Sn bidentate Lewis acid

Inorganica Chimica Acta 364 (2010) 162–166

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Binding of a 2-pyridyl moiety to a B/Sn bidentate Lewis acid Ramez Boshra, Ami Doshi, Krishnan Venkatasubbaiah, Frieder Jäkle ⇑ Department of Chemistry, Rutgers University-Newark, 73 Warren Street, Newark, NJ 07102, USA

a r t i c l e

i n f o

Article history: Available online 17 September 2010 This paper is dedicated to Prof. Arnold L. Rheingold. Keywords: Bidentate Lewis acids Boron Ferrocene Heterocycles Reverse chelate Tin

a b s t r a c t Reaction of the bidentate Lewis acid 1,2-Fc(SnMe2Cl)(B(Me)Cl) (1, Fc = 1,2-ferrocenylene) with 2-trimethylstannylpyridine gave a complex (2), in which the 2-pyridyl group forms a bridge between the Lewis acidic tin and boron centers. The structure of 2 was confirmed by multinuclear and two-dimensional NMR, single crystal X-ray diffraction, and elemental analysis. The crystal structure shows that the pyridyl moiety is positioned endo relative to the substituted Cp ring and UV–Vis spectroscopy revealed an enhancement of the ferrocene-centered dd transition in comparison to related complexes with pyridine as a terminal ligand. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Bidentate Lewis acids continue to attract much interest due to their superior characteristics in the binding of anions and neutral Lewis basic substrates [1]. For instance, highly Lewis acidic bifunctional organoborane Lewis acids are advantageous in the activation of catalysts due to their outstanding ability to effectively abstract ligands from transition metal complexes [2], whereas polydentate organotin, organoboron, organoaluminum, and organomercury Lewis acids have proven useful for binding and ultimately the recognition of anions such as fluoride or cyanide [3]. Moreover, strong Lewis acid–base interactions have been shown to lead to the formation of stable heterocycles with unusual electronic or optical properties [4,5]. An interesting sub-class among these ‘‘reverse chelate complexes” [6] is derived from heteronuclear bidentate Lewis acids, in which two different Lewis acidic sites act in concert in the binding of a given substrate [7]. We have been exploring the binding behavior of the heteronuclear bidentate Lewis acid 1,2Fc(SnMe2Cl)(B(Me)Cl) and related ferrocene-based planar chiral Lewis acids [8–13]. With respect to the binding of neutral nucleophiles we showed that complexes A are formed upon treatment with an equimolar amount of pyridine. Of interest is that the pyridine base selectively attacks from the exo-side and the boron-bound chlorine atom is weakly bridging to the Lewis acidic tin center, thereby defining the stereochemistry at the chiral boron center generated [10]. We also explored the coordination of the ⇑ Corresponding author. E-mail address: [email protected] (F. Jäkle). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.09.010

fluoride anion, which preferentially adopts a bridging position between boron and tin centers [11,12]. In this paper we report on the formation of a complex, in which a 2-pyridyl moiety is bridging the two Lewis acidic sites. We also demonstrate that one isomer is preferentially formed, in which the nitrogen donor atom forms a dative bond to boron and the ligand system is placed in the endo-position of the ferrocene framework.

2. Results and discussion In an effort to introduce a ligand that is able to bridge the Lewis acidic boryl and stannyl groups, we initially treated the bidentate Lewis acid 1 with pyridazine. However, a single crystal X-ray structure of the resulting complex 1pyridazine revealed that the structure is almost identical to that of 1pyridine (complex A with R@H in Chart 1) [10], with the pyridazine as terminal ligand attached to the more Lewis acidic boron center (see Fig. S3, Supplementary Material). Hence, 1pyridazine was not further studied. We then reasoned that covalent attachment of the homologous 2-pyridyl ligand through a B–C bond might favor formation of a bridging structure with creation of a N–Sn dative bond. Thus, we reacted the racemate of 1 with 2-Me3SnPy. Unexpectedly, the transmetalation reaction took place at the supposedly less reactive Sn–Cl rather than the B–Cl bond, and the Sn bound pyridine adduct 2 was obtained as dark orange crystals in ca. 61% isolated yield after recrystallization (Scheme 1). The formation of 2 was confirmed by multinuclear NMR spectroscopy, single-crystal X-ray diffraction, and elemental analysis.

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R

Me Cl

Sn

Cl

Me

N B

X

Me

Me

Sn

F

R B Me

Me

Fe

Fe

A R = H, NMe2

B X = Cl, F R = F, Ph Chart 1.

The unusual substitution pathway is likely a result of initial complexation of pyridine to the boron center (cf. compounds A in Chart 1), which places the 2-Me3SnPy ligand in close proximity to the tin center. Evidence for formation of an adduct 1PySnMe3 was gathered from 1H and 11B NMR analysis of the reaction mixture in CD3CN immediately after mixing the two components. Most importantly, an upfield shifted 11B NMR signal at 15.4 ppm is consistent with Lewis acid–base complexation. Notable is also that one of the Sn–Me groups of the ferrocene species is strongly upfield shifted to 0.25 ppm, while the Cp signals are in a similar range as for the starting material, suggesting that some structural reorganization at the boron and tin environments occurs. On the path to product formation, the pyridyl ring may then bridge to the tin center via p-bonding, effectively replacing the bridging chlorine substituent. This type of interaction has been observed in the crystal structure of 1,2-Fc(SnMe3)(B(Me)Ph) [9] and proposed to be critically important in the fluoride-promoted aryl group transfer from boron to tin in the reaction of 1,2-Fc(SnMe2Cl)(B(Me)Ar) with KF to give complexes 1,2-Fc(SnMe2Ar)(B(Me)F) [12]. The final step involves formation of the Sn–C bond to the pyridyl ring with concomitant release of Me3SnCl, which was generated as a by-product of the reaction based on NMR studies on the crude mixture. The above mechanistic proposal nicely explains the relatively low temperature at which the reaction takes place, facilitated by the intramolecular rearrangement [14]. X-ray quality single crystals of 2 were obtained from a concentrated solution in CH3CN and Et2O at 37 °C. Selected geometric

Me B Cl

N

Sn Me Fe Cl

Me

Table 1 Comparison of selected geometric parameters of compounds 2 and 1pyridine [10].

B1–N1 (Å) B1–Cl1 (Å) C1–C2–B1 (deg) C2–C1–Sn1 (deg) 180° – angle (CpCent–C2–B1)a 180° – angle(CpCent–C1–Sn1)a (C1–C5)//(C6–C10) (deg)b

Fe

N

SnMe3

SnMe3 B Me Cl Sn Me Fe Cl Me

–Me3SnCl

Me

Reflections collected Independent reflections Absorption correction Refinement method Data/restraints/parameters Goodness-of-fit (GOf) on F2 Final R indices [I > 2r(I)]a R indices (all data)a Peak/hole (e Å3)

Cl

B N Me Sn SnMe3 Fe Cl Me

1. PySnMe3 Scheme 1. Synthesis of compound 2 and proposed mechanism.

1.635(3) 1.923(3) 127.6(2) 126.77(12) 4.0 (up) 2.9 (dn) 2.79

2 Empirical formula Molecular weight T (K) Wavelength (Å) Crystal system Space group a (Å) b (Å) c (Å) b (°) V (Å3) Z qcalc (g cm3) l (mm1) F(0 0 0) Crystal size (mm) h range (°) Limiting indices

2

N

1pyridine [10]

1.629(4) 1.962(3) 127.6(3) 117.3(2) 3.4 (up) 10.9 (dn) 0.91

Table 2 Crystal data and structure refinement details of 2.

Me

1

2

a The position of the boryl and stannyl group relative to the Cp ring is indicated as ‘‘dn” for bending towards Fe and as ‘‘up” for bending away from Fe; CpCent refers to the centroid of the substituted Cp ring. b Cp tilt angle.

Cl Me B Me N Sn

SnMe3

–Me3SnCl

parameters for 2 are summarized in Table 1 and structure refinement details are provided in Table 2. The molecular structure features the 6-membered heterocycle pointing in the direction of the free Cp ring (Fig. 1). Although intramolecular complexation of pyridyl-type ligands to boron has been previously described [15], selective formation of a compound with the pyridyl ring in endoposition, pointing toward the Fe center is unusual and interesting in its own right. Moreover, pyridine complexation in 2 offers a rare opportunity to compare the inter- versus intramolecular complexation modes to the boron center in 1-stannyl-2-borylferrocenes. Table 1 provides a comparison of compounds 1pyridine (A with R@H in Chart 1) and 2. The B–N bond distance for 2 (1.629(4) Å) is similar to that of 1.635(3) Å for 1pyridine. However, some characteristic differences between the complexation modes are apparent. Most notably, as a result of the intramolecular pyridine complexation, the bond distances and angles around the Sn center are distorted. The C2–C1–Sn1 bond angle of 117.3(2)° is significantly less than the 126.77(12)° measured for 1pyridine (versus 126° expected for the substituent of a 5-membered ring system). Furthermore, the Sn center in 2 is sharply bent downward with

a

C18H21BClFeNSn 472.16 100(2) 1.54178 monoclinic P2(1)/n 10.1523(2) 9.8596(2) 18.2482(4) 105.1190(10) 1763.38(6) 4 1.778 19.210 (Cu Ka) 936 0.34  0.26  0.22 4.55–68.10 10 6 h 6 12 11 6 k 6 11 21 6 l 6 21 13711 3177 [Rint = 0.0410] numerical full-matrix least-squares on F2 3177/0/212 1.139 R1 = 0.0339, wR2 = 0.0870 R1 = 0.0340, wR2 = 0.0872 1.823 and 0.664

R1 = ||Fo|  |Fc||/|Fo|; wR2 ¼ ðR½wðF 2o  F 2c Þ2 =R½wðF 2o Þ2 Þ1=2 .

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3 4 5

Fe

Cl Me B Me 6 N Sn 5 Me

3

2-major

4

Me' Me' 3' B Cl 6' N 4' Sn 5' 5' Fe Me' 3' 4' 2-minor

Chart 2. Proposed structures of two isomers of 2 in solution. Strong NOE signals are indicated with arrows.

Fig. 1. Molecular structure of 2 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å), interatomic distances (Å), and angles (deg): Sn1–C14 2.199(3), Sn1–C1 2.110(3), Sn1–C12 2.136(4), Sn1–C13 2.130(3), B1–C2 1.601(5), B1–C11 1.608(5), B1–N1 1.629(4), B1– Cl1 1.962(3), C1–Sn1–C12 116.25(13), C12–Sn1–C13 109.28(14), C1–Sn1–C13 121.59(13), C1–Sn1–C14 95.01(12), C12–Sn1–C14 105.07(13), C13–Sn1–C14 106.71(12), C2–B1–Cl1 106.5(2), C2–B1–C11 114.1(3), C2–B1–N1 113.0(3), C11– B1–N1 112.7(3), C11–B1–Cl1 108.1(2), N1–B1–Cl1 101.2(2), C1–C2–B1 127.6(3), C2–C1–Sn1 117.3(2).

the dip angle (180° – angle(CpCent–C1–Sn1)) measuring 10.9° compared with 2.9° for 1pyridine. We further examined the structure of 2 by solution NMR spectroscopy. For this purpose, single crystals of 2 were dissolved in CDCl3. The 11B NMR in CDCl3 displays a single signal at 6.7 ppm, which confirms complexation of pyridine to the boron center in solution [16]. However, in the 119Sn NMR two sharp signals are found at 34.3 (small) and 37.7 ppm (large), both of which are in a region typical of dialkyldiaryltin compounds [17]. Similarly, the 1H NMR shows two sets of signals with a ratio of ca. 9:1 due to the presence of two different isomers in solution [18]. There are several possible reasons for such an observation: (1) The pyridyl complexation to boron might be dynamic and one set of signals corresponds to the open form and another to the bridged pyridyl complex. However, the absence of a second downfield-shifted 11B NMR resonance speaks against an open-chain form. (2) The chloride substituent on boron reversibly dissociates with formation of a cationic borane species. Again, a second 11B NMR signal would be expected in the downfield region of the spectrum. To further address this possibility we also recorded 1H NMR spectra in different solvents. At ambient temperature, the ratio of the major to the minor isomer increased with increasing solvent polarity from 7:1 in C6D6 to 9:1 in CDCl3, and 20:1 in CD3CN. If the minor isomer corresponded to an ionic species, the opposite trend would have been expected. (3) The chirality at the boron center, in combination with the inherent planar chirality of 2, gives rise to diastereomers with different orientation of the B–Cl and B–Me substituents relative to the Cp plane (exo or endo-position; 2-major and 2-minor in Chart 2). The following arguments further support the notion that this is most likely the origin of the observed isomer formation in solution. Careful inspection of the 1H NMR spectrum in CDCl3, aided by an H,H-COSY experiment, shows that the Cp proton adjacent to the boron center experiences a downfield shift from 4.54 ppm for the major isomer to 4.84 ppm for the minor isomer, while the B– Me signal shows a significant upfield shift from 0.90 to 0.43 ppm for the minor isomer. At the same time, the pyridyl proton in 6-position moves from 9.12 to 9.86 ppm for the minor isomer. Given that most other signals are not shifted significantly, these changes clearly suggest that the two isomers only differ in the

substitution pattern on boron as illustrated in Chart 2. Consistent with the presence of two isomers of very similar structure is also that the free Cp ring resonances for the major and minor isomers are found at 3.50 and 3.77 ppm, respectively. They are both upfield shifted in comparison to that of the starting material (3.83 ppm), presumably due to the shielding effect of the pyridine ring in endo-position. This observation speaks against the formation of an isomer in which the pyridyl moiety is in the exo-position of ferrocene. H,H-NOESY NMR spectroscopy was used to further study the two isomers. From the spectrum acquired at room temperature, using a mixing time of 500 ms, exchange peaks between the pyridyl protons H6/H60 , the Cp–H3/Cp–H30 protons, and the B–Me/B– Me0 protons are clearly evident (Fig. 2). While this further confirms the presence of two interchanging isomers, information on the conformation of the individual isomers is masked by the fact that exchange-relayed NOE peaks are also present (see Fig. S2 in the Supporting Information). We therefore acquired another dataset at 50 °C. At that temperature, no exchange was evident on the NMR time scale. However, the isomer ratio changed to 97:3,

Fig. 2. H,H-NOESY spectrum of 2 in CDCl3 acquired at 25 °C with a 500 ms mixing time. The most prominent exchange and NOE peaks are marked.

R. Boshra et al. / Inorganica Chimica Acta 364 (2010) 162–166

Fig. 3. Comparison of the absorption spectra of 1, 1pyridine, and 2 in CH2Cl2.

essentially precluding further studies on the minor isomer. On the other hand, unequivocal assignments for the major isomer are possible. A strong NOE crosspeak between the pyridyl proton H6 and the B–Me group is consistent with the proposed cyclic structure, whereas crosspeaks between the Cp protons and the B–Me and one of the Sn–Me groups support the configuration depicted in Chart 2 for the isomer 2-major. A final comment concerns the noticeably darker orange color observed for 2 in dichloromethane solution. The UV–Vis spectrum reveals an absorption band for both complexes, 1pyridine and 2, at kmax = 448 nm (Fig. 3). However, a hyperchromic effect is observed with the molar absorptivity (e) for 2 measuring 390 L mol1 cm1 (versus 190 for 1pyridine). Such an effect is typically observed when charge transfer contributes significantly to the primarily ferrocene-centered dd transition. However, in the case of charge transfer contributions, a bathochromic shift typically accompanies the hyperchromic effect. Therefore, the observed hyperchromic effect is more likely a result of lowering of the symmetry and its effect on the strict adherence to the Laporte selection rule. 3. Conclusion We have successfully used the 2-pyridyl ligand as a bridging group between the Lewis acidic tin and boron centers of the bidentate Lewis acid 1. Formation of a covalent Sn–C bond in 2 appears to be critical for stabilization of the resulting heterocycle, as the related complex with pyridazine as a bifunctional Lewis base adopts an open structure. Indeed, the geometric parameters deduced from the X-ray crystal structure of complex 2 reveal a certain degree of strain, which is reflected in unusual bond angles around the tin center, including the bending of the stannyl group out of the Cp plane toward the Fe center. An enhancement of the dd-transition band at ca. 450 nm relative to that for 1pyridine with its open structure is also evident.

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dual broadband gradient probe (Nalorac, Varian Inc., Martinez, CA). 1H NMR spectra were referenced internally to the residual solvent signal; 119Sn and 11B NMR spectra were referenced externally to SnMe4 (d = 0) and BF3. Et2O (d = 0) in C6D6, respectively. A mixing time of 500 ms was used for the two-dimensional NOESY measurements, which were obtained with the standard pulse sequence that was followed by a 90° pulse flanked by two 5 G/cm gradient for dephasing any residual transverse magnetization and suppressing potential artifacts, before the relaxation delay. UV–Vis absorption data were acquired on a Varian Cary 500 UV–Vis–NIR spectrophotometer. Solutions (103 and 104 M) were prepared using a microbalance (±0.1 mg) and volumetric glassware and then charged into quartz cuvettes with sealing screw caps (Starna) inside the glove box. The elemental analysis was performed by Quantitative Technologies Inc. Whitehouse, NJ.

4.2. Reaction of compound 1 with 2-Me3SnPy: Synthesis of 2 2-Me3SnPy (80 mg, 0.33 mmol) in CD3CN (2 mL) was added dropwise to compound 1 (160 mg, 0.37 mmol) in CD3CN (2 mL) while stirring. A sample was withdrawn, diluted with CD3CN, and studied by NMR spectroscopy. 1H NMR (500 MHz, CD3CN, 25 °C): d = 8.40 (br, 1 H, Py–H), 7.00 (d, J = 7.0 Hz, 1 H, Py–H), 7.88 (br, 1 H, Py–H), 7.37 (br, 1 H, Py–H), 4.54 (br, 1 H, Cp–H), 4.57 (br, 1 H, Cp–H), 4.53 (br, 1 H, Cp–H), 4.21 (s, 5 H, Cp), 0.88 (br, 3 H, Sn– Me), 0.61 (s, 3 H, B–Me), 0.53 (s/d, J (117/119Sn, H) = 56/58 Hz, 9 H, SnMe3), 0.25 (br, 3 H, Sn–Me). 11B NMR (160.3 MHz, CD3CN, 25 °C): d = 15.4 (w1/2 = 320 Hz). The reaction mixture was kept stirring for 24 h at RT, followed by removal of all volatile components under high vacuum. The residual solid material was washed with hexanes (3  2 mL) and then extracted with Et2O (3  3 mL). Dark orange crystals were obtained from the combined extracts after 2 days at 38 °C. Yield: 95 mg (0.20 mmol; 61% yield). For 2major: 1H NMR (500 MHz, CDCl3, 25 °C): d = 9.12 (d, J = 5.5 Hz, 1 H, Py–H6), 7.97 (dd, J = 7.0 Hz, 1.0 Hz, 1 H, Py–H3), 7.95 (m, 1 H, Py–H4), 7.58 (m, 1 H, Py–H5), 4.54 (dd, J = 1.0, 2.5 Hz, 1 H, Cp– H3), 4.46 (pst, J = 2.5 Hz, 1 H, Cp–H4), 4.35 (dd, J = 1.0, 2.5 Hz, 1 H, Cp–H5), 3.50 (s, 5 H, Cp), 0.90 (s, 3 H, B–Me), 0.76 (s/d, J (117/119Sn, H) = 63/65 Hz, 3 H, exo Sn–Me), 0.66 (s/d, J (117/119Sn, H) = 51/53 Hz, 3 H, endo Sn–Me); 119Sn NMR (186.4 MHz, CDCl3, 25 °C): 37.7. For 2-minor: 1H NMR (500 MHz, CDCl3, 25 °C): d = 9.86 (d, J = 6.0 Hz, 1 H, Py–H6), 7.86 (m, 2 H, Py–H3,4), 7.59 (overlapped, 1 H, Py–H5), 4.82 (dd, J = 1.0, 2.0 Hz, 1 H, Cp–H3), 4.45 (overlapped, 1 H, Cp–H4), 4.27 (dd, J = 1.0, 2.0 Hz, 1 H, Cp–H5), 3.77 (s, 5 H, Cp), 0.80 (s, 3 H, Sn–Me), 0.55 (s, 3 H, Sn– Me), 0.43 (s, 3 H, B–Me); 119Sn NMR (186.4 MHz, CDCl3, 25 °C): 34.3. For 2 (isomer mixture): 11B NMR (160.3 MHz, CDCl3, 25 °C): d = 6.7 (w1/2 = 310 Hz); UV–Vis (CH2Cl2): kmax = 448 (e = 390 L mol1 cm1), 329 nm (sh, e = 310 L mol1 cm1). Elemental Anal. Calc. C, 45.79; H, 4.48; N, 2.97. Found: C, 45.91; H, 4.34; N, 2.98.

4. Experimental 4.3. Crystallography 4.1. Reagents and general methods Compound 1 [8,9] and 2-Me3SnPy [19] were prepared according to literature procedures. All reactions and manipulations were carried out under an atmosphere of prepurified nitrogen using either Schlenk techniques or an inert-atmosphere glove box (Innovative Technologies). Acetonitrile was dried over CaH2 and diethyl ether was distilled from Na/benzophenone prior to use. The 499.9 MHz 1H NMR, 186.4 MHz 119Sn NMR, and 160.3 MHz 11 B NMR spectra were recorded on a Varian INOVA NMR spectrometer (Varian Inc., Palo Alto, CA) equipped with a 5 mm

Data for 1pyridazine and 2 were collected on a Smart Apex CCD diffractometer at 100 K using Cu Ka (1.54178 Å) radiation. The structures were solved by direct methods and refined by full-matrix least squares based on F2 with all reflections (SHELXTL V5.10; G. Sheldrick, Siemens XRD, Madison, WI). Non-hydrogen atoms were refined with anisotropic displacement coefficients, and hydrogen atoms were treated as idealized contribution. SADABS (Sheldrick, G.M. SADABS (2.01), Bruker/Siemens Area Detector Absorption Correction Program; Bruker AXS: Madison, WI, 1998) absorption correction was applied.

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5. Supplementary material Details of the single crystal X-ray analyses and structure plot of 1pyridazine; 1H NMR of compound 2 and expansions of the twodimensional NOESY NMR spectrum of 2. Acknowledgements

[4]

[5]

We are grateful to the Petroleum Research Fund administered by the American Chemical Society and the National Science Foundation for financial support (CHE-0809642, NSF-CRIF 0116066). F.J. thanks the Alfred P. Sloan Foundation for a research fellowship and the Alexander von Humboldt Foundation for a Friedrich Wilhelm Bessel Award. We thank Dr. Chengzhong Cui for the preparation of 2-PySnMe3, Jiawei Chen for help with some of the experimental work, and Dr. Lazaros Kakalis for the acquisition of the 2D NMR spectra. Appendix A. Supplementary material CCDC 775665 and 775666 contain the supplementary crystallographic data for 2 and 1pyridazine, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2010.09.010.

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