Metallacyclic Zinc Complexes of Alkylidene-Linked Bitopic Bis

Apr 20, 2007 - Synthesis, Crystal Structure, and Characterization of three New Letrozole Complexes. Yun-Zhi Tang , Man Zhou , Jun Huang , Zan Cao ...
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Metallacyclic Zinc Complexes of Alkylidene-Linked Bitopic Bis(pyrazolyl)methane Ligands: Unusual Exocyclic Bridging Fluoride Ligands Daniel L. Reger,* Russell P. Watson, Mark D. Smith, and Perry J. Pellechia

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 6 1163-1170

Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed January 23, 2007; ReVised Manuscript ReceiVed March 7, 2007

ABSTRACT: The alkylidene-linked, bitopic bis(pyrazolyl)methane ligands 1,1,3,3-tetrakis(1-pyrazolyl)propane ([CH(pz)2]2CH2; L1; pz ) 1-pyrazolyl), 1,1,4,4-tetrakis(1-pyrazolyl)butane ([CH(pz)2]2(CH2)2; L2), and 1,1,5,5-tetrakis(1-pyrazolyl)pentane ([CH(pz)2]2(CH2)3; L3) react with Zn(BF4)2‚5H2O in tetrahydrofuran (THF) to afford complexes with the empirical formula [ZnL](BF4)2 (L ) L1 (1), L2 (2), and L3 (3)). 1H NMR studies indicate that all three complexes are fluxional in solution. Recrystallization of the complexes from acetonitrile yields the crystallographically characterized compounds [Zn(µ-[CH(pz)2]2CH2)(H2O)(CH3CN)]3(BF4)6‚2.5CH3CN (1a), {[Zn2(µ2-[CH(pz)2]2[CH2]2)2(µ2-F)](BF4)3‚CH3CN‚0.5H2O}∞ (2a), and [Zn3(µ-[CH(pz)2]2(CH2)3)3(H2O)5](BF4)6‚4CH3CN (3a). Compounds 1a and 3a are trimeric metallacycles in which acetonitrile and water molecules complete the coordination spheres of the octahedral or trigonal bipyramidal Zn2+ ions. Compound 2a consists of one-dimensional chains of dimeric metallacycles connected through the trigonal bipyramidal zinc cations by exocyclic bridging fluoride ligands, the origin of which must be fluoride abstraction from the BF4- counterions. The polymeric chains of 2a are also supported by π-π stacking between adjacent pyrazolyl groups, and the compound contains highly ordered solvent molecules that alternate in layers with the intricate gridlike structure of the cationic polymers. Introduction The coordination chemistry of poly(pyrazolyl)methane compounds continues to expand into the broad applicability developed for their poly(pyrazolyl)borate counterparts.1 Like the borate compounds, improved synthetic methods have led to derivatives of the original poly(pyrazolyl)methane compounds by either substitution of the pyrazolyl rings by bulky groups (second-generation ligands)1,2 or by substitution of the carbon “backbone” of the ligands by a variety of functional groups (third generation ligands).3 Second- and third-generation ligands provide finer control of the electronic and steric properties of the discrete or polymeric complex molecules and ions formed from these ligands as well as increasing predictive power over the extended covalent or noncovalent supramolecular structures of many complexes.4,5 We are developing series of third-generation poly(pyrazolyl)methane compounds in which two or more ligating sites are linked by organic moieties of varying flexibilities to form polytopic ligands.6 The choice of linkers combined with the properties of the pyrazolyl ligating groups are designed to allow the study of the factors that influence both the molecular and the supramolecular structures of these complexes, particularly in the solid state. One of our goals is to enable crystal engineering of these complexes through the geometric preferences and potential for noncovalent interactions specifically designed into the poly(pyrazolyl)methane ligands.7 Recently, we have contrasted the molecular and supramolecular coordination chemistries of flexible ligands with those of more rigid ones. We designed bis(pyrazolyl)methane compounds linked by alkylidene groups to give the series [CH(pz)2]2(CH2)n (pz ) 1-pyrazolyl), where the ligands in which n ) 1, 2, and 3 are denoted L1, L2, and L3, respectively.6a,6b In addition, we prepared the arene-linked ligands m-[CH(pz)2]2C6H4 * To whom correspondence should be addressed. E-mail: reger@ mail.chem.sc.edu.

(Lm),6c p-[CH(pz)2]2C6H4 (Lp),6c and 1,3,5-[CH(pz)2]3C6H3 (L3).6d The arene-linked ligands as well as the methylene-linked L1 are categorized as “fixed” because the respective distances between the central carbon atoms of the CH(pz)2 donor groups are nearly constant when comparing analogous families of their metal complexes. These ligands, however, are not “rigid” because of the flexibility imparted by rotation about the methine-arene (Ar-CH(pz)2) or methine-methylene (CH2CH(pz)2) bond. The ligands L2 and L3, on the other hand, do have the conformational freedom to vary the intermetallic distances in their complexes and are therefore considered flexible ligands. We found that with weakly coordinating anions (BF4-, SO3CF3-, and PF6-), the m-arene-linked ligands and the alkylidene-linked ligands, regardless of length, consistently direct formation of argentacyclic dimeric cations [Ag2(µ-L)2]2+. In contrast, the complex structure in the presence of the more strongly coordinating NO3- anion in the alkylidene systems is variable, providing discrete acyclic chains, polymeric cycles, or acyclic polymers, depending on the length of the linker. In the fixed systems, the use of p-arene ligands also results in acyclic coordination polymers.6a,6b,8 Equally intriguing are the reactions of the m-arene-linked ligands with BF4- salts of some divalent transition metals, which afford the cyclic metallacycles [M2(µ-Lm)2(µ-F)](BF4)3 (M ) Fe, Zn), [Zn2(µ-L3)2(µ-F)](BF4)3, and [Cd2(µ-Lm)2(µ-F)2](BF4)2.9 These complexes are similar to those in the silver(I) compounds but also contain fluoride ligands bridging the two metal ions as in Figure 1. The origin of fluoride ligand must be from abstraction from the BF4- counterions by the dinuclear Lewis acid fragments. The m-arene-linked ligands thus provide a reliable and convenient entry into transition metal fluoride coordination chemistry. Having attributed the fluoride abstraction observed in these reactions to a combination of the dominance of the bimetallic metallacyclic structure and the desire of the divalent cations for higher coordination numbers, we were interested in whether the alkylidene-linked ligands

10.1021/cg070078s CCC: $37.00 © 2007 American Chemical Society Published on Web 04/20/2007

1164 Crystal Growth & Design, Vol. 7, No. 6, 2007

Figure 1. Cationic metallacycle of [M2(µ-Lm)2(µ-F)](BF4)3 (M ) Fe, Zn; Lm ) m-[CH(pz)2]2C6H4; see ref 9). Color scheme: metal cation ) pink; F ) green; C ) yellow; N ) blue; H ) white.

would provide analogous fluoride abstraction and bridging, given that these ligands are also known to direct metallacycle formation. We report here the reactions of the alkylidene-linked ligands L1, L2, and L3 with Zn(BF4)2 and a discussion of the solution and solid-state structures of the resulting complexes.

Experimental Section General Considerations. Air-sensitive materials were handled under a nitrogen atmosphere using standard Schlenk techniques or in a Vacuum Atmospheres HE-493 dry box. All solvents were dried by conventional methods prior to use. The compounds [CH(pz)2]2CH2,6b [CH(pz)2]2(CH2)2,6a and [CH(pz)2]2(CH2)36a were prepared as previously described. Zn(BF4)2‚5H2O was purchased from Aldrich and used as received. Reported melting points are uncorrected. 1H and 19F NMR spectra were recorded on a Varian Mercury/VX 300, Varian Mercury/ VX 400, or Varian INOVA 500 spectrometer. All chemical shifts are in ppm. 1H NMR spectra were referenced to residual undeuterated solvent signals. 19F NMR spectra were externally referenced to CF3CO2H, and the chemical shifts are reported vs CCl3F. Mass spectrometric measurements were obtained on a MicroMass QTOF spectrometer. Elemental analyses were performed on vacuum-dried samples by Robertson Microlit Laboratories (Madison, NJ). Synthesis of Zinc Complexes. The zinc complexes were prepared by the following general procedure: A 100-mL THF solution of Zn(BF4)2‚5H2O was added by cannula to a 20-mL THF solution of the ligand. A white solid precipitated during the addition, and the resulting suspension was stirred at room temperature overnight. The solid was isolated by gravity filtration, washed with 20 mL of THF and 20 mL of Et2O, and dried in vacuo. Recrystallization by the vapor diffusion of diethyl ether into concentrated acetonitrile solutions of the crude solid yielded pure, microcrystalline powder of the complex. [Zn([CH(pz)2]2CH2)](BF4)2 (1). When using 0.50 g (1.6 mmol) of Zn(BF4)2‚5H2O and 0.48 g (1.6 mmol) of [CH(pz)2]2CH2 (L1), the yield was 0.68 g (80%) of 1. mp: >300 °C. 1H NMR (400 MHz, CD3CN, 25 °C): δ 7.93 (br s), 7.73 (br s), 6.43 (br s). 1H NMR (500 MHz, CD3CN, -40 °C): δ 8.29 (d, J ) 2.2 Hz, 2 H, 3/5-pz), 8.05 (d, J ) 2.0 Hz, 2 H, 3/5-pz), 7.93 (d, J ) 2.4 Hz, 2 H, 3/5-pz), 7.78 (d, J ) 2.4 Hz, 2 H, 3/5-pz), 6.76 (t, J ) 2.4 Hz, 2 H, 4-pz), 6.54 (t, J ) 2.3 Hz, 2 H, 4-pz), 6.40 (t, J ) 8.1 Hz, 2 H, CH(pz)2), 2.88 (t, J ) 8.1 Hz, 2 H, -CH2-). 19F NMR (376 MHz, CD3CN, 25 °C): δ -145 (BF3‚CD3CN), -150 (BF4-). MS ESI(+) m/z (rel % abund) [assgn]: 939 (1) [L2Zn2F(BF4)2]+, 767 (15) [L2ZnBF4]+, 699 (20) [L2ZnF]+, 391 (10) [LZnF]+, 340 (40) [L2Zn]2+, 309 (60) [L + H]+, 241 (100) [L - pz]+. Recrystallization by the slow diffusion of Et2O into dilute 1-mL acetonitrile solutions of 1 afforded X-ray quality crystals of the compound [Zn(µ-[CH(pz)2]2CH2)(H2O)(CH3CN)]3(BF4)6‚2.5CH3CN (1a). Elemental analysis revealed that upon removal of the crystals from the mother liquor and drying in vacuo, inner and outer sphere water and acetonitrile molecules were lost, leaving unsolvated 1: Anal. Calcd.

Reger et al. for C15H16B2F8N8Zn: C, 32.92; H, 2.95; N, 20.47. Found: C, 32.81; H, 2.75; N, 20.49. [Zn([CH(pz)2]2(CH2)2)](BF4)2 (2). When using 1.00 g (3.04 mmol) of Zn(BF4)2‚5H2O (in 450 mL of THF) and 1.00 g (3.10 mmol) of [CH(pz)2]2(CH2)2 (L2; in 100 mL of THF), the yield was 1.54 g (90%). mp: 258 °C dec. 1H NMR (300 MHz, CD3CN, 25 °C): δ 8.09 (br s), 7.58 (br s), 6.97 (br s), 6.51 (br s), 2.04 (br s, -CH2CH2-). 1H NMR (500 MHz, CD3CN, -40 °C): δ 8.13 (d, J ) 2.0 Hz, 2 H, 3/5-pz), 8.01 (d, J ) 2.0 Hz, 2 H, 3/5-pz), 7.87 (d, J ) 2.5 Hz, 2 H, 3/5-pz), 7.46 (d, J ) 1.5 Hz, 2 H, 3/5-pz), 6.85 (br s, 1 H, CH(pz)2), 6.83 (br s, 1 H, CH(pz)2), 6.60 (t, J ) 2.3 Hz, 2 H, 4-pz), 6.30 (t, J ) 2.5 Hz, 2 H, 4-pz), 1.93-1.89 (m, overlap with residual undeuterated solvent, -CH2CH2-). 19F NMR (376 MHz, CD3CN, 25 °C): δ -143 (BF3‚CD3CN), -149 (BF-4-). MS ESI(+) m/z (rel % abund) [assgn]: 967 (1) [L2Zn2F(BF4)2]+, 795 (15) [L2ZnBF4]+, 727 (20) [L2ZnF]+, 405 (20) [LZnF]+, 354 (50) [L2Zn]2+, 323 (75) [L + H]+, 255 (100) [L - pz]+. Recrystallization by the slow diffusion of Et2O into dilute 1-mL acetonitrile solutions of 2 afforded X-ray quality crystals of the compound {[Zn2(µ2-[CH(pz)2]2[CH2]2)2(µ2-F)](BF4)3‚CH3CN‚0.5H2O}∞ (2a). A quantity of crystals insufficient for elemental analysis was obtained. Rapid recrystallization from acetonitrile/ether yielded a microcrystalline solid that when dried was consistent with unsolvated 2: Anal. Calcd. for C16H18B2F8N8Zn: C, 34.23; H, 3.23; N, 19.96. Found: C, 33.97; H, 3.34; N, 19.66. [Zn([CH(pz)2]2(CH2)3)](BF4)2 (3). When using 0.50 g (1.6 mmol) of Zn(BF4)2‚5H2O and 0.52 g (1.5 mmol) of [CH(pz)2]2(CH2)3 (L3), the yield was 0.74 g (81%). mp: 225 °C dec. 1H NMR (400 MHz, CD3CN, 25 °C): δ 8.08 (br s), 7.88 (br s), 6.76 (br s), 6.54 (br s). 1H NMR (500 MHz, CD3CN, -40 °C): δ 8.19-7.74 (m, 3/5-pz), 6.626.51 (m, 4-pz and CH(pz)2), 2.02 (br s, -CH2CH2CH2-), 1.60 (br s, -CH2CH2CH2-). 19F NMR (376 MHz, CD3CN, 25 °C): δ -144 (BF3‚CD3CN), -149 (BF4-). MS ESI(+) m/z (rel % abund) [assgn]: 995 (1) [L2Zn2F(BF4)2]+, 755 (5) [L2ZnF]+, 419 (10) [LZnF]+, 368 (25) [L2Zn]2+, 337 (90) [L + H]+, 269 (100) [L - pz]+. Recrystallization by the slow diffusion of Et2O into dilute 1-mL acetonitrile solutions of 3 afforded X-ray quality crystals of the compound [Zn3(µ-[CH(pz)2]2(CH2)3)3(H2O)5](BF4)6‚4CH3CN (3a). Elemental analysis revealed that upon removal of the crystals from the mother liquor and drying in vacuo, inner and outer sphere water and acetonitrile molecules were lost, leaving unsolvated 3: Anal. Calcd. for C17H20B2F8N8Zn: C, 35.49; H, 3.50; N, 19.47. Found: C, 35.45; H, 3.43; N, 19.38. X-ray Structure Analysis. Details of the data collections for 1a, 2a, and 3a are given in Table 1. X-ray diffraction intensity data were measured at 150(1) K on a Bruker SMART APEX diffractometer (Mo KR radiation, λ ) 0.71073 Å).10 Raw area detector data frame integration was performed with SAINT+.10 Direct methods structure solution, difference Fourier calculations, and full-matrix least-squares refinement against F2 were performed with SHELXTL.11 Important notes regarding the solution and refinement for all three structures follow. In compound 1a, there is extensive disorder of the counterions and solvent molecules. Three of the six independent BF4- groups (B(4), B(5), B(6)) were modeled as disordered over two (B(4), B(5)) or three (B(6)) orientations. Occupancies for each disorder component were fixed at values providing reasonable displacement parameters, and their geometries were restrained to be similar to the ordered BF4- anions. Atoms in close proximity were assigned equal displacement parameters. In general, only atoms with occupancies g0.5 were refined anisotropically; all others were refined isotropically. Of the independent acetonitrile molecules identified, N(141) and N(142) behave normally; N(143)/N(144) is disordered about an inversion center. Two independent orientations with occupancies 1/4 each (N(143)/N(144)) were refined with the aid of six distance restraints. These atoms were assigned a common isotropic displacement parameter, and their hydrogen atoms were idealized. Another region of electron density was located, but the disorder was too severe to be modeled satisfactorily. These species are assumed to be additional acetonitrile molecules. The contribution of these species was removed from the structure factor calculations using SQUEEZE.12 Note that the reported FW, dcalc, and F(000) refer to the identified species only. All non-hydrogen atoms were refined with anisotropic displacement parameters except where noted. Hydrogen atoms bonded to carbon atoms were placed in geometrically idealized positions and included as riding atoms. The water hydrogen atoms were located in difference maps and their coordinates were adjusted to give

Metallacyclic Zn Complexes

Crystal Growth & Design, Vol. 7, No. 6, 2007 1165 Table 1. Crystal Data and Refinement Details for 1a, 2a, and 3a

formula formula weight (g mol-1) crystal system space group a (Å) b (Å) c (Å) β (˚) V (Å3) Z D (calc), Mg‚m-3 T (K) crystal shape crystal size (mm3) final R indices [I > 2σ(I)] R1(F) wR2(F2)

1a

2a

3a

C56H70.50B6F24N29.50O3Zn3 1921.89 monoclinic P21/n 17.1422(8) 27.0769(12) 19.0568(9) 95.0930(10) 8810.4(7) 4 1.449 150(1) block 0.24 × 0.20 × 0.15 0.0492 0.1187

C34H40B3F13N17O0.50Zn2 1105.00 monoclinic C2/c 16.3869(6) 12.7873(5) 43.2225(17) 91.9010(10) 9052.0(6) 8 1.622 150(1) block 0.22 × 0.20 × 0.14 0.0463 0.1031

C59H82B6F24N28O5Zn3 1980.50 monoclinic C2/c 20.5797(12) 15.8645(9) 28.6555(17) 96.2860(10) 9299.4(9) 4 1.415 150(1) block 0.36 × 0.24 × 0.18 0.0694 0.2060

d(O-H) ) 0.84 Å. These atoms were treated as riding on the parent oxygen atom with Uiso, H ) 1.5Ueq, O. In compound 2a, there is extensive disorder of all four of the BF4sites. Anion B(1) is disordered over two closely separated sites B(1A)/ B(1B) in the refined ratio A/B ) 0.81(1)/0.19(1). Anion B(2) is disordered about a 2-fold rotational axis, and as such only half is present in the asymmetric unit. The disorder of this anion was modeled by placing four F atoms around the central boron (B(2) on the C2 axis), and restraining the four B-F bond distances to be close to 1.4 Å. Fluorine atom occupancies were adjusted to give reasonable displacement parameters, provided they sum to unity. The disorder of anion B(3) was modeled with three independent BF4- groups B(3A), B(3B), B(3C), with fixed occupancies of 0.5, 0.25, and 0.25, respectively. Anion B(4) is not positionally disordered but refined to half-occupancy. Another large electron density peak in this region could not be assigned to a BF4- disorder group. This peak refined well to half of an oxygen atom, and was assumed to be a 1/2-occupied water molecule (O(1S)) disordered commensurate with B(4). No hydrogen atoms were located or calculated for O(1S). For all BF4- anions, only the major occupancy (g0.5) atoms were refined anisotropically. Minor occupancy atoms were refined isotropically. A total of 70 distance restraints (SHELX DFIX, DANG, and SADI instructions) were used to maintain chemically reasonable geometries for these groups. In compound 3a, two of the three BF4- anions are disordered (B(2) and B(3)). Atom B(2) was modeled as occupying two distinct, equally populated orientations; B(3) was modeled as occupying three distinct orientations in the fixed proportions A/B/C ) 0.5/0.3/0.2. These values were chosen to give acceptable thermal parameters. There were several electron density peaks in the vicinity of O(1) that could be not modeled successfully. These were removed from the structure factor calculations with SQUEEZE/PLATON (714.4 Å3 solvent-accessible void volume per unit cell, 26 electrons per cell).12 Atoms of anions B(2) and B(3) were refined isotropically; all other non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms bonded to carbon atoms were placed in geometrically idealized positions and included as riding atoms. Reasonable positions for oxygen-bound hydrogens were located in difference maps, modified to give d(O-H) near 0.84 Å, and subsequently treated as riding on the parent oxygen atom. The hydrogen atoms of O(3) are common to both disorder sites. A total of 48 distance restraints were used to assist in modeling the anion disorder. It must be noted that the displacement parameters for all refined atoms in 3a including the trizinc cation are larger than usual for a structure determination at 150 K. This is likely due to slight positional disorder of all species, including the entire trizinc cation, with the disorder being absorbed by the anisotropic refinement. Two data sets for two different crystals were collected to rule out an anomalous crystal; both were similar. Considering the difficulties encountered, the precision of the refinement is of course lower than expected, as reflected in the R-values and high standard uncertainties of the derived quantities.

Results and Discussion Syntheses. The combination of THF solutions of Zn(BF4)2‚ 5H2O and [CH(pz)2]2CH2 (L1), [CH(pz)2]2(CH2)2 (L2), or [CH-

(pz)2]2(CH2)3 (L3) afforded a product whose elemental analysis corresponded to the empirical formula [ZnL](BF4)2 (L ) L1, L2, or L3), as shown in eq 1. THF

Zn(BF4)2 + L 98 [ZnL](BF4)2 L ) L1 (1), L2 (2), L3 (3) (1) The 1H NMR spectra of 1-3 in acetonitrile at room temperature show severely broadened ligand resonances, but at lower temperatures, the spectra of 1 and 2 show two equalintensity sets of chemically distinct pyrazolyl groups belonging to the respective ligand. At -40 °C, for example, the spectrum of 2 contains sharp doublets or triplets of equal integration for protons in the two pyrazolyl environments (8.13, 8.02, 7.87, and 7.45 ppm for the 3/5-pyrazolyl protons; 6.60 and 6.30 ppm for the 4-pyrazolyl protons) as well as two sets of methine protons (6.85 and 6.83 ppm; see Experimental Section) with equal but half as much intensity as those from the pyrazolyl rings. The 1H NMR spectrum of 3 remained broad even at low temperatures. Raising the temperatures of solutions of all three complexes does not result in significant sharpening of the broad resonances observed at room temperature; in all cases, the signals remain broad up to 80 °C. The 19F NMR solution spectra of the pure solids show the expected BF4- signals at ca. -150 ppm and also much weaker signals corresponding to solvated BF3.13 No F- resonances are observed, but because the observation of BF3 implies the presence of F-, it is likely that low concentrations combined with longer relaxation times (T1) prevent the detection of the fluoride anion, presumably coordinated to zinc. Further evidence of Zn-F species in solutions of all three compounds is supplied by their mass spectra. Positive ion electrospray mass spectra of all three complexes show clusters of low abundance corresponding to the fragments [L2Zn2F(BF4)2]+, [L2ZnF]+, [L2ZnBF4]+, [LZnF]+, and [L2Zn]2+, where L ) L1, L2, or L3. The protonated ligand signal [L + H]+ is also observed, and the base peak of each spectrum is [L - pz]+. Solid-State Structures. When complex 1 is slowly recrystallized from acetonitrile/ether, the compound [Zn(µ-[CH(pz)2]2CH2)(H2O)(CH3CN)]3(BF4)6‚2.5CH3CN (1a) is isolated. An ORTEP diagram of the cation in 1a is shown in Figure 2 (top left), and selected bond lengths and angles are given in Table 2. Compound 1a consists of discrete cationic trimeric metallacycles in which three Zn2+ ions are bridged by three L1 ligands. Each ligand molecule is coordinated in a bidentate fashion at both ligating ends to two different zinc ions. All three Zn2+ ions reside in an octahedral environment, four sites of which are occupied by the bridging L1 ligands. The remaining two

1166 Crystal Growth & Design, Vol. 7, No. 6, 2007

Reger et al.

Figure 2. ORTEP and space-filling representations of the trimetallic cations in [Zn(µ-[CH(pz)2]2CH2)(H2O)(CH3CN)]3(BF4)6‚2.5CH3CN (1a; top) and [Zn3(µ-[CH(pz)2]2(CH2)3)3(H2O)5](BF4)6‚4CH3CN (3a; bottom). ORTEP ellipsoids are drawn at the 40% (1a) and 20% (3a) probability levels. The water molecule bound to Zn(2) in 3a is disordered about the 2-fold axis of the cation and is shown in only one position. For clarity, some hydrogen atoms are omitted from the ORTEP drawings. Table 2. Selected Bond Distances (Å) and Angles (°) for 1a Zn(1)-N(11) Zn(1)-N(21) Zn(1)-N(1) Zn(1)-O(1) Zn(2)-N(31) Zn(2)-N(41) Zn(2)-N(2) Zn(2)-O(2) Zn(3)-N(71) Zn(3)-N(81) Zn(3)-N(3) Zn(3)-O(3)

2.162(3) 2.139(3) 2.215(4) 2.103(3) 2.135(3) 2.172(3) 2.140(4) 2.137(3) 2.134(3) 2.170(3) 2.214(4) 2.091(3)

N(11)-Zn(1)-N(21) N(1)-Zn(1)-N(111) N(1)-Zn(1)-O(1) N(1)-Zn(1)-N(11) N(41)-Zn(2)-N(61) N(2)-Zn(2)-N(51) N(2)-Zn(2)-O(2) N(2)-Zn(2)-N(31) N(81)-Zn(3)-N(101) N(3)-Zn(3)-N(91) N(3)-Zn(3)-O(3) N(3)-Zn(3)-N(71)

86.09(13) 168.35(13) 83.87(13) 93.66(13) 175.93(13) 85.96(14) 83.31(13) 169.16(14) 179.92(15) 171.89(13) 84.95(13) 84.24(13)

sites, which are cis to one another, are occupied by a molecule each of acetonitrile and water. Both inner and outer sphere

solvent molecules are lost upon removal from the mother liquor and drying in vacuo (see Experimental Section). In the crystals, ligands cis to each other are separated by angles ranging from 83° to 105°, and those trans to each other are 168° to 180° apart. Zinc-nitrogen bond distances involving the ligands (2.13-2.17 Å) are consistent with distances measured in previously reported zinc complexes of related arene-linked ligands.9 A space-filling model of the cation in 1a is shown in Figure 2 (top right). The cavity at the center of the metallacycle measures approximately 1.5 Å in diameter as determined by the distance between the hydrogen atoms H(2) and H(8) (attached to C(2) and C(8)) and taking into account the van der Waals radius for the hydrogen atom. The lack of solvent molecules filling this void is likely due to its small size. In the extended structure of 1a, intermolecular π-π stacking between pyrazolyl groups creates one-dimensional (1D) networks of cyclic cations that extend in the ac-plane, as shown in Figure 3. The shortest perpendicular distance between pyrazolyl rings involved in these interactions is 3.55 Å, and the ring centroids are offset by 29°. Although half of the counterions in the structure suffer from disorder, some of the ordered BF4- species are positioned close enough to pyrazolyl ring centroids to be considered long anion-π interactions (3.23,

Metallacyclic Zn Complexes

Crystal Growth & Design, Vol. 7, No. 6, 2007 1167

Figure 3. Illustration of the π-π stacking in 1a (red dashed lines) among trimetallic cations. Table 3. Selected Bond Distances (Å) and Angles (°) for 3a Zn(1)-N(11) Zn(1)-N(21) Zn(1)-N(51) Zn(1)-N(61) Zn(1)-O(1) Zn(1)-O(2) Zn(2)-N(31) Zn(2)-N(41) Zn(2)-O(3)

2.131(4) 2.122(5) 2.155(4) 2.114(5) 2.144(4) 2.174(4) 2.061(5) 2.150(7) 2.027(8)

N(11)-Zn(1)-N(21) N(11)-Zn(1)-O(2) N(21)-Zn(1)-N(61) N(51)-Zn(1)-N(61) O(1)-Zn(1)-O(2) N(31)-Zn(2)-N(31a) N(31)-Zn(2)-O(3) N(31)-Zn(2)-N(41) N(41)-Zn(2)-N(41a) N(41)-Zn(2)-O(3)

88.37(17) 166.95(16) 179.50(18) 87.85(18) 82.78(16) 125.0(3) 122.6(7) 87.5(2) 175.9(3) 78.0(3)

3.24, and 3.46 Å). The role of such interactions in directing supramolecular structures of many compounds is becoming increasing important in the rational design of extended solidstate structures,14 and long anion-π interactions were pointed out in silver(I) complexes of related arene-linked bis(pyrazolyl)methane ligands.8 In the present work, however, anion-π interactions do not appear to be dominant factors in organizing extended structure. Recrystallization of 3 from acetonitrile/ether yields the compound [Zn3(µ-[CH(pz)2]2(CH2)3)3(H2O)5](BF4)6‚4CH3CN (3a). The cationic unit of 3a is similar to the metallacycle of 1a in that 3a is also trimetallic with three bridging L3 ligands that are bidentate at both ends and ligated to two different zinc cations (Figure 2, bottom left). An unusual feature of the cation in 3a is that only two of the three zinc cations adopt octahedral coordination environments, the third, Zn(2), being fivecoordinate and trigonal bipyramidal. The cis and trans bond angles about the octahedral zinc cations (83°-108° for cis, 167°-179° for trans) resemble those in 1a, and the angles about Zn(2) are only slightly distorted from an ideal trigonal bipyramid. The equatorial (N(31), N(31a), O(3)) bond angles are 111°-125°; the apical bond angle is 176° and these donor atoms have 78°-95° bond angles with the equatorial plane donor atoms (Table 3). Zinc-nitrogen (2.06-2.16 Å) and Zn-O (2.03-2.18 Å) distances are also similar to 1a. Compound 3a is further distinct in its lack of acetonitrile ligands; each zinc ion is instead coordinated by one or two water molecules. A space-filling model of the cation in 3a is shown in Figure 2 (bottom right). As might be expected from the increase in ligand size from 1a to 3a, the cavity at the center of 3a (ca. 3.0 Å) is approximately twice as large as the cavity in 1a, and two acetonitrile molecules are in this cavity, as shown in Figure 4. Analysis of the extended structure of crystalline 3a is hindered

Figure 4. Space-filling model of the cation in 3a showing acetonitrile molecule (green atoms) in the central cavity.

Figure 5. ORTEP drawing of a segment of the infinite 1D chain in {[Zn2(µ2-[CH(pz)2]2[CH2]2)2(µ2-F)](BF4)3‚CH3CN‚0.5H2O}∞ (2a). Displacement ellipsoids are drawn at the 40% probability level.

by positional disorder of all the species present, as described in the Experimental Section. The analytically pure powder isolated by the rapid recrystallization of 2 from acetonitrile/ether has the empirical formula [ZnL2](BF4)2, as discussed above. To date, however, no crystals of this empirical formula, analogous to 1a and 3a, that are suitable for X-ray studies have been isolated from solutions of 2. Instead, when dilute acetonitrile/ether solutions of 2 are allowed to sit for extended periods of time, the compound {[Zn2(µ2-[CH(pz)2]2[CH2]2)2(µ2-F)](BF4)3‚CH3CN‚0.5H2O}∞ (2a) is isolated. Compound 2a consists of 1D chains of dimeric metallacycles connected through the zinc cations by bridging fluoride ligands. The Zn2+ ions adopt a somewhat distorted trigonal bipyramidal geometry, with N(11) and N(81) occupying axial positions on Zn(1) and N(41) and N(51) in the axial positions of Zn(2). A segment of these chains is shown in Figure 5, and selected bond distances and angles are given in Table 4. Orthogonal views of polymeric segments in 2a are given in Figure 6. As indicated by the NMR and mass spectrometry experiments described above, fluoride abstraction from the BF4leads to the major crystalline product 2a in this system. The existence of dimeric metallacycles in 2a has precedents in the structures of silver(I) coordination polymers formed from

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Table 4. Selected Bond Distances (Å) and Angles (°) for 2a Zn(1)-F(1) Zn(1)-N(11) Zn(1)-N(21) Zn(1)-N(71) Zn(1)-N(81) Zn(2)-F(1a) Zn(2)-N(31) Zn(2)-N(41) Zn(2)-N(51) Zn(2)-N(61) Zn(1)-F(1)-Zn(2b) F(1)-Zn(1)-N(21) F(1)-Zn(1)-N(71) N(21)-Zn(1)-N(71) N(11)-Zn(1)-N(81) N(11)-Zn(1)-F(1) N(11)-Zn(1)-N(21) N(11)-Zn(1)-N(71) F(1a)-Zn(2)-N(31) F(1a)-Zn(2)-N(61) N(31)-Zn(2)-N(61) N(41)-Zn(2)-N(51) N(31)-Zn(2)-N(41) F(1a)-Zn(2)-N(41)

1.966(2) 2.145(3) 2.040(3) 2.053(3) 2.103(3) 1.953(2) 2.051(3) 2.123(3) 2.134(3) 2.036(3) 150.6(1) 104.4(1) 137.5(1) 118.1(1) 168.5(1) 85.0(1) 91.7(1) 92.6(1) 137.2(1) 105.6(1) 117.2(1) 168.6(1) 88.6(1) 85.3(1)

Figure 6. Orthogonal views of segments of the 1D chains in 2a. Red, dashed lines illustrate π-π interactions. Hydrogen atoms are omitted.

L2.8 Furthermore, zinc complexes of the fixed, arene-linked ligands m-[CH(pz)2]2C6H4 (Lm) and 1,3,5-[CH(pz)2]3C6H3 (L3) also exhibit fluoride bridges.9 Unique to 2a, however, is that whereas the bimetallic zinc cations formed from Lm and L3 were supported by a single fluoride ligand bridging the two zinc cations of each individual metallacycle, the bridging fluoride ligands in 2a are exocyclic, serving to link two adjacent metallacycles within which there are no bridging fluoride ligands. Thus, instead of forming discrete cations, associated only by weak noncovalent interactions in the solid state, as in the complexes of the fixed ligands Lm and L3, compound 2a exists as an infinite coordination network of cations held together by Zn-F bonds. Another difference between 2a and the fluoride-bridged complexes formed from the fixed ligands is that the Zn-F-Zn angles in the latter are 180°, thus forming a linear bridge. In contrast, the Zn-F-Zn bonds in 2a are bent at an angle of 151°. As shown by both views in Figure 6, the fluoride bridging in 2a is supported by π-π interactions between pyrazolyl rings of adjacent metallacycles. These interactions comprise two crystallographically distinct sets with centroid-centroid distances of 3.55 and 3.57 Å and perpendicular distances of 3.44 and 3.33 Å, respectively. Given the short distances of these interactions and the flexibility of the ethylene-linked ligands L2, which likely allows a large number of metallacyclic

Figure 7. Schematic representations of the crystal packing of the 1D chains in 2a. Metallacycles are shown as single red lines (ligand molecules) capped by red spheres (Zn2+ ions). Fluoride ligands and Zn-F bonds are green spheres and lines, respectively. Tetrafluoroborate ions are shown as orange spheres, water molecules as small red spheres, and acetonitrile molecules as gray cylinders. The top view is perpendicular to the layers of chains; the bottom view is parallel to the layers.

conformations, the π-π interactions appear to play an important role in organizing the structure of the polymeric chains. Another interesting feature of the stacking of the polymeric chains in 2a is illustrated in Figure 7. The ligands of the metallacycles are represented as lines, and the zinc ions connected by them as capping spheres. The fluoride ligands are shown as spheres bridging the zinc ions. Both views in Figure 7 display the two symmetry-related sets of 1D chains in the structure, each forming two adjacent layers of chains parallel to the ab-plane. One set of two layers runs in the [110] direction

Metallacyclic Zn Complexes

Crystal Growth & Design, Vol. 7, No. 6, 2007 1169

and alternates along the b-axis with the second set running in the [1h10] direction. In the top view of Figure 7, it can be seen that each chain makes an angle of 75.9° with respect to the direction of the chains in the other set, resulting in a gridlike pattern. Tetrafluoroborate anions and solvent molecules suffer from disorder, but their positions within the structure just described adopt an ordered pattern overall. As Figure 7 (bottom) shows, acetonitrile molecules (gray cylinders) are only present between layers of two distinct polymeric chains and not between layers running in the same direction. The BF4- ions (large orange spheres) and water molecules (small red spheres) are present between each layer of complex chains.

dimers. Of the three crystalline compounds studied here, only 2a displays significant noncovalent interactions. The π-π stacking of the polymeric dimers of 2a may be as important in organizing the extended structure of that compound as the bridging fluoride ligands.

Conclusions

Supporting Information Available: X-ray crystallographic files in CIF format for compounds 1a, 2a, and 3a. This information is available free of charge via the Internet at http://pubs.acs.org.

The results described above represent some of the most significant differences in reactivity between the arene- and alkylidene-linked bis(pyrazolyl)methane ligands we have investigated. The silver(I) chemistry of both groups of ligands is relatively consistent; all the alkylidene- and m-arene-linked ligands promote formation of argentacyclic dimers when weakly coordinating anions such as BF4- and SO3CF3- are used.6a,6b,8 The strong tendency of the m-arene-linked ligands to form these dimeric metallacycles is clearly indicated with the chemistry of divalent iron, zinc, and cadmium where fluoride abstraction from BF4- gives fluoride-bridged compounds of this type. In contrast, although the alkylidene-linked ligands employed here formed dimeric metallacycles with silver(I), clearly the driving force for this arrangement is not as strong with these more flexible ligands. We observe here the formation of very different trimeric metallacycles for the methylene (L1) and propylene (L3) linked ligands. The complex initially formed with the ethylene linked L2 is also not a metallacycle containing bridging fluoride ligands, as observed with the m-arene-linked ligands. Although powder samples of this product have the same [ZnL](BF4)2 empirical formula as the materials observed by X-ray crystallography for the other two ligands, a very unusual exocyclic fluoride bridged structure containing a polymer of bimetallic metallacycles bridged by the fluoride ligands crystallizes from solutions of this compound. This exocyclic fluoride bridging arrangement is supported by π-π stacking of pyrazolyl rings from the adjacent dimers. The possibility for the formation of fluoride bridged species is indicated by solution studies of all three of the initially synthesized [ZnL](BF4)2 complexes which show by NMR low concentrations of BF3 in solutions of the pure compounds (and thus F-) and fluoride containing metallic species, including [L2Zn2F(BF4)2]+, in the ES mass spectrometry results. Despite this indication of solution abstraction of fluoride in all cases, no fluoride ion-containing crystalline compound has been isolated from solutions of 1 or 3. NMR studies show that the structures of 1-3 are fluxional in acetonitrile and that the dynamic exchange processes that occur can be slowed significantly by decreasing the temperature, affording for 1 and 2 solution structures in which two nonequivalent environments can be distinguished, results that fit the solid-state structures. It is not clear whether the observation of low concentrations of BF3 and F- in solution play any part in the dynamic exchange process, a process not observed up to 80 °C in [Zn2(µ-Lm)2(µ-F)](BF4)3. The differences between the complexes of the alkylidenelinked ligands and their more rigid counterparts illustrate a general principle in supramolecular chemistry5 that flexible building blocks, here the alkylidene-linked ligands, provide less predictable structures than more rigid tectons, such as the arenelinked bis(pyrazolyl)methane ligands, all of which provide

Acknowledgment. We thank the National Science Foundation (CHE-0414239) for financial support. We also thank the Alfred P. Sloan Foundation for support of R.P.W. The Bruker CCD single-crystal diffractometer was purchased using funds provided by the NSF Instrumentation for Materials Research Program through Grant DMR:9975623.

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