Dual role of acetate as a nucleophile and as an internal base in cycloplatination reaction of sym - N, N ', N '-triarylguanidines

Article abstract of DOI:10.1021/ic302058u

Reaction of cis-[Cl₂Pt(S(O)Me₂)₂] with 1 equiv of sym-N,N',N'-triarylguanidines, ArN=C(NHAr)₂ (sym = symmetrical; Ar = 2-MeC₆H₄ (LH₂ ^2-tolyl), 2-(MeO)C₆H

Full text of DOI:10.1021/ic302058u

Article
pubs.acs.org/IC
Dual Role of Acetate as a Nucleophile and as an Internal Base in
Cycloplatination Reaction of sym-N,N′,N″‑Triarylguanidines
Palani Elumalai,† Natesan Thirupathi,*,† and Munirathinam Nethaji‡
†Department of Chemistry, University of Delhi, Delhi 110 007, India
‡Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
S
* Supporting Information
ABSTRACT: Reaction of cis-[Cl2Pt(S(O)Me2)2] with 1 equiv of
sym-N,N′,N″-triarylguanidines, ArNC(NHAr)2 (sym = sym-
metrical; Ar = 2-MeC6H4 (LH22‑tolyl), 2-(MeO)C6H4 (LH22‑anisyl),
4-MeC6H4 (LH24‑tolyl), 2,5-Me2C6H3 (LH22,5‑xylyl), and 2,6-
Me2C6H3 (LH22,6‑xylyl)) in toluene under reflux condition for 3
h afforded cis- or trans-[Cl2Pt(S(O)Me2)(ArNC(NHAr)2)]
(Ar = 2-MeC6H4 (1), 2-(MeO)C6H4 (2), 4-MeC6H4 (3), 2,5-
Me2C6H3 (4), and 2,6-Me2C6H3 (5), respectively) in 83−96%
2‑tolyl
yield. Reaction of cis-[Cl2Pt(S(O)Me2)2] with 1 equiv of LH2
and LH2
4‑tolyl
in the presence of 1 equiv of NaOAc in methanol
under reflux condition for 3 h afforded acetate-substituted products, cis-[(AcO)ClPt(S(O)Me2)(ArNC(NHAr)2)] (Ar = 2-
MeC6H4 (6) and 4-MeC6H4 (7)) in 83% and 84% yields, respectively. Reaction of cis-[Cl2Pt(S(O)Me2)2] with 1 equiv of
2‑anisyl
2‑tolyl
LH2
and LH2
in the presence of 1 equiv of NaOAc in methanol under reflux condition for 3 and 12 h afforded six-
membered [C,N] platinacycles, [Pt{κ2(C,N)-C6H3R-3(NHC(NHAr)(NAr))-2}Cl(S(O)Me2)] (Ar = 2-RC6H4; R = OMe (8)
and Me (9)), in 92% and 79% yields, respectively. The new complexes have been characterized by analytical and spectroscopic
techniques, and further the molecular structures of 1, 2, 4, 5, 6, and 8 have been determined by single-crystal X-ray diffraction.
The platinum atom in 1, 4, and 5 exhibited the trans configuration, while that in 2, 6, and 8 exhibited the cis configuration.
Complex 6 is shown to be the precursor for 9, and the former is suggested to transform to the latter possibly via an
intramolecular C−H activation followed by elimination of AcOH. The solution behavior of new complexes has been studied by
multinuclear NMR (1H, 195Pt, and 13C) spectroscopy. The new complexes exist exclusively as a single isomer (trans (1 and 5)
and cis (6 and 7)), a mixture of cis and trans isomers with the former isomer being predominant in the case of 2 and the latter
isomer being predominant in the case of 3. Complex 5 in the trans form revealed the presence of one isomer at 0.007 mM
1
concentration and two isomers in about 1.00:0.12 ratio at 0.154 mM concentration as revealed by H NMR spectroscopy, and
this has been ascribed to the restricted Pt−S bond rotation at higher concentration. Platinacycle 8 exists as one isomer, while 9
exists as a mixture of seven isomers in solution. The influence of steric factor, π-acceptor property of the guanidine, subtle solid-
state packing forces upon the configuration of the platinum atom, and the number of isomers in solution have been outlined.
Factors that accelerate or slow down the cycloplatination reaction, the role of NaOAc, and a plausible mechanism of this reaction
have been discussed.
Only two platinum(II) complexes of sym-N,N′,N″-triphenyl-
guanidine, PhNC(NHPh)2 (LH2Ph), have been structurally
characterized,23,24 although synthesis, reactivity studies, and
structural aspects of platinum(II) complexes of other classes of
guanidines have been steadily increasing since 1988.25−29
Steric factor and basicity of primary amines have been shown
to promote the cycloplatination reaction.30 We wanted to study
how the subtle variations in steric, basic, and conformational
properties of sym-N,N′,N″-triarylguanidines, ArNC(NHAr)2
(Ar = 2-MeC6H4 (LH22‑tolyl), 2-(MeO)C6H4 (LH22‑anisyl), 4-
MeC6H4 (LH24‑tolyl), 2,5-Me2C6H3 (LH22,5‑xylyl), and 2,6-
Me2C6H3 (LH22,6‑xylyl)), influence the course of the cyclo-
platination reaction. The aforementioned properties of sym-
INTRODUCTION
■
Platinum(II) imine complexes and imine-derived [C,N] and
[C,N,N′] types of platinacycles have been studied intensively in
the past due to their relevance in inorganic and organometallic
syntheses, reactivity pattern, and structural aspects.1−12 Further,
these complexes exhibit interesting biological activity,13,14
materials properties such as mesomorphism, liquid crystalline
property,15,16 and photophysical17,18 and catalytic properties.19
sym-N,N′,N″-Trisubstituted guanidines (RNH)2CNR (sym =
symmetrical; R = alkyl and aryl) are one of the interesting
classes of nitrogen donor ligands, capable of binding the metal
ion in their (i) neutral form in monodentate (κ1N)
coordination mode, (ii) monoanionic form (i.e., guanidi-
nate(1−)) in chelating (κ2N,N′) and bridging (μ2-κ1N;κ1N′)
coordination modes, and (iii) dianionic form (i.e., guanidi-
nate(2−)) in chelating coordination mode (see Chart 1).20−22
Received: September 24, 2012
Published: February 7, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Article
C, 42.80; H, 4.34; N, 6.24; S, 4.76. Found: C, 42.96; H, 4.18; N, 6.12;
S, 4.40.
Chart 1
Complex 2. Complex 2 was prepared from cis-[Cl2Pt(S(O)Me2)2]
2‑anisyl
(50.0 mg, 0.118 mmol) and LH2
(45.0 mg, 0.119 mmol) in
toluene (15 mL) and purified as described previously for 1. Yield: 83%
(82.0 mg, 0.098 mmol). Mp: 165.0 °C (decomp). FT-IR (KBr, cm−1):
ν(NH) 3449 (br, vs); ν(CN) 1615 (vs); ν(SO) 1116 (m). Two
isomers were observed in about a 1.0:2.1 ratio as estimated from the
1
integrals of OCH3 and NH protons of the guanidine unit. H NMR
(CDCl3, 300 MHz): δ 3.23 (br, 6H, (CH3)2S(O), major isomer), 3.26
(s, 6H, (CH3)2S(O), minor isomer), 3.59 (br, 3H, OCH3, major
isomer), 3.62 (s, 3H, OCH3, minor isomer), 3.68 (br, 3H, OCH3,
major isomer), 3.71 (s, 3H, OCH3, minor isomer), 3.82 (s, 3H, OCH3,
minor isomer), 3.86 (br, 3H, OCH3, major isomer), 6.47−7.19 (m,
24H, ArH), 7.81, 7.84 (each s, 2H, NH, minor isomer), 8.04 (s, 2H,
NH, major isomer). 195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −2721
(major isomer), −2771 (minor isomer). MS (TOF-ES+), m/z
(intensity %), [ion]: 761 (5), [M + K]+; 744 (7), [M + Na]+; 724
(9) [M + K − HCl], 648 (74), [M − 2HCl]+; 378 (30), [LH32‑anisyl]+,
377 (100), [LH22‑anisyl]+. Anal. Calcd for C24H29N3O4SCl2Pt·CHCl3
(Mw 840.96): C, 35.71; H, 3.60; N, 5.00; S, 3.81. Found: C, 35.68; H,
3.65; N, 4.94; S, 3.92. Pale yellow crystals of 2·1.5CHCl3 suitable for
X-ray diffraction were grown from toluene/CHCl3 mixture in a 5 mm
NMR tube at ambient temperature over a period of 1 week.
Complex 3. Complex 3 was prepared from cis-[Cl2Pt(S(O)Me2)2]
(100 mg, 0.237 mmol) and LH24‑tolyl (78.0 mg, 0.237 mmol) in toluene
(15 mL) and purified as described previously for 1. Yield: 93% (158
mg, 0.220 mmol). Mp (DSC): 161.7 °C. FT-IR (KBr, cm−1): ν(NH)
3472 (br), 3402 (br), ν(N−H···Cl) 3262 (m); ν(CN) 1619 (vs);
ν(SO) 1129 (m). 1H NMR (CDCl3, 400 MHz): δ 2.15, 2.20 (each
br, 2 × 3H, CH3), 2.30 (s, 3 H, CH3), 3.27, 3.34 (each s, 2 × 3H,
(CH3)2S(O)), 5.89 (br, 1H, NH), 6.72, 6.82, 6.94, 6.97 (each br, 8H,
ArH), 7.12, 7.39 (each d, JH,H = 8.0 Hz, 2 × 2H, ArH), 8.00 (br, 1H,
NH). 13C{1H} NMR (CDCl3, 100.5 MHz): δ 20.8 (br), 21.1 (CH3),
43.6 ((CH3)2S(O)), 122.5, 123.8, 126.7, 129.3, 129.5, 130.2, 134.3,
134.7, 135.2, 136.0, 141.8 (ArC and ArCH), 153.6 (CN). Two
carbon resonances were observed for CH3 carbons and 11 carbon
resonances for aryl carbons of the guanidine unit rather than the
expected 3 and 12 resonances, respectively, due to overlapping peaks.
Two days old CDCl3 solution of 3 revealed the formation of two
isomers in about a 1:0.2 ratio as estimated from the integrals of CH3
and NH protons of the guanidine unit. 1H NMR (CDCl3, 400 MHz):
δ 2.14 (br, 3H, CH3, major isomer), 2.17 (s, 3H, CH3, minor isomer),
2.20 (br, 3H, CH3, major isomer), 2.23 (s, 3H, CH3, minor isomer),
2.27 (s, 3H, CH3, minor isomer), 2.30 (s, 3H, CH3, major isomer),
3.28 (s, 3H, (CH3)2S(O), major isomer), 3.32 (s, 2 × 3H,
(CH3)2S(O), minor isomer), 3.34 (s, 3H, (CH3)2S(O), major isomer),
5.84 (br, 1H, NH, minor isomer), 5.92 (br, 1H, NH, major isomer),
6.72, 6.80, 6.82, 6.94, 6.97, 6.99 (each br, 2 × 8H, ArH, major and
minor isomers), 7.09 (d, JH,H = 8.0 Hz, 2H, ArH, minor isomer), 7.12
(d, JH,H = 8.4 Hz, 2H, ArH, major isomer), 7.35 (d, JH,H = 8.0 Hz, 2H,
ArH, minor isomer), 7.39 (d, JH,H = 8.4 Hz, 2H, ArH, major isomer),
7.86 (br, 1H, NH, minor isomer), 8.02 (br, 1H, NH, major isomer).
195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −2898 (minor isomer),
−3009 (major isomer). MS (TOF-ES+), m/z (intensity %), [ion]: 661
(8), [M + Na − Cl]+; 639 (18), [M + H − Cl]+; 603 (20), [M −
2Cl]+ ; 330 (100), [LH3 4 ‑ t o l y l ]+ . Anal. Calcd for
C24H29N3SOCl2Pt·0.5C7H8 (Mw 719.64): C, 45.90; H, 4.62; N,
5.84; S, 4.46. Found: C, 45.80; H, 4.57; N, 5.89; S, 4.46.
N,N′,N″-triarylguanidines permitted us to incorporate them as
κ2N,N′ guanidinate(1−) around the ruthenium(II) atom and as
κ2C,N monoanionic scaffolds around the palladium(II) atom in
the past.31 Further, the substituent(s) on the aryl rings of sym-
N,N′,N″-triarylguanidine units in palladium(II) and ruthenium-
(II) complexes have been shown to substantially influence the
solid-state structures and solution behavior of these complexes.
Herein, we report the high-yield synthesis and character-
ization of three classes of platinum(II) guanidine complexes,
namely, cis- or trans-[Cl2Pt(S(O)Me2)(ArNC(NHAr)2)] (Ar
= 2-MeC6H4 (1), 2-(MeO)C6H4 (2), 4-MeC6H4 (3), 2,5-
Me2C6H3 (4), and 2,6-Me2C6H3 (5)), acetate-substituted
products, cis-[(AcO)ClPt(S(O)Me2)(ArNC(NHAr)2)] (Ar
= 2-MeC6H4 (6) and 4-MeC6H4 (7)), and six-membered [C,N]
platinacycles, cis-[Pt{κ2(C,N)-C6H3R-3(NHC(NHAr)(
NAr))-2}Cl(S(O)Me2)] (Ar = 2-RC6H4; R = OMe (8) and
Me (9)). The molecular structures of seven compounds
(LH22,5‑xylyl, 1, 2, 4, 5, 6, and 8) have been determined by single-
crystal X-ray diffraction. Factors that determine the time scale
of cycloplatination, the role of NaOAc, and a plausible
mechanism of this reaction with sym-N,N′,N″-triarylguanidines
have been outlined.
EXPERIMENTAL SECTION
Full experimental details pertinent to general considerations may be
found in the Supporting Information.
■
Complex 1. cis-[Cl2Pt(S(O)Me2)2] (100 mg, 0.237 mmol) and
LH22‑tolyl (78.0 mg, 0.237 mmol) were dispersed in toluene (15 mL) in
a 50 mL round-bottom flask that was fitted to a water condenser
capped with an anhydrous CaCl2 guard tube. The reaction mixture in
the flask was simultaneously stirred and refluxed for 3 h. During the
course of the reaction, the solid slowly dissolved to afford a clear
yellow solution. Subsequently, the reaction mixture was cooled and
concentrated under vacuum to about 5 mL. The solution was stored at
ambient temperature to afford a solid. The solid was crystallized from
CHCl3/toluene mixture at ambient temperature over a period of
several hours to afford 1 as pale yellow crystals. Yield: 96% (153 mg,
0.227 mmol). Mp (DSC): 195.8 °C. FT-IR (KBr, cm−1): ν(NH) 3462
(br, m), 3377 (m); ν(CN) 1620 (vs); ν(SO) 1142 (m). 1H
NMR (CDCl3, 300 MHz): δ 1.97, 2.34, 2.60 (each s, 3 × 3H, CH3),
3.31 (s, 6H, (CH3)2S(O)), 5.62 (s, 1H, NH), 6.83−7.21 (m, 11H,
ArH), 7.76 (d, JH,H = 7.8 Hz, 1H, ArH), 7.90 (s, 1H, NH). 13C{1H}
NMR (CDCl3, 100.5 MHz): δ 17.64, 18.20, 19.39 (CH3), 43.63
((CH3)2S(O)), 124.61, 125.07, 126.19, 126.26, 126.38, 126.44, 126.95,
127.10, 129.16, 130.45, 130.62, 131.63, 131.86, 132.50, 134.36, 135.41,
135.56, 143.01 (ArC and ArCH), 154.54 (CN). 195Pt{1H} NMR
(85.8 MHz, CDCl3): δ −2967. MS (TOF-ES+), m/z (intensity %),
[ion]: 674 (37), [M]+; 618 (12), [M + Na − S(O)Me2]+; 586 (39),
[M − (2HCl, Me)]+. Anal. Calcd for C24H29N3SOCl2Pt (Mw 673.57):
Complex 4. Complex 4 was synthesized from cis-[Cl2Pt(S(O)-
Me2)2] (101 mg, 0.239 mmol) and LH22,5‑xylyl (89.0 mg, 0.240 mmol)
in toluene (15 mL) and purified as described previously for 1. The
sample was crystallized from CH2Cl2/toluene mixture at ambient
temperature over a period of several hours to afford 4 as pale yellow
crystals. Yield: 94% (161 mg, 0.225 mmol). Mp (DSC): 199.6 °C. FT-
IR (KBr, cm−1): ν(NH) 3447 (br, m), 3374 (m); ν(CN) 1602 (vs);
ν(SO) 1139 (m). The number of isomers in solution varied
1
depending upon the concentration of the sample. H NMR (CDCl3,
300 MHz, 0.007 mM): δ 1.94, 2.14, 2.18, 2.30, 2.31, 2.54 (each s, 6 ×
3H, CH3), 3.30, 3.31 (each s, 2 × 3H, (CH3)2S(O)), 5.61 (s, 1H,
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Inorganic Chemistry
Article
NH), 6.64 (d, JH,H = 7.8 Hz, 1H, ArH), 6.71−6.75 (m, 2H, ArH), 6.84
(s, 1H, ArH), 6.89−6.93 (m, 2H, ArH), 6.95 (s, 1H, ArH), 7.07 (d,
JH,H = 7.5 Hz, 1H, ArH), 7.55 (s, 1H, ArH), 7.69 (s, 1H, NH).
13C{1H} NMR (CDCl3, 75.5 MHz, 0.084 mM): δ 17.1, 17.6, 18.9,
20.6, 20.7, 21.0 (CH3), 43.5 ((CH3)2S(O)), 125.2, 125.6, 126.7, 126.8,
127.6, 128.4, 129.0, 129.4, 130.0, 130.1, 131.0, 131.2, 135.0, 135.1,
136.0, 136.7, 142.8 (ArC and ArCH), 154.1 (CN). Note: Only 17
carbon resonances were observed for aryl carbons of the guanidine
unit rather than the expected 18 peaks, presumably due to overlapping
peaks. 195Pt{1H} NMR (CDCl3, 85.8 MHz): δ −2959. Two isomers
were observed in about a 1.00:0.12 ratio at 0.154 mM concentration as
estimated from the integrals of CH3 and NH protons of the guanidine
mmol) were charged into a 25 mL round-bottom flask in methanol
(10 mL), and the flask was fitted to a water condenser capped with an
anhydrous CaCl2 guard tube. The reaction mixture in the flask was
simultaneously stirred and refluxed for 3 h and cooled to ambient
temperature. The volatiles from the reaction mixture were removed
under vacuum to afford a solid. The solid was dissolved in CH2Cl2 and
filtered. The filtrate was layered with toluene and stored at ambient
temperature over a period of several days to afford 6·S(O)Me2 as pale
yellow crystals in 83% (78.0 mg, 0.101 mmol) yield. Mp: 198.6 °C.
FT-IR (KBr, cm−1): ν(NH) 3440 (br m), 3384 (m); νa(OCO) 1623
(vs); ν(CN) 1603 (vs); νs(OCO) 1384 (m); ν(SO) 1132 (m).
1H NMR (CDCl3, 300 MHz): δ 1.96, 2.13, 2.30, 2.52 (each s, 4 × 3H,
CH3), 3.34, 3.53 (each s, 2 × 3H, (CH3)2S(O)), 5.64 (s, 1H, NH),
6.76 − 6.82 (m, 3H, ArH), 6.87 (t, JH,H = 7.2 Hz, 2H, ArH), 6.98 (d,
JH,H = 6.9 Hz, 1H, ArH), 7.05 (d, JH,H = 7.9 Hz, 1H, ArH), 7.16−7.18
(m, 4H, ArH), 7.67 (d, JH,H = 7.8 Hz, 1H, ArH), 10.60 (s, 1H, NH).
13C{1H} NMR (CDCl3, 100.5 MHz): δ 17.60, 18.15, 19.35, 20.88
(CH3), 43.50 ((CH3)2S(O)), 124.58, 125.07, 126.17, 126.20, 126.33,
126.40, 126.89, 127.04, 129.12, 130.41, 130.57, 131.57, 131.84, 132.52,
134.32, 135.39, 135.54, 142.98 (ArC and ArCH), 154.55 (CN),
176.80 (OC(O)). 195Pt{1H} NMR (85.8 MHz, CDCl3): δ −2718. MS
(TOF-ES+), m/z (intensity %), [ion]: 736 (8), [M + K]+; 698 (25),
[M + H]+; 662 (20), [M − Cl]+; 610 (20), [M + Li − (OAc, Cl)]+;
331 (100), [LH32‑tolyl]+. Anal. Calcd for C26H32N3O3SClPt·(CH3)2S-
(O) (Mw 775.34): C, 43.38; H, 4.94; N, 5.42; S, 8.27. Found: C, 43.35;
H, 4.91; N, 5.42; S, 8.27. Greenish yellow crystals of 6·S(O)Me2 for X-
ray diffraction were grown from methanol/CHCl3 mixture at ambient
temperature over a period of several days.
1
unit (see the Supporting Information). H NMR (CDCl3, 400 MHz,
0.154 mM): δ 1.90 (s, 3H, CH3, minor isomer), 1.94 (s, 3H, CH3,
major isomer), 2.12 (s, 3H, CH3, minor isomer), 2.14 (s, 2 × 3H, CH3,
major and minor isomers), 2.18 (s, 3 H, CH3, major isomer), 2.30,
2.31 (each s, 3 × 3H, CH3, major isomer (6H), minor isomer (3H)),
2.35 (s, 2 × 3H, CH3, minor isomer), 2.55 (s, 3H, CH3, major isomer),
3.26, 3.27 (each s, 2 × 3H, (CH3)2S(O), major isomer), 3.39, 3.45
(each s, 2 × 3H, (CH3)2S(O), minor isomer), 5.63 (s, 1H, NH, major
isomer), 5.74 (s, 1H, NH, minor isomer), 6.57 (d, JH,H = 7.8 Hz, 1H,
ArH, minor isomer), 6.64 (d, JH,H = 7.8 Hz, 1H, ArH, major isomer),
6.67−6.75 (m, 2 × 2H, ArH, major and minor isomers), 6.85, 6.90,
6.92, 6.96 (each s, 4H, ArH, major isomer), 7.07 (d, JH,H = 7.8 Hz, 1H,
ArH, major isomer), 7.13−7.18 (m, 3H, ArH, minor isomer), 7.24 (d,
JH,H = 7.3 Hz, 1H, ArH, minor isomer), 7.43 (s, 1H, ArH, minor
isomer), 7.56 (s, 1H, ArH, major isomer), 7.71 (s, 1H, NH, major
isomer), 7.94 (s, 1H, NH, minor isomer), 7.97 (s, 1H, ArH, minor
isomer). The 13C{1H} signals of the minor isomer are indicated by the
asterisk (*). 13C{1H} NMR (CDCl3, 100.5 MHz, 0.154 mM): δ 16.8*,
17.0, 17.4*, 17.6, 18.1*, 18.9, 19.0*, 20.6, 20.7, 21.0, 21.1* (CH3),
43.4, 43.5, 44.5*, 45.4* ((CH3)2S(O)), 123.8*, 124.3*, 124.8*, 125.2
(CH), 125.5 (CH), 125.8*, 126.4*, 126.7 (CH), 126.8 (CH), 127.1*,
127.6 (CH), 127.8*, 127.9*, 128.1*, 128.4 (C), 128.9 (C), 129.4
(CH), 129.8*, 129.9 (CH), 130.1 (CH), 130.2*, 131.0 (C), 131.2
(CH), 131.4*, 132.0*, 134.7*, 134.9 (C), 135.1 (C), 135.8*, 135.9
(C), 136.0 (C), 136.2*, 136.5*, 136.6 (C), 142.8 (C), 142.9*, 154.0
(CN), 154.1*. Only 5 resonances were observed for CH3 carbons of
the minor isomer rather than the expected 6 due to overlapping peaks
at δ 20.7. MS (TOF-ES+), m/z (intensity %), [ion]: 834 (12), [M +
Cs − Me]+; 679 (5), [M − HCl]+; 565 (9), [M − 2HCl −
Me2S(O)]+; 373 (40), [LH32,5‑xylyl]+; 372 (100), [LH22,5‑xylyl]+; 370
(35), [L2,5‑xylyl]+. Anal. Calcd for C27H35N3OSCl2Pt (Mw 715.66): C,
45.31; H, 4.93; N, 5.87; S, 4.48. Found: C, 45.42; H, 4.89; N, 5.59; S,
4.22.
Method 2. Complex 1 (100 mg, 0.148 mmol) and NaOAc (13.0
mg, 0.158 mmol) were charged into a 25 mL round-bottom flask and
dispersed in methanol (10 mL), and the flask was fitted to a water
condenser capped with an anhydrous CaCl2 guard tube. The reaction
mixture in the flask was simultaneously stirred and refluxed for 3 h and
cooled to ambient temperature. Subsequently, the volatiles from the
reaction mixture were removed under vacuum to afford a solid. The
solid was dissolved in CH2Cl2 and filtered. The filtrate was layered
with toluene and stored at ambient temperature over a period of
several days to afford 6·S(O)Me2 as pale yellow crystals in 85% (105
mg, 0.135 mmol) yield. The authenticity of 6·S(O)Me2 prepared by
1
this method was verified by H and 13C{1H} NMR spectroscopy.
Complex 7. Complex 7 was prepared from cis-[Cl2Pt(S(O)Me2)2]
(101 mg, 0.239 mmol), LH24‑tolyl (80.0 mg, 0.243 mmol), and NaOAc
(20.0 mg, 0.244 mmol) and purified as described previously for 6
(method 1). Yield of 7·S(O)Me2: 84% (156 mg, 0.201 mmol). Mp:
239.0 °C (decomp). FT-IR (KBr, cm−1): ν(NH) 3381 (m); νa(OCO)
1626 (s); ν(CN) 1604 (vs); νs(OCO) 1377 (m); ν(SO) 1134
Complex 5. Complex 5 was synthesized from cis-[Cl2Pt(S(O)-
Me2)2] (50.0 mg, 0.118 mmol) and LH22,6‑xylyl (45.0 mg, 0.121 mmol)
in toluene (15 mL) and purified as described previously for 1. The
sample was crystallized from CHCl3/toluene mixture at ambient
temperature over a period of several hours to afford 5·C7H8 as light
brown crystals. Yield: 93% (79.0 mg, 0.110 mmol). Mp (DSC): 191.2
°C. FT-IR (KBr, cm−1): ν(NH) 3442 (br, m), 3341 (m); ν(CN)
1
(m). H NMR (CDCl3, 400 MHz): δ 2.10 (s, 3H, CH3), 2.14, 2.20
(each br, 2 × 3H, CH3), 2.30 (s, 3H, CH3), 3.27, 3.34 (each s, 2 × 3H,
(CH3)2S(O)), 5.90 (s, 1H, NH), 6.72, 6.80, 6.94, 6.97 (each br, 4 ×
2H, ArH), 7.11 (d, JH,H = 8.1 Hz, 2H, ArH), 7.39 (d, JH,H = 8.8 Hz, 2H,
ArH), 8.02 (s, 1H, NH). 13C{1H} NMR (CDCl3, 100.5 MHz): δ 20.9
(br), 21.1 (CH3), 43.7 ((CH3)2S(O)), 122.6, 123.4, 123.6, 123.9,
126.4, 126.7, 128.3, 129.4, 129.6, 130.0, 130.2, 134.3, 134.8, 135.3,
136.1, 136.8, 141.8, 142.1 (ArC and ArCH), 153.7 (CN), 177.4
(OC(O)). Note: Only two carbon resonances were observed for CH3
carbons of the guanidine unit rather than the expected three
resonances, presumably due to overlapping peaks. 195Pt{1H} NMR
(85.8 MHz, CDCl3): δ −2891. MS (TOF-ES+), m/z (intensity %),
[ion]: 1312 (10), [2 M + K − 2AcOH]+, 638 (26), [M − OAc]+; 602
(53), [M − Cl − OAc]+; 564 (15), [M + K − Cl − S(O)Me2 −
OAc]+ , 330 (100), [LH2 4 ‑ t o l y l ]+ . Anal. Calcd for
C26H32N3O3SClPt·(CH3)2S(O) (Mw 775.34): C, 43.38; H, 4.94; N,
5.42; S, 8.27. Found: C, 43.40; H, 4.93; N, 5.45; S, 8.29.
1
1618 (vs); ν(SO) 1157 (s). H NMR (CDCl3, 300 MHz): δ 2.06,
2.33, 2.82 (each s, 6 × 3H, CH3), 3.33 (s, 6H, (CH3)2S(O)), 5.73 (s,
1H, NH), 6.68, 6.76 (each d, JH,H = 7.4 Hz, 2 × 2H, ArH), 6.80−6.90
(m, 2H, ArH), 7.05−7.23 (m, 3H, ArH), 8.56 (br, 1H, NH). 13C{1H}
NMR (CDCl3, 75.5 MHz): δ 17.5, 18.1, 19.2 (CH3), 43.5
((CH3)2S(O)), 124.5, 124.9, 126.1, 126.3, 126.8, 127.0, 129.0, 130.3,
130.5, 131.5, 131.7, 132.4, 134.2, 135.3, 135.4, 142.8 (ArC and ArCH),
154.4 (CN). Note: Only 16 carbon resonances were observed for
aryl carbons of the guanidine unit rather than the expected 18 peaks,
presumably due to overlapping peaks. 195Pt{1H} NMR (CDCl3, 85.8
MHz): δ −2893. MS (TOF-ES+), m/z (intensity %), [ion]: 1454 (10),
[2M + Na]+; 1396 (10), [2M − Cl]+; 776 (7), [M + Cs − 2HCl]+;
739 (8), [M + Na]+; 680 (22), [M − Cl]+; 565 (22), [M − 2HCl −
S(O)Me2]+; 372 (100), [LH22,6‑xylyl]+. Anal. Calcd for
C27H35N3OSCl2Pt (Mw 715.66): C, 45.31; H, 4.93; N, 5.87; S, 4.48.
Found: C, 45.52; H, 4.96; N, 5.85; S, 4.68.
Complex 8. cis-[Cl2Pt(S(O)Me2)2] (101 mg, 0.239 mmol),
2‑anisyl
LH2
(90.1 mg, 0.241 mmol), and NaOAc (20.0 mg, 0.244
mmol) were charged into a 25 mL round-bottom flask in methanol
(10 mL), and the flask was fitted to a water condenser capped with an
anhydrous CaCl2 guard tube. The reaction mixture in the flask was
simultaneously stirred and refluxed for 3 h and subsequently cooled to
ambient temperature. The volatiles from the reaction mixture were
Complex 6. Method 1. cis-[Cl2Pt(S(O)Me2)2] (51.0 mg, 0.121
mmol), LH22‑tolyl (40.0 mg, 0.121 mmol), and NaOAc (10.0 mg, 0.122
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Table 1. Crystallographic Data for 1, 2·1.5CHCl3, and 4
1
2·1.5CHCl3
4
formula
fw
C24H29Cl2N3OPtS
673.55
C51H61Cl13N6O8Pt2S2
1801.21
C27H35Cl2N3OPtS
715.63
temp. (K)
wavelength (λ)
cryst syst
space group
a (Å)
100(2)
120(2)
100(2)
0.71073
0.71073
0.71073
triclinic
triclinic
triclinic
P1
̅
P1
P1
̅
̅
10.504(4)
11.299(5)
12.294(4)
111.917(5)
103.026(5)
101.580(4)
1251.7(7)
2
12.658(5)
16.391(5)
16.940(4)
79.543(5)
75.782(5)
76.201(4)
3280.4(19)
2
10.154(5)
11.260(5)
12.970(4)
106.736(5)
99.459(4)
97.130(5)
1377.5(11)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
vol. (Å3)
Z
ρcalcd (g cm−3
)
1.787
1.824
1.725
F(000)
660
1764
708
μ(Mo Kα) (mm−1
θ range (deg)
)
5.924
4.907
5.388
1.89−26.00
7116
2.96−26.37
48 345
1.68−26.37
26 064
no. of reflns collected
no. of reflns used
parameters
4947
13 401
5631
295
749
325
a
R1 [I > 2σ(I)]
0.0496
0.0313
0.0253
b
wR2 (all reflns)
0.1242
0.0708
0.0732
c
GooF on F2
1.206
1.092
1.309
largest diff peak/hole (e·Å−3
)
3.387/−1.872
1.873/−1.599
2.764/−1.427
a
b
c
R1 = Σ||Fo| − |Fc||/Σ|Fo|. wR2 = {Σ[w(Fo − Fc2)2]/Σ[w(Fo )2]}1/2. S = {Σ[w(Fo − Fc2)2]/(n − p)}1/2
.
2
2
2
removed under vacuum to afford a solid. The solid was dissolved in
CH2Cl2 and filtered. The filtrate was layered with toluene to afford 8
as pale brown crystals in 92% (151 mg, 0.220 mmol) yield. Mp
(DSC): 225.6 °C. FT-IR (KBr, cm−1): ν(NH) 3436 (sh), 3392 (m);
ν(CN) 1618 (vs); ν(SO) 1117 (m). 1H NMR (CDCl3, 400
MHz): δ 3.35, 3.48 (each br, 2 × 3H, (CH3)2S(O)), 3.79, 3.85, 4.08
(each s, 3 × 3H, OCH3), 6.67 (d, JH,H = 8.1 Hz, 1H, ArH), 6.90−6.99
(m, 5H, ArH), 7.11−7.23 (m, 4H, ArH), 7.53 (d, JH,H = 8.1 Hz, 1H,
ArH), 7.87, 7.90 (each s, 2H, NH). 13C{1H} NMR (CDCl3, 100.5
MHz): δ 46.2, 46.8 ((CH3)2S(O)), 55.2, 55.8, 56.1 (OCH3), 106.2,
111.0, 111.3, 119.4, 120.7, 121.2, 122.0, 123.4, 125.5, 126.6, 126.7,
127.5, 130.0, 131.0, 134.8, 146.6, 148.1, 150.8 (ArC and ArCH), 154.3
(CN). 195Pt{1H} NMR (85.8 MHz, CDCl3): δ −3737. MS (TOF-
ES+), m/z (intensity %), [ion]: 608 (53), [M + H − S(O)Me2]+; 572
(100), [M − Cl − S(O)Me2]+; 555 (72), [M − S(O)Me2 − HCl −
Me]+; 493 (69), [L2‑anisyl + Cs − Me]+; 477 (20), [L2‑anisyl + Cs −
OMe]+; 378 (33), [LH32‑anisyl]+; 376 (28), [LH2‑anisyl]+. Anal. Calcd for
C24H28N3O4SClPt (Mw 685.10): C, 42.08; H, 4.12; N, 6.13; S, 4.68.
Found: C, 41.94; H, 4.17; N, 5.93; S, 4.60. Pale yellow crystals of 8 for
X-ray diffraction were grown from methanol/CH2Cl2 mixture at
ambient temperature over a period of several days.
estimated from the integrals of CH3 protons of the guanidine unit. 1H
NMR (CDCl3, 400 MHz): δ 1.64 (s, 3H, CH3, major isomer), 1.89,
1.90 (each s, 2 × 3H, CH3, minor isomer 2), 1.94, 1.96 (each s, 2 ×
3H, CH3, major isomer), 1.98 (s, 3H, CH3, minor isomer 2), 2.04, 2.06
(each s, 2 × 3H, CH3, major isomer), 2.10 (s, 3H, CH3, intermediate
isomer), 2.12 (s, 3H, CH3, major isomer), 2.19 (s, 3H, CH3,
intermediate isomer), 2.24, 2.28, 2.33 (each s, 3 × 3H, CH3, major
isomer), 2.36 (s, 3H, CH3, intermediate isomer), 2.43 (s, 3H, CH3,
minor isomer 1), 2.49 (s, 3H, CH3, major isomer), 2.58 (s, 3H, CH3,
minor isomer 1), 2.59, 2.62 (each s, 2 × 3H, CH3, major isomer), 2.70
(s, 3H, CH3, minor isomer 1), 3.26, 3.27 (each s, 2 × 3H,
(CH3)2S(O), intermediate isomer), 3.29 (s, 3H, (CH3)2S(O), minor
isomer 2), 3.30 (s, 2 × 3H, (CH3)2S(O), one major isomer and minor
isomer 2), 3.32 (s, 3 × 3H, (CH3)2S(O), three major isomers), 3.40 (s,
3H, (CH3)2S(O), minor isomer 1), 3.42 (s, 3H, (CH3)2S(O), major
isomer), 3.44 (s, 3H, (CH3)2S(O), minor isomer 1), 3.49 (s, 3H,
(CH3)2S(O), major isomer), 3.52 (s, 2 × 3H, (CH3)2S(O), two major
isomers), 5.56 (s, 1H, NH, minor isomer 2), 5.61 (s, 3 × 1H, NH, two
major isomers and one intermediate isomer), 6.23 (s, 1H, NH, major
isomer), 6.32 (s, 1H, NH, minor isomer 1), 6.41 (s, 1H, NH, major
isomer), 6.72−6.81 (m, 13H, ArH), 6.83−6.92 (m, 13H, ArH), 6.94−
7.09 (m, 16H, ArH), 7.12−7.21 (m, 16H, ArH), 7.26−7.32 (m, 11H,
ArH), 7.43 (d, JH,H = 8.1 Hz, 2H, ArH), 7.61−7.69 (m, 6H, ArH (5H)
and NH (1H) for major isomer), 7.75 (d, JH,H = 7.4 Hz, 1H, ArH),
7.79 (s, 1H, NH, major isomer), 8.03 (s, 1H, NH, minor isomer 2),
9.82 (s, 1H, NH, intermediate isomer), 10.15 (s, 1H, NH, minor
isomer 1), 10.56 (s, 2H, NH, two major isomers). 195Pt{1H} NMR
(CDCl3, 85.8 MHz): δ −2645 (minor), −2710 (major), −2760
(minor), −2839 (minor), −2851 (minor), −2958 (major), and −2995
(minor). MS (TOF-ES+), m/z (intensity %), [ion]: 641 (20), [M + K
− Cl]+; 638 (22), [M + H]+; 621 (15), [M − H, Me]+; 601 (100) [M
Complex 9. cis-[Cl2Pt(S(O)Me2)2] (100 mg, 0.237 mmol),
2‑tolyl
LH2
(82.0 mg, 0.249 mmol), and NaOAc (20.0 mg, 0.244
mmol) were charged into a 25 mL round-bottom flask in methanol
(10 mL), and the flask was fitted to a water condenser capped with an
anhydrous CaCl2 guard tube. The reaction mixture in the flask was
simultaneously stirred and refluxed for 12 h and subsequently cooled
to ambient temperature. The volatiles from the reaction mixture were
removed under vacuum to afford a solid. The solid was dissolved in
CH2Cl2 and filtered, and the filtrate was layered with toluene to afford
9·S(O)Me2 as pale-brown crystalline material in 79% (134 mg, 0.187
mmol) yield. Mp: 181.0 °C (decomp). FT-IR (KBr, cm−1): ν(NH)
3433 (br, s), 3378 (sh, s), 3294 (m); ν(CN) 1620 (vs); ν(SO)
1142 (m). FT-IR (CHCl3, cm−1): ν(NH) 3387 (br, m); ν(CN)
1622 (vs); ν(SO) 1129 (br, m). The 1H NMR spectrum of 9
indicated the presence of four major isomers, one intermediate isomer,
and two minor isomers in about 1:1:1:1:0.23:0.12:0.11 ratios as
−
Cl]+ ; 330 (15), [LH3 2 ‑ t o l y l ]+ . Anal. Calcd for
C24H28N3OSClPt·(CH3)2S(O) (Mw 715.23): C, 43.66; H, 4.79; N,
5.88; S, 8.97. Found: C, 43.59; H, 4.83; N, 5.85; S, 8.92.
Complex 9 was crystallized from a CHCl3/methanol mixture at
ambient temperature over a period of 1 week to afford crystals of one
1
of the major isomers. The H NMR spectral data of this isomer is
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Table 2. Crystallographic Data for 5·C7H8, 6·S(O)Me2, and 8
5·C7H8
6·S(O)Me2
8
formula
fw
C34H43Cl2N3OPtS
807.76
C28H38ClN3O4PtS2
775.27
C24H28ClN3O4PtS
685.09
temp. (K)
wavelength (λ)
cryst syst
space group
a (Å)
100(2)
100(2)
100(2)
0.71073
0.71073
0.71073
triclinic
triclinic
triclinic
P1
̅
P1
̅
P1
̅
8.968(5)
11.004 (5)
15.910(4)
88.421(5)
76.519(4)
81.102(5)
1508.4(12)
2
10.8407(8)
11.8760(9)
12.3107(9)
92.791(4)
106.147(5)
93.814(4)
1515.30(19)
2
8.8240(8)
10.6295(10)
13.7122(13)
75.839(4)
82.686(5)
84.765(4)
1234.5(2)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
vol. (Å3)
Z
ρcalcd (g cm−3
)
1.779
1.699
1.843
F(000)
808
772
672
μ(Mo Kα) (mm−1
θ range (deg)
)
4.932
4.894
5.912
1.32−26.37
24 915
6146
1.72−26.37
31 530
1.98−26.37
17 879
no. of reflns collected
no. of reflns used
parameters
6157
5017
337
343
321
a
R1 [I > 2σ(I)]
0.0275
0.0784
1.271
0.0348
0.0228
b
wR2 (all reflns)
0.0955
0.0591
c
GooF on F2
1.231
1.401
largest diff peak/hole (e·Å−3
)
2.429/−0.971
2.702/−1.421
2.056/−1.475
a
b
c
R1 = Σ||Fo| − |Fc||/Σ|Fo|. wR2 = {Σ[w(Fo − Fc2)2]/Σ[w(Fo )2]}1/2. S = {Σ[w(Fo − Fc2)2]/(n − p)}1/2
.
2
2
2
given below. 1H NMR (CDCl3, 400 MHz): δ 1.96, 2.33, 2.60 (each s,
3 × 3H, CH3), 3.30, 3.32 (each s, 2 × 3H, (CH3)2S(O)), 5.61 (s, 1H,
NH), 6.83 (s, 1H, ArH), 6.84−6.87 (m, 1H, ArH), 6.89−6.93 (m, 1H,
ArH), 6.93−6.96 (m, 1H, ArH), 6.98−7.02 (m, 2H, ArH), 7.06 (d, JHH
= 7.3 Hz, 1H, ArH), 7.10−7.14 (m, 1H, ArH), 7.17−7.21 (m, 2H,
ArH), 7.75 (d, JH,H = 6.6 Hz, 1H, ArH), 7.79 (s, 1H, NH). Other data
could not be obtained for this isomer due to paucity of the sample.
X-ray Crystallography. Details of data collections, structure
solutions, and refinements are presented in Tables 1 and 2. Further
details may be found in the Supporting Information.
solid from either CHCl3/toluene mixture (1, 2, 3, 5) or
CH2Cl2/toluene mixture (4) at ambient condition. Complexes
1−5 are soluble in CHCl3 and CH2Cl2 and sparingly soluble in
acetone, methanol, and toluene.
Single-Crystal X-ray Diffraction. Molecular structures of 1,
2, 4, and 5 were determined by single-crystal X-ray diffraction
and are depicted in Figure 1. Selected bond distances and bond
angles are listed in Table 3. Two molecules are observed in an
asymmetric unit of 2, but the molecular structure and bond
parameters are given only for molecule 1. The platinum atom is
surrounded by two chlorides, the imine nitrogen of guanidine,
and the sulfur atom of DMSO and displayed a slightly distorted
square planar geometry. Two chlorides are located trans to each
other around the platinum atom as are the sulfur atom of
DMSO and the imine nitrogen of guanidine in 1, 4, and 5. On
the contrary, the aforementioned pairs of substituents are
located cis to each other around the platinum atom in 2.
Complexes of the type [Cl2Pt(S(O)R2)2] have been shown to
be stable in cis form due to the effective (d−d) π bonding, but
the trans form becomes stable with sterically more demanding
R substituent, such as isoamyl.32 In the present study, the
guanidine in 2 is sterically less hindered and perhaps more π
accepting than the guanidine in 1, 4, and 5. Thus, the
configuration of the platinum atom in 1, 4, and 5 is
predominantly influenced by steric factor, while the electronic
factor dictates the configuration of the platinum atom in 2. The
guanidine units in complexes displayed anti−anti conformation
(τ(CNCN) −146.7(8)°, −154.7(8)° (1); −147.4(4)°,
−147.1(4)° (2); 150.0(4)°, 151.1(4)° (4), and −151.1(4)°,
−150.2(4)° (5)) irrespective of their conformations in
RESULTS AND DISCUSSION
■
i. Platinum(II) Guanidine Complexes (1−5). Synthesis.
2‑tolyl
Reaction of cis-[Cl2Pt(S(O)Me2)2] with 1 equiv of LH2
,
LH22‑anisyl, LH24‑tolyl, LH22,5‑xylyl, and LH2
carried out
2,6‑xylyl
separately in toluene under reflux condition for 3 h afforded
1, 2, 3, 4, and 5, respectively, in high yield as illustrated in
Scheme 1. The products from the reaction mixture were
isolated by evaporation of the mother liquor at ambient
condition to afford a solid followed by crystallization of the
Scheme 1. Syntheses of Complexes 1−5
2‑tolyl
2,5‑xylyl
uncoordinated form (LH2
and LH2
anti−anti and
33
2‑anisyl
2,6‑xylyl
LH2
and LH2
syn−anti). The anti−anti conforma-
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Figure 1. Molecular structures of 1, 2, 4, and 5 at the 50% probability level. Only hydrogen atoms of the amino moieties are shown for clarity.
Table 3. Selected Bond Distances (Angstroms) and Bond Angles (degrees) for 1, 2, 4, and 5
1
2
4
5
Pt1−Cl1
Pt1−Cl2
Pt1−S1
Pt1−N1
N1−C1
N2−C1
N3−C1
S1−O1/S1−O4
Cl1−Pt1−Cl2
S1−Pt1−Cl1
S1−Pt1−Cl2
N1−Pt1−Cl1
N1−Pt1−Cl2
N1−Pt1−S1
2.297(3)
2.294(3)
2.210(3)
2.057(7)
1.327(13)
1.374(14)
1.330(14)
1.468(8)
2.306(1)
2.318(1)
2.206(1)
2.033(3)
1.318(5)
1.362(5)
1.350(5)
1.468(3)
90.47(5)
90.13(5)
177.14(4)
177.7(1)
88.1(1)
2.301(1)
2.297(1)
2.209(1)
2.041(4)
1.318(5)
1.359(5)
1.355(5)
1.472(3)
170.57(4)
93.57(5)
90.52(5)
88.0(1)
2.326(1)
2.300(1)
2.224(1)
2.086(4)
1.318(6)
1.361(5)
1.366(5)
1.462(4)
175.95(4)
90.21(5)
88.95(5)
90.8(1)
173.5(1)
90.5(1)
92.8(1)
88.1(2)
88.7(2)
178.0(2)
88.3(1)
90.2(1)
91.4(1)
176.87(9)
178.0(1)
tion of the guanidine unit in complexes probably imparts
minimum steric pressure on the platinum atom.
Δ
CN, ΔCN′, and ρ parameters34,35 defined in Chart 2 can be
used to understand the structure and bonding of the guanidine
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that δ(195Pt) values of cis-[Cl2Pt(S(O)R2)L] (L = σ-donor/π-
acceptor ligands) are downfield shifted with respect to their
trans analogues, and this has been ascribed to a better (d−d)π
bonding in the former isomer.37 Hence, the major δ(195Pt)
signal of 2 and the minor δ(195Pt) signal of 3 are assigned to the
cis isomer, and the minor δ(195Pt) signal of 2 and the major
δ(195Pt) signal of 3 are assigned to the trans isomer. The
Δδ(δcis − δtrans) values for 2 and 3 are 50 and 111, respectively.
Upon comparing the δ(195Pt) value of the trans isomer of 2
with that of 1 it appears that the guanidine in the former
complex is a better π acceptor than that present in the latter
complex, although the steric factor can also influence δ(195Pt)
values to some extent.38
Chart 2. Definition of ΔCN, ΔCN′, and ρ Parameters for 1, 2,
a
4, and 5
a
ΔCN = y − x; ΔCN′ = z − x; ρ = 2x/(y + z); [Pt] Cl2Pt(S(O)Me2).
unit in 1, 2, 4, and 5. The ΔCN and ΔCN′ value of 0.0 and the ρ
value of 1.0 indicate a maximum n−π conjugation involving the
lone pair of the amino nitrogen atoms with the CN π*
orbital, and any deviation from these values indicate a
hampered n−π conjugation. ΔCN and ΔCN′ values in complexes
(ΔCN, ΔCN′ = 0.047(19), 0.003(19) Å (1); 0.044(5), 0.032(5)
Å (2); 0.041(5), 0.037(5) Å (4); and 0.043(8), 0.048(6) Å
(5)) are smaller than the corresponding values known for the
respective uncoordinated guanidines (ΔCN, ΔCN′ = 0.095(6),
0.098(6) Å (LH22‑tolyl); 0.100(6), 0.111(6) Å (LH22‑anisyl);
0.097(3), 0.098(3) Å (LH22,5‑xylyl); and 0.083(6), 0.090(6) Å
(LH22,6‑xylyl))33 (see also the Supporting Information). The ρ
value in complexes (ρ = 0.98 (1) and 0.97 (2, 4, and 5)) is
greater than that known for the respective uncoordinated
guanidines (ρ = 0.93 (LH22‑tolyl, LH22,5‑xylyl, and LH22,6‑xylyl) and
0.92 (LH22‑anisyl)).33 Moreover, the amino nitrogen atoms are
planar in complexes.
The aforementioned structural parameters indicate a greater
n−π conjugation in complexes as compared with the respective
guanidines. However, n−π conjugation in complexes is not at
its maximum, and this feature is partly ascribed to the
interaction of the lone pair of the amino nitrogen atoms with
the π electrons of the aryl ring and partly ascribed to the steric
clash between the central CN3 unit and the NC2H unit of the
amino nitrogen atoms caused by the aryl substituent of the
latter unit. The angle between two mean planes constituted by
the CN3 unit and the NC2H unit are 33.7(6)°, 24.8(7)° (1),
32.5(2)°, 32.9(3)° (2), 29.8(2)°, 29.1(2)° (4), and 29.2(2)°,
29.5(2)° (5).
1
The H and 13C{1H} NMR spectra of 1 and 5 indicated the
presence of a single isomer in solution. However, the 1H NMR
spectrum of 2 revealed the presence of two isomers in about a
1.0:2.1 ratio, as estimated from the integrals of OCH3 and NH
protons of the guanidine unit. The major set of signals is
assigned to the cis isomer as observed in the solid state, and the
minor set of signals is assigned to the trans isomer as also
1
suggested by 195Pt{1H} NMR spectroscopy. The H NMR
spectrum of a freshly prepared solution of 3 revealed the
presence of one isomer only, but upon storing the solution for
48 h, formation of two isomers in about a 1.0:0.2 ratio was
noticed. From the 1H and 195Pt{1H} NMR spectroscopic
studies, the major set of signals is assigned to the trans isomer
and the minor set of signals to the cis isomer. The cis−trans
isomerization event and isomeric composition of 2 and 3 are
summarized in Scheme 2.
Scheme 2. Cis−Trans Isomerization of 2 and 3
IR and NMR Spectroscopy. IR spectra of 1−5 revealed
characteristic band(s) for the NH and CN moieties of
guanidines. Further, the DMSO molecule in 1−5 revealed one
band in the 1116−1157 cm−1 interval assignable to the
ν(S(O)) stretch, and these values are greater than that reported
for free DMSO (ν(S(O)) 1055 cm−1 in the solid state and 1070
cm−1 in CCl436), indicating sulfur coordination of DMSO to
the platinum atom in the complexes.
Platinum(II) imine complexes and imine-derived platina-
cycles are amenable to 195Pt{1H} NMR studies. The δ(195Pt)
values are sensitive to the coordination environment around the
platinum atom and steric/electronic properties of the
surrounding ligands.37 The δ(195Pt) values are useful to
distinguish not only cis and trans configurations of the
platinum but also E and Z configurations of the imine unit in
[Cl2Pt(S(O)Me2)(ferrocenyl imine)].5a,b,8a The 195Pt{1H}
NMR spectra of 1, 4, and 5 revealed one signal at δ(195Pt)
−2967, −2959, and −2893, respectively, and these δ(195Pt)
values agree with those reported for the 195Pt nucleus
surrounded by “N,Cl2,SDMSO” set of donor atoms.5b,8a The
195Pt{1H} NMR spectrum of 2 revealed a closely separated pair
of resonances at δ(195Pt) −2721 (major) and −2771 (minor),
while that of 3 revealed a widely separated pair of resonances at
δ(195Pt) −2898 (minor) and −3009 (major). It has been shown
cis-[Cl2Pt(S(O)R2)(L)] (L = π-acceptor ligand such as
sulfoxide32 and pyrimidine;38 R = alkyl and aryl) is more stable
than its trans analogue due to enhanced (d−d)π bonding.
However, with sterically more demanding sulfoxide such as
diphenyl sulfoxide only trans-[Cl2Pt(S(O)Ph2)(L)] (L =
pyrimidine) has been isolated, but even this complex in
CD2Cl2 has been shown to slowly isomerize to its cis analogue
after a few weeks.38 The solution behavior of 3 closely
resembles the solution behavior of trans-[Cl2Pt(S(O)Ph2)-
(pyrimidine)].
1
The H NMR spectrum of 4 measured at 0.004, and 0.042
mM (400 MHz) revealed the presence of one major isomer and
a very small quantity of minor isomer as identified from the
signals of CH3 protons of DMSO or from those of guanidine
and DMSO. 1H NMR patterns did not change even after 48 h.
The ratio of the two isomers at 0.154 mM (400 MHz) as
estimated from the integrals of the alkyl protons was 1.00:0.12.
1H NMR spectral features of 4 could arise due to formation of
(i) cis and trans isomers as discussed previously for 2 and 3, (ii)
trans-configurated 4 with anti−anti and syn−anti conforma-
tions of the guanidine unit, and (iii) three rotamers, namely, A,
B, and C, that arise due to the restricted Pt−S bond rotation of
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the trans configurated 4 as illustrated in Scheme 3. Formation
of the cis isomer is ruled out due to the mutual repulsive
Single-Crystal X-ray Diffraction. The molecular structure
and selected bond parameters of 6 are depicted in Figure 2.
Scheme 3. Restricted Pt−S Bond Rotation That Leads to
Formation of Three Rotamers of 4 in Solution
interaction between DMSO and the guanidine unit. Formation
of the syn−anti conformer for the guanidine unit is also ruled
out as it involves C−N(H) single-bond rotation, and we believe
1
that this high-energy process is less likely. Thus, the H NMR
spectral behavior of 4 could be attributed to the presence of
one major rotamer A as found in the solid state and the minor
rotamer B or C at higher concentration. The presence of two
rotamers in solution could arise due to the change in
intermolecular hydrogen-bonding pattern associated with the
oxygen atom of DMSO with concentration.
Figure 2. Molecular structure of 6 at the 50% probability level. Only
hydrogen atoms of the amino moieties are shown for clarity. Selected
bond distances (Angstroms) and bond angles (degrees): Pt1−Cl1
2.306(2), Pt1−O2 2.046(4), Pt1−S1 2.188(1), Pt1−N1 2.028(4),
N1−C1 1.308(7), N2−C1 1.369(7), N3−C1 1.349(7), O2−C25
1.297(7), O3−C25 1.228(7); Cl1−Pt1−O2 89.1(1), S1−Pt1−Cl1
91.92(5), S1−Pt1−O2 178.3(1), N1−Pt1−Cl1 175.4(1), N1−Pt1−
O2 87.7(2), N1−Pt1−S1 91.4(1).
ii. Substitution versus Cycloplatination. Synthesis.
2‑tolyl
Reaction of cis-[Cl2Pt(S(O)Me2)2] with 1 equiv of LH2
and LH24‑tolyl in the presence of 1 equiv of NaOAc in methanol
carried out separately under reflux condition for 3 h afforded
acetate-substituted products 6 and 7 in 83% and 84% yields,
respectively. Interestingly, the circuitous route involving
reaction of 1 with 1 equiv of NaOAc in methanol under reflux
condition for 3 h also afforded 6 in 85% yield. Reaction of cis-
[Cl2Pt(S(O)Me2)2] with 1 equiv of LH22‑anisyl in the presence of
1 equiv of NaOAc in methanol under reflux condition for 3 h
The platinum atom is surrounded by the imine nitrogen of the
guanidine, oxygen atom of the monodentate acetate, chloride,
and the sulfur atom of DMSO. The bond parameters around
the platinum atom indicate a slightly distorted square planar
geometry. The oxygen atom of the acetate moiety and the
sulfur atom of DMSO are placed trans to each other around the
platinum atom as are the imine nitrogen of the guanidine and
the chloride. Thus, the platinum atom in 6 revealed a cis
configuration that contrasts with the trans configuration of the
platinum atom observed in 1.
afforded platinacycle 8 in 92% yield, and the analogous reaction
2‑tolyl
carried out with bulkier LH2
for 12 h afforded 9 in 79%
yield. Syntheses of 6−9 are illustrated in Scheme 4.
The oxygen atom O3 of the acetato ligand in 6 acts as a
trifurcated hydrogen-bond acceptor by forming one intra-
molecular N−H···O hydrogen bonding with the hydrogen
atom H3 of the amino moiety and two intermolecular C−H···O
hydrogen bondings, one with H13 of the tolyl unit of the
adjacent molecule and the other with H24B of DMSO of
another adjacent molecule, both the neighbors being related to
the reference molecule by inversion symmetry (see the
Supporting Information). The aforementioned noncovalent
interactions are absent in 1 and believed to be at least partly
responsible for the cis configuration of the platinum atom in 6.
The ΔCN′ (0.061(7) Å), ΔCN (0.041(7) Å), and ρ (0.96) values
in conjunction with the planarity of the amino nitrogen atoms
in 6 suggest a partial n−π conjugation between the lone pair of
the amino nitrogen atoms with the CN π* orbital. To the
best of our knowledge, the molecular structure of 6 represents
the first crystallographically characterized acetate-substituted
product formed prior to formation of [C,N] platinacycles from
Scheme 4. Syntheses of Complexes 6−9
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reactions of nitrogen donor ligands with Pt(II) precursor
carried out in the presence of NaOAc39 (see later).
The molecular structure and selected bond parameters of 8
are depicted in Figure 3. The platinum atom is surrounded by
1623 and 1384 cm−1 and 1626 and 1377 cm−1, respectively, for
the acetate moiety assignable to νa(OCO) and νs(OCO)
stretches, respectively. The Δν = 239 (6) and 249 cm−1 (7)
values clearly indicated monodentate acetate coordination
mode.41 Complexes 6−9 revealed one band in the 1117−
1142 cm−1 interval assignable to the ν(S(O)) stretch of the
sulfur-coordinated DMSO.36
The 195Pt{1H} NMR spectrum of 6 revealed one resonance
at δ(195Pt) −2718, and this value is more deshielded than that
observed for 1 (δ(195Pt) −2967). The 195Pt{1H} NMR
spectrum of 7 revealed one resonance at δ(195Pt) −2891, and
this value is comparable with that revealed by the cis isomer of
3 but more deshielded than that revealed by its trans analogue
(δ(195Pt) −2898 (cis) and −3009 (trans)). 195Pt{1H} NMR
spectroscopy thus indicated the cis configuration for the
platinum atom in 7. The Δδ(δOAc − δCl) value of 7 for the 3/7
pair favorably matches with that known for the cis-[Cl2Pt-
(N,N′)]/cis-[(AcO)ClPt(N,N′)] pair (N,N′ = (η5-C5H5)Fe-
[(η5-C5H4)CHN(CH2)3NMe2; Δδ 7) wherein OAc has
been suggested to lie trans to the imine nitrogen.5c Thus, the
Δδ(δOAc − δCl) value of 249 for the 1/6 pair appears to arise
largely from the change in the configuration of the platinum
atom in conjunction with the steric factor rather than from the
1
substituent effect. H NMR spectra of 6 and 7 indicated the
Figure 3. Molecular structure of 8 at the 50% probability level. Only
hydrogen atoms of the amino moieties are shown for clarity. Selected
bond distances (Angstrsoms) and bond angles (degrees): Pt1−Cl1
2.404(1), Pt1−N1 2.060(4), Pt1−S1 2.213(1), Pt1−C14 2.014(4),
N1−C1 1.317(6), N2−C1 1.352(6), N3−C1 1.364(6), N2−C9
1.412(5), C9−C14 1.402(6); N1−Pt1−Cl1 89.6(1), N1−Pt1−S1
174.8(1), N1−Pt1−C14 87.9(2), S1−Pt1−C14 94.4(1), Cl1−Pt1−
C14 176.9(1), S1−Pt1−Cl1 87.98(4).
presence of three sets of signals in the alkyl region in about
3:2:1 ratios assignable to CH3 protons of the guanidine,
DMSO, and acetate moiety, respectively. The diagnostic
13C{1H} NMR signals for CH3 and OC(O) carbon nuclei of
6 and 7 further corroborated the structures determined by X-
ray diffraction and 195Pt{1H} NMR studies.
The 195Pt{1H} NMR spectrum of 8 revealed one resonance
at δ(195Pt) −3737, and this value is upfield shifted relative to
that observed for 2 (cis isomer; δ(195Pt) −2721). This trend is
in line with the literature trend reported for the [Cl2PtL(S-
(O)Me2)]/[ClPt(C,N)(S(O)Me2)] pair (L = imine ligands,
and C,N = imine-derived monoanionic κ2C,N scaffolds).4b,5a,8a
1H and 13C{1H} NMR spectra of 8 revealed the presence of
one isomer in solution, and these spectral features contrast with
those reported for I and II wherein two boat conformers
interconvert through a planar intermediate via the six-
membered “[C,N]Pd” ring inversion as revealed by a detailed
1H NMR studies.31c The presence of a single isomer of 8 in
solution could arise due to the bulky nature of DMSO or its
more strongly π-accepting nature, but we believe that the
former feature predominantly dictates the number of solution
isomers.
the imine nitrogen, anisyl carbon, sulfur atom of DMSO, and
chloride and thus revealed a slightly distorted square planar
geometry. In 8, DMSO is coordinated to the platinum atom in
the cis relation with respect to the Pt−C bond, and this feature
contrasts with that observed in the guanidine-derived six-
membered palladacycles, [Pd{κ2(C,N)-C6H3(OMe)-3(NHC-
(NHAr)(NAr))-2}Br(L)] (Ar = 2-(MeO)C6H4; L =
NC5H3Me2-2,6 (I) and NC5H3Me2-2,4 (II)) wherein lutidine
is placed trans to the Pd−C bond.31c In a hypothetical trans
isomer of 8, there would be a repulsive interaction between
DMSO and the anisyl moiety of the NAr unit. Thus, the
steric factor appears to predominantly dictate the cis
configuration of the platinum atom in 8 (see above).
The six-membered “[C,N]Pt” ring in 8 revealed a pseudo-
boat α conformation with N1, C1, C9, and C14 atoms defining
the basal plane of the boat while Pt1 and N2 atoms defining the
tips of the boat.40 The Pt1−N1 distance, 2.060(4) Å, is
comparable to the predicted value of 2.07 Å, but the Pt1−C14
distance, 2.014(4) Å, is slightly shorter than the predicted value
of 2.09 Å (covalent radii r(Pt(II)) 1.36 Å, r(N) 0.71 Å,
r(C(sp2)) 0.73 Å), indicating some degree of Pt−C multiple-
bond character. The ΔCN′ (0.047(6) Å) and ΔCN (0.035(6) Å)
values for the guanidine unit of 8 are smaller than those
previously reported for I (ΔCN′ = 0.077(6) Å; ΔCN = 0.067(6)
The 1H NMR spectrum of 9 indicated the presence of seven
isomers in about 1:1:1:1:0.23:0.12:0.11 ratios as estimated from
the integrals of CH3 protons of the guanidine moiety. Further,
the 195Pt{1H} NMR spectrum of 9 revealed seven resonances at
δ(195Pt) −2645, −2710, −2760, −2839, −2851, −2958, and
−2995. Platinacycle 9 upon fractional crystallization in CHCl3/
methanol mixture once afforded one of the isomers as revealed
by 1H NMR spectroscopy (see Experimental Section).
Intriguingly, the δ(195Pt) values of 9 are more deshielded
than that observed for 8. IR and 1H NMR spectroscopic data of
metal sulfoxides were shown to be useful to assign the
coordination modes of sulfoxides.36,42 The IR spectrum of 9
was also measured in CHCl3, which revealed one broad band at
1129 cm−1, and this value is greater than that reported for free
DMSO (see above). Also, the 1H NMR chemical shifts of CH3
protons of the coordinated DMSO in 9 lie in the interval δ
2‑anisyl
Å), II (ΔCN′ = 0.065(4) Å; ΔCN = 0.064(4) Å), and LH2
(see above).31c,33 The endocyclic amino nitrogen atom is
planar, while the exocyclic amino nitrogen atom deviates
significantly from planarity (∑N2 = 360°; ∑N3 = 343°).
IR and NMR Spectroscopy. IR spectra of 6−9 revealed
characteristic band(s) for the NH and CN moieties of
guanidines. Complexes 6 and 7 also revealed a pair of bands at
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Scheme 5. Plausible Mechanism of Cycloplatination
3.26−3.52, and these values are more downfield shifted from
the 1H NMR chemical shift of free DMSO (δ 2.53), suggesting
the presence of S-bonded DMSO in solution. For O-bonded
DMSO, a smaller downfield shift (δ 2.59−3.03) has been
suggested.42
product prior to formation of [C,N] platinacycle in a reaction
of nitrogen-donor ligand with platinum(II) precursor in the
presence of NaOAc was only detected by 1H NMR
spectroscopy,3 and in one publication such products were
isolated in unspecified low yields.5c In this context, the high-
yield syntheses of 6 and 7 and the molecular structure of the
former represent a significant advancement in the chemistry of
[C,N] platinacycles. Thermolysis of 6 carried out in methanol
for 9 h followed by solvent evaporation afforded a brown solid
In principle, the platinum atom in platinacycle such as 9 can
have either the cis or the trans configuration, and hence two
configurational isomers are possible. Each configurational
isomer in turn can have six conformational isomers either
due to three distinct orientations of DMSO with respect to the
square plane of the platinum or due to two distinct orientations
of o-Me substituent of the NAr unit. The structures of 12
possible isomers of 9 are illustrated in Charts S1 and S2,
Supporting Information. It is difficult to assign specific
structures for seven isomers detected by NMR studies in the
present investigation from those given in Charts S1 and S2,
1
whose H NMR spectrum revealed the formation of 9. This
experiment suggests that 9 is formed from the platinum
precursor, the guanidine, and NaOAc possibly via 6. Other
crucial factors that are responsible for formation of five-
membered [C,N] platinacycles include the cis configuration
rather than the trans configuration for the platinum atom and
the E configuration rather than the Z configuration for the
imine unit in the precursor, [Cl2Pt(S(O)Me2)(arylimine)].2d,5b
In case the arylimine in cis-[Cl2Pt(N,N′)] (N,N′ = PhCH
N(CH2)2NMe2 and ArCHN(CH2)2NCHAr; Ar = 9-
C14H9) possesses the Z configuration then isomerization to E
configuration precedes cycloplatination.3,7
1
Supporting Information. H NMR spectra of 8 and that of 9
that contained one major isomer along with a small proportion
of other isomers did not change significantly with temperature
(see the Supporting Information).
iii. Stereochemical Requirements and Mechanistic
Aspects of Cycloplatination Reaction. The mechanism of
cyclometalation in general9a,b,43 and the role of metal-bound
carboxylate in such reaction44 in particular have been
investigated in detail by both experimental and accurate
computational methods. Palladium acetate is one of the useful
precursors for the synthesis of a range of [C,N] palladacycles,
but synthesis of the corresponding [C,N] platinacycles was
hampered mainly due to commercial unavailability and
difficulty associated with preparation of platinum(II) acetate.45
In 1974, Shaw and co-workers for the first time used NaOAc as
an external base in the cyclometalation reaction to accelerate
the reaction rate.46 Since then NaOAc has became one of the
essential reagents in both stoichiometric4a and recently in
catalytic C−H activation processes.47 Only rarely was cyclo-
platination of imine ligands carried out in the absence of
NaOAc.48 One of the convenient routes for [C,N] platinacycles
involves reaction of cis-[Cl2Pt(S(O)Me2)2] with N-donor
ligands in the presence of a stoichiometric amount of NaOAc
in methanol under reflux condition. In this reaction, it has been
proposed that NaOAc plays a dual role, as a nucleophile and as
an internal base.4a,d Formation of an acetate-substituted
2‑anisyl
2,6‑xylyl
Though LH2
and LH2
revealed the syn−anti
conformation in the solid state, the former was successfully
cycloplatinated while the latter was not. Probably, the acetate-
substituted product [(AcO)ClPt(S(O)Me2)(ArNC-
(NHAr)2)] (Ar = 2-(MeO)C6H4; D) was formed transiently
2,6‑xylyl
before forming platinacycle, 8. The inability of LH2
to
form acetate-substituted product [(AcO)ClPt(S(O)Me2)-
(ArNC(NHAr)2)] (Ar = 2,6-Me2C6H3; E) is ascribed to
greater steric encumbrance around the platinum atom in the
supposedly formed intermediate 5. Formation of primary
amine-derived five-membered platinacycles is promoted by
steric factor.30a However, sterically more hindered 9 is formed
in 12 h, while sterically less hindered 8 is formed in 3 h, and
this difference in time scale of cycloplatination could be
ascribed to the difficulty with which the C−N(H) single bond
rotates in the acetate-substituted product 6 before forming 9 as
compared with hitherto unknown D in the formation of 8 as
outlined below. Reaction of 1/2, for example, with NaOAc in
methanol could produce acetate-substituted products 6/D, and
these intermediates are presumed to undergo the C−N(H)
single-bond rotation, solvent dissociation, followed by an
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intramolecular C−H activation process either through inter-
mediates F/G via ambiphilic metal ligand activation
(AMLA)44b or through intermediates H/I via a solvent-assisted
concerted metalation deprotonation (CMD) pathway49 to
afford 8/9 as shown in Scheme 5.
Cyclopalladation of dimethylbenzylamine with Pd(OAc)2
was suggested to involve a Wheland-type intermediate, but
this experimental finding was later refined by accurate DFT
calculations and subsequently shown to involve an agostic
intermediate rather than a Wheland intermediate.50,51 The cis
configuration of the platinum atom in 6 to form 9 is probably
ascribed to the flexible nature of two six-membered rings
spanned by the platinum atom in intermediates F/G or H/I.
ACKNOWLEDGMENTS
■
The authors acknowledge the (i) Department of Science and
Technology, New Delhi, for a research grant (grant no. SR/S1/
IC-04/2010), (ii) University Science Instrumentation Center,
Univerisity of Delhi, (iii) NMR Research Center, Indian
Institute of Science, Bangalore, and (iv) Indian Institute of
Technology Roorkee for analytical, spectroscopic, and X-ray
diffraction measurements.
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CONCLUSION
■
Judicious choice of aryl substituents of sym-N,N′,N″-triarylgua-
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* Supporting Information
́
Crystallographic data in CIF format, packing diagram of 6,
general considerations, materials, experimental details pertinent
to X-ray diffraction, synthesis, analytical, spectroscopic data of
sym-N,N′-bis(2,5-xylyl)thiourea and LH22,5‑xylyl, figure showing
the molecular structure, table of crystallographic parameters,
and selected bond parameters of LH22,5‑xylyl, charts illustrating
12 probable solution isomers of 9, 195Pt{1H} NMR spectra of
1−9, 1H, 13C{1H}, 1H−13C HETCOR, DEPT 90 NMR spectra
(9) (a) Otto, S.; Chanda, A.; Samuleev, P. V.; Ryabov, A. D. Eur. J.
Inorg. Chem. 2006, 2561−2565. (b) Ryabov, A. D.; Otto, S.; Samuleev,
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1
of 4, H NMR spectrum of one of the seven isomers of 9
(11) Michelin, R. A.; Sgarbossa, P.; Mazzega Sbovata, S.; Gandin, V.;
Marzano, C.; Bertani, R. ChemMedChem 2011, 6, 1172−1183.
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crystallized from CHCl3/methanol mixture, and variable-
1
temperature H NMR spectra of 8 and 9. This material is
available free of charge via the Internet at http://pubs.acs.org.
(13) Cortes
D.; Messeguer, R.; Calvis, C.; Baldoma,
Calvet, T.; Cascante, M. Eur. J. Med. Chem. 2012, 54, 557−566.
(14) Navarro-Ranninger, C.; Lopez-Solera, I.; Gonzalez, V. M.; Per
J. M.; Alvarez-Valdes, A.; Martín, A.; Raithby, P. R.; Masaguer, J. R.;
́
, R.; Crespo, M.; Davin, L.; Martín, R.; Quirante, J.; Ruiz,
̀
L.; Badia, J.; Font-Bardía, M.;
AUTHOR INFORMATION
■
́
́
́
ez,
Corresponding Author
́
*E-mail: tnat@chemistry.du.ac.in, thirupathi_n@yahoo.com.
Alonso, C. Inorg. Chem. 1996, 35, 5181−5187.
Notes
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Liq. Cryst. 2007, 34, 1463−1472. (b) Bilgin Eran, B.; Tschierske, C.;
The authors declare no competing financial interest.
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2010)) on ‘Cl2Pt(S(O)C2)(N)’ revealed 83 structures as outlined with
reference code numbers in the Supporting Information, whereas the
search on ‘(AcO)ClPt(S(O)C2)(N)’ revealed none.
(40) The symbol α indicates an upward orientation of the ortho
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plane of the boat, while the symbol β indicates a downward orientation
of the same moiety.
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