Cu(SO3)47-: A Readily Accessible Building Block for New

Mar 5, 2008 - Cu(SO3)47-: A Readily Accessible Building Block for New Coordination Polymers ... Phone: 61 3 8344 6469 (R.R.); 61 3 8344 0341 (B.F.A.)...
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CRYSTAL GROWTH & DESIGN

Cu(SO3)47-: A Readily Accessible Building Block for New Coordination Polymers

2008 VOL. 8, NO. 4 1288–1293

Brendan F. Abrahams,* Marissa G. Haywood, and Richard Robson* School of Chemistry, UniVersity of Melbourne, Victoria 3010, Australia ReceiVed October 17, 2007; ReVised Manuscript ReceiVed December 12, 2007

ABSTRACT: The combination of Cu(II) with excess sodium sulfite or a mixture of sodium sulfite and sodium hydrogensulfite produces a colorless solution consistent with the reduction of Cu(II) to Cu(I). The addition of divalent metal ions such as Mn2+, Co2+, Ni2+ and Zn2+ to such solutions, leads to the generation of coordination networks in which the divalent metal ions link together anions of formula Cu(SO3)47-. This symmetrical anion consists of a tetrahedrally coordinated Cu(I) center bound to the sulfur atoms of four sulfite anionic ligands. Isostructural 1D coordination polymers of composition Na3{[CuI(SO3)4][ZnII(H2O)2]2} · H2O and Na3{[CuI(SO3)4][CoII(H2O)2]2} · H2O, isostructural 2D coordination polymers of composition [Na4(H2O)17][Ni(H2O)6]2{[Cu(SO3)4]2[Ni(H2O)2]3} and [Na4(H2O)17][Co(H2O)6]2{[Cu(SO3)4]2[Co(H2O)2]3}, and a 3D coordination polymer of composition Na(H2O)6{[CuI(SO3)4][MnII(H2O)2]3} are reported. Introduction We believe that a number of simple, readily available anions, despite the extensive study devoted to them over the years, still provide rich prospects for the discovery of new coordination chemistry, in particular, new coordination polymers. For instance, it was recently shown that the simple carbonate ion, when acting in concert with the guanidinium cation, affords a very extensive family of new, highly symmetrical anionic coordination networks.1,2 Sulfite ion, the bridging ligand used in the work reported below, presents the intriguing possibility that, in contrast to its dianionic relative CO32-, all four of its atoms can in principle provide coordinate bonds to metal centers, the M-S-O-M* bridges thus formed bringing M and M* into close electronic communication. The structures of a number of 1D, 2D and 3D µ-sulfito-metal coordination polymers have been reported. Among these are electrically neutral chains and networks, some containing only metal and sulfite as in the “binary” metal sulfites Ma(SO3)b, whereas in others, neutral coligands such as H2O, NH3 and N-heterocycles are also bonded to the metal;3 there are also a few examples of anionic [Mx(SO3)y]z- arrangements.4 In the present report, we describe the very simple generation of the CuI(SO3)47- ion, in which the metal center is in a tetrahedral environment of four sulfur donors, and demonstrate that it provides a versatile building block for the construction of mixed metal 1D, 2D and 3D coordination polymers. Experimental Section Na3{[Cu (SO3)4][ZnII(H2O)2]2} · H2O (1). Fifteen milliliters of an aqueous solution containing Cu(NO3)2 · 3H2O (220 mg, 0.925 mmol) was mixed with 5 mL of an aqueous solution containing 880 mg (8.0 mmol) of Na2SO3 and 220 mg (2.5 mmol) of NaHSO3 to produce a colorless solution. A solution of Zn(NO3)2 · 6H2O (550 mg, 1.85 mmol) in H2O (10 mL) was added to the colorless solution. After 2 weeks, pale yellow crystals were collected, washed with water, and dried at the pump. Yield, 406 mg, 64%. Anal. Calcd for H10CuNa3O17S4Zn2 (%): S, 19.0; H, 1.5. Found: S, 19.3; H, 1.6. Na3{[Cu(SO3)4][CoII(H2O)2]2} · H2O (2) and [Na4(H2O)17][Co(H2O)6]2{[Cu(SO3)4]2[Co(H2O)2]3} (3). A 15 mL aqueous solution of Co(NO3)2 · 6H2O (1.00 g, 3.45 mmol) was added to 20 mL of a colorless I

* Corresponding authors. Fax: 61 3 9347 5180 (R.R.); 61 3 9347 5180 (B.F.A.). Phone: 61 3 8344 6469 (R.R.); 61 3 8344 0341 (B.F.A.). E-mail: [email protected] (R.R.); [email protected] (B.F.A.).

solution that was prepared by mixing Cu(NO3)2.3H2O (278 mg, 1.15 mmol), and Na2SO3 (1.30 g, 10.35 mmol) in H2O (20 mL). A mixture of pink crystals of [Na4(H2O)17][Co(H2O)6]2{[Cu(SO3)4]2[Co(H2O)2]3} and purple crystals of Na3{[Cu(SO3)4][CoII(H2O)2]2} · H2O grew from an initially formed pale pink amorphous precipitate. The crystals were collected on a Hirsch funnel after 10 days, washed with water and dried in air. The pink and purple crystals were separated under a microscope for analysis. Elemental anal. Calcd for H70Cu2Co5S8Na4O59 (pink compound) (%): S, 14.4; H, 4.0. Found: S, 14.3; H, 4.0. Anal. Calcd for H10Co2CuNa3O17S4 (purple compound) (%): S, 19.4; H, 1.5. Found: S, 19.3; H, 1.6. [Na4(H2O)17][NiII(H2O)6]2{[CuI(SO3)4]2[NiII(H2O)2]3} (4). Fifteen milliliters of an aqueous solution of Ni(NO3)2 · 6H2O (1.00 g, 3.45 mmol) was added to 20 mL of a colorless aqueous solution prepared by mixing Cu(NO3)2 · 3H2O (278 mg, 1.15 mmol) with Na2SO3 (1.30 g, 10.4 mmol). A pale green precipitate that formed initially slowly transformed into green platelike crystals that were collected on a Hirsch funnel after 10 days, washed with water and dried in air. Yield: 550 mg, 34%. Anal. Calcd for H70Cu2Ni5S8Na4O59 (%): S, 14.4; H, 4.0. Found: S, 14.6; H, 4.2. Na(H2O)6{[CuI(SO3)4][MnII(H2O)2]3} (5). Ten milliliters of an aqueous solution of Mn(CH3COO)2 · 4H2O (986 mg, 4.025 mmol) was added to 20 mL of a colorless aqueous solution prepared by mixing Cu(NO3)2 · 3H2O (278 mg, 1.15 mmol) and Na2SO3 (1.74 g, 13.8 mmol). An amorphous solid precipitated immediately, which transformed into pale pink crystals over a period of several weeks. Yield: 834 mg, 92%. Anal. Calcd for H24CuMn3NaO24S4 (%): S, 17.1; H, 2.2. Found: S, 17.5; H, 3.3. X-ray Crystallography. The crystal structures of 1-5 were determined by single crystal X-ray crystallography. All data were collected using a Bruker CCD diffractometer and the structures were solved using direct methods and refined using a full-matrix least-squares procedure.5 Crystallographic data and structural refinement parameters are presented Table 1. Full details of crystal data and the structure refinements, in CIF format, are available as Supporting Information.

Results and Discussion The addition of blue aqueous solutions of CuII to aqueous solutions containing excess SO32- (or SO32-/HSO3-) yields colorless solutions which we presume contain species of the type CuI(SO3)n(2n-1)-. We surmised that, with a sufficient excess of sulfite, the predominant Cu-containing species might be CuI(SO3)47-, in which the metal center might reasonably be expected, on the basis of the known geometrical and ligand atom preferences of CuI, to have a tetrahedral environment of four sulfur donors. We considered that such solutions, which we refer to below as “CuI/SO32- solutions”, might prove to be very

10.1021/cg7010259 CCC: $40.75  2008 American Chemical Society Published on Web 03/05/2008

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Table 1. Crystallographic Data and Structure Refinement Details for 1–5

formula formula mass T (K) cryst syst space group a (Å) b (Å) c (Å) R (°) β (deg) γ (deg) V (Å3) Z D calcd (g cm-3) no. of reflns collected no. of independent reflns Rint data/restraints/params GOF on F2 R1 [I > 2σ(I)] wR2 (all data)

1

2

3

4

5

H10CuNa3O17S4Zn2 673.6 130 monoclinic P21/m 7.4909(9) 14.041(2) 7.7846(9) 90 99.963(2) 90 806.4(2) 2 2.766 5037 1895 0.0273 1895/4/150 1.090 0.0332 0.0778

H10Co2CuNa3O17S4 660.69 130 monoclinic P21/m 7.5101(8) 14.1390(15) 7.7786(8) 90 100.225(2) 90 812.86(15) 2 2.691 5068 1914 0.0273 1914/4/150 1.145 0.0419 0.0945

H70Co5Cu2Na4O59S8 1784.7 130 trigonal P-3c1 11.3771(6) 11.3771(6) 23.847(2) 90 90 120 2673.2(3) 2 2.775 15325 2057 0.0511 2057/8/53 1.114 0.0487 0.1201

H70Ni5Cu2Na4O59S8 1783.6 130 trigonal P-3c1 11.3081(6) 11.3081(6) 23.591(2) 90 90 120 2612.5(3) 2 2.279 15304 2019 0.0479 2019/8/152 1.033 0.0417 0.1146

H24CuMn3NaO24S4 787.8 130 trigonal R3c 12.2069(9) 12.2069(9) 25.324(2) 90 90 120 3267.9(3) 6 2.402 4681 1485 0.0558 1485/9/129 1.064 0.0293 0.0650

convenient sources of new, easily accessible products containing many other metals together with copper. Because of its high overall negative charge, CuI(SO3)47-, appeared a good candidate for binding other metal cations strongly, offering numerous conceivable chelating modes, two very symmetrical possibilities being shown in Figure 1. Depending on the orientation of the SO3 moieties around the Cu-S bonds the CuI(SO3)47- unit could conceivably act as a bidentate chelating agent for up to six metal cations in which case they would be located at the corners of an octahedron as in Figure 1a or it might act as a tridentate chelating agent for up to four metal cations which would be located at the corners of a tetrahedron as in Figure 1b. Connectivities of these sorts could lead to interesting coordination architectures, including coordination networks, e.g., if octahedral bis-tridentate metal centers were used to link CuI(SO3)47- units together in the manner seen in Figure 1b with no deformation, a diamond-related network would inevitably result. Of course, under appropriate circumstances CuI(SO3)47might be capable of binding any number of cations from one to six (or even more). The addition of Zn2+ to the “CuI/SO32- solution” yields crystals of composition Na3{[CuI(SO3)4][ZnII(H2O)2]2} · H2O (1).

Figure 1. (a) Possible metal-binding mode for CuI(SO3)47-, in which six metal centers are attached at six separate bidentate sites. In the most symmetrical form possible, the six metal centers would be located at the corners of a regular octahedron. b) The possible binding of four metal centers to CuI(SO3)47- at four separate tridentate sites. In the most symmetrical form possible, the four metal centers would be located at the corners of a regular tetrahedron. Color code: Cu, blue; S, yellow; O, red; metal center, green.

When Co2+ is used in place of Zn2+, a mixture of pink and purple crystals is obtained, the two types being easily distinguished and manually separated. The purple crystals, of composition Na3{[CuI(SO3)4][CoII(H2O)2]2}.H2O (2) are isostructural with the Zn compound, the CuI(SO3)47- units being components of a 1D polymeric anion, {[Cu(SO3)4][MII(H2O)2]2}3-, (M ) Zn or Co) whose structure is shown, for the Zn case, in Figure 2. As can be seen, pairs of cis diaqua Zn2+ centers link one CuI(SO3)47- unit to the next. Within each CuI(SO3)47- unit, two sulfite ligands (A and A′ in Figure 2) are equivalent, sulfite A providing one oxygen donor to Zn(A) and sulfite A′ providing one oxygen donor to Zn(A′). A third sulfite, B in Figure 2, is associated with this same pair of Zn centers, providing a ZnOSOZn bridge. The fourth sulfite ligand, C, is associated with the next pair of Zn centers along the chain by simultaneously forming two 4-membered chelate rings, as can be seen in Figure 2. The metal centers, each with two cis aqua ligands thereby acquire a distorted octahedral environment. The anionic polymers extend in the a direction with sodium ions providing links between parallel chains. The addition of Ni2+ to the “CuI/SO32- solution” affords crystals of composition [Na4(H2O)17][NiII(H2O)6]2{[CuI(SO3)4]2[NiII(H2O)2]3} (3). As indicated earlier, Co2+ with the

Figure 2. The structure of the 1D polymeric anion {[CuI(SO3)4][ZnII(H2O)2]2}3- in Na3{[CuI(SO3)4][ZnII(H2O)2]2} · H2O. Selected bond lengths and angles Cu-S 2.2401(9)–2.262(1) Å, S-Cu-S 100.82(5)– 116.24(3)°, Zn-O 2.028(2)-2.327(3) Å. Color code: Cu, blue; S, yellow; O, red; zinc, green.

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Figure 3. Coordination environments of the metal ions in the 2D anion {[CuI(SO3)4]2[CoII(H2O)2]3}8- in [Na4(H2O)17][CoII(H2O)6]2{[CuI(SO3)4]2[CoII(H2O)2]3} a) Three CoII centers bound to each CuI(SO3)47unit. b) A trans diaqua CoII center connecting two CuI(SO3)47- units. Selected bond lengths and angles Cu-S 2.250(1)–2.265(2) Å, S-Cu-S 101.21(5)–115.63(3)°, Co-O 2.057(3)-2.267(3) Å. Color code: Cu, blue; S, yellow; O, red; Co, pink.

“CuI/SO32- solution” gives a mixture of pink and purple crystals, the latter having the 1D polymeric structure described immediately above. The pink crystals, having the composition [Na4(H2O)17][CoII(H2O)6]2{[CuI(SO3)4]2[CoII(H2O)2]3}(4) are isostructural with the Ni compound (trigonal space group P-3c1). The anionic polymer {[CuI(SO3)4]2[CoII(H2O)2]3}8- has an undulating 2D structure. All CuI(SO3)47- units are equivalent, providing three bidentate sites for cobalt binding as shown in Figure 3a, where it can be seen that one of the sulfite ligands bridges all three Co centers, whereas the other three sulfite units are each associated with only one Co center. All trans diaqua Co2+ centers are equivalent, serving to link one CuI(SO3)47unit to another as shown in Figure 3b. A (6,3) net (or hexagonal grid net) is thus formed as shown in structures a and b in Figure 4, with Cu providing the 3-connecting nodes. The mode of binding the three Co2+ centers to the CuI(SO3)47- unit seen here is closely related to the mode of binding six metal centers shown in Figure 1a. The three Co2+ centers can be regarded as a “facial” group of three members of the octahedral set seen in Figure 1a and the Co · · · Cu · · · Co angles are consequently close to 90° (88.5(1)°). The hexagonal grid is therefore highly puckered, as shown in Figure 4c. The countercations for this anionic 2D network are Co(H2O)62+, Na(H2O)6+, and unusual Na3(H2O)113+ clusters. Na(H2O)6+ cations fit beautifully at the center of each hexagonal ring of the puckered (6,3) net, forming twelve hydrogen bonds, six to aqua ligands on the Co2+ within the 2D network and six to sulfite oxygen centers, as shown in Figure 5a. The remaining Na+ cations are present in the intersheet regions as unusual triangular [Na3(H2O)11]3+ clusters indicated in structures b and c in Figure 5. This triangular cation is situated on a 3-fold axis which also passes

Figure 4. Structure of the 2D hexagonal grid polymeric anion {[CuI(SO3)4]2[CoII(H2O)2]3}8- in [Na4(H2O)17][CoII(H2O)6]2{[CuI(SO3)4]2[CoII(H2O)2]3}. (a) The hexagonal grid (or (6,3) net) topology. (b) The hexagonal grid topology showing only Cu and Co centers. (c) A “side view” of the hexagonal grid revealing the highly puckered geometry. Selected bond lengths and angles Cu-S 2.264(1)–2.268(2) Å,S-Cu-S103.01(3)–115.09(2)°,Co-O(anionicnetwork)2.069(3)-2.123(3) Å. Color code: Cu, blue; S, yellow; O, red; Co, pink.

through the intrasheet Na(H2O)6+ cations. Two water molecules lie on a 3-fold axis and each bridges three sodium centers. These two water molecules occupy cis positions in a distorted octahedral environment around each of the three sodium ions. Three doubly bridging water molecules are situated at the vertices of an equilateral triangle. Pairs of these water molecules occupy trans positions on each sodium ion. The coordination of each sodium ion is completed by two nonbridging cis water molecules. The doubly bridging and nonbridging water molecules of this tricationic cluster form hydrogen bonds with sulfite oxygen atoms. The doubly

Cu(SO3)47-: Building Block for New Coordination Polymers

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Figure 5. Hydrated counteractions in [Na4(H2O)17][CoII(H2O)6]2{[CuI(SO3)4]2[CoII(H2O)2]3}. (a) The complementary incorporation of Na(H2O)6+ into the hexagonal rings of the (6,3) net. Striped bonds represent hydrogen bonds between the Na(H2O)6+ ion and the anionic framework. (b) A view slightly off the 3-fold axis showing the [Na3(H2O)11]3+ and Co(H2O)62+ cations. (c) A view almost normal to the 3-fold axis showing the [Na3(H2O)11]3+ and Co(H2O)62+ cations sandwiched between the anionic sheets. For clarity, only the metal ions of the anionic 2D networks are shown in (b) and (c). Color code: Cu, blue; S, yellow; O, red; Co in anionic sheet, pink; Co in Co(H2O)62+, orange; Na, green.

bridging and terminal water molecules of these cations form hydrogen bonds to sulfite oxygen atoms belonging to both upper and lower anionic sheets. The triply bridging water molecules do not participate in hydrogen bonding. To the best of our knowledge, this [Na3(H2O)11]3+ has not been observed previously; however, we note that similar arrangements of sodium ions involving mixtures of oxygen donor ligands have been reported.6 Hexaaqua cobalt(II) cations also lie between the anionic sheets but instead of lying on the 3-fold axis that passes through the hexagonal rings, these cations are situated on the 3-fold axes that pass through the Cu centers. The Co(H2O)62+ cations lie on two distinct planes in the inter sheet region as illustrated in Figure 5c. Like the trisodium cluster, the water molecules of the hexaaqua cobalt(II) complex form hydrogen bonds between sulfite oxygen atoms belonging to upper and lower anionic networks. Thus both types of intersheet cation serve to cement adjacent anionic sheets together through charge-assisted hydrogen bonds. Despite the fact that the [Na3(H2O)11]3+ and

Co(H2O)62+ cations share the intersheet region, no hydrogen bonds are formed between the hydrated cations. The addition of Mn2+ to a Cu/SO32- mixture (Mn to Cu ratio of 3.5:1), leads to the formation of crystals of composition Na(H2O)6{[CuI(SO3)4][MnII(H2O)2]3} (5). As with the case of compounds 2-4, the formation of this compound has been achieved with a slight excess of the divalent metal ion in the reaction mixture. In this case the CuI(SO3)47- unit is essentially playing the 6-connecting role represented in Figure 1a, although there are minor distortions from a regular octahedral disposition of the six Mn centers, as shown in Figure 6a. The cis diaqua Mn2+ centers effectively serve as 2-connectors, linking one CuI(SO3)47- unit to another within an infinite 3D network, part of which (a pseudo square of Cu centers, actually a rhombus) is shown in Figure 6b. This network has the topology of the R-Po net, represented in Figure 6c. In a manner that parallels the highly complementary incorporation of Na(H2O)6+ into the hexagonal cavities of the {[CuI(SO3)4]2[CoII(H2O)2]3}8- 2D sheets discussed

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Figure 6. Structure of the 3D, R-Po-like {[CuI(SO3)4][MnII(H2O)2]3}- network. (a) One CuI(SO3)47- unit providing bidentate sites for six MnII centers. (b) Cis diaqua MnII centers linking CuI(SO3)47- units to form a pseudosquare within the R-Po-like network. The thin back lines highlight the connectivity between Cu and Mn centers. (c) The R-Po-like net showing only 6-connecting Cu centers and 2-connecting Mn centers. (d) Complementary incorporation of Na(H2O)6+ into the pseudocubic cavities; blue bonds represent the Na-O interactions within the Na(H2O)6+ ion; striped bonds represent hydrogen bonds between the Na(H2O)6+ ion and the anionic framework. Selected bond lengths and angles Cu-S 2.272(1)–2.2973(8) Å, S-Cu-S 108.40(3)–110.52(2)°, Mn-O 2.133(3)-2.256(3) Å. Color code: Cu, blue; S, yellow; O, red; Mn, pink; Na, green.

above and shown in Figure 5a, the Na(H2O)6+ cations in 5 fit equally neatly into the pseudocubic cavities of the 3D R-Po-like {[CuI(SO3)4][MnII(H2O)2]3}- network, making no less than 18 H-bonds either to sulfito oxygen centers or to water molecules coordinated to MnII as shown in Figure 6d. Conclusion We have shown that (a) the new building block CuI(SO3)47is very easily generated; (b) it does have the expected tetrahedral disposition of sulfur centers; (c) it affords a range of metal derivates, including discrete molecular species as well as 1D, 2D, and 3D polymers, in which it remains intact (and there is every indication the range of compounds could be readily extended); (d) the SO3 moieties are able to rotate around the Cu-S bonds allowing the CuI(SO3)47- unit to act either as a tridentate ligand binding “facially” to an octahedral metal center or as a bidentate ligand, 3D R-Po-type networks becoming accessible when all six of the available bidentate metal binding

sites are utilized. The Na+ cation, which is present in all the compounds reported here, is clearly structurally noninnocent. Replacing Na+ by a host of other available cocations, that are readily envisaged, will almost certainly provide new structural types. In addition, poly sulfito complexes other than CuI(SO3)47promise interesting structural variations (e.g., the readily available Pd(SO3)46- provides a square arrangement of the four sulfur centers in contrast to the tetrahedral arrangement in CuI(SO3)47-). Examples reported here indicate that the sulfite unit is quite capable of simultaneously binding metal cations at all four of its constituent atoms; the short MSOM bridges generated in this way may afford facile electronic communication between the M centers, possibly leading, with appropriate metals, to useful cooperative electronic/magnetic effects. It is now more generally appreciated that coordination polymers hold great promise as a source of new solids with useful properties. In prospecting for useful new materials within this coordination polymer field, it is important to have cheap,

Cu(SO3)47-: Building Block for New Coordination Polymers

readily available building blocks at our disposal: the general approach described here, not necessarily restricted to the specific case of CuI(SO3)47-, could therefore be of great value. Our preliminary results hint at the wide range of sulfite-based coordination polymers that could be easily accessible. Acknowledgment. The authors gratefully acknowledge the support of the Australian Research Council. Supporting Information Available: Crystallographic data for compounds 1-5 (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

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Crystal Growth & Design, Vol. 8, No. 4, 2008 1293 Naturforsch., B 1994, 49, 60. (g) Buchmeier, W.; Engelen, B.; Mueller, H. Z. Naturforsch., B 1998, 53, 131. (h) Engelen, B. Z. Naturforsch., B 1994, 49, 127. (i) Baggio, R. F.; Baggio, S. Acta Crystallogr., Sect. B 1976, 32, 1959. (j) Engelen, B.; Frieburg, C. Z. Naturforsch., B 1979, 34, 1495. (k) Nyberg, B. Acta Chem. Scand. 1973, 27, 1541. (l) Quinones, H.; Baggio, S. J. Inorg. Nucl. Chem., 1972, 34, 2153. (m) Larsson, L. O. Acta Chem. Scand. 1969, 23, 2261. (n) Engelen, B.; Buchmeier, W.; Lutz, H. D Z. Naturforsch., B 1987, 42, 37. (o) Lutz, H. D.; Buchmeier, W.; Engelen, B. Z. Naturforsch., B 1983, 38, 523. (p) Magnusson, A.; Johansson, L. G. Acta Crystallogr., Sect. B 1981, 37, 1400. (q) Magnusson, A.; Johansson, L. G. Acta Crystallogr., Sect. B 1981, 37, 1108. (r) Solovyov, L. A.; Bloklim, A. I.; Mulagaleev, R. F.; Kirik, S. D. Acta Crystallogr., Sect. C 1999, 55, 293. (4) (a) Nyberg, B. Acta Crystallogr., Sect B 1978, 34, 1418. (b) Niinisto, L.; Larsson, L. O. Acta Crystallogr., Sect. B 1973, 29, 623. (c) Messer, D.; Breitinger, D. K.; Haegler, W. Acta Crystallogr., Sect. B 1979, 35, 815. (d) Nyberg, B.; Kierkegaard, P. Acta Chem. Scand. 1968, 22, 581. (e) Nyberg, B.; Cynkier, I. Acta Chem. Scand. 1972, 26, 4175. (f) Lutz, H. D.; Buchmeier, W. Z. Anorg. Allg. Chem. 1983, 507, 199. (5) Sheldrick, G. M., SHELX97 (Release 97–2); University of Göttingen: Göttingen, Germany, 1998. (6) (a) Long, Y.-X.; Long, L.-S.; Huang, R.-B.; Zheng, L.-S.; Ng, S. W. Acta Crystallogr., Sect. E 2005, 61, m790. (b) Klaui, W.; Muller, A.; Eberspach, W.; Boese, R.; Goldberg, I. J. Am. Chem. Soc. 1987, 109, 164.

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