Poly[μ-bromido-μ-(2,2-dimethylpropane-1,3-diyl diisocyanide)-silver(I)]: a powder diffraction study

In the title compound, [AgBr(C7H10N2)]n, adjacent Ag(I) atoms are bridged by bidentate CNCH2C(CH3)2CH2NC ligands via the NC groups, forming [Ag{CNCH2C(CH3)2CH2NC}]n chains with the metal atom in a distorted tetrahedral coordination. The bromide counter-anions cross-link the Ag(I) atoms of the chains, forming a two-dimensional polymeric network {[AgI(CNCH2C(CH3)2CH2NC)]Br}n extending parallel to (010). The polymeric structure is similar to that of the very recently reported Cl−, I− and NO3 − analogues. This gives a strong indication that 2,2-dimethylpropane-1,3-diyl diisocyanide is a potential ligand for giving polymeric structures on treatment with AgX (X = Cl−, Br−, I− or NO3 −) regardless of the counter-anion used.


Comment
In order to better understand and further explore the chemistry of 2,2-dimethylpropane-1,3-diyl diisocyanide, the synthesis and solid state characterization of the polymeric complex {[Ag I (CNCH 2 C(CH 3 ) 2 CH 2 NC)]Br} n is presented. Treatment of AgBr with two equimolar amount of 2,2-dimethylpropane-1,3-diyl diisocyanide in dry EtOH at room temperature afforded a highly insoluble white powder (I) even in polar or coordinate solvents. These strongly gave an indication that the obtained compound (I) have a polymeric structure, which is very similar to the polymeric structure of {[Ag I (CNCH 2 C(CH 3 ) 2 CH 2 NC)]X} n (X = Clor I -) Rukiah & Al-Ktaifani, 2009).
The solid state structure of (I) was confirmed by X-ray powder diffraction study exhibiting, as expected, a polymeric structure, which is very similar to the analogous Cl -, Iand NO 3 polymers. In the obtained structure, the Ag I centers are bridged with each of the two adjacent Ag neighbours by the bidentate ligands CNCH 2 C(CH 3 ) 2 CH 2 NC via the NC groups to form {Ag I (CNCH 2 C(CH 3 ) 2 CH 2 NC)} n chains. The Brcounterpart anions are cross linked the Ag centres of the chains to form a polymeric 2-D network {[Ag I (CNCH 2 C(CH 3 ) 2 CH 2 NC)]Br} n (Fig. 1). In the same manner to the polymeric structure of Cl -, Iand NO 3 analogues, the CNCH 2 C(CH 3 ) 2 CH 2 NC in the complex just behaves as bis-monodentate and the chelate behaviour is completely absent. This is undoubtedly expected for steric reason as the distance between the two isocyanide groups in the CNCH 2 C(CH 3 ) 2 CH 2 NC molecule are relatively too short to allow chelate complexing (Chemin et al., 1996) ( Fig.2).
As the conformation of the bidentate ligand (CNCH 2 C(CH 3 ) 2 CH 2 NC) in the three polymeric structures are almost alike, it can be concluded that their molecular structures are very similar. Therefore it can be stated the counterpart anion (Cl -, Bror I -) have no effective role in changing the polymeric structure of the complex. It is also noteworthy, that the bidentate ligand exhibits a very strong tendency to form polymeric complexes rather than dimeric or trimeric complexes suggesting the 2,2-dimethylpropane-1,3-diyl diisocyanide to be a potential bidentate ligand in the syntheses of organometallic polymers of different transition metals.

Experimental
All reactions and manipulations were carried out under inert atmosphere by using two fold vacuum line and schlenk technique. Solvents were dried and distilled over sodium wire; glassware dried and flamed before used. AgBr was a commercial sample and was used as received. IR spectra were operated on FTIR Jasco 300 E. Microanalysis was performed using EURO EA. Powder X-ray diffraction was performed by Stoe Transmission diffractometre (Stadi P). C, 27.12%; H, 3.25%; N, 9.03%. IR (KBr) νcm -1 : 2201.6 (N≡ C).

Refinement
The powder sample was slightly ground in a mortar, loaded into two foils of Mylar and fixed in the sample holder with a mask of suitable internal diameter (7.0 mm). Data were collected at room temperature and pressure in transmission geometry employing Cu K α1 radiation. Indexing was performed using the program DICVOL04 (Boultif & Louër, 2004)  The powder pattern was truncated to 55° in 2θ (Cu K α1 ), corresponding to real-space resolution of 1.67 Å. The Monte Carlo simulated annealing (parallel tempering algorithm) used to solve the crystal structure of compound (I) from powder pattern in direct space. One molecule of CNCH 2 C(CH 3 ) 2 CH 2 NC ligand and two free atoms of Ag and Br were introduced randomly in the orthorhombic cell calculated by Le Bail refinement. The H atoms can be ignored during the structure solution process because they do not contribute significantly to the powder diffraction pattern, due to their low X-ray scattering power. During the parallel tempering calculations, the ligand had the possibility to translate, to rotate around its centre of mass and to modify its torsion angles and the atoms Ag and Br had the possibility to modify its position in the unit cell. The model found by FOX was introduced in the program GSAS (Larson & Von Dreele, 2004), interfaced by EXPGUI (Toby, 2001) for Rietveld refinements as a starting point. The background was refined using a shifted Chebyshev polynomial with 20 coefficients. The Thompson-Cox-Hastings (Thompson et al., 1987) pseudo-Voigt profile function was used with an axial divergence asymmetry correction of (Finger et al., 1994). The two asymmetry parameters of this function S/L and D/L were both fixed at 0.0215 during the Rietveld refinement.
Geometric soft restraints were applied to the C°N, N-C and C-C distances to guide them towards their normal values, but no restrains were imposed on the Ag-C and Ag-Br distances. Likewise, no restraints were imposed on bond angles.
The hydrogen atoms were introduced at theoretical positions with CH 2 and CH 3 distances constrained to be 0.97 Å for CH 3 and 0.98 Å for CH 2 . They were refined with restrains on their bonds distances and bond angles to their normal values. One isotropic atomic displacement parameter was introduced per types of atoms C, N and H. The final refinement cycles were performed using anisotropic displacement parameters for Ag and Br atoms. Intensities were corrected for absorption effects with a function for a flat plate sample in transmission geometry (function number 4 in GSAS). The value of with m.d was 0.8.
The plate normal can be either perpendicular to the diffraction vector or tilted by some fixed angle φ in the diffraction plane.
The preferred orientation was modeled using a spherical-harmonics description (Von Dreele, 1997) with 18 coefficients. In the course of the refinement, the structure of AgBr has been introduced in the final refinement. The unit-cell parameters, the atomic displacement parameters of Ag and Br and the profile parameters were allowed to vary of this compound. The amount of this impurity was about 0.1%. The observed and calculated diffraction patterns for the refined crystal structure are shown in Fig. 3. Fig. 1. A view, along the c axis of the crystal structure of compound (I), H atoms are not shown for clarity. Fig. 2. The asymmetric unit of (I) and atom-numbering are shown. Poly[µ-bromido-µ-(2,2-dimethylpropane-1,3-diyl diisocyanide)-silver(I)]