Polyhedron 21 (2002) 2283 /2291 www.elsevier.com/locate/poly
Base hydrolysis kinetics and equilibria of [bis(2-pyridylmethyl)amine]chloroplatinum(II) and crystal and molecular structures of [Pt(bpma)Cl]Cl × H2O and [Pt(bpma)(OH2)](ClO4)2 × 2H2O /
/
Bruno Pitteri a,*, Giuliano Annibale a, Giampaolo Marangoni a, Valerio Bertolasi b, Valeria Ferretti b b
a Dipartimento di Chimica, Universita` Ca’ Foscari di Venezia, Calle Larga S. Marta 2137, 30123 Venezia, Italy Dipartimento di Chimica e Centro di Strutturistica Diffrattometrica, Universita` di Ferrara, Via Borsari 46, 44100 Ferrara, Italy
Received 19 April 2002; accepted 11 July 2002
Abstract The kinetics of base hydrolysis of [Pt(bpma)Cl]Cl [bpma/bis(2-pyridylmethyl)amine] have been studied (5 /10 4 5/[OH ]5/ 0.2 mol dm 3) at 25 8C and I /0.2 mol dm3 by UV /Vis spectrophotometry. The derived rate law is consistent with a mechanism involving a rapid preequilibrium between the substrate [Pt(bpma)Cl] and the amido species [Pt(bpmaH 1)Cl] (pKa1 /12.3) followed by slow solvolysis to give the corresponding aqua species [Pt(bpma)(OH2)]2 [ks /(2.89/0.2)/10 4 s 1] and [Pt(bpmaH 1)(OH2)] [ks ? /(3.289/0.02) /10 3 s 1] which undergo deprotonation and are converted to unreactive hydroxo species. The aqua complex [Pt(bpma)(OH2)](ClO4)2 ×/2H2O, independently isolated in the solid, behaves as a dibasic acid in water solution (25 8C, I /0.2 mol dm3), the two ionization constants being related to the dissociation of the water molecule in the aquoamine (pK1 /5.49/0.1) and of the amino group in the hydroxo-amino species (pK2 /11.59/0.1), respectively. The X-ray crystal structures of [Pt(bpma)Cl]Cl ×/H2O and [Pt(bpma)(OH2)](ClO4)2 ×/2H2O are reported. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Base hydrolysis kinetics; Equilibria; Crystal and molecular structures
1. Introduction The effect of the deprotonation of an amine ligand coordinated to a transition metal ion on the substitutional lability of the other ligands has been extensively studied in the case of octahedral Co(III) complexes. Rate enhancements by factors ranging from 105 to 1013 in favour of the amido species have been observed [1]. As far as square planar d8 substrates are concerned, investigations on this subject are very limited. Baddley and Basolo [2] studied chloride substitution by bromide on the system [Au(dien)Cl]2/[Au(dienH 1)Cl] (dien /1,5-diamino-3-azapentane) and found that the
* Corresponding author. Tel.: /39-041-257 8558; fax: /39-041-257 8517 E-mail address:
[email protected] (B. Pitteri).
reactivity was halved in the amido complex. On the contrary, in the solvolysis reactions of the system [Au(Et4dien)Cl]2/[Au(Et4dienH 1)Cl] a factor of 60 in favour of the amido species was found [3]. A similar factor (30) has been reported for the hydrolysis of the [Pd(Et4dien)Cl]/[Pd(Et4dienH 1)Cl] system [4]. Romeo et al. studied the displacement of dimethylsulfoxide (dmso) from [Pt(en)(dmso)2]2 [5], [Pt(dien)(dmso)]2 [5], [PtMe(dpa)(dmso)] (en /ethylendiamine, dpa / bis(2-pyridyl)amine) [6] and the corresponding amido species. In these cases the differences in reactivity between the amino and the amido species were small and not always in the same direction. We have recently reported the synthesis of the cationic species [Pt(bpma)Cl] [bpma /bis(2-pyridylmethyl)amine] and studied the chloride substitutions with nucleophiles [7]. In aqueous basic solution the complex undergoes deprotonation of the secondary
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 1 1 6 9 - 5
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amine to give the conjugate amido species [Pt(bpmaH 1)Cl]. Following on these observations and considering the paucity of kinetic data on amine deprotonation of Pt(II) systems and the effect of amine deprotonation on associatively activated substitution reactions, we thought of interest to carry out a kinetic study on the base hydrolysis of [Pt(bpma)Cl]. An investigation on the acid /base behaviour of the aqua species [Pt(bpma)(OH2)]2 has also been performed in order to elucidate the nature and the distribution of the species formed in solution. The crystal structures of [Pt(bpma)Cl]Cl ×/H2O and [Pt(bpma)(OH2](ClO4)2 ×/2H2O are also included.
2. Experimental
2.1. Materials Pure reagent grade NaBr, LiClO4, NaCH3SO3, AgClO4 and AgCF3SO3 (Fluka and Aldrich) were dried over P2O5 in a vacuum desiccator and used without further purification. Deuterated solvents were furnished by Euriso-top. Bis(2-pyridylmethyl)amine was prepared according to a published method [8] and its identity confirmed by 1H NMR.
2.3.2. Aqua[bis(2-pyridylmethyl)amine]platinum(II)trifluoromethanesulfonate, [Pt(bpma)(OH2)](CF3SO3)2 (2) Silver trifluoromethanesulfonate (0.257 g, 1 mmol) was added to a warm solution (70 8C) of [Pt(bpma)Cl]Cl ×/H2O (0.241 g, 0.5 mmol) in water (20 cm3) and the mixture stirred in the dark for 15 min. The AgCl formed was filtered off and the solution evaporated to dryness. The title compound was obtained as a white powder in almost quantitative yield. (Anal . Found: C, 23.6; H, 1.96; N, 5.95. C14H15F6N3O7PtS2 requires: C, 23.7; H, 2.13; N, 5.91%). LM (dmf / dimethylformamide) /139 V1 cm2 mol 1; lmax/nm (water) 267, 274 (sh) (o /dm3 mol1 cm 1 /14 100); dH (200 MHz; solvent D2O; protonated signals of the solvent downfield from SiMe4 as reference) 8.20 [2H, d, J /5.3 Hz, J(Pt /H) /28 Hz, H6], 7.99 (2H, t, J /7.8 Hz, H4), 7.45 (2H, d, J /7.7 Hz, H3), 7.38 (2H, t, J / 7.5, 5.3 Hz, H5), 4.45 (4H, m, 2CH2, partially masked by solvent peak). 2.3.3. Aqua[bis(2-pyridylmethyl)amine]platinum(II)perchlorate dihydrate, [Pt(bpma)(OH2)](ClO4)2 ×/ 2H2O The complex was obtained by dissolving 2 (50 mg) in hot water (80 8C, 10 cm3) containing an excess of LiClO4 (100 mg). Slow cooling of the solution at room temperature resulted in the formation of colourless crystals, which were filtered off, washed with cold water (2 /3 cm3) and dried in vacuo. Yield: /80%.
2.2. Instruments 2.4. Kinetics Infrared spectra (4000 /250 cm1, KBr discs and Nujol mulls; 400 /150 cm 1, polyethylene pellets) were recorded on a Nicolet Magna FT IR 750 spectrometer. Electronic spectra and kinetics measurements were obtained on a Perkin/Elmer Lambda 15 spectrophotometer. Proton NMR spectra were taken on a Bruker AC 200 F spectrometer. pH measurements were obtained using an Amel combined glass electrode on an Amel 334-B instrument. Conductivity measurements were carried out with a CDM 83 Radiometer Copenhagen conductivity meter and a CDC 334 immersion cell. Elemental analyses were performed by the Microanalytical Laboratory of the University of Padua.
2.3. Preparation of complexes
2.3.1. Chloro[bis(2-pyridylmethyl)amine]platinum(II)chloride monohydrate, [Pt(bpma)Cl]Cl×/H2O (1) It was prepared as previously reported [7]. Crystals suitable for X-ray structure determination was grown from a hot concentrated aqueous solution of the compound by slow cooling.
The reactions were followed spectrophotometrically by measuring the changing absorbance at a suitable wavelength (296 nm) as a function of time. They were initiated by adding 2/20 ml of a 0.015 mol dm 3 water solution of the substrate, [Pt(bpma)Cl]Cl ×/H2O, to 3 cm3 of the required NaOH or NaBr solution previously brought to the reaction temperature (25 8C) in a thermostated cell in the spectrophotometer. The ionic strength was kept constant at 0.2 mol dm 3 with NaCH3SO3. Pseudo first-order rate constants (kobs/ s 1) were obtained either from the gradients of plots of log(Dt/D) versus time or from a non-linear leastsquares fit of experimental data to the equation Dt / D/(Do/D)exp(/kobst), where Do, D and kobs are the parameters to be optimized (Do, absorbance after mixing of reactants; D, absorbance at completion of reaction). 2.5. Determination of the pKa’s values of the complex [Pt(bpma)(OH2)](CF3SO3)2 Aliquots of a 0.01 mol dm 3 aq. solution of [Pt(bpma)(OH2)](CF3SO3)2 (I/ 0.2 mol dm 3,
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2.6. X-ray crystallography
Fig. 1. ORTEP view and atom numbering of the asymmetric unit of 1 showing the thermal ellipsoids at 30% probability level.
NaCH3SO3) thermostated at 25 8C were titrated with standard CO2-free 0.1 mol dm 3 NaOH solution (I/ 0.2 mol dm 3, NaCH3SO3). The electrode was calibrated with buffer solutions of the same ionic strength prepared according to Ref. [9]. The ionization constants pK1 and pK2 have been computed by a non-linear regression of the titration data by means of the Marquardt algorithm.
Fig. 2. ORTEP view and atom numbering of the asymmetric unit of 2 showing the thermal ellipsoids at 30% probability level.
The data for both structures 1 and 2 were collected on a Nonius Kappa CCD area detector diffractometer using graphite monochromated Mo Ka radiation (l/ ˚ ). The reflections were corrected for Lp and 0.7107 A absorption [10] effects. The crystal parameters and other experimental details of the data collections are summarized in Table 3. The structures were solved by direct and Fourier methods. Full-matrix least-squares refinement was performed with all non-hydrogen atoms anisotropic and hydrogens in calculated positions. For structure 1, the positions of both hydrogens belonging to the water molecule were found in the difference-Fourier map and kept fixed during the refinement. The N2 atom of the secondary amine group was found to be disordered over two positions with occupancy factors of 0.62(4) and 0.38(4), respectively. For structure 2, the hydrogens belonging to the water molecules O1 and O10 were found in the difference-Fourier map and refined isotropically, while the positions of those linked to the water oxygen O11 were not determined. Programs used and sources of scattering factor data are given in Ref. [11].
Fig. 3. The dimeric arrangement in the crystal of 1 showing the N2 atom disordered over two positions and the interactions between two square-planar Pt(II) complexes.
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3. Results and discussion 3.1. Structures of [Pt(bpma)Cl]Cl×/H2O and [Pt(bpma)(OH2)](ClO4)2 ×/2H2O ORTEP [12] views of the asymmetric units of 1 and 2 are shown in Figs. 1 and 2, respectively. Selected bond distances and angles are reported in Table 4. The asymmetric unit of compound 1 consists of a square-planar [Pt(bpma)Cl] cation, a Cl anion and a water molecule. The bpma molecule binds the central metal with its three N atoms; the coordination around Pt is completed by a Cl atom in trans position with respect to N2 atom, which is found to be disordered over two positions, N2 and N2?, below and above the mean coordination plane with deviations of /0.45(2) ˚ , respectively. Pt /N and Pt/Cl distances and /0.34(5) A (Table 4) compare well with the mean values of 2.01 [4] ˚ , respectively, obtained from a systematic and 2.29 [1] A analysis of several PtCl2L2 complexes (L /N-donor ligand) [13]. Due to the strain induced by the small bite of the chelate ligand both the N /Pt /N(cis ) angles have a value of 83.38, rather smaller than 908. The Cl counterion acts as a bridge between the cation and the water molecule being involved in two hydrogen bonds with O1 and N2 atoms (Table 5). The squareplanar cations are assembled in dimeric units, through a crystallographic centre of symmetry, by means of Pt Pt ˚ and N2 /H Cl1 hydrogen interactions of 3.7467(4) A bonds. This arrangement is shown in Fig. 3 and the intraand intermolecular H-bonds are listed in Table 5. Intermolecular short contacts between Pt atoms are not infrequent in crystals. Taken as reference-value the van der Waals radius given by Bondi [14] for Pt atom of ˚ , a search has been made on the Cambridge 1.80 A Crystallographic Database [15] of all structures of Pt(II) complexes containing at least a pyridine ligand, where the ˚. Pt metal is involved in Pt Pt contacts less than 4.0 A Thirty-eight structures have been retrieved, giving a mean ˚ [2]. A list of CSD refcodes and Pt /Pt distance of 3.5 A Pt Pt distances is available in Section 4. The mutual orientation of the complexes joined by means of Pt Pt contacts has been evaluated by calculating the angle f formed by the least-squares coordination plane and the line connecting the two central metal. The shortest ˚ [16] are Pt Pt distances of 3.235, 3.237 and 3.244 A associated with angles of 85.28, 80.68 and 85.78, very close to 908, while longer distances, similar to that found in the ˚ [17] are present compound, of 3.755, 3.787 and 3.852 A associated with smaller angles of 64.08, 67.08 and 61.78, respectively. In the present compound the f angle is 73.0(1)8. It can be concluded that the Pt Pt interaction is as stronger as the f angle is closer to 908, in agreement with the presence of an interaction between two filled dz2 and two empty pz orbitals of the two adjacent Pt atoms [18].
The asymmetric unit of compound 2 is built up by a square-planar [Pt(bpma)H2O]2 cation, two ClO4 anions and two water molecules. Pt1, the three N coordinated atoms and O1 lie almost perfectly on a plane; the deviations from the best plane are as follows: Pt1, 0.024(1); N1, /0.001(6); N2, /0.011(6); N3, ˚ . Pt1/N (pyridine) dis/0.002(6); O1, /0.011(6) A tances of 2.010(5) and 2.014(6) are in agreement with those observed in compound 1, in [Pt(bpma)pyridine]2 [7] and in other Pt(II) square-planar complexes [13,19]. ˚ , however, is remarkPt /N2(sp3) distance of 1.952(7) A ably shorter than those found both in [Pt(II)bpma] ˚] complexes [2.009(12) in 1, 2.008(5) [7], 2.11(2) [19] A and in square-planar Pt(II) complexes where an N(sp3) atom is in trans position to a water molecule [Pt /N in ˚ ] [20]. From a search on CSD files the range 2.01 /2.02 A [15], a number of structures containing a water molecule directly co-ordinated to Pt(II) has been retrieved. From these structural data it is possible to determine the trans influence exerted by different atoms on the water oxygens. For instance, in structures where the water molecule is in trans position with respect to a nitrogen ˚ [20], in atoms [Pt /OH2 distances from 2.05 to 2.10 A
Fig. 4. Absorption spectra of 5/10 5 mol dm 3 [Pt(bpma)Cl] (25 8C, I/0.2 mol dm 3): in water solution (A); aged with [OH ]/ 5/10 4 mol dm 3 (B); immediately after dissolution in the presence of [OH ]/10 1 mol dm 3 (C); aged in the presence of [OH ]/ 10 1 mol dm 3 (D). Absorption spectrum of 5 /10 5 mol dm 3 [Pt(bpma)(OH2)](CF3SO3)2 (25 8C, I/0.2 mol dm 3) at pH 4 (HClO4) (E).
B. Pitteri et al. / Polyhedron 21 (2002) 2283 /2291
˚ reported for the agreement with the value of 2.048(7) A present structure], the trans influence exerted by the nitrogen can be evaluated by comparing these distances ˚ observed in a trans -diaqua Pt(II) with those of 2.015 A complex [21] where the trans influences on Pt /O bonds are mutually balanced. The maximum lengthening is observed in Pt(II) complexes where the water molecule is in trans position to a carbon atom [22] [Pt /O distances ˚ ] while similar effects to that from 2.17 to 2.27 A observed for nitrogen are found for P /Pt(II)/OH2 [23] and Cl/Pt(II) /OH2 [24] systems where Pt/O lengths are ˚. in the range 2.08 /2.12 A The analysis of the crystal packing of complex 2 shows that all the water molecules, the perchlorate anions and the N2 /H amine group are involved in the formation of a complex network of hydrogen bonds listed in Table 3, and that no Pt Pt contacts less than ˚ are present in the crystal. 7.5 A
3.2. Solution chemistry The UV /Vis spectrum of an aqueous solution of [Pt(bpma)Cl]Cl (curve A in Fig. 4) does not change appreciably even after a long period of time. Addition of NaOH to this solution leads to an immediate change in the spectrum. The new spectrum depends to some extent on [OH ], but for [OH ]]/0.16 mol dm 3 it becomes pH independent (curve C). Immediate reacidification of the basic solutions leads to a spectrum which is identical with that of the starting chloro species. Without reacidification, subsequent slow spectral changes are observed which is pH dependent. The final spectra for [OH ] /5 /10 4 and 0.1 mol dm 3 are shown as curves B and D, respectively. Addition of NaCl up to 0.1 mol dm 3 has no effect on these spectra. The above observations can be accounted for by the reactions shown in Scheme 1. The amino species 1 undergoes reversible deprotonation of the secondary amino group to give the conjugate amido species 4. This rapid process (spectral changes from A to C) is followed by slow solvolysis of 1 and 4 to give the corresponding aqua complexes which, under our experimental conditions, are fully converted to hydroxo species which are known to be inert toward nucleophilic substitution [25].
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Table 1 Pseudo first-order rate constants, kobs, for the overall process (Scheme 1) in water at 25 8C 102[OH ] (mol dm3)
103kobs
0.05 0.08 0.10 0.15 0.20 0.25 0.30 0.40 0.50 0.60 0.80 1.00 1.50 2.00 2.50 3.00 4.00 5.00 7.00 8.00 9.00 10.00 12.00 14.00 15.00 16.00 18.00 19.00 20.00
0.35490.001 0.39990.002 0.43490.004 0.51790.003 0.59490.004 0.65490.006 0.7290.01 0.8390.01 0.9690.02 1.1090.04 1.3290.03 1.4790.03 1.8090.01 1.9690.04 2.1690.05 2.2490.06 2.4490.06 2.5790.05 2.7990.05 2.8890.04 2.9690.03 3.0790.05 3.1490.05 3.2190.04 3.2390.05 3.2590.07 3.2790.05 3.2690.07 3.2790.06
a
a
(s 1)
I 0.2 mol dm3 NaCH3SO3NaOH.
Fig. 5. Plot of 1/(c/kobs) against [OH ] for the base hydrolysis of [Pt(bpma)Cl] .
3.3. Kinetics of the base hydrolysis
Scheme 1.
The kinetics of the base hydrolysis were carried out in presence of at least 10-fold excess of sodium hydroxide (5 /104 5/[OH ] 5/0.2 mol dm 3) with respect to the concentration of the substrate. The individual runs gave
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Fig. 6. Plot of pseudo first-order rate constants kobs against [OH ] for the base hydrolysis of [Pt(bpma)Cl] .
first-order plots. The pseudo first-order rate constants, kobs, has been reported in Table 1. kobs increases with increasing [OH ], but for [OH ] ]/0.16 mol dm 3 it is independent of [OH ], the mean value of four determinations being c /(3.269/ 0.01) /103 s 1. A plot of 1/(c/kobs) against [OH ] is linear up to [OH ]/0.05 mol dm 3 (Fig. 5), with finite intercept a/(3309/3) s and slope b/ (2.269/0.02) /104 s mol1 dm3 according to Eq. (1) 1=(ckobs ) ab[OH ]
(1)
Eq. (1) can be rearranged to Eq. (2) to give the explicit dependance of kobs from [OH ] kobs (c1=a[OH ]bc=a)=(1[OH ]b=a)
(2)
puted (Fig. 6). An independent determination of the solvolytic rate constant ks has been achieved by carrying out the kinetics of the reaction [Pt(bpma)Cl] /Br 0/ [Pt(bpma)Br] /Cl in water solution at 25 8C and I /0.2 mol dm 3 under pseudo first-order conditions. The two terms rate law kobs /ks/kBr[Br ], usually found in nucleophilic substitution in square-planar complexes [25], is obeyed (Fig. 7) with kBr /(4.19/ 0.1) /10 2 s1 mol 1 dm3 and ks /(2.69/0.3) /104 s 1, in close agreement with the value obtained from the kinetics of the base hydrolysis. The pseudo first-order rate constants, kobs, has been reported in Table 2. The higher reactivity of the amido species 4 with respect to the amino species 1 (ks?/ks /11.7) is in
From reaction Scheme 1, assuming the preequilibrium condition between 1 and 4, and for [OH ] /[substrate], the following expression for kobs is derived kobs (ks [OH ]ks ?Ka1 =Kw )=(1[OH ]Ka1 =Kw )
(3)
where ks and ks? are the rate constants for the solvolysis of 1 and 4, respectively, Ka1 the protolysis constant of 1, and Kw /[H ][OH ]. Eq. (3) is in complete agreement with the observed kinetics, the equilibrium and kinetic constants being related to those extracted from the empirical rate law (2) through the expressions c/ks?, a /1/(ks?/ks), and b /a/Ka1/Kw. From a non-linear best fit of all the experimental data to Eq. (3) with ks, ks? and Ka1 the parameters to be optimized (Kw /1014 mol2 dm 6) the values ks /(2.89/0.2) /104 s 1, ks?/ (3.289/0.02) /103 s 1, pKa1 /12.3 have been com-
Fig. 7. Plot of pseudo first-order rate constants kobs against [Br ] for the reaction [Pt(bpma)Cl] /Br 0/[Pt(bpma)Br] /Cl .
B. Pitteri et al. / Polyhedron 21 (2002) 2283 /2291 Table 2 Pseudo first-order rate constants, kobs, for the reaction: [Pt(bpma)Cl] Br 0 [Pt(bpma)Br] Cl in water at 25 8C 102[Br] (mol dm 3)
103kobs (s 1)
0.7 0.9 2.0 2.5 4.0 5.0 6.0 7.5 10
0.6090.01 0.6690.01 1.0490.01 1.3390.02 1.8990.01 2.3890.07 2.7190.02 3.3490.02 4.5090.04
Table 3 Crystallography data for [Pt(bpma)(OH2)]ClO4 × 2H2O (2) Complex Empirical formula M Temperature (K) Crystal system Space group ˚) a (A ˚) b (A ˚) c (A b (8) ˚ 3) U (A Z Dcalc (g cm 3) m (Mo Ka) (cm 1) F (000) Crystal size (mm) umin /umax (8) Unique reflections Refined parameters Observed reflections [I ] 2s (I )] R on F2 (observed reflections) wR on F2 (all reflections) Goodness-of-fit on F2
[Pt(bpma)Cl]Cl× H2O
1
Table 4 ˚ ) and angles (8) with estimated standard Selected bond distances (A deviations (e.s.d.s) in parentheses
Bond distances Pt1 N1 Pt1 N2 Pt1 N3 Pt1 Cl1 Pt1 O1 N1 C5 C5 C6 N2 C6 N2 C7 C7 C8 N3 C8
I 0.2 mol dm3 NaCH3SO3.
(1)
and
2
C12H15Cl2N3OPt 483.26 295 monoclinic P 21/c 8.4991(2) 12.7963(3) 13.5678(3) 103.359(1) 1435.67(6) 4 2.24 101.40 912 0.26 0.23 0.15 2.93 /25 2516 183 2481
C12H19Cl2N3O11Pt 647.29 295 monoclinic P 21/c 7.7675(3) 14.9477(3) 17.2872(5) 100.755(1) 1971.9(1) 4 2.18 74.49 1248 0.33 0.17 0.14 3 /30 5715 278 4652
0.0291 0.0741 1.285
0.0456 0.1133 1.165
keeping with the results found by Romeo et al. [5] in the study of the system [Pt(enH 1)(dmso)2] /[Pt(en)(dmso)2]2 (ks?/ks #/10). If one assumes that the different charge on the substrates has no a significative effect on the rate of hydrolysis, as found by Martin and coworkers [26] for a number of Pt(II) chloroammine complexes, the difference in reactivity could be ascribed to the greater trans effect of the amido group. Unfortunately, no study has been performed on other comparable systems and a comparison with data relative to substitution reactions with anionic nucleophiles in other amino /amido systems [2,5], for which a reversal of behaviour with the conjugate base becoming less labile than the amine species was observed, cannot be properly done as charge neutralization, absent in the hydrolysis
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1
2
2.008(6) 2.009(12) 2.011(5) 2.301(2)
2.010(5) 1.952(7) 2.014(6)
1.349(10) 1.529(11) 1.472(15) 1.458(13) 1.497(11) 1.356(9)
Bond angles N1 Pt1 N2 N1 Pt1 N3 N1 Pt1 Cl1 N1 Pt1 O1 N2 ZPt1 N3 N2 Pt1 Cl1 N2 Pt1 O1 N3 Pt1 Cl1 N3 Pt1 O1
83.3(3) 166.0(2) 97.2(2)
2.048(7) 1.338(9) 1.511(10) 1.475(9) 1.484(9) 1.491(10) 1.351(9) 83.6(2) 166.7(2) 93.4(3) 83.2(2)
83.3(3) 168.7(6)
176.4(3) 97.8(2) 99.8(3)
Table 5 ˚) Hydrogen bond distances (A Donor acceptor Symmetry operation
1 O1 Cl2 N2 Cl2 O1 Cl2 N2? Cl1 2 O1 O10 O1 O11 N2 O2 O11 O7 O10 O4 O10 O5 O11 O9
x1, y , z1 x , y , z
x , y1/2, 1/ 2z x , 1/2y , 1/2z x , 1y , z
Distance (on acceptor atom)
3.202(7) 3.16(2) 3.237(8) 3.38(5) 2.547(11) 2.550(11) 3.003(9) 2.715(16) 2.748(12) 2.892(12) 2.890(15)
reactions, could overcome the effect of the deprotonation. Therefore, further investigations are necessary before any reasonable conclusion can be drawn. The nature and the distribution of the species formed in solution have been elucidated on investigating the acid /base behaviour of the aqua species [Pt(bpma)(OH2)]2. In Fig. 4, the UV spectrum of the aqua-complex [Pt(bpma)(OH2)](CF3SO3)2 (2) in perchloric acid solution (pH 4) is shown as curve E. When titrated with NaOH, 2 behaves as a dibasic acid and two acid
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Acknowledgements We thank the Italian Ministry of University for financial support and Tatiana Bobbo for technical assistance.
References Scheme 2.
dissociation constants, pK1 /5.49/0.1 and pK2 /11.59/ 0.1, are obtained which can be discussed on considering the acid /base equilibria occurring in solution, as depicted in Scheme 2. Not all of the microscopic equilibrium constants in Scheme 2 can be derived from the two experimentally determined K1 and K2 constants unless at least one piece of additional information is available. Literature data on similar systems indicate that the dissociation constant of a /NH / group (K2,5 in the present case) is always some six order of magnitude smaller than the acid dissociation constant of a coordinated water molecule (K2,3 in the present case). For instance, the pKa values for the deprotonation of the secondary amino group in the cationic species [Pt(dien)(Me2SO)]2 [5] and [PtMe(dpa)(Me2SO)] [6] are 11.949/0.02 and 12.19/0.2, respectively, while a pKa / 6.13 for the protolysis of co-ordinated water in the aquo-cation [Pt(dien)(OH2)]2 has been reported [27]. It is therefore reasonable to assume K2,3 /K2,5 and since [28] K1 /K2,3/K2,5 and K2 /(K3,6 /K5,6)/(K3,6/ K5,6), it follows that K1 $/K2,3 and K2 $/K3,6. Consequently, it can be concluded that the reaction products formed in the base hydrolysis of 1 are the hydroxo species 3 and 6, the anpholite 5 being completely dismutated. Up to [OH ]$/3/104 mol dm 3, the prevailing product is the amino-hydroxo species 3, whose spectrum is shown as curve B in Fig. 4.
4. Supplementary material Crystallographic data of the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 168846 /168847 for the compounds [Pt(bpma)Cl]Cl ×/H2O and [Pt(bpma)(OH2)](ClO4)2 ×/2H2O. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: /441223-336033; e-mail:
[email protected] or www: http://www.ccdc.cam.ac.uk).
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