Adaptability of the SiNN Pincer Ligand in Iridium and Rhodium Complexes Relevant to Borylation Catalysis

Article abstract of DOI:10.1021/acs.organomet.5b00125

A comparison of Rh and Ir complexes of the SiNN ligand (combining Si-H, amido, and quinoline donors) reveals its great degree of adaptability. The amido donor can function as a boryl group acceptor, and the Si-H/metal interaction is highly variable. In contrast to Ir analogues, complexes of Rh do not catalyze dehydrogenative borylation of terminal alkynes but do act as modest benzene borylation catalysts.

Full text of DOI:10.1021/acs.organomet.5b00125

Communication
pubs.acs.org/Organometallics
Adaptability of the SiNN Pincer Ligand in Iridium and Rhodium
Complexes Relevant to Borylation Catalysis
Chun-I Lee, Nathanael A. Hirscher,† Jia Zhou,‡ Nattamai Bhuvanesh, and Oleg V. Ozerov*
Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States
S
* Supporting Information
ABSTRACT: A comparison of Rh and Ir complexes of the
SiNN ligand (combining Si−H, amido, and quinoline donors)
reveals its great degree of adaptability. The amido donor can
function as a boryl group acceptor, and the Si−H/metal
interaction is highly variable. In contrast to Ir analogues,
complexes of Rh do not catalyze dehydrogenative borylation of
terminal alkynes but do act as modest benzene borylation
catalysts.
quinoline-type donors with the Si−H functionality. Complex 2-
ransition-metal-catalyzed borylation of C−H bonds has
Tbecome an important synthetic tool over the last two Ir can be rapidly converted to 3a-Ir in the presence of HBpin,
decades.1−3 The products of C−H borylation, organoboronate
and either 2-Ir or 3a-Ir can be used as a precatalyst (Scheme
1). The present work explores Rh analogues of these Ir SiNN
complexes. Although the new Rh compounds turned out to be
inactive as DHBTA catalysts, they brought to light an unusual
dual noninnocence6 behavior of the SiNN ligand. The (SiNN)
Rh system also turned out to be a modest benzene borylation
catalyst.
esters, are versatile building blocks in organic synthesis,
enabling facile formation of new carbon−carbon and carbon−
heteroatom bonds. In particular, much attention has been
dedicated to Ir-catalyzed borylation of aromatic C−H bonds
where considerable success has been achieved.4−6 Our group
recently reported a complementary Ir catalyst capable of
dehydrogenative C−H borylation of terminal alkynes
(DHBTA; Scheme 1).7 This Ir system was based on a novel
SiNN pincer ligand that combines nitrogenous amido and
By analogy with the synthesis of 2-Ir, 1 smoothly reacted
with 1/2 equiv of [(COE)2RhCl]2 in fluorobenzene at ambient
temperature to produce a blue solution of 2-Rh (Scheme 1).
Workup of the reaction mixture by filtration and recrystalliza-
tion allowed isolation of 2-Rh in 66% yield as an analytically
pure blue solid. 1H and 13C{1H} NMR spectra of 2-Rh
displayed the expected features that were similar to those of 2-
Ir, with the resonances corresponding to the organic framework
Scheme 1. Synthesis of SiNN Complexes of Ir and Rh and
DHBTA Catalysis by Ir Complexes
1
of SiNN and of COE readily identifiable. An upfield H NMR
resonance was observed at −16.9 ppm corresponding to the
Si−H hydrogen bound to Rh (cf. δ −21.1 ppm in 2-Ir). This
resonance displayed coupling to 103Rh (JRh−H = 31 Hz) and
possessed satellites from coupling to 29Si (JSi−H = 51 Hz). A
single-crystal X-ray diffraction study (Figure 1) established that
2-Rh is isomorphous with 2-Ir, crystallizing in the same space
group (P21/n) with unit cell volumes within 0.5% of each other.
On the molecular level, 2-Rh and 2-Ir are nearly super-
imposable,9 with only small differences in the metrics associated
with the coordination sphere of the metals.
Treatment of 2-Rh with 5 equiv of HBpin resulted in the
predominant formation of a new product (3b-Rh) that could
be isolated as a yellow solid in 59% yield. It contained 1H NMR
Received: February 14, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.5b00125
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Organometallics
Communication
and borylated diarylamine); the silyl ligand is trans to the
empty site.13 DFT-calculated free energies of the 3a-M/3b-M
isomers are consistent with the experimental facts: for Ir, 3a-Ir
is preferred by 1.8 kcal/mol, while for Rh, 3b-Rh is favored by
5.2 kcal/mol. The free energy barrier14 for the direct migration
of the boryl from M in 3a-M to N was calculated to be 16.9
kcal/mol for M = Ir and 12.0 kcal/mol for M = Rh (i.e., 17.2
kcal/mol barrier from the ground state 3b-Rh to 3a-Rh); both
are consistent with reversible migration on the time scale of
experimental handling at ambient temperature.
Figure 1. ORTEP drawings of 2-Rh (left, 80% probability ellipsoids)
and 3b-Rh (right, 50% probability ellipsoids) showing selected atom
labeling. Hydrogen atoms are omitted for clarity. Selected bond
distances (Å) and angles (deg) for 2-Rh: Rh−Si, 2.3787(13); C1−C2,
1.388(8); Si−Rh−N2, 140.87(11). Selected bond distances (Å) and
angles (deg) for 3b-Rh: Rh−Si, 2.2223(9); Rh−B2, 2.007(3); N1−B1,
1.459(4); Si−Rh−N2, 106.51(7); N1−Rh−B2, 175.33(11).
Figure 3 depicts the geometries of the M/Si/H triangles in
compounds 2 and 3. The variation of the geometric parameters
resonances expected for all of the constituent groups in
“(SiNN)Rh(Bpin)2”; however, several features distinguishing it
from 3a-Ir were immediately apparent. The yellow color of 3b-
Rh was suspect, as 3a-Ir is purple whereas both 2-Ir and 2-Rh
are blue. Instead of a single resonance for the Bpin methyl
groups (as is the case for 3a-Ir), 3b-Rh exhibited several.10 In
the 11B NMR spectrum, 3b-Rh exhibited two resonances at
41.3 and 23.8 ppm (3a-Ir shows a single resonance at 28.9
ppm). Finally, the JSi−H value for 3b-Rh was not discernible
(assumed <1 Hz). This was inconsistent with 3a-Rh, the exact
structural analogue of 3a-Ir (JSi−H = 32 Hz), because the change
from Ir to Rh dramatically increased the JSi−H value in 2-Rh (51
Hz) vs 2-Ir (8 Hz). These observations led us to envision a
different, isomeric structure for 3b-Rh, which was confirmed by
a single-crystal X-ray diffraction study (Figure 1). The
connectivity in 3b-Rh differs from that in 3a-Rh by migration
of one of the boryl groups onto the central N of the SiNN
ligand. The hydride in 3b-Rh was not located by XRD
Figure 3. Metric parameters in the M/Si/H triangles (M = Rh, Ir) in
compounds 2 and 3. DFT calculated distances (Å) are given in blue,
Si−M−H angles (deg) in red, and XRD-determined M−Si distances in
black.
within these structures can be viewed as reflective of the
continuum between Si−H σ complexes and silyl/hydride
oxidative addition.15 2-Rh and 3a-Rh clearly belong to the
former, while 3b-Ir and 3b-Rh belong to the latter. As
discussed previously,7 2-Ir is probably best viewed as a silyl/
hydride complex while 3a-Ir is a borderline structure. The
geometrical changes in 2 and 3 are not limited only to the Si−
H distances. As the Si−H distances increase, the Si−M−H
angles necessarily open up while the M−Si and M−H distances
decrease. This is consistent with the strengthening of M−Si and
M−H interactions upon diminishment of the Si−H inter-
actions. The complexity of interactions of Si−H bonds with
transition metals has been thoroughly studied and analyzed,15
but Si−H as an adaptable spectator donor site in a chelating
ligand has not been widely used.16 The migration of boryl from
M (3a-M) to N (3b-M) can be viewed as B−N reductive
coupling which happens in concert with Si−H oxidative
cleavage. Thus, the two adaptable features of the SiNN ligand
serve to smooth out the electronic changes at the metal. The
structure 3a maximizes the number and the strength of metal−
ligand bonds, which is more favorable for the 5d metal
iridium.17
methods, but its presence is apparent from the doublet (JRh−H
=
30 Hz) at −15.0 ppm in the 1H NMR spectrum. We previously
observed B−Namido bond formation in addition reactions of
HBcat and B2cat2 (cat = catecholate) with the cationic amido/
bis(phosphine) [(PNP)Pd]+ complex.11
Density functional theory (at the M06/SDD/6-311G**
level)12 was used to calculate the structures of 2-Rh, 3a-Rh, and
3b-Rh. Figure 2 shows the immediate coordination environ-
Figure 2. Drawings of the DFT calculated structures of 2-Rh (left),
3b-Rh (middle), and 3a-Rh (right). N2 is the quinoline N.
In contrast to its iridium analogue, 2-Rh showed no DHBTA
activity: treatment of 2-Rh with HBpin in C6D6, followed by 4-
MeC6H4CCH, did not give rise to any alkynylboronate after 3
days at room temperature. Instead, incomplete and unselective
hydroboration of 4-MeC6H4CCH to various alkenylboronates
took place.
When pure 3b-Rh was dissolved in C6D6, it underwent
degradation over time, with C6D5Bpin signals detectable in the
mixture. Pure 3b-Rh also slowly decomposed in a cyclohexane-
d12 solution, with release of free HBpin. Thermolysis of 2-Rh or
ment in the three calculated Rh structures. The non-H atom
positions reproduce those in the available XRD determinations
well. The calculated structures of 2-Ir and 3a-Ir were reported
by us previously,7 using the same DFT methods. The three
distinct X-type ligands (boryl, silyl, hydride) in 3b-Rh are each
of a very strong trans influence and adopt a facial arrangement.
The approximately square pyramidal coordination sphere about
Rh is completed by two neutral nitrogenous donors (quinoline
B
DOI: 10.1021/acs.organomet.5b00125
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Organometallics
Communication
3b-Rh in C6D6 in the presence of excess HBpin resulted in
catalytic production of C6D5Bpin, with ca. 30 turnovers after 48
h at 80 °C (Table 1). Using B2pin2 in place of HBpin resulted
files is also available for free from the Cambridge Crystallo-
graphic Data Centre: CCDC 1027428 (2-Rh); CCDC
1027429 (3b-Rh).
AUTHOR INFORMATION
Table 1. Catalytic Borylation of Neat C6D6 (80 °C, 48 h)
■
using 2-Rh and 3b-Rh
Corresponding Author
*E-mail for O.V.O.: ozerov@chem.tamu.edu.
a
b
Rh catalyst
boron source
C6D5Bpin yield (%)
Present Addresses
2-Rh
HBpin
HBpin
B2pin2
27
33
7
†Division of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, California 91125.
‡Department of Chemistry, Harbin Institute of Technology,
Harbin 150001, China.
3b-Rh
2-Rh
3b-Rh
B2pin2
b
5
a
Rh:Bpin ratio of 1:100. Yield based on equivalents of Bpin,
determined by H NMR vs C6Me6 internal integration standard.
1
Notes
The authors declare no competing financial interest.
in a smaller number of turnovers of C6D6 under the same
conditions. Although there are examples of Rh C−H borylation
catalysts in the literature,18 their efficiency is far outstripped by
the best examples of Ir catalysis.4−6 The catalytic reactivity of
the (SiNN)Rh system is in contrast with the lack of arene
borylation with (SiNN)Ir.
Interestingly, recent work on the “traditional” iridium
aromatic C−H borylation catalysts supported by bipyridine-
type ligands highlighted the usefulness of an Si−H moiety as a
directing group in the substrate.19 The intermediate A
proposed19a by Hartwig et al. (Figure 4) bears a structural
ACKNOWLEDGMENTS
■
We are grateful to the following agencies for support of this
research: the National Science Foundation (grant CHE-
1300299 to O.V.O.) and the Welch Foundation (grant A-
1717 to O.V.O.). We are grateful to Prof. T. Don Tilley and
Mark Lipke (UC Berkeley) for advice on and to Dr. Steven K.
Silber (Texas A&M) for experimental assistance with the
double quantum filter NMR experiments. We thank Ms. Linda
Redd for editorial assistance.
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ASSOCIATED CONTENT
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■
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