Dehydrocoupling of tBu2PH·BH3
Inorganic Chemistry, Vol. 40, No. 17, 2001 4331
refined with the SHELXTL-PC V5.1 software package.26 The data
frames were integrated and scaled using the Denzo-SMN package.27
Refinement was by full-matrix least squares on F2 using all data
(negative intensities included). Molecular structures are presented with
ellipsoids at a 30% probability level. In all structures hydrogens bonded
to carbon atoms were included in calculated positions and treated as
riding atoms. Hydrogens attached to boron and phosphorus atoms were
refined with isotropic thermal parameters, except for hydrogens attached
to B2 in the mixture of compounds 2 and 1, which were also included
in calculated positions. Crystallographic data were deposited in the
Cambridge Crystallographic Data Centre with codes CCDC-166925
(tBu2PH‚BH3), CCDC-166926 (1), and CCDC-166927 (2 and 1).
tBu2PH-BH2-tBu2P-BH3 (1). Neat tBu2PH‚BH3 (0.84 g, 5.25
mmol) and Rh6(CO)16 (ca. 20 mg, 2 mol % Rh) were stirred at 160 °C
for 63 h (Table 1, entry 8). The dark brown reaction mixture became
liquid upon heating and solidified when cooled to room temperature.
Recrystallization from diethyl ether gave colorless crystals of 1 which
were suitable for single-crystal X-ray analysis. Isolated yield: 0.50 g
Figure 4. Molecular structure of the mixture of tBu2PH-BH2-tBu2P-
BH2Cl (2) and tBu2PH-BH2-tBu2P-BH3 (1). Selected bond lengths
(Å) and angles (deg): P2-B1 1.954 (2), P1-B1 1.967(2), P1-B2
1.955(3), P2-H1P 1.305(19), B2-Cl1 1.898(4); P2-B1-P1 119.23-
(11), B2-P1-B1 120.98(11), H1P-P2-B1 115.2(8), P1-B2-Cl1
110.32(16). Hydrogen atoms attached to carbon atoms are omitted.
1
(60%). Mp: 130-131 °C. H NMR (300 MHz, CDCl3): δ ) 4.57
(dm, JPH ) 371 Hz, PH), 1.37 (d, JPH ) 14.0 Hz, tBu), 1.22 (d, JPH
)
11.8 Hz, tBu), 1.10 to -0.10 (br q, JBH ca. 95 Hz, BH3), BH2 not
observed. 11B{1H} NMR (96 MHz, CDCl3): δ ) -37.2 to -40.8 (br
m, BH2 and BH3). 31P{1H} NMR (121 MHz, CDCl3): δ ) 39.5 (br,
tBu2PH), 13.2 (br, tBu2P). MS (EI, 70 eV): m/z (%) 317 (19) [M+
-
H], 304 (35) [M+ - BH3], 146 (100) tBu2PH. Anal. Calcd for
shown in Figure 4. It is the least precise of those reported here,
and only the major component 2 is discussed.
C16H42B2P2: C, 60.4; H, 13.3. Found: C, 59.7; H, 12.8.
The terminal bond lengths P2-B1 (1.954(2) Å) and P1-B2
(1.955(3) Å) are comparable to those observed in 1, while the
internal bond length P1-B1 (1.967(2) Å) is slightly shorter.
The phosphorus and boron environments in 2 also exhibit
distortion from the ideal tetrahedral geometry (P1, 102.42(13)-
120.98(11)°; P2, 101.4(8)-115.2(8)°; B1, 103.4(10)-119.23-
(11)°); the P1-B2-Cl1 angle is 110.32(16)°. The B-Cl bond
in 2 of 1.898(4) Å is somewhat longer than that reported for
the related four-membered chain Ph2PCl-BH2-Ph2P-BH2Cl
(B-Cl 1.877(7) Å).24
Thermal Decomposition of tBu2PH-BH2-tBu2P-BH3 (1). A
sample of 1 (0.10 g) was loaded into a sublimator and heated and
maintained at 175-180 °C for 16 h. A small amount of a colorless
sublimate was noted on the coldfinger and was subsequently identified
as tBu2P(O)H (31P NMR). The off-white residue in the sublimator was
analyzed by 31P NMR spectroscopy without further purification. 31P-
{1H} NMR (121 MHz, CDCl3): δ ) 67.0 (s, tBu2P(O)H), 53.8 (br),
48.9 (q, tBu2PH‚BH3), 39.5 (br, tBu2PH, 1), 34.0 (br), 26.5 (br), 21.0
(s, tBu2PH), 13.2 (br, tBu2P, 1), 8.5 (br).
tBu2PH-BH2-tBu2P-BH2Cl (2) and tBu2PH-BH2-tBu2P-BH3
(1). Neat tBu2PH‚BH3 (0.27 g, 1.69 mmol) and RhCl3 hydrate (ca. 45
mg, 10 mol % Rh) were stirred at 160 °C for 16 h (Table 1, entry 11).
After cooling to room temperature the dark brown reaction mixture
was recrystallized from diethyl ether to give colorless crystals of a
mixture of compounds 2 and 1 which were suitable for single-crystal
X-ray analysis. Isolated yield 0.18 g. These compounds could not be
separated by fractional crystallization and could not be distinguished
Summary
With the synthesis of tBu2PH-BH2-tBu2P-BH3 (1) and
tBu2PH-BH2-tBu2P-BH2Cl (2) we have demonstrated again
the usefulness of rhodium complexes in the catalytic formation
of phosphorus-boron bonds. In contrast to the catalytic de-
hydrocoupling of Ph2PH‚BH3, tBu2PH‚BH3 reacts much slower
and does not yield appreciable amounts of cyclic species. This
may primarily be due to the high steric requirements of the tBu2
groups but may also be attributed to the low polarity of the
P-H bond in tBu2PH‚BH3.
1
by their crystal habits. H NMR (300 MHz, CDCl3): δ ) 5.01 (dm,
JPH ) 380 Hz, PH, 2), 4.56 (dm, JPH ) 371 Hz, PH, 1), 1.39 (d, JPH
)
14.0 Hz, tBu, 2), 1.37 (d, JPH ) 14.0 Hz, tBu, 1), 1.31 (d, JPH ) 12.1
Hz, tBu, 2), 1.22 (d, JPH ) 11.8 Hz, tBu, 1), 1.10 to -0.10 (br q, JBH
ca. 95 Hz, BH3), BH2 not observed. 11B{1H} NMR (96 MHz, CDCl3):
δ ) -15.8 (br, BH2Cl, 2), -36.0 to -41.5 (br m, BH2 and BH3, 2 and
1). 31P{1H} NMR (121 MHz, CDCl3): δ ) 39.3 (br, tBu2PH, 1), 33.5
(br, tBu2PH, 2), 13.0 (br, tBu2P, 1), -3.3 (br, tBu2P, 2). MS (EI, 70
eV): m/z (%) 352 (5) [M+(2)], 318 (16) [M+(1)], 57 (100) tBu.
Experimental Section
General Information. All reactions were performed under an
atmosphere of dry nitrogen while workup procedures were carried out
in air. tBu2PH, [Rh(1,5-cod)2][OTf], Rh6(CO)16 (Strem), and RhCl3
hydrate (Pressure Chemical Co.) were purchased and used as received.
[{Rh(µ-Cl)(1,5-cod)}2] was prepared following a literature procedure.25
tBu2PH‚BH3 was prepared following a procedure analogous to that for
iBuPH2‚BH3.13 NMR spectra were recorded on a Varian Gemini or
Mercury 300 MHz spectrometer. Chemical shifts are referenced to
solvent peaks (1H) or external BF3‚Et2O (11B) or H3PO4 (31P). Mass
spectra were obtained with a VG 70-250S mass spectrometer operating
in electron impact (EI) mode. Elemental analyses were performed by
Quantitative Technologies, Inc., Whitehouse, NJ.
X-ray Structural Characterization. Crystal data and details of the
data collection are provided in Table 2. Diffraction data were collected
on a Nonius Kappa-CCD using graphite-monochromated Mo KR
radiation (λ ) 0.71073 Å). A combination of 1° φ and ω (with κ offsets)
scans were used collect sufficient data. The structures were solved and
Acknowledgment. This research was supported by the
Natural Science and Engineering Research Council of Canada
(NSERC), and the Petroleum Research Fund (PRF) administered
by the American Chemical Society (ACS). H.D. thanks the
Deutsche Forschungsgemeinschaft (DFG) for a postdoctoral
fellowship.
1
Supporting Information Available: Figures giving H and 31P-
{1H} NMR spectra of compound 1. Crystallographic data in CIF format.
This material is available free of charge via the Internet at http://
pubs.acs.org.
IC0100626
(26) Sheldrick, G. M. SHELXTL-PC, V5.1; Bruker Analytical X-ray
Systems Inc.: Madison, 1997.
(25) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1979, 19, 218.
(27) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.