Powder study of poly[(μ₂-2,2-di­methyl­propane-1,3-diyl diisocyanide)-μ₂-iodido-silver(I)]

In order to explore the chemistry of the bidentate ligand 2,2-dimethyl­propane-1,3-diyl diisocyanide and to investigate the effect of counter-ions on the polymeric structure of (2,2-dimethyl­propane-1,3-diyl diisocyanide)silver(I) complexes, the title polymeric compound, [AgI(C₇H₁₀N₂)]_n, was synthesized by treatment of 2,2-dimethyl­propane-1,3-diyl diisocyan­ide with AgI. X-ray powder diffraction studies show, as expected, a polymeric structure, similar to the very recently reported Cl⁻ and NO₃⁻ analogues [AgX(C₇H₁₀N₂)]_n (X = Cl⁻ or NO₃⁻). In the title structure, the Ag^I centre is bridged to two adjacent Ag^I neighbours by bidentate 2,2-dimethyl­pro­pane-1,3-diyl diisocyanide ligands via the NC groups to form [Ag{CNCH₂C(CH₃)₂CH₂NC}]_n chains. The iodide counter-ions crosslink the Ag^I centres of the chains to form a two-dimensional polymeric {[Ag{CNCH₂C(CH₃)₂CH₂NC}]I}_n network. This study also shows that this bidentate ligand forms similar polymeric structures on treatment with AgX, regardless of the nature of the counter-ion X⁻, and also has a strong tendency to form polymeric complexes rather than dimeric or trimeric ones.

S1. Comment

Diisocyanides have received considerable attention in coordination chemistry in recent years (Harvey, 2001; Sakata et al., 2003; Espinet et al., 2000; Moigno et al., 2002). Some diisocyanides have been used as bidentate ligands in the synthesis of bi-, tri- and tetranuclear complexes and organometallic polymers, which have potential practical applications as new materials in areas such as hydrogen gas production (Mann et al., 1977; Sigal et al.,1980), semi- and photoconductivity and photovoltaic cells (Fortin et al., 2000). Recently, the bidentate ligand 2,2-dimethylpropane-1,3-diyl diisocyanide was used for the first time in the synthesis of the organometallic polymers [Ag(C₇H₁₀N₂)(X)]_n (X = Cl^- or NO₃^-) (Al-Ktaifani et al., 2008; Rukiah & Al-Ktaifani, 2008), which have been fully characterized by X-ray powder diffraction studies, IR spectroscopy and microanalysis. In both polymeric structures [Ag(C₇H₁₀N₂)(X)]_n (X = Cl^- or NO₃^-), the bidentate ligand behaves only in a bis-monodentate manner to form highly insoluble polymeric compounds, rather than chelating to form dimeric or trimeric compounds. The bidentate ligands bridge the Ag^I centres to form [Ag(CNCH₂C(CH₃)₂CH₂NC)]_n chains, while the counterions crosslink the Ag^I centres of the chains to form a two-dimensional polymeric network structure. It is therefore of interest to examine how changing the counterion or the metal centre might affect the structures and properties of the products obtained. Therefore, the syntheses of new organometallic polymers of Ag and Cu with different anions (I^-, CN^-, SO₄^2-, Cl^-, NO₃^-) using the bidentate ligand 2,2-dimethylpropane-1,3-diyl diisocyanide, and their solid-state characterization, are in progress. In this article, the synthesis of the title compound, (I), and its solid-state structure are presented.

Compound (I) was prepared by the treatment of AgI with two equivalents of 2,2-dimethylpropane-1,3-diyl diisocyanide in dry EtOH. The product is a highly insoluble white powder, even in polar or coordinate solvents. This is an indication that (I) has a polymeric structure. The FT–IR spectrum of the powder of (I) shows a characteristic sharp absorption at 2196.3 cm^-1, which is readily assigned to the N≡C stretching mode. The slight increase of the N≡C stretching frequency in the present Ag complex, (I) (2196.3 cm^-1), compared with the free ligand, 2,2-dimethylpropane-1,3-diyl diisocyanide (2148.3 cm^-1), is consistent with the coordination of the NC groups of the ligands to the Ag^I centres. This is quite in accord with the propensity of the N≡C group to be a good donor and show relatively weak back-bonding (Mathieson et al., 2001).

An X-ray powder diffraction study was carried out to solve and refine the crystal structure of the powder compound, (I). As expected, the study reveals a polymeric structure, which is very similar to the analogous Cl^- and NO₃^- polymers, in which the Ag^I centres are bridged to each of the two adjacent Ag^I neighbours by the bidentate CNCH₂C(CH₃)₂CH₂NC ligand via the NC groups, at Ag—C distances of 2.078 (11) and 2.043 (12) Å, to form [Ag(CNCH₂C(CH₃)₂CH₂NC)]_n chains. The I^- counterions cross-link the Ag^I centres of the chains at Ag—I distances of 2.9564 (24) and 2.8915 (23) Å to form a polymeric two-dimensional network, {[Ag(CNCH₂C(CH₃)₂CH₂NC)]I}_n (Fig. 1). As observed in the polymeric structures of the Cl^- and NO₃^- analogues, the CNCH₂C(CH₃)₂CH₂NC ligand in complex (I) just behaves as bis-monodentate, and chelating behaviour is completely absent. This is undoubtedly expected for steric reasons, as the distance between the two isocyanide groups in the CNCH₂C(CH₃)₂CH₂NC molecule is rather too short to allow chelate complexing (Chemin et al., 1996) (Fig. 2).

In complex (I), the average bond distances and angles of {Ag^I[CNCH₂C(CH₃)₂CH₂NC]I}_n are comparable with their analogues in {Ag^I[CNCH₂C(CH₃)₂CH₂NC]Cl}_n, and the conformation of the bidentate CNCH₂C(CH₃)₂CH₂NC ligand in both polymeric structures is almost the same. Hence it can be concluded that both molecular structures are very similar. Thus, the counterion (NO₃^-, Cl^- or I^-) plays no effective role in changing the polymeric structure of the complex. The C—C [1.527 (5), 1.529 (5), 1.528 (5) and 1.532 (5) Å], C≡N [1.159 (6) and 1.158 (6) Å] and N—C [1.472 (6) and 1.473 (6) Å] bond lengths are in their normal ranges [Reference for standard values?] and comparable with their counterparts in the reported polymeric structures {[AgCNCH₂C(CH₃)₂CH₂NC]X}_n (X = Cl^- or NO₃^-; Al-Ktaifani et al., 2008; Rukiah & Al-Ktaifani, 2008), in {[Ag(dmb)₂]NO₃.0.7H₂O}_n (dmb = 1,8-diisocyano-p-menthan [-methane?]; Fortin et al., 1997), and in the dinuclear complexes Ag(dmb)₂X₂ (X = Cl^-, Br^- or I^-; Perreault et al., 1993) (these distances are restrained to their normal values in the Rietveld refinement). In complex (I), the two Ag—C≡ N angles [165.0 (22) and 161.1 (20)°] in the Ag(CNCH₂C(CH₃)₂CH₂NC)Ag unit of the polymer are comparable with their counterparts in {[AgCNCH₂C(CH₃)₂CH₂NC]X}_n (X = Cl^- or NO₃^-). Excluding H···H contacts, four short contacts (less than the sum of the van der Waals radii) exist, namely C4···N2 = 2.941 Å, C5···N1 = 3.086 Å, C5···N2 = 2.784 Å and C6···N1 = 3.052 Å.

In summary, the synthesis of organometallic polymers using the bidentate ligand 2,2-dimethylpropane-1,3-diyl diisocyanide, giving the title complex (I) as well as the previously reported polymers {Ag^I[CNCH₂C(CH₃)₂CH₂NC]X}_n (X = Cl^- and NO₃^-), has been shown to give similar polymeric structures regardless of the counterions. It is also noteworthy that the bidentate ligand exhibits a very strong tendency to form polymeric complexes rather than dimeric or trimeric ones, suggesting that it is a potential bidentate ligand in the synthesis of organometallic polymers of different transition metals.

S2. Experimental

All reactions and manipulations were carried out under an inert atmosphere using a twofold vacuum line and Schlenk techniques. Solvents were dried and distilled over sodium wire; glassware was dried and flamed before used. AgI was obtained commercially and used as received. IR and electronic spectra were recorded on an FT–IR Jasco 300 E spectrometer and a Shimadzu UV-3100 spectrophotometer, respectively. Microanalysis was performed using a EURO EA.

To a suspension of AgI (0.28 g, 1.19 mmol) in dry EtOH (15 ml) was added dropwise a solution of CNCH₂C(CH₃)₂CH₂NC (0.30 g, 2.45 mmol) in EtOH with rapid stirring at room temperature. The resulting solution was stirred overnight and then filtered, and the volatiles removed in vacuo. The resulting product was washed with ether to afford a white powder, (I) [0.35 g, yield 85%; m.p. 440 K (starts to decompose)]. Analytical data for AgC₇H₁₀N₂I, found: C 24.47, H 3.01, N 7.41%; required: C 23.55, H 2.82, N 7.84%. IR (KBr, ν, cm^-1): 2196.3 (N≡C).

S3. Refinement

The powder sample was ground slightly in a mortar, loaded between two Mylar foils and fixed in the sample holder with a mask of suitable internal diameter. Data were collected at room temperature and pressure in transmission geometry employing Cu Kα₁ radiation. Indexing was performed using the program DICVOL04 (Boultif & Louër, 2004) with standard options. Confidence figures of merit M₂₀ = 28.8, F₂₀ = 64.8 (0.0049, 63) were obtained for an orthorhombic unit cell of reasonable volume (assuming Z = 8). Cell parameters were a = 17.098 (3), b = 16.113 (3), c = 7.677 (1) Å and V = 2115.1 ų. The most probable space group was Pcab, which was obtained using the program CHECK-GROUP interfaced by WINPLOTR (Roisnel & Rodriguez-Carvajal, 2001). The parameters a and b were interchanged for working with the standard space group Pbca. The program FOX (Favre-Nicolin & Černý, 2002) was employed for structure solution. The powder pattern was truncated to 45.3° in 2θ (Cu Kα₁), corresponding to a real-space resolution of 2.00 Å. Monte Carlo simulated annealing (parallel tempering algorithm) was used to solve the crystal structure of compound (I) in direct space. One molecule of 2,2-dimethylpropane-1,3-diyl diisocyanide and two free atoms of Ag and I were introduced randomly in the orthorhombic cell obtained by the program DICVOL04. The H atoms were not introduced during the structure solution process. During the parallel tempering calculations, the ligand had the possibility of translating, rotating around its centre of mass and modifying its torsion angles. The two atoms Ag and I had the possibility of modifying their positions in the unit cell. The model obtained by FOX was used as the starting point for Rietveld refinements in the program GSAS (Larsen & Von Dreele, 2004), interfaced by EXPGUI (Toby, 2001). The profile function used was a pseudo-Voigt function convoluted with an axial divergence asymmetry function (Finger et al., 1994), and with S/L and D/L both fixed at 0.0225.

H atoms were included in calculated positions and treated in the subsequent refinement as riding atoms, with CH₂ and CH₃ distances constrained to be 0.98 and 0.97 Å, respectively. Geometric soft restraints were applied to C≡N, N—C and C—C distances to their normal values, but no restraints were imposed on the Ag—C and Ag—I distances. Likewise, no restraints were imposed on bond angles.

The background was refined using a shifted Chebyshev polynomial with 15 coefficients, whereas the preferred orientation was modelled using a spherical-harmonics description (Von Dreele, 1997). One isotropic atomic displacement parameter for C, N and H atoms was used, and this was fixed at 0.035 Ų for C and N and 0.05 Ų for H. The final refinement cycles were performed using anisotropic atomic displacement parameters for Ag and I atoms. A joint refinement strategy was implemented, in which the structure of AgI was included to take account of the impurity peaks arising from the presence of a small amount (estimated at less than 1%) of this compound in the sample. In the course of the refinement the AgI unit-cell parameters were allowed to vary, whilst all other parameters were fixed. The observed and calculated diffraction patterns for the refined crystal structure are shown in Fig. 3.

Computing details

Data collection: WinXPOW (Stoe & Cie, 1999); cell refinement: GSAS (Larsen & Von Dreele, 2004); data reduction: WinXPOW (Stoe & Cie, 1999); program(s) used to solve structure: FOX (Favre-Nicolin & Černý, 2002); program(s) used to refine structure: GSAS (Larsen & Von Dreele, 2004); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: GSAS (Larsen & Von Dreele, 2004).

Figures

	Fig. 1. A view, along the a axis, of the crystal structure of compound (I). H atoms have been omitted.
	Fig. 2. The asymmetric unit of (I), with the atom-numbering scheme.
	Fig. 3. Final observed (points), calculated (line) and difference profiles for the Rietveld refinement of (I).

poly[(µ₂-2,2-dimethylpropane-1,3-diyl diisocyanide)-µ₂-iodido-silver(I)]

Crystal data

[AgI(C7H10N2)]	F(000) = 1328.0
Mr = 356.94	Dx = 2.238 Mg m^−3
Orthorhombic, Pbca	Cu Kα1 radiation, λ = 1.54060 Å
Hall symbol: -P 2ac 2ab	µ = 37.72 mm^−1
a = 16.1168 (2) Å	T = 298 K
b = 17.1119 (2) Å	Particle morphology: fine powder (visual estimate)
c = 7.68115 (7) Å	white
V = 2118.38 (6) Å^3	flat sheet, 7.0 × 7.0 mm
Z = 8	Specimen preparation: Prepared at 298 K and 101.3 kPa

Data collection

Stoe STADI P transmission diffractometer	Data collection mode: transmission
Radiation source: sealed X-ray tube, C-Tech	Scan method: step
Ge 111 monochromator	Absorption correction: for a cylinder mounted on the φ axis GSAS absorption/surface roughness correction: function number 5. Flat-plate sample in transmission geometry. Absorption correction term (µ.r) = 0.62300. Correction is not refined.
Specimen mounting: drifted powder between two Mylar foils	Tmin = 0.209, Tmax = 0.258

Refinement

Least-squares matrix: full	Profile function: pseudo-Voigt (Thompson et al., 1987) with asymmetry correction (Finger et al., 1994)
Rp = 0.035	96 parameters
Rwp = 0.045	40 restraints
Rexp = 0.037	H-atom parameters constrained
RBragg = 0.046	(Δ/σ)max = 0.01
R(F^2) = 0.04640	Background function: GSAS background function number 1 with 15 terms. Shifted Chebyshev function of 1st kind 1: 586.311 2: -519.475 3: 267.033 4: -102.404 5: 6.91776 6: 12.7341 7: -9.54797 8: 9.05230 9: -4.87263 10: 1.67672 11: -3.02510 12: 8.16164 13: -3.66437 14: 3.25240 15: 2.09897
Excluded region(s): none	Preferred orientation correction: spherical harmonics function

Special details

Experimental. none

Refinement. Restraints weighting factor: 1.0

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ag1	0.40361 (15)	0.81127 (12)	1.0926 (2)	0.05534
I1	0.32906 (11)	0.67068 (11)	0.9326 (2)	0.0526
C1	0.3405 (11)	0.8935 (12)	0.944 (3)	0.035*
C2	0.2131 (3)	0.95359 (9)	0.78388 (18)	0.035*
C3	0.1395 (3)	0.9010 (3)	0.8280 (6)	0.035*
C4	0.06172 (10)	0.93279 (7)	0.73869 (16)	0.035*
C5	0.11397 (7)	0.90847 (6)	1.0189 (2)	0.035*
C6	0.15127 (15)	0.8136 (3)	0.79199 (16)	0.035*
C7	0.0219 (10)	0.7274 (11)	0.867 (3)	0.035*
N1	0.2905 (7)	0.9293 (8)	0.8703 (14)	0.035*
N2	0.0781 (6)	0.7689 (8)	0.8486 (13)	0.035*
H2A	0.22159	0.95249	0.65888	0.05*
H2B	0.2002	1.00665	0.81913	0.05*
H4A	0.0297	0.89013	0.69317	0.05*
H4B	0.0291	0.96126	0.82184	0.05*
H4C	0.0775	0.96695	0.64522	0.05*
H5A	0.06055	0.88474	1.03515	0.05*
H5B	0.15405	0.88234	1.09072	0.05*
H5C	0.11127	0.96271	1.05036	0.05*
H6A	0.15972	0.80594	0.66814	0.05*
H6B	0.19968	0.79521	0.85482	0.05*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ag1	0.058 (3)	0.052 (2)	0.0561 (18)	0.0114 (18)	−0.005 (2)	−0.0024 (18)
I1	0.075 (3)	0.0341 (18)	0.0486 (15)	−0.0059 (16)	0.0099 (18)	−0.0062 (17)

Geometric parameters (Å, º)

Ag1—I1	2.956 (2)	C4—C3	1.529 (5)
Ag1—I1^i	2.892 (2)	C4—H4A	0.9600
Ag1—C1	2.078 (11)	C4—H4B	0.9600
Ag1—C7^ii	2.043 (12)	C4—H4C	0.9601
C1—N1	1.159 (6)	C5—C3	1.528 (5)
C2—C3	1.527 (5)	C5—H5A	0.9600
C2—N1	1.473 (6)	C5—H5B	0.9600
C2—H2A	0.9701	C5—H5C	0.9601
C2—H2B	0.9700	C6—C3	1.532 (5)
C3—C2	1.527 (5)	C6—N2	1.472 (6)
C3—C4	1.529 (5)	C6—H6A	0.9700
C3—C5	1.528 (5)	C6—H6B	0.9700
C3—C6	1.532 (5)	C7—N2	1.158 (6)
I1—Ag1—I1^i	107.07 (9)	C3—C4—H4B	109.32
I1—Ag1—C1	97.1 (6)	C3—C4—H4C	109.57
I1—Ag1—C7^iii	100.3 (6)	H4A—C4—H4B	109.50
I1^i—Ag1—C1	102.8 (6)	H4A—C4—H4C	109.45
I1^i—Ag1—C7^iii	106.5 (6)	H4B—C4—H4C	109.50
C1—Ag1—C7^iii	139.5 (9)	C3—C5—H5A	109.31
Ag1—I1—Ag1^iv	96.87 (8)	C3—C5—H5B	109.36
Ag1—C1—N1	165 (2)	C3—C5—H5C	109.54
C3—C2—N1	113.0 (7)	H5A—C5—H5B	109.37
C3—C2—H2A	108.5	H5A—C5—H5C	109.61
C3—C2—H2B	108.8	H5B—C5—H5C	109.64
N1—C2—H2A	108.7	C3—C6—N2	110.8 (6)
N1—C2—H2B	108.7	C3—C6—H6A	109.09
H2A—C2—H2B	108.9	C3—C6—H6B	109.11
C2—C3—C4	109.1 (3)	N2—C6—H6A	109.3
C2—C3—C5	111.9 (3)	N2—C6—H6B	109.2
C2—C3—C6	116.0 (3)	H6A—C6—H6B	109.33
C4—C3—C5	100.4 (3)	Ag1^v—C7—N2	161 (2)
C4—C3—C6	111.6 (3)	C1—N1—C2	163 (2)
C5—C3—C6	106.7 (3)	C6—N2—C7	168.5 (17)
C3—C4—H4A	109.49
N1—C2—C3—C4	177.7 (7)	C2—C3—C6—N2	176.9 (5)
N1—C2—C3—C5	67.5 (6)	C4—C3—C6—N2	−57.3 (5)
N1—C2—C3—C6	−55.3 (6)	C5—C3—C6—N2	51.4 (5)

Symmetry codes: (i) x, −y+3/2, z+1/2; (ii) x+1/2, −y+3/2, −z+2; (iii) x+3/2, −y+5/2, −z+2; (iv) x, −y+3/2, z−1/2; (v) x+1/2, −y+5/2, −z+2.