Thermodynamic and kinetic studies of H atom transfer from HMo(CO) 3(η5-C5H5) to Mo(N[t-Bu]Ar) 3 and (PhCN)Mo(N[t-Bu]Ar)3: Direct insertion of benzonitrile into the Mo-H bond of HMo(N[t-Bu]A

Article abstract of DOI:10.1021/ic800945m

Synthetic studies are reported that show that the reaction of either H ₂SnR₂ (R = Ph, n-Bu) or HMo(CO)₃(Cp) (1-H, Cp = η⁵-C₅H₅) with Mo(N[t-Bu]Ar)₃ (2, Ar = 3,5-C₆H

Full text of DOI:10.1021/ic800945m

Inorg. Chem. 2008, 47, 9380-9389
Thermodynamic and Kinetic Studies of H Atom Transfer from
HMo(CO)3(η5-C5H5) to Mo(N[t-Bu]Ar)3 and (PhCN)Mo(N[t-Bu]Ar)3: Direct
Insertion of Benzonitrile into the Mo-H Bond of HMo(N[t-Bu]Ar)3
forming (Ph(H)CdN)Mo(N[t-Bu]Ar)3
Manuel Temprado,†,‡ James Eric McDonough,§ Arjun Mendiratta,† Yi-Chou Tsai,†,|
George C. Fortman,§ Christopher C. Cummins,*,† Elena V. Rybak-Akimova,*,‡ and Carl D. Hoff*,§
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139, Department of Chemistry, Tufts UniVersity, 62 Talbot AVenue,
Medford, Massachusetts, 02155, and Department of Chemistry, UniVersity of Miami,
1301 Memorial DriVe, Coral Gables, Florida, 33146
Received May 25, 2008
Synthetic studies are reported that show that the reaction of either H2SnR2 (R ) Ph, n-Bu) or HMo(CO)3(Cp) (1-H, Cp
5
) η -C5H5) with Mo(N[t-Bu]Ar)3 (2, Ar ) 3,5-C6H3Me2) produce HMo(N[t-Bu]Ar)3 (2-H). The benzonitrile adduct
(PhCN)Mo(N[t-Bu]Ar)3 (2-NCPh) reacts rapidly with H2SnR2 or 1-H to produce the ketimide complex (Ph(H)CdN)Mo(N[t-
Bu]Ar)3 (2-NC(H)Ph). The X-ray crystal structures of both 2-H and 2-NC(H)Ph are reported. The enthalpy of reaction of
1-H and 2 in toluene solution has been measured by solution calorimetry (∆H ) -13.1 ( 0.7 kcal mol-1) and used to
estimate the Mo-H bond dissociation enthalpy (BDE) in 2-H as 62 kcal mol-1. The enthalpy of reaction of 1-H and
2-NCPh in toluene solution was determined calorimetrically as ∆H ) -35.1 ( 2.1 kcal mol-1. This value combined with
the enthalpy of hydrogenation of [Mo(CO)3(Cp)]2 (12) gives an estimated value of 90 kcal mol-1 for the BDE of the
ketimide C-H of 2-NC(H)Ph. These data led to the prediction that formation of 2-NC(H)Ph via nitrile insertion into 2-H
would be exothermic by ∼36 kcal mol-1, and this reaction was observed experimentally. Stopped flow kinetic studies of
the rapid reaction of 1-H with 2-NCPh yielded ∆Hq ) 11.9 ( 0.4 kcal mol-1, ∆Sq ) -2.7 ( 1.2 cal K-1 mol-1.
Corresponding studies with DMo(CO)3(Cp) (1-D) showed a normal kinetic isotope effect with kH/kD ≈ 1.6, ∆Hq ) 13.1
( 0.4 kcal mol-1 and ∆Sq ) 1.1 ( 1.6 cal K-1 mol-1. Spectroscopic studies of the much slower reaction of 1-H and
2 yielding 2-H and 1/212 showed generation of variable amounts of a complex proposed to be (Ar[t-Bu]N)3Mo-Mo(CO)3(Cp)
(1-2). Complex 1-2 can also be formed in small equilibrium amounts by direct reaction of excess 2 and 12. The
presence of 1-2 complicates the kinetic picture; however, in the presence of excess 2, the second-order rate constant
for H atom transfer from 1-H has been measured: 0.09 ( 0.01 M-1 s-1 at 1.3 °C and 0.26 ( 0.04 M-1 s-1 at 17 °C.
Study of the rate of reaction of 1-D yielded kH/kD ) 1.00 ( 0.05 consistent with an early transition state in which
formation of the adduct (Ar[t-Bu]N)3Mo · · · HMo(CO)3(Cp) is rate limiting.
Introduction
to form substituted ketimide complexes of general form
(Ph(X)CdN)Mo(N[t-Bu]Ar)3. Some of these complexes
undergo clean extrusion of nitrile to form XMo(N[t-Bu]Ar)3.
Known thermodynamic parameters corresponding to such
reactions are summarized in the thermochemical cycle in
Scheme 1 showing that nitrile extrusion is exothermic only
for X ) OC(O)Ph and that the driving force for this reaction
is primarily entropic for the other complexes studied.1 These
data suggest that the reverse of the ꢀ-X elimination reaction,
nitrile insertion, should be favorable for suitable X groups.
Lending credence to this hypothesis is the observation that
Nb(H)(η2-t-Bu(H)CdNAr)(N[CH2-t-Bu]Ar)2, which is the
The addition of benzonitrile to Mo(N[t-Bu]Ar)3 (2, Ar )
3,5-C6H3Me2) results in the formation of equilibrium amounts
of (PhCN)Mo(N[t-Bu]Ar)3 (2-NCPh), which reacts rapidly
with X2 (X ) SC6H5, SC6F5, SeC6H5, TeC6H5, OC(O)Ph)
* To whom correspondence should be addressed. E-mail: ccummins@
mit.edu (C.C.C.), erybakak@tufts.edu (E.V.R.-A), c.hoff@miami.edu
(C.D.H).
†
Massachusetts Institute of Technology.
Tufts University.
University of Miami.
Current address: Department of Chemistry, National Tsing Hua
‡
§
|
University, Hsinchu 30013, Taiwan, Republic of China.
9380 Inorganic Chemistry, Vol. 47, No. 20, 2008
10.1021/ic800945m CCC: $40.75  2008 American Chemical Society
Published on Web 09/13/2008

H Atom Transfer
the method of Grubbs.5 THF was passed through a MBraun solvent
purification system, further dried by stirring with sodium metal and
then filtered through alumina before use. Distilled solvents were
transferred under vacuum into vacuum-tight vessels before being
transferred into a Vacuum Atmospheres drybox. C6D6 was pur-
chased from Cambridge Isotopes and was degassed and dried over
4 Å molecular sieves. The 4 Å molecular sieves and Celite were
dried in vacuo overnight at a temperature above 200 °C. Compounds
Scheme 1. Thermodynamic Data for Enthalpies of Reaction (kcal
mol-1) for the Assembly of Ketimide Complexes and for Nitrile
Extrusion Reactions
9
2,6,7 HN(t-Bu)Ar,8 and MoCl3(THF)3 were prepared following
literature methods. HMo(CO)3(Cp) (1-H, Cp ) η5-C5H5) was
prepared by reaction of Mo(CO)3(p-xylene) and C5H6 in THF10
and purified by high-vacuum sublimation. 1-D was obtained by
stirring a pentane solution of 1-H over a large excess of D2O for
several hours, separating the organic phase, evaporating it to
dryness, and then subliming 1-D. Analysis by NMR showed it to
be ∼95% D and 5% H substituted. PhCN was distilled under
reduced pressure before use and stored in a nitrogen-filled drybox.
1
All other compounds were used as received. H and 13C NMR
spectra were recorded on Unity 300, Mercury 300, Bruker
AVANCE-400, or Varian INOVA-500 spectrometers at room
temperature. 13C NMR spectra are proton decoupled. Chemical
shifts are reported with respect to internal solvent: 7.16 and 128.38
1
(t) ppm (C6D6) for H and 13C, respectively. FTIR spectra were
obtained using a Perkin-Elmer 2000 FTIR/Microscope system that
has been described elsewhere.11 CHN analyses were performed by
H. Kolbe Mikroanalytisches Laboratorium (Mu¨lheim, Germany)
or Midwest Microlabs LLC (Indianapolis, IL).
C-H oxidative addition form of the trisamide species
Nb(N[CH2-t-Bu]Ar)3, reacts with mesityl nitrile to form η2-
(MesCN)Nb(N[CH2-t-Bu]Ar)3, whereas reaction with t-
BuCN results in insertion of the nitrile into the Nb-H bond
forming the ketimide complex (H(t-Bu)CdN)Nb(η2-t-Bu(H)-
CdNAr)(N[CH2-t-Bu]Ar)2.2
Synthesis of HMo(N[t-Bu]Ar)3 (2-H). A solution of H2Sn(n-
Bu)2 (1.346 g, 5.80 mmol) or H2SnPh2 (0.480 g, 1.74 mmol) in 5
mL of Et2O was added dropwise to a 50 mL round-bottom flask
containing an orange-brown solution of 2 (1.864 g, 2.90 mmol) in
20 mL of Et2O. The resulting solution turned dark red rapidly and
was stirred for 1 h at 20 °C. The solution was filtered through a
plug of Celite. Upon removal of all volatile components, a brick-
red solid was obtained, extracted into a minimum amount of
n-pentane, and stored at -35 °C to give 1.718 g (2.75 mmol) of
brick-red powder (92% yield).
In addition, 2-H was synthesized using 1-H as the H-atom donor.
In a scintillation vial, a solution of 1-H (119 mg, 0.48 mmol) in 2 mL
of pentane was added to a solution of 2 (300 mg, 0.48 mmol) in 15
mL of n-pentane giving a dark red solution. After it was stirred for
2 h at T ) 20 °C, the reaction mixture was filtered. Volatile materials
Legzdins and co-workers3 have reported direct insertion
of acetonitrile into the W-H bond of the 16-valence electron
complex (η5-C5Me5)W(NO)(CH2SiMe3)H forming (η5-
C5Me5)W(NO)(CH2SiMe3)(NdC(H)CH3); however, there
are relatively few reports of nitrile insertion into metal-hydride
or metal-alkyl bonds in transition-metal complexes. Recent
studies of nitrile insertion into lanthanide and actinide
complex bonds have been reported,4 such as that shown for
the thorium (and the uranium analog) complexes in eq 1.
(1) Mendiratta, A.; Cummins, C. C.; Kryatova, O. P.; Rybak-Akimova,
E. V.; McDonough, J. E.; Hoff, C. D. J. Am. Chem. Soc. 2006, 128,
4881.
(2) Figueroa, J. S.; Cummins, C. C. J. Am. Chem. Soc. 2003, 125, 4020.
(3) Debad, J. D.; Legzdins, P.; Lumb, S. A.; Batchelor, R. J.; Einstein,
F. W. D. Organometallics 1995, 14, 2543.
(4) (a) Schelter, E. J.; Yang, P.; Scott, B. L.; Da Re, R. E.; Jantunen,
K. C.; Martin, R. L.; Hay, P. J.; Morris, D. E.; Kiplinger, J. L. J. Am.
Chem. Soc. 2007, 129, 5139. (b) Schelter, E. J.; Yang, P.; Scott, B. L.;
Thompson, J. D.; Martin, R. L.; Hay, P. J.; Morris, D. E.; Kiplinger,
J. L. Inorg. Chem. 2007, 46, 7477. (c) Jantunen, K. C.; Burns, C. J.;
Castro-Rodriguez, I.; Da Re, R. E.; Golden, J. T.; Morris, D. E.; Scott,
B. L.; Taw, F. L.; Kiplinger, J. L. Organometallics 2004, 23, 4682.
(5) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. Organometallics 1996, 15, 1518.
The present work reports synthetic, thermodynamic, and
kinetic studies relevant to reactions shown in Scheme 1, for
X ) H. In this case, it is shown that nitrile insertion, rather
than extrusion, is the thermodynamically favored and ob-
served reaction.
(6) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim,
E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem.
Soc. 1996, 118, 8623.
(7) Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins, C. C. J. Am.
Chem. Soc. 1995, 117, 4999.
Experimental Section
(8) Tsai, Y. -C.; Stephens, F. H.; Meyer, K.; Mendiratta, A.; Gheorghiu,
M. D.; Cummins, C. C. Organometallics 2003, 22, 2902.
(9) Stoffelbach, F.; Saurenz, D.; Poli, R. Eur. J. Inorg. Chem. 2001, 2699.
(10) Hoff, C. D. J. Organomet. Chem. 1985, 282, 201.
(11) Ju, T. D.; Capps, K. B.; Roper, G. C.; Hoff, C. D. Inorg. Chim. Acta
1998, 270, 488.
General Considerations. Unless stated otherwise, all operations
were performed in a Vacuum Atmospheres or MBraun drybox under
an atmosphere of purified nitrogen or argon. Diethyl ether, toluene,
n-pentane, n-hexane, and THF were dried and deoxygenated by
Inorganic Chemistry, Vol. 47, No. 20, 2008 9381

Temprado et al.
were removed under reduced pressure, and 235 mg (0.38 mmol, 78%
yield) of pure 2-H was isolated. H NMR (400 MHz, T ) 20 °C,
C6D6): δ ) 28.13 (s, 27H, C(CH3)3), -7.54 (s, 18H, C6H3(CH3)2),
Enthalpy of Reaction of 2 and 1-H. In the glovebox, a solution
of 2 (0.1545 g, 0.247 mmol) was prepared in 5 mL of freshly
distilled toluene and transferred via syringe through a Teflon syringe
filter into the calorimeter cell under argon atmosphere. A sample
of freshly sublimed 1-H (0.0242 g, 0.098 mmol) was loaded into
the solid sample holder of the calorimeter cell. The cell was sealed,
taken from the glovebox, and loaded into a Setaram C-80
calorimeter. After thermal equilibration, approximately two hours,
the reaction was initiated and followed to completion at 30 °C.
Following the return to baseline, the cell was taken back into the
glovebox, and its contents examined by FTIR to confirm conversion
of 1-H to 12. A small residual band at 1864 cm-1 indicated a trace
amount of the proposed intermediate complex (Ar[t-
1
-18.28 (br, s, 6H, o-ArH), -26.71 (s, 3H, p-ArH) ppm. IR: νMo-H
)
1860 cm-1, νMo-D ) 1334 cm-1. µeff ) 3.09 µB (Evans’ method,12
C6D6, 19.8 °C). Anal. Calcd for C36H55N3Mo: C, 63.17; H, 6.83; N,
4.91. Found: C, 62.97; H, 6.90; N, 4.81.
Synthesis of (Ph(H)CdN)Mo(N[t-Bu]Ar)3 (2-NC(H)Ph). A
scintillation vial was charged with 2 (200 mg, 0.32 mmol) and 5
mL of n-pentane. PhCN (33 mg, 0.32 mmol) was added resulting
in a rapid color change to purple. To this solution was added solid
1-H (79 mg, 0.32 mmol), and the solution was allowed to stir for
40 min. After this time, the solution was dark blue, and copious
amounts of red precipitate had formed. The reaction mixture was
filtered through Celite, and the volatile components were removed
under dynamic vacuum. Recrystallization from Et2O (-35 °C)
furnished 2-NC(H)Ph as large blue blocks (2 crops, 123 mg, 53%
Bu]N)
3Mo-Mo(CO)3(Cp) (1-2). A separate calorimetric study of
the enthalpy of reaction of 2 and 12 was undertaken and indicated
that ∆H ) -12 ( 2 kcal mol-1 for formation of the proposed
metal-metal bonded complex. Estimated corrections for the trace
amounts of (Ar[t-Bu]N)3Mo-Mo(CO)3(Cp) detected in the calo-
rimetric reactions of 2 and 1-H were below experimental error limits
and are not included in the reported average value based on six
independent determinations of ∆H ) -10.0 ( 0.7 kcal mol-1 for
the enthalpy of reaction based on solid 1-H. The enthalpy of solution
of 1-H in toluene is ∆H ) 3.1 ( 0.1 kcal mol-1 yielding a value
1
yield). H NMR (500 MHz, T ) 20 °C, C6D6): δ ) 1.30 (s, 27H,
C(CH3)3), 2.18 (s, 18H, C6H3(CH3)2), 6.55 (d, 2H, o-PhH), 6.67
(s, 6H, o-ArH), 6.67 (t, 1H, p-PhH), 6.69 (s, 3H, p-ArH), 7.17 (t,
2H, m-Ph), 7.44 (s, 1H, N ) CH) ppm. 13C NMR (75 MHz, T )
20 °C, C6D6): δ ) 21.31 (C6H3(CH3)2), 31.27 (C(CH3)3), 62.28
(N-C(CH3)3), 122.28, 124.42, 127.04, 127.30, 129.03, 135.77,
137.23, 150.27, 156.49 ppm. UV-vis (toluene, 20 °C): λmax ) 410,
585 nm. Anal. Calcd for C43H60N4Mo: C, 70.86; H, 8.30; N, 7.69.
Found: C, 70.66; H, 8.23; N, 7.31.
with all species in solution of ∆H ) -13.1 ( 0.7 kcal mol-1
.
Enthalpy of Reaction of 2-NCPh and 1-H. In the glovebox, a
solution of 2 (3.7 g, 14.9 mmol) was prepared in 200 mL of toluene.
The filtered solution was then taken from the glovebox and
transferred under argon atmosphere to an isoperibol calorimeter
system with an internal capacity of 300 mL. The calorimeter was
thermally equilibrated at 20 °C for a period of 2 h. At that time, 10
mL of freshly distilled benzonitrile was added, and the solution
turned into the dark purple color characteristic of 2-NCPh.
Following an equilibration time of about 10 min, the calorimeter
was electrically calibrated. Sealed glass ampoules containing 0.1949,
0.1828, and 0.1524 g of 1-H were broken into the solution.
Following that, additional electrical calibrations were performed.
It is known that 2-NCPh forms a dimer slowly in solution under
these conditions; however the rapid nature of the reaction and the
use of a large excess of 2-NCPh allowed reasonable instrument
performance because any heat evolved during the relatively slow
dimerization was absorbed into the baseline for both calorimetric
runs, as well as electrical calibrations. The values determined, ∆H
) -34.25, -30.22, and -31.77 kcal mol-1 yield an average value
of the enthalpy of reaction based on solid 1-H of ∆H ) -32.0 (
2.0 kcal mol-1 giving a value with all species in solution of ∆H )
-35.1 ( 2.1 kcal mol-1 using the enthalpy of solution of 1-H in
toluene (∆H ) 3.1 ( 0.1 kcal mol-1).
Stopped Flow Kinetic Measurements. Toluene solutions of the
reagents were prepared in a MBraun glovebox filled with argon
and placed in Hamilton gastight syringes. Time-resolved spectra
(400-700 nm) were acquired at temperatures from -30 to -10
°C using a Hi-Tech Scientific (Salisbury, Wiltshire, U.K.) SF-43
Multi-Mixing CryoStopped-Flow Instrument in diode array mode.
The stopped-flow instrument was equipped with stainless steel
plumbing, a 1.00 cm3 stainless steel mixing cell with sapphire
windows, and an anaerobic gas-flushing kit. The instrument was
connected to an IBM computer with IS-2 Rapid Kinetic software
(Hi-Tech Scientific). The temperature in the mixing cell was
maintained to (0.1 °C, and the mixing time was 2-3 ms. The
driving syringe compartment and the cooling bath, filled with
heptane (Fisher), were flushed with argon before and during the
experiments, using anaerobic kit flush lines. All flow lines of the
SF-43 instrument were extensively washed with degassed, anhy-
Insertion of PhCN into 2-H. Four different tubes with a mixture
of 2 and 0.7 equiv of 1-H were prepared. To three of the samples,
2 equiv of PhCN were added after 3, 20, and 60 min, and then all
of them were flame-sealed under vacuum for NMR study. The
sample without any added PhCN was used as a control to check
the stability of 2-H over the time period studied. It appeared to be
stable; however, after six days, some decomposition occurred based
1
on analysis of the integration data. The initial H NMR data run
on day 1 showed that the tube that had nitrile added after 3 min
had copious amount of 2-NC(H)Ph and the other two tubes had
sequentially more 2-H and smaller and finally traces of 2-NC(H)Ph.
NMR spectra were taken also after 1, 2, and 5 days. At the end of
day 6, conversion of 2-NC(H)Ph via nitrile insertion was complete.
Figure SF1 shows decay of 2-H over time for the tube in which
the benzonitrile was added after 20 min.
The possibility that the insertion may have occurred slowly by
a direct process catalyzed by free 2 forming 2-NC(H)Ph was
investigated by comparing the rates of formation of 2-NC(H)Ph in
the presence of excess added 2 to 2-H. No significant rate
enhancement was observed.
Crystallographic Structure Determinations. X-ray diffraction
data were collected using a Siemens Platform three-circle diffractometer
coupled to a Bruker-AXS Smart Apex CCD detector with graphite-
monochromated Mo KR radiation (λ ) 0.71073 Å), performing φ-
and ω-scans. The structure was solved by direct methods using
SHELXS13 and refined against F2 on all data by full-matrix least-
squares with SHELXL-97.14 All non-hydrogen atoms were refined
anisotropically; all hydrogen atoms were included into the model at
their geometrically calculated positions and refined using a riding model
except in the case of 2-H, where the hydride position was located from
the electronic density difference map.
(12) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Bottomley, F.; Lin,
I. J. B. J. Chem. Soc., Dalton Trans. 1981, 271.
(13) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990,
A46, 467.
(14) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
9382 Inorganic Chemistry, Vol. 47, No. 20, 2008

H Atom Transfer
Similarly, the sum of the same angles in complex 27 and
ClMo(N[t-Bu]Ar)318 are 357.7(3)° and 350(1)° respectively.
The small deviation from planarity at Mo in 2-H can be
attributed to the lack of π interaction between molybdenum
and the hydride ligand. The sum of the three N-Mo-N
angles in ClMo(N[t-Bu]Ar)3 is slightly smaller than that
observed in 2-H because of the presence of π-bonding
between molybdenum and the chloride ligand, as well as
the relative bulk of the Cl atom. The three 3,5-C6H3Me2
substituents are on one side of the trigonal plane and the
three t-Bu groups on the other. The experimentally deter-
mined Mo-H interatomic distance of 1.58(3) Å is similar
to that reported for the W-H bond (between 1.57 and 1.59
Å) in the compound [(Me3SiCH2CH2N)3N]WH.19 However,
since the hydride position was located from the electron
density difference map, we do not expect this value to be
accurate. Several neutron diffraction studies of molybdenum
hydrides have been reported,20-23 giving values for the
Mo-H bond of 1.69,20 1.685,21 1.716,22 and 1.789 (aver-
age)23 Å for the complexes Mo(H)(η2-Me2CdNAr)(N[i-
Pr]Ar)2, H2Mo(η5-C5H5)2, H3Mo(η5-1,2,4-C5H2-(t-Bu)3)-
(PMe3)2, and HMo(CO)3(η5-C5Me5), respectively. A similar
underestimated Mo-H bond length of 1.58 Å was obtained
by X-ray diffraction by Poli and co-workers.22
The Mo-H and Mo-D (which was prepared using
D2SnR2) stretching frequencies were located at 1860 and
1334 cm-1 for Mo-H and Mo-D, respectively, in ap-
proximate agreement with harmonic oscillator predictions.
The magnetic susceptibility of complex 2-H, determined
using Evans’ method,12 was found to be µeff ) 3.09 µB, close
to the spin-only value for a d2 complex (2.83 µB). This
magnetic property can be explained in terms of the electronic
structure of 2-H. Since no π-interaction exists between the
hydride ligand and molybdenum, the half-occupied π orbitals
(dxz and dyz, taking the Mo-H vector as the z axis in the C3
point group) remain degenerate in the complex. Hence, the
C3 symmetric compound is expected to remain paramagnetic
with two unpaired electrons in the degenerate dxz and dyz
orbitals.
Figure 1. Thermal ellipsoid plot (50% probability) of HMo(N[t-Bu]Ar)3.
Selected distances (Å) and angles (deg): Mo-H, 1.58(3); Mo-N1, 1.967(2);
Mo-N2, 1.960(2); Mo-N3, 1.957(2); N1-Mo-N2, 119.46(8); N2-Mo-N3,
116.75(8); N3-Mo-N1, 120.02(8); H-Mo-N1, 98(1); H-Mo-N2, 96(1);
H-Mo-N3, 95(1).
drous toluene before charging the driving syringes with reactant
solutions. The reactions were studied by rapid scanning spectro-
photometry under pseudo-first-order conditions (0.3 mM for 2 and
premixed solution of 50 mM for PhCN and 4 mM for 1-H or 1-D).
The rate dependence on [XMo(CO)3(C5H5)] (X ) H, D) was studied
at -30 °C varying [XMo(CO)3(C5H5)] from a 1- to 15-fold excess,
and the reaction order in XMo(CO)3(C5H5) was established. All of
the experiments were performed in a single-mixing mode of the
instrument, with a 1:1 (v/v) mixing ratio. The reactions were
monitored for three to five half-lives. A series of three to six
measurements at each temperature gave an acceptable standard
deviation (within 10%). Data analysis was performed with IS-2
Rapid Kinetic software from Hi-Tech Scientific or Spectfit/32
Global Analysis System software from Biologic. Single-exponential
kinetic profiles were observed in all experiments.
Results and Discussion
Preparation and X-Ray Structure of 2-H. Preparation
of 2-H was first achieved with organotin dihydrides15 as
shown in reaction 2
2Mo(N[t-Bu]Ar)3 +
H2SnR2
8 2HMo(N[t-Bu]Ar)3 + 1/n(R2Sn)n (2)
(R ) n-Bu, Ph)
H/D Exchange in Reaction of H2 and DMo(N[t-Bu]-
Ar)3. The deuterated isotopomer, 2-H, reacts with H2 (4 atm)
at T ) 20 °C to produce the hydride complex, which was
identified by IR spectroscopy. A reasonable mechanism for
this reaction is proposed in eq 3
It was observed that addition of 2 equiv of H2Sn(n-Bu)2
was necessary to achieve complete formation of the para-
magnetic brick-red Mo(IV) complex, 2-H, whereas only 0.6
equiv of H2SnPh2 were needed. This is consistent with the
better H-donor ability of H2SnPh2 over H2Sn(n-Bu)2.16
Attempts to directly hydrogenate 2 under 1000 psi H2 at T
) 20 °C show slow reaction with e4% conversion to 2-H
in 16 h despite the expected thermochemical favorability
(vide infra).
The solid-state structure of 2-H is shown in Figure 1. It
clearly shows a trigonal-monopyramidal complex with C3
symmetry.17 The geometry of the Mo-N3 unit is almost
planar with the sum of N-Mo-N angles ) 356.23°.
Schrock and co-workers prepared the W(VI) trihydride
complex, [(Me3SiCH2CH2N)3N]WH3, by treating [(Me3Si-
CH2CH2N)3N]WH with H2.19 Despite the failure to spec-
troscopically detect H3Mo(N[t-Bu]Ar)3, this provides rea-
sonable precedent for a trihydride intermediate being formed,
albeit in low concentrations, as part of the H/D exchange
mechanism. While a trihydride complex such as that proven
in ref 19 seems to be the most logical intermediate in this
(15) Tsai, Y.-C. PhD thesis, Massachusetts Institute of Technology,
Cambridge, MA, 2002.
(16) Kuivila, H. G. Synthesis 1970, 499.
(17) Cummins, C. C.; Lee, J.; Schrock, R. R.; Davis, W. D. Angew. Chem.,
Int. Ed. 1992, 31, 1501.
Inorganic Chemistry, Vol. 47, No. 20, 2008 9383

Temprado et al.
reaction sequence, other mechanisms (such as σ-bond
metathesis)24 cannot be ruled out.
Preparation and X-Ray Structure of 2-NC(H)Ph. Com-
pound 2-NCPh has been shown to behave as a carbon-based
radical.1,8,25 Because hydrogen atom abstraction is a typical
reaction of carbon-based radicals,26 it was decided to examine
the reactivity of 2-NCPh with a variety of H-atom donors
as shown in reaction 4.
PhCN-Mo(N[t-Bu]Ar)3 [H] 8 Ph(H)CN-Mo(N[t-Bu]Ar)3
Et2O
[H] ) HMo(CO)3(C5H5), H2Sn(n-Bu)2
no reaction with HSn(n-Bu)3 (4)
When a freshly prepared, purple, pentane solution of
2-NCPh is treated with 1 equiv of crystalline 1-H,27 a color
change to deep blue, along with precipitation of a red solid
is observed over a period of 5 min. Removal of the red solid
(12), followed by crystallization from Et2O, resulted in the
isolation of 2-NC(H)Ph as large blue blocks in 53% yield.
Compound 2-NC(H)Ph is diamagnetic and exhibits a H
NMR resonance at δ ) 7.41 ppm attributed to the ketimide
proton.
Figure 2. Solid-state structure (50% ellipsoids) of Ph(H)CN-Mo(N[t-
Bu]Ar)3 (2-NC(H)Ph). Selected bond distances (Å): Mo-Namide, 1.973
(average); Mo-N4, 1.7896(12); N4-C4, 1.3055(17); distance between
centroids (a-b), 4.113. Selected bond angles (deg): Mo-N4-C4,
166.84(11).
1
The solid-state structure of 2-NC(H)Ph is shown in Figure
2. Of note is the short Mo-N(4) distance of 1.7896(12) Å
as compared to the average Mo-amide distance of 1.973
Å. This contraction is likely a consequence of the hybridiza-
tion change upon going from an amide to a ketimide nitrogen,
as well as both the π-acceptor and π-donor character of the
ketimide ligand. The ability of the N-C π* orbital to act as
a π acceptor is facilitated by the nearly linear (166.84(11)°)
Mo-N-C angle. A relevant structural comparison is to the
parent ketimide complex (H2CdN)Mo(N[t-Bu]Ar)3 prepared
by deprotonation of the cationic methylimido complex.28
Compared to 2-NC(H)Ph, (H2CdN)Mo(N[t-Bu]Ar)3 features
similar Mo-N and N-C distances (1.777(4) and 1.300(7)
Å, respectively), although the Mo-N-C unit is considerably
closer to linearity (Mo-N-C ) 178.04(4)°). In addition,
the nitrosyl complex (OdN)Mo(N[Ad]Ar)3 displays an
Mo-N-O angle of 179.7°.29 One possible rationalization
for the slight deviation from linearity in 2-NC(H)Ph is that
there may be a π-stacking interaction between the phenyl
residue on the ketimide and the aryl substituent on one of
the amido groups bound to molybdenum. The aromatic rings
are neither parallel to each other nor in an eclipsed sandwich
configuration but are displaced and tilted to form an angle
of 19.94°. The distance between the centroids is 4.113 Å,
and the shortest distance between the rings is 3.267 Å. This
compares to the observation that in the solid crystalline form
many aromatic molecules form stacks with displaced rings
in approximately parallel molecular planes separated by
3.3-3.6 Å30 and in the case of tilted compounds form angles
between 15 and 40° with a distance of 4.4-4.7 Å between
centroids.31 This interaction results in reversal of the con-
figuration of one of the anilides, a situation that has been
25
previously noticed in (Ph(PhX)CdN)Mo(N[t-Bu]Ar)3 (X
) S, Se) and ((H3C)3SiO(Ph)CdN)Mo(N[t-Bu]Ar)332 where
the anilide ligands also adopt an up-down-sideways con-
formation. However, in cases where this interaction is not
possible, such as in the parent ketimide (H2CdN)Mo(N[t-
(18) Fu¨rstner, A.; Mathes, C.; Lehmann, C. W. J. Am. Chem. Soc. 1999,
121, 9453.
(19) Dobbs, A. A.; Schrock, R. R.; Davis, W. M. Inorg. Chim. Acta 1997,
263, 171.
(20) Tsai, Y. -C.; Johnson, M. J. A.; Mindiola, D. J.; Cummins, C. C. J. Am.
Chem. Soc. 1999, 121, 10426.
(21) Schultz, A. J.; Stearley, K. L.; Williams, J. M.; Mink, R.; Stucky,
G. D. Inorg. Chem. 1977, 16, 3303.
28
Bu]Ar)3 or ((H3C)3SiO(t-Bu)CN)Mo(N[t-Bu]Ar)3,33 the
conformation of the Mo(N[t-Bu]Ar)3 moiety is one of near
C3 symmetry, in agreement with this proposition.
Compound 2-NC(H)Ph can also be prepared by reaction
of 2-NCPh with 0.5 equiv H2Sn(n-Bu)2. While the reaction
(22) Baya, M.; Houghton, J.; Daran, J. -C.; Poli, R.; Male, L.; Albinati,
A.; Gutman, M. Chem.sEur. J. 2007, 13, 5347.
1
appears to be quantitative by H NMR, the presence of the
Sn side product hinders isolation, rendering the former
method more attractive for synthetic purposes. Interestingly,
2-NCPh shows no reaction with HSn(n-Bu)3, consistent with
the previous observation that HSn(n-Bu)3 is a less active
H-atom donor than H2Sn(n-Bu)2.16
(23) Brammer, L.; Zhao, D.; Bullock, R. M.; McMullan, R. K. Inorg. Chem.
1993, 32, 4819.
(24) Thompson, M. E.; Baxter, S. M.; Ray Bulls, A.; Burger, B. J.; Nolan,
M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem.
Soc. 1987, 109, 203.
(25) Mendiratta, A.; Cummins, C. C.; Kryatova, O. P.; Rybak-Akimova,
E. V.; McDonough, J. E.; Hoff, C. D. Inorg. Chem. 2003, 42, 8621.
(26) Free Radicals, Vol. II; Kochi, J. K., Ed.; Wiley: New York, 1973.
(27) (a) Landrum, J. T.; Hoff, C. D. J. Organomet. Chem. 1985, 282, 215.
(b) Amer, S.; Kramer, G.; Poe, A. J. Organomet. Chem. 1981, 220,
75.
(30) Dahl, T. Acta Chem. Scand. 1994, 48, 95.
(31) Sony, S. M. M.; Ponnuswamy, M. N. Cryst. Growth Des. 2006, 6,
737.
(28) Sceats, E. L.; Figueroa, J. S.; Cummins, C. C.; Loening, N. M.; Van
der Wel, P.; Griffin, R. G. Polyhedron 2004, 23, 2751.
(29) Cummins, C. C. Prog. Inorg. Chem. 1998, 47, 768.
(32) Curley, J. J.; Sceats, E. L.; Cummins, C. C. J. Am. Chem. Soc. 2006,
128, 14036.
(33) Curley, J. J.; Cummins, C. C. Unpublished results.
9384 Inorganic Chemistry, Vol. 47, No. 20, 2008

H Atom Transfer
Benzonitrile Insertion into 2-H. On the basis of estimates
of bond strengths (vide infra), it was predicted that benzoni-
trile insertion into complex 2-H should be thermodynamically
favorable. Reaction 5 was observed to occur; however it was
extremely slow even in the presence of 20 equiv of
benzonitrile. The reaction requires about one week to go to
completion at T ) 20 °C. NMR data supporting this
conclusion are shown in Supporting Information Figure SF1.
2Mo(N[t-Bu]Ar)3 + H2 f 2HMo(N[t-Bu]Ar)3 (10)
These data yield an estimate for the Mo-H bond strength in
2-H of 62 kcal mol-1. The Mo-H bond dissociation energy in
1-H has been estimated previously using calorimetric measure-
ments (BDE ) 66 kcal mol-1)10,27a and by employing a
combination of the pKa with the experimentally determined
oxidation potential of 1- (BDE ) 70 kcal mol-1).36 The Mo-H
bond strengths in 2-H and 1-H are surprising because they
indicate a stronger Mo-H bond in 1-H. This is in contrast to
other Mo-X BDE values in these systems where the 2 molecule
generally forms stronger Mo-X bonds. For example, the
(PhS)Mo(N[t-Bu]Ar)3 Mo-S bond strength37 of 55 kcal mol-1
compares to data for the (PhS)Mo(CO)3(C5H5) bond strength38
of 38 kcal mol-1. Because the spontaneous direction of reaction
8 produces a complex with a weaker H-Mo bond, it is
formation of the metal-metal bond in 12 that provides the
thermochemical driving force. As is well known, because of
steric factors, 2 does not form the expected metal-metal triple
bond,39 which would be anticipated to significantly alter the
thermochemical profile of the hydrogenation reaction in eq 10.
Thermochemistry of Reaction of 1-H and 2-NCPh. As
discussed in a later section, reaction 11 is rapid and
quantitative even at low temperatures.
HMo(N[t-Bu]Ar)3 + PhCN f (Ph(H)CdN)Mo(N[t-Bu]Ar)3 (5)
The slow nature of this reaction led us to investigate its
catalysis. Provided that H atom transfer in reaction 7 is rapid,
addition of free 2 to 2-H would be expected to increase the
rate of reaction 5 by the combined sequence of reactions 6
and 7
Mo(N[t-Bu]Ar)3 + PhCN h (PhCN)Mo(N[t-Bu]Ar)3 (6)
(
)
(
[
]
)
(
(
)
)
(
[
]
)
PhCN Mo N t-Bu Ar 3
Ph H C)N Mo N t-Bu Ar 3
+
f
+
(
[
]
)
(
[
]
)
HMo N t-Bu Ar 3
Mo N t-Bu Ar 3
(7)
Contrary to our expectations, no significant rate enhance-
ment was observed for nitrile insertion into the H-Mo bond
in the presence of free added 2. Instead, the previously
reported slow dimerization of 2-NCPh was observed.8
The most plausible mechanism for the slow insertion of
nitrile into the H-Mo bond involves coordination of nitrile
at 2-H in a transition state geometry similar to that shown
in Figure 3. This transition state configuration resembles that
proposed for the thermodynamically favored ꢀ-X elimination
step in the reactions shown in Scheme 1. Additional support
for this is gleaned from the fact that H2 undergoes H/D
exchange with DMo(N[t-Bu]Ar)3 (vide supra). This implies
some ability on the part of 2-H to coordinate added ligands.
The fact that there is a spin change in going from a ground-
state triplet in 2-H to a singlet in 2-NC(H)Ph would be
expected to further increase the activation energy34 for this
thermodynamically favorable reaction.
(
)
(
[
]
)
(
(
)
)
(
[
]
)
PhCN Mo N t-Bu Ar 3
Ph H C)N Mo N t-Bu Ar 3
+
f
+
(
) (
)
[
(
) (
)]
2
HMo CO 3 Cp
1/2 Mo CO 3 Cp
(11)
The enthalpy of reaction 11 with all species in toluene
solution at 20 °C was determined as ∆H ) -35.1 ( 2.1
kcal mol-1. The addition of twice the enthalpy of reaction
11 to that of reaction 9 yields directly a value of ∆H ) -76.5
kcal mol-1 for reaction 12
2(PhCN)Mo(N[t-Bu]Ar)3 +
H2 f 2(Ph(H)CdN)Mo(N[t-Bu]Ar) (12)
These data in combination with the BDE of H2 lead to an
estimate of 90 kcal mol-1 for the C-H bond dissociation
energy in 2-NC(H)Ph. The C-H bond strength in the
[Ph(H)CdN] radical has not been reported, but data for the
H2CdN radical indicate a C-H bond strength of 33 ( 6
kcal mol-1.40 The large difference in C-H bond strength
for the complex 2-NC(H)Ph is attributed to the strength of
the Mo-N bond. This is illustrated in the thermochemical
cycle in reaction eq 13.
Thermochemistry of Reaction of 1-H and 2. In the
presence of 1-H, conversion of 2 to 2-H occurs quantitatively
in toluene solution at 30 °C according to reaction 8
(
[
]
)
(
[
]
)
Mo N t-Bu Ar 3
HMo N t-Bu Ar 3
+
f
+
(8)
(
) (
)
[
(
) (
)]
2
HMo CO 3 Cp
1/2 Mo CO 3 Cp
These data lead to the estimate that the bond between 2
As discussed in a later section, this reaction involves
generation and disappearance of small amounts of complex
1-2, but the enthalpy of reaction at 30 °C with all species
in toluene solution was determined as ∆H ) -13.1 ( 0.7
kcal mol-1. The enthalpy of hydrogenation of 12 as shown
in reaction 9 has been previously reported as +6.3 kcal
mol-1:27a,35
and (Ph(H)CdN · ) is on the order of 71 ( 8 kcal mol-1.
[Mo(CO)3(Cp)]2 + H2 f 2HMo(CO)3(Cp)
(9)
The addition of twice reaction 8 to reaction 9 yields
directly a computed enthalpy of hydrogenation for reaction
10 of ∆H ) -19.9 ( 1.4 kcal mol-1.
Figure 3. Proposed transition state for insertion of benzonitrile into
HMo(N[t-Bu]Ar)3.
Inorganic Chemistry, Vol. 47, No. 20, 2008 9385

Temprado et al.
The enthalpy of binding of the isoelectronic formal three
electron donor ligand ·NO to 2 was measured experimentally
as -82.5 kcal mol-1.41 It seems most reasonable to view
these complexes as approaching reduced Mo(II) complexes
of the cationic NO+ and [Ph(H)CdN]+ ligands and as having
an approximate ModNdX (X ) O, C(H)Ph) formulation.42
Despite the relatively large error in the estimate in reaction
13, it appears that the metal-ligand bond strength decreases
in going from a nitrosyl to a ketimide for binding to 2.
Figure 4. Thermochemical cycle for insertion of benzonitrile.
Combination of the data for the enthalpy of binding of
nitrile determined from Keq studies (ca. -14 kcal mol-1)43
and the enthalpy of reactions 8 and 11 measured in this work
leads to a calculated exothermic insertion of benzonitrile of
-36 kcal mol-1 as shown in the thermochemical cycle in
Figure 4. Compared to insertion into the Mo-X bond [X )
OC(O)Ph (+2.4 kcal mol-1), SePh (-5.8 kcal mol-1), SPh
(-10 kcal mol-1)], the reverse of the nitrile elimination step
shown in Scheme 1), and considering the unfavorable entropy
associated with nitrile insertion, it is only for X ) H (-36
kcal mol-1) that insertion is predicted to be thermodynami-
cally favored at room temperature and above. As discussed
previously, this reaction was observed to occur, albeit slowly
over a period of days. In terms of how changing X influences
the thermochemistry, it is clear that for X ) H, the relatively
weak Mo-H bond compared to the strong C-H bond
formed would serve to favor this reaction.
Figure 5. Spectral changes observed at -25 °C upon mixing 2 (0.3 mM)
with a premixed solution of PhCN (50 mM) and 1-H (0.3 mM), time scale
) 95 s. The concentrations are given before mixing in the stopped flow
cell. The UV-vis spectra of all the compounds involved in the reaction
are shown in Supporting Information Figure SF3.
Kinetic Study of Reaction of 1-H with 2-NCPh. In
contrast to the relatively slow reaction of 1-H with 2 (vide
infra), reaction of 1-H with 2-NCPh was found to be too
rapid to study by conventional kinetic methods. Stopped flow
(34) Poli, R. Acc. Chem. Res. 1997, 30, 494.
(35) Nolan, S. P.; Lopez de la Vega, R.; Mukerjee, S. L.; Gonzalez, A. A.;
Zhang, K.; Hoff, C. D. Polyhedron l988, 7, l49l.
(36) Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1989, 111, 6711. (b) Tilset,
M.; Parker, V. D. J. Am. Chem. Soc. 1990, 112, 2843.
(37) McDonough, J. E.; Weir, J. J.; Sukcharoenphon, K.; Hoff, C. D.;
Kryatova, O. P.; Rybak-Akimova, E. V.; Scott, B.; Kubas, G. J.;
Stephens, F. H.; Mendiratta, A.; Cummins, C. C. J. Am. Chem. Soc.
2006, 128, 10295.
(38) Mukerjee, S. L.; Gonzalez, A. A.; Nolan, S. P.; Ju, T. D.; Lang, R. F.;
Hoff, C. D. Inorg. Chim. Acta 1995, 240, 175.
(39) Multiple Bonds between Metal Atoms, 3rd ed.; Cotton, F. A., Murillo,
C. A., Walton, R. A., Eds.; Springer: Berlin, 2005.
(40) (a) Estimate made based on a revised estimate of the enthalpy of
formation of H2CN (ref 40b) based on experimental data (ref 40c), as
well as enthalpies of formation for H and HCN taken from NIST
webbook. (b) Nesbitt, F. L.; Marston, G.; Stief, L. J.; Wickramaaratchi,
W.; Tao, R. B.; Klemm, J. Phys. Chem. 1991, 95, 7613. (c) Cowles,
D. C.; Travers, M. J.; Frueh, J. L.; Ellison, G. B. J. Chem. Phys. 1991,
94, 3517.
(41) Johnson, A. R.; Baraldo, L. M.; Cherry, J. P. F.; Tsai, Y. C.; Cummins,
C. C.; Kryatov, S. V.; Rybak-Akimova, E. V.; Capps, K. B.; Hoff,
C. D.; Haar, C. M.; Nolan, S. P. J. Am. Chem. Soc. 2001, 123, 7271.
(42) Landry, V. K.; Pang, K.; Quan, S. M.; Parkin, G. Dalton Trans. 2007,
820.
Figure 6. Cross section of the absorbance changes at 580 nm as a function
of time for the reaction: 2-NCPh + 1-H (T ) -25 °C).
kinetic methods were used to study the rate of reaction 11
in toluene solution over the temperature range of -10 to
-30 °C. The reaction was studied by combination of a
mixture of 1-H and a large excess of PhCN and a solution
of 2. Typical spectral changes observed are shown in Figure
5.
Under the conditions used, the reaction of 2 with excess
PhCN to form 2-NCPh was too fast to be observed. The
good fit to kinetic data (Figure 6) and presence of a clean
isosbestic point at ∼550 nm (Figure 5) argue for a clean
reaction process which was found to be first order in both
1-H and the in situ generated 2-NCPh (Figure SF2). In
contrast to the slow reaction of 1-H and 2, in which the
(43) Kryatova, O. P.; Rybak-Akimova, E. V.; Mendiratta, A.; Tsai, Y. C.;
Cummins, C. C.; McDonough, J. E.; Hoff, C. D. Manuscript in
preparation.
9386 Inorganic Chemistry, Vol. 47, No. 20, 2008

H Atom Transfer
activation parameters ∆Hq ) 11.9 ( 0.4 kcal mol-1, ∆Sq )
-2.7 ( 1.2 cal K-1 mol-1 and ∆Hq ) 13.1 ( 0.4 kcal mol-1,
∆Sq ) 1.1 ( 1.6 cal K-1 mol-1 for 1-H and 1-D, respectively,
were obtained. The uncertainties assigned to the ∆Hq and
∆Sq values were twice the calculated errors in the slope and
intercept of the Eyring plot.
The activation parameters obtained in this work can be
directly compared with those obtained previously for the
reaction of 2-NCPh with other reagents1,25,43 (Table 1).
There is a correlation between ∆Hq and the strength of
the bond broken. Surprisingly for a bimolecular reaction, the
apparent activation entropy calculated from the Eyring plot
for reaction 11 is very small. The rates and activation
parameters for reaction of 1-H with different organic radicals
(reaction 14) have been previously reported by the Franz48
and Norton49 groups.
Figure 7. Eyring plot of (Ph(X)CdN)Mo(N[t-Bu]Ar)3 formation (X ) H,
D).
Table 1. Activation Parameters for Reaction of (PhCN)Mo(N[t-Bu]Ar)3
with Substrate and BDE of Bond Broken
BDE
substrate
BzOOBz
PhTeTePh
PhSSPh
∆Hq (kcal mol-1
)
∆Sq (cal mol-1 K-1
)
(kcal mol-1
)
·R + HMo(CO)3(Cp) f RH + · Mo(CO)3(Cp) (14)
2.0
3.7
7.5
-31
-29
-26
-2.7
19
33
46
For R ) hex-5-enyl, Franz and co-workers found k ) 4.2
× 108 M-1 s-1 at T ) 298 K, ∆Hq ) 0.47 kcal mol-1, and
∆Sq ) -17.5 cal mol-1 K-1, and for R ) Ph3C · , Norton et
al. determined k ) 514 M-1 s-1 at T ) 298 K, ∆Hq ) 5.76
kcal mol-1, and ∆Sq ) -26.8 cal mol-1 K-1. Interaction of
a free organic radical with the molybdenum hydride has a
low enthalpy of activation and, even for the relatively
sterically unencumbered hex-5-enyl radical, a significant
negative entropy of activation.
The higher activation enthalpy and near-zero activation
entropy observed in reactivity of the bound nitrile radical
in 2-NCPh may be rationalized in terms of a multistep
process, such as a two-step process shown in reaction eq
15.
HMo(CO)3(Cp)
11.9
66,10,27a 7036
complex 1-2 was observed, no bands attributable to a
significant concentration of an intermediate complex were
found for reaction 11 in either UV-vis or FTIR spectra.
Kinetic parameters were determined by monitoring the
spectral changes in the 400-700 nm range under pseudo-
first-order conditions (excess of 1-H or 1-D), yielding
essentially wavelength-independent rate constants. Figure
6 shows a typical cross section of the absorbance changes
at 580 nm as a function of time for the reaction studied;
an excellent fit to a single-exponential rate law was
obtained.
Because of the low stability of dilute solutions of 2,
different batches of freshly prepared samples of 2 were
used, and reproducible data for this highly air-sensitive
reaction system could be obtained. A complete list of all
individual data obtained for kobs as a function of temper-
ature for reaction 11 is collected in Supporting Information
Tables ST1 (1-H) and ST2 (1-D).
In contrast to studies of reaction 8 (vide infra), reaction
11 was found to show a normal kinetic isotope effect with
kH/kD ≈ 1.6 with slight variation as a function of temperature.
A kinetic isotope effect for 1-H/1-D of 3.38 has been
calculated by semiclassical theory.44 However, maximum
isotope effects are expected for symmetric linear transition
states, and they are generally lower when steric constraints
prevent linearity45 such as in our current case. A kinetic
isotope effect of 1.8, similar to that measured here, has been
determined for the H/D atom transfer from HW(CO)3(Cp)/
DW(CO)3(Cp) to 4-phenyl-3-penten-1-yl radical,46 and the
same result was obtained for the H-/D- transfer from
HMo(CO)3(Cp)/DMo(CO)3(Cp) to Ph3C+.47 The slower rate
of reaction for 1-D indicates that cleavage of the (H/D)-Mo
bond is important in the transition state. Eyring plots for both
1-H and 1-D are shown in Figure 7, from which the derived
The first step may involve a pre-equilibrium intramo-
lecular rearrangement of 2-NCPh leading to a more
reactive form of the nitrile complex, in which there is some
“loosening” of the molybdenum to nitrile bond. This may
be necessary to break, in the second step, the relatively
strong H-Mo bond of 1-H. The higher than expected
enthalpy of activation and less negative entropy of
activation could result from the fact that the derived
(44) Bullock, R. M. In Transition Metal Hydrides; Dedieu, A., Ed.; VCH:
New York, 1992; Chapter 8.
Inorganic Chemistry, Vol. 47, No. 20, 2008 9387

Temprado et al.
activation parameters correspond to the sum of the
activation parameters for the pre-equilibrium (k1/k-1) and
H-atom transfer (k2) steps.
ratios showed similar behavior with 1-2 formation favored
as this ratio increased.
(
[
]
)
Mo N t-Bu Ar 3
(
[
]
)
(
) (
)
An alternative mechanism to that shown in reaction 15
would involve proton coupled electron transfer50 as shown
in eq 16.
+
h Ar t-Bu N 3Mo-Mo CO 3 Cp
[
(
) (
)]
2
1/2 Mo CO 3 Cp
(17)
All attempts to isolate crystalline 1-2 failed because it
was found to be stable only in the presence of a large excess
of 2. Its formulation is based on spectroscopic evidence.
Despite steric crowding around the Mo atom in 2, precedent
for formation of metal-metal-bonded complexes of the type
proposed here exists because a stable derivative with an
established metal-metal bond has been proven in the
previously prepared complex (Ar[t-Bu]N)3Mo-Sn(C6H5)3.15
A plausible mechanism for the reaction of 1-H and 2 is
outlined in Scheme 2. Full resolution of this kinetic scheme
is beyond the scope of the current work.
Information regarding the H atom transfer step could be
obtained under conditions of excess 2, where it would be
expected that ·Mo(CO)3(Cp) produced in reaction 17 should
be trapped. Under pseudo-first-order conditions of 5- to 20-
fold excess 2, plots of ln[1-H] versus time yielded linear
plots through 2-3 half-lives. The reaction was observed to
obey the rate law d[P]/dt ) kobs[2][1-H] with kobs ) 0.09 (
0.01 M-1 s-1 at 1.3 °C and 0.26 ( 0.04 M-1 s-1 at 17 °C.
These data yield an estimate of Ea ≈ 10.7 kcal mol-1 for
the H atom transfer. The activation energy for binding of
ligands to the sterically congested ground-state quartet metal
site of 2 may make up a significant portion of this activation
energy. For example, binding of AdNtC to 2 was found to
have ∆Hq ) 5.5 ( 0.5 kcal mol-1.54 Furthermore, thermo-
chemical estimates for the bond dissociation energy of
H-Mo in both 1-H and 2-H discussed earlier show that the
bond in 1-H is about 4-8 kcal mol-1 stronger than that in
2-H. Thus, the Ea of ∼10.7 kcal mol-1 is close to the sum
of enthalpy of activation for bringing a ligand into the
coordination sphere of 2 plus the energy difference between
the two Mo-H bonds.
Comparison to studies done at 17 °C of the rate of reaction
of 1-D yielded kH/kD ) 1.00 ( 0.05 as shown in Supporting
Information (Figure SF4). The low or zero kinetic isotope
effect is consistent with an early transition state in which
formation of the adduct (Ar[t-Bu]N)3Mo · · · HMo(CO)3(Cp)
is the rate limiting step.
The reaction of 2 and 1-H is much slower than that
observed for 2-NCPh and 1-H. This may be because of the
greater accessibility at a site removed from the crowded metal
center in the latter reaction. A similar rate acceleration was
previously observed1 for the reaction of benzoyl peroxide
with 2-NCPh compared to 2.
The first step would involve initial H+ transfer from 1-H
to 2-NCPh, followed by electron transfer to ultimately yield
products. The normal kinetic isotope effect found would
appear to argue against rate-determining electron transfer but
does not rule out a proton-coupled electron transfer pathway.
Work by Mayer,51 Abu-Omar,52 and Bullock53 has shown
that, in some cases, small changes in reactants and conditions
can cause a shift in mechanism. It is clear from the data in
Table 1 that there is a dramatic difference in the reactivity
of 1-H with 2-NCPh compared to the other reagents.
Additional work aimed at discerning if reaction occurs by
direct H atom transfer as shown in eq 15 or proton-mediated
electron transfer as shown in eq 16 is planned.
Kinetic Studies of reaction of 1-H with 2. Qualitative
spectroscopic studies of the reaction of 1-H and 2 were
investigated by FTIR spectroscopy and gave evidence for
buildup and decay of an intermediate complex proposed to
be 1-2 (Figure 8). The concentration of the intermediate
complex was found to increase with increasing [2]. The
intermediate showed an identical infrared spectroscopic
signature to that observed in reaction 17 between 12 and 2
as shown in Figure 9. Reactions performed at different 2/12
(45) More O’Ferral, R. A. J. Chem. Soc. B 1970, 785.
(46) Bullock, R. M.; Samsel, E. G. J. Am. Chem. Soc. 1990, 112, 6886.
(47) Cheng, T. -Y.; Bullock, R. M. J. Am. Chem. Soc. 1999, 121, 3150.
(48) Franz, J. A.; Linehan, J. C.; Birnbaum, J. C.; Hicks, K. W.; Alnajjar,
M. S. J. Am. Chem. Soc. 1999, 121, 9824.
(49) Eisenberg, D. C.; Lawrie, C. J. C.; Moody, A. E.; Norton, J. R. J. Am.
Chem. Soc. 1991, 113, 4888.
(50) Huynh, M. H. V.; Meyer, T. J. Chem. ReV. 2007, 107, 5004.
(51) Mayer, J. M.; Rhile, I. J. Biochim. Biophys. Acta 2004, 1655, 51.
(52) Zdilla, M. J.; Dexheimer, J. L.; Abu-Omar, M. M. J. Am. Chem. Soc.
2007, 129, 11505.
Despite the abundant reports in the literature regarding
hydrogen atom transfer from metal carbonyl hydrides to
carbon-based radicals46,48,49 and to olefins,45,55 works related
to hydrogen atom transfer between metal complexes56 are
(53) For a review comparing hydride, proton and hydrogen atom transfer
reactions of metal hydrides, see: Bullock, R. M. Comments Inorg.
Chem. 1991, 12, 1.
(54) Stephens, F. H.; Figueroa, J. S.; Cummins, C. C.; Kryatova, O. P.;
Kryatov, S. V.; Rybak-Akimova, E. V.; McDonough, J. E.; Hoff, C. D.
Organometallics 2004, 23, 3126.
9388 Inorganic Chemistry, Vol. 47, No. 20, 2008

H Atom Transfer
strong bonds found previously for the Mo(N[t-Bu]Ar)3
system. In contrast, the C-H bond of (Ph(H)CdN)Mo(N[t-
Bu]Ar)3 of ∼90 kcal mol-1 and the metal to ketimide bond
strength (Ph(H)CdN)Mo(N[t-Bu]Ar)3 of ∼71 kcal mol-1 are
both high. The combination of these factors make benzoni-
trile insertion into the XMo(N[t-Bu]Ar)3 bond thermody-
namically favorable for X ) H in contrast to cases where
nitrile extrusion is observed (X ) OC(O)Ph, TePh, SeC6F5,
SC6F5).
The transfer of an H atom from 1-H to 2 is uphill by
4-8 kcal mol-1. The driving force for atom transfer is
the formation of a Mo-Mo bond in the dimerization of
the metal radical 1. The reaction proceeds with initial
trapping of · Mo(CO)3(Cp) by 2 to form a complex
proposed as (Ar[t-Bu]N)3Mo-Mo(CO)3(Cp) which ulti-
mately goes on to form 12. The rate constant for this
reaction at 17 °C of 0.26 M-1 s-1 is nearly 4 orders of
Figure 8. Reaction of 1-H and 2. The bands at 2024 and 1935 cm-1 are
from the starting 1-H; those at 2015 (w) 1956, 1912 cm-1 are from the
final product 12, and an intermediate assigned as 1-2 has bands that
rise and then shrink at 1942 and 1862 cm-1. Qualitative study of the
reaction products performed at room temperature over a period of three
hours.
magnitude slower than that for the H atom transfer from
57
1-H to 2-NCPh of 1.8 × 103 M-1 s-1
.
However the
reaction rate is nearly 5 orders of magnitude slower than
the value of 3.9 × 108 M-1 s-1 for H atom transfer from
1-H to the hex-5-enyl radical.58
Acknowledgment. The authors are grateful for support
of this work by the National Science Foundation Grants CHE
0615743, 0719157, and 0750140, the Department of Energy
Grant DE-FG02-06ER15799, and the Spanish Ministry of
Education and Science and the Fulbright commission under
Fellowship FU-2006-1738. Stopped-flow instrumentation at
Tufts was supported by the NSF CRIF Grant (CHE-
0639138). We wish to thank John J. Curley for crystal-
lographic assistance.
Figure 9. Reaction of 2 (0.098 M) and 12 (0.015 M) in toluene solution at
room temperature. The bands at 1943 and 1863 cm-1 assigned to (Ar[t-
Bu]N)3Mo-Mo(CO)3(Cp) grow with time, while those assigned to 12 at
2015 (w), 1956, 1912 cm-1 shrink. Spectra obtained starting at 2, 15, 25,
95, and 960 min.
Scheme 2. Proposed Mechanism for the Reaction of 1-H and 2
Supporting Information Available: NMR spectra showing the
nitrile insertion in the Mo-H bond of 2-H, concentration depen-
dence from 1-H for formation of 1-NC(Ph)H, UV-vis spectra for
12, 2-NCPh, and 2-NC(H)Ph, a complete list of all individual data
obtained for kobs as a function of temperature for reaction 11, and
crystallographic information files (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
IC800945M
(55) Choi, J.; Tang, L.; Norton, J. R. J. Am. Chem. Soc. 2007, 129, 234,
and references reported therein.
(56) (a) Nappa, M. J.; Santi, R.; Halpern, J. Organometallics 1985, 4, 34.
(b) Kiss, G.; Zhang, K.; Mukerjee, S. L.; Hoff, C. D. J. Am. Chem.
Soc. 1990, 112, 5657. (c) Song, J.-S.; Bullock, R. M.; Creutz, C. J. Am.
Chem. Soc. 1991, 113, 9863.
scarce, and no detailed studies of kinetic isotope effects or
activation parameters could be found for comparison to our
results.
(57) The value at T )17 °C was obtained by extrapolation of data in Figure
7 of this paper.
(58) This value was extrapolated using the activation parameters reported
by Franz et al, see ref 10. Data at T ) 17 °C was chosen for
comparison since extrapolation of data for k1 of reaction 17 in this
work based on Ea alone, was not deemed valid. The data at 17 °C are
also the closest to the literature values. This comparison illustrates
the wide range of H atom transfer rates from HMo(CO)3(Cp).
Conclusions
The Mo-H bond dissociation energy in 2-H of ∼62 kcal
mol-1 was lower than expected on the basis of the generally
Inorganic Chemistry, Vol. 47, No. 20, 2008 9389