Coordination polymers and polygons using di-pyridyl-thiadiazole spacers and substituted phosphorodithioato NiII complexes: potential and limitations for inorganic crystal engineering

Coordinatively unsaturated P-substituted dithiophosphonato, dithiophosphato, and dithiophosphito complexes {[Ni((MeO)2PS2)2] (1), [Ni((EtO)2PS2)2] (2), [Ni(MeOdtp)2] (3), and [Ni((Ph)2PS2)2] (4)} were reacted with the bis-functional ligands 3,5-di-(4-pyridyl)-1,2,4-thiadiazole (L1) and 3,5-di-(3-pyridyl)-1,2,4-thiadiazole (L2) to give the coordination polymers (1–4·L1)∞, (3·L2)∞, and (4·L2·2C7H8)∞ and the discrete dimers (1–2·L2)2, all characterised by single crystal X-ray diffraction. A comparison of the structures shows that L1 can be exploited for the predictable assembly of undulating chains independent of the nature of the NiII complex, while L2 allows for the existence of different supramolecular constructs ensuing from different ligand conformations deriving from the rotation of the pyridyl rings.


Introduction
The past decades experienced an impressive number of novel multidentate bridging ligands designed to construct a variety of discrete metal-organic polygons and polyhedra (MOPs) or metal-organic frameworks (MOFs) with high dimensionality. These supramolecular assemblies are of great appeal due to a combination of their intrinsic beauty and promising applications in various fields such as gas storage, ion exchange, chemical sensing, catalysis, energy transfer, and separation. 1

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The properties of these crystalline materials are critically dependent on their network structures, and the deliberate creation of a crystalline network, planned using properly designed building blocks, remains, nowadays, a challenge. Several aspects need to be evaluated in developing a network based on coordination polymers: the building blocks, i.e. metal nodes and organic spacers, metal coordination environments, formation conditions, and weak secondary interactions. Multi-topic organic molecules are commonly used as spacers, and an opportune choice of the number and position of donor atoms can be used to direct the network assembly, although factors such as flexibility of the ligand and different accessible conformations need to be examined and taken into account. The metal coordination environment is exceptionally difficult to control when "naked" metal ions are used as nodes, and in view of that, the use of neutral coordination complexes held together by additional donor molecules or secondary bonding interactions has gained striking importance. 2 In fact, by reducing the degrees of freedom of the system, for instance by using cis-protected metal blocks in place of the naked metal ions or by using a neutral, coordinatively unsaturated metal complex, less uncertainty can be expected. Moreover, the use of neutral synthons leads to self-reliant supramolecular assemblies which do not require the presence of a counterion and thus reduce the occurrence of isomerism.
In this respect, we have been developing a synthetic program based on the ability of neutral dithiophosphonato Ni II complexes [NiĲROdtp) 2 ] [ROdtp = (RO)(4-MeOC 6 H 4 )PS 2 particular 4,4′-bipyridine and its analogues. § ,4 This assembly process is based on the capability of the coordinatively unsaturated Ni II ion of these square-planar complexes to axially bind monodentate ligands, such as pyridine, to yield discrete octahedral complexes. 5,6 In addition, suitable N-L-N bidentate bipyridyl-based spacers yield 1D coordination polymers of the type [Ni(ROdtp) 2 (N-L-N)] ∞ . 7,8 The primary structural motif of these polymers mainly depends on the features of the spacers such as length, rigidity and orientation of the donor atoms, as recently confirmed by the deliberate stereospecific generation of homochiral polymeric helices built from a designed enantiopure binaphthyl-based ligand. 9 The substituents on the phosphorus atoms are responsible for the connection of the polymers through hydrogen bonds and face-to-face or edge-to-face π-π interactions, thus influencing the final 3D architecture. 7 As a consequence, coordination polymers and 3D assemblies with different structures and architectures can be built up by varying either the bridging ligands or the substituents on the P atom of the initial Ni complexes. In order to better understand the process of molecular recognition between components and how the steric information contained in the P-substituents and in the orientation of ligand binding sites combine to give the final structure, differently P-substituted dithiophosphonato, dithiophosphato, and dithiophosphito complexes {[NiĲ(MeO) 2 - (3), and [Ni((Ph) 2 PS 2 ) 2 ] (4)} were reacted with the bis-functional ligands 3,5-di-(4-pyridyl)-1,2,4-thiadiazole (L1) 10 and 3,5-di-(3pyridyl)-1,2,4-thiadiazole (L2) 10 (Scheme 1).
We have recently investigated the reactivity of L1 and L2 with I 2 and IBr, in both polar and apolar media, thereby elucidating the role of specific directional interactions, namely, NH + ⋯N and N⋯I, combined with geometrical features of the molecules, in the formation of different supramolecular constructs. 10 The investigation of these ligands is extended here to the formation of coordination polymers.

Materials
All commercially available compounds were used as received.

Results
The bis-functional ligands L1 and L2 (Scheme 1) were first isolated by Meltzer et al. in 1955, 21 but their complexing ability towards metal ions, or as Lewis donors, has not been investigated to date. In fact, a search in the Cambridge Crystallographic Database revealed only a recent work by Ondrejkovicova et al. 22 Both L1 and L2 contain two pyridyl groups linked by a 1,2,4-thiadiazole ring acting as a rigid, non-reactive spacer. Compared with the popular 4,4′bipyridine linker, the pyridyl rings in L1 and L2 feature different geometries and separation lengths (9.95 and 9.60 Å, respectively). Moreover, due to the different positions of the nitrogen atoms in L1 and L2 and to the different rotational conformations possible for the pyridyl rings, several orientations of the binding sites can be expected. A potential energy surface (PES) analysis carried out on L1 and L2 at the DFT level in the gas phase by rotating one pyridyl substituent by an angle τ ranging between −180 and 180°clearly shows that both donors display an energy minimum when the central 1,2,4-thiadiazole ring is coplanar with the pyridine moieties ( Fig. 1). In the case of L2, two planar isomers, cisoid and transoid, are possible, differing in energy by less than 1 kcal mol −1 and showing similar metric parameters. As previously described, L1 and L2 feature the highest Kohn-Sham molecular orbitals localized on the negatively charged nitrogen atoms of the two pyridine moieties, which are therefore available to behave as donor sites towards Lewis unsaturated metal complexes. 10 The (4)} complexes under solvothermal conditions afforded solid, crystalline compounds, which were isolated and identified by means of single crystal X-ray diffraction as coordination polymers of formulae (1·L1) ∞ , (2·L1) ∞ , (3·L1) ∞ , and (4·L1) ∞ , respectively. Crystallographic data and selected bond lengths and angles are reported in Tables S1 † and 1, respectively. With the exception of compound (4·L1) ∞ , the L1 molecules are found in two different but essentially superimposable orientations, arising from a 180°rotation of the molecule about the direction passing through the midpoint of the N-S bond and through the remaining nitrogen atom of the penta-atomic ring (Fig. S1-S3 †). As a consequence, the nitrogen and the adjacent sulfur atoms show fractional This journal is © The Royal Society of Chemistry 2016 occupancies refined at values of about 60% and 40%, respectively, and only the major orientation of the molecule is illustrated in the figures. In the case of compound (2·L1) ∞ , the L1 molecule is located about a crystallographic two-fold axis, and therefore, the resulting disorder is modelled with occupancies of 50% for each orientation.
Coordination polymers (1-4·L1) ∞ show similar coordination environments: the nickel ions display distorted octahedral geometries with the equatorial plane occupied in isobidentate bonds with two dithiophosphoric ligands and the pyridine rings belonging to spacer L1 axially bridging adjacent Ni II ions to form infinite polymeric chains. The relevant bond lengths and angles (Table 1) are similar to those found in analogous coordination polymers. [7][8][9] The structures of the coordination polymers share the presence of neutral undulated polymeric chains with very similar pitches: Ni-Ni distances, through coordinate bonds, of 13.70, 14.15, 13.97, and 14.02 Å for (1·L1) ∞ , (2·L1) ∞ , (3·L1) ∞ , and (4·L1) ∞ , respectively (Fig. 2). These chains are closely analogous to the C 1 1 (12) chains of type [⋯ĲPy∩PyH + ⋯Py∩PyH + ) n ⋯] built up through the moderate NH + ⋯N bonds derived by the protonation of L1 molecules, reported for comparison in Fig. 2e. 10 This confirms that the orientation of the nitrogen atoms parapositioned in the outwards pyridyl rings of L1 self-governs the geometry of the resulting supramolecular aggregates, leading to similar shapes independent of the nature of the interacting Lewis acid. The asymmetric unit of compound (1·L1) ∞ (Fig. S2 † and 2a) contains two independent Ni II ions, namely, Ni1 and Ni2, lying on crystallographic inversion centres with coordination sites differing in the orientation of the methoxy substituents at the phosphorous atoms. The two coordination sites are bridged by L1 molecules, thus forming 1D parallel chains propagating along the 101 direction. These chains interact with each other through C-H⋯S short contacts, involving the pyridine rings and the coordinated sulphur atoms, leading to the 2D layers shown in Fig. 3a. The polymeric chains pack in two different orientations (coloured blue and yellow in Fig. 3b and S3 †) generated by inversion along the screw axes parallel to the 010 direction. As a consequence, symmetry related 2D layers formed by differently oriented chains alternate when packed and interact through synergic C-H⋯O bonds ("d" and "e" in Fig. 3b and S3a †) involving the pyridine rings and the methoxy P-substituents belonging to the Ni1b coordination sphere in an R 2 2 (7) motif. This packing arrangement enables the formation of "rippled" void channels of about 60 Å 3 corresponding to 2.2% of the unit cell volume (Fig. S3b †). The presence of OEt Table 1 Selected bond lengths (Å), bond and torsion angles (°), and angles between pyridyl (Py)/thiadiazole (Tdz) ring mean planes (°) for (1·L1) ∞ , (2·L1) ∞ , (3·L1) ∞ , and (4·L1) ∞ . Numbering scheme according to Fig. S1 (9) 90.39 (5) 89.12 (5) 88.76(5) S1-Ni-S2 82.04(4) b 82.010 (17) 82.55 (2) 110.95 (7)  a In order to compare structural parameters of the polymers which reside in different space groups a common artificial numbering scheme has been adopted as shown in Fig. S1. b Average of the bond parameters for the two symmetry independent fragments (P 1 S 1 S 2 Ni 1 ) and (P 2 S 3 S 4 Ni 2 ).  Fig. 4) directions, respectively, interact through weak H bonds involving the P-substituents and the central thiadiazole ring of L1 ("a" in Fig. 4) and form 2D layers which stack along the [010] direction leading to a very dense packing with no voids. The coordination polymers (3·L1) ∞ and (4·L1) ∞ ( Fig. 2c  and d, S5 and S6 †) are characterised by the presence of aryl substituents at the phosphorous atoms which are expected to engender additional intra-and inter-molecular aromatic interactions. Similarly to those described previously herein, the asymmetric units of compounds (3·L1) ∞ and (4·L1) ∞ contain two independent Ni II ions located on crystallographic inversion centres and differing only in the orientation of the aryl P-substituents. In the coordination polymer (3·L1) ∞ , the interactions involving the aromatic rings are intramolecular in na-ture (edge-to-face interaction "a" in Fig. 5). Weak interactions between the methoxy groups (interactions "b" and "c" in Fig. 5) connect the chains in layers which pack parallel to each other, forming a three-dimensional network through C-H⋯S interactions mainly involving the coordinated sulphur atoms and the methoxy substituents (interactions "d"-"g" in Fig. S5b †).
Aromatic interactions become prevalent in polymer (4·L1) ∞ due to the presence of phenyl substituents at the phosphorous atoms, so that the polymeric chains pack in layers built up by face-to-face and edge-to-face interactions only ("a"-"g" in Fig. 6). The layers pack parallel to the a axis through weak C-H⋯S interactions ("h"-"j" in Fig. S6b †). Despite the fact that all the coordination polymers featuring L1 as a spacer exhibit the same primary motif found in the supramolecular aggregates of L1H + (Fig. 2e), the presence and orientation of the P-substituents influence the final architecture of the polymers via aromatic or C-H⋯S intramolecular  interactions. The main consequence is the loss of planarity of the bridging L1 ligands (Table 1), probably ascribable to the conformational arrangement the pyridine rings adopt to optimize these interactions. It is interesting to note that in the H-bonded chains built up by protonated L1 molecules, where such interactions are not present, the ligands retain planarity. 10 The reactions of L2 with nickel complexes 1-4 under solvothermal conditions afforded crystalline compounds recognised by means of single crystal X-ray diffraction as the dimers (1·L2) 2 and (2·L2) 2 and the coordination polymers (3·L2) ∞ and (4·L2) ∞ in compound (4·L2·2C 7 H 8 ) ∞ , respectively ( Fig. 7-10 and S8-S11 †). The two different constructs reflect the different conformations of L2 behaving as a convergent linker in the dimeric structures and as a divergent linker in the polymers. Crystallographic data and selected bond lengths and angles are reported in Tables S2 † and 2. Similarly to the previously discussed cases of polymers (1-3·L1) ∞ , in the crystal structures of the dimer (1·L2) 2 and the polymers (3-4·L2) ∞ , the spacer L2 is located in two essentially superimposable orientations. As a consequence, the nitrogen and the adjacent sulfur atoms show fractional occupancies refined at values of 68.8 and 31.2% for (1·L2) 2 and 61.5 and 31.5% for (4·L2) ∞ (the figures represent the molecules in the major orientation only).
In the case of compound (3·L2) ∞ , the L2 molecule is located about a crystallographic two-fold axis, and therefore, the resulting disorder is modelled with occupancies of 50% for each orientation. The asymmetric unit of the dimer (2·L2) 2 contains two independent units featuring differently oriented ligands L2 (Fig. 7b). Both dimers (1-2·L2) 2 and polymers (3-4·L2) ∞ display octahedral Ni II coordination environments similar to those previously discussed, with two isobidentate dithiophosphoric ligands in the equatorial plane and the axial positions occupied by the pyridine rings of L2 bridging spacers.
The bond lengths and angles (Table 2) are also similar to those found in the analogous coordination polymers  3.90, 19; f, C34-H34a⋯Cnt iii PyĲN1) 3.58, 109; g, C19 iv -H19a iv ⋯Cnt PyĲN1) 3.90 Å, 129°. Symmetry codes: H atoms not involved in the shown interactions have been omitted for clarity. The labels used for the described interactions refer to the original numbering scheme (Fig. S6 †). (1-4·L1) ∞ . Notwithstanding similarities with the results obtained with L1, the use of L2 as a linker leads to different constructs. In fact, as evidenced by DFT calculations (see above), L2 can exist either as a transoid or as a cisoid isomer, and a convergent or a divergent conformation can be distinguished depending on the orientation of the N atoms of the pyridyl rings that can point inwards or outwards with respect to the pinch angle. In (1·L2) 2 and (2·L2) 2 , the pyridyl rings of ligand L2 are oriented in a convergent fashion leading to closed rings rather than polymeric chains ( Fig. 7a and b). It is interesting to note that this construct is far from predictable in its behaviour since a convergent fashion does not necessarily lead to closed rings. This is demonstrated by the formation of the mono-dimensional C 1 1 (10) spirals found in the two structures of (HL2)I 5 and (HL2)IBr 2 assembled through NH + ⋯N bonds between protonated L2 molecules exhibiting a convergent cisoid conformation. 10 In dimers (1·L2) 2 and (2·L2) 2 , two spacers bridge two dithiophosphato nickel complexes through axial coordination generating eicos-atomic planar wheels with openings of about 8 × 8 Å 2 and inner Ni⋯Ni distances of 7.73, 7.77, and 7.58 Å for (1·L2) 2 and the two independent units of (2·L2) 2 , respectively.
In the crystal packing of (2·L2) 2 , the two symmetrical independent units (containing Ni1 and Ni2 ions) that, as previously observed differ in the orientation of the thiadiazole ring (Tdz), interact with each other through π-π interactions involving the pyridine and Tdz rings forming puckered ribbons (Fig. 9a). A view along the a axis (Fig. 9b) shows the ribbons aligned in a parallel arrangement through H bonds and weaker interactions mainly involving the sulphur atoms and the pyridine hydrogens. A cisoid but divergent conformation of L2 leads to the coordination polymers (3·L2) ∞ and (4·L2) ∞ ( Fig. 7c and d and S10-S12 †). The asymmetric units of both compounds contain one independent Ni II ion situated on a crystallographic inversion centre in the first case and on a mirror plane in the latter. The polymeric supramolecular constructs present close analogies with the C 1 1 (10) chains formed by head-to-tail NH + ···N bonds between adjacent pyridine rings of protonated L2 molecules featuring a trans conformation. 10 Both complexes 3 and 4 feature aromatic substituents that are involved in π-type interactions which govern the assembly of the relevant polymers. In (3·L2) ∞ , the aromatic P-substituents are engaged in intramolecular π-π interactions with the facing pyridine rings and intermolecular C-H···O interactions with the oxygen belonging to the MeO P-substituents which connect the chains in layers (interactions a and b in Fig. S12a †). The layers pack in parallel through C-H···S interactions involving the para-methoxy group of the Ar-P substituents and the coordinated sulphur atoms (interactions c-e in Fig. S12b †), thus forming an overall three-dimensional net-work. Solvent accessible voids of 69 Å 3 (in blue in the inset of Fig. S12 †) correspond to 4% of the unit cell volume. In polymer (4·L2) ∞ , the exclusive presence of phenyl substituents at the phosphorous atoms results in the aromatic interactions becoming prevalent such that the parallel polymeric chains pack through edge-to-face interactions only (Fig. 10a). The resulting network features empty channels suitable for the inclusion of toluene molecules which engage in π-interactions with the phenyl rings ( Fig. 10b and c). It is interesting to note that, in contrast to the previously discussed structures (see also Tables 1 and 2), (4·L2) ∞ contains perfectly planar L2 ligands.

Conclusions
The reaction of L1 with the differently P-substituted dithiophosphonato, dithiophosphato, and dithiophosphito complexes 1-4 yielded the corresponding coordination polymers (1-4·L1) ∞ featuring polymeric [⋯ĲPy∩Py⋯M⋯Py∩Py)⋯] chains built from coordination to axial vacant sites on the nickel complexes. These results indicate that L1 can be used as a spacer for the predictable assembly of smoothly undulating chains independent of the nature of the interacting Lewis acid, since the orientation of the nitrogen atoms parapositioned in the outwards pyridyl rings of L1 self-governs the geometry of the resulting supramolecular construct. On the contrary, L2 allows for the existence of different supramolecular constructs ensuing from different ligand conformations deriving from the rotation of the pyridyl rings.
In particular, the results suggest that aromatic P-substituents capable of π-interacting with the aromatic rings of the ligand tend to favour divergent constructs. The influence of secondary interactions involving the P-substituents is confirmed by the loss of planarity of L1 and L2 in order to enhance inter-molecular packing interactions. a Average of the bond parameters for the two fragments (P 1 S 1 S 2 Ni) and (P 2 S 3 S 4 Ni). b Average of the bond parameters for the symmetry independent coordination environment around Ni1 and Ni2.