Crystal structures of chlorido[dihydroxybis(1-iminoethoxy)]arsanido-κ3 N,As,N′]platinum(II) and of a polymorph of chlorido[dihydroxybis(1-iminopropoxy)arsanido-κ3 N,As,N′]platinum(II)

The square-planar and trigonal–bipyramidal coordination environment around the Pt and As atom, respectively, each are distorted, with a τ5 value of 0.794 and 0.711 for arsenic in (1) and (2), respectively.

Each central platinum(II) atom in the crystal structures of chlorido[dihydroxybis(1-iminoethoxy)arsanido-3 N,As,N 0 ]platinum(II), [Pt(C 4 H 10 AsN 2 O 4 )Cl] (1), and of chlorido[dihydroxybis(1-iminopropoxy)arsanido-3 N,As,N 0 ]platinum(II), [Pt(C 6 H 14 AsN 2 O 4 )Cl] (2), is coordinated by two nitrogen donor atoms, a chlorido ligand and to arsenic, which, in turn, is coordinated by two oxygen donor ligands, two hydroxyl ligands and the platinum(II) atom. The square-planar and trigonal-bipyramidal coordination environments around platinum and arsenic, respectively, are significantly distorted with the largest outliers being 173.90 (13) and 106.98 (14) for platinum and arsenic in (1), and 173.20 (14) and 94.20 (9) for (2), respectively. One intramolecular and four classical intermolecular hydrogen-bonding interactions are observed in the crystal structure of (1), which give rise to an infinite three-dimensional network. A similar situation (one intramolecular and four classical intermolecular hydrogen-bonding interactions) is observed in the crystal structure of (2). Various -interactions are present in (1) between the platinum(II) atom and the centroid of one of the five-membered rings formed by Pt, As, C, N, O with a distance of 3.7225 (7) Å , and between the centroids of five-membered (Pt, As, C, N, O) rings of neighbouring molecules with distances of 3.7456 (4) and 3.7960 (6) Å . Likewise, weak -interactions are observed in (2) between the platinum(II) atom and the centroid of one of the five-membered rings formed by Pt, As, C, N, O with a distance of 3.8213 (2) Å , as well as between the Cl atom and the centroid of a symmetry-related five-membered ring with a distance of 3.8252 (12) Å . Differences between (2) and the reported polymorph . Angew. Chem. Int. Ed. 52, 10749-10752] are discussed.

Chemical context
Platinum and arsenic compounds have shown great versatility in terms of applications in the biological and medicinal fields (Reedijk, 2009). Platinum compounds are still the most widely used drugs in the fight against cancer in spite of the serious side effects and the resistance of some types of cancers (Miller et al., 2002;Basu & Krishnamurthy, 2010;Jakupec et al., 2003;Kauffman et al., 2010;Wheate et al., 2010;Rosenberg et al., 1965;Marino et al., 2017;Aabo et al., 1998;Kelland, 2007;Shi et al., 2019). Tumoral malignancies have a high lethality rate and are among the most widespread and difficult diseases to treat. The need for the development of new drugs and treatment alternatives has increased as many of the available effective drugs are comparable and similar to each other (Ott, 2009;Burchenal, 1978). Platinum-based antitumour agents have guided and constructed the current tumor chemotherapy treatment, but the side effects complicate and inhibit their clinical application (Rosenberg et al., 1965;Marino et al., 2017;Basu & Krishnamurthy, 2010;Aabo et al., 1998;Kelland, 2007;Shi et al., 2019). Drug resistance is a major limiting factor in terms of the range of tumours that can be treated and the improvement of the therapy (Marino et al., 2017). Arsenic trioxide was approved by the FDA in 2000 for the treatment of acute promyelocytic leukemia, and since then several studies have shown that the combinatorial employment of arsenic and platinum-based cancer drugs has shown significant therapeutic potential Shen et al., 2004;Emadi & Gore, 2010;Zhang et al., 2009Zhang et al., , 2010. These results led to the synthesis of complexes containing both platinum and arsenic (Swindell et al., 2013;Miodragović et al., 2013Miodragović et al., , 2019, which were called arsenoplatins. Initial results indicate that these complexes are able to bypass drug-resistance mechanisms that lower the effect of cisplatin and have higher cytotoxicity than cisplatin in some cases. To date, the studies of Miodragović et al. ( , 2019 are the only crystallographic data available in the CCDC (Groom et al., 2016).
The structures reported here, [Pt(C 4 H 10 AsN 2 O 4 )Cl] (1), and [Pt(C 6 H 14 AsN 2 O 4 )Cl], (2), expand on this work and form part of an ongoing study on arsenoplatins, their solid-and solution-state behaviour and evaluation thereof.

Database survey
Two crystal structures similar to (1) were found after a search of the Cambridge Structure Database (CSD, Version 5.40, update of November 2019; Groom et al., 2016), both of which (ODOHAS, ODOHEW) were reported by Miodragović et al. (2013Miodragović et al. ( , 2019. They consist of the same arsenoplatin complex as (1), accompanied by an acetamide hemihydrate and acetamide solvent species in the unit cell, and crystallize in the P1 and P2 1 /n space groups, respectively. The search also revealed that (2) represents a polymorph, with the first crystal structure determination (ODOGOF; Miodragović et al., 2013) in the orthorhombic space group type Pbca, in contrast to space group type P2 1 /c of (2).

Structural commentary
In (1) the square-planar coordination environment around platinum(II) is defined by two nitrogen donor atoms, a chlorido ligand and the coordination to arsenic. In turn, arsenic is coordinated by two oxygen donor atoms, two hydroxyl ligands and by platinum(II), completing a trigonalbipyramidal coordination sphere (Fig. 1). The first (ODOHAS) of the other two structure reports with a chlorido[dihydroxybis(1-iminoethoxy)]arsanido]platinum(II) molecule  is different from (1) because of an acetamide solvent molecule in the unit cell and a different space group (P2 1 /n), and the second (ODOHEW) crystallizes in the same space group as (1) (P1) but with acetamide and hemihydrate solvent molecules in the unit cell. The bond lengths in the title compound compare very well with those in the two structures in literature. The Pt-As bond length of 2.2730 (12) Å and the Pt-Cl bond length of 2.3401 (15) Å are similar to 2.2732 (3) and 2.3272 (8) Å for ODOHEW, and 2.2729 (2) and 2.3328 (6) Å for ODOHAS. The Pt-N bond lengths vary between 1.999 (4) and 2.005 (4) Å , the As-O bond lengths between 1.898 (3) and 2.107 (3) Å , and the As-OH bond lengths between 1.722 (3) and 1.738 (3) Å . Overall, these molecular structures compare well. When comparing the Pt-As and Pt-Cl bond lengths to those of other platinum(II) complexes where As and Cl are in trans positions, it is clear that the Pt-As bond lengths do not vary significantly and range between 2.3333 (6) and 2.3599 (2) Å , while for the Pt-Cl bond lengths a greater variation is seen, in a range from 2.2917 (4) to 2.3927 (5) Å (Reinholdt & Bendix, 2017;Clegg, 2016;Dube et al., 2016;Imoto et al., 2017;Muessig et al., 2019). While the Pt-Cl length in (1) compares well with these trans complexes, the Pt-As bond length is somewhat smaller. The square-planar coordination around the central platinum(II) atom is distorted with N1-Pt1-N2 and N1-Pt1-As1 being 173.90 (13) and 85.18 (11) , respectively, deviating from the expected 180 and 90 . The trigonal-bipyramidal coordination around the arsenic atom is significantly distorted with O4-As1-O6, O1-As1-O2 and O1-As1-Pt1 being 106.98 (14) Molecular structures of (1) and (2), indicating the numbering schemes. Displacement ellipsoids are drawn at a probability level of 50%.
tively. Considering arsenic with a coordination number of 5, the index 5 parameter can be used to calculate any potential distortion (Addison et al., 1984). The 5 parameter is defined as ( -)/60 with the largest and the second largest angle in the coordination sphere and was calculated as 0.794 for (1), suggesting a significantly distorted trigonal-bipyramidal shape around arsenic ( 5 = 0 for an ideal square pyramid and 1 for an ideal trigonal bipyramid).
The coordination environments of the platinum and arsenic atoms in (2) are the same as in (1), i.e. Pt1 is coordinated by a chlorido ligand, two nitrogen donor atoms and arsenic, that is additionally bonded to two hydroxyl ligands and two oxygen donor atoms (Fig. 1). The Pt-As and Pt-Cl bond lengths of 2.2672 (8) Å and 2.3387 (11) Å in (2) are virtually identical with the bond lengths of 2.2687 (4) Å and 2.3361 (9) Å , respectively, in the orthorhombic polymorph reported by Miodragović et al. (2013). Again, these Pt-As and Pt-Cl bond lengths fit well into the ranges reported for other structures where As and Cl are in trans positions. The squareplanar coordination environment around the platinum(II) atom is similarly distorted in the structures of the two polymorphs, with the ideal 180 (N-Pt-N) and 90 (N-Pt-Cl) angles deviating at 173.59 (13) and 94.68 (9) for the structure determined by Miodragović et al. (2013) and 173.20 (14) (N1-Pt1-N2) and 94.16 (11) (N1-Pt1-Cl1) for (2), respectively. The largest deviation of the trigonal-bipyramidal coordination sphere of the arsenic atom in the polymorphic structures pertains to the Pt-As-OH angle, with reported values of 129.78 (10) and 124.67 (9) for the orthorhombic structure  and of 130.05 (11) and 124.46 (9) for (2). The 5 parameter for (2) is calculated as 0.711.

Figure 3
Illustration of the infinite three-dimensional frameworks formed by the hydrogen-bonding interactions in (1) and (2). Blue dashed lines indicate the infinite networks along the a axes, purple dashed lines along the b axes and gold dashed lines along the c axes. Hydrogen atoms not involved in the interactions were omitted for clarity.

Refinement
Crystal data and details of data collections and structure refinements are summarized in Table 3. Methyl and methylene hydrogen atoms were placed in geometrically idealized positions (C-H = 0.95-0.97 Å ) and constrained to ride on their parent atoms [U iso (H) = 1.5U eq (C) and 1.2U eq (C)]. The OH and NH hydrogen atoms were located in a difference-Fourier map and their positional parameters were constrained with O-H = 0.84 (2) Å and N-H = 0.89 (2) Å for (1), and N-H = 0.87 (2) Å for (2) with O-H distances fixed at 0.82 Å and with U iso (H) = 1.5U eq (O). For (2), the F c versus F o plot proved ten reflections to be outliers, and they were removed from the refinement as systematic errors.

Funding information
We would like to thank the University of the Free State and the South African National Research Foundation (NRF) for financial support. Part of this work is based on the research supported by the National Research Foundation.

Chlorido[dihydroxybis(1-iminoethoxy)arsanido-κN,As,N′]platinum(II) (1)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Pt1 0.07024 (2) 0.05975 (2) 0.24814 (2) 0.00525 (6)  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 1.53 e Å −3 Δρ min = −1.97 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.