Partially and Fully Reversible Solvation-Controlled Borylene
Swapping and Metal-Only Lewis Pair Formation
Stefanie Bertsch, Holger Braunschweig,* Rian D. Dewhurst, Krzysztof Radacki, Christian Saalfrank,
Benedikt Wennemann, and Qing Ye
Institut fü r Anorganische Chemie, Julius-Maximilians-Universitä t Wü rzburg, Am Hubland, 97074 Wü rzburg, Germany
* S Supporting Information
ABSTRACT: New borylene-containing metal-only Lewis pairs based on
group 6 metals and ruthenium are prepared by a partially reversible
borylene transfer from group 6 borylene complexes [M{BN(SiMe
3
)
2
}-
(CO)
5
] (M = Mo, W). The complexes contain strong Ru → Mo/W
dative interactions. As in a previous report on related compounds,
redissolving these complexes resulted in reversion back to the precursors,
enabled by a sacrificial CO donor. It was observed that this reversion
process becomes less selective as the group 6 metal becomes heavier, leading to greater amounts of unidentified byproducts as
the group is descended.
T
he borylene transfer reaction, discovered in 2003,
1
has
become a useful tool in expanding the range of known
transition-metal borylene complexes.
2
However, until recently,
the reaction had failed with precursors containing group 8
metals ruthenium and osmium. Consequently, the chemistry of
ruthenium borylene complexes was limited to a handful of
examples from the groups of Sabo-Etienne and Aldridge,
3
only
one of which was a terminal, base-free example.
3a
The state of
osmium borylene chemistry was even less advanced, with only
one base-stabilized example being known, from the group of
Roper.
4
Our recent report detailed the first osmium (3d, Figure 1),
and second ruthenium (3a) terminal, base-free borylene
complexes, both prepared by intermetallic borylene transfer
from [Cr{BN(SiMe
3
)
2
}(CO)
5
](1a).
5
However, in contrast to
all previous borylene transfer processes, the group 6 metal was
retained in the complex, leading to metal-only Lewis pairs
(MOLPs)
6
in which a zerovalent group 8 fragment donates a
pair of electrons to the group 6 pentacarbonyl fragment.
Although the mechanism of the conventional borylene transfer
reaction has not been definitively determined, a number of
bridging dinuclear borylene complexes have been isolated from
the process.
7
Indeed, bridging motifs appear to be highly
favored with the aminoborylene ligand: BN(SiMe
3
)
2
; thus, the
dinuclear, nonbridging borylene MOLP complexes prepared in
this earlier report
5
were highly unusual. This MOLP formation
was, therefore, presumed to be an offshoot of the conventional
borylene transfer mechanism.
The MOLP-forming reactions were also found to be
reversible,
5
driven in the forward direction by the crystallization
of the MOLP-borylene complexes 3a and 3d from hexanes,
and in the reverse by dissolution in more polar solvents. This
unprecedented phenomenon suggested that the energetic
profile of the reaction must be relatively flat. In this note, we
further explore this reactivity pattern and expand this family of
unusual borylene-containing MOLPs by preparing molybde-
num and tungsten examples, thus completing the chromium
triad.
Similarly to the preparation of 3a and 3d,
5
hexane solutions
containing [Ru(CO)
3
(PMe
3
)
2
](2a) and a slight excess of
group 6 borylene complexes [M{BN(SiMe
3
)
2
}(CO)
5
](1b:M
= Mo; 1c: M = W) in quartz NMR tubes were photolyzed for
15 h using a 550 W Hg/Xe UV lamp (Figure 1). During this
time, yellow needles of trans,trans-[(OC)
5
M←Ru(CO)
2
-
(PMe
3
)
2
{BN(SiMe
3
)
2
}]·(0.5 n-C
6
H
14
)(3b: M = Mo; 3c: W)
formed, which were used for single-crystal X-ray diffraction
studies.
Single-crystal X-ray crystallographic study of 3b and 3c
(Figure 2) showed the new MOLPs to be isostructural both to
each other and to the previously published 3a. Both complexes
were found to cocrystallize with half a molecule of hexane in
the unit cell. Both compounds possess RuB double bonds
(3b: 1.941(3) Å; 3c: 1.949(7) Å), ruthenium-group 6 metal
Received: March 10, 2014
Published: June 20, 2014
Figure 1. Synthesis of borylene-containing MOLPs by interrupted
intermetallic borylene transfer.
Note
pubs.acs.org/Organometallics
© 2014 American Chemical Society 3649 dx.doi.org/10.1021/om5002414 | Organometallics 2014, 33, 3649-3651