research papers Acta Crystallographica Section B

Structural Science, Crystal Engineering and Materials

Linear alkaline earth metal phosphinate coordination polymers: synthesis and structural characterization

ISSN 2052-5206

Jeffrey A. Rood,a* Ashley L. Huttenstine,a Zachery A. Schmidt,a Michael R. Whitea and Allen G. Oliverb Department of Chemistry and Biochemistry, Elizabethtown College, Elizabethtown, PA 17022, USA, and bDepartment of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA

Reaction of alkaline earth metal salts with diphenylphosphinic acid in dimethylformamide solvent afforded four coordination polymers: [Mg3(O2PPh2)6(DMF)2]2DMF (I), [Ca(O2PPh2)2(DMF)2] (II), [Sr(O2PPh2)2(DMF)2] (III) and [Ba(O2PPh2)2(DMF)2] (IV) (where DMF is N,N-dimethylformamide). Single-crystal X-ray diffraction revealed that all four compounds produce linear chain structures in the solid state, with the Ca, Sr and Ba forming isostructural crystals. The bulk materials were characterized by FT–IR and 1H NMR spectroscopy and elemental analyses.

Correspondence e-mail: [email protected]

1. Introduction:

# 2014 International Union of Crystallography

In the broad field of extended solids, hybrid inorganic–organic materials have received tremendous interest in recent times. Categorized by a variety of names including metal–organic frameworks (MOFs) or coordination polymers, these materials are of interest for a variety of applications including separation science, optics and magnetism (Batten et al., 2012; Rao et al., 2008; Dincˇa & Long, 2008; Fletcher et al., 2005; Chen et al., 2006; Choi & Suh, 2004). The area has largely been dominated by the use of carboxylate ligands as linker molecules in framework design (Tranchemontagne et al., 2009). Our interest lies in fundamental investigations into the structures of new coordination polymers composed of metal ions and less conventional organic linker molecules. Specifically we have been investigating the use of phosphinic acids in the synthesis of extended materials. The structures of metal phosphinates in the solid state have been fairly well studied for the transition metals, with detailed examples of copper (Bino & Sissman, 1987), manganese (Ruettiger et al., 1999; Du et al., 1991), cobalt (Liu et al., 1992) and nickel (Annan et al., 1991). Orlandini and co-workers have expanded on this area with numerous reported structures involving diphosphinate anions (Costantino, Ienco et al., 2008; Bataille et al., 2008; Costantino, Midollini & Orlandini, 2008; Midollini et al., 2004, 2006; Ciattini et al., 2005; Cecconi et al., 2004; Berti et al., 2002). Their work also extended to the p-block with reports of lead(II) (Cecconi et al., 2003) and tin(II) (Beckmann et al., 2004, 2005) compounds. Comparatively, less is known about the solid-state structures of alkaline earth metal phosphinates. Decades ago, investigations into such materials focused on IR studies of phosphinates composed of beryllium, magnesium and calcium ions to gain insight into the binding modes of the phosphinate moiety (Katzin et al., 1978; Mikulski et al., 1981). Since that time, little focus has been given to alkaline earth metal phosphinates as potentially useful coordination polymers. One report exists of magnesium and calcium extended structures formed from reaction with P,P0 -diphenylmethylenediphos-

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doi:10.1107/S2052520614004247

Received 26 November 2013 Accepted 24 February 2014

Acta Cryst. (2014). B70, 602–607

research papers phinic acid (Midollini et al. 2006). Recently, Westerhausen reported on the isolation of a linear calcium diphenylphosphinate isolated from the reaction of diphenylphosphinic acid and calcium bis(hexamethyldisilazide) and a series of other oxidation products for calcium and strontium bis(diphenylphosphanides) (Al-Shboul et al., 2012).

To recognize the full potential of phosphinates as organic linkers for coordination polymers, a systematic study of the solid-state structures of metal phosphinates is warranted. Herein, we discuss the synthesis and crystal structures of four alkaline earth phosphinates: [Mg3(O2PPh2)6(DMF)2]2DMF (I) and the isostructural series [Ca(O2PPh2)2(DMF)2] (II), [Sr(O2PPh2)2(DMF)2] (III) and [Ba(O2PPh2)2(DMF)2] (IV).

12H, —O2PPh2), 7.676 (m, 8H, —O2PPh2), 7.92 (s, 1H, O C—H). FT–IR (cm1): 3050 w, 3030 w, 2930 w, 1667 s, 1437 s, 1380 m, 1262 m, 1211 s, 1136 s, 1069 s, 1026 s, 795 m, 726 s, 695 s. 2.3. Synthesis of [Ca(O2PPh2)2(DMF)2] (II)

Ca(NO3)24H2O (0.054 g, 0.25 mmol) and diphenylphosphinic acid (0.109 g, 0.5 mmol) were added to DMF (4 ml) in a 15 ml scintillation vial. The mixture was stirred until a clear solution formed. The vial was tightly capped and left at room temperature. Over a period of 3 weeks, crystals were deposited on the walls of the vial. Yield: 0.068 g (44% based on Ca). Anal.: calc. for (I): C 58.12, H 5.53, N 4.51; found: C 58.04, H 5.45, N 4.57. 1H NMR (400 MHz, D2O):  = 2.863 (s, 3H, N— CH3), 3.018 (s, 3H, N—CH3), 7.478 (m, 12H, —O2PPh2), 7.706 (m, 8H, —O2PPh2), 7.89 (s, 1H, O C—H). FT–IR (cm1): 3050 w, 2960 w, 1650 s, 1525 w, 1436 m, 1392 s, 1188 s, 1134 s, 1100 m, 1053 m, 1035 w, 790 m, 723 s, 701 s. 2.4. Synthesis of [Sr(O2PPh2)2(DMF)2] (III)

SrCl2 (0.068 g, 0.25 mmol) and diphenylphosphinic acid (0.111 g, 0.5 mmol) were added to DMF (4 ml) in a 15 ml scintillation vial. The vial was tightly capped and placed in a silicone oil bath at 333 K until the solids dissolved. The vial was removed from the oil bath and left at room temperature. Over a period of 3 weeks, crystals were deposited on the walls of the vial. Yield: 0.058 g (41% based on Sr). Anal.: calc. for (I): C 53.92, H 5.23, N 4.21; found: C 53.77, H 4.99, N 4.08. 1H NMR (400 MHz, D2O):  = 2.857 (s, 3H, N—CH3), 3.009 (s, 3H, N—CH3), 7.469 (m, 12H, —O2PPh2), 7.706 (m, 8H, — O2PPh2), 7.93 (s, 1H, O C—H). FT–IR (cm1): 3051 w, 2930 w, 1648 s, 1492 m, 1436 m, 1391 s, 1258 w, 1179 s, 1133 s, 1098 m, 1050 m, 1023 w, 997 w, 757 m, 722 s, 699 s.

2. Experimental 2.1. General

Magnesium acetate tetrahydrate, calcium acetate hydrate, strontium chloride, barium nitrate and dimethyl formamide were obtained commercially and used as received. Diphenylphosphinic acid (99%) was purchased from Alpha Aesar and used without further purification. FT–IR spectra were recorded on a Nicolet Magna 760 FTIR spectrometer. 1H NMR was carried out on a Varian 400 MR spectrometer. Elemental analyses were completed by Midwest Microlabs. The reported yields are based on the first crop of crystals that were isolated from the mother liquor. 2.2. Synthesis of [Mg3(O2PPh2)6(DMF)2]2DMF (I)

Mg(CH3COO)24H2O (0.054 g, 0.25 mmol) and diphenylphosphinic acid (0.109 g, 0.5 mmol) were added to DMF (4 ml) in a 15 ml scintillation vial. The vial was tightly capped and placed in a silicone oil bath at 333 K. Over a period of 7 d crystals were deposited on the walls of the vial. Yield: 0.12 g (29% based on Mg). Anal.: calc. for (I): C 60.53, H 5.31, N 3.42; found: C 58.49, H 5.17, N 3.16. 1H NMR (400 MHz, D2O):  = 2.853 (s, 3H, N—CH3), 3.002 (s, 3H, N—CH3), 7.471 (m, Acta Cryst. (2014). B70, 602–607

2.5. Synthesis of [Ba(O2PPh2)2(DMF)2] (IV)

Ba(NO3)2 (0.063 g, 0.25 mmol) and diphenylphosphinic acid (0.109 g, 0.5 mmol) were added to DMF (7 ml) in a 15 ml scintillation vial. The vial was tightly capped and placed in a silicone oil bath at 388 K until the solids dissolved. The temperature was reduced to 313 K and held constant throughout the course of the reaction. Over a period of 7 d crystals were deposited on the walls of the vial. Yield: 0.076 g (43% based on Ba). Anal.: calc. for (I): C 50.19, H 4.77, N 3.90; found: C 49.69, H 4.99, N 3.96. 1H NMR (400 MHz, D2O):  = 2.874 (s, 3H, N—CH3), 3.028 (s, 3H, N—CH3), 7.504 (m, 12H, —O2PPh2), 7.668 (m, 8H, —O2PPh2), 7.95 (s, 1H, O C—H). FT–IR (cm1): 3051 w, 2930 w, 1648 s, 1492 m, 1436 m, 1391 s, 1258 w, 1179 s, 1133 s, 1098 m, 1050 m, 1023 w, 997 w, 757 m, 722 s, 699 s. 2.6. X-ray crystallography

All crystals were mounted at cryogenic temperatures on MiTeGen loops in Paratone-N oil. Data were recorded in a routine fashion using a combination of ! and ’ scans. Crystals

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research papers Table 1 X-ray crystallography details. For all structures: Z = 2. No restraints were used in the refinements. H-atom parameters were constrained. (I)

(II)

(III)

(IV)

C78H74Mg3N2O14P62C3H7NO 1668.33 Monoclinic, P21/n

C30H34CaN2O6P2

C30H34N2O6P2Sr

C30H34BaN2O6P2

620.61 Monoclinic, P21/c

668.15 Monoclinic, P21/c

717.85 Monoclinic, P21/c

 ( ) ˚ 3) V (A Radiation type  (mm1) Crystal size (mm)

150 12.7538 (5), 22.3294 (9), 15.0157 (6) 103.175 (2) 4163.7 (3) Mo K 0.22 0.34  0.15  0.14

120 9.7975 (2), 5.79149 (16), 25.7052 (7) 97.272 (3) 1446.84 (7) Mo K 0.38 0.38  0.21  0.19

150 9.7098 (17), 5.8964 (10), 26.165 (5) 97.635 (2) 1484.8 (4) ˚ Synchrotron,  = 0.77490 A 0.82 0.16  0.01  0.01

120 9.6068 (7), 6.0118 (4), 26.6405 (19) 98.033 (1) 1523.50 (19) Mo K 1.45 0.67  0.12  0.05

Data collection Diffractometer

Bruker APEXII

Bruker APEXII

Bruker X8 APEXII CCD

Multi-scan (Blessing, 1995); SADABS (Sheldrick, 2008) 0.96, 0.97 57 065, 8517, 6996

SuperNova, Dual, Cu at zero, Atlas Multi-scan (Blessing, 1995); SADABS (Sheldrick, 2008) 0.98, 1.00 9758, 3496, 2745

SADABS (Sheldrick, 2008) 0.92, 0.94 15 927, 3020, 2425

Multi-scan (Blessing, 1995); SADABS (Sheldrick, 2008) 0.811, 0.930 22 314, 3153, 2694

0.038 26.4 0.626

0.033 29.3 0.689

0.054 26.4 0.573

0.028 26.5 0.627

0.039, 0.095, 1.06 3496 189 0.45, 0.35

0.030, 0.078, 1.02 3020 189 0.40, 0.35

0.018, 0.041, 1.02 3153 189 0.44, 0.38

Crystal data Chemical formula Mr Crystal system, space group Temperature (K) ˚) a, b, c (A

Absorption correction Tmin, Tmax No. of measured, independent and observed [I > 2(I)] reflections Rint max ( ) ˚ 1) (sin /)max (A

Refinement R[F2 > 2(F2)], wR(F2), S 0.036, 0.095, 1.03 No. of reflections 8517 No. of parameters 515 3 ˚ )
max,
min (e A 0.69, 0.43

Computer programs: APEX2, SAINT, XPREP (Bruker, 2008), CrysAlis PRO (Agilent, 2012).

of (I) and (IV) were examined on a Bruker APEX-II instrument. An optimal data collection strategy requiring a minimum of fourfold redundancy was derived (Bruker, 2008). X-ray data for a crystal of (II) were measured on an Agilent Technologies SuperNova instrument (Agilent, 2012) and data for crystal (III) were recorded at beamline 11.3.1 (Bruker, 2008) in the Advanced Light Source at Lawrence Berkeley National Laboratory. Cell parameters were refined using reflections harvested from the data collection with I > 10(I). All data were corrected for Lorentz and polarization effects, and inter-frame scaling by SADABS (Bruker, 2008) or CrysAlisPro (Agilent, 2012). Anomalous dispersion values for data recorded using synchrotron radiation were calculated using the Brennan and Cowan method as executed in PLATON (Brennan & Cowan, 1992; Spek, 2009). The structures were solved from partial data sets by using the Autostructure option in APEX2 (Bruker, 2008). This option employs an iterative application of direct methods, Patterson synthesis and dual-space routines of SHELXTL (Sheldrick, 2008). H atoms were placed at calculated geometries and allowed to ride on the position of the parent atom. Hydrogen displacement parameters were set to 1.2 times the

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equivalent isotropic U value of the parent atom. All crystallographic details are available in Table 1.1

3. Results and discussion 3.1. Synthesis and spectroscopy

The coordination polymers (I)–(IV) were prepared by reaction of the appropriate metal salt and diphenylphosphinic acid in dimethylformamide at elevated temperatures. The IR spectra and 1H NMR spectra of (I)–(IV) were consistent with the proposed formulations. In the FT–IR spectra, strong P—O stretching bands for all compounds were evident in the range 1100–1200 cm1. Additionally, stretches in the range 1600 cm1 were seen in each case resulting from the carbonyl moiety of dimethylformamide. The 1H NMR spectra in D2O revealed resonances corresponding to the aryl protons of diphenylphosphinate anions as well as the aldehyde and methyl protons of dimethylformamide. These resonances are likely due to the formation of the dissociated forms of the 1 Supporting information for this paper is available from the IUCr electronic archives (Reference: RY5058).

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Acta Cryst. (2014). B70, 602–607

research papers Table 2

lengths and angles involving the Mg atoms is given in Table 2.

˚ ) and angles ( ) for (I). Selected bond distances (A Mg1—O4 Mg1—O6 Mg1—O2 Mg1—O5

1.9155 (13) 1.9286 (13) 1.9290 (13) 1.9327 (13)

Mg2—O3 Mg2—O1 Mg2—O7

2.0252 (13) 2.0365 (11) 2.1424 (13)

O4—Mg1—O6 O4—Mg1—O2 O6—Mg1—O2 O4—Mg1—O5 O6—Mg1—O5 O2—Mg1—O5

109.21 (6) 110.24 (6) 107.69 (6) 112.01 (6) 105.08 (6) 112.36 (6)

O3—Mg2—O1 O3—Mg2—O1 O1—Mg2—O7 O1—Mg2—O7 O3—Mg2—O7 O3—Mg2—O7

89.19 (5) 90.81 (5) 90.72 (5) 89.28 (5) 93.22 (5) 86.78 (5)

metal phosphinate in D2O and are not reflective of the coordination polymers themselves. 3.2. Solid-state structure of [Mg3(O2PPh2)6(DMF)2]2DMF (I)

A single type of bridging mode for the diphenylphosphinate ligand was observed in (I). In all cases the ligand bridges between two Mg atoms in a Mg–O–P–O–Mg fashion. Such bridging results in a series of eight-membered spiro rings that join through the Mg centers and create a one-dimensional chain running along the crystallographic c-axis (Fig. 1). An alternating pattern of a six-coordinate Mg atom followed by two four-coordinate Mg atoms proliferates throughout the chain. As shown in Fig. 2, within the chains, the Mg atoms reside in two distinct coordination geometries. Mg1 exhibits distorted tetrahedral geometry via coordination to four O atoms from four distinct phosphinate ligands. Mg2 sits on an inversion center. When inversion symmetry is applied, Mg2 connects to four O atoms of four phosphinates and additionally coordinates to two dimethylformamide molecules, producing a distorted octahedral geometry. DMF molecules also exist within the lattice as solvent of crystallization. The Mg—O bond distances are in the range 1.9155 (13)– ˚ around the tetrahedral magnesium center and 1.9327 (13) A ˚ around the octahedral magnesium 2.0252 (13)–2.1424 (13) A center. O—Mg—O bond angles range from 105.08 (6) to 112.36 (6) around Mg1. The cis O—Mg2—O angles range from 86.78 (5) to 93.22 (5) and the trans O—Mg2—O angles are 180.0 due to inversion symmetry. A complete list of bond

3.3. Solid-state structures of [Ca(O2PPh2)2(DMF)2] (II), [Sr(O2PPh2)(DMF)2] (III) and [Ba(O2PPh2)(DMF)2] (IV)

Single crystals for the calcium, strontium and barium compounds in the series were isolated from reactions in DMF solution. The bridging mode of the phosphinate ligand in these compounds resembles that found in (I) where the metal centers are linked together in an M–O–P–O–M fashion. Unlike the magnesium centers in (I), the calcium, strontium and barium centers in (II)–(IV) all exhibit distorted octahedral geometry with contacts to four O atoms from four phosphinate ligands and two O atoms from two DMF molecules. Each metal center lies on an inversion center. Again as with (I), eight-membered rings connected through shared metal centers proliferate through the structure creating onedimensional chains. As shown in Fig. 3, isostructural chains result in the calcium, strontium and barium compounds, with each material crystallizing in the monoclinic space group, P21/c. Such chain-type structures containing octahedral metals have been noted for cobalt, nickel and manganese compounds as well (Ruettiger et al., 1999; Du et al., 1991; Liu et al., 1992; Annan et al., 1991). Additionally, Westerhausen and coworkers recently reported on the chain-like structure of [Ca(O2PPh2)2(DMSO)2] (where DMSO is dimethyl sulfoxide), which is structurally analogous to (II) as the DMSO solvate (Al-Shboul et al., 2012). Relevant bond lengths and angles for (II)–(IV) are listed in Table 3. Within (II) the Ca—O bond lengths range from ˚ . Angles around the distorted 2.3152 (13) to 2.4095 (13) A

Figure 1 A single chain in (I). The inversion center operation within the crystal structure generates a pattern of one octahedral magnesium center followed by two tetrahedral magnesium centers within the chain. H atoms have been omitted for clarity. Acta Cryst. (2014). B70, 602–607

Figure 2 Coordination environment around the two types of Mg center in (I). H atoms have been omitted for clarity

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research papers octahedral calcium center range from 85.60 (5) to 93.50 (5) . Small single crystals of (III) were isolated and suitable for synchrotron X-ray diffraction studies. Within (III) the Sr—O ˚ and the bond lengths range from 2.4499 (15) to 2.5619 (15) A O—Sr—O angles range from 84.46 (5) to 95.56 (5) . As expected, the longest metal–oxygen bond distances within the isostructural series exist within (IV), with Ba—O lengths of ˚ . Bond angles around the barium 2.6111 (12) to 2.7268 (13) A center range from 83.06 (4) to 95.54 (5) , and produce the largest deviation from ideal octahedral geometry in the series. The isolation of isostructural crystals upon progressing from calcium to strontium to barium is rather unique. For instance, in the more widely studied area of carboxylates, it has been noted for a series of alkaline earth metal formates synthesized under consistent conditions that the coordination geometry and resulting extending structure changes dramatically with a change in the metal ion size. Additionally, the carboxylate anion is able to adopt a variety of bridging modes between the metal centers (Watanabe´ & Matsui, 1978; Matsui et al., 1980; Viertelhaus et al., 2005; Rood et al., 2006). In the case of (II)– (IV), the M–O–P–O–M bridging mode of the diphenylphosphinate ligand is fixed and seems to play a dominant role in the formation of the chain-like coordination polymers.

Figure 3 The chain structures isolated for compounds (II)–(IV). H atoms have been omitted for clarity.

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Table 3 ˚ ) and angles ( ) for (II)–(IV). Selected bond distances (A M = Ca (II), Sr (III), Ba (IV). (II)

(III)

(IV)

M1—O1 M1—O2 M1—O3

2.3152 (13) 2.3294 (13) 2.4095 (13)

2.4637 (14) 2.4504 (14) 2.5614 (14)

2.6111 (12) 2.6241 (12) 2.7268 (13)

O1—M1—O1i O2—M1—O1i O1—M1—O3i O3—M1—O1i O2—M1—O3i O3—M1—O2i

87.21 (5) 92.79 (5) 85.60 (5) 94.40 (5) 86.50 (5) 93.50 (5)

88.73 (5) 91.27 (5) 86.05 (5) 93.95 (5) 84.44 (5) 95.56 (5)

88.73 (5) 91.27 (5) 86.05 (5) 93.95 (5) 84.46 (5) 95.54 (5)

i

Symmetry code: (i) x; y; z.

4. Conclusions A series of alkaline earth metal phosphinates has been synthesized and structurally characterized in an effort to further extend the classes of organic molecules used in hybrid coordination polymers. The crystal structures obtained from this study indicate that a single bridging mode of diphenylphosphinate predominates in all structures, leading to the isolation of a series of one-dimensional chains in the solid state. While compound (I) contains Mg atoms in two different coordination geometries, the remainder of the series, (II)– (IV), produced isostructural crystals containing Ca, Sr or Ba atoms in distorted octahedral geometries. This study has provided useful information regarding the bridging mode that the diphenylphosphinate anion tends to adopt in the presence of alkaline earth metals. Further studies aim to build upon this work by incorporating other organic linker molecules in an effort to isolate two- and three-dimensional materials with potentially useful properties.

We gratefully acknowledge a Research Corporation Cottrell College Science Award (#19730), the National Science Foundation (grant CHE-0958425) for instrument support, and Elizabethtown College and the Department of Chemistry and Biochemistry for start-up funds and support. Special thanks to Dr Jeanette Krause, Director of the Richard C. Elder X-ray Crystallography Facility at the University of Cincinnati for collection of synchrotron diffraction data. Samples for crystallographic analysis at the synchrotron were submitted through the SCrALS (Service Crystallography at Advanced Light Source) program. Crystallographic data were collected at the Small-Crystal Crystallography Beamline 11.3.1 at the Advanced Light Source (ALS). The ALS is supported by the US Department of Energy, Office of Energy Sciences Materials Sciences Division, under contract DE-AC0205CH11231 at Lawrence Berkeley National Laboratory. Images were generated using CrystalMaker (CrystalMaker Software, 2010).

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Linear alkaline earth metal phosphinate polymers

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