Comparison of molecular structures of cis-bis[8-(dimethylphosphanyl)quinoline]nickel(II) and -platinum(II) complex cations

Owing to the intramolecular steric hindrance between the mutually cis-positioned 8-(dimethylphosphanyl)quinoline (Me2Pqn) ligands, the nickel(II) complex of cis-[Ni(Me2Pqn)2]2+ shows a remarkable tetrahedral distortion of the NiII coordination geometry. In contrast, the corresponding platinum(II) complex of cis-[Pt(Me2Pqn)2]2+ exhibits a typical square-planar PtII coordination center, but large envelope-type distortions of the five-membered chelate planes are observed.

1. Chemical context 8-Quinolylphosphanes are an intriguing class of ligands because they form a planar asymmetrical five-membered chelate ring via coordination through a phosphane-P atom having a strong -donating character and an imine-N atom incorporated in a -conjugated quinoline ring (Salem & Wild, 1992;Scattolin et al., 2017;Cai et al., 2018). The electronic properties of these ligands, in particular their -bonding nature, may stabilize unusual electronic states of their transition-metal complexes Hashimoto et al., 2010;Hopkins et al., 2019). In addition, the steric requirement from the planar quinoline moiety often has a significant influence on the properties of their metal complexes. For example, the nickel(II), palladium(II) and platinum(II) complexes containing two 8-(diphenylphosphanyl)quinoline (Ph 2 Pqn) in the cis(P,P) configuration exhibit a severe distortion of the square-planar coordination geometry around M II (M = Ni, Pd or Pt; Suzuki, 2004;Hashimoto et al., 2010;. The dimethylphosphanyl analogue, 8-(dimethylphosphanyl)quinoline (Me 2 Pqn), is an interesting derivative, because it would give a stronger trans influence, ISSN 2056-9890 which could affect the steric congestion between the intramolecular ligands. However, the transition-metal complexes bearing Me 2 Pqn are limited to only those listed in section 4: Database survey, all of which were reported by our group. In 1995 we reported the preparation and crystal structure of (SP-4-2)-[Pd(Me 2 Pqn) 2 ](BF 4 ) 2 , but the crystal structures of the corresponding Ni II and Pt II complexes were not compared.
In the crystal structure of 2, three F atoms of one of the BF 4 À anions show disorder over two sets of positions: (F2A, F3A and F4A) and (F2B, F3B and F4B). The occupancy parameters of these atoms were refined with suitable restrictions and found to be 0.573 (10) and 0.427 (10) for the A-set atoms and the B-set atoms, respectively.
In both crystals, two Me 2 Pqn ligands coordinate to a metal(II) center in the bidentate 2 P,N mode to form a cis- An ellipsoid plot of the molecular structures in [Pt(Me 2 Pqn) 2 ](BF 4 ) 2 Á-CH 3 CN (2), showing the atom-numbering scheme, with ellipsoids drawn at the 50% probability level. The minor component atoms (F2B, F3B and F4B) of the positionally disordered F atoms are omitted for clarity. Table 1 Selected geometric parameters (Å , ) for 1.

Figure 1
An ellipsoid plot of the molecular structures in [Ni(Me 2 Pqn) 2 ]-(ClO 4 ) 2 ÁCH 3 NO 2 (1), showing the atom-numbering scheme, with ellipsoids drawn at the 50% probability level. = Ni or Pt), having a roughly square-planar coordination geometry (Figs. 1 and 2, Tables 1 and 2). For the group 10 metal(II) complexes bearing 8-quinolylphosphanes, it was revealed that most of the bis( 2 P,N)-type complexes have a similar geometrical structure, except for those of the halide complexes (Suzuki, 2004;, because the strong trans influence of the phosphane donor groups makes the mutually trans(P,P) configuration thermodynamically unstable. The Ni-N bond lengths in 1 are 1.970 (2) (Yang et al., 2007) of 0.199 (1) . This is due to the steric requirement from the planar quinoline moiety located in the mutually cis positions around the Ni II center. In the analogous MePhPqn and Ph 2 Pqn complexes, the 4 values are 0.273 (1) and 0.189 (2) , respectively. By contrast, the 4 value of the Pt II complex in 2 is only 0.014 (1) , indicating a nearly perfect planar coordination geometry around the Pt II center. The corresponding value in [Pt(Ph 2 Pqn) 2 ](BF 4 ) 2 is 0.149 (2) . It is obvious that the present planar structure of the Pt II center in 2 is a rare example. In this complex, the interligand steric interaction expected for the mutually cis-positioned quinoline groups could be reduced by envelope-type bending of the planar Me 2 Pqn chelate coordination, that is, the displacement of the Pt II metal center from the ideal plane defined by the chelate ring of 8-quinolylphosphanes. The dihedral angle, ' C , between the [Pt,P,N] coordination plane and the [PCCN] phosphanylquinoline planes in 2 are 21.53 (16) and 24.76 (16) , and the displacement of the Pt1 atom from the ideal quinoline [C 9 H 6 N] planes is 0.579 (5) and 0.550 (5) Å . The two quinoline planes are nearly parallel, with the dihedral angle between them being only 7.99 (10) . Such a synchronized bending deformation of two chelate coordination (Fig. 4) acts to reduce the steric congestion effectively. The corresponding ' C values for 1 are 17.44 (9) and 19.76 (9) , and the dihedral angle between the two quinoline planes is obviously large, at 33.35 (6) . Interestingly, the analogous palladium(II) complex, [Pd(Me 2 Pqn) 2 ](BF 4 ) 2 , has a 4 value of 0.096 (2) , which is in between those of the present Ni II and Pt II complexes.      A packing drawing of [Ni(Me 2 Pqn) 2 ](ClO 4 ) 2 CH 3 NO 2 (1) along the crystallographic b axis (two unit cells are shown). Color code: Ni, dark green; Cl, light green; P, orange; O, red; N, blue; C, black and H, gray.

Supramolecular features
In the crystal structure of 1, there are two ClO 4 À anions and a CH 3 NO 2 solvent molecule, in addition to the [Ni(Me 2 Pqn) 2 ] 2+ complex cation in the asymmetric unit. The asymmetric unit of the Pt II complex, 2, contains a [Pt(Me 2 Pqn) 2 ] 2+ complex cation, two BF 4 À anions (in one of which the positions of three F atoms are disordered) and a CH 3 CN solvent molecule. In the crystal structures of both 1 and 2 (Figs. 5 and 6, respectively), no remarkable intermolecular stacking or hydrogen-bonding interactions are observed.

Synthesis and crystallization
The ligand, Me 2 Pqn, and the nickel(II) complexes, [Ni(Me 2 Pqn) 2 ](ClO 4 ) 2 , were prepared according to the method reported previously . Single crystals of 1 suitable for an X-ray diffraction study were obtained by recrystallization from nitromethane by diffusion of diisopropyl ether. The platinum(II) complex, [Pt(Me 2 Pqn) 2 ](BF 4 ) 2 , was prepared by the following method. A methanol (5 ml) solution of Me 2 Pqn (0.76 mmol) was added dropwise with stirring to a dichloromethane solution (10 ml) of [PtCl 2 (EtCN) 2 ] (0.105 g, 0.278 mmol), and the mixture was stirred for 24 h at room temperature. After removal of the resulting precipitate, the filtrate was concentrated to ca 5 ml using a rotary evaporator. A large excess amount of a methanol solution of NaBF 4 was added, and the mixture was stirred for 1 h at room temperature. The resulting pale-yellow precipitate was collected by filtration, washed with water (5 ml) and diethyl ether (10 ml), and dried in vacuo. Colorless platelet-shaped crystals of [Pt(Me 2 Pqn) 2 ](BF 4 ) 2 ÁCH 3 CN (2) were obtained by recrystallization from an acetonitrile solution by diffusion of diisopropyl ether. Yield: 0.126 g (61%). Analysis calculated for C 24 H 27 B 2 F 8 N 3 P 2 Pt: C, 35.37; H, 3.24; N, 3.75%. Found (after completely drying): C, 35.39; H, 2.89; N, 3.74%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were refined using a riding model, with C-H = 0.95 (aromatic) or 0.98 (methyl) Å and U iso (H) = 1.2U eq (C). In the analysis of 2, two sets of F atoms for one of the two BF 4 À anions were introduced as positionally disordered atoms, and their occupation parameters were refined with suitable restrictions [the final major:minor occupancy ratio was 0.573 (10):0.427 (10)].

Funding information
This work was partly supported by JSPS KAKENHI Grant No. 18 K05146.

(SP-4-2)-cis-Bis[8-(dimethylphosphanyl)quinoline-κ 2 N,P]nickel(II) bis(perchlorate) nitromethane monosolvate
(1)  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.

(SP-4-2)-cis-Bis[8-(dimethylphosphanyl)quinoline-κ 2 N,P]platinum(II) bis(tetrafluoroborate) acetonitrile monosolvate (2)
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 )