Hydrogen Bonding Motifs of N,N',N' - American Chemical Society

Hydrogen Bonding Motifs of N,N′,N′′-Trisubstituted Guanidinium Cations with Spherical and Rodlike Monoanions: Syntheses and Structures of I-, I3-, and SCN- Salts Farouq F. Said,*,‡ Patrick Bazinet,§ Tiow-Gan Ong,§ Glenn P. A. Yap,† and Darrin S. Richeson*,§

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 1 258-266

Department of Chemistry, Al al-bayt UniVersity, Mafraq, Jordan, 25113, Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware, 19716, and Department of Chemistry, UniVersity of Ottawa, Ottawa, ON K1N 6N5, Canada ReceiVed June 29, 2005

ABSTRACT: The trisubstituted guanidines N,N′,N′′-triisopropylguanidine (TPG) and N,N′-diisopropyl-N′′-2,6-dimethylphenylguanidine (DPArG) react with ammonium iodide in water to produce the iodide salts (TPGHNH3)I (1) and (DPArGH)I (2). Compound 1 displayed a complex cation that consisted of a 1:1 ratio of TPG and ammonium cation, and the solid-state structure of this compound is a three-dimensional hydrogen-bonded network in which all three guanidine nitrogens are involved in hydrogen bonding with a three-coordinate iodide anion. The orientation of the N-substituents of the cation in 2 led to hydrogen-bonded cation/anion chains. Triiodide salts, TPGH+I3- (3) and DPArGH+I3- (4), are also reported. The primary structures of 3 and 4 consist of hydrogenbonded dimers. The shortest NH‚‚‚I interactions for 3 are with one terminal and the central I atoms, while in 4 the two hydrogenbonding interactions are to the terminal I centers. A third, slightly longer NH‚‚‚I interaction in 3 led to an extended double layer chain structure. In 4, an NH interaction with the aromatic ring of an adjacent dimeric unit generated chains of dimers. The reaction between TPG and NH4SCN produced TPGH+SCN- (5) with a structure displaying two cystallographically unique cations, each with three hydrogen bonds to the thiocyanate anions. Two structural subunits were identified, a dimeric (TPGH+SCN-)2 unit and a cation/anion chain, and these are cross-linked via hydrogen-bonding interactions to yield the 3-D structure for 5. Introduction Harnessing intermolecular forces for the rational assembly of molecular building blocks with the ultimate goal of designing extended supramolecular structures is a fundamental chemical challenge. Intermolecular hydrogen-bonding and ionic interactions represent essential forces for the self-organization of individual charged units into robust supramolecules.1-6 For example, the hydrogen bond strengths when accompanied by an ionic component range between 10 and 45 kcal mol-1 compared to 4-15 kcal mol-1 for hydrogen bonds between neutral molecules. In addition, studies of organic ammonium halides have demonstrated that the N-H+‚‚‚X- interactions play a significant role in influencing the structural motif adopted in the solid state.7 These concepts have been elegantly combined with the ability of the planar C3-symmetric guanidinium cation (C(NH2)3+) (X) to form multiple hydrogen bonds with well-

* To whom correspondence should be addressed. E-mail: darrin@ science.uottawa.ca. ‡ Al al-bayt University. § University of Ottawa. † University of Delaware.

defined 3-fold spatial orientation for the preparation of novel extended frameworks.8-11 In contrast to the successful applications of guanidinium for the generation of a variety of interesting crystalline motifs, there are few reports employing N,N′,N′′trisubstituted analogues.12 Recent reports of modified triaminoguanidinium species demonstrate the potential of these species.13-17 With these facts in mind and in conjuction with our exploration of new routes to novel guanidines,18 we have initiated an examination of the charge-assisted hydrogen-bonding configurations in the extended structures of N,N′,N′′-trisubstituted guanidinium cations, and we recently reported the preparation and extended structures of the chloride and bromide salts of N,N′,N′′-triisopropylguanidinium (TPGH+) (A) and N,N′diisopropyl-N′′-dimethylphenylguanidium (DPArGH+) (B).19 To

elaborate on our initial examination of small spherical anions, we now report the preparation and extended structures of TPGH+ and DPArGH+ with the larger spherical iodide anion (I-), along with two linear triatomic anions represented by triiodide anion (I3-) and the less symmetric thiocyanate anion (SCN-). The TPGH+ cation retains the C3-symmetry in the guanidinium cation, while the implementation of a symmetryreducing aryl group in the case of DPArGH+ will alter the steric

10.1021/cg050303a CCC: $33.50 © 2006 American Chemical Society Published on Web 11/15/2005

H-Bonding Motifs of Substituted Guanidinium Cations

Crystal Growth & Design, Vol. 6, No. 1, 2006 259

Table 1. Crystal Data and Structure Refinement for {TPGHNH3}+I- (1), {DPArGH}+I- (2), {TPGH}+I3- (3), {DPArGH}+I3- (4), and {TPGH}+SCN- (5) compound empirical formula formula weight temp (K) λ (Å) crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z F(calc) (Mg/m3) µ (mm-1) absorption correction final R indices [I > 2σ(I)] R1a wR2b a

1 C10H27IN4 330.26

2 C15H26IN3 375.29

tetragonal P42/n 18.0542(9) 18.0542(9) 9.6549(9)

monoclinic P21/c 8.231(2) 18.798(6) 11.582(4) 92.43(4)

3147.1(4) 8 1.394 2.018

1790.3(10) 4 1.392 1.782

0.0328 0.0735

0.0532 0.0801

3 C10H24I3N3 567.02 203 0.71073 triclinic P1h 8.731(3) 9.906(3) 11.485(3) 74.633(3) 82.483(4) 82.632(4) 945.0(5) 2 1.993 4.948 semiempirical from equivalents 0.0391 0.1069

4 C15H26I3N3 629.09

5 C11H24N4S 244.40

triclinic P1h 8.8864(19) 10.103(2) 13.598(3) 79.331(3) 78.375(3) 64.249(3) 1070.4(4) 2 1.952 4.379

monoclinic P21/c 8.8610(19) 26.644(6) 13.442(3)

3116.8(11) 8 1.042 0.193

0.0402 0.0831

0.0695 0.1802

100.850(3)

R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) (∑w(|Fo| - |Fc|)2/∑w|Fo|2)1/2.

Scheme 1

and electronic features of the cation while preserving the potential for 3-fold hydrogen-bonding arrays. By revealing the intermolecular interactions that affect the fundamental structural features of solids designed around these hydrogen-bonding arrays, we hope to design molecular components with specific symmetries and functional group disposition that can be applied to the synthesis of solids with targeted supramolecular interactions. Experimental Section General. N,N′,N′′-Triisopropylguanidine (TPG) and N,N′-diisopropyl-N′′-2,6-dimethylphenylguanidine (DPArG) were prepared according to literature methods.18 All other reagents were purchased from Aldrich Chemical Co. and used without further purification. Elemental analyses were run on a Perkin-Elmer PE CHN 4000 elemental analysis system. Synthesis and Crystallization of N,N′,N′′-Triisopropylguanidine Ammonium Iodide, {C(NHiPr)2iPrNHNH3}+I-, (TPGHNH3)I, 1. In a round-bottom flask, a combination of 0.100 g (0.690 mmol) of ammonium iodide and 0.129 g (0.696 mmol) of N,N′,N′′-triisopropylguanidine were suspended in 10 mL of distilled water. The reaction mixture was heated and maintained at the boiling point for 5 min. Offwhite cubic crystals were deposited overnight upon slow cooling of the solution (0.180 g, 0.545 mmol, 79%). Anal. Calcd for C10H27N4I: C, 36.37; H, 8.24; N, 16.96. Found: C, 36.15; H,8.55; N,17.04.

Synthesis and Crystallization of N,N′-Diisopropyl-N′′-2,6-dimethylphenylguanidinium Iodide, {C(NHiPr)2(NH2,6-Me2C6H3)}+I-, DPArGHI, 2. In a round-bottom flask, 0.050 g (0.202 mmol) of N,N′diisopropyl-N′′-2,6-dimethylphenylguanidine was dissolved in 10 mL of hot distilled water. To this solution was added 0.033 g (0.228 mmol) of ammonium iodide, and the resulting mixture was boiled for 15 min and cooled to room temperature and then to 5 °C in a refrigerator. The following day, crystalline white solid 2 was isolated by filtration (0.064 g, 0.170 mmol, 84%). Anal. Calcd for C15H26N3I: C, 48.01; H, 6.98; I, 33.81; N, 11.20. Found: C, 48.35; H, 6.55; N,11.04. Synthesis and Crystallization of N,N′,N′′-Triisopropylguanidinium Triiodide, {C(NHiPr)3}+I3-, TPGHI3, 3. To a sample of 0.040 g (0.216 mmol) of N,N′,N′′-triisopropylguanidine in a vial was added 1 mL of HI solution (55% aq, 0.731 mmol). The mixture was agitated for several minutes during which time the colorless solid dissolved and the solution became dark brown and homogeneous. The vial was sealed and placed in a refrigerator (5 °C). White crystalline plates of 3 formed over the next week and were isolated by filtration (0.096 g, 78%). Anal. Calcd for C10H24N3I3: C, 21.18; H, 4.27; N, 7.41. Found: C, 20.96; H, 3.98; N, 7.55. Synthesis and Crystallization of N,N′-Diisopropyl-N′′-2,6-dimethylphenylguanidinium Triiodide, {C(NHiPr)2(NH2,6-Me2C6H3)}+I3-, DPArGHI3, 4. To a vial containing 0.050 g (0.202 mmol) of N,N′diisopropyl-N′′-2,6-dimethylphenylguanidine was added 1 mL of HI (55% aq, 0.731 mmol). A brown precipitate was formed over the next few minutes and was isolated by filtration. This solid was dissolved in

260 Crystal Growth & Design, Vol. 6, No. 1, 2006

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Figure 1. Molecular structure and atom numbering scheme for {TPGHNH3}+I-, 1. Table 2. Selected Bond Lengths [Å] and Angles [deg] for {TPGHNH3}+I- (1) and {DPArGH}+I- (2) Distances compound 1 N(1)-C(10) N(1)-C(1) N(2)-C(10) N(2)-C(4) N(3)-C(10) N(3)-C(7)

compound 2 1.342(4) 1.476(4) 1.346(4) 1.476(4) 1.344(4) 1.490(3)

N(1)-C(15) N(1)-C(1) N(2)-C(15) N(2)-C(4) N(3)-C(15) N(3)-C(12)

1.329(6) 1.470(7) 1.328(6) 1.466(7) 1.337(6) 1.454(6)

Angles compound 1 C(10)-N(1)-C(1) C(10)-N(2)-C(4) C(10)-N(3)-C(7) N(1)-C(10)-N(3) N(1)-C(10)-N(2) N(3)-C(10)-N(2)

compound 2 125.6(3) 124.4(2) 123.7(2) 120.3(3) 120.1(3) 119.6(3)

C(15)-N(1)-C(1) C(15)-N(2)-C(4) C(15)-N(3)-C(12) N(3)-C(15)-N(2) N(3)-C(15)-N(1) N(2)-C(15)-N(1)

130.0(5) 126.8(5) 125.3(4) 120.8(5) 121.3(5) 117.8(5)

Torsion Angles compound 2 C(15)-N(1)-C(1)-C(2) C(15)-N(2)-C(4)-C(5) C(15)-N(3)-C(12)-C(7) C(12)-N(3)-C(15)-N(2)

138.8(6) -102.6(7) 119.1(7) 156.6(5)

methanol and cooled to -5 °C. Crystals of 4 deposited over the next week (0.115 g, 0.182 mmol 90% yield). Anal. Calcd for C15H26N3I3: C, 28.64; H, 4.17; N, 6.68. Found: C, 28.96; H, 3.94; N, 7.05. Synthesis and Crystallization of N,N′,N′′-Triisopropylguanidinium Thiocyanate, {C(NHiPr)3}+SCN-, TPGHSCN, 5. To a solution of 0.060 g (0.324 mmol) of N,N′,N′′-triisopropylguanidine dissolved in 3 mL of water was added 120 mg (1.58 mol) of ammonium thiocyanate previously dissolved in 2 mL of water. A colorless precipitate formed immediately upon combination of the solutions. The solid was isolated, dissolved in hot water, and cooled to 5 °C in a refrigerator. Colorless crystals of 5 were obtained (0.055 g, 0.225 mmol 69% yield). Anal. Calcd for C11H24N4S: C, 54.06; H, 9.90; N, 22.92. Found: C, 53.89; H, 10.14; N, 23.28. Structural Determination for Compounds 1-5. Single crystals were mounted on thin glass fibers using viscous oil and then cooled to the data collection temperature. Crystal data and details of the measurements are summarized in Table 1. Data were collected on a Bruker AX SMART 1k CCD diffractometer using 0.3° ω-scans at 0°, 90°, and 180° in φ. Unit-cell parameters were determined from 60 data frames collected at different sections of the Ewald sphere. Semiempirical absorption corrections based on equivalent reflections were applied. The structures were solved by direct methods, completed with difference Fourier syntheses, and refined with full-matrix least-squares procedures based on F2. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were treated as idealized

Figure 2. Representations of the hydrogen-bonding and short contact interactions for {TPGHNH3}+I-, 1. In all cases, the guanidine and ammonium are represented as stick figures with the iodide anion as a purple sphere. For panel c, CH bonds have been omitted for clarity. Panels a and b emphasize the hydrogen bonding of the cation, and panel c emphasizes that of the anion. contributions. All scattering factors and anomalous dispersion factors are contained in the SHELXTL 5.1 program library.

Results and Discussion We recently demonstrated that the two trisubstituted guanidines employed in this study, TPG and DPArG, readily react with NH4X (X ) Cl, Br) to give the guanidinium halides and that these species exhibited interesting intermolecular interactions



Table 3. Selected Bond Lengths [Å] and Angles [deg] for {TPGH}+I3- (3), {DPArGH}+I3- (4), and {TPGH}+SCN- (5) Distances compound 3

compound 4

I(1)-I(3) I(1)-I(2) N(1)-C(10) N(1)-C(1) N(2)-C(10) N(2)-C(4) N(3)-C(10) N(3)-C(7)

2.9316(8) 2.9484(8) 1.327(6) 1.474(7) 1.338(6) 1.485(5) 1.346(6) 1.477(6)

I(1)-I(2) I(2)-I(3) N(1)-C(15) N(1)-C(1) N(2)-C(15) N(2)-C(4) N(3)-C(15) N(3)-C(12)

compound 5 2.9196(7) 2.9404(7) 1.330(8) 1.478(7) 1.324(7) 1.471(7) 1.351(7) 1.448(8)

N(1)-C(1) N(1)-C(2) N(2)-C(1) N(2)-C(5) N(3)-C(1) N(3)-C(8) N(4)-C(11) N(4)-C(12) N(5)-C(11) N(5)-C(15) N(6)-C(11) N(6)-C(18) N(7)-C(21) N(8)-C(22) S(1)-C(21) S(2)-C(22)

1.339(6) 1.457(6) 1.323(6) 1.466(7) 1.335(6) 1.456(6) 1.310(10) 1.444(11) 1.369(10) 1.393(11) 1.334(11) 1.395(11) 1.159(9) 1.146(8) 1.644(9) 1.663(8)

Angles compound 3 I(3)-I(1)-I(2) C(10)-N(1)-C(1) C(10)-N(2)-C(4) C(10)-N(3)-C(7) N(1)-C(10)-N(2) N(1)-C(10)-N(3) N(2)-C(10)-N(3)

compound 4 178.629(14) 126.9(4) 125.3(4) 128.1(4) 120.5(4) 119.6(4) 119.9(4)

I(1)-I(2)-I(3) C(15)-N(1)-C(1) C(15)-N(2)-C(4) C(15)-N(3)-C(12) N(2)-C(15)-N(1) N(2)-C(15)-N(3) N(1)-C(15)-N(3)

compound 5 178.30(2) 126.8(5) 126.5(5) 124.4(5) 121.3(5) 118.6(6) 120.1(5)

C(1)-N(1)-C(2) C(1)-N(2)-C(5) C(1)-N(3)-C(8) C(11)-N(4)-C(12) C(11)-N(5)-C(15) C(11)-N(6)-C(18) N(2)-C(1)-N(3) N(2)-C(1)-N(1) N(3)-C(1)-N(1) N(4)-C(11)-N(6) N(4)-C(11)-N(5) N(6)-C(11)-N(5) N(7)-C(21)-S(1) N(8)-C(22)-S(2)

127.3(5) 129.8(5) 129.1(5) 130.3(10) 130.1(10) 134.4(12) 119.7(5) 120.8(5) 119.5(5) 122.0(8) 119.6(8) 117.9(8) 179.1(8) 179.3(7)

Torsion Angles compound 4 C(15)-N(1)-C(1)-C(3) C(15)-N(3)-C(12)-C(7) C(12)-N(3)-C(15)-N(1)

compound 5 145.7(6) 76.7(8) -176.3(6)

leading to 3-, 2- and 1-D stuctures.19 Both TPG and DPArG react smoothly with ammonium iodide in aqueous solution to give excellent yields of two new crystalline materials, 1 and 2 (Scheme 1). While compound 2 provided elemental analysis data consistent with the anticipated formulation of DPArGH+I-, the analytical results for compound 1 indicated excess nitrogen and suggested the incorporation of an ammonium cation in this material. These results were confirmed by single-crystal X-ray analyses, which revealed the structures of these two materials (Table 1). Compound 1 crystallized in the cubic space group P42/n, and a diagram of the asymmetric unit is presented in Figure 1. Rather than the initially anticipated guanidinium iodide, TPGH+I-, compound 1 exhibits an extended solid-state structure with a complex cation comprised of a 1:1 ratio of guanidine (TPG) and ammonium cations and an iodide counterion. Bond distances and angles for the structure of 1 are summarized in Table 2. The {TPGHNH3}+ cation is emphasized in Figure 2a and confirms that the guanidine is present with a pseudo-C3 propeller-like orientation of the NiPr groups. The complex {TPGHNH3}+ cation interacts with the iodide anion through hydrogen bonding that is enhanced by the opposite charges of the two components. The overall effect is a three-dimensional hydrogen-bonding network in this solid. Three views of the hydrogen-bonding network of 1 are presented in Figure 2a-c, and a summary of the metrical parameters of hydrogen bonding is presented in Table 4. The guanidine participates in two kinds of hydrogen bonding, which are demonstrated in Figure 2b. Each of the two guanidine NHi-

-172.4(5) 81.4(15)

C(5)-N(2)-C(1)-N(3) C(11)-N(4)-C(12)-C(14)

Table 4. Hydrogen Bonds and Short Contacts for {TPGHNH3}+I(1), {DPArGH}+I- (2), {TPGH}+I3- (3), {DPArGH}+I3- (4), and {TPGH}+SCN- (5)a D-H‚‚‚A

d(D-H), Å

d(H‚‚‚A), Å

d(D‚‚‚A), Å

∠(DHA), deg

N(1)-H(1)‚‚‚I N(2)-H(2)‚‚‚I N(4)-H(4D)‚‚‚N(3) N(4)-H(4B)‚‚‚I

Compound 1 0.832 3.203 0.840 2.977 0.767 2.497 0.863 2.660

3.989 3.882 3.143 3.508

158.53 167.74 142.78 167.71

N(1)-H(1A)‚‚‚I#1 N(2)-H(2A)‚‚‚I#1 N(3)-H(3A)‚‚‚I

Compound 2 0.87 2.98 0.87 2.96 0.87 2.82

3.669(5) 3.675(5) 3.677(4)

137.6 140.7 168.7

N(1)-H(1B)‚‚‚I(3) N(2)-H(2D)‚‚‚I(2) N(3)-H(3D)‚‚‚I(1)

Compound 3 0.87 3.182 0.87 3.155 0.87 3.160

3.963 3.843 3.964

150.61 137.69 155.49

N(1)-H(1A)‚‚‚I(3) N(2)-H(2A)‚‚‚I(1)

Compound 4 0.87 3.04 0.87 3.01

3.818(5) 3.789(5)

149.1 149.4

N(1)-H(1A)‚‚‚N(8) N(2)-H(2A)‚‚‚S(1) N(3)-H(3A)‚‚‚S(2) N(4)-H(4D)‚‚‚N(7) N(5)-H(5B)‚‚‚S(2) N(6)-H(6D)‚‚‚S(1)

Compound 5 0.87 2.404 0.87 2.715 0.87 2.768 0.87 2.408 0.87 2.796 0.87 2.853

3.214 3.538 3.580 2.985 3.554 3.621

155.12 158.04 156.04 148.48 146.34 148.14

a van der Waals radii taken from Bondi, A. J. Phys. Chem. 1964, 68, 441. Pertinent radii: C ) 1.70 Å; H ) 1.2 Å; I ) 1.98 Å; N ) 1.55 Å; S ) 1.80 Å.

Pr groups interacts with an independent I- center, while the third guanidine nitrogen (i.e., N(3)) is interacting with the NH4+


Said et al.

Figure 3. Molecular structure and atom numbering scheme for {DPArGH}+I-, 2. Hydrogen atoms have been omitted for clarity.

cation through an N(3)guanidine‚‚‚H‚‚‚NH3 hydrogen-bonding interaction. The TPGHNH3+ cation also forms a hydrogen bond to the I- anion and via the bridging N(3)guanidine‚‚‚H‚‚‚NH3 moiety. The three hydrogen bonds that constitute the coordination sphere of the iodide anion are shown in Figure 2c. The net result is that all three guanidine nitrogens are involved in hydrogen bonding to the anion with one case being mediated through an ammonium cation. The structural consequences of replacing one of the isopropyl groups in TPG with an aryl moiety was examined by employing DPArG in an analogous reaction with NH4I. In this case, reaction proceeded as expected to yield DPArGH+I-, 2, as confirmed by microanalysis and the crystal structure of this material. A summary of bond distances and angles for 2 is provided in Table 2, and a structural representation is given in Figure 3. The guanidinium CN3 core remains planar but, in contrast to the pseudo-C3 arrangement observed for 1, the DPArGH+ cation in 2 adopts a conformation in which the aryl group is oriented cisoid to the isopropyl group on N(1) (configuration C). The orientation of the aromatic ring perpen-

dicular to the planar CN3 core reduces the steric interactions of the 2,6-dimethylphenyl and isopropyl groups that are experienced when the guanidinium cation has a C orientation. A consequence of this particular orientation of the cation is that the two NH(iPr) groups of the DPArGH+ cations (i.e., N(1) and N(2)) are positioned to form hydrogen bonds in a chelating fashion to the I- anion as shown in Figure 4. The third NH group of an adjacent DPArGH+ cation completes the hydrogen bond environment of the iodide anions. The result is a set of hydrogen-bonded cation/anion chains that run along the crystallographic a axis of the structure as shown in Figure 4. There are no significant short contacts between these chains in the extended packing of 2 (Figure 4b). Both 2 and its bromide analogue, DPArGH+Br-, possess very similar structures.19 In both cases, the cation adopts the C orientation for the substituents, and they display identical

Figure 4. Representations of the hydrogen-bonding scheme for {DPArGH}+I-, 2. The guanidinium cation is shown in stick figure representation with the iodide anion as a purple sphere. Panel a shows a single chain running along the crystallographic a axis. Panel b displays the packing of the hydrogen-bonded chains viewed perpendicular to the crystallographic c axis (a axis oriented horizontally). In this view, the CH groups have been omitted for clarity.

hydrogen-bonded anion/cation chains. Interestingly, these results contrast with the structure observed for DPArGH+Cl- in which the cation was found with configuration D. A recent examination of charge-assisted hydrogen bonding (CAHB) for N-H‚‚‚I interactions classified them according to whether the interaction was between oppositely charged ions [(()CAHB], was between a neutral and charged species [(+)CAHB or (-)CAHB], or was without charge assistance.20 The average observed (N)H‚‚‚I distance for (()CAHBs was, as expected, the shortest at 2.808 Å, while the average for N-H‚ ‚‚I hydrogen bonding without charge assistance was the longest at 2.958 Å. On the basis of these observations, the N-H‚‚‚I interactions in 1 and 2 are most similar to those hydrogen bonds that do not experience charge assistance. This may be partially attributed to the delocalized positive charge of the guanidinium cation. In an effort to expand upon the hydrogen-bonded structural motifs for substituted guanidinium salts, we have extended our examination beyond spherical halide anions to include linear rodlike triatomic anions. The triiodide anion (I3-) is a symmetrical linear anion that is often employed as the counterion for large cations. The charge distribution in I3- is nonuniform with the terminal iodines possessing charges of -0.419 compared to the a charge of -0.163 on the central atom.21 As a


Crystal Growth & Design, Vol. 6, No. 1, 2006 263 Scheme 2

class of compounds, polyiodides have a well-deserved reputation for fascinating structural chemistry.22 Air oxidation of aqueous HI solutions is known to generate triiodide anions, and we therefore examined the direct reactions of TPG and DPArG with an aqueous HI to yield the corresponding guanidinium triiodide salts (Scheme 2). The structures of TPGH+I3-, 3, and DPArGH+I3-, 4, were obtained by single-crystal X-ray diffraction analysis (Table 1). The molecular structure of 3 is presented in Figure 5 with a summary of bonding parameters given in Table 3. The structure consists of a planar C3-symmetric N,N′,N′′-triisopropylguanidinium cation with features similar to those observed in 1 and in the Cl- and Br- analogues of TPGH+.19 The targeted linear I3- is the counteranion in this compound. The hydrogen-bonding contacts between the TPGH+ and I3- are displayed in Figure 6 and summarized in Table 4. The shortest contacts are those between N(3)H‚‚‚I(1) (3.157 Å) and N(2)H‚‚‚I(2) (3.155 Å), and these lead to a primary structure consisting of {TPGHI3}2 dimers, which is shown in Figure 6a. Hydrogen bonding to the central iodine atom of the I3- anion of 3 is contrary to the charge distribution of the anion and to general hydrogen-bonding behavior of triiodide.22,23 When the hydrogen-bonding interactions in 3 are expanded to include interactions that are 0.1 Å longer than the sum of van der Waals radii, a third interaction between N(1)H and I(3) is obtained and results in an extended double layer chain structure (Figure 6b). These chains runs along the crystallographic b axis and do not exhibit any short interchain contacts. The resultant structure is unusual in that all three of the guanidinium NH groups are involved with hydrogen bonding to each of the unique I centers of the triiodide anion. A structural diagram of DPArGH+I3- 4 is presented in Figure 7 with a summary of the metrical parameters obtained for this structure in Table 3. Similar to 3, this structure displayed a

Figure 5. Molecular structure and atom numbering scheme for {TPGH}+I3-, 3. Hydrogen atoms have been omitted for clarity.

guanidinium cation with a planar CN3 core and with the N-substituents oriented in a propeller orientation. The counterion was the anticipated linear I3- anion. The pseudo-C3 orientation of the N-substituents contrasts with our observations for compound 2 (DPArGH+I-) and with the structures obtained for DPArGH+X- (X ) Cl, Br).19 The cation/anion hydrogen bond interactions for 4 are, in this case, limited to involve only the N(iPr)-H groups and the termini of the triiodide anion as shown in Figure 8. The net result of the two hydrogen-bonding interactions is the formation of dimeric {DPArGHI3}2 units with large nonplanar rings

Figure 6. Representations of the hydrogen-bonding schemes for compound {TPGH}+I3-, 3. The guanidinium cation is shown in stick figure representation with the triiodide anion as purple spheres. The CH groups have been omitted for clarity. Panel a displays the dimeric species formed from the shortest NH‚‚‚I interactions. Additional NH‚ ‚‚I interactions that are 0.1 Å longer generate the double layer chains shown in panel b. The view in panel b is perpendicular to the crystallographic a axis, and the b (horizontal) and c axes are shown on the right side in the diagram.


Said et al.

Figure 7. Molecular structure and atom numbering scheme for {DPArGH}+I3-, 4. Hydrogen atoms have been omitted for clarity.

perpendicular to the crystallographic b axis. While the N(3)ArH groups are not involved in this network an examination of short intermolecular contacts does reveal an interaction between this moiety and the same unit in an adjacent dimer. The shortest of these interactions is between N(3)H and the C(10) center on the neighboring aromatic ring at a distance of 2.86 Å. The methyl groups on C(7) and C(11) are nested together and lead to a close approach of C(7) and C(14) (3.71 Å) and position H(14) to within 2.9 Å of C(7) (van der Waals radii of C ) 1.70 Å, H ) 1.20 Å). This additional interaction is displayed in Figure 8b and leads to the extended chain formation shown in this figure. The thiocyanate anion, NCS-, is a linear triatomic anion that is known to form strong and multiple hydrogen-bonding interactions with both the N and S ends of the ion.24 In addition, thiocyanate is of reduced symmetry compared with I3- (C∞ν vs D∞h), which should influence the extended hydrogen-bonding structure with guanidinium. Therefore, we chose to target the preparation of TPGH+SCN- by employing the reaction of the symmetrically substituted guanidine TPG with NH4SCN. This reaction produced an immediate precipitate when the aqueous solutions of reagents were combined, and the solid that was crystallized in high yield gave an elemental analysis consistent with the target formulation, TPGH+SCN-, 5. The structure of 5 was determined (Table 1), and the selected bond distances and angles are presented in Table 3 with structural diagrams given in Figures 9 and 10. The asymmetric unit consisted of two independent TPGH+ cations and two SCN- anions with no remarkable differences between their structural parameters. The guanidinium cations can be labeled by their central C atoms as G-C1 and G-C11. Similarly the thiocyanates are conveniently labeled as S1-SCN and S2-SCN. Both of the guanidinium cations adopt C3 -orientations of the iPr groups. In addition, each cation makes three hydrogen bonds with three independent anions. Two of these bonds are with the S end of the SCN- anion and one is with the N end of the anion (Figure 10a,b). Dissecting the extended structures for the two independent cations, G-C1 and G-C11, reveals that they exhibit two different extended bonding motifs. The G-C1 cation is organized into pseudo-dimer units as shown in Figure 11a. These dimeric units are similar to those observed in the case of compound 4, {DPArGHI3}2. These G-C1/S2-SCN dimers are bridged through the thiocyanate anions to the G-C11 cations.

Figure 8. Representations of the hydrogen-bonding schemes for compound {DPArGH}+I3-, 4. The guanidinium cation is shown in stick figure representation with the triiodide anion as purple spheres. Panel a displays the dimeric species formed from the NH‚‚‚I interactions with a view that is perpendicular to the crystallographic b axis (the c axis is horizontal in the diagram). The CH groups have been omitted for clarity. Panel b shows the extended short contacts between adjacent N(3)ArH groups with the same view as that in panel a. In this diagram, the H atoms on the iPr groups have been omitted for clarity.

Figure 9. Molecular structure and atom numbering scheme for {TPGH}+SCN-, 5. Hydrogen atoms have been omitted for clarity.

In contrast, the G-C11 cations form chains with the S1-SCN anions that run along the crystallographic c axis as shown in Figure 11b. These chains are cross-linked to the G-C1 cations through hydrogen bonding mediated by the thiocyanate ion. The packing of these two subunits is shown in Figure 11c, which


Figure 10. Representations of the hydrogen-bonding schemes for compound {TPGH}+SCN-, 5. The guanidinium cation is shown in stick figure representation with the thiocyanate anion as a ball-and-stick model (yellow ) S, gray ) C, and blue ) N). Panel a displays the hydrogen-bonding interactions of the G-C1 cation, and panel b displays the hydrogen-bonding interactions of the G-C11 cation. The CH groups have been omitted for clarity. Panel c shows the packing diagram for 5.

displays the 3-D relationship between the G-C11/S1-SCN chains and the G-C1/S2-SCN dimers. Conclusions In this repor,t we have further elucidated the intermolecular interactions that influence the fundamental structural features of solids designed around novel N,N′,N′′-trisubstituted guanidinium cations. As anticipated, charge-assisted hydrogen bonds, NH+‚‚‚X-, appear to dominate the extended structures that are observed. In addition, weaker interactions between the Nsubstituents have been revealed. A comparison of the structures


Figure 11. Representations of the bonding patterns for the two crystallographically independent guanidinium cations in 5. Panel a displays the G-C1 cation as a green stick figure (CH groups eliminated for clarity) and emphasizes the formation of dimeric subunits in the extended structure of this material. The purple spheres in this diagram represent the location of the G-C11 cations. Panel b displays the G-C11 cation as a purple stick figure (CH groups eliminated for clarity) and emphasizes the extended chain substructure adopted by this cation. In both panels, the S1-SCN anion is red and the S2-SCN anion is yellow. Panel c employs the same color scheme to provide a view of the packing for the two cations. This view is oriented perpendicular to the crystallographic a axis with the b axis vertical and the c axis horizontal.

of DPArGH+X- (X ) Cl, Br, I) with DPArGH+I3- revealed a potential role for cation/anion matching in the structures of these compounds and an influence of the anion on the observed isomeric forms for the substituted guanidinium cations. A common structural motif for the DPArGH+ cation with X- is the formation of hydrogen-bonded chains. The factors that influence the observed conformation need to be further examined and are a point of our continuing investigations. Our initial investigation of the supramolecular structure of TPGH+ and DPArGH+ with rodlike linear triatomic anions, represented by I3- and SCN--, reveals the potential for a rich 1-, 2-, and 3-D structural chemistry of these species. Trisubstituted guanidinium cations certainly represent interesting 3-fold hydrogen-bonding moieties for crystal design, and our continuing efforts are attempting to unravel the influence of substituents and identity of the acceptor in their extended structures.


Acknowledgment. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). Supporting Information Available: Full details for the structural determinations of compounds 1-5 (cif files). This material is available free of charge via the Internet at http://pubs.acs.org.

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CG050303A

Hydrogen Bonding Motifs of N,N',N' - American Chemical Society

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