Synthesis and metal coordination chemistry of (phenyl)(pyridin-2-ylmethyl) phosphinodithioic acid, [2-C5H4N]CH2P(S)(SH) (Ph)

Article abstract of DOI:10.1016/j.poly.2011.11.041

A three-step synthesis for the bifunctional ligand (phenyl)(pyridin-2- ylmethyl)phosphinodithioic acid, [2-C₅H₄N]CH ₂P(S)(SH)(Ph) (3-H), was developed. The molecule was characterized by spectroscopic methods, and single cr

Full text of DOI:10.1016/j.poly.2011.11.041

Polyhedron 33 (2012) 327–335
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and metal coordination chemistry of
(phenyl)(pyridin-2-ylmethyl)phosphinodithioic acid, [2-C5H4N]CH2P(S)(SH)(Ph)
Manab Chakravarty a,1, Sylvie Pailloux a,2, S. Ouizem a, K.A. Smith a, Eileen N. Duesler a, Robert T. Paine a,
,
⇑
Neil J. Williams b, Robert D. Hancock b
a Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, NM 87131, USA
b Department of Chemistry and Biochemistry, University of North Carolina-Wilmington, Wilmington, NC 28403, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A three-step synthesis for the bifunctional ligand (phenyl)(pyridin-2-ylmethyl)phosphinodithioic acid,
[2-C5H4N]CH2P(S)(SH)(Ph) (3-H), was developed. The molecule was characterized by spectroscopic meth-
ods, and single crystal X-ray diffraction analysis, and the molecule found to exist in a zwitterionic form (30).
Protonation constants for 30 were measured spectrophotometrically in 0.1 M NaClO4 aqueous solution. The
coordination chemistry of 30 toward CdCl2 and PtCl2 was explored, and the complexes [Cd(30)Cl2]2 and
[Pt(3ꢀ)2]ꢁCHCl3 were isolated and structurally characterized by single crystal X-ray diffraction methods.
Complexation constants for 30 with Cd(II), Zn(II) and La(III), as a function of pH, were measured by titration
techniques. An initial effort to prepare the [pyridin-bis(2,6-yl-methylphosphinodithioic acid)] analog of
3-H led to the formation and isolation of (phenyl)(6-methylpyridin-2-ylmethyl)phosphinodithioic acid,
[2-C5H3N(6-CH3)]CH2P(S)(SH)(Ph) (6-H), that was characterized by spectroscopic methods and single crys-
tal X-ray diffraction analysis. As found with 3-H, 6-H, adopts a zwitterionic structural form (60).
Ó 2011 Elsevier Ltd. All rights reserved.
Received 29 September 2011
Accepted 22 November 2011
Available online 29 November 2011
Keywords:
Pyridyl
Dithiophosphinic acid
Cd(II)
Pt(II) complexes
Protonation constants
Complexation constants
X-ray crystal structures
1. Introduction
to organic platforms that also carry additional harder or softer do-
nor sites such that unique asymmetric coordination fields may be
The syntheses and chemistry of mono-dithiophosphoric, (RO)2P
(S)(SH), -dithiophosphonic, (RO)(R)P(S)(SH), and -dithiophosphi-
nic, R2P(S)(SH), acids have been widely studied, and numerous
applications for the reagents have been developed [1–3]. The coor-
dination chemistry of the respective anions has also attracted
much attention as these species display a remarkable diversity in
coordination modes with s-, p-, d- and f-block metal cations, and
much of that activity has been reviewed [1–6]. Few bis-dithio-
phosphorus acids or their conjugate bases have been reported,
but Kuchen and coworkers [7] have outlined a preparation for
the bis(dithiophosphinic acids), [R2P(S)(SH)]2(CH2)n (n = 4–10). Da-
vies and coworkers [8] have also described a synthesis and coordi-
nation chemistry for a bis(dithiophosphinate), Li2[PhP(S)2(CH2)]2.
In nearly all examples, the organyl groups, R, in the acids and their
anionic conjugate bases are simple alkyl or aryl fragments that do
not contain additional donor site functionality. In our own case, we
are interested in attachment of dithiophosphorus acid fragments
produced. We report here the synthesis and selected coordination
chemistry of one such target ligand type, [2-C5H4N]CH2P(S)(SH)-
(Ph) (3-H), that contains a pyridin-2-ylmethyl fragment and a
dithiophosphinic acid group.
2. Experimental
2.1. Materials and general procedures
Organic reagents were purchased from Aldrich Chemical Co. and
used as received. Organic solvents were rigorously dried according
to standard procedures. The CdCl2 and PtCl2 for metal complex prep-
arations were purchased from Ventron. The metal perchlorates for
the titration analyses were purchased from VWR and Strem Chemi-
cals. Titrations were performed with solutions prepared by using
deionized water (Milli-Q, Waters Corp) of >18 M
X
cmꢀ1 resistivity.
Infrared spectra were recorded with a Tensor 200 FTIR spectrometer,
and NMR spectra were recorded with Bruker FX-250, Avance 300
and Avance 500 NMR spectrometers. External chemical shift stan-
⇑
Corresponding author. Tel.: +1 505 277 1661.
E-mail address: rtpaine@unm.edu (R.T. Paine).
Current address: Chemistry Department, Birla Institute of Technology and
dards, Me4Si (1H, 13C) and 85% H3PO4 31P), were employed. Mass
(
1
spectra were measured in the positive ion mode unless noted other-
wise at the UNM Mass Spectrometry Facility, and elemental analyses
were performed by Galbraith Laboratories.
Sciences, Pilani-Hyderabad Campus, Hyderabad 500078, A.P., India.
2
Current address: Department of Chemistry, University of California-Berkeley,
Berkeley, CA, USA.
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.11.041

328
M. Chakravarty et al. / Polyhedron 33 (2012) 327–335
2.2. Experimental procedures
was treated dropwise under nitrogen with DIBAL-H solution
(Aldrich, 1 M in THF, 25.4 mL) over 10 min (23 °C). The mixture
was stirred (23 °C, 24 h) and diethyl ether (15 mL) was added. This
mixture was cooled (0 °C) and quenched cautiously with 6 M
NaOH (40 mL). Following vigorous gas evolution, the layers were
separated, and the organic layer was transferred via syringe to an-
other flask, dried over Na2SO4 and then transferred to a flask con-
taining sulfur (0.27 g, 8.4 mmol). The mixture was refluxed (12 h),
filtered and the filtrate evaporated. The remaining residue was
purified by column chromatography (SiO2, CHCl3/MeOH, 90/10).
(Phenyl)(6-methylpyridin-2-ylmethyl)phosphinodithioic acid (6-
H) was recovered as a colorless solid that was crystallized from
MeOH/CHCl3, 5/1. Yield: 0.12 g (20%). Mp 212–214 °C. IR (KBr,
2.2.1. Ligand syntheses
2.2.1.1. (Phenyl)(pyridin-2-ylmethyl)phosphinodithioic acid (3-H). A
sample of PhP(OEt)2 (0.78 g, 3.9 mmol) was added to a solution of
2-bromomethylpyridine (0.68 g, 3.9 mmol) in dry CH3CN (15 mL),
and the mixture was heated under reflux (4 h). The CH3CN was vac-
uum evaporated, the residue dissolved in CH2Cl2 (20 mL), and the
resulting solution washed with water (2 ꢂ 10 mL). The organic
phase was separated, vacuum evaporated, and the residue purified
by column chromatography (SiO2, 70–230 mesh, elution with
CH2Cl2/MeOH, 80/20). (Phenyl)(pyridine-2-ylmethyl)phosphinic
acid ethyl ester, 2-[(Ph)(EtO)P(O)CH2]C5H4N (1), was obtained as a
colorless liquid. Yield: 0.90 g, 87%. IR (KBr, cmꢀ1):
m
= 1226 (s,
mPO).
cmꢀ1): PS). 31P{1H} NMR (DMSO-d6):
m = 646 (vs, mPS), 549 (s, m
31P{1H} NMR (CDCl3): d = 39.6. 1H NMR (CDCl3): d = 1.27 (t,
JHH = 7.0 Hz, 3H, OCH2CH3), 3.57 (d, JPH = 18.0 Hz, 2H, PCH2), 3.88–
3.98 (m, 1H, OCHAHBCH3), 4.05–4.15 (m, 1H, OCHACHBCH3), 7.10 (t,
JHH = 6.0 Hz, 1H, Ar-H), 7.27–7.71 (m, 7H, Ar-H), 8.2 (d, JHH = 4.7 Hz,
1H, Ar-H). 13C{1H} NMR (CDCl3): d = 16.3 (OCH2CH3), 40.8 (d,
JPC = 92.3 Hz, PCH2), 61.0 (d, JPC = 6.2 Hz, OCH2CH3), 121.6 (d,
JPC = 2.5 Hz, Ar), 124.4 (d, JPC = 3.9 Hz, Ar), 128.3 (d, JPC = 12.5 Hz,
Ar), 130.2 (d, JPC = 127.6 Hz, Ar), 131.6 (d, JPC = 9.8 Hz, Ar), 132.2 (d,
JPC = 1.9 Hz, Ar), 136.2(Ar), 149.2(Ar), 152.4 (d, JPC = 7.2 Hz, Ar). Mass
spectrum (ESI): m/z = 262.09 [M+H+], 284.07 [M+Na+]. Anal. Calc. for
d = 63.8. 1H NMR (DMSO-d6): d = 2.65 (s, 3H, CH3), 3.71 (d,
JPH = 13.0 Hz, 2H, PCH2), 7.22 (d, JHH = 8.0 Hz, 1H, Ar), 7.39 (br,
3H, Ar), 7.66 (d, JHH = 7.5 Hz, 1H, Ar), 7.99–8.07 (m, 2H, Ar), 8.20
(t, JHH = 7.5 Hz, 1H, Ar), 14.89 (br, 1H, NH). 13C{1H} NMR (DMSO-
d6): d = 19.1 (CH3), 51.7 (d, JPC = 35.7 Hz, PCH2), 124.2, 124.6,
127.1 (d, JPC = 11.6 Hz), 129.4, 130.6 (d, JPC = 10.4 Hz), 142.8 (d,
JPC = 76.0 Hz), 143.8, 150.4, 151.4. Mass spectrum (ESI):
m/z = [M+H+] 280.03, [M+Na+] 302.02. Anal. Calc. for C13H14NPS2:
C, 55.89; H, 5.05; N, 5.01; S, 22.95. Found: C, 55.46; H, 5.05; N,
4.92; S, 23.13%.
C
14H16NO2P: C, 64.36; H, 6.17; N, 5.36. Found: C, 61.18; H, 6.04; N,
5.19%.
A 100 mL three-necked flask, outfitted with an addition funnel,
2.2.2. Coordination complex syntheses
2.2.2.1. Cadmium(II) complex. Compound 30 (0.052 g, 0.19 mmol)
and CdCl2 (0.018 g, 0.1 mmol) were combined in a mixture of
CHCl3 (3 mL) and MeOH (3 mL), and the solution refluxed (12 h).
A solid, [Cd(30)Cl2]2, formed during heating. After cooling (23 °C),
the solid was collected by filtration. Yield: 0.04 g (90%). IR (KBr,
was charged with a sample of 1 (0.561 g, 2.15 mmol), frozen with
liquid nitrogen, evacuated and backfilled with nitrogen. Dry tolu-
ene (10 mL) was added followed by dropwise addition (10 min,
23 °C) of DIBAL-H solution (Aldrich, 1 M in toluene, 8.6 mL). During
the addition, the solution became light yellow in color. The mixture
was then heated (50 °C) and stirred (12 h). The resulting mixture
was cooled (ꢀ78 °C) and cautiously quenched with 4 M NaOH
solution (4 mL). The phases were allowed to separate, and the or-
ganic phase was removed under nitrogen by syringe to another
Schlenk vessel containing sulfur (0.137 g, 4.39 mmol). The remain-
ing aqueous phase was extracted with fresh toluene (2 ꢂ 5 mL),
and the organic layers were added to the vessel containing the first
organic fraction and sulfur. The mixture was stirred (23 °C, 1 h) and
then gently heated (70 °C, 4 h). A solid formed that was recovered
by filtration and washed with toluene leaving a white solid
(phenyl)(pyridin-2-ylmethyl)phosphinodithioic acid, (3-H), that
was crystallized by slow evaporation from MeOH/CH3CN
(1/1)solution. Yield: 0.38 g (67%). Mp 238–240 °C. The compound
is soluble in DMSO and DMF, slightly soluble in MeOH, EtOH,
CHCl3, CH3CN and THF, and it has slight solubility in water. IR
cmꢀ1):
m = 629 (vs, mPS), 534 (s, m
PS). 31P{1H} NMR (DMSO-d6):
d = 66.9. 1H NMR (DMSO-d6): d = 3.79 (d, JPH = 13.0 Hz, 2H, PCH2),
4.45 (br, 1H, NH), 7.42 (br, 3H, Ph), 7.49 (br, 1H, Pyr), 7.79 (br,
1H, Pyr), 8.04–8.08 (m, 2H, Ph), 8.29 (br, 1H, Pyr), 8.75 (d,
JHH = 5 Hz, 1H, Pyr). Mass spectrum (HRMS-negative ion mode):
m/z = [MꢀHꢀ] 447.8478; C12H11114Cd35ClNP32S requires 447.8481.
Anal. Calc. for C12H12CdCl2NPS2: C, 32.13; H, 2.70; N, 3.12. Found:
C, 34.18; H, 3.10, N, 2.91%.
2.2.2.2. Platinum(II) complex. Compound 30 (0.099 g, 0.37 mmol)
and PtCl2 (0.050 g, 0.19 mmol) were initially combined in CH3CN
(10 mL), and the solution refluxed (12 h). The resulting yellow
solution was evaporated, the residue was dissolved in CHCl3/MeOH
(1:5), and the resulting solution refluxed (12 h). A clear reddish
brown solution was obtained that was allowed to slowly evaporate
leaving yellow crystals, [Pt(3ꢀ)2]. IR spectra (KBr, cmꢀ1):
m = 653
(KBr, cmꢀ1):
m
= 652 (vs,
mPS), 543 (s,
m
PS). 31P{1H} NMR (DMSO-
(vs, mPS), 523 (s, m
PS). 31P{1H} NMR (CDCl3): d = 55.5. Mass spectrum
d6): d = 64.0. 1H NMR (DMSO-d6): d = 3.69 (d, JPH = 12.5 Hz, 2H,
PCH2), 7.36 (br, 3H, Ph), 7.48 (d, JHH = 8.0 Hz, 1H, Pyr-H3), 7.79
(dd, JHH = 6.0 Hz, 1H, Pyr-H4), 7.99–8.03 (m, 2H, Ph), 8.31 (dd,
JHH = 7 Hz, 1H, Pyr-H5), 8.72 (d, JHH = 5 Hz, 1H, Pyr-H6). 1H{31P}
NMR (DMSO-d6): d = 3.69 (s, 2H, PCH2), 7.36 (br, 3H, Ph), 7.48 (d,
JHH = 7 Hz, 1H, Pyr-H3), 7.79 (dd, JHH = 6 Hz, 1H, Pyr-H4), 8.02 (d,
JHH 3 Hz, 2H, Ph), 8.31 (dd, JHH = 7 Hz, 1H, Pyr-H5), 8.72, (br, 1H,
Pyr-H6), 15.34 (br, s, 1H, NH). 13C{1H} NMR (DMSO-d6): d = 52.1
(d, JPC = 36.4 Hz, PCH2), 124.0. 127.1 (d, JPC = 11.9 Hz), 127.7,
129.4, 130.5 (d, JPC = 10.7 Hz), 140.0, 142.5 (d, JPC = 76 Hz), 143.9,
150.8 (d, JPC = 6.8 Hz). Mass spectrum (HRESI): m/z = [M+H+]
266.0223; C12H13NP32S2 requires 266.0227, [M+Na+] 288.0045;
(HRMS): m/z = [M+H+] 723.9883; C24H23N2P232S4195Pt requires
723.9867.
2.2.3. Ligand pKa and metal stability constant determinations
The spectrophotometric titration experiments were carried out
with a 1.0 cm quartz flow cell (VWR) placed in a Varian 300 Cary
1E UV–Vis spectrophotometer controlled by Cary Win UV Scan
Application version 02.00(5) software. This was connected to an
external, temperature controlled titration cell maintained at
25.0 0.1 °C. The solution was continually circulated from the
external cell to the flow cell in the spectrophotometer using a peri-
staltic pump. A VWR sympHony™ SR60IC pH meter with a VWR
sympHony™ gel epoxy semi-micro combination pH electrode
was used for all pH readings, which were recorded in the external
cell. Equilibration and mixing were allowed for by circulating the
solution for 15 min before each new spectrum was recorded. Fit-
ting of theoretical absorbance versus pH or log[M] curves was
accomplished using the SOLVER module of EXCEL [10]. For a set
C
12H12NNaP32S2 requires 288.0047. Anal. Calc. For C12H12NPS2: C,
54.32; H, 4.56; N, 5.28; S, 24.17. Found: C, 54.80, H, 4.65; N,
5.33; S, 23.95%.
2.2.1.2. (Phenyl)(6-methyl-pyridin-2-ylmethyl)phosphinodithioic acid
(6-H). A sample of 4 (0.941 g, 2.21 mmol) [9] in dry THF (15 mL)

M. Chakravarty et al. / Polyhedron 33 (2012) 327–335
329
Table 1
Crystallographic data.
30
60
[Cd(30)Cl2]2
[Pt(3ꢀ)2]ꢁCHCl3
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
C12H12NPS2
265.32
monoclinic
C2/c
C13H14NPS2
279.34
monoclinic
P21/c
C12H12CdCl2NPS2
448.62
C13H12Cl3NPPt0.5S2
481.22
monoclinic
C2/c
triclinic
ꢀ
P1
12.1486(4)
24.3471(9)
8.7351(3)
90
101.726(1)
90
8.0467(2)
10.6324(3)
15.9585(4)
90
101.249(2)
90
8.2646(3)
8.6321(3)
12.8299(4)
106.785(2)
90.200(2)
112.724(2)
801.39(5)
2
19.6310 (3)
12.9702(2)
16.4569(4)
90
125.164(1)
90
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
Volume
Z
2529.8(2)
8
1339.11(6)
4
3425.5(1)
8
T (K)
188(2)
1.393
0.518
183(2)
1.386
0.493
202(2)
1.859
2.040
223(2)
1.866
4.924
Dcalc (g cmꢀ3
)
Absorption coefficient (mmꢀ1
)
Minimum/maximum transmission
Reflection collected
0.8800/0.9003
32488
0.8804/0.9777
37396
0.7468/0.8996
16884
0.9162/0.8052
60338
Independent reflections [Rint
Final R indices [I > 2 (I)] R1 (wR2)
Final R indices (all data) R1(wR2)
]
5486[0.0236]
0.0282(0.0686)
0.0345(0.0724)
5602[0.0315]
0.0308(0.0796)
0.0433(0.0862)
5215[0.0296]
0.0278(0.0518)
0.0451(0.0561)
8316[0.0293]
0.0205(0.0479)
0.0342(0.0537)
r
of spectra of any one metal ion with 30, SOLVER was used to fit
formation constants and molar absorbances for the species in solu-
tion. The standard deviations given for logK values in Table 1 were
calculated using the SOLVSTAT macro [10].
its formation by this approach requires the preparation of the
intermediate secondary phosphine, [2-C5H4N]CH2P(H)(Ph), 2. A
synthesis for 2, from the combination of 2-(chloromethyl)pyridine
and NaP(H)Ph, has been described in the literature [18,19]; however,
the yield was found to vary. Therefore, an alternative approach, that
does not involve isolation of the intermediate phosphine, was em-
ployed as summarized in Scheme 1. Initially, a solvent free Arbusov
synthesis for the starting reagent (phenyl)(pyridin-2-ylmethyl
)phosphine acid ethyl ester, 1, was explored. Mixtures of 2-(bromo-
methyl)pyridine and PhP(OEt)2, were heated without solvent, in a
fashion similar to that described by Horner and coworkers [20]. This
synthesis proved to be unsatisfactory as the formation of 1 was
accompanied by several unidentified products resulting from ther-
mal decomposition of 2-(bromomethyl)pyridine. Use of 2-(chloro-
methyl)pyridine also gave poor results. However, when the
Arbusov reaction of 2-(bromomethyl)pyridine and PhP(OEt)2 was
performed in refluxing CH3CN solution, the conversion was efficient,
and [2-C5H4N]CH2P(O)(OEt)(Ph), 1, was isolated in good yield (87%)
as a colorless liquid. The compound displays a parent ion, [M+H+], in
its ESI mass spectrum and a very strong absorption in the IR spec-
trum, 1226 cmꢀ1, that is assigned to the mPO terminal phosphoryl
stretching frequency. The band position is intermediate between
2.2.4. Single crystal X-ray diffraction analyses
Single crystals of 30 (colorless prism, 0.25 ꢂ 0.21 ꢂ 0.21 mm)
and 60 (pale yellow thin platelet, 0.05 ꢂ 0.07 ꢂ 0.27 mm) were se-
lected from preparative samples described above. Single crystals of
[Cd(30)Cl2]2 (colorless prism, 0.15 ꢂ 0.11 ꢂ 0.05 mm) were ob-
tained by slow cooling of a hot CHCl3/MeOH (1:1) solution of the
crude complex. Single crystals of [Pt(3ꢀ)2]ꢁCHCl3 (pale yellow nee-
dle, 0.046 ꢂ 0.069 ꢂ 0.437 mm) were grown by slow evaporation
of a solution of [Pt(3ꢀ)2] in CHCl3/MeOH (1:5). The X-ray diffrac-
tion data were collected with a BRUKER X8 Apex II CCD system
equipped with a graphite monochromator, MoK
a sealed X-ray
tube (k = 0.71073 Å) and Oxford Cryostream 700 low temperature
device. Full spheres of diffraction data were collected and inte-
grated with SAINT [11] using a narrow frame algorithm. Data were
corrected for adsorption (SADABS) [12], and the structures solved
by direct methods and refined with full-matrix least-squares on
F2 using SHELXTL [13]. Selected crystal data are summarized in Table
1. All structure solutions and refinements were well behaved. The
H-atoms on C-atoms were included in idealized positions, and they
were allowed to vary in position with Uiso = 1.2 Ueq of the parent
atom. For compounds 30 and 60, the H atom on the N-atom was lo-
cated in difference maps, and it was allowed to vary in position and
Uiso. For [Pt(3ꢀ)2]ꢁ2CHCl3, the CHCl3 solvent molecule in the asym-
metric unit was disordered over two positions with occupancies
58.5% (C13H13ACl1Cl2Cl3) and 41.5% (C13H13BCl1Cl4Cl5). Atoms
C13 and Cl1 are common to both positions.
CH3CN
PhP(OEt)2
N
4h
N
OEt
Ph
reflux
P
Br
O
1
DIBAL-H
toluene
12h, 50°C
3. Results and discussion
S8, toluene
2h, 70°C
3.1. Ligand synthesis and characterization
N
H
N
N
Ph
S
A variety of methods have been reported for the synthesis of
dithiophosphinic acids [1–3,14]. In particular, the reaction of sulfur
with secondary phosphines is known to provide good access to both
alkyl and aryl substituted dithiophosphinic acids [15–17]. Given
that [2-C5H4N]CH2P(S)(SH)(Ph), 3-H, is the target ligand molecule,
Ph
SH
P
P
P
S
H
Ph
S
3'
3-H
2
Scheme 1. Synthesis of compound 3-H and 30.

330
M. Chakravarty et al. / Polyhedron 33 (2012) 327–335
values of mPO for [2-C5H4N]CH2P(O)Ph2, 1190 cmꢀ1 [21] and
[2-C5H4N]CH2P(O)(OEt)2, 1248 cmꢀ1. A similar shift with substitu-
ent group modifications occurs in the series Ph2P(O)CH2COOH,
1168 cmꢀ1, Ph(EtO)P(O)CH2COOH, 1177 cmꢀ1 and (EtO)2P(O)CH2-
COOH, 1257 cmꢀ1 [22,23]. These trends reflect the greater electron
withdrawing effect exerted by the ethoxy group relative to the phe-
nyl group. The 31P{1H} NMR spectrum of 1 shows a single resonance,
d 39.6, that is typical for phosphonate esters [24], and the 1H and
13C{1H} NMR spectra are consistent with the assumed structure.
As evidenced by the CHN microanalytical data and a low intensity
1H NMR signal (d 3.09), a minor organic impurity is present in sam-
ples of 1, but it does not complicate the subsequent reduction
chemistry.
Several reagents have been described in the literature as effective
reducing agents for organyl phosphinates, and much of that chemis-
try has been summarized by Busacca and coworkers [25–27]. In the
present work, DIBAL-H is found to be most effective for reduction of
1, although slightly different reagent stoichiometry and reaction
time conditions are employed compared to typical literature
descriptions. For example, Busacca reported that phenylmethylm-
ethyl phosphinate, (Ph)(Me)P(O)(OMe), is reduced by 2.2 equiva-
lents of DIBAL-H in toluene at 50 °C over 2 h [27]. Applications of
these conditions to the reduction of 1 leads to only 50% conversion
to 2 based upon 31P NMR analysis of the aqueous NaOH quenched
reaction mixture. However, increasing the amount of DIBAL-H
reductant to 4.5 equivalents and the reaction time to 12 h give 2 in
80–90% yield. The synthesis outlined in the Experimental section
calls for use of 4 eq. of DIBAL-H and a 4 h reaction time. The forma-
tion of 2 is supported by 31P NMR analysis that shows a doublet res-
onance, d ꢀ45.3, JPH = 208.9 Hz, in toluene solution, and this shift
and coupling constant compare favorably with reported data for 2
recorded from Et2O solution: dꢀ48.0, JPH = 209 Hz [18]. Without iso-
lation, the toluene solution of 2 was combined with sulfur flowers,
and the combination was gently heated at 70 °C for 4 h. The product,
3-H, is obtained as a white solid. Multiple CHN analyses show small
variances between theoretical and observed values that may result
either from a minor impurity or difficulties with the combustion
analyses for the molecule. Spectroscopic data do not show evidence
for impurities although they may exist below the detection limits of
the data. The compound displays a parent ion, [M+H+], in its high res-
olution ESI mass spectrum, and a IR spectrum shows strong absorp-
tions at 652 and 582 cmꢀ1 that are tentatively assigned to mPS asym
mode [17,29,31]. The 31P{1H} NMR spectrum for 3-H contains a sin-
gle sharp, symmetrical resonance, d 64.1, that is typical of shifts re-
ported for other secondary dithiophosphinates and phosphin
odithioic acids [17,24]. The proton coupled 31P NMR spectrum dis-
plays a nearly symmetrical 1, 4, 6, 4, 1 pentet-like pattern that sim-
ulation suggests arises from nearly equivalent coupling of the
phosphorus with the two methylene protons (JHP = 12.5 Hz) and
two ortho-phenyl protons (JHP = 13.0 Hz). The 1H and 13C{1H} NMR
spectra are consistent with the proposed structure except that a
1H resonance that might be assigned to a S–H proton is not detected.
However, a broad resonance, with relative intensity of one, is ob-
served at d 15.34 that is assigned to a pyridinium proton. It is also
noted that the proton resonances for the H4 and H5 protons of the
pyridyl ring appear as pseudo-triplets (overlapping doublet of dou-
blets) due to nearly equivalent neighboring H–H couplings. There-
fore, the IR and 1H NMR data suggest that 3-H exists in a
zwitterionic form, 30 as illustrated in Scheme 1. If correct, the mole-
cule in this form should display a N–H stretching frequency. Unfor-
tunately, this vibration was not confidently assigned since it can
appear over a large frequency range depending upon the strength
of the N–H+ bond which is impacted by hydrogen bonding interac-
tions [32,33]. Alternative evidence for the zwitterionic form, as well
as an estimate of the strength of the N–H interaction, could be pro-
vided by resolution of the 15N–H coupling constant. Limbach and
coworkers [34–36], in a series of elegant studies, have assessed the
strength of N–H+ interactions in a number of pyridine–acid com-
plexes by measuring JNH for 15N enriched samples, at low tempera-
ture, in fluorocarbon solvent systems. Parallel studies with 30
would be very informative, but the synthesis of the 15N enriched
molecule would be time consuming, and it has not been undertaken.
In a parallel synthetic effort, the chemistry summarized in
Scheme 2 was explored in an attempt to convert 4 into the
bis(phosphinodithioic acid), 8-H2. However, under the conditions
examined, DIBAL-H addition to 4 results in partial reductive
dephosphorylation with formation of 5. The intermediate diphos-
phine, 7, also forms as indicated by a 31P NMR resonance, d
ꢀ45.5, JPH = 206 Hz. Addition of sulfur to the crude reaction mix-
ture gives 6-H as a pale yellow solid with modest, unoptimized
yield (20%). The compound displays a parent ion [M+H+] in its high
resolution ESI mass spectrum, and the IR spectrum is very similar
to that for 3-H with strong bands at 646 and 548 cmꢀ1 that are
tentatively assigned to mPS modes. As found with 3-H, there
are no absorptions in the region 2600–2200 cmꢀ1 that can be as-
cribed to a mSH mode in a monomeric –P(S)(SH) containing mole-
cule or to associated, hydrogen bonded species [17,29,31]. The
and mPS
stretching modes based upon prior literature assign-
sym
ments [17,28–30]. It is noted that no band is observed in the region
2600–2200 cmꢀ1 that could be attributed to a free or associated mSH
DIBXAL-H
S8
N
N
N
Ph
HS
Ph
SH
THF, 24h
EtO
Ph
OEt
Ph
P
P
P
P
P
P
S
S
Ph
H
H
Ph
O
O
4
7
8
DIBAL-H
THF
24h
S8
Et2O/THF H3C
H3C
N
H
H3C
N
Ph
S
N
P
Ph
SH
P
S
P
H
Ph
S
5
6-H
6'
Scheme 2. Synthesis of 6-H and 60.

M. Chakravarty et al. / Polyhedron 33 (2012) 327–335
331
31P{1H} NMR spectrum displays a symmetrical singlet, d 64.5, that
is split into a pentet-like pattern in the 31P proton coupled spec-
trum. The 1H and 13C{1H} NMR spectra are similar to those of 3-
H except a resonance for the o-Me group also appears: 1H d 2.54;
13C{1H} d 19.1. In addition, a broad, low field 1H NMR resonance,
assigned to a pyridinium proton, is found at d 14.89. As noted,
the synthesis of 6-H, described in Scheme 2, is not optimized,
and it is likely that the compound would be more efficiently pre-
pared by reduction of [C5H3(o-CH3)N]CH2P(O)(Ph)(OEt) with
DIBAL-H followed by reaction of the intermediate phosphine with
sulfur. Given reports in the literature [37,38], it is not surprising
that the conditions described here for the reduction of 4 lead to
partial dephosphorylation of 7. A modified procedure that pro-
duces the bis(phosphinodithioic acid), 8-H2, will be reported sepa-
rately along with its coordination chemistry.
3.2. Ligand pKa and metal stability constant determinations
The solubility of 3-H in water is quite low (ꢃ10ꢀ4 M); however,
Fig. 2. Absorbance as a function of pH for 10ꢀ4 M 3-H(30) in aqueous 0.1 M NaClO4
solution as a function of pH between pH 2.27 and 6.13. Points are experimental
points, solid lines are theoretical curves of absorbance versus pH calculated as
described in the text.
the molecule displays intense electronic absorption bands in the
region 200–350 nm due to p–
p⁄ transitions involving the pyridine
fragment. Therefore, it is possible to determine the ligand proton-
ation constants and to assess metal ion complexation equilibria in
0.1 M NaClO4 solutions as described previously for other heterocy-
clic ligands [39–45]. The spectra for acid titrations of 3-H(30) be-
tween pH 2.42 and 8.17 are shown in Fig. 1, and the variation in
absorbance at five wavelengths for 3-H(30) as a function of pH is
shown in Fig. 2. Analysis of these spectra provides the protonation
constants summarized in Table 2. The value for pK1 = 5.40(3) is as-
signed to protonation of the pyridyl N-atom based upon known
constants for pyridine (pK = 5.24 at ionic strength 0.1) and substi-
tuted pyridines [46]. Protonation constants for dithiophosphinic
acids are sparse in the literature, but they are known to be rela-
tively strong acids and not strongly influenced by the nature of
substituent groups [17]. For example, Kabachnik and coworkers
[47] have reported values between 1.74 and 2.64 for a series of
dithiophosphinic acids, RR0P(S)(SH), in water/alcohol mixtures,
and Zucal and coworkers [48] list values of 0.08 to 0.2 for several
dialkyldithiophosphinic acids in CCl4/H2O mixtures. Therefore,
the measured second protonation constant, pK2 = 1.2(1), is as-
signed with confidence to the dithiophosphinic acid fragment. A
reviewer quite correctly questioned how a pK2 = 1.2 could be
determined if the titration went down to only pH 2.42, as
suggested by Fig. 1. In fact, as seen in Fig. 2, the titration went
down to pH 2.27 although the spectra at lower pH are not included
in Fig. 1, as they obscure the isosbestic points. One sees the change
in absorbance produced by the small amount of diprotonated
ligand present near pH 2 in Fig. 2. Because the diprotonated form
of the ligand is only of the order of 10% at the lowest pH values at-
tained, the pK2 is reported with a relatively large standard devia-
tion in Table 2.
The logK values derived from metal ion titrations of 3-H(30)
with Cd(II), Zn(II) and La(III) in 0.1 M NaClO4 aqueous solutions
at 25 °C are also listed in Table 2. As would be expected for sul-
fur-donors that are soft in the HSAB sense of Pearson [49,50], the
hard La(III) ion in aqueous solution appears to have little affinity
for 3-H as indicated by no change in the spectrum of the ligand
in the presence of even 10ꢀ3 M La(III). Zinc(II) is intermediate in
the HSAB classification [49,50], and this ion also displays only a
very low affinity for 3-H(30) in aqueous solution. The weak interac-
tion indicated by logK = 2.1(1) probably results from Zn(II) binding
to the pyridyl donor fragment. Only the soft Cd(II) shows a reason-
able affinity for 3-H(30) in water (logK = 4.33(5)), and this might be
expected to result from Cd(II) binding to both the pyridyl N-atom,
and the S-atom(s) of the ligand.
3.3. Selected nonaqueous metal ion coordination chemistry
Given the results of the metal/ligand titration analyses, the
coordination chemistry of 3-H(30) was initially examined with
the soft cations Cd(II) and Pt(II). Combinations of 3-H with CdCl2
in a 2:1 ratio in CHCl3/MeOH mixtures gives a colorless, crystalline
complex, and the same complex is obtained from a 1:1 reactant
combination. The CHN analytical data for the complex are most
consistent with a 1:1 metal:ligand composition, Cd(30)Cl2, although
the agreement with the theoretical composition is not acceptable
indicating that either the sample is impure or there are difficulties
Table 2
Protonation and formation constants for 3-H (L) determined in 0.1 M NaClO4 at 25 °C.
Lewis acid
Equilibrium
logK
Reference
H+
H+ + OHꢀ ¡ H2O
Lꢀ + H+¡ LH
13.78
5.40(3)
1.2(1)
4.33(5)
2.1(1)
<2
[46]
H+
this work
this work
this work
this work
this work
H+
LH + H+¡ LH2
+
Cd2+
Zn2+
La3+
Cd2+ + Lꢀ¡ CdL+
Zn2+ + Lꢀ¡ ZnL2+
La3+ + Lꢀ¡ LaL2+
Fig. 1. Variation of the spectra of 10ꢀ4 M 3-H(30) in aqueous 0.1 M NaClO4 solution
as a function of pH between pH 2.42 and 8.17.

332
M. Chakravarty et al. / Polyhedron 33 (2012) 327–335
with the combustion analysis. A high resolution, electrospray ion-
ization mass spectrum (negative ion mode) of the complex, on the
other hand, displays an intense negative ion envelop with the max-
imum intensity ion at m/z = 447.8478; a negative ion composition
C
12H11NP32S235Cl2112Cd requires m/z = 447.8481. This is consistent
with the proposed Cd(30)Cl2 composition. An IR spectrum for the
complex shows two strong absorptions at 629 cmꢀ1
and 533 (
(mPS
)
asym
m
PS sym) cmꢀ1, both strongly shifted from the respective
bands (652 and 582 cmꢀ1) in 3-H(30). The 31P NMR spectrum for
the complex shows a single resonance at d 66.9. These data suggest
that the ligand is present in its zwitterionic form, and, based on the
magnitudes of the IR shifts, it appears to be bound to Cd(II) as a
bidentate S,S0 phosphinate in the solid state.
Very little relevant formation constant data are available for
Pt(II) [46], but an estimate of how strong a Lewis acid Pt(II) is
can be obtained by comparison with its more labile and more eas-
ily studied congener Pd(II). The logK1 for Pd(II) with pyridine is 8.5
[46], indicating that Pd(II) is a stronger Lewis acid than the proton
(pKa for pyridine is 5.24), Cd(II) or Zn(II). Further, Pt(II) should be at
least equally as strong a Lewis acid as Pd(II). Indeed, a 2:1 combi-
nation of 3-H(30) with PtCl2, first in CH3CN solution and then in
CHCl3/MeOH solution, produces a reddish brown solution that,
upon standing, deposits yellow crystals. Elemental analyses were
not collected, but a high resolution mass spectrum for this complex
shows an intense parent ion envelop with a maximum intensity
ion, m/z = 723.9883 consistent with a 2:1 ligand: metal composi-
tion C24H23N2P232S4195Pt, m/z = 723.9867. An IR spectrum of the
Fig. 3. Molecular structure and atom labeling scheme for 30. Thermal ellipsoids are
shown at the 20% level.
crystals contains two strong absorptions at 653 and 523 cmꢀ1
.
The absence of a significant shift in the higher frequency band sug-
gests a monodentate coordination mode for the dithiophosphinate
ligand [29,30]. The 31P NMR spectrum displays a single resonance
at d 55.5. While Cd(II) is a fairly weak Lewis acid, and thus not as
readily able to drive the proton off the pyridine N-atom in 3-
H(30) at the acidity levels present in the synthesis of the Cd(II)/3-
H(30) complex, the more powerful Lewis acid Pt(II) apparently is
able to displace the proton from the pyridinium fragment. Further,
the spectroscopic data, suggest that the anion 3ꢀ acts as a mono-
dentate ligand toward Pt(II). Single crystal X-ray analyses were
undertaken in order to reveal the details of the ligand–metal inter-
actions in these complexes.
Fig. 4. Molecular structure and atom labeling scheme for 60. Thermal ellipsoids are
shown at the 20% level.
109(1)° and for N1–H1ꢁ ꢁ ꢁS1A, H1–S1A 2.69(2) Å, N1ꢁ ꢁ ꢁS1A
3.465(1), angle 153(1)°. van Zyl and coworkers [52] have reported
a similar but more symmetric bifurcated N–Hꢁ ꢁ ꢁ(S,S0) hydrogen
bonding interaction in a dithiophosphonate decorated cholesteryl
fragment, and a similar structure has been proposed by Swan
and coworkers [53,54] for an o-aminobenzylphenyldithiophosphi-
nic acid.
3.4. X-ray crystal structure analyses for ligands and complexes
The molecular structures for 3-H(30) and 6-H(60) have been
determined by single crystal X-ray diffraction analyses, and views
of the molecules are shown in Figs. 3 and 4, respectively. Selected
bond lengths are summarized in Table 3. The structure determina-
tions confirm that both molecules exist in the respective zwitter-
ionic forms, 30 and 60, wherein the dithioic acid proton has
migrated to the pyridinyl N-atom. The P atom in both molecules
has a distorted tetrahedral geometry in which the C–P–C and the
S–P–S bond angles are the smallest and largest, respectively, of
the set of angles about the P-atom: 30, C6–P–C7 102.06(4), S1–P–
S2 118.15(2); 60, C7–P–C8 103.10(4), S1–P–S2 118.74(2). The P–S
bond lengths are expected to be equivalent in an anionic dithiolate
structure and intermediate in length between a typical P@S double
bond (1.91 Å) and a P–S single bond (2.1 Å) [51]. The measured
bond lengths in 30 and 60 are found to be intermediate in length,
but they are slightly different within each molecule: 30, P1–S1
1.9763(3) Å and P1–S2 1.9955(3) Å; 60, P1–S1 1.9863 Å and P1–
S2 1.9785(3) Å. The differences may result from weak intermolec-
ular hydrogen bonding (30) or a combination of intramolecular and
intermolecular hydrogen bonding (60) interactioꢀns that occur be-
tween the pyridinium N–H+ proton and the PS2 unit: 30 for N1–
H1ꢁ ꢁ ꢁS2A, H1ꢁ ꢁ ꢁS2A 2.39(2) Å, Nꢁ ꢁ ꢁS2A 3.2447(8) Å, angle 168(1)°;
60 for N1–H1ꢁ ꢁ ꢁS2, H1ꢁ ꢁ ꢁS2 2.85(2) Å, N1ꢁ ꢁ ꢁS2 3.2194(9) Å, angle
The molecular structure for the complex formed between
3-H(30) and CdCl2 is shown in Fig. 5 and selected bond lengths
are presented in Table 3. In the solid state, the proposed 1:1 me-
tal:ligand composition is confirmed, but the complex is clearly di-
meric, [Cd(30)Cl2]2. The structure is centrosymmetric, and the
ligands are in the neutral zwitterionic form, 30. Each Cd(II) is five
coordinate with coordination positions occupied by a terminal Cl
atom, two bridging Cl atoms and two sulfur atoms from one S,S0
chelating bidentate dithiophosphinate fragment. This results in a
highly distorted square pyramidal-like coordination geometry
about the Cd(II) ion. The terminal Cd1–Cl2 bond length,
2.4599(5) Å is shorter than the bridging interactions, Cd1–Cl1
2.5004(5) Å and Cd–Cl1A 2.6714(5) Å, and the Cd–S distances,
Cd1–S1 2.5004(5) Å and Cd1–S2 2.8670(5) Å are quite different.
As expected, the shorter Cd–S interaction involves the S-atom
with a longer P–S bond length, 2.0160(7) Å, and the longer Cd–S
interaction is associated with a shorter P–S distance, P1–S2,
1.9836(7) Å. The dimeric molecular units are weakly associated
in the unit cell along the b-axis via intermolecular N–Hꢁ ꢁ ꢁCl
hydrogen bond interactions: H1ꢁ ꢁ ꢁCl2D 2.26(2) Å, N1ꢁ ꢁ ꢁCl2D,
3.107(2) Å, angle 176(2)°. For comparison, structures for numer-

M. Chakravarty et al. / Polyhedron 33 (2012) 327–335
333
Table 3
Selected bond lengths (Å).
Bond type
P–S
30
60
[Cd(30)2Cl2]2
[Pt(3ꢀ)2]ꢁCHCl3
P1–S1 2.0476(6)
P1–S2 1.9615(7)
P1–S1 1.9763(3)
P1–S2 1.9955(3)
P1–S1 1.9863(3)
P1–S2 1.9785(3)
P1–S1 2.0160(8)
P1–S2 1.9836(7)
P–C
P1–C6 1.8571(9)
P1–C7 1.8200(9)
P1–C8 1.8239(9)
P1–C9 1.857(1)
P1–C7 1.846(2)
P1–C1 1.809(2)
P1–C6 1.837(2)
P1–C7 1.816(2)
C–N
C1–N1 1.345(1)
C5–N1 1.347(1)
C1–N1 1.351(1)
C5–N1 1.351(1)
C8–N1 1.331(3)
C12–N1 1.339(3)
C1–N1 1.354(2)
C5–N1 1.356(2)
C–C
C5–C6 1.483(1)
C5–C7 1.481(1)
C7–C8 1.491(3)
C5–C6 1.489(2)
M–S
Cd1–S1 2.5519(5)
Cd1–S2 2.8670(5)
Pt1–S1 2.3249(4)
Pt1–S1A 2.3249(4)
M–Cl
M–N
Cd1–Cl1 2.5004(5)
Cd1–Cl2 2.4599(5)
Cd1–Cl1A 2.6714(5)
Cd1A–Cl1 2.6713(5)
Pt1–N1 2.026(1)
Pt1–N1A 2.026(1)
Fig. 5. Molecular structure and atom labeling scheme for [Cd(30)Cl2]2. Thermal
ellipsoids are shown at the 20% level.
Fig. 6. Molecular structure and atom labeling scheme for [Pt(3ꢀ)2]ꢁ2CHCl3 with the
outer sphere CHCl3 molecules omitted for clarity. Thermal ellipsoids are shown at
the 20% level.
ous Cd(II) complexes with anionic dithiophosphorus ligands have
been reported. In most cases the Cd(II) is six coordinate, and the
ligand is bonded in a symmetrical S,S0 bidentate manner [1–6,55].
Two structures are particularly relevant to [Cd(30)Cl2]2. Shimoi
and coworkers [56] reported the structure for Cd[(EtO)2P(S)2]2
(hexamethylenetetramine)2, and Jian and coworkers [57] de-
scribed the structure of Cd[(iPrO)2P (S)2]2(pyridine)2. In both
cases, the Cd(II) ion has a six coordinate, octahedral geometry
with the monodentate amine N-atoms in trans positions, and
the dithiophosphates bonded in a much more symmetrical biden-
tate fashion: Cd[(EtO)2P(S)2]2(hexamethylenetetramine)2, Cd–S1
2.704(1) Å, Cd–S2 2.682(1) Å, Cd–N1 2.595(2) Å, P–S1 1.982(1) Å,
N,S mode is the fact that the logK1 values for the Cd(II) and Zn(II)
with 3-H(30) are greater than for pyridine itself (logK1 (pyridine)
for Cd(II) = 1.37 and for Zn(II) = 1.05 [46]). This would not be the
case if the S-donor of 3-H(30) was not also involved in
coordination.
The molecular structure for the complex formed between3-H(30)
and PtCl2 is shown in Fig. 6 and selected bond lengths are summa-
rized in Table 3. The structure contains monomeric [Pt(3ꢀ)2] units
in which the ligands are deprotonated and bind as anionic dith-
iophosphinates, RR0P(S)(S)ꢀ, 3ꢀ. The Pt(II) ion has a square planar
geometry, and each ligand interacts with the Pt(II) via a bidentate
N,S chelate interaction: Pt1–N1 2.026(1) Å and Pt–S1 2.3249(4) Å.
The coordinated sulfur has a significantly longer phosphorus–sulfur
bond length, P1–S1 2.0476(6) Å, compared to the terminal uncoordi-
nated sulfur, P1–S2 1.9615(7) Å. The composition and N,S chelate
mode are, therefore, consistent with the MS and IR data. There are
few structural reports for Pt(II) complexes of anionic dithiophospho-
rus ligands available for comparison, but in most cases the ligand
acts as a symmetrical S,S0 bidentate chelate. For example, the struc-
tures of Pt[(EtO)2P(S)2]2 [58]and Pt[(EtO)(Ph)P(S)2]2 [59] have
square planar geometries with symmetrical bidentate Pt(S,S0) inter-
actions: Pt[(EtO)2P(S)2]2 Pt–S1 2.332(4) Å, Pt–S2 2.325(3) Å, P–S1
2.007(4) Å, P–S2 1.998(4) Å and Pt[(EtO)(Ph)P(S)2]2 Pt–S1 2.333(3)
Å, Pt–S2 2.343(3) Å. Examples also exist with mixed bis (biden-
tate)(monodentate) coordination as in [Pt(Cy2PS2)2(Cy2PH)] [29]
that contains two anionic dithiophospinate ligands, one of which
P–S2
1.979(1) Å
and
Cd[(iPrO)2P(S)2]2(pyridine)2,
Cd–S1
2.704(1) Å, Cd–S2 2.694(1) Å, Cd–N1 2.399(3) Å, P–S1 1.993(1) Å,
P–S2 1.996 Å. It is interesting that the MS and IR data and the
X-ray crystal structure determination for [Cd(30)Cl2]2 indicate that
the Cd(II) does not displace the proton from the pyridine N-atom.
However, in aqueous solution there is considerable change in the
electronic spectra that involve
p–
p⁄ transitions within the pyri-
dine fragment upon formation of the complex as the pH is in-
creased. This suggests that the Cd(II) may displace the
pyridinium proton, and coordinate to the N-atom as well as to
a S-donor center of 3ꢀ in aqueous solution. Similarly, the changes
in the p–
p⁄ transitions within the pyridine upon formation of the
Zn(II) complex suggest that the Zn(II) may coordinate via an N,S
interaction. Supporting the conclusion that the ligand binds in a

334
M. Chakravarty et al. / Polyhedron 33 (2012) 327–335
is symmetrical S,S0 bidentate, Pt–S1 2.342(2) Å, Pt–S2 2.413(2) Å,
and the second is monodentate, Pt–S3 2.302(2) Å. The P–S bond
lengths involving the Pt-bonded S-atoms are longer, P1–S1
2.024(2) Å, P1–S2 2.017(2) Å, P2–S3 2.062(2) Å, than the terminal
P–S bond length, 1.964(2) Å. Apparently the neutral phosphine,
Cy2PH, out-competes with the second bidentate chelate interaction
for the fourth coordination position on Pt(II). In another interesting
example, Pt[(EtO)2P(S)2](ppy) (ppy = 2-phenylpyridine), the dith-
iophosphonate binds in a symmetrical bidentate S,S0 mode with
Pt–S1 2.402(2) Å and Pt–S2 2.311(2) Å, and the ppyꢀ is orto-metal-
lated with Pt–N1 2.018(5) Å [60].
References
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4. Conclusion
A convenient, high yield synthesis for a bifunctional pyridin-
2-ylmethylphosphinodithioic acid, 3-H(30), has been developed.
The 1H NMR spectra and X-ray crystal structure determination re-
veal that the strongly acidic phosphinodithioic acid proton mi-
grates from a sulfur atom to the basic pyridinyl N-atom resulting
in a zwitterionic structure, 30. Selected metal binding affinities
for 3-H(30) as a function of pH have been accessed spectrophoto-
metrically with La(III), Zn(II) and Cd(II) in water solution. Not sur-
prisingly, harder cations such as La(III) and Zn(II), under these
conditions, show low affinity for the neutral ligand. However, the
ligand shows modest coordination strength with the softer cation
Cd(II), and in water the ligand may be coordinated in its anionic
form as a bidentate N,S-chelator. On the other hand, the X-ray crys-
tal structure determination for the complex [Cd(30)Cl]2, formed in
CHCl3/MeOH solution, indicates that, under this condition, the li-
gand binds in its neutral 30 form utilizing an asymmetric bidentate
S,S0 chelate interaction. In contrast, the crystal structure for
[Pt(3ꢀ)2], shows that Pt(II) displaces the proton from 3-H(30), and
the resulting anionic ligand acts as a bidentate N,S-chelator.
Given the interesting acid–base and complexation properties of
the neutral dithioic acid, 3-H, its zwitterionic isomer, 30, and the
monoanion form, 3ꢀ, it is anticipated that a rich variety of coordi-
nation complex structures will evolve depending on the HSAB
character of the coordinated metal, metal coordination geometry
preferences, steric effects introduced by ligand substituent groups
and solvent. Further coordination chemistry with soft d-block me-
tal cations as well as with f-block cations in the absence of strongly
bonded water will be forthcoming. The latter will provide some
interesting experimental results to consider in light of the reveal-
ing computational studies reported by Boehme and Wipff [61,62]
regarding the formation of thiophosphoryl ligand complexes with
lanthanide ions.
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Acknowledgements
Financial support for this study was provided by the U.S.
Department of Energy (DoE), Chemical Sciences, Geosciences and
Biosciences Office, Office of Basic Energy Sciences (Grant DE-
FG02-03ER15419) (RTP). In addition, funds from the National Sci-
ence Foundation assisted the purchase of the X-ray diffractometer
(CHE-0443580) and NMR spectrometers (CHE-0840523 and
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Appendix A. Supplementary data
CCDC 832336, 832337, 832338 and 832339 contains the supple-
mentary crystallographic data for compounds (30), (60), [Cd(30)Cl2]2
and [Pt(3ꢀ)2]ꢁCHCl3. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.

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