Syntheses, structural characterization, and thermal behaviour of metal complexes with 3-aminopyridine as co-ligands

Six mixed metal complexes with 3-aminopyridine (3-ampy) as a co-ligand have been synthesized: catena-{[M(μ2-3-ampy)(H2O)4]SO4·H2O} (M=Ni (1) and Co (2)), [Co(3-ampy)4(NCS)2] (3), [Co(3-ampy)2(NCS)2] (4), [Co(3-ampy)4(N3)2] (5) and mer-[Co(3-ampy)3(N3)3] (6), (NCS−=isothiocyanate ion, N3− azide ion), and characterized by physio-chemical and spectroscopic methods as well as single crystal X-ray and powder diffraction. In the isostructural complexes 1 and 2 single μ2-3-ampy links the Ni(II) and Co(II) centers into polymeric chains. The mononuclear Co(II) and Co(III) pseudohalide complexes 3–6 reveal only terminal 3-ampy ligands. The 3-ampy ligands form supramolecular hydrogen bonded systems via their NH2-groups and non-covalent π-π ring-ring interactions via their pyridine moieties. Thermoanalytical properties were investigated for 1–3.


Introduction
Aminopyridine ligands have been extensively studied in the synthesis of many coordination metal compounds ranging from simple mononuclear to coordination polymeric compounds (CPs) with different dimensionality. The construction of CPs is attributed to the intriguing structural diversity and the dual functionality of these ligands, which may lead to the formation of 1D or 3D structures [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], especially in the presence of other potentially bridging ligands such as pseudohalides [1][2][3]13]. Another feature incorporated in this class of compounds is their affinity to show different hydrogen bonds, N-H⋯X between the amino NH and a coordinated pseudohalide (X), which tends to extend the structure from 1D to a 3D network. Extra stability can also be generated through π-π stacking interactions between the aromatic pyridyl ligands, which play an important role in stabilizing the resultant polymeric architectures [1-3, 9, 15, 16].

Materials and physical measurements
3-aminopyridine was purchased from Aldrich. All other materials were reagent grade quality. Infrared spectra of the solid complexes were performed on a Bruker Alpha P (platinum-ATR-cap). UV-Vis-NIR spectra were recorded with a LS950 Perkin-Elmer Lambda-spectrometer. Thermal analyses were performed on solid samples with using NETSCH STA (N 2 atmosphere; heating rate 10 °C/min). PXRD measurements of the microcrystalline bulk material were performed with a Bruker D8 Advance powder diffractometer. Elemental CHN microanalyses were carried out with an Elementar Vario EN3 analyzer.

X-ray crystal structure analysis
The X-ray single-crystal data of the six title compounds were collected on a Bruker-AXS APEX II CCD diffractometer at 100(2) K. The crystallographic data, conditions retained for the intensity data collection and some features of the structure refinements are listed in Table 1. Data collections were performed with Mo-Kμ radiation (λ = 0.71073 Å); data processing, Lorentz-polarization and absorption corrections were performed using APEX and the SADABS computer programs [47,48]. The structures were solved by direct methods and refined by fullmatrix least-squares methods on F 2 , using the SHELX [49,50] program library. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were located from difference Fourier maps, assigned with isotropic displacement factors. Geometrical constraints (HFIX) were applied only for H atoms bonded to C atoms. Further programs used: Mercury, PLATON and ToposPro [51][52][53]. Packing plots are given in the supplementary section (Figs. S1-S6).

Synthesis, IR and UV-VIS spectroscopy
The synthesis of the complexes 3-5 was straightforward. Reactions of corresponding metal(II) salts with 3-ampy and KSCN (3 and 4) or NaN 3 (5) in aqueous or aqueous methanol, afforded the corresponding title compounds. Complexes 1 and 2 were obtained by reaction of equimolar amounts of metal(II) sulfate hydrates with 3-ampy from aqueous solutions. The Co(III) azido complex 6 was obtained as by-product in the synthesis of 5 due to air oxidation of the Co(II) in the mother liquor of 5. The phase purity of the bulk material of solid compounds was checked by XRPD patterns of 1-6 (Figs S7-S12, supplementary section).
In addition to the vibrations of the pyridine moiety of the 3-ampy molecule in the complexes, the IR spectra show the characteristic bands of the pseudohalide ligands. The strong or very strong absorption bands of ν as (SCN) and ν as (N 3 ) in complexes 3-6 are found at 2083 cm −1 (3), at 2090 and 2071 cm −1 (4), at 2052 cm −1 (5) and at 2004 cm −1 (6), respectively. The weak to medium strong vibrations of the -NH 2 group occur in the region 3100-3500 cm −1 . In case of 1 and 2, these bands are buried by the broad band at ~ 3180 cm −1 arising from O-H vibrations of the lattice water molecule and aqua ligands. In 1 and 2, the ionic nature of the sulfate group is indicated by the appearance of a very strong broad band (v 3 ) centered around 1077 cm −1 (1), 1077 cm −1 (2) and a sharp band (ν 4 ) at 609 cm −1 for 1 and 613 cm −1 for 2 [2,28,29,54].

Thermoanalytical behaviour of compounds 1-3
The thermogravimetric heating curves for compounds 1 and 2 are shown in Fig. 7. The Ni(II) compound 1 shows the following steps with weight losses: (195-340 °C); step 3: Δm = 13.43% (340-442 °C); step 4: Δm = 26.45% (442-975 °C). The first step of weight loss is accompanied with a sharp DSC signal at 174.2C and 138.8 °C for 1 and 2, respectively. The weight losses of 26.63% and 26.38% matches quite well with release of one lattice water molecule and four aqua ligands (theoretical: 26.58% and 26.56%) for 1 and 2, respectively. The anhydrous products show further steps of weight losses by releasing the 3-ampy ligand and the decomposition of the sulfate anion. The residual mass of the Ni(II) sample at 975 °C is 27.87%. Its PXRD pattern indicate a phase mixture with main microcrystalline components identified as Ni 3 S 2 and NiO (Fig. S13). The residual mass of the Co(II) sample of 25.16% at 975° C matches well with Co 9 S 8 (theoretical 24.90%) as identified from the PXRD pattern. (Fig. S14).
The thermogravimetric heating curve for compound 3 is shown in Fig. 8. [Co(3-ampy) 4 (NCS) 2 ] shows the following steps with weight losses: step 1: Δm = 34.25% (110-220 °C); step 2: Δm = 34.17% (220-470 °C); step 3: Δm = 15.25% (470-975 °C). The first two steps of weight loss are accompanied with sharp DSC signals at 195.2 °C and 341.3 °C, respectively. The first step of weight loss is accompanied by color change from orange to intensive blue and PXRD pattern of sample separated at 220 °C is identical with that of [Co(3-ampy) 2 (NCS) 2 ] (4), (Fig. S15) confirming the release of two 3-ampy molecules (theoretical 34.13%). The weight loss of second decomposition step indicates further release of remaining two 3-ampy molecules to form intermediate Co(NCS) 2 , followed by further decomposition to Co 9 S 8 at 975 °C as confirmed by PXRD (Fig. S16). (residual mass: 16.33%, theoretical mass: 15.31%).     (6), where in the latter case Co(II) was oxidized to Co(III). The isolation of the monomeric complexes through the coordination of the most basic pyridine-N-nitrogen is highly predictable. However, the 3-ampy molecule does behave as an "innocent" ligand as it looks. Surprisingly, it is the less basic amino group has very high tendency to coordinate to other metal ion and propagate the formation of polymeric chains with different dimensionality, as this was obvious in complexes 1 and 2.
In fact, the formation of CPs with 3-ampy reflects the general trend of this molecule to act as a bridging ligand through its two N-atoms. This trend was demonstrated by the isolation of many CPs with different metal ions such as Cd(II), Cu(II), Ni(II), Co(II), Ag(I) and Ag(II) but still the highest majority were obtained with Cd(II) ( Table 1). Pseudohalides used here and or those reported in Table 2 behaved as simple terminal monodentate ligands. Other carboxylate (acetate, benzoate, isophthalate, adipate) or oxyanions (nitrate, sulfonylate,…) seem to be more effective as bridging ligands in the presence of 3-ampy in expanding the network chains (Table 2). Another interesting feature is provided by the 3-ampy co-ligand in its complexes is its ability to form supramolecular hydrogen bonded systems via the NH 2 -group and non-covalent π-π ring-ring interactions via its pyridine moiety, which adds extra stability to the resulted compounds.