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Solvent exchange, solvent interchange, aquation and isomerisation reactions of cis- and trans-[Co(tmen)2(NCMe)2]3+ in water, Me2SO and MeCN: kinetics and stereochemistry
Inorganica Chimica Acta 357 (2004) 665–676 www.elsevier.com/locate/ica
Solvent exchange, solvent interchange, aquation and isomerisation reactions of cis- and trans-[Co(tmen)2(NCMe)2]3þ in water, Me2SO and MeCN: kinetics and stereochemistry W. Gregory Jackson *, A.F.M. Mokhlesur Rahman, M. Anthony Wong School of Chemistry, University College, The University of New South Wales, Australian Defence Force Academy, Canberra, ACT 2600, Australia Received 17 March 2003; accepted 21 May 2003
Abstract The synthesis and characterisation of cis- and trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 are described. Solvolysis rates have been measured by both 1 H NMR spectroscopy and UV–Vis spectrophotometry in dimethyl sulfoxide at 298.2 K. The cis isomer undergoes solvolysis by consecutive first-order reactions, k1 ¼ 5.61 104 and k2 ¼ 5.35 104 s1 , each with steric retention. The measured solvolysis rate (single step reaction) for the trans isomer is k ¼ 1.54 105 s1 . The solvent exchange rates have been measured by 1 H NMR spectroscopy in CD3 CN at 298.2 K: kex (cis) ¼ kct + kcc ¼ 2.0 105 and kex (trans) ¼ ktc + ktt ¼ 4.56 106 s1 . From these data, the measured cis–trans isomerisation rate (1.71 106 s1 ) and equilibrium position in CH3 CN (17% trans), the steric course for substitution in the exchange processes has been determined: trans reactant – 69% trans product; cis reactant – 99% cis product. Aquation rates for cis- and trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 have also been determined spectrophotometrically and by NMR; kcis ¼ 1.3 104 and ktrans ¼ 2.7 105 s1 . In both cases the steric course for the primary aquation step is indeterminate because the subsequent steps are faster. Where data are available, the [Co(tmen)2 X2 ]nþ complexes are found to be consistently much more reactive than their [Co(en)2 X2 ]nþ analogues. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Cobalt(III); Synthesis; Kinetics; Mechanism; Acetonitrile; Hydrolysis
1. Introduction Ethylenediamine (en) has been one of the most used ligands in coordination chemistry, especially in Co(III) chemistry for kinetic and other studies [1,2]. A newer and similar ligand is tetramethylethylenediamine or 2,3dimethylbutane-2,3-diamine (tmen), which is en with all four backbone CHs replaced by methyl groups. It has been rarely employed as a ligand in coordination chemistry, no doubt due to its tedious multi-step synthesis [3–5] and also because there was little expectation it would bring surprising new chemistry. This has proved far from the truth. Ethylenediamine easily undergoes oxidative dehydrogenation when it coordinates with reactive high*
Corresponding author. Fax: +61-2-62688090. E-mail address: [email protected] (W.G. Jackson).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2003.05.010
valent ruthenium and osmium-oxo complexes [6], and tmen has been employed as a means of circumventing this difficulty because per-methyl substitution at carbon removes the source of the problem. This has resulted in the discovery that tmen can stabilise otherwise very reactive species for a variety of high valency ruthenium and osmium complexes [7–10]. Other interesting chemistry of tmen has been observed for the hexamine Co(III) complex [Co(tmen)3 ]3þ [5,11–13]. This pink ion exhibits two ligand-field bands at 504 and 362 nm, substantially red-shifted compared to the classic yellow [Co(en)3 ]3þ species, with maxima at 470 and 340 nm [5]. Further, the base catalysed hydrolysis rate for this ion was found to be at least nine orders of magnitude larger than for the parent [Co(en)3 ]3þ complex. The labile kinetic behaviour of [Co(tmen)3 ]3þ towards base catalysed hydrolysis was attributed to steric
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crowding imposed by the 12 peripheral methyl groups [11]. Further support for the strain-induced enhancement of reactivity was reflected in the data obtained for the less sterically hindered ion [Co(tmen)2 en]3þ (which incidentally is not pink, but the yellow colour typical of normal hexaaminecobalt(III) complexes). The partial relief of strain was reflected in the kinetics of its base hydrolysis; the complex is reported to be two orders of magnitude less labile than the [Co(tmen)3 ]3þ complex. Other research into Co(III)–tmen complexes has exposed the unexpected. Amongst these were the synthesis of stable hydrido and sideways-bonded peroxo complexes involving a Co(tmen)2 backbone [14], previously unknown for any other tetraaminecobalt(III) template. Few kinetic studies have been carried out on Co(III)– tmen complexes. The earliest and perhaps the only other example was the work by Pearson et al. [15]. They studied the kinetics of aquation of a series of compounds of the type trans-[Co(AA)2 Cl2 ]þ , in which the steric properties of the bidentate diamine ligand (AA) were varied (ring size and C-alkyl substitution). It was found that an increase in ring size or steric crowding of the bidentate ligand was accompanied by an increase in the rate of aquation. For [Co(tmen)2 Cl2 ]þ , the aquation was so rapid that it could not be measured even at low temperatures. Our work has confirmed this observation. The present study on the substitution reactions of [Co(tmen)2 X2 ]nþ complexes was undertaken to quantify the kinetic effects of backbone C–Me substitution. To slow the rates to those we could measure, we chose MeCN as the leaving group. This normally binds tightly to Co(en)2 moieties and is substituted on the timescale of days to weeks. With tmen, the timescale is more conveniently min. We report herein a variety of rates for substitution, for H2 O, Me2 SO and MeCN as solvents, and including trans to cis isomerisation and solvent exchange. The stereochemistry of substitution, in so far as it could be determined, is also reported.
2. Experimental All chemicals were AnalaR or an equivalent grade. Carbon-13 and proton NMR spectra were recorded on Varian XL-300 and Unity Plus 400 MHz instruments at 20 °C. Solvents used were D2 O with dioxane as the internal reference (13 C, d 69.27 relative to DSS), and Me2 SO-d6 with the central peak of the CD3 septet as the reference (13 C, d 39.37 relative to SiMe4 ). Full visible absorption spectra and absorbance-time traces were recorded on a HP8452A diode array UV–Vis spectrophotometer thermostated to 25.00 0.05 °C with use of a Lauda RM6 circulating water bath. Spectra saved as. kd files were converted to .txt files and read using SpecfitÒ to perform Global Spectral Analysis to extract the rate and spectral data. The cation exchange medium
used was Dowex 50Wx2 (Hþ form, 200–400 mesh; BioRad). Carbon dioxide free Milli-Q water was used for all physical measurements. Evaporations were carried out on a B€ uchi rotary evaporator at temperatures less than 40 °C. Kinetic studies were performed on both the cis and trans isomers of [Co(tmen)2 (NCCH3 )2 ](ClO4 )3 using NMR and UV–Vis spectrophotometry in Me2 SO-d6 , MeCN-d3 and in acidified D2 O/dioxane using welldocumented techniques [16]. Kinetic data obtained from successive 1 H NMR spectra utilised peak heights of selected resonances which were normalised with respect to a reference signal or set of signals (usually the sum of the peak heights for bound and free MeCN). Rate constants were calculated using standard nonlinear least squares fitting procedures (with IgorÒ or KaleidographÒ ). 2.1. Synthesis of 2,3-Dimethylbutane-2,3-diamine (tmen) The following is a modification of the procedure described by Hendry and Ludi [5]. 2.1.1. [O2 NC(CH3 )2 C(CH3 )2 NO2 ] 2-Nitropropane (356 g, 4.0 mol) was stirred with NaOH (8 M, 500 mL) in a round-bottomed flask cooled in ice. Aqueous bromine (320 g, 4.0 mol) was added from a pressure equalised dropping funnel over a period of 3–4 h, and after a further 12 h ethanol was added (300 mL) and the mixture was refluxed for 2 h. Upon cooling, fine white crystals of 2,3-dimethyl-2,3-dinitrobutane were obtained. They were filtered, washed with cold ethanol and air dried. Yield: 86%. 2.1.2. [H3 NC(CH3 )2 C(CH3 )2 NH3 ](NO3 )2 The crude dinitro compound (70.4 g, 4.0 mol) was placed in a round-bottomed flask equipped with a condenser and conc. HCl (300 mL) was added. The slurry was vigorously stirred and heated to 60 °C. Tin (300 g; 20 mesh) was gradually added and conc. HCl (300 mL) was used to rinse the metal from the sides. The resulting mixture was boiled for 30 min under reflux, and then cooled in ice and NaOH (320 g) was added while thoroughly stirred. The resultant tmen was distilled into an ice cooled flask containing HNO3 (8 M, 100 mL). The distillate was rotaevaporated to a third volume and the resultant fine white crystals of the acid nitrate salt were collected, washed with cold ethanol and ether, and air dried. Yield: 82%. 1 H NMR (Me2 SO-d6 ): d 1.30 (12H, CH3 ), d 8.13 (4H, NH2 ); 13 C NMR: d 21.8 (4C, CH3 ), d 57.7 (2C). 2.2. cis- and trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 2.2.1. Na3 [Co(O2 CO)3 ][17,18] Co(NO3 )2 6H2 O (29.1 g, 0.10 mol) in water (50 mL) and excess hydrogen peroxide (30%, 10 mL) was added
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dropwise to a slurry of sodium bicarbonate (42.0 g, 0.50 mol) in H2 O (50 mL). The mixture was allowed to stand at 0 °C for 1 h with continuous stirring. The olive product was filtered and washed cold distilled water (5 10 mL), then thoroughly washed with absolute ethanol and dry ether. Yield: 85%. 2.2.2. [Co(tmen)2 Cl2 ]Cl Excess Na3 [Co(CO3 )3 ]0 (65 g, 0.2 mol) was added to tmen nitrate (24.2 g, 0.10 mol) in water (500 mL). The mixture was stirred on a water bath (60 °C) overnight. The mixture was filtered to remove excess Na3 [Co(CO3 )3 ] yielding a reddish-purple solution. Conc. HCl (25 mL) was added to the solution and the mixture warmed on a water bath (60 °C) for 2 h. After evaporation to dryness, the purple solid was dissolved in distilled water and loaded onto a Dowex column. After flushing thoroughly with water, the reddish-purple band of the bis(tmen) compound was eluted using 3 M HCl (the diaqua, aquachloro and dichloro ions elute together because they are in rapid equilibrium). The eluate was evaporated to dryness on a rotary evaporator, and the dried solid was placed in a Soxhlet extractor thimble and the [Co(tmen)2 (Cl2 )]Cl extracted using methanol (ca. 500 mL). The resultant green solution was evaporated to dryness to yield green/ purple cis-/trans-[Co(tmen)2 Cl2 ]Cl. No isomer separation was necessary for the subsequent synthesis, although it has been achieved. Yield: 80%. 1 H NMR (CD3 OD; 100% trans): d 1.32 (24H, CH3 ), d 4.93 (8H, NH2 ); 13 C NMR: d 28.9 (8C, CH3 ), d 65.5 ppm (4C). 2.2.3. [Co(tmen)2 (OSO2 CF3 )2 ]CF3 SO3 [Co(tmen)2 Cl2 ]Cl (6 g, 0.02 mol) was gradually added to triflic acid (25 mL). The reddish-purple solution was stirred continuously and warmed (60 °C) to drive out HCl until there was no residual Cl in solution (Agþ test). The cooled solution was added dropwise to ether (1 L) to yield a gelatinous red precipitate that was filtered and washed generously with more ether and dried in vacuo. Yield: 92%. The bound triflate is easily substituted by solvent, thus the 1 H and 13 C NMR spectra correspond to the respective cis-[Co(tmen)2 (sol)2 ]3þ species in D2 O or Me2 SO-d6 .
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(5 °C) for 10 min were filtered, washed with isopropanol and ether, and air dried. Yield: 89%. The product is invariably contaminated with about 15% of the less soluble trans isomer which is difficult to completely remove by direct recrystallisation from water. The product was extracted with dry methanol to leave a residue of insoluble trans isomer. The solvent was quickly removed in vacuo by rotaevaporation and the product recrystallised from water. A small sample of pure trans isomer was also obtained by a similar recrystallisation. Anal. Calc. for CoC16 H42 N6 O12 Cl3 H2 O: C, 27.70; H, 6.39; N, 12.11. Found: C, 27.4; H, 6.1; N, 12.1%. UV–Vis (MeCN): e (max) 473 nm, 180.5; e (max) 351 nm, 144.5 M1 cm1 . 1 H NMR (Me2 SOd6 ): d 6.95, 2H, d (NH); 5.88, 2H, d (NH); 5.80, 2H, d (NH); 3.40, 2H, s (lattice H2 O); 2.68, 6H, s (CH3 –CN); 1.38, s, 6H, (CH3 ); 1.25, s, 6H (CH3 ); 1.15, 6H, s (CH3 ); 1.10, 6H, s (CH3 ). 13 C NMR (Me2 SO-d6 ): d 132.0, 2C (CN); 68.0, 2C (Ctert ); 63.5, 2C (Ctert ); 26.2, 2C (CH3 ); 26.0, 2C (CH3 ); 25.0, 2C (CH3 ); 24.8, 2C (CH3 ); 4.0 ppm, 2C (CH3 –CN).
2.2.5. trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 cis-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 (0.5 g) was added to acetonitrile (5 mL) in a test tube to form a slurry. The test tube was placed in a waterbath at 70 °C and the slurry was allowed to evaporate slowly to dryness by loosely sealing the tube with parafilm. Solid samples so obtained were tested for trans isomer content using 1 H NMR (Me2 SO-d6 ). The evaporation process was repeated until a purity of greater than 95% trans was obtained. The final solid was recrystallised as described for the cis isomer to yield yellow crystals of pure trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 . Anal. Calc. for CoC16 H42 N6 O12 Cl3 H2 O: C, 27.70; H, 6.39; N, 12.11. Found: C, 27.8; H, 5.9; N, 12.0%. UV–Vis (MeCN): e (max) 448 nm, 100.5; e (sh) 350 nm, 94.5 M1 cm1 . 1 H NMR (Me2 SO-d6 ): d 5.97, 8H, s (NH); 3.41, 2H, s (lattice H2 O); 2.63, 6H, s (CH3 –CN); 1.30, s, 24H, (CH3 ). 13 C NMR (Me2 SO-d6 ): d 138.0, 2C (CN); 65.8, 4C (Ctert ); 25.8, 8C (CH3 ); 5.1, 2C (CH3 – CN) ppm.
3. Results 2.2.4. cis-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 Dried [Co(tmen)2 (OSO2 CF3 )2 ]CF3 SO3 (10 g, 0.02 mol) was dissolved in acetonitrile (200 mL) and warmed for 2 h to yield an orange solution. The solution was evaporated to dryness to yield an orange solid that was recrystallised by dissolving in a minimal amount of water and adding one fifth volume of NaClO4 solution (5 M). The orange crystals after cooling
3.1. Aquation reactions 3.1.1. cis-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 Sharp, well-defined isosbestic points were observed at 380, 407 and 509 nm, which were unshifted throughout reaction through to the cis-[Co(tmen)2 (OH2 )2 ]3þ species, indicative of a single reaction step (Fig. 1).
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Fig. 1. UV–Vis spectra of cis-[Co(tmen)2 (NCCH3 )]3þ in 0.1 M HClO4 at 25 °C. The absorbance decreases with time at 450 nm.
cis-½CoðtmenÞ2 ðNCCH3 Þ2 3þ k;H2 O
! ½CoðtmenÞ2 ðOH2 ÞðNCCH3 Þ slow
3þ
þ CH3 CN
3þ
½CoðtmenÞ2 ðOH2 ÞðNCCH3 Þ H2 O
3þ ! cis-½CoðtmenÞ2 ðOH2 Þ2 þ CH3 CN fast
The 1 H NMR spectra (Fig. 2) confirmed the absence of any intermediate [Co(tmen)2 (NCCH3 )(OH2 )]3þ species. The rate constants calculated from both UV–Vis and NMR data are summarised in Table 1. 3.1.2. trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 The aquation of the trans isomer occurs with complete rearrangement of the trans configuration, i.e., the final product is cis-[Co(tmen)2 (OH2 )2 ](ClO4 )3 . This was confirmed by the 1 H NMR spectra (Fig. 3). However, the actual steric course for the primary aquation step is unknown, because all subsequent steps following the loss of the first MeCN ligand must be faster. The UV–Vis spectra show the presence of three sharp isosbestic points at 357, 400 and 478 nm (Fig. 4). The presence of isosbestic points suggests a single product or multiple products produced in constant proportions. In the present case the isosbestic points correspond precisely to the overlap in the spectra of the reactant trans isomer and cis-[Co(tmen)2 (OH2 )2 ]3þ . Thus, again, the loss of the first MeCN ligand is rate determining, and subsequent trans to cis isomerisation and hydrolysis of the second MeCN ligand are much faster. The rate data are summarised in Table 2.
Fig. 2. Representative 1 H NMR spectra for the reaction of cis[Co(tmen)2 (NCCH3 )2 ]3þ in acidified D2 O/dioxane at different time intervals. A (bound Me for CH3 CN) and B (free CH3 CN) represent the peaks used in the kinetic analysis.
3.2. Solvolysis reactions in DMSO 3.2.1. cis-[Co(tmen)2 (NCCH3 )2 ]3þ Repeated scans of the UV–Vis spectrum of cis[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 reacting in DMSO
revealed initial isosbestic points at 355, 380, 430 and 500 nm. However, all three Ôisosbestic pointsÕ shifted towards shorter wavelengths at a later stage, revealing a two-stage reaction (Fig. 5).
W.G. Jackson et al. / Inorganica Chimica Acta 357 (2004) 665–676 Table 1 Rate constants for the aquation of cis-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 in 0.01 M HClO4 Method
Temperature (°C)
k 104 (s1 )
NMR UV–Vis
25.0 0.1 25.00 0.05
1.41 0.04 1.31 0.01
The rate constants for each of the two steps were obtained by following the changes in absorbance with time, at specific wavelengths, on the UV–Vis spectro-
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photometer. The rate constants calculated are summarised in Table 3. The relative magnitudes of the rate constants clearly indicate that k1 k2 . We have noted previously [19,20] that such a kinetic situation requires special consideration, since absorbance-time data can be reproduced quite accurately by a range of k1 and k2 values that differ by less than a factor of about two. Indeed, in the limit as k2 ! k1 , the rate equation for [B] and the derived absorbance-time equation become:
Fig. 3. 1 H NMR spectra for trans-[Co(tmen)2 (NCCH3 )2 ]3þ in acidified D2 O/dioxane at different time intervals. A and B represent the methyl peaks due to bound and free CH3 CN, respectively, employed in the calculation of the rate constant.
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Fig. 4. UV–Vis spectra of trans-[Co(tmen)2 (NCCH3 )]3þ in 0.1 M HClO4 at 25 °C. The absorbance decreases with time at 450 nm.
Table 2 Rate constants for the aquation of trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 in 0.01 M HClO4 Method
Temperature (°C)
k 105 (s1 )
NMR UV–Vis
25 1.0 25.00 0.05
2.5 0.2 2.7 0.2
Table 3 Spectrophotometrically determined rate constants for the sequential substitution of CH3 CN in cis-[Co(tmen)2 (NCCH3 )2 ]3þ by DMSO
104 kslow (s1 ) 104 kfast (s1 )
Run 1
Run 2
Run 3
Average
5.26 6.32
4.96 6.59
5.47 6.12
5.2 0.3 6.3 0.2
½B ¼ A0 kt ekt D D1 ¼ A0 ½ðea ec Þ þ ktðeb ec Þ kt The fit to the data (470 nm) for the present system under the assumption k1 ¼ k2 is excellent (Fig. 6); the rate constant calculated was 5.71 104 s1 , which lies between the actual true values for kfast and kslow (see below). This is yet another good example of a kinetic system that illustrates the point that a good data fit does
not guarantee that you have analysed the system correctly [19]. An additional but related problem is that it is not straightforward to determine if k1 or k2 is kfast or kslow [20]. One resolution of the ambiguity relies upon all species in the reacting system being observed simultaneously, i.e., individually. The reaction was followed using NMR spectroscopy to resolve the ambiguity. As expected, the
Fig. 5. UV–Vis spectra of cis-[Co(tmen)2 (NCCH3 )]3þ in Me2 SO, illustrating the shift in Ôisosbestic pointsÕ. The absorbance decreases at 450 nm with time.
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Fig. 6. Graph of the changes in absorbance with time for the stepwise reaction of cis-[Co(tmen)2 (NCCH3 )2 ]3þ in Me2 SO. The fits to the absorbance-time data were made assuming k1 ¼ k2 . The results reveal a good fit to the data, with k ¼ (5.71 0.03) 104 s1 (m4 ). The good fit is misleading, and the result is incorrect (refer text). 1
H NMR spectra of cis-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 in Me2 SO-d6 show clearly the presence of the intermediate species, B (Fig. 7). The two rate constants k1 and k2 could thus be obtained by measuring the changes in the peak
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Fig. 8. Graph of the changes of concentration of cis-[Co(tmen)2 (OH2 )(NCCH3 )]3þ with time. The data were obtained by measuring the normalised peak heights of the species A, B and C in the 1 H NMR spectra (Fig. 7). The results were fitted for k1 and k2 using the relevant rate equation. The line shows a good fit to the data, with rate constants, k1 and k2 shown as m2 and m3 , respectively (in min1 ).
height of the intermediate B by the method discussed previously. The rate constant k1 was obtained independently by analysing the decay of A. The rate constants so calculated were k1 ¼ (5.61 0.06) 104 and k2 ¼ (5.35 0.02) 104 s1 (Fig. 8). Thus k1 is actually slightly greater than k2 . 3.2.2. trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 In contrast to the data for the reaction of cis[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 in DMSO, the UV–Vis spectra show the presence of sharp, well-defined isosbestic points at 424 and 478 nm for the entire reaction of the trans isomer in DMSO (Fig. 9), as for 0.1 M HClO4 as solvent. The final spectrum corresponds to pure cis[Co(tmen)2 (OSMe2 )2 ]3þ . This indicates that the solvolysis of the trans isomer in DMSO is a simple A ! C reaction scheme. The trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 complex shows characteristic changes in the 1 H and 13 C NMR spectra as it undergoes substitution by Me2 SO-d6 and isomerises to cis-[Co(tmen)2 (OS(CD3 ))2 ](ClO4 )3 without any observable intermediates (Figs. 10 and 11). The rate constant k was derived from the changes of the peak heights of coordinated acetonitrile or free acetonitrile with respect to time. The rate constants calculated from both UV–Vis spectral and NMR spectroscopic data are summarised in Table 4. trans-½CoðtmenÞ2 ðNCCH3 Þ2 k
3þ
! cis-½CoðtmenÞ2 ðMe2 SO-d6 Þ2
3þ
3.3. Solvent exchange reactions in CD3 CN 1 Fig. 7. Representative H NMR spectra for cis[Co(tmen)2 (NCCH3 )2 ]3þ in Me2 SO-d6 recorded at different time intervals. A, B and C represent the peak height measured for species A, B and C and used in the calculation of the rate constants.
3.3.1. cis-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 The rate of exchange of the coordinated acetonitrile ligand was followed by NMR. There is little cis to trans
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Fig. 9. Changes in absorption spectra for the solvolysis of trans-[Co(tmen)2 (NCMe)2 ]3þ in DMSO. The absorbance increases with time at 550 nm.
Fig. 11. Representative 13 C NMR spectra for trans[Co(tmen)2 (NCCH3 )2 ]3þ in Me2 SO-d6 at different time intervals. The spectra clearly show the isomerisation from a trans to a cis isomer without build-up of any intermediate species.
Fig. 10. Representative 1 H NMR spectra for trans-[Co(tmen)2 (NCCH3 )2 ]3þ in Me2 SO-d6 recorded at different time intervals. A and C are the peaks used in the calculation of the rate constant, k.
isomerisation and so the changes in UV–Vis spectra were not examined. The 1 H NMR spectra (Fig. 12) show that as time progresses, the peak height of the coordinated Co-
Table 4 Rate constants for the spontaneous [Co(tmen)2 (NCCH3 )2 ](ClO4 )3 in DMSO
substitution
of
trans-
Method
Temperature (°C)
k 105 (s1 )
NMR UV–Vis
25 1.0 25.00 0.05
1.03 0.06 1.54 0.03
NCCH3 ligand decreases as it is replaced by CD3 CN, while the signal for free CH3 CN grows. The exchange process is a stepwise reaction,
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Ht =Ht0 ¼ A0 ðek1 t f1 þ ½k1 =2ðk2 k1 Þg fk1 =2ðk2 k1 Þgek2 t Þ For MeCN exchange, k1 ¼ 2k2 (in the absence of isotope effects), therefore Ht =Ht0 ¼ A0 ek2 t Thus, the rate constant measured by using the coordinated MeCN peak is actually the rate constant, k2 . The exchange rate constant, kex is twice this rate constant, i.e., kex ¼ 2k2 [21,22]. The same result is obtained if the normalised area of the free MeCN peak is measured with respect to time. In the 1 H NMR spectra, the peaks for bound (*) and free (+) MeCN are labelled. The changes in the normalised peak heights with respect to time were fitted using the appropriate function, yielding k2 ¼ (1.00 0.06) 105 s1 . Thus, the MeCN exchange rate constant, kexðcÞ , for cis-[Co(tmen)2 (NCCH3 )2 ]3þ is (2.0 0.1) 105 s1 .
Fig. 12. Representative 1 H NMR spectra for cis-[Co(tmen)2 (NC CH3 )2 ]3þ in CD3 CN at different time intervals, 25 °C. * and + denote signals due to bound and free acetonitrile, respectively.
3þ
½CoðtmenÞ2 ðNCCH3 Þ2 A k1
3þ
! ½CoðtmenÞ2 ðNCCH3 ÞðNCCD3 Þ B k2
3þ
! ½CoðtmenÞ2 ðNCCD3 Þ2 C where A ¼ A0 ek1t and B ¼ ½k1 =ðk2 k1 Þðek1 t ek2 t Þ
The coordinated MeCN signal in the 1 H NMR is proportional to 2A + B. Using peak heights as a measure of area, Ht ¼ cð2½A þ ½BÞ Ht1 ¼ 0 Ht0 ¼ c2A0 where c is the proportionality constant. Thus Ht =Ht0 ¼ ½A=A0 þ ½B=2A0 Substituting, we get
3.3.2. trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 The rate of solvent exchange, kexðtÞ , was measured analogously to that for the cis isomer. The measured rate constant was (1.37 0.09) 104 min1 , thus the rate constant kexðtÞ for solvent exchange is (2.74 0.18) 104 min1 , or (4.56 0.30) 106 s1 . The isomerisation rate constant, ki (where ki ¼ kct + ktc ), was measured by UV–Vis spectroscopy utilising the changes in the absorbance as trans-[Co(tmen)2 (NC CH3 )2 ]3þ isomerises to cis-[Co(tmen)2 (NCCH3 )2 ]3þ in acetonitrile. The trans ! cis isomerisation was followed for 3t1=2 . The changes in absorbance were followed at two wavelengths, 350 and 490 nm (where these changes were relatively large). Data collected at these two wavelengths were used to calculate ki . The equilibrium position (Ktc ¼ ktc /kct ) was determined from the final spectrum using the known spectra for the pure cis and trans isomers in this solvent. The UV–Vis spectra of trans-[Co(tmen)2 (NCCH3 )2 ]3þ in acetonitrile show two not-so-well-defined isosbestic points, at 322 and 405 nm (not shown). However, it was clear from the NMR data that only trans- and cis[Co(tmen)2 (NCMe)2 ]3þ are present at any time. The rate constant, ki determined spectrophotometrically was (2.01 0.05) 106 s1 . The rate constant, ki was also determined from the 1 H NMR data (Fig. 13) by following the loss of the trans-methyl peak with time (this was normalised to the sum of the methyl peak heights for the cis- and transions and that for the free ligand; the sum is constant). The calculated rate constant was ki ¼ (1.71 0.17) 106 s1 , Fig. 14. These data, and also the 13 C NMR spectra (Fig. 15), indicate ca. 17% residual trans isomer at equilibrium.
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1 Fig. 13. Representative H NMR spectra of trans[Co(tmen)2 (NCCH3 )2 ]3þ in CD3 CN at different time intervals; * and + denote bound and free acetonitrile, respectively.
3.4. Solvent exchange and steric course of substitution
Fig. 15. Representative 13 C NMR spectra of trans-[Co(tmen)2 (NC CH3 )2 ]3þ in CD3 CN at different time intervals. The peaks marked * are the free solvent peaks. The 13 C NMR spectra clearly show the isomerisation from the trans ! cis isomer, with some (ca. 15%) residual trans-isomer at equilibrium.
The 17% equilibrium percentage of trans product determined both spectrophotometrically and by NMR compares favourably with the ca. 15% trans produced in the synthesis of cis-[Co(tmen)2 (NCCH3 )2 ]3þ .
Using the NMR value of ki ¼ 1:71 106 s1 we have, ktc þ kct ¼ ki ¼ 1:71 106 s1 ktc =kct ¼ Ktc ¼ 83=17 Thus solving for ktc and kct , we get, ktc ¼ 1:41 106 s1 kct ¼ 3:00 107 s1 From the NMR data, kexðtÞ ¼ 4:56 106 s1 kexðtÞ ¼ ktt þ ktc ktt ¼ 3:15 106 s1 transð%Þ ¼ 102 ktt =kexðtÞ ¼ 69%:
Fig. 14. Graph showing the depletion of the trans-methyl peak with time. The rate constant ki has the value of 1.02 104 min1 (m3 ), or 1.7 106 s1 , in reasonable agreement with the spectrophotometric value. The calculations also show that as t ! 1, the normalised peak height ! m1 . This shows that at equilibrium 17% trans-isomer remains.
Thus the steric course of MeCN substitution of trans[Co(tmen)2 (NCCH3 )2 ]3þ is 69% trans product. The steric course of substitution for cis[Co(tmen)2 (NCCH3 )2 ]3þ is deduced similarly,
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kex ¼ kct þ kcc ¼ 2:00 105 s1 kct ¼ 3:00 107 s1
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in water strongly favours the cis form (99%), yet all trans is obtained on slow evaporation [26]. 4.2. Equilibria
Thus, kcc ¼ 1:97 105 s1 2
cisð%Þ ¼ 10 kcc =kex ¼ 99% Thus the steric course of MeCN substitution of cis[Co(tmen)2 (NCCH3 )2 ]3þ is 99 % cis product.
4. Discussion 4.1. Synthesis The ligand was obtained as described [5], save for the use of HNO3 to directly quench the steam distilled diamine to prevent substantial losses. The synthetic methods for the complexes are an extension of those successful in bis(ethylenediamine) chemistry [23,24]. Thus cis- or trans-[Co(tmen)2 Cl2 ]Cl, in neat CF3 SO3 H readily yielded [Co(tmen)2 (O3 SC F3 )2 ]CF3 SO3 which is all cis. The complex in acetonitrile gave [Co(tmen)2 (NCCH3 )2 ](CF3 SO3 )3 quantitatively. Recrystallisation from water as the perchlorate salt consistently yielded cis complex contaminated with a little trans isomer. We found that the cis isomer is selectively extracted by methanol, leaving the insoluble trans form. After the rapid removal of solvent (the cis decomposes on standing in alcohol for a few hours), each was recrystallised to purity from water. We discovered a quantitative synthesis of trans[Co(tmen)2 (NCCH3 )2 ]3þ some years ago but which we could not repeat. It transpires that the cis/trans equilibrium in acetonitrile favours the cis form, but not exclusively so (ca. 17% at 25 °C, and somewhat more, ca. 30%, at reflux temperature). Cis/trans isomerisation is not fast, but if a solution at equilibrium is allowed to slowly evaporate, the less soluble trans isomer crystallises preferentially, and the entire mixture can be milked of the minor trans form if the conditions are controlled. Rapid evaporation gives largely the more abundant cis form. This process is akin to a second-order asymmetric transformation, whereby the less soluble form can be crystallised quantitatively if crystallisation is slow compared to the rate of diastereomer interconversion. The chiral [Co(en)2 (R)cys]þ system is a good example; the Kisomer is obtained quantitatively from a D=K mixture which under equilibrium conditions favours the D form (ca. 70%) [25]. The principle is precisely the same for achiral systems, as here, where two chemically distinct species are in rapid equilibrium, but the minor and less soluble component crystallises first. The K[Cr(ox)2 (OH2 )2 ] system is the classic example – the equilibrium
The extraordinary lability of the K-[Co(tmen)3 ]3þ ion in aqueous base has been attributed to steric congestion arising from interligand Me Me interactions [11]. Based on our recent work with the tmen complexes [27], we no longer believe the reactivity is steric in origin. The [Co(tmen)2 X2 ]þ complexes are not anomalous in their thermodynamic preference for the cis or trans configuration. In water and in Me2 SO, the cis-[Co(tmen)2 (sol)2 ]3þ complex predominates over trans-[Co(tmen)2 (sol)2 ]3þ which is undetectable, whereas in CH3 CN as solvent, the preference is not quite as pronounced (83% cis) [28]. In methanol, the reverse is found, with the trans-[Co(tmen)2 Cl2 ]þ isomer (>95%) predominating over the cis. This behaviour closely mimics the [Co(en)2 X2 ]nþ chemistry [29], and the relative isomer stabilities in both systems can be accounted for in terms of solvent properties, i.e., solvation [22,30]. The two tmen ligands are remote in trans-[Co(tmen)2 X2 ]nþ and it now seems clear that there are also no significant steric interactions for the cis configuration. In summary, en and tmen show little difference in the relative stabilities of their cis and trans forms; the equilibria can be swung totally from trans predominant to cis predominant merely by raising the polarity of the solvent (which favours the dipolar cis isomer through increased solvation) [31]. There is therefore no support from relative stability considerations for the perceived steric congestion in the cis-[Co(tmen)2 X2 ]nþ complexes and likely the [Co(tmen)3 ]3þ species as well. 4.3. Reactivity Trans-[Co(tmen)2 Cl2 ]þ is over 106 -fold more reactive than trans-[Co(en)2 Cl2 ]þ . For cis-[Co(tmen)2 Cl2 ]þ , a similar rate enhancement is evident; this complex also hydrolyses ÔinstantlyÕ in dilute aqueous acid [14]. Prior to this work, there were no quantified rate parameters for substitution reactions of tmen complexes. We chose acetonitrile as the leaving group because it binds tightly and thus the rate of substitution of its tmen complexes might be slowed to the point of being measurable. This proved to be the case. The cis- and trans-[Co(tmen)2 (NCCH3 )2 ]3þ complexes solvolyse in DMSO and water sequentially, at rates which are on the minutes–hours timescales at 25 °C. They were followed spectrophotometrically and also by 1 H and 13 C NMR; the kinetic data obtained by the different techniques were in excellent agreement. The first formed product, cis-/trans-[Co(tmen)2 (NC CH3 )(sol)]3þ , is observed directly for the reaction of cis[Co(tmen)2 (NCCH3 )2 ]3þ in DMSO, and in this case the
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product is 100% cis-[Co(tmen)2 (NCCH3 )(OSMe2 )]3þ , which decays further to cis-[Co(tmen)2 (OSMe2 )2 ]3þ at coincidentally almost the same rate, despite the statistical advantage (2) that the bis(acetonitrile) complex has. For all the other reactions studied (except the exchange processes), the first formed product reacted much more rapidly than it was generated, in losing the second CH3 CN ligand. The role of the Ôorientating groupÕ A in [Co(en)2 (A)X]nþ complexes has been studied extensively in the past [21,32] and has a dramatic effect on the rate at which X is substituted. For the present set of bis(tmen) complexes, replacement of CH3 CN by H2 O increases substantially (>10-fold) the rate of loss of the residual CH3 CN leaving group in the cis complex, whereas DMSO replacement of CH3 CN raises the rate only by a factor of about two. cis-[Co(tmen)2 (NCCH3 )2 ]3þ isomer is more reactive than the trans isomer by a factor of about 5 for the water and acetonitrile solvent systems, and about 40 for DMSO as solvent. The apparent single step substitution reactions for trans-[Co(tmen)2 (NCCH3 )2 ]3þ in water and DMSO therefore are not surprising, since any first formed trans-[Co(tmen)2 (NCCH3 )(sol)]3þ now bears a good leaving group, H2 O or DMSO, and such species can rapidly isomerise, through water or DMSO exchange as appropriate, to generate the more reactive cis[Co(tmen)2 (NCCH3 )(sol)]3þ which then loses the nitrile ligand to generate the observed cis-[Co(tmen)2 (sol)2 ]3þ . The isomerisation and exchange reactions for the [Co(tmen)2 (NCCH3 )2 ]3þ isomers in acetonitrile as solvent, provide meaningful steric course data. 1 H NMR spectroscopy was used to probe the exchange process and UV–Vis spectrophotometry to measure the rates for the trans–cis isomerisation processes. The cis isomer substitutes with substantial retention in dissociating its coordinated CH3 CN, whereas for the trans isomer there is substantial rearrangement (31%) in the CD3 CN reentry process. There are no rate or stereochemical data for the corresponding cis- and trans-[Co(en)2 (NCCH3 )2 ]3þ complexes, although they are known [24] and they are extremely unreactive in water, DMSO and acetonitrile. The tmen complexes are certainly more reactive, but to what extent remains unknown. Given the lack of significant interaction between the two tmen ligands, it has become apparent that one tmen ligand may be sufficient to labilise the metal ion towards substitution reactions. Indeed, the trans[Co(tmen)(en)Cl2 ]þ ion and the four isomers of [Co(dien)(tmen)Cl]2þ have recently been synthesised [27]. They are strikingly reactive compared to their en analogues and it is anticipated that detailed studies on these complexes might elucidate the reason(s) for the dramatic kinetic effect of per-C-methylation on the chelate ring.
Acknowledgements Financial support from the Australian Research Council is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6]
[7] [8] [9] [10] [11] [12]
[13] [14] [15] [16] [17] [18] [19] [20] [21]
[22]
[23] [24] [25] [26] [27] [28]
[29] [30] [31] [32]
A. Werner, Ber. Dtsch. Chem. Ges. 45 (1912) 121. S.M. Jorgenson, J. Prakt. Chem. 39 (1889) 1. P.N. Becker, G.B. Bergman, Organometallics 2 (1983) 787. P. Krajnik, R.H. Grabtree, Inorg. Chem. 32 (1993) 1028. P. Hendry, A. Ludi, Helv. Chim. Acta 71 (1988) 1966. F.C. Anson, J.A. Christe, T.J. Collins, R.J. Coots, T.T. Furtani, S.L. Gipson, J.T. Keech, T.I. Kraft, B.D. Santarsiero, G.A. Spies, J. Am. Chem. Soc. 106 (1984) 4460. C.-M. Che, H.-W. Lam, R.J. Wang, C.W. Mak, J. Chem. Soc., Chem. Commun. (1990) 820. C.-M. Che, W.H. Chiu, S.M. Peng, Inorg. Chem. 35 (1996) 3369. K.Y. Wong, C.-M. Che, C.K. Li, W.H. Chiu, Z.Y. Zhou, C.W. Mak, J. Chem. Soc., Chem. Commun. (1992) 754. K.Y. Wong, C.-M. Che, H.W. Lam, K.F. Chin, Z.Y. Zhou, C.W. Mak, J. Chem. Soc., Dalton Trans. (1993) 857. P. Hendry, A. Ludi, H. Piab, Inorg. Chim. Acta 175 (1990) 83. P. Hendry, A. Ludi, in: A.G. Sykes (Ed.), Advances in Inorganic and Bioinorganic Chemistry, vol. 35, Academic Press, New York, 1990, p. 117. P. Hendry, A. Ludi, Helv. Chim. Acta 71 (1986) 1966. W.G. Jackson, A.F.M.M. Rahman, A.D. Rae, A.C. Willis, D.C.R. Hockless, J. Chem. Soc., Chem. Commun. (2003) 2748. R.G. Pearson, C.R. Boston, F. Basolo, J. Am. Chem. Soc. 75 (1953) 3089. W.G. Jackson, Aust. J. Chem. 34 (1981) 215. H.F. Bauer, W.C. Drinkard, J. Am. Chem. Soc. 82 (1960) 5031. M. Shibata, in: H. Yamatera (Ed.), Modern Syntheses of Cobalt(III) Complexes, vol. 110, Springer, Berlin, 1983, p. 118. W.G. Jackson, A.M. Sargeson, Inorg. Chem. 17 (1978) 1348. W.G. Jackson, J.M. Harrowfield, P.D. Vowles, Int. J. Chem. Kin. 9 (1977) 535. W.G. Jackson, A.M. Sargeson, in: P. de Mayo (Ed.), Rearrangement in Coordination Complexes, vol. 2, Academic Press, New York, 1980, p. 273. W.G. Jackson, in: I. Bernal (Ed.), The Stereochemistry of the Bailar Inversion and Related Substitution Processes, vol. 1, Elsevier, Amsterdam, 1986, p. 255. P. Comba, N.J. Curtis, W.G. Jackson, A.M. Sargeson, Aust. J. Chem. 39 (1986) 1297. N.E. Dixon, W.G. Jackson, M.J. Lancaster, G.A. Lawrance, A.M. Sargeson, Inorg. Chem. 20 (1981) 470. H.C. Freeman, C.J. Moore, W.G. Jackson, A.M. Sargeson, Inorg. Chem. 17 (1978) 3513. G.B. Kauffman, D. Faoro, Inorg. Synth. 17 (1977) 147. W.G. Jackson, Z. Tao, unpublished results, 2000. W.G. Jackson, unpublished data. Compared to other dipolar aprotic media such as DMSO or DMF, there is a much reduced tendency towards the stabilisation of the polar cis isomers in CH3 CN. B. Bosnich, C. Ingold, M.L. Tobe, J. Chem. Soc. (1965) 4074. A.M. Sargeson, G.H. Searle, Inorg. Chem. 6 (1967) 787. W.R. Fitzgerald, A.J. Parker, D.W. Watts, J. Am. Chem. Soc. 90 (1968) 5744. M.L. Tobe, in: A.G. Sykes (Ed.), Mechanism of Base Hydrolysis, vol. 2, Academic Press, London, 1983, p. 1.
Solvent exchange, solvent interchange, aquation and isomerisation reactions of cis- and trans-[Co(tmen)2(NCMe)2]3þ in water, Me2SO and MeCN: kinetics and stereochemistry W. Gregory Jackson *, A.F.M. Mokhlesur Rahman, M. Anthony Wong School of Chemistry, University College, The University of New South Wales, Australian Defence Force Academy, Canberra, ACT 2600, Australia Received 17 March 2003; accepted 21 May 2003
Abstract The synthesis and characterisation of cis- and trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 are described. Solvolysis rates have been measured by both 1 H NMR spectroscopy and UV–Vis spectrophotometry in dimethyl sulfoxide at 298.2 K. The cis isomer undergoes solvolysis by consecutive first-order reactions, k1 ¼ 5.61 104 and k2 ¼ 5.35 104 s1 , each with steric retention. The measured solvolysis rate (single step reaction) for the trans isomer is k ¼ 1.54 105 s1 . The solvent exchange rates have been measured by 1 H NMR spectroscopy in CD3 CN at 298.2 K: kex (cis) ¼ kct + kcc ¼ 2.0 105 and kex (trans) ¼ ktc + ktt ¼ 4.56 106 s1 . From these data, the measured cis–trans isomerisation rate (1.71 106 s1 ) and equilibrium position in CH3 CN (17% trans), the steric course for substitution in the exchange processes has been determined: trans reactant – 69% trans product; cis reactant – 99% cis product. Aquation rates for cis- and trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 have also been determined spectrophotometrically and by NMR; kcis ¼ 1.3 104 and ktrans ¼ 2.7 105 s1 . In both cases the steric course for the primary aquation step is indeterminate because the subsequent steps are faster. Where data are available, the [Co(tmen)2 X2 ]nþ complexes are found to be consistently much more reactive than their [Co(en)2 X2 ]nþ analogues. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Cobalt(III); Synthesis; Kinetics; Mechanism; Acetonitrile; Hydrolysis
1. Introduction Ethylenediamine (en) has been one of the most used ligands in coordination chemistry, especially in Co(III) chemistry for kinetic and other studies [1,2]. A newer and similar ligand is tetramethylethylenediamine or 2,3dimethylbutane-2,3-diamine (tmen), which is en with all four backbone CHs replaced by methyl groups. It has been rarely employed as a ligand in coordination chemistry, no doubt due to its tedious multi-step synthesis [3–5] and also because there was little expectation it would bring surprising new chemistry. This has proved far from the truth. Ethylenediamine easily undergoes oxidative dehydrogenation when it coordinates with reactive high*
Corresponding author. Fax: +61-2-62688090. E-mail address: [email protected] (W.G. Jackson).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2003.05.010
valent ruthenium and osmium-oxo complexes [6], and tmen has been employed as a means of circumventing this difficulty because per-methyl substitution at carbon removes the source of the problem. This has resulted in the discovery that tmen can stabilise otherwise very reactive species for a variety of high valency ruthenium and osmium complexes [7–10]. Other interesting chemistry of tmen has been observed for the hexamine Co(III) complex [Co(tmen)3 ]3þ [5,11–13]. This pink ion exhibits two ligand-field bands at 504 and 362 nm, substantially red-shifted compared to the classic yellow [Co(en)3 ]3þ species, with maxima at 470 and 340 nm [5]. Further, the base catalysed hydrolysis rate for this ion was found to be at least nine orders of magnitude larger than for the parent [Co(en)3 ]3þ complex. The labile kinetic behaviour of [Co(tmen)3 ]3þ towards base catalysed hydrolysis was attributed to steric
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crowding imposed by the 12 peripheral methyl groups [11]. Further support for the strain-induced enhancement of reactivity was reflected in the data obtained for the less sterically hindered ion [Co(tmen)2 en]3þ (which incidentally is not pink, but the yellow colour typical of normal hexaaminecobalt(III) complexes). The partial relief of strain was reflected in the kinetics of its base hydrolysis; the complex is reported to be two orders of magnitude less labile than the [Co(tmen)3 ]3þ complex. Other research into Co(III)–tmen complexes has exposed the unexpected. Amongst these were the synthesis of stable hydrido and sideways-bonded peroxo complexes involving a Co(tmen)2 backbone [14], previously unknown for any other tetraaminecobalt(III) template. Few kinetic studies have been carried out on Co(III)– tmen complexes. The earliest and perhaps the only other example was the work by Pearson et al. [15]. They studied the kinetics of aquation of a series of compounds of the type trans-[Co(AA)2 Cl2 ]þ , in which the steric properties of the bidentate diamine ligand (AA) were varied (ring size and C-alkyl substitution). It was found that an increase in ring size or steric crowding of the bidentate ligand was accompanied by an increase in the rate of aquation. For [Co(tmen)2 Cl2 ]þ , the aquation was so rapid that it could not be measured even at low temperatures. Our work has confirmed this observation. The present study on the substitution reactions of [Co(tmen)2 X2 ]nþ complexes was undertaken to quantify the kinetic effects of backbone C–Me substitution. To slow the rates to those we could measure, we chose MeCN as the leaving group. This normally binds tightly to Co(en)2 moieties and is substituted on the timescale of days to weeks. With tmen, the timescale is more conveniently min. We report herein a variety of rates for substitution, for H2 O, Me2 SO and MeCN as solvents, and including trans to cis isomerisation and solvent exchange. The stereochemistry of substitution, in so far as it could be determined, is also reported.
2. Experimental All chemicals were AnalaR or an equivalent grade. Carbon-13 and proton NMR spectra were recorded on Varian XL-300 and Unity Plus 400 MHz instruments at 20 °C. Solvents used were D2 O with dioxane as the internal reference (13 C, d 69.27 relative to DSS), and Me2 SO-d6 with the central peak of the CD3 septet as the reference (13 C, d 39.37 relative to SiMe4 ). Full visible absorption spectra and absorbance-time traces were recorded on a HP8452A diode array UV–Vis spectrophotometer thermostated to 25.00 0.05 °C with use of a Lauda RM6 circulating water bath. Spectra saved as. kd files were converted to .txt files and read using SpecfitÒ to perform Global Spectral Analysis to extract the rate and spectral data. The cation exchange medium
used was Dowex 50Wx2 (Hþ form, 200–400 mesh; BioRad). Carbon dioxide free Milli-Q water was used for all physical measurements. Evaporations were carried out on a B€ uchi rotary evaporator at temperatures less than 40 °C. Kinetic studies were performed on both the cis and trans isomers of [Co(tmen)2 (NCCH3 )2 ](ClO4 )3 using NMR and UV–Vis spectrophotometry in Me2 SO-d6 , MeCN-d3 and in acidified D2 O/dioxane using welldocumented techniques [16]. Kinetic data obtained from successive 1 H NMR spectra utilised peak heights of selected resonances which were normalised with respect to a reference signal or set of signals (usually the sum of the peak heights for bound and free MeCN). Rate constants were calculated using standard nonlinear least squares fitting procedures (with IgorÒ or KaleidographÒ ). 2.1. Synthesis of 2,3-Dimethylbutane-2,3-diamine (tmen) The following is a modification of the procedure described by Hendry and Ludi [5]. 2.1.1. [O2 NC(CH3 )2 C(CH3 )2 NO2 ] 2-Nitropropane (356 g, 4.0 mol) was stirred with NaOH (8 M, 500 mL) in a round-bottomed flask cooled in ice. Aqueous bromine (320 g, 4.0 mol) was added from a pressure equalised dropping funnel over a period of 3–4 h, and after a further 12 h ethanol was added (300 mL) and the mixture was refluxed for 2 h. Upon cooling, fine white crystals of 2,3-dimethyl-2,3-dinitrobutane were obtained. They were filtered, washed with cold ethanol and air dried. Yield: 86%. 2.1.2. [H3 NC(CH3 )2 C(CH3 )2 NH3 ](NO3 )2 The crude dinitro compound (70.4 g, 4.0 mol) was placed in a round-bottomed flask equipped with a condenser and conc. HCl (300 mL) was added. The slurry was vigorously stirred and heated to 60 °C. Tin (300 g; 20 mesh) was gradually added and conc. HCl (300 mL) was used to rinse the metal from the sides. The resulting mixture was boiled for 30 min under reflux, and then cooled in ice and NaOH (320 g) was added while thoroughly stirred. The resultant tmen was distilled into an ice cooled flask containing HNO3 (8 M, 100 mL). The distillate was rotaevaporated to a third volume and the resultant fine white crystals of the acid nitrate salt were collected, washed with cold ethanol and ether, and air dried. Yield: 82%. 1 H NMR (Me2 SO-d6 ): d 1.30 (12H, CH3 ), d 8.13 (4H, NH2 ); 13 C NMR: d 21.8 (4C, CH3 ), d 57.7 (2C). 2.2. cis- and trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 2.2.1. Na3 [Co(O2 CO)3 ][17,18] Co(NO3 )2 6H2 O (29.1 g, 0.10 mol) in water (50 mL) and excess hydrogen peroxide (30%, 10 mL) was added
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dropwise to a slurry of sodium bicarbonate (42.0 g, 0.50 mol) in H2 O (50 mL). The mixture was allowed to stand at 0 °C for 1 h with continuous stirring. The olive product was filtered and washed cold distilled water (5 10 mL), then thoroughly washed with absolute ethanol and dry ether. Yield: 85%. 2.2.2. [Co(tmen)2 Cl2 ]Cl Excess Na3 [Co(CO3 )3 ]0 (65 g, 0.2 mol) was added to tmen nitrate (24.2 g, 0.10 mol) in water (500 mL). The mixture was stirred on a water bath (60 °C) overnight. The mixture was filtered to remove excess Na3 [Co(CO3 )3 ] yielding a reddish-purple solution. Conc. HCl (25 mL) was added to the solution and the mixture warmed on a water bath (60 °C) for 2 h. After evaporation to dryness, the purple solid was dissolved in distilled water and loaded onto a Dowex column. After flushing thoroughly with water, the reddish-purple band of the bis(tmen) compound was eluted using 3 M HCl (the diaqua, aquachloro and dichloro ions elute together because they are in rapid equilibrium). The eluate was evaporated to dryness on a rotary evaporator, and the dried solid was placed in a Soxhlet extractor thimble and the [Co(tmen)2 (Cl2 )]Cl extracted using methanol (ca. 500 mL). The resultant green solution was evaporated to dryness to yield green/ purple cis-/trans-[Co(tmen)2 Cl2 ]Cl. No isomer separation was necessary for the subsequent synthesis, although it has been achieved. Yield: 80%. 1 H NMR (CD3 OD; 100% trans): d 1.32 (24H, CH3 ), d 4.93 (8H, NH2 ); 13 C NMR: d 28.9 (8C, CH3 ), d 65.5 ppm (4C). 2.2.3. [Co(tmen)2 (OSO2 CF3 )2 ]CF3 SO3 [Co(tmen)2 Cl2 ]Cl (6 g, 0.02 mol) was gradually added to triflic acid (25 mL). The reddish-purple solution was stirred continuously and warmed (60 °C) to drive out HCl until there was no residual Cl in solution (Agþ test). The cooled solution was added dropwise to ether (1 L) to yield a gelatinous red precipitate that was filtered and washed generously with more ether and dried in vacuo. Yield: 92%. The bound triflate is easily substituted by solvent, thus the 1 H and 13 C NMR spectra correspond to the respective cis-[Co(tmen)2 (sol)2 ]3þ species in D2 O or Me2 SO-d6 .
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(5 °C) for 10 min were filtered, washed with isopropanol and ether, and air dried. Yield: 89%. The product is invariably contaminated with about 15% of the less soluble trans isomer which is difficult to completely remove by direct recrystallisation from water. The product was extracted with dry methanol to leave a residue of insoluble trans isomer. The solvent was quickly removed in vacuo by rotaevaporation and the product recrystallised from water. A small sample of pure trans isomer was also obtained by a similar recrystallisation. Anal. Calc. for CoC16 H42 N6 O12 Cl3 H2 O: C, 27.70; H, 6.39; N, 12.11. Found: C, 27.4; H, 6.1; N, 12.1%. UV–Vis (MeCN): e (max) 473 nm, 180.5; e (max) 351 nm, 144.5 M1 cm1 . 1 H NMR (Me2 SOd6 ): d 6.95, 2H, d (NH); 5.88, 2H, d (NH); 5.80, 2H, d (NH); 3.40, 2H, s (lattice H2 O); 2.68, 6H, s (CH3 –CN); 1.38, s, 6H, (CH3 ); 1.25, s, 6H (CH3 ); 1.15, 6H, s (CH3 ); 1.10, 6H, s (CH3 ). 13 C NMR (Me2 SO-d6 ): d 132.0, 2C (CN); 68.0, 2C (Ctert ); 63.5, 2C (Ctert ); 26.2, 2C (CH3 ); 26.0, 2C (CH3 ); 25.0, 2C (CH3 ); 24.8, 2C (CH3 ); 4.0 ppm, 2C (CH3 –CN).
2.2.5. trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 cis-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 (0.5 g) was added to acetonitrile (5 mL) in a test tube to form a slurry. The test tube was placed in a waterbath at 70 °C and the slurry was allowed to evaporate slowly to dryness by loosely sealing the tube with parafilm. Solid samples so obtained were tested for trans isomer content using 1 H NMR (Me2 SO-d6 ). The evaporation process was repeated until a purity of greater than 95% trans was obtained. The final solid was recrystallised as described for the cis isomer to yield yellow crystals of pure trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 . Anal. Calc. for CoC16 H42 N6 O12 Cl3 H2 O: C, 27.70; H, 6.39; N, 12.11. Found: C, 27.8; H, 5.9; N, 12.0%. UV–Vis (MeCN): e (max) 448 nm, 100.5; e (sh) 350 nm, 94.5 M1 cm1 . 1 H NMR (Me2 SO-d6 ): d 5.97, 8H, s (NH); 3.41, 2H, s (lattice H2 O); 2.63, 6H, s (CH3 –CN); 1.30, s, 24H, (CH3 ). 13 C NMR (Me2 SO-d6 ): d 138.0, 2C (CN); 65.8, 4C (Ctert ); 25.8, 8C (CH3 ); 5.1, 2C (CH3 – CN) ppm.
3. Results 2.2.4. cis-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 Dried [Co(tmen)2 (OSO2 CF3 )2 ]CF3 SO3 (10 g, 0.02 mol) was dissolved in acetonitrile (200 mL) and warmed for 2 h to yield an orange solution. The solution was evaporated to dryness to yield an orange solid that was recrystallised by dissolving in a minimal amount of water and adding one fifth volume of NaClO4 solution (5 M). The orange crystals after cooling
3.1. Aquation reactions 3.1.1. cis-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 Sharp, well-defined isosbestic points were observed at 380, 407 and 509 nm, which were unshifted throughout reaction through to the cis-[Co(tmen)2 (OH2 )2 ]3þ species, indicative of a single reaction step (Fig. 1).
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Fig. 1. UV–Vis spectra of cis-[Co(tmen)2 (NCCH3 )]3þ in 0.1 M HClO4 at 25 °C. The absorbance decreases with time at 450 nm.
cis-½CoðtmenÞ2 ðNCCH3 Þ2 3þ k;H2 O
! ½CoðtmenÞ2 ðOH2 ÞðNCCH3 Þ slow
3þ
þ CH3 CN
3þ
½CoðtmenÞ2 ðOH2 ÞðNCCH3 Þ H2 O
3þ ! cis-½CoðtmenÞ2 ðOH2 Þ2 þ CH3 CN fast
The 1 H NMR spectra (Fig. 2) confirmed the absence of any intermediate [Co(tmen)2 (NCCH3 )(OH2 )]3þ species. The rate constants calculated from both UV–Vis and NMR data are summarised in Table 1. 3.1.2. trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 The aquation of the trans isomer occurs with complete rearrangement of the trans configuration, i.e., the final product is cis-[Co(tmen)2 (OH2 )2 ](ClO4 )3 . This was confirmed by the 1 H NMR spectra (Fig. 3). However, the actual steric course for the primary aquation step is unknown, because all subsequent steps following the loss of the first MeCN ligand must be faster. The UV–Vis spectra show the presence of three sharp isosbestic points at 357, 400 and 478 nm (Fig. 4). The presence of isosbestic points suggests a single product or multiple products produced in constant proportions. In the present case the isosbestic points correspond precisely to the overlap in the spectra of the reactant trans isomer and cis-[Co(tmen)2 (OH2 )2 ]3þ . Thus, again, the loss of the first MeCN ligand is rate determining, and subsequent trans to cis isomerisation and hydrolysis of the second MeCN ligand are much faster. The rate data are summarised in Table 2.
Fig. 2. Representative 1 H NMR spectra for the reaction of cis[Co(tmen)2 (NCCH3 )2 ]3þ in acidified D2 O/dioxane at different time intervals. A (bound Me for CH3 CN) and B (free CH3 CN) represent the peaks used in the kinetic analysis.
3.2. Solvolysis reactions in DMSO 3.2.1. cis-[Co(tmen)2 (NCCH3 )2 ]3þ Repeated scans of the UV–Vis spectrum of cis[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 reacting in DMSO
revealed initial isosbestic points at 355, 380, 430 and 500 nm. However, all three Ôisosbestic pointsÕ shifted towards shorter wavelengths at a later stage, revealing a two-stage reaction (Fig. 5).
W.G. Jackson et al. / Inorganica Chimica Acta 357 (2004) 665–676 Table 1 Rate constants for the aquation of cis-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 in 0.01 M HClO4 Method
Temperature (°C)
k 104 (s1 )
NMR UV–Vis
25.0 0.1 25.00 0.05
1.41 0.04 1.31 0.01
The rate constants for each of the two steps were obtained by following the changes in absorbance with time, at specific wavelengths, on the UV–Vis spectro-
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photometer. The rate constants calculated are summarised in Table 3. The relative magnitudes of the rate constants clearly indicate that k1 k2 . We have noted previously [19,20] that such a kinetic situation requires special consideration, since absorbance-time data can be reproduced quite accurately by a range of k1 and k2 values that differ by less than a factor of about two. Indeed, in the limit as k2 ! k1 , the rate equation for [B] and the derived absorbance-time equation become:
Fig. 3. 1 H NMR spectra for trans-[Co(tmen)2 (NCCH3 )2 ]3þ in acidified D2 O/dioxane at different time intervals. A and B represent the methyl peaks due to bound and free CH3 CN, respectively, employed in the calculation of the rate constant.
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Fig. 4. UV–Vis spectra of trans-[Co(tmen)2 (NCCH3 )]3þ in 0.1 M HClO4 at 25 °C. The absorbance decreases with time at 450 nm.
Table 2 Rate constants for the aquation of trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 in 0.01 M HClO4 Method
Temperature (°C)
k 105 (s1 )
NMR UV–Vis
25 1.0 25.00 0.05
2.5 0.2 2.7 0.2
Table 3 Spectrophotometrically determined rate constants for the sequential substitution of CH3 CN in cis-[Co(tmen)2 (NCCH3 )2 ]3þ by DMSO
104 kslow (s1 ) 104 kfast (s1 )
Run 1
Run 2
Run 3
Average
5.26 6.32
4.96 6.59
5.47 6.12
5.2 0.3 6.3 0.2
½B ¼ A0 kt ekt D D1 ¼ A0 ½ðea ec Þ þ ktðeb ec Þ kt The fit to the data (470 nm) for the present system under the assumption k1 ¼ k2 is excellent (Fig. 6); the rate constant calculated was 5.71 104 s1 , which lies between the actual true values for kfast and kslow (see below). This is yet another good example of a kinetic system that illustrates the point that a good data fit does
not guarantee that you have analysed the system correctly [19]. An additional but related problem is that it is not straightforward to determine if k1 or k2 is kfast or kslow [20]. One resolution of the ambiguity relies upon all species in the reacting system being observed simultaneously, i.e., individually. The reaction was followed using NMR spectroscopy to resolve the ambiguity. As expected, the
Fig. 5. UV–Vis spectra of cis-[Co(tmen)2 (NCCH3 )]3þ in Me2 SO, illustrating the shift in Ôisosbestic pointsÕ. The absorbance decreases at 450 nm with time.
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Fig. 6. Graph of the changes in absorbance with time for the stepwise reaction of cis-[Co(tmen)2 (NCCH3 )2 ]3þ in Me2 SO. The fits to the absorbance-time data were made assuming k1 ¼ k2 . The results reveal a good fit to the data, with k ¼ (5.71 0.03) 104 s1 (m4 ). The good fit is misleading, and the result is incorrect (refer text). 1
H NMR spectra of cis-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 in Me2 SO-d6 show clearly the presence of the intermediate species, B (Fig. 7). The two rate constants k1 and k2 could thus be obtained by measuring the changes in the peak
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Fig. 8. Graph of the changes of concentration of cis-[Co(tmen)2 (OH2 )(NCCH3 )]3þ with time. The data were obtained by measuring the normalised peak heights of the species A, B and C in the 1 H NMR spectra (Fig. 7). The results were fitted for k1 and k2 using the relevant rate equation. The line shows a good fit to the data, with rate constants, k1 and k2 shown as m2 and m3 , respectively (in min1 ).
height of the intermediate B by the method discussed previously. The rate constant k1 was obtained independently by analysing the decay of A. The rate constants so calculated were k1 ¼ (5.61 0.06) 104 and k2 ¼ (5.35 0.02) 104 s1 (Fig. 8). Thus k1 is actually slightly greater than k2 . 3.2.2. trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 In contrast to the data for the reaction of cis[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 in DMSO, the UV–Vis spectra show the presence of sharp, well-defined isosbestic points at 424 and 478 nm for the entire reaction of the trans isomer in DMSO (Fig. 9), as for 0.1 M HClO4 as solvent. The final spectrum corresponds to pure cis[Co(tmen)2 (OSMe2 )2 ]3þ . This indicates that the solvolysis of the trans isomer in DMSO is a simple A ! C reaction scheme. The trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 complex shows characteristic changes in the 1 H and 13 C NMR spectra as it undergoes substitution by Me2 SO-d6 and isomerises to cis-[Co(tmen)2 (OS(CD3 ))2 ](ClO4 )3 without any observable intermediates (Figs. 10 and 11). The rate constant k was derived from the changes of the peak heights of coordinated acetonitrile or free acetonitrile with respect to time. The rate constants calculated from both UV–Vis spectral and NMR spectroscopic data are summarised in Table 4. trans-½CoðtmenÞ2 ðNCCH3 Þ2 k
3þ
! cis-½CoðtmenÞ2 ðMe2 SO-d6 Þ2
3þ
3.3. Solvent exchange reactions in CD3 CN 1 Fig. 7. Representative H NMR spectra for cis[Co(tmen)2 (NCCH3 )2 ]3þ in Me2 SO-d6 recorded at different time intervals. A, B and C represent the peak height measured for species A, B and C and used in the calculation of the rate constants.
3.3.1. cis-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 The rate of exchange of the coordinated acetonitrile ligand was followed by NMR. There is little cis to trans
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Fig. 9. Changes in absorption spectra for the solvolysis of trans-[Co(tmen)2 (NCMe)2 ]3þ in DMSO. The absorbance increases with time at 550 nm.
Fig. 11. Representative 13 C NMR spectra for trans[Co(tmen)2 (NCCH3 )2 ]3þ in Me2 SO-d6 at different time intervals. The spectra clearly show the isomerisation from a trans to a cis isomer without build-up of any intermediate species.
Fig. 10. Representative 1 H NMR spectra for trans-[Co(tmen)2 (NCCH3 )2 ]3þ in Me2 SO-d6 recorded at different time intervals. A and C are the peaks used in the calculation of the rate constant, k.
isomerisation and so the changes in UV–Vis spectra were not examined. The 1 H NMR spectra (Fig. 12) show that as time progresses, the peak height of the coordinated Co-
Table 4 Rate constants for the spontaneous [Co(tmen)2 (NCCH3 )2 ](ClO4 )3 in DMSO
substitution
of
trans-
Method
Temperature (°C)
k 105 (s1 )
NMR UV–Vis
25 1.0 25.00 0.05
1.03 0.06 1.54 0.03
NCCH3 ligand decreases as it is replaced by CD3 CN, while the signal for free CH3 CN grows. The exchange process is a stepwise reaction,
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Ht =Ht0 ¼ A0 ðek1 t f1 þ ½k1 =2ðk2 k1 Þg fk1 =2ðk2 k1 Þgek2 t Þ For MeCN exchange, k1 ¼ 2k2 (in the absence of isotope effects), therefore Ht =Ht0 ¼ A0 ek2 t Thus, the rate constant measured by using the coordinated MeCN peak is actually the rate constant, k2 . The exchange rate constant, kex is twice this rate constant, i.e., kex ¼ 2k2 [21,22]. The same result is obtained if the normalised area of the free MeCN peak is measured with respect to time. In the 1 H NMR spectra, the peaks for bound (*) and free (+) MeCN are labelled. The changes in the normalised peak heights with respect to time were fitted using the appropriate function, yielding k2 ¼ (1.00 0.06) 105 s1 . Thus, the MeCN exchange rate constant, kexðcÞ , for cis-[Co(tmen)2 (NCCH3 )2 ]3þ is (2.0 0.1) 105 s1 .
Fig. 12. Representative 1 H NMR spectra for cis-[Co(tmen)2 (NC CH3 )2 ]3þ in CD3 CN at different time intervals, 25 °C. * and + denote signals due to bound and free acetonitrile, respectively.
3þ
½CoðtmenÞ2 ðNCCH3 Þ2 A k1
3þ
! ½CoðtmenÞ2 ðNCCH3 ÞðNCCD3 Þ B k2
3þ
! ½CoðtmenÞ2 ðNCCD3 Þ2 C where A ¼ A0 ek1t and B ¼ ½k1 =ðk2 k1 Þðek1 t ek2 t Þ
The coordinated MeCN signal in the 1 H NMR is proportional to 2A + B. Using peak heights as a measure of area, Ht ¼ cð2½A þ ½BÞ Ht1 ¼ 0 Ht0 ¼ c2A0 where c is the proportionality constant. Thus Ht =Ht0 ¼ ½A=A0 þ ½B=2A0 Substituting, we get
3.3.2. trans-[Co(tmen)2 (NCCH3 )2 ](ClO4 )3 The rate of solvent exchange, kexðtÞ , was measured analogously to that for the cis isomer. The measured rate constant was (1.37 0.09) 104 min1 , thus the rate constant kexðtÞ for solvent exchange is (2.74 0.18) 104 min1 , or (4.56 0.30) 106 s1 . The isomerisation rate constant, ki (where ki ¼ kct + ktc ), was measured by UV–Vis spectroscopy utilising the changes in the absorbance as trans-[Co(tmen)2 (NC CH3 )2 ]3þ isomerises to cis-[Co(tmen)2 (NCCH3 )2 ]3þ in acetonitrile. The trans ! cis isomerisation was followed for 3t1=2 . The changes in absorbance were followed at two wavelengths, 350 and 490 nm (where these changes were relatively large). Data collected at these two wavelengths were used to calculate ki . The equilibrium position (Ktc ¼ ktc /kct ) was determined from the final spectrum using the known spectra for the pure cis and trans isomers in this solvent. The UV–Vis spectra of trans-[Co(tmen)2 (NCCH3 )2 ]3þ in acetonitrile show two not-so-well-defined isosbestic points, at 322 and 405 nm (not shown). However, it was clear from the NMR data that only trans- and cis[Co(tmen)2 (NCMe)2 ]3þ are present at any time. The rate constant, ki determined spectrophotometrically was (2.01 0.05) 106 s1 . The rate constant, ki was also determined from the 1 H NMR data (Fig. 13) by following the loss of the trans-methyl peak with time (this was normalised to the sum of the methyl peak heights for the cis- and transions and that for the free ligand; the sum is constant). The calculated rate constant was ki ¼ (1.71 0.17) 106 s1 , Fig. 14. These data, and also the 13 C NMR spectra (Fig. 15), indicate ca. 17% residual trans isomer at equilibrium.
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1 Fig. 13. Representative H NMR spectra of trans[Co(tmen)2 (NCCH3 )2 ]3þ in CD3 CN at different time intervals; * and + denote bound and free acetonitrile, respectively.
3.4. Solvent exchange and steric course of substitution
Fig. 15. Representative 13 C NMR spectra of trans-[Co(tmen)2 (NC CH3 )2 ]3þ in CD3 CN at different time intervals. The peaks marked * are the free solvent peaks. The 13 C NMR spectra clearly show the isomerisation from the trans ! cis isomer, with some (ca. 15%) residual trans-isomer at equilibrium.
The 17% equilibrium percentage of trans product determined both spectrophotometrically and by NMR compares favourably with the ca. 15% trans produced in the synthesis of cis-[Co(tmen)2 (NCCH3 )2 ]3þ .
Using the NMR value of ki ¼ 1:71 106 s1 we have, ktc þ kct ¼ ki ¼ 1:71 106 s1 ktc =kct ¼ Ktc ¼ 83=17 Thus solving for ktc and kct , we get, ktc ¼ 1:41 106 s1 kct ¼ 3:00 107 s1 From the NMR data, kexðtÞ ¼ 4:56 106 s1 kexðtÞ ¼ ktt þ ktc ktt ¼ 3:15 106 s1 transð%Þ ¼ 102 ktt =kexðtÞ ¼ 69%:
Fig. 14. Graph showing the depletion of the trans-methyl peak with time. The rate constant ki has the value of 1.02 104 min1 (m3 ), or 1.7 106 s1 , in reasonable agreement with the spectrophotometric value. The calculations also show that as t ! 1, the normalised peak height ! m1 . This shows that at equilibrium 17% trans-isomer remains.
Thus the steric course of MeCN substitution of trans[Co(tmen)2 (NCCH3 )2 ]3þ is 69% trans product. The steric course of substitution for cis[Co(tmen)2 (NCCH3 )2 ]3þ is deduced similarly,
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kex ¼ kct þ kcc ¼ 2:00 105 s1 kct ¼ 3:00 107 s1
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in water strongly favours the cis form (99%), yet all trans is obtained on slow evaporation [26]. 4.2. Equilibria
Thus, kcc ¼ 1:97 105 s1 2
cisð%Þ ¼ 10 kcc =kex ¼ 99% Thus the steric course of MeCN substitution of cis[Co(tmen)2 (NCCH3 )2 ]3þ is 99 % cis product.
4. Discussion 4.1. Synthesis The ligand was obtained as described [5], save for the use of HNO3 to directly quench the steam distilled diamine to prevent substantial losses. The synthetic methods for the complexes are an extension of those successful in bis(ethylenediamine) chemistry [23,24]. Thus cis- or trans-[Co(tmen)2 Cl2 ]Cl, in neat CF3 SO3 H readily yielded [Co(tmen)2 (O3 SC F3 )2 ]CF3 SO3 which is all cis. The complex in acetonitrile gave [Co(tmen)2 (NCCH3 )2 ](CF3 SO3 )3 quantitatively. Recrystallisation from water as the perchlorate salt consistently yielded cis complex contaminated with a little trans isomer. We found that the cis isomer is selectively extracted by methanol, leaving the insoluble trans form. After the rapid removal of solvent (the cis decomposes on standing in alcohol for a few hours), each was recrystallised to purity from water. We discovered a quantitative synthesis of trans[Co(tmen)2 (NCCH3 )2 ]3þ some years ago but which we could not repeat. It transpires that the cis/trans equilibrium in acetonitrile favours the cis form, but not exclusively so (ca. 17% at 25 °C, and somewhat more, ca. 30%, at reflux temperature). Cis/trans isomerisation is not fast, but if a solution at equilibrium is allowed to slowly evaporate, the less soluble trans isomer crystallises preferentially, and the entire mixture can be milked of the minor trans form if the conditions are controlled. Rapid evaporation gives largely the more abundant cis form. This process is akin to a second-order asymmetric transformation, whereby the less soluble form can be crystallised quantitatively if crystallisation is slow compared to the rate of diastereomer interconversion. The chiral [Co(en)2 (R)cys]þ system is a good example; the Kisomer is obtained quantitatively from a D=K mixture which under equilibrium conditions favours the D form (ca. 70%) [25]. The principle is precisely the same for achiral systems, as here, where two chemically distinct species are in rapid equilibrium, but the minor and less soluble component crystallises first. The K[Cr(ox)2 (OH2 )2 ] system is the classic example – the equilibrium
The extraordinary lability of the K-[Co(tmen)3 ]3þ ion in aqueous base has been attributed to steric congestion arising from interligand Me Me interactions [11]. Based on our recent work with the tmen complexes [27], we no longer believe the reactivity is steric in origin. The [Co(tmen)2 X2 ]þ complexes are not anomalous in their thermodynamic preference for the cis or trans configuration. In water and in Me2 SO, the cis-[Co(tmen)2 (sol)2 ]3þ complex predominates over trans-[Co(tmen)2 (sol)2 ]3þ which is undetectable, whereas in CH3 CN as solvent, the preference is not quite as pronounced (83% cis) [28]. In methanol, the reverse is found, with the trans-[Co(tmen)2 Cl2 ]þ isomer (>95%) predominating over the cis. This behaviour closely mimics the [Co(en)2 X2 ]nþ chemistry [29], and the relative isomer stabilities in both systems can be accounted for in terms of solvent properties, i.e., solvation [22,30]. The two tmen ligands are remote in trans-[Co(tmen)2 X2 ]nþ and it now seems clear that there are also no significant steric interactions for the cis configuration. In summary, en and tmen show little difference in the relative stabilities of their cis and trans forms; the equilibria can be swung totally from trans predominant to cis predominant merely by raising the polarity of the solvent (which favours the dipolar cis isomer through increased solvation) [31]. There is therefore no support from relative stability considerations for the perceived steric congestion in the cis-[Co(tmen)2 X2 ]nþ complexes and likely the [Co(tmen)3 ]3þ species as well. 4.3. Reactivity Trans-[Co(tmen)2 Cl2 ]þ is over 106 -fold more reactive than trans-[Co(en)2 Cl2 ]þ . For cis-[Co(tmen)2 Cl2 ]þ , a similar rate enhancement is evident; this complex also hydrolyses ÔinstantlyÕ in dilute aqueous acid [14]. Prior to this work, there were no quantified rate parameters for substitution reactions of tmen complexes. We chose acetonitrile as the leaving group because it binds tightly and thus the rate of substitution of its tmen complexes might be slowed to the point of being measurable. This proved to be the case. The cis- and trans-[Co(tmen)2 (NCCH3 )2 ]3þ complexes solvolyse in DMSO and water sequentially, at rates which are on the minutes–hours timescales at 25 °C. They were followed spectrophotometrically and also by 1 H and 13 C NMR; the kinetic data obtained by the different techniques were in excellent agreement. The first formed product, cis-/trans-[Co(tmen)2 (NC CH3 )(sol)]3þ , is observed directly for the reaction of cis[Co(tmen)2 (NCCH3 )2 ]3þ in DMSO, and in this case the
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product is 100% cis-[Co(tmen)2 (NCCH3 )(OSMe2 )]3þ , which decays further to cis-[Co(tmen)2 (OSMe2 )2 ]3þ at coincidentally almost the same rate, despite the statistical advantage (2) that the bis(acetonitrile) complex has. For all the other reactions studied (except the exchange processes), the first formed product reacted much more rapidly than it was generated, in losing the second CH3 CN ligand. The role of the Ôorientating groupÕ A in [Co(en)2 (A)X]nþ complexes has been studied extensively in the past [21,32] and has a dramatic effect on the rate at which X is substituted. For the present set of bis(tmen) complexes, replacement of CH3 CN by H2 O increases substantially (>10-fold) the rate of loss of the residual CH3 CN leaving group in the cis complex, whereas DMSO replacement of CH3 CN raises the rate only by a factor of about two. cis-[Co(tmen)2 (NCCH3 )2 ]3þ isomer is more reactive than the trans isomer by a factor of about 5 for the water and acetonitrile solvent systems, and about 40 for DMSO as solvent. The apparent single step substitution reactions for trans-[Co(tmen)2 (NCCH3 )2 ]3þ in water and DMSO therefore are not surprising, since any first formed trans-[Co(tmen)2 (NCCH3 )(sol)]3þ now bears a good leaving group, H2 O or DMSO, and such species can rapidly isomerise, through water or DMSO exchange as appropriate, to generate the more reactive cis[Co(tmen)2 (NCCH3 )(sol)]3þ which then loses the nitrile ligand to generate the observed cis-[Co(tmen)2 (sol)2 ]3þ . The isomerisation and exchange reactions for the [Co(tmen)2 (NCCH3 )2 ]3þ isomers in acetonitrile as solvent, provide meaningful steric course data. 1 H NMR spectroscopy was used to probe the exchange process and UV–Vis spectrophotometry to measure the rates for the trans–cis isomerisation processes. The cis isomer substitutes with substantial retention in dissociating its coordinated CH3 CN, whereas for the trans isomer there is substantial rearrangement (31%) in the CD3 CN reentry process. There are no rate or stereochemical data for the corresponding cis- and trans-[Co(en)2 (NCCH3 )2 ]3þ complexes, although they are known [24] and they are extremely unreactive in water, DMSO and acetonitrile. The tmen complexes are certainly more reactive, but to what extent remains unknown. Given the lack of significant interaction between the two tmen ligands, it has become apparent that one tmen ligand may be sufficient to labilise the metal ion towards substitution reactions. Indeed, the trans[Co(tmen)(en)Cl2 ]þ ion and the four isomers of [Co(dien)(tmen)Cl]2þ have recently been synthesised [27]. They are strikingly reactive compared to their en analogues and it is anticipated that detailed studies on these complexes might elucidate the reason(s) for the dramatic kinetic effect of per-C-methylation on the chelate ring.
Acknowledgements Financial support from the Australian Research Council is gratefully acknowledged.
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