Selective dehydrocoupling of phosphines by triamidoamine zirconium catalysts

Article abstract of DOI:10.1021/om070189o

New zirconium triamidoamine complexes (N₃N)ZrR (N₃N = N(CH₂CH₂NSiMe₃)₃^3- R = Me, 1; PHPh, 2; PHCy, 4) are effective catalysts for the dehydrocoupling of primary and secondary pho

Full text of DOI:10.1021/om070189o

2492
Organometallics 2007, 26, 2492-2494
Selective Dehydrocoupling of Phosphines by Triamidoamine
Zirconium Catalysts
Rory Waterman*
Department of Chemistry, UniVersity of Vermont, Burlington, Vermont 05405-0125
ReceiVed February 28, 2007
Summary: New zirconium triamidoamine complexes (N3N)ZrR
(N3N ) N(CH2CH2NSiMe3)33-; R ) Me, 1; PHPh, 2; PHCy,
4) are effectiVe catalysts for the dehydrocoupling of primary
and secondary phosphines and select for the diphosphine
product. Mechanistic analysis reVealed that metal-catalyzed
P-P bond formation occurs Via σ-bond metathesis steps.
anticipated that the electronic structure of triamidoamine-
supported zirconium would present unique and favorable
properties for phosphine dehydrocoupling catalysis. This com-
munication describes the preparation of new triamidoamine
zirconium complexes that exhibit high reactivity in the dehy-
drocoupling catalysis of primary and secondary phosphines and
selectivity to form the diphosphine product in dehydrocoupling
reactions of primary phosphines.
Metal-catalyzed dehydrocoupling is rapidly becoming a
mainstay reaction in the formation of element-element bonds.1
In particular, dehydrocoupling of phosphines has been of
heightened interest. Two families of rhodium catalysts have been
reported, and in dehydrocoupling reactions of primary phos-
phines, the diphosphine products are produced.2,3 Early transi-
tion-metal catalysts, particularly those of the group 4 metals,
are tremendously promising in phosphine dehydrocoupling due
to their high reactivity and low cost.4-6 However, the observed
selectivity has been poor. Reported catalysts form only mixtures
of (RPH)2 and (PR)n (n ) 4, 5, or 6) unless the substrate is a
bulky 2,4,6-substituted arylphosphine.4e To make this catalysis
viable for synthetic applications, new metal complexes that
improve on the current selectivity are required. Additionally,
developing a greater understanding of the P-P bond forming
event will aid in improving selectivity.
Triamidoamine complexes of the early transition metals are
well known.7 Limited attention, however, has been seen with
triamidoamine-supported zirconium.8 Such complexes present
two orthogonal π-symmetric frontier orbitals,7 which would
allow for some degree of π-donation from a phosphido ligand
and also provide a potentially vacant orbital adjacent to the
phosphorus center. Given that terminal phosphido ligands
display π-bonding with group 4 metallocene complexes,9 it was
Reaction of (N3N)ZrCl (N3N ) N(CH2CH2NSiMe3)33-) with
MeLi in Et2O afforded analytically pure, colorless crystals of
(N3N)ZrMe (1) in 82% yield.10 Complex 1 displayed pseudo-
C3-symmetry with resonances at δ 0.37 and 36.4 for the methyl
1
ligand in the H and 13C NMR spectra, respectively. Heating
benzene solutions of 1 with excess phenylphosphine results in
1
liberation of methane, as observed by H NMR spectroscopy,
and formation of the primary phosphido complex (N3N)Zr-
(PHPh) (2) as analytically pure, yellow crystals in 91% yield
(eq 1). The phosphido ligand of complex 2 was observed at δ
-48.2 in the 31P NMR spectrum with JPH ) 203 Hz. The
phosphido proton of 2 displayed a resonance at δ 3.97 in the
1H NMR and νPH ) 2273 in the infrared. The spectroscopic
data of 2 differ little from the range of values seen for other
monomeric zirconium phosphido complexes.4,11,12
* Corresponding author. E-mail: rory.waterman@uvm.edu.
(1) (a) Gauvin, F.; Harrod, J. F.; Woo, H. G. AdV. Organomet. Chem.
1998, 42, 363-405. (b) Corey, J. Y. AdV. Organomet. Chem. 2004, 51,
1-52.
(2) Bohm, V. P. W.; Brookhart, M. Angew. Chem., Int. Ed. 2001, 40,
4694-4696.
Complex 1 is an effective catalyst precursor for the dehy-
drocoupling of primary and secondary phosphines, and the
results of catalytic runs are summarized in Table 1. Primary
alkyl (Cy, tBu), primary aryl (Ph; p-Tol, Tol ) tolyl; 2-EtC6H4),
and diphenyl phosphines are tolerated in this catalysis. Mesityl
phosphine (MesPH2) was very slow to react with complex 1,
and a limited quantity of (MesPH)2 formed only upon extended
heating. The lack of reactivity associated with MesPH2 appears
not to be an electronic effect, as p-TolPH2 was readily
dehydrocoupled by 1 to give (p-TolPH)2. Reaction of 1 with
2-EtC6H4PH2 is initially rapid, but formation of diphosphine
(2-EtC6H4PH)2 requires prolonged heating. This steric effect is
pronounced as Ph2PH was efficiently converted to (Ph2P)2 but
only after long reaction times.
(3) Han, L.-B.; Tilley, T. D. J. Am. Chem. Soc. 2006, 128, 13698-13699.
(4) (a) Fermin, M. C.; Stephan, D. W. J. Am. Chem. Soc. 1995, 117,
12645-12646. (b) Etkin, N.; Fermin, M. C.; Stephan, D. W. J. Am. Chem.
Soc. 1997, 119, 2954-2955. (c) Hoskin, A. J.; Stephan, D. W. Angew.
Chem., Int. Ed. 2001, 40, 1865-1867. (d) Stephan, D. W. Angew. Chem.,
Int. Ed. 2000, 39, 314-329. (e) Masuda, J. D.; Hoskin, A. J.; Graham, T.
W.; Beddie, C.; Fermin, M. C.; Etkin, N.; Stephan, D. W. Chem.-Eur. J.
2006, 12, 8696-8707.
(5) Xin, S.; Woo, H. G.; Harrod, J. F.; Samuel, E.; Lebuis A.-M. J. Am.
Chem. Soc. 1997, 119, 5307-5313.
(6) Catalytic dehydrocoupling of phosphine-borane adducts has been
reported with early transition-metal catalysts: (a) Dorn, H.; Singh, R. A.;
Massey, J. A.; Nelson, J. M.; Jaska, C. A.; Lough, A. J.; Manners, I. J.
Am. Chem. Soc. 2000, 122, 6669-6678. (b) Jaska, C. A.; Manners I. In
Inorganic Chemistry in Focus II; Meyer, G., Naumann, D., Wesemann, L.,
Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2005;
pp 53-64.
(7) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9-16.
(8) (a) Duan, Z.; Naiini, A. A.; Lee, J.-H.; Verkade, J. G. Inorg. Chem.
1995, 34, 5477-5482. (b) Morton, C.; Munslow, I. J.; Sanders, C. J.; Alcock,
N. W.; Scott, P. Organometallics 1999, 18, 4608-4613.
(9) Baker, R. T. Whitney, J. F. Wreford, S. S. Organometallics 1983, 2,
1049-1051.
10.1021/om070189o CCC: $37.00 © 2007 American Chemical Society
Publication on Web 04/07/2007

Communications
Organometallics, Vol. 26, No. 10, 2007 2493
1
Phosphido complex 2 was identified by H and 31P NMR
Table 1. Results of Dehydrocoupling Reactions with
Catalytic (N3N)ZrMe (1)a
spectroscopy as the resting state of the catalyst during dehy-
drocoupling. Additionally, complex 2 was found to be catalyti-
cally competent for dehydrocoupling of PhPH2. Under catalytic
conditions, the disappearance of PhPH2 obeyed second-order
kinetics in [PhPH2] over at least 3 half-lives as monitored by
1H and 31P NMR spectroscopy in benezene-d6. At times greater
than 4 half-lives, some decomposition of catalyst is observed.
A first-order dependence on zirconium (added as 2) was
established by a plot of [2] versus kobs, which was linear for
phosphine
productb
timec
yieldd
e
f
PhPH2
PhPH2
(PhPH)2, (PhP)x
(PhPH)2
(MesPH)2
(p-TolPH)2
(2-EtC6H4PH)2
<1.5
>95
92
<5
90
7
g
MesPH2
p-TolPH2
>21
h
7
10
2
2-EtC6H4PH2
86
i
j
CyPH2
(CyPH)2, (CyP)4
>98
>98
90
i
tBuPH2
(tBuPH)2
3
12
8
Ph2PH
PhPH2/CyPH2
(Ph2P)2
PhHP-PHCyk
55
0.5-8 mol % of complex 2. An overall rate law of rate ) kobs
-
[2][PhPH2]2 can be proposed where kobs ) 2.46(4) × 10-5 M-2
s-1 at 90 °C. Activation parameters ∆Hq ) 13.4(2) kcal/mol
and ∆Sq ) -35.7(2) eu (T ) 67.5-119.8 °C) were obtained
from an Eyring analysis. A kinetic isotope effect (KIE) kH/kD
) 3.1(5) was obtained by measuring the ratio of PhHP-PDPh
to (PhPH)2 for the first equivalent of diphosphine product
formed in the reaction of 2 with 30 equiv of PhPHD.13 These
data are consistent with an ordered transition state and support
the working hypothesis that the P-P bond forming step proceeds
via σ-bond metathesis.14 It was further observed that performing
the catalysis under an atmosphere of hydrogen led to a reduced
rate of diphosphine formation.
a Catalysis was performed in a sealed vessel in degassed benzene or
benzene-d6 solution at 90 °C with 5 mol % catalyst loading. b Character-
ization data for new compounds can be found in the Supporting Information.
c Reaction time in days. d Percent formation of product(s) measured by
integration vs internal standard (31P NMR). e Run at 120 °C, 1 mol %
catalyst. f Mixture of x ) 4, 5, and 6. g Mes ) mesityl. h Tol ) tolyl. i Run
at 70 °C. j (CyPH)2 to (CyP)4 ratio was highly variable, ranging from 4:1
to 1:2. k 40% of PhPH2 was consumed to competitively form (PhPH)2.
At 1 mol %, complex 1 completely consumed PhPH2 in
benzene-d6 solution in less than 36 h at 120 °C as observed by
31P NMR spectroscopy. This qualitative measure implies that 1
is a faster catalyst than anionic zirconocene catalysts under these
conditions.4a The products of this dehydrocoupling were identi-
fied by 31P NMR spectroscopy as a mixture of (PHPh)2, (PPh)4,
(PPh)5, and (PPh)6. It has been suggested that the formation of
(PPh)5 results from the high reaction temperature, as titanocene-
catalyzed dehydrocoupling of PhPH2 forms a mixture of only
(PHPh)2, (PPh)4, and (PPh)6 at ambient temperature.5
Repeating the catalysis with 1 at 90 °C affords (PHPh)2 as
the exclusive dehydrocoupling product in a nearly 1 to 1 ratio
of the rac and meso isomers. Despite this high selectivity for
the diphosphine, the reaction is markedly slower at this
temperature, proceeding to completion after 7 days.
A σ-bond metathesis reaction between phenylphosphine and
2 to afford (PhPH)2 would presumably generate a zirconium
hydride product, (N3N)ZrH. Such a complex was sought
synthetically. Exposure of benzene-d6 solutions of 1 to 1 atm
of hydrogen cleanly formed methane and a new zirconium
complex where the C3-symmetry of the triamidoamine ligand
1
had been lifted. Analysis of the H and 13C NMR spectra and
an HMQC experiment revealed that the product of hydrogena-
tion is the metalated complex [η5-(Me3SiNCH2CH2)2NCH2CH2-
NSiMe2CH2]Zr (3, eq 2). It was found that thermolysis of
benzene solutions of 1 in the absence of H2 gave analytically
pure, colorless crystals of 3 in 47% yield. The limited isolated
yield of 3 is the result of the high solubility of 3 in hydrocarbon
(10) See Supporting Information for complete experimental and char-
acterization details. Selected spectral data for new zirconium compounds:
(N3N)ZrCl: 1H (C6D6, 500.1 MHz) δ 3.230 (t, CH2, 2 H), 2.243 (t, CH2,
2 H), 0.315 (s, CH3, 27 H). (N3N)ZrMe (1): 1H (C6D6, 500.1 MHz) δ
3.228 (t, CH2, 2 H), 2.171 (t, CH2, 2 H), 0.370 (s, CH3, 3 H), 0.264 (s,
CH3, 27 H); 13C{1H} (C6D6, 125.8 MHz) δ 62.19 (s, CH2), 47.47 (s, CH2),
36.41 (s, ZrCH3), 1.23 (s, CH3). (N3N)Zr(PHPh) (2): 1H (C6D6, 500.1 MHz)
δ 7.622 (t, C6H5, 2 H), 7.110 (m, C6H5, 1 H), 6.914 (t, C6H5, 2 H), 3.974
(d, PH, JPH ) 203 Hz, 1 H), 3.212 (t, CH2, 6 H), 2.160 (t, CH2, 6 H), 0.275
(s, CH3, 27 H); 31P{1H} (C6D6, 202.4 MHz) -48.16 (s). [η5-(Me3SiNCH2-
CH2)2NCH2CH2NSiMe2CH2]Zr (3): 1H (C6D6, 500.1 MHz) δ 3.605 (t, CH2,
2 H), 3.126a (m, CH2, 4 H), 2.400b (m, CH2, 6 H), 0.416 (s, CH3, 6 H),
0.276 (s, CH3 and ZrCH2, 20 H); 13C{1H} (C6D6, 125.8 MHz) δ 56.64 (s,
CH2), 56.08 (s, CH2), 49.96 (s, CH2), 46.80 (s, CH2), 32.96 (s, ZrCH2),
2.65 (s, CH3), 0.48 (s, CH3). (N3N)Zr(PHCy) (4): 1H (C6D6, 500.1 MHz)
δ 3.246 (t, CH2, 6 H), 2.824 (dd, PH, JPH ) 212 Hz, JHH ) 4.3 Hz, 1 H),
2.626 (m, C6H11, 1 H), 2.232 (m, C6H11, 2 H), 2.160 (t, CH2, 6 H), 1.803
(m, C6H11, 2 H), 1.612 (m, C6H11, 3 H), 1.435 (m, C6H11, 2 H), 1.234 (m,
C6H11, 1 H), 0.355 (s, CH3, 27 H); 31P{1H} (C6D6, 202.4 MHz) -26.19
(s).
1
solvents. Monitoring the reaction in benzene-d6 by H NMR
spectroscopy showed complete conversion of 1 to complex 3
under the thermolysis conditions. Complex 3 is highly related
t
to the dimethyl-tert-butylsilyl derivative, [η5-(Me2 BuSiNCH2-
CH2)2NCH2CH2NSitBuMeCH2]Zr, reported by Scott and co-
workers.8b The presence of tert-butyl substituents in the latter
complex gave well-resolved 1H NMR data, whereas significant
overlap of chemically inequivalent methylene resonances of the
ethyl groups as well as overlap of the metalated methyl group
and the trimethylsilyl substituents was observed in the 1H NMR
spectrum of complex 3.
(11) (a) Benac, B. L; Jones, R. A. Polyhedron 1989, 8, 1774-1777. (b)
Baker, R. T.; Fultz, W. C.; Marder, T. B.; Williams, I. D. Organometallics
1990, 9, 2357-2367. (c) Larsonneur, A.-M.; Choukroun, R.; Daran, J.-C.;
Cuenca, T.; Flores, J. C.; Royo, P. J. Organomet. Chem. 1993, 444, 83-
89. (d) Danopoulos, A. A.; Edwards, P. G.; Harman, M.; Hursthouse, M.
B.; Parry, J. S. J. Chem. Soc., Dalton Trans. 1994, 7, 977-985. (e)
Lindenberg, F.; Shribman, T.; Sieler, J.; Hey-Hawkins, E.; Eisen, M. S. J.
Organomet. Chem. 1996, 515, 19-25. (f) Breen, T. L.; Stephan, D. W.
Organometallics 1996, 15, 4509-4514. (g) Urne˘zˇius, E.; Klippenstein, S.
J.; Protasiewicz, J. D. Inorg. Chim. Acta 2000, 297, 181-190. (h) Urne˘zˇius,
E.; Kin-Chung, L.; Rheingold, A. L.; Protasiewicz, J. D. J. Organomet.
Chem. 2001, 630, 193-197.
(12) Other group 4 metals with phosphido ligands display similar
spectroscopic properties: (a) Roddick, D. M.; Santarsiero, B. D.; Bercaw,
J. E J. Am. Chem. Soc. 1985, 107, 4670-4678. (b) Vaughn, G. A.;
Hillhouse, G. L.; Rheingold, A. L. Organometallics 1989, 8, 1760-1765.
(c) Low-coordinate Ti-primary phosphido complexes display large JPH
values: Basuli, F.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J. J.
Am. Chem. Soc. 2003, 125, 10170-10171.
Complex 3 reacted cleanly with PhPH2 in benzene-d6 at
ambient temperature to quantitatively form phosphido 2.
(13) Interestingly, a solution of equimolar PhPH2 and PhPD2 underwent
conproportionation in the presence of catalytic 2 to give a mixture of PhPH2
(δ -123.0), PhPD2 (δ -125.4), and PhPHD (δ -124.2) in a 1:2:1 ratio.
Pure PhPHD was prepared by careful treatment of PhPHLi with 1 equiv of
D2O followed by distillation. A description of this competition experiment
can be found in the Supporting Information.
(14) (a) Fendrick, C. M.; Marks, T. J. J. Am. Chem. Soc. 1986, 108,
425. (b) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan,
M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem.
Soc. 1987, 109, 203-219. (c) Tilley, T. D. Comments Inorg. Chem. 1990,
10, 37-46.

2494 Organometallics, Vol. 26, No. 10, 2007
Communications
Scheme 1. Catalytic Cycle for the Dehydrocoupling of
Phenylphosphine.
Complex 3 is catalytically competent in the dehydrocoupling
of PhPH2, and the rate of dehydrocoupling by 3 was identical
to that of 2 within experimental error. Treatment of complex 1
with PhPD2 gave only methane and no methane-d1 as observed
1
2
by H and H NMR spectroscopy, suggesting that complex 1
initially decomposes to complex 3 before reaction with phen-
ylphosphine (eq 3). This hypothesis is supported by the
observation of deuterium incorporation into the trimethylsilyl
2
substituents in the H NMR spectrum of the product (N3N)-
ZrPDPh (2-d).
(eq 1). Complex 4 is spectroscopically analogous to phen-
ylphosphido derivative 2 and exhibited resonances at δ 2.82
1
and -26.2 (JPH ) 212 Hz) for the phosphido ligand in the H
and 31P NMR spectra, respectively.
Reaction of 1 with an excess of equimolar quantities of PhPH2
and CyPH2 (20 equiv each) in benzene-d6 gave exclusively 2
as the sole zirconium-containing product. Upon extended heating
at 90 °C, the reaction produced a new diphosphine product,
PhHP-PHCy, as identified by 31P and 1H NMR spectroscopy.
Approximately 40% of the phenylphosphine was converted to
(PhPH)2 during the course of the reaction. For the first three
turnovers of PhHP-PHCy, (PhPH)2 was not observed, and no
(CyPH)2 was observed until all phenylphosphine was consumed.
If an excess of PhPH2 was present in the reaction, then it was
consumed to form (PhPH)2 until the ratio of phosphines was
equal, at which time formation of PhHP-PHCy commenced.
Efficient separation of these products has not yet been achieved,
and efforts to improve this selectivity are underway. To the best
of our knowledge, this reaction is unique, as catalytic hetero-
dehydrocoupling of two different phosphines has not been
previously reported.
In summary, new triamidoamine zirconium complexes have
been prepared that effectively dehydrocouple primary and
secondary phosphines. These catalysts select cleanly for the
diphosphine product, a feature that may be attributable to the
constraints imparted by the triamidoamine ligand, as other early
transition-metal catalysts readily form LnM(PxRx) complexes (M
) Ti, Zr; x ) 2, 3) not observed with the triamidoamine ligand
used here.4e,5 Kinetic data and activation parameters indicate a
σ-bond metathesis mechanism for P-P bond formation in the
dehydrocoupling of phenylphosphine. The metalated complex
3 is important in this catalysis, and it appears to be an
appropriate starting point to prepare (N3N)Zr-ERn complexes
(E ) main group element). Further exploration of these reactive
zirconium species and related catalytic processes are underway.
Current evidence suggests that complex 3 is involved in the
dehydrocoupling catalysis. Benzene-d6 solutions of the deuter-
ated phosphido complex 2-d gradually exchanged the deuterium
of phosphido ligand for hydrogen from the trimethylsilyl
substituents at ambient temperature, demonstrating that eq 3 is
reversible, though it lies heavily to the right. In the catalytic
dehydrocoupling of PhPD2 (98% PhPD2, 2% PhPHD) by 3 in
benezene-d6, (PhPD)2, PhDP-PHPh, and (PhPH)2 are formed,
and the proteo products are observed in much greater proportions
than expected from the PhPD2 starting material. A steady-state
concentration of PhPH2 is formed during the course of the
catalysis, and the methyl resonances from the trimethylsilyl
substituents of 2 decreased in intensity over time, as seen by
2
1H NMR spectroscopy. In H NMR spectra of the reaction
mixture, a signal corresponding to deuterated trimethylsilyl
groups increased in intensity during the course of the catalysis.
A mechanism for the catalysis that incorporates the kinetic
data and these observations is shown in Scheme 1. First, this
dehydrocoupling requires that 2 equiv of phenylphosphine be
consumed per catalytic cycle by mass balance, which is borne
out in the kinetic data (vida supra). Additionally, the inhibition
by hydrogen suggests a reversible formation of complex 2.
However, the equilibrium between metalated derivative 3 and
phosphido 2, the facile formation of 3 under catalysis conditions,
and the inability to observe a hydride complexseven under an
atmosphere of hydrogenssuggest that complex 3 is present in
the catalytic cycle.
The catalytic dehydrocoupling of CyPH2 by 1 in benzene-d6
proceeded to (CyPH)2 at 70 °C, but that product began to convert
to (CyP)4 upon extended reaction times or at higher tempera-
tures.15 It is known that many diphosphines (PHR)2 thermally
decompose to RPH2 and (PR)n (n ) 4, 5), a facile reaction for
(CyPH)2.15b The current evidence suggests that (CyP)4 is forming
thermally, but a metal-catalyzed process cannot yet be fully
discounted. The catalyst resting state was observed to be (N3N)-
Zr(PHCy) (4) by 1H and 31P NMR spectroscopy, and the identity
of complex 4 was confirmed through an independent synthesis
Acknowledgment. This work was supported by the Uni-
versity of Vermont. Dr. Bruce Deker is acknowledged for
assistance with NMR experiments.
Supporting Information Available: Complete experimental
procedures and characterization data, catalysis details, and repre-
sentative kinetic plots. This material is available free of charge via
the Internet at http://pubs.acs.org.
(15) (a) Blaurock, S.; Hey-Hawkins, E. Zeit. Anorg. Allg. Chem. 2002,
628, 37-40. (b) Baudler, M.; Gruner, C.; Tschaebunin, H.; Hahn, J. Chem.
Ber. 1982, 115, 1739-1745.
OM070189O