Differences in the stability of zirconium(IV) complexes related to catalytic phosphine dehydrocoupling reactions

Article abstract of DOI:10.1039/c1dt10105f

Relating to the catalytic dehydrocoupling of secondary phosphine substrates, zirconium phosphide complexes supported by triamidoamine and pentamethylcyclopentadienyl ligands exhibit different stability that is attributed to β-hydride elimination.

Full text of DOI:10.1039/c1dt10105f

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COMMUNICATION
Differences in the stability of zirconium(IV) complexes related to catalytic
phosphine dehydrocoupling reactions†
Michael B. Ghebreab, David K. Newsham and Rory Waterman*
Received 18th January 2011, Accepted 3rd March 2011
DOI: 10.1039/c1dt10105f
Relating to the catalytic dehydrocoupling of secondary phos-
phine substrates, zirconium phosphide complexes supported
by triamidoamine and pentamethylcyclopentadienyl ligands
exhibit different stability that is attributed to b-hydride
elimination.
Dehydrogenative bond forming catalysis has become an in-
creasingly general and important synthetic method for main-
group systems (Scheme 1).1–2 Recent important advances utilizing
dehydrocoupling catalysis include the preparation of materials and
Scheme 2 Dehydrocoupling of ( )-methylphenylphosphine by zirconium
pre-catalysts supported by triamidoamine and pentamethylcyclopentadi-
chemicals for hydrogen storage.3–6 Complexes of d0 metals have
enyl ligands.
stood at the forefront of this chemistry for some time. One example
gas afforded (PPhMe)221 inconversions exceeding 95% as a mixture
of this phenomenon is the high reactivity and selectivity of d0 metal
of diastereomers as observed by 31P NMR spectroscopy. The
complex catalysts in the dehydrocoupling of phosphines.7 This
final diphosphine product can be isolated from the reaction
reactivity follows from well developed catalytic dehydrocoupling
in 42% yield as a 55 : 45 ratio of diastereomers. Han and
chemistry of silanes by group 4 metal complexes.8–11
Tilley reported a slight but consistent diastereoselectivity in
3
the formation of meso-diphosphines with (dippe)Rh(h -CH2Ph)
(dippe = 1,2-bis(diisopropylphosphino)ethane) as a dehydrocou-
pling catalyst.13 Interestingly, the reductive coupling of MePhP-
fragments gives a similar ratio of diastereomers.22
Scheme 1 Dehydrocoupling catalysis.
During the catalysis, a complex tentatively assigned to the
Despite the substantial contributions to phosphine dehydro-
zirconium phosphido derivative was observed with a 31P NMR
coupling catalysis from rhodium and recently tin catalysts,12–14 the
resonance at d 35.3. The identity of this complex was confirmed
dominant catalyst type in this transformation has been those of d0
in an independent synthesis of (N3N)ZrPMePh (2, N3N =
group 4 metals, zirconium in particular.7 The premier catalyst in
N(CH2CH2NSiMe3)33-)‡ by reaction of complex 1 with one
the field remains [Cp*2Zr(H)3]- as well as related phosphinimide
equiv. of MePhPH in benzene solution (eqn (1)). Analytical
derivatives discovered by Stephan and coworkers.15–17 We have
and spectroscopic data for 2 are consistent with the formulation
reported on the utility of triamidoamine-supported zirconium
given as well as the family of terminal phosphido complexes of
complexes in this transformation.18–20 Herein, we report initial
the (N3N)Zr fragment that are known.18,20,23 Complex 2 is also
work in a comparative study of metallocene and triamidoamine-
competent for the dehydrocoupling of ( )-methylphenylphosphine
supported zirconium(IV) complexes that reveals the importance
in both benzene and THF solvent and appears to be the resting
of b-hydrogen elimination for phosphine substrates in dehydro-
state of the catalyst during the catalytic reaction.
coupling catalysis.
Reaction of ( )-methylphenylphosphine with 5 mol% of [k -
5
N,N,N,N,C -(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH2]Zr (1) in
(1)
benzene-d6 solution resulted in evolution of hydrogen gas and
consumption of phosphine as observed by NMR spectroscopy
(Scheme 2). Persistent heating at 90 ◦C with venting of hydrogen
Reaction of ( )-cyclohexylmethylphosphine with 5 mol% of
complex 1 in benzene-d6 did not give (PCyMe)2 (Cy = cyclohexyl)
upon extended heating. A new resonance was observed at d -38.2
in the 31P NMR spectra of the reaction mixture. However, attempts
to prepare (N3N)ZrPCyMe (3), the putative phosphido complex
Department of Chemistry, University of Vermont, Cook Physical Sciences
Building, Burlington, VT, USA. E-mail: rory.waterman@uvm.edu; Fax: 01
802 656-8705; Tel: 01 802 656-0278
† Electronic supplementary information (ESI) available: Experimental and
spectroscopic data. See DOI: 10.1039/c1dt10105f
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7683–7685 | 7683
©

that the resonance is tentatively assigned to, were not successful.
It is hypothesized that P–H activation at complex 1 is disfavored
because this phosphine is too sterically encumbered and that the
observation of 3 is only a function of high concentrations of
CyMePH, a phenomenon that has been seen for reaction of 1 with
bulky primary phosphines.20,23 Moreover, the importance of the
equilibrium between the metallacyclic complex 1 and E–H (E = C,
P) bonds has been described in the context of hydrophosphination
catalysis.24 Therefore, alternative catalysts with broader substrate
scope that might also give diastereoselectivity were sought, and
metallocene derivatives were approached as potential candidates.
Reaction of MePhPH with 5 mol% of [Li][Cp*2Zr(H)3] (4)
in THF solution gave (PPhMe)2 in conversions greater than
80% with a ratio of diastereomers of approximately 55 : 45 as
observed by 31P NMR spectroscopy (Scheme 2). Immediately upon
addition of the reagents, formation of LiPMePh was observed. In
contrast, complex 4 was found not to catalyze the dehydrocoupling
of CyMePH to any detectable extent under similar conditions.
Steric factors seem less likely in thwarting this reactivity given
In contrast, it has been observed that (N3N)Zr complexes
are stable with respect to b-hydride elimination. For example,
(N3N)ZrPH(CH2)2PH2 is isolable, and heating this complex
prompts loss of phosphine and formation of complex 1 followed
by dehydrocoupling rather than decomposition.18
Based on prior study, there are significant mechanistic dif-
ferences between these two catalysts. Reported data support
a s-bond metathesis mechanism for P–P bond formation by
triamidoamine-zirconium complexes regardless of primary or
secondary phosphine substrate.20 From work by Stephan, the
dehydrocoupling of primary phosphines by group 4 metallocene
complexes proceeds with two key features: (1) formation of di-
and tri-phosphinato intermediates and (2) formation of terminal
phosphinidene ligands.16–17 The dehydrocoupling of secondary
phosphines by 4 has been described, but it is less well un-
derstood. The nature of the substrate can substantially affect
the mechanism. For example, a difference in dehydrocoupling
mechanism between primary and secondary arsines has been
reported for the triamidoamine-zirconium complex 1.29 Therefore,
it is possible that both catalysts may be operating with a s-
bond metathesis mechanism for the dehydrocoupling of secondary
phosphine substrates, but the differences in stability of the
phosphido complexes may be the result of b-hydrogen elimination
at the zirconocene catalyst. These observations suggest that such
reactivity considerations are important in catalyst selection for
dehydrocoupling applications, and further investigation of these
two different catalysts is underway.
that Cp*2ZrH(PCy2)2 is known.16 Anionic zirconocene hydride
-
complexes were found by Stephan and coworkers to effectively
dehydrocouple Ph2PH in high yield. However, the dehydrocou-
pling of Et2PH with the same family of catalysts gave a poor yield
(10%).17
Treatment of Cp*2ZrCl2 with LiPMePh resulted in a green
solution. Attempts to isolate a crystalline product from the
reaction routinely failed due to decomposition (eqn (2)). Crude
reaction mixtures contained a predominate product with a new 31
P
This work was supported by the U. S. National Science
Foundation (CHE-0747612). R. W. is a fellow of the Alfred P.
Sloan Foundation and a Cottrell Scholar of Research Corporation
for Science Advancement.
NMR resonance at d 95.6 that is potentially the bis(phosphide)
Cp*2Zr(PMePh)2. Decomposition products included small quan-
tities of MePhPH and (PPhMe)2. Similar reactivity was observed
with LiPCyMe, but these reactions were not explored in detail.
Notes and references
‡ (N3N)ZrPMePh (2) 60% isolated yield. Anal. Calcd for C22H47N4PSi3Zr:
1
(2)
C, 46.03; H, 8.25; N, 9.76. Found: C, 46.16; H, 8.25; N, 9.76. H NMR
(500.1 MHz): d 7.56 (t, C6H5, 2 H), 7.22 (t, C6H5, 2 H), 6.95 (vt, C6H5,
1 H), 3.26 (t, CH2, 6 H), 2.22 (t, CH2, 6 H), 2.11 (d, JPH = 25 Hz, CH3,
1
3 H), 0.26 (s, CH3, 27 H). 31P{ H} NMR (202.5 MHz): d 35.3 (s, PCH3).
For complete experimental and spectroscopic details, see ESI.†
Reductive elimination of a diphosphine from a bis(phosphido)
metallocene complex has been reported.25 The limited quantity of
(PMePh)2 that was produced in the decomposition of the putative
Cp*2Zr(PMePh)2 suggested that decomposition occurs here by an
alternative mechanism. Our working hypothesis is that unstable
phosphorus-containing compounds are generated via b-hydride
elimination. This hypothesis is supported by the observation
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of Cp*2Zr(PPhMe)2 by salt metathesis and by the observation
of a broad resonance at d 167.6 by 31P NMR spectroscopy,
which is the majority of the phosphorus-containing product and
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reaction of Cp*2Zr(H)2 with MePhPH behaved similarly. However,
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Cp*2Zr(PPhMe)2 provide an active catalyst for the dehydrocou-
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©