Catalytic dehydrogenative coupling of stannanes by alkoxysilyl heterobimetallic complexes: Influence of the assembling ligand

Article abstract of DOI:10.1021/om9906619

The heterobimetallic complexes [(OC)₃(R′₃Si)Fe(μ-P-Z)Pd(η ³-allyl)] (1, P-Z = Ph₂PNHPPh₂ (dppa); 2, P-Z = Ph₂P(o-C₅H₄N) (Ph₂Ppy); 3, P-Z = Ph₂PCH₂PPh₂ (dppm); R′₃ = (OMe)₃, Me(OSiMe₃)₂) are very efficient catalysts for the dehydrogenative coupling of tin hydrides R₃SnH (R = Ph, ⁿBu). Besides establishing that siloxy complexes are more active catalysts than their alkoxysilyl counterparts, the present study shows that modifications in the nature of the assembling functional phosphine ligand P-Z result in variations of catalyst efficiency, expressed by the TON (overall turnover number) and TOF (maximum turnover frequency) values and lifetime. The Ph₂Ppy-bridged complexes are the most efficient catalysts, and in the case of ⁿBu₃SnH, TON and TOF values higher than 1.2 × 10⁶ and 2 × 10⁸, respectively, have been observed. A marked difference in reactivity is also observed between the diphosphine ligands dppa and dppm, with the dppa-bridged complexes being more active than their dppm analogues. The new complex [(OC)₃{(Me₃SiO)₂MeSi}Fe(μ-Ph ₂Ppy)Pd(η³-allyl)] (2b) prepared in the course of this work has been structurally characterized by X-ray diffraction.

Full text of DOI:10.1021/om9906619

444
Organometallics 2000, 19, 444-450
Ca ta lytic Deh yd r ogen a tive Cou p lin g of Sta n n a n es by
Alk oxysilyl Heter obim eta llic Com p lexes: In flu en ce of
th e Assem blin g Liga n d †
Pierre Braunstein,*,‡ J e´roˆme Durand,‡ Xavier Morise,‡ Antonio Tiripicchio,§ and
Franco Ugozzoli§
Laboratoire de Chimie de Coordination, UMR CNRS 7513, Universite´ Louis Pasteur,
4, rue Blaise Pascal, F-67070 Strasbourg Ce´dex, France, and Dipartimento di Chimica
Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Universita` di Parma,
Centro di Studio per la Strutturistica Diffrattometrica del CNR, Parco Area delle Scienze 17A,
I-43100 Parma, Italy
Received August 16, 1999
The heterobimetallic complexes [(OC)3(R′3Si)Fe(µ-P-Z)Pd(η3-allyl)] (1, P-Z ) Ph2PNHPPh2
(dppa); 2, P-Z ) Ph2P(o-C5H4N) (Ph2Ppy); 3, P-Z ) Ph2PCH2PPh2 (dppm); R′3 ) (OMe)3,
Me(OSiMe3)2) are very efficient catalysts for the dehydrogenative coupling of tin hydrides
n
R3SnH (R ) Ph, Bu). Besides establishing that siloxy complexes are more active catalysts
than their alkoxysilyl counterparts, the present study shows that modifications in the nature
of the assembling functional phosphine ligand P-Z result in variations of catalyst efficiency,
expressed by the TON (overall turnover number) and TOF (maximum turnover frequency)
values and lifetime. The Ph2Ppy-bridged complexes are the most efficient catalysts, and in
the case of nBu3SnH, TON and TOF values higher than 1.2 × 106 and 2 × 108, respectively,
have been observed. A marked difference in reactivity is also observed between the
diphosphine ligands dppa and dppm, with the dppa-bridged complexes being more active
than their dppm analogues. The new complex [(OC)3{(Me3SiO)2MeSi}Fe(µ-Ph2Ppy)Pd(η3-
allyl)] (2b) prepared in the course of this work has been structurally characterized by X-ray
diffraction.
In tr od u ction
cross-coupling reactions for fine chemical synthesis.16-19
Although numerous procedures are known to allow tin-
tin bond formation,2,3,20 new syntheses of distannanes
are still being sought. The dehydrogenative coupling of
tin hydrides appears as an efficient method. It is known
to be facilitated by reagents such as amines19 and
amides.21 Surprisingly, examples of this reaction being
catalyzed by transition-metal complexes remain
scarce.22-27 This approach is, however, of current inter-
est owing to recent advances in the synthesis of high-
The development of the chemistry of compounds
containing a tin-tin bond mainly results from the
numerous industrial,1-4 biological,2-7 and synthetic
applications of distannanes. In the last case they are
usually employed as radical sources,8-13 as precursors
to tin-metal complexes14,15 or in palladium-catalyzed
* To whom correspondence should be addressed. E-mail: braunst@
chimie.u-strasbg.fr.
† Dedicated to our friend and colleague Professor H. Vahrenkamp,
on the occasion of his 60th birthday, with our warmest congratulations
and good wishes.
(14) (a) Quintard, J . P.; Pereyre, M. Rev. Silicon, Germanium, Tin
Lead Compd. 1980, 4, 154. (b) Abel, E. W.; Moorhouse, S. J . Organomet.
Chem. 1970, 24, 687.
‡ Universite´ Louis Pasteur.
§ Universita` di Parma.
(1) Evans, C. J . In Chemistry of Tin; Harrison, P. G., Ed.; Blackie:
Glasgow, Scotland, 1989; pp 421-453, and references therein.
(2) Davies, A. G.; Smith, P. G. In Comprehensive Organometallic
Chemistry; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Perga-
mon: Oxford, U.K., 1982; Vol. 2, pp 519, and references therein.
(3) Wardell, J . L. In Encyclopedia of Inorganic Chemistry; King, R.
B., Ed.; Wiley: New York, 1994; Vol. 8, pp 4172, and references therein.
(4) Weis, R. W. Organometallic Compounds, 2nd ed.; Springer: New
York, 1973; Vol. II, first supplement, pp 826-832, and references
therein.
(15) Holt, M. S.; Wilson, W. L.; Nelson, J . H. Chem. Rev. 1989, 89,
11.
(16) Diederich, F.; Stang, P. J . Metal-Catalyzed Cross-Coupling
Reactions; Wiley-VCH: Weinheim, Germany, 1998.
(17) Farina, V.; Krishnamurthy, V.; Scott, W. J . Organic Reactions;
Wiley: New York, 1997; Vol. 50, pp 1-652, and references therein.
(18) Stille, J . K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508 and
references therein.
(19) Davies, A. G.; Osei-Kissi, D. K. J . Organomet. Chem. 1994, 474,
C8.
(20) Sita, L. R. Adv. Organomet. Chem. 1995, 38, 189 and references
therein.
(5) Davies, A. G.; Smith, P. G. Adv. Inorg. Chem. Radiochem. 1980,
23, 1 and references therein.
(6) Cusack, P.; Smith, P. G. Rev. Silicon, Germanium, Tin Lead
Compd. 1983, 7, 1.
(21) Puff, H.; Breuer, B.; Gehrke-Brinkmann, G.; Kind, P.; Reuter,
H.; Schub, W.; Wald, W.; Weideinbru¨ck, G. J . Organomet. Chem. 1989,
363, 265.
(22) Bugamin, N. A.; Gulevich, Y. V.; Beletskaya, I. P. Izv. Akad.
Nauk SSSR, Ser. Khim. 1982, 2639.
(23) Mitchell, T. N.; Amamria, A.; Killing, H.; Rutschow, D. J .
Organomet. Chem. 1986, 304, 257.
(24) Zhang, H. X.; Guibe´, F.; Balavoine, G. J . Org. Chem. 1990, 55,
1859.
(7) Selwyn, M. J . In Chemistry of Tin; Harrison, P. G., Ed.;
Blackie: Glasgow, Scotland, 1989; pp 359-396, and references therein.
(8) Pereyre, M.; Pommier, J . C. J . Organomet. Chem. 1976, 1, 161.
(9) Keck, G. E.; Byers, J . H. J . Org. Chem. 1985, 50, 5442.
(10) Storck, G.; Sher, P. J . Am. Chem. Soc. 1983, 105, 6765.
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Commun. 1984, 133.
(12) Keck, G. E.; Tafesh, A. M. J . Org. Chem. 1989, 54, 5845.
(13) Snider, B. B.; Buckman, B. O. J . Org. Chem. 1992, 57, 4883.
(25) Voskoboynikov, A. Z.; Beletskaya, I. P. New J . Chem. 1995, 19,
723.
10.1021/om9906619 CCC: $19.00 © 2000 American Chemical Society
Publication on Web 01/22/2000

Dehydrogenative Coupling of Stannanes
Organometallics, Vol. 19, No. 4, 2000 445
Ch a r t 1
molecular-weight polystannanes, a promising class of
new polymers, by dehydropolymerization of tin
dihydrides.27-30
We have recently reported that trialkoxysilyl and
siloxy heterobimetallic Fe-Pd complexes of the types
A and B were efficient catalysts for the dehydrogenative
coupling of stannanes.26,27 Interestingly, this reaction
enabled us to evaluate the role of the heterobimetallic
structure of the catalysts and the influence of the ligand
environment on the active metal center.27
Although elementary transformations appear to take
place at palladium, the silyl ligand bound to the iron
center plays a key role in the catalytic process.27 This
ligand is involved in the stabilization of reactive inter-
mediates via formation of OfPd interactions, whereas
owing to its hemilabile behavior a vacant coordination
site at the Pd center may be released for an incoming
substrate molecule (equilibrium B h C, Scheme 1). We
have observed that substituent modifications at silicon
resulted in considerable variations of catalyst perfor-
mances and lifetime. We have also established that the
heterobimetallic Fe-Pd silyl derivatives were more
efficient catalysts than mononuclear Pd complexes and
retained their bimetallic structure throughout the pro-
cess.27
as catalysts for the dehydrogenative coupling of Ph3-
n
SnH and Bu3SnH.
We have previously reported the synthesis of 1a ,31
2a ,32 and 3a ,b.32,33 The new complex [(OC)3{(Me3-
SiO)2MeSi}Fe(µ-Ph2Ppy)Pd(η3-allyl)] (2b) was obtained
as an air-stable yellow-orange powder in 64% yield
(based on Pd) by reaction of the dinuclear complex [Pd-
(η3-C3H5)(µ-Cl)]2 with 2 molar equiv of the iron metalate
K[Fe{SiMe(OSiMe3)2}(CO)3(Ph2Ppy-P)] (5)34 (obtained
by deprotonation of the hydrido complex 4; Scheme 2).
Its IR spectrum (KBr) shows three ν(CO) vibrations, at
1955 m, 1895 s, and 1873 vs cm-1, indicative of the mer
arrangement of the three carbonyl ligands around the
Fe center. The 31P{1H} NMR spectrum contains a
singlet at δ 73.3 ppm. Other spectroscopic data are given
in the Experimental Section. The structure of 2b was
confirmed by an X-ray diffraction study (Figure 1). Se-
lected bond distances and angles are given in Table 1.
The coordination geometry around the Fe atom is
distorted octahedral; that of the Pd atom involves the
Fe atom, the N atom from the Ph2Ppy ligand, and the
C(9), C(10), and C(11) carbon atoms from the allylic
group interacting in a η3 fashion. When the allylic group
is considered as occupying two coordination sites, the
environment around the Pd atom is distorted square
planar. Whereas the N-Pd-Fe angle (92.8(4)°) is
regular, the P-Fe-Pd angle of 75.6(2)° is very acute,
Sch em e 1
Herein, we report our investigations aimed at evalu-
ating the influence of the nature of the assembling
ligand on the dehydrogenative coupling of Ph3SnH and
nBu3SnH catalyzed by Fe-Pd-allyl heterobimetallic
complexes. For this purpose, the catalytic activity of
dppa (Ph2PNHPPh2), dppm (Ph2PCH2PPh2), and Ph2-
Ppy (Ph2P{o-C5H4N}) bridged derivatives has been
determined. Important variations in catalyst efficiency
and lifetime have been observed.
Resu lts
1. Ca ta lysts. The Fe-Pd heterobimetallic complexes
pictured in Chart 1 have been used in the present study
(31) Braunstein, P.; Durand, J .; Kickelbick, G.; Knorr, M.; Morise,
X.; Pugin, R.; Tiripicchio, A.; Ugozzoli, F. J . Chem. Soc., Dalton Trans.
1999, 4175.
(32) Braunstein, P.; Faure, T.; Knorr, M.; Balegroune, F.; Grandjean,
D. J . Organomet. Chem. 1993, 462, 271.
(33) Braunstein, P.; Knorr, M.; Piana, H.; Schubert, U. Organome-
tallics 1991, 10, 828.
(34) Knorr, M.; Braunstein, P. Bull. Soc. Chim. Fr. 1992, 129, 663.
(26) Braunstein, P.; Morise, X.; Blin, J . J . Chem. Soc., Chem.
Commun. 1995, 14, 1455.
(27) Braunstein, P.; Morise, X. Organometallics 1998, 17, 540.
(28) Imori, T.; Lu, V.; Cai, H.; Tilley, T. D. J . Am. Chem. Soc. 1995,
117, 9931.
(29) Babcock, J . R.; Sita, L. R. J . Am. Chem. Soc. 1996, 118, 12481.
(30) Lu, V.; Tilley, T. D. Macromolecules 1996, 29, 5763.

446 Organometallics, Vol. 19, No. 4, 2000
Braunstein et al.
Sch em e 2
F igu r e 1. View of the molecular structure of complex 2b.
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 2b
Pd-Fe
Pd-C9
Pd-C11
Fe-P
Fe-C1
Fe-C3
Si1-O5
Si2-O4
N-C4
O1-C1
O3-C3
2.678(4)
2.20(3)
2.18(3)
2.210(6)
1.71(2)
1.77(2)
1.66(2)
1.55(2)
1.32(2)
1.20(2)
1.18(2)
Pd-N
2.114(16)
2.12(4)
1.44(4)
2.331(6)
1.75(2)
1.65(2)
1.84(2)
1.58(2)
1.34(3)
1.17(3)
1.35(4)
Pd-C10
C10-C11
Fe-Si1
Fe-C2
probably because of the narrow P-C(4)-N angle (109.7-
(13)°) and the rather long Fe-Pd bond (2.678(4) Å). The
value of the Fe-Pd bond length falls in the range (2.544-
(4)-2.850(2) Å) of those reported for similar structures
containing a P,P (dppm or dppa) bridging ligand.31-33,35,36
2. Deh yd r ogen a tive Cou p lin g of R3Sn H. The data
reported below refer to average values determined for a
minimum of three catalytic runs. Note that in the
presence of a large excess of radical scavenger (TEMPO,
galvinoxyl) no significant variations of the catalysis
results were observed.
Si1-O4
Si1-C24
Si3-O5
N-C8
O2-C2
C9-C10
Fe-Pd-N
92.8(4)
Pd-Fe-P
75.6(2)
67.4(6)
108.0(9)
81.6(7)
84.1(6)
98.7(8)
178.0(2)
152.0(10)
176.1(22)
109.7(13)
C2-Fe-Pd
C2-Fe-C3
C1-Fe-C2
Si1-Fe-C2
P-Fe-C3
82.7(7)
140.7(10)
105.9(10)
82.8(7)
98.0(7)
94.2(6)
172.5(13)
178.0(17)
174.2(18)
129(3)
C3-Fe-Pd
C1-Fe-C3
Si1-Fe-C3
Si1-Fe-C1
P-Fe-C2
P-Fe-Si1
Si1-O5-Si3
Fe-C2-O2
P-C4-N
(i) P h 3Sn H. The catalytic activity may be monitored
by either the formation of Ph6Sn2 (3a ) or the volume of
H2 released (2a ,b) (eq 1),26,27 as described in the
Experimental Section. Note that, under the reaction
P-Fe-C1
Si1-O4-Si2
Fe-C1-O1
Fe-C3-O3
C9-C10-C11
lifetimes, are observed, as evidenced by the plateau
being reached after ca. 48 h. The overall catalytic
activity of the dppa complex 1a (TON ) 1550) is,
however, more than twice that of its dppm analogue 3a
(TON ) 700). This may be related to the higher
maximum reaction rate (TOF; see Table 2) observed for
1a . The situation for the Ph2Ppy-bridged complex 2a
is, however, somewhat different. The reaction is much
faster (the plateau is reached after only ca. 30 min) and
the TOF value is considerably higher (ca. 1.70 × 105,
Table 2). However, this rate is only maintained for a
few seconds after an induction period of ca. 5 s, resulting
in the total catalytic activity being in the same range
as that of 1a (TON ) 1460; Table 2). For comparison,
we found it of interest to determine the catalytic activity
conditions used, the decrease in reaction rate and
eventually the end of the reaction are due to the fading
of the catalyst activity and not to a decrease in substrate
concentration.26
Figure 2 shows the plot of TON vs time for the
trimethoxysilyl complexes 1a , 2a , and 3a , and selected
data are given in Table 2. For the diphosphine-contain-
ing catalysts 1a and 3a long reaction times, or catalyst
(35) Braunstein, P.; Knorr, M.; Schubert, U.; Lanfranchi, M.;
Tiripicchio, A. J . Chem. Soc., Dalton Trans. 1991, 1507.
(36) Braunstein, P.; Knorr, M.; Tiripicchio, A.; Tiripicchio-Camellini,
M. Angew. Chem., Int. Ed. Engl. 1989, 28, 1361.

Dehydrogenative Coupling of Stannanes
Organometallics, Vol. 19, No. 4, 2000 447
Ta ble 3. Selected Ca ta lysis Da ta for th e
Deh yd r ogen a tive Cou p lin g of Bu 3Sn H Ca ta lyzed
by 1a , 2a , or 3a
catalyst
TON maxa
plateau, minb
TOFc
1a
ca. 500 000
>1 200 000
ca. 114 000
120
d
1
ca. 5 × 106
ca. 23 × 106
2a
d
3a 27
a
Total catalytic activity expressed in turnover numbers (aver-
b
age value of a minimum of three experiments). Time after which
the plateau is reached. c Maximum turnover frequency expressed
in TON h-1
.
Not determined.
d
F igu r e 2. Catalytic dehydrogenative coupling of Ph3-
SnH: plot of TON vs time for the complexes 1a , 2a , and
3a .
Ta ble 2. Selected Ca ta lysis Da ta for th e
Deh yd r ogen a tive Cou p lin g of P h 3Sn H Ca ta lyzed
by 1a , 2a ,b, or 3a ,b
catalyst
TON maxa
plateau, hb
TOFc
1a
1550
1460
700
1850
1630
48
0.5
48
0.25
1
ca. 260
2a
ca. 1.70 × 105
3a 27
2b
ca. 100
ca. 2.10 × 105
ca. 1.98 × 105
F igu r e 4. Catalytic dehydrogenative coupling of nBu3-
SnH: plot of TON vs time for the complexes 1a and 3a .
3b27
a
Total catalytic activity expressed in turnover numbers (aver-
b
age value of a minimum of three experiments). Time after which
the plateau is reached. c Maximum turnover frequency expressed
for 1a and 3a are shown in Figure 4, which illustrates
the difference between the dppa and dppm complexes.
Very high reaction rates and TON values, accompanied
by a short catalyst lifetime (ca. 1 min), have been
reported for the dppm complex 3a (Table 3).27 For the
dppa complex 1a , the reaction proceeds more slowly, the
TOF value is ca. 5 × 106, and the plateau is only reached
after 2 h (Figure 4, Table 3). This enhanced catalyst
lifetime combined with a slow decrease of the reaction
rate (after 8 min the turnover frequency is still >1 ×
106) account for 1a being a much more efficient catalyst
than 3a (TON values of 500 000 and 114 000, respec-
tively). In the case of the Ph2Ppy complex 2a no reliable
catalysis data could be obtained, the reaction being
extremely fast. We observed the consumption of more
in TON h-1
.
n
than 2.5 × 106 molar equiv of Bu3SnH within a few
seconds.
F igu r e 3. Catalytic dehydrogenative coupling of Ph3-
SnH: comparative plot of TON vs time for the pairs of
complexes 2a and 2b (same assembling ligand) and 2b and
3b (same siloxy ligand).
Discu ssion
Mechanistic aspects of the dehydrogenative coupling
of tin hydrides catalyzed by heterobimetallic Fe-Pd
alkoxysilyl and siloxy complexes have been discussed
in a previous paper.27 Despite the very high reaction
rates observed in the case of alkylstannanes, a σ-bond
metathesis type mechanism or a succession of oxidative-
addition and reductive-elimination steps were preferred
to a radical chain mechanism. This is corroborated by
the fact that radical scavengers, such as TEMPO or
galvinoxyl, did not affect the catalysis measurements.
Although it is always difficult to be sure of the true
nature of a catalytic species, particularly in systems that
display activities as high as those encountered in this
work, we believe from in situ spectroscopic measure-
ments on the less active systems (which allowed a
higher catalyst concentration) that the bimetallic frame-
work of the catalyst is retained throughout the reaction
of the Ph2Ppy-bridged siloxy complex 2b. It is a more
efficient catalyst than its trimethoxysilyl counterpart
2a (higher TON and TOF values; Table 2). However, it
displays a shorter lifetime (plateau reached after ca. 15
min). Figure 3 illustrates the comparison between 2a
and 2b, which have the same assembling ligand, and
between 2b and 3b, which have the same siloxy ligand.
The last complex has been investigated previously.27
(ii) n Bu 3Sn H. The reactivity of the trimethoxysilyl
complexes 1a and 2a has also been evaluated with
regard to the dehydrogenative coupling of nBu3SnH. The
volume of H2 released was monitored and converted into
turnover numbers. Selected catalysis data are given in
Table 3 and compared with those previously reported
for 3a .27 The plots of turnover numbers vs time obtained

448 Organometallics, Vol. 19, No. 4, 2000
Braunstein et al.
and that elementary transformations take place at Pd.
The role of the iron fragment is therefore to provide the
Pd center with a suitable ligand environment through
the metal-metal bonding, the assembling phosphine,
and the silyl ligand. The last species is involved in the
stabilization of reactive intermediates by its ability to
develop bridging SiOfPd interactions. As a result,
substituent modifications at silicon should lead to
important variations in catalyst performances and
lifetime. In the present study, we have indeed found
that siloxy complexes are much more active catalysts
(precursors) in the dehydrogenative coupling of Ph3SnH
than their trialkoxysilyl counterparts: higher TONs and
TOFs are obtained for 2b and 3b than for their tri-
alkoxysilyl analogues 2a and 3a , respectively. This
would be related to the oxygen donor capacity of siloxy
ligands (Fe-Si-O-SiR3) being lower than that of
alkoxysilyl ligands (Fe-Si-O-CR3), owing to the more
electropositive character of the Si atom. Consequently,
the SiOfPd interactions are less stabilizing in the
former case, providing a more accessible coordination
site at Pd for an incoming substrate molecule (stan-
nane). This results in higher reaction rates and TONs.
However, a shorter catalyst lifetime is observed, owing
to this lower stabilization (see, for example, catalysis
data for 3a and 3b; Table 2).
center is changed. Thus, complexes 2, in which a
pyridine ligand is bound to Pd, are much more active
than their dppa and dppm counterparts (Figures 2-4,
Tables 2 and 3). Not only are higher TONs obtained,
but improved TOFs are also observed. However, this
higher reactivity is accompanied by a shorter lifetime
of the catalysts. In the case of the dehydrogenative
coupling of nBu3SnH, the reaction rate is too high to be
determined. We estimate that it is >2 × 108 TON h-1
.
Although various factors may be invoked, it seems
reasonable to assume that the variations of reactivity
of complexes 1-3 for the dehydrogenative coupling of
tin hydrides result from the assembling ligands having
different electronic and steric influences on the Pd
center. Thus, the higher reactivity and shorter lifetime
of 2, in comparison to 1 and 3, may probably be ascribed
to the lower electron donating property37 and/or the
lower steric hindrance of the pyridine ligand vs the Ph2P
groups of dppm and dppa. When 2b is compared with
its dppm analogue,32 the electronic effect of the pyridine
compared to the Ph2P group also manifests itself in the
downfield shift of the 1H NMR resonances of the protons
H1 and H2 of the CH2 group trans to the nitrogen (see
Experimental Section). The relative effects of the diphos-
phine ligands in complexes 1 and 3 appear more subtle.
One factor might be the lower electron-donating prop-
erty of dppa compared to that of dppm. Indeed, the ν-
(CO) vibrations of the iron carbonyls are observed at
higher wavenumbers for dppa-containing Fe mono-
nuclear or Fe-Pd heterobimetallic complexes compared
to their dppm analogues.31 Structural aspects at Pd may
also be considered. Indeed, we have observed slight
We have verified the purity of the distannanes
isolated after catalysis by 119Sn NMR spectroscopy (see
Experimental Section). Analysis of the reaction mixtures
did not indicate the presence of products resulting from
rearrangement of the substituents at tin. Only in the
case of nBu3SnH were traces (<2%) of (nBu3Sn)2O
detected, which originate from adventitious oxidation.
differences in the bond distances and angles of [(OC)3Fe-
The nature of the assembling phosphine in these
bimetallic complexes plays a major role in providing
additional stability.35,36 It is certainly involved in the
retention of the bimetallic structure of the catalysts
during the reaction process, a factor which may largely
contribute to the higher efficiency of Fe-Pd complexes
in the catalytic dehydrogenative coupling of stannanes
compared to Pd mononuclear complexes. Furthermore,
the assembling ligand exerts a direct influence on the
catalyst performances through its functional group
coordinated to the active Pd center. This accounts for
the important variations observed in the TON and TOF
values as well as in catalyst lifetime for the dppa,
Ph2Ppy, and dppm complexes 1-3, respectively. In the
case of complexes 1a and 3a , replacement of the CH2
group in 3a by the amino function NH leads to improved
catalyst efficiency. Indeed, the TON and TOF values
obtained with 1a for the dehydrogenative coupling of
Ph3SnH are more than twice those found for 3a , where-
as long lifetimes are noted for both catalysts (Table 2).
The situation is slightly different with nBu3SnH. Sur-
prisingly, the TOF observed for 1a , although it is very
high (ca. 5 × 106), is now lower than that obtained with
3a (ca. 23 × 106). However, a much longer lifetime and
only a slow decrease in the reaction rate (a turnover
frequency >1 × 106 is still observed after 8 min) com-
pensate for this drawback and account for the former
being a more efficient catalyst than the latter (Table 3).
{µ-Si(OMe)2(OMe}(µ-P,P)PdCl] (6, P,P ) dppm;36 7, P,P
) dppa).31 For example, Pd-O and Pd-Cl bond dis-
tances are longer in 7, whereas the P-N-P angle in 7
(121.8(12)°) is more obtuse than the P-C-P angle in 6
(112.0(1)°). It is noteworthy that these complexes are
stabilized by a SiOfPd interaction and are thus analo-
gous to intermediates formed during the catalytic
process.27
Besides the fact that we have developed highly active
catalysts for the formation of distannanes, the dehy-
drogenative coupling of tin hydrides represents a re-
markable tool for studying the reactivity of heterobi-
metallic Fe-Pd silyl systems. Comparisons between
catalysts have involved modification of one of the
variables while maintaining the others constant. After
having witnessed the effect of the metal-metal bonding
and substituent modifications at Si on the catalytic
activity and reaction kinetics, we have now observed
that the catalyst performances are also dependent on
the nature of the assembling difunctional ligand. We
have thus established that Ph2Ppy-bridged systems are
more active than their dppa analogues, themselves
being more active than dppm-bridged complexes. We
have now the possibility of tuning the reactivity of the
Pd center, in heterobimetallic Fe-Pd silyl complexes,
by modifying the nature of the ligands carried by the
Fe fragment. This work represents a further insight into
As expected, more important effects on catalytic
performances are observed when the nature of the donor
group of the assembling ligand coordinated to the Pd
(37) Basolo, F.; Pearson, R. G. Mechanisms of Inorganic Reactions,
2nd ed.; Wiley: New York, 1967; pp 351-453.

Dehydrogenative Coupling of Stannanes
Organometallics, Vol. 19, No. 4, 2000 449
reduced pressure, and the residue was dissolved in diethyl
ether and filtered over a 5 cm Celite pad placed in a pipet.
The solvent was then removed in vacuo, affording complex 2b
as a yellow-orange air-stable powder (1.33 g, yield 72%). IR
homogeneous bimetallic catalysis and contributes to a
better understanding of the influence of a metal center
(Fe) on the reactivity of the adjacent metal (Pd).38
1
(KBr): 1955 m, 1895 s, 1873 vs cm-1. H NMR (C6D6, 298 K;
Exp er im en ta l Section
for the labeling of the allyl protons, see below): δ 0.49 (s, 18H,
3
OSiMe3), 2.82 (d, 1H, J (H-H) ) 11.25 Hz, allyl H3), 3.11 (d,
All the reactions and manipulations were carried out under
an inert atmosphere of purified nitrogen using standard
Schlenk-tube techniques. Solvents were dried and distilled
under nitrogen before use: hexane and toluene over sodium,
tetrahydrofuran and diethyl ether over sodium-benzophenone,
dichloromethane over phosphorus pentoxide. Nitrogen (Air
liquide, R-grade) was passed through BASF R3-11 catalyst and
molecular sieve columns to remove residual oxygen and water.
Elemental C, H, and N analyses were performed by the Service
de microanalyses du CNRS (ULP Strasbourg). Infrared spectra
were recorded on a IFS 66 Bruker FT-IR spectrometer. The
1H and 31P{1H} spectra were recorded at 300.1 and 121.5 MHz,
respectively, on a Bruker AM300 instrument and the 119Sn
NMR spectra at 149 MHz on a Bruker AC400 instrument. Fe-
(CO)5 was obtained from Aldrich, and HSiMe(OSiMe3)2 was
provided by Rhoˆne-Poulenc Spe´cialite´s Chimiques; both were
used as received. The complexes [Pd(η3-allyl)(µ-Cl)]2,39 1a ,31
2a ,32 3a ,33 and 3b32 were prepared according to published
procedures. Ph3SnH was obtained by reduction of Ph3SnCl
with LiAlH4 in diethyl ether.40 nBu3SnH was a commercial
sample (Lancaster) and was used as received. Experimental
errors on the TON and TOF values have been estimated to
5%; values given have been rounded off.
1H, 3J (H-H) ) 12.85 Hz, allyl H2), 3.66 (d, 1H, 3J (H-H) )
6.26 Hz, allyl H4), 4.29 (br, 1H, allyl H1), 5.04 (m, 1H, allyl
H5), 6.12-8.30 (m, 14H, aromatics). 31P{1H} NMR (C6D6, 298
K): 73.3. Anal. Calcd for C30H40FeNO5PPdSi3: C, 46.67; H,
5.22; N, 1.81. Found: C, 46.94; H, 5.25; N, 1.86.
Deh yd r ogen a tive Cou p lin g of P h 3Sn H Ca ta lyzed by
1a , 2a ,b , a n d 3a ,b . Gen er a l P r oced u r e. A Schlenk flask
equipped with a stirring bar and a serum cap was charged
with 4.00 g of Ph3SnH (11.4 mmol) in 15 mL of CH2Cl2 and
was placed in a water bath at 293 K.
Syn th esis of [(OC)3F e{SiMe(OSiMe3)2}(µ-P h 2P p y)P d -
(η3-a llyl)] (2b). A magnetically stirred solution of [Fe(CO)5]
(1.57 mL, 12 mmol) and HSiMe(OSiMe3)2 (5.00 g, 22.5 mmol)
in hexane (100 mL) was irradiated in a 2-propanol-cooled
photochemical reactor at 283 K for ca. 4 h. Irradiation with a
mercury lamp (180 W, TQ 150, Hereaus) was stopped when
the ν(CO) absorptions of [Fe(CO)5] (2001 and 2023 cm-1) had
almost completely disappeared. The resulting complex, cis-
[HFe{SiMe(OSiMe3)2}(CO)4] (ν(CO) (hexane) 2096 m, 2032 sh,
2026 vs, 2013 vs cm-1), was not isolated, and the resulting
pale yellow mixture was added immediately to a toluene
solution (50 mL) of (2-diphenylphosphino)pyridine (2.89 g, 11
mmol) in two portions. After each addition the CO evolved was
removed under reduced pressure for 1 min. The solution was
stirred for 5 min at room temperature and then placed at -20
°C for 2-3 days, which afforded mer-[HFe(CO)3{SiMe(O-
SiMe3)2}(Ph2Ppy-P)] (4) as a pale brown air- and moisture-
sensitive solid (4.40 g, yield 64%), which was rapidly engaged
in further reactions. IR (toluene): 2051 w, 1987 s, 1975 vs
H2 Mon itor in g (2a ,b a n d 3b). When the volume of H2
released was monitored, the procedure was as follows: the
Schlenk flask was fitted onto a gas buret and 1.0 mL of a 0.1
mM solution of catalyst (10-4 mmol) was rapidly added to the
reaction mixture via syringe through the serum cap. The
volume of H2 released was directly read on the graduated
buret, taking into account the content of the syringe. The
turnover numbers were calculated by the following equation:
TON ) moles of H2/mol of catalyst, with moles of H2 being
determined by applying the gas equation: PV ) nRT. Each
experiment was repeated at least three times, and the mean
values were used for the plots. When the reaction was over,
the volatiles were removed in vacuo and the residue was
washed with Et2O, affording Ph6Sn2 as a white solid (mp 225-
1
235 °C;41,42 the H NMR spectrum only showed signals in the
aromatic region). The TON determined from the amount of
Ph6Sn2 recovered was in all cases in good accordance with that
determined from the volumes of H2.
P h 6Sn 2 Mon itor in g (1a a n d 3a ). When the formation of
Ph6Sn2 was monitored, the procedure was as follows: a battery
of five Schlenk tubes was prepared as described above, except
that they were not connected to gas burets. The catalyst was
then added, and the reaction mixtures were successively
quenched at t ) 1, 4, 8, 24, and 72 h by addition of water. The
volatiles were removed under reduced pressure, and the
residue was washed with Et2O, affording pure Ph6Sn2 as a
white solid, which was weighed. The turnover numbers were
determined by the following equation: TON ) (moles of Ph6-
Sn2)/(moles of catalyst). The purity of Ph6Sn2 was controlled
by 119Sn NMR spectroscopy (149 MHz, CH2Cl2/C6D6): δ -143.7
ppm with satellites, similar to the literature value of -144
ppm (CDCl3).43 A catalytic reaction was carried out in only 5
mL of CH2Cl2 in order to obtain a concentrated supernatant
solution, which was examined by 119Sn NMR spectroscopy
before quenching. It contained more than 88% Ph6Sn2 and
small amounts of Ph3SnCl (δ -44.7 ppm) and Ph3SnH (δ -165
cm-1. H NMR (C6D6, 298 K): δ -8.87 (d, J (P-H) ) 26 Hz,
1H, Fe-H), 0.31 (s, 18H, OSiMe3), 0.93 (s, 3H, Fe-SiMe), 6.3-
8.5 (m, 14H, aromatics). 31P{1H} NMR (C6D6, 298 K): 63.2.
Solid complex 4 (1.50 g, 2.4 mmol) was added to a suspen-
sion of excess KH in THF (20 mL). An immediate gas evolution
was noticed (H2), whereupon the color darkened. The formation
of K[Fe{SiMe(OSiMe3)2}(CO)3(Ph2Ppy-P)] (5) was complete
after ca. 30 min. The solution was then filtered over a 1 cm
Celite pad and slowly added to a THF solution (10 mL) of [Pd-
(η3-allyl)(µ-Cl)]2 (0.44 g, 1.2 mmol) at 233 K. The reaction
mixture was warmed to room temperature and stirred for 1 h
before it was filtered. The volatiles were removed under
1
2
(38) (a) Braunstein, P.; Rose´, J . In Comprehensive Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Pergamon: Oxford, U.K., 1995; Vol. 10, pp 351-385. (b) Braunstein,
P.; Rose´, J . In Metal Clusters in Chemistry; Braunstein, P., Oro, L. A.,
Raithby, P. R., Eds.; Wiley-VCH: Weinheim, Germany, 1999; Vol. 2,
pp 616-677. (c) Braunstein, P.; Knorr, M.; Stern, C. Coord. Chem. Rev.
1998, 178-180, 903.
(39) Sakakibara, M.; Takahashi, Y.; Sakai, S.; Ishii, Y. J . Chem. Soc.,
Chem. Commun. 1969, 396.
(40) Kuivila, H. G.; Sawyer, A. K.; Armour, A. G. J . Org. Chem. 1961,
26, 1426.
(41) Tamborski, C.; Ford, F. E.; Soloski, E. J . J . Org. Chem. 1963,
28, 237.
(42) Gilman, H.; Rosenberg, S. D. J . Org. Chem. 1953, 18, 1554.
(43) Adams, S.; Dra¨ger, M.; Mathiasch, B. J . Organomet. Chem.
1987, 326, 173.

450 Organometallics, Vol. 19, No. 4, 2000
Braunstein et al.
Ta ble 4. Cr ysta llogr a p h ic Da ta for Com p lex 2b
intensities were corrected for Lorentz and polarization. An
empirical absorption correction was applied (maximum and
minimum transmission factors 1.00-0.598).44 The structure
was solved by direct methods using SIR9245 and refined by
blocked full-matrix least squares based on F using SHELX76.46
Only the Pd, Fe, P, and Si atoms were refined anisotropically.
All the hydrogen atoms, except those of the allyl group, were
placed at their at their geometrically calculated positions and
refined “riding” on their corresponding carbon atoms. All the
calculations were carried out on the GOULD ENCORE91
computer of the Centro di Studio per la Strutturistica Diffrat-
tometrica of the CNR, Parma, Italy.
Atomic coordinates of the non-hydrogen atoms (Table S1;
Supporting Information), thermal parameters for the non-
hydrogen atoms (Table S2), atomic coordinates of the hydrogen
atoms (Table S3), and all bond distances and angles (Table
S4) have been deposited at the Cambridge Crystallographic
Data Centre.
mol formula
mol wt
cryst syst
space group
a, Å
C30H41FeNO5PdSi3
773.14
orthorhombic
Pna21
17.888(2)
23.967(4)
8.652(2)
3709(1)
4
b, Å
c, Å
V, Å3
Z
D
calcd, g cm-3
1.384
F(000)
1588
µ(Cu KR), cm-1
no. of measd rflns
no. of obsd data measd (I g 2σ(I))
no. of unique data obsd
no. of variables
GOFa
10.51
6616
1807
1024
200
1.008
0.037
0.039
R(Fo)b
Rw(Fo)c
a
b
GOF ) [∑w(|Fo| - |Fc|)2/(Nobservns - Nvariables)]1/2
.
R ) ∑||Fo|
Ack n ow led gm en t. Financial support from the Cen-
tre National de la Recherche Scientifique (Paris), the
Consiglio Nazionale delle Ricerche (Roma), and the
Ministe`re de l′Education Nationale, de la Recherche et
de la Technologie (Paris) (Ph.D. grant to J .D.) are
gratefully acknowledged. We also thank Rhoˆne-Poulenc
Spe´cialite´s Chimiques for a gift of HSiMe(OSiMe3)2.
- |Fc||/∑|Fo|. c Rw ) [∑w(|Fo| - |Fc|)2/∑w(Fo)2]1/2
.
ppm); no resonance for (Ph3Sn)2O (δ -82.7 ppm), which could
have formed by adventitious oxidation, was observed.
Deh yd r ogen a tive Cou p lin g of n Bu 3Sn H. A procedure
similar to that described above for the H2 monitoring in
dehydrogenative coupling of Ph3SnH was used, with typically
5.00 g of nBu3SnH (17.2 mmol) and 0.5 mL of a 3.45 × 10-5
M
Su p p or tin g In for m a tion Ava ila ble: Tables of X-ray
crystallographic data for 2b. This material is available free of
charge via the Internet at http://pubs.acs.org.
solution of the catalyst ((moles of nBu3SnH)/(moles of catalyst)
) ca. 1 000 000), except in the case of catalyst 3a , which was
used as a 6.9 × 10-6 M solution. A vigorous H2 evolution was
immediately observed in all cases. Analysis of the catalytic
mixture by 119Sn NMR spectroscopy showed the very intense
signal of nBu6Sn2 at δ -83 ppm and a very weak signal (<2%)
at δ 83 ppm, which corresponds to the value for (nBu3Sn)2O.
X-r a y Str u ctu r e Deter m in a tion of 2b. X-ray data were
collected on a Philips PW 1100 diffractometer using graphite-
monochromated Mo KR radiation (λ ) 0.710 73 Å). Crystal
data and experimental details are summarized in Table 4. The
OM9906619
(44) Walker, N.; Stuart, D. Acta Crystallogr., Sect. A 1983, 39, 158.
Ugozzoli, F. Comput. Chem. 1987, 11, 109.
(45) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giaco-
vazzo, C.; Guagliardi, A.; Polidori, G. SIR92. J . Appl. Crystallogr. 1994,
27, 435.
(46) Sheldrick, G. SHELX76, Program for Crystal Structure Deter-
minations; University of Cambridge, Cambridge, U.K., 1976.