Recent advances in metal phosphonate chemistry II

Current Opinion in Solid State and Materials Science 6 (2002) 495–506

Recent advances in metal phosphonate chemistry II Abraham Clearfield* Department of Chemistry, Texas A& M University, College Station, TX 77842 -3012, USA Received 26 June 2002; accepted 5 November 2002

Abstract Phosphonic acids are more acidic than carboxylic acids. They are non-planar and possess twice the number of protons. These attributes lead to strong hydrogen bond formation that can be utilized in design of crystal-engineered structures and self-assembled supramolecular arrays. Their superior cation complexing ability leads to formation of a vast number of metal complexes and their use in lanthanide and actinide separations. Recent work, largely drawn from the work of the author’s research group, is summarized in this update of the author’s 1996 review [Curr Opin Solid State Mater Sci 1 (1996) 268].  2003 Elsevier Science Ltd. All rights reserved.

1. Introduction The first article in this series appeared in 1996 [**1]. Since then, the field has expanded in a remarkable way, so that not all aspects of the subject can be covered. A perspective article will shortly appear in the Dalton Transaction that serves to bring the reader up to date on the subject of porous hybrid inorganic–organic porous materials [*2]. We have successfully prepared microporous hybrids with biphenyl and terphenyl pillars with pores in ˚ range. Surface areas are |400 m 2 / g for the the 10–20 A biphenyl and |250–300 m 2 / g for the terphenyl derivatives. Sulfonation of the aromatic rings yields highly acid materials that have potential applications as catalysts and proton conductors. On a broader scale if the method proves to be general, irrespective of the nature of the pillar, then a new family of porous hybrids in which the functionality can be designed, should ensue. In this review, we will focus on two areas of metal phosphonate chemistry: crown ether immobilization derivatives and the crystal engineering / hydrogen bonding aspects of self-assembled compounds, as well as some unusual coordination compounds that we synthesized.

leads to a relatively new class of compounds with a wide range of applications. These applications include efficient separations, ion transportation, anion activation, sensoring and switching, and catalysis [3,4]. A recent new approach has been to immobilize azacrown ethers into inorganic layered phases. This is done by first converting the crown ether to a phosphonic acid followed by a topotactic ester interchange reaction with g-zirconium phosphate, Zr(PO 4 )(H 2 PO 4 )?2H 2 O [*5,6]. We have used a different synthetic strategy in that the phosphonic acid was allowed to react directly with the metal cation or to a mixture of the cation and phosphoric acid. In the case where Zr(IV) was the cation of choice, layered compounds were formed [*7]. However, with divalent metals, linear chain compounds were obtained. These latter compounds have been termed ‘macrocyclic leaflets’ because the crown ethers, attached to either a hydrogen-bonded or metal-bonded backbone, resemble leaves attached to a twig. We have reported on the structures of two of these compounds in a short communication [**8]. In this paper, we provide details on further additions to this family of compounds. The first step in preparing one- or two-dimensional arrays of crown ethers is to convert the desired crown ether to a phosphonic acid. This is most readily accomplished by a Mannich-type reaction with an azacrown ether [*7].

2. Immobilized crown ether compounds Incorporation of macrocycles into polymeric matrices *Tel.: 11-979-845-2936; fax: 11-979-845-2370. E-mail address: [email protected] (A. Clearfield).

C 12 H 22 O 5 NH 1 CH 2 O 1 H 3 PO 3 HCl

→C 12 H 22 O 5 NCH 2 PO 3 H 2 1 H 2 O

The resultant phosphonic acid exists as a Zwitter ion.

1359-0286 / 03 / $ – see front matter  2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-0286(02)00151-1

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Zwitter ion formation increases the tendency to dimer formation as shown in Scheme 2b. Treatment of an ethanol solution of the N-(phosphonomethyl)-aza-18-crown-6 with Co(ClO 4 ) 2 ?6H 2 O in dilute HCl did not result in formation of a cobalt complex. Rather, a proton was transferred to the phosphonic acid dimers, one proton for every two phosphonic acid molecules [**8,*9]. Co(ClO 4 ) 2 1 HCl 1 [C 12 H 22 O 5 NH 1 PO 3 H] 2 → ↓[C 12 H 22 O 5 NH 1 PO 3 H 2 ] 2 H 1 ? ClO 42 1 Co(ClO 4 )Cl (1)

Fig. 1. Schematic representation of the self-assembled N-(phosphonomethyl)-aza-18-crown-6 linear chain. The backbone is formed by hydrogen bonding of the phosphonic acid dimers (dashed lines) and the O–H–O region indicated by a solid gray line. Each azacrown ether ring contains a water molecule.

The structure of this compound is that of linear arrays as shown in Fig. 1. In the Zwitter ion dimer, each phosphonic acid group contains a single proton. Two such groups form the dimers through P–O–H? ? ?O–P hydrogen bonds in ˚ The proton that is which the O–O distance is |2.50 A. transferred is shared equally by a set of dimers by bonding to the third oxygen of adjacent dimers. This proton sits on a center of symmetry to form O–H–O type hydrogen ˚ These protons are bonds at an O–O distance of 2.415 A. responsible for the linear chain formation shown in Fig. 1. A water molecule is encapsulated within the azacrown ring through NH? ? ?O and OH? ? ?O hydrogen bonding. The perchlorate anions sit between the chains in electrostatic fashion. The arrangement of the crown ethers in this and other linear chain compounds resemble leaves on a twig, hence the title, ‘Macrocyclic Leaflets.’ Another example of the leaflet construction was obtained with N-(phosphonomethyl)-aza-15-crown-5, (C 10 H 20 O 4 )NHCH 2 PO 3 H. This compound crystallizes in ] the triclinic system, P1, a55.627(1), b58.922(1), c5 ˚ 15.108(2) A, a 577.495(2)8, b 584.520(2)8, g 5 72.537(2)8. This compound also forms dimers through POH hydrogen bonds but in the absence of proton transfer uses N–H? ?OP hydrogen bonding to self-assemble into linear chain leaflet form as shown in Fig. 2. There are no water molecules within the ring [*9]. Two examples of linear chain macrocyclic leaflets, where the phosphonate groups are covalently bonded to metals, have also been prepared [**8,*9,*10]. Both the zinc and cadmium compounds were synthesized by a layering technique so as to obtain single crystals. The metal nitrates were dissolved in water while the crown ether phosphonic acids were utilized as ethanol solutions on top of which were added the aqueous solutions. The cadmium compound forms a very complex central chain (twig) from which the crown ethers are appended. The formula is [Cd 4 (O 3 PCH 2 NHC 12 H 24 O 5 ) 3 (NO 3 ) 4 (H 2 O) 5 ? NO 3 ?3H 2 O. Surprisingly, the space group is the familiar ˚ b5 P21 /c, a517.136(2), b520.181(3), c520.858(3) A, 104.937(2)8. Two of the cadmium cations are six-coordinate and two of them are seven-coordinate. Each phosphonate group bonds to four cadmium atoms in which a Cd–O–P–O–Cd bridge is formed and the third phosphonate oxygen bonds to two cadmiums (Fig. 3). This connectivity forms infinite chains in the a-axis direction in which the Cd(II) and phosphonate groups are the inner cores of the chain. Cd3 connects to Cd39 through two single oxygen bridges and two Cd–O–P–O–Cd bridges. The remainder of the coordination sphere for these metal atoms is furnished by a water molecule and a monodentate nitrate group. Cd1 and Cd2 are connected through two single oxygen bridges originating from P2 and P3 and also by an O–P1–O type bridge. Cd1 completes its coordination sphere by bonding to two water molecules and to a chelating nitrate group. Cd2 bonds to only one water oxygen and is chelated by a nitrate group. The seventh

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Fig. 2. One-dimensional self-assembled array of N-(phosphonomethyl)aza-15-crown-5. The O–H–O hydrogen bonds responsible for dimer ˚ distant between oxygen atoms and the NH? ? ?O formation are 2.52(1) A ˚ There are no water molecules within the inter dimer bonds are 2.66(1) A. rings.

bond arises from a nitrate oxygen involved in chelation to Cd4 that also bonds to Cd2 through electron pair donation. Cd4 is also bridged to a symmetry equivalent cadmium atom. However, unlike the Cd3–Cd39 connectivity, these two cadmiums are connected to each other through two OPO-type bridges and no-single-oxygen bridges. Both Cd4 atoms are also chelated by a nitrate group and bonded to one water molecule. A coordination number of six is achieved by formation of a coordinate covalent bond from a nitrate group oxygen already involved in chelation to a neighboring Cd atom. The sequence along the chain is Cd3–Cd39, Cd2–Cd1, Cd4–C49, Cd1–Cd2, Cd3–Cd39, etc. The single oxygen ˚ for bridges result in short Cd–Cd distances of 3.288(3) A ˚ Cd3–Cd39 and for Cd2–Cd1 3.470(2) A. The cadmium– ˚ oxygen distances range from 2.179(11) to 2.478(16) A The azacrown rings are attached to the linear chains at

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Fig. 3. Ball and stick representation of the cadmium aza-18-crown-6 backbone repeat unit. The nitrate N atoms are represented by the small black spheres and the oxygens and water molecules are light gray. The aza-crown ether rings are bonded through P–CH 2 –N connectivity and in the orientation shown would be positioned alternately above and below the chain.

phosphorus atom positions through CH 2 –N bonding on both sides of the chains. The macrocycle ring contains a neutral water molecule, giving rise to multiple hydrogen bonds. There are five O? ? ?O contacts in the range 2.75– ˚ in each of the three crystallographically distinct 3.0 A crown ethers and in addition, three N–H? ? ?O hydrogen ˚ are present in bonds (N? ? ?O, 2.73(2); 2.71(2); 2.72(2) A) each macrocycle ring. The zinc compound formed with N-(phosphonomethyl)aza-15-crown-5 [ZnO 3 PCH 2 NH 1 C 10 H 20 O 4 ]?NO 3 ? 0.5H 2 O, is orthorhombic a59.780(7), b517.353(13), c5 ˚ Pnna, Z58. The zinc atoms are four-coordinate 21.118 A, bonding to three phosphonate oxygens and one nitrate oxygen. Each phosphonate group bridges two zinc atoms in pairs forming eight-membered rings as shown in Fig. 4. The third phosphonate oxygen bridges to Zn in another eight-membered ring, forming a second set of eight-membered rings. The two sets of rings are inclined to each other in a rough, notch-like fashion forming

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Fig. 4. A line drawing of the zinc N-(phosphonomethyl)-aza-15-crown-5 nitrate hydrate showing the stacking of the rings attached to the central core of eight-membered zinc phosphonate rings centered at the corners and center of the unit cell. Nitrate groups are monodentately bonded to Zn above and below the eight-membered rings.

crenulated chains. We note that the phosphorus atoms in the horizontal eight-rings are diagonally across from each other and are alternately above and below the plane of the ring in alternate rings. As a result, the azacrown rings stack in rows along the a-axis in a staggered fashion, alternately above and below the eight-membered rings, forming four rows of crown ethers. The azacrown conformation is cradle-like with the bowls of the rings facing each other both above and below the octagonal-shaped ring, but staggered at ]12 a apart. The azacrown rings in any one row ˚ apart (the a-axis distance). This large spacing is are 9.78 A required because the azacrown rings are stacked edge to edge. It appears that this particular conformation of the azacrown rings results from intermolecular hydrogen bonds between the NH group and the ether oxygens (N– ˚ 1168; O2, 3.21 A, ˚ 126.58; O3, 3.64 A, ˚ H? ? ?O, O1, 2.75 A, ˚ 135.78; O4, 2.78(2) A, 109.78). The nitrate groups also stack along a. There is also a series of H-bonds that arise from the positioning of the water and nitrate groups, ˚ O–H? ? ?O12, 2.92(2) A. Cerium(III) formed a discrete dimer with phophonomethyl-aza-15-crown-5, [Cd(HO 3 PCH 2 NHC 10 H 20 O 4 )(NO 3 ) 3 (H 2 O) 2 ] 2 .It should be noted that none of the phosphonic acid protons were displaced by the cerium

cation, requiring that three nitrate ions neutralize each Ce 31 . The two cerium ions are bridged by two phosphonate groups with the remaining two uncoordinated protonated oxygens of the bridging ligands pointing in opposite directions [*9]. The cerium atoms are each chelated by three nitrate groups and also bonded to two water molecules for a total coordination number of 10. The dimers self-assemble into linear chain psuedo-leaflets through hydrogen bonding between the PO–H group as donor and a coordinated water molecule as acceptor (O–O, 2.83(3) ˚ A coordinate water also acts as donor to one of the A). ˚ chelating nitrate groups (O–O, 2.78(3) A). Previously, we had reported on the preparation and structure of polymeric zirconium phosphonomethyl azacrown ethers [*7,11]. These compounds are layered with the layers having the a-zirconium phosphate structure or a mixed a-, g-type layer. Pure g-type analogues have also been prepared [*5,6]. These two-dimensional polymeric azacrown compounds, together with the one-dimensional type reported here, constitute a new class of immobilized macrocyclic complexes with interesting potential. These studies suggest novel routes for grafting crown ethers onto a polymeric matrix. For example, in the present study, only a one-to-one ratio of metal to phosphonic acid was used as

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well as very mild conditions. By changing this ratio and utilizing more severe conditions as well as a range of different anions, we expect many new structure types to form. Our present studies clearly demonstrate that steric and directional interactions of the crown ether moiety and the phosphonic acid in phosphorylated crown ether complexes result in the formation of predictable one-dimensional networks. Hydrogen bonding and the metal coordination environment dictate the subtle structural details of the 1D structures (conformation, relative orientation and separation distance between crown ethers). The complexes presented here bring out several interesting features that are relevant to the rational design of supramolecular routes for grafting crown ethers into a polymeric matrix for achieving selective ion exchange, transportation and catalytic properties using either organic solids or inorganic solids. No doubt, additional features will arise as more compounds are synthesized. Probably, the leaflet structures reported here represent the first ever-direct structural characterization of polymeric crown ether assemblies, where crown ethers are present in regular arrays and available for metal ion recognition. The overlapping nature of crown ethers in these compounds indicates that one can design artificial ionic channels using leaflet structures as model compounds. The structural information provided by these crystalline polymeric crown ethers present a unique opportunity to study the metal ion binding selectivity of polymeric arrays considering cooperative effects and dimensionality. Successful design of insoluble crystalline crown ethers provides an invaluable route for making potentially inexpensive, reusable and highly selective polymeric crown ether-based catalysts, ion exchangers and metal ion separators. Before leaving the subject of crown ethers, it should be pointed out that we obtained reactions in which the crown ether coordinates to cations and this feature determined the crystal structure of the N,N9-bis(phosphonomethyl)-1,10diaza-18-crown-6(H 4 L) complex [11]. A copper complex, Cu(HO 3 PCH 2 NC 12 H 24 O 4 NCH 2 PO 3 H), was obtained by slow evaporation of an alcohol–water solution of H 4 L1 Cu(II) [12]. The copper ion sits in the center of the diazacrown ring bonded to two ring oxygens and both aza nitrogens. The phosphonate groups bend backwards so that one oxygen from each group bonds to Cu in the axial

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positions of an octahedron. The four bonding ring atoms constitute the equatorial plane. The axial bonds are significantly longer than the equatorial bonds, satisfying the Jahn–Teller theorem. This bonding arrangement leaves one protonated oxygen and one negatively charged oxygen remaining as part of the phosphonate groups. As a result, these groups spontaneously form one-dimensional chains through hydrogen bonding as shown in Fig. 5. A very different complex was formed with cadmium nitrate by a synthetic procedure similar to the one utilized for the copper complex. The empirical formula for the cadmium compound is [Cd 2.75 (L)(H 2 O) 7 ]?1.5NO 3 ?7H 2 O? CH 3 OH, where L is the diazacrown phosphonate in which 21 Cd replaced the four phosphonic acid protons. The unit ˚ space cell dimensions are a520.815(1), c518.585(1) A, group P42 /n, Z58. There are four unique cadmiums in the asymmetric portion of the unit cell, two are eight-coordinate and two are six-coordinate. Cd3 is bonded to all four oxygens and both aza nitrogens of the diazacrown ether ring. The coordination is completed by oxygens from the two phosphonic acid groups, which in this case are in cis conformation. The other eight-coordinate cadmium is chelated by four different phosphonate groups while the six-coordinated cadmiums are bonded to three and four water molecules, respectively, the remaining coordination sites being filled by bridging phosphonate oxygens. The connectivity of the cadmiums is such as to form large channels parallel to the c-axis direction as shown in Fig. 6. Water and methanol molecules fill the channels.

3. Crystal engineered structures

3.1. Hydrogen bonded self-assembled compounds We have shown in the previous section that the structure of crown ether phosphonic acids is controlled by hydrogen bonding into self-assembled linear arrays. Even when metals are complexed, the larger assembly of basic units is somewhat controlled by residual hydrogen bonding. The form of the assembly taken depends on how many hydrogens remain, either on the nitrogen atom or on the phosphonate groups. Up until recently, the use of phosphonic acids has been concentrated on their reactions with

Fig. 5. One-dimensional hydrogen bonded Cu(II) N,N9-bis(phosphonomethyl)-1,10-diaza-18-crown-6 groups along the a-axis. The Cu, O, N, P and C atoms are drawn as open (large), crossed, octanded, hatched and black circles, respectively. Hydrogen atoms of the carbon atoms are omitted for clarity. Hydrogen bonds are represented by dotted lines.

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Fig. 6. View of the structure of Cd 2.75 [N,N9-bis(phosphonomethyl)-1,10-diaza-18-crown-6)(H 2 O) 7 ?1.5(NO 3 ]?7H 2 O?MeOH down the c-axis. The phosphonate groups are represented by tetrahedra. The Cd, O, N and C atoms are drawn as open, crossed, hatched and black circles, respectively. There is a second set of interconnecting tunnels parallel to the b-axis direction.

metals [**13]. Although we did publish the structures of two phosphonic acids detailing their propensity to hydrogen bond [11], their use in organic chemistry has generally been neglected. A glimpse of what is in store for the venturesome experimenter has been provided in a recent paper in which phenylphosphonic acid has been reacted with a series of amines [*14]. Intricate layers form, held together totally by hydrogen bonds. While a great deal of work has been carried out with carboxylic acid, the use of phosphonic acids adds to the dimensionality of the ligand, resulting from the tetrahedral nature of the phosphonic acid group with two available protons.

We have begun a systematic examination of the Scheme 1 ligands shown below:

3.2. Nitrilotris(methylphosphonic acid), N( CH2 PO3 H2 )3 , represented as I-H6 N Because of the Zwitter ion form of the molecule, it can be also represented as I-H 5 NH. Some of the ways that hydrogen bonding may occur with I-NH 6 are given in Scheme 1. We have already seen examples of types a and b when discussing the self-assembly of crown ether phosphonic

Scheme 1.

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acids. We recently synthesized a series of compounds utilizing divalent metals Mn, Co, Ni, Zn and Cd. These cations displaced two protons from I-H 6 N to obtain compounds of composition M(I-H 3 NH)(H 2 O) 3 where M represents the metal cations listed in the previous sentence [*15]. Three water molecules and three oxygen atoms from two phosphonate anions of I-H 3 NH octahedrally coordinate to the metal centers in the isomorphic complexes. The phosphonate anions chelate to the metal center to form an eight-membered ring and one of them additionally interconnects the neighboring metal centers to form a onedimensional helical coordination polymer. The hydrogenbonding sites of the triphosphonates anions on the onedimensional polymer are oriented at an angle, |738, as the presence of three-coordinate water molecules demands such a configuration. Consequently, the complementary hydrogen-bonding recognition sites of the one-dimensional helical polymer, form a two-dimensional corrugated sheet structure through divergent hydrogen bonds, instead of a double helix through convergent hydrogen bonds, O? ? ?O, ˚ N? ? ?O, 2.723(3) A. ˚ A more complete descrip2.588(3) A, tion of the structure is in print as well as that of an anhydrous complex of the same metals [16]. The fact that each of the metals, even combinations of these metals, yielded the same structure indicates that it is the hydrogen bonding that controls the formation of the layers. Rather than displacing two protons with a divalent cation, we also replaced a single proton by a 1:1 combination of I-H 5 NH and an amine. Evaporation of an ethanol– water mixture of the phosphonic acid with 1,7-phenanthro-

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line resulted in the formation of condensed hexagonal rings of the type in Scheme 2d [17]. The proton was transferred to the amine, which is then encapsulated within the ring. The hexagonal ring is formed by type a hydrogen bonds ˚ and has internal dimensions of (O? ? ?O, 2.45–2.47 A) 2 ˚ 9.7312 A . There are a total of five protons per nitrilotris(phosphonic acid) group that must be distributed among the three phosphonate groups. In the 1,7-phenanthroline framework, the grouping is 2, 1.5 and 0.5 protons and one proton bonded to the nitrogen. These protons further link the hexagons through symmetrical hydrogen bonds in the third dimension resulting in a very rigid three-dimensional cooperative hydrogen-bonded network with hydrophilic cavities along the walls of the supramolecular hexagon ˚ The two hydration water (O? ? ?O, 2.464(3), 2.455(3) A). molecules occupy these cavities and form hydrogen bonds ˚ with phosphonic acids (O–H? ? ?O, 2.723(3), 2.839(3) A). Additional H-bonding to the amine groups further strengthens the three-dimensional network. Several variants of this hexagonal ring structure were obtained by the use of other amines, 1,10-phenanthroline, acridine and tripropylamine. The distribution of protons on the phosphonic acid groups was found to be 1.5, 1.5, 1 for the phenanthroline and 2, 1, 1 for the other amines. The H-atom positions were determined from difference Fourier maps and from the observation of the significant P–O bond length increase over the average bond length. Because of the different arrangement of the protons in the several templated structures, the frameworks, while still hexagonal, make use of the motifs shown in Scheme 2 in different

Scheme 2.

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combinations giving rise to variations in the hexagonal shape. However, they are all three-dimensional in nature [17]. It is interesting to note that the transfer of a proton to 1,7-phenanthroline and its encapsulation in the hexagonal framework changes its color from pale yellow ( l, 368 and 384 nm) to deep orange red. Apparently, this change is due to the presence of a broad charge transfer band in the protonated species. A similar transformation does not occur with the 1,10-phenanthroline complex. Double deprotonation of I-H 6 N is effected using a 2:1 ratio of amine to acid. This loss of an additional proton prevents formation of the three-dimensional structures and leads to complex one-dimensional chains as shown in Fig. 7. The crystal packing with the amines produces twodimensional layers of alternating negatively charged hydrogen-bonded chains and positively charged amines as

shown in the figure for the 1,7-phenanthroline complex [17]. In order to remove more than two protons from I-H 6 N, we turned to the use of metal ions. In a hydrothermal reaction at 180 8C, we obtained a coordination complex of Zn(II), Zn 2 [HO 3 PNH(CH 2 PO 3 ) 2 ]. We note here that four of the six phosphonic acid protons have been displaced by zinc atoms. This complex is hexagonal, P61 , Z56 with ˚ [18]. In this structure, the a58.3553(8), c526.657(4) A nitrogen remains protonated and thus does not coordinate to zinc so that only one phosphonic acid group contains the remaining proton. The two unique Zn atoms are tetrahedrally coordinated by four oxygens from four different phosphonate groups. The ligand I-H 5 NH uses eight of its nine oxygens to bond to eight different Zn atoms. In doing so, it forms a three-dimensional network in which opened 12-membered rings are present as shown in Fig. 8.

Fig. 7. Depiction of layer formation through double deprotonation of nitrilotri(methylphosphonic acid) by 1,7-phenanthroline. The phosphonic acid forms a linear chain structure that is negatively charged and pillared by the positively charged amine.

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Fig. 8. View of the Zn 2 [HO 3 PCH 2 NH(CH 2 PO 3 ) 2 ] structure down the a-axis showing the open framework resulting from formation of 12-membered rings. C–PO 3 tetrahedra are shaded in black. Zn and amine N atoms are shown as large open and octanded circles, respectively.

3.3. Complexes of N,N9 bis( phosphonomethyl)aminoacetic acid, ( H2 O3 PCH2 )2 NCH2 COOH,II-H5 N A zinc compound of II-H 5 N was prepared in a very similar hydrothermal procedure as for the Zn derivative of I-H 6 N just described [18]. This new compound was also hexagonal P61 with a58.0677(12), c527.283(6), Z56. Displacement of four protons by Zn(II) leaves only one proton on the nitrogen remaining on the ligand. Each of the two unique Zn atoms is tetrahedrally coordinated through three phosphonate oxygens and one carboxylate oxygen. The ZnO 4 tetrahedra are interconnected by the bridging phosphonate and carboxylate groups resulting in a threedimensional structure much the same as was produced by the Zn–I-H 6 N complex. In this compound, there are also two sets of 12-membered rings formed, one ring consisting of three Zn, three P and six O and the other by three Zn, two P, one C, six O. The zinc compound reported here contrasts to a platinum derivative reported by Galanski et al. [19]. The H 5 N ligand forms a five-membered chelate ring with Pt(II) by bonding through N and the carboxyl group. The phosphonic acid groups remain uncoordinated, requiring a composition Pt[O 2 C–CH 2 N(CH 2 PO 3 H 2 ) 2 ] 2 .

3.4. Complexes formed by H4 PMIDA, H4 N Our earlier results with H 4 PMIDA compounds of Zr have been published and summarized in a recent review [*9] and in Part I of this series [**1]. These earlier results were obtained with Zr(IV) as the metal ion. Two addition-

al novel metal complexes were obtained with divalent ions, Co and Zn [20]. Their metal acetates were refluxed for 30 min with H 4 PMIDA in the presence of urea. Single crystals were obtained by diffusing absolute alcohol into the filtered solutions. The ratio of metal to H 4 PMIDA was 2:1, resulting in displacement of all four protons of the ligand. The cobalt compound, [Co(PMIDA)(H 2 O) 5 ]?H 2 O, also produced a voluminous pink gel as well as the single crystals. There are two unique cobalt atoms in the structure. One is coordinated to four water molecules and two phosphonate oxygens from two PMIDA groups, forming two OPO bridges across two Co1 creating an eight-membered ring. The third oxygen of the phosphonate group bonds to Co2 as does the PMIDA nitrogen, forming a five-membered ring. The two carboxylate groups are also bonded to Co2, again forming two additional five-membered rings. The free carboxylate oxygens bond to adjacent Co2 atoms to form a linear chain. A water molecule occupies the sixth coordination site. The Co1 octahedra forming dimers through phosphonate bridging are terminated by the four water molecules around the periphery of each octahedron. These water molecules bind the dimers together, through a network of hydrogen bonds. The dimers connect to the Co2 octahedra through O3 of the phosphonate group in both the positive and negative a-axis direction. The result is illustrated in Fig. 9. The dimers are grouped about an a-axis value of zero and hydrogen bonded to each other in the bc plane. The dimers are, in turn, also connected to the chelated octahedra of Co2 at approximately 1 ]31 a and 2 ]31 a and are thus, sandwiched between the chains. Chains of Co2 octahedra facing each other are connected through a

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Fig. 9. ORTEP representation of the fundamental unit in Co 2 [O 3 PCH 2 N(CH 2 COO) 2 (H 2 O) 5 ]?H 2 O illustrating the bonding motif (a) and view of the crystal structure down the c-axis direction (b). The phosphonate groups are represented by tetrahedra and Co, N, O, C atoms are shown as open, gray, crossed, and black circles, respectively.

hydrogen-bonding network that ties the entire structure together three-dimensionally. The structure of the zinc compound, [Zn 2 (PMIDA)(CH 3 COOH)]?2H 2 O, is very different from that of the cobalt compound [20]. One Zn is tetrahedrally coordinated by two phosphonate oxygens, O2, O3 (Fig. 10a), and two carboxylate oxygens, O4, O6, from four PMIDA ligands. The second Zn is five-coordinate formed from a chelating PMIDA group creating three five-mem-

Fig. 10. ORTEP representation of the fundamental unit of Zn 2 (O 3 PCH 2 N(CH 2 COO) 2 (CH 3 COOH)]?2H 2 O (a) and (b) view of the structure down the b-axis. The phosphonate groups are represented by tetrahedra and Zn, N, O and C atoms are shown as open, gray, crossed and black circles, respectively. Dotted lines indicate H-bonds.

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Scheme 3.

bered rings and bonded to a neutral acetic acid molecule [C7–O9, 1.344(6); C7–O8, 1.275(5)]. The coordination about Zn2 is that of a distorted trigonal bipyramid with O1, O5, O7 atoms forming a trigonal plane with N1 and O8 in the axial positions (N1–Zn–O8, 1658). The two types of polyhedra are linked together into layers (Fig. 10b) along the k202l plane of the unit cell with the acetate group protruding into the interlayer space.

3.5. Complexes of Nmethyliminobis(methylenephosphonic acid), CH3 N( CH2 PO3 H)2 , H4 NC The several ways in which this ligand interacts with divalent cations is shown in Scheme 3. Structure a is just the Zwitter ion form of the ligand represented as H 3 NHC. We have prepared three different compound types, Mn(H 2 NHC) 2 ?2H 2 O [21] and an isostructural Cd compound [22]; Zn 3 (NHC) 2 [21] and Zn(HNHC) [22]. The first two compounds are of type b (Scheme 3); the Zn 3 (NHC) 2 compound is of type c and the last compound has a type d structure. All of these compounds have protonated amine groups. However, an additional series of compounds in which the amino proton is removed, allowing the nitrogen to bond to the metal, should also be achievable.

4. Conclusions In the recent past we have prepared a number of complexes of mono-, di- and triphosphonic acids, with nitrilotris(methylphosphonic acid) (I-H 6 N) representing the triphosphonic acid. It is clear that the degree of deprotonation and the resultant hydrogen bonding determines the type of complex formed. The phosphorylated crown ethers, N-(phosphonomethyl)-aza-18-crown-6 and N-(phosphonomethyl)-aza-15-crown-5 form one-dimensional polymeric arrays through hydrogen bonded and coordination

bonded self-assembly. I-H 6 N forms unique three-dimensional hexagonal structures when a single proton is transferred to an amine. This reaction triggers formation of a hexagonal three-dimensional hydrogen-bonded array that encapsulates the amines as templates. If two protons are removed, one-dimensional chain structures are obtained. By choice of the proper template, layered compounds may also be prepared. The exploration of divalent metal complexes with these ligands has produced several new structure types as described here. Further systematic removal of the protons sequentially from these and other phosphonic acid ligands is in progress to uncover other possible structure types. With this knowledge, it is now possible to design structures or to synthesize new phosphonic acids as ligands for this purpose in much the same manner that has been so fruitfully exploited with carboxylic acid ligands. It has been pointed out by Jensen et al. [23] and Nash [24] that the presence of an additional oxygen donor atom in phosphonic versus carboxylic acids gives each phosphonate group a binding strength equivalent to 1.5 carboxylate groups. Furthermore, phosphonic acids are more highly acidic than carboxylic acids and can form metal complexes of trivalent and tetravalent metals in a acid solutions. Complexes of divalent metals are generally soluble at pH values below 2. This superior complexation ability of diphosphonic acids in acidic solutions for high valent cations has been utilized in lanthanide and actinide separation [23] and in the preparation of polymer-based cation exchange resins [24].

Acknowledgements We acknowledge with thanks the financial support from the Robert A. Welch Foundation through Grant No. A673 and the Department of Energy, Basic Sciences Division through Grant No. DE-FG03-00ER15086 and the National

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Science Foundation for grants DMR-9707151 and DMR0080040.

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