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• 0   Introduction

  There is currently a fundamental interest in the reactivity of organolanthanide complexes toward unsa-turated small molecules, because this is the source for design of new catalytic reactions and catalysts^[1-6]. The selective insertion of an organic functional group into a lanthanide-ligand bond has proved to be a successful and pervasive method for the formation of carbon-carbon and carbon-heteroatom bonds in synthesis of organolanthanide derivatives and organic compounds^[6-13]. For example, by applying this type of insertions as the key step, many organolanthanide-catalyzed reactions such as hydrosilylation^[7], hydroa-mination^{[2b, 8]}, hydroalkoxylation^[9], hydrothiolation^[10], hydrophosphination^[11], hydroboration^[12], and C-H bond addition^{[2c, 13]} of diverse C-C unsaturated substrates have been developed. In contrast, the development of metathesis reactions of trivalent organolanthanide complexes with unsaturated substrates leading to the complete cleavage of the unsaturated chemical bond, lags far behind, despite their fundamental scientific interest and the potential utility in organic synthesis. The major reason might be attributed to the absence of conventional oxidative-addition/reductive-elimination processes and the difficulties encountered in the formation of rare earth metal-carbon and rare earth metal-heteroatom multiply-bonded intermediates, which often involve in transition metal-catalyzed metathesis reactions of unsaturated substrates^[14]. Recent pioneering works show that reaction of tetranuclear rare earth metal polyhydrido complexes with CO yielded ethylene and the corresponding tetraoxo cubane complexes^[15a]. Chen and coworkers reported that yttrium dihydride reacted with Ph₃P=Se to give an yttrium selenide complex in the company of the liberation of Ph₃P and H₂^[15b]. Furthermore, methylidene complexes of rare earth metals have proven their ability to cleave the C=O, N=N and C=N double bonds^[16]. Despite these achievements, the use of trivalent organolanthanide complexes as reagents for the transformation of an unsaturated functional group into another unsaturated one in the context of metathesis reactions remains underutilized. The question of whether or not rare earth metal amido complexes can promote formation and transformation of unsaturated carbon-heteroatom bonds as readily as rare earth metal polyhydrido and methylidene complexes mediate C=O bond metathesis reaction is still an open issue.

  The chemical nature of isocyanates, with an electrophilic center at carbon and nucleophilic centers at oxygen and nitrogen, allows a diverse variety of reactions to occur, thus leading to a great potential for constructing higher organic structures^[17]. The selectivity of isocyanate transformations has been profitably controlled by metal species, since their participation allows the fine-tuning of bond forming processes and reactivity of isocyanates^[18]. In attempts to establish more selective catalyst systems and to have a better understanding of the mechanistic aspects as well as to develop new reactions, extensive studies on the reaction of organometallic complexes with isocyanates have been carried out. Noticeably, in contrast to transition metals, selective C=N or C=O bond cleavage of isocyanates mediated by rare earth metals remains a significant challenge, mainly due to the instability of the resulting nitrene and carbonyl complexes and to the facile C=O addition pathways available^[18]. The stoichiometric reaction of trivalent organolanthanide compounds with isocyanates gives generally the addition products (Scheme 1)^[19].

  

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  On the other hand, using isocyanates as CO precursors in organic synthesis remains little explored^[20]. Hou and coworkers reported in 2004 that yttrocene tetrahydrido complex [(C₅Me₄SiMe₃)Y(μ-H)]₄(L) (L=Me₃SiCCHCHCSiMe₃) could abstract the oxygen from aryl isocyanate to form the corresponding μ₃-oxo complex [(C₅Me₄SiMe₃)Y]₄(μ₃-O)(μ-H)₂(L) or [(C₅Me₄SiMe₃)Y]₄(μ₃-O)₂(L), depending on the substrate ratio^[21]. In the course of our ongoing studies involving the reaction of organolanthanide complexes with isocya-nates, we found that the further interaction of the neighboring NH₂ group with an isocyanate unit inserted into the lanthanide-sulfur bond could lead to unique reactivity and selectivity trends, allowing a mild and efficient construction of the coordinated benzothiazole 2-oxide ligand^[20a] (Scheme 2). So far, no general method for rare earth metal-based carbonyla-tion has been reported. Given that the above reaction provides an alternative route to carbonylation of organic substrates, we were interested in broadening the reaction. Herein, we report the synthesis and structure of lanthanocene complexes containing o-aminobenzamido dianion ligand, and a new reactivity pattern of amido complexes toward isocyanates: carbonylative coupling/cyclization, which provides a new method for the construction of a coordinated quinazolyldiolate ring skeleton.

  

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  1   Experimental

  1.1   General remarks

  All operations involving air-and moisture-sensitive compounds were carried out under an inert atmosphere of purified nitrogen using standard Schlenk techniques. All organic solvents such as THF, toluene, and n-hexane were refluxed and distilled over sodium benzophenone ketyl under N₂ prior to use. o-Aminobenzamide, HMPA (hexamethyl-phosphorictriamide), N, N′-diisopropyl carbodiimide (DIC) and phenyl isocyanate (PhNCO) were purchased from commercial sources and were used without further purification. Elemental analyses for C, H, and N were carried out by using a Rapid CHN-O analyzer. Metal analyses were accomplished using the literature method^[22]. Infrared spectra for air-and moisture-sensitive compounds were obtained on a Nicolet FTIR 360 spectrometer with samples prepared as Nujol mulls. ¹H NMR data were obtained on a Bruker DMX-400 NMR spectrometer.

  1.2   Preparation

  1.2.1   Synthesis of [CpYb(μ-η²:η²-NHC₆H₄CONH)(μ₃-η¹:η¹:η²-NHC₆H₄CONH)YbCp(HMPA)]₂ (1a)

  To a toluene solution (20 mL) of Cp₃Yb (0.331 g, 0.898 mmol) was added o-aminobenzamide (0.122 g, 0.898 mmol) at room temperature. The reaction mixture was stirred at room temperature overnight. Then, HMPA (0.161 g, 0.898 mmol) was added to the resulting yellow turbid solution. After stirring for 2 h, the reaction mixture was changed to a clear solution. The solution was then cooled at -10 ℃ overnight to give 1a as orange crystals. Yield: 0.388 g (94%). Anal. Calcd. for C₆₀H₈₀N₁₄O₆P₂Yb₄(%): C, 39.01; H, 4.36; N, 10.61; Yb, 37.47. Found(%): C, 39.12; H, 4.41; N, 10.53; Yb, 37.33. IR(Nujol, cm^-1): 3 365 m, 3 309 w, 1 601 m, 1 526 m, 1 457 vs, 1 378 m, 1 327 m, 1 145 s, 990 s, 873 w, 758 s.

  1.2.2   Synthesis of [CpEr(μ-η²:η²-NHC₆H₄CONH)(μ₃-η¹:η¹:η²-NHC₆H₄CONH)ErCp(HMPA)]₂ (1b)

  Following the described method for synthesis of 1a, using Cp₃Er (0.496 g, 1.37 mmol), o-aminobenza-mide (0.187 g, 1.37 mmol) and HMPA (0.246 g, 1.37 mmol) afforded 1b as light pink crystals. Yield: 0.534 g (86%). Anal. Calcd. for C₆₀H₈₀N₁₄O₆P₂Er₄(%): C, 39.50; H, 4.42; N, 10.75; Er, 36.67. Found(%): C, 39.55; H, 4.43; N, 10.69; Er, 36.58. IR(Nujol, cm^-1): 3 365 m, 3 310 w, 1 600 m, 1 525 m, 1 457 vs, 1 379 m, 1 327 m, 1 144 s, 989 s, 872 w, 757 s.

  1.2.3   Synthesis of [CpY(μ-η²:η²-NHC₆H₄CONH)(μ₃-η¹:η¹:η²-NHC₆H₄CONH)YCp(HMPA)]₂ (1c)

  Following the described method for synthesis of 1a, using Cp₃Y (0.86 g, 1.01 mmol), o-aminobenza-mide (0.137 g, 1.01 mmol) and HMPA (0.181 g, 1.01 mmol) afforded 1c as a light yellow crystalline powder. Yield: 0.281 g (74%). Anal. Calcd. for C₆₀H₈₀N₁₄O₆P₂Y₄(%): C, 47.69; H, 5.34; N, 12.98; Y, 23.54. Found(%): C, 47.74; H, 5.37; N, 12.90; Y, 23.52. IR(Nujol, cm^-1): 3 364 m, 3 309 w, 1 600 m, 1 524 m, 1 458 vs, 1 378 m, 1 327 m, 1 143 s, 988 s, 758 s. ¹H NMR (C₆D₆, 400 MHz, 25 ℃): δ 7.92 (s, NH, 2H), 7.90 (s, NH, 2H), 7.08 ~6.43 (m, C₆H₄, 16H), 6.29 (s, C₅H₅, 20H), 3.71 (s, NH, 2H), 3.56 (s, NH, 2H), 2.28 (d, J=8.0 Hz, N(CH₃)₂, 36H).

  1.2.4   Synthesis of [Cp₂Yb(μ₃-η²:η²:η¹-Quo)]₃Yb(HMPA)₂ (2a)

  To a toluene solution (20 mL) of 1a (0.412 g, 0.225 mmol) was added PhNCO (0.107 g, 0.901 mmol) at 0 ℃. The reaction mixture was stirred at room temperature overnight. The solution was concentrated and cooled at -15 ℃ for several days to give 2a as a yellow powder. Crystals of 2a suitable for X-ray analysis were obtained by recrystallization in a mixed solvent of toluene and THF. Yield: 0.230 g (53%). Anal. Calcd. for C₆₆H₇₈N₁₂O₈P₂Yb₄(%): C, 41.25; H, 4.09; N, 8.75; Yb, 36.02. Found(%): C, 41.29; H, 4.14; N, 8.69; Yb, 35.96. IR(Nujol, cm^-1): 3 411 w, 3 339 s, 2 192 m, 1 598 m, 1 537 s, 1 465 s, 1 373 s, 1 300 m, 1 149 m, 991 s, 751 s.

  1.2.5   Synthesis of [Cp₂Er(μ₃-η²:η²:η¹-Quo)]₃Er(HMPA)₂ (2b)

  Following the method described for synthesis of 2a, using 1b (0.552 g, 0.30 mmol) and PhNCO (0.144 g, 1.21 mmol) afforded 2b as a pink powder. Yield: 0.268 g (46%). Anal. Calcd. For C₆₆H₇₈N₁₂O₈P₂Er₄(%): C, 41.76; H, 4.14; N, 8.85; Er, 35.24. Found(%): C, 41.47; H, 4.04; N, 8.69; Er, 35.16. IR(Nujol, cm^-1): 3 410 w, 3 339 s, 2 191 m, 1 597 m, 1 537s, 1 463 s, 1 373 s, 1 300 m, 1 149 m, 991 s, 753 s.

  1.2.6   Synthesis of [Cp₂Y(μ₃-η²:η²:η¹-Quo)]₃Y(HMPA)₂ (2c)

  Following the method described for synthesis of 2a, using 1c (0.408 g, 0.27 mmol) and PhNCO (0.128 g, 1.08 mmol) afforded 2c as a light yellow powder. Yield: 0.214 g (50%). Anal. Calcd. for C₆₆H₇₈N₁₂O₈P₂Y₄(%): C, 50.01; H, 4.96; N, 10.60; Y, 22.44. Found: C, 49.96; H, 4.91; N, 10.49; Y, 22.39. IR(Nujol, cm^-1): 3 407 w, 3 337 s, 2 192 m, 1 597 m, 1 536s, 1 463 s, 1 373 s, 1 301 m, 1 240 w, 1 147 m, 990 s, 756 s. ¹H NMR (C₆D₆, 400 MHz, 25 ℃): δ 7.34~7.08 (m, C₆H₄, 12H), 6.15 (s, C₅H₅, 30H), 2.12 (s, NMe₂, 36H).

  1.2.7   Synthesis of {Cp₂Yb[μ-η¹:η¹:η³-ⁱPrNC(NHⁱPr)NC₆H₄-CONH]}₃Yb(HMPA)₃ (3a)

  To a toluene solution (20 mL) of 1a (0.412 g, 0.225 mmol) was added DIC (0.141 g, 1.12 mmol) at 0 ℃. The reaction mixture was stirred at room tempera-ture overnight. The solution was concentrated and cooled at -15 ℃ for several days to give 3a as a yellow powder. The yellow crystals of 3a·THF suitable for X-ray analysis were obtained by recrystallization in a mixed solvent of toluene and THF. Yield: 0.389 g (70%). Anal. Calcd. for C₉₀H₁₄₄N₂₁O₆P₃Yb₄(%): C, 45.01; H, 6.04; N, 12.25; Yb, 28.82. Found(%): C, 45.14; H, 6.07; N, 12.17; Yb, 28.75. IR (Nujol, cm^-1): 3 410 m, 3 381 w, 3 280 w, 1 600 w, 1 522 w, 1 460 vs, 1 377 s, 1 127 m, 837 w, 727 s.

  1.2.8   Synthesis of {Cp₂Er[μ-η¹:η¹:η³-ⁱPrNC(NHⁱPr)NC₆H₄-CONH]}₃Er(HMPA)₃ (3b)

  Following the described method for synthesis of 3a, using 1b (0.552 g, 0.30 mmol) and DIC (0.152 g, 1.20 mmol) afforded 3b·THF as light pink crystals. Yield: 0.375 g (50%). Anal. Calcd. for C₉₀H₁₄₄N₂₁O₆P₃Er₄(%): C, 45.45; H, 6.10; N, 12.37; Er, 28.13. Found(%): C, 45.44; H, 6.13; N, 12.31; Er, 28.05. IR(Nujol, cm^-1): 3 409 m, 3 381 w, 3 279 w, 1 599 w, 1 556 w, 1 459 vs, 1 379 s, 1 158 w, 1 126 m, 988 w, 729 s.

  1.2.9   Synthesis of {Cp₂Y[μ-η¹:η¹:η³-ⁱPrNC(NHⁱPr)NC₆H₄-CONH]}₃Y(HMPA)₃ (3c)

  Following the described method for synthesis of 3a, using 1c (0.468 g, 0.31 mmol) and DIC (0.154 g, 1.22 mmol) afforded 3c as light yellow crystalline solid. Yield: 0.285 g (43%). Anal. Calcd. for C₉₀H₁₄₄N₂₁O₆P₃Y₄(%): C, 52.35; H, 7.03; N, 14.25; Y, 17.22. Found(%): C, 52.39; H, 7.09; N, 14.23; Er, 17.20. IR(Nujol, cm^-1): 3 406 m, 3 380 w, 3 280 w, 1 598 w, 1 554 w, 1 460 vs, 1 377 s, 1 155 w, 1 128 m, 993 w, 726 w. ¹H NMR (C₆D₆, 400 MHz, 25 ℃): δ 8.31 (s, NH, 3H), 7.23~6.99 (m, C₆H₄, 12H), 6.09 (s, C₅H₅, 30H), 3.75 (m, CH(CH₃)₂, 3H), 3.55 (s, NH, 3H), 3.33 (m, CH(CH₃)₂, 3H), 2.49(d, J=8.0 Hz, N(CH₃)₂, 54H), 1.34 (d, J=6.4 Hz, CH(CH₃)₂, 18H), 0.93 (d, J=6.4 Hz, CH(CH₃)₂, 18H). Crystals of 3c·THF suitable for X-ray analysis were obtained by layering n-hexane to its concentrated toluene/THF solution.

  1.2.10   Synthesis of {CpYb[μ-η¹:η²:η²-ⁱPrNC(NHⁱPr)NC₆H₄-CONH]}₂ (4a)

  To a toluene solution (20 mL) of Cp₃Yb (0.238 g, 0.646 mmol) was added o-aminobenzamide (0.088 g, 0.646 mmol) at room temperature. After stirred at room temperature overnight, DIC (0.082 g, 0.646 mmol) was added to the resulting yellow turbid solution at 0 ℃. Then, the reaction mixture was heated at 110 ℃ for 12 h. The hot clear solution was slowly cooled to room temperature, affording 4a as yellow crystals. Yield: 0.300 g (93%). Anal. Calcd. for C₃₈H₅₀N₈O₂Yb₂(%): C, 45.78; H, 5.06; N, 11.24; Yb, 34.71. Found(%): C, 45.84; H, 5.02; N, 11.20; Yb, 34.65. IR(Nujol, cm^-1): 3 396 m, 3 381 w, 1 598 w, 1 463 vs, 1 377 s, 1 327 m, 1 170 m, 1 124 w, 875 w, 763 s.

  1.2.11   Synthesis of {CpEr[μ-η¹:η²:η²-ⁱPrNC(NHⁱPr)NC₆H₄-CONH]}₂ (4b)

  Following the method described for synthesis of 4a, using Cp₃Er (0.326 g, 0.898 mmol), o-aminobenza-mide (0.123 g, 0.898 mmol) and DIC (0.113 g, 0.898 mmol) afforded 4b as light pink crystals. Yield: 0.400 g (91%). Anal. Calcd. for C₃₈H₅₀N₈O₂Er₂(%): C, 46.32; H, 5.11; N, 11.37; Er, 33.95. Found(%) C, 46.39; H, 5.15; N, 11.28; Er, 33.82. IR(Nujol, cm^-1): 3 399 m, 3 379 w, 1 598 w, 1 548 w, 1 459 vs, 1 377 s, 1 172 m, 1 124 m, 874 w, 764 s.

  1.2.12   Synthesis of {CpY[μ-η¹:η²:η²-ⁱPrNC(NHⁱPr)NC₆H₄CO-NH]}₂ (4c)

  Following the method described for synthesis of 4a, using Cp₃Y (0.317 g, 1.11 mmol), o-aminobenzamide (0.152 g, 1.11 mmol) and DIC (0.140 g, 1.11 mmol) afforded 4c as light yellow crystals. Yield: 0.321 g (70%). Anal. Calcd. for C₃₈H₅₀N₈O₂Y₂(%): C, 55.08; H, 6.08; N, 13.52; Y, 21.46. Found(%): C, 55.09; H, 6.12; N, 13.50; Y, 21.45. IR(Nujol, cm^-1): 3 399 m, 3 375 w, 1 741 w, 1 598 w, 1 547 w, 1 460 vs, 1 377 s, 1 171 m, 1 124 m, 763 s. ¹H NMR (C₆D₆, 400 MHz, 25 ℃): δ 7.28~7.11 (m, C₆H₄, 8H), 6.94 (s, NH, 2H), 6.45 (s, C₅H₅, 10H), 6.12 (s, NH, 2H), 3.35~3.22 (m, CH(CH₃)₂, 4H), 1.28 (d, J=6.4 Hz, CH₃, 6H), 0.86 (d, J=6.4 Hz, CH₃, 6H).

  1.2.13   Transformation from 4a to 3a

  To a toluene solution (20 mL) of 4a (0.254 g, 0.255 mmol) was added HMPA (0.091 g, 0.510 mmol) at room temperature. After stirring for 12 h, the solution was concentrated and cooled at -15 ℃ for several days to give 3a as yellow crystals. Yield: 0.165 g (54%).

  1.3   X-ray data collection, structure determination and refinement

  Suitable single crystals were sealed under N₂ in thin-walled glass capillaries for X-ray structural analysis. X-ray diffraction data were collected on a SMART APEX CCD diffractometer (graphite-mono-chromated Mo Kα radiation, φ-ω scan technique, λ=0.071 073 nm). The intensity data were integrated by means of the SAINT program^[23]. SADABS^[24] was used to perform area-detector scaling and absorption corrections. The structures were solved by direct methods and were refined against F² using all reflec-tions with the aid of the SHELXTL package^[25]. All non-hydrogen atoms were found from the difference Fourier syntheses and refined anisotropically. The H atoms were included in calculated positions with isotropic thermal parameters related to those of the supporting carbon atoms but were not included in the refinement. All calculations were performed using the Bruker Smart program. Details of this SQUEEZE are given in the cif files. Crystal data, data collection, and processing parameters for the complexes are summarized in Table 1~2.

  Table 1.  Crystal and data collection parameters of complexes 1a, 1b and 4a~4c

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  	1a	1b	4a	4b	4c
  Formula	C60H80N14O6P2Yb4	C60H80N14O6P2Er4	C38H50N8O2Yb2	C38H50N8O2Er2	C38H50N8O2Y2
  Formula weight	1 847.48	1 824.36	996.94	985.38	828.68
  Crystal size/mm	0.25×0.22×0.14	0.15×0.12×0.10	0.30×0.25×0.15	0.35×0.18×0.08	0.19×0.10×0.08
  Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
  Space group	C2/c	C2/c	P21/n	P21/n	P21/n
  a/nm	2.837 8(9)	3.091 3(10)	1.388 2(5)	1.390 9(3)	1.395 5(4)
  b/nm	1.256 2(4)	1.330 0(4)	0.990 0(4)	0.993 2(2)	0.995 5(3)
  c/nm	2.539 3(8)	2.466 4(8)	1.474 2(5)	1.477 2(3)	1.481 8(5)
  β/(°)	106.834(4)	112.082(4)	102.548(5)	102.550(2)	102.724(4)
  V/nm3	8.664(5)	9.397(5)	1.977 7(12)	1.991 9(7)	2.007 9(10)
  Z	4	4	2	2	2
  Dc/(g·cm-3)	1.416	1.290	1.674	1.643	1.371
  μ/mm-1	4.360	3.611	4.742	4.226	2.917
  F(000)	3 584	3 552	980	972	856
  Reflection measured	19 258	19 109	8 648	8 713	8 896
  Unique reflection	8 420(Rint=0.046 0)	8 311(Rint=0.038 5)	3 822(Rint=0.076 8)	3 890(Rint=0.032 1)	2 916(Rint=0.068 7)
  Goodness-of-fit on F2	1.002	1.006	1.015	0.993	0.997
  Final R indices	R1=0.040 3,	R1=0.039 1,	R1=0.051 3,	R1=0.033 3,	R1=0.038 6,
  [I > 2σ(I)]	wR2=0.079 0	wR2=0.105 6	wR2=0.134 4	wR2=0.088 2	wR2=0.055 7
  R indices (all data)	R1=0.074 0, wR2=0.082 7	R1=0.062 0, wR2=0.111 2	R1=0.061 4, wR2=0.140 7	R1=0.040 2, wR2=0.093 1	R1=0.083 2, wR2=0.059 3

  Table 2.  Crystal and data collection parameters of complexes 2a and 3·THF

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  	2a	3a·THF	3b·THF	3с·THF
  Formula	C66H78N12O8P2Yb4	C94H152N21O7P3Yb4	C94H152Er4N21O7P3	C94H152N21O7P3Y4
  Formula weight	1 921.50	2 473.44	2 450.32	2 136.92
  Crystal size/mm	0.30×0.20×0.15	0.15×0.06×0.05	0.40×0.15×0.10	0.20×0.12×0.12
  Crystal system	Trigonal	Triclinic	Triclinic	Triclinic
  Space group	P63/m	P1	P1	P1
  a/nm	1.563 4(4)	1.579 2(5)	1.586 9(4)	1.590 4(11)
  b/nm	1.563 4(4)	1.858 6(6)	1.806 1(4)	1.869 4(13)
  c/nm	2.074 7(8)	2.461 4(7)	2.494 5(6)	2.471 2(16)
  α/(°)	90	77.664(5)	77.505(3)	77.669(11)
  β/(°)	90	78.642(5)	77.840(3)	78.692(10)
  γ/(°)	120	66.029(4)	66.919(3)	65.831(10)
  V/nm3	4.392(2)	6.400(3)	6.358(3)	6.500(8)
  Z	2	2	2	2
  Dc/(g·cm-3)	1.453	1.284	1.242	1.092
  μ/mm-1	4.305	2.983	2.698	1.854
  F(000)	1 864	2 488	2 390	2 240
  Reflection measured	18 196	26 698	25 656	27 071
  Unique reflection	2 661(Rint=0.067 8)	22 124(Rint=0.057 8)	21 540(Rint=0.063 1)	22 493(Rint=0.063 2)
  Goodness-of-fit on F2	1.002	1.016	1.052	1.204
  Final R indices [I > 2σ(I)]	R1=0.051 6, wR2=0.150 8	R1=0.059 8, wR2=0.140 1	R1=0.076 3, wR2=0.145 5	R1=0.072 4, wR2=0.155 0
  R indices (all data)	R1=0.067 3, wR2=0.159 7	R1=0.116 1, wR2=0.150 0	R1=0.185 8, wR2=0.162 7	R1=0.175 5, wR2=0.165 6

  CCDC: 951003, 1a; 951104, 1b; 951102, 2a; 951004, 3a; 951106, 3b; 951005, 3c; 951103, 4a; 951107, 4b; 951105, 4c.

  2   Results and discussion

  2.1   Syntheses and characterization of lanthanocene derivatives with the o-aminobenzamido dianion ligand

  Considering that the ureido resulting from addition of amine to isocyanate might potentially undergo the nucleophilic substitution or addition^[26-27] (Scheme 3), our interest in the rare earth metal-mediated C=N bond cleavage of isocyanates prompted us to explore the possibility of reacting two coordinated NH anions with one isocyanate molecule.

  

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  On the basis of this idea, we focused our attention on using the linked NH/CONH dianion lanthanide complexes as the template of the target reaction as the anionic NH and CONH groups offer a degree of selectivity, in which the two NH groups can react either with a reagent having two functionalities or with two separate reagents bearing the same or different functionality. Thus, it may be expected that the different reactivity between coordinated NH and CONH anions toward the same functionality would control reaction degree of advancement, for less reactive substrates only the NH addition takes place, whereas the more reactive ones lead to the occurrence of a tandem reaction involved the two kinds of amido groups. Three o-aminobenzamido dianion lanthanide complexes [CpLn(μ-η²:η²-NHC₆H₄CONH)(μ₃-η¹:η¹:η²-NHC₆H₄CONH)LnCp(HMPA)]₂ (Ln=Yb, 1a; Er, 1b; Y, 1c) as the designed starting materials were synth-esized in good yields by protonolysis of Cp₃Ln with o-aminobenzamide followed by crystallization in a HMPA and toluene mixture (Scheme 4).

  

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  Complexes 1a~1c are air-and moisture-sensitive, and readily dissolved in toluene and THF. They are thermally stable at room temperature. Complexes 1a~1c were characterized by elemental analysis and spectroscopic methods. In the IR spectra, all of them show two sharp N-H stretching vibration peaks at about 3 310 and 3 365 cm^-1 attributable to the different chemical environments of NH groups. ¹H NMR spectra of complex 1c show four NH resonance peaks at 7.92, 7.90 3.71, and 3.56, respectively. The solid-state structures of 1a and 1b were determined by X-ray analysis.

  As shown in Fig. 1, X-ray analysis results show that 1a and 1b are isostructural, four o-aminobenza-mido dianion ligands connect with two CpLn and two CpLn(HMPA) fragments via two different μ-η²:η²-and μ₃-η¹:η¹:η²-bonding modes, respectively. These bond parameters indicate that the amido moiety acts as both a bridging and side-on chelating group, in which the negative charge is delocalized over the OCN unit. Characteristically, both of bridging O and N atoms have two distinctive metal-oxygen (nitrogen) distances. One is between those expected for an Ln³⁺-O(N) single bond and an Ln³⁺←:O(N) donor bond, while another falls in the range of the Ln³⁺←:O(N) donating bond lengths for neutral oxygen or nitrogen donor ligands^[28]. This difference may be attributed to the chelating coordination effect caused by the amido ligand.

  图1 Thermal ellipsoid (30%) plot of complexes {CpLn(μ-η²:η²-NHC₆H₄CONH)[μ³-η¹:η¹:η²-NHC₆H₄CONH]LnCp-(HMPA)}₂ (Ln=Yb, 1a; Er, 1b) Figure1. Thermal ellipsoid (30%) plot of complexes {CpLn(μ-η²:η²-NHC₆H₄CONH)[μ³-η¹:η¹:η²-NHC₆H₄CONH]LnCp-(HMPA)}₂ (Ln=Yb, 1a; Er, 1b)

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  2.2   Reaction of 1 with phenyl isocyanate

  With dianionic o-aminobenzamido organolantha-nides 1 in hand, we next explored their reactivity toward phenyl isocyanate. As expected, treatment of 1 with PhNCO(n_PhNCO/n₁=4) in toluene led to the occurr-ence of an unusual tandem addition/cyclization/amine elimination/ligand redistribution reaction, giving the dianionic quinazolyldiolate lanthanide complexes {Cp₂Ln[μ₃-η²:η²:η¹-Quo]}₃Ln(HMPA)₂ (Ln=Yb, 2a; Er, 2b; Y, 2c) in moderate isolated yields (Scheme 5). The elimination product PhNH₂ was unambiguously identified by GC-MS, but attempts to isolate another metal-containing product were unsuccessful. The metathesis reactions of isocyanates with organolan-thanide complexes are rarely observed^[20-21]. This is the first example of the amido complex-mediated C=N bond cleavage of isocyanates, and provides an effective method for the construction of quinazolyldiol skeleton^[29].

  

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  Complexes 2a~2c are readily dissolved in THF but slightly soluble in toluene. They were chara-cterized by elemental analysis and IR spectroscopy. The 1H NMR spectra of 2c was also determined and showed that there is no N-H resonance absorption peak. The structure of 2a was identified by X-ray single-crystal diffraction analysis. As shown in Fig. 2, 2a is a tetranuclear structure possessing three bridging dianionic quinazolinyldione ligands. 2a has a three-fold rotation axis on the O-O alignment, and the three ytterbium and two HMPA oxygen atoms occupy an equatorial plane and an apical position of the hexahedral skeletal structure, respectively (Fig. 2). The four Yb atoms locate in two different coordinate environments. Atoms Yb(2), Yb(2A) and Yb(2B) are coordinated by one chelating η²-N, O unit, one non-bridging oxygen atom and two η⁵-Cp groups, respectively, to form a distorted triagonal-bipyramidal geometry, while the central Yb(1) is coordinated by three η²-N, O units from different quinazolyldiolate ligands and two HMPA oxygen atoms to form a hexagonal bipyramidal geometry. The bond parameters indicate that the π-electrons of C=O and C=N double bonds are delocalized over the pyrimidinyldionate skeleton. The Yb-O distances are in normal range^[5c].The rather long distances between ytterbium and N atoms may be attributed to both the rigidity of the quinazolyldiolate ring and the steric crowding^[30].

  图2 Thermal ellipsoid (30%) plot of complex {Cp₂Yb[μ³-η²:η²:η¹-Quo]}₃Yb(HMPA)₂ (2a) Figure2. Thermal ellipsoid (30%) plot of complex {Cp₂Yb[μ³-η²:η²:η¹-Quo]}₃Yb(HMPA)₂ (2a)

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  2.3   Reactivity of 1 toward diisopropylcarbodii-mide

  To obtain additional insight into the mechanism and scope of the reaction, we further examined the behavior of 1 toward carbodiimide. In markedly contrast to isocyanate, treatment of 1 with ⁱPrN=C=NⁱPr (n_ligand/n₁=4) provided {Cp₂Ln[μ-η¹:η¹:η²-ⁱPrNC(NHⁱPr)NC₆H₄ CONH]}₃Ln(HMPA)₃ (Ln=Yb, 3a; Er, 3b; Y, 3c) as the main metal-containing products, even with a prolonged heating at 110 ℃ (Scheme 6). In addition, 3a~3c are inactive to excess ⁱPrN=C=NⁱPr. These results indicate that ⁱPrN=C=NⁱPr is preferentially added to NH rather than CONH group. The coor-dinated CONH group is inert, and undergoes neither addition to ⁱPrN=C=NⁱPr nor nucleophilic attack to the newly formed guanidinate ligand under the conditions involved. This difference might be attributed to the strong chelating interaction between CONH and metal, leading to the lower nucleophilicity compared with the NH group.

  

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  Crystallographic data for 3 show that there are two independent molecules (Fig. 3), crystallizing in one asymmetrical unit. It is noteworthy that the π-electrons of the C=N double bond of two guanidinate groups are delocalized over the N-C-N unit (N(2)-C(1) and N(4)-C(1), 0.135(2) and 0.134(2) nm; C(49)-N(10) and C(49)-N(12), 0.136(2) and 0.135(2) nm), whereas the bond parameters (N(6)-C(25) and N(8)-C(25)) of a third guaninate group suggest a tendency toward CN double and single bonds (cf. mean values of 0.136 nm for C(sp²)-N and 0.129 nm for C(sp²)=N)^[31].

  图3 Thermal ellipsoid (30%) plot of complex {Cp₂Ln[μ-η¹:η²:η²-ⁱPrNC(NHⁱPr)NC₆H₄CONH]}₃Ln(HMPA)₃(Ln=Yb, 3a; Er, 3b; Y, 3c) Figure3. Thermal ellipsoid (30%) plot of complex {Cp₂Ln[μ-η¹:η²:η²-ⁱPrNC(NHⁱPr)NC₆H₄CONH]}₃Ln(HMPA)₃(Ln=Yb, 3a; Er, 3b; Y, 3c)

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  In order to exclude the effect of HMPA, the HMPA-free reaction was examined. As shown in Scheme 6, the treatment of Cp₃Ln with o-aminobenza-mide followed by reacting with ⁱPrN=C=NⁱPr gave {CpLn[μ-η¹:η²:η²-NHCOC₆H₄NC(NHⁱPr)NⁱPr]}₂ (Ln=Yb, 4a; Er, 4b; Y, 4c), indicating that the CONH group in 4 is also inert. Interestingly, 4 can be transformed into 3 in the presence of HMPA, revealing an unusual HMPA-induced ligand redistribution of organolantha-nides.

  Crystals suitable for a single-crystal X-ray study were grown by slow cooling a hot toluene solution of 4. As shown in Fig. 4, compounds 4a, 4b and 4c are isostructural, and each of the lanthanide atoms is coordinated by one η⁵-C₅H₅ ligand, one chelating η²-CONH with side donating coordination, one chelating guanidinate group and one bridging oxygen atom, indicating that only NHAr groups have combined with ⁱPrN=C=NⁱPr. There are no informal bond lengths and angles compared to other guanidinate lanthanide complexes^[27].

  图4 Thermal ellipsoid (30%) plot of complex {CpLn[μ-η¹:η²:η²-OCNHC₆H₄NC(NHⁱPr)NⁱPr]}₂ (Ln=Yb, 4a; Er, 4b; Y, 4c) Figure4. Thermal ellipsoid (30%) plot of complex {CpLn[μ-η¹:η²:η²-OCNHC₆H₄NC(NHⁱPr)NⁱPr]}₂ (Ln=Yb, 4a; Er, 4b; Y, 4c)

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  Based on these experimental results, a possible reaction pathway for the formation of 2 is proposed in Scheme 7. Coordination insertion of PhNCO into the Ln-NHAr bond followed by a ligand redistribution give the intermediate Ⅱ, which places the CONH group and the resulting ureido into close proximity, as observed in the formation of 3. A sequential addition of the activated N-H to the C=N double bond leads to the occurrence of the cyclization^[13]. Reductive elimination of PhNH₂ generates the quinazolyldiolate ring. Obviously, the formation of a more stable aromatic quinazolyldiolate skeleton might contribute to the occurrence of the cyclization/amine-elimination reaction of the putative amido intermediate(Ⅱ). But attempts to isolate the intermediates Ⅰ and Ⅱ have been unsuccessful due to their thermodynamic instability. A tandem addition/cyclization/elimination reaction is observed in the treatment of 1 with PhNCO, whereas for 3 no subsequent reactions are observed even with a longer reaction time and higher temperature. It is possible that both the larger steric hindrance and the weaker electrophilic reactivity of guanidinate compared to ureido prevent the subsequent cyclization.

  

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  3   Conclusions

  In conclusion, three new tetranuclear lanthano-cene derivatives incorporating the o-aminobenzamido dianion ligand, [CpLn(μ-η²:η²-NHC₆H₄CONH)(μ₃-η¹:η¹:η²-NHC₆H₄CONH)LnCp-(HMPA)}₂ (Ln=Yb, Er, Y), have been synthesized by reaction of Cp₃Ln with o-aminobenzamide followed by crystallization in a mixed HMPA and toluene solvent. The first example of tandem reaction of two coordinated NH moieties with an isocyanate molecule resulting in the formation of a dianionic quinazolyldiolate fragment is described. Furthermore, these o-aminobenzamido complexes can also undergo the single NH addition to carbodiimide selectively. In addition, an unusual HMPA-induced ligand redistribution of organolanthanides is observed in these processes. The present results demonstrate that o-aminobenzamido organolanthanide complexes can be expected to have synthetic potential because they provide a reaction system in which the two coordinated NH moieties show different reactivity toward the same functionality. In this case, after the first reaction one surviving NH group could undergo cyclization across the first reaction product. On the other hand, this work provides an alternative route to carbonylation of organic substrates, which is very difficult to achieve by rare earth metal system due to the mismatch in the orbital energy of the hard rare earth metal ion and the soft CO ligand. The catalytic synthesis of heterocycles, which is based on such reactions, is undergoing.

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• 

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  Figure 1  Thermal ellipsoid (30%) plot of complexes {CpLn(μ-η²:η²-NHC₆H₄CONH)[μ³-η¹:η¹:η²-NHC₆H₄CONH]LnCp-(HMPA)}₂ (Ln=Yb, 1a; Er, 1b)

  Hydrogen atoms, Cp rings and HMPA ligands are omitted for clarity; Selected bond lengths (nm) and angles (°): for 1a: Yb(1)-N(2) 0.228 5(6), Yb(1)-O(1) 0.226 0(5), Yb(1)-O(2A) 0.226 0(4), Yb(1)-N(3) 0.247 0(6), Yb(1)-N(4) 0.255 7(5), Yb(2)-N(1) 0.232 7(6), Yb(2)-O(1) 0.238 2(5), Yb(2)-O(3) 0.220 6(4), Yb(2)-N(3) 0.231 2(6), N(2)-Yb(1)-O(1) 74.6(2), N(3)-Yb(2)-N(4) 68.1(2); For 1b: Er(1)-N(2) 0.231 3(6), Er(1)-O(1) 0.226 9(5), Er(1)-O(2A) 0.227 9(5), Er(1)-N(3) 0.249 1(5), Er(1)-N(4) 0.253 8(6), Er(2)-N(1) 0.236 0(6), Er(2)-O(1) 0.241 5(5), Er(2)-O(3) 0.219 3(5), Er(2)-N(3) 0.234 3(6), N(1)-C(1) 0.128 1(9), O(1)-C(1) 0.130 6(8), N(4)-C(8) 0.134 5(9), O(2)-C(8) 0.126 4(8), N(2)-Er(1)-O(1) 74.4(2), N(3)-Er(2)-N(4) 67.4(2), N(1)-C(1)-O(1) 115.3(6), N(4)-C(8)-O(2) 123.8(6); Symmetry codes: A:-x, y, 0.5-z

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  Figure 2  Thermal ellipsoid (30%) plot of complex {Cp₂Yb[μ³-η²:η²:η¹-Quo]}₃Yb(HMPA)₂ (2a)

  Hydrogen atoms, Cp rings and HMPA ligands are omitted for clarity; Selected bond lengths (nm) and angles (°): Yb(1)-N(1) 0.254(1), Yb(1)-O(2) 0.234(1), Yb(1)-O(3) 0.217(1), Yb(2)-N(2) 0.244(1), Yb(2)-O(2) 0.228(1), Yb(2)-O(1B) 0.224(1), C(2)-N(2) 0.136(2), C(1)-N(2) 0.127(2), C(1)-N(1) 0.130(2), C(1)-O(2) 0.136(2), C(8)-N(1) 0.134(2), C(8)-O(1) 0.121(2), N(1)-C(1)-N(2) 136(1), C(1)-N(1)-C(8) 118(1), N(1)-C(8)-C(7) 107(1); Symmetry codes: A:-x+y, -x+1, -z+1.5; B: -y+1, x-y+1, z

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  Figure 3  Thermal ellipsoid (30%) plot of complex {Cp₂Ln[μ-η¹:η²:η²-ⁱPrNC(NHⁱPr)NC₆H₄CONH]}₃Ln(HMPA)₃(Ln=Yb, 3a; Er, 3b; Y, 3c)

  Hydrogen atoms and HMPA ligands are omitted for clarity; Selected bond lengths (nm) and angles (°) for 3a: Yb(1)-O(1) 0.214(1), Yb(1)-O(4) 0.222(1), Yb(2)-N(2) 0.241(1), Yb(2)-N(4) 0.231(1), Yb(2)-N(1) 0.242(1), C(1)-N(2) 0.131(2), C(1)-N(4) 0.133(2), Yb(3)-N(6) 0.241(1), Yb(3)-N(8) 0.228(1), Yb(3)-N(5) 0.234(1), C(25)-N(6) 0.132(1), C(25)-N(8) 0.141(2), Yb(4)-N(10) 0.246(1), Yb(4)-N(12) 0.232(1), Yb(4)-N(9) 0.235(1), C(49)-N(10) 0.131(2), C(49)-N(12) 0.134(1), N(2)-C(1)-N(4) 112(1); For 3b: Er(1)-O(1) 0.222(1), Er(1)-O(4) 0.218(1), Er(2)-N(3) 0.230(1), Er(2)-N(4) 0.233(1), Er(2)-N(1) 0.241(1), C(1)-N(2) 0.126(2), C(1)-N(4) 0.142(2), Er(3)-N(6) 0.242(2), Er(3)-N(8) 0.233(1), Er(3)-N(5) 0.235(1), C(25)-N(6) 0.136(2), C(25)-N(8) 0.135(2), C(25)-N(7) 0.135(2), Er(4)-N(10) 0.240(1), Er(4)-N(12) 0.228(1), Er(4)-N(9) 0.238(1), C(49)-N(10) 0.139(2), C(49)-N(12) 0.148(2), N(2)-C(1)-N(4) 112(1); For 3c: Y(1)-O(1) 0.216(1), Y(1)-O(4) 0.228(1), Y(2)-N(2) 0.247(1), Y(2)-N(4) 0.233(1), Y(2)-N(1) 0.238(1), C(1)-N(2) 0.131(1), C(1)-N(4) 0.141(1), Y(3)-N(6) 0.246(1), Y(3)-N(8) 0.2339(8), Y(3)-N(5) 0.2414(7), C(25)-N(6) 0.132(1), C(25)-N(8) 0.138(1), C(25)-N(7) 0.136(1), Y(4)-N(10) 0.244(1), Y(4)-N(12) 0.235(1), Y(4)-N(9) 0.235(1), C(49)-N(10) 0.130(1), C(49)-N(12) 0.138(1), C(49)-N(11) 0.138(1), N(2)-C(1)-N(4) 115 (1)

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  Figure 4  Thermal ellipsoid (30%) plot of complex {CpLn[μ-η¹:η²:η²-OCNHC₆H₄NC(NHⁱPr)NⁱPr]}₂ (Ln=Yb, 4a; Er, 4b; Y, 4c)

  Hydrogen atoms are omitted for clarity; Selected bond lengths (nm) and angles (°) for 4a: Yb-N(3) 0.233(1), C(3)-N(2) 0.137(1), N(2)-Yb-N(3) 57(2), N(1)-C(1)-O(1) 114(1), Yb-O(1)-Yb(A) 112(2), N(2)-C(8)-N(3) 114(1); For 4b: Er-N(3) 0.234(1), C(3)-N(2) 0.138(1), N(2)-Er-N(3) 57(2), N(1)-C(1)-O(1) 115(1), Er-O(1)-Er(A) 111(2), N(2)-C(8)-N(3) 112(1); For 4c: Y-N(3) 0.235(1), C(3)-N(2) 0.138(1), O(1)-Y-O(1A) 69(1), N(1)-C(1)-O(1) 114(1), N(1)-C(1)-C(2) 125(1), Y-O(1)-Y(A) 111(1), N(2)-C(8)-N(3) 112(1); Symmetry codes: A: 0.5-x, 0.5+y, 0.5-z

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  Table 1.  Crystal and data collection parameters of complexes 1a, 1b and 4a~4c

  	1a	1b	4a	4b	4c
  Formula	C60H80N14O6P2Yb4	C60H80N14O6P2Er4	C38H50N8O2Yb2	C38H50N8O2Er2	C38H50N8O2Y2
  Formula weight	1 847.48	1 824.36	996.94	985.38	828.68
  Crystal size/mm	0.25×0.22×0.14	0.15×0.12×0.10	0.30×0.25×0.15	0.35×0.18×0.08	0.19×0.10×0.08
  Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
  Space group	C2/c	C2/c	P21/n	P21/n	P21/n
  a/nm	2.837 8(9)	3.091 3(10)	1.388 2(5)	1.390 9(3)	1.395 5(4)
  b/nm	1.256 2(4)	1.330 0(4)	0.990 0(4)	0.993 2(2)	0.995 5(3)
  c/nm	2.539 3(8)	2.466 4(8)	1.474 2(5)	1.477 2(3)	1.481 8(5)
  β/(°)	106.834(4)	112.082(4)	102.548(5)	102.550(2)	102.724(4)
  V/nm3	8.664(5)	9.397(5)	1.977 7(12)	1.991 9(7)	2.007 9(10)
  Z	4	4	2	2	2
  Dc/(g·cm-3)	1.416	1.290	1.674	1.643	1.371
  μ/mm-1	4.360	3.611	4.742	4.226	2.917
  F(000)	3 584	3 552	980	972	856
  Reflection measured	19 258	19 109	8 648	8 713	8 896
  Unique reflection	8 420(Rint=0.046 0)	8 311(Rint=0.038 5)	3 822(Rint=0.076 8)	3 890(Rint=0.032 1)	2 916(Rint=0.068 7)
  Goodness-of-fit on F2	1.002	1.006	1.015	0.993	0.997
  Final R indices	R1=0.040 3,	R1=0.039 1,	R1=0.051 3,	R1=0.033 3,	R1=0.038 6,
  [I > 2σ(I)]	wR2=0.079 0	wR2=0.105 6	wR2=0.134 4	wR2=0.088 2	wR2=0.055 7
  R indices (all data)	R1=0.074 0, wR2=0.082 7	R1=0.062 0, wR2=0.111 2	R1=0.061 4, wR2=0.140 7	R1=0.040 2, wR2=0.093 1	R1=0.083 2, wR2=0.059 3

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  Table 2.  Crystal and data collection parameters of complexes 2a and 3·THF

  	2a	3a·THF	3b·THF	3с·THF
  Formula	C66H78N12O8P2Yb4	C94H152N21O7P3Yb4	C94H152Er4N21O7P3	C94H152N21O7P3Y4
  Formula weight	1 921.50	2 473.44	2 450.32	2 136.92
  Crystal size/mm	0.30×0.20×0.15	0.15×0.06×0.05	0.40×0.15×0.10	0.20×0.12×0.12
  Crystal system	Trigonal	Triclinic	Triclinic	Triclinic
  Space group	P63/m	P1	P1	P1
  a/nm	1.563 4(4)	1.579 2(5)	1.586 9(4)	1.590 4(11)
  b/nm	1.563 4(4)	1.858 6(6)	1.806 1(4)	1.869 4(13)
  c/nm	2.074 7(8)	2.461 4(7)	2.494 5(6)	2.471 2(16)
  α/(°)	90	77.664(5)	77.505(3)	77.669(11)
  β/(°)	90	78.642(5)	77.840(3)	78.692(10)
  γ/(°)	120	66.029(4)	66.919(3)	65.831(10)
  V/nm3	4.392(2)	6.400(3)	6.358(3)	6.500(8)
  Z	2	2	2	2
  Dc/(g·cm-3)	1.453	1.284	1.242	1.092
  μ/mm-1	4.305	2.983	2.698	1.854
  F(000)	1 864	2 488	2 390	2 240
  Reflection measured	18 196	26 698	25 656	27 071
  Unique reflection	2 661(Rint=0.067 8)	22 124(Rint=0.057 8)	21 540(Rint=0.063 1)	22 493(Rint=0.063 2)
  Goodness-of-fit on F2	1.002	1.016	1.052	1.204
  Final R indices [I > 2σ(I)]	R1=0.051 6, wR2=0.150 8	R1=0.059 8, wR2=0.140 1	R1=0.076 3, wR2=0.145 5	R1=0.072 4, wR2=0.155 0
  R indices (all data)	R1=0.067 3, wR2=0.159 7	R1=0.116 1, wR2=0.150 0	R1=0.185 8, wR2=0.162 7	R1=0.175 5, wR2=0.165 6

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