjusting the pH to ∼7-8 to deprotonate the
ligand precursor with aqueous 1 M NaOH or NEt3 (Schemes
2 and 3). For the pyranthiones Htma and Hetma, metal nitrate
salts were used, and the reactions were carried out in aqueous
solutions. Although the indium(III) complexes could be
obtained by reacting the indium salt and the ligand precursor
in a 1:3 ratio, an excess of ligand precursor was required
for the gallium(III) complexes (1:6). This can be explained
by the slightly softer (HSAB) nature of indium(III) and,
hence, the slightly higher affinity of indium(III) for sulfur
as compared to that of gallium(III). All of the complexes
were obtained in moderate to high yields (∼60-90%).
For the pyridinethiones Hmppt and Hdppt, the reactions
were carried out in dry, degassed acetonitrile, and NEt3
replaced NaOH as the base (Scheme 3). These conditions
were required to avoid pyridinethione dimerization, which
occurs in the presence of a strong base (such as OH-) and
molecular oxygen in aqueous solutions. Because metal nitrate
salts dissolve sparingly in MeCN, metal perchlorate salts
were used for these reactions. Unlike the Ga(III)-pyranthione
complexes, only the stoichiometric 1:3 ratio of metal salt to
ligand precursor was required to obtain the pure, neutral tris-
(ligand) complexes with Ga(III), as well as with In(III). This
implies that the deprotonated pyridinethiones are better
ligands than are the pyranthiones and is consistent with the
Overall, the IR spectra of all of the ML3 complexes are
almost superimposable, indicating similar structures in the
solid state. For the Ga(III) and In(III) complexes, the
pyranthione vibrations undergo a larger shift (∼40-50 cm-1)
(52) Kice, J. L In Sulfur in Organic and Inorganic Chemistry; Senning,
(53) Monga, V.; Thompson, K. H.; Yuen, V. G.; Sharma, V.; McNeill, J.
A. Ed.; Marcel Dekker: New York, 1971; Vol. 1, p 155.
H.; Patrick, B. O.; Orvig, C. Inorg. Chem. 2005, 44, 2678-2688.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2673

Monga et al.
Figure 2. 1H NMR spectra of In(tma)3, Ga(tma)3 and Htma (300 MHz, DMSO-d6, room temperature, * represents the solvent peak).
than do the pyridinethione peaks, which shift by only ∼10-
With the pyridinethiones, the involvement of the nitrogen
atom in the ring current affected Ha more than Hb (Ha ∆δ ≈
0.15-0.30 ppm, Hb ∆δ ≈ 0.06-0.19 ppm) as Ha is closer
to the nitrogen atom. The ring hydrogens for the group 13
metal complexes with mppt- and dppt- shift upfield [except
for Ga(mppt)3]. For the corresponding oxygen analogues, an
upfield shift was observed for Ha (Ha ∆δ ≈ 0.10 ppm), but
Hb shifted slightly downfield. The contrast with the thio
ligands could be attributed to the presence of the more
polarizable and less electronegative sulfur, which pulls less
electron density toward itself as compared to the oxygen atom
in CdO. Comparing the mppt- complexes with those of
dppt-, it was noted that a greater shift is observed for the
N-CH3 (dppt-) complexes.
23 cm-1 to lower energies. This bathochromic shift was also
observed for the oxygen analogues.12 The absence of νNO
3
(∼1384 cm-1) or νClO (∼1007 cm-1) in the IR spectra
4
indicated the absence of any corresponding counterions in
the complexes. The appearance of a new peak around ∼521-
664 cm-1 for M-O stretches [where M ) Ga(III) or In-
(III)] confirmed the complexation of the ligand to the
respective metal ions. Compared to the oxygen analogues,
these complexes had lower νM-O stretching frequencies, as
expected.
1
To study the solution structures of these complexes, H,
13C, and 2D-HMBC NMR spectra were obtained for all of
the group 13 metal complexes. For the group 13 metal
complexes with tma- and etma-, both ring hydrogens shifted
downfield (Figure 2). This was expected because binding to
the electropositive metal ion pulls electron density away from
these hydrogens. A similar shift was also seen for the ring
hydrogens in the Al(III) and Ga(III) complexes with the
The anomalous behavior of Ga(mppt)3 can be explained
by the more electropositive Ga(III) [as compared to In(III)]
deshielding the ring hydrogens more strongly than those for
In(mppt)3. Because the signals for the ring hydrogens are
broad for both Ga(III) and In(III) complexes, the chemical
shift could be an average value for rapidly interconverting
geometric (fac and mer) isomers. This could also explain
the small change in δ (∆δ ≈ 0.01-0.10 ppm). A similar
(small) change was also observed with Ga(dpp)3, the oxygen
analogue of Ga(dppt)3.16
corresponding oxygen analogue, the maltolato ligand.12
A
larger downfield shift was detected for Hb (0.11-0.23 for
Hb, 0.07-0.10 for Ha) because it is closer to the coordinating
CdS group. The greater shift for Hb in the In(III) complexes
(∼0.22 ppm for InL3 vs ∼0.10 ppm for GaL3) confirms the
stronger binding of the softer sulfur donor to the softer
In(III) ion as compared to that to Ga(III) metal ion.
The 13C NMR spectra were very useful in predicting the
molecular structures of the complexes in solution. The most
2674 Inorganic Chemistry, Vol. 44, No. 8, 2005

Complexes with Mixed O,S Ligands DeriWed from Maltol
Table 3. Selected Bond Lengths (Å) and Bond Angles for
fac-Ga(tma)3
Bond Lengths (Å)
C(1)-S(1)
C(7)-S(2)
C(13)-S(3)
C(2)-O(1)
C(8)-O(3)
C(14)-O(5)
1.684(4)
1.693(4)
1.699(4)
1.322(4)
1.293(4)
1.318(4)
O(1)-Ga(1)
O(3)-Ga(1)
O(5)-Ga(1)
S(1)-Ga(1)
S(2)-Ga(1)
S(3)-Ga(1)
1.941(3)
1.963(2)
1.959(2)
2.4822(11)
2.4610(11)
2.4594(11)
Bond Angles (deg)
92.00(4)
85.27(4)
93.13(4)
95.28(9)
S(3)-Ga(1)-S(2)
S(3)-Ga(1)-S(1)
S(2)-Ga(1)-S(1)
O(1)-Ga(1)-S(3)
O(5)-Ga(1)-S(1)
O(3)-Ga(1)-S(3)
O(3)-Ga(1)-S(2)
O(3)-Ga(1)-S(1)
O(1)-Ga(1)-O(5)
O(1)-Ga(1)-O(3)
O(5)-Ga(1)-O(3)
168.78(8)
175.10(8)
83.60(8)
97.09(8)
O(1)-Ga(1)-S(2) 172.14(9)
92.16(11)
89.22(11)
93.59(10)
Figure 3. ORTEP diagrams of fac-Ga(tma)3 (50% thermal ellipsoids).
O(1)-Ga(1)-S(1)
O(5)-Ga(1)-S(3)
O(5)-Ga(1)-S(2)
84.56(8)
84.34(8)
91.47(8)
significant shifts were observed for the CdS (∆δ ≈ 10 ppm)
and C-O-M (∆δ ≈ 5-7 ppm) peaks for all group 13
complexes. 2D-HMBC NMR spectra (Figure S3) were
obtained to distinguish between C-O-M and C-CH3 as
both are quaternary carbon atoms and these two peaks appear
very close together in the 13C NMR spectra of most of these
complexes.
Scheme 4. Syntheses of Tris(pyranthionato)Ln(III) Complexes
An X-ray crystal structure for Ga(tma)3 was obtained
(Figure 3). Yellow chip crystals of Ga(tma)3 suitable for
X-ray diffraction were grown by mixing warm solutions of
tris(acetylacetonato)gallium(III), Ga(acac)3, in EtOH and
Htma in THF (Ga3+/Htma in a 1:3 ratio) followed by slow
evaporation of the resulting solution. The material crystallizes
as the fac isomer with both enantiomers, ∆ and Λ, in the
asymmetric unit (Tables S8-S10). Other group 13 com-
plexes with similar ligands have been found to crystallize
out as fac54 or mer isomers.36 For the Al(III) complex of the
corresponding oxygen analogue (maltol), Al(ma)3, only the
mer isomer was isolated.55 On the other hand, the pyridinone
complexes with Al(III) and Ga(III) were isolated as the fac
isomers.16 In both enantiomers of Ga(tma)3, the three
thiomaltolato ligands are arranged in an octahedral arrange-
ment, with two sulfur atoms and two oxygen atoms in a cis
arrangement in the equatorial plane and one sulfur and one
oxygen atom along the perpendicular axis. Table 3 lists
selected bond lengths and bond angles for fac-Ga(tma)3. The
Ga-O [1.95(2) Å] and Ga-S [avg 2.46(1) Å] distances for
the fac isomer of Ga(tma)3 are shorter than are those for the
mer isomer of an In(III) complex with a similar coordination
sphere [mer-In(O3S3), In-O )2.20(4) Å, In-S ) 2.53(2)
Å for tris(1-oxopyridine-2-thionato)indium(III), In(PT)3].36
The longer bonds in In(PT)3 are an outcome of increased
electron density in the system due to the ring nitrogen
(instead of oxygen). Compared to the structure of its oxygen
analogue, Al(ma)3, the M-O (hydroxyl oxygen) bond length
in Ga(tma)3 is slightly longer [1.888(2) Å]55 than that in Al-
(ma)3 [1.874(2) Å], suggesting a slightly weaker M-O bond.
A range of lanthanide complexes was synthesized by
reacting the respective metal perchlorate salts with the two
pyranthiones, Htma and Hetma, in ethanol at room temper-
Table 4. Elemental Analyses of the Ln(III) Complexes of tma- and
etma-
complex
C% Calcd (found)
H% Calcd (found)
La(tma)3‚2H2O
La(etma)3‚H2O
Pr(tma)2(OH)‚H2O
Pr(etma)3‚0.5H2O
Nd(tma)2(OH)‚H2O
Nd(etma)3‚2H2O
Sm(tma)3
Sm(etma)3‚0.5H2O
Gd(tma)2(OH)‚H2O
Gd(etma)3‚2.5H2O
Ho(tma)3‚1.5H2O
Ho(etma)3‚2H2O
Lu(tma)3
36.13 (36.03)
40.52 (40.26)
31.45 (31.31)
40.98 (40.89)
31.23 (31.05)
39.05 (38.91)
37.67 (37.47)
39.23 (39.33)
30.37 (30.10)
37.77 (37.75)
35.13 (35.16)
37.84 (38.07)
36.12 (36.24)
32.88 (32.84)
3.20 (2.96)
3.72 (3.40)
2.86 (2.88)
3.60 (3.66)
2.84 (2.81)
3.90 (3.58)
2.63 (2.96)
3.76 (3.46)
2.76 (2.74)
3.92 (3.49)
2.95 (3.08)
3.78 (3.48)
2.53 (2.91)
3.15 (3.07)
Lu(etma)2(OH)‚H2O
ature (Scheme 4). All of the Ln(III) complexes with both
pyranthiones were yellow solids, but the Ln(etma)3 com-
plexes had a shiny texture and were obtained in moderate to
high yields (53-95%, Table S11). Several attempts to
recrystallize the precipitated solids resulted in decomposition
of the product and hydrolysis of the Ln(III) ions.
The elemental analyses of the lanthanide complexes gave
the empirical formula LnL3‚xH2O or LnL2(OH)‚xH2O (see
Table 4). The formation of two different species with similar
ligands can be explained by the extremely sensitive nature
of the Ln(III) ions toward the pH of a solution and the ability
to hydrolyze easily with increasing pH.56 Although it is
difficult to predict the exact structure of the complex without
a crystal structure, these formulas and the two different
species are consistent with the Ln complexes reported
previously with the oxygen analogue of thiomaltol (mal-
tol)21,22 and other bidentate O,S donor ligands with Ln such
as ethylthioacetate,57 acetoanilide,58,59 and thiohydroxamic
(54) Rose, D. J.; Da Chang, Y.; Chen, Q.; Kettler, P. B.; Zubieta, J. Inorg.
Chem. 1995, 34, 3973.
(55) Finnegan, M. M.; Rettig, S. J.; Orvig, C. J. Am. Chem. Soc. 1986,
108, 5033.
(56) Baes, C. F., Jr.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley
& Sons: New York, 1986; p 129.
(57) Dutt, N. K.; Rahut, S. J. Inorg. Nucl. Chem. 1970, 32, 1033.
Inorganic Chemistry, Vol. 44, No. 8, 2005 2675

Monga et al.
Figure 4. 1H NMR spectra of Lu(etma)2(OH), La(etma)3 and Hetma (400 MHz, DMSO-d6, room temperature, * represents the solvent peak).
Table 5. 13C NMR Chemical Shift (δ) Data in ppm for the All of the
Ln(III) Complexes (400 MHz, DMSO-d6, Room Temperature)
acids.60 Characteristic data from the mass and IR spectra for
the Ln complexes are given in Table S11 and Figure S4. As
seen in Figure S4, all of the Ln-ethylthiomaltolato complexes
are almost superimposable with identical shifts in the
frequencies with respect to the free ligands (∼40-50 cm-1).
1H, 13C, and 2D-HMBC NMR spectra were also obtained
for the diamagnetic lanthanide complexes La(tma)3, Lu(tma)3,
La(etma)3, and Lu(etma)2(OH)‚H2O. Figure 4 shows the
1
relative shift in the H NMR signals for the La(III) and
compd
Htma
C-1
C-2
C-3
C-4
C-5
C-6a C-7
Lu(III) metal complexes with respect to the free ligand
precursor (also see Table S12). Lack of free ligand precursor
peaks and observed shifts in both C-H hydrogens conclu-
sively indicate binding of the ligand to the Ln(III) ions. For
the lanthanide complexes, the relative chemical shifts in the
13C NMR spectra (with respect to the free ligand) are not as
prominent as for the group 13 complexes but still show that
the ligands are bound to the Ln(III) ion (Table 5). This was
also confirmed by obtaining 2D-HMBC NMR spectra. The
146.31 150.09 185.45 124.31 148.51 14.81
146.64 163.27 186.62 123.11 144.10 14.92
147.62 161.52 185.28 122.76 144.90 15.04
150.07 149.44 185.73 124.30 148.66 21.78 10.49
150.94 162.79 186.91 123.01 144.10 21.80 10.92
-
-
-
La(tma)3
Lu(tma)3
Hetma
a
a
La(etma)3
Lu(etma)2(OH)a 151.53 161.41 185.65 122.56 144.55 21.86 10.96
a C-6 is CH3 group for Htma and Ln-tma.
CdS peak shifts by only ∼0.3-1.2 ppm, a behavior that
has been observed in other ketonic oxygen-61 and sulfur-
bound group 13 and Ln complexes.62 The broadening and
(58) Dutt, N. K.; De, R. J. Indian Chem. Soc. 1971, 48, 981.
(59) Sen, D. N.; Umapathy, P.; Budhkar, A. P. Indian J. Chem. 1971, 9,
376.
(60) Mathur, S. P.; Sharma, B. K. J. Macromol. Sci.-Chem. 1984, A21,
(61) Green, D. E. Ph.D. Thesis, University of British Columbia, Vancouver,
BC, Canada, 2004; p 62.
833.
2676 Inorganic Chemistry, Vol. 44, No. 8, 2005

Complexes with Mixed O,S Ligands DeriWed from Maltol
the small chemical shift of the peaks in the 13C NMR spectra
of the Ln(III) metal complexes could also be indicative of
monodentate (from O-) freely rotating ligands in a strongly
coordinating solvent such as DMSO. Because Ln(III) ions
have a lower affinity for sulfur (than the group 13 metal
ions), the S-Ln covalent coordinate bond might be getting
replaced by DMSO-Ln. The NMR spectra for the La(III)
and Lu(III) complexes are comparable, indicating that the
ligands bind in a similar manner with metal ions at either
end of the lanthanide series.
similar characterization of these thio ligands with Ga(III)
and In(III) ions suggests that these thio compounds with
mixed O,S donors are good ligands for these metal ions. It
would be interesting to further evaluate the in vivo stability
of these complexes for comparison to that of the correspond-
ing oxygen analogues so that their potential as diagnostic or
therapeutic pharmaceutical agents can be tested in the future.
Preliminary results are also presented for the synthesis of
a range of Ln(III)-pyranthione complexes across the series.
Some complexes were isolated only as LnL2(OH) complexes
rather than the expected LnL3 species, demonstrating that
the coordination chemistry of these thio ligands with Ln-
(III) metal ions is very sensitive to pH.
Conclusions
Four new mixed O,S ligand precursors, Htma, Hetma,
Hmppt, and Hdppt, were synthesized successfully in good
yields; various characterization techniques, including X-ray
crystallographic analysis, were used to verify the structures
in solid state and in solution. The characterization data were
comparable to those for their respective oxygen analogues,
and the shifts (in IR and NMR spectra) were reasonable on
the basis of replacing CdO with CdS.
New Ga(III) and In(III) complexes with two pyranthiones
(Htma and Hetma) and two pyridinethiones (Hmppt and
Hdppt) were successfully synthesized and characterized. The
characterization indicates that In(III) binds to these thio
ligands better than does Ga(III) and, hence, forms more stable
complexes with these ligands. Successful complexation and
Acknowledgment. The authors thank the Natural Sci-
ences and Engineering Research Council (NSERC) and the
Canadian Institutes of Health Research (CIHR) for financial
support. Dr. Bin Song is thanked for his help with the
potentiometric titrations.
Supporting Information Available: Complete characterization
data for the thiomaltol derivatives including a table of NMR data;
crystallographic data and complete bond lengths and angles for the
four X-ray structures; yields and mass spectrometry data for the
lanthanide complexes, as well as NMR data for the La and Lu
derivatives; ORTEP diagrams for Htma and Hetma; 2-D NMR data
for Hetma and Ga(etma)3; and IR data for Hetma and etma-
lanthanide complexes. This material is available free of charge via
the Internet at http://pubs.acs.org.
(62) Kaushik, A.; Singh, Y.; Rai, A. K. Indian J. Chem. A 1996, 35, 704.
(b) Kaushik, A.; Singh, Y. P.; Rai, A. K. Phosphorus, Sulfur Silicon
Relat. Elem. 2001, 169, 591.
IC048693Y
Inorganic Chemistry, Vol. 44, No. 8, 2005 2677