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A.M. Kirillov et al. / Journal of Organometallic Chemistry 691 (2006) 4153–4158
(as known [1,4] for Clꢀ) on the ligation of N2. Moreover,
its EL value is also comparable to those of some alkynyl
ligands, e.g. C„CPhꢀ at trans-{OsCl(dppm)2}+ (dppm =
Ph2PCH2PPh2) [16].
The electrochemical experiments were carried out on an
EG&G PAR 273A potentiostat/galvanostat connected to
a personal computer through a GPIB interface. Cyclic vol-
tammetry (CV) studies were undertaken in a two-compart-
ment three-electrode cell, at platinum wire working
(d = 0.5 mm) and counter electrodes. A Luggin capillary
connected to a silver-wire pseudo-reference electrode was
used to control the working electrode potential. The solu-
tions were saturated with N2 by bubbling this gas before
each run, and the oxidation potentials of the complexes
were measured by CV, in the presence of ferrocene as the
internal standard, and the redox potential values are
3. Conclusions
We have shown that N2 and a bound carboxylate N,O-
ligand can be compatible at a single metal centre, and have
prepared, by a convenient route, the first example of such a
type of complex with a genuine neutral N„N ligand. The
complex is stable in air, either as a solid or in solution, and
the carboxylate arm of the picolinate ligand is shown to be
an electron-donor similar to chloride, presenting, as Clꢀ, a
noteworthy stabilizing effect on the trans Re–N2 bond.
Hence, carboxylate groups of biological molecules can
co-exist with N2 at a common binding metal centre, what
is in accord with the hypothesis [4] of coordination of dini-
trogen at the Mo centre (which has a homocitrate ligand)
of FeMoco of nitrogenase.
quoted relative to the SCE by using the [Fe(g5-C5H5)2]0/+
ox
1=2
(E ¼ 0:525 vs. SCE) redox couple in 0.2 M CH2Cl2/
[Bu4N][BF4] solution [17]. The obtained potentials vs.
SCE were converted to the NHE scale by addition of
0.245 V.
4.2. Synthesis of [ReCl(N2)(CO)2(PPh3)2] (1)
Preliminary studies on the reactivity of the dinitrogen
complex 2 show that N2 can be displaced by imidazole
(C3H4N2) to give [Re(pic)(C3H4N2)(CO)(PPh3)2]. The
combination, at a single metal centre, of different ligands
with recognized biological significance and the possibility
to replace N2 by imidazole and potential biological carriers
at a Re-picolinate centre (bearing a metal and a N,O-
ligand, both with pharmacological applications) are also
interesting features of the obtained complex, although the
lack of solubility in water is a drawback. They deserve fur-
ther exploration and encourage the search for the synthesis
of more N,O and carboxylate dinitrogen complexes and,
for those soluble in water, with a potential application in
medicinal chemistry.
Complex 1 was prepared by a published method [18].
CV (CH2Cl2, [Bu4N][BF4], v = 0.2 V sꢀ1, vs. SCE): E1=2
¼ 0:94 V, IIEopx ¼ 1:79 V. X-ray quality crystals were grown
by slow evaporation at 5 ꢁC of C6H6/MeOH or C6H6/
EtOH solutions.
ox
I
4.3. Synthesis of [Re(pic)(N2)(CO)(PPh3)2] (2)
To a cloudy solution of 1 (100 mg, 0.12 mmol) in MeOH
(20 mL)/C6H6 (20 mL) an excess of picolinic acid (148 mg,
1.20 mmol) was added and the reaction mixture was
refluxed for 15 h under dinitrogen. The resulting orange
clear solution was concentrated under reduced pressure
to give an orange oily solid which was treated with
10 mL MeOH to produce a suspension. The solid was then
filtered off (see below for the use of filtrate), washed with
MeOH (3 · 5 mL) and Et2O (3 · 5 mL), whereafter it was
dissolved in CH2Cl2 (5 mL) to form a clear solution which
was taken to dryness under reduced pressure yielding an
orange oil. The addition of Et2O (40 mL) followed by
freezing the obtained mixture in liquid nitrogen (freeze-
thaw method) led to the precipitation of a solid which
was isolated by filtration, washed with Et2O (3 · 5 mL)
and dried in vacuo to yield complex 2 as an orange micro-
crystalline solid (55 mg, 51%). The filtrate from the first fil-
tration (see above) was left to evaporate in air for 2 d.
During this time, orange crystals were separated out from
the solution. They were collected, washed with MeOH
(3 · 3 mL), Et2O (3 · 3 mL) and dried in vacuo to give a
second crop of 2 (15 mg, 14%; i.e. total isolated yield of
65%). Further purification of 2 can be achieved by recrys-
tallization from C6H6/MeOH(EtOH), C6H6/n-C5H12 or
CH2Cl2/n-C5H12 mixtures. Anal. Calc. for C43H34N3O3-
P2Re: C, 58.10; H, 3.86; N, 4.73. Found: C, 58.19; H,
4.01; N, 4.40. IR (KBr mmax/cmꢀ1): 3054w m(CH), 2040m
m(N2), 1926m and 1855s m(CO), 1659m mas(COO), 1599m
ms(COO), 747m and 695s m(PPh); 1H NMR (300 MHz,
4. Experimental
4.1. General materials and experimental procedures
All synthetic and electrochemical work was performed
under dinitrogen using standard Schlenk techniques. The
solvents were dried and degassed by standard methods.
Potassium perrhenate (Merck), triphenylphosphine
(Aldrich), benzoylhydrazine (Aldrich), carbon monoxide
(Air Products) and picolinic acid (Aldrich) were obtained
from commercial sources and used as received. C, H and
N elemental analyses were carried out by the Microanalyt-
´
ical Service of the Instituto Superior Tecnico. Positive-ion
FAB mass spectra were obtained on a Trio 2000 instru-
ment by bombarding 3-nitrobenzyl alcohol (m-NBA)
matrices of the samples with 8 keV (ca. 1.18 · 1015 J) Xe
atoms. Mass calibration for data system acquisition was
achieved using CsI. Infrared spectra (4000–400 cmꢀ1) were
recorded on a Jasco FT/IR–430 instrument in KBr pellets.
1
For TLC, Merck UV 254 SiO2 plates have been used. H,
31P{1H} and 13C{1H} NMR spectra were measured on a
Varian UNITY 300 spectrometer at ambient temperature.
Kirillov, Alexander M.
