Kinetics and mechanism of complex formation of nickel(II) with tetra-N-alkylated cyclam in N,N-dimethylformamide (DMF): Comparative study on the reactivity and solvent exchange of the species Ni(DMF)62+ and Ni(DMF)5Cl

Article abstract of DOI:10.1021/ic991139h

¹³C NMR was used to study the rate of DMF exchange in the nickel(II) cation Ni(DMF)₆²⁺ and in the monochloro species Ni(DMF)₅Cl⁺ with ¹³C-labeled DMF in the temperature range of 193-395 K i

Full text of DOI:10.1021/ic991139h

Inorg. Chem. 2000, 39, 1721-1727
1721
Kinetics and Mechanism of Complex Formation of Nickel(II) with Tetra-N-alkylated
Cyclam in N,N-Dimethylformamide (DMF): Comparative Study on the Reactivity and
Solvent Exchange of the Species Ni(DMF)6 and Ni(DMF)5Cl+
2+
Horst Elias,* Ru1diger Schumacher, Jo1rg Schwamberger, and Thorsten Wittekopf
Institut fu¨r Anorganische Chemie, Technische Universita¨t Darmstadt, Petersenstrasse 18,
D-64287 Darmstadt, Federal Republic of Germany
Lothar Helm, Andre´ E. Merbach,* and Stefan Ulrich
Institut de Chimie Minerale et Analytique, Universite´ de Lausanne, BCH,
CH-1015 Lausanne, Switzerland
ReceiVed September 23, 1999
13C NMR was used to study the rate of DMF exchange in the nickel(II) cation Ni(DMF)62+ and in the monochloro
species Ni(DMF)5Cl+ with 13C-labeled DMF in the temperature range of 193-395 K in DMF (DMF ) N,N-
dimethylformamide). The kinetic parameters for solvent exchange are kex ) (3.7 ( 0.4) × 103 s-1, ∆Hq ) 59.3
( 5 kJ mol-1, and ∆Sq ) +22.3 ( 14 J mol-1 K-1 for Ni(DMF)62+ and kex ) (5.3 ( 1) × 105 s-1, ∆Hq ) 42.4
( 4 kJ mol-1, and ∆Sq ) +6.7 ( 15 J mol-1 K-1 for Ni(DMF)5Cl+. Multiwavelength stopped-flow
2+
spectrophotometry was used to study the kinetics of complex formation of the cation Ni(DMF)6 and of the
100-fold more labile cation Ni(DMF)5Cl+ with TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and
TEC (1,4,8,11-tetraethyl-1,4,8,11-tetraazacyclotetradecane) in DMF at 298 K and I ) 0.6 M (tetra-n-
butylammoniumperchlorate). Equilibrium constants K for the addition of the nucleophiles DMF, Cl-, and Br- to
the complexes Ni(TMC)2+ and Ni(TEC)2+ were determined by spectrophotometric titration. Formation of the
complexes Ni(TMC)2+ and Ni(TEC)2+ was found to occur in two stages. In the initial stage, fast, second-order
nickel incorporation with rate constants k1(TMC) ) 99 ( 5 M-1 s-1 and k1(TEC) ) 235 ( 12 M-1 s-1 leads to
2+
the intermediates Ni(TMC)int and Ni(TEC)int2+, which have N4-coordinated nickel. In the second stage, these
intermediates rearrange slowly to form the stereochemically most stable configuration. First-order rate constants
2+
2+
for the one-step rearrangement of Ni(TMC)int and the two-step rearrangment of Ni(TEC)int are presented.
Because of the rapid formation of Ni(DMF)5Cl+, the reactions of Ni(DMF)62+ with TMC and TEC are accelerated
upon the addition of tetra-n-butylammoniumchloride (TBACl) and lead to the complexes Ni(TMC)Cl+ and Ni-
(TEC)Cl+, respectively. For initial concentrations such that [TBACl]o/[nickel]o g 20, intermediate formation is
230 times (TMC) and 47 times (TEC) faster than in the absence of chloride. The mechanism of complex formation
is discussed.
Introduction
The reactivity of a metal cation correlates with the lability
of the coordinated solvent S, as characterized by the rate constant
kex according to eq 2.
As reviewed recently,11 the formation of complexes of tetraaza
cyclic ligands L such as cyclam with divalent transition metal
cations M2+ such as Ni2+ and Cu2+ in dipolar aprotic solvents
is a two-stage process.
kex
\
MSnq+ + nS* y
z MSn*q+ + nS
(2)
It is well-known that, for a given solvated metal ion, the size
of kex is affected by the solvent S and by changes in the solvation
shell of Mq+, as achieved by partial substitution of the
coordinated solvent. As an example, water exchange in Ni-
2+
(H2O)5(NH3)2+, Ni(H2O)3(NH3)32+, and Ni(H2O)(NH3)5 is
The reaction sequence presented in eq 1 shows schematically
that there is an initial fast stage of metal incorporation, leading
to an N4-coordinate intermediate MLint2+, which is followed by
a consecutive slow phase, in which stereochemical rearrange-
ment takes place to form the thermodynamically most stable
stereoisomer ML2+. Depending on the nature of the macrocyclic
ligand L, both the fast and the slow stages can comprise several
steps.1,2
faster by factors of about 8,3a 78,3a and 134,3b respectively,
2+
compared to the exchange in the hexaaqua ion Ni(H2O)6
.
The present contribution considers two kinetic aspects of the
system Ni2+/Cl-/DMF and, to a lesser extent, of the system
(2) Sanzenbacher, R.; Elias, H. Inorg. Chim. Acta 1996, 246, 267.
(3) (a) Burgess, J. Metal Ions in Solution; Ellis Horwood: Chichester,
U.K., 1978; p 335. (b) Wilkins, R. Kinetics and Mechanism of
Reactions of Transition Metal Complexes; VCH Verlagsgesellschaft:
Weinheim, Germany, 1991; p 214.
(1) Elias, H. Coord. Chem. ReV. 1999, 187, 37.
10.1021/ic991139h CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/28/2000

1722 Inorganic Chemistry, Vol. 39, No. 8, 2000
Elias et al.
Ni2+/Br-/DMF (DMF ) N,N-dimethylformamide). On one
hand, the reaction of nickel ions with the macrocyclic ligands
L ) TMC (tetra-N-methylated cyclam) and L ) TEC (tetra-
N-ethylated cyclam) in DMF according to eq 3 is investigated
by stopped-flow spectrophotometry in the absence and in the
presence of chloride and bromide ions, respectively, to obtain
information about the kinetics and mechanism of macrocyclic
complex formation with the mono halo species Ni(DMF)5Cl+
The concentration of the solutions used in kinetic work is given in
molarity M (mol L-1).
Instrumentation. UV/vis spectra were recorded on a diode array
spectrophotometer (Zeiss, type Specord S10), and UV/vis/NIR spectra
were recorded on a double-beam spectrophotometer (Perkin-Elmer, type
Lambda 900). Complex formation kinetics measurements were per-
formed on a diode array spectrophotometer with a two-chamber quartz
cell (t1/2 > 10 min) and on a rapid-scan stopped-flow spectrophotomter7
(t1/2 < 1 min). 13C NMR spectra were recorded on a Bruker spectrometer
(type Avance 400).
and Ni(DMF)5Br+, as compared to the fully solvated cation
2+
Ni(DMF)6
.
Spectrophotometric Titration. Equilibrium constants K for addition
reactions according to eq 5 with X ) DMF, Cl-, or Br- were
determined by spectrophotometric titration.
Ni(DMF)62+ + L f NiL(DMF)x2+ + (6 - x)DMF (3)
K
NiL2+ + X y
\
z NiLX2+
(5)
The absorbance/[X] data were computer-fit to eq 6 to obtain K.
A ) (Ao + A∞K[X])/(1 + K[X])
(6)
The symbols Ao and A∞ refer to the absorbance of the species NiL2+
and NiLX2+, respectively, at [NiL2+]o.
Kinetic Investigation of Complex Formation. Complex formation
according to eq 3 was studied spectrophotometrically in DMF at 298
K and I ) 0.6 M (TBAClO4). The experiments were carried out either
under 1:1 conditions ([metal]o ) [ligand]o) or under pseudo-first-order
conditions ([metal]o . [ligand]o or vice versa). Rate constants were
obtained by multiwavelength analysis in the range l ) 320-620 nm.
The absorbance/time data, as obtained for the fast stage of complex
formation, were computer-fit to either eq 7 (1:1 conditions), eq 7a (mild
excess of nickel or ligand), or eq 8 (pseudo-first-order conditions) to
obtain the corresponding rate constant k (second-order) or kobsd (pseudo-
first-order).
This reaction is investigated by stopped-flow spectrophotometry
in the absence and in the presence of chloride and bromide ions,
respectively, to obtain information about the kinetics and
mechanism of macrocyclic complex formation with the mono
halo species Ni(DMF)5Cl+ and Ni(DMF)5Br+, as compared to
the fully solvated cation Ni(DMF)62+. On the other hand, the
rate of solvent exchange in the mono halo species Ni(DMF)5Cl+
and Ni(DMF)5Br+ according to eq 4 is studied by the variable-
temperature 13C NMR technique and compared to the hexa
2+
solvated cation Ni(DMF)6
.
A ) {(Ao - A∞)/(1 + k[metal]ot)} + A∞
A ) {(Ao - A∞)(z - 1) exp(-ât)}/{z - exp(-ât)} + A∞ (7a)
A ) (Ao - A∞) exp(-kobsdt) + A∞ (8)
(7)
kex
Ni(DMF)5Cl+ + 5DMF* y
\
z Ni(DMF*)5Cl+ + 5DMF (4)
Experimental Section
Equation 7a was used to fit the absorbance/time data obtained under
moderate excess conditions [z ) [nickel]o/[ligand]o or [ligand]o/[nickel]o,
respectively, with z in the range 1-10 and â ) k[excess partner]o(1 -
z-1) ) kobsd(1 - z-1)]. Under certain conditions (see Results), the time
dependence of the absorbance was biphasic or triphasic so that the data
had to be fit to the sum of two (m ) 2) or three (m ) 3) exponentials
according to eq 9 (Ao and A∞ refer to t ) 0 and t ) ∞, respectively).
Chemicals. TBAClO4 (tetra-n-butylammoniumperchlorate), TBABr
(tetra-n-butylammoniumbromide), TBACl‚H2O (tetra-n-butylammoni-
umchloride monohydrate), 4iBuM (4-isobutylmorpholin), and DMF
(N,N-dimethylformamide) were commercially available in reagent grade
quality and were used without further purification. [13C2]DMF (N,N-
dimethyl[13C2]formamide; 99%) was obtained from Isotec. Ni(DMF)6-
(ClO4)2 was prepared on the basis of the procedure described by Fee
et al.4
m
Warning! Perchlorate salts and organic solutions of such salts are
potentially explosiVe. They should be handled in small quantities and
with caution.
A ) [ Ai exp(-kobsd,it)] + A∞
(9)
∑
i)1
Ligands. TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetra-
decane)5a and TEC‚4HBr (1,4,8,11-tetraethyl-1,4,8,11-tetraazacyclotet-
radecane tetrahydrobromide)6 were prepared as described in the litera-
ture. Treatment of the hydrobromide with NaOH, extraction with CHCl3,
and distillation in vacuo led to the free ligand TEC (slightly yellow
oil).
Complexes. Trans I-Ni(TMC)(ClO4)2, obtained by the reaction of
an aqueous solution of nickel perchlorate with a solution of TMC in
methanol, was recrystallized in methanol/water. Trans III-Ni(TMC)-
(ClO4)2 was prepared as described in the literature.5b The preparation
of Ni(TEC)(ClO4)2 according to the literature procedure6 led to the violet
form of this complex as the major product.
NMR Investigation of Solvent Exchange. Solutions for 13C NMR
spectroscopy were prepared using [13C2]DMF diluted with normal DMF
to 27% 13C enrichment. A stock solution was prepared by adding 214.50
mg (0.308 mmol) of Ni(DMF)6(ClO4)2 to 3.014 g of DMF, resulting
in a solution 0.102 m in Ni(DMF)6(ClO4)2 (Sol. 1). Two solutions
containing chloride anions were prepared as follows: (i) 27.17 mg
(0.098 mmol) of TBACl was added to 1.087 g of the stock solution,
giving a solution 0.10 m in TBACl (Sol. 2), and (ii) 83.52 mg (0.03
mmol) of TBACl was added to 1.074 g of stock solution, giving a
solution 0.30 m in TBACl (Sol. 3). One solution containing bromide
anion was prepared by adding 32.08 mg (0.01 mmol) of TBABr to
0.969 g of stock solution, giving a solution 0.11 m in TBABr (Sol. 4).
Benzene (1%) was added to all of the solutions as a chemical shift
standard.
Solutions. The solutions for NMR work were prepared by weighing,
and their concentration is therefore given in molality m (mol kg-1).
The kinetics of DMF exchange on Ni(DMF)6 and Ni(DMF)5Cl+
2+
(4) Fee, W. W.; McElholum, D. E.; McPherson, A. J.; Rundle, D. L. Aust.
J. Chem. 1973, 26, 1207.
(5) (a) Barefield, E. K.; Wagner, F. Inorg. Chem. 1973, 12, 2435. (b)
Barefield, E. K.; Wagner, F. Inorg. Chem. 1976, 15, 408.
(6) Oberholzer, M. R.; Neuburger, M.; Zehnder, M.; Kadeb, T. A. HelV.
Chim. Acta 1995, 78, 505.
were followed by 13C NMR as a function of temperature (13C resonance
frequency ) 100.61 MHz). The acquisition parameters for the variable-
(7) Wannowius, K. J.; Sattler, F.; Elias, H. GIT Fachz. Lab. 1985, 11,
1138.

Complex Formation of Nickel(II)
Inorganic Chemistry, Vol. 39, No. 8, 2000 1723
temperature measurements were as follows: pulse length of 8 ms,
quadrature detection mode with 64K data points resulting from 2500
scans accumulated over a spectral width of 150 kHz. No 1H decoupling
was applied. Chemical shifts were measured with respect to the high-
field part of the apparent benzene doublet, fixed to +127.205 ppm.
The temperature was controlled within (0.2 K using a Bruker B-VT
3000 digital instrument and was measured before and after spectral
accumulation by substituting the sample with a calibrated platinum
resistance fit into an NMR tube.8 Standard 5-mm NMR tubes were
used.
Table 1. Kinetic Parameters Obtained from the
Variable-Temperature 13C NMR Data for Solutions of the Cations
Ni(DMF)6 and Ni(DMF)5Cl+ in DMF
2+
Ni(DMF)6
Ni(DMF)5Cl+
this worke
2+
1H/17O NMR
literature results
this workd
kex298, s-1
3.8 × 103 a
6.9 × 103 b
7.7 × 103 c
62.9 a
(3.7 ( 0.4) × 103 (5.3 ( 1) × 105
∆Hq, kJ mol-1
54.4 b
59.3 ( 5
42.4 ( 4
39.3 c
Results
∆Sq, J mol-1 K-1 +33.5 a
Formation of Nickel Halo Complexes in the Systems Ni2+
/
+25.0b
-37.7 c
+22.3 ( 14
+6.7 ( 15
Cl-/DMF and Ni2+/Br-/DMF. The calorimetric and spectro-
photometric study of Ishigura et al.9a on the formation of chloro
complexes of nickel(II) in the system Ni(DMF)6(ClO4)2/Et4NCl/
DMF showed that stepwise coordination of one, two, three, and
finally four Cl- ions to the nickel takes place, the overall
formation constants being log â1 ) 2.85 (â1 ) 708 M-1), log
â2 ) 3.76, log â3 ) 5.53, and log â4 ) 7.40 (298 K; I ) 0.4
M Et4NClO4). Pilarczyk and Klinszporn9b reported â1 ) 1380
M-1 for the formation of the octahedral monochloro species
Ni(DMF)5Cl+ according to eq 10 at 298 K.
a Ref 12. b Ref 13. c Ref 14. d Solution that is 0.102 m in Ni(DMF)6-
(ClO4)2. e Mean value of two solutions that are 0.026 m (0.004 m) in
Ni(DMF)62+, 0.076 m (0.098 m) in Ni(DMF)5Cl+, and 0.024 m (0.202
m) in Cl- in DMF.
Ni(DMF)62+ + Cl- h Ni(DMF)5Cl+ + DMF
[Ni(DMF)5Cl+]
[Ni(DMF)52+][Cl-]
â1 )
(10)
One calculates that, for â1 ) 708 M-1, [nickel]:[chloride] )
1:2 and [nickel]tot ) 1 × 10-3 M, 53% of the nickel is present
in the form of the cation Ni(DMF)5Cl+, approximately 46% in
the form of the cation Ni(DMF)62+, and 1% in the form of the
species Ni(DMF)4Cl2. One should note that the formation of
the species Ni(DMF)5Cl+ according to eq 10 is a fast process,
as controlled by the rate of solvent exchange in the cation Ni-
2+
(DMF)6
.
The equilibrium constant â1 for the formation of the mono-
bromo species Ni(DMF)5Br+ in the system Ni2+/Br-/DMF is
reported to be 631 M-1 at 298 K.9b Spectrophotometric titration
Figure 1. Part of 13C NMR spectra of solutions that are 0.102 m Ni-
(DMF)6(ClO4)2 in DMF with added (a) chloride (0.100 m) and (b)
bromide (0.110 m) recorded at 215 K. The equilibrium concentrations
obtained by integration are provided on the left. Solid bars represent
the 13C shifts of the trans- and cis-methyl carbons of the corresponding
species.
2+
of Ni(DMF)6 with TBABr in DMF led to the considerably
smaller value of â1 ) 133 ( 10 M-1 10
.
13C NMR Investigation of Solvent Exchange at Variable
Temperature. The 13C NMR spectra of the cation Ni(DMF)6
2+
(Sol. 1) were recorded between 193 and 395 K. At lower
temperatures (<294 K), a doublet was observed at 161.9/162.9
ppm that was assigned to the formyl carbon of bulk DMF. Two
quartets centered at 35.0 and 29.4 ppm were assigned to the
cis- and trans-methyl groups of bulk DMF, respectively. In the
temperature range 193-294 K, signals of the methyl carbons
of coordinated DMF were visible (at 116.2 and 77.7 ppm at
193 K). These signals do not show a multiplet structure because
of the strong relaxation enhancement of the methyl protons
caused by the paramagnetic Ni2+ cation. The transverse
relaxation enhancement and chemical shift of bulk DMF, both
as a function of temperature, were analyzed simultaneously using
the standard Swift and Connick formalism.11 The results as
presented in Table 1 agree very well with those obtained
1
previously by H NMR.1212-14
In addition to the resonances attributable to bulk DMF and
to Ni(DMF)62+, the 13C NMR spectra of the cation Ni-
(DMF)5Cl+ (Sols. 2 and 3) show two additional signals at 108.1
and 87.9 ppm. The coordinated DMF in the species Ni-
(DMF)5Cl+ can be classified as one axial and four equatorial
DMF molecules. Keeping in mind that the cis- and trans-methyl
carbons have different chemical shifts, one should observe two
resonance pairs with a ratio of 4:1. Furthermore, a certain
2+
amount of Ni(DMF)6 is always present in Sols. 2 and 3,
leading to two 13CH3 resonances as described above. Figure 1a
shows the spectral region of the coordinated methyl in Sol. 2
at 215 K. Only four resonance lines are found instead of six, as
theoretically predicted. It follows from integration that we can
attribute the signal at 117.3 ppm to the superposition of the
(8) Ammann, C.; Meier, P.; Merbach, A. E. J. Magn. Reson. 1982, 46,
319.
(9) (a) Ishiguro, S.; Ozutsumi, K.; Ohtaki, H. Bull. Chem. Soc. Jpn. 1987,
11, 531. (b) Pilarczyk, M.; Klinszporn, L. Bull. Pol. Acad. Sci., Chem.
1986, 34, 53.
(10) Wittekopf, T. Dr.-Ing. Dissertation, Technische Universita¨t Darmstadt,
D17, Darmstadt, Federal Republic of Germany, submitted in 1999.
(11) Swift, T. J.; Connick, R. E. J. Chem. Phys. 1962, 37, 307.
(12) Matwiyoff, N. A. Inorg. Chem. 1966, 5, 788.
(13) Frankel, L. S. Inorg. Chem. 1971, 10, 2360.
(14) Babiec, J. S.; Langford, C. H.; Stengle, T. R. Inorg. Chem. 1966, 5,
1362.

1724 Inorganic Chemistry, Vol. 39, No. 8, 2000
Elias et al.
enhancements and chemical shifts of bulk DMF resonances
using a simultaneous nonlinear least-squares fit with the simple
Swift and Connick approach.11 One can further expect that the
exchange rate of Ni(DMF)4Cl2 is much faster than that of Ni-
(DMF)5Cl+. Therefore, even small amounts of Ni(DMF)4Cl2
could contribute to the line broadening of the bulk DMF signals.
The contribution of this process can be excluded, however,
because of the fact that the Swift and Connick data obtained in
a large temperature range are consistent with those observed at
low temperatures on the bound DMF signals (Figure 2). The
results are summarized in Table 1 (the error limits correspond
to one standard deviation).
Because the concentration of the species Ni(DMF)5Br+ in
the temperature range covered is <10-3 m, DMF exchange on
this complex could not be measured. The transverse relaxation
enhancements and chemical shifts of the bulk DMF resonances
of Sol. 4 can therefore be fit in fair approximation by parameters
2+
Figure 2. Temperature dependence of the transverse relaxation rates
of methyl groups: (b, O) trans-CH3 of equatorial DMF in Ni-
(DMF)5Cl+ and (9, 0) cis-CH3 of axial DMF in Ni(DMF)5Cl+. Solid
(open) symbols correspond to Sol. 2 (Sol. 3).
obtained for solutions of the cation Ni(DMF)6
.
State of Coordination of the Complexes NiL2+ (L ) TMC,
TEC) in DMF and in the Presence of Halide Ions. Nickel-
(II) complexes of cyclam and derivatives of cyclam with a more
or less square-planar N4 coordination geometry tend to increase
their coordination number from four to five or six. Titration of
the complexes trans I-Ni(TMC)2+ and Ni(TEC)2+ with DMF
in nitromethane leads to the equilibrium constants K ) 3.9 and
0.23 m-1, respectively, for the addition of one molecule of DMF
according to eq 5 (see Table 2). The addition of a second mole-
cule of DMF (that is, the formation of octahedrally coordinated
complexes) is not observed. A comparison of the UV/vis spectra
of the complexes in nitromethane and in DMF confirms that,
in DMF, the five-coordinate species prevails [98% trans I-Ni-
(TMC)(DMF)2+ vs 2% trans I-Ni(TMC)2+ and 75% Ni(TEC)-
(DMF)2+ vs 25% Ni(TEC)2+].
2+
cis-CH3 of DMF in Ni(DMF)6 and the cis-CH3 of the
equatorial DMF in Ni(DMF)5Cl+. Likewise, we attribute the
peak at 77.6 ppm to the trans-CH3 of DMF in Ni(DMF)62+ and
the trans-CH3 of the axial DMF in Ni(DMF)5Cl+. The reso-
nances at 108.1 and 87.9 ppm are due to the cis-CH3 of axial
DMF in Ni(DMF)5Cl+ and the trans-CH3 of equatorial DMF
in Ni(DMF)5Cl+, respectively. Sol. 4, containing bromide ions,
shows primarily two 13CH3 resonances that are due to Ni-
2+
(DMF)6 (Figure 1b). A small peak at 80.6 ppm can be
attributed to the equatorial DMF of trans-Ni(DMF)5Br+.
Raising the temperature results in a broadening of the 13CH3
resonances and a shift toward bulk DMF signals because of
the chemical exchange of DMF molecules. At temperatures
above 294 K, the signals of the coordinated DMF disappear in
the baseline because of the large broadening caused by
exchange.
Titration of the complexes with halide ions in DMF shows
that there is a strong tendency to form the five-coordinate halo
species NiLX+ (Table 2). Chloride ions are bound more firmly
than bromide ions, and the five-coordinate halo TMC complexes
are several orders of magnitude more stable than the corre-
sponding TEC complexes. The analogous behavior of the
complexes trans I-Ni(TMC)(DMF)2+ and Ni(TEC)(DMF)2+
upon addition of nucleophiles suggests that the cation Ni(TEC)2+
has the trans-I configuration in DMF.
Kinetics of Nickel Complex Formation with TMC. As part
of a systematic study,15 we reported earlier that the reaction of
the cation Ni(DMF)62+ with TMC in DMF is biphasic according
to eq 11, with an initial second-order incorporation step (rate
constant k1) followed by a first-order isomerization step (rate
constant k2).
Equilibrium constant â1 for the formation of the chloro
complex according to eq 10 was obtained from the integration
of the 13CH3 resonances of the spectra recorded at 193-218 K.
Because of the uncertainty in the integration and the small
temperature range covered, a significant temperature dependence
of â1 could not be found. The best nonlinear least-squares fit
of all of the data resulted in a value of â1 ) 116 ( 22 m-1
,
independent of temperature. The concentrations of the species
Ni(DMF)62+, Ni(DMF)5Cl+, and Cl- in Sol. 2 (Sol. 3) are 0.026
m (0.004 m), 0.076 m (0.098 m), and 0.024 m (0.202 m),
respectively. The concentration of the cation Ni(DMF)5Br+ is
estimated to be <10-3 m.
-1
-1
k2 (s
k1 (M-1 s
2+
Figure 2 shows the transverse relaxation rates of the unique
resonances attributed to the cis-CH3 of axial DMF and to the
trans-CH3 of equatorial DMF in the chloro complex. At
temperatures above 217 K (1000/T ≈ 4.6 K-1), these relaxation
rates are directly related to the lifetimes of DMF molecules in
the equatorial and the axial positions, respectively. The observed
lifetimes are the same within experimental error, and we
conclude that the exchange rate constants of equatorial and axial
DMF in Ni(DMF)5Cl+ are the same. We can therefore treat
coordinated DMF in Ni(DMF)5Cl+ as one single exchanging
species. Furthermore, the concentration of Ni(DMF)5Cl+ is
Ni2+ + TMC
)8 Ni(TMC)int
)8 Ni(TMC)2+
fast
slow
(11)
In this earlier investigation, the kinetic data were based on
absorbance/time measurements recorded at 20 wavelengths
(494-509 nm) lying within one of the absorption bands (λmax
) 512 nm) of the product trans I-Ni(TMC)(DMF)2+
.
The kinetic analysis of the absorbance/time data of the present
study is based on multiwavelength analysis in the wide range
of 320-620 nm. Application of this more extensive analytical
procedure to the renewed study of reaction 11 confirmed the
final first-order isomerization step (rate constant k2) but surpris-
2+
much larger than that of Ni(DMF)6 (at least for Sol. 3), and
the exchange of DMF on Ni(DMF)5Cl+ is about 500 times faster
(at 250 K) than that of DMF on the hexacoordinated cation
Ni(DMF)62+. We therefore analyzed the transverse relaxation
(15) (a) Ro¨per, R.; Elias, H. Inorg. Chem. 1992, 31, 1202. (b) Ro¨per, R.;
Elias, H. Inorg. Chem. 1992, 31, 1210.

Complex Formation of Nickel(II)
Inorganic Chemistry, Vol. 39, No. 8, 2000 1725
Table 2. Equilibrium Constants K (M-1) for the Addition of Nucleophiles X to Complexes NiL2+ According to Equation 5 at 298 K
complex
X ) DMFa
X ) Cl- b
X ) Br- b
trans III-Ni(TMC)2+
0.47 ( 0.06
3.9 ( 0.4
0.23 ( 0.02
(8.4 ( 0.8) × 103
(3.5 ( 0.4) × 103
(2.5 ( 0.3) × 103
(3.1 ( 0.3) × 105
(4.9 ( 0.4) × 103
trans I-Ni(TMC)2+
>106
Ni(TEC)2+ (violet form)
a Solvent nitromethane. b Solvent DMF.
Table 3. Rate Constants for the Reaction of Nickel(II) with the Ligands L ) TMC and TEC in DMF at 298 K and I ) 0.6 M (TBAClO4)
According to Equation 11 in the Absence and in the Presence of Chloride Ions
system
[Ni(DMF)6(ClO4)2]o, M
[L]o, M
[TBACl], M
[4iBuM]o, M
k1a, M-1 s-1
240 ( 19
k1 () k1b), M-1 s-1
k2 × 104, s-1
L ) TMC
1
2
3
0.01-0.1
0.001
0.001
0.001
0.001
0.001
99 ( 5
15 ( 1a
5 ( 0.8
11 ( 1d
0.02
99 ( 9b
0.00001-0.1
(23 ( 3) × 103 c
L ) TEC
4
5
6
7
0.001
0.001
230 ( 10
410 ( 21 (k2a)e
64 ( 5 (k2b)e
420 ( 24 (k2a)e
64 ( 6 (k2b)e
480 ( 30 (k2a)e
63 ( 5 (k2b)e
405 ( 20 (k2a)e
63 ( 5 (k2b)e
0.005-0.05
0.001
0.001
237 ( 12
0.005-0.05
0.001
237 ( 13
0.001
0.0001-0.05
(11 ( 1) × 103 c
a Mean of k2 as obtained at five different concentrations of nickel perchlorate. b Mean of k2 as obtained at five different concentrations of the
base 4iBuM. c As obtained from the saturation value of kobsd,1 (see Figure 3 and Discussion). d Mean of k2 as obtained at 11 different concentrations
of TBACl. e The slow isomerization stage of the reaction consists of two consecutive steps, as characterized by first-order rate constants k2a and k2b;
the data listed are the mean of k2a and k2b as obtained at four different concentrations of the excess partner nickel and ligand, respectively.
ingly showed two parallel second-order processes (k1a ) 240
( 19 M-1 s-1 and k1b ) 99 ( 5 M-1 s-1 at 298 K; see system
1 in Table 3 and Table S11) for the initial stage of the reaction.
It followed from some additional experiments16 that the faster
second-order reaction (k1a) is obviously caused by residual water
in DMF. In the presence of the nearly noncoordinating base
4-isobutylmorpholin (4iBuM), the k1a path disappeared, and k1b
) k1 ) 99 ( 8.5 M-1 s-1 remained as a second-order rate
constant describing the initial reaction of nickel with TMC (see
system 2 in Table 3 and Table S12). The various rate constants
and reaction conditions are summarized in Table 3.
In the presence of chloride ions, complex formation according
to eq 11 is substantially accelerated (see system 3 in Table 3
Figure 3. Dependence of the first-order rate constants kobsd,1 (s-1) on
and Table S13). As shown in Figure 3 for the condition [TMC]o
the concentration of TBACl for the reactions of nickel(II) perchlorate
) [nickel]o, the experimental rate constant kobsd,1, obtained by
fitting the absorbance/time data to eq 7a with z ) [TMC]o/
[Ni(DMF)5Cl+]o,17 increases approximately linearly18 with
[TBACl]o to arrive at a plateau for [TBACl]o/[Ni(DMF)5Cl+]o
g 20 at which kobsd,1 ) 23 ( 3 s-1, which corresponds to k1 )
23 × 103 M-1 s-1 (see Table 3). This means that, at an
approximately 20-fold or higher excess of chloride over nickel,
the rate of formation of the intermediate nickel complex with
TMC is a factor of approximately 230 faster than the rate in
the absence of chloride.
(10-3 M) with (9) TMC (10-3 M) and (.) TEC (10-3 M) in DMF
according to reaction 11 at 298 K (the value for kobsd,1 at [TBACl] )
7 × 10-3 M was neglected; see Discussion).
In the presence of chloride, the spectrum of the intermediate
is clearly different from the spectrum of the intermediate Ni-
(TMC)int+ obtained in the absence of chloride, and the spectrum
of the product corresponds to the spectrum of Ni(TMC)Cl+, as
obtained from titration studies (see Table S17). These findings
are in line with the interpretation that the species Ni(DMF)5Cl+
2+
reacts with TMC to form the intermediate Ni(TMC)Clint
which isomerizes to form the complex Ni(TMC)Cl2+
It should be mentioned that chloride ions, admixed after the
formation of the intermediate Ni(TMC)int
2+ 19 led to rate constant
,
.
(16) As controlled by Karl Fischer titration, the commercial reagent grade
DMF had a mean water content corresponding to [H2O] ) 1.5 × 10-2
M. In dried DMF (CaH2) the k1a path disappeared, as it also did in
commercial DMF after the addition of the base 4iBuM as a sponge
for the water at a concentration of [4iBuM] g 10-2 M. We think that
the residual water interacts with the base TMC, but we do not have a
convincing interpretation at hand that explains the mechanistic details
,
of k2 ) (11 ( 1) × 10-4 s-1, which is in fair agreement with
the data obtained for k2 with chloride present from the beginning.
The final absorption spectrum obtained in this experiment with
delayed admixing of chloride corresponded to the species Ni-
(TMC)Cl+.
2+
of the k1a path. For the kinetic comparison of the species Ni(DMF)6
and Ni(DMF)5Cl+ reacting with TMC, the base 4iBuM was added to
suppress the water-initiated pathway.
(17) The calculation of z ) [TMC]o/[Ni(DMF)5Cl+] ) 1.9 was based on
â1 ) 708 m-1 and â2 ) 5750 m-1 (see ref 9).
(19) This experiment was performed with a four-syringe stopped-flow unit
(Biologic, type SFM-4). After generation of the intermediate Ni-
(18) The initial increase of kobsd,1 with [TBACl]o is approximately linear.
Computer fitting of the data obtained for kobsd,1 in the range [TBACl]o
) 1 × 10-4 M to [TBACl]o ) 7 × 10-3 M leads to a slope of m )
1.0 ( 0.3.
2+
(TMC)int within approximately 90 s, the third syringe was used to
conduct a stopped-flow experiment with the intermediate and a solution
of chloride ions in DMF.

1726 Inorganic Chemistry, Vol. 39, No. 8, 2000
Elias et al.
Compared to chloride ions, bromide ions enhance the rate of
complex formation between nickel and TMC much less ef-
fectively. Under comparable conditions, rate enhancement by
bromide is a factor of approximately 10 smaller than that for
chloride. The kinetic effect of bromide appears to be more
complex20 and was therefore not studied in detail.
The Eigen-Wilkins mechanism implies that second-order rate
constants k, describing complex formation reactions with
monodentate ligands, are composite parameters according to k
) kiKos. Fast initial formation of an outer-sphere complex
(equilibrium constant Kos) is followed by rate-controlling
interchange (first-order rate constant ki). In the interchange step,
a coordinated solvent molecule is replaced by the incoming
ligand. It is well documented3 that solvent exchange on the
solvated nickel(II) cation follows the Id mechanism. If so, the
Kinetics of Nickel Complex Formation with TEC. To our
knowledge, complex formation of nickel with TEC has not been
studied so far. The results obtained confirm that formation of
the complex Ni(TEC)2+ follows the reaction pattern shown in
eq 11 for TMC, in that fast, second-order nickel incorporation
is followed by slow, first-order isomerization of the initially
2+
rate of complex formation of the cation Ni(DMF)6 with
monodentate ligands should correlate with the rate of solvent
exchange on this cation, as characterized by kex ) 3.7 × 103
s-1 (see Table 1). This means that the size of the experimentally
obtained second-order rate constant k for complex formation
reactions with monodentate ligands is expected to lie in the range
formed intermediate Ni(TEC)int2+. In contrast to the intermediate
2+
Ni(TMC)int2+, the species Ni(TEC)int
isomerizes in two
consecutive steps (see first-order rate constants k2a and k2b in
Table 3).
from 0.6 × 103 s-1 (Kos ) 0.15 M-1 22
labile cation Ni(DMF)5Cl+, the range expected for k is from
0.8 × 105 s-1 (Kos ) 0.15 M-1) to 5 × 105 s-1 (Kos ) 1.0
M-1).
)
to 4 × 103 s-1 (Kos
)
1.0 M-1 22
at 298 K, according to k ) kexKos. For the more
)
Complex formation with TEC was studied under 1:1 condi-
tions {[nickel]o ) [TEC]o; see system 4 in Table 3}, as well as
with an excess of either nickel (see system 5 in Table 3 and
Table S14) or TEC (see system 6 in Table 3 and Table S15).
The data obtained for k1, k2a, and k2b under these various
conditions agree very satisfyingly (see Table 3). It follows from
the comparison of k1(TMC) with k1(TEC) that intermediate
formation with TEC is approximately 2.4 times faster than with
TMC. Both of the consecutive steps describing the first-order
Kinetics of Complex Formation: The Comparison of
TMC with TEC and the Kinetic Effect of Chloride Ions.
The results obtained for the tetra-N-ethylated cyclam derivative
TEC confirm the general experience1 that (i) complex formation
with N4 macrocycles of the cyclam type takes place in two stages
according to eq 1, and (ii) both stages can comprise several
steps. First, TEC incorporates the nickel(II) cation 2.4 times
faster than TMC (see data for k1 in Table 3), which indicates a
somewhat greater flexibility of TEC. Second, in contrast to
complex formation with TMC, the slow, first-order isomerization
isomerization of the intermediate formed with TEC are faster
2+
than the one isomerization step observed for Ni(TMC)int
.
In the presence of chloride ions, complex formation with TEC
according to eq 11 is significantly accelerated (see system 7 in
Table 3 and Table S16). As shown in Figure 3 for the condition
2+
of the intermediate Ni(TEC)int is biphasic. The similarity in
[TEC]o ) [nickel]o, the experimental rate constant kobsd,1
,
kinetic behavior and absorption spectra (see Table S17) suggests
that, as in the case of TMC,1,15a the configuration of the
intermediate Ni(TEC)int2+ is trans-II and that of the product Ni-
(TEC)2+ (violet form) trans-I. The stereochemistry of the species
formed in the first isomerization step (rate constant k2a ) 437
× 10-4 s-1) is not known.
obtained by fitting the absorbance/time data to eq 7a with z )
[TEC]o/[Ni(DMF)5Cl+]o,17 increases approximately linearly21
with [TBACl]o to arrive at a plateau for [TBACl]o/[Ni-
(DMF)5Cl+]o g 20 at which kobsd,1 ) 11 ( 1 s-1, which
corresponds to k1 ) 11 × 103 M-1 s-1 (see Table 3). This means
that, at a 20-fold or higher excess of chloride over nickel, the
rate of formation of the intermediate nickel complex with TEC
is a factor of approximately 47 faster than the rate in the absence
of chloride.
Depending on the estimated size of Kos (see above), second-
order rate constants k1 ) 99 M-1 s-1 and k1 ) 235 M-1 s-1 for
nickel incorporation with TMC and TEC, respectively, are
approximately 1 order of magnitude smaller than expected for
complex formation with monodentate ligands according to the
Eigen-Wilkins mechanism. In line with earlier work,1,15 it is
reasonable to assume for both TMC and TEC that the rate
constant k1 describes the rate-controlling formation of the second
Ni-N bond according to eqs 12-14 with k1 ) KosKkint (M )
Ni2+, S ) DMF, and L ) TMC or TEC). In the species MS5-L
the tetradentate ligand L is singly bonded.
Discussion
Lability of Coordinated DMF in the Cation Ni(DMF)5Cl+.
298
The rate constant at room temperature kex
for solvent
exchange in the cation Ni(DMF)62+, as determined by 13C NMR
in the present study, agrees well with the results obtained earlier
by H NMR12 (see Table 1). Relative to the chloride position,
1
the species Ni(DMF)5Cl+ has four equatorial (cis) DMF
positions and one axial (trans) position. The 13C NMR data
obtained do not support the expectation that there is preferential
labilization of the trans-orientated DMF molecule. The data
suggest instead that the rate of exchange of DMF is the same
in all five of the DMF positions. The results presented in Table
1 for 298 K show that the replacement of one of the six DMF
MS6 + L h {MS6, L}
{MS6, L} h MS5-L + S
MS5-L h MSxLint + (5 - x)S
Kos
(12)
(13)
(14)
K
kint
The monochloro species Ni(DMF)5Cl+ is more than 100-fold
more labile (see Table 1) and hence more reactive than the cation
Ni(DMF)62+. When the macrocyclic ligand L is exposed to a
mixture of Ni(DMF)5Cl+ and Ni(DMF)62+, as generated by
2+
ligands in the octahedral ligand field of the cation Ni(DMF)6
by chloride enhances the rate of DMF exchange by a factor of
143.
(20) The rate constant kobsd,1 for the initial incorporation step increases
linearly with [Cl-]o (see ref 18). In the case of bromide, admixed in
the concentration range 0.0001-0.18 M, the slope of the plot of kobsd,1
vs [Br-]o is approximately 0.5 instead of 1.0.
(21) The initial increase of kobsd,1 with [TBACl]o is approximately linear.
Computer fitting of the data obtained for kobsd,1 in the range [TBACl]o
) 1 × 10-4 M to [TBACl]o ) 2 × 10-2 M leads to a slope of m )
1.1 ( 0.1.
(22) One can estimate, on the basis of theoretically deduced expressions
(see ref 3), that Kos ) 0.15 M-1 for a divalent cation such as Ni2+
interacting with an uncharged species in water. In organic solvents,
Kos can be on the order of approximately 1 M-1 (see ref 23).
(23) Wilkins, R. G. The Study of Kinetics and Mechanism of Reactions of
Transition Metal Complexes; Allyn & Bacon: Boston, MA, 1974; p
183.

Complex Formation of Nickel(II)
Inorganic Chemistry, Vol. 39, No. 8, 2000 1727
chloride ions according to the equilibrium in eq 10, it will
therefore react preferentially with the cation Ni(DMF)5Cl+.
Again, it is found that second-order rate constants k1 ) 23 ×
103 M-1 s-1 (TMC) and k1 ) 11 × 103 M-1 s-1 (TEC) for the
incorporation step in the presence of chloride are approximately
1 order of magnitude smaller than expected for the reaction of
the monochloro species Ni(DMF)5Cl+ with monodentate ligands
according to the Eigen-Wilkins mechanism. From the mecha-
nistic point of view, it is therefore reasonable to assume that,
in the case of Ni(DMF)5Cl+ instead of Ni(DMF)62+, the reaction
sequence in eqs 12-14 applies analogously with MSxLClint
being the intermediate formed in reaction 14.
proximately 10 upon going from z ) +2 (X ) water) to z )
+1 (X ) acetate), 0 (X ) oxalate), and -1 (X ) tricarballylate).
This rate enhancement was explained on electrostatic grounds.
For the series of nickel species NiX2+ ) Ni(H2O)62+, Ni(H2O)5-
NH32+, Ni(H2O)4en2+, and Ni(H2O)3dien2+, the change in
complexation rate with cyclam was found to be quite minimal.
The authors correlated the reactivity of MXz toward monopro-
tonated cylam with the thermodynamic stability of MXz. In
comparison with the dipolar aprotic solvent DMF, though, the
medium water is very different, in that protonation of the
macrocyclic ligand occurs, a fact that complicates the compari-
son of data.
As a matter of fact, the rate constants kobsd,1 plotted in Figure
3 were obtained by fitting the absorbance/time data to eq 7a
with z ) [ligand]o/[Ni(DMF)5Cl+]o. This means that the
variation of [Ni(DMF)5Cl+] with [TBACl]o was taken into
account for the determination of kobsd,1. The approximately linear
increase of kobsd,1 with [TBACl]o (see Figure 3) indicates the
participation of a chloride-dependent equilibrium. As shown in
the reaction sequence in eqs 15-18 (M ) Ni2+, S ) DMF, X
) Cl-, and L ) TMC or TEC), we suggest that the species
MS4X-L with singly bonded L is subject to dissociation
according to eq 17.
Conclusions
The formation of complexes of nickel(II) with tetra-N-
ethylated cyclam (TEC) in DMF follows the general reaction
pattern observed for cyclam and other cyclam derivatives, in
that fast, second-order nickel incorporation leads to an inter-
mediate with full N4 coordination, which rearranges slowly to
form a thermodynamically more stable nickel TEC complex.
The formation of the intermediate with TEC and also its two-
step rearrangement are somewhat faster than in the case of tetra-
N-methylated cyclam (TMC) studied earlier. Mechanistically,
formation of the second Ni-N bond controls the rate of
intermediate formation. The results obtained suggest that the
MS5X + L h {MS5X, L}
{MS5X, L} h MS4X-L + S
MS4X-L + S h MS5-L + X
MS4X-L f MSxXLint + (4 - x)S
Kos
Ka
Kb
kint
(15)
(16)
(17)
(18)
complex formed, Ni(TEC)2+, has the trans-I configuration.
Compared to that of the fully solvated cation Ni(DMF)6
2+
,
solvent exchange of the DMF molecules coordinated in the
monochloro nickel(II) species Ni(DMF)5Cl+ is more than 100-
fold faster at 298 K. The 13C NMR data obtained are in line
with the interpretation that the rate of DMF exchange is the
same for equatorially and axially coordinated DMF in the cation
Ni(DMF)5Cl+.
Second-order rate constant k1 would thus also depend on the
size of Kb according to k1 ) KosKaKbkint. The reaction sequence
in eqs 15-18 postulates that the rate-controlling formation of
the second Ni-N bond occurs with the species MS4X-L, the
concentration of which is affected by chloride ions according
to the equilibrium in eq 17. At adequate excess concentrations
of chloride, this equilibrium is shifted to the left, and the
dependence of kobsd,1 on [TBACl]o arrives at saturation (see
Figure 3).
The sum of the experimental data confirms for both ligands
that both the intermediate formed in the first, fast stage of the
reaction and the final product contain chloride bound to the
nickel.
To our knowledge, there are no reports so far on the kinetic
effect of coordinating anions or other ligands on complex for-
mation reactions of transition metals with macrocyclic N-donor
or S-donor ligands in nonaqueous media. Wu and Kaden studied
the kinetics of complexation reactions of nickel(II)24a and
copper(II)24b with cyclam in the presence of additional ligands
in aqueous solution. They found that the rate of the reaction of
the partially complexed, hydrated metal species NiXz and CuXz
with monoprotonated cyclam increases by a factor of ap-
The reactions of Ni(DMF)5Cl+ with TMC and TEC lead to
the macrocyclic chloro complexes Ni(TMC)Cl+ and Ni(TEC)-
Cl+, respectively. For both TMC and TEC, the rate of
intermediate formation with Ni(DMF)5Cl+ is considerably
higher than that with Ni(DMF)62+. The rate enhancement by
factors of 230 (TMC) and 47 (TEC) is on the order of the
increased lability of coordinated DMF in Ni(DMF)5Cl+ com-
pared to Ni(DMF)62+. Mechanistically, formation of the second
Ni-N bond controls the rate of intermediate formation with
the species Ni(DMF)5Cl+.
Acknowledgment. Sponsorship of this work by the Deutsche
Forschungsgemeinschaft, Swiss National Science Foundation,
and Verband der Chemischen Industrie e.V. is gratefully
acknowledged.
Supporting Information Available: Material concerning solvent
exchange: calculation of equilibrium constant, Tables S1 and S2,
equations used for treatment of 13C NMR relaxation and chemical shift
data with fitting results, Figure S1 and Tables S3-S10. Material
concerning the kinetics of complex formation and spectroscopic data:
Tables S11-S17. This material is available free of charge via the
Internet at http://pubs.acs.org.
(24) (a) Wu, Y.; Kaden, T. A. HelV. Chim. Acta 1984, 67, 1868. (b) Wu,
Y.; Kaden, T. A. HelV. Chim. Acta 1985, 68, 1611.
IC991139H