Structural Probing on the Sn-CC5 ring Bond of the Sn(II) Metallocenes in Both the Solid State and the Temperature-dependent Solution Relaxation State①

ZHOU Pan LI Jian-Cheng ZHANG Yi-Wei LI Bin ZHU Hong-Ping



Structural Probing on the Sn-CC5 ringBond of the Sn(II) Metallocenes in Both the Solid State and the Temperature-dependent Solution Relaxation State①

ZHOU Pan LI Jian-Cheng ZHANG Yi-Wei LI Bin ZHU Hong-Ping②

(College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China)

Various bond modes of the M-CC5 ringexist in metallocene compounds of group 14 heavier elements, mostly due to an intricate interaction between the lone electron pairs at the M center and the 6-electrons of the C5ring. The tin(II) metallocene complexes LSnR (L = HC[CMe(N-2,6-Pr2C6H3)]2, R = cyclopentadienyl, C5H5(1); indenyl, C9H7(2); fluorenyl, C13H9(3)) stabilized by the-diketiminato ligand were prepared and utilized in the study on their solid and solution state structures. X-ray single-crystal diffraction data revealed an1-mode of the Sn-CC5 ringbond in each 1～3. However, the room temperature1H NMR spectral studies disclosed such a fluxional bonding mode in solution. The119Sn NMR studies suggested a quadruple coordination nature of the Sn center in 1 while the triple coordination manner was for the Sn atom in both 2 and 3. Then the variable-temperature (25～–75 ℃)1H NMR spectral studies for each 1～3 were performed, which detected the relaxation state structures of 1～3 at lower temperature. All of these results indicate a stereochemical activity of the lone electron pairs at the tin(II) atom that definitely has an electronic interaction with the 6-electrons of the C5ring. The observed Sn-CC5 ringbond modes appear influenced by either the metallocene size or the compound state existed.

Sn(II) metallocene, Sn–CC5 ringbond mode, single-crystal structure, NMR solution structure, stereochemical activity of the lone pair electrons;

1 INTRODUCTION

Since the disclosure of Cp2Fe, the first ‘sand- wich’ compound, in the early 1950s[1, 2], the bon- ding character of the M-C5-ring (abbreviated as M-C5) has always been the research curiosity and interest because of the diverse M-C5- andn(n = 1～5) bonding modes. The monoanionic C5-ring contains 6-electrons in the valent shell and has been widely utilized as the ligand to particularly stabilize the metal center in a mononuclear state[3, 4]. The molecular orbital (MO) calculations have revealed the intricate orbital hybridizations for this Cp and the derivatives[5, 6]. It has been shown that an M-5-C5bonding mode usually dominates in theandblock metal coordination compounds[7], whereas the varied bond modes are found in theandblock elemental complexes. We recently have reported the synthesis of the-diketiminato ligand stabilized germylene cyclopentadienyl compound LGeCp (L = HC[C(Me)N-2,6-Pr2C6H3]2) and its tellurized compound LGe(Cp)=Te and further the GeCl2adduct LGe(Cp)=Te(GeCl2)[8]. Solid state structural characterization confirmed a Ge–1–Cp bond in the former one compound while the Ge–– Cp bond in the latter two complexes, whose formation occurred through the distinguishable 1,2-H- and 1,3-H-shifts over the Cp ring.Probably, there existed a strong interaction between the lone electron pair at the Ge center and the 6-electrons over the Cp ring in LGeCp, as in the latter two complexes the lone pair had been used up to form the Ge=Te bond. We became interested in detecting such an interaction, that is, in what kind of chemical phenomena observed is such an interaction shown for forming the related compound. Studies on this aspect are rare probably due to the lack of suitable species. We now focused on the stannylene metal- locene compounds. Currently, only a few stannylene metallocenes have been reported. Complexes (5-C5Ph5)2SnII(180°)[9], (5-C5Pr5)2SnII(180°)[10], and [2-3-1,3-(SiMe3)2C9H5]2SnII(175.9°)[11]have been documented with the CC5 ring(centroid)–Sn–CC5 ring(centroid)bond angles close to the linear angle. This suggests the burying of the Sn(II) lone pair in function. However, compounds (5-C5R5)2Sn (R = H, 148.0 and 143.7[12]; R = Me, 143.6 and 144.6° (two independent molecules in each))[13]and (5- C5Me5)Sn[5-1,3-(SiMe3)2C9H5] (151.6°)[11]were repor- ted to have the notably nonlinear CC5 ring(centroid)–Sn–CC5ring(centroid)angles, indicating the stereochemical activity of the Sn(II) lone pair. Moreover, various Sn–n-Cp bond fashions were found in (5-C5H5)SnCl[14], [(3-C5H5)SnN=C(NMe2)2]2[15], and [(1-C5H5)SnN(SiMe3)P(Pr2)=CH]2-2,3-C4H2N2[16]. Herein, we continued to utilize the-diketiminato ligand and prepared the new Sn(II) C5-ring com- pounds LSnR (R = C5H5(1), C9H7(2), C13H9(3)). Both the solid and solution state structures of these compounds were studied in detail.

2 EXPERIMENTAL

All manipulations were carried out under a dry argon or nitrogen atmosphere by using Schlenk line and glovebox techniques. The organic solvents toluene and-hexane were dried by refluxing with sodium/potassium benzophenone under nitrogen atmosphere prior to use. C6D6was dried with a sodium/potassium alloy, whereas CDCl3was dried with CaH2.1H (400 MHz) and13C (100 MHz) NMR spectra were recorded on a Bruker Avance II 400 MHz spectrometer.119Sn (186 MHz)NMR spectra were recorded on a Bruker Avance II 500 MHz spectrometer. Melting point of the compound was measured in a sealed glass tube using a Büchi-540 instrument. Elemental analysis was performed on a Thermo Quest Italia SPA EA 1110 instrument. Commercially available reagents were purchased from Aldrich, Acros or Alfa-Assar Chemical Co. and used as received.

2. 1 Synthesis of LSn(C5H5) (1)

To a solution of LH (1.045 g, 2.5 mmol) in toluene (50 mL) at –30 ℃ was added dropwiseBuLi (1 mL 2.5 M solution in-hexane, 2.5 mmol). The mixture was stirred and left to warm to room temperature. The ligand lithium salt LLi was formed and subsequently added to a vigorously stirring suspension of SnCl2(1.430 g, 2.5 mmol) in toluene (30 mL) at –78 ℃. The mixture was left to warm to room temperature and stirred for additional 12 h. Compound LSnCl was formed, which was not isolated but used for further reaction. Thus, the LSnCl containing mixture was cooled to –78 ℃, and to it was added CpNa (1.25 mL 2 M THF solution, 2.5 mmol, Cp = cyclopentadienyl). The mixture was left to warm to room temperature and further stirred for 20 h. By filtration to remove the insoluble LiCl generated, the filtrate was evaporated to dryness under reduced pressure. The residue was collected and washed with-hexane (10 mL) to give an orange-yellow crystalline solid of 1 (1.10 g). The-hexane washing solution was stored at –20 ℃ for 24 h, yielding orange-yellow crystals of 1 (0.21 g) that were of X-ray diffraction analysis quality. Total yield: 1.31 g, 87%. m.p: 203 ℃.1H NMR (400 MHz, C6D6, 298 K, ppm):1.16 (d,3HH= 6.8 Hz, 12 H, CHMe2), 1.39 (d,3HH= 6.8 Hz, 12 H, CHMe2), 1.54 (s, 6 H, CMe), 3.37 (sept,3HH= 6.8 Hz, 4 H, CHMe2), 4.60 (s, 1 H,-CH), 5.82 (s, 5 H, C5H5), 7.17 (m, 6 H, C6H3);13C NMR (100 MHz, C6D6, 298 K, ppm):23.62, 25.02, 28.48 (CMe and CHMe2), 95.93 (-), 110.15 (C5H5), 124.37, 126.35, 142.39, 143.52 (C6H3), 164.09 (CN). Anal. Calcd. (%) for C34H46N2Sn (r= 601.45): C, 67.90; H, 7.71; N, 4.66. Found (%): C, 67.79; H, 7.69; N, 4.71.

2. 2 Synthesis of LSn(C9H7) (2)

The LSnCl (2.5 mmol) was freshly prepared in a similar manner to that used in the synthesis of 1. The indenyl lithium salt IndLi (2.5 mmol, Ind = indenyl) was prepared from the reaction of indene (0.29 mL, 2.5 mmol) withBuLi (1 mL 2.5 M solution in-hexane, 2.5 mmol) in toluene (20 mL) from –30 ℃ to room temperature within 12 h. Then, the LSnCl containing mixture was cooled to –78 ℃ and to it the IndLi solution was added dropwise. The mixture was left to warm to room temperature and stirred for additional 20 h. The insoluble LiCl was removed by filtration and the filtrate was evaporated to dryness under reduced pressure. The residue was collected and washed with-hexane (10 mL) to give an orange-yellow crystalline solid of 2 (1.15 g). The-hexane washing solution was stored at –20 ℃for 24 h, yielding orange-yellow crystals of 2 (0.18 g) that were of X-ray diffraction analysis quality. Total yield: 1.33 g, 82%. m.p: 206 ℃.1H NMR (400 MHz, C6D6, 298 K, ppm):1.16 (br, 24 H, CH2), 1.50 (s, 6 H, CMe), 3.48 (br, 4 H, CHMe2), 4.61 (s, 1 H,-C), 5.45 (m, 1 H, Sn–CHC9H7), 5.94 (m, 2 H, C9H7), 6.66 (m, 2 H, C9H7), 6.94 (m, 2 H, C9H7), 7.17～7.24 (m, 6 H, C6H3);13C NMR (100 MHz, C6D6, 298 K, ppm):23.2, 24.8, 28.5 (CMe and CHMe2), 94.3 (Sn–CHC9H7), 96.5 (-), 121.1, 121.7, 124.8, 126.5, 126.7, 140.3, 142.5, 143.6 (C6H3and C9H7), 165.8 (CN).119Sn NMR (186 MHz, C6D6, 298 K, ppm):–253.6. Anal. Calcd. (%) for C38H48N2Sn (M= 651.51): C, 70.05; H, 7.43; N, 4.30. Found (%): C, 70.18; H, 7.50; N, 4.38.

2. 3 Synthesis of LSn(C13H9) (3)

A: Preparation of FluLi (Flu = fluorenyl). To a solution of fluorene (3.32 g, 20 mmol) in toluene (80 mL) at –30 ℃ was added dropwiseBuLi (8 mL 2.5 M solution in-hexane, 20 mmol). The mixture was stirred and left to warm to room temperature. After additional stirring for 6 h, an orange solution was formed, which was stored at –20 ℃. 24 h later, a yellow solid of FluLi was precipitated, which was collected and washed with-hexane (4 mL) (2.44 g, 71% yield).

B: Preparation of LSn(C13H9) (3). The LSnCl (2.5 mmol) was freshly prepared in a similar manner to that used for the synthesis of 1. Then, the LSnCl containing mixture was cooled to –78 ℃ and to it a solution of FluLi (0.43 g, 2.5 mmol) in toluene (30 mL) was added. The mixture was stirred and left to warm to room temperature. After additional stirring for 20 h, the insoluble LiCl generated was removed by filtration, and the filtrate was evaporated to dryness under reduced pressure. The residue was collected and washed with- hexane (9 mL) to give an orange-yellow crystalline solid of 3 (1.21 g). The-hexane washing solution was stored at –20 ℃ for 72 h, yielding orange- yellow crystals of 3 (0.18 g) that are of the X-ray diffraction analysis quality. Total yield: 1.39 g, 79%. m.p: 213 ℃.1H NMR (400 MHz, CDCl3, 298 K, ppm):1.21 (d,3HH= 6.8 Hz, 6 H, CHMe2), 1.32 (d,3HH= 6.8 Hz, 6 H, CHMe2), 1.33 (d,3HH= 6.8 Hz, 12 H, CHMe2), 1.94 (s, 6 H, CMe), 3.11 (sept,3HH= 6.8 Hz, 2 H, CHMe2), 3.70 (sept,3HH= 6.8 Hz, 2 H, CHMe2), 4.89 (s, 1 H,-C), 5.40 (s, 1 H, Sn–CC13H9), 7.01 (br, 2 H, C13H9), 7.13 (m, 2 H, C13H9), 7.25 (m, 2 H, C13H9), 7.70 (m, 2 H, C13H9), 7.29～7.36 (m, 6 H, C6H3);13C NMR (100 MHz, CDCl3, 298 K, ppm):24.0, 24.2, 24.7, 24.8, 27.6, 27.8, 29.2 (CMe and CHMe2), 49.9 (Sn–CHC13H9), 100.4 (-), 119.7, 123.8, 124.2, 125.1, 126.8, 127.3, 127.4, 140.8, 141.6, 142.8, 144.7, 145.9 (C6H3and C13H9), 165.8 (CN).119Sn NMR (186 MHz, CDCl3, 298 K, ppm):–238.3. Anal. Calcd. (%) for C42H50N2Sn (M= 701.57): C, 71.90; H, 7.18; N, 3.99. Found (%): C, 71.62; H, 7.53; N, 3.89.

2.4 X-ray crystallographic analysis

Crystallographic data for compound 1 were collected on a Rigaku R-Axis Spider IP Instru- ment and the data for compounds 2 and 3 were on an Oxford Gemini S Ultra system. During measurements a graphite-monochromatic Mo-radiation (= 0.71073 Å) was used. Absorp- tion corrections were applied by using the spherical harmonics program (multi-scan type). All structures were solved by direct methods (SHELXS-96)[17]and refined against2using SHELXL-97[18]. In general, the non-hydrogen atoms were located by difference Fourier syn- thesis and refined anisotropically, and hydrogen atoms were included using a riding model withisotied to theisoof the parent atoms unless otherwise specified. In 1, two independent molecules were disclosed. A summary of cell parameters, data collection, and structure solu- tion and refinements is given in Table 1 (See Supporting Information).

For compound 1, a single crystal with dimen- sions of 0.50mm × 0.40mm × 0.30mm was selected for X-ray diffraction analysis. A total of 49898 reflections were collected in therange of 3.08<<24.78º, of which 13772 wereindepen- dent withint= 0.0300.

For compound 2, a single crystal with dimen- sions of 0.20mm × 0.20mm × 0.10mm was selected for X-ray diffraction analysis. A total of 12564 reflections were collected in therange of 2.95<<26.00º, of which 7069 wereindependent withint= 0.0528.

For compound 3, a single crystal with dimen- sions of 0.10mm × 0.10mm × 0.05mm was selected for X-ray diffraction analysis. A total of 14303 reflections were collected in therange of 3.21<<26.00º, of which6671wereindependent withint= 0.0302.

2.5 Variable-temperature 1H NMR measurement

The variable-temperature1H NMR studies of compounds 1～3 were performed on the Bruker Avance II 400 MHz spectrometer. The sample was dissolved in CD2Cl2in a well-sealed standard 5 mm NMR tube.Temperature inside the rotor was controlled by liquid nitrogen under dry nitrogen flow. Five temperature points 298, 273, 248, 223 and 198 K accurate to ±0.5 K were selected for collecting the1H NMR data.

3 RESULTS AND DISCUSSION

3. 1 Synthesis of LSnR (L = HC[C(Me)N-2,6-iPr2C6H3]2, R = C5H5 (1), C9H7 (2), C13H9 (3))

-Diketiminatotin(II) chloride LSnCl was pre- pared from the reaction of LLi and SnCl2from-78℃ to room temperature, where LLi wasgenerated from the reaction of LH andBuLi from-30℃ to room temperature. The LSnCl was not isolated but used for further reaction with C5H5Na,C9H7Li and C13H9Li, respectively. In comparison with commercial availability of C5H5Na, C9H7Li and C13H9Li were freshly prepared by the reaction of the respective indene and fluorene withBuLi from-30℃ to room temperature. Compound LSn(C5H5) (1) was obtained as a combination of orange-yellow crystalline solid and crystals in a total yield of 87%(Scheme 1). However, the production of LSn(C9H7) (2) and LSn(C13H9) (3) depended on the source form of C9H7Li and C13H9Li. The formation of 2 was from the reaction of LSnCl with thegenerated C9H7Li while that of 3 was throughthe reaction with the isolated C13H9Li(Scheme 1). The reaction by using thegenerated C13H9Li led to the formation of messy products and the isolation of the target was failed. Both 2 and 3were obtained as the combined orange-yellow crystalline solid and crystals in the total yields of 82% and 79%, respectively. Com- pounds1～3all are air and moisturesensitive. Melting point (mp) measurement indicated that they are thermally stable and have the mp at 203, 206 and 213 ℃, respectively.

Scheme 1. Syntheses of compounds 1～3

3. 2 Solid state structure analysis of compounds 1～3

The solid state structures of compounds 1～3 were disclosed by X-ray crystallography. It is clearly revealed that the Sn atom is coordinated by the L ligand andfurther by the cyclopentadienyl (Cp) for 1 (Fig. 1), indenyl (Ind) for 2 and fluorenyl (Flu) for 3 (Fig. 2), respectively. The L ligand,-chelates the Sn atom with the Sn–N bond lengths of 2.239(2)～2.250(2) for 1, 2.220(2) and 2.223(2) for 2, and 2.227(5)～2.241(4) Å for 3. The corresponding N–Sn–N biting angles are found by 81.21(6)° (81.71(6)° is for another independent molecule) for 1, 83.18(7)° for 2, and 82.09(16)° for 3, respectively. These data are comparable to those found in similar tin(II) compounds LSnMe (2.209(2) and 2.218(2) Å, 84.69(7)°)[19]and LSnC6F5(2.186(1) and 2.164(2) Å, 86.05(5)°)[20].

Fig. 1. X-ray crystal structure of 1 with thermal ellipsoids at 50% possibility (along theaxis). Hydrogen atoms at L are omitted for clarity. Selected bond distances (Å) and bond angles (°) (the values in the bracket are for those of another independent molecule): Sn(1)–N(1) 2.241(2) (2.246(2)), Sn(1)–N(2) 2.239(2) (2.250(2)), Sn(1)–C(6) 2.380(2) (2.395(2)), Sn(1)∙∙∙C(7) 2.683 (2.675), Sn(1)∙∙∙C(8) 3.076 (3.064), Sn(1)∙∙∙C(9) 3.104 (3.094), Sn(1)∙∙∙C(10) 2.723 (2.726), Sn(1)∙∙∙CCp(centroid)2.540 (2.538); N(2)–Sn(1)–N(1) 81.21(6) (81.71(6) )

Focusing on the Sn-Cp bonding moiety, it is found that the Sn–CC5H5bond lengths in 1 vary remarkably, and one contact (2.380(2) Å) is signi- ficantly shorter than the others (2.683, 2.726, 3.076 and 3.104 Å). The Sn∙∙∙CC5H5(centroid)distance is 2.538 Å. It has been reported that in (5-C5Ph5)2Sn the Sn–CC5Ph5bond distances are 2.686(6)Å, 2.687(7), 2.688(7), 2.689(7) and 2.705(7) Å (Sn∙∙∙CC5H5(centroid), 2.401 Å)[9]and in Sn(5-C5H5)2the Sn–CC5H5distances are 2.58(2), 2.62(2), 2.68(3), 2.71(3) and 2.75(3) Å[12]. It is also seen that in Sn(5-C5H5)Cl the Sn–CC5H5separations of 2.45(2)～2.74(3) Å are displayed with the Sn∙∙∙CC5H5(centroid)of 2.30 Å[14]. Furthermore, compound [(3-C5H5)SnN=C(NMe2)2]2exhibits almost the same separation for the shortest Sn– CC5H5bond and the Sn∙∙∙CCp(centroid)one (2.432(4) Å)[15], whereas in [(1-C5H5)SnN(SiMe3)P(Pr2)=CH)]2-2,3-C4H2N2the Sn–CC5H5distances are found by 2.359(9) Å versus 2.708, 2.703, 3.126 and 3.147 Å[16].Accordingly, the Sn–Cp bonding in 1 is better ascribed in an1-mode. However, the shortest Sn–CC5H5separation in 1 is longer than that of the Sn–-Cp bond (2.18 Å) and also longer than those of the Sn–CMebond in LSnMe (2.253(2) Å)[19]and the Sn–CC6F5bond in LSnC6F5(2.304(2) Å)[20]. Responding to the Sn–1-Cp bond nature in 1, the location of the H(6A) atom attached at C(6) under the geometric H-addition is away from the C(6)C(7)H(7A)C(8)H(8A)C(9)H(9A)C(10)H(10A) plane (the least-squares plane ∆ = 0.0121 Å) by 0.3777 Å. It is strikingly noted that in this situation the sum of the peripheral angles around the Sn atom in 1is calculated to be ca. 273.98(7)° (271.40(8)° for another molecule), which is very close to those observed in three-coordinate LSnMe (270.33(8)°)[17]and LSnH(271.1(8)°)[21]. This structure character implies a location of one lone pair electrons at the Sn center, being stereochemically active. The presence of the Sn–1-Cp bond may be probably caused due to an electronic repulsion between the Sn(II) lone pair and the Cp 6electrons. Calcu- lations indicated similar interaction observed for the Ge–1-Cp bond in LGeCp[8].

Fig. 2. X-ray crystal structures of 2 (left) and 3 (right) with thermal ellipsoids at 50% possibility (along theaxis). Hydrogen atoms at L are omitted for clarity. Selected bond distances (Å) and bond angles (°) for 2: Sn(1)–N(1) 2.220(2), Sn(1)–N(2) 2.223(2), Sn(1)–C(31) 2.307(3), Sn(1)∙∙∙C(32) 2.963, Sn(1)∙∙∙C(33) 3.653, Sn(1)∙∙∙C(34) 3.682, Sn(1)∙∙∙C(39) 2.988, Sn(1)∙∙∙CInd(centroid)2.919; N(1)–Sn(1)–N(2) 83.18(7). For 3: Sn(1)–N(1) 2.241(4), Sn(1)– N(2) 2.227(5), Sn(1)–C(31) 2.363(5), Sn(1)∙∙∙C(32) 2.961, Sn(1)∙∙∙C(37) 3.478, Sn(1)∙∙∙C(38) 3.459, Sn(1)∙∙∙C(43) 2.931, Sn(1)∙∙∙CFlu(centroid)2.807; N(2)–Sn(1)–N(1) 82.09(16)

Similar to 1, the Sn–C9H7in 2 is considered to adopt the1-mode as well because the corres- ponding one short contact (2.307(3) Å) versus other longer ones (2.963～3.682 Å) were observed with the Sn∙∙∙CC9H7(centroid)of 2.919 Å. Referring to the “slip parameter”[22, 23]for describing the so-called “indenyl effect”[24, 25]as an indicator of the hapticity, Δ(Sn∙∙∙Cindenyl(centroid)) is calculated to be ca. 1.01 Å. These data are significantly different from the lower values by the 0.69～0.79 Å range for the3-bond complexes, 0.125 Å for the3-2intermediate coordination complexes, and 0 Å for the perfect5-bond compounds[26, 11]. The sum of the peripheral angles around the Sn atom is 277.74(8)°. Therefore, the Sn(II) lone pair electrons in 2 are stereochemi- cally active as well. The Sn–1-CC13H9bond is also formed in 3, as indicated by the remarkably differed bond distances (2.363(5) versus 2.931～3.478 Å; Sn∙∙∙CC13H9(centroid)separation of 2.807 Å) and significantly 360°-deviated peripheral angle around the Sn atom (288.55(18)°). Due to different sizes for the Cp, Ind, and Flu, the differed Sn–CC5 ringbond distances as well as the Sn peripheral angles are actually displayed.

3. 3 Solution structure analysis of compounds 1～3

Compounds 1～3 are well soluble in toluene, benzene, chloroform, and dichloromethylene and they are also characterized by the1H,13C and119Sn NMR spectroscopy. The1H NMR spectra show clearly the presence of the L ligand in 1～3 by exhibiting the characteristic CMe2septet (3.37 ppm for 1,3.48 ppm for 2, and3.11 and 3.70 ppm for 3) and the correlating CH2doublet resonances (1.38 and 1.16 ppm for 1,1.16 and 1.50 ppm for 2,and1.21, 1.32, and 1.33 ppm for 3) from the L-aryl isopropyl substituents. The singlet resonances for the respective C(1.54 ppm for 1,1.50 ppm for 2, and1.94 ppm for 3) and-C(4.60 ppm for 1,4.61 ppm for 2, and4.89 ppm for 3) from the L backbone are also presented.

Fig. 3. Structure array of compounds 1～3 along theaxis. (In solution a Sn-CC5 ringfluxional bond is suggested to form for each compound)

In the1H NMR spectrum of 1 one singlet resonance at5.82 ppm is shown assignable to the C55as similar resonance is found in the lighter congener LGe(C5H5) (5.68 ppm)[8]. The obser- vance of one resonance can be considered as a consequence for the time-averaged, equivalent dynamic solution behavior of the C55five protons. This indicates the formation of fluxional Sn–CC5H5bonding in solution (Fig. 6), in sharp contrast to the solid stateof the Sn–1–CC5H5bond observed. The C97protons in 2 exhibit resonances at5.45, 5.94, 6.66, and 6.94 ppm, respectively, with an integral intensity ratio of 1:2:2:2, whereas the C139protons in 3 present resonances at5.86, 6.88, 7.08, 7.24 and 7.88 ppm, with the integral intensity ratio of 1:2:2:2:2. The C9H7contains four groups for the seven protons and C13H9five groups for the nine protons, and therefore they display respective four and five resonances, significantly different from one resonance for the highly symmetric C5H5group. These two resonance patterns indicate a fluxional bonding for the Sn–C9H7in 2 and Sn–C13H9in 3 (Fig. 6), respectively, in solution against their solid state Sn–1–CC9H7and Sn–1–CC13H9bonds as well. The solution1H NMR of 1-(Bu3Sn)C9H7has also been studied, which suggested a rapid ‘slipping’ of the Ind ring at the 2- and 2΄-positions by theBu3Sn part[27]. With respect to compound 2, such rapid ‘slipping’ behaviour for the Ind ring by the LSn should be also possible (Fig. 6), because this dynamic pattern is able to give four resonances for the C97in the integral intensity of 1:2:2:2 as well.

The119Sn NMR spectra exhibit the resonance at–646.4 ppm for 1,–253.6 ppm for 2, and–238.3 ppm for 3, respectively. It was reported that the119Sn chemical shifts usually varied with the coor- dination number; the resonance occurred at the range of–950 to –730 ppm is for the six-coor- dinate tin complex, of–730 to –650 ppm for the four-coordinate tin complex, and of–350 to –270 ppm for the three-coordinate tin complex[28]. It has also been documented that LSnR exhibited the tin resonance at–224 ppm for R = Cl,–259 ppm for R =Bu,–239 ppm for R = OSO2CF3, and–237 ppm for R = N3[29]. The119Sn NMR data of 2 and 3 are close to those of the three-coordinate Sn(II) complexes, but that of 1 is near to those of the four-coordinate Sn(II) complexes. Such differed119Sn NMR data suggest a subtle distinction existing among the Sn–C5H5in 1, the Sn–C9H7in 2, andthe Sn–C13H9in 3, although the Sn-CC5 ringfluxional bonding is exhibited for all of 1～3in solution. It is necessary to get further insight into the solution feature ofthese bonds in 1～3.

We performed the low temperature1H NMR studies. As shown in Fig. 4 for 1 recorded in CD2Cl2, with the temperature lowered from 25 to –75 ℃, the C55singlet resonance showed almost no change. This implies a temperature-unresolved, fast rotational, dynamic behavior of the Cp ring around the fluxional Sn–C5H5bond. However, the L-aryl CMe2septet resonance underwent a broadening (to a maximum at –25 ℃) and then splitting into two resonances (3.45 and 2.85 ppm at –75 ℃). This indicates a gradual separation of four isopropyl substituents into two groups following a decrease of the temperature during which a rotation of the L-aryls slows down along the N-C bond (note that a little change of the CH2septet resonances was also observed), and implies a functioning of the inequivalent electronic environment for these substituents to face up. As indicated by the solid-state X-ray structure, such environment is induced due to the presence of the stereochemically active Sn(II) lone pair electrons with the different Cp group, where an electronic repulsion interaction between the lone pair and the 6-electrons of the C5 ring actually exists over the Sn–C5H5bond, although in solution the Cp group is in a rotational state.

Fig. 4. Variable-temperature1H NMR studies of 1 in CD2Cl2

Fig. 5. Variable-temperature1H NMR studies of 2 in CD2Cl2

Fig. 6. Variable-temperature1H NMR studies of 3 in CD2Cl2

It was seen that inFig. 4 the C97resonances in 2 yet did not show change at lower temperature. However, the CMe2septet resonance shifted from a broad peak to two well-separated signals (3.71 and 2.77 ppm at –75 ℃) and meanwhile the CH2doublet resonance from a broad peak to four signals (1.46, 1.23, 1.12, and 0.71 ppm at –75 ℃). Similarly, lowering the temperature allows the enhancement of the stereochemical activity of the Sn(II) lone pair for function in influencing the L ligand CHMe2proton resonance modes.

In viewing Fig. 6 on the1H NMR study of 3, it was found that there displayed two CMe2septet and three CH2doublet resonances at 25 ℃, and this resonance pattern seems unchanged when lowering to –75 ℃. Instead, at –75 ℃, a sudden change of the C139resonances from the five groups to well-resolved nine groups (5.36, 5.90, 6.67, 7.08, 7.18, 7.50, 7.56, 7.80, and 7.98 ppm) is observed. In this circumstance the Flu group is better to rotate at the orthogonal position domi- nantly relative to the LSn array, against the orientation shown in Fig. 3 (for 3), because in this situation the nine C139resonances were able to display when towards the inequivalent environment caused due to the stereochemical activity of the Sn(II) lone pair.

4 CONCLUSION

In summary, we have prepared the-diketiminato ligand stabilized monocyclopentadienyl (1), -indenyl (2), and -fluorenyl tin(II) (3) compounds. The solid state structures of1～3 were established by X-ray crystallography, which clearly revealed an1-mode for the Sn-C5H5, Sn-C9H7, and Sn-C13H9bonds, respectively, all under support by the LSn, and proved the stereochemical activity of the lone pair electrons located at the Sn(II) center. However, each structure is only the presence of one kind state since the detailed1H NMR studies at variable temperature (25～–75 ℃) disclosed time-averaged, complicated dynamic states of 1～3 in solution. The fluxional bonding for the Sn-C5H5in 1, Sn-C9H7in 2 and Sn-C13H9in 3 actually exists in solution. The observance of the resonance change from either the L-aryl isopropyl substituents (CMe2and CH2) in 1 and 2 or the C139group in 3 at low tem- perature may, however, verify reasonably the stereochemical activity of the Sn(II) lone pair. The presence of their solid state structures indicates the inequivalent environment for these groups to face up. The states corresponding at such resonance change temperature (ca.-50～–75 ℃ for 1,0～–75 ℃ for 2, and –75 ℃ for 3) may be consi- dered as the solution relaxation states, which were able to be detected by means of lowering the temperature1H NMR measurement. Such tempera- ture-dependent resonance changes reflect an influence of the C5ring size (Cp in 1, Ind in 2, and Flu in 3) on the corresponding Sn-CC5 ringbond in solution. This is, to the best of our knowledge, for the first time to provide the experimental evidence for illustrating the solution Sn-CC5 ringbond and further understanding an electronically static interaction between the Sn(II) lone pair electrons and the 6-electrons of the C5 ring.

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2 June 2017;

7 November 2017 (CCDC 997153 for 1, 997154 for 2 and 997155 for 3)

① This project was supported by the National Natural Science Foundation of China (21473142 and 21673191) and the National Innovative Research Team of China (IRT_14R31 and J1310024)

. E-mail: hpzhu@xmu.edu.cn

10.14102/j.cnki.0254-5861.2011-1742