Facile cleavage of phenyl groups from BiPh3 in its reactions with Os3(CO)10(NCMe)2 and evidence for localization of π-bonding in a bridging benzyne ligand
Graphical abstract
Five products Os2(CO)8(μ-BiPh), 2, Os3(CO)10(μ3-C6H4)–, 3, Os3(CO)10Ph(μ-η2-OCPh), 4, HOs5(CO)18(μ-η2-C6H4)(μ4-Bi), 5 and HOs6(CO)20(μ-η2-C6H4)(μ4-Bi), 6 were obtained by the facile cleavage of phenyl groups from BiPh3 in its reaction of Os3(CO)10(NCMe)2, 1 with BiPh3. A fifth compound Os3(CO)10(μ-η2-OCPh)2, 7 was obtained by thermal transformation of 3.
Introduction
The cleavage of phenyl groups from the triphenylphosphine, PPh3, in its reactions with triosmium dodecacarbonyl goes back to some of the very first reactions of this complex that were studied [1], [2]. Cleavage of phenyl groups from AsPh3 and SbPh3 in triosmium carbonyl complexes is also facile [3], [4]. Recently, complexes containing transition metal–bismuth bonds have attracted attention [5], [6]. Transition metal–bismuth catalysts have been shown to exhibit high activity and selectivity for the oxidation and ammoxidation of hydrocarbons [7], [8], [9]. We have recently reported the synthesis of dirhenium carbonyl complexes containing SbPh2 and BiPh2 ligands by the cleavage of phenyl rings from SbPh3 and BiPh3 in their reactions with Re2(CO)8[μ-η2-C(H)C(H)Bun](μ-H), Eq. (1) [10]–(2) [11]. These products have been shown to be good catalysts for the ammoxidation of 3-picoline to 3-cyanopyridine [12].
We have also shown that phenyl groups are readily cleaved from BiPh3 in its reaction with Os3(CO)11(NCMe), Scheme 1 [13]. In fact, only one product, Os2(CO)8(μ-BiPh), contained any phenyl groups bonded to a Bi atom. Products containing phenyl groups or ligands derived from them, such as C6H4 and PhCO, were abundant.
In this follow up study, we have now investigated the reaction of BiPh3 with Os3(CO)10(NCMe)2, 1. These studies have yielded yet another new osmium–bismuth complex as well as some new osmium carbonyl complexes containing phenyl, benzoyl and benzyne ligands derived from the phenyl groups that were cleaved from the BiPh3. Most interestingly, a high resolution structure analysis of one of the new products Os3(CO)10(μ3-C6H4), 3 which contains a triply-bridging benzyne ligand shows a distinct pattern of long and short C–C bonds around the ring of the benzyne ligand. Molecular orbitals for 3 were obtained by density functional theory (DFT) calculations and indicate that this bonding pattern can be attributed to a partial localization of the π-bonding in the ring which is enhanced at the shorter C–C bonds. The results of these studies are reported herein.
Section snippets
General data
Reagent grade solvents were dried by the standard procedures and were freshly distilled under nitrogen prior to use. Unless indicated otherwise, all reactions were performed under an atmosphere of nitrogen. Infrared spectra were recorded on a Thermo Nicolet Avatar 360 FT-IR spectrophotometer. 1H NMR spectra were recorded on a Varian Mercury 300 spectrometer operating at 300.1 MHz. Mass spectral (MS) measurements were performed by a direct-exposure probe by using electron impact ionization (EI)
Thermal transformations of 4
A 7.4 mg (0.0072 mmol) amount of 4 was dissolved in 10 mL of hexane in a 50 mL three neck flask. The solution was heated up to reflux for 5 h. After cooling, the solvent was removed in vacuo, and the products were then isolated by TLC by using a 6/1 hexane/methylene chloride elution solvent mixture to yield in order of elution: 0.7 mg of 3 (10.5% yield); 3.2 mg of Os3(CO)10(μ-η2-OCPh)2, 7 [15] (42% yield). Spectral data for 7: IR vCO (cm−1 in hexane): 2099(w), 2068(vs), 2048(m), 2016(vs),
Results
Five products: Os2(CO)8(μ-BiPh) [13], 2 (2.5% yield), Os3(CO)10(μ3-C6H4)–, 3 (3.6% yield), Os3(CO)10Ph(μ-η2-OCPh), 4 (11% yield), HOs5(CO)18(μ-η2-C6H4)(μ4-Bi) [13], 5 (8.9% yield) and HOs6(CO)20(μ-η2-C6H4)(μ4-Bi), 6 (14% yield) were obtained all in low yields from the reaction of 1 with BiPh3 in a methylene chloride solution at reflux for 2 h. Three of the products, 3, 4 and 6 are new and were characterized by a combination of IR, NMR, mass spec and single-crystal x-ray diffraction analyses.
Discussion
A summary of the reactions and products obtained in this study is shown in the Scheme 2. Five products 2–6 were formed from the reaction of 1 with BiPh3. Compounds 5 and 6 contain spiro-μ4-Bi atoms formed by the cleavage of all of the phenyl groups from the BiPh3. Products 3 and 4 contain the phenyl rings that were cleaved from the BiPh3, although the ring in 3 is actually a triply-bridging C6H4 ligand formed by the cleavage and elimination of one hydrogen atom from a phenyl ring. A high
Conclusions
The cleavage of phenyl groups from BiPh3 by activated osmium carbonyl cluster complexes is facile and leads to products containing naked bridging bismuth atoms and coproducts containing the phenyl rings. Similar results were found in our studies of the reactions of BiPh3 with activated rhenium carbonyl complexes [11]. Evidence has been presented that shows a pattern of alternating long and short C–C distances in the bridging benzyne ligand in 3 that is consistent with a significant amount of
Acknowledgments
This research was supported by the National Science Foundation under grants No. CHE-1111496 and CHE-1048629 for the purchase of the computational facility.
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