A non-classical hydrogen bond in the molybdenum arene complex [η6-C6H5C6H3(Ph) OH]Mo(PMe3)3: Evidence that hydrogen bonding facilitates oxidative addition of the O-H bond

Article abstract of DOI:10.1039/b208678f

Mo(PMe₃)₆ reacts with 2,6-Ph₂C₆H₃OH to give the η⁶-arene complex [η⁶-C₆H₅C₆H₃(Ph) OH]Mo(PMe₃)₃ which exhibits a non-classical Mo...H-OAr hydrogen bond; DFT calculations indicate that the hydrogen bonding interaction facilitates oxidative addition of the O-H bond to give [η⁶,η¹-C₆H₅C₆ H₃(Ph)O]Mo(PMe₃)₂H.

Full text of DOI:10.1039/b208678f

A non-classical hydrogen bond in the molybdenum arene complex
[h6-C6H5C6H3(Ph)OH]Mo(PMe3)3: evidence that hydrogen bonding
facilitates oxidative addition of the O–H bond†
Tony Hascall, Mu-Hyun Baik, Brian M. Bridgewater, Jun Ho Shin, David G. Churchill, Richard A.
Friesner* and Gerard Parkin*
Department of Chemistry, Columbia University, New York, New York 10027, USA.
E-mail: parkin@chem.columbia.edu; Fax: 212 932 1289; Tel: 212 854 8247
Received (in Purdue, IN, USA) 5th September 2002, Accepted 25th September 2002
First published as an Advance Article on the web 11th October 2002
1
Mo(PMe3)6 reacts with 2,6-Ph2C6H3OH to give the h6-arene
complex [h6-C6H5C6H3(Ph)OH]Mo(PMe3)3 which exhibits a
non-classical Mo…H–OAr hydrogen bond; DFT calcula-
tions indicate that the hydrogen bonding interaction facili-
tates oxidative addition of the O–H bond to give [h6,h1-
C6H5C6H3(Ph)O]Mo(PMe3)2H.
solution; thus, not only is the H resonance of the OH group
shifted substantially downfield from that of d 5.2 for
6
2,6-Ph2C6H3OH to d 8.2 in [h -C6H5C6H3(Ph)OH]Mo(PMe3)3,
but the hydroxylic proton also exhibits coupling to the three
phosphorus nuclei of the PMe3 ligands [JP–H = 2 Hz]. Variable
temperature H NMR spectroscopic studies indicate that the
1
signal for the OH group is also temperature dependent, ranging
from d 8.0 at ca. 70 °C to d 8.5 at 290 °C.
Hydrogen bonding interactions are pervasive in chemistry and
biology. For example, having recently been described as the
most important of all directional intermolecular interactions,1
hydrogen bonds play critical roles in determining the structures
of biological systems and are used extensively as control
elements in crystal engineering,1,2 While the proton acceptors
of hydrogen bonds are most commonly electronegative atoms
such as oxygen or fluorine, it has recently become recognized
that there are several types of non-classical hydrogen bonds in
which the acceptors are either electron rich metal centers,
hydride ligands, or organic p-systems.3 The full potential of
these non-classical hydrogen bonds, however, is yet to be
realized. Therefore, in this paper, we address structural and
reactivity aspects of a complex that features a non-classical
Mo…H–O hydrogen bond, and demonstrate that this interaction
facilitates the oxidative addition of the O–H bond to the metal
center.
It is important to emphasize that 3-center-4-electron M…H–
X interactions are quite distinct from the 3-center-2-electron
interaction present in, for example, agostic complexes, hydro-
carbon s-complexes and dihydrogen complexes.7,8 Specifi-
cally, the metal center in M…H–X sigma bond complexes acts
as a Lewis acid towards HX (i.e. M / H–X), whereas the metal
in a hydrogen bond complex acts as a Lewis base towards HX
(i.e. M ? H–X). In this regard, the linearity of the Mo…H–O
interaction [172(3)°] is similar to that of other hydrogen bonds,
and contrasts with the markedly bent geometries that are
typically observed for 3-center-2-electron interactions.7,8
For a metal center to participate as the proton acceptor in a
hydrogen bond, it is essential for it to be electron rich. Indeed,
examples of such complexes have only previously been
recognized for those with d8 and d10 metal centers.3e Thus, the
6
existence of a hydrogen bond interaction within [h -
Mo(PMe3)6 reacts rapidly with 2,6-Ph2C6H3OH at 70 °C to
C6H5C6H3(Ph)OH]Mo(PMe3)3 is noteworthy since it only
possesses a d6 molybdenum center. The zerovalent nature of the
molybdenum and the strong donor nature of the PMe3 ligands
are presumably the factors responsible for enhancing the
basicity of the metal center in this complex.
6
6
yield the h -arene complex [h -C6H5C6H3(Ph)OH]Mo(PMe3)3
(Scheme 1).
6
Although [h -C6H5C6H3(Ph)OH]Mo(PMe3)3 is not unusual
6
in the sense that half-sandwich h -arene complexes of the type
6
6
(h -ArH)ML3 (e.g. L = CO, PR3) have featured prominently in
Although stable at room temperature, [h -C6H5C6H3(Ph)OH]-
6
1
the organometallic chemistry of chromium, molybdenum and
Mo(PMe3)3 is converted to the aryloxy-hydride species [h ,h -
6
tungsten, the formation of [h -C6H5C6H3(Ph)OH]Mo(PMe3)3
C6H5C6H3(Ph)O]Mo(PMe3)2H at 80 °C (Scheme 1). The
6
1
by arene displacement of three PMe3 ligands is unexpected in
view of the fact that Mo(PMe3)6 reacts preferentially with the
O–H bond of alkyl substituted phenols ArOH to give aryloxide
molecular structure of [h ,h -C6H5C6H3(Ph)O]Mo(PMe3)2H
has been determined by X-ray diffraction, as has that of the
6
1
tungsten analogue [h ,h -C6H5C6H3(Ph)O]W(PMe3)2H which
2
derivatives Mo(PMe3)4(OAr)H (Ar
=
2,4,6-C6H2Me3,
is obtained from the reaction of W(PMe3)4(h -CH2PMe2)H
2,6-C6H3Pri2).4 The most important feature of the structure of
with 2,6-Ph2C6H3OH.6,9
6
[h -C6H5C6H3(Ph)OH]Mo(PMe3)3 is the presence of an intra-
Brammer has previously postulated that hydrogen bonded
species may be intermediates for the oxidative addition of H–X
to a metal center, but has noted that there are no definitive
molecular Mo…H–OAr hydrogen bond (Fig. 1), characterized
by the following bond lengths and angle: d(Mo…H) = 2.76(3)
Å, d(Mo…O) = 3.571(2) Å, and Mo–H–O = 172(3)°.5,6
In addition to the diffraction study, spectroscopic studies
indicate that the hydrogen bonding interaction also persists in
examples in which hydrogen bonded complexes undergo
6
oxidative
addition.3b
The
conversion
of
[h -
6
1
C6H5C6H3(Ph)OH]Mo(PMe3)3 to [h ,h -C6H5C6H3(Ph)O]-
Scheme 1
† Electronic supplementary information (ESI) available: synthesis of the
Mo arene complex and its W analogue and of the respective aryloxy-hydride
species. X-Ray structure determination, computational details and cartesian
coordinates of optimized geometries. See http://www.rsc.org/suppdata/cc/
b2/b208678f/
6
Fig. 1 Molecular structure of [h -C6H5C6H3(Ph)OH]Mo(PMe3)3.
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CHEM. COMMUN., 2002, 2644–2645
This journal is © The Royal Society of Chemistry 2002

Fig. 2 Electronic energy surface for various mechanisms for oxidative addition of the O–H bond (values in kcal mol21).
3 (a) R. H. Crabtree, O. Eisenstein, G. Sini and E. Peris, J. Organomet.
Chem., 1998, 567, 7; (b) L. Brammer, D. Zhao, F. T. Ladipo and J.
Mo(PMe3)2H, therefore, provides the first well-defined exam-
ple of a hydrogen bonded [M]…H–OAr system transforming to
Braddock-Wilking, Acta Crystallogr. Sect. B-Struct. Commun., 1995,
a product of overall oxidative addition, i.e. [M](H)(OAr).
51, 632; (c) R. Custelcean and J. E. Jackson, Chem. Rev., 2001, 101,
However, this observation does not address the issue of whether
1963; (d) M. J. Calhorda, Chem. Commun., 2000, 801; (e) A. Martin, J.
the hydrogen bonded [M]…H–OAr species is actually on the
direct pathway to forming [M](H)(OAr) from reaction of [M]
Chem. Educ., 1999, 76, 578; (f) L. M. Epstein and E. S. Shubina, Coord.
Chem. Rev., 2002, 231, 165.
with H–OAr, or whether it represents an unproductive side
reaction. This issue has been addressed by performing a series
of DFT calculations (B3LYP)10 to establish the feasibility of
several mechanistic scenarios. Significantly, the calculations
indicate that the [Mo]…H–OAr hydrogen bonding interaction11
does indeed facilitate oxidative addition by providing a pathway
4 T. Hascall, V. J. Murphy and G. Parkin, Organometallics, 1996, 15,
3910.
6
5 The tricarbonyl analogue, [h -C6H5C6H3(Ph)OH]Mo(CO)3 also ex-
hibits a Mo…O distance of 3.6 Å, but the possibility of a hydrogen
bonding interaction was not discussed. See: J. L. Kerschner, E. M.
Torres, P. E. Fanwick, I. P. Rothwell and J. C. Huffman, Organome-
tallics, 1989, 8, 1424.
6
that enthalpically stabilizes the intermediate {[h -
6 CCDC reference numbers 193307–09. See http://www.rsc.org/supp-
data/cc/b2/b208678f/ for crystallographic data in CIF or other electronic
format.
C6H5C6H3(Ph)OH]Mo(PMe3)2} resulting from dissociation of
PMe3;12 specifically, intermediate A with a hydrogen bond
(Fig. 2, bold line) is more stable than intermediate B without a
hydrogen bond (Fig. 2, plain line).13 We also considered the
possibility that the oxidative addition could occur via a
mechanism that involves initial protonation followed by
internal nucleophilic attack (Fig. 2, dashed surface). However,
although the initial protonation step is only 12.9 kcal mol21
uphill enthalpically, the barrier for subsequent displacement of
the PMe3 ligand (both associatively and dissociatively) from
7 R. H. Crabtree, Angew. Chem., Int. Ed. Engl., 1993, 32, 789.
8 Metal Dihydrogen and s-Bond Complexes: Structure, Theory, and
Reactivity, ed. G. J. Kubas, Kluwer Academic/Plenum Publishers, New
York, 2001.
6
1
9 The PMePh2 analogues, [h ,h -C6H5C6H3(Ph)O]M(PMePh2)2H (M =
Mo, W) have been reported. See ref. 5.
10 All calculations were carried out using DFT as implemented in the
Jaguar 4.1 suite of ab initio quantum chemistry programs. Geometry
optimizations were performed with the B3LYP functional and the
6-31G** basis set for C, H, O and P, while Mo was represented using the
Los Alamos LACVP** basis set. The energies of the optimized
structures were reevaluated by additional single point calculations on
each optimized geometry using the cc-pVTZ(-f) basis set for C, H, O
and P and the LACV3P** basis set for Mo.
6
{[h -C6H5C6H3(Ph)O]Mo(PMe3)3H} is substantial and there-
fore not favored.14
6
6
In summary, the h -arene complex [h -C6H5C6H3(Ph)OH]-
Mo(PMe3)3 exhibits a non-classical Mo…H–O hydrogen bond.
DFT calculations provide evidence that this interaction facili-
tates oxidative addition of the O–H bond, thereby resulting in
the formation of [h ,h -C6H5C6H3(Ph)O]Mo(PMe3)2H.
We thank the U. S. Department of Energy, Office of Basic
Energy Sciences (DE-FG02-93ER14339 to G. P. and the NIH
(GM 40526 to R. A. F.) for support of this research, and
Professor Lee Brammer for helpful comments.
11 For recent calculations on intramolecular M…H–O interactions, see: G.
Orlova and S. Scheiner, Organometallics, 1998, 17, 4362.
6
1
6
12 It should also be noted that the hydrogen bonded structure for {[h -
C6H5C6H3(Ph)OH]Mo(PMe3)2} is calculated to be 3.9 kcal mol21
lower in energy than the structure in which the oxygen acts as a donor
to the 16-electron molybdenum center (with a distance of 2.45 Å).
13 The calculations also indicate that the oxidative addition from the
6
hydrogen bonded complex [h -C6H5C6H3(Ph)OH]Mo(PMe3)3 is sig-
nificantly more exothermic for tungsten (27.2 kcal mol21) than for
molybdenum (20.6 kcal mol21).
Notes and references
1 T. Steiner, Angew. Chem., Int. Ed., 2002, 41, 48.
2 G. R. Desiraju and T. Steiner, The Weak Hydrogen Bond in Structural
Chemistry and Biology, Oxford University Press, Oxford, 1999.
14 Further evidence which suggests that the mechanism does not involve
internal proton transfer is that the rate constant for oxidative addition of
the O–H bond is not strongly influenced by solvent polarity: 5.32(8) 3
1025 s21 (benzene) and 1.54(2) 3 1024 s21 (THF) at 100 °C.
CHEM. COMMUN., 2002, 2644–2645
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