1,4-Bis(4-nitrosophenyl)piperazine: Novel bridging ligand in dinuclear complexes of rhodium(iii) and iridium(iii)

Article abstract of DOI:10.1039/c1dt11696g

The synthesis, spectroscopic characterization and crystal structures of the first 1,4-bis(4-nitrosophenyl)piperazine (BNPP) (4) bridged dinuclear complexes of rhodium(iii) and iridium(iii) are presented. The reaction of the μ₂-halogenido-bridged dimers [(η⁵-C ₅Me₅)IrX₂]₂ [X = Cl (5a), Br (5b), I (5c)] and [(η⁵- C₅Me₅)RhCl ₂]₂ (6a) with 4 yields the dinuclear complexes [(η⁵-C₅Me₅)IrX₂] ₂-BNPP (7a-c) and [(η⁵-C₅Me ₅)RhCl₂]₂-BNPP (8a). All new compounds were characterized by their NMR, IR and mass spectra. The X-ray structure analyses of the obtained half-sandwich complexes revealed a slightly distorted pseudo-octahedral configuration ("three-legged pianostool") for the metal(iii) centers. The bridging BNPP ligand is σ-N coordinated by both nitroso groups and shows different conformations of the piperazine ring depending on the solvent used for crystallization. Moreover the crystal structures of 1,4-bis(4-nitrosophenyl)piperazine (4) and its precursor 1,4-diphenylpiperazine (3) are reported.

Full text of DOI:10.1039/c1dt11696g

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PAPER
1,4-Bis(4-nitrosophenyl)piperazine: novel bridging ligand in dinuclear
complexes of rhodium(III) and iridium(III)†
Stefan Wirth, Florian Barth and Ingo-Peter Lorenz*
Received 7th September 2011, Accepted 11th November 2011
DOI: 10.1039/c1dt11696g
The synthesis, spectroscopic characterization and crystal structures of the first 1,4-bis(4-nitrosophenyl)-
piperazine (BNPP) (4) bridged dinuclear complexes of rhodium(III) and iridium(III) are presented. The
5
5
reaction of the m2-halogenido-bridged dimers [(h -C5Me5)IrX2]2 [X = Cl (5a), Br (5b), I (5c)] and [(h -
5
C5Me5)RhCl2]2 (6a) with 4 yields the dinuclear complexes [(h -C5Me5)IrX2]2-BNPP (7a–c) and
[(h -C5Me5)RhCl2]2-BNPP (8a). All new compounds were characterized by their NMR, IR and mass
5
spectra. The X-ray structure analyses of the obtained half-sandwich complexes revealed a slightly
distorted pseudo-octahedral configuration (“three-legged pianostool”) for the metal(III) centers. The
bridging BNPP ligand is s-N coordinated by both nitroso groups and shows different conformations of
the piperazine ring depending on the solvent used for crystallization. Moreover the crystal structures of
1,4-bis(4-nitrosophenyl)piperazine (4) and its precursor 1,4-diphenylpiperazine (3) are reported.
There are only few metal complexes containing two s-N
Introduction
coordinated C-nitroso groups and all of them are stabilized by
a chelate system in some way. Vicinal dinitroso compounds s-
N coordinated to the same metal atom via both NO functions
are especially known for cobalt because of their application in
organic synthesis,26–37 but some examples with ruthenium38,39 or
platinum40 also exist. If the second C-NO unit is located not close
enough other functional neighbors,9 for example a deprotonated
amino group,41,42 are able to stabilize the chelate as well. In this
context it should be mentioned that the presence of two C-nitroso
and additional functional neighbors does not guarantee s-N
coordination or interaction at both possible positions as shown
by the different results published for 2,4-dinitrosoresorcinol,
unfortunately without crystallographic data.43–47
In our attempts dimeric halogenido bridged half-sandwich
complexes of Ir(III) (5a–c) and Rh(III) (6a) were employed because
these compounds are easily accessible and remarkably robust;
they show good solubility in standard organic solvents and their
organic p-ligand which is relatively inert towards substitution
reactions can be used to fine-tune their chemical properties.
These are also the reasons why organometallic half-sandwich
complexes have been already extensively examined and reviewed
as building blocks in supramolecular chemistry during the last
decade.48–52 Since the first approaches for cubic compounds in
the late 1990s53,54 many polynuclear complexes with different two-
and three-dimensional geometries were synthesized using various
ligands with N-, O-, S-, P- or NC-donor groups. Recently also
M–C bonding via C–H activation was added to the toolbox for
the design of organometallic macrocycles55–59 and cages60 which
can lead to an increased structural stability compared to M–O or
M–N bonds. However, until now C-nitroso compounds were not
applied as linking or bridging molecules in this field of research.
Since the first reports on the synthesis of aliphatic1,2 or aromatic3–5
C-nitroso compounds were published in 1874 a considerable
variety of synthetic routes to high-yield preparations of C-nitroso
compounds have been developed.6,7 Comprehensive reviews are
also available for their rich coordination chemistry,8,9 their biolog-
ical activities,10 possible applications in organic synthesis11 or the
historical development12 of this compound class.
This article is concerned with 1,4-bis(4-nitrosophenyl)-
piperazine (BNPP) (4), an example ofadistinctfamilyof molecules
in which two C-nitroso groups are to be found not interacting
with each other neither intra- nor intermolecularly. Although the
first report on 4 is dated to 187913 and an improved synthesis
of its hydrochloride has existed since 191814 only few references
about BNPP are available. Most deal with organic molecules like
piperazine,14 its derivatives15 or 14C-labelled related compounds16–19
where the hydrochloride of 4 appears as an intermediate in their
synthesis. Some applications in polymer chemistry20–23 or as a
fungicide24 are also known. The only complex containing 4 was
published 1994.25 Cameron et al. anticipated the possibility of a
coordinative polymerization at Sn-centers. This, however, did not
occur as proved by several spectroscopic techniques; coordination
to the applied tin complex took place with only one of the two
C-nitroso groups. Until today, this was the only other attempt to
use BNPP in coordination chemistry.
Department of Chemistry, Ludwig-Maximilian-University Munich, Bute-
nandtstraße 5–13 (House D), D-81377, Munich, Germany. E-mail: ipl@cup.
uni-muenchen.de; Fax: +49-(0)89-2180-77867
† Electronic supplementary information (ESI) available: CCDC reference
numbers 838208–838214. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt11696g
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In the following, the synthesis, spectroscopic charac-
terization and crystal structures of the first 1,4-bis(4-
nitrosophenyl)piperazine (4) bridged dinuclear complexes are de-
scribed. To the best of our knowledge these compounds presented
here are the first ones with two not interacting C-nitroso groups
which coordinate in the s-N mode without any chelate assistance
to two different metal centers.
rotation of the nitroso group along the C–N bond and its large
magnetic anisotropy. “Freezing” this rotation at -60 C leads to
◦
a significant splitting of the broad signal (d = 6.94 and 9.00 ppm)
whereas that at 9.00 ppm is ascribed to the proton in the “anti
position” of the nitroso-O caused by the strong deshielding effect
of this functional group.25,61 The 13C NMR spectra of 4 show the
same effect. In solution only at -60 ◦C is the full set of signals
detectable. The signal splitting for the carbon atoms in the ortho
position of the nitroso group is enhanced to D = 31.1 ppm (d =
110.0 and 141.1 ppm).61 All other protons and carbon atoms are
detected in solution in the expected areas. The solid state 13C NMR
spectrum of 4 is comparable to the low temperature spectrum in
solution in the signal position and count. The significant areas of
the different NMR spectra in solution and in the solid state are
depicted in Fig. 1.
The IR spectra of 4 (KBr pellets) show weak n(C–H) absorp-
tions between 3100 and 2850 cm-1. At wavenumbers smaller than
1597 cm-1 the fingerprint of 4 is observed. The n(N O) absorption
at 1358 cm-1 was located by comparison with the IR spectra
of N,N-dimethyl-4-nitrosoaniline (n(N O) 1363 cm-1)9,62 whose
structure and IR spectra are very similar to 4. Mass spectrometric
investigation in the DEI mode shows the expected [M+] peak with
its characteristic isotope pattern and an assignable fragmentation
pattern for compound 4.
Results and discussion
Synthesis and characterization of BNPP (4)
The synthesis of 4 is derived from a literature method.14 The
precursor 1,4-diphenylpiperazine (3) is obtained by condensation
from aniline (1) and 1,2-dibromoethane (2) which are heated with
sodium carbonate to 130 ◦C (Scheme 1). For nitrosation 3 was
powdered and suspended in conc. HCl at 0 ◦C and a solution of
NaNO2 was added. BNPP (4)precipitatedas agreen hydrochloride
(¥ 2HCl) which was filtered off. The modifications carried out to
this method14 are described in the experimental section. To get pure
4 its hydrochloride was dissolved in methanol and triethylamine
was added to remove HCl before column chromatography on silica
gel. This synthesis yielded 4 as air stable, green crystals, soluble
for example in dichloromethane or acetone and nearly insoluble
in n-pentane.
Since no crystallographic data were available, the molecular
structures of 3 and 4 were determined using single crystal X-
ray diffraction. Isothermic diffusion of n-pentane into a solution
of 3 or 4 in acetone resulted in the formation of X-ray quality
single crystals. The molecular structures with atom numbering
are illustrated in Fig. 2 and 3, selected bond lengths and angles
are listed in Table 1. Details of the crystallographic data and
refinement are summarized in Table 3.
The piperazine ring in 3 shows chair conformation and an
inversion center, so both phenyl rings exhibit identical geometry.
C–N bond lengths lie within the expected range for single bonds
while the N1–C1 bond to the aromatic ring is slightly shorter
than the N1–C7 and N1–C8 within the piperazine ring. The sp3
hybridization of N1 is confirmed by angles around 109◦ enclosed
by N1 and its neighboring carbon atoms C1, C7 and C8i (Table 1).
C–C bond lengths in the phenyl ring range from 1.371(3) to
˚
1.395(2) A and prove their aromatic character. The parallel planes
of theses aromatic rings show an intramolecular vertical plane-to-
˚
plane distance of 0.558 A.
Crystallographic characterization of 4 exhibits two molecules
with different conformations of the piperazine ring (chair and
boat/Fig. 3) within the unit cell. 4 in chair conformation possesses
an inversion center with similar bonding situations in both
aromatic rings (Table 1), as already discussed for 3. The N
O
bond length is consistent with the expectation for an aromatic
9
˚
C-nitroso compound (1.13–1.29 A) but is a little bit longer than
that found for N,N-dimethyl-4-nitrosoaniline (1.131 and 1.212
A).63 The nitroso group is located in the plane of the aromatic ring
˚
(deviation 0.2◦). N6–C20 and N5–C17 are a little bit shorter than
expected for single bonds and indicate a quinoid contribution to
the C17–C22 ring. This is supported by shortened C18–C19 and
C21–C22 bonds in comparison to 3 or to the other C–C bond
lengths in the ring. C–N and C–C bonds in the piperazine unit
are single bonds without any doubt. Nitrosation of 3 also effects
the geometry of the piperazine-N which is noticeable flattened in
Scheme 1 Synthesis of 1,4-bis(4-nitrosophenyl)piperazine (4).14
In contrast to 3 the analytical data available for 4 is fragmentary.
1
Therefore BNPP (4) was characterized by H and 13C NMR, IR
1
and mass spectra. In the H NMR spectra at room temperature
a broad signal at d = 7.77 ppm is assigned to the protons in the
ortho-position of the NO-group. The broadening is caused by the
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1
Fig. 1 Significant areas of the H NMR (A: (CD3)2SO, B: CDCl2) and the 13C NMR spectra (C: (CD3)2SO, D: CDCl2, E: solid state) of 4 at room
temperature (A, C, E) and at -60 ◦C (B, D) (solvent signals are omitted for clarity: B: 5.32 ppm (CDCl2); C: 39.5 ppm ((CD3)2SO); D: 53.5 ppm (CDCl2);
spinning sidebands at E are marked with *).
4. That becomes evident by the sum of angles which is increased
from 342.8◦ for N1 in 3 to 353.8◦ for N5 in 4. This flattening also
explains the decline of the intramolecular vertical plane-to-plane
slightly increased. Surprisingly in the boat conformation three
of the four C–N bonds to the aromatic rings (N1–C1, N2–C11,
N4–C14) are distinctly shorter than in the chair conformation.
Shorter bond lengths at these positions suggest a higher quinoid
contribution to the aromatic system for this conformation.
(Fig. 3)
This is as well indicated by an almost planar geometry of the
piperazine nitrogen atoms (sum of angles N1 359.7◦, N2 360.0◦).
All other C–N and C–C bonds within the heterocycle are in the
expected range for single bonds and are comparable to those of 3
(Table 1).
˚
distance of the parallel phenyl rings to only 0.287 A.
The most obvious differences between the two molecular units
of 4 is the orientation of the phenyl rings which are not parallel
in the boat conformation but enclose an angle of 56.27◦. By
lack of an inversion center within the piperazine ring both sides
vary a little in their bonding situation (Table 1). N O bond
lengths lie in the same range as in the chair conformation whereas
the deviation of the nitroso groups out of the phenyl planes is
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◦
˚
Table 1 Selected bond lengths [A] and angles [ ] of compounds 3 and 4
Angles [◦]
Torsion angles [◦]
˚
Bond lengths [A]
3 (Chair conformation)
N1–C1
1.412(2)
1.459(2)
1.46(3)
C2–C3
C5–C6
C7–C8
1.375(2)
1.386(2)
1.507(2)
C1–N1–C7
C1–N1–C8#1
C7–N1–C8#1
117.0(2)
115.6(2)
110.2(2)
C2–C1–N1
C6–C1–N1
119.8(2)
122.5(2)
C7–N1–C1–C6
C8#1–N1–C1–C2
N1–C7–C8–N1#1
1.6(2)
-48.1(2)
-56.9(2)
N1–C7
N1–C8#1
4 (Chair conformation)
O3–N6
1.245(2)
1.395(2)
1.376(2)
1.460(2)
N5–C23
1.463(2)
1.492(3)
1.364(2)
1.369(2)
O3–N6–C20
C19–C20–N6
C21–C20–N6
C17–N5–C23
116.7(2)
125.2(2)
116.2(2)
121.1(2)
C17–N5–C24
C24–N5–C23
N5–C23–C24#2
N5–C24–C23#2
120.8(2)
111.9(2)
113.1(2)
112.4(2)
O3–N6–C20–C19
C23–N5–C17–C22
C24–N5–C17–C18
N5–C23–C24–N5#2
-0.2(3)
29.0(3)
-3.2(3)
52.2(2)
N6–C20
N5–C17
N5–C24
C23–C24#2
C18–C19
C21–C22
4 (Boat conformation)
O1–N3
N3–C4
N1–C1
N1–C10
N1–C7
C7–C8
C2–C3
C5–C6
1.241(2)
1.412(2)
1.363(2)
1.456(2)
1.463(2)
1.513(3)
1.364(2)
1.371(2)
O2–N4
1.254(2)
1.389(2)
1.350(2)
1.461(2)
1.471(2)
1.516(2)
1.362(2)
1.360(2)
O1–N3–C4
C3–C4–N3
C5–C4–N3
C1–N1–C7
C1–N1–C10
C10–N1–C7
N1–C7–C8
N1–C10–C9
115.7(2)
124.5(2)
116.2(2)
120.9(2)
123.1(2)
115.7(2)
110.7(2)
111.0(2)
O2–N4–C14
C13–C14–N4
C15–C14–N4
C11–N2–C9
C11–N2–C8
C8–N2–C9
115.9(2)
125.4(2)
115.9(2)
121.6(2)
123.2(2)
115.2(2)
110.9(2)
110.8(2)
O1–N3–C4–C3
C7–N1–C1–C6
C10–N1–C1–C2
N1–C7–C8–N2
O2–N4–C14–C13
C8–N2–C11–C12
C9–N2–C11–C16
N2–C9–C10–N1
-2.9(3)
-8.2(3)
-14.6(3)
-59.4(2)
3.8(3)
-8.7(3)
-6.4(3)
-58.4(2)
N4–C14
N2–C11
N2–C8
N2–C9
C9–C10
C12–C13
C15–C16
N2–C8–C7
N2–C9–C10
Symmetry transformations used to generate equivalent atoms: #1 -x+1,-y,-z+1; #2 -x+2,-y+1,-z+1
◦
˚
Table 2 Selected bond lengths [A] and angles [ ] of 7a–c and 8a compared to equivalent data of 4
˚
Bond lengths [A]
M–X1
M–X2
M–N(O) N–O
N(O)–C
N1–C1
N1–C7
N1–C8
C2–C3
C5–C6
4 (Chair)b
4 (Boat)b
—
—
—
2.388(2)
—
—
—
—
—
—
1.245(2)
1.395(2)
1.412(2)
1.389(2)
1.397(5)
1.39(2)
1.376(2)
1.363(2)
1.350(2)
1.360(5)
1.35(2)
1.463(2)
1.463(2)
1.471(2)
1.458(6)
1.47(2)
1.49(2)
1.48(2)
1.460(2)
1.456(2)
1.461(2)
1.461(5)
1.49(2)
1.364(2)
1.364(2)
1.362(2)
1.345(6)
1.37(2)
1.35(2)
1.363(9)
1.35(2)
1.369(2)
1.371(2)
1.360(2)
1.363(6)
1.37(2)
1.37(2)
1.348(9)
1.38(2)
1.241(2)
1.254(2)
1.245(4)
1.256(9)
1.270(9)
1.252(8)
1.250(7)
1.246(3)
7a (Chair)
2.420(2) 2.069(3)
2.414(2) 2.061(7)
2.407(2) 2.088(8)
2.538(2) 2.064(7)
7a¢ (Boat)a,b 2.399(2)
2.419(2)
2.513(2)
2.7132(6) 2.7109(7) 2.072(6)
2.3950(7) 2.4308(7) 2.097(2)
1.38(2)
1.34(2)
1.47(2)
7b (Chair)
7c (Boat)
8a (Chair)
1.432(9)
1.387(9)
1.389(3)
1.366(9)
1.351(9)
1.355(3)
1.454(9)
1.483(9)
1.459(3)
1.476(9)
1.464(4)
1.352(4)
1.365(4)
Angles [◦]
Torsion angles [◦]
X1–M–X2 N–M–X1 N–M–X2 C1–N1–C7 C1–N1–C8 C7–N1–C8 O1–N2–C4–C3 C2–C1–N1–C8 C6–C1–N1–C7
4 (Chair)b
4 (Boat)b
—
—
—
—
—
121.1(2)
120.9(2)
121.6(2)
124.0(4)
120.7(7)
121.7(7)
122.9(7)
123.3(6)
124.1(2)
120.8(2)
123.1(2)
123.2(2)
124.4(4)
123.3(7)
123.2(7)
124.7(7)
122.2(6)
124.6(2)
111.9(2)
115.7(2)
115.2(2)
110.7(3)
115.1(7)
114.7(7)
111.5(6)
114.5(6)
110.7(2)
-0.2(3)
-2.9(3)
3.8(3)
8.1(6)
9(2)
-3.2(3)
-14.6(3)
-8.7(3)
5.6(7)
23(2)
29.0(3)
-8.2(3)
-6.4(3)
-6.8(7)
9(2)
—
—
—
—
7a (Chair)
87.12(4)
85.5(2)
88.9(2)
88.2(2)
84.8(2)
87.7(2)
93.27(9)
93.2(2)
90.5(2)
94.5(2)
95.4(2)
7a¢ (Boat)a,b 87.21(9)
87.63(9)
-3(2)
11(2)
3(2)
15(2)
2(2)
2(2)
5.8(4)
8(2)
-10(2)
1(2)
7b (Chair)
7c (Boat)
8a (Chair)
88.12(4)
89.02(2)
89.28(3)
86.47(6) 95.12(6)
7.7(4)
-5.5(4)
a If more than one complex molecule in similar conformation was present in the assmyetric unit, only the data for one unit are listed. b Column headings
are derived from the metal complexes with inversion center (7a, 7b, 7c, 8a), for compounds with different atom numbering (4, 7a¢) comparable bond
lengths and angles are given.
Synthesis and characterization of the BNPP bridged complexes
7a–c and 8a
in nonpolar solvents addition of n-pentane does not cause this
effect.
Due to the described decomposition in unsaturated solution
reasonable characterization by NMR spectroscopy was possible
only as 13C NMR in the solid state. As an example, two of these
spectra (7a and c) are depicted with that of 4 in Fig. 4. Surprisingly
the spectra differ in the signal count for equivalent atoms at some
positions. For example the “CH2” carbons of the piperazine rings
show four peaks for uncoordinated 4 whereas only two signals are
detected in 7a and just one in 7c (Fig. 4). A different kind and
count of conformations in the solid state may be an explanation
for this result. Due to the described decompositions in solvents for
According to Scheme 2 the reaction of BNPP (4) with the starting
5
5
complexes [(h -C5Me5)IrX2]2 [X = Cl (5a), Br (5b), I (5c)] or [(h -
C5Me5)RhCl2]2 (6a) was carried out in dry dichloromethane (sat-
urated solutions) at room temperature. The nucleophilic cleavage
of the dimers by 4 yields the products as green (7a–c) or brown
(8a) solids which decompose slowly when exposed to moist air.
If dissolved in polar solvents such as DMF, DMSO, acetone
or dichloromethane the ligand is released and both starting
materials are formed back. Since the complexes are insoluble
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Scheme 2 Synthesis of the iridium(III) (7a–c) and rhodium(III) (8a) complexes with 4 as a bridging ligand.
the loss of the halogenido ligands only with weak intensity. For
such high molecular masses (900 to 1500 g mol-1) this finding
is not astonishing. Despite this low intensity the unique isotope
pattern is observed for all peaks up to [M+] of 7c at 1459.0 g
mol-1. Complex 8a does not show an [M+] peak at all, only the
characteristic fragmentation and isotope patterns are detected.
Single crystals suitable for X-Ray diffraction analysis were
obtained by isothermic diffusion of n-pentane into a solution
of DMF (7a, 7b, 8a), chloroform (7a¢) or dichloromethane (7c).
Crystallographic data and details of the structure refinements are
summarized in Table 3. An interesting side effect of the attempts
to gain crystals in good quality under different conditions is the
discovery of the solvent dependent conformation of the piperazine
ring. If DMF was employed to solve the complexes the chair
conformation was always found (7a, 7b, 8a). Use of chloroform
(7a’) or dichloromethane (7c) leads to a piperazine ring only in
the boat conformation. For example the two units with different
conformation in the molecular structure obtained for complex 7a,
depending on the different solvent used for crystallization (DMF
or CHCl3), are depicted with atom numbering in Fig. 5. ORTEP64
plots of all molecular structures with the closest interacting solvent
molecules and the molecular packing within the unit cell are
available as ESI.† Only for the crystals grown in DMF direct
interaction of the solvent molecules with the piperazine ring via
hydrogen bonding is observed. The data for these hydrogen bonds
are listed in Table 6 as ESI.† Selected bond lengths and angles of
7a–c and 8a are listed and compared with those of 4 in Table 2.
1,4-Bis(4-nitrosophenyl)piperazine (4) acts in all complexes
without any doubt as a bridging ligand between two slightly
distorted pseudo-octahedral rhodium(III) or iridium(III) centers,
respectively. This is important because for the only other com-
pound containing 4 as ligand terminal monocoordination via one
C-nitroso group to tin was published.25 The coordination sphere
(“three-legged pianostool” configuration) of each metal consists
Fig. 2 Molecular structure of 1,4-diphenylpiperazine (3). Hydrogen
atoms are omitted for clarity. Displacement ellipsoids are drawn at 30%
probability level.
7a–c and 8a neutralizing this effect by measurement in the liquid
state is only feasible for uncoordinated 4 (Fig. 1). A consistent
trend for the coordination induced shifts of the signals can only
be detected for a few atoms. For example the ortho-carbon atom
(anti position nitroso-O) shows after coordination a little shift
to a higher field compared to 4. A detailed comparison of the
corresponding signals is available in tabular form as ESI (Table
4).†
In the IR spectra of 7a–c and 8a (KBr pellets) several weak n(C–
H)absorptions are located between3095and 2860cm-1. Within the
fingerprint area (wavenumbers smaller than 1597 cm-1) n(N O)
absorptions are observed from 1351 to 1347 cm-1 which are shifted
to marginally lower frequencies compared to 4 (1358 cm-1). This
small shift lies in the expected scale for s-N coordination of C-
nitroso compounds.9,62
The mass spectra of 7a–c in the FAB+ mode exhibit the parent
peaks as well as comparable fragmentation patterns resulting from
5
of one h -pentamethylcyclopentadienyl, two terminal halogenido
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Fig. 3 Molecular structure of 1,4-bis(4-nitrosophenyl)piperazine (4) in its chair (a) and boat (b) conformation. Hydrogen atoms are omitted for clarity.
Displacement ellipsoids are drawn at 30% probability level.
Fig. 4 13C NMR solid state spectra of 4 (A), 7a (B) and 7c (C) at room temperature (spinning sidebands are marked with *).
2182 | Dalton Trans., 2012, 41, 2176–2186
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Fig. 5 Molecular structures of 7a crystallized from DMF (chair conformation/structure 7a) and from CHCl3 (boat conformation/structure 7a¢).
Hydrogen atoms, solvent molecules (7a: 2 DMF, 7a¢: 10 CHCl3) and a second molecular unit (7a¢) are omitted for clarity. Displacement ellipsoids are
drawn at 30% probability level.
and the s-N coordinated nitroso ligand. The M–X bond lengths
found in 7a–c and 8a are almost identical to those observed
for the terminal halogenido ligands in the starting complexes
Experimental
Materials and methods
65
66
66
˚
˚
˚
5a (2.387(4) A), 5b (2.519(5) A), 5c (2.694(1) A) and 6a
All experiments for the preparation of the complexes 7a–c and 8a
were carried out under a dry argon atmosphere using Schlenk and
67
˚
(2.397(2) A) The angles enclosed by those or one halogenido
5
ligand and the nitroso-N only differ by a few degrees from
vacuum-line techniques. The starting complexes [(h -C5Me5)IrX2]2
90◦ and so prove the slightly distorted geometry (Table 2). The
(X = Cl (5a),75 Br (5b),76 I (5c)77) and [(h -C5Me5)RhCl2]2 (6a)77
5
5
distance between the h -Cp*-ring and the corresponding metal
were prepared according to literature procedures. The published
synthesis of the hydrochloride of BNPP (4)14 had to be modified
to yield the pure compound. Solvents were purified by standard
procedures; dichloromethane was distilled from calcium hydride
and n-pentane from lithium aluminum hydride. Acetone and
center is increased in 7a–c and 8a compared to 5a–c and 6a
due to dimer cleavage and nitroso coordination (5a: 1.7563(4)
65
66
66
67
˚
˚
˚
˚
A), 5b: 1.771(2) A, 5c: 1.8013(3) A and 6a: 1.7558(3) A.
A list of plane to plane distances and angles within 7a–c and
8a is available as ESI (Table 5).† The Ir–N bonds found in 7a–
˚
chloroform were dried dynamically in a column with a 3 A
molecular sieve. All dried solvents were stored under a dry
˚
c range from 2.061(7) to 2.088(8) A and are a little longer than
˚
those recently reported for a comparable Ir(III) complex (1.948(6)
argon atmosphere with a 3 A molecular sieve (dichloromethane,
68
˚
A ) in which 1-chloro-2-nitroso-1,2-dihydrodicyclopentadiene is
chloroform) or sodium pieces (acetone, n-pentane), respectively.
DMF (reagent grade) was purchased from VWR and used without
further purification.
s-N coordinated. Crystallographic data for Rh(III) with s-N
coordination of a C-nitroso ligand are not available. The only
almost similar structurally characterized compound contains a
Rh(I) center and 1-bromo-4-nitrosobenzene as a ligand with a Rh–
NMR spectra in solution were recorded with a JEOL Eclipse
400 spectrometer at ambient temperature unless stated otherwise.
Solid-state NMR measurements were carried out at room temper-
ature with a Bruker DSX Avance 500 FT spectrometer equipped
with a commercial 2.5 mm MAS NMR double-resonance probe
(samples contained in a 4 mm ZrO2 rotor) at a magnitude field
of 11.7 T; the rotation frequency was 9 kHz. All chemical shifts
were determined in parts per million relative to TMS. IR spectra
(KBr) were recorded using a Perkin Elmer Spectrum One FT-IR
spectrometer in the range of 4000–400 cm-1. Mass spectra were
obtained with a JEOL MStation JMS-700. Elemental analyses
were performed by the Microanalytical Laboratory of the Depart-
ment of Chemistry and Biochemistry using a Heareus elementar
varioEL. Single crystal X-ray diffraction data were collected using
a Nonius Kappa CCD and an Oxford Diffraction Xcalibur S with
Sapphire CCD detector, both using graphite-monochromatized
Mo-Ka radiation. Structures were solved by direct methods
N(O) bond length of 1.968(8) A69 which is somewhat shorter than
˚
that found in 8a. Thus, the M–N bond lengths observed in 7a–c
and 8a are in the range of Rh(III) and Ir(III) complexes, respectively,
with bidentate sp2-N-donor ligands.70–74 The structural changes in
4 induced by the coordination process are by far not as strong as
assumed. The N O bond length is only marginally effected. But
few trends can be found in our crystallographic data (Table 2).
N1–C7 and N1–C8 are slightly longer in 7a–c and 8a compared to
4. The sum of the angles of the piperzine-N, already around 360◦
in 4 for boat but 353.8◦ for chair conformation, rises to values
from 359.1 to 360.0◦ regardless which conformation is found
in 7a–c and 8a. This indicates a stronger quinoid contribution
in the aromatic ring after coordination. The NO group is
located in all complexes within the phenyl plane with only small
deviations.
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with the SHELXS software and refined on F2 by full-matrix
least-squares with SHELXL-97.78 The PLATON79/SQUEEZE80
software was applied to subtract the contribution of heavily
disordered solvent from diffraction data if no reasonable modeling
was possible during refinement. SQUEEZE details are appended
to the deposited final refinement CIF. CCDC numbers in Table 3
contain the supplementary crystallographic data for this paper. †
(CH), 142.5 (CH), 154.6 (C1), 162.2 (C4), 162.8 (C4). MS (DEI):
m/z 296.2 (100%) [M+], 148.2 (8) [21 M+], 134.2 (24) [21 M+ -CH2],
120.2 (10) [12 M+ -2CH2].
Synthesis of the complexes 7a–c and 8a: general procedure
A saturated solution of BNPP (4) in dry dichloromethane (4–
6 ml) was added slowly to a saturated solution of the starting
5
complexes [(h -C5Me5)IrX2]2 [X = Cl (5a), Br (5b), I (5c)] or
Synthesis of 1,4-bis(4-nitrosophenyl)piperazine (BNPP) (4)
5
[(h -C5Me5)RhCl2]2 (6a) in dry dichloromethane (4–11 ml) and
The modified synthesis of 4 is based on a literature method.14
The crude precursor 1,4-diphenylpiperazine (3) is obtained from
aniline (1) and 1,2-dibromethane (2) by condensation where
they (no solvent) are stirred with sodium carbonate (to quench
generated HBr) at 130 ◦C for 5 h. For analytically pure 3
column chromatography (silica gel, dichloromethane/n-pentane)
is necessary. Colorless crystals of 3 suitable for X-ray analysis are
obtained by isothermic diffusion of n-pentane into a solution of 3
in acetone.
For nitrosation 0.654 g (2.74 mmol) crude 3 was powdered
and suspended in 20 ml conc. HCl at 0 ◦C. Then, a solution of
0.757 g (10.98 mmol) NaNO2 in 5 ml distilled water was added
slowly through a cannula to the bottom of the flask. The mixture
turned brown immediately and was stirred at 0 ◦C for 10 min.
The resulting precipitate (1,4-bis(4-nitrosophenyl)piperazine (4)
¥ 2HCl) was filtered off, washed with cold diluted hydrochloric
acid (HCl : water = 1 : 1) and cold acetone. Then the crude
hydrochloride of 4 was dried in vacuo and stored in argon. Yield 4
¥ 2HCl: 0.853 g (2.31 mmol, 84%).
stirred at room temperature. The color of the combined solution
instantly turned red. After the reaction was completed (see below)
dichloromethane was evaporated to a volume of about 5 ml. Then
10 ml of dry n-pentane were added and the products precipitated as
green (7a–c) or brown (8a) solids. After filtration (schlenk fritt) the
complexes were washed carefully with 2 ml dry dichloromethane
(cold), three times with 5 ml dry n-pentane and dried in vacuo.
[l2 -(1,4-Bis(4-nitrosophenyl)piperazin-N,N¢)bis(dichlorido-(g5 -
pentamethyl-cyclopentadienyl)-iridium(III))] (7a). Reagents: 86.0
mg (0.108 mmol) 5a, 32.0 mg (0.108 mmol) 4. Reaction time:
72 h. Yield: 63.7 mg (0.058 mmol, 54%), dark green powder.
Crystals were obtained by slow isothermic diffusion of n-pentane
into a nearly saturated solution of 7a in DMF (structure 7a, red
platelets) or chloroform (structure 7a¢, violet rods). Anal. Calc. for
C36H46Cl4Ir2N4O2 (1093.022 g mol-1): C, 39.56; H, 4.24; N, 5.13.
Found: C, 39.30; H, 4.43; N, 4.87%. IR: nmax/cm-1 (KBr) 3095w,
3044w, 2963w, 2914w, 2866w, 1591vs, 1543w, 1517 m, 1450 m,
1411 m, 1351 m n(N O), 1327 s, 1308 s, 1223 s, 1126vs, 1023 s,
989 m, 940 m, 874 m, 835w, 723 m, 646w, 624w, 590w. 13C NMR
(solid state): d = 8.9 (CH3-C5Me5), 45.2 + 48.1 (C7+C8), 88.6 (Cq-
C5Me5), 111.0 + 113.0 + 122.7 (C2+C5+C6), 140.5 (C3), 153.9 (C1),
155.6 (C1), 165.4 (C4), 167.2 (C4). MS (FAB+): m/z 1093.1 (<1%)
[M+ +H], 1057.1 (<1) [M+ -Cl], 1022.2 (<1) [M+ -2Cl], 694.1 (2)
[M+ -IrCp*Cl2], 659.3 (18) [M+ -IrCp*Cl2 -Cl], 643.2 (5) [M+
-IrCp*Cl2 -Cl -O], 624.2 (4) [M+ -IrCp*Cl2 -2Cl], 363.2 (100)
[IrCp*Cl].
To remove HCl the hydrochloride of 4 was dissolved in methanol
and triethylamine was added till the solution changed its color
from red to light green. Then the mixture was absorbed onto 25.0 g
R
R
ISOLUTEꢀ (Biotage) and dried in vacuo. Loading on ISOLUTEꢀ
is comparable to loading on silica, but it offers very sharp bands
and does not affect the following chromatographical process. The
pure green product 4 was isolated after column chromatography
on silica gel (EtOAc/n-pentane) and dried in vacuo. Yield: 0.481
g (1.62 mmol, 59%). Green crystals (rods) were obtained by slow
isothermic diffusion of n-pentane into a solution of 4 in acetone.
Anal. Calc. for C16H16N4O2 (296.324 g mol-1): C, 64.85; H, 5.44;
N, 18.91. Found: C, 64.46; H, 5.50; N, 18.41%. IR: nmax/cm-1
(KBr) 3093w, 3055w, 2908w, 2872w, 2853w, 1597vs, 1557w, 1515vs,
1467w, 1446 m, 1373 s, 1358vs n(N O), 1325vs, 1398 s, 1283 s,
1242 s, 1210vs, 1142vs, 1113vs, 1024 s, 941 m, 831 m, 824 m, 797w,
718 m, 644 m, 516w, 503w. 1H NMR (400 MHz, (CD3)2SO): d =
3.87 (s, 8H, H7+H8), 7.03 (d, 3JH,H = 8.8 Hz, 4H, H2+H6), 7.77 (br
s, 4H, H3+H5). 1H NMR (400 MHz, CD2Cl2, -60 ◦C): d = 3.87 (s,
[l2 -(1,4-Bis(4-nitrosophenyl)piperazin-N,N¢)bis(dibromido-(g5 -
pentamethyl-cyclopentadienyl)-iridium(III))] (7b). Reagents: 98
mg (0.101 mmol) (5b), 29.9 mg (0.101 mmol) (4). Reaction time:
93 h. Yield: 116 mg (0.091 mmol, 90%), green powder. Red crystals
(platelets) were obtained by slow isothermic diffusion of n-pentane
into a nearly saturated solution of 7b in DMF. Anal. Calc. for
C36H46Br4Ir2N4O2 (1270.826 g mol-1): C, 34.02; H, 3.65; N, 4.41.
Found: C, 33.78; H, 3.77; N, 4.24%. IR: nmax/cm-1 (KBr) 3086w,
3041w, 2962w, 2910w, 2860w, 1593vs, 1541w, 1521 m, 1445 m,
1410 m, 1367 m, 1349 m n(N O), 1327 s, 1303 s, 1249 m, 1222 m,
1136vs, 1126vs, 1025 s, 988 m, 943w, 872 m, 820w, 724 m, 643w,
622w, 590w. 13C NMR (solid state): d = 9.7 (CH3-C5Me5), 44.8
+ 46.3 + 46.5 (C7+C8), 88.8 + 89.3 (Cq-C5Me5), 110.5 + 110.8
+ 112.3 + 120.4 + 121.0 (C2+C5 +C6), 137.4 + 140.6 (C3), 154.8
(C1), 154.8 (C1), 156.8 (C1), 164.9 (C4), 165.0 (C4), 168.2 (C4). MS
(FAB+): m/z 1273.2 (<1%) [M+ +H], 1190.1 (<1) [M+ +H -Br],
1110.3 (<1) [M+ -2Br], 783.2 (2) [M+ -IrCp*Br2 -H], 703.3 (11)
[M+ -IrCp*Br2 -Br], 488.1 (6) [Ir +BNPP], 407.2 (100) [IrCp*Br].
3
4
8H, H7+H8), 6.61 (dd, JH,H = 9.4 Hz, JH,H = 2.0 Hz, 2H, H2 or
H6), 6.65 (dd, 3JH,H = 9.4 Hz, 4JH,H = 1.9 Hz, 2H, H2 or H6), 6.94
(dd, 3JH,H = 9.0 Hz, 4JH,H = 1.9 Hz, 2H, H3 or H5), 9.00 (dd, 3JH,H
4
= 8.9 Hz, JH,H = 1.7 Hz, 2H, H3 or H5). 13C NMR (100 MHz,
(CD3)2SO): d = 44.8 (C7+C8), 111.1 (C2+C6), 154.6 (C1), 163.0
◦
(C4).‡ 13C NMR (100 MHz, CD2Cl2, -60 C): d = 43.9 (C7+C8),
109.1 + 109.4 + 110.0 (C2+C5+C6), 141.1 (C3), 153.7 (C1), 162.3
(C4). 13C NMR (solid state): d = 38.6 (CH2), 43.9 (CH2), 45.0
(CH2), 46.3 (CH2), 110.0 (CH), 110.1 (CH), 112.2 (CH), 112.7
[l2 -(1,4-Bis(4-nitrosophenyl)piperazin-N,N¢)bis((g5 -pentameth-
yl-cyclopentadienyl)-diiodido-iridium(III))] (7c). Reagents: 118
mg (0.102 mmol) (5c), 30.2 mg (0.102 mmol) (4). Reaction time:
26 h. Yield: 107 mg (0.073 mmol, 72%), green powder. Black
‡ The signal of C3/C5 is missing in solution at room temperature due to
rotation of the NO-group.
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crystals (rods) were obtained by slow isothermic diffusion of n-
pentane into a nearly saturated solution of 7c in dichloromethane.
Anal. Calc. for C36H46I4Ir2N4O2 (1458.828 g mol-1): C, 29.64; H,
3.18; N, 3.84. Found: C, 29.28; H, 3.42; N, 3.75%. IR: nmax/cm-1
(KBr) 3084w, 2985w, 2960w, 2910w, 2870w, 1590vs, 1539w, 1518
m, 1452w, 1410 m, 1378 m, 1347 m n(N O), 1325 m, 1298 s,
1244 m, 1217 m, 1127vs, 1022 s, 985w, 941w, 871 m, 824w, 722 m,
640w, 621w, 591w. 13C NMR (solid state): d = 12.5 (CH3-C5Me5),
45.5 (C7+C8), 90.3 (Cq-C5Me5), 111.3 + 123.8 (C2+C5+C6), 140.8
(C3), 155.2 (C1), 166.2 (C4). MS (FAB+): m/z 1459.0 (<1%) [M+
+H], 1331.2 (<1) [M+ -I], 1204.2 (<1) [M+ -2I], 877.2 (<1)
[M+ -IrCp*I2 +H], 751.3 (14) [M+ -IrCp*I2 -I], 735.3 (<1) [M+
-IrCp*I2 -I -O], 623.4 (<1) [M+ -IrCp*I2 -2I -H], 455.2 (100)
[IrCp*I].
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[l2 -(1,4-Bis(4-nitrosophenyl)piperazin-N,N¢)bis(dichlorido-(g5 -
pentamethyl-cyclopentadienyl)-rhodium(III))]
(8a). Reagents:
81.6 mg (0.132 mmol) (6a), 39.1 mg (0.132 mmol) (4). Reaction
time: 74 h. Yield: 109 mg (0.119 mmol, 90%), brown powder. Red
crystals (platelets) were obtained by slow isothermic diffusion of
n-pentane into a nearly saturated solution of 8a in DMF. Anal.
Calc. for C36H46Cl4N4O2Rh2 (914.399 g mol-1): C, 47.29; H, 5.07;
N, 6.13. Found: C, 46.78; H, 5.10; N, 5.87%. IR: nmax/cm-1 (KBr)
3056w, 2987w, 2963w, 2916w, 2868w, 1597vs, 1535 m, 1521 s, 1442
m, 1414 m, 1368 m, 1349 s n(N O), 1327vs, 1304vs, 1284 s, 1262
s, 1215vs, 1149vs, 1116vs, 1026 s, 986 m, 942 m, 821 m, 718 m, 645
m, 619w, 559w. 13C NMR (solid state): d = 10.4 (CH3-C5Me5),
43.5 + 44.6 + 48.0 (C7+C8), 94.3 + 98.8 (Cq-C5Me5), 107.9 + 110.4
+ 114.8 + 122.6 (C2+C5+C6), 137.6 (C3), 139.3 (C3), 153.5 (C1),
156.4 (C1), 162.3 (C4), 163.0 (C4). MS (FAB+): m/z 842.4 (<1%)
[M+ -2Cl], 807.3 (<1) [M+ -3Cl], 673.1 (10) [M+ -Cp* -3Cl],
637.1 (1) [M+ -Cp* -4Cl], 604.1 (1) [M+ -RhCp*Cl2], 273.2 (100)
[RhCp*Cl].
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In the present paper the synthesis, characterization and crystal
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nitrosophenyl)piperazine (4) as bridging ligand are reported.
These are the first structurally characterized complexes of BNPP
reported. Additionally X-ray analyses were also carried out of 4
and its precursor 1,4-diphenylpiperazine (3). There is only one
earlier paper that claims the coordination of 4 to a metal center,
but in contrast to our results via only one nitroso group.25 In
view of the convenient synthesis of pure BNPP reported here
and the crystallographic proof of stable complexes with BNPP
coordinated on both sides, completely new fields of applications
for example in metal organic frameworks arise. First experiments
carried out with simple metallic salts already show promising
results and will be published soon.
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Acknowledgements
Financial support by the Ludwig-Maximilians University Munich
and the Dr Klaus Ro¨mer Foundation is gratefully acknowledged.
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