Metal(II) complexes of the carbonylnitronate ligands O2NCHC(O)R- (R = Ph, OMe or OEt) and X-ray structure of (DMANH)[Ni(O2NCHC(O)Ph)3] (DMAN = 1,8-bis(dimethylamino)naphthalene)

Article abstract of DOI:10.1016/S0020-1693(98)00256-4

New coordination compounds involving carbonylnitronate ligands O₂NCHC(O)R^- (R = Ph, OMe or OEt) are prepared by a synthetic procedure based on the reaction of a nickel(II) or palladium(II) salt with benzoylnitromethane or alkyl nitroacetate in the presence of a deprotonating agent. The use of the proton sponge 1,8-bis(dimethylamino)naphthalene (DMAN) in tetrahydrofuran affords, in high yields, the anionic complexes (DMANH)[Ni(O₂NCHC(O)R)₃] as brilliant green solids. The benzoylnitronate complex (R = Ph) crystallises in a monoclinic unit cell [space group P2₁/n, a = 13.061(2), b = 16.415(2), c = 17.827(2) A, β = 102.7(2)°], which contains four octahedral anionic nickel complexes each close to a big DMANH⁺ cation. One nitronate ligand is easily replaced by two molecules of ethanol to give the neutral complexes [Ni(O₂NCHC(O)R)₂(EtOH)₂]. The nickel is O,O′-coordinated to the ligand through one carbonyl and one nitro oxygen atom. A different coordination mode, in which only the nitro group is bonded to the metal, is, however, observed in the palladium complexes [PdCl(O₂NCHC(O)R)]₂ (R = OMe, OEt).

Full text of DOI:10.1016/S0020-1693(98)00256-4

Inorganica Chimica Acta 285 (1999) 18±24
Metal(II) complexes of the carbonylnitronate ligands
O2NCHC(O)R (R ˆ Ph, OMe or OEt) and X-ray
structure of (DMANH)[Ni(O2NCHC(O)Ph)3]
(DMAN ˆ 1,8-bis(dimethylamino)naphthalene)
a,
a
a
b
c
Ã
Marino Basato , Nicola Detomi , Marco Meneghetti , Giovanni Valle , Augusto C. Veronese
a
Á Á
Centro di Studio sulla Stabilita e Reattivita dei Composti di Coordinazione, CNR, Dipartimento di Chimica Inorganica, Metallorganica e Analitica,
University of Padova, via Marzolo 1, 35131 Padua, Italy
b Centro di Studio sui Biopolimeri, CNR, Dipartimento di Chimica Organica, University of Padova, via Marzolo 1, 35131 Padua, Italy
c Dipartimento di Scienze Farmaceutiche, University of Ferrara, via Fossato di Mortara 17, 44100 Ferrara, Italy
Received 12 March 1998; received in revised form 4 May 1998; accepted 13 May 1998
Abstract
New coordination compounds involving carbonylnitronate ligands O2NCHC(O)R (R ˆ Ph, OMe or OEt) are prepared by a synthetic
procedure based on the reaction of a nickel(II) or palladium(II) salt with benzoylnitromethane or alkyl nitroacetate in the presence of a
deprotonating agent. The use of the proton sponge 1,8-bis(dimethylamino)naphthalene (DMAN) in tetrahydrofuran affords, in high yields,
the anionic complexes (DMANH)[Ni(O2NCHC(O)R)3] as brilliant green solids. The benzoylnitronate complex (R ˆ Ph) crystallises in a
Ê
monoclinic unit cell [space group P21/n, a ˆ 13.061(2), b ˆ 16.415(2), c ˆ 17.827(2) A, ꢀ ˆ 102.7(2)8], which contains four octahedral
‡
anionic nickel complexes each close to a big DMANH cation. One nitronate ligand is easily replaced by two molecules of ethanol to give
the neutral complexes [Ni(O2NCHC(O)R)2(EtOH)2]. The nickel is O,O0-coordinated to the ligand through one carbonyl and one nitro
oxygen atom. A different coordination mode, in which only the nitro group is bonded to the metal, is, however, observed in the palladium
complexes [PdCl(O2NCHC(O)R)]2 (R ˆ OMe, OEt). # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Nickel complexes; Palladium complexes; Carbonylnitronato complexes
1. Introduction
sphere) plays an important role, as expected for a mechan-
ism in which the ®rst stage of the catalytic cycle is repre-
sented by coordination to the metal centre of the anion of the
C±H acid substrate. The O,O0 coordination, in which the
anion acts as a bidentate ligand in a six-member chelate ring,
is common with b-carbonylenolates, as shown by the very
extensive studies, in particular, on the acetylacetonato com-
plexes [5±11]. By contrast, the coordinating ability of
carbonylnitronates has been checked only for the anion of
a-nitroketones, with the exclusion of nitroesters [12]. This is
rather surprising as, for example, Michael additions of a-
nitroesters are often catalysed by transition metal centres
[13,14].
In consideration of the intrinsic interest in these types
of complexes and of their catalytic relevance, we report
here the synthesis and characterisation of a series of new
nickel(II) and palladium(II) complexes with the nitro
ligands O2NCHC(O)R (R ˆ Ph, OMe or OEt), having
either the keto or ester function.
In the frame of our extensive work on the reactivity of C±
H acid compounds towards nitriles, it has been found that the
metal-catalysed C±C bond forming reaction
Mn‡
RCꢀO†CH2Z ‡ R0CN ! RCꢀO†ꢀZ†CˆCꢀR0†NH2
is not limited to b-dicarbonyls (Z ˆ RCO), but is also
characteristic of other pronucleophiles as b-ketoamides
(Z ˆ RHNCO), b-ketophosphonates or phosphonoacetates
(Z ˆ PO(OR)2) [1±3].
We have found recently that the same reaction can be
extended to benzoylnitromethane and alkyl nitroacetates [4].
The nature of the catalyst (type of metal and of coordination
*Corresponding author. Tel.: 39-49-827-5217; fax: 39-49-827-5223;
e-mail: basato@chim02.unipd.it
0020-1693/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(98)00256-4

M. Basato et al. / Inorganica Chimica Acta 285 (1999) 18±24
19
2. Experimental
drous nickel chloride with the nitroester and triethylamine
1:2:2 molar ratio.
2.1. General
[Ni(O2NCHC(O)OMe)2(EtOH)2] (5): NiCl2 (0.31 g,
2.3 mmol) suspended in anhydrous ethanol (20 cm3) was
treated with an ethanol solution of O2NCH2C(O)OMe
(0.43 cm3, 4.6 mmol). To this suspension was added drop-
wise a solution of freshly distilled Et3N (0.65 cm3,
4.6 mmol) in ethanol (20 cm3). The mixture was stirred
for 3 days and the unreacted nickel chloride ®ltered. The
resulting green solution was slowly concentrated in vacuo
and the precipitate washed with ethyl ether, yield 53%, m.p.
1168C (Found: C, 31.64; H, 5.47; N, 7.39. C10H20N2NiO10
requires: C, 31.04; H, 5.21; N, 7.24%). Paramagnetic,
Infrared spectra were recorded on a Bruker IFS 66
1
FT; H and 13C NMR spectra on a Jeol FX 90Q FT or a
Bruker AC (200 MHz) spectrometer. The high-resolution
solid-state 13C measurements were performed on a
Bruker AM 250 with the setting conditions described
in Ref. [15]. Thermal analyses employed the Perkin-
Elmer TGS-2 equipment. The reagents were high
purity products and used as received. Solvents were
dried before use and the reaction apparatus carefully
deoxygenated. Reactions were performed under argon
at room temperature and all operations were under an
inert atmosphere.
1
ꢁ ˆ 3.3 BM; ꢂ~max=cm (KBr) 3325 s [ꢂ(O±H)], 1648 s
[ꢂ(C O)], 1469 s, 1375 w, 1309 s, 1202 s, 1179 s, 1069 s,
966 m, 944 m, 776 s. It loses the two molecules of ethanol
between 908C and 1508C.
2.2. Syntheses
[Ni(O2NCHC(O)OEt)2(EtOH)2] (6): NiCl2 (0.36 g,
2.7 mmol) suspended in anhydrous ethanol (20 cm3) was
treated with an ethanol solution of O2NCH2C(O)OEt
(0.63 cm3, 5.5 mmol). To this suspension was added drop-
wise a solution of freshly distilled Et3N (0.77 cm3,
5.5 mmol) in ethanol (20 cm3). The reaction mixture was
stirred for 3 days and the unreacted nickel chloride ®ltered.
The resulting green solution was slowly concentrated in
vacuo and the precipitate washed with ethyl ether, yield
65%, m.p. 1128C (Found: C, 35.06; H, 5.72; N, 6.87.
C12H24N2NiO10 requires: C, 34.73; H, 5.83; N, 6.75%).
Paramagnetic, ꢁ ˆ 3.3 BM; ꢂ~max=cm 1 (KBr) 3360 s [ꢂ(O±
H)], 1636 s [ꢂ(C O)], 1480 s, 1376 m, 1312 s, 1184 s, 1062
s, 953 m, 767 s. It loses the two molecules of ethanol
between 908C and 1508C.
Synthesis of complexes (DMANH)[Ni(O2NCHC(O)R)3]
(1±3): The starting nickel(II) compound (5 mmol, usually
anhydrous NiCl2) was suspended in THF (30 cm3); the
ligand O2NCH2C(O)R (7.5 mmol) and the deprotonating
agent DMAN (7.5 mmol) in THF (30 cm3) were added
dropwise and the mixture kept under stirring for 5 days.
The blue by-product (DMANH)2[NiCl4] was ®ltered, the
solution was concentrated and treated with ether to give the
complex (DMANH)[Ni(O2NCHC(O)R)3] as a green solid.
Shorter reaction times were needed starting from the more
soluble [NiCl2(PPh3)2]; in this case the by-product was
mainly (DMANH)[NiCl3(PPh3)].
(DMANH)[Ni(O2NCHC(O)Ph)3] (1): Green crystals,
yield 95%, m.p. 1898C (Found: C, 59.00; H, 4.85; N,
9.03. C38H37N5NiO9 requires: C, 59.5; H, 4.85; N,
The alternative procedure (b) involves dissolution of
complexes 1±3 in acetone and addition of ethanol.
[Ni(O2NCHC(O)Ph)2(EtOH)2] (4) and [Ni(O2NCDC(O)-
Ph)2(EtOD)2] (4-d): Complex 1 (0.38 g, 0.50 mmol) was
dissolved in the minimum quantity of acetone (15 cm3),
then ethanol (20 cm3) was added causing a green precipitate
of 4, yield 96%; ꢂ~max=cm 1 (KBr) 3312 s [ꢂ(O±H)], 1591 m
[ꢂ(C O)], 1547 s, 1500 m, 1467 s, 1438 m, 1353 m
[ꢂ(NO2)], 1300 s,br [ꢂ(NO2)], 1226 m, 1113 w, 1049 w,
975 m, 903 m, 775 w, 701 w. The same reaction conducted
by adding C2H5OD gave the deuterated complex 4-d;
1
9.10%). Paramagnetic, ꢁ ˆ 3.2 BM; ꢂ~max=cm
(KBr)
3340 br [ꢂ(N±H)], 1591 m [ꢂ(C O)], 1549 s, 1497 m,
1463 s, 1358 m [ꢂ(NO2)], 1292 s [ꢂ(NO2)], 1220 m, 976
m, 904 m, 831 w, 766 m, 706 m.
(DMANH)[Ni(O2NCHC(O)OMe)3] (2): Green crystals,
yield 72%, m.p. 1568C (Found: C, 44.91; H, 5.13; N, 10.80.
C23H31N5NiO12 requires: C, 44.37; H, 5.13; N, 11.15%).
Paramagnetic, ꢁ ˆ 3.2 BM; ꢂ~max=cm (KBr) 3340 br
[ꢂ(N±H)], 1630 m [ꢂ(C O)], 1466 s, br, 1364 m, 1306 s,
1177 s, br, 1062 s, 965 s, 840 m, 766 s.
1
1
ꢂ~max=cm (KBr) 2431 m [ꢂ(O±D)], 1589 m [ꢂ(C O)],
(DMANH)[Ni(O2NCHC(O)OEt)3] (3): Green crystals,
yield 66%, m.p. 1578C (Found: C, 46.58; H, 5.52; N,
10.11. C26H37N5NiO12 requires: C, 46.59; H, 5.56; N,
1546 s, 1447 m, 1385 s, br, 1356 m [ꢂ(NO2)], 1284 s
[ꢂ(NO2)], 1172 m, 1050 w, 966 m, 924 m, 873 w, 701 w.
[Ni(O2NCHC(O)OMe)2(EtOH)2] (5) and [Ni(O2NCH-
C(O)OEt)2(EtOH)2] (6): Complex 2 (or 3) (0.5 mmol)
was dissolved in ethanol (5±10 cm3) and diethyl ether
(20 cm3) was slowly added. The slow crystallisation (few
days) afforded small green crystals of 5 (yield 30%) (or of 6,
yield 40%).
Synthesis of complex [Pd(O2NCHCOPh)2] (7): This
complex was prepared by extending to palladium a reported
procedure [12], using acetone instead of ethanol as solvent.
Palladium acetate (0.23 g, 1.0 mmol) and O2NCH2COPh
1
10.54%). Paramagnetic, ꢁ ˆ 3.3 BM; ꢂ~max=cm (KBr)
3340 br [ꢂ(N±H)], 1615 m [ꢂ(C O)], 1470 s, br, 1376 w,
1307 s, 1177 s, br, 1061 s, 966 s, 844 m, 769 s.
Synthesis of complexes [Ni(O2NCHC(O)R)2(EtOH)2]
(4±6): Complex 4 (R ˆ Ph) was prepared by reaction of
nickel acetate with benzoyl nitromethane in ethanol [12].
The same procedure gave no products with the nitroesters
for which two alternative routes were used. The more
straightforward one (a) implies reaction in ethanol of anhy-

20
M. Basato et al. / Inorganica Chimica Acta 285 (1999) 18±24
were dissolved in dry acetone (20 cm3) and left under
stirring for 20 h. The volume was reduced in vacuo and
the yellow±brown precipitate washed with diethyl ether,
yield 72%, m.p. (dec.) 1718C (Found: C, 44.11; H, 2.75; N,
6.37. C16H12N2O6Pd requires: C, 44.21; H, 2.78; N, 6.44%);
2.3. X-ray crystal structure determination of
(DMANH)[Ni(O2NCHC(O)Ph)3]
Suitable crystals of 1 were obtained by slow crystal-
lisation at 48C from a solution in acetone. The X-ray work
was carried out on a Philips four-circle diffractometer with
monochromatic Mo Ka radiation. Crystal data: C38H37N5-
NiO9, M ˆ 766.45, F(000) ˆ 1600, monoclinic, space group
1
ꢂ~max=cm (KBr) 1596 m and 1586 m [ꢂ(C O)], 1511 s,
1468 s, 1393 s [ꢂ(NO2)], 1264 s [ꢂ(NO2)], 1228 m, 1186 w,
1101 w, 1000 w, 942 m, 916 m, 774 w, 694 w; ꢃH (DMSO-
d6) 6.30 and 6.54 (s, 1H, CH), 7.3±8.1 (m, 5H, Ph); ꢃC (solid
state) 116.3 (CH), 128.6, 130.6 and 136.4 (Ph), 183.9
(C O).
Synthesis of complexes [PdCl(O2NCHC(O)R)]2 (8, 9):
They were prepared by reaction of anhydrous palladium
chloride with K(O2NCHC(O)R) in THF.
K(O2NCHC(O)R): An equimolar quantity of the nitro
ester O2NCH2C(O)R and of potassium metal were sus-
pended in anhydrous THF, under vigorous stirring at
258C. After ca. 3 days the potassium disappeared and the
resulting white suspension was ®ltered. The product was
never dried, because, carbonylnitronates of alkali metals are
Ê
P21/n, a ˆ 13.061(2), b ˆ 16.415(2), c ˆ 17.827(2) A, ꢀ ˆ
Ê 3
102.7(2)8, V ˆ 3728(3) A , Dc ˆ 1.37 g cm 3, Z ˆ 4, ꢄ(Mo
Ka) ˆ 0.71069 A,ꢁ ˆ 5.79 cm 1.Intensitiesweremeasured
Ê
by the ꢅ±2ꢅ method up to ꢅ ˆ 288 yielding 8291 unique
re¯ections of which 2702 were signi®cantly above back-
ground [F > 4ꢆ(F)]. The structure was solved by a direct
method using the SIR 92 program [17]. The positions of the
C, N, O and Ni atoms were re®ned with cycles of blocked
least-squaresusingthermalanisotropicparameters.Allhydro-
gen atoms were identi®ed in the difference Fourier map and
introduced with isotropic thermal parameters in the ®nal re-
®nement cycles (SHELX 76). Convergence was reached at
R ˆ 0.071 and Rw ˆ 0.069 (w ˆ 1/[ꢆ2(F) ‡ 0.001154 F2]).
potentially explosive [16]. They were identi®ed from their
1
infrared spectra; K(O2NCHCOOMe): ꢂ~max=cm (KBr)
1676 s [ꢂ(CO)], 1452 s, br, 1279 s, 1142 s, 1071 s, 766
1
m; K(O2NCHCOOEt): ꢂ~max=cm (KBr) 1686 s [ꢂ(CO)],
1472 s, br, 1291 s, 1148 s, 1071 s, 770 m.
3. Results and discussion
[PdCl(O2NCHC(O)OMe)]2Á0.2THF (8): [PdCl2(PhCN)2]
(0.76 g, 2.0 mmol) in THF (15 cm3) was treated with a
solution of K(O2NCHCOOMe) (0.69 g, 4.0 mmol) in THF.
The mixture was left under stirring at room temperature for
4 days and traces of palladium metal ®ltered. The red±
brown solution was evaporated to dryness and the resulting
yellow±brown solid crystallised from THF±ether, yield
95%, m.p. (dec.) 1468C (Found: C, 16.92; H, 1.83; N,
5.27. C3.8H5.6ClNO4.2Pd requires: C, 16.64; H, 2.06; N,
The reaction of a metal(II) salt with benzoylnitromethane
or alkyl nitroacetates gives, in the presence of a deprotonat-
ing agent, complexes in which the monoanion of the nitro
derivative is coordinated to the metal centre. The success of
the synthesis and the nature of the obtained complexes
markedly depend on many variables: the metal centre,
the nitro pronucleophile, the base.
The proton sponge 1,8-bis(dimethylamino)naphthalene
(DMAN) proved to be very ef®cient in reactions with nickel
chloride. In nonalcoholic solvents, as tetrahydrofuran, the
anionic complexes (DMANH)[Ni(O2NCHC(O)R)3] 1±3 are
obtained in good yield, according to Eq. (1).
1
5.11%); ꢂ~max=cm (KBr) 1720 s [ꢂ(C=O)], 1620 w, 1564
w, 1515 s, 1466 m, 1436 m, 1340 s, 1176 s, 1067 m, 907 w;
far infrared (in Nujol, cm 1): 330 br; ꢃH (DMSO-d6) 3.60
(s, 3H, CH3O), 2.81 (s, 1H, CH); ꢃC (solid state) 54.2
(CH3O), ca. 67 (CH), 69.4 (THF), 172.0 (CO). It explodes
violently at temperatures above 1468C.
[PdCl(O2NCHC(O)OEt)]2 (9): Anhydrous PdCl2 (0.36 g,
2.0 mmol) suspended in THF (20 cm3) was treated with a
solution of K(O2NCHCOOEt) (0.69 g, 4.0 mmol) in THF.
The reaction mixture was left under stirring at room tem-
perature for 4 days and the unreacted PdCl2 ®ltered off. The
red±brown solution was evaporated to dryness and the
resulting yellow±brown solid crystallised from THF±ether,
yield 85%, m.p. 1438C (Found: C, 17.71; H, 2.14; N, 4.97.
2NiCl2 ‡ 3O2NCH2CꢀO†R ‡ 3DMAN
! ꢀDMANH†‰NiꢀO2NCHCꢀO†R† Š ꢀ1 3†
3
‡ ꢀDMANH† ‰NiCl4Š
(1)
2
R ˆ Ph ꢀ1†; OMe ꢀ2†; OEt ꢀ3†
So DMAN not only ef®ciently deprotonates the nitro
carbonyl O2NCH2C(O)R, but also acts, in its protonated
form, as countercation. The ef®ciency of the synthesis is
due, at least in part, to the formation of (DMANH)2[NiCl4]
as by-product, which is almost insoluble in the reaction
medium and drives the reaction towards the products. The
molecular structure of the complex (DMANH)[Ni(O2NCH-
C(O)Ph)3] (1) shows an octahedral geometry around the
metal centre. The benzoylnitronate moiety is coordinated to
the nickel(II) through one keto and one nitro oxygen atom
(Fig. 1). The planarity of the ligand (excluding the phenyl
substituent) and the bond distances (Tables 1 and 2, average
values: Ni±O 2.02, C O 1.24, C C 1.40, C N 1.38, N O
C4H6ClNO4Pd requires: C, 17.54; H, 2.21; N, 5.11%);
1
ꢂ~max=cm
(KBr) 1714 s [ꢂ(C=O)], 1616 w, 1565 m,
1516 s, 1472 m, 1324 s, 1187 s, 1092 m, 1033 m, 911 w,
863 w; far infrared (in Nujol, cm 1): 330 br; ꢃH (DMSO-d6)
1.24 (t, 3H, CH3CH2O), 4.14 (q, 2H, CH3CH2O), 2.81 (s,
1H, CH); ꢃC (solid state) 17.4 (CH3CH2O), 62.9
(CH3CH2O), ca. 67 (CH), 173.0 (CO). It explodes violently
at temperatures above 1438C.

M. Basato et al. / Inorganica Chimica Acta 285 (1999) 18±24
21
Fig. 1. ORTEP view of the anionic complex [Ni(O2NCHC(O)Ph)3] .
Ê
1.27, N=O 1.23 A) clearly indicate that electron delocalisa-
tion involves all bonds.
These distances can be compared with those reported
for: [Cu(O2NCHC(O)Ph)2] and its adducts with 2- and
4-methylpyridine [18], cis-dipyridine complexes of [Zn-
(O2NCHC(O)Ph)2] and [Mn(O2NCHC(O)Ph)2] [19,20],
and other chelate complexes having nitro and keto groups
as Ag[Ni(acac)3]Á2AgNO3ÁH2O [21], trans-[Pd(bzac)2]
[22], cis-[Pd(bzac)2] [23,24], trans-[Cu(bzac)2] [25],
K2[Cu(O2NCHCOO)2]ÁH2O [26], [Ni(O2NCHCOO)N4]
(N4 ˆ cyclic tetradentate ligand) [27], (C5H5)Zr(Cl)[O2N=
C(CH3)2] [28], [Li(O2N=CHPh)ÁEtOH]n [29].
Ê
It is seen that the average C±O bond length (1.24 A) in 1
is slightly shorter than in the Cu (1.27), Zn (1.25) and Mn
Ê
(1.25 A) benzoylnitronates. Furthermore, the observed
value is lower than that reported in benzoylacetonato com-
Ê
plexes (1.29 A); thus it appears that the nitro group favours,
compared with the acetyl, an enhanced double bond char-
acter of the benzoyl carbonyl (typical value for C(sp2)=O
Ê
1.21 A) [30,31]. The N±O distance for the metal-coordi-
Ê
nated oxygen has a value (1.27 A) between that found in
Ê
organic nitro compounds (1.22 A) [31] and in alkanenitro-
Ê
nate complexes (1.30±1.34 A, N±O single bond) [28,29]. At
the same time, the N±O bond for the oxygen not involved in
coordination maintains a high degree of double bond char-
Table 1
Ê
Selected bond distances (A) for 1
Ni±O(1)
Ni±O(4)
2.046(7)
2.030(6)
2.019(6)
1.27(1)
1.24(1)
1.23(1)
1.275(9)
1.236(9)
1.41(1)
1.45(1)
1.47(1)
1.49(1)
1.42(1)
1.38(1)
1.41(1)
1.37(1)
1.39(2)
1.41(1)
1.41(1)
1.36(1)
1.35(1)
Ni±O(3)
Ni±O(6)
2.011(5)
1.996(6)
2.016(6)
1.24(1)
1.28(1)
1.25(1)
1.23(1)
1.36(1)
1.38(1)
1.50(1)
1.48(1)
1.50(1)
1.504(9)
1.51(1)
1.501(9)
1.44(1)
1.36(1)
1.42(1)
1.43(1)
1.41(1)
Ni±O(7)
Ni±O(9)
O(1)±N(1)
O(3)±C(2)
O(5)±N(2)
O(7)±N(3)
O(9)±C(18)
N(2)±C(9)
N(4)±C(25)
N(4)±C(36)
N(5)±C(37)
C(1)±C(2)
C(9)±C(10)
C(17)±C(18)
C(25)±C(26)
C(26)±C(27)
C(28)±C(29)
C(29)±C(34)
C(31)±C(32)
C(33)±C(34)
O(2)±N(1)
O(4)±N(2)
O(6)±C(10)
O(8)±N(3)
N(1)±C(1)
N(3)±C(17)
N(4)±C(35)
N(5)±C(31)
N(5)±C(38)
C(2)±C(3)
C(10)±C(11)
C(18)±C(19)
C(25)±C(30)
C(27)±C(28)
C(29)±C(30)
C(30)±C(31)
C(32)±C(33)
Ê
acter (1.23 A). Thus the chelate ring can be sketched as in
(a), whereas (b) and (c) represent the two more important
limit forms (R ˆ Ph).

22
M. Basato et al. / Inorganica Chimica Acta 285 (1999) 18±24
Table 2
Selected bond angles (8) for 1
O(7)±Ni±O(9)
O(6)±Ni±O(7)
88.5(3)
90.1(2)
O(6)±Ni±O(9)
O(4)±Ni±O(9)
94.7(3)
93.4(3)
O(4)±Ni±O(7)
177.0(3)
172.7(3)
92.0(2)
O(4)±Ni±O(6)
87.3(3)
O(3)±Ni±O(9)
O(3)±Ni±O(7)
88.6(2)
O(3)±Ni±O(6)
O(1)±Ni±O(9)
O(3)±Ni±O(4)
O(1)±Ni±O(7)
89.9(3)
90.6(3)
87.2(3)
O(1)±Ni±O(6)
178.0(3)
86.1(3)
O(1)±Ni±O(4)
91.9(3)
O(1)±Ni±O(3)
Ni±O(1)±N(1)
123.5(5)
125.9(5)
126.0(6)
117.8(8)
124.0(8)
120.1(9)
117.6(9)
122.0(8)
112.4(8)
112.1(8)
111.2(7)
126.2(8)
117.1(7)
117.0(7)
127.0(8)
117.2(7)
122.7(6)
124.5(8)
117.8(7)
117.0(6)
122.2(8)
122.7(9)
121.0(9)
119.4(8)
118.4(8)
124.8(8)
121.5(8)
120.2(8)
120.7(8)
Ni±O(3)±C(2)
Ni±O(6)±C(10)
125.1(5)
123.8(6)
126.7(5)
118.2(9)
119.8(8)
120.1(8)
120.2(8)
109.1(7)
111.8(6)
114.1(6)
123.8(9)
116.7(8)
123.0(6)
126.1(9)
115.7(7)
117.2(6)
127.1(9)
117.7(7)
123.0(5)
119.0(8)
118.7(8)
120(1)
Ni±O(4)±N(2)
Ni±O(7)±N(3)
Ni±O(9)±C(18)
O(1)±N(1)±O(2)
O(1)±N(1)±C(1)
O(5)±N(2)±C(9)
O(7)±N(3)±O(8)
O(7)±N(3)±C(17)
C(25)±N(4)±C(36)
C(37)±N(5)±C(38)
C(31)±N(5)±C(37)
O(3)±C(2)±C(1)
O(3)±C(2)±C(3)
C(2)±C(3)±C(4)
O(6)±C(10)±C(9)
O(6)±C(10)±C(11)
C(10)±C(11)±C(12)
O(9)±C(18)±C(17)
O(9)±C(18)±C(19)
C(18)±C(19)±C(20)
N(4)±C(25)±C(26)
C(25)±C(26)±C(27)
C(27)±C(28)±C(29)
C(28)±C(29)±C(30)
C(25)±C(30)±C(29)
C(25)±C(30)±C(31)
C(30)±C(31)±C(32)
C(31)±C(32)±C(33)
C(29)±C(34)±C(33)
O(2)±N(1)±C(1)
O(4)±N(2)±O(5)
O(4)±N(2)±C(9)
O(8)±N(3)±C(17)
C(35)±N(4)±C(36)
C(25)±N(4)±C(35)
C(31)±N(5)±C(38)
N(1)±C(1)±C(2)
C(1)±C(2)±C(3)
C(2)±C(3)±C(8)
N(2)±C(9)±C(10)
C(9)±C(10)±C(11)
C(10)±C(11)±C(16)
N(3)±C(17)±C(18)
C(17)±C(18)±C(19)
C(18)±C(19)±C(24)
N(4)±C(25)±C(30)
C(26)±C(25)±C(30)
C(26)±C(27)±C(28)
C(28)±C(29)±C(34)
C(30)±C(29)±C(34)
C(29)±C(30)±C(31)
N(5)±C(31)±C(30)
N(5)±C(31)±C(32)
C(32)±C(33)±C(34)
‡
Fig. 2. View of complex 1 including the countercation DMANH .
nated carbonyl [10,15], which can be compared with that at
1700 cm 1 found in the free ligand. The identi®cation of the
NO2 stretching bands is less reliable, but it may be proposed
on the basis of the deuteration test made on complex 4 (see
later); thus the asymmetric and symmetric stretching bands
of the ligand at 1555 and 1330 cm 1 become in the complex
two bands at 1358 and 1292 cm 1 corresponding to the N±O
stretching of the uncoordinated and coordinated oxygen.
The marked shift to lower wavenumber for both the carbo-
nyl and nitro groups is indicative of a reduction of their
double bond character as a consequence of coordination to
the metal.
Complexes (DMANH)[Ni(O2NCHC(O)R)3] (2) (R ˆ
OMe) and 3 (R ˆ OEt), derived from methyl and ethyl
nitroacetate respectively, are also easily obtained in good
yield as green microcrystals. Their infrared spectra show
coordination of the carbonyl oxygen of the ester; in fact the
CO stretching frequency moves from 1755 to 1630 (2) and
from 1755 to 1615 (3) [37]. The two medium intensity bands
of the nitro group in the free ligand at 1563 (2), 1563 (3) and
120.3(8)
120.2(8)
116.8(7)
120.2(7)
118.4(8)
120.6(8)
The importance of structure (c) in which a negative charge
is localised on the methine carbon supports our mecha-
nistic proposals on the metal-catalysed C±C bond forming
reaction between C±H acid nitro derivatives and nitriles [4].
The deprotonating agent DMAN is not only a strong base
(pKa 12) [32,33], but also a very hindering molecule and the
big cation, which is formed during the reaction, can favour
the crystallisation of complex 1. Fig. 2 shows that, albeit in
the absence of de®nite bond interactions, the protonated
base lies rather close to the anionic complex, thus stabilising
the crystal structure. As a matter of fact the use of other
deprotonating agents like TlOEt or K metal gives intractable
‡
mixtures. The bond distances and angles in DMANH are
‡
1343 (2), 1335 (3) are absent in the complexes and no
1
absorptions are observable between 1630 and 1480 cm
very similar to those found in DMANH ClM (ClMH ˆ
.
1,2-dichloromaleic acid) [34±36] and, in particular, the
cation is characterised by an unsymmetrical N±HÁ Á ÁN
hydrogen bond, whose geometric parameters [N±H ˆ
Thus, despite the dif®culty of a safe attribution for the NO2
stretchings in the absence of labelling experiments, they
surely have moved to lower energies, supporting for com-
plexes 2, 3 the same type of coordination to the metal, via
one carbonyl and one nitro oxygen atom, fully demonstrated
for the w-nitroacetophenonates.
Ê
0.80(1), NÁ Á ÁH ˆ 1.86(1) A, N±HÁ Á ÁN ˆ 168.7(5)8] ®t well
with ab initio calculations [34].
The infrared spectrum of 1 is consistent with the
molecular structure determined by X-rays and shows, in
Complexes [Ni(O2NCHC(O)R)2(EtOH)2] (4±6) can be
synthesised in more than one way; a clear distinction has to
1
particular, a band at 1591 cm attributable to the coordi-

M. Basato et al. / Inorganica Chimica Acta 285 (1999) 18±24
23
be made between the keto and the ester nitro derivatives. It
has been reported that 4 (R ˆ Ph) and the analogous ML2
complexes of various bivalent transition metals (M ˆ Mn,
Fe, Co, Ni, Cu, Zn) are easily obtained by reaction of their
acetates with benzoylnitromethane in ethanol [12]. This
procedure cannot be extended to the synthesis of the nitro-
ester complexes 5 (R ˆ OMe) and 6 (R ˆ OEt); in fact, the
reacting nickel acetate and the ester are recovered
unchanged after 4 h at re¯ux in ethanol. Thus, the reaction
is better carried out using anhydrous nickel chloride in the
presence of a base like triethylamine to deprotonate the
nitroacetate (Eq. (2)).
a marked shift to lower ®eld is also observed for the 13C CH
resonance [116.3 versus 98.4 (cis) and 98.6 (trans) ppm].
These data indicate that the methino group has an increased
vinylic character and that the benzoyl and nitro substituents
exhibit a strong deshielding effect. The carbonyl resonance
at 183.9 ppm has a lower chemical shift with respect to
organic carbonyls (ca. 200 ppm) [38] and also to benzoyl
carbonyls coordinated to palladium [188.4 (cis) and 188.6
(trans)], thus indicating an enhanced reduction of the double
C=O bond [15].
Attempts to prepare the ester complexes analogous to 7
by reaction of palladium acetate with the nitro ligands in
ethanol were unsuccessful, because of an extensive hydro-
lysis of the ester group. Coordination of the carbonylnitro-
nate ligand O2NCHC(O)R (R ˆ OMe, OEt) is achievable
under very anhydrous conditions by reaction in THF of
PdCl2 (or [PdCl2(PhCN)2]) with K(O2NCHC(O)R). This
synthetic procedure gives complexes of formula
[PdCl(O2NCHC(O)R)]2, in which only one chlorine atom
has been replaced by the carbonylnitronate.
The bands at 1720 (8) and 1714 (9) cm 1 are attributable
to the stretching of an uncoordinated ester carbonyl. These
values are only slightly lower (ca. 30 cm 1) than in the free
ligand, so coordination to the metal has to be ruled out,
because, as reported [37] and observed in complexes 2, 3, 5
and 6, the resulting reduction of the double bond character
in C=O would give stretching values slightly above
1600 cm 1. On the other hand a small energy decrease is
expected anyway, because of weak electron delocalisation
in the coordinated anionic ligand extended to all functional
groups. The attribution for the NO2 stretching bands [1340
and 1176 (8), 1324 and 1187 (9) cm 1] is tentative, as no
well de®ned values are reported in the literature [39], and
would imply for the coordinated nitro group shifts to lower
energy of ca. 230 and ca. 150 cm 1. In addition one broad
band is observed at ca. 330 cm 1 in the far IR attributable to
a Pd±Cl stretching in bridged complexes [40].
NiCl2 ‡ 2O2NCH2CꢀO†R ‡ 2Et3N
EtOH
! ‰NiꢀO2NCHCꢀO†R†2ꢀEtOH† Š ‡ 2ꢀEt3NH†Cl
(2)
2
Complexes 4±6 are also obtained by dissolving 1±3 in
acetone or dichloromethane and treating them with ethanol
to displace one carbonylnitronate ligand (Eq. (3)). Treat-
ment with water gives extended hydrolysis of the coordi-
nated ester.
ꢀDMANH†‰NiꢀO2NCHCꢀO†R† Š ꢀ1 3† ‡ 2EtOH
3
! ‰NiꢀO2NCHCꢀO†R†2ꢀEtOH† Š ꢀ4 6†
2
‡ ꢀDMANH†ꢀO2NCHCꢀO†R†
(3)
The easy replacement by ethanol would indicate that
these anionic bidentate ligands have moderate bonding
ability, at least with ester substituents, whereas in the case
of benzoylnitromethane the substitution may be favoured by
the limited solubility of 4.
Complex 1, when treated with C2H5OD, gives
[Ni(O2NCDC(O)Ph)2(EtOD)2] (4-d), as the result of an
H/D exchange involving the methine hydrogen. Comparison
of the infrared spectra of 4 and 4-d shows, in particular, that
1
in the region 1600±1200 cm only the bands at 1353 and
1300 cm 1 are scarcely affected by the deuteration, so being
attributable to the two NO stretchings. This hypothesis is
con®rmed by a similar study on the square planar
[Cu(O2NCDC(O)Ph)2] [4]. The spectra of complexes 4±6
show the expected bands at 1591 (4), 1648 (5), 1636 (6)
The NMR spectra were obtained in deuteroacetone solu-
tions for the proton and in the solid state for 13C , because the
limited stability in solution of complexes 8 and 9 is incom-
patible with the accumulation times required for 13C. The
methino group shows the proton resonance at 2.81 (8 and 9);
this value is completely different from that observed in 7
(6.30 and 6.54 ppm). Also the 13C resonance [ca. 67 (8) and
(9)] occurs at much higher ®elds, thus suggesting a different
1
cm of the coordinated carbonyl and are similar, as a
whole, to those of the corresponding complexes 1±3, so
it is easy to envisage the same type of O,O0 coordination via
CO and NO2 oxygen atoms.
The diamagnetic square planar palladium complex
[Pd(O2NCHC(O)Ph)2] (7) can be prepared in acetone from
palladium acetate by an extension of the reported procedure
[12]. Its infrared spectrum is characterised by two bands at
1596 and 1586 cm 1 attributable to the CO stretching of the
coordinated carbonyl (cis and trans isomers) and by the
bands relative to the NO2 group at 1393 and 1264 cm 1. The
NMR data are particularly interesting because they give
novel information on a coordinated carbonylnitronate moi-
ety. The values of 6.30 and 6.54 ppm for the methine
hydrogen can be compared with the values of 6.06 and
6.04 found in cis- and trans-[Pd(MeC(O)CHC(O)Ph)2] [24];
1
coordination mode. The H and 13C signals of the C±H
group are fairly close to those of organic nitroacetates,
suggesting in particular that the carbon maintains an sp3
hybridization. The alkanenitronate anion can present more
limit formulas whose weight depends on the various sub-
stituents and type of coordinated metal.

24
M. Basato et al. / Inorganica Chimica Acta 285 (1999) 18±24
It has been observed that the ®rst limit formula (a) is
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This work was partially supported by the Ministero
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