Linear One-Dimensional Coordination Polymers Constructed by Dirhodium Paddlewheel and Tetracyanido-Metallate Building Blocks

: In this article, we describe the preparation of anionic heteronuclear one-dimensional coordination polymers made by dirhodium paddlewheels and tetracyanido-metallatate building blocks. A series of complexes of (PPh 4 ) 2n [{Rh 2 ( µ -O 2 CCH 3 ) 4 }{M(CN) 4 }] n (M = Ni ( 1 ), Pd ( 2 ), Pt ( 3 )) formulae were obtained by reaction of [Rh 2 ( µ -O 2 CCH 3 ) 4 ] with (PPh 4 ) 2 [M(CN) 4 ] in a 1:1 or 2:1 ratio. Crystals of 1 − 3 suitable for single crystal X-ray di ﬀ raction were grown by slow di ﬀ usion of a dichloromethane solution of the dirhodium complex into a chloroform solution of the corresponding tetracyanido–metallatate salt. Compounds 1 and 2 are isostructural and crystallize in the triclinic P -1 space group, while compound 3 crystallizes in the monoclinic P 2 1 / n space group. A detailed description of the structures is presented, including the analysis of the packing of anionic chains and PPh 4 + cations.


Introduction
Dinuclear complexes with a paddlewheel structure show a rich chemistry and different electronic configurations as a consequence of the distribution of the energy levels and the number of electrons in the dimetallic unit [1,2]. Dirhodium complexes and, in particular, the tetracarboxylato derivatives, are among the most important and versatile paddlewheel compounds [1,2], and their reactivity has been explored in several fields such as catalysis [3][4][5][6][7][8][9] or bioinorganic chemistry [10][11][12][13]. The ground state electron configuration for most of these complexes is σ 2 π 4 δ 2 δ* 2 π* 4 for a diamagnetic Rh 2 4+ unit, which, therefore, displays a single metal-metal bond order [1,2]. Many dirhodium molecular complexes have been reported owing to the facility of the rhodium ions to coordinate monodentate donor ligands at the axial positions of the paddlewheel structure [1,2,[14][15][16][17]. Moreover, dirhodium units have been used to form one-dimensional coordination compounds using bridging ligands between the dimetallic cores [18][19][20][21]. The use of other metal complexes as connectors between the paddlewheel units can lead to the formation of heterometallic one-dimensional coordination polymers, whose versatile chemical and physical properties, such as temperature dependent luminescence or modulation of their electronic structures, make those polymers promising materials [22][23][24][25]. An interesting approach to obtain this kind of heterometallic one-dimensional polymers is the use of platinum complexes to form {[Rh 2 ]-[Pt 2 ]-[Pt 2 ]} n chains, in which the different dimetallic units show direct Rh-Pt metal-metal bonds [26][27][28][29]. Paramagnetic [Rh 2 ]-[Pt-Cu-Pt] chains can be synthesized, also inserting a copper complex [30,31]. In addition, several heterometallic one-dimensional polymers using dicyanidoaurate, [Au(CN) 2 ] − [32], or dicyanidoargentate, [Ag(CN) 2 ] − [33], as bridging axial ligands have been reported by our research group. The use of cyanide-bridged cyanidometallates to form heterometallic complexes has been widely reported in the literature, owing to the diversity in the topologies and dimensionalities found in those complexes and to the interesting magnetic or optical properties showed by some of them [34][35][36][37][38][39][40].
The use of charged cyanidometallates to bridge the neutral Rh(II)-Rh(II) units requires the presence of counter-cations to compensate the negative charge, which can simply be alkali cations [32] or bulkier groups like tetraphenylphosphonium [33]. The interest of the latter lays in its higher arrangement diversity owing to the supramolecular structures that can be formed by phenyl-phenyl interactions through double, quadruple, or sextuple phenyl embraces [33,[41][42][43][44].
In this work, we examine the reaction of [   4 ] was prepared by the following procedure: a solution of 0.100 g of K 2 [PtCl 4 ] (0.24 mmol) in 10 mL of water was mixed with a solution of 0.120 g of NaCN (2.45 mmol) in 10 mL of water. The mixture was stirred for 24 h and a solution of 0.200 g of PPh 4 Br (0.48 mmol) in 15 mL of water was added, immediately obtaining a white precipitate. The stirring was kept for 30 min and the solid was filtered and washed with water and diethyl ether. Yield: 0.170 g (72%). The rest of the reactants and solvents were obtained from commercial sources and used as received.

Materials and Physical Measurements
The elemental analysis measurements were carried out by the Elemental Analysis Service of the Universidad Complutense of Madrid. The FTIR spectra were collected with a Perkin-Elmer Spectrum 100 Spectrometer equipped with a universal ATR accessory in the 4000-650 cm −1 range. The mass spectra were collected by the Mass Spectrometry Service of the Universidad Complutense of Madrid using the electrospray ionization (ESI) technique and an ion trap-Bruker Esquire-LC spectrometer. Single crystal X-ray diffraction data were collected using an Oxford Diffraction Atlas diffractometer with Mo K α (λ = 0.71073 Å) radiation at room temperature. CCDC 1954793-1954795 contain the crystallographic data. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. A summary of some crystal and refinement data can be found in Table 1, and more information is collected in the Supplementary Material in Tables S1-S6.

Synthesis, Spectroscopic Characterization, and Spectrometric Characterization
The synthesis of compounds 1-3 was initially attempted using  4 ] units, leading to 1D arrangements instead. The reactions were repeated using a 1:1 ratio, obtaining products whose elemental analyses and IR spectra indicate both similar composition and coordination modes to those found in the crystal structures. Unfortunately, the lack of crystallinity in the obtained products from the 1:1 reactions does not allow the structural comparison of the solid state arrangement with the crystals from the 2:1 reactions. Compounds 1-3 can be obtained by stirring a mixture of the reactants at room temperature. However, following this procedure, the products are obtained as solids that cannot be recrystallized because of the low solubility of the polymeric species in common solvents. In order to obtain single crystals of the products that allowed the structural determination of the compounds, slow diffusion of a solution of [Rh 2 (µ-O 2 CCH 3 ) 4  The IR spectra of all the compounds display bands corresponding to the symmetric and asymmetric stretching modes of the carboxylate groups: COO as (1597-1584 cm −1 ); COO sym (1450−1400 cm −1 ). The C-H stretching bands of the aliphatic and aromatic groups appear below and above 3000 cm −1 , respectively. Moreover, two bands are observed between 2167 and 2120 cm −1 in all cases corresponding to the CN − groups. The lower energy band corresponds to terminal CN − ligands (not coordinated to Rh 2 units), while the higher energy band corresponds to bridging CN − ligands, coordinated to Rh 2 units.
The base peak of the ESI − mass spectra of 1-3 corresponds to [(PPh 4 )M(CN) 4 ] − . A peak close to the base peak that corresponds to [M-(PPh 4 )] − is also present in all the spectra, as well as a peak assigned to [M+Rh 2 (O 2 CCH 3 ) 4 -(PPh 4 )] − . This last peak indicates polymerization of the complexes, although it could also be caused by an association process during the measurements.

Structural Description
The crystal structures of 1-3 were determined by single crystal X-ray diffraction. Complexes 1 and 2 are isostructural and crystallize in the P-1 space group, while compound 3 crystallizes in the P2 1 /n space group ( Table 1) Figures S4 and S5 show their coordination environments, and Table 2 shows a selection of bond lengths and angles in 1-3; collected in more detail in Sections S4 and S5, Tables S7-S10. There are highly disordered solvent molecules in the crystals, which could not be satisfactorily modelled, and were removed with SQUEEZE software [45].  [48]. The Rh-O distances in 1-3 (2.019(5)−2.052(4)Å) are also very similar to the values found in these analogous compounds.
In the tetracyano-metallate units, the metal atom is located at an inversion center, and these fragments bridge two dimetallic units by two CN − groups in trans disposition coordinated to the axial positions of the rhodium atoms. The Rh-N distances range from 2.209 (5) Å for 1, 13.095 Å for 2, and 13.096 Å for 3. This distance is logically much longer than that found for similar chains with a zigzag disposition, for example, Kn[{Rh2(μ-O2CCH3)4}{Au(CN)2}]n (Au-Au distance = 8.365 Å) [32]. An important structural difference is observed in the angle between the {M(CN) 4 } plane and one of the planes in the paddlewheel molecule: in compounds 1 and 2, the two are almost parallel (10.30 • in 1 and 6.23 • in 2), while in compound 3, the value is considerably higher (22.64 • ), as can be seen in Figure 1 (right).
However, the main difference between the two structural types lies in the arrangement of the anionic chains and the molecular cations in the crystal (see Figure 2). The packing is achieved in the three compounds by C-H···π interactions between hydrogen atoms from the acetate ligands and some of the rings in the PPh 4 + cations (Figures S6 and S7, and Tables S11 and S12). In the structure of compounds 1 and 2, only four hydrogen atoms in each paddlewheel unit participate in these interactions, while in compound 3, six C-H···π interactions are established, resulting in a denser packing.

Conclusions
The Probably, the presence of bulky PPh4 + groups hinders the formation of two-dimensional networks. The crystal structure of these polymers consists of almost linear chains formed by {Rh2(μ-O2CCH3)4} paddlewheel units axially bridged by two trans cyanide groups of the tetracyano-metallate moieties. The phenyl rings of the PPh4 + cations are involved in C-H•••π interactions with the hydrogen atoms from the acetate ligands, so the higher the number of involved hydrogen atoms, the denser the resulting packing.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Conclusions
The The crystal structure of these polymers consists of almost linear chains formed by {Rh 2 (µ-O 2 CCH 3 ) 4 } paddlewheel units axially bridged by two trans cyanide groups of the tetracyano-metallate moieties. The phenyl rings of the PPh 4 + cations are involved in C-H···π interactions with the hydrogen atoms from the acetate ligands, so the higher the number of involved hydrogen atoms, the denser the resulting packing.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.