Article pubs.acs.org/Organometallics
Synthesis of Unsaturated Starburst Compounds with a Boron Atom in the Core Jędrzej Walkowiak and Bogdan Marciniec* Center of Advanced Technologies and Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznań, Poland S Supporting Information *
ABSTRACT: A highly effective method for regio- and stereoselective synthesis of unsaturated starburst compounds with a boron atom core and a trans configuration of all double bonds via silylative coupling of selected olefins (styrenes, vinylboronates, and vinylsilanes) with boron tris(dimethylvinylsiloxide) catalyzed by [Ru(CO)ClH(PCy3)2] is described. All products could be potential precursors for new highly branched materials with unique mechanical, thermal, and optoelectronic properties.
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complexes containing or generating hydride ([TM]-H) or silyl ([TM]-Si) ligands (silicometallics) (where TM = Ru, Rh, Ir, Co). This mode of reactivity seems to be general and is also exhibited by vinylboranes (trans-borylation)9 and vinylgermanes (trans-germylation).10 These reactions have become valuable synthetic tools in the preparation of vinyl-functionalized organometalloid compounds. The general scheme is shown by eq 1.6
endrimers constitute a class of highly branched, symmetric and precisely designed molecules, whose properties depend on the core of the molecule as well as on the types of functional groups attached.1 Over the last few decades a new class of dendrimers, called metallodendrimers, containing main-group elements and transition metals, has attracted much attention because of their unique chemical and physical properties. Metallodendrimers can be used in many branches of chemistry, biology, medicine, and materials science. Their structure determines potential applications of these highly branched compounds in catalysis, photoelectronic, lightharvesting, host−guest chemistry, and new hybrid inorganic/ organic materials.2 There are only a few known examples of dendrimers containing a boron atom in the core or at the ends of dendrimer arms. Carboborane-modified star-shaped molecules synthesized via cobalt-catalyzed cycloaddition are used in neutron capture therapy in cancer disease.3 Boron-modified dendrimers are also used as active Lewis acid catalysts for, among others, hydrosilylation reactions of ketones.4 Such catalysts can be easily separated from the reaction mixture, recycled, and used in the subsequent catalytic process. The presence of boron and silicon atoms in dendrimer structures determines the physical and chemical properties of new materials. Such dendrimers enhance the thermal and mechanical properties and can be used as precursors for new types of ceramic or optoelectronic materials.5 The investigation of new, highly selective protocols for the synthesis of branched organometallic compounds containing boron and silicon is of great interest. The contribution of our group was the development of a unique, convenient, effective, highly regio- and stereoselective method for the functionalization of molecular and macromolecular compounds containing one or a few vinyl groups connected to the silicon atom.6−8 The process, named silylative coupling (SC), occurs via cleavage of the C−Si bond in vinylsilane and the C−H bond in olefin and is catalyzed by © 2012 American Chemical Society
The regio- and stereoselectivities of these processes are strongly dependent on the reaction conditions and the type of reagents. For example, for the silylative coupling of vinylsilanes with vinylboranes at low (0−25 °C) temperatures the formation of 1,1-borylsilylethene is strongly recommended. A higher temperature increases the amount of 1-boryl-2silylethene.7c,11 Silylative coupling is a very powerful tool for the synthesis of unsaturated, highly conjugated compounds. The process was used to modify tris(dimethylvinylsilyl)benzene with substituted styrenes, giving as a product starburst unsaturated compounds with silylene−vinylene−arylene sequences with high yields and selectivities. Such compounds show photo- and electroluminescence properties.12 Modification of multi-vinyl-substituted cyclosiloxanes and silsesquioxanes with highly π-conjugated olefins was recently carried out using this coupling reaction.13 This coupling reaction is often an alternative to metathesis for the synthesis of silyl-substituted olefins, especially when methyl-substituted vinylsilanes are applied as reagents. As was Received: November 9, 2011 Published: May 8, 2012 3851
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molar excess of styrene (and 1.5 molar excess of vinylboronate) for each vinyl group of tris(dimethylvinylsilyl)borate (I) is sufficient for total conversion of the substrate. Such an excess causes no problem during the isolation of the products 1−7. The best results were obtained when 2 mol % of the ruthenium catalyst in relation to I was used. A lower concentration of the catalyst extends the time necessary for total conversion of the substrate. When 3 mol % of ruthenium complex was used in the reaction of I with vinyloboronate, the presence of bisborylethene was detected in the reaction mixture. This is a product of a competitive homocoupling reaction (trans-borylation), in which the C−B bond in one molecule and the C−H bond in another vinylboronate molecule are catalytically activated. It was also shown that the reactions should be carried out for 24 h if a high yield of the products is to be expected.19 In the reactions with vinylsilanes, a 2- or 3-fold excess of vinylsilane for each vinyl group in boron siloxide I is required to obtain a desirable product, but difficulties in the process selectivity arise (Table 2). When vinylsilane is used as a reagent, two parallel reactions of homocoupling of vinylsilane (eq 3) and its cross-coupling with tris(dimethylvinylsilyl)borate (I) are observed.
previously proved, when the number of methyl substituents in vinylsilanes is increased, the amount of the desired metathetic product decreases. Substitution of OEt or OSiMe3 by methyl groups increases competition to metathesis β-SiR3 elimination in β-SiR3-substituted ruthenacyclobutanes followed by reductive elimination and formation of allylsilane as the main product. Such a transformation reduces significantly the number of species active in metathesis and slows down or almost completely inhibits the formation of ethenylsilanes.14,15 Palladium-catalyzed Heck reactions of vinylsilanes with olefins are other routes to silyl-substituted ethenes. This coupling protocol, which has found many applications in organic synthesis and industry, can be applied to any kind of alkene, but for vinylsilanes it is quite problematic. When typical Heck conditions were applied to the coupling of vinylsilanes with aryl or alkenyl iodides, the main products were styrene derivatives instead of silyl-substituted alkenes. Silicon−carbon bond cleavage, in the presence of Pd complexes, is responsible for the formation of undesirable products. This effect was suppressed by the addition of silver salts (i.e., AgNO3), but the parallel formation of biaryls made them the major product. Also, the addition of strong bases and other components necessary for the proper course of the process might be responsible for the cleavage of the bonds to the silicon atom.16−18
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RESULTS AND DISCUSSION In this paper we present the application of the silylative coupling protocol for the stereo- and regioselective synthesis of branched products with a boron atom in the core and πconjugated substituents or functional organometallic groups at the ends of branches (eq 2).
The formation of homocoupling products follows from two reasons. When two different vinyl-substituted silyl molecules are used in the reaction, a variety of silylative coupling products (cross-coupling as well as homocoupling) is possible. The excess of another vinylsilane molecule is necessary for the complete conversion of all vinyl groups in tris(dimethylvinylsilyl)boronate, and therefore homocoupling products are also observed in the reaction mixture, often as a major product. On the other hand, the occurrence of homocoupling products is related to the fact that the smaller vinylsilane molecule more readily undergoes insertion into the ruthenium−hydrogen bond than does the more sterically hindered tris(dimethylvinylsilyl)borate (I). Subsequently the insertion of the second vinylsilane molecule into the Ru−Si bond occurs more quickly and bissilylethene is observed as a main product. The chemical similarity of both coupling productsbissilylethene and boronateis responsible for difficulties in the isolation of silyl-substituted boron siloxide products by column chromatography (only one product was isolated with high purity, 8). The presence of vinylmetalloid compounds (vinylsilane, vinylboronate, or vinylgermane) is essential for the coupling procedure, because the formation of the TM−E bond (where E = Si, B, Ge) is required for such catalysis, and therefore the reaction between two organic terminal olefins does not occur. All possible catalytic routes that occurred in the Ru−Hcatalyzed coupling of tris(dimethylvinylsilyl)borate (I) with a variety of vinyl compounds are presented in Scheme 2. Pathway A shows the cross-coupling of I with organic olefins or vinylboronates. In this process the insertion of I begins the catalytic cycle and no side reactions are observed under optimized reaction conditions. When vinylsilanes are used as the reagents, the competitive parallel homocoupling reaction of vinylsilanes occurs, which often becomes the main catalytic
A spectrum of olefins was tested for the functionalization of tris(dimethylvinylsilyl)borate (I), including styrene, styrenes substituted at the para position, 9-vinylanthracene, 4-vinylbiphenyl, 2-vinyl-1,3,2-dioxaborinane, and a broad range of vinylsilanes. Scheme 1 shows the number of isolated trisubstituted products, which were characterized by NMR spectra and elemental analysis. The coupling reaction was examined in the presence of complexes containing the Ru−H bond, particularly [Ru(CO)ClH(PCy3)2]. This five-coordinate ruthenium complex is known as the most effective catalyst in silylative coupling reactions.6 The influence of reaction conditions (temperature, molar ratio of reagents, and concentration of catalyst) on the conversion of I with select olefins (styrene and vinylboronate) was investigated. The reactions were carried out in toluene or 1,4-dioxane at elevated temperatures.19 The optimized results of the efficient silylative coupling of tris(dimethylvinylsilyl)borate (I) with styrene, substituted styrenes as well as other highly π-conjugated olefins, and vinylboronate are compiled in Table 1. The yield of desired products is influenced by the molar ratio of the reagents. It has been proved that for olefins only a 1.1 3852
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Scheme 1. Isolated Functionalized Borates Synthesized via a Silylative Coupling Reaction
Table 1. Isolated Yields of Starburst Compounds with a Boron Siloxide Core and a Trans Configuration of the CC Bonda
Table 2. Silylative Coupling of Tris(dimethylvinylsilyl)borate (I) with Vinylsilanes no.
SiR3
conversn of silane (%)b
conversn of I (%)b
selectivity of CC/HC productsc,d
1 2 3
SiEt3 SiMe2Ph Si(OSiMe3)3
95 97 94
91 (8)e 94 97
1:1.5 1:1.6 1:1.4
Reaction conditions: [Ru]:[(I)]:[vinylsilane] = (2 × 10−2):1:6; argon; open system; T = 80 °C; t = 24 h; [Ru(CO)ClH(PCy3)2]; toluene. bDetermined by GC. cDetermined by GC and GC-MS. dCC = cross-coupling product; HC = homocoupling product. eThe numbering of isolated products described in the Experimental Section is given in parentheses. a
process (pathway C). The formation of a desirable starburst product is still possible (pathway B), but the process is much less selective in comparison to the reaction with organic olefins (pathway A). The scheme, for simplicity, presents only the coupling of one of the vinylsilyl groups in tris(dimethylvinylsilyl)borate (I), but the following coupling with the remaining groups occurs analogously. Most products with functional groups at the end of the branch of the starburst compound (see Scheme 1, products 1− 8) with a boron atom in the core were isolated and purified by liquid chromatography, and their structures were confirmed using NMR spectroscopy (1H, 13C, 29Si, 11B NMR). The chemical shifts in 11B NMR spectra for boron bonded to three
Reaction conditions: [Ru]:[I] = (2 × 10−2):1; argon; open system; T = 80 °C; t = 24 h; [Ru(CO)ClH(PCy3)2]; toluene. bAll isolated products have an E configuration at the double CC bond. cThe numbering of isolated products described in the Experimental Section is given in parentheses. a
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Scheme 2. Proposed Mechanism of the Ruthenium-Catalyzed Coupling of Tris(dimethylvinylsilyl)borate (I) with Organic Olefins, Vinylsilanes, and Vinylboronate
functionalized borates can be potentially used as precursors for new highly branched materials with unique mechanical, thermal, and optoelectronic properties. The physicochemical properties and reactivity of the products are now under investigation.
oxygen atoms are in the typical range (∼27 ppm) for all products 1−7 and are similar to the signal due to tris(vinyldimethylsilyl)borate. In case of product 7, the borinane group has a characteristic shift for a boron atom attached to ethenyl and two alkoxy groups (31.7 ppm). Signals in the 29Si NMR spectra in tris(dimethylvinylsilyl)borate as well as in the products have positive shift values due to the presence of a boron atom close to a silyl group, which causes changes in the shielding of the silicon nucleus. The J(H−H) coupling constant (approximately 19−20 Hz) for protons connected to sp2 carbon atoms shows exactly in the 1H NMR spectrum that only E isomers are obtained during the coupling process under select reaction conditions. This confirms that the chosen method distinguishes high regio- and stereoselectivity and is a clean way (only gaseous ethylene is obtained as a byproduct) to functionalize boron siloxide. The preferential formation of the E isomer can be explained by the higher thermodynamic stability of this isomer, which is formed exclusively in the process when elevated temperatures are applied. It is commonly known that at high temperatures Z isomers often undergo thermal isomerization to the more stable E isomer. As was already mentioned, the regio- and stereoselectivity of some of the silylative coupling procedures (i.e., with vinylboronates) are temperature dependent and different isomers can be obtained by controlling the process parameters. The steric hindrance of the substituents at the silicon atom and the type of olefin can also influence the formation of a specific stereoisomer. The alternative metathesis route cannot be used due to deactivation of ruthenium alkylidene Grubbs catalysts, when methyl groups are directly attached to the silicon atom.14,15 The Heck coupling with the corresponding aryl iodides can also be problematic because of the necessity of using the base and additives that on one hand accelerate the reaction but on the other hand can cleave the B−O−Si bond.
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EXPERIMENTAL SECTION
General Considerations. 1H NMR (300 MHz), 13C NMR (75 MHz), 11B NMR (96 MHz), and 29Si NMR (79 MHz) spectra were recorded on a Varian XL 300 MHz spectrometer in CDCl3 solution. Chemical shifts are reported in ppm with reference to the residual solvent (CH3Cl) peak for 1H and 13C, to BF3·Et2O for 11B, and to TMS for 29Si. Analytical gas chromatographic (GC) analyses were performed on a Varian Star 400CX instrument with a DB-5 fused silica capillary column (30 m × 0.15 mm) and TCD. Mass spectra were obtained by GC-MS analysis (VarianSaturn 2100T instrument, equipped with a BD-5 capillary column (30 m) and an ion trap detector). Elemental analyses were carried out on a Vario EL III instrument. Thin-layer chromatography (TLC) was carried out on plates coated with 250 μm thick silica gel (Aldrich and Merck), and the column chromatography was performed with silica gel 60 (70−230 mesh; Fluka). Toluene and 1,4-dioxane were dried by distillation from sodium, and hexane was dried by distillation from sodium hydride. Liquid substrates were also dried and degassed by bulb-to-bulb distillation. The reactions were carried out under a dry argon atmosphere. The chemicals were obtained from the following sources: toluene, 1,4-dioxane, dodecane, hexane, benzene-d6, chloroform-d, styrene, 4-chlorostyrene, 4-bromostyrene, 4-methoxystyrene, 9-vinylanthracene and 4-vinylbiphenyl from Aldrich, ethyl acetate from ChemPur, tris(dimethylvinylsilyl)borate (I)20 from ABCR GmbH & Co.KG. 2-Vinyl-1,3-dioxaborinane was synthesized according to the literature procedure with some modifications.21,22 The ruthenium complex [Ru(CO)ClH(PCy3)2] (A)23 was prepared according to literature procedures. General Procedure for the Silylative Coupling of Tris(dimethylvinylsilyl)borate (I). In a typical test, the ruthenium catalyst [Ru(CO)ClH(PCy3)2)] (A; 2 mol %) was dissolved in toluene or 1,4-dioxane and placed in a glass vessel under argon. The reagents and dodecane as internal standard (5% by volume all components) used in the appropriate molar ratios (see Tables 1 and 2) were added. The reaction was conducted in an open system, under argon. Subsequently, the vessel was heated at 80 °C and maintained at this temperature for 6−48 h. The progress of the reaction was monitored by GC (GC-MS). The conversion of reagents and chemoselectivity of the reactions was calculated by using the internal standard method. After the reaction, the crude product was purified by silica gel modified with 5% of HMDS column chromatography with hexane/ethyl acetate
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CONCLUSIONS A highly selective method for modification of tris(dimethylvinylsilyl)borate (I) with styrenes and other πconjugated olefins is presented. The reactions take place with high regio- and stereoselectivitythe trans product is observed exclusively. For vinylsilanes, due to their high reactivity, two parallel reactions of homocoupling of vinylsilanes and their cross-coupling with I are observed. Olefin-, boroxine-, or silyl3854
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(1/1) as eluent, and the structure of the product was proved by 1H, 13 C, 11B, and 29Si NMR and elemental analysis. S pec tros copic Data o f Isolated Products. Tris(dimethylstyrylsilyl)borate (1). Isolated yield: 88%. 1H NMR (300 MHz, CDCl 3 ): δ 0.31 (18H, s, Si(CH 3 ) 2 ), 6.46 (3H, d, (CH3)2SiCHCHPh, JHH = 19.5 Hz), 7.02 (3H, d, (CH3)2SiCH CHPh, JHH = 19.3 Hz), 7.27−7.40 (9H, m, C6H5), 7.44−7.47 (6H, m, C6H5). 13C NMR (75 MHz, CDCl3): δ 0.2 (Si(CH3)2), 126.5 (C6H5), 127.3 (C6H5), 128.4 (C6H5), 137.7 (C6H5), 140.5 ((CH3)2SiCH CHPh), 145.0 ((CH3)2SiCHCHPh). 11B NMR (96 MHz, CDCl3): δ 26.9. 29Si NMR (79 MHz, CDCl3): δ 2.1. Anal. Calcd for C30H39BO3Si3: C, 66.39; H, 7.24. Found: C, 66.57; H, 7.30. Tris((4-chlorostyryl)silyldimethyl)borate (2). Isolated yield: 81%. 1 H NMR (300 MHz, CDCl3): δ 0.30 (18H, s, Si(CH3)2), 6.42 (3H, d, (CH3)2SiCHCH, JHH = 19.2 Hz), 6.95 (3H, d, (CH3)2SiCHCH, JHH = 19.2 Hz), 7.29−7.39 (12H, m, C6H4Cl). 13C NMR (75 MHz, CDCl3): δ 0.18 (Si(CH3)2), 127.7 (C6H4Cl), 128.6 (C6H4Cl), 133.9 (C6H4Cl), 136.2 (C6H4Cl), 128.2 ((CH3)2SiCHCH), 143.6 ((CH3)2SiCHCH). 11B NMR (96 MHz, CDCl3): δ 28.6. 29Si NMR (79 MHz, CDCl3): δ 7.3 . Anal. Calcd for C30H36BCl3O3Si3: C, 55.77; H, 5.72. Found: C, 55.67; H, 5.74. Tris((4-bromostyryl)dimethylsilyl)borate (3). Isolated yield: 84%. 1 H NMR (300 MHz, CDCl3): δ 0.30 (18H, s, Si(CH3)2), 6.43 (3H, d, (CH3)2SiCHCH, JHH = 19.3 Hz), 6.91 (3H, d, (CH3)2SiCHCH, JHH = 19.5 Hz), 7.19−7.47 (12H, m, C6H4Br). 13C NMR (75 MHz, CDCl3): δ 0.17 (Si(CH3)2), 128.0 (C6H4Br), 128.4 (C6H4Br), 131.6 (C6H4Cl), 136.8 (C6H4Br), 143.2 ((CH3)2SiCHCH), 143.7 ((CH3)2SiCHCH). 11B NMR (96 MHz, CDCl3): δ 26.9. 29Si NMR (79 MHz, CDCl3): δ 7.2. Anal. Calcd for C30H36BBr3O3Si3: C, 46.23; H, 4.66. Found: C, 46.19; H, 4.60. Tris((4-methoxystyryl)dimethylsilyl)borate (4). Isolated yield: 82%. 1 H NMR (CDCl3, 300 MHz): δ 0.34 (18H, s, Si(CH3)2), 3.82 (9H, s, OCH3), 6.45 (3H, d, (CH3)2SiCHCH, JHH = 19.7 Hz), 6.95 (3H, d, (CH3)2SiCHCH, JHH = 19.5 Hz), 7.35−7.48 (4H, m, C6H4OMe). 13 C NMR (CDCl3, 75 MHz): δ 0.18 (Si(CH3)2), 55.2 (OCH3), 113.8 (C6H4), 127.3 (C6H4), 130.4 (C6H4), 136.2 (CHCH), 143.5 (CH CH), 159.2 (C6H4). 11B NMR (96 MHz, CDCl3): δ 27.4. 29Si NMR (79 MHz, CDCl3): δ 7.0. Anal. Calcd for C33H45BO6Si3: C, 62.64; H, 7.17. Found: C, 62.10; H, 7.34. Tris(((E)-2-(anthracen-9-yl)vinyl)dimethylsilyl)borate (5). Isolated yield: 73%. 1H NMR (300 MHz, CDCl3): δ 0.24 (18H, s, Si(CH3)2), 6.44 (d, 3H, SiCHCH, JHH = 19.4 Hz), 7.52 (d, 3H, SiCHCH, JHH = 19.4 Hz), 7.49−7.56 (m, 9H, anthracene), 8.00−8.04 (m, 3H, anthracene), 8.35−8.41 (m, 6H, anthracene). 13C NMR (75 MHz, CDCl3): δ 0.3 (Si(CH3)2), 130.2 (SiCHCH), 127.2, 127.6, 128.0, 128.4, 132.1, 136.1, 139.0 (anthracene), 146.2 (SiCHCH). 11B NMR (96 MHz, CDCl3): δ 27.1. 29Si NMR (79 MHz, CDCl3): δ 5.4. Anal. Calcd for C54H51BO3Si3: C, 76.93; H, 6.10. Found: C, 76.66; H, 6.31. Tris(((E)-2-(biphenyl-4-yl)ethenyl)dimethylsilyl)borate (6). Isolated yield: 77%. 1H NMR (300 MHz, CDCl3): δ 0.49 (18H, s, (Si(CH3)2), 6.78 (d, 3H, SiCHCH, JHH = 19.3 Hz), 7.70 (d, 3H, SiCHCH, JHH = 19.3 Hz), 7.35−7.90 (m, 27H, C6H4−Ph). 13C NMR (75 MHz, CDCl3): δ 0.16 (Si(CH3)2), 128.4 (SiCHCH), 127.1, 128.2, 128.7, 129.0, 136.74, 140.5, 141.2 (C6H4-C6H5), 144.1 (SiCHCH). 11B NMR (96 MHz, CDCl3): δ 27.0. 29Si NMR (79 MHz, CDCl3): δ 4.8. Anal. Calcd for C48H51BO3Si3: C, 74.78; H, 6.67. Found: C, 74.55; H, 6.89. Tris(((E)-2-(1,3,2-dioxaborinan-2-yl)ethenyl)dimethylsilyl)borate (7). Isolated yield: 76%. 1H NMR (300 MHz, CDCl3): δ 0.18 (18H, s, Si(CH3)2), 1.86 (6H, br, BOCH2CH2), 3.99 (t, BOCH2), 6.23 (3H, d, (CH3)2SiCHCH, JHH = 17.8 Hz), 6.51 (3H, d, (CH3)2SiCHCH, JHH = 19.3 Hz). 13C NMR (75 MHz, CDCl3): δ 0.24 (Si(CH3)2), 27.2 (BOCH2CH2), 62.7 (BOCH2CH2), 139.9 (SiCHCH). 11B NMR (96 MHz, CDCl3): δ 27.1, 31.6. 29Si NMR (79 MHz, CDCl3): δ 7.9. Anal. Calcd for C21H42B4O9Si3: C, 44.56; H, 7.48. Found: C, 44,68; H, 7.56. Tris(dimethyl((E)-2-(triethylsilyl)ethenyl)silyl)borate (8). Isolated yield: 63%. 1H NMR (300 MHz, CDCl3): δ 0.23 (18H, s, Si(CH3)2), 0.57 (q, 18H, SiCH2CH3), 0.92 (t, 27H, SiCH2CH3), 6.31 (3H, d,
CH3SiCHCH, JHH = 19.8 Hz), 6.53 (3H, s, CH3SiCHCH, JHH = 20 Hz). 13C NMR (75 MHz, CDCl3): δ 0.15 (Si(CH3)2), 3.2 (SiCH2CH3), 7.3 (SiCH2CH3), 140.5 (Et3SiCHCH), 148.8 (Et3SiCHCH). 11B NMR (96 MHz, CDCl3): δ 27.2. 29Si NMR (79 MHz, CDCl3): δ 4.1, 6.2. Anal. Calcd for C30H69BO3Si6: C, 54.83; H, 10.58. Found: C, 55.10; H, 10.73.
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ASSOCIATED CONTENT
S Supporting Information *
A table detailing the optimization of the silylative coupling reaction of tris(dimethylvinylsilyl)borate (I) with styrene and 2-vinyl-1,3,2-dioxaborinane by the influence of the process temperature, time, molar ratio of reagents, and catalyst on the substrate conversion and product selectivity. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +48 (61) 82 91 508. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the Ministry of Science and Higher Education of Poland (Grant NN 204265538) is gratefully acknowledged.
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REFERENCES
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Organometallics
Article
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dx.doi.org/10.1021/om201098w | Organometallics 2012, 31, 3851−3856
