Organometallic benzylidene anilines: Donor-acceptor features in NCN-pincer Pt(ii) complexes with a 4-(E)-[(4-R-phenyl)imino]methyl substituent

Article abstract of DOI:10.1039/c4dt01023j

A series of organometallic 4,4′-substituted benzylidene aniline complexes 4-ClPt-3,5-(CH₂NMe₂)₂C ₆H₂CHNC₆H₄R′-4′, abbreviated as PtCl[NCN(CHNC₆H₄R′-4′)-4], with R′ = NMe₂, Me, H, Cl, CN (1-5, respectively), was synthesized via a Schiff-base condensation reaction involving reaction of PtCl[NCN(CHO)-4] (7) with the appropriate 4-R′-substituted aniline derivative (6a-e) in toluene. The resulting arylplatinum(ii) products were obtained in 75-88% yield. Notably, product 2 was also obtained in 68% yield from a reaction in the solid state by grinding solid 7 with aniline 6b. The structures of 2, 4, and 5 in the solid state (single crystal X-ray diffraction) showed a non-planar geometry, in particular for compound 5. The electronic interaction between the donor benzylidene fragment PtCl(NCN-CH) and the para-R′ aniline substituent through the azomethine bridge was studied with NMR and UV/Vis spectroscopy. Linear correlations were found between the azomethine ¹H, the ¹⁹⁵Pt NMR and various ¹³C NMR chemical shifts, and the substituent parameters σ_F and σ_R of R′ at the aniline site. In common with organic benzylidene anilines, the azomethine ¹H NMR chemical shift showed anomalous substituent behavior. The ¹⁹⁵Pt NMR chemical shift of the platinum center can be used as a probe for the electronic properties of the delocalized π-system of the benzylidene aniline framework, to which it is connected. The dual substituent parameter treatment of the azomethine ¹³C NMR shift gave important insight into the unique behaviour of the Pt-pincer group as a substituent. Inductively, it is a very strong electron-withdrawing group, whereas mesomerically it behaves like a very strong electron donating group.

Full text of DOI:10.1039/c4dt01023j

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Organometallic benzylidene anilines:
donor–acceptor features in NCN-pincer Pt(II)
complexes with a 4-(E)-[(4-R-phenyl)imino]-
methyl substituent†‡
Cite this: Dalton Trans., 2014, 43,
12200
Guido D. Batema,a Martin Lutz,§b Anthony L. Spek,¶b Cornelis A. van Walree,a,c
a
Gerard P. M. van Klinkk and Gerard van Koten*a
A series of organometallic 4,4’-substituted benzylidene aniline complexes 4-ClPt-3,5-(CH2NMe2)2C6H2CHv
NC6H4R’-4’, abbreviated as PtCl[NCN(CHvNC6H4R’-4’)-4], with R’ = NMe2, Me, H, Cl, CN (1–5, respect-
ively), was synthesized via a Schiff-base condensation reaction involving reaction of PtCl[NCN(CHvO)-4] (7)
with the appropriate 4-R’-substituted aniline derivative (6a–e) in toluene. The resulting arylplatinum(II)
products were obtained in 75–88% yield. Notably, product 2 was also obtained in 68% yield from a
reaction in the solid state by grinding solid 7 with aniline 6b. The structures of 2, 4, and 5 in the solid state
(single crystal X-ray diffraction) showed a non-planar geometry, in particular for compound 5. The
electronic interaction between the donor benzylidene fragment PtCl(NCN–CH) and the para-R’ aniline
substituent through the azomethine bridge was studied with NMR and UV/Vis spectroscopy. Linear corre-
1
lations were found between the azomethine H, the 195Pt NMR and various 13C NMR chemical shifts, and
the substituent parameters σF and σR of R’ at the aniline site. In common with organic benzylidene anilines,
the azomethine 1H NMR chemical shift showed anomalous substituent behavior. The 195Pt NMR chemical
shift of the platinum center can be used as a probe for the electronic properties of the delocalized
π-system of the benzylidene aniline framework, to which it is connected. The dual substituent parameter
treatment of the azomethine 13C NMR shift gave important insight into the unique behaviour of the
Pt-pincer group as a substituent. Inductively, it is a very strong electron-withdrawing group, whereas
mesomerically it behaves like a very strong electron donating group.
Received 6th April 2014,
Accepted 27th June 2014
DOI: 10.1039/c4dt01023j
www.rsc.org/dalton
Introduction
aOrganic Chemistry and Catalysis, Faculty of Science, Utrecht University,
Universiteitsweg 99, 3584CG Utrecht, The Netherlands. E-mail: g.vankoten@uu.nl
bCrystal and Structural Chemistry, Faculty of Science, Utrecht University,
The introduction of transition metals in organic materials
with important optical properties continues to attract the inter-
est of current research.1 Herein, the possibility to engineer the
redox properties of a metal center embedded in its ligand sur-
rounding is an important tool. In the presence of other
electro-active groups it is possible to transport charge from the
metal center (donor) to the ligand backbone (acceptor), or vice
versa. The nature of the backbone or spacer separating the
electroactive groups, including the metal center, is very impor-
tant in this process.
Within our current research on stilbenoid NCN-pincer plati-
num complexes A, PtCl[NCN(C2H2C6H4-R′-4′)-4] (R′ = NMe2,
OMe, SiMe3, H, I, CN, NO2; Fig. 1),2,3 it has been shown
that the functional group R′ tunes the electronic properties of
the metal center, and that the PtCl fragment4,5 can be
regarded as an electron donating group. Previous results using
Padualaan 8, 3584CH Utrecht, The Netherlands
cSchool of Chemical and Physical Sciences, Flinders University, GPO Box 2100,
Adelaide 5001, Australia
†Dedicated to Professor William (Bill) C. Kaska on the occasion of his 80th
birthday. Gerard van Koten thanks Bill for exciting times during various joint
“Pincer Metal” projects in Utrecht and Santa Barbara.
‡Electronic supplementary information (ESI) available: Table S1 for the UV/Vis
data of PtCl[NCN(CHvNC6H4R′-4′)-4], 1–5. Table S2 containing the experimental
details for the X-ray crystal structure determinations of 2, 4 and 5 and Table S3
with Selected NMR and IR data of PtCl[NCN(CHvNC6H4R′-4′)-4] (1–5) and a
comment on the ATR IR data. CCDC 649683–649685. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c4dt01023j
§Address correspondence pertaining to crystallographic studies to this author.
E-mail: m.lutz@uu.nl
¶Retired.
kPresent address: Avantium Chemicals, Zekeringstraat 29, 1014 BV Amsterdam,
The Netherlands.
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Results and discussion
Synthesis
Reactions of 4-formyl-2,6-bis[(dimethylamino)methyl]phenyl
platinum(II) chloride (7)2 with the respective p-substituted ani-
lines 6a–f (Scheme 1) were performed in dry toluene with a drop
of formic acid as catalyst. Molecular sieves (4 Å) were used to
capture the released water (Na2SO4 and MgSO4 were not
effective). The respective complexes 1–5 were obtained pure, and
in crystalline form after an easy work-up procedure in moderate
to good yield (75–88%). Compounds 2–5 were stable when stored
under air, but 1 (R′ = NMe2) slowly hydrolysed upon several
weeks of storage. Complexes 1–5 were fully characterized by
NMR, UV/Vis, ATR-IR, and mass spectroscopy, as well as elemen-
tal analysis. The synthesis of 8 (R′ = NO2) was also attempted, but
under the above-described reaction conditions complete conver-
sion of 7 did not occur. Furthermore, attempts to separate pure 8
from the reactants failed. Prolonged reaction times, higher reac-
tion temperatures as well as the addition of extra 4-nitroaniline
to the reaction mixture did not drive the reaction to completion.
It is interesting to note that both Schiff-base condensation
reactions22,23 and the synthesis of pincer-metal compounds24
can be performed using solvent-free conditions. The applica-
bility of this potentially Green method was tested for the syn-
thesis of 2 involving of grinding of a mixture of solid
organometallic aldehyde 7 (1 equiv.) and aniline 6b (1.2
equiv.) in a Schlenk flask at room temperature, see Experi-
mental section. Monitoring of the reaction progress by 1H
NMR indicated that gradual formation of 2 (1 h 27%, 5 days
68%) occurred. Neither decomposition nor formation of side
products due to the high pressures exerted on the reactants
during grinding was observed.
Fig. 1 NCN-pincer platinum(II) chloride benzylidene fragment in
R’-substituted stilbene, A, and a benzylidene aniline, B.
a
PtX(NCN–COOH-4) (X = Cl, I) had already indicated that, quali-
tatively, the Hammett σp parameter of a PtI substituent can be
considered as to be of comparable strength to that of a NMe2
group, i.e. acts as a π-donor.6 In the stilbenoid complexes the
donor–acceptor groups are connected via alternating carbon–
carbon single and double bonds only. Although benzylidene
anilines are isoelectronic with stilbenes, regarding their π-elec-
tronic structure, their properties can be strongly different.7,8
Therefore it became of interest to study how the physical and
electronic properties of these complexes would change when
the ethen-1,2-diyl linker in the stilbenoid complexes (A) is
replaced by an isoelectronic carbon–nitrogen double bond (B)
as the linker, see Fig. 1.
It must be noted that a great variety of ortho-cyclometalated
imine complexes,9,10 and complexes with
a benzylidene
aniline as a monodentate coordinating ligand11–13 have been
reported but only a limited number of para-metalated benzyli-
dene aniline complexes incorporating
a metal-carbon σ-
bond.14–17 Ferrocenylimines are well known organometallic
benzylidene aniline-type complexes which find use in cyclo-
metalation reactions.18–20 However, to our knowledge, the syn-
thesis and structures of para-metalated benzylidene aniline
complexes in the solid state have not been reported thus far.
Previous studies revealed that the para-aldehyde functional-
ity of PtCl(NCN–CHvO-4),2 7, can be readily converted into R′
substituted stilbenes (see A in Fig. 1). In the present study we
report its direct conversion into a [(4-R′-phenyl)imino]methyl
one using substituted anilines 6 in a Schiff-base condensation
reaction (Scheme 1).
Structures in the solid state
For 2, 4 and 5 crystals suitable for single crystal X-ray structure
determinations were obtained by slow concentration of CD2Cl2
solutions of the respective compounds (evaporation in air).
Displacement ellipsoid plots of the molecules are depicted in
Fig. 2 and typical bond lengths and angles are listed in Table 1.
Compounds 2 and 4 are isostructural and possess an
overall bent shape, i.e. the (Pt)C1 carbon atoms are 0.3700(17)
and 0.3824(19) Å (for residues 2 and 4, respectively) beneath
the least-squares plane of the central double bond (C4–C13–
N3–C14) (Fig. 2). For carbon atom C17 the distances are 0.2808(18)
We describe the synthesis and structural characterization of
a series of these 4-platinum benzylidene aniline complexes
and compare their physical and electronic properties with
those of their organometallic stilbenoid and corresponding
organic benzylidene anilines7,21 analogues.
Scheme 1 Schiff-base condensation reaction of 6a–f and 7, giving 1–5.
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Fig. 2 Displacement ellipsoid plot (50% probability level) of 2 (top left and bottom left), 4 (top right), and 5 (bottom right). Hydrogen atoms have
been omitted for clarity. For 5 the minor disorder component is drawn with dashed lines.
Table 1 Selected bond lengths [Å], angles [°] and torsion angles [°] of 2, 4 and 5
2 [R′ = Me]
4 [R′ = Cl]
5 [R′ = CN]
Pt1–C1
Pt1–N1
Pt1–N2
Pt1–Cl1
C4–C13
C13–N3
C14–N3
C1–Pt1–N1
C1–Pt1–N2
C1–Pt1–Cl1
N1–Pt1–N2
Pt1–N1–C7–C2
Pt1–N2–C10–C6
C4–C13–N3–C14
C3–C4–C13–N3
C5–C4–C13–N3 (φ)
C13–N3–C14–C15 (θ)
C13–N3–C14–C19
1.9139(16)
2.0922(14)
2.0865(14)
2.4116(4)
1.467(2)
1.273(2)
1.416(2)
1.919(2)
2.0931(19)
2.0867(18)
2.4132(6)
1.463(3)
1.279(3)
1.413(3)
81.88(8)
82.11(8)
173.85(7)
163.98(7)
−29.7(2)
−32.9(2)
−173.3(2)
173.7(2)
−1.7(3)
1.915(5)
2.089(4)
2.083(4)
2.4037(12)
1.462(7)
1.285(8)
1.402(8)
82.13(19)
82.3(2)
176.56(15)
164.45(16)
32.1(5)
82.03(6)
82.06(6)
173.95(5)
164.07(5)
−29.35(16)
−32.72(15)
−173.86(16)
173.52(17)
−1.5(3)
26.6(6)
−177.8(5)
177.0(6)
−1.1(9)
17.2(3)
−166.67(18)
22.1(4)
46.8(8)a/−57.9(12)b
−134.7(6)a/123.5(12)b
−162.6(2)
Angle between planes [C1–C2–C3–C4–C5–C6] and [C14–C15–C16–C17–C18–C19] [°]
20.52(8)
24.59(11)
44.7(3)a/58.5(5)b
a Major disorder component (81.4(7)% occupancy). b Minor disorder component (18.6(7)% occupancy).
and 0.289(3) Å, respectively. A bent shaped conformation in complexes2,25–28 show a similar distorted square planar geome-
benzylidene anilines has been observed previously for Ni(η5- try around the Pt nucleus, in which the carbon atoms of the
C5H5)(PBu3)(SC6H4NvCHC6H4CH3-4′) where the methylbenzy- benzylic substituents (C7 and C10) are positioned above and
lidene group is bent away from the rest of the molecule to below the plane defined by C1–N1–Pt1–N2–Cl1. For the two
avoid steric crowding.16 The same is likely to be the case here. five-membered metallacycles, which are slightly puckered,
Despite rather similar cell parameters, compound 5 is not iso- torsion angles for Pt1–N1–C7–C2 and Pt1–N2–C10–C6 have
structural with 2 and 4, and has different packing. Compound magnitudes between 26.6(6) and 32.9(2)°. The Pt1–C1 dis-
5 crystallizes with a disordered aniline fragment (Fig. 2). This tances of 2, 4, and 5 are in the range of 1.9139(16) to 1.919(2) Å.26
type of disorder is rather common in para-substituted benzyli-
dene aniline derivatives and will be commented below.
It is known that the molecular structure of substituted
organic benzylidene anilines in the solid state can be non-
Comparison of the structural features of the NCNPtCl frag- planar, since both the aniline fragment A and benzylidene
ments of 2, 4, and 5 with earlier reported [PtCl(NCN)] fragment C (Fig. 3) can rotate with respect to the azomethine
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The presence of a less EW or moderate ED substituent on
the aniline ring reduces the twist angle θ, as is found in 2 and
4. In 5 the bond length N3–C14 is slightly shorter while the
distance N3–C13 is larger, compared to 2 and 4. Probably in 5
the strong intramolecular charge transfer interaction between
the lone-pair electrons of the azomethine nitrogen atom and
the cyano group results in a substantial contribution of a
quinoid resonance structure.
From the aromatic ring interaction analysis of the three
independent structures, close interactions between aromatic
planes with distances around 3.5 Å (or shorter) appeared not
to be present in the crystal. Therefore we conclude that there is
no attractive aromatic ring π–π-stacking between neighboring
molecules. In contrast, all three structures show an intermole-
cular interaction of an aromatic hydrogen atom with the
π-system of a neighboring molecule (Table 2), referred to as an
intermolecular Ar–H⋯π-interaction34 (X–H⋯Cg(π-ring)). Such
interactions were also observed in solid state structures of
other substituted organic benzylidene anilines.35
Fig. 3 Representation of the three fragments of the benzylidene aniline
pincer platinum complex; R’-substituted aniline ring A, azomethine
bridge B, and NCN-pincer platinum fragment C. Torsion angles θ and φ.
fragment B (torsion angles θ and φ, respectively).8,29 The con-
formational changes are controlled by the electronic and/or
steric effects induced by the substituents on the aniline or
benzylidene ring.30 Indeed, for complexes 2 (R′ = Me), 4 (R′ =
Cl) and 5 (R′ = CN) significant rotation of fragments A (θ (C13–
N3–C14–C15 and C13–N3–C14–C19) 17.2(3)–57.9(12)°) but
quite small rotations of C (φ (C3–C4–C13–N3 and C5–C4–C13–
N3) 1.1(9)–1.7(3)°) are observed (Table 1).
The marked difference between these torsion angles θ and φ
results from a combination of steric, inductive and mesomeric
effects, and is in agreement with earlier obtained crystallo-
graphic data for other substituted organic benzylidene
anilines.29,31–33 When going through the series from the more
electron donating (ED) Me-group to the more electron withdraw-
ing (EW) CN-group, φ hardly changes while θ becomes larger.
Particularly noteworthy is the large θ angle between fragments A
and B in 5. According to Nakai and coworkers33 such large twist
angles of θ can be explained by the strong intramolecular
charge transfer interaction between the lone-pair electrons of
the azomethine nitrogen atom and the EW group at the aniline
ring. The energetically unfavorable twisting from molecular pla-
narity, which affects the intramolecular charge transfer stabili-
zation between the para-substituents, is compensated for by the
stabilization resulting from the nitrogen lone-pair electron inter-
action with the electron deficient aniline ring.
In Fig. 4 this is shown as an example for 4, in which the
aromatic proton of C19–H19 interacts with the neighboring
Fig. 4 Intermolecular Ar–H⋯π-interaction of H(19) with neighboring
aromatic plane for 4.
Table 2 Selected intermolecular distances [Å] and angles [°] for 2, 4 and 5
X–H⋯π interactions
Compnd
X(I)–H(I) → Cg(J)f
H⋯Cga
H_perpb
γc
9.90
9.25
10.43
X–H⋯Cgd
X⋯Cge
2
4
5
C(19)–H(19) → Cg(3)[f1]
C(19)–H(19) → Cg(3)[f1]
C(12)–H(12A) → Cg(3)[f2]
2.77
2.74
2.71
2.724
2.700
2.662
159
160
168
3.6664(19)
3.641(3)
3.670(7)
Potential H bondsg
Compnd
D–H⋯A
D–Hh
H⋯Ah
D⋯Ah, f
D–H⋯Ai
2
4
5
C(10)–H(10A)⋯Cl(1)[f3]
C(10)–H(10B)⋯Cl(1)[f1]
C(10)–H(10A)⋯Cl(1)[f3]
C(10)–H(10B)⋯Cl(1)[f1]
C(9)–H(9A)⋯Cl(1)[f1]
C(11)–H(11B)⋯Cl(1)[f4]
0.99
0.99
0.99
0.99
0.98
0.98
2.76
2.71
2.75
2.69
2.82
2.83
3.7429(17)
3.5286(18)
3.726(2)
3.517(3)
3.735(5)
3.650(7)
171
140
169
141
156
142
a Distance between H(I) and ring centroid Cg(J) (Å). b Perpendicular distance of H(I) on ring J (Å). c Angle between H(I) → Cg(J) vector and normal
plane J (°). d Angle defined by X(I), H(I), and Cg(J) (°). e Distance between X(I) and Cg(J) (Å). f Symmetry operations: [f1] x, 1/2 − y, 1/2 + z; [f2] x,
3/2 − y, −1/2 + z; [f3] −x, 1 − y, 1 − z; [f4] 2 − x, 1/2 + y, 1/2 − z. g D = donor, H = hydrogen, A = acceptor. h Distance [Å]. i Angle [°].
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Fig. 5 Intermolecular C–H⋯Cl–M interaction displayed for 2 which are commonly observed in NCN-pincer platinum(II)2 and other metal
complexes.36,37
1
aromatic ring. In all the structures intermolecular C–H⋯Cl–M
SCS are the H, 13C{1H}, or 195Pt{1H} NMR chemical shifts
interactions are also found between both the CH2 (in 2 and 4) of the atom of interest from the substituted compound, in the
or the CH3 (in 5) protons of the (dimethylamino)methyl present study 1 (R′ = NMe2), 2 (R′ = Me), 4 (R′ = Cl) and 5 (R′ =
groups of fragment C with a neighboring metal-bound chloride CN) relative to that observed for compound 3 (R′ = H), while σF
and vice versa, with distances ranging from 2.71 to 2.83 Å and σR are the field inductive and resonance parameters,
(Table 2; example shown for 2 in Fig. 5).
respectively, for the aromatic ring para-substituent (Table 4).
As an example, the azomethine proton shift in the 1H NMR
spectra of 1–5 (Table 3) is strongly influenced by the electronic
character of the substituents present on the benzylidene
aniline backbone. The effect is much stronger in C6D6 than
in CD2Cl2.
Spectroscopic characterization
Selected NMR data of Pt complexes 1–5 have been compiled in
Table 3. The substituent induced chemical shifts (SCS)
1
observed in the H, 13C{1H}, and 195Pt{1H} NMR spectra have
been examined by both a single substituent (eqn (1)) and a
dual substituent parameter (DSP) method (eqn (2)),21 using
Hammett constant σp and modified Swain–Lupton constants
(σF, σR),38 respectively.
Linear correlations were found between the SCS and the
single substituent parameter σp displayed in Table 4 and
Fig. 6, reflecting the electronic interaction between substi-
tuent R′ and the molecular backbone, either inductively or by
resonance.
SCS ¼ ρσp þ constant
ð1Þ
ð2Þ
The correlations improved upon using the DSP method,
accounting for the individual influences of the field inductive
SCS ¼ ρFσF þ ρRσR þ constant
Table 3 Selected NMR data of PtCl[NCN(CHvNC6H4R’-4’)-4] (1–5)a
δ 1H (HCvN)
13C (C–Pt)b
δ
13C (HCvN)b
δ
195Pt (Pt)b,d
σp (σF,σR) Hammette
b
c
Compound
CD2Cl2
C6D6
δ
1 (R′ = NMe2)
2 (R′ = Me)
3 (R′ = H)
4 (R′ = Cl)
5 (R′ = CN)
8.40
8.36
8.35
8.33
8.31
8.58
8.41
8.33
8.17
7.98
151.4
152.4
152.7
153.2
154.3
157.03
160.62
161.46
161.87
163.47
−3125
−3115
−3111
−3105
−3090
−0.83 (0.15, −0.98)
−0.17 (0.01, −0.18)
0.00 (0.03, 0.00)
0.23 (0.42, −0.19)
0.66 (0.51, 0.15)
a δ in ppm. b 43 mM solution in CD2Cl2. c 2.0 mM solution in C6D6. d Na2PtCl6 as external reference. e Values from ref. 39.
Table 4 Correlation of the 1H, 13C{1H} and 195Pt{1H} NMR SCS values versus the Hammett constant σp, and the dual substituent parameters σF and
σR for 1–5 in CD2Cl2
Entry
Atom
ρ (σp)
rb
ρF (σF)
ρR (σR)
rb
ρF/ρR
1
2
3
4
5
H (CHvN)
H (CHvN)a
C (CHvN)
C (ipso to Pt)
Pt
−0.062 0.003
−0.41 0.05
4.3 0.4
0.9971
0.9790
0.9858
0.9889
0.9746
−0.068 0.008
−0.61 0.03
2.9 0.4
−0.059 0.004
−0.331 0.015
4.85 0.20
0.9974
0.9993
0.9988
0.9946
0.9878
1.16
1.85
0.59
1.45
1.68
1.92 0.17
2.4 0.4
1.71 0.19
23
3
33
6
19
3
a In C6D6. b Correlation coefficient.
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study for the isoelectronic stilbenoid pincer platinum com-
plexes PtCl[NCN(C2H2C6H4-R′-4′)-4].2 This shows that the elec-
tronic influence exerted by the R′-substituents on the platinum
center in both the benzylidene aniline and stilbenoid pincer
platinum compounds is almost the same, despite the struc-
tural and conformational differences of these compounds.
Interestingly, the Hammett relations found allow the evalu-
ation of the substituent character and strength of the Pt group,
as it has been reported that the substituent at the benzylidene
moiety affects the sensitivity of the –CvN– 13C NMR chemical
shift to the substituent at the aniline moiety.21 Thus, in plots of δ
13C (CvN) versus σF and σR of substituents at the aniline site,
ρF was found to vary from 1.99 0.28 (4-Me2N group at the
benzylidene site) to 2.81 0.50 (4-nitro group at the benzyli-
dene site). Parameter ρR varied from 5.20 0.37 in presence of
a benzylidene 4-Me2N donor to 8.49 0.65 in presence of a
4-benzylidene nitro acceptor.21 In this context, the ρF parameter
of 2.9 0.4 found here for the molecules with the 4-Pt substi-
tuent at the benzylidene site suggests that inductively a Pt sub-
stituent behaves like an EW group stronger than a nitro group.
Given the formal +2 charge on Pt this is not unexpected. For
the mesomeric case, the ρR value of 4.85 0.20 suggests that
the Pt substituent is stronger than a dimethylamino group.
The Pt atom thus very readily donates electrons from its d orbi-
tals to the organic π-system. Together, these two features
render a Pt containing substituent an extraordinary and
schizophrenic substituent: simultaneously it is positioned
among the strongest electron donors and among the strongest
electron acceptors. When attached to a π system like in the
present case, the mesomeric effect dominates.
An UV/Vis spectroscopic study was performed in solvents of
different polarity to further examine the properties of the ben-
zylidene aniline pincer complexes 1–5 in solution (Table S1,‡
Fig. 8). The complexes show a typical, strong absorption band
at wavelengths around 350–392 nm, assigned to a π–π* tran-
sition.49 At higher energies, around 290 (for 1 at 339 nm) and
260 nm weaker absorption bands are found for 1–5, referred to
in literature as superimposed absorptions from local exci-
tations of the aniline and benzylidene fragments, respect-
ively.49 Indeed the absorptions for substituted anilines and
PtCl(NCN) complexes are found around these wavelengths.49,50
In MeCN a strong absorption band for 1–5 in the UV region
was found around 202–207 nm, also observed by others.31
Focusing on the long wavelength transitions, the highest
energy maxima are found for the unsubstituted compound
and the compound with the weakly donating methyl group. As
usual, introduction of stronger para-substituents (either ED or
Fig. 6 Hammett correlation of the 1H NMR chemical shift of the
azomethine proton (CHvN) of 1–5 in CD2Cl2 (squares) and C6D6 (dots).
and resonance effect of the substituent on the electronic inter-
actions in the molecule. The ρF/ρR values are close to one, indi-
cating that field inductive and mesomeric effects are about
equally strong. With ρF/ρR being larger than one, in most cases
the inductive effect is slightly dominant. The exception is the
chemical shift of the azomethine carbon atom (entry 3), which
is more influenced by the mesomeric effect.
The positive ρ values in entries 3–5 (Table 4) reflect that the
NMR signals of the respective atoms shift to lower field when
R′ becomes more EW in character, caused by decreased shield-
ing of the atoms. In contrast, the azomethine proton shows an
opposite shift behavior, also referred to as the anomalous shift
behavior of benzylidene anilines.38,40–42 An explanation for
this behavior is that the molecules have a non-planar geometry
in solution. As pointed out above, the more EW R′, the larger
the twist angle θ is. Upon twisting, the π-cloud of the aniline
ring induces shielding of the azomethine hydrogen, shifting
the proton signal to higher field (Fig. 7). This reasoning is sup-
ported by the observation that the anomalous shift behavior is
stronger in deuterated benzene (entry 2, Table 4). Twisting
makes the azomethine hydrogen atom better accessible to
solvent molecules, inducing an aromatic solvent-induced shift
(ASIS)43 by the magnetic anisotropy of benzene.44–46 It is,
however, also possible that bond polarization effects are
responsible for the occurrence of anomalous NMR shift
behavior.21,47,48
Notable is the ρ = 23 value of the correlation of δ 195Pt{1H}
with the Hammett σp (entry 5), which is in close agreement
with the ρ = 25 value, which was found in a similar correlation
Fig. 7 Out of plane ring-twist increases shielding of the azomethine proton due to increased overlap of the π-cloud with the free electron pair of
the azomethine nitrogen.
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platinum(II) chloride substituted benzylidene grouping con-
densed with a R′ substituted aniline, is reported. Interestingly,
the involved Schiff base condensation reaction occurs with
retention of the Pt–C bond of the NCN-pincer platinum(II)
chloride fragment. Preliminary experiments confirmed that these
reactions can also be carried out under solvent-free conditions.
In common with other benzylidene anilines these platinum
substituted benzylidene anilines exhibit bent and twisted
structures in the solid state. The extent of twisting of the R′-
substituted aniline ring with respect to the azomethine group
depends on the nature of the substituent R′.
Despite the introduction of a nitrogen atom into the ethy-
lene bridge and the occurrence of the twisting the electronic
properties resemble those of stilbenoid pincer platinum com-
plexes, the complete absence of fluorescence being a notable
exception. The lowest energy absorptions in the electronic
absorption spectra in the two types of compounds occur at not
too different positions, and according to the dependence of
the 195Pt{1H} NMR shift on the Hammett σp parameters, the
sensitivity of the metal to the electronic properties of substitu-
ents at the other site of the molecule is highly comparable.
The dual substituent parameter treatment of the azomethine
13C NMR shift gave important insight into the unique behaviour
of the Pt-halide-moiety as a substituent.6,55 Inductively, it is a
very strong electron withdrawing group, whereas mesomerically
it behaves like a very strong electron donating group.
Fig. 8 UV/Vis absorption spectra for 1 (NMe2), 2 (Me), 3 (H), 4 (Cl) and
5 (CN) in THF.
EW) on the aniline fragment results in a bathochromic shift.
No distinct solvatochromic behavior for 1–5 is observed,
suggesting a non-polar ground state. When comparing the
long wavelength maxima with those of benzylidene anilines
bearing the same substituent at the aniline 4-position but
without the Pt-substituent, a bathochromic shift is noted.
Maxima of 376 (in chloroform), 321 (in cyclohexane), 312 (in
chloroform) and 336 nm (in heptane) have been reported for
benzylidene anilines bearing a Me2N,7 Me,51 H7 or Cl52 group
at the aniline 4-position, respectively. This indicates that the
Pt-group affects the energy levels of the π electron system, pre-
sumably by virtue of its mesomeric electron donating pro-
perties. Note that this effect is much more pronounced for the
compounds with weakly donating and accepting substituents.
The Pt substituent induces a shift of 3720 cm−1 (312 to
353 nm) for unsubstituted benzylidene aniline, while the shift
is only 690 cm−1 (376 to 386 nm) for the system with the
strong dimethylamino donor.
The UV-Vis maxima of the Pt-benzylidene anilines are not
too different from those of the 4′-substituted Pt-stilbenes. In
dichloromethane as solvent, maxima of 371 (R′ = NMe2), 350
(R′ = H), and 376 (R′ = CN) have for instance been reported for
the latter series of compounds.2 Hence, for a given substituent,
the maxima do not differ by more than ca. 15 nm. The details
of the differences are not easy to disclose, as they involve
effects as planarity, the stabilization of the LUMO by the elec-
tronegative imine nitrogen7 and specific interactions, like the
one between the azomethine nitrogen lone pair and the cyano
substituent.
Experimental section
General
All reactions involving air- or moisture-sensitive reagents were
performed by standard Schlenk techniques unless stated
otherwise. Toluene and pentane were distilled from Na/benzo-
phenone, CH2Cl2 was distilled from CaH2 prior to use. The
platinum precursor [PtCl(NCN–CHO-4)] (7) was prepared
according to a published procedure.2 All other reagents were
obtained commercially and were used without further purifi-
cation. 1H and 13C{1H} NMR spectra were recorded at 25 °C on
Bruker AC 300 NMR or Varian Inova 300 or Varian 400 spec-
trometers (operating frequencies: for 1H spectra at 400 and
300 MHz; for 13C spectra at 101 and 75 MHz), chemical shifts
are reported in ppm and referenced to residual solvent reso-
nances. 195Pt{1H} NMR spectra were recorded on a Varian Inova
300 MHz NMR spectrometer (operating at 64.4 MHz), refer-
enced to external Na2PtCl6 (1 M in D2O, δ = 0 ppm).56 Elemen-
tal analyses were performed by Kolbe, Mikroanalytisches
Laboratorium (Mülheim a.d. Ruhr, Germany). ES-MS spectra
were obtained from the Biomolecular Mass Spectrometry
Group at the Utrecht University. Infrared spectra were recorded
with a Perkin Elmer Spectrum 1 FT-IR spectrometer. UV/Vis
spectra were collected on Cary 1 or Cary 5 spectrophotometers
in spectrophotometric grade solvents (Acros), see Table S1.‡
None of the solutions containing 1–5 showed fluorescence,
probably due to the twisting around the azomethine bond (E–Z
isomerization), which was shown to be responsible for the
absence of fluorescence in other organic benzylidene anilines.53,54
Conclusions
General procedure for the synthesis of 1–5
A useful and efficient synthetic route toward organometallic To a sealed reaction tube containing 4 Å molecular sieves and
benzylidene anilines, comprising
a
common NCN-pincer aldehyde 7 in dry toluene (30 mL), the respective aniline 6a–f
12206 | Dalton Trans., 2014, 43, 12200–12209
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Paper
was added. At room temperature formic acid (one drop) was formation. No reaction workup was performed, yielding a
added and the reaction mixture was heated to 70 °C and slowly mixture of compounds 2, 6b and 7 as a powder. 1H NMR
stirred at this temperature for 3 days. The mixture was filtered (300 MHz, C6D6): δ = 9.90 (s; CHO), 8.42 (s; NvCH), 7.34
3
3
and the residual molecular sieves were extracted with dichloro- (m; ArH), 7.08 (m; ArH), 6.90 (d, J = 8.0 Hz; ArH), 6.34 (d, J =
methane. The combined organic fractions were evaporated to 8.3 Hz; ArH) 3.27 (s, 3J(H–Pt) = 45.1 Hz; CH2), 3.12 (s, 3J(H–Pt) =
dryness in vacuo, and the residue was dissolved in a minimal 45.7 Hz; CH2), 2.67 (s, 3J(H–Pt) = 37.4 Hz; NCH3), 2.60 (s,
amount of dichloromethane. Subsequently the product was 3J(H–Pt) = 38.5 Hz; NCH3), 2.16 (s; ArCH3), 2.15 (s; ArCH3);
precipitated with the addition of pentane. The product was
[PtCl(NCN–CHvNC6H5-4)] (3). Aniline 6c: 38 μl
isolated after centrifugation of the resulting suspension. (0.42 mmol); aldehyde 7: 125 mg (0.28 mmol). Yield: 118 mg
When necessary the last steps were repeated to remove traces (0.22 mmol, 81%) of 3 as an off-white powder. 1H NMR
of residual aniline.
(300 MHz, CD2Cl2): δ = 8.35 (s, 1H; NvCH), 7.41–7.36 (m, 4H;
3
[PtCl(NCN–(CHvNC6H4-NMe2-4′)-4)] (1). 4-(Dimethylamino)- ArH), 7.22–7.17 (m, 3H; ArH), 4.09 (s, JH,Pt = 46.2 Hz, 4H;
aniline 6a: 45 mg (0.42 mmol); aldehyde 7: 125 mg CH2), 3.07 (s, JH,Pt = 38.1 Hz, 12H; CH3); 13C{1H} NMR
3
(0.28 mmol). Yield: 132 mg (0.24 mmol, 88%) of 1 as a brown (75 MHz, CD2Cl2): δ = 161.46 (CvN), 153.0, 152.7 (Cipso), 144.5
1
powder. H NMR (300 MHz, CD2Cl2): δ = 8.40 (s, 1H; NvCH), (2JC,Pt = 77.4 Hz; Cortho), 132.5, 129.5, 125.7, 121.0, 120.4
3
7.36 (s, 2H; ArH), 7.20 (d, JH,H = 9.0 Hz, 2H; ArH), 6.75 (d, (3JC,Pt = 36.0 Hz; Cmeta), 77.7 ((2JC,Pt = 64.3 Hz; NCH2), 54.6
3
3JH,H = 8.7 Hz, 2H; ArH), 4.08 (s, JH,Pt = 44.1 Hz, 4H; CH2), (N(CH3)2); 195Pt{1H} NMR (64 MHz, CD2Cl2): δ = –3111; IR
3
3.06 (s, JH,Pt = 38.1 Hz, 12H; NCH3), 2.97 (s, 6H; ArN(CH3)2); (ATR): ˜ν = 3011, 2984, 2921, 1616 (CvN), 1578, 1564, 1487,
13C{1H} NMR (75 MHz, CD2Cl2): δ = 157.03 (CvN), 151.4 1469, 1453, 1433, 1346, 1335, 1318, 1312, 1275, 1206, 1171,
(Cipso), 149.7, 144.4 (2JC,Pt = 77.4 Hz; Cortho to Pt), 141.7, 133.2, 1152, 1106, 1087, 1025, 1015, 974, 961, 945, 912, 883, 855,
122.2, 119.9 (3JC,Pt = 34.9 Hz; Cmeta), 113.3, 77.7 (2JC,Pt = 63.2 836, 772, 737, 710, 699 cm−1; MS (ES+; CH2Cl2) m/z: 525.11
Hz; NCH2), 54.6 (N(CH3)2), 40.9 (ArN(CH3)2); 195Pt{1H}NMR [M + H]+; elemental analysis calcd (%) for C19H24ClN3Pt
(64 MHz, CD2Cl2): δ = –3125; IR (ATR): ˜ν = 2977, 2918, 2798 (524.94): C 43.47, H 4.61, N 8.00; found: C 43.38, H 4.70,
1676, 1613 (CvN), 1590, 1559, 1511, 1446, 1345, 1291, 1221, N 7.95.
1164, 1123, 1085, 1014, 943, 882, 834, 756, 724, 710 cm−1; MS
[PtCl(NCN–(CHvNC6H4-Cl-4′)-4)] (4). 4-Chloroaniline 6d:
(ES+; CH2Cl2) m/z: 568.11 [M + H]+; elemental analysis calcd (%) 53 mg (0.42 mmol); aldehyde 7: 125 mg (0.28 mmol). Yield:
for C21H29ClN4Pt (568.01): C 44.40, H 5.15, N 9.86; found: 136 mg (0.24 mmol, 87%) of 4 as a light yellow powder.
C 44.61, H 5.11, N 9.93.
[PtCl(NCN–(CHvNC6H4-Me-4′)-4)] (2). 4-Methylaniline 6b: 2H; ArH), 7.35 (d, JH,H = 8.7 Hz, 2H; ArH), 7.14 (d, JH,H
45 mg (0.42 mmol); aldehyde 7: 125 mg (0.28 mmol). Yield: 8.7 Hz, 2H; ArH), 4.09 (s, JH,Pt = 45.9 Hz, 4H; CH2), 3.07 (s,
132 mg (0.24 mmol, 88%) of 2 as a light yellow powder. 3JH,Pt = 38.1 Hz, 12H; CH3); 13C{1H} NMR (75 MHz, CD2Cl2):
1H NMR (300 MHz, CD2Cl2): δ = 8.36 (s, 1H; NvCH), 7.38 δ = 161.87 (CvN), 153.2 (Cipso), 151.6, 144.6 (2JC,Pt = 78.5 Hz;
1H NMR (300 MHz, CD2Cl2): δ = 8.33 (s, 1H; NvCH), 7.38 (s,
3
3
=
3
3
3
(s, 2H; ArH), 7.19 (d, JH,H = 8.1 Hz, 2H; ArH), 7.10 (d, JH,H
=
C
ortho), 132.2, 130.9, 129.5, 122.4, 120.5 (3JC,Pt = 34.9 Hz; Cmeta),
8.1 Hz, 2H; ArH), 4.09 (s, JH,Pt = 45.6 Hz, 4H; CH2), 77.6 ((2JC,Pt = 62.1 Hz; NCH2), 54.6 (N(CH3)2); 195Pt{1H} NMR
3
3
3.07 (s, JH,Pt = 38.4 Hz, 12H; NCH3), 2.36 (s, 3H; ArCH3); (64 MHz, CD2Cl2): δ = –3105; IR (ATR): ˜ν = 3003, 2973, 2920,
13C{1H} NMR (75 MHz, CD2Cl2): δ = 160.62 (CvN), 152.4 1611 (CvN), 1587, 1573, 1558, 1483, 1452, 1430, 1421, 1402,
(Cipso), 150.3, 144.5 ((2JC,Pt = 79.6 Hz; Cortho), 135.6, 132.6, 1366, 1332, 1312, 1289, 1166, 1149, 1085, 1007, 971, 960,
130.1, 120.9, 120.3 (3JC,Pt = 34.9 Hz; Cmeta), 77.7 (2JC,Pt
=
945, 909, 872, 839, 816, 745, 714, 673 cm−1; MS (ES+; CH2Cl2)
64.3 Hz; NCH2), 54.6 (N(CH3)2), 21.0 (ArCH3); 195Pt{1H} NMR m/z: 559.09 [M + H]+; elemental analysis calcd (%) for
(64 MHz, CD2Cl2): δ = –3115; IR (ATR): ˜ν = 3004, 2974, 2920, C19H23Cl2N3Pt (559.39): C 40.80, H 4.14, N 7.51; found:
1617 (CvN), 1577, 1560, 1503, 1466, 1452, 1432, 1421, 1402, C 40.69, H 4.18, N 7.46.
1366, 1313, 1291, 1277, 1212, 1169, 1149, 1108, 1085, 1027,
[PtCl(NCN–(CHvNC6H4-CN-4′)-4)] (5). 4-Cyanoaniline 6e:
1014, 964, 945, 909, 875, 863, 844, 834, 815, 780, 736, 49 mg (0.42 mmol); aldehyde 7: 125 mg (0.28 mmol). Yield:
715 cm−1; MS (ES+; CH2Cl2) m/z: 540.14 [M + 2H]+; elemental 115 mg (0.21 mmol, 75%) of 5 as a yellow powder. 1H NMR
3
analysis calcd (%) for C20H26ClN3Pt (538.97): C 44.57, H 4.86, (300 MHz, CD2Cl2): δ = 8.31 (s, 1H; NvCH), 7.67 (d, JH,H
=
3
N 7.80; found: C 44.45, H 4.93, N 7.72.
8.7 Hz, 2H; ArH), 7.40 (s, 2H; ArH), 7.22 (d, JH,H = 8.4 Hz, 2H;
3
3
[PtCl(NCN–(CHvNC6H4-Me-4′)-4)] (2, solvent free proce- ArH), 4.10 (s, JH,Pt = 45.6 Hz, 4H; CH2), 3.07 (s, JH,Pt
=
dure). Solid aldehyde 7 (30 mg, 0.067 mmol) and solid 37.8 Hz, 12H; CH3); 13C{1H} NMR (75 MHz, CD2Cl2): δ = 163.47
4-methylaniline 6b (8.6 mg, 0.08 mmol) were ground together (CvN), 157.0 (CN), 154.3 (Cipso), 151.6, 144.7 (2JC,Pt = 77.4 Hz;
in a Schlenk flask using an egg shaped magnetic stirrer.
C
ortho), 133.7, 131.8, 121.8, 120.8 (3JC,Pt = 36.0 Hz; Cmeta), 119.5,
Reaction progress was followed by analyzing samples with 108.7, 77.6 (2JC,Pt = 62.1 Hz; NCH2), 54.6 (N(CH3)2); 195Pt{1H}
1H NMR (δ aldehyde-H/δ imine-H; 9.90/8.42 ppm, respect- NMR (64 MHz, CD2Cl2): δ = –3090; IR (ATR): ˜ν = 3008, 2983,
ively). After 1 h stirring of the solid reaction mixture 27% of 2 2922, 2219 (CuN), 1615 (CvN), 1594, 1576, 1558, 1494, 1452,
was formed that increased to 68% after 5 days. Subsequent 1433, 1424, 1403, 1345, 1317, 1218, 1171, 1149, 1105, 1087,
stirring (16 h) of the mixture at 50 °C, followed by (16 h) at the 1017, 986, 974, 959, 947, 882, 869, 841, 765, 727, 711 cm−1; MS
same temperature under vacuum did not increase product (ES+; CH2Cl2) m/z: 551.14 [M + 2H]+; elemental analysis calcd
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(%) for C20H23ClN4Pt (549.95): C 43.68, H 4.22, N 10.19; found:
C 43.74, H 4.29, N 10.12.
[PtCl(NCN–(CHvNC6H4-NO2-4′)-4)] (8) (e.g. by repeated pre-
cipitations of the impure reaction product with pentane from
dichloromethane followed by crystallization from toluene).
7 C. A. van Walree, O. Franssen, A. W. Marsman, M. C. Flipse
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X-ray crystal structure determinations
X-ray intensities were measured on a Nonius KappaCCD dif-
fractometer with rotating anode (graphite monochromator, λ =
0.71073 Å) up to a resolution of (sin θ/λ)max = 0.65 Å−1 at a
temperature of 150 K. The structures were solved with auto-
mated Patterson methods (program DIRDIF-99,57 compounds
2 and 5). The initial coordinates of 4 were taken from the iso-
structural 2. Refinement was performed with SHELXL-9758
against F2 of all reflections. Non-hydrogen atoms were refined
with anisotropic displacement parameters. All hydrogen atoms
were located in a difference Fourier map and refined with a
riding model. In 5 the phenyl ring C14–C19 was refined with a
disorder model with occupancy of 81.4(7) vs. 18.6(7)%. Geome-
try calculations and checking for higher symmetry was per-
formed with the PLATON program.59
10 L. Diez, P. Espinet, J. A. Miguel and M. P. Rodriguez-
Medina, J. Organomet. Chem., 2005, 690, 261–268.
11 M. Lee, Y. S. Yoo and M. G. Choi, Macromolecules, 1999, 32,
2777–2782.
12 M. Lee, Y. S. Yoo, M. G. Choi and H. Y. Chang, J. Mater.
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13 Y. S. Yoo, J. H. Im, B. H. Han, M. Lee and M. G. Choi, Bull.
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14 R. M. Moutloali, J. Bacsa, W. A. Ddamba and J. Darkwa,
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15 R. M. Moutloali, F. A. Nevondo, J. Darkwa, E. I. Iwuoha and
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17 L. A. van de Kuil, H. Luitjes, D. M. Grove, J. W. Zwikker,
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468–477.
Further details about the crystal structure determinations
are given in Table S2.‡
CCDC 649683 (compound 2), 649684 (compound 4), and
649685 (compound 5) contain the supplementary crystallo-
graphic data for this paper.
18 A. Houlton, N. Jasim, R. M. G. Roberts, J. Silver,
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Acknowledgements
This work was partially supported (GDB, ML, and ALS) by the
Council for Chemical Sciences of the Netherlands Organi-
zation for Scientific Research (NWO/CW).
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