Synthesis, characterization, and reactivity of a d2, Mo(IV) complex supported by a new OCO-trianionic pincer ligand

Article abstract of DOI:10.1021/ja074901k

The new terphenyl pincer precursor 3,3″-di-tert-butyl-1,1′:3′,1″-terphenyl-2,2″-diol (1) is examined for its ability to support early transition metals as a trianionic ligand. Metalation proceeds smoothly at -35 °C with Mo(NMe₂)₄ to

Full text of DOI:10.1021/ja074901k

Published on Web 01/09/2008
Synthesis, Characterization, and Reactivity of a d2, Mo(IV) Complex
Supported by a New OCO-Trianionic Pincer Ligand
Soumya Sarkar, Adam R. Carlson, Melanie K. Veige, Joseph M. Falkowski, Khalil A. Abboud, and
Adam S. Veige*
Department of Chemistry, Center for Catalysis, UniVersity of Florida, GainesVille, Florida 32611
Received July 3, 2007; E-mail: veige@chem.ufl.edu
Scheme 1. Classic Pincer (A, E ) P, N: S-H-S) and New
Trianionic Pincer Ligands (B, C: H-H-H)
Early (groups 3-6) and late (groups 7-10) transition metal
complexes have distinct structural and reactivity properties. Early
metals favor high oxidation states, high coordination numbers, are
typically electrophilic and are intolerant of many functional groups.
The opposite is true for late metal complexes, in which low
coordinate and low oxidation states prevail and a wide range of
functional groups are tolerated.1 By altering the properties of
ancillary ligands, chemists are able to manipulate the inherent
properties of a specific metal to promote and amplify reactions.
Pincer ligands are easily modified and offer an excellent
opportunity to manipulate the electronic and geometric properties
of metal complexes.2 Classic pincer ligands (A, Scheme 1)3 are
complementary to late transition metals due to their soft-hard-
soft arrangement of donor atoms. However, only a few early metals
have been supported by this original ligand design.4 Our approach
is to match the harder early transition metals with a harder pincer
ligand. Instead of a soft-hard-soft pincer motif, we have
synthesized a series of new hard-hard-hard pincer ligands based
on amido-arylide-amido linkages (B, Ar ) 2,6-iPrC6H3, and 3,5-
MeC6H3, Scheme 1) and attached them to group IV metals,5 but
significant challenges remain.
Scheme 2
Lithium amides and arylides readily reduce metal substrates, thus
complicating metalation. Alternatively, starting with neutral ligands
requires the activation of distinctly different N-H and C-H bonds
during metalation which can be difficult as separate problems, let
alone in concert.6 To access group VI complexes supported by a
trianionic pincer ligand, we began synthesizing ligands based on
an OCO arrangement of donor atoms (C, Scheme 1).7 Herein we
report a successful metalation via ArO-H and Ar-H bond
activation to generate a d2, Mo(IV) complex supported by a new
trianionic OCO terphenyl pincer ligand and its subsequent reactivity.
O-H and C-H bond activation to metalate 1 is achieved with
remarkable ease under mild conditions. The Mo(IV), d2 (µeff ) 3.06
µB)8 amido-dimethylamine complex, [3,3′′-di-tert-butyl-2,2′′-di-
(hydroxy-κO)-1,1′:3′,1′′-terphenyl-2′-yl-κC2′](N-methylmethanami-
nato)bis(N-methylmethanamine)molybdenum(IV), abbreviated as
[tBuOCO]Mo(NMe2)(NHMe2)2 (2), is formed at -35 °C upon
combining pentane solutions of 1 and Mo(NMe2)4 (Scheme 2).
Compound 2 precipitates in 80% yield as an orange powder and
can be further purified by recrystallization from concentrated
solutions of 1,2-dimethoxyethane (DME) at -35 °C. The 1H NMR
spectrum of paramagnetic 2 revealed broad resonances that
prohibited a detailed analysis, but some information could be
gleaned. For example, tBu protons appear at 2.83 ppm, the NHMe2
protons are observed at 2.01 ppm, and three sets of aryl-H
resonances are observed, with one appearing extremely downfield
at 66.7 ppm while the others resonate at 9.74 ppm and upfield at
-9.85 ppm. Ultimately, an X-ray structural analysis on a single
crystal obtained from cooled concentrated DME solutions of 2
confirmed its identity.
The solid-state structure of 2 is presented in Figure 1 and
validates the assignment of a trianionic tridentate pincer ligand.
The structure consists of an octahedral Mo(IV) center bound to
the pincer ligand, two dimethylamines (d(Mo1-N2) ) 2.390(3)
Å, d(Mo-N3) ) 2.430(3) Å) and one dimethylamido (d(Mo-N1)
) 1.928(3) Å). The amido and amine ligands are easily distin-
guished since N1 is trigonal and N2 and N3 are pyramidal. In
addition, the protons on N3 and N4 were located from the difference
Fourier map, and the M-N bond lengths are significantly different.
The Mo-NMe2 ligand is twisted away from the N2-Mo-N3 plane
by 35° to break the otherwise perfect solid-state Cs symmetry. A
space-filling model indicates the twist is due to packing forces that
place a DME solvent molecule atop the amido group.9a
Considerable strain is imparted to the pincer backbone and is
attributable to congestion caused by the dimethylamine ligand trans
to C1. The N-Me groups are nearly parallel to the O1-C1-O2
t
plane which forces them into the Bu’s of the pincer. As a result,
t
the Bu groups are strained creating 33 and 32° torsion angles
between the aryl backbone C2-C7 and C6-C17 connections,
respectively, and the central ring is bent up by 30°.
9
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10.1021/ja074901k CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. ORTEP molecular structures of compounds 2, 3, and 5 (half of dimer) with ellipsoids drawn at the 50% probability level. Hydrogen atoms and
solvent molecules for 2 (DME), 3 (benzene), and 5 (DMF) are removed for clarity.
Scheme 3
the apical position of a distorted square pyramid (d(MotN) )
1.6601(15) Å), but unlike 4, the OCO ligand is now attached as a
trianionic pincer. To balance charge, a Na ion, solvated with DMF,
is present 2.4453(17) Å from the nitride. Multinuclear NMR
spectroscopic, elemental, and IR characterization of 5 are consistent
with the X-ray findings.
Considering the facile C-H bond activation to form 2 and the
observed attached/detached ring in 5 and 4, we are currently
exploring the reversible activation of the backbone C-H bond. The
detached ring C-H bond represents a masked metal hydride that
may be activated when needed. These preliminary results indicate
that new OCO trianionic pincer ligands can support high oxidation
state transition metal complexes.
When benzene solutions of 2 are treated with 2,6-lutidine‚HCl
and left undisturbed, large purple-red crystals deposit within 2 h
1
Acknowledgment. The authors thank the University of Florida,
the donors of the ACS Petroleum Research Fund-G (#44063-G3),
and the Camille and Henry Dreyfus Foundation.
(Scheme 2). A H NMR spectrum of the crystals revealed aryl-H
resonances downfield at 28.81 ppm (ν1/2 ) 12 Hz) and upfield at
-1.10, -1.98, and -5.34 ppm (ν1/2 ) 7 Hz). The N-Me protons
are broad and located at -2.18 ppm (ν1/2 ) 153 Hz).
Supporting Information Available: Full experimental procedures,
spectroscopic, and crystallographic (CIF) data. This material is available
free of charge via the Internet at http://pubs.acs.org.
With the aid of a single-crystal X-ray diffraction experiment,
the crystals were identified as the d2 (µeff ) 2.56 µB)8 Mo(IV)
chloride complex [tBuOCO]MoCl(NHMe2)2 (3) presented in Figure
1. The octahedral Mo(IV) center is coordinated by the pincer ligand,
trans-dimethylamines, and a chloride. C2 symmetry now results
from a 33° twist in the backbone along the Cl1-Mo-C1 axis, due
to the smaller Cl- ligand. The dimethylamine ligands orient off
axis by 57° and are rotated 88° with respect to each other, which
again can be attributed to sterics.9a
The dimethylamines on 3 are bound tightly and do not release
under vacuum nor substitute with THF, DME, or CO, even at
elevated temperatures (80 °C). Attempts to reduce 3 with Na/Hg
or alkylate with MeMgCl were unsuccessful, providing only
intractable mixtures and protonated 1.
[tBuOCO]MoCl(NHMe2)2 (3) did not react with Me3SiN3 or
NaN3 in refluxing THF. Instead, the more polar DMF solvent had
to be used. Red solutions of 3 in DMF turn yellow-orange when
treated with NaN3 at 25 °C and release N2 to provide a mixture of
products that are inseparable. A single crystal of one species was
procured from a mixture of solids and was identified as the yellow
Mo(VI) nitride complex [tBuOCHO]MotN(NMe2)(DMF) (4,
Scheme 2).9b
Since 4 is produced in low yield as part of a mixture of
unidentifiable products, we sought other routes to nitrido species,
and treating 2 with NaN3 in DMF proved effective. Nitrido anion
dimer {[tBuOCO]MotN(NMe2)Na(DMF)}2 5 is synthesized in
excellent yield (86%) and purity according to Scheme 3; the
molecular structure is presented in Figure 1. The nitride occupies
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