Synthesis, structures, and linkage isomerism of (allylbenzylmalonate)platinum(II) complexes

Article abstract of DOI:10.1016/S0020-1693(99)00394-1

New platinum(II) complexes A₂Pt(ABM) (A = triethylphosphine (TEP); A₂ = 2,2-dimethyl-1,3-propanediamine (DMPDA), trans-(±)-1,2-diaminocyclohexane (DACH); ABM = allylbenzylmalonate) have been synthesized in aqueous solution and characterized by means of X-ray analysis and multinuclear NMR spectroscopy. The crystal structures of cis-(TEP)₂Pt(ABM) (monoclinic P2₁/n, a = 12.179(3), b = 16.869(7), c = 12.870(3) A, β = 93.27(2)°, V = 2757(1) A³, Z = 4, R = 0.0390) and (DACH)-Pt(ABM)·2H₂O (triclinic P1, a = 7.649(3), b = 14.064(2), c = 19.190(8) A, α = 98.98(2), β = 90.05(3), γ = 105.73(2)°, V = 1961(1) A³, Z = 2, R = 0.0455) have been solved. The platinum atom in both complexes adopts a typical square planar arrangement with each coligand in cis positions. In the solid state, the ABM ligand exhibits different chelation modes depending on the coligands: (O,O′)-chelation in the phosphine analog and (O,alkene)-chelation in the amine analogs. For the phosphine complex, the (O,O′)-chelation mode of the ABM ligand is impregnable even in solutions at variable temperatures, probably due to the strong π-bonding between the phosphorus and platinum atoms. However, (diamine)platinum(II) complexes have shown interesting linkage isomerism between the (O,O′)- and (O,alkene)-chelation modes in solutions depending on solvent and temperature.

Full text of DOI:10.1016/S0020-1693(99)00394-1

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Inorganica Chimica Acta 295 (1999) 214–221
Synthesis, structures, and linkage isomerism of
(allylbenzylmalonate)platinum(II) complexes
Young-A Lee a,b, Young Keun Chung b, Youn Soo Sohn a,*
a Inorganic Chemistry Laboratory, Korea Institute of Science and Technology, Seoul 130-650, South Korea
b Department of Chemistry, Seoul National Uni6ersity, Seoul 151-742, South Korea
Received 18 March 1999; accepted 9 August 1999
Abstract
New platinum(II) complexes A2Pt(ABM) (A=triethylphosphine (TEP); A2=2,2-dimethyl-1,3-propanediamine (DMPDA),
trans-(9)-1,2-diaminocyclohexane (DACH); ABM=allylbenzylmalonate) have been synthesized in aqueous solution and charac-
terized by means of X-ray analysis and multinuclear NMR spectroscopy. The crystal structures of cis-(TEP)2Pt(ABM)
3
,
,
(monoclinic P21/n, a=12.179(3), b=16.869(7), c=12.870(3) A, i=93.27(2)°, V=2757(1) A , Z=4, R=0.0390) and (DACH)-
(
,
Pt(ABM)·2H2O (triclinic P1, a=7.649(3), b=14.064(2), c=19.190(8) A, h=98.98(2), i=90.05(3), k=105.73(2)°, V=1961(1)
A , Z=2, R=0.0455) have been solved. The platinum atom in both complexes adopts a typical square planar arrangement with
3
,
each coligand in cis positions. In the solid state, the ABM ligand exhibits different chelation modes depending on the coligands:
(O,O%)-chelation in the phosphine analog and (O,alkene)-chelation in the amine analogs. For the phosphine complex, the
(O,O%)-chelation mode of the ABM ligand is impregnable even in solutions at variable temperatures, probably due to the strong
p-bonding between the phosphorus and platinum atoms. However, (diamine)platinum(II) complexes have shown interesting
linkage isomerism between the (O,O%)- and (O,alkene)-chelation modes in solutions depending on solvent and temperature. © 1999
Elsevier Science S.A. All rights reserved.
Keywords: X-ray structures; Linkage isomerism; Platinum(II) complexes; Allylbenzylmalonate ligand; NMR spectroscopy
[1,3–9]. In some cases, the steric factor may play an
important role in determining the relative stability of the
linkage isomers [10]. According to our previous work, a
unique isomerism between the (O,alkene)- and (O,O%)-
chelates was observed for allyl- and diallylmalonatoplat-
inum(II) complexes, which depends on solvent and
temperature [11].
In order to expand the coordination chemistry of
platinum(II) complexes, another malonate derivative
containing one alkene group, allylbenzylmalonate, has
been used as a multidentate anionic ligand for the
synthesis of new platinum(II) complexes. The ligand may
coordinate to the platinum atom via either mode of
(O,O%)-, or (O,alkene)-chelation. Phosphine and amine
were employed as a coligand in order to examine the
coligand effect on the linkage isomerism. Here we report
the synthesis and structures of novel platinum(II) com-
plexes A2Pt(ABM) (A2=phosphine or amine, ABM=
allylbenzylmalonate) along with their linkage isomerism
dependent on coligand, solvent, and temperature.
1. Introduction
Among the properties of the metal complexes involv-
ing multidentate ligands, linkage isomerism is increas-
ingly important in inorganic chemistry. It is an expanding
field that offers a potential for rational control of
important biological molecules [1,2], and the results have
been extended beyond the initial studies to such diverse
areas as quantum mechanical calculations, molecular
switches, isomeric catalysts, the design of therapeutic
reagents, the imaging agents in the body, and the
separation of diastereomers. Although the coordination
mode of multidentate ligands may be vaguely predicted
via electronic effect, the coordination selectivity is sensi-
tive to various factors such as central metals, coligands,
reaction time, temperature, the solvent properties, etc.
* Corresponding author. Tel.: +82-2-958 5081; fax: +82-2-958
5089.
E-mail address: yssohn@kistmail.kist.re.kr (Y.S. Sohn)
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00394-1

Y.-A Lee et al. / Inorganica Chimica Acta 295 (1999) 214–221
215
of the crude product in ethanol–ethyl ether (1:1) af-
forded colorless crystals suitable for X-ray analysis.
Yield 82%. M.p. 136–137°C. Anal. Calc. for
C25H42O4P2Pt: C, 45.2; H, 6.38. Found: C, 45.0; H,
6.32%. IR (KBr, cm−1): w(COO)asym, 1646, 1616;
w(COO)sym, 1360, 1346. 1H NMR (Me2SO-d6, ppm):
1.04 (t, CH3, 9H, J=7.8 Hz), 1.10 (t, CH3, 9H, J=7.8
Hz), 1.90 (q, CH2P, 12H, J=7.8 Hz), 2.49 (d, CH2, 2H,
J=6.2 Hz), 3.42 (s, CH2, 2H), 4.84 (d, ꢀCH2, 1H,
J=17.6 Hz), 4.91 (d, ꢀCH2, 1H, J=10.2 Hz), 5.69–
5.74 (m, ꢀCH, 1H), 7.11–7.23 (m, C6H5, 5H). 13C
NMR (Me2SO-d6, ppm): 7.6, 13.4, 13.9, 40.0, 42.2,
60.1, 116.5 (CꢀC), 125.9, 127.5, 129.7, 136.0, 138.3
(CꢀC), 176.0 (CꢀO). 31P NMR (Me2SO-d6, ppm): 27.7
(1JPtꢁP=3536.0 Hz). 195Pt NMR (Me2SO-d6, ppm):
−4198.6 (1JPtꢁP=7056.5 Hz).
2. Experimental
2.1. Materials and instrumentation
Potassium tetrachloroplatinate(II) was used as re-
ceived from Kojima, and triethylphosphine (TEP), 2,2-
dimethyl-1,3-propanediamine (DMPDA), and trans-
(9)-1,2-diaminocyclohexane (DACH) from Aldrich
were also used without further purification. Diethyl
allylbenzylmalonate (ABM) was prepared by the litera-
ture procedure [12] and then hydrolyzed with 1.5 equiv.
of Ba(OH)2·8H2O in 95% methanol to obtain the
barium salt. The acid form of ABM was obtained by
the standard method [13]. cis-Bis(triethylphosphine)-
platinum(II) nitrate and cis-(diamine)platinum(II) sul-
fates were also prepared by literature methods [14–16].
Elemental analyses were performed by the Advanced
Analysis Center at KIST. The infrared spectra in the
5000–400 cm−1 region were measured as KBr pellets
on a Perkin–Elmer 16F PC model FT-IR spectrometer.
Multinuclear (1H, 13C, 31P, and 195Pt) NMR spectra
were recorded on a Varian Gemini-300 NMR spec-
trometer operating at 300.00 MHz (1H), 75.48 MHz
(13C), 121.44 MHz (31P) and 64.39 MHz (195Pt) in pulse
2.2.2. (DACH)Pt(ABM)
To a solution of (DACH)PtSO4·H2O (0.85 g, 2.0
mmol) in water (50 ml) was added Ba(ABM)·2H2O
(0.81 g, 2.0 mmol) in water (50 ml), and the resulting
mixture was then stirred for 3 h at r.t. After barium
sulfate was filtered off, the filtrate was evaporated to
dryness. The crude white solid was recrystallized from a
mixture of water–acetone (1:1) to obtain colorless crys-
tals suitable for X-ray crystallography (74% yield).
M.p. 204°C (dec.). Anal. Calc. for C19H26N2O4Pt·H2O:
C, 40.8; H, 5.04; N, 5.01. Found: C, 40.5; H, 5.00; N,
4.97%. IR (KBr, cm−1): w(COO)asym, 1664, 1634, 1596,
1564; w(COO)sym, 1374, 1318, 1290, 1254, 1242. 1H
NMR (D2O, ppm): 1.10–1.48 (m, 8H), 1.54–1.72 (m,
6H), 1.96–2.04 (m, 2H), 2.07–2.18 (m, 2H), 2.47–2.70
(m, 6H), 2.90 (d, 2H, J=13.7 Hz), 3.25 (d, 2H, J=
13.4 Hz), 4.18 (d, 1H, J=14.8 Hz), 4.27 (d, ꢀCH2, 1H,
J=14.8 Hz), 4.74 (d, ꢀCH2, 1H, J=12.4 Hz), 4.99 (d,
ꢀCH2, 1H, J=7.8 Hz), 5.43–5.67 (q, ꢀCH, 2H), 7.22–
7.40 (m, C6H5, 10H). 1H NMR (Me2SO-d6, ppm):
0.93–1.09 (m, CH2 in DACH, 2H), 1.12–1.31 (m, CH2
in DACH, 2H), 1.39–1.48 (m, CH2 in DACH, 2H),
1.76–1.89 (m, CH2 in DACH, 2H), 2.03–2.15 (m,
CHN in DACH, 2H), 3.32 (s, CH2, 2H), 3.66 (dd, CH2,
2H, J=13.3/10.5 Hz), 4.89 (s, ꢀCH2, 1H), 4.94 (d,
ꢀCH2, 1H, J=4.7 Hz), 5.13–5.31 (m, NH, 2H), 5.72–
5.99 (m, ꢀCH and NH, 3H), 7.04–7.26 (m, C6H5, 5H).
1
mode with Fourier transform. Variable temperature H
NMR spectra were measured on a Varian Unity Plus
600 MHz NMR spectrometer. The chemical shifts were
relative to SiMe4 (1H and 13C), (C6H5O)3PO (31P), and
Na2PtCl6 (195Pt) as an internal or external standard for
the indicated nuclei.
2.2. Synthesis
2.2.1. cis-(TEP)2Pt(ABM)
cis-(TEP)2Pt(NO3)2 (2.22 g, 4.0 mmol) dissolved in
water (100 ml) was eluted through an anion exchange
resin (Amberlite IRA-400 (OH−)) to obtain a dihy-
droxy species, cis-(TEP)2Pt(OH)2, and an additional
amount of water (400 ml) was passed through the
column. To the dihydroxy solution (pH 11.0) was
added dropwise an equimolar aqueous solution of
H2ABM (0.94 g, 4.0 mmol) in water (30 ml). The
reaction mixture was stirred for 16 h at room tempera-
ture (r.t.). The resultant solution was condensed to give
a white product in quantitative yield. Recrystallization
2.2.3. (DMPDA)Pt(ABM)
This compound was prepared in 72% yield by the
same procedure used for (DACH)Pt(ABM). M.p.
171°C (dec.). Anal. Calc. for C18H26N2O4Pt·2H2O: C,
38.2; H, 5.35; N, 4.95. Found: C, 38.2; H, 5.30; N,
4.97%. IR (KBr, cm−1): w(COO)asym, 1630, 1574;
1
w(COO)sym, 1284, 1238. H NMR (D2O, ppm): 0.98 (s,
CH3, 3H), 0.99 (s, CH3, 3H), 1.71 (dd, CH2, 1H,
J=13.2/7.6 Hz), 2.29 (s, CH2N, 2H), 2.61–2.78 (m,
CH2 and CH2N, 3H), 2.93 (d, CH2, 1H, J=13.8 Hz),
3.28 (d, CH2, 1H, J=13.8 Hz), 4.25 (d, ꢀCH2, 1H,

216
Y.-A Lee et al. / Inorganica Chimica Acta 295 (1999) 214–221
J=15.0 Hz), 4.66 (d, ꢀCH2, 1H, J=7.7 Hz), 5.51 (q,
(CꢀO, uncoordinated to Pt). 13C NMR (DMF-d7,
ppm): 23.7, 24.1, 42.0, 44.2, 54.1, 63.3, 115.9 (CꢀC),
126.4, 128.1, 130.9, 138.3, 140.0 (CꢀC), 177.5 (CꢀO).
195Pt NMR (D2O, ppm): −1162.0. 195Pt NMR (DMF-
d7, ppm): −1944.6. 195Pt NMR (CD3OD, ppm):
−1146.9, −1936.1.
1
ꢀCH, 1H), 7.29–7.43 (m, C6H5, 5H). H NMR (DMF-
d7, ppm): 0.94 (s, CH3, 6H), 2.34 (s, CH2N, 4H), 2.63
(d, CH2, 2H, J=6.6 Hz), 3.67 (s, CH2, 2H), 4.87–5.00
(m, ꢀCH2, 2H), 5.37–5.54 (m, NH2, 4H), 5.86–6.01 (m,
ꢀCH, 1H), 7.12–7.33 (m, C6H5, 5H). 1H NMR
(Me2SO-d6, ppm): 0.80 (s, CH3, 6H), 2.05 (s, CH2N,
4H), 2.50 (s, CH2, 2H), 3.63 (s, CH2, 2H), 4.89 (s,
ꢀCH2, 1H), 4.94 (d, ꢀCH2, 1H, J=7.4 Hz), 5.21–5.33
(br, s, NH2, 4H), 5.75–5.89 (m, ꢀCH, 1H), 7.08–7.26
2.3. X-ray analyses of cis-(TEP)2Pt(ABM) and
(DACH)Pt(ABM)·2H2O
1
A single crystal of each compound was wedged in a
Lindemann capillary with mother liquor. The X-ray
data were collected on an Enraf–Nonius CAD4 auto-
matic diffractometer with graphite-monochromated Mo
(m, C6H5, 5H). H NMR (CD3OD, ppm): (O,O%-iso-
mer); 0.85 (s, CH3, 3H), 0.87 (s, CH3, 3H), 3.39 (s, CH2,
2H), 4.98 (d, ꢀCH2, 1H, J=9.9 Hz), 5.05 (d, ꢀCH2, 1H,
J=17.6 Hz), 5.71–5.86 (m, ꢀCH, 1H), 7.13–7.39 (m,
C6H5, 5H), (O,alkene-isomer); 0.94 (s, CH3, 3H), 0.96
(s, CH3, 3H), 1.41 (dd, CH2, 1H, J=12.6/7.4 Hz), 2.58
(dd, CH2, 1H, J=12.7/7.0 Hz), 3.11 (d, CH2, 1H,
J=13.4 Hz), 3.20 (d, CH2, 1H, J=13.4 Hz), 4.00 (d,
ꢀCH2, 1H, J=14.6 Hz), 4.43 (d, ꢀCH2, 1H, J=7.7
Hz), 5.51–5.64 (m, ꢀCH, 1H), 7.13–7.39 (m, C6H5,
5H). 13C NMR (D2O, ppm): 24.0, 26.6, 36.1, 36.2, 44.8,
53.8, 54.2, 68.5, 79.5 (CꢀC, coordinated to Pt), 98.2
(CꢀC, coordinated to Pt), 129.6, 129.7, 130.8, 131.1,
133.3, 140.3, 179.1 (CꢀO, coordinated to Pt), 183.3
,
Ka (u=0.71073 A) at ambient temperature. Unit cell
dimensions were based on 25 well-centered reflections
by using a least-squares procedure. During the data
collection, three standard reflections monitored every
hour did not show any significant intensity variation.
The data were corrected for Lorentz and polarization
effects. Absorption effects were corrected by the em-
pirical C-scan method. The structures were solved by
the Patterson method and were refined by full-matrix
least-squares techniques (SHELX-97) [17]. All non-hy-
drogen atoms were refined anisotropically and hydro-
gen atoms were added at calculated positions. The
largest peak in the final difference synthesis was close to
the heavy platinum atom. Crystal parameters and pro-
cedural information corresponding to data collection
and structure refinement are given in Table 1. See also
Section 5.
Table 1
Crystallographic data for cis-(TEP)2Pt(ABM) and (DACH)-
Pt(ABM)·2H2O
cis-(TEP)2Pt(ABM) (DACH)Pt(ABM)
Formula
Formula weight
T (°C)
C25H42O4P2Pt
663.62
25 (2)
C38H49N4O8Pt2·2H2O
1116.02
25 (2)
3. Results
,
u (A)
0.71073
monoclinic
P21/n (no. 14)
12.719(3)
16.869(7)
12.870(3)
90.0
93.27(2)
90.0
2757(2)
4
0.71073
triclinic
Crystal system
Space group
a (A)
b (A)
c (A)
h (°)
i (°)
k (°)
(
P1 (no. 2)
3.1. Synthesis
,
7.649(3)
14.064(2)
19.190(8)
98.98(2)
90.05(3)
105.73(2)
1961(1)
2
,
The reaction of cis-bis(triethylphosphine)platin-
um(II) dihydroxide with allylbenzylmalonic acid in
aqueous solution at r.t. smoothly afforded the title
complex, cis-(TEP)2Pt(ABM), in quantitative yield. The
complex is a colorless crystalline solid with a sharp
melting point. It is soluble in common organic solvents
such as alcohols, chloroform, and dichloromethane,
and even slightly soluble in water, but insoluble in ether
or hexane. The reaction of (diamine)platinum(II) sul-
fates with barium salts of the ABM ligand in aqueous
solution at r.t. produced the title complexes in high
yields (72ꢀ74%) with precipitation of barium sulfate.
Recrystallization of the complexes in a solvent pair of
water and acetone resulted in colorless crystalline solid
products, which decompose in the range of 171ꢀ
204°C. The products are moderately soluble in water
and in polar organic solvents such as methanol,
ethanol, dimethylformamide, Me2SO, etc., presumably
due to the low hydrophilicities of the anionic ligand.
,
3
,
V (A )
Z
Dcalc (g cm−3
)
1.599
1.890
Absorption coefficient 5.232
(mm−1
F(000)
7.189
)
1328
1090
Crystal size (mm)
q Range (°)
Index ranges
0.30×0.35×0.35
1.99–24.98
h, k,9l
0.35×0.45×0.30
2.15–22.48
h,9k,9l
4520
Reflections collected
3824
Independent reflections 3652
Parameters refined 289
Goodness-of-fit on F2 1.092
4298
477
1.231
Final R indices
[I\2|(I)] a
R1=0.0390, wR2= R1=0.0455, wR2=
0.0931 0.1377
a R1=SꢀꢀFoꢀ−ꢀFcꢀꢀ/SꢀFoꢀ; wR2={Sw(Fo2−Fc2)2/SwFo4}1/2, where
w=1/{|2Fo2+(0.10P)2+0.00P} and P={max(Fo2, 0)+2Fc2}/3.

Y.-A Lee et al. / Inorganica Chimica Acta 295 (1999) 214–221
217
angles are listed in Table 2. As expected, the local
geometry around the platinum(II) atom approximates
to a typical square plane with two phosphorus atoms in
cis positions. The anionic ABM ligand chelated the
platinum atom through two carboxylate groups, result-
ing in (O,O%)-chelation. The bond lengths of C(1)ꢁO(1)
,
,
(1.290(9) A) and C(3)ꢁO(2) (1.292(9) A) are longer than
,
those of C(1)ꢁO(3) (1.203(9) A) and C(3)ꢁO(4)
,
(1.217(9) A), being consistent with typical monodentate
carboxylates in other platinum(II) complexes. The
,
C(5)ꢁC(6) distance of the alkene group (1.33(2) A) is
,
close to that of the normal double bond (1.33 A) [18].
Fig. 1. ORTEP drawing of cis-(TEP)2Pt(ABM) showing the atomic
,
labeling scheme and thermal ellipsoids at the 50% level.
The bond lengths of PtꢁP (2.227(2), 2.234(2) A) are
similar to those of PtꢁP bonds in other phosphine–plat-
inum(II) complexes [16]. The angle of P(1)ꢁPtꢁP(2)
(99.87(8)°) is splayed out due to the steric crowding of
triethyl groups on the phosphorus atom. Thus the
bulkiness of the phosphine ligand is partially responsi-
ble for the distortion of the square planar geometry
around the platinum atom.
For (DACH)Pt(ABM)·2H2O, there are two indepen-
dent molecules in an asymmetric region of a triclinic
unit cell and the structures of the two molecules are
within error of being identical. One of the molecules
and its labeling scheme are depicted in Fig. 2, and the
selected bond distances and angles are listed in Table 2.
The local geometry around the platinum(II) atom also
approximates to a square plane. However, an interest-
ing feature is the bonding mode of the anionic ABM
ligand which chelated the platinum atom through one
Table 2
,
Selected bond lengths (A) and angles (°) for cis-(TEP)2Pt(ABM) and
(DACH)Pt(ABM)·2H2O
cis-(TEP)2Pt(ABM)
PtꢁO(1)
PtꢁO(2)
PtꢁP(1)
2.059(5)
2.066(5)
2.227(2)
2.234(2)
1.290(9)
1.292(9)
1.203(9)
1.217(9)
1.44(1)
O(1)ꢁPtꢁO(2)
O(1)ꢁPtꢁP(1)
O(2)ꢁPtꢁP(2)
87.2(2)
88.5(2)
84.4(2)
99.87(8)
121.5(7)
103.8(6)
118.8(6)
120.4(7)
126(1)
PtꢁP(2)
P(1)ꢁPtꢁP(2)
O(1)ꢁC(1)
O(2)ꢁC(3)
O(3)ꢁC(1)
O(4)ꢁC(3)
C(4)ꢁC(5)
C(5)ꢁC(6)
O(4)ꢁC(3)ꢁO(2)
C(1)ꢁC(2)ꢁC(3)
C(8)ꢁC(7)ꢁC(2)
O(3)ꢁC(1)ꢁC(2)
C(6)ꢁC(5)ꢁC(4)
1.33(2)
(DACH)Pt(ABM)·2H2O
Pt(1)ꢁO(1)
Pt(1)ꢁN(1)
Pt(1)ꢁN(2)
Pt(1)ꢁC(8)
Pt(1)ꢁC(7)
O(1)ꢁC(11)
O(2)ꢁC(11)
O(3)ꢁC(12)
O(4)ꢁC(12)
C(7)ꢁC(8)
1.979(9)
2.02(1)
2.09(1)
2.16(1)
2.17(1)
1.31(3)
1.21(2)
1.22(2)
1.26(2)
1.34(2)
O(1)ꢁPt(1)ꢁC(8)
O(1)ꢁPt(1)ꢁC(7)
O(1)ꢁPt(1)ꢁN(1)
N(1)ꢁPt(1)ꢁN(2)
C(8)ꢁPt(1)ꢁC(7)
O(1)ꢁC(11)ꢁO(2)
O(3)ꢁC(12)ꢁO(4)
C(7)ꢁC(8)ꢁC(9)
90.0(5)
93.0(5)
171.8(3)
84.2(4)
36.1(5)
121(1)
,
of the two carboxylate groups (Pt(1)ꢁO(1), 1.979(9) A)
and the alkene group (Pt(1)ꢁC(7), 2.17(1); Pt(1)ꢁC(8),
,
2.16(1) A), resulting in (O,alkene)-chelation. Thus an-
124(1)
124(1)
other carboxylate group is uncoordinated in contrast to
the (O,O%)-chelation mode of cis-(TEP)2Pt(ABM). For
the coordinated carboxylate group, the bond length of
,
C(11)ꢁO(1) (1.31(2) A) is longer than that of C(11)ꢁ
,
O(2) (1.21(2) A). The corresponding bond lengths of
the dangling carboxylate group are 1.22(2) (C(12)ꢁO(3))
,
and 1.26(2) A (C(12)ꢁO(4)), which indicate the rela-
tively delocalized charge distribution. The C(7)ꢁC(8)
,
bond length of the alkene group (1.34(2) A) is similar to
,
that of the normal double bond (1.33 A). The alkene
group is positioned perpendicularly to the platinum
plane and the C(7)ꢁPtꢁC(8) angle of 36.1(5)° is smaller
than the values of 40 to 43° observed in other known
alkene–platinum(II) complexes [19,20]. Intermolecular
hydrogen bonds exist between the carboxylate group
Fig. 2. ORTEP drawing of (DACH)Pt(ABM)·2H2O showing the
atomic labeling scheme and thermal ellipsoids at the 50% level.
,
and the amine NH (O···N% distances, 2.79–2.86 A).
These intermolecular hydrogen bonds may give rise to
additional stability for the (O,alkene)-chelate in the
solid state.
They are stable in aqueous and organic solutions at
least for a few weeks at r.t.
The chelation mode of the anionic ligand in the solid
state can also be clearly discerned by the stretching
frequencies of the carboxylate groups in the IR spectra
[21]. For cis-(TEP)2Pt(ABM), two asymmetric carboxy-
3.2. Solid state structures
The X-ray crystal structure of cis-(TEP)2Pt(ABM) is
depicted in Fig. 1, and relevant bond distances and

218
Y.-A Lee et al. / Inorganica Chimica Acta 295 (1999) 214–221
late stretching frequencies at 1646 and 1616 cm−1 and
two corresponding symmetric stretching frequencies at
1360 and 1346 cm−1 indicate that the two carboxylate
groups are not equivalent, but the large separation
(Dw\270 cm−1) between the asymmetric and symmet-
ric stretching bands implies that both carboxylates of
the ABM ligand are coordinated in a monodentate
fashion to the platinum atom, which is in accord with
the aforementioned X-ray data. For (DMPDA)-
Pt(ABM), two asymmetric carboxylate bands were also
observed at 1630 and 1574 cm−1, but the band at 1630
cm−1 corresponds to the carboxylate group coordi-
nated to the platinum atom, while the band at 1574
cm−1 is due to the uncoordinated carboxylate, which
lies in the same region as its free ligand (1568 cm−1).
Thus, the IR spectra for (DMPDA)Pt(ABM) disclose
coexistence of a coordinated and a dangled carboxylate
group, i.e. (O,alkene)-chelation. The spectral similarity
of the carboxylate bands of (DACH)Pt(ABM) to that
of (DMPDA)Pt(ABM) suggests that the bonding mode
of the anionic ligand is not dependent on these kinds of
amine coligands.
platinum species in the solution. Furthermore, these
chemical shifts are similar to those of cis-
(PMe3)2Pt(oxalate) (31P NMR (D2O) l= −26.7; 195Pt
NMR (D2O) l= −4334), which were already eluci-
1
dated as cis-(P,P)Pt(O,O) [22]. The H NMR spectra
disclose that the allybenzylmalonate group is coordi-
nated to the platinum(II) atom in (O,O%)-chelation
mode in various polar solvents such as CD3OD,
Me2SO, DMF, and D2O. Thus, for the present phos-
phine complex, only the (O,O%)-chelate was obtained in
the solid state and the (O,O%)-bonding mode of the
allybenzylmalonate ligand is retained without dissocia-
tion or isomerization in these solutions.
However, for the (diamine)Pt(ABM) complexes,
multinuclear (1H, 13C, and 195Pt) NMR studies have
shown that the chelation mode of the anionic ABM
ligand is prominently solvent-dependent, in contrast to
the above phosphine analog. In D2O solution, the 13C
NMR spectrum of (DMPDA)Pt(ABM) exhibits two
resonances at 183.3 and 179.1 ppm, corresponding to a
free carboxylate and a coordinated carboxylate, respec-
tively. The alkene resonances at 98.2 and 79.5 ppm are
significantly shifted upfield compared to those of the
potassium salt of ABM (139.3 and 117.8 ppm). Such
results clearly indicate that the (O,alkene)-chelation
mode of the ABM ligand is retained in aqueous solu-
tion. In DMF solution, however, the 13C NMR spec-
trum of (DMPDA)Pt(ABM) shows a quite different
resonance pattern. The 13C chemical shift of the car-
boxylate groups shows a single resonance at 176.1 ppm,
and the alkene resonances (140.0 and 115.9 ppm) of the
ABM ligand are similar to those of its potassium salt.
Thus, for (DMPDA)Pt(ABM), the ABM ligand
chelates the platinum atom through two carboxylate
groups in DMF solution, in contrast to its (O,alkene)-
mode in the solid state and in aqueous solution. The
1H NMR spectra of (DMPDA)Pt(ABM) in different
solvents at r.t. are presented in Fig. 3. The ꢀCH2
protons of ABM in D2O (Fig. 3(a)) show two doublets
at 4.66 (3Jtrans=7.7 Hz) and 4.25 (3Jcis=15.0 Hz) ppm
that shift to higher field compared to those of a free
alkene group. The methylene proton resonances of the
ABM and DMPDA ligands are complicated due to the
unsymmetrical ligation of the ABM ligand, i.e.
3.3. Sol6ent effect
The 31P NMR spectrum of cis-(TEP)2Pt(ABM) in
Me2SO solution exhibits a single signal flanked by 195Pt
satellites (1JPtꢁP=3536.0 Hz) at 27.7 ppm, and the 195Pt
NMR spectrum shows only a triplet at −4198.6 ppm
(1JPtꢁP=7056.5 Hz), which reflects the presence of one
1
(O,alkene)-chelation. However, its H NMR spectrum
(Fig. 3(c)) in Me2SO-d6 is prominently different from
that in D2O. The ꢀCH2 protons of ABM in D2O (Fig.
3(a)) show a doublet at 4.94 ppm and a singlet at 4.89
ppm, disclosing that the alkene group of the ABM
ligand is not coordinated in Me2SO. The complex has
the same bonding mode in DMF as that in Me2SO.
The chemical shifts and the resonance pattern of the
complex in CD3OD (Fig. 3(b)) indicate an equilibrium
between the two chelation modes at r.t. For
(DACH)Pt(ABM), similar spectral patterns were ob-
served in each solution.
Fig. 3. 1H NMR spectra of (DMPDA)Pt(ABM) in D2O (a), CD3OD
(b), and Me2SO-d6 (c). The star-marked peaks are due to solvent
molecules.

Y.-A Lee et al. / Inorganica Chimica Acta 295 (1999) 214–221
219
4(b)) shows two resonances at −1936 and
−1147 ppm with the ratio of approximately 1:2 at r.t.,
supporting that the two isomers coexist in methanol
solution. The signal at −1936 ppm corresponds to the
(O,O%)-chelate while the chemical shift at −1147 ppm
is assignable to the (O,alkene)-chelate. The 195Pt NMR
spectrum in D2O (Fig. 4(c)) shows a single resonance at
−1162 ppm, indicating the presence of only one
(O,alkene)-isomer. Thus, for the present platinum com-
plexes, variation of medium significantly affects the
coordination mode of the anionic ligand.
3.4. Temperature effect
Fig. 4. 195Pt NMR spectra of (DMPDA)Pt(ABM) in DMF-d7 (a),
CD3OD (b), and D2O (c).
The proton spectrum of (DMPDA)Pt(ABM) in
CD3OD exhibits a marked temperature dependence in
the range of 188–328 K (Fig. 5). The spectrum
of (DMPDA)Pt(ABM) in CD3OD at r.t. (at 298 K)
shows two sets of resonance signals, indicating coexis-
tence of the two linkage isomers, (O,O%)- and
(O,alkene)-chelates, in the ratio of approximately 1:2.
When the solution was cooled down to 188 K, the ratio
of two linkage isomers did not change, and only some
peaks were broadened. However, warming up the solu-
tion resulted in gradual increasing of the (O,alkene)-
chelate (indicated as ¡ in Fig. 5), and at 328 K (the limit
temperature of CH3OH), the ratio of the (O,O%)- and
(O,alkene)-chelates became approximately 2:1. An
The 195Pt NMR spectra demonstrate more clearly the
structural variation dependent on the solvent. The 195Pt
NMR spectra of (DMPDA)Pt(ABM) were measured in
various solvents and shown in Fig. 4. The spectrum in
DMF solution (Fig. 4(a)) exhibits only one 195Pt reso-
nance, reflecting the presence of one platinum species in
the solution. Furthermore, the chemical shift at −1945
ppm in DMF-d7 lies in a similar region to that of
(DMPDA)Pt(DAM) (−1932 ppm), which were already
elucidated as (N,N%-DMPDA)Pt(O,O%-DAM) [11].
However, the 195Pt NMR spectrum in CD3OD (Fig.
Fig. 5. Variable-temperature 1H NMR spectra of (DMPDA)Pt(ABM) in CD3OD (500 MHz). The star-marked peak is due to water molecule.

220
Y.-A Lee et al. / Inorganica Chimica Acta 295 (1999) 214–221
Therefore, it appears that the (O,alkene)-mode is more
stable in the protic solvent, whereas the (O,O%)-mode is
relatively more stable in aprotic solvents such as Me2SO
and dimethylformamide. Such a result proves that sol-
vent molecules play an important role in the determina-
tion of the coordination mode. In polarity, dielectric
constant, and hydrogen bonding ability, MeOH is a
medium between the two extremes, H2O and Me2SO,
and as such the two chelation modes coexist in MeOH
solution, its equilibrium being dependent on tempera-
ture. Temperature-dependent NMR spectra have been
used to determine the transition temperature and
reversibility. For instance, (DMPDA)Pt(ABM) in
CD3OD exhibited an equilibrium between the two spe-
cies at r.t., but warming up the sample, the (O,O%)-mode
is favored because the hydrogen bonding is weakened at
high temperature. This result is an evidence for the
stabilization by hydrogen bonding. For the (diamine)-
platinum complexes, the two chelation modes seem to
be energetically close to each other.
In conclusion, it has been shown in this study that
the electronic effect of the coligand on the linkage
isomerism was dominant compared with the solvent and
temperature effects. However, solvent and temperature
are also important in controlling the linkage isomerism
that may be useful in molecular control. In particular,
the interconversion properties between the (O,O%)- and
(O,alkene)-modes appear to be associated with proxim-
ity in energy between the two chelation modes of the
allylbenzylmalonate ligand. Various factors should be
understood prior to molecular design of new platinum
compounds that exhibit desirable molecular properties.
interesting feature is that such a switching is essentially
reversible on temperature: at low temperature the ABM
ligand has a chelation mode of (O,alkene) whereas at
elevated temperature the equilibrium shifts to the
(O,O%)-chelate. For the phosphine analog such a linkage
isomerism was not observed and only the (O,O%)-chela-
tion mode was locked in the temperature range of 188
to 340 K.
4. Discussion
Structural characterization of the platinum(II) com-
plexes containing the ABM ligand has shown that
different linkage isomers were observed that depended
on the subtle change of various factors. Different bond-
ing fashions of the anionic ABM ligand were clearly
observed depending on different coligands: the ABM
complexes have shown (O,O%)- and (O,alkene)-chelation
modes in the solid state with phosphine and amine
coligands, respectively. Such a difference between the
chelation modes seems to be due to the stabilization
induced by the electronic effect of the coligands. The
phosphine ligand is known to form a strong p bond
with transition metals by back-donation (dp–dp) [23].
Therefore, it is presumed that the back-donation from
the two phosphorus atoms (3dxz, 3dyz) to the platinum
atom (5dxz, 5dyz) excludes p-bonding by the alkene
group. Such a strong electronic effect seems to be
superior to the solvent and temperature effects, and
consequently, only (O,O%)-chelation mode is locked for
the (diphosphine)platinum(II) complex.
However, for the amine analogs without competition
for p-bonding (amine can not form a p bond), the
ethylene group is preferentially bonded. Moreover, the
intermolecular hydrogen bonds between the carboxylate
and amine NH groups may give rise to additional
stability for (O,alkene)-chelation in the solid state. In
solutions, the present ABM complexes exhibit linkage
isomerism between the (O,O%)- and (O,alkene)-chelates
similarly to the allyl- and diallylmalonate complexes
previously reported [11], and the hydrogen bonding
ability of solvent with the platinum complex molecules
seems to play an important role in controlling the
coordination mode of the ABM ligand. The (O,alkene)-
chelated species has a zwitterionic form, which interacts
with the solvent molecules through hydrogen bonding.
5. Supplementary material
Details of atomic coordinates, anisotropic thermal
parameters, lists of bond lengths and angles, and 13C
NMR spectra of (DMPDA)Pt(ABM) are supplied. Ad-
ditional information, such as final atomic coordinates
and isotropic thermal parameters of cis-(TEP)2-
Pt(ABM) and (DACH)Pt(ABM)·2H2O, is available
from the author on request.
Acknowledgements
This research was financially supported by the Min-
istry of Science and Technology in Korea.
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