Reactions of platinum(iv)-bound nitriles with isomeric nitroanilines: Addition vs. substitution

Article abstract of DOI:10.1039/c2dt30986f

The platinum(iv) complex trans-[PtCl₄(EtCN)₂] reacts smoothly and under mild conditions with isomeric o-, m- and p-nitroanilines (NAs) yielding two different types of products depending on the NA isomer, viz. the nitroaniline complexes cis/trans-[PtCl₄(NA)₂] (cis/trans-1-3) and the amidine species trans-[PtCl₄{NHC(Et)NHC ₆H₄NO₂-m}(EtCN)] (4), trans-[PtCl ₄{NHC(Et)NHC₆H₄NO₂-m}₂] (5) and trans-[PtCl₄{NHC(Et)NHC₆H₄NO ₂-p}(EtCN)] (6). Complexes 4 and 5 undergo cyclometalation, furnishing mer-[PtCl₃{NHC(Et)NHC₆H₃NO ₂-m}(EtCN)] (7) and mer-[PtCl₃{NHC(Et)NHC ₆H₄NO₂-m}{NHC(Et)NHC₆H ₃NO₂-m}] (8), respectively. Moreover, 8 both in the solid state and in solution undergoes the second step of the cyclometalation, generating [PtCl₂{NHC(Et)NHC₆H₃NO ₂-m}₂] (9). In 4, the nitrile ligand is highly reactive toward nucleophilic addition and it undergoes facile hydration accompanied by the elimination of the nitrile, thus producing cis-[PtCl₄(NH ₂C₆H₄NO₂-m){NHC(OH)Et}] (10), or methanol addition providing trans-[PtCl₄{NHC(Et)NHC₆H ₄NO₂-m}{NHC(Et)OMe}] (11). All compounds, besides 9, were characterized by C, H, and N elemental analyses, high-resolution ESI-MS, IR, ¹H and ¹³C{¹H} NMR spectroscopic techniques. Complex 9, which was not isolated as a pure compound, was identified in the reaction mixture by ESI-MS and ¹H and ¹³C{¹H} NMR spectroscopies. Complexes trans-1, trans-2, 4, 5, 6, 8, 10, and 11 were additionally studied by X-ray diffraction.

Full text of DOI:10.1039/c2dt30986f

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PAPER
Reactions of platinum(IV)-bound nitriles with isomeric nitroanilines: addition
vs. substitution†
Alexander N. Chernyshev,a,b Nadezhda A. Bokach,*a Pavel V. Gushchin,a Matti Haukka*b and
Vadim Yu. Kukushkin*a,c
Received 6th May 2012, Accepted 24th August 2012
DOI: 10.1039/c2dt30986f
The platinum(IV) complex trans-[PtCl4(EtCN)2] reacts smoothly and under mild conditions with isomeric
o-, m- and p-nitroanilines (NAs) yielding two different types of products depending on the NA isomer,
viz. the nitroaniline complexes cis/trans-[PtCl4(NA)2] (cis/trans-1–3) and the amidine species trans-
[PtCl4{NHvC(Et)NHC6H4NO2-m}(EtCN)] (4), trans-[PtCl4{NHvC(Et)NHC6H4NO2-m}2] (5) and
trans-[PtCl4{NHvC(Et)NHC6H4NO2-p}(EtCN)] (6). Complexes 4 and 5 undergo cyclometalation,
furnishing mer-[PtCl3{NHvC(Et)NHC6H3NO2-m}(EtCN)] (7) and mer-[PtCl3{NHvC(Et)NHC6H4NO2-m}-
{NHvC(Et)NHC6H3NO2-m}] (8), respectively. Moreover, 8 both in the solid state and in solution
undergoes the second step of the cyclometalation, generating [PtCl2{NHvC(Et)NHC6H3NO2-m}2] (9).
In 4, the nitrile ligand is highly reactive toward nucleophilic addition and it undergoes facile hydration
accompanied by the elimination of the nitrile, thus producing cis-[PtCl4(NH2C6H4NO2-m){NHvC(OH)-
Et}] (10), or methanol addition providing trans-[PtCl4{NHvC(Et)NHC6H4NO2-m}{NHvC(Et)OMe}]
(11). All compounds, besides 9, were characterized by C, H, and N elemental analyses, high-resolution
ESI-MS, IR, 1H and 13C{1H} NMR spectroscopic techniques. Complex 9, which was not isolated
as a pure compound, was identified in the reaction mixture by ESI-MS and 1H and 13C{1H} NMR
spectroscopies. Complexes trans-1, trans-2, 4, 5, 6, 8, 10, and 11 were additionally studied by X-ray
diffraction.
subject see ref. 2c,d) shows that the largest fraction of research is
Introduction
devoted to the creation of a C–N bond by the addition sp3- and
sp2-(H)N-nucleophiles, as well as by coupling of some nitrogen
In general, interest in the conversion of nitriles at metal centers
heterocycles5 with ligated RCN species. Recent examples
stems from the possibilities (i) to use nitriles as versatile syn-
thons for the preparation of new compounds, often unreachable
in pure organic synthesis, via C–O, C–N, C–C, C–P, and C–S
include nucleophilic addition of di- and triamines to (nitrile)PtIV
species,6 coupling of primary amines,4b,7 ferrocenyl-functiona-
bond making;1,2 (ii) to provide environmentally friendly metal-
catalyzed hydrolytic transformations of RCN species to amides,
e.g., of industrial and pharmacological significance;1a,b,2a,3 (iii)
to synthesize, via nucleophilic addition, diverse imino com-
plexes, e.g., exhibiting unusual structure–antitumor properties
relationships.2d,4
lized amine,8 and nucleobases9 with PtII-bound nitriles.
Reactions of metal-activated nitriles and such strong sp3-(H)-
N-nucleophiles as ammonia, unfunctionalized primary and
secondary amines4b–d,7,10 and some functionalized (with OH,11
NH2,5d,6,12 or CO2R13 moieties) amines are well documented
and this coupling comprises one of the primary routes for the
direct generation of ligated and metal-free amidines HNvC(R)-
NR′R′′.1a,2c,d,4c,13,14 These species serve as useful building blocks
for syntheses of many biologically important materials2c,d,4a–c,10a
and, in particular, some (amidine)PtIV complexes exhibit cyto-
static activity, e.g., providing significant sensitization for the
antitumor actions of the known drugs cyclophosphamide and
doxorubicine.2d,6
The analysis of experimental material collected to date (for
our reviews see ref. 1a,d; reviews from other groups on the
aDepartment of Chemistry, St. Petersburg State University,
Universitetsky Pr. 26, 198504 Stary Petergof, Russian Federation.
E-mail: bokach@nb17701.spb.edu, kukushkin@vk2100.spb.edu
bUniversity of Jyväskylä, Department of Chemistry, P.O. Box 35,
FI-40014 University of Jyväskylä, Finland. E-mail: matti.o.haukka@jyu.fi
cSaint Petersburg State Forest Technical University, Institutsky per. 5,
194021 St. Petersburg, Russian Federation
†Electronic supplementary information (ESI) available: Tables with
crystal data; molecular structures of trans-[PtCl4(EtCN)2], trans-1,
trans-2, 6, 10, and 11; experimental details and characterization of cis/
trans-1–3. CCDC 881311–881319. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2dt30986f
In contrast to the coupling of the strong sp3-(H)N-nucleo-
philes, the reactions of weak sp3-(H)N-nucleophiles have not
been studied. Being interested in the amplification of an on-
going project on reactions of metal-activated nitriles (for recent
works see ref. 15) and, in particular, on the metal-mediated
additions of various HN-nucleophiles to the CuN moiety,6 we
focused our attention on the reaction of (nitrile)PtIV species with
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 12857–12864 | 12857

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the isomeric nitroanilines as weak HN-nucleophiles having pre-
dominantly sp3-character of the NH2 group.16 The nitroanilines
(NA) were employed for this study as easily accessible and com-
mercially available representatives of weak N-nucleophiles,
where the electron-withdrawing group NO2 decreases the nucleo-
philic properties of NAs to such a degree that nitroanilines react
only with highly activated RCN species at platinum(IV) centers.
Furthermore, the unsubstituted nitroaniline, C6H5NH2, reduced
PtIV complexes and could not be applied for the nucleophilic
addition study.
Our goal was at least 3-fold (i) to verify how the isomerism of
nitroanilines affects the interplay between these species and the
(nitrile)PtIV moiety; (ii) to get an easy entry to platinum(IV)
N-aryl amidine complexes; (iii) to study the reactivity of the
iminoacylated N-arylamidine functionality.
interface. A semi-empirical or numerical absorption correction
(SADABS27 or Xprep in Shelxtl v.6.14-128) was applied to
all data. Structural refinements were carried out using
SHELXL-97.22 In the structure of 8, the H2O molecules and CH3
group (C12 and C12B) were disordered over two sites with
occupancy ratio 0.58/0.42 and 0.79/0.21 respectively. The C–C
distances C11–C12 and C11–C12B were set to be similar. The
anisotropic displacement parameters of the disordered oxygens
were restrained so that their Uij components approximate to
isotropic behavior. The anisotropic displacement parameters of
the disordered carbons were set to be similar. The H2O and NH
hydrogen atoms were located from the difference Fourier map
but constrained to ride on their parent atom, with Uiso = 1.5Ueq
(parent atom). Other hydrogen atoms were positioned geometri-
cally and were also constrained to ride on their parent atoms,
with C–H = 0.95–0.98 Å, and Uiso = 1.2–1.5Ueq (parent atom).
The crystallographic details are summarized in Tables S1 and S2
(ESI†) and selected bond lengths and angles for trans-
[PtCl4(EtCN)2], trans-1, trans-2, 4, 5, 6, 8, 10, and 11 are listed
in the captions to Fig. 1–3 and Fig. S1–S6 (ESI†).
Experimental
Materials and methods
NAs and solvents used in syntheses were obtained from com-
mercial sources (Aldrich) and used as received. The solvents
employed for preparation of 7–9 were purified by distillation and
kept over zeolites. Synthesis of trans-[PtCl4(EtCN)2] was pro-
duced by the known methodology17 by chlorination of the plati-
num(II) complex trans-[PtCl2(EtCN)2].18 Yields are based on
platinum; if it is not specified, isolated yields are given. The
spectroscopy yields were determined via signal integration in the
1H NMR spectra. For TLC, Merck 60 F254 SiO2 plates were
used. Elemental analyses for carbon, hydrogen, and nitrogen
were carried out with a 185V Carbon Hydrogen Nitrogen Analy-
zer Hewlett Packard apparatus and an Elenentar Vario Micro
instrument. Electrospray ionization mass-spectra were obtained
on a Bruker micrOTOF spectrometer equipped with electrospray
ionization (ESI) source and MeOH was used as the solvent (in
all cases unless specifically indicated). The instrument was oper-
ated both in positive and negative ion modes using a m/z range
of 50–3000. The capillary voltage of the ion source was set at
−4500 V (ESI+-MS) or 3500 V (ESI−-MS) and the capillary
exit at (70–150) V. In the isotopic pattern, the most
intensive peak is reported. Infrared spectra were recorded on
Shimadzu FTIR-8400S and Bruker Vertex 70 spectrometers
Synthetic work
The experimental details and characterization of nitroaniline
complexes cis/trans-1–3 are given in the ESI.†
Fig. 1 Molecular structure of 4 with the atomic numbering scheme.
Thermal ellipsoids are given at the 50% probability level. Selected bond
lengths (Å) and angles (°): Pt(1)–N(1) 2.001(8), Pt(1)–N(2) 1.989(8),
Pt(1)–Cl(1) 2.320(3), Pt(1)–Cl(2) 2.326(3), Pt(1)–Cl(3) 2.317(3), Pt(1)–
Cl(4) 2.316(3), N(1)–C(1) 1.288(14), N(2)–C(10) 1.160(12), N(3)–C(1)
1.330(15), N(2)–Pt(1)–N(1) 175.4(4), Cl(3)–Pt(1)–Cl(1) 179.56(10),
Cl(4)–Pt(1)–Cl(2) 177.75(9).
1
(4000–400 cm−1) in KBr pellets. The H and 13C{1H} NMR
spectra were measured on Bruker-DPX300 and Bruker 400 Ultra
Shield instruments at ambient temperature.
X-ray determination
The crystals of trans-[PtCl4(EtCN)2], trans-1, trans-2, 4, 5, 6, 8,
10, and 11 were obtained by a slow evaporation of solvents at
4–5 °C. All crystals were immersed in cryo-oil, mounted in a
Nylon loop, and measured at a temperature of 100 K. The X-ray
diffraction data were collected on a Nonius KappaCCD, Bruker
Axs Smart Apex II, Bruker Axs KappaApexII diffractometers
using Mo Kα radiation (λ = 0.710 73 Å). The Apex2,19
EvalCCD20 and Denzo/Scalepack21 program packages were used
for cell refinements and data reductions. The structures were
solved by direct methods using Shelxs 97,22 SUPERFLIP,23
SIR9724 or SIR200825 program with the Olex 226 graphical user
Fig. 2 Molecular structure of 5 with the atomic numbering scheme.
Thermal ellipsoids are given at the 50% probability level. Selected
bond lengths (Å) and angles (°): Pt(1)–N(1) 2.0278(16), Pt(1)–Cl(1)
2.3197(4), Pt(1)–Cl(2) 2.3176(4), N(1)–C(1) 1.301(2), N(2)–C(1)1.342(2),
N(1)–Pt(1)–Cl(1) 93.88(5), Cl(2)–Pt(1)–Cl(1) 89.004(17).
12858 | Dalton Trans., 2012, 41, 12857–12864
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added to the solution. The resulting suspension was stirred for
2 d at 20–25 °C. The obtained orange solution was evaporated to
dryness at RT. The resulting orange precipitate was trans-
[PtCl4{NHvC(Et)NHC6H4NO2-m}2] (5) (58.4 mg, 95%).
trans-[PtCl4{NHvC(Et)NHC6H4NO2-m}(EtCN)] (4). Found:
C, 24.55; H, 2.69; N, 9.53 (Calcd for C12H16N4Cl4O2Pt:
C, 24.63; H, 2.76; N, 9.57). ESI−-MS, m/z: 582.9653; calcd for
[M − H]− 582.9548. TLC: Rf = 0.70 (Me2CO–CHCl3 = 1 : 20,
v/v). IR spectrum in KBr (selected bands, cm−1): 3350 m and
3295 m ν(N–H), 3060 w and 2920 w ν(C–H), 2331 m ν(CuN),
1641 s ν(CvN), 1610 m ν(CvCAr), 1540 s and 1534 s
ν(CvCAr and/or asymmNvO), 1350 s νsymm(NvO), 744 w δ(C–
HAr). 1H NMR spectrum (CDCl3, δ): 9.65 (s, 1H, NHamine), 8.29
(d, 4.0 Hz, 1H), 8.14 (s, 1H), 7.71 (t, 1H), 7.61 (d, 4.0 Hz, 1H)
(Ar), 5.90 (s, JPt–H = 14,5 Hz, 1H from NHimine), 3.10 (q,
3.6 Hz, 2H, CH2 from NHvCEt), 2.55 (q, 3.6 Hz, 2H, CH2
from NuCEt), 1.54 (t, 3.6 Hz, 3H, CH3 from NHvCEt), 1.27
(t, 3.6 Hz, 3H, CH3 from NuCEt). 13C{1H} spectrum (CDCl3,
δ): 170.8 (CvN), 149.4 and 136.9 (Cipso), 132.7, 131.3, 123.7,
121.9 (CAr), 120.0 (CuN), 27.4 (NHvCCH2CH3), 13.2
(CuNCH2CH3), 10.7 (NHvCCH2CH3), 10.0 (CuNCH2CH3).
Fig. 3 Molecular structure of 8 with the atomic numbering scheme.
Thermal ellipsoids are given at the 50% probability level. Selected bond
lengths (Å) and angles (°): Pt(1)–N(1) 2.043(4), Pt(1)–N(2) 2.003(4),
Pt(1)–C(18) 2.014(5), Pt(1)–Cl(1) 2.3015(12), Pt(1)–Cl(2) 2.4545(12),
Pt(1)–Cl(3) 2.3113(13), N(1)–C(1) 1.299(6), N(2)–C(10) 1.299(6), N(3)–
C(1) 1.323(7), N(4)–C(10) 1.349(6), N(2)–Pt(1)–C(18) 90.57(18), N(2)–
Pt(1)–N(1) 173.98(18), Cl(1)–Pt(1)–Cl(2) 90.06(4), Cl(1)–Pt(1)–Cl(3)
177.89(5).
Reaction of trans-[PtCl4(EtCN)2] with isomeric NAs
(i) In a molar ratio of 1 : 1. The NA (30.9 mg, 0.224 mmol) was
dissolved in CH2Cl2 (2 mL), whereupon the solid trans-
[PtCl4(EtCN)2] (100.0 mg, 0.224 mmol) was added to the solu-
tion. The resulting suspension was stirred at 20–25 °C for 3 d for
o- or 1 d for m- and p-NAs. The yellow solid residue of
cis/trans-[PtCl4(NA)2] (o-NA, cis/trans-1; m-NA, cis/trans-2;
p-NA, cis/trans-3) was precipitated, separated by filtration and
dried in air at room temperature (RT). Yields were 64.4 mg, 47%
(cis/trans-1), 4.1 mg, 3%, (cis/trans-2), and 60.2 mg, 44%,
(cis/trans-3), correspondingly. The filtrates from the reactions
with m-NA and p-NA were evaporated at RT to dryness, furnish-
ing the yellow solid residue of mono-amidine complexes 4
(m-NA, NMR yield 90%) or 6 (p-NA, NMR yield 8%), respect-
ively. The crude material was purified by chromatography on
SiO2 (CHCl3–Me2CO, 10 : 1, v/v) to give, in the first fraction,
pure complexes 4 (81.0 mg, 62%) or 6 (6.6 mg, 5%). The for-
mation of amidine complexes in the reaction involving o-NA
was not observed.
(ii) In a molar ratio of 1 : 2. The NA (30.9 mg, 0.224 mmol)
was dissolved in CH2Cl2 (2 mL), whereupon the solid trans-
[PtCl4(EtCN)2] (50.0 mg, 0.112 mmol) was added to the solu-
tion. The resulting suspension was stirred at 20–25 °C for 5 d for
o- and 1 d for m- and p-NAs. The yellow precipitate of cis/trans-
1–3 was filtered off, washed with CHCl3 (two 1 mL portions)
and dried in air at RT. Yields are 65.2 mg, 95% (cis/trans-1),
4.5 mg, 6.5%, (cis/trans-2), and 63.0 mg, 92% (cis/trans-3). The
formation of 5 was observed only in the case of the reaction with
m-NA. The filtrate from the reaction with m-NA was evaporated
at RT to dryness and the resulting yellow solid residue was sepa-
rated by chromatography on SiO2 (CHCl3–Me2CO, 10 : 1, v/v)
to give, in the first fraction, bis-amidine complex 5 (58.5 mg, 72%).
trans-[PtCl4{NHvC(Et)NHC6H4NO2-m}2] (5). Found: C,
30.06; H, 2.99; N, 11.70 (Calcd for C18H22N6Cl4O4Pt: C, 29.89;
H, 3.07; N, 11.62). ESI−-MS, m/z: 720.9893; calcd for [M − H]−
720.9975. TLC: Rf = 0.70 (Me2CO–CHCl3 = 1 : 25, v/v). IR
spectrum in KBr (selected bands, cm−1): 3380 m and 3300 m
ν(N–H), 3092 w ν(C–H), 1632 s ν(CvN), 1613 m ν(CvCAr),
1583 m ν(CvC), 1526 s νasymm(NvO), 1347 s νsymm(NvO),
1
741 w δ(C–HAr). H NMR spectrum ((CD3)2CO), δ): 9.49 (s,
2H, NHamine), 8.27 (d, 4.7 Hz, 2H), 8.21 (s, 2H), 7.84 (m, 4H)
(Ar), 6.92 (s, br, 2H, NHimine), 2.87 (q, 7.4 Hz, 4H), 1.19 (t, 7.4
Hz, 6H)(Et). 13C{1H} NMR spectrum ((CD3)2CO), δ): 172.2
(CvN), 149.9 and 138.6 (Cipso), 132.9, 132.0, 123.0, 121.4
(CAr), 27.1, 11.7 (Et).
trans-[PtCl4{NHvC(Et)NHC6H4NO2-p}(EtCN)] (6). Found:
C, 24.70; H, 2.87; N, 9.70 (Calcd for C12H16N4Cl4O2Pt:
C, 24.63; H, 2.76; N, 9.57). ESI−-MS, m/z: 582.9597; calcd for
[M − H]− 582.9548. TLC: Rf = 0.68 (eluent Me2CO–CHCl3 =
1 : 25, v/v). IR spectrum in KBr (selected bands, cm−1): 3425 m
and 3323 w ν(N–H), 2948 w ν(C–H), 2341 m ν(CuN), 1659 s
ν(CvN), 1603 s ν(CvCAr), 1530 s νasymm(NvO), 1348 m
1
νsymm(NvO), 674 w δ(C–HAr). H NMR spectrum (CDCl3, δ):
9.73 (s, br, 1H, NHamine), 8.27 (d, 8.4 Hz, 2H), 7.75 (d, 8.4 Hz,
2H)(Ar), 5.95 (s, br, 1H from NHimine), 2.84 (q, 7.5 Hz, 2H,
CH2 from NHvCEt), 2.71 (q, 7.0 Hz, 2H, CH2 from NuCEt),
1.39 (t, 7.6 Hz, 3H, CH3 from NHvCEt), 1.16 (t, 7.2 Hz, 3H,
CH3 from NuCEt). 13C{1H} spectrum (CDCl3, δ): 171.7
(CvN), 148.6, 137.2 (Cipso), 126.9, 124.6 (CAr), 119.1 (CuN);
26.8
(NHvCCH2CH3);
12.9
(NuCCH2CH3);
10.3
(NHvCCH2CH3); 9.8 (NuCCH2CH3).
Cyclometalation of the amidine in 4
Reaction of trans-[PtCl4{NHvC(Et)NHC6H4NO2-m}(EtCN)] (4)
and m-NA
(i) In solution. Complex 4 (50.0 mg, 0.085 mmol) was dissolved
in the freshly distilled CH2Cl2 (2 mL), dinitrogen was passed
through the solution (30 min), and the reaction mixture was left
to stand for 10 d at 30 °C in a closed flask. The yellow
Complex 4 (50.0 mg, 0.085 mmol) was dissolved in CH2Cl2
(2 mL), whereupon the solid m-NA (11.7 mg, 0.085 mmol) was
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precipitate of mer-[PtCl3{NHvC(Et)NHC6H3NO2-m}(EtCN)]
(7) (26.3 mg, 56%) was separated by filtration, washed with
CH2Cl2 (1 mL) and dried in air at 20–25 °C. (ii) In the solid
state. Complex 4 (50.0 mg, 0.085 mmol) was heated in the solid
sate under dinitrogen for 7 d at 50 °C. The resulting orange-
brown powder was dissolved in acetone (1 mL) and 7 (18.0 mg,
38%) was separated by column chromatography (Me2CO–
CHCl3 = 1 : 20, v/v, second fraction).
formed powder contains some degradation products along with
[PtCl2{NHvC(Et)NHC6H3NO2-m}2] (9) (NMR yield 25%).
(ii) In the solid phase. The powder of mer-[PtCl3{NHvC(Et)-
NHC6H4NO2-m}{NHvC(Et)NHC6H3NO2-m}] (2) (70.0 mg,
0.102 mmol) was heated under dinitrogen for 3 d at 100 °C. The
resulting solid contains 9 (NMR yield 20%) along with some
degradation products, which were not separated.
[PtCl2{NHvC(Et)NHC6H3NO2-m}2] (9). ESI−-MS, m/z:
649.0529; calcd for [M − H]− 649.0444. 1H NMR spectrum
((CD3)2CO, δ): 9.83 (s, 2H, NHamine), 8.47 (d, 8.1 Hz, 2H), 8.11
(d, 8.1 Hz, 2H), 7.73 (s, 2H)(Ar), 4.22 (s, 2H NHimine), 2.78
(q, 7.4 Hz, 4H), 1.21 (t, 7.4 Hz, 6H)(Et). 13C{1H} NMR spec-
trum ((CD3)2CO, δ): 165.7 (CvN), 147.4, 144.8 (Cipso), 125.4,
120.7, 113.1, 109.4 (CAr), 45.3, 12.5 (Et).
mer-[PtCl3{NHvC(Et)NHC6H3NO2-m}(EtCN)] (7). Found:
C, 26.12; H, 2.67; N, 10.14 (Calcd for C12H15N4Cl3O2Pt: C,
26.27; H, 2.76; N, 10.21). ESI−-MS, m/z: 546.9807; calcd for
[M − H]− 546.9782. TLC: Rf = 0.41 (Me2CO–CHCl3 = 1 : 20,
v/v). IR spectrum in KBr (selected bands, cm−1): 3251 m ν(N–
H), 3005 w and 2979 w ν(C–H), 2326 m ν(CuN), 1658 s
ν(CvN), 1573
m ν(CvCAr), 1527 s ν(CvCAr and/or
1
asymmNvO), 1384 s νsymm(NvO), 731 w δ(C–H from Ar). H
NMR spectrum (CD3OD, δ): 9.78 (s, 1H, NHamine), 8.28 (d,
8.0 Hz 1H), 8.09 (d, 8.0 Hz 1H), 7.76 (s, 1H)(Ar), 4.42 (s, 1H,
NHimine), 2.58 (q, 7.2 Hz, 2H, CH2 from Et), 2.41 (q, 7.4 Hz,
2H, CH2 from Et), 1.31 (m, 6H, 2CH3 from 2Et). 13C{1H}
NMR spectrum (CD3OD, δ): 168.2 (CvN), 150.9, 141.14
(Cipso); 129.5, 126.8, 122.4, 120.9 (CAr); 117.3 (CuN); 38.2
(CH2 from Et), 21.9 (CH2 from Et), 11.8 (CH3 from Et), 10.3
(CH3 from Et).
Hydration of the EtCN in 4
Complex 4 (50.0 mg, 0.085 mmol) was dissolved in undried
CHCl3 (2 mL) and left to stand for 8 h at 50 °C. The resulting
solution was evaporated under vacuum at RT and the obtained
residue was washed with Et2O (two 1 mL portions). Yield of 10
was 31.8 mg, 68%.
cis-[PtCl4(NH2C6H4NO2-m){NHvC(OH)Et}] (10). Found: C,
19.68; H, 2.31; N, 7.60 (Calcd for C9H13N3Cl4O3Pt: C, 19.72;
H, 2.39; N, 7.67). ESI−-MS, m/z: 545.9354; calcd for [M − H]−
546.9310. TLC: Rf = 0.38 (eluent Me2CO–CHCl3 = 1 : 20, v/v).
IR spectrum in KBr (selected bands, cm−1): 3308 w and 3289 m
ν(N–H), 3010 w, 2954 w, and 2918 w ν(C–H), 1648 m and
1615 m ν(CvN), 1639 s ν(C–O), 1552 s ν(CvC), 1537 s
ν(CvCAr and/or asymmNvO), 1348 s νsymm(NvO), 742 w
δ(C–H from Ar). 1H NMR spectrum (CDCl3, δ): 9.85 (s, br, 1H,
OH), 8.64 (s, br, 2H, NH2amine), 8.25 (d, 6.9 Hz, 1H), 7.88 (d,
6.9 Hz, 1H), 7.98 (s, 1H), 7.81 (t, 1H, 6.8 Hz)(Ar), 5.78 (s, 1H
from NHimine), 2.14 (q, 7.2 Hz, 2H, CH2 from Et), 1.21 (t,
7.3 Hz, 3H, CH3 from Et). 13C{1H} NMR spectrum (CDCl3, δ):
164.8 (CvN), 148.3 and 147.7 (Cipso); 130.8, 124.7, 120.2,
118.1 (CAr); 22.7 (CH2 from Et), 8.8 (CH3 from Et).
Cyclometalation of the amidine in 5
Complex 5 (50.0 mg, 0.069 mmol) was dissolved in dry distilled
MeOH (3 mL) and stirred for 1 d at 50 °C. The resulting solution
was evaporated and the obtained yellow residue of mer-
[PtCl3{NHvC(Et)NHC6H4NO2-m}{NHvC(Et)NHC6H3NO2-m}]
(8) (31.7 mg, 67%) was washed with CHCl3 (three 1 mL por-
tions), Et2O (1 mL) and dried under vacuum at RT.
mer-[PtCl3{NHvC(Et)NHC6H4NO2-m}{NHvC(Et)NHC6H3-
NO2-m}] (8). Found: C, 31.37; H, 2.98; N, 12.09 (Calcd for
C18H21N6Cl3O4Pt: C, 31.48; H, 3.08; N, 12.24). ESI−-MS, m/z:
685.0249; calcd for [M − H]− 685.0211. TLC: Rf = 0.51
(Me2CO–CHCl3 = 1 : 20, v/v). IR spectrum in KBr (selected
bands, cm−1): 3435 m ν(N–H), 2925 w ν(C–H), 1635 s
Addition of methanol to the EtCN ligand in 4
ν(CvN), 1559
w
ν(CvC), 1529
s
ν(CvCAr and/or
asymmNvO), 1384 s νsymm(NvO), 739 w δ(C–H from Ar).
1H NMR spectrum ((CD3)2CO, δ): 9.92 (s, 1H, NHamine), 8.50
(s, 1H, NHamine), 8.40 (d, 7.8 Hz, 1H), 8.15 (d, 8.7 Hz, 1H),
8.03 (m, 2H), 7.90 (m, 2H), 7.65 (d, 8.7 Hz, 1H)(Ar), 6.13 (s,
1H, NHimine from amidine), 4.14 (s, 1H, NHimine from metalla-
cycle), 2.93 (q, 7.4 Hz, 2H, CH2 from Et), 2.84 (q, 7.5 Hz, 2H,
CH2 from Et), 1.32 (m, 6H, 2CH2 from 2Et). 13C{1H} NMR
spectrum ((CD3)2CO, δ): 174.6, 160.1 (CvN), 149.6, 148.8,
147.6, 138.5 (Cipso); 132.2, 130.9, 125.1, 122.7, 121.1, 120.6,
116.7, 111.7 (CAr); 49.7 (CH2 from Et), 34.2 (CH2 from Et),
12.3 (CH3 from Et), 12.0 (CH3 from Et).
Complex 4 (25.0 mg, 0.043 mmol) was dissolved in MeOH
(2 mL) and stirred at 20–25 °C for 1 d, whereupon the resulting
solution was evaporated at RT giving the yellow trans-
[PtCl4{NHvC(Et)NHC6H4NO2-m}{NHvC(Et)OMe}]
(11)
(24.7 mg, 94%).
trans-[PtCl4{NHvC(Et)NHC6H4NO2-m}{NHvC(Et)OMe}]
(11). Found: C, 25.15; H, 3.14; N, 9.23 (Calcd for
C13H20N4Cl4O3Pt: C, 25.30; H, 3.27; N, 9.08); ESI+-MS, m/z:
638.9714; calcd for [M + Na]+ 638.9786. TLC: Rf = 0.70
(eluent Me2CO–CHCl3 = 1 : 40). IR spectrum in KBr (selected
bands, cm−1): 3337 m ν(N–H), 3038 w, 2924 m, and 2854 w
ν(C–H), 1654 m and 1631 m ν(CvN), 1604 s ν(CvCAr),
1528 s and 1504 s ν(CvCAr and/or asymmNvO), 1311 m
Cyclometalation of the amidine in 8
1
(i) In solution. 8 (70.0 mg, 0.102 mmol) was dissolved in dry
acetone (2 mL) and left to stand for 12 d at 50 °C. The resulting
orange-brown solution was evaporated in air at 4–5 °C. The
νsymm(NvO), 771 w δ(C–HAr). H NMR spectrum ((CD3)2CO,
δ): 9.58 (s, br, 1H, NHamine), 8.37 (t, 9.5 Hz, 1H), 8.23 (d,
9.5 Hz, 1H), 7.86 (m, 2H)(Ar), 7.05 (s, br, 1H, NHimine), 6.94
12860 | Dalton Trans., 2012, 41, 12857–12864
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(s, br, 1H, NHimine), 4.13 (s, 3H, CH3 from OMe), 3.10 (q,
7.1 Hz, 2H, CH2 from Et), 2.85 (q, 8.4 Hz, 2H, CH2 from Et),
1.22 (t, 8.2 Hz, 3H, CH3 from Et), 1.18 (t, 7.5 Hz, 3H, CH3
from Et). 13C{1H} NMR spectrum ((CD3)2CO, δ): 177.8
(CvN), 172.4 (CvN), 149.1 and 138.0 (Cipso), 133.2, 132.1,
123.6, 121.8 (CAr), 56.0 (MeO), 27.3 (CH2 from Et), 27.1 (CH2
from Et), 11.4 (CH3 from Et), 10.0 (CH3 from Et).
of unidentified products. Therefore, the reaction was conducted
at RT.
Thus, the reaction of trans-[PtCl4(EtCN)2] with o-NA in
CH2Cl2 at both 1 : 1 and 1 : 2 molar ratios leads exclusively to
the known29 disubstitution product cis/trans-1 and this reaction
proceeds longer (3 d) than the reactions with m- (1 d) and p-NAs
(1 d). The reaction of trans-[PtCl4(EtCN)2] and m-NA at a
1 : 1 molar ratio proceeds smoothly at RT and completes for 1 d,
providing previously unreported amidine complex 4 as the major
product (NMR yield ca. 90%; Scheme 1, A), while disubstitu-
tion product cis/trans-2 (Scheme 1, D) is formed in minor quan-
tities (NMR yield ca. 5%; isolated yield ca. 3%). When this
reaction is performed in a 1 : 2 molar ratio of the reactants,
it also requires 1 d at 20–25 °C to complete and leads to bis-
amidine complex 5 (72%; Scheme 1, B) along with small
amounts of cis/trans-2 (ca. 3%).
Eventually, p-NA reacts with trans-[PtCl4(EtCN)2] in a
1 : 1 molar ratio for 1 d, furnishing 6 (NMR yield ca. 8%) and
disubstitution product cis/trans-329 (NMR yield ca. 46%; iso-
lated yield 44%). Increasing the molar ratio to 1 : 2 leads to
selective generation of cis/trans-3 (92%). The experimental
details and characterization of nitroaniline complexes cis/trans-
1–3 are given in the ESI.†
Results and discussion
For this study we addressed the model complex trans-
[PtCl4(EtCN)2] bearing a platinum(IV) center, which provides a
very high activation of the ligated nitriles1b,6,15a and exhibits
rather good solubility in the most common organic solvents; an
X-ray structure of this complex was determined in the framework
of this project (Fig. S1, ESI†). As weak (H)N-nucleophiles we
employed the isomeric o-, m-, and p-nitroanilines (o-, m-, and
p-NAs) that are commercially available and soluble in organic
solvents.
Reactions of trans-[PtCl4(EtCN)2] with NAs
The complex trans-[PtCl4(EtCN)2] reacts with all isomeric NAs
at RT giving different products depending on the molar ratio of
the reactants and the position of the NO2 substituent in NAs
(Scheme 1). We observed two main directions of the reaction
between trans-[PtCl4(EtCN)2] and NAs, viz., nucleophilic
addition to the CuN bond (Scheme 1, routes A and B) and sub-
stitution of the nitrile ligand (route D, see ESI†). Lowering the
reaction temperature to −14 °C leads to decreasing the reaction
rate with no substantial changes in the ratio between the substi-
tution and the addition products. Increasing the temperature to
38 °C gives rise to the increase of the substitution product
fraction. However, the reaction at the higher temperature is
accompanied with other side processes, giving a broad mixture
Reactivity of the amidine complexes
(i) Cyclometalation. Despite the fact that amidine complexes 4
and 5 are stable below 4 °C they undergo cyclometalation both
in the solid phase (50 °C, 7 d) and in solution (CH2Cl2, 30 °C,
10 d for 4 and MeOH, 50 °C, 1 d for 5) (Scheme 2); this trans-
formation was monitored by TLC and H NMR. Liberation of
the propiononitrile or generation of hydration products were not
observed.
Complex 5, bearing two amidine ligands, undergoes a step-
wise cyclometalation via generation of mono-cyclometalated
product 8 (67%) followed by cyclization of the second ligand to
furnish bis-cyclometalated complex 9 (yield 25% in acetone
solution, 20% in the solid phase) (Scheme 3).
1
The first stage of the cyclometalation proceeds rather rapidly
(MeOH solution, 50 °C, 1 d), while the second step requires
longer time (MeOH, 50 °C, 12 d or in the solid phase at 100 °C
for 3 d), yielding a broad mixture of degradation products, where
1
9 was detected by ESI-MS, H and 13C{1H} NMR methods, but
was not isolated as a pure compound.
It is worth mentioning that a similar cyclization furnishing
cyclometalated N-aryl amidines was observed earlier for the
reaction of MeCN with the anilines NH2C6H4R-p (R = H, CH3,
OCH3) at a RuII center.30 The cycloruthenation proceeds faster
(1.5 h) than the cycloplatination of 5, but requires elevated
Scheme 1 Reactions of trans-[PtCl4(EtCN)2] with NAs.
Scheme 2 The cyclometalation of 4.
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Scheme 5 Methanolysis of the EtCN ligand in 4.
The hydration of the nitrile ligand is platinum(IV)-mediated.
It is well known that nitriles bound to a PtIV center are highly
reactive toward hydration. Thus, the complex [PtCl5(EtCN)]−
hydrolyzed in the solid phase even in wet air at RT,33 and the
hydration of [PtCl4(RCN)2] (R = Me, Et) occurs with water in
undried solvents. Nevertheless, as stated above, complex 4
bearing the nitrile ligand is stable in air below 4 °C, in contrast
to the starting [PtCl4(EtCN)2]. Probably its inertness is caused
by the amidine ligand, which seems to be a better electron donor
than the nitrile. This makes the platinum center of 4 less
electrophilic.
Scheme 3 The stepwise cyclometalation of 5.
(iii) Addition of Methanol. Compound 4 dissolved in neat
MeOH converts to mixed-ligand complex 11 (RT, 1 d, 94%)
(Scheme 5). The formation of [PtCl4(NH2C6H4NO2-m){NHv
C(OMe)Et}], which is structurally similar to 10, was not
observed at RT, or at 50 °C.
Scheme 4 The hydration of 4 accompanied by the elimination of the
nitrile.
The literature data on other platinum complexes15e,34 indicates
that the R′OH–RCN coupling depends upon, first, the oxidation
state of the metal and, second, the ligand environment in the
Pt species. In the case of higher oxidation states (i.e. platinum(IV)
center), the R′OH–RCN coupling does not require the use
of a base, which is instead required for the lower oxidation
state (i.e. platinum(II) center34b). The observed addition of
MeOH gives an another example of the metal-mediated addition
of alcohols to nitriles activated through coordination to PtIV, and
the reaction is conducted under mild conditions and it does not
require addition of a base for activation of the nucleophile.34b
temperatures (100–110 °C) and a considerable excess of the
appropriate aniline, which was used as a media for the reaction.
In addition, the cycloruthenation – similar to the vast majority of
cyclometalations – proceeds only in the liquid phase.30,31 In con-
trast, the solid state cyclometalation is studied very little and, to
the best of our knowledge, the solid state cyclometalation of
amidines was not previously observed.32 Hence, the solid state
conversion of N-aryl amidine in 4 and 8 represents the first
example of a solid state cyclometalation in amidine chemistry
and the second example in imine chemistry.32
(ii) Nitrile hydration in 4. Complex 4 heated at 50 °C in
undried CHCl3 undergoes hydration accompanied with the elim-
ination of the nitrile (Scheme 4). Complex 10, bearing both
m-NA and the iminole ligands in the cis-position, was isolated
as the final product (68%). Concurrently, the free propiononitrile
Characterization of 4–11
Compounds 4–8, 10, and 11 were characterized by C, H, and N
elemental analyses, high resolution ESI-MS, IR, 1H and 13C
{1H} NMR spectroscopic techniques. Complex 9, which was not
isolated as a pure compound, was identified in the reaction
mixture by ESI-MS and 1H and 13C{1H} NMR spectroscopies.
In the ESI−-MS of 4–10, the observed peaks were attributed
to [M − H]−, while in the ESI+-MS of 11 the peaks of
[M + Na]+ were detected; in all cases, the experimental isotopic
patterns agree well with the calculated ones. In the IR spectrum
of amidine complexes 4–8, 10, 11, strong stretches in the range
1615–1659 cm−1 were assigned to the ν(CvN) vibrations and
medium intensity bands at 3289–3302 and 3337–3435 cm−1 to
ν(N–H).
1
was detected in the reaction mixture by H NMR. Bis-amidine
complex 5 does not undergo a similar elimination.
At least two mechanisms for generation of 10 and the liber-
ation of EtCN are possible but experimental evidence supporting
any of them have not yet been obtained and these works require
further investigation. The first plausible mechanism consists of
the elimination of the aniline via cleavage of the C–N bond in
the amidine ligand followed by replacement of the coordinated
nitrile by free m-NA. The PtIV-mediated hydration of the other
nitrile ligand occurs as an independent process. This mechanism
suggests the reversibility of the nucleophilic addition of m-NA,
but if the equilibrium exists, it is strongly shifted to the addition
products, because no products originating from the retro-addition
were observed when 4 and 6 were kept in anhydrous solvents. A
relevant reversibility was observed earlier for another imino
ligand derived from a platinum-mediated nitrile–pyrazole cou-
pling.5a The second plausible mechanism suggests the nucleo-
philic attack of H2O on the amidine ligand followed by
formation of the platinum-bound carboxamide and free nitro-
aniline, which coordinates to PtIV with liberation of EtCN.
One weak band at ca. 2330 cm−1 was attributed to ν(CuN) in
the nitrile containing species 4, 5, and 7. In the 1H NMR spectra
of 4–6, 10, 11, the signals from the imine NH’s were detected at
1
6.94–5.78 ppm. High-field shift of NH’s was observed in the H
NMR spectra for cyclometalated complexes 7–9 (NH’s signals
were detected at 4.42–4.14 ppm).
Complexes trans-1 (Fig. S2, ESI†), trans-2 (Fig. S3, ESI†), 4
(Fig. 1), 5 (Fig. 2), 6 (Fig. S4, ESI†), 8 (Fig. 3), 10 (Fig. S5,
12862 | Dalton Trans., 2012, 41, 12857–12864
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ESI†), and 11 (Fig. S6, ESI†) were also characterized by X-ray
diffraction.
the major (or even the only) products of the reaction. Two com-
petitive processes are possible in the [PtCl4(EtCN)2] and NA
reaction – the kinetically controlled addition to the CuN bond
and thermodynamically favorable substitution of the nitrile
ligand by NA. The increasing nucleophilicity of NAs generates
the addition products, and, on the contrary, decreasing nucleo-
philicity leads to the substitution products.
Third, we described the second example of cyclometalation of
N-aryl substituted amidine, which, to the best of our knowledge,
were previously observed only for a RuII-based system.30
All compounds, apart from 8, have slightly distorted octa-
hedral geometries with two N-donor ligands in the trans-posi-
tion. Complex 8 exhibits a slightly distorted octahedral geometry
with three chlorides in the mer-orientation. In 4, 5, 6, 8, and 11,
the C(1)N(1) (1.288(14)–1.303(7) Å) bond lengths of the
amidine ligand are slightly longer than the typical CvN dis-
tances [1.269(2) Å] and C(1)N(3) [1.323(7)–1.343(5) Å] shorter
than the single C–N bond [1.443(4) Å],35 correspondingly. Com-
parison of these values suggests a moderate degree of delocaliza-
tion in the NCN amidine fragment, which was also indicated
earlier for relevant (amidine)PtIV complexes.6,11 The NCN frag-
ments are planar with a sum of angles around the C(1) carbon
close to 360°. In 4, 5, 6, 8, and 11, the amidine ligands adopt the
(Z)-configuration, which is stabilized by the Cl(1)⋯HN hydro-
gen bonding. The C(10)N(2) bond of the nitrile ligands in 4
[1.160(12) Å] and in 6 [1.128(8) Å] exhibits values typical for
the nitrile CuN triple bond [1.139(4)].36 In 8, the metalacycle
is approximately planar. The bond length of the double C(10)N(2)
bond is 1.299(6) Å which is equal, within 3σ, to the typical
value of 1.269(2) Å,35 the bond length of the single C(10)N(4)
bond is 1.349(6) Å, which is slightly smaller than the typical
value of 1.443(4) Å.35 In 4, 5, 6, 8, and 11, the hydrogen atoms
from the amido groups are involved in an intramolecular
N–H⋯Cl hydrogen bonding [the shortest N1–H1⋯Cl1 of
2.7083(12) Å is in 6, the longest N1–H1⋯Cl3 of 2.9729(27) is
in 4]. In 10, the hydrogen of the hydroxo group is involved in
intra- and intermolecular H-bondings with the chloride ligands;
the shortest donor–acceptor distances (N⋯Cl) are between
3.0179(25) and 3.0238(23) Å); another H-bond (2.7487(33) Å)
is between the hydrogens of the NH2 group and the oxygen
atom of the hydroxo group. In trans-1, there are two H-bonds
through the coordinated NH2 group with the oxygen atoms from
the o-NO2 group of the same NA and the nitro-group of the
other molecule (with donor–acceptor N⋯O distances 2.6854(23)
and 3.0615(26) Å, respectively), while in trans-2, the H-bond of
2.9682(27) is between the Cl ligands and NH2.
Acknowledgements
The authors express their gratitude to Federal Grant-in-Aid
Program “Human Capital for Science and Education in Innova-
tive Russia” (Governmental Contract No. P676 from 20/05/2010
and P1294 from 09/06/2010, and Agreement No. 14.
B37.21.0794), Russian Fund of Basic Research (grants 11-03-
00262 and 00483), Saint-Petersburg State University (research
grant 2011–2013), Government of the Saint-Petersburg (grant
2.5/27-04/012). The financial support from the Visby Program
through the Swedish Institute is gratefully acknowledged.
Dr A.V. Khripun is thanked for experimental assistance at an
early stage of the project.
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Final remarks
The results from this work may be considered from a few
perspectives. First, we demonstrated that platinum(IV), known as
one of the strongest activators of the nitrile functionality,6,15a,37
accelerates coupling of propiononitrile ligands even with such
weak nucleophiles as nitroanilines. It should be noticed that the
metal-free coupling of nitriles even with strong H(N)-nucleo-
philes proceeds only when the nitrile bears a strong electron-
acceptor group R, e.g., CCl3.1a
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38
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