Crystal structures of [IrCl2(NHCHPh)((dppm)(C(N2dppm))-κ3 P,C,P′)]Cl·5.5MeCN and [IrI(NHCHPh)(((dppm)C(N2))-κ2 P,C)(dppm-κ2 P,P′)]I(I3)·0.5I2·MeOH·0.5CH2Cl2: triazene fragmentation in a PCN pincer iridium complex

In this communication, two compounds and their respective crystal structures, obtained via fragmentation of the triazene moiety in a PCN pincer iridium complex, are discussed. One showcases a novel (dppm)C(N2dppm) PCP pincer, the other contains a (dppm)C(N2) diazomethylenephosphorane moiety.


Chemical context
A peculiarity of triazenes is that their N-N bonds are comparatively easily cleaved. This may result, other than N 2 extrusion reactions, in diazonium and quaternary ammonium moieties, in diazo compounds and amines (Baumgarten, 1967;Schroen & Brä se, 2005), or in diazo compounds and amides (Myers & Raines, 2009) depending on the triazene substitution pattern. By taking advantage of their reactivity, the transformation of organic azides into diazo compounds via triazene intermediates has developed into a broad synthetic route to diazo compounds (Myers & Raines, 2009).
When the intermediate (in the case of benzyl azide) is brought into contact with air, pale-yellow crystals of compound 3 separate within a few hours. It contains a novel PCP pincer system involving one seven-and one fivemembered ring. The difference to the PCP pincer ligand of the starting complex 1 is that the pincer of 3 has an N 2 moiety inserted into one P-C bond of the CDP functionality of C(dppm) 2 . Regarding the reaction mechanism, we propose that first, the Ir I center of the intermediate is oxidized by atmospheric oxygen. This is presumably followed by a homolytic cleavage of the N2-N3 bond (numbering according to the crystal structure of 3) of (BnN 3 )C(dppm), producing the diazomethylenephosphorane (dppm)C(N 2 ) involving N1 and N2, and a benzylnitrene moiety containing N3.
Via an intramolecular Staudinger reaction (Staudinger & Meyer, 1919a,b) of the diazo functionality of (dppm)C(N 2 ) with the pendent phosphine of the monodentate dppm ligand, the phosphazine (dppm)C(N 2 dppm) is formed and subsequently acts as PCP pincer ligand. In this ligand, the central divalent carbon (Petz & Frenking, 2010) of (dppm)C(N 2 dppm) connects to one tertiary phosphine of the dppm unit, and to a diazophosphorane (Murahashi et al., 2005). The benzylnitrene undergoes a 1,2-hydride shift, thus producing a benzaldimine moiety that remains in the coordination sphere of iridium. In this context, it is very noteworthy that the scission of the N1-N2 bond occurs in the course of the aforementioned transformation of organic azides into diazo compounds via triazenes (Myers & Raines, 2009).
In a related fragmentation reaction, compound 4 was obtained through treatment of [Ir((BnN 3 )C(dppm)-3 P,C,N)(dppm-2 P,P 0 )]Cl (2) with hydroiodic acid. It is apparent that a rupture of the N1-N2 bond (numbering as in the structure of 4) of (BnN 3 )C(dppm) occurred again, resulting in the formation of a diazomethylenephosphorane (dppm)C(N 2 ) and a benzylnitrene part. However, in this case, the diazo functionality remains unchanged, since in contrast to the formation of 3, no free phosphine functionality is available. The benzylnitrene unit again undergoes a 1,2-hydride shift and, as a benzaldimine, coordinates to the Ir metal center.

Structural commentary
The structure of 3 ( Fig. 1) shows a six-coordinate monocationic Ir III complex and one chloride counter-ion. The asymmetric unit contains one formula unit and 5.5 molecules of MeCN. Selected bond lengths and bond angles of 3 are given in Table 1. The most significant intramolecular interactions are listed in Table 2. The iridium center is coordinated by the facial PCP pincer system, which involves one sevenmembered IrC(N 2 dppm) ring and one five-membered IrC(dppm) ring. A benzaldimine ligand is positioned trans to the phosphorus donor of the five-membered ring, the remaining two coordination sites being occupied by chlorido ligands cis to each other. The deviations of the angles C1-Ir1-Cl1 = 170.06 (13) and N3-Ir1-P1 = 169.02 (11) from a regular octahedral geometry indicate some strain in the pincer system. Both the N1-C1 bond length [1.280 (5) Å ] and the N1-N2 bond length [1.445 (5) Å ] are typical for a C N double bond and an N-N single bond, respectively. The P3-N2 bond length [1.586 (4) Å ] is in the range of P N double bonds observed for iminophosphoranes (Ireland et al., 2010;Peng et al., 2011;Sun et al., 2011). Corresponding bond lengths in other phosphazene systems exhibit values of 1.62-1.64 Å for P-N, 1.36-1.39 Å for N-N and 1.31 Å for C-N. (Bethell et al., 1992;Supurgibekov et al., 2011;Galina et al., 2013;Nikolaev et al., 2016). The P2-C1 bond length [1.836 (4) Å ] indicates a single bond. The environment around C1 is strictly planar (sum of the angles amounts to 359.7 ). Examination of the C4-N3 bond length within the benzaldimine ligand [1.270 (6) Å ] indicates a double bond and is almost identical to that observed in compound 4 [1.267 (8) Å ] and a previously reported iridium benzaldimine complex [1.260 (6) Å ] involving a phosphorus donor atom trans to the benzaldimine nitrogen donor (Albertin et al., 2008). The most striking intramolecular interaction of 3 is the hydrogen bond N3-H3NÁ Á ÁN2 [HÁ Á ÁA 2.15 (5) Å , D-HÁ Á ÁA 138 (4) ], while other intramolecular interactions involve atoms N1 and Cl1 and the various phenyl rings ( Table 2).
The structure of 4 ( Fig. 2) consists of a six-coordinate dicationic Ir III complex, one iodide and one triiodide counterion. The asymmetric unit contains one half molecule of dichloromethane and iodine and one molecule of methanol. Selected bond lengths and angles of 4 are summarized in Table 1. The most significant intramolecular interactions are listed in Table 3. The iridium center is coordinated by the bidentate ligand (dppm)C(N 2 ), which forms a five-membered chelate ring via one C and one P donor atom. A fourmembered ring is formed by a bidentate dppm ligand and is oriented perpendicular to the plane of the five-membered ring A view of the molecular structure of the cation of compound 4, with displacement ellipsoids drawn at the 30% probability level and atom labelling. Only the ipso carbon atoms of the dppm phenyl groups are shown, the anions and solvate molecules have been omitted for clarity.  Symmetry code: (i) Àx þ 3 2 ; y þ 1 2 ; Àz þ 1 2 .

Figure 1
A view of the molecular structure of the cation of compound 3, with displacement ellipsoids drawn at the 30% probability level and atom labelling. Only the ipso carbon atoms of the dppm phenyl groups are shown, and solvate molecules have been omitted for clarity.

Supramolecular features
In the crystal of 3, the cationic complexes are interconnected through the chloride anions via essentially C-HÁ Á ÁCl3 hydrogen bonds. The most significant hydrogen-bonding interactions are given in Table 2. Of these, two stem from phenyl groups and one from a methylene group of the PCP pincer's dppmN 2 part (H3AÁ Á ÁCl3 2.63 Å ). It is worth mentioning that such interactions are frequently observed in dppm and related ligands (Jones & Ahrens, 1998). A graphical representation of these interactions is given in Fig. 3. Effectively, the C-HÁ Á ÁCl3 hydrogen bonds link the cationic complexes, forming chains propagating along the b-axis direction.

Synthesis and crystallization
The syntheses of the title compounds are summarized in the reaction scheme. 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 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 a* axis of the crystal packing of 3, highlighting some of the intra-and intermolecular interactions. For clarity, solvate molecules and non-involved H atoms have been omitted, and for uninvolved phenyl moieties, only the ipso carbon atoms are displayed.

Figure 4
A view along the c axis of the crystal ordering of 4, highlighting some of the intermolecular interactions. For clarity, uninvolved solvate molecules and H atoms have been omitted, and for non-involved phenyl groups, only the ipso carbon atoms are displayed.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The acetonitrile solvent molecules in the crystal lattice of 3 are severely disordered in position and occupation. At least 5.5 molecules in the asymmetric units were refined. Occupation values were varied to give a reasonable isotropic displacement factor. All C-and N-atoms of solvent molecules were refined isotropically with bond restraints, the hydrogen atoms were omitted. The proton on N3 was freely refined.
The hydrogen atom at N1 of 4 was found and refined with a bond restraint of 0.87 (2) Å . The I 3 À anion (I4-I6) is positionally disordered (ratio roughly 1:1), as is the I À anion with a ratio I2:I2A of 9:1. The dichloromethane solvent molecule lies near a twofold rotation axis (disorder) and was refined with an occupancy of 0.5. Another disorder occurs for the solvent methanol with a ratio of 1:1. The C and O atoms of methanol   were refined isotropically with bond restraints of 1.40 Å . The hydrogen atoms of methanol were calculated, those of dichloromethane omitted. All other H atoms were positioned geometrically (C-H = 0.94-0.98 Å ) and refined as riding with U iso (H) = 1.2-1.5 U eq (C). For both structures, data collection: COLLECT (Nonius, 1998); data reduction: DENZO and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); software used to prepare material for publication: Mercury (Macrae et al., 2008) and publCIF (Westrip, 2010).

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 at N3 was found and refined normally with isotropic displacement parameters. The solvent molecules of acetonitrile in the crystal lattice are strongly disordered in position and occupation. At least a sum of 5.5 molecules in the asymmetric units were refined. Occupation values were varied to a more or less reasonable isotropic displacement factor. All C and N-atoms of solvents were isotropically refined with a sum of bond restraints and hydrogen atoms were omitted.

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 good approximation, semi-empirical absorption correction methods (programs such as 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. Hydrogen at atom at N1 was found and refined isotropically with a bond restraint of 87 pm. The I 3anion (I4-I6) is approx. 1:1 positionally disordered, as is the Ianion with an I2:I2A ratio of 9:1. The solvent dichloromethane lies nearby a twofold rotation axis (disorder) and was refined with an occupancy of 0.5. A further disorder occurs for the solvent methanol with ratio 1:1. C-and O-atoms of methanol were refined isotropically with bond restraints of 140 pm. Hydrogen atoms at dichloromethane were omitted.