Assigning Hapticity to Cyclopentadienyl Derivatives of Antimony and

Aug 1, 2013 - Benjamin M. Day and Martyn P. Coles*. ,†. Department of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ, U.K.. •S Supporti...
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Assigning Hapticity to Cyclopentadienyl Derivatives of Antimony and Bismuth Benjamin M. Day and Martyn P. Coles*,† Department of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ, U.K. S Supporting Information *

ABSTRACT: The chelating diamide [Me2Si{NAr}2]2− (Ar = 2,6-i-Pr2C6H3) has been used as an ancillary ligand in antimony and bismuth chemistry. In contrast to the monomeric antimony chloride compound Sb(Me2Si{NAr}2) Cl (1a), the bismuth derivative (1b) exists as a trimer in the solid state, linked by Bi···(arene) π-bonding and Bi···Cl interactions. Compounds 1a,b are used as an entry point into rare antimony and bismuth compounds incorporating Cp and Cp* ligands. Reactions of the chlorides with LiCp and KCp* afford the organometallic species E(Me2Si{NAr}2)(C5R5) (2a, R = H, E = Sb; 2b, R = H, E = Bi; 3a, R = Me, E = Sb; 3b, R = Me, E = Bi). Variable-temperature NMR experiments indicate different fluxional processes occur in solution involving facile haptotropic shifts of the [C5R5]− ring. Structural characterization of 2b and 3a,b reveals monomeric compounds in which the cyclopentadienyl-derived ligand adopts a low hapticity in the solid state.

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INTRODUCTION The cyclopentadienyl anion and its derivatives, [C5R5]−, are ubiquitous in organometallic chemistry and have been used as ligands to elements from across the periodic table. In contrast to compounds of the transition elements in which π bonding of the C5 ring predominates, those of the main-group elements contain a range of hapticities varying from symmetrical η5 through the “slipped” ηx structures (x = 3−1) to purely σbound ligands.1 In addition to the structural diversity for these compounds, their ability to undergo fluxional processes in solution is well known.2 Despite their widespread application in main-group chemistry, the use of cyclopentadienyl anions as ligands for the heavier group 15 elements antinomy and bismuth is relatively underexplored. To prevent complications arising from multiple metal···cyclopentadienyl interactions, we targeted compounds incorporating a single Cp-derived group. We therefore chose a chelating, dianionic ligand to provide a stable ancillary framework that satisfies the remaining valencies of the E(III) element. Work from Veith and co-workers indicated that the chelating diamide [Me2Si{Nt-Bu}2]2− may be suitable. The chlorides E(Me2Si{Nt-Bu}2)Cl (E = As, Sb (A), Bi (B)) have been reported,3 and their chemistry has been briefly explored.4 The solid-state structures of A and B, however, show intermolecular E···Cl interactions that linked the molecules into 1-D chains (Figure 1), suggesting access to the metal is still possible, which may give rise to intermolecular interactions in the proposed Cp derivatives. The N-aryl analogue [Me2Si{NAr}2]2− (Ar = 2,6-i-Pr2C6H3) is a bulky alternative to Veith’s diamide that has demonstrated favorable properties as a chelating, dianionic ligand, with structurally characterized © XXXX American Chemical Society

Figure 1. Schematic of the extended chain structure of E(Me2Si{NtBu}2)Cl (E = Sb, Bi).

examples of transition-metal (Mn,5 Zr6), lanthanide (Y, Nd, Sm, Yb),7 and main-group (Li,6 Mg,8 Ca,9 Sr,9 Ba,9 Zn,5,10 Cd,5 B,11 Ge,12 Sn,12,13 Pb12) compounds. We report herein the use of this ligand as a stable platform at antimony and bismuth for the study of isolated metal···[C5R5]− (R = H, Me) interactions. During the preparation of this paper, the synthesis and crystal structure of the antimony derivative Sb(Me2Si{NAr}2)Cl (1a) was reported by the group of Ma and Roesky.14

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out under dry nitrogen using standard Schlenk-line and cannula techniques or in a conventional nitrogen-filled glovebox. Solvents were dried over appropriate drying agents and degassed prior to use. NMR spectra were recorded using a Bruker Avance DPX 300 MHz spectrometer at 300.1 (1H) and 75.4 MHz (13C{1H}) or a Varian VNMRS 500 MHz spectrometer at 500.1 (1H) and 125.4 MHz (13C{1H}). Proton and carbon chemical shifts were referenced Received: May 16, 2013

A

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internally to residual solvent resonances. S. Boyer at London Metropolitan University performed elemental analyses. Sb(Me2Si{NAr}2)Cl (1a) was made by a modified literature procedure,14 using isolated [Me2Si{N(Ar)Li}2].6 The high air and moisture sensitivity and possible photolability of bismuth compounds 2b and 3b prevented accurate elemental analyses from being obtained. Preparation of Bi(Me2Si{NAr}2)Cl (1b). A THF solution of [Me2Si{N(Ar)Li}2] (750 mg, 1.8 mmol) was added dropwise to a THF solution of BiCl3 (568 mg, 1.8 mmol) at −78 °C with the exclusion of light. The resultant solution was left to stirred for 3 h while it was warmed to ambient temperature. Removal of the volatiles in vacuo afforded an orange solid, from which the product was extracted into toluene. Concentration and storage of this solution at −20 °C for 24 h afforded orange crystals of 1a. Yield: 0.800 g, 68%. Anal. Calcd for C26H40N2BiClSi (653.1): C, 47.81; H, 6.17; N, 4.29. Found: C, 47.69; H, 6.27; N, 4.17. 1H NMR: δ 7.23 (br, 4H, m-CH), 6.86 (t, 3JHH = 7.6 Hz, 2H, p-CH), 4.41 (m br, 2H, CHMe2), 3.79 (m br, 2H, CHMe2), 1.65−0.90 (br, 24H, CHMe2), 0.60, 0.07 (s, 3H, SiMe). 13C NMR: δ 137.3 (i-C), 125.9 (p-CH), 122.8 (br, m-CH), 26.9 (CHMe2), 25.0 (br, CHMe2), 11.8, 6.7 (SiMe); o-C not observed. Formation of Sb(Me2Si{NAr}2)Cp (2a). A solution of 1a (112 mg, 0.20 mmol) in 0.5 mL of C6D6 was added to a suspension of LiCp (14 mg, 0.20 mmol) in d6-benzene (0.1 mL). The resultant solution was stirred for 5 h before being filtered. Analysis of the filtrate by 1H NMR spectroscopy indicated complete consumption of 1a with 1H and 13C resonances consistent with the formation of 2a. 1H NMR: δ 7.17 (partially obscured by solvent peak, 4H, m-CH), 7.08 (t, 3JHH = 7.5 Hz, 2H, p-CH), 6.26 (s, 5H, C5H5), 3.93 (sept, 3JHH = 6.9 Hz, 4H, CHMe2), 1.32, 1.29 (d, 3JHH = 6.9 Hz, 12H, CHMe2), 0.30 (s, 6H, SiMe2). 13C NMR: δ 146.4 (o-C), 138.8 (i-C), 124.3 (p-CH), 123.7 (m-CH), 114.3 (C5H5), 27.7 (CHMe2), 25.8, 25.0 (CHMe2); SiMe2 not observed. Preparation of Bi(Me2Si{NAr}2)Cp (2b). Compound 2b was prepared in a manner analogous to that described for 2a, using 150 mg of 1b (0.23 mmol) and 17 mg of LiCp (0.23 mmol). The reaction mixture was filtered and the solvent allowed to slowly evaporate, affording 2b as a red crystalline solid. Yield: 94 mg, 60%. 1H NMR: 7.28 (d, 3JHH = 7.7 Hz, 4H, m-CH), 6.85 (t, 3JHH = 7.7 Hz, 2H, p-CH), 6.26 (s, 5H, C5H5), 3.94 (sept, 3JHH = 6.9 Hz, 4H, iPr-CH), 1.29 (br, 24H, iPr-CH3), 0.24 (s, 6H, SiCH3). 13C NMR (C6D6, 100 MHz): 148.1 (o-C), 138.2 (i-C), 125.3 (p-CH), 123.0 (m-CH), 113.6 (C5H5), 27.5 (CHMe2), 27.2 (CHMe2), 25.3 (CHMe2), 9.4 (SiCH3). Preparation of Sb(Me2Si{NAr}2)Cp* (3a). Compound 3a was prepared in a manner analogous to that described for 2a, using 150 mg of 1a (0.27 mmol) and 46 mg of KCp* (0.28 mmol). The solvent was allowed to slowly evaporate from the filtrate in a glovebox, resulting in the isolation of 3a as colorless crystals. Yield: 135 mg, 77%. Anal. Calcd for C36H55N2SiSb (665.7): C, 65.00; H, 8.33; N, 4.21. Found: C, 64.92; H, 8.48; N, 4.12. 1H NMR: δ 7.15 (d, 3JHH = 7.5 Hz, 4H, mCH), 7.05 (t, 3JHH = 7.5 Hz, 2H, p-CH), 4.05 (br, 4H, CHMe2), 1.80 (s, 15H, C5Me5), 1.41, 1.29 (d, 3JHH = 6.6 Hz, 12H, CHMe2), 0.41 (br, 6H, SiMe2). 13C NMR: δ 145.7 (o-C), 141.9 (i-C), 124.3 (p-CH), 123.7 (m-CH), 121.3 (C5Me5), 28.3 (CHMe2), 26.8, 25.8 (CHMe2), 10.7 (C5Me5); SiMe2 not observed. Preparation of Bi(Me2Si{NAr}2)Cp* (3b). Compound 3b was prepared in a manner analogous to that described for 2a, using 180 mg of 1b (0.28 mmol) and 48 mg of KCp* (0.28 mmol). The reaction mixture was filtered and the solvent allowed to slowly evaporate, affording 2b as a red crystalline solid. Yield: 145 mg, 70%. 1H NMR: 7.25 (d, 3JHH = 7.6 Hz, 4H, m-CH), 6.86 (t, 3JHH = 7.6 Hz, 2H, p-CH), 4.02 (sept, 3JHH = 6.8 Hz, 4H, CHMe2), 2.08 (s, 15H, C5Me5), 1.36, 1.30 (d, 3JHH = 6.8 Hz, 12H, CHMe2), 0.35 (s, 6H, SiMe2). 13C NMR: 147.0 (o-C), 141.1 (i-C), 123.9 (p-CH), 123.3 (m-CH), 121.1 (C5Me5), 27.5 (CHMe2), 26.8, 26.7 (CHMe2), 10.6 (SiMe2), 10.3 (C5Me5). Crystallographic Data Collection and Refinement Procedures. Crystals were covered in inert oil, and suitable single crystals were selected under a microscope and mounted on a Bruker AXS diffractometer (1b) or an Enraf Nonius Kappa CCD diffractometer

(2b, 3a,b). Data were collected at 173(2) K using Mo Kα radiation at 0.71073 Å. The structures were refined with SHELXL-97.15

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RESULTS The lithium diamide [Me2Si{N(Ar)Li}2]6 reacts with ECl3 to afford E(Me2Si{NAr}2)Cl (1a, E = Sb;14 1b, E = Bi) as colorless (1a) or orange (1b) crystals (Scheme 1). This Scheme 1. Synthesis of E(Me2Si{NAr}2)Cl (1) and Conversion to Cyclopentadienyl (2) and Pentamethylcyclopentadienyl (3) Derivatives, E(Me2Si{NAr}2)(C5R5) (a, E = Sb; b, E = Bi)

method is similar to that used for the synthesis of the N-tertbutyl derivatives A and B3 and the recently reported synthesis of the antimony compound 1a,14 although we found it convenient to isolate the dilithio salt.6 Initially, low yields (∼30%) of the bismuth compound 1b were obtained, with formation of an insoluble black solid observed as the reaction mixture warmed to room temperature. Repeating the reaction with the exclusion of light increased the yield to 68%. Given that, once isolated, compound 1b is stable to ambient light under inert conditions in both the solid state and solution, this may indicate the formation of light-sensitive intermediates during its formation. The room-temperature 1H NMR spectra of 1b shows inequivalent SiMe2 groups (δH 0.60, 0.07) consistent with C1 symmetry and inequivalent faces of the metallacycle caused by a pyramidal metal geometry; this is consistent with data reported for 1a.14 In contrast to the single broad resonance observed for the CHMe2 proton in 1a, however, the peaks corresponding to the methine protons are resolved into separate broad resonances for the bismuth derivative, prompting a study by variable-temperature NMR. At high temperature (383 K), a symmetrical environment is observed for the diamide ligand consisting of one septet and one doublet resonance for the i-Pr substituents and a single resonance for the SiMe2 groups. In contrast, at low temperature (193 K) two septets and four doublets are noted for the i-Pr groups, with distinct silicon− methyl resonances. Assuming the diamide remains bidentate in solution, the single SiMe2 environment is consistent with averaged C2h symmetry for the metallacycle, which can be attained through a number of possible pathways. Rapid pyramidal inversion of the bismuth atom would result in a single averaged silicon methyl resonance in the NMR spectrum (Figure 2, pathway i). A similar inversion has been postulated in the chiral organobismuth chloride Bi[2-(Me2NCH2)C6H4][(Me3Si)2CH]Cl (C),16 although this process required the use a strongly coordinating solvent (DMSO) that promoted “edge” inversion17 via a T-shaped transition state. Inversion of C was not observed in C6D6, suggesting a different mechanism B

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Figure 3. Thermal ellipsoid plot (30%) of one of the independent molecules of Bi(Me2Si{NAr}2)Cl (1b). Hydrogen atoms and hexane solvate are omitted.

Table 1. Selected Bond Lengths (Å) and Angles (deg) for Bi(Me2Si{NAr}2)Cl (1b) Bi1−N1 Bi2−N3 Bi2−Cl2 Si1−N2 N1−Bi1−N2 N2−Bi1−Cl1 N3−Bi2−N3′ N3−Si2−N3′

Figure 2. Solution-state processes affording equivalent SiMe 2 resonances observed in the high-temperature 1H NMR spectrum of 1b.

occurs in 1b. Ionization of compound 1b can potentially occur to afford the corresponding C2h cation [Bi(Me2Si{NAr}2)]+. The related [Bi(Me2Si{Nt-Bu}2)]+ cation derived from B has been isolated and characterized as the [AlCl4]− salt, showing a single resonance for the SiMe2 protons in the 1H NMR spectrum.4a However, spontaneous chloride dissociation has not been reported in B and the cation is only produced in the presence of the group 13 chlorides MCl3 (M = Al, Ga, In). Finally an intramolecular chloride transfer process to the opposite face of the metallacycle also explains the coalescence of the SiMe2 resonances (pathway iii). Although long-range Bi···Cl interactions that would facilitate chloride transfer have been observed in the solid state (see Figure 1 and discussion below), their retention in solution has not been documented. We also note that while pathways i−iii explain coalescence of the silicon methyl singlets at high temperature, additional rotation of the aryl substituents must take place to account for the observed changes to the i-Pr resonances. The molecular structure of 1b is shown in Figure 3; selected bond lengths and angles are collected in Table 1. The compound crystallizes with one and a half molecules in the unit cell, with the latter component on a mirror plane; the independent molecules are essentially identical. Each molecule consists of a three-coordinate bismuth atom with a chelating diamide and terminal chloride. The geometry at the metal atoms is pyramidal with bond angles in the range 71.78(11)− 97.89(9) and 70.74(16)−96.37(9)° for Bi1 and Bi2, respectively. The most acute angle is described by the bite of the ligand and is comparable to that in B (71.8(4)°). The smaller N−Bi−N value at Bi2 reflects the increased Bi−N distances attributed to multiple intermolecular Bi···Cl interactions (vide infra). Bismuth atoms Bi1 and Bi2 are displaced 0.294(6) and 0.229(8) Å “below” the N2Si least-squares plane (defined as being on the same side as the third ligand), which generates fold angles across the nitrogen···nitrogen vector of −170.2(2) and −172.6(3)°, respectively (where the negative

2.132(3) 2.181(3) 2.4857(16) 1.733(3) 71.78(11) 96.34(9) 70.74(16) 93.3(2)

Bi1−N2 Bi1−Cl1 Si1−N1 Si2−N3 N1−Bi1−Cl1 N1−Si1−N2 N3−Bi2−Cl2

2.144(3) 2.5560(10) 1.735(3) 1.736(3) 97.89(9) 92.58(15) 96.37(9)

value indicates a fold below the N2Bi plane). By analogy with a recent definition applied to pyramidal, three-coordinate group 14 metal(II) centers with a ligating β-diketiminate ligand,18 this corresponds to an endo configuration. The crystal structure of 1b shows two types of long-range interactions involving bismuth, forming trimeric [1b]3 units (Figure 4a). Bi1 is involved in bismuth···arene π bonding to the aryl substituent from an adjacent molecule, generating a pseudo piano-stool geometry (Figure 4b). The metal is 3.194(2) Å from the C6 plane with an angle of 7.08(9)° between the ring normal and the metal to ring center vector corresponding to a ring slippage of 0.40 Å. Comparing these data to a series of crystal structures [BiCl3·(C6H6‑nMen)] (n = 0,19 n = 2,20 n = 3 (1,2,3-21 and 1,3,5-22,23) and n = 622,24), we note that the distortion from η6 bonding in 1b is less than in 1,2,3Me3C6H321 (ring slippage 0.75 Å), despite having the same substitution pattern about the aromatic ring. This is attributed to a comparatively large distance between bismuth and the C6 plane in 1b (Bi···C distances 3.350(4)−3.674(4) Å) in comparison with [BiCl3·(1,2,3-Me3C6H3)] (Bi···C distances 3.168(7) to 3.751(8) Å), reflecting the bulk of the amide substituents. There are two intermolecular Bi···Cl interactions between Bi2 and Cl1/Cl1′ (3.199(1) Å; cf. Bi−Cl bonds of 2.556(1) and 2.4857(16) Å), generating a square-based-pyramidal geometry (Figure 4b). Compound B shows a similar intermolecular Bi···Cl interaction that generates a pseudofour-coordinate metal (Figure 1), which was described as an acid−base interaction between an sp3d-hybridized Bi acceptor and a chloride donor from a neighboring molecule.3 Extending this acid−base model to 1b, the bonding is consistent with sp3d2 hybridization of the bismuth, with two chloride donors and a lone-pair of electron trans to the Bi−Cl bond. The bond order (BO) can be calculated using eq 1,25 where r is the experimental distance between the bismuth and chlorine atoms, C

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Figure 5. Schematic representations of haptotropic shifts.

hence this pathway accounts for both observations described above. The NMR spectra of Cp* derivatives 3a,b suggest fluxional processes involving the [C5Me5]− ring similar to those noted for the Cp derivatives. Only one environment is observed for the ring methyl groups for both compounds (3a, δH 1.80 ppm; 3b, δH 2.08 ppm), although in contrast to the data for 2a,b, the SiMe2 resonance for 3a was broad at room temperature. Cooling a sample to 193 K resolved this peak into two singlets, with no apparent broadening of the C5Me5 signal (Figure 6). It was not possible to resolve the peaks in the corresponding bismuth compound at this temperature, although broadening

Figure 4. (a) Association of three Bi(Me2Si{NAr}2)Cl units into [1b]3, showing Bi···π-aryl and Bi···Cl interactions. (b) Local environment at Bi1 and Bi2 in [1b]3.

⎡ (r − r ) ⎤ BO = exp⎢ 0 ⎥ ⎣ B ⎦

(1)

and using values of r0 = 2.423 Å26 and B = 0.39.27 These data give a BO value of 0.14 for the intramolecular Bi···Cl interaction in 1b, notably less than the corresponding interaction in B (BO = 0.20). The cyclopentadienyl derivatives E(Me2Si{NAr}2)(C5R5) (a, E = Sb; b, E = Bi; 2, R = H; 3, R = Me) were synthesized from the reaction of the metal chlorides 1a,b with LiCp or KCp* (Scheme 1). Compound 2a was characterized in solution by 1H and 13C NMR; the remaining derivatives were isolated as colorless (3a) or red (2b and 3b) crystalline solids. The high sensitivity of these compounds to air/moisture (and possibly light) prevented accurate elemental analyses from being obtained. This was particularly evident for the bismuth derivatives, for which sealed NMR samples were observed to deposit black solid material on exposure to ambient light conditions. The 1H and 13C NMR spectra of cyclopentadienyl derivatives 2a,b show a single proton and carbon resonance for the [Cp]− ligand. Both compounds also show one singlet for the SiMe2 groups in the 1H NMR spectrum (2a, δH 0.30 ppm; 2b, δH 0.24 ppm). Assuming the ligands remain coordinated to the metal in solution and only considering intramolecular processes, these results can be explained by haptotropic shifts of the C5 ring, ubiquitous in main-group cyclopentadienyl compounds (Figure 5).2,28 For example, if a high-hapticity intermediate is adopted in which the metal passes through a C 2h -symmetric intermediate, shown in pathway iv, the metal can undergo sigmatropic shifts scrambling the Cp positions. In this case, the average structure will also have equivalent SiMe2 groups and

Figure 6. VT 1H NMR spectra of Sb(Me2Si{NAr}2)(Cp*) (3a): (a) CHMe2; (b) C5Me5; (c) CHMe2; (d) SiMe2; (*) d8-toluene. D

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was observed, suggesting a similar process was occurring, albeit with a lower energy barrier. These data are consistent with a [1,n]-Sb/Bi sigmatropic shift in which the C5 ligand remains bonded to the same face of the metallacycle, illustrated for a [1,2]-Sb/Bi shift in pathway v. This process accounts for the equivalent Cp* resonances and would result in inequivalent SiMe2 groups through retention of the C1 symmetry of the complex. The i-Pr resonances of the aromatic rings are also resolved at low temperature, consistent with multiple fluxional processes occurring in solution (vide supra). The molecular structures of 2b and 3a,b are presented in Figures 7−9, with the primary metal−carbon interaction to the

Figure 9. Thermal ellipsoid plot (30%) of Bi(Me2Si{NAr}2)(Cp*) (3b). Hydrogen atoms are omitted.

Table 2. Selected Bond Lengths (Å) and Angles (deg) for Bi(Me2Si{NAr}2)Cp (2b) Bi−N1 Si−N1 Bi−C27 Bi···C29 Bi···C31 N1−Bi−N2 Bi−N1−Si N1−Bi−C27

Figure 7. Thermal ellipsoid plot (30%) of Bi(Me2Si{NAr}2)(Cp) (2b). Hydrogen atoms are omitted.

2.180(2) 1.728(2) 2.468(8) 3.121(3) 2.708(3) 70.55(8) 97.74(10) 110.23(11)

Bi−N2 Si−N2 Bi···C28 Bi···C30 N1−Si−N2 Bi−N2−Si N2−Bi−C27

2.176(2) 1.729(2) 2.784(3) 3.073(3) 93.37(11) 97.85(9) 99.89(11)

Table 3. Selected Bond Lengths (Å) and Angles (deg) for Sb(Me2Si{NAr}2)(Cp*) (3a) and Bi(Me2Si{NAr}2)(Cp*) (3b) Complex 3a Sb−N Sb−C15 Sb···C17 N−Sb−N′ Sb−N−Si Bi−N1 Si−N1 Bi−C27 Bi···C29 Bi···C31 N1−Bi−N2 Bi−N1−Si N1−Bi−C27

Figure 8. Thermal ellipsoid plot (30%) of Sb(Me2Si{NAr}2)(Cp*) (3a). Hydrogen atoms and benzene solvate are omitted.

2.096(2) Si−N 2.281(3) Sb···C16 3.115(2) 73.36(9) N−Si−N′ 97.03(7) N−Sb−C15 Complex 3b 2.204(4) 1.729(5) 2.397(6) 3.052(6) 2.707(6) 70.11(17) 97.1(2) 102.9(2)

Bi−N2 Si−N2 Bi···C28 Bi···C30 N1−Si−N2 Bi−N2−Si N2−Bi−C27

1.735(2) 2.691(2) 92.39(11) 103.22(7) 2.214(4) 1.733(5) 2.702(6) 3.083(6) 94.3(2) 96.7(2) 105.9(2)

with the values for the bismuth derivatives 2b (31.9(1)°) and 3b (31.6(3)°) being considerably greater than that in the antimony compound 3a (23.25(4)°). This is partially explained by the greater distance between the bismuth atom and the C5 plane (2b, 2.446(2) Å; 3b 2.382(4) Å) in comparison with the corresponding distance in the antimony derivative (2.302(2) Å). There is also an apparent relationship between the orientation of the aromatic rings with respect to the metallacycle, based on whether the organometallic ligand is Cp or Cp*. Large dihedral angles are found in 2b (75.2(1) and 70.5(1)°) and smaller values in 3a (66.2(1)°) and 3b (65.4(2) and 63.5(2)°). These parameters are also influenced by the overall hapticity of the C5 ring at the metal center, discussed in detail below.

C5 ring shown in each case. Selected bond lengths and angles are collected in Tables 2 and 3. The basic molecular unit is common to all structures and consists of a pyramidal metal center bonded to a chelating diamide and a [C5R5]− anion. The combination of bulky ligands is sufficient to shield the metal from long-range interactions, and each compound is monomeric in the solid state. In each case the metallacycle is essentially planar, with the metal atoms located above the least-squares plane defined by N1−Si−N2 (2b, 0.218(4) Å; 3a, 0.129(4) Å; 3b, 0.425(8) Å), corresponding to an exo configuration18 with positive fold angles of +173.0(2), +175.62(6), and +166.4(4)°, respectively. The dihedral angle between the metallacycle and the plane described by the C5 ring appears to be dependent on the metal, E

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bridging chlorides (Figure 11a).27 The cyclopentadienyl ligand was disordered by pseudorotation, and the bonding was assigned as a mixture of η2 and η3 coordination. The crystal structure of the red crystalline modification of BiCp3 shows η1 coordination of each cyclopentadienyl ligand, with weak intermolecular binding of one Cp group to a neighboring bismuth (Figure 11b).32 The primary metal−carbon interaction in 2b (Bi−C(α), 2.468(3) Å) is at the long end of the range observed for η1coordinated Cp ligands in BiCp332 and is more closely matched to the metal−carbon distances in Bi(η2-Cp)Cl2 (Table 4).27 These distances will, however, also be influenced by the relative bulk of the other ligands at the metal center. Thus in 2b, where a bulky diamide ligand is present, longer metal−carbon distances are predicted. Examining the remaining metal− carbon distances, we note that the [Cp]− anion is not symmetrically bonded to the metal center. The Bi···C31 distance in 2b (2.708(3) Å) is considerably shorter than the Bi···C28 distance (2.784(3) Å), tilting the C5 plane of the organometallic ligand relative to the SiN2Bi metallacycle (Figure 12a). The smaller distance is in the 3σ range attributed to a bond in the η3-haptotropic isomer of BiCpCl2 (longest Bi− C bond 2.64(3) Å) but is a distance not considered as contributing to the overall hapticity in the η2-haptotropic isomer (shortest nonbonding Bi···C interaction 2.76(4) Å).27 The assignment of the Cp ligand in 2b as η1 or η2 is therefore ambiguous if it is based only on metal−carbon distances. Sitzmann et al. examined the hapticity of cyclopentadienylsubstituted bismuth dihalides, examining the position of the metal atom relative to the C(α) atom and the pattern of metal−carbon distances, rather than their absolute values.33 Using these criteria, a bismuth atom located above the C(α)···Z vector (Figure 10) with three shorter and two longer Bi···C distances corresponds to η3 coordination. In contrast, the metal shifted “to one side” with two shorter and three longer Bi···C distances is designated as η2 coordination.34 Extending this classification to η1 coordination, a regular increase in the Bi···C distances with C(α) < C(β/β′) < C(γ/γ′) would be anticipated. The Bi···C distances in 2b are consistent with this pattern, suggesting an η1 description of the bonding is appropriate. This criterion does not, however, distinguish between η1(π) or η1(σ) (Figure 13), and previous work has shown the distinction between the two can be small.35 According to Budzelaar et al. the classification of hapticity can be quantified according to the ring slippage d of the metal

DISCUSSION The ηx hapticity of cyclopentadienyl ligands at main-groupmetal centers has been examined using many criteria derived from crystallographic data.1a,29 The area was comprehensively reviewed by Jutzi and Burford in 19991c and more recently by Budzelaar and co-workers1a and continues to elicit interest as new applications for the Cp ligands are developed.29 The key parameters used for comparison in this section have been summarized in Figure 10,30 with the relevant values for compounds 2b and 3a and 3b collected in Tables 4−6, along with data taken from examples of related structures.

Figure 10. Parameters used to analyze the hapticity of the metal−C5 interactions: (a) assignment of hapticity according to displacement of the metal along the C(α)···Z vector, with values of d, according to Budzelaar;1a (b) X = C5 ring centroid, N = normal to C5 least-squares plane (lsp), a, b, c = C−C bond lengths, d = ring slippage, θ = angle between C5 lsp and metal···centroid, ϕ = angle between C5 lsp and C(α) methyl substituent in Cp* derivatives.

Bismuth Cyclopentadienyl Derivatives. Cp−bismuth compounds date back to studies conducted by Fischer in 1960;31 compound 2b is, however, only the third Cp derivative of bismuth to be structurally characterized. The first example, BiCpCl2, forms polymeric chains in the solid state linked by

Table 4. Geometric Parameters for the Bi−Cp Bonding in Bi(Me2Si{NAr}2)(Cp) (2b) and Related Bismuth−Cp Structures 2b E−C(α)/Å E···C(β/β′)/Å E···C(γ/γ′)/Å E···X/Å E···C5(lsp)/Å d/Å θ/deg a, a′/Å b, b′/Å c/Å ΔC−C/Åa

2.468(3) 2.708(3), 3.073(3), 2.584(2) 2.446(2) 0.83 108.8(1) 1.412(5), 1.351(5), 1.385(6) 0.08

2.784(3) 3.121(3)

1.428(6) 1.371(6)

Bi(η1-Cp)3 (i)b

Bi(η1-Cp)3 (ii)b

Bi(η1-Cp)3 (iii)b,c

2.34(2) 2.86(2), 2.86(2) 3.39(2), 3.37(2) 2.738(8) 2.39(1) 1.34 119.3 1.46(2), 1.40(3) 1.40(3), 1.34(3) 1.43(2) 0.12

2.37(2) 3.04(2), 3.14(2) 3.79(2), 3.89(3) 3.065(9) 2.21(3) 2.12 133.7 1.47(3), 1.43(3) 1.40(2), 1.33(2) 1.41(2) 0.14

2.41(2) 2.94(2), 3.05(2) 3.66(2), 3.68(2) 2.952(7) 2.36(2) 1.78 127.1 1.44(3), 1.39(3) 1.40(2), 1.36(2) 1.43(3) 0.08

Bi(η2-Cp)Cl2 2.47(3) 2.48(3), 2.76(4), 2.40(2) 2.34(2) 0.53 102.7 1.36(4), 1.47(4), 1.43(5) 0.11

2.81(4) 2.90(4)

1.38(5) 1.47(4)

Bi(η3-Cp)Cl2 2.38(3) 2.61(3),, 2.64(3) 2.91(3), 2.92(3) 2.39(2) 2.31(3) 0.60 104.5 1.50(5), 1.50(4) 1.44(5), 1.44(4) 1.50(4) 0.06

a

Maximum difference in C−C bond lengths in the C5 ring. b(i), (ii), and (iii) represent the three Cp groups in BiCp3. cCp ring involved in longrange intermolecular π-bonding to a second Bi atom (see Figure 11a). F

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Table 5. Geometric Parameters for the Sb−Cp* Bonding in Sb(Me2Si{NAr}2)(Cp*) (3a) and Related Structures E−C(α)/Å E···C(β/β′)/Å E···C(γ/γ′)/Å E···X/Å E···C5(lsp)/Å d/Å θ/deg a, a′/Å b, b′/Å c/Å ΔC−C/Åb ϕ/deg a

3a

[Sb(η1-Cp*)]4

2.281(3) 2.691(2) 3.115(2) 2.520(1) 2.302(2) 1.03 114.0(6) 1.463(3) 1.381(3) 1.424(4) 0.08 17.6

2.260(4) 3.121(5), 2.921(5) 3.835(6) 3.740(4) 2.990(3) 2.087(5) 2.14 135.7 1.501(6), 1.493(7) 1.361(7), 1.344(7) 1.441(8) 0.16 38.5

Sb(η2-Cp*){Cr(CO)5}2 2.343(3) 3.058(3), 3.446(3), 2.636(2) 2.198(3) 1.46 123.5 1.487(4), 1.412(4), 1.412(4) 0.09

2.394(3) 3.105(3)

1.428(4) 1.397(5)

Sb(η3-Cp*)Cl2 2.254(9) 2.595(10),\ 2.620(11) 2.988(12), 2.951(11) 2.402(5) 2.249(6) 0.84 110.6 1.468(14), 1.496(14) 1.414(15), 1.404(15) 1.405(17) 0.09 14.8

(η3-Cp*)2SbCl (i)a

(η3-Cp*)2SbCl (ii)a

2.507(4) 2.630(4), 2.810(4), 2.409(3) 2.377(3) 0.39 99.3 1.428(7), 1.409(6), 1.427(7) 0.02 8.0

2.527(4) 2.633(5), 2.813(4), 2.396(2) 2.371(2) 0.34 98.2 1.419(8), 1.419(8), 1.394(7) 0.03 9.0

2.671(4) 2.846(6)

1.429(7) 1.418(6)

2.625(4) 2.787(4)

1.408(7) 1.411(6)

(i) and (ii) represent the two Cp* groups in SbCp*2Cl. bMaximum difference in C−C bond lengths in the C5 ring.

Table 6. Geometric Parameters for the Bi−Cp* Bonding in Bi(Me2Si{NAr}2)(Cp*) (3b) and Related Structures 3b E−C(α)/Å E···C(β), E···C(β′)/Å E···C(γ), E···C(γ′)/Å E···X/Å E···C5(lsp)/Å a, a′/Å b, b′/Å c/Å d/Å θ/deg ΔC−C/Åb ϕ/deg a

2.397(6) 2.707(6), 3.083(6), 2.528(3) 2.382(4) 1.445(9), 1.370(9), 1.395(9) 0.84 109.6(2) 0.08 13.9

2.702(6) 3.052(6)

1.454(10) 1.403(11)

Bi(η2-Cp*)I2

Bi(η3-Cp*)Br2

Bi(η5-Cp*)2Cl (i)a

Bi(η5-Cp*)2Cl (ii)a

2.531(11) 2.541(4), 2.668(5) 2.686(4), 2.742(3) 2.349(6) 2.335(7) 1.39(2), 1.42(2) 1.41(2), 1.38(2) 1.42(2) 0.26 96.5 0.04 6.2

2.466(8) 2.584(8), 2.560(8) 2.754(8), 2.771(8) 2.334(4) 2.303(4) 1.444(11), 1.459(11) 1.405(12), 1.418(12) 1.398(12) 0.38 99.3 0.06 9.4

2.625(12) 2.709(12), 2.704(12) 2.831(12), 2.798(12) 2.449(7) 2.438(7) 1.42(2), 1.434(19) 1.43(2), 1.426(16) 1.43(2) 0.23 95.4 0.01 6.1

2.638(12) 2.677(12), 2.719(13) 2.880(13), 2.828(12) 2.452(6) 2.471(5) 1.43(2), 1.43(2) 1.423(18), 1.398(17) 1.44(2) 0.30 97.0 0.04 7.0

(i) and (ii) represent the two Cp*-groups in BiCp*2Cl. bMaximum difference in C−C bond lengths in the C5 ring.

Figure 11. Structurally characterized bismuth−Cp compounds (see text for references).

atom along the C(α)···Z vector;1a such haptotrophic shifts were first examined for lighter organometallic fragments by Hoffman and co-workers.36 The extent of this slippage will influence the angle θ. Thus, an η5 coordination would have a value of d = 0 Å (θ = 90°), with increasing d values attributed to lower hapticities (Figure 11). The slippage for σ-bonded ligands will depend on the radius of the metal, and the overall distance of the metal from the Cp ligand will dictate the magnitude of the angle θ. Both d and θ values in 2b are much less than noted in the η1(σ)-bonded ligands in BiCp3 (Table 4), indicative of a π component to the bonding. The ring slippage (d = 0.83 Å) matches that predicted by Budzelaar for η3 coordination but is greater than that observed in Bi(η3-Cp)Cl2 (0.60 Å). Viewing the structure along the normal (N) to the C5 plane (Figure 12b), we note the bismuth atom deviates from the C(α)···Z vector toward C(31), indicating bias toward η2 coordination. Finally, the C−C bond distances of the C5 ring are considered. In 2b they alternate around the C5 ring, with a range of 1.351(5)−1.428(6) Å corresponding to a ΔC−C value of 0.08 Å. This parameter has been used to indicate the relative delocalization of π-electron density in arsenic and antimony

Figure 12. (a) View along the Bi···Si vector of 3a showing the twisting of the Cp ring relative to the N2SiBi metallacycle. (b) View perpendicular to the C5 ring.

G

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between the C(α) methyl group and the plane of the C5 ring is inconsistent with a σ bond between the metal and the carbon atom, for which a much higher value would be expected (e.g. ϕ = 38.5° for the η1(σ) bonding in [SbCp*]4). We therefore conclude that the Cp* ligand in 3a is bonded to the antimony in a η1(π) bonding mode. There have been three previous reports of structurally characterized compounds containing the Bi−Cp* fragment;38f,40 related neutral and cationic bismuth compounds containing other alkyl-substituted cyclopentadienyl derivatives are known.33,41 Most pertinent to this discussion are BiCp*Br2 and BiCp*I2,40b assigned as containing η3 and η2 coordination of the Cp* rings, respectively, and BiCp*2Cl,38f in which both rings are η5 coordinated. Data derived from these structures are included in Table 6 alongside those for 3b. As for 3a, the pattern of metal···carbon distances in 3b that matches that expected for η1 or η3 bonding. Comparing data with Bi(η3-Cp*)Br2, we note that the Bi···C(β) distances are considerably longer in 3b, with a greater ring slippage (d = 0.84 Å); these data suggest a η1 coordination. The low ΔC−C (0.08 Å) and ϕ (13.9°) values indicate π delocalization in the Cp* ring, consistent with η1(π) bonding. The view in Figure 14, however, shows that there is a more pronounced bias toward η3 bonding in comparison with the analogous antimony compound.

Figure 13. Different geometries of classical η1(π)- and η1(σ)-bonded Cp rings.

compounds of the tetramethylcyclopentdienyl ligand.37 The ΔC−C value for 2b is less than for the η1(σ)-bonded rings of BiCp3 (except the ligand involved in π bonding to a second Bi atom of a neighboring molecule, Figure 11a), indicating greater delocalization around the ring, consistent with a π component to the metal−Cp bond. It is apparent from these data that it is not appropriate to assign a specific hapticity to the bonding of the cyclopentadienyl ligand in 2b, with data indicating an intermediate between η3 coordination and η1(π) and with a bias toward η2 coordination of the Cp ligand. This likely reflects the flat potential energy surface for the rotation of the Cp ring and the influence that minor packing forces can have on the bonding of the Cp rings. Antimony and Bismuth Pentamethylcyclopentadienyl Derivatives:1b Derivatives of the cyclopentadienyl anion have been used previously in the organometallic chemistry of antimony, and structurally characterized examples of neutral35,37,38 and cationic38f,39 compounds have been reported, showing a range of hapticities for the [C5R5]− ring. Most relevant to this discussion are the cyclotetrastibine [SbCp*]4,38c the trimetallic Sb/Cr compound Sb(Cp*){Cr(CO)5}2,38e and the chloro species SbCp*Cl235 and SbCp*2Cl;38f geometric data for these compounds are presented alongside that of 3a in Table 5. Compound 3a crystallizes in the Pnma space group with the molecular unit on a mirror plane that passes through the metal and the SiMe2 group and bisects the Cp* ligand. This enforces a symmetric bonding of the Cp* ligand with respect to the C(α)···Z vector, with a pattern of Sb···C distances in 3a which matches that for η1 or η3 bonding. The ring slippage (d = 1.03 Å) is significantly less than that for the η1(σ)-bonded ligand in [SbCp*]4 (d = 2.14 Å),38c approaching the value of 1.2 Å defined by Budzelaar for a η1(π)-ligated ring. A projection perpendicular to the plane of the C5 ring shows the Sb atom directly above the C(α) position consistent with η 1 coordination of the Cp* ligand (Figure 14). The relatively small ΔC−C value (0.08 Å) suggests delocalization of electron density in the C5 ring consistent with a π component to the bonding. In addition, the relatively acute angle ϕ of 17.6°

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CONCLUSIONS In conclusion, we have isolated a series of antinomy and bismuth compounds supported by the chelating diamide [Me2Si{NAr}2]2−. NMR data for the chlorides E(Me2Si{NAr}2)Cl (E = Sb, Bi) are consistent with a pyramidal metal at room temperature, although at high temperature fluxional processes are occurring that render both faces of the metallacycle equivalent on the NMR time scale. The bismuth chloride associates into trimers in the solid state through a combination of bismuth···arene π bonding and long-range Bi···Cl interactions. Conversion to the Cp and Cp* derivatives was facile, although the resultant organometallic compounds are highly sensitive. VT NMR data are consistent with a number of fluxional processes taking place in solution, most likely involving haptotropic shifts of the C5 ring. The molecular structures of both Cp and Cp* derivatives show a tendency toward low-hapticity rings in the solid state. Detailed structural analysis has shown that assigning a specific hapticity to the ring is not straightforward, due to structures that are intermediate between “classical” ηx-bonding situations.

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ASSOCIATED CONTENT

S Supporting Information *

CIF files giving crystallographic data for 1b, 2b, and 3a,b and a table giving crystal structure and refinement data for 1b, 2b, and 3a,b. 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 for M.P.C.: [email protected]. Present Address

† School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand.

Notes

Figure 14. Views perpendicular to the C5 ring in 3a (left) and 3b (right).

The authors declare no competing financial interest. H

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R. J.; Howard, J. A. K.; Puschmann, H. J. Appl. Crystallogr. 2009, 42, 339−341. (31) Fischer, E. O.; Schreiner, S. Chem. Ber. 1960, 93, 1417−1424. (32) Lorberth, J.; Massa, W.; Wocadlo, S.; Sarraje, I.; Shin, S.-H.; Li, X.-W. J. Organomet. Chem. 1995, 485, 149−152. (33) Sitzmann, H.; Wolmershäuser, G.; Boese, R.; Bläser, D. Z. Anorg. Allg. Chem. 1999, 625, 2103−2107. (34) This definition of η2 coordination, with the metal atom located above the midpoint of the C(α)−C(β) bond, corresponds to a 144° anticlockwise rotation of the η2 coordination illustrated in Figure 10, with a renaming of the carbon atoms such that α → γ′, β → β, β′ → γ,γ → α, and γ′ → β. (35) Bartlett, R. A.; Cowley, A.; Jutzi, P.; Olmstead, M. M.; Stammler, H.-G. Organometallics 1992, 11, 2837−2840. (36) Anh, N. T.; Elian, M.; Hoffman, R. J. Am. Chem. Soc. 1978, 100, 110−116. (37) Chmely, S. C.; Hanusa, T. P.; Rheingold, A. L. Organometallics 2010, 29, 5551−5557. (38) (a) Birkhahn, M.; Krommes, P.; Massa, W.; Lorberth, J. J. Organomet. Chem. 1981, 208, 161−167. (b) Frank, W. J. Organomet. Chem. 1991, 406, 331−341. (c) Kekia, O. M.; Jones, R. L., Jr.; Rheingold, A. L. Organometallics 1996, 15, 4104−4106. (d) Ehleiter, Y.; Wolmershäuser, G.; Sitzmann, H. Z. Anorg. Allg. Chem. 1996, 622, 923−930. (e) Schiffer, M.; Johnson, B. P.; Scheer, M. Z. Anorg. Allg. Chem. 2000, 626, 2498−2504. (f) Wiacek, R. J.; Jones, J. N.; Macdonald, C. L. B.; Cowley, A. H. Can. J. Chem. 2002, 80, 1518− 1523. (g) Konchenko, S. N.; Pushkarevsky, N. A.; Virovets, A. V.; Scheer, M. Dalton Trans. 2003, 581−585. (39) Sitzmann, H.; Ehleiter, Y.; Wolmershäuser, G.; Ecker, A.; Uffing, C.; Schnockel, H. J. Organomet. Chem. 1997, 527, 209−213. (40) (a) Ü ffing, C.; Ecker, A.; Baum, E.; Schnöckel, H. Z. Anorg. Allg. Chem. 1999, 625, 1354−1356. (b) Monakhov, K. Y.; Zessin, T.; Linti, G. Organometallics 2011, 30, 2844−2854. (41) (a) Sitzmann, H.; Wolmershäuser, G. Chem. Ber. 1994, 127, 1335−1342. (b) Sitzmann, H.; Wolmershäuser, G. Z. Naturforsch., B: Chem. Sci. 1997, 52, 398−400.

ACKNOWLEDGMENTS We thank the EPSRC UK National Crystallographic Service at the University of Southampton for the collection of the crystallographic data for 1b and Dr. S. Mark Roe (University of Sussex) for the collection of the crystallographic data for 2b.

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REFERENCES

(1) (a) Budzelaar, P. H. M.; Engelberts, J. J.; van Lenthe, J. H. Organometallics 2003, 22, 1562−1576. (b) Jutzi, P.; Reumann, G. Dalton Trans. 2000, 2237−2244. (c) Jutzi, P.; Burford, N. Chem. Rev. 1999, 99, 969−990. (2) Jutzi, P. Chem. Rev. 1986, 86, 983−996. (3) Veith, M.; Bertsch, B. Z. Anorg. Allg. Chem. 1988, 557, 7−22. (4) (a) Veith, M.; Bertsch, B.; Huch, V. Z. Anorg. Allg. Chem. 1988, 559, 73−88. (b) Caminade, A.-M.; Veith, M.; Huch, V.; Malisch, W. Organometallics 1990, 9, 1798−1802. (5) Lu, D.-Y.; Yu, J.-S. K.; Kuo, T.-S.; Lee, G.-H.; Wang, Y.; Tsai, Y.C. Angew. Chem., Int. Ed. 2011, 50, 7611−7615. (6) Hill, M. S.; Hitchcock, P. B. Organometallics 2002, 21, 3258− 3262. (7) Zhu, X.; Fan, J.; Wu, Y.; Wang, S.; Zhang, L.; Yang, G.; Wei, Y.; Yin, C.; Zhu, H.; Wu, S.; Zhang, H. Organometallics 2009, 28, 3882− 3888. (8) Pi, C.; Wan, L.; Gu, Y.; Wu, H.; Wang, C.; Zheng, W.; Weng, L.; Chen, Z.; Yang, X.; Wu, L. Organometallics 2009, 28, 5281−5284. (9) Yang, D.; Ding, Y.; Wu, H.; Zheng, W. Inorg. Chem. 2011, 50, 7698−7706. (10) Tsai, Y.-C.; Lu, D.-Y.; Lin, Y.-M.; Hwang, J.-K.; Yu, J.-S. K. Chem. Commun. 2007, 4125−4127. (11) Luthin, W.; Stratmann, J.-G.; Elter, G.; Meller, A.; Heine, A.; Gornitzka, H. Z. Anorg. Allg. Chem. 1995, 621, 1995−2000. (12) Yang, D.; Guo, J.; Wu, H.; Ding, Y.; Zheng, W. Dalton Trans. 2012, 41, 2187−2194. (13) Day, B. M.; Dyer, P. W.; Coles, M. P. Dalton Trans. 2012, 41, 7457−7460. (14) Ma, X.; Ding, Y.; Roesky, H. W.; Sun, S.; Yang, Z. Z. Anorg. Allg. Chem. 2013, 639, 49−52. (15) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal Structures; University of Göttingen, Göttingen, Germany, 1997. (16) Breunig, H. J.; Ghesner, I.; Ghesner, M. E.; Lork, E. Inorg. Chem. 2003, 42, 1751−1757. (17) Dixon, D. A.; Arduengo, A. J., III; Fukunaga, T. J. Am. Chem. Soc. 1986, 108, 2461−2462. (18) (a) Harris, L.-A.; Coles, M. P.; Fulton, J. R. Inorg. Chim. Acta 2011, 369, 97−102. (b) Tam, E. C. Y.; Maynard, N. A.; Apperley, D. C.; Smith, J. D.; Coles, M. P.; Fulton, J. R. Inorg. Chem. 2012, 51, 9403−9415. (19) Frank, W.; Schneider, J.; Müller-Becker, S. J. Chem. Soc., Chem. Commun. 1993, 799−800. (20) Müller-Becker, S.; Frank, W.; Schneider, J. Z. Anorg. Allg. Chem. 1993, 619, 1073−1082. (21) Frank, W.; Reiland, V. Acta Crystallogr. 1998, C54, 1626−1628. (22) Schier, A.; Wallis, J. M.; Müller, G.; Schmidbaur, H. Angew. Chem., Int. Ed. Engl. 1986, 25, 757−759. (23) Schmidbaur, H.; Wallis, J. M.; Nowak, R.; Huber, B.; Müller, G. Chem. Ber. 1987, 120, 1837−1843. (24) Schmidbaur, H.; Nowak, R.; Schier, A.; Wallis, J. M.; Huber, B.; Müller, G. Chem. Ber. 1987, 120, 1829−1835. (25) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244− 247. (26) Haaland, A.; Hougan, J.; Samdal, S.; Tremmel, J. Acta Chem. Scand. 1988, 42A, 409−412. (27) Frank, W. J. Organomet. Chem. 1990, 386, 177−186. (28) Jutzi, P. Adv. Organomet. Chem. 1986, 26, 217−295. (29) Mebs, S.; Chilleck, M. A.; Grabowsky, S.; Braun, T. Chem. Eur. J. 2012, 18, 11647−11661. (30) The values of the parameters were calculated from crystallographic data using OLEX-2: Dolomanov, O. V.; Bourhis, L. J.; Gildea, I

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