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Study of Hydroboration of (μ-H)2Os3(CO)10 with Various Borane Complexes(BH3·L: L=Lewis base)

(μ-H)2Os3(CO)10와 Borane Complexes(BH3·: L=Lewis base)의 수소 붕소화 반응성 연구

  • 정장훈 (명지대학교 이과대학 화학과)
  • Published : 2003.12.20

Abstract

Keywords

Hydroboration;Lewis acid base;Osmium borylidyne carbonyl cluster

EXPERIMENTAL SECTION

General data

All reactions were prerformed under inert-atmosphere conditions. Standard vacuum line and inert-atmosphere techniques were employed.5 Os3(CO)12(Strem) was used as received. (μ-H)2Os3(CO)10 was prepared according to the literature method.6 Diborane was prepared by literature methods and stored at -196℃ in a glass tube.7 O(CH3)2 (Matheson Scientific Products) was dried and stored over Na at -78℃. BH3.L (L=N(CH3)3, N(C2H5)3, Pyridine, THF, S(CH3)2, P(C4H9)3, PPh3) (Aldrich) was stroed 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 chemical shifts are referenced to Si(CH3)4 (1H, δ=0.00 ppm) and BF3.OEt2 (11B, δ=0.00 ppm).

Reaction of (-H)2Os3(CO)10 with BH3.S(CH3)2

In the glovebox, (μ-H)2Os3(CO)10 (200 mg, 0.235 mmol) was weighed into a 50 mL long-neck flask equipped with a Kontes vacuum line adaptor. Borane complex. BH3.S(CH3)2 (9.38 mmol) was added to the flask and 25 mL volume of CH2CL2 was condensed into the flask at -78℃. After being warmed to room tmeperature, the solution was stirred for 7 days during which time yellow solution formed. The solvent and excess BH3.L was removed by means of dynamic high vacuum leaving a yellow solid. An extractor and receiver flask were attached to the flask including the yellow solid in glovebox. A 20 mL volume of hexane was vacuum trans-ferred into the flask and the solution was srirred at room temperature for 1 hour. The solution was flitered to give a yellow filtrate. The solid left on the frit was then washed a couple of times with hexane. The solvent was removed by means of dynamic high vacuum leaving a light yellow solid and then the solid was recrystallyzed in CH3CI. This yellow product was identified as (μ-H)2Os3(CO)9(μ-H)2BH by 1H & 11B NMR spectroscopies. The yield of product is 86.5 mg (0.013 mmol). a 44.0% yield based on (μ-H)2Os3(CO)10. (μ-H)2Os3(CO)9(μ-H)2BH:1H NMR (CDCI3, 30C) δ 4.7(br, 1 B-H), -13.6 (br, 2 B-H-Os), -20.5 (q, 2 Os-H-Os) ppm; 11B NMR (CDCl3, 30C) δ 18.5 (br) ppm.

Reactions of (μ-H)2Os3(CO)10 with BH3.L (L=N (CH3)3, N(C2H5)3, Pyridine, THF, P(C4H9)3, PPh3)

(μ-H)2Os3(CO)10(200 mg, 0.235 mmol) was weighed into a 50 mL long-neck flask equipped with a Kontes vacuum line adaptor. Borane complex BH3.L (9.38 mmol; (L=N (CH3)3, N(C2H5)3, Pyridine, THF, P(C4H9)3, PPh3) was added to the flask in the glovebox and 25 mL volume of CH2CL2 was condensed into the flask at -78 ℃. After being warmed to room temperature, the solution was stirred for 7 days. The volatile components were removed by means of dynamic high vacum leaving brown residue. The products were separated by preparative TLC on 2 mm silica using a mixed solvent toluene/hexane as an eluent. A light yellow band was identified from its NMR spectrum, as reported above, as (μ-H)2Os3(CO)9(μ-H)2BH.

Reactions of (μ-H)2Os3(CO)10 with BH3.L (L=O (CH3)2, O(C2H5)2

(μ-H)2Os3(CO)10 (200 mg, 0.235 mmol) was weighed into a 50 mL long-neck flask. The flask was topped with a Kontes vacuum line adaptor and evacuated. 25 mL volume of CH2CL2 was condensed into the flask at -78 ℃. Lewis base (0.235 mmol: O(CH3)2, O(C2H5)2) and B2H6 (0.118 mmol) was measured in a calibrated bulb and condensed into the flask. The solution was stirred at room temperature for 7 days. The volatile components were removed leaving a light yellow residue. The yellow product was recrystallized from CH2Cl2. The product was identified as (μ-H)3Os3(CO)9(3μ-BCO) by 1H & 11B NMR spectroscopies. (μ-H)3Os3(CO)9(μ3-BCO): 1H NMR (CDCl3, 30C) δ - 19.8 (q, 3 Os-H-Os) ppm; 11B NMR (CDCl3, 30C) δ 18.5 ppm.

 

REULTS AND DISCUSSIONS

The hydroboration reactions of the unsaturated cluster (μ-H)2Os3(CO)10 with various borane complexes BH3.L (L=O (CH3)2, O(C2H5)2, THF, N(CH3)3, N(C2H5)3, Pyridine, S(CH3)2, P(C4H9)3, PPh3) were investigated.

As shown in Table 1, (μ-H)2Os3(CO)9(μ-H)2BH was produced in 44.0% yield from the reaction of (μ-H)2Os3(CO)10 with BH3.S(CH3)2. Meanwhile, the cluster was produced in yield of less than 1% from thr reactions of (μ-H)2Os3(CO)10 with BH3.L (L=N (CH3)3, N(C2H5)3, Pyridine, P(C4H9)3, PPh3) and not from thr reactions of (μ-H)2Os3(CO)10 with BH3.L (L=O(CH3)2, O(C2H5)2, THF).

Although thr reactions wrer followed by means of NMR spectroscopy, it was not successful to observe intermediates in the reactions of (μ-H)2Os3(CO)10 with various borane complexes. However, on the basis of the known chemistry of boranes and thr products obtained, the reaction pathways for the formation of (μ-H)2Os3(CO)9(μ-H)2BH in Scheme 1 can be proposed. BH3.L functions as an electron pair donor through a B-H bond, adding to the unsaturated cluster (μ-H)2Os3(CO)10 by forming two hydrogen bridged Os-H-B bonds of Ib. The ability of the borane complexes BH3.L to add to transition metals through the formation of metal-H-B bonds is well-konwn.8 For the formation of (μ-H)2Os3(CO)9(μ-H)2BH, a

Table 1.Yield of (μ-H)2Os3(CO)9(μ-H)2BH in the reaction of H2Os3(CO)10 with Borane complexes

Scheme 1.Proposed reaction pathways for the formation of (μ-H)2Os3(CO)9(μ-H)2BH.

CO of Os(CO)4 unit of the intermaediate Ib should be eliminated to form Os(CO)3. The CO shifts to an electronically unsaturated neighboring Os atom to form a CO bridged Os-CO-Os bond and two terminal hydrogens do to neighboring Os atoms to form H bridged Os-H-Os bonds of Ic. The intermediate Ic was not observed but Fe analogue of Ic including the CO bridged Os-CO-Os bond was prepared by Fehlner.9 The formations of the bridge bonds are followed by elimination of CO from Os-CO-Os bond, thereby inducing formation of another Os-B bond due to the electron deficiency of the osmium atom. As shown in final step of the proposed pathways, B atom shifts to the Os atom and thus BH3.L unit is incorporated into Os3 triangle by formation of a Os-B bond. Finally, release of L from the adduct produces the (μ-H)2Os3(CO)9(μ-H)2BH.

A key step in the formation of the cluster is release of Lewis base from the adduct. In the scheme of the reaction pathways, release of Lewis base from thr adducth occurs in the last step. However, Shore reported that for the formation of another tri-osmium borylidyne carbonyl cluster (μ-H)3Os3(CO)9(μ3-BCO) containing Os3B core in the hydroboratin reaction of (μ-H)2Os3(CO)10 with B2H6 in presence of O(CH3)2, release of lewis base O(CH3)2 from the adduct occurred in the early step of the reaction pathways, inducing migration of a CO of Os(CO)4 to B atom to form BCO unit capping Os3 core.3,10 The distinct difference between the pathways for the formation of (μ-H)2Os3(CO)9(μ-H)2BH and (μ-H)3Os3(CO)9(μ3-BCO) is when the release of Lewis base L from the adduct of BH3.L occurs. The results of the reactions in Table 1 can be rationaled by the proposed reaction pathways and the hard-soft acid-base principle.11,12 The bond strength in donor-acceptor complexes such as BH3.L (L=Lewis base) can be interpretated on the basis of the theory of hard and soft acids and bases suggested by Pearson.13,14 The reaction of (μ-H)2Os3(CO)10 with BH3.S(CH3)2, produces the cluster in the largest yield of 44%. According to the theory, soft acid BH3 combines with soft base S(CH3)2 to form very stable borane complexes BH3.S(CH3)2 and thus the base S(CH3)2 hardly dissociate from the complex until the final step of the reaction pathway so that migration of CO to B atom for the formation of (μ-H)3Os3(CO)9(μ3-BCO) is inhibited. Instead, the large stability of the complex induces decarbonylation to form (μ-H)2Os3(CO)9(μ-H)2BH. That is why BH3.S(CH3)2 reacts with (μ-H)2Os3(CO)10 to produce (μ-H)2Os3(CO)9(μ-H)2BH in the largest yield.

Soft bases P(C4H9)3 and PPh3 strongly combime with soft acid BH3. However, the reactions of the (μ-H)2Os3(CO)10 with the borane complexes produce little (μ-H)2Os3(CO)9(μ-H)2BH, because the borane complexes hardly add to the cluster due to the steric hindrance of bulky phosphins. In the reactions of (μ-H)2Os3(CO)10 with BH3.L (L=O(CH3)2, O(C2H5)2, THF), (μ-H)2Os3(CO)9(μ-H)2BH is not produced because O(CH3)2, O(C2H5)2, and THF are typical hard bases and thus the relaease of Lewis bases easily occurs in the early step of the reaction pathways due to weak combination of BH3 with the Lewis bases. Although N(CH3)3, N(C2H5)3. and Pyrdine are a little bit softer than ether, those are hard bases and thus the borane complexes easily dissociate in the early step of the reaction pathways. Therefore, the reactions of (μ-H)2Os3(CO)10 with BH3.L (L=N(CH3)3, N(C2H5)3, Pyridine) produce little (μ-H)2Os3(CO)9(μ-H)2BH.

In summary, the proposed reaction pathways for the formation of (μ-H)2Os3(CO)9(μ-H)2BH on basis of the hard-soft acid-base princlple is consistent with the results of the hydroboration reactions of (μ-H)2Os3(CO)10 with various borane complexes. The yield of the cluster in the hydroboration reaction depends on the stability of the borane complexes.

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