Journal of the Korean Chemical Society (대한화학회지)

Korean Chemical Society (대한화학회)
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Improved Synthesis of Tetra- and Hexaosmium Carbonyl Boride Cluster HOs4(CO)12BH2 and H3Os6(CO)16B

테트라- 그리고 헥사 오스뮴 카보닐 붕소 클러스터, HOs4(CO)12BH2 and H3Os6(CO)16B의 효율적 합성 방법 연구

  • 류택형 (명지대학교 이과대학 화학과) ;
  • 정장훈 (명지대학교 이과대학 화학과)
  • Published : 2004.12.20
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Abstract

Keywords

EXPERIMENTAL SECTION

General Data

All reactions were performed under inert atmosphere conditions. Standard vacuum line and inertatmosphere techniques were employed.8 Os3(CO)12 (Strem) was used as received. (μ-H)2Os3(CO)10 was prepared according to the literature method.9 BH3 · N(C2H5)3 (Aldrich) was stored in a glovebox refrigerator and used as received. Solvents were dried with P2O5 or Na, distilled and stored in a sealed flask. Thin-layer chromatography plates (J.T. Baker, 250 m) were activated at 45 ℃ for 24 hours before use. 1H and 11B NMR spectra were obtained using a brucker AM-300. NMR chemical shifts are referenced to Si(CH3)4 (1H, δ=0.00 ppm) and BF3 · OEt2 (11B, δ=0.00 ppm). Infrared spectra were recorded on a Mattson Polaris Fourier transition spectrometer with 2 cm−1 resolution.

Formation of HOs4(CO)12BH2 and H3Os6(CO)16B from the reaction of (μ-H)2Os3(CO)10 with BH3 · NEt3

(μ-H)2Os3(CO)10 (300mg, 0.353 mmol) was placed in a 100 mL flask equipped with a Kontes vacuum adaptor and 30 mL volume of hexane was condensed into the flask at −78 ℃. In the glovebox, borane complex BH3 · NEt3 (1.00 mmol) was added to the flask via syringe. The contents of the flask were frozen and the flask was evacuated several times to ensure that no nitrogen from the glovebox atmosphere remained. After being warmed to room temperature, the solution was stirred at 70 ℃ for 1 h during which time the solution became yellow. The volatile components were removed by means of dynamic high vacuum leaving brown residue. The products were seperated by preparative TLC on 2 mm silica using a mixed solvent toluene/hexane as an eluent. A light yellow band (Rf = 0.74) was identified by NMR and IR spectroscopy as the previously reported tetraosmium carbonyl boride cluster HOs4(CO)12BH2 (98 mg, 0.088 mmol, 33% yield based on (μ-H)2Os3(CO)10: 1H NMR (CDCl3, 30 ℃) -9.5 (q, JBH=65 Hz), -24.3(s) ppm; 11B NMR (CDCl3, 30 ℃) 119.7 (t, JHB=65 Hz) ppm; IR (C6H12, vCO) 2075 vs, 2054 m, 2028 m, 2015 m, 2010 m, 1996 m cm−1.

A light brown band (Rf = 0.50) in the preparative TLC on silica above was identified by NMR and IR spectroscopy as hexaosmium carbonyl boride cluster H3Os6(CO)16B (63 mg, 0.039 mmol, 21% yield based on (μ-H)2Os3(CO)10: 1H NMR (CDCl3, 30 ℃) -9.5 (t, JBH=5 Hz), -16.9 (d, JBH=5 Hz) ppm; 11B NMR (CDCl3, 30 ℃) 188.4 (s) ppm; IR (C6H12, vCO) 2080 m, 2062 s, 2035 m, 2026 m cm−1.

 

RESULTS AND DISCUSSIONS

Earlier, we found that the thermolysis of (μ-H)3Os3(CO)9(μ-BCO) in toluene at 110 ℃ for 3 days produced the tetra- and hexaosmium carbonyl boride clusters, HOs4(CO)12BH26 and H3Os6(CO)16B7 in 4.1% and 2.3% yields, respectively. These observations are consistent with the well-known tendency for osmium carbonyl clusters to condense to higher nuclearity clusters upon thermolysis.10 However, the synthetic method is required to improve the very low yields. The previously reported clusters HOs4(CO)12BH2 and H3Os6(CO)16B are formed in 33% and 21% yields when the reaction of (μ-H)2Os3(CO)10 with BH3 · NEt3 occurs at 70 ℃ for 1 h. This new procedure is superior to the previous one in that the latter guarantees higher yields with reduced reaction time is required and the yields are much higher.

The observation of the intermediates in the formation of HOs4(CO)12BH2 was not successful. However, the reaction pathways for the formation of HOs4(CO)12BH2 can be proposed as shown in Scheme 1 based on known chemistry of boranes and the product obtained. BH3 · NEt3 can function as an electron pair donor through a B-H bond, adding to the unsaturated cluster (μ-H)2Os3(CO)10 by forming as Os-H-B bond and then the formation of the initial adduct Ia occurs. The ability of BH3 · L (L=Lewis base) type complexes to add to transition metals through the formation of metal-H-B bonds is well-known.11 BH3 · NEt3 is significantly dissociated at 70 ℃, thereby inducing the formation of intermediate Ib with two Os-H-B bonds. The initial adducts occuring through the formation of Os-H-B in the Scheme 1 are based on the reaction pathways for the formation of triosmium carbonyl borylidyne cluster (μ-H)3Os3(CO)9(μ -BCO) in the reaction of (μ-H)2Os3(CO)10 with B2H6 and BH3 · NEt3 at room temperature.12 The proposed intermediate Ib is similar in structure to that assigned to its analogue Os3(CO)10CH4.13

Scheme 1.Proposed reaction pathways for the formation of HOs4(CO)12BH2.

While insertion of boron atom into an osmiumcarbonyl bond of the Os(CO)4 unit with concomitant reductive elimination of H2 produces (μ-H)3Os3(CO)9(μ-BCO), the complex is not observed in this reaction. This result shows the insertion of boron atom into the osmium-carbonyl bond does not occur in the reaction. Instead, reductive elimination of H2 from the intermediate Ib would lead to intermediate Ic which has a bridge B-H-Os hydrogen and a bridge Os-CO-Os carbonyl. The intermediate Ic combines with Os(CO)3, which is a fragment of (μ-H)2Os3(CO)10, through the formation of B-H-Os bond and a Os-Os bond between two electron deficient Os(CO)3 units. Although intermediate Ic was not observed, Fehlner has prepared the iron analogue of Ic.14 Finally, the dissociation of the Os-COOs bridge carbonyl induces the formation of another Os-Os bond to produce tetraosmium carbonyl boride cluster HOs4(CO)12BH2.

Hexaosmium carbonyl boride cluster H3Os6(CO)16B contains 86 valence electrons with seven skeletal electron pairs.15 However, the cluster does not possess one of the geometries previously observed for Os6 cluster with 86 valence electrons which conform to electron counting rule.16 The Os6 framework of H3Os6(CO)16B can be viewed as being derived from a pentagonal bipyramid that is missing one equatorial vertex in Fig. 1. It contains an interstitial boron atom. Many high nuclearity clusters were originally prepared by unplanned routes or by-products. Meanwhile, H3Os6(CO)16B was formed efficiently in the reaction of (μ-H)2Os3(CO)10 with BH3 · NEt3 in high yield. Although the systematic procedures for the formation of the cluster are too complicated, it can be deduced that Os3B unit such as intermediate Ic in Scheme 1 combines with an electron deficient Os3 unit to produce H3Os6 (CO)16B through dissociation of CO and reductive elimination of H2.

Fig. 1.Molecular structure of H3Os6(CO)16B.

In summary, the clusters HOs4(CO)12BH2 and H3Os6(CO)16B were produced in the reaction of (μ-H)2Os3(CO)10 with BH3 · NEt3 at 70 ℃ for 1 hour which is superior to previous one because significantly less reaction time is required and the yields are much higher. The reaction pathways for the formation of HOs4(CO)12BH2 can be proposed as shown in Scheme 1.

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