Rh-catalyzed P-P bond activation

Article abstract of DOI:10.1039/b712972f

A Rh-catalyst derived from (NacNac)Rh(COE)(N₂) effects the hydrogenation and silylation of P-P bonds to give secondary phosphines and silylphosphines, (Ph₂PH) and (Ph₂PSiRR′₂) respectively; the latter process is shown to also involve the silylation of secondary phosphines. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712972f

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Rh-catalyzed P–P bond activation{
Stephen J. Geier and Douglas W. Stephan*
Received (in Cambridge, UK) 23rd August 2007, Accepted 28th September 2007
First published as an Advance Article on the web 16th October 2007
DOI: 10.1039/b712972f
(NacNac = HC(CMeN(iPr2C6H2))2) favor non-productive P–H
A Rh-catalyst derived from (NacNac)Rh(COE)(N2) effects the
hydrogenation and silylation of P–P bonds to give secondary
phosphines and silylphosphines, (Ph2PH) and (Ph2PSiRR92)
respectively; the latter process is shown to also involve the
silylation of secondary phosphines.
elimination over dehydrocoupling to give P–P bond formation.
This supposition prompted us to probe the catalytic activity of 1 in
reactions with diphosphine substrates. Treatment of P2Ph4 under
4 atm of H2 with 10 mol% of 1 at 50 uC resulted in the
hydrogenation of the P–P bond to give Ph2PH in 95% yield in 12 h
(Scheme 1). In a similar fashion, the product Ph2P(SiPh2H) was
also prepared in 98% yield using 10 mol% 1 to catalyze the
activation of P2Ph4 in the presence of 5 equivalents of Ph2SiH2 at
100 uC for 48 h (Scheme 1). Analogous use of PhMe2SiH gave the
phosphine Ph2P(SiPhMe2) in 95% yield with 5% of the by-product
Ph2PH, while bulkier silanes resulted in lower yields of the
silylphosphine. For instance, use of Ph2MeSiH or Ph3SiH gave
76% and 74% of the P–Si coupled products Ph2P(SiPh2Me) and
Ph2P(SiPh3) together with 17% and 25% yield of Ph2PH,
respectively. Alkylsilanes proved to be less reactive. Et3SiH
resulted in conversion of P2Ph4 to 16% of Ph2PSiEt3 with 44%
Ph2PH, while iPr3SiH afforded no P–Si coupling product and only
29% Ph2PH.
The impact of organometallic chemistry and catalysis on organic
chemistry has been dramatic, as is evidenced by the awarding of
Nobel Prizes in 2001 and 2005. A related area, ripe for impact,
involves the application of the concepts of organometallic
chemistry to main group synthesis and materials chemistry. Such
‘‘inorganometallics’’ has drawn some recent attention.1 While
much of the work to date has involved stoichiometric transforma-
tions, some catalytic processes are emerging.2–5 For example,
we previously reported the use of [Cp*2ZrH3]26–8 and
CpTi(NPt-Bu3)(C2H4)2 as precatalysts for the dehydrocoupling
of primary and secondary phosphines, affording a variety of P–P
bonded oligomers.9 In a similar fashion, Harrod and co-workers
showed that titanocene derivatives can act as catalysts for the
heterocoupling of silanes and phosphines.10,11 A recent report by
Waterman and co-workers highlights the catalytic heterocoupling
of primary phosphines and silanes by a Zr(IV) triamidoamine
complex.15 Waterman and Tilley have also reported similar use of
zirconocene and hafnocene complexes to effect catalytic dehy-
drocoupling of stibines, affording oligostibines.12 Brookhart and
Bohm have also probed similar dehydrocoupling of secondary
phosphines using the Rh precatalyst Cp*Rh(CH2LCHSiMe3)2,13
although this system gave only high yields for select phosphines
under harsh conditions. Most recently, Han and Tilley have
reported dehydrocoupling of phosphines using a catalyst based on
the Rh(iPr2PCH2CH2PiPr2) fragment.14 In related chemistry,
stoichiometric functionalizations of P–P bonds have been
investigated to some extent;16 metal-mediated reactions
involving the activation of P–P bonds has drawn less attention.
We have recently examined the stoichiometric reactivity of Ni
and Fe b-diketiminates with P–H and P–P bonds.17 In this
paper, we report the discovery of a catalyst for the activation of
P–P bonds derived from the b-diketiminate-complex
Rh(NacNac)(C8H14)N2 1.18
To garner further information regarding these P–P bond
activations, the catalyst precursor 1 was reacted stoichiometrically
with P2Ph4 in toluene and allowed to stir overnight at 25 uC.
Subsequent work-up gave a dark-red residue which was recrys-
tallized from pentane to give dark-red crystals of 2 in 50% yield.{ It
should be noted that this diminished yield is due to loss during
crystallization as NMR data for reaction mixtures show near
quantitative formation of 2. This product exhibits a single 31P{1H}
resonance at 251.4 ppm with a Rh–P coupling of 140 Hz. The 1H
NMR data for 2 was consistent with the presence of a 1 : 1 ratio of
NacNac and P2Ph4. An X-ray diffraction study confirmed the
formulation of 2 as Rh(NacNac)(P2Ph4) (Scheme 2, Fig. 1). The
N2P2 coordination sphere about Rh is a distorted square-plane.
The Rh–N distances of 2.054(3) s and 2.065(3) s are slightly
shorter than those seen in the precursor 1 (2.076(3) s and
2.074(3) s) suggesting that the (P2Ph4) fragment has weaker trans
In initial trials, we attempted to probe the utility of 1 in the
dehydrocoupling of Ph2PH. Over a 12 h period at 70 uC
compound 1 proved to be a poor catalyst for such dehydrocou-
pling, giving P2Ph4 in only 30% yield. We hypothesized
that the steric demands of the NacNac ancillary ligand
Department of Chemistry and Biochemistry, University of Windsor,
Windsor, Ontario, Canada N9B 3P4. E-mail: Stephan@uwindsor.ca;
Fax: +1 519-973-7098
{ Electronic supplementary information (ESI) available: Experimental and
spectroscopic details. See DOI: 10.1039/b712972f
Scheme 1 Hydrogenation and silylation of P2Ph4 catalyzed by 1.
Chem. Commun., 2008, 99–101 | 99
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(Scheme 2). The metric parameters in 3 were found to be similar to
those in 2, with Rh–N distances of 2.064(2) s and 2.067(2) s
and Rh–P bond lengths of 2.2229(11) s and 2.2466(10) s, while
the P–P distance was determined to be 2.1254(14) s. These latter
observations suggest that the smaller, more basic diphosphine
binds to Rh more strongly, precluding the partial dissociation that
initiates subsequent P–P cleavage reactions.
Scheme 2 Synthesis of 2, 3 and 4.
Addition of H2 to solutions of 2 resulted in no observable
reactions at ambient temperatures, further suggesting that reaction
of P2Ph4 requires thermal dissociation of the g2-P2Ph4 to at least
an g1-P2Ph4 to permit oxidative addition at Rh.
In the case of H2 reactions, the transient Rh(NacNac)(g1-P2Ph4)
has a vacant coordination site permitting oxidative addition of H2
to Rh. Subsequent reductive elimination of the secondary
phosphine via P–P bond cleavage is proposed. Elimination of a
second equivalent of phosphine regenerates the Rh(NacNac)
fragment which then re-enters the catalytic cycle by reaction with
P2Ph4.
In the case of the silylation reactions, an analogous catalytic
cycle would generate equal amounts of Ph2PH and Ph2PSiR3 for
each P2Ph4 molecule activated. However, only low concentrations
of Ph2PH are observed by 31P{1H} NMR spectroscopy of the
catalytic silylation reaction mixtures. This suggests that the second
part of the catalytic cycle, where Ph2PH must dehydrocouple with
a second equivalent of silane, is more rapid than the initial P2Ph4
activation. Reaction of P2Ph4 and two equivalents of Ph2SiH2 in
the presence of 10 mol% of 1 at 50 uC for 5 days affords a 74%
yield of the silylphosphine Ph2P(SiPh2H) and a 23% yield of
Ph2PH.
Independent experiments demonstrated such activation of P–H
bonds. For example, exposure of Ph2PH to 4 atm of D2 in the
presence of 5 mol% 1 led to hydrogen for deuterium exchange
resulting in 85% conversion to Ph2PD. While treatment of 1 with
excess Ph2PH generates a species in solution formulated as
Rh(NacNac)(Ph2PH)2, dissociation of phosphine and oxidative
addition of D2, followed by reductive elimination of HD and
Ph2PD accounts for the observed deuteration. Similarly, hetero-
nuclear dehydrocoupling of Ph2PH and silanes proceeds using
5 mol% of species 1. For example, reaction of Ph2PH and Ph2SiH2
gave quantitative yield of Ph2P(SiPh2H) in 18 h at 50 uC. Similarly,
Ph2P(SiPhMe2) is formed in 84% yield under similar conditions,
while Ph2P(SiPh2Me) is formed in 85% yield in 18 h at 100 uC. As
with the diphosphines, bulkier silanes afford lesser yields as
Ph2P(SiPh3) is formed in only 40% yield from Ph2PH and Ph3SiH
and the homo-dehydrocoupling by-product P2Ph4 is observed in
10% yield.
Fig. 1 ORTEP depictions of (a) 2, and (b) 3.
influence than cis-cyclooctene and N2. The NacNac ligand bite
angle in 2 is 90.53(11)u, while Rh–P distances are 2.2301(10) s and
2.2423(10) s. The P–P bond in 2 is 2.1389(14) s and is comparable
to the P–P bond in [Ni(NacNac)2(P2Ph2)] (2.125(3) s).17 The RhP2
ring gives rise to a P–Rh–P angle of 57.14(4)u and thus the square-
planar geometry about Rh is distorted with pseudo-trans N–Rh–P
angles of 160.02(8)u and 162.37(9)u and pseudo-cis N–Rh–P angles
of 107.38(9)u and 106.23(8)u. The steric crowding of the Rh
coordination sphere is further evidenced from the twisting of the
RhP2 plane with respect to the RhN2 plane by 14.8u. The presence
of the g2-P2Ph4 to a single metal center in 2 appears to be unique in
that literature precedent demonstrates the propensity of P2Ph4 to
bind in a monodentate fashion, or to bridge two metal centers.19–29
In addition, the related complex Ni(NacNac)(Ph2PH)17 binds only
one phosphine ligand yielding the three coordinate species,
presumably a result of the slightly larger atomic radius of Rh as
well as the aforementioned geometry distortions which accom-
modate the g2-P2Ph4.
Overall, these data support a mechanism in which hydrosilyla-
tion of P2Ph4 affords both Ph2PH and the silylphosphine; however
the phosphine reacts further to form another equivalent of
silylphosphine (Scheme 3). This latter P–Si dehydrocoupling is
apparently faster than the initial P–P bond activation. Evidence
for the nature of an intermediate was derived from
stoichiometric reactions of 1 with Ph2PH and Ph2SiH2. In this
1
case a short-lived species was observed spectroscopically. The H
Reactions with P2Et4 seem to support partial dissociation of the
P2 fragment as a key step in the reaction. While attempts to carry
out catalytic P–P activation reactions with P2Et4 proved
unsuccessful, the species Rh(NacNac)(P2Et4) 3, the analog of 2,
was readily formed in a stoichiometric reaction of P2Et4 with 1
NMR resonance at 213.5 ppm was indicative of a Rh–hydride
species, while the doublet at 5.05 ppm indicated the presence of
the PH bond of coordinated phosphine. These data together with
the 31P{1H} resonance at 47.4 ppm and the 29Si{1H} signal at
21.4 ppm were consistent with the formulation of
4 as
100 | Chem. Commun., 2008, 99–101
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¯
solution at 230 uC. M = 698.73, space group: monoclinic, P1, a =
10.566(3), b = 11.676(4), c = 17.636(6) s, a = 102.412(4), b = 93.045(4), c =
114.125(4)u, V = 1914.7(11) s3, Z = 2, T = 296(2) K, data: variables 6726:
381, R = 0.0459, Rw = 0.1233, GOF = 0.969; CCDC 658231. For
crystallographic data in CIF or other electronic format, see DOI: 10.1039/
b712972f
Generation of 4: to 20 mg 1 (0.032 mmol) in 2 mL toluene was added
12 mg Ph2SiH2 (0.064 mmol). The solution was cooled to 230 uC upon
which 6 mg Ph2PH (0.032 mmol) was added. The solution was allowed to
warm to room temperature, volatiles were removed in vacuo and the
residue was washed with pentane (2 6 2 mL), leaving 12 mg of 4
1
(0.013 mmol, 43% yield). H NMR (C6D6) d: 213.5 (1H, d of d, JH–P
=
28.4 Hz, JH–Rh = 15.4 Hz), 0.23 (3H, d, J = 6.6 Hz), 0.62 (3H, d, J =
6.6 Hz), 0.71 (3H, d, J = 6.6 Hz), 1.04 (3H, d, J = 6.6 Hz), 1.06 (3H, d,
J = 6.6 Hz), 1.11 (3H, d, J = 6.6 Hz), 1.43 (3H, d, J = 6.6 Hz), 1.47 (3H, d,
J = 6.6 Hz), 1.65 (3H, s), 1.88 (3H, s), 2.76 (1H, sept, J = 6.6 Hz), 2.82 (1H,
sept, J = 6.6 Hz), 4.07–4.16 (2H, ov sept, J = 6.6 Hz), 5.00 (1H, d, JH–Rh
=
36 Hz), 5.05 (1H, d, JP–H = 361 Hz), 5.34 (1H, s), 6.35–7.61 (26H, ov m).
31P{1H} NMR (C6D6) d: 47.4 (d of d, JP–H = 361 Hz, JP–Rh = 137 Hz).
29Si{1H} NMR (C6D6) d: 21.4 (d of d, J = 23.5 Hz, J = 34.1 Hz).
1 T. P. Fehlner, Inorganometallics, Plenum Press, New York, 1992.
2 F. Gauvin, J. F. Harrod and H. G. Woo, Adv. Organomet. Chem., 1998,
42, 363.
Scheme 3 Proposed mechanism of silylation of P2Ph4 and PPh2H.
3 J. Y. Corey, Adv. Organomet. Chem., 2004, 51, 1.
4 T. D. Tilley, Comments Inorg. Chem., 1990, 10, 37.
5 T. D. Tilley, Acc. Chem. Res., 1993, 26, 22.
6 N. Etkin, M. C. Fermin and D. W. Stephan, J. Am. Chem. Soc., 1997,
119, 2954.
(NacNac)RhH(SiHPh2)(PHPh2) (Scheme 2). This Rh(III) inter-
mediate is analogous to the Ir complexes (NacNac)IrH2(PR3)
isolated previously by Chirik and co-workers.30 This intermediate
is also related to CpRhH(SiR3)(PR3) complexes reported by
Marder and co-workers.31 Loss of H2 from this species would yield
the proposed intermediate (NacNac)Rh(SiHPh2)(PPh2) which is
proposed to undergo reductive elimination of the silyl–phosphide
product. This proposition suggests that reductive Si–P elimination
occurs more readily than ligand redistribution reactions.
In summary, the Rh-catalyst derived from 1 effects the catalytic
hydrogenation and silylation of diphosphines. Inherent in this
chemistry is the dehydrocoupling of secondary phosphines with
silanes. Further studies of the mechanism, catalyst optimization
and applications of these processes are ongoing.
7 N. Etkin, A. J. Hoskin and D. W. Stephan, J. Am. Chem. Soc., 1997,
119, 11420.
8 M. C. Fermin and D. W. Stephan, J. Am. Chem. Soc., 1995, 117, 12645.
9 J. D. Masuda, A. J. Hoskin, T. W. Graham, C. Beddie, M. C. Fermin,
N. Etkin and D. W. Stephan, Chem.–Eur. J., 2006, 12, 8696.
10 R. Shu, L. Hao, J. F. Harrod, H.-G. Woo and E. Samuel, J. Am. Chem.
Soc., 1998, 120, 12988.
11 S. Xin, H. G. Woo, J. F. Harrod, E. Samuel and A.-M. Lebuis, J. Am.
Chem. Soc., 1997, 119, 5307.
12 R. Waterman and T. D. Tilley, Angew. Chem., Int. Ed., 2006, 45, 2926.
13 V. P. W. Bohm and M. Brookhart, Angew. Chem., Int. Ed., 2001, 40,
4694.
14 L. Han and T. D. Tilley, J. Am. Chem. Soc., 2006, 128, 13698.
15 A. J. Roering, S. N. MacMillan, J. M. Tanski and R. Waterman, Inorg.
Chem., 2007, 46, 6855.
16 J. D. Masuda, W. W. Schoeller, B. Donnadieu and G. Bertrand, Angew.
Chem., Int. Ed., 2007, 46, 7052, and references therein.
17 G. Bai, P. Wei, A. Das and D. W. Stephan, Dalton Trans., 2006, 1141.
18 J. D. Masuda and D. W. Stephan, Can. J. Chem., 2005, 83, 324.
19 L. J. Arnold, L. Main and B. K. Nicholson, Appl. Organomet. Chem.,
1990, 4, 503.
Financial support from NSERC of Canada is gratefully
acknowledged. SJG is grateful for the award of an Ontario
Graduate Scholarship.
Notes and references
{ Preparation of 2 and 3: these compounds were prepared in a similar
fashion and thus only one preparation is detailed. To a solution of 30 mg
(0.081 mmol) P2Ph4 in 5 mL of toluene was added a solution of 50 mg 1
(0.079 mmol) in 5 mL of toluene. The mixture was allowed to stir overnight
upon which the solvent was removed in vacuo. The dark-red residue was
washed with 10 mL cold pentane to give 35 mg of the product 2
(0.039 mmol, 50% yield). 1H NMR (C6D6) d: 0.84 (6H, d, J = 6.5 Hz), 1.20
(6H, d, J = 6.5 Hz), 1.71 (6H, s), 4.45 (4H, sept, J = 6.5 Hz), 5.15 (1H, s),
6.73–7.19 (26H, br m). 31P{1H} NMR (C6D6) d: 251.4 (d, JP–Rh = 140 Hz).
13C NMR (C6D6) d: 24.0, 24.4, 28.6, 98.0, 124.0, 124.1, 127.5–128.6 (m,
obscured by C6D6), 128.9, 134.9 (app. t, J = 7.5 Hz), 157.7, 159.6. EA anal.
calcd for RhP2N2C53H61 (%) C: 71.21, H: 7.22, N: 3.13; found: C: 70.91, H:
7.22, N: 2.72. X-Ray quality crystals were grown by slow evaporation from
a pentane solution. M = 890.89, space group: monoclinic, P21/n, a =
10.8591(11), b = 35.296(4), c = 12.6312(13) s, b = 94.769(2)u, V =
4824.6(9) s3, Z = 4, T = 273(2) K, data: variables 8472: 525, R = 0.0484,
20 M. R. Churchill, C. Bueno and D. A. Young, J. Organomet. Chem.,
1981, 213, 139.
21 D. Rehder, J. Organomet. Chem., 1977, 137, C25.
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23 A. J. M. Caffyn and M. J. Mays, J. Organomet. Chem., 2005, 690, 2209.
24 H. Adams, A. Biebricher, S. R. Gay, T. Hamilton, P. E. McHugh,
M. J. Morris and M. J. Mays, J. Chem. Soc., Dalton Trans., 2000, 2983.
25 A. Martin, M. J. Mays, P. R. Raithby and G. A. Solan, J. Chem. Soc.,
Dalton Trans., 1993, 1431.
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G. Conole, M. McPartlin and H. R. Powell, J. Chem. Soc., Dalton
Trans., 1991, 3103.
27 G. Conole, M. McPartlin, M. J. Mays and M. J. Morris, J. Chem. Soc.,
Dalton Trans., 1990, 2359.
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D. Vitali, J. Chem. Soc., Chem. Commun., 1979, 576.
29 G. Fachinetti, C. Floriani and H. Stoeckli-Evans, J. Chem. Soc., Dalton
Trans., 1977, 2297.
1
Rw = 0.1099, GOF = 1.053; CCDC 658230. 3: H NMR (C6D6) d: 0.74
(8H, app. pent, J = 7.8 Hz), 1.26 (12H, t of d, J = 7.3 Hz, 2.9 Hz), 1.35
(12H, d, J = 6.9 Hz), 1.63 (12H, d, J = 6.9 Hz), 1.84 (6H, s), 4.19 (4H, sept,
J = 6.9 Hz), 5.19 (1H, s), 7.15–7.29 (6H, m). 31P NMR (C6D6) d: 264.51 (d,
J = 127 Hz). 13C NMR (C6D6) d: 9.8, 12.9, 22.4, 23.9 (d, J = 9.6 Hz), 28.3,
97.0, 123.2, 123.8, 127.3–130.5 (m, obscured by C6D6), 140.3, 156.7, 159.4.
EA anal. calcd for RhP2N2C37H55 (%) C: 63.32, H: 9.19, N: 3.99; found: C:
63.55, H: 9.24, N: 4.12. X-Ray quality crystals were grown from a pentane
30 W. H. Bernskoetter, E. Lobkovsky and P. J. Chirik, Organometallics,
2005, 24, 4367.
31 M. P. Campian, J. L. Harris, N. Jacin, R. N. Perutz, T. B. Marder and
A. C. Whitwood, Organometallics, 2006, 25, 5093.
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