Organometallics 2009, 28, 929–932
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Stepwise Boron-to-Zinc C6F5 Group Transfer in a Zn-Calixarene System Natalie Kotzen, Israel Goldberg, and Arkadi Vigalok* School of Chemistry, The Sackler Faculty of Exact Sciences, Tel AViV UniVersity, Tel AViV 69978, Israel ReceiVed September 17, 2008 Summary: Methylzinc calixarene complex (1) reacts with one equiValent of B(C6F5)3 to giVe the ion-pair (calixarene)Znδ+-Me-B(C6F5)3δ- complex (2), which readily undergoes the pentafluorophenyl group transfer to the zinc center. Reaction between 1 and H2O-B(C6F5)3 results in the formation of the ion-pair (calixarene)Znδ+-HO-B(C6F5)3δ- complex (4). In contrast to 2, complex 4 requires harsher reaction conditions to undergo the C6F5 group transfer, with the reaction taking place under the autocatalytic conditions.
Introduction Interactions of B(C6F5)3 and its derivatives with transition metal alkyl complexes received a great deal of attention in the development of highly active catalysts for alkene polymerization.1 Alkyl, usually methyl, groups can be readily abstracted by this strong Lewis acid generating an ion-pair with R-B(C6F5)3- as anion.2 The chemistry of such ion-pairs has been extensively studied, particularly with regard to the catalyst activity and stereoselectivity of the polymerization process.3 The degree of coordination of the R-B(C6F5)3- anion with the cation can significantly influence the catalysts’ activity, while the pentafluorophenyl group transfer from the anion to the transition metal center often appears as an important contributor to the catalyst deactivation.4 In contrast to transition metals, such group transfer represents a very common reactivity pathway in the reaction of nontransition metal alkyls with B(C6F5)3. For example, various alkylzinc and alkylaluminum precursors react with B(C6F5)3 to produce the corresponding pentafluorophenyl metal complexes (eq 1).5a,b Interestingly, in coordinating solvents or in the presence of multidentate ligands the reactions * Corresponding author. E-mail:
[email protected]. (1) For general references see: (a) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391–1434. (b) Luo, L.; Marks, T. J. Top. Catal. 1999, 7, 97–106. (c) Bochmann, M. Top. Catal. 1999, 7, 9–22. (2) For a review see Piers, W. E. AdV. Organomet. Chem. 2005, 52, 1–76. (3) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113, 3623–3625. (b) Bochmann, M.; Lancaster, S. J.; Hursthouse, M. B.; Abdul Malik, K. M. Organometallics 1994, 13, 2235–2243. (c) Landis, C. R.; Rosaaen, K. A.; Sillars, D. R. J. Am. Chem. Soc. 2003, 125, 1710– 1711. (d) Zhang, S.; Piers, W. E.; Gao, X.; Parvez, M. J. Am. Chem. Soc. 2000, 122, 5499–5509. (e) Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255–270. (4) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10015–10031. (b) Thorn, M. G.; Etheridge, Z. C.; Fanwick, P. E.; Rothwell, I. P. J. Organomet. Chem. 1999, 591, 148–162. (c) Scollard, J. D.; McConville, D. H.; Rettig, S. J. Organometallics 1997, 16, 1810–1812. (d) Jime´nez Pindado, G.; Thornton-Pett, M.; Bochmann, M. J. Chem. Soc., Dalton Trans. 1997, 3115–3127. (e) Jime´nez Pindado, G.; Thornton-Pett, M.; Bouwkamp, M.; Meetsma, A.; Hessen, B.; Bochmann, M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2358–2361. For theoretical studies see: (f) Wondimagegn, T.; Xu, Z.; Vanka, K.; Ziegler, T. Organometallics 2004, 23, 3847–3852.
proceed differently, giving products of the alkyl abstraction rather than C6F5 transfer (eq 2).5,6
RnM-R + B(C6F5)3 h RnM-C6F5 + R-B(C6F5)2 (1) LnM-R + B(C6F5)3 h LnM+R-B(C6F5)3-
(2)
We recently reported the preparation and reactivity of a series of bimetallic (zinc) calixarene inclusion complexes (Figure 1).7 In these complexes, one of the alkylzinc centers (exocyclic) is accessible for interactions with other organic and inorganic reagents, while the second one (endocyclic) is located deep inside the hydrophobic calixarene cavity that protects it from reacting with other reagents.7c The X-ray structures of several zinc calixarene complexes showed weak complexation of the ether groups of the calixarene lower rim to the exocyclic metal center. Here we present the results of our studies of reactivity of these complexes with perfluoroaryl Lewis acids, including the unexpected pattern in the pentafluorophenyl group transfer to Zn.
Results and Discussion Preparation of the Zwitterionic Calixarene Zinc Complexes. We recently showed that the reaction between 1 and B(C6F5)3 resulted in the methyl group removal from the exocyclic zinc center and formation of a new calixarene zinc complex, assigned the structure of 2 (Scheme 1).8 The 1H NMR spectrum of 2 showed the signal of the methyl group of the CH3-B(C6F5)3anion at 0.2 ppm. Complex 2 was relatively unstable in solution (vide supra), however, when the bipyridine (bipy) ligand was added, the quantitative formation of the corresponding thermally stable adduct 3 was observed (Scheme 1).8 The latter was fully spectroscopically characterized and showed typical NMR data (5) (a) Walker, D. A.; Woodman, T. J.; Hughes, D. L.; Bochmann, M. Organometallics 2001, 20, 3772–3776. (b) Klosin, J.; Roof, G. R.; Chen, E. Y.-X.; Abboud, K. A. Organometallics 2000, 19, 4684–4686. (c) Amo, V.; Andres, R.; de Jesus, E.; de la Mata, F. J.; Flores, J. C.; Gomez, R.; Gomes-Sal, M. P.; Turner, J. F. C. Organometallics 2005, 24, 2331–2338. (d) Dagorne, S.; Guzei, I. A.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 2000, 122, 274–289. (e) Qian, B.; Ward, D. L.; Smith, M. R., III. Organometallics 1998, 17, 3070–3076. (f) Milione, S.; Grisi, F.; Centore, R.; Tuzi, A. Organometallics 2006, 25, 266–274. (g) Spitzmesser, S. K.; Gibson, V. C. J. Organomet. Chem. 2003, 673, 95–101. (h) Hannant, M. D.; Schormann, M.; Bochmann, M. J. Chem. Soc., Dalton Trans. 2002, 4071– 4073. (i) Robson, D. A.; Rees, L. H.; Mountford, P.; Schro¨der, M. Chem. Commun. 2000, 1269–1270. (6) For a comparative analysis of B(C6F5)3 reactivity toward transition and nontransition metal alkyls, see: Bochmann, M.; Lancaster, S. J.; Hannant, M. D.; Rodriguez, A.; Schormann, M.; Walker, D. A.; Woodman, T. J. Pure Appl. Chem. 2003, 75, 1183–1195. (7) (a) Bukhaltsev, E.; Goldberg, I.; Vigalok, A. Organometallics 2004, 23, 4540–4543. (b) Bukhaltsev, E.; Goldberg, I.; Vigalok, A. Organometallics 2005, 24, 5732–5736. (c) Bukhaltsev, E.; Frish, L.; Cohen, Y.; Vigalok, A. Org. Lett. 2005, 7, 5123–5126. (8) Kotzen, N.; Goldberg, I.; Vigalok, A. Inorg. Chem. Commun. 2005, 8, 1028–1030.
10.1021/om800904y CCC: $40.75 2009 American Chemical Society Publication on Web 01/09/2009
930 Organometallics, Vol. 28, No. 3, 2009
Notes
Figure 1
Figure 4. ORTEP view of a molecule of 6 with thermal ellipsoids shown at 50% probability. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Zn(1)-C(1) 1.932(3), Zn(2)-C(2)1.968(2),Zn(1)-O(1)1.9529(16),Zn(2)-O(1)1.9552(16), Zn(2)-O(2) 2.3384(17), Zn(1)-Zn(2) 3.0134, O(3)-Zn(2)-O(1) 80.38(7), O(4)-Zn(2)-O(2) 162.14(6).
Figure 2. ORTEP view of a molecule of 3 with thermal ellipsoids shown at 50% probability. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Zn(1)-C(1) 1.925(4), Zn(1)-O(1) 1.957(3), Zn(2)-O(1) 2.000(3), Zn(2)-O(2) 2.323(3), Zn(2)-N(1) 2.070(4), B(1)-C(2) 1.627(7), Zn(1)-Zn(2) 3.0329, O(3)-Zn(2)-O(1) 80.08(10), O(4)-Zn(2)-O(2) 160.44(9), N(1)Zn(2)-N(2) 79.09(14).
Figure 3. ORTEP view of a molecule of 5b with thermal ellipsoids shown at 50% probability. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Zn(1)-C(1) 1.936(7), Zn(1)-O(1) 1.956(4), Zn(2)-O(1) 1.980(4), Zn(2)-O(2) 2.269(5), Zn(2)-N(1) 2.082(5), B(1)-O(5) 1.451(9), Zn(1)-Zn(2) 3.0565, O(3)-Zn(2)-O(1) 78.69(16), O(4)-Zn(2)-O(2) 160.61(14), N(1)-Zn(2)-N(2) 80.1(2).
for the out-of-sphere R-B(C6F5)3- and bimetallic calixarene cation. The X-ray structure of 3 is shown in Figure 2. The formation of 2 in reaction between 1 and B(C6F5)3 was in agreement with the reactivity pattern of alkylzinc complexes
bearing etheric ligands, calixarene lower rim providing the intramolecular ether complexation. The composition of 2, obtained initially in the reaction between 1 and B(C6F5)3, was clearly established by NMR spectroscopy and its reactivity with bipy or phen to give 3. However, our numerous attempts to reproduce the experiments were only “partially successful”, as the obtained complex 4 showed different spectroscopic properties than 2. In particular, we were puzzled by the fact that no 1H, 13C and 11B NMR signals could be assigned to the anion of 4. In contrast, all NMR signals of the cationic part of the molecule were sharp and wellresolved at room temperature and appeared to be similar to those in 2, with the exception of the B-Me signal that was clearly missing. For example, the 1H NMR signal of the internal MeZn group showed a typical high-field signal at -2.50 ppm, while the CH2Ph group signals of the calixarene ligand gave rise to a singlet at 5.11 ppm, a slight downfield shift from the same signal in starting 1. In addition, a new broad signal at 5.15 ppm with relative integration of 1 appeared in the 1H NMR spectrum. The 19 F spectrum of 4 in C6D6 showed the signals at -134.00 (d), -155.82 (t), and -162.79 (t) ppm, the latter two corresponding to the m- and p-fluoro substituents. The large difference in the chemical shifts between these atoms (6.97 ppm in the case of 4) is often used as an indication of R-B(C6F5)3- attached to the metal cation in the ion pair-like complex.9 Unfortunately, while several batches of good looking crystals of 4 were prepared, the crystals showed extremely high sensitivity to the environment change and decomposed upon the attempts to analyze them by X-ray crystallography. Moreover, the addition of bipy or phenanthroline (phen) resulted in the formation of 5a,b, which also showed no signals due to the methyl group in either 1H or 13C NMR although the NMR signals due to the calixarene-based cation were nearly identical to those in 3. Finally, we were able to crystallize complex 5b which unexpectedly showed the presence of an OH group at the boron anion (9) (a) Horton, A. D.; de With, J.; van der Linden, A. J.; van de Weg, H. Organometallics 1996, 15, 2672–2674. (b) Horton, A. D.; de With, J. Chem. Commun. 1996, 1375–1376.
Notes
Organometallics, Vol. 28, No. 3, 2009 931 Scheme 1
Scheme 2
instead of the methyl group (Figure 3).10 This led us to verify the purity of the starting B(C6F5)3 purchased from different suppliers. Although the 19F NMR spectra of all batches of the commercial compound testified for the presence of a single material, there were some differences in the chemical shifts in the compounds obtained from different suppliers. Close examination of these differences showed that while one of the suppliers indeed provided B(C6F5)3, batches obtained from two other suppliers contained H2O-B(C6F5)3 instead.11 Thus, the reaction between 1 and H2O-B(C6F5)3 resulted in the formation of the zwitter-ionic 4, which upon the reaction with bipy or phen gave complexes 5a and 5b, respectively (Scheme 2). The formation of 4 likely proceeds via the cleavage of the metalmethyl bond by the acidic hydrogen of the coordinated water molecule.10a,e Although no X-ray structure of 4 is available, the large downfield shift of the HO-B(C6F5)3 hydrogen atom in the 1H NMR spectrum at 5.15 ppm suggests that the anion is strongly coordinated to the Zn center. For comparison, the same signal appears at ca. 2.37 ppm in 5a or 5b.12 Pentafluorophenyl Group Transfer to Zinc. As mentioned previously, complex 2 was unstable at room temperature. We found that, after several hours at 25 °C, complex 2 was completely converted to the pentafluorophenyl zinc complex 6 (Scheme 3), with the reaction following simple first order kinetics (t1/2 ≈ 60 min). The X-ray structure of 6 is shown in Figure 4 and represents the first example of the crystallographically characterized bimetallic calixarene inclusion complexes with two different organozinc groups inside and outside the cavity. The bonding distances and angles in 6 are very similar to those in other alkylzinc calixarene complexes. We note (10) For a first example of a metal complex with the HO-B(C6F5)3anion, see: (a) Hill, G. S.; Manojlovic-Muir, L.; Muir, K. W.; Puddephatt, R. J. Organometallics 1997, 16, 525–530. See also: (b) Bergquist, C.; Parkin, G. J. Am. Chem. Soc. 1999, 121, 6322–6323. (c) Doerrer, L. H.; Green, M. L. H. J. Chem. Soc., Dalton Trans. 1999, 4325–4329. (d) Danopoulos, A. A.; Galsworthy, J. R.; Green, M. L. H.; Cafferkey, S.; Doerrer, L. H.; Hursthouse, M. B. Chem. Commun. 1998, 2529–2530. (e) Zhang, F.; Kirby, C. W.; Hairsine, D. W.; Jennings, M. C.; Puddephatt, R. J. J. Am. Chem. Soc. 2005, 127, 14196–14197. (11) For comparative NMR analyses of the two compounds, see: Beringhelli, T.; Maggioni, D.; D’Alfonso, G. Organometallics 2001, 20, 4927–4938. (12) There is little literature information available regarding the position of the HO-B(C6F5)3- signal in metal complexes (ref 10). In some cases, the signal could not be observed. Bergquist, C.; Fillebeen, T.; Morlok, M. M.; Parkin, G. J. Am. Chem. Soc. 2003, 125, 6189–6199.
Scheme 3
Scheme 4
however, a slightly shorter Zn1-Zn2 distance of 3.013 Å compared with 3.06 Å typically observed in other similar complexes.7a,b Unlike 2, complex 4 showed significantly higher thermal stability and was stable in solution at room temperature. However, heating of 4 in benzene resulted in the formation of the same C6F5 group transfer product 6 (Scheme 4). Surprisingly, following up the conversion of 4 to 6 by 1H NMR spectroscopy showed a clear induction period in this group-transfer reaction. Heating pure samples of 4 at different temperatures showed no decomposition for long periods of time, followed by rapid conversion to 6 (Figure 5). In contrast, when 4 was not purified thoroughly, prior to heating, the conversion to 6 was significantly faster with no induction period observed. This made us consider the possibility of a Lewis acid-catalyzed C6F5 transfer in 4, since free three-coordinate boron byproduct is formed during the (13) The resulting boron species were found to participate in a series of equilibria in solution, with the monomeric form dominant in dry nonpolar solvents: (a) Beringhelli, T.; D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercandelli, P.; Sironi, A. Organometallics 2003, 22, 1588–1590. (b) Beringhelli, T.; D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercandelli, P.; Sironi, A. Organometallics 2004, 23, 5493–5502. (c) Beringhelli, T.; D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercandelli, P.; Sironi, A. Organometallics 2007, 26, 2088–2095.
932 Organometallics, Vol. 28, No. 3, 2009
Figure 5. Kinetic profiles of conversion of 4 to 6 at different temperatures.
reaction.13 Indeed, when small amounts of free (C6F5)3B were added to a solution of pure 4 in C6D6, very rapid conversion to 6 was observed, the reaction being completed in less than 20 min at 63 °C. To our knowledge, such Lewis acid accelerated group transfer reaction is unprecedented. We believe that the strong interaction of the hydroxy group of the anion with the zinc center hinders the latter from reacting with one of the C6F5 groups. Thus, the Lewis acid, which is likely to coordinate to the oxygen atom of HO-B(C6F5)3,10c causes the anion dissociation from the zinc coordination sphere, making the C6F5 group transfer significantly more facile. In summary, we reported the preparation of zwitter-ionic calixarene zinc complexes with Me-B(C6F5)3- and HOB(C6F5)3- as anions. The complexes undergo the transmetalation reaction to give the neutral pentafluorophenyl Zn complexes. While this reaction is very rapid with Me-B(C6F5)3 anion and obeys the first-order kinetics, the reaction with HO-B(C6F5)3as the anion requires heating and shows long induction period. The reaction is autocatalytic as it is accelerated by the presence of a three-coordinate boron reagent, which is formed as a byproduct. We believe that these findings will be useful for studies of the boron-to-metal group transfer transformations in main group and transition metal complexes.
Experimental Part General Data. All operations with air- and moisture-sensitive compounds were performed in a nitrogen-filled Innovative Technology glovebox. All solvents were degassed and stored under highpurity nitrogen over activated 4Å molecular sieves. All deuterated commercially available reagents were degassed and used as received. The NMR spectra were recorded in C6D6 on Bruker AC 200 MHz or Bruker AMX 400 MHz spectrometers. Kinetic measurements were recorded by 1H NMR on Bruker AC 200 MHz after prior heating of the apparatus to the desired temperature. 1H and 13C NMR signals are reported in ppm downfield from TMS. 19 F chemical shifts are reported in ppm upfield from CFCl3. Elemental analysis was performed in the laboratory for microanalysis at the Hebrew University of Jerusalem. Compounds 1-3 have been reported previously.7,8 Synthesis of 4. One equivalent of B(C6F5)3 · H2O (10.4 mg, 0.02 mmol) was added to a solution of 1 (20 mg, 0.02 mmol) in C6D6 at room temperature. The reaction was instantaneous as confirmed by 1H and 19F NMR spectroscopy. After addition of toluene, the solution was partially evaporated and the product was precipitated by addition of nitromethane at room temperature, filtered and dried in vacuum (88%).
Notes 1 H NMR: -2.53 (3H, s, ZnMe), 0.96 (18H, s, tBu), 1.24 (18H, s, tBu), 2.87 (4H, d, JHH ) 14.0 Hz, ArCH2Ar), 3.91 (4H, d, JHH ) 14.0 Hz, ArCH2Ar), 5.11 (4H, s, CH2Ph), 5.15 (1H, s, HOB), 6.85-7.21 (18H, m, Ph and Ar). 13C NMR: -18.59, 31.22, 31.75, 34.20, 34.25, 34.32, 78.42, 120.25 (m), 126.16, 127.16, 128.41, 129.34, 129.78, 130.36, 131.46, 133.75, 137.89 (dm), 140.48 (dm), 143.27, 148.44 (dm), 149.26, 149.72, 155.82. 19F NMR: -134.00 (d, JFF ) 19.6 Hz), -155.82 (t, JFF ) 20.7 Hz), -162.79 (t, JFF ) 19.3 Hz). Anal. for C77H70BF15O5Zn2: Found (calcd): C 62.10 (61.72), H 5.00 (4.72). Synthesis of 5. One equivalent of bipy or phen in C6D6 was added to a solution of Zn2Me2Bn2cax (20 mg, 0.02 mmol) in C6D6, followed by addition of 1 equiv of B(C6F5)3 (10.4 mg, 0.02 mmol). Changing the addition order did not significantly alter the product yield. The product was purified by crystallization from toluene/ pentane solution (90%).
5a: 1H NMR: 8.87 (d, J ) 8.2 Hz, 2H, bipy); 7.71 (d, J ) 4.9 Hz, 2H, bipy); 7.52 (t, J ) 7.8 Hz, 2H, bipy); 7.29 (s, 4H, Ar); 7.18 (s, 4H, Ar); 6.88 (t, J ) 7.5 Hz, 2H, Ph); 6.50 (t, J ) 7.7 Hz, 4H, Ph); 6.25 (t, J ) 6.5 Hz, 2H, bipy); 6.02 (d, J ) 7.5 Hz, 4H, Ph); 3.99 (d, J ) 13.6 Hz, 4H, Ar-CH2-Ar); 4.00 (s, 4H, CH2Ph); 3.25 (d, J ) 13.6 Hz, 4H, Ar-CH2-Ar); 1.35 (s, 18H, t-Bu); 1.09 (s, 18H, t-Bu); -2.35 (s, 3H, ZnMe). 13C NMR: 158.16, 152.03, 149.66, 148.26, 147.10, 142.73, 141.95, 133.02, 132.72, 130.06, 129.62, 128.69, 128.02, 127.81, 127.62, 126.54, 125.37 (aromatic), 78.79 (CH2-Ph); 34.76 (Ar-CH2-Ar); 34.47, 34.35 (C(CH3)3); 31.92, 31.32 (C(CH3)3); -18.39 (ZnMe). 19F NMR: -135.39 (d, J ) 22 Hz), -162.22 (t, J ) 20 Hz), -165.95 (t, J ) 20 Hz). 5b: 1H NMR: 8.04 (m, 4H, phen); 7.70 (s, 2H, phen); 7.39 (s, 4H, Ar); 7.24 (s, 4H, Ar); 6.74 (m, 2H, phen); 6.37 (t, J ) 7.5 Hz, 2H, Ph); 6.05 (t, J ) 7.64 Hz, 4H, Ph); 5.73 (d, J ) 7.4 Hz, 4H, Ph); 4.16 (d, J ) 13.8 Hz, 4H, Ar-CH2-Ar); 3.86 (s, 4H, CH2Ph); 3.38 (d, J ) 13.8 Hz, 4H, Ar-CH2-Ar); 1.40 (s, 18H, t-Bu); 1.11 (s, 18H, t-Bu); -2.26 (s, 3H, ZnMe).13C NMR: 147.92, 132.67, 130.06, 129.10, 126.71, 126.13 (aromatic); 78.80 (CH2-Ph); 34.86, 34.43 (C(CH3)3); 31.96, 31.35 (C(CH3)3), -8.15 (ZnMe).19F NMR: -135.18 (d, J ) 22 Hz), -163.00 (t, J ) 20 Hz), -166.37 (t, J ) 20 Hz). FAB-MS: found (calcd) 1153 (1153). Preparation of 6. Complex 6 was obtained quantitatively upon reacting 1 (20 mg, 0.02 mmol) with B(C6F5)3 (10.4 mg, 0.02 mmol) in benzene (1 mL) at RT for 5 h. It could also be obtained by heating 4 (20 mg, 0.013 mmol) at 60 °C for 8 h. After the completion, the solvent was partially evaporated and pentane was added. The solution was cooled to -30 °C for overnight and solid 6 was filtered and dried in vacuum. 1 H NMR: 6.75-7.28 (18H, m, Ph and Ar), 4.56 (4H, s, CH2Ph), 4.41 (4H, d, JHH ) 13.8 Hz, ArCH2Ar), 3.26 (4H, d, JHH ) 13.8 Hz, ArCH2Ar), 1.28 (18H, s, tBu), 1.08 (18H, s, tBu), -2.31 (3H, s, ZnMe). 13C NMR: 157.50, 152.07, 148.90, 148.59 (dm), 141.83, 140.63 (dm), 136.99 (dm), 134.75, 133.38, 130.58, 130.45, 129.04, 128.43, 127.29, 126.29, 117.31 (t), 78.07, 34.86, 34.46, 34.21, 31.94, 31.44, -17.88 (ZnMe). 19F NMR: -114.61 (m), -156.47 (t, JFF ) 19.7 Hz), -161.2 (m). FAB-MS: measured (calcd) 1140 (1140).
Acknowledgment. We thank the Israel Science Foundation for supporting this work. Supporting Information Available: Crystallographic data in CIF format for complexes 3, 5b, and 6. This material is available free of charge via the Internet at http://pubs.acs.org. OM800904Y
