Dialkylcyanamides are more reactive substrates toward metal-mediated nucleophilic addition than alkylcyanides

Article abstract of DOI:10.1039/c3dt51137e

The dialkylcyanamide complexes Q[PtCl₃(NCNR₂)] (Q = Ph₃PCH₂Ph, R₂ = Me₂1, Et ₂2, C₅H₁₀3, C₄H₈O 4; Q = NMe₄, R₂ = Me₂5; Q = NEt₄, R ₂ = Me₂6) were synthesized either by dissolving Q ₂[Pt₂(μ-Cl)₂Cl₄] in neat NCNR₂ (1-4) or by substitution of a NCNR₂ ligand with Cl^- in [PtCl₂(NCNR₂)₂] by its treatment with QCl (5, 6). Nucleophilic addition of dibenzylhydroxylamine, HON(CH₂Ph)₂, to 1-6 results in the formation of the complexes Q[PtCl₃{NHC(NR₂)ON(CH₂Ph) ₂}] (Q = Ph₃PCH₂Ph, R₂ = Me ₂, 7; Et₂, 8; C₅H₁₀, 9; C ₄H₈O, 10; Q = Me₄N, R₂ = Me ₂11; Q = Et₄N, R₂ = Me₂, 12) that further convert at room temperature in the solid state (1-24 h) or in a solution (0.5-2 h) to the imine complexes Q[PtCl₃{N(CH₂Ph)C(H)Ph}] (Q = Ph₃PCH₂Ph, 13; Me₄N, 14; Et₄N, 15) and the corresponding dialkylureas H₂NC(O)NR₂. The competitive reactivity study of the nucleophilic addition of HON(CH ₂Ph)₂ to (Ph₃PCH₂Ph)[PtCl ₃(NCR′)] (R′ = Ph, NR₂, CH₂Ph) indicated that the reactivity of the coordinated NCNR₂ is comparable to NCPh, while NCCH₂Ph appeared to be much less reactive than the former two ligands. Compounds 1-6 and 13 were fully characterized by elemental analyses (C, H, N), high resolution ESI-MS, IR, and ¹H and ¹³C{¹H} NMR spectroscopy. The structure of 1 was additionally verified by a single-crystal X-ray diffraction.

Full text of DOI:10.1039/c3dt51137e

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Dialkylcyanamides are more reactive substrates toward
metal-mediated nucleophilic addition than
alkylcyanides†
Cite this: DOI: 10.1039/c3dt51137e
Tatyana B. Anisimova,a,b Nadezhda A. Bokach,*a Fedor M. Dolgushinc and
Vadim Yu. Kukushkin*a,d
The dialkylcyanamide complexes Q[PtCl3(NCNR2)] (Q = Ph3PCH2Ph, R2 = Me2 1, Et2 2, C5H10 3, C4H8O 4;
Q = NMe4, R2 = Me2 5; Q = NEt4, R2 = Me2 6) were synthesized either by dissolving Q2[Pt2(μ-Cl)2Cl4] in
neat NCNR2 (1–4) or by substitution of a NCNR2 ligand with Cl− in [PtCl2(NCNR2)2] by its treatment with
QCl (5, 6). Nucleophilic addition of dibenzylhydroxylamine, HON(CH2Ph)2, to 1–6 results in the formation
of the complexes Q[PtCl3{NHC(NR2)ON(CH2Ph)2}] (Q = Ph3PCH2Ph, R2 = Me2, 7; Et2, 8; C5H10, 9; C4H8O,
10; Q = Me4N, R2 = Me2 11; Q = Et4N, R2 = Me2, 12) that further convert at room temperature in the
solid state (1–24 h) or in a solution (0.5–2 h) to the imine complexes Q[PtCl3{N(CH2Ph)vC(H)Ph}] (Q =
Ph3PCH2Ph, 13; Me4N, 14; Et4N, 15) and the corresponding dialkylureas H2NC(vO)NR2. The competitive
reactivity study of the nucleophilic addition of HON(CH2Ph)2 to (Ph3PCH2Ph)[PtCl3(NCR’)] (R’ = Ph, NR2,
CH2Ph) indicated that the reactivity of the coordinated NCNR2 is comparable to NCPh, while NCCH2Ph
appeared to be much less reactive than the former two ligands. Compounds 1–6 and 13 were fully
characterized by elemental analyses (C, H, N), high resolution ESI-MS, IR, and 1H and 13C{1H} NMR
spectroscopy. The structure of 1 was additionally verified by a single-crystal X-ray diffraction.
Received 1st May 2013,
Accepted 2nd July 2013
DOI: 10.1039/c3dt51137e
www.rsc.org/dalton
NCNR2 ligands furnish different products,18–20,29 or at quanti-
tative level when kinetic experiments showed that NCNR2
Introduction
In contrast to the wealth of chemistry associated with conven- species exhibit an unexpectedly high activation toward metal-
tional NCR′ (R′ = Alk, Ar) nitriles,1–17 the coordination chem- mediated 1,3-dipolar cycloaddition of nitrones as compared to
istry of dialkylcyanamides, NCNR2—ligands that fall into the NCAlk.30,31 In addition, synthetic experiments indicate that
category of the so-called push–pull nitriles—is still a very little dialkylcyanamide ligands, despite a strong donor character of
explored field. The data, gradually accumulated in the litera- the NR2 group (that becomes evident upon inspection of the
ture (for reviews see ref. 1–3, 5 and 6; for recent works see ref. Pickett and Lever parameters for NCNR2 32), exhibit high reac-
18–27), demonstrated that reactivity modes of NCNR2 ligands tivity toward nucleophilic addition.33 However, all these differ-
can substantially vary from those of the conventional ences were observed exclusively at a qualitative level and no
nitriles.18–20,28 These distinctions were recognized at qualitat- convincing quantitative or semi-quantitative data were
ive level when the same metal-mediated reactions of NCR′ and reported.
In the framework of our ongoing project on the reactivity of
metal-activated substrates with the CuN bond,1–6 we focused
our attention on kinetic studies34,35 of platinum-mediated
nucleophilic addition to nitrile species and verified the effects
of the oxidation state and the nature of a HON-nucleophile on
reaction rates. In this work, we attempted to compare the
nitrile ligands and to establish a relative degree of activation
for platinum(II)-bound NCNR2 and NCR′ (R′ = Alk, Ph) ligands.
The scenario of this work was the following: we intended,
first, to synthesize the previously unknown platinum(II)
anionic complexes [PtCl3(NCNR2)]− featuring only one dialkyl-
cyanamide ligand; second, to study the nucleophilic addition
of HON(CH2Ph)2 to a platinum(II)-activated NCNR2; third, to
aDepartment of Chemistry, Saint Petersburg State University, Universitetsky Pr. 26,
198504 Stary Petergof, Russian Federation
bCentro de Química Estrutural, Complexo I, Instituto Superior Técnico, TU Lisbon,
Av. Rovisco Pais, 1049-001 Lisbon, Portugal
cA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of
Sciences, 119991 Moscow, Russian Federation
dInstitute of Macromolecular Compounds of the Russian Academy of Sciences, V.O.
Bolshoi Pr. 31, 199004 Saint Petersburg, Russian Federation.
E-mail: bokach@nb17701.spb.edu, kukushkin@vk2100.spb.edu
†Electronic supplementary information (ESI) available: Tables with crystal data
of 1; experimental details and characterization of 5, 6, 15, and 20–22. CCDC
936511. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3dt51137e
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conduct a comparative kinetic study for [PtCl3(NCR′)]− (R′ =
NR2, CH2Ph, Ph) complexes with an aim to verify the reactivity
differences between the dialkylcyanamide and the convention-
al nitrile ligands.
Table 1 Numbering of 1–6
i
R′
Q
1
2
3
4
5
6
NMe2
NEt2
NC5H10
NC4H8O
NMe2
Ph3PCH2Ph
Ph3PCH2Ph
Ph3PCH2Ph
Ph3PCH2Ph
NMe4
NMe2
NEt4
Results and discussion
Synthesis and characterization of Q[PtCl3(NCNR2)]
For this study, we addressed the platinum(II) compounds
Q[PtCl3(NCNR2)] (Q = Ph3PCH2Ph, Me4N, Et4N). These com-
plexes have only one ligand that can be involved in the nucleo-
philic addition and this reaction can be easily monitored by
1H NMR spectroscopy. In addition, the kinetic inertness of
platinum(II) complexes minimizes the possibility of ligand
exchange. Moreover, the anionic mono-nitrile complexes
Q[PtCl3(NCNR2)] do not have geometric isomers and do not
contain other organic ligands that could complicate the reac-
tions with the nitrile functionality. The idea behind the syn-
thesis of different anionic complexes was to obtain platinum
species suitable for kinetic studies that could be easily moni-
tored by a NMR method. For this purpose, complexes bearing
different cations, having different solubilities in the commonly
used deuterated solvents, were synthesized.
Scheme 2 Trimerization of dimethylcyanamide.
Method C led to the trimerization of the dimethylcyan-
amide (Scheme 2) along with the generation of (Ph3PCH2Ph)-
[PtCl3(NCNR2)]. The major product of the reaction is hexa-
methylmelamine (HRESI+-MS, m/z: 509.3335 [2M + H2BF3]+
[509.3407 calcd]; 1H NMR in CDCl3, δ: 3.23 (s, 9H, Me).
13C{1H} NMR in CDCl3, δ: 30.1; lit. ESI+-MS, m/z: 210 [M]+;
1H NMR in CDCl3, δ: 3.25 (s, 9H, Me)42), which is known for its
antitumor properties.43 This trimerization was previously
studied by Dornan et al. using aluminum amide as a Lewis
acid catalyst.42
The complexes (Ph3PCH2Ph)[PtCl3(NCNR2)] (R2 = Me2, 1;
R2 = Et2, 2; R2 = C5H10, 3; R2 = C4H8O, 4) were prepared from
(Ph3PCH2Ph)2[Pt2(µ-Cl)2Cl4]41 by suspending the bridged
complex in neat NCNR2 and further heating to 110–125 °C
until homogenization of the reaction mixture (3–5 min) by the
literature method reported for the generation of similar
platinum(II) NCR′ species (Scheme 1, A).39 Compounds 1–4
were isolated in good (ca. 90%) yields as pale orange solids.
In the synthesis of Q[PtCl3(NCNR2)] (Q = Et4N, Me4N), the
best yields were achieved using method B (see ESI†). The com-
plexes Q[PtCl3(NCNMe2)] (Q = Me4N, 5; Q = Et4N, 6) were
prepared from the corresponding cis-[PtCl2(NCNMe2)2] via
substitution of one coordinated NCNMe2 by treatment with
[Q]Cl (Q = Me4N, Et4N) in refluxing MeNO2 (Scheme 1, B).40
Compounds 5 and 6 were isolated in good (ca. 80%) yields as
bright orange solids.
Anionic complexes of the type [PtCl3(L)]−, analogous to the
Cossá salt K[PtCl3(NH3)],36–38 are important species in the
coordination chemistry of platinum(II). Complexes of this type
are known for the conventional nitriles (L = NCAlk, NCAr) and
were unknown until now for dialkylcyanamides (L = NCNR2).
The reported complexes bearing the conventional nitriles, viz.
Q[PtX3(NCR′)], were generated via (i) splitting the
Q2[Pt2(μ-Cl)2Cl4] species by their dissolution in NCR′
(Scheme 1, A; Q = Et4N, R′ = Me, X = Br),39 (ii) substitution of a
NCR′ ligand with X− in [PtX2(NCR′)2] by treatment with Q[X]
(Scheme 1, B; Q = Et4N, R′ = Me, CH2Ph, Ph, CH2CO2Et, X = Cl;
Q = Ph3PCH2Ph, R′ = Me, X = Cl; Q = Et4N, R′ = Me, X = Br),40
or (iii) substitution of the X− ligand with NCMe in Q2[PtX4] in
acetonitrile in the presence of BF3·Et2O as the halide-abstract-
ing reagent (Scheme 1, C; Q = Ph3PCH2Ph, R′ = Me, X = Cl).41
All these methods were examined in the syntheses of the
target Q[PtCl3(NCNR2)] species (Scheme 1, Table 1). In the case
of Q = Ph3PCH2Ph, the best yields were achieved by using
method A (reflux in NCNR2 for 3–5 min) and B (reflux in
EtNO2 for 5–7 min).
Compounds 1–6 were characterized by C, H, and N elemen-
tal analyses, high resolution ESI+/−-MS, IR, 1H and 13C{1H}
NMR spectroscopic techniques, and single-crystal X-ray diffrac-
tion (for 1). The HRESI− mass spectra of 1–6 display the peaks
from [M − Q]− with the characteristic isotopic distribution.
The IR spectra of 1–6 exhibit one strong band in the range
2270–2297 cm−1 attributed to the CuN stretching vibrations.
These values agree well with those observed for cis/trans-
[PtCl2(NCNR2)2] (ν(CuN) 2284–2294 cm−1 18,44
and are higher
)
than those for the corresponding uncomplexed NCNR2 species
(ν(CuN) ca. 2215 cm−1) indicating the electrophilic activation
1
of the nitrile upon its coordination.13 In the H NMR spectra
of 1–6, the protons of the NR2 groups were detected in the
Scheme 1 Routes to Q[PtX3(NCR’)] complexes.
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high oxidation state metal centers, viz. PtIV 48 and ReIV.49 In
addition, this coupling was used for the synthesis of metal-
containing or metal-free phthalocyanines.50 In this work, we
addressed, on the one hand, new platinum(II) compounds
Q[PtCl3(NCNR2)] and, on the other hand, dibenzylhydroxyl-
amine as a nucleophile referring to the system we studied in
our previous works.34,35 This hydroxylamine has two equi-
valent methylene groups that resonate at ca. 4 ppm (1H NMR),
viz. in the area that is free from other signals, and therefore
the application of HON(CH2Ph)2 facilitates the kinetic studies
(see later). In addition, this compound is stable, solid and can
easily be weighted.
We observed that addition of excess HON(CH2Ph)2 to any
one of 1–6 in CHCl3 at RT resulted in the generation of
the new imino species Q[PtCl3{NHC(NR2)ON(CH2Ph)2}] (Q =
Ph3PCH2Ph, R2 = Me2, 7; Et2, 8; C5H10, 9; C4H8O, 10; Q = Me4N,
R2 = Me2 11; Q = Et4N, R2 = Me2, 12) (Scheme 3, D; Table 2).
In the 1H NMR spectra of 9–12, the NCH protons of the NR2
group were detected in the range of 3.27–3.60 ppm that
is deshielded than those of starting complexes 1–4
(2.78–3.33 ppm). The proton of the newly formed NH group
was detected in the range of 5.86–6.14 ppm as a broad singlet.
Compounds 9–12 gave satisfactory C, H, and N elemental ana-
lyses, which are consistent with their formulae. Although these
compounds are rather reactive and undergo further conver-
sion, the elemental analyses agree well with the composition
of the nucleophilic addition product rather than with the sub-
stitution product.
Fig. 1 View of 1 with the atomic numbering scheme. Thermal ellipsoids are
drawn at the 30% probability level. Selected bond lengths (Å) and angles (°):
Pt(1)–Cl(1) 2.303(1), Pt(1)–Cl(2) 2.281(1), Pt(1)–Cl(3) 2.295(1), Pt(1)–N(1)
1.971(4), N(1)–C(1) 1.132(6), C(1)–N(2) 1.301(7), N(1)–Pt(1)–Cl(2) 176.29(14),
N(1)–Pt(1)–Cl(3) 88.7(1), Cl(1)–Pt(1)–N(1) 89.7(1), Cl(2)–Pt(1)–Cl(3) 90.59(4),
Cl(2)–Pt(1)–Cl(1) 91.20(4), Cl(3)–Pt(1)–Cl(1) 176.46(6), Pt(1)–N(1)–C(1) 169.7(5),
N(1)–C(1)–N(2) 179.5(6).
ranges 1.23–1.76 and 2.78–3.77 ppm. The PCH2 moiety
appeared as a doublet in the interval between 5.11 and 5.24
(for 1–4), and the NR4 cation emerges as a singlet at 3.48 ppm
(R = Me) (for 5) or as triplet and quartet at 1.43 and 3.51 ppm,
correspondingly (R = Et) (for 6). In the 13C{1H} NMR spectra of
1–4, the carbons of the NR2 cation were observed in the ali-
phatic region, while the PCH2 carbon was detected as a
doublet at 118.0 ppm (for 1–4).
Molecular structure of 1 (Fig. 1) is built up by the cation
[Ph3PCH2Ph]+ and the anionic complex [PtCl3(NCNMe2)]−. The
coordination polyhedron has one dialkylcyanamide and three
Cl− ligands resulting in a typical square-planar geometry. All
bond angles around the platinum(II) center are close to 90°
varying from 88.68(13) to 91.20(4)°. The Pt–Cl distances
(2.2808(11)–2.3026(12) Å) are typical for the single Pt–Cl
bonds.18,29,44–46 The Pt–N(1)–C(1)–N(2) fragment is not linear
(Pt–N(1)–C(1) 169.7(5)° and N(1)–C(1)–N(2) 179.5(6)°) similarly
to the bent NCNMe2 in trans-[PtCl2(NCNMe2){S(O)Me2})].46
The Pt–N(1) distance (1.971(4) Å) agrees well with the corres-
ponding values observed in trans-[PtCl2(NCNMe2)2]18 and cis-
[PtCl2(NCNMe2){S(O)Me2}]29 (1.973(8) and 1.970(6) Å, respecti-
vely). The C(1)–N(1) distance (1.132(6) Å) is the normal CuN
bond47 and it is comparable to the values observed for the
similar platinum(II) complexes trans-[PtCl2(NCNMe2)2]18 and
cis-[PtCl2(NCNMe2){S(O)Me2}]29 (1.129(14) and 1.137(9) Å,
correspondingly). The NMe2 group is disordered over two posi-
tions with equal occupancies.
A benzyl group in the hydroxylamine can be easily deproto-
nated. We attempted the application of the other hydroxyl-
amine, viz. HONEt2, as a nucleophile in our reactions, and
found that its reaction with Q[PtCl3(NCNR2)] (NR2 = Me2, Et2,
C5H10, C4H8O) does not proceed selectively. Based on ESI-MS−
and NMR data both nucleophilic addition and coordination of
Scheme 3 Studied reactions.
Table 2 Numbering of the compounds
i
R
Q
i
Q
i
R
7
8
9
10
11
NMe2
NEt2
NC5H10
NC4H8O
NMe2
Ph3PCH2Ph
Ph3PCH2Ph
Ph3PCH2Ph
Ph3PCH2Ph
NMe4
13
14
15
Ph3PCH2Ph
NMe4
NEt4
16
17
18
19
NMe2
NEt2
NC5H10
NC4H8O
Dialkylcyanamide–hydroxylamine coupling
The nucleophilic addition of N,N-substituted hydroxylamines
to coordinated nitriles was previously studied by our group for 12
NMe2
NEt4
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HONEt2 was detected. Importantly, reaction E (Scheme 3) bearing the strong donor substituents NAlk2 as compared to
specific for dibenzylhydroxylamine was not observed.
conventional nitriles NCR′ with donor groups R′ (as confirmed
by considering their Pickett PL and Lever EL parameters32,63,64
toward the nucleophilic addition. As was revealed from the
)
Dehydration of iminoacylated dibenzylhydroxylamine
In the current work, complexes with iminoacylated dibenzyl- preparative experiments, the reactivity of the NCNR2 ligands is
hydroxylamine 7–9 appeared to convert in the solid state at RT even comparable to that of benzonitrile bearing the moderate
for 4–24 h (R2 = Me2, 6 h; R2 = Et2, 4; R2 = C5H10, C4H8O, 24 h) acceptor group Ph.
to the imino complexes Q[PtCl3{E/Z-N(CH2Ph)vC(H)Ph}]
In the previous work, we studied the kinetics of the nucleo-
(ca. 1 : 1 E/Z mixture; Q = Ph3PCH2Ph, 13; Q = Me4N, 14, Q = philic addition of dibenzylhydroxylamine to coordinated con-
Et4N, 15) and the corresponding dialkylureas H2NC(vO)NR2 ventional nitriles at PtIV and PtII centers.35 In the current work,
(R2 = Me2, 16; R2 = Et2, 17; R2 = C5H10, 18; R2 = C4H8O, 19) the conventional kinetic study of the nucleophilic addition of
(Scheme 3, E) that were detected in the 1H NMR (signals of the dibenzylhydroxylamine to the coordinated push–pull nitriles is
NCH protons of the NR2 group in the range of 3.19–3.95 ppm) not straightforward because of the observed dehydration (see
and HRESI+ mass spectra ([M + H]+ or [M + Na]+). Compound above). Therefore, to get a quantitative estimate of the depen-
13 was purified by column chromatography on silica gel and dence for the reactivity on the nature of the R group and in
characterized by C, H, and N elemental analyses, HRESI+/−-MS, order to compare the differences between the reactivity of co-
IR, 1H and 13C{1H} NMR spectroscopic techniques, while 14 ordinated and uncomplexed NCR′ species, we conducted a
and 15 were identified by HRESI+/−-MS and 1H NMR methods.
competitive reactivity study of the nucleophilic addition to
1
In the H NMR spectrum of 13, the characteristic signals at NCR′ (R′ = NMe2, NEt2, NC5H10, NC4H8O) and NCR′ (R′ = Ph,
9.30 and 9.33 ppm are from the protons of the NvCH group CH2Ph) ligands. The method employed includes the reaction
in the Z and E forms. Protons of the NCH2 group were detected between the equimolar mixture of two complexes and the
at 5.38 and 5.39 ppm as two singlets corresponding to the Z nucleophile, viz. HON(CH2Ph)2, in a 2 : 1 molar ratio (see the
and E forms. In the 13C{1H} NMR spectrum, the resonance of Experimental section) (Scheme 4).
the NvCH carbon was found at 168.0 ppm, while the carbon
The competitive reactivity study demonstrates that the reac-
of the NCH2 group emerged at 66.0 ppm.
tivity of the NCR′ species decreases in the following order: NR2
Herein we observed the first example of the formation of an ≈ Ph > CH2Ph, which points out the unexpectedly high reactiv-
imine complex via elimination of substituted ureas from imi- ity of the dialkylcyanamide ligand bearing the strong donor
noacylated hydroxylamine ligand at the platinum(II) center. NR2 substituent toward the nucleophilic addition. A similar
Complexes bearing imines of this type, e.g. cis/trans-[PtCl2- result was obtained earlier in our group for 1,3-dipolar cyclo-
{N(CH2C6H4Cl-4′)vCH(4-ClC6H4)}{S(O)Me2}], were previously addition of nitrones to coordinated dialkylcyanamides and
obtained by the substitution of a Me2SO ligand in cis-[PtCl2- conventional nitriles.30
{S(O)Me2}2] with the imine (4-ClC6H4)CHvNCH2(4′-ClC6H4).51–53
We also observed the coupling between the uncomplexed
A similar imine complex, viz. [Pt(pip2NCN){N(Me)vCMe2}]- conventional NCR′ (R′ = Ph, CH2Ph) and the push–pull nitrile
[CF3SO3] (pip2NCN = the pincer 1,3-(MeNC6H9)2C6H3 ligand), NCNR2 and HON(CH2Ph)2 followed by the dehydration of
was produced upon the addition of MeNH2 to the [Pt- the resulting HNvC(R)ON(CH2Ph)2 (20–22). Although the
(pip2NCN)(acetone)]2+ in an acetone solution.54 The complex addition of the hydroxylamine to coordinated and un-
with the same imine, trans-[Pt(MeNH2)2(MeNvCMe2)2]I2, was complexed nitrile species proceeds under different conditions
synthesized by the reaction of acetone with the amine in (which preclude a quantitative comparison of the reaction
[Pt(MeNH2)4]I2.55
A
similar imine complex, viz. [PtCl- rates), we made a qualitative estimate of the accelerating
(NH2CH2CH2NvCMe2)(PBun3)]+, was generated in the reaction effect of the coordination. Thus, the nucleophilic addition of
between [Pt2(μ-Cl)2Cl2(PBun3)2] and ethanediamine in
acetone.56
It is also worthwhile noting that only two examples of
metal-mediated dehydration of substituted hydroxylamines
were previously reported and they include the TiIII-mediated
dehydration of N,N-disubstituted hydroxylamines giving
imines57 and stepwise conversion of N-arylhydroxylamines to
1,4-naphthoquinone imines after the oxidation of the former
with Ag2O or air giving 1-naphthyl phenyl nitroxides, which
then convert to the corresponding imines.58
Competitive reactivity study
The synthetic data of the current work and those previously
obtained for the nucleophilic addition to platinum complexes
with the conventional nitrile and dialkylcyanamide
ligands59–62 indicate unexpectedly high reactivity of NCNR2 Scheme 4 Competitive reactivity study.
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HON(CH2Ph)2 proceeds more slowly in the case of uncom- commercial sources and used as received. The complexes
plexed NCR′ (in C6D6 at 65 °C the full conversion was reached (Ph3PCH2Ph)2[Pt2(μ-Cl)2Cl4]41 and cis/trans-[PtCl2(NCR′)2]18
after 2 d for R′ = NR2, or 7 d for R′ = Ph; 5 mmol of NCR′, were prepared by the reported procedures. The compounds
5 mmol of HON(CH2Ph)2 in 0.5 mL of C6D6) rather than in the (Ph3PCH2Ph)[PtCl3(NCR′)] (R′ = Ph, CH2Ph) were synthesized
case of the coordinated NCR′ (20 °C, the full conversion was from the corresponding cis-[PtCl2(NCR′)2] according to the
reached after 40 min for R′ = NMe2, or 30 min for R′ = Ph; method described earlier.40 For column chromatography silica
0.5 mmol of (Ph3PCH2Ph)[PtCl3(NCR′)], 0.5 mmol of dibenzyl- gel 60 F254, 0.063–0.200 mm (Merck) was used.
hydroxylamine in 0.5 mL of CD2Cl2). Therefore, these synthetic
Elemental analyses were performed using EuroVector 3000
data indicate that coordination to the metal dramatically and 185B Carbon Hydrogen Nitrogen Analyzer Hewlett-
affects the reactivity of the nitrile species.
Packard instruments. Electrospray ionization mass spectra
were obtained using Bruker micrOTOF spectrometer
a
equipped with an electrospray ionization source and MeCN or
MeOH was used as the solvent. The instrument was operated
both in positive and negative ion modes using an m/z range of
Final remarks
The result of this work can be considered from a few perspec- 50–3000. The capillary voltage of the ion source was set at
tives. First, we succeeded in synthesis of the Cossá-type com- −4500 V (ESI+-MS) or 3500 V (ESI−-MS) and the capillary exit
plexes [PtCl3(NCNR2)]− featuring the dialkylcyanamide
(70–150) V. In the isotopic pattern, the most intensive peak is
ligands. In the platinum chemistry, complexes [PtCl3(L)]− play reported. Infrared spectra were recorded using a Shimadzu
an important role in synthesis of the compounds having two FTIR 8400S instrument in KBr pellets. 1H and 13C{1H} NMR
different neutral ligands, viz. [PtCl2(L)(L′)], that otherwise are spectra were measured using Bruker-DPX 300 and Bruker
difficult-to-obtain. We believe that an easy access to Avance II + 400 MHz (UltraShieldtm Magnet) spectrometers at
[PtCl3(NCNR2)]− should stimulate progress in the synthetic ambient temperature.
chemistry of dialkylcyanamides and open up a route to various
yet unknown [PtCl2(L)(NCNR2)] species.
X-ray determination
Second, although the nucleophilic addition of dibenzyl-
hydroxylamine to platinum(II)-activated conventional nitriles
was thoroughly studied,35 this reaction with metal-bound dia-
lkylcyanamides has a specific feature consisting in further
transformation of the iminoacylated hydroxylamine producing
ligated imine and uncomplexed urea (Scheme 3). It is antici-
pated that the generation of the latter drives the reaction and
the observed transformation represents yet another example of
different reactivity modes between dialkylcyanamides and con-
ventional nitriles.
Third, the competitive reactivity study of the nucleophilic
addition of HON(CH2Ph)2 to NCR′ (R′ = Ph, NR2, CH2Ph)
ligands demonstrates that the reactivity changes in the follow-
ing order: NR2 ≈ Ph > CH2Ph. Dimethylcyanamide appears to
be highly activated by the platinum(II) center and its reactivity
toward HON(CH2Ph)2 is comparable to benzonitrile ligand
bearing the moderate electron acceptor group. In addition,
NCNR2 ligands behave like stronger σ-donors32 and, conse-
quently, they are better ligands than conventional nitriles.
These complementary properties of dialkylcyanamides, i.e.
better donor properties of the nitrile N atom and high electro-
philic activation of the nitrile C atom, make them perfect can-
didates for the nucleophilic addition studies and open up new
A single-crystal X-ray diffraction experiment was carried out
with a Bruker SMART APEX II diffractometer65 (graphite mono-
chromated Mo-Kα radiation, l = 0.71073 Å, ω-scan technique,
T = 100(2) K). A multi-scan absorption correction based on
equivalent reflections (SADABS66) was applied to the data. The
structure was solved by direct methods and refined by the full-
matrix least-squares technique against F2 with the anisotropic
thermal parameters for all non-hydrogen atoms using the
SHELXTL program package.67 Hydrogen atoms were positioned
geometrically and included in the structure factor calculations
in the riding motion approximation. The crystallographic
details are summarized in Table S1 (ESI†).
Synthetic work
Preparation of (Ph3PCH2Ph)[PtCl3(NCNR2)]. Suspension of
(Ph3PCH2Ph)2[Pt2(μ-Cl)2Cl4] (80 mg, 0.061 mmol) in NCNR2
(0.244 mmol) was refluxed until the starting complex was dis-
solved (ca. 5 min). The reaction mixture was then cooled to RT
and the product was precipitated by addition of benzene
(1 mL), washed with Et2O (three 1 mL portions) and dried in air
at RT. Yields: 84 mg, 95% (R2 = Me2, 1), 78 mg, 86% (R2 = Et2,
2), 87 mg, 93% (R2 = C5H10, 3), 89 mg, 93% (R2 = C4H8O, 4).
(Ph3PCH2Ph)[PtCl3(NCNMe2)]
(1). Anal.
Calcd
for
horizons for further work in the area of metal-activated C28H28N2Cl3PPt: C, 46.39; H, 3.89; N, 3.86%. Found: C, 46.06;
nitriles.1,2
H, 3.84; N, 3.58%. HRESI+-MS, m/z: 353.1514 [Ph3PCH2Ph]+
[353.1459 calcd]. HRESI−-MS, m/z: 370.9254 [PtCl3(NCNMe2)]−
[370.9196 calcd]. IR spectrum in KBr, selected bands, cm−1
:
1
2289 s ν(CuN). H NMR in CDCl3, δ: 2.78 (s, 5H, NCH3), 5.11
(d, JPH 14.0 Hz, 2H, PCH2), 7.03–7.13 and 7.67–7.77 (m, 20H,
Experimental section
Materials and instrumentation
2
1
Ph). 13C{1H} NMR in CDCl3, δ: 40.1 (NCH3), 117.9 (d, JPC
The dialkylcyanamides NCNR2 (R2 = Me2, Et2, Acros; R2
=
335 Hz, PCH2), 127.4–132.0 (Ph); the NCN carbon was not
C4H8O, C5H10, Aldrich) and solvents were obtained from detected.
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(Ph3PCH2Ph)[PtCl3(NCNEt2)]
Dalton Transactions
(2). Anal.
Calcd
for
(Ph3PCH2Ph)[PtCl3{NHC(NC5H10)ON(CH2Ph)2}] (9). HRESI+-
C30H32N2Cl3PPt: C, 47.85; H, 4.28; N, 3.72%. Found: C, 47.83; MS, m/z: 353.1425 [Ph3PCH2Ph]+ [353.1459 calcd]. HRESI−-MS,
H, 4,05; N, 3.74%. HRESI+-MS, m/z: 353.1635 [Ph3PCH2Ph]+ m/z: 624.0894 [PtCl3(NCNC5H10)]− [624.0662 calcd]. 1H NMR in
[353.1459 calcd]. HRESI−-MS, m/z: 398.9464 [PtCl3(NCNEt2)]− CDCl3, δ: 1.49–1.66 (m, 6H, β-CH2 and γ-CH2), 3.33–3.38 (m, 4H,
[398.9509 calcd]. IR spectrum in KBr, selected bands, cm−1
:
α-CH2), 4.01 (s, 4H, NCH2), 5.35 (d, JPH 14.0 Hz, 2H, PCH2),
2
1
3
2280 s ν(CuN). H NMR in CD2Cl2, δ: 1.23 (t, JHH 7.2 Hz, 6H, 5.92 (s, br, 1H, NH), 7.09–7.76 (m, 20H, Ph). 13C{1H} NMR was
3
2
CH3), 3.08 (q, JHH 7.2 Hz, 4H, NCH2), 5.02 (d, JPH 13.7 Hz, not recorded because of the fast conversion of 9 in solution.
(Ph3PCH2Ph)[PtCl3{NHC(NC4H8O)ON(CH2Ph)2}] (10). HRESI+-
2H, PCH2), 7.05–7.35 (m, 5H, Ph from CH2Ph), 7.67–7.88 (m,
MS, m/z: 353.1413 [Ph3PCH2Ph]+ [353.1459 calcd]. HRESI−-MS,
m/z: 626.0740 [PtCl3(NCNC4H8O)]− [626.0455 calcd]. 1H NMR in
CDCl3, δ: 3.57–3.60 (m, 4H, NCH2 from NC4H8O), 4.15–4.18 (m,
for 4H, OCH2), 4.03 (s, 4H, NCH2), 5.25 (d, 2JPH 13.7 Hz, 2H, PCH2),
15H, PPh3). 13C{1H} NMR in CD2Cl2, δ: 13.1 (CH3), 46.1
1
(NCH2), 117.7 (d, JPC 330 Hz, PCH2), 127.3–135.6 (Ph); the
NCN carbon was not detected.
(Ph3PCH2Ph)[PtCl3(NCNC5H10)]
(3). Anal.
Calcd
C31H32N2Cl3PPt: C, 48.67; H, 4.22; N, 3.66%. Found: C, 48.63; 6.14 (s, br, 1H, NH), 7.06–7.77 (m, 20H, Ph). 13C{1H} NMR was
H, 4.26; N, 3.66%. HRESI+-MS, m/z: 353.1427 [Ph3PCH2Ph]+ not recorded because of the fast conversion of 10 in solution.
[353.1459 calcd]. HRESI−-MS, m/z: 410.9458 [PtCl3(NCNC5H10)]−
Products 7–10 are unstable and they convert at RT after
1–24 h (in solid state) or 0.5–2 h (in a solution) to the imine
[410.9509 calcd]. IR spectrum in KBr, selected bands, cm−1
:
2274 s ν(CuN). 1H NMR in CDCl3, δ: 1.52–1.76 (m, br, 6H, complex (Ph3PCH2Ph)[PtCl3{(PhCH2)NvCHPh}] (13) and the
2
β-CH2 and γ-CH2), 3.12–3.33 (m, br, 4H, α-CH2), 5.12 (d, JPH N,N-dialkyl urea H2NCO(NR2) (R2 = Me2, 16, R2 = Et2, 17, R2 =
14.0 Hz, 2H, PCH2), 7.03–7.28 and 7.68–7.80 (m, 20H, Ph). 13C C5H10, 18, R2 = C4H8O, 19). Complex 13 was separated by
{1H} NMR in CDCl3, δ: 23.0 (γ-CH2), 25.0 (β-CH2), 49.8 (α-CH2), column chromatography on SiO2.
1
(Ph3PCH2Ph)[PtCl3{N(CH2Ph)vCH)Ph}] (13). Anal. Calcd for
C39H35NCl3PPt·1/5[PtCl2{N(CH2Ph)vC(H)Ph}2]: C, 52.10; H,
for 3.97; N, 2.12%. Found: C, 51.95; H, 3.96; N, 2.45%. [PtCl2-
118.0 (d, JPC 347 Hz, PCH2), 127.5–135.4 (Ph); the NCN carbon
was not detected.
(Ph3PCH2Ph)[PtCl3(NCNC4H8O)]
(4). Anal.
Calcd
C30H30N2Cl3OPPt: C, 46.97; H, 3.94; N, 3.65%. Found: C, 47.02; {N(CH2Ph)vCHPh}2] additive was detected by 1H NMR and
H, 4.10; N, 3.78%. HRESI+-MS, m/z: 353.1589 [Ph3PCH2Ph]+ ESI−-MS spectroscopic techniques, and the ratio 13 : [PtCl2-
[353.1459 calcd]. HRESI−-MS, m/z: 412.9309 [PtCl3(NCNC4H8O)]− {N(CH2Ph)vCHPh}2] = 5 : 1 was determined by using H NMR
1
[412.9301 calcd]. IR spectrum in KBr, selected bands, cm−1
:
(the ratio does not change in repeated syntheses). It could not
2285 s ν(CuN). 1H NMR in CDCl3, δ: 3.19–3.28 (m, br, be separated from 13 by column chromatography (due to close
2
4H, NCH2), 3.63–3.77 (m, br, 4H, OCH2), 5.12 (d, JPH 14.1 Hz, retention indexes) or by recrystallization from CHCl3–Et2O or
2H, PCH2), 7.04–7.23 and 7.67–7.80 (m, 20H, Ph). 13C{1H} MeNO2–Et2O mixtures in various ratios of their components.
NMR in CDCl3, δ: 48.2 (NCH2), 66.0 (OCH2), 117.9 (d, JPC HRESI+-MS, m/z: 353.1427 [Ph3PCH2Ph]+ [353.1459 calcd].
1
338 Hz, PCH2), 127.6–135.4 (Ph); the NCN carbon was not HRESI−-MS,
m/z:
495.9759
[PtCl3{N(CH2Ph)vCHPh}]−
detected.
Reaction of Q[PtCl3(NCNR2)] with HON(CH2Ph)2. (PhCH2)2NOH
(53 mg, 0.25 mmol) was added to a solution of any one of 1–4
[495.9713 calcd]. IR spectrum in KBr, selected bands, cm−1
:
3057–2893 s ν(C–H), 1586 m ν(CvN). 1H NMR in CDCl3, δ: 5.11
2
(d, JPH 13.8 Hz, 2H, PCH2), 5.38 and 5.39 (E/Z ratio is 1 : 1, s +
(0.05 mmol) in CHCl3 (0.5 mL) at RT. After 5 min the reaction
s, br, 2H, NCH2), 7.02–7.84 (m, 20H, Ph), 9.30 and 9.33 (s + s,
mixture was evaporated under a stream of air and an oily 1H, NvCH). 13C{1H} NMR in CDCl3, δ: 66.0 (NCH2), 117.7 (d,
residue was formed. The product was crystallized under Et2O 1JPC 335 Hz, PCH2), 128.5–135.6 (Ph), 168.0 (NvCH).
Identification of the ureas formed upon dehydration.
NH2C(vO)NMe2 (16). HRESI+-MS, m/z: 111.0491 [M + Na]+
[111.0534 calcd], 129.0570 [M + H2O + Na]+ [129.0640 calcd].
NH2C(vO)NEt2 (17). HRESI+-MS, m/z: 117.1074 [M + H]+ [117.1028
calcd. NH2C(vO)NC5H10 (18). HRESI+-MS, m/z: 129.1130 [M + H]+
[129.1028 calcd]. NH2C(vO)NC4H8O (19). HRESI+-MS, m/z:
131.0892 [M + H]+ [131.0821 calcd].
(1 mL) and washed with Et2O (two 1 mL portions). Yields:
43 mg, 92% (R2 = Me2, 7), 46 mg, 95% (R2 = Et2, 8), 43 mg,
89% (R2 = C5H10, 9), 46 mg, 94% (R2 = C4H8O, 10).
(Ph3PCH2Ph)[PtCl3{NHC(NMe2)ON(CH2Ph)2}] (7). HRESI+-MS,
m/z: 353.1424 [Ph3PCH2Ph]+ [353.1459 calcd]. HRESI−-MS, m/z:
584.0314 [PtCl3(NCNMe2)]− [584.0349 calcd]. 1H NMR in
CDCl3, δ: 3.41 (s, 6H, NMe), 4.03 and 4.05 (s + s, br, 4H,
2
Competitive reactivity studies. The reaction of (Ph3PCH2Ph)-
[PtCl3(NCNR2)] (0.5 mmol) and (Ph3PCH2Ph)[PtCl3(NCR′)]
(0.5 mmol, R′ = Ph, CH2Ph) with HON(CH2Ph)2 (0.5 mmol)
was performed in a CD2Cl2 solution (0.5 mL) at 20 °C and was
monitored by 1H NMR spectroscopy. The spectra were registered
NCH2), 5.29 (d, JPH 13.9 Hz, 2H, PCH2), 5.87 (s, br, 1H, NH),
7.50–7.80 (m, 20H, Ph). 13C{1H} NMR was not recorded
because of the fast conversion of 7 in solution.
(Ph3PCH2Ph)[PtCl3{NHC(NEt2)ON(CH2Ph)2}] (8). HRESI+-MS,
m/z: 353.1425 [Ph3PCH2Ph]+ [353.1459 calcd]. HRESI−-MS, m/z:
612.0766 [PtCl3(NCNEt2)]− [612.0662 calcd]. 1H NMR in CDCl3, immediately after the addition of HON(CH2Ph)2 to the reaction
3
3
mixture and then after 5, 10, 15, 20, and 30 min; after 80 min
HON(CH2Ph)2 was not detected in the 1H NMR spectra. The
signals of the imine complex (Ph3PCH2Ph)PtCl3-{N(CH2Ph)v
CHPh} were found in the spectrum recorded 20 min after the
addition of HON(CH2Ph)2. The mean value k1/k2 was calculated
δ: 1.28 (t, JHH 6.3 Hz, 6H, Me), 3.27 (q, JHH 6.4 Hz, 4H, CH2
from Et), 3.95 and 4.02 (s + s, br, 4H, NCH2), 5.24 (d,
2JPH 14.0 Hz, 2H, PCH2), 5.86 (s, br, 1H, NH), 7.04–7.75 (m,
20H, Ph). 13C{1H} NMR was not recorded because of the fast
conversion of 8 in solution.
Dalton Trans.
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based on the time points for 5, 10, and 15 min; the k1/k2 ratio
was stable within the specified time period.
The ratio k1/k2 (R = CH2Ph) was calculated according to the
formula
2 V. Y. Kukushkin and A. J. L. Pombeiro, Inorg. Chim. Acta,
2005, 358, 1–21.
3 N. A. Bokach, Russ. Chem. Rev., 2010, 79, 89–100.
4 N. A. Bokach, M. L. Kuznetsov and V. Yu. Kukushkin,
Coord. Chem. Rev., 2011, 255, 2946–2967.
0
k1 ðSðNÞ ꢀ SðNÞtÞ
5 N. A. Bokach and V. Y. Kukushkin, Russ. Chem. Rev., 2005,
74, 153–170.
¼
;
0
k2
ðSðXÞ ꢀ SðXÞtÞ
6 A. J. L. Pombeiro and V. Y. Kukushkin, Comprehensive
Coordination Chemistry II, 2004, 1, 639–660.
7 T. J. Ahmed, S. M. M. Knapp and D. R. Tyler, Coord. Chem.
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19 P. V. Gushchin, M. L. Kuznetsov, M. Haukka, M. J. Wang,
A. V. Gribanov and V. Y. Kukushkin, Inorg. Chem., 2009, 48,
2583–2592.
20 P. V. Gushchin, M. R. Tyan, N. A. Bokach, M. D. Revenco,
M. Haukka, M. J. Wang, C. H. Lai, P. T. Chou and
V. Y. Kukushkin, Inorg. Chem., 2008, 47, 11487–11500.
21 K. V. Luzyanin, A. G. Tskhovrebov, M. C. Carias,
M. F. C. Guedes da Silva, A. J. L. Pombeiro and
V. Y. Kukushkin, Organometallics, 2009, 28, 6559–6566.
22 K. V. Luzyanin, A. G. Tskhovrebov, M. F. C. Guedes da Silva,
M. Haukka, A. J. L. Pombeiro and V. Y. Kukushkin,
Chem.–Eur. J., 2009, 15, 5969–5978.
23 A. G. Tskhovrebov, N. A. Bokach, M. Haukka and
V. Y. Kukushkin, Inorg. Chem., 2009, 48, 8678–8688.
24 N. A. Bokach, M. L. Kuznetsov, M. Haukka,
V. I. Ovcharenko, E. V. Tretyakov and V. Y. Kukushkin, Orga-
nometallics, 2009, 28, 1406–1413.
where S(X)t and S(N)t are integral intensities of signals of the
protons of the NMe2 or NCH2 group of NCNR2 and the CCH2
group of NCCH2Ph, respectively, and S(X)0 and S(N)0 are the
starting integral intensities of these signals in the spectrum
recorded immediately after the addition of HON(CH2Ph)2. All
the intensities were reduced to the number of protons in the
group (reduced to 6 for NMe2, 4 for N(CH2–)2 and 2 for CCH2).
Because of signal overlap of the aromatic protons of the
NCPh ligand (7.01–7.22 ppm) and the resulting imine ligand
NHC(Ph)ON(CH2Ph)2 (7.01–7.50 ppm), NMR integrations in
the calculations of the k1/k2 (R = Ph) ratio were not possible.
Also, taking into account that the imino complexes generated
during the reaction were unstable, it was necessary to use the
formula that allows the calculation of the conversion of NCPh
knowing the difference in conversions of HON(CH2Ph)2 and
NCNMe2 (formula (a)) or NCNR2 (R = NEt2, NC5H10, NC4H8O;
formula (b)):
0
0
1
1
2 ðSðNÞ ꢀ SðNÞtÞ ꢀ 3 ðSðXÞ ꢀ SðXÞtÞ
k1
k2
¼
ðaÞ
ðbÞ
0
1
3 ðSðXÞ ꢀ SðXÞtÞ
0
0
k1 ðSðNÞ ꢀ SðNÞtÞ ꢀ ðSðXÞ ꢀ SðXÞtÞ
¼
0
k2
ðSðXÞ ꢀ SðXÞtÞ
where S(X)t and S(N)t are integral intensities of signals of the
protons of the NMe2 or NCH2 group of the NCNR2 and the
NCH2 group of HON(CH2Ph)2, respectively, and S(X)0 and S(N)0
are starting integral intensities of these signals in the spec-
trum recorded immediately after the addition of HON(CH2Ph)2
(Scheme 4).
Acknowledgements
The authors express their gratitude to the Russian Fund of
Basic Research for grants 12-03-33071, 12-03-00076 and 13-03-
12411 and the RAS Presidium subprogram 8P (coordinated by
acad. N.T. Kuznetsov) for financial support. The authors
also acknowledge Saint Petersburg State University for
research grants (2011–2013, 12.37.133.2011 and 2012–2013,
12.39.1050.2012). TBA and NAB are much obliged to the
Fundação para Ciência e Tecnologia (FCT), Portugal – project
PTDC/QUI-QUI/109846/2009.
25 K. V. Luzyanin, A. J. L. Pombeiro, M. Haukka and
V. Y. Kukushkin, Organometallics, 2008, 27, 5379–5389.
26 M. R. Tyan, N. A. Bokach, M. J. Wang, M. Haukka,
M. L. Kuznetsov and V. Y. Kukushkin, Dalton Trans., 2008,
5178–5188.
27 P. V. Gushchin, K. V. Luzyanin, M. N. Kopylovich,
M. Haukka, A. J. L. Pombeiro and V. Y. Kukushkin, Inorg.
Chem., 2008, 47, 3088–3094.
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