Synthesis and characterization of dirhodium(II,II) formamidinate complexes containing short-bite nitrogen ligands

Article abstract of DOI:10.1016/S0020-1693(02)01303-8

The Rh₂⁴⁺ complex [Rh₂(form)₂(O₂CCF₃)₂ (H₂O)₂] (form=N,N′-di-p-tolylformamidinate anion) easily reacts with the short-bite ligand pyrido[2,3-b]pyrazine, molar ratio 1:2, yielding the complex [Rh₂(form)₂(pyrido[2,3-b]pyrazine)₂ (O₂CCF₃)₂] which has been characterized by conductivity measurements and IR and ¹H NMR spectroscopy. The complex exhibits a lantern type structure with the two nitrogen ligands coordinated, in a cisoid arrangement, at the equatorial positions across the dirhodium unit while the axial sites are occupied by two monoligated trifluoroacetate groups. Conductivity and IR data show that in DMSO the complex undergoes dissociation of both the axial ligands leading to the species [Rh₂(form)₂(pyrido[2,3-b]pyrazine)₂(DMSO) ₂]²⁺. The parent compound reacts also with 2-chloro-5,7-dimethyl-1,8-naphtyridine and 2,7-dichloro-5-methyl-1,8-naphtyridine leading to [Rh₂(form)₂(O₂CCF₃) (C₁₀H₉N₂O)(H₂O)] and [Rh₂(form)₂(O₂CCF₃) (C₉H₆N₂ClO)(H₂O)], respectively, in which the dirhodium unit is supported by three different bridging ligands. Microanalysis, ¹H NMR and X-ray data unambiguously show that the reaction proceeds, unexpectedly, with elimination of a trifluoroacetate group and the substitution of a chlorine atom of the naphtyridine ligands by oxygen.

Full text of DOI:10.1016/S0020-1693(02)01303-8

Inorganica Chimica Acta 344 (2003) 190ꢂ196
/
www.elsevier.com/locate/ica
Synthesis and characterization of dirhodium(II,II) formamidinate
complexes containing short-bite nitrogen ligands
Giuseppe Tresoldi *, Sandra Lo Schiavo, Francesco Nicolo`, Paola Cardiano,
Pasquale Piraino *
Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universita` di Messina, Salita Sperone n. 31, Vill. S. Agata, 98166 Messina,
Italy
Received 8 July 2002; accepted 6 September 2002
Abstract
The Rh/42ꢀ complex [Rh2(form)2(O2CCF3)2(H2O)2] (formꢁ
N,N?-di-p-tolylformamidinate anion) easily reacts with the short-bite
ligand pyrido[2,3-b]pyrazine, molar ratio 1:2, yielding the complex [Rh2(form)2(pyrido[2,3-b]pyrazine)2(O2CCF3)2] which has been
/
1
characterized by conductivity measurements and IR and H NMR spectroscopy. The complex exhibits a ‘‘lantern’’ type structure
with the two nitrogen ligands coordinated, in a cisoid arrangement, at the equatorial positions across the dirhodium unit while the
axial sites are occupied by two monoligated trifluoroacetate groups. Conductivity and IR data show that in DMSO the complex
undergoes dissociation of both the axial ligands leading to the species [Rh2(form)2(pyrido[2,3-b]pyrazine)2(DMSO)2]2ꢀ. The parent
compound reacts also with 2-chloro-5,7-dimethyl-1,8-naphtyridine and 2,7-dichloro-5-methyl-1,8-naphtyridine leading to [Rh2(for-
m)2(O2CCF3)(C10H9N2O)(H2O)] and [Rh2(form)2(O2CCF3)(C9H6N2ClO)(H2O)], respectively, in which the dirhodium unit is
supported by three different bridging ligands. Microanalysis, 1H NMR and X-ray data unambiguously show that the reaction
proceeds, unexpectedly, with elimination of a trifluoroacetate group and the substitution of a chlorine atom of the naphtyridine
ligands by oxygen.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Dirhodium complexes; Equatorial reactivity; Nitrogen ligands
1. Introduction
of two or four anionic N-donor bidentate nitrogen
ligands into the ‘‘lantern structure’’ of dirhodium(II,II)
carboxylate complexes dramatically affects the redox
properties of these species facilitating the thermody-
namic access to mixed-valent dirhodium(II,III) [9] and
charge-transfer complexes [10]. It also causes a con-
siderable improvement in their catalytic and biological
activity. As an example the complex [Rh2(5(S)-Mepy)4]
Complexes containing the Rh/42ꢀ core are the center of
many experimental and theoretical studies since they
represent a unique example of compounds involved in
catalytic as in biological processes. Dirhodium(II,II)
tetracarboxylates catalyze, in fact, the synthesis of a-
hydroxycarbonyl compounds via a Rh-carbene inter-
mediate [1], decomposition [2] and CꢂH insertion [3] of
/
(Mepyꢁmethyl-2-pyrrolidone-5(S)-carboxylate) and its
/
diazocarbonyl compounds as well as cyclization of a-
diazoesters [4]. They are also able to interact with GG
and AA sites on single stranded oligonucleotides [5],
single stranded and duplex DNA [6] and to promote
DNA photocleavage [7] while exhibiting cytostatic
activity against human carcinoma [8]. The introduction
enantiomeric form promote high enantioselectivity in
intramolecular cyclopropanation of allyl diazoacetates
[11], intermolecular cyclopropenation of 1-alkynes with
diazoesters and diazoamides [12], in lactone synthesis
via intramolecular Cꢂ
/
H insertion [13,14]. In addition
the species [Rh2(form)2(O2CCF3)2(H2O)2] (formꢁ
/
N,N?-di-p-tolylformamidinate anion) gives rise to a
significant increase in survival time of mice bearing
Ehrlich ascite or Leukaemia 1210 [15]. So the introduc-
tion into the lantern structure of ligands other than
* Corresponding author. Tel.: ꢀ
756.
E-mail address: tresoldi@chem.unime.it (G. Tresoldi).
/
39-090-676 5710; fax: ꢀ39-090-393
/
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 3 0 3 - 8

G. Tresoldi et al. / Inorganica Chimica Acta 344 (2003) 190ꢂ
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191
carboxylates allows a greater variation in the molecular
patterns and hence the chemical reactivity of dirhodiu-
m(II,II) complexes. Recently dirhodium(II,II) com-
plexes were also involved, as linear connectors or
angular building blocks, in the synthesis of redox-active
square molecular boxes [16].
H(7) (10.14 ppm) with respect to the corresponding
values exhibited by free pyrido[2,3-b]pyrazine (9.07 and
9.21 ppm, respectively). On the contrary the H(3)
proton, which could be affected by the N(4) coordina-
tion, is virtually unchanged.
Although pyrido[2,3-b]pyrazine in principle may act
as a ‘‘short bite’’ or exobidentate ligand, the downfield
shift of the H(2) and H(7) protons, very likely due the
magnetic anisotropy of the rhodium centers, indicates
that the nitrogen atoms N(1) and N(8) are involved in
coordination. So the ligands prefer to adopt the
N(1),N(8) bridging coordination mode across the dir-
hodium core with consequent displacement of the two
trifluoroacetate groups from the equatorial sites.
Following our interest in the chemistry of Rh/24ꢀ
complexes we focused our attention on the synthesis of
dirhodium(II,II) complexes in which the Rh/42ꢀ core is
supported by nitrogen containing ligands only. We wish
to report on the reaction of the complex [Rh2(for-
m)2(O2CCF3)2(H2O)2] (1) with the short-bite nitrogen
ligands pyrido[2,3-b]pyrazine, 2-chloro-5,7-dimethyl-
1,8-naphtyridine (chloronaphy) and 2,7-dichloro-5-
methyl-1,8-naphtyridine (dichloronaphy) (Fig. 1). The
parent complex is an ideal starting material for the
synthesis of eight-nitrogen coordinated dirhodium(II,II)
complexes since the trifluoroacetate groups are very
labile owing to the trans labilizing effect of the
formamidinate groups. So complex 1 exhibits, besides
the well known axial reactivity, a remarkable equatorial
reactivity too. This leads to mild conditions for the
coordination of bidentate ligands at the equatorial
position, being completely unaffected by the
Rh(form)2Rh fragment. We report also on the X-ray
analysis of the complex [Rh2(form)2(O2CCF3)(C10H9-
N2O)(H2O)], a rare example of a dirhodium(II,II)
complex in which the Rh/42ꢀ core is supported by three
different bridging ligands.
Complex 2a, which is soluble only in polar organic
solvents, is non conducting in dichloromethane while in
acetonitrile it behaves as a 1:2 electrolyte displaying a
value of 260 Vꢃ1 cm2 molꢃ1. Solid IR spectrum of 2a
shows, in addition to the bands associated to the
formamidine linkages, an absorption at 1668 cmꢃ1
consistent with the presence of monodentate fluorocar-
boxylate groups, s-coordinated at the axial positions.
The occurrence of a single n(CO) band in the solid IR
spectrum suggests that both trifluoroacetates are co-
ordinated in a similar manner. IR spectra also reveal
that the compound adopts in solution a different
configuration, being the n(CO) absorption detected in
CH3CN or DMSO at 1695 cmꢃ1. The combined
conductivity and IR data suggest that in coordinating
solvents the monoaxial ligated trifluoroacetate groups
are displaced from the axial sites and the ionic bis-
adducts
(LꢁCH3CN, DMSO) are generated [17]. The adduct
[Rh2(form)2(C7H5N3)2(DMSO)2][O2CCF3]2×DMSO
[Rh2(form)2(pyrido[2,3-b]pyrazine)2(L)2]2ꢀ
2. Results and discussion
/
/
Treatment
of
the
complex
[Rh2(for-
(2b) (Fig. 2) was isolated as an air-stable crystalline solid
for which satisfactory analytical data have been ob-
tained but data from crystal of 2b did not allow
complete refinement. Notwithstanding the bad quality
m)2(O2CCF3)2(H2O)2] (1) with pyrido[2,3-b]pyrazine,
molar ratio 1:2, yields, after minimum work-up, the
green complex [Rh2(form)2(pyrido[2,3-b]pyrazine)2-
(O2CCF3)2] (2a). The complex is an air-stable crystalline
solid for which satisfactory analytical data have been
obtained. The 1H NMR (CDCl3, r.t.) shows the
expected signals of the methyl and methyne protons of
the formamidinate fragments, as single resonances, at
2.23(s) and 7.43(t) ppm pointing out the symmetric
arrangement of the two formamidinates across the Rh/24ꢀ
core. Inspection of the pyridyl protons region clearly
shows five sets of resonances with the right integral
ratio. In particular the spectrum experiences the sig-
nificant downfield shift of the H(2) (10.19 ppm) and
Fig. 1.
Fig. 2. Proposed structure for 2b.

192
G. Tresoldi et al. / Inorganica Chimica Acta 344 (2003) 190ꢂ196
/
of crystal data the structure depicted in Fig. 2 is clearly
detected.1
When the pyrido[2,3-b]pyrazine is allowed to react
with the tetracarboxylate complex [Rh2(acetate)4], a
ligand. The upfield shift of H(3) and H(4) protons may
be due to increased shielding of the protons, caused by
back-donation from dp orbitals of the metal to p*
orbitals of the ligands, as well as from the substitution of
a chlorine atom by an oxygen. The combined IR and 1H
redꢂorange insoluble material, which could not to be
/
NMR data are consistent with a RhꢂRh unit supported
/
fully characterized, was obtained. The material, which
analyses as [Rh2(acetate)4(pyrido[2,3-b]pyrazine)] (3),
very likely corresponds to one dimensional linear chains
of Rh2(acetate)4 fragments linked by the pyrido[2,3-
b]pyrazine ligand, axially coordinated to the dirhodium
units through N(1), N(4) or N(8), N(4) atoms. So it acts
as an exobidentate ligand. Similar structures have been
by two cis formamidinate groups, one bridging trifluor-
oacetate and the naphtyridine bridged to the dimetal
core through the N1 and N8 atoms.
When the reaction is performed by using dichlorona-
phy, the compound [Rh2(form)2(O2CCF3)(C9H6N2-
ClO)(H2O)] (5), similar to 4, is obtained. Once again
microanalytical data confirm that the reaction proceeds
with elimination of a trifluoroacetate group and sub-
stitution of a chlorine of dichloronaphy by an oxygen
atom. Conductivity measurements, spectroscopic 1H
NMR and IR data suggest that a similar structure can
reported for the complexes [Rh2(acetate)4L] (Lꢁ
2,3,5,6-tetramethylbenzene-1,4-diammine or phenazine)
[18] and [Rh2(form)2(O2CCF3)2L] (Lꢁpyrazine or 4,4-
bipyridine) [19].
/
/
Addition of chloronaphy to a diethyl ether solution of
1 yields, after the workup described in Section 3, a
1
be envisaged for 4 and 5. The H NMR spectrum of 5,
which is non-conducting, exhibits four different 1H
NMR signals for the methyl protons of the formamidi-
nate fragment pointing out the asymmetrical arrange-
ment of the two formamidinate ligands around the
dirhodium core. Furthermore the protons of the di-
chloronaphy ring resonate at frequencies comparable to
those detected for the chloronaphy ring of 4 (Table 1),
yellowꢂbrown microcrystalline solid which corresponds
/
to the empirical formula [Rh2(form)2(O2CCF3)(C10H9-
N2O)(H2O)] (4). Unexpectedly the microanalytical data
show that the crystals contain only a trifluoroacetate
group for dimeric unit and an oxygen atom has replaced
1
a chloro group of the naphtyridine. Infrared and H
NMR spectra were not of much help in solving this
problem but they were useful to suggest the structure
further confirmed by the X-ray analysis. Complex 4,
which is non-electrolyte even in coordinating solvents,
experiences the n(CO) absorption at 1641 cmꢃ1, the
value being consistent with those of bridging trifluoro-
supporting the bridging N(1)Ã
/N(8) coordination of
dichloronaphy.
The reaction leading to 4 and 5 takes place with an
obscure course and we do not feel confident in making
unsupported speculations. We can only exclude that the
formation of 4 may be caused by 5,7-dimethyl-2-
hydroxy-1,8-naphtyridine (OHnaphy) impurities present
in the chloronaphy because its purity has been carefully
checked. Furthermore from the 1:1 reaction of 1 with
Ohnaphy we recovered a mixture of products that we
were unable to isolate in a pure form 4.
1
acetate groups while the H NMR spectra give a clear
indication of the coordination mode of chloronaphy. It
is worth mentioning that 1,8 naphtyridines display
geometries ranging from monodentate, chelating biden-
tate and bridging bidentate [19], being its ability to act in
a bridging bidentate mode favored by the presence of
functional groups in positions 2 and 7 [20]. The 1H
NMR spectrum of 4 shows four different signals for the
methyl protons (2.13, 2.23, 2.28, and 2.37 ppm) and two
for the methyne protons (7.8 and 6.98 ppm) of
formamidinate fragments [21], as expected for a low
symmetry molecule. The methyl protons region (Table
1) also experiences two singlets at 3.01 and 2.50 ppm
assigned to Me(7) and Me(5) of chloronaphy, respec-
tively. Both the signals are downfield shifted with
respect to those of uncoordinated chloronaphy (2.66
and 2.73 ppm) strongly suggesting that N(8) is involved
in the coordination to the dimetal center. The analysis of
the chloronaphy protons clearly shows that H(6) proton
is poorly affected by the coordination while H(3) and
H(4) lie at high field compared to the signals of the free
We suggest that, as previously found [16ꢂ20], the first
/
step of the reaction consists in the coordination of
chloronaphy at the axial site of the dirhodium unit
followed by axialꢂequatorial isomerization and conse-
/
quent displacement of one trifluoroacetate group from
the equatorial to the axial site. So a mixture of isomers is
not unexpected bearing also in mind that OHnaphy may
act as a bridge through N(1) and the oxygen atom. It is
worth mentioning that the complex Rh2(O2CCH3)4, that
does not exhibit equatorial reaction, reacts with OHna-
phy leading to the bis axial adduct Rh2(O2CCH3)4(OH-
naphy)2 [22].
2.1. X-ray structure of
1
[Rh2(form)2(O2CCF3)(C10H9N2O)(H2O)]×/2
(C4H10O) (4)
/
The molecular structure of 4, including the atomic
numbering scheme, is illustrated in Fig. 3. Selected bond
distances and angles are listed in Table 2. The crystal
1
Unpublished structural analysis of 2b gave Rh(1)Ã
O ꢁ2.335(9), Rh(1) ÃN(1?) ꢁ2.073(12), Rh(1) Ã
2.068(11), Rh(2)ÃN(8?)ꢁ2.066(11), Rh(2)ÃN(1)ꢁ2.077(11).
/
Oꢁ
/
2.263(9),
Rh(2) Ã
/
/
/
/
/
N(8) ꢁ
/
/
/
/
/

G. Tresoldi et al. / Inorganica Chimica Acta 344 (2003) 190ꢂ
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193
Table 1
1H NMR chemical shifts (ppm) and coupling constants (Hz)
H(3)
H(4)
H(6)
Me(5)
Me(7)
2.73
Chloronaphy
Dichloronaphy
OHnaphy
7.43(d, Jꢁ
7.51(d, Jꢁ
6.66(d, Jꢁ
6.09(d, Jꢁ
6.22(d, Jꢁ
/
8.60)
9.20)
10.50)
8.01)
8.37)
8.24(d)
8.28(d)
7.87(d)
7.06(d)
7.13(d)
7.21(s)
7.34(s)
6.89(s)
7.05(s)
7.00(s)
2.66
2.70
2.50
2.50
2.54
/
/
2.60
3.01
4
5
/
/
Table 2
structure is constituted by the co-crystallization of
discrete molecules [Rh2(form)2(O2CCF3)(C10H9-
N2O)(H2O)] with diethylꢂether in the ratio 2:1. The
˚
Selected bond lengths (A) and angles (8) for 4
/
˚
Bond lengths (A)
isolated solvent molecule is placed at a crystallographic
center of symmetry and its alkyl branches are very
disordered. The complex consists of a dirhodium(II)
unit bridged by three different type of ligands (two
adjacent formamidinates (form), one trifluoracetate and
one naphthyridine derivative) while a water molecule is
bound to one Rh center on the axial position. Therefore
the Rh/42ꢀ unit is asymmetric and shows a distorted
octahedral coordination of Rh(1) and a square pyrami-
dal arrangement for Rh(2) having the axial site empty,
Rh(1)Ã
Rh(1)Ã
Rh(1)Ã
Rh(1)Ã
Rh(1)Ã
N(1)Ã
C(1)Ã
C(2)Ã
N(3)Ã
N(4)Ã
N(5)Ã
N(6)Ã
C(41)Ã
/
O(1)
N(5)
N(3)
O(3)
N(1)
2.186(4) Rh(2)Ã
2.017(5) Rh(2)Ã
2.029(5) Rh(2)Ã
2.095(4) Rh(2)Ã
2.108(5) Rh(2)Ã
/
Rh(1)
N(6)
N(4)
O(4)
N(2)
2.4487(9)
/
/
1.994(5)
2.026(5)
2.094(4)
2.030(5)
1.388(7)
1.347(7)
1.323(7)
1.315(7)
1.320(7)
1.329(7)
1.254(7)
/
/
/
/
/
/
/
C(2)
1.384(7) N(1)Ã
1.380(7) N(2)Ã
1.260(7) N(3)Ã
1.435(7) C(11)Ã
1.425(7) N(5)ÃC(26)
1.432(7) C(26)ÃN(6)
C(41)
/
C(1)
C(8)
C(11)
/
N(2)
/
/
O(2)
/
/
C(12)
C(19)
C(27)
C(34)
/N(4)
/
/
/
/
/
1.430(7) O(3)Ã
/
while each metal carries ꢀ
surroundings have a significant influence on the corre-
sponding coordination shell: while RhÃOacet distances
are similar (2.095(4) vs. 2.094(4) A), the RhÃNform
(2.029(5) and 2.017(5) vs. 2.026(5) and 1.994(5) A,
respectively) and the RhÃNnaph bond lengths (2.108(5)
/2 formal charge. The different
/O(4)
1.273(7)
Bond angles (8)
/
N(5)Ã
N(5)Ã
N(5)Ã
O(3)Ã
N(6)Ã
N(4)Ã
N(2)Ã
N(5)Ã
/
Rh(1)Ã
Rh(1)Ã
Rh(1)Ã
Rh(1)Ã
Rh(2)Ã
Rh(2)Ã
C(1)Ã
C(26)Ã
/
N(3)
90.6(2) N(3)Ã
/
Rh(1)Ã
/
88.6(2)
90.3(2)
87.7(2)
97.2(2)
88.6(2)
90.5(2)
124.3(6)
127.4(6)
˚
O(3)
/
/
/N(1)
90.0(2) O(3)Ã
N(1)
O(1) 101.2(2) N(3)Ã
O(1)
/
Rh(1)Ã
Rh(1)Ã
Rh(1)Ã
/
˚
/
/
/
/
/
˚
vs. 2.030(5) A) show an elongation balancing the
electronic and steric effects of the higher coordination.
Whereas the formamidinate bites are symmetric and
/
/O(1)
83.5(2) N(1)Ã
/
/
O(1)
/
/N(4)
91.4(2) N(6)Ã
/
Rh(2)Ã
/
equal (the average values are CÃ
/
Nꢁ
/
1.320(7) and
N(2)
˚
NÁ Á ÁNꢁ
/
2.337(8) A), both the RhÃ
/
Nform distances of
/
/O(4)
89.4(2) N(2)Ã
/
Rh(2)Ã
/
O(4)
/
/
N(1)
115.9(5) N(4)Ã
/
C(11)Ã
/
N(3)
/
/
N(6) 124.1(6) O(3)Ã
/
C(41)Ã
/
O(4)
Hydrogen bonds
DHA
d(DÃ
H)
0.94 1.91
/
d(HÁ Á ÁA)
d(DÁ Á ÁA) DÃ
/
HÁ Á ÁA
O(1)Ã
H(1A)Á Á ÁO(2)
O(1)Ã
H(1B)Á Á ÁO(2)?
/
2.517(6) 119.7
/
0.94 2.17
3.022(5) 149.5
The equivalent position of atom O(2) is obtained by ꢃ
/
x, ꢃ
/
y, 1ꢃz
/
symmetry operation.
the formamidinate trans to the trifluoracetate are
slightly shorter than the corresponding ones of the other
bridge opposite to the naphthyridine, due to the
different trans-effect.
The bond lengths within the naphtyridine ring are
irregular while the distance from C(2) to the alleged O(2)
Fig. 3. Perspective view and atom numbering scheme of molecule 4.
Thermal ellipsoids are drawn at 25% of probability, while H size is
arbitrary. Dashed and dotted lines represent intra- and inter-molecular
(sym. op. ꢃ
/
x, ꢃ
/
y, 1ꢃz) H interactions. The co-crystallized diethyl-
/
˚
[1.252(7) A] is very short and far from the usual
ether molecule has been omitted for clarity.

194
G. Tresoldi et al. / Inorganica Chimica Acta 344 (2003) 190ꢂ
/196
˚
1.38 A. These
˚
separation (2.266(7) A) of the unique acetate bridge. In
˚
keeping a light longer inter-metallic distance [2.453(1) A]
was detected for the analogous cis complex
[Rh2(form)2(C7H5N3)2(DMSO)2][O2CCF3].2DMSO (2b)
Caromatic
Ã
/
O single bond distance of ꢀ
/
data may be explained assuming that the exo-cyclic
O(2), involved in the conjugation with the naphtyridine
system, exhibits a predominant pyridone character. The
C(2)Ã
/
O(2) bond length, although larger than the value
(DMSOꢁdimethylsulfoxide, axial coordinated via the
/
of 1.233(6) found in the dirhodium (II,II) complex
Rh2(O2CCH3)4(OHnaphy)2 [22], denotes a high percen-
tage of double bond. A study on all the 69 1,8-
naphthyridine-2-oxo fragments reported in the CSD
[23] has evidenced bond length values ranging from
O-atom) where both the acetate were replaced by the
pyrido[2,3-b]pyrazine. Although the X-ray diffraction
data were very bad and the model refinement did not
reach a good enough level to being published, due to the
very high disorder of the counter-ion trifluoracetate and
the co-crystallized DMSO molecules in the crystal
packing,1 the cationic complex is sufficiently well-
defined to evidence the significant agreement of its
geometry with that of compound 4.
˚
1.21(3) [24] up to 1.37(1) A [25] without any clear
correlation between the geometry of the bicyclical ring
and the delocalization degree of the double-bond. The
oxygen atom O(2) is located at a very short distance
[2.517(6)] from the oxygen O(1) of the axially coordi-
nated water molecule (intramolecular hydrogen interac-
tion, see Table 2) and simultaneously has relatively short
interactions of 3.022(5) to the axially coordinated water
molecule of a neighboring unit (intermolecular hydro-
gen interaction). The steric hindrance of the axial water
molecule with the surrounding phenyl group and the
naphthyridine oxygen push O(1) toward the acetate side
as evidenced by the enlargement of the angles N(5)Ã
Rh(1)ÃO(1)ꢁ101.2(2) and N(1)ÃRh(1)ÃO(1)ꢁ
97.2(2)8 as compared to the contraction of O(3)Ã
Rh(1)ÃO(1) up to 83.5(2)8. This hindrance might be
related to the different rotation of the phenyl rings of the
formamidinate: in the bridge opposite to the acetate the
two p-tolyl groups are equally rotated with respect to
3. Experimental
3.1. General remarks
The starting complex [Rh2(form)2(CF3COO)2(H2O)2]
[9a], 5,7-dimethyl-2-chloro-1,8-naphtyridine (chlorona-
phy) and 2,7-dichloro-5-methyl-1,8-naphtyridine (di-
/
/
/
/
/
/
chloronaphy)
and
5,7-dimethyl-2-hydroxy-1,8-
/
naphtyridine were prepared according to literature
data [27] and their purity checked by elemental analysis
1
and H NMR spectra. All other chemicals are commer-
/
cially available and used as supplied. None of the
compounds here reported are air sensitive, but all
reactions were carried out under an atmosphere of dry
nitrogen. Elemental analyses were performed by RE-
DOX snc Laboratorio di Microanalisi, Cologno Monz-
ese (Milano), Italy. Infrared spectra were recorded on
the bite plane [C(26)Ã
C(26)ÃN(6)ÃC(34)ÃC(39)ꢁ
cent formamidinate the ring close to the axial water is
more rotated than the other one [C(11)ÃN(3)ÃC(12)Ã
C(17)ꢁ68.9(8) vs. C(11)ÃN(4)ÃC(19)ÃC(20)ꢁ
/
N(5)Ã
/
C(27)Ã
/
C(32)ꢁ44.6(8) and
/
/
/
/
/
48.9(9)8] while in the adja-
/
/
/
/
/
/
/
/
KBr pellets with a PerkinꢂElmer FT 1720X spectro-
/
32.7(9)8]. In fact the distorsion of the Rh(1) coordina-
˚
tion geometry is also evidenced by the 0.370(4) A
meter. The NMR measurements were performed with a
Bruker AMX 300 spectrometer using standard pulse
sequences.
deviation of O(1) on the acetate side from the mean
plane passing through the Rh/42ꢀ unit and the bite of the
naphthyridine and of the opposite formamidinate
bridge. However both the formamidinate bites are
almost flat as evidenced by the corresponding torsion
3.2. Preparations
3.2.1. [Rh2(form)2(C7H5N3)2(O2CCF3)2] (2a)
angles NÃ
/
RhÃ
/
RhÃ
/
N being close to 08, while in other
Solid pyrido[2,3-b]pyrazine (0.047 g, 0.36 mmol) was
added to a benzene solution (20 ml) of [Rh2(form)2(CF3-
COO)2(H2O)2] (1) (0.160 g, 0.175 mmol) and the
resulting dark-green solution was left to stir for
approximately 4 h. After this time the solution was
filtered and the solvent removed in vacuo affording a
green solid that was recrystallized from CHCl3/n-
heptane affording 2a as green crystals. Yield 85%.
Anal. Found: C, 50.30; H, 3.51; N, 12.30; F, 10.05.
Calc. for C48H40F6N10O4Rh2 (1140.72): C, 50.54; H,
Rh2(form)4 complexes the ligand torsion along the
intermetallic axis is significant [10].
˚
Rh bond length of 2.4487(9) A is comparable
The RhÃ
/
˚
with that [2.434(1) A] found in the homoleptic tetra-
formamidinate complex [Rh2(form)4] [9b] but longer
than those detected in the complexes [Rh2(form)2-
(O2CCF3)2(H2O)2] [9a] and [Rh2(O2CCF3)2(H2O)2] [26]
˚
(2.426(1) and 2.394(3) A, respectively). Being the RhÃ
/
Rh separation dependent on the number and the bite
size of the bridges, the above data may be explained
taking into account that the bite of the naphthyridine
and the two formamidinate in 4 are almost similar
3.53; N, 12.28; F, 9.99%. LM (CH3CN, 10ꢃ3 mol dmꢃ3
,
20 8C)ꢁ
/
260 Vꢃ1 cm2 molꢃ1. IR (nujol mulls, cmꢃ1):
1577s [n(NCN)], 1668vs [nasym(CO2)]; IR (CH3CN,
(NÁ Á ÁN is 2.347(8) vs. the average 2.336(8) A, respec-
cmꢃ1): 1695vs [nasym(CO2)]. H NMR (CDCl3, 300.13
1
˚
3
2.23 [s, 12H, Me(form)], 7.43 (t, JHRh
tively) while both are significantly larger than OÁ Á ÁO
MHz): dꢁ
/
ꢁ3.6
/

G. Tresoldi et al. / Inorganica Chimica Acta 344 (2003) 190ꢂ
/196
195
Hz, 2H, [CH(form)], 7.78 (m, br, H6), 8.50 (dd, 3Jꢁ
/
6.2
6.2
the air. After approximately 24 h a red solid was formed.
It was recrystallized from CHCl3/n-heptane affording 5
as dark red microcrystals. Yield 78%. Anal. Found: C,
50.38; H, 3.90; N, 8.58; Cl, 3.70. Calc. for
C41H38ClF3N6O4Rh2 (977.06): C, 50.40; H, 3.92; N,
8.60; Cl, 3.63%. IR (nujol mulls, cmꢃ1): 1577s
[n(NCN)], 1645vs [nasym(CO2)]. 1H NMR (CDCl3,
4
3
Hz, Jꢁ
/
1.9 Hz, H5), 8.89 (d, H3), 10.14 (dd, Jꢁ
/
4
3
Hz, Jꢁ
/
1.9 Hz, H7), 10.19 (d, Jꢁ2.6 Hz, H2).
/
3.2.2. [Rh2(form)2(C7H5N3)2(DMSO)2][O2CCF3]2×
DMSO (2b)
/
2a (0.114g, 0.1 mmol) was dissolved in 20 ml of
CHCl3/n-heptane solution (1/1) in the presence of 0.5 ml
of DMSO. After a few days, the green crystalline
product was collected and dried in air. Yield 90%.
Anal. Found: C, 47.00; H, 4.30; N, 10.20; F, 8.20. Calc.
for C54H58F6N10O7Rh2S3 (1375.12): C, 47.17; H, 4.25;
N, 10.19; F, 8.29%. IR (DMSO, cmꢃ1): 1695vs
[nasym(CO2)].
300.13 MHz): dꢁ2.24, 2.29, 2.38, 2.46 [4 s, 12H,
/
3
Me(form)], 7.80 [t, JHRh
ꢁ
/
3.5 Hz, 2H, CH(form)],
3
7.28 [t, JHRh
ꢁ
/
3.7 Hz, 2H, CH(form)].
3.2.5. X-ray crystallographic study
Suitable crystals of 4 were obtained by addition of n-
heptane to a chloroform/diethyl ether solution of the
compound. Single-crystal X-ray diffraction measure-
ments were performed at room temperature on a
Siemens P4 automatic four-circle diffractometer using
3.2.2.1. [Rh2(O2CCH3)4(C7H5N3)]
(3).
[Rh2(O2CCH3)4] (0.108 g, 0.488 mmol) was dissolved
in 30 ml of an acetone/water solution (1/1) and to the
resulting mixture was added 0.064 g (0.488 mmol) of
crude pyrido[2,3-b]pyrazine. The starting light-blue
solution immediately turned green. It was left under
stirring for approximately 3 h, during which time 3
precipitated as a rust solid. This was separated by
filtration, washed several times with water and dried in
vacuo. Yield 80%. Anal. Found: C, 31.24; H, 3.10; N,
7.35. Calc. for C15H17N3O8Rh2 (573.13): C, 31.43; H,
2.99; N, 7.33%. IR (nujol mulls, cmꢃ1): 1561vs
[nasym(CO2)].
graphite-monochromatized Mo Ka radiation (lMoKaꢁ
/
˚
0.71073 A).
1
3.2.5.1. Crystal data of 4. (C42H41F3N6O4Rh2)×/2
/
¯
(C4H10O), triclinic, space group P1, aꢁ
/
11.043(3), bꢁ
105.76(3), bꢁ 101.45(2),
2, Dcalcꢁ1.533 Mg
0.10ꢄ0.07 mm, l(Mo
0.0450/0.0976 and wR2ꢁ
0.0870/0.0973 for 3951 (obs with I ꢁ2s(I))/7533 (all
the independent) reflections, goodness-of-fitꢁ0.776,
Tꢁ298(2) K. Lattice parameters were obtained from
least-squares refinement of the setting angles of 30
reflections within 1252u 5298 range. No crystal decay
/
˚
11.969(4), cꢁ
gꢁ 90.78(3)8, Uꢁ
mꢃ3, crystal dimensions 0.12ꢄ
Ka)ꢁ
0.841 mmꢃ1, R1ꢁ
/
17.093(5) A, aꢁ
/
/
3
˚
/
/
2126(1) A , Zꢁ
/
/
/
/
/
/
/
/
/
/
3.2.3. [Rh2(form)2(O2CCF3)(C10H9N2O)(H2O)] (4)
A diethyl ether solution (20 ml) of 5,7-dimethyl-2-
chloro-1,8-naphtyridine (0.021 g, 0.110 mmol) was
added to a solution of 1 (0.100 g, 0.109 mmol) in the
same solvent (30 ml). The resulting green-shining
solution was left standing overnight, during which
time a small amount of an unidentified green precipitate
was formed. This was removed by filtration and the
mother liquors were taken to dryness in vacuo. The dark
green residue, dissolved in CHCl3 (30 ml), by the time
turns dark yellow and by addition of n-heptane (20 ml)
/
/
was evidenced by the measurement of three standard
reflections, monitored every 197 measurements. The
intensities of the 8169 reflections (collected up to 2uꢁ
/
508) were evaluated by a learnt-profile-fitting procedure
[28] among 2u shells and then corrected for Lorentz-
polarization and absorption (semi-empirical method by
azimuthal scan data [29]: min. and max. transmission
factors were 0.742 and 0.900) effects. No account was
taken for extinction effects. The structure was solved by
standard methods and subsequently completed by a
combination of least-squares technique and Fourier
Syntheses. Data-collection and reduction has been
performed by using the SHELXTL 5.05 system. All non-
hydrogen atoms were refined anisotropically. Whereas
the final difference Fourier maps showed several hydro-
gen positions, H atoms were placed in calculated
positions (the idealized geometry depending on the
parent atom type) and included in the refinement among
the ‘‘riding model’’ method. The coordinated H2O
molecule has been treated as a ‘‘rigid body’’ fragment
whose position has been optimized by the model
refinement lsq-cycles. The terminal trifluoromethyl of
the acetate bridge appeared to be affected by rotational
disorder and it was restrained and split into two
orientations with 0.6 and 0.4 of occupancy, respectively.
affords compound 4 as a brownꢂyellow crystalline solid.
/
Yield 63%. Anal. Found: C, 53.00; H, 4.53; N, 8.59; F,
6.00. Calc. for C42H41F3N6O4Rh2 (956.65): C, 52.73; H,
4.32; N, 8.78; F, 5.96%. IR (nujol mulls, cmꢃ1): 1577s
[n(NCN)], 1641vs [nasym(CO2)]. 1H NMR (CDCl3,
300.13 MHz): dꢁ2.13, 2.23, 2.28, 2.37 [4 s, 12 H,
/
Me(form)], 6.98, 7.8 [2 t, br, 2 H, CH(form)].
3.2.4. [Rh2(form)2(O2CCF3)(C9H6N2ClO)(H2O)]
(5)
A diethyl ether solution (40 ml) of 2,7-dichloro-5-
methyl-1,8-naphtyridine (0.040 g, 0.188 mmol) was
added to a solution of 1 (0.172 g, 0.187 mmol) in the
same solvent (30 ml). From the resulting solution, left
standing overnight, a light green solid precipitated. The
mother liquors were pipetted off and left to evaporate at

196
G. Tresoldi et al. / Inorganica Chimica Acta 344 (2003) 190ꢂ196
/
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Inorg. Chem. 36 (1997) 2458.
Further large electron residuals were localized around
the crystallographic center of symmetry on the DF maps
and were accounted for a very disordered diethylether
molecule. The oxygen atom lies in special position and
its ethyl fragment is split into two conformations of 0.5
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during the refinement. The model optimization was
2
carried out by minimizing the function aw(F/o2
/
ꢃ
/
F/2c
with the full-matrix least-square technique based on all
)/
(b) P. Piraino, G. Bruno, S. Lo Schiavo, F. Laschi, P. Zanello,
Inorg. Chem. 26 (1987) 2205.
7533 independent F2 [Rint
program [30]. The last difference map showed no
ꢁ
/
0.0202] with SHELXL97
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significant electron density residuals (ranging from
0.758 to 0.616 e Aꢃ3). Neutral-atom scattering factors
˚
ꢃ
and anomalous dispersion corrections are those in-
cluded in the program [31]. Final geometrical calcula-
tions and drawings were carried out with the PARST
program [32] and the XPW utility of the Siemens
package, respectively.
/
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Chem. Soc. 121 (1999) 4538;
4. Supplementary material
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Piraino, Eur. J. Inorg. Chem. (2002) 79;
(c) S. Lo Schiavo, S. Serroni, F. Puntoriero, G. Tresoldi, P.
Piraino, Eur. J. Inorg. Chem., in press.
Crystallographic data (excluding structure factors) for
the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre,
CCDC No. 181823. Copies of this information may be
obtained free of charge from The Director, CCDC, 12
[17] E. Rotondo, G. Bruno, F. Nicolo`, S. Lo Schiavo, P. Piraino,
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Union Road, Cambridge CB2 1EZ, UK (fax: ꢀ44-
/
1223-336-033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
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Acknowledgements
This work was supported by the Consiglio Nazionale
delle ricerche (CNR) and Ministero dell’Universita` e
della Ricerca Scientifica e Tecnologica (MIUR) of Italy.
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