X-ray crystal structure of [L 2Ag3]+[OTf]−·5C6D6: a monoanionic bisphosphinimine ligand supported trisilver complex

A trisilver complex stabilized by two monoanionic bisphosphinimine ligands is reported. Noteworthy asymmetry at the cluster core is observed. This is the first example of a trisilver complex supported by monoanionic bisphosphinimine ligands.


Structural commentary
The title compound [L 2 Ag 3 ] + [OTf] À Á5C 6 D 6 (L = 2,5-(4-i PrC 6 H 4 N PPh 2 )C 4 H 2 N; OTf = OSO 2 CF 3 ) (I) has three silver cations coordinated by two tridentate, anionic, pyrrolebased bisphoshinimine ligands (Fig. 1). The trisilver complex is highly asymmetric with all three silver distances significantly ISSN 2056-9890 different ( Table 1). The likely cause of this is that two of the silver ions are coordinated to one anionic pyrrole nitrogen (N4 and N6) and one phosphinimine nitrogen (N3 and N5), whereas the remaining silver atom is coordinated to two phosphinimine nitrogens (N1 and N2) (Fig. 1). The Ag-N bond lengths are similar and range from 2.127 (2) to 2.173 (2) Å , significantly shorter than the average found in a CSD search of 2.318 (20) Å . As a result of the silver atoms being coordinated to different nitrogen atoms, the Ag1-Ag3 and Ag1-Ag2 distances are substantially longer than Ag2-Ag3, which in turn causes an acute Ag2-Ag1-Ag3 angle of 52.186 (5), far less than the average value of 60.0 (2) found in the CSD.
Compared to the neutral, protonated, ligand precursor HL, key bond distances and angles are similar with some slight distortions. The C-N-C bond angle of the pyrrole ring changes from 109.0 (2) (CSD refcode NAYMIL; Johnson et al. 2012) to 105.6 (2) , likely as a consequence of coordination to Ag2 and Ag3 (Table 1). Additionally, although the C-P-N bond angles remain significantly different on one side of the ligand to the other, the C-P-N angle of the phosphinimine functionalities coordinated to Ag2 and Ag3 are significantly smaller than that of HL [111.63 (3) from 119.37 (12) ]. There is not a substantial difference between the more acute C-P-N bond angle in HL [106.29 (12) ] compared to the C-P-N angle of the phosphinimine moieties coordinated to Ag1 [105.3 (6) ]. Lastly, there is no major change in the P N distance of the phosphinimine groups [1.61 (3) Å from 1.57 (7) Å ]. Johnson et al. (2009) reported a trisilver complex Ag 3 L 3 {L = 2 -1,3-bis[2,6-(diisopropylphenyl)triazenide]} and described their complex as a silver triangle possessing equilateral geometry such that their Ag-Ag bond distances are approximately the same within error (CSD refcode OGOHIC). The current compound does not show this equilateral geometry. McKee et al. (2001) describe their trisilver complex (CSD refcode ACUWAW) as 'near-linear' without considering the AgÁ Á ÁAg short contacts. The current complex (I) also shows a distorted linear geometry when only considering Ag-N contacts with N-Ag-N angles averaging 172.8 (2) (

Figure 1
Displacement ellipsoid plot (50% probability) of (I) showing the atomic labelling scheme. Hydrogen atoms, the trifluoromethanesulfonate counter-ion, and the deuterated benzene solvent molecules have been removed for clarity.

Supramolecular features
The title compound recrystallized in the monoclinic space group C2/c with five deuterated benzene solvent molecules in the asymmetric unit (Fig. 2 Fig. 3). Numerous reports of trisilver complexes contain interactions between the silver atoms and the triflate counter-ion, although they are often weak interactions [CSD refcodes ACUWAW (McKee et al., 2001), MEMSOM (Su et al., 2000) and VIGNEF (Martin et al., 2007)]. No silver-anion interactions were observed for (I). Consideration of a space-filling model of the asymmetric unit reveals that the bulky monoanionic pincer-ligand shields the silver atoms from any inter-actions with the triflate oxygen atoms. These oxygen atoms do, however, display short contacts with meta hydrogens on two separate phenyl groups (H14 and H52) as well as one hydrogen on a deuterated benzene solvent molecule (HM), as shown in Fig. 3.

Synthesis and crystallization
In an NMR tube, two equivalents of NaL [L = 2,5-(4-i PrC 6 H 4 N PPh 2 )C 4 H 2 N] and three equivalents of AgOTf were dissolved in benzene-d 6 . Crystals were grown in the NMR tube from benzene-d 6 . The synthesis of NaL has been previously published and utilizes a modified Staudinger reaction (Hä nninen et al., 2016;Staudinger & Meyer, 1919).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The structure was solved with intrinsic phasing using SHELXT and refined with SHELXL. Problems with large residual peaks indicated the need to investigate the reciprocal lattice. Evidence of a cracked crystal with a four-component multi-crystal model was developed in CrysAlis PRO v41.113a. Twin refinement and finalization produced an HKLF4 file with completeness greater than 99% and acceptable I/. A sufficient refinement was obtained and removed the large residual peaks. After the silver cluster and trifluoromethanesulfonate were accurately modelled, no less than five deuterated benzene solvent molecules were revealed in difference maps, two of which were well ordered and three were positionally disordered. The latter deuterated benzene solvent molecules were split into two independent units and SADI and RIGU restraints were applied during refinement. Additionally, one of the solvent molecules was restrained with an ISOR restraint.  Table 2 Summary of short contacts (Å ) for (I).

Figure 3
Representation of short contacts with only key hydrogen atoms and deuterated benzene solvent molecules shown for clarity Packing diagram of (I) viewed down the b axis with deuterated benzene solvent molecules removed for clarity. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.005 Δρ max = 1.11 e Å −3 Δρ min = −0.83 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.