A R T I C L E S
Du et al.
7.70 (t, J ) 16.2 Hz, 1H, ReH), 2.8-1.2 (m, 66H, PCy3). 31P NMR
(CD2Cl2, 121.4 MHz): δ 10.0 ppm (s). FT-IR (KBr): 2034 (ResH),
992 (RetO) cm-1. Anal. Calcd (found) for C36H67Cl2OP2Re: C, 51.78
(51.40); H, 8.09 (7.75).
a refutation of possible mechanisms rather than proof of a given
reaction mechanism. By this scientific standard results from this
study have shown decisively that the terminal multiple bonds
RetX (X ) O, NAr, or N) in Re(X)Cln(PR3)2 (where n ) 2 or
3) hydrosilylation catalysts neither are directly involved in silane
activation nor undergo [2 + 2] cycloaddition with silane
(SisH). Furthermore, we show that the observable and isolable
intermediates are not the major players in the catalytic pathway.
This underscores the importance of obtaining quantitative kinetic
information in elucidating reaction mechanisms, particularly
those of catalytic reactions because frequently what is seen is
not what is responsible for catalysis, even in cases when
stoichiometric reactivity for each of the intermediates could be
independently established.22,23 By process of elimination, com-
plexation of η2-silane cis to the RetX bond followed by
heterolytic cleavage is the most viable catalytic pathway for
the series Re(X)Cln(PR3)2 (X ) O, NAr, or N; n ) 2 or 3; and
R ) Ph or Cy).
Re(NMes)Cl2(H)(PPh3)2 (5a). To a green solution of Re(NMes)-
Cl3(PPh3)2 (155 mg, 0.151 mmol) in CH2Cl2 was added Et3SiH (120
µL, 0.751 mmol). The reaction mixture was stirred at ambient
temperature for 18 h in which the color changed from green to yellow
to dark red. The solvent was removed under vacuum, and the residue
was triturated with pentane. The light brown powder was collected by
filtration and washed with pentane. Yield: 110 mg (80%). X-ray quality
crystals were grown by gas-phase diffusion of pentane into a benzene
1
solution. H NMR (CD2Cl2, 300 MHz): δ 7.75 (m, 12H), 7.27 (m,
18H), 6.37 (s, 1H), 6.16 (s, 1H), 4.74 (t, 1H, J ) 17.1 Hz, ReH), 1.98
(s, 3H), 1.77 (s, 3H), 1.40 (s, 3H). 13C NMR (CD2Cl2, 75 MHz): δ
138.3, 137.3, 134.8, 134.2, 133.8, 133.3, 130.3, 129.7, 129.1, 128.1,
22.9, 20.1, 19.7. 31P NMR (CD2Cl2, 121.4 MHz): δ 10.1 ppm (s).
FT-IR (KBr): 2058 cm-1 (ResH). ESI-MS: 912/914/916 (M - H)+,
878/880 (M - Cl)+ with correct isotopic pattern. Anal. Calcd (found)
for C45H42Cl2NP2Re‚1.5C6H6: C, 62.78 (62.60); H, 4.98 (5.04); N, 1.36
(1.35).
Experimental Section
Re(NMes)Cl3(PPh3)(NH2Mes) (7). A suspension of ReOCl3(PPh3)2
(721 mg, 0.865 mmol) and 2,4,6-trimethylaniline (1 mL, 7.10
mmol) was allowed to reflux for 48 h in dry benzene (35 mL). The
resulting dark green solution was stripped off the solvent and treated
with a mixture of acetic acid-H2O (1:1). The green precipitate was
collected by filtration and redissolved in hot benzene. After filtration,
the filtrate was allowed to stand for several days. The green crystals
were collected by filtration. Yield: 361 mg (48%). X-ray quality crystals
were grown from a benzene solution of 7. In the filtrate, Re(NMes)-
Materials and Instruments. All manipulations were performed
under air unless stated otherwise. Solvents were purified with a solvent
purification system (Anhydrous Engineering Inc.) prior to use. Orga-
nosilanes and carbonyl compounds were obtained from commercial
sources and used as received. NMR spectra were recorded on a Varian
Inova 300 or Mercury 200 instrument at 20 °C. Proton resonances were
referenced internally to residual solvent peaks (CHDCl2, δ 5.32).
Infrared spectra were recorded as KBr pellets on a Perkin-Elmer 2000
FT-IR spectrometer. UV-vis data were recorded on a Shimadzu 2501
spectrophotometer and reported as λmax in nm (log ꢀ). Mass spectrometry
was performed by the Purdue University Campus Wide Mass Spec-
trometry Center using a Hewlett-Packard Engine mass spectrometer
(GC/MS). Elemental microanalyses were done by the Purdue University
Microanalytical Laboratory. Kinetic runs were carried out on a
Shimadzu 2501 spectrophotometer or a Varian Inova 300 NMR
instrument at 25 °C. Data analyses were performed using KaleidaGraph,
version 3.6.
1
Cl3(PPh3)2 could be isolated. H NMR (CD2Cl2, 300 MHz): δ 7.67
(m, 6H), 7.29 (m, 9H), 7.18 (s, 2H, NH2), 6.71 (s, 2H), 6.39 (s, 2H),
2.30 (s, 6H), 2.21 (s, 3H), 2.11 (s, 3H), 1.60 (s, 6H). 31P NMR
(CD2Cl2, 121.4 MHz): δ -17.0 ppm (s). ESI-MS: 823/825 (M + 1)+,
785/787/789 (M - Cl)+ with correct isotopic pattern. Anal. Calcd
(found) for C36H39Cl3N2PRe‚0.5C6H6: C, 54.32 (54.41); H, 4.91 (4.71);
N, 3.25 (3.17).
Kinetics. The reaction of ReOCl3(PPh3)2 (1a) with Et3SiH in
CH2Cl2 was monitored by UV-vis at 430 nm where ReOCl3(PPh3)2
has a shoulder absorbance while ReOCl2(H)(PPh3)2 does not. Attempts
to monitor this reaction by 31P NMR were unsuccessful. The reac-
tion of Re(NMes)Cl3(PPh3)2 with Et3SiH in CH2Cl2 was monitored by
Re(O)Cl2(H)(PPh3)2 (4a). To a yellow suspension of ReOCl3(PPh3)2
(203 mg, 0.244 mmol) in CH2Cl2 (10 mL) was added Et3SiH (200 µL,
1.25 mmol). After being stirred at ambient temperature for 4.5 h, the
mixture was filtered and the yellow-green filtrate was dried under
vacuum. The resulting material was washed with pentane to give 4a.
Yield: 180 mg (92%). X-ray quality crystals were grown by gas-phase
diffusion of pentane into a CH2Cl2 solution in the presence of a small
1
UV-vis at 360 nm. Other reactions were monitored by H NMR in
CD2Cl2 in the presence of an internal standard (Ph3CH). The initial
rates of the 1a-Et3SiH reaction were obtained from a fifth-order
polynomial fit of the concentration-time profile, as in [1a]t
)
1
amount of Et3SiH. H NMR (CD2Cl2, 300 MHz): δ 7.78 (m, 12H),
[1a]0 - m1t - m2t2 - ...-m5t5, from which Vi ) m1. Data analysis
was performed by a nonlinear least-squares fitting using KaleidaGraph.
X-ray Data Collection and Structure Solution. A summary of
structure determination is provided here and further details can be
found in the Supporting Information. Crystallographic data for 4a:
C36H31Cl2OP2Re, FW ) 798.70, 0.38 × 0.35 × 0.28 mm3 crystal
dimension, orthorhombic, space group Pbca (# 61), a ) 18.0285(3)
Å, b ) 16.6735(3) Å, c ) 21.7549(4) Å, V ) 6539.5(2) Å3, Z ) 8,
Fcalcd ) 1.622 g/cm3, u ) 40.5/cm, 58 558 reflections collected, 8408
unique (Rint ) 0.067), R1 ) 0.055, R2 ) 0.146, for 5437 reflections
7.47 (m, 19H, aromatic CH and ReH). 31P NMR (CD2Cl2, 121.4
MHz): δ 8.0 ppm (s). FT-IR (KBr): 2018 (ResH), 990 (RetO) cm-1
.
Anal. Calcd (found) for C36H31Cl2OP2Re: C, 54.14 (54.50); H, 3.91
(4.03). Re(O)Cl2(D)(PPh3)2 was prepared similarly from ReOCl3(PPh3)2
and Et3SiD (Aldrich, 97 atom % D).
ReOCl2(H)(PCy3)2 (4b). To a yellow suspension of ReOCl3(PCy3)2
(180 mg, 0.207 mmol) in CH2Cl2 (10 mL) was added Et3SiH (190 µL,
1.19 mmol). After being stirred at ambient temperature for 5.5 h, the
mixture was filtered and the yellow-green filtrate was dried under
vacuum. This resulting material was triturated and washed with pentane
2
2
with Fo > 2σ(Fo ). Crystallographic data for 5a: C45H42Cl2NP2Re,
1.5(C6H6), FW ) 1033.07, 0.35 × 0.35 × 0.15 mm3 crystal dimension,
monoclinic, space group P121/n1(# 14), a ) 15.3680(3) Å, b )
19.1680(3) Å, c ) 15.9227(4) Å, â ) 103.2930(7)°, V ) 4564.74(16)
Å3, Z ) 4, Fcalcd ) 1.50 g/cm3, u ) 29.2/cm, 42 473 reflections collected,
11 165 unique (Rint ) 0.054), R1 ) 0.042, R2 ) 0.069, for 7192
1
to give 4b. Yield: 137 mg (79%). H NMR (CD2Cl2, 300 MHz): δ
(22) (a) Halpern, J. Science, 1982, 217, 401-7. (b) Chan, A. S. C.; Pluth, J. J.;
Halpern, J. J. Am. Chem. Soc. 1980, 102, 5952-4.
(23) This truism has been referred to as the Halpern axiom or Halpern’s rule.
For example, see: (a) Darensbourg, D. J.; Darensbourg, M. Y.; Goh, L.
Y.; Luvig, M.; Wiegreffe, P. J. Am. Chem. Soc. 1987, 109, 7539-40. (b)
Yin, C.; Xu, Z.; Yang, S.-Y.; Ng, S. M.; Wong, K. Y.; Lin, Z.; Lau, C. P.
Organometallics 2001, 20, 1216-22. (c) Hagen, C. M.; Vieille-Petit, L.;
Laurenczy, G.; Suss-Fink, G.; Finke, R. G. Organometallics 2005, 24,
1819-31. (d) Tschan, M. J. L.; Suss-Fink, G.; Cherioux, F.; Therrein, B.
Chem.sEur. J. 2007, 13, 292-9.
2
2
reflections with Fo > 2σ(Fo ). Crystallographic data for 7: C36H39-
Cl3N2PRe, 0.5(C6H6), FW ) 862.32, 0.27 × 0.20 × 0.11 mm3 crystal
dimension, triclinic, space group P-1(# 2), a ) 11.8396(5) Å, b )
12.8071(5) Å, c ) 13.6137(7) Å, R ) 72.664(2)°, â ) 76.118(2)°, γ
9
5186 J. AM. CHEM. SOC. VOL. 129, NO. 16, 2007