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)62+ and Ni(DMF)5Cl+ Horst Elias,* Ru1 diger Schumacher, Jo1 rg 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 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,Ndimethylformamide). 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 spectrophotometry was used to study the kinetics of complex formation of the cation Ni(DMF)62+ 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-nbutylammoniumperchlorate). 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 the intermediates Ni(TMC)int2+ 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 for the one-step rearrangement of Ni(TMC)int2+ and the two-step rearrangment of Ni(TEC)int2+ 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.
13C
Introduction 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.
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 rearrangement 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 (1) Elias, H. Coord. Chem. ReV. 1999, 187, 37.
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. 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(H2O)5(NH3)2+, Ni(H2O)3(NH3)32+, and Ni(H2O)(NH3)52+ is faster by factors of about 8,3a 78,3a and 134,3b respectively, compared to the exchange in the hexaaqua ion Ni(H2O)62+. 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.
10.1021/ic991139h CCC: $19.00 © 2000 American Chemical Society Published on Web 03/28/2000
1722 Inorganic Chemistry, Vol. 39, No. 8, 2000 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 (tetraN-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+ and Ni(DMF)5Br+, as compared to the fully solvated cation Ni(DMF)62+.
Ni(DMF)62+ + L f NiL(DMF)x2+ + (6 - x)DMF (3)
Elias et al. 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 performed 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). Spectrophotometric Titration. Equilibrium constants K for addition reactions according to eq 5 with X ) DMF, Cl-, or Br- were determined by spectrophotometric titration. 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])
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 variabletemperature 13C NMR technique and compared to the hexa solvated cation Ni(DMF)62+. kex
Ni(DMF)5Cl+ + 5DMF* y\z Ni(DMF*)5Cl+ + 5DMF (4) Experimental Section Chemicals. TBAClO4 (tetra-n-butylammoniumperchlorate), TBABr (tetra-n-butylammoniumbromide), TBACl‚H2O (tetra-n-butylammoniumchloride 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,Ndimethyl[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 Warning! Perchlorate salts and organic solutions of such salts are potentially explosiVe. They should be handled in small quantities and with caution. Ligands. TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)5a and TEC‚4HBr (1,4,8,11-tetraethyl-1,4,8,11-tetraazacyclotetradecane tetrahydrobromide)6 were prepared as described in the literature. 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. Solutions. The solutions for NMR work were prepared by weighing, and their concentration is therefore given in molality m (mol kg-1). (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.
(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 (pseudofirst-order).
A ) {(Ao - A∞)/(1 + k[metal]ot)} + A∞ A ) {(Ao - A∞)(z - 1) exp(-βt)}/{z - exp(-βt)} + A∞ A ) (Ao - A∞) exp(-kobsdt) + A∞
(7) (7a) (8)
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 β ) 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). m
∑A exp(-k
A)[
i
obsd,it)]
+ A∞
(9)
i)1
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. The kinetics of DMF exchange on Ni(DMF)62+ and Ni(DMF)5Cl+ 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 highfield 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)62+ and Ni(DMF)5Cl+ in DMF Ni(DMF)62+ NMR literature results 298, s-1
kex
∆Hq, kJ mol-1
Results Formation of Nickel Halo Complexes in the Systems Ni2+/ Cl-/DMF and Ni2+/Br-/DMF. The calorimetric and spectrophotometric 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 β1 ) 2.85 (β1 ) 708 M-1), log β2 ) 3.76, log β3 ) 5.53, and log β4 ) 7.40 (298 K; I ) 0.4 M Et4NClO4). Pilarczyk and Klinszporn9b reported β1 ) 1380 M-1 for the formation of the octahedral monochloro species Ni(DMF)5Cl+ according to eq 10 at 298 K.
Ni(DMF)5Cl+
1H/17O
∆Sq, J mol-1 K-1
this workd
this worke
3.8 × 6.9 × 103 b (3.7 ( 0.4) × 103 (5.3 ( 1) × 105 7.7 × 103 c 62.9 a 54.4 b 59.3 ( 5 42.4 ( 4 39.3 c +33.5 a +25.0b +22.3 ( 14 +6.7 ( 15 -37.7 c 103 a
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 β1 )
[Ni(DMF)5Cl+] [Ni(DMF)52+][Cl-]
(10)
One calculates that, for β1 ) 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(DMF)62+. The equilibrium constant β1 for the formation of the monobromo species Ni(DMF)5Br+ in the system Ni2+/Br-/DMF is reported to be 631 M-1 at 298 K.9b Spectrophotometric titration of Ni(DMF)62+ with TBABr in DMF led to the considerably smaller value of β1 ) 133 ( 10 M-1.10 13C NMR Investigation of Solvent Exchange at Variable Temperature. The 13C NMR spectra of the cation Ni(DMF)62+ (Sol. 1) were recorded between 193 and 395 K. At lower temperatures (
