Article pubs.acs.org/Organometallics
Structural and Magnetic Studies of a Quasi-Inverse Sandwich Cyclooctatetraene Complex with Two High-Spin Chromium(II) Ions Bound Anti-Facially Jessica N. Boynton,† Owen T. Summerscales,† Fernande Grandjean,‡ Gary J. Long,§ James C. Fettinger,† and Philip P. Power*,† †
Department of Chemistry, University of California, Davis, California 95616, United States University of Liège, B-4000 Sart Tilman, Belgium § Department of Chemistry, Missouri University of Science and Technology, University of Missouri, Rolla, Missouri 65409-0010, United States ‡
S Supporting Information *
ABSTRACT: Reaction of K2COT (COT = 1,3,5,7-cyclooctatetraene, C8H8) with the aryl chromium(II) halide [AriPr4Cr(μCl)]2 (AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2) gave (CrAriPr4)2(μ2-η3:η4-COT) (1), in which a nonplanar COT ring is complexed between two CrAriPr4 moieties, a configuration previously unknown for chromium complexes of COT. One Cr2+ ion is bonded primarily to three COT carbons (Cr−C = 2.22−2.30 Å) as well as an ipso carbon (Cr−C ≈ 2.47 Å) from a flanking aryl ring of its terphenyl substituent. The other Cr2+ ion bonds to an ipso carbon (Cr−C ≈ 2.53 Å) from its terphenyl substituent as well as four COT carbons (Cr−C = 2.24−2.32 Å). The COT carbon−carbon distances display an alternating pattern, consistent with the nonplanarity and nonaromatic character of the ring. The magnetic properties of 1 indicate that the Cr2+ ions have a high-spin d4 configuration with S = 2. The temperature dependence of the magnetism indicates that their behavior is due to zero-field splitting of the S = 2 state. Attempts to prepare 1 by the direct reaction of quintuple-bonded (CrAriPr4)2 with COT were unsuccessful.
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INTRODUCTION Recent work has shown that the treatment of AriPr4MMAriPr4 (M = Ge or Sn) with 1,3,5,7-cyclooctatetraene (COT, C8H8) yielded the inverted sandwich complexes (AriPr4M)2(COT). In these, the COT ring is reduced to (COT)2−, which has an almost planar 10-π-electron [C8H8]2− structure with short C− C ring distances consistent with aromaticity.1,2 In contrast, the reaction of AriPr4GaGaAriPr4 with COT gave a complex in which each GaAriPr4 unit had added across a C−C double bond on opposite sides of the ring to form two Ga−C σ-bonds with no further interactions between the Ga atoms and ring carbons.3 These very different results prompted us to explore additions to other metals. In contrast to Ge and Sn species, octahapto COT structures of transition metals are rare and limited to the Ti species (η5-C5H5)Ti(η8-C8H8).4 Calculations5 on metal complexes C5H5MC8H8 (M = Ti, V, Cr, Mn, Fe, Co) confirmed the Ti structure, but for V, Cr, and Mn, hexahapto structures of the type (η5-C5H5)M(η6-C8H8) were more stable, consistent with experimental results for Cr and Mn.6,7 In order to promote the possibility of octahapto COT coordination, we attempted to © 2012 American Chemical Society
synthesize a Cr complex similar to the Ge and Sn species by the reaction of COT with AriPr4CrCrAriPr4. Unfortunately, this reaction was unsuccessful. Nonetheless, the reaction of the aryl chromium(II) chloride dimer [AriPr4Cr(μ-Cl)]2 with K2COT afforded the paramagnetic, quasi-inverted sandwich compound (CrAriPr4)2(μ2-η3:η4-COT), 1, which was characterized by magnetometry, electronic spectroscopy, and X-ray crystallography. These show that it is paramagnetic and has a structure that is unique for transition metal COT derivatives.
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EXPERIMENTAL SECTION
General Procedures. All manipulations were carried out under anaerobic and anhydrous conditions by using modified Schlenk techniques under a dinitrogen atmosphere or in a Vacuum Atmospheres HE-43 drybox. All solvents were dried by the method of Grubbs et al. and then stored over potassium.8 All physical measurements were carried out under anaerobic and anhydrous Received: October 4, 2012 Published: December 3, 2012 8556
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Scheme 1. Synthetic Route to (CrAriPr4)2(μ2-η3:η4-COT) (1)
measurements. A diamagnetic correction of −0.000 689 emu/mol, from Pascal’s constants, was used.10 The same sample was cooled to 5 K, and its magnetization was measured in a field of ±5 T; no magnetic hysteresis was observed.
conditions. IR spectra were recorded as Nujol mulls between KBr plates. UV−visible spectra were recorded as dilute hexane solutions in 3.5 mL quartz cuvettes. Melting points were determined on a Meltemp II apparatus using glass capillaries sealed with vacuum grease and are uncorrected. Unless otherwise stated, all materials were obtained from commercial sources and used as received. [AriPr4Cr(μ-Cl)]2 was prepared by a literature method. Preparation of (CrAriPr4)2(μ2-η3:η4-COT), 1. Cyclooctatetraene (41 μL, 0.36 mmol) was added to potassium (0.031 g, 0.8 mmol) in THF (30 mL) via syringe at ca. 25 °C, which after stirring overnight afforded a solution of K2COT. This was added dropwise at 25 °C to a solution of [AriPr4Cr(μ-Cl)]2 (0.35 g, 0.36 mmol) in PhMe (20 mL). During addition, the blue color of the [AriPr4Cr(μ-Cl)]2 solution became dark red. The reaction was stirred overnight at ca. 25 °C, and the THF/PhMe solvent was removed under reduced pressure. The residue was extracted with hexane (50 mL) to afford a dark red-black solution. Filtration and concentration to ca. 10 mL and storage for three days at ca. 4 °C gave dark red-black cubes of 1. Yield: 0.082 g (23%), mp 234 °C−dec. Anal. Calcd for C34H41Cr: C, 81.40; H, 8.24. Found: C, 81.89; H, 8.01. UV−vis, nm (ε, M−1 cm−1): 417 (1037), 304 (1848), and 267 (2650). IR in Nujol mull (cm−1) in KBr: νC−H 3000−2800 (s), νC−H (aromatic) 850−750 (m). Direct Reaction of COT with AriPr4CrCrAriPr4. Refluxing a solution of (CrAriPr4)2 (0.09 g, 0.10 mmol) and 110 μL of ca. 1 mmol of COT in toluene for 3 days afforded no change in color or apparent reaction by 1 H NMR spectroscopy. Exposure of this solution (in a quartz Schlenk tube) to UV radiation for 3 days, followed by concentration to ca. 5 mL under reduced pressure, gave dark red-black crystals after ca. 1 week at ambient temperature. Yield: 0.073 g (81%). The crystal was confirmed to be unreacted starting material, (CrAriPr4)2, by X-ray crystallography and 1H NMR spectroscopy. X-ray Crystallography. Details of the structure determination and refinement9 can be found in the Supporting Information. Crystal data: Empirical formula, C68H82Cr2; formula weight (g/mol), 1003.34; temperature, 90(2) K; wavelength, 0.71073 Å; crystal system, orthorhombic; space group, Pca21; unit cell dimensions, a = 17.0108(17) Å, b = 19.7941(19) Å, c = 16.8614(17) Å, α = β = γ = 90°; volume, 5677.5(10) Å3; Z, 4; density (calculated), 1.174 Mg/m3; absorption coefficient, 0.422 mm−1; F(000), 2152; crystal size, 0.36 × 0.26 × 0.23 mm3; crystal color and habit, dark red-black block; diffractometer, Bruker APEX-II CCD; theta range for data collection, 2.61−27.48°; index ranges, −22 ≤ h ≤ 22, −25 ≤ k ≤ 25, −21 ≤ l ≤ 21; reflections collected, 12 967; independent reflections, 12 967 [R(int) = 0.0000]; observed reflections (I > 2σ(I)), 12 010; completeness to theta = 27.48°, 99.9%; absorption correction, semiempirical from equivalents, Twinabs; max. and min. transmission, 0.9102 and 0.8635; data/restraints/parameters, 12 967/15/694; goodness-of-fit on F2, 1.028; final R indices [I > 2σ(I)], R1 = 0.0297, wR2 = 0.0724; R indices (all data), R1 = 0.0337, wR2 = 0.0739. Magnetic Studies. A powdered sample of 1 was sealed under vacuum in a 3 mm diameter quartz tube. The magnetic properties were measured using a Quantum Design MPMSXL7 superconducting quantum interference magnetometer; the sample was first zero-field cooled to 2 K, and its moment was measured upon warming from 2 to 300 K in an applied field of 0.01 T. In order to ensure thermal equilibrium between the sample in the quartz tube and the temperature sensor, the moment was measured at each temperature until it reached a constant value; ca. 15 h was required for the
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RESULTS AND DISCUSSION Synthesis. The addition of the dipotassium salt of cyclooctatetraene to the chloro-bridged dimer [AriPr4Cr(μCl)]211 (Scheme 1) gave (MAriPr4)2(μ2-η3:η4-COT) as dark redblack crystals in ca. 23% yield with elimination of KCl. Other products from this reaction could not be readily characterized. Attempts to synthesize 1 by direct reaction of COT and the quintuple bonded dimer (CrAriPr4)2,12 in a similar manner to that used for the (MAriPr4)2 (M = Ge or Sn) derivatives, were unsuccessful.1,2 Unreacted (CrAriPr4)2 was the only characterizable product (by X-ray crystallography and 1H NMR spectroscopy) recovered from the reaction. Structure. The structure of (AriPr4Cr)2(μ2-η3:η4-COT) is illustrated in Figure 1, where it is apparent that the COT ring,
Figure 1. Thermal ellipsoid plot (30%) of (CrAriPr4)2(μ2-η3:η4-COT) (1), without H atoms. Selected bond distances and angles are given in Table 1.
sandwiched between the two CrAriPr4 moieties, is puckered. The structural data in Table 1 show that the C−C distances within the COT ring have an alternating pattern. The Cr(1) and Cr(2) ions are bound on opposite faces of the C8H8 ring to carbons C(1), C(2), and C(3) and to C(2), C(3), C(4), and C(5), respectively. As a result, C(2) and C(3) interact with both chromium(II) ions, whereas there are no metal interactions with C(6), C(7), and C(8). This is reflected in the C−C distances, of which the longest is the C(2)−C(3) bond at 1.477(2) Å and the shortest is the C(6)−C(7) bond at 1.385(3) Å. The Cr−C distances involving the C8H8 ring carbons lie in the range 2.2229(18)−2.3179(10) Å for C(1), 8557
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Magnetic Properties. The magnetic properties of 1 indicate that both Cr2+ ions are present in high-spin d4 (S = 2) configurations. A plot of the average, μeff, per Cr(II) ion for 1 from 2 to 300 K (see Figure S1 in the SI) reveals an increase in μeff from 4.24 μB at 2 K to a maximum of 4.79 μB at ca. 70 K and then a gradual decrease from 4.79 to 4.56 μB at 300 K. The low-temperature changes may result from either zero-field splitting of the S = 2 state or from magnetic exchange between the two Cr2+ ions. As a result, the 1/χM is linear only above ca. 100 K, and a Curie−Weiss law fit of 1/χM between 100 and 300 K yields an average θ of 13.8 K, an average C of 5.00 emu K/ mol, and corresponding average μeff of 6.69 μB per mole of dimer or 4.92 μB per mole of Cr2+ ions (see the inset to Figure S1 in the SI). The increase in μeff and χMT between ca. 2 and 100 K may result from either zero-field splitting of the S = 2 state of each Cr2+ ion or magnetic exchange between the two Cr2+ ions. Because of its compatibility with the 5 K magnetization results (see below), the zero-field splitting approach is the most reasonable and is discussed in more detail here, even though a fit15 with the Heisenberg isotropic exchange coupling Hamiltonian, H = −2JS1·S2, is also successful for χMT from 2 to 100 K; see Figure S2. A fit of χMT as a f(T) between 2 and 100 K is shown in Figure 2, which also shows the parallel and perpendicular
Table 1. Selected Interatomic Distances and Angles for 1 atoms
distance (Å)
atoms
angle (deg)
Cr(1)−C(9) Cr(1)−CCOT(1) Cr(1)−CCOT(2) Cr(1)--CCOT(3) Cr(1)−CAryl(15) Cr(2)−C(39) Cr(2)−CCOT(2) Cr(2)−CCOT(4) Cr(2)−CCOT(3) Cr(2)−CCOT(5) Cr(2)−C(45) C(1)−C(8) C(1)−C(2) C(2)−C(3) C(3)−C(4) C(4)−C(5) C(5)−C(6) C(6)−C(7) C(7)−C(8)
2.0746(16) 2.2237(17) 2.2901(17) 2.2229(18) 2.4736(16) 2.0816(15) 2.2370(17) 2.2390(16) 2.3179(16) 2.4789(18) 2.529(16) 1.382(3) 1.425(3) 1.477(2) 1.426(3) 1.393(3) 1.403(3) 1.385(3) 1.400(4)
C(9)−Cr(1)−C(3) C(9)−Cr(1)−C(1) C(10)−C(9)−Cr(1) C(14)−C(9)−C(10) C(14)−C(9)−Cr(1) C(39)−Cr(2)−C(2) C(39)−Cr(2)−C(4) C(39)−Cr(2)−C(3) C(39)−Cr(2)−C(5) C(40)−C(39)−Cr(2) C(44)−C(39)−Cr(2) C(44)−C(39)−C(40)
105.35(6) 168.56(7) 100.59(11) 116.78(14) 142.58(13) 107.71(6) 175.45(7) 140.54(6) 150.66(6) 102.02(10) 141.18(12) 116.78(13)
C(2), C(3), and C(4) with a longer distance to C(5) of 2.4789(18) Å. There are also long Cr(1)---C(15) interactions at 2.4736(16) Å and Cr(2)---C(45) at 2.529(16) Å to the i-C of the terphenyl flanking aryl rings, which are accompanied by large differences in the Cr−C(ipso)−-C(ortho) angles of 41.99(13)° at C(9) and 39.16(12)° at C(39). The internal angles at the C(9) and C(39) ring carbons (both 116.78(14)°) are less than 120° and are consistent with the electropositive nature of the chromium(II) ions. The Cr−C(ipso) distances involving C(9) and C(39) are 2.0746(16) and 2.0816(15) Å, respectively, which are marginally longer than the 2.041(3) Å in the precursor [AriPr4Cr(μ-Cl)]211 but are shorter than the 2.131(4) Å found in the quintuple-bonded compound AriPr4CrCrAriPr4.12 The disparity in the Cr−C(ipso)−C(ortho) angles is apparently characteristic of Cr(II)-terphenyl structures and arises from the electron deficiency of the chromium(II) site, favoring further interactions between the Cr(II) and electron-rich groups, as well as the tendency of the high-spin d4 configuration to favor square-planar coordination.10 No structures similar to that of (AriPr4Cr)2(μ2-η3:η4-COT) are available for comparison. However, there have been experimental and theoretical5,13 interest in the bonding of COT in other Cr species. For example, some complexes have been structurally characterized in which two Cr atoms are bound syn-facially to COT14 as well as various hexahaptochromium species15 and complexes in which a C−C bond of the COT ring has been cleaved.16 In these, which generally involve Cr(0) interacting with the double bonds of the COT ring, the Cr−C distances lie within a relatively wide range, ca. 2.05−2.35 Å. In addition, there exist structurally characterized electronically related dichromium pentalene complexes.17 Electronic Spectroscopy. The UV−visible absorption spectra of the intensely colored complex 1 in hexane revealed three weak electronic transitions. UV−vis absorptions (λmax, nm (ε, M−1 cm−1)) were observed at 417 (1000), 304 (1800), and 267 (2700). Definitive assignments are not possible because of the lack of both comparison data and detailed computational work on the energy states of 1.
Figure 2. Plot of χMT vs T between 2 and 120 K in 0.01 T and fit by assuming axial zero-field splitting for S = 2 with gav = 1.950(1) and Dav = +2.29(2) cm−1.
components of the calculated χMT. This fit,18,19 which assumes a random powder, an axial zero-field splitting, and S = 2, yields gav = 1.950(1) and Dav = +2.29(2) cm−1; because of the 5 K magnetization results, see below, this fit with a positive D-value is preferred. However, a fit with gav = 1.950(1) and Dav = −2.12(4) cm−1 (see Figure S3) affords an only slightly poorer fit. The reason20 for this difference is discussed in the Supporting Information and illustrated in Figures S4 and S5. Whether a positive or negative D-value is used, the fit reflects the average values for the two Cr(II) ions. Because the Dav is an average value for two slightly different Cr(II) sites and the second-order influence of the nonaxial E zero-field parameter is small, no attempt was made to include this parameter in the fit. The magnetization of 1 was measured at 5 K; see Figure 3. As a check of the internal consistency of the results, the slope of 8558
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dependence of χMT is best described by the presence of an axial zero field splitting Dav of +2.29(2) cm−1 for the Cr(II) ions with S = 2 and gav of 1.950(1). Transition metal complexes in which a cycloalkene not derived from delocalized rings bridges two metal centers are rare22 and may yield interesting magnetic results in the future.
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ASSOCIATED CONTENT
* Supporting Information S
Details of the structural determination and refinement and CIF as well as magnetic data for 1 are provided. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Address
Chemistry Department, University of California, Davis, California 95616, United States.
Figure 3. Molar magnetization of 1 measured at 5 K. Inset: Initial magnetization of 1 used to obtain the slope and thus the molar magnetic susceptibility, χM, at 5 K.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-0948417) and Peter Klavins for assistance with the magnetization measurements.
the molar magnetization in emu/mol versus applied field from 0 to 10 000 Oe was found to yield a 5 K χM of 1.126 emu/mol, which is in good agreement with the χM of 1.068 emu/mol obtained above at 5 K and 100 Oe. It is obvious from Figure 3 that the magnetization does not saturate and reaches only 4.57 μB at an applied field of ±5 T. In the absence of any magnetic anisotropy, the magnetization of 1 would be expected to saturate at ca. 8 μB for two Cr(II) ions each with S = 2; this clearly is not observed. The positive D value of +2.29 cm−1 obtained from the fit of χMT indicates that, in a zero applied field, the ground state of the Cr(II) ions is S = 0 and the excited states for S = ±1 and ±2 are then at S2D, or 2.29 and 9.16 cm−1, respectively, above the ground state. In an applied field of ±5 T, the Zeeman effect strongly perturbs the order of these states, such that the ground state is MS = −1 and the excited states with MS = −2 and 0 are at ca. 2.23 and 2.35 cm−1, respectively, above the ground state. Thus, at 5 K or 3.48 cm−1, the thermal population of these three states leads, parallel to the applied field, to an average ⟨Sz⟩ for each Cr(II) ion of ca. 1, and a magnetization of ca. 4 Nβ for the two Cr(II) ions is expected. This value is in good agreement with the observed magnetization of 4.57 μB observed at 5 K in an applied field of ±5 T. For this reason the analysis of χMT as a f(T) in terms of zero-field splitting with D = +2.29 cm−1 is preferred over both a negative D-value and magnetic exchange coupling. Currently the reason for the small decrease in μeff, from ca. 80 to 300 K, is unknown. Neither the zero-field model nor the exchange coupling approach explains it, but it may result from a combination of the low-symmetry crystal field, spin−orbit coupling, and electron delocalization21 or from changes in the structure of 1 as the temperature increases from 90 K, the temperature of the X-ray structural study, to 300 K.
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REFERENCES
(1) Summerscales, O. T.; Wang, X. P.; Power, P. P. Angew. Chem., Int. Ed. 2010, 49, 4788. (2) Summerscales, O. T.; Jimenez-Halla, J. O. C.; Merino, G.; Power, P. P. J. Am. Chem. Soc. 2011, 133, 180. (3) Caputo, C. A.; Guo, J. D.; Nagase, S.; Fettinger, J. C.; Power, P. P. J. Am. Chem. Soc. 2012, 134, 7155. (4) Van Oven, H. R.; de Liefde Meijer, H. J. J. Organomet. Chem. 1969, 19, 373. (5) Wang, H.; Chen, X.; Xie, Y.; King, R. B.; Schaefer, H. F. Organometallics 2010, 29, 1934. (6) (a) Angermund, K.; Betz, P.; Dohring, A.; Jolly, P. W.; Kruger, C.; Schonfelder, K. U. Polyhedron 1993, 12, 2663. (b) Müller, J.; Menig, H. J. Organomet. Chem. 1975, 96, 83. (c) Kreiter, C. G.; Maasbol, A.; Anet, F. A. L.; Kaesz, H. D.; Winstein, S. J. Am. Chem. Soc. 1966, 88, 3444. (7) Pauson, P. L.; Segal, J. A. J. Chem. Soc., Dalton Trans. 1975, 2387. (8) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518. (9) (a) Bruker SAINT, 7.68a; Bruker AXS Inc.: Madison, WI, USA, 2009. (b) Bruker SMART APEX, 2010.9-1; Bruker AXS Inc.: Madison, WI, USA, 2010. (10) Bain, G. A.; Berry, J. F. J. Chem. Educ. 2008, 85, 532. (11) Sutton, A. D.; Ngyuen, T.; Fettinger, J. C.; Olmstead, M. M.; Long, G. J.; Power, P. P. Inorg. Chem. 2007, 46, 4809. (12) Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long, G. J.; Power, P. P. Science 2005, 310, 844. (13) Wang, H. Y.; Du, Q. A.; Xie, Y. M.; King, R. B.; Schaefer, H. F. J. Organomet. Chem. 2010, 695, 215. (14) (a) Elschenbroich, C.; Heck, J.; Massa, W.; Schmidt, R. Angew. Chem., Int. Ed. Engl. 1983, 22, 330. (b) Heck, J.; Rist, G. J. Organomet. Chem. 1988, 342, 45. (c) Maurice, P.; Hermans, J. A.; Scholten, A. B.; Vandenbeuken, E. K.; Bussaard, H. C.; Roeloffsen, A.; Metz, B.; Reijerse, E. J.; Beurskens, P. T.; Bosman, W. P.; Smits, J. M. M.; Heck, J. Chem. Ber. 1993, 126, 553. (d) Brauer, D. J.; Kruger, C. Inorg. Chem. 1976, 15, 2511. (15) Munro, J. D.; Pauson, P. L. J. Chem. Soc. 1961, 3475.
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CONCLUSION A stable quasi-inverse sandwich species in which two crystallographically distinct high-spin Cr(II) ions bond on opposite sides of a nondelocalized C8H8 ring has been synthesized and characterized. In agreement with the field dependence of the 5 K magnetization, the temperature 8559
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(16) Geibel, W.; Wilke, G.; Goddard, R.; Kruger, C.; Mynott, R. J. Organomet. Chem. 1978, 160, 139. (17) Summerscales, O. T.; Cloke, F. G. N. Coord. Chem. Rev. 2006, 250, 1122. (18) Kahn, O. Molecular Magnetism; VCH Publishers: New York, 1993. (19) O’Connor, C. J. Prog. Inorg. Chem. 1982. (20) Skokol, J. J. University of California, Berkeley, 2003. (21) Figgis, B. N. Introduction to Ligand Fields; Wiley-Interscience: New York, 1966. (22) (a) Giannini, L.; Solari, E.; Dovesi, S.; Floriani, C.; Re, N.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1999, 121, 2784. (b) Wadepohl, H.; Galm, W.; Pritzkow, H. Angew. Chem., Int. Ed. 1990, 29, 686.
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