Crystal structures of four new iridium complexes, each containing a highly flexible carbodiphosphorane PCP pincer ligand

The synthesis and crystal structures of four iridium–PCP pincer complexes, each containing a highly flexible carbodiphosphorane PCP pincer ligand, are discussed.


Chemical context
The syntheses of the title compounds are summarized in the Scheme. The substitution of the bridging chlorido ligands of [Ir(Cl)(cod)] 2 by the cationic PCP pincer ligand [CH(dppm) 2 ]Cl quantitatively affords the five-coordinate Ir(I) PCP pincer complex [Ir(cod)(CH(dppm) 2 -3 P,C,P)](Cl) 2 (1a). The central carbon of the PCP ligand is part of a protonated ISSN 2056-9890 carbodiphosphorane (CDP) functionality. Metathesis with Tl(OTf) gave the corresponding OTf salt (1b). These products represent the first examples of a non-meridional coordination mode of the PCP pincer ligand [CH(dppm) 2 ] + .
The central carbon of CDPs carries two lone electron pairs and is able to interact with one or two Lewis acids (Petz & Frenking, 2010). Consequently, the central carbon of the PCP pincer ligand of 2 is able to interact with another Lewis acid and can be converted to the conjugate CH acid [Ir(Cl) 2 (H)(CH(dppm) 2 -3 P,C,P)]Cl (3), upon treatment with aqueous hydrochloric acid.
The reaction of 2 with carbon monoxide results in the substitution of the chlorido ligand positioned trans to the hydrido ligand and affords [Ir(Cl)(H)(CO)(CH(dppm) 2 -3 P,C,P)]Cl (4). The isomer of 4 with the CO ligand positioned trans to the carbodiphosphorane carbon of the PCP pincer ligand has been synthesized via reaction of Vaska's complex with [CH(dppm) 2 ]Cl (Reitsamer et al., 2018). Related [Ir(Cl)(H)(CO)(PCP)] complexes with the H and CO ligands in a trans configuration have been obtained via addition of CO to the corresponding five-coordinated complexes [Ir(Cl)(H)(PCP)] (Goldberg et al., 2015;Segawa et al., 2009;Jonasson et al., 2015;Kuklin et al., 2006), or, in one case, by bubbling CO through a solution of [Ir(Cl)(H)(MeCN)(PCP)] in dichloromethane, with the H and Cl ligands being in a trans configuration (Silantyev et al., 2014). Both isomers of [Ir(Cl)(H)(CO)(PCP)], either with H and CO or H and Cl in a trans configuration have been structurally characterized for a triptycene-based PCP pincer ligand (Silantyev et al., 2014;Azerraf & Gelman, 2009) and a cyclohexyl-based PCP pincer ligand (Jonasson et al., 2015).

Structural commentary
The molecular structures of the four complexes are illustrated in Figs. 1-4, and selected bond distances and bond angles are given in Table 1. The structure of 1b ( Fig. 1) establishes an 18electron five-coordinate dicationic Ir I complex with two OTf À Figure 1 Structure of compound 1b, with atom labelling and 30% probability displacement ellipsoids. For clarity, only the ipso carbon atoms of the phenyl groups are shown, and the solvent molecules have been omitted.

Figure 2
Structure of compound 2, with atom labelling and 30% probability displacement ellipsoids. For clarity, only the ipso carbon atoms of the phenyl groups are shown, and the solvent molecules have been omitted.
The structure of 2 ( Fig. 2) consists of an octahedral Ir III coordination compound. The Ir center is coordinated by the PCP pincer, one hydrido and two chlorido ligands. The [C(dppm) 2 ] unit coordinates in a meridional manner; the Cl1 ligand is located trans to the central CDP carbon C1, ligands H1 and Cl2 are positioned normal to this plane and are trans to each other. The Ir1-C1 bond length amounts to 2.101 (5) Å and is comparatively short according to the weak trans influence of a chlorido ligand. With a P4-Ir1-P1 angle of 173.09 (5) , [C(dppm) 2 ] also showcases high structural flexibility. Both the planar environment of C1 and the P-C bond lengths within the CDP functionality are in keeping with CDPs interacting with one Lewis acid (Petz & Frenking, 2010). The configuration of the two five-membered rings of the PCP pincer system is somewhat dissimilar, as evidenced by a comparison of the corresponding angles which differ up to ca 8 (see Table 1). The structure of 3 (Fig.  3) exhibits a [Ir(Cl) 2 (H)(CH(dppm) 2 -3 P,C,P)] + complex cation, accompanied by a chloride counter-ion. Protonation of the CDP carbon C1 results in a distorted tetrahedral environment. The bond angles P2-C1-P3, P2-C1-Ir1 and P3-C1-Ir1 are reduced by ca 5-7 , as compared to the values for compound 2. As expected, due to protonation, the C1-P2/P3 bond lengths are now characteristic of P-C single bonds (Petz & Frenking, 2010). The orientation of the proton on C1 relative to the hydrido ligand H1 is anti-periplanar. Protonation of the CDP carbon yields a heterogeneous effect on Ir-donor distances: while the Ir1-C1 bond length is longer than in 2 [2.132 (4) Å cf. 2.101 (5) Å ], the Ir1-Cl1 bond length is shorter [2.405 (1) Å cf. 2.441 (2) Å ]. The two rings of the PCP pincer system are different as has been emphasized for compound 2.
The structure of compound 4 consists of a [Ir(Cl)(H)-(CO)(C(dppm) 2 -3 P,C,P)] + complex cation and a chloride counter-ion (Fig. 4)  Structure of compound 3, with atom labelling and 30% probability displacement ellipsoids. For clarity, only the ipso carbon atoms of the phenyl groups are shown, and the solvent molecules have been omitted.

Figure 4
Structure of compound 4, with atom labelling and 30% probability displacement ellipsoids. For clarity, only the ipso carbon atoms of the phenyl groups are shown, and the solvent molecules have been omitted. Table 1 Selected bond lengths (Å ) and bond angles ( ) for compounds 1b-4.  (14) 106.83 (17) PCP pincer in a meridional mode, with one chlorido ligand trans to the central CDP carbon atom and one hydrido and one carbonyl ligand trans to each other. Compared to compound 2, the CO ligand causes a lengthening of the Ir1-C1 and the Ir-P bonds, while both the Ir1-C1 and the Ir1-H1 bonds are shortened (Table 1). In contrast to 2 and 3, the angles formed by the two rings of the pincer system are quite similar. The planarity around atom C1 and the C1-P2/P3 bond lengths confirms a CDP with one Lewis acid attached.

Supramolecular features
In all four crystal structures the CH 2 groups and the central CH group of the [CH(dppm) 2 ] + unit interact with solvate molecules and anions. It has been pointed out that such C-HÁ Á ÁX interactions are a common feature of complexes containing dppm or related ligands (Jones & Ahrens, 1998). The most significant hydrogen-bonding interactions in the crystals of the four compounds are given in Tables 2-5, and illustrated in Figs. 5-8. In the crystal of 1b (Fig. 5), two neighbouring molecules are linked via C-HÁ Á ÁO hydrogen bonds involving two O atoms (O4 and O5) of two inversion-related OTf À anions. Each complex cation is linked to the ethyl acetate solvate molecule by a C3A-H3AÁ Á ÁO7 hydrogen bond and to the other OTf À anion by three (trifurcated) C-HÁ Á ÁO3 hydrogen bonds.
In the crystal of 2 (Fig. 6), molecules are linked by C-HÁ Á ÁCl hydrogen bonds, forming a 2 1 helix propagating along the b-axis direction. The acetone solvate molecule is linked to the complex molecule by a C-HÁ Á ÁO hydrogen bond.
In the crystal of 3 (Fig. 7), the free Cl À anion is linked to the complex cation by three C-HÁ Á ÁCl hydrogen bonds.
In the crystal of 4 ( Fig. 8 A view along the a axis of the crystal packing of compound 1b. Only the H atoms involved in the most significant intermolecular interactions (Table 2) have been included. The ethyl acetate solvate molecule is shown in ball-and-stick mode.

Synthesis and crystallization
The syntheses of the title compounds are summarized in the Scheme. All preparations were carried out under an inert atmosphere (N 2 ) by the use of standard Schlenk techniques. The 1 H, 13 C and 31 P NMR spectra were recorded on a Bruker DPX 300 NMR spectrometer (300 MHz) and were referenced against 13 C/ 1 H solvent peaks of the solvents or an external 85% H 3 PO 4 standard, respectively. The phosphorus atoms in the NMR data are labelled in the same way as in the figures. A view along the c axis of the crystal packing of compound 3. Only the H atoms involved in the most significant intermolecular interactions (Table 4) have been included. The free Cl À anions and the disordered water molecules are shown in ball-and-stick mode.

Figure 8
A view along the a axis of the crystal packing of compound 4. Only the H atoms involved in the most significant intermolecular interactions (Table 5) have been included. Table 4 Hydrogen-bond geometry (Å , ) for 3.  Table 5 Hydrogen-bond geometry (Å , ) for 4. Synthesis of complexes 1a and 1b: [IrCl(cod)] 2 (8.5 mg; 0.0125 mmol) and [CH(dppm) 2 ]Cl (20.5 mg; 0.025 mmol) (Reitsamer et al., 2012) were dissolved in CHCl 3 (0.6 ml), whereupon 1a formed instantaneously. Immediately after, a solution of TlOTf (17.7 mg; 0.05 mmol) in MeOH (0.1 ml) was added and the mixture was stirred for 15 min. The TlCl precipitate was removed and the volatiles evaporated in vacuo. Single crystals of 1b were obtained by layering a solution of the residue in CH 2 Cl 2 with EtOAc.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. The hydrogen atoms at C1, C4=C5 and C8=C9 of 1b were located in a difference-Fourier map and refined with bond restraints (C-H = 0.96 Å for H1 and 0.93 Å for H4, H5, H8 and H9). Both OTf À anions show positional disorder in the occupancy ratio of 0.7:0.3. The solvent molecule CH 2 Cl 2 also shows positional disorder with the ratio 0.7:0.3; the hydrogen atoms of this disordered molecule were omitted.
In 2, the metal-bound hydrogen atom was located in a difference-Fourier map and refined with the bond restraint Ir-H = 1.6 Å , since free refinement resulted in an unrealistically long bond distance of 1.88 Å . The solvent acetone molecule is slightly disordered with a solved positional disorder for one methyl group, namely C6:C6A (ratio 0.5:0.5). Solvent hydrogen atoms could not be localized and were omitted.
In 3, positional disorder of the anion Cl3:Cl3A was found in an occupancy ratio of 0.667:0.333. Hydrogen atoms H1 and H1A were located in a difference-Fourier map and freely refined. The water solvent molecules show higher temperature factors and are slightly disordered, but this disorder was not solved; therefore the oxygen atoms (O5 and O6 with half occupancy) were refined isotropically and their hydrogen atoms were omitted.
In 4, atom H1 was located in a difference-Fourier map and refined with bond restraint Ir-H = 1.6 Å . Hydrogen atoms of the MeOH and H 2 O solvate molecules were omitted. One chloride anion is positionally disordered with an occupancy ratio of 0.5:0.5 for Cl2 and Cl2A. Possibly because of this disorder, two MeOH positions C6-O3 and C7-O4 are only half occupied; also, a water molecule is split over four positions (O5, O5A, O5B and O5C) with an occupancy of 0.25 for each; they were refined isotropically.

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. Refinement. Small crystal with low diffraction at higher 2 theta angles. Hydrogen atom at Ir was localized and refined isotropically with bond restraint (d = 1.6 Å), because of a too long bond distance of 1.88 Å by free refinement. The solvent molecule aceton is slightly disordered with one solved positional disorder for a methyl group C6:C6A at ratio 1:1. Hydrogen atoms at solvent could not be localized and were omitted.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.85 e Å −3 Δρ min = −1.45 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. Refinement. Hydrogen H1 at Ir1 was found and refined with bond restraint (d = 1.6 Å). Hydrogen atoms at solvent MeOH and H2O were omitted. One Cl-anion is positionally disordered in ratio 1:1 for Cl2 and Cl2A. Maybe because of this disorder two MeOH positions C6-O3 and C7-O4 are only half occupied, also a water molecule, which is split in four positions with occupancy of 0.25 for each position. O5, O5A, O5B and O5C were refined with isotropic displacement par..