Nickel(II)-complexes of aliphatic and aromatic nitro compounds: Preparation and crystal structures of [Ni2L(μ1,3-O 2NC6H4O)][ClO4] and [Ni 2L(μ1,3-O2P(OC<su

Article abstract of DOI:10.1002/zaac.200800407

The dinuclear Ni^II complex [Ni₂LCl][ClO₄] (1[ClO₄]), where L^2- is a 24-membered macrocyclic hexaamine-dithiophenolate ligand, has been examined regarding its ability to coordinate organic nitro compou

Full text of DOI:10.1002/zaac.200800407

ARTICLE
DOI: 10.1002/zaac.200800407
Nickel(II)-Complexes of Aliphatic and Aromatic Nitro Compounds:
Preparation and Crystal Structures of [Ni2L(μ1,3-O2NC6H4O)][ClO4] and
[Ni2L(μ1,3-O2P(OC6H4NO2)2)][BPh4] (L ؍ macrocyclic hexaamine-
dithiophenolate ligand)
Gunther Steinfeld[a] and Berthold Kersting*[a]
Dedicated to Professor Gerald Henkel on the Occasion of His 60th Birthday
Keywords: Nitro compounds; Nickel; Crystal structure
Abstract. The dinuclear NiII complex [Ni2LCl][ClO4] (1[ClO4]),
where L2Ϫ is
24-membered macrocyclic hexaamine-dithio-
phenolate ligand, has been examined regarding its ability to
coordinate organic nitro compounds (i.e. para-nitrophenolate,
nitromethane, and bis(para-nitrophenyl)phosphate). Complex
1[ClO4] reacts with para-nitrophenolate in methanol to produce
ligand in its aci-nitro resonance form. The reaction of 1[ClO4] with
nitromethane in basic solution affords the known nitrite complex
[Ni2L(μ1,2-NO2)][ClO4] (3[ClO4]), rather than an aci-nitro-
methanid complex [Ni2L(μ1,2-O2NϭCH2)]ϩ (4). The reaction
a
of
1[ClO4]
with
bis(para-nitrophenyl)phosphate
yielded
[Ni2L(μ1,3-O2P(OC6H4NO2)2)]ϩ (5), which reveals a μ1,3-bridging
[Ni2L(μ1,3-O2NC6H4O)][ClO4] (2[ClO4]), the crystal structure bis(para-nitrophenyl)phosphate-ligand.
determination of which reveals a bridging para-nitrophenolate
try of this macrocyclic ligand, we decided to prepare and
Introduction
characterize [M2L(μ-LЈ)]ϩ complexes bearing organic nitro
compounds as coligands.
In previous work, we have described a series of macro-
cyclic dinickel complexes of the type [Ni2L(μ-LЈ)]nϩ bearing
Herein we describe the synthesis, isolation and
characterization of two novel dinickel complexes,
various oxoanions of main-group elements as coligands [1]
Ϫ
(Figure 1). These include complexes with LЈ ϭ NO2
,
namely [Ni2L(μ1,3-O2NC6H4O)]ϩ (2) and [Ni2L(μ1,3
-
Ϫ
Ϫ
Ϫ
Ϫ
2Ϫ
NO3 [2], HCO3 [3], RCO2 [4, 5], H2PO4 [3], SO4
O2P(OC6H4NO2)2)]ϩ (5), which are coligated by nitro-
phenolate and bis(para-nitrophenyl)phosphate ligands. We
also describe the [Ni2LCl]ϩ mediated nitrite release
from nitromethane to give the known nitrito complex
[Ni2L(μ1,2-NO2)]ϩ (3). A survey of the literature reveals
little is known of such complexes [7, 8].
and ClO4Ϫ [6]. To further develop the coordination chemis-
Results and Discussion
The red microcrystalline complex [Ni2L(μ1,3-O2NC6H4O)]-
[ClO4] (2[ClO4]) was obtained in good yields by the reaction
of the chlorido-briged complex [Ni2L(μ-Cl)][ClO4] (1[ClO4])
with para-nitrophenolate (prepared in situ from para-nitro-
phenol and triethylamine) in methanol, followed by
addition of an excess of LiClO4. The UV/Vis spectrum of
2[ClO4] in MeCN reveals an intense absorption at 467 nm,
most likely a π-π* transition of the coordinated para-nitro-
phenolate group (free para-nitrophenolate absorbs at
390 nm [9]). The other two bands at 647 and 1056 nm are
Figure 1. Complexes described in this study and their labels.
attributable to the two spin-allowed d-d transitions 3A2g
Ǟ
* Prof. Dr. B. Kersting
E-Mail: b.kersting@uni-leipzig.de
[a] Institut für Anorganische Chemie der Universität Leipzig
Johannisallee 29
3
3
3T1g and A2g Ǟ T2g, respectively, of an octahedral nickel-
(II) ion. In the IR spectrum of 2[ClO4], the bands at 1343
and 1157 cmϪ1 can be assigned to the asymmetric and sym-
04103 Leipzig, Germany
260
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Z. Anorg. Allg. Chem. 2009, 635, 260Ϫ264

Nickel(II)-Complexes of Aliphatic and Aromatic Nitro Compounds
metric stretching modes of a μ1,3-bridging RNO2 group [2]. C(42)ϪO(3) and C(39)ϪN(7) distances and the coplanarity
Crystals of 2[ClO4]·4MeOH picked from the reaction of the planes through the Ni2O2N chelate ring and the C6
mixture are monoclinic, space group P21/n. Figure 2 dis- ring. It should be noted however that the two N-O distances
plays the structure of the [Ni2L(μ1,3-O2NC6H4O)]ϩ cation. are different at 1.323(3) and 1.281(3) A. Hence the molecu-
˚
The para-nitrophenolate ion bridges the two Ni ions via its lar structure of 2 indicates also a contribution of the non
NO2 group in a symmetric fashion. Thus, both NiII ions aci-nitro form. The average NiϪN (2.209(2) A) and NiϪS
˚
˚
are six-coordinate with three N and two S atoms from the (2.4503(9) A) distances show no unsual features and com-
supporting ligand L2Ϫ and one O(nitro) atom of the para- pare well with those reported for the nitrato-bridged com-
nitrophenolate moiety.
plex [Ni2L(μ1,3-O2NO)]ϩ (6, Figure 3) [2]. Complex 2 has
relatively long NϪO distances (1.302(3) A). The corre-
˚
sponding distances in 6 are significantly shorter (1.261(7)).
˚
The average NiϪO distances (2.107(2) A) in 2 are also sig-
˚
nificantly longer than in 6 averaging 2.071(4) A, indicating
that the para-nitrophenolate interacts less strongly with the
divalent nickel(II) ions than the nitrate ion. The structure
is further stabilized by intermolecular hydrogen bonding in-
teractions involving the MeOH solvate molecules and the
˚
carbonyl oxygen atom O(3) (O(3)···O(8) ϭ 2.693(4) A,
˚
O(8)···O(9) 2.714(4) A).
Figure 2. Molecular structure of the [Ni2L(μ1,3-O2NC6H4O)]ϩ
cation 2 in crystals of 2[ClO4]·4MeOH (30 % probability thermal
ellipsoids). All hydrogen atoms except those involved in hydrogen
bonding interactions (indicated by the dashed lines) are omitted
for clarity.
˚
Selected bond lengths /A and angles /°: Ni1ϪO1 2.127(2), Ni1ϪN1 2.297(2),
Ni1ϪN2 2.124(2), Ni1ϪN3 2.215(2), Ni1ϪS1 2.4588(8), Ni1ϪS2 2.4290(9),
Ni2ϪO2 2.087(2), Ni2ϪN4 2.195(2), Ni2ϪN5 2.127(2), Ni2ϪN6 2.296(2),
Ni2ϪS1 2.4955(10), Ni2ϪS2 2.4179(9), Ni1···Ni2 3.494(1), O1-N7-C39
123.8(2), O1-N7-O2 116.9(2), C39-N7-O2 118.0(2).
Figure 3. Comparison of bond lengths and angles in [Ni2L(μ1,3
O2NC6H4O)]ϩ (2) and [Ni2L(μ1,3-O2NO)]ϩ (6) [2].
-
The p-nitrophenolate ion can exist in two resonance
forms, the nitro form and the aci-nitro form [10, 11]. The
nitro-form is generally more stable than its aci-nitro tau-
tomer, since formation of the latter is accompanied by an
unfavourable dearomatization [12].
A number of metal complexes with nitrophenolate li-
gands have been reported in the literature [13, 14]. As far
as we are aware, 2[ClO4] is the first discrete Ni2 complex
with a μ1,3-coordinated nitroarene moiety in the aci-nitro
form [15].
In view of the above findings, the coordination of the
nitromethanid anion (pKs(CH3NO2) ϭ 10.2) was sought
(Scheme 1). Indeed, when a mixture of 2[ClO4] and triethyl-
amine was heated in a CH3NO2/CH3OH solvent system, a
gradual color change from yellow to green took place over
the course of a few minutes. A green product could be iso-
lated in 80 % yield upon addition of an excess of LiClO4
In the present case, the observed non-equivalence of the and filtration. This material, however, turned out to be the
CϪC bond lengths (see Figure 3 for the individual values) known nitrito complex [Ni2(μ1,2-NO2)][ClO4] (3[ClO4]) [2]
indicates a considerable contribution of the aci-nitro reson- (by virtue of identical spectroscopic IR und UV/Vis data)
ance form. This is further supported by the short and not the desired nitromethanid complex “[Ni2L(μ1,3
-
Z. Anorg. Allg. Chem. 2009, 260Ϫ264
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de 261

G. Steinfeld, B. Kersting*
ARTICLE
NO2CH2)]ϩ” (4). Attempts to access the nitromethanide
complex 4 by halide abstraction from 2[ClO4] with
Pb(ClO4)2 in nitromethane also invariably led to the nitrito
complex 3[ClO4].
Scheme 2. Preparation of the bis(para-nitrophenyl)phosphato com-
plex 5.
˚
lar to those in 2. The Ni···Ni distance is at 3.577(1) A. The
coordination about the phosphorus atom is not perfectly
tetrahedral. The mean PϪO (1.480(4) A) and PϪOR
˚
Ϫ
Scheme 1. Attempted preparation of the CH2ϭNO2 complex 4.
˚
˚
(1.614(4) A) bond lengths differ by ca. 0.13 A and the O-P-
O bond angles range from 101.2(2) to 120.5(3)°. These val-
ues are very similar to those of the dihydrogenphosphato-
bridged complex 7 [3]. It is also worth mentioning that the
Precedence for nitrite elimination from nitroalkanes exist
in the literature. Nitromethane, for example, has been re-
ported to decompose in the presence of a base to form the
NO2 anion [16]. The formation of the nitrite complex 3 is
thus most likely a matter of trapping this species from solu-
˚
N-O distances in 5 (mean 1.229(9) A) are significantly
shorter than in 2 (1.302(4) A), an effect that can be traced
Ϫ
˚
to the presence of the aci-nitro form in 2.
tion.
We finally examined the reaction of 1[ClO4] with
bis(para-nitrophenyl)phosphate. In principle, this ligand
could bind either via its NO2 function or via its phosphate
group to the [Ni2L]2ϩ fragment [3] (Scheme 2). The reaction
of 1[ClO4] with sodium bis(para-nitrophenyl)phosphate
proceeded smoothly producing a green solution, from
which after addition of LiClO4 a green microcrystalline
solid of composition [Ni2L(μ1,3-O2P(OC6H4NO2)2)][ClO4]
(5[ClO4]) was obtained in good yields. A crystal structure
determination was undertaken using crystals of 5[BPh4]
grown by recrystallization from MeOH. The solid state
structure confirmed unambiguously the formulation of the
product as a phosphato-bridged complex as represented in
Figure 4. From this it can be concluded that neutral R-NO2
groups have only a weak affinity for the [Ni2L]2ϩ fragment,
and that such coligands will be easily replaced by many
other weakly donating species (which include common sol- Figure 4. Molecular structure of the [Ni2L(O2P(O-C6H4NO2)2)]ϩ
cation in crystals of 5[BPh4]·MeOH (30 % probability thermal
ellipsoids). Hydrogen atoms are omitted for clarity.
vents).
The structure of 5[BPh4]·MeOH consists of [Ni2L(μ1,3
-
O2P(OC6H4NO2)2)]ϩ complex cations and MeOH solvate
molecules. There are no intermolecular interactions be-
tween the components. Figure 4 provides an ORTEP view
of the structure of the cation 5. The macrocycle adopts a
bowl-shaped C2v-symmetric conformation, as observed in
2. Each nickel atom is coordinated by two S and three N
˚
Selected bond lengths /A: Ni1ϪO1 2.051(4), Ni1ϪN1 2.219(5), Ni1ϪN2
2.136(5), Ni1ϪN3 2.258(5), Ni1ϪS1 2.5042(17), Ni1ϪS2 2.4459(18),
Ni2ϪO2 2.050(4), Ni2ϪN4 2.202(5), Ni2ϪN5 2.150(5), Ni2ϪN6 2.289(5),
Ni2ϪS1 2.4447(18), Ni2ϪS2 2.4910(18), Ni1···Ni2 3.577(1), P1ϪO1
1.477(4), P1ϪO2 1.482(4), P1ϪO3 1.603(4), P1ϪO4 1.624(5), O5ϪN7
1.216(9), O6ϪN7 1.237(9), O7ϪN8 1.231(10), O8ϪN8 1.232(10).
atoms from the supporting ligand and an O atom of a μ1,3
-
Experimental Section
bridging phosphodiester in a severely distorted octahedral
fashion. The distortions from the ideal octahedral coordi-
nation are manifested in the cis and trans L-Ni-L bond
angles, which deviate by as much as 17.5° from their ideal
Unless otherwise noted the preparations of the metal complexes
were carried out under an argon atmosphere by using standard
Schlenk techniques. The starting material [Ni2LCl][ClO4] (1[ClO4])
values. The Ni-metal ligand bond lengths in 5 are very simi- was prepared as described in the literature [17]. All other reagents
262 www.zaac.wiley-vch.de
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2009, 260Ϫ264

Nickel(II)-Complexes of Aliphatic and Aromatic Nitro Compounds
were purchased from commercial sources. Elemental analyses were
performed on a Vario EL analyzer (Elementaranalysensysteme
GmbH). IR spectra were taken on a Bruker VECTOR 22 FT-IR-
spectrophotometer as KBr pellets. Electronic absorption spectra
were recorded on a JASCO V670 UV/vis/near spectrometer.
Crystal Structure Determinations
Single crystals of 2[ClO4]·4MeOH were picked from the reaction
mixture. Crystals of 5[BPh4]·MeOH were grown by recrystalliza-
tion from MeOH. The data sets were collected at 210(2) K on a
Bruker AXS X-ray diffractometer using graphite-monochromated
˚
Mo-Kα radiation (λ ϭ 0.71073 A). The data were processed with
Caution! Perchlorate salts of transition metal complexes are hazard-
ous and may explode. Only small quantities should be prepared and
great care taken.
SAINT [18] and corrected for absorption using SADABS [19]. The
structures were solved by direct methods [20] and refined by full-
matrix least-squares on the basis of all data against F2 using
SHELXL-97 [21]. PLATON was used to search for higher sym-
metry [22]. All non-hydrogen atoms were refined anisotropically,
except for the C and O atoms of the disordered MeOH solvate
molecules. H atoms were placed at calculated positions and refined
as riding atoms with isotropic displacement parameters.
Preparation of [Ni2L(μ1,3-O2N-C6H4O)][ClO4] (2[ClO4]): To a solu-
tion of [Ni2LCl][ClO4] (1[ClO4]) (92 mg, 0.10 mmol) in methanol
(30 mL) was added solid para-nitrophenol (16.7 mg, 0.120 mmol),
followed by a solution of NEt3 (20.3 mg, 0.200 mmol) in MeOH
(2 mL) to produce an immediate color change from yellow to red.
After stirring for 30 min at r.t. solid LiClO4 ·3H2O (160 mg,
1.00 mmol) was added to give a red solid which was isolated by
filtration, washed with little methanol (5 mL) and dried in vacuum.
Yield: 76 mg (74 %). M.p. 289-291 °C (decomposition). IR (KBr):
ν/cmϪ1: 2961, 2866 (C-H), 1617, 1596, 1343, 1157 (NO2), 1097vs
ν3(ClO4). UV/Vis (CH3CN): λmax/nm (ε/MϪ1cmϪ1): 467 (2900), 647
(36), 1056 (50). Elemental analysis calcd. for C44H68ClN7Ni2O7S2
(1024.03): C 51.61, H 6.69, N 9.57, S 6.26; found: C 51.87, H 7.02,
N 9.34, S 6.42 %. This compound was additionally characterized
by X-ray crystallography.
2[ClO4]·4MeOH: C48H84ClN7Ni2O11S2 (1152.21 g·molϪ1), mono-
clinic, space group P21/n, a ϭ 15.706(3), b ϭ 16.033(3), c ϭ
3
˚
˚
22.209(4) A, β ϭ 93.10(3)°, V ϭ 5584.4(18) A , Z ϭ 4, ρ ϭ
1.370 g·cmϪ3, μ(Mo-Kα) ϭ 0.858 mmϪ1, crystal size: 0.28 ϫ 0.22
ϫ 0.10 mm. Of 35496 reflections (Rint ϭ 0.0309), 13586 were un-
ique and 8985 observed (I > 2σI). Two of the MeOH solvate mol-
ecules were found to be disordered over two sites. The site occu-
pancy factors were refined as follows: 0.61(1)/0.39(1) for
O(10a)C(47a)/O(10b)C(47b) and 0.39(1)/0.61(1) for O(11a)C(48a)/
O(11b)C(48b). The C and O atoms of these two solvate molecules
were refined isotropically. Final residuals: R1 ϭ 0.0432, wR2
0.1181 (for observed data), R1 ϭ 0.0760, wR2 ϭ 0.1389 for all data.
ϭ
Preparation of [Ni2L(μ1,2-NO2)][ClO4] (3[ClO4]). Method A: To a
solution of complex [Ni2LCl][ClO4] (92 mg, 0.10 mmol) in a
CH3NO2/MeOH (1:1) solvent system (30 mL) was added triethyl-
amine (20.2 mg, 0.20 mmol), and the mixture was heated under re-
flux for 10 min to produce a color change from yellow to green.
After the mixture was cooled to room temperature, solid
LiClO4 ·3H2O (160 mg, 1.00 mmol) was added. The solution was
stirred for further 2 h during which time an olive-green solid pre-
cipitated. The solid was isolated by filtration, washed with little
methanol and dried in vacuum. Yield: 73 mg (78 %). IR (KBr):
ν/cmϪ1: 1183 ν(NO2Ϫ), 1094vs ν(ClO4Ϫ), 624 ν(ClO4Ϫ); UV/Vis
(CH3CN); λmax/nm (ε/MϪ1cmϪ1): 623 (42), 1104 (63). Elemental
analysis calcd. for C38H64ClN7Ni2O6S2 (929.28): C 48.97, H 6.92,
N 10.52, S 6.88; found: C 48.76, H 6.84, N 10.57, S 6.72 %. Method
B: To a solution of [Ni2LCl][ClO4] (92 mg, 0.10 mmol) in CH3NO2
(10 mL) was added solid Pb(ClO4)2 (20.5 mg, 0.50 mmol). The re-
sulting green solution was filtered off from PbCl2 and left to stand
in a closed vessel for several days. Green crystals of 3[ClO4] were
isolated by filtration. Yield: 56 mg (60 %). The analytical data of
this compound are identical with those of 3[ClO4] prepared by
method A.
5[BPh4]·MeOH: C75H96BN8Ni2O9PS2 (1476.92 g·molϪ1), mono-
clinic, space group P21/n, a ϭ 14.102(2), b ϭ 16.913(2), c ϭ
3
˚
˚
32.513(4) A, β ϭ 95.083(3), V ϭ 7723.7(19) A , Z ϭ 4, ρ ϭ
1.270 g·cmϪ3, μ(Mo-Kα) ϭ 0.621 mmϪ1, crystal size: 0.33 ϫ 0.25
ϫ 0.18 mm. Of 23781 reflections (Rint ϭ 0.0669), 7207 were unique
and 4450 observed (I > 2σI). The MeOH solvate molecule was
found to be disordered over two sites. The site occupancy factors
were fixed at 0.50 and the C and O atoms were refined isotropically.
Final residuals: R1 ϭ 0.0629, wR2 ϭ 0.1758 (for observed data),
R1 ϭ 0.1032, wR2 ϭ 0.1901 for all data.
CCDC-693320 (for 2[ClO4]) and CCDC-693646 (for 5[BPh4]) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data request/cif.
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Received: July 4, 2008
Accepted: September 23, 2008
264
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Z. Anorg. Allg. Chem. 2009, 260Ϫ264