Synthesis, structure and magnetic properties of 2-D and 3-D [cation]{M[Au(CN)2]3} (M = Ni, Co) coordination polymers

Polyhedron 26 (2007) 2189–2199 www.elsevier.com/locate/poly

Synthesis, structure and magnetic properties of 2-D and 3-D [cation]{M[Au(CN)2]3} (M = Ni, Co) coordination polymers Julie Lefebvre, Daniel Chartrand, Daniel B. Leznoff

*

Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, BC, Canada V5A 1S6 Received 19 October 2006; accepted 21 October 2006 Available online 7 November 2006

Abstract In order to study the templating effect of the cation and the resulting impact on the magnetic properties, reactions of M(II) salts with [cation][Au(CN)2] were conducted, yielding a series of coordination polymers of the form [cation]{M[Au(CN)2]3} (cation = nBu4N+, PPN+ (bis(triphenylphosphoranylidene)ammonium); M = Ni(II) and Co(II)). The structures of nBu4N{M[Au(CN)2]3} and PPN{M[Au(CN)2]3} (M = Ni and Co) contain two distinct 3-D anionic frameworks of {M[Au(CN)2]3}, hence the framework was sensitive to the cation, but not to the identity of the metal center. In nBu4N{M[Au(CN)2]3}, the metal centers are connected by [Au(CN)2] units to form six 2-D (4, 4) rectangular grids that are fused through the M centers to yield a complex three-dimensional framework which accommodates the nBu4N+ cations. In PPN{M[Au(CN)2]3}, the framework adopts a simpler non-interpenetrated Prussian-blue-type pseudo-cubic array, with the PPN+ cations occupying each cavity; no reduction in dimensionality occurs despite the large cation size. In the presence of water, {Co(H2O)2[Au(CN)2]2} Æ nBu4N[Au(CN)2] was obtained, a 2-D layered polymer that contains neutral sheets ˚ and hydrogen-bondof {Co(H2O)2[Au(CN)2]2} which are separated by nBu4N[Au(CN)2] layers; aurophilic interactions of 3.4250(13) A ing connect the layers. The magnetic properties of all compounds were investigated by SQUID magnetometry. The Ni(II) polymers have similar magnetic behaviour, which are dominated by zero-field splitting with very weak antiferromagnetic interactions at low temperature (D  2–3 cm1, zJ < 1 cm1). The magnetic behaviour of all of the Co(II) polymers were found to be very similar, and dominated by single-ion effects (i.e. a large first-order orbital contribution). No significant magnetic coupling is observed in any of these coordination polymers, suggesting that the [Au(CN)2] bridging unit behaves as a poor mediator of magnetic exchange in these high-dimensionality systems.  2006 Elsevier Ltd. All rights reserved. Keywords: Coordination polymer; Magnetic properties; Dicyanoaurate; Template effects; Aurophilic interactions; Polymorphism

1. Introduction The structural diversity and associated properties exhibited by coordination polymers are key features that are driving research into these multidimensional moleculebased materials [1,2]. Ideally, using judiciously chosen building blocks, coordination polymers with specific desired structures and properties can be obtained [3–5]. Cyanometallates of the form [M(CN)x]n have received particularly close attention as useful building blocks in syn*

Corresponding author. E-mail address: dleznoff@sfu.ca (D.B. Leznoff).

0277-5387/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.10.045

thesizing molecule-based magnetic [6–11] and other materials [12–15] since the cyanide ligand readily binds to metals at both the C- and N-termini and facilitates electronic interactions between them. Also, a wide range of coordination geometries can be accessed (x = 2–9) and the central metal M can be changed in a systematic, property-determining fashion [9,16,17]. Other cyanide-containing bridging ligands such as dicyanamide (dca, NCNCN) have also been extensively utilized to form coordination polymers [18,19]. In addition to the geometry and preferred coordination modes of the building blocks, in cases where non-coordinating ions are incorporated, the potential to use their

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Scheme 1.

physical (e.g. shape) and chemical (e.g. hydrogen-bonding) properties to influence or ideally, to template structures has been a subject of considerable interest [19–21]. For example, counterions that contain phenyl-groups often assemble using ‘‘phenyl embraces’’ of p–p stacking interactions that influence the accompanying coordination polymer geometry [22]. In anionic metal oxalate networks the choice of cation influences the formation of 2-D (6, 3) sheets or 3-D chiral (10, 3) networks [23,24]. The structural effect of such cations in metal dicyanamide (dca) polymers of the form [cation]{M(dca)3} has been particularly well-studied, yielding a wide range of different structural motifs as a function of the size and shape of the cations [20,25–28]. For example, increasing the size and p–p stacking ability of the cations in 3-D systems such as [EtPh3P]{Ni(dca)3} to larger ones such as [Ph4P]+ induces a reduction in structural dimensionality as the 3D system cannot flex to accommodate the cation bulk [27,28]. The dca anion is a divergent, bent bridging unit that can also bind metals via the central nitrogen donor (Scheme 1a); the five-atom bridge is not linear but subtends c.a. 120–125 angles through the central nitrogen and M– ˚ when only the ends are M distances range from 7 to 8 A bound (l1,5-binding) [29]. In comparison, the cyanometallate [Au(CN)2] also contains a five-atom bridging unit but is essentially linear (C–Au–C angles are generally ˚ through >175), leading to M–M distances of 10–11 A the dicyanoaurate. Also, while the central Au-atom cannot bind to other transition metals, it can readily form attractive aurophilic interactions that can increase the structural dimensionality of the polymer (Scheme 1b) [30]. As part of our investigations into coordination polymers containing the [Au(CN)2] building block [12,13,31–33], and given the diverse structural motifs generated by modifying countercations in the related dca-based systems, we initiated an examination of the analogous [organic cation]{M[Au(CN)2]3} system. We hereby report the first structures of metal tris-cyanoaurate systems with organic cations and their magnetic properties. 2. Experimental 2.1. Reagents and general procedures All reagents were purchased from commercial sources and used as received. Infrared spectra were recorded on a Thermo Nicolet Nexus 670 FT-IR spectrometer with samples prepared as KBr pressed pellets. Microanalyses (C, H, N) were performed by Miki Yang using a com-

puter-controlled Carlo Erba (Model 1106) CHN analyzer. Thermogravimetric analyses (TGA) were performed in an air atmosphere on a Shimadzu TGA-50 instrument at a rate of 5 C per minute. Solid-state UV–Vis–NIR absorption spectra of 1–5 were measured by reflectance using an Ocean Optics SD2000 spectrophotometer equipped with tungsten halogen and deuterium lamps. Prior to analysis, the samples were ground into fine powders, dispersed in water, deposited on a glass plate and left to dry completely. Magnesium oxide powder, prepared in the same fashion, was used as a reference. 2.2. Synthesis of nBu4N[Au(CN)2] Æ 0.5H2O A 30 mL aqueous solution of nBu4NBr (2.325 g, 7.21 mmol) was added to a 30 mL aqueous solution of KAu(CN)2 (2.002 g, 6.95 mmol) while stirring. A white solid immediately precipitated and was stirred for 30 min. The powder of nBu4N[Au(CN)2] Æ 0.5H2O was isolated by filtration and dried overnight. Yield: 3.409 g (97%). Anal. Calc. for C18H37N3AuO0.5: C, 43.20; H, 7.45; N, 8.40. Found: C, 43.24; H, 7.48; N, 8.11%. IR (KBr, cm1): 2962 (s), 2932 (s), 2875 (m), 2865 (sh), m(CN) 2146 (s), 1630 (w), 1488 (m), 1463 (m), 1384 (w), 1156 (w), 1111 (w), 1054 (w), 1030 (w), 882 (m), 805 (w), 740 (m), 490 (s). 2.3. Synthesis of nBu4N{Ni[Au(CN)2]3} (1) A 5 mL ethanolic solution of Ni(NO3)2 Æ 6H2O (28 mg, 0.096 mmol) was added to a 5 mL ethanolic solution of n Bu4N[Au(CN)2] Æ 0.5 H2O (153 mg, 0.31 mmol). The solution was left to evaporate for several days. When c.a. 5 mL of solvent were left, a purple powder of nBu4N{Ni[Au(CN)2]3} (1) was collected by filtration and air dried. Yield: 44 mg (44%). Anal. Calc. for C22H36N7Au3Ni: C, 25.21; H, 3.46; N, 9.35. Found: C, 25.52; H, 3.59; N, 9.62%. IR (KBr, cm1): 2961 (s), 2933 (s), 2887 (m), m(CN) 2195 (s), 1479 (m), 1377 (m), 1150 (w), 1106 (w), 1051(w), 1024 (w), 880 (m), 800 (w), 736 (m), 491 (s). UV–Vis–NIR (nm): 305, 355, 564 (broad) and 910 (broad). Crystals of 1 suitable for single crystal X-ray diffraction analysis were obtained by recrystallization of the purple powder under hydrothermal conditions in a sealed glass ampoule inserted into a stainless steel vessel and heated in a Lindberg Heavy-Duty furnace equipped with a programmable temperature controller. Specifically, a sample of nBu4N{Ni[Au(CN)2]3} (90 mg in 3 mL of water) was heated to 125 C over a period of 2 h, maintained at this temperature for 6 h, and slowly cooled to 25 C at a rate of 1 C per hour. The crystals and the powder had identical powder X-ray diffractograms and IR spectra. 2.4. Synthesis of Co[Au(CN)2]2(H2O)2 A 5 mL acetonitrile solution of nBu4N[Au(CN)2] Æ 0.5 H2O (98 mg, 0.20 mmol) was added to a 5 mL acetonitrile

J. Lefebvre et al. / Polyhedron 26 (2007) 2189–2199

solution of Co(ClO4)2 Æ 6H2O (38 mg, 0.10 mmol). An immediate pink precipitate was obtained but was left covered overnight. The powder of Co[Au(CN)2]2(H2O)2 was then collected by filtration and air dried. Yield: 44 mg (38%). Anal. Calc. for C4H4N4Au2CoO2: C, 8.10; H, 0.68; N, 9.45. Found: C, 8.37; H, 0.77; N, 9.33%. IR (KBr, cm1): 3143 (b), m(CN) 2204 (s), m(CN) 2168 (s), m(13CN) 2118 (w), 1530 (m), 886 (w), 755 (w), 542 (w), 467 (m). 2.5. Synthesis of nBu4N{Co[Au(CN)2]3} (2) n

Bu4N[Au(CN)2] Æ 0.5H2O (40 mg, 0.080 mmol) and Co[Au(CN)2]2(H2O)2 (31 mg, 0.052 mmol) were dissolved completely in 15 mL of ethanol by stirring for 10 min. Once dissolved, the stirring was discontinued and the peach solution was left to evaporate to dryness at room temperature, yielding peach crystals covered with white precipitate. To this mixture 3 mL of ethanol was added and the solution was lightly stirred and then filtered after 3 min. The filtrate was separated and the pink precipitate of nBu4N{Co[Au(CN)2]3} (2) was washed with 3 mL of water and 3 mL of cold ethanol. The separated filtrate was evaporated to dryness and subjected to the same treatment to increase the yield. Yield: 29 mg (52%). Anal. Calc. for C22H36N7Au3Co: C, 25.20; H, 3.46; N, 9.35. Found: C, 25.42; H, 3.54; N, 9.39%. IR (KBr, cm1): 2960 (s), 2933 (m), 2873 (m), m(CN) 2189 (s), m(13CN) 2157 (vw), 1479 (m), 1462 (w), 1377 (w), 1175 (vw), 1149 (w), 1104 (vw), 1053 (vw), 1024 (w), 879 (w), 800 (vw), 734 (w), 484 (w). UV–Vis–NIR (nm): 470 (broad), 490 (sh), 515 (sh), 1030. 2.6. Synthesis of {Co[Au(CN)2]2(H2O)2} Æ Bu4N[Au(CN)2] (3)

n

n

Bu4N[Au(CN)2] Æ 0.5H2O (40 mg, 0.080 mmol) and Co[Au(CN)2]2(H2O)2 (35 mg, 0.059 mmol) powders were mixed together, suspended in a 1 mL mixture of ethanol/ water (9:1, v/v) and sealed with parafilm. The precipitate slowly turned pink, and formed small pink rectangular prism crystals of {Co[Au(CN)2]2(H2O)2} Æ nBu4N[Au(CN)2] (3) after three days. The solution was then filtered and the crystals isolated. Yield: 49 mg (77%). Anal. Calc. for C22H38N7O2Au3Co: C, 24.37; H, 3.72; N, 9.04. Found: C, 24.55; H, 3.79; N, 9.26%. IR (KBr, cm1): 3454 (b), 3207 (m), 2960 (s), 2930(m), 2872 (m), m(CN) 2183 (s), m(CN) 2147 (s), 2108 (vw), 1600 (m), 1469 (m), 1380 (w), 1164 (vw), 1104 (vw), 1068 (vw), 1032 (vw), 926 (vw), 881 (w), 731 (vw), 490 (m). UV–Vis–NIR (nm): 470, 500, 1040. Crystals of 3 suitable for single crystal X-ray diffraction analysis were obtained by a different route: A 5 mL ethanol/water (1:1 v/v) solution of nBu4N[Au(CN)2] Æ 0.5H2O (105 mg, 0.210 mmol) was added slowly to a 5 mL ethanol/water solution of CoðClO4 Þ2  6H2 O (26 mg, 0.071 mmol). The pink solution immediately became peach colored, and was left to slowly evaporate. Pink/red rectangular prism crystals of 3 and white nBu4N(ClO4) crystals formed as the ethanol evaporated. The solution was then

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filtered, and the crystals of 3 were separated by hand; they can be cleaned with ethanol, but are somewhat soluble. Yield: 25 mg (33%). The crystals and the powder had identical powder X-ray diffractograms and IR spectra. 2.7. Synthesis of PPN[Au(CN)2] A 10 mL aqueous solution of KAu(CN)2 (251 mg, 0.872 mmol) was added to a 20 mL mixture of ethanol/ water (1:1 v/v) solution of bis(triphenylphosphoranylidene)ammonium chloride (PPNCl) (525 mg, 0.915 mmol) while stirring. An immediate white precipitate formed. After standing for 30 min, the solid of PPN[Au(CN)2] was filtered, washed with water and air-dried overnight. Yield: 666 mg (97%). Anal. Calc. for C38H30N3AuP2: C, 57.95; H, 3.84; N, 5.38. Found: C, 57.71; H, 3.86; N, 5.17%. IR (KBr, cm1): 3088 (vw), 3075 (w), 3055 (m),3038 (w), 3022 (w), 3008 (vw), 2990 (vw), m(CN) 2145 (s), 1589 (m), 1483 (m), 1456 (m), 1439 (s), 1434 (s), 1285 (s), 1267 (s), 1180 (m), 1160 (w), 1116 (s), 997 (m), 796 (m), 743 (s), 726 (s), 690 (s), 548 (s), 531 (s), 496 (s). A different preparation of this salt, via [AuCl2], has previously been reported [34]. 2.8. Synthesis of PPN{Ni[Au(CN)2]3} (4) A 5 mL ethanolic solution of PPN[Au(CN)2] (100 mg, 0.127 mmol) was added slowly to a 5 mL ethanolic solution of Ni(NO3)2 Æ 6H2O (13 mg, 0.044 mmol). A solid immediately precipitated from the solution, which changed from blue to completely colorless. A very fine pale blue powder of PPN{Ni[Au(CN)2]3} (4) was obtained upon filtration. Yield: 39 mg (69%). Anal. Calc. for C42H30N7Au3NiP2: C, 37.53; H, 2.25; N, 7.29. Found: C, 37.30; H, 2.34; N, 7.19%. IR (KBr, cm1): 3076 (w), 3055 (m), 3021 (w), 3009 (w), 2998 (vw), 2971 (vw), m(CN) 2192 (s), m(CN) 2151 (vw), 1588 (w), 1476 (w), 1436 (m), 1383 (s), 1330 (m), 1302 (w), 1272 (w), 1186 (w), 1160 (vw), 1117 (s), 1026 (w), 997 (w), 738 (w), 723 (s), 688 (w), 550 (m), 531 (s), 499 (m). UV–Vis–NIR (nm): 335, 562 (broad) and 895 (broad). Crystals suitable for X-ray crystallography were obtained using an H-tube technique. On one side of a 55 mL H-shaped tube filled with ethanol, a 2 mL ethanolic solution of PPN[Au(CN)2] (96 mg, 0.121 mmol) was deposited at the bottom. On the other side, a 0.5 mL ethanolic solution of Ni(NO3)2 Æ 6H2O (15 mg, 0,052 mmol) was deposited. The H-shaped tube was sealed, and after 10 days, light purple cubic crystals of 4 formed. The crystals and the powder had identical powder X-ray diffractograms and IR spectra. 2.9. Synthesis of PPN{Co[Au(CN)2]3} (5) A 7 mL ethanolic solution of PPN[Au(CN)2] (150 mg, 0.190 mmol) was added slowly to a 7 mL ethanolic solution of Co(ClO4)2 Æ 6H2O (23 mg, 0.060 mmol). A peach powder

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immediately precipitated from the solution, which changed from pink to completely colorless. A very fine peach powder of PPN{Co[Au(CN)2]3} (5) was obtained upon filtration. Yield: 40 mg (71%). Anal. Calc. for C42H30N7Au3CoP2: C, 37.52; H, 2.25; N, 7.29. Found: C, 37.77; H, 2.15; N, 7.47%. IR (KBr, cm1): 3145 (vw), 3076 (w), 3055 (m), 3021 (w), 3009 (w), 2989 (vw), m(CN) 2187 (s), m(CN) 2175 (sh), 1588 (w), 1476 (w), 1438 (m), 1384 (s), 1330 (m), 1320 (m), 1302 (w), 1272 (w), 1186 (w), 1160 (vw), 1117 (s), 1026 (w), 997 (w), 881 (vw), 840 (vw), 800 (vw), 738 (w), 723 (s), 690 (w), 550 (m), 531 (s), 499 (m). UV–Vis–NIR (nm): 475 (broad), 490 (sh), 510 (sh) and 990. 2.10. X-ray crystallographic analysis Powder X-ray diffractograms of all compounds were measured on a Rigaku RAXIS-Rapid Auto diffractometer ˚ ). Samples equipped with a Cu Ka source (k = 1.54056 A were mounted on a glass fiber using grease and were exposed, as the phi axis was spun (10 s1), for a period of 40–60 min. Single crystals of 1, 3 and 4 were mounted on glass fibers using epoxy adhesive. Single crystal X-ray diffraction data for 1, 3 and 4 were recorded on an Enraf Nonius CAD4F diffractometer equipped with a Mo ˚ ) and controlled by the DIFRAC Ka source (k = 0.71073 A program [35] in the following ranges: 1, 4 < 2h < 51; 3, 4 < 2h < 48; 4, 4 < 2h < 55. The NRCVAX Crystal Structure System was used to perform a psi-scan absorption correction (in the transmission range 0.0135–0.0409 for 1, 0.1068–0.1707 for 3 and 0.0303–0.1154 for 4) and data reduction, including Lorentz and polarization corrections. The crystal structure of each compound was solved using the Sir 92 routine and refined through Fourier techniques using the CRYSTALS software package [36]. Crystallographic data for compounds 1, 3 and 4 are collected in Table 1. For 1, the Ni and Au atoms were refined anisotropically while the C and N atoms were kept isotropic due to the limited number of observed data. Full matrix least-squares refinement for 1 (1540 reflections included) on F (156 parameters) converged to R1 = 0.0483, wR2 = 0.0529 (Io > 2.5r(Io)). For 3, all non-hydrogen atoms were refined anisotropically except for the C atoms of the nBu-groups that were kept isotropic due to their very large thermal motion. Full matrix least-squares refinement for 3 (689 reflections included) on F (87 parameters) converged to R1 = 0.0406, wR2 = 0.0469 (Io > 2.5r(Io)). For 4, all non-hydrogen atoms were refined anisotropically. Full matrix leastsquares refinement for 4 (1172 reflections included) on F (87 parameters) converged to R1 = 0.0303, wR2 = 0.0411 (Io > 2.5r(Io)). Hydrogen atoms were placed in geometric positions in all compounds and their coordinates allowed to ride with their associated atoms. Diagrams were prepared using ORTEP-3 (version 1.076) [37] and POV-RAY (version 3.6.0) [38]. Selected bond lengths

Table 1 Crystallographic data and structural refinement details for 1, 3 and 4 Compound

1

3

4

Formula

C22H36N7Au3Ni 1048.19 purple block 0.42 · 0.42 · 0.28 tetragonal I41c d 16

C22H40N7Au3CoO2 1095.52 red/dark pink rectangular prism 0.22 · 0.20 · 0.14 tetragonal P42/mnm 4

C42H30N7Au3NiP2 1344.30 pale purple cube 0.34 · 0.31 · 0.17 rhombohedral R3c 6

fw Colour Shape Dimension (mm3)

Crystal system Space group Z Unit cell dimensions ˚) a (A 23.532(4) 14.444(2) 15.284(3) ˚) b (A 23.532 14.444 15.284 ˚) c (A 23.181(3) 15.453(2) 31.530(10) a () 90 90 90 b () 90 90 90 c () 90 90 120 ˚ 3) V (A 12,837(3) 3223.9(6) 6379(2) 2.169 2.232 2.100 qcalc (g cm3) l (mm1) 14.270 14.143 10.867 R1 (I > 2.5r(I)) 0.0483 0.0420 0.0303 0.0529 0.0351 0.0411 wR2 (I > 2.5r(I)) Goodness-of-fit 1.3778 1.4542 1.888 P P R1 ¼ P jðjF o j  jF c jÞj= jF P for observed data (Io > 2.5r(I)), o j, wR2 ¼ ½ ðwðjF o j  jF c jÞ2 Þ= ðwF o Þ2 1=2 , for observed data (Io > 2.5r(I)), P GOF ¼ ½ ðwðjF o j  jF c jÞ2 Þ=degrees of freedom1=2 .

and angles for 1, 3 and 4 are reported, respectively, in Tables 2–4. 2.11. Magnetometry Magnetization measurements were performed with a Quantum Design MPMS-XL-7S SQUID magnetometer with an Evercool-equipped liquid helium dewar. Microcrystalline samples of 1–5 were packed in gelatin capsules and mounted in diamagnetic plastic straws. Direct current (dc) magnetization was measured for all samples upon

Table 2 ˚ ) and angles () for nBu4N{Ni[Au(CN)2]3} (1) Selected bond lengths (A Bond lengths Ni(1)–N(1) Ni(1)–N(2) Ni(1)–N(3) Angles N(1)–Ni(1)–N(3) N(1)–Ni(1)–N(4) N(1)–Ni(1)–N(5) N(1)–Ni(1)–N(6) N(3)–Ni(1)–N(5) N(3)–Ni(1)–N(6) C(1)–Au(1)–C(6 0 ) C(2)–Au(2)–C(500 )

2.02(3) 2.07(3) 2.07(3) 85.1(10) 94.0(12) 90.9(11) 92.8(11) 93.1(10) 85.9(11) 173.1(16) 173.5(14)

Ni(1)–N(4) Ni(1)–N(5) Ni(1)–N(6) N(2)–Ni(1)–N(3) N(2)–Ni(1)–N(4) N(2)–Ni(1)–N(5) N(2)–Ni(1)–N(6) N(4)–Ni(1)–N(5) N(4)–Ni(1)–N(6) C(3)–Au(3)–C(4*)

2.07(3) 2.04(3) 2.00(3) 92.4(12) 88.4(12) 88.8(11) 87.5(12) 86.9(12) 94.1(13) 175.0(16)

Symmetry transformation: ( 0 ) y, x  1/2, z + 1/4; (00 ) y + 1/2, x, z  1/4; (*) y, x  1/2, z  1/4.

J. Lefebvre et al. / Polyhedron 26 (2007) 2189–2199 Table 3 ˚ ) and angles () for {Co(H2O)2[Au(CN)2]2} Æ Selected bond lengths (A n Bu4N[Au(CN)2] (3) Bond lengths Co(1)–N(1) Co(1)–O(1) Angles O(1)–Co(1)–N(1) N(1)–Co(1)–N(1 0 ) Au(1)–Au(2)–Au(1)

2.096(12) 2.089(16) 85.1(4) 90.42(7) 163.30(6)

Au(1)–Au(2) O(1)  N(2) C(1)–Au(1)–C(1*) C(2)–Au(2)–C(200 )

3.4250(13) 2.898 171.5(10) 179.7(13)

Symmetry transformation: ( 0 ) y + 1/2, x + 1/2, z + 5/2; (*) y, x, z; (00 ) y, x, z + 3; () x, y, z + 3.

Table 4 ˚ ) and angles () for PPN{Ni[Au(CN)2]3} (4) Selected bond lengths (A Bond lengths Ni(1)–N(1) Au(1)–C(1) Angles N(1)–Ni(1)–N(1 0 ) C(1)–Au(1)–C(1*)

2.072(5) 1.988(6) 88.9(2) 177.3(4)

P(1)–N(2) C(1)–N(1) N(1)–Ni(1)–N(00 )

1.548(3) 1.114(8) 91.1(2)

Symmetry transformation: (*) x  y, y, z + 1/2; ( 0 ) x  y + 1/3, x + 1/3, z + 2/3; (00 ) y + 1, x  y, z.

cooling from 300 to 1.8 K under an applied dc field of 1 kOe. Measurements for all compounds were also performed under an applied field of 100 Oe and no field dependency was observed. The magnetic susceptibility of each compound was corrected for the diamagnetic contribution of the constituent atoms using Pascal’s constants [39]. 3. Results 3.1. Synthesis and structural characterization of n Bu4N{Ni[Au(CN)2]3} (1) and nBu4N{Co[Au(CN)2]3}(2) The reaction of Ni(II) with nBu4N[Au(CN)2] Æ 0.5H2O in ethanol afforded, over a period of several hours, a purple precipitate whose composition was consistent with nBu4N{Ni[Au(CN)2]3} (1) by elemental analysis. When the same reaction was performed with Co(II), a mixture of nBu4Nsalts and Co-containing products were obtained, probably due to the higher solubility of the Co-analogue. To eliminate the resulting separation problem, Co[Au(CN)2]2(H2O)2 was dissolved in ethanol and used as the Co(II) source, to which was added one equivalent of nBu4N[Au(CN)2] Æ 0.5H2O, thereby yielding the byproduct-free nBu4N{Co[Au(CN)2]3} (2). The FT-IR spectra of 1 and 2 show only one cyanide vibration frequency at 2195 and 2189 cm1, respectively, blue-shifted with respect to the mCN frequency of 2146 cm1 for nBu4N[Au(CN)2] Æ 0.5 H2O, consistent with all N-cyano groups binding to the transition metal [9].

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Dark blue crystals of nBu4N{Ni[Au(CN)2]3} (1) suitable for single crystal X-ray diffraction analysis were obtained by hydrothermal recrystallization. The solid-state structure determined for nBu4N{Ni[Au(CN)2]3} (1) is shown in Fig. 1. The Ni(II) centers are coordinated to six N-cyano ˚ groups, with an average bond length of 2.04(3) A (Fig. 1a, Table 2), yielding a distorted octahedral geometry. Each [Au(CN)2] unit in 1 bridges two Ni(II) centers, generating a three dimensional network (Fig. 1). The structure is built up by a series of two-dimensional (4, 4) rectangular grids, each oriented in a different direction. Each rectangular unit in each grid contains six Ni(II) centers connected by [Au(CN)2] units: one Ni(II) center at each corner and two Ni(II) centers along one pair of opposite edges (Fig. 1b). Each Ni(II) center occupies the edge position on three intersecting fused grids (shown in black in Fig. 1c). Additional [Au(CN)2] units allow the formation of another set of three grids that intersect at the same Ni(II) center (edge-position of the first set of grids) which now occupies the corner position in each of this latter set of grids (shown in grey in Fig. 1c). In other words, six rectangular grids intersect at one Ni center: each Ni center occupies an edge position on three grids and a corner position on the other three grids. No interpenetration of additional networks occurs. The pores generated by this network accommodate the n Bu4N+ cations. The central N atom lies above an [Au(CN)2] unit while the n-butyl groups spread on both sides to occupy the empty space. Thus, the structure of n Bu4N{Ni[Au(CN)2]3} (1) does not contain any empty pores. In addition, no Au–Au (aurophilic) interactions are present in this system, perhaps a result of the ‘‘insulating’’ n-butyl groups that surround the [Au(CN)2] units. Although no crystal of 2 suitable for single crystal X-ray diffraction analysis could be obtained, the powder diffractogram of 2 was found to be superimposable to that generated from the single-crystal structure of 1 (Fig. 2); this is excellent evidence to suggest that 2 is isostructural with 1. The solid state UV–Vis–NIR absorption spectra of 1 and 2 (Figure S1) show maxima attributable to d  d transitions at 305, 355, 564 (br) and 910 nm for 1 and 470 (with shoulder peaks at 490 and 515 nm) and 1030 nm for 2. These spectra are as expected for octahedral Ni(II) and Co(II) systems with moderate ligand fields [40]. 3.2. Synthesis and structural characterization of {Co(H2O)2[Au(CN)2]2} Æ nBu4N[Au(CN)2] (3) When water was added to the reaction of pale pink Co[Au(CN)2]2(H2O)2 with nBu4N[Au(CN)2] in ethanol, a product with a different colour was obtained. The composition of this dark red-pink product was consistent with {Co(H2O)2[Au(CN)2]2} Æ nBu4N[Au(CN)2] (3) by elemental analysis; hence, it was different than 2, which is anhydrous. The FT-IR spectrum of {Co(H2O)2[Au(CN)2]2} Æ nBu4N[Au(CN)2] (3) shows two different cyanide vibration frequencies, 2182 and 2147 cm1, which suggests the presence

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Fig. 1. (a) Coordination sphere around the Ni(II) center (only two cations are shown; butyl groups were simplified for clarity); (b) Extended network showing a two dimensional rectangular (4, 4) grid and the position of one n Bu4N+ cation; (c) Six rectangular grids intersecting at one Ni(II) center (labelled): the Ni center occupies an edge position on three grids (shown in black) and a corner position on the other three grids (shown in grey) (C and N atoms were removed for clarity and Au atoms are shown smaller and pale grey).

of bound N-cyano groups as well as free or weakly interacting N-cyano groups; complex 2 had only one mCN band at 2189 cm1.

The structure of {Co(H2O)2[Au(CN)2]2} Æ nBu4N[Au(CN)2] (3) contains octahedral Co(II) centers that are coordinated to four N-cyano groups in the equatorial plane ˚ ) and two water molecules (Co– (Co–N = 2.096(12) A ˚ O = 2.089(16) A) in the axial sites (Fig. 3, Table 3); thus, 3 is not a true polymorph of 2 but rather a pseudo-polymorph [41] or, more appropriately, a solvent adduct. A 2-D square grid is formed through the cobalt-bridging [Au(CN)2] units with the water molecules lying above and below the grid (Fig. 3a). Free [Au(CN)2] units are also present and lie above and parallel to opposing edges of the Co[Au(CN)2]2 square array. Each free [Au(CN)2] unit interacts with the underlying grid both through hydrogen-bonding between the terminal N-cyano atoms and the hydrogen of the water molecules, and also through aur˚, ophilic (Au–Au) interactions (Au1–Au2 = 3.4250(13) A Fig. 3b). Aurophilic interactions are considered to exist when the distance between two Au atoms is smaller than ˚ [42–44]. The the sum of the van der Waals radii, 3.6 A alignment of each layer of free [Au(CN)2] units alternates 90 with the two pairs of parallel edges of the square arrays. Hence, through aurophilic interactions and hydrogen-bonding, a 3-D network is obtained. The nBu4N+ cations occupy the empty sites in the layer of unbound [Au(CN)2] units. Overall, {Co(H2O)2[Au(CN)2]2} Æ nBu4N[Au(CN)2] (3) can be considered to be a layered polymer that contains neutral sheets of {Co(H2O)2[Au(CN)2]2} which are separated by nBu4N[Au(CN)2] layers. The geometry around the Au atom in the bridging [Au(CN)2] unit (C(1)–Au(1)–C(1*) = 171.5(10)) differs from that observed in the free unit (C(2)–Au(2)– C(200 ) = 179.7(13)): a clear bending of the [Au(CN)2] units within the 2-D grid can be observed (Fig. 3). Theoretically, an angle of 173.6 was calculated for an anionic dimer of ˚ [Au(CN)2] units, with a Au–Au interaction of 3.742 A and a torsion angle of 90 [45]. Hence, the experimentally observed acute C–Au–C angle could reflect the presence of Au–Au interactions, even though the torsion angle is different, or this unusually large distortion from linearity around the Au atom could reflect the need to accommodate the large nBu4N+ cation incorporated between the grids. The basic 2-D grid structure of {Co(H2O)2[Au(CN)2]2} Æ nBu4N[Au(CN)2] (3) is similar to that reported for other solvent adducts, such as {Co(dmf)2[Au(CN)2]2} [45], {Mn(H2O)2[Au(CN)2]2} [46] and {Cu(pyridine)2[Au(CN)2]2} [12]. In these polymers, no extra nBu4N[Au(CN)2] layer is present and the solvent molecules, binding through their donor atom, replace the water molecules in 3. The packing of the 2-D square grids is different in these other systems as the grids are offset to allow direct ˚ between two grids [45,46] aurophilic interactions of 3.1 A or p–p interactions between the pyridine rings [12]. Interestingly, heating a sample of 3 at 150 C for 90 min to remove the bound water molecules cleanly generated the dehydrated 3-D array of 2 as indicated by X-ray diffractograms and FT-IR spectra that were superimposable with those of independently synthesized 2. This significant

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Fig. 2. Powder X-ray diffractograms generated for n Bu4N{Co[Au(CN)2]3} (2).

n

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Bu4N{Ni[Au(CN)2]3} (1) from the single crystal diffraction data and measured for

structural rearrangement in the solid state is a strong indication of the flexible nature of cyanoaurate-based coordination polymers [12]. Note that 3 is not regenerated in the solid state by exposure to water vapour. 3.3. Synthesis and structural characterization of PPN{Ni[Au(CN)2]3} (4) and PPN{Co[Au(CN)2]3} (5) The reaction of PPN[Au(CN)2] with M(II) salts (M = Ni and Co) in ethanol immediately yielded very fine precipitates. Elemental analysis confirmed that the product compositions were consistent with PPN{M[Au(CN)2]3} (M = Ni (4); Co (5)). FT-IR spectroscopy showed one cyanide vibration frequency for 4 at 2192 cm1 and two barely resolved frequencies for 5 at 2187 and 2175 cm1. This suggests that all the cyanide groups are in a similar environment in 4 while a slight structural difference among the cyanide groups exists in 5, likely due to broken symmetry. No mCN band corresponding to free [Au(CN)2] (2140 cm1) could be observed in the FT-IR spectrum of either 4 or 5, suggesting that all N-cyano units are coordinated to the metal centers. Pale purple crystals of PPN{Ni[Au(CN)2]3} (4) were obtained through slow diffusion of reagents in an H-shaped tube. The structure determined for 4 is shown in Fig. 4. As for 1, the Ni(II) centers have an octahedral geometry and coordinate to six N-cyano groups (Table 4). Each [Au(CN)2] unit bridges two Ni(II) centers to create a 3-D Prussian-Blue-type pseudo-cubic array. The PPN+ cation occupies the space in the center of each cube, preventing the interpenetration of a second network. This ‘‘expanded ˚ edges (Ni–Ni), Prussian Blue’’ analogue, with 10.269 A can be compared with other examples such as Fe4[Re6-

Te8(CN)6]3 Æ xH2O [47] and [Ni(en)2]3[Fe(CN)6](PF6)2 [48] ˚ , respectively. which have edges of 14.1285 and 9.908 A The powder X-ray diffractograms of 4 and 5 are nearly superimposable (Fig. 5), which indicates that an identical arrangement of the building blocks is present in the two polymers. This is also consistent with the FT-IR spectra; the small difference in cyanide vibration frequency can be attributed to the difference between N-cyano group binding to Ni or Co. The solid-state UV–Vis–NIR absorption spectra of 4 and 5 (Figure S1) were found to have two d–d maxima at 562 and 895 nm for 4 and 475 nm (with shoulder peaks at 490 and 510 nm) and 990 nm for 5; absorptions below 400 nm are obscured by strong PPN-based peaks. The absorption spectra of the PPN{M[Au(CN)2]3} polymers are very similar to those of their respective nBu4N{M[Au(CN)2]3} analogues, consistent with the observation that the geometry and coordination sphere of the metal centers are similar in both pairs of compounds. 3.4. Templating effects of the cation The structures of 1–5 can be contrasted with the related K{Fe[Au(CN)2]3} [46], K{Co[Au(CN)2]3} [49] and K{Mn[Ag(CN)2]3} [50] structures, which contain triply interpenetrated pseudo-cubic 3-D cyanometallate arrays. The switch of the non-steric K+ cation for the nBu4N+ cation has eliminated the interpenetration and modified the form of the 3-D network, but has not caused a reduction in dimensionality. Hydration of the Co(II) in 3 does, however, cause a reduction in dimensionality in terms of the coordinate bond network. Finally, the PPN+ cation rather surprisingly also does not force any reduction in

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Fig. 4. Extended structure of PPN{Ni[Au(CN)2]3} showing the coordination sphere around the Ni(II) centers and the Prussian-Blue-type pseudo-cubic array.

Fig. 3. (a) 2-D square grid of {Co(H2O)2[Au(CN)2]2} Æ nBu4N[Au(CN)2] (3) showing the 2-D square grids viewed down the c-axis; (b) Coordination sphere around a Co(II) center in 3; (c) Interactions between the grids and the free [Au(CN)2] units along the c-axis. nBu groups were omitted for clarity.

dimensionality; indeed, this non-interpenetrated structures is strain-free and nearly identical to a single pseudo-cubic network found in the K+-analogues.

Compounds 1–3 decompose within the same narrow temperature range, losing the six cyanide groups and the cation all at once. The nickel compounds have slightly higher decomposition temperatures, with 1 being stable until 295 C, then losing cyanides and the nBu4N+ cation, leaving gold and nickel oxide at 330 C (observed: 64.0%; calculated: 63.5%). Complex 4 starts decomposing at 325 C, with a slow decomposition of the PPN+ cation occurring up to 750 C, obtaining Ni(P2O3) and gold (final mass observed: 55.0%; calculated: 56.5%). In the case of the cobalt compounds, 2 is stable until to 260 C, and then loses the cyanides and nBu4N+ cation until 338 C, leaving gold and cobalt oxide (observed: 62.5%; calculated: 63.9%). Hydrated 3 first loses its two water molecules between 100 C and 120C (observed: 4.0%; calculated: 3.3%) then further decomposes along the same path as 2, losing all cyanides and nBu4N+ in the same 260–338 C temperature range (observed: 60.3%; calculated: 61.8%). Complex 5 follow the same pattern as 4, slowly decomposing after 275 C up until 730 C, forming Co(P2O3) and gold as the end products (final mass observed: 56.7%; calculated: 56.5%). Thus, the tris-cyanoaurate frameworks are in general stable until 260–325 C. These decomposition temperatures are typical for cyanometallate systems [51] but lower than for cyanoaurate arrays that show substantial aurophilic interactions such as M(pyrazine)[Au(CN)2]2, which loses its cyanides at a remarkable 406 C for M = Ni and 360 C for Co [32].

3.5. Thermal stability

3.6. Magnetic properties of nBu4N{M[Au(CN)2]3} (Ni, 1; Co, 2)

Thermogravimetric analysis data from 25 to 500 C were collected for 1, 2, 3 and up to 820 C for 4 and 5.

The magnetization of nBu4N{M[Au(CN)2]3} (M = Ni (1); Co (2)) was measured upon cooling from 300 to

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Fig. 5. Comparison between the powder X-ray diffractogram of PPN{Co[Au(CN)2]3} (5) and the diffractogram predicted from the single-crystal structure of PPN{Ni[Au(CN)2]3} (4).

1.8 K in a 1 kOe applied magnetic field.pffiffiffiffiffiffiffiffiffi The calculated effective magnetic moments ðleff ¼ 2:828 vM T Þ of 1 and 2 as a function of temperature are shown in Fig. 6. At 300 K, a value of 3.03 lB is observed for 1, which is consistent with isolated spin-only S = 1 Ni(II) centers.[52] As the temperature is lowered, the leff of 1 remains constant until 30 K and then decreases to reach 2.22 lB at 1.8 K. The inverse of the susceptibility v1 M of 1 can be fit to the

Curie–Weiss expression, [52] yielding a g of 2.166(2) and a h value of 1.27(5) K. The drop in the leff of 1 at low temperature could be due to either zero field splitting (zfs) or weak antiferromagnetic interactions mediated through the [Au(CN)2]-units or a combination of both. The product of the susceptibility by the temperature (vMT) for 1 can be fitted to the zfs expression for an S = 1 system, [52] yielding a D of 5.4(1) cm1 and a g of 2.13(1). The fit of vMT for 1 was improved by introducing a molecular field approximation to estimate the magnetic coupling between the Ni(II) centers: vzfs  vmf ¼  ð1Þ 1  Ng2zJ 2 l2 vzfs B

Fig. 6. Temperature dependence of the effective magnetic moment (leff) for nBu4N{Ni[Au(CN)2]3} (1, s) and nBu4N{Co[Au(CN)2]3} (2, D). The solid line represents the theoretical fit to the zfs expression with a molecular field approximation for the Ni(II) analogue (see text).

where zJ is the exchange coupling constant with the neighboring Ni(II) centers. This second fit yielded a D of 2.7(1) cm1, a g of 2.173(2) and zJ/kB of 0.85(2) cm1. Values of D for related octahedral Ni(II) centers, such as in Ni(tren)[Au(CN)2]2, range from 1 to 4 cm1 [53]. The value for a symmetric octahedral Ni(II) center such as that found in 1 should be on the low side of this range. Hence, these values are reasonable and suggest very weak interactions between the Ni(II) centers may be present at best. For 2, an effective moment of 4.93 lB is observed at 300 K (Fig. 6), which is larger than the spin-only value of an S = 3/2 system, but is in the range expected for an octahedral high-spin Co(II) center (4.1–5.2 lB), [40] for which a large first-order orbital contribution is present. As the temperature decreases, the leff remains constant down to 175 K, after which it decreases steadily and reaches 3.51 lB at 1.8 K. The decrease in the leff of 2 can be due to a

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combination of single-ion effects and very weak antiferromagnetic interactions; no maximum was observed in the magnetic susceptibility (vM) versus temperature plot. No attempt was made to model this behaviour as any coupling through the [Au(CN)2] units would have a very small magnitude and would be difficult to distinguish from the singleion effects. In related Co(II) dicyanamide complexes with l1,5-bridging dca units, any coupling was shown to be below 1 cm1, with single-ion effects dominating [29]. 3.7. Magnetic properties of {Co(H2O)2[Au(CN)2]2} Æ n Bu4 N[Au(CN)2] (3) Despite the different structural arrangement described for 2 and 3, the temperature dependence of the effective magnetic moment of both compounds is nearly identical: 3 has an effective moment of 4.88 lB at 300 K and 3.57 lB at 1.8 K (not shown). The magnetic behaviour of {Co(H2O)2[Au(CN)2]2} Æ n Bu4N[Au(CN)2] (3) is similar to that reported for other coordination polymers containing Co(II) centers linked via [Au(CN)2]-units in a 2-D grid array. {Co(dmf)2[Au(CN)2]2} [45] and {Co(pyrazine)2[Au(CN)2]2} [32] have effective moments of 4.92 and 5.21 lB, respectively, at 300 K, which decrease to reach 3.79 and 3.64 lB at 1.8 K. 3.8. Magnetic properties of PPN{M[Au(CN)2]3} (M = Ni (4), Co (5)) The magnetization of 4 and 5 was also measured upon cooling from 300 to 1.8 K in a 1 kOe external magnetic field and the temperature dependence of the effective magnetic moments are shown in Fig. 7. At 300 K, a value of 3.05 lB is observed for 4, as expected for an S = 1 Ni(II)

Fig. 7. Temperature dependence of the effective magnetic moment of PPN{M[Au(CN)2]3 (M = Ni (4, s) and Co (5, h)) with a 1 kOe applied magnetic field.

center. The leff remains constant until 20 K and then decreases slightly to a value of 2.22 lB at 1.8 K. The inverse of the susceptibility of 4 was fitted to the Curie–Weiss expression and yielded a g of 2.173(1) and a h of 1.11(4) K. As for 1, the vMT of 4 could be fitted to the zfs expression and yielded a D of 5.0(1) cm1 and a g of 2.145(4). When a molecular field approximation was added to the fit (Eq. (1)), a D of 2.6(1) cm1, a g of 2.180(2) and a zJ/kB of 0.75(2) cm1 were obtained. As for 1, these values indicate the presence of very weak interactions between the Ni(II) centers. Similarly, the effective moment of 5.06 lB determined for 5 at 300 K, which is consistent with an octahedral Co(II) center, drops to 3.69 lB at 1.8 K. As for 2 and 3, the behaviour of 5 is dominated by Co(II) single-ion effects. 3.9. Magneto-structural conclusions and comparison with other complexes To summarize, these results indicate that [Au(CN)2] is a poor mediator of magnetic exchange in these 2-D and 3-D systems, although in other reported [Au(CN)2]-based coordination polymers, significant coupling can be observed [31,54]. It is possible that, in these high dimensionality/high symmetry systems, competing ferromagnetic and antiferromagnetic interactions might contribute to cancel out the overall coupling. The lack of any significant magnetic interactions, even in the structurally cohesive, pseudo-cubic 3-D cyanoaurate polymers 4 and 5 is comparable to the observation that magnetic coupling through l1,5-dca bridges is generally very weak [18,19,29]. 4. Conclusion This preliminary study has shown that 3-D {M[Au(CN)2]3} coordination polymers can be readily prepared. The structures of nBu4N{M[Au(CN)2]3} and PPN{M[Au(CN)2]3} (M = Ni and Co) contain two distinct 3-D anionic frameworks of {M[Au(CN)2]3}, thus the framework was sensitive to the cation, but not to the identity of the metal center. Despite using large cations such as n Bu4N+ or PPN+, a 3-D structure is maintained (except when the metal remains solvated), contrary to the reduction in structural dimensionality observed in dicyanamide (dca) systems [18,19]. The longer reach of linear [Au(CN)2] compared to bent dca may account for this difference. A further increase in cation bulkiness or the introduction of hydrogen-bonding moieties into the cations is likely to eventually result in a significant superstructure rearrangement [55,56] and we are pursuing these avenues. Unfortunately, despite the presence of 2-D and 3-D networks with strongly N-bound cyanoaurates, including a pseudo-cubic Prussian Blue-type array, no magnetic ordering was detected above 1.8 K; indeed, almost no significant coupling through the [Au(CN)2] units was observed, indicating that the linear d10-Au(CN)2 bridge is a very weak mediator of magnetic exchange in these systems.

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