Oxidative addition of dihydrogen to (η6-arene)Mo(PMe 3)3 complexes: Origin of the naphthalene and anthracene effects

Article abstract of DOI:10.1021/ja058107f

In contrast to the benzene and naphthalene compounds (η⁶- PhH)Mo(PMe₃)₃ and (η⁶-NpH)Mo-(PMe ₃)₃, the anthracene complex (η⁶-AnH) Mo(PMe₃)₃ reacts with H₂ to undergo a haptotropic shift and give the η⁴-anthracene compound (η⁴-AnH)Mo(PMe₃)₃H₂. Density functional theory calculations indicate that the increased facility of naphthalene and anthracene to adopt η⁴-coordination modes compared to that of benzene is a consequence of the fact that the Mo-(η⁴-ArH) bonding interaction increases in the sequence benzene < naphthalene < anthracene, while the Mo-(η⁶-ArH) bonding interaction follows the sequence benzene > naphthalene ≈ anthracene.

Full text of DOI:10.1021/ja058107f

Published on Web 04/01/2006
6
Oxidative Addition of Dihydrogen to (η -Arene)Mo(PMe
3
)
3
Complexes:
Origin of the Naphthalene and Anthracene Effects
Guang Zhu, Kevin E. Janak, Joshua S. Figueroa, and Gerard Parkin*
Contribution from the Department of Chemistry, Columbia UniVersity,
New York, New York 10027
Received November 29, 2005; E-mail: parkin@columbia.edu
6
6
Abstract: In contrast to the benzene and naphthalene compounds (η -PhH)Mo(PMe
)
3 3
and (η -NpH)Mo-
6
(
PMe
3
)
3
, the anthracene complex (η -AnH)Mo(PMe
3
)
3
reacts with H
2
to undergo a haptotropic shift and
4
4
give the η -anthracene compound (η -AnH)Mo(PMe
3
)
3
H
2
. Density functional theory calculations indicate
4
that the increased facility of naphthalene and anthracene to adopt η -coordination modes compared to
4
that of benzene is a consequence of the fact that the Mo-(η -ArH) bonding interaction increases in the
sequence benzene < naphthalene < anthracene, while the Mo-(η -ArH) bonding interaction follows the
6
sequence benzene > naphthalene ≈ anthracene.
Introduction
Scheme 1
It has long been known that indenyl complexes of the tran-
sition metals typically exhibit much more facile associative
reactions than their cyclopentadienyl counterparts.1,2 Indeed, the
magnitude of the rate enhancement is so substantial that a special
term was coined by Basolo to describe it, viz. the “indenyl
3
effect”. The origin of the indenyl effect is associated with the
fact that the indenyl ligand shows a greater tendency to adopt
3
an η -coordination mode than does the cyclopentadienyl ligand,
and the coordinative unsaturation that is provided by the ring
slippage facilitates an associative mechanism. Not only are such
haptotropic shifts now widely recognized to play a prevalent
role in the chemistry of cyclopentadienyl, indenyl, and fluorenyl
4
complexes, but more facile ring slippage has also been proposed
to account for the increased lability of naphthalene compared
have been concerned with complete arene displacement by donor
to that of benzene in metal-arene complexes, and this phe-
nomenon has been referred to as the “naphthalene effect”.
ligands such as CO and PR , and we are aware of none involving
3
5
,6
oxidative addition of H , a reaction that is of considerable
2
8
However, despite the many observations that naphthalene is
more readily displaced than benzene, there are no experimental
studies that quantify the influence of ring fusion on a well-
significance with respect to arene hydrogenation. In this paper,
we (i) provide evidence that ring fusion to an arene promotes
the oxidative addition of H to a metal center via facilitating an
2
6
4
7
6
4
defined reversible η /η haptotropic shift. Furthermore, the
η -to-η haptotropic shift and (ii) provide an explanation for
majority of studies pertaining to the influence of ring fusion
the effect.
(
1) For early examples, see: (a) Hart-Davis, A. J.; Mawby, R. J. J. Chem.
Soc. (A) 1969, 2403-2407. (b) Hart-Davis, A. J.; White, C.; Mawby, R. J.
Inorg. Chim. Acta 1970, 4, 441-446. (c) Jones, D. J.; Mawby, R. J. Inorg.
Chim. Acta 1972, 6, 157-160.
Results and Discussion
6
1
. Synthesis of (η -ArH)Mo(PMe3)3 and Reactivity to-
6
6
9
6
wards H2. The η -arene compounds (η -PhH)Mo(PMe3)3, (η -
(
2) (a) Basolo, F. Polyhedron 1990, 9, 1503-1535. (b) Basolo, F. Inorg. Chim.
Acta 1985, 100, 33-39. (c) Basolo, F. New J. Chem. 1994, 18, 19-24.
3) (a) Rerek, M. E.; Ji, L. N.; Basolo, F. J. Chem. Soc., Chem. Commun.
10
6
NpH)Mo(PMe3)3, and (η -AnH)Mo(PMe3)3 may be conve-
niently obtained via reaction of Mo(PMe3)6 with the appropriate
arene, namely PhH (benzene), NpH (naphthalene), and AnH
(anthracene), as illustrated in Scheme 1. The molecular structures
(
1
983, 1208-1209. (b) Rerek, M. E.; Basolo, F. J. Am. Chem. Soc. 1984,
06, 5908-5912.
1
(
4) (a) O’Connor, J. M.; Casey, C. P. Chem. ReV. 1987, 87, 307-318. (b) Alt,
H. G.; Samuel, E. Chem. Soc. ReV. 1998, 27, 323-329. (c) Calhorda, M.
J.; Veiros, L. F. Coord. Chem. ReV. 1999, 185-186, 37-51.
6
6
of (η -NpH)Mo(PMe3)3 and (η -AnH)Mo(PMe3)3 have been
(
5) (a) Thi, N. P. D.; Spichiger, S.; Paglia, P.; Bernardinelli, G.; K u¨ ndig, E.
P.; Timms, P. L. HelV. Chim. Acta 1992, 75, 2593-2607. (b) K u¨ ndig, E.
P.; Perret, C.; Spichiger, P. S.; Bernardinelli, G. J. Organomet. Chem. 1985,
determined by X-ray diffraction, as illustrated in Figures 1 and
2
. Examination of the Mo-C bond length data (Table 1)
2
86, 183-200.
4
(
6) The ability to access an η -coordination mode is also of importance with
respect to the product release step of the transition-metal-promoted
functionalization of arenes. See, for example: (a) Semmelhack, M. F.;
Chlenov, A.; Ho, D. M. J. Am. Chem. Soc. 2005, 127, 7759-7773. (b)
Butler, I. S.; Uhm, H. L. Comments Inorg. Chem. 1988, 7, 1-16.
6
4
(7) For examples of η -to-η haptotropic shifts of arene ligands, see: (a)
Bennett, M. A.; Lu, Z.; Wang, X.; Bown, M.; Hockless, D. C. R. J. Am.
Chem. Soc. 1998, 120, 10409-10415. (b) Boncella, J. M.; Green, M. L.
H. J. Organomet. Chem. 1987, 325, 217-231.
5452
9
J. AM. CHEM. SOC. 2006, 128, 5452-5461
10.1021/ja058107f CCC: $33.50 © 2006 American Chemical Society

Addition of H
2
to (η6-Arene)Mo(PMe
)
3 3
Complexes
A R T I C L E S
Scheme 2
6
Figure 1. Molecular structure of (η -NpH)Mo(PMe3)3.
6
6
4
slipped” η -arene compound is provided by Ti(η -AnH)(η -
6
4
AnH)(dmpe), which contains both η - and η -coordinated
anthracene ligands.
arene ligands in (η -NpH)Mo(PMe3)3 and (η -AnH)Mo(PMe3)3
has been addressed by using computational methods and is
described in section 2.
1
5,16
The origin of the displacement of the
6
6
6
In view of the fact that the zerovalent (η -ArH)Mo(PMe3)3
derivatives are electron-rich (i.e., have high-energy occupied
1
7
metal-based orbitals), we considered that each would be
susceptible towards oxidative addition of dihydrogen, cf. the
6
18
Figure 2. Molecular structure of (η -AnH)Mo(PMe3)3 (only one of the
facile formation of Mo(PMe3)4H4 from Mo(PMe3)6. However,
two independent molecules is shown).
6
in contrast to Mo(PMe3)6, (η -PhH)Mo(PMe3)3 is unreactive
towards H2 (1 atm) in benzene solution at 80 °C (Scheme 2).19
6
Table 1. Selected Bond Lengths (Å) for (η -NpH)Mo(PMe3)3 and
6
(
η -AnH)Mo(PMe3)3
(
14) M u¨ ller, P.; Bernarddinelli, G.; Jacquier, Y. HelV. Chim. Acta 1992, 75,
1995-2008.
a
3 3
( )
η6-AnH)Mo(PMe
b,c
(
η6-NpH)Mo(PMe
3
)
3
6
(15) The η -anthracene ligand is characterized by Ti-C bond lengths of 2.31-
Mo(X)-C(Y2)
Mo(X)-C(Y3)
Mo(X)-C(Y4)
Mo(X)-C(Y5)
Mo(X)-C(Y1)
Mo(X)-C(Y6)
Mo(X)-P(Y′1)
Mo(X)-P(Y′2)
Mo(X)-P(Y′3)
2.263(12)
2.245(16)
2.220(12)
2.270(10)
2.378(11)
2.404(8)
2.381(3)
2.418(3)
2.393(3)
2.297(3)
2.261(3)
2.285(3)
2.268(3)
2.405(3)
2.424(3)
2.4274(8)
2.3930(9)
2.4350(7)
2.307(3)
2.270(3)
2.285(3)
2.267(3)
2.422(3)
2.419(3)
2.4342(8)
2.4354(8)
2.3976(9)
2
.36 Å, while the Ti-C distances for the ring junction average 2.63 Å.
See: Seaburg, J. K.; Fischer, P. J.; Young, V. G.; Ellis, J. E. Angew. Chem.,
Int. Ed. 1998, 37, 155-157.
4
(
16) For other examples of η -anthracene complexes, see: (a) Brennessel, W.
W.; Ellis, J. E.; Pomije, M. K.; Sussman, V. J.; Urnezius, E.; Young, V.
G., Jr. J. Am. Chem. Soc. 2002, 124, 10258-10259. (b) Brennessel, W.
W.; Ellis, J. E.; Roush, S. N.; Strandberg, B. R.; Woisetschl a¨ ger, O. E.;
Young, V. G., Jr. Chem. Commun. 2002, 2356-2357. (c) Mashima, K.;
Nakayama, Y.; Kaidzu, M.; Ikushima, N.; Nakamura, A. J. Organomet.
Chem. 1998, 557, 3-12. (d) M u¨ ller, J.; Gaede, P. E.; Hirsch, C.; Qiao, K.
J. Organomet. Chem. 1994, 472, 329-335. (e) Boese, R.; Stanger, A.;
Stellberg, P.; Shazar, A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1475-
a
X ) Y ) Y′ ) no value. b X ) 1, Y ) 10, Y′ ) 1. c X ) 2, Y ) 20,
1
477. (f) Protasiewicz, J. D.; Bianconi, P. A.; Williams, I. D.; Liu, S.;
Y′ ) 2.
Rao, C. P.; Lippard, S. J. Inorg. Chem. 1992, 31, 4134-4142. (g) Bennett,
M. A.; Bown, M.; Hockless, D. C. R. Aust. J. Chem. 2000, 53, 507-515.
6
(
17) Evidence for electron richness of (η -arene)Mo(PR
)
3 3
derivatives is provided
indicates that the arene ligands are displaced from symmetric
η -coordination towards a “flat” η -coordination mode. Similar
flat-slipped” coordination modes have been observed for (η -
6
by the observations that (i) the molybdenum center in [η -C
6 5 6 3
H C H (Ph)-
6
4
11
OH]Mo(PMe ) serves as a hydrogen bond acceptor such that the complex
3
3
a
6
6
possesses an intramolecular Mo‚‚‚HsOAr hydrogen bond, (ii) (η -arene)-
“
6
Mo(PR
(PR
3
H
)
2
3
]
complexes may be doubly protonated to give [(η -arene)Mo-
12
6
2+ b 6
AnH)Cr(CO)3 and other (η -ArH)Cr(CO)3 complexes which
3
)
3
3 3
, and (iii) (η -arene)Mo(PR ) derivatives have low ionization
c,d
feature fused polycyclic arenes.1
3,14
potentials. (a) Hascall, T.; Baik, M.-H.; Bridgewater, B. M.; Shin, J. H.;
Another example of a “flat-
Churchill, D. G.; Friesner, R. A.; Parkin, G. J. Chem. Soc., Chem. Commun.
2002, 2644-2645. (b) Green, M. L. H.; Mitchard, L. C.; Silverthorn, W.
(
8) (a) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chemical 2003, 191,
87-207. (b) Amer, I.; Amer, H.; Ascher, R.; Blum, J.; Sasson, Y.;
Vollhardt, K. P. C. J. Mol. Catal. 1987, 39, 185-194.
E. J. Chem. Soc., Dalton Trans. 1974, 1361-1363. (c) Green, M. L. H.;
Hughes, A. K.; Lincoln, P.; Martin-Polo, J. J.; Mountford, P.; Sella, A.
Wong, L.-L.; Bandy, J. A.; Banks, T. W.; Prout, K.; Watkin, D. J. J. Chem.
Soc., Dalton Trans. 1992, 2063-2069. (d) Asirvatham, V. S.; Gruhn, N.
E.; Lichtenberger, D. L.; Ashby, M. T. Organometallics 2000, 19, 2215-
2227.
1
6
(
9) (η -PhH)Mo(PMe
Asirvatham, V. S.; Kowalski, A. S.; Khan, M. A. Organometallics 1999,
8, 5004-5016. (b) Canestrari, M.; Green, M. L. H.; Izquierdo, A. J. Chem.
Soc., Dalton Trans. 1984, 2795-2801.
3 3
) has been recently reported. See: (a) Ashby, M. T.;
1
(18) 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.
6
(
(
3 3
10) (η -NpH)Mo(PMe ) has been previously reported. See ref 5a.
11) For a review of the structures of metal-arene compounds, see: Hubig, S.
M.; Lindeman, S. V.; Kochi, J. K. Coord. Chem. ReV. 2000, 200-202,
2
(19) Oxidative addition of H may, nevertheless, be induced photochemically
6
8
31-873.
3 2 2
to yield the dihydride complex (η -PhH)Mo(PMe ) H . The latter com-
6
(
(
12) Hanic, F.; Mills, O. S. J. Organomet. Chem. 1968, 11, 151-158.
13) Arrais, A.; Diana, E.; Gervasio, G.; Gobetto, R.; Marabello, D.; Stanghellini,
P. L. Eur. J. Inorg. Chem. 2004, 1505-1513.
pound, however, reacts with PMe
3
at room temperature to regenerate (η -
6
PhH)Mo(PMe
PhH)Mo(PMe
3
)
)
3
, thereby indicating that oxidative addition of H
is reversible.
2
to (η -
3
3
J. AM. CHEM. SOC.
9
VOL. 128, NO. 16, 2006 5453

A R T I C L E S
Zhu et al.
6
Figure 3. Geometry-optimized structures of (η -ArH)Mo(PH3)3.
6
The naphthalene complex (η -NpH)Mo(PMe3)3 is likewise inert
that ring only, the thermodynamics of the isomerization has been
towards H2. It is, therefore, interesting that the anthracene
postulated to be a consequence of the variation in resonance
6
2c,23
derivative (η -AnH)Mo(PMe3)3 reacts readily with H2 at room
energies of the individual arenes.
Thus, since the incremental
4
20
temperature to yield (η -AnH)Mo(PMe3)3H2, a transformation
increase in resonance energy of “linear” fused arenes diminishes
6
4
24
that is accompanied by an η -to-η haptotropic shift of the
as the length of the arene increases, the loss of resonance
anthracene ligand. The haptotropic shift is reversible, and
removal of the H2 atmosphere results in regeneration of (η -
AnH)Mo(PMe3)3. The ability of the anthracene ligand to
promote a transformation that is not observed for either the
benzene or naphthalene counterparts may be described as an
energy associated with the folding of the arene decreases in the
sequence benzene > naphthalene > anthracene, and this
variation has been proposed to rationalize the more facile
6
2c,23
displacement of the larger arene.
While this simple argument,
based only on energetic changes involving the arene ligand,
6
“
anthracene effect”.
In addition to forming an η -arene compound, anthracene is
unique in that the ligand may be hydrogenated upon prolonged
could certainly rationalize the experimental observation that (η -
4
AnH)Mo(PMe3)3 reacts more exothermically with H2 than does
6
6
either (η -PhH)Mo(PMe3)3 or (η -NpH)Mo(PMe3)3, we have
performed density functional theory (DFT, B3LYP) calculations
to procure a more detailed understanding of the influence of
ring fusion on the haptotropic shifts of arene compounds.
2
1
heating at 90 °C to liberate 1,2,3,4-tetrahydroanthracene.
Polycyclic arenes are typically easier to hydrogenate than
monocyclic arenes, and the homogeneous hydrogenation of
8
6
anthracene has been well documented. The formation of 1,2,3,4-
The geometry-optimized structures of (η -ArH)Mo(PH3)3 and
4
tetrahydroanthracene rather than 9,10-dihydroanthracene is
(η -ArH)Mo(PH3)3H2 are illustrated in Figures 3 and 4, respec-
indicative of a nonradical mechanism,22 and thereby provides
tively, with selected bond lengths listed in Tables 2 and 3.
4
6
good evidence that hydrogenation occurs via (η -AnH)Mo-
Significantly, the calculated structures of (η -NpH)Mo(PH3)3
6
(PMe3)3H2.
and (η -AnH)Mo(PH3)3 reproduce the “flat-slipped” coordina-
2
. Origin of the Naphthalene and Anthracene Effects. As
tion mode of the arenes in the experimentally derived structures
of (η -NpH)Mo(PMe3)3 and (η -AnH)Mo(PMe3)3. In addition,
the calculations also indicate that the arene ligands of the
6
6
noted above, ring fusion to benzene facilitates displacement of
an arene. For example, the ease of substitution of the arene
ligand of (η -ArH)Cr(CO)3 by PR3 increases in the sequence
benzene < naphthalene < anthracene, a reaction that has been
postulated to occur via an (η -ArH)Cr(CO)3(PR3) intermediate.
This trend has previously been rationalized in terms of the loss
of resonance energy upon folding the arene from the planar
geometry in (η -ArH)Cr(CO)3 to that of the bent geometry in
the proposed (η -ArH)Cr(CO)3(PR3) intermediate. Specifically,
6
4
4
dihydrides (η -ArH)Mo(PH3)3H2 exhibit an η -folded config-
uration. The variation in the C-C bond lengths associated with
4
23
6
4
the coordinated arene rings of (η -ArH)Mo(PH3)3 and (η -ArH)-
Mo(PH3)3H2 is summarized in Figure 5. The most notable
feature, which is exemplified for the benzene derivatives, is that
the C-C bond lengths for the uncoordinated “double bonds”
in the η -complexes are shorter than the corresponding values
in the η -derivatives, while the adjacent bonds are longer.
6
4
4
4
6
25
assuming that folding of the arene and adoption of an η -
coordination mode causes a total loss of resonance energy for
These changes may be viewed in terms of the four carbon atoms
involved in the η -coordination becoming “deconjugated” from
4
4
4
(
6 8 3 3 2
20) A closely related η -cyclohexadiene compound, (η -C H )Mo(PMe ) H ,
has been reported. See: Green, M. L. H.; Parkin, G. J. Chem. Soc., Chem.
Commun. 1985, 771-773.
(24) Calculated resonance energies: benzene, 20.0 kcal mol-1; naphthalene, 30.5
kcal mol- ; and anthracene, 36.9 kcal mol . See: Dewar, M. J.; Liano,
1
-1
(
3 4 4
21) Mo(PMe ) H is the major molybdenum-containing product, but insoluble
C. D. J. Am. Chem. Soc. 1969, 91, 789-795.
unidentified species are also formed.
(25) For representive papers describing the bonding variations in indenyl
complexes, see: (a) Westcott, S. A.; Kakkar, A. K.; Stringer, G.; Taylor,
N. J.; Marder, T. B. J. Organomet. Chem. 1990, 394, 777-794. (b) Kakkar,
A. K.; Jones, S. F.; Taylor, N. J.; Collins, S.; Marder, T. B. J. Chem. Soc.,
Chem. Commun. 1989, 1454-1456. (c) Trnka, T. M.; Bonanno, J. B.;
Bridgewater, B. M.; Parkin, G. Organometallics 2001, 20, 3255-3264.
(d) Cadierno, V.; Diez, J.; Gamasa, M. P.; Gimeno, J.; Lastra, E. Coord.
Chem. ReV. 1999, 193-195, 147-205. (e) Calhorda, M. J.; F e´ lix, V.;
Veiros, L. F. Coord. Chem. ReV. 2002, 230, 49-64. (f) Reference 4.
(
22) (a) Halpern, J. Pure Appl. Chem. 1979, 51, 2171-2182. (b) Landis, C. R.;
Halpern, J. Organometallics 1983, 2, 840-842. (c) Grey, R. A.; Pez, G.
P.; Wallo, A. J. Am. Chem. Soc. 1980, 102, 5948-5949. (d) Fordyce, W.
A.; Wilczynski, R.; Halpern, J. J. Organomet. Chem. 1985, 296, 115-
1
1
25. (e) Lynch, T. J.; Banah, M.; Kaesz, H. D.; Porter, C. R. J. Org. Chem.
984, 49, 1266-1270.
(
23) Zhang, S.; Shen, J. K.; Basolo, F.; Ju, T. D.; Lang, R. F.; Kiss, G.; Hoff,
C. D. Organometallics 1994, 13, 3692-3702.
5454 J. AM. CHEM. SOC.
9
VOL. 128, NO. 16, 2006

Addition of H
2
to (η6-Arene)Mo(PMe
)
3 3
Complexes
A R T I C L E S
4
Figure 4. Geometry-optimized structures of (η -ArH)Mo(PH3)3H2.
4
Table 2. Average Mo-C Bond Lengths for the Outer, Central,
Mo(PH3)3 to that in (η -ArH)Mo(PH3)3H2. As expected, the
and Ring Junction Carbon Atoms of Geometry-Optimized
facility of folding the arene fragment does indeed increase in
the sequence benzene (33.9 kcal mol ) < naphthalene (30.6
kcal mol ) < anthracene (26.8 kcal mol ), as summarized in
Table 4 and Figure 6. However, the magnitude of the effect is
sufficiently small that it does not determine the overall
thermodynamics (Figure 7). Thus, the energy required to fold
6
(
η -ArH)Mo(PH3)3
-
1
benzenea
naphthalene
anthracene
2 6 4
1,2-Me C H
-
1
-1
d(Mo-Couter)/Å
d(Mo-Ccentral)/Å
d(Mo-Cjunction)/Å
2.323
2.324
2.325
2.314
2.310
2.475
2.312
2.319
2.533
2.319
2.321
2.354
a
-1
The outer, central, and junction designations for benzene are arbitrary.
the arene decreases by only 7.1 kcal mol from benzene to
anthracene, whereas the exothermicity of the overall reaction
-
1
increases by 25.3 kcal mol . It is, therefore, evident that other
factors are responsible for the observed effect, of which two
are (i) energetic changes due to structural modifications of the
Table 3. Average Mo-C Bond Lengths for the Outer, Central,
and Ring Junction Carbon Atoms of Geometry-Optimized
4
(
η -ArH)Mo(PH3)3H2
benzene
naphthalene
anthracene
[
Mo(PH3)3] and [Mo(PH3)3H2] moieties in the various arene
6
4
d(Mo-Couter)/Å
d(Mo-Ccentral)/Å
d(Mo-Cjunction)/Å
2.299
2.467
3.265
2.284
2.424
3.220
2.282
2.416
3.190
complexes and (ii) variation of Mo-(η -ArH) and Mo-(η -
ArH) bond energies; a simple thermodynamic cycle that
incorporates these factors is illustrated in Figure 8.
Of these factors, the structural differences of the [Mo(PH3)3]
6
4
and [Mo(PH3)3H2] moieties in (η -ArH)Mo(PH3)3 and (η -ArH)-
Mo(PH3)3H2, respectively, are sufficiently small that they
contribute little to the variation in overall thermodynamics, as
illustrated by the similar energies associated with the oxidative
addition of H2 (∆E+H ) to the various [Mo(PH3)3] fragments
2
(Table 4 and Figure 9). The variation in overall thermodynamics
does, however, correlate well with changes predicted by
6
4
consideration of Mo-(η -ArH) and Mo-(η -ArH) bond ener-
6
gies (Figure 10). Specifically, the intrinsic “snap” Mo-(η -ArH)
bond dissociation energies (BDEs) decrease in the sequence
-
1
-1
benzene (67.1 kcal mol ) > naphthalene (62.2 kcal mol ) ≈
-
1
26
4
anthracene (64.0 kcal mol ), whereas the Mo-(η -ArH)
BDEs exhibit the opposite trend and increase in the sequence
Figure 5. Variation of C-C bond lengths for the coordinated arene rings
-
1
-1
6
4
benzene (64.4 kcal mol ) < naphthalene (77.5 kcal mol ) <
of (η -ArH)Mo(PH3)3 and (η -ArH)Mo(PH3)3H2.
-
1
anthracene (82.1 kcal mol ). As a result of these opposing
trends, of which the latter is dominant, the oxidative addition
of H2 becomes more exothermic in the sequence benzene <
naphthalene < anthracene. In this regard, it is pertinent to note
that a recent analysis of the indenyl effect has concluded that
the remainder of the arene. In this regard, the C-C bond lengths
4
of the uncoordinated “double bonds” in (η -ArH)Mo(PH3)3H2
are marginally shorter than the corresponding calculated values
in the free arenes (benzene, 1.397 Å; naphthalene, 1.432 Å;
anthracene, 1.439 Å), while the respective C-C bond lengths
5
3
the more facile η -to-η haptotropic shift is a consequence of
6
3
3
in (η -AnH)Mo(PH3)3 are marginally longer.
(i) the M-(η -Ind) bond being stronger than the M-(η -Cp)
bond and (ii) the M-(η -Ind) bond being weaker than the
5
The calculations are also in accord with the experimental
5
27
6
M-(η -Cp) bond.
observations concerned with the reactivity of (η -AnH)Mo-
PMe3)3 towards H2. Specifically, the calculated exothermicity
6
(
The key to understanding the influence of ring fusion on η -
to-η haptotropic shifts of arene ligands is, therefore, concerned
with why the Mo-(η -ArH) and Mo-(η -ArH) bond energies
6
4
of the oxidative addition of H2 to (η -ArH)Mo(PH3)3 increases
substantially in the sequence benzene (21.3 kcal mol ) <
naphthalene (1.4 kcal mol ) < anthracene (-4.0 kcal mol ),
as summarized in Table 4. To evaluate the proposal that the
variation in the thermodynamics is dictated by a loss of
resonance energy, we have calculated the energy required to
-
1
6
4
-
1
-1
6
(
26) This trend in M-(η -ArH) BDEs has been observed experimentally. For
6
example, the Cr-arene bond energy in (η -PhH)Cr(CO) is respectively
3
4.9 and 5.9 kcal mol- stronger than that in (η -NpH)Cr(CO)
1
6
and (η -
6
3
AnH)Cr(CO) . See ref 23.
3
(
27) (a) Veiros, L. F. Organometallics 2000, 19, 3127-3136. (b) Calhorda, M.
J.; Rom a˜ o, C. C.; Veiros, L. F. Chem. Eur. J. 2002, 8, 868-875.
6
fold the isolated arene (∆Efold) with the geometry in (η -ArH)-
J. AM. CHEM. SOC.
9
VOL. 128, NO. 16, 2006 5455

A R T I C L E S
Zhu et al.
SCF
6
Table 4. DFT-Calculated Energy Values (H ) Pertaining to the Thermodynamic Cycle for Oxidative Addition of H2 to (η -AnH)Mo(PH3)3
-
1
(
Values in kcal mol )
benzene
naphthalene
anthracene
6
4
∆Eoveralla
BDE[Mo-(η -ArH)]
∆Efold
∆E+H2
-BDE[Mo-(η -ArH)]
(
(
η -ArH)Mo(PH3)3 + H2 f (η -ArH)Mo(PH3)3H2
21.27
67.07
33.87
-15.28
-64.39
1.37
62.22
30.55
-13.90
-77.50
-3.99
64.00
26.83
-12.70
-82.12
6
6
singlet
6
η -ArH)Mo(PH3)3 f ArH[η -geom] + [Mo(PH3)3]
6
4
ArH[η -geom] f ArH[η -geom]
singlet
singlet
[
(
Mo(PH3)3]
+ H2 f [Mo(PH3)3H2]
4
4
singlet
4
η -ArH)Mo(PH3)3H2 f ArH[η -geom] + [Mo(PH3)3H2]
a
6
4
∆
Eoverall ) {BDE[Mo-(η -ArH)] - BDE[Mo-(η -ArH)]} + ∆Efold + ∆E+H .
2
6
Figure 6. Energy changes for folding an isolated arene with the geometry
in (η -ArH)Mo(PH3)3 to that in (η -ArH)Mo(PH3)3H2.
Figure 8. Thermodynamic cycle for oxidative addition of H2 to (η -ArH)-
6
4
Mo(PH ) decomposed in terms of intrinsic “snap” Mo-(η6-ArH) and Mo-
3 3
4
(
η -ArH) bond energies, the energy required to fold the arene (∆Efold), and
the energy associated with the oxidative addition of H2 (∆E+H ) to the
2
6
various [Mo(PH3)3] fragments with the geometries in (η -ArH)Mo(PH3)3.
Figure 7. Comparison of the energy required to fold an isolated arene
6
with the overall energy of oxidative addition to (η -ArH)Mo(PH3)3.
Figure 9. Comparison of the energy associated with the oxidative addition
exhibit opposing trends. It is, therefore, pertinent to discuss the
nature of the bonding in these complexes. In this regard, the
transition metal-(η -arene) interaction has been the subject of
of H2 (∆E+H ) to the various [Mo(PH3)3] fragments with the overall energy
2
6
of oxidative addition to (η -ArH)Mo(PH3)3.
6
the different nature of the HOMOs of the various arenes (Figure
many theoretical calculations, and on this basis it is generally
recognized that π-donation from the arene highest occupied
molecular orbital (HOMO) to an appropriate metal dπ-orbital
provides an important component that is supplemented by a
δ-back-bonding interaction with the arene lowest unoccupied
1
2). For benzene, the HOMO is a doubly degenerate e1g orbital,
6
the symmetry of which enforces symmetric η -coordination. In
6
3
(29) For calculations on (η -arene)ML complexes, see: (a) Howell, J. A. S.;
Ashford, N. F.; Dixon, D. T.; Kola, J. C.; Albright, T. A.; Kwang, S. K.
Organometallics 1991, 10, 1852-1864. (b) Albright, T. A.; Hofmann, P.;
Hoffmann, R.; Lillya, C. P.; Dobosh, P. A. J. Am. Chem. Soc. 1983, 105,
3396-3411. (c) Byers, B. P.; Hall, M. B. Organometallics 1987, 6, 2319-
2
8,29
molecular orbital (LUMO);
(
a molecular orbital diagram for
6
η -PhH)Mo(PH3)3 which illustrates these interactions is shown
2325. (d) Rogers, R. D.; Atwood, J. L.; Albright, T. A.; Lee, W. A.; Rausch,
in Figure 11. Focusing on the π-donor interaction, the effect of
M. D. Organometallics 1984, 3, 263-270. (e) Albright, T. A.; Hofmann,
P.; Hoffmann, R. J. Am. Chem. Soc. 1977, 99, 7546-7557. (f) Albright,
T. A.; Carpenter, B. K. Inorg. Chem. 1980, 19, 3092-3097. (g) Cohen,
R.; Weitz, E.; Martin, J. M. L.; Ratner, M. A. Organometallics 2004, 23,
2315-2325. (h) Albright, T. A. Acc. Chem. Res. 1982, 15, 149-155. (i)
Reference 17d.
6
ring fusion on Mo-(η -ArH) bond energies may be traced to
(
28) Muetterties, E. L.; Bleeke, J. R.; Wucherer, E. J.; Albright, T. A. Chem.
ReV. 1982, 82, 499-525.
5456 J. AM. CHEM. SOC.
9
VOL. 128, NO. 16, 2006

Addition of H
2
to (η6-Arene)Mo(PMe
)
3 3
Complexes
A R T I C L E S
6
4
Figure 10. Variation of intrinsic “snap” Mo-(η -ArH) and Mo-(η -ArH)
bond energies. Note that the difference in bond energies (∆BDE) correlates
6
well with the overall energy of oxidative addition to (η -ArH)Mo(PH3)3.
contrast, the HOMO of naphthalene is asymmetric with respect
to each six-membered ring, with no p-orbital coefficient on
carbon atoms of the ring junction.28 As such, the most efficient
overlap is achieved if the naphthalene ligand slips to maximize
the interaction with the four nonjunction atoms. Thus, while
6
the benzene ring of (η -PhH)Mo(PH3)3 is coordinated sym-
metrically to molybdenum, with Mo-C bond lengths in the
6
range 2.32-2.34 Å, the naphthalene ring of (η -NpH)Mo(PH3)3
4
is displaced and approaches a “flat” η -coordination mode (see
Figure 3 and Table 2). For example, the Mo-C distances of
6
the nonjunction atoms of (η -NpH)Mo(PH3)3 (2.31-2.32 Å) are
distinctly shorter than those of the ring junction (2.48 Å).
Evidence that this displacement is undoubtedly an electronic
and not steric effect is provided by the observation that the 1,2-
6
dimethylbenzene ligand in (η -1,2-Me2C6H4)Mo(PH3)3 exhibits
6
little displacement from symmetric η -coordination (2.32-2.35
6
Å; Table 2). The displacement of the naphthalene ligand of (η -
NpH)Mo(PH3)3 from symmetric coordination is in accord with
a reduced bonding interaction compared to that of benzene in
6
(
η -PhH)Mo(PH3)3.
6
Figure 11. Qualitative molecular orbital diagram for (η -PhH)Mo(PH3)3
Whereas the HOMO of naphthalene has a node on the carbon
(the z-axis is defined as the Mo-benzene centroid).
atoms of the ring junction, the corresponding carbon atoms of
30
4
anthracene have a small contribution that is out-of-phase with
respect to the metal dπ-orbital and thereby further reduces the
π-bonding interaction. The anthracene ligand, therefore, slips
to a further degree than does the naphthalene ligand, such that
the Mo-C distances for the ring junction increase to 2.54 Å.
In essence, the dxz orbital of (η -ArH)Mo(PH3)3H2 (with x
representing the slip direction) interacts not only with an
occupied arene orbital but also with the arene LUMO,32 resulting
in a standard three-orbital interaction pattern. The energy of
the occupied midlevel orbital is determined by the relative
energy of the arene LUMO, a simple estimate of which is
provided by the HOMO-LUMO gap of (η -ArH)Mo(PH3)3,
and decreases in the sequence benzene (4.4 eV) > naphthalene
(3.3 eV) > anthracene (2.7 eV). This trend is consistent with
an increased back-bonding interaction and hence a corresponding
increase in Mo-(η -ArH) bond energy across the series benzene
< naphthalene < anthracene. While slipping alone provides a
6
The Mo-(η -AnH) interaction is, however, slightly stronger
6
6
than the Mo-(η -NpH) bond, presumably because anthracene
is a better acceptor31 and back-bonding is more efficient (vide
infra).
3
3
4
The stability of (η -ArH)Mo(PH3)3H2 depends critically on
4
the energy of the metal-arene antibonding orbital derived by
interaction of occupied arene and metal orbitals. The energy of
this occupied metal-arene antibonding orbital is reduced
substantially via a back-bonding interaction with the arene
LUMO, an interaction that becomes significant upon folding
mechanism by which the arene LUMO may participate in a
π-back-bonding interaction, the arene ligand of (η -ArH)Mo-
(PH3)3H2 folds in order to stabilize the midlevel component of
the three-orbital interaction by minimizing the antibonding
4
4
the arene and slipping to an η -coordination mode (Figure 13).
(
30) Zilberg, S.; Haas, Y.; Shaik, S. J. Phys. Chem. 1995, 99, 16558-16565.
31) For example, the HOMO-LUMO gaps of the free arenes are as follow:
benzene, 6.64 eV; naphthalene, 4.74 eV; and anthracene, 3.54 eV (B3LYP
and cc-pVTZ(-f) basis set).
(32) Note that the arene LUMO also participates in a δ-back-bonding interaction
(
with the metal center.
(33) The HOMO-LUMO gaps of the free arenes exhibit the same trend. See
ref 31.
J. AM. CHEM. SOC.
9
VOL. 128, NO. 16, 2006 5457

A R T I C L E S
Zhu et al.
Figure 12. Interaction of the various arene HOMOs with a dπ-orbital. The degenerate nature of the benzene HOMO enforces symmetric coordination, while
the asymmetric nature of the naphthalene and anthracene HOMOs promotes a ring-slip in order to maximize the dπ-interaction with the nonjunction carbon
atoms.
Figure 13. Simplified illustration of the role of the benzene LUMO in stabilizing an occupied M-benzene antibonding dπ-orbital upon displacing the
6
benzene ligand along the x-axis (for clarity, the interaction between only one component of the doubly degenerate LUMO is illustrated). For an η -benzene
4
complex (left), the dxz orbital cannot interact with the benzene LUMO, and if the dxz orbital is occupied (as it is in a “20-electron” (η -ArH)Mo(PH3)3H2
4
complex), the interaction with the occupied benzene orbital is unfavorable. However, the reduced symmetry associated with η -benzene coordination (right)
enables the benzene LUMO to stabilize the occupied antibonding component of the filled-filled interaction between the occupied dxz orbital and occupied
4
benzene orbital. As such, a “20-electron” complex adopts an η -coordination mode.
interaction between dxz and the orbital component on the two
considerably smaller. Thus, to a first approximation, the
variation in the exothermicity of the oxidative addition of H2
nonbonded atoms, as illustrated in Figure 14. The observation
3
6
that the M-(η -Ind) bond is calculated to be stronger than the
to (η -ArH)Mo(PH3)3 may be rationalized in terms of the
3
4
M-(η -Cp) bond has been rationalized in similar terms, with
differing back-bonding interactions within (η -ArH)Mo(PH3)3H2.
2
7
the HOMO being less antibonding for the indenyl derivative.
Experimental Details
A more quantitative analysis of the back-bonding interactions
6
4
General Considerations. All manipulations were performed using
a combination of dry glovebox, high-vacuum, and Schlenk techniques
under an argon atmosphere.34 Solvents were purified and degassed by
in (η -ArH)Mo(PH3)3 and (η -ArH)Mo(PH3)3H2 has been
obtained by evaluation of the energy difference and overlap
between the occupied metal-based orbitals and unoccupied arene
orbitals, as summarized in Tables 5 and 6. Examination of these
data indicates that the back-bonding interaction, as crudely
evaluated by the overlap/∆E ratio, increases in the sequence
standard procedures. Mo(PMe
method.35
3 6
) was prepared by the literature
1H and 13C NMR spectra were measured on Bruker 300wb DRX,
Bruker Avance 400 DRX, and Bruker Avance 500 DMX spectrometers.
6
benzene < naphthalene < anthracene for both (η -ArH)Mo-
Chemical shifts are reported in ppm relative to SiMe
4
(δ ) 0) and
4
(
PH3)3 and (η -ArH)Mo(PH3)3H2, but the interaction is much
were referenced internally with respect to the protio solvent impurity
1
3
more significant for the latter because the energy gap is
(δ ) 7.15 for C
6
D
5
H and δ ) 2.09 for C
6
D
5
CD
2
H) and the
C
5458 J. AM. CHEM. SOC.
9
VOL. 128, NO. 16, 2006

Addition of H
2
to (η6-Arene)Mo(PMe
)
3 3
Complexes
A R T I C L E S
4
Figure 14. Occupied midlevel component of the three-orbital interaction for (η -ArH)Mo(PH3)3H2. The arrow indicates the out-of-phase component with
the dxz orbital that promotes folding of the arene in these complexes.
Table 5. Analysis of the Principal Back-Bonding Interactions in
(1 mL) was heated at 140 °C for 2 days, after which period the volatile
components were removed in vacuo. The solid residue was extracted
6
(η -ArH)Mo(PH3)3
E/eV
into pentane (1 mL) and filtered, and the volatile components were
6
removed from the filtrate in vacuo to give (η -NpH)Mo(PMe
3
)
3
as a
Mo(PH
occ)
3
)
3
arene
∆
eV
E/
overlap
(au)
overlap/
1
6 6
dark brown solid (18 mg, 40% yield). H NMR data (C D ): 1.11 [br,
(
(unocc)
∆E
9
6
Me of 3 P(CH
3
)
3
], 3.71 [br, 2 H of C10
H
8
], 4.32 [br, 2 H of C10
H
8
],
].
benzenea
-4.380
4.376
-2.262
-2.260
-3.120
3.662
2.118
2.116
1.231
0.666
1.22 × 10
1.22 × 10
1.50 × 10
1.23 × 10
-1
0.06
0.06
0.12
0.18
3
3
.70 [m, JH-H ) 3, 2 H of C10
H
8
], 7.21 [m, JH-H ) 3, 2 H of C10
H
8
-
-
-
1
1
1
-
13
1
C{ H} NMR data (C
D
6 6
): 26.0 [m, 9 Me of 3 P(CH
], 72.5 [s, 2 CH of C10 ], 97.1 [s, 2 C of C10
s, 2 CH of C10 ], 129.2 [s, 2 CH of C10
(C ): 4.8 [br, 3 P of 3 P(CH
3
)
3
], 68.5 [s, 2
naphthalene
anthracene
-4.351
-4.328
CH of C10
H
8
H
8
H
8
], 121.3
31
1
[
H
8
H
8
]. P{ H} NMR data
a
-1
In contrast to the naphthalene and anthracene derivatives, the two back-
D
6 6
3 3
) ]. IR data (KBr disk, cm ): 3044
6
bonding interactions in (η -ArH)Mo(PH3)3 are almost degenerate due to
(w), 2985 (w), 2956 (s), 2895 (vs), 2797 (w), 2366 (vw), 2345 (vw),
the higher symmetry.
1
419 (m), 1412 (m), 1346 (m), 1286 (m), 1269 (s), 1237 (w), 1078
Table 6. Analysis of the Principal Back-Bonding Interactions in
(w), 985 (w), 944 (vs), 928 (vs), 840 (s), 799 (m), 786 (m), 773 (m),
4
(η -ArH)Mo(PH3)3H2
738 (s), 701 (m), 688 (s), 658 (s), 650 (s), 620 (m), 570 (w).
6
Synthesis of (η -AnH)Mo(PMe
3
)
3
. A mixture of Mo(PMe
3
)
6
(55
E/eV
mg, 0.10 mmol) and anthracene (18 mg, 0.10 mmol) in cyclohexane
2 mL) was heated at 140 °C for 4 days, after which period the volatile
Mo(PH
3
)
3
H
2
arene
∆
eV
E/
overlap
(au)
overlap/
(
(occ)
(unocc)
∆E
components were removed in vacuo. The solid residue was extracted
into pentane (1 mL) and filtered, and the solvent was removed in vacuo.
-
-
-
1
1
1
benzene
naphthalene
anthracene
-4.357
-4.312
-4.264
-3.339
-3.719
-3.979
1.018
0.593
0.285
2.21 × 10
2.21 × 10
2.07 × 10
0.22
0.37
0.73
6
The residue was dissolved in benzene and lyophilized to give (η -AnH)-
1
Mo(PMe )
3 3
as a green solid (30 mg, 60% yield). H NMR spectroscopy
indicates that the crude sample contains ca. 15% residual anthracene.
Crystals suitable for X-ray diffraction were grown from pentane by
resonances (δ ) 128.0 for C
referenced relative to 85% H
reference (δ ) 141.0). All coupling constants are reported in hertz.
Variable-temperature T measurements were made at 300 MHz using
6
D
6
), respectively. 31P NMR spectra were
PO (δ ) 0) using P(OMe) as an external
1
2
3
4
3
slow evaporation. H NMR data (C
6
D
6
): 1.17 [d, JP-H ) 4, 9 Me of
10], 4.59 [s, 2 H of C14
10], 7.54 [s, 2 H of C14 10], 7.57 [m, 2 H of
10]. C{ H} NMR data (C ): 27.2 [s, 9 Me of 3 P(CH ], 67.6
[s, 2 CH of C14 10], 74.8 [s, 2 CH of C14 10], 104.3 [s, 2 C of C14
121.6 [s, 2 CH of C14
of C14
3
3 3
3 P(CH ) ], 4.00 [d, JH-H ) 4, 2 H of C14
H
H10],
1
7.03 [m, 2 H of C14
H
H
13
1
the inversion recovery method. IR spectra were recorded on a Perkin-
C
14
H
D
6 6
3 3
)
Elmer Paragon 1000 spectrophotometer.
H
H
H10],
6
Synthesis of (η -PhH)Mo(PMe
3
)
3
. A suspension of Mo(PMe
)
3 6
(50
H
10], 122.5 [s, 2 CH of C14
H
10], 126.4 [s, 2 CH
10], 130.7 [s, 2 C of C14 ): 7.1
3 3
[br, 2 P of 3 P(CH ], 9.3 [br, 1 P of 3 P(CH ) ]. IR data (KBr disk,
3
1
1
mg, 0.09 mmol) in benzene (2 mL) was heated at 120 °C for 1 day,
after which period the volatile components were removed in vacuo.
The solid residue was extracted into benzene (1 mL) and filtered, and
H
6 6
H10]. P{ H} NMR data (C D
3
)
3
-
1
cm ): 3048 (m), 2959 (vs), 2896 (vs), 2799 (m), 1609 (w), 1570 (w),
1454 (w), 1414 (s), 1327 (m), 1288 (m), 1269 (s), 1238 (m), 1120 (w),
944 (vs), 927 (vs), 836 (s), 687 (s), 649 (s).
6
the filtrate was lyophilized to give (η -PhH)Mo(PMe
3
)
3
as a dark brown
): 1.18 [br, 9 Me of 3
]. C{ H} NMR data (C ): 27.2 [m,
]. P{ H} NMR data
1
6 6
solid (30 mg, 82% yield). H NMR data (C D
1
3
1
6
6
P(CH
Me of 3 P(CH
): 3.3 [br, 3 P of 3 P(CH
m), 3024 (m), 2957 (vs), 2895 (vs), 2800 (w), 1418 (m), 1287 (w),
269 (s), 946 (vs), 926 (vs), 836 (m), 758 (m), 690 (s), 649 (vs), 594
w).
Synthesis of (η -NpH)Mo(PMe
mg, 0.10 mmol) and naphthalene (13 mg, 0.10 mmol) in cyclohexane
)
3 3
], 3.69 [m, 6 H of C
], 71.8 [s, 6 CH of C
]. IR data (KBr disk, cm ): 3045
6
H
6
6
D
6
Reactivity of (η -PhH)Mo(PMe ) towards H . A solution of (η -
3
3
2
3
1
1
9
)
3 3
6
H
6
PhH)Mo(PMe3)3 (5 mg) in C D6 (0.5 mL) in an NMR tube was
6
-
1
(C D
6 6
3
)
3
saturated with H (1 atm) and heated at 80 °C for 2 days. The sample
2
1
(
1
was monitored by H NMR spectroscopy, thereby demonstrating that
6
there was no reaction between (η -PhH)Mo(PMe ) and H . Upon
3
3
2
(
heating at 120 °C for 1 day, the sample was observed to decompose
6
3
)
3
. A mixture of Mo(PMe
3
)
6
(55
with the formation of a black precipitate.
6
. A solution of (η6-
Reactivity of (η -NpH)Mo(PMe
NpH)Mo(PMe (3 mg) in C
saturated with H (1 atm) and heated at 80 °C for 2 days. The sample
was monitored by H NMR spectroscopy, thereby demonstrating that
3 3 2
) towards H
)
3 3
6 6
D (0.5 mL) in an NMR tube was
(
34) (a) McNally, J. P.; Leong, V. S.; Cooper, N. J. In Experimental
Organometallic Chemistry; Wayda, A. L., Darensbourg, M. Y., Eds.;
American Chemical Society: Washington, DC, 1987; Chapter 2, pp 6-23.
2
1
(
b) Burger, B. J.; Bercaw, J. E. In Experimental Organometallic Chemistry;
6
3 3 2
there was no reaction between (η -NpH)Mo(PMe ) and H . Upon
Wayda, A. L., Darensbourg, M. Y., Eds.; American Chemical Society:
Washington, DC, 1987; Chapter 4, pp 79-98. (c) Shriver, D. F.; Drezdzon,
M. A. The Manipulation of Air-SensitiVe Compounds, 2nd ed.; Wiley-
Interscience: New York, 1986.
heating at 120 °C for 1 day, the sample was observed to decompose
with the formation of a black precipitate.
6
Reactivity of (η -AnH)Mo(PMe
3
)
3
towards H
2
. (a) Generation
(
35) (a) Murphy, V. J.; Parkin, G. J. Am. Chem. Soc. 1995, 117, 3522-3528.
4
6
(b) Hascall, T. Ph.D. Thesis, Columbia University, New York, 1999.
of (η -AnH)Mo(PMe
3 3 2
) H . A solution of (η -AnH)Mo(PMe (5 mg)
3 3
)
J. AM. CHEM. SOC.
9
VOL. 128, NO. 16, 2006 5459

A R T I C L E S
Zhu et al.
in C
6
D
6
(1 mL) in an NMR tube was saturated with H
2
(1 atm). The
Table 7. Crystal Data, Data Collection, and Refinement
Parameters
sample was monitored by H and 31P NMR spectroscopy, thereby
1
4
η6-NpH)Mo(PMe3)3
(
η6-AnH)Mo(PMe3)3
demonstrating the formation of (η -AnH)Mo(PMe
of 1 day at room temperature. Low-temperature H NMR spectroscopy
) H
3 3 2
over the period
(
1
lattice
formula
formula weight
space group
orthorhombic
C19H35MoP3
452.32
Pbca
monoclinic
C23H37BMoP3
502.38
P21
4
at -70 °C reveals two conformations of (η -AnH)Mo(PMe
3
)
H
3 2
with
at 25 °C): -4 [very broad, 2 MoH],
.01 [d, JP-H ) 7, 9 Me of 3 P(CH ], 2.76 [br, 2 H of C14 10], 4.78
10], 7.12 [m, 2 H of C14 10],
CD at 90 °C, hydride
1
a ca. 5:1 ratio. H NMR data (C
1
D
6 6
2
3
)
3
H
a/Å
b/Å
c/Å
16.0464(19)
9.5126(12)
29.522(4)
90
90
90
9.5584(7)
16.3202(13)
16.2667(13)
90
94.116(1)
90
[br, 2 H of C14
H
10], 6.80 [s, 2 H of C14
H
H
1
7
.43 [m, 2 H of C14
H
10]. H NMR data (C
6
D
5
3
2
1
region only): -3.87 [q, JP-H ) 44, 2 H of MoH
2 6 5
]. H NMR data (C D -
R/°
2
2
CD at -70 °C, hydride region only): -3.99 [tt, JP-H ) 40, JP-H
3
)
â/°
1
2
2
2
9
)
, JH-H ) 9, 1 H of MoH
2
], -2.47 [m, JP-H ) 65, JP-H ) 42, JP-H
γ/°
32, 1JH-H ) 9, 1 H of MoH
] (major species); -8.49 [tt, JP-H
2
)
V/Å3
4506.3(10)
8
2531.0(3)
4
2
6, 2JP-H ) 10, JH-H ) 10, 1 H of MoH
minor species). The T minimum values of the hydride signals of the
major species in toluene-d are 210 ms (δ -3.99 ppm) and 199 ms (δ
2.47 ppm) at 300 MHz and -60 °C, consistent with a dihydride,
1
Z
5
(
2
2
], -3.71 [m, 1H of MoH ]
temperature (K)
radiation (λ, Å)
243
0.71073
1.333
0.794
25.0
243
0.71073
1.318
0.714
28.2
1
8
-3
F(calcd), g cm
µ(Mo KR), mm
θmax, °
-
-1
3
1
1
rather than dihydrogen, structure. P{ H} NMR data (C
6
D
6
at 25 °C):
3
1
1
no. of data
3945
10652
3
.4 [s, 3 P of 3 P(CH
3 3 6 5 3
) ]. P{ H} NMR data (C D CD at -70 °C),
2
no. of parameters
209
488
two conformations are observed: ABC pattern 5.3 [P
A
, JP-P ) 11,
3 3 B
P-P ) 24, 1 P of 3 P(CH ) ], 6.1 [P , JP-P ) 11, JP-P ) 34, 1 P of
R1
wR2
GOF
0.0925
0.1584
1.210
0.0267
0.0631
1.033
2
2
2
J
, JP-P ) 24, 2JP-P ) 34, 1 P of 3 P(CH
2
) ]
3 3
3
(
[
P(CH
3
3
) ], and 15.9 [P
C
major species); -2.2 [t, 2JP-P ) 15, 1 P of 3 P(CH
)
] and 5.7
3
3
2
d, JP-P ) 15, 2 P of 3 P(CH
)
3 3
] (minor species). IR data (KBr disk,
-
1
cm ): 3049 (s), 2964 (vs), 2900 (vs), 2801 (w), 1783 (m), 1727 (m)
data collection, and refinement parameters are summarized in Table 7.
The structures were solved using direct methods and standard difference
map techniques and were refined by full-matrix least-squares procedures
[ν(Mo-H)], 1618 (m)**, 1464 (s), 1407 (s), 1315 (m)**, 1297 (m),
1
7
274 (s), 1233 (m), 1145 (m)*, 938 (vs), 881 (vs)*, 853 (s), 738 (s),
22 (vs), 663 (s), 471 (s); absorptions labeled * are present in
6
2
38
on F with SHELXTL (version 6.10).
(
η -AnH)Mo(PMe
3
)
3
, while those labeled ** are present in anthra-
Computational Details. All calculations were initially carried out
cene.
using DFT as implemented in the Jaguar 4.0, 4.1, and 6.0 suites of ab
initio quantum chemistry programs. Initial geometries were obtained
from crystal structures when available, and in other cases the desired
molecule was built through modification of the coordinates of a similar
4
39
(b) Reductive Elimination of H
2
from (η -AnH)Mo(PMe
3
)
3
H
2
.
4
A sample of (η -AnH)Mo(PMe
described above, was frozen and evacuated to remove the H
3
)
3
H
2
in C
6
D
6
(0.5 mL), prepared as
atmo-
2
sphere. The sample was allowed to warm to room temperature and
compound with known structure. The PH
3
ligand was used instead of
1
was monitored by H NMR spectroscopy, thereby demonstrating that
PMe for computational expediency. Geometry optimizations were
3
4
6
40
(
η -AnH)Mo(PMe
) H
3 3 2
converted to an equilibrium mixture with (η -
performed with the B3LYP density functional and the 6-31G** basis
set for C, H, N, and P, while Mo was represented using the Los Alamos
AnH)Mo(PMe and H
3
)
3
2
over a period of 1 day.
4
41
(
c) Hydrogenation of (η -AnH)Mo(PMe
3
)
3
H
2
in C
(0.5 mL), prepared as described
(1 atm), was heated at 90 °C for one week
and monitored by H NMR spectroscopy, thereby demonstrating the
D
6 6
. A sample
LACVP basis set that includes relativistic effective core potentials.
4
of (η -AnH)Mo(PMe
above in the presence of H
3
)
3
H
2
in C
6
D
6
No structure constraint was used during the geometry optimization
unless otherwise specified. The energies of the optimized structures
were re-evaluated by additional single-point calculations on each
optimized geometry using the cc-pVTZ(-f) correlation-consistent triple-ú
basis set for C, H, N, and P, and LACV3P for Mo.
2
1
3
6
conversion to 1,2,3,4-tetrahydroanthracene (ca. 67% yield based on
6
(
3 3
η -AnH)Mo(PMe ) using mesitylene as an internal standard); a small
3
7
6
6
quantity of 9,10-dihydroanthracene (ca. 4% yield) is also obtained.
The structures of (η -AnH)Mo(PH
obtained by replacing PMe ligands in the crystal structures of (η -
and (η -NpH)Mo(PMe with PH ligands, respec-
tively, followed by geometry optimization. The structure of (η -
3
)
3
and (η -NpH)Mo(PH
3 3
) were
6
The reaction is accompanied by liberation of anthracene (ca. 29% yield)
3
6
6
and the formation of Mo(PMe
3
)
4
H
4
, (η -PhH-d
6
)Mo(PMe
3
)
3
, and a small
AnH)Mo(PMe
)
3 3
3
)
3
3
6
6
amount of unidentified precipitate. The ratio of Mo(PMe
)Mo(PMe
converts to (η -PhH-d
d) Hydrogenation of (η -AnH)Mo(PMe
3
)
4
4
H :(η -PhH-
6
d
6
)
3 3
decreases over the course of the reaction as Mo(PMe
) H
3 4 4
PhH)Mo(PH
3
)
3
was obtained by modifying the structure of (η -
6
)Mo(PMe
3
)
3
(ca. 95% yield after one week).
in C 12. A sample
12 (0.5 mL) was treated with PMe (ca.
. The sample
NpH)Mo(PH
3
)
3
as a starting point prior to geometry optimization.
6
4
4
The structure of (η -AnH)Mo(PMe ) H was obtained by adding two
(
3
) H
3 2
6
D
3 3
2
6
6
hydride ligands to the molybdenum center of (η -AnH)Mo(PH ) prior
to geometry optimization. The structure of (η -AnH)Mo(PH ) H so
obtained was used as a starting point to evaluate the stability of other
conformations that differ by the relative positions of anthracene, PH
of (η -AnH)Mo(PMe
3
)
3
in C
6
D
3
3 3
4
4
3
0 µL) and H (1 atm) to generate (η -AnH)Mo(PMe
2
3 3
) H
2
3
3
2
1
was heated at 80 °C for 25 days and analyzed by H NMR spectroscopy,
3
,
thereby demonstrating the conversion to 1,2,3,4-tetrahydroanthracene
4
6
and hydride ligands. A total of 13 distinct structures of (η -AnH)Mo-
PH were obtained, with the lowest energy structure being 2.0
(
ca. 78% yield based on (η -AnH)Mo(PMe
3
)
3
using mesitylene as an
, 9,10-dihydroan-
(
3 3 2
) H
internal standard); in contrast to the reaction in C
6
D
6
thracene was not observed and the amount of liberated anthracene was
reduced to ca. 15%. The reaction was accompanied by the formation
(
38) Sheldrick, G. M. SHELXTL, An Integrated System for SolVing, Refining
and Displaying Crystal Structures from Diffraction Data; University of
G o¨ ttingen: G o¨ ttingen, Germany, 1981.
of a black precipitate and Mo(PMe
3 4 4
) H (ca. 14% yield).
6
(39) Schr o¨ dinger, LLC, Portland, OR.
X-ray Structure Determinations. X-ray diffraction data for (η -
(
40) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652. (b) Becke, A. D.
Phys. ReV. A 1988, 38, 3098-3100. (c) Lee, C. T.; Yang, W. T.; Parr, R.
G. Phys. ReV. B 1988, 37, 785-789. (d) Vosko, S. H.; Wilk, L.; Nusair,
M. Can. J. Phys. 1980, 58, 1200-1211. (e) Slater, J. C. Quantum Theory
of Molecules and Solids, Vol. 4: The Self-Consistent Field for Molecules
and Solids; McGraw-Hill: New York, 1974.
6
NpH)Mo(PMe
3
)
3
and (η -AnH)Mo(PMe
3 3
) were collected on a Bruker
P4 diffractometer equipped with a SMART CCD detector; crystal data,
(
36) NMR data are reported in ref 8b.
(
37) 1H NMR data (C
6
D
6
): 3.63 [s, 4 H of C14
12): 3.83 [s, 4 H of C14
.14 [m, 4 H of C14H12].
H
12], 7.08 [br, 8 H of C14
H
12].
12],
(41) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270-283. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284-298. (c) Hay, P. J.; Wadt,
W. R. J. Chem. Phys. 1985, 82, 299-310.
1
H NMR data (C
6
D
H12], 7.05 [m, 4 H of C14
H
7
5460 J. AM. CHEM. SOC.
9
VOL. 128, NO. 16, 2006

Addition of H
2
to (η6-Arene)Mo(PMe
)
3 3
Complexes
A R T I C L E S
kcal mol- more stable than the second most stable geometry; the other
1
Mo-arene complex of interest was partitioned into its constituent
1
1
6
conformations examined are considerably less stable (7.8-18.1 kcal
mol ) than the lowest energy structure. The computational result that
there is a conformer of (η -AnH)Mo(PH
fragments, e.g., ArH ( A
Single-point calculations were performed using the BLYP functional
with a triple-ú quality basis set augmented with one polarization function
1
) and Mo(PH
)
3 3
( A
1
3 3
) for (η -ArH)Mo(PH ) .
-
1
40b,c
4
3 3 2
) H which is only slightly
50-52
higher in energy than the ground state is consistent with low-temperature
that also includes scalar relativistic effects (ADF basis ZORA/TZP).
1
31
H and P NMR spectroscopic studies (vide infra). The NMR
spectroscopic data also indicate that the lowest energy structure
possesses C symmetry, whereas the higher energy structure possesses
For the Mo atoms, the 1s2s2p3s3p3d core of electrons was treated with
the frozen-core approximation. To ascertain the binding interactions
between the fragments, recombination of the original complex was
performed via a single-point calculation using the constituent fragments
to describe the final Kohn-Sham orbitals as linear combinations of
fragment orbitals. The composition and interaction between Mo and
arene fragment orbitals within the individual molecular orbitals of the
complex was determined by correlating the individual symmetrized
fragment orbital (SFO) energies and overlap populations calculated as
part of the standard ADF output.
1
C
s
symmetry, in accord with the calculations.
The structures of (η -PhH)Mo(PH
4
4
3
)
H
3 2
3 3 2
and (η -NpH)Mo(PH ) H
4
were obtained by modifying each of the 13 geometries of (η -AnH)-
Mo(PH followed by geometry optimization. In both cases, the most
stable structures of (η -PhH)Mo(PH
3 3 2
) H
4
4
3
)
3
H
2
4
and (η -NpH)Mo(PH
3
)
3
H
2
display an overall similarity to that of (η -AnH)Mo(PH
Molecular orbital analyses were performed with the aid of JIMP,
which employs Fenske-Hall calculations and visualization using
MOPLOT. Analysis of the back-bonding interactions in (η -ArH)-
3
3 2
) H .
4
2
4
3
Conclusions
4
4
6
4
Mo(PH
fragment partitioning method
Density Functional program suite (ADF 2004.01).
optimizations were performed with the BP86 functional,
3
)
3
and (η -ArH)Mo(PH
3
5,46
)
3
H
2
was performed by utilizing the
In conclusion, the increased facility of naphthalene and
4
as employed in the Amsterdam
4
anthracene to adopt η -coordination modes compared to that
4
7,48
ADF geometry
of benzene is not a consequence of a reduced loss of resonance
4
0b,49
and the
energy for the arene fragment, but is rather a consequence of
all-electron, Slater-type orbital (STO) basis sets employed were of
triple-ú quality augmented with two polarization functions and incor-
porated relativistic effects using the zero-order regular approximation
4
the fact that the Mo-(η -ArH) bonding interaction increases
in the sequence benzene < naphthalene < anthracene, while
6
5
0-52
the Mo-(η -ArH) bonding interaction exhibits the opposite
(
ADF basis ZORA/TZ2P).
After full geometry optimization, the
6
trend. As a consequence, oxidative addition of H2 to (η -AnH)-
(
42) Manson, J.; Webster, C. E.; Hall, M. B. JIMP Development Version 1.1.5,
Mo(PMe3)3 is much more thermodynamically favored than
oxidative addition to (η -PhH)Mo(PMe3)3.
2
005 (built for Windows PC and Redhat Linux 7.3); Department of
6
Chemistry, Texas A&M University, College Station, TX 77842 (http://
www.chem.tamu.edu/jimp/).
(
43) Hall, M. B.; Fenske, R. F. Inorg. Chem. 1972, 11, 768-775.
44) Lichtenberger, D. L. Version 2.0, June 1993; Department of Chemistry,
University of Arizona, Tuscon, AZ 85721.
Acknowledgment. We thank the U.S. Department of Energy,
Office of Basic Energy Sciences (DE-FG02-93ER14339), for
support of this research and Professor Daniel G. Nocera for
generous access to his computational facility. Professor Mu-
Hyun Baik, Professor Todd Marder, and the reviewers are
thanked for their helpful comments and advice which signifi-
cantly improved the quality of this article.
(
(45) (a) Morokuma, K. J. Chem. Phys. 1971, 55, 1236-1244. (b) Morokuma,
K. Acc. Chem. Res. 1977, 10, 294-300.
(
46) (a) Ziegler, T.; Rauk, A. Theor. Chim. Acta 1977, 46, 1-10. (b) Ziegler,
T.; Rauk, A. Inorg. Chem. 1979, 18, 1558-1565. (c) Ziegler, T.; Rauk, A.
Inorg. Chem. 1979, 18, 1755-1759.
(
47) ADF 2004.01; SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam,
The Netherlands (http://www.scm.com).
(48) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S. J. A.; Fonseca Guerra,
C.; Baerends, E. J.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22,
Supporting Information Available: Cartesian coordinates for
9
31-967 and references therein.
(
(
(
(
49) (a) Perdew, J. P. Phys. ReV. B 1986, 34, 7406-7406. (b) Perdew, J. P.
geometry-optimized structures (PDF); crystallographic data for
Phys. ReV. B 1986, 33, 8822-8824.
6
6
(
η -NpH)Mo(PMe3)3 and (η -AnH)Mo(PMe3)3 (CIF). This
50) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99,
4
597-4610.
material is available free of charge via the Internet at
http://pubs.acs.org.
51) van Lenthe, E. The ZORA Equation. Thesis, Vrije Universiteit Amsterdam,
The Netherlands, 1996.
52) Faas, S.; Snijders, J. G.; van Lenthe, J. H.; van Lenthe, E. Baerands, E. J.
Chem. Phys. Lett. 1995, 246, 632-640.
JA058107F
J. AM. CHEM. SOC.
9
VOL. 128, NO. 16, 2006 5461