Molybdenum(VI) Complexes of a 2,2′-Biphenyl-bridged
Bis(amidophenoxide): Competition between Metal-Ligand and
Metal-Amidophenoxide π Bonding
Jason A. Kopec, Sukesh Shekar, and Seth N. Brown*
Department of Chemistry and Biochemistry, 251 Nieuwland Science Hall, University of Notre Dame, Notre Dame, Indiana
46556-5670, United States
* S Supporting Information
ABSTRACT: The 2,2′ -biphenyl-bridged bis(2-aminophenol) ligand 4,4′ -di-tert-butyl-N, N′ -bis(3,5-di-tert-butyl-2-hydroxyphenyl)-2,
2′ -diaminobiphenyl (
t
BuClipH
4
) reacts with MoO
2
(acac)
2
to form (
t
BuClipH
2
)MoO
2
, where the diarylamines remain protonated and
bind trans to the terminal oxo groups. This complex readily loses water on treatment with pyridine or 3,5-lutidine to form mono-oxo
complexes (
t
BuClip)MoO(L), which exhibit predominantly a cis-β geometry with an aryloxide trans to the oxo group. Exchange of the
pyridine ligands is rapid and takes place by a dissociative mechanism, which occurs with retention of stereochemistry at
molybdenum. Oxo-free alkoxide complexes (
t
BuClip)Mo(OR)
2
are formed from (
t
BuClipH
2
)MoO
2
and ROH. Treatment of
NMo(O
t
Bu)
3
with
t
BuClipH
4
results in complete deprotonation of the bis(aminophenol) and formation of a dimolybdenum
complex (
t
BuClip)Mo(μ-N)(μ-NH
2
)Mo(
t
BuClip) containing both a bridging nitride (Mo-N = 1.848 Å, Mo-N-Mo =
109.49°) and a bridging amide group. The strong π bonding of this bis(amidophenoxide) ligand allows the molybdenum center
to interconvert readily among species forming three, two, one, or zero π bonds from multiply bonded ligands.
■
INTRODUCTION
The chemistry of molybdenum(VI) is dominated by oxo
ligands, and to a lesser extent other strong π-donors such as
imido, nitrido, alkylidene, and alkylidyne ligands. The MoO
homolytic bond dissociation energy is among the highest
known.
1
While loss of a single oxo from a dioxomolybdenum com-
plex is often observed, complete deoxygenation of oxomolybde-
num complexes is rare, and indeed molybdenum(VI) compounds
devoid of metal -ligand multiple bonds are uncommon regardless
of their synthetic provenance.
2
The propensity of high-valent
molybdenum to form metal-ligand multiple bonds stems from
the metal’ s high Lewis acidity and its d
0
configuration. Multiply
bonded ligands are required to achieve high valence electron
counts while maintaining modest coordination numbers.
One exception to the hegemony of oxo and related ligands in
the chemistry of molybdenum(VI) is provided by catecholate.
Catecholates can be considered as isolobal to oxo, with the two
σ bonds of the bidentate ligands paralleling the σ and one of the
π bonds of the oxo ligand (Figure 1). The second π interaction
of the oxo is analogous to donation from the B
1
-symmetric
donor orbital of the catecholate, which is raised in energy by its
interaction with a filled benzene π-bonding orbital (the out-of-
phase combination is lowered in energy because of its overlap
with the benzene π* orbital and is not an effective π donor).
3
Congruent with the oxo/catecholate analogy, a variety of
molybdenum(VI) tris(catecholates) are known,
4
and mixed
oxo-catecholate complexes are also common.
5-7
In some cases,
catecholate spectator ligands appear to enable the complete loss
of ancillary terminal oxo ligands, which would otherwise be
difficult to achieve at Mo(VI). For example, oxobis(3,6-di-tert-
butylcatecholato)molybdenum(VI) is isolated as a tetramer
with bridging rather than terminal oxo groups, and the oxo
groups are lost entirely on reaction with isopropanol to give
(3,6-
t
Bu
2
C
6
H
2
O
2
)
2
Mo(O
i
Pr)
2
.
6
Here we describe the preparation, characterization, and
reactivity of molybdenum complexes of a tetradentate bis-
(amidophenoxide) ligand. 2-Amidophenoxides are isoelectronic
Received: August 9, 2011
Published: January 19, 2012
Article
pubs.acs.org/IC
© 2012 American Chemical Society 1239 dx.doi.org/10.1021/ic201736h | Inorg. Chem. 2012, 51, 1239-1250