Crystal structure of an iridium(III) complex of the [C(dppm)2] PCP pincer ligand system and its conjugate CH acid form

The crystal structures of [IrIII(CO)(C(dppm)2-κ 3P,C,P′)ClH]Cl and [IrIII(CO)(CH(dppm)2-κ 3P,C,P′)ClH]Cl2 have been determined. Both complexes show a slightly distorted octahedral coordinated IrIII centre. The PCP pincer ligand system is arranged in a meridional manner.

After the successful creation of the newly designed PCP carbodiphosphorane (CDP) ligand [Reitsamer et al. (2012). Dalton Trans. 41, 3503-3514; Stallinger et al. (2007). Chem. , the treatment of this PCP pincer system with the transition metal iridium and further the analysis of the structures by single-crystal diffraction and by NMR spectroscopy were of major interest. Two different iridium complexes, namely (bis{[(diphenylphosphanyl)methyl]di-phenylphosphanylidene}methane-3 P,C,P 0 )carbonylchloridohydridoiridium(III) chloride dichloromethane trisolvate, [Ir III (CO){C(dppm) 2 -3 P,C,P 0 }ClH]ClÁ-3CH 2 Cl 2 (1) and the closely related (bis{[(diphenylphosphanyl)methyl]di-phenylphosphanylidene}methanide(1+)-3 P,C,P 0 )carbonylchloridohydridoiridium(III) dichloride-hydrochloric acid-water (1/2/5.5), [Ir III (CO){CH(dppm) 2 -3 P,C,P 0 )ClH]Cl} 2 (2), have been designed and both complexes show a slightly distorted octahedral coordinated Ir III centre. The PCP pincer ligand system is arranged in a meridional manner, the CO ligand is located trans to the central PCP carbon and a hydride and chloride are located perpendicular above and below the P 2 C 2 plane. With an Ir-C CDP distance of 2.157 (5) Å , an Ir-CO distance of 1.891 (6) Å and a quite short C-O distance of 1.117 (7) Å , complex 1 presents a strong carbonyl bond. Complex 2, the corresponding CH acid of 1, shows an additionally attached proton at the carbodiphosphorane carbon atom located antiperiplanar to the hydride of the metal centre. In comparison with complex 1, the Ir-C CDP distance of 2.207 (3) Å is lengthened and the Ir-C-O values indicate a weaker trans influence of the central carbodiphosphorane carbon atom.

Chemical context
Based on a great number of investigations of iridium complexes in organic synthesis (Oro & Claver, 2009), on the large variety of metal-pincer ligand interactions and reactivities (Morales-Morales & Jensen, 2007;Choi et al., 2011), the catalytic and stoichiometric organometallic chemistry of iridium PCP pincer complexes attracted our attention.
Up to now, diverse PCP pincer systems have been generated and these systems are, in general, classified according to the charge of the central carbon atom. Both an anionic sp 2 or sp 3 hybridization of the central carbon atom is possible (Table 1) and the charge arises from the metallation of the pertinent C-H functionalities of the non-coordinated ligand subunits. Furthermore, neutral PCP pincer ligands containing a divalent carbon(II) donor atom, for instance an alkylidene carbene or a NHC, are well known (Table 1; Crocker et al., 1982). Moreover, PCP pincer complexes based on tropylium backbones have been reported. The cationic central carbon atom is part of a seven-membered six-electron arene fragment and because of the C-C bond lengths, designation as a cycloheptatrienylidene carbene is allowed (Table 1).
Our focus is on the creation of new iridium complexes containing a PCP ligand system with a neutral or a cationic central carbon atom, respectively. The central carbon is part of a carbodiphosphorane (CDP) functionality and can be described as a naked carbon atom or as a divalent carbon(0) atom in an excited singlet (1D) state stabilized by two tertiary phosphines via donor-acceptor interactions. Consequently, this central atom disposes of two lone-electron pairs and is able to interact with one or two Lewis acids (Petz & Frenking, 2010).
The protonated CDP ligand system [CH(dppm) 2 ]Cl enters an oxidative addition reaction with Vaska's compound [Ir I (CO)Cl(PPh 3 ) 2 ], forming the iridium PCP pincer CDP complex [Ir III (CO)(C(dppm) 2 -3 P,C,P 0 )ClH]Cl (1) (see reaction scheme). During this reaction sequence, the central carbon atom is deprotonated, becomes neutral and coordinates the iridium transition metal. Treatment of complex 1 with hydrochloric acid leads to the protonation of the central carbon atom and consequently to the formation of the conjugate CH acid of 1, the [Ir(CO)(CH(dppm) 2 -3 P,C,P 0 )-ClH]Cl 2 complex 2 (see reaction scheme). Relative to the hydrido ligand at the iridium transition metal, the additionally attached proton adopts a syn-or anti-periplanar conformation. In solution, the existence of both isomers can be demonstrated by the use of NMR spectroscopy. However, the examination of several crystals revealed only the anti-periplanar configuration of complex 2. Whether this is incidental or the crystallization is accompanied by the isomerization of the syn-periplanar to the anti-periplanar conformation is unclear.

Structural commentary
Complex 1 (Fig. 1) crystallizes in the monoclinic space group P2 1 /n and the asymmetric unit consists of one formula unit of 1 and three molecules of CH 2 Cl 2 . The structure can be divided into two parts, the [Ir III (CO)(C(dppm) 2 -3 P,C,P 0 )ClH] + monocation and the chloride counter-ion. The iridium transition metal centre exhibits an octahedral ligand system, formed by a meridional arranged C(dppm) 2 , relative to the C1 atom, a trans-coordinated carbonyl unit, and a chlorido and hydrido ligand located perpendicular to the meridional plane or more precisely trans to each other. The P1-Ir1-P4 angle of 170.69 (5) indicates a small deviation from the octahedral geometry and this value is larger compared to many related

Figure 1
Structure of complex 1 with displacement ellipsoids drawn at the 30% probability level. Solvent residues are omitted.
Iridium PCP pincer complexes. The environment of the CDP carbon atom C1 is strictly planar (sum of angles at C1 = 360 ; Table 2) and the C1-P2 and C1-P3 bond lengths are 1.697 (5) and 1.711 (5) Å , respectively. Not only the geometry, but also the bond lengths are characteristic for a carbodiphosphorane atom, which interacts with one Lewis acid (Petz & Frenking, 2010). In general, bond lengths are directly connected with the valence-bond structure of a carbon atom and an increasing of the valence state causes a significant expansion of the bond gaps [Csp 2 < C(carbene) < Csp 3 ]. Consequently, the Ir1-C1 separation of 2.157 (5) Å indicates an sp 3 hybridization of the carbodiphosphorane carbon atom, which is substantiated by the data collected in Table 1. Additionally, interactions (Table 3) between the chloride counter-ion and the methylene groups of the PCP pincer ligand system can be detected and the bond lengths of about 2.60 Å [Cl2Á Á ÁH2B(1 + x, y, z)] and 2.62 Å [Cl2Á Á ÁH3B(1 + x, y, z)] illustrate the location within the van der Waals radii. These C-HÁ Á ÁX interactions are a common feature of complexes containing dppm or related ligands (Jones & Ahrens, 1998). Moreover, the chloride counter-ion interacts with the hydrogen atoms of the CH 2 Cl 2 molecules as well, forming distances of about 2.59 Å [Cl2Á Á ÁH5B( 1 2 + x, 1 2 À y, 1 2 + z)] and 2.47 Å [Cl2Á Á ÁH6B(À 1 2 À x, 1 2 + y, 3 2 À z)]. The asymmetric unit of 2 comprises two [Ir III (CO)-(CH(dppm) 2 -3 P,C,P 0 )ClH]Cl 2 complex molecules (Fig. 2), four molecules of HCl and eleven molecules of water in total. Both complex molecules are distinctly asymmetric in the solid state. As a result of the threefold coordination of the transition metal by the PCP pincer ligand system, two fivemembered metallacycles are formed, each adopting an approximately envelope conformation. One methylene group (C3) and one phosphorus atom (P2) are positioned at the flap positions above the plane generated by the C1-C2-P1-Ir1 and C1-Ir1-P3-P4 atoms. Complex 2 crystallizes in the monoclinic space group P2 1 /n and the complex molecule can be described as one [Ir III (CO)(CH(dppm) 2 -3 P,C,P 0 )ClH] 2+ dication stabilized by two chloride counter-ions. Overall, complex 2 represents the conjugate CH acid of the [Ir III (CO)(C(dppm) 2 -3 P,C,P 0 )ClH]Cl complex (1). The carbodiphosphorane carbon atom additionally coordinates a second Lewis acid, the proton H1, which adopts an anti-periplanar conformation relative to the hydrido ligand H11. As a consequence, atom C1 forms a distorted tetrahedron with the directly coordinated atoms (sum of angles = 344.3 ). In comparison with complex 1, the values of the angles P2-C1-Ir1 and P3-C1-Ir1 are significantly reduced, whereas the P2-C1-P3 angles differs to a lesser extent ( Table 2). The coordination of a second Lewis acid causes a lengthening of the C1-P distances by about 0.098 Å , resulting in bond lengths in the range of P-C single bonds. Moreover, the Ir1-C1 distance [2.207 (3) Å ] is markedly longer compared to that of the conjugate base 1 [2.157 (5) Å ], as has also been observed in other carbodiphosphorane complexes (Petz et al., 2009;Reitsamer et al., 2012;Tonner et al., 2006). Furthermore, the protonation of the C1 atom leads to a decrease of the trans influence of the carbodiphosphorane carbon donor atom, confirmed by an shortening of the Ir-CO distance and an increasing of the carbonyl bond gap. Besides, C-HÁ Á ÁO and C-HÁ Á ÁCl interactions (Table 4) between the methylene groups of the dppm moieties and the water or HCl molecules can be detected, causing for example separations in the range of 2.61 Å [H2AÁ Á ÁO4(1 À x, 1 À y, 1 À z)], 2.89 Å [H2BÁ Á ÁCl7(x À 1 2 , Ày + 1 2 , z + 1 2 )], 2.51 Å [H3AÁ Á ÁCl8(x, 1 + y, z)] and 2.57 Å [H3BÁ Á ÁCl5(x, 1 + y, z)].

Synthesis and crystallization
All preparations were carried out under an inert atmosphere (N 2 ) using standard Schlenk techniques. The 1 H, 13 C and 31 P NMR spectra were recorded on a Bruker DPX 300 NMR spectrometer and were referenced against the 13 C/ 1 H solvent peaks of the solvents chloroform, methanol or the external 85% H 3 PO 4 standard, respectively. The phosphorus atoms in the NMR data are labelled as in Figs. 1 and 2.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. Refinement of complex 1 resulted in the location of the hydride hydrogen atom. The bond length was restrained to a distance of 1.6 Å and a fixed isotropic displacement parameter of 1.5U eq of iridium was applied. The hydrido ligand of complex 2 was also detected and refined isotropically without the use of bond restraints. Furthermore, the proton of the CDP carbon atom was spotted and refined with bond restraints of 0.98 Å . The hydrogen atoms of the water and solvent molecules could only be partially detected and were omitted. A determination of a 1:1 positional disorder of one water molecule (O4 and O4A) and one HCl or chloride (Cl10 and Cl1A) was possible. Eight chloride positions can be detected, which are occupied by a total of four chlorides and four hydrochloric acid units. The hydrogen-atom positions of the phenyl subunits and methylene groups were refined with calculated positions (C-H = 0.94 and 0.98 Å ) using a riding model with U iso (H) = 1.2U eq (C).
Acta Cryst. (2018). E74, 620-624 research communications where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 1.68 e Å −3 Δρ min = −0.91 e Å −3 Special details Experimental. All data sets were measured with several scans to increase the number of redundant reflections. In our experience this method of averaging redundant reflections replaces in a good approximation semi-empirical absorptions methods (absorption correction programs like SORTAV lead to no better data sets). 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. The hydrogen atom at Ir1 were found and must be refined with bond restraint of 1.6 angs. and a fixed isotropc displacement parameter of 1.5 times higher than Ueq of Ir1.

Special details
Experimental. All data sets were measured with several scans to increase the number of redundant reflections. In our experience this method of averaging redundant reflections replaces in a good approximation semi-empirical absorptions methods (absorption correction programs like SORTAV lead to no better data sets). 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. Two molecules in the asymmetric unit. Hydrogen atoms at Ir1 and Ir2 were found and refined isotropically. Hydrogens at C1 and C5 were also found but refined with bond restraints (d=0.98 angs.). Between the molecules is a network of hydrogen bonded water and hydrochloric acid molecules and chloride anions. The hydrogen atoms of these molecules could only partially found and were omitted. One water molecule (O4 and O4A) and one Hydrochloric acid or chloride (Cl10 and Cl1A) have a 1:1 position disorder. There are 8 Cl-positions in the network represented 4 chloride and 4 acid units.