Comparison of the Reactivity of the Low Buried-Volume Carbene Complexes (ITMe)2Pd(PhC≡CPh) and (ITMe)2Pd(PhN=NPh)

Article abstract of DOI:10.1021/acs.organomet.8b00112

The novel Pd(0)-azobenzene complex (ITMe)₂Pd(PhN=NPh) (5) (ITMe = 1,3,4,5-tetramethylimidazol-2-ylidene) has been isolated and characterized in the solid state and by cyclic voltammetry. Its reactivity toward E-E′ bonds (E, E′ = Si, B, Ge) has

Full text of DOI:10.1021/acs.organomet.8b00112

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Comparison of the Reactivity of the Low Buried-Volume Carbene
Complexes (ITMe)2Pd(PhCCPh) and (ITMe)2Pd(PhNNPh)
Melvyn B. Ansell,† Samantha K. Furfari,† F. Geoffrey N. Cloke,† S. Mark Roe,† John Spencer,*,†
and Oscar Navarro*,‡
†Department of Chemistry, University of Sussex, Falmer, Brighton, BN19QJ, U.K.
‡Biomaterials and Biomechanics, School of Dentistry, Oregon Health & Science University, 2730 SW Moody Avenue, Portland,
Oregon 97239, United States
S
* Supporting Information
ABSTRACT: The novel Pd(0)-azobenzene complex
(ITMe)2Pd(PhNNPh) (5) (ITMe = 1,3,4,5-tetramethyli-
midazol-2-ylidene) has been isolated and characterized in the
solid state and by cyclic voltammetry. Its reactivity toward E−
E′ bonds (E, E′ = Si, B, Ge) has been compared with that of
the known carbene complex (ITMe)2Pd(PhCCPh) (2).
Whereas 2 reacts with all E−E′ bonds studied, 5 only reacted
with B−B and B−Si moieties, echoing our previous catalytic
studies on azobenzenes.
INTRODUCTION
RESULTS AND DISCUSSION
■
■
(NHC)-containing palladium complexes (NHC = N-hetero-
cyclic carbene) are very well-documented in the scientific
literature. Dominated by bulky ligands, these complexes display
tremendous versatility in high turnover number (TON)
catalytic reactions.1−4 Recent reports from our groups have
focused on the much smaller carbene ITMe (ITMe = 1,3,4,5-
tetramethylimidazol-2-ylidene), and its low percentage buried
volume Pd(0) complex Pd(ITMe)2 (1), as well as the isolable
alkyne-bearing complex (ITMe)2Pd(PhCCPh) (2).5−7 Both
can easily cleave nonactivated E−E′ bonds (E, E′= Si, B),
resulting in Pd(II) complexes that can undergo an atom-
economical, Z-selective addition to internal alkynes affording
disilylated or silaborated alkenes. These studies have been
underpinned by X-ray structural determinations of potential
intermediates and mechanistic studies. Complex 2 and its
oxidative addition products 3 and 4 (Scheme 1) are involved in
stoichiometric reactions of Si−Si, Si−B, and B−B bonds.
Catalytic reactions under mild conditions using very low
loadings of 1 or 2 include silaborations, disilylations, and
diborations of alkynes and silaborations and diborations of
azobenzenes.8
We decided to exploit the lability of the alkyne ligand exchange
in 2. The new complex (ITMe)2Pd(PhNNPh), 5, was easily
synthesized from 2 in the presence of azobenzene and at room
temperature (Scheme 2, a). Alternatively, complex 5 could also
be prepared directly from the Pd(II)-allyl precursor 6 in a two-
step, one-pot procedure in a slightly higher yield (Scheme 2, b).
1
The H NMR spectrum of 5 in C6D6 shows clear upfield
shifts in the resonances for the N-(1,3)-methyl protons, from
3.49 ppm in 2 vs 3.31 ppm in 5, and for the C-(4,5)-methyl
protons, from 1.54 ppm in 2 vs 1.38 ppm in 5 (see the
Supporting Information). In the same way, the 13C{1H} NMR
resonance for the carbenic carbon in 5 is significantly shifted
upfield (189.0 ppm) in comparison to 2 (198.7 ppm). These
data suggest that the coordination of PhNNPh versus
diphenylacetylene results in a much greater electron density at
the palladium center corresponding to a higher degree of back-
bonding. Complex 5 represents, to the best of our knowledge,
the first NHC-Pd coordination complex of azobenzene.
Single crystal X-ray crystallography of complex 5 revealed a
Y-shaped structure with a clear elongation of the NN bond
[1.412(6) Å] when compared to free PhNNPh [1.25 Å]
(Figure 1). The lengthening of the NN bond is consistent
with single bond character between the two nitrogen atoms.
This feature is commonly observed among other transition
metal-azobenzene complexes in a zero-oxidation state, includ-
ing platinum [1.430(13) Å],10 nickel [1.385(5) Å],11 and iron
[1.398(2) Å] analogues.12 The Pd−Ccarbenic bond lengths in 5
These encouraging findings have led us to further explore the
scope and reactivity of 2. First, we wished to explore the
synthesis of NHC-containing azobenzene complexes related to
2 to shed some light on the aforementioned catalytic additions
of E−E′ bonds to azobenzenes. Next, given our success in
achieving low catalyst loadings and mild conditions in a number
of E−E′ additions to alkynes, we wished to investigate the use
of complexes such as 2 in the activation of Ge−Ge bonds.9
Received: February 23, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00112
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Scheme 1. Additions of E−E′ Bonds to Alkynes Mediated by
2
Figure 1. Solid state structure of 5. Hydrogen atoms are omitted for
clarity. Selected bond lengths [Å] and angles [deg]: N2−N5:
1.412(5), Pd1−N2: 2.066(4), Pd1−N5: 2.093(4), Pd1−CA:
2.075(5), Pd1−CD: 2.057(5), N2−Pd1−N5: 39.69(17), CA−Pd1−
N5: 113.94(18), CD−Pd1−N2: 110.4(2).
presumably in part associated with the NHCs. In the case of
5, we noted an apparently reversible feature (E1/2 −2.02 V),
assignable to an essentially pure ligand-based process associated
with azobenzene. This may reflect dissociation in THF
solution, a process comparable to those observed for a range
of free azobenzene derivatives (−1.7 to −1.98 V).19
Having recently reported the catalytic diboration and
silaboration of azobenzenes using 2, complex 5 was an ideal
candidate on which to carry out stoichiometric reactions.
Hence, the oxidative addition of silaboranes and disilanes to 5
afforded, as expected, the known hydrazine analogues 7 and 8,
respectively (Scheme 3).
Scheme 2. Two Routes for the Synthesis of Azobenzene
Complex 5
To study further the capabilities of these (NHC)2-Pd(0)
complexes, we decided to explore analogous reactions with a
digermane substrate. The palladium-catalyzed addition of Ge−
Ge bonds to alkynes often requires relatively high palladium
loadings, auxiliary ligands, or the use of activated or strained
germanium species.20 Platinum catalyzed processes are also
documented.21,22 To our delight, the oxidative addition of
hexamethyldigermane to in situ prepared 2, from complex 6, at
room temperature, led to the novel Pd(II) complex 9, although
we were unable to easily extract the digermylated alkene 10
(Scheme 4). However, a 1H NMR experiment showed
quantitative conversion of 2 to 10 after 3 days. Attempts to
replicate this Ge−Ge reactivity on the azobenzene complex 5
invariably failed, even at higher temperature (100 °C).
Single crystals of 9 suitable for X-ray analysis were isolated
from a saturated toluene solution at −30 °C. However, it was
not possible to refine the data for lengthy discussions on bond
lengths and angles (R1 = 8.8% and wR2 = 27.7%) apart from the
molecular structure, displaying connectivity only and showing a
cis-configuration (see the Supporting Information). Similarly,
the solid-state structure of 10 was determined (see the
Supporting Information).
are also longer [2.057(5) and 2.075(5) Å] than those in 2
[2.029(3) and 2.033(3) Å].
To further support our assignment of a zero-oxidation state
to palladium, both 2 and 5 were subjected to cyclic
voltammetry experiments. In both cases, an irreversible
oxidation is observed (−0.660 V for 2; −0.860 V for 5) in a
region consistent with previous reports for the oxidation of
Pd(0).13,14 In contrast, there is no evidence for the reverse
process nor, indeed, reduction of Pd(II), which has been
typically observed in the region −1.5 to −2.0 V for related
systems.13−18 Both complexes do exhibit an irreversible
reduction at strongly cathodic potential (below −3.20 V)
which we tentatively assign to the reduction of the parent
[Pd(NHC)2L] complex (L = alkyne or azobenzene),
Encouraged by these results, the catalytic reaction of
Me3GeGeMe3 with diphenylacetylene was carried out, affording
10 in nearly quantitative yield (Scheme 5). This is an
interesting reaction because germyl groups attached to sp2
carbons can be readily transformed into iodides with retention
B
DOI: 10.1021/acs.organomet.8b00112
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Hexamethyldigermane undergoes oxidative addition to 2 to
afford Pd(II) complex 9 and adds stereoselectively to
diphenylacetylene using catalytic amounts of 2. These findings
underline again the excellent potential of the low-buried
volume complexes 1 and 2 in the catalytic activation of E−E′
bonds. We are actively investigating further applications of
these complexes in the synthesis of high value end products.
Scheme 3. Reactivity of Azobenzene Complex 5 with (Bpin)2
and pinB-SiMe2Ph
EXPERIMENTAL SECTION
■
The manipulation of air-sensitive compounds and their spectroscopic
measurements were undertaken using standard Schlenk line
techniques under predried Ar (using a BASF R3-11(G) catalyst and
4 Å molecular sieves), or in a MBraun glovebox under N2 (O2 < 10.0
ppm). All glassware was dried in a 160 °C oven prior to use. Celite was
predried in a 200 °C oven and then dried with a heat gun under a
dynamic vacuum prior to use. Filter cannulae equipped with microfiber
filters were dried in an oven at 160 °C prior to use. Solvents employed
in air-sensitive reactions were dried using vacuum distillation, followed
by distillation over potassium, e.g., hexane, toluene, or stored over
activated 4 Å molecular sieves under an Ar atmosphere (e.g., 2-methyl
tetrahydrofuran). Dried solvents were degassed and stored over Ar in
ampules containing activated molecular sieves. For reactions carried
out under ambient conditions, solvents were used as purchased.
Deuterated benzene, C6D6, was degassed and dried by refluxing over
potassium for 3 days, vacuum transferred into ampules, and stored
under N2. Deuterated chloroform, CDCl3, was used as purchased.
NMR spectra were recorded on a Varian VNMRS 400 (1H 399.5
MHz; 13C{1H}n 100.5 MHz; 11B{1H} 128.2 MHz; 19F 375.9 MHz;
29Si{1H} 79.4 MHz) or 500 (1H 499.9 MHz; 13C{1H} 125.7 MHz).
Chemical shifts are reported in ppm. The spectra were referenced to
the corresponding protic solvent (1H) or signals of the solvent (13C).
11B{1H}, 19F, and 29Si{1H} NMR spectra were referenced externally
relative to BF3·OEt2, CFCl3 (10%), and SiMe4 respectively.
Scheme 4. Reactivity of Complex 2 with (GeMe3)2
Single crystal X-ray diffraction data were collected at the University
of Sussex on an Agilent Technologies Xcalibur Gemini Ultra
diffractometer (λCu(Kα) = 1.54184 or λMo(Kα) = 0.71073)
equipped with an Eos CCD area detector. Data were collected at
173 K using an Oxford Cryosystems Cobra low temperature device.
Data were processed using CrysAlisPro, and the unit cell parameters
were refined against all data. Semiempirical absorption corrections
were carried out using the MULTI-SCAN program.24 The structures
were solved by using an intrinsic phasing method (SHELXT),25 and
refined F0 by full matrix least-squares refinement using −2013,26
within OLEX2.27 All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atoms were added at
calculated positions and refined using riding models with isotropic
displacement parameters based on the equivalent isotropic displace-
ment parameter (Ueq) of the parent atoms. The crystal data of 5 were
collected on a BRUKER X8-APEC II diffractometer with a CCD area
detector and multilayer mirror monochromated Mo(Kα) radiation.
The structure was solved using an intrinsic phasing method
(SHELXT),25 refined with the SHELXL program,28 and expanded
using Fourier techniques. All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were included in structure factor
calculations. All hydrogen atoms were assigned to idealized geometric
positions.
Scheme 5. Catalytic Digermylation of Diphenylacetylene
The following structures have been assigned CCDC numbers
(CCDCs 1824672, 5; 1824670, 9; 1824671, 10).
[Pd(ITMe)2(PhNNPh)] (5). Complex 2 (0.032 g, 0.06 mmol)
and PhNNPh (0.019 g, 0.10 mmol) were dissolved in toluene (5.0
mL). The resulting mixture was stirred at room temperature for 3 h. At
this stage, the volatiles were removed in vacuo and the crude material
was washed and recrystallized in a toluene/hexane solution (3:1, 2 ×
of stereochemistry, allowing for the stereoselective synthesis of
tetra-substituted alkenes.23
CONCLUSION
■
2.0 mL). The resulting yellow powder was washed with pentane (3 ×
1
We have extended the chemistry of the low-buried volume
complexes 1 and 2 to include the isolation of the novel
azobenzene complex 5, which was shown to react with E−E′
bonds (B−Si and B−B) to afford substituted hydrazines.
2.0 mL). Yield: 0.020 g (62%). H NMR (399.5 MHz, C6D6): δH
=
7.80 (m, 4H, Ph), 7.26 (m, 4H, Ph), 6.87 (pseudo-t, J = 7.2 Hz, 2H, p-
Ph), 3.31 (s, 12H, N(1,3)-CH3), 1.38 (s, 12H, C(4,5)-CH3). 13C{1H}
NMR (100.5 MHz, C6D6): δC = 189.0 (NCN), 163.0 (i-Ph), 128.9
C
DOI: 10.1021/acs.organomet.8b00112
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(Ph), 124.1 (C(4,5)-CH3), 120.5 (Ph), 117.0 (p-Ph), 35.2 (N(1,3)-
CH3), 8.7 (C(4,5)-CH3) Elem. Anal. Calcd for C26H34N6Pd: C,
58.15%; H, 6.38%; N, 15.65%. Found: C, 58.10%; H, 6.47%; N,
15.60%.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: j.spencer@sussex.ac.uk (J.S.).
*E-mail: navarrof@ohsu.edu (O.N.).
Scale-Up Synthesis of 5. Isopropanol (12.2 μL, 0.16 mmol) was
added to a stirred mixture of [(ITMe)Pd(methallyl)Cl] (6) (0.051 g,
0.16 mmol), ITMe (0.024 g, 0.19 mmol), and potassium tert-butoxide
(0.019 g, 0.17 mmol) suspended in toluene (5.0 mL). The reaction
mixture was stirred at room temperature under a N2 atmosphere for
4.5 h, at which point PhNNPh (0.036 g, 0.20 mmol) was added and
the solution was stirred for 17 h at room temperature. The resulting
reaction mixture was filtered, the filtrate’s volatiles were removed in
vacuo, and the crude material was recrystallized in a toluene/hexane
solution (2:1, 2 × 15.0 mL) at −35 °C. A yellow solid was obtained
after filtration and subsequently washed with hexane (3 × 5.0 mL).
Yield: 0.060 g (70%).
ORCID
Samantha K. Furfari: 0000-0001-6131-0529
F. Geoffrey N. Cloke: 0000-0001-7822-6593
John Spencer: 0000-0001-5231-8836
Author Contributions
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
Synthesis of cis-[Pd(ITMe)2(GeMe3)2] (9). Isopropanol (10.0 μL,
0.14 mmol) was added to a suspension of [(ITMe)Pd(methallyl)Cl]
(6) (0.046 g, 0.14 mmol), ITMe (0.019 g, 0.16 mmol), and potassium
tert-butoxide (0.017 g, 0.15 mmol) in toluene (10.0 mL). The resulting
reaction mixture was stirred at room temperature under a N2
atmosphere for 4.5 h. At this stage, (GeMe3)2 (58.0 μL, 0.29 mmol)
was added and the reaction mixture was stirred for 19 h at room
temperature. The solution was then filtered via cannula, the filtrate’s
volatiles were removed in vacuo, and the resulting off-white solid was
ACKNOWLEDGMENTS
■
EPSRC (EP/L505109/1) and the University of Sussex
(M.B.A.), and EPSRC (EP/N016785/1) (S.K.F.), are thanked
for funding. Dr. Ian Crossley (University of Sussex) is thanked
for invaluable assistance and advice. High resolution mass
spectrometry was carried out by Dr. A. Abdul-Sada (University
of Sussex) and by the EPSRC UK National Mass Spectrometry
Facility, University of Swansea. Elemental analyses were carried
out by Stephen Boyer, London Metropolitan University. M.B.A.
would like to dedicate this publication to the memory of
Maureen Rayner.
1
washed with hexane (3 × 5.0 mL). Yield: 0.050 g (59%). H NMR
(399.5 MHz, C6D6): δH = 3.33 (s, 12H, N(1,3)-CH3), 1.42 (s, 12H,
C(4,5)-CH3), 0.64 (s, 18H, GeMe3). 13C{1H} NMR (100.5 MHz,
C6D6): δC = 193.6 (NCN), 123.5 (C(4,5)-CH3), 35.0 (N(1,3)-CH3),
8.6 (C(4,5)-CH3), 7.6 (GeMe3). Elem. Anal. Calcd for C20H42-
N4Ge2Pd: C, 40.70%; H, 7.17%; N, 9.49%. Found: C, 41.13%; H,
7.44%; N, 9.46%
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1
reveal an off-white solid. Yield 0.057 g (98%). H NMR (399.5 MHz,
CDCl3): δH = 6.99 (m, 4H, Ph), 6.87 (m, 2H, p-Ph), 6.66 (m, 4H,
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00112.
1
Scanned H and 13C NMR spectra for all compounds
synthesized herein (PDF)
Accession Codes
CCDC 1824670−1824672 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
D
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Organometallics XXXX, XXX, XXX−XXX