Ligand trans-effect in octahedral complexes of 3d metals and its manifestation in the synthesis of metalloporphyrins in solution

Article abstract of DOI:10.1134/S003602360907016X

We study features of the indicator kinetic reaction of meso (tetraphenyl)tetrabenzoporhine with cobalt(II), nickel(II), and manganese(II) acetates in electron-donating organic solvents ( N , N -dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and pyrid

Full text of DOI:10.1134/S003602360907016X

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2009, Vol. 54, No. 7, pp. 1090–1094. © Pleiades Publishing, Inc., 2009.
Original Russian Text © O.V. Toldina, D.B. Berezin, B.D. Berezin, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 7, pp. 1151–1155.
COORDINATION
COMPOUNDS
Ligand trans-Effect in Octahedral Complexes of 3d Metals
and its Manifestation in the Synthesis
of Metalloporphyrins in Solution
O. V. Toldinaa, D. B. Berezinb, and B. D. Berezinb
a Institute of Solution Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, Ivanovo, 153045 Russia
b Ivanovo State University of Chemical Technology, ul. Engelsa 7, Ivanovo, 153000 Russia
Received December 17, 2007
Abstract—We study features of the indicator kinetic reaction of meso(tetraphenyl)tetrabenzoporhine with
cobalt(II), nickel(II), and manganese(II) acetates in electron-donating organic solvents (N,N-dimethylforma-
mide (DMF), dimethyl sulfoxide (DMSO), and pyridine (Py)) and in their binary mixtures at temperatures in
the range 348–368 K. Specific features of the trans-effect of solvent molecules on the rate and activation param-
eters of formation of manganese(II), cobalt(II), nickel(II), and copper(II) complexes with porphyrin are noted.
DOI: 10.1134/S003602360907016X
The trans-effect of molecular and acido ligands
since it was discovered by Chernyaev more than one
hundred years ago in platinum(II) complexes [1] has
been widely used for the synthesis of mixed-ligand
complexes of the platinum-group metals and cobalt(III)
[2] and substantiated theoretically [3–6]. trans-Effect,
which consists in the labilization of the M–L (metal–
ligand) donor–acceptor bond located in the trans-posi-
tion of a square-planar [ML4] or octahedral [ML6] com-
plex relative to the trans-influencing ligand LT (scheme 1),
is manifested only in very stable complexes of the plat-
inum-group metals (Ru, Rh, Pd, Os, Ir, and Pt). Later
[7], the trans-effect was discovered in very stable
cobalt(III) complexes in dissociation reactions in polar
electron-donating solvents.
LT
M
L
LT
M
L
Scheme 1.
EXPERIMENTAL
Porphyrin for use in indicator kinetic reactions was
prepared as described in [14]; solvents and salts were
washed and dried after [15, 16]. The rate of the indica-
tor reaction of metal phorphyrin formation was derived
from the change of electronic absorption spectra in the
visible measured on an SF-46 instrument. The experi-
mental procedures and calculations of kinetic parame-
ters did not differ from those described in [7].
2+
'
Some kinetic features of the behavior of [MLmLp ]
mixed solvates discovered several decades later [8–12]
implied the existence of the ligand trans-effect in the
coordination sphere of these labile complexes [13]. The
trans-effect is a complex phenomenon; to understand it,
one should study mixed coordination spheres of other
metals than platinum-group metals and cobalt(III).
Such complexes include mixed-ligand complexes of 3d,
4d, and 5d metals; these complexes have not been stud-
ied, unlike platinum metal complexes.
RESULTS AND DISCUSSION
Tables 1 and 2 list effective (keff) and true (bimolec-
ular k2v98 ) rate constants, activation energies (Ea), and
activation entropies (∆S≠) of reaction (1) in which phor-
1090

LIGAND trans-EFFECT IN OCTAHEDRAL COMPLEXES OF 3d METALS
1091
Table 1. Rates and activation parameters of reaction (1) in which the complex CoTPTBP is formed in DMF, DMSO, Py, and
their binary mixtures
k2v98 , L/(mol s)
keff × 104, s–1
∆S≠, J/(mol K)
–43
*
Ea, kJ/mol
Solvent
T, K
Py
308
318
328
3.35
7.28
15.3
0.59
0.77
0.96
2.50
3.0
63
55
61
58
58
53
2
3
4
5
3
2
6
DMSO
DMF
308
318
328
4.18
8.19
15.85
–68 12
–46 13
–50 16
–48 10
308
318
328
4.90
10.2
21.3
DMF-Py (50%–50%)
308
318
328
12.4
26.2
50.1
DMSO-Py (50%–50%)
DMF-DMSO (50%–50%)
308
318
328
15.8
31.2
63.8
308
318
328
16.5
31.2
58.8
3.2
–64
7
* Calculated from the Arrhenius equation. Precision: 10%.
phyrin complexes are produced from mixed solvate
complexes of cobalt(II) and nickel(II) acetates and NH-
active tetraphenyltetrabenzoporphine (I):
The results can be interpreted on the basis of the the-
oretical inferences made in [17, 18], where H2TPTBP
was shown to be one of the most reactive phorphyrins
in the reactions used to study the trans-activity of sol-
298
k
, L/(mol s)
3.5
1
3.0
2.5
2.0
1.5
1.0
0.5
N
NH
N
2
3
HN
0
50
Py, DMSO, DMF, %
100
(I)
[MAc2(S1)m(S2)n – m – 2] + H2TPTBP
Fig. 1. trans-Influence in the kinetics of reaction (1) of
cobalt(II) acetate in binary solvents: (1) DMSO–DMF, (2)
Py–DMSO, and (3) Py–DMF.
(1)
TPTBP + 2HAc + S….
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TOLDINA et al.
Table 2. Rates and activation parameters of reaction (1) in which the complex NiTPTBP is formed in DMF, DMSO, Py, and
their binary mixtures
k2v98 , L/(s mol)
keff × 104, s–1
∆S≠, J/(mol K)
–32
*
Ea, kJ/mol
Solvent
T, K
Py
348
358
368
0.825
1.71
3.51
0.010
77
78
2
4
6
DMSO
DMF
348
358
368
2.99
6.04
13.1
0.033
–18 15
368
Reaction does not occur
DMF-Py (50%–50%)
DMSO-Py (50%–50%)
DMF-DMSO (50%–50%)
348
358
368
1.88
4.11
8.10
0.023
0.063
0.019
78
75
71
4
3
4
–23 11
–24 10
–48 12
348
358
368
4.84
10.3
19.9
348
358
368
2.38
4.92
9.00
* Calculated from the Arrhenius equation. Precision: 10%
vated salts of 3d metals whose 3d shell is unfilled.
A very important inference [18] is that trans-activity
trans-Activity was also found in Py, DMSO, and DMF increases in going from the homogeneous coordinate
molecules along S–Cu–S (homosolvates) and S2–Cu–S2 LT–M–LT to heterogeneous coordinate L–M–LT.
(mixed solvates) trans-coordinates in DMF–Py, DMF–
The results of our study displayed in Tables 1 and 2
DMSO, and DMSO–Py binary solvents.
and Figs. 1 and 2 imply that [MAc2(S)n – 2] solvated
salts, where M = Co or Ni, have individual features.
Whereas CoAc2 reacts with phorphyrin at moderate and
roughly equal rates in all three net solvents (avg., kv298
0.77 0.12 L/(mol s)), NiAc2 in DMF does not form Ni-
porphyrin even at 368 K and reaction rates with Py (kv298
=
298
k
, L/(mol s)
0.07
0.06
0.05
0.04
0.03
0.02
0.01
=
1
0.010 L/(mol s)) and DMSO (k2v98 = 0.033 L/(mol s)) differ
by a factor of three. The absence of reaction (1) for
NiAc2 in DMF can arise from the dominance of
NiAc2(DMF)2 tetrahedral complex in this solvent; the
trans-effect cannot appear in these complex because of
the absence of a trans-coordinate along axes x, y, and z
[5, 17, 18]. In DMSO, Py, and DMSO–Py, DMF–Py,
and DMF–DMSO (1 : 1) mixed solvents, there are suf-
ficient concentrations of octahedral solvated salts
[NiAc2(S)4], in which the trans-effect is manifested.
The degree of manifestation is dictated by the ratio of
concentrations of octahedral and tetrahedral solvates.
2
3
0
50
100
Py, DMSO, DMF, %
In the NiAc2–Py–DMSO(DMF) system, the trans-
influence is pronounced, especially in DMF–Py. The
reaction rate in this system is about twofold the rate in
net pyridine. trans-Activity along the Py–Ni–DMF
Fig. 2. trans-Influence in the coordination sphere of
NiAc (S) in binary solvents: (1) DMSO–DMF, (2) Py–
DMF, and (3) Py–DMSO.
2
4
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LIGAND trans-EFFECT IN OCTAHEDRAL COMPLEXES OF 3d METALS
1093
coordinate evidently dominates in Py. In DMF–DMSO
A
binary solvent, the trans-activity of DMSO along the
DMF–Ni–DMSO coordinate is as along the Py–Ni–
DMSO coordinate in the Py–DMSO system. These
inferences are made from analysis of the data displayed
in Table 2 and Fig. 2. Very close values of the activation
energies (avg., 76 2 kJ/mol) and activation entropies
(avg., –29 9 J/(mol K)) in five systems studied show
that reaction (1) between porphyrin and NiAc2 follows
the same activation mechanism SEN2 [19–21]. A rather
similar scenario is observed in cobalt acetate (CoAc2)
systems in the same solvents (Table 1, Fig. 1), except
for the fact that the reaction rate between CoAc2 and
TPTBP in net and mixed binary solvents is tens of times
that for NiAc2.
2.5
2.0
1.5
1.0
0.5
0
400
500
600
700 λ, nm
For mixed solvates of cobalt acetate based on
Fig. 3. Destruction of the H TPTBP ligand in DMF in the
2
CH3COOH,
the
stability
constants
of
presence of MnAc in 120 min.
2
[CoAc2(HAc)3(DMF)], [CoAc2(HAc)3(DMSO)], and
[CoAc2(HAc)3Py] are 14.7, 15.0, and 240, respectively.
For the corresponding mixed solvates of ZnAc2 Kst =
190, 204, and 1700. For [MnAc2(HAc)3(DMF)] Kst =
9.9, whereas for [NiAc2(HAc)3(DMF)] Kst = 5.4. From
these values and the data compiled in Tables 1 and 2,
the weaker the coordination link with an electron-
donating solvent molecule, the weaker the trans-influ-
ence enhancing reaction (1). For this reason, the reac-
tion rate for NiAc2 is far lower than for CoAc2. Proba-
bly, the same reason is responsible for the roughly
slowest. Destruction reasons and mechanisms have not
been studied.
The inertness of MnAc2 in complex-formation reac-
tion (1) with tetraphenyltetrabenzoporphine is unex-
pected, the more so as Golubchikov et al. [22] found the
trans-influence in the coordination sphere of
[MnAc2(HAc)4 – m(DMF)m] in the complex-formation
reaction with tetraphenylporphine. Where 1.2 mol/L
equal enhancement of reaction (1) in mixed solvents, DMF is present in CH3COOH, the MnTPP complex is
compared to net solvents, on account of trans-influence
along S1–Co–S2 axes; the enhancement is ~2.5–4 times
in DMF–Py, ~4–5 times in DMSO–Py, and ~4 times in
DMF–DMSO. Presumably, for CoAc2 the same mech-
anism of the enhancing trans-effect on reaction (1)
operates in all six solvents (Table 1), as the mean acti-
vation energy is 58 3 kJ/mol and the mean activation
entropy is –53 8 J/(mol K). The activation energy for
NiAc2 is 18 kJ/mol higher and the activation entropy is
24 J/(mol K) higher than for CoAc2. The solvatokinetic
compensation effect strictly holds; this effect is charac-
teristic of all phorphyrin coordination reactions
[19−21] and caused by the high role of the solvent in
forming the transition states of phorphyrin systems.
formed three times as rapidly as in glacial CH3COOH.
Golubchikov et al. [22] concluded that, in the MAc2–
H2TPP–CH3COOH–DMF systems, the trans-influence of
DMF along coordinates x and y in the octahedral complex
changes in the order Mn2+ < Co2+ > Ni2+, Cu2+ ꢀ Zn2+ (for
Zn2+, it is zero). Toldina et al. [23] suggested that the
trans-influence is impossible in high-spin manga-
nese(II), cobalt(II), iron(II), and nickel(II) solvated
salts. trans-Influence is manifested in these cations
only in the low-spin state, which can appear in the tran-
sition state of reaction (1) and similar reactions with
other phorphyrins. Avoiding the transition state, phor-
phyrin-coordinated manganese(II) and other metal(II)
cations can return to the high-spin state.
We also studied reaction (1) with manganese acetate
MnAc2 in the same solvents as we chose for NiAc2 and
CoAc2. Unexpectedly, MnAc2 was completely inert in
Py, DMSO, and DMF. The destruction of the H2TPTBP
ligand was observed in all three solvents at 360°C, with
the disappearance of strongest Soret band in the elec-
tronic absorption spectrum in the presence of MnAc2
(Fig. 3). The ligand in DMF or DMSO was half-
destructed in 2 h; in DMSO–DMF (1 : 1), the ligand
The trans-influence mechanism in the coordination
sphere of octahedral complexes has not been discussed
in the literature. Bersuker and Ablov [5] assumed that
the stronger steric crowding of the reaction site (M2+),
enhanced influence of cis-ligands, and the like are char-
acteristic of this type of complex. However, Bersuker
and Ablov [5] dealt with very strongly bound com-
plexes of cobalt(III), ruthenium(III), rhodium(III), plat-
was far more stable; and in Py, its destruction was the inum(IV), and other metals.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 7 2009

1094
TOLDINA et al.
In [17, 18, 22, 23], we studied the trans-influence of
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