Triamidoamine-supported zirconium complexes in the catalytic dehydrocoupling of 1,2-bisphosphinobenzene and -ethane

Article abstract of DOI:10.1016/j.poly.2009.05.074

The readily prepared zirconium complex, [κ⁵-(Me₃SiNCH₂CH₂)₂NCH₂CH₂NSiMe₂CH₂]Zr (1), is an effective precatalyst in the dehydrocoupling of o-bisphosphinobenzene and 1,2-bisphosphinoethane to the known intercalated products. The phosphanido complex (N₃N)Zr[κ²-1,2-PH(PH₂)C₆H₄] (N₃N=N(CH₂CH₂NSiMe₃)₃^3-), 2 was prepared independently by reaction of 1 with o-bisphosphinobenzene. Complex 2 was identified as an intermediate zirconium complex in the catalytic dehydrocoupling of o-bisphosphinobenzene. Likewise, previously reported (N₃N)Zr(PHCH₂CH₂PH₂) (3) was identified in the catalytic dehydrocoupling of 1,2-bisphosphinoethane. Investigation of the thermal decomposition of 2 and the reactivity of 2 with stoichiometric o-bisphosphinobenzene suggest that the catalysis proceeds via sequential P{single bond}P bond forming steps. The solid state structure of 2, which features a six-coordinate (N₃N)Zr-complex, is reported.

Full text of DOI:10.1016/j.poly.2009.05.074

Polyhedron 29 (2010) 42–45
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Triamidoamine-supported zirconium complexes in the catalytic dehydrocoupling
of 1,2-bisphosphinobenzene and -ethane
Michael B. Ghebreab a, Tamila Shalumova b, Joseph M. Tanski b, Rory Waterman a,
*
a Department of Chemistry, University of Vermont, Burlington, VT 05405, United States
b Vassar College, Poughkeepsie, NY 12604, United States
a r t i c l e i n f o
a b s t r a c t
The readily prepared zirconium complex, [j
5-(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH2]Zr (1), is an effective
Article history:
Available online 8 June 2009
precatalyst in the dehydrocoupling of o-bisphosphinobenzene and 1,2-bisphosphinoethane to the known
intercalated products. The phosphanido complex (N3N)Zr[
j
2-1,2-PH(PH2)C6H4] (N3N=N(CH2CH2NSiM-
This work is dedicated to Prof. William
Geiger – an innovative chemist, valued
colleague, and wonderful person – on the
occasion of his 65th birthday.
e3)33ꢀ), 2 was prepared independently by reaction of 1 with o-bisphosphinobenzene. Complex 2 was
identified as an intermediate zirconium complex in the catalytic dehydrocoupling of o-bisphosphinoben-
zene. Likewise, previously reported (N3N)Zr(PHCH2CH2PH2) (3) was identified in the catalytic dehydro-
coupling of 1,2-bisphosphinoethane. Investigation of the thermal decomposition of 2 and the reactivity
of 2 with stoichiometric o-bisphosphinobenzene suggest that the catalysis proceeds via sequential PAP
bond forming steps. The solid state structure of 2, which features a six-coordinate (N3N)Zr-complex, is
reported.
Keywords:
Zirconium
Dehydrocoupling catalysis
Phosphine
Ó 2009 Elsevier Ltd. All rights reserved.
Phosphanido complex
Sigma-bond metathesis
1. Introduction
[16,17]. Furthermore, thisdehydrocoupling catalysis bears a striking
similarity to main-group-mediated processes reported by Wright
Catalytic dehydrocoupling of phosphines has witnessed ex-
panded interest over the last several years [1], and this reaction
is assuming a more prominent role among metal-catalyzed trans-
formations that feature PAH bond activation [2–9]. However, a
limited understanding of the mechanism of dehydrocoupling and
a short list of catalysts suggest there is unrealized potential for this
transformation [1].
and co-workers [15]. On the other hand, current evidence supports
-bond metathesis reactivity at a single active site for complex 1
r
[11]. The differences between the triamidoamine-supported zirco-
nium complexes and cyclopentadienyl-supported species led to an
investigation of similar dehydrocoupling substrates, namely o-bis-
phosphinobenzene and 1,2-bisphosphinoethane. It has already been
ꢀ
shown that these complexes can be dehydrocoupled by Cp*2Zr(H)3
A readily prepared, triamidoamine-supported zirconium com-
[17,18]. Ourgoal in thisstudy was to test the capacityof complex1 to
engage in the similar reactivity while a Zr@P moiety is unlikely to be
at play and to see what mechanistic understanding could be gar-
nered about the dehydrocoupling process.
plex [
j
5-(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH2]Zr (1) [10] and re-
3ꢀ
lated (N3N)ZrX (N3N=N(CH2CH2NSiMe3)3
, X = Me, PHR, PR2)
derivatives have been shown to be effective catalysts in the dehy-
drocoupling of primary and secondary phosphines to the respec-
tive diphosphine products [11]. This family of complexes also
affects the heterodehydrocoupling of phosphines with silanes
and germanes and the dehydrocoupling of arsines [12,13].
2. Experimental
All manipulations were performed under a nitrogen atmo-
sphere with dry, oxygen-free solvents using an M. Braun glovebox
or standard Schlenk techniques. Benzene-d6 was purchased from
Cambridge Isotope Laboratory then degassed and dried over NaK
The differences between complex 1 and Cp*2Zr(H)3ꢀ, the first
phosphine dehydrocoupling catalyst discovered by Stephan and
co-workers [14], are substantial despite both being zirconium based
[15]. The latter may undergo PAH bond activation across a phosphi-
alloy. Celite-454 was heated to
a temperature greater than
2ꢀ
nidene (Zr@PR) moiety, and di- and triphosphinato (P2R2 and
180 °C under dynamic vacuum for at least 8 h. Elemental analysis
was performed on an Elementar varioMICRO cube. A Bruker AXR
500 MHz was used to collect 1D data, and a Varian 500 spectrom-
eter was used to record HMQC and HMBC spectra using standard
Varian pulse sequences with modified P-90 values and relaxation
P3R32ꢀ) ligated zirconium appear to be important intermediates
* Corresponding author. Tel./fax: +1 802 656 0278.
E-mail address: rory.waterman@uvm.edu (R. Waterman).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.05.074

M.B. Ghebreab et al. / Polyhedron 29 (2010) 42–45
43
times. Spectra were collected in benzene-d6 solution and are re-
ported with reference to residual solvent resonances (d 7.16 and
d 128.0) or external 85% H3PO4 (d 0.0). Infrared spectra were col-
lected on a Perkin–Elmer System 2000 FT-IR spectrometer at a res-
olution of 1 cmꢀ1. Mass spectra were collected on an Applied
Biosystems 4000QTrap Pro. Complex 1 was prepared according to
literature procedures [10]. Phosphanes were purchased from Strem
Chemicals and used without further purification.
crystal of complex 2 was mounted in a nylon loop with Paratone-N
cryoprotectant oil. The structure was solved using direct methods
and standard difference map techniques and refined by full-matrix
least squares procedures on F2 with SHELXTL (version 6.14) [19]. All
non-hydrogen atoms were refined anisotropically. Hydrogen
atoms on carbon were included in calculated positions and were
refined using a riding model. The hydrogen atoms on the phospho-
rus atoms of 2, H(1), H(2), and H(3), were located in the Fourier dif-
ference map and refined freely. Crystal data and refinement details
are presented in Table 1.
2.1. Preparation of (N3N)Zr[j
2-1,2-PH(PH2)C6H4] (2)
A scintillation vial was charged with 1 (100 mg, 0.22 mmol) and
3 mL of benzene. To the solution of 1, o-bisphosphinobenzene
(31 mg, 0.22 mmol) was added, and the resulting pale yellow solu-
tion was stirred at ambient temperature for 30 min then frozen
and lyophilized. The resultant powder was dissolved in Et2O, and
the solution was filtered through a bed of Celite and concentrated
until incipient crystallization. Yellow crystals of 2 formed upon
cooling to ꢀ30 °C overnight (83 mg, 0.14 mmol, 64%). 1H
(500.1 MHz): d 7.44 (t, C6H5, 1 H), 7.17 (obscured by solvent,
C6H4, ꢁ1 H), 6.88 (t, C6H4, 1 H), 6.68 (t, C6H4, 1 H), 4.67 (dd,
3. Results and discussion
3.1. Catalysis
Heating benzene-d6 solutions of o-bisphosphinobenzene with
5 mol% of complex 1 resulted in the liberation of hydrogen and for-
mation of several new products and consumption of 1. Over time, a
new phosphorus-containing product emerges, [(C6H4)PPH]2, and
forms in 97% yield based on 31P NMR spectroscopy (Eq (1)). The
product is known and was previously prepared by the catalytiꢀc
dehydrocoupling of o-bisphosphinobenzene by Cp*2Zr(H)3
[17,18]. It is known that in the dehydrocoupling of o-bisphosphino-
benzene reported by Stephan, a high molecular weight product,
[(C6H4)P2]8, is formed [18]. However, extended heating of reaction
mixtures of 1 with the phosphine substrate or of the completed cat-
alytic reaction gave no evidence for theformation of thismacrocycle.
JPH = 221.5 Hz, JPH = 31.6 Hz, PH,
1 H), 4.61 (dd, JPH = 270 Hz,
J = 5.3 Hz, PH2, 2 H), 3.20 (t, CH2, 6 H), 2.28 (t, CH2, 6 H), 0.27 (s,
CH3, 27 H). 13C{1H} (125.8 MHz): d 147.2 (m, C6H4), 143.3 (m,
C6H4), 136.3 (m, C6H4), 132.2 (m, C6H4), 130.2 (s, C6H4), 122.7 (s,
C6H4), 63.2 (s, CH2), 47.6 (s, CH2), 2.2 (s, CH3). 31P (202.46 MHz):
d ꢀ24.1 (dd, JPP = 95 Hz, JPH = 221.5 Hz, PH), ꢀ92.1 (ddt, JPP = 95 Hz,
JPH = 270 Hz, JPH = 32 Hz, PH2). IR (KBr, Nujol): 2918 s, 2849 s, 2722
w (mPH), 2673 w (mPH), 1461 s, 1377 s, 1299 w, 1260 w, 1153 w,
1094 w, 1022 w, 928 w, 803 w, 720 w cmꢀ1. Anal. Calc. for
C21H46N4P2Si3Zr: C, 42.60; H, 7.83; N, 9.46. Found: C, 42.78; H,
7.94; N, 9.46%.
ð1Þ
2.2. Data for phosphine intermediates
2.2.1. [(C6H4)PH2(
1H (500.1 MHz):
JPH = 201.5 Hz, JPH = 15 Hz, PH2,
l
-PH)]2 (4a)
d
6.84–7.35 (m, C6H4,
8 H), 3.99 (dd,
H), 3.43 (dd, JPH = 201 Hz,
4
Likewise, 5 mol% of complex 1 dehydrocoupled 1,2-bisphosphi-
noethane to the highly related, known phosphine, [(C2H4)PPH]2, in
95% yield, with liberation of hydrogen gas (Eq (2)) [18]. In both
JPH = 16 Hz, PH,
2
H). 31P{1H} (202.46 MHz): d, ꢀ120.5 (d,
JPP = 73.5 Hz, JPH = 201.5 Hz, JPH = 15 Hz, PH2), ꢀ123.7 (d,
JPP = 73.3 Hz, JPH = 201 Hz, JPH = 16 Hz, PH). MS (m/z): 282.0.
Table 1
2.2.2. C6H4(PH)2 (4b)
1H (benzene-d6, 500.1 MHz): d 7.11 (obscured, C6H4, ꢁ2 H), 6.63
Crystal and refinement parameters for complex 2.
(m, C6H4,
2 H), 4.34 (d, JPH = 352 Hz, PH, 2
H). 31P{1H}
2
(202.46 MHz): d ꢀ33.5 (s, P H). MS (m/z): 140.0.
Formula
Molecular weight
Crystal system
Color
a (Å)
b (Å)
C21H46N4P2Si3Zr
592.05
monoclinic
yellow
15.958(2)
9.212(1)
21.320(2)
90
100.767(2)
90
3078.9(6)
P21/c
4
1.94–28.28
0.594
33 171
2.3. General conditions for catalytic dehydrocoupling of primary
phosphines
An NMR tube fitted with PTFE stopcock was charged with
10 mol% of 1 (20 mg, 0.022 mmol), the appropriate bisphosphino
reagent (0.22 mmol), and sufficient benzene-d6 to bring the vol-
ume to greater than 0.5 mL. The solution was then heated for 4–
6 weeks at 100 °C in an oil bath, and the progress of the reaction
was monitored by 31P NMR spectroscopy. The initial color of the
solution changed from light yellow and to a gradually darker yel-
low upon heating. During the reaction, the head space of the
NMR tube was evacuated of any H2 gas generated at daily intervals.
The products were identified by comparison to the chemical shifts
in the literature data [18].
c (Å)
a
(°)
b (°)
(°)
c
Unit cell volume (Å3)
Space group
Z
h Range (°)
l
(mmꢀ1
)
N
Nind
Rint
7646
0.0351
0.0271
0.0613
a
R1 (I > 2
r
(I))
(I))
qmin (e Å3)
b
wR2 (I > 2
qmax
r
D
;
D
0.431; ꢀ0.252
2.4. X-ray structure of complex 2
GoF on R1
1.036
a
R1 = ||Fo| ꢀ |Fc||/
R|Fo|.
X-Ray diffraction data were collected on a Bruker APEX 2 CCD
platform diffractometer (Mo Ka, k = 0.71073 Å) at 125 K. A suitable
b
2
wR2 = {
R
[w(Fo ꢀ Fc2)2]/
R .
[w(Fo2)2]}1/2

44
M.B. Ghebreab et al. / Polyhedron 29 (2010) 42–45
cases, hydrogen gas inhibited phosphorus product formation, and
unless the H2 byproduct, which was observed by 1H NMR spectros-
copy, was vented from the reaction, the catalysis was stunted.
ð2Þ
In each catalytic reaction, a single new zirconium complex was
observed to be formed that persisted throughout the catalysis. For
the dehydrocoupling of o-bisphosphinobenzene, this complex was
unknown. However, reaction complex 1 with 1 equiv of o-bis-
phosphinobenzene resulted in quantitative formation of a new,
pseudo-C3-symmetric zirconium product that was identical to
the complex observed in the catalytic reaction as measured by
1H and 31P NMR spectroscopy in benzene-d6. The product is
(N3N)Zr[j
2-1,2-PH(PH2)C6H4] (2), which was readily isolated as
analytically pure, yellow crystals in 64% yield upon crystallization
from Et2O (Eq (3)). Routine characterization by NMR and IR
spectroscopy confirmed the basic formulation of the complex. A
1H-coupled 31P NMR experiment allowed assignment of the phos-
phanido and phosphine 31P nuclei at d ꢀ24.1 and ꢀ92.1, respec-
tively. Interestingly, no evidence of proton exchange was
observed by NMR spectroscopy in benzene-d6. This observation
may be the result of a limited sample of temperatures available
in this solvent, slow proton exchange, or very rapid proton ex-
change on the NMR timescale.
Fig. 1. Perspective view of complex 2 with thermal ellipsoids drawn at the 35%
probability level and hydrogen atoms, except those located on the two phosphorus
atoms, omitted for clarity.
Table 2
Selected bond lengths (Å) and angles (°) for (N3N)Zr[j
2-1,2-PH(PH2)C6H4] (2).
Zr–P(1)
Zr–P(2)
P(1)–C(16)
P(2)–C(21)
2.9089(6)
2.7375(5)
1.821(2)
1.828(2)
Zr–N(1)
Zr–N(2)
Zr–N(3)
Zr–N(4)
2.085(1)
2.112(1)
2.088(2)
2.469(2)
C(16)–P(1)–Zr
Zr–P(1)–H(1)
Zr–P(1)–H(2)
H(1)–P(1)–H(2)
104.19(6)
129.6(9)
117.6(9)
96.2(13)
C(21)–P(2)–Zr
Zr–P(2)–H(3)
P(2)–Zr-P(1)
105.19(6)
106.6(9)
68.81(2)
ð3Þ
The interaction of the phosphine group with zirconium is the
likely result of the proximity that the rigid phenylene backbone
places this substituent to the metal. This arrangement stands in
contrast with complex 3, in which the phosphine donor is nearly
as far from the zirconium as possible being separated by the ex-
tended ethylene backbone [10]. If the six-coordinate complex were
more stable, presumably complex 3 would have adopted a similar
configuration in the solid state.
In the latter catalytic reaction, all of the observable zirconium
was rapidly converted to (N3N)Zr(PHCH2CH2PH2) (3), which was
the only zirconium complex observed by NMR spectroscopy
throughout the course of the reaction. An independent preparation
of complex 3 was recently reported via reaction of 1 with 1,2-bis-
phosphinoethane [10].
3.2. Molecular structure of 2
3.3. Mechanistic study
Single crystals of complex 2 were grown from concentrated
Et2O solution cooled to ꢀ30 °C. The molecular structure of complex
To gain some additional insight into these reactions, studies on
the decomposition of 2 were undertaken. Heating benzene-d6 solu-
tions of 2 at 70 °C resulted in several changes. In all experiments,
[(C6H4)PPH]2 product was formed as identified by 31P NMR spec-
troscopy. The amount of product varied by run and never appeared
in more than 10% yield based on complex 2. Two intermediate
complexes were formed rapidly in the reaction, which dominated
the phosphorus-containing species. The first complex appeared
2 features a
j
2-phosphanido/phosphine ligand that results in a
six-coordinate complex (Fig. 1). Isolated six-coordinate (N3N)Zr
species are rare. However, such complexes are important in the
formation of element–element bonds via
r-bond metathesis, as
have been proposed for the dehydrocoupling of phosphines by
these (N3N)Zr-complexes [11].
In the solid state, these two ZrAP bonds are different. The bond
between the phosphanido phosphorus, P(2), and zirconium is
markedly shorter than that for the phosphine phosphorus, P(1),
and zirconium (ca. 0.17 Å, Table 2). With the located hydrogen
atom, P(2) is pyramidal, and the short bond length of 2.7375(5) Å
to be [(C6H4)PH2(l
-PH)]2 (4a) based on 31P and 1H NMR spectros-
copy, 31P–1H HMBC and HMQC experiments, and mass spectrome-
try. The spectroscopic data shows a single product rather than a
mixture of rac and meso forms. From our data, it is not clear if
these two forms coincidentally exhibit the same chemical shifts
or if there is some selectivity. Formation of 4a would be the
product of reacting complex 2 with 1 equiv of o-bisphosphinoben-
zene. The free phosphine would likely be generated through the
may be due to electrostatic attraction rather than any
p-bonding
component. Indeed, this is consistent with the highly ionic bonding
model for triamidoamine–zirconium complexes developed in re-
cent theoretical investigations [10].

M.B. Ghebreab et al. / Polyhedron 29 (2010) 42–45
45
Scheme 1. Thermal reaction pathways of complex 2.
equilibrium of 2 with 1 and o-bisphosphinobenzene (Scheme 1).
Kinetic accessibility of 1 and phosphine from (N3N)Zr-phosphanido
complexes is a common feature of this system [10,11]. Compound
4a is observed in low concentration during the catalytic reactions.
The other major phosphorus-containing product is tentatively
assigned to be C6H4(PH)2 (4b) based on similar experiments. For-
mation of 4b would likely be the result of an intramolecular phos-
phine dehydrocoupling to give a putative zirconium–hydride
complex, which would rapidly eliminate hydrogen to form com-
plex 1 (Scheme 1). Observation of hydrogen by 1H NMR spectros-
copy lends indirect support to this reaction path. If this
hypothesis is correct, it suggests that this intramolecular dehydro-
coupling is competitive with the intermolecular dehydrocoupling
that forms 4a at 70 °C. Phosphines 4a and 4b form in similar con-
centrations, which further suggests that the sequence of reactions
to form 4a is relatively rapid or the intramolecular dehydrocou-
pling to afford 4b is relatively slow. Current data do not allow a dis-
tinction between these possibilities to be made.
Several other phosphorus-containing intermediates were ob-
served, but the concentration of these species could not be suffi-
ciently enhanced to provide definitive assignment. Prolonged
heating of complex 2 at 70 °C or heating to temperatures >100 °C
ultimately resulted in decomposition to a myriad of minor prod-
ucts and insoluble materials.
Heating benzene-d6 solutions of complex 2 with an equimolar
quantity of o-bisphosphinobenzene resulted in predominate for-
mation of 4a with very little of 4b observed. This observation sug-
gests that the formation of these two compounds is based on
competitive reactivity in the decomposition of complex 2, and that
the added phosphine established favorable conditions for the for-
mation of 4a. Further heating of this reaction mixture yielded con-
siderably more of the final dehydrocoupling product [(C6H4)PPH]2.
phosphine. These observations suggest that 4a reacts with complex
1 to form final product.
Supplementary data
CCDC 729412 contains the supplementary crystallographic data
for complex 2. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgments
This work was supported by the U.S. National Science Founda-
tion (CHE-0747612 to R.W.). X-ray crystallographic facilities were
provided by the U.S. National Science Foundation (CHE-0521237
to J.M.T). The Applied Biosystems 4000QTrap Pro mass spectrome-
ter was purchased with NSF funding (CHE-0821501). R.W. is a Re-
search Fellow of the Alfred P. Sloan Foundation. The authors thank
Dr. Bruce Deker and Bruce O’Rourke for assistance with 2-D NMR
and MS experiments, respectively.
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products. The identity of an intermediate complex (N3N)Zr[j
2-1,2-
PH(PH2)C6H4] (2) was confirmed by independent preparation and
an X-ray diffraction study. The initial step in the catalysis appears
to be a PAP bond forming event to give complex 4a as identified in
the decomposition of 2 and the reaction of 2 with one equiv of
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