Metal-organic layers stabilize earth-abundant metal-terpyridine diradical complexes for catalytic C-H activation

Article abstract of DOI:10.1039/c7sc03537c

We report the synthesis of a terpyridine-based metal-organic layer (TPY-MOL) and its metalation with CoCl₂ and FeBr₂ to afford CoCl₂·TPY-MOL and FeBr₂·TPY-MOL, respectively. Upon activation with NaEt₃

Full text of DOI:10.1039/c7sc03537c

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Metal-Organic Layers Stabilize Earth-Abundant Metal-Terpyridine
Diradical Complexes for Catalytic C-H Activation
Received 00th January 20xx,
Accepted 00th January 20xx
Zekai Lin,a,† Nathan C. Thacker,a,† Takahiro Sawano,a,† Tasha Drake,a Pengfei Ji,a Guangxu Lan,a
Lingyun Cao,b Shubin Liu,c Cheng Wang,b and Wenbin Lina,b,
*
DOI: 10.1039/x0xx00000x
We report the synthesis of a terpyridine-based metal-organic layer (TPY-MOL) and its metalation with CoCl2 and FeBr2 to
afford CoCl2·TPY-MOL and FeBr2·TPY-MOL, respectively. Upon activation with NaEt3BH, CoCl2·TPY-MOL catalyzed benzylic
C-H borylation of methylarenes whereas FeBr2·TPY-MOL catalyzed intramolecular Csp3-H amination of alkyl azides to afford
pyrrolidines and piperidines. X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure
(EXAFS), X-ray photoelectron spectroscopy, UV-Vis-NIR spectroscopy, and electron paramagnetic spectroscopy (EPR)
measurements as well as density functional theory (DFT) calculations identified M(THF)2·TPY-MOL (M = Co or Fe) as the
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active catalyst with a MII-(TPY•• 2- electronic structure featuring divalent metals and TPY diradical dianions. We believe that
)
site isolation stabilizes novel MII-(TPY••)2- (M = Co or Fe) species in the MOLs to endow them with unique and enhanced
catalytic activities for Csp3-H borylation and intramolecular amination over their homogeneous counterparts. The MOL
catalysts are also superior to their metal-organic framework analogs owing to the removal of diffusion barriers. Our work
highlights the potential of MOLs as a novel 2D molecular material platform for designing single-site solid catalysts without
diffusional constraints.
with larger channels and pores,34 only moderate success has been
achieved to date. MOFs constructed from elongated ligands tend to
suffer from interpenetration as well as framework distortion,
Introduction
Over the past two decades, metal-organic frameworks (MOFs) have
attracted great interest among scientists and engineers owing to
their potential in various applications including gas storage and
separation,1-6 heterogeneous catalysis,7-16 nonlinear optics,17,18
chemical sensing,19-21 biomedical imaging,22,23 and drug delivery.24,25
In particular, MOFs have provided an excellent platform for
designing single-site solid catalysts for many important organic
transformations.26-32 By shutting down intermolecular deactivation
pathways via spatial isolation of active sites, MOFs have afforded
turnover numbers (TONs) several orders of magnitude higher than
their homogeneous analogs.26,29 The catalytic performance of MOFs
is, however, still limited by the diffusion rates of large substrates
and products within the 3D frameworks.33 Although many
strategies have been devised to overcome this diffusion limitation
of MOFs, for example, by elongating functional ligands26 or diluting
them with catalytically inactive spectator ligands to construct MOFs
whereas MOFs built from mixed functional and spectator ligands
have diminished atom efficiency.
We recently showed that diffusional constraint of MOFs could
be lifted by reducing one dimension of the MOF crystals to only a
few nanometers in thickness to afford a new category of 2D
materials, metal-organic layers (MOLs).35 Unlike 3D MOFs, the
active sites in ultrathin 2D MOLs are readily accessible to substrates
during catalytic reactions. On the other hand, MOLs still inherit the
heterogeneous nature, ordered structure, and molecular tunability
of MOF catalysts,36-38 and have the potential to provide a rare 2D
molecular material platform for designing a new class of single-site
solid catalysts without diffusional constraints. We report here the
synthesis of a new metal-organic layer, TPY-MOL, based on Hf6(µ3-
O)4(µ3-OH)4(HCO2)6 secondary building units (SBUs) and 4'-(4-
carboxyphenyl)-[2,2':6',2''-terpyridine]-5,5''-dicarboxylate
(TPY)
bridging ligands and the metalation of TPY ligands in TPY-MOL with
CoCl2 and FeBr2 to afford highly effective recyclable and reusable
MOL catalysts for challenging benzylic C-H borylation and
intramolecular sp3 C-H amination reactions (Figure 1). Spectroscopic
and computational studies identified unprecedented CoII/FeII-
terpyridine diradical complexes as catalytic active sites for the
borylation and amination reactions.
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Results and Discussion
Synthesis and Postsynthetic Metalation of TPY-MOL
TPY-MOL was synthesized in 76% yield by heating a mixture of
HfCl4, H3TPY, and formic acid in DMF and water at 120 °C for 24 h.
The PXRD pattern of TPY-MOL matched the simulated pattern
based on the (hk0) reflections only that are characteristic of 2D
MOL structures and aligned well with that of isostructural BTB-MOL
(BTB is 1,3,5-benzenetribenzoate, Figure 2a).35 Transmission
Electron Microscopy (TEM) images showed ultra-thin films of TPY-
MOL whereas the high resolution TEM (HRTEM) images of TPY-MOL
showed clear lattice with the dark spots corresponding to Hf6
clusters (Figure 2b,c). The distances between adjacent spots on the
HRTEM image (20.1 Å) matched well with that between two
adjacent Hf6 SBUs (20.0 Å) in the MOL structural model. Atomic
Force Microscopy (AFM) images of TPY-MOL indicated monolayer
thickness for many nano-sheets with an average measured
thickness of 1.2 nm, corresponding to the van der Waals size of Hf6
SBUs (Figure 2d,e).
Figure 1. M•TPY-MOLs, constructed from Hf6 SBUs and TPY and
then metalated with Co and Fe, were used for benzylic C-H
borylation and intramolecular sp3 C-H Amination reactions,
respectively.
Owing to their distinct coordination, redox, and photophysical
properties, terpyridines (tpy) and their metal complexes have been
explored for potential applications in many fields, including polymer
science,39,40
optoelectronics,41,42
medicinal
chemistry,43,44
nanotechnology,45 and molecular catalysis.41,46,47 Although tpy
derivatives provide a potentially interesting ligand platform for
designing earth-abundant metal catalysts, few examples have been
reported in the literature,47-50 in part due to their strong propensity
to undergo disproportionation reactions to form catalytically
inactive M(tpy)2 complexes.48,49 Installation of bulky groups on the
6,6”-positions of tpy could prevent such bimolecular deactivation
processes in M-tpy catalysts but often at the expense of their
catalytic activities.48 By incorporating a tpy derivative into the TPY-
MOL, we effectively shut down the disproportionation
decomposition pathway without relying on steric protection at the
6,6” positions and obtained highly effective MOL catalysts based on
M-tpy complexes (M = Co or Fe) for benzylic C-H borylation and
intramolecular sp3 C-H amination reactions. The MOL-based M-tpy
catalysts displayed at least 20 times higher catalytic activity and
distinct chemoselectivity in benzylic C-H borylation reactions and 50
times higher TONs in intramolecular sp3 C-H amination reactions
over their homogeneous analogs.
Figure 2. (a) PXRD patterns of TPY-MOL, CoCl2•TPY-MOL,
FeBr2•TPY-MOL, Co(THF)2•TPY-MOL, and Fe(THF)2•TPY-MOL in
comparison to simulated PXRD pattern for TPY-MOL; (b) TEM image
of TPY-MOL; (c) HRTEM image and fast Fourier transform (FFT)
2 | J. Name., 2012, 00, 1-3
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pattern of TPY-MOL; (d) Tapping-mode atomic-force microscope We first investigated C-H borylation of m-xylene by Co•TPY-MOL.
(AFM) topographic image of TPY-MOL; (e) Height profile along the Organoboronic compounds are a useful class of intermediates for
white line of TPY-MOL.
forming carbon-carbon and carbon-heteroatom bonds through
coupling reactions. C-H borylation with boron reagents such as
TPY-MOL was readily metalated with CoCl2 or FeBr2(THF)2 (1.05 B2pin2 is one of the most direct and convenient methods for the
eqv. w.r.t TPY) to afford CoCl2•TPY-MOL or FeBr2•TPY-MOL with synthesis of organoboronic compounds. Although C-H borylation
100% metal loading, as determined by inductively coupled plasma- with arenes has been developed in the past two decades, benzylic
mass spectrometry (ICP-MS). X-ray absorption near edge structure C-H borylation is still rare (Table S7, SI).27,51-56 Upon activation with
(XANES) analysis revealed +2 oxidation state for CoCl2•TPY-MOL NaEt3BH, CoCl2•TPY-MOL (0.5 mol%) catalyzed m-xylene borylation
and FeBr2•TPY-MOL (Figure 3a, b). The oxidation state assignments with B2pin2 at 100 °C over 3 days to afford 42% yield of borylated
were further confirmed by X-ray photoelectron spectroscopy (XPS, products, with a 4.2:1 selectivity favoring the benzylic position
Figure S15, SI). Extended X-ray absorption fine structure (EXAFS) (Table 1, entry 1). The borylated products were obtained in 95%
fitting indicated the coordination of Co(II) to three N atoms of TPY yield with a slightly higher selectivity for benzylic borylation (4.6:1)
and two chlorides in CoCl2•TPY-MOL and the coordination of Fe(II) when the catalyst loading increased to 1.0 mol% (Table 1, entry 2).
to three N atoms of TPY and two bromides in FeBr2•TPY-MOL The activation of CoCl2•TPY-MOL with NaEt3BH is necessary for the
(Figure 3c,d). The similarity of EXAFS-derived bond distances in borylation reaction (Table 1, entry 3). Under identical conditions, a
CoCl2•TPY-MOL (Co-Nc = 2.09 ± 0.01Å, Co-Nt = 2.16 ± 0.01Å and Co- TPY-MOF control, which is isostructural to the previously reported
Cl = 2.28 ± 0.01Å) and crystallographically determined CoCl2•tpy BTB-MOF in which 2D layers stack in a staggered arrangement to
distances (Co-Nc = 2.071Å, Co-Nt = 2.139Å and Co-Cl = 2.298Å) result in a 3D MOF,35 gave no conversion, likely due to slow
validates the EXAFS fitting results.
diffusion of the substrates and products. (Table 1, entry 4). The
homogeneous analog gave 2% borylated products with a 5.7:1
selectivity favoring the arene C-H bond (Table 1, entry 5). Such
moderate arene borylation activity was recently reported for
homogenous tpy-Co derivatives.49 Active site isolation in MOLs thus
not only increases the TON by more than 20 times (over the
homogeneous analog) but also afforded unusual selectivity of
borylation for the benzylic C-H bond.
a)
b)
Pre-edge
Rising edge
Pre-edge
7705
7710
7715
7110
7115
7120
CoCl2
CoCl2
•
TPY-MOL
TPY-MOL
TPY-MOL
FeBr2(THF)2
FeBr2 TPY-MOL
Fe(THF)2 TPY-MOL
Co(THF)2
Co(Bpin)2
•
•
•
•
Co
•TPY-MOL-post
7100
7110
7120
7130
7140
7150
7700
7710
7720
Energy(eV)
7730
7740 7750
Energy(eV)
Table 1. Cobalt-catalyzed C-H borylation of m-xylene
c)
d)
6
4
0.5
2
0
0.0
-2
-4
-6
|F.T. (Exp)|
|F.T. (Fit)|
Real (Exp)
Real (Fit)
|F.T. (Exp)|
|F.T. (Fit)|
Real (Exp)
Real (Fit)
Entry
1
2b
Catalyst
Yielda (%) (Bn : Ar)
42 (4.2 : 1)
-0.5
CoCl2•TPY-MOL
CoCl2•TPY-MOL
0
2
4
6
0
2
4
6
R(Å)
R(Å)
95 (4.6:1)
f)
e)
1.0
3c
CoCl2•TPY-MOL
CoCl2•TPY MOF
“Homogeneous”
CoCl2•tpy
0
2
0.5
0.0
4
0
5
2 (1 : 5.7)
0
|F.T. (Exp)|
|F.T. (Fit)|
Real (Exp)
Real (Fit)
NMR yield based on CH3NO2 as an internal standard; b1.0 mol%
Co; c without the addition of NaEt3BH.
a
-0.5
-1.0
|F.T. (Exp)|
|F.T. (Fit)|
Real (Exp)
Real (Fit)
-2
0
2
4
6
0
2
4
6
R(Å)
R(Å)
We further investigated the substrate scope for Co(THF)2•TPY-
MOL catalyzed C-H borylation reactions. Benzylic borylated
products were produced exclusively for p-xylene, 1-t-butyl-4-
methylbenzene, and mesitylene in >90% yields (Table 2, entries 2-
4). For p-methoxytoluene, a high selectivity of 59 : 6 : 1 was
obtained for the benzylic borylated product (Table 2, entry 5). For
toluene, borylated products were obtained in 92% yield, but the
selectivity for the benzylic borylation product was moderate (Table
2, entry 6). These results indicate the influence of steric hindrance
on the selectivity of benzylic vs. aromatic broylation by
Co(THF)2•TPY-MOL.
Figure 3. (a) XANES spectra of CoCl2, CoCl2•TPY-MOL, Co(THF)2•TPY-
MOL, Co(Bpin)2•TPY-MOL, and Co•TPY-MOL-post; (b) XANES
spectra of FeBr2(THF)2, FeBr2•TPY-MOL, and Fe(THF)2•TPY-MOL; (c-
f) Experimental EXAFS spectra and fits of CoCl2•TPY-MOL, R factor =
0.006 (c), FeBr2•TPY-MOL, R factor = 0.011 (d), Co(THF)2•TPY-MOL,
R factor = 0.013 (e) and Fe(THF)2•TPY-MOL, R factor = 0.015 (f) in R
space showing the magnitude of Fourier Transform (black hollow
squares, black solid line) and real components (blue hollow squares,
blue solid line).
Co-TPY-MOL Catalyzed Benzylic C-H Borylation
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reported [CoI(tpy)2]57 (Co-Nc = 2.003 Å, Co-Nt = 2.130Å), arguing
against the +1 oxidation state for Co(THF)2•TPY-MOL. Furthermore,
Co(THF)2•TPY-MOL has shorter Co-N bond distances than those for
CoIICl2•TPY-MOL (Co-Nc = 1.90 ± 0.01 Å, Co-Nt = 2.09 ± 0.01 Å), but
similar Co-N bond distances to a reported low-spin CoII(tpy)(BH4)
complex with the (tpy•)- ligand (Co-Nc = 1.810 Å, Co-Nt = 1.925 Å).58
The Co-N bond distance analysis thus supports the formulation of
the CoII-(tpy••)2- electronic structure for Co(THF)2•TPY-MOL.
Table 2. Co•TPY-MOL catalyzed C-H borylation of arenesa
1.0 mol% [Co]a
R
O
O
O
O
Bpin
R
+
B
B
R
+
100 °C, 3 d
-HBpin
Bpin
(60 equiv.)
Entry Substrate
1
Product
Yield (%)
(Bn : Ar)
95 (4.6 : 1)
(73:14)c
Signal
Smoothed
Signal
Smoothed
Co 2p3/2
Fe 2p3/2
BE = 781.2 eV
2
93 (88)c
BE = 709.2 eV
Co 2p1/2
Fe 2p1/2
3b
4
91
BE = 797.0 eV
BE = 722.7 eV
shake-up
satellite
shake-up
satellite
shake-up
satellite
shake-up
satellite
91 (84)c
5b
6
86 (59 : 6 : 1)
815
810
805
800
795
790
785
780
775 745
740
735
730
725
720
715
710
705
700
Binding Energy(eV)
Binding Energy(eV)
92 (0.91 : 1.4 :
1)
Figure 4. Co 2p and Fe 2p XPS spectra of Co(THF)2•TPY-MOL (left)
and Fe(THF)2•TPY-MOL (right).
a[Co] = Co(THF)2•TPY-MOL, NMR yield with CH3NO2 as an internal
standard; b4 mol% [Co]; cisolated yields are shown in parentheses.
Wavenumber(103cm-1)
Co•TPY-MOL was recovered and used for at least 10 times
without any loss of activity in C-H borylation of p-xylene (Figure S32,
SI). We conducted several tests to demonstrate the heterogeneity
of Co•TPY-MOL. First, we showed that the PXRD of Co•TPY-MOL
recovered from C-H borylation of p-xylene remained the same as
that of freshly prepared Co•TPY-MOL (Figure S33, SI). Second, we
used ICP-MS to show that the amounts of Co and Hf leaching into
the supernatant during the C-H borylation of p-xylene were less
0.092% and 0.037% respectively. Finally, we observed that the
removal of Co•TPY-MOL from the reaction mixture after several
hours stopped the C-H borylation of p-xylene (Scheme S2, SI).
30
20
10
0
CoCl2
Co(THF)2
FeBr2 TPY-MOL
Fe(THF)2 TPY-MOL
•
TPY-MOL
•TPY-MOL
•
•
Identification of the Co(THF)2•TPY-MOL Catalyst
We studied the catalytically active species by hydrogen
quantification, Infrared (IR), UV-Vis-NIR, XPS, and electron
paramagnetic resonance (EPR) spectroscopy, XANES, EXAFS, and
Density functional theory (DFT) calculations. One equiv. of H2 was
generated upon treatment of CoCl2•TPY-MOL with NaEt3BH,
suggesting the formation of Co(THF)x•TPY-MOL via reductive
elimination of H2 from the putative CoH2•TPY-MOL intermediate.
This 2-electron reduction process was also confirmed by titration of
Co(THF)x•TPY-MOL with ferrocenium hexafluorophosphate which
resulted in the generation of two equiv. of ferrocene w.r.t to CoTPY-
MOL (Figure S6, SI). IR spectra showed no characteristic band of
N≡N, ruling out the coordination of dinitrogen to Co. XANES
analysis indicated +2 oxidation state for the Co center (Figure 3a).
This oxidation state assignment was further supported by XPS
spectroscopy which gave a Co 2p3/2 binding energy of 781.2 eV with
the expected shake-up peak for the CoII centers (Figure 4).
400
600
800
1000
1200
1400
1600
Wavelength (nm)
Figure 5. UV-Vis-NIR absorption spectra of CoCl2•TPY-MOL,
Co(THF)2•TPY-MOL, FeBr2•TPY-MOL, and Fe(THF)2•TPY-MOL in THF
at 25 °C.
We used UV-Vis-NIR spectroscopy to discern the diradical
nature of TPY ligands in CoTPY-MOLs (Figure 5). Co(THF)2•TPY-MOL
exhibited two intense, broad bands centered at 552 and 759 nm
and a weak but broad band at 1105 nm, indicative of π to π* and π*
to π* transitions for the reduced tpy ligand.59-63 In contrast, these
bands are absent in CoCl2•TPY-MOL with the neutral TPY ligand
(Figure 5). The proposed (tpy••)2- species was previously observed in
reduced M(tpy)2 complexes, such as CrIII(tpy)2, VIV(tpy)2, and
TiIV(tpy)2, by Wieghardt and coworkers.62,63 However, we are not
aware of any example of M-tpy complexes featuring the (tpy••)2-
species.
The EXAFS spectra at the Co K-edge were well fitted with a
structural model in which Co coordinates with three N atoms of TPY
and two THF molecules (Figure 3e). Co-N bond distances (Co-Nc =
1.81 ± 0.02 Å, Co-Nt = 1.92 ± 0.02 Å) are shorter than those of the
4
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Our XANES, EXAFS, and XPS results clearly indicate the CoII dianion is not expected to give any EPR signal. Interestingly, the
oxidation state for Co(THF)2•TPY-MOL whose electronic structure is energy of quartet state of Co(THF)2•tpy is calculated to be only 0.40
best described as CoII(THF)2•(TPY••)2--MOL. The (tpy••)2- diradical kcal/mol higher than that of the doublet GS. This small energy gap
dianion can have either a singlet (S=0) or a triplet (S=1) ground is consistent to that deduced from temperature-dependent EPR
state, which can potentially be experimentally differentiated by EPR signals of Co(THF)2•tpy. The charge distribution of the quartet state
spectroscopy. EPR spectroscopy of Co(THF)2•TPY-MOL gave an is similar to that of the doublet GS with positive charge (1.29) on
isotropic signal with giso = 2.003 at r. t. in toluene suspension. The the Co center and negative charge (-1.40) on tpy (Table S9, SI). The
same MOL sample frozen at 20 K exhibits a stronger isotropic signal calculated bond distances are similar between the quartet state and
with giso = 2.003, confirming that the same species was detected at the doublet GS (Table S11, SI). A Mulliken spin density population
r.t. and 20K (Figure 6). More interestingly, the g value falls in the and spin density plot of the quartet state revealed the residence of
range of 2.003 – 2.005,59,64,65 where radicals in extended organic π the 1.091 unpaired spin on Co center and 1.887 unpaired spins on
systems were often observed. The EPR signal intensity was tpy, affording a low-spin CoII, d7 doublet (SCo = 1/2) and a tpy triplet
temperature-dependent, which can be fitted with the Bleaney and diradical dianion (Stpy = 1) (Figure 7). The energetically accessible
Bowers equation66 typically used for organic diradicals (Figure 6). low-lying triplet excited state of (tpy••)2- was previously proposed
The fitting of temperature-dependent EPR signals indicates that the for the hypothetical [ZnII(tpy2-)(NH3)2]0.62 DFT calculations thus
(TPY••)2- diradical has a singlet ground state with singlet-to-triplet support the origin of the experimental tpy diradical dianion EPR
energy gap of 0.04 kcal/mol. The observed EPR signal is thus signal as thermally populated quartet state of CoII(THF)2•tpy••.
attributed to the thermally populated TPY triplet excited state.67 Moreover, we believe that conjugation of Hf6 SBU to TPY can
Moreover, a weak signal giso ≈ 2.04 was observed at 20 K, consistent further stabilize TPY diradical dianion and lower energy difference
with low-spin CoII centers. Therefore, our EPR data provide strong between doublet and quartet states of CoII(THF)2•TPY••-MOL.
support to our proposed electronic structure CoII(THF)2•(TPY••)2--
MOL We have ruled out the possibility of SBU-based free radicals
because TPY-MOL treated with NaEt3BH exhibited no signal at r.t. or
20K (Figure S16, SI).
Co(THF)2•TPY-MOL_r.t.
Fe(THF)2•TPY-MOL_r.t.
Co(THF)2•TPY-MOL_20K
Fe(THF)2 TPY-MOL_20K
•
g = 2.003
g = 2.003
g = 2.003
g = 2.003
Figure 7. Proposed electronic structure of MII(THF)2•(TPY••)2--MOL,
M = Co or Fe (left); calculated Mulliken spin density distribution and
spin density plots (blue: positive; green: negative) of Co(tpy)(THF)2
quartet state (middle) and Fe(tpy)(THF)2 quintet state (right).
Experimental
Fitting
Experimental
Fitting
20
40
60
80
100
120
140
160
20
40
60
80
T(K)
100
120
140
160
T(K)
3300
3400
3500
3600
3700
3300
3400
3500
3600
3700
Filed(G)
Field(G)
We also investigated the activation of CoCl2•tpy molecular
complex with NaEt3BH. Upon treating CoCl2•tpy in THF with 10
equiv of NaEt3BH, the mixture turned dark green immediately with
concomitant formation of Co nanoparticles as black precipitate
(Figures S7 and S9, SI). The solution was filtered through Celite and
evaporated to afford Co(tpy)2 (HR-MS calculated for C30H22N6Co
[M+]: 525.1238, found: 525.1257).
Figure 6. X-band EPR spectra of Co(THF)2•TPY-MOL (left) and
Fe(THF)2•TPY-MOL (right) suspended in toluene at r.t. and 20K.
Microwave frequency: 9.629 GHz for Co(THF)2•TPY-MOL at r.t.;
9.629 GHz for Co(THF)2•TPY-MOL at 20K; 9.634 GHz for
Fe(THF)2•TPY-MOL at r.t.; 9.630 GHz for Fe(THF)2•TPY-MOL at 20K.
Insets are temperature-dependent EPR intensity plots and their fits
to the Bleaney and Bowers equation. The fitting results gave a
singlet to triplet (TPY••)2- energy gap of 0.04 and 0.10 kcal/mol for
Co(THF)2•TPY-MOL (left) and Fe(THF)2•TPY-MOL, respectively.
Mechanistic Studies of Co(THF)2•TPY-MOL Catalyzed C-H
Borylation
To gain insight into the mechanism of the C-H borylation reaction,
we carried out several experiments. First, we performed kinetic
isotope effect (KIE) studies in order to afford information on the
rate-determining step of the C-H borylation reactions. The initial
rates of C-H borylations with p-xylene and p-xylene-d8 were
determined by running parallel reactions in separate vessels, and
the comparison of the initial rates gave a KIE value of 2.7 (Scheme
S3, SI). Such a primary KIE indicates the involvement of the C-H
bond breaking in the rate-determining step.
Density functional theory (DFT) calculations and natural
population analyses with the B3LYP/6-311G(d) basis set on
Co(THF)2•tpy gave a doublet ground state (GS) with high positive
charge distribution (1.24) on the Co center and negative charge
distribution (-1.34) on tpy (Table S9, SI). A comparison charge
distribution on CoCl2•tpy revealed that the Co center in
Co(THF)2•tpy maintains +2 oxidation state.
A Mulliken spin
population analysis and spin density plot revealed that 0.996
unpaired electron resides on the Co center, affording a ground state
with a low-spin CoII, d7 doublet (SCo = 1/2) and a tpy diradical
dianion singlet (Stpy = 0) (Figure S47, SI). The singlet tpy diradical
Second, we detected the presence of HBpin by gas
chromatography-mass spectrometry (GC-MS) at the end of the C-H
borylation reactions. Third, we determined the resting state of the
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catalyst by EXAFS studies. By treating Co(THF)2•TPY-MOL with 20 FeBr2•TPY-MOL was treated with 10 equiv of NaEt3BH,
equiv of B2pin2, we obtained the Co(Bpin)2•TPY-MOL product in Fe(THF)2•TPY-MOL was generated along with 1 equiv. of H2. This 2-
which Co coordinates to three N atoms of TPY and two Bpin groups electron reduction process was also confirmed by titration of
according to EXAFS fitting (Figure S13, SI). To determine the resting Fe(THF)2•TPY-MOL with ferrcenium hexafluorophosphate which
state of the catalyst, the C-H borylation reaction was stopped at resulted in the generation of two equiv. of ferrocene. EXAFS fitting
70% conversion and the organic volatiles were evaporated. EXAFS indicates Fe coordinates to three N from TPY and two THF
studies indicated that the remaining residue had the same structure molecules for Fe(THF)2•TPY-MOL (Figure 3f) while infrared
as Co(Bpin)2•TPY-MOL (Figure S14, SI). Finally, EPR spectra of spectroscopy indicates no coordination of dinitrogen to Fe centers.
Co(Bpin)2•TPY-MOL did not show any signals corresponding to a The oxidation state of Fe(THF)2•TPY MOL was determined to be +2
TPY-based radical EPR signal (Figure S16, SI), suggesting a typical by XANES analysis since the pre-edge position for Fe(THF)2•TPY-
CoII•TPY complex with negative charge localized on the Bpin MOL (7111.6 eV) aligned well with FeBr2(THF)2 (7111.5 eV),
ligands.
FeBr2•TPY-MOL (7111.5 eV) and two reported five-coordinate
species (iPrPDI)FeCl2 (7111.8eV) and (iPrPDI)Fe(N2)2 (7111.9 eV).68
Interestingly, a second feature at 7113.2 eV was observed for
Fe(THF)2•TPY-MOL, assignable to the 1s to ligand π* transitions.
This feature was also seen in a reported (iPrPDI2-)FeII(N2)2 species
(7114.0 eV). It is worth mentioning that [Fe(tpy)2]n+ (n = 0, 1, 2)
were all determined to have FeII centers.69 Furthermore, XPS
spectroscopy clearly shows FeII oxidation state for Fe(THF)2•TPY-
MOL based on characteristic Fe 2P3/2 binding energy of 709.2 eV
and shake-up peaks (Figure 4). The electronic spectrum of
Fe(THF)2•TPY-MOL is very similar to that of CoII(THF)2•(TPY••)2--
MOL, indicating the presence of (TPY••)2- diradical dianion on
Fe(THF)2•TPY-MOL (Figure 5). Fe(THF)2•TPY-MOL gave an EPR signal
with giso = 2.003 at r. t. in a toluene suspension. The same MOL
sample frozen at 20 K exhibited a stronger signal with giso = 2.003
(Figure 4). The fitting of temperature-dependent EPR signals
indicates that the (TPY••)2- diradical has a singlet ground state with
singlet-to-triplet energy gap of 0.10 kcal/mol. The observed EPR
signal is thus attributed to the thermally populated TPY triplet
excited (Figure 6).67 Therefore, the EPR data provide strong
evidence of our proposed electronic structure of the
FeII(THF)2•(TPY••)2--MOL catalyst.
Scheme 1. Proposed Mechanism for the Co(THF)2•TPY-MOL
catalyzed C-H borylation of arenes with B2pin2.
DFT calculations and natural population analyses with the
B3LYP/6-311G(d) basis set on Fe(THF)2•tpy gave a triplet GS with
high positive charge distribution (1.29) on the Fe center and
negative charge distribution (-1.39) on tpy (Table S10, SI). Spin
density plot of the GS revealed that 2.013 unpaired electrons reside
On the basis of these experimental and calculation results, we
propose
a catalytic cycle for the C-H active borylation of
methylarenes as shown in Scheme 1. The CoCl2•TPY-MOL (I) is
activated by NaEt3BH in THF to give the CoH2•TPY-MOL (II)
intermediate, which quickly undergoes reductive elimination of H2
to produce the CoII(THF)2•(TPY••)2--MOL (III) catalyst. Oxidative
addition of B2(pin)2 to III results in Co(Bpin)2•TPY-MOL (IV), which is
the catalyst resting state for the C-H borylation reactions. σ-Bond
metathesis between IV and methylarene proceeds as a rate-
determining step to form Co(H)(Bpin)•TPY (V) and the benzylic
borylated product. The reaction of V with B2pin2 regenerates the
intermediate IV and forms HBpin as a byproduct via σ-Bond
metathesis. The transformation of V to IV could alternatively
involve a two-step process of reductive elimination of HBpin from V
followed by oxidative addition of B2Pin2 to the intermediate to form
IV. We are not able to differentiate between the concerted on-step
on the Fe center, affording an intermediate-spin FeII, d6 center (SFe
=
1), and a tpy singlet diradical dianion antiferromagnetically coupled
to each other (Stpy = 0) (Figure S51, SI). The GS of Fe(THF)2•tpy again
is not expected to give any organic radical EPR signal, which
contradicts our experimental results. We believe that the
experimental tpy EPR signal comes from thermal population of the
quintet state of Fe(THF)2•tpy which is only 5.26 kcal/mol higher in
energy than that of triplet GS, consistent to our EPR analysis The
charge distribution of the quintet state is similar to that of triplet GS
with positive charge (1.34) on the Fe center and negative charge (-
1.44) on tpy (Table S10, SI). A Mulliken spin population analysis and
spin density plot revealed that 2.094 unpaired spins reside on the
Fe center and 1.887 unpaired spins on tpy, affording an
intermediate-spin FeII, d6 compound (SFe = 1), and a tpy triplet
diradical dianion (STPY = 1) (Figure 7), which is consistent with our
experimental EPR results. The coordination of Hf6 SBUs to TPY is
expected to further stabilize TPY diradical dianion and lower energy
difference between triplet and quintet states of FeII(THF)2•(TPY••)2--
MOL.
σ-bond
metathesis
and
the
two-step
reductive
elimination/oxidative addition process.
Fe•TPY-MOL Catalyzed Intramolecular sp3 C-H Amination
TPY-MOL was also metalated with FeBr2(THF)2 to generate
FeBr2•TPY-MOL. Similar to the Co(THF)2•TPY-MOL case, when
6
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Upon activation with NaEt3BH, 2 mol% of FeBr2•TPY-MOL
catalyzed intramolecular Csp3-H amination of 1-azido-4-
phenylbutane (1a) in the presence of two equivalents of di-tert-
butyl dicarbonate (Boc2O) at 90 °C to form Boc-protected α-phenyl
pyrrolidine (2a) in 89% yield. This level of activity is 9 times as high
as that of the MOF control (Table 3, entry 4). Under identical
conditions, the homogeneous tpy-Fe catalyst only afforded the
product in 3% yield, probably due to the deactivation of tpy-Fe
catalyst via bimolecular pathways (Table 3, entry 5). Indeed,
treatment of FeBr2•tpy with 10 equiv. of NaEt3BH produced a
mixture Fe(tpy)2 and Fe nanoparticles; such a disproportionation
Figure 8. Substrate scope for
a
α
synthesis. Reaction conditions: Fe (2 mol%), Boc2O (2 equiv.);
-substituted pyrrolidine
c
d
bisolated yields. Fe (2 mol%), Boc2O (5 equiv.); Fe (5 mol%),
Boc2O (2 equiv.).
reaction was previously observed for
complexes.69,70
a series of (PDI)FeBr2
Table 3. Iron catalyzed intramolecular C-H amination
entry
Catalyst
Yield (%)a
TON
44.5
76
1
2b
3c
4
FeBr2•TPY MOL
FeBr2•TPY MOL
FeBr2•TPY MOL
FeBr2•TPY MOF
89
76
16
10
3
8
5
Figure 9. Substrate scope for α-substituted piperidine synthesis.
5
“Homogeneous”
Fe(tpy)Br2
1.5
Piperidines can also be formed via C-H amination with the
Fe-TPY-MOL catalyst (Figure 9). For example, 7-azidohept-1-
ene was converted to the exclusively six-member ring product
1-Boc-2-vinylpiperidine in 34% yield. By comparison, Betley’s
Fe-dipyrrinato homogenous catalyst required a stoichiometric
equivalent of catalyst to obtain 45% yield. Furthermore, the 1-
Boc-2,2-dimethylpiperidine and 1-Boc-2-phenylpiperidine
could also be formed from alkyl azides. In these examples, the
pyrrolidine products were also observed.
a
b
NMR Yield with MeNO2 as an internal standard; 1 mol% Fe
loading; c without addition of NaEt3BH.
A higher TON of 76 was achieved when the Fe loading was
decreased to 1 mol% (Table 3, entry 2). With a much simpler ligand,
Fe-TPY-MOL outperformed Betley’s Fe-dipyrrinato homogenous
catalyst by 13 times71 and our recently reported NacNac-MOF
catalysts by 4 times28 in TONs. It is worth noting that FeBr2•TPY-
MOL, without activation with NaEt3BH, showed low activity (Table
3, entry 3), suggesting that the formation of Fe-nitrene compound
might be a key elementary step of the intramolecular Csp3-H
PXRD pattern of Fe•TPY-MOL catalysts recovered from
Csp3-H amination reactions suggested that the integrity of the
MOL maintained under reaction conditions. ICP-MS of the
supernatant showed < 0.1% of Fe and < 0.1% of Hf had leached
into the supernatant. Furthermore, The Fe•TPY MOL catalyst
could be recovered and reused four times (Scheme S4, SI).
amination reaction.71-77
3
We further explored the substrate scope of intramolecular Csp
-
H amination reactions (Figure 8). At 2 mol% catalyst loading and in
the presence of 2 equiv. of Boc2O, the 2, 2-dimethylpyrrolidine (2b)
was formed in 57% yield. Due to reactivity of the vinyl substituent in
2c, 5 eq of Boc2O was required to give modest yield at 2 mol% Fe.
Since the MOL catalysts are free from diffusion constraints,
substrates with a bulky substituent such as 3, 5-diphenylphenyl was
also tolerated and gave 75% yield at 5 mol% Fe and 2 eq of Boc2O.
Conclusions
We have synthesized
a terpyridine-based TPY-MOL and
metalated TPY-MOL with CoCl2 and FeBr2 to generate M•TPY-
MOL catalysts for benzylic C-H borylation and Csp3-H amination
reactions. Interestingly, M•TPY-MOL catalysts showed
significantly higher activity and different chemo-selectivity
than homogeneous and MOF controls. Spectroscopic studies
Boc
R1
Fe(THF)2•TPY-MOL (2-5 mol% Fe)
R1
N
Boc2O (2-5 eq)
N3
R2
and DFT calculations indicated the formation of
2-
R2
1a-1d
90 oC, 2 d, C6H6
unprecedented MOL-stabilized MII-(TPY••
)
species featuring
2a-2d
Boc
divalent metals and TPY diradical dianions. We believe that the
2-
Ph
formation of novel MII-(TPY••
) (M = Co or Fe) species endows
Boc
Boc
Boc
N
them with unique and enhanced catalytic activities in Csp3-H
borylation and intramolecular amination reactions. Our work
demonstrates the ability to engineer MOLs as single-site solid
Me
Me
2a: 89%a (84%)b 2b: 57%a
N
N
N
Ph
Ph
2d: 75%d (69%)b
2c: 44%c
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20 Wanderley, M. M.; Wang, C.; Wu, C.-D.; Lin, W. J. Am. Chem.
Soc. 2012, 134, 9050.
catalysts without diffusional constraints and to elucidate
intricate electronic structures of MOL-stabilized metal
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complexes
.
Conflicts of interest
There are no conflicts to declare.
25 He, C. B.; Lu, K. D.; Liu, D. M.; Lin, W. B. J. Am. Chem. Soc. 2014,
136, 5181.
26 Zhang, T.; Manna, K.; Lin, W. J. Am. Chem. Soc. 2016, 138, 3241.
27 Manna, K.; Ji, P.; Lin, Z.; Greene, F. X.; Urban, A.; Thacker, N. C.;
Lin, W. Nat. Commun. 2016, 7, 12610.
28 Thacker, N. C.; Lin, Z.; Zhang, T.; Gilhula, J. C.; Abney, C. W.; Lin,
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29 Sawano, T.; Lin, Z.; Boures, D.; An, B.; Wang, C.; Lin, W. J. Am.
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30 Xiao, D. J.; Bloch, E. D.; Mason, J. A.; Queen, W. L.; Hudson, M.
Acknowledgements
We thank NSF (CHE-1464941) and the National Natural Science
Foundation of the P.R.China (21671162, 21471126) for
financial support and K. Ni, W. Han, M. Piechowicz, Dr. A. S.
Filatov and Prof. J. Anderson for experimental help. XAFS data
were collected at the APS at ANL on Beamline 10BM-A, B, R.; Planas, N.; Borycz, J.; Dzubak, A. L.; Verma, P.; Lee, K.; Bonino, F.;
Crocella, V.; Yano, J.; Bordiga, S.; Truhlar, D. G.; Gagliardi, L.; Brown,
C. M.; Long, J. R. Nat. Chem. 2014, 6, 590.
31 Mondloch, J. E.; Katz, M. J.; Isley Iii, W. C.; Ghosh, P.; Liao, P.;
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32 Comito, R. J.; Fritzsching, K. J.; Sundell, B. J.; Schmidt-Rohr, K.;
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supported by the Materials Research Collaborative Access
Team (MRCAT). MRCAT operations are supported by the
Department of Energy and the MRCAT member institutions.
This research used resources of the Advanced Photon Source,
a U.S. Department of Energy (DOE) Office of Science User
Facility operated for the DOE Office of Science by Argonne
National Laboratory under Contract No. DE-AC02-06CH11357.
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