B-C CleaVage by Iron Hydride
Organometallics, Vol. 26, No. 13, 2007 3225
4H could be BEt2 CH2, m-H, or iPr methine. Peaks for the backbone
H and bridging H were not observed.) UV-vis (pentane): 299 (ꢀ
) 27.3 mM-1 cm-1), 398 (ꢀ ) 15.0 mM-1 cm-1), 554 (ꢀ ) 2.9
mM-1 cm-1) nm. µeff (C6D6, 25 °C): 5.1(1) µB. Anal. Calcd for
C33H53N2BFe: C, 72.80; H, 9.81; N, 5.15. Found: C, 73.52; H,
9.52; N, 5.62.
informative, showing that the most likely mechanism proceeds
through attack of a ring-opened iron hydride species on the
borane in the rate-determining transition state. In this mecha-
nism, the hydride ligand acts as a nucleophile, apparently
activating the borane toward transfer of the R group to iron. In
addition, the hydridoborate product of the reaction reacts to
cleave the N-N bond of hydrazine, releasing the two bridging
hydrides as H2. The B-C and N-N cleavage reactions
demonstrate the ability of low-coordinate iron complexes to
perform difficult bond transformations under mild conditions.
LMeFe(µ-H)2BPh2 (2b). 1a (127 mg, 0.13 mmol) and triphenyl-
borane (32 mg, 0.13 mmol) were dissolved in 20 mL of toluene.
The solution was heated at 80 °C for 18 h. Volatile materials were
removed under vacuum, and the residue was extracted with toluene
(15 mL), filtered, and concentrated to 5 mL. Crystallization from
1
pentane at -26 °C gave red blocks (56 mg, 67%). H NMR (500
Experimental Section
MHz, C6D6): 78 (6H, backbone CH3), 32 (4H), 17 (4H+4H), -4
i
i
(12H, Pr CH3), -8 (2H), -40 (12H, Pr CH3), -46 (4H), -52
(2H) ppm. (Peaks integrated as 4H could be iPr methine, aryl m-H,
phenyl o-H, or phenyl m-H. Peaks integrated as 2H could be aryl
p-H or phenyl p-H. Peaks for the backbone H and bridging H were
not observed.) UV-vis (toluene): 291 (ꢀ ) 14.6 mM-1 cm-1),
398 (ꢀ ) 6.3 mM-1 cm-1), 567 (ꢀ ) 1.2 mM-1 cm-1) nm. µeff
(C6D6, 25 °C): 4.0(1) µB. Anal. Calcd for C41H53BFeN2: C, 77.12;
H, 8.05; N, 4.39. Found: C, 76.88; H, 7.85; N, 4.38.
General Considerations. All manipulations were performed
under a nitrogen atmosphere using standard Schlenk techniques or
in an M. Braun glovebox maintained at or below 1 ppm of O2 and
1
H2O. Glassware was dried at 150 °C overnight. H NMR spectra
were recorded on a Bruker Avance 500 spectrometer (500 MHz)
at 22 °C and referenced internally to residual protiated solvent
(C6D5H at 7.15 ppm). Resonances are broad singlets unless
otherwise specified. Infrared spectra (450-4000 cm-1) were
recorded on KBr pellet samples in a Shimadzu FTIR spectro-
photometer (FTIR-8400S). A total of 32 scans at 2 cm-1 resolution
were collected in each case. Electronic spectra were recorded
between 280 and 1000 nm on a Cary 50 UV-visible spectro-
photometer, using screw-cap quartz cuvettes of 1 cm optical path
length. Elemental analyses were determined by Desert Analytics
(Tucson, AZ). Pentane, tetrahydrofuran (THF), and toluene were
purified by passage through activated alumina and “deoxygenizer”
columns obtained from Glass Contour Co. (Laguna Beach, CA).
Deuterated benzene and THF were dried over CaH2, then over Na/
benzophenone, and then vacuum transferred into a storage container.
Before use, an aliquot of each solvent was tested with a drop of
sodium benzophenone ketyl in THF solution. Celite was dried
overnight at 200 °C under vacuum. Hydrazine, purchased from
Aldrich, was dried over KH and vacuum transferred before use.
KH, purchased for Aldrich as suspensions in oil, was washed with
pentane three times and dried. BEt3 (1.0 M in hexane) was
purchased from Aldrich and used without further purification. The
preparation and properties of [LMeFeCl]2, [LMeFeH]2 (1), and
KHBEt3 were previously reported.39
LtBuFe(µ-H)2BEt2 (2c). A Schlenk flask was loaded with a
mixture of LtBuFeCl (722 mg, 1.21 mmol) and KHBEt3 (167 mg,
1.21 mmol) in toluene (50 mL). The mixture was stirred at 80 °C
for 18 h. Volatile materials were removed under vacuum, and the
residue was extracted with pentane (50 mL), filtered, and concen-
trated to 10 mL. Crystallization at -26 °C gave dark red blocks
1
(224 mg, 59%). H NMR (500 MHz, C6D6): 71 (6H, BEt2 CH3),
42 (1H, backbone), 35 (18H, backbone CH3), 16 (4H), -13 (12H,
i
iPr CH3), -61 (12H, Pr CH3), -66 (4H), -89 (2H, p-H), -313
(4H) ppm. (Peaks integrated as 4H could be BEt2 CH2, m-H, or iPr
methine. Peaks for the bridging H were not observed.) UV-vis
(pentane): 333 (ꢀ ) 9.42 mM-1 cm-1), 414 (ꢀ ) 6.82 mM-1 cm-1),
597 (ꢀ ) 1.27 mM-1 cm-1) nm. µeff(C6D6, 25 °C): 5.7(1) µB. Anal.
Calcd for C39H65N2BFe: C, 74.52; H, 10.42; N, 4.46. Found: C,
74.60; H, 10.20; N, 4.34.
Complexes 3a and 3c have been reported in ref 12.
LMeFePh (3b). The compound was typically synthesized in one
flask from LMeLi and FeCl2(THF)1.5 as follows. A resealable flask
was loaded with a mixture of LMeLi (592 mg, 1.4 mmol) and
FeCl2(THF)1.5 (329 mg, 1.4 mmol) in toluene (20 mL). The mixture
was stirred at 80 °C for 22 h. Phenylmagnesium chloride (2.0 M
in THF, 0.7 mL, 1.4 mmol) was added, and the mixture was stirred
for 4 h at room temperature. Solvent was removed under vacuum.
The residue was extracted with pentane (20 mL), filtered through
Celite, concentrated to 3 mL, and cooled to -45 °C to give bright
yellow crystals (487 mg, 63% yield). 1H NMR (C6D6, 21 °C): 163
(2H, phenyl m-CH), 126 (1H, phenyl p-CH), 47 (6H, backbone
LMeFe(µ-H)2BEt2 (2a) from 1a. A sample of 1a (139 mg, 0.147
mmol) was dissolved in 15 mL of toluene. Triethylborane (0.29
mL of a 1.0 M solution in hexane) was added via syringe to the
bright red solution. The solution was heated at 70 °C for 14 h.
Volatile materials were removed under vacuum, and the residue
was extracted with pentane (15 mL), filtered, and concentrated to
5 mL. Crystallization from pentane at -35 °C gave bright red
needles (68 mg, 72%).
i
CH3), 28 (1H, backbone CH), -10 (4H, m-CH), -21 (12H, Pr
LMeFe(µ-H)2BEt2 (2a) from FeCl2. The compound was typically
synthesized in one flask from LMeLi and Fe(THF)1.5Cl2 as follows.
A Schlenk flask was loaded with a mixture of LMeLi (1.69 g, 3.98
mmol) and Fe(THF)1.5Cl2 (0.94 g, 4.0 mmol) in toluene (50 mL).
The mixture was stirred at 80 °C for 22 h. All solvent was removed
under vacuum, and a solution of KHBEt3 (0.56 g, 4.0 mmol) in
toluene (50 mL) was added. The mixture was heated at 80 °C for
4 h. Volatile materials were removed under vacuum, and the residue
was extracted with pentane (100 mL), filtered, and concentrated to
20 mL. Crystallization at -35 °C gave bright red needles (777 mg,
CH3), -82 (2H, p-CH), -118 (4H, iPr CH), -125 (12H, iPr CH3).
(Phenyl o-CH was not observed. Peak assignments were based on
1
integrations and H NMR spectra of other known LMeFeR com-
plexes.) UV-vis (pentane): 325 (ꢀ ) 15.6 mM-1 cm-1), 368 (ꢀ )
11.4 mM-1 cm-1), 494 (ꢀ ) 1.0 mM-1 cm-1) nm. µeff (C6D6, 25
°C): 5.2(1) µB. Anal. Calcd for C35H46N2Fe: C, 76.35; H, 8.42;
N, 5.09. Found: C, 76.06; H, 8.20; N, 4.99.
LMeFe(H2NNH2)(µ-H)2BEt2 (4). LMeFe(µ-H)2BEt2 (221 mg,
0.407 mmol) was dissolved in diethyl ether (10 mL). Hydrazine
(13 µL, 0.41 mmol) was added via syringe to the bright red solution.
The solution was shaken, causing an immediate color change from
bright red to orange-pink. The solution was concentrated to 2 mL
and cooled to -35 °C to give bright pink-yellow needles of 4 (212
mg, 90%). 1H NMR (400 MHz, C6D6): 24 (6H), 19 (4H), 3 (12H,
iPr CH3), 1 (6H), -15 (12H, iPr CH3), -24 (4H), -40 (2H, p-H),
-62 (1H, backbone C-H), -260 (4H) ppm. (Peaks integrated as
1
72%). H NMR (400 MHz, C6D6): 81 (6H, BEt2 CH3), 46 (6H,
backbone CH3), 17 (4H), -4 (12H, iPr CH3), -36 (12H, iPr CH3),
-40 (4H), -50 (2H, p-H), -208 (4H) ppm. (Peaks integrated as
(39) Eckert, N. A.; Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Inorg.
Chem. 2004, 43, 3306-3321. Smith, J. M.; Lachicotte, R. J.; Holland, P.
L. J. Am. Chem. Soc. 2003, 125, 15752-15753. Vela, J.; Smith, J. M.; Yu,
Y.; Ketterer, N. A.; Flaschenriem, C. J.; Lachicotte, R. J.; Holland, P. L. J.
Am. Chem. Soc. 2005, 127, 7857-7870.
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4H could be BEt2 CH2, m-H, or Pr methine. Peaks integrated as
6H could be BEt2, CH3, or backbone CH3. Peaks for the bridging