Nickel(II), Palladium(II), and Platinum(II) η3-Allyl Complexes Bearing a Bidentate Titanium(IV) Phosphinoamide Ligand: A Ti←M2 Dative Bond Enhances the Electrophilieity of the π-Allyl

Article abstract of DOI:10.1021/om8011085

Three Ti-M₂ (M₂ = Ni, Pd, Pt) heterobimetallic complexes, [η³-methallyl)M₂(Ph₂PN ^tBu)₂TiCl₂](OTf), were synthesized in which a Ti^IV←M₂ interactio

Full text of DOI:10.1021/om8011085

1988
Organometallics 2009, 28, 1988–1991
Nickel(II), Palladium(II), and Platinum(II) η3-Allyl Complexes
Bearing a Bidentate Titanium(IV) Phosphinoamide Ligand: A
TirM2 Dative Bond Enhances the Electrophilicity of the π-Allyl
Moiety
Hironori Tsutsumi,† Yusuke Sunada,‡ Yoshihito Shiota,‡ Kazunari Yoshizawa,‡ and
Hideo Nagashima*,†,‡
Institute for Materials Chemistry and Engineering and Graduate School of Engineering Sciences,
Kyushu UniVersity, Kasuga, Fukuoka 816-8580, Japan
ReceiVed NoVember 21, 2008
Chart 1
Summary: Three Ti-M2 (M2 ) Ni, Pd, Pt) heterobimetallic
complexes, [(η3-methallyl)M2(Ph2PNtBu)2TiCl2](OTf), were syn-
thesized in which a TiIVrM2 interaction was suggested by
crystallography and DFT calculations. The TiIVrM2 interaction
enhanced electrophilicity of the η3-methallyl ligand of M2,
leading to high reactiVity of the η3-methallyl moiety with Et2NH
compared with that of the dppp analogue.
The importance of bidentate phosphine ligands in modern
organometallic chemistry is well recognized, particularly for
molecular catalysts useful for organic and polymer synthesis.1
The structural design of metal-phosphine complexes is estab-
lished by controlling the bulkiness of the substituents on the
phosphorus atoms, the cone angle, and the bite angle.1 In
contrast, the electronic properties of the bidentate phosphine
ligands are often tuned by the donor property of the substituents
bonded to the phosphorus atoms, which is estimated by
Tolman’s ꢀ value (Chart 1, A).1 A unique bidentate phosphine
ligand, which is not explained by the ꢀ value, has been
developed by Yoshifuji and Ozawa, in which effective use of a
phosphorus analogue of a R-diimine ligand results in emphasiz-
ing back-donation from the metal center to the ligand (Chart 1,
B).2 The third approach for the electronic control is use of the
metalloligand of type C in Chart 1, in which a metallic species
is incorporated in a structure of the phosphines. For instance, a
bidentate phosphorus ligand having a Lewis acidic boron center
in the ligand backbone coordinates to a Rh or Au atom; existence
of a M2fB dative bond in C-1 is suggested in the literature.3,4
C-2 type complexes are also synthesized, which are proposed
to have M2fM1 (M1 ) Ti, Zr, M2 ) Rh, Pt) interactions.5
In these preceding papers, the M2fM1 interaction has been
discussed on the basis of the metal-metal bond distance of the
crystal structure, 11B and/or 31P NMR spectroscopy, and
(4) A notable related complex of C-1, [R2B-(CH2PPh2)2Pt+R(THF)],
with no direct M-B bond interaction was reported by Peters et al. In this
complex, the electron-donating property of the borate moiety enhanced the
electron density of phosphorus atoms, which resulted in higher electron
density of the Pt center, leading to successful C-H bond activation of arenes.
See: (a) Thomas, S. J.; Peters, J. C. J. Am. Chem. Soc. 2001, 123, 5100. (b)
Thomas, S. J.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 8870.
(5) (a) Choukroun, R.; Iraq, A.; Gervais, D.; Daran, J. -C.; Jeannin, Y.
Organometallics 1987, 6, 1197. For closely related examples, see: (b)
Ferguson, G. S.; Wolczanski, P. T.; Pa´rka´nyi, L.; Zonnevylle, M. C.
Organometallics 1988, 7, 1967. (c) Baxter, S. M.; Ferguson, G. S.;
Wolczanski, P. T. J. Am. Chem. Soc. 1988, 110, 4231.
(6) Hydroformylation of alkenes catalyzed by C-2 has been well
investigated as a typical reaction of heterobimetallic complexes. However,
the rate enhancement observed is usually explained by bimetallic activation
of CO through a Cp2ZrrOdC-M2(CO)n intermediate, and nothing has
been discussed about the effect of the ZrrM2 interaction, which reduces
the electron density of M2 and enhances the electrophilicity of the CO ligand
on M2. See: (a) Cornelissen, C.; Erker, G.; Kehr, G.; Fro¨hlich, R.
Organometallics 2005, 24, 214. (b) Bosch, B. E.; Bru¨mmer, I.; Kunz, K.;
Erker, G.; Fro¨hlich, R.; Kotila, S. Organometallics 2000, 19, 1255. (c)
Cornelissen, C.; Erker, G.; Kehr, G.; Fro¨hlich, R. Dalton Trans. 2005, 24,
4059. (d) See ref 5.
* To whom correspondence should be addressed. E-mail: nagasima@
cm.kyushu-u.ac.jp.
† Graduate School of Engineering Sciences.
‡ Institute for Materials Chemistry and Engineering.
(1) For reviews: (a) Tolman, C. A. Chem. ReV. 1977, 77, 313. (b)
Dierkes, P.; van Leeuwen, P. W. N. M. J. Chem. Soc., Dalton Trans. 1999,
1519. (c) van Leeuwen, P. W. N. M.; Kamer, P. C. J.; Reek, J. N. H.;
Dierkes, P. Chem. ReV. 2000, 100, 2741. (d) Appleby, T.; Woollins, J. D.
Coord. Chem. ReV. 2002, 235, 21. (e) Freixa, Z.; van Leeuwen, P. W. N. M.
Dalton Trans. 2003, 1890. (f) Minahan, D. M. A.; Hill, W. E.; McAuliffe,
C. A. Coord. Chem. ReV. 1984, 55, 31.
(2) (a) Minami, T.; Okamoto, H.; Ikeda, S.; Tanaka, R.; Ozawa, F.;
Yoshifuji, M. Angew. Chem., Int. Ed. 2001, 40, 4501. (b) Ozawa, F.;
Okamoto, H.; Kawagishi, S.; Yamamoto, S.; Minami, T.; Yoshifuji, M.
J. Am. Chem. Soc. 2002, 124, 10968. (c) Hayashi, A.; Yoshitomi, T.; Umeda,
K.; Okazaki, M.; Ozawa, F. Organometallics 2008, 27, 1970. (d) Hayashi,
K.; Nakatani, M.; Hayashi, A.; Takano, M.; Okazaki, M.; Toyota, K.;
Yoshifuji, M.; Ozawa, F. Organometallics 2008, 27, 2321. (e) Ozawa, F.;
Yoshifuji, M. Dalton Trans. 2006, 4987.
(3) (a) Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.;
Bourissou, D. Angew. Chem., Int. Ed. 2006, 45, 1611. (b) Bontemps, S.;
Sircoglou, M.; Bouhadir, G.; Puschmann, H.; Howard, J. A. K.; Dyer, P. W.;
Miqueu, K.; Bourissou, D. Chem. Eur. J. 2008, 14, 731. (c) Sircoglou, M.;
Bontemps, S.; Mercy, M.; Saffon, N.; Takahashi, M.; Bouhadir, G.; Maron,
L.; Bourissou, D. Angew. Chem., Int. Ed. 2007, 46, 8583.
(7) For other examples of early-late heterobimetallic complexes which
can act as catalysts for hydroformylation, see: (a) Choukroun, R.; Gervais,
D.; Kalck, P.; Senocq, F. J. Organomet. Chem. 1987, 335, C9. (b)
Choukroun, R.; Gervais, D.; Rifai, C. Polyhedron 1989, 8, 1760. (c)
Trzeciak, A. M.; Zio´lkowski, J. J.; Choukroun, R. J. Organomet. Chem.
1991, 420, 353. (e) Choukroun, R.; Dahan, F.; Gervais, D.; Rifai, C.
Organometallics 1990, 9, 1982. (f) Mattheis, C.; Braunstein, P.; Fischer,
A. Dalton Trans. 2001, 800. (g) Priya, S.; Balakrishna, M. S.; Mague, J. T.
J. Organomet. Chem. 2004, 689, 3335.
10.1021/om8011085 CCC: $40.75
2009 American Chemical Society
Publication on Web 03/11/2009

Communications
Organometallics, Vol. 28, No. 7, 2009 1989
Figure 2. Gaussview depiction of the HOMO for the full molecule
of 2a. See the Supporting Information for the molecular orbitals
of 2b,c.
Scheme 1
Figure 1. Molecular structure of 2a with 50% probability ellipsoids.
The counteranion (OTf) and hydrogen atoms are omitted for clarity.
ORTEP drawings of 2b,c are given in the Supporting Information.
theoretical calculations;3 however, there are a few reports on
the effect of the interaction from the viewpoint of reactivity of
the complex.6,7 The M2fM1 interaction is expected to reduce
the electron density of M2. This would increase the electrophi-
licity of the ligand bonded to M2, leading to its facile reaction
with nucleophiles. However, to the best of our knowledge, there
has been no report which demonstrates that complex C is
susceptible to facile transformation of the ligand on M2 by
reaction with nucleophiles.8,9 We have recently reported the
synthesis of titanium phosphinoamides, (Ph2PNtBu)2TiCl2, the
reaction of which with Pd(II), Pt(II), and other metallic species
affords complexes of the C-3 type.10 This prompted us to
synthesize (η3-allyl)palladium derivatives of C-3, because (η3-
allyl)palladium(II) complexes are known to undergo facile
nucleophilic attack at the η3-allyl ligand. As we expected, the
palladium complex 2b, the Ti(IV)rPd interaction of which is
proved by the Ti-Pd bond distance determined by crystal-
lography, NMR spectroscopy, and DFT calculations, is much
more reactive with Et2NH or aniline than (dppp)Pd(η3-methallyl)
(3b). Similar studies on the nickel and platinum homologues
of 2b showed similar enhancement of the electrophilicity of
the η3-methallyl ligand. The results clearly demonstrate that
the M1rM2 interaction produced by the metalloligand
(Ph2PNtBu)2TiCl2 is a good entry for facile nucleophilic
transformation of the ligand on M2.
Table 1. Ti-M2 Bond Distances of 2a-c, Determined by X-ray
Diffraction Analysis, Sum of the Covalent Radii of Ti and M2, and
Bond Order Estimated by DFT Calculation
Ti-Ni
(2a)
Ti-Pd
(2b)
Ti-Pt
(2c)
Ti-M2 dist determined by
X-ray: R1 (Å)
2.604(2)
2.8155(5)
2.765(2)
sum of the covalent radii of
Ti and M2: R2 (Å)
2.84
2.99
2.96
deviation: (R1/R2) × 100 (%)
91.7
351.1
0.33
94.2
352.5
0.29
93.4
353.5
0.35
a
∑TiR
bond order
a Sum of the three angles around the titanium atom (N(1)-Ti-N(2),
N(1)-Ti-Cl(1), and N(2)-Ti-Cl(1)).
afforded the Ti-M2 heterobimetallic complexes [(η3-methal-
lyl)M2(Ph2PNtBu)2TiCl2](OTf) (M2 ) Ni (2a), Pd (2b), Pt (2c))
in 57, 73, and 81% yield, respectively (Scheme 1). In the same
manner, [(η3-methallyl)M2(dppp)]OTf complexes (M2 ) Ni, Pd,
Pt) were synthesized.
Treatment of the titanium phosphinoamide (Ph2PNtBu)2TiCl2
with [M2(η3-methallyl)(acetone)2](OTf) (M2 ) Ni, Pd, Pt)
The molecular structures of 2a-c were unequivocally deter-
mined by X-ray diffraction analysis; the molecular structure of
2a is described in Figure 1, and detailed bond distances and
angles are given in the Supporting Information. The complexes
2a-c have a six-membered dimetallacyclohexane skeleton with
a boat conformation. The titanium center is bound to two
nitrogen and two chloride atoms assuming a tetrahedral coor-
dination geometry, and the M2 center adopts the square-planar
coordination geometry. These structural features are nearly
identical with those of our previously reported Ti-Pt hetero-
bimetallic complexes.10 The Ti-Ni, Ti-Pd, and Ti-Pt bond
lengths (2.604(2) Å for 2a, 2.8155(5) Å for 2b, and 2.765(2) Å
for 2c) are shorter than the sum of the covalent radii of Ti and
M2 (91.7% (M2 ) Ni), 94.2% (M2 ) Pd), and 93.4% (M2 )
(8) Complexes of type C including early and late transition elements
have been investigated in the chemistry of early-late heterobimetallic
(ELHB) complexes. There are several ELHB complexes with a dative bond,
in which the M1-M2 interaction is discussed from the point of view of
crystallography and theoretical calculations.9 However, only a few papers
have been reported on the reactivities.
(9) (a) Stephan, D. W. Coord. Chem. ReV. 1989, 95, 41. (b) Adams,
R. D.; Cotton, F. A. Catalysis by Di- and Polynuclear Metal Cluster
Complexes; Wiley-VCH: New York, 1998. (c) Braunstein, L. A.; Oro, L. A.;
Raithby, P. R. Metal Clusters in Chemistry; Wiley-VCH: New York, 1999;
Vol. 2. (d) Wheatley, N.; Kalck, W. Chem. ReV. 1999, 99, 3379. (e) Gade,
L. H. Angew. Chem., Int. Ed. 2000, 39, 2658.
(10) (a) Nagashima, H.; Sue, T.; Oda, T.; Kanemitsu, A.; Matsumoto,
T.; Motoyama, Y.; Sunada, Y. Organometallics 2006, 25, 1987. (b) Sunada,
Y.; Sue, T.; Matsumoto, T.; Nagashima, H. J. Organomet. Chem. 2006,
691, 3176. (c) Sue, T.; Sunada, Y.; Nagashima, H. Eur. J. Inorg. Chem.
2007, 18, 2897.

1990 Organometallics, Vol. 28, No. 7, 2009
Communications
Scheme 2
Pt) of the sum of the radii, respectively) (Table 1).11 The M2
atom deviates by 0.2123, 0.1622, and 0.1466 Å from the least-
squares plane around the M2 atom defined by two phosphorus
atoms and the methallyl moiety due to the Ti-M2 interaction.
The sum of the four angles around the basal plane of M2 is
357.7° for 2a, 357.8° for 2b, and 359.0° for 2c, which is slightly
distorted from the theoretical value of 360° for a square-planar
coordination geometry. In contrast, the sum of the three angles
around the titanium atom (N(1)-Ti-N(2), N(1)-Ti-Cl(1), and
N(2)-Ti-Cl(1): 351.1° for 2a, 352.5° for 2b, and 353.5° for
2c) showed that the geometry of the titanium centers of 2a-c
deviates significantly from tetrahedral and is rather close to
trigonal bipyramidal. These structural features can be ascribed
to the presence of the Ti-M2 bond interaction.
NMR spectroscopy of these heterobimetallic complexes
suggests that the Ti(IV)rM2 interaction in 2a-c leads to lower
electron density of M2 in comparison with that of the dppp
analogues 3a-c, which affects the 13C resonances of the η3-
methallyl moiety as well as the 31P signal of the phosphorus
ligand. Two 13C signals appeared for the η3-methallyl moiety
of 2b, which are due to the terminal and central carbons; both
of them are shifted to a lower field than those of the dppp
analogues, 3b. The 13C NMR spectrum of 2b showed two signals
Scheme 3
All of the above results indicate that the M2 center donates
the electron density of the d orbital to the titanium center via a
Ti(IV)rM2(II) dative bond, leading to reduction of the electron
density of M2, which should enhance the electrophilicity of the
η3-methallyl ligand in 2a-c compared with that of their dppp
analogues. The following experiments clearly demonstrate that
this is true. Treatment of 2a with 10 equiv of HNEt2 in CD2Cl2
at room temperature resulted in quantitative formation of a
corresponding diethylmethallylamine (4a) within 5 min (Scheme
2).14 This is in sharp contrast to the fact that no formation of
4a was observed in the reaction of HNEt2 with 3a for a
prolonged reaction time (2 h). Similarly, reaction of 2b,c with
HNEt2 was completed within 5 min in CD2Cl2 to give 4a in
quantitative yield, whereas only a trace amount (ca. ∼4%) of
4a was obtained by treatment of 3b,c with HNEt2 under the
same conditions. The reaction of 2b with 10 equiv of aniline
also proceeded effectively to afford the corresponding N-
methallylaniline (4b) in 23% yield after 5 min and 85% yield
after 12 h at room temperature. In contrast, 3b was totally
unreactive toward aniline under the same reaction conditions.
Ozawa and Yoshifuji have described similar enhancement of
the electrophilicity of the allyl moiety in their (DPCB)Pd(η3-
allyl) complexes; treatment of the Pd complex with HNEt2 led
to the instant formation of diethylallylamine in 82% yield.
Similarly, the reaction of aniline afforded a 45% yield of
N-allylaniline and a 72% yield after 24 h.2a
at 83.0 (t, JC-P ) 11.6 Hz, CH2dC(Me)) and 141.1 (t, JC-P
)
4.3 Hz, CH2dC(Me)) which are ca. 9 and 3 ppm shifted to
lower field compared with that of 3b. A single 31P resonance is
visible for 2a (δ 14.1), 2b (δ 9.75), and 2c (δ 0.60), which is
shifted lower by 20, 1.1, and 0.3 ppm, respectively, compared
with those of 3a-c.12
Computational studies on the full models of 2a-c were
carried out using density functional theory (DFT) with the
B3LYP functional and the LanL2DZ basis set based on the
geometry determined by X-ray diffraction analysis. This sug-
gests that the HOMO (for 2a,c) or HOMO-1 (for 2b) may
involve some Ti-M2 bonding interaction by donating the
The high reactivity of our Ti-M2 heterobimetallic complexes
was also demonstrated in the catalytic allylation of HNEt2.
Reaction of 2 equiv of HNEt2 with methallyl chloride catalyzed
by 2b (10 mol %) was performed in CD2Cl2 at room temper-
ature, from which quantitative formation of 4a was confirmed
within 5 min by the NMR spectrum (Scheme 3).15 The same
reaction was examined either in the absence of catalyst or in
the presence of 3b (10 mol %), revealing that the yields of 4a
were 7% and 11%, respectively, after 1.5 h.
2
electrons from the dz and dyz orbitals of M2 to the titanium center
(Figure 2).13 Mayer atomic bond order analysis revealed that
each M2-Ti bond was found to have a bond order of 0.33 for
2a, 0.29 for 2b, and 0.35 for 2c, respectively, suggesting weak
bonding between the two metal centers. Similar to the bond
distances, the bond orders decrease in the trend Ti-Ni ≈ Ti-Pt
> Ti-Pd, suggesting that the Ti-Pd bond is somewhat weaker
than the Ti-Ni and Ti-Pt interactions.
(14) The reactions of 2a-c with Et2NH or aniline gave the corresponding
methallylamine and the product derived from the metallic component. The
1H and 31P NMR spectra of the latter are complicated, suggesting
decomposition of 2a-c under these conditions.
(15) Catalytic allylation with nucleophiles is generally performed by
using allyl carboxylates or allyl carbonates.16 Since the complexes 2a-c
are generally sensitive toward compounds containing oxygen atoms,
exposure of 2a-c to allyl carboxylates or allyl carbonates instantly results
in decomposition of the complexes. Interestingly, 2a-c are stable toward
the compounds containing halogen atoms, and the reactions with allyl
chloride proceed smoothly as described in the text.
(16) (a) Tsuji, J. Palladium Reagents and Catalysts; Wiley: Chichester,
U.K., 1995. (b) Harrington, P. J. Transition Metal Allyl Complexes: Pd,
W, Mo-assisted Nucleophilic Attack. In ComprehensiVe Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon
Press: New York, 1995; Vol. 12,Chapter 8.2. (c) Handbook of Organopal-
ladium Chemistry for Organic Synthesis; Negishi, E., Ed.; Wiley: New York,
2002. (d) Frost, C. G.; Howarth, J.; Williams, J. M. J. Tetrahedron:
Asymmetry 1992, 3, 1089.
(11) Cordero, B.; Go´mez, V.; Platero-Prats, A. E.; Reve´s, M.; Echeverr´ıa,
J.; Cremades, E.; Barraga´n, F.; Alvarez, S. Dalton Trans. 2008, 2832.
(12) 31P NMR spectra suggest the presence of a Ti-M2 bond interaction.
Single 31P resonances are visible for 2a (δ 14.1), 2b (δ 9.75), and 2c (δ
0.60), which are shifted lower by 20, 1.1, and 0.3 ppm, respectively,
compared with those of 3a-c. The JPt-P value (3216 Hz) in 2c is smaller
than that of 3c (JPt-P ) 3512 Hz). Although the possibility of showing a
significant difference in 31P NMR spectra owing to the presence of nitrogen
atoms in the bidentate phosphine backbone could not be ruled out, these
spectral features could indicate that the electron densities around the M2
center of 2a-c were reduced compared with those of 3a-c.
(13) One of the reviewers suggested that NBO analysis may be a better
tool for understanding the bonding situation. According to this suggestion,
we carried out the NBO calculations of complexes 2a-c at various levels
of theory. However, the Ti-M interaction is too weak to describe the
bonding situation in 2a-c for NBO calculations.

Communications
Organometallics, Vol. 28, No. 7, 2009 1991
In conclusion, we have demonstrated that the electrophilicity
of the η3-methallyl moiety can be significantly enhanced due
to the TirM2 dative bond by introducing the Lewis acidic
titanium center in the bidentate phosphine backbone. In other
words, this work opens a new and alternative way to enhance
the reactivity of the η3-allyl moiety. This may lead to new ideas
in exploring the new transition-metal catalysts which have
unique reactivities and chemical properties.
Synergy of Elements) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan.
Supporting Information Available: Text, figures, tables, and
CIF files giving a detailed experimental section, the molecular
structures of 2a-c, details of crystallographic studies (2a-c),
Gaussview depiction of HOMO or HOMO-1 for the full molecules
1
of 2a-c, and the actual H, 13C, 19F, and 31P NMR data and ESI-
tof-MS spectra of 2a-c. This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by a Grant-
in-Aid for Science Research on Priority Areas (No. 18064014,
OM8011085