Reactions of H2 and R3SiH with electrophilic cobalt(III) alkyl complexes: Spectroscopic characterization, dynamics, and chemistry of [Cp*Co(L)(H)(η2-H2)][B(Ar F)4] and [Cp*Co(L)(H)η2-HSiR 3][B(ArF)4]

Article abstract of DOI:10.1021/om700620w

Co(III) agostic alkyl complexes [Cp*Co(L)(CH₂CH ₂-μ-H)][B(Ar_F)₄] (Cp* = C ₅(CH₃)₅, L = P(OCH₃)₃, 1a, or P(CH₃)₃, 1b; Ar_F = 3,5-(CF ₃)₂C₆H₃) react with H₂ to yield ethane and trihydride complexes characterized as η²- dihydrogen hydride species [Cp*Co(L)(H)(η²-H ₂)][B(Ar_F)], 2a and 2b, in which there is rapid scrambling between the η²-H₂ ligand and the terminal Co-H. Complexes 2a and 2b react with a variety of neutral donor ligands (L′ = RCN, PMe₃, P(OMe)₃, H₂O, CH₃OH) to yield [Cp*Co(L)(L′)(H)][B(Ar^F)₄] complexes. Reaction of silanes with either 1a,b in the presence of traces of water or 2a,b yields η²-silane hydrides, [Cp*Co(L)(H)(η²- HSiR₃)][B(Ar_F)₄]. Analysis of the dynamics of these species by NMR spectroscopy provides evidence for an extremely rapid process involving silyl migration between hydrogens and a slower process in which a cobalt-silyl η²-H₂ complex is formed as an intermediate and results in hydrogen scrambling between the two diastereomers of [Cp*Co(L)(H)(η²-H-SiHMePh)][B(Ar_F) ₄]. The structures and dynamics of 2a,b and the η²- silane complexes imply that cleavage of the cobalt alkyl bonds of la,b in hydrogenation and hydrosilation catalytic cycles occurs by the σ-CAM (σ-complexassisted metathesis) process.

Full text of DOI:10.1021/om700620w

5950
Organometallics 2007, 26, 5950-5960
Reactions of H2 and R3SiH with Electrophilic Cobalt(III) Alkyl
Complexes: Spectroscopic Characterization, Dynamics, and
Chemistry of [Cp*Co(L)(H)(η2-H2)][B(ArF)4] and
[Cp*Co(L)(H)(η2-HSiR3)][B(ArF)4]
Mark D. Doherty, Brian Grant, Peter S. White, and Maurice Brookhart*
Department of Chemistry, UniVersity of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599
ReceiVed June 21, 2007
Co(III) agostic alkyl complexes [Cp*Co(L)(CH2CH2-µ-H)][B(ArF)4] (Cp* ) C5(CH3)5, L ) P(OCH3)3,
1a, or P(CH3)3, 1b; ArF ) 3,5-(CF3)2C6H3) react with H2 to yield ethane and trihydride complexes
characterized as η2-dihydrogen hydride species [Cp*Co(L)(H)(η2-H2)][B(ArF)], 2a and 2b, in which there
is rapid scrambling between the η2-H2 ligand and the terminal Co-H. Complexes 2a and 2b react with
a variety of neutral donor ligands (L′ ) RCN, PMe3, P(OMe)3, H2O, CH3OH) to yield [Cp*Co(L)(L′)-
(H)][B(ArF)4] complexes. Reaction of silanes with either 1a,b in the presence of traces of water or 2a,b
yields η2-silane hydrides, [Cp*Co(L)(H)(η2-HSiR3)][B(ArF)4]. Analysis of the dynamics of these species
by NMR spectroscopy provides evidence for an extremely rapid process involving silyl migration between
hydrogens and a slower process in which a cobalt-silyl η2-H2 complex is formed as an intermediate and
results in hydrogen scrambling between the two diastereomers of [Cp*Co(L)(H)(η2-H-SiHMePh)][B(ArF)4].
The structures and dynamics of 2a,b and the η2-silane complexes imply that cleavage of the cobalt-
alkyl bonds of 1a,b in hydrogenation and hydrosilation catalytic cycles occurs by the σ-CAM (σ-complex-
assisted metathesis) process.
These formally 16-electron alkyl complexes, 1, exhibit â-agostic
interactions (R ) ethyl or greater) and are highly dy-
namic.15,19,21,25 Rotation around the CR-Câ bond is rapid (∆Gq
) 11-12 kcal/mol), and the metal can migrate along the carbon
chain through a series of â-H elimination/olefin rotation/
reinsertion reactions, now commonly termed chain-walking
processes.15-17,19
Introduction
Interest in the structure and chemistry of highly electrophilic
late transition metal complexes has escalated over the past two
decades due to their increasing application in catalytic trans-
formations as well as the ability to stabilize and characterize
highly reactive unsaturated cationic late metal complexes
through the use of inert, weakly coordinating counteranions,
particularly fluorinated aryl borate anions. Notable examples
include Ir(III) complexes of the type Cp*Ir(PMe3)R+, which
are active for sp3-C-H bond activations,1,2 and Ni(II) and Pd(II)
complexes of general structure (L-L)MR+, many of which are
active catalysts for olefin dimerizations,3-5 oligomerizations,6,7
and polymerizations,8,9 olefin/CO copolymerizations,10-13 and
hydrosilation14 reactions.
These species serve as living ethylene polymerization
We have had an ongoing interest in the chemistry of elec-
catalysts,16,20,21,25-27 R-olefin oligomerization catalysts,15 and
trophilic complexes of the general structure (C5R5)(L)CoR+.15-27
* Corresponding author. E-mail: mbrookhart@unc.edu.
(1) Golden, J. T.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc.
2001, 123, 5837-5838.
(14) LaPointe, A. M.; Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1997,
119, 906-917.
(15) Broene, R. D.; Brookhart, M.; Lamanna, W. M.; Volpe, A. F. J. J.
Am. Chem. Soc. 2005, 127, 17194-17195.
(2) Klei, S. R.; Tilley, T. D.; Bergman, R. G. J. Am. Chem. Soc. 2000,
122, 1816-1817.
(16) Brookhart, M.; DeSimone, J. M.; Grant, B. E.; Tanner, M. J.
Macromolecules 1995, 28, 5378-5380.
(17) Brookhart, M.; Grant, B. E. J. Am. Chem. Soc. 1993, 115, 2151-
2156.
(18) Brookhart, M.; Grant, B. E.; Volpe, A. F. J. Organometallics 1992,
11, 3920-3922.
(19) Brookhart, M.; Lincoln, D. M.; Volpe, A. F. J.; Schmidt, G. F.
Organometallics 1989, 8, 1212-1218.
(3) Ledford, J.; Shultz, C. S.; Gates, D. P.; White, P. S.; DeSimone, J.
M.; Brookhart, M. Organometallics 2001, 20, 5266-5276.
(4) Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 1137-1138.
(5) Shultz, C. S.; DeSimone, J. M.; Brookhart, M. Organometallics 2001,
20, 16-18.
(6) Svejda, S. A.; Brookhart, M. Organometallics 1999, 18, 65-74.
(7) Killian, C. M.; Johnson, L. K.; Brookhart, M. Organometallics 1997,
16, 2005-2007.
(20) Brookhart, M.; Volpe, A. F. J. Late transition metal catalysts for
olefin polymerization. EP19910200928 19910418, 1991.
(21) Brookhart, M.; Volpe, A. F. J.; Lincoln, D. M.; Horvath, I. T.; Millar,
J. M. J. Am. Chem. Soc. 1990, 112, 5634-5636.
(22) Cracknell, R. B.; Orpen, A. G.; Spencer, J. L. J. Chem. Soc., Chem.
Commun. 1984, 326-328.
(8) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414-6415.
(9) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169-1203.
(10) Brookhart, M.; Wagner, M. I. J. Am. Chem. Soc. 1996, 118, 7219-
7220.
(23) Cracknell, R. B.; Orpen, A. G.; Spencer, J. L. J. Chem. Soc., Chem.
Commun. 1986, 1005-1006.
(11) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996,
118, 4746-4764.
(24) Lenges, C. P.; Brookhart, M.; Grant, B. E. J. Organomet. Chem.
1997, 528, 199-203.
(12) Shultz, C. S.; Ledford, J.; DeSimone, J. M.; Brookhart, M. J. Am.
Chem. Soc. 2000, 122, 6351-6356.
(25) Schmidt, G. F.; Brookhart, M. J. Am. Chem. Soc. 1985, 107, 1443-
1444.
(13) Drent, E.; Budzelaar, P. H. M. Chem. ReV. 1996, 96, 663-682.
10.1021/om700620w CCC: $37.00 © 2007 American Chemical Society
Publication on Web 10/19/2007

Electrophilic Cobalt(III) Alkyl Complexes
Organometallics, Vol. 26, No. 24, 2007 5951
Scheme 1. Mechanism of Co(III)-Catalyzed Olefin
Hydrosilation
Figure 1. ORTEP representations of [Cp*Co(P(OMe)3)(CH2CH2-
µ-H)][B(ArF)4], 1a, and [Cp*Co(PMe3)(CH2CH2-µ-H)][B(ArF)4],
1b. The [B(ArF)4] anion has been omitted for clarity. Selected bond
lengths (Å) and angles (deg) for 1a: Co(1)-C(8) 1.969(7), Co(1)-
C(7) 2.119, Co(1)-P(1) 2.1637(18), C(8)-C(7) 1.477(13); Co(1)-
C(8)-C(7) 74.4(5), Co(1)-P(1)-O(1) 118.2(2), Co(1)-P(1)-O(3)
109.84(19), Co(1)-P(1)-O(5) 123.6(2). Selected bond lengths (Å)
and angles (deg) for 1b: Co(1)-C(1) 1.961(3), Co(1)-P(1)
2.1976(9), C(1)-C(2) 1.462(5); Co(1)-C(1)-C(2) 75.16(19),
Co(1)-P(1)-C(3) 113.22(12), Co(1)-P(1)-C(4) 114.95(12), Co(1)-
P(1)-C(5) 120.24(12).
olefin hydrogenation and hydrosilation catalysts.17,28 Chain
transfer in the ethylene polymerization reaction can be achieved
by cleavage of the cobalt-alkyl bond with H2 or triethylsilane
(eq 1).16 This cleavage reaction employing H2 is also a key step
in the hydrogenation of olefins.
characterized by low-temperature 1H and 13C NMR spectroscopy
and were shown to possess â-agostic C-H bonds exhibiting
C-H coupling constants of 61 Hz for 1a and 63 Hz for 1b.
X-ray structural analysis of complexes 1a and 1b (see Figure 1
and Table 4) now also support the assignment as â-agostic
structures, as indicated by the small Co-CR-Câ angles (74.4-
(5)° for 1a and 75.16(19)° for 1b) and short CR-Câ bond lengths
(1.477(13) Å for 1a and 1.462(5) Å for 1b). These angles and
bond lengths are consistent with solid-state structures of related
â-agostic complexes reported by Spencer22,29-31 and Tanner
(Cp*Co(P(OMe)3)(CH2CR-µ-H)+, R ) C4H9, 1c)26 and with
the calculated structure of CpCo(PH3)(CH2CHR-µ-H)+.32
The catalytic cycle for the hydrosilation reaction is shown in
Scheme 1. An unusual feature of this mechanism is that the
migration of cobalt to the terminal carbon is rate-determining.17
Again, cleavage of the cobalt-alkyl bond provides the key step
in catalyst turnover.
This article reports studies aimed at elucidating the details
of the cleavage of the cobalt-alkyl bonds of [Cp*Co(L)(CH2-
CH2-µ-H)][B(ArF)4] (L ) P(OCH3)3, 1a, or P(CH3)3, 1b) with
hydrogen and silanes and identifying the intermediates that result
from these cleavage reactions. NMR evidence suggests that
cleavage of the cobalt alkyl complexes with H2 results in the
formation of trihydride complexes that are best described as
η2-H2/H complexes (Cp*(L)Co(H2)H+); the corresponding silane
complexes exist as η2-silane hydrides (Cp*(L)Co(HSiR3)H+).
Spectroscopic as well as dynamic properties of these species
provide insight into the mode of cleavage of the cobalt-alkyl
bonds.
Hydrogenolysis of [Cp*Co(L)(CH2CH2-µ-H)][B(ArF)4] (Cp*
) C5(CH3)5; L ) P(OMe)3, 1a; L ) PMe3, 1b). Exposure of
methylene chloride-d2 solutions of 1a to excess hydrogen at
-30 °C results in the liberation of an equivalent of ethane and
the formation of 2a as the major product after several minutes.
1
The upfield portion of the H NMR spectrum of 2a exhibits a
2
broad doublet (δ -10.46, JHP ) 29 Hz) integrating for three
protons versus the corresponding P(OMe)3 and Cp* signals,
indicating an empirical formula of [Cp*Co(P(OMe)3)(H)3]-
[B(ArF)4] for complex 2a (eq 2). A minor additional upfield
signal (δ -10.99, 2JHP ) 113 Hz) integrating for one proton is
accompanied by a corresponding set of P(OMe)3 and Cp*
signals and has been assigned to the coordinated water complex
[Cp*Co(P(OMe)3)(H)(H2O)][B(ArF)4], 3a, resulting from co-
ordination of trace amounts of water present in solution.
Addition of H2 to 1a in the presence of excess H2O leads to
quantitative formation of a single species with 1H NMR signals
matching those of the minor product 3a, confirming the above
assignment. Similar results were observed upon exposure of
complex 1b to hydrogen in methylene chloride-d2 at -30 °C,
generating complex 2b, [Cp*Co(PMe3)(H)3][B(ArF)4] (δ -10.70,
2JHP ) 27 Hz), as the major product and complex 3b, [Cp*Co-
Results and Discussion
Synthesis and Characterization of [Cp*Co(L)(CH2CH2-
µ-H)][B(ArF)4] (Cp* ) C5(CH3)5; L ) P(OMe)3, 1a; L )
PMe3, 1b). As previously reported,18,19 low-temperature pro-
tonation of the Cp*Co(L)(η2-ethylene) (L ) P(OMe)3, PMe3)
complexes with [H(OEt2)2][B(ArF)4] followed by precipitation
with hexane results in the isolation of the cationic ethyl
complexes 1a and 1b, respectively. These complexes have been
2
(PMe3)(H)(H2O)][B(ArF)4] (δ -11.27, JHP ) 97.5 Hz), as a
(29) Carr, N.; Dunne, B. J.; Orpen, A. G.; Spencer, J. L. J. Chem. Soc.,
Chem. Commun. 1988, 926-928.
(26) Tanner, M. J. Mechanistic Studies of Co(III)-Catalyzed Reactions:
Living Polymerization of Ethylene. Ph.D. Thesis, University of North
Carolina at Chapel Hill, Chapel Hill, 1997.
(27) Tanner, M. J.; Brookhart, M.; DeSimone, J. M. J. Am. Chem. Soc.
1997, 119, 7617-7618.
(28) Brookhart, M.; Grant, B. E.; Lenges, C. P.; Prosenc, M. H.; White,
P. S. Angew. Chem., Int. Ed. 2000, 39, 1676-1679.
(30) Carr, N.; Mole, L.; Orpen, A. G.; Spencer, J. L. J. Chem. Soc.,
Dalton Trans. 1992, 2653-2662.
(31) Conroy-Lewis, F. M.; Mole, L.; Redhouse, A. D.; Litster, S. A.;
Spencer, J. L. J. Chem. Soc., Chem. Commun. 1991, 1601-1603.
(32) Han, Y.; Deng, L.; Ziegler, T. J. Am. Chem. Soc. 1997, 119, 5939-
5945.

5952 Organometallics, Vol. 26, No. 24, 2007
Doherty et al.
Scheme 2. Substitution of the η2-H2 Ligand in
Co(III)-(H)(η2-H2)+ Complexes 2a and 2b
1
Figure 2. Upfield region of the H NMR spectrum of partially
deuterated 2a.
minor species. All attempts to isolate 2a or 2b have led only to
oils that readily decompose at room temperature.
29.1 Hz for 2b. These values are fully consistent with an η2-H2
structural assignment.38-43 This calculation makes the assump-
tion that H and D statistically distribute between bridging and
terminal hydride positions and there is negligible coupling
between the terminal hydride and the η2-H2 protons.44
These η2-dihydrogen complexes, Cp*Co(L)(η2-H2)(H)+, can
be added to the list of previously reported complexes possessing
the general formula (C5R5)M(L)(H)3+ (R ) H, CH3, M ) Ir45-48
and Rh38). Like 2a/b, all members of this family exhibit a single
1
resonance in the upfield region of their H NMR spectra at
The spin-lattice relaxation time, T1(min), of metal-bound
hydrides has been used to differentiate between classical
dihydride and nonclassical η2-H2 structures in transition metal
polyhydride complexes.33-36 Measurement of T1 at a variety of
temperatures gives minimum values at -80 °C of 22 ms for 2a
and 52 ms for 2b, suggesting that these complexes are likely to
be η2-H2 complexes.33,34,36 Since quadrapolar contributions to
relaxation rates can be expected for hydrides bound to cobalt
(59Co I ) 7/2, 100% natural abundance, γ ) 6.3015 × 10-7
rad T-1 s-1),37 these T1(min) values are better viewed as upper
limits of the relaxation rate due to dipole-dipole contributions
and thus do not provide unambiguous proof of an η2-H2
structure.35
ambient temperatures. The analogous Rh complexes have been
assigned an (η2-H2)(H) structure based on T1(min) measurements,
H-D coupling constants, and isotopic labeling studies.38
However, an absence of observable H-D coupling has elimi-
nated an η2-H2 formulation for the reported Ir complexes, which
have been characterized as classical Ir(V) trihydrides.47,48 Arrest
of the site exchange process between the unique trans hydride
and the two equivalent cis hydrides has been achieved for these
1
complexes using low-temperature H NMR techniques.47 Ex-
tremely large, temperature-dependent H-H couplings have been
observed in these low-temperature spectra, a phenomenon that
has been attributed to a quantum mechanical exchange
process.47-49
In order to better define the structural nature of these cobalt-
bound hydrides, the H-D coupling constants of partially
Having established the structures of 2a and 2b as η2-H2
complexes, we turned to investigating the reactivity of the
coordinated H2 ligand. As shown in Scheme 2, complexes 2a
1
deuterated samples of 2a and 2b were measured by H NMR
spectroscopy. Removal of the H2 atmosphere from screw cap
NMR tubes containing methylene chloride-d2 solutions of 2a
or 2b via two freeze-pump-thaw cycles and addition of an
atmosphere of deuterium gas results in the formation of an
isotopic mixture of hydrides (H3 (2a/b), H2D (2a/b-d1), HD2
(2a/b-d2), and D3 (2a/b-d3) isotopologues). The signals for 2a/
b-d1 and 2a/b-d2 are partially obscured by the broad doublet
corresponding to 2a/b (shown for 2a in Figure 2); however an
H-D coupling constant of 9.5 Hz for 2a and 9.7 Hz for 2b
could be measured. Assuming a Co(H)(η2-H2) structure and
accounting for statistical exchange of the deuterium into terminal
(Co-H) and bridging (η2-H-H) positions, the calculated 1JHD
coupling constants in the η2-HD ligands are 28.5 Hz for 2a and
(38) Taw, F. L.; Mellows, H.; White, P. S.; Hollander, F. J.; Bergman,
R. G.; Brookhart, M.; Heinekey, D. M. J. Am. Chem. Soc. 2002, 124, 5100-
5108.
(39) Heinekey, D. M.; Oldham, W. J. J. Chem. ReV. 1993, 93, 913-
926.
(40) Kubas, G. J. Acc. Chem. Res. 1988, 21, 120-128.
(41) Kubas, G. J. J. Organomet. Chem. 2001, 635, 37-68.
(42) Morris, R. H. Non-classical Hydrogen Bonding along the Pathway
to the Heterolytic Splitting of Dihydrogen. In Recent AdVances in Hydride
Chemistry, 1st ed.; Peruzzini, M., Poli, R., Eds.; Elsevier Science Ltd.:
Amsterdam, NY, 2001; pp 1-38.
(43) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes: Structure,
Theory and ReactiVity; Kluwer Academic/Plenum Publishers: Boston, 2001;
p 472.
(44) Gusev, D. G.; Kuhlman, R. L.; Renkema, K. B.; Eisenstein, O.;
Caulton, K. G. Inorg. Chem. 1996, 35, 6775-6783.
(45) Gilbert, T. M.; Bergman, R. G. J. Am. Chem. Soc. 1985, 107, 3502-
3507.
(33) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789-805.
(34) Crabtree, R. H.; Hamilton, D. G. AdV. Organomet. Chem. 1988,
28, 299-338.
(46) Gilbert, T. M.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc.
1985, 107, 3508-3516.
(35) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am.
Chem. Soc. 1991, 113, 4173-4184.
(47) Heinekey, D. M.; Hinkle, A. S.; Close, J. D. J. Am. Chem. Soc.
1996, 118, 5353-5361.
(36) Hamilton, D. G.; Crabtree, R. H. J. Am. Chem. Soc. 1988, 110,
4126-4133.
(48) Heinekey, D. M.; Millar, J. M.; Koetzle, T. F.; Payne, N. G.; Zilm,
K. W. J. Am. Chem. Soc. 1990, 112, 909-919.
(37) Friebolin, H. Basic One- and Two-Dimensional NMR Spectroscopy,
3rd ed.; Wiley-VCH: New York, 1998; p 3.
(49) Sabo-Etienne, S.; Chaudret, B. Chem. ReV. 1998, 98, 2077-2091.

Electrophilic Cobalt(III) Alkyl Complexes
Organometallics, Vol. 26, No. 24, 2007 5953
Table 1. 1H NMR Data for Cp*Co(L)(H)(L′)+ Complexes
2-6, 8, and 10
hydrogen of 9a′ appears as a quartet at δ 3.90 with a
corresponding methyl doublet at δ 1.15, while the methylene
hydrogens of the 1,2-insertion product 9a appear as four separate
multiplets between 2.2 and 2.9 ppm, indicating two sets of
diastereotopic methylene groups as a result of chelation of the
carbonyl group and the chiral center at cobalt. It is likely the
carbonyl group is coordinated to Co as shown in 9a′; however,
there is no spectroscopic evidence to support such an assump-
tion. The alternative structure would involve an oxa-π-allyl
structure, i.e., coordination as an enolate.55,56
complex
δ Co-H (ppm)
2JHP (Hz)
Cp*Co(P(OMe)3)(H)(η2-H2)+ 2aa
Cp*Co(PMe3)(H)(η2-H2)+ 2ba
Cp*Co(P(OMe)3)(H)(OH2)+ 3aa
Cp*Co(PMe3)(H)(OH2)+ 3ba
Cp*Co(P(OMe)3)(H)(NCCH3)+ 4aa
Cp*Co(PMe3)(H)(NCCH3)+ 4ba
Cp*Co(P(OMe)3)(H)(NCArF)+ 5aa
Cp*Co(PMe3)(H)(NCArF)+ 5ba
Cp*Co(P(OMe)3)2(H)+ 6ab
-10.46, d
-10.70, d
-10.99, d
-11.27, d
-12.54, d
-13.58, d
-12.11, d
-13.14, d
-14.29, t
-16.60, t
-15.30, dd
-11.52, d
-11.63, d
-17.23, d
-18.09, d
29
27
113
98
110
101
109
99
81
80
76 and 86
121
108
98
Cp*Co(PMe3)2(H)+ 6ba
Cp*Co(P(OMe)3)(PMe3)(H)+ 6ca
Cp*Co(P(OMe)3)(H)(CH3OH)+ 8ac
Cp*Co(PMe3)(H)(CH3OH)+ 8ba
Cp*Co(P(OMe)3)(H)2 10aa
Cp*Co(PMe3)(H)2 10ba
89
b
a CD2Cl2 solution at -30 °C at 500 MHz. CD2Cl2 solution at 25 °C at
c
300 MHz. CD2Cl2 solution at 20 °C at 500 MHz.
Over a period of 24 h at room temperature, this isomeric
mixture converts completely to 9a, which can then be isolated
as a stable red-brown solid. Concomitant formation of methyl
and 2b can be considered a convenient source of the [Cp*CoL-
(H)]+ fragment. Addition of 2 equiv of acetonitrile to solutions
of 2a or 2b quantitatively generates the corresponding [Cp*Co-
(L)(H)(NCCH3)][B(ArF)4] complexes 4a and 4b, while addition
of ∼1 equiv of 3,5-bis(trifluoromethyl)benzonitrile to solutions
of 2a or 2b quantitatively generates the corresponding [Cp*Co-
(L)(H)(NC(3,5-(CF3)2C6H3))][B(ArF)4] complexes 5a and 5b.
Similarly, addition of 1 equiv of P(OMe)3 to 2a or PMe3 to 2b
quantitatively generates the corresponding bisphosphite and
bisphosphine hydride complexes, 6a and 6b, respectively. Also,
addition of 1 equiv of P(OMe)3 to 2b results in the formation
of the mixed phosphine/phosphite hydride complex 6c. Con-
sistent with our findings that these Co(III) complexes function
as olefin hydrogenation catalysts, addition of an olefin such as
propene to 2a generates the previously reported â-agostic propyl
complex, 7a (R ) CH3).19 Finally, addition of 3 equiv of
methanol to a solution of 2a or 2b quantitatively generates the
corresponding O-bound methanol hydride adducts 8a and 8b,
as evidenced by the observation in the 1H NMR spectrum of a
1
propionate 9c is observed by H NMR spectroscopy, resulting
from hydrogenation of methyl acrylate by the H2 released upon
olefin coordination. Insertion of methyl acrylate into the Co-H
bond of 2b results in a similar mixture of regioisomeric products,
which convert to the 1,2-insertion product 9b.
The source of the methyl propionate appears to be hydro-
genolysis of the 2,1-insertion product 9a′. Chelate 9a is
unreactive toward hydrogen, and furthermore the apparently
tight chelate interaction is not disrupted by strong ligands
including acetonitrile, PMe3, and P(OMe)3. While one could
envision a mechanism of conversion of 9a′ to 9a involving
continual hydrogenolysis of 9a′ and ultimately funneling all
product to the unreactive 9a, the amount of hydrogen present
and the amount of methyl propionate formed are inconsistent
with such a process. Furthermore, isomerization of 9a′ to 9a
occurs at a similar rate when hydrogen is completely removed
from the -30 °C solution via three freeze-pump-thaw cycles.
Thus, conversion of 9a′ to 9a is proposed to occur via a standard
â-elimination, olefin rotation, reinsertion mechanism. As in
similar cases,50-52,54,57 the five-membered chelates (9a/9b) are
thermodynamically favored over the four-membered chelates
(9a′/9b′).
3
methyl doublet (8a: δ 3.00, JHH ) 4.5 Hz; 8b: δ 2.91 broad
doublet) and a 1H quartet (8a: δ 3.94 broad quartet; 8b: δ
3.94 broad quartet) corresponding to coordinated methanol. The
chemical shift of the OH of bound methanol (δ 3.94) should
be a good model for the chemical shifts of bound OH2 in 3a
and 3b; however, no signals for 3a and 3b are observed in this
range. Their absence may be the result of exchange broadening.
The 1H NMR data for the Co-H signals of complexes 2-6 and
8 and 10 are presented in Table 1 along with the relevant JPH
coupling constants. The low JPH coupling constants for the cobalt
η2-H2 monohydride complexes 2a and 2b (29 and 27 Hz)
relative to the monohydride complexes (3a/3b-6a/6b, 8a/8b)
no doubt arise from averaging of a high JPH for the terminal
hydride with a low JPH value for the η2-H2 ligand. Assuming a
JPH of ca. 80-90 Hz for the terminal hydride, the JPH for the
η2-H2 ligand must be near zero. Upon addition of methyl acrylate
to a solution of 2a at -30 °C, hydrogen is displaced and
products from both 1,2- and 2,1-insertion of acrylate into the
Co-H bond result. Rather than agostic species analogous to
7a, complexes 9a and 9a′ (initial ratio ) 1:11) are produced,
which exhibit chelation of the carbonyl oxygen to complete the
18-electron configuration at Co(III) (eq 3). Structures similar
to both 9a and 9a′ are well-precedented.50-54 The methine
Single crystals of 9a suitable for X-ray diffraction analysis
were obtained by vapor diffusion of pentane into a concentrated
solution of isolated 9a in toluene. An ORTEP representation
and key bond distances and angles for 9a are shown in Figure
3 (crystallographic data are summarized in Table 4). The
carbonyl group is coordinated to Co through the oxygen (Co-
1
(1)-O(4) ) 1.974(3) Å), as was suggested on the basis of H
NMR data.
In addition to H2 substitution and olefin insertion chemistry,
2a and 2b also react as acids upon exposure to amine bases, as
is shown in eq 4. Addition of ∼1 equiv of triethylamine to
(52) Hauptman, E.; Sabo-Etienne, S.; White, P. S.; Brookhart, M.; Garner,
J. M.; Fagan, P. J.; Calabrese, J. C. J. Am. Chem. Soc. 1994, 116, 8038-
8060.
(53) Johnson, L.; Bennett, A.; Dobbs, K.; Hauptman, E.; Ionkin, A.; Ittel,
S.; McCord, E.; McLain, S.; Radzewich, C.; Yin, Z.; Wang, L.; Wang, Y.;
Brookhart, M. PMSE Preprints 2002, 86, 319.
(54) Yi, C. S.; Liu, N. J. Organomet. Chem. 1998, 553, 157-161.
(55) Burkhardt, E. R.; Doney, J. J.; Bergman, R. G.; Heathcock, C. H.
J. Am. Chem. Soc. 1987, 109, 2022-2039.
(50) Brookhart, M.; Hauptman, E. J. Am. Chem. Soc. 1992, 114, 4437-
4439.
(51) Brookhart, M.; Sabo-Etienne, S. J. Am. Chem. Soc. 1991, 113,
2777-2779.
(56) Doney, J. J.; Bergman, R. G.; Heathcock, C. H. J. Am. Chem. Soc.
1985, 107, 3724-3726.
(57) Gottfried, A. C.; Brookhart, M. Macromolecules 2003, 36, 3085-
3100.

5954 Organometallics, Vol. 26, No. 24, 2007
Doherty et al.
Figure 4. (A) Upfield region of the 1H NMR spectrum of Cp*Co-
(P(OMe)3)(H)2(SiPhMeH)+ (14a) at -30 °C showing the Co-H
signal (δ -12.60, 2JHP ) 46.5 Hz). (B) 29Si satellites obtained after
the 1H/29Si HMQC of 14a (δ -12.60, 1JSiH(observed) ) 33.5 Hz, 2JHP
) 46.5 Hz).
Figure 3. ORTEP representation of [Cp*Co(P(OMe)3)(CH2CH2C-
(O)OCH3)][B(ArF)4], 9a. The [B(ArF)4] anion has been omitted for
clarity. Selected bond lengths (Å) and angles (deg): Co(1)-C(1)
2.013(6), C(1)-C(2) 1.511(8), C(2)-C(3) 1.488(8), C(3)-O(4)
1.232(6), Co(1)-O(4) 1.974(3), C(3)-O(7) 1.310(6), O(7)-C(6)
1.447(7), Co(1)-P(1) 2.1339(16), Co(1)-C(8) 2.135(7), Co(1)-
C(9) 2.117(8), Co(1)-C(10) 2.077(9), Co(1)-C(11) 2.037(8), Co-
(1)-C(12) 2.123(8); O(4)-Co(1)-C(1) 84.9(2), Co(1)-C(1)-C(2)
108.4(4), C(1)-C(2)-C(3) 110.5(5), C(2)-C(3)-O(4) 121.6(5),
C(3)-O(4)-Co(1) 114.2(3), C(2)-C(3)-O(7) 116.7(5), C(3)-
O(7)-C(6) 116.8(4), Co(1)-P(1)-O(18) 114.36(16), Co(1)-P(1)-
O(20) 118.79(17), Co(1)-P(1)-O(22) 111.71(16).
Scheme 3. Reaction of 1a and Et3SiH
methylene chloride-d2 solutions of 2a and 2b quantitatively
generates the neutral Co(III) dihydride complexes 10a and 10b.
Similar heterolytic cleavage of η2-H2 ligands has been reported
by Crabtree58 for the deprotonation of [IrH(η2-H2)(bq)(PPh3)2]+
(bq ) benzoquinone) by alkyl lithium reagents and by Hei-
nekey59 for the deprotonation of [CpRu(dmpe)(η2-H2)]+ (dmpe
) 1,2-bis(dimethylphosphino)ethane) by triethylamine to gener-
ate the corresponding Ru-H complex.
formula Cp*Co(P(OMe)3)(SiEt3)(H)2+ (D, Scheme 3) was also
considered. Due to overlapping metal hydride signals between
11a and the other major product, distinguishing between these
possible structures on the basis of signal integration was not
possible. Prolonged reaction times lead to signals due to 2a and
2b, indicating further hydrolysis of the Co-Si moiety.
The η2-dihydrogen hydride complex 2a serves as a convenient
source of Cp*Co(P(OMe)3)(H)+ (Vide supra) and as such is a
logical starting point for the independent synthesis of compounds
with the general formula Cp*Co(P(OMe)3)(SiEt3)(H)2+. Addi-
tion of a slight excess of Et3SiH to a CD2Cl2 solution of 2a
resulted in the loss of hydrogen and quantitative formation of
Silanolysis of [Cp*Co(L)(CH2CH2-µ-H)][B(ArF)4] (Cp* )
C5(CH3)5; L ) P(OMe)3, 1a; L ) PMe3, 1b). Reaction of 1a
with triethylsilane followed a less straightforward route relative
to the hydrogenolysis reaction. Monitoring the reaction of 1a
with Et3SiH (5 equiv) at 25 °C by 1H NMR spectroscopy
showed the formation of two major cobalt hydride products and
the liberation of 1 equiv of ethane. The first of these, 11a,
exhibited a doublet (δ -13.30, 2JHP ) 51 Hz) that was initially
believed to correspond to Cp*Co(P(OMe)3)(SiEt3)2(H)+ (B,
Scheme 3), the expected product from reaction of Cp*Co-
(P(OMe)3)(SiEt3)+ (intermediate A, Scheme 3) with an ad-
ditional equivalent of silane. However, recognizing that cationic
metal silyl complexes are highly electrophilic and extremely
sensitive to water, the possibility that 11a resulted from
hydrolysis of Cp*Co(P(OMe)3)(SiEt3)2(H)+ (B) and had the
1
a species with H NMR signals matching those of 11a. The
Co-H signal integrates for two protons versus the corresponding
Cp* and P(OMe)3 signals, consistent with the formula Cp*Co-
(P(OMe)3)(SiEt3)(H)2+. Close examination of the Co-H signal
(δ -13.30) revealed the presence of 29Si satellites (I ) 1/2,
4.7% natural abundance);37 however due to short T1(min) values
and the quadrupole moment of 59Co, which broadens the hydride
signal, these satellites are not sufficiently resolved to obtain
accurate 29Si-1H coupling constants. Enhancement of the 29Si
1
satellites was achieved using a 1D H/29Si HMQC experiment
(500.13 MHz), providing a 29Si-1H coupling constant of 29
Hz (shown for the PhMeSiH2 complex 14a in Figure 4).
Knowing this coupling constant, we were also able to employ
a 29Si{1H} DEPT 45 experiment to obtain a 29Si chemical shift
of δ 20.9 (2JSiP ) 11 Hz) relative to Me4Si (δ 0.00).
(58) Crabtree, R. H.; Lavin, M. J. Chem. Soc., Chem. Commun. 1985,
794-795.
(59) Chinn, M. S.; Heinekey, D. M. J. Am. Chem. Soc. 1987, 109, 5865-
5867.

Electrophilic Cobalt(III) Alkyl Complexes
Organometallics, Vol. 26, No. 24, 2007 5955
+
Figure 6. Possible structures for Cp*Co(L)(SiR3)H2
.
crease in the intensity of the Co-H signal; however no H-D
coupling was observed. The presence of deuterium in the
terminal Co-H position as indicated by the decreased signal
intensity and the lack of H-D coupling clearly indicate that
the silyl η2-H2 structure II is not the correct structure of 12a.
Distinguishing between the classical silyl dihydride structure
I and the η2-silane hydride structure III can be accomplished
using the 29Si-1H coupling constants. The JSiH values for η2-
silane complexes typically fall in the range 40-160 Hz, while
JSiH values between terminal hydride and terminal silyl sub-
stituents are very much smaller.60-70 1H NMR spectra of
complexes 11-14 exhibit a single cobalt-hydride resonance
of intensity 2H down to -80 °C (complexes 14a and 14b exhibit
dynamic behavior below -30 °C, which will be discussed
below) and observed 29Si-1H coupling constants in the range
29-32 Hz (see Table 2), implying dynamic behavior whereby
terminal (i.e., Co-H) and bridging (i.e., η2-H-Si) hydrides
undergo site exchange.71-73 As a result, the observed 29Si-1H
coupling constants represent a time-averaged value between the
terminal (i.e., 2JSiH) and bridging (i.e., 1JSiH) hydrides.74,75 Under
the assumption that there is no observed coupling between the
terminal hydrides and 29Si (i.e., 2JSiH ) 0 Hz),28,75 the 29Si-1H
coupling constant (1JSiH) is calculated to be 58 Hz for 11a and
58 Hz for 11b, in good agreement with expected values.63,67-69
The observed values of 29Si-1H coupling constants for com-
plexes 12-14 are displayed in Table 2 and range from 29 to
34 Hz, implying η2-Si-H structures for all of these species with
1JSiH values of 58-68 Hz.
1
Figure 5. Variable-temperature H NMR of 14a.
The same procedure can be used to generate in situ a variety
of complexes with the general formula Cp*Co(L)(SiR3)H2
+
starting from 2a or 2b (eq 5). Addition of Ph2SiH2, PhSiH3,
and Ph(Me)SiH2 to 2a generates the corresponding phosphite
complexes 12a, 13a, and 14a, respectively; while addition of
Et3SiH, Ph2SiH2, PhSiH3, and Ph(Me)SiH2 to 2b quantitatively
generates the corresponding phosphine complexes 11b, 12b,
13b, and 14b. Complexes 12-14 contain terminal Si-H bonds
which are not coordinated to cobalt and do not undergo
interchange with the metal-bound hydrides. They exhibit slightly
higher 29Si-1H coupling constants (216-222 Hz), relative to
the 29Si-1H coupling constants in the free silanes (Ph2SiH2:
1JSiH ) 200 Hz, PhSiH3: JSiH ) 200 Hz, PhMeSiH2: JSiH
)
1
1
193 Hz).
1
The H NMR spectrum of a CD2Cl2 solution of 14a below
-31 °C shows broadening of the Co-H doublet at δ -12.60
(Figure 5), while complete decoalescence into two doublets is
observed at -78 °C. By contrast, the Co-H signals for 11a-
The 29Si{1H} NMR spectra of these η2-silane complexes were
also examined and the chemical shifts of 11-14 along with
1
13a do not exhibit any appreciable broadening at -80 °C. H
(60) No 29Si-1H coupling was observed in the related neutral dihydri-
dodisilyl Co(V) complexes: See ref 28.
NMR data for the Co-H signals of complexes 11-14 are
presented in Table 2 along with the relevant 31P and 29Si
coupling constants.
(61) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 13794-13807.
(62) Colomer, E.; Corriu, R. J. P.; Marzin, C.; Vioux, A. Inorg. Chem.
1982, 21, 368-373.
(63) Corey, J. Y.; Braddock-Wilking, J. Chem. ReV. 1999, 99, 175-
292.
(64) Hashimoto, H.; Hayashi, Y.; Aratani, I.; Kabuto, C.; Kira, M.
Organometallics 2002, 21, 1534-1536.
(65) Karshtedt, D.; Bell, A. T.; Tilley, T. D. Organometallics 2006, 25,
4471-4482.
(66) Luo, X.; Kubas, G. J.; Bryan, J. C.; Burns, C. J.; Unkefer, C. J. J.
Am. Chem. Soc. 1994, 116, 10312-10313.
(67) Nikonov, G. I. AdV. Organomet. Chem. 2005, 53, 217-309.
(68) Schneider, J. J. Angew. Chem., Int. Ed. 1996, 35, 1068-1075.
(69) Schubert, U. AdV. Organomet. Chem. 1990, 30, 151-187.
(70) Yong, L.; Hofer, E.; Wartchow, R.; Butenscho¨n, B. Organometallics
2003, 22, 5463-5467.
Three structures having the formula Cp*Co(P(OMe)3)(SiR3)-
(H)2+ were considered for complex 11a (Figure 6): a classical
Co(V) silyl dihydride structure Cp*Co(P(OMe)3)(SiR3)(H)2+ (I),
a Co(III) silyl η2-dihydrogen structure Cp*Co(P(OMe)3)(SiR3)-
(η2-H2)+ (II), and a Co(III) η2-silane hydride structure Cp*Co-
(P(OMe)3)(η2-HSiR3)(H)+ (III). The silyl η2-H2 structure II was
ruled out using a deuterium labeling experiment. In structure
II the hydrogen atoms always reside in an η2-H2 ligand, and
thus in the d1-labeled complexes, JHD values of ca. 30 Hz should
be observed. Addition of Ph2SiH2 to samples of 2a-dn to form
Cp*Co(P(OMe)3)(SiPh2H)(D)2-n(H)n+, 12a-dn, exhibited a de-
(71) Atheaux, I.; Delpech, F.; Donnadieu, B.; Sabo-Etienne, S.; Chaudret,
B.; Hussein, K.; Barthelat, J.; Braun, T.; Duckett, S. B.; Perutz, R. N.
Organometallics 2002, 21, 5347-5357.
(72) Hussein, K.; Marsden, C. J.; Barthelat, J.; Rodriguez, V.; Conejero,
S.; Sabo-Etienne, S.; Donnadieu, B.; Chaudret, B. J. Chem. Soc., Chem.
Commun. 1999, 1315-1316.
(73) Lachaize, S.; Sabo-Etienne, S. Eur. J. Inorg. Chem. 2006, 2115-
2127.
(74) Ng, S. M.; Lau, C. P.; Fan, M.; Lin, Z. Organometallics 1999, 18,
2484-2490.
(75) Taw, F. L.; Bergman, R. G.; Brookhart, M. Organometallics 2004,
23, 886-890.

5956 Organometallics, Vol. 26, No. 24, 2007
Scheme 4. Possible Hydride Exchange Pathways in η2-Silane Complexes
Doherty et al.
Table 2. 1H NMR Data for Co(III) Complexes 11-14
basis of the trihydride structures, the Rh complexes, for example,
Cp*(PMe3)Rh(H)(η2-HSiPh3)+,75 exist as Rh(III) η2-silane hy-
drides with structures analogous to 12a/b, whereas the Ir systems
exist as classical Ir(V) aryl silyl hydrides as a result of ortho-
C-H activation of a Si-Ph substituent.2 Along with complexes
1a/b and 2a/b, η2-coordination of a Si-H bond to a Cp*Co-
(L)(R)+ fragment completes a series of compounds featuring
η2-CâH, η2-H2, and η2-SiH ligands.41 One noteworthy aspect
of these Co-(η2-silane) cations is that the Si center is extremely
electrophilic. Despite the most rigorous attempts to dry NMR
solvents, the presence of 2a/b in samples of 11-14 generated
by addition of silane to 1a/b indicates that these species react
rapidly with trace amounts of moisture. This is reminiscent of
the acute moisture sensitivity of Cp*Co(L)(CH2CH(SiR3)-µ-
H)+, a previously reported compound that likewise hydrolyzes
to give 1a/b.17
2JHP 1JSiH(obsd)
a
δ Co-H
(ppm)
complex
(Hz)
(Hz)
Cp*Co(P(OMe)3)(H)(η2-HSiEt3)+, 11ab
Cp*Co(PMe3)(H)(η2-HSiEt3)+, 11bc
Cp*Co(P(OMe)3)(H)(η2-H2SiPh2)+, 12ad
Cp*Co(PMe3)(H)(η2-H2SiPh2)+, 12bd
Cp*Co(P(OMe)3)(H)(η2-H3SiPh)+, 13ab
Cp*Co(PMe3)(H)(η2-H3SiPh)+, 13bd
-13.30, d 51
-13.96, d 45
-12.14, d 46
-13.06, d 41
-12.34, d 44
-13.14, d 43.5
29
29
31.5
30
29
29
33.5
28.5
Cp*Co(P(OMe)3)(H)(η2-H2Si(Me)Ph)+, 14ad -12.60, d 46.5
Cp*Co(PMe3)(H)(η2-H2Si(Me)Ph)+, 14bd
-13.15, d 44.5
a Calculated values of the 29Si-1H coupling constants in the range 58-
1
1
68 Hz, JSiH(calc.), are obtained by doubling the JSiH(obsd) values reported
b
c
here. CD2Cl2 solution at 25 °C at 400 MHz. CD2Cl2 solution at -19 °C
at 500 MHz. CD2Cl2 solution at -30 °C at 500 MHz.
d
Table 3. 29Si{1H} NMR Data for Co(III) Complexes 11-14a
b
δ
29Si
2JSiP
(Hz)
∆
(ppm)
complex
(ppm)
Hydride Exchange Mechanisms in [Cp*Co(L)(H)(η2-
HSiR3)][B(ArF)4] Complexes 11-14. The observation of a
single Co-H signal in the 1H NMR spectra of η2-silane hydride
complexes 11-13 down to -80 °C shows that the terminal (i.e.,
Co-H) and bridging (i.e., η2-H-Si) hydrides are exchanging
rapidly on the NMR time scale with barriers certainly less than
ca. 7-8 kcal/mol. Two possible mechanisms for this dynamic
process can be envisioned that exchange hydrides between
bridging and terminal positions. The first of these processes (eq
6, Scheme 4) involves full cleavage of the µ-Ha-Si bond and
formation of a Co(V) silyl dihydride in which the two hydrides
are equivalent. The second mechanism, which we favor, involves
what can be described as a “silyl slide”, a process in which
there is no increase in Co-Si bonding as Si “slides” from one
hydrogen to the adjacent one and a formal Co(V) oxidation state
is avoided (eq 7, Scheme 4). This latter mechanism has been
invoked in many similar processes involving late metals.14,78 It
has been analyzed and discussed in detail by Perutz and Sabo-
Etienne, who have proposed that such a mechanism applies to
σ-bond metathesis reactions of late metal complexes, a process
they term σ-complex-assisted metathesis (σ-CAM).78
Cp*Co(P(OMe)3)(H)(η2-HSiEt3)+, 11a
Cp*Co(PMe3)(H)(η2-HSiEt3)+, 11b
Cp*Co(P(OMe)3)(H)(η2-H2SiPh2)+, 12a
Cp*Co(PMe3)(H)(η2-H2SiPh2)+, 12b
Cp*Co(P(OMe)3)(H)(η2-H3SiPh)+, 13a
Cp*Co(PMe3)(H)(η2-H3SiPh)+, 13b
Cp*Co(P(OMe)3)(H)(η2-H2Si(Me)Ph)+, 14a
Cp*Co(PMe3)(H)(η2-H2Si(Me)Ph)+, 14b
20.9, d
20.7, br
-2.5, d
0.7, d
-28.4, br
-26.7, d
-8.9, d
-9.0, d
10.9
20.6
20.4
30.6
33.8
31.0
32.7
26.7
26.6
13.0
14.0
14.0
12.3
13.5
a CD2Cl2 solution at -30 °C at 99.36 MHz (relative to external (CH3)4Si
b
at δ ) 0.00). ∆ (ppm) ) δcoord - δfree
.
29Si{1H} NMR δfree Et3SiH )
0.29, PhSiH3 ) -59.4, Ph2SiH2 ) -33.1, PhMeSiH2 ) -35.6.
the change in chemical shift upon η2-coordination to the metal
(∆ (ppm) ) δcoord - δfree) are displayed in Table 3. Regardless
of the phosphorus ligand employed, the chemical shifts of the
η2-coordinated silanes are shifted approximately 20-30 ppm
downfield relative to the uncoordinated silanes, consistent with
reported 29Si chemical shifts of other η2-silane complexes.74 The
limited number of examples containing 29Si NMR data of Co-
silyl complexes exhibit 29Si chemical shifts that are shifted
significantly downfield (>50 ppm) relative to the uncoordinated
silanes. Related Co(I) silyl complexes, (CO)3(L)Co(SiEt3),
reported by Cutler exhibit 29Si chemical shifts of δ 51.9 (L )
PPh2Me) and 52.9 (L ) PPh3),76 while the neutral silyl hydride
complex CpCo(C2H4)(SiEt3)H recently reported by Perutz
exhibits a 29Si chemical shift of δ 38.9 (relative to external
(CH3)4Si at δ 0.00).77 (Note that this neutral, Co(III) species
exists as the oxidative addition adduct, not the Co(I) η2-silane
complex.)
The dynamic behavior of 14a is somewhat more complex
than that of 11-13. Coordination of the prochiral silane,
PhMeSiH2, results in generation of both a chiral Co center and
a chiral Si center, and thus two diastereomers can be formed,
14a and 14a′, as shown in Scheme 5. A rapid silyl slide process
interconverts diastereomers and will interconvert Ha and Ha′ and
Hb and Hb′ in the two diastereomers, but will not scramble Ha/
Ha′ with Hb/Hb′. Yet at -31 °C only one signal is seen for the
As in the trihydride cases, Rh and Ir silyl dihydride analogues
of 11-14 have been reported with Ph3SiH. As expected on the
(77) Ampt, K. A. M.; Duckett, S. B.; Perutz, R. N. Dalton Trans. 2007,
2993-2996.
(78) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46,
2578-2592.
(76) Gregg, B. T.; Cutler, A. R. Organometallics 1992, 11, 4276-4284.

Electrophilic Cobalt(III) Alkyl Complexes
Organometallics, Vol. 26, No. 24, 2007 5957
Scheme 5. Interconversion of Diastereomers 14a and 14a′
for the dynamic processes studied above followed by loss of
alkane, thus avoiding a formal Co(V) intermediate. This overall
process conforms exactly to the σ-CAM process discussed by
Perutz and Sabo-Etienne and is assumed to operate in a variety
of catalytic processes including the activation of dihydrogen,79,80
silanes,81 boranes,82,83 and alkanes.79,80,84
Summary
Addition of H2 to electrophilic Co(III) agostic alkyl complexes
1a/b results in the formation of Co(III) η2-dihydrogen hydride
complexes, as shown by analysis of JHD values in partially
deuterated species. Rapid exchange between the terminal hydride
and the hydrogens of the η2-dihydrogen results in a single cobalt
hydride resonance. These substitutionally labile dihydrogen
complexes react with a variety of neutral donor ligands including
nitriles, phosphines, and methanol to generate the corresponding
Co(III)(L)(L′)H+ complexes. Insertion of methyl acrylate into
the Co-H bond of 2a/b results in the formation of a mixture
of 1,2- and 2,1-insertion products 9a/9a′ and 9b/9b′, exhibiting
either a four-membered chelate (kinetic product, 9a′/9b′) or five-
membered chelate structure. These mixtures ultimately rearrange
at 20 °C to form the more stable five-membered chelates 9a
and 9b. Complex 9a has been independently synthesized and
characterized by single-crystal X-ray diffraction.
average of all four hydrogens (Figure 5, one doublet, average
JHP ) 46.5 Hz). At low temperatures it is evident that the signal
decoalesces into two doublets of nearly equal intensity, which
we assign to Ha/Ha′ and Hb/Hb′ of the rapidly interconverting
diastereomers.
We propose that the interconversion of Ha/Ha′ with Hb/Hb′
occurs through the intermediacy of an η2-dihydrogen complex,
as shown for one diastereomer in Scheme 6. Rotation of the
silane brings Ha and Hb into a “cis” orientation. A hydrogen
“slide” (analogous to the silyl slide) results in formation of a
η2-H2 silyl species, 15a. Rotation of the η2-H2 ligand inter-
changes Ha and Hb and results in exchange of Ha and Hb in
diastereomer 14a. A similar process can occur in 14a′, and
coupled with a rapid silyl slide that interchanges diastereomers,
all four hydrogens undergo site exchange. Since the η2-H2 silyl
isomer is higher in energy than the η2-silane hydride isomer,
the barrier for this dynamic process is now in a range measurable
by NMR spectroscopy. The overall barrier for the process can
be roughly estimated from the coalescence temperature (ca. -75
°C) to be ca. 9.4 kcal/mol. (It is significant to note here that
the terminal Si-H does not scramble with the bridging Si-H
on an NMR time scale. A similar feature applies to 12a/12b
and 13a/13b.)
Cobalt-Alkyl Bond Cleavage Mechanism. The spectro-
scopic characterization of complexes 2a/b and 11-14 as
nonclassical structures [Cp*Co(L)(H)(η2-H2)][B(ArF)4] and
[Cp*Co(L)(H)(η2-HSiR3)][B(ArF)4] and the dynamic processes
these species exhibit provide insight into the mechanism of the
cleavage of the metal alkyl bond in Co alkyl agostic complexes
by silanes and H2, key steps in hydrogenation and hydrosilation
(see Scheme 1).17 The agostic complex is assumed to react with
H2 or silanes to yield initially the η2-H2 or η2-silane complexes
16 and 17 (eqs 8 and 9, Scheme 7). Cleavage of the cobalt
alkyl bond likely occurs as shown in Scheme 7, where an η2-
alkane complex is formed by the same mechanism as proposed
Reaction of triethylsilane with these Co(III) alkyl complexes
generates two products, the major one of which has been
identified as the η2-silane hydride complex 11a on the basis of
the value of its 29Si-1H coupling constant of 58 Hz. Several
other η2-silane hydride complexes, 11-14, have been generated
by addition of silane to the aforementioned η2-dihydrogen
hydride complexes 2a and 2b. Only a single hydride resonance
for complexes 11-13 is seen even at -80 °C, supportive of a
rapid dynamic process in which the silyl group slides between
hydrides, avoiding a true Co(V) intermediate. Variable-temper-
1
ature H NMR studies of 14a containing a prochiral silane,
PhMeSiH2, suggest the presence of two distinct pathways for
exchange of the terminal Co-H and bridging η2-SiH. The higher
energy process can be frozen out at -78 °C and is proposed to
involve silane rotation and formation of a η2-H2 ligand, while
the lower energy pathway is proposed to involve rapid sliding
of the silyl substituent between bridging and terminal hydrides.
These new η2-H2 and η2-silane complexes are suggestive of
similar η2-coordination occurring prior to the cleavage of Co-
Scheme 6. Mechanism Proposed for Hydrogen Scrambling in Diastereomer 14a

5958 Organometallics, Vol. 26, No. 24, 2007
Doherty et al.
Scheme 7. Proposed Mechanism for Co(III) Alkyl Bond Cleavage in Cp*Co(L)(CH2CH2-µ-H)+ Species by H2 and Silanes, Key
Steps in Catalytic Hydrogenation and Hydrosilation (see Scheme 1)
Table 4. Crystallographic Data
Polymer grade ethylene (99.9%) and hydrogen were purchased from
National Welders Supply Co. and used as received. Methylene
chloride-d2 was purchased from Cambridge Isotope Laboratories,
dried over CaH2, and degassed using freeze-pump-thaw tech-
niques. Deuterium gas was purchased from Cambridge Isotope
Laboratories and used as received. All 1H, 13C, and 29Si{1H} NMR
spectra were recorded on Bruker Avance 300, 400, or 500 MHz
spectrometers and calibrated at low temperature using MeOH.
Chemical shifts are reported relative to internal CHDCl2 (δ 5.32
for 1H) and CD2Cl2 (δ 54.00 for 13C) and external (CH3)4Si (δ 0.00
for 29Si).
1a‚H2O
1b
9a
formula
fw
cryst syst
space group
a (Å)
C
47H43F24O4PBCo C47H41F24PBCo
C49H43F24O5PBCo
1268.54
monoclinic
P21/c
1228.52
triclinic
P1h
1162.51
orthorhombic
Pbca
10.3961(8)
15.3779(10)
17.9619(13)
93.743(5)
104.803(5)
105.259(5)
2651.2(3)
2
18.1005(4)
19.3274(4)
27.7687(6)
90
90
90
12.8214(4)
21.0585(6)
19.7828(8)
90
98.726(2)
90
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
9714.5(4)
8
5279.5(3)
4
NMR Data for [B(ArF)4]- Anion. The 1H and 13C NMR
-
D
calcd (Mg/m3)
1.539
1.590
1.596
chemical shifts of the B(ArF)4 counteranion do not change
λ (Å)
1.54178 (Cu)
3.979
0.71073 (Mo)
0.513
0.71073 (Mo)
0.486
significantly with temperature and are reported here for all
complexes. 1H NMR (CD2Cl2): δ 7.70 (8H, s, ArF o-H), 7.55 (4H,
s, ArF p-H). 13C NMR (CD2Cl2): δ 162.0 (q, Cipso, 1JCB ) 67 Hz),
µ (mm-1
)
cryst dimens
0.20 × 0.15 × 0.15 0.25 × 0.20 × 0.05 0.25 × 0.10 × 0.10
(mm3)
1
2
135.0 (d, Cortho, JCH ) 163 Hz), 129.0 (q, Cmeta, JCF ) 47 Hz),
T (K)
θ range (deg)
no. of rflns
no. of indep rflns 8524
R1
wR2
Rall
GOF
100(2)
2.57-65.00
28 674
100(2)
1.47-25.06
42 497
8596
0.0420
0.0950
0.0714
1.004
100(2)
1.42-25.00
37 245
9297
0.0744
0.1898
0.1139
1.034
1
1
125.0 (q, CF3, JCF ) 378 Hz), 117.5 (d, Cpara, JCH ) 163 Hz).
[Cp*Co(P(OMe)3)(C2H4-µ-H)][B(ArF)4], 1a. This complex was
synthesized according to previously published procedures.18 X-ray
quality crystals of 1a were obtained by slow vapor diffusion of
hexane into a concentrated methylene chloride solution of 1a.
[Cp*Co(PMe3)(C2H4-µ-H)][B(ArF)4], 1b. The BF4- salt of this
complex has been previously reported;19 however the B(ArF)4- salt
has been isolated here using a procedure analogous to that used to
0.0831
0.2127
0.1111
1.050
alkyl bonds via a σ-CAM pathway when these agostic alkyl
complexes serve as hydrogenation and hydrosilation catalysts.
1
prepare 1a, and the H NMR data for the Cp*Co(PMe3)(C2H4-µ-
-
H)+ are consistent with that reported for the BF4 complex.18
A
Experimental Section
flame-dried Schlenk flask was charged with Cp*Co(PMe3)(C2H4)
(70 mg, 0.235 mmol) and [H(OEt2)2][B(ArF)4] (214 mg, 0.211
mmol) at room temperature. Methylene chloride (4 mL) was added,
and the mixture was stirred until all solids had dissolved (∼2-3
min) before cooling to -78 °C for 1-2 h. Hexane (10 mL) was
added to precipitate the product, and the reaction mixture was
allowed to settle for ∼30 min at -78 °C. The red-brown solid was
isolated via cannula filtration, washed with hexane (2 × 5 mL),
and dried in Vacuo at room temperature for ∼30 min before being
stored in the glovebox at -35 °C (219 mg, 89%). X-ray quality
crystals were grown by slow evaporation of methylene chloride-d2
General Methods. All manipulations of air- and/or moisture-
sensitive compounds were conducted using standard Schlenk
techniques. Argon was purified by passage through columns of
BASF R3-11 catalyst (Chemalog) and 4 Å molecular sieves.
Toluene, pentane, hexane, methylene chloride, and diethyl ether
were deoxygenated and dried over a column of activated alumina.85
Materials. Acetonitrile, 3,5-bis(trifluoromethyl)benzonitrile, metha-
nol (anhydrous), triethyl amine, 1,2,3,4,5-pentamethylcyclopenta-
diene (Cp*H), and iodine were used as received from Aldrich.
Dicobalt octacarbonyl was purchased from Strem and used as
received. Na[B(ArF)4]86 (ArF ) 3,5-(CF3)2C6H3), [H(OEt2)2B-
(ArF)4],18 7a,19 Cp*Co(P(OMe)3)(C2H4).87,88 and Cp*Co(PMe3)-
(C2H4)87,88 were prepared according to literature procedures.
(84) Cleavage of a Ti(IV)-alkyl bond by silanes has been shown to
produce a titanium hydride and an alkyl silane, suggesting a very different
polarity of the Ti-R bond (relative to the Co-R bonds studied here) with
a much higher negative charge on carbon and higher positive charge on
the metal, Ti(IV): Koo, K.; Marks, T. J. J. Am. Chem. Soc. 1999, 121,
8791-8802.
(85) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518-1520.
(86) Yakelis, N. A.; Bergman, R. G. Organometallics 2005, 24, 3579-
3581.
(87) Beevor, R. G.; Frith, S. A.; Spencer, J. L. J. Organomet. Chem.
1981, 221, C25-C27.
(88) Hofmann, L.; Werner, H. J. Organomet. Chem. 1985, 289, 141-
155.
(79) Lam, W. H.; Jia, G.; Lin, Z.; Lau, C. P.; Eisenstein, O. Chem.-
Eur. J. 2003, 9, 2775-2782.
(80) Ng, S. M.; Lam, W. H.; Mak, C. C.; Tsang, C. W.; Jia, G.; Lin, Z.;
Lau, C. P. Organometallics 2003, 22, 641-651.
(81) Lachaize, S.; Sabo-Etienne, S.; Donnadieu, B.; Chaudret, B. Chem.
Commun. 2003, 214-215.
(82) Hartwig, J. F.; Cook, K. S.; Hapke, M.; Incarvito, C. D.; Fan, Y.;
Webster, C. E.; Hall, M. B. J. Am. Chem. Soc. 2005, 127, 2538-2552.
(83) Webster, C. E.; Fan, Y.; Hall, M. B.; Kunz, D.; Hartwig, J. F. J.
Am. Chem. Soc. 2003, 125, 858-859.

Electrophilic Cobalt(III) Alkyl Complexes
Organometallics, Vol. 26, No. 24, 2007 5959
from a concentrated solution of 1b. 1H NMR (CD2Cl2, 500.13 MHz,
-82 °C): δ 2.64 (1H, s, C-Hâ), 1.58 (15H, s, C5(CH3)5), 1.22
[Cp*Co(P(OMe)3)2(H)][B(ArF)4], 6a. This complex was gener-
ated quantitatively using the above procedure by adding P(OMe)3
3
1
(1H, s, C-Hâ), 0.993 (9H, d, P(CH3)3, JHP ) 9.5 Hz), -0.228
(1 µL, 0.0083 mmol, 1.0 equiv) to a CD2Cl2 solution of 2a. H
(1H, s, C-HR), -0.508 (1H, s, C-HR), -12.74 (1H, br s, Co-
NMR (CD2Cl2, 300.13 MHz, 25 °C): δ 3.62 (18H, virtual t,
H-C). 13C NMR (CD2Cl2, 100.59 MHz, -80 °C): δ 95.0 (s,
P(OCH3)3, Japparent ) 12.5 Hz), 1.78 (15H, s, C5(CH3)5), -14.29
3
C5(CH3)5), 25.2 (t, CR, 1JCH ) 159 Hz), 12.9 (dq, P(CH3)3, 1JCH
)
(1H, t, Co-H, JHP ) 81 Hz).
2
129 Hz, 2JHP ) 30 Hz), 8.90 (q, C5(CH3)5, 1JCH ) 129 Hz), -5.47
[Cp*Co(P(OMe)3)(H)(MeOH)][B(ArF)4], 8a. This complex was
generated quantitatively using the above procedure by adding
MeOH (1 µL, 0.025 mmol, 3 equiv) to a CD2Cl2 solution of 2a.
1H NMR (CD2Cl2, 500.13 MHz, 20 °C): δ 3.94 (1H, br q, CH3OH,
1
1
(td, Câ, JCHâ ) 139 Hz, JC-H-Co ) 64 Hz).
[Cp*Co(P(OMe)3)(H)(η2-H2)][B(ArF)4], 2a. A flame-dried screw
cap NMR tube was charged with [Cp*Co(P(OMe)3)(Et)][B(ArF)4]
(1a; 10 mg, 0.0083 mmol) and CD2Cl2 (600 µL) and cooled to
-78 °C. H2 (∼20 equiv, 4.0 mL, 0.165 mmol) was added via
syringe, and the tube was shaken several times before being placed
in a precooled NMR probe at -30 °C. The desired product was
generated as the major species in solution (85-100% yield) within
3
3JHH ) 4.5 Hz), 3.80 (9H, d, P(OCH3)3, JHP ) 11.0 Hz), 3.00
(3H, d, CH3OH, 3JHH ) 4.5 Hz), 1.56 (15H, s, C5(CH3)5), -11.52
2
(1H, d, Co-H, JHP ) 121 Hz).
[Cp*Co(P(OMe)3)(CH2CH2C(O)OCH3)][B(ArF)4], 9a, and
[Cp*Co(P(OMe)3)(CH(CH3)C(O)OCH3)][B(ArF)4], 9a′. These
complexes were generated as a 11:1 mixture of 9a′:9a using the
above procedure by adding methyl acrylate (1.1 µL, 0.12 mmol,
1.5 equiv) to a CD2Cl2 solution of 2a at -30 °C and monitoring
olefin insertion at 0 °C. The ratio of 9a′:9a changed to 1:21 after
24 h at room temperature. 9a: 1H NMR (CD2Cl2, 300.13 MHz, 20
°C) δ 3.78 (9H, d, P(OCH3)3, 3JHP ) 10.8 Hz), 3.64 (3H, s, OCH3),
1
30 min at -30 °C and has been characterized in situ. H NMR
(CD2Cl2, 500.13 MHz, -30 °C): δ 3.54 (9H, d, P(OCH3)3, 3JHP
)
11.5 Hz), 1.87 (15H, s, C5(CH3)5), -10.46 (3H, d, Co(H)(η2-H2),
2JHP ) 28.5 Hz). The minor species in solution (0-15% yield) has
been identified as [Cp*Co(P(OMe)3)(H)(OH2)][B(ArF)4] (3a) and
has also been characterized in situ. 1H NMR (CD2Cl2, 500.13 MHz,
-30 °C): δ 3.72 (9H, d, P(OCH3)3), 3JHP ) 11.0 Hz), 1.61 (15H,
3
3
2.84 (1H, q, CH2, JHH ) 9 Hz), 2.61 (1H, q, CH2, JHH ) 9 Hz),
2.4 - 2.2 (2H, overlapping m, CH2), 1.42 (15H, C5(CH3)5). 9a′:
1H NMR (CD2Cl2, 500.13 MHz, 0 °C) δ 3.93 (1H, br s, CHCH3)
2
s, C5(CH3)5), -10.99 (1H, d, Co-H, JHP ) 112.8 Hz). The
resonance of the bound H2O protons has not been located; this signal
is either masked by other resonances or exchange-broadened. Both
of these complexes decompose above 0 °C, and attempts to isolate
them have been unsuccessful.
3
3.77 (9H, d, P(OCH3)3, JHP ) 11 Hz), 3.48 (3H, s, OCH3), 1.49
(15H, s, C5(CH3)5), 1.17 (3H, d, CHCH3, JHH ) 7 Hz).
3
[Cp*Co(P(OMe)3)(H)(η2-HSiEt3)][B(ArF)4], 11a. This complex
was generated quantitatively using the above procedure by adding
Et3SiH (2 µL, 0.012 mmol, 1.5 equiv) to a CD2Cl2 solution of 2a.
1H NMR (CD2Cl2, 400.05 MHz, 25 °C): δ 3.67 (9H, d, P(OCH3)3,
3JHP ) 12.0 Hz), 1.86 (15H, s, C5(CH3)5), 1.00 (9H, br t, (CH3-
CH2)3Si), 0.55 (6H, br q, (CH3CH2)3Si), -13.3 (2H, d, Co(H)(η2-
[Cp*Co(PMe3)(H)(η2-H2)][B(ArF)4], 2b. This complex was
generated following the previously described procedure for the
[Cp*Co(P(OCH3)3)(H)(η2-H2)][B(ArF)4] complex starting from
[Cp*Co(PMe3)(Et)][B(ArF)4] (1b, 10 mg, 0.0086 mmol). 1H NMR
(CD2Cl2, 500.13 MHz, -30 °C): δ 1.87 (15H, s, C5(CH3)5), 1.33
3
(9H, d, P(CH3)3, JHP ) 11.0 Hz), -10.70 (3H, d, Co(H)(η2-H2),
1
2
SiH), JHSi(observed) ) 29.0 Hz, JHP ) 51 Hz). 29Si{1H} DEPT 45
2JHP ) 27.0 Hz). The minor species in solution (0-15% yield) has
been identified as [Cp*Co(PMe3)(H)(OH2)][B(ArF)4] (3b) and has
2
(CD2Cl2, 99.36 MHz, -30 °C): δ 20.9 (d, JSiP ) 10.9 Hz).
[Cp*Co(P(OMe)3)(H)(η2-HSiPh2H)][B(ArF)4], 12a. This com-
plex was generated quantitatively using the above procedure by
adding Ph2SiH2 (2.5 µL, 0.012 mmol, 1.5 equiv) to a CD2Cl2
solution of 2a. 1H NMR (CD2Cl2, 500.13 MHz, -30 °C): δ 7.8-
7.3 (10H, overlapping m, (C6H5)2Si), 5.71 (1H, s, Si-Hterminal, 1JHSi
) 222 Hz), 3.39 (9H, d, P(OCH3)3, 3JHP ) 11.5 Hz), 1.77 (15H, s,
C5(CH3)5), -12.14 (2H, d, Co(H)(η2-HSi), 1JHSi(observed) ) 31.5 Hz,
2JHP ) 46 Hz). 29Si{1H} DEPT 45 (CD2Cl2, 99.35 MHz, -30 °C):
1
also been characterized in situ. H NMR (CD2Cl2, 500.13 MHz,
-30 °C): δ 1.61 (15H, s, C5(CH3)5), 1.41 (9H, d, P(CH3)3, 3JHP
)
2
10.5 Hz), -11.27 (1H, d, Co-H, JHP ) 97.5 Hz). The chemical
shift of the bound H2O protons has not been located; this signal is
either masked by other resonances or exchange-broadened. Both
of these complexes decompose above 0 °C, and attempts to isolate
them have been unsuccessful.
2
In Situ Generation of [Cp*Co(L)(H)(L′)][B(ArF)4] Complexes
from the Corresponding [Cp*Co(L)(H)(η2-H2)][B(ArF)4] Com-
plexes. The desired ligand, L′, was added via syringe to a screw
cap NMR tube at -30 °C containing the appropriate [Cp*Co(L)-
(H)(η2-H2)][B(ArF)4] complex (generated in situ according to the
above procedures). The tube was shaken several times before being
placed in a precooled NMR probe at -30 °C. The desired products
δ -2.5 (d, JSiP ) 13 Hz).
[Cp*Co(P(OMe)3)(H)(η2-HSiPhH2)][B(ArF)4], 13a. This com-
plex was generated quantitatively using the above procedure by
adding PhSiH3 (2 µL, 0.017 mmol, 2 equiv) to a CD2Cl2 solution
of 2a. 1H NMR (CD2Cl2, 400.05 MHz, 25 °C): δ 7.65-7.25 (5H,
1
overlapping m, (C6H5)Si), 4.69 (2H, s, Si(Hterminal)2, JSiH ) 218
3
Hz), 3.63 (9H, d, P(OCH3)3, JHP ) 11.6 Hz), 1.78 (15H, s, C5-
1
1
(CH3)5), -12.34 (2H, d, Co(H)(η2-SiH), JHSi(observed) ) 29.0 Hz,
were generated and characterized in situ by H NMR due to their
2JHP ) 44 Hz). 29Si{1H} DEPT 45 (CD2Cl2, 99.35 MHz, -30 °C):
δ -28.4 (br s).
instability above 0 °C.
[Cp*Co(P(OMe)3)(H)(NCCH3)][B(ArF)4], 4a. This complex
was generated quantitatively using the above procedure by adding
CH3CN (1 µL, 0.019 mmol, 2 equiv) to a CD2Cl2 solution of 2a.
1H NMR (CD2Cl2, 500.13 MHz, -30 °C): δ 3.61 (9H, d,
[Cp*Co(P(OMe)3)(H)(η2-HSiPhMeH)][B(ArF)4], 14a. This com-
plex was generated quantitatively using the above procedure by
adding PhMeSiH2 (2 µL, 0.012 mmol, 1.5 equiv) to a CD2Cl2
solution of 2a. 1H NMR (CD2Cl2, 500.13 MHz, -30 °C): δ 7.6-
3
P(OCH3)3, JHP ) 11.5 Hz), 2.25 (3H, s, CH3CN), 1.64 (15H, s,
C5(CH3)5), -12.54 (1H, d, Co-H, 2JHP ) 110 Hz). 13C NMR (CD2-
Cl2, 125.76 MHz, -33 °C): δ 128.5 (s, CH3CN), 97.73 (s, C5-
1
7.3 (5H, overlapping m, (C6H5)Si), 5.04 (1H, s, Si-Hterminal, JSiH
3
) 216 Hz), 3.64 (9H, d, P(OCH3)3, JHP ) 12 Hz), 1.69 (15H, s,
2
C5(CH3)5), 0.54 (3H, s, (CH3)Si), -12.60 (2H, d, Co(H)(η2-SiH),
1JSiH(observed) ) 33.5 Hz, 2JHP ) 45 Hz). 1H NMR (CD2Cl2, 500.13
MHz, -76 °C): δ 7.7-7.2 (5H, overlapping m, (C6H5)Si), 4.97
(CH3)5), 52.91 (d, P(OCH3)3, JCP ) 5.4 Hz), 9.64 (s, C5(CH3)5),
8.71 (s, CH3CN).
[Cp*Co(P(OMe)3)(H)(NCArF)][B(ArF)4], 5a (ArF ) 3,5-(CF3)2-
C6H3). This complex was generated quantitatively using the above
procedure by adding 3,5-bis(trifluoromethyl)benzonitrile (1.5 µL,
0.0089 mmol, 1.1 equiv) to a CD2Cl2 solution of 2a. 1H NMR (CD2-
Cl2, 500.13 MHz, -30 °C): δ 8.23 (1H, s, ArF p-H), 7.98 (2H, s,
3
(1H, s, Si-Hterminal), 3.60 (9H, d, P(OCH3)3, JHP ) 12 Hz), 1.59
(15H, s, C5(CH3)5), 0.40 (3H, s, (CH3)Si), -12.66 (1H, d, Co(HA)-
(η2-SiHB), 2JHP ) 43.5 Hz), -12.81 (1H, d, Co(HB)(η2-SiHA), 2JHP
) 43.5 Hz). 29Si{1H} DEPT 45 (CD2Cl2, 99.35 MHz, -30 °C): δ
-8.9 (d, 2JSiP ) 12.3 Hz) Variable-temperature 1H NMR behavior
of 14a is discussed in the text.
3
ArF o-H), 3.68 (9H, d, P(OCH3)3, JHP ) 11.5 Hz), 1.71 (15H, s,
2
C5(CH3)5), -12.11 (1H, d, Co-H, JHP ) 109 Hz).

5960 Organometallics, Vol. 26, No. 24, 2007
Doherty et al.
1
[Cp*Co(PMe3)(H)(NCCH3)][B(ArF)4], 4b. This complex was
Si(Hterminal)2, JSiH ) 218 Hz), 1.69 (15H, s, C5(CH3)5), 1.51 (9H,
2
generated quantitatively using the above procedure by adding CH3-
d, P(CH3)3, JHP ) 7.5 Hz), -13.14 (2H, d, Co(H)(η2-SiH),
1
2
CN (1 µL, 0.019 mmol, 2 equiv) to a CD2Cl2 solution of 2b. H
1JSiH(observed) ) 29 Hz, JHP ) 43.5 Hz). 29Si{1H} DEPT 45 (CD2-
2
NMR (CD2Cl2, 500.13 MHz, -30 °C): δ 2.28 (3H, s, CH3CN),
Cl2, 99.35 MHz, -30 °C): δ -26.7 (d, JSiP ) 14 Hz).
2
1.62 (15H, s, C5(CH3)5), 1.36 (9H, d, P(CH3)3, JHP ) 10 Hz),
[Cp*Co(PMe3)(H)(η2-HSiPhMeH)][B(ArF)4], 14b. This com-
plex was generated in situ quantitatively using the above procedure
by adding PhMeSiH2 (4.7 µL, 0.0344 mmol, 2 equiv) to a CD2Cl2
2
-13.58 (1H, d, Co-H, JHP ) 101 Hz).
[Cp*Co(PMe3)(H)(NCArF)][B(ArF)4], 5b (ArF ) 3,5-(CF3)2-
C6H3). This complex was generated quantitatively using the above
procedure by adding 3,5-bis(trifluoromethyl)benzonitrile (2 µL,
0.012 mmol, 1.4 equiv) to a CD2Cl2 solution of 2b. 1H NMR (CD2-
Cl2, 500.13 MHz, -30 °C): δ 8.22 (1H, s, NCArF p-H), 8.02 (2H,
s, NCArF o-H), 1.69 (15H, s, C5(CH3)5), 1.44 (9H, d, P(CH3)3, 2JHP
1
solution of 2b (20 mg, 0.0172 mmol of 1b). H NMR (CD2Cl2,
500.09 MHz, -30 °C): δ 7.70-7.30 (5H, overlaping m, PhSi),
1
5.03 (1H, s, SiHterminal, JSiH ) 219 Hz), 1.71 (15H, s, C5(CH3)5),
2
1.40 (9H, d, P(CH3)3, JHP ) 10.5 Hz), 0.81 (3H, br s, CH3Si),
-13.15 (2H, d, Co(H)(η2SiH), 1JSiH(observed) ) 28.5 Hz, 2JHP ) 44.5
Hz). 29Si{1H} DEPT 45 (CD2Cl2, 99.35 MHz, -30 °C): δ -9.0
2
) 10 Hz), -13.14 (1H, d, Co-H, JHP ) 98.5 Hz).
2
(d, JSiP ) 13.5 Hz).
[Cp*Co(PMe3)2(H)][B(ArF)4], 6b. This complex was generated
quantitatively using the above procedure by adding PMe3 (1 µL,
0.010 mmol, 1.2 equiv) to a CD2Cl2 solution of 2b. 1H NMR (CD2-
Cl2, 500.13 MHz, -30 °C): δ 1.73 (15H, s, C5(CH3)5), 1.39 (18H,
Deprotonation of [Cp*Co(L)(η2-H2)(H)][B(ArF)4], 2a and 2b.
A solution of 2a or 2b was generated in an NMR tube as described
previously in CD2Cl2. NEt3 (1 - 1.5 equiv) was added to these
solutions at 20 °C via syringe, and the tubes were briefly shaken
to ensure complete mixing. Cp*Co(L)(H)2 complexes 10a and 10b
were generated quantitatively and characterized in situ. Cp*Co-
(P(OMe)3)(H)2, 10a: 1H NMR (CD2Cl2, 500.13 MHz, -30 °C) δ
3.41 (9H, d, P(OCH3)3, 3JHP ) 11.5 Hz), 1.85 (15H, s, C5(CH3)5),
-17.23 (2H, d, Co(H)2, 2JHP ) 97.5 Hz). Cp*Co(PMe3)(H)2, 10b:
1H NMR (CD2Cl2, 500.13 MHz, -30 °C), δ 1.84 (15H, s,
2
br s, P(CH3)3), -16.6 (1H, t, Co-H, JHP ) 79.5 Hz).
[Cp*Co(PMe3)(H)(P(OMe)3)][B(ArF)4], 6c. This complex was
generated quantitatively using the above procedure by adding
P(OMe)3 (1 µL, 0.012 mmol, 1.2 equiv) to a CD2Cl2 solution of
1
2b. H NMR (CD2Cl2, 500.13 MHz, -30 °C): δ 3.72 (9H, d,
P(OCH3)3, 3JHP ) 10 Hz), 1.55 (15H, s, C5(CH3)5), 1.32 (9H, br s,
P(CH3)3), -15.30 (1H, dd, Co-H, 2JHP ) 76 Hz, 2JHP ) 85.5 Hz).
[Cp*Co(PMe3)(H)(MeOH)][B(ArF)4], 8b. This complex was
generated quantitatively using the above procedure by adding
MeOH (1 µL, 0.025 mmol, 3 equiv) to a CD2Cl2 solution of 2a.
1H NMR (CD2Cl2, 400.09 MHz, -30 °C): δ 3.94 (1H, br s,
CH3OH), 2.91 (3H, br s, CH3OH), 1.51 (15H, s, C5(CH3)5), 1.34
2
C5(CH3)5), 1.17 (9H, d, P(CH3)3, JHP ) 9.5 Hz), -18.09 (2H, d,
Co(H)2, JHP ) 88.5 Hz).
2
General Procedure for Generating Partially Deuterated [Cp*-
(L)Co(H)(η2-H2)][B(ArF)4] Complexes, 2a-dn and 2b-dn. A solu-
tion of the corresponding [Cp*Co(L)(H)(η2-H2)][B(ArF)4] complex
in methylene chloride-d2 was degassed via two freeze-pump-thaw
cycles, back-filled with D2 gas, and stored at -78 °C. The tube
was then briefly shaken before being placed in a precooled NMR
probe at -71 °C, at which point the reaction was monitored by 1H
NMR.
2
(9H, br s, P(CH3)3), -11.63 (1H, d, Co-H, JHP ) 108 Hz).
[Cp*Co(PMe3)(CH2CH2C(O)OCH3)][B(ArF)4], 9b, and [Cp*-
Co(PMe3)(CH(CH3)C(O)OCH3)][B(ArF)4], 9b′. These complexes
were generated as a 2:1 mixture of 9b′:9b using the above procedure
by adding methyl acrylate (1.2 µL, 0.13 mmol, 1.5 equiv) to a CD2-
Cl2 solution of 2b at -30 °C and monitoring olefin insertion at
that temperature. Decomposition occurred before complete isomer-
ization could take place. 9b: 1H NMR (CD2Cl2, 500.13 MHz, -15
°C) δ 3.76 (3H, s, OCH3), 3.04 (1H, q, CH2, JHH ) 10 Hz), 2.85
(1H, br t, CH2, JHH ) 12 Hz), 2.63 (1H, dd, CH2, JHH ) 8.7 Hz,
JHH ) 19.5 Hz) 2.4 - 2.3 (1H, CH2, hidden under CH2 signal for
[Cp*Co(P(OMe)3)(CH2CH2C(O)OCH3)][B(ArF)4], 9a. A flame-
dried Schlenk flask was charged with 1a (200 mg, 0.165 mmol),
methyl acrylate (297 µL, 3.30 mmol, 20 equiv), and methylene
chloride (5 mL) under an atmosphere of Ar at room temperature.
The solution was allowed to stir for 5-10 min before purging with
H2 for 15 min, at which point the solution was sealed off from Ar
and H2 and allowed to stir at room temperature for 2 h. The solution
was then concentrated under vacuum to ∼ 1-2 mL, and hexane (8
mL) was added. Volatiles were removed in Vacuo with stirring to
give a brown powder (178 mg, 0.140 mmol, 85%), which was then
dried under vacuum for an additional 1 h before being stored in a
glovebox at -35 °C. X-ray quality crystals were grown by vapor
diffusion of pentane into a concentrated 50:50 mixture of toluene
and methylene chloride solution of 9a. 1H NMR data are identical
to those obtained upon in situ formation of 9a from 2a. 13C{1H}
NMR (CD2Cl2, 125.77 MHz, 20 °C): δ 191.5 (s, CdO), 96.6 (s,
C5(CH3)5), 55.7 (s, OCH3), 54.9 (d, P(OCH3)3, 2JCP ) 8 Hz), 38.5
(s, â-CH2), 9.2 (s, C5(CH3)5), 7.3 (s, R-CH2). Anal. Calcd for
C49H43F24O5BPCo: C, 46.39; H, 3.42. Found: C, 46.15; H, 3.17.
Isomerization of 9a′ to 9a in the Absence of H2. A ∼8:1
mixture of 9a′:9a was generated from 2a and methyl acrylate in a
J-Young NMR tube as described above. Upon consumption of 2a,
excess H2 was removed from the reaction via three freeze-pump-
4
methyl propionate), 1.50 (15H, d, C5(CH3)5, JHP ) 1.8 Hz), 1.30
2
(9H, d, P(CH3)3, JHP ) 9.8 Hz). 9b′: 1H NMR (CD2Cl2, 500.13
MHz, -15 °C) δ 3.67 (1H, br s, CHCH3), 3.56 (3H, s, OCH3),
1.48 (15H, s, C5(CH3)5), 1.43 (3H, br s, CHCH3), 1.37 (9H, d,
2
P(CH3)3, JHP ) 9.4 Hz).
[Cp*Co(PMe3)(H)(η2-HSiEt3)][B(ArF)4], 11b. This complex
was generated in situ in ∼50% conversion using the above
procedure by adding Et3SiH (14 µL, 0.086 mmol, 10 equiv) to a
CD2Cl2 solution of 2b. 1H NMR (CD2Cl2, 500.13 MHz, -19 °C):
δ 1.80 (15H, s, C5(CH3)5), 1.45 (9H, d, P(CH3)3, 2JHP ) 10.5 Hz),
-13.96 (2H, d, Co(H)(η2-SiH), 1JHSi(observed) ) 29.0 Hz, 2JHP ) 45
Hz). 29Si{1H} DEPT 45 (CD2Cl2, 99.35 MHz, -30 °C): δ 20.7
(br s).
[Cp*Co(PMe3)(H)(η2-HSiPh2H)][B(ArF)4], 12b. This complex
was generated in situ quantitatively using the above procedure by
adding Ph2SiH2 (6.4 µL, 0.0344 mmol, 2 equiv) to a CD2Cl2
1
1
solution of 2b (20 mg, 0.0172 mmol of 1b). H NMR (CD2Cl2,
thaw cycles. The reaction was then monitored via H NMR over
500.09 MHz, -30 °C): δ 7.65 - 7.39 (10H, overlapping m, Ph2-
Si), 5.80 (1H, s, SiHterminal, 1JSiH ) 222 Hz), 1.72 (15H, s, C5(CH3)5),
1.14 (9H, d, P(CH3)3, 2JHP ) 10.5 Hz), -13.07 (2H, d, Co(H)(η2-
the course of 24 h at 20 °C resulting in a 1:11 ratio of 9a′:9a.
Acknowledgment. We thank the National Science Founda-
tion (CHE-0615704) for support of this work.
1
2
SiH), JSiH(observed) ) 30 Hz, JHP ) 44 Hz). 29Si{1H} DEPT 45
2
(CD2Cl2, 99.35 MHz, -30 °C): δ 0.72 (d, JSiP ) 14 Hz).
Supporting Information Available: CIF files giving complete
crystallographic data for complexes 1a‚H2O, 1b, and 9a. This
material is available free of charge via the Internet at http://
pubs.acs.org.
[Cp*Co(PMe3)(H)(η2-HSiPhH2)][B(ArF)4], 13b. This complex
was generated in situ quantitatively using the above procedure by
adding PhSiH3 (4.3 µL, 0.0344 mmol, 2 equiv) to a CD2Cl2 solution
of 2b (20 mg, 0.0172 mmol of 1b). 1H NMR (CD2Cl2, 500.09 MHz,
-30 °C): δ 7.80-7.30 (5H, overlapping m, PhSi), 4.69 (2H, s,
OM700620W