Niobium/rhodium bimetallic complexes: synthesis, structure, and catalytic hydrosilylation of acetophenone and benzaldehyde

Article abstract of DOI:10.1021/om900363r

Reactions of a new imido phosphido complex, Cp₂Nb(=NBu ^t)(PPh₂) (1), with olefin complexes of rhodium afford two types of compounds. With [(μ-C1)Rh(C₂H₄) ₂]₂, an imido/phosphido-

Full text of DOI:10.1021/om900363r

4500 Organometallics 2009, 28, 4500–4506
DOI: 10.1021/om900363r
Niobium/Rhodium Bimetallic Complexes: Synthesis, Structure, and
Catalytic Hydrosilylation of Acetophenone and Benzaldehyde
Somying Leelasubcharoen,† Pavel A. Zhizhko,† Lyudmila G. Kuzmina,‡
Andrei V. Churakov,‡ Judith A. K. Howard,§ and Georgii I. Nikonov*,†
†Chemistry Department, Brock University, 500 Glenridge Avenue, St. Catharines, L2S 3A1, Ontario,
Canada, ‡Institute of General and Inorganic Chemistry RAS, Leninskii Prosp. 31, 119991 Moscow, Russian
Federation, and §Chemistry Department, University of Durham, South Road, Durham DH1 3LE, U.K.
Received May 7, 2009
Reactions of a new imido phosphido complex, Cp2Nb(dNBut)(PPh2) (1), with olefin complexes of
rhodium afford two types of compounds. With [(μ-Cl)Rh(C2H4)2]2, an imido/phosphido-bridged
complex, Cp(Cl)Nb(μ-NBut)(μ-PPh2)RhCp (2), characterized by NMR and X-ray diffraction, was
prepared. Formation of 2 represents a rare example of a Cp/Cl exchange reaction in organometallic
chemistry. When 1 reacts with [(μ-Cl)Rh(COD)]2, the target phosphido-bridged bimetallic complex
Cp2Nb(dNBut)(μ-PPh2)Rh(Cl)(COD) (4) is formed. The structure of this product was established
by NMR and an X-ray diffraction study. Both 2 and 4 are catalyst precursors for hydrosilylation of
benzaldehyde and acetophenone. The activity of the neutral complex 4 exceeds that of the cation
[Cp2Nb(NBut)(PPh2)Rh(COD)]þ derived from 4 by chloride anion abstraction.
Introduction
was based on unsaturated group 4 metal moieties,1,4 and
relatively little is known about the behavior of group 5
derivatives.4b,5
Heterobimetallic complexes1 have recently received re-
newed attention because the cooperative action of both
metal atoms allows them to activate robust bonds and
mediate complex transformations.2,3 All successful examples
of active bimetallic systems are almost exclusively based on
metals from the right (Rh, Pt, Cu, etc.) or middle part of
transition series.3 Despite the fact that complexes featuring
both early and late transition metals, i.e., early late hetero-
bimetallics (ELHB), have been the subject of intensive
research over the last 25 years,1 relatively little is known
about their catalytic activity.1b,4 Previous work in this area
was primarily based on the idea that electron-deficient early
transition metals should serve as intramolecular Lewis acidic
centers of ELHB, assisting the transformation of organic
molecules such as carbonyls in the coordination sphere of
late transition metals (LTM), thus facilitating catalysis.1a,4c
For this reason, the earlier design of most ELHB complexes
Another approach to a rational design of a bimetallic
catalyst consists in the incorporation of one of the metal
atoms into the ligand framework of the other catalytically
active center, which is usually a LTM center. This can
facilitate chemical transformation on the LTM by means
of fine adjustment of ligand bonding capabilities. Such an
approach has previously recommended itself in the catalytic
Suzuki-Miyaura reaction mediated by phosphido-bridged
Re/Pd and Ru/Pd complexes.6 In this case, the polarization
of the Mδþ-Pδ- bond (M = Re or Ru) makes the phos-
phorus atom a better donor to the palladium atom. One can
expect that metallophosphides, MPR2, based on the electro-
positive early transition metals will result in even greater
polarization of the metal-phosphorus bond,6a which will
further increase the bonding capabilities of the phosphido
ligand toward a catalytically active late transition metal.
Surprisingly, although a number of phosphido-bridged
ELHBs are known,1a,1b,7 their application to organic synth-
esis and catalysis is scarce.4a
*To whom correspondence should be addressed. Tel: (905)6885550,
ext 3350. Fax: (905)9328846. E-mail: gnikonov@brocku.ca.
(1) Reviews: (a) Stephan, D. W. Coord. Chem. Rev. 1989, 95, 41–107.
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Angew. Chem., Int Ed. 2000, 39, 2659–2678.
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Organometallics 2003, 22, 2944–2945. (b) Torkelson, J. R.; Antwi-Nsiah, F.
H.; McDonald, R.; Cowie, M.; Pruis, J.; Jalkanen, K. J.; DeKock, R. L. J. Am.
Chem. Soc. 1999, 121, 3666–3683. (c) Trepanier, S. J.; Sterenberg, B. T.;
McDonald, R.; Cowie, M. J. Am. Chem. Soc. 1999, 121, 2613–2614.
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r
pubs.acs.org/Organometallics
Published on Web 07/13/2009
2009 American Chemical Society

Article
Hemilabile ligands play a prominent role in modern
Organometallics, Vol. 28, No. 15, 2009 4501
chloride substitution by MeLi to give Cp2Nb(dNBut)(Me)
has been previously described.12 Although the 31P NMR
signal of 1 was not detectable due to the fast relaxation on the
93Nb nucleus (spin I = 9/2, natural abundance 100%), its
position can be established from a cross-peak in the 1H-31P
HSQC NMR between the ortho-protons of the phenyl group
and the phosphorus signal at δ = -0.2 ppm.
catalysis.8 They usually feature a soft center (usually based
on C, S, or P), which ensures strong binding to the metal, and
a hard center (O or N based), which can bind loosely to
a vacant metal site, thus creating a masked form of an
unsaturated complex. We suggested that the attachment of
hard and soft ligands such as imido and phosphido to an
early transition metal would give rise to a new type of
“spectator” metalloligand9 that could be used as a hemi-
labile ligand to a catalytically active late transition metal.
Herein we report the application of this concept to the
preparation of imido phosphido niobocene Cp2Nb(dNBut)-
(PPh2) (1) and the synthesis of some Nb/Rh heterobimetal-
lics derived therefrom. In the course of our studies, we
encountered an unusual exchange of Cp and chloride ligands
between two metal centers. Application of the new Nb/Rh
ELHB to catalytic hydrosilylation of acetophenone and
benzaldehyde is also reported.
Preparation of Bimetallic Complexes: Reaction with [(μ-
Cl)(Rh(C2H4)2]2. Diolefin complexes of Rh(I) in combination
with phosphine, NHC carbenes, or related two-electron do-
nors are common catalysts of hydrosilylation of carbonyls.14,15
Several families of related hemilabile ligands were applied in
this catalytic system.15c,15d Bearing this in mind, we targeted
the preparation of an adduct between the imido-phosphide 1
and [(μ-Cl)(Rh(C2H4)2]2. To our surprise, the reaction between
1 and 0.5 equiv of [(μ-Cl)(Rh(C2H4)2]2 in toluene resulted in
the exchange compound Cp((Cl)Nb(μ-NBut)(μ-PPh2)RhCp
(2) featuring the Cp ligand at rhodium and the chloride at
niobium (eq 2). This is a very rare example of Cp ring
substitution at an early transition metal center.16 The 1H
NMR spectrum of 2 exhibits two Cp resonances, 4.88 and
5.42 ppm, of equal intensity that integrate as five protons each
relative to the signals of the Ph and But groups. Correlation of
these proton signals with the 13C NMR signals at 87.3 and
Results and Discussion
Preparation of Cp2Nb(dNBut)(PPh2) (1). Our initial strat-
egy to prepare complex 1 was based on the insertion/depro-
tonation method developed in this laboratory10 and by the
group of Moıse.11 Namely, we anticipated that the reaction
between Cp2Nb(dNBut)(H)12 and ClPR2 would give the
insertion product [Cp2Nb(dNBut)(PHR2)]Cl, which could
be then deprotonated to furnish the phosphide 1. Related
cationic P-H phosphine complexes have been previously
prepared by reactions of Cp2MH3 and Cp2MH(L) (M = Nb,
Ta; L = two-electron ligand) with chlorophosphines.10,11,13
In reality, it happens that Cp2Nb(dNBut)(H) reacts with
ClPEt2 to give Cp2Nb(dNBut)(Cl) and HPEt2. We attribute
the difference in chemical behavior between our imido
hydride system and the previously studied trihydrides and
monohydrides to the ease of phosphine displacement from
the intermediate [Cp2Nb(dNBut)(PHR2)]þ, which generates
an unsaturated intermediate, [Cp2Nb(dNBut)]þ, amenable
to coupling with the chloride.
1
104.4 ppm, respectively, supports their assignment. The H
signal of the But group at 1.72 ppm in 2 is shifted downfield
relative to the corresponding resonance in 1 (1.02 ppm),
indicating the presence of a bridging imido group in the former.
Eventually, complex 1 was prepared in moderate yield by
reacting Cp2Nb(dNBut)(Cl) with LiPPh2 Et2O in ether
(eq 1). 1 was isolated as a yellow crystalline solid and
characterized by multinuclear NMR spectroscopy. Similar
3
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4502 Organometallics, Vol. 28, No. 15, 2009
Leelasubcharoen et al.
formation of a black precipitate, the benzene-soluble part
contained only the starting phosphide 1 and a small amount
of Cp2Nb(NBut)Cl. Similar pretreatment of [Rh(μ-Cl)-
(C2H4)2]2 with Me2PhSiH affords a new, fluxional Cp com-
pound characterized by the 1H NMR (C6D6) signal at
6.21 ppm. This product however decomposes overnight to
furnish Cp2Nb(NBut)Cl. No signals were observed in the 31
P
NMR, suggesting that the phosphorus-containing co-pro-
duct is either fluxional or insoluble in benzene.
When [Rh(μ-Cl)(C2H4)2]2 was pretreated with PPh3, the
fluxional complex [Rh(μ-Cl)(C2H4)(PPh3)]2 characterized
by the Rh-coupled 31P signal at 53.9 ppm (dd, J(Rh-P) =
184.6 Hz, J(Rh-P) = 6.0 Hz) was formed. Subsequent 1:1
reaction of this product with 1 results in a fast formation of a
mixture containing a new ethylene compound, Cp2Nb-
(dNBut)(μ-PPh2)Rh(PPh3)(C2H4)Cl (3), the exchange pro-
duct 2, and Cp2Nb(NBut)Cl in the ratio 10:3.5:3.0. Complex
3 is unstable. After 1 h at room temperature, the signals due
to 2 and 3 diminish, leaving Cp2Nb(NBut)Cl as the main
niobium product. Complex 3 is characterized by its 1H NMR
Cp signal at 6.06 ppm integrated as 10 protons, a broad
ethylene signal at 2.13 ppm integrated as four protons, and a
singlet at 0.66 ppm (But group) integrated as nine protons.
The position of the latter signal suggests that the imido group
remains terminal.
Figure 1. Molecular structure of compound 2. Selected bond
distances (in A) and angles (in deg): Nb1-Rh1 2.6744(2), Nb1-
N1 1.8643(17), Nb1-P1 2.4136(5), Nb1-Cl1 2.4349(5), Nb1-
Cp 2.67(6), Rh1-N1 2.0817(16), Rh1-P1 2.2516(5), Rh1-Cp
2.06(10), N1-Nb1-Cl1 106.71(5), P1-Nb1-Cl1 100.998(18),
N1-Nb1-P1 103.08(5), N1-Rh1-P1 101.91(5). Hydrogen
atoms are omitted for clarity.
Preparation of Bimetallic Complexes: Reaction with [(μ-
Cl)(Rh(COD)]2. Reaction of 1 with 0.5 equiv of [(μ-Cl)(Rh-
(COD)]2 (COD = 1,5 cyclooctadiene) in toluene afforded
the target bimetallic complex Cp2Nb(dNBut)(μ-PPh2)Rh-
(Cl)(COD) (4) (Scheme 1). Complex 4 was isolated from
toluene/hexane solution in the form of orange crystals of its
X-ray structure analysis proved unequivocally the formula-
tion of 2 as the Cp/Cl exchange product Cp((Cl)Nb(μ-
NBut)(μ-PPh2)RhCp, but not the target complex Cp2Nb-
(dNBut)(μ-PPh2)RhCl(C2H4)2. The molecular structure of
2 exhibits a three-leg piano-stool geometry for niobium and a
two-leg piano-stool geometry for rhodium; both metal atoms
are bridged by the imido and phosphido ligands. Our search
in the Cambridge Structural Database17 did not reveal any
bimetallic rhodium complex with a μ2-bridging imido ligand;
thus complex 2 is the first representative of this class.
However, its Rh-N bond of 2.082(2) A is comparable with
Rh-imido bonds in trimetallic complexes M2Rh(μ3-NR), in
which the imido ligand bridges three metal centers (1.980-
2.102 A).17 The Rh-P bond of 2.2516(5) A in 2 is at the
shorter end of a very wide spectrum of rhodium-phosphido
distances (2.202-2.840 A). Similarly, the Nb-N distance of
1.864(2) A is the shortest in the class of imido-bridged Nb
complexes (range 1.952-2.135 A), and the Nb-P bond of
2.4136(5) is the shortest in the class of phosphido-bridged Nb
complexes (range 2.487-2.717 A). These structural features
may be the consequence of a very short Nb-Rh distance
(2.6744(2) A), which indicates the presence of a direct Nb-
Rh bond (compare with the sum of metal radii, 2.774 A) and
thus accounts for the diamagnetism of this complex.
Complex 2 is stable in the solid state but gradually
decomposes in solution at room temperature to give un-
identified products. It does not react with H3SiPh over the
course of 29 h. Attempted Cl/H exchange in 2 under the
action of L-Selectride (Li[BH(CH(CH3)C2H5)3]) resulted in
no reaction.
In an attempt to remove the chloride and to stop the Cp/Cl
exchange leading to 2, solutions of [Rh(μ-Cl)(C2H4)2]2 in
THF were pretreated with L-Selectride before the addition
of 1. Although a reaction occurred, accompanied by the
hexane solvate (4 0.5hexane), and its structure was estab-
lished by multinuclear NMR and X-ray diffraction analysis.
3
1
The H NMR spectrum of 4 in C6D6 shows, in addition to
other signals, a doublet at 6.14 ppm (J(P-H) = 1.2 Hz) for
the Cp ring integrated as 10 protons, two broad singlets
at 5.62 and 3.15 ppm for nonequivalent olefin protons
integrated as four each, and a singlet at 0.66 ppm for the
terminal But-imido group. The downfield-shifted olefin sig-
1
nal at 5.62 ppm is coupled in the H-31P HSQC spectrum
with the broad phosphorus resonance at 26.3 ppm (d, J(P-
Rh) = 114.2 Hz), which suggests the mutual trans arrange-
ments of these groups. The upfield olefin signal at 3.15 ppm,
therefore, comes from the other double bond of COD that is
trans to the chloride ligand, which is a weaker trans ligand
than phosphide. These spectroscopic features are consistent
with the presence of a four-coordinate Rh center bound to
niobium via the phosphide bridge.
Complex 4 is stable in the solid state but gradually
decomposes in solutions at room temperature (half-life
approximately 4 days) to give Cp2Nb(NBut)Cl and the
known complex [Rh2(μ-Cl)(μ-PPh2)(COD)2], identified by
the 31P NMR.18 After a few days, the latter product partially
rearranges to [Rh(μ-PPh2)(COD)]2.19
The X-ray structure analysis of 4 confirms the phosphido-
bridged bimetallic structure (Figure 2). The niobium atom is
in a typical disubstututed niobocene geometry.20 The imido
ligand remains terminal, forming a short Nb-N bond
of 1.776(4) A, which is close to the mean NbdN bond of
1.782 A found for the wide range of niobium-imido bonds
(18) Burkhardt, E. W.; Mercer, W. C. Inorg. Chem. 1984, 23, 1779.
(19) Kreter, P. E.; Meek, D. W. Inorg. Chem. 1983, 22, 319.
(20) Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98, 1729.
(17) Cambridge Structural Database: Allen, F. H. Acta Cryst. 2002,
B58, 380–388.

Article
Organometallics, Vol. 28, No. 15, 2009 4503
Scheme 1. Preparation of Complexes 4 and 5
COD ligand is positioned trans to the phosphide, and the
other one is trans to the chloride ligand. The Rh-CCOD
distances trans to the phosphide (2.183(5) and 2.209(4) A)
are longer than those trans to the chloride (2.102(5) and
2.137(5) A), which highlights the stronger trans influence of
the phosphide ligand.
Catalytic Studies. On the basis of literature precedents on
catalytic hydrosilylation of carbonyls by cationic Rh com-
plexes,15b,15c,15e we initially sought to study the catalytic
activity of the cationic complex [Cp2Nb(μ-NBut)(μ-PPh2)-
Rh(COD)]þ (5), which could be prepared by chloride
abstraction from 4. We anticipated that the imido ligand
would be able to occupy the bridging position, so that the
fragment Cp2Nb(NBut)(PPh2) would serve as a hemilabile
ligand to the cation [Rh(COD)]þ.
The cation 5 was generated in CD2Cl2 by reacting 4 with
AgBF4 (Scheme 1). Upon mixing the reagents, the color of
the solution changed immediately from yellow to brown,
accompanied by the formation of a brown precipitate. The
1H NMR showed the formation of a new species, which
however turned out to be too unstable to allow isolation. In
light of this, hydrosilylation of acetophenone by PhSiH3
catalyzed by the in situ formed 5 was attempted. The course
of the reaction in CD2Cl2 was monitored by 1H NMR
spectroscopy. In 5 h, complete conversion was achieved,
giving a mixture of mono- and bis(hydrosilylation) pro-
ducts (PhSiH2OCH(Me)Ph and PhSiH(OCH(Me)Ph)2,
respectively) in the ratio 38:62 (Table 1).
However, to our surprise, the parent neutral complex 4
showed even greater activity. The reaction was complete in
2 h, showing the same product composition (Table 1, entries
2 and 3). The change of solvent from methylene chloride to
benzene had no effect on the rate of hydrosilylation or
product distribution (Table 1, entries 2 and 3), whereas the
nature of the silane does affect the reaction. With a second-
ary silane (PhMeSiH2), the reaction took more time and
up to 30% of a dehydrogenative silylation product, PhSi-
HMeOC(Ph)dCH2, was formed (Table 1, entry 4). Tertiary
silanes were not active in this reaction (Table 1, entries 5-7).
For comparison, hydrosilylation of acetophenone was
attempted with the parent Rh complex [Rh(μ-Cl)(COD)]2
in the absence of any added phosphide. [Rh(μ-Cl)(COD)]2
turned out to be a better catalyst than 4 for the hydrosilyla-
tion with PhMeSiH2 (Table 1, entry 4 vs entry 15), but a
worse one for the hydrosilylation with PhSiH3 (Table 1,
entry 14 vs entry 2).
Figure 2. Molecular structure of the compound 4 0.5hexane.
3
Selected bond distances (in A) and angles (in deg): Nb1-P1
2.6854(13), Rh1-P1 2.3790(12), Nb1-N1 1.776(4), Rh1-Cl1
2.3968(13), Rh1-C24 2.102(5), Rh1-C23 2.137(5), Rh1-C28
2.183(5), Rh1-C27 2.209(4), N1-Nb1-P1 94.10(13), C31-
N1-Nb1 166.3(4), P1-Rh1-Cl1 92.27(4), C24-Rh1-P1
94.92(14), C23-Rh1-P1 93.28(13), C28-Rh1-P1 152.75(13),
C27-Rh1-P1 170.35(14), C24-Rh1-Cl1 149.93(15), C23-
Rh1-Cl1 168.88(14), C28-Rh1-Cl1 89.02(14), C27-Rh1-
Cl1 86.80(14). Hydrogen atoms and the hexane solvate are
omitted for clarity.
(1.672-2.051 A).17 For related niobocene imido complexes,
a narrow range of NbdN distances has been determined
(1.737(6)-1.804(5) A).12,21 The bond angle Nb-N-But
of 166.2(3)° is also quite normal for niobocene imido com-
plexes.21a The Nb-P bond of 2.685(1) A and the Rh-P bond
of 2.379(1) A are noticeably longer than the corresponding
distances in compound 2 discussed above, which can be
accounted for by the absence of direct Nb-Rh bonding at
the long Nb-Rh separation of 4.266 A. The Rh atom is
ligated by the chloride ligand, two double bonds of COD,
and the bridging phosphide, which together comprise the
usual, square-planar geometry around the d8 Rh center.
Consistent with the NMR data, one double bond of the
Even more surprisingly, the Cp/Cl exchange complex 2
also showed significant, although much reduced catalytic
activity in the hydrosilylation of acetophenone (Table 1,
entries 9-12). Again, the primary silane PhSiH3 performed
much better than the secondary one (Table 1, entry 8 vs
entry 9), while tertiary silanes were not active (entries 10-12).
Complex 2 does not catalyze the addition of Me2PhSiH to
1-hexene. The isomerization to 2-hexene takes place instead.
ꢀ
ꢀ
ꢀ
~
(21) (a) Garces, A.; Perez, Y.; Gomez-Ruiz, S.; Fajardo, M.; Antinolo,
A.; Otero, A.; Lopez-Mardomingo, C.; Gomez-Sal, P.; Prashar, S.
ꢀ
ꢀ
~
J. Organomet. Chem. 2006, 691, 3652. (b) Antinolo, A.; Fajardo, M.;
ꢀ
ꢀ
ꢀ
Lopez-Mardomingo, C.; Lopez-Solera, I.; Otero, A.; Perez, Y.; Prashar, S.
Organometallics 2001, 20, 3132. (c) Green, M. L. H.; James, J. T.; Chernega,
A. N. J. Chem. Soc., Dalton Trans. 1997, 1719. (d) Herrman, W. A.; Baratta,
W.; Herdtweck, E. Angew. Chem., Int. Ed. Engl. 1996, 35, 1951.
(e) Chernega, A. N.; Green, M. L. H.; Suarez, A. G. Can. J. Chem. 1995, 73,
1157. (f) Schmidt, S.; Sundermeyer, J. J. Organomet. Chem. 1994, 472, 127.

4504 Organometallics, Vol. 28, No. 15, 2009
Table 1. Catalytic Hydrosilylation of Acetophenone by 2, 4, and 5 (3%) in C6D6
Leelasubcharoen et al.
entry
1
catalyst
silane
time
5 ha
yield (%)
5
4
PhSiH3
PhSiH2OCH(Me)Ph: 32%
PhSiH(OCH(Me)Ph)2: 68%
2
3
4
PhSiH3
2 ha
2 h
5 h
PhSiH2OCH(Me)Ph: 40%
PhSiH(OCH(Me)Ph)2: 60%
PhSiH2OCH(Me)Ph: 32%
PhSiH(OCH(Me)Ph)2: 68%
PhSiHMeOCH(Me)Ph: 60%
PhSiHMeOC(Ph)dCH2: 30%
NR
PhSiH3
MePhSiH2
5
6
7
Me2PhSiH
Et3SiH
(OEt)3SiH
1 d
1 d
1 d
NR
NR
8
2
PhSiH3
7 h 10 min
PhSiH2OCH(Me)Ph: 20%
PhSiH(OCH(Me)Ph)2: 80%.
PhSiHMeOCH(Me)Ph 100%
NR
NR
NR
9
MePhSiH2
Me2PhSiH
Et3SiH
1 d 22 h
1 d
1 d
10
11
12
(OEt)3SiH
1 d
14
15
[Rh(μ-Cl)(COD)]2
[Rh(μ-Cl)(COD)]2
PhSiH3
7 h 30 min
2 h
PhSiH2OCH(Me)Ph: 19%
PhSiH(OCH(Me)Ph)2: 81%
PhSiHMeOCH(Me)Ph: 63%
PhSiHMeOC(Ph)dCH2: 34%.
MePhSiH2
a In CD2Cl2.
Complex 4 was also found to catalyze hydrosilylation of
benzaldehyde with PhSiH3 at a low catalyst load (1.5 mol %,
50 min for completeness), with the products being mono- and
bis(addition) derivatives, PhSiH2OCH2Ph and PhSiH-
(OCH2Ph)2,22 in the ratio 36:64. At 3.0% catalyst load, the
reaction was complete in 5 min, showing the selective for-
mation of PhSiH(OCH2Ph)2 (94% yield).
In an attempt to gain more insight into these catalytic
transformations, stoichiometric reactions were attempted.
When complex 4 reacts with PhSiH3 in a 1:1 ratio, decom-
position occurs, leading to the formation of Cp2Nb(NBut)Cl
and Cp2Nb(NBut)(PPh2). With a stoichiometric amount of
PhCHO, 4 decomposes to Cp2Nb(NBut)Cl, but no new
detectable Rh species could be identified in the reaction
mixture by 1H and 31P NMR.
of the starting imido-phopshido complex of Nb to make it a
better hemilabile metalloligand.
Experimental Details
All manipulations were carried out, using conventional inert
atmosphere glovebox and Schlenk techniques. Solvents were
dried by Grubbs-type columns or by distillation from appro-
priate drying agents. NMR spectra were obtained with Bruker
DPX-300 and Bruker DPX-600 instruments (1H: 300 and
600 MHz; 13C: 75.5 and 151 MHz; 29Si: 119.2 MHz; 31P: 121.5
and 243 MHz). IR spectra were measured on an ATI Mattson
FTIR spectrometer. L-Selectride was purchased from Alfa
Aesar, and [Rh(μ-Cl)(COD)]2 and [Rh(μ-Cl)(C2H4)]2 were from
Strem. Silanes PhSiH3, MePhSiH, and MePh2SiH were pre-
pared by reducing the corresponding chlorides with LiAlH4, and
Et3SiH and (OEt)3SiH were obtained from Aldrich. Complexes
Cp2Nb(NtBu)Cl12 and Ph2PLi Et2O23 were prepared according
3
Conclusions
to the literature methods.
Synthesis of Cp2Nb(NBut)(PPh2) (1). Ph2PLi Et2O (0.177 g,
The new imido phosphido complex Cp2Nb(NBut)(PPh2)
(1) serves as a metallophosphine ligand to rhodium olefin
fragments {ClRhL2}, affording phosphido-bridged bimetal-
lic complexes. The outcome of the reaction depends on the
nature of the olefin L. With L = ethylene, the main product
is an unusual Cp/Cl exchange complex, Cp(Cl)Nb(μ-NBut)-
(μ-PPh2)RhCp (2), featuring a bridging imido group in
addition to bridging phosphido group. When L2 is the
chelating diolefin COD, the reaction of 1 with 0.5 equiv of
[(μ-Cl)RhCOD]2 proceeds as a simple phosphide addition to
furnish the bimetallic product Cp2Nb(dNBut)(μ-PPh2)Rh-
(Cl)(COD) (4). Both 2 and 4 were found to be active
precatalysts for hydrosilylation of acetophenone and ben-
zaldehyde. The neutral complex 4 exhibits enhanced reactiv-
ity in comparison with the cationic derivative [Cp2-
Nb(NBut)(PPh2)Rh(COD)]þ, presumably because the latter
has the imido ligand too tightly coordinating to the Rh
center. Future research will be directed at the optimization
3
0.665 mmol) dissolved in Et2O (20 mL) was added under stirring
to a solution of Cp2Nb(NBut)Cl (0.213 g, 0.646 mmol) in Et2O
(20 mL) at -30 °C. The color changed first from yellow to brown
and then gradually turned to brown-green in the course of 2 h.
The solution was allowed to warm to room temperature and
then was stirred overnight, affording a brown suspension. After
filtration through a microglass filter paper, volatiles were re-
moved from the yellow solution in vacuo to give a yellow solid.
The latter was washed with hexane and recrystallized from Et2O
at -20 °C, affording a yellow microcrystalline material in 77%
yield (0.245 g, 0.511 mmol). 1H NMR (300 MHz, C6D6, ppm):
δ 1.02 (s, 9H, But), 5.52 (d, J(P-H) = 0.6 Hz, 10H, Cp), 7.05
(t, J(H-H) = 7.2 Hz, 2H, p-Ph), 7.19 (t, J(H-H) = 7.2 Hz, 4H,
m-Ph), 7.80 (dd, J(H-H) = 7.2 Hz, J(P-H) = 9.3 Hz, 4H,
o-Ph). 13C NMR (75.5 MHz, C6D6, ppm): δ 31.0 (But), 108.0 (d,
J(P-C) = 2.3 Hz, Cp), 125.7 (p-Ph), 127.8 (m-Ph), 135.0 (d,
J(C-P) = 16.6 Hz, o-Ph), 148.9 (d, J(C-P) = 35.4 Hz, i-Ph).
1H-31P HSQC NMR (C6D6, ppm): δ -0.2 (s, PPh2). Anal.
Calcd for C26H29NNbP: C, 65.14; H, 6.10; N, 2.92. Found: C,
65.04; H, 6.16; N, 2.80.
(22) (a) Yun, S. S.; Lee, J.; Lee, S. Bull. Korean Chem. Soc. 2001, 22,
623. (b) Yun, S. S.; Kin, T. S.; Kin, C. H. Bull. Korean Chem. Soc. 1994, 15,
522.
(23) Bartlett, R. A.; Olmstead, M. M.; Power, P. P. Inorg. Chem.
1986, 25, 1243.

Article
Synthesis of CpNbCl(μ-NBut)(μ-PPh2)RhCp (2). A solution
Organometallics, Vol. 28, No. 15, 2009 4505
Table 2. X-ray Crystal Data and Structure Refinement for 2 and
4 0.5hexane
of complex 1 (0.101 g, 0.211 mmol) in toluene (5 mL) was added
to a stirred solution of [Rh(μ-Cl)(C2H4)2]2 (0.035 g, 0.105 mmol)
in toluene (5 mL). The resulting dark green solution was stirred
for 15 min, after which time the volatiles were removed in vacuo.
The green solid produced was washed with hexane and recrys-
tallized from Et2O, giving a green microcrystalline material
in 81% yield (0.105 g, 0.170 mmol). 1H NMR (300 MHz,
C6D6, ppm): δ 1.72 (s, 9H, But), 4.88 (s, 5H, Cp), 5.42 (d,
J(P-H) = 0.6 Hz), 5H, Cp), 6.99 (m, 4H, m-Ph), 7.05 (m, 2H,
p-Ph), 7.28 (m, 2H, o-Ph), 8.06 (dd, J(H-H) = 8.3 Hz, J(P-H) =
13.0 Hz, 2H, o-Ph). 13C NMR (75.5 MHz, C6D6, ppm): δ 35.4
(But), 87.3 (Cp), 104.4 (Cp), 133.2, 134.1 (Ph). Anal. Calcd for
C26H29ClNNbPRh: C, 50.55; H, 4.73; N, 2.27. Found: C, 51.04;
H, 5.25; N, 1.83.
3
2
4 0.5hexane
3
formula
fw
C26H29ClNNbPRh
617.74
C37H48ClNNbPRh
769.00
color, habit
cryst size, mm
cryst syst
space group
a, A
green, needle
0.20 ꢀ 0.08 ꢀ 0.08
monoclinic
P21/n
yellow-orange, block
0.38 ꢀ 0.12 ꢀ 0.10
monoclinic
P21/c
9.1504(3)
16.5399(6)
16.5001(6)
103.540(1)
2427.83(15)
4
9.1766(6)
18.6877(12)
20.1635(12)
98.312(3)
3421.5(4)
4
b, A
c, A
β, deg
V, A3
Z
Generation of Cp2Nb(dNBut)(μ-PPh2)Rh(PPh3)(C2H4)Cl (3).
To [Rh(μ-Cl)(C2H4)2]2 (0.003 g, 0.009 mmol) in C6D6 (0.6 mL)
was added PPh3 (0.005 g, 0.019 mmol) to give an orange solution
of[Rh(μ-Cl)(C2H4)(PPh3)]2 (31P NMR(121.5 MHz, C6D6, ppm):
δ 53.9 ppm (dd, J(Rh-P) = 184.6 Hz, J(Rh-P) = 6.0 Hz)).
Then 0.009 g (0.019 mmol) of complex Cp2Nb(NtBu)(PPh2) (1)
was added to the mixture, causing the color change to brown.
NMR spectra revealed the formation of a mixture of compounds
containing Cp2Nb(dNBut)(μ-PPh2)Rh(PPh3)(C2H4)Cl (3), 2,
and Cp2Nb(NBut)Cl in the ratio 10:3.5:3.0. Complex 3 decom-
poses in benzene solutions in the course of 1 h.
T, °C
150
1.690
123
1.493
F
calc, g/cm3
F(000)
1240
1.342
1580
0.968
μ, mm-1
2θmax, deg
58.00
26 584
56.00
23 855
total no. of reflns
unique reflns
Rint
no. with I g 2σ(I)
no. of variables
R1 (I g 2σ(I))
wR2 (all data)
GOOF
6445
0.0324
8234
0.0531
5497
280
6317
379
0.0239
0.0586
1.050
0.0587
0.1113
1.164
Selected NMR data for Cp2Nb(dNBut)(μ-PPh2)Rh(PPh3)-
(C2H4)Cl (3): 1H NMR (300 MHz, C6D6, ppm): δ 6.06 (s, 10, H,
Cp), 2.13 (bd, J(Rh-H) = 1.8 Hz, 4 H, C2H4), 0.66 (s, 9H, But).
31P NMR (121.5 MHz, C6D6, ppm): δ 36.4 (d, J(Rh-P) =
128.8 Hz, PPh3).
The silane (16 μL, 0.130 mmol) was added into the solution,
followed by the carbonyl compound (0.128 mmol). The course
of the reaction was monitored by 1H NMR spectroscopy. The
hydrosilylation products PhSiH2OCH2Ph,24 PhSiH(OC-
H2Ph)2,24 PhSiH2(OCH(Me)Ph),25 and PhSiH(OCH(Me)-
Synthesis of [Cp2NbCl(NBut)(μ-PPh2)RhCl(COD)] 0.5hexane
3
3
Ph)225 were characterized by NMR.
First Stereoisomer of PhSiHMeOCH(Me)Ph.
(4 0.5hexane). A solution of complex 1 (0.100 g, 0.209 mmol) in
26
1H NMR
toluene (5 mL) was added to a stirred solution of [Rh(μ-Cl)-
(COD)]2 (0.052 g, 0.106 mmol) in toluene (5 mL). The resulting
orange solution was stirred for 15 min, after which time the
solvent was removed in vacuo. The orange solid produced was
washed with hexane and recrystallized by layer diffusion of
(300 MHz, C6D6, ppm): δ 0.30 (d, 3H, SiMe), 1.38 (d, 3H, Me),
1
5.29 (q, 1H, SiH), 7.61 (m, SiPh). H-29Si HSQC NMR (1H:
300 MHz; 29Si: 59.6 MHz, C6D6, J = 7 Hz, ppm): δ -4. 29Si
INEPTþ NMR (119.2 MHz, C6D6, J = 200 Hz, ppm): δ -4.3
1
1
3
(d, JSi-H = 210). H-13C HSQC NMR (1H: 300 MHz; 13C:
75.5 MHz, C6D6, J = 145 Hz, ppm): δ -1.0 (OMePh), 22.0
(SiMe).
hexane intoa toluenesolution, giving orange crystalsof 4 0.5hex-
1
ane in 91% yield (0.146 g, 0.190 mmol). H NMR (600 MHz,
toluene-d8, 237 K, ppm): δ 0.69 (s, 9H, But), 0.95 (t, J(H-H) =
7.2 Hz, 3H, 0.5 hexane), 1.25 (m, 4H, 0.5 hexane) 1.65 (m, 2H,
COD), 1.80 (m, 2H, COD), 2.34 (m, 4H, COD), 3.17 (s, 2H,
COD), 5.62 (s, 4H, COD), 6.14 (s, 10H, Cp), 6.98 (m, 2H, p-Ph),
7.01 (m, 2H, Ph), 7.22 (m, 2H, Ph), 7.25 (m, 2H, Ph), 8.95 (bs, 2H,
Second Stereoisomer of PhSiHMeOCH(Me)Ph. 1H NMR
(300 MHz, C6D6, ppm): δ 0.33 (d, 3H, Me), 1.41 (d, 3H, MeSi),
1
5.42 (q, 1H, SiH), 7.52 (m, SiPh). H-29Si HSQC NMR (1H:
300 MHz; 29Si: 59.6 MHz, C6D6, J = 7 Hz, ppm): δ -4.8 (d,
1
1
1JSi-H = 208 Hz). H-13C HSQC NMR (1H: 300 MHz; 13C:
o-Ph). H NMR (300 MHz, 295 K, C6D6, ppm): δ 0.66 (s, 9H,
But), 1.59 (m, 2, COD), 1.72, (m, 2H, COD), 2.27 (m, 4H, COD),
3.15 (bs, 2H, COD), 5.62 (bs, 2H, COD), 6.14 (d, J(P-H) =
1.2 Hz, 10H, Cp), 6.98 (t, J(H-H) = 7.4 Hz, 2H, p-Ph), 7.01
(t, J(H-H) = 7.4 Hz, 4H, m-Ph), 8.11 (bs, 4H, o-Ph). 1H NMR
(300 MHz, 295 K, CD2Cl2, ppm): δ 0.81 (s, 9H,But), 1.77 (m, 2,
COD), 1.92, (m, 2H, COD), 2.37 (m, 4H, COD), 2.93 (bs, 2H,
COD), 5.09 (bs, 2H, COD), 6.21 (d, J(P-H) = 1.5 Hz, 10H, Cp),
7.17 (m, 2H, p-Ph), 7.25 (m, 4H, m-Ph), 7.8 (dd, J(H-H) =
7.5 Hz, J(H-P) = 7.5 Hz, 4H, o-Ph). 13C NMR (150.9 MHz,
C6D6, 239 K, ppm): δ 14.5 (hexane), 23.2 (hexane), 29.4 (bs,
COD), 30.7 (But), 32.2 (hexane), 33.0 (bs, COD), 70.7 (bs, COD),
98.3 (bs, COD), 110.0 (Cp), 126.8 (p-Ph), 127.4 (m, Ph), 127.7
75.5 MHz, C6D6, J = 145 Hz, ppm): δ -1.0 (OMePh), 22.0
(SiMe).
27
PhSiHMeOC(Ph)dCH2.
1H NMR (300 MHz, C6D6,
ppm): δ 0.40 (d, 3H, SiMe), 4.57 (d, 1H, CdCH2), 4.90 (d,
1
1H, CdCH2), 5.47 (q, 1H, PhSiHMe). H-29Si HSQC NMR
(1H: 300 MHz; 29Si: 59.6 MHz, C6D6, J = 7 Hz, ppm): δ -22.
1H-13C HSQC NMR (1H: 300 MHz; 13C: 75.5 MHz, C6D6, J =
145 Hz, ppm): δ 90 (CdCH2).
General Procedure for the Hydrosilylation of PhC(O)CH3 by
PhSiH3 Catalyzed by 4 or 4/AgBF4 in CD2Cl2. Complex 4 (4.0 mg,
0.005 mmol, 3.0 mol %) or a mixture of 4 (4.0 mg, 0.005 mmol,
3.0 mol %) and AgBF4 (1.0 mg, 0.005 mmol) was dissolved in
0.6 mL of CD2Cl2. PhSiH3 (23 μL, 0.186 mmol) and PhC(O)CH3
(22.0 μL, 0.188 mmol) were charged into the mixture. The course
of the reaction was monitored by 1H NMR spectroscopy.
(m-Ph), 129.2 (Ph), 135.1 (o-Ph), 143.7 (d, J = 12.9 Hz, i-Ph). 31
P
NMR (243.0 MHz, toluene-d8, 239 K, ppm): δ26.3 (d, J(Rh-P) =
114.2 Hz). Anal. Calcd for C37H48ClNNbPRh: C, 57.79; H, 6.29;
N, 1.82. Found: C, 57.85: H, 6.18; N, 1.83.
General Procedure for the Hydrosilylation of PhC(O)CH3
and PhC(O)H Catalyzed by 2 or 4 in Benzene. The catalyst
(0.003 mmol, 2.3 mol %) was dissolved in 0.6 mL of C6D6.
(25) (a) Khalimon, A. Y.; Simionescu, R.; Kuzmina, L. G.; Howard,
J. A. K.; Nikonov, G. I. Angew. Chem., Int. Ed. 2008, 47, 7701. (b) Yun, S.
S.; Yang, Y. S.; Lee, S. Bull. Korean Chem. Soc. 1997, 18, 1058.
(26) (a) Glushkova, N. E.; Kochina, T. A.; Kharitonov, N. P. Zhur.
Obshch. Khim. 1984, 54, 2741. (b) Tamio, H.; Yamamoto, K.; Kumada, M. J.
Organomet. Chem. 1976, 112, 253.
(27) Iretskii, A. V.; Zhidkova, O. B.; Dolgushina, T. S.; Labeish, N.
N.; Skvortsova, N. K.; Galishev, V. A. Zhur. Obshch. Khim. 1993, 63,
1681.
(24) (a) Yun, S. S.; Lee, J.; Lee, S. Bull. Korean Chem. Soc. 2001, 22,
623. (b) Yun, S. S.; Kim, T. S.; Kim, C. H. Bull. Korean Chem. Soc. 1994, 15,
522. (c) Peterson, E.; Khalimon, A. Y.; Simionescu, R.; Kuzmina, L. G.;
Howard, J. A. K.; Nikonov, G. I. J. Am. Chem. Soc. 2009, 131, 908.

4506 Organometallics, Vol. 28, No. 15, 2009
Leelasubcharoen et al.
Crystal Structure Determinations. X-ray quality crystals of 2
were obtained by low-temperature crystallization from ether,
and crystals of 4 0.5hexane were grown by layer diffusion of
were solved by direct methods29 and refined by full-matrix least-
squares procedures, using w(|F2o| - |Fc2|)2 as the refined function.
In both structures, all non-hydrogen atoms were refined with
anisotropic thermal parameters and all hydrogen atoms were
placed in calculated positions and refined using a riding model.
3
hexane into a toluene solution of 4. In both cases, crystals were
covered by polyperfluoro oil and mounted directly onto the
Bruker Smart three-circle diffractometer with CCD area detec-
tor at 150 and 123 K, respectively. Experimental intensities were
collected using Mo KR radiation (0.71073 A) in ω-scan mode.
The crystallographic data and characteristics of structure solu-
tion and refinement are assembled in Table 2. The Bruker
SAINT program28 was used for data reduction. The structures
Acknowledgment. This research was supported by
Research Corporation and NSERC grants to G.I.N. L.
G.K. and J.A.K.H. thank the Royal Society of Chemistry
and the University of Durham for support.
Supporting Information Available: Crystallographic informa-
tion files (CIF) for compounds 2 and 4. This material is available
free of charge via the Internet at http://pubs.acs.org.
(28) SAINT, Version 6.02A; Bruker AXS Inc.: Madison, WI, 2001.
(29) SHELXTL-Plus, Release 5.10; Bruker AXS Inc.: Madison, WI,
1997.