Phosphane-functionalized cycloheptatrienyl-cyclopentadienyl titanium sandwich complexes: Phosphorus ligands with an integrated reducing agent for palladium(0) catalyst generation

Article abstract of DOI:10.1002/anie.201304252

A picture of (non)innocence: Non-innocent and bulky phosphanes bearing a reducing cycloheptatrienyl-cyclopentadienyl titanium moiety swiftly convert Pd^II into Pd⁰ species at room temperature (see scheme). The resulting complex cataly

Full text of DOI:10.1002/anie.201304252

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Angewandte
Communications
DOI: 10.1002/anie.201304252
À
C C Coupling
Phosphane-Functionalized Cycloheptatrienyl–Cyclopentadienyl
Titanium Sandwich Complexes: Phosphorus Ligands with an
Integrated Reducing Agent for Palladium(0) Catalyst Generation**
Alain C. Tagne Kuate, Soheila Sameni, Matthias Freytag, Peter G. Jones, and Matthias Tamm*
The Nobel prize winning[1] palladium-catalyzed cross-cou-
pling methodology has become ubiquitous and indispensable
for the formation of carbon–carbon and carbon–heteroatom
bonds in organic synthesis and materials science.[2,3] The most
common ligands employed in combination with a suitable
palladium(0) or palladium(II) complex are phosphane-based,
with Buchwaldꢀs biaryl and Bellerꢀs N-arylpyrrolyl (or
indolyl) dialkylphosphanes of the type A and B, respectively,
representing particularly prominent classes of electron-rich
phosphorus ligands (Scheme 1).[4,5] Ferrocenyl mono- and
diphosphanes have also become firmly established as ancil-
lary ligands for transition metal catalyzed cross-couplings,[6]
and sterically demanding phosphanes such as C and D have
corresponding half-sandwich complexes [(h5-C5H4PR2)TiCl3]
with magnesium in the presence of cycloheptatriene.[10] These
phosphanes can be accessed more conveniently by treatment
of troticene (1) with n-butyl lithium in the presence of
N,N’,N’,N’’,N’’-pentamethyldiethylenetriamine (pmdta), and
the
resulting
lithium
complex
[(h7-C7H7)Ti(h5-
C5H4Li·pmdta)] allows selective phosphane functionalization
of 1 at the five-membered ring by reaction with chlorophos-
phanes (ClPR2; Scheme 1).[11]
With 2 and 3 in hand, a few rhodium(I), platinum(II), and
gold(I) complexes were isolated and structurally character-
ized,[10,11] and none of them showed any evidence for
secondary interactions involving the titanium atom, despite
a valence electron count of 16 for the sandwich moiety. In
contrast, weak intramolecular interactions were found for
metal complexes of the heavier group 4 analogues [(h7-
C7H7)M(h5-C5H4PR2)] (M = Zr, Hf; R = Ph, iPr), which can
also dimerize in the solid state through weak intermolecular
phosphorus–metal contacts.[10,12] These findings are in agree-
ment with the reactivity of the unaltered Cht-Cp sandwich
complexes [(h7-C7H7)M(h5-C5H5)] (M = Ti, Zr, Hf), where
only the zirconium and hafnium derivatives are susceptible to
coordination of additional two-electron-donor ligands such as
isocyanides, phosphanes, and N-heterocyclic carbenes.[9,13,14]
In view of these results, we envisaged that troticenyl
phosphanes might act as innocent ligands in organotransi-
tion-metal catalysis, with the sandwich moiety merely acting
as a more bulky ferrocene analogue.[15] This assumption,
however, was drastically refuted by the study reported herein.
Since cyclohexyl (Cy) and tert-butyl (tBu) groups repre-
sent the favorite substituents (R) in the ligands A–D, the
corresponding phosphanes 4 and 5 were synthesized as green
crystalline solids in approximately 80% yield according to the
established procedure by use of ClPCy2 and ClPtBu2,
À
À
À
been used for numerous C C, C N, and C O bond forming
reactions.[7,8]
Scheme 1. Selected monophosphane ligands for palladium-catalyzed
cross-coupling reactions. Preparation of troticenyl phosphanes. Cy=
cyclohexyl, pmdta=N,N’,N’,N’’,N’’-pentamethyldiethylenetriamine.
Another class of phosphorus ligands derived from the
cycloheptatrienyl–cyclopentadienyl (Cht-Cp) titanium sand-
wich moiety (troticene; 1) was recently introduced by our
group,[9] and the troticenyl monophosphanes 2 (R = Ph) and 3
(R = iPr) were initially prepared by the reduction of the
1
respectively (Scheme 1).[11] The H and 13C NMR spectra are
in agreement with exclusive substitution at the Cp ring, and
the 31P NMR spectra give rise to signals at d = À4.7 (4) and
32.2 ppm (5), which is in agreement with the values reported
for the analogous ferrocenes [(h5-C5H5)M(h5-C5H4PR2)] (M =
Cy, tBu).[7b,16] The molecular structures of 4 and 5 were
additionally confirmed by X-ray diffraction analysis
(Figure 1). Both compounds crystallize with four independent
molecules in the asymmetric unit with very similar structural
characteristics (see the Supporting Information), which
closely resemble those in many other troticene deriva-
tives.[9–11]
[*] Dr. A. C. Tagne Kuate, Dr. S. Sameni, Dr. M. Freytag,
Prof. Dr. P. G. Jones, Prof. Dr. M. Tamm
Institut fꢀr Anorganische und Analytische Chemie
Technische Universitꢁt Braunschweig
Hagenring 30, 38106 Braunschweig (Germany)
E-mail: m.tamm@tu-bs.de
Homepage: http://www.tu-braunschweig.de/iaac
[**] A.C.T.K. is grateful to the Alexander von Humboldt foundation for
a postdoctoral fellowship.
To exploit the potential of 4 and 5 as ancillary ligands in
metal-catalyzed reactions, the Suzuki–Miyaura coupling
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304252.
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Chemie
presence of 0.1 mol% Pd(OAc)2, thus indicating remarkably
fast coupling in comparison with related catalyst systems.[4b]
We assumed that the short reaction times in Table 1
indicate fast generation of a catalytically active species and
therefore sought to isolate the palladium(0) complexes from
the reactions of Pd(OAc)2 with two equivalents of either 4 or
5 in toluene; the complexes [Pd(4)2] (6) and [Pd(5)2] (7)
resulted as dark green solids in approximately 50% yield after
stirring at room temperature for 3 hours (Scheme 2). The
formation of 6 and 7 was unequivocally confirmed by NMR
spectroscopy, for example, by observation of the 31P NMR
chemical shifts at significantly lower field in comparison with
those of the free ligands (6: d = 23.8 ppm, Dd = 28.5 ppm; 7:
d = 57.9 ppm, Dd = 25.7 ppm).
Figure 1. ORTEP drawings of one of the four independent molecules of
4 (left) and 5 (right) with thermal displacement parameters drawn at
50% probability.[31]
between aryl bromides and phenyl boronic acid was chosen as
a test reaction. Initial screening of various bases and
palladium precursors (see the Supporting Information) indi-
cated optimum performance for palladium(II) acetate, [Pd-
(OAc)2], in toluene together with potassium hydroxide as the
base. Under the reaction conditions summarized in Table 1
Table 1: Suzuki–Miyaura coupling of aryl bromides with phenyl boronic
acid catalyzed by Pd(OAc)2 in combination with the troticenyl phos-
phanes 4 or 5.[a]
Scheme 2. Preparation of palladium(0) diphosphane complexes.
The molecular structures were additionally established by
X-ray diffraction analyses, thus revealing the expected
formation of linear diphosphane complexes with Pd-P bond
lengths similar to those reported for other [Pd(PR3)2] systems
(Figure 2).[17] Both structures exhibit a couple of unremark-
Entry
Ar
t [min]
Conv. [%]
Yield [%]
4
5
4
5
4
5
1
2
3
4
Ph
1
1
1
1
1
1
1
1
100
100
100
100
100
100
100
100
93
90
92
95
90
88
98
88
4-MeC6H4
4-MeOC6H4
4-tBuC6H4
5
5
5
60
5
10
20
97
99
72
98
99
97
90
95
69
80
95
92
6[b]
7[c]
2,4,6-Me3C6H2
8
60
60
180
10
60
180
99
98
71
91
76
74
99
94
69
81
74
70
9[b]
10[c]
2,4,6-iPr3C6H2
[a] Pd(OAc)2 (10À5 mol, 1 mol%), 4 or 5 (2ꢃ10À5 mol, 2 mol%), ArBr
(1 mmol), PhB(OH)2 (2 mmol), KOH (2 mmol), toluene (1.5 mL), n-
decane (0.050 mL), T=1008C. The conversion was monitored by GC
with n-decane as an internal standard. The yields were determined after
isolation of the pure products by column chromatography. [b] 0.1 mol%
Pd(OAc)2. [c] 0.01 mol% Pd(OAc)2.
Figure 2. ORTEP drawings of 6 (left) and 7 (right) with thermal
displacement parameters drawn at 50% probability. 6 displays crystal-
lographic inversion symmetry. Selected bond lengths [ꢂ] and angles [8]
for 6: Pd–P 2.2648(4), P-Pd-PA 180.0; for 7: Pd–P1 2.2682(5), Pd–P2
2.2760(5), P1-Pd-P2 179.06(2).[31]
(toluene, T= 1008C, 1 mol% Pd, L:Pd ratio = 2), very rapid
conversions and high yields of isolated products were
obtained for less sterically demanding, unactivated aryl
bromides (reaction times ca. 1 min, entries 1–4). Fast coupling
was also observed for the more hindered substrates such as
mesityl and 2,4,6-triisopropylphenyl bromide (entries 5 and
8), thus providing high yields (ꢀ 90%) in the case of 4. For the
last two substrates, the reactions were also accomplished in
a satisfactory manner upon lowering the catalyst concentra-
tion to 0.1 (entries 6 and 9) and 0.01 mol% (entries 7 and 10),
and it is particularly noteworthy that 2,4,6-triisopropyl-1,1’-
biphenyl was isolated, after 60 minutes, in 94% yield in the
able intramolecular C-H···Pd van der Waals contacts of about
2.9 ꢁ, with the shortest distances involving the C7H7 ring in 6
(Pd-H7 = 2.84 ꢁ) and a tert-butyl group in 7 (Pd-H40B =
2.89 ꢁ).[17b–d] The structural parameters of the troticene
units in 6 and 7 are similar to those in the free phosphane
ligands 4 and 5 and in other troticene derivatives.[9–11]
It should be noted that numerous complexes of the type
[Pd(PR3)2] were previously prepared by the reaction of
tertiary phosphanes with palladium(0) complexes such as
[Pd(dba)2] or [Pd2(dba)3] (dba = dibenzylideneacetone)[17bc,18]
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or with suitable palladium(II) precursors such as [Pd(h3-
C3H5)(h5-C5H5)],[19] [Pd(h3-1-PhC3H4)(h5-C5H5)],[17e]
process, in which the trinegative Cht ligand delivers three
electrons to afford the neutral C7H7 radical and subsequently
its dimer (C7H7)2 (9). Hence, assignment of a formal + 4
oxidation state to the titanium atom in 1 gives a tricationic
(CpTi)3+ fragment, which furnishes 8 by coordination of three
acetate ions. This oxidation can also be accomplished with
silver(I) acetate, which reacts cleanly with 1 in a 1:3 ratio as
shown in Scheme 3. It should be noted that comparison of the
formal redox potential of the tropylium ion C7H7+ (e. g. E0’ =
À0.65 V in CH3CN)[27] with those of the Ag+/Ag and Pd2+/Pd
couples suggests that further oxidation of ditropyl (9) should
occur in the presence of Ag(OAc) or Pd(OAc)2. However,
[(tmeda)PdMe2],[20] [Pd(h3-2-methylallyl)Cl]2,[21] and [(h2:h2-
1,5-octadiene)PdBr2],[22] which provide palladium(0) by
reductive elimination steps. For palladium(II) salts such as
Pd(OAc)2, however, the phosphane itself has to act as the
reducing agent with formation of the corresponding phos-
phane oxide.[23] The system Pd(OAc)2/PPh3 was particularly
well studied, and it was suggested that the intermediate
palladium(II) complex [Pd(OAc)2(PPh3)2] slowly rearranges
through an intramolecular mechanism to form initially the
palladate(0) [Pd(PPh3)(OAc)]À and the phosphonium ion
[Ph3P(OAc)]+, which are subsequently converted into the
catalytically active species [Pd(PPh3)2(OAc)]À and Ph3PO by
reaction with excess phosphane or residual water, respec-
tively. In contrast, a very recent study involving [Pd(OAc)2-
(PCy3)2] showed that this complex cannot be reduced to
[Pd(PCy3)2] in the presence of an excess of PCy3, but follows
a base-promoted pathway to form Cy3PO and a monophos-
phane-palladium(0) species, which is trapped in the presence
of PhBr and water to form the dinuclear acetato- and
À
this process, which involves C C bond cleavage, seems to be
unfavorable under these reaction conditions, since no indica-
tion of 2,4,6-cycloheptatrienyl acetate[28] formation was found
by treatment of 9 (or 1) with a large excess of metal acetate.
In contrast, four-electron oxidation of 1 can indeed be
accomplished by use of silver(I) trifluoromethanesulfonate
(triflate) [AgOTf], in CH2Cl2, and tropylium tetrakis(trifluor-
omethanesulfonato)titanate(IV) (10) can be isolated in high
yield (81%, Scheme 3). This salt is composed of (C7H7)+
cations and [CpTi(OTf)4]À anions, and the structure of the
latter is best described as a pseudosquare pyramid with the h5-
C5H5 ligand in the apical position and with the four CF3SO3-
kO ligands forming the basal plane (Figure 3). We believe
hydroxo-bridged
complex
[{(Cy3P)Pd(Ph)}2(m-OAc)(m-
OH)].[24]
To elucidate a possible mechanism for the palladium
reduction by 4 and 5, the reaction of Pd(OAc)2 with two
equivalents of the phosphane ligand was monitored by
NMR spectroscopy in [D8]toluene solution. The 31P NMR
spectra showed rapid formation of the corresponding palla-
dium(0) complexes with no indication of phosphane oxide
formation, while the 1H NMR spectrum showed characteristic
multiplets in a 2:2:2:1 ratio at d = 6.54, 6.11, 5.22 and
2.03 ppm, which point to the formation of a cycloheptatriene
species. It was therefore supposed that the redox process for
palladium(0) generation might involve the troticene rather
than the phosphane moiety. Accordingly, 1 was treated with
excess Pd(OAc)2 in CH2Cl2, thus instantaneously furnishing
a brownish precipitate of elemental palladium. Filtration and
evaporation of the reaction mixture gave a yellow solid, from
which 7,7’-bi-1,3,5-cycloheptatriene (9; ditropyl) was isolated
by extraction with n-pentane,[25] and the remaining solid was
identified as practically pure [(h5-C5H5)Ti(OAc)3] (8) by
NMR spectroscopy and X-ray structure determination
(Scheme 3).[26]
Figure 3. ORTEP drawing of 10 with thermal displacement parameters
drawn at 50% probability. Selected bond lengths [ꢂ] and angles [8]:
Ti–O1 2.017(2), Ti–O4 2.005(2), Ti–O7 2.001(2), Ti–O10 1.963(2);
O1-Ti-O7 138.68(8), O4-Ti-O10 133.34(8).[31]
that the different reactivity of 1 towards AgOAc and AgOTf
can be ascribed to the different nature of the two counter-
ions.[27] Whereas the acetate can be regarded as a relatively
strongly coordinating anion, which binds to the Ti atom in
a bidentate fashion and would afford a covalent ester with the
tropylium cation, the triflate anion is a more weakly
coordinating anion and therefore capable of forming a ther-
modynamically favorable tropylium salt. Thereby, the high
electrophilicity of the cyclopentadienyl–titanium fragment
will require coordination of four triflate ions and conse-
quently result in the formation of the ate-complex 10.
Scheme 3. Oxidation of troticene (1) by palladium(II) or silver(I)
acetate and by silver trifluoromethanesulfonate (AgOTf).
Considering the stoichiometry of the above redox process
(Scheme 3), the reactions between the phosphanes 4 and 5
and Pd(OAc)2 were repeated with an L:Pd ratio of 8:3 (2.67-
fold phosphane excess according to a three-electron redox
process with 0.67 equiv of L providing two electrons). In fact,
the palladium(0) complexes were isolated in significantly
Since it has been demonstrated several times that the
substantially covalently bound Cht ligand in 1 is better
conceived as a À3 ligand rather than a + 1 ligand,[9] the
formation of 8 and 9 can be formally described as an oxidation
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c) J. P. Wolfe, S. L. Buchwald, Angew. Chem. 1999, 111, 2570 –
higher yields of 70% (6) and 78% (7); Scheme 2). We were
able to confirm that these palladium(0) species are indeed
able to serve as (pre-)catalysts for Suzuki–Miyaura coupling
of mesityl and 2,4,6-triisopropylphenyl bromide with phenyl
boronic acid, and these reactions proceed at a similar rate
under the reaction conditions used for Pd(OAc)2 in combi-
nation with the phosphanes 4 and 5.[29] It should be noted,
however, that these results do not imply that the isolable
L2Pd0 complexes 6 and 7 themselves represent the catalyti-
cally active species, since the corresponding adducts L1Pd0 are
usually considered to be the active catalysts for many
phosphane/palladium systems, and their formation can be
rationalized by an equilibrium between the L2Pd0 and L1Pd0
species.[2,30] Furthermore, variation of the L:Pd ratio from 2.0
(as shown in Table 1) to 2.67 and 1.67 (see the Supporting
Information)[29] did not reveal a clear impact on the catalytic
performance, so that, at this stage, we are unable to provide
clear evidence for L2Pd0 or L1Pd0 species operating as the
active catalysts.
In conclusion, a new type of phosphane ligand has been
introduced, and the cycloheptatrienyl–cyclopentadienyl tita-
nium fragment serves as an integrated reducing agent. The
sandwich moiety is able to provide three or four electrons
depending on the oxidant. Consequently, palladium(0) spe-
cies are efficiently formed from Pd(OAc)2 in the presence of
the phosphanes 4 and 5, which results in exceptionally short
reaction times observed for biaryl formation by Suzuki–
Miyaura cross-coupling. This concept will be further exploited
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À
for other palladium-catalyzed C X coupling reactions, and its
applicability towards catalytic processes involving other
(more or less) noble metals will be also investigated.
Received: May 17, 2013
Revised: June 6, 2013
Published online: July 1, 2013
À
Keywords: C C coupling · palladium · phosphane ligands ·
sandwich complexes · titanium
.
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data for this paper. These data can be obtained free of charge
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[25] The molecular structure was additionally established by X-ray
structure determination (see the Supporting Information).
Ditropyl, (C7H7)2, had previously been isolated on several
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