Cooperative transition metal/lewis acid bond-activation reactions by a bidentate (boryl)iminomethane complex: A significant metal-borane interaction promoted by a small bite-angle lz chelate

Article abstract of DOI:10.1021/ja505843g

The synthesis of a three-coordinate Pt-borane complex featuring a bidentate LZ (boryl)iminomethane (BIM) ligand is reported. Unlike other LZ-type borane ligands featuring a single-donor buttress, the small bite angle enforced by the BIM ligand is shown to promote a significant metal-borane reverse-dative σ-interaction akin to multiply strapped metalloboratranes. The steric accessibility of the reactive Pt → B bond fostered by the BIM ligand allows for a rich reactivity profile toward small molecules that exploit metal-borane cooperative effects. The unligated (boryl)iminomethane BIM is also synthetically accessible and functions as a Frustrated Lewis Pair (FLP). The ability of the free BIM to effect bond activation reactions is contrasted with the behavior seen in the corresponding platinum-bound complexes.

Full text of DOI:10.1021/ja505843g

Communication
pubs.acs.org/JACS
Cooperative Transition Metal/Lewis Acid Bond-Activation Reactions
by a Bidentate (Boryl)iminomethane Complex: A Significant Metal−
Borane Interaction Promoted by a Small Bite-Angle LZ Chelate
Brandon R. Barnett, Curtis E. Moore, Arnold L. Rheingold, and Joshua S. Figueroa*
Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, Mail Code 0358, La Jolla,
California 92093-0358, United States
S
* Supporting Information
ligand frameworks possessing only a single L-type buttress have
ABSTRACT: The synthesis of a three-coordinate Pt−
borane complex featuring a bidentate “LZ” (boryl)imino-
methane (BIM) ligand is reported. Unlike other LZ-type
borane ligands featuring a single-donor buttress, the small
bite angle enforced by the BIM ligand is shown to promote
a significant metal−borane reverse-dative σ-interaction
akin to multiply strapped metalloboratranes. The steric
accessibility of the reactive Pt → B bond fostered by the
BIM ligand allows for a rich reactivity profile toward small
molecules that exploit metal−borane cooperative effects.
The unligated (boryl)iminomethane BIM is also syntheti-
cally accessible and functions as a Frustrated Lewis Pair
(FLP). The ability of the free BIM to effect bond
activation reactions is contrasted with the behavior seen in
the corresponding platinum-bound complexes.
thus far shown limited ability to foster a significant metal-to-
borane interaction.16,17 This follows from the fact that borane-
containing LZ ligands most closely mimic the coordinative
abilities of a free borane. However, such bidentate LZ chelates
are of interest for the development of isolable metal−borane
complexes that offer increased coordinative unsaturation and
flexibility toward incoming ligands and substrates. Accordingly,
herein we report platinum complexes featuring a (boryl)imino-
methane ((R2B)(H)CNR; BIM) ligand that enables the
formation of significant metal−borane interactions within a
bidentate LZ chelate. We also demonstrate a rich and
cooperative reaction chemistry of the Pt-to-borane linkage
with a host of small-molecule substrates. The ability of the
(boryl)iminomethane ligand to provide a significant metal−
borane interaction within a bidentate framework arises from a
small bite angle between the Z-type borane and L-type N-imino
coordinating groups.
ransition metal−borane complexes (LnM→BR3) featuring
The zero-valent bis(m-terphenyl isocyanide) platinum
complex, Pt(CNArDipp2
)
(ArDipp2 = 2,6-(2,6-(i-Pr)2C6H3)2-
T
so-called reverse-dative σ-interactions have received
increasing attention in coordination and small-molecule
activation chemistry.1−7 It is now established that coordination
of a Lewis acidic borane (Z-type ligand) can significantly
modulate the electronic and geometric structure properties of a
transition metal center in a manner distinct from traditional
Lewis basic, two-electron donor ligands (L-type ligand).4,8,9
Increasingly, reactivity profiles of metal−borane complexes with
small molecule substrates have been uncovered that signifi-
cantly diverge from those of either free boranes (BR3) or
transition metal fragments featuring only σ-donating, L-type
ligands.5,7 These studies have led to new catalytic processes that
exploit the “reverse polarity” of the metal−borane unit10,11 and
have also demonstrated the ability of coordinated boranes to
function in a hemilabile fashion to modulate the electronic
structure of a metal center during multielectron transformations
of small molecules.12−14
2
C6H3), is accessed via Mg-metal reduction of the dichloride,
PtCl2(CNArDipp2)2 (mixture of cis- and trans- isomers), in a
manner that parallels the synthesis of the palladium congener
Pd(CNArDipp2)2.18 Treatment of Pt(CNArDipp2)2 with dicyclo-
hexylborane (HBCy2) results in the clean formation of three-
coordinate Pt(κ2-N,B-Cy2BIM)(CNArDipp2) (1, Cy2BIM = Cy2B-
(H)CNArDipp2), which features a chelating LZ (boryl)imino-
methane moiety, via formal 1,1-hydroboration of one
CNArDipp2 ligand (Figure 1). Despite the bidentate nature of
the (boryl)iminomethane ligand in Pt(κ2-N,B-Cy2BIM)-
(CNArDipp2) (1), evidence for a significant Pt→B reverse-
dative σ-interaction is provided by both its solid-state structure
and solution spectroscopic properties.
In the solid state, complex 1 features a distorted T-shaped
geometry with a Pt−B distance of 2.314(6) Å and N1−Pt−B
and C2−Pt−B angles of 66.01(17)° and 105.63(17)°,
respectively. The N1−Pt−B angle in 1 is of particular note,
as this small bite-angle between the rigid Cy2BIM ligand and Pt
enables a short Pt−borane interaction in the absence of
additional donor groups. In comparison, the related platinum
tris-(o-phosphinophenylene)−borane complex, Pt[κ4-B,P3-
(o-iPr2PC6H4)3B], features an average P−Pt−B bite angle of
Ligand design strategies that enable a significant primary
interaction between a transition metal and a borane have relied
on the presence of two or more L-type ligands to buttress the
metal-to-borane σ-interaction (i.e., L2Z or L3Z).4−7 Tradition-
ally, multiple donor groups have been incorporated within a
borane−ligand framework to overcome the inherently low
coordinative ability of free borane molecules (BR3). Indeed,
transition metal−borane complexes that lack additional donor
groups and are also devoid of any secondary coordinative
interactions have yet to be fully authenticated.15 In addition,
Received: June 10, 2014
Published: July 9, 2014
© 2014 American Chemical Society
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Pt(CNArDipp2)2 that would lead to 1,1-hydroboration of free
CNArDipp2 by HBCy2, followed by combination of the newly
formed Cy2BIM ligand with the zero-valent [Pt(CNArDipp2)]
fragment. Instead, we presently favor a tandem sequence
involving H−B bond oxidative addition to the Pt center in
Pt(CNArDipp2)2, followed by α-H migration and B−C bond
reductive elimination from the putative boryl-hydride inter-
mediate [HPt(BCy2)(CNArDipp2)2] to furnish complex 1.23,24
The absence of a multiple-donor buttress in 1 allows it to
react readily with a variety of substrates in a manner that
demonstrates cooperation between the Pt and borane units in
bond activation processes. For example, exposure of 1 to H2 (1
atm) in C6H6 leads to the eradication of the Pt→B σ-
interaction and irreversible formation of the hydride−
borohydride complex, PtH(η2-H,B-κ1-N-HCy2BIM)(CNArDipp2
)
(4, Scheme 1), as determined by X-ray diffraction.25 The
Scheme 1. Reaction Pinwheel for Complex 1
Figure 1. Synthesis (top) and molecular structure (bottom) of Pt(κ2-
N,B-Cy2BIM)(CNArDipp2) (1).
85.7(2)° and accordingly requires a three-donor buttress to
stabilize a short Pt−B interaction (2.224(2) Å).19 Near-90° bite
angles are also present in the gold mono-(o-phosphino-
phenylene)−borane LZ chelate complexes ClAu(κ2-B,P-
(o-iPr2PC6H4)BCy2) and ClAu(κ2-B,P-(o-iPr2PC6H4)B(Flu))
(Flu = fluorene),16 which have been shown by crystallographic
and 11B NMR spectroscopic analyses to possess significantly
attenuated Au−B interactions relative to more highly buttressed
borane frameworks.5 In contrast, 1 gives rise to a 11B NMR
signal at +18 ppm that is substantially upfield of the
corresponding resonance of the free Cy2BIM ligand (+74
ppm; 2, Scheme 2) and strongly indicates an increase in the
coordination number at boron upon ligation to the Pt center.20
The presence of a significant reverse-dative σ-interaction is also
indicated by NBO calculations on the model complex Pt(κ2-
N,B-Me2BIM)(CNMe), which reveal a fully occupied bonding
orbital comprised of 81% Pt character and 19% boron
character.21 Importantly, the boron contribution in this NBO
is an admixture of both 2s- and 2pz-orbital character, thereby
indicating a rehybridization of the boron center from sp2 to sp3
upon interaction with Pt. This hybridization change also
rationalizes the observed pyramidalization at boron in the solid-
state structure of 1 (Σ(∠(C−B−C)) = 348.4°).
structural parameters of the Pt−(HB) contact in complex 4 are
consistent with its formulation as an η2-H,B-borohydride σ-
complex and are corroborated by a 11B NMR chemical shift of
−7 ppm, which is slightly downfield of those found for four-
coordinate, sp3-hybridized hydridoborates.20 Importantly, free
Cy2BIM also reacts with H2, but in a fashion distinct from Pt-
ligated 1. Thus, treatment of Cy2BIM with H2 affords the
methylene-bridged aminoborane, Cy2BCH2N(H)ArDipp2 (5,
Scheme 2), in an apparent H2-activation/intramolecular imine
hydrogenation sequence reminiscent of untethered imine-
borane Frustrated Lewis Pairs (FLPs).26 Accordingly, ligation
to the low-valent Pt center modulates the H2-reativity of
Cy2BIM in favor of 1,2-addition across the Pt−B interaction
rather than internal hydrogenation.
With respect to the formation of 1, it is important to note
that the reaction between Pt(CNArDipp2)2 and HBCy2 in C6H6
solution is complete in ca. 30 min and no intermediates are
1
observed by H NMR spectroscopy during the course of the
reaction. Furthermore, the free Cy2BIM ligand 2 is easily
prepared and isolated by 1,1-hydroboration of CNArDipp2 upon
the addition of HBCy2. Free Cy2BIM (2) reacts readily with
BCl3 to form an imino-borane/bridging-chloride double Lewis
acid/base adduct that has been structurally characterized (3,
Scheme 2), thus demonstrating the generality of the framework
to serve as an ambiphilic donor−acceptor species.22 However,
free Cy2BIM (2) does not react with Pt(CNArDipp2)2 in C6D6
solution over the course of several days, which is an observation
we attribute to a slow rate of isocyanide dissociation from two-
1
coordinate Pt(CNArDipp2)2. FTIR and 2D EXSY H NMR
Free Cy2BIM and 1 show similarly divergent reactivity toward
H2O. Whereas Cy2BIM reacts with H2O to yield boronate-
amine 6 via H−O bond cleavage and a 1,2-cyclohexyl shift
spectroscopic studies are consistent with this proposal and do
not indicate a fast isocyanide dissociation process from
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Communication
Scheme 2. Reactivity of Free Cy2BIM (2)
the methoxide and p-nitroanilide ligands bridge the Pt and B
centers in a manner analogous to the hydroxide ligand in 7. In
contrast, the addition of phenylacetylene (HCCPh) to 1
provides the hydride complex, PtH(η2-C,C-κ1-N-
PhCC-Cy2BIM)(CNArDipp2) (10, Scheme 1), which possesses
an acetylide group σ-bound to the Cy2BIM boron center and η2-
C,C ligated to platinum (Figure 2). Most notably, the η2-C,C-
acetylide coordination mode found in 10 is in direct contrast to
the large number of σ-bound Pt(II) acetylides reported in the
literature32,33 and demonstrates how borane ligation to a
transition metal not only facilitates substrate activation but also
significantly influences the structural properties of resultant
products.
Finally, cooperative bond activation by the Pt and B centers
in 1 is also apparent in its reactivity toward organic carbonyl
compounds. Treatment of 1 with acetone or benzaldehyde
results in CO bond reduction and formation of complexes
11 and 12, respectively, featuring oxymethyl groups that are C-
bound to a formally Pt(II) center and O-bound to boron
(d(C−O) = 1.429(3) Å for 11, Figure 2; d(C−O) = 1.419(2) Å
for 12). The CO bond reductions leading to complexes 11
and 12 are accompanied by a 1,2-cyclohexyl shift, which
transforms the Cy2BIM ligand into a dianionic amidoboronate.
This cyclohexyl migration mirrors the reactivity of free Cy2BIM
with H2O and, likewise, reasonably occurs to compensate for an
increase of charge on the borane center. In this respect,
cooperative carbonyl reduction by the Pt−borane unit in 1 is
reminiscent of the CO bond insertion chemistry available to
some late transition metal σ-boryl complexes.34,35 Accordingly,
the Cy2BIM system provides a platform to directly probe the
principles differentiating the reactivity of M→B reverse-dative
interactions and σ-bound boryls.36 These investigations, as well
as others aimed at further exploiting accessible M→B reverse-
dative interactions fostered by the small bite-angle (boryl)-
iminomethane framework, are in progress.
(Scheme 2), addition of H2O to 1 results in a formal Pt-
centered H−O bond oxidative addition to afford PtH(μ-
OH)(Cy2BIM)(CNArDipp2) (7, Scheme 1), in which a hydroxide
group bridges the Cy2BIM-borane and newly formed Pt−H
units. Crystallographic characterization of complex 7 (Figure 2)
revealed a B−O bond distance of 1.541(3) Å, which is
considerably longer than the average B−O distance of four-
coordinate, O-coordinated borates (i.e., ROBR3; d(B−O)av
=
1.481( 0.041) Å) contained within the Cambridge Structural
Database.27 This long B−O bond in 7, which undoubtedly
results from coordination of the hydroxide ligand to the Lewis
acidic Pt(II) center, likely lessens a buildup of charge on the
boron atom and obviates the need for cyclohexyl-group
migration within the Cy2BIM fragment. It is also noteworthy
that well-defined H−O bond oxidative addition of H2O to low-
valent transition metal centers is still limited to only a few
examples.28−31 However, it has been shown that pendant
hydrogen-bond donor groups in the ligand periphery can
promote H2O oxidative addition to zerovalent Group 10
metals.28 Accordingly, the reaction between 1 and H2O offers a
complement to this approach, wherein O−H bond activation is
facilitated by direct coordination of a Lewis acidic group to a
transition metal center.
The activation of H−X bonds by the Pt→B unit in 1 can also
be extended to other substrates. Addition of methanol or p-
nitroaniline to Pt(κ2-N,B-Cy2BIM)(CNArDipp2) (1) results in
formal H−X oxidative addition and formation of the platinum−
hydride complexes PtH(μ-OMe)(Cy2BIM)(CNArDipp2) (8) and
PtH(μ-N(H)C6H4NO2)(Cy2BIM)(CNArDipp2) (9), respectively
(Scheme 1). Structural characterization of 8 and 9 revealed that
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic, computational and crystallographic details (PDF and
CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
jsfig@ucsd.edu
■
Notes
The authors declare no competing financial interest.
Figure 2. Molecular structures of complexes 7, 10, and 11. The hydride ligand in 10 was not found in the electron-density difference map. Complex
11 possesses an η1-ipso interaction between Pt and one flanking Dipp ring in the solid state.
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ACKNOWLEDGMENTS
■
We are grateful to the National Science Foundation for support
(CHE-0954710 and a Graduate Research Fellowship to B.R.B.)
and to Prof. Carlos A. Guerrero and Matthew Del Bel for a
donation of HBCy2. J.S.F. is a Camille Dreyfus Teacher-Scholar
(2012−2017).
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