Rhodium Complexes Bearing PAlP Pincer Ligands

Article abstract of DOI:10.1021/jacs.8b04199

We report rhodium complexes bearing PAlP pincer ligands with an X-type aluminyl moiety. IR spectroscopy and single-crystal X-ray diffraction analysis of a carbonyl complex exhibit the considerable σ-donating ability of the aluminyl ligand, whose Lewis acidity is confirmed through coordination of pyridine to the aluminum center. The X-type PAlP-Rh complexes catalyze C2-selective monoalkylation of pyridine with alkenes.

Full text of DOI:10.1021/jacs.8b04199

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Communication
Rhodium Complexes Bearing PAlP Pincer Ligands
Naofumi Hara, Teruhiko Saito, Kazuhiko Semba, Nishamol
Kuriakose, Hong Zheng, Shigeyoshi Sakaki, and Yoshiaki Nakao
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04199 • Publication Date (Web): 24 May 2018
Downloaded from http://pubs.acs.org on May 24, 2018
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Rhodium Complexes Bearing PAlP Pincer Ligands
Naofumi Hara,† Teruhiko Saito,† Kazuhiko Semba,† Nishamol Kuriakose,‡ Hong Zheng,‡ Shigeyoshi Sakaki,‡,*
and Yoshiaki Nakao†,*
†Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan, and ‡Fukui Institute for
Fundamental Chemistry, Kyoto University, Sakyo-ku, Kyoto 606-8103, Japan
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Supporting Information Placeholder
Chart 1. Transition Metal Complexes with X-type Group 13
Ligands.
Abstract: We report rhodium complexes bearing PAlP pincer lig-
ands with an X-type aluminyl moiety. IR spectroscopy and single-
crystal X-ray diffraction analysis of a carbonyl complex exhibit
the considerable -donating ability of the aluminyl ligand, whose
Lewis acidity is confirmed through coordination of pyridine to the
aluminum center. The X-type PAlP–Rh complexes catalyze C2-
selective mono-alkylation of pyridine with alkenes.
Group 13 X-type ligands that form a σ-bond to a transition
metal center have received considerable attention as they formally
possess a lone pair of electrons and a vacant π-orbital, resulting in
unique bifunctional nucleophilic/electrophilic properties1 to coop-
erate with transition metal centers. Recently, such reactive species
have been integrated into multi-dentate supporting ligands for
transition metals.2 For example, Yamashita and Nozaki reported
the synthesis and the reactivity of group 9 transition metal com-
plexes that bear tridentate pincer-type ligands with an X-type
boryl donor (PBP) ligand [Chart 1(a)].2a–d Remarkably, the corre-
sponding PBP–Rh complex can activate carbon–carbon bonds of
cyclobutanone derivatives at room temperature, demonstrating the
high reactivity of the rhodium center resulting possibly from the
highly electron-donating nature of the PBP pincer ligand.2d They
also revealed a strong trans influence of the PBP ligand by X-ray
diffraction analysis. Furthermore, nickel and cobalt complexes
bearing the ligand have been shown to catalyze hydrogenation of
olefins through a unique mode of H2 activation by the B–Co/Ni
bonds.2e
Compared with the rich chemistry of boryl ligands, analogous
X-type aluminyl ligands have received less attention, probably
due to prospective synthetic difficulties associated with their high-
ly ionic character.3 To the best of our knowledge, merely four
examples of X-type aluminyl ligands have been reported [Chart
1(b)].4 A very recent example by Takaya and Iwasawa is particu-
larly intriguing because the palladium complex bearing an alumi-
nyl pincer ligand shows very high activity towards catalytic re-
duction of CO2.4d The σ-donation3b of aluminyl ligands in combi-
nation with Lewis acidity possibly higher than that of boryl ligand
should enable cooperative and/or site-selective activation of or-
ganic compounds by transition metal centers to result in unique
and useful transformations. Aiming at the synthesis of such transi-
tion metal complexes, we designed a new diphosphine ligand with
an X-type aluminyl moiety bearing a tridentate NNN backbone
(PAlP), which was based on related triphosphine ligands intro-
duced by Lu.5a This ligand should be able to stabilize the highly
ionic aluminum moiety, considering that its structure should be
sufficiently flexible to accommodate both tetra- and penta-
coordinate aluminum species. Herein, we report the synthesis,
characteristics and reactivity of rhodium complexes bearing an X-
type PAlP pincer ligand [Chart 1(c)].5
The reaction of diphosphine 1 with 2.2 equivalents of tBuLi in
Et2O at room temperature afforded lithiated 1, which was subse-
quently added to a suspension of AlCl3 in toluene to furnish alu-
minum-containing diphosphine 2 in 80% yield (Scheme 1). The
single-crystal X-ray diffraction analysis of 2 showed that the alu-
minum center adopted distorted trigonal bi-pyramidal geometry
composed of three nitrogen, one chlorine, and one phosphorus
atoms. A resonance at  10.6 ppm was observed in 31P{1H} NMR
at room temperature in C6D6 due to a fast equilibrium between
coordination and dissociation of the phosphines to the aluminum
center.6
The reaction between 2 and 0.50 equivalents of [Rh(nbd)(μ-
Cl)]2 (nbd = 2,5-norbornadiene) at 80 °C afforded Al/Rh hetero-
bimetallic complex 3 (Scheme 1). 31P{1H} NMR of 3 in CDCl3
afforded two broad signals at room temperature and four magneti-
cally inequivalent phosphorus signals at  60.0/68.7 ppm (JP–Rh
163/166, JP–P = 28/26 Hz, major) and  57.5/69.6 ppm (JP–Rh
=
=
161/163, JP–P = 31/28 Hz, minor) at –40 °C.7 These results indi-
cated that there was a slow equilibrium in an NMR time-scale.
DFT calculations suggested they were derived from two unsym-
metrical structures of 3, which could be interconverted because of
a low energy difference of 2.7 kcal/mol (Figure S4). The crystal
structure of 3 was unequivocally determined by X-ray diffraction
analysis, demonstrating its symmetrical structure in a solid state
due possibly to better packing. The Al–Rh bond [2.5487(8) Å] is
longer than that of a reported rhodium complex bearing a typical
Z-type aluminum ligand (2.425 Å).5b The Wiberg bond index of
the Al–Rh bond in 3 was calculated to be 0.27, which could sup-
port the bonding interaction but was smaller than that (0.39) of the
reported complex.5b The relatively smaller value can be ascribed
to charge-transfer interaction between two nitrogen and alumi-
num, increasing the electron density of aluminum to weaken the
Al–Rh bond (Figure S5 and Table S1). Besides the Wiberg bond
index, the geometry of aluminum and rhodium centers could also
support the bonding interaction to conclude that 3 has a Z-type
aluminum center.
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Scheme 1. Preparation of Rhodium Complex Bearing an X-type
Aluminyl Ligand and Crystal Structure of 2 and 3
forded dicarbonyl complex 5 (Scheme 2). 13C{1H} NMR showed
two kinds of doublets of triplets at  202.3 ppm (dt, JRh–C = 73 Hz,
JP–C = 20 Hz) and  207.7 ppm (dt, JRh–C = 58 Hz, JP–C = 12 Hz).
The IR spectroscopy of 5 featured two absorption bands at 1973
cm–1 and 1907 cm–1, which were assigned to the carbonyl ligands.
These suggested high electron density of the rhodium center in 5,
compared with carbonyl vibration frequencies reported of dicar-
bonyl PSiP–rhodium (νCO = 2025/1974 cm–1)2f and monocarbonyl
PBP-rhodium (νCO = 1933 cm–1).2c The solid structure of 5 deter-
mined by single-crystal X-ray diffraction analysis again revealed
the X-type coordination of the aluminyl ligand. The aluminum
and rhodium centers adopt distorted tetrahedral and distorted trig-
onal bi-pyramidal geometries, respectively. The Al–Rh bond
length [2.4378 Å] of 5 is considerably shorter than that of 3, and
the Wiberg bond index of 5 (0.47) is larger than that of 3 (0.27).9
The Rh–C1 distance is 1.930 Å, suggesting that the PAlP ligand
has almost the same trans-influence as the PSiP ligand, in which
the distance of Rh–CO bond trans to silyl is reported to be
1.9397(3) Å.2f
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Scheme 2. Synthesis and Crystal Structure of 5
Crystal structures of 2 and 3 (atomic displacement parameters set
at 30% probability; all hydrogen atoms and solvent molecules are
omitted for clarity). See Table S7 for details.
Crystal structure of 5 (atomic displacement parameters set at 30%
probability; all hydrogen atoms and solvent molecules are omitted
for clarity). See Table S8 for details.
Reduction of 3 with 2.0 equivalents of KC8 in the presence of
3.0 equivalents of norbornadiene afforded rhodium complex 4
(Scheme 1),8 in which the aluminum center served as an X-type
aluminyl ligand. A signal at   ppm (JP–Rh = 153 Hz) was
majorly observed in 31P{1H} NMR, while unidentified signals at
   ppm (JP–Rh = 140, 144 Hz) were also noted as minor
components, which could be a structural or conformational isomer
of 4 (vide infra). 1H NMR showed two kinds of olefinic protons of
the coordinating norbornadiene at  3.04 and 5.32 ppm, suggest-
ing monodentate coordination. Unfortunately, the single crystals
of 4 suitable for X-ray diffraction analysis could not be obtained.
Thus, we conducted DFT calculations to gain some more insights
into the structural and electronical characters of 4. A calculated
structure of 4 exhibited a substantial bonding overlap between the
rhodium d and the lone pair orbitals of aluminum (Figure S13),
suggesting a covalent bond between them. It was also supported
by the Wiberg bond index (0.45).9 As the d orbital (–5.4 eV) of
neutral [Rh(nbd)] is energetically located below that of the va-
lence orbital of aluminum (–3.4 eV) of the PAlP ligand, the Al–Rh
bond is polarized (Al+/Rh−) (Figure S14). This coordination
mode is unusual and thus represents another characteristic of the
complex. It should furthermore be noted that the coordinating
double bond of the norbornadiene ligand is not located trans to
aluminum (side view of 4 in Figure S17), but above it, which is
uncommon for such diene ligands due probably to the trans-
influence by aluminum. The PiPr2 moieties are located not to be
trans each other, with the PRhP angle being 107.92° (Figure S13).
This should be attributed to geometrical flexibility imposed by the
Al–N–CH2–P chain structure. As the coordinating double bond
prefers strong -back donation from the rhodium(I) center, it oc-
cupies the coordination site trans to the phosphines to overlap
well with the d orbital, which is destabilized by two phosphines
bearing the narrow PRhP angle.10
To further understand the electronic character of the X-type
PAlP pincer ligand, we conducted a series of DFT calculations.
Initially, we examined L1 as a model for the PAlP ligand, in
which the PiPr2 moieties were omitted to focus on the frontier
orbitals on the aluminum atom [Figure 1(a)]. The HOMO of L1 is
located on the aluminum center [Figure 1(b)] and corresponds to a
lone pair orbital.11 The empty 3p orbital on the aluminum atom,
which is oriented perpendicular to the N1AlN3 plane, is identified
as the LUMO+9, which participates in the coordination of Lewis
bases (vide infra).12 Another characteristic is the Al+–N– bonds,
which are heavily polarized due to the large difference of electro-
negativity between aluminum and nitrogen [Figure 1(c)]. Consid-
ering these aspects, L1 should be best represented by its canonical
diamido–Al+ resonance structure [Figure 1(d)]. A similar electron-
ic structure was previously proposed for the anionic five-
membered aluminum(I) heterocycle [: Al{N(H)C(H)}2].3c,13
Figure 1. Coordination geometry (a), frontier orbitals (b), electron
distribution (c), and dominant canonical form (d) for the anionic
aluminum model ligand L1.
We then investigated the derivatization of 4 to further gain
experimental evidences and characteristics of X-type PAlP–Rh
complexes. Exposure of 4 under a CO atmosphere (1.0 atm) af-
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To confirm the Lewis acidity of the aluminum in the X-type
Scheme 4. C2-selective mono-alkylation of pyridine catalyzed by
6
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3
4
5
6
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8
PAlP ligand, the reaction of 4 with pyridine was conducted. Small
broad multiplet signals were observed in the 31P{1H} NMR, and
the subsequent addition of HSiEt3 to the mixture afforded rhodi-
1
um complex 6, that was characterized by H, 13C and 31P NMR
spectroscopy and single-crystal X-ray diffraction analysis,
through the oxidative addition of the C(2)–H bond of pyridine
coordinating to the aluminum center (Scheme 3). Hydrosilylation
and hydropyridylation (vide infra) of the norbornadiene ligand
were also detected by a GC-MS analysis of the reaction mixture.
Similar reaction was recently reported by Ozerov with an iridium
complex bearing an X-type boryl ligand.2g Rhodium hydride was
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observed at  –7.17 ppm (td, JP–H = 24 Hz, JRh–H = 11 Hz) in H
aDetermined by 1H NMR.
NMR and 2048 cm–1 in IR spectrums.14 13C{1H} NMR revealed a
resonance at  224.6 ppm (dt, JRh–C = 34 Hz, JP–C = 9.3 Hz,),
which was assigned to the pyridyl C-2 bonding to the rhodium
center. Finally, the crystal structure of 6 was confirmed by X-ray
diffraction analysis. The formation of 6 reveals that the PAlP
pincer ligand has enough Lewis acidity to interact with a Lewis
base and that the rhodium center is electron-rich enough to acti-
vate a proximal C–H bond. A pure sample of 6 was treated with
norbornadiene to show NMR spectra similar to those obtained
with the reduction of 3 (Eq. S3).
In conclusion, we have successfully synthesized and charac-
terized rhodium complexes bearing the X-type PAlP ligand,
demonstrating that the newly developed ligand has moderate
Lewis acidity as well as possible -donating nature to enable a
catalytic C2-selective mono-alkylation of pyridine with alkenes
through the oxidative addition of C(2)–H bond to rhodium(I).
Further studies on the development of other catalytic reactions are
ongoing in our laboratory.
ASSOCIATED CONTENT
Scheme 3. Synthesis and Crystal Structure of 6
Supporting Insformation: Detailed experimental proce-
dures, as well as spectroscopic and analytical data. CIF files con-
taining crystallographic data for 2, 3, 5, and 6. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Crystal structure of 6 (atomic displacement parameters set at 30%
probability; all hydrogen atoms and solvent molecules are omitted
for clarity). See Table S8 for details.
sakaki.shigeyoshi.47e@st.kyoto-u.ac.jp
nakao.yoshiaki.8n@kyoto-u.ac.jp
The observed stoichiometric oxidative addition of C(2)–H
bond of pyridne prompted us to investigate possible C2-selective
functionalizations of pyridine by 6 as a catalyst. As a model reac-
tion, we conducted the reaction of pyridine with alkenes to expect
the C2-selective alkylation,15 which generally requires a substitu-
ent at the C6 position to hamper deactivation of metal catalysts by
coordination of pyridine.15a Meanwhile, the reaction with parent
pyridine often gives a mixture of C2- and C6-dialkylation prod-
ucts with a limited scope of alkenes.15b,15c The reaction of pyridine
(7) with styrene (8a) in the presence of 5 mol% of 6 at 110 °C for
20 h afforded a mixture of C2-alkylated products (linear/branch =
1/4) in 80% yield (Scheme 4). Although the yields of the corre-
sponding products were moderate, 1-octene (8b) and trimethylvi-
nylsilane (8c) were also viable substrates to give linear alkylation
products.16 Trace amounts of dialkylation products were observed
only in the case of 8a. Use of complex 2 and 3 or [RhCl(nbd)]2, in
combination with several different phosphine ligands and AlEt2Cl
as an external Lewis acid, did not afford the product, whereas 4
gave alkylation products in similar yields (Table S2–S4). NMR
studies and the reaction using 7-d5 suggested that 6 was the rest-
ing state and that the exclusive C(2)–H bond activation was fol-
lowed by alkene insertion in a reversible manner (See Supporting
Information) .
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by the “JST CREST program Grant
Number JPMJCR14L3 in Establishment of Molecular Technology
towards the Creation of New Functions”
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Journal of the American Chemical Society
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ACS Paragon Plus Environment