[AU(DIEN)Cl]Cl2: Exchange Phenomena Observed by 1H and 13C NMR Spectroscopy

The solution behaviour of the square-planar gold(III) complex [Au(dien)Cl]Cl2 (dien = 1,5- diamino-3-azapentane) was investigated by 13H and 13C NMR spectroscopy. We have found that 1H NMR spectra of [Au(dien)Cl] Cl2 are characterised by exchange behaviour over the whole pH range, and some exchange effects are also seen in 13C NMR spectra of the deprotonated hydroxo derivative of the complex in alkaline solution. An exchange rate of > 378 s-1 was determined from 1H NMR spectra at pH∗2 (coalescence temperature 40C°). In slightly acidic solutions of the complex, 1H chemical shifts are in accordance with the known deprotonation of the central amine group of the complexed diethylenetriamine ligand. In 13C NMR spectra, two separate sets of resonances are observed for the chloro and the hydroxo complex of gold(III) diethylenetriamine. The hydroxo complex [Au(dien-H)OH+ shows exchange effects in 13C NMR spectra. Variable temperature studies show the carbon atoms next to the central secondary amine to be inequivalent and each present in two different environments that are in intermediate to fast exchange on the NMR time-scale.

INTRODUCTION Gold(Ill) has recently received much attention because of its possible involvement in the 01 biological action/side-effects of anti-arthritic gold(I) drugs'. It has been shown that gold(Ill) is a 2 3 reactive metabolite of gold(I)in mice and humans. The oxidation of gold(I) to gold(Ill) has been demonstrated in vitro and in vivo4''6.
The known gold (111) chelate complex [Au(dien)CI]Cl is of special interest because of its potential as a probe for gold(Ill) binding sites on biological molecules, e.g. nucleotides 7 and peptides .
Chlorodiethylenetriaminegold(lll) chloride was first prepared in 19639 and the crystal 10 structure was solved in 1986 In the solid state, the complex is pseudooctahedral with the nitrogens of the diethylenetriamine ligand occupying three of the equatorial coordination sites around gold(Ill) and one chlorine atom occupying the fourth coordination site in the plane. Two additional chlorine atoms are situated in axial positions with considerably longer Au-CI bond lengths In aqueous solution, the complex [Au(dien)] / has been shown to take part in both acidbase and hydrolytic equilibria . The complex loses a proton from one of the nitrogen ligands with a pK a valueof 4.0 in 0.5 M CIO4 or 4.7 in 0.5 M CI according to equation 1. [Au(dien)CI] + e-> [Au(dien-H)CI] + H (1) It has been confirmed that deprotonation occurs from the central, secondary nitrogen in a single-crystal X-ray diffraction   Proton-decoupled C NMR spectra of [Au(dien)CI]Cl at selected pH* values are shown in Figure 2 and a plot of chemical shift values of the various aC resonances versus pH* is shown in Figure 3. Up to pH* 5.2, only two C signals were observed indicating that carbon atoms C and C e plus C and Co are equivalent. The more downfield of the two resonances (peak 1) was assigned to the equivalent carbon atoms C and Co next to the secondary amine. Peak 2 was assigned to the equivalent carbon atoms C and Ce next to the primary amine groups. This was based on the previous assignment of resonances for the palladium(ll)-diethylenetriamine complex on the basis of chemical shift data for CHNHCHCHNHCH and NHCHCHNH and is in agreement with some previously reported carbon-13 data for the complex6. Both the resonances were pH dependent with peak showing a downfield shift of 4.9 ppm and peak 2 an upfield shift of 1.5 ppm upon pH increase from pH* 3.0 to pH* 6.5 (Figures 2 and 3  In spectra recorded at pH* 6.1 and higher, new resonances (two broad peaks 3a, 3b and one sharp peak 4) appeared and increased in intensity with increasing pH until they were the only resonances observed at pH* 10.0. From previous studies , the complex would be expected to be fully hydrolysed by pH 10, so that the new peaks appearing at pH* > 6.1 were assigned to the deprotonated hydroxo derivative [Au(dien-H)OH] /. From their chemical shift values, the downfield (broad) resonances (peaks 3a and 3b) were again assigned to the carbon atoms C and Cc next to the secondary, central amine, while the upfield, sharp peak (peak 4) was assigned to the carbon atoms Ca and Ca next to the primary amines. The changes in the spectra of [Au(dien)CI] + were reversible as lowering the pH from pH* 10.0 to pH* 5.1 yielded the same spectrum as before the pH increase.

C NMR Studies
Increasing the temperature of a solution of [Au(dien)CI]Cl in DO, pH* 9.9 from 22 C to 60 C led to a clear sharpening of peaks 3a and 3b in 3C-{-H} NMR spectra (Figure 4).
Temperatures higher than 60 C could not be employed due to the instability of the complex.
Vol. 6, Noso [4][5]1999 [  5). In the pH* range 3.0 to 5.9, only two broad peaks were seen which showed pH-dependent shifts. Peak (h5 1.09 ppm in the pH* range 3.0 to 7.0) was broad over the whole investigated pH range. Peak 2 (z5 0.27 ppm in the pH* range 3.0 to 7.0) was broad at low pH values but turned into a relatively sharp triplet in spectra of pH* 4 and above.
The pH-dependent shifts of those peaks were again attributed to the deprotonation of the central secondary amine group. Peak was assigned to the methylene protons of C and Co because of its large pH-dependent shift upon deprotonation of the central amine; peak 2 with a smaller shift difference was assigned to the protons attached to Ca and Cd. An increase of the temperature from 22 C to 60 C of a solution of [Au(dien)CI]Cl in D=O at pH* 3.2 resulted in the appearance of two triplets in place of the very broad resonances seen at this pH* value at 22 C ( Figure 6). Upon lowering the pH* to 2.0 at 22 C, both peak and peak 2 split into two broad resonances (peaks la, lb and 2a, 2b respectively, Figure 7). Upon temperature increase, those peaks broadened and coalesced ( Figure 7) with coalescence temperatures of ca. 30 C for peak 2 and ca. 40 C for peak 1. The calculated exchange rates at these temperatures from the chemical shift difference at 22 C were > 118 s for peak 2 and > 378 s 1 for peak 1. At pH* values of 7.0 and above, additional H NMR resonances were observed. Contrary to the C spectra, where a complete set of new peaks appeared for the hydroxo species at higher pH values, the difference between chloro and hydroxo species was not as clear in H NMR spectra. Peak was observable as a broad resonance up to pH* 12.1 and must therefore represent protons in the chloro-as well as the hydroxo species. Two very broad resonances (peaks 3a and 3b) appeared on either side of peak in spectra between pH* 7.0 and 10.9 ( Figure 5). New resonances also appeared around peak 2 at pH* values of 7.9 and higher (peak 4). They were not of equal intensity and were difficult to follow. Variable temperature experiments of a solution at pH* 9.9 ( Figure 8) showed peak sharpening to form a still broad triplet at higher temperatures, while the very broad peaks 3a and 3b broadened and coalesced at ca. 40 C. The exchange rate at this temperature was > 443 s from the chemical shift difference at 22 C. At the same pH* value, the resonances for the methylene protons of C a and Ce (peaks 2 and 4) broadened upon temperature increase.  (Figure 9). The former two crosspeaks can be assigned to the two -CH-NH groups (Ca and Ca) and the latter two to the -NHCH resonances in the one-dimensional spectra. Exchange processes in the hydroxo species [Au(dien-H)(OH)] / account for the broad appearance of peaks 3a and 3b in the aC spectra ( Figure 2). Upon increase in temperature from 22 C to 60 C (Figure 4), the two broad peaks become sharper. The sharpening of the resonances suggests that the two resonances are not exchanging with each other, otherwise signals would be expected to become broader at higher temperatures and eventually coalesce to a single peak. In contrast to this, an increasing sharpening of the peaks is observed that can only be explained by assuming that each of the two broad peaks is already an average of two resonances in intermediate to fast exchange on the NMR time-scale. The carbon atoms next to the terminal amines, Ca and C stay equivalent under the conditions used. This behaviour suggests that the carbon atoms C and Co next to the central amine in [Au(dien-H)] + (peaks 3a and 3b) are inequivalent in the hydroxo species and that each of the inequivalent carbon atoms C and Co is present in two different environments that are in intermediate to fast exchange on the NMR time-scale. The different environments may be due to different conformations of the five-membered chelate rings which exist in two enantiomeric forms (X and .). with the interconversion between these structures proceeding via an envelope conformation '.  However, the interconversion is usually believed to be too rapid to be observed . Various explanations are possible for the inequivalence of C and Co in the hydroxo-species. It might be due to the presence of a hydroxo-bridged dimer. The crystal structure of dimethylgold(lll) hydroxide, a hydroxo-bridged tetrameric gold(Ill) complex, has been reported 2 and hydroxobridged complexes are well known for palladium(ll) and platinum(ll) complexes. Restricted rotation in a possible dimer could account for the inequivalence of the carbon atoms. Another possibility is hydrogen-bonding with axial hydroxide substituents around gold(Ill) which could render the molecule less symmetrical. It must be noted that the carbon atoms are equivalent in the chloro species [Au(dien-H)CI] /. In this complex, the axial substituents are chloride ions and they are thought to be replaced by hydroxide upon increasing the pH of the solutionS.
Proton NMR spectra of [Au(dien)CI]Cl show exchange-broadening over the whole pH range, i.e. both chloro and hydroxo complexes are affected. The broadening at low and at higher pH is likely to be caused by different mechanisms. At low pH (pH* 2.0, Figure 7), broadening could be explained most easily by a ring-opening and -closing equilibrium involving protonation of one of the terminal NH groups. An equilibrium between unprotonated ringclosed and protonated ring-opened species of [Au(dien)CI] + has been suggested to occur in acidic solution on the basis of kinetic studies . Ring opening at one end of the molecule in [Au(dien)CI]Cl_ would render the methylene groups of the two five-membered chelate rings inequivalent and broadening would occur if the exchange between ring-opened and ringclosed species was at an intermediate rate on the NMR time-scale. As the H concentration decreases, ring-opening would be suppressed, consistent with the sharpening of the resonances for the methylene protons of C, and Ce at pH* > 3 (peak 2 in Figure 5). However, the observation of only two carbon resonances at pH* 1.5 suggests that no ring-opening reaction occurs at that pH* value. The inequivalence of the H resonances in strongly acidic solution must therefore be due to a different mechanism, possibly again an interconversion of different conformations of the five-membered chelate rings.
The exchange rates calculated from H NMR temperature studies at pH* 2 and 10 are estimates because of the broadness of the resonances at the lowest temperatures studied. The maximum chemical shift difference of the exchanging protons in the two environments might be larger and the exchange rates are therefore lower limits Additional resonances in'H NMR spectra of [Au(dien)CI]Cl (peaks 3a, 3b and 4, Figure 5) occur above pH* 7 in solutions of the complex alone, but not in spectra of a mixture of [Au(III)(Gly-Gly-L-His-H.2)] + and the complex, in which the major species between pH* 6 and 12 is the bridged imidazole complex9. Therefore, those additional resonances are a characteristic of the hydroxo complex, possibly due to a hydroxo-bridged complex, rather than a feature of the diethylenetriamine ligand.
We have shown in this work that the seemingly simple complex of gold(Ill) with diethylenetriamine exhibits an extremely complex solution behaviour. More work is certainly needed to unravel the basis of the various exchange processes observed in [Au(dien)] +.