Kinetics and Mechanism of the Formation and Acid Dissociation of Cobalt(II), Nickel(II), and Copper(II) Complexes with the Highly Enolized β-Diketone 3-(N-Acetylamido)pentane-2,4-dione (=Hamac) in Aqueous Solution

Article abstract of DOI:10.1021/ic950272g

The β-diketone Hamac = 3-(N-acetylamido)pentane-2,4-dione was characterized by potentiometric, spectrophotometric, and kinetic methods. In water, Hamac is very soluble (2.45 M) and strongly enolized, with [enol]/ [ketone] = 2.4 ± 0.1. The pK_a of Hamac is 7.01 ± 0.07, and the rate constants for enolization, k_e, and ketonization, k_k, at 298 K are 0.0172 ± 0.0004 s^-1 and 0.0074 ± 0.0015 s^-1, respectively. An X-ray structure analysis of the copper(II) complex Cu(amac)₂·toluene (=C₂₁H₂₈CuN₂C₆; monoclinic, C2/c; a = 20.434(6), b = 11.674(4), c = 19.278(6) A? β = 100.75(1)° Z = 8; R_w = 0.0596) was carried out. The bidentate anions amac^- coordinate the copper via the two diketo oxygen atoms to form a slightly distorted planar CuO₄ coordination core. Rapid-scan stopped-flow spectrophotometry was used to study the kinetics of the reaction of divalent metal ions M²⁺ (M = Ni,Co,Cu) with Hamac in buffered aqueous solution at variable pH and 1 = 0.5 M (NaClO₄) under pseudo-first-order conditions ([M²⁺]₀ ? [Hamac]₀) to form the mono complex M(amac)⁺. For all three metals the reaction is biphasic. The absorbance/time data can be fitted to the sum of two exponentials, which leads to first-order rate constants k_f (fast initial step) and k_s (slower second step). The temperature dependence of k_f and k_s was measured. It follows from the kinetic data that (i) the keto tautomer of Hamac, HK, does not react with the metal ions M²⁺, (ii) the rate constant k_f increases linearly with [M²⁺]₀ according to k_f = k₀ + k₂[M²⁺]₀, and (iii) the rate constant k_s does not depend on [M²⁺]₀ and describes the enolization of the unreactive keto tautomer HK. The pH dependence of the second-order rate constant k₂ reveals that both the enol tautomer of Hamac, HE, and the enolate, E^-, react with M²⁺ in a second-order reaction to form the species M(amac)⁺. At 298 K rate constants k_HE are 18 ± 6 (Ni), 180 ± 350 (Co), and (9 ± 5) × 10⁴ (Cu) M^-1 s^-1 and rate constants k^E are 924 ± 6 (Ni), (7.4 ± 0.6) ± 10⁴ (Co), and (8.4 ± 0.2) × 10⁸ (Cu) M^-1 s^-1. The acid dissociation of the species M(amac)⁺ is triphasic. Very rapid protonation (first step) leads to M(Hamac)²⁺, which is followed by dissociation of M(Hamac)²⁺ and M(amac)⁺, respectively (second step). The liberated enol Hamac ketonizes (third step). The mechanistic implications of the metal dependence of rate constants k_HE, k_E, k_-HE, and k_-E are discussed.

Full text of DOI:10.1021/ic950272g

Inorg. Chem. 1996, 35, 2343-2351
2343
Kinetics and Mechanism of the Formation and Acid Dissociation of Cobalt(II),
Nickel(II), and Copper(II) Complexes with the Highly Enolized â-Diketone
3-(N-Acetylamido)pentane-2,4-dione ()Hamac) in Aqueous Solution†
Ju1rgen Hirsch,‡ Helmut Paulus,§ and Horst Elias*,‡
Institut fu¨r Anorganische Chemie and Fachbereich Materialwissenschaft, Technische Hochschule
Darmstadt, Petersenstrasse 18, D-64289 Darmstadt, Federal Republic of Germany
ReceiVed March 8, 1995X
The â-diketone Hamac ) 3-(N-acetylamido)pentane-2,4-dione was characterized by potentiometric, spectropho-
tometric, and kinetic methods. In water, Hamac is very soluble (2.45 M) and strongly enolized, with [enol]/
[ketone] ) 2.4 ( 0.1. The pKa of Hamac is 7.01 ( 0.07, and the rate constants for enolization, ke, and ketonization,
kk, at 298 K are 0.0172 ( 0.0004 s-1 and 0.0074 ( 0.0015 s-1, respectively. An X-ray structure analysis of the
copper(II) complex Cu(amac)2‚toluene ()C21H28CuN2O6; monoclinic, C2/c; a ) 20.434(6), b ) 11.674(4), c )
19.278(6) Å; â ) 100.75(1)°; Z ) 8; Rw ) 0.0596) was carried out. The bidentate anions amac- coordinate the
copper via the two diketo oxygen atoms to form a slightly distorted planar CuO4 coordination core. Rapid-scan
stopped-flow spectrophotometry was used to study the kinetics of the reaction of divalent metal ions M2+ (M )
Ni,Co,Cu) with Hamac in buffered aqueous solution at variable pH and I ) 0.5 M (NaClO4) under pseudo-first-
order conditions ([M2+]0 . [Hamac]0) to form the mono complex M(amac)+. For all three metals the reaction
is biphasic. The absorbance/time data can be fitted to the sum of two exponentials, which leads to first-order rate
constants kf (fast initial step) and ks (slower second step). The temperature dependence of kf and ks was measured.
It follows from the kinetic data that (i) the keto tautomer of Hamac, HK, does not react with the metal ions M2+
,
(ii) the rate constant kf increases linearly with [M2+]0 according to kf ) k0 + k2[M2+]0, and (iii) the rate constant
ks does not depend on [M2+]0 and describes the enolization of the unreactive keto tautomer HK. The pH dependence
of the second-order rate constant k2 reveals that both the enol tautomer of Hamac, HE, and the enolate, E-, react
with M2+ in a second-order reaction to form the species M(amac)+. At 298 K rate constants kHE are 18 ( 6 (Ni),
180 ( 350 (Co), and (9 ( 5) × 104 (Cu) M-1 s-1 and rate constants kE are 924 ( 6 (Ni), (7.4 ( 0.6) × 104 (Co),
and (8.4 ( 0.2) × 108 (Cu) M-1 s-1. The acid dissociation of the species M(amac)+ is triphasic. Very rapid
protonation (first step) leads to M(Hamac)2+, which is followed by dissociation of M(Hamac)2+ and M(amac)+,
respectively (second step). The liberated enol Hamac ketonizes (third step). The mechanistic implications of the
metal dependence of rate constants kHE, kE, k-HE, and k-E are discussed.
Introduction
lot of mechanistic information concerning reactions 2a-c. It is
The reaction of metal ions with â-diketones are more
complicated than many other complex formation reactions in
which the rate of complexation is controlled by the rate of
solvent exchange. The complications arise from the fact that
â-diketones HL are subject to a keto/enol tautomerism according
to (1). The keto tautomer HK is in equilibrium with the enol
M
q+ + HK f ME(q-1)+ + H+
q+ + HE f ME(q-1)+ + H+
q+ + E- f ME(q-1)
(2a)
(2b)
(2c)
M
M
nevertheless still a matter of dispute1 as to which ligand-based
and/or metal-based factors control the intimate mechanism of
these reactions.
ke
HK y
\
kz HE
(1a)
(1b)
k
HK h H+ + E- h HE
(5) Hynes, M. J.; O’Regan, B. D. J. Chem. Soc., Dalton Trans. 1976,
1200.
HE, and both tautomers can dissociate to form the enolate E-,
so that a given metal ion can react with either HK, HE, or E-.
The kinetics and mechanisms of complex formation with
â-diketones have been thoroughly discussed by Hynes in a
recent review.1 In particular, the work carried out by Pearson
and Anderson,2 Sutin et al.,3,4 and Hynes et al.5-18 provides a
(6) Hynes, M. J.; O’Regan, B. D. Proc. R. Irish Acad. 1977, 77B, 285.
(7) Hynes, M. J.; O’Regan, B. D. J. Chem. Soc., Dalton Trans. 1979,
162.
(8) Hynes, M. J.; O’Regan, B. D. J. Chem. Soc., Dalton Trans. 1980, 7.
(9) Hynes, M. J.; O’Regan, B. D. J. Chem. Soc., Dalton Trans. 1980,
1502.
(10) Hynes, M. J.; O’Shea, M. T. J. Chem. Soc., Dalton Trans. 1983, 331.
(11) Hynes, M. J.; O’Shea, M. T. Inorg. Chim. Acta 1983, 73, 201.
(12) Ando, I.; Yoshizumi, K.; Kazuhisa, I.; Ujimoto, K.; Kurihara, H. Bull.
Chem. Soc. Jpn. 1983, 56, 1368.
(13) Hynes, M. J.; O’Mara, C.; Kelly, D. F. Inorg. Chim. Acta 1986, 120,
131.
(14) Hernando, J. M.; Blanco, C.; Prieto, T. Bull. Soc. Chim. Fr. 1987,
775.
(15) Hynes, M. J.; Kelly, D. F. J. Chem. Soc., Dalton Trans. 1988, 905.
(16) Hynes, M. J.; Mooney, M. T. Proc. R. Irish Acad. 1989, 89 B, 441.
(17) Blanco, C.; Hynes, M. J. Bull. Soc. Chim. Fr. 1989, 611.
(18) Blanco, C.; Hynes, M. J. J. Chim. Phys. 1989, 86, 1989.
† Dedicated to Prof. Dr. K.-H. Lieser on the occasion of his 75th birthday.
‡ Institut fu¨r Anorganische Chemie.
§ Fachbereich Materialwissenschaft.
X Abstract published in AdVance ACS Abstracts, March 1, 1996.
(1) Hynes, M. J. ReV. Inorg. Chem. 1990, 11, 21.
(2) Pearson, R. G.; Anderson, O. P. Inorg. Chem. 1970, 9, 39.
(3) Jaffe, M. R.; Fay, D. P.; Cefola, M.; Sutin, N. J. Am. Chem. Soc.
1971, 93, 2878.
(4) Fay, D. P.; Nichols, A. R., Jr.; Sutin, N. Inorg. Chem. 1971, 10, 2096.
0020-1669/96/1335-2343$12.00/0 © 1996 American Chemical Society

2344 Inorganic Chemistry, Vol. 35, No. 8, 1996
Hirsch et al.
Our work19 on the coordination chemistry of “isonitroso-
acetylacetone” ()3-(hydroxyimino)pentane-2,4-dione) led us to
the substituted â-diketone 3-(N-acetylamido)pentane-2,4-dione
) Hamac,20 the copper complex of which was prepared by
pKa of Hamac. The pKa of the ligand in water (I ) 0.5 M, NaClO4)
was determined by potentiometric and spectrophotometric titration with
NaOH at 25 °C. The potentiometric method26 led to pKa ) 7.08 and
the spectrophotometric titration,27 based on λmax(Hamac) ) 278 nm (ꢀ
) 6700 M-1 cm-1) and λmax(amac-) ) 296 nm (ꢀ ) 15 400 M-1 cm-1),
led to pKa ) 6.93. The mean of these two independent determinations
is thus pKa ) 7.01 ( 0.07.
Stability Constants. Equilibrium constant K1 for mono complex
formation according to (3a) was determined by spectrophotometric
K1
\
M
2+ + amac- y
z M(amac)+
(3a)
Fackler and Cotton.21 This ligand is a remarkable â-diketone
in that it is very soluble in water, much more strongly enolized
than any of the â-diketones studied so far, and rather acidic.
We decided to study the kinetics of mono complex formation
according to (3) in aqueous solution for M ) Co, Ni, and Cu
titration of solutions of Hamac and the metal perchlorate (M ) Co,
Ni, Cu) and NaOH at I ) 0.5 M (NaClO4) and 298 K. Complex
formation was monitored in the UV range as well as in the d-d range,
as based on the following absorption data [λmax, nm (ꢀmax, M-1 cm-1)]:
Hamac, 278 (6700); amac-, 296 (15 400); Co(amac)+, 298 (9400) and
508 (14); Ni(amac)+, 302 (12 350) and 648 (3.4); Cu(amac)+, 302
(11 850) and 724 (32); Cu(amac)2, 298 (22 600) and 648 (44).
Computer fitting of the data28 led to the following stability constants.
M
2+ + Hamac h M(amac)+ + H+
(3)
and thus contribute to the understanding of the mechanism of
the enol-based and enolate-based reactions 2b and 2c.
M
K1, M-1 (UV)
K1, M-1 (d-d)
K1, M-1 (mean)
Co
Ni
Cu
2.44 × 104
7.02 × 104
1.70 × 107
1.93 × 104
6.57 × 104
1.17 × 107
(2.18 + 0.25) × 104
(6.79 + 0.22) × 104
(1.43 ( 0.27) × 107
Experimental Section
Chemicals. The various metal perchlorates and buffers (HMOPS,20
HMES,20 HLUS20,22) were reagent grade. The solvent water was
purified by double distillation of deionized water in a quartz apparatus.
The ligand Hamac (mp 99-100 °C) was prepared from Hacac20 in a
two-step procedure as described.21,23
Cu(amac)2 and Cu(amac)2‚toluene. A solution of 0.65 g of Cu-
(AcO)2‚H2O (3.25 mmol) in 25 mL of warm ethanol was slowly added
to a solution of 1.02 g of Hamac (6.5 mmol) in 25 mL of warm ethanol.
After 10 min of stirring at 60 °C, the hot green solution was filtered.
Cooling of the filtrate led to a green precipitate, which was recrystallized
from ethanol, washed with cyclohexane, and dried in vacuo over silica
gel at 60 °C to yield crystalline Cu(amac)2 (yield 40%; dec pt >150
°C). Cu(amac)2‚toluene was prepared analogously, with a 1:1 mixture
of ethanol/toluene instead of ethanol serving as solvent. Cooling of
the reaction mixture led to green needles of Cu(amac)2‚toluene (yield
40%; dec pt >180 °C).
The results of elemental analysis (C,H,N) of Hamac and the two
copper complexes were in good agreement with calculated data. The
magnetic moments of Cu(amac)2 and Cu(amac)2‚toluene at 20 °C were
found to be 1.91 and 1.93 µB, respectively. The solubility of Hamac
(2.45 M) and Cu(amac)2 (0.54 M) in water at 25 °C was determined
spectrophotometrically.
Instrumentation. Magnetic susceptibility: magnetic susceptibility
balance (Johnson-Matthey, type MSB-MKI). UV/vis spectra: diode
array spectrophotometer (Hewlett-Packard, type 8451). pH: pH meter
(Metrohm, type 654) in combination with a glass electrode (Ingold,
type 405-88-S7), calibrated at I ) 0.5 M (NaClO4) to yield the
relationship -log[H+] ) pH - 0.11. Potentiometric titration: apparatus
(Metrohm) consisting of potentiograph, automatic titrator, and Ag/AgCl
reference glass electrode. Kinetic measurements: modified24 stopped-
flow spectrophotometer (Durrum, D 110) and rapid-scan stopped-flow
spectrophotometer.25
Titration of the system Cu2+/Hamac and NaOH up to pH 7 led to
K2 ) (5.32 ( 3.2) × 105 M-1 for the formation of the bis complex
Cu(amac)2. In the case of cobalt and nickel, bis complex formation
was not detectable.
Rate of Enolization/Ketonization and the Ratio [enol]/[ketone].
The rate constant ke for enolization was determined by the bromination
method described elsewhere.3 The bromination of the enol is too fast
to be monitored by the stopped-flow technique at ambient temperature.
The stopped-flow spectrophotometric investigation of the reaction of
Hamac with Br2 at 452 nm (ꢀmax(Br2) ) 103 M-1 cm-1 3) led therefore
to a sharp initial drop in absorbance, ∆A ()Ainit - A0), followed by a
first-order decay of A from A0 to A∞. This first-order reaction, which
corresponds to the enolization of the keto tautomer initially present,
was studied at [Hamac]0 ) [Br2]0 ) 2.5 × 10-3 M in the pH range
0.80-1.68 (I ) 0.5 M, NaClO4). The absorbance/time data were
computer-fitted to eq 4, to obtain the parameters ke, A0, and A∞ (Table
A ) (A0 - A∞) exp(-ket) + A∞
(4)
1). Rate constant ke is pH-independent and averages to ke ) 0.0172 (
0.0004 s-1 at 25 °C. The ratio [enol]/[ketone], following from the ratio
of amplitudes (see Table 1), averages to 2.4 ( 0.1.
The rate constant of ketonization, kk, was also determined by stopped-
flow spectrophotometry. A solution of Hamac at pH 8.5, containing
exclusively the enolate, was reacted with dilute HClO4 to produce
(25) Drexler, C.; Elias, H.; Fecher, B.; Wannowius, K. J. Fresenius J. Anal.
Chem. 1991, 340, 605.
(26) Stepwise titration with NaOH changes the pH, from which the degree
of protonation, n ) [Hamac]/[Hamac]0, was calculated. Computer-
fitting of the n/[H+] data22 to n ) ([H+]/Ka)/(1 + [H+]/Ka) led to Ka.
(27) The species Hamac and amac- differ in their UV absorption. Stepwise
titration with NaOH led to A/[H+] data (A ) absorbance), which were
computer-fitted to A ) [A0(Hamac) + A0(amac-)Ka/[H+]]/(1 + Ka/
[H+]). The symbols A0(Hamac) and A0(amac-) stand for the absor-
bance of the species Hamac and amac- at [Hamac]0.
(19) Elias, H.; Wiegand, D.; Wannowius, K. J. Inorg. Chim. Acta 1992,
197, 21.
(20) Abbreviations: Hamac ) 3-(N-acetylamido)pentane-2,4-dione; HMOPS
) 3-(N-morpholino)propanesulfonic acid; HMES ) 2-(N-morpholino)-
ethanesulfonic acid; HLUS ) 2,6-dimethylpyridine-3-sulfonic acid;
Hacac ) acetylacetone ) pentane-2,4-dione; Htfpd ) 1,1,1-trifluo-
ropentane-2,4-dione; Hhptd ) heptane-3,5-dione; Htftbd ) 4,4,4-
trifluoro-1-(2-thienyl)butane-1,3-dione.
(21) Fackler, J. P., Jr.; Cotton, F. A. Inorg. Chem. 1963, 2, 102.
(22) Bips, U.; Elias, H.; Hauro¨der, M.; Kleinhans, G.; Pfeifer, S.;
Wannowius, K. J. Inorg. Chem. 1983, 22, 3862.
(23) Wolff, L.; Bock, P.; Lorentz, G.; Trappe, P. Liebigs Ann. Chem. 1902,
325, 139.
(24) Elias, H.; Fro¨hn, U.; von Irmer, A.; Wannowius, K. J. Inorg. Chem.
1980, 19, 869.
(28) The absorbance/[H+] data were fitted to eq I (monitoring in the UV
range) or eq II
A ) (A0HL + A0MLK1[M2+]0Ka/[H+])/(1 + K1[M2+]0Ka/[H+]) (I)
A ) (A0M + A0MLK1[HL]0Ka/[H+])/(1 + K1[HL]0Ka/[H+]) (II)
(monitoring in the d-d range) with A0HL, A0ML, and A0 being the
M
absorbance of the species HL, ML, and M at concentrations [M2+
(eq I) and [HL]0 (eq II), respectively.
]
0

Co, Ni, and Cu Complexes with Hamac
Inorganic Chemistry, Vol. 35, No. 8, 1996 2345
Table 1. Rate Constants for the Enolization and Ketonization of
Hamac at 298 K (I ) 0.5 M)
Table 3. Atomic Parameters (×104) for Cu(amac)2‚toluene
(Excluding H)
(ke + kk),
∆A,
A0 - A∞, ∆A/(A0 - A∞) )
atom
x/a
y/b
z/c
Ueq, Å2 a
c
c
pH
ke, s-1 a
s-1 b
%
%
[enol]/[ketone]
Cu1
O1
C1
C2
C3
C4
C5
O2
N1
C6
O3
C7
4846(1)
4209(2)
3250(4)
3628(4)
3301(3)
3617(4)
3237(4)
4206(2)
2638(3)
2093(3)
2118(3)
1438(3)
5517(2)
6539(4)
6150(4)
6451(3)
6120(3)
6478(4)
5481(2)
7170(3)
7528(4)
7285(2)
8268(3)
4721(1)
5911(5)
6987(7)
5854(7)
4849(7)
3776(7)
2728(7)
3586(4)
4931(6)
4816(7)
4752(6)
4797(8)
3567(4)
2648(7)
3714(7)
4766(8)
5769(7)
6884(7)
5849(4)
4826(6)
4899(7)
4838(6)
5051(7)
4150(1)
3991(2)
3525(5)
3646(3)
3361(3)
3450(3)
3123(5)
3776(2)
2964(3)
3270(4)
3895(2)
2739(4)
4232(2)
4320(4)
4377(3)
4541(3)
4616(3)
4784(4)
4553(2)
4672(3)
4160(4)
3535(2)
4417(3)
417(5)
473(31)
785(56)
472(40)
412(39)
463(43)
741(54)
498(31)
467(35)
516(44)
821(38)
714(49)
492(31)
663(51)
417(42)
400(38)
426(42)
662(50)
512(31)
449(34)
481(43)
682(35)
508(43)
0.80
1.06
1.68
0.0175
0.0167
0.0175
69.6
70.9
71.2
30.4
29.1
28.8
2.29
2.44
2.47
1.09
1.45
2.34
3.02
0.0247
0.0240
0.0240
0.0256
a Obtained by fitting the A/t data to eq 4; [Hamac]0 ) [Br2]0 ) 2.5
× 10-3 M. b Obtained by fitting the A/t data to eq 5; [Hamac]0 ) 5 ×
10-5 M. c The parameters ∆A and (A0 - A∞) are described in the
Experimental Section.
O4
C8
C9
Table 2. Crystallographic Data for Cu(amac)2‚toluene
C10
C11
C12
O5
N2
C13
O6
C14
C21H28CuN2O6
fw ) 467.98
Z ) 8
T ) 26.0 °C
λ ) 0.710 69 Å
F(calcd) ) 1.38 g cm-3
µ ) 10.0 cm-1
R(Fo)a ) 0.0742
Rw(Fo)b ) 0.0596
space group: C2/c (No. 15)
a ) 20.434(6) Å
b ) 11.674 (4) Å
c ) 19.278 (6) Å
â ) 100.75(1)°
V ) 4517.85 ( 4.2 Å3
a Ueq ) /3∑i∑jUijai*a*aiaj.
1
j
a R(Fo) ) ∑|Fo - Fc|/∑Fo. b Rw(Fo) ) ∑w1/2|Fo - Fc|/w1/2∑Fo.
Technische Hochschule Darmstadt. Scattering factors f0, f ′, and f ′′
for C, H, N, and O are stored in SHELX-76.29 The final positional
parameters are given in Table 3.
exclusively the enol. The ketonization of the enol, which is a reversible
first-order reaction with kobsd ) ke + kk, was followed at different pH
values (I ) 0.5 M, NaClO4). The A/t data were fitted to eq 5 to obtain
Results
A ) (A0 - Aeq) exp[-(ke + kk)t] + Aeq
(5)
The â-Diketone Hamac. Hamac is a colorless solid which,
due to the polar acetamido group in 3-position, is well soluble
in water. The solubility (2.45 M) exceeds that of Hacac by a
factor of 1.5. Moreover, the complex Cu(amac)2 can be
dissolved in water up to 0.56 M, which is 700 times higher
than for Cu(acac)2. The pKa of Hamac, as determined inde-
pendently by potentiometric and spectrophotometric titration,
is 7.01 ( 0.07 (mean of 7.08 and 6.93). Hamac is thus by
almost two pK units more acidic than Hacac (see Table 4).
the pH-independent sum of ke and kk (Table 1). The averaged values
of (ke + kk) ) 0.0246 ( 0.0015 s-1 and ke ) 0.0172 s-1 (see above)
led to kk ) 0.0074 ( 0.0015 s-1 at 25 °C.
Kinetics of Complex Formation. Reaction 3 was followed by
stopped-flow or rapid-scan-stopped-flow spectrophotometry at I ) 0.5
M (NaClO4) and variable pH in buffered solution under pseudo-first-
order conditions ([M2+]0 g 10[Hamac]0). The A/t data, resulting from
the increase in absorbance at 300-315 nm with time, were computer-
fitted to eq 6 (irreversible biphasic consecutive reaction). The fitting
The rate of enolization of Hamac (ke ) 0.0172 s-1) is rather
close to that of Hacac and Htfpd20 (ke ) 0.015 s-1), but the
rate of ketonization is considerably smaller (Table 4). This
means that the enol/ketone equilibrium is much more shifted
toward the enol tautomer, with K ) ke/kk ) 2.3 ( 0.4. The
spectrophotometric investigation of the kinetics of enolization
according to the bromination method leads to the reaction
amplitudes ∆A and (A0 - A∞), the ratio of which corresponds
to K (see Table 1). This independently obtained value amounts
to K ) 2.4 ( 0.1 and is in good agreement with the value
following from the ratio ke/kk. To our knowledge, the ligand
Hamac is thus the most strongly enolized â-diketone so far
studied kinetically.
A ) af[exp(-kft)] + as[exp(-kst)] + A∞
(6)
procedure, based on the least-squares method, led to af, as, A∞, kf, and
ks (kf, ks ) rate constants for the fast and slow reaction; af, as )
corresponding amplitudes in units of absorbance).
Kinetics of Acid Hydrolysis. Reaction 7 was followed as described
for complex formation according to (3), with HClO4 being the excess
M(amac)+ + H+ f M2+ + Hamac
(7)
partner ([HClO4]0 g 10[M(amac)+]0). The A/t data of the biphasic
process were computer-fitted with eq 6.
X-ray Structure Determination. The green crystal of Cu(amac)2‚
toluene, as grown from the ethanol/toluene reaction mixture, was needle-
shaped and had the dimensions 0.15 × 0.16 × 1.2 mm. Intensities
were measured on a four-circle diffractometer (Stoe-Stadi-4) using
graphite-monochromatized Mo KR radiation (λ ) 0.710 69 Å; scan
2θ:ω ) 1:1). Cell constants were determined by the least-squares
method from the 2θ angles of 94 reflections (T ) 299 K) on the same
instrument. Lp and background corrections and a numerical absorption
correction (EMPIR and REDU4 (Stoe); Tmin ) 0.935 and Tmax ) 1.000)
were applied.
The structure was solved by direct methods with SHELXS-86
(PATT) and refined by least-squares to the R values given in Table 2.
Hydrogen atoms were positioned geometrically (C-H distance ) 0.96
Å) and not refined. An empirical extinction correction was applied.
All crystallographic calculations were performed with the programs
SHELX-76 and SHELXS-86 on an IBM 3090 computer at the
Structure of the Complex Cu(amac)2‚toluene. The ligand
Hamac carries three donor oxygen atoms in total so that the
question of O(acetyl) coordination cannot be excluded. It
follows from Figure 1, however, that in the complex unit Cu-
(amac)2 the ligand prefers the normal coordination with the two
â-diketone oxygens being bound to the copper. The complex
is centrosymmetric and crystallizes in the monoclinic space
group C2/c with 8 formula units per unit cell. Half of the
toluene is located on a 2-fold axis; the other half is located close
(29) Data for Cu was taken from: International Tables for X-ray Crystal-
lography; Kynoch Press: Birmingham, England, 1974, Vol. IV.
(30) Pearson, R. G.; Moore, J. W. Inorg. Chem. 1966, 5, 1528.

2346 Inorganic Chemistry, Vol. 35, No. 8, 1996
Hirsch et al.
Table 4. Parameters Characterizing the â-Diketone Hamaca
â-diketone20
solubility, M
2.45
pKa
ke, s-1
kk, s-1
K ) ke/kk ) [enol]/[ketone]
ref
Hamac
7.01 ( 0.07
0.0172 ( 0.0004
0.0074 ( 0.0015
2.3 ( 0.4
2.4 ( 0.1b
0.13
0.01
0.12
this work
Hacac
Htfpd
Hhptd
1.6
9.00
6.06
0.015
0.015
0.0058
0.114
1.5
0.048
30
10
7
10.0
a The data refer to the solvent water, I ) 0.5 M, and 298 K. b As determined from the ratio of amplitudes (see Table 1).
Table 5. Selected Distances (pm) and Bond Angles (deg) for
Cu(amac)2‚toluene
Distances
Cu-O1
Cu-O2
Cu-O4
Cu-O5
O1-C2
O2-C4
O4-C9
O5-C11
C2-C3
C3-C4
C9-C10
C10-C11
188.9(5)
190.6(5)
190.7(5)
191.1(5)
124.9(8)
126.9(8)
128.3(8)
129.1(8)
141.0(10)
140.4(10)
138.3(10)
137.3(10)
C1-C2
152.8(10)
152.1(10)
149.3(10)
149.8(10)
142.7(8)
144.5(8)
135.6(9)
133.6(8)
152.6(9)
151.1(9)
120.0(8)
121.8(8)
328.6(2)
C4-C5
C8-C9
C11-C12
C3-N1
C10-N2
N1-C6
N2-C13
C6-C7
Figure 1. View of the coordination geometry in the complex
Cu(amac)2.
C13-C14
C6-C3
C13-O6
Cu-Cua
Angles
O1-Cu-O2
Cu-O2-C4
Cu-O1-C2
O2-C4-C3
O1-C2-C3
C2-C3-C4
92.7(2)
125.9(5)
127.2(5)
126.0(7)
125.9(7)
121.4(6)
O4-Cu-O5
Cu-O4-C9
Cu-O5-C11
O4-C9-C10
O5-C11-C10
C9-C10-C11
91.7(2)
127.1(5)
125.9(5)
123.8(7)
124.1(7)
125.0(6)
a Distance between neighboring complex units.
distances lie in between those for single and double bonds and
reflect delocalization of electron density within the chelate rings
formed, which is typical for â-diketone complexes.
Kinetics of Complex Formation. The reaction of Hamac
with an excess of the divalent metal ions M2+ (M ) Co, Ni,
Cu) leads to the species M(amac)+. These species absorb
strongly in the UV and have weak d-d transitions in the visible
range (see Experimental Section). The strong absorption at
about 300 nm corresponds to the π f π* transition of the
enolate amac-, which is bathochromically shifted and intensified
upon coordination to the metal. Monitoring of the formation
of the species M(amac)+ at 300-315 nm should therefore result
in an increase in absorbance.
Figure 2. Projection of the unit cell of the complex Cu(amac)2‚toluene
along [010].
to a center of symmetry and is highly disordered, which limited
the refinement to Rw ) 0.0596 (Table 2).
The copper coordinates the two ligands in a planar fashion.
The planes of the two chelate rings are slightly bent off the
CuO4 coordination plane though, with the two N-acetylamido
groups pointing into opposite directions (Figure 1). As shown
in Figure 2, the units of Cu(amac)2 form dimers (Cu-Cu
distance ) 329 pm; shortest intermolecular Cu-O distance )
279 pm). The dimeric units are stabilized by weak intermo-
lecular hydrogen bonds between N2 and O3 (distance ) 291
pm) and the dimers are stacked along [001] via weak hydrogen
bonds between N1 and O6 (distance ) 292 pm). The toluene
located on the twofold axis is sitting between the stacked dimers.
The Cu-Cu distance of 329 pm and the normal magnetic
moment of 1.93 µB point to the absence of metal-metal
interactions. The planar arrangement of the CuO4 coordination
core is slighly distorted (see angles and distances in Table 5).
The average Cu-O distance (190.3 pm), C-O distance (127.3
pm), and C-C distance within the chelate rings (139.3 pm; see
Table 5) is rather close to what is found for similar â-diketone
copper complexes of the type bis(3-R-pentane-2,4-dionato)-
copper(II) with R ) phenyl31 or methyl.32 The C-C and C-O
As an example, Figure 3a shows the series of spectra obtained
for the reaction of an excess of Ni2+ ions with Hamac at pH
6.2. The first 50 spectra were recorded every second, the second
set of 50 spectra every 5 s. The reaction is obviously biphasic.
As shown in Figure 3b, the first set of spectra is characterized
by a sharp isosbestic point at 288 nm, with a decrease in
absorbance below 288 nm and an increase above. This initial
reaction step is followed by a slower reaction (see Figure 3c),
associated with a gradual increase of the absorption peak at 302
nm, typical for Ni(amac)+. The A/t data can be fitted to the
sum of two exponentials (eq 6), which leads to rate constants
kf (fast initial step) and ks (slower second step). For M ) Co,
Cu the same type of biphasic kinetics is observed.
Variation of [M2+
] at constant pH leads to the data
0
summarized in Table 6. Rate constant kf increases with
increasing [M2+]0 and, for a given concentration of the metal,
its size depends on the nature of the metal according to kf(Cu)
> kf(Co) > kf(Ni). Rate constant ks is independent of [M2+]0,
nearly of the same size for M ) Ni, Co and somewhat larger
for M ) Cu. These findings suggest that the kr step describes
complex formation and that the ks step has to do with the
(31) Carmichael, J. W., Jr.; Steinrauf, L. K.; Belford, R. L. J. Chem. Phys.
1965, 43, 3959.
(32) Robertson, J.; Truter, M. R. J. Chem. Soc. A 1967, 309.

Co, Ni, and Cu Complexes with Hamac
Inorganic Chemistry, Vol. 35, No. 8, 1996 2347
Table 7. First-Order Rate Constant for the Reaction of Ni2+ Ions
with Exclusively the Enol Tautomer of Hamac33 and for the Reac-
tion of Co2+ Ions with an Excess of Hamac at 298 K (I ) 0.5 M)
[Ni2+], mMa
pHb
k, s-1 c
[Hamac], Md
pHe
k, s-1 f
1.00
2.50
5.00
10.0
25.0
50.0
5.50
5.51
5.51
5.43
5.45
5.50
0.0831
0.116
0.179
0.295
0.660
1.26
0.0375
0.050
0.075
0.10
4.65
4.67
4.60
4.61
4.55
21.4
25.2
35.3
43.4
51.8
0.15
a [Hamac]0 ) 5 × 10-5 M. b Adjusted with HMES. c Obtained by
fitting eq 4 to the A/t data recorded at 300 nm. d [Co2+]0 ) 0.00375
M. e Adjusted with HLUS. f Obtained by fitting eq 4 to the A/t data
recorded at 500 nm.
of Hamac at pH 5.5.33 The A/t data thus obtained could be
well fitted to a single exponential according to (4); i.e. only
one step was observed. The first-order rate constant resulting
from this experiment increases with increasing [Ni2+]0 (Table
7), as does kf. This result indicates that the ks step obviously
describes the (slow) enolization of the keto tautomer of Hamac
after the enol tautomer has been consumed in (fast) complex
formation with the nickel present in excess. In addition, the
fact that ks * f([Ni]0) is consistent with the interpretation that
the keto tautomer as such does not add to the nickel. The second
experiment concerns the reaction of Co2+ ions with an excess
of Hamac, as monitored in the d-d range. In this case the A/t
data could also be fitted to a single exponential according to
(4). The resulting first-order rate constant increases with
increasing [Hamac]0 (Table 7). These findings suggest that,
for [Hamac]0 g 10[Co2+]0, there is an excess of enolic Hamac
present to completely convert the Co2+ ions into the species
Co(amac)+ in a pseudo-first-order reaction.
The reaction of Hamac with copper is much faster than that
with nickel and cobalt. For experimental reasons the concentra-
tion of excess copper had therefore to be kept low. As shown
in Table 6, for some of the copper experiments the pseudo-
first-order condition ([Cu2+]0 g 10[Hamac]0) was not fulfilled.
The surprising result was however that even for [Cu2+]0:
[Hamac]0 ) 1:1 and 2.5:1 the A/t data could be well fitted to
eq 6.34 A simple interpretation of this finding would come from
the assumption that the species Cu(amac)+ is mainly formed
via the reaction of the Cu2+ ion with the enolate amac-, for
which the condition [Cu2+]0 . [amac-] is of course fulfilled
in the pH range 3-4.7.
Figure 3. Spectral changes in the UV range associated with the reaction
of Ni2+ ions ([Ni2+]0 ) 5 × 10-3 M) with Hamac ([Hamac]0 ) 5 ×
10-5 M) in water at pH 6.2, 298 K, and I ) 0.5 M (NaClO4) Key: (a)
100 consecutive spectra, with ∆t ) 1 s for spectra 1-50 and ∆t ) 5
s for spectra 51-100 (every second spectrum omitted for clarity); (b)
spectra 1-12; (c) spectra 25, 50, 60, 70, 80, 90, and 100.
Table 6. Rate Constants kf and ks for the Reaction of Hamac with
Ni2+, Cu2+, and Co2+ Ions According to Eq 3 at 298 Ka,b
[M2+], mM
pHc
kf, s-1
ks, s-1
M ) Ni
0.50
1.25
2.50
3.75
5.00
12.5
25.0
4.94
4.87
4.85
4.84
4.84
4.85
4.83
0.0485
0.0654
0.0850
0.104
0.121
0.233
0.396
0.0104
0.0231
0.0237
0.0249
0.0243
0.0246
0.0248
Figure 4 shows the plot of the dependence kf ) f([Ni2+]0)
and ks ) f([Ni2+]), respectively, for the system Ni2+/Hamac at
three different pH values. The rate constant ks for the slower
second reaction step is independent of [Ni2+]0 (Figure 4b) and
increases weakly with increasing pH. The averaged size of ks
is close to that of the rate constant for the enolization of Hamac,
ke ) 0.0172 s-1 (Table 4), but is somewhat larger. An
explanation for this comes from the study of the rate of
enolization at variable pH in the presence of buffers.35 This
study shows that the enolization of Hamac is subject to general
base catalysis. Hydroxyl ions and the B- ions of added buffers
HB affect ke according to ke ) kHOH[H2O] + kOH[OH-] +
kB[B-]. The term kHOH[H2O] amounts to 0.017 ( 0.001 s-1 at
M ) Cu
M ) Co
0.050
0.125
0.250
0.500
1.00
3.49
3.47
3.48
3.47
3.47
3.45
18.2
20.3
23.7
28.8
42.7
64.7
0.144
0.162
0.154
0.150
0.138
0.128
2.00
0.50
1.25
2.50
3.75
5.00
12.5
25.0
4.89
4.87
4.84
4.86
4.83
4.81
4.80
0.0066
0.0115
0.0237
0.0238
0.0255
0.0259
0.0265
8.67
9.70
10.1
11.2
15.7
23.4
a Conditions [Hamac]0 ) 5 × 10-5 M; I ) 0.5 M (NaClO4). b kf
and ks obtained by computer-fitting of the A/t data to eq 6. c The pH
was adjusted with the buffers HLUS20 (Ni, Co) and formic acid (Cu)
at [buffer] ) 0.1 M.
(33) The solution of Hamac used for this stopped-flow experiment was set
to pH 8.4 to convert Hamac completely to amac-. This solution was
reacted with solutions of nickel perchlorate of pH 5.5 (as adjusted
with the buffer HMES20) to produce reaction mixtures of pH 5.5
containing only the enol tautomer of Hamac upon mixing.
(34) In the case of the experiment with the stoichiometry [Cu2+]0:[Hamac]0
) 1:1 it was attempted to fit the A/t data for the fast initial step to the
equation valid for a 1:1 second-order reaction, A ) A∞ + (A0 - A∞)/
(1 + [Cu2+]0kt). The quality of the fit was very poor though.
tautomerism of the liand Hamac. Convincing support for this
interpretation comes from two experiments. In the first one,
excess Ni2+ ions were reacted exclusively with the enol tautomer

2348 Inorganic Chemistry, Vol. 35, No. 8, 1996
Hirsch et al.
Table 8. Rate Constants k2, k0, and ks for the Formation of the
Species M(amac)+ According to Eq 3 at Variable pH, 298 K, and I
) 0.5 Ma
pHb
k2, M-1 s-1 c
k0, s-1 d
ks, s-1 c
M ) Ni
4.8
5.3
5.5
5.7
6.0
6.2
6.4
6.7
7.0
7.7
14.2
27.4
47.0e
43.4
79.1
100
164
253
418
659
0.049
0.058
0.056e
0.119
0.082
0.144
0.089
0.189
0.177
0.738
0.0242
0.0268
0.0439
0.0578
0.0743
0.0846
0.109
0.155
0.518
M ) Cu
M ) Co
3.0
3.5
4.3
4.7
17 000
24 000
150 000
309 000
23.7
17.4
11.0
5.67
0.0904
0.146
0.0222
0.0248
4.6
4.9
5.3
5.6
6.0
6.2
275f
618
1510
1920
6050
8060
12.6f
7.89
7.59
6.99
3.11
10.0
0.0251
0.0298
0.0465
0.0656
0.0809
Figure 4. Plot of the experimental rate constants kf and ks vs the
concentration of nickel ions for reaction 3 at variable pH and 298 K
([Hamac]0 ) 5 × 10-5M).
a Each set of rate constants k2, k0, and ks at a given pH is based on
a series of stopped-flow experiments at five to seven different
concentrations of excess M2+ ions at [Hamac]0 ) 5 × 10-5M (as shown,
for example, in Table 6), leading to five to seven pairs of rate constants
kf and ks at the given pH. Fitting of the kf data to eq 8 for a given pH
leads to rate constants k2 and k0. b As adjusted with the buffers formic
acid (pH 3.0-3.5), HLUS20 (pH 4.3-5.3), HMES20 (pH 5.5-6.7), and
HMOPS20 (pH 7.0-7.7). c The error limits for k2 and ks are of the order
(5%. d Due to the fact that k0()intercept) is determined by extrapola-
tion the error limits for k0 are very high. This is especially true for
“steep” dependencies, i.e. for large k2 values. e Experiment with
exclusively the enol tautomer of Hamac (see Table 7).33 f Experiment
with an excess of Hamac (see Table 7).
298 K, and the increase in ks with increasing pH and with
different buffers (see Figure 4b) is thus due to increasing base
catalysis.
As shown in Figure 4a for M ) Ni, the experimental rate
constant kf, describing the formation of the mono complex
M(amac)+, increases linearly with [M2+
] at a given pH
0
according to (8). Fittings of eq 8 to the kf/[M2+]0 data leads to
rate constants k0 and k2, as summarized in Table 8.
kf ) k0 + k2[M2+]0
(8)
Scheme 1. Schematic Description of the Various Steps
Controlling the Formation and Acid Dissociation of the
Species M(amac)+ ) ME+
The size of k2 is pH-dependent (see Figure 4a). If, according
to Scheme 1, complex formation takes place with enolic Hamac
()HE) as well as with the enolate amac- ()E-), the rate of
complex formation should follow eq 9 (in view of the excess
d[ME+]/dt ) kHE[HE][M2+]0 -
k-HE(KMEH)-1[ME+][H+] + kE[E-][M2+]0 (9)
Table 9 presents the data for kHE and kE, as obtained by
computer-fitting.
condition, [M2+]0 . [Hamac]0, the reverse reaction, as char-
acterized by the rate constant k-E, is neglected).37 Integration
of (9) leads to an exponential with an experimental rate constant,
kobsd ≡ kf, as described by eq 10. For a given pH (i.e., [H+] )
Table 10 presents the temperature dependence of rate
constants kf and ks and the corresponding activation enthalpies
∆Hq. These activation enthalpies are not pure activation
parameters since both kf and ks are not single rate constants but
are instead composite experimental parameters.
kf ) k-HE(KMEH)-1[H+] + (kHE + kEKa/[H+])(1 + Ka/
[H+])-1[M2+]0 (10)
(35) The rate law for the enolization of the keto tautomer HK of Hamac at
variable pH in the presence of buffers HB was found to be a three-
term rate law, V ) (kHOH[H2O) + kOH[OH-] + kB[B-])[HK], with
kHOH ) (3.07 ( 0.09) × 10-4 M-1 s-1, kOH ) (1.5 ( 0.1) × 105 M-1
s-1, and kB on the order of 1 M-1 s-1 for the buffers HMES and
HMOPS.36 In the absence of buffers and at pH < 4.8, the rate is
given by V ) kHOH [H2O][HK].
constant), eq 10 corresponds to eq 8 with k0 ) k-HE(KMEH)-1[H+]
and k2 ) (kHE + kEKa/[H+])/(1 + Ka/[H+]). The plot of k2(1 +
Ka/H+) vs Ka/[H+] according to (11) should therefore give a
k2(1 + Ka/[H+]) ) kHE + kEKa/[H+]
(11)
(36) Dr. Ing. Thesis, submitted to Technische Hochschule Darmstadt (D17)
by J.H. in 1994.
(37) The equilibrium MEH2+ h ME+ + H+ is a fast equilibrium. At pH
> pKMEH the formation of the species MEH2+ corresponds therefore
to the formation of the species ME+.
linear dependence with a slope of kE and an intercept kHE
.
Figure 5 shows this type of plot for M ) Ni, Cu, and Co and

Co, Ni, and Cu Complexes with Hamac
Inorganic Chemistry, Vol. 35, No. 8, 1996 2349
Table 10. Temperature Dependence of Rate Constants kf and ks
for the Formation of the Species M(amac)+ According to Eq 3 at I
) 0.5 M and Activation Parameter ∆Hq
M ) Nia
M ) Cob
M ) Cuc
T, K
kf, s-1 ks, s-1 kf, s-1 ks, s-1 kf, s-1 ks, s-1
285
288
291
293
298
303
308
313
0.204
0.261
0.353
0.439
0.687
1.08
0.0297
0.0325 12.9
0.0411
0.0543 19.3
0.0703 29.0
0.0995 43.6
0.136
0.188
0.0416 9.98
0.00739
0.0594 13.6
0.0832 17.6
0.0166
0.0241
0.0366
0.0444
0.0605
0.120
0.164
0.231
24.2
29.2
44.2
1.60
2.40
60.3
86.3
∆Hq, kJ mol-1 d 63.4 ( 1 47.6 ( 2 54.7 ( 2 48.7 ( 1 40.6 ( 2 46.4 ( 3
a At pH 6.2 (buffer: HMES), [Ni2+]0 ) 0.005 M, [Hamac]0 ) 5 ×
10-5 M. b At pH 6.2 (buffer: HMES), [Co2+]0 ) 0.0025 M, [Hamac]0
) 5 × 10-5 M. c At pH 4.2 (buffer: HLUS), [Cu2+]0 ) [Hamac]0 ) 5
× 10-5 M. d Obtained from the slope of the plot of ln(kf/T) and ln(ks/
T), respectively, vs 1/T. The data for ∆Sq are not presented. Due to
the fact that both kf (see eqs 10 and 12) and ks ()ke)35 are composite
parameters the data for ∆Sq are rather meaningless.
the complex M(amac)+ ) ME+. The concentration of the
species MEH2+ thus formed is governed by the pH. The decay
of ME+ and MEH2+ liberates the enol HE (second step; kf),
which finally ketonizes (third step; ks ) ke + kk). If so, the
experimental rate constant kf for the observable fast step of acid
hydrolysis should follow relationship 12. The plot of kf ) f(pH)
Figure 5. Plot of the term k2(1 + Ka/[H+]) vs Ka/[H+] according to eq
11 for nickel (a), copper (b), and cobalt (c).
Table 9. Rate and Equilibrium Constants for the Formation and
Acid Dissociation of the Species M(amac)+ (M ) Ni, Co, Cu) at
298 K and I ) 0.5 M (NaClO4)
kf ) (k-E + k-HE[H+]/KMEH)/(1 + [H+]/KMEH
)
(12)
for M ) Co, as shown in Figure 6, indicates the expected
saturation type behavior of kf.38 Computer-fitting of eq 12 to
the data leads to the parameters k-E, k-HE and KMEH for M )
Co and Cu, albeit with large errors for k-HE and KMEH (see Table
9). In the case of nickel the dependence of kf on [H+] is linear
up to pH 1.3. A linear dependence of kf on [H+] follows from
eq 12 for the condition [H+]/KMEH e 0.1. This in turn means
for pH 1.3 that the condition KMEH g 0.5 has to be fulfilled. It
follows from the numbers obtained for KMEH that, compared to
the complex cation Co(amac)+, the species Ni(amac)+ and Cu-
(amac)+ are only weakly protonated.
M ) Ni
kHE, M-1 s-1 18 ( 6
M ) Co
180 ( 350
M ) Cu
(9 ( 5) × 104
(1.3 ( 0.7) × 104 a
kE, M-1 s-1 924 ( 6
kOH, M-1 s-1
(7.4 ( 0.6) × 104 (8.4 ( 0.2) × 108
(3.0 ( 0.1) × 108 a
k-HE, s-1
k-E, s-1
g10.3b
240 ( 43
1330 ( 210
15.7 ( 1.6
0.054 ( 0.005 9.5 ( 2.3
K
MEH, M
g0.5b
0.0038 ( 0.0013 0.22 ( 0.05
kE/k-E, M-1 1.7 × 104
0.91 × 104
2.18 × 104
3.2 × 106
0.039
5.4 × 107
1.6 × 107
4.4 × 109
0.32
K1, M-1 c
kex, s-1 d
ki,E/kex
6.79 × 104
3.2 × 104
0.048
e
f
ki,HE/kex
1.9 × 10-3
1.9 × 10-4
6.8 × 10-5
Discussion
a As obtained by fitting eq 16b to the k2/[H+] data. b Estimated on
the basis of the condition [H+]/KMEH e 0.1. c Spectrophotometrically
determined formation constant for the species M(amac)+ according to
eq 3a. d Rate constant for water exchange at 298 K.44,45 e Calculated
In aqueous solution the ligand Hamac is present mainly in
its enolic form (70%). In the complex Cu(amac)2‚toluene (see
Figure 1) and, most probably, in all of the species M(amac)+
studied the ligand behaves like an unsubstituted â-diketone and
coordinates the metal via the two diketo oxygens. The kinetic
results are consistent with the interpretation that the keto
tautomer of Hamac does not add to the metal. The coordination
of the enol tautomer, as studied with an excess of metal ion, is
biphasic. There is a fast initial step (kf), describing “fast”
M(amac)+ formation, and a slower consecutive step (ks),
describing “slow” M(amac)+ formation, as controlled by the
enolization of the “unreactive” keto tautomer. The rate of the
latter process is subject to general base catalysis. The activation
enthalpies ∆Hq for the slower second step (ks) of the reaction
with nickel, cobalt, and copper agree within the limits of error
(see Table 10). This result supports the interpretation that the
ks step describes the enolization of the keto tautomer of Hamac.
For a given pH, rate constant kf for complex formation increases
linearly with the metal concentration according to eq 8, which
on the basis of ki,E ) kE/KOS for kOS ) 0.6 M-1 46 f Calculated on the
.
basis of ki,HE ) kHE/KOS for KOS ) 0.3 M-1 46
.
Kinetics of Acid Hydrolysis. The acid hydrolysis of the
mono complex M(amac)+ (M ) Co, Ni, Cu), as monitored
spectrophotometrically in the UV range, was found to be a three-
step process. The first step is a sharp change in absorbance,
∆A, too fast to be monitored by the stopped-flow technique.
This initial rapid step is followed by a change in absorbance
with time to which eq 6 can be fitted satisfyingly. The fitting
procedure leads to first-order rate constants kf for a fast second
step and ks for a third slower step (see Table 11). The rate
cosntant ks is pH-independent and found to the same for M )
Co, Ni, and Cu; its size corresponds to (ke + kk) (see Table 1).
This means that the ks step is indeed the final third step
describing the ketonization of enolic Hamac liberated by the
acid hydrolysis of the species M(amac)+.
The very rapid initial change in absorbance, ∆A, increases
with decreasing pH. It is therefore suggested that, according
to Scheme 1, the first step consists in the fast protonation of
(38) It would have been most desirable to extend the measurements to
higher proton concentrations. This was however limited by the kf step
becoming too fast for observation.

2350 Inorganic Chemistry, Vol. 35, No. 8, 1996
Table 11. Experimental Rate Constants for the Acid Hydrolysisa of the Species M(amac)+ at Variable pH, 298 K, and I ) 0.5 M (NaClO4)
M ) Cob M ) Cuc M ) Nid
Hirsch et al.
pHe
kf, s-1 f
ks, s-1 f
∆Ag
pHe
kf, s-1 f
ks, s-1 f
∆Ag
pHe
kf, s-1 f
ks, s-1 f
∆Ag
3.81
3.56
3.45
3.12
2.91
2.60
2.29
18.8
24.7
30.3
45.1
68.3
0.0283
0.0266
0.0258
0.0258
0.0262
0.0266
0.0263
0.054
0.103
0.079
0.100
0.101
0.112
0.184
2.86
2.50
2.27
2.04
1.89
1.64
1.48
1.36
22.9
34.5
47.9
66.8
86.4
136
187
229
0.0220
0.0236
0.0235
0.0242
0.0234
0.0234
0.0225
0.0231
0.030
0.114
0.119
0.218
0.236
0.307
0.374
0.487
3.70
2.63
2.34
1.61
1.30
0.065
0.101
0.148
0.548
1.09
0.0194
0.0241
0.0252
0.0269
0.0248
0.017
0.049
0.045
0.051
0.085
101
a In the stopped-flow experiment, the solution containing the ligand Hamac and an excess of the metal perchlorate at pH 5-6 was mixed with
perchloric acid of variable concentration. b [Co2+] ) 1 × 10-2 M, [Hamac] ) 7.5 × 10-5 M, monitoring at 275 nm. c [Cu2+] ) 5 × 10-4 M,
[Hamac] ) 7.5 × 10-5 M, monitoring at 325 nm. d [Ni2+] ) 7.5 × 10-3 M, [Hamac] ) 7.5 × 10-5 M, monitoring at 275 nm. e pH after mixing.
f As obtained by computer-fitting of eq 6 to the exponential absorbance/time data. g Initial drop in absorbance; too fast to be followed by the
stopped-flow technique.
which is too high to make sense.42 The interpretation of the
pH dependence of k2 on the basis of reactions (13)-(15) is
therefore rejected. The interpretation based on Scheme 1 is
obviously more appropriate. Support for this comes also from
the comparison of the parameters K1 and kE/k-E. Complex
formation constant K1, as determined by spectrophotometric
titration (see Experimental Section), should agree with the ratio
kE/k-E, as obtained kinetically from the study of complex
formation and dissociation, respectively. The data listed in
Table 9 show that K1 and kE/k-E differ by factors of about 4
(Ni), 2.4 (Co), and 3.4 (Cu), which are acceptable in view of
the different methods applied.
Figure 6. Plot of the experimental rate constant kf vs log[H+] according
to eq 12 for the fast step of the acid dissociation of the species Co-
(amac)+ at 298 K.
In a general sense, the results of the present study are in line
with those of earlier studies on complex formation with
â-diketones.1 It is found that the enolate of Hamac reacts faster
with a given metal than the enol, kE > kHE, and that there is a
strong metal effect on the rate of complexation with both species,
namely Cu2+ . Co2+ > Ni2+. The more specific questions to
be addressed concern the aspect of rate-control through water
exchange on the metal (Eigen-Wilkins mechanism) and the
aspect of first bond formation versus second bond formation
(chelate ring closure).
leads to rate constants k2 and k0.39 According to Scheme 1, the
observed pH dependence of k2 is due to the participation of the
enolate in complex formation. Alternatively, the dependence
of k2 on pH could also be due to the mono hydroxo species
M(OH)+ being involved. The acidity of the hydrated cations
M
2+, as described by their pK* values,40 increases from Ni2+
-
(10.3) to Co2+(9.7) to Cu2+(7.7).41 At least for copper it is
therefore conceivable that complex formation takes place
according to (13)-(15). For [M2+] ) constant ) [M2+]0, [H+]
Rate constants kex for water exchange on the metal are listed
in Table 9. In the framework of the Eigen-Wilkins mechanism
for complex formation with monodentate ligands X it is expected
that ki, the rate constant for the interchange step (X substitutes
coordinated H2O), is close to kex. In the present study, ki,HE
can be estimated according to kHE ) ki,HEKos (see Table 9). The
ratio ki,HE/kex ranges from 1.9 × 10-3 (Ni) to 6.8 × 10-5 (Cu).
It is much too small to be indicative of solvent exchange
controlling the rate of the reaction of HE with the metal. Rate
constant kE however leads to ki,E/kex ) 0.32 for copper, which
would allow us to invoke solvent exchange to control the
reaction of the enolate with copper. Surprisingly, for cobalt
K*
M
2+ y
z M(OH)+ + H+
(13)
(14)
(15)
kHE
M
2+ + HE 8 ME+ + H+
kOH
M(OH)+ + HE 8 ME+ + H2O
) constant g 10-5 M, and [M(OH)+] , [M2+], the correspond-
ing rate law is given by (16). Rate constants kHE and kOH, as
(42) Rate constant kex for the exchange of coordinated water is a measure
for the reactivity of hydrated metal ions Mq+ toward ligand substitution.
The ratio k(M(OH)2+)/kex(M3+) is found to be on the order of 500 for
M ) Cr, Fe, and Ga.43 To our knowledge there are no data available
for water exchange in divalent metal cations and the corresponding
monohydroxo species, i.e. for the ratio kex(M(OH)+)/kex(M2+).
(43) Van Eldik, R. Ed. Inorganic High Pressure Chemistry; Studies in
Inorganic Chemistry 7; Elsevier: Amsterdam, 1986; pp 82 and 85.
(44) Ducommun, Y.; Zbinden, D.; Merbach, A. E. HelV. Chim. Acta 1982,
65, 1385.
(45) Powell, D. H.; Helm, L.; Merbach, A. E. J. Chem. Phys. 1991, 95,
9258.
(46) The calculated ion pair formation constants KOS for divalent 3d cations
pairing with singly charged anions are 0.81 M-1 for I ) 0.1 M and
0.54 M-1 for I ) 1.0 M for the medium water and 298 K.47
(47) Jordan, R. B. Reaction Mechanisms of Inorganic and Organometallic
Systems; Oxford University Press: Oxford, England, 1991; p 33.
d[ME+]/dt ) k2[M2+]0([HE]0 - [ME+])
k2 ) kHE + kOHK*/[H+]
(16a)
(16b)
obtained from the plot of k2 vs K*/[H+], are also listed in Table
9 for M ) Cu. The ratio kOH/kHE is found to be 2.3 × 104,
(39) Rate constant k0, describing the back reaction (k0 ≡ k-HE[H+]/KMEH),
is obtained from the intercept with large limits of error, so that the
detailed discussion of its size does not make much sense.
(40) The parameter pK* describes the equilibrium M2+(aq) h M(OH)+
+
H+.
(41) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum
Press: New York, 1976; Vol. 4, p 6.

Co, Ni, and Cu Complexes with Hamac
Inorganic Chemistry, Vol. 35, No. 8, 1996 2351
and nickel the ratio ki,E/kex is smaller by about one order of
magnitude (0.039 and 0.048, respectively). Considering the pK*
values of the aqua metal ions under study, one is tempted to
suggest that the reaction of the enolate with the M2+ cation is
“acidity-assisted” in the sense that formation of the first M-O
bond with the enolate is initiated and facilitated by the formation
of a more or less strong hydrogen bond between a coordinated
water molecule and the attacking enolate. This would at least
explain as to why the ratio ki,E/kex is highest for copper (pK* )
7.7) and lower for cobalt (pK* ) 9.7) and nickel (pK* ) 10.3).48
In summary, the ratio ki,E/kex ) 0.32 for copper could suggest
the operation of the Eigen-Wilkins mechanism with formation
of the first Cu-O bond with the enolate E- being controlled
by the rate of water exchange on the copper. The low values
of ki,E/kex obtained for cobalt and nickel however are not in line
with this interpretation and point to intermolecular hydrogen
bond formation as being rate-affecting.
easily the mono hydroxo species, which is able to support the
breaking of the hydrogen bond stabilized cyclic enol.
The acid hydrolysis of the species M(amac)+ ) ME+ is
triphasic. Very fast proton addition, leading to the cation
MEH2+, is followed by the dissociation of MEH2+ and ME+ to
form M2+ and the enol HE, which finally ketonizes. The pH
dependence of the dissociation reaction leads to the acid
dissociation constant KMEH (see Scheme 1) for copper (KMEH
) 0.22 M) and cobalt (KMEH ) 0.0038 M). In the case of nickel
one can estimate KMEH g 0.5 M. These data mean that the
species Co(amac)+ is the most basic one and that the species
Ni(amac)+ and Cu(amac)+ are much weaker proton acceptors.
From the chemical point of view one has to assume that the
cation MEH2+ corresponds to a species in which the enol Hamac
coordinates in a monodentate fashion. This unstable species is
involved in complex formation as well as in complex dissocia-
tion as an intermediate.
For the reaction of the enol HE with the metal it is found
that ki,HE/kex , 1, which means that the rate of this reaction is
obviously not governed by the rate of solvent exchange on the
metal cation. In addition, the ratio ki,HE/kex is more strongly
metal-dependent than the ratio ki,E/kex (see Table 9). The
existence of the kHE pathway as such indicates, however, that
the rate-determining step occurs prior to the formation of the
second M-O bond with the entering ligand. The question of
why the formation of the first M-O bond with the enol HE
occurs so slowly is as open to speculation as it has been all the
time.1 It is reasonable to assume that the enol HE is substan-
tially stabilized by intramolecular hydrogen bond formation,
producing a six-membered heterocyclic ring. The symmetry
of this ring and the charge distribution within it will be rather
close to the symmetry and charge delocalization in the mono
complex species ME+. This means that the reaction of the enol
HE with the metal corresponds to the substitution of the cation
H+ by the cation M2+. From the kinetic point of view, this
process is found to be a second-order reaction. Its slowness
cannot simply be due to the fact that the enol is a poor entering
ligand.49 If that were the case, the ratio kHE/kex should not be
so much metal-dependent as it is. Some characteristic property
of the metal has to be involved, and one might again suggest
that the different acidity of the hydrated metal cations is
essential. The most acidic copper ion is the one providing most
Conclusions
The â-diketone Hamac, a modified acetylacetone with a
N-acetylamido group in the 3-position, is very soluble in water,
strongly enolized ([enol]/[ketone] ) 2.4), and rather acidic (pKa
) 7.1). Hamac coordinates metal ions such as Cu2+ in a normal
fashion, i.e. via the two diketo oxygens. It follows from the
kinetic investigation of the formation of the mono complex
M(amac)+ with M ) Ni, Co, and Cu at variable pH that the
enol tautomer of Hamac, HE, and the enolate anion, E-, are
the reactive species to form M(amac)+, with second-order rate
constants kE > kHE. In both, kHE and kE are strongly metal-
dependent, the order of reactivity being Ni < Co , Cu. The
kinetic data suggest that (i) in the reaction of the divalent metal
ions M2+ with both the enol HE and the enolate E- formation
of the first M-O bond is rate-controlling and (ii) the acidity of
the coordinated water in the cations Maq2+ is rate-affecting. The
study of the acid dissociation leads to the equilibrium constant
KMEH for the protonation of the complex ME+. The protonated
species MEH2+ with Hamac coordinated in a monodentate
fashion is an intermediate in both complex formation and
complex dissociation.
Acknowledgment. Sponsorship of this work by the Deutsche
Forschungsgemeinschaft, Verband der Chemischen Industrie,
e.V., and Otto-Ro¨hm-Stiftung is gratefully acknowledged. The
authors appreciate the expertise of Dr. Klaus Ju¨rgen Wannowius
in computer handling of the kinetic data.
(48) Interestingly, the ratio kE/kHE also follows qualitatively the acidity of
the aqua cations in that kE/kHE for copper is by a factor of 183 and 23
larger than for nickel and cobalt, respectively.
Supporting Information Available: Tables S1-S4, listing com-
plete crystallographic data, calculated coordinates of hydrogens,
distances and angles, and thermal parameters (5 pages). Ordering
information is given on any current masthead page.
(49) It is of interest to note that rate constants kHE for the reaction of Ni2+
ions with the enol tautomers of Hacac, Hamac, Hhptd,20 Htfpd,20 and
Htftbd20 lie within the narrow range of 1.7 (Htfpd) to 19.3 (Hacac)
M-1 s-1 (298 K).50
(50) The data for kHE were taken from Table 2 of ref 1.
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