Crystal structure of a palladium(II) complex containing the wide bite-angle bis(selenium) ligand, cis-[(tBuNH)(Se)P(μ-NtBu)2P(Se)(NHtBu)]

A palladium(II) complex containing a bis(selenium) ligand based on cyclodiphosph(V)azane, cis-[(tBuNH)(Se)P(μ-NtBu)2P(Se)(NHtBu)] has been synthesized and structurally characterized. The crystal structure revels chelation of ligand through selenium donors with a natural bite-angle of 110.54 (1)°


Chemical context
Cyclodiphosph(III)azanes are four-membered P III -N ring systems with general formula, cis-[RP(-N t Bu) 2 PR]. The planar nature of the four-membered ring favors a bridging bidentate coordination mode through phosphorus donors rather than chelation, to afford structurally interesting macrocyclic and polymeric complexes (Balakrishna, 2016). The main-group chemistry of the corresponding P V analogue cyclodiphosph(V)azanes, cis-[R(E)P(-N t Bu) 2 P(E)R] (E = O, S, Se, and Te; R = NH t Bu) and its amide derivatives has been studied extensively by Stahl (2000) and Briand and co-workers (Briand et al., 2002). While examples of coordination of cyclodiphosph(V)azanes with transition metal ions are scarce, the sulfur and selenium derivatives are especially interesting as they have a special affinity for soft metals and have the potential to form complexes with wide natural bite-angles through chelation (Chivers et al., 2001). Several late transitionmetal complexes containing wide natural bite-angle chelating ligands (L-M-L = 100-134 ) have been developed over the years and have shown promising catalytic activity for several reactions (Kamer et al., 2001). The majority of these wide biteangle ligands are phosphorus and/or nitrogen donor ligands (Motolko et al., 2017;Czauderna et al., 2015). Herein we report the synthesis and crystal structure of the palladium(II) complex (II) with a wide bite-angle selenium ligand based on cyclodiphosph(V)azane cis-[( t BuNH)(Se)P(-N t Bu) 2 P(Se)-(NH t Bu)], (I).

Supramolecular features
In the crystal, the molecules are connected through weak N-HÁ Á ÁCl and C-HÁ Á ÁCl hydrogen-bonding interactions (Fig. 2, Table 1). Interestingly, in the solid-state structure II, the exocyclic nitrogen substitutents are arranged in an endo, endo fashion, whereas in ligand I they are arranged in exo, endo orientations . An overlay plot of the free ligand molecule I with the ligand fragment of II is shown in Fig. 3. This conformational change upon coordination is possibly caused by the formation of intermolecular hydrogenbonding interactions. A similar conformational change influenced by hydrogen-bonding interactions has previously been noted (Chandrasekaran et al., 2011).
A dichloromethane solution (10 mL) of [Pd(COD)Cl 2 ] (100 mg, 0.35 mmol) was added dropwise to a solution of cis- Perspective view of palladium complex II with displacement ellipsoids drawn at the 50% probability level. All H atoms have been omitted for clarity except at N3 and N4. Only the major occupancy component of the disordered t-butyl group is shown. Table 1 Hydrogen-bond geometry (Å , ).  (7) for 6 h. The solution was then concentrated to 10 mL, diluted with 10 mL of pentane, and stored at 248 K for a day to afford the analytically pure orange crystalline product. X-ray quality crystals were obtained by slow evaporation from a DMF solution at room temperature. Yield: 76% (206 mg, 0.067 mmol), m.p. 455-457 K.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms attached to carbon were placed in calculated positions (C-H = 0.98 Å ), while those attached to nitrogen were placed in locations derived from a difference-Fourier map and their coordinates adjusted to give N-H = 0.91 Å . All were included as riding contributions with U iso (H) = 1.2-1.5 times those of the parent atoms. The t-butyl group attached to N4 was modeled as rotationally disordered over two sites of approximately equal population. These were refined with restraints so that the geometries of the two components of the disorder are comparable. Overlay of the uncoordinated ligand I (gray) with the coordinated ligand fragment in complex II (purple).  Computer programs: APEX2 and SAINT (Bruker, 2013), SHELXT (Sheldrick, 2015a), SHELXL2014 (Sheldrick, 2015b), Mercury (Macrae et al., 2006) and SHELXTL (Sheldrick, 2008).

Special details
Experimental. The diffraction data were obtained from 3 sets of 400 frames, each of width 0.5° in ω, collected at φ = 0.00, 90.00 and 180.00° and 2 sets of 800 frames, each of width 0.45° in φ, collected at ω = -30.00 and 210.00°. The scan time was 10 sec/frame. 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. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. Hatoms attached to carbon were placed in calculated positions (C-H = 0.98 Å) while those attached to nitrogen were placed in locations derived from a difference map and their coordinates adjusted to give N-H = 0.91 Å. All were included as riding contributions with isotropic displacement parameters 1.2 -1.5 times those of the attached atoms. The tbutyl group attached to N4 is rotationally disordered over two sites of approximately equal population. These were refined with restraints that the geometries of the two components of the disorder be comparable.