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A new tris(2-furyl) substituted pyrazolylborate ligand and its zinc complex chemistry
Inorganica Chimica Acta 359 (2006) 4079–4086 www.elsevier.com/locate/ica
A new tris(2-furyl) substituted pyrazolylborate ligand and its zinc complex chemistry Jose´ A. Maldonado Calvo, Heinrich Vahrenkamp
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Institut fu¨r Anorganische und Analytische Chemie der Universita¨t Freiburg, Albertstr. 21, D-79104 Freiburg, Germany Received 22 March 2006; accepted 10 April 2006 Available online 22 April 2006
Abstract The new ligand hydrotris(3-(2 0 -furyl)-5-methylpyrazolyl)borate (TpFu,Me) was prepared by the usual procedure. With zinc salts, it forms the TpFu,MeZn–X complexes (X = Cl, Br, I, NCS, CH3COO, CF3COO). With zinc perchlorate, the bis-ligand complex Zn(TpFu,Me)2 is formed preferrably, but by carefully controlling the reaction conditions, the ‘‘enzyme model’’ TpFu,MeZn–OH could be obtained. The latter models carbonic anhydrase by inserting CO2 and CS2 in methanol producing TpFu,MeZn–OCOOMe and TpFu,MeZn–SCSOMe. It models hydrolases by the hydrolytic cleavage of tris(p-nitrophenyl)phosphate and c-thiobutyrolactone. It does not hydrolyse trifluoroacetamide, but instead deprotonates it, yielding TpFu,MeZn–NHCOCF3. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Zinc complexes; Pyrazolylborate ligands; Hydroxide complexes; Biomimetic reactions
1. Introduction The tris(pyrazolyl)borate ligands belong to the most suitable ligands for a biomimetic zinc complex chemistry [1,2]. They owe this advantage to the accessibility of such species, in which various substituents at the 3-positions of the pyrazole rings provide a favourable encapsulation of the functional Zn–X units. In most cases, the particular 3-substituents have been hydrocarbon species, creating a hydrophobic cavity around the Zn–X centers. In order to further approximate the biological environment of zinc, it would be desirable to employ such pyrazolylborates with polar substituents at the 3-positions. This should improve the modeling of hydrolytic zinc enzymes, it should allow to study the influence of hydrogen bonding interactions in the vicinity of the metal, and it should lead the way to a biomimetic chem-
*
Corresponding author. Tel.: +49 761 203 6120; fax: +49 761 203 6001. E-mail address: [email protected] (H. Vahrenkamp).
0020-1693/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.04.016
istry with pyrazolylborate–zinc complexes in aqueous media. There has been a limited number of reports in the literature on the zinc complex chemistry of pyrazolylborate ligands with polar 3-substituents. We described those with 3-pyridyl groups [3,4], Carrano reported on those with 3-carboxyester groups [5,6]. Promising examples which have not yet been exploited for zinc complexes are Trofimenko’s pyrrolidinocarbonyl- [7] and Ward’s 2-pyridyl-substituted ligands [8]. It seems to be a common feature of these polar pyrazolylborates that they favour higher coordination numbers than 4 for the zinc ion. The present paper reports a new member of this group of polar pyrazolylborate ligands, the 2-furyl-substituted TpFu,Me. The furyl substituent was chosen because of the easy accessibility of the furyl-substituted pyrazole. The main motivation for the study of its zinc complexes was the hope that the more polar environment of zinc would improve the efficiency of the complexes as models for hydrolytic zinc enzymes.
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H B
N
N
N
N
N
N
O
O
O
[ TpFu,Me ]-
2. Results and discussion 2.1. Ligand synthesis The synthesis of the furyl-substituted pyrazole precursor from acetyl-2-furyl-methane and hydrazine hydrate was straightforward. Its reaction with KBH4 in a melt at 200 °C produced the potassium salt KTpFu,Me in good yield, which was purified by recrystallization from acetonitrile. The ligand as a potassium salt is soluble in polar organic solvents, but not in water. Its characteristic spectral features are the m(BH) band in the IR at 2494 cm1 and the pyrazole-H resonance in the 1H NMR in CDCl3 at 6.04 ppm. The furyl group’s proton resonances appear as multiplets.
˚ ): Zn– Fig. 1. Structure of the thiocyanate complex 4. Bond distances (A N1 2.036(3), Zn–N2 2.022(4), Zn–N3 2.011(3), Zn–N(thiocyanate) 1.892(4). Bond angle: Zn–N–C 155.8(1)°.
2.2. Simple zinc complexes The basic zinc complex chemistry of TpFu,Me was probed by reacting the potassium salt of the ligand with zinc salts. This way the halide complexes 1–3, the thiocyanate complex 4 and the two acetate complexes 5 and 6 were obtained. They are all colorless and crystalline, easily identified by IR and 1H NMR spectroscopy. X= Nr.:
Cl 1
Br 2
TpFu,MeZn–X I NCS OCOCH3 3 4 5
OCOCF3 6
For comparative purposes, the crystal structures of 4 and 5 were determined. Fig. 1 shows that the thiocyanate in 4 is bound to zinc via the nitrogen atom, as usual [9], and slightly bent at the Zn–N–C array. The acetate ligand in 5 is attached in a semibidentate fashion, just halfway between the extremes of a purely monodentate [9] and almost perfect bidentate fashion [10]. Compared to the multitude of pyrazolylborate–Zn–X complexes, the Zn–N bond lengths in 4 and 5 are in the normal range, and the orientation of the furyl rings is comparable to that of the phenyl rings in the TpPh,MeZn complexes. In both cases, two of the furyl oxygen atoms point to the center of the complexes, while the third one points to the outside (see Figs. 1 and 2).
˚ ): Zn–N1 Fig. 2. Structure of the acetate complex 5. Bond lengths (A 2.038(5), Zn–N2 2.044(5), Zn–N3 2.091(5), Zn–O 1.923(3), Zn O 2.534(3).
2.3. The zinc-hydroxide complex The essential reagents in biomimetic pyrazolylborate– zinc complex chemistry are the Zn–OH species, the proper enzyme models [1]. As a rule they form easily from the Tp
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ligand and a zinc salt with non-coordinating anions, e.g. Zn(ClO4)2, in the presence of KOH. This proved difficult here, and initial attempts resulted only in the formation of the bis-ligand complex 7, which is presumed to result from dismutation of the desired 8 into 7 and Zn(OH)2. Lowering the reaction temperature and carefully controlling the reaction conditions finally allowed the isolation of the hydroxide complex 8. ZnðTpFu;Me Þ2 7
TpFu;Me Zn–OH 8
Complex 8 was too labile to grow single crystals for a structure determination, producing 7 again. 7 was easy to crystallize, yielding the molecular structure displayed in Fig. 3. In 7 the coordination of the zinc ion is octahedral to a very good approximation. Surprisingly, there is no steric hindrance between the furyl substituents of the two opposing TpFu,Me ligands, due to the coplanarity of the pyrazole and furan rings and the resulting perfect interlocking of the two molecular halves. Complex 7 shares this property with the ZnTp2 complex in which the Tp ligands bear 3-pyridyl substituents [4]. 2.4. Reactions with CO2 and CS2 In our previous studies we could demonstrate that the TpZn–OH complexes are suitable models of carbonic anhydrase, in that they insert CO2 to form a labile bicarbonate complex which in the presence of methanol is converted to the stable methylcarbonate complex [1,11,12]. This was also observed here. Bubbling CO2 into a solution of 8 in methanol/dichloromethane produced the methylcarbonate complex 9, which persists in a CO2 atmosphere. 9 is
Fig. 4. Structure of the methyldithiocarbonate complex 10. Bond lengths ˚ ): Zn–N1 2.081(2), Zn–N2 2.063(2), Zn–N3 2.057(2), Zn–S 2.268(1), (A Zn O 2.683(2), C–S 1.724(3), C@S 1.655(3).
easily recognized by its typical 1H NMR spectrum, but attempts to recrystallize it led to partical re-conversion to 8 with subsequent decomposition. TpFu;Me Zn–OCOOMe 9
TpFu;Me Zn–SCSOMe 10
The reaction of 8 with CS2 as a substitute for CO2 resulted in a more stable product, the methyldithiocarbonate complex 10, also in analogy to previously observed reactions [12]. 10 yielded crystals suitable for a structure determination, the result of which is shown in Fig. 4. Just like in complex 5 there is a slight tendency of the zinc ion to become five-coordinate, this time evident from the rather short Zn O distance involving the methoxy group. All molecular details of 10 are very similar to those of the complex TpPh,MeZn–SCSOMe [13]. 2.5. Cleavage of esters There are many reports in the literature on the cleavage of esters by hydrolytically active zinc complexes as models of esterases and phosphatases [2], including our own work [1]. It could therefore be expected that complex 8 would react with the preferred substrate for such studies, tris(pnitrophenyl)phosphate, PO(ONit)3, according to Eq. (1). So it did.
˚ ): Fig. 3. Structure of the bis-ligand complex 7. Zn–N bond lengths (A 2.116(5), 2.126(5), 2.223(5), 2.226(5), 2.248(6), 2.256(6).
2TpFu;Me Zn–OH þ POðONitÞ3 8 ! TpFu;Me Zn–OPOðONitÞ2 þ TpFu;Me Zn–ONit þ H2 O 12 11 ð1Þ
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As observed before [14], the product mixture of reaction (1) was difficult to separate, but 31P NMR spectroscopy indicated complete conversion. To confirm this, the two reaction products were prepared by the standard acid–base reaction between 8 and bis(p-nitrophenyl)phosphoric acid and p-nitrophenol, respectively. This way both 11 and 12 were obtained as yellow crystalline materials. As a rule, the TpZn–OH complexes are not reactive enough to cleave simple aliphatic esters [15]. Therefore we chose lactones as activated esters for cleavage reactions with 8. We observed that b-butyrolactone was consumed upon reaction with 8, but the reaction products remain unidentified as yet. In contrast, a clean cleavage occurred with c-thiobutyrolactone in dichloromethane/methanol, but the reaction product was not the expected TpFu,MeZn-carboxylate complex, but rather the thiolate 13 in which the carboxylate terminus of the substrate has been esterified. This esterification resembles that of the heterocumulene substrates in 9 and 10 and points to a catalytic function of the TpZn–OH complexes. TpFu;Me Zn–SðCH2 Þ3 COOMe 13 The constitution of 13 was confirmed by a structure determination, see Fig. 5. The bonding situation in 13 is completely analogous to that in the many TpZn–thiolate complexes. It is noteworthy that the carboxylate terminus
˚ ): Zn–N1 Fig. 5. Structure of the thiolate complex 13. Bond lengths (A 2.084(2), Zn–N2 2.070(2), Zn–N3 2.075(2), Zn–S 2.220(1).
of the thiolate ligand does not interact with the zinc ion, as is also not the case in the most closely related complex TpPh,MeZn–S(CH2)2SCSOMe [16]. 2.6. Attempts at the cleavage of amides Several years ago we had reported that the ‘‘enzyme model’’ TpCum,MeZn–OH (Cum = p-isopropylphenyl) effects the hydrolytic cleavage of trifluoroacetamide to yield ammonia and the acetate complex TpCum,MeZn– OCOCF3 [17]. Later, however, we observed that the preferred reaction is the deprotonation of the amide, for instance with TpPh,MeZn–OH yielding the amidate complex TpPh,MeZn–NHCOCF3 [18,19]. In order to clarify this situation, we used complex 8 as a hydrolytic agent for trifluoroacetamide. Our observation was that 8 does not cleave the amide, but instead deprotonates it, producing the amidate complex 14. TpFu;Me Zn–NHCOCF3 14 As the data in the experimental section show, it is virtually impossible to distinguish 14 from the trifluoroacetate complex 6 by IR or 1H NMR spectroscopy. The only significant difference lies in the amide band of 14 (1690 cm1) versus the carboxylate band of 6 (1715 cm1), in both cases in analogy with authentic reference complexes [18,19]. The constitution of 14, i.e. the attachment of NH rather than O to zinc, could then be confirmed by a structure determination, see Fig. 6. Although crystallography does not allow to
Fig. 6. Structure of the amidate complex 14 (one of two molecules in the ˚ ): Zn–N1 2.035 and 2.071(5), Zn–N2 asymmetric unit). Bond lengths (A 2.021 and 2.029(5), Zn–N3 2.011 and 2.030(5), Zn–N(amidate) 1.876 and 1.894(5), N(amidate) O(furyl) 3.055 and 3.112(8).
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distinguish NH from O, the location of the NH group is evident from the orientation of one of the furyl substituents, generating a short N O contact which should result from a N–H O hydrogen bond. Unlike the acetate ligand in complex 5 the amidate ligand in complex 14 is clearly monodentate, providing another distinguishing feature for these two complex types. 2.7. Conclusions The new ligand TpFu,Me has allowed the investigation of a new zinc enzyme model TpFu,MeZn–OH. Compared to our established models, TpPh,MeZn–OH and TpCum,MeZn–OH [1], it is more labile, but also somewhat more reactive. It can be subjected to the established model reactions, as there are heterocumulene insertion and ester cleavage. With respect to the controversial question of amide cleavage it has clarified the case of trifluoroacetamide, showing that this substrate is not cleaved but rather deprotonated. Compared to TpPh,MeZn–OH and TpCum,MeZn–OH, the new TpFu,MeZn–OH has a stronger tendency for dismutation into Zn(OH)2 and Zn(TpFu,Me)2. This corresponds to our previous observation that pyrazolylborates with more polar substituents are no longer ‘‘tetrahedral enforcers’’, allowing coordination numbers 5 and 6 [4]. In accord with this, the acetate ligand in complex 5 is characteristically bidentate, and the TpFu,MeZn unit readily picks up bidentate enzyme inhibitors [20]. In a general sense, the zinc complexes of the furyl-substituted Tp ligand resemble those of the phenyl-substituted one, both in structure and reactivity. This relates to the geometrical and electronic similarity of benzene and furan. Hence the increase of polarity of the Tp ligand due to the furyl substituents is limited, which is also evident from the fact that the new ligand has not increased the water-solubility of its zinc complexes. We are therefore engaged in the synthesis of further and more polar pyrazolylborate systems. 3. Experimental The general working and measuring procedures were as described [21]. All reagents were obtained commercially. The starting material acetyl-2-furyl-methane was prepared according to the literature procedure [22] from acetone, 2-furylethylester and sodium hydride. Its reported reaction with hydrazine hydrate [23] yielded 3furyl-5-methylpyrazole.
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ing to room temperature, the glassy residue was carefully powdered and treated for 30 min with 100 ml of chloroform in an ultrasound bath. After filtration, the residue was dried in vacuo and then heated to reflux in 200 ml of acetonitrile. The mixture was filtered hot and the filtrate kept at 5 °C for crystallization. 3.94 g (64%) of KTp2-Fu,Me was obtained as colourless crystals, m.p. 196 °C. IR(KBr): 2494 m (BH). 1 H NMR (CDCl3): d (ppm) = 1.94 [s, 3H, CH3CN], 2.38 [s, 9H, Me(pz)], 6.05 [s, 3H, H(pz)], 6.35 [m, 6H, Fu3 and Fu4], 7.26 [d, J = 6.2 Hz, 3H, Fu5]. Anal. Calc. for C24H22BKN6O3 Æ 2CH3CN (492.39 + 82.10): C, 58.54; H, 4.91; N, 19.50. Found: C, 58.11; H, 5.14; N, 19.35%. 3.2. Simple zinc complexes 3.2.1. Complex 1 A solution of 27 mg (0.20 mmol) of ZnCl2 in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 97 mg (88%) of 1 as colourless crystals, m.p. 180 °C (dec.). IR(KBr): 2545 m (BH). 1 H NMR (CDCl3): d (ppm) = 2.49 [s, 9H, Me(pz)], 6.32 [s, 3H, H(pz)], 6.46 [dd, J = 3.5 and 1.8 Hz, 3H, Fu4], 7.44 [d, J = 1.5 Hz, 3H, Fu5], 7.48 [d, J = 3.5 Hz, 3H, Fu3]. Anal. Calc. for C24H22BClN6O3Zn (554.13): C, 52.02; H, 4.00; N, 15.17. Found: C, 51.98; H, 4.45; N, 14.73%. 3.2.2. Complex 2 A solution of 45 mg (0.20 mmol) of ZnBr2 in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 95 mg (81%) of 2 as colourless crystals, m.p. 205 °C. IR(KBr): 2546 m (BH). 1 H NMR (CDCl3): d (ppm) = 2.48 [s, 9H, Me(pz)], 5.22 [s, 1H, CH2Cl2], 6.29 [s, 3H, H(pz)], 6.42 [dd, J = 3.6 and 1.8 Hz, 3H, Fu4], 7.35 [d, J = 3.2 Hz, 3H, Fu3], 7.37 [d, J = 1.4 Hz, 3H, Fu5]. Anal. Calc. for C24H22BBrN6O3Zn Æ 0.5CH2Cl2 (588.48 + 42.47): C, 45.90; H, 3.62; N, 13.11. Found: C, 45.91; H, 3.65; N, 13.02%.
3.1. Synthesis of the ligand A mixture of 7.41 g (50.0 mmol) of 3-(furan-2-yl)-5methyl-pyrazole and 0.675 g (12.5 mmol) of KBH4 was slowly heated to 200 °C, the temperature being controlled by an immersed thermometer. The resulting melt was kept at 200 °C until it started to turn yellow (ca. 3 h). After cool-
3.2.3. Complex 3 A solution of 64 mg (0.20 mmol) of ZnI2 in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness.
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Recrystallization from dichloromethane/methanol (1:1) yielded 111 mg (86%) of 3 as colourless crystals, m.p. 240 °C. IR(KBr): 2526 m (BH). 1 H NMR (CDCl3): d (ppm) = 2.49 [s, 9H, Me(pz)], 5.29 [s, 1H, CH2Cl2], 6.25 [s, 3H, H(pz)], 6.43 [dd, J = 3.4 and 1.8 Hz, 3H, Fu4], 7.10 [d, J = 3.4 Hz, 3H, Fu3], 7.47 [d, J = 1.6 Hz, 3H, Fu5]. Anal. Calc. for C24H22BIN6O3Zn Æ 0.5CH2Cl2 (645.58 + 42.47): C, 42.77; H, 3.37; N, 12.21. Found: C, 42.85; H, 3.36; N, 12.23%. 3.2.4. Complex 4 A solution of 74 mg (0.20 mmol) of Zn(ClO4)2 Æ 6H2O in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 15 min at 0 °C. To this solution, 19 mg (0.20 mmol) of KSCN dissolved in 15 ml of methanol was added dropwise with stirring for a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 101 mg (85%) of 4 as colourless crystals m.p. 256 °C (dec.). IR(KBr): 2544 m (BH), 2093 vs (SCN). 1 H NMR (CDCl3): d (ppm) = 2.45 [s, 9H, Me(pz)], 6.23 [s, 3H, H(pz)], 6.42 [dd, J = 3.4 and 1.8 Hz, 3H, Fu4], 6.87 [d, J = 3.3. Hz, 3H, Fu3], 7.67 [d, J = 1.1 Hz, 3H, Fu 5]. Anal. Calc. for C25H22BN7O3SZn (576.76): C, 52.06; H, 3.84; N, 17.00; S, 5.56. Found: C, 51.75; H, 3.94; N, 16.84; S, 5.54%.
tallization from dichloromethane/methanol (1:1) yielded 85 mg (67%) of 6 as a white powder, m.p. 196 °C. IR(KBr): 2557 m (BH), 1715 vs (CO). 1 H NMR (CDCl3): d (ppm) = 2.48 [s, 9H, Me(pz)], 6.24 [s, 3H, H(pz)], 6.44 [m, 3H, Fu4], 6.84 [d, J = 3.2 Hz, 3H, Fu3], 7.43 [d, J = 1.8. Hz, 3H, Fu5]. 19F NMR (CDCL3): d (ppm) = 75.25. Anal. Calc. for C26H22BF3N6O5Zn (631.69): C, 49.44; H, 3.51; N, 13.30. Found: C, 49.16; H, 3.50; N, 13.26%. 3.2.7. Complex 7 A solution of 74 mg (0.20 mmol) of Zn(ClO4)2 Æ 6H2O in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 15 min at room temperature. To the solution, 17 mg (0.30 mmol) of KOH dissolved in 25 ml of methanol was added dropwise with stirring over a period of 2 h at room temperature. Then the solution was filtered and the filtrate was evaporated to dryness. The residue was recrystallized from dichloromethane/methanol (1:1). 88 mg (91%) of 7 was obtained as colourless crystals, m.p. 320 °C (dec.). IR(KBr): 2552 m (BH). 1 H NMR (CDCl3): d (ppm) = 2.55 [s, 18H, Me(pz)], 4.92 [d, J = 2.8 Hz, 6H, Fu3], 5.72 [dd, J = 3.4 and 1.6 Hz, 6H, Fu4], 5.92 [s, 6H, H(pz)], 6.79 [d, J = 1.6 Hz, 6H, Fu5]. Anal. Calc. for C48H44B2N12O6Zn (971.96): C, 57.32; H, 4.56; N, 17.29. Found: C, 57.03; H, 4.47; N, 16.41%.
3.2.5. Complex 5 A solution of 44 mg (0.20 mmol) of Zn(OAc)2 Æ 2H2O in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. The residue was recrystallized from dichloromethane/methanol (1:1). 106 mg (92%) of 5 was obtained as colourless crystals, m.p. 230 °C. IR(KBr): 2541 m (BH), 1627s (CO). 1 H NMR (CDCl3): d (ppm) = 2.00 [s, 3H, CH3], 2.47 [s, 9H, Me(pz)], 6.23 [s, 3H, H(pz)], 6.42 [dd, J = 3.4 and 1.8 Hz, 3H, Fu4], 7.00 [d, J = 1.8 Hz, 3H, Fu5], 7.40 [d, J = 3.4 Hz, 3H, Fu3]. Anal. Calc. for C26H25BN6O5Zn (577.72): C, 54.05; H, 4.36; N, 14.55. Found: C, 53.70; H, 4.51; N, 14.34%.
3.2.8. Complex 8 A solution of 74 mg (0.20 mmol) of Zn(ClO4)2 Æ 6H2O in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 15 min at 0 °C. To this solution, 17 mg (0.30 mmol) of KOH dissolved in 25 ml of methanol was added dropwise with stirring over a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. The residue was recrystallized from dichloromethane/methanol (1:1). 68 mg (64%) of 8 was obtained as a white powder, m.p. 220 °C (dec.). IR(KBr): 3412 w (OH), 2540 m (BH). 1 H NMR (CDCl3): d (ppm) = 2.47 [s, 9H, Me(pz)], 6.27 [s, 3H, H(pz)], 6.47 [dd, J = 3.4 and 1.8 Hz, 3H, Fu4], 7.42 [d, J = 1.8 Hz, 3H, Fu5], 7.67 [d, J = 3.4 Hz, 3H, Fu3]. Anal. Calc. for C24H23BN6O4Zn (535.68): C, 53.81; H, 4.33; N, 15.69. Found: C, 53.72; H, 4.15; N, 15.61%.
3.2.6. Complex 6 A solution of 74 mg (0.20 mmol) of Zn(ClO4)2 Æ 6H2O in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 15 min and then cooled to 0 °C. To this solution, 30 mg (0.20 mmol) of KOC(O)CF3 dissolved in 15 ml of methanol was added dropwise with stirring for a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrys-
3.2.9. Complex 9 100 mg (0.19 mmol) of 8 was dissolved in 50 ml of methanol/dichloromethane and maintained at 0 °C. A slow stream of CO2 was passed through the solution until all solvent was evaporated. The residue was taken up in 15 ml of benzene, filtered and a slow stream of CO2 was passed again through the solution until the volume was reduced to one-third. At 5 °C, 106 mg (51%) of 9 precipitated as white powder, m.p. 244 °C.
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1
IR(KBr): 2544 m (BH), 1707 m (CO). 1 H NMR (CDCl3): d (ppm) = 2.41 [s, 9H, Me(pz)], 2.93 [s, 3H, OCH3], 6.19 [s, 3H, H(pz)], 6.39 [m, 3H, Fu4], 6.92 [d, J = 3.0 Hz, 3H, Fu3], 7.41 [m, 3H, Fu5]. Anal. Calc. for C26H25BN6O6Zn (593.72): C, 52.60; H, 4.24; N, 14.14; Zn, 11.01. Found: C, 53.61; H, 4.39; N, 14.18; Zn, 10.45%.
H NMR (CDCl3): d (ppm) = 2.51 [s, 9H, Me(pz)], 6.25 [dd, J = 3.7 and 2.6 Hz, 3H, Fu4], 6.28 [s, 3H, H(pz)], 6.44 [d, J = 9.2 Hz, 2H, C6H4], 7.03 [m, 6H, Fu3, 5], 7.83 [d, J = 9.2 Hz, 2H, C6H4]. Anal. Calc. for C30H26BN7O6Zn (672.78): C, 54.86; H, 3.99; N, 14.93; Zn, 9.96. Found: C, 53.91; H, 4.14; N, 14.64; Zn, 9.21%.
3.2.10. Complex 10 100 mg (0.19 mmol) of 8 was dissolved in 50 ml of methanol/dichloromethane. After addition of 15 ll (19 mg, 0.25 mmol) of CS2, the solution was stirred for 2 h at 0 °C. Then all volatiles were removed in vacuo and the residue was taken up in dichloromethane, filtered and the filtrate was evaporated to dryness in vacuo. Recrystallization from dichloromethane/methanol (1:1) yielded 93 mg (78%) of 10 as colourless crystals, m.p. 268 °C (dec.). IR(KBr): 2534 m (BH). 1 H NMR (CDCl3): d (ppm) = 2.50 [s, 9H, Me(pz)], 3.22 [s, 3H, OCH3], 6.24 [s, 3H, H(pz)], 6.40 [dd, J = 3.3 and 1.8 Hz, 3H, Fu4], 6.93 [d, J = 3.3 Hz, 3H, Fu3], 7.46 [d, J = 1.2 Hz, 3H, Fu5]. Anal. Calc. for C26H25BN6O4S2Zn (625.85): C, 49.90; H, 4.03; N, 13.43; S, 10.25. Found: C, 49.36; H, 4.09; N, 13.25; S, 10.12%.
3.2.13. Complex 13 100 mg (0.19 mmol) of 8 was dissolved in 50 ml of methanol/dichloromethane/water (5:4:1). After addition of 19 mg (0.19 mmol) of c-thiobutyrolactone dissolved in 10 ml of dichloromethane, the solution was stirred for 3 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 94 mg (74%) of 13 as colourless crystals, m.p. 170 °C (dec.). IR(KBr): 2547 m (BH), 1739 vs (CO). 1 H NMR (CDCl3): d (ppm) = 1.70 [m, 2H, CH2], 2.08 [t, J = 7.6 Hz, 2H, CH2], 2.22 [t, J = 7.4 Hz, 2H, CH2], 2.48 [s, 9H, Me(pz)], 3.68 [s, 3H, OMe], 6.28 [s, 3H, H(pz)], 6.46 [dd, J = 3.2 and 1.8 Hz, 3H, Fu4], 7.47 [m, 6H, Fu3, 5]. Anal. Calc. for C29H31BN6O5SZn Æ 0.5CH2Cl2 (651.87): C, 53.43; H, 4.79; N, 12.89; S, 4.92. Found: C, 50.18; H, 4.51; N, 12.03; S, 4.62%.
3.2.11. Complex 11 100 mg (0.19 mmol) of 8 was dissolved in 50 ml of methanol/dichloromethane (1:1). After addition of 65 mg (0.19 mmol) of HOP(O)(p-nitrophenolate)2 dissolved in 10 ml of dichloromethane, the solution was stirred for 3 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 63 mg (38%) of 11 as a slight yellow powder, m.p. 228 °C. IR(KBr): 2557 m (BH), 1345 vs (NO), 1180 s (PO). 1 H NMR (CDCl3): d (ppm) = 2.51 [s, 9H, Me(pz)], 5.30 [s, 2H, CH2Cl2], 6.28 [s, 3H, H(pz)], 6.48 [dd, J = 3.4 and 1.8 Hz, 3H, Fu4], 6.79 [d, J = 3.2 Hz, 3H, Fu4], 7.24 [d, J = 8.6 Hz, 4H, C6H4], 7.54 [d, J = 1.0 Hz, 3H, Fu5], 8.07 [d, J = 8.6 Hz, 4H, C6H4]. 31 P NMR(CDCl3): d (ppm) = 13.7. Anal. Calc. for C36H30BN8O11PZn Æ CH2Cl2 (857.86 + 84.93): C, 47.14; H, 3.42; N, 11.89. Found: C, 47.154; H, 3.85; N, 11.62%. 3.2.12. Complex 12 100 mg (0.19 mmol) of 8 was dissolved in 50 ml of methanol/dichloromethane (1:1). After addition of 26 mg (0.19 mmol) of p-nitrophenol dissolved in 10 ml of dichloromethane, the solution was stirred for 3 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 83 mg (65%) of 12 as yellow crystals, m.p. 210 °C. IR(KBr): 2557 m (BH), 1308 vs (NO).
3.2.14. Complex 14 100 mg (0.19 mmol) of 8 was dissolved in 50 ml of methanol/dichloromethane (1:1). After addition of 21 mg (0.19 mmol) of trifluoroacetamide dissolved in 10 ml of methanol/water (3:1), the solution was stirred for 3 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 51 mg (43%) of 14 as colourless crystals, m.p. 240 °C. Drying in vacuo removed the cocrystallized solvents and converted the crystals to a powder. IR(KBr): 3433 m (NH), 2554 m (BH), 1690 vs (amide). 1 H NMR (CDCl3): d (ppm) = 2.48 [s, 9H, Me(pz)], 6.22 [s, 3H, H(pz)], 6.44 [m, 3H, Fu4], 6.74 [d, J = 3.3 Hz, 3H, Fu3], 7.44 [d, J = 0.5 Hz, 3H, Fu5]. Anal. Calc. for C26H23BF3N7O4Zn (630.71): C, 49.51; H, 3.68; N, 15.55. Found: C, 48.38; H, 3.61; N, 15.02%. 3.2.15. Structure determinations Crystals were obtained as described in the experimental section. Data sets were obtained on a Bruker Smart CCD diffractometer at 30 °C and subjected to an empirical absorption correction (program SADABS). The structures were solved with direct methods and refined anisotropically [24]. Hydrogen atoms were included with fixed distances and isotropic temperature factors 1.5 times those of their attached atoms. Parameters were refined against F2. In case of 14, the cocrystallized solvent molecules were treated as a diffuse contribution using the program SQUEEZE [25]. Drawings were produced with SCHAKAL [26]. Crystallographic data are listed in Table 1.
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Table 1 Crystallographic details 4
5
7
10
13
14
Formula
C25H22BN7O3SZn
C26H25BN6O5Zn
C48H44B2N12O6Zn
C26H25BN6O4S2Zn
C29H31BN6O5SZn
MW Space group Z ˚) a (A ˚) b (A ˚) c (A a (°) b (°) c (°) ˚ 3) V (A
576.76 P21/c 4 14.316(2) 8.436(1) 22.175(3) 90 101.496(3) 90 2624.3(7) 1.46 1.06 0.059 0.158
577.72 P 1 2 8.61(1) 10.73(1) 15.21(1) 76.78(3) 86.75(3) 74.49(3) 1318(4) 1.46 0.98 0.055 0.156
971.96 P212121 4 13.20(3) 13.51(3) 26.43(7) 90 90 90 4714(21) 1.37 0.58 0.053 0.136
625.85 P21/c 4 11.475(3) 20.174(5) 12.763(3) 90 105.889(5) 90 2841.7(12) 1.46 1.06 0.040 0.118
651.87 P 1 2 10.660(2) 12.227(2) 13.697(2) 68.156(2) 84.962(3) 65.185(2) 1499.2(4) 1.44 0.94 0.043 0.130
C26H23BF3N7O4Zn Æ 0.5H2O Æ CH3OH 671.76 P 1 4 14.146(3) 15.009(3) 15.482(3) 105.695(4) 101.480(4) 100.599(4) 3001.1(11) 1.40 0.88 0.070 0.199
d (calc) (g cm3) l(Mo Ka) (mm1) R1 (observed reflections) wR2 (all reflections)
4. Supplementary material The crystallographic data of the structures described in this paper were deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-601978 (for 4), CCDC-601979 (for 5), CCDC601980 (for 7), CCDC-601981 (for 10), CCDC-601982 (for 13) and CCDC-601983 (for 14). Copies of these data are available free of charge from the following address: The Director, CCDC, 12 Union Road, GB-Cambridge CB2 1EZ (fax: +44 12 23/3 36 0 33; e-mail: [email protected]). Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. We are indebted to Drs. H. Brombacher and C. Pe´rez Olmo for help with the structure determinations. References [1] [2] [3] [4] [5]
H. Vahrenkamp, Acc. Chem. Res. 32 (1999) 589. G. Parkin, Chem. Rev. 104 (2004) 699. K. Weis, H. Vahrenkamp, Inorg. Chem. 36 (1997) 5589. K. Weis, H. Vahrenkamp, Inorg. Chem. 36 (1997) 5592. B.S. Hammes, M.W. Carrano, C. Carrano, J. Chem. Soc. Dalton Trans. (2001) 1448.
[6] B.S. Hammes, X.M. Luo, M.W. Carrano, C. Carrano, Inorg. Chim. Acta 341 (2002) 33. [7] A.L. Rheingold, C.D. Incarvito, S. Trofimenko, J. Chem. Soc. Dalton Trans. (2000) 1233. [8] P.L. Jones, A.J. Amoroso, J.C. Jefferey, J.A. McCleverty, E. Psillakis, L. Rees, M.D. Ward, Inorg. Chem. 36 (1997) 10. [9] A. Looney, R. Han, I.B. Gorell, M. Cornebise, K. Yoon, G. Parkin, Organometallics 14 (1995) 274. [10] M. Osawa, M. Tanaka, K. Fujisawa, N. Kitajima, Y. Moro-Oka, Chem. Lett. (1996) 397. [11] R. Alsfasser, M. Ruf, S. Trofimenko, H. Vahrenkamp, Chem. Ber. 126 (1993) 703. [12] M. Ruf, H. Vahrenkamp, Inorg. Chem. 35 (1996) 6571. [13] M. Rombach, H. Brombacher, H. Vahrenkamp, Eur. J. Inorg. Chem. (2002) 153. [14] J.A. Maldonado Calvo, H. Vahrenkamp, Inorg. Chim. Acta 358 (2005) 4019. [15] M. Rombach, C. Maurer, K. Weis, E. Keller, H. Vahrenkamp, Chem. Eur. J. 5 (1999) 1013. [16] H. Brombacher, H. Vahrenkamp, Inorg. Chem. 43 (2004) 6050. [17] M. Ruf, K. Weis, H. Vahrenkamp, J. Chem. Soc. Chem. Commun. (1994) 135. [18] F. Groß, H. Vahrenkamp, unpublished. [19] H. Brombacher, H. Vahrenkamp, Inorg. Chem. 43 (2004) 6054. [20] J.A. Maldonado Calvo, H. Vahrenkamp, unpublished. [21] M. Fo¨rster, R. Burth, A.K. Powell, T. Eiche, H. Vahrenkamp, Chem. Ber. 126 (1993) 2643. [22] S.R. Harris, R. Levine, J. Am. Chem. Soc. 71 (1949) 1120. [23] I.I. Grandberg, J. Gen. Chem. USSR 30 (1960) 2896. [24] G.M. Sheldrick, Bruker AXS, SHELXTL Program Package, Version 5.1, Universita¨t Go¨ttingen, 1998. [25] A.L. Spek, J. Appl. Crystallogr. 36 (2003) 7. [26] E. Keller, SCHAKAL for Windows, Universita¨t Freiburg, 2001.
A new tris(2-furyl) substituted pyrazolylborate ligand and its zinc complex chemistry Jose´ A. Maldonado Calvo, Heinrich Vahrenkamp
*
Institut fu¨r Anorganische und Analytische Chemie der Universita¨t Freiburg, Albertstr. 21, D-79104 Freiburg, Germany Received 22 March 2006; accepted 10 April 2006 Available online 22 April 2006
Abstract The new ligand hydrotris(3-(2 0 -furyl)-5-methylpyrazolyl)borate (TpFu,Me) was prepared by the usual procedure. With zinc salts, it forms the TpFu,MeZn–X complexes (X = Cl, Br, I, NCS, CH3COO, CF3COO). With zinc perchlorate, the bis-ligand complex Zn(TpFu,Me)2 is formed preferrably, but by carefully controlling the reaction conditions, the ‘‘enzyme model’’ TpFu,MeZn–OH could be obtained. The latter models carbonic anhydrase by inserting CO2 and CS2 in methanol producing TpFu,MeZn–OCOOMe and TpFu,MeZn–SCSOMe. It models hydrolases by the hydrolytic cleavage of tris(p-nitrophenyl)phosphate and c-thiobutyrolactone. It does not hydrolyse trifluoroacetamide, but instead deprotonates it, yielding TpFu,MeZn–NHCOCF3. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Zinc complexes; Pyrazolylborate ligands; Hydroxide complexes; Biomimetic reactions
1. Introduction The tris(pyrazolyl)borate ligands belong to the most suitable ligands for a biomimetic zinc complex chemistry [1,2]. They owe this advantage to the accessibility of such species, in which various substituents at the 3-positions of the pyrazole rings provide a favourable encapsulation of the functional Zn–X units. In most cases, the particular 3-substituents have been hydrocarbon species, creating a hydrophobic cavity around the Zn–X centers. In order to further approximate the biological environment of zinc, it would be desirable to employ such pyrazolylborates with polar substituents at the 3-positions. This should improve the modeling of hydrolytic zinc enzymes, it should allow to study the influence of hydrogen bonding interactions in the vicinity of the metal, and it should lead the way to a biomimetic chem-
*
Corresponding author. Tel.: +49 761 203 6120; fax: +49 761 203 6001. E-mail address: [email protected] (H. Vahrenkamp).
0020-1693/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.04.016
istry with pyrazolylborate–zinc complexes in aqueous media. There has been a limited number of reports in the literature on the zinc complex chemistry of pyrazolylborate ligands with polar 3-substituents. We described those with 3-pyridyl groups [3,4], Carrano reported on those with 3-carboxyester groups [5,6]. Promising examples which have not yet been exploited for zinc complexes are Trofimenko’s pyrrolidinocarbonyl- [7] and Ward’s 2-pyridyl-substituted ligands [8]. It seems to be a common feature of these polar pyrazolylborates that they favour higher coordination numbers than 4 for the zinc ion. The present paper reports a new member of this group of polar pyrazolylborate ligands, the 2-furyl-substituted TpFu,Me. The furyl substituent was chosen because of the easy accessibility of the furyl-substituted pyrazole. The main motivation for the study of its zinc complexes was the hope that the more polar environment of zinc would improve the efficiency of the complexes as models for hydrolytic zinc enzymes.
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H B
N
N
N
N
N
N
O
O
O
[ TpFu,Me ]-
2. Results and discussion 2.1. Ligand synthesis The synthesis of the furyl-substituted pyrazole precursor from acetyl-2-furyl-methane and hydrazine hydrate was straightforward. Its reaction with KBH4 in a melt at 200 °C produced the potassium salt KTpFu,Me in good yield, which was purified by recrystallization from acetonitrile. The ligand as a potassium salt is soluble in polar organic solvents, but not in water. Its characteristic spectral features are the m(BH) band in the IR at 2494 cm1 and the pyrazole-H resonance in the 1H NMR in CDCl3 at 6.04 ppm. The furyl group’s proton resonances appear as multiplets.
˚ ): Zn– Fig. 1. Structure of the thiocyanate complex 4. Bond distances (A N1 2.036(3), Zn–N2 2.022(4), Zn–N3 2.011(3), Zn–N(thiocyanate) 1.892(4). Bond angle: Zn–N–C 155.8(1)°.
2.2. Simple zinc complexes The basic zinc complex chemistry of TpFu,Me was probed by reacting the potassium salt of the ligand with zinc salts. This way the halide complexes 1–3, the thiocyanate complex 4 and the two acetate complexes 5 and 6 were obtained. They are all colorless and crystalline, easily identified by IR and 1H NMR spectroscopy. X= Nr.:
Cl 1
Br 2
TpFu,MeZn–X I NCS OCOCH3 3 4 5
OCOCF3 6
For comparative purposes, the crystal structures of 4 and 5 were determined. Fig. 1 shows that the thiocyanate in 4 is bound to zinc via the nitrogen atom, as usual [9], and slightly bent at the Zn–N–C array. The acetate ligand in 5 is attached in a semibidentate fashion, just halfway between the extremes of a purely monodentate [9] and almost perfect bidentate fashion [10]. Compared to the multitude of pyrazolylborate–Zn–X complexes, the Zn–N bond lengths in 4 and 5 are in the normal range, and the orientation of the furyl rings is comparable to that of the phenyl rings in the TpPh,MeZn complexes. In both cases, two of the furyl oxygen atoms point to the center of the complexes, while the third one points to the outside (see Figs. 1 and 2).
˚ ): Zn–N1 Fig. 2. Structure of the acetate complex 5. Bond lengths (A 2.038(5), Zn–N2 2.044(5), Zn–N3 2.091(5), Zn–O 1.923(3), Zn O 2.534(3).
2.3. The zinc-hydroxide complex The essential reagents in biomimetic pyrazolylborate– zinc complex chemistry are the Zn–OH species, the proper enzyme models [1]. As a rule they form easily from the Tp
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ligand and a zinc salt with non-coordinating anions, e.g. Zn(ClO4)2, in the presence of KOH. This proved difficult here, and initial attempts resulted only in the formation of the bis-ligand complex 7, which is presumed to result from dismutation of the desired 8 into 7 and Zn(OH)2. Lowering the reaction temperature and carefully controlling the reaction conditions finally allowed the isolation of the hydroxide complex 8. ZnðTpFu;Me Þ2 7
TpFu;Me Zn–OH 8
Complex 8 was too labile to grow single crystals for a structure determination, producing 7 again. 7 was easy to crystallize, yielding the molecular structure displayed in Fig. 3. In 7 the coordination of the zinc ion is octahedral to a very good approximation. Surprisingly, there is no steric hindrance between the furyl substituents of the two opposing TpFu,Me ligands, due to the coplanarity of the pyrazole and furan rings and the resulting perfect interlocking of the two molecular halves. Complex 7 shares this property with the ZnTp2 complex in which the Tp ligands bear 3-pyridyl substituents [4]. 2.4. Reactions with CO2 and CS2 In our previous studies we could demonstrate that the TpZn–OH complexes are suitable models of carbonic anhydrase, in that they insert CO2 to form a labile bicarbonate complex which in the presence of methanol is converted to the stable methylcarbonate complex [1,11,12]. This was also observed here. Bubbling CO2 into a solution of 8 in methanol/dichloromethane produced the methylcarbonate complex 9, which persists in a CO2 atmosphere. 9 is
Fig. 4. Structure of the methyldithiocarbonate complex 10. Bond lengths ˚ ): Zn–N1 2.081(2), Zn–N2 2.063(2), Zn–N3 2.057(2), Zn–S 2.268(1), (A Zn O 2.683(2), C–S 1.724(3), C@S 1.655(3).
easily recognized by its typical 1H NMR spectrum, but attempts to recrystallize it led to partical re-conversion to 8 with subsequent decomposition. TpFu;Me Zn–OCOOMe 9
TpFu;Me Zn–SCSOMe 10
The reaction of 8 with CS2 as a substitute for CO2 resulted in a more stable product, the methyldithiocarbonate complex 10, also in analogy to previously observed reactions [12]. 10 yielded crystals suitable for a structure determination, the result of which is shown in Fig. 4. Just like in complex 5 there is a slight tendency of the zinc ion to become five-coordinate, this time evident from the rather short Zn O distance involving the methoxy group. All molecular details of 10 are very similar to those of the complex TpPh,MeZn–SCSOMe [13]. 2.5. Cleavage of esters There are many reports in the literature on the cleavage of esters by hydrolytically active zinc complexes as models of esterases and phosphatases [2], including our own work [1]. It could therefore be expected that complex 8 would react with the preferred substrate for such studies, tris(pnitrophenyl)phosphate, PO(ONit)3, according to Eq. (1). So it did.
˚ ): Fig. 3. Structure of the bis-ligand complex 7. Zn–N bond lengths (A 2.116(5), 2.126(5), 2.223(5), 2.226(5), 2.248(6), 2.256(6).
2TpFu;Me Zn–OH þ POðONitÞ3 8 ! TpFu;Me Zn–OPOðONitÞ2 þ TpFu;Me Zn–ONit þ H2 O 12 11 ð1Þ
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As observed before [14], the product mixture of reaction (1) was difficult to separate, but 31P NMR spectroscopy indicated complete conversion. To confirm this, the two reaction products were prepared by the standard acid–base reaction between 8 and bis(p-nitrophenyl)phosphoric acid and p-nitrophenol, respectively. This way both 11 and 12 were obtained as yellow crystalline materials. As a rule, the TpZn–OH complexes are not reactive enough to cleave simple aliphatic esters [15]. Therefore we chose lactones as activated esters for cleavage reactions with 8. We observed that b-butyrolactone was consumed upon reaction with 8, but the reaction products remain unidentified as yet. In contrast, a clean cleavage occurred with c-thiobutyrolactone in dichloromethane/methanol, but the reaction product was not the expected TpFu,MeZn-carboxylate complex, but rather the thiolate 13 in which the carboxylate terminus of the substrate has been esterified. This esterification resembles that of the heterocumulene substrates in 9 and 10 and points to a catalytic function of the TpZn–OH complexes. TpFu;Me Zn–SðCH2 Þ3 COOMe 13 The constitution of 13 was confirmed by a structure determination, see Fig. 5. The bonding situation in 13 is completely analogous to that in the many TpZn–thiolate complexes. It is noteworthy that the carboxylate terminus
˚ ): Zn–N1 Fig. 5. Structure of the thiolate complex 13. Bond lengths (A 2.084(2), Zn–N2 2.070(2), Zn–N3 2.075(2), Zn–S 2.220(1).
of the thiolate ligand does not interact with the zinc ion, as is also not the case in the most closely related complex TpPh,MeZn–S(CH2)2SCSOMe [16]. 2.6. Attempts at the cleavage of amides Several years ago we had reported that the ‘‘enzyme model’’ TpCum,MeZn–OH (Cum = p-isopropylphenyl) effects the hydrolytic cleavage of trifluoroacetamide to yield ammonia and the acetate complex TpCum,MeZn– OCOCF3 [17]. Later, however, we observed that the preferred reaction is the deprotonation of the amide, for instance with TpPh,MeZn–OH yielding the amidate complex TpPh,MeZn–NHCOCF3 [18,19]. In order to clarify this situation, we used complex 8 as a hydrolytic agent for trifluoroacetamide. Our observation was that 8 does not cleave the amide, but instead deprotonates it, producing the amidate complex 14. TpFu;Me Zn–NHCOCF3 14 As the data in the experimental section show, it is virtually impossible to distinguish 14 from the trifluoroacetate complex 6 by IR or 1H NMR spectroscopy. The only significant difference lies in the amide band of 14 (1690 cm1) versus the carboxylate band of 6 (1715 cm1), in both cases in analogy with authentic reference complexes [18,19]. The constitution of 14, i.e. the attachment of NH rather than O to zinc, could then be confirmed by a structure determination, see Fig. 6. Although crystallography does not allow to
Fig. 6. Structure of the amidate complex 14 (one of two molecules in the ˚ ): Zn–N1 2.035 and 2.071(5), Zn–N2 asymmetric unit). Bond lengths (A 2.021 and 2.029(5), Zn–N3 2.011 and 2.030(5), Zn–N(amidate) 1.876 and 1.894(5), N(amidate) O(furyl) 3.055 and 3.112(8).
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distinguish NH from O, the location of the NH group is evident from the orientation of one of the furyl substituents, generating a short N O contact which should result from a N–H O hydrogen bond. Unlike the acetate ligand in complex 5 the amidate ligand in complex 14 is clearly monodentate, providing another distinguishing feature for these two complex types. 2.7. Conclusions The new ligand TpFu,Me has allowed the investigation of a new zinc enzyme model TpFu,MeZn–OH. Compared to our established models, TpPh,MeZn–OH and TpCum,MeZn–OH [1], it is more labile, but also somewhat more reactive. It can be subjected to the established model reactions, as there are heterocumulene insertion and ester cleavage. With respect to the controversial question of amide cleavage it has clarified the case of trifluoroacetamide, showing that this substrate is not cleaved but rather deprotonated. Compared to TpPh,MeZn–OH and TpCum,MeZn–OH, the new TpFu,MeZn–OH has a stronger tendency for dismutation into Zn(OH)2 and Zn(TpFu,Me)2. This corresponds to our previous observation that pyrazolylborates with more polar substituents are no longer ‘‘tetrahedral enforcers’’, allowing coordination numbers 5 and 6 [4]. In accord with this, the acetate ligand in complex 5 is characteristically bidentate, and the TpFu,MeZn unit readily picks up bidentate enzyme inhibitors [20]. In a general sense, the zinc complexes of the furyl-substituted Tp ligand resemble those of the phenyl-substituted one, both in structure and reactivity. This relates to the geometrical and electronic similarity of benzene and furan. Hence the increase of polarity of the Tp ligand due to the furyl substituents is limited, which is also evident from the fact that the new ligand has not increased the water-solubility of its zinc complexes. We are therefore engaged in the synthesis of further and more polar pyrazolylborate systems. 3. Experimental The general working and measuring procedures were as described [21]. All reagents were obtained commercially. The starting material acetyl-2-furyl-methane was prepared according to the literature procedure [22] from acetone, 2-furylethylester and sodium hydride. Its reported reaction with hydrazine hydrate [23] yielded 3furyl-5-methylpyrazole.
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ing to room temperature, the glassy residue was carefully powdered and treated for 30 min with 100 ml of chloroform in an ultrasound bath. After filtration, the residue was dried in vacuo and then heated to reflux in 200 ml of acetonitrile. The mixture was filtered hot and the filtrate kept at 5 °C for crystallization. 3.94 g (64%) of KTp2-Fu,Me was obtained as colourless crystals, m.p. 196 °C. IR(KBr): 2494 m (BH). 1 H NMR (CDCl3): d (ppm) = 1.94 [s, 3H, CH3CN], 2.38 [s, 9H, Me(pz)], 6.05 [s, 3H, H(pz)], 6.35 [m, 6H, Fu3 and Fu4], 7.26 [d, J = 6.2 Hz, 3H, Fu5]. Anal. Calc. for C24H22BKN6O3 Æ 2CH3CN (492.39 + 82.10): C, 58.54; H, 4.91; N, 19.50. Found: C, 58.11; H, 5.14; N, 19.35%. 3.2. Simple zinc complexes 3.2.1. Complex 1 A solution of 27 mg (0.20 mmol) of ZnCl2 in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 97 mg (88%) of 1 as colourless crystals, m.p. 180 °C (dec.). IR(KBr): 2545 m (BH). 1 H NMR (CDCl3): d (ppm) = 2.49 [s, 9H, Me(pz)], 6.32 [s, 3H, H(pz)], 6.46 [dd, J = 3.5 and 1.8 Hz, 3H, Fu4], 7.44 [d, J = 1.5 Hz, 3H, Fu5], 7.48 [d, J = 3.5 Hz, 3H, Fu3]. Anal. Calc. for C24H22BClN6O3Zn (554.13): C, 52.02; H, 4.00; N, 15.17. Found: C, 51.98; H, 4.45; N, 14.73%. 3.2.2. Complex 2 A solution of 45 mg (0.20 mmol) of ZnBr2 in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 95 mg (81%) of 2 as colourless crystals, m.p. 205 °C. IR(KBr): 2546 m (BH). 1 H NMR (CDCl3): d (ppm) = 2.48 [s, 9H, Me(pz)], 5.22 [s, 1H, CH2Cl2], 6.29 [s, 3H, H(pz)], 6.42 [dd, J = 3.6 and 1.8 Hz, 3H, Fu4], 7.35 [d, J = 3.2 Hz, 3H, Fu3], 7.37 [d, J = 1.4 Hz, 3H, Fu5]. Anal. Calc. for C24H22BBrN6O3Zn Æ 0.5CH2Cl2 (588.48 + 42.47): C, 45.90; H, 3.62; N, 13.11. Found: C, 45.91; H, 3.65; N, 13.02%.
3.1. Synthesis of the ligand A mixture of 7.41 g (50.0 mmol) of 3-(furan-2-yl)-5methyl-pyrazole and 0.675 g (12.5 mmol) of KBH4 was slowly heated to 200 °C, the temperature being controlled by an immersed thermometer. The resulting melt was kept at 200 °C until it started to turn yellow (ca. 3 h). After cool-
3.2.3. Complex 3 A solution of 64 mg (0.20 mmol) of ZnI2 in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness.
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Recrystallization from dichloromethane/methanol (1:1) yielded 111 mg (86%) of 3 as colourless crystals, m.p. 240 °C. IR(KBr): 2526 m (BH). 1 H NMR (CDCl3): d (ppm) = 2.49 [s, 9H, Me(pz)], 5.29 [s, 1H, CH2Cl2], 6.25 [s, 3H, H(pz)], 6.43 [dd, J = 3.4 and 1.8 Hz, 3H, Fu4], 7.10 [d, J = 3.4 Hz, 3H, Fu3], 7.47 [d, J = 1.6 Hz, 3H, Fu5]. Anal. Calc. for C24H22BIN6O3Zn Æ 0.5CH2Cl2 (645.58 + 42.47): C, 42.77; H, 3.37; N, 12.21. Found: C, 42.85; H, 3.36; N, 12.23%. 3.2.4. Complex 4 A solution of 74 mg (0.20 mmol) of Zn(ClO4)2 Æ 6H2O in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 15 min at 0 °C. To this solution, 19 mg (0.20 mmol) of KSCN dissolved in 15 ml of methanol was added dropwise with stirring for a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 101 mg (85%) of 4 as colourless crystals m.p. 256 °C (dec.). IR(KBr): 2544 m (BH), 2093 vs (SCN). 1 H NMR (CDCl3): d (ppm) = 2.45 [s, 9H, Me(pz)], 6.23 [s, 3H, H(pz)], 6.42 [dd, J = 3.4 and 1.8 Hz, 3H, Fu4], 6.87 [d, J = 3.3. Hz, 3H, Fu3], 7.67 [d, J = 1.1 Hz, 3H, Fu 5]. Anal. Calc. for C25H22BN7O3SZn (576.76): C, 52.06; H, 3.84; N, 17.00; S, 5.56. Found: C, 51.75; H, 3.94; N, 16.84; S, 5.54%.
tallization from dichloromethane/methanol (1:1) yielded 85 mg (67%) of 6 as a white powder, m.p. 196 °C. IR(KBr): 2557 m (BH), 1715 vs (CO). 1 H NMR (CDCl3): d (ppm) = 2.48 [s, 9H, Me(pz)], 6.24 [s, 3H, H(pz)], 6.44 [m, 3H, Fu4], 6.84 [d, J = 3.2 Hz, 3H, Fu3], 7.43 [d, J = 1.8. Hz, 3H, Fu5]. 19F NMR (CDCL3): d (ppm) = 75.25. Anal. Calc. for C26H22BF3N6O5Zn (631.69): C, 49.44; H, 3.51; N, 13.30. Found: C, 49.16; H, 3.50; N, 13.26%. 3.2.7. Complex 7 A solution of 74 mg (0.20 mmol) of Zn(ClO4)2 Æ 6H2O in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 15 min at room temperature. To the solution, 17 mg (0.30 mmol) of KOH dissolved in 25 ml of methanol was added dropwise with stirring over a period of 2 h at room temperature. Then the solution was filtered and the filtrate was evaporated to dryness. The residue was recrystallized from dichloromethane/methanol (1:1). 88 mg (91%) of 7 was obtained as colourless crystals, m.p. 320 °C (dec.). IR(KBr): 2552 m (BH). 1 H NMR (CDCl3): d (ppm) = 2.55 [s, 18H, Me(pz)], 4.92 [d, J = 2.8 Hz, 6H, Fu3], 5.72 [dd, J = 3.4 and 1.6 Hz, 6H, Fu4], 5.92 [s, 6H, H(pz)], 6.79 [d, J = 1.6 Hz, 6H, Fu5]. Anal. Calc. for C48H44B2N12O6Zn (971.96): C, 57.32; H, 4.56; N, 17.29. Found: C, 57.03; H, 4.47; N, 16.41%.
3.2.5. Complex 5 A solution of 44 mg (0.20 mmol) of Zn(OAc)2 Æ 2H2O in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. The residue was recrystallized from dichloromethane/methanol (1:1). 106 mg (92%) of 5 was obtained as colourless crystals, m.p. 230 °C. IR(KBr): 2541 m (BH), 1627s (CO). 1 H NMR (CDCl3): d (ppm) = 2.00 [s, 3H, CH3], 2.47 [s, 9H, Me(pz)], 6.23 [s, 3H, H(pz)], 6.42 [dd, J = 3.4 and 1.8 Hz, 3H, Fu4], 7.00 [d, J = 1.8 Hz, 3H, Fu5], 7.40 [d, J = 3.4 Hz, 3H, Fu3]. Anal. Calc. for C26H25BN6O5Zn (577.72): C, 54.05; H, 4.36; N, 14.55. Found: C, 53.70; H, 4.51; N, 14.34%.
3.2.8. Complex 8 A solution of 74 mg (0.20 mmol) of Zn(ClO4)2 Æ 6H2O in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 15 min at 0 °C. To this solution, 17 mg (0.30 mmol) of KOH dissolved in 25 ml of methanol was added dropwise with stirring over a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. The residue was recrystallized from dichloromethane/methanol (1:1). 68 mg (64%) of 8 was obtained as a white powder, m.p. 220 °C (dec.). IR(KBr): 3412 w (OH), 2540 m (BH). 1 H NMR (CDCl3): d (ppm) = 2.47 [s, 9H, Me(pz)], 6.27 [s, 3H, H(pz)], 6.47 [dd, J = 3.4 and 1.8 Hz, 3H, Fu4], 7.42 [d, J = 1.8 Hz, 3H, Fu5], 7.67 [d, J = 3.4 Hz, 3H, Fu3]. Anal. Calc. for C24H23BN6O4Zn (535.68): C, 53.81; H, 4.33; N, 15.69. Found: C, 53.72; H, 4.15; N, 15.61%.
3.2.6. Complex 6 A solution of 74 mg (0.20 mmol) of Zn(ClO4)2 Æ 6H2O in 20 ml of methanol was dropped with stirring to a solution of 100 mg (0.20 mmol) of KTp2-Fu,Me in 20 ml of dichloromethane over a period of 15 min and then cooled to 0 °C. To this solution, 30 mg (0.20 mmol) of KOC(O)CF3 dissolved in 15 ml of methanol was added dropwise with stirring for a period of 2 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrys-
3.2.9. Complex 9 100 mg (0.19 mmol) of 8 was dissolved in 50 ml of methanol/dichloromethane and maintained at 0 °C. A slow stream of CO2 was passed through the solution until all solvent was evaporated. The residue was taken up in 15 ml of benzene, filtered and a slow stream of CO2 was passed again through the solution until the volume was reduced to one-third. At 5 °C, 106 mg (51%) of 9 precipitated as white powder, m.p. 244 °C.
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1
IR(KBr): 2544 m (BH), 1707 m (CO). 1 H NMR (CDCl3): d (ppm) = 2.41 [s, 9H, Me(pz)], 2.93 [s, 3H, OCH3], 6.19 [s, 3H, H(pz)], 6.39 [m, 3H, Fu4], 6.92 [d, J = 3.0 Hz, 3H, Fu3], 7.41 [m, 3H, Fu5]. Anal. Calc. for C26H25BN6O6Zn (593.72): C, 52.60; H, 4.24; N, 14.14; Zn, 11.01. Found: C, 53.61; H, 4.39; N, 14.18; Zn, 10.45%.
H NMR (CDCl3): d (ppm) = 2.51 [s, 9H, Me(pz)], 6.25 [dd, J = 3.7 and 2.6 Hz, 3H, Fu4], 6.28 [s, 3H, H(pz)], 6.44 [d, J = 9.2 Hz, 2H, C6H4], 7.03 [m, 6H, Fu3, 5], 7.83 [d, J = 9.2 Hz, 2H, C6H4]. Anal. Calc. for C30H26BN7O6Zn (672.78): C, 54.86; H, 3.99; N, 14.93; Zn, 9.96. Found: C, 53.91; H, 4.14; N, 14.64; Zn, 9.21%.
3.2.10. Complex 10 100 mg (0.19 mmol) of 8 was dissolved in 50 ml of methanol/dichloromethane. After addition of 15 ll (19 mg, 0.25 mmol) of CS2, the solution was stirred for 2 h at 0 °C. Then all volatiles were removed in vacuo and the residue was taken up in dichloromethane, filtered and the filtrate was evaporated to dryness in vacuo. Recrystallization from dichloromethane/methanol (1:1) yielded 93 mg (78%) of 10 as colourless crystals, m.p. 268 °C (dec.). IR(KBr): 2534 m (BH). 1 H NMR (CDCl3): d (ppm) = 2.50 [s, 9H, Me(pz)], 3.22 [s, 3H, OCH3], 6.24 [s, 3H, H(pz)], 6.40 [dd, J = 3.3 and 1.8 Hz, 3H, Fu4], 6.93 [d, J = 3.3 Hz, 3H, Fu3], 7.46 [d, J = 1.2 Hz, 3H, Fu5]. Anal. Calc. for C26H25BN6O4S2Zn (625.85): C, 49.90; H, 4.03; N, 13.43; S, 10.25. Found: C, 49.36; H, 4.09; N, 13.25; S, 10.12%.
3.2.13. Complex 13 100 mg (0.19 mmol) of 8 was dissolved in 50 ml of methanol/dichloromethane/water (5:4:1). After addition of 19 mg (0.19 mmol) of c-thiobutyrolactone dissolved in 10 ml of dichloromethane, the solution was stirred for 3 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 94 mg (74%) of 13 as colourless crystals, m.p. 170 °C (dec.). IR(KBr): 2547 m (BH), 1739 vs (CO). 1 H NMR (CDCl3): d (ppm) = 1.70 [m, 2H, CH2], 2.08 [t, J = 7.6 Hz, 2H, CH2], 2.22 [t, J = 7.4 Hz, 2H, CH2], 2.48 [s, 9H, Me(pz)], 3.68 [s, 3H, OMe], 6.28 [s, 3H, H(pz)], 6.46 [dd, J = 3.2 and 1.8 Hz, 3H, Fu4], 7.47 [m, 6H, Fu3, 5]. Anal. Calc. for C29H31BN6O5SZn Æ 0.5CH2Cl2 (651.87): C, 53.43; H, 4.79; N, 12.89; S, 4.92. Found: C, 50.18; H, 4.51; N, 12.03; S, 4.62%.
3.2.11. Complex 11 100 mg (0.19 mmol) of 8 was dissolved in 50 ml of methanol/dichloromethane (1:1). After addition of 65 mg (0.19 mmol) of HOP(O)(p-nitrophenolate)2 dissolved in 10 ml of dichloromethane, the solution was stirred for 3 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 63 mg (38%) of 11 as a slight yellow powder, m.p. 228 °C. IR(KBr): 2557 m (BH), 1345 vs (NO), 1180 s (PO). 1 H NMR (CDCl3): d (ppm) = 2.51 [s, 9H, Me(pz)], 5.30 [s, 2H, CH2Cl2], 6.28 [s, 3H, H(pz)], 6.48 [dd, J = 3.4 and 1.8 Hz, 3H, Fu4], 6.79 [d, J = 3.2 Hz, 3H, Fu4], 7.24 [d, J = 8.6 Hz, 4H, C6H4], 7.54 [d, J = 1.0 Hz, 3H, Fu5], 8.07 [d, J = 8.6 Hz, 4H, C6H4]. 31 P NMR(CDCl3): d (ppm) = 13.7. Anal. Calc. for C36H30BN8O11PZn Æ CH2Cl2 (857.86 + 84.93): C, 47.14; H, 3.42; N, 11.89. Found: C, 47.154; H, 3.85; N, 11.62%. 3.2.12. Complex 12 100 mg (0.19 mmol) of 8 was dissolved in 50 ml of methanol/dichloromethane (1:1). After addition of 26 mg (0.19 mmol) of p-nitrophenol dissolved in 10 ml of dichloromethane, the solution was stirred for 3 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 83 mg (65%) of 12 as yellow crystals, m.p. 210 °C. IR(KBr): 2557 m (BH), 1308 vs (NO).
3.2.14. Complex 14 100 mg (0.19 mmol) of 8 was dissolved in 50 ml of methanol/dichloromethane (1:1). After addition of 21 mg (0.19 mmol) of trifluoroacetamide dissolved in 10 ml of methanol/water (3:1), the solution was stirred for 3 h at 0 °C. Then the solution was filtered and the filtrate was evaporated to dryness. Recrystallization from dichloromethane/methanol (1:1) yielded 51 mg (43%) of 14 as colourless crystals, m.p. 240 °C. Drying in vacuo removed the cocrystallized solvents and converted the crystals to a powder. IR(KBr): 3433 m (NH), 2554 m (BH), 1690 vs (amide). 1 H NMR (CDCl3): d (ppm) = 2.48 [s, 9H, Me(pz)], 6.22 [s, 3H, H(pz)], 6.44 [m, 3H, Fu4], 6.74 [d, J = 3.3 Hz, 3H, Fu3], 7.44 [d, J = 0.5 Hz, 3H, Fu5]. Anal. Calc. for C26H23BF3N7O4Zn (630.71): C, 49.51; H, 3.68; N, 15.55. Found: C, 48.38; H, 3.61; N, 15.02%. 3.2.15. Structure determinations Crystals were obtained as described in the experimental section. Data sets were obtained on a Bruker Smart CCD diffractometer at 30 °C and subjected to an empirical absorption correction (program SADABS). The structures were solved with direct methods and refined anisotropically [24]. Hydrogen atoms were included with fixed distances and isotropic temperature factors 1.5 times those of their attached atoms. Parameters were refined against F2. In case of 14, the cocrystallized solvent molecules were treated as a diffuse contribution using the program SQUEEZE [25]. Drawings were produced with SCHAKAL [26]. Crystallographic data are listed in Table 1.
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Table 1 Crystallographic details 4
5
7
10
13
14
Formula
C25H22BN7O3SZn
C26H25BN6O5Zn
C48H44B2N12O6Zn
C26H25BN6O4S2Zn
C29H31BN6O5SZn
MW Space group Z ˚) a (A ˚) b (A ˚) c (A a (°) b (°) c (°) ˚ 3) V (A
576.76 P21/c 4 14.316(2) 8.436(1) 22.175(3) 90 101.496(3) 90 2624.3(7) 1.46 1.06 0.059 0.158
577.72 P 1 2 8.61(1) 10.73(1) 15.21(1) 76.78(3) 86.75(3) 74.49(3) 1318(4) 1.46 0.98 0.055 0.156
971.96 P212121 4 13.20(3) 13.51(3) 26.43(7) 90 90 90 4714(21) 1.37 0.58 0.053 0.136
625.85 P21/c 4 11.475(3) 20.174(5) 12.763(3) 90 105.889(5) 90 2841.7(12) 1.46 1.06 0.040 0.118
651.87 P 1 2 10.660(2) 12.227(2) 13.697(2) 68.156(2) 84.962(3) 65.185(2) 1499.2(4) 1.44 0.94 0.043 0.130
C26H23BF3N7O4Zn Æ 0.5H2O Æ CH3OH 671.76 P 1 4 14.146(3) 15.009(3) 15.482(3) 105.695(4) 101.480(4) 100.599(4) 3001.1(11) 1.40 0.88 0.070 0.199
d (calc) (g cm3) l(Mo Ka) (mm1) R1 (observed reflections) wR2 (all reflections)
4. Supplementary material The crystallographic data of the structures described in this paper were deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-601978 (for 4), CCDC-601979 (for 5), CCDC601980 (for 7), CCDC-601981 (for 10), CCDC-601982 (for 13) and CCDC-601983 (for 14). Copies of these data are available free of charge from the following address: The Director, CCDC, 12 Union Road, GB-Cambridge CB2 1EZ (fax: +44 12 23/3 36 0 33; e-mail: [email protected]). Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. We are indebted to Drs. H. Brombacher and C. Pe´rez Olmo for help with the structure determinations. References [1] [2] [3] [4] [5]
H. Vahrenkamp, Acc. Chem. Res. 32 (1999) 589. G. Parkin, Chem. Rev. 104 (2004) 699. K. Weis, H. Vahrenkamp, Inorg. Chem. 36 (1997) 5589. K. Weis, H. Vahrenkamp, Inorg. Chem. 36 (1997) 5592. B.S. Hammes, M.W. Carrano, C. Carrano, J. Chem. Soc. Dalton Trans. (2001) 1448.
[6] B.S. Hammes, X.M. Luo, M.W. Carrano, C. Carrano, Inorg. Chim. Acta 341 (2002) 33. [7] A.L. Rheingold, C.D. Incarvito, S. Trofimenko, J. Chem. Soc. Dalton Trans. (2000) 1233. [8] P.L. Jones, A.J. Amoroso, J.C. Jefferey, J.A. McCleverty, E. Psillakis, L. Rees, M.D. Ward, Inorg. Chem. 36 (1997) 10. [9] A. Looney, R. Han, I.B. Gorell, M. Cornebise, K. Yoon, G. Parkin, Organometallics 14 (1995) 274. [10] M. Osawa, M. Tanaka, K. Fujisawa, N. Kitajima, Y. Moro-Oka, Chem. Lett. (1996) 397. [11] R. Alsfasser, M. Ruf, S. Trofimenko, H. Vahrenkamp, Chem. Ber. 126 (1993) 703. [12] M. Ruf, H. Vahrenkamp, Inorg. Chem. 35 (1996) 6571. [13] M. Rombach, H. Brombacher, H. Vahrenkamp, Eur. J. Inorg. Chem. (2002) 153. [14] J.A. Maldonado Calvo, H. Vahrenkamp, Inorg. Chim. Acta 358 (2005) 4019. [15] M. Rombach, C. Maurer, K. Weis, E. Keller, H. Vahrenkamp, Chem. Eur. J. 5 (1999) 1013. [16] H. Brombacher, H. Vahrenkamp, Inorg. Chem. 43 (2004) 6050. [17] M. Ruf, K. Weis, H. Vahrenkamp, J. Chem. Soc. Chem. Commun. (1994) 135. [18] F. Groß, H. Vahrenkamp, unpublished. [19] H. Brombacher, H. Vahrenkamp, Inorg. Chem. 43 (2004) 6054. [20] J.A. Maldonado Calvo, H. Vahrenkamp, unpublished. [21] M. Fo¨rster, R. Burth, A.K. Powell, T. Eiche, H. Vahrenkamp, Chem. Ber. 126 (1993) 2643. [22] S.R. Harris, R. Levine, J. Am. Chem. Soc. 71 (1949) 1120. [23] I.I. Grandberg, J. Gen. Chem. USSR 30 (1960) 2896. [24] G.M. Sheldrick, Bruker AXS, SHELXTL Program Package, Version 5.1, Universita¨t Go¨ttingen, 1998. [25] A.L. Spek, J. Appl. Crystallogr. 36 (2003) 7. [26] E. Keller, SCHAKAL for Windows, Universita¨t Freiburg, 2001.