Low-spin manganese(n) and cobalt(in) complexes of N-aryl-2pyridylazophenylamines: New tridentate N,N,N-donors derived from cobalt mediated aromatic ring amination of 2-(phenylazo)pyridine. Crystal structure of a manganese(ii) complex f

Article abstract of DOI:10.1039/a909769d

The Royal Society of Chemistry 2000 Two new tridentate ligands of the type NH4C5N=NC6H4N(H)C6H4(R) (R = H (HL1) or CH3 (HL2)) have been synthesized by the cobalt mediated direct phenyl ring amination of co-ordinated NH4C5N=NC6H5. These bind to metal ions as monoanionic N,N,N-donors (L^-), affording [Mn^IIL2] and [Co^IIIIL2]ClO4 complexes in very high yields. The compounds have low-spin electronic configurations. While the manganese complexes are paramagnetic with one unpaired electron (1.65-1.70 //B) the cobalt complexes are diamagnetic. Crystal structure determination of [Mn(L')2] has revealed the presence of a distorted octahedral MnN6 co-ordination sphere. The two aza nitrogens of the anionic tridentate ligands approach the metal centre closest with Mn-N(aza) ca. 1.89 A. The other two Mn-N distances are: Mn-N(py), 2.00; Mn-N(amido), 1.95 A. There is a significant degree of ligand backbone conjugation in the co-ordinated ligands which has resulted in shortening of the C-N bond distances and also in lengthening of the diaza (N=N) distances. In fluid solution, [MnL2] species exhibit six line EPR spectra with low hyperfine constant (A = 75 G). In frozen dichloromethane-toluene (77 K), rhombic EPR spectra are observed, consisting of an isolated signal gj (ca. 1.91) and two relatively close signals g, and g2 (ca. 2.06 and 2.03 respectively). Both manganese as well as cobalt complexes display multiple redox responses. The manganese complexes show Mn^II . Mnm oxidation at ca. 0.38 V and the cobalt analogues display reversible Co^III-Co^II reduction at -0.40 V. Electrogenerated [MnIL2]II shows transitions in the near IR region.

Full text of DOI:10.1039/a909769d

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Low-spin manganese(II) and cobalt(III) complexes of N-aryl-2-
pyridylazophenylamines: new tridentate N,N,N-donors derived from
cobalt mediated aromatic ring amination of 2-(phenylazo)pyridine.
Crystal structure of a manganese(II) complex†
Amrita Saha, Partha Majumdar and Sreebrata Goswami*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Calcutta 700 032, India. Fax: 91-33-473 2805; E-mail: icsg@mahendra.iacs.res.in
Received 13th December 1999, Accepted 6th April 2000
Two new tridentate ligands of the type NH C N᎐NC H N(H)C H (R) (R = H (HL1) or CH (HL2)) have been
᎐
4
5
6
4
6
4
3
synthesized by the cobalt mediated direct phenyl ring amination of co-ordinated NH C N᎐NC H . These bind to
᎐
4
5
6
5
metal ions as monoanionic N,N,N-donors (LϪ), affording [MnIIL2] and [CoIIIL2]ClO4 complexes in very high yields.
The compounds have low-spin electronic configurations. While the manganese complexes are paramagnetic with one
unpaired electron (1.65–1.70 µB) the cobalt complexes are diamagnetic. Crystal structure determination of [Mn(L1)2]
has revealed the presence of a distorted octahedral MnN6 co-ordination sphere. The two aza nitrogens of the anionic
tridentate ligands approach the metal centre closest with Mn–N(aza) ca. 1.89 Å. The other two Mn–N distances
are: Mn–N(py), 2.00; Mn–N(amido), 1.95 Å. There is a significant degree of ligand backbone conjugation in the
co-ordinated ligands which has resulted in shortening of the C–N bond distances and also in lengthening of the
diaza (N=N) distances. In fluid solution, [MnL2] species exhibit six line EPR spectra with low hyperfine constant
(A = 75 G). In frozen dichloromethane–toluene (77 K), rhombic EPR spectra are observed, consisting of an isolated
signal g3 (ca. 1.91) and two relatively close signals g1 and g2 (ca. 2.06 and 2.03 respectively). Both manganese as well
as cobalt complexes display multiple redox responses. The manganese complexes show MnII
MnIII oxidation at
ca. 0.38 V and the cobalt analogues display reversible CoIII
[MnIIIL2]ϩ shows transitions in the near IR region.
CoII reduction at Ϫ0.40 V. Electrogenerated
Introduction
The low spin state (S = ¹) of manganese() is generally an
¯
²
uncommon entity1–5 because of its highest spin-pairing
energy6 amongst bivalent 3d ions. Ligands with very strong
ligand fields can induce low-spin character on this metal ion.
Herein we describe a few low-spin manganese() and cobalt()
complexes of a new anionic tridentate N,N,N donor (LϪ). The
low-spin complexes are of type [MnIIN6] and [CoIIIN6]ϩ.
The crystal structure of one member of the manganese
complexes has been determined. Spectroelectrochemistry of
each of these characterizes the existence of corresponding
trivalent manganese and bivalent cobalt complexes in
solutions.
The ligand 2-(2-(arylamino)phenyl)azopyridine (HL) was
obtained7 from an unusual cobalt-promoted fusion of aromatic
amines to the pendant aryl ring of co-ordinated 2-(phenyl-
azo)pyridine. It readily loses one Hϩ and binds as a tridentate
monoanionic donor. This report constitutes the first explor-
ation of the co-ordination chemistry of this ligand which
appears to be rich and versatile.
which reacts smoothly with neat ArNH2 to produce7 a green
cobalt() complex, [Co(L)2]ϩ, [1]ϩ (eqn. (1)). The cationic com-
plex was isolated as its perchlorate salt in >80% yield.
Results and discussion
A. Ligand synthesis
(1)
(i) Cobalt-promoted aromatic ring amination of co-ordinated
2-(phenylazo)pyridine (pap). The neutral N,N-donor pap is
known8 to form a brown cationic tris chelate [Co(pap)3]2ϩ
1
† Supplementary data available: H NMR spectrum of HL1 in CDCl3,
available from BLDSC (SUPP. NO. 57703, 2 pp.). See Instructions for
Authors, Issue 1 (http://www.rsc.org/dalton).
DOI: 10.1039/a909769d
J. Chem. Soc., Dalton Trans., 2000, 1703–1708
This journal is © The Royal Society of Chemistry 2000
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Table 1 EPR g valuesa and derived energy parameters of [Mn(L)2]
(ii) Isolation of HL from [1]؉ and its characterization. The
reduction of cationic complex [1]ϩ by dilute N2H4 followed by
removal of Co2ϩ as CoS led to the isolation of HL in moderate
yield (ca. 60%) (Scheme 1). The ligands HL were obtained as
Derived energy
parametersb/cmϪ1
Compound
g1
g2
g3
∆
V
∆E1
∆E2
N2H4
(NH4)2Sx
[1]ϩ
[1]
HL ϩ CoS
[Mn(L1)2]
[Mn(L2)2]
2.043
2.057
2.022
2.033
1.897
1.916
1490
1660
200
280
1375
1510
1710
1900
Scheme 1
a In dichloromethane–toluene glass at 77 K. b Taking the value of the
spin–orbit coupling constant (λ) for low-spin manganese() as equal to
crystalline orange solids which melt between 82 and 84 ЊC. The
ligands, HL1 and HL2 showed resolved 1H NMR in CDCl3. The
N–H resonance appeared9 as a broad signal at δ 10.58. The four
pyridyl protons resonate10 in the range δ 7.5 to 8.7. Signals due
to the rest of the aryl protons appear as overlapping resonances
300 cmϪ1
.
1
between δ 6.85 and 7.40. The H NMR spectrum of HL1 is
deposited as supplementary material.
The solution spectrum of HL in CH3CN consists of a strong
absorption at 470–490 nm which is associated with two more
transitions at 280 and 320 nm respectively. While the lowest
energy transition11 is due to n → π*, the rest are π → π*
transitions. For comparison, the parent ligand pap shows12 two
transitions at 445 and 320 nm respectively. The ligand HL can
be deprotonated very easily. The pKa for HL1 is quite low,
8.5, and the ligand [L]Ϫ binds as an anionic N,N,N-donor
through the deprotonation of amine nitrogen. Notably, the
2-(phenylazo)pyridine part of [L]Ϫ is a strong acid12 (soft)
while the co-ordinating nitrogen of the [NAr]Ϫ part is a hard
base. Thus the combination of the soft and hard donor char-
acters of different binding sites in [L]Ϫ is capable of stabilizing
transition metal ions in different oxidation states.
B. Low spin [MnII(L)2] and [CoIII(L)2]ClO4
(i) Isolation and characterization. The 2:1 reaction of HL
with manganese() chloride tetrahydrate in methanol contain-
ing NEt3 furnished dark brown [MnII(L)2] 2 in excellent yields
(80–85%). The cationic green cobalt() complexes were simi-
larly obtained from cobalt() chloride hexahydrate (eqn. (2)).
Fig. 1 EPR spectra of [Mn(L1)2] in (a) 1:1 dichloromethane–toluene
solution at 300 K, (b) frozen 1:1 dichloromethane–toluene solution at
77 K, showing computed splittings of t2 orbitals. DPPH = Diphenyl-
picrylhydrazyl.
methanol
(2)
MCl2ؒnH2O ϩ 2HL
[M(L)2]zϩ
NEt3
M
z
Compound
[1]ϩ
2
K) dichloromethane–toluene solution the [Mn(L1)2] complex
displays a well resolved rhombic spectrum. The EPR data are
collected in Table 1 and representative spectra of 2a are shown
in Fig. 1. The three g components for the spectra of both 2a and
2b at 77 K further split due to hyperfine coupling to give com-
plex spectral features characteristic of the low spin state of the
manganese() complexes. These spectral features can be con-
trasted13 with those of high spin manganese() complexes
which show only very broad room temperature spectra spread-
ing over g = 2–5 and very complex low temperature (77 K) spec-
tra over a very wide range due to zero field splitting in
the distorted metal environment. In no case 55Mn hyperfine
splitting is observable.
The rhombicity of the EPR spectra in the present cases indi-
cates14 the asymmetry of the electronic environment around
manganese() in the [Mn(L)2] complexes. The spectra may be
considered as pseudo-axial, consisting of a rather isolated sig-
nal g3 (g|| in the true axial case) and two relatively close signals
g1 and g2 (rhombic components of g⊥). Accordingly, the axial
distortion (∆), that splits t2 levels into a and e components, is
expected to be larger than the rhombic distortion (V ), which
splits e (Fig. 1). Spin–orbit coupling causes further changes in
the energy gap. Thus, two electronic transitions (transition
energies ∆E1 and ∆E2, ∆E1 < ∆E2) are, in principle, probable
within these three levels. All these energy parameters have been
computed (Table 1) using the observed g values, the g-tensor
theory15 of low-spin d5 complexes and a reported method.16
The axial distortion is indeed much stronger than the rhombic
CoIII
1
0
MnII
These cationic cobalt complexes were isolated as their perchlor-
ate salts in almost quantitative yields. In solution, while the
manganese() complexes are non-electrolytes, the cobalt com-
plexes behave as 1:1 electrolytes (Λ = 120–130 ΩϪ1 cm2 molϪ1 in
acetonitrile).
The cobalt complexes are diamagnetic and the room tem-
perature (298 K) magnetic moments of solid [Mn(L)2] lie in the
range 1.65–1.70 µB. Thus, both the species are low spin, ideal-
ized t2 (CoIII) and t2 (MnII). For comparison, the magnetic
moments of low spin manganese() complexes are known to
span the range1–5 1.7 to 2.3 µB. The diamagnetic cobalt()
6
5
1
complexes exhibited highly resolved H NMR spectra. In con-
trast the [Mn(L)2] complexes are EPR active. In fluid solution
each of the complexes 2a and 2b exhibits a six-line (55Mn, I =
5/2) spectrum with g = 2.043 and 2.058 respectively. A notable
feature of the spectra is the relatively low value of the hyperfine
coupling constant A, 70–85 G, which is characteristic1 of a low
spin state of MnII. For example, the A values for MnII(RxL)
[H RxL = HON᎐C(R)N᎐NC H S(CH ) SC H N᎐NC(R)᎐
᎐
᎐
᎐
᎐
2
6
4
2
x
6
4
NOH (x = 2 or 3; R = Ph or α-naphthyl)] are ca. 70 G and those
for other reported complexes3b,c,4a,5 span the range 75–100 G.
Thus the hyperfine coupling constants for the present mangane-
se() complexes lie in the lower limit of the above range. This is
expected due to strong dπ–pπ M–L interactions. In frozen (77
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Table 2 Selected bond lengths (Å) for [Mn(L1)2]
Table 3 Cyclic voltammetric dataa
Mn–N(1)
Mn–N(3)
Mn–N(4)
Mn–N(5)
Mn–N(7)
Mn–N(8)
N(5)–C(22)
N(6)–C(22)
N(6)–N(7)
N(7)–C(23)
C(23)–C(28)
C(28)–N(8)
2.003(2)
1.8877(19)
1.957(2)
1.997(2)
1.8894(19)
1.950(2)
1.361(3)
1.362(3)
1.324(3)
1.388(3)
1.410(3)
1.357(3)
N(8)–C(29)
C(23)–C(24)
C(24)–C(25)
C(25)–C(26)
C(26)–C(27)
C(27)–C(28)
N(5)–C(18)
C(18)–C(19)
C(19)–C(20)
C(20)–C(21)
C(21)–C(22)
1.424(3)
1.395(3)
1.369(4)
1.394(4)
1.368(4)
1.411(4)
1.351(3)
1.370(4)
1.383(4)
1.359(4)
1.399(4)
Compound
Oxidation E1/2/V
Reduction ϪE1/2/V
[Mn(L1)2]
[Mn(L2)2]
[Co(L1)2]ClO4
[Co(L2)2]ClO4
HL1
0.38, 1.10,b 1.56c
0.35, 1.05,b 1.52c
1.21,b 1.56c
0.96, 1.45, 1.88d
0.98, 1.47, 1.89d
0.40, 0.81, 1.52d
0.41, 0.80, 1.49d
1.11, 1.51d
1.17,b 1.51c
1.05,c 1.36c
a Experiments were carried out in CH3CN and CH2Cl2 at 298 K using
NEt4ClO4 as supporting electrolyte. The reported data correspond to
a scan rate of 50 mV sϪ1 b ipa > ipc. c Irreversible anodic response;
.
the potential corresponds to Epa. d Irreversible cathodic response; the
potential corresponds to Epc.
Fig. 2 An ORTEP17 diagram of [Mn(L1)2], 2a.
Fig. 3 Cyclic voltammograms (scan rate 50 mV sϪ1) of a 10Ϫ3 M solu-
tion in acetonitrile (0.1 M NEt4ClO4) at 298 K at a platinum working
electrode of (a) [Co(L1)2]ϩ, (b) [Mn(L1)2].
one. The calculated values of ∆E1 and ∆E2 are 1375–1510 and
1710–1900 cmϪ1 respectively which lie in the infrared region and
could not be detected. The analysis of EPR spectral data thus
indicates that the [Mn(L)2] complexes are significantly distorted
from an ideal octahedral arrangement. This is further revealed
from the structural analysis as discussed below.
Mn–N(aza) bonds, either through an increase in the electron
density on N(aza) and a concomitant increased basicity or
through enhanced π-bonding capability of the more conjugated
ligand.
(ii) Structure of [Mn(L1)2]. Fig. 2 shows the molecular struc-
ture of the complex [Mn(L1)2] along with the atomic numbering
scheme used. Selected bond distances are listed in Table 2. The
two ligands bind the metal in the N6 fashion using pairs of
pyridyl-N, azo-N, and amido-N atoms. The relative orientations
within the pairs are cis, trans and cis, respectively. The chelate
bite angles N(py)–Mn–N(azo), i.e. N(1)–Mn–N(3) and N(5)–
Mn–N(7), are similar (ca. 77Њ) and smaller than N(azo)–Mn–
N(amido) bite angles (ca. 81Њ). The corresponding cobalt()
complex also showed7 a similar trend. The two aza nitrogens of
the anionic tridentate ligands approach the metal centre closest,
with Mn–N(3)/N(7) ca. 1.89 Å. The Mn–N(py) (ca. 2.00 Å)
distances are longest.
There is an indication of significant backbone conjugation in
the co-ordinated anionic ligand. The average distance N(aza)–
C(phenyl) 1.38 Å is shorter than the value of 1.421(5) Å found
for the corresponding distance18 in the salt [Hpap]ClO4. Simi-
larly, the amido nitrogen of [L]Ϫ binds to the phenyl group of
pap at a shorter distance than is found for a N–C(phenyl) single
C. Redox and spectroelectrochemistry
Redox properties of the cobalt and manganese complexes have
been studied by cyclic voltammetry (CV) using a platinum
working electrode. Voltammetric data are collected in Table 3
and representative voltammograms are shown in Fig. 3.
The cobalt complexes, [1]ϩ, display three major reversible to
quasireversible cathodic responses in the range Ϫ0.0 to Ϫ1.5 V.
These also show two high potential (>1.0 V) anodic responses,
of which the first is quasireversible (ipa > ipc) and the other one
is irreversible. The manganese complexes 2, on the other hand,
show a low potential anodic response at ca. 0.38 V in addition
to two high potential responses occurring at similar potentials
to those for the cobalt analogues. The cathodic scans for 2 show
one reversible response at Ϫ0.96 V which lies in between the
second and third cathodic responses for the cobalt complex. It
may be worthwhile first to consider the redox potentials of a
representative “free” ligand, viz. HL1. It displays two irrevers-
ible anodic and a reversible cathodic response at 1.05, 1.36, and
Ϫ1.11 V respectively (Table 3). A comparison of the nature of
the voltammograms of [Co(L1)2]ϩ, [Mn(L1)2], and HL1 confirms
that the first cathodic response for the cobalt complex and the
least potential anodic response for the manganese complex
bond distance. The N᎐N aza distances also indicate elongation
᎐
which is consistent with the notion of greater electron delocal-
ization along the ligand backbone in the anionic [L]Ϫ. Some
degree of conjugation along the ligand backbone of the bis-
chelate system could be implicated in the shortening of the
J. Chem. Soc., Dalton Trans., 2000, 1703–1708
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Table 4 Electronic spectral data
Compound
Absorptiona λmax/nm(ε/MϪ1 cmϪ1
)
[Mn(L1)2]
1170(2060), 760(10360), 575(10940), 525b(9360),
375(15030), 322b(25300), 290(32320), 240(40430)
2390b(1270), 2260b(2220), 2230b(2380), 1750b
(1110), 745b(5546), 640(13310), 515(11410),
400b(15700), 325b(24250)
[Mn(L1)2]ϩ c
[Mn(L1)]Ϫ c
[Mn(L2)2]d
[Co(L1)2]PF6
680b(7610), 580b(7290), 535b(6500), 340(24900),
300(24560)
760(11340), 575(11800), 523b(10040),
370(10180), 320b(27700), 290(34060), 245(43110)
885(7650), 795(11460), 715(10550), 650b(8600),
400b(17474), 375b(17600), 310b(22170),
250(48140)
[Co(L1)2]c
800b(7450), 730b(6880), 525b(5160), 475(9740),
395b(17770), 420(18340), 255(41840)
970b(3440), 890b(4580), 860b(4580), 630b(4580),
500b(9740), 408(25790), 295b(30950), 260(40120)
795(10600), 720(10300), 660b(8580),
375b(21580), 350b(21000), 320b(21700),
250(46500)
[Co(L1)2]Ϫ c
[Co(L2)2]ClO4
Fig. 5 Solution spectra of [Mn(L1)2] (- - - -), [Mn(L1)2]ϩ (——) and
d
[Mn(L1)2]Ϫ (– –). Inset: OTTLE cell experiments, time dependence
·
spectra, applied potential 0.75 (i), Ϫ1.30 V (ii).
HL1 d
HL2 d
490(7820), 420b(5640), 325(15800), 280(19800),
220b(23800)
495(7970), 325(15100), 280(18210), 220b(15370),
215(16520)
the range 900–650 nm. On the basis of their high intensities,
these transitions are assigned as charge transfer in nature. Since,
6
CoIII in complex [1]ϩ is in the low-spin t2 configuration, these
a Data obtained from absorption spectra: solvent CH3CN. b Shoulder.
c Spectroelectrochemically generated compound in an OTTLE cell (see
text). d Spectral data in UV and visible range.
low energy bands may be due19 to dπ(CoIII) to ligand acceptor
orbitals. The multiple nature of the transitions originates from
the lack of symmetry of the co-ordination environment as well
as the presence of different acceptor levels. Upon successive
reductions, [1]ϩ → 1 → [1]Ϫ, the transition energies as well
as the intensities both decrease appreciably (Table 4). However,
the intensities are still high for allowed charge transfer transi-
tions. This may be due to considerable mixing of metal and
ligand orbitals. There are two more major transitions at 400
and 250 nm. This part of the spectrum (400–220 nm) of
[Co(L1)2]ϩ closely matches with the spectrum of [Mn(L1)2]ϩ (see
below). These transitions may be assigned as intra-ligand
transitions. The “free” ligand HL1 shows multiple transitions in
the region 500–220 nm. Its conjugate base, [L]Ϫ, generated in
solution by the addition of dilute NaOH(aq) showed almost
identical spectral features.
The transitions in the range 1400–450 nm for the mangan-
ese() complex [Mn(L1)2] are vastly different from those
observed for [Co(L1)2]ϩ. The lowest energy transition at 1170
nm is broad and least intense (ε 2060 MϪ1 cmϪ1). There are three
more highly intense (ε >10000 MϪ1 cmϪ1) major transitions in
the aforesaid region. Upon oxidation of 2a to [2a]ϩ, the 1170
nm transition disappeared with concomitant appearance of two
broad transitions at 1750 and 2260 nm. These transitions may
be assigned to partially allowed transitions originating20 from
the splitting of low-spin d4 MnIII. However, the intensities of
these are very high as compared to usual d–d transitions which
may be attributed to considerable mixing of metal and ligand
orbitals. Upon reduction of the starting manganese() complex
the broad low energy transition disappeared. The visible range
spectra of the manganese complexes consist of multiple intense
charge transfer transitions. The bivalent complex, 2a, showed
two major transitions at 760 and 575 nm respectively which may
be assigned as metal to ligand charge transfer [dπ(MnII)→L].
Upon oxidation to the corresponding trivalent state [2a]ϩ these
two transitions are blue shifted. The UV-range transitions are
dominated by strong intra-ligand charge transfer. The spectral
changes due to oxidation and reduction of [Mn(L1)2] are shown
in Fig. 5.
Fig. 4 Solution spectra of [Co(L1)2]ϩ (- - - -), [Co(L1) ] (– –) and
·
2
[Co(L1)2]Ϫ (——). Inset: OTTLE cell experiments, time dependence
spectra, applied potential Ϫ0.65 (i), Ϫ1.0 V (ii).
may be assigned to CoIII
CoII and MnII
MnIII redox
couples in the respective complexes. The nature of the other
responses is not certain and may be due to redox processes at
the co-ordinated [L]Ϫ centres.
In order to study the spectra of the electrogenerated
cobalt() and manganese() complexes, two representative
examples viz. [Co(L1)2]ClO4 and [Mn(L1)2] were chosen for
spectroelectrochemical studies. These were performed in an
optically transparent thin layer electrode (OTTLE) cell. The
compounds are more soluble in dichloromethane (dcm) than
in acetonitrile and hence dcm was used as the solvent for a
meaningful record of relatively low intensity spectral changes
especially in the low energy region. It is to be noted here that the
nature as well as the potentials of the redox responses of both
the above cobalt() and manganese() complexes are similar in
these two solvents.
Fig. 4 shows the solution spectra of [Co(L1)2]ϩ [1]ϩ along
with electrogenerated [Co(L1)2] 1 and [Co(L1)2]Ϫ [1]Ϫ. The spec-
tral data are collected in Table 4. Both reduction processes are
reversible and the spectra of the starting cationic [1]ϩ, neutral
1, and anionic [1]Ϫ compounds may quantitatively be repro-
duced by application of the appropriate potentials. The starting
complex shows an envelope of four closely spaced transitions in
Conclusion
We have successfully achieved the synthesis of some new low-
spin manganese() and cobalt() complexes. The ligands (HL)
used in this work are obtained from novel cobalt assisted
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J. Chem. Soc., Dalton Trans., 2000, 1703–1708

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organic ring amination processes via C–H activation. The pres-
ence of the combination of hard and soft binding sites in these
N,N,N-donors makes them excellent ligands for soft as well as
hard metal ions. The bond distances in the manganese com-
plexes are indicative of extensive delocalization along the ligand
backbone. This effect is also reflected in the unusual low hyper-
fine constant, A, values in their EPR spectra. The metal oxid-
ation potentials for [MnL2] are low. The spectroelectrochemical
C, 58.98; H, 4.09; N, 15.29%. ΛM = 130 ΩϪ1 cm2 molϪ1 (1 × 10Ϫ3
Ϫ
M in CH CN). IR(KBr): ν(N᎐N) 1240, ν(C–N) 1590, ν(ClO
)
4
᎐
3
1090, 620 cmϪ1
.
Isolation of 2-[2-(phenylamino)phenyl]azopyridine (HL1) from
complex [1a]ϩ. The compound [Co(L1)2]ClO4 (0.0834 g, 0.1184
mol) was dissolved in ethanol (30 ml) and to it hydrazine
hydrate (5 ml) and yellow ammonium sulfide (5 ml) were added.
The mixture was then stirred for 30 min at room temperature.
The resulting orange-yellow solution was evaporated under
vacuum, then extracted with benzene and subjected to column
chromatography on a silica gel (1 × 15 cm) column. An orange-
yellow band was eluted with benzene–chloroform (90:10),
which on evaporation yielded orange crystals of HL1 in 60%
yield. Found: C, 74.09; H, 5.38; N, 19.36. Calc. for C17H14N4:
studies on [MnL2]
[MnL2]ϩ and [Co(L)2]ϩ
[Co(L)2]
redox processes confirm the existence of the corresponding tri-
valent manganese and bivalent cobalt complexes respectively in
solution. The co-ordination chemistry involving anionic [L]Ϫ
appears to be very rich with many uncommon features which are
under scrutiny.
C, 74.45; H, 5.10; N, 20.43%. IR(KBr): ν(N᎐N) 1240, ν(C–N)
᎐
Experimental
1
1595 cmϪ1. mp 82 ЊC. pKa = 8.5 1. H NMR: δ 10.58 (N–H),
8.70 (d, 4 H), 8.00 (d, 1 H), 7.87 (t, 2 H), 7.15 (t, 7 H), 6.90
(t, 8 H) and 7.77 (d, 9 H).
Materials
The starting complex [Co(pap)3][ClO4]2 was prepared by the
reported method.8 Solvents and chemicals used for synthesis
were of analytical grade. The supporting electrolyte tetraethyl-
ammonium perchlorate and solvents for electrochemical work
were obtained as before.21 CAUTION: perchlorate salts of metal
complexes are generally explosive. Although no detonation
tendencies have been observed, care is advised and handling
of only small quantities recommended.
Similarly, HL2 was isolated as orange needles from [Co-
(L2)2]ClO4, 1b. Yield 58%. Found: C, 75.26; H, 5.73; N, 19.55.
Calc. for C18H16N4: C, 75.00; H, 5.55; N, 19.44%. IR(KBr):
1
ν(N᎐N) 1240, ν(C–N) 1510 cmϪ1. mp 84 ЊC. pKa = 7.5 1. H
᎐
NMR: 10.58 (N–H), 8.70 (d, 4 H), 8.00 (d, 1 H), 7.88 (t, 2 H),
7.35 (t, 7 H), 6.90 (t, 8 H) and 7.80 (d, 9 H).
[Co(L1)2]ClO4 from preformed HL1. The ligand HL1 (0.157 g,
0.573 mmol) was dissolved in 25 ml of methanol and to it 1–2
drops of triethylamine were added. To the deprotonated lig-
and solution a methanolic solution of CoCl2ؒ6H2O (0.06819 g,
0.287 mmol) was added and the mixture stirred for 1 hour at
room temperature. The solution changed from orange-yellow to
green. The resulting solution was filtered and to it diluted aque-
ous NaClO4 was added. The dark crystalline compound thus
obtained was recrystallized from dichloromethane–hexane.
Yield 93%. The physicochemical properties of this compound
exactly correspond to those of an authentic sample obtained
from the reaction of [Co(pap)3]2ϩ and PhNH2 as described
above.
Physical measurements
A Shimadzu UV 21000 uv/vis spectrophotometer was used to
record electronic spectra. The IR spectra were obtained with
a Perkin-Elmer 783 spectrophotometer, H NMR spectra in
1
CDCl3 with a Bruker Avance DPX300 spectrophotometer and
SiMe4 as internal standard. A Perkin-Elmer 240C elemental
analyser was used to collect microanalytical data (C,H,N).
Electrochemical measurements were performed under a dry
nitrogen atmosphere on a PAR model 370-4 electrochemistry
system as described earlier.18 All potentials reported in this
work are referenced to the Ag–AgCl electrode and uncorrected
for junction contribution. The pH measurements were made
with a µ-pH system 361 Systronics pH meter, standardized with
buffers 7.0 and 9.2. Electrical conductivities were measured by
using a Systronic Direct Reading Conductivity meter 304. The
pK value was determined pH-metrically as described before.22
EPR measurements were made with a Varian 109C E-line
X-band spectrometer fitted with a flat cell. Spectra were cali-
brated with DPPH (g = 2.0037). Magnetic moment measure-
ments were carried out by using a PAR 155 vibrating
sample magnetometer fitted with a Walker Scientific L75FBAL
magnet. Spectroelectrochemical studies were carried out in
an OTTLE cell mounted in the sample compartment of a
Perkin-Elmer Lambda 19 spectrophotometer, as described
earlier.23
The complex [Co(L2)2]ClO4 [1b]ϩ was obtained similarly in
95% yield by treating HL2 with hydrated cobalt() chloride.
Bis{phenyl[2-(2-pyridylazo)phenyl]amido}manganese(II),
[Mn(L1)2] 2a. The ligand HL1 (0.1455 g, 0.53 mmol) was dis-
solved in 25 ml of methanol and to it 1–2 drops of triethyl-
amine were added. To the deprotonated ligand solution a
methanolic solution of MnCl2ؒ4H2O (0.06834 g, 0.26 mmol)
was added and the mixture stirred for 1 hour at room temper-
ature. The solution changed from orange-yellow to dark brown.
The resultant mixture was filtered and the solvent evaporated
under vacuum. The compound thus obtained was finally
crystallized from dichloromethane–hexane. Yield 85%. Found:
C, 67.58; H, 4.42; N, 18.70. Calc. for C34H26MnN8: C, 67.89; H,
4.32; N, 18.63%. µeff (298 K): 1.65 µ . IR(KBr): ν(N᎐N) 1230,
᎐
B
ν(C–N) 1590 cmϪ1
.
Syntheses
Similarly MnCl2ؒ4H2O reacted with HL2 in 1:2 ratio in
methanolic solution in the presence of 1–2 drops of triethyl-
amine as base to give crystalline [Mn(L2)2] 2b. Yield 80%.
Found: C, 68.48; H, 5.26; N, 17.56. Calc. for C36H30MnN8:
C, 68.68; H, 4.77; N, 17.80%. µeff (298 K): 1.70 µB. IR(KBr):
Bis{phenyl[2-(2-pyridylazo)phenyl]amido}cobalt(III)
per-
chlorate, [Co(L1)2]ClO4, [1a]ϩ. A mixture of [Co(pap)3][ClO4]2
(0.2 g, 0.248 mmol) and aniline (0.5 ml) was heated on a steam
bath for 1 hour. The initial brown colour gradually changed to
intense green. The cooled green mixture was throughly washed
with diethyl ether. Crystallization of the crude product from a
dichloromethane–hexane mixture yielded [1a]ϩ (83%) as green
crystals. Found: C, 57.35; H, 3.86; N, 15.45. Calc. for C34H26Cl-
CoN8O4: C, 57.92; H, 3.69; N, 15.89%. ΛM = 120 ΩϪ1 cm2 molϪ1
(1 × 10Ϫ3 M in CH CN). IR(KBr): ν(N᎐N) 1250, ν(C–N) 1590,
ν(N᎐N) 1230, ν(C–N) 1595 cmϪ1
.
᎐
Crystal structure of [Mn(L1)2] 2a
An X-ray quality crystal of complex 2a was obtained by slow
diffusion of a dichloromethane solution of the compound into
hexane. The crystal data are collected in Table 5. Intensity data
for a brown cubic crystal with dimensions 0.51 × 0.48 × 0.47
mm were measured on an Enraf-Nonius MACH3 automatic
diffractometer equipped with graphite-monochromated Mo-Kα
᎐
3
ν(ClO4Ϫ) 1100, 625 cmϪ1
.
The complex [Co(pap)3][ClO4]2 reacts similarly with p-
toluidine to yield crystalline [Co(L2)2]ClO4, [1b]ϩ. Yield (80%).
Found: C, 58.93; H, 3.90; N, 14.91. Calc. for C36H30ClCoN8O4:
J. Chem. Soc., Dalton Trans., 2000, 1703–1708
1707

View Article Online
2 U. Knof, T. Weyhermüller, T. Wolter and K. Weighardt, J. Chem.
Soc., Chem. Commun., 1993, 726.
Table 5 Crystallographic data and details of refinement of [Mn(L1)2]
3 (a) W. P. Griffith Coord. Chem. Rev., 1975, 17, 177; (b) P. T.
Manoharan and H. B. Gray, Chem. Commun., 1965, 324; (c)
D. A. C. McNeil, J. B. Raynor and M. C. R. Symons, J. Chem. Soc.,
1965, 410.
4 (a) P. Fantucci, L. Naldini, F. Cariati, V. Valenti and C. Bussetto,
J. Organomet. Chem., 1974, 64, 109; (b) D. S. Matteson and R. A.
Bailey, J. Am. Chem. Soc., 1969, 91, 1975.
5 N. G. Connelly, K. A. Hassard, B. J. Dunne, A. G. Orpen, S. J.
Raven, G. A. Carriedo and V. Riera, J. Chem. Soc., Dalton Trans.,
1988, 1623; G. A. Carriedo, V. Riera, N. G. Connelly and S. J. Raven,
J. Chem. Soc., Dalton Trans., 1987, 1769.
6 A. B. P. Lever, Inorganic Electronic Spectroscopy, 2nd edn., Elsevier,
New York, 1984, p. 750.
Chemical formula
Formula weight
T/K
Crystal system
Space group
a/Å
b/Å
c/Å
C34H26MnN8
601.57
293(2)
Monoclinic
P21/n
10.3746(17)
16.061(6)
16.726(6)
β/Њ
90.15(2)
2786.9(15)
4
V/Å3
Z
µ/mmϪ1
0.514
Reflections collected/unique
Final R indices [I > 2σ(I )]
5381/4901 [R(int) = 0.0000]
R1 = 0.0332, wR2 = 0.0740
7 A. Saha, A. K. Ghosh, P. Majumdar, K. N. Mitra, S. Mondal, K. K.
Rajak, L. R. Falvello and S. Goswami, Organometallics, 1999, 18,
3772.
8 A. K. Mahapatra, Ph.D. Thesis, Jadavpur University (India),
1989.
9 K. N. Mitra, S. Choudhury, A. Castiñeiras and S. Goswami,
J. Chem. Soc., Dalton Trans., 1998, 2901.
10 A. K. Mahapatra, B. K. Ghosh, S. Goswami and A. Chakravorty,
J. Indian Chem. Soc., 1986, 63, 101.
11 K. C. Kalia and A. Chakravorty, J. Org. Chem., 1970, 35, 2231.
12 S. Goswami, A. R. Chakravarty and A. Chakravorty, Inorg. Chem.,
1981, 20, 2246.
13 P. Chakravorty, S. K. Chandra and A. Chakravorty, Inorg. Chem.,
1993, 32, 5349; R. Rajan, R. Rajaram, B. U. Nair, T. Ramasami and
S. K. Mondal, J. Chem. Soc., Dalton Trans., 1996, 2019.
14 P. H. Rieger, Coord. Chem. Rev., 1994, 135/136, 203.
15 B. Bleaney and M. C. M. O’Brien, Proc. Phys. Soc. London,
Sect. B, 1956, 69, 1216; J. S. Griffith, The Theory of Transition Metal
Ions, Cambridge University Press, London, 1961, p. 364.
16 S. Bhattacharya and A. Chakravorty, Proc. Indian Acad. Sci., Chem.
Sci., 1985, 95, 159.
radiation, λ = 0.71073 Å. Data were corrected for Lorentz-
polarization effects.21 A total of 5381 reflections were collected,
of which 4901 were unique and 3584 satisfied the I > 2σ(I )
criterion and were used in the subsequent analysis. The
structure was solved by employing the SHELXS 86 program
package24 which revealed the positions of all non-hydrogen
atoms, and refined by full-matrix least squares based on
F2.25 All hydrogen atoms of the ligands were exactly located
in calculated positions.
CCDC reference number 186/1928.
See http://www.rsc.org/suppdata/dt/a9/a909769d/ for crystal-
lographic files in .cif format.
Acknowledgements
Financial support received from the Department of Science
and Technology, New Delhi, is acknowledged. X-Ray data on
complex 2a were collected at the National Single Crystal X-Ray
Facilities at the School of Chemistry, University of Hyderabad.
We thank Dr S. Paul and Dr S. Bhattacharya for their help. SG
is grateful to Professor J. A. McCleverty and Dr M. D. Ward
for allowing him to perform the OTTLE cell experiments at
their laboratory. He also thanks the RSC for its financial sup-
port which allowed him to visit the University of Bristol.
17 C. K. Johnson, ORTEP II, Report ORNL-5138, Oak Ridge
National Laboratory, Oak Ridge, TN, 1976.
18 P. Majumdar, S.-M. Peng and S. Goswami, J. Chem. Soc., Dalton
Trans., 1998, 1569.
19 B. K. Santra and G. K. Lahiri, J. Chem. Soc., Dalton Trans., 1997,
1883.
20 F. A. Cotton, G. Wilkinson, C. A. Murillo and M. Bochmann,
Advanced Inorganic Chemistry, 6th edn., John Wiley and Sons, Inc.,
New York, 1999, p. 764.
21 S. Choudhury, A. K. Deb and S. Goswami, J. Chem. Soc., Dalton
Trans., 1994, 1305.
22 P. Ghosh and A. Chakravorty, J. Chem. Soc., Dalton Trans., 1985,
361.
23 S.-M. Lee, M. Marcaccio, J. A. McCleverty and M. D. Ward, Chem.
Mater., 1998, 10, 3272.
24 G. M. Sheldrick, SHELXS 86, Program for the solution of crystal
structures, University of Göttingen, 1993.
25 G. M. Sheldrick, SHELXL 97, Program for the refinement of crystal
structure, University of Göttingen, 1997.
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J. Chem. Soc., Dalton Trans., 2000, 1703–1708