DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 2171–2175 2171 Synthesis and electrochemistry of [Pt(tame)2]41: crystallographic analysis of bis[1,1,1-tris(aminomethyl)ethane-N,N9]platinum(II) bis(tetrachlorozincate) dihydrate Kylie N. Brown,a David C. R. Hockless a and Alan M. Sargeson b a Research School of Chemistry, Australian National University, A.C.T., 0200 Australia b Chemistry Department, Australian National University, A.C.T., 0200 Australia Received 4th March 1999, Accepted 7th May 1999 Reaction of K2PtCl6 with the tripodal ligand tame [1,1,1-tris(aminomethyl)ethane] in dimethylformamide catalysed by K2PtCl4 aVorded the octahedral bis(tridentate ligand) [Pt(tame)2]41 ion.The cyclic voltammogram of [Pt(tame)2]41 in aqueous media showed an irreversible reduction of the six-co-ordinate platinum(IV) species to PtII and bulk electrochemical reduction of the [PtIV(tame)2]41 ion quantitatively produced a square planar platinum(II) complex [Pt(tame)2]21.Crystallographic analysis of the protonated complex [PtII(Htame)2][ZnCl4]2?2H2O, showed two dissociated nitrogen atoms on opposite sides of the PtN4 21 co-ordination plane which has typical PtII–N bond lengths (2.042(6) Å). The routes to the formation of the [PtIV(tame)2]41 ion and its reduced product are addressed. Introduction Platinum(IV) sarcophagine† cage complexes, namely [Pt(sep)]41, [Pt((NO2)2sar)]41, and [Pt((NHOH)2sar)]41, have been synthesized previously using [Pt(en)3]41 as a template,1 in analogy to the aqueous capping methods devised for [Co(en)3]31 encapsulation 2 (Scheme 1).It was also anticipated that the platinum(IV) cage complexes with expanded cavities, such as [Pt(tricosaneN6)]41, could be synthesized using the capping strategy described for [Rh(tricosaneN6)]31 complexes, where tris(propane-1,3-diamine)rhodium(III) ([Rh(tn)3]31) and [1,1,1- tris(5-amino-2-azapentyl)ethane]rhodium(III) ([Rh(stn)]31) ions were used as templates (Scheme 2).3 Since the anticipated PtIV– N bond length for the saturated PtN6 41 cage (ª 2.07 Å) is comparable to that of Rh–N in saturated RhN6 31 systems (2.09 Å),3 the synthesis of platinum(IV) tricosaneN6 complexes should be feasible using either [Pt(tn)3]41 or [Pt(stn)]41 complexes as templates.However, the syntheses of the necessary precursors for the platinum(IV) capping reactions, namely [Pt(tn)3]41 and [Pt(stn)]41, have not been reported and surprisingly attempts to synthesize the [Pt(tn)3]41 ion were not fruitful using modified syntheses which had been successful for [Pt(en)3]414 and [Pt(en)x- (pn)3 2 x]41 (x = 3, 2, 1 or 0).5 These routes produced mostly Scheme 1 Capping [Pt(en)3]41.(i) CH3NO2 or NH3, CH2O and base, in water. † Abbreviated ligand names used: sar, sarcophagine = 3,6,10,13,16,19- hexaazabicyclo[6.6.6]icosane; sep, sepulchrate = 1,3,6,8,10,13,16,19- octaazabicyclo[6.6.6]icosane; tricosaneN6 = 3,7,11,15,18,22-hexaazabicyclo[ 7.7.7]tricosane; Me5-tricosanetriimineN6 = 1,5,9,13,20-pentamethyl- 3,7,11,15,18,22-hexaazabicyclo[7.7.7]tricosa-3,14,18-triene; tame = 1,1,1-tris(aminomethyl)ethane; [9]aneN3 = 1,4,7-triazacyclononane; pn = propane-1,2-diamine. [Pt(tn)Cl2], [Pt(tn)2]21, trans-[Pt(tn)2Cl2]21 and red polymeric species of unknown constitution, regardless of the temperature and solvents including dimethylformamide, water, dimethyl sulfoxide and ethanol.In no case was the target [Pt(tn)3]41 ion isolated even after using ion-exchange chromatography.6 Similar attempts to synthesize [Pt(stn)]41 were also unsuccessful. Red oils were obtained when free stn was treated with K2[PtCl6] in the presence of K2[PtCl4] in a variety of solvents (e.g.water, DMF, DMSO, alcohols). NMR Spectroscopy and ion exchange chromatography also showed these oils to be complex mixtures. An alternative route to the synthesis of bicyclic platinum(IV) tricosaneN6 cage complexes was envisaged by strapping the template [Pt(tame)2]41 from top to bottom (Scheme 3).A precedent for such a path existed for the analogous cobalt(III) template, which when treated with formaldehyde and a range of aldehydes in acetonitrile gave the [Co(tricosanetriimineN6)]31 framework.7 The synthesis for the simple [Pt(tame)2]41 complex had not been reported, however, and this paper addresses its synthesis and redox properties. The tripodal tame ligand coordinates facially to the metal ion, in analogy to the [9]aneN3 and [9]aneS3 (trithiacyclononane) ligands, but is more flexible.Scheme 2 Capping [Rh(tn)3]31 and [Rh(stn)]31. (i) CH3NO2, CH2O and base, in water.2172 J. Chem. Soc., Dalton Trans., 1999, 2171–2175 The reactivities of the three bis(tridentate ligand) complexes are compared. Results and discussion A good yield of [Pt(tame)2]Cl4 (75%) was obtained after 40 h when free tame was added to a solution of K2[PtCl6] dissolved in warm dimethylformamide at 40 8C with a catalytic amount of K2[PtCl4].When the reaction was performed in ethanol and heated for three hours at 70 8C a slightly lower yield of the desired product was obtained. The 1H and 13C NMR spectra for [Pt(tame)2]Cl4 in D2O were consistent with that for an octahedral bis(tridentate ligand) complex with D3h symmetry. For example, the methyl, quaternary and methylene carbon atom resonances occurred respectively at d 21.80, 45.15 and 47.20 in D2O. The 195Pt coupling constants with the methylene and quaternary carbon atoms are typical for complexes of this type (2JPt–C = 8.4; 3JPt–C = 44.2 Hz, respectively).8 The nitrogen protons rapidly exchange with D2O and were not observed.This is consistent with the first pKa (7.0 ± 0.1) which is characteristic for simple hexaamine platinum(IV) complexes.9 The formation of the [Pt(tame)2]41 template is likely to be initiated by tame substitution at the catalytic platinum(II) ion to form a square planar complex, as outlined in Scheme 4.Oxidation by PtCl6 22 then takes place via a bridged intermediate, accompanied by co-ordination of the pendant amines on the two tame residues, to generate the six-co-ordinate hexaamine geometry preferred by PtIV. An equivalent amount of the catalyst PtCl4 22 ion is also regenerated. This proposal parallels the mechanism for ligand substitution about the platinum(IV) ion in the presence of PtII described previously for didentate ligands.10–13 It was hoped that the PtIV–PtIII couple would show a degree of reversibility, given the bis(tridentate ligand) nature of the complex.However, the cyclic voltammetry (CV) of the parent [Pt(tame)2]Cl4 in aqueous 1 M HClO4 using a hanging mercury drop electrode showed an irreversible electrochemical reduction wave at Epc = 10.02 V (vs. SHE, 100 mV s21, Fig. 1). The reduction potential was largely insensitive to electrolyte, acid and scan rate up to 0.5 V s21, and was slightly more positive than that for [Pt(en)3]41,1 presumably because of the larger bite size of the tame chelate.The response was also irreversible in acetone at 295 K, using edge-plane pyrolytic graphite and gold disc electrodes. No response was observed using a platinum disc electrode in acetone. Formation of the [Pt(tame)2]31 complex was not evident in either aqueous media or acetone. The response was also irreversible up to 5 V s21. The electrochemical data are attributed to a two-electron reduction of PtIV, accompanied by dissociation of two nitrogen donor atoms, to give a square planar platinum(II) complex as shown in Scheme 5.This is supported by the coulometric reduction of [Pt(tame)2]Cl4 at 20.80 V vs. SCE in 0.1 M CF3CO2H and also in 0.1 M NaClO4 using a mercury pool working electrode which clearly showed a two-electron process. The 1H and 13C NMR spectra of the bulk electrolysis product were relatively simple and indicated that only one product was formed. The microanalysis of the isolated Scheme 3 Strapping [Pt(tame)2]41 to form [Pt(Me5-tricosanetriimineN6)] 41.(i) CH3CH2CHO, CH2O and base, in acetonitrile. tetrachlorozincate salt also implied that the two dissociated amine groups were protonated. The 1H NMR spectrum of [PtII(Htame)2]Cl4 in D2O showed that the signals for the methylene protons on the dissociated strand of the tame ligand were poorly resolved, and vicinal platinum-195 coupling was not discernible. The splitting pattern of the methylene protons of the co-ordinated methylene amine strands was consistent with that expected for an AA9 system, presumably due to the less flexible environment induced by co-ordination.However, the configuration about the platinum(II) ion was not defined by the NMR data. Two amine donor groups clearly had dissociated cis or trans as a result of the reduction, to form a complex with either C2h or C2v Fig. 1 Cyclic voltammogram of [Pt(tame)2]41 in 1 M HClO4 (100 mV s21, vs.Ag–AgCl–KCl (sat.), hanging mercury drop electrode). Scheme 4 Proposed mechanism for [Pt(tame)2]41 synthesis (some amine protons have been omitted for clarity).J. Chem. Soc., Dalton Trans., 1999, 2171–2175 2173 symmetry respectively. However, both isomers would give rise to the same number of 13C signals and similar 1H NMR splitting patterns. An X-ray crystallographic analysis of a single crystal of the tetrachlorozincate salt of the product was therefore necessary to determine its structure. The structural analysis confirmed that two trans nitrogen atoms had dissociated. The ORTEP14 plot of the cation in [PtII(Htame)2][ZnCl4]2?2H2O is presented in Fig. 2. Similar trans stereochemistry has been observed for the two ligands in [Pt([9]aneN3H)2]41.15 The interatomic distances and angles are listed in Table 1 and other relevant crystallographic data are in Table 2. The Pt–N bond lengths are consistent with those of platinum(II) saturated amine complexes. The six-membered chelate rings of both tame residues are in a chair conformation with their dissociated pendant amines in equatorial positions and far from the feasible trans bonding sites on the Pt.Scheme 5 Electrochemical reduction of [PtIV(tame)2]41 (amine protons have been omitted for clarity). Fig. 2 An ORTEP diagram of the cation in [PtII(Htame)2][ZnCl4]2? 2H2O. Table 1 Selected interatomic distances (Å) and bond angles (8) for [PtII(Htame)2][ZnCl4]2?2H2O Pt(1)–N(1) Pt(1)–N(2) N(3)–C(5) C(2)–C(3) C(2)–C(5) N(1)–Pt(1)–N(1) N(1)–Pt(1)–N(2) Pt(1)–N(1)–C(3) C(1)–C(2)–C(3) C(1)–C(2)–C(5) C(3)–C(2)–C(5) N(1)–C(3)–C(2) N(3)–C(5)–C(2) 2.042(6) 2.042(6) 1.51(1) 1.53(1) 1.53(1) 180.0 87.9(3) 116.3(5) 111.8(7) 110.3(7) 108.8(6) 113.3(7) 115.1(7) N(1)–C(3) N(2)–C(4) C(1)–C(2) C(2)–C(4) N(1)–Pt(1)–N(2) N(2)–Pt(1)–N(2) Pt(1)–N(2)–C(4) C(1)–C(2)–C(4) C(3)–C(2)–C(4) C(4)–C(2)–C(5) N(2)–C(4)–C(2) 1.50(1) 1.49(1) 1.53(1) 1.53(1) 92.1(3) 180.0 118.6(5) 111.3(6) 110.0(6) 104.4(6) 114.6(7) It is not surprising that the two amine groups trans to each other dissociated during the reduction process since this is the pathway that requires the least rearrangement.It is also well known that square planar platinum(II) complexes interact weakly with most nucleophiles at these axial sites. It is instructive therefore to look at the reduction of the [Pt(en)3]41 ion. NMR Spectrometry was used to monitor the hydrogenation of [Pt(en)3]41 in the presence of Pd/C in 1 M DCl.The experiment was undertaken under acidic conditions in order to trap the dissociated amines and limit their reco-ordination to the platinum( II) centre. After 20 min of hydrogenation at pH ª 1 the 1H NMR spectrum showed four signals: three triplets at d 2.68 (ª 1 H), 3.11 (1 H) and 3.32 (1 H) and a singlet at d 3.38 (0.2 H). Four signals were observed in the 13C NMR spectrum at d 37.4, 40.1, 44.6 and 48.1.None of these spectra had signals consistent with those of the parent [Pt(en)3]41 ion, whose 1H and 13C resonances occur at d 3.26 and 49.4, respectively. These data are consistent with the sample containing mostly [Pt(en)- (NH2CH2CH2NH3)2]41 ion with about 20% [Pt(en)2]21 and free ethylenediamine: the triplet at d 2.68 is attributed to the methylene protons of the chelate in [Pt(en)(NH2CH2CH2NH3)2]41 and the two triplets at d 3.11 and 3.32 are attributed to the methylene protons of the unidentate ethylenediamine ligands.The singlet at d 3.38 is ascribed to the methylene protons of the dissociated and protonated ethylenediamine; the ethylenediamine protons of [Pt(en)2]21 (at d 2.65) overlap with those of the chelate in [Pt(en)(NH2CH2CH2NH3)2]41. Likewise, the assignment of the 13C spectrum is as follows: d 37.4 (free ethylenediamine), 40.1 and 44.6 (unidentate ethylenediamine) and 48.1 (didentate ethylenediamine for the [Pt(en)(NH2CH2CH2- NH3)2]41 ion overlapped with those of the [Pt(en)2]21 ion, d 47.6).The 1H and 13C NMR spectra of the products from the hydrogenation performed under neutral conditions were very diVerent. After 30 min of hydrogenation, followed by addition of DCl, only two major signals were evident in the 1H NMR spectrum, a triplet at d 2.67 and a singlet at d 3.39 and are attributed to [Pt(en)2]21 and protonated ethylenediamine, respectively. The 13C NMR spectrum also showed two major resonances, at d 47.6 ([Pt(en)2]21) and 37.4 (free ethylenediamine).However, in both the NMR spectra, signals of very low intensity were observed which were consistent with traces of the [Pt(en)(NH2CH2CH2NH3)2]41 ion. The implication of these two results is that PtIV will preferentially dissociate two trans groups. The formation of [Pt(en)2]21 and ethylenediamine in the neutral reduction may seem to be an exception to this requirement. However, this product almost certainly arises from rapid intramolecular amine addition, rearrangement and diamine dissociation in the initially formed complex [PtII(en)(NH2CH2CH2NH2)]21 ion.There will also be other occasions when such facile subsequent events obscure the initial product, particularly with multidentate systems.16 Formation of the platinum(III) ion was, likewise, not evident in attempts electrochemically to oxidise the platinum(II) Table 2 Crystal data for [PtII(Htame)2][ZnCl4]2?2H2O Chemical formula M Crystal system Space group a/Å b/Å c/Å b/8 V/Å3 Z m(Cu-Ka)/cm21 No.reflections measured unique Rint Residuals R, R9 C10H36Cl8N6O2PtZn2 881.91 Monoclinic P21/n (no. 14) 12.012(2) 10.194(2) 12.012(1) 112.276(7) 1361.1(3) 2 185.33 2284 2171 0.049 0.036, 0.0472174 J. Chem. Soc., Dalton Trans., 1999, 2171–2175 counterpart in aqueous media and in acetone, in contrast to the behaviour of the analogous [Pt([9]aneS3)2]31/21 couple in acetonitrile, where [Pt([9]aneS3)2]31 was produced from the bulk oxidation of [Pt([9]aneS3)2]21.17 The more polarisable sulfur donor atoms appear to help to stabilise PtIII.The diYculty in reco-ordinating nitrogen atoms compared to the sulfur atoms in the transformation from square planar PtII to octahedral PtIII might be explained by initial oxidation of the sulfur atoms followed by their intramolecular reduction by the platinum(II) ion to form PtIII. The same type of path is much less accessible for the nitrogen-containing ligands.Attempts chemically to generate the platinum(III) complex were also unsuccessful: the absorbance changes of a solution containing equimolar amounts of [PtII(Htame)2]Cl4 and the one electron oxidant ferrocenium tetrafluoroborate were negligible over 120 min at 298 K. The inability to regenerate the [PtIV(tame)2]41 from [PtII(Htame)2]41 was also evident when a solution of [PtII- (Htame)2]Cl4 in D2O was purged with oxygen for 3 d at 323 K. This behaviour, however, is not inconsistent with that of the similar complex [PtII([9]aneN3H)2]41 which took 20 h to oxidise to its related platinum(IV) hexaamine complex at 90 8C.15 The CV of the bulk-reduced [PtII(Htame)2]41 solution in 0.1 M aqueous NaClO4 showed an irreversible anodic response at 0.22 V vs.SCE using the edge-plane pyrolytic graphite electrode which disappeared after a second sweep. This is akin to passivation of the electrode which has been observed with more complicated platinum(II) tetraamine complexes.7,18 Bulk electrochemical oxidation of the same reduced solution using a carbon rod electrode at 0.4 V vs.SCE for 12 h was likewise unsuccessful: the 1H and 13C NMR spectra of the desalted electrolysed solution showed that no oxidation had occurred. It is not clear why the oxidation to reform the platinum(IV) hexaamine complex was unsuccessful, especially since the steric demands of the tame ligand are not high and access of oxidants is not hindered.It might be that eVective oxidation requires an inner-sphere pathway and the oxidants used here were not suitable. Despite the inability to regenerate the platinum(IV) hexaamine complex readily, it is clear that the synthetic pathway involves cycling between platinum-(II) and -(IV) species and this process requires further investigation. The use of the [Pt(tame)2]41 ion in encapsulation reactions will be described in subsequent publications. Experimental Syntheses All chemicals (AR grade) were used as received unless otherwise specified.Bio-Rad analytical grade Dowex 50W-X2 (200– 400 mesh, H1 form) was used in the cation exchange chromatography. All evaporations were conducted with Buchi rotatory evaporators at Torr ª 16–20 using a water-bath (< 50 8C). 1,1,1-Tris(aminomethyl)ethane (tame). The compound tame?3HCl (25.5 g) prepared as described previously19 was suspended in hot ethanol (1 L) and added slowly to a warm solution of NaOH (13.5 g) in ethanol (200 mL).The mixture was heated at reflux under a stream of nitrogen for 3 h. The ethanol was evaporated and the tame ligand extracted from the white residue using hot chloroform. The suspension was filtered and the solvent evaporated to yield a pale yellow oil. NMR (D2O): 1H, d 0.80 (s, 1 H, CH3) and 2.45 (s, 2 H, CH2); 13C, d 18.64 (CH3), 39.81 (quaternary carbon) and 45.64 (CH2). Bis[1,1,1-tris(aminomethyl)ethane-N,N9,N0]platinum(IV) tetrachloride monohydrate, [Pt(tame)2]Cl4?H2O.Free tame (2 equivalents, 1.0 g) was added dropwise to a stirring suspension of K2PtCl6 (2 g) in dimethylformamide (15 mL) in the dark; K2PtCl4 (ª 5 mg) was added to catalyse the reaction. A clear orange solution formed after the tame was added and within ª5 min a colourless precipitate was evident. The reaction was heated at 40 8C for 30 h. The reaction mixture was diluted to 500 mL with water, the pH adjusted to 4–5 with HCl and then the solution was sorbed onto a column (15 × 3 cm) of Dowex 50W-X2 cation exchange resin.The column was washed with water (500 mL) and 2 M HCl (500 mL) and the complex then eluted with 6 M HCl. Evaporation of the eluate to near dryness yielded a colourless powder which was filtered oV, washed with ethanol and then 2-propanol. The powder was dried in vacuo over molecular sieves. Yield: 75% (Calc. for C10H30Cl4N6Pt? H2O: C, 20.38; H, 5.47; Cl, 24.06; N, 14.26; Pt, 33.11. Found: C, 20.35; H, 5.71; Cl, 24.67; N, 14.13; Pt, 32.95%). pKa1 7.0 ± 0.1, pKa2 11 ± 0.1 at 25 8C (0.1 mmol titrated with 0.100 M NaOH in 10 mL H2O potentiometrically; data were analysed with SUPERQUAD).20 NMR (D2O): 1H, d 1.10 (s, 1 H, CH3) and 2.87 (t, 1 H, CH2, 3JPt–H = 10.3 Hz); 13C, d 21.80 (s, CH3), 45.15 (t, quaternary, 3JPt–C = 44.2) and 47.20 (t, CH2, 2JPt–C = 8.4 Hz).[PtII(Htame)2]Cl4. Controlled potential electrolysis of [Pt(tame)2]Cl4?H2O (207 mg) in 0.1 M NaClO4 (ª 20 mL) at 2800 mV vs.SCE using a mercury pool working electrode indicated that 1.96 electrons per platinum(IV) ion were consumed. After electrolysing for 12 h the solution was decanted. The mercury pool was rinsed three times with water ( ª10 ml). The combined washings and the electrolysed solution were loaded onto a 2 × 5 cm column of Dowex cation exchange resin which was then washed with water (500 mL), 1 M HCl (500 mL) and the product eluted with 6 M HCl. The 6 M eluate was evaporated to dryness to yield a cream-coloured powder (90%).Crystals of trans-[PtII(Htame)2][ZnCl4]2?2H2O were slowly grown from an aqueous solution containing two drops of a saturated solution of ZnCl2 in 4 M HCl. NMR (D2O): 1H, d 1.20 (s, 3 H, CH3), 2.74 (m, 4 H, CH2), 3.10 (s, 2 H, CH2), 5.08 (broad s, NH2) and 5.35 (broad s, NH2); 13C, d 18.4 (CH3), 37.3 (quaternary), 46.1 (CH2), 48.9 (CH2) and 49.2 (CH2). Hydrogenation of [Pt(en)3]Cl4. The complex [Pt(en)3]Cl4 (20 mg) was dissolved in 1 M DCl (0.7 mL) in an NMR tube and Pd/C catalyst (10%, ª3 mg) was introduced. The suspension was purged gently with hydrogen for 20 min and then with nitrogen for 5 min.The sample was sealed and centrifuged for 5 min before the 1H and 13C NMR spectra were acquired. Assignment of the signals was aided by doping the sample with ethylenediamine after the spectrum had been acquired and by comparing the NMR spectra of separate samples of the precursors and products in 1 M DCl.NMR (1 M DCl): 1H, d 2.68 (t, ª1 H, co-ordinated ethylenediamine of [Pt(en)(NH2CH2- CH2NH3)2]41, overlapping a small signal from [Pt(en)2]21), 3.11 (t, 1 H, unidentate ethylenediamine of [Pt(en)(NH2CH2CH2- NH3)2]41), 3.32 (t, 1 H, unidentate ethylenediamine of [Pt(en)- (NH2CH2CH2NH3)2]41) and 3.38 (s, 0.2 H, unco-ordinated ethylenediamine); 13C, d 37.4 (unco-ordinated ethylenediamine), 40.1 (unidentate ethylenediamine of [Pt(en)(NH2CH2CH2- NH3)2]41), 44.6 (unidentate ethylenediamine of [Pt(en)(NH2- CH2CH2NH3)2]41) and 48.1 (co-ordinated ethylenediamine of [Pt(en)(NH2CH2CH2NH3)2]41, overlapping a small signal from [Pt(en)2]21).The above procedure was repeated in neutral D2O, using 20 mg [Pt(en)3]Cl4 and hydrogenating for 30 min instead. Assignment of the signals was aided by doping the sample with ethylenediamine after the spectrum had been acquired and by comparing the NMR spectra of separate samples of the precursors and products in neutral D2O.NMR after acidification (ª 0.1 M DCl): 1H, d 2.67 (t, 2 H, [Pt(en)2]21) and 3.39 (s, 1 H, uncoordinated ethylenediamine); 13C, d 37.4 (unco-ordinated ethylenediamine) and 47.6 ([Pt(en)2]21). Physical methods All 1H and 13C NMR spectra were acquired using a Varian Gemini 300 MHz spectrometer and standard Varian software.J. Chem. Soc., Dalton Trans., 1999, 2171–2175 2175 The solvents D2O and DCl (Merck) were used without further purification. All spectra were referenced internally against 1,4-dioxane (d 3.744 vs.(CH3)4Si for the 1H NMR spectra and d 67.3 vs. (CH3)4Si for 13C NMR spectra).21 The electrolytes used in the aqueous electrochemistry were of AR grade. The electrolyte concentration was typically 0.1 or 1.0 M. The concentration of the electroactive species was ª1 mM. The samples were purged for ª15 min with a continuous flow of argon or nitrogen prior to data acquisition. Measurements were acquired under a blanket of dinitrogen or argon at ª293 ± 1 K unless otherwise specified.The cyclic voltammograms using a mercury drop working electrode were recorded using a Princeton Applied Research Model-170 Polarographic Analyser or Model-173 Universal Programmer in conjunction with a Model-175 Potentiostat/Galvanostat (PAR-175). Both systems were interfaced with a Hewlett-Packard 7046A (X,Y) plotter. The mercury electrode (Metrohm 663 VA stand with an RSC Model-411 interface unit) was generally used in the hanging mercury drop mode.The three-electrode configuration included an auxiliary electrode, which was a carbon rod (ª0.4 cm diameter, ª8 cm in length), and the reference electrode, which was either a Ag–AgCl–KCl(sat) (199 mV vs. SHE)22 or a saturated calomel electrode (SCE, 241 mV vs. SHE).22 Structure determination The X-ray crystallographic analysis of a single crystal of [Pt(Htame)2][ZnCl4]2?2H2O was made using a Rigaku AFC-6R diVractometer with graphite monochromated Cu-Ka (l= 1.54178 Å) radiation and a rotating anode generator.The data (Table 2) were collected using the w–2q scan technique to a maximum 2q value of 120.18. No decay correction was applied. The refinement reflections, 1799 [I > 3s(I )], were corrected for Lorentz-polarisation eVects. The structure was solved by direct methods23 and expanded using Fourier techniques.24 The nonhydrogen atoms were refined anisotropically. Hydrogen atoms were included but not refined. Neutral atom scattering factors were taken from Cromer and Waber.25 Anomalous dispersion eVects were included in Fcalc;26 the values for Df 9 and Df 0 were those of Creagh and McAuley.27 The values of the mass attenuation coeYcients were those of Creagh and Hubbel.28 CCDC reference number 186/1455.Acknowledgements This research was supported in part by the Australian Research Council. The authors are also grateful to the ANU Microanalytical Service and D. Bogsanyi for the pKa determination.References 1 H. A. Boucher, G. A. Lawrance, P. A. Lay, A. M. Sargeson, A. M. Bond, J. C. Sangster and J. C. Sullivan, J. Am. Chem. Soc., 1983, 105, 4562. 2 R. J. Geue, T. W. Hambley, J. M. Harrowfield, A. M. Sargeson and M. R. Snow, J. Am. Chem. Soc., 1984, 106, 5478 and refs. therein. 3 R. J. Geue, M. B. McDonnell, A. 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