Tris(imidazolyl) cadmium and zinc complexes: structural and spectroscopic characterization of M–OH2 motifs

Polyhedron 23 (2004) 405–412 www.elsevier.com/locate/poly

Tris(imidazolyl) cadmium and zinc complexes: structural and spectroscopic characterization of M–OH2 motifs Janis K. Voo, Christopher D. Incarvito, Glenn P.A. Yap, Arnold L. Rheingold, Charles G. Riordan * Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA Received 12 June 2003; accepted 6 September 2003 Dedicated to Jerry Trofimenko, colleague and friend, for his contributions to tripodal coordination chemistry

Abstract The tris(imidazolyl) cavitand ligand, 1,3,5-triethyl-2,4,6-tris[N-methyl-imidazol-2-yl-thiomethyl]benzene, TriMIm, is used to prepare monomeric aqua complexes of cadmium and zinc with the general formula, [(TriMIm)M(OH2 )]X2 (M ¼ Cd, X ¼ BF4 ; M ¼ Zn, X ¼ BF4 or NO3 ). The molecular structures of each species are characterized by trigonal pyramidal metal ion coordination and hydrogen-bonding between the bound water molecule and the non-coordinating counter ions. The binuclear zinc hydroxobridged complex, [{(TriMIm)Zn}2 (l-OH)](BF4 )3 , has been characterized by X-ray diffraction methods. Ó 2003 Elsevier Ltd. All rights reserved.

1. Introduction 0

The tris(pyrazolyl)hydroborato ligand, [TpR;R ], developed by Jerry Trofimenko more than 35 years ago [1,2] has found great utility as a scaffold for modeling a number of important metallobiochemistry motifs [2–4]. The tripodal [N3 ] donor environment of the ligand mimics the nitrogen-rich ligation sphere found at the active site of numerous non-heme proteins. Leading examples include KitajimaÕs dicopper peroxo complex [5], [(Tp2iPr0 )Cu]2 (O2 ) and ParkinÕs and VahenkampÕs [TpR;R ]Zn(OH) species [6]. The former was prepared and authenticated as a structural and spectroscopic precedent of the oxygenated form of the O2 -binding protein hemocyanin in advance of evidence of such l-g2 :g2 -O2 coordination at the protein active site [7]. The mononuclear zinc hydroxide species provided the opportunity to assess the inherent nucleophilicity, and more recently, basicity [8] of a [N3 ]Zn(OH) species. Inherent in each of these examples is the utility of systematic variation of the pyrazole substituents in controlling structure and reactivity at the attendant metal complexes, clearly one of the strengths of the [Tp] ligand family. *

Corresponding author. E-mail address: [email protected] (C.G. Riordan).

0277-5387/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2003.11.006

Recently, we have prepared a new tris(imidazolyl)based cavitand ligand and reported aspects of its coordination chemistry with copper(I) [9]. The ligand, TriMIm, 1 is preorganized to ligate a single metal ion in a trigonal planar array of imidazoles. The enforced geometry is distinct from the [Tp] ligands where more acute N–M–N angles are observed. Herein, we report the synthesis and structural characterization of cadmium- and zinc-aqua complexes, [(TriMIm)M(OH2 )]2þ .

1 Abbreviation: TriMIm, 1,3,5-triethyl-2,4,6-tris[N -methyl-imidazol2-yl-thiomethyl]benzene.

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2. Experimental 2.1. Materials 2-Mercapto-1-methylimidazole, NaH, MgSO4 , Cd(BF4 )2
6H2 O, Cd(NO3 )2
4H2 O, Zn(BF4 )2
xH2 O, and Zn(NO3 )2
6H2 O were used as supplied by commercial vendors. [Cu(MeCN)4 ]BF4 [10], and TriMIm [9] were prepared according to literature procedures. Pentanes, tetrahydrofuran, and diethyl ether were freshly distilled from sodium/benzophenone. Methylene chloride was freshly distilled from calcium hydride. Acetone was freshly distilled over MgSO4 . Acetonitrile (UV Grade) was purchased from Burdick and Jackson and
molecular sieves and stored under N2 . stored over 4 A Deuterated solvents were purchased from Cambridge
molecular sieves Isotope Laboratories, dried over 4 A and stored under N2 . 2.2. Physical methods Proton and carbon NMR spectra were recorded either on a Bruker AC-250, AM-250, or DRX-400 NMR spectrometer. Chemical shifts were referenced to the residual protio solvent signal. Chemical shifts are quoted in d (ppm) and coupling constants in Hz. Infrared spectra were recorded on a Mattson Genesis Series FTIR spectrophotometer under dinitrogen purge at ambient temperature, unless stated otherwise. FT-IR samples were prepared as KBr pellets. Melting points were determined with a Melt-Temp melting point determination apparatus and are reported uncorrected. Elemental analyses (C, H, N) were performed by Desert Analytics, Inc., Tucson, AZ. 2.3. Synthesis of [(TriMIm)Cd(OH2 )](BF4 )2 (1) To a stirred solution of Cd(BF4 )2
6H2 O (196 mg, 0.5 mmol) in distilled water, 0.5 ml, was added a solution of TriMIm (272 mg, 0.5 mmol) in acetone, 3 ml, at ambient temperature. The colorless solution was stirred overnight during which time it turned cloudy. Removal of volatiles under reduced pressure followed by extraction into acetonitrile, 1 ml, and filtration through Celite gave a colorless solution. Removal of solvent under reduced pressure yielded a white solid, which was collected on a glass frit and washed with diethyl ether and pentanes. Yield: 200 mg, 48%. Mp (dec.) 296–300 °C. 1 H NMR (250 MHz, CD3 CN, 298 K): d 0.9 (t, 9H, 3 J (HH) 7.5 Hz, CH2 CH3 ), 1.1 (m, 3H, CH2 CH3 ), 2.1 (m, 3H, CH2 CH3 ), 3.7 (d, 3H, 2 J (HH) 15 Hz, SCH2 ), 4.0 (s, 9H, NCH3 ), 4.2 (d, 3H, 2 J (HH) 15 Hz, SCH2 ), 7.1 (d, 3H, 3 J (HH) 1.5 Hz, {CN(Me)CHCHN}), 7.6 (d, 3H, 3 J (HH) 1.5 Hz, {CN(Me)CHCHN}). 13 C{1 H} NMR (100 MHz, CD3 CN, 298 K): d 15.6, 21.9, 34.1, 36.4, 127.5, 131.0, 133.2, 141.4, 143.8. FT-IR (KBr)

tOH ¼ 3475 and 3530 cm1 . Anal. Calc. for C27 H38 N6 S3 OCdB2 F8 : C, 38.4; H, 4.53; N, 9.95. Found: C, 38.7; H, 4.36; N, 10.0%. 2.4. Synthesis of [(TriMIm)Cd(NCCH3 )](BF4 )2 (2) To a stirred solution of Cd(BF4 )2
6H2 O (374 mg, 0.95 mmol) in distilled water, 0.8 ml, was added a solution of TriMIm (515 mg, 0.95 mmol) in acetonitrile, 5 ml, at ambient temperature. The colorless solution was stirred overnight. Removal of volatiles under reduced pressure followed by extraction into acetonitrile, 10 ml, and filtration through Celite gave a colorless solution. Removal of solvent under reduced pressure yielded a white solid, which was collected on a glass frit and washed with diethyl ether and pentanes. Yield: 500 mg, 61%. Mp, 252–256 °C. 1 H NMR (250 MHz, CD3 CN, 298 K): d 0.9 (t, 9H, 3 J (HH) 7.5 Hz, CH2 CH3 ), 1.1 (m, 3H, CH2 CH3 ), 1.9 (s, 3H, NCCH3 ), 2.1 (m, 3H, CH2 CH3 ), 3.7 (d, 3H, 2 J (HH) 15 Hz, SCH2 ), 4.0 (s, 9H, NCH3 ), 4.2 (d, 3H, 2 J (HH) 14 Hz, SCH2 ), 7.1 (d, 3H, 3 J (HH) 2.0 Hz, {CN(Me)CHCHN}), 7.6 (d, 3H, 3 J (HH) 2.0 Hz, {CN(Me)CHCHN}). 13 C{1 H} NMR (100 MHz, CD3 CN, 298 K): d 15.6, 21.9, 34.1, 36.5, 127.5, 131.0, 133.2, 141.4, 143.8. FT-IR (KBr) tCN ¼ 2286 cm1 . Anal. Calc. for C29 H39 N7 S3 CdB2 F8 : C, 40.1; H, 4.53; N, 11.3. Found: C, 40.0; H, 4.57; N, 11.4%. 2.5. Synthesis of [(TriMIm)Cd(ONO2 )](NO3 ) (3) To a stirred solution of Cd(NO3 )2
4H2 O (118 mg, 0.38 mmol) in acetonitrile, 20 ml, was added a solution of TriMIm (213 mg, 0.39 mmol) in acetonitrile, 5 ml, at ambient temperature. The colorless solution was stirred overnight. Filtration through Celite and removal of volatiles under reduced pressure yielded a white solid, which was collected on a glass frit and washed with diethyl ether and pentanes. Yield: 210 mg, 68%. Mp, 202– 206 °C. 1 H NMR (250 MHz, CD3 CN, 298 K): d 0.9 (t, 9H, 3 J (HH) 7.3 Hz, CH2 CH3 ), 1.1 (m, 3H, CH2 CH3 ), 2.0 (m, 3H, CH2 CH3 ), 3.7 (d, 3H, 2 J (HH) 14 Hz, SCH2 ), 4.0 (s, 9H, NCH3 ), 4.2 (d, 3H, 2 J (HH) 14 Hz, SCH2 ), 7.1 (d, 3H, 3 J (HH) 1.5 Hz, {CN(Me)CHCHN}), 7.5 (d, 3H, 3 J (HH) 1.5 Hz, {CN(Me)CHCHN}). 13 C{1 H} NMR (62.5 MHz, CD3 CN, 298 K): d 15.8, 21.6, 34.2, 36.2, 126.8, 131.6, 132.6, 140.8, 142.4. Anal. Calc. for C27 H36 N8 S3 O6 Cd: C, 41.7; H, 4.67; N, 14.4. Found: C, 41.1; H, 4.73; N, 13.5%. 2.6. Synthesis of [(TriMIm)Zn(OH2 )](BF4 )2 (4) To a stirred solution of Zn(BF4 )2
xH2 O (133 mg, 0.56 mmol) in distilled water, 0.5 ml, was added a solution of TriMIm (300 mg, 0.56 mmol) in methylene chloride, 8 ml, at ambient temperature. The colorless

J.K. Voo et al. / Polyhedron 23 (2004) 405–412

solution was stirred overnight during which time it turned cloudy gradually. Removal of volatiles under reduced pressure followed by extraction into acetonitrile, 2 ml, and filtration through Celite gave a colorless solution. Removal of solvent under reduced pressure resulted in a white solid, which was collected on a glass frit and washed with diethyl ether and pentanes. Yield: 240 mg, 60%. Mp (dec.) 276–280 °C. 1 H NMR (250 MHz, CD3 CN, 298 K): d 0.9 (t, 9H, 3 J (HH) 7.5 Hz, CH2 CH3 ), 1.2 (m, 3H, CH2 CH3 ), 2.0 (m, 3H, CH2 CH3 ), 3.7 (d, 3H, 2 J (HH) 14 Hz, SCH2 ), 4.0 (s, 9H, NCH3 ), 4.2 (d, 3H, 2 J (HH) 15 Hz, SCH2 ), 7.1 (s, 3H, {CN(Me)CHCHN}), 7.5 (s, 3H, {CN(Me)CHCHN}). 13 C{1 H} NMR (62.5 MHz, CD3 CN, 298 K): d 16.0, 21.7, 34.2, 36.3, 126.9, 131.4, 131.7, 141.2, 143.4. FT-IR (KBr) tOH ¼ 3445 cm1 . Anal. Calc. for C27 H38 N6 S3 OZnB2 F8 : C, 40.7; H, 4.80; N, 10.5. Found: C, 40.9; H, 4.87; N, 10.6%. 2.7. Synthesis of [(TriMIm)Zn(OH2 )](NO3 )2 (5) To a stirred solution of Zn(NO3 )2
6H2 O (127 mg, 0.43 mmol) in acetonitrile, 50 ml, was added a solution of TriMIm (230 mg, 0.43 mmol) in acetonitrile, 5 ml, at ambient temperature. The colorless solution was stirred overnight. Filtration through Celite and removal of volatiles under reduced pressure yielded a white solid, which was collected on a glass frit and washed with diethyl ether and pentanes. Yield: 230 mg, 72%. Mp, 206– 210 °C. 1 H NMR (250 MHz, CD3 CN, 298 K): d 0.9 (t, 9H, 3 J (HH) 7.5 Hz, CH2 CH3 ), 1.2 (m, 3H, CH2 CH3 ), 2.0 (m, 3H, CH2 CH3 ), 3.7 (d, 3H, 2 J (HH) 14 Hz, SCH2 ), 4.0 (s, 9H, NCH3 ), 4.2 (d, 3H, 2 J (HH) 14 Hz, SCH2 ), 7.2 (d, 3H, 3 J (HH) 1.5 Hz, {CN(Me)CHCHN}), 7.5 (d, 3H, 3 J (HH) 1.5 Hz, {CN(Me)CHCHN}). 13 C{1 H} NMR (62.5 MHz, CD3 CN, 298 K): d 16.0, 21.6, 34.3, 36.2, 126.3, 131.5, 132.0, 140.6, 143.3. FT-IR (KBr) tOH ¼ 3398 cm1 . Anal. Calc. for C27 H38 N8 S3 O7 Zn: C, 43.3; H, 5.12; N, 15.0. Found: C, 43.8; H, 5.15; N, 14.9%. 2.8. Synthesis of [{(TriMIm)Zn}2 (l-OH)](BF4 )3 (6) To a stirred solution of Zn(BF4 )2
xH2 O (440 mg, 1.84 mmol) in distilled water, 2 ml, was added a solution of TriMIm, (1.0 g, 1.85 mmol) in acetone, 50 ml, at ambient temperature. The colorless solution was stirred overnight during which time it turned cloudy. Removal of volatiles under reduced pressure followed by extraction into acetonitrile, 10 ml, and filtration through Celite gave a colorless solution. Removal of solvent under reduced pressure yielded a white solid, which was collected on a glass frit and washed with diethyl ether and pentanes. Yield: 830 mg, 60%. Mp (dec.) 280–284 °C. 1 H NMR (250 MHz, CD3 CN, 298 K) d 0.8 (t, 18H, 3 J (HH) 7.5 Hz, CH2 CH3 ), 1.1 (m, 6H, CH2 CH3 ), 1.9

407

(m, 6H, CH2 CH3 ), 3.7 (d, 6H, 2 J (HH) 14 Hz, SCH2 ), 3.9 (s, 18H, NCH3 ), 4.1 (d, 6H, 2 J (HH) 15 Hz, SCH2 ), 6.3 (s, 6H, {CN(Me)CHCHN}), 7.1 (s, 6H, {CN(Me)CHCHN}). 13 C{1 H} NMR (62.5 MHz, CD3 CN, 298 K): d 15.9, 21.6, 34.1, 36.0, 126.1, 126.7, 131.1, 139.8, 143.4. FT-IR (KBr) tOH ¼ 3450 cm1 . Anal. Calc. for C54 H73 N12 S6 OZn2 B3 F12 : C, 43.5; H, 4.94; N, 11.3. Found: C, 43.9; H, 5.08; N, 11.2%. 2.9. X-ray structural solution and refinement Suitable crystals were selected and mounted on glass fibers with epoxy cement. Data were collected on a Siemens P4 diffractometer equipped with a SMART CCD detector with graphite Mo Ka radiation (Ka
The unit cell parameters and orientation k ¼ 0:7107 A). matrices were determined by harvesting reflections from three orthogonal sets of 15 frames using 0.3°x scans. The systematic absences in the diffraction data are uniquely consistent with the reported space group for the compound identified. Crystallographic analysis (direct methods) of the reduced data using the S H E L X T L (version 5.1) package of programs located all non-hydrogen atoms, which were refined anisotropically using the conventional full-matrix least-squares method. Hydrogen atoms were treated as idealized contributions, except for the hydrogen atoms on O1 in 1, which were located in the difference Fourier map. In 2, the coordinated CH3 CN was found to be disordered over two positions. Attempts to model the disorder were not successful due to insufficient data resolution.

3. Results and discussion 3.1. Synthesis of Cd and Zn aqua complexes Reaction of equimolar mixtures of TriMIm and a hydrated metal salt in the appropriate solvent yielded the corresponding monomeric metal aqua complex, [(TriMIm)M(OH2 )]X2 (M ¼ Cd, X ¼ BF4 (1); M ¼ Zn, X ¼ BF4 (4), and M ¼ Zn, X ¼ NO3 (5)), in moderate to good yields as a white, microcrystalline solid. The materials possess good solubility in acetone, acetonitrile, and methanol but, are only sparingly soluble in hydrocarbons and chlorinated solvents. Spectroscopic (1 H, 13 C NMR, FT-IR) and combustion analytical data (experimental) support the indicated compositions, which were verified by X-ray diffraction analyses. Judicious selection of the reaction solvent is required to generate the desired complexes with differing behavior observed between Zn and Cd. TriMIm complexation reactions with Zn favor formation of the aqua complexes, 4 and 5, even in the donor solvent acetonitrile. However, the analogous reaction in acetone yields the hydroxo-bridged dimer, 6.

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The Cd-aqua complex 1 was prepared in acetone. When the same reaction was conducted in acetonitrile, with BF4 as counter ion, the Cd–NCCH3 adduct, 2, was generated. However, the reaction in acetonitrile with Cd(NO3 )2 as the metal source yielded the O-nitrato complex, 3. It appears that the fates of the reactions are driven by insolubility of the products. 3.2. Molecular structures The molecular structures of the aqua complexes 1, 4 and 5 are depicted in Fig. 1 with crystallographic data collected in Table 1. Relevant, selected metric parameters are contained in Table 2. Each complex consists of a monomeric core, coordinated to three imidazolyl nitrogens of the TriMIm and one water molecule. The geometry at each divalent ion can be described as trigonal pyramidal with the three nitrogens defining the basal plane and the oxygen in the apical position. The imidazolyl rings are all canted in the same direction inducing an asymmetry to the structures that is maintained in solution as evidenced in the 1 H NMR spectra recorded at room temperature. That is, there are two enantiomers that differ in the handedness of the canting (clockwise versus counter clockwise). In contrast, the three coordinate Cu(I), [(TriMIm)Cu]BF4 , exhibited facile interconversion between the two diastereomers with DGz ¼ 13 kcal/mol [9]. The sum of the three N–M–N angles is essentially 360°, thus requiring the metal and three nitrogens to be coplanar. The deviations of the individual N–M–N angles from 120° are small, less than
in 1 is within 3.5°. The Cd–OH2 distance of 2.340(2) A the range of values reported for the limited number of
The four coordinate Cd aqua complexes, 2.31–2.38 A.
distance is somewhat shorter than that of 2.41(1) A found for [Cd(TPA)(OH2 )(NO3 )]NO3 [11]. The two zinc aqua complexes, differing only in the identity of the counter anions, show similar molecular structures despite crystallizing in distinct space groups. Further,
while the Zn–N distances are all approximately 2.00 A,
for 4 the Zn–OH2 distances differ by 0.10, 2.130(4) A
for 5. The clear difference in Zn–OH2 and 2.033(1) A distances reflect the variable degrees of hydrogenbonding between the bound water and the anions. The nitrate ion is a better H-bond acceptor than tetrafluoroborate thus, more effectively polarizing the O–H bond and consequently, shortening the Zn–O distance. The Zn–O distances in 4 and 5 are significantly longer than those found in two recently characterized [N3 ]Zn(OH2 ) complexes in which [N3 ] is either [TptBu;Me ] [8] or the tris(imidazolyl) calix[6]arene [12,13]. The latter
two complexes exhibit Zn–O distances below 2.00 A,
similar despite Zn–N distances of approximately 2.00 A, to 4 and 5. In contrast, the Zn–O distances reported for the (presumably) aqua forms of the enzymes carbonic
[14] and adamalysin II (2.35(2) anhydrase II (2.05(2) A)

Fig. 1. Structures of metal-aqua complexes, 1, 4 and 5.


[15] are similar to 4 and 5. Further, the geometries of A) these complexes are best described as pseudo-tetrahedral, with more acute N–Zn–N angles. Perhaps the most notable feature in the structures of the metal–aqua complexes is the hydrogen-bonds

J.K. Voo et al. / Polyhedron 23 (2004) 405–412

409

Table 1 Crystallographic data for 1–6

Lattice Empirical formula Formula weight Color, habit Crystal size (mm) Space group
a (A)
b (A)
c (A) a (°) b (°) c (°)
3 ) V (A Z Density (g/cm3 ) Temperature (K) 2h range (°) l (Mo Ka) (cm1 ) RðF Þ; Rw ðF Þ Goodness of fit on F 2

1

2
2CH3 CN

3
CH3 CN

4

5CH3 CN

6
7CH3 CN

orthorhombic C27 H38 B2 CdF8 N6 OS3 844.83 colorless, rod 0.30  0.30  0.20 P 21 21 2l 11.5115(17) 15.475(2) 19.060(3) 90 90 90 3395.4(9) 4 1.653 173(2) 4.14–56.7 9.05 0.0284, 0.0684 1.025

monoclinic C33 H45 B2 CdF8 N9 S3 949.98 colorless, block 0.30  0.20  0.20 P 21 =c 14.2483(11) 13.3074(10) 22.7875(18) 90 103.574(2) 90 4200.0(6) 4 1.502 173(2) 3.68–50.0 7.41 0.0529, 0.1566 1.440

monoclinic C29 H39 N9 06 S3 Cd

orthorhombic C27 H3g B2 F8 N6 OS3 Zn 797.80 colorless, block 0.14  0.08  0.08 P 21 21 21 11.5115(17) 15.475(2) 19.060(3) 90 90 90 3395.4(9) 4 1.561 173(2) 3.40–56.74 9.86 0.0711,0.1249 1.164

rhombohedral C32 H41 N6 07 S3 Zn

monoclinic C68 H94 B3 F12 N19 OS6 Zn2 1777.15 colorless, block 0.25  0.25  0.20 P 21 =c 15.1881(14) 25.418(2) 21.487(2) 90 102.424(2) 90 8101.1(13) 4 1.457 173(2) 3.74–54.0 8.29 0.0647, 0.1507 0.924

Quantity minimized: RðwF 2 Þ ¼ MaxðFo ; 0Þ=3.

818.27 colorless, block 0.20  0.20  0.15 Ia 19.833(3) 15.971(2) 22.557(3) 90 98.622(2) 90 7064.2(16) 8 1.539 173(2) 3.14–54.0 8.50 0.0609, 0.1615 1.100

783.26 colorless, block 0.10  0.10  0.10 R-3 12.153(3) 12.153(3) 12.153(3) 84.494(2) 84.494(2) 84.494(2) 1771.5(9) 2 1.468 150(2) 2.28–28.18 9.25 0.0519,0.1160 1.025

P P P P ½wðFo2  Fc2 Þ2 = ½wðFo2 Þ2 1=2 ; R ¼ D= ðFo Þ, D ¼ jðFo  Fc Þj, w ¼ 1=½r2 ðFo2 Þ þ ðaP Þ2 þ bP , P ¼ ½2Fc2 þ

formed between the bound water and each of the counter anions. In 1 the H-bonding interactions were established clearly by locating the hydrogen atoms in the difference Fourier maps. In 4 and 5, the hydrogen atoms were not located, but they were inferred by the characteristically short heavy atom distances, Table 2, and supported by FT-IR spectroscopic data, detailed in the following section. In complexes 1 and 4 the distances between the aqua oxygen and one of the fluorides of
the BF4 are similar, with each distance below 2.9 A. For comparison, in the Cu(II) aqua complex, [Cu(pmdien)(OH2 )F]BF4 , the O–FBF3 distance is 2.87
[16]. In the nitrato complex, 5, the two nitrates are A located at the same distance from the aqua ligand as required by the imposed crystallographic symmetry. The
H-bonding to Zn–OH2 O1- - -O2 distance is 2.779(2) A. units is common to the three other structurally authenticated [N3 ]Zn(OH2 ) species that predate this report. In the Tp supported examples of Vahrenkamp [17] and Parkin [8], novel H-bond acceptors, [TpAr;Me ]Zn(OH) and (C6 F5 )3 BOH, respectively are required, with O- - -O
In contrast, Reindistances in the range of 2.4–2.5 A. audÕs calix[6]arene derivative [13] shows the Zn-bound aqua ligand H-bound to a second water molecule in the heart of the calix[6]arene cavity with a O- - -O distance
The exceptional stability of this Zn aqua of 2.539(7) A. complex has been ascribed to the unique combination of the calixarene cavity and the H-bonding to a second water molecule. Identification of complexes 4 and 5 demonstrates clearly that neither special counter ions

nor novel hydrophobic ligand cavity environments are requisite to stabilize [N3 ]Zn(OH2 ) motifs. It is plausible the enforced trigonal pyramidal geometry in the [(TriMIm)M(OH2 )]2þ ions modulates the Lewis acidity of the metals permitting for the isolation of aqua, as opposed to hydroxo, adducts. Deprotonation of the zinc-aqua complexes is elicited facilely by changing the reaction conditions as described later. The aqua ligand in 1 is replaced with acetonitrile by changing the reaction solvent from acetone to acetonitrile. A thermal ellipsoid representation of [(TriMIm)Cd(NCCH3 )](BF4 )2 , (2) is contained in Fig. 2. Selected bond lengths and angles are given in Table 2. The environment of the Cd ion is that of a trigonal pyramid, with the imidazolyl nitrogens lying in the basal plane and an acetonitrile nitrogen in the axial position.
is within the The Cd–NCCH3 distance of 2.328(5) A range of distances for the limited number of crystallographically defined Cd–NCCH3 derivatives. The three imidazolyl donors and the Cd constitute a trigonal plane with the sum of the three N–Cd–N angles of 359.9°. The
is similar to Cu–NIm average distance in 2 at 2.190(7) A
The that observed in the aqua derivative 1 at 2.184(3) A. acetonitrile ligand is positioned on the three-fold axis as indicated in the narrow range of N(7)–Cd–NIm angles, 87.5(2)°–94.2(2)°. The nitrato complex 3 prepared by reaction of Cd(NO3 )2
6H2 O and TriMIm in acetonitrile indicates that the nitrato ligand binds to Cd preferentially over H2 O and CH3 CN, the latter two ligands binding only

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Table 2
and angles (°) for complexes 1–6 Selected distances (A) Complex 1 Bond lengths Cd1–N1 Cd1–N3 Cd1–N5 Cd1–O1 O1–F3 O1–F7 Bond angles N1–Cd1–N3 N1–Cd1–N5 N3–Cd1–N5 N1–Cd1–O1 N3–Cd1–O1 N5–Cd1–O1 Complex 2 Bond lengths Cd1–N1 Cd1–N3 Cd1–N5 Cd1–N7 Bond angles N1–Cd1–N3 N1–Cd1–N5 N3–Cd1–N5 N1–Cd1–N7 N3–Cd1–N7 N5–Cd1–N7 Cd1–N7–C28 N7–C28–C29 Complex 3 Bond lengths Cd1–N1 Cd1–N3 Cd1–N5 Cd1–O1 Cd1–O2 Bond angles N1–Cd1–N3 N1–Cd1–N5 N3–Cd1–N5 N1–Cd1–O1 N3–Cd1–O1 N5–Cd1–O1 Complex 4 Bond lengths Zn1–N1 Zn1–N3 Zn1–N5 Zn1–O1 O1–F3 O1–F7 Bond angles N1–Zn1–N3 N1–Zn1–N5 N3–Zn1–N5 N1–Zn1–O1 N3–Zn1–1 N5–Zn1–O1

2.183(2) 2.185(2) 2.184(2) 2.340(2) 2.717(3) 2.894(3)

122.10(8) 117.82(9 120.02(8) 87.50(9) 86.13(9) 99.19(9)

2.197(4) 2.196(4) 2.177(4) 2.328(5)

119.1(2) 119.4(2) 121.2(2) 87.5 (2) 93.1(2) 94.2(2) 173.4(8) 159.5(14)

2.188(7) 2.213(9) 2.205(10) 2.342(9) 2.731(9)

115.2(4) 120.4(4) 122.4(3) 110.9(4) 79.6(3) 93.9(3)

2.00(1) 2.01(1) 2.00(1) 2.13(1) 2.735(3) 2.805(3)

123.1(2) 116.5(2) 118.3(2) 91.2(2) 90.1(3) 101.5(3)

Table 2 (continued) Complex 5 Bond lengths Zn–N1 Zn–O1 O1–O2

1.983(1) 2.033(1) 2.779(2)

Bond angles N1–Zn–N1A N1–Zn–O1

119.033(8) 95.68(2)

Complex 6 Bond lengths Zn1–N1 Zn1–N3 Zn1–N5 Zn2–N7 Zn2–N9 Zn2–N11 Zn1–O1 Zn2–O1 Zn1–Zn2

1.98(1) 1.99(1) 1.99(1) 1.99(1) 1.98(1) 2.01(1) 1.95(1) 1.94(1) 3.84(2)

Bond angles Zn1–O1–Zn2

160.0(2)

when BF4 is the counter ion. The structure of [(TriMIm)Cd(NO3 )](NO3 ), (3) is depicted in Fig. 2 with selected metric parameters contained in Table 2. Of interest is the coordination mode of the nitrato group as such ligands can coordinate via unidentate, bidentate and anisobidentate modes. The Cd–O distances are
while the Cd–O–N angles are 2.342(9) and 2.731 (9) A 87.7(8)° and 107.4(8)°. Parkin and coworkers proposed that the nature of the interaction between the bicarbonate intermediate and the metal ion influences the efficiency of carbonic anhydrase enzymes [18–23]. Accordingly, metals showing greater tendency to form bidentate bicarbonate intermediates are anticipated to be slow catalysts. As there are significant synthetic challenges in isolating mononuclear bicarbonate adducts, metal nitrates have been pursued as structural analogs due to the similar coordination modes available to nitrates and bicarbonates. The criteria established to classify the nitrato coordination mode rely on the differences in M–O interaction as defined by the differences in the two M–O distances and M–O–N angles [24]. The values determined for 3 are in accord with anisobidentate nitrato coordination. Comparison with a number of Cd(NO3 ) complexes, Table 3, reveals 3 shows the greatest degree of asymmetry in nitrato binding. The trigonal planar coordination defined by the TriMIm accounts for this observation, as it is not sufficiently flexible to accommodate neither the trigonal bipyramidal nor square pyramidal geometries required for (more) symmetric nitrato coordination. The zinc hydroxo dimer, 6, was produced via the simple change of reaction solvent from acetone to acetonitrile. While a similar change in the synthesis of the

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411

Table 2. Each Zn ion lies in the basal plane defined by the three imidazolyl nitrogens, where the sums of N– 0 0 Zn(1)–N and N–Zn(2)–N angles are 355.8(4)° and 355.1(3)°, respectively. Both zinc atoms are bridged by a hydroxo group with a substantially linear Zn(1)–O(1)– Zn(2) angle of 160.0(2)°. The steric demands of the TriMIm ligand require that the single atom bridge forms with the wide M–O–M angle. The bridging hydroxide is
symmetric with Zn–O distances of Zn(1)–O(1), 1.95(1) A
and Zn(2)–O(1), 1.94(1) A. The Zn–O distances support assignment of the bridging group as hydroxide. Although the hydrogen atom of the bridging unit was not located in the difference Fourier map, its presence was supported further by the identification of three BF4 per cation as required for charge balance and by infrared analysis, m(OH) at 3450 cm1 . Unlike the aqua complexes, the hydroxide does not appear to participate in hydrogen bonding with a BF4 anion as the closest O- -
F distance is 4.19(1) A. As the Zn-aqua complex is formed exclusively in acetone, it is likely the first species generated in acetonitrile. However, due to the higher polarity of the latter solvent, the aqua complex then ionizes to a mononuclear Zn–OH. While the identity of the base has not been established, TriMIm could serve this role. Under acidic conditions the free acid form, [H3 TriMIm]3þ , is accessible. The [(TriMIm)Zn(OH)]þ , reacts subsequently with a second Zn-aqua complex to generate the binuclear product. The presence of a [N3 Zn]2 –OH unit serves as a model compound for the hydrolytic function of hydroxo bridging zinc enzymes such as aminopeptidase and phospholipase C [25]. 3.3. Spectroscopic data for metal aqua complexes

Fig. 2. Structures of the cations of 2, 3 and 6.

corresponding Cd complexes resulted in isolation of 2, such a solvento species was not evident for Zn. Presumably, the higher polarity of the acetonitrile permits for generation of 6. X–ray structural analysis provided unequivocal evidence for the hydroxo-bridged dimer, 6, Fig. 2. Selected bond angles and distances are listed in

Proton and C-13 NMR spectroscopic data were consistent with the indicated formulations for 1, 4 and 5. However, even in aprotic solvents proton signals for the bound water were not evident. Infrared spectra of solid samples prepared as KBr pellets revealed broad features ascribed to the m(OH) modes above 3400 cm1 . Similar features were absent from the spectra of 2 and 3 further supporting the assignment. In 1 the band is split due to solid state effects with values of 3530 and 3475 cm1 . The single broad m(OH) bands for 4 and 5 occur at 3445 and 3398 cm1 , respectively. The relative ordering for the Zn aqua species is consistent with stronger Hbonding between the bound water and the nitrato anion in 5, as stronger H-bonds exhibit shifts to lower energy. Despite the apparent acidity of the metal aqua complexes, attempts to generate the conjugate metal hydroxo species via deprotonation have failed. Addition of a range of bases including NEt3 , LiNEt2 , KOH, KOEt, and BuLi, resulted in either direct replacement of the aqua ligand or other, more catastrophic degradation to ill-defined species. Alternatively, efforts to generate 4 via

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Table 3 Comparison of metric parameters for Cd-nitrato complexes Compound þ

[TriMIm]Cd(NO3 )] [PimiPrtBu ]Cd(NO3 )]þ [23] [TptBu;Me ]Cd(NO3 )] [23] [TpPh;Me ]Cd(NO3 )] [26] [16aneN4 ]Cd(NO3 )]þ [27]


d1 /A


d2 /A

h1 /°

h2 /°


Dd/A

Dh/°

Classification

2.34 2.25 2.27 2.33 2.40

2.73 2.38 2.29 2.35 2.46

107.4 97.8 102.9 95.2 95.2

87.7 90.4 101.1 95.0 96.9

0.39 0.13 0.02 0.02 0.06

19.7 7.4 1.8 0.2 1.7

anisobidentate bidentate bidentate bidentate bidentate

Dd ¼ jd2  d1 j, Dh ¼ jh2  h1 j. PimiPrtBu , tris(1-isopropyl-4-tert-butylimidazolyl)phosphine. 16aneN4 , 1,5,9,13-tetramethyl-1,5,9,13-tetra-azacyclohexadecane.

protonation of 6 have yet to be successful. Efforts to verify that species 4 and 6 are a conjugate acid/base pair continue in this laboratory. Acknowledgements We thank the National Institutes of Health (GM59191) for support of this work.

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