Reversible C-H bond activation in coordinatively unsaturated molybdenum aryloxy complexes, Mo(PMe3)4(OAr)H: Comparison with their tungsten analogs

Article abstract of DOI:10.1021/om960553b

Mo(PMe₃)₆ reacts with ArOH(Ar = C₆H₂Me₃, C₆H₃Prⁱ₂) to give aryloxy-hydride derivatives Mo(PMe₃)₄-(OAr)H in contrast to the cyclometalated derivatives 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 to yield cyclometalated derivatives but that the products so obtained are thermodynamically unstable with respect to the aryloxy-hydride derivatives.

Full text of DOI:10.1021/om960553b

3910
Organometallics 1996, 15, 3910-3912
Rever sible C-H Bon d Activa tion in Coor d in a tively
Un sa tu r a ted Molybd en u m Ar yloxy Com p lexes,
Mo(P Me3)4(OAr )H: Com p a r ison w ith Th eir Tu n gsten
An a logs
Tony Hascall, Vincent J . Murphy, and Gerard Parkin*
Department of Chemistry, Columbia University, New York, New York 10027
Received J uly 8, 1996X
Sch em e 1
Summary: Mo(PMe3)6 reacts with ArOH (Ar ) C6H2Me3,
C6H3Pri2) to give aryloxy-hydride derivatives Mo(PMe3)4-
(OAr)H in contrast to the cyclometalated derivatives that
are obtained for the corresponding tungsten system.
Deuterium labeling and magnetization transfer studies,
however, demonstrate that the coordinatively unsatu-
rated molybdenum complexes Mo(PMe3)4(OAr)H are in
fact kinetically capable of intramolecular oxidative ad-
dition of a C-H bond to yield cyclometalated derivatives
but that the products so obtained are thermodynamically
unstable with respect to the aryloxy-hydride derivatives.
The activation of carbon-hydrogen bonds by transi-
tion metal centers continues to be an important area of
research, with the ultimate goal of achieving selective
functionalization of hydrocarbons.1 Of the most com-
mon nonradical mechanisms for C-H bond activation,
namely (i) σ-bond metathesis, (ii) addition to metal-
ligand multiple bonds, and (iii) oxidative addition, the
lattermost is especially favored for electron-rich transi-
tion metal complexes. We have a particular interest in
the chemistry of such complexes and have demonstrated
that W(PMe3)6 and W(PMe3)4(η2-CH2PMe2)H are ca-
pable of selective intramolecular sp2 and sp3 C-H bond
activation of phenol and its alkyl-substituted deriva-
tives, thereby resulting in the formation of novel four-
and five-membered oxametallacycles, e.g. W(PMe3)4(η2-
OC6H4)H2 and W(PMe3)4[η2-OC6H2Me2(CH2)]H2.2 Since
an appreciation of the factors that promote C-H bond
activation is important, we sought to compare analogous
reactions of the molybdenum counterpart Mo(PMe3)6.3
Significantly, the chemistry of these molybdenum and
tungsten systems proved to be quite distinct with
respect to their ability to achieve C-H bond activation.
ligands.6 Mo(PMe3)2(OPh)4 provides a striking contrast
to the product of the corresponding tungsten system
which results from sp2 C-H bond activation: specifi-
cally, both W(PMe3)6 and W(PMe3)4(η2-PMe2CH2)H
react with phenol to give the four-membered oxa-
metallacycle W(PMe3)4(η2-OC6H4)H2.2
The formation of a tetrakis(aryloxy) derivative Mo-
(PMe3)2(OAr)4 may be inhibited by increasing the steric
demands of ArOH. Indeed, incorporation of substitu-
ents into both ortho-positions allows aryloxy-hydride
complexes Mo(PMe3)4(OAr)H to be isolated. For ex-
ample, Mo(PMe3)6 reacts with 2,4,6-trimethylphenol and
2,6-diisopropylphenol to give the purple diamagnetic
(4) The formation of aryloxy derivatives accompanied by elimination
of H2 has also been observed in a related tungsten system, namely
the reactions of W(PMePh2)4Cl2 with phenol or p-cresol to give WCl2-
(PMePh2)2(OAr)2. See: J ang, S.; Atagi, L. M.; Mayer, J . M. J . Am.
Chem. Soc. 1990, 112, 6413-6414.
Mo(PMe3)6 reacts with an excess of phenol in benzene
to give the orange paramagnetic tetrakis(phenoxy)
complex Mo(PMe3)2(OPh)4, accompanied by elimination
of H2 (Scheme 1).4 Mo(PMe3)2(OPh)4 has been charac-
terized by X-ray diffraction,5 identifying an octahedral
geometry about the metal center, with trans-PMe3
(5) Mo(PMe3)2(OPh)4 is monoclinic, P21/ c (No. 14), with
a )
10.212(2) Å, b ) 10.606(3) Å, c ) 14.607(2) Å, â ) 98.11(2)°, and Z )
2. Mo(PMe3)4(OC6H2Me3) is triclinic, P1h (No. 2), with a ) 9.514(2) Å,
b ) 12.220(2) Å, c ) 12.820(3) Å, R ) 87.24(2)°, â ) 77.80(2)°, γ )
81.99(2)°, and Z ) 2. Mo(PMe3)4(OC6H3Pri2)H is orthorhombic, Pnma
(No. 62), with a ) 20.055(5) Å, b ) 17.076(5) Å, c ) 9.400(3) Å, and Z
) 4.
(6) The Mo-O bond lengths in Mo(PMe3)2(OPh)4 [1.958(3) and
1.966(3) Å] are comparable to those in the related complex Mo-
(NHMe2)2(O-2-C6H4Ph)4 [1.972(3) and 1.963(3) Å].6a For reference, the
mean Mo-OAr bond length for complexes listed in the Cambridge
Structural Database (version 5.11) is 1.99 Å. See, for example: (a)
Bartos, M. J .; Kriley, C. E.; Yu, J . S.; Kerschner, J . L.; Fanwick, P. E.;
Rothwell, I. P. Polyhedron 1989, 8, 1971-1977. (b) Kerschner, J . L.;
Yu, J . S.; Fanwick, P. E.; Rothwell, I. P.; Huffman, J . C. Organo-
metallics 1989, 8, 1414-1418. (c) Walborsky, E. C.; Wigley, D. E.;
Roland, E.; Dewan, J . C.; Schrock, R. R. Inorg. Chem. 1987, 26, 1615-
1621. (d) Kerschner, J . L.; Torres, E. M.; Fanwick, P. E.; Rothwell, I.
P.; Huffman, J . C. Organometallics 1989, 8, 1424-1431.
X Abstract published in Advance ACS Abstracts, August 15, 1996.
(1) (a) Activation and Functionalization of Alkanes; Hill, C. L., Ed.;
J ohn Wiley and Sons: New York, 1989. (b) Arndtsen, B. A.; Bergman,
R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154-
162. (c) Ryabov, A. D. Chem. Rev. 1990, 90, 403-424. (d) J ones, W.
D.; Feher, F. J . Acc. Chem. Res. 1989, 22, 91-100. (e) Special issue:
J . Organomet. Chem. 1996, 504(1,2).
(2) (a) Rabinovich, D.; Zelman, R.; Parkin, G. J . Am. Chem. Soc.
1990, 112, 9632-9633. (b) Rabinovich, D.; Zelman, R.; Parkin, G. J .
Am. Chem. Soc. 1992, 114, 4611-4621.
(3) (a) Murphy, V. J .; Parkin, G. J . Am. Chem. Soc. 1995, 117, 3522-
3528. (b) Brookhart, M.; Cox, K.; Cloke, F. G. N.; Green, J . C.; Green,
M. L. H.; Hare, P. M.; Bashkin, J .; Derome, A. E.; Grebenik, P. D. J .
Chem. Soc., Dalton Trans. 1985, 423-433.
S0276-7333(96)00553-5 CCC: $12.00 © 1996 American Chemical Society

Communications
Organometallics, Vol. 15, No. 19, 1996 3911
Sch em e 2
F igu r e 1. Molecular structure of Mo(PMe3)4(OC6H2Me3)H.
complexes Mo(PMe3)4(OC6H2Me3)H and Mo(PMe3)4-
(OC6H3Pri )H, respectively (Scheme 1). Decisive char-
2
acterization of the products as aryloxy-hydride com-
plexes is provided by NMR spectroscopy.7 For example,
the 1H NMR spectra of Mo(PMe3)4(OC6H2Me3)H and
Mo(PMe3)4(OC6H3Pri )H exhibit triplet of triplet reso-
2
nances attributable to Mo-H ligands at δ -6.59 (J P-H
) 23, 84 Hz) and -6.53 (J P-H ) 22, 85 Hz), respectively.
Furthermore, the observation of doublet resonances in
the selectively decoupled 31P{1H-P(CH3)3} spectra pro-
vides convincing evidence that each complex contains
only a single Mo-H ligand.8
Me3, 2.065(3) Å; Ar ) C6H3Pri , 2.072(6) Å] are inter-
2
mediate between those in the formally 14-electron
complex Mo(PMe3)2(OPh)4 [1.958(3) and 1.966(3) Å] and
the 18-electron derivative Mo(PMePh2)2(O-2,6-C6H3-
Ph2)H [2.164(5) Å].6d,14,15
The molecular structures of Mo(PMe3)4(OC6H2Me3)H
and Mo(PMe3)4(OC6H3Pri )H have been determined by
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 the intermediates responsible for the C-H bond
activation reactions in the corresponding tungsten
system. For example, the five-membered oxametalla-
cycle W(PMe3)4[η2-OC6H2Me2(CH2)]H2 was proposed to
be formed via intramolecular oxidative addition of one
of the ortho-methyl C-H bonds to the tungsten center
in the intermediate [W(PMe3)4(OC6H2Me3)H].2 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.16
Consequently, the energy required to break the C-H
bond is not compensated by the formation of Mo-C and
2
X-ray diffraction (Figure 1).5 Neglecting the hydride
ligand, the coordination environment about molybde-
num is based on a trigonal bipyramid, with axial PMe3
ligands; the hydride ligand appears to be located close
to the equatorial plane and trans to the aryloxy sub-
stituent.9 As such, the trans arrangement of the aryloxy
and hydride ligands contrasts with the cis geometries
observed for the related 18-electron octahedral com-
plexes Ru(PMe3)4(OC6H4Me)H10 and [Ir(PMe3)4(OMe)H]-
[PF6].11 Moreover, consistent with the suggestion that
the hydride ligands of Mo(PMe3)4(OC6H2Me3)H and
Mo(PMe3)4(OC6H3Pri )H are best described as trans to
2
an aryloxy rather than a PMe3 ligand, the hydride
1
signals in the H NMR spectra do not exhibit the large
trans J P-H couplings of ca. 100 and 145 Hz that are
observed for Ru(PMe3)4(OC6H4Me)H10 and [Ir(PMe3)4-
(OMe)H][PF6],11 respectively.12
Since the aryloxy complexes Mo(PMe3)4(OAr)H are
formally 16-electron (d4), lone pair π donation from
oxygen is presumably operative.13 Consequently, the
Mo-OAr bond lengths in Mo(PMe3)4(OAr)H [Ar ) C6H2-
(13) Consistent with the occurence of lone pair donation, the
aryloxy-hydride complexes Mo(PMe3)4(OAr)H are diamagnetic and
provide a contrast to the paramagnetic nature of closely related
Mo(PR3)4X2 (e.g. X ) Cl, I). See ref 3b and: Carmona, E.; Mar´ın, J .
M.; Poveda, M. L.; Atwood, J . L.; Rogers, R. D. Polyhedron 1983, 2,
185-193.
(14) Rothwell has also noted a correlation between electron count
and W-OAr bond length in a series of tungsten aryloxide complexes.
See: Kerschner, J . L.; Fanwick, P. E.; Rothwell, I. P. Inorg. Chem.
1989, 28, 780-786.
(15) Furthermore, the bond angles at oxygen for Mo(PMe3)4(OC6H2-
Me3)H and Mo(PMe3)4(OC6H3Pri2)H are 159.7(3) and 145.9(6)°, respec-
tively; however it should be noted that on its own such a parameter
cannot be reliably used as an indicator of π-bonding. See ref 14 and:
(a) Howard, W. A.; Trnka, T. M.; Parkin, G. Inorg. Chem. 1995, 34,
5900-5909. (b) Steffey, B. D.; Fanwick, P. E.; Rothwell, I. P. Polyhe-
dron 1990, 9, 963-968. (c) Coffindaffer, T. W.; Steffy, B. D.; Rothwell,
I. P.; Folting, K.; Huffman, J . C.; Streib, W. E. J . Am. Chem. Soc. 1989,
111, 4742-4749.
(16) (a) Bonding Energetics in Organometallic Compounds; Marks,
T. J ., Ed.; ACS Symposium Series; American Chemical Society:
Washington, DC, 1990; Vol. 428. (b) Martinho Simo˜es, J . A.; Beau-
champ, J . L. Chem. Rev. 1990, 90, 629-688. (c) Eisenberg, D. C.;
Norton, J . R. Israel J . Chem. 1991, 31, 55-66. (d) Skinner, H. A.;
Connor, J . A. Pure Appl. Chem. 1985, 57, 79-88.
(7) See Supporting Information.
(8) Such data clearly exclude the possibility of the complexes being,
in fact, 18-electron trihydride complexes Mo(PMe3)4(OAr)H3.
(9) The location of the hydride ligands were observed in electron
density difference maps; however, as with most hydrides coordinated
to heavy metals, their location as determined by X-ray diffraction is
subject to considerable uncertainty.
(10) (a) Osakada, K.; Ohshiro, K.; Yamamoto, A. Organometallics
1991, 10, 404-410. (b) Hartwig, J . F.; Andersen, R. A.; Bergman, R.
G. Organometallics 1991, 10, 1875-1887.
(11) Milstein, D.; Calabrese, J . C.; Williams, I. D. J . Am. Chem. Soc.
1986, 108, 6387-6389.
(12) The hydride and thiolate ligands in Mo(dppe)2(SC6H2Pri3)H
have also been reported to adopt a cis disposition, but the geometry is
grossly distorted from octahedral. The structure is also fluxional on
the NMR time scale, and the hydride resonance appears as a quintet
with J P-H ) 47 Hz. See: Henderson, R. A.; Hughes, D. L.; Richards,
R. L.; Shortman, C. J . Chem. Soc., Dalton Trans. 1987, 1115-1121.

3912 Organometallics, Vol. 15, No. 19, 1996
Communications
Mo-H bonds.17 Thus, for molybdenum, the equilibrium
between the aryloxy derivative and its cyclometalated
counterpart lies in favor of the former species, while for
tungsten the latter prevails. The relative energetics of
these two equilibria resembles that of the cyclometala-
tion equilibria involving M(PMe3)6 and M(PMe3)4(η2-
CH2PMe2)H, for which the equilibrium constant for the
tungsten system is a factor of ca. 2 × 103 greater than
that for molybdenum at 30 °C.3a
Evidence that the molybdenum aryloxy-hydride com-
plexes are indeed in equilibrium with their cyclo-
metalated derivatives, and that C-H bond activation
is kinetically feasible, is provided by magnetization
transfer and deuterium exchange studies. For example,
proton magnetization transfer is observed between the
Mo-H ligands and the ortho-methyl substituents of the
aryloxy group in Mo(PMe3)4(OC6H2Me3)H. Likewise,
scrambling of deuterium into the ortho-methyl groups
is observed in the reaction of Mo(PMe3)6 with 2,4,6-
Me3C6H2OD. Both of these processes can be rational-
ized in terms of a mechanism that involves the revers-
ible formation of a five-membered oxametallacycle-
dihydride complex via sp3 C-H bond activation, thereby
permitting site exchange, as illustrated in Scheme 2.18
In summary, the reactions of Mo(PMe3)6 with phenol
and its derivatives provide some interesting contrasts
with the corresponding reactions of the tungsten com-
plexes W(PMe3)6 and W(PMe3)4(η2-CH2PMe2)H. Specif-
ically, the tendency to achieve C-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 cy-
clometalated derivatives, e.g. W(PMe3)4(η2-OC6H4)H2
and W(PMe3)4[η2-OC6H2Me2(CH2)]H2, are obtained for
the tungsten system. Such differences are attributed
to weaker M-C and M-H bond energies for molybde-
num compared to tungsten.
Ack n ow led gm en t. We thank the U.S. Department
of Energy, Office of Basic Energy Sciences (Grant No.
DE-FG02-93ER14339), for support of this research. G.P.
is the recipient of a Camille and Henry Dreyfus Teacher-
Scholar Award (1991-1996) and a Presidential Faculty
Fellowship Award (1992-1997).
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details, tables of spectroscopic data and crystal struc-
ture data, and ORTEP diagrams for Mo(PMe3)2(OPh)4,
Mo(PMe3)4(OC6H2Me3), and Mo(PMe3)4(OC6H3Pri2)H (26 pages).
Ordering information is given on any current masthead page.
OM960553B
(18) It is, of course, possible that the six-coordinate complexes
Mo(PMe3)4(OAr)H may dissociate PMe3 prior to the C-H bond activa-
tion step; cf. the mechanism for oxidative addition of H2 to W(PMe3)4I2.
See: Rabinovich, D.; Parkin, G. J . Am. Chem. Soc. 1993, 115, 353-
354.
(17) It must also be recognized that the oxidative addition of the
C-H bond will be accompanied by loss of oxygen to metal π-donation.