Germanium(iii) corrole complex: Reactivity and mechanistic studies of visible-light promoted N-H bond activations

Article abstract of DOI:10.1039/c3sc52326h

The [(TPFC)Ge(TEMPO)] (1, TPFC = tris(pentafluorophenyl)corrole, TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxyl) complex was characterized by X-ray diffraction and spectroscopic studies. EPR studies indicate that the weak Ge-O bond in 1 is photo-cleaved to form a tetra-coordinated germanium(iii) radical, [(TPFC)Ge(iii)]. DFT calculations show that the spin density on the germanium center in [(TPFC)Ge(iii)] has a significant s character. Under visible-light irradiation, 1 reacts rapidly with ammonia, primary/secondary aliphatic amines and aniline to produce (TPFC)Ge-NR¹R² (R¹R ² = HH, HⁿPr, HⁱPr, H^tBu, HPh, Et₂, ⁱPr₂) complexes in high yields (65-95%). The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3sc52326h

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EDGE ARTICLE
Germanium(III) corrole complex: reactivity and mechanistic studies of
visible-light promoted N–H bond activations
Huayi Fang,a Zhen Ling,a Ke Lang,a Penelope J. Brothers,b Bas de Bruinc,* and Xuefeng Fua,*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The [(TPFC)Ge(TEMPO)] (1, TPFC = tris(pentafluorophenyl)corrole, TEMPO = (2,2,6,6ꢀ
tetramethylpiperidinꢀ1ꢀyl)oxyl) complex was characterized by Xꢀray diffraction and spectroscopic studꢀ
ies. EPR studies indicated that the weak Ge–O bond in 1 was photoꢀcleaved to form a tetraꢀcoordinated
germanium (III) radical, [(TPFC)Ge(III)]•. The DFT calculation showed that the spin density on the gerꢀ
10 manium center in [(TPFC)Ge(III)]• had significant s character. Under visibleꢀlight irradiation, 1 reacted
rapidly with ammonia, primary/secondary aliphatic amines and aniline to produce (TPFC)Ge–NR1R2
(R1R2 = HnPr, HiPr, HtBu, HPh, Et2, iPr2) complexes in high yields (65% ꢀ 95%).
been prepared by other routes, photoꢀpromoted N–H bond activaꢀ
Introduction
tion by main group element radical species has only been rarely
50 encountered, yet offers interesting prospects to activate ammonia
and amines. From this perspective, we decided to investigate the
activation of N–H bonds with germanium radicals. In this paper
we report our first results in this new research area, in which we
demonstrate the photolysis of [(TPFC)Ge(TEMPO)] 1 (TPFC =
Metal catalyzed coupling of ammonia with arenes and the hyꢀ
15 droamination of olefins with ammonia have been listed among
the top ten challenges for catalysis.1 Related studies on N–H bond
activation can be considered as the foundation stone for both
developing new catalytic amine transformations for the industrial
production of bulk chemicals and for understanding biochemical
20 processes in living cells.2 In recent years, N–H bond activation by
transition metal complexes is receiving increasing attention3 in
which the formation of metal amides is the key step. Therefore,
detailed insights into this elementary step will shed new light on
the development of new catalytic processes.
55 tris(pentafluorophenyl)ꢀcorrole,
tetramethylpiperidinꢀ1ꢀyl)oxyl) to generate a tetraꢀcoordinated
germanium (III) complex, [(TPFC)Ge(III)]• (TPFC
TEMPO
=
(2,2,6,6ꢀ
=
tris(pentafluorophenyl)corrole. Under visibleꢀlight irradiation, 1
reacted rapidly with N–H bonds in ammonia, primary/secondary
60 aliphatic amines to form germanium amides in high yields at
room temperature. These reactions proceed via discrete radical
pathways with lower activation barriers than those normally
observed for N–H bond activation reactions proceeding via twoꢀ
electron pathways in either mainꢀgroup or transition metal chemꢀ
65 istry.
25
In order for the N–H bonds of ammonia and amines to be
activated by transition metals, the logical first step is the coordiꢀ
nation of the lone electron pair of these substrates to the vacant
coordination site of the transition metals. This leads to facile
formation of Wernerꢀtype complexes, but subsequently hampers
30 N–H bond activation. On the other hand, radical reaction pathꢀ
ways offer a promising new tool for N–H bond activation and
subsequent functionalization while this possibility has been bareꢀ
ly explored.3e Finally, the use of main group elements rather than
transition metals seems a viable approach towards N–H bond
35 functionalization reactions as the main group elements with filled
dꢀorbitals have a much weaker affinity for the lone pairs of
amines, thus preventing the formation of Wernerꢀtype complexꢀ
es.4
Results and discussion
Complex [(TPFC)Ge(TEMPO)] (1) was prepared by reacting
(TPFC)Ge–H11 with two equivalents of TEMPO (Scheme 1).
Recently, the activation of N–H bonds using lowꢀvalent diaꢀ
40 rylstannylene,5 diarylgermylene,5a,6 Nꢀheterocyclic silylene7 and
germylene8 species has been reported. Silanone9 and a nickelꢀ
coordinated Nꢀheterocyclic silylene10 species are also capable of
activating N–H bonds. The activation of N–H bonds by silicon
and its lowꢀvalent heavier congeners typically proceeds via twoꢀ
45 electron pathways including direct oxidative addition of the N–H
bond to the lowꢀvalent metal center and metalꢀligand cooperative
processes. Although main group element amide complexes have
70
Scheme 1. Synthesis of [(TPFC)Ge(TEMPO)] (1)
This leads to hydrogen atom transfer (HAT) from the germaniꢀ
um hydride to the TEMPO radical (Scheme 1, step a), a reaction
similar to the previously reported HAT from Ph3GeH to TEMꢀ
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PO.12 The resulting germanium radical [(TPFC)Ge]• underwent
radical coupling with another molecule of TEMPO radical to
in the EPR spectrum (Figure 3) to that observed in Figure 2. The
EPR spectra analysis thus revealed that the paramagnetic
[(TPFC)Ge]• radical has a gꢀvalue of 2.0028. Therefore, the weak
55 broad singlet appearing in between the second and third peak of
the TEMPO radical signal (Figure 2) was ascribed to the signal of
[(TPFC)Ge]• radical.
form
1 (Scheme 1, step b). The formation of 2,2,6,6ꢀ
tetramethylpiperidinꢀNꢀol (TEMPOH) as a sideꢀproduct of this
reaction was observed by ESIꢀMS and NMR spectroscopy.
5
The molecular structure of 1, as determined by Xꢀray diffracꢀ
tion, showed that its 6ꢀmembered piperidinyl ring is locked in a
chair form (Figure 1a). The Ge(1)–O(1) bond length (1.786(3) Å)
is nearly identical to those in (TPFC)Ge–OH (1.785(4) Å) (see
a)
10 ESI, Figure S4) and (TPFC)Ge–OCH2CH3 (1.789(8) Å),11 and is
a little longer than the Ge–O bond in [(TPC)Ge]2O (in the range
1.718(11) to 1.773(13) Å).13 However, the Ge(IV)–O bond in 1 is
slightly shorter than the lowꢀvalent Ge(II)–O bond (1.804(2) Å)
in ArGe(TEMPO) (Ar = C6H3ꢀ2,6ꢀ(C6H3ꢀ2,6ꢀiPr2)2)14 and the
15 Ge(IV)–O bond (averaging 1.825 Å) in a dialkylgermylene bisꢀ
TEMPO adduct R2Ge(TEMPO)2.15 The germanium center proꢀ
trudes slightly (0.50 Å) from the N4ꢀplane of the corrole ligand,
comparable with (TPFC)Ge–OH (0.48 Å) and (TPFC)Ge–
OCH2CH3 (0.49 Å).11 As a direct result of the presence of the
20 bulky axial TEMPO ligand, the coordination geometry around the
germanium center in 1 is distorted away from an idealized square
pyramid, leading to a significant decrease in the angle of the Ge–
O bond relative to the N4ꢀplane (Figure 1b). The N–O bond
length (1.477(4) Å) in 1 is nearly identical to those in
25 ArGe(TEMPO) (1.476(2) Å)14 and the dialkylgermylene bisꢀ
TEMPO adduct R2Ge(TEMPO)2 (1.482(av) Å),15 consistent with
the presence of a classical N–O single bond, and is significantly
longer than the N–O bonds of stable nitroxide radicals (typically
1.23~1.30 Å and 1.296(3) Å for the TEMPO radical).16
b)
a)
b)
Figure 2. a) EPR monitoring of the hydrogen transfer reaction with
60 TEMPO•/(TPFC)Ge–H ratio of 1/3000; weak signal assigned to
[(TPFC)Ge]• expanded to the right and b) EPR spectrum of the reaction
o
solution after heating at 80 C for 70 minutes. (3 mg (TPFC)Ge–H in 0.6
mL toluene; microwave frequency: 9.047662 GHz; microwave power: 1
mW).
30
Figure 1. a) Thermal ellipsoid plot (50% probability level) of 1 (hydroꢀ
gen atoms omitted for clarity); b) Angles (O) between the Ge–O bond and
the mean N4ꢀplane in complex 1 (top), (TPFC)Ge–OCH2CH3 (middle)11
and (TPFC)Ge–OH (bottom, this work).
35
The formation of the [(TPFC)Ge]• intermediate was observed
by monitoring the reaction of (TPFC)Ge–H with TEMPO radical
by EPR spectroscopy. In order to reduce the signal intensity of
TEMPO radical to give a clear observation of the [(TPFC)Ge]•
radical by EPR spectroscopy, a minute amount of TEMPO was
40 mixed with (TPFC)Ge–H ([TEMPO]:[(TPFC)Ge–H] = 1:3000).
o
The sample was heated at 80 C and monitored every 10 or 20
minutes by room temperature EPR spectroscopy (Figure 2a). The
EPR spectrometer was equipped with a Mn marker which was
used as the reference signal. The intensity of the TEMPO radical
45 signal decayed fast and the weak broad singlet which can be
assigned to the [(TPFC)Ge]• radical kept growing and became
clearly observable (Figure 2b).
65
Figure 3. EPR spectrum of [(TPFC)Ge]• formed via the homolysis of Ge–
H in vacuum (microwave frequency: 9.047070 GHz; microwave power: 4
mW).
The [(TPFC)Ge]• radical could also be independently prepared
by homolysis of the Ge–H bond of (TPFC)Ge–H by leaving this
50 compound stirring in toluene solution under vacuum in a sealed
EPR tube for a few days. This sample showed an identical singlet
In the presence of one equivalent of TEMPOH, about 6% of
70 complex 1 (in d8ꢀtoluene, 8.8 mmol Lꢀ1) underwent thermal disꢀ
sociation to form 2,2,6,6,ꢀtetramethylpiperidine and (TPFC)Ge–
OH within 48 hours at 100 oC, observed by 1H NMR spectroscoꢀ
2
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py, resulting from cleavage of the N–O bond. This reactivity is
similar to that previously reported for TEMPO adducts of siliꢀ
con,12 iron,17 uranium18 and rhenium.19 However, in the absence
of TEMPOH no observable decomposition of complex 1 was
conditions (Figure S7). It is worth noting that this appears to be
the first experimental observation of a resolved hydrogen hyperꢀ
fine splitting pattern in an EPR spectrum of TEMPO, for which
the previously reported proton hyperfine values could apparently
1
5
observed under identical conditions, which implies that TEMꢀ 65 only be determined by H NMR methods.22 The EPR proton and
POH acts as a hydrogen source for the formation of 2,2,6,6,ꢀ
tetramethylpiperidine and (TPFC)Ge–OH. Furthermore, activaꢀ
tion of the ringꢀmethyl C–C bond of TEMPO20 was not observed
at temperatures up to 150 oC.
It is quite interesting that when complex 1 was exposed to
excess ammonia (3 atm, in d8ꢀtoluene), it slowly transformed
over a period of 12 hours at room temperature in the dark to the
germanium amide complex (TPFC)Ge–NH2 (2) and a stoichioꢀ
metric amount of TEMPOH. More prolonged reaction times
nitrogen hyperfine coupling constants determined by spectral
simulations are listed in Table S1 (ESI).
a)
10
15 resulted in the observation of a new set of corrole hydrogen resoꢀ
1
nances in the H NMR spectrum (52% yield), ascribed to the
formation of [(TPFC)Ge]2NH (3) according to mass spectrometry
([M]+ observed at m/z 1747.95, calc. 1747.97, ESI, Figure S6).
The 1H NMR spectra for 2 and 3 are similar to those observed for
20 (TPC)Ge–OH and [(TPC)Ge]2O.13 The ratio of complex 2 to
complex 3 remained constant over 9 days in d8ꢀtoluene at room
temperature, allowing an estimation of the equilibrium constant
Keq = 53.8 ± 1.5 at 298 K for the interconversion of 2 and 3.
Similar experiments at 40 oC in the dark with two aliphatic
25 amines allowed us to roughly estimate the equilibrium constants
for the reactions of 1 with nꢀpropylamine (Keq(nPr) ≈ 0.2) and iꢀ
propylamine (Keq(iPr) ≈ 0.1). Amines with bulky substituents
showed much slower reaction rates and poor yields even at eleꢀ
vated reaction temperatures (Table 1).
b)
30
Remarkably, photochemical reactions of 1 with ammonia,
aliphatic amines and aniline, using a 500 W highꢀpressure xenon
lamp equipped with a 420 ~ 780 nm filter, led to much more
rapid N–H bond activation reactions to form TEMPOH and the
amide products (TPFC)Ge–NR1R2 (R1R2 = HPr, HiPr, HtBu, HPh,
Figure 4. a) EPR spectrum of 1 in toluene solution at room temperature
70 (microwave frequency 9.044484 GHz; microwave power 1 mW) and b) at
238 K (microwave frequency 9.049704 GHz; microwave power 0.1 mW).
35 Et2, iPr2) in almost quantitative yields (Table 1).21 Steric effects
were proved to have no significant influence on the N–H bond
activation of primary aliphatic amines (Table 1, entries 2ꢀ4). N–H
bond activation of the very bulky diisopropylamine required 128
hours to reach 65% yield (Table 1, entry 7). The reaction with
40 aniline also took a longer reaction time, around 6 hours, to reach
almost quantitative yields (> 95% ), which is likely due to the
electronꢀwithdrawing property of the phenyl ring which makes
the nitrogen atom more electron positive than other amine subꢀ
strates (Table 1, entry 5).
Photolysis of 1 by prolonged exposure to visible light at room
temperature led to a clear increase in intensity of the EPR signal
at g = 2.0063 (S = ½) characteristic of the free TEMPO radical
75 (Figure 5).
45
The distorted Ge–O bond in 1 is relatively weak, and photoꢀ
induced homolytic splitting of this bond (the reverse of the proꢀ
cess shown in Scheme 1, step b) may be responsible for the obꢀ
served rapid and smooth N–H bond activation by 1 under visible
light irradiation. Moreover, the EPR spectrum of 1 recorded in
50 toluene at room temperature showed a clear hyperfineꢀsplit signal
characteristic of the TEMPO radical as well as a relatively weak,
broad singlet (Figure 4a). The weak broad singlet appearing in
between the second and third peak of the TEMPO signal (Figure
4a) was identical to the signal of [(TPFC)Ge]• radical observed in
55 Figure 2. The closedꢀshell complex 1 is diamagnetic, thus the
observed EPR signal was most likely a consequence of a small
extent of homolysis of the Ge–O bond. The isotropic solution
phase EPR spectrum of 1 at 238 K (Figure 4b) showed a hyperꢀ
fine splitting pattern which was identical to the EPR spectrum of
60 free TEMPO in dilute toluene solution recorded under similar
80
Figure 5. In-situ EPR spectrum of 1 in toluene with continuous
irradiation by visible light at room temperature (microwave freꢀ
quency: 9.044484 GHz; microwave power: 1 mW).
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Table 1. The reactions of complex 1 with amines and aniline in the dark and under visible light irradiation.
Although an increased intensity of the [(TPFC)Ge]• singlet (S
= ½, g = 2.0028) was not clearly observable (most likely a result
of either its relatively weak signal intensity or its high chemical
reactivity), evidence for its formation was provided by a lightꢀ
radical, which facilitates the photoꢀcleavage of the Ge–O bond in
(TPFC)Ge(OMeTEMPO). Therefore, the equilibrium constant for
the reaction shown in Scheme 2 deviates slightly from one.
The photochemical reaction of 1 in the presence of ethylene
5
induced axial ligand exchange reaction. Visibleꢀlight irradiation 30 led to exclusively formation of (TPFC)GeꢀCH2CH2ꢀY (Y =
of a mixture of (TPFC)Ge(TEMPO) and OMeTEMPO radical
(2,2,6,6ꢀtetramethylpiperidinꢀ1ꢀyl)oxyl, 10) in 65% yield. Ethꢀ
ylene effectively inserts into the Ge–O bond of 1, which further
confirmed the formation of the TEMPO and [(TPFC)Ge]• radicals
upon photolysis of 1. Complex 10 is formed by radical coupling
10
(
OMeTEMPO = (4ꢀmethoxyꢀ2,2,6,6ꢀtetramethylpiperidinꢀ1ꢀyl)oxyl)
in d8ꢀtoluene at room temperature led to formation of
(TPFC)Ge(OMeTEMPO) with an equilibrium constant Keq ≈ 0.5
resulting from the dissociation of (TPFC)Ge(TEMPO) and reꢀ 35 of ethylene with the [(TPFC)Ge]• and TEMPO radicals.
coupling of [(TPFC)Ge]• with the OMeTEMPO radical. In comparꢀ
Computational studies using DFT methods confirmed that
homolysis of the Ge–O bond of complex 1 is thermodynamically
feasible. The calculation revealed a rather small BDE of 35.4 kcal
molꢀ1 of the Ge–O bond of 1 in the gas phase (see ESI). Forꢀ
40 mation of [(TPFC)Ge]• and TEMPO radical is calculated to be
endergonic by ꢀGotol = 20.6 kcal molꢀ1 (ꢀGogas = 19.6 kcal molꢀ1).
In comparison, Ge−O bond heterolysis to produce cationic
[(TPFC)Ge(IV)] and the TEMPO− anion is endergonic to a much
larger amount as ꢀGotol = +103.7 kcal molꢀ1 (ꢀGogas = +151.8 kcal
45 molꢀ1). In the HOMO of 1 (Figure S9a), the conjugated πꢀsystem
of the TPFC ligand is mixed with the N–O π* orbital. The LUMO
of 1 (Figure S9b) is essentially a π*ꢀorbital of the TPFC ligand.
Therefore, Ge−O bond photoꢀcleavage most likely occurs via
its HOMOꢀLUMO electronic transition, in which the electron
50 density migrates from the π*ꢀorbital of the N–O bond of the axial
TEMPO ligand to the [(TPFC)Ge] moiety. The calculations
further showed that the tetraꢀcoordinated germanium center in
[(TPFC)Ge]• had a domed coordination geometry. 51% of the
spin density of [(TPFC)Ge]• is located on the germanium center,
15 ison, no axial ligand exchange was observed in the dark for
weeks under similar reaction conditions.
Scheme 2. Visible light promoted axial ligand exchange.
The OMeTEMPO and TEMPO radicals are structurally similar
20 and differ only in the substituent on the paraꢀposition of the sixꢀ
membered piperidinyl ring, so that the exchange process should
be nearly degenerate (ꢀG° ≈ 0) and the equilibrium constant
should approach one. However, the methoxyl group in OMeTEMꢀ
PO radical induces the configuration of Nꢀcontaining sixꢀ
25 membered ring to be closer to a chair form than that of TEMPO
4
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while the rest of the spin density is delocalized over the corrole
ligand (Figure 6).
25 Summary and Conclusions
Most previously reported N–H bond activation processes
involve mononuclear direct twoꢀelectron concerted oxidative
addition reactions of the N–H bond to a lowꢀvalent metal cenꢀ
ter,3b external base assisted N–H bond cleavage,25 or metalꢀligand
30 cooperative effects.3d N–H bond activations through radicaloid
pathways provide an alternative route with an expected lower
activation energy, but are far more rarely encountered.3e The
photochemical N–H bond activation reactions by 1 are strongly
suggestive of a radical pathway. The experimental observations
35 including the EPR measurement of 1 under visible light irradiaꢀ
tion, lightꢀpromoted axial ligand exchange, single insertion of
ethylene into the Ge–O bond to form a Ge–CH2–CH2–OTempo
moiety, as well as DFT calculations providing clear evidence for
the formation of a reactive TEMPO•/[(TPFC)Ge]• radical pair
40 which activates the N–H bond in a concerted way to form
(TPFC)Ge–NR1R2 (R1R2 = HPr, HPr, HtBu, HPh, Et2, iPr2) prodꢀ
ucts. Further reactivity studies of the tetraꢀcoordinated germaniꢀ
um radical with other small molecules were in progress. We hope
that the findings reported in this paper will stimulate a broader
45 development of the thus far underꢀinvestigated area of substrate
activation with radicalꢀchemistry and mainꢀgroup element chemꢀ
istry.
Figure 6. Spin density of the [(TPFC)Ge]• radical (isovalue=0.0004).
5
The SOMO of [(TPFC)Ge]• (Figure S10) together with the
corresponding spin density plot showed the spin density on the
germanium center has distinct s character (i.e. a sp3 hybrid orbital
of the germanium center), while the spin density on the center of
most of the previously reported stable germanium radical with a
10 formal oxidation state of +3 and a tripleꢀcoordianted metal center
was in orbitals with mainly p character.23
Solvent Cage
Acknowledgements
This work was supported by the Natural Science Foundation of
50 China (21171012) and the Netherlands Organization for Scienꢀ
tific Research (NWOꢀCW VICI project 016.122.613). We thank
Dr. Shengfa Ye (MaxꢀPlanck Institute for Chemical Energy
Conversion) for his helpful discussions.
N
N
400-800 nm
O
O
Ge
Ge
Notes and references
55 a Beijing National Laboratory for Molecular Sciences, College of Chemis-
try and Molecular Engineering, Peking University, Beijing 100871,
China. Fax: (86) 10-6275-6035; E-mail: fuxf@pku.edu.cn
b School of Chemical Sciences, University of Auckland, Private Bag
92019, Auckland 1042, New Zealand.
R1
R2
N
Solvent Cage
Ge
R1R2NH
R. T.
60 c Homogeneous & Supramolecular Catalysis, van ’t Ho
ff Institute for
Ge
N
R1
H
R2
O
N
OH
N
Molecular Sciences (HIMS), University of Amsterdam (UvA), Science
Park 904, 1098 XH Amsterdam, The Netherlands
† Electronic Supplementary Information (ESI) available: Details of UVꢀ
Vis spectrum of (TPFC)Ge(TEMPO), solid state structure of (TPFC)Ge–
65 OH, CSIꢀMS spectra and its simulation of (TPFC)Ge–NH2 and
[(TPFC)Ge]2NH, simulation of EPR spectrum of TEMPO at 238K,
kinetic study of the light promoted reaction of (TPFC)Ge(TEMPO) with
nꢀpropylamine, and other experiment and computational details. CCDC
reference numbers 945438ꢀ945439. See DOI: 10.1039/b000000x/
R1R2=HH, HnPr, HiPr, HtBu, HPh, Et2, iPr2
Scheme 3. N–H bond activation through a termolecular pathway.
Combining the aforementioned observations, N–H bond actiꢀ
15 vation by 1 most likely proceeds via a termolecular transition
state (Scheme 3) similar to that proposed for methane activation
by rhodium porphyrin complexes24 and the activation of NH3 by a
palladium pincer dimer.3e Accordingly, aniline (which has more
contracted frontier orbitals as a consequence of the electron withꢀ
20 drawing effect of the phenyl ring), and the bulkier secondary
aliphatic amines are less reactive. Furthermore, a stepwise radical
chain pathway (i.e. with radicals escaping the solvent cage) can
be excluded by considering the formation of 10 without the genꢀ
eration of (TPFC)Ge–CH2CH2–Ge(TPFC) or other byproducts.
70
75
80
1
2
J. Haggin, Chem. Eng. News, 1993, 71(22), 23.
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