摘要:
Engineering crystals of ionic complexes. A deliberate synthesis of an acetato bridged zinc dimer (2) Goutam K. Patra,a Golam Mostafa,b Michael G. B. Drewc and Dipankar Datta*a a Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India. E-mail: icdd@mahendra.iacs.res.in b Department of Solid State Physics, Indian Association for the Cultivation of Science, Calcutta 700 032, India c Department of Chemistry, University of Reading, Whiteknights, Reading, UK RG6 6AD Received 27th April 2000, Accepted 27th June 2000, Published 10th July 2000 In the 1 : 1 : 1.1 reaction of Zn(CH3CO2)2·2H2O, diethylenetriamine (dien) and NaX in ethanol, X = PF6– gives rise to the polymer [Zn(dien)(CH3CO2)]n(PF6)n and X = BPh4– yields the desired dimer [Zn(dien)(CH3CO2)]2(BPh4)2 in the solid.Carboxylate-bridged dinuclear zinc cores occur in the active sites of phosphatases1,2 and aminopeptidases3–5 and consequently these are of much biological importance. However, until recently, not many examples of carboxylato bridged dinuclear zinc cores have been isolated in the laboratory.6–13 Here, we report a novel diacetate-bridged cationic zinc dimer which has been synthesised deliberately by making use of some observations made earlier by us14 in connection with engineering crystals of ionic complexes. 4 – Earlier we have demonstrated that in reaction (1), where dien is diethylenetriamine, the product P� is a quasi-onedimensional polymer [Cu(dien)(CH3CO2)]n(ClO4)n for X = –, it is dimeric [Cu(dien)(CH ClO 3CO2)]2(PF6)2 for X = PF6 and it is the mononuclear complex [Cu(dien)2](BPh4)2·3H2O for X = BPh4–.14 (1) 4 Thus, the nuclearity of the cationic part of the product P� in reaction (1) is found to depend on the size of the noncoordinating anion X.With this lead, we have performed reaction (1) using Zn(CH3CO2)2·2H2O instead of Cu(CH3CO2)2·H2O in the hope of isolating a rare, biologically important acetate-bridged dinuclear zinc core with dien as the supporting ligand [reaction (2)].† When X = ClO –, the product P in reaction (2) is an acetato bridged quasi-one-dimensional polymer [Zn(dien)(CH3CO2)]n(ClO4)n (1). 6 This we have reported elsewhere.15 When X = PF –, the product of reaction (2) is still a polymer: [Zn(CH3COO)(dien)]n(PF6)n (2).Its X-ray crystal structure (Fig. 1) shows that it is isostructural with 1 (Table 1). But when X = BPh4–, we obtain our desired dimer [Zn(dien)(CH3CO2)]2(BPh4)2 (3). Its structure, as determined by X-ray crystallography, is shown in Fig. 2 (Table 1). The cation in 3 is centrosymmetric. The metal DOI: 10.1039/b003412g coordination sphere is a distorted trigonal bipyramid. While 1 and its copper(II) analogue are isostructural,15 3 has a structure different from the copper(II) dimer [Cu(dien)(CH3CO2)]2(PF6)2 where the acetate groups bridge the two copper(II) centers in the monoatomic mode. It should be noted that in complexes 1–3, the anions do not coordinate with the metal centers.Fig. 1 The structure of the one-dimensional polymeric cation in 2 with ellipsoids at 30% probability. Selected bond lengths (Å): Zn– N31 2.139(7), Zn–N341 2.114(10), Zn–O11 2.019(7), Zn–O13 2.787(14), Zn–O13(0.5 + x, 1.5 – y, 1.5 – z) 2.037(8). Click image or here to view the 3D crystal structure of 2. Fig. 2 The structure of the cation in 3 with ellipsoids at 30% probability. Axial bond lengths (Å): Zn1–N1 2.070(3), Zn1–N4 2.237(4). Equatorial bond lengths (Å): Zn1–N7 2.087(4), Zn1–O11 2.134(3), Zn1–O12(1 – x, –y, –z) 1.962(3). Click image or here to view the 3D crystal structure of 3. CrystEngComm, 2000, 19Table 1 Crystal data for compounds 2 and 3a Parameter C6H16F6N3O2PZn 2 372.56 1093.65 Orthorhombic Monoclinic Pnma P21/n 8.536(9) 9.896(12) 8.899(11) 9.911(12) 18.52(2) 28.43(2) — 93.11(1) 1407 2784 4 2 1.928 1.759 3951 1279 0.0698 0.1960 Mw Crystal system Space group a/Å b/Å c/Å b/° V/Å3 Zm/mm–1 Dc/g cm–3 Data collected Unique R1 wR2 a The data were collected with Mo-Ka radiation using the MARresearch Image Plate System.The crystals were positioned at 70 mm from the Image Plate. 95 frames were measured at 2° intervals with a counting time of 7 min. Data analyses were carried out with the XDS program.22 The structures were solved using direct methods with the SHELXS-86 program.23 The non-hydrogen atoms in the cations were refined with anisotropic thermal parameters. The hydrogen atoms were included in geometric positions and given thermal parameters equivalent to 1.2 times those of the atom to which they were attached.In 2, the structure had crystallographic Cs symmetry with the metal atom on the mirror plane. The nitrogen atom on the mirror plane was disordered over two sites with occupancy factors of 0.71 and 0.29, respectively. In 3, the structure was a centrosymmetric dimer. Both structures were refined on F2 using SHELXL.24 Click here for full crystallographic data (CCDC no. 1350/25). The Zn–Zn distance in 2 is 4.584 Å while that in 3 is 4.266 Å. In the zinc enzymes, this distance is less than 3.5 Å. This is because of the fact that in the biological dimeric zinc core, the carboxylate groups bridge in the monoatomic and/or syn–syn mode. The syn–anti bridging mode present in 3 understandably increases the separation between the two metal ions.For the various bridging modes of a carboxylate group, see ref. 15. Further, in all the synthetic examples of carboxylate-bridged zinc dimer known to date, the carboxylate fragment, unlike in 3, bridges the two zinc centers in the syn–syn mode. We now try to rationalise our results obtained in reactions (1) and (2) in terms of the relative sizes of the various ions involved in the ionic products P and P� in order to formulate a principle for engineering crystals of ionic complexes. Experimental and theoretical aspects of the crystallisation of ionic complexes have received little attention earlier. In this context, Basolo first observed16 in 1968 that "solid salts separate from aqueous solution easiest for combinations of either small cation–small anion or large cation–large anion, preferably with systems having the same but opposite charges on the counterions".To explain this, Basolo used some examples from the preceding literature. Examining additional examples of this phenomenon, McDaniel in 1972 concluded that there is a lower critical limit for the size of the cation, but no upper limit.17 In 1991, Mingos and Rohl18 first calculated the volumes of a number of anions and cations theoretically, using the Gavezzotti method,19 to examine the observations of Basolo and McDaniel in a quantitative manner. They concluded that "the precise matching of the sizes of the cations and anions in molecular salts is not a dominating consideration in crystallization...".These studies indicate that the crystallisation of ionic complexes is still not well understood. We have calculated C60H72N6O4Zn2B2 3 0.914 1.307 9167 4048 0.0535 0.1390 6 4 4 2]2+, 207.4; the volumes of the various ions involved in the products of reactions (1) and (2) by using STERIC, a computer program developed by Taverner,20 following the method of Gavezzotti. These are as follows (in Å3): ClO –, 51.3; PF –, 69.7; BPh –, 306.6; [Cu(dien) [Cu(dien)(CH3CO2)]22+, 324.4; [Zn(dien)(CH3CO2)]22+, 4 4 316.7. In reaction (1), though the size of [Cu(dien)(CH3CO2)]22+ matches with BPh4– much more than PF6–, it does not crystallise with BPh –.On the other hand, it is really interesting to find that the ratio of the sizes of BPh – and [Zn(dien)(CH3CO2)]22+ in 3 is 0.97, i.e. almost a perfect match of the counterion size that occurs in 3. These results indicate that there are two complementary factors operating in tandem in the reactions of type (1) and (2): (i) there is an unproliferating effect exerted by the larger anions on the countercations; the tendencylymerisation in the cation decreases as the size of the counteranion increases; (ii) there is a tendency to achieve a matching of the sizes of the counterions. 4 The sort of "unproliferating effect", discussed here, has been observed earlier in connection with the isolation of the anionic species SbF4–.It has been found that SbF –, which has a tendency to polymerise, can be isolated in solid by large cations.21 Thus it seems that we can formulate a general principle in connection with the crystallisation of ionic complexes that the larger the ion, the less is the tendency of polymerisation in the counterion. This in a way means that in the process of crystallisation of the ionic complexes, a matching of the sizes of the counterions is approached. It should be remembered that crystal engineering refers to the solid state. Consequently, our zinc complex 3 may not have a dimeric structure in solution. To that extent, it is not intended to be a model of biologically occurring carboxylate-bridged dimeric zinc cores.Acknowledgements M.G. B. D. thanks EPSRC and the University of Reading for funds for the Image Plate System. D. D. thanks the Department of Science and Technology, New Delhi, India for financial support. References 1 Y. Zhang, J. Y. Liang, S. Huang, H. Ke and W. N. Lipscomb, Biochemistry, 1993, 32, 1844. 2 C. G. Dealwis, L. Chen, C. Brennan, W. Mandecki and C. Abad-Zapatero, Protein Eng., 1995, 8, 865. 3 S. K. Burley, P. R. David, A. Taylor and W. N. Lipscomb, Proc. Natl. Acad. Sci. USA, 1990, 87, 6878. 4 S. L. Roderick and B. W. Matthews, Biochemistry, 1993, 32, 3907. 5 B. Chevrier, C. Schalk, H. D’orchymont, J. M. Rondeau, D. Moras and C. Tarnus, Structure, 1994, 2, 283. 6 A. Birnbaum, F. A. Cotton, Z. Dori and M. Kapon, Inorg. Chem., 1984, 23, 1617.7 W. Clegg, I. R. Little and S. P. Straughan, J. Chem. Soc., Dalton Trans., 1986, 1283. 8 S. Uhlenbrock and B. Krebs, Angew. Chem., Int. Ed. Engl., 1992, 31, 1647. 9 P. Chaudhuri, C. Stockheim, K. Wieghardt, W. Deck, R. Gregorzik, H. Vahrenkamp, B. Nuber and J. Weiss, Inorg. Chem., 1992, 31, 1451. 10 X. -M. Chen, Y. -X. Tong and T. C. W. Mak, Inorg. Chem., 1994, 33, 4586. 11 M. Yamami, M. Tanaka, H. Sakiyama, T. Koga, K. Kobayashi, H. Miyasaka, M. Ohba and H. Okawa, J. Chem. Soc., Dalton Trans., 1997, 4595. 12 H. Sakiyama, R. Mochizuki, A. Sugawara, M. Sakamoto, Y. Nishida and M. Yamasaki, J. Chem. Soc., Dalton Trans., 1999, 997. 13 J. Skorsepa, K. Györyová, M. Melník, K. Smolander and M. Ahlgrén, Acta Crystallogr., Sect.C, 1995, C51, 1069 and references therein. 14 G. K. Patra, G. Mostafa, D. A. Tocher and D. Datta, Inorg. Chem. Commun., 2000, 3, 56. 15 D. A. Tocher, G. K. Patra, J. P. Naskar and D. Datta, Indian J. Chem., Sect. A, 1999, 38, 870. 16 F. Basolo, Coord. Chem. Rev., 1968, 3, 213. 17 D. H. McDaniel, Annu. Rep. Inorg. Gen. Synth., 1972, 293. 18 D. M. P. Mingos and A. L. Rohl, Inorg. Chem., 1991, 30, 3769. 19 A. Gavezzotti, J. Am. Chem. Soc., 1983, 105, 5220. 20 B. C. Taverner, STERIC, Program for Calculation of Molecular Steric parameters, University of the Witswatersrand, South Africa, 1995. 21 B. E. Douglas, D. H. McDaniel and J. J. Alexander, Concepts and Models of Inorganic Chemistry, Wiley, New York, 3rd edn., 1994, sect. 5.8.5.22 W. Kabsch, J. Appl. Crystallogr., 1988, 21, 916. 23 G. M. Sheldrick, SHELXL-86, Program for crystal structure solution, Acta Crystallogr., Sect. A, 1990, A46, 467. 24 G. M. Sheldrick, SHELXL-93, University of Göttingen, 1993. 4 Footnote † 0.88 g ( 4 mmol) of pulverised Zn(CH3CO2)2·2H2O was dissolved in 120 ml of ethanol. To this solution, 0.44 ml (4 mmol) of dien was added dropwise with constant stirring. The colourless reaction mixture was stirred for 1 h, after which it was filtered. To the filtrate, 4.4 mmol of NaX (X = PF6– and BPh –) dissolved in minimum volume of ethanol was added dropwise with constant stirring. The reaction mixture was then left in the air for days till the volume was reduced to ~20 ml. The colourless crystals deposited were filtered off, washed with 5 ml of 9 : 1 ethanol–water mixture and dried in air. [Zn(dien)(CH3CO2)]n(PF6)n (2): yield, 1.1 g (74%). Anal. found (calc.): C, 19.38 (19.34); H, 4.29 (4.33); N, 11.20 (11.28)%. LM (CH3OH): 101 W–1 cm2 mol–1 (1 : 1 electrolyte per formula unit). [Zn(dien)(CH3CO2)]2(BPh4)2 (3): yield, 1.4 g (62%). Anal. found (calc.): C, 65.97 (65.89); H, 6.72 (6.64); N, 7.66 (7.68)%. LM (CH3OH): 216 W–1 cm2 mol–1 (1 : 2 electrolyte). CrystEngComm © The Royal Society of Chemistry 2000
ISSN:1466-8033
DOI:10.1039/b003412f
出版商:RSC
年代:2000
数据来源: RSC