Comparative regiochemistry of protonation of (η6-C6H 5R)(1,1′,1″-tris(2-diphenylphosphinomethyl)ethane)Mo(0) (R=H, Me and SiMe3)

Article abstract of DOI:10.1016/S0022-328X(01)00785-9

Our previous mechanistic studies of 1 (R=H) demonstrate it is protonated to give a metal-hydride via a mechanism that involves indirect exo attack on the arene ligand followed by endo proton transfer to the metal. The toluene derivative 1 (R=Me) reacts via a similar mechanism with unusual ortho/meta/para=61:36:3% selectivity. In contrast, the first equivalent of proton hydrolyzes the TMS group of 1 (R=SiMe₃) under surprisingly mild conditions and the second equivalent reacts with the product 1 (R=H) to give the metal-hydride via the aforementioned indirect mechanism.

Full text of DOI:10.1016/S0022-328X(01)00785-9

Journal of Organometallic Chemistry 628 (2001) 275–279
www.elsevier.nl/locate/jorganchem
Comparative regiochemistry of protonation of
(h6-C6H5R)(1,1%,1¦-tris(2-diphenylphosphinomethyl)ethane)Mo(0)
(R=H, Me and SiMe3)
Victor S. Asirvatham, Masood A. Khan, Michael T. Ashby *
Department of Chemistry and Biochemistry, The Uni6ersity of Oklahoma, Room 208, 620 Parrington O6al, Norman, OK 73019, USA
Received 12 January 2001; accepted 22 March 2001
Abstract
Our previous mechanistic studies of 1 (R=H) demonstrate it is protonated to give a metal-hydride via a mechanism that
involves indirect exo attack on the arene ligand followed by endo proton transfer to the metal. The toluene derivative 1 (R=Me)
reacts via a similar mechanism with unusual ortho/meta/para=61:36:3% selectivity. In contrast, the first equivalent of proton
hydrolyzes the TMS group of 1 (R=SiMe3) under surprisingly mild conditions and the second equivalent reacts with the product
1 (R=H) to give the metal-hydride via the aforementioned indirect mechanism. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Protonation; Molybdenum; Phosphine; Regioselectivity; Mechanism
1. Introduction
pirical to density functional theory) [3]. We have
contemplated the effect that arene substituents might
have on the mechanism of protonation. Herein we
compare the mechanisms of protonation of three arene
derivatives: (h5-C6H5R)Mo(TRIPOD) (1) (R=H, Me,
The study of protolytic mechanisms under kinetic
control is problematic because most Brønsted reactions
are very fast and reversible. We have recently explored
the protonation of (h6-arene)Mo(phosphine)3 to give
the metal-hydrides [(h6-arene)Mo(phosphine)3(H)]+
which takes place via two distinct mechanisms: (1)
indirect exo protonation of the arene ligand to give a
h5-cyclohexadienyl transient that transfers its endo pro-
ton to the metal; and (2) direct protonation of the metal
[1,2]. In a previous study we reported the effect of
changing the steric demand of the ancillary phosphine
ligands on partitioning between these two mechanisms
(see Scheme 1) [2]. Although a correlation is generally
observed between the size of the phosphine ligand and
the mechanism of protonation, partitioning is better
described by mapping the computed frontier orbital
density of the complexes onto total electron density
surfaces, thereby revealing the sterically accessible fron-
tier orbital density [2,3]. The qualitative conclusions
from such calculations were not sensitive to the meth-
ods that were employed (which ranged from semi-em-
* Corresponding author. Fax: +1-405-3256111.
E-mail address: mtashby@ou.edu (M.T. Ashby).
Scheme 1.
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00785-9

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V.S. Asir6atham et al. / Journal of Organometallic Chemistry 628 (2001) 275–279
SiMe3;
TRIPOD=1,1%,1¦-tris(2-diphenylphosphi-
and basic conditions. For alkyl arene substituents un-
der basic conditions, the HIE of the ortho arene pro-
tons is disfavored, ortho/meta/para=25:33:42% for
(h6-C6H5CH3)Cr(CO)3 [9], and the coordinated arene
ligands are more reactive than the free arenes. While it
is known that coordinated arene ligands in (h6-
C6H5R)Cr(CO)3 undergo HIE more slowly than the
free arenes under acidic conditions [10], the regiochem-
istry of HIE for the metal complexes is not known
because the ortho, meta and para resonances fortu-
nomethyl)ethane). Derivatives of compound 1 were se-
lected for this study because the parent compound 1
(R=H) is known to react with proton exclusively via
the arene mechanism, thus the regioselectivity of proto-
nation at the arene ligands of 1 is reported here.
2. Results and discussion
1
2.1. Protonation of 1 (R=Me)
itously overlap in the H-NMR spectra [10]. Further-
more, the latter reactions are studied under equilibrium
conditions, but the protonation of 1 occurs irreversibly,
which simplifies interpretation of the HIE of 1. The
arene resonances of [(h6-C6H5CH3)Mo(TRIPOD)(H)]+
, 2 (R=Me), have been assigned as: 1H-NMR (CD2Cl2,
500 MHz, 20°C): l 5.44 (br d, 2H, J=6 Hz), 5.38 (br
t, 2H, J=7 Hz), 5.27 (br t, 1H, J=8 Hz). Protonation
of the arene ligand of 1 (R=Me) occurs with different
selectivity, ortho/meta/para=61:36:3%, compared to
free toluene, ortho/meta/para=37:1:62%. To our
knowledge, this is the first electrophilic aromatic substi-
tution of toluene that occurs meta in preference to para.
The origin of the difference may be due to several
variables, including different rotational orientations of
the arene ligands of the carbonyl and phosphine deriva-
tives, different reactivities of the arene C centers that
are eclipsed and staggered with respect to the tripodal
ligands, or a difference in reactivity of the Cr and Mo
derivatives.
Regioselectivity of electrophilic aromatic substitution
is often influenced by the substitution pattern and the
electronic nature of the substituents on the ring [4].
Generally, electron-withdrawing groups are meta-di-
recting and electron-releasing groups are ortho/para-di-
recting. However, the directing influence of substituents
can be significantly affected when the arene is h6-bound
to a transition metal. It has been argued for three-
legged piano-stool complexes, (h6-C6H5R)ML3, that the
R-group is staggered with respect to the ML3 moiety
for large substituents and eclipsed for small substituents
[5]. Thus, the regiochemistry of Friedel–Crafts acetyla-
tion of (h6-C6H5CH3)Cr(CO)3, ortho/meta/para=
43:17:40%, is very different from free toluene,
ortho/meta/para=1:2:97% [6,7]. Since electrophilic at-
tack on the arene groups of (h6-arene)ML3 complexes
is apparently favored at the arene C centers that are
staggered with respect to the ML3 moiety, the steric
demands of arene substituents play an important role in
determining the regioselectivity of electrophilic substitu-
tion of such complexes [7]. Thus, the percentage of
meta acetylation of (h6-C6H5R)Cr(CO)3 increases with
the steric demand of the arene substituent, R=Me/Et/
iPr/tBu=17:33:59:87.
2.2. Protonation of 1 (R=SiMe3)
Coordination of arenes to Cr(CO)3 is known to
activate the arene toward nucleophilic attack. Thus,
base cleavage of aryl–silyl bonds is known to be facili-
tated by an electron-withdrawing group. Accordingly,
the arene–Si bond of (h6-C6H5SiMe3)Cr(CO)3 is readily
cleaved by nucleophiles [11]. However, complexation of
arenes to Cr(CO)3 is known to deactivate the arenes
toward electrophilic attack. Alcohol solutions of (h6-
C6H5SiMe3)Cr(CO)3 can apparently be extracted after
acidification with HCl without cleavage of the arene–Si
bond [11]. Condensed HCl and high pressures are re-
quired to cleave some silyl derivatives of (h6-
arene)Cr(CO)3 [12]. Furthermore, addition of HBF4 to
Unlike acetylation, protonation should not exhibit a
significant steric effect. In an effort to explore the
stereoelectronic effect of substituting the arene ligand of
1 on the regioselectivity of protonation, we have at-
i
tempted to synthesize the series 1 (R=H, Me, Et, Pr,
tBu). Toward this goal, we have synthesized the first
two members of this series via arene exchange of the
corresponding bis-arene precursors by TRIPOD. We
have also synthesized the bis-arene derivatives (h6-
i
t
C6H5R)2Mo (R=Et, Pr, Bu) via a novel arene-ex-
change methodology [8], but our effort to synthesize the
Mo(h6-C6H5SiHMe2)2
produces
[Mo(h6-C6H5Si-
i
corresponding half-sandwich derivatives 1 (R=Et, Pr,
HMe2)2]+ with no evidence of cleavage of the arene–Si
bonds [13]. In contrast, 1 (R=SiMe3) reacts with one
equivalent of acid at r.t. to rapidly produce 1 (R=H).
In the presence of excess acid, 1 (R=SiMe3) yields 2
(R=H).
tBu) have been thwarted by problems associated with
i
t
their purification. Although 1 (R=Et, Pr, Bu) were
identified spectroscopically, analytically pure samples
(as indicated by combustion analysis) were not ob-
tained. Accordingly, no effort was made to carry out
tracer studies on these derivatives.
There have been many previous crystal structures of
metal complexes of arene ligands that bear silyl groups,
but only two of C6H5SiMe3, [(h6-C6H5SiMe3)Ru(h5-
C5Me5)]+ and (h6-C6H5SiMe3)Mo(CO)3 [14,15]. A
Hydrogen isotope exchange (HIE) of (h6-
C6H5R)Cr(CO)3 has been explored under both acidic

V.S. Asir6atham et al. / Journal of Organometallic Chemistry 628 (2001) 275–279
277
this steric stress together with the electron richness of 1
contribute to the comparative facility of hydrolysis of 1
(R=SiMe3) compared to its tricarbonyl analog.
3. Experimental
3.1. Chemicals and sol6ents
All operations were carried out using Schlenk or
glovebox techniques under Ar or N2. Hydrocarbon
solvents were distilled from sodium/benzophenone ke-
tal, CH2Cl2 was distilled from CaH2, and MeOH was
refluxed over Mg and distilled [16]. All solvents were
degassed by three freeze–pump–thaw cycles before use.
The phosphine ligands were used as received from
Aldrich and Strem Chemicals. Mo(h6-C6H6)2 [17] and
CDCl2F [18] were synthesized according to published
procedure.
Fig. 1. Molecular structure of 1 (Z=SiMe3) showing the atom
labeling scheme and the thermal vibration ellipsoids (30% probabil-
ity). Selected interatomic distances and angles: MoꢀC(1)=2.296(3),
MoꢀC(2)=2.259(3),
MoꢀC(5)=2.312(3),
MoꢀC(3)=2.361(3),
MoꢀC(6)=2.278(3),
MoꢀC(4)=2.314(3),
MoꢀP(1)=2.4305(7),
,
MoꢀP(2)=2.4050(7), MoꢀP(3)=2.4143(7) A; P(1)ꢀMoꢀP(2)=
82.92(2), P(1)ꢀMoꢀP(3)=82.60(2), P(2)ꢀMoꢀP(3)=85.20(2)°.
3.2. Instruments and references
1H- and 31P-NMR spectra were recorded on a Varian
XL-500 spectrometer. The NMR samples were pre-
pared in tubes that had been glass-blown onto Schlenk
adapters. The solutions were freeze–pump–thawed be-
fore the tubes were flame-sealed under vacuum. 1H-
NMR spectra were referenced to residual solvent peaks
CDHCl2 (5.32 ppm), CHCl2F (7.47 ppm, d, JHF=50
Hz) and C6D5H (7.24 ppm). 31P-NMR spectra were
referenced to external 85% H3PO4. Combustion analy-
ses were performed by Midwest Microlabs, Indiana-
polis, IN.
Table 1
,
Comparison of selected interatomic distances (A), angles (°), and
torsion angles (°) for 1 (Z=SiMe3) a
1 (Z=H)
1 (Z=SiMe3) (h6-C6H5SiMe3)-
Mo(CO)3
MoꢀL(1)
MoꢀL(2)
MoꢀL(3)
2.412(1)
2.3866(9)
2.3843(9)
2.4305(7)
2.4050(7)
2.4143(7)
1.957(6)
1.963(5)
1.954(6)
L(1)ꢀMoꢀL(2)
L(1)ꢀMoꢀL(3)
L(2)ꢀMoꢀL(3)
84.81(3)
83.50(3)
83.13(3)
82.92(2)
82.60(2)
85.20(2)
86.2(2)
84.3(3)
88.6(3)
MoꢀArcent
1.816(3)
1.814(3)
1.908(3)
3.3. Synthesis of Mo(p6-C6H5Me)2
ArcentꢀMoꢀL(1)
ArcentꢀMoꢀL(2)
ArcentꢀMoꢀL(3)
129.2(1)
128.4(1)
131.0(1)
136.3(1)
128.0(1)
124.6(1)
129.2(3)
125.4(3)
128.9(3)
The apparatus that was used in this procedure is
reported in the literature [19]. Mo wire (3 g) was heated
and the vapor was co-condensed with toluene (50 ml)
over a 2 h period. The product was isolated by vacuum
transfer of the solvent and sublimation of the product.
A known amount (ca. 400 mg) of wire vaporized to
C3ꢀSi/Ar
C3ꢀSi/MoꢀArcent
MoꢀArcent/Ar
–
–
–
19.7(1)
110.5(1)
91.4(1)
0.3(3)
90.1(3)
90.4(3)
a L is the phosphorus donor atom of 1 (Z=H) and 1 (Z=SiMe3)
and the carbon donor of the carbonyl ligands of (h6-
C6H5SiMe3)Mo(CO). Arcent is the centroid of the arene ligand
C1ꢀC6.
1
give 50 mg of the sublimed product. H-NMR (C6D6,
500 MHz, 20°C): l 4.41 (m, 5H, C6H5CH3), 1.85 (s, 3H,
C6H5CH3). 13C{1H}-NMR (C6D6, 125 MHz, 20°C): l
89.65 (s, ipso), 78.68 (s, ortho), 76.02 (s, para), 75.08 (s,
meta), 21.63 (s, CH3).
comparison of the crystal structure of 1 (R=SiMe3)
and the corresponding tricarbonyl analog [14] evidences
considerable strain as a result of steric conflict between
the TMS group and the TRIPOD ligand (Fig. 1 and
Table 1). In particular, note the CipsoꢀSi bond of (h6-
C6H5SiMe3)Mo(CO)3 is co-planar with the arene ring
whereas the corresponding bond of 1 (R=SiMe3)
forms an angle of about 20° with the least-squares
plane that best defines the h6-arene ring. We suggest
3.4. Synthesis of Mo(p6-C6H5SiMe3)2
The procedure employed to synthesize Mo(h6-
C6H5SiMe3)2 was essentially the same as that reported
by Green et al.; however, our yield was inexplicably
better [13]. (h6-C6H6)2Mo (227 mg, 0.9 mmol) was
dissolved in cyclohexane (30 ml). To a second Schlenk
flask was added cyclohexane (100 ml), n-BuLi (2.8 ml

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V.S. Asir6atham et al. / Journal of Organometallic Chemistry 628 (2001) 275–279
of 1.6 M in cyclohexane, 4.5 mmol), and TMEDA (0.68
ml, 4.5 mmol). The green solution of Mo(h6-C6H6)2
was transferred via a Teflon tubing to the lithiating
mixture. The resulting green solution was stirred for 3 h
at 50°C during which time it turned dark and a brown
precipitate was formed. The solution was cooled to 0°C
and Me4SiCl (1.15 ml, 9.0 mmol) was added. The
resulting mixture was warmed to room temperature
(r.t.) and stirred overnight during which time it became
yellow-green in color. Degassed water was added to
quench excess lithium reagent, the cyclohexane layer
was separated, washed with water (2×25 ml), and the
solvent was removed under vacuum. The green residue
was redissolved in hexane, filtered through Celite, con-
centrated, and cooled to obtain the crystalline green
product (172 mg, 67% yield). 1H-NMR (C6D6, 500
MHz, 20°C): l 4.63 (t, 2H, JHH=6 Hz, para-H), 4.50
(t, 4H, JHH=7 Hz, ortho-H), 4.42 (d, 4H, JHH=6.5
Hz, meta-H), 0.10 (s, 18H, CH3). 13C{1H}-NMR
(C6D6, 125 MHz, 20°C): l 79.91 (s, ortho), 79.66 (s,
ipso), 77.95 (s, meta), 75.58 (s, meta), −0.20 (s, CH3).
3.7. Crystal structure determination of
(p6-C6H5SiMe3)Mo(TRIPOD)
The conditions that were used to collect X-ray data
for 1 (R=SiMe3) are summarized in Table 1. The
X-ray data were collected with a Siemens P4 diffrac-
tometer [20], corrected for Lorentz and polarization
effects, and an empirical absorption correction based
on „ scans was applied [21]. The structure was deter-
mined by the heavy atom method and refined using the
SHELXTL (Siemens) system by full-matrix least-squares
on F2 using all reflections [22]. All of the non-hydrogen
atoms were refined with anisotropic displacement
parameters and all of the hydrogen atoms were in-
cluded in the refinements with idealized parameters.
For all the hydrogen atoms except the methyl atoms,
the temperature factors were assigned to be 1.2 times
the Ueq value of the carbon atom on which they ride;
for the methyl hydrogens the values were 1.5 times that
of the Ueq values of the corresponding carbon atoms.
Crystal data: C50H53MoP3Si, MW=870.86, dark red
parallelpipeds, crystal size 0.48×0.44×0.28 mm3,
monoclinic, temp. 188(2) K, space group P21/n, a=
3.5. Synthesis of (p6-C6H5Me)Mo(TRIPOD)
10.7247(13), b=19.510(3), c=20.433(3) A, i=
,
3
−1
,
90.097(6)°, V=4275.4(10) A , Z=4, v=0.482 mm
,
Mo(h6-C6H5Me)2 (1.35 mmol) and TRIPOD (0.76 g,
1.22 mmol) were heated in a sealed glass tube under
vacuum at 160°C for 48 h. The tube was opened under
nitrogen, the contents were extracted with 1:1 benzene/
heptane (ꢀ30 ml), and the extract was filtered. The
red-orange product crystallized from the solvent upon
R (I\2|(I))=0.0377, wR (I\2|(I))=0.0936, R (all
data)=0.0468, wR (all data)=0.1008, 7524 indepen-
dent reflections, data/restraints/parameters=7517/0/
500. Selected metric data for 1 (R=SiMe3) and related
compounds are compared in Table 1. A thermal ellip-
soid drawing of 1 (R=SiMe3) is illustrated in Fig. 1.
Tables of atomic coordinates and equivalent isotropic
displacement parameters, anisotropic displacement
parameters, hydrogen coordinates and isotropic
parameters, and bond length and angles are available as
supporting information.
1
cooling to 5°C (typical yields of 40–60%). H-NMR
(C6D6, 400 MHz, 20°C): l 7.05 (m, 12H, Ph), 6.93 (t,
6H, J=7 Hz, Ph), 6.83 (t, 12H, J=7 Hz, Ph), 4.41 (br,
2H, ortho-C6H5Me), 4.36 (br, 2H, meta-C6H5Me), 4.26
(br, 1H, para-C6H5Me), 2.16 (s, 6H, CH2), 1.86 (s, 3H,
C6H5Me), 1.10 (s, 3H, TRIPOD CH3). 31P{1H}-NMR
(C6D6, 161 MHz, 20°C): l 46.58 (s). Anal. Found: C,
70.95; H, 5.90. Calc. for C48H47P3Mo (MW 812.77): C,
70.93, H, 5.83%.
4. Supplementary Material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC no. 155775 for compound (h6-
C6H5SiMe3)Mo(TRIPOD). Copies of this information
may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
3.6. Synthesis of (p6-C6H5SiMe3)Mo(TRIPOD)
A procedure analogous to that used to synthesize
(h6-C6H5Me)Mo(TRIPOD) was employed (typical
yields of 40–60%). 1H-NMR (C6D6, 400 MHz, 20°C): l
7.16 (m, 11H, Ph), 7.03 (m, 1H, Ph), 6.92 (t, 6H, J=7
Hz, Ph), 6.85 (t, 12H, J=7 Hz, Ph), 4.90 (d, 2H,
(Fax:
+44-1223-336-033; e-mail: deposit@ccdc.
cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
JHH=6 Hz, ortho-C6H5SiMe3), 4.70 (m, 1H, para-
C6H5SiMe3), 4.17 (t, 2H, HH=5 Hz, meta-
J
C6H5SiMe3), 2.20 (s, 6H, TRIPOD CH2), 1.07 (s, 3H,
TRIPOD CH3), 0.11 (s, 9H, C6H5SiMe3). 31P{1H}-
NMR (C6D6, 161 MHz, 20°C): l 46.20 (s). Anal.
Found: C, 68.99, H, 6.15. Calc. for C50H53P3SiMo
(870.93): C, 68.95, H, 6.13%.
Acknowledgements
Partial financial support of the Petroleum Research
(PRFc 29900-AC3) and the National Science Founda-
tion (CHE-9612869) is gratefully acknowledged.

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