The syntheses and structures of coordinatively unsaturated aryloxy-hydride complexes of molybdenum, Mo(PMe3)4(OAr)H: Reversible C-H bond activation and comparison with their tungsten analogues

Article abstract of DOI:10.1016/S0022-328X(02)01306-2

Mo(PMe₃)₆ reacts with ArOH (Ar = 2,4,6-C₆H₂Me₃, 2,6-C₆H₃Pr₂ⁱ) to give coordinatively unsaturated aryloxy-hydride derivatives Mo(PMe₃)₄(OAr)H. The formation of Mo(PMe₃)₄(OAr)H is in marked contrast to the cyclometalated products of C-H bond activation that are obtained for the corresponding tungsten system. Deuterium labeling and magnetization transfer studies, however, demonstrate that the coordinatively unsaturated molybdenum complexes Mo(PMe₃)₄(OAr)H are in fact kinetically capable of intramolecular oxidative addition of a C-H bond of the ortho substituents to yield cyclometalated derivatives that are thermodynamically unstable with respect to the aryloxy-hydride derivatives.

Full text of DOI:10.1016/S0022-328X(02)01306-2

Journal of Organometallic Chemistry 652 (2002) 37ꢁ49
/
www.elsevier.com/locate/jorganchem
The syntheses and structures of coordinatively unsaturated aryloxy-
hydride complexes of molybdenum, Mo(PMe3)4(OAr)H: reversible
CÃ
/
H bond activation and comparison with their tungsten analogues
Tony Hascall, Vincent J. Murphy, Kevin E. Janak, Gerard Parkin *
Department of Chemistry, Columbia University, 3000 Broadway (Box 3115), New York, NY 10027, USA
Received 13 November 2001
Abstract
Mo(PMe3)6 reacts with ArOH (Arꢀ
Mo(PMe3)4(OAr)H. The formation of Mo(PMe3)4(OAr)H is in marked contrast to the cyclometalated products of CÃ
/
2,4,6-C6H2Me3, 2,6-C6H3Pri2) to give coordinatively unsaturated aryloxy-hydride derivatives
H bond
/
activation that are obtained for the corresponding tungsten system. Deuterium labeling and magnetization transfer studies,
however, demonstrate that the coordinatively unsaturated molybdenum complexes Mo(PMe3)4(OAr)H are in fact kinetically
capable of intramolecular oxidative addition of a CÃ
/
H bond of the ortho substituents to yield cyclometalated derivatives that are
thermodynamically unstable with respect to the aryloxy-hydride derivatives. # 2002 Published by Elsevier Science B.V.
Keywords: Molybdenum; Tungsten; Trimethylphosphine; Aryloxy; Hydride; Carbonꢁhydrogen bond cleavage
/
1. Introduction
The reactivity of carbonꢁ
[5]. Significantly, the chemistry of these molybdenum
and tungsten systems proved to be quite distinct with
H bond activation
/
hydrogen bonds towards
respect to their ability to achieve CÃ
/
[6].
transition metals continues to be an important area of
research due to its relevance to the selective functiona-
lization of hydrocarbons [1]. We are particularly inter-
ested in the chemistry of electron-rich transition metal
complexes due to their potential to react with a variety
of bonds by oxidative-addition. For example, previous
studies have demonstrated that W(PMe3)6 and
2. Results and discussion
2.1. Reactivity of Mo(PMe3)6 towards methanol: CÃ
/H
bond activation and the synthesis of the formaldehyde
complex Mo(PMe3)4H2(h2-CH2O)
W(PMe3)4(h2-CH2PMe2)H react with the carbonꢁ
/
hy-
drogen bonds of alcohols. Thus, W(PMe3)4(h2-
CH2PMe2)H reacts with MeOH to give an h2-formal-
dehyde complex [2], while phenol derivatives react to
give four- and five-membered oxametallacycles [3], as
illustrated in Schemes 1 and 2 [4]. Most noteworthy of
the latter reactions is the selectivity towards the forma-
tion of four-membered rings [3]. Since it is important to
expand our understanding of the factors that promote
The reaction of Mo(PMe3)6 with methanol in pentane
results in the oxidative addition of both an OÃH and CÃ
bond to give the formaldehyde complex
/
/
H
Mo(PMe3)4H2(h2-CH2O), as illustrated in Scheme 1,
analogous to the synthesis of the tungsten complex
W(PMe3)4H2(h2-CH2O) by reaction of W(PMe3)4(h2-
CH2PMe2)H with methanol [2]. A related Mo formal-
dehyde complex, (dppe)2MoH2(h2-CH2O), has recently
been reported to be formed from the decomposition of
(dppe)2MoH2(OMe)2 [7,8]. The formaldehyde ligand of
Mo(PMe3)4H2(h2-CH2O) is characterized by a singlet in
the 1H-NMR spectrum at d 3.51 and a triplet in the 13C-
CÃ
/
H bond activation, we sought to compare analogous
reactions of the molybdenum counterpart Mo(PMe3)6
* Corresponding author.
E-mail address: parkin@chem.columbia.edu (G. Parkin).
NMR spectrum at d 63.4 (1JCÃH
ꢀ159 Hz) [9]. Further-
/
0022-328X/02/$ - see front matter # 2002 Published by Elsevier Science B.V.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 3 0 6 - 2

38
T. Hascall et al. / Journal of Organometallic Chemistry 652 (2002) 37ꢁ49
/
Scheme 1.
more, the hydride ligands are observed as a quintet in
1
the H-NMR spectrum at d ꢂ
/
2.65 (2JPÃH
ꢀ38 Hz).
/
The molecular structure of Mo(PMe3)4H2(h2-CH2O)
has been determined by X-ray diffraction (Fig. 1),
confirming the h2-coordination mode of the formalde-
hyde ligand moiety. However, the formaldehyde and
hydride ligands are disordered so that detailed discus-
sion of the bonding is unwarranted.
Fig. 1. Molecular structure of Mo(PMe3)4H2(h2-CH2O). Only one of
the disordered conformations is shown.
aryl CÃ/H bonds of phenol. The molecular structure of
Mo(PMe3)2(OPh)4 has been determined by X-ray dif-
fraction, as illustrated in Fig. 2. Also in contrast to the
tungsten system, the formation of Mo(PMe3)2(OPh)4
involves elimination of H2. Precedence for such a
reaction is, nevertheless, provided by the observation
that W(PMePh2)4Cl2 reacts with phenol or p-cresol to
give W(PMePh2)2Cl2(OAr)2 and H2 [11].
2.2. Reactivity of Mo(PMe3)6 towards phenols:
aryloxide formation without CÃ/H bond activation
While the molybdenum and tungsten systems behave
similarly with respect to the reactions with methanol, the
reactions with phenols differ considerably. For example,
Mo(PMe3)6 reacts with phenol to give the orange
paramagnetic [10] tetrakis(phenoxide) complex Mo(P-
Me3)2(OPh)4 (Scheme 3), in contrast to the diamagnetic
yellow metallacycle W(PMe3)4H2(h2-OC6H4) observed
for the tungsten system (Scheme 2) [3]. Thus, in striking
In an effort to achieve CÃ/H bond activation by the
molybdenum system, it was considered essential to
prevent the formation of a tetrakis(aryloxy) derivative
Mo(PMe3)2(OAr)4 by using bulky substituents in the
ortho positions to restrict the reaction of Mo(PMe3)6 to
a single equivalent of the phenol. Indeed, Mo(PMe3)6
was observed to react with 2,4,6-trimethylphenol and
contrast to the tungsten system which achieves CÃH
bond activation, the molybdenum system is inert to the
/
Scheme 2.

T. Hascall et al. / Journal of Organometallic Chemistry 652 (2002) 37ꢁ
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39
Scheme 3.
Decisive characterization of the products as aryloxy-
hydride complexes, Mo(PMe3)4(OC6H2Me3)H and
Mo(PMe3)4(OC6H3Pri2)H, is provided by NMR spectro-
scopy. For example, the 1H-NMR spectra of Mo(P-
Me3)4(OC6H2Me3)H and Mo(PMe3)4(OC6H3Pri2)H
exhibit triplet of triplet resonances at
(2JPÃH 6.53 (2JPÃH
23 and 84 Hz) and ꢂ
H ligands, respectively. In
d
ꢂ
/
6.59
ꢀ
/
/
ꢀ/22 and 85
Hz) attributable to the MoÃ
/
addition to the integration of the 1H-NMR spectra,
evidence that the compounds are monohydrides, rather
than 18-electron trihydrides Mo(PMe3)4(OAr)H3, is
provided by comparison of the fully 31P{1H} and
selectively decoupled 31P{1HÃ
/
P(CH3)3} spectra. Speci-
fically, the latter spectra exhibit an additional doublet
coupling, thereby providing convincing evidence that
each complex contains only a single MoÃ
/H ligand.
Fig. 2. Molecular structure of Mo(PMe3)2(OPh)4. Selected bond
Additional characterization of Mo(PMe3)4(OC6H2-
Me3)H and Mo(PMe3)4(OC6H3Pri2)H is provided by
X-ray diffraction, with the molecular structures being
illustrated in Figs. 3 and 4. Neglecting the hydride
ligand, the coordination environment about molybde-
num is based on a trigonal bipyramid, with axial PMe3
ligands. The hydride ligands were observed in electron
density difference maps and are located close to the
equatorial plane and trans to the aryloxy substituent.
This trans arrangement of the aryloxy and hydride
ligands contrasts with the cis geometries observed for
the related 18-electron octahedral complexes Ru(P-
Me3)4(OC6H4Me)H [14] and [Ir(PMe3)4(OMe)H][PF6]
[16]. While the position of hydrogen atoms determined
by X-ray diffraction, particularly those bound to heavy
metals, is subject to considerable uncertainty, the
location of the hydride ligand trans to the aryloxy
group in Mo(PMe3)4(OAr)H is reasonable based on the
arrangement of the other ligands. To illustrate this
˚
lengths (A) and angles (8): MoÃ
O(2) 1.958(3); PÃMoÃO(1) 97.8(1), PÃ
O(2) 90.2(1), PÃMoÃP? 180.0; O(1)ÃMoÃ
94.2(1), O(1)ÃMoÃ(O1?) 180.0, O(2)ÃMoÃ
C(11) 14.3(3), MoÃO(2)ÃC(21) 137.4(3).
/
P 2.644(2), MoÃ
MoÃO(2) 85.8(1), O(1)Ã
P? 82.2(1), O(2)ÃMoÃ
(O2?) 180.0, MoÃO(1)Ã
/
O(1) 1.966(3), MoÃ
MoÃ
P?
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
2,6-diisopropylphenol to give the purple diamagnetic
complexes Mo(PMe3)4(OC6H2Me3)H and Mo(P-
Me3)4(OC6H3Pri2)H, respectively (Scheme 3). Mono-
nuclear aryloxy-hydride complexes of Mo are not
common and structurally characterized examples appear
to be limited to the 18-electron complexes [h6,h1-
C6H5C6H3-
(Ph)O]Mo(PMe3)2H [12] and [h6,h1-C6H5C6H3(Ph)O]-
Mo(PMePh2)2H [13], and the catecholate derivative
Mo(dppe)2H2(h2-O2C6H4) [7]. Related 18-electron octa-
hedral M(PMe3)4(OAr)H complexes are also known for
ruthenium [14,15].

40
T. Hascall et al. / Journal of Organometallic Chemistry 652 (2002) 37ꢁ49
/
Fig. 3. Molecular structure of Mo(PMe3)4(OC6H2Me3)H. Selected
˚
bond lengths (A) and angles (8): MoÃ
MoÃP(3) 2.455(1), MoÃP(4) 2.473(1), MoÃ
109.3(1), P(1)ÃMoÃP(3) 88.9(1), P(2)ÃMoÃ
P(4) 97.4(1), P(2)ÃMoÃP(4) 101.4(1), P(3)ÃMoÃ
MoÃO 121.2(1), P(2)ÃMoÃO 129.0(1), P(3)ÃMoÃ
MoÃO 79.6(1), MoÃOÃC(1) 159.7(3).
/
P(1) 2.337(1), MoÃ
O 2.065(3); P(1)Ã
P(3) 88.4(1), P(1)Ã
P(4) 165.7(1), P(1)Ã
O 86.1(1), P(4)Ã
/
P(2) 2.348(1),
MoÃP(2)
MoÃ
Fig. 5. Comparison of the molecular structures of Mo(P-
Me3)4(OC6H2Me3)H and Ru(PMe3)4(OC6H4Me)H (for which the
hydride ligand was not located) emphasizing the trans and cis
aryloxy-hydride dispositions, respectively.
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
thiolate complex Mo(dppe)2(SC6H2Pri3)H which has
also been reported to adopt a cis-disposition of hydride
and thiolate ligands, but the geometry is grossly
distorted from octahedral [17].
The diamagnetic nature of the 16-electron d4 Mo(P-
Me3)4(OAr)H species is in contrast to related paramag-
netic octahedral trans-Mo(PMe3)4X2 (XꢀCl, Br, I)
/
complexes. A principal distinction between these two
sets of complexes is that the four p-symmetry orbitals of
the two trans-X atoms in M(PMe3)4X2 belong to two
sets of E symmetry orbitals (Fig. 6) [18]. As a
consequence of this symmetry, the halide p-orbitals are
required to interact with a pair of non-bonding metal
orbitals (of the ‘t2g set’ in an idealized octahedral
geometry), with the result that a d4 metal center gives
rise to a triplet. In contrast to trans-Mo(PMe3)4X2, the
p-donor orbitals of a single bent aryloxide ligand of
Mo(PMe3)4(OAr)H are not degenerate, and so it is
permissible to interact with a single orbital of the ‘t2g set’
(Fig. 6) [19]. The result of this selective interaction with
a single metal orbital is that the four non-bonding
electrons fully occupy two d-orbitals and the compound
is diamagnetic.
Fig. 4. Molecular structure of Mo(PMe3)4(OC6H3Pri2)H. Selected
˚
bond lengths (A) and angles (8): MoÃ
/
P(1) 2.460(3), MoÃ
/P(2)
2.464(3), MoÃ
168.4(1), P(1)ÃMoÃ
O 79.1(2), P(2)ÃMoÃ
108.9(1), MoÃOÃC(1) 145.9(6).
/
P(3) 2.338(2), MoÃ
P(3) 90.4(1), P(2)Ã
O 89.3(2), P(3)ÃMoÃ
/
O
2.072(6); P(1)Ã
MoÃP(3) 96.3(1), P(1)Ã
O 124.9(1), P(3)ÃMoÃ
/
MoÃ
/
P(2)
/
/
/
/
/
MoÃ
/
/
/
/
/
/
/P(3?)
/
/
point, the coordination environments of the metal
centers in Mo(PMe3)4(OC6H2Me3)H and Ru(P-
Me3)4(OC6H4Me)H (in which the hydride ligand was
not located) [14a,14b] are compared in Fig. 5, from
which it is evident that the coordination site for the
hydride ligand is trans to the aryloxy ligand in the
former complex, and cis in the latter species. In addition
to the structural evidence, spectroscopic evidence that
the hydride ligands of Mo(PMe3)4(OC6H2Me3)H and
Mo(PMe3)4(OC6H3Pri2)H are best described as trans to
an aryloxy rather than a PMe3 ligand is provided by the
observation that the hydride signals in the 1H-NMR
Evidence that lone pair p donation from oxygen is
operative in Mo(PMe3)4(OAr)H is provided by the
observation that the MoÃ/OAr bond lengths are inter-
mediate between those in the formally 14-electron
complex Mo(PMe3)2(OPh)4 and the 18-electron deriva-
tives, [h6,h1-C6H5C6H3(Ph)O]Mo(PMe3)2H and [h6,h1-
˚
C6H5C6H3(Ph)O]Mo(PMePh2)2H [2.164(5) A] [13] (Ta-
ble 1). Thus, the MoÃ
/
O bond length increases with
dp donation from
electron count, presumably as ppꢁ
/
2
spectra do not exhibit the large trans JPÃH couplings of
oxygen diminishes with increasing saturation of the
metal center. Indeed, Rothwell has previously examined
a series of tungsten chloro aryloxy complexes and found
ca. 100 and 145 Hz that are observed for Ru(P-
Me3)4(OC6H4Me)H and [Ir(PMe3)4(OMe)H][PF6], re-
spectively. It is worth noting that the structures of
Mo(PMe3)4(OAr)H are different from that of the
a good correlation between electron count and WÃ
/
O
˚
bond length [20a]. The distances varied from 1.84 A in

T. Hascall et al. / Journal of Organometallic Chemistry 652 (2002) 37ꢁ
/49
41
Fig. 6. Rationalization of the paramagnetic and diamagnetic natures of Mo(PMe3)4X2 and Mo(PMe3)4(OAr)H.
˚
12-electron cis-WCl4(OC6H3Ph2)2 to 2.14 A in 18-
electron
Rothwell concluded that the WÃ
deuterium into the ortho-methyl groups of the mesityl
ring may be rationalized by a mechanism that involves
the reversible formation of a five-membered oxametalla-
[h6,h1-C6H5C6H3(Ph)O]W(dmpe)Cl
and
O bond length appears
/
cycle-dihydride complex via sp3 CÃ
/H bond activation,
to be an excellent probe of the electronic demands of the
metal center [20a]. Bond angles at oxygen, however,
cannot alone be used reliably as an indicator of p-
bonding, since they are also influenced by steric inter-
actions [20].
thereby permitting site exchange, as illustrated in
Scheme 5 [22].
Reversible CÃ
/
H bond activation within Mo(P-
Me3)4(OAr)H is further demonstrated by 1H-NMR
magnetization transfer experiments [23] which demon-
strate that exchange between the hydride and ortho-
methyl groups of Mo(PMe3)4(OC6H2Me3)H is rapid on
the NMR time-scale [24]. The magnetization transfer
experiment also permits determination of the barrier for
2.3. Reversible CÃ
/
H bond activation by molybdenum in
the aryloxy-hydride complexes Mo(PMe3)4(OAr)H
2.3.1. Kinetics of CÃ/H bond activation
oxidative addition of the methyl CÃ
molybdenum center (Table 2 and Fig. 7): DH%ꢀ
kcal molꢂ1 and DS%ꢀ
8(2) e.u. [25].
/
H bond to the
The isolation of the molybdenum aryloxy-hydride
complexes Mo(PMe3)4(OAr)H is of particular signifi-
cance since the tungsten analogues were postulated to be
/
13.0(4)
/
ꢂ
/
Magnetization transfer studies were also performed
on Mo(PMe3)4(OC6H3Pri2)H, demonstrating exchange
between both the methyl and methine protons of the
isopropyl groups and the hydride group, indicating that
the intermediates responsible for the CÃ
/
H bond activa-
tion reactions in the corresponding tungsten system. For
example, [W(PMe3)4(OC6H2Me3)H] was proposed to be
the intermediate that underwent intramolecular oxida-
both types of CÃ/H bonds reversibly add to the metal
tive-addition of one of the ortho-methyl CÃ
/
H bonds to
give the five-membered oxametallacycle W(PMe3)4[h2-
OC6H2Me2(CH2)]H2.
(Scheme 6). However, because of the competing ex-
change mechanisms, the analysis of the data to obtain
quantitative rate constant information is complex.
Nevertheless, due to the fact that it was necessary to
heat the sample to 340 K in order to observe magnetiza-
tion transfer, it is evident that the barrier for oxidative
Although the 18-electron cyclometalated molybde-
num complexes were not detected spectroscopically,
evidence for their existence in equilibrium with Mo(P-
Me3)4(OAr)H is provided by deuterium labeling and
magnetization transfer studies. For example, scrambling
of deuterium into the ortho-methyl groups of the mesityl
ring is observed in the reaction of Mo(PMe3)6 with
2,4,6-Me3C6H2OD (Scheme 4) [21]. Incorporation of
addition of both types of the isopropyl CÃ
/
H bonds is
greater than that for the ortho-methyl groups of
Mo(PMe3)4(OC6H2Me3), for which magnetization
transfer was detected as low as 250 K.
Table 1
MoÃ
/
O bond lengths and MoÃ
/
OÃC bond angles as a function of electron count
/
˚
O) (A)
Electron count
d(MoÃ
/
MoÃ
/
OÃ/C (8)
References
Mo(PMe3)2(OPh)4
Mo(PMe3)4(OC6H2Me3)H
Mo(PMe3)4(OC6H3Pri2)H
[h6,h1-C6H5C6H3(Ph)O]Mo(PMe3)2H
14
16
16
18
1.958(3), 1.966(3)
2.065(3)
2.072(6)
137.4(3), 141.3(3)
159.7(3)
145.9(6)
This work
This work
This work
a
2.187(3)
118.3(3)
a
T. Hascall, G. Parkin, unpublished results.

42
T. Hascall et al. / Journal of Organometallic Chemistry 652 (2002) 37ꢁ49
/
Scheme 4.
Table 2
cally capable of undergoing intramolecular CÃ
activation of the aryl substituents. As such, the inability
to isolate the product of CÃH bond activation implies
that the process is not thermodynamically favored for
molybdenum. The ability to isolate molybdenum species
that correspond to reactive intermediates for tungsten is
presumably a consequence of the fact that a second row
transition metal typically forms weaker bonds to carbon
and hydrogen than does the corresponding third row
metal [26]. Consequently, the energy required to break
/
H bond
Rate constants for intramolecular CÃH bond activation of Mo(P-
Me3)4(OC6H2Me3)H
/
/
Temperature (K)
k (sꢂ1
)
266
272
279
284
290
303
313
1.6
3.1
5.4
8.7
13.1
32.4
90.5
the CÃ
/
H bond is not compensated by the formation of
H bonds [27]. Thus, for molybdenum,
MoÃC and MoÃ
/
/
the equilibrium between the aryloxy derivative and its
cyclometalated counterpart lies in favor of the former
species, while for tungsten the latter prevails (Scheme 7).
The relative energetics of these two equilibria resembles
that of the cyclometalation equilibria involving
M(PMe3)6 and M(PMe3)4(h2-CH2PMe2)H, for which
the equilibrium constant for the tungsten system is a
factor of ca. 2ꢃ
103 greater than that for molybdenum
/
at 30 8C (Scheme 7) [5a,28].
In order to evaluate further the notion that the
difference in the molybdenum and tungsten systems
may be rationalized by the respective bond energies, a
knowledge of their values is essential. Experimental data
on metalꢁ
are, however, relatively scarce [26]. Nevertheless, one
system for which MÃH and MÃC bond energies have
been measured for both Mo and W is Cp2MH2 and
Cp2M(CH3)2 [26b,29]. The bond energies for these
complexes are listed in Table 3, from which it can be
/
carbon and metalꢁ
/
hydrogen bond strengths
Fig. 7. Eyring plot for oxidative-addition of an ortho methyl CÃ
/H
/
/
bond in Mo(PMe3)4(OC6H2Me3)H.
2.3.2. Thermodynamics of CÃ
/
H bond activation
The above deuterium labeling and magnetization
transfer studies indicate that the six-coordinate ary-
loxy-hydride complexes Mo(PMe3)4(OAr)H are kineti-
seen that the sum of the WÃ
strengths is considerably greater than that of Mo, by 26
kcal molꢂ1. Thus, if the differences in MÃ
H and MÃ
/
H and WÃ
/
CH3 bond
/
/C
Scheme 5.

T. Hascall et al. / Journal of Organometallic Chemistry 652 (2002) 37ꢁ
/49
43
Table 3
MÃH and MÃ
Mo, W)
/
/
C bond energies in Cp2MH2 and Cp2M(CH3)2 (Mꢀ
/
a
Bond energy (kcal molꢂ1
)
Mo
W
D(MÃ
D(MÃ
D(MÃ
/
C)
61
40
74
53
/H)
/H)ꢄ
/
D(MÃ/C)
101
127
a
Data taken from: J.A. Martinho Simoes, J.L. Beauchamp, Chem.
˜
688; M.J. Calhorda, A.R. Dias, M.E. Minas da
Piedade, M.S. Salema, J.A. Martinho Simoes, Organometallics 6
Rev. 90 (1990) 629ꢁ
/
˜
738; A.R. Dias, J.A. Martinho Simoes, Polyhedron 7
(1987) 734ꢁ
/
˜
(1988) 1531ꢁ/1544.
M(PMe3)4H2[h2-OC6H2Me2(CH2)] (Mꢀ
/Mo, W). The
optimized structures of M(PMe3)4(OC6H2Me3)H and
M(PMe3)4H2(h2-OC6H2Me2CH2) are similar for each
metal and are comparable to the experimental structures
of Mo(PMe3)4(OC6H2Me3)H and W(PMe3)4H2[h2-
OC6H2Me2(CH2)]. Selected metrical data for the calcu-
lated structures are given in Tables 4 and 5, along with
experimental data for comparison. From these data, it is
evident that there is reasonable agreement between the
calculated and experimental structures, although the
DFT calculations systematically overestimate the
Scheme 6.
metalꢁligand bond lengths, as has been previously
/
observed [30]. The relative energies [31] of the two
isomers for each metal are given in Table 6, demonstrat-
ing that the calculations agree with the experimental
observations that the aryloxy-hydride species is favored
for Mo while the CÃ/H bond activated isomer predomi-
nates for tungsten. In each case, the preferred isomer is
favored by ca. 6 kcal molꢂ1. Ignoring entropy changes,
such an energy difference corresponds to an equilibrium
constant of ca. 2ꢃ
104 at 298 K, a value sufficiently
large to explain why the disfavored isomers have not
/
been observed spectroscopically for either system. If the
strength of the ortho CÃ
taken to be the same as the methyl CÃ
toluene, 88 kcal molꢂ1 [32], then the calculated sum of
H bonds formed by the
/
H bond of the aryloxy ligand is
/H bond in
the energies of the MÃ
/
C and MÃ
/
cyclometallation of M(PMe3)4(OC6H2Me3)H are 82 and
94 kcal molꢂ1 for Mo and W, respectively [33].
Table 4
Selected calculated metrical data for M(PMe3)4(OC6H2Me3)H (Mꢀ
/
Scheme 7.
Mo, W); experimental values for Mo(PMe3)4(OC6H2Me3)H given in
parentheses
bond energies for the metallocene fragments translate to
the Mo and W systems described here, intramolecular
Mo
W
CÃ
/
H bond activation formation would be predicted to
be thermodynamically more favored for the tungsten
˚
O (A)
MÃ
MÃ
MÃ
MÃ
/
2.118 (2.065)
150.0 (159.7)
1.743
2.073
151.5
1.751
2.530
2.529
2.416
2.417
/
OÃ
/
C (8)
˚
H (A)
system.
/
˚
Pax (A)
/
2.530 (2.455)
2.549 (2.473)
2.419 (2.337)
2.418 (2.348)
To provide a more quantitative evaluation and
understanding of the energetics of these CÃ
activation reactions, we have performed DFT (B3LYP)
calculations on M(PMe3)4(OC6H2Me3)H and
/H bond
˚
Peq (A)
MÃ
/

44
T. Hascall et al. / Journal of Organometallic Chemistry 652 (2002) 37ꢁ49
/
Table 5
Selected calculated metrical data for M(PMe3)4H2[h2-OC6H2-
2.4. Oxidative addition of H2 to
Mo(PMe3)4(OC6H2Me3)H and reductive elimination of
ArOH
Me2(CH2)] (Mꢀ
Mo, W); experimental values for W(PMe3)4H2[h2-
OC6H2Me2(CH2)] given in parentheses
/
Mo
W
A further distinction between the molybdenum and
tungsten systems is that the tungsten compound
W(PMe3)4H2[h2-OC6H2Me2(CH2)] is stable to H2 at
110 8C [3], whereas at 80 8C Mo(PMe3)4(OC6H2-
Me3)H liberates the phenol C6H2Me3OH and forms
˚
MÃ
MÃ
MÃ
/
O (A)
2.195
2.333
1.720
1.726
2.578
2.571
2.582
2.495
2.176 (2.145)
2.327 (2.290)
1.738
˚
/
C (A)
˚
H (A)
/
1.732
˚
Pax (A)
MÃ
/
2.559 (2.494)
2.571 (2.496)
2.493 (2.426)
2.568 (2.479)
Mo(PMe3)4H4
(Scheme
8).
Likewise,
Mo(P-
Me3)4(OC6H3Pri2)H reacts with H2 to give Mo(P-
Me3)4H4 and C6H3Pri2OH. The reaction between
Mo(PMe3)4(OAr)H and H2 is postulated to occur via
initial oxidative addition and formation of Mo(P-
Me3)4(OAr)H3. Support for this proposal is not only
provided by the fact that several analogous tungsten
complexes W(PMe3)4(OAr)H3 have been reported [3],
but also by deuterium labeling and magnetization
transfer experiments. For example, treatment of Mo(P-
Me3)4(OC6H2Me3)H with D2 at room temperature
results in deuterium incorporation into both the ortho-
˚
Peq (A)
MÃ
/
Table 6
Calculated energies (kcal molꢂ1
)
of M(PMe3)4H2[h2-OC6H2-
Me2(CH2)] relative to M(PMe3)4(OC6H2Me3)H (MꢀMo, W)
/
Mo
W
M(PMe3)4(OC6H2Me3)H
M(PMe3)4H2(h2-OC6H2Me2CH2)
0
0
ꢄ
/
5.9
ꢂ
/
6.0
1
methyl groups and the hydride sites, as judged by H-
2
NMR and H-NMR spectroscopy. More interestingly,
magnetization transfer is observed between the reso-
nances of the ortho-methyl groups, the hydride and
dissolved H2, indicating that all three types of hydrogen
exchange with each other rapidly on the NMR time-
scale [34]. However, due to the competing rapid
exchange between three sites, quantitative rate informa-
tion was not extracted from the magnetization transfer
experiment. The deuterium labeling and magnetization
transfer studies are consistent with the mechanism
illustrated in Scheme 9.
In order to assess the validity of the calculations on
M(PMe3)4(OC6H2Me3)H and M(PMe3)4H2[(h2-OC6H2-
Me2(CH2)], DFT studies were also performed on
M(PMe3)6 and M(PMe3)4(h2-CH2PMe2)H (Mꢀ
/Mo,
W), which represent a related system in which experi-
mental values for DH have been measured. The results
are shown in Table 7, from which it can be seen that the
calculated energies for the cyclometallation of
M(PMe3)6 are significantly less than the experimental
values, such that the reaction is predicted to be
exothermic, whereas experimentally, DH was found to
be positive. The calculations do, however, reproduce the
result that formation of M(PMe3)4(h2-CH2PMe2)H is
In a similar fashion, deuterium labeling studies also
demonstrate that both types of CÃ/H bonds (methyl and
methine) of the ortho-isopropyl substituents of Mo(P-
Me3)4(OC6H3Pri2)H undergo reversible activation in the
presence of D2, prior to reductive elimination of
C6H3Pri2OH and formation of Mo(PMe3)4H4. Magneti-
zation transfer studies indicate that exchange between
H2 and the hydride sites occurs rapidly at room
temperature. Since exchange with the isopropyl hydro-
gens does not occur at room temperature, it permits
determination of the rate constant for oxidative addition
of H2 (6.2 Mꢂ1 sꢂ1 at 303 K) [25].
more favored for W than for Mo, by ca. 9 kcal molꢂ1
,
compared with the measured difference of 6.6 kcal
molꢂ1. It should be noted, however, that the formation
of M(PMe3)4(h2-CH2PMe2)H also involves dissociation
of a phosphine ligand from M(PMe3)6 and so is not a
direct measure of the difference in MÃ
/
H and MÃ/C bond
strengths.
Table 7
Calculated and experimental energies (kcal molꢂ1) for cyclometalla-
tion of M(PMe3)6
3. Experimental
Mo
W
MoÃW
/
3.1. General considerations
a
b
DH (experimental)
DE (calculated)
15.9
1.3
9.3
10.7
6.6
9.4
ꢂ
/
ꢂ/
All manipulations were performed using a combina-
tion of glovebox, high-vacuum or Schlenk techniques
[35]. Solvents were purified and degassed by standard
procedures and all commercially available reagents were
used as received, unless otherwise noted in the experi-
a
V.J. Murphy, G. Parkin, J. Am. Chem. Soc. 117 (1995) 3522ꢁ
/
3528.
b
D. Rabinovich, R. Zelman, G. Parkin, J. Am. Chem. Soc. 114
(1992), 4611ꢁ4621.
/

T. Hascall et al. / Journal of Organometallic Chemistry 652 (2002) 37ꢁ
/49
45
Scheme 8.
and all manipulations of it were performed under an
argon atmosphere.
3.2. Synthesis of Mo(PMe3)4H2(h2-CH2O)
A suspension of Mo(PMe3)6 (0.43 g, 0.78 mmol) in
pentane (25 ml) was treated with excess methanol (0.9
ml, 22 mmol). The reaction was stirred at room
temperature (r.t.) overnight, yielding a golden-brown
solution. The volatile components were then removed
under reduced pressure, the residue extracted into
pentane (20 ml), filtered, and the pentane removed
under vacuum, yielding Mo(PMe3)4H2(h2-CH2O) as a
1
brown solid (0.21 g, 62%). H-NMR (C6D6): 3.51 [s,
CH2O], 1.20 [s, 4 P(CH3)3], ꢂ
/
2.65 [quint, JPÃH
ꢀ
/
38, 2
¯
¯
MoH]. 13C-NMR (C6D6): 63.4 [t, JPÃC
ꢀ
/
6; t, JCÃH
ꢀ
/
¯
159; CH2O], 22.6 [br m, 4 P(CH3)3]. 31P-NMR (C6D6):
¯
¯
3.5 [br s]. Anal. Calc. for C13H40OP4Mo: C, 36.1; H,
9.3%. Found: C, 35.5; H, 9.3%. IR data (KBr, cmꢂ1):
2965 (m), 2902 (m), 2807 (w), 1734 (m), 1420 (m), 1294
(m), 1276 (m), 1153 (m), 937 (s), 845 (m), 696 (m).
Scheme 9.
mental procedures. IR spectra were recorded as KBr
pellets or neat samples on Perkinꢁ
spectrophotometers and are reported in cmꢂ1. Elemen-
Elmer 2400
/
Elmer 1430 or 1600
3.3. Synthesis of Mo(PMe3)2(OPh)4
tal analyses were measured using a Perkinꢁ
/
CHN Elemental Analyzer. 1H-NMR spectra were
recorded on Varian VXR-200 (200.057 MHz), VXR-
300 (299.943 MHz), VXR-400 (399.95 MHz) and Bruker
Avance DPX 300, DRX 300 and DMX 500 spectro-
A mixture of Mo(PMe3)6 (0.69 g, 1.25 mmol) and
phenol (1.00 g, 11.3 mmol) was stirred in benzene (15
ml) for 2 h. The volatile components were removed
under reduced pressure and the resulting tan solid
washed with pentane (3ꢃ
Me3)2(OPh)4 as an orange solid (0.33 g, 43%). 1H-
NMR (C6D6): 20.3, [s, OC6H5], ꢂ17 [br s, 2 P(CH3)3],
/50 ml) to give Mo(P-
1
meters. H and 13C chemical shifts are reported in ppm
relative to SiMe4 (dꢀ
with respect to the protio solvent impurity (dꢀ
C6D5H) and the 13C resonances (dꢀ
128.0 for C6D6),
respectively. 31P-NMR chemical shifts are reported in
ppm relative to 85% H3PO4 (dꢀ0) and were referenced
using P(OMe3) (dꢀ141.0) as external standard.
Mo(PMe3)6 was prepared by the literature method [5a]
/0) and were referenced internally
/
/7.15 for
¯
¯
ꢂ
/
18 [br s, OC6H5], ꢂ22 [br s, OC6H5]. Anal. Calc. for
/
/
¯
¯
C30H38O4P2Mo: C, 58.1; H, 6.2%. Found: C, 58.2; H,
6.0%. IR Data (KBr, cmꢂ1): 3058 (w), 2905 (w), 1583
(s), 1478 (s), 1416 (w), 1252 (s), 1157 (m), 1065 (w), 1019
(w), 994 (w), 948 (m), 854 (s), 754 (m), 694 (m), 624 (m),
603 (m), 514 (w), 462 (m).
/
/

46
T. Hascall et al. / Journal of Organometallic Chemistry 652 (2002) 37ꢁ49
/
3.4. Synthesis of Mo(PMe3)4(OC6H2Me3)H
Mo(PMe3)6 with C6H2Me3OD [36] was monitored by
1
both H and H-NMR spectroscopy.
2
A mixture of Mo(PMe3)6 (0.38 g, 0.69 mmol) and
2,4,6-trimethylphenol (0.078 g, 0.57 mmol) in benzene (5
ml) was stirred for 1 h at 80 8C, over which period a
deep purple solution was obtained. The volatile compo-
nents were removed under reduced pressure and the
residue was extracted into pentane (ca. 20 ml). The
3.5. Synthesis of Mo(PMe3)4(OC6H3Pri2)H
A mixture of Mo(PMe3)6 (0.44 g, 0.80 mmol) and 2,6-
diisopropylphenol (0.13 ml, 0.70 mmol) in benzene (5
ml) was stirred for 1.5 h at 60 8C, over which period a
deep purple solution was obtained. The volatile compo-
nents were removed under reduced pressure and the
residue extracted into pentane (ca. 20 ml). The filtrate
filtrate was concentrated to ca. 5 ml and cooled to ꢂ
/
78 8C, giving Mo(PMe3)4(OC6H2Me3)H as a purple
crystalline solid which was isolated by filtration and
dried in vacuo (0.16 g); further concentration of the
was concentrated to ca. 2 ml and cooled to ꢂ78 8C,
/
giving Mo(PMe3)4(OC6H3Pri2)H as a purple crystalline
solid which was isolated by filtration and dried in vacuo.
filtrate followed by storage at ꢂ78 8C yielded an
/
additional 0.02 g. Total yield: 0.18 g (59% based on
1
ArOH). H-NMR (C6D6): 6.99 [s, OC6H2Me3], 2.35 [s,
1
Yield: 0.24 g (59% based on ArOH). H-NMR (C6D6):
¯
7.21 [d, JHÃH
H], 3.22 [septet, JHÃH
14, 2 P(CH3)3], 1.37 [d, JHÃH
ꢀ
/
7.5, 2 meta-H], 6.86 [t, JHÃH
ꢀ7.5, para-
/
para-CH3], 2.08 [2 ortho-CH3], 1.37 [s, 2 P(CH3)3], 1.01
¯
¯
¯
ꢀ
/
7, 2 CH(CH3)2], 1.39 [d, JPÃH
ꢀ
/
[s, 2 P(CH3)3], ꢂ
/
6.59 [t, JPÃH
ꢀ
/
23; t, JPÃH
ꢀ
/
84; MoH].
¯
¯
ꢀ
/
7, 2 CH(CH3)2], 1.00 [s, 2
¯
¯
13C-NMR (C6D6): 167.3 [s, OC6H2Me3], 129.0 [d,
¯
P(CH3)3], ꢂ
/
6.53 [t, JPÃH
ꢀ
/
22; t, JPÃH
ꢀ
/
85; MoH]. 13C-
¯
JCÃH
ꢀ150, OC6H2Me3], 121.9 [s, OC6H2Me3], 121.1
/
¯
¯
NMR (C6D6): 167.5 [s, OC6H3Pri2], 133.5 [s, OC6H3Pr2i ],
¯
¯
[s, OC6H2Me3], 30.9 [d, JPÃC
ꢀ
/
17; q, JCÃH
128; 2 P(CH3)3], 21.0 [q,
ꢀ129; 2
/
¯
¯
122.5 [d, JCÃH
ꢀ
/
150, OC6H3Pri2], 114.7 [d, JCÃH
ꢀ
/
157,
18;
125, 2 CH(CH3)2], 24.9 [q,
¯
P(CH3)3], 21.3 [q, JCÃH
ꢀ
/
¯
OC6H3Pri2], 30.5 [q, JCÃH
ꢀ
/
127; d, JPÃC
ꢀ
/
2
¯
¯
JCÃH
ꢀ
/
128; 2 ortho-CH3], 17.7 [q, JCÃH
CH3]. 31P{1H, P(CH3)3} NMR (C6D6): 45.6 [t, JPÃP
ꢀ
/
128; para-
¯
P(CH3)3], 25.7 [d, JCÃH
ꢀ
/
¯
¯
¯
ꢀ
/
JCÃH
ꢀ
/
125, 2 CH(CH3)2], 22.4 [q, JCÃH
ꢀ126, 2
/
¯
¯
¯
P(CH3)3]. 31P{1H, P(CH3)3} NMR (C6D6): 48.3 [t,
23; d, JPÃH
JPÃH
ꢀ
/
84, 2 P(CH3)3], 1.01 [t, JPÃP
23, 2 P(CH3)3]. Anal. Calc. for C21H48OP4Mo:
ꢀ23; d,
/
¯
¯
¯
ꢀ
/
JPÃP
ꢀ
/
23; d, JPÃH
ꢀ
/
85, 2 P(CH3)3], 1.13 [t, JPÃP
ꢀ23;
/
¯
¯
C, 47.0; H, 9.0%. Found: C, 46.9; H, 9.0%. IR Data
(KBr, cmꢂ1): 2964 (m), 2900 (m), 1750 (w) [nMoÃH],
1603 (w), 1472 (m), 1421 (m), 1369 (w), 1312 (m), 1292
(m), 1269 (m), 1156 (w), 934 (s), 847 (m), 821 (m), 734
(m), 696 (m), 676 (m), 645 (m), 509 (m). The reaction of
d, JPÃH
ꢀ/223,
2
P(CH3)3]. Anal. Calc. for
¯
C24H54OP4Mo: C, 49.8; H, 9.4%. Found: C, 49.8; H,
9.3%. IR Data (KBr, cmꢂ1): 2964 (m), 2903 (m), 1751
(w) [nMoÃH], 1585 (m), 1425 (m), 1337 (m), 1274 (m),
1108 (w), 1044 (w), 930 (s), 740 (m), 692 (m), 643 (m).
Table 8
Crystal, intensity collection and refinement data
Mo(PMe3)4H2(h2-CH2O)
Mo(PMe3)2(OPh)4
Mo(PMe3)4(OC6H2Me3)H
Mo(PMe3)4(OC6H3Pri2)H
Lattice
Formula
Monoclinic
C13H40MoOP4
432.27
P2/c
Monoclinic
C30H38MoOP4
620.5
Triclinic
C21H48MoOP4
Orthorhombic
C24H54MoOP4
578.5
Formula weight
Space group
536.4
¯
P21/c
P/
1/
Pnma
˚
a (A)
16.4296(15)
9.6195(9)
15.7796(15)
90
10.212(2)
10.606(3)
14.607(2)
90
9.514(2)
12.220(2)
12.820(3)
87.24(2)
77.80(2)
81.99(2)
1442.3(5)
2
20.055(5)
17.076(5)
9.400(3)
90
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
117.756(2)
90
98.11(2)
90
90
90
3
˚
V (A )
Z
Temperature (K)
2206.9(4)
4
223
1567.5(8)
2
295
3219.0(16)
4
295
295
˚
Radiation (l, A)
0.71073
1.301
0.878
0.71073
1.315
0.551
0.71073
1.235
0.686
0.71073
1.194
0.619
rcalc (g cmꢂ3
)
m (Moꢁ
umax (8)
Number of data
/
Ka), (mmꢂ1
)
28.3
5122
22.5
2045
25.0
3226
22.5
2934
Number of parameters
R1
216
0.0427
0.0812
170
0.0378
0.0522
249
0.0327
0.0428
152
0.0544
0.0606
wR2

T. Hascall et al. / Journal of Organometallic Chemistry 652 (2002) 37ꢁ
/49
47
3.6. Computational details
5. Supplementary material
All DFT calculations were performed using Jaguar
[37]. Geometries were optimized starting from the X-ray
crystal structure coordinates of Mo(PMe3)4(OC6H2-
Me3)H, W(PMe3)4H2[h2-OC6H2Me2(CH2)], W(PMe3)6
and W(PMe3)4(h2-CH2PMe2)H at the BLYP level using
the LACVP** basis set. No symmetry constraints were
The crystallographic data for Mo(PMe3)4H2(h2-
CH2O) (CCDC 173846), Mo(PMe3)2(OPh)4 (CCDC
TORDIN), Mo(PMe3)4(OC6H2Me3)H (CCDC TOR-
DOT), and Mo(PMe3)4(OC6H3Pri2)H (CCDC TOR-
DUZ) have been deposited with the Cambridge
Crystallographic Data Center. Copies of this informa-
tion may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
used. The structures of M(PMe3)6 (Mꢀ
verged to S6 symmetry and those of M(PMe3)4(h2-
CH2PMe2)H (MꢀMo, W) converged to Cs symmetry.
/Mo, W) con-
/
(Fax: ꢄ44-1223-336033; e-mail: deposit@ccdc.cam.a-
/
Single point energy calculations were then performed on
the optimized structures with appropriate symmetry
constraints at the B3LYP level using triple-z basis sets
(LACV3P** for the transition metals, cc-pVTZ(-f) for
all other elements). PMe3 was optimized at the B3LYP
level with the cc-pVTZ(-f) basis set.
c.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
We thank the US Department of Energy, Office of
Basic Energy Sciences (#DE-FG02-93ER14339) for
support of this research.
3.7. X-ray structure determinations
X-ray diffraction data for Mo(PMe3)4H2(h2-CH2O)
were collected on a Bruker P4 diffractometer equipped
with a SMART CCD detector; data for all other complexes
were collected using a Siemens P4 diffractometer.
Crystal data, data collection and refinement parameters
are summarized in Table 8. The structures were solved
using direct methods and standard difference map
techniques, and were refined by full-matrix least-squares
procedures using SHELXTL [38]. Hydrogen atoms on
carbon were included in calculated positions.
References
[1] (a) C.L. Hill (Ed.),Activation and Functionalization of Alkanes,
Wiley, New York 1989;
(b) B.A. Arndtsen, R.G. Bergman, T.A. Mobley, T.H. Peterson,
Acc. Chem. Res. 28 (1995) 154;
(c) A.D. Ryabov, Chem. Rev. 90 (1990) 403;
(d) W.D. Jones, F.J. Feher, Acc. Chem. Res. 22 (1989) 91;
(e) Special issue J. Organomet. Chem. 504 (1996).;
(f) A.E. Shilov, G.B. Shul’pin, Chem. Rev. 97 (1997) 2879;
(g) R.H. Crabtree, J. Chem. Soc. Dalton Trans. (2001) 2437.
[2] M.L.H. Green, G. Parkin, K.J. Moynihan, K. Prout, J. Chem.
Soc. Chem. Commun. (1984) 1540.
4. Summary
[3] (a) D. Rabinovich, R. Zelman, G. Parkin, J. Am. Chem. Soc. 112
(1990) 9632;
In summary, Mo(PMe3)6 reacts with methanol to give
the formaldehyde complex Mo(PMe3)4(h2-CH2O), ana-
logous to that of the tungsten system, whereas the
reactions with phenol and its derivatives yield signifi-
cantly different products. Specifically, the tendency to
(b) D. Rabinovich, R. Zelman, G. Parkin, J. Am. Chem. Soc. 114
(1992) 4611.
[4] The reactivity of related tungsten complexes towards alcohols has
been reported, but CÃH bond activation reactions have not been
/
observed. For example (a) W(PMe3)4Cl2 and W(PMe3)4H2Cl2
react with alcohols to yield W(PMe3)3(O)Cl2 and liberate the
hydrocarbon, (b) while W(PMe3)5H2 reacts with PhCH2OH to
yield W(PMe3)4H3(OCH2Ph), thermolysis of which yields
W(PMe3)4(CO)H2 and benzene. (a) T.J. Crevier, J.M. Mayer, J.
Am. Chem. Soc. 119 (1997) 8485.(b) T.J. Crevier, J.M. Mayer,
Inorg. Chim. Acta 270 (1998) 202.
achieve CÃ
/
H bond activation for a molybdenum center
is reduced considerably from that for a tungsten center,
so that the products of the molybdenum system are
simple aryloxy derivatives, e.g. Mo(PMe3)2(OPh)4 and
Mo(PMe3)4(OAr)H, whereas cyclometalated CÃ
/
H clea-
[5] (a) V.J. Murphy, G. Parkin, J. Am. Chem. Soc. 117 (1995)
3522;
vage products, e.g. W(PMe3)4(h2-OC6H4)H2 and
W(PMe3)4[h2-OC6H2Me2(CH2)]H2, are obtained for
the tungsten system. Such differences are attributed to
(b) M. Brookhart, K. Cox, F.G.N. Cloke, J.C. Green, M.L.H.
Green, P.M. Hare, J. Bashkin, A.E. Derome, P.D. Grebenik, J.
Chem. Soc. Dalton Trans. (1985) 423.
weaker MÃ
/
C and MÃ
/
H bond energies for molybdenum
H bond
[6] Some of this work has been communicated: T. Hascall, V.J.
Murphy, G. Parkin, Organometallics 15 (1996) 3910.
[7] M. Minato, H. Sakai, Z.-G. Weng, D.-Y. Zhou, S. Kurishima, T.
Ito, M. Yamasaki, M. Shiro, M. Tanaka, K. Osakada, Organo-
metallics 15 (1996) 4863.
[8] Furthermore, Cp2Mo(h2-CH2O) has been reported. See: (a) G.E.
Herbereich, J. Okuda, Angew. Chem. Int. Ed. Engl. 24 (1985)
402.(b) S. Gambarotta, C. Floriani, A. Chiesi-Villa, C. Guastini,
J. Am. Chem. Soc. 107 (1985) 2985.
compared with tungsten, such that the CÃ
/
activation products are thermodynamically unstable.
Despite this instability, deuterium labeling and magne-
tization transfer studies demonstrate that the coordina-
tively unsaturated molybdenum complexes Mo(PMe3)4-
(OAr)H are nevertheless kinetically capable of intramo-
lecular CÃ
/
H bond activation.

48
T. Hascall et al. / Journal of Organometallic Chemistry 652 (2002) 37ꢁ49
/
[9] The triplet is further split into a triplet of triplets due to coupling
to phosphorus.
bond activation step, cf. the mechanism for oxidative-addition
of H2 to W(PMe3)4I2. See: D. Rabinovich, G. Parkin, J. Am.
Chem. Soc. 115 (1993) 353.
[10] Interestingly, both paramagnetic and diamagnetic tungsten(IV)
complexes of the type W(PR3)2(OAr)2X2 are known, depending
on the nature of the substituents. See: (a) L.M. Atagi, J.M.
Mayer, Angew. Chem. Int. Ed. Engl. 32 (1993) 439.(b) C.E.
Kriley, P.E. Fanwick, I.P. Rothwell, J. Am. Chem. Soc. 116
(1994) 5225.(c) J.L. Kerschner, P.E. Fanwick, I.P. Rothwell, J.C.
Huffman, Inorg. Chem. 28 (1989) 780.
[23] (a) R. Freeman, A Handbook of Nuclear Magnetic Resonance,
Longman, Harlow, UK 1988;
(b) D.C. Roe, P.M. Kating, P.J. Krusic, B.E. Smart, Top. Catal. 5
(1998) 133.
[24] Exchange between the two PMe3 ligands of Mo(PMe3)4(OC6H2-
Me3)H and free PMe3 was also observed by magnetization
transfer experiments. The PMe3 exchange, however, appears to
be independent from the cyclometallation as the rate of the
hydride/ortho-methyl exchange was not noticeably affected by the
presence of added PMe3.
[11] S. Jang, L.M. Atagi, J.M. Mayer, J. Am. Chem. Soc. 112 (1990)
6413.
[12] T. Hascall, G. Parkin, unpublished results.
[13] (a) J.L. Kerschner, P.E. Fanwick, I.P. Rothwell, J. Am. Chem.
Soc. 109 (1987) 5840;
[25] It is important to note that appropriate statistical factors have
been included in order to relate the observed magnetization
transfer rate constants to those of the actual chemical processes
involved. See, for example: M.L.H. Green, L.-L. Wong, A. Sella,
Organometallics 11 (1992) 2660.
(b) J.L. Kerschner, P.E. Fanwick, I.P. Rothwell, J.C. Huffman,
Organometallics 8 (1989) 1424.
[14] (a) K. Osakada, K. Ohshiro, A. Yamamoto, Organometallics 10
(1991) 404;
(b) J.F. Hartwig, R.A. Andersen, R.G. Bergman, Organometallics
10 (1991) 1875;
[26] (a) T.J. Marks (Ed.), Bonding Energetics in Organometallic
Compounds, ACS Symposium Series, 1990, pp. 428;
(b) J.A. Martinho Simoes, J.L. Beauchamp, Chem. Rev. 90 (1990)
(c) M.J. Burn, M.G. Fickes, F.J. Hollander, R.G. Bergman,
Organometallics 14 (1995) 137.
˜
629;
(c) D.C. Eisenberg, J.R. Norton, Israel J. Chem. 31 (1991) 55;
[15] For some other structurally characterized transition metal ar-
yloxy-hydride complexes containing phosphine ligands, see: (a)
J.R. Bleeke, T. Haile, P.R. New, M.Y. Chiang, Organometallics
12 (1993) 517.(b) A.L. Seligson, R.L. Cowan, W.C. Trogler,
Inorg. Chem. 30 (1991) 3371.(c) R.L. Cowan, W.C. Trogler, J.
Am. Chem. Soc. 111 (1989) 4750.(d) F.T. Ladipo, M. Kooti, J.S.
Merola, Inorg. Chem. 32 (1993) 1681.(e) B.C. Ankianiec, P.E.
Fanwick, I.P. Rothwell, J. Am. Chem. Soc. 113 (1991) 4710.(f)
V.M. Visciglio, P.E. Fanwick, I.P. Rothwell, J. Chem. Soc. Chem.
Commun. (1992) 1505.(g) C. Di Bugno, M. Pasquali, P. Leoni, P.
(d) H.A. Skinner, J.A. Connor, Pure Appl. Chem. 57 (1985)
79;
(e) J.A. Martinho Simoes (Ed.), Energetics of Organometallic
˜
Species, NATO ASI series, 1991, pp. 367;
(f) Halpern, J. Acc. Chem. Res. 15 (1982) 238.
[27] It must also be recognized that the oxidative-addition of the CÃ
bond will be accompanied by loss of oxygen to metal p-donation.
[28] For another example in which intramolecular CÃH bond activa-
/
H
/
tion is observed in a dinuclear tungsten complex, but not in the
analogous Mo species, see: M.H. Chisholm, J.-H. Huang, J.C.
Huffman, I.P. Parkin, Inorg. Chem. 36 (1997) 1642.
Sabatino, D. Braga, Inorg. Chem. 28 (1989) 1390.(h) H. Noth, M.
¨
Schmidt, Organometallics 14 (1995) 4601.
[16] D. Milstein, J.C. Calabrese, I.D. Williams, J. Am. Chem. Soc. 108
(1986) 6387.
[29] (a) M.J. Calhorda, A.R. Dias, M.E. Minas da Piedade, M.S.
Salema, J.A. Martinho Simoes, Organometallics
˜
734;
6 (1987)
[17] The structure is fluxional on the NMR time-scale and the hydride
resonance appears as a quintet with JPÃH
Henderson, D.L. Hughes, R.L. Richards, C. Shortman, J. Chem.
Soc. Dalton Trans. (1987) 1115.
ꢀ
/
47 Hz. See: R.A.
(b) A.R. Dias, J.A. Martinho Simoes, Polyhedron 7 (1988) 1531.
˜
[30] See for example, reference [18].
[31] The electronic energies are the enthalpies at 0 K uncorrected for
zero point energy contributions.
[18] T. Hascall, D. Rabinovich, V.J. Murphy, M.D. Beachy, R.A.
Friesner, G. Parkin, J. Am. Chem. Soc. 121 (1999) 11402.
[19] Furthermore, the departure from octahedral symmetry of Mo(P-
Me3)4(OAr)H also promotes the selective p-interaction with a
single metal orbital.
[32] D.F. McMillen, D.M. Golden, Ann. Rev. Phys. Chem. 33 (1982)
493.
[33] It is worth noting that the sum of the MÃ
/
C and MÃH bond
/
energies for the Mo and W complexes is significantly less for
Mo(PMe3)4H2[h2-OC6H2Me2(CH2)] than those for the Cp2MX2
system. In part, this difference may be attributed to a more
crowded environment in the former system.
[20] (a) J.L. Kerschner, P.E. Fanwick, I.P. Rothwell, Inorg. Chem. 28
(1989) 780;
(b) W.A. Howard, T.M. Trnka, G. Parkin, Inorg. Chem. 34
(1995) 5900;
[34] For some other examples of magnetization transfer between H2
and hydride ligands, see: (a) B.E. Hauger, D. Gusev, K.G.
Caulton, J. Am. Chem. Soc. 116 (1994) 208.(b) R.L. Miller, R.
Toreki, R.E. LaPointe, P.T. Wolczanski, G.D. Van Duyne, D.C.
Roe, J. Am. Chem. Soc. 115 (1993) 5570.(c) J. Halpern, L. Cai,
P.J. Desrosiers, Z. Lin, J. Chem. Soc. Dalton Trans. (1991)
717.
(c) B.D. Steffey, P.E. Fanwick, I.P. Rothwell, Polyhedron 9
(1990) 963;
(d) T.W. Coffindaffer, B.D. Steffy, I.P. Rothwell, K. Folting, J.C.
Huffman, W.E. Streib, J. Am. Chem. Soc. 111 (1989)
4742.
[21] It should be noted that deuterium incorporation is also observed
in the PMe3 resonances of Mo(PMe3)4(OC6H2Me3)H. In this
regard, the reaction of W(PMe3)4(h2-CH2PMe2)H with C6H5OD
results in deuterium incorporation into the PMe3 groups of
W(PMe3)4H2(h2-OC6H4) as well as in the free phosphine liberated
in the reaction. This observation was explained by a mechanism
[35] (a) J.P. McNally, V.S. Leong, N.J. Cooper, in: A.L. Wayda, M.Y.
Darensbourg (Eds.), Experimental Organometallic Chemistry
(chapter 2), American Chemical Society, Washington, DC 1987,
pp. 6ꢁ23;
/
(b) B.J. Burger, J.E. Bercaw, in: A.L. Wayda, M.Y. Darensbourg
involving a direct attack by phenol of the WÃC bond of
/
(Eds.), Experimental Organometallic Chemistry (chapter 4),
W(PMe3)4(h2-CH2PMe2)H (reference [3]). Thus deuterium incor-
poration into the PMe3 ligands of Mo(PMe3)4(OC6H2Me3)H
American Chemical Society, Washington, DC 1987, pp. 79ꢁ98;
/
(c) D.F. Shriver, M.A. Drezdzon, The Manipulation of Air-
Sensitive Compounds, 2nd ed., Wiley-Interscience, New York
1986.
suggests
a competitive mechanism involving reaction of
W(PMe3)4(h2-CH2PMe2)H with the phenol.
[22] It is, of course, possible that the six-coordinate complexes
[36] C6H2Me3OD was obtained by stirring a solution of 2,4,6-
trimethylphenol (0.48 g) in D2O (4 ml) and CH3CN (15 ml)
Mo(PMe3)4(OAr)H may dissociate PMe3 prior to the CÃH
/

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/49
49
containing a catalytic amount of HCl (aq) overnight. The volatile
components were removed in vacuo to give C6H2Me3OD [2H-
NMR (C6H6): d 3.9]. Deuteration was found to be ca. 90%
complete by 1H-NMR spectroscopy.
[38] G.M. Sheldrick, SHELXTL, An Integrated System for Solving,
Refining and Displaying Crystal Structures from Diffraction
Data, University of Gottingen, Gottingen, Federal Republic of
¨ ¨
Germany 1981.
[37] JAGUAR Version 3.5, Schrodinger, Inc., Portland, OR, 1998.
¨