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Zinc complexes of hydrogen bond accepting ester substituted trispyrazolylborates
Inorganica Chimica Acta 341 (2002) 33 /38 www.elsevier.com/locate/ica
Zinc complexes of hydrogen bond accepting ester substituted trispyrazolylborates Brian S. Hammes, Xuemei Luo, Mary W. Carrano, Carl J. Carrano * Department of Chemistry and Biochemistry, Southwest Texas State University, San Marcos, TX 78666, USA Received 8 February 2002; accepted 8 April 2002 Dedicated to Professor K.N. Raymond on the occasion of his 60th birthday
Abstract As models for the labile water ligands so ubiquitous in zinc metalloprotein active sites, aquo and hydroxo complexes of zinc with the ester substituted trispyrazolylborate ligand, [(TpCO2Et,Me) have been isolated and crystallographically characterized. These complexes are stabilized by internal hydrogen bonding between the water or hydroxide and the ester carbonyls of the ligand. The dinuclear hydroxo complex, which maintains its structure in solution, appears to catalyze self-transesterification reactions in alcoholic solvents. However, the expansion of the coordination sphere from four- to five-coordinate in 2 or the dimerization that occurs with 3 suggest that a more sterically restrictive ester substituted ligand will be needed to enforce the desired mononuclear four-coordinate pseudotetetrahedral geometry most commonly seen in zinc metalloenzyme active sites. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Zinc complexes; Hydrogen bond; Ester substituted trispyrazolylborates
1. Introduction The tris(pyrazolyl)borate ligands pioneered by Trofimenko in the late 1960’s, have arguably become the preeminent platform for making complexes in coordination, organometallic and bioinorganic chemistries [1]. Modifications to the parent ligand over the last few decades have been primarily directed towards the synthesis of highly sterically congested analogs that have allowed for the stabilization of species not heretofore accessible [2 /5]. More recently attempts have been made to modify the electronic and metal binding properties of these ligands, an example of which are the perflourinated derivatives of TpMe,Me which lead to chemistry quite different from that of the parent [6]. An alternate approach is to replace one or more of the pyrazole nitrogens by another type of donor atom (thiol, phenol, carboxylate, etc.) to produce a heteroscorpionate ligand. We and others have recently succeeded in
* Corresponding author. Tel.: /1-512-245 3117; fax: /1-512-245 2374 E-mail address: [email protected] (C.J. Carrano).
providing a general route into this class of ligands and have begun to examine their coordination chemistry with a variety of metal ions [7 /9]. Finally, an area of recent interest concerns modifications designed to increase the denticity of the Tp ligand from h3 to h6 using different N or O donors attached to the pyrazole rings [10 /12]. Thus it is clear that the utility of these ligands is being further enhanced by a new generation of modifications designed to address other than steric issues. Based on the enormous success of the TpR, platform [2 /5] as a ligand for metalloenzyme modeling studies, we have recently described a first generation of simple 5carboxyester substituted analogs of Tp which simultaneously provide both steric bulk and hydrogen bonding groups which can be directed towards the center of the metal binding cavity. The presence of strong intramolecular H-bonding between the ligand carbonyls and water molecules leads to the stabilization of discrete, well characterized, divalent metal aquo complexes of the general type in [TpCO2Et,MeM(H2O)x ] where M / Co(II), Ni(II), Mn(II), or Cu(II) and x /2 or 3 depending on the metal [13,14]. These complexes are
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 1 7 7 - 5
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both, unprecedented in Tp chemistry, and good structural models for enzymes of the vicinal oxygen chelate (VOC) superfamily [15]. Since water, hydroxide and other small exogenous ligands are also ubiquitous in pseudutetrahedral zinc metalloenzyme centers [16] and clearly important to enzymatic activity, zinc complexes of tripodal ligands have been extensively studied by numerous workers including Kimura, Parkin, Vahrenkamp, and others and several review articles are available [17 /19]. Some examples of hydrogen bond stabilized pentacoordinate Zn /OH complexes have recently been reported [20,21]. In the present work we have sought to stabilize pseudotetrahedral zinc aquo or hydroxo species by internal hydrogen bonding using the ester substituted Tp ligand [TpCO2Et,Me] and describe the isolation and structural characterization of three such complexes.
2. Experimental All syntheses were carried out in air and the reagents and solvents purchased from commercial sources and used as received unless otherwise noted. Methanol was distilled under argon over CaH2. Potassium tris(3carboxyethyl-5-methyl)pyrazolylborate was prepared using the previously reported procedures [13]. 2.1. [(TpCO2Et,Me)ZnNO3] (1) A slurry of potassium tris(3-carboxyethyl-5methyl)pyrazolylborate (1) (0.30 g, 0.59 mmol) in 30 ml of CH2Cl2 was treated with solid Zn(NO3)2 ×/H2O (0.18 g, 0.59 mmol). The resulting solution was stirred for 24 h, filtered to remove a small amount of solid (KNO3), and concentrated under reduced pressure to a volume of 6 ml. Crystals for an X-ray diffraction study were grown by the slow diffusion of hexane into a CH2Cl2 solution giving 0.18 g (51%) of [(TpCO2Et,Me)ZnNO3] (1) as colorless plates. Anal . Calc. (Found) for [(TpCO2Et,Me)ZnNO3], C21H28N7O9BZn ×/0.125 C6H14: C, 42.86 (42.60); H, 4.93 (4.71); N, 16.09 (15.74)%. 1H NMR (CDCl3) d 6.57 (s, 1H, PzH ), 4.45 (q, 2H, J /7 Hz, /OCH2 /), 2.49 (s, 3H, Pz /CH3), 1.40 (t, 3H, J /7 Hz, /CH3). 13C NMR (CDCl3) d 160.52, 146.51, 144.51, 108.75, 61.87, 14.22, 12.67. FTIR (KBr, cm 1): n /2565 (B/H), 1729 (C /O), 1515 (N /O). 2.2. [(TpCO2Et,Me)Zn(H2O)(OAc)] (2) A slurry of potassium tris(3-methyl-5-carboxyethyl)pyrazolylborate (0.73 g, 1.4 mmol) in 30 ml of CH2Cl2 was treated with solid Zn(OAc)2 ×/4H2O (0.47 g, 1.4 mmol). The resulting solution was stirred for 24 h, filtered to remove a small amount of solid (KOAc), and concentrated under reduced pressure to a volume of 5
ml. Crystals for an X-ray diffraction study were grown by the slow diffusion of hexane into a CH2Cl2 solution giving 0.42 g (50%) of [(TpCO2Et,Me)ZnOAc] (2) as colorless needles. Anal . Calc. (Found) for [(TpCO2Et,Me)ZnOAc] ×/0.5H2O, C23H32N6O8.5BZn: C, 45.68 (45.56); H, 5.34 (5.22); N, 13.90 (13.79)%. 1H NMR (CDCl3) d 6.51 (s, 1H, PzH), 5.57 (br, 1H, BH), 4.38 (q, 2H, J /7 Hz, /OCH2 /), 2.46 (s, 3H, Pz /CH3), 2.07 (s, 3H, /C(O)2CH3), 1.38 (t, 3H, J /7 Hz, /CH3). 13 C NMR (CDCl3) d 179.70, 161.06, 145.81, 144.25, 108.43, 61.31, 22.50, 14.14, 12.65. FTIR (KBr, cm 1): n /2559 (B/H), 1727 (C /O), 1703 (C /O), 1633 (C /O), 1614 (C /O). 2.3. [{(TpCO2Et,Me)Zn}2OH]ClO4 (3) A slurry of potassium TpCO2Et,Me (0.253 g) in 15 ml CH3OH was treated dropwise with a solution of Zn(ClO4)2 ×/6H2O in CH3OH. The resulting mixture was stirred for 6 h, and concentrated under vacuum after removal of a small amount of white solid. Crystallization was achieved by slow diffusion of ether into a concentrated methanolic solution. Caution! This and the other perchlorate salts isolated in this work are potentially explosive and should be handled with care. Anal . Calc. (Found) for [{(TpCO2Et,Me)Zn}2OH]ClO4, C42H57O17N12B2Zn2Cl: C, 42.36 (42.29); H, 4.79(4.68); N, 14.12(14.07); Cl, 2.98 (3.08)%. 1H NMR (CD3OD): d 6.633 (s, 2H, PzH), 3.970 (m, 4H, /CH2 /), 2.581 (s, 6H, Pz /CH3), 1.005 (t, 6H, /CH3). 13C NMR (CD3OD): d 166.7, 149.8, 145.3, 110.2, 59.0, 19.0, 13.3. FTIR (KBr, cm 1): n /2566 (B/H), 1725 (C /O). 2.4. [(TpCO2Et,Me)Zn(H2O)3]ClO4 (4) After the deposition of crystals from the reaction mixture that produced 3, addition of more ether to the mother liquor and storage for several days at 4 8C yielded a crop of crystals of 4 as colorless blocks. Anal . Calc. (found) for TpCO2Et,MeZn(H2O)3ClO4, C21H34N6O13BZnCl: C, 36.53; H, 4.93; N, 12.18%. Found: C, 37.88; H, 4.17; N, 12.54%. 1H NMR (CD3OD) d 6.636 (s, 1H, PzH), 4.422(m, 2H, /CH2 /), 2.541(s, 3H, Pz /CH3), 1.379 (t, 3H, /CH3). 13 C NMR: d 166.5, 149.7, 145.3, 110.3, 63.8, 15.1, 13.5. FTIR (KBr, cm 1): n /2569 (B/H), 1724 (C /O). 2.4.1. Physical methods Elemental analyses were obtained from Quantitative Technologies, Inc., Whitehouse, NJ. All samples were dried in vacuo prior to analysis. The presence of solvates was corroborated by FTIR, 1H NMR, or X-ray crystallography. 1H and 13C NMR spectra were collected on a Varian UNITY INOVA 400 MHz NMR spectrometer. Chemical shifts are reported in ppm relative to an internal standard of TMS. The 13C quaternary carbon
B.S. Hammes et al. / Inorganica Chimica Acta 341 (2002) 33 /38
peaks that are not observed are a result of either poor solubility and/or overlapping signals. Electrospray mass spectra (ESI MS) were recorded on a Finnigan LCQ ion-trap mass spectrometer equipped with an ESI source (Finnigan MAT, San Jose, CA). A gateway PC with NAVIGATOR software version 1.2 (Finnigan Corp., 1995 /1997), was used for data acquisition and plotting. Isotope distribution patterns were calculated using the program ISOPRO 3.0. IR spectra were recorded as KBr disks on a Perkin /Elmer spectrum one FTIR spectrometer equipped with a Dell Optiplex PC. 2.4.2. X-ray crystallography Crystal, data collection, and refinement parameters for 1, 2 and 3 are given in Table 1. Crystals of all complexes were sealed in thin-walled quartz capillaries, mounted on a Siemens P4 diffractometer with a sealed˚ ) controlled via PC tube Mo X-ray source (l /0.71073 A computer (data collected at 203 K for 1 and 2 and 298 K for 3) Automatic searching (Siemens XSCANS 2.1), centering, indexing, and least-squares routines were carried out for each crystal with at least 25 reflections
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in the range, 205/2u 5/258 used to determine the unit cell parameters. During the data collection, the intensities of three representative reflections were measured every 97 reflections, and any decay observed was empirically corrected for by the software during data processing. The systematic absences in the diffraction data are consistent with the space group P/1¯ for 2, and 3 and for P 21/n for 1. The structures were all solved using direct methods or via the Patterson function, completed by subsequent difference Fourier syntheses, and refined by full-matrix least-squares procedures on F2. For 2 the metal complex crystallized with two crystallographically independent molecules per unit cell only one of which showed internal H-bonding. The metrical parameters quoted in the text refer only to this molecule but structural parameters of both molecules are available in the Supporting Information. All non-hydrogen atoms were refined with anisotropic displacement coefficients, while hydrogen atoms were treated as idealized contributions using a riding model except were noted. All software and sources of the scattering factors are contained in the SHELXTL 5.0 program library (G.
Table 1 Summary of crystallographic data and parameters for 1, [(TpCO2Et,Me)ZnNO3] 2, [(TpCO2Et,Me)Zn(OAc)H2O)] and 3, [{(TpCO2Et,Me)Zn}2OH]ClO4
Molecular formula Formula weight Temperature (K) Crystal system Space group Cell constants ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) Z ˚ 3) V (A Absorption coefficient, mcalc (mm 1) dcalc (g cm 3) F (000) Crystal dimensions (mm) ˚) Radiation (l , A h , k , l Ranges collected u Range (8) Number of reflections collected Number of unique reflections Number of parameters Data/parameter ratio Refinement method R (F ) a Rw(F2) b GOFw c ˚ 3) Largest difference peak and hole (e A a b c
R [ajDF j/ajFoj]. Rw [aw (DF )2/awFo2]. Goodness-of-fit on F2.
1
2
3
C21H28BN7O9Zn 598.68 203(2) monoclinc P 21/n
C46H63N12O17B2Zn2 604.22 203(2) triclinic ¯/ /P1
C42H57B2N12ClO17.5Zn2 1197.81 293(2) triclinic ¯/ /P1
12.077(3) 15.581(2) 15.426(3) 90 110.57(2) 90 4 2717.7(9) 0.964 1.462 1240 0.4 0.7 0.6 Mo Ka (0.71073) 00 11, 00 16, 150 15 1.85 /22.50 3490 3302 352 9.35 full-matrix least-squares of F2 0.0621 0.1476 1.065 0.692 and 0.717
11.730(5) 15.270(3) 16.294(4) 84.76(1) 73.84(2) 88.54(2) 4 2792(2) 0.938 1.438 1258 0.8 0.5 0.5 Mo Ka (0.71073) 0 0 10, 16 0 16, 160 17 1.79 /22.50 7222 6794 711 9.54 full-matrix least-squares of F2 0.0515 0.1315 1.029 1.022 and 0.709
11.399(3) 15.230(5) 17.562(3) 79.46(3) 72.14(2) 73.38(3) 2 2765.5(13) 0.993 1.438 1240 0.3 0.2 0.5 Mo Ka (0.71073) 110 0, 160 15, 180 17 1.76 /22.50 7521 7092 689 10.29 full-matrix least-squares of F2 0.067 0.1594 1.020 0.706 and 0.374
B.S. Hammes et al. / Inorganica Chimica Acta 341 (2002) 33 /38
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Table 2 ˚ ) and bond angles (8) for 1 Selected bond lengths (A
Table 4 ˚ ) and bond angles (8) for 3 Selected bond lengths (A
Bond lengths Zn(1) O(7) Zn(1) N(3) Zn(1) O(8)
1.934 (8) 2.051 (7) 2.457 (8)
Zn(1) N(1) Zn(1) N(5) Zn(1) N(1)
2.031(7) 2.061(7) 2.482(7)
Bond angles O(7) Zn(1) N(1) N(1) Zn(1) N(3) N(1) Zn(1) N(5) O(7) Zn(1) O(8) N(3) Zn(1) O(8)
123.8 91.2 94.7 55.3 176.7
O(7) Zn(1) N(3) O(7) Zn(1) N(5) N(3) Zn(1) N(5) N(1) Zn(1) O(8) N(5) Zn(1) O(8)
122.5(3) 126.2(3) 88.4(3) 88.5(3) 94.8(3)
(3) (3) (3) (3) (3)
Symmetry transformations used to generate equivalent atoms.
Sheldrick, Siemens XRD, Madison, WI). Selected bond distances and angles for these complexes are shown in Tables 2 /4; Figs. 1 /3 contain the thermal ellipsoid diagrams.
Bond lengths Zn(1) O(13) Zn(1) N(1) Zn(2) O(13) Zn(2) N(7)
1.867 2.078 1.875 2.069
(6) (7) (6) (7)
Zn(1) N(3) Zn(1) N(5) Zn(2) N(9) Zn(2) N(11)
2.026(7) 2.104(7) 2.037(7) 2.116(8)
Bond angles O(13) Zn(1) N(3) N(3) Zn(1) N(1) N(3) Zn(1) N(5) O(13) Zn(2) N(9) N(9) Zn(2) N(7) N(9) Zn(2) N(11)
136.0 94.4 87.5 138.3 92.4 85.1
(3) (3) (3) (3) (3) (3)
O(13) Zn(1) N(1) O(13) Zn(1) N(5) N(1) Zn(1) N(5) O(13) Zn(2) N(7) O(13) Zn(2) N(11) N(7) Zn(2) N(11)
121.3(3) 115.4(3) 89.1(3) 124.2(3) 111.0(3) 90.8(3)
Symmetry transformations used to generate equivalent atoms.
3. Results and discussion 3.1. Solid state structures The nature of the complex formed is highly dependent on the zinc salt utilized. Thus with zinc nitrate only a simple pseudotetrahedral nitrate complex, analogous to that seen in the normal alkyl substituted TpR,R analogs, is obtained. No water molecules stabilized by internal H-bonding with the ligand are found in the coordination sphere and consequently the carbonyl oxygens are all pointed away from the metal yielding a narrow and relatively crowded binding pocket stabilizing the pseudotetrahedral geometry around the zinc. The nitrate ion is coordinated to the zinc in an anisobidentate fashion as expected for such a binding environment [22]. The structure of the zinc complex formed when the acetate salt is used reveals two independent molecules in the unit cell. One contains a simple pseudotetrahedral
Fig. 1. ORTEP diagram (20% probability ellipsoids) of [(TpCO2Et,Me )ZnNO3] showing complete atomic labeling.
Table 3 ˚ ) and bond angles (8) for 2 Selected bond lengths (A Bond lengths Zn(1) O(9) Zn(1) N(1) Zn(1) N(3) Zn(2) N(7)
1.966 2.040 2.376 2.076
(4) (5) (5) (4)
Zn(1) O(8) Zn(1) N(5) Zn(2) O(17)
2.014(4) 2.055(4) 1.899(4)
Bond angles O(9) Zn(1) O(8) O(8) Zn(1) N(1) O(8) Zn(1) N(5) O(9) Zn(1) N(3) N(1) Zn(1) N(3) O(17) Zn(2) N(9) N(9) Zn(2) N(11) N(9) Zn(2) N(7)
93.3 103.4 92.4 87.8 85.9 132.7 94.0 86.8
(2) (2) (2) (2) (2) (2) (2) (2)
O(9) Zn(1) N(1) O(9) Zn(1) N(5) N(1) Zn(1) N(5) O(8) Zn(1) N(3) N(5) Zn(1) N(3) O(17) Zn(2) N(11) O(17) Zn(2) N(7) N(11) Zn(2) N(7)
119.8(2) 141.9(2) 95.3(2) 168.5(2) 79.8(2) 121.8(2) 119.7(2) 90.2(2)
Symmetry transformations used to generate equivalent atoms.
Fig. 2. ORTEP diagram (20% probability ellipsoids) of one of the crystallographically independent molecules of [(TpCO2Et,Me)Zn(OAc)H2O)] with partial atomic labeling. Dotted lines represent internal hydrogen bonds.
Tp zinc /acetate complex, 2a similar in all respects to the nitrate analog. However, the second molecule in the cell is the five-coordinate aquo complex [(TpCO2Et,Me)Zn(OAc)(H2O)] (2b) where a coordinated water is stabi-
B.S. Hammes et al. / Inorganica Chimica Acta 341 (2002) 33 /38
Fig. 3. ORTEP diagram (20% probability ellipsoids) of the cationic portion of [{(TpCO2Et,Me)Zn}2OH]ClO4. For clarity only partial atomic labeling is shown. Dotted lines represent internal hydrogen bonds between the bridging hydroxide and the weak interaction between the ester carbonyl oxygens and the zinc atoms.
lized by two strong internal H-bonds (Fig. 2). One such bond is between the water and the acetate ligand for which the hydrogen atom was located and refined. More germane, however, is that between the water and the ester carbonyl O3 that is now directed into the metal binding cavity. Although the hydrogen atom in question could not be located unambiguously on the difference ˚ ) is maps, the heavy atom-heavy atom distance (2.66 A indicative of strong hydrogen bonding. While O5 is also pointed inwards, which indicates some interaction with ˚ ). the water, the distance between them is long (3.5 A Ester carbonyl O1 is pointed outside, indicating no interaction within the metal/ligand cavity. Although a pentacoordinate aquo complex of zinc is by no means unprecedented for the Tp family of ligands [23 /25] the internal H-bonding interactions cause a number of important structural changes. In particular in 2b, the water is found in the equatorial plane of the trigonalbipyramid rather than in the axial position it occupies in the other known examples. Perhaps more importantly, the Zn /Owater bond, while clearly appropriate for a ˚ ), is on coordinated water rather than hydroxide (1.966 A ˚ the short end of the expected range and is about 0.1 A shorter than that found in its non H-bonding analogs, suggesting a stabilization by the former. When a zinc salt of a poorly coordinating counter ion is used, i.e. zinc perchlorate, two distinct species were formed: (1) a mononuclear six-coordinate complex [(TpCO2Et,Me)Zn(H2O)3] which displays strong Hbonding between the ester carbonyls and three facially coordinated waters (completely analogous to the previously reported Ni, Co and Mn analogs [13]) (2) the four-coordinate, binuclear, mono-hydroxo bridged, [(TpCO2Et,Me)Zn(m-OH)Zn(TpCO2Et,Me)]ClO4. Crystals
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of the first complex are isomorphous with those of the previously reported Ni(II) derivative hence its structure is not reported in detail here. The second complex shows two (TpCO2Et,Me)Zn units bridged by a single hydroxide ion with a Zn /O /Zn angle of 134.48 and a Zn /Zn ˚ The bridging ion is unequivocally distance of 3.424 A identified as hydroxide based on charge considerations and the short Zn /O bond lengths of 1.867(6) and ˚ . While mono hydroxo bridged dinuclear Zn 1.875(6) A complexes are certainly not unprecedented, they are also not common with only 11 examples found in the Cambridge Structural Data Base. In these structurally characterized complexes the Zn /O /Zn angle ranges from 115 to 1498 with an average 135.4(63), very similar to that found for 3. However, the average Zn /OH bond length seen in 3 of 1.871(2) is considerably shorter than that found in any of the previously characterized ˚. dinuclear complexes which bonds average 1.930(30) A but is similar to values seen in mononuclear fourcoordinate species. In addition to the facial coordination of the Tp ligand and the bridging hydroxide ion, each Zn ion shows a rather strong interaction with one of the ester carbonyls of the opposite ligand (Zn /Oco /2.576(6)) so that the coordination geometry around the metal can be viewed as distorted trigonal-bipyramidal. The hydroxyl hydrogen, which was located on the difference map, is Hbonded to two ester carbonyl moieties with heavy atom˚ heavy atom distances of 2.902 (O13 /O11) and 2.842 A (O13/O5) and O /H /O angles of 152.1 (O13 /H13d / O11) and 96.78 (O13 /H13D /O5). 3.2. Solution studies In acetonitrile or freshly prepared methanol solutions positive ion mode electrospray mass spectra (ESI MS) on 3 show two clean mass clusters. The first has an m /e centered around 536 amu and an isotope pattern indicative of the mononuclear [(TpCO2Et,Me)Zn] cation. The second has an envelope centered about 1092 amu and an isotope pattern that is consistent with the [(TpCO2Et,Me)Zn(m-OH)Zn(TpCO2Et,Me)] cation. Thus the binuclear species represented by the solid-state structure appears to maintain its integrity in solution. However, allowing a methanol solution of 3 to stand for a few hours reveals complete conversion to a new set of species with molecular masses of 493 and 1008 amu, which correspond to the complexes where the ethyl esters of the ligand have been quantitatively replaced by methyl esters, i.e. a transesterification has occurred. A small amount of the simple pseudotetrahedral MeOH adduct [(TpCO2Me,Me)Zn(MeOH)] is also observable. The transesterification reaction can also be followed by 1 H or 13C NMR spectroscopy from which a pseudo first order rate constant in pure CD3OD at 25 8C of 4.1(4) /10 2 h 1 was determined.
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B.S. Hammes et al. / Inorganica Chimica Acta 341 (2002) 33 /38
In summary both aquo and hydroxo complexes of zinc displaying internal hydrogen bonding with the ester substituted trispyrazolylborate ligand, TpCO2Et,Me have been isolated and crystallographically characterized. The hydroxo complex appears to catalyze self-transesterification reactions in alcoholic solvents. However, the expansion of the coordination sphere from four- to fivecoordinate in 2 or the dimerization that occurs with 3 suggest that a more sterically restrictive ester substituted ligand will be needed to enforce the desired mononuclear four-coordinate pseudotetrahedral geometry most commonly seen in zinc metalloenzyme active sites.
4. Supplimentary material Crystallographic data for 1 /3 have been deposited with the Cambridge Structural Data Center under CCDC Nos. 159089, 159088 and 178596, respectively. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: /44-1223-336-033; email: [email protected] or www: http:// www.ccdc.cam.ac.uk).
Acknowledgements This work was supported in part by Grants AI-1157 from the Robert A. Welch Foundation and CHE9726488 from the NSF. The NSF-ILI program grant USE-9151286 is acknowledged for partial support of the X-ray diffraction facilities at the Southwest Texas State University.
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Zinc complexes of hydrogen bond accepting ester substituted trispyrazolylborates Brian S. Hammes, Xuemei Luo, Mary W. Carrano, Carl J. Carrano * Department of Chemistry and Biochemistry, Southwest Texas State University, San Marcos, TX 78666, USA Received 8 February 2002; accepted 8 April 2002 Dedicated to Professor K.N. Raymond on the occasion of his 60th birthday
Abstract As models for the labile water ligands so ubiquitous in zinc metalloprotein active sites, aquo and hydroxo complexes of zinc with the ester substituted trispyrazolylborate ligand, [(TpCO2Et,Me) have been isolated and crystallographically characterized. These complexes are stabilized by internal hydrogen bonding between the water or hydroxide and the ester carbonyls of the ligand. The dinuclear hydroxo complex, which maintains its structure in solution, appears to catalyze self-transesterification reactions in alcoholic solvents. However, the expansion of the coordination sphere from four- to five-coordinate in 2 or the dimerization that occurs with 3 suggest that a more sterically restrictive ester substituted ligand will be needed to enforce the desired mononuclear four-coordinate pseudotetetrahedral geometry most commonly seen in zinc metalloenzyme active sites. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Zinc complexes; Hydrogen bond; Ester substituted trispyrazolylborates
1. Introduction The tris(pyrazolyl)borate ligands pioneered by Trofimenko in the late 1960’s, have arguably become the preeminent platform for making complexes in coordination, organometallic and bioinorganic chemistries [1]. Modifications to the parent ligand over the last few decades have been primarily directed towards the synthesis of highly sterically congested analogs that have allowed for the stabilization of species not heretofore accessible [2 /5]. More recently attempts have been made to modify the electronic and metal binding properties of these ligands, an example of which are the perflourinated derivatives of TpMe,Me which lead to chemistry quite different from that of the parent [6]. An alternate approach is to replace one or more of the pyrazole nitrogens by another type of donor atom (thiol, phenol, carboxylate, etc.) to produce a heteroscorpionate ligand. We and others have recently succeeded in
* Corresponding author. Tel.: /1-512-245 3117; fax: /1-512-245 2374 E-mail address: [email protected] (C.J. Carrano).
providing a general route into this class of ligands and have begun to examine their coordination chemistry with a variety of metal ions [7 /9]. Finally, an area of recent interest concerns modifications designed to increase the denticity of the Tp ligand from h3 to h6 using different N or O donors attached to the pyrazole rings [10 /12]. Thus it is clear that the utility of these ligands is being further enhanced by a new generation of modifications designed to address other than steric issues. Based on the enormous success of the TpR, platform [2 /5] as a ligand for metalloenzyme modeling studies, we have recently described a first generation of simple 5carboxyester substituted analogs of Tp which simultaneously provide both steric bulk and hydrogen bonding groups which can be directed towards the center of the metal binding cavity. The presence of strong intramolecular H-bonding between the ligand carbonyls and water molecules leads to the stabilization of discrete, well characterized, divalent metal aquo complexes of the general type in [TpCO2Et,MeM(H2O)x ] where M / Co(II), Ni(II), Mn(II), or Cu(II) and x /2 or 3 depending on the metal [13,14]. These complexes are
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 1 7 7 - 5
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both, unprecedented in Tp chemistry, and good structural models for enzymes of the vicinal oxygen chelate (VOC) superfamily [15]. Since water, hydroxide and other small exogenous ligands are also ubiquitous in pseudutetrahedral zinc metalloenzyme centers [16] and clearly important to enzymatic activity, zinc complexes of tripodal ligands have been extensively studied by numerous workers including Kimura, Parkin, Vahrenkamp, and others and several review articles are available [17 /19]. Some examples of hydrogen bond stabilized pentacoordinate Zn /OH complexes have recently been reported [20,21]. In the present work we have sought to stabilize pseudotetrahedral zinc aquo or hydroxo species by internal hydrogen bonding using the ester substituted Tp ligand [TpCO2Et,Me] and describe the isolation and structural characterization of three such complexes.
2. Experimental All syntheses were carried out in air and the reagents and solvents purchased from commercial sources and used as received unless otherwise noted. Methanol was distilled under argon over CaH2. Potassium tris(3carboxyethyl-5-methyl)pyrazolylborate was prepared using the previously reported procedures [13]. 2.1. [(TpCO2Et,Me)ZnNO3] (1) A slurry of potassium tris(3-carboxyethyl-5methyl)pyrazolylborate (1) (0.30 g, 0.59 mmol) in 30 ml of CH2Cl2 was treated with solid Zn(NO3)2 ×/H2O (0.18 g, 0.59 mmol). The resulting solution was stirred for 24 h, filtered to remove a small amount of solid (KNO3), and concentrated under reduced pressure to a volume of 6 ml. Crystals for an X-ray diffraction study were grown by the slow diffusion of hexane into a CH2Cl2 solution giving 0.18 g (51%) of [(TpCO2Et,Me)ZnNO3] (1) as colorless plates. Anal . Calc. (Found) for [(TpCO2Et,Me)ZnNO3], C21H28N7O9BZn ×/0.125 C6H14: C, 42.86 (42.60); H, 4.93 (4.71); N, 16.09 (15.74)%. 1H NMR (CDCl3) d 6.57 (s, 1H, PzH ), 4.45 (q, 2H, J /7 Hz, /OCH2 /), 2.49 (s, 3H, Pz /CH3), 1.40 (t, 3H, J /7 Hz, /CH3). 13C NMR (CDCl3) d 160.52, 146.51, 144.51, 108.75, 61.87, 14.22, 12.67. FTIR (KBr, cm 1): n /2565 (B/H), 1729 (C /O), 1515 (N /O). 2.2. [(TpCO2Et,Me)Zn(H2O)(OAc)] (2) A slurry of potassium tris(3-methyl-5-carboxyethyl)pyrazolylborate (0.73 g, 1.4 mmol) in 30 ml of CH2Cl2 was treated with solid Zn(OAc)2 ×/4H2O (0.47 g, 1.4 mmol). The resulting solution was stirred for 24 h, filtered to remove a small amount of solid (KOAc), and concentrated under reduced pressure to a volume of 5
ml. Crystals for an X-ray diffraction study were grown by the slow diffusion of hexane into a CH2Cl2 solution giving 0.42 g (50%) of [(TpCO2Et,Me)ZnOAc] (2) as colorless needles. Anal . Calc. (Found) for [(TpCO2Et,Me)ZnOAc] ×/0.5H2O, C23H32N6O8.5BZn: C, 45.68 (45.56); H, 5.34 (5.22); N, 13.90 (13.79)%. 1H NMR (CDCl3) d 6.51 (s, 1H, PzH), 5.57 (br, 1H, BH), 4.38 (q, 2H, J /7 Hz, /OCH2 /), 2.46 (s, 3H, Pz /CH3), 2.07 (s, 3H, /C(O)2CH3), 1.38 (t, 3H, J /7 Hz, /CH3). 13 C NMR (CDCl3) d 179.70, 161.06, 145.81, 144.25, 108.43, 61.31, 22.50, 14.14, 12.65. FTIR (KBr, cm 1): n /2559 (B/H), 1727 (C /O), 1703 (C /O), 1633 (C /O), 1614 (C /O). 2.3. [{(TpCO2Et,Me)Zn}2OH]ClO4 (3) A slurry of potassium TpCO2Et,Me (0.253 g) in 15 ml CH3OH was treated dropwise with a solution of Zn(ClO4)2 ×/6H2O in CH3OH. The resulting mixture was stirred for 6 h, and concentrated under vacuum after removal of a small amount of white solid. Crystallization was achieved by slow diffusion of ether into a concentrated methanolic solution. Caution! This and the other perchlorate salts isolated in this work are potentially explosive and should be handled with care. Anal . Calc. (Found) for [{(TpCO2Et,Me)Zn}2OH]ClO4, C42H57O17N12B2Zn2Cl: C, 42.36 (42.29); H, 4.79(4.68); N, 14.12(14.07); Cl, 2.98 (3.08)%. 1H NMR (CD3OD): d 6.633 (s, 2H, PzH), 3.970 (m, 4H, /CH2 /), 2.581 (s, 6H, Pz /CH3), 1.005 (t, 6H, /CH3). 13C NMR (CD3OD): d 166.7, 149.8, 145.3, 110.2, 59.0, 19.0, 13.3. FTIR (KBr, cm 1): n /2566 (B/H), 1725 (C /O). 2.4. [(TpCO2Et,Me)Zn(H2O)3]ClO4 (4) After the deposition of crystals from the reaction mixture that produced 3, addition of more ether to the mother liquor and storage for several days at 4 8C yielded a crop of crystals of 4 as colorless blocks. Anal . Calc. (found) for TpCO2Et,MeZn(H2O)3ClO4, C21H34N6O13BZnCl: C, 36.53; H, 4.93; N, 12.18%. Found: C, 37.88; H, 4.17; N, 12.54%. 1H NMR (CD3OD) d 6.636 (s, 1H, PzH), 4.422(m, 2H, /CH2 /), 2.541(s, 3H, Pz /CH3), 1.379 (t, 3H, /CH3). 13 C NMR: d 166.5, 149.7, 145.3, 110.3, 63.8, 15.1, 13.5. FTIR (KBr, cm 1): n /2569 (B/H), 1724 (C /O). 2.4.1. Physical methods Elemental analyses were obtained from Quantitative Technologies, Inc., Whitehouse, NJ. All samples were dried in vacuo prior to analysis. The presence of solvates was corroborated by FTIR, 1H NMR, or X-ray crystallography. 1H and 13C NMR spectra were collected on a Varian UNITY INOVA 400 MHz NMR spectrometer. Chemical shifts are reported in ppm relative to an internal standard of TMS. The 13C quaternary carbon
B.S. Hammes et al. / Inorganica Chimica Acta 341 (2002) 33 /38
peaks that are not observed are a result of either poor solubility and/or overlapping signals. Electrospray mass spectra (ESI MS) were recorded on a Finnigan LCQ ion-trap mass spectrometer equipped with an ESI source (Finnigan MAT, San Jose, CA). A gateway PC with NAVIGATOR software version 1.2 (Finnigan Corp., 1995 /1997), was used for data acquisition and plotting. Isotope distribution patterns were calculated using the program ISOPRO 3.0. IR spectra were recorded as KBr disks on a Perkin /Elmer spectrum one FTIR spectrometer equipped with a Dell Optiplex PC. 2.4.2. X-ray crystallography Crystal, data collection, and refinement parameters for 1, 2 and 3 are given in Table 1. Crystals of all complexes were sealed in thin-walled quartz capillaries, mounted on a Siemens P4 diffractometer with a sealed˚ ) controlled via PC tube Mo X-ray source (l /0.71073 A computer (data collected at 203 K for 1 and 2 and 298 K for 3) Automatic searching (Siemens XSCANS 2.1), centering, indexing, and least-squares routines were carried out for each crystal with at least 25 reflections
35
in the range, 205/2u 5/258 used to determine the unit cell parameters. During the data collection, the intensities of three representative reflections were measured every 97 reflections, and any decay observed was empirically corrected for by the software during data processing. The systematic absences in the diffraction data are consistent with the space group P/1¯ for 2, and 3 and for P 21/n for 1. The structures were all solved using direct methods or via the Patterson function, completed by subsequent difference Fourier syntheses, and refined by full-matrix least-squares procedures on F2. For 2 the metal complex crystallized with two crystallographically independent molecules per unit cell only one of which showed internal H-bonding. The metrical parameters quoted in the text refer only to this molecule but structural parameters of both molecules are available in the Supporting Information. All non-hydrogen atoms were refined with anisotropic displacement coefficients, while hydrogen atoms were treated as idealized contributions using a riding model except were noted. All software and sources of the scattering factors are contained in the SHELXTL 5.0 program library (G.
Table 1 Summary of crystallographic data and parameters for 1, [(TpCO2Et,Me)ZnNO3] 2, [(TpCO2Et,Me)Zn(OAc)H2O)] and 3, [{(TpCO2Et,Me)Zn}2OH]ClO4
Molecular formula Formula weight Temperature (K) Crystal system Space group Cell constants ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) Z ˚ 3) V (A Absorption coefficient, mcalc (mm 1) dcalc (g cm 3) F (000) Crystal dimensions (mm) ˚) Radiation (l , A h , k , l Ranges collected u Range (8) Number of reflections collected Number of unique reflections Number of parameters Data/parameter ratio Refinement method R (F ) a Rw(F2) b GOFw c ˚ 3) Largest difference peak and hole (e A a b c
R [ajDF j/ajFoj]. Rw [aw (DF )2/awFo2]. Goodness-of-fit on F2.
1
2
3
C21H28BN7O9Zn 598.68 203(2) monoclinc P 21/n
C46H63N12O17B2Zn2 604.22 203(2) triclinic ¯/ /P1
C42H57B2N12ClO17.5Zn2 1197.81 293(2) triclinic ¯/ /P1
12.077(3) 15.581(2) 15.426(3) 90 110.57(2) 90 4 2717.7(9) 0.964 1.462 1240 0.4 0.7 0.6 Mo Ka (0.71073) 00 11, 00 16, 150 15 1.85 /22.50 3490 3302 352 9.35 full-matrix least-squares of F2 0.0621 0.1476 1.065 0.692 and 0.717
11.730(5) 15.270(3) 16.294(4) 84.76(1) 73.84(2) 88.54(2) 4 2792(2) 0.938 1.438 1258 0.8 0.5 0.5 Mo Ka (0.71073) 0 0 10, 16 0 16, 160 17 1.79 /22.50 7222 6794 711 9.54 full-matrix least-squares of F2 0.0515 0.1315 1.029 1.022 and 0.709
11.399(3) 15.230(5) 17.562(3) 79.46(3) 72.14(2) 73.38(3) 2 2765.5(13) 0.993 1.438 1240 0.3 0.2 0.5 Mo Ka (0.71073) 110 0, 160 15, 180 17 1.76 /22.50 7521 7092 689 10.29 full-matrix least-squares of F2 0.067 0.1594 1.020 0.706 and 0.374
B.S. Hammes et al. / Inorganica Chimica Acta 341 (2002) 33 /38
36
Table 2 ˚ ) and bond angles (8) for 1 Selected bond lengths (A
Table 4 ˚ ) and bond angles (8) for 3 Selected bond lengths (A
Bond lengths Zn(1) O(7) Zn(1) N(3) Zn(1) O(8)
1.934 (8) 2.051 (7) 2.457 (8)
Zn(1) N(1) Zn(1) N(5) Zn(1) N(1)
2.031(7) 2.061(7) 2.482(7)
Bond angles O(7) Zn(1) N(1) N(1) Zn(1) N(3) N(1) Zn(1) N(5) O(7) Zn(1) O(8) N(3) Zn(1) O(8)
123.8 91.2 94.7 55.3 176.7
O(7) Zn(1) N(3) O(7) Zn(1) N(5) N(3) Zn(1) N(5) N(1) Zn(1) O(8) N(5) Zn(1) O(8)
122.5(3) 126.2(3) 88.4(3) 88.5(3) 94.8(3)
(3) (3) (3) (3) (3)
Symmetry transformations used to generate equivalent atoms.
Sheldrick, Siemens XRD, Madison, WI). Selected bond distances and angles for these complexes are shown in Tables 2 /4; Figs. 1 /3 contain the thermal ellipsoid diagrams.
Bond lengths Zn(1) O(13) Zn(1) N(1) Zn(2) O(13) Zn(2) N(7)
1.867 2.078 1.875 2.069
(6) (7) (6) (7)
Zn(1) N(3) Zn(1) N(5) Zn(2) N(9) Zn(2) N(11)
2.026(7) 2.104(7) 2.037(7) 2.116(8)
Bond angles O(13) Zn(1) N(3) N(3) Zn(1) N(1) N(3) Zn(1) N(5) O(13) Zn(2) N(9) N(9) Zn(2) N(7) N(9) Zn(2) N(11)
136.0 94.4 87.5 138.3 92.4 85.1
(3) (3) (3) (3) (3) (3)
O(13) Zn(1) N(1) O(13) Zn(1) N(5) N(1) Zn(1) N(5) O(13) Zn(2) N(7) O(13) Zn(2) N(11) N(7) Zn(2) N(11)
121.3(3) 115.4(3) 89.1(3) 124.2(3) 111.0(3) 90.8(3)
Symmetry transformations used to generate equivalent atoms.
3. Results and discussion 3.1. Solid state structures The nature of the complex formed is highly dependent on the zinc salt utilized. Thus with zinc nitrate only a simple pseudotetrahedral nitrate complex, analogous to that seen in the normal alkyl substituted TpR,R analogs, is obtained. No water molecules stabilized by internal H-bonding with the ligand are found in the coordination sphere and consequently the carbonyl oxygens are all pointed away from the metal yielding a narrow and relatively crowded binding pocket stabilizing the pseudotetrahedral geometry around the zinc. The nitrate ion is coordinated to the zinc in an anisobidentate fashion as expected for such a binding environment [22]. The structure of the zinc complex formed when the acetate salt is used reveals two independent molecules in the unit cell. One contains a simple pseudotetrahedral
Fig. 1. ORTEP diagram (20% probability ellipsoids) of [(TpCO2Et,Me )ZnNO3] showing complete atomic labeling.
Table 3 ˚ ) and bond angles (8) for 2 Selected bond lengths (A Bond lengths Zn(1) O(9) Zn(1) N(1) Zn(1) N(3) Zn(2) N(7)
1.966 2.040 2.376 2.076
(4) (5) (5) (4)
Zn(1) O(8) Zn(1) N(5) Zn(2) O(17)
2.014(4) 2.055(4) 1.899(4)
Bond angles O(9) Zn(1) O(8) O(8) Zn(1) N(1) O(8) Zn(1) N(5) O(9) Zn(1) N(3) N(1) Zn(1) N(3) O(17) Zn(2) N(9) N(9) Zn(2) N(11) N(9) Zn(2) N(7)
93.3 103.4 92.4 87.8 85.9 132.7 94.0 86.8
(2) (2) (2) (2) (2) (2) (2) (2)
O(9) Zn(1) N(1) O(9) Zn(1) N(5) N(1) Zn(1) N(5) O(8) Zn(1) N(3) N(5) Zn(1) N(3) O(17) Zn(2) N(11) O(17) Zn(2) N(7) N(11) Zn(2) N(7)
119.8(2) 141.9(2) 95.3(2) 168.5(2) 79.8(2) 121.8(2) 119.7(2) 90.2(2)
Symmetry transformations used to generate equivalent atoms.
Fig. 2. ORTEP diagram (20% probability ellipsoids) of one of the crystallographically independent molecules of [(TpCO2Et,Me)Zn(OAc)H2O)] with partial atomic labeling. Dotted lines represent internal hydrogen bonds.
Tp zinc /acetate complex, 2a similar in all respects to the nitrate analog. However, the second molecule in the cell is the five-coordinate aquo complex [(TpCO2Et,Me)Zn(OAc)(H2O)] (2b) where a coordinated water is stabi-
B.S. Hammes et al. / Inorganica Chimica Acta 341 (2002) 33 /38
Fig. 3. ORTEP diagram (20% probability ellipsoids) of the cationic portion of [{(TpCO2Et,Me)Zn}2OH]ClO4. For clarity only partial atomic labeling is shown. Dotted lines represent internal hydrogen bonds between the bridging hydroxide and the weak interaction between the ester carbonyl oxygens and the zinc atoms.
lized by two strong internal H-bonds (Fig. 2). One such bond is between the water and the acetate ligand for which the hydrogen atom was located and refined. More germane, however, is that between the water and the ester carbonyl O3 that is now directed into the metal binding cavity. Although the hydrogen atom in question could not be located unambiguously on the difference ˚ ) is maps, the heavy atom-heavy atom distance (2.66 A indicative of strong hydrogen bonding. While O5 is also pointed inwards, which indicates some interaction with ˚ ). the water, the distance between them is long (3.5 A Ester carbonyl O1 is pointed outside, indicating no interaction within the metal/ligand cavity. Although a pentacoordinate aquo complex of zinc is by no means unprecedented for the Tp family of ligands [23 /25] the internal H-bonding interactions cause a number of important structural changes. In particular in 2b, the water is found in the equatorial plane of the trigonalbipyramid rather than in the axial position it occupies in the other known examples. Perhaps more importantly, the Zn /Owater bond, while clearly appropriate for a ˚ ), is on coordinated water rather than hydroxide (1.966 A ˚ the short end of the expected range and is about 0.1 A shorter than that found in its non H-bonding analogs, suggesting a stabilization by the former. When a zinc salt of a poorly coordinating counter ion is used, i.e. zinc perchlorate, two distinct species were formed: (1) a mononuclear six-coordinate complex [(TpCO2Et,Me)Zn(H2O)3] which displays strong Hbonding between the ester carbonyls and three facially coordinated waters (completely analogous to the previously reported Ni, Co and Mn analogs [13]) (2) the four-coordinate, binuclear, mono-hydroxo bridged, [(TpCO2Et,Me)Zn(m-OH)Zn(TpCO2Et,Me)]ClO4. Crystals
37
of the first complex are isomorphous with those of the previously reported Ni(II) derivative hence its structure is not reported in detail here. The second complex shows two (TpCO2Et,Me)Zn units bridged by a single hydroxide ion with a Zn /O /Zn angle of 134.48 and a Zn /Zn ˚ The bridging ion is unequivocally distance of 3.424 A identified as hydroxide based on charge considerations and the short Zn /O bond lengths of 1.867(6) and ˚ . While mono hydroxo bridged dinuclear Zn 1.875(6) A complexes are certainly not unprecedented, they are also not common with only 11 examples found in the Cambridge Structural Data Base. In these structurally characterized complexes the Zn /O /Zn angle ranges from 115 to 1498 with an average 135.4(63), very similar to that found for 3. However, the average Zn /OH bond length seen in 3 of 1.871(2) is considerably shorter than that found in any of the previously characterized ˚. dinuclear complexes which bonds average 1.930(30) A but is similar to values seen in mononuclear fourcoordinate species. In addition to the facial coordination of the Tp ligand and the bridging hydroxide ion, each Zn ion shows a rather strong interaction with one of the ester carbonyls of the opposite ligand (Zn /Oco /2.576(6)) so that the coordination geometry around the metal can be viewed as distorted trigonal-bipyramidal. The hydroxyl hydrogen, which was located on the difference map, is Hbonded to two ester carbonyl moieties with heavy atom˚ heavy atom distances of 2.902 (O13 /O11) and 2.842 A (O13/O5) and O /H /O angles of 152.1 (O13 /H13d / O11) and 96.78 (O13 /H13D /O5). 3.2. Solution studies In acetonitrile or freshly prepared methanol solutions positive ion mode electrospray mass spectra (ESI MS) on 3 show two clean mass clusters. The first has an m /e centered around 536 amu and an isotope pattern indicative of the mononuclear [(TpCO2Et,Me)Zn] cation. The second has an envelope centered about 1092 amu and an isotope pattern that is consistent with the [(TpCO2Et,Me)Zn(m-OH)Zn(TpCO2Et,Me)] cation. Thus the binuclear species represented by the solid-state structure appears to maintain its integrity in solution. However, allowing a methanol solution of 3 to stand for a few hours reveals complete conversion to a new set of species with molecular masses of 493 and 1008 amu, which correspond to the complexes where the ethyl esters of the ligand have been quantitatively replaced by methyl esters, i.e. a transesterification has occurred. A small amount of the simple pseudotetrahedral MeOH adduct [(TpCO2Me,Me)Zn(MeOH)] is also observable. The transesterification reaction can also be followed by 1 H or 13C NMR spectroscopy from which a pseudo first order rate constant in pure CD3OD at 25 8C of 4.1(4) /10 2 h 1 was determined.
38
B.S. Hammes et al. / Inorganica Chimica Acta 341 (2002) 33 /38
In summary both aquo and hydroxo complexes of zinc displaying internal hydrogen bonding with the ester substituted trispyrazolylborate ligand, TpCO2Et,Me have been isolated and crystallographically characterized. The hydroxo complex appears to catalyze self-transesterification reactions in alcoholic solvents. However, the expansion of the coordination sphere from four- to fivecoordinate in 2 or the dimerization that occurs with 3 suggest that a more sterically restrictive ester substituted ligand will be needed to enforce the desired mononuclear four-coordinate pseudotetrahedral geometry most commonly seen in zinc metalloenzyme active sites.
4. Supplimentary material Crystallographic data for 1 /3 have been deposited with the Cambridge Structural Data Center under CCDC Nos. 159089, 159088 and 178596, respectively. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: /44-1223-336-033; email: [email protected] or www: http:// www.ccdc.cam.ac.uk).
Acknowledgements This work was supported in part by Grants AI-1157 from the Robert A. Welch Foundation and CHE9726488 from the NSF. The NSF-ILI program grant USE-9151286 is acknowledged for partial support of the X-ray diffraction facilities at the Southwest Texas State University.
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