Synthesis and characterization of new imidorhenium(V) and imidorhenium(VI) complexes of pyridyltriazines and pyrazinyltriazine with halide coligands including rare iodide

Article abstract of DOI:10.1016/j.ica.2008.02.001

The blue colored imido complexes [Re(NC₆H₄Cl)X₃(L)] have been synthesized by three methods: (i) reaction of [Re^VOX₃(L)] with p-ClC₆H₄NH₂, (ii) reaction of [Re^III<

Full text of DOI:10.1016/j.ica.2008.02.001

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Inorganica Chimica Acta 361 (2008) 2815–2820
www.elsevier.com/locate/ica
Synthesis and characterization of new imidorhenium(V) and
imidorhenium(VI) complexes of pyridyltriazines
and pyrazinyltriazine with halide coligands including rare iodide
Samir Das *
Indian Association for the Cultivation of Science, Department of Inorganic Chemistry, Jadavpur, Kolkata 700 032, India
Received 20 July 2007; received in revised form 27 January 2008; accepted 4 February 2008
Available online 8 February 2008
Abstract
The blue colored imido complexes [Re(NC6H4Cl)X3(L)] have been synthesized by three methods: (i) reaction of [ReVOX3(L)] with
p-ClC6H4NH2, (ii) reaction of [ReIII(OPPh3)X3(L)] with p-ClC6H4NH2 and (iii) reaction of [ReVOX3(PPh3)2] with L followed by the
addition of p-ClC6H4NH2 in boiling toluene. Here, X = Cl, Br, I and L are 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine (L2) and its dimethyl
(L1) and pyrazinyl (L3) analogues. The [Re(NC6H4Cl)Cl3(L1)] (1a), [Re(NC6H4Cl)Cl3(L2)] (1b), [Re(NC6H4Cl)Br3(L2)] (1c),
[Re(NC6H4Cl)I3(L2)] (1d), [Re(NC6H4Cl)Cl3(L3)] (1e), [Re(NC6H4Cl)Br3(L3)] (1f), [Re(NC6H4Cl)I3(L3)] (1g), complexes have been char-
acterized electrochemically and spectroscopically. The X-ray structures of [Re(NC6H4Cl)Cl3(L2)] and [Re(NC6H4Cl)I3(L3)] reveal that
the ReCl3 fragment is meridionally disposed and that the L ligand is N,N-coordinated such that the pyridine/pyrazine nitrogen lies trans
to the imide nitrogen. The feasibility of generating the rhenium(VI) congener of the imidorhenium(V) complex is also examined with the
help of six-line EPR spectra at room temperature.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Imidorhenium(V); Imidorhenium(VI); Pyridyltriazines; Pyrazinyltriazine; Iodo species; Crystal structure
1. Introduction
determination has authenticated the presence of various
halide coligands including rare iodide. The complexes dis-
playing two bands in the visible region near 590 and
This work stems from our previous interest in the tran-
sition-metal chemistry of bipyridine-like ligands in general
and pyridyltriazines and pyrazinyltriazine, in particular [1].
The richness of this chemistry in the cases of iron [2], ruthe-
nium [3] and molybdenum [4] prompted us to explore par-
allel developments for the neighboring element rhenium. In
the present work, the successful synthesis of rhenium(V)
arylimides complexes of 5,6-dimethylpyridyltriazine, 5,6-
diphenylpyridyltriazine and 5,6-diphenylpyrazinyltriazine
with various halide coligands is explored. The complexes
have been isolated in the solid state and crystal structure
1
750 nm, are diamagnetic and show well defined H NMR
lines. They are electroactive in acetonitrile solution and
exhibit a well defined ReVI/ReV couple near 1.0 V versus
SCE. In general, the reduction potentials of pyrazinyltri-
azine complexes are greater than pyridyltriazine complexes
reflecting the superior electron withdrawing ability of the
pyrazinyl fragment [5]. Similarly, phenyl substituted tri-
azine complexes are superior electron withdrawal than
the corresponding methyl substituted ones. The feasibility
of generating the rhenium(VI) congener of the imidorheni-
um(V) complex is examined with the help of electrochemi-
cal and EPR techniques [6]. The dianionic imido nitrogen
in NAr2ꢀ is formally isoelectronic with the oxo ligand
and their rhenium(V) chemistry has received significant
attention [7–9].
*
Present address: Department of Chemistry, Washington State Univer-
sity, Pullman, WA 99164-4630, United States. Tel.: +1 509 335 5334; fax:
+1 509 335 8867.
E-mail address: sdas@wsu.edu
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.02.001

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S. Das / Inorganica Chimica Acta 361 (2008) 2815–2820
2. Experimental
2.1. Materials
2.3. Crystal structure determination
Single crystals of the complexes [Re(NC6H4Cl)Cl3(L2)],
1b and [Re(NC6H4Cl)I3(L3)] (1g) were grown by slow diffu-
sion of hexane into dichloromethane solutions of the
respective compounds. Data were collected on a Nicolet
R3m/V four-circle diffractometer with graphite-monochro-
[ReOX3(PPh3)2], [ReOX3(L)], [Re(OPPh3)X3(L)] and L
were prepared as reported methods [8]. p-Chloroaniline
was purchased from Aldrich (USA) and used as obtained.
For electrochemical work, HPLC grade acetonitrile was
used. All other chemicals and solvents were of reagent
grade and were used as received.
˚
mated Mo Ka radiation (k = 0.71073 A) by the x-scan
technique in the range 3° 6 2h < 47° and 3° 6 2h < 50°,
respectively, in the case of 1b and 1g complexes. All data
were corrected for Lorentz-polarization and absorption
[10]. The metal atoms were located from Patterson maps
and the rest of the non-hydrogen atoms emerged from suc-
cessive Fourier syntheses. The structures were then refined
by full-matrix least-squares procedure on F2. All non-
hydrogen atoms were refined anisotropically. All hydrogen
atoms were included in calculated positions. Calculations
were performed using the SHELXTLTM V 5.03 program pack-
age [11].
2.2. Physical measurements
UV–Vis spectra (CH2Cl2 solution), IR spectra (KBr
1
disk), and H NMR spectra (CDCl3) were recorded on a
Shimadzu UVPC 1601 spectrophotometer, a Nicolet
Magna IR 750 Series II spectrometer, and a Bruker FT
300 MHz spectrometer, respectively. The numbering
1
scheme used for H NMR assignment is the same as in
crystallography. Spin–spin structures are abbreviated as
follows: s, singlet; d, doublet; t, triplet. Microanalyses (C,
H, N) were performed using a Perkin–Elmer 2400 Series
II elemental analyzer. EPR spectra were recorded on a
Varian E-109C X-band spectrometer. Electrochemical
measurements were performed under nitrogen atmosphere
using a CH 620A electrochemical analyzer, with platinum
working electrode. The supporting electrolyte was tetrae-
thylammonium perchlorate (TEAP), and the potentials
are referenced to the saturated calomel electrode (SCE)
without junction correction. Mass spectra were measured
with Q-TOF mass spectrometer (MeCN).
2.4. [Re(NC6H4Cl)Cl3(L2)] (1b)
Empirical formula C52H36Cl8N10Re2, crystal system
monoclinic; space group P21/c; a = 10.956(7) A, b =
˚
˚
˚
26.150(10) A, c = 18.252(5) A, b = 93.38(4)°; V = 5220(4)
3
˚
A ; Z = 4, 7634 unique reflections. Final residuals R1 =
0.0575; wR2 = 0.1335 [I > 2r(I)].
2.5. [Re(NC6H4Cl)I3(L3)] (1g)
Empirical formula C25H17ClI3N6Re, crystal system
monoclinic; space group P21/c; a = 12.534(7) A, b =
˚
˚
˚
15.180(5) A, c = 16.487(7) A, b = 112.00(4)°; V = 2909(2)
3
˚
A ; Z = 4. 5584 unique reflections. Final residuals R1 =
NAr
0.0494; wR2 = 0.1180 [I > 2r(I)].
N
N
N
N
X
X
X
V
Re
R
N
N
R
R
N
E
3. Results and discussion
R
1
N
E = CH, R = Me, L
3.1. Synthesis
2
E
E = CH, R = Ph, L
Upon boiling [ReOX3(L)] with p-ClC6H4NH2 in tolu-
ene a blue solution is formed and on work-up, this
afforded blue colored imido complexes of type 1. The
reaction is stated in Eq. (1). Here, the ReVO moiety acts
as a base
Ar = C H Cl(p)
3
6 4
E = N, R = Ph, L
E = CH, R = Me, X = Cl, 1a
E = CH, R = Ph, X = Cl, 1b
E = CH, R = Ph, X = Br, 1c
E = CH, R = Ph, X = I, 1d
E = N, R = Ph, X = Cl, 1e
E = N, R = Ph, X = Br, 1f
E = N, R = Ph, X = I, 1g
½ReVOX3ðLÞꢁ þ p-ClC6H4NH2
! ½ReVðNC6H4ClÞX3ðLÞꢁ þ H2O
ð1Þ
2.2.1. Synthesis of complexes
and the oxo function is transferred formally as an oxide
dianion which then combines with amine protons generat-
ing bound imide and free water. This is to be contrasted
with the transfer of an oxygen atom from ReVO to the oxo-
philic substrates [1].
It was observed that the same imido species could also
be prepared by boiling [ReIII(OPPh3)X3(L)] with p-ClC6-
H4NH2 in toluene in air. In effect the OPPh3 ligand is
The X = Cl, Br imido complexes, i.e., 1a–c, 1e and 1f
were synthesized by the three general procedures and
details are given in the case of one complex while the iodo
species 1d and 1g are quite stable unlike their oxo precur-
sors were isolated by procedure (iii) only. The details of
syntheses and characterization of the compounds are given
in the Supplementary material.

S. Das / Inorganica Chimica Acta 361 (2008) 2815–2820
2817
replaced by NAr2ꢀ and the metal oxidation state increases
by two units [1].
It was subsequently observed that reaction between
[ReVOX3(PPh3)2] and L in toluene followed by the addition
of p-ClC6H4NH2 afforded the imido complexes 1 in a one-
pot reaction. Another advantage of this procedure is that
the X = I species could also be isolated. These iodo species
are quite stable unlike their oxo precursors, [ReVOI3(L)],
which could not be isolated. The methods by which the
complexes of type 1 are synthesized are shown in Scheme 1.
3.2. Spectra and electrochemistry
Fig. 1. Cyclic voltammogram of [Re(NC6H4Cl)Cl3(L3)] (1e) in acetonitrile
solution at platinum working electrode (scan rate 100 mV sꢀ1).
The UV–Vis spectra of the imido complexes are qualita-
tively similar to those of the oxo precursors and are char-
acterized by two bands in the visible region near 590 and
750 nm, precluding the possible observation of ligand field
transitions of the type dxy ! dxz, dyz [8]. Such transitions
may be the origin of the two relatively weak bands
observed in the visible spectrum of [Re(MeNH2)4-
(MeN)Cl]2+ [6]. The complexes are diamagnetic and dis-
dized metal, the X-band EPR spectra in dried acetonitrile
were examined. The spectrum of 1e+ is shown in Fig. 2.
3.4. Structures
3.4.1. [Re(NC6 H4 Cl)Cl3 (L2)] (1b)
The complex crystallizes with two metrically similar but
crystallographically distinct molecules (Molecule 1 and
1
play well resolved H NMR lines in the aromatic region.
The NMR data are given in the Supplementary material.
The complexes are electroactive in acetonitrile solution
and exhibit a well defined ReVI/ReV couple near 1.0 V ver-
sus SCE. In general, the potentials of the L3 species are
slightly higher than the corresponding L2 species. Similarly,
phenyl substituted L2 complexes have superior electron
withdrawing ability than the corresponding L1 ligated
ones. There is a relatively large decrease (by 0.6 V) in the
reduction potential in going from [ReOX3(L)] to
[Re(NAr)X3(L)], signifying a better stability of the ReVI
state under imide coordination [1,8,9].
3.3. Electrochemical oxidation of type 1 species
Blue colored type 1 complexes undergo one-electron oxi-
dation electrochemically affording green colored 1+ com-
plexes at room temperature. 1+ complexes are very
unstable and spontaneously reduce to type 1 complexes.
The representative case of 1e, which forms relatively stable
electrooxidized congener, will be considered. Constant
potential coulometry of this complex at 1.15 V in dry ace-
tonitrile furnished the cation [ReVI(NC6H4Cl)Cl3(L3)]+
(Scheme 1) whose cyclic voltammogram (initial scan catho-
dic) is the same as that of the parent complex (initial scan
anodic), Fig. 1. In order to confirm that 1e+ contains oxi-
Fig. 2. X-band EPR spectrum of [Re(NC6H4Cl)Cl3(L3)]+, 1e+ in aceto-
nitrile solution (300 K). Instrument settings power, 30 dB; modulation,
100 KHz; sweep center, 3200 G, At center field g = 1.97, Average values
A = 459 G.
[ReVOX3(L)]
i)
V
[ReVOX3(PPh3)2]
[Re (NC6H4Cl)X3(L)]
iii)
[ReVI(NC6H4Cl)X3(L)]+
iv)
ii)
[ReIII(OPPh3)X3(L)]
Scheme 1. (i) H2NC6H4Cl, X = Cl, Br; (ii) H2NC6H4Cl, X = Cl, Br; (iii) L, H2NC6H4Cl, X = Cl, Br, I; (iv) electrochemical oxidation.

2818
S. Das / Inorganica Chimica Acta 361 (2008) 2815–2820
Table 1
Molecule 2) in the asymmetric unit. Perspective views of
the molecules are shown in Fig. 3 and selected bond param-
eters are listed in Table 1.
˚
Selected bond distances (A), angles (°) and their estimated standard
deviations for [Re(NC6H4Cl)Cl3(L2)]
Molecule 1
Molecule 2
In both the molecules, the distorted octahedral ReCl3N3
coordination sphere has meridional disposition for the
chloride ligands. The heterocyclic ligand is N,N-coordi-
nated and the pyridine nitrogen (N1/N10) lies trans to the
imido nitrogen (N5/N50). In Molecule 1, the equatorial
atoms Cl1, Cl2, Cl3 and N2 constitute a good plane (mean
Distance
Re1–N1
Re1–N2
Re1–Cl1
Re1–Cl2
Re1–Cl3
Re1–N5
2.235(11)
2.070(11)
2.355(4)
2.382(4)
2.389(4)
1.720(12)
2.222(11)
2.061(11)
2.377(4)
2.386(4)
2.389(4)
1.717(11)
˚
deviation 0.01 A) and the Re1 atom is displaced from this
˚
plane by 0.30 A towards imido nitrogen N5. In Molecule 2,
Angles
Cl10, Cl20, Cl30 and N20 define only a satisfactory plane
N5–Re1–N2
N5–Re1–N1
N5–Re1–Cl1
N5–Re1–Cl3
N1–Re1–Cl1
N1–Re1–Cl2
N2–Re1–Cl2
N1–Re1–Cl3
N2–Re1–Cl3
Cl1–Re1–Cl3
Cl2–Re1–Cl3
90.7(5)
165.1(5)
104.4(4)
98.3(4)
90.5(3)
84.0(3)
90.6(3)
81.5(3)
89.2(3)
88.39(14)
164.93(14)
90.4(5)
161.5(4)
104.5(4)
91.7(4)
91.8(3)
82.5(3)
86.0(3)
80.4(3)
95.7(3)
87.38(18)
161.54(14)
0
˚
(mean deviation 0.05 A) from which Re1 atom is displaced
˚
by 0.32 A. The mean deviations of the pyridine, triazine
and chelate rings from the planarity are, respectively,
˚
0.01, 0.05 and 0.02 A in Molecule 1 and 0.01, 0.04 and
˚
0.06 A, respectively, in Molecule 2. When all three rings
are taken together the corresponding mean deviations are
˚
0.06 and 0.12 A, respectively, for two molecules. Overall,
Molecule 2 is slightly more distorted than Molecule 1.
˚
The Re–N(imide) lengths Re1–N5, 1.720(12) A and
N.B. The scheme of atom numbering of both the molecules is the same.
The prime sign(0)s as shown in the crystal structure of Molecule 2 (Fig. 3)
are omitted from the table for clarity.
0
0
˚
Re1 –N5 , 1.717(11) A are only slightly longer than the ide-
˚
alized triple bond value 1.69 A, the corresponding double
bond being 1.84 A [12,13]. The Re1–N5–C21 fragment
˚
(Molecule 1) is significantly more linear than the Re10–
N50–C210 fragment (Molecule 2), as can be seen from the
angle subtended at the nitrogen atom – 170.0(10)° and
159.0(10)°, respectively. In hexacoordinated ReV-imido
species, the Re–N(imido) bond have been found to span
the range 1.67–1.79 A and the Re–N–C angle to span the
range 160–180° [1,9,12–20].
sphere the I1, I2, I3 and N2 atoms define a good plane with
˚
mean deviation 0.02 A and the metal atom is displaced
˚
towards the N6 atom by 0.29 A from the plane. Here, the
pyrazine, triazine and chelate rings are all planar (mean
˚
deviation, <0.02 A). The entire chelated ligand (metal
˚
included) excluding the phenyl rings makes a satisfactory
˚
˚
plane with mean deviation 0.05 A.
The Re–N6 length, 1.717(9) A is again consistent with
3.4.2. [Re(NC6 H4 Cl)I3 (L3)] (1g)
triple bonding. The trans influence of the triple bonds is
reflected in lengthening of the Re–N1 bond compared to
A molecular view of 1g is shown in Fig. 4 along with the
atom numbering scheme. Selected bond parameters are
listed in Table 2. In the meridional ReI3N3 coordination
˚
Re–N2 bond by ꢂ0.17 A. A similar lengthening due to
trans influence also occurs in the structure of [Re-
Fig. 3. Molecular view and atom labeling scheme for Molecule 1 and Molecule 2 of [Re(NC6H4Cl)Cl3(L2)] (1b). All non-hydrogen atoms are represented
by 30% thermal probability ellipsoids. Hydrogen atoms are omitted for clarity.

S. Das / Inorganica Chimica Acta 361 (2008) 2815–2820
2819
Fig. 4. Molecular view and atom labeling scheme for [Re(NC6H4Cl)-
I3(L3)] (1g). All non-hydrogen atoms are represented by 30% thermal
probability ellipsoids. Hydrogen atoms are omitted for clarity.
Table 2
Fig. 5. Nonbonded Clꢃ ꢃ ꢃI contacts in [Re(NC6H4Cl)I3(L3)] (1g).
˚
Selected bond distances (A), angles (°) and their estimated standard
deviations for [Re(NC6H4Cl)I3(L3)]
Distances
X-ray structure determination of [Re(NC6H4Cl)Cl3(L2)]
Re–N1
Re–I1
Re–I3
2.258(9)
2.711(2)
2.735(2)
Re–N2
Re–I2
Re–N6
2.087(9)
2.730(2)
1.717(9)
and [Re(NC6H4Cl)I3(L3)] has revealed Re„N(imide) triple
˚
bonding (distance ꢂ 1.72 A) with consequent lengthening
˚
Angles
(by ꢂ0.17 A) of Re–N lying trans to the imide nitrogen
N6–Re–N2
N6–Re–I1
N6–Re–I3
N2–Re–I1
N1–Re–I2
N1–Re–I3
I1–Re–I3
C20–N6–Re
91.5(4)
101.5(3)
94.0(3)
166.5(2)
83.2(2)
83.6(2)
89.34(5)
168.5(9)
N6–Re–N1
N6–Re–I2
N2–Re–N1
N1–Re–I1
N2–Re–I2
N2–Re–I3
I2–Re–I3
164.6(4)
and a shift of the metal atom towards the imide nitrogen
away from the equatorial plane of the halide ligands. The
structural characterization of rare iodo complexes of
ReVNAr is also interesting.
99.2(3)
73.2(3)
93.7(2)
91.5(3)
86.1(3)
166.70(3)
Acknowledgments
The author is grateful to Prof. A. Chakravorty for
technical advice and useful suggestions. This work is finan-
cially supported by the Department of Science and Tech-
nology and Council of Scientific and Industrial Research,
India.
(NC6H4Cl)Cl3(L2)] (Table 1). In the lattice, a nonbonded
˚
Clꢃ ꢃ ꢃI interaction 3.81(4) A occurs between symmetry
related molecules as depicted in Fig. 5.
4. Conclusion
Appendix A. Supplementary material
The seven imido complexes of type [ReV(NC6H4Cl)-
X3(L)] (X = Cl, Br, I; and L = L1–L3) have been synthe-
sized and characterized. These display ReVI/ReV couple
near 1.0 V as compared to 1.6 V in [ReVOX3(L)] signifying
a better stabilization of the rhenium(VI) state under imide
coordination. The imido complexes are nearly reversible
electrooxidized in solution to EPR-active congeners of
the hexavalent metal.
CCDC 624556 and 292213 contain the supplementary
crystallographic data for 1b and 1g. These data can be
obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.ica.2008.02.001.

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S. Das / Inorganica Chimica Acta 361 (2008) 2815–2820
(f)S. Banerjee, S. Bhattacharyya, I. Chakraborty, B.K. Dirghangi, Acta
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