Steric influence on platinum(II) ascorbate complexes

www.elsevier.com/locate/ica Inorganica Chimica Acta 329 (2002) 66 – 70

Steric influence on platinum(II) ascorbate complexes Hidetaka Yuge, Takeshi Ken Miyamoto * Department of Chemistry, School of Science, Kitasato Uni6ersity, Kitasato, Sagamihara, Kanagawa 228 -8555, Japan Received 20 April 2001; accepted 15 November 2001

Abstract From the reaction between dihydroxoplatinum(II) and L-ascorbic acid, two types of platinum(II) ascorbate complexes were obtained and structurally characterized with ethylenediamine (en), N,N-dimethylethylenediamine (dmen) and N,N,N%trimethylethylenediamine (trimen) as stabilizing ligands. In [Pt(en)(asc-C,O)] (1), [Pt(dmen)(asc-C,O)] (2) and [Pt(trimen)(ascC,O)] (4), the ascorbate dianion forms a five-membered chelate ring, coordinating to the Pt(II) ion at the 2-carbon and the 5-oxygen atoms (C,O-chelate). From the same mother solution, crystals of [Pt(trimen)(asc-O,O%)] (3) were obtained during the precipitation of 4; in 3 the ascorbate is bound to the Pt at the 2- and 3-oxygen atoms (O,O%-chelate). Compounds 3 and 4 are the first well-characterized linkage isomers among the transition-metal ascorbate complexes. The O,O%-chelated 3 slowly changes to the C,O-chelated 4 in an aqueous solution. Bulkiness of the stabilizing ligand, i.e. en, dmen and trimen has an influence on the formation of the C,O-chelated species, 1, 2 and 4. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Platinum(II) complexes; Ascorbate; PtC bond; Crystal structures

1. Introduction L-Ascorbic acid, one of the most important biomolecules, acts as a two-electron reductant in the electron transport systems. It has been supposed to bind to a transition-metal ion at two hydroxy groups of the 2- and 3-positions [1]. However, among the ascorbate complexes of the transition metals, several examples have been reported on platinum(II). Hollis et al. showed the ascorbate dianion (asc2 − ) in the platinum(II) compound, [Pt(cis-dach)(asc)] (dach=1,2-diaminocyclohexane), was bound to the Pt at the 2-C and the 5-O atoms (C,O-chelate) [2]. As its precursor the oxygen-bound ascorbate compound was also proposed by the 195Pt and 13C NMR investigation. In a previous paper we have reported the new synthetic method of the platinum(II) ascorbate complex through neutralization of each equivalent of ascorbic acid and cis-[Pt(OH)2L2] (L =am(m)ines or PMe3) in an aqueous solution [3]. In [Pt(PMe3)2(asc)], one of the resulting compound, the ascorbate dianion coordinated to the Pt atom at the 2- and 3-O atoms (O,O%-chelate),

* Corresponding author. Tel.: +81-42-778 8305; fax: +81-42-778 9953. E-mail address: [email protected] (T.K. Miyamoto).

while in the other, [Pt(R,R-dach)(asc)], the asc2 − ligand was bound at the 2-C and the 5-O atoms [2–4]. It was inferred that the different coordination modes of the ascorbate dianion was affected by the steric bulkiness of the stabilizing ligand, L. The present work demonstrates that the coordination modes of the ascorbate dianion about the transition metals are controlled by the stabilizing ligands with ethylenediamine derivatives, as ethylenediamine (en), N,N-dimethylethylenediamine (dmen), N,N,N%-trimethylethylenediamine (trimen) and N,N,N%,N%-tetramethylethylenediamine (tmeda).

2. Experimental

2.1. Preparation 2.1.1. [PtL2(ONO2)2] (L2 = en, dmen, trimen and tmeda) A series of water-soluble platinum(II) nitrate complexes, [PtL2(ONO2)2] (L2 = en, dmen, trimen and tmeda) were prepared from K2PtCl4 and en derivatives, L2, according to Refs. [5,6]. The following synthetic procedures were carried out using dihydroxo species [PtL2(OH)2] rather than the dinitrato [PtL2(ONO2)2] for

0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 1 ) 0 0 8 1 0 - 6

H. Yuge, T.K. Miyamoto / Inorganica Chimica Acta 329 (2002) 66–70

the reactions to proceed cleanly without producing some salts such as NaNO3 [3].

2.1.2. [Pt(en)(asc-C,O)] ·2H2O (1 ·2H2O) and [Pt(dmen)(asc-C,O)] ·2H2O (2 ·2H2O) The 0.05 mol dm − 3 aqueous solution of [Pt(en)(ONO2)2] was passed through a column packed with an anion-exchange resin (DIAION SA10AOH) according to Ref. [7]. The aqueous solution of equimolar H2asc was added to the eluate of [Pt(en)(OH)2]. The resulting solution was evaporated to 0.3 mol dm − 3 and allowed to stand in air at room temperature (r.t.) for 1 day. Pale-yellow crystalline precipitates of 1·2H2O were obtained. Typical yields were 40%. Similar procedures were applied replacing [Pt(en)(ONO2)2] with [Pt(dmen)(ONO2)2]. When the solutions were allowed to stand for 3 days, pale-yellow crystals of 2·2H2O were obtained. Typical yields were 18%. 1H NMR (l, D2O) of 1: 2.37– 2.58 [H(en), m, 4H], 3.38 [H(C6), d, 2H], 4.01 [H(C5), t, 1H],4.23 [H(C4), d, 1H]; 2: 2.56–2.69 [H(dmen), m, 10H], 3.44 [H(C6), d, 2H], 4.12 [H(C5), t, 1H], 4.19 [H(C4), d, 1H]. The 13C NMR chemical shifts due to the ascorbate moieties in 1 and 2 corresponded to those reported in Ref. [4]. 2.1.3. [Pt(trimen)(asc-O,O%)] ·2H2O (3 ·2H2O) and [Pt(trimen)(asc-C,O)] ·H2O (4 ·H2O) Similar procedures described in Section 2.1.2 were applied for [Pt(trimen)(ONO2)2]. When allowed to stand in air at r.t. for 1 day, yellow plate-like crystals of 3·2H2O were collected by filtration. After further 3 days, pale-yellow crystals of 4·H2O were obtained from the filtrate. Typical yields were 4 and 9%, respectively. 1 H NMR (l, D2O) of 3: 2.43– 2.84 [H(trimen), m, 13H], 3.59 [H(C6), d, 2H], 3.75 [H(C5), t, 1H], 4.50 [H(C4), d, 1H]; 4: 2.41– 2.81 [H(trimen), m, 13H], 3.40 [H(C6), d, 2H], 4.04 [H(C5), t, 1H], 4.16 [H(C4), d, 1H]. 13C NMR (l, D2O) of 3: 176.75 (C1), 134.85 (C2), 168.73 (C3), 74.14 (C4), 69.84 (C5), 66.12 (C6), 61.94, 55.49, 54.02, 51.83 and 41.88 (C7– C11). The 13C NMR chemical shifts due to the ascorbate moieties in 4 corresponded to those reported in Ref. [4]. As for tmeda complex, no crystalline precipitate was obtained even by further evaporation. 2.2. NMR spectroscopy and X-ray crystallography 1

H and 13C NMR spectra were measured in a Bruker ARX300 (1H: 300 MHz and 13C: 75.5 MHz spectrometer) equipped with a 5 mm diameter tube at 23 °C. The concentration of each D2O solution was approximately 0.1 mol dm − 3. Intensity data collections of the X-ray diffraction were carried out in a Rigaku AFC-7R automated fourcircle diffractometer by … – 2q scan technique at 23 °C. The cell dimensions were refined using 25 reflections in

67

28B 2qB 35° range. The crystal data are summarized in Table 1. Three standard reflections were monitored after every 150 measurements; no remarkable decay was observed. The crystal structures were solved by the direct method using SHELXS97 [8], and refined using SHELXL97 [9] through the full-matrix least-squares on F 2.

3. Results and discussion

3.1. General obser6ation and NMR spectroscopy Pale-yellow crystals of 1·2H2O, 2·2H2O and 4·H2O for en, dmen and trimen have been obtained after 1, 3 and 4 days with 40, 18 and 9% yields, respectively. In the 1H NMR spectra of 1, 2 and 4, the chemical shifts for the asc2 − ligand agreed well with those of [Pt(R,Rdach)(asc-C,O)] at l 3.39, 4.02 and 4.26 [3]. The results of the X-ray structure analysis proved that they were C,O-chelated species in common (Section 3.2). As the more bulky stabilizing ligand, i.e. en, dmen and trimen, was employed, longer time was needed to precipitate the C,O-chelated platinum(II) ascorbate complexes and the less amounts of the compounds were materialized. The methyl groups of the dmen or trimen ligands appear to hinder the formation of the C,O-chelate, compared with the en. During the precipitation of 4·H2O, a small amount (a 4% yield) of yellow plate-like crystals of 3·2H2O, in which the ascorbate ligand formed an O,O%-chelate (Section 3.2), was obtained. The 1H NMR chemical shifts due to the asc2 − ligand in 3 were similar to those of [Pt(PMe3)2(asc-O,O%)] at l 3.66, 3.77 and 4.54 [3]. The 13C NMR spectra of the D2O solutions of 3 and 4 had been observed for a month. After a month, signals of 4 were observed along with those of 3 in the spectrum of 3, while no remarkable change had been found in the spectra of 4. It was proved that the O,O%-chelated species changed to the C,O-chelate in the solution. Rearrangement from 3 to 4 is so slow that the O,O%chelated species, 3 would be crystallized because of its relatively low solubility, as illustrated in Scheme 1. In the case of the en and dmen complexes, signals due to the O,O%-chelated species were also observed in the 1H NMR spectra of their mother solutions before precipitation of 1·2H2O and 2·2H2O, respectively. There seemed O,O%-chelated species in the solution without precipitation of tmeda complex [3].

3.2. Molecular structures As shown in Figs. 1 and 2, the X-ray structure analyses reveal that compounds 1–4 commonly contain PtL2 moieties and asc2 − ligands in a 1:1 ratio. However, the asc2 − in 3 is bound to the Pt atom at the

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Table 1 Crystal data and structure refinement parameters for 1–4 1 C8H18N2O8Pt Empirical formula Formula weight 465.33 Crystal size 0.08×0.12×0.15 (mm) Crystal system monoclinic Space group P21 (No. 4) Unit cell dimensions a (A, ) 6.1355(8) b (A, ) 16.065(1) c (A, ) 6.752(1) i (°) 107.15(1) V (A, 3) 635.9(2) Z 2 Dcalc (g cm−3) 2.430 v (Mo Ka) 11.072 (mm−1) hkl ranges −85h58, −225k522, −95l50 Reflections 3997 measured Unique 3727, 0.0170 reflections, Rint Parameters 185 Goodness-of-fit 1.043 on F 2 R1 [F 2o \2|(F 2o )] 0.0221 R1 (all data) 0.0265 wR2 (all data) 0.0488 Dzmin, Dzmax −0.96 and 0.79 (e A, −3)

2

3

4

C10H22N2O8Pt

C11H24N2O8Pt

C11H22N2O7Pt

493.39 0.12×0.14×0.14

507.41 0.10×0.12×0.18

489.40 0.10×0.14×0.14

orthorhombic P212121 (No. 19)

monoclinic P21 (No. 4)

monoclinic P21 (No. 4)

11.513(1) 13.375(2) 9.802(2) 90 1509.2(4) 4 2.171 9.337

8.8350(9) 17.975(1) 10.3396(7) 93.621(7) 1638.7(2) 4 2.057 8.602

6.0558(9) 15.8611(8) 7.930(1) 105.27(1) 734.8(2) 2 2.212 9.583

−165h516, 05k518, 05l513 4998

05h512, −255k525, −145l514 10 102

05h58, −225k522, −115l510 4628

4403, 0.0279

9550, 0.0289

4276, 0.0166

203 1.017

434 1.015

203 1.053

0.0278 0.0352 0.0670 −1.05 and 1.10

0.0317 0.0448 0.0709 −1.46 and 0.73

0.0184 0.0224 0.0421 −0.98 and 0.70

2- and 3-O atoms of the enediol (O,O%-chelate; Fig. 2), while those in 1, 2 and 4 are at the 2-C of the lactone ring and the 5-O of the deprotonated hydroxy group (C,O-chelate; Fig. 1). Compounds 3 and 4 are the first linkage isomers of the transition-metal ascorbate complexes isolated from the same mother solution. Except for the stabilizing ligands, the molecular structures of 1, 2 and 4 are quite similar to [Pt(cisdach)(asc-C,O)] and [Pt(trans-R,R-dach)(asc-C,O)] [2,3]; the most of the bond lengths and angles correspond within their standard deviations (Table 2). The longer PtN1 bonds trans to the PtC2 are observed: 2.083(4), 2.109(4) and 2.126(3) A, for 1, 2 and 4, respectively. The lactone ring of each ascorbate is regarded to involve the diketone form rather than the original enediol owing to the elongated C2C3 and the shortened C3O3 lengths. In 2 the PtC bond is formed at the cis position of the amino group of the dmen ligand, hindered by the more bulky dimethylamino group. As shown in Fig. 3, assuming a model with the reverse configuration, the hydroxy O2 atom of the asc2 − ligand would be closer to the dimethylamino group of the dmen with approximately 1.5 A, of CH3···OH. The isomer has not been found by NMR spectroscopy and

X-ray crystallographic study. Similarly in 4 the PtC bond is restricted at the cis position of the less bulky methylamino group of the trimen ligand. In addition, the geometry about the methylamino N2 atom is absolutely in an R conformation, also caused by the short contact between the O2 atom of the asc2 − ligand and the methylamino group of the trimen. As for tmeda complex, the asc2 − ligand would not coordinate with the Pt at the C2 and O5 atoms owing to the short contact with the methyl groups of the tmeda ligand. This is consistent with the 1H NMR result that shows the presence of the O,O%-chelated species in the solution. Two crystallographically independent molecules are found in 3·2H2O; no significant differences are observed

Scheme 1.

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tion in contrast to the C24C25C26O26 with trans, which perhaps is caused by differences of hydrogenbond network and/or crystal packing. The ascorbate moieties of 3·2H2O and [Pt(PMe3)2(asc-O,O%)] have so similar structural features in the bond lengths and angles (Table 3), that the O12C12C13O13 and O22C22C23O23 chains in 3·2H2O can be regarded as enediols as well as in [Pt(PMe3)2(asc-O,O%)]. The dimethylamino group of the trimen ligand in each Table 2 Selected bond lengths (A, ) and bond angles (°) for 1, 2 and 4

Fig. 1. ORTEP [10] drawings of (a) [Pt(en)(asc-C,O)] (1), (b) [Pt(dmen)(asc-C,O)] (2) and (c) [Pt(trimen)(asc-C,O)] (4) shown with the atomic notations at 50% probability level.

1

2

4

Bond lengths PtN1 PtN2 PtC2 PtO5 C1O1 C2O2 C3O3 C1O4 C4O4 C5O5 C1C2 C2C3 C3C4

2.083(4) 2.043(4) 2.117(5) 2.026(4) 1.198(6) 1.394(6) 1.222(6) 1.354(6) 1.449(7) 1.421(6) 1.491(7) 1.457(7) 1.522(7)

2.109(4) 2.034(5) 2.107(5) 2.021(4) 1.215(8) 1.395(6) 1.232(7) 1.363(8) 1.447(7) 1.411(6) 1.476(8) 1.451(8) 1.540(8)

2.126(3) 2.042(3) 2.110(4) 2.024(3) 1.208(5) 1.396(5) 1.220(6) 1.364(6) 1.458(5) 1.410(4) 1.488(6) 1.475(5) 1.529(6)

Bond angles N1PtN2 N1PtC2 N2PtC2 N1PtO5 N2PtO5 C2PtO5 O1C1O4 C2C1O1 C2C1O4 PtC2C1 PtC2O2 PtC2C3 C3C2O2 C1C2O2 C1C2C3 C2C3O3 C4C3O3 C2C3C4 PtO5C5

83.0(2) 173.2(2) 90.2(2) 90.7(2) 173.7(2) 96.0(2) 120.5(5) 129.6(5) 109.8(4) 109.1(3) 112.7(3) 87.3(3) 123.0(4) 115.4(4) 105.7(4) 131.3(5) 124.4(5) 104.2(4) 115.8(3)

83.7(2) 173.7(2) 90.1(2) 89.5(2) 172.1(2) 96.6(2) 119.6(6) 129.6(6) 110.8(5) 102.5(3) 112.5(3) 96.4(3) 120.8(5) 117.3(5) 104.0(5) 130.7(5) 123.7(5) 105.7(5) 118.6(3)

84.0(1) 173.4(1) 89.6(1) 90.9(1) 174.9(1) 95.5(1) 120.3(5) 129.0(5) 110.6(4) 109.0(3) 113.4(3) 90.0(2) 120.8(4) 116.0(4) 104.4(4) 130.7(4) 124.6(4) 104.7(3) 115.4(2)

Fig. 2. Two crystallographically independent molecules of [Pt(trimen)(asc-O,O%)] (3) shown with the atomic notations.

in the bond lengths and angles between them (Table 3), except for each conformation of the side chain (Fig. 2). The C14C15C16O16 chain takes gauche conforma-

Fig. 3. Assumed model with reverse configuration for 2.

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Table 3 Selected bond lengths (A, ) and bond angles (°) for 3

4. Conclusions

Bond lengths Pt1N1 Pt1N2 Pt1O12 Pt1O13 C11O11 C12O12 C13O13 C11O14 C14O14 C11C12 C12C13 C13C14

2.022(5) 2.037(5) 2.060(4) 2.044(4) 1.219(8) 1.363(7) 1.311(7) 1.375(9) 1.456(8) 1.432(9) 1.341(9) 1.497(8)

Pt2N3 Pt2N4 Pt2O22 Pt2O23 C21O21 C22O22 C23O23 C21O24 C24O24 C21C22 C22C23 C23C24

2.047(5) 2.031(5) 2.056(4) 2.056(5) 1.213(9) 1.368(8) 1.326(7) 1.366(8) 1.467(8) 1.448(9) 1.326(9) 1.488(8)

Bond angles N1Pt1N2 N1Pt1O12 N1Pt1O13 N2Pt1O12 N2Pt1O13 O12Pt1O13 Pt1O12C12 Pt1O13C13 O11C11O14 O11C11C12 C12C11O14 O12C12C13 C11C12C13 C11C12O12 C12C13O13 C14C13O13 C12C13C14 C13C14O14

85.0(2) 176.8(3) 92.6(2) 96.4(2) 177.5(2) 86.0(2) 102.8(4) 105.1(4) 119.6(6) 131.4(7) 109.0(6) 123.1(6) 108.8(6) 128.1(6) 123.0(6) 127.1(6) 109.9(5) 103.0(5)

N3Pt2N4 N3Pt2O22 N3Pt2O23 N4Pt2O22 N4Pt2O23 O22Pt2O23 Pt2O22C22 Pt2O23C23 O21C21O24 O21C21C22 C22C21O24 O22C22C23 C21C22C23 C21C22O22 C22C23O23 C24C23O23 C22C23C24 C23C24O24

84.3(2) 177.9(3) 94.0(2) 95.9(2) 178.2(2) 85.8(2) 103.2(4) 104.6(4) 120.1(7) 131.1(7) 108.7(6) 123.2(6) 108.2(6) 128.5(6) 123.0(6) 126.0(6) 111.1(6) 102.4(5)

Bulkiness of the stabilizing ligand has a steric influence on PtC bond formation of platinum(II) ascorbate complex. The more bulky stabilizing ligand being employed, time to precipitate the C,O-chelate has increased and its yield has decreased. Investigation on the molecular structures shows that the PtC bond formation would be hindered by the short contacts between the 2-hydroxy group of the ascorbate dianion and the methyl groups of the en derivatives, dmen, trimen or tmeda. The O,O%-chelate changes slowly to the thermodynamically stable C,O-chelate, unless the short contact inhibits the PtC bond formation. The reaction between dihydroxoplatinum(II) and L-ascorbic acid is summarized in Scheme 2. It is confirmed that the bulkiness of the stabilizing ligands is one of the most important factors in the determination of the coordination mode, an O,O%-chelate or a C,O-chelate.

5. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC nos. 177077–177080. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: + 44-1223-336-033; e-mail: [email protected] or www: http://www.ccdc. cam.ac.uk). The observed and calculated structural factors are available from the author on request.

References

Scheme 2.

molecule is situated at the trans position of the 2-O atom. The geometry about the N2 and N4 atoms also takes an R conformation. It might be possible for compound 3 to take the reverse configuration or conformation, as any short contacts between the asc2 − and the trimen ligands have not been observed, assuming some models. No reason has been found to restrict the formation of any isomers from the molecular structure viewpoint.

[1] P.A. Seib, B.M. Tolbert (Eds.), Ascorbic acid: chemistry, metabolism, and uses. In: Advances in Chemistry, vol. 200, American Chemical Society, Washington, DC, 1982. [2] (a) L.S. Hollis, A.R. Amundsen, E.W. Stern, J. Am. Chem. Soc. 107 (1985) 274; (b) L.S. Hollis, E.W. Stern, A.R. Amundsen, A.V. Miller, S.L. Doran, J. Am. Chem. Soc. 109 (1987) 3596. [3] (a) H. Yuge, T.K. Miyamoto, Chem. Lett. (1996) 375; (b) H. Yuge, T.K. Miyamoto, Acta Crystallogr., Sect. C 52 (1996) 3002. [4] L.S. Hollis, E.W. Stern, Inorg. Chem. 27 (1988) 2826. [5] S.G. Dhara, Indian J. Chem. 8 (1970) 193. [6] D.S. Gill, B. Rosenberg, J. Am. Chem. Soc. 104 (1982) 4598. [7] T. Totani, K. Aono, M. Komura, Y. Adachi, Chem. Lett. (1986) 429. [8] G.M. Sheldrick, Acta Crystallogr., Sect. A 46 (1990) 467. [9] G.M. Sheldrick, SHELXL97, Program for Crystal Structure Refinement, University of Go¨ ttingen, Go¨ ttingen, Germany, 1997. [10] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.