Supramolecular Architectures of Metal Complexes Controlled by a

Oct 9, 2009 - Department of Chemistry, Eastern Illinois University Charleston, Illinois 61920. Received August 21, 2009; Revised Manuscript Received ...
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DOI: 10.1021/cg901000d

Supramolecular Architectures of Metal Complexes Controlled by a Strong π 3 3 3 π Stacking, 1,8-Naphthalimide Functionalized Third Generation Tris(pyrazolyl)methane Ligand

2010, Vol. 10 386–393

Daniel L. Reger,* Eric Sirianni, Jacob J. Horger, and Mark D. Smith Department of Chemistry and Biochemistry, University of South Carolina Columbia, South Carolina 29208

Radu F. Semeniuc Department of Chemistry, Eastern Illinois University Charleston, Illinois 61920 Received August 21, 2009; Revised Manuscript Received September 24, 2009

ABSTRACT: A new hetero-bifunctional ligand designed to form supramolecular structures with a combination of covalent and noncovalent forces has been prepared. The ligand, (1,8-naphthalimide)CH2C6H4CH2OCH2C(pz)3 (Ltris, pz = pyrazolyl ring), contains both a tris(pyrazolyl)methane coordination unit and a 1,8-naphthalimide strong π 3 3 3 π stacking unit. The reactions of iron(II), copper(II), and cadmium(II) tetrafluoroborate salts with 2 equiv of Ltris yield [Fe(Ltris)2](BF4)2 (1), [Cd(Ltris)2](BF4)2 (2), and [Cu(Ltris)2](BF4)2 (3), respectively. Vapor diffusion crystallization yielded two pseudopolymorphs for 1. In the solid-state structures of both forms, the iron is in an octahedral environment with bond distances expected for lowspin iron(II); both tris(pyrazolyl)methane donor sets are in a κ3-coordination mode with each Ltris ligand in a U-shaped, syn arrangement. In one pseudopolymorph, the 1,8-naphthalimide side chains are on the same side of the molecule when viewed down the metal axis (syn-1), twisted only 30° with respect to each other. In the second pseudopolymorph, the side chains are oriented in opposite directions at 180° (anti-1). Both structures are one-dimensional, organized by intermolecular π 3 3 3 π stacking interactions of the 1,8-naphthalimide units. Each naphthalimide unit reaches past the naphthalimide unit on the adjacent cation, leading to a π 3 3 3 π stack using the face of the naphthalimide unit directed toward its metal complex, interlocking the U-shaped groups. The interaction of the two naphthalimide groups is directionally specific; they are oriented head to tail with substantial overlap of the parallel rings. The twisting of the side chains at 30° for syn-1 leads to a helical structure, whereas the 180° orientation of the side chains in anti-1 leads to a linear structure. The cadmium(II) complex is isostructural to syn-1 and the copper(II) is isostructural to anti-1. Although the supramolecular structures of these two compounds are similar to the matched iron(II) complexes because of the similar orientations of the side-chains and the consistent noncovalent interactions of the naphthalimide synthon, the larger size of cadmium(II) leads to a highly distorted structure about the metal. The copper(II) structure is also distorted, as expected for a d 9 complex.

Introduction 1

Important to crystal engineering is the elucidation of the factors that govern the noncovalent assembly of molecules (or other species) into their ultimate solid-state architectures.2 An essential key in this effort is the identification of reliable, robust, supramolecular synthons that can be transferred from one system to another.3 Hydrogen bonding is a widely used synthon in crystal engineering because the directionality and strength of its associative protocol enable facile and reliable transfer to other systems.4 In contrast, π 3 3 3 π stacking has a less predictable directional associative protocol because the interactions are generally weaker than with hydrogen bonding and that variable orientations of the moieties involved often occur.5,6 We have recently shown that it is possible to design, synthesize, and implement the use of strong and directional π 3 3 3 π stacking for crystal engineering, by building the 1,8-naphthalimide synthon into bis(pyrazolyl)methane ligands, Lbis in Chart 1. The coordination chemistry of these ligands with Re(I)7a and Ag(I)7b centers has demonstrated

that the 1,8-naphthalimide group leads to association into dimers and polymers by strong π 3 3 3 π stacking interactions, as expected for the interaction of two π-deficient aromatic systems.5 Importantly, the interaction is frequently directionally specific; in the π 3 3 3 π stacking interactions the dipole vectors (which run from the center of the fused aromatic group through the nitrogen atom of the 1,8-naphthalimide groups) are generally oriented at 180°, antiparallel.7 To date, the ligands used by us in studying the noncovalent organization imparted by the 1,8-naphthalimide synthon have been built on bidentate donor groups and have limited Chart 1

To whom correspondence should be addressed. *E-mail: reger@mail. chem.sc.edu. pubs.acs.org/crystal

Published on Web 10/09/2009

r 2009 American Chemical Society

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possible orientations (see Lbis in Chart 1) due to the short link (one CH2 group) between the naphthalimide moiety and the donor group.7 In related work with polytopic ligands based on joining multiple tris(pyrazolyl)methane units with a rigid central arene core through a flexible ether linkage, we have shown that these ligands are “structurally adaptive” because this ligand design allows them to be versatile in the self-assembly process.8 We report here the synthesis of a new ligand, Ltris in Chart 1, that has a similar arene core and flexible ether linkage joining the versatile tris(pyrazolyl)methane donor unit and the 1,8-naphthalimide synthon. This synthon introduces into our structurally adaptive poly(pyrazolyl)methane based ligands the new feature of organizing structures via noncovalent interactions. The new ligand has a number of special features: (a) when bonded to metals with octahedral coordination preferences, the rigid architecture of facial bonding is encoded in each tris(pyrazolyl)methane group; (b) all three rigid parts, the tris(pyrazolyl)methane group, the central arene ring and the naphthalimide moiety are connected by flexible chains, conferring to the ligand structurally adaptive properties; (c) molecular mechanics calculations show that the overall length and structure of Ltris, after coordination to an octahedral metallic center, prevent the formation of an “intramolecular” π 3 3 3 π stacking of the 1,8-naphthalimide group in the resulting covalent architecture. The chemistry of this ligand is featured here by the structures of four [(Ltris)2M]2þ complexes of metals (M = Fe, Cd, Cu) with octahedral preferences. Experimental Section General. All manipulations were carried out under a nitrogen atmosphere using standard Schlenk techniques or a Vacuum Atmospheres HE-493 drybox unless otherwise noted. Solvents for airsensitive synthetic procedures were dried by conventional methods and distilled under a dry N2 atmosphere immediately prior to use. NMR spectra were recorded by using a Varian Gemini 300. All reagents are commercially available (Aldrich) and were used without further purification. The 2,2,2-tris(1-pyrazoyl)ethanol starting material was synthesized according to published procedures.9 Preparation of 4-((2,2,2-Tris(1-pyrazoyl)ethoxy)methyl)benzonitrile. A mixture of 4-(bromomethyl)benzonitrile (2.00 g, 10.2 mmol) and 2,2,2-tris(1-pyrazolyl)ethanol (2.69 g, 11.0 mmol) was dissolved in dry THF (50 mL). This solution was added dropwise to a suspension of NaH (275 mg, 11.5 mmol) in THF (50 mL) over a 30 min period. This mixture was heated at reflux for 12 h, and then allowed to cool to room temperature. Water (ca. 25 mL) was carefully added to the reaction mixture to consume excess NaH and to dissolve all the solids. The mixture was then extracted with methylene chloride (50 mL), the organic layer was separated, dried over magnesium sulfate, and filtered, and the solvent was removed using rotary evaporation to afford a light yellow oil. This oil was triturated with a 20% NaOH solution (125 mL) for 6 h to remove excess 2,2,2-tris(1-pyrazolyl)ethanol. The waxy off-white solid was filtered, washed well with water, and dried under a vacuum overnight (2.62 g, 73%). IR (cm-1) 3137, 2860 (pz), 2226 (CN); 1H NMR (CHCl3) δ 4.59 (s, 2H, CH2), 5.17 (s, 2H, CH2), 6.35 (dd, 3H, J = 2.6, 1.8 Hz, 4-pz), 7.25 (d, J = 7.8 Hz, 2H, Ar), 7.39, 7.66 (dd, dd; 3H, 3H; J = 0.8, 2.6; 0.6, 1.8 Hz; 3,5-pz), 7.58 (d, J = 8.5 Hz, 2H, Ar). 13C NMR (CHCl3) δ 73.4 (CH2), 74.2 (CH2), 89.9 (C(pz)3), 106.9 (pz), 111.9 (Ar), 118.9 (CN), 127.9 (Ar), 130.9 (pz), 132.4 (Ar), 141.7 (pz), 142.8 (Ar). HRMS (ESIþ) for [C19H17N7OH (M þ H)]þ Calcd 360.1573, Obsd 360.1575. MS ESIþ m/z (rel. % abund.) [assgn]: 292 (100) [M - pz]þ, 360 (12) [M þ H]þ, 382 (12) [M þ Na]þ, 398 (5) [M þ K]þ, 423 (18) [M þ CH3CN þ Na]þ. Preparation of 4-((2,2,2-Tris(1-pyrazoyl)ethoxy)methyl)benzyl1,8-naphthalimide (Ltris). Into a 250 mL Schlenk flask, lithium aluminum hydride (450 mg, 11.9 mmol) was added under a nitrogen atmosphere. Dry THF (25 mL) was added to the Schlenk flask, and

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then 4-((2,2,2-tris(1-pyrazoyl)ethoxy)methyl)benzonitrile (1.41 g, 3.93 mmol) was dissolved in dry THF (25 mL) and added to the Schlenk flask dropwise and the resulting mixture was allowed to stir overnight. This suspension was diluted with diethyl ether (125 mL), cooled in an ice bath, and water (0.5 mL), 20% NaOH solution (0.5 mL), and additional water (1.5 mL) were added dropwise. The reaction was allowed to warm to room temperature and stirred for several hours. Volatiles were removed by rotary evaporation, and the residue was taken up in water (20 mL). This mixture was extracted with methylene chloride (3  25 mL). The combined organic extracts were dried over magnesium sulfate and filtered and the solvent was removed via rotary evaporation. The residue was dried under a vacuum to afford 1.36 g of crude 4-((2,2,2-tris(1pyrazoyl)ethoxy)-methyl)benzylamine as a viscous orange oil. This crude product was dissolved in ethanol (25 mL) and added into a 100 mL round-bottom flask. 1,8-Naphthalic anhydride (694 mg, 3.5 mmol) was added, the flask was equipped with a reflux condenser and the solution was heated at reflux for 24 h. The solution was allowed to cool to r.t. and filtered. The filtrate was washed three times with ethanol and three times with ether to afford an off-white powder (787 mg, 37%). IR (cm-1) 3137, 2874 (pz), 1698, 1655, 1626, 1591 (naphthalimide); 1H NMR (CHCl3) δ 4.46 (s, 2H, CH2), 5.08 (s, 2H, CH2), 5.36 (s, 2H, CH2), 6.30 (dd, 3H, J = 2.1, 1.6 Hz, 4-pz), 7.12 (d, J = 7.9 Hz, 2H, Ar), 7.40, 7.62 (d, d; 3H, 3H; J = 2.6, 1.8 Hz; 3,5-pz), 7.51 (d, J = 8.2 Hz, 2H, Ar), 7.73 (t, J = 7.9 Hz, 2H, nphth), 8.14 (d, J = 8.2 Hz, 2H, nphth), 8.59 (d, J = 6.5 Hz, 2H, nphth); 13C NMR (CHCl3) 43.4 (CH2), 73.6 (CH2), 74.1 (CH2), 89.9 (C(pz)3), 106.6 (pz), 122.8 (nphth), 127.1 (nphth), 128.0 (Ar), 128.3 (nphth), 129.4 (Ar), 131.1 (nphth), 131.6 (nphth), 131.8 (pz), 134.3 (nphth), 136.5 (Ar), 137.3 (Ar), 141.5 (pz), 164.4 (C = O). HRMS (ESIþ) for [Ltris þ H]þ Calcd 544.2097, Obsd 544.2101. MS ESIþ m/z (rel. % abund.) [assgn]: 408 (41) [M - H-(pz)2]þ, 476 (100) [M - pz]þ, 544 (39) [M þ H]þ, 566 (87) [M þ Na]þ, 582 (23) [M þ K]þ. Preparation of [Fe(Ltris)2](BF4)2 (1). A THF (20 mL) solution of Fe(BF4)2 3 6H2O (34 mg, 0.15 mmol) was treated dropwise with a THF solution (25 mL) of Ltris (108 mg, 0.19 mmol). After the solution was stirred for 30 min, the pink precipitate that formed was collected and dried (71 mg, 32%). ESþ/MS for {[(Ltris)2Fe][BF4]}þ: Calcd 1229.3429, Obsd 1229.3390. MS ESIþ m/z (rel. % abund.) [assgn]: 571 (100) [(Ltris)2Fe]2þ, 1187 (15) {[(Ltris)2Fe][formate]}þ. Preparation of [Cd(Ltris)2](BF4)2 (2). The same procedure was followed as with the iron complex using [Cd2(thf)5][BF4]410 in acetone followed by addition of hexanes at the end of the reaction to ensure complete precipitation (49 mg, 20%). ESþ/MS for {[(Ltris)2Cd][BF4]}þ: Calcd 1287.3126, Obsd 1287.3131. MS ESIþ m/z (rel. % abund.) [assgn]: 600 (2) [(Ltris)2Cd]2þ, 1245 (2) {[(Ltris)2Cd][formate]}þ. Preparation of [Cu(Ltris)2](BF4)2 (3). The same procedure was followed as with the iron complex using Cu(BF4)2 3 3H2O (63 mg, 27%). ESþ/MS for {[(Ltris)2Cu][BF4]}þ: Calcd 1236.3373, Obsd 1236.3351. MS ESIþ m/z (rel. % abund.) [assgn]: 575 (9) [(Ltris)2Cu]2þ, 1149 (65) [(Ltris)2Cu]þ, 1245 (2) {[(Ltris)2Cd][formate]}þ. X-ray Structure Determinations. Suitable crystals for X-ray crystal structure determinations were grown by vapor diffusion of diethyl ether into an acetonitrile solution of the compounds. General crystal data and structure refinement details can be found in Table 1. X-ray diffraction intensity data from a reddish needle (syn-1), orange needle (anti-1), colorless needle (syn-2), and pale-blue needle (anti-3) were measured at 150(1) K on a Bruker SMART APEX diffractometer (Mo KR radiation, λ = 0.71073 A˚).11 Raw area detector data frame integration was performed with SAINTþ.11 Compounds syn-1 and syn-2 diffracted weakly because of anion and solvent disorder (discussed below), and these data sets were truncated at 2θmax = 45. Final unit cell parameters were determined by least-squares refinement of large sets of reflections taken from each data set. All non-hydrogen atoms were refined with anisotropic displacement parameters, except where noted. Hydrogen atoms were placed in geometrically idealized positions and included as riding atoms. The structures were solved by either direct methods using SHELXS (anti-1 and anti-3),12 or with SIR2002 (syn-1 and syn-2).13 All difference Fourier calculations and full-matrix least-squares refinements against F2 were performed with SHELXTL.12

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Table 1. Selected Crystal Data and Structure Refinement for [Fe(Ltris)2](BF4)2 (syn-1), [Fe(Ltris)2](BF4)2 3 4(CH3CN) (anti-1), [Cd(Ltris)2](BF4)2 (syn-2), and [Cu(Ltris)2](BF4)2 3 3(CH3CN) 3 0.5(C4H10O) (anti-3) syn-1 anti-1 syn-2 anti-3 formula formula weight cryst syst space group a, A˚ b, A˚ c, A˚ β, deg V, A˚3 Z no. data (I > 2σ(I)) 2θmax R1 I > 2σ(I) wR2 I > 2σ(I) ΔF (max/min)

C62H50B2F8FeN14O6 1316.63 orthorhombic Pbcn 19.8574(9) 20.9036(10) 14.9561(7) 6208.1(5) 4 4087 (2641) 45.1 0.048 0.093 þ0.38/-0.22

C70H62B2F8FeN18O6 1480.85 monoclinic C2/c 29.4569(12) 9.4286(4) 26.0582(11) 106.960(1) 6922.6(5) 4 6135 (4062) 50.1 0.050 0.124 þ0.73/-0.33

Compounds syn-1 and syn-2 crystallize in the space group Pbcn as determined uniquely by the pattern of systematic absences in the intensity data. The asymmetric unit of each consists of half of one [M(Ltris)2]2þ cation (M = Fe, Cd) located on a 2-fold axis of rotation, one disordered BF4- anion, and a region of disordered solvent species. The BF4- disorder was modeled with two independent orientations with refined populations 0.795(6)/0.205(6) (syn-1) and 0.83(1)/0.17(1) (syn-2), both constrained to sum to unity). The geometry of the minor components were restrained to be similar to that of the major components (SHELX SAME instruction, 10 total restraints), and the minor component atoms were refined isotropically. No reasonable disorder model was achieved for the diffusely scattering solvent species in either structure. Their contribution to the structure factors was therefore removed with the SQUEEZE program.14 The disorder regions are centered at (0,1/2,0) and equivalent positions and occupies a total volume per unit cell of 499.5 A˚3 (8.0% of the unit cell volume) for syn-1 and 585.5 A˚3 per unit cell (9.1% of the total unit cell volume) for syn-2. The reported dcalc, F.W., and F(000) correspond to known unit cell contents only. Compounds anti-1 and anti-3 crystallize in the space group C2/c as determined by the pattern of systematic absences in the intensity data and by the successful solutions and refinements of the structures. The asymmetric unit of each consists of half of one [M(Ltris)2]2þ cation (M = Fe, Cu) located on a crystallographic inversion center, one BF4- anion, and a variable amount of acetonitrile and/or diethyl ether solvent molecules, which were modeled as follows. In anti-1, two acetonitrile molecules are present. Both display inflated displacement parameters, indicating minor disorder of these groups, which was not modeled. The largest peak in the final difference map is located near C54 of an acetonitrile molecule. In anti-3, approximately two acetonitrile molecules at positions similar to anti-1 were observed, along with additional nonnegligible electron density peaks in the same region. These peaks were modeled as 1/4 of a diethylether molecule over both sites. The occupancies of the acetonitrile molecules were each manually fixed at 3/4 to make physical sense. A total of 11 distance restraints were used to maintain a chemically reasonable geometry for the disordered species. Atoms of the ether molecule were refined with a common isotropic displacement parameter. It should be emphasized that the identity and quantity of the disordered species in anti-3, although reasonably modeled, are less precisely determined than the other components of the structure. The largest peak remaining in the final difference map is located in the disorder region.

Results and Discussion Syntheses and Characterization. Scheme 1 shows the synthesis of the new ligand Ltris. Starting from 2,2,2-tris(1pyrazolyl)ethanol and 4-(bromomethyl)benzonitrile, the tris(pyrazolyl)methane substituted nitrile was readily prepared under basic conditions. The reduction of this new nitrile with LiAlH4 afforded the amine, which reacts with

C62H50B2F8CdN14O6 1373.18 orthorhombic Pbcn 20.3130(7) 20.8605(8) 15.1162(6) 6405.3(4) 4 4222 (3121) 45.1 0.066 0.182 þ2.18/-1.00

C70H64B2F8CuN17O6.5 1484.54 monoclinic C2/c 29.3169(16) 9.5421(5) 26.2643(14) 108.627(1) 6962.4(6) 4 5546 (4120) 48.3 0.042 0.106 þ0.44/-0.28

Scheme 1. Synthesis of the New Ligand Ltris and Its Possible Conformations before and after Coordination to an Octahedral Metallic Center; i: NC-Ph-CH2Br, NaH, THF; ii: LiAlH4, THF; iii: naphthalic anhydride (C12H6O3), ethanol

1,8-naphthalic anhydride to afford the desired ligand Ltris in good overall yield. The metal complexes were obtained by the reaction of iron(II), cadmium(II), and copper(II) tetrafluoroborate salts with two equivalents of Ltris. This procedure afforded three new compounds, [Fe(Ltris)2](BF4)2 (1), [Cd(Ltris)2](BF4)2 (2), and [Cu(Ltris)2](BF4)2 (3). These compounds are insoluble in common organic solvents and only slightly soluble in acetonitrile. Crystallization by vapor diffusion of diethyl ether into an acetonitrile solution of the compounds yielded two pseudopolymorphs for 1, but only one forms each of 2 and 3. Solid-State Structures. Examination of ligand Ltris in Scheme 1 shows two limiting orientations of the C(pz)3 (pz = pyrazolyl ring) and naphthalimide functional groups,

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Figure 1. Structural diagram of asymmetric unit of the cation in syn-1.

either a syn or anti conformation. In all four structures, the ligands adopt the U-shaped, syn conformation shown in the middle-right of Scheme 1. After coordination to a metal ion, two orientations of the syn ligands are possible; a syn or anti orientation of the side chain of the ligand with respect to the C(pz)3-M-(pz)3C, pseudo-3-fold axis of the metal complex, as shown in the bottom of Scheme 1. In the case of 1, two pseudopolymorphs formed in the same crystallization procedure, one with a syn orientation and the other with an anti orientation of the ligands with respect to the C(pz)3-M(pz)3C axis, hereafter denoted as syn-1 and anti-1. In the case of 2, only the syn-2 form was identified, and with 3 only the anti-3 form was present. Syn-[Fe(Ltris)2](BF4)2 (syn-1). Figure 1 shows a structural diagram of the asymmetric unit of the cation, which consists of half of one [Fe(Ltris)2]2þ cation located on a 2-fold axis of rotation. The numbering scheme shown applies to all four structures reported herein, although the symmetry equivalent N-coordinating atoms differ because of C2 and Ci point symmetry of the syn and anti compounds, respectively. In the syn structures, the symmetry-equivalent N(11a) and N(21a) are oriented cis with respect to N(11) and N(21) because of their positions relative to the 2-fold rotational axis, while (N31a) is trans. All three matched symmetry equivalent nitrogen atoms in the centrosymmetric anti structures are oriented trans. The ligand in all four structures is in the syn configuration with respect to the central arene ring, as pictured to the right in Figure 1. The structure of the cationic building blocks features two ligands bonded to one Fe(II) center with both tris(pyrazolyl)methane donor sets in a κ3-coordination mode, Figure 2. The iron center is in an almost perfect octahedral environment (see Table 2), with an average Fe-N bond length of 1.94 A˚, a typical distance for a low spin Fe(II) compound.15 The low spin, diamagnetic nature of the complex was confirmed by a magnetic measurement on a SQUID magnetometer. The side chains on the two Ltris ligands are in a syn orientation with respect to the C(pz)3-Fe-(pz)3C axis, with the two groups twisted about 30° around the axis, as pictured at the bottom of Figure 2. As anticipated from molecular modeling studies, intramolecular π 3 3 3 π stacking is not observed, even in this orientation of the 1,8-naphthalimide groups. The cationic building blocks of syn-1 are associated into one-dimensional (1D) supramolecular spirals by the anticipated intermolecular π 3 3 3 π stacking interaction of the 1,8naphthalimide units, see Figure 3. Each naphthalimide unit

Figure 2. The structure of the cationic syn-1 complex, showing the syn orientation of the ligand side chains with a rotation angle of about 30°.

reaches past the naphthalimide unit on the adjacent cation, leading to a π 3 3 3 π stack using the face of the naphthalimide unit directed toward its metal complex, interlocking each pair of U-shaped ligands. We have not observed this arrangement for the π 3 3 3 π stack previously, but this ligand separates the naphthalimide synthon from the poly(pyrazolyl)methane unit to a much greater degree than in our previous ligands. Figure 4 shows details of the π 3 3 3 π stacking interaction. The naphthalimide groups are stacked with their dipole vectors oriented at 180° with the two rings exactly parallel. The perpendicular distance between the two naphthalimide units is 3.45 A˚. As seen in Figure 4a, while the rings are stacked virtually on top of each other along the dipole vector direction, there is lateral slippage of the aromatic rings. We have defined a parameter, χ, that quantifies the amount of slippage one ring involved in the π 3 3 3 π stack has with respect to the other, a combined parameter for both directions of possible slippage. The slippage parameter is the third side of the right triangle formed with the average perpendicular distance between the rings and the line joining the central carbon atoms of the two naphthalimide rings.16 For syn-1, χ is 2.22 A˚. For comparison, in other systems we have observed χ values to range from 0.62 to 2.90 A˚, with values much above 2.4 A˚ indicating substantial slippage and a weaker interaction.16 An interesting consequence of this interlocking arrangement with moderate slippage of the naphthalimide synthons, coupled with the presence of the linking arene group in the ligand, is that the π 3 3 3 π stacking interaction is supported by a cooperative pair of C-H 3 3 3 π interactions of one of the aromatic hydrogen atoms on each naphthalimide with the linking arene group of the other ligand, as clearly shown in both pictures in Figure 4. The

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Table 2. Selected Bond Distances and Angles

M-N(11) M-N(21) M-N(31)

syn-1

anti-1

syn-2

anti-3

1.931(3) 1.958(3) 1.948(3)

Bond Distances (A˚) 1.957(2) 1.948(2) 1.956(2)

2.295(5) 2.306(6) 2.268(5)

2.089(2) 2.195(2) 2.005(2)

Bond Angles (deg) N(11)-M-N(21) N(11)-M-N(31) N(21)-M-N(31) N(11)-M-N(11a) N(11)-M-N(21a) N(11)-M-N(31a) N(21)-M-N(21a) N(21)-M-N(31a) N(31)-M-N(31a) MN-NC(1) average torsion

87.36(12) 87.79(13) 86.55(13) 92.74(17) 178.44(14) 91.90(13) 92.58(17) 93.77(13) 179.55(19) 5.7

Figure 3. Two syn-1 cationic complexes associated by an interlinked, π 3 3 3 π stacking interaction of the naphthalimide moieties.

metrics of the interaction are H 3 3 3 centroid and C 3 3 3 centroid distances of 2.68 and 3.54 A˚, respectively, with a C-H 3 3 3 centroid angle of 151°. The combination of all the parameters indicates that the stacking interaction is strong. These noncovalent interactions combined with the 30° degree twist of the 1,8-naphthalimide group with respect to the coordinated tris(pyrazolyl)methane moieties generate a helical structure, Figures 3 and 5. Anti-[Fe(Ltris)2](BF4)2 3 4(CH3CN) (anti-1). As with syn-1, the asymmetric unit consists of half of one [Fe(Ltris)2]2þ cation, only in this case located on a crystallographic inversion center rather than a 2-fold axis of rotation. Two Ltris ligands are bonded to one Fe(II) center with the tris(pyrazolyl)methane donor sets acting in the κ3-coordination mode, Figure 6. All bond lengths and angles, Table 2, are about the same as with syn-1. The iron center is again in a low-spin octahedral environment, with an average Fe-N bond length of 1.95 A˚. Another similarity of this compound with syn-1 is the syn conformation of the ligand (the naphthalimide and [C(pz)3] moieties) with respect to the central arene ring. The major difference between syn-1 and anti-1 is, of course, the anti orientation of the side chains in the two Ltris groups with respect to the C(pz)3-Fe-(pz)3C

87.35(10) 86.50(9) 87.37(9) 180 92.65(10) 93.50(9) 180 92.63(9) 180 1.7

79.20(19) 78.92(19) 81.00(18) 88.2(3) 161.4(2) 110.2(2) 116.1(3) 92.51(18) 167.8(3) 27.2

83.49(8) 83.57(8) 84.31(9) 180 96.51(9) 96.44(8) 180 95.69(9) 180 3.5

Figure 4. Views of the naphthalimide π 3 3 3 π stack of syn-1 perpendicular (a) and parallel (b) to the arene plane.

axis, Figure 6. As shown, the two side arms of the ligands are oriented in opposite directions, as imposed by the crystallographic inversion center. The cationic building blocks of anti-1 are associated into 1D supramolecular chains by intermolecular π 3 3 3 π stacking interaction of the 1,8-naphthalimide units; see Figure 7. Again, each naphthalimide unit reaches past the naphthalimide unit on the adjacent cation, leading to a π 3 3 3 π stack using the face of the naphthalimide unit directed toward its metal complex. This interaction is similar to the one for syn-1 shown in Figure 4: the 180° orientation of the naphthalimide group dipole vectors, a perpendicular distance between two naphthalimide units of 3.49 A˚, and a dihedral angle of 0°. In this case, there is less lateral slippage of the aromatic rings with χ = 1.74 A˚. Again, as clearly seen in Figure 7, the π 3 3 3 π stacking interaction is supported by a cooperative pair of C-H 3 3 3 π interaction of one of the aromatic hydrogen atoms on each naphthalimide with the linking arene group of the other ligand. The metrics of the interaction are H 3 3 3 centroid and C 3 3 3 centroid distances of 2.52 and 3.43 A˚, respectively, with a C-H 3 3 3 centroid angle of 160°. In contrast to syn-1, the 180° degree orientation of the 1,8naphthalimide side chains generates a linear, interlocking chain type structure, Figure 7.

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Figure 7. Chain formation of anti-1. Figure 5. Helical chain formation of syn-1.

Figure 6. The structure of the cation in anti-1, showing the anti orientation of the ligand side chains with a rotation angle of 180°.

Syn-[Cd(Ltris)2](BF4)2 (syn-2). This compound is isostructural with syn-1. The π 3 3 3 π stacking interaction has the following metrics: the naphthalimide groups are stacked with their dipole vectors oriented at 180°; the dihedral angle is 0°; the perpendicular distance between two naphthalimide units is 3.45 A˚ and χ = 2.15. The metrics of the C-H 3 3 3 π interaction are H 3 3 3 centroid and C 3 3 3 centroid distances of 2.71 and 3.59 A˚, respectively, with a C-H 3 3 3 centroid angle of 155°. Although the geometry about the cadmium center is approximately octahedral, the much larger average Cd-N bond length of 2.29 A˚ when compared to the iron(II) complexes causes structural distortions. As we and others have noted previously, because of the inflexibility of the sp3 hybridized central methine carbon atom (C(1)), κ3-poly(pyrazolyl)methane (and poly(pyrazolyl)borate, where the central atom is B) ligands need to distort to bond to metal atoms that yield M-N bond distances much larger than 2 A˚.17 The most often observed distortion is ring twisting with respect to ideal C3v symmetry, as measured by the

MN-NC(1) torsion angle. In an ideal arrangement, this angle would be 0° and the metal would be in the plane of the pyrazolyl ring (as observed in the bottom drawings of Figures 2 and 6, where the orientation is down the 3-fold axis, for the iron(II) complexes). As seen in Table 2, the average torsion angle for syn-2 is large, 27.2°, whereas in the other three compounds, with M-N bond distances in the “ideal” range for a tris(pyrazolyl)methane ligand, the angles are much smaller. This ring tilting, coupled with the restrictions of the bite angle of each tris(pyrazolyl)methane unit, even when tilted, leads to the distortions in octahedral angles shown in Table 2. It is interesting that these distortions in the metal environment do not impact the overall supramolecular structure. Anti-[Cu(Ltris)2](BF4)2 3 3(CH3CN) 3 0.5(C4H10O) (anti-3). This compound is isostructural with anti-1. The metrics of the π 3 3 3 π stacking interaction of the 1,8-naphthalimide units are as follows: the naphthalimide groups are stacked with their dipole vectors oriented at 180°; the dihedral angle is 0°; the perpendicular distance between two naphthalimide units is 3.49 A˚ and χ = 1.93 A˚. The metrics of the C-H 3 3 3 π interaction are H 3 3 3 centroid and C 3 3 3 centroid distances of 2.52 A˚ and 3.42 A˚ respectively, with a C-H 3 3 3 centroid angle of 158°. In this case, the copper atom is in a distorted octahedral environment (see Table 2); there are four Cu-N bond lengths of about 2.0 A˚ and two longer axial Cu-N bond lengths of about 2.2 A˚. These differences in the coordination environment of the copper center are expected because of the Jahn-Teller effect and do not influence the remaining structural characteristics of this compound. It is interesting to note that a κ3-tris(pyrazolyl)methane ligand with one longer M-N bond, as observed here, does not distort by ring twisting; the average twist angle is only 3.5°. Surprisingly, the most tilted ring 1 at 6.1° does not correlate with the longest Cu-N(21) bond distance, where ring 2 associated with this longest bond has a tilt of 4.1°. The 1,8-Naphthalimide Synthon. The common structural denominator of all four of these structures is the directional π 3 3 3 π stacking of the naphthalimide groups with their dipole vectors oriented at 180°. In all four cases, this interaction leads to a 1D supramolecular structure whose shapes depend on the relative orientation (syn or anti) of the side chains of the ligands with respect to the C(pz)3-M-(pz)3C axis. In the syn cases, helical coordination polymers were

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obtained whereas in the anti cases linear coordination polymers were obtained. An interesting difference in the π 3 3 3 π stacking interaction observed here and that reported earlier in our analogous methylene-linked bis(pyrazolyl)methane ligands such as Lbis, Scheme 1, is that with Ltris the longer linking piece favors the stack where each naphthalimide group reaches past the neighboring one and interacts on the side that faces its metal complex, producing an interlocked structure. In contrast, the rings in the metal complexes of Lbis interacted on the “outside” face as necessitated by the shorter link. The “interlocking” arrangement of the π 3 3 3 π stack observed here leads to additional C-H 3 3 3 π interactions that support the stacking arrangement. Interestingly, in both arrangements, the distances between the metal centers are fairly constant at ca. 15 A˚. The structural characteristics of these compounds definitely demonstrate the robustness of the 1,8-naphthalimide synthon. While the basic characteristics of this interaction (strength and directionality) are maintained throughout all the structures reported here, there are small differences between the π 3 3 3 π stacking of the naphthalimide groups in the case of the syn compounds when compared to the anti compounds. One difference is slighter greater perpendicular distances in the cases of the anti compounds (around 3.5 A˚) compared to the cases of syn compounds (around 3.4 A˚) and a second is the greater lateral slippage of the naphthalimide units in the case of syn compounds (average χ = 2.18 A˚) when compared with the anti compounds (χ = 1.83 A˚). These small differences in the interaction parameters are most likely a result of crystal packing forces and appear to be independent of the nature of the metallic center. Conclusion This work successfully demonstrates the use of π 3 3 3 π stacking interactions in a rational manner in crystal engineering by building a ligand that joins a 1,8-naphthalimide synthon with a tris(pyrazolyl)methane coordination unit through an arene core and flexible ether linkage. The strong π 3 3 3 π stacking of the 1,8-naphthalimide functional group dominates the noncovalent structures of the four compounds reported here. By connecting the two functional groups with a relatively long chain, we have observed a new type of π 3 3 3 π stacking for the 1,8-naphthalimide groups where they reach past each other and interact on the sides that face the metals to which they are attached, forming a strong “interlocking” arrangement. In this configuration, the π 3 3 3 π stacking is supported by a C-H 3 3 3 π interaction between one of the aromatic hydrogen atoms on each naphthalimide with the linking arene group of the other ligand. We note that the 1,8-naphthalimide group can be easily incorporated into a large variety of compounds (with donor properties or not) that have specially designed functions. From a crystal engineering point of view, the ease of the synthetic procedures involving introduction of the naphthalimide group and the facile transfer of this directional associative protocol from one system to another opens the door for the synthesis of new supramolecular, discrete species or coordination polymers. Acknowledgment. The authors acknowledge the National Science Foundation (CHE-0715559) for financial support. We thank Dr. William Cotham for obtaining the mass spectrometry data.

Reger et al. Supporting Information Available: X-ray crystallographic files in CIF format. This information is available free of charge via the Internet at http://pubs.acs.org.

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