A Facile Route to Phosphanylborohydrides : Synthesis , Crystal Structure and Spectroscopic Properties of 1 , 2-Bis ( Diphenylphosphinoborane ) Ethane

A novel and simple synthetic way using NaBH4 in the mixture of H2O-THF was applied to prepare 1,2-bis(diphenylphosphinoborane)ethane, dppe(BH3)2, in high yield and purity. The phosphanylborohydride compound dppe(BH3)2 was isolated in the form of colorless crystals and characterized by single crystal X-ray diffraction, H, C, P and B NMR spectroscopy. Prismatic colorless crystals of dppe(BH3)2 were obtained in monoclinic crystal system and space group P21 with two asymmetric units in the unit cell. Lattice parameters were: a = 11.657(2), b = 17.237(2), c = 12.764(2) Å,  = 98.735(14) ̊, 2535.0(7) Å.


Introduction
A recent study [1] has reported the catalytic activity of ruthenium (III) acetylacetonate in the presence of different phosphorus compounds such as 1,2-bis(diphenylphosphino)ethane, dppe, in the hydrolysis of sodium borohydride.At the end of catalytic reaction, in addition to the unreacted dppe, unexpectedly we isolated a new species which contains two BH 3 molecules coordinated to dppe.Obviously, in this catalytic reaction, NaBH 4 acts not only as a substrate to generate hydrogen, but also as a BH 3 supplier in forming phosphanylborohydrides such as 1,2-bis(diphenylphosphinoborane)ethane, dppe(BH 3 ) 2 .In literature, phosphanylborohydrides have been prepared by using the mixture of sodium borohydride and iodine in monoglyme [2] or using the other borane sources: dppe(BH 3 ) 2 by complexation of dppe with BH 3 •S(CH 3 ) 2 [3], rac/meso-[HP(BH 3 )(Ph)CH 2 ] 2 from the reaction of BH 3 .thf[4] or reaction of phosphine oxides with diborane [5], from the reaction of trialkylphosphines with bromoboranes or bromochloroboranes [6].In addition following phosphanylborohydrides have been reported: tertiary mono and diphosphine-borane complexes [7][8][9], cyclic phosphine-boranes [10], phosphine-carborane clusters [11], phosphinyl-borane radicals [12] and phosphine alkylene boranes [13].It is noteworthy that the phosphanylborohydride [P(BH 3 )Ph 2 ] -forms dative bonds of higher p character and establish more stable σ adducts towards the acceptor orbital of the Lewis acid in comparison with its neutral counterpart P(CH 3 )Ph 2 [14].A similar phenomenon was observed in the study of chalcogenated phosphanylborohydrides K[EP(BH 3 )R 2 ] (E: O, S, Se, Te; R: Ph, t-Bu) with a certain degree of E=P multiple bond character [15].Borane complexes of phosphorus compounds, a very common oxidation free relay for catalytic ligands (phosphines, phosphites or phosphinites) can be easily deprotected by treatment with polymer-supported piperazine, N-methylpiperazine [16] or pyrrol derivatives [3].Phosphanylborohydrides supported by amines such as polypyrroles, are very useful for homogeneous catalysis due to more efficient recovery and purification [3].Despite the known examples given above, the chemistry of phosphanylborohydrides is still largely undeveloped [17][18][19][20][21][22].Herein we report a new and simple synthetic way using NaBH 4 in an homogeneous aqueous-organic s o l u t i o n t o y i e l d

Equipment
All reactions involving air sensitive compounds were performed under argon or nitrogen atmospheres. 1 H, 13  e ion mass spectrometry data was obtained on a Bruker Micro TOF-LC/ESI/Ms system.
The experimental setup consists of a 75 mL jacketed reaction flask containing a Teflon-coated stir bar placed on a magnetic stirrer (Heidolph MR-301) which can be thermostated to 25.0˚C by circulating water through its jacket from a constant temperature bath (RL6 LAUDA water bath).Note that drolysis of sodium borohydride was released from the flask through a bubbler.

Synthesis of 1,2-Bis(Diphenylphosphinoborane)Ethane, DPPE(BH 3 ) 2
For the preparation of 1,2-bis(diphenylphosphinoborane) ethane, dppe(BH 3 ) 2 , 140 mg (0.35 mmol) of 1,2-bis(diphenylphospino)ethane, dppe, was dissolved in 10 mL of THF by vigorous stirring.Then, the solution was transferred into a 75 mL jacketed reaction flask containing 30 mg (0.79 mmol) NaBH 4 dissolved in 40 mL water and thermostated at 25.0˚C.The reaction was started by turning on the magnetic stirrer (Heidolph MR-301) at 1000 rpm under inert atmosphere (argon or nitrogen).After 3 h stirring, the mixture was extracted with dichloromethane and the combined organic extracts were cooled in order to precipitate out traces of sodium borohydride or metaborate remaining in organic extracts.
Then, the solution was dried over magnesium sulfate, filtered and evaporated in vacuum giving 144 mg of pure dppe(BH 3 ) 2 complex (96% yield).Colorless crystals of dppe(BH 3 ) 2 were obtained by crystallization from the hexane-dichloromethane solution at 0˚C after one week, which were separated by filtration.

Crystal Structure Analysis
X-ray diffraction measurements were performed with MoK α radiation on an Enraf-Nonius CAD4 diffractometer [23] equipped with a graphite monochromator.Intensity data were collected by /2 scan mode.The cell parameters were determined from a least-squares refinement of 18 centered reflections in the range of 10.12˚    18.03˚.Cell refinement was carried out using CAD-4 EXPRESS.Data reduction was carried out using XCAD4 [24].The structures were solved by Patterson methods and refined using the program SHELX [25].A full-matrix least-squares refinement on F 2 was done.For all non-hydrogen atoms anisotropic displacement parameters were refined.Borane( g hyound were placed geometrically and a riding model was used with ( ) 1.5 ( ) , respectively.Methylene hydrogen's were taken from a difference Fourier map and refined.Single crystal X-ray diffraction analysis of the colorless crystal shows the crystallization in monoclinic system with space group P2 1 and two asymmetric units with a formula of C 26 H 30 B 2 P 2 and four molecules per unit cell.Table 1 shows the crystal data and crystal refinement of dppe(BH 3 ) 2 .The atomic coordinates and isotropic displacement parameters are listed in Table 2. Selected bond lengths and angles are given in Table 3. ORTEP [26] drawing of the dppe(BH 3 ) 2 complex with the atomic numbering scheme is given in Figure 1.The unit cell of the structure as shown in Figure 2. The conformations of molecules and molecular packing geometry were analyzed using PLATON [27].The structure includes several pi-ring interactions between two asymng interaction geome-metric moieties.Details of the pi-ri try are given in Table 4.

Results and Discussion
When an aqueous solution of sodium borohydride is dded to a solution of 1,2-bis(diphenylphospino)ethane, a 3 pe, in tetrahydrofurane unde action occurs, along with the Copyright © 2011 SciRes.JCPT In the crystal structure of the title compound, the monoclinic unit cell contains two molecules of [C 26 H 30 B 2 P 2 ] and four molecules per unit cell.All molecular properties of two asymmetric units of the molecule I are similar of the molecule II as given in Table 3.The average bond 112.9˚, 113.1˚, 105.8˚, and 105.5˚ for B-P-C, B'-P'-C', C-P-C and C'-P'-C', respectively.Study of the interactions between P atoms and contact atoms in coordination sphere with average distances (P-B: 1.876 Å for the molecule I, 1.905 Å for II and P-C 1.804 Å for I, 1.809 Å for II) reveals that the P atoms are surrounded by four atoms (one boron and three carbon atoms) in nearly ideal tetrahedral geometry.
Both molecules in an asymmetric unit have a similar three-dimensional conformation.Dihedral angles between the least square planes of phe -Sc nation sphere of P atoms are Cg4 = 88.2(3)Å for molecu Å, Cg7-Cg8 = 71.5(3)Å for molecule II. Figure 2 shows the unit cell of dppe(BH 3 ) 2 .
The soluti len )ethane, dppe(BH 3 ) 2 , he single crystal stru worthy that the H NMR spectrum gives two doublets at 2.15 and 2.38 ppm for the prochiral methylene groups [28] while the 13 C NMR spectrum exhibits only one signal at 22.93 ppm for the methylene carbons.Moreover, two separated multiplets at 7.54 and 7.61 ppm are observed for ortho-hydroge and similar to hydrogens of meth tho-hydrogens of phenyl rings are not identical due to their different positions with respect to borane moieties. 31P NMR spectrum shows a peak at -12.5 ppm which is about 10 ppm m sponding peak of meso-[HP(BH 3 )(Ph)CH 2 ] 2 complex [4], indicating that dppe(BH 3 ) 2 complex containing more phenyl rings has more electron-rich phosphorus atoms.However, 11 B NMR gives a peak at -40.06 ppm for borane groups comparable to the value of -41.6 ppm reported for meso-[HP(BH 3 )(Ph 2 2 [ 4].MS does not show the molecular ion peak expected at m/z = 425.Instead, it shows peaks at m/z = 429 or 431 due to oxidation of dppe(BH 3 ) 2 during the sampling/ionization whereby BH 3 groups are replaced by the oxo groups.

Conclusions
In conclusion, sodium borohydride is a in the synthesis of phosphanylborohydride compounds such as 1,2-bis(diphenylphosphinoborane)ethane, dppe (BH 3 ) 2 , in high yield.Sodium borohydride and metaborate are easily separated by extraction with dich methane, providing an easy separation for the preparation of phosphanylborohydrides with high purity.The adduct dppe(BH 3 ) 2 crystallizes in monoclinic system with space group P2 1 and two asymmetric units c ideal tetrahedral phosphorus atoms.