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Russian Chemical Bulletin, International Edition, Vol. 63, No. 12, pp. 2630—2634, December, 2014

Synthesis, structure, and catalytic activity of new aluminum complexes formed with sterically bulky ligands K. V. Zaitsev,a E. A. Kuchuk,a B. N. Mankaev,a A. V. Churakov,b G. S. Zaitseva,a D. A. Lemenovskii,a and S. S. Karlova aM.

V. Lomonosov Moscow State University, Department of Chemistry, Build. 3, 1 Leninskie Gory, 119991 Moscow, Russian Federation. Fax: +7 (495) 932 8846. Email: [email protected] bN. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 31 Leninsky prosp., 119991 Moscow, Russian Federation A reaction of pyridine related ligands containing two hydroxyalkyl groups, 2,6(HO—Z— CH2)2C5H3N (Z is 1,1cyclohexylene, 2,2adamantylene) with trimethylaluminum and tri methoxyaluminum leads to three new aluminum complexes with the Me—Al or MeO—Al bond: 2,6C5H3N(CH2—Z—O)2Al—X (Z is 1,1cyclohexylene, X = Me (5), OMe (6); Z is 2,2adamantylene, X = Me (7)). The structures of new derivatives were confirmed by NMR spectroscopy and elemental analysis. The structure of complex 5 was studied by Xray diffrac tion analysis. Complex 5 is a dimer in the solid state due to the formation of a fourmembered ring ...Al—O...Al—O.... Complexes 5 and 6 were studied as initiators of the ringopening polymerization of caprolactone. Key words: aluminum, complexes, polymerization, biodegradable polymers, Xray diffrac tion analysis, tridentate ligands.

Nowadays, biodegradable and biocompatible synthetic polymers derived from aliphatic esters, such as polylactide, polyglycolide, polycaprolactone, become an important class of polymeric compounds used in applied areas. They are used in different branches, first of all, for production of food packaging, as well as in medicine and pharmacology (for development of drug delivery systems, fabrication of implants and scaffolds for biological tissues). In contrast to other plastics, biodegradable polymers can be degraded by microorganisms in different natural media within rela tively short period of time (for example, within half a year), at the same time, most of industrial methods used in the manufacture of products from standard plastics can be used in their processing. The synthesis of this type of bio degradable polymers is carried out by a ringopening poly merization (ROP) of cyclic esters.1 The metal complex initiated ringopening polymerization (in Scheme 1, an example of polymerization of caprolactone upon treat ment with aluminum derivatives is given) allows to con trol the process and leads to the material with a required structure, stereochemistry, and molecularmass distri bution.1,2 It should be noted that the properties of an obtained polymer strongly depend on the structure of the initiator used. At the present time, a toxic tin bis(2ethylhexanoate) is used as an initiator in the industrial synthesis of poly mers based on aliphatic esters, therefore, it seems very

Scheme 1

Reagents and conditions: i. LAlMe/ROH; ii. 1) LAlOMe, 2) ROH.

actual to obtain new nontoxic initiators of polymeriza tion.1 The purpose of the present work is to synthesize new aluminum complexes, to study structure of these com pounds, including their oligomerity degree, to examine a possibility of their use in polymerization of capro lactone. Results and Discussion It is obvious that the structure of initiator has a crucial effect on the properties of synthesized polymers. To ob tain the reproducible results, it is necessary for the initia tor molecule to be monomeric or dimeric. The oligomeri

Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2630—2634, December, 2014. 10665285/14/63122630 © 2014 Springer Science+Business Media, Inc.

Al complexes in polymerization of caprolactone

Russ.Chem.Bull., Int.Ed., Vol. 63, No. 12, December, 2014

ty degree in this case is controlled by the ligand design. An introduction of sterically bulky substituents in the ligand fragment stabilizes the desired complex geometry. At the same time, it should be noted that the use of too bulky groups can complicate polymerization, hindering a nucleo philic attack on the electrophilic metal center. We believe that at present, one of the most structurally promising ligand systems for these purposes are 2,6di hydroxyalkylpyridines, which were earlier successfully used for the synthesis of complexes of different metals (germa nium, tin, titanium), including for the purpose of stabili zation of unusual oxidation states and coordination poly hedrons of metal atoms.3—6 The advantage of this ligand system consists in the relative simplicity of the synthesis of compounds with different groups. Besides, we believe that such systems can efficiently stabilize the most suitable for polymerization geometry of complexes. Analysis of literature data showed that by now no alu minum complexes based on dihydroxyalkylpyridines are described, whereas only several aminodiphenolate deriva tives are known for another aluminum compounds based on the ONOtype tridentate ligands.7—9 The ligand 1 was synthesized according to the following scheme: a sequen tial treatment of 2,6lutidine with nbutyllithium (1.1 equiv.) and cyclohexanone gave a pyridinecontain ing monoalcohol 2. Its further sequential treatment with nbutyllithium (2.1 equiv.) and a carbonyl compound gave dialcohol 1 (Scheme 2). The earlier described ligand 3 was obtained by a similar scheme using adamantanone as the carbonyl compound.10 Compounds 1 and 2 were obtained for the first time. Scheme 2

Z=

(1, 2),

(3, 4)

The reaction of ligands 1 and 3 with 1 equivalent of trimethylaluminum or trimethoxyaluminum (generated in situ from AlMe3 and methanol) furnished three earlier unknown target aluminum complex 5—7 in high yields (Scheme 3).

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Scheme 3

Z=

(1, 5, 6),

(3, 7)

X = Me (5, 7), OMe (6)

The structures of these compounds in solution (for 5 and 7 in CDCl3, for 6 in THFd8) were established by multinuclear NMR spectroscopy. The NMR spectra of the aluminum methyl derivatives 5 and 7 are character ized by the presence of one set of signal, that unambigu ously indicates the monomeric structure of these com pounds (since a dimeric compound in this case should be formed by the reaction between the oxygen and the alumi num atoms, which would have lead to the nonequivalence of two symmetric "halves" of the ligand). In the case of compound 6 containing a OMe group at the Al atom, the NMR spectra are also characterized by the presence of one set of signals, that indicates that only one species exists in the solution. However, in this case we cannot unambiguously draw a conclusion on the association type of this derivative (monomeric or dimeric), since the dimer ization can occur via the MeOAl interaction without disturbance of the ligand symmetry. The structure of complex 5 was studied by Xray dif fraction analysis (Fig. 1). Selected bond distances and bond angles are given in Tables 1 and 2, respectively. The structure 5 contain two independent molecules of the com plex with close geometric parameters. The main differ ence is observed in the conformations of cyclohexane rings at atoms C(21) and C(41). Xray diffraction studies show that complex 5 is a dimer in the solid state due to the formation of a fourmembered ring ...Al—O...Al—O.... The dimerization occurs via the formation of a coordination bond between the aluminum atom and the oxygen atom of the second monomeric unit, with both methyl groups being arranged on the same side of the fourmembered ring (cisisomer). The coordination number (CN) of aluminum atom in complex 5 is five, whereas the coordination polyhedron of the central atom is a distorted trigonal bipyramid, with the nitrogen and oxygen atoms of the second monomeric unit occupying the axial positions. The Al—O(CN 2) bond distances, i.e., between the aluminum atom and the oxygen atom not involved in the formation of additional interactions, are predictably shorter, than the Al—O(CN 3) bond distances.

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C(24)

Table 2. Principal bond angles in the structure 5*

C(25)

C(22)

C(61)

C(23) C(26)

C(62)

C(65)

C(67) C(66)

A

C(63) C(64) C(59) C(60) C(58) O(21)

C(2) N(2) Al(2)

C(28)

/deg

Angle

C(21)

C(27)

Zaitsev et al.

O(12)

C(29)

O—Al—O

122.10(6) 93.82(5) 77.69(5) 121.54(6) 93.26(5) 77.07(5) 124.17(7) 113.69(7) 99.90(6) 124.67(7) 113.77(7) 100.30(7) 89.96(6) 91.42(6) 168.86(6) 89.33(5) 90.53(5) 166.79(6) 86.55(6) 88.79(7)

C(19)

O(22)

O—Al—C C(18) C(69) C(70) C(68)

C(56)

C(57)

Al(1)

C(1)

N(1) O(11)

C(17)

C(52)

C(16)

C(53)

O—Al—N

C(51)

C(54)

C(15)

C(13)

C(55) C(12)

C(14)

Fig. 1. Molecular structure of compound 5. One independent molecule is shown. Thermal ellipsoids are given with 50% prob ability. Hydrogen atoms and toluene solvent molecule are omitted.

B

C—Al—N

123.60(6) 92.82(5) 77.74(5) 121.68(6) 90.74(5) 77.76(5) 124.00(7) 112.39(6) 99.92(6) 125.17(7) 113.15(7) 101.39(7) 90.05(6) 91.11(5) 168.16(6) 89.03(6) 91.23(5) 166.88(6) 87.94(6) 89.42(7)

* Two independent molecules, A and B.

To sum up, a dimeric structure of complex 5 in the solid state differs from the monomeric structure of this complex in solution. The activities of complexes 5 and 6 as initiators of polymerization were studied using a model reaction with caprolactone (Table 3). For the methyl complex 5, the polymerization was carried out in solution (in the pres ence of benzyl alcohol, BnOH, as an external nucleo phile), in the case of methoxy complex 6, it was carried out in bulk (in the monomer melt). All the compounds studied were found to be active in the ringopening poly

merization of caprolactone and demonstrated high con version degrees of the monomer. For compound 6, the polymer molecular weight was shown to increase as the [initiator]/[monomer] ratio increased. In conclusion, in the course of this study we synthe sized and structurally characterized new aluminum com plexes based on sterically bulky 2,6dihydroxyalkylpyr idines, which possess catalytic activity in the ringopening polymerization of cyclic esters.

Table 1. Principal interatomic distances in the struc ture 5a

Table 3. Ringopening polymerization of caprolactone initiat ed by complexes 5 and 6

Distance

Ina

Al—O(CN 2)

d/Å

b

Al—O(CN 3)b

Al—Al Al—C Al—N a b

A

B

1.7457(12) 1.7552(12) 1.8239(12) 1.9276(11) 1.8347(12) 1.9422(12) 2.9379(7) 2.0196(17) 1.9897(19) 2.1584(15) 2.1732(14)

1.7479(12) 1.7429(13) 1.8225(12) 1.9426(12) 1.8284(12) 1.9366(12) 2.9320(7) 2.0182(17) 1.9841(18) 2.1620(14) 2.1655(15)

Two independent molecules, A and B. Coordination number of oxygen atom.

5d 6e 6f a

[In]/ [BnOH]0

t/h

1 : 1.7 —

1 2 3 0.5 1

— —

Cb (%) >99 40 >99 42 92

Mnc/g mol–1 GPC

NMR

10673 — 20007 — 35341

16524 12572 34232 — 66322

Mw/Mn

1.25 — 1.56 — 2.02

Initiator. Conversion. c M is the number mass of polymer determined by gel perme n ation chromatography and 1H NMR spectroscopy. d Polymerization in a solution in toluene (5 mL), 100 C, [In] = 0.02 mmol L–1; [M]0/[In] = 300 : 1, M is the monomer. e Polymerization in bulk: 100 C, [M] /[In] = 300 : 1. 0 f Polymerization in bulk: 130 C, [M] /[In] = 600 : 1. 0 b

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Experimental All manipulations with aluminum derivatives were carried out under dry argon using standard Schlenk technique. 1H and 13C NMR spectra were recorded on Bruker Avance 400 or Agi lent 400 MR spectrometers (400 MHz and 100 MHz, respec tively) at room (25 C) temperature in CDCl3 or THFd8, using signals of residual protons of deuterosolvents as references; chem ical shifts are given relative to Me4Si. Elemental analysis was performed in the Laboratory of Organic Microanalysis of De partment of Chemistry, M. V. Lomonosov Moscow State Uni versity. Compounds 5 and 7 appeared to be extremely hydrolyti cally unstable, which made it difficult to obtain satisfactory ele mental analysis data for these compounds (see the experimental data). Gel permeation chromatography was carried out on a HPLC chromatograph (a Styrogel column HR 5E, HR 4E, 300h7, 8 mm, refractometric detector), THF was the solvent, the flow rate was 1 mL min–1, the sample concentration was 1 mg mL–1, the sample amount was 100 L, polystyrene stan dards were used for calibration. Column chromatography was carried out on Merck silica gel (60—100 mesh) Solvents were purified according to the standard procedures; THF was main tained over KOH, then refluxed and distilled over metallic sodi um in the presence of benzophenone. Cyclohexanone and 2,6lutidine were purified by distillation at atmospheric pres sure. Adamantanone (Aldrich), nbutyllithium (2.5 M solution in hexane) (Aldrich), AlMe3 (2 M solution in toluene) (Aldrich) were used without additional purification. 2[(6Methylpyridin 2yl)methyl]adamantan2ol (4)10 and 2,2´[pyridin2,6di yldi(methylene)]diadamantan2ol (3)10 were obtained accord ing to the known procedures. 1[(6Methylpyridin2yl)methyl]cyclohexanol (2). nButyl lithium (37.2 mL, 0.09 mol, 2.5 M solution in hexane) was added dropwise to a solution of 2,6lutidine (10.00 g, 0.09 mol) in THF (200 mL) at –60 C. The mixture was stirred for 1 h at –60 C, followed by a dropwise addition of a solution of cyclohexanone (8.83 g, 0.09 mol) in THF (25 mL) at –50 C. The mixture was slowly warmedup to room temperature and stirred for 16 h. Then, the solution was acidified with 2 M HCl to pH = 1 (univer sal indicator) and stirred for 1 h. After addition of 2 M NaOH to neutrality, the aqueous layer was twice extracted with ethyl acet ate, the combined organic layers were dried with Na2SO4. The solvent was evaporated to obtain a yellow oil. The target product was isolated by column chromatography (eluent light petrole um—dichloromethane—triethylamine = 4 : 1 : 0.1; Rf(2) = 0.4). Alcohol 2 (12.35 g, 65%) was obtained as a dense colorless oil. 1H NMR (CDCl ), : 7.45 (t, 1 H, C(4)H , J = 7.6 Hz); 6.95 3 Py (d, 1 H, C(3)HPy or C(5)HPy, J = 7.8 Hz); 6.86 (d, 1 H, C(3)HPy or C(5)HPy, J = 7.6 Hz); 6.05 (s, 1 H, OH); 2.80 (s, 2 H, CH2Py); 2.46 (s, 3 H, CH3Py); 1.23—1.66 (m, 10 H, Hcyclohex). 13C NMR (CDCl3), : 158.83, 157.14, 136.87, 121.24, 120.8 (CPy), 71.38 (HOCCH2), 46.68 (CCH2Py), 37.93 (CH3Py), 25.82, 24.23, 22.26 (Ccyclohex). Found (%): C, 76.40; H, 9.15; N, 6.77. C13H19NO. Calculated (%): C, 76.06; H, 9.33; N, 6.82. 1,1´[Pyridin2,6diyldi(methylene)]dicyclohexanol) (1). nButyllithium (32 mL, 0.08 mol, 2.5 M solution in hexane) was added to monoadduct 2 (8.25 g, 0.04 mol) in THF (100 mL) at –10 C. The mixture was stirred for 4 h, then a solution of cyclohexanone (3.92 g, 0.04 mol) in THF (10 mL) was rapidly added at 0 C, and the mixture was allowed to stand for 16 h.

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Saturated aqueous NH4Cl was added. The aqueous layer was extracted with CH2Cl2 (3×30 mL), the combined organic layers were washed with NaClsaturat and dried with Na2SO4. The sol vent was evaporated to obtain a yellowish oil, which was purified by column chromatography (diethyl ether—light petroleum, 2 : 1; Rf(1) = 0.4). Dialcohol 1 (9.22 g, 76%) was obtained as a dense colorless oil. 1H NMR (CDCl3), : 7.53 (t, 1 H, C(4)HPy, J = 7.4 Hz); 7.00 (d, 2 H, C(3)HPy or C(5)HPy, J = 7.4 Hz); 4.05 (s, 2 H, OH); 2.87 (s, 4 H, CH2Py); 1.61, 1.46, 1.25 (all m, 20 H, Hcyclohex). 13C NMR (CDCl3), : 158.13, 136.85, 122.38 (CPy), 71.60 (HOCCH2), 48.22 (CCH2Py), 37.85, 25.73, 22.23 (Ccyclohex). Found (%): C, 75.43; H, 9.71; N, 4.40. C19H29NO2. Calculat ed (%): C, 75.21; H, 9.63; N, 4.62. 5´Methyl4´,6´dioxa13´aza5´alumadispiro[cyclohex ane1,3´bicyclo[7.3.1]tridecane7´,1cyclohexane]1´(13´), 9´,11´triene (5). Trimethylaluminum (1.29 mL, 2.57 mmol, 2.0 M solution in toluene) was added dropwise to a solution of ligand 1 (0.78 g, 2.57 mmol) in toluene (20 mL) keeping the temperature within (–40—–30 C). The mixture was stirred at room temperature for three days. Volatile components were evap orated to obtain a light yellow powder, which was recrystallized from a mixture of hexane—toluene to obtain complex 5 (0.57 g, 65%) as a white friable powder. 1H NMR (CDCl3), : 7.55 (t, 1 H, C(4)HPy, J = 7.4 Hz); 6.98 (d, 2 H, C(3)HPy or C(5)HPy, J = 7.4 Hz); 3.04 (d, 2 H, CH2Py, J = 5.5 Hz); 0.88—1.91 (20 H, Hcyclohex); –1.02 (s, 3 H, AlMe). 13C NMR (CDCl3), : 159.24, 157.32, 122.99 (Carom), 75.19 (AlOCCH2), 70.01 (CCH2Py), 38.44, 24.68, 23.81 (Ccyclohex), no signal for the AlMe was ob served. Found (%): C, 69.15; H, 8.60. C20H30AlNO2. Calculat ed (%): C, 69.94; H, 8.80. 5´Methyl4´,6´dioxa13´aza5´alumadispiro[cyclohex ane1,3´bicyclo[7.3.1]tridecane7´,1cyclohexane]1´(13´), 9´,11´triene (6). A solution of methanol (0.61 mL, 0.015 mol) in toluene (7 mL) was added dropwise to a solution of Me3Al (2.50 mL, 5.00 mmol, 2 M solution in toluene) at –70 C. The mixture was stirred for 1 h, the temperature was gradually raised to ambient, followed by a dropwise addition of a solution of ligand 1 (1.52 g, 5.00 mmol) in toluene (5 mL). The reaction mixture was heated to 95 C, stirred for 20 h, formation of a white precipitate was observed. The precipitate was filtered off, washed with some toluene and dried in vacuo to obtain complex 6 (1.53 g, 85%) as a white powder. 1H NMR (THFd8), : 7.81 (t, 1 H, C(4)HPy, J = 7.8 Hz); 7.37 (d, 2 H, C(3)HPy or C(5)HPy, J = 7.4 Hz); 3.89 (s, 4 H, CH2Py); 3.13 (s, 3 H, OCH3); 1.54—2.03 (m, 20 H, Hcyclohex). 13C NMR (THFd8), : 159.52, 137.12, 123.25 (C Py), 71.59 (AlOCCH2 ), 50.46 (CCH 2Py), 39.09 (AlOCH3), 27.16, 25.66 23.24 (Ccyclohex). Found (%): C, 66.42; H, 8.18; N, 3.50. C20H30NO3Al. Calculated (%): C, 66.83; H, 8.41; N, 3.90. 5´Methyl4´,6´dioxa13´aza5´alumadispiro[tricyclo [3.3.1.13,7]decane2,3´bicyclo[7.3.1]tridecane7´,2tricyclo [3.3.1.13,7]decane]1´(13´),9´,11´triene (7). Trimethylalumi num (0.50 mL, 1.0 mmol, 2 M solution in toluene) was added dropwise to a solution of ligand 3 (0.41 g, 1.0 mmol) in anhy drous toluene (20 mL) at –30 C. The temperature was slowly raised to ambient. The reaction mixture was stirred for 1 day. Toluene was evaporated in vacuo, diethyl ether was added to the residue, which was stirred for 30 min. A precipitate formed was filtered off, washed with diethyl ether (2×2 mL), and dried in vacuo to obtain complex 7 (0.42 g, 93%) as a white solid com pound. 1H NMR (CDCl3), : 7.80 (t, 1 H, C(4)HPy, J = 7.8 Hz);

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7.23 (d, 2 H, C(3)HPy or C(5)HPy, J = 7.8 Hz); 3.65 (d, 2 H); 2.80 (d, 2 H); 2.56 (d, 2 H, J = 14.3 Hz); 2.35 (d, 2 H, J = 14.3 Hz); 1.42—1.85 (m, 24 H, HAd (Ad is the adaman tylene)); –0.76 (s, 3 H, AlCH3). 13C NMR (CDCl3), : 27.53, 32.72, 34.67, 37.26, 38.70 (CAd), 44.72 (PyCH2), 75.22 (COAl), 123.84, 140.64, 159.19 (CPy), no signal for the AlMe group was observed. Found (%): C, 74.55; H, 8.18. C28H38NO2Al. Calcu lated (%): C, 75.14; H, 8.56. Xray diffraction analysis of compound 5 was performed on a Bruker SMART APEX II automated diffractometer at 150 K (MoK radiation,  = 0.71073 Å, graphite monochromator). Crystals 5 (C87H128Al4N4O8, M = 1465.85) are monoclinic, space group Р21/n, a = 21.9971(8), b = 12.7385(4), c = 28.6355(10) Å,  = 90.426(1), V = 8023.7(5) Å3, Z = 4, dcalc = 1.213 g cm–3, (MoK) = 0.116 mm–1, F(000) = 3176. Intensities of 72209 reflections (19361 of them were independent, Rint = 0.0533) were measured using scan technique in the range of 1.16 <  < 28.00 (–29  h  29, –16  k  16, –37  l  33). Adsorption correction was included based on the measurements of intensities of equivalent reflections.11 The structure was solved by direct method; all the nonhydrogen atoms (except those of the disordered toluene molecules) were refined by the fullma trix anisotropic least squares method on F 2 (SHELXTL12). One of the solvent toluene molecules (C(91)—C(94)) is disorder ed over two positions with equal occupancies on a crystallo graphic inversion center. The second solvent toluene molecule (C(101)—C(107), C(201)—C(207)) is also disordered on the in version center, but rather over four positions (the occupancies are 0.34/0.34/0.17/0.17). All the hydrogen atoms were placed in the calculated positions and refined using a riding model. The final R factors are as follows: R1 = 0.0480 for 13416 reflections with I > 2(I) and wR2 = 0.1217 on all the data using 940 para meters of refinement, GOOF = 1.014, min/max = –0.276/0.322. The structure 5 was deposited with the Cambridge Structural Database (CCDC 1034218).

This work was financially supported by the Russian Scientific Foundation (Project No. 143300017; synthe

Zaitsev et al.

sis of compounds 2, 4, and 7, Xray diffraction and poly merization studies) and the Russian Foundation for Basic Research (Project No. 120300206; synthesis of com pounds 1, 3, 5, and 6). References 1. O. DechyCabaret, B. MartinVaca, D. Bourissou, Chem. Rev., 2004, 104, 6147. 2. J. Wu, T.L. Yu, C.T. Chen, C.C. Lin, Coord. Chem. Rev., 2006, 250, 602. 3. M. Huang, E. Kh. Lermontova, K. V. Zaitsev, A. V. Churak ov, Y. F. Oprunenko, J. A. K. Howard, S. S. Karlov, G. S. Zaitseva, J. Organomet. Chem., 2009, 694, 3828. 4. K. V. Zaitsev, M. V. Bermeshev, S. S. Karlov, Y. F. Oprunen ko, A. V. Churakov, J. A. K. Howard, G. S. Zaitseva, Inorg. Chim. Acta, 2007, 360, 2507. 5. R. Fandos, B. Gallego, M. I. LуpezSolera, A. Otero, A. Rodríguez, M. J. Ruiz, P. Terreros, T. van Mourik, Organometallics, 2009, 28, 1329. 6. E. Gуmez, R. Flores, G. Huerta, C. AlvarezToledano, R. A. Toscano, V. Santes, N. Nava, P. Sharma, J. Organomet. Chem., 2003, 672, 115. 7. C.T. Chen, C.A. Huang, B.H. Huang, Dalton Trans., 2003, 3799. 8. C.T. Chen, C.A. Huang, B.H. Huang, Macromolecules, 2004, 37, 7968. 9. G. Szigethy, A. F. Heyduk, Dalton Trans., 2012, 8144. 10. B. Koning, J. Buter, R. Hulst, R. Stroetinga, R. M. Kellogg, Eur. J. Org. Chem., 2000, 2735. 11. G. M. Sheldrick, SADABS. Program for scaling and correc tion of area detector data. University of Göttingen. Ger many, 1997. 12. G. M. Sheldrick, Acta. Crystallogr., 2008, A64, 112. Received November 17, 2014