A novel series of rhenium-bipyrimidine complexes: Synthesis, crystal structure and electrochemical properties

Article abstract of DOI:10.1039/b615136a

Four novel rhenium complexes of formula [ReCl₄(bpym)] (1), [ReBr₄(bpym)] (2) PPh₄[ReCl₄(bpym)] (3) and NBu₄[ReBr₄(bpym)] (4) (bpym = 2,2′-bipyrimidine, PPh₄ = tetraphenylphosphon

Full text of DOI:10.1039/b615136a

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www.rsc.org/dalton | Dalton Transactions
A novel series of rhenium-bipyrimidine complexes: synthesis, crystal structure
and electrochemical properties†
Rau´l Chiozzone,*a Ricardo Gonza´lez,a Carlos Kremer,a Mar´ıa Fernanda Cerda´,b Donatella Armentano,c
Giovanni De Munno,c Jose´ Mart´ınez-Lillod and Juan Faus*d
Received 18th October 2006, Accepted 30th November 2006
First published as an Advance Article on the web 2nd January 2007
DOI: 10.1039/b615136a
Four novel rhenium complexes of formula [ReCl4(bpym)] (1), [ReBr4(bpym)] (2) PPh4[ReCl4(bpym)] (3)
and NBu4[ReBr4(bpym)] (4) (bpym = 2,2ꢀ-bipyrimidine, PPh4 = tetraphenylphosphonium cation and
NBu4 = tetrabutylammonium cation), have been synthesized and their crystal structures determined by
single-crystal X-ray diffraction. The structures of 1 and 2 consist of [ReX4(bpym)] molecules held
together by van der Waals forces. In both complexes the Re(IV) central atom is surrounded by four
halide anions and two nitrogen atoms of a bpym bidentate ligand in a distorted octahedral
environment. The structures of 3 and 4 consist of [ReX4(bpym)]− anions and PPh4 (3) or NBu4 (4)
cations. The coordination sphere of the Re(III) metal ion is the same as in 1 and 2, respectively.
However, whereas the Re–X bonds are longer the Re–N bonds are shorter than in 1 and 2. This fact
reveals that the bpym ligand forms a stronger bond with Re(III) than with Re(IV) resulting in a
stabilisation of the lower oxidation state. [ReX4(bpym)] complexes are easily reduced, chemically and
electrochemically, to the corresponding [ReX4(bpym)]− anions. A voltammetric study shows that the
electron transference is a reversible process characterized by formal redox potentials of +0.19 V (1) and
+0.32 V (2) vs. NHE, in acetonitrile as solvent.
+
+
transmitting electronic and magnetic interactions between metal
centers.2
Only a few rhenium complexes containing bpym as the
Introduction
In the past few years we have devoted our efforts to the synthesis,
through ligand substitution reactions of the [ReX6]2− anions
(X = Cl, Br), of rhenium(IV) complexes with potential bridging
ligands and we have used them as reagents in the preparation of
heteropolynuclear coordination compounds. In particular, we have
explored the possibilities of Re(IV)-oxalate and Re(IV)-malonate
complexes acting as ligands towards 3d metal ions. Following
this approach numerous oxalato- and malonato-bridged bimetallic
assemblies Re–M (M = Cu, Ni, Co, Fe, Mn) have been reported
and their magnetic properties investigated.1
Recently we have embarked on a research program for the
synthesis of rhenium(IV) complexes with nitrogen-donor ligands
and we have focused our interest on exploring the possibilities of
the use of 2,2ꢀ-bipyrimidine (bpym). This bis-chelating heterocyclic
molecule has proven to be an efficient bridging ligand for the
stepwise synthesis of polynuclear complexes and is capable of
ligand have been reported up to now. Among them, a num-
ber of rhenium(I) tricarbonyl complexes of general formula
[Re(CO)3(bpym)L]n+ (L = CO, CH3CN, pyridine, N-methyl-4,4ꢀ-
bipyridinium ion; n = 0–2) have been synthesized and structurally
characterized.3 Also several bipyrimidine-bridged homo and het-
eropolynuclear complexes of Re(I) and different metal ions such as
Ru(II), Rh(III), Ir(III) and Ln(III) (Ln = Yb, Er, Eu, Gd, Nd) have
been prepared and their photophysical properties studied.4 But
the only example of higher valence rhenium bipyrimidine com-
pounds described up until now is the oxorhenium(V) compound
[ReO(CH3)(edt)(bpym)] (edt = 1,2-ethanedithiolate).5
The chemistry of Re(IV) with N-heterocyclic ligands is scarcely
developed compared with that of other oxidation states. In fact,
very few complexes of Re(IV) containing this type of ligand are
known. The compounds [ReCl4(pzH)(PPh3)] (pzH = pyrazole)
and [ReCl4(pyz)(PPh3)] (pyz = pyrazine) were synthesized by lig-
and substitution onto [ReCl4(PPh3)2] in chloroform.6 On the other
hand, [ReX4(bpy)] (X = Cl, Br; bpy = 2,2ꢀ-bipyridine)7 and other
reported Re(IV) complexes with the substituted N-heterocyclic
ligands dimethyladenine,8 N–p-tolyl-2-picolinamida,9 2–[(6-
benzothiazol-2-yl-pyridin-2-ylmethyl)-amino]-benzenethiol10 and
2,9-dimethyl-1,10-phenantroline11 have been prepared from Re(III)
or Re(V) complexes. Finally, the compound trans-[ReCl4(py)2]
(py = pyridine) has been obtained by reaction of (pyH)2[ReCl6]
with pyridine in a sealed tube.12
aCa´tedra de Qu´ımica Inorga´nica, Facultad de Qu´ımica de la Universidad de
la Repu´blica, Avda. General Flores 2124, CC 1157, Montevideo, Uruguay.
E-mail: rchiozzo@fq.edu.uy
bLaboratorio de Biomateriales, Facultad de Ciencias, Universidad de la
Repu´blica, Igua´ 4225, Montevideo, Uruguay
cDipartimento di Chimica, Universita` della Calabria, via P. Bucci 14/c,
87036, Rende, Cosenza, Italy
dDepartamento de Qu´ımica Inorga´nica/Instituto de Ciencia Molecular,
Facultad de Qu´ımica de la Universidad de Valencia, Dr. Moliner 50, 46100,
Burjassot, Valencia, Spain. E-mail: juan.faus@uv.es
Here we report the synthesis and crystal structure of
four novel complexes, [ReCl4(bpym)] (1), [ReBr4(bpym)] (2),
† Electronic supplementary information (ESI) available: Fig. S1–S4. See
DOI: 10.1039/b615136a
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(PPh4)[ReCl4(bpym)] (3) and (NBu4)[ReBr4(bpym)] (4). An easy
interconversion of the ReIII/ReIV bpym complexes was notorious in
the course of the synthesis of the complexes. With this in mind, we
also investigated the electrochemical behavior of the chloro and
bromo complexes in two solvents used during the preparation,
namely, acetonitrile and dimethylformamide.
Results and discussion
Synthesis of the complexes
The reaction of [ReX6]2− anions (X = Cl, Br) with bpym in
hot dry N,N-dimethylformamide (DMF) gave rise after some
work up to the monosubstituted complexes [ReX4(bpym)]. Ligand
substitution reactions of the inert hexahalorhenates in this solvent
have been previously reported for the preparation of [Re(CN)7]3−13
and a series of mono and bis(oxalato) complexes.14 In the latter
case, it was also noted that a complete halide substitution could
not be achieved due to kinetic factors. The main reason for the
choice of this solvent was its high boiling point, but the role of
the DMF molecules in the reaction path may be important, as the
isolation of the [ReX5(DMF)]− anion suggests.15
Neutral compounds 1 and 2 are readily soluble in most organic
solvents but insoluble in water. They can be crystallized as solvates,
[ReX4(bpym)]·S (S = solvent), from chloroform as well as benzene,
ethyl acetate or acetone. The solvent molecules are lost slowly in
the air at room temperature and more quickly by heating at 100 ◦C
in the oven. Well-formed crystals without solvent molecules are
isolated from chloroform solutions, provided that evaporation of
solvent is slow enough.
Both Re(IV) complexes undergo easily an one-electron re-
duction, by reaction with ascorbic acid in ethanol solution
for example, giving the [ReX4(bpym)]− anions. The reaction is
visually detected due to the intense dark violet colour of these
Re(III) complexes. Solutions of [ReX4(bpym)] in alcohols and
other solvents such as tetrahydrofuran evolve over time a violet
colour, showing that the solvent can act as a reductant of Re(IV).
We have found that these reactions, which are very slow at
room temperature and usually require heating, are catalysed very
effectively by activated alumina. This result was first observed
accidentally during the attempts of purification of the rhenium(IV)
complexes by column chromatography and it is the best procedure
to synthesize the chloro complex 3. The bromo analog 4 can be
obtained in the same way. Although the synthesis of 3 needs the
previous isolation of 1, 4 can also be obtained by direct reaction of
(NBu4)2[ReBr6] and bpym in isopropanol in an “one-pot” process
(see Experimental).
Fig. 1 Perspective drawing of the structure of [ReX4(bpym)] [X = Cl (1)
and Br (2)] showing the atom numbering scheme. Thermal ellipsoids are
drawn at 30% of probability level.
a Re(IV) atom in a distorted octahedral geometry, being bonded
to a bidentate bpym molecule and four chloride/bromide anions.
The main distortion of the ideal octahedral geometry is due to
the short bite angle of the bidentate bpym ligand [76.3(3) and
75.6(2)◦ for N(1)–Re–N(2) in 1 and 2, respectively]. The average
˚
Re–X and Re–N bond distances [2.322(2) and 2.133(7) A in 1 and
˚
2.4595(7) and 2.145(4) A in 2] are in agreement with those found in
the literature.7,8,12,16 The Re–X bond lengths in trans position to the
nitrogen donors are slightly shorter than those in cis positions. This
effect is more evident in compound 2 [average value 2.311(3) and
˚
˚
2.333(2) A in 1 and 2.4406(7) and 2.4634(6) A in 2]. The N(1), N(2),
X(2), and X(3) set of atoms constitute the best equatorial plane
˚
around Re, the largest deviation from planarity being 0.009(3) A
˚
for N(1) and 0.017(2) A for N(2) in 1 and 2, respectively. The Re
atom is practically placed in this plane. The pyrimidyl rings are
planar [largest deviation from the mean planes not greater than
˚
˚
0.038(6) A for C(4) in 1 and 0.030(4) A for C(3) in 2] and the C–C
and C–N bond lengths are as expected. On the contrary the bpym
molecule as a whole shows evident deviation from planarity, largest
deviations from the mean planes being 0.183(8) (1) and 0.209(6)
˚
(2) A. There is an appreciable tilt between the two pyrimidyl rings,
the dihedral angles between them being 16.0(3) and 16.8(2)◦ in 1
and 2, respectively (Fig. S1†). This is probably the main reason
for which both compounds crystallize in an acentric space group.
˚
The metal atom is −0.246(9) (1) and 0.264(5) A (2), respectively,
out of the mean plane of the whole bpym molecule. The dihedral
angles between previous plane and mean equatorial one are 6.9(2)
(1) and 8.7(2)◦ (2).
The rhenium(III) complexes are stable in air as solids but in so-
[ReX4(bpym)] molecules are packed through weak forces, even
if there are aromatic groups no p–p interactions are present. It is
possible to define different planes of rhenium atoms in the crystal
packing: pair of parallel planes, with shortest Re · · · Re distance
lution are slowly oxidised by O2 giving a mixture of [ReX4(bpym)]
−
and ReO4
.
Description of the structures of [ReCl4(bpym)] (1) and
[ReBr4(bpym)] (2)
˚
between them of 6.766 in 1 and 6.886 A in 2 [Re(1) · · · Re(1a); a =
x − 1/2, −y + 1/2, −z], spaced by other planes forming a dihedral
angle with previous pair of sheets of 37.2 and 39.2◦ in 1 and 2,
respectively, giving rise to a herringbone motif (Fig. S2†). The
shortest Re · · · Re distances between the two not parallel planes
Compounds 1 and 2 are isostructural and consist of discrete
[ReX4(bpym)] molecules held together by means of van der Waals
interactions. A perspective drawing of the structure showing the
atom numbering is depicted in Fig. 1, while selected bond lengths
and angles are listed in Table 1. Each [ReX4(bpym)] unit contains
˚
are 6.550 and 6.728 A [Re(1) · · · Re(1b); b = −x + 1/2 + 1, −y,
z − 1/2].
654 | Dalton Trans., 2007, 653–660
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◦
a
˚
Table 1 Selected bond lengths (A) and angles ( ) for compounds 1–4
1 (X = Cl) 2 (X = Br) 3 (X = Cl)
4 (X = Br)
Re(1)–N(1)
Re(1)–N(2)
Re(1)–X(1)
Re(1)–X(2)
Re(1)–X(3)
Re(1)–X(4)
Re(2)–N(5)
Re(2)–N(6)
Re(2)–X(5)
Re(2)–X(6)
Re(2)–X(7)
Re(2)–X(8)
2.134(7)
2.131(7)
2.344(2)
2.317(2)
2.306(3)
2.322(2)
2.147(4)
2.144(4)
2.4964(7)
2.4418(6)
2.4394(7)
2.4604(6)
2.0545(14)
2.0438(15)
2.3717(5)
2.3766(6)
2.3780(5)
2.3711(5)
2.08(2)
2.08(2)
2.514(3)
2.523(3)
2.531(3)
2.536(3)
2.07(2)
2.09(2)
2.520(2)
2.519(2)
2.540(2)
2.521(2)
N(1)–Re(1)–N(2)
N(1)–Re(1)–X(1)
N(2)–Re(1)–X(1)
N(1)–Re(1)–X(3)
N(2)–Re(1)–X(3) 171.3(2)
N(1)–Re(1)–X(2) 170.2(2)
N(2)–Re(1)–X(2)
X(1)–Re(1)–X(2)
X(1)–Re(1)–X(3)
76.3(3)
85.8(2)
87.2(2)
95.0(2)
75.6(2)
85.1(1)
87.1(1)
76.90(6)
171.2(4)
94.29(4)
89.80(5)
91.73(4)
94.60(5)
171.16(4)
94.16(2)
89.80(5)
90.62(2)
90.68(2)
89.92(2)
178.06(2)
88.32(5)
87.41(4)
74.9(7)
92.6(5)
90.5(5)
169.1(5)
94.7(5)
97.0(5)
171.8(5)
91.2(1)
90.8(1)
177.4(1)
93.3(1)
89.6(1)
91.6(1)
84.9(5)
88.3(5)
75.7(8)
88.4(5)
94.8(5)
172.3(6)
92.5(5)
89.4(5)
170.3(5)
97.0(5)
86.2(5)
89.12(8)
92.60(8)
92.37(8)
92.32(8)
175.14(9)
89.40(8)
Fig. 2 Perspective drawing of the [ReCl4(bpym)]− anions in 3 showing
the atom numbering scheme. Thermal ellipsoids are drawn at 30% of
probability level.
96. 0(1)
171.5(1)
170.3(1)
94.7(1)
93.88(2)
93.20(3)
169.23(2)
93.69(3)
94.14(2)
93.43(3)
85.8(1)
93.9(2)
93.9(2)
92.6(1)
X(1)–Re(1)–X(4) 170.5(1)
X(2)–Re(1)–X(3)
X(2)–Re(1)–X(4)
X(3)–Re(1)–X(4)
N(1)–Re(1)–X(4)
N(2)–Re(1)–X(4)
N(5)–Re(2)–N(6)
N(5)–Re(2)–X(5)
N(5)–Re(2)–X(6)
N(5)–Re(2)–X(7)
N(5)–Re(2)–X(8)
N(6)–Re(2)–X(5)
N(6)–Re(2)–X(6)
N(6)–Re(2)–X(7)
N(6)–Re(2)–X(8)
X(5)–Re(2)–X(7)
X(6)–Re(2)–X(7)
X(6)–Re(2)–X(8)
X(5)–Re(2)–X(6)
X(5)–M(1)–X(8)
X(7)–M(1)–X(8)
94.8(1)
93.4(1)
93.5(1)
86.4(2)
85.7(2)
93.88(2)
Fig. 3 Perspective drawing of the [ReBr4(bpym)]− anions of the asym-
metric unit in 4 showing the atom numbering scheme. Thermal ellipsoids
are drawn at 30% of probability level.
a Estimated standard deviations in the last significant digits are given in
parentheses.
˚
˚
A in 3 and 2.526(2) and 2.07(2) A in 4] are in agreement with those
found in the literature for compounds containing rhenium(III),17,18
and it can be noted that while Re–X bond distances enlarge, Re–
N bond distances shorten in 3 and 4 when compared to 1 and 2.
Shorter Re–N bond distances indicate that a stronger bond occurs
in compounds 3 and 4 where rhenium atom is in a lower oxidation
state, evidencing a higher affinity of Re(III) respect to Re(IV) for
bpym, and reflecting the p-accepting ability of this heterocyclic
ligand.
Description of the structures of (PPh4)[ReCl4(bpym)] (3) and
(NBu4)[ReBr4(bpym)] (4)
The structure of compounds 3 and 4 consists of discrete
[ReX4(bpym)]− anions and tetraphenylphosphonium (3) or tetra-
butylammonium (4) cations. Two crystallographically inequiv-
alent complex anions are present in the asymmetric units of
compound 4. A perspective drawing of the structures showing
the atom numbering is depicted in Fig. 2 and 3, and selected
bond lengths and angles are summarized in Table 1. Each
[ReX4(bpym)] unit contains a Re(III)atom in the same environment
of 1 and 2, being bonded to a bidentate bpym molecule and four
chloride/bromide anions in a distorted octahedral geometry. As
in all compounds with chelating ligands, the main distortion of
the ideal octahedral geometry is due to the short bite angle of the
bidentate bpym ligand [76.90(6) in 3 and 75.0(6) and 75.7(7)◦ in 4].
The average Re–X and Re–N bond distances [2.374(1) and 2.049(1)
Unlike structures 1 and 2, no significant variations in the Re–X
bond lengths in trans and cis positions to the nitrogen donors were
observed in 3 and 4. As in compounds 1 and 2, the best equatorial
plane around the Re atom contains the nitrogen atoms of bpym
ligands [the largest deviation from planarity being 0.018(1) for
˚
N(2) and 0.047(8) A for N(5) in 3 and 4, respectively] and forms a
dihedral angle with the mean plane of the bpym molecule of 12.0(1)
in 3 and 11.2(5) and 7.6(6)◦ in 4. The Re atom is practically placed
in this plane. The pyrimidyl rings are planar [largest deviation
from the mean planes not greater than 0.033(1) A for C(4) in 3
and 0.04(1) A for N(2) in 4] and the C–C and C–N bond lengths
˚
˚
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are as expected. As found in the structures of 1 and 2, the bpym
molecule as a whole shows evident deviation from planarity, largest
Irreversible couple II appears at Eꢀ0 = +1.35 V, with DEp
=
0.18 V, and ipa/ipc < 1, and is assigned to a Re(IV)/Re(V)
pair. This process is controlled by the slow transference of the
electron, according to the linear dependence between Epa and ln m
(slope 29 mV decade−1). Both oxidation processes (couples I and
II) involve electroactive species diffusing onto the electrode, as
assessed by a linear plot for anodic current peak versus the square
root of the scan rates.
˚
deviations from the mean planes being 0.217(2) (3) and 0.15(2) A
(4) and again there is an appreciable tilt between the two pyrimidyl
rings, the dihedral angles between them being 16.0(1) in 3 and 13(1)
and 14(1)◦ in 4.
˚
The metal atom is 0.425(2) A out of the mean plane of the whole
˚
bpym molecule in 3 and 0.37(2) and 0.28(2) A out of the previous
plane, respectively, for Re(1) and Re(2) in 4.
The third signal (couple III) has a noticeable cathodic contri-
bution detected at −0.47 V with a small anodic counterpart at
−0.33 V (ipa/ipc ꢁ 1). A comparison with the profile of Au-pc
in 1 mM bpym solution in acetonitrile (Fig. 4, inset) allows us
to assign these waves to a redox process involving the ligand,
superimposed to the reduction of the supporting electrolyte.
Fig. 5a shows the voltammetric profile of Au-pc in 1 mM solu-
tion of [ReBr4(bpym)] in acetonitrile, obtained at m = 0.050 Vs−1
for the first potential scan (solid line). At first sight, this profile
presents more complexity when compared to the chloro analogue.
However, it is possible to recognize couples I and II, corresponding
to the successive oxidations Re(III) → Re(IV) and Re(IV) → Re(V).
[ReX4(bpym)]− anions are well separated by the bulky PPh4
+
+
(3) or NBu4 (4) cations in the resulting three-dimensional ionic
lattices. The values of the shortest interionic Re · · · Re distances
are 8.412(1) (3) [Re(1) · · · Re(2a); (a) = 1 − x, 2 − y, 2 − z] and
˚
8.974 (1) A (4) [Re(1) · · · Re(2a); (a) = 1 + x, y, z].
+
The PPh4 cation in 3 shows the expected tetrahedral shape
and its bond lengths and bond angles are in agreement with
those previously reported in other compounds.19–22 Each cation is
involved in a parallel-fourfoldphenyl-embrace (P4PE), in which two
phenyl rings are approximately parallel and the motif comprises
one offset-face-to-face (off) and two edge-to-face (ef) attractive
phenyl interactions.23 The N–C and C–C distances and the N–
The reversible first couple is located at Eꢀ0 = +0.32 V (DEp
=
+
C–C and C–C–C angles of each tetrahedral NBu4 cation in 4
0.06 V), and irreversible couple II has an Ea = +1.25 V and Ec =
+0.95 V. Contributions originated in the system ligand–supporting
electrolyte–solvent are also present.
are affected by same scatterings. This doubtless arises from the
considerable thermal motion, which is quite a common feature
observed in most of the crystals containing this organic cation.
Fig. 5a also shows some other new contributions that did not
appear in the first potential scan, but from recording the profile
after some minutes of work, cycling between −0.10 V and + 0.90 V
at the same scan rate (dashed line). The new peaks are labeled
IVa (+0.43 V), Va (+0.67 V), VIc (+0.26 V) and VIIc (0.05 V).
As can be seen in Fig. 5b, after 1 h of work, the couple I is
still detected, but anodic current peaks IVa and Va predominate.
These anodic contributions that were not detected during the first
cycles of the potential scan, arose during the working time and
their intensity current peaks reached a maximum when couple I
disappeared. Irreversible oxidations waves IVa and Va correspond
Electrochemistry
Fig. 4 shows the voltammetric profile of Au-pc in 1 mM solution
of [ReCl4(bpym)] in acetonitrile obtained at m = 0.050 Vs−1. Three
remarkable redox couples can be detected. Couple I is a reversible,
one electron process located at Eꢀ0 = +0.19 V, with DEp = 0.06 V,
and ipa/ipc = 1, where ipa and ipc denote the intensities of the anodic
and cathodic current peaks respectively, DEp represents the anodic
to cathodic peak separation (Epa − Epc), and Eꢀ0 corresponds to
the formal redox potential of the couple calculated as (Epa
+
−
to the formation of ReO4 from adsorbed ReO2 monolayers or
Epc)/2. This couple can be assigned to a Re(III)/Re(IV) pair,
due to the alternating appearance and disappearance of the red–
purple coloration of the Re(III)-complex on the surface of the gold
electrode.
multilayers, respectively, while the cathodic peaks VIc and VIIc
−
are consequences of the formation of ReO2 from ReO4 and
subsequent steps of reduction to species such [ReIIIO]+ and ReO.24
In order to confirm these assignments, the working electrode was
then rinsed with acetonitrile and stored without further cleaning
in a solution of the supporting electrolyte, and the voltammetric
profile recorded again. This is shown in Fig. 5c, which shows
only the characteristic signals corresponding to the formation of
soluble perrhenate from ReO2 deposited on the electrode.
A second set of experiments was performed in DMF as solvent.
The voltammetric profile of complex 1 is not so well resolved
in DMF as in acetonitrile (Fig. S3†). In any case, couple I is
clearly located at Eꢀ0 = +0.10 V (DEp = 0.10 V, ipa/ipc < 1). For
this couple, peak potential values are not dependant on the scan
rate, indicating a fast electron exchange. The process involves the
electroactive species diffusing onto the electrode, according to the
ip versus v1/2 linear adjustment. Couple I is partially overlapped
to the redox contributions ascribed to electron exchanges of the
system ligand–supporting electrolyte–solvent. Couple II, corre-
sponding to the Re(IV)/Re(V) pair, is poorly defined having an
anodic contribution at +1.30 V, and a cathodic one at ca. +0.8 V.
The profile of complex 2 in DMF (Fig. S4†) shows a quasi re-
versible couple I located at Eꢀ0 = +0.22 V (DEp = 0.08 V, ipa/ipc = 1
Fig. 4 Voltammetric profile of Au-pc in 1 mM [ReCl4(bpym)] in 0.04 M
tetraethylammonium perchlorate in acetonitrile. Voltammetric profile
of Au-pc in 1 mM 2,2ꢀ-bipyrimidine in 0.04 M tetraethylammonium
perchlorate in acetonitrile (inset). T = 25 ◦C, m = 0.050 V s−1
.
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Fig. 5 Voltammetric profile of Au-pc in 1 mM [ReBr4(bpym)] in 0.04 M tetraethylammonium perchlorate in acetonitrile. (a) Profile for the first potential
scan recorded at m = 0.050 V s−1(solid line) and profile obtained after some minutes of work at the same potential scan rate (dashed line). (b) Profile
after 1 h of work, recorded at low potential scan rates (m = 0.025 V s−1). (c) Voltammetric profile of Au-pc in 0.04 M tetraethylammonium perchlorate in
acetonitrile: electrode stored 24 h without cleaning.
at m = 0.050 V s−1). This process is also diffusion controlled, as ip
Experimental
adjusts linearly with m1/2. It is possible to detect the couple II, with
Preparation of compounds
an anodic current peak at +1.15 V, and a cathodic contribution
at ca. + 0.8 V. The potential peak value for the anodic wave is
independent of m and ipa adjusted linearly with m1/2, involving the
diffusion of the electroactive species. Other redox contributions
from the ligand and/or the supporting electrolyte are not ob-
served, probably due to overlap by the Re(III)/Re(IV) couple.
All reagents, metal salts and ligands were used as received from
commercial sources. Ammonium hexachlororhenate and potas-
sium hexabromorhenate were prepared from the corresponding
perrhenate salts following literature procedures.25 The K2[ReBr6]
was transformed into the n-tetrabutylammonium salt by meta-
thetic reaction in cold 0.5 M HBr. Solvents were dried by standard
methods when necessary. N,N-Dimethylformamide (water con-
˚
tent 0.1%) was stored with Linde type 4 A molecular sieves.
Conclusions
[ReCl4(bpym)] 1. Ammonium hexachlororhenate (0.65 g,
1.5 mmol) and bpym ◦(0.36 g, 2.25 mmol) were stirred in DMF
(4 mL) for 4 h at 110 C. After cooling the dark brown solution
was filtered and evaporated at 60 ◦C until the complete elimination
of solvent. The brown residue was washed with acidified H2O
(3 × 15 mL H2O with 2–3 drops of 0.5 M HCl) and left to dry
in the air. This solid was then shaken with CH2Cl2 (50 mL) for
30 min and the resulting solution was filtered and the insoluble
residue was discarded. The filtrate was washed several times with
portions of H2O (25 mL) in a separation funnel until the aqueous
layer became almost colourless. The dark orange dichloromethane
layer was then dried and the solvent evaporated under reduced
pressure to leave an orange–yellow microcrystalline solid which
was finally dried at 110 ◦C. Yield: 20%. If all the pink aqueous
fractions are collected and left to stand in air, the Re(III) complex
is reoxidized to Re(IV) and a further crop of crystals of 1 can be
obtained, increasing the yield by up to 25% (Found: C, 19.6; H,
1.23; N, 11.1. C8H6N4Cl4Re requires C, 19.8; H, 1.24; N, 11.5%);
kmax/nm (CH3CN) 240 (e/dm3 mol−1 cm−1 20 300), 273sh, 290
(12000), 660sh, 719 (31) and 769 (36); IR bands associated with
coordinated bpym at mmax(KBr disc)/cm−1 1579vs, 1547m, 1406vs,
819m, 746s and 667m. Golden crystals of 1 suitable for X-ray
determination were grown from a solution in CHCl3 by very slow
evaporation at room temperature.
We have reported on the synthesis of the first examples of Re(IV)-
bpym complexes, 1 and 2, by ligand substitution onto [ReX6]2−
anions in DMF, and Re(III)-bpym complexes, 3 and 4, by reduction
of the former compounds.
Rhenium–nitrogen bond distances shorter in 3 and 4 than in 1
and 2, respectively, indicate a stronger bond in the lower oxidation
state, which can be explained on the basis of the p-acceptor
character of bpym. The higher affinity of Re(III) for bpym stabilizes
oxidation state III relative to IV, and then complexes 1 and 2 can
be easily reduced, chemically and electrochemically.
Electrochemical reduction of 1 to 3 is reversible in acetoni-
trile; reduction of 2 to 4 is reversible in acetonitrile and quasi
reversible in DMF. Complex 2 evidences a notorious instability in
acetonitrile towards formation of rhenium dioxide and perrhenate.
Voltammograms also show that 1 and 2 could be oxidized, but
these processes are not reversible, probably due to the expected
high reactivity of the Re(V) species. It can be noted that for the
oxidation process Re(IV) → Re(V), the anodic peak is less positive
for 2 than for 1 in both solvents, so 2 can be more easily oxidized
than 1. On the contrary, the bromine complex 2 is ca. 0.12 V easier
to reduce to Re(III) than the chlorine complex 1 in acetonitrile and
DMF, that suggests that bromo ligands contribute to stabilizing
the oxidation state III with respect to IV better than chloro ligands.
◦
For both complexes, Eꢀ values in CH3CN are ca. 0.10 V more
positive than the related potential values in DMF, and DEp are
[ReBr4(bpym)] 2. Potassium hexabromorhenate (223 mg,
smaller in CH3CN than in DMF, due to a faster electron transfer.
0.3 mmol) and bpym (47.4 mg, 0.3 mmol) were mixed in DMF
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(1 mL) and heated at 90 ◦C over 2 h with continuous stirring.
The resulting dark brown solution was evaporated at 60 ◦C
until complete elimination of DMF. The residue was extracted
with CHCl3 to give a reddish brown solution which was filtered
and evaporated to dryness. The last residue was then extracted
with dry methanol, filtered and evaporated to dryness again.
The crude product was finally dried at 100 ◦C to give 2 as a
dark powder. Yield: 40% (Found: C, 14.3; H, 1.05; N, 8.40.
C8H6N4Br4Re requires C, 14.5; H, 0.91; N, 8.44%); kmax/nm
(CH3CN) 227 (e/dm3 mol−1 cm−1 16100), 245 (16100), 308sh, 356
(4600), 688 (110), 741 (110) and 822 (110); IR bands associated
with coordinated bpym at mmax(KBr disc)/cm−1 1576vs, 1544m,
1404vs, 814m, 743s and 665m. Red crystals of 2 suitable for X-ray
determination were grown from a solution in CHCl3 by very slow
evaporation at room temperature.
ethanol under inert atmosphere was treated with ascorbic acid
(50 mg, 0.28 mmol). The colour changed slowly from yellow to
violet as the rhenium(IV) complex was dissolved. Then an excess
of tetraphenylphosphonium chloride (150 mg, 0,33 mmol) was
added. The solution was filtered and left to evaporate slowly in an
inert atmosphere. Small polyhedric crystals of 3 appeared after a
few days.
(NBu4)[ReBr4(bpym)] 4. Tetrabutylammonium hexabromo-
rhenate (230 mg, 0.2 mmol) and bpym (64 mg, 0.4 mmol) were
refluxed in 2-propanol : acetone 10 : 1 (20 mL) for 1 h. After
cooling, the solution was filtered and allowed to stand, producing
well-formed dark red–purple crystals of complex 4 in 15% yield
after 3 days. Alternatively, if the mentioned solution was first
shaken with ascorbic acid (176 mg, 1.0 mmol) for 24 h and then
filtered and allowed to stand for a few days, the yield rose to
up to 30% (Found: C, 31.85; H, 4.72; N, 7.67. C24H42N5Br4Re
requires C, 31.80; H, 4.67; N, 7.73%); kmax/nm (CH3CN) 240
(e/dm3 mol−1 cm−1 20 600), 352 (7200), 390sh, 524 (4500) and 786
(2400); IR bands associated with coordinated bpym at mmax(KBr
disc)/cm−1: 1571m, 1392vs, 1170s, 999s, 734m.
(PPh4)[ReCl4(bpym)] 3. Complex 1 (48.6 mg, 0.1 mmol) was
dissolved in the minimum amount of acetonitrile and the resulting
yellow solution was loaded onto an alumina column partly
deactivated with 10% water. At once, a colour change from
yellow to violet was observed at the top of the column. The
sample was then eluted with tetraphenylphosphonium chloride
0.013 M in acetonitrile, and the violet fraction was collected.
The solvent was then evaporated to dryness at room temperature
to give a mixture of an almost black crystalline solid (3) and a
large excess of PPh4Cl. The latter was dissolved by the addition
of water and the insoluble crystals of 3 filtered, washed with
water and dried in air. Yield: >80% (Found: C, 46.4; H, 3.11;
N, 6.30. C32H26N4Cl4PRe requires C, 46.6; H, 3.17; N, 6.78%);
kmax/nm (CH3CN) 431 (e/dm3 mol−1 cm−1 3400), 521 (6000),
592sh and 798 (2300); IR bands associated with coordinated
bpym at mmax(KBr disc)/cm−1: 1387vs, 1179s, 997s. Better crystals
of 3 for crystallographic studies could be obtained by a second
procedure: A slurry of complex 1 (10 mg, 0.02 mmol) in absolute
Crystallography
X-Ray diffraction data were collected using a Bruker R3m/V
automatic four-circle for compounds 1, 4 and a Bruker-Nonius
X8APEXII CCD area detector diffractometer for the complexes
2, 3. Graphite-monochromated Mo-Ka radiation (k = 0.71073
˚
A) was used in both cases. Lorentz-polarization and empirical
absorption corrections through the w-scan program26 was applied
for compound 4, whereas the data of compound 1 was corrected by
the XABS program.27 The maximum and minimum transmission
factors were 0.0931 and 0.1965 for 1, and 0.1919 and 0.1267 for 4.
The data for compounds 2, 3 were processed through the SAINT28
Table 2 Crystal data and structure refinement for [ReX4(bpym)] where X = Cl (1) and X = Br (2), P(C6H5)4[ReCl4(bpym)] (3) and N(C4H9)4[ReBr4(bpym)]
(4)a
Compound
1
2
3
4
Formula
C8H6Cl4N4Re
486.17
Orthorhombic
P212121
4
9.154(2)
10.703(3)
13.401(2)
90.0
90.0
90.0
1313.0(5)
2.459
900
C8H6Br4N4Re
664.01
Orthorhombic
P212121
4
9.6216(9)
10.8583(10)
13.4431(11)
90.0
90.0
90.0
C32H26Cl4N4PRe
1651.08
C48H84Br8N10Re2
1812.93
Monoclinic
P21
Formula weight
Crystal system
Space group
Z
Triclinic
¯
P1
2
2
˚
a/A
9.0661(9)
12.6350(12)
14.2539(13)
89.361(2)
86.606(2)
82.236(2)
1615.0(3)
1.698
16.227(3)
11.094(2)
18.502(4)
90.0
105.87(3)
90.0
3203.8(11)
1.879
1744
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
1404.5(2)
3.140
1188
Dc/g cm−3
F(000)
808
41.72
l/cm−1
100.49
200.15
88.02
refln. collected/obs.
Goodness-of-fit on F2
R1 [I > 2r(I)]b
Absolute structure parameter
4577/3835
1.058
0.0422
0.008(15)
0.1059
1.362 and −1.844
17330/4872
0.989
0.0299
20306/10366
1.027
0.0200
6237/5992
0.892
0.0501
0.03(2)
0.1143
−0.030(14)
c,d
wR2
0.0646
0.0473
0.851 and −0.660
−3
˚
Largest diff. peak and hole/e A
)
1.551 and −1.140
1.485 and −1.762
a Details in common: T = 293(2) K. b R1 = RꢂFo| − |Fcꢂ/R |Fo|. c wR2 = {R [w(Fo − Fc2)2]/[(w(Fo2)2]}
.
d w = 1/[r2(Fo2) + (aP)2 + bP] with P =
2
1/2
[Fo + 2Fc2]/3, a = 0.0656 (1), 0.0289 (2), 0.0226 (3) and 0.0720 (4), b = 4.6686 (1), 0.3058 (3) and 0 (2, 4).
2
658 | Dalton Trans., 2007, 653–660
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reduction and SADABS29 absorption software. The unit cell
parameters, which are listed in Table 2 together with a summary
of the structure refinement data, in 1 and 4 were determined from
least-squares refinement of the setting angles of 25 reflections in the
2h range 15–30◦, while in 2 and 3, were based upon least-squares
refinement of 8317 (2) and 6489 (3) reflections. The structures
were solved by Patterson methods and subsequently completed
by Fourier recycling using the SHELXTL software package.30
Non-hydrogen atoms were refined anisotropically except the side
chain carbon atoms C(21)–C(22)–C(23)–C(24) of the n-butyl in
compound 4 that are affected by a large thermal motion, therefore
were set on DF map and not refined. Hydrogen atoms were set
in calculated positions and refined as riding atoms. Full-matrix
least-squares refinements on F2, carried out by minimizing the
function Rw(|Fo| − |Fc|)2, reached convergence with values of
the discrepancy indices given in Table 2. The final geometrical
calculations were carried out with the PARST program.31 The
graphical manipulations were performed using the XP utility of
the SHELXTL system.
D. Armentano, G. De Munno, F. Lloret, J. Cano and J. Faus, Inorg.
Chem., 2004, 43, 7823.
2 (a) G. De Munno and M. Julve, in Metal Ligand Interactions. Structure
and Reactivity, ed. N. Russo and D. Salahub, NATO ASI Series C474,
Kluwer: Dordrecht, 1996, p. 139 and references therein; (b) G. De
Munno, F. Lloret and M. Julve, in Magnetism: A Supramolecular
Function, ed. O. Kahn, NATO ASI Series C484, Kluwer: Dordrecht,
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K. R. Dunbar, Inorg. Chem., 1999, 38, 5841; (g) S. R. Marshall, C. D.
Incarvito, J. L. Manson, A. L. Rheingold and J. S. Miller, Inorg. Chem.,
2000, 39, 1969; (h) D. Armentano, G. De Munno, J. Faus, F. Lloret and
M. Julve, Inorg. Chem., 2001, 40, 655; (i) D. Armentano, G. De Munno,
F. Lloret, M. Julve, J. Curely, A. M. Babb and J. Y. Lu, New J. Chem.,
2003, 27, 161; (j) J. A. Real, A. B. Gaspar, C. M. Mun˜oz, P. Gu¨tlich,
V. Ksenofontov and H. Spiering, in Topics in Current Chemistry, ed.
P. G u¨tlich and H. A. Goodwin, Springer: Berlin, 2004, vol. 233, pp.
167–193; (k) S. Triki, F. The´tiot, F. Vandevelde, J. Sala-Pala and C. J.
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J. Sala-Pala and S. Golhen, Inorg. Chim. Acta, 2005, 358, 3277 and
references therein.
Testing the two enantiomeric models of the crystal 1, 2 and
4 with a final refined Flack parameter of 0.01(1), 0.03(1), and
0.03(2), respectively, shows that the handedness was uniquely
determined. CCDC reference numbers are 623923–623926 for
complexes 1–4, respectively. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b615136a
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Other measurements
Electronic absorption spectra were recorded with a Shimadzu
UV-1603 spectrophotometer. Infrared spectra (KBr pellets) were
obtained on a Perkin-Elmer 1750 FT-IR spectrometer. Elemental
analyses for carbon, hydrogen and nitrogen were carried out in a
Carlo Erba EA1108 elemental analyzer.
The voltammetric profile of the systems were evaluated in ace-
tonitrile or DMF with tetraethylammonium perchlorate (0.04 M)
as supporting electrolyte, at potential scan rates m varying between
0.01 V s−1 ≤ m ≤ 0.10 V s−1. Measurements were carried out in a one
compartment conic cell, using a polycrystalline (pc) Au disc (3 mm
diameter) as working electrode, a Pt sheet as counter electrode and
Ag/AgNO3 0.1 M in the supporting electrolyte (E = 0.60 V vs.
NHE) as the reference. All potentials in the text are referenced to
the NHE.
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Acknowledgements
The authors thank the financial support from the EU (Project
QueMolNa MRTN-CT-2003-504880), the Ministerio Espan˜ol de
Educacio´n y Ciencia (Project CTQU2004-03633), the Comisio´n
Sectorial de Investigacio´n Cient´ıfica (Project 290) and the Pro-
grama de Desarrollo de Ciencias Ba´sicas (Uruguay) and the Italian
Ministero dell’Istruzione, dell’Universita` e della Ricerca.
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660 | Dalton Trans., 2007, 653–660
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