Crystal polymorphism in 2,2'-bipyrimidine-based iridium(III) complexes

Abstract Mononuclear iridium(III) complexes, NBu4[IrBr4(bpym)] (1) and [IrBr3(bpym)(MeCN)] (2a and 2b) [NBu4 + = tetra(n-butyl)ammonium cation; bpym = 2,2'-bipyrimidine], have been prepared and characterized. Their crystal structures were determined by single-crystal X-ray diffraction. 1 and 2a crystallize in the monoclinic crystal system with space groups P21/c and P21/n, respectively. The 2b phase crystallizes in the orthorhombic crystal system with space group P212121. In these complexes, each iridium(III) is six-coordinate and bonded to two (1) or three (2a and 2b) nitrogen atoms from bpym and MeCN (2a and 2b) molecules and three (2a and 2b) or four (1) bromides. In all the cases, they exhibit a distorted octahedral environment. 2a and 2b display a packing polymorphism containing π···Br, C–H···Br and Br···Br intermolecular interactions. Further analyses of their crystal structures were performed through the SHAPE and CrystalExplorer programs. The electrochemical properties of 1 and 2 were investigated through cyclic voltammetry (CV) technique, with 1 showing a reversible iridium(III)–iridium(IV) redox process.

Regarding the phenomenon of crystal polymorphism (and pseudo-polymorphism) in Ir III chemistry, only a few works dealing with this topic have been reported [2,18,19]. Polymorphic species display different packing arrangements of the same constituent molecules and, despite containing identical chemical composition, each phase constitutes a singular crystalline material with its own chemical and physical properties [20,21]. Similar effects can also be observed in pseudo-polymorphic phases [22]. Thus, small changes in the synthesis and crystallization can lead to different polymorphic species, which can provide useful information on the factors that govern the molecular self-assembling process in the crystalline state as well as on structure-property relationships [23,24].
Herein we report the synthesis and crystal structure of three iridium(III) complexes, NBu 4 (1), which was isolated and characterized as the tetra(n-butyl)ammonium salt. 1 was used as a starting material for the preparation of both 2a and 2b.
In order to explore the chemical reactivity of these new Ir(III) complexes toward their oxidation to Ir(IV), several attempts were made by adding stoichiometric mixtures of different oxidants, such as NH 4 [Ce(SO 4 ) 4 ]Á2H 2 O, Br 2 , and H 2 O 2 , in their acidic aqueous solutions. However, no Ir(IV) complex was isolated by means of these oxidizing agents. The subsequent products were the initial Ir(III) complexes, indicating their stability against chemical oxidation. For that reason, electrochemical studies were performed on these Ir(III) complexes.

Physical measurements
Elemental analyses (C, H, N) were performed on a CE Instruments EA 1110 CHNS analyzer. Infrared spectra were recorded on a Thermo-Nicolet 6700 FT-IR spectrophotometer from 4000 to 400 cm À1 . Ir/Br molar ratios were analyzed through a Hitachi S-4800 field emission scanning electron microscope (SEM-EDX) equipped with a system of Xray microanalysis. Electrospray Ionization Mass Spectrometry (ESI-MS) spectra of 1 and 2 were performed on a SCIEX TripleTOF 6600þ mass spectrometer by using a direct infusion electrospray ionization source (ESI). 1 was dissolved in CHCl 3 and its scans were realized over positive and negative ions ( Figure S1), whereas 2 was dissolved in MeCN and recorded for negative ions ( Figure S2). All these measurements were performed in the Central Service for the Support to Experimental Research (SCSIE) at the University of Valencia. Cyclic voltammetry (CV) measurements were carried out with an Autolab/PGSTAT204 and a three-electrode measuring cell with Metrohm electrodes, a 3 mm diameter Pt disc as working electrode, a glassy carbon rod as auxiliary electrode and Ag/AgCl (3 M KCl) as reference electrode. CV curves were obtained with 1 mM solution of 1 and 2 in dry CH 2 Cl 2 , and the conductivity of the system was improved with a 0.25 M NBu 4 [PF 6 ] solution as supporting electrolyte. Argon gas was directly bubbled through the solution in the cell and stirred before each CV experiment. All the CV curves were recorded at 20 C. The studied potential range was from À1.0 to þ1.7 V vs. Ag/AgCl at a scan rate between 10 and 500 mVÁs À1 . Ferrocene (Fc) was added at the end of the experiment as an internal standard.

Crystallographic data collection and structure determination
X-ray diffraction data of single crystals of 1, 2a and 2b [with dimensions 0.69 Â 0.31 Â 0.17 (1), 0.43 Â 0.11 Â 0.09 (2a) and 0.30 Â 0.13 Â 0.09 mm 3 (2b)] were collected on a Bruker D8 Venture diffractometer with PHOTON II detector using graphitemonochromated Mo-K a radiation (k ¼ 0.71073 Å). The structure was solved by standard direct methods and subsequently completed by Fourier recycling using SHELXTL [46]. The final full-matrix least-squares refinements on F 2 , minimizing the function Rw(jF o j-jF c j) 2 , reached convergence with values of the discrepancy indices given in Table 1. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms of the bpym and MeCN molecules were set in calculated positions and refined as riding atoms. Graphical manipulations were performed using DIAMOND [47]. CCDC 2178030 (1), 2178031 (2a) and 2178032 (2b). À0.014(6)  (1) and 2.487(1) Å for 2b, respectively. As in 1, the rings of the bpym molecules in 2a and 2b are planar, but they exhibit dihedral angle values somewhat lower than that of 1 [approximately 2.7(1) and 3.9(1) for 2a and 2b, respectively]. In the three compounds, the C-C and C-N bond lengths of the bpym ligand are as expected for this molecule when coordinated to a metal center [33,34,37,41].
The coordination environment and geometry of the Ir(III) ions of this family of mononuclear complexes were further analyzed through the SHAPE program [48,49]. In all the cases, the Ir(III) ions display a coordination number equal to six (Figures 1, 2 and S3). The lower SHAPE values computed for these Ir(III) ions were 1.086, 1.135, 1.168 for 1, 2a and 2b, respectively, 0 being the perfect match for the ideal polyhedron ( Table 2). As expected, these calculated values can be assigned to an octahedral geometry, hence indicating the same geometry for the Ir(III) ions in 1, 2a, and 2b. The most regular octahedron in this family would be that of 1. In any case, this is an approximation, because the Ir(III) ions exhibit a slightly distorted octahedral geometry generated mainly by the short bite angle of the bpym ligand (see above).

Analysis of the Hirshfeld surfaces
Hirshfeld surfaces of [IrBr 4 (bpym)] À (1) and neutral [IrBr 3 (bpym)(MeCN)] (2a and 2b) complexes were calculated and their close intermolecular interactions were analyzed through the CrystalExplorer program [52][53][54][55][56][57]. These surfaces were mapped taking into account the distance from a point on the surface to the nearest atom outside (d e ) and inside (d i ) their surface. To overcome limitations related to the size of atoms, a normalized contact distance (d norm ) was also considered [52][53][54][55][56][57]. Hirshfeld surfaces for 1, 2a and 2b are shown in Figures 6-8, respectively. In general, the shorter contacts are displayed by using red color, whereas white indicates contacts around the van der Waals separation, and blue is for longer contacts [52]. In 1, the most important contacts are the C-HÁÁÁBr interactions, which mainly involve (NBu 4 ) þ cations and bromides of the [IrBr 4 (bpym)] À complexes and represent approximately 51% of the complete fingerprint plot ( Figure 6). Moreover, intermolecular C-HÁÁÁN interactions generated by bpym ligands are highlighted from the full fingerprint as ca.

Electrochemical properties
The electrochemical properties of 1 and 2 were studied employing cyclic voltammetry (CV) technique ( Figure 9). For 1, only a redox couple was detected and assigned to the Ir(III)/Ir(IV) pair ( Figures S5 and S6), whereas no reversible peaks were observed for 2. Figure 9 shows the reversibility of this system, which was tested from 10 to 500 mVÁs À1 . It also shows a position of the anodic (E pa ) and cathodic peaks (E pc ) which is independent from the scanning rate, as expected for a reversible redox process. The value of the half-wave potential, defined by the half sum of the cathodic and anodic peaks, [E 1/2 ¼ (E pa þ E pc )/2], was þ0.758(3) V versus Fc/Fc þ for the Ir(III)/Ir(IV) couple in 1. This value is in agreement with those reported for similar compounds of general formula [IrBr 4 (L) 2 ] À (L ¼ solvent), which have been reported in MeCN and within the range from þ0.53 to þ1.14 V versus Fc/Fc þ [58]. In addition, the peak-to-peak separation stands for the absolute value of the potential difference between cathodic and anodic peaks, (DE p ¼ jE pa À E pc j), which supports the reversibility of the process, having in mind a value of ca. 56 mV for the ideal one-electron exchange reversible process at 20 C. In the studied scan rate, the values obtained for 1 are from 72 to 90 mV. Although these results are somewhat greater than those expected for the one-electron exchange of the Ir(III)/Ir(IV) pair, this deviation is mainly caused by the Ohmic drop of the non-aqueous solvent, that confers a high resistivity medium, which is only partially compensated by the supporting electrolyte [59][60][61][62].
The one-electron exchange assigned to the Ir(III)/Ir(IV) pair in 1 is also supported by the fact that the value of DEp found for the Fc/Fc þ system under the same conditions was ca. 102 mV at 10 and 100 mV s À1 . On the other hand, the intensity of the anodic (I pa ) and cathodic peaks (I pc ) show a good linear relationship with the square root of the scan rate, indicating that both processes are controlled by diffusion mechanisms in the studied scan rate range ( Figure S7). The reversibility of a redox couple can also be proved through the absolute value of the ratio between peak currents when these are steady and equal to one (jI pa /I pc j ¼ 1), as observed for 1 (Table S1). þ ¼ tetra(n-butyl)ammonium cation; bpym ¼ 2,2 0 -bipyrimidine], have been synthesized and characterized. The study of their crystal structure through single-crystal X-ray diffraction revealed that 2a and 2b are polymorphic species containing pÁÁÁBr, C-HÁÁÁBr and BrÁÁÁBr intermolecular interactions with different crystallographic values in their packings. A further analysis of their crystal structure was performed through the SHAPE and CrystalExplorer programs. In addition, the electrochemical properties of 1 and 2 were investigated by cyclic voltammetry (CV). 1 exhibits a reversible one-electron exchange process, assigned to the Ir(III)/Ir(IV) pair, with a half-wave potential of ca. þ0.758(3) V. Complex 2 displays different electrochemical behavior, with no detected reversible process. The results of this study could be very useful for obtaining paramagnetic bpym-based Ir(IV) species that allow us to investigate their magnetic properties. Finally, these complexes are new members of the short list of mononuclear bpym-based Ir(III) systems.