Electronically Unsaturated Complexes of Fe, Co, Ni
A R T I C L E S
mmol) in THF (5 mL) and stirred for 4 h. Volatile materials were
removed from the dark green mixture under vacuum, and the residue
was extracted with 2:1 diethyl ether/pentane (17 mL), filtered, and
concentrated to 8 mL. Cooling to -35 °C gave brown crystals of
LCo(µ-Cl)2Li(ether)2 (458 mg, 48% yield). 1H NMR (400 MHz,
C6D6): δ 50 (br), 22.2, 20 (br), 12.0, 8.8, 3.8, 2.1, 1.5, 0.4, -7, -40,
-43. µeff (C6D6, 295 K)) 4.7 µB. IR (toluene): 1619 (w), 1535 (m),
1492 (s), 1432 (m), 1384 (vs), 1362 (vs), 1317 (vs), 1217 (m), 1102
(w), 1049 (m) cm-1. UV-vis (toluene, ꢀ in mM-1 cm-1): 338 (12),
512 (0.50), 538 (0.47), 689 (0.40) nm. Anal. Calcd for C43H71N2O2-
CoLiCl2: C, 65.81, H, 9.12, N, 3.57. Found: C, 65.86, H, 9.02, N,
3.35.
analogues, an observation that may be relevant to the mechanism
of enzymatic methanogenesis.
Experimental Section
General Considerations. All manipulations were performed under
a nitrogen atmosphere by standard Schlenk techniques or in an M. Braun
Unilab N2-filled glovebox maintained at or below 1 ppm of O2 and
H2O. Glassware was dried at 150 °C overnight. Proton NMR data were
recorded on Bruker Avance 400 or Bruker AMX-400 spectrometers
(400 MHz) at 22 °C. Shifts are reported in ppm, relative to residual
protiated solvent in C6D6 (δ 7.15), CH2Cl2 (δ 5.31), or THF-d8 (δ 1.73,
3.58). In parentheses are listed: integrations, T2 values in ms calculated
LTl. LLi(THF) (742 mg, 1.28 mmol) was added to a stirred solution
of thallium ethoxide (Aldrich, 319 mg, 1.28 mmol) in pentane (13 mL).
After several hours, the yellow mixture was pumped down, extracted
with toluene (60 mL), and filtered. Volatile materials were removed
from the filtrate, and the residue was washed with cold pentane to give
550 mg (61% yield) of yellow powder. This powder appeared pure by
NMR spectroscopy, but sensitivity to trace air or to light prevented
trustworthy elemental analysis (the material turned black in transit).
1H NMR (400 MHz, C6D6): δ 7.03(m, 6, aryl), 5.31(s, 1, backbone),
3.34 (septet, 4, iPr), 1.29(d, 12, iPr), 1.27(s, 18, tBu), 1.2(br s, 12, iPr).
13C NMR (400 MHz, C6D6): δ 169.7, 140.3, 123.6, 33.2, 27.7, 23.2.
IR (toluene): 1533 (m), 1390 (s), 1366 (m), 1327 (w), 1218 (w), 1146
(w), 1096 (w) cm-1. UV-vis (toluene, ꢀ in mM-1 cm-1): 376(18) nm.
A preliminary X-ray crystal structure showed monomeric LTl but had
apparent twinning problems that prevented accurate solution of the
structure.
-1 20
as (π∆ν1/2
)
,
and assignments. Carbon-13 NMR spectra of the
paramagnetic compounds showed substantially fewer peaks than
expected and were not pursued. T1 measurements and DQF-COSY
were recorded on a Varian INOVA-500 spectrometer (500 MHz) at
30.0 °C; T1 values were calculated using a curve-fitting procedure.37
IR spectra were recorded on a Mattson Instruments 6020 Galaxy Series
FTIR; solution spectra used a cell with CsF windows. EPR spectroscopy
at 77 K used a quartz Dewar suspended in the cavity of a Bruker ESP-
300 spectrometer; parameters are given in the Supporting Information.
UV-vis spectra were measured on a Cary 50 spectrophotometer, using
cuvettes sealed to a Schlenk-type stopcock. Solution magnetic suscep-
tibilities were determined by Evans’ method,38 and are considered
accurate to (0.3 µB. Microanalysis was performed by Desert Analytics
(Tucson, AZ).
Pentane, diethyl ether, methylene chloride, tetrahydrofuran (THF),
and toluene were purified by passage through activated alumina and
“deoxygenizer” columns from Glass Contour Co. (Laguna Beach, CA).
Deuterated benzene, THF-d8, and 2-methyltetrahydrofuran were dried
over CaH2 and then over Na and then vacuum distilled into a storage
container or directly into the NMR tube. Carbon monoxide (CP grade)
was obtained from Air Products, and 13CO (99 atom %) was from
Aldrich Chemical Co. Anhydrous metal salts were prepared by treatment
of the hydrates with thionyl chloride.39 The MCl2(THF)n complexes
were prepared using the method of Kern,40 and NiCl2 was converted
to a THF adduct using an analogous method.41 Celite was dried at 200
°C under vacuum. The lithium salt LLi(THF) was prepared by treating
a solution of LH42 in THF with 1.0 equiv of butyllithium, removing
solvent, and crystallizing from pentane. Methyllithium (Aldrich, 1.4
M in diethyl ether) was transferred in a 8-10 mL portion to a 20-mL
scintillation vial, treated with several drops of dioxane to precipitate
halide impurities,43 filtered through Celite, and pumped down to a white
solid for use. Methylmagnesium chloride (Aldrich, 3 M in THF) was
used as received. The preparation of LFeCl and LFeCH3 has been
described previously.16,17 A solution of CD3MgI for the preparation of
LFeCD3 and LCoCD3 was generated by stirring a solution of CD3I
(Aldrich) in diethyl ether with Mg metal. Methyl-d3 compounds
synthesized from this Grignard reagent solution were spectroscopically
identical to their protiated analogues.
LCoCl. A solution of LTl (126 mg, 0.178 mmol) in THF (5 mL)
was added to a slurry of CoCl2(THF)1.5 (42 mg, 0.176 mmol) in THF
(5 mL), stirred for 2.5 h, and then filtered. Volatile materials were
removed to leave a brown residue that was dissolved in diethyl ether
(2 mL). Crystals formed upon standing at room temperature for several
hours. Total yield: 42 mg (40%). 1H NMR (400 MHz, C6D6): δ 60.8
(4, 4, m-aryl), 27.6 (18, 5, tBu), -2.8 (12, 6, iPr “back” methyl), -46.0
i
i
(2, 6, p-aryl), -57.4 (4, 1, Pr methine), -84.4 (12, 2, Pr “front”
methyl), -90.7 (1, 3, backbone). µeff (C6D6, 295 K) ) 4.7 µB. UV-
vis (toluene, ꢀ in mM-1 cm-1): 358(6.8), 510 (sh, ∼0.2), 652 (0.10)
nm. Mp: 240 °C (dec). Anal. Calcd for C35H53N2CoCl: C, 70.51, H,
8.96, N, 4.70. Found: C, 69.30, H, 9.12, N, 4.81.
LCoCH3. Methylmagnesium chloride (0.12 mL, 0.36 mmol) was
added to a solution of LCo(µ-Cl)2Li(ether)2 (216 mg, 0.276 mmol) in
diethyl ether (7 mL), leading to immediate formation of a white
precipitate. After 0.5 h, the mixture was filtered, volatile materials were
removed from the filtrate under vacuum, and the residue was extracted
with 1:1 diethyl ether/pentane (20 mL). The solution was concentrated
to 8 mL, filtered, and cooled to -35 °C to give brown crystals of
1
LCoCH3. H NMR (400 MHz, C6D6): δ 78.1 (4, 11, m-aryl), 27.2
i
(18, 23, tBu), 4.1 (12, 27, Pr “back” methyl), -27.6 (2, 18, p-aryl),
i
i
-53.8 (4, 2, Pr methine), -67.1 (1, 6, backbone), -118 (12, 4, Pr
“front” methyl). µeff (C6D6, 295 K) ) 4.9 µB. IR (toluene): 1511 (m),
1433 (m), 1383 (vs), 1363 (vs), 1319 (s), 1220 (m), 1198 (w), 1098
(w) cm-1. UV-vis (toluene, ꢀ in mM-1 cm-1): 335(15), 565(0.13),
725(0.16) nm. Anal. Calcd for C36H56CoN2: C, 75.10, H, 9.80, N, 4.87.
Found: C, 74.78, H, 9.68, N, 4.80.
Assignment of 1H NMR Signals in LFeCH3.17 [shift in ppm
(integration, T1 in ms, T2 in ms, assignment)] δ 133.2 (1, <1, 0.4,
backbone), 42.8 (18, 2, 2, tBu), -3.1 (4, 6, 5, m-aryl), -28.3 (12, 6,
5, “back” iPr methyl), -108.1 (2, 4, 3, p-aryl), -118.2 (4, <1, 0.2, iPr
i
methine), -129.5 (12, <1, 0.4, “front” Pr methyl).
LNiCl. A Schlenk flask was loaded with NiCl2(THF)0.7 (1.75 g, 10.2
mmol), LLi(THF) (3.72 g, 6.41 mmol), and THF (30 mL) and heated
to 70 °C overnight. Volatile materials were removed from the dark
green mixture under vacuum, and the residue was extracted with CH2Cl2
(70 mL), filtered, and concentrated to 15 mL. Addition of diethyl ether
(10 mL) and cooling to -35 °C gave green crystals of LNiCl. A second
crop of crystals was collected to give a total yield of 2.5 g (66%).1H
LCo(µ-Cl)2Li(ether)2. A solution of LLi(THF) (722 mg, 1.24 mmol)
in THF (5 mL) was added to a slurry of CoCl2(THF)1.5 (291 mg, 1.22
(37) Evaluation of T1 from an inversion-recovery experiment is described:
Levy, G. C.; Peat, I. R. J. Magn. Reson. 1975, 18, 500-521.
(38) Schubert, E. M. J. Chem. Educ. 1992, 69, 62.
(39) Pray, A. R. Inorg. Synth. 1990, 28, 321-323.
(40) Kern, R. J. J. Inorg. Nucl. Chem. 1962, 24, 1105-1109.
(41) Eckert, N. A.; Bones, E. M.; Lachicotte, R. L.; Holland, P. L., submitted.
(42) Budzelaar, P. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J. Inorg. Chem.
1998, 1485-1494.
i
NMR (400 MHz, CD2Cl2): δ 29.3 (4, 14, m-aryl), 18.5 (4, 11, Pr
methine), 5.6 (12, 19, iPr methyl), 4.1 (12, 18, iPr methyl), 2.5 (18, 21,
tBu), -14.7 (2, 14, p-aryl), -123.5 (1, 11, backbone). µeff (CD2Cl2,
295 K) ) 3.1 µB. IR (CH2Cl2): 2967 (vs), 2869 (m), 1587 (w), 1521
(43) Holland, P. L.; Smith, M. E.; Andersen, R. A.; Bergman, R. G. J. Am.
Chem. Soc. 1997, 119, 12815-12823.
9
J. AM. CHEM. SOC. VOL. 124, NO. 48, 2002 14423