Syntheses, crystal structures and supramolecular topologies of nickel(II)–s/p/d¹⁰/NH₄⁺ complexes derived from a compartmental ligand

Abstract

The syntheses, characterization and crystal structures of ten complexes of composition [{Ni^IILLi^I(H₂O)₂}{Ni^IIL}₂](ClO₄) (1), [Ni^IILNa^I(ClO₄)(CH₃OH)] (2), [(Ni^IIL)₂Na^I](BPh₄)·CH₃COCH₃ (3), [(Ni^IIL)₂Rb^I](BPh₄)·CH₃COCH₃ (4), [(Ni^IIL)₂Cs^I](ClO₄)·CH₃CN (5), [{Ni^IILMg^II(H₂O)₃}{Ni^IIL}₂](ClO₄)₂ (6), [Ni^IILCa^II(NO₃)₂(H₂O)]·1.5H₂O (7), [Ni^IILCd^II(NO₃)₂]·CH₃CN (8), [Ni^IILPb^II(NO₃)₂] (9) and [(Ni^IIL)₂(NH₄)](PF₆) (10) are reported, where H₂L = N,N′-ethylenebis(3-ethoxysalicylaldimine). The crystal systems and space groups are: triclinic Pī for 1, 4, 5 and 8, monoclinic P2₁/c for 2, 3, 7 and 9, monoclinic C2/c for 6 and monoclinic P2₁/n for 10. In 1–10, the salen type N₂O₂ compartment of [L]²⁻ is occupied by a Ni^II ion, while the larger and open O(phenoxo)₂O(ethoxy)₂ compartment interacts with Li^I in 1, Na^I in 2 and 3, Rb^I in 4, Cs^I in 5, Mg^II in 6, Ca^II in 7, Cd^II in 8, Pb^II in 9 and NH₄⁺ in 10. Compounds 1 and 6 are [2 × 1 + 1 × 2] tetrametallic cocrystals of one diphenoxo-bridged dinuclear Ni^IILi^I (for 1) and Ni^IIMg^II (for 6) unit and two mononuclear [Ni^IIL] moieties. Compounds 2, 7, 8 and 9 are diphenoxo-bridged dinuclear Ni^IINa^I, Ni^IICa^II and Ni^IICd^II and Ni^IIPb^II systems, respectively. On the other hand, compounds 3, 4 and 5 are trinuclear Ni^II₂Na^I, Ni^II₂Rb^I and Ni^II₂Cs^I systems, respectively, in which each of the two adjacent pairs of metal ions is diphenoxo-bridged. The rubidium(I) and cesium(I) analogues are also double-decker sandwich systems. The ammonium ion in [(Ni^IIL)₂(NH₄)](PF₆) (10) is sandwiched in between two [Ni^IIL] moieties due to hydrogen bonds between ammonium hydrogen atoms and O₄ compartments. Weak interaction assisted following self-assemblies are observed: dimeric in 1 and one-dimensional in 10 due to Ni⋯Ni weak interaction; one-dimensional in 2 due to C–H⋯π and O–H⋯O hydrogen bonds; nice cyclic topology in 5 due to Ni⋯Ni interaction and C–H⋯O hydrogen bonds; one-dimensional in 6 due to Ni⋯Ni interaction and C–H⋯O hydrogen bond; one-dimensional in 8 due to π···π stacking.

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Introduction

On the basis of coordinate/covalent bonds and in terms of the composition/nuclearity/topology, the coordination compounds may be classified as: (1) discrete (mono- and dinuclear and also oligo- and polynuclear clusters);^1–6 (2) polymeric (1-D, 2-D and 3-D).^7–9 Both these two broad types may be self-assembled by several types of noncovalent interactions, namely, strong and weak hydrogen bonds, C–H⋯π interactions and π⋯π stacking and also halogen⋯halogen, sulfur⋯sulfur, metal⋯metal weak interactions, etc.^5c,9,10 Weak interaction directed self-assemblies of metal–organic systems have been attracting vast fascination in recent years. Aesthetic beauty, development of new topologies and utilization as functional materials of the supramolecular aggregates are the major aspects in this research area of crystal engineering/supramolecular chemistry.

In general, polynuclear clusters and polymeric metal systems are formed in the environment of both blocking organic ligand (s) and bridging organic or inorganic ligand (s).^6–9 On the other hand, discrete mono/di/oligonuclear systems are stabilized in most of the cases in the environment of a preorganized blocking ligand. The following such preorganized ligands may be mentioned as examples: salen type family (for mononuclear);¹ tetraimino/tetraaminodiphenolate macrocyclic ligands (for dinuclear);^2,3 tetraimino/tetraaminotetraphenolate macrocyclic ligands (for tetranuclear);⁴ 3-methoxy/ethoxysalicylaldehyde-diamine Schiff bases (for mono-, di- and oligonuclear).^5c,11–23 Among these preorganized organic moieties, the 3-methoxy/ethoxysalicylaldehyde-diamine Schiff bases are compartmental acyclic ligands consisting of two significantly different compartments, a N(imine)₂O(phenoxo)₂ and a O(phenoxo)₂O(methoxy/ethoxy)₂. A 3d metal ion is more favourable to be incorporated into the N₂O₂ compartment to produce a mononuclear compound, the O₄ compartment of which can accommodate 3d, 4f, s, and p block metal ions to produce di/tri/oligonuclear systems.^5c,11–23 The 3-methoxysalicylaldehyde-diamine and 3-ethoxysalicylaldehyde-diamine Schiff bases are closely similar ligands and therefore expected to stabilize similar metal complexes, as observed in some cases (3d–4f systems for example).^11f,12 On the other hand, remarkable differences have also been observed between these two ligand systems in recent years. The key reason of such differences is the tendency of a water molecule to be encapsulated into the O(phenoxo)₂O(ethoxy)₂ compartment, resulting in the stabilization of inclusion products and two-component and even three-component cocrystals.^13–22 Again, some other cocrystals are produced where water encapsulation does not take place,^15,16 seemingly indicating the inherent potentiality of 3-ethoxysalicylaldehyde-diamine Schiff bases to stabilize cocrystals. These cocrystals, in turn, are tetra,^13–17 penta,¹⁸ hexa,^16,19 hepta²⁰ and octametallic^15,16 and also polymeric,²¹ thus represent interesting structural diversity. Some of the discrete or cocrystalline metal complexes are further self-assembled because of the existence of noncovalent interactions such as hydrogen bonds, π⋯π stacking and metal⋯metal weak interactions.^17,20 In brief, some of the systems derived from 3-ethoxysalicylaldehyde-diamine Schiff bases can be well considered as the weak interaction directed self-assemblies and thus interesting in the frontier research area of crystal engineering/supramolecular chemistry. We have been therefore motivated to explore this area further. In contrast to the several products obtained on reacting mononuclear copper(II) complexes with a second metal salt, only few products obtained from a mononuclear nickel(II) compounds have been reported. The aim of the present investigation is to explore the nickel(II)–main group metal and nickel(II)–ammonium systems derived from N,N′-ethylenebis(3-ethoxysalicylaldimine) (H₂L; Scheme 1). Accordingly, we have reacted [Ni^IIL ⊂ (H₂O)] with Li(ClO₄)₂·3H₂O, NaClO₄·H₂O, NaBPh₄, RbBPh₄, CsClO₄, Mg(ClO₄)₂·6H₂O, Ca(NO₃)₂·4H₂O, Cd(NO₃)₂·4H₂O, Pb(NO₃)₂ and (NH₄)PF₆. Herein, we report the syntheses, characterization and crystal and supramolecular structures of the ten compounds obtained from these reactions.

Scheme 1

Experimental

Materials and physical measurements

All the reagents and solvents were purchased from commercial sources and used as received. The mononuclear inclusion product [Ni^IIL ⊂ (H₂O)] was synthesized by the reported procedure.¹⁴ Elemental (C, H and N) analyses were performed on a Perkin-Elmer 2400 II analyzer. IR spectra were recorded from KBr disks of the samples over the range 400–4000 cm⁻¹ on a Bruker-Optics Alpha–T spectrophotometer.

Syntheses

[{Ni^IILLi^I(H₂O)₂}{Ni^IIL}₂](ClO₄) (1), [Ni^IILNa^I(ClO₄)(CH₃OH)] (2), [(Ni^IIL)₂Na^I](BPh₄)·CH₃COCH₃ (3), [(Ni^IIL)₂Rb^I](BPh₄)·CH₃COCH₃ (4), [(Ni^IIL)₂Cs^I](ClO₄)·CH₃CN (5), [{Ni^IILMg^II(H₂O)₃}{Ni^IIL}₂](ClO₄)₂ (6), [Ni^IILCa^II(NO₃)₂(H₂O)]·1.5H₂O (7), [Ni^IILCd^II(NO₃)₂]·CH₃CN (8), [Ni^IILPb^II(NO₃)₂] (9) and [(Ni^IIL)₂(NH₄)](PF₆) (10). These ten compounds were prepared by reacting [Ni^IIL ⊂ (H₂O)] with appropriate salts of metal ions, for 1–9, or ammonium ion, for 10. The salts used are as follows: Li(ClO₄)₂·3H₂O for 1, NaClO₄·H₂O for 2, Na(BPh₄) for 3, Rb(BPh₄) for 4, CsClO₄ for 5, Mg(ClO₄)₂·6H₂O for 6, Ca(NO₃)₂·4H₂O for 7, Cd(NO₃)₂·4H₂O for 8, Pb(NO₃)₂ for 9 and (NH₄)PF₆ for 10. For compounds 1, 2, 3, 4, 5 and 7, a suspension of [Ni^IIL ⊂ (H₂O)] in a solvent (acetone for 1, 2, 3, 4 and 7; acetonitrile for 5) was treated with a solution of the metal salt in the same (acetone for 7, acetonitrile for 5) or a different (methanol for 1, 2, 3 and 4) solvent to result in a solution from which the product was isolated. On the other hand, for 6, 8, 9 and 10, a suspension of [Ni^IIL ⊂ (H₂O)] in a solvent (acetone for 6; acetonitrile for 9 and methanol for 8 and 10) was treated with a solution of the metal salt or ammonium salt in the same (acetone for 6; methanol for 8 and 10) or a different (water for 9) solvent to produce a precipitate, which was dissolved in either methanol (for 6) or acetonitrile (for 8, 9 and 10) and the product was isolated from the resulting solution. As representative examples for the two sets, the syntheses of two compounds, 1 and 10, are described below in details.

[{Ni^IILLi^I(H₂O)₂}{Ni^IIL}₂](ClO₄) (1). A methanol solution (5 mL) of LiClO₄·3H₂O (0.83 g, 0.5 mmol) was added dropwise to a suspension of [Ni^IIL ⊂ (H₂O)] (0.215 g, 0.5 mmol) in acetone (15 mL) under stirring. After stirring for half an hour, the dark red solution was filtered to remove any suspended particles and the filtrate was kept at room temperature and allowed to evaporate slowly. After a few days, a red crystalline compound containing diffraction quality single crystals deposited, which was collected by filtration and washed with cold methanol. Yield: 0.196 g (85%). Anal. Calcd for C₆₀H₇₀N₆O₁₈ClNi₃Li: C 52.16, H 5.11, N 6.08. Found: C 51.86, H 5.23, N 6.18. IR (cm⁻¹, KBr): ν(H₂O), 3492 w; ν(C–H), 3054 w, 2979 w, 2928 w, 2870 w; ν(C=N), 1621 vs; ν(ClO₄), 1090 s, 622 w.

[(Ni^IIL)₂(NH₄)](PF₆) (10). A methanol solution (5 mL) of NH₄PF₆ (0.81 g, 0.5 mmol) was added slowly to a suspension of [Ni^IIL ⊂ (H₂O)] (0.215 g, 0.5 mmol) in methanol (15 mL). Immediately after the addition, an orange coloured precipitate started to deposit. After stirring the solution for one hour, requisite amount of acetonitrile was added dropwise and the mixture was heated gently to dissolve the precipitate. The resulting solution was cooled, filtered to remove any suspended particle and then kept at room temperature for slow evaporation. After a few hours a dark red coloured crystalline compound containing single crystals suitable for X-ray diffraction was collected by filtration and dried in vacuum. Yield: 0.173 g (70%). Anal. Calcd for C₄₀H₄₈N₅O₈F₆PNi₂: C 48.55, H 4.89, N 7.08. Found: C 48.32, H 4.98, N 7.23. IR (cm⁻¹, KBr): ν(C–H), 3052 w, 2973 w, 2932 w, 2876 w; ν(C=N), 1618 vs; ν(PF₆), 843 vs.

Data for 2 follows: Colour: Red. Yield: 0.227 g (80%). Anal. Calcd for C₂₁H₂₆N₂O₉ClNiNa: C 44.44, H 4.62, N 4.94. Found: C 44.68, H 4.72, N 5.16. IR (cm⁻¹, KBr): ν(C–H), 3057 w, 2931 w, ν(C=N), 1614 vs; ν(ClO₄), 1089 vs, 625 w.

Data for 3 follows: Colour: Red. Yield: 0.251 g (82%). Anal. Calcd for C₆₇H₇₀N₄O₉BNi₂Na: C 65.61, H 5.75, N 4.57. Found: C 65.98, H 5.44, N 4.84. IR (cm⁻¹, KBr): ν(C–H), 3047 w, 2984 w, 2897 w; ν(acetone), 1709 w; ν(C=N), 1615 vs; ν(BPh₄), 736 s, 703 m.

Data for 4 follows: Colour: Red. Yield: 0.258 g (80%). Anal. Calcd for C₆₇H₇₀N₄O₉B₁Ni₂Rb: C 62.43, H 5.47, N 4.35. Found: C 62.58, H 5.68, N 4.42. IR (cm⁻¹, KBr): ν(C–H), 3054 w, 2979 w, 2924 w, 2877 w; ν(C=N), 1617 vs; ν(BPh₄), 737 s, 707 m.

Data for 5 follows: Colour: Red. Yield: 0.214 g (78%). Anal. Calcd for C₄₂H₄₇N₅O₁₂ClNi₂Cs: C 45.85, H 4.31, N 6.37. Found: C 45.52, H 4.52, N 6.20. IR (cm⁻¹, KBr): ν(C–H), 3054 w, 2972 w, 2930 w, 2876 w; ν(C=N), 1621 vs; ν(ClO₄), 1084 vs, 623 w.

Data for 6 follows: Colour: Red. Yield: 0.191 g (75%). Anal. Calcd for C₆₀H₆₆N₆O₂₄Cl₂Ni₃Mg: C 47.19, H 4.36, N 5.51. Found: C 46.95, H 4.52, N 5.34. IR (cm⁻¹, KBr): ν(H₂O), 3424 w; ν(C–H), 3054 w, 2975 w, 2929 w, 2875 w; ν(C=N), 1622 vs; ν(ClO₄), 1089 s, 623 w.

Data for 7 follows: Colour: Red. Yield: 0.249 g (80%). Anal. Calcd for C₂₀H₂₇N₄O_12.5NiCa: C 38.61, H 4.37, N 9.00. Found: C 38.98, H 4.78, N 9.24. IR (cm⁻¹, KBr): ν(H₂O), 3397 m; ν(C–H), 3066 w, 2982 w, 2924 w; ν(C=N), 1625 vs; ν(NO₃), 1467 s, 1388 s, 1307 s, 1232 s.

Data for 8 follows: Colour: Red. Yield: 0.286 g (83%). Anal. Calcd for C₂₂H₂₅N₅O₁₀NiCd: C 38.24, H 3.65, N 10.14. Found: C 38.46, H 3.69, N 10.32. IR (cm⁻¹, KBr): ν(C–H), 2970 w, 2934 w; ν(C=N), 1622 s; ν(NO₃), 1458 s, 1384 s, 1299 s.

Data for 9 follows: Colour: Red. Yield: 0.279 g (75%). Anal. Calcd for C₂₀H₂₂N₄O₁₀NiPb: C 32.25, H 2.98, N 7.53. Found: C 32.48, H 3.16, N 7.67. IR (cm⁻¹, KBr): ν(C–H), 3064 w, 2929 w; ν(C=N), 1631 vs; ν(NO₃), 1455 s, 1384 s, 1291 w.

Safety note. The perchlorate salts of metal complexes with an organic ligand are potentially explosive and should be handled with care.

Crystal structure determination of 1–10

The crystallographic data for the ten compounds are summarized in Table 1. Diffraction data of 1–10 were collected on a Bruker-APEX II SMART CCD diffractometer at 296 K using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). For data processing and absorption correction the packages SAINT^24a and SADABS^24b were used. The structures were solved by direct and Fourier methods and refined by full-matrix least-squares based on F² using SHELXTL^24c and SHELXL-97 packages.^24d Compound 7 lost crystalline nature due to removal of solvent immediately after isolation and therefore diffraction data of 7 were collected on mounting a crystal dipped in it's mother liquor in a capillary.

Table 1 Crystallographic data for 1–10

	1	2	3	4	5	6	7	8	9	10
a R 1 = [∑||Fo| − |Fc||/∑|Fo|]. b wR2 = [∑w(Fo^2 − Fc^2)^2/∑wFo^4]^1/2.
empirical formula	C60H70N6O18ClNi3Li	C21H26N2O9ClNiNa	C64H64N4O8BNi2Na	C64H64N4O8B1Ni2Rb1	C42H47N5O12ClNi2Cs	C60H66N6O24Cl2 Ni3Mg	C40H44N8O23Ni2Ca2	C22H25N5O10NiCd	C40H44N8O20Ni2Pb2	C40H48N5O8F6PNi2
fw	1381.74	567.59	1168.41	1230.89	1099.63	1526.53	1202.41	690.58	1488.63	989.22
crystal colour	red	red	red	red	red	red	red	red	red	red
crystal system	Triclinic	monoclinic	Monoclinic	Triclinic	Triclinic	Monoclinic	Monoclinic	Triclinic	Monoclinic	Monoclinic
space group	Pī	P21/c	P21/c	Pī	Pī	C2/c	P21/c	Pī	P21/c	P21/n
a/Å	11.8610(16)	8.2000(6)	18.056(3)	16.0343(19)	13.5600(19)	15.4418(13)	16.101(5)	8.0768(13)	14.2881(12)	15.934(3)
b/Å	15.811(2)	21.5329(14)	19.572(3)	16.0435(19)	14.089(2)	22.4552(18)	13.981(5)	12.074(2)	13.7472(11)	14.920(2)
c/Å	17.483(2)	14.1163(10)	18.290(3)	25.379(3)	14.473(2)	18.8479(14)	13.162(4)	14.348(2)	12.8579(10)	18.255(3)
α/°	84.901(2)	90.00	90.00	83.810(2)	68.785(6)	90.00	90.00	67.471(5)	90.00	90.00
β/°	73.443(2)	104.627(3)	109.934(2)	83.857(2)	68.942(5)	94.555(2)	113.378(4)	89.983(5)	105.288(2)	92.114(6)
γ/°	80.682(2)	90.00	90.00	68.346(2)	67.311(5)	90.00	90.00	83.029(6)	90.00	90.00
V/Å^3	3098.2(7)	2411.7(3)	6076.1(17)	6016.6(12)	2299.1(6)	6514.8(9)	2719.7(15)	1281.3(4)	2436.2(3)	4337.0(12)
Z	2	4	4	4	2	4	2	2	2	4
T/ K	296(2)	296(2)	296(2)	296(2)	296(2)	296(2)	296(2)	296(2)	296(2)	296(2)
2θ/°	2.44–50.00	3.54–70.76	2.40–52.00	2.74–52.66	3.12–55.00	3.20−47.38	2.76–52.00	3.08–54.32	2.96–53.98	3.34–65.40
μ(Mo Kα)/mm^−1	1.022	0.987	0.683	1.487	1.722	1.035	0.964	1.630	7.736	0.988
ρ calcd/g cm^−3	1.481	1.563	1.277	1.359	1.588	1.556	1.468	1.790	2.029	1.515
F(000)	1440	1176	2448	2552	1116	3160	1240	696	1440	2048
absorption-correction	multi-scan	multi-scan	multi-scan	multi-scan	multi-scan	multi-scan	multi-scan	multi scan	multi-scan	multi scan
T min	0.7916	0.8121	0.8532	0.7553	0.7245	0.8519	0.7108	0.7580	0.2479	0.8270
T max	0.8618	0.8423	0.8986	0.8078	0.7822	0.8858	0.8159	0.8039	0.3365	0.8580
index ranges	−14≤ h ≤ 14	−13≤ h ≤ 12	−22≤ h ≤ 22	−19≤ h ≤ 19	−17≤ h ≤ 16	−16≤ h ≤ 17	−19≤ h ≤ 19	−10≤ h ≤ 10	−16≤ h ≤ 18	−24≤ h ≤ 19
	−18 ≤ k ≤ 18	−28 ≤ k ≤ 35	−22 ≤ k ≤ 24	−19 ≤ k ≤ 19	−18 ≤ k ≤ 15	−24 ≤ k ≤ 24	−16 ≤ k ≤ 17	−15 ≤ k ≤ 15	−17 ≤ k ≤ 17	−22 ≤ k ≤ 22
	−20 ≤ l ≤ 20	−22 ≤ l ≤ 22	−21 ≤ l ≤ 22	−31≤ l ≤ 31	−18 ≤ l ≤ 18	−20 ≤ l ≤ 20	−15 ≤ l ≤ 15	−17 ≤ l ≤ 15	−16 ≤ l ≤ 15	−27 ≤ l ≤ 27
reflections collected	22237	42069	45408	61237	29152	19136	26444	16171	24691	80861
independent reflections	10803	10820	11842	23600	10139	4592	5305	5414	5205	15792
R int	0.0565	0.0412	0.0446	0.0477	0.0448	0.0596	0.0284	0.0819	0.0515	0.0374
R 1 / wR2^b [I > 2σ(I)]	0.0524/0.0981	0.0478/0.1270	0.0452/ 0.1309	0.0480/0.0968	0.0402/0.1191	0.0512/ 0.1356	0.0514/0.1603	0.0679/ 0.1724	0.0363/ 0.0929	0.0436/0.1263
R 1 / wR2^b (for all Fo^2)	0.1460/0.1349	0.1054/0.1493	0.0760/ 0.1483	0.1184 /0.1130	0.0769/ 0.1540	0.1042/ 0.1661	0.0717/ 0.1705	0.1169/ 0.1968	0.0643/ 0.1110	0.0866/0.1546

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During the development of the structures of 1, 3, 4, 7 and 9, it became apparent that few atoms were each disordered over two sites. These disordered atoms were the four perchlorate oxygen atoms (O15, O16, O17 and O18) in 1, three diimino side chain carbon atom (C9, C28 and C29) in 3, two diimino side chain carbon atoms (C28 and C29) in 4 and two nitrate oxygen atoms (O8 and O9 in 7 and O5 and O6 in 9) in both 7 and 9. The disorder of all these atoms was modeled with following final occupancy parameters: 0.60 and 0.40 for O15, O16, O17 and O18 in 1; 0.50 and 0.50 for C9, C28 and C29 in 3, C28 and C29 in 4 and O8 and O9 in 7; 0.80 and 0.20 for O5 and O6 in 9. Of the two disordered sites of O5 in 9, the minor component (O5B) is coordinated to Pb1, while the major component (O5A) remains noncoordinated. It may be noted that Ueq for one perchlorate oxygen atom (O6) in 2 is high giving an indication of disorder but could not be modelled successfully. In 7, the presence of an oxygen atom (O12) was determined on the basis of a q-peak in the difference Fourier map consistent with ∼½ of an oxygen atom in a geometrically sensible location.

The following hydrogen atoms were located from difference Fourier maps: four hydrogen atoms of the two water molecules in 1 and four ammonium hydrogen atoms in 10. It was not possible to locate all the six hydrogen atoms of the three water molecules in 6 and the three water hydrogen atoms (two of the coordinated water and one of the solvated water molecule) in 7. All the other hydrogen atoms for the compounds 1–10 were inserted on geometrical calculated positions with fixed thermal parameters.

The O–H distances in 1 and two of the four N–H distances and all the H–H distances of the ammonium cation in 10 were restrained to 0.82 Å, 0.85 and 1.35 Å, respectively. In 3, the similar bond lengths involving C9A, C9B, C28A, C28B, C29A and C29B varied significantly and therefore C–C and C–N distances involving these carbon atoms were restrained to get standard and consistent values.

The disordered carbon atoms (C9A, C9B, C28A, C28B, C29A and C29B) in 3 and the disordered oxygen atoms (O5A, O5B, O6A and O6B) in 9 had to be refined isotropically because of nonpositive definite problem. All other nonhydrogen atoms were refined anisotropically. On the other hand, all hydrogen atoms which were either located or inserted were refined isotropically. For better refinement of the structure of 9, the distances between C19 and C20 and also between C17 and C18 of an ethoxy moiety were restrained to 1.5 Å. The fluctuating C–C bond distances in an aromatic ring (containing C111, C112, C113, C114, C115 and C116) of the counter anion tetraphenylborate in the structure of 4 were fixed by rigid body constraint.

It was not possible to assign properly all or some of the solvent molecules in the structures of 3, 4 and 7 and therefore these solvent molecules were eliminated by using the SQUEEZE facility of PLATON to improve the refinement.²⁵ Electron count per unit cell for the eliminated solvent molecules in 7 is 62 indicating the possibility of one acetone and three water molecules, in addition to assigned 0.5H₂O. As already mentioned, crystals of 7 lose solvents quickly. However, of the squeezed one acetone and three water molecules, one water molecule is considered in the formula because elemental analyses are matched with a formula containing 1.5H₂O. Electron count per unit cell for the eliminated solvent molecules in 4 is 32 indicating the possibility of one acetone molecule as solvent of crystallization, which is well matched with elemental analyses (vide infra). Electron count per unit cell for the eliminated solvent molecules in 3 is 119 indicating the possibility of the presence of several solvent molecules. Crystals of 3 also lose solvent molecules within a few hours to become powder. However, we are considering one acetone molecule in the formula of 3 because the elemental analyses and FT-IR spectrum (vide infra) indicate the presence of one acetone molecule.

The refinements converged to the R₁ values [I > 2σ(I)] of 0.0524, 0.0478, 0.0452, 0.0480, 0.0402, 0.0512, 0.0514, 0.0679, 0.0363 and 0.0436 for 1–10, respectively.

Results and discussion

Description of the structures of 1–10

All the ten compounds 1–10 contain one or more deprotonated ligand, [L]²⁻, the salen type N₂O₂ compartment of each of which is occupied by a Ni^II ion to result in the formation of a [Ni^IIL] moiety. On the other hand, the larger and open O(phenoxo)₂O(ethoxy)₂ compartment of the ligand in one or more [Ni^IIL] moieties interact (s) with Li^I in 1, Na^I in 2 and 3, Rb^I in 4, Cs^I in 5, Mg^II in 6, Ca^II in 7, Cd^II in 8, Pb^II in 9 and NH₄⁺ in 10. Two phenoxo and two ethoxy oxygen atoms for a particular O(phenoxo)₂O(ethoxy)₂ compartment in all these compounds form a good plane as evidenced by small mean deviation, <0.07 Å, from the corresponding least-squares O₄ plane. These deviations along with the displacement of the metal ion from the corresponding O₄ plane in 1–9 are compared in Table 2.

Table 2 Comparison of the bond lengths (Å) and angles (°) of the second metal environment along with the displacement (d_M in Å) of the second metal centre and average deviation (d_O in Å) of the constituent atoms from the corresponding least-squares O(phenoxo)₂O(ethoxy)₂ plane in 1–9

Compound No.	M–O(phenoxo)	M–O(ethoxy)	M–O(water)	M–O(nitrate)	M–O(perchlorate)	M–O(methanol)	O–M–O angles	d O	d M
1	2.008(9)–2.024(10)		1.983(9)–1.993(9)				73.6(3)–149.7(5)	0.02	0.010
2	2.3027(14)–2.3182(15)	2.5312(15)–2.5610(14)			2.365(2)	2.3471(19)	64.40(5)–166.48(6)	0.005	0.035
3	2.409(2)–2.443(2)	2.505(2)–2.580(2)					60.84(7)–172.42(8)	0.033–0.058	0.108–0.148
4	2.772(2)–2.886(2)	2.960(3)–3.134(2)					51.10(6)–176.55(6)	0.01–0.068	1.558–1.758
5	2.963(3)–3.047(3)	3.119(3)–3.183(3)					49.29(7)–163.54(9)	0.038–0.053	1.699–1.83
6	2.191(4)		2.108(9)–2.112(4)				67.2(2)–178.0(3)	0.045	0.001
7	2.367(3)–2.370(2)	2.502(3)–2.519(3)	2.355(3)	2.292(9)−2.690(8)			26.7(3)–169.13(9)	0.031	0.124
8	2.288(5)–2.291(5)	2.524(6)–2.544(6)		2.343(7)–2.467(7)			52.5(2)–175.8(2)	0.050	0.026
9	2.448(5)–2.478(5)	2.684(6)–2.699(5)		2.631(6)–2.940(12)			41.5(4)–174.09(19)	0.005	0.092

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[{Ni^IILLi^I(H₂O)₂}{Ni^IIL}₂](ClO₄) (1; Fig. 1) and [{Ni^IILMg^II(H₂O)₃}{Ni^IIL}₂](ClO₄)₂ (6; Fig. 2) are [2 × 1 + 1 × 2] tetrametallic cocrystals of one diphenoxo-bridged dinuclear Ni^IILi^I (1) or Ni^IIMg^II (6) unit and two mononuclear [Ni^IIL] moieties, the composition of the dinuclear unit is [Ni^IILLi^I(H₂O)₂]⁺ for 1 and [Ni^IILMg^II(H₂O)₃]²⁺ for 6. Li^I is tetracoordinated by two phenoxo and two water oxygen atoms, while Mg^II is pentacoordinated by two phenoxo and three water oxygen atoms. The displacement of Li^I and Mg^II from the corresponding least-squares O(phenoxo)₂O(ethoxy)₂ plane is 0.01 and 0.00 Å, respectively, indicating that Li^I and Mg^II are well incorporated in the O(phenoxo)₂O(ethoxy)₂ compartment. Two coordinated water molecules in 1 and two of the three coordinated water molecules in 6 interact with the O(phenoxo)₂O(ethoxy)₂ compartment of two [Ni^IIL] moieties by forming bifurcated hydrogen bonds to stabilize the self-assembled tetrametallic cocrystals. The geometries of these hydrogen bonds are summarized in Table 3; the donor⋯acceptor contacts of the hydrogen bonds lie in the ranges 2.980–3.095 Å and 2.787–3.021 Å, respectively, indicating that hydrogen bonds are moderately strong.

Fig. 1 Crystal structure of [{Ni^IILLi^I(H₂O)₂}{Ni^IIL}₂](ClO₄) (1). Hydrogen atoms, except those of water molecules, are omitted. The perchlorate anion and the ethoxy carbon atoms are also not shown for clarity. Symmetry code: C, 1 − x, 1 − y, 1 − z.

Fig. 2 Crystal structure of [{Ni^IILMg^II(H₂O)₃}{Ni^IIL}₂](ClO₄)₂ (6). Hydrogen atoms are omitted. The perchlorate anion and the ethoxy carbon atoms are also not shown for clarity. Symmetry code: A, 1 − x, y, 0.5 − z.

Table 3 Geometries (distances in Å and angles in °) of the hydrogen bonds in 1, 2,5,6 and 10

Compound No.	D⋯A/D–H⋯A	H⋯A	D⋯A	D–H⋯A
1	O(9)–H(9C)⋯O(1)	2.418	2.980	126.6
	O(9)–H(9C)⋯O(4)	2.258	3.057	161.8
	O(9)–H(9D)⋯O(2)	2.260	3.001	151.2
	O(9)–H(9D)⋯O(3)	2.371	3.040	140.0
	O(10)–H(10C)⋯O(11C)	2.270	3.017	152.6
	O(10)–H(10C)⋯O(13C)	2.491	3.095	131.9
	O(10)–H(10D)⋯O(12C)	2.411	2.992	128.9
	O(10)–H(10D)⋯O(14C)	2.257	3.046	162.5
2	O(5)–H(5A)⋯O(8B)	2.16	2.89	164.88
5	C(8A)–H(8AA)⋯O(9A)	2.584	3.385	139.97
	C(9A)–H(9AA)⋯O(12A)	2.581	3.114	114.67
	C(14B)–H(14B)⋯O(10A)	2.513	3.297	142.17
	C(28)–H(28A)⋯O(5A)	2.57	3.30	132.5
6	O(7)⋯O(1A)		2.855
	O(7)⋯O(2A)		2.787
	O(7)⋯O(3A)		2.907
	O(7)⋯O(4A)		3.021
	C(8B)–H(8C)⋯O(1)	2.60	3.36	136.0
10	N(5)–H(5B)⋯O(2)	2.047	2.787	145.13
	N(5)–H(5B)⋯O(4)	2.398	3.128	144.22
	N(5)–H(5C)⋯O(3)	2.167	2.981	172.18
	N(5)–H(5D)⋯O(5)	2.278	2.767	119.65
	N(5)–H(5D)⋯O(7)	2.260	3.050	166.58
	N(5)–H(5A)⋯O(6)	2.211	2.826	131.29
	N(5)–H(5A)⋯O(8)	2.333	3.081	150.61

---

[Ni^IILNa^I(ClO₄)(CH₃OH)] (2; Fig. S1), [Ni^IILCa^II(NO₃)₂(H₂O)]·1.5H₂O (7; Fig. 3), [Ni^IILCd^II(NO₃)₂]·CH₃CN (8; Fig. S2) and [Ni^IILPb^II(NO₃)₂] (9; Fig. S3) are diphenoxo-bridged dinuclear Ni^IINa^I (2), Ni^IICa^II (7), Ni^IICd^II (8) and Ni^IIPb^II (9) compounds. All the four oxygen atoms of the O₄ compartment coordinate to Na^I, Ca^II, Cd^II and Pb^II. The sodium(I) in 2 is hexacoordinated in which the remaining two coordination positions are occupied by a perchlorate oxygen atom and a methanol oxygen atom. The cadmium(II) in 8 and lead(II) in 9 are octacoordinated; both the metal ions are additionally bonded with four oxygen atoms of two chelating nitrates. On the other hand, the calcium(II) in 7 is nonacoordinated by additional coordination of four oxygen atoms of two chelating nitrates and one water oxygen atom. The displacement of Na^I, Ca^II , Cd^II and Pb^II from the corresponding least-squares O(phenoxo)₂O(ethoxy)₂ plane is 0.03, 0.12, 0.03 and 0.09 Å, respectively, indicating that Na^I_, Ca^II, Cd^II and Pb^II are almost incorporated in the O(phenoxo)₂O(ethoxy)₂ compartment.

Fig. 3 Crystal structure of [Ni^IILCa^II(NO₃)₂(H₂O)]·1.5H₂O (7). Hydrogen atoms are omitted. The water molecules of crystallization and the ethoxy carbon atoms are also not shown for clarity. Of the two disordered positions of one oxygen atoms of one nitrate anion, one is shown.

[(Ni^IIL)₂Na^I](BPh₄)·CH₃COCH₃ (3; Fig. 4), [(Ni^IIL)₂Rb^I](BPh₄) (4; Fig. 5), [(Ni^IIL)₂Cs^I](ClO₄)·CH₃CN (5; Fig. 6) are trinuclear systems. Compound 4 has two independent units, shown in Fig. 5. The sodium(I) in 3, rubidium(I) in 4 and cesium(I) in 5 are octacoordinated to two phenoxo and two ethoxy oxygen atoms of each of the two [Ni^IIL] moieties and thus the structures consist of two diphenoxo-bridged Ni^IINa^I (3), Ni^IIRb^I (4) or Ni^IICs^I (5) moieties. Of these three trinuclear compounds, Ni^II₂Rb^I (4) and Ni^II₂Cs^I (5) are double-decker sandwich systems.

Fig. 4 Crystal structure of [(Ni^IIL)₂Na^I](BPh₄)·CH₃COCH₃ (3). Hydrogen atoms are omitted. The tetraphenylborate anion, acetone molecule and the ethoxy carbon atoms are also not shown for clarity. Of the two disordered positions of three carbon atoms of the amine moieties, one is shown.

Fig. 5 Crystal structure of [(Ni^IIL)₂Rb^I](BPh₄)·CH₃COCH₃ (4). Hydrogen atoms are omitted. The tetraphenylborate anion and the ethoxy carbon atoms are also not shown for clarity. Of the two disordered positions of two carbon atoms of the amine moiety, one is shown.

Fig. 6 Crystal structure of [(Ni^IIL)₂Cs^I](ClO₄)·CH₃CN (5). Hydrogen atoms are omitted. The perchlorate anion, The acetonitrile molecule of crystallization and the ethoxy carbon atoms are also not shown for clarity.

[(Ni^IIL)₂(NH₄)](PF₆) (10; Fig. 7) is a self-assembly of one NH₄⁺ ion and two [Ni^IIL] moieties. Two hydrogen atoms of ammonium ion interacts with one [Ni^IIL] moiety; in this case, each of the two hydrogen atoms, H5A and H5B, forms bifurcated hydrogen bonds with one phenoxo and one ethoxy oxygen atoms. Remaining two other hydrogen atoms interact with the second [Ni^IIL] moiety; in this case, one hydrogen atom, H5D, forms bifurcated hydrogen bonds with one phenoxo and one ethoxy oxygen atoms, while H5C interacts only with an ethoxy oxygen atom. The geometries of the hydrogen bonds are summarized in Table 3. The donor⋯acceptor contacts lie in the range 2.767–3.128 Å, indicating that hydrogen bonds are moderately strong. As Rb^I in 4 and Cs^I in 5 are sandwiched, the ammonium ion in 10 is also sandwiched in between two [Ni^IIL] moieties. However, the sandwich compounds 4 and 5 are formed due to strong metal–ligand chemical bonds, while the sandwich compound 10 is generated due to hydrogen bonds.

Fig. 7 Crystal structure of [(Ni^IIL)₂(NH₄)](PF₆) (10). Hydrogen atoms, except those of ammonium ion, are omitted. The hexafluorophosphate anion and the ethoxy carbon atoms are also not shown for clarity.

The nickel(II) ion in all the compounds 1–10 adopts only slightly distorted square planar environment. The Ni–O/N bond distances, cisoid angles and transoid angles lie in the ranges 1.820(3)–1.862(5) Å, 82.48(15)–95.70(14)° and 174.09(19)–178.99(17)°, respectively (Table S1–S10†). The ranges of the average deviation of the constituent atoms and the ranges of the displacement of the nickel(II) centre from the corresponding least-squares N₂O₂ plane are 0.01–0.08 Å and 0.00–0.02 Å, respectively. All these structural parameters are in the ranges observed in the previously published nickel(II) complexes derived from H₂L.^14,20

The bond distances and bond angles of the coordination environments of Li^I in 1, Na^I in 2 and 3, Rb^I in 4, Cs^I in 5, Mg^II in 6, Ca^II in 7, Cd^II in 8 and Pb^II in 9 are listed in Tables S1–S9,† while the ranges of bond lengths and angles of these metal centres are compared in Table 2. The order of the bond distances of alkali metal ions or alkaline metal ions involving a particular type of ligand centre follows the order of their ionic radii. For example: the order of the metal–phenolate bond distances of alkali metal ions: Li–O [2.008(9) and 2.024(10) Å in 1] < Na–O [2.3027(14) and 2.3182(15) Å in 2] and [2.409(2)–2.443(2) Å in 3] < Rb–O [2.772(2)–2.886(2) Å in 4] < Cs–O [2.963(3)–3.047(3) Å in 5]; the order of the metal–ethoxy bond distances of alkali metal ions: Na–O [2.5312(15) and 2.5610(14) Å in 2 and 2.505(2)–2.580(2) Å in 3] < Rb–O [2.960(3)–3.134(2) Å in 4] < Cs–O [3.119(3)–3.183(3) Å in 5]; the order of metal–phenolate bond distances of alkaline metal ions: Mg–O [2.191(4) Å in 6] < Ca–O [2.367(3) and 2.370(2) Å in 7]. The Pb–phenolate and Pb–ethoxy bond distances, 2.448(5)–2.478(5) Å and 2.684(6)–2.699(5) Å, respectively, lie in between the corresponding bond lengths involving Ca^II/Cd^II and Rb^I, which are in line with the order of the ionic radii, 8-coordinated Rb^I (1.61 Å) > 9-coordinated Pb^II (1.35 Å) > 8-coordinated Cd^II (1.1 Å) / 9-coordinated Ca^II (1.18 Å). As already discussed, the Li^I centre in 1, Mg^II centre in 6 and Ca^II centre in 7 are coordinated with two, three and one water molecules, respectively. The metal–water bond lengths [1.983(9)–1.993(9) Å for Li^I, 2.108(9)–2.112(4) Å for Mg^II and 2.355(3) Å for Ca^II] are shortest bond distances in their coordination environments. The sodium(I) centre in 2 is coordinated to a methanol and a perchlorate oxygen atom. The Na−perchlorate [2.365(2) Å] and Na−methanol [2.3471(19) Å] bond distances lie in between the Na−phenolate and Na−ethoxy separations. Each of the calcium(II) in 7, cadmium(II) in 8 and lead(II) in 9 is coordinated to two chelating nitrates. The ranges of the Pb^II–nitrate, Ca^II–nitrate and Cd^II–nitrate bond distances are 2.631(6)–2.940(12) Å, 2.292(9)–2.690(8) Å and 2.343(7)–2.467(7) Å, respectively. The nickel⋯alkali metal separations follow the order of ionic radii: Ni⋯Cs (3.908 and 3.909 Å) > Ni⋯Rb (3.679–3.772 Å) > Ni⋯Na (3.324 Å in 2, 3.434 and 3.456 Å in 3) > Ni⋯Li (2.992 Å); Ni⋯Ca (3.403 Å) > Ni⋯Mg (3.206 Å) and Ni⋯Pb (3.484 Å) > Ni⋯Cd (3.301 Å). The Ni–O(phenoxo)–Li/Na/Rb/Cs/Mg/Ca/Cd/Pb bridge angles lie in the range 101.61(10)–107.96(9)°, indicating that the bridge angles in 1–9 are not changed significantly.

Except that of the Mg^II centre in 6, the coordination environment of other metal ions, Li^I, Na^I, Rb^I, Cs^I, Ca^II, Cd^II and Pb^II, in the title compounds can not be modeled by any regular or distorted geometry. The geometry of the Mg^II centre, on the other hand, can be described as intermediate between distorted square pyramidal and distorted trigonal bipyramidal; the value of τ for this environment is 0.53.¹³ In the case of the distorted square pyramidal geometry, one phenoxo oxygen atom (O5) and three water oxygen atoms (O7, O7A and O8) define the basal plane, while the remaining bridging phenoxo oxygen atom (O5A) occupies the axial position. The average deviation of the constituent atoms and the displacement of the metal centre from the least-squares O₄ basal plane are significantly large, 0.30 and 0.31 Å, respectively, which indicate the appreciable distortion of the square pyramidal geometry. Although one transoid angle [O7–Mg1–O7A = 178.0(3)°] in the basal plane deviates by only a small amount from the ideal value, the value of the second transoid angle [O5–Mg1–O8 = 146.40(10)°] in the basal plane is much smaller. In addition, the wide range of the cisoid angles [67.2(2)–91.39(15)°] are in line with significant distortion. On the other hand, in the case of the distorted trigonal bipyramidal environment, one water oxygen atom (O8) and two bridging phenoxo oxygen atoms (O5 and O5A) define the trigonal plane, while the two remaining water oxygen atoms (O7 and O7A) occupy the axial positions. Although the bond angle [178.0(3)°] involving two axial atoms is not significantly deviating from the ideal value, the wide range of other bond angles [67.2(2)–146.40(10)°] is indicative of appreciable distortion of the trigonal bipyramidal coordination environment.

The weak interactions like π⋯π stacking, hydrogen bonds and Ni⋯Ni interactions^10f,20 in compounds 1–10 have been checked to understand possible supramolecular topologies. As shown in Fig. 8, two tetrametallic Ni^II₃Li^I moieties in 1 are self-assembled due to a Ni⋯Ni weak interaction to result in the generation of a dimeric aggregate. In the Ni^IINa^I compound 2, the alcohol hydrogen atom (H5A) and one methyl hydrogen atom (H21B) of the coordinated methanol molecule interact, respectively, with one oxygen atom (O8B) of the coordinated perchlorate anion and one aromatic ring of a neighbouring molecule (Fig. 9). These O–H⋯O and C–H⋯π hydrogen bonds result in the generation of a one-dimensional self-assembly in the structure of 2. As shown in Fig. 10, four sandwich Ni^II₂Cs^I units in 5 are interlinked due to one Ni⋯Ni interaction and four C–H⋯O hydrogen bonds involving three hydrogen atoms (H28A, H8AA and H9AA) of the lateral diimino side chains, one hydrogen atom (H14B) of an aromatic ring and one phenoxo (O5A) and three perchlorate (O9A, O10A and O12A) oxygen atoms to result in the generation of a nice cyclic self-assembly. In 6, the [2 × 1 + 1 × 2] tetrametallic Ni^II₃Mg^II units are self-assembled to one-dimensional polymeric topology (Fig. S4†) due to one Ni⋯Ni interaction and one C–H⋯O hydrogen bond involving a hydrogen atom (H8C) of the lateral diimino side chain and a phenoxo oxygen atom (O1). The supramolecular structure of the Ni^IICd^II compound 8 is also one-dimensional; however, the weak interaction in this case is π⋯π stacking (Fig. 11). In 10, the neighbouring individual Ni^II₂(NH₄) sandwich moieties are self-assembled to one-dimensional topology due to Ni⋯Ni interaction (Fig. 12). The geometries of the C–H⋯O/O–H⋯O hydrogen bonds in 2, 5 and 6 are summarized in Table 3, indicating that the C–H⋯O interactions are weak but the O–H⋯O interactions are strong or moderately strong. On the other hand, the H⋯π distance of the C–H⋯π hydrogen bond in 2 is 2.99 Å. The distances between the metal ions exhibiting Ni⋯Ni interaction in 1, 5, 6 and 10 are, respectively, 3.32, 3.30, 3.45 and 3.43 Å, while the chemical environments of the two interacting metal ions are perfectly planar as evidenced by the dihedral angle (0°) between the corresponding least-squares N₂O₂ planes and therefore the existence of Ni⋯Ni interaction in these molecules can be well considered.^10f,20 The centroid to centroid distance and dihedral angle between the aromatic rings responsible for π⋯π stacking interaction in 8 are 3.56 Å and 1.37°, respectively.

Fig. 8 Perspective view of [{Ni^IILLi^I(H₂O)₂}{Ni^IIL}₂](ClO₄) (1) demonstrating the dimeric self-assembly generated due to Ni⋯Ni weak interaction. Hydrogen atoms, except those of water molecules, are omitted. The perchlorate anion and the ethoxy carbon atoms are also not shown for clarity. Symmetry codes: C, 1 − x, 1 − y, 1 − z; D, −1 + x, y, z; E, −x, 1 − y, 1 − z.

Fig. 9 Perspective view of [Ni^IILNa^I(ClO₄)(CH₃OH)] (2) demonstrating the one-dimensional supramolecular topology generated due to C–H⋯π and O–H⋯O hydrogen bonds. Hydrogen atoms, except those of methanol molecule, are omitted. The ethoxy carbon atoms are also not shown for clarity. Symmetry codes: A, −1 + x, y, z; B, 1 + x, y, z.

Fig. 10 Perspective view of [(Ni^IIL)₂Cs^I](ClO₄).CH₃CN (5) demonstrating cyclic self-assembly generated due to Ni⋯Ni interaction and C–H⋯O hydrogen bonds. Hydrogen atoms, except those participating in hydrogen bonds, are omitted. The ethoxy carbon atoms are also not shown for clarity. Symmetry codes: A, −x, 1 − y, 1 − z; B, −x, −y, 1 − z; C, x, −1 + y, z.

Fig. 11 Perspective view of [Ni^IILCd^II(NO₃)₂]·CH₃CN (8) demonstrating the one-dimensional supramolecular topology generated due to π⋯π stacking. Hydrogen atoms are omitted. The acetonitrile molecule of crystallization and the ethoxy carbon atoms are also not shown for clarity. Symmetry codes: A, 1 + x, y, z; B, −1 + x, y, z.

Fig. 12 Perspective view of [(Ni^IIL)₂(NH₄)](PF₆) (10) demonstrating the one-dimensional supramolecular topology generated due to Ni⋯Ni weak interaction. Hydrogen atoms, except those of ammonium ion, are omitted. The hexafluorophosphate anion and the ethoxy carbon atoms are also not shown for clarity. Symmetry codes: A, 1 − x, 1 − y, 1 − z.

Comparison of the structures of 1–10 and related compounds: significant aspects

Previously 3d–3d [2 × 1 + 1 × 2] tetrametallic cocrystals [{M_A^IILM_B^II(H₂O)₃}{M_A^IIL}₂](ClO₄)₂ (M_A = Cu^II and M_B = Cu^II, Co^II, Mn^II; M_A = Ni^II and M_B = Cu^II, Ni^II, Co^II, Fe^II , Mn^II) as well as 3d–s [2 × 1 + 1 × 2] tetrametallic cocrystals [{Cu^IILM_s^I(H₂O)₂}{Cu^IIL}₂](X) (M_s = Li^I, X = ClO₄; M_s = Na^I, X = NO₃; M_s = Na^I, X = dicyanamide) and [{Cu^IILMg^II(H₂O)₃}{Cu^IIL}₂](NO₃)₂ have been reported.^13–16 The Ni^II–Li^I compound [{Ni^IILLi^I(H₂O)₂}{Ni^IIL}₂](ClO₄) (1) and Ni^II–Mg^II compound [{Ni^IILMg^II(H₂O)₃}{Ni^IIL}₂](ClO₄)₂ (6) in the present investigation are similar [2 × 1 + 1 × 2] tetrametallic cocrystals. The governing force for the stabilization of all these cocrystals is the encapsulation of two coordinated water molecules of the dinuclear cores in the O₄ compartment of two [Cu^II/Ni^IIL] moieties. Therefore, On the basis of [2 × 1 + 1 × 2] cocrystallization behaviour, the metal ions Li^I, Na^I and Mg^II can be considered to resemble 3d metal ions. In contrast to Li^I and Na^I, Mg^II centre resembles 3d metal ions also in terms of coordination number, set of coordinated atoms (two bridging phenoxo and three water oxygen atoms) and coordination geometry (intermediate between square pyramidal and trigonal bipyramidal). Therefore, Mg^II resembles 3d metal ions exactly. We proposed these resemblances previously^15,16 but strengthened due to the composition of compounds 1 and 6.

The role of 3d metal ion in the N₂O₂ compartment of [L]²⁻ on the nature of final product has been observed previously. For example, the reaction of [Cu^IIL ⊂ (H₂O)] with NaNO₃ produces the [2 × 1 + 1 × 2] tetrametallic cocrystal [{Cu^IILNa^I(H₂O)₂}{Cu^IIL}₂](NO₃), while the three component [3 × 1 + 2 × 1 + 1 × 2] cocrystal [(Ni^IIL)₂Na^I(NO₃)]·[{Ni^IILNa^I(H₂O)₂}{Ni^IIL}₂(NO₃)] is produced from the reaction of [Ni^IIL ⊂ (H₂O)] with NaNO₃.^15,20 The nickel(II)–rubidium(I) compound [(Ni^IIL)₂Rb^I](BPh₄).CH₃COCH₃ (4) is a double-decker system, while the copper(II)–rubidium(I) compound [{(Cu^IIL)₂Rb^I(μ-H₂O)}₂](BPh₄)₂·2CH₃COCH₃ is a quadruple decker system in which two double-decker topologies are bridged by two water molecules.¹⁶ Both these 3d–Rb^I compounds are produced on using the same rubidium(I) salt, RbBPh₄. Clearly, the different compositions of the copper(II)–rubidium(I) and nickel(II)–rubidium(I) systems arise due to the difference of the metal ion, Cu^II or Ni^II, in the N₂O₂ compartment. On the other hand, formation of three different types of products, the three component trinuclear-dinuclear-mononuclear cocrystal [(Ni^IIL)₂Na^I(NO₃)]·[{Ni^IILNa^I(H₂O)₂}{Ni^IIL}₂(NO₃)],²⁰ trinuclear compound [(Ni^IIL)₂Na^I](BPh₄)·CH₃COCH₃ (3) and dinuclear compound [Ni^IILNa^I(ClO₄)(CH₃OH)] (2) obtained on reacting [Ni^IIL ⊂ (H₂O)] with NaNO₃, NaBPh₄ and NaClO₄ respectively, indicates the role of anion on the nuclearity, composition and topology of the product. Again, both the 3d metal ion in the N₂O₂ compartment and the anion may have the roles for the formation of a [2 × 1 + 1 × 2] Cu^II₃Cd^II cocrystal¹⁷ [{Cu^IILCd^II(H₂O)₂(CH₃CN)}{Cu^IIL}₂](ClO₄)₂ and a dinuclear Ni^IICd^II compound [Ni^IILCd^II(NO₃)₂]·CH₃CN (8) on reacting [Cu^IIL ⊂ (H₂O)] with cadmium(II) perchlorate and [Ni^IIL ⊂ (H₂O)] with cadmium(II) nitrate, respectively. On the other hand, while the copper(II)–lead(II) compound [{Cu^IILPb^II(μ-NO₃)(NO₃)}₂] is a dimer of dinuclear type tetrametallic system,¹⁹ the nickel(II)–lead(II) compound [Ni^IILPb^II(NO₃)₂] (9) is a dinuclear system. It may be mentioned that the trinuclear sandwich type Ni^II₂Cs^I and dinuclear Ni^IICa^II complexes, [(Ni^IIL)₂Cs^I](ClO₄)·CH₃CN (5) and [Ni^IILCa^II(NO₃)₂(H₂O)]·1.5H₂O (7), are the new systems in terms of 3d–s metal ion combination, derived from H₂L. Previously, a compound of composition [{Cu^IIL}₂ ⊂ {H₃N(ethylene)NH₃}](NO₃)₂ has been reported in which two [NH₃]⁺ hydrogen atoms of each of the two sites form hydrogen bonds with the O₄ compartment of one [Cu^IIL] moiety to result in the formation of a supramolecular dimer.²² The compound [(Ni^IIL)₂(NH₄)](PF₆) (10) is of similar type in terms of supramolecular self-assembly except that in this case two sets of hydrogen atoms interacting with two O₄ compartments come from a NH₄⁺ ion, while in [{Cu^IIL}₂ ⊂ {H₃N(ethylene)NH₃}](NO₃)₂ these two sets come from two RNH₃⁺ sites. Therefore, compound 10 may also be considered as a supramolecular dimer. At this point, it is relevant to mention that a similar copper(II)–ammonium compound derived from a similar ligand has been recently reported.²³

Conclusions

The aim of the present investigation has been to explore the structure and topology of the products obtained on reacting [Ni^IIL ⊂ (H₂O)] with salts of s, p and d¹⁰ metal ions and ammonium ion, where H₂L = N,N′-ethylenebis(3-ethoxysalicylaldimine). Two tetrametallic [2 × 1 + 1 × 2] cocrystals, Ni^II₃Li^I (1) and Ni^II₃Mg^II (6), four dinuclear compounds, Ni^IINa^I (2), Ni^IICa^II (7), Ni^IICd^II (8) and Ni^IIPb^II (9), one trinuclear compound, Ni^II₂Na^I (3), two trinuclear double-decker sandwich compounds, Ni^II₂Rb^I (4) and Ni^II₂Cs^I (5) and one supramolecular dimer, Ni^II₂(NH₄) (10), have been reported. The structures have been compared with previously reported related compounds derived from H₂L. The major outcomes of the present investigation can be summarized as: Strengthening our previous proposal of showing a structural resemblance between the structures formed with Li^I, Na^I and Mg^II and those with 3d metal ions, in the Mg^II case this is essentially an exact structural resemblance; the anion dependent topology of the nickel(II)–sodium(I) complexes; the role of 3d metal ion (Cu^II or Ni^II) in the N₂O₂ compartment on the composition and topology of 3d–rubidium^I system; probable role of 3d metal ion (Cu^II or Ni^II) in the N₂O₂ compartment and also anion on composition and topology of 3d–cadmium^II system; similar role of diprotonated diamines and ammonium ion in terms of the generation of similar self-assembly, i.e., supramolecular dimers of [Cu^IIL/Ni^IIL] moieties. All in all, the structural diversity of the metal complexes derived from H₂L ligand system has been further enriched by some interesting structural observations in the present investigation.

Acknowledgements

Financial support from the Government of India through Department of Science and Technology (SR/S1/IC–12/2008) and University Grants Commission (Fellowship to S. Sarkar) is gratefully acknowledged. Crystallography was performed at the DST-FIST, India-funded Single Crystal Diffractometer Facility at the Department of Chemistry, University of Calcutta.

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Footnote

† Electronic supplementary information (ESI) available: Tables S1–10 and Fig. S1–S4. CCDC reference numbers 813955 for 1, 813956 for 10 and 813957–813964 for 2–9, respectively. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ra00144b

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