Chemistry of azopyrimidine chelates of ReVO, ReIIIOPPh3 and ReVNAr

Article abstract of DOI:10.1016/S0277-5387(03)00023-8

The concerned azo ligands are L¹, L² and L³ (general abbreviation L) in which the aryl groups are phenyl, p-chlorophenyl and o,p-dichlorophenyl, respectively. The reaction of L¹ with KReO₄ in hot concentrated HCl affording [Re^VOCl₃(L²)] (2a), is attended with aryl chlorination. A similar reaction with L² furnished [Re^VOCl₃(L³)] (2b). The oxo complexes spontaneously reacted with PPh₃ and ArNH₂ furnishing [Re^III(OPPh₃)Cl₃(L)] (3) and [Re^V(NAr)Cl₃(L)] (4), respectively. Unlike 2 and 4, 3 displays large paramagnetic ¹H NMR shifts. The type 2 species exhibit an irreversible electrochemical Re^V→Re^VI oxidation near 2.2 V versus SCE. In type 4 species the Re^VI/Re^V couple occurs near 1.5 V and in type 3 species the Re^IV/Re^III couple lies near 1.2 V. Structure determination of 2b, [Re^III(OPPh₃)Cl₃(L¹)] (3a) and [Re^V(NC₆H₄Cl-p)Cl₃n (L²)] (4b) have revealed uniform meridional geometry. In 2b the Re-O and in 4b the Re-N (imide) distances are 1.666(10) and 1.719(6) ?, respectively, consistent with triple bonding. The Re-N bonds lying trans to the oxo oxygen 2b and to the imide nitrogen in 4b are substantially lengthened. The Re-N(azo) lengths are 2.072(13), 1.957(9) and 2.003(6) ? in 2b, 3a and 4b, the corresponding N-N distances being 1.25(2), 1.318(11) and 1.305(9) ?, respectively. In 3a dπ(Re)-π* (azo) back-bonding is clearly present and in 4b electron flow to the azo (π*) orbital occurs via the route ArN → Re^V → azo(π*). The corresponding route in 2b is less effective because the oxo group is a poorer donor compared with the imido group. In the lattice of 4b noncovalent Cl···Cl and aromatic π-stacking are present.

Full text of DOI:10.1016/S0277-5387(03)00023-8

Polyhedron 22 (2003) 901Á907
/
www.elsevier.com/locate/poly
6
7
Chemistry of azopyrimidine chelates of ReVO, ReIIIOPPh3 and
ReVNAr
8
Samir Das a, Indranil Chakraborty a, Animesh Chakravorty a,b,
*
9
10
a Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India
b Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 064, India
11
Received 15 October 2002; accepted 6 January 2003
Abstract
The concerned azo ligands are L1, L2 and L3 (general abbreviation L) in which the aryl groups are phenyl, p-chlorophenyl and
o,p-dichlorophenyl, respectively. The reaction of L1 with KReO4 in hot concentrated HCl affording [ReVOCl3(L2)] (2a), is attended
with aryl chlorination. A similar reaction with L2 furnished [ReVOCl3(L3)] (2b). The oxo complexes spontaneously reacted with
PPh3 and ArNH2 furnishing [ReIII(OPPh3)Cl3(L)] (3) and [ReV(NAr)Cl3(L)] (4), respectively. Unlike 2 and 4, 3 displays large
1
paramagnetic H NMR shifts. The type 2 species exhibit an irreversible electrochemical ReV0
/
ReVI oxidation near 2.2 V versus
SCE. In type 4 species the ReVI/ReV couple occurs near 1.5 V and in type 3 species the ReIV/ReIII couple lies near 1.2 V. Structure
determination of 2b, [ReIII(OPPh3)Cl3(L1)] (3a) and [ReV(NC6H4Cl-p)Cl3(L2)] (4b) have revealed uniform meridional geometry. In
˚
2b the Reꢀ
Reꢀ
are 2.072(13), 1.957(9) and 2.003(6) A in 2b, 3a and 4b, the corresponding Nꢀ
/
O and in 4b the Reꢀ
/
N (imide) distances are 1.666(10) and 1.719(6) A, respectively, consistent with triple bonding. The
/
N bonds lying trans to the oxo oxygen 2b and to the imide nitrogen in 4b are substantially lengthened. The Reꢀ
/
N(azo) lengths
˚
N distances being 1.25(2), 1.318(11) and 1.305(9) A,
˚
/
respectively. In 3a dp(Re)-p* (azo) back-bonding is clearly present and in 4b electron flow to the azo (p*) orbital occurs via the route
ArN0 azo(p*). The corresponding route in 2b is less effective because the oxo group is a poorer donor compared with the
/
ReV0
/
imido group. In the lattice of 4b noncovalent Clꢀ ꢀ ꢀCl and aromatic p-stacking are present.
# 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Azopyrimidine chelates; Rhenium azo chelates; Rhenium pyrimidine chelates; Rhenium azopyrimidine chelates
1. Introduction
aryl group. The oxo chelates undergo facile transforma-
tion to ReIIIOPPh3 and ReVNAr species upon treatment
with PPh3 and aromatic amines, respectively. The
structure and properties of the azopyrimidine chelate
family are described in this work.
This work stems from our interest in the synthesis,
structure and reactions of new oxorhenium(V) reagents
incorporating chelation by heterocyclic donor sites [1,2].
Recently we initiated a programme on the rhenium
chemistry of azoheterocyclic ligands [3,4] which have
pronounced p-acidic [5Á
/
10] properties. In the present
2. Experimental
work we report the first instances of the binding of
rhenium by 2-(arylazo)pyrimidines. Oxorhenium(V)
chelates have been isolated. The synthesis is attended
with spontaneous and regiospecific chlorination of the
2.1. General considerations
[ReOCl3(PPh3)2] [11], and 2-(arylazo)pyrimidine [12]
were prepared by reported methods. Potassium perrhe-
nate and triphenyl phosphine were purchased from
Aldrich (USA). For electrochemical work HPLC grade
* Corresponding author. Tel.: ꢀ
2473-2805.
E-mail address: icac@mahendra.iacs.res.in (A. Chakravorty).
/
91-33-2472-4436; fax: ꢀ91-33-
/
0277-5387/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0277-5387(03)00023-8

902
S. Das et al. / Polyhedron 22 (2003) 901Á907
/
acetonitrile was used. All other chemicals and solvents
were of reagent grade and were used as received.
thus obtained was subjected to column chromatography
on a silica-gel column (60Á120 mesh). The yellowish
brown band was eluted using benzeneÁacetonitrile (25:
1.5) mixture, and from the eluate 3b was obtained in
pure form. Yield: (108 mg, 70%). Anal. Calc. for
C28H22Cl4N4OPRe: C, 42.60; H, 2.81; N, 7.10. Found:
/
UVÁ
/
Vis spectra (CH2Cl2 solution), IR spectra (KBr
/
disk), and 1H 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
C, 42.65; H, 2.96; N, 7.19%. UVÁ
Mꢁ1 cmꢁ1)): 443 (4800); 334 (9260). IR (cmꢁ1, KBr
pellet): 1120 (OꢁP), 1397 (Nꢁ
N).1H NMR (CDCl3) d:
H1, ꢁ39.5 (ill resolved); H2, 50.6 (ill-resolved); H3, ꢁ
25.5 (ill-resolved); H6, H10, 35.5 (d, J 7.8 Hz); H7, H9,
9.45 (d, J 7.5Hz); aromatic multiplets, 7.69Á6.29.
/
Vis (lmax (nm) (o,
1
scheme used for H NMR assignment is the same as in
crystallography. SpinÁ/spin structures are abbreviated as
/
/
follows: s, singlet; d, doublet; t, triplet; m, multiplet.
/
/
Cyclic voltammetry (in MeCN, 0.1 M TEAP) was done
at
a
platinum working electrode under nitrogen
/
atmosphere on a PAR model 370-4 electrochemistry
system [13]. Microanalyses (C, H, N) were performed
2.3.1. [Re(OPPh3)Cl3(L3)] (3c)
Anal. Calc. for C28H21Cl5N4OPRe: C, 40.80; H, 2.57;
N, 6.80. Found: C, 40.76; H, 2.64; N, 6.89%. UVÁVis
(lmax (nm) (o, Mꢁ1 cmꢁ1)): 445 (4550); 339 (8760). IR
(cmꢁ1, KBr pellet): 1121 (Oꢁ N). 1H NMR
P), 1396 (Nꢁ
(CDCl3) d: H1, ꢁ39.7 (ill-resolved); H2, 50.8 (ill-
resolved); H3, ꢁ25.6 (ill-resolved); H6, 35.9 (d, J 8.0
Hz); H7, 11.9 (d, J, 8.0 Hz); H9, 9.41 (s); aromatic
multiplets; 7.62Á6.23.
using a PerkinÁElmer 2400 Series II elemental analyser.
/
/
2.2. Synthesis of [ReOCl3(L)] (2)
/
/
The complexes were prepared by the same general
methods. Details are given for [ReOCl3(L2)] (2a).
/
/
To a hot solution of KReO4 (250 mg, 0.86 mmol) in
10 ml of concentrated hydrochloric acid was added 962
mg (5.2 mmol) of L1. The resulting mass was stirred
magnetically and heated to reflux for 1.5 h. A dark solid
started depositing from the solution which was then
cooled to room temperature (r.t.). The solid collected by
filtration was made acid-free by washing several times
with water. It was finally washed with n-hexane and
then dried in vacuo over fused calcium chloride. Yield:
(362 mg, 80%). Anal. Calc. for C10H7Cl4N4ORe: C,
22.78; H, 1.34; N, 10.63. Found: C, 22.88; H, 1.39; N,
/
2.4. Synthesis of [Re(OPPh3)Cl3(L1)] (3a) from
[ReOCl3(PPh3)2]
To a suspension of [ReOCl3(PPh3)2] (100 mg, 0.12
mmol) in 20 ml of benzene was added 30 mg (0.16
mmol) of L1, and the mixture was stirred for 30 min.
The solvent was then removed under reduced pressure.
The solid thus obtained was dissolved in 5 ml of
dichloromethane and subjected to column chromato-
graphy in the same manner as described earlier. An
10.67%. UVÁ
(2900); 375 (5760). IR (cmꢁ1, KBr pellet): 1000 (Reꢀ
N). 1H NMR (CDCl3) d: H1 9.26 (d, J 4.5
/
Vis (lmax (nm) (o, Mꢁ1 cmꢁ1)): 531
/
O), 1341 (Nꢁ
/
yellowish brown band was eluted out with benzeneÁ
/
Hz); H2 6.71 (t, J 5.2Hz); H3 8.88 (d, J, 5.4 Hz); H6,
H10 7.93 (d, J 8.8 Hz); H7 H9 7.57 (d, J 7.8 Hz).
acetonitrile (25:1.5) mixture. The required complex 3a
was then obtained from the eluate as yellowish micro-
crystals. Yield: (46 mg, 50%). Anal. Calc. for
C28H23Cl3N4OPRe: C, 44.54; H, 3.07; N, 7.42. Found:
2.2.1. [ReOCl3(L3)] (2b)
Anal. Calc. For C10H6Cl5N4ORe: C, 21.38; H, 1.08;
C, 44.63; H, 3.18; N, 7.54%. UVÁVis (lmax (nm) (o,
/
N, 9.98. Found: C, 21.49; H, 1.12; N, 9.90%. UVÁ
(lmax (nm) (o, Mꢁ1 cmꢁ1)): 529 (2750), 372 (5980). IR
(cmꢁ1, KBr pellet): 1000 (Reꢀ N). 1H
O), 1340 (Nꢁ
/Vis
Mꢁ1 cmꢁ1)): 448 (4500); 337 (9730). IR (cmꢁ1, KBr
1
pellet): 1122 (Oꢁ
/
P), 1395 (Nꢁ
/
N). H NMR (CDCl3) d:
/
/
H1, ꢁ
/
39.4 (ill-resolved); H2, 50.2 (ill resolved); H3, ꢁ
/
NMR (CDCl3) d: H1 9.24 (d, J 4.5 Hz); H2 6.65 (t, J,
4.8 Hz); H3 8.86 (d, J 6.0 Hz); H6 7.77 (d, J 8.7 Hz); H7
7.40 (d, J 8.7 Hz); H9 7.51 (s).
25.7 (ill-resolved); H6, H10, 35.7(d, J 8.0 Hz); H7, H9,
9.51 (t, J 7.3 Hz); H8, 7.23 (t, J 7.5 Hz).
2.5. Synthesis of [Re(NC6H4Cl-p)Cl3(L2)] (4b) from
[ReOCl3(L2)] (2a)
2.3. Synthesis of [Re(OPPh3)Cl3(L)] (3)
The same general methods were used for all the
complexes. The case for [Re(OPPh3)Cl3(L2)] (3b) is
detailed below.
To a solution of (2a) (100 mg, 0.19 mmol) in 15 ml of
dichloromethane was added 252 mg (0.96 mmol) of
PPh3. The pink solution turned yellowish brown in-
stantly, and it was stirred for 5 min at r.t. The solvent
was then removed under reduced pressure, and the solid
To a warm solution of 2a (100 mg, 0.19 mmol) in 10
ml of toluene was added an excess of p-chloro aniline
(121 mg, 0.95 mmol), and the mixture was heated to
reflux for 1 h. During this time the colour changed from
pinkish red to redÁviolet. The solvent was then removed
/
under reduced pressure, and the solid thus obtained was
subjected to column chromatography. The violet band
was eluted using tolueneÁacetonitrile (25:1) mixture,
/

S. Das et al. / Polyhedron 22 (2003) 901Á
/907
903
and from the eluate 4b was obtained in pure form. Yield:
(85 mg, 70%). Anal. Calc. for C16H11Cl5N5Re: C, 30.18;
H, 1.74; N, 11.00. Found: C, 30.29; H, 1.67; N, 11.10%.
Table 1
Crystallographic data for [ReOCl3(L3)] (2b), [Re(OPPh3)Cl3(L1)]×
(3a)×
H2O and [Re(NC6H4Cl-p)Cl3(L2)] (4b)
/H2O
/
UVÁ
/
Vis (lmax (nm) (o, Mꢁ1 cmꢁ1)): 510 (2700); 326
2b
3a
4b
(6450). IR (cmꢁ1, KBr pellet): 1396 (Nꢁ
/
N). H NMR
1
Empirical
formula
Formula
weight
C10H6Cl5N4ORe C28H25Cl3N4O2PRe C16H11Cl5N5Re
(CDCl3) d: H1, 9.34 (d, J 6.0 Hz); H2, 6.68 (t, J 5.1 Hz);
H3, 9.16 (d, J 4.1 Hz); H6, H10, 7.48 (d, J 9.0 Hz); H7,
H9, 7.32 (d, J 9.0 Hz); H12, H16, 7.23 (ill resolved);
H13, H15, 7.17 (d, J 8.7 Hz).
561.64
773.04
636.75
Crystal
system
Space
orthorhombic
Pbca
monoclinic
P21/n
triclinic
¯
P1
/
2.6. Synthesis of [Re(NC6H5)Cl3(L1)] (4a) from
[ReOCl3(PPh3)2]
group
˚
a (A)
˚
10.450(4)
11.596(5)
25.549(9)
9.390(5)
21.014(9)
15.023(5)
9.604(3)
10.834(3)
11.383(3)
63.40(2)
71.32(2)
84.59(2)
1001.6(5)
2
b (A)
˚
c (A)
To a suspension of [ReOCl3(PPh3)2] (100 mg, 0.12
mmol), in 20 ml of toluene was added 56 mg (0.30
mmol) of L1, and the mixture was allowed to reflux for 2
a (8)
b (8)
g (8)
V (A )
Z
96.25(4)
3
˚
3096(2)
8
2.410
2947(2)
4
1.742
h. The resulting redÁviolet solution was evaporated
/
under reduced pressure. The solid mass thus obtained
was dissolved in 5 ml of dichloromethane and was
subjected to column chromatography on a silica-gel
Dcalc
(Mg mꢁ3
m (mmꢁ1
2.111
)
)
8.713
4.483
6.745
column as mentioned above. A redÁ
/
violet band was
u Range (8) 1.59Á
/
25.00
1.67Á
/
23.50
0.0434, 0.1147
2.11Á23.50
/
a, wR2
[I ꢀ2s(I)]
0.0549, 0.0924
0.0300, 0.0843
0.762
b
R
eluted using tolueneÁacetonitrile (25:1) mixture. Solvent
/
/
removal from the eluate afforded 4a in pure crystalline
form. Yield (47 mg, 65%). Anal. Calc. for
C16H13Cl3N5Re: C, 33.84, H, 2.31; N, 12.33. Found:
Goodness- 1.071
of-fit on F2
0.905
a
Rꢂ
wR2ꢂ
/
jjFojꢁ
/
jFcjj/ajFoj.
Fc2)2/aw(Fo2)2]1/2
C, 33.77; H, 2.35; N, 12.38%. UVÁ
Mꢁ1 cmꢁ1)): 512 (2550); 323 (6800). IR (cmꢁ1, KBr
pellet): 1397 (Nꢁ
N). 1H NMR (CDCl3) d: H1, 9.36 (d, J
/
Vis (lmax (nm) (o,
b
/
[aw(Fo2ꢁ
/
.
/
6.0 Hz); H2, 6.69 (t, J 4.7 Hz); H3, 9.14 (d, J 4.5 Hz);
H6, H10, 7.54 (d, J 6.0 Hz); H7, H9, 7.28 (multiplet);
H8, 7.28 (multiplet); H12, H16, 7.20 (multiplet); H13,
H15, 7.20 (multiplet).
3. Results and discussion
2.7. X-ray crystallography
3.1. Synthesis
Single crystals of 2b, 3a and 4b were grown by slow
diffusion of hexane into dichloromethane solution. Cell
parameters were determined by a least-squares fit of 30
The three azopyrimidine ligands [12], (1), concerning
this work are L1, L2 and L3. Ligand L1 reacted virtually
quantitatively with KReO4 in hot concentrated hydro-
chloric acid furnishing the pink coloured complex 2a of
ligand L2. In this reaction the metal oxidation state
machine-centred reflections (1485
collected by the v scan technique in the range 385
508 for 2b and 3852u 5478 for 3a and 4b on a Nicolet
R3m/V four-circle diffractometer with graphite mono-
/
2u 5
/
288). Data were
/
2u 5
/
/
/
changes from ꢀ
/
70
/
ꢀ5 and the aryl group is chlori-
/
˚
chromated Mo Ka (lꢂ0.71073 A) radiation at 25 8C.
/
nated at para-position. Upon using the L2 ligand in
the above synthesis, chlorination occurs at the ortho-
position and the complex 2b is isolated. Aryl
chlorination is an integral part of this synthesis. In
the strongly acidic reaction medium metal reduction in
Two check reflections after each 198 reflections showed
no significant intensity reduction. All data were cor-
rected for Lp effects and absorption [14].
In all the cases metal atoms were located from
Patterson maps, and the rest of the nonhydrogen atoms
emerged from successive Fourier synthesis. The struc-
tures were refined by full-matrix least-squares proce-
dures on F2. All nonhydrogen atoms were refined
anisotropically. Hydrogen atoms were included at
presence of
L
appears to be attended with
incipient generation of electrophilic Clꢀ which chlor-
inates. The net observed reaction in the case of L2
is stated in Eq. (1) where L3 is a chlorinated derivative
of L2. One instance of this type of chlorination
was observed earlier in the case of 2-(phenylazo)pyridine
[4].
2
˚
0.08 A ]. Calculations were
calculated positions [Uꢂ
/
performed using the SHELXTL V5.03 [15] program
package. Significant crystal data are listed in Table 1.

904
S. Das et al. / Polyhedron 22 (2003) 901Á907
/
rhenium(III) complexes 3 are paramagnetic and show
large shift [3,4,16] in their 1H NMR spectra. The
protons of the L ligand span the chemical shift range
from ꢀ
/
51 to ꢁ40 ppm.
/
3.3. Electrochemistry
All the complexes reported in this work are electro-
active in acetonitrile solution. Reduction potential data
are listed in Table 2 and representative voltammograms
are shown in Fig. 1. The oxo complexes display an
irreversible one-electron ReV0
/
ReVI oxidation near 2.2
V versus SCE. The imido ligand is a superior electron
donor than the oxo ligand and the metal is more easily
oxidised in the imido species of type 4, the reduction
potential of the quasireversible ReVI/ReV couple being
Â
1.5 V. In the phosphine oxide species the ReIV/ReIII
/
Table 2
Cyclic voltametric data in acetonitrile solution (0.1 M TEAP)
a
Compound
E1/2 (V) (DEp (mV))
az/azꢁ
Renꢀ1/Ren
ReVIIOꢁ4 ꢀL2 ꢀ5HCl 0 [ReOCl3(L3)]ꢀ3H2OꢀClꢁ (1)
b
2a
2b
3a
3b
3c
4a
4b
0.18(80)
0.17(70)
2.16
2.18
2b
b
c
The type 2 species are potent oxygen atom transfer
reagents which react, Eq. (2), smoothly with PPh3 in
benzene affording the yellowish brown phosphine oxide
complex 3. The complex 3a incorporating L1 was
afforded by the reaction of [ReOCl3(PPh3)2]
ꢁ
/
0.46
0.42
0.40
1.20(100)
1.19(60)
1.18(80)
1.52(80)
1.56(80)
c
c
ꢁ
/
ꢁ/
ꢁ
/
0.31(60)
ꢁ
/
0.22(80)
a
Conditions: working electrode, platinum; reference electrode,
1/2(Epa Epc) where Epa and Epc
are anodic and cathodic peak potentials, respectively, DEpꢂEpc Epa
2ꢀPPh3 0 [ReIII(OPPh3)Cl3(L)]
(2)
SCE; scan rate 50 mV sꢁ1. E1/2
ꢂ/
ꢀ/
3
/
ꢁ/
;
nꢂ
/
5 for compound 2 and 4 and nꢂ3 for compound 3.
/
with the ligand, thus avoiding aryl chlorination.
b
c
The ReVO oxygen atom in 2 can be eliminated in the
oxide form by reacting with excess arylamines, in
toluene, Eq. (3) affording violet coloured imido species
of type 4. The complex [Re(NC6H5)Cl3(L1)] has been
prepared from [ReOCl3(PPh3)2] and L1, the imido group
arising via azo splitting [4].
Epa
.
Epc
.
2ꢀArNH2 0 [ReV(NAr)Cl3(L)]ꢀH2O
(3)
4
3.2. Spectra
Spectral data are given in Section 2. The type 2 and
type 3 complexes, respectively, display ReꢂO and Pꢁ
stretches near 1000 and 1120 cmꢁ1, respectively. The
N stretch in the complexes generally occur near 1390
/
/O
Nꢁ
/
cmꢁ1. The type 2, 3 and 4 species display a charge
transfer band in the visible region near 530, 445 and 510
nm, respectively. The rhenium(V) species 2 and 4 are
diamagnetic and display well-resolved 1H NMR spectra
Fig. 1. Cyclic voltammograms of :
10ꢁ3 mol dmꢁ3 solutions of (a)
/
[ReOCl3(L2)] (2a), (b) [Re(OPPh3)Cl3(L2)] (3b) and (c)
[Re(NC6H5)Cl3(L1)] (4a) in acetonitrile solution at a platinum working
electrode.
in the region 5Á10 ppm. On the other hand the
/

S. Das et al. / Polyhedron 22 (2003) 901Á
/907
905
Table 3
Selected bond distances (A) and angles (8) for 2b, 3a and 4b
˚
2b
3a
4b
Bond distances
Reꢀ
Reꢀ
Reꢀ
Reꢀ
Reꢀ
Reꢀ
Reꢀ
N3ꢀ
Pꢀ
/
O
1.666(10)
2.021(6)
/
N5
N4
N1
Cl1
Cl2
Cl3
1.719(6)
2.003(6)
2.173(6)
2.350(2)
2.393(2)
2.331(2)
1.305(9)
/
2.072(13)
2.218(13)
2.317(4)
2.337(5)
2.336(5)
1.25(2)
1.957(9)
2.022(8)
2.341(3)
2.326(3)
2.336(3)
1.318(11)
1.501(7)
/
/
/
/
/
N4
/
O
Bond angles
OꢀReꢀN4
N5ꢀReꢀN4
OꢀReꢀN1
N5ꢀReꢀ
N4ꢀReꢀ
OꢀReꢀ
N5ꢀReꢀ
N4ꢀReꢀ
N1ꢀReꢀ
OꢀReꢀ
N5ꢀReꢀ
N4ꢀReꢀ
N1ꢀReꢀ
Cl1ꢀReꢀ
OꢀReꢀCl3
N5ꢀReꢀ
N4ꢀReꢀ
N1ꢀReꢀ
Cl1ꢀReꢀ
Cl2ꢀReꢀ
C11ꢀ
/
/
90.4(5)
100.6(3)
174.6(3)
/
/
93.5(3)
/
/
160.7(5)
/
/N1
162.4(3)
72.5(2)
/
/N1
70.4(5)
109.3(4)
74.0(4)
91.5(2)
/
/Cl1
Fig. 2. Molecular view and atom labelling scheme for [ReOCl3(L3)]
(2b). All nonhydrogen atoms are represented by 30% thermal prob-
ability ellipsoids. Hydrogen atoms are omitted for clarity.
/
/
Cl1
Cl1
Cl1
103.7(2)
162.8(2)
90.8(2)
/
/
159.5(4)
90.0(3)
99.3(4)
167.0(3)
93.9(3)
88.1(2)
/
/
/
/Cl2
/
/
Cl2
Cl2
Cl2
90.0(2)
93.8(2)
80.6(2)
87.41(8)
couple occur near 1.2 V. The metal reduction potentials
/
/
97.0(4)
81.6(3)
85.6(2)
98.8(4)
95.6(2)
92.7(3)
89.56(13)
88.5(2)
of the present complexes are significantly higher (by Â
/
/
/
0.2 V) than those of corresponding complexes incorpor-
ating chelation by 2-(arylazo)pyridine [4] and 2-(aryla-
zo)imidazole [3] reflecting the milder inferior overall
donor ability of the 2-(arylazo)pyrimidine ligand.
All the complexes also display a response at negative
potentials assignable to azo reduction [5,6] consistent
with the relatively low-lying nature of the azo p* orbital
which can thus participate in back-bonding (see struc-
ture of (3a)).
/
/
Cl2
/
/
/
/
Cl3
Cl3
Cl3
102.9(2)
98.4(2)
88.0(2)
/
/
85.3(4)
82.1(3)
86.2(2)
161.7(2)
87.0(2)
90.9(3)
/
/
/
/
Cl3
Cl3
88.57(13)
176.04(10)
85.65(8)
166.50(8)
164.3(5)
/
/
/
N5ꢀRe
/
3.4. Structure
The structure of [ReOCl3(L3)] (2b) shown in Fig. 2
authenticates the chlorination reaction of Eq. (1). In the
distorted octahedral coordination sphere the ReCl3
fragment is meridionally disposed. The Cl1, Cl2, Cl3
and N4 atoms constitute an excellent plane with mean
˚
deviation 0.01 A. The metal atom is displaced from the
˚
plane towards the oxo oxygen atom by 0.36 A. The
˚
chelate ring is planar (mean deviation 0.03 A) and the
pyrimidine ring is coplanar with it. The pendant
dichlorophenyl ring makes a dihedral angle of 658 with
this plane. Selected bond parameters are collected in
˚
O distance in 2b, 1.666(10) A lies well
Table 3. The Reꢀ
/
˚
within the range 1.689
by ReVO species [1Á3]. Idealised ReVꢂ
lengths are estimated to be 1.60 and 1.75 A, respectively,
[17,18]. Thus the ReꢀO bond order in 2b is close to three
(s2p4). The Reꢀ
N1 distance, 2.218(13) A is significantly
longer than Reꢀ
trans influence of the oxo oxygen.
/
0.03 A which is usually spanned
/
/
O and ReVꢁ
/O
˚
/
Fig. 3. Molecular view and atom labelling scheme for
[Re(OPPh3)Cl3(L1)] (3a). All nonhydrogen atoms are represented by
30% thermal probability ellipsoids. Hydrogen atoms are omitted for
clarity.
˚
/
˚
N4 distance, 2.072 (13) A reflecting the
/

906
S. Das et al. / Polyhedron 22 (2003) 901Á907
/
with the chelate ring. The pendant phenyl ring is inclined
˚
to the chelate ring by 468. The Reꢀ
/
N1 length, 2.022(8) A
0.2 A shorter than that in 2b due to
elimination of trans influence. The ReꢀN4 length,
0.1 A) in 3a reflecting
˚
(Table 3) in 3a is Â
/
/
˚
˚
1.957(9) A is also shorter (by Â
/
the p-basicity of trivalent rhenium [2,3,19] which
engages in substantial back-bonding with the p*(azo)
˚
N length in 3a is 0.06 A
orbital. It is significant that Nꢀ
/
longer than that in 2b. Lastly, among the centroids of
the four triangular faces with O as the common vertex,
that of the face defined by O1, Cl1 and Cl3 lie closest to
˚
the P atom: 2.88 A (the other distances are in the range
˚
3.09Á
/
3.19 A). The initial nucleophilic attack of PPh3 on
ReVO may have occurred from the side of the face above
noted which is also the least hindered.
A view of [Re(NC6H4Cl-p)Cl3(L2)] (4b) is shown in
Fig. 4. Like 2b and 3a it also has a meridional ReCl3
fragment. The Cl1, Cl2, Cl3 and N4 atoms constitute a
˚
good plane (mean deviation 0.03 A) from which the
metal atom is displaced towards imide nitrogen by 0.29
˚
A (in 2b the corresponding displacement towards oxo
˚
oxygen was 0.36 A). The chelate ring is planar (mean
˚
deviation 0.03 A) and the pyrimidine ring makes a small
Fig. 4. Molecular view and atom labelling scheme for [Re(NC6H4Cl-
p)Cl3(L2)] (4b). All nonhydrogen atoms are represented by 30%
thermal probability ellipsoids. Hydrogen atoms are omitted for clarity.
dihedral angle (6.68) with it. The pendant chlorophenyl
group is inclined to the chelate ring by 458. The Reꢀ
/
N5
The structure of [Re(OPPh3)Cl3(L1)] (3a) is shown in
˚
length, 1.719(6) A (Reꢀ
/
N5ꢀ
/
C11 angle is 164.38) is
consistent with the triple bonding [1,20] (Table 3). The
Fig. 3. It has the same gross meridional geometry as
˚
O, 2.021(6) A, length has
˚
observed in 2b. The Reꢀ
/
Reꢀ
/
N1 distance is Â
/
0.2 A longer than the Reꢀ
/
N4
˚
increased by 0.36 A compared with that in 2b due to
the decrease of bond order from 3 to 1. In effect the
metal atom has descended away from the oxygen atom
into a pseudo-plane defined by C11, Cl2, Cl3 and N4
length due to the trans influence of the imide nitrogen.
˚
N bond in 4b, 1.305(9) A, is significantly longer
The Nꢀ
than that in 2b. This is ascribed to significant electron
flow to the azo (p*) via the route ArN0 azo(p*)
ReV0
[3,4,20]. The imido group is a stronger donor than the
oxo group and O0 azo(p*) is a less effective
ReV0
route for electron flow.
/
/
/
˚
(mean deviation 0.15 A). The chelate ring is only
˚
approximately planar (mean deviation 0.06 A) and the
/
/
planar pyrimidine ring makes a dihedral angle of 6.98
Fig. 5. Nonbonded Clꢀ ꢀ ꢀCl contacts and p-stacking in (4b).

S. Das et al. / Polyhedron 22 (2003) 901Á
/907
907
Neither 2b nor 3a displayed any unusual intermole-
cular contacts. In 4b, however, noncovalent Clꢀ ꢀ ꢀCl and
aromatic p stacking are present leading to interlinked
dimers as depicted in Fig. 5.
Acknowledgements
We thank the Department of Science and Technology
and the Council of Scientific and Industrial Research
New Delhi, for financial support.
References
4. Conclusion
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