Highly strained heterometallacycles of group 4 metallocenes with bis(diphenylphosphino)amide ligands

Article abstract of DOI:10.1002/chem.201201114

A study regarding coordination chemistry of the bis(diphenylphosphino)amide ligand Ph₂P-N-PPh₂ at Group 4 metallocenes is presented herein. Coordination of N,N-bis(diphenylphosphino)amine (1) to [(Cp ₂TiCl)₂] (Cp=I·⁵-cyclopentadienyl) generated [Cp₂Ti(Cl)P(Ph₂)N(H)PPh₂] (2). The heterometallacyclic complex [Cp₂Ti(κ²-P,P-Ph ₂P-N-PPh₂)] (3 Ti) can be prepared by reaction of 2 with n-butyllithium as well as from the reaction of the known titanocene-alkyne complex [Cp₂Ti(I·²-Me₃SiC ₂SiMe₃)] with the amine 1. Reactions of the lithium amide [(thf)₃Li{N(PPh₂)₂}] with [Cp ₂MCl₂] (M=Zr, Hf) yielded the corresponding zirconocene and hafnocene complexes [Cp₂M(Cl){κ²-N,P-N(PPh ₂)₂}] (4 Zr and 4 Hf). Reduction of 4 Zr with magnesium gave the highly strained heterometallacycle [Cp₂Zr(κ ²-P,P-Ph₂P-N-PPh₂)] (3 Zr). Complexes 2, 3 Ti, 4 Hf, and 3 Zr were characterized by X-ray crystallography. The structures and bondings of all complexes were investigated by DFT calculations. All roads lead to Rome: Heterometallacyclic complexes of the type [Cp₂M(κ ²-P,P-Ph₂P-N-PPh₂)] (Cp= I·⁵-cyclopentadienyl; M=Ti, Zr) can be prepared by using four different synthetic pathways (see scheme). All routes yielded the corresponding metallacycles in very high yields. Analysis of the structure and bonding of these complexes revealed that in-plane aromaticity plays an important role for the stabilization of these species. Copyright

Full text of DOI:10.1002/chem.201201114

FULL PAPER
DOI: 10.1002/chem.201201114
Highly Strained Heterometallacycles of Group 4 Metallocenes with
Bis(diphenylphosphino)amide Ligands
Martin Haehnel, Sven Hansen, Anke Spannenberg, Perdita Arndt,
Torsten Beweries,* and Uwe Rosenthal*[a]
Dedicated to Prof. Heinz Berke on the occasion of his 65th birthday
Abstract: A study regarding coordina-
tion chemistry of the bis(diphenylphos-
G
corresponding zirconocene and hafno-
cene complexes [Cp2M(Cl)
{k2-N,P-N-
(PPh2)2}] (4Zr and 4Hf). Reduction of
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
Group 4 metallocenes is presented
herein. Coordination of N,N-bis(diphe-
nylphosphino)amine (1) to [(Cp2TiCl)2]
(Cp=h5-cyclopentadienyl) generated
4Zr with magnesium gave the highly
strained heterometallacycle [Cp2Zr(k2-
P,P-Ph2P-N-PPh2)] (3Zr). Complexes
2, 3Ti, 4Hf, and 3Zr were character-
ized by X-ray crystallography. The
structures and bondings of all com-
plexes were investigated by DFT calcu-
lations.
[Cp2Ti(Cl)PACHTUNGTRENNUNG(Ph2)N(H)PPh2] (2). The
heterometallacyclic complex [Cp2Ti(k2-
P,P-Ph2P-N-PPh2)] (3Ti) can be pre-
pared by reaction of 2 with n-butyl-
Introduction
Ring-strained metallacycles of Group 4 metallocenes have
been of great interest in our studies over the last decades.[1–3]
In this context, the synthesis of three-membered and five-
membered rings, such as metallacyclopropenes, 1-metallacy-
clopent-3-ynes, or 1-metallacyclopenta-2,3,4-trienes, could
be realized. The preparation of the corresponding all-carbon
four-membered metallacyclobuta-2,3-diene (Scheme 1, A)
was not successful until now; however, a theoretical study
predicted its existence and described various approaches for
its synthesis.[4] Additionally, the incorporation of hetero-
ACHTUNGTRENNUNGatoms into the ring systems was found to be an elegant way
Scheme 1. Different four-membered metallacycles.
for stabilizing highly strained structures.[1,4] Therefore, we
focused on the synthesis of four-membered heterometalla-
A
substituent
CyN=C=NCy (Cy=cyclohexyl) with [Cp2Ti(h2-Me3SiC2-
SiMe3)], a bimetallic complex was formed, which displayed
R
and the metal. In the reaction of
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG
L=pyridine) led to the formation of four-membered metal-
lacycles containing the metal, nitrogen, and sulfur
(Scheme 1, B).[5] By using carbodiimides as substrates, the
reactivity becomes more diverse and depends on both the
coordination of the metal fragments to the two nitrogen
atoms as well as to the central carbon atom of the substrate
(Scheme 1, D).[6] This coordination type was described
before for a bimetallic iron complex.[7] By using the zircono-
cene source [Cp2Zr(py)(h2-Me3SiC2SiMe3)] in the reaction
[a] M. Haehnel, S. Hansen, Dr. A. Spannenberg, Dr. P. Arndt,
Dr. T. Beweries, Prof. Dr. U. Rosenthal
Leibniz-Institut fꢀr Katalyse e. V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29A, 18059 Rostock (Germany)
Fax : (+49)381-1281-51176
with carbodiACTHNUGTRNEUNGimides, the formation of five-membered hetero-
metallacycloallenes occurred as we have recently de-
scribed.[8]
These results prompted us to investigate reactions with
precursors for allylic heterometallacycles, such as N,N-bis-
AHCTUNGTREN(NGUN diphenylphosphino)amine Ph2P-N(H)-PPh2 (1), which to
Fax : (+49)381-1281-51104
E-mail: uwe.rosenthal@catalysis.de
torsten.beweries@catalysis.de
date has only been used as a ligand in its deprotonated form
in a few complexes.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201114.
Chem. Eur. J. 2012, 00, 0 – 0
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In general, N,N-bis(diphenylphosphino)amide as a ligand
can exist in three different binding modes (Scheme 2).
N-Binding (Scheme 2, a) is only known in the complexes
Results and Discussion
Reaction of N,N-bis(diphenylphosphino)amine (1) with
[([18]crown-6)Cs{N
G
G
[(Cp2TiCl)2] at elevated temperatures in THF resulted in the
formation of the titanoceneACTHNUTRGENUGN(III) monochloride complex 2
(Scheme 3). N,N-bis(diphenylphosphino)amine acts as
a donor ligand to fill up the deficient coordination number
of the {Cp2TiCl} fragment. The paramagnetic complex 2 was
characterized by mass spectrometry, its molecular-ion peak
was found at m/z 599. Green crystals suitable for an X-ray
analysis were obtained from a saturated toluene solution at
88C.
The molecular structure of complex
2 is shown in
Scheme 2. Different coordination modes of the Ph2P-N-PPh2 moiety at
a metal center.
À
Figure 1. The Ti1 P1 bond length was found to be
2.6224(5) ꢃ, which is in the expected range compared with
other trivalent titanoceneACHTNUTRGENN(UG III) complexes [Cp2TiClACHTUGNTERN(NUGN PMe2R)]
(R=Me,[22]
,
SiMe3[23]). The structural parameters (N1
À
À
(PMDTA = N,N,N’,N’’,N’’-pentamethyldiethylenetriamine),
in which additional interactions of the metal with one or
two Ph groups, respectively, were observed.[9a,b] N-Binding
with an additional P-coordination (Scheme 2, b) was de-
scribed for numerous rare earth[10–13] and some alkaline
earth metal complexes[12–15] as well as for the sodium com-
H1A···Cl1 unit: N1 H1A 0.80(2), H1A···Cl1 2.62(2),
plex [(PMDTA)Na{N
was also discussed for the lithium complex [(thf)3Li{N-
(PPh2)2}].[9c] Roesky and co-workers described [Cp2Zr(Cl)-
(PPh2)2}].[16] This coordination mode
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG ACHTUNGTRENNUNG(PPh2)2}]—the only transition-metal complex with
{k2-N,P-N
coordination mode b.[17]
Complexes containing the chelating k2-P,P-(Ph2P-N-PPh2)
fragment (Scheme 2, c) have been isolated before. However
besides the rubidium complex [([18]crown-6)Rb{N-
ACHTUNGTRENNUNG ACHTUNGTRENNUGN
(PPh2)2}][9b] and the indium complex [In{N(PPh2)2}3],[9c] only
examples with late transition metals, such as iron, nickel,
palladium, and platinum, were reported in this context. El-
lermann and Wend have prepared [CpFe(CO)(k2-P,P-Ph2P-
N-PPh2)] as the first metallacycle, displaying a cyclic Fe-P-
N-P unit at the metal center by salt-metathesis reaction.[18]
However, the obtained product was only characterized spec-
troscopically. Kornev and co-workers published a NiII com-
plex [Ni(k2-P,P-Ph2P-N-PPh2)2] containing two PNP moieties
coordinated at a single metal center by disproportionation
Scheme 3. Synthesis of the titanocene complexes 2 and 3Ti.
of [(Ph3P)2NiNACHTUNGTRENNUNG(SiMe3)2] in the presence of Ph2P-N(H)-PPh2
along with the formation of [(Ph3P)2Ni{h2-Ph2P-N(H)-
PPh2}].[19] The platinum and iron complexes [{h2-Ph2PNP-
ACHTUNGTRENNUNG
(Ph2)NPPh2}M(k2-P,P-Ph2P-N-PPh2)2] (M=Fe, Pt) reported
by Ellermann et al. were synthesized by the reaction of
MCl2 and [(thf)3LiN
(PPh2)2] in boiling toluene.[20] Adding
CO to the iron complex resulted in the formation of
[{h2-Ph2PNP(Ph2)NPPh2}(CO)2Fe(k2-P,P-Ph2P-N-PPh2)2].
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
The first complex of this type, which was characterized by
X-ray crystal-structure analysis, was the palladium complex
[(Et3P)(Cl)Pd(k2-P,P-Ph2P-N-PPh2)], obtained by the reac-
(PPh2)2}].[21] In contrast to
tion of [PdCl2ACHTUNGTRENNUNG(PEt3)2] with [Li{NACHTUNGTRENNUGN
the lithium complex, in this case, the complexation of the
ligand to the metal takes place through a h2-coordination
with the relatively soft, in terms of the HSAB concept,
phosphorus atoms.
Figure 1. Molecular structure of complex 2. Hydrogen atoms, except
H1A, are omitted for clarity. The thermal ellipsoids correspond to 30%
probability.
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Heterometallacycles of Group 4 Metallocenes
FULL PAPER
Scheme 4. Possible resonance forms of complexes 3M.
À
À
N1···Cl1 3.283(2) ꢃ; N1 H1A···Cl1 141(2)8) indicate a weak
hydrogen bond between the chlorine and the hydrogen
tetrahedral geometry. The Ti P distances (2.5997(6)–
2.6293(6) ꢃ) are comparable to those in complex 2. Due to
the chelating effect of the ligand, the P-N-P angle (av
104.78) is rather small for an sp2-hybridised nitrogen atom.
The very small P-Ti-P angle (av 60.18) proves the high strain
of the metallacycle, being even smaller than the P-M-P
angle (64.5–67.48) in other transition-metal complexes men-
tioned above.[20] This is notable, because the square-planar
or octahedral geometry of these complexes and the tetrahe-
dral geometry of the titanium center would suggest the op-
posite (ideal: square planar/octahedral 908, tetrahedral
109.58).
atom of the amino group.[24]
Most interestingly, the titanocene monochloride complex
2 reacted with n-butyllithium in THF in a well-predicted
way to give complex 3Ti as the first highly strained four-
membered metallacycle of an early transition metal with
a N,N-bis(diphenylphosphino)amide ligand (Scheme 3). For-
mation of the dark green TiIII metallacyclic amide [Cp2Ti(k2-
P,P-Ph2P-N-PPh2)] (3Ti) occurred selectively and in high
yield (85%); oxidation to TiIV was not observed. Complex
3Ti was characterized by mass spectrometry, its molecular-
ion peak was found at m/z 562.
Additionally, we investigated other ways to obtain the
metallacycle 3Ti (Scheme 3). When we applied the known
titanocene source [Cp2Ti(h2-Me3SiC2SiMe3)][25] in combina-
tion with the amine 1, we found that formation of the de-
sired TiIII complex occurred readily. Moreover, compound
3Ti can be prepared from the monochloride complex
À
Compared with the P N bond lengths in free N,N-bis(di-
phenylphosphino)amine (1) (1.692 ꢃ, P N single bond),
the P N bond lengths (av. 1.653(3) ꢃ) in the deprotonated
[26]
À
À
ligand bound to a metal are shortened, but longer than a typ-
ical P=N double bond (1.599 ꢃ).[27] This indicates a bond
order between 1 and 2, which can be attributed to the possi-
ble resonance forms for the complex, depicted in Scheme 4.
The reaction of hafnocene dichloride with one equivalent
of lithiated N,N-bis(diphenylphosphino)amine led, not sur-
prisingly, to the formation of a metallocene(IV) amide com-
plex, which features additional coordination of one of the
phosphorus atoms of the ligand (Scheme 5). Complex 4Hf is
[(Cp2TiCl)2] and the lithium amide [(thf)3Li{NACHTNUGRTNEUNG(PPh2)2}]. It
should be noted that in both cases, formation of the metalla-
cycle occurred selectively (yields of 85 and 95%, respective-
ly).
The molecular structure of complex 3Ti is shown in
Figure 2. The titanium center is coordinated by two Cp li-
gands and the chelating PNP moiety in a strongly distorted
Scheme 5. Synthesis of complexes 4Zr and 4Hf.
isostructural to the zirconocene complex [Cp2Zr(Cl)
(PPh2)2}] (4Zr), which was described by Roesky and co-
ACHTUNGTRENNUNG
{k2-N,P-
NACHTUNGTRENNUNG
workers[17] and characterized by NMR spectroscopy. In com-
plex 4Hf in solution, two different phosphorus resonances
were found at d=60.3 and À4.7 ppm with the downfield
signal indicating a strong coordination of the phosphorus
atom to the hafnium center even in solution. At room tem-
perature, rapid exchange of the coordinating phosphorus
atom can be observed by 31P NMR NOESY spectroscopy,
indicating a “flapping” of the N,N-bis(diphenylphosphino)-
1
amide ligand at the metal center. In H NMR spectra, the
3
resonance of the Cp rings appears as a triplet due to a J
coupling (3JP, H =8.4 Hz) of the Cp protons with the coordi-
nating phosphorus atom. At T=348 K, both 31P NMR reso-
Figure 2. Molecular structure of 3Ti. Hydrogen atoms, the second mole-
cule of the asymmetric unit, and the solvent molecules (THF) are omit-
ted for clarity. The thermal ellipsoids correspond to 30% probability.
Chem. Eur. J. 2012, 00, 0 – 0
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U. Rosenthal, T. Beweries et al.
Table 1. Comparison of selected calculated bond lengths (interatomic
distances, respectively) [ꢃ] and angles [8] of complexes 4Ti, 4Zr, and
4Hf with experimental data.
nances disappeared, indicating coalescence of the phospho-
rus atoms at elevated temperatures. Additionally, complex
4Hf was characterized by mass spectrometry, its [M+H]+
peak was found at m/z 730.
4Ti
4Zr
4Zr
4Hf
4Hf
A
E
E
N
ACHTUNGTRENNUNG
Colorless crystals of 4Hf suitable for an X-ray diffraction
analysis were obtained from a saturated toluene solution at
room temperature. The molecular structure of complex 4Hf
is shown in Figure 3. The hafnium center is coordinated by
À
P1 M
P2···M
2.572
3.776
1.660
1.740
2.166
2.481
2.132
2.114
128.71
81.79
56.76
83.37
39.88
124.26
150.18
25.43
À175.06
2.705
3.874
1.678
1.748
2.269
2.532
2.290
2.273
127.02
85.09
56.70
85.11
38.19
123.97
149.11
24.11
À174.24
2.649(2)
3.836
2.713
3.826
1.689
1.752
2.221
2.497
2.270
2.249
126.75
86.06
54.82
86.77
38.41
122.69
148.48
25.86
À173.60
2.6248(6)
3.781
À
P1 N
1.642(5)
1.708(5)
2.249(5)
2.518(2)
2.239(3)
2.246(3)
125.94(1)
84.70(14)
57.6(2)
1.653(2)
1.708(2)
2.208(2)
2.5085(6)
2.224
À
P2 N
À
M N
À
M Cl
À
Ct1 M
À
Ct2 M
2.211
127.1
Ct1-M-Ct2
Cl-M-N
84.50(5)
56.83(7)
84.36(8)
38.81(5)
123.74(12)
149.62(11)
26.7(2)
M-P1-N
P1-N-M
N-M-P1
P1-N-P2
M-N-P2
84.3(2)
38.08(1)
124.2(3)
151.3(3)
7.94
Cl-M-N-P2
Cl-M-N-P1
À177.44
À174.06(7)
Figure 3. Molecular structure of complex 4Hf. Hydrogen atoms, the
second position of the disordered Ph group, and the solvent molecule
(toluene) are omitted for clarity. The thermal ellipsoids correspond to
30% probability.
Scheme 6. Synthesis of the metallacycle 3Zr from complex 4Zr.
two Cp units, a chloride ligand, and the N,N-bis(diphenyl-
phosphino)amide moiety through N1 and P1 in a h2-fashion,
similar to the Zr complex 4Zr.[17] Hf1, N1, and P1 form
a plane with the chloride ligand being 0.26 ꢃ above and the
molecular structure of complex 3Zr, isostructural to com-
plex 3Ti, is depicted in Figure 4. A comparison of selected
bond lengths and angles of 3Ti and 3Zr is shown in Table 2.
The zirconium center is also coordinated in a strongly dis-
À
phosphorus atom P2 0.31 ꢃ below this plane. A Hf1 P1
À
bond length of 2.6248(6) ꢃ was obtained. (cf. [Cp2Hf-
torted tetrahedral geometry. The P N bond lengths in com-
2
(PMe3)(h -Me3SiC2SiMe3)] Hf P 2.660(1) ꢃ[28]).
plex 3Zr are equivalent and the same as were found in 3Ti.
À
Analysis of the bonding in complex 4Hf by using DFT
À
methods showed Wiberg bond indices of 0.48 for Hf P1 and
À
À
À
0.49 for Hf N as well as 0.94 for N P1 and 0.80 for N P2.
À
Compared with the free ligand, the P N bond orders are
significantly decreased (cf. 1.03). Moreover, these values in-
dicate a significant bonding interaction between the metal
center and the P and N atoms, respectively. Similar results
À
were found for the isostructural complex 4Zr (Zr P1 0.52,
À
À
À
Zr N 0.51, N P1 0.96, N P2 0.81; Table 1).
The reaction of complex 4Zr with magnesium at elevated
temperatures in THF resulted in the formation of a very
rare ZrIII complex, the four-membered metallacycle 3Zr
(Scheme 6), which is isostructural to complex 3Ti. The reac-
tion occurs even with an excess of Mg, impressively corrobo-
rating the stability of this ZrIII complex. This is noteworthy,
because normally ZrIII compounds are very sensitive to pos-
sible reduction or disproportionation reactions.[29] Addition-
ally, complex 3Zr was characterized by mass spectrometry,
the molecular ion peak was found at m/z 603.
Figure 4. Molecular structure of complex 3Zr. Hydrogen atoms and the
solvent molecules (toluene) are omitted for clarity. The thermal ellipsoids
correspond to 30% probability.
Complex 3Zr can be obtained as a dark orange com-
pound from concentrated toluene solutions at À788C. The
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Heterometallacycles of Group 4 Metallocenes
FULL PAPER
Table 2. Comparison of selected calculated and experimental bond
Table 3. NBO occupancies, polarizations and Wiberg bond indices
lengths (inter
A
(WBI) of the calculated compounds [(Ph2P)2N]À, 3Ti, 3Zr, and 3Hf.
plexes 3Ti, 3Zr, and the computed complex 3Hf. For complex 3Ti, all
experimental values are averaged.
Occu-
pancy [e] of YNBO
Polarization
WBI
3Ti
3Ti
3Zr
3Zr
3Hf
(sp2.65)+0.555
(sd6.67)+0.837
(sd6.21)+0.853
(sp2.47)+0.552
(sp3.00)+0.560
(sd4.86)+0.875
(sd4.83)+0.885
(sp2.44)+0.554
(sp4.33
(sp2.23
(sp2.17
(sp3.48
(sp3.55
(sp1.94
(sp1.90
(sp3.67
)
)
)
)
)
)
)
)
)
)
)
)
)
1.03
0.60
À
G
1.975
0.948
0.947
0.986
0.981
0.832
0.547
0.522
0.834
0.829
0.484
0.466
0.833
0.826
0.471
0.455
0.834
0.826
E
G
G
R
ACHTUNGTRENNUNG
a
3Ti
3Zr
3Hf
ACHTUNGTRENNUNG
À
À
P1 M
À
P2 M
2.637
2.637
1.683
1.683
3.298
2.046
2.046
136.08
60.85
104.98
97.09
97.09
À6.06
À3.39
1.40
2.622
2.604
1.652
1.654
3.270
2.034
2.043
136.2
60.1
2.758
2.761
1.685
1.685
3.436
2.211
2.213
137.24
58.06
105.25
98.40
98.27
À4.43
À4.40
1.04
2.7036(6)
2.7130(5)
1.655(2)
1.652(2)
3.379
2.723
2.726
1.686
1.686
3.410
2.173
2.176
136.44
58.53
104.38
98.59
98.47
À3.76
À4.60
0.98
Ti P
ACHTUNGTRENNUNG
À
N P
T
1.01
0.58
1.01
0.58
1.00
À
À
P1 N
N P
ACHTUNGTRENNUNG
À
À
P2 N
Zr P 0.957
ACHTUNGTRENNUNG
À
M···N
Zr P 0.956
ACHTUNGTRENNUNG
À
À
Ct1 M
2.179
2.179
137.7
N P
0.986
0.981
ACHTUNGTRENNUNG
À
À
Ct2 M
N P
À
Ct1-M-Ct2
P1-M-P2
Hf P 0.959
G
À
57.69(2)
104.43(10)
98.81(6)
98.54(6)
Hf P 0.959
E
À
P1-N-P2
N-P1-M
N-P2-M
NICS0 ꢃ [ppm]
NICS1 ꢃ [ppm]
Mulliken spin
density (at M)
104.7
97.21
97.84
N P
0.985
0.980
ACHTUNGTRENNUNG
À
N P
ACHTUNGTRENNUNG
tions on the basis of NBO are summarized in Table 3. The
NICS0 ꢃ
values of À6.06 ppm for Ti, À4.43 ppm for Zr,
and À3.76 ppm for Hf indicate in-plane aromaticity in the
heterometallacycle, similarly as we observed before in a the-
oretical study of Group 4 metallocene sulfurdiimide com-
plexes[5] and Group 4 metallocene bis(trimethylsilyl)acety-
lene complexes.[34] The NBO analysis suggests that there are
The P-Zr-P angle (57.69(2)8) represents the high strain of
the heterometallacyle 3Zr as shown in 3Ti and deviates
highly from the ideal tetrahedral angle (109.478).
À
Attempts to perform a similar reduction reaction starting
from the hafnocene complex 4Hf to get a full set of isostruc-
tural heterometallacyclic complexes 3M failed until now,
thus indicating that the reactivity of the hafnocene species is
more diverse than for its zirconocene analogue. However,
the hypothetical complex 3Hf was identified as stationary
point by DFT analysis and was found to be a minimum from
the absence of imaginary harmonic vibrational frequencies
(see below), suggesting that this compound should be in
principle stable. Further experimental studies to address this
issue are under way; the results will be published in due
course. In this context, it should be noted that unexpected,
yet significant changes in reactivity were observed before
when going from zirconocenes to hafnocenes. Examples in-
two highly polarized Ti P s bonds with occupancies of ap-
À
proximately 95% and two P N s bonds, each having a high
occupancy of 98%. This, along with the Wiberg bond indices
(Table 3), reveals the presence of a four-membered s skele-
ton for complex 3Ti. An interannular bonding contribution
between the metal center and the central N can be excluded
by comparing the contact distances and the sum of the cova-
lence radii (rcov; calculated contacts: Ti···N 3.30 ꢃ, Zr···N
À
3.44 ꢃ and Hf···N 3.41 ꢃ, respectively; Srcov: Ti N 2.07 ꢃ,
Zr N 2.25 ꢃ, and Hf N 2.23 ꢃ, respectively[35]). It should
be noted that the small differences between Ti on one side
and Zr and Hf on the other side are in good agreement with
other theoretical studies of Group 4 metallocenes
(Table 3).[34]
À
À
À
À
clude C H and Si C bond activations in the synthesis of
decamethylhafnocene alkyne complexes,[30] THF ring-open-
ing reactions[31] as well as more recently dinitrogen activa-
tion and functionalization at Group 4 metallocenes.[32]
Conclusion
In summary, we presented a detailed experimental and theo-
retical study on the coordination chemistry of the known
N,N-bis(diphenylphosphino)amide ligand to Group 4 metal-
locene fragments {Cp2M} (M=Ti, Zr, Hf). Depending on
the metal, its oxidation state, and the nature of the ligand
precursor (i.e., free amine vs. its Li salt), different products
are obtained, including MIII and MIV chlorides as well as the
metallacycles 3M (M=Ti, Zr). For M=Ti, the latter species
can be generated in several ways, namely reduction of the
amine with [Cp2Ti], salt metathesis from [(Cp2TiCl)2] and
the Li salt or by reaction of the titanocene monochloride 2.
Complex 3Zr was readily obtained by reduction of the ZrIV
chloro complex 4Zr. DFT analysis of the metallacycles 3M
revealed that the stability of these species can be attributed
to a strong cyclic delocalization of electrons in the ring, sim-
DFT study of the metallacycles 3M: To get a more detailed
picture of the bonding situation in the complexes 3M, DFT
calculations on the BVP86/ACTHNUTRGENNG(U 6-311+GACHUTNGTRENN(NGU d,p)/LANL2DZ) level
of theory were performed. The computed structural parame-
ters are in good agreement with those found by X-ray dif-
fraction analysis (Table 2). For comparison, we also deter-
À
mined P N bond lengths and the P-N-P angle in the free
À
À
ligand [(Ph2P)2N] (P N 1.684 ꢃ, P-N-P 116.918).
In complex 3Ti, the TiIII center exhibits a Mulli
A
density of 1.40 (cf. Zr 1.04, Hf 0.98). Examination of the
charge distribution in the ligand by means of natural-bond
orbitals (NBO) analysis revealed that the negative charge is
localized at the N atom in all complexes 3M (for details, see
the Supporting Information, Table S1). The bond composi-
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U. Rosenthal, T. Beweries et al.
yellow. The mixture was dropped slowly into a solution of [Cp2ZrCl2]
(288 mg, 0.99 mmol) in THF (15 mL). After stirring for three days, all
volatiles were removed in vacuum, and the yellow residue was suspended
in toluene (40 mL), filtered, and dried in vacuum to give a pale yellow
product. Yield: 504 mg (80%). 1H NMR ([D6]benzene, 300 MHz, 296 K):
d=7.93 (p, 3J=8.1 Hz, 8H, Ph), 7.22 (t, 3J=6.9 Hz, 4H, Ph), 7.00 (m,
8H, Ph), 5.81 ppm (s, 10H, Cp). 31P NMR ([D6]benzene, 300 MHz,
296 K): d=61.24, À4.49 ppm.
ilarly as observed before for titanocene and zirconocene sul-
furdiimide complexes.
Experimental Section
General: All manipulations were carried out in an oxygen- and moisture-
free argon atmosphere by using standard Schlenk and glovebox tech-
niques. All solvents were dried over sodium/benzophenone and freshly
distilled from sodium tetraethylaluminate prior to use. N,N-bis(diphenyl-
phosphino)amine was purchased from ABCR chemicals and used as re-
ceived. [Cp2TiCl2] and [Cp2ZrCl2] were purchased from Sigma–Aldrich
and used as received. [(Cp2TiCl)2] and [Cp2HfCl2] were purchased from
MCAT (Metallocene Catalysts & Life Science Technologies, Konstanz,
Germany) and used without further purification. [Cp2Ti(h2-
Preparation of 4Hf: To a stirred solution of N,N-bis(diphenylphosphino)-
amine (410 mg, 1.06 mmol) in THF (20 mL), a solution of n-butyllithium
in hexane (2.5m, 0.47 mL, 1.17 mmol) was added at RT. After stirring the
reaction mixture for 2 h, the color turned to light yellow. The mixture
was dropped slowly into a solution of [Cp2HfCl2] (404 mg, 1.06 mmol) in
THF (15 mL). After stirring for three days, all volatiles were removed in
vacuum, and the yellow residue was suspended in toluene (40 mL), fil-
tered, and concentrated to a volume of 4 mL in vacuum. The precipitated
product was dissolved at 808C. Slow cooling to RT gave colorless crystals
suitable for X-ray diffraction analysis; crystals were filtered, washed with
cold toluene, and dried in vacuum. The mother liquor was cooled to
À788C to complete crystallization. Overall yield: 695 mg (90%); m.p.
ACTHNUTRGNEUNG
Me3SiC2SiMe3)][25] and [(thf)3Li{N(PPh2)2}][9] were prepared according to
described procedures. For NMR analysis, Bruker AV300 spectrometer
was used. 1H and 13C NMR chemical shifts are given in ppm (d) and were
referenced by using the chemical shifts of residual protio solvent reso-
nances: [D6]benzene (dH =7.16, dC =128.0 ppm). For elemental analysis,
Leco CHNS-932 elemental-analyzer machine was used. For melting
points determination, Bꢀchi 535 apparatus was used. Melting points are
uncorrected and were measured in sealed capillaries.
1
1538C (decomp under Ar); H NMR ([D6]benzene, 300 MHz, 296 K): d=
7.92 (t, 3J=7.5 Hz, 8H, Ph), 7.07 (m, 12H, Ph), 5.78 ppm (t, 3J=8.4 Hz,
10H, Cp). 13C NMR ([D6]benzene, 300 MHz, 296 K): d=133.16–128.79
(Ph), 110.62 ppm (s, Cp); 31P{1H} NMR ([D6]benzene, 300 MHz, 296 K):
d=60.26, À4.72 ppm; elemental analysis calcd (%) for C34H30ClHfNP2
(728.50 gmolÀ1): C 56.06, H 4.12, N 1.92; found: C 56.08, H 4.15, N 2.06;
MS (CI, iso-butane): m/z (%): 730 [M+H]+.
Preparation of complex 2 from [(Cp2TiCl)2]: To a stirred solution of
[(Cp2TiCl)2] (269 mg, 0.63 mmol) in THF (10 mL), a solution of N,N-bis(-
diphenylphosphino)amine (1; 520 mg, 1.35 mmol) in THF (15 mL) was
added. While the reaction mixture was stirred at 508C for 16 h, its color
turned to dark green. After cooling to RT, the solvent was removed in
vacuum. The dark green residue was dried in vacuum. Yield: 720 mg
(95%); m.p. 1398C (decomp under Ar); MS (CI; iso-butane): m/z (%):
599 [M]+, 563 [MÀCl]+; elemental analysis calcd (%) for C34H31ClNP2Ti
(598.88 gmolÀ1): C 68.19, H 5.22, N 2.34; found: C 68.22, H 5.31, N 2.49;
Crystals suitable for X-ray diffraction analysis were grown from a saturat-
ed toluene solution at 88C.
Preparation of 3Zr from 4Zr: Complex 4Zr (126 mg, 0.197 mmol) and
magnesium (5 mg, 0.206 mmol) were suspended in THF (10 mL) and
heated to 508C for 3.5 h. The color slowly changed from pale yellow to
orange and later to dark brown. After cooling to RT, all volatiles were
removed in vacuum, and the dark brown precipitate was suspended in
toluene (20 mL) and filtered. The solution was concentrated to 10 mL
and stored at À788C to give dark orange crystals of 3Zr suitable for X-
ray analysis; crystals were filtered, washed with cold toluene, and dried
in vacuum. Yield: 65 mg (55%); m.p. 1468C (decomp under Ar); ele-
mental analysis calcd (%)for C34H30NP2Zr (605.78 gmolÀ1): C 67.41, H
4.99, N 2.31; found: C 66.18, H 5.28, N 2.17; MS (EI, 70 eV): m/z (%):
604 [M]+, 385 [MÀCp2Ti]+, 262 [MÀNPPh2]+.
Preparation of complex 3Ti from [Cp2Ti(h2-Me3SiC2SiMe3)]: To a stirred
solution of [Cp2Ti(h2-Me3SiC2SiMe3)] (542 mg, 1.56 mmol) in toluene
(10 mL),
a solution of N,N-bis(diphenylphosphino)amine (1; 628 mg,
1.63 mmol) in toluene (10 mL) was added. After stirring the reaction
mixture for three days at 858C, the color turned from light brown to dark
brown. The mixture was cooled to RT, and all volatiles were removed in
vacuum. The dark brown residue was dissolved in mixture of THF/n-
hexane (3:1; 20 mL) and stored at À308C for three days to give dark
green crystals, which were filtered, washed with cold n-hexane (3 mL),
and dried in vacuum. Yield: 746 mg (85%); m.p. 1118C (decomp under
Ar); MS (EI, 70 eV): m/z (%): 562 [M]+, 385 [MÀCp2Ti]+, 377
[MÀPPh2]+, 178 [Cp2Ti]+; elemental analysis calcd (%) for
C34H30NP2Ti·THF (562.42 gmolÀ1): C 71.93; H 6.04, N 2.21; found: C
72.03, H 6.04, N 2.27. Crystals suitable for X-ray diffraction analysis were
obtained from a saturated solution of THF at 88C.
Computational details: DFT calculations on basis of the BVP86[36] level
of theory were performed applying the ECP basis set LANL2DZ[37] on
the Group 4 metals and 6–311+GACTHUNRTGNEUNG(d,p) to the other elements, respective-
ly, followed by NBO 5.9[38] analyses. All calculations have been carried
out with the Gaussian 09, Rev. A0.2[39] suite of programs.
Crystallographic details: Data were collected using graphite-monochro-
mated MoKa radiation (l=0.71073 ꢃ) on the following diffractometers:
STOE IPDS II (complex 2) and Bruker Kappa APEX II Duo (complexes
3Ti, 4Hf, and 3Zr), respectively. The structures were solved by direct
methods (SHELXS-97) and refined by full-matrix least-squares proce-
dures on F2 (SHELXL-97).[40] CCDC-873829 (2), CCDC-873826 (3Ti),
CCDC-873827 (4Hf), and CCDC-873828 (3Zr) contain the supplementa-
ry crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Preparation of complex 3Ti from [(Cp2TiCl)2]: To a stirred solution of
[(Cp2TiCl)2] (245 mg, 0.574 mmol) in THF (10 mL),
a solution of
[(thf)3Li{N(PPh2)2}] (442 mg, 1.15 mmol) in THF (10 mL) was added. The
ACHTUNGTRENNUNG
reaction mixture was stirred at 508C for 8 h. After cooling to RT, the sol-
vent was removed in vacuum. The dark green residue was suspended in
toluene (20 mL) and filtered. All volatiles were removed in vacuum, and
the green precipitate was dried in vacuum. Yield: 716 mg (95%).
Crystal data for 2: C34H31ClNP2Ti; M=598.89 gmolÀ1; triclinic; space
¯
group P1; a=9.3644(4), b=9.7193(4), c=17.3054(8) ꢃ; a=84.095(4), b=
80.708(3), g =70.022(3)8; V=1458.9(1) ꢃ3; T=150(2) K; Z=2, 1calcd
=
1.363 gcmÀ3; m=0.519 mmÀ1; absorption correction: numerical (max. and
min. transmission: 0.96 and 0.86); 27795 reflections measured; 7855 inde-
pendent reflections (Rint =0.0376); final R values [I>2s(I)]: R1 =0.0316;
wR2 =0.0669; final R values (all data): R1 =0.0529; wR2 =0.0701; GOF on
F2 =0.856, 356 parameters.
Preparation of 3Ti from 2: To
a stirred solution of 2 (152 mg,
0.254 mmol) in THF (10 mL), a solution of n-butyllithium in hexane
(1.6m, 0.24 mL, 0.38 mmol) was added. The light green solution turned
dark green immediately. The solvent was removed in vacuum, and the
dark green residue was suspended in toluene (20 mL of) and filtered. All
volatiles were removed in vacuum, and the dark green precipitate was
dried in vacuum. Yield: 122 mg (85%).
Alternative synthesis of 4Zr:[17] To a stirred solution of N,N-bis(diphenyl-
phosphino)amine (380 mg, 0.99 mmol) in THF (20 mL), a solution of n-
butyllithium in n-hexane (1.6m, 0.74 mL, 1,18 mmol) was added at RT.
After stirring the reaction mixture for one hour, the color turned to light
Crystal data for 3Ti: C34H30NP2Ti·THF; M=634.53 gmolÀ1; monoclinic;
space group Cc; a=26.7399(5), b=16.9826(3), c=17.4040(6) ꢃ; b=
126.169(1)8; V=6380.2(3) ꢃ3; T=150(2) K; Z=8; 1calcd =1.321 gcmÀ3
;
m=0.400 mmÀ1; absorption correction: multiscan (max. and min. trans-
mission: 0.75 and 0.67); 56620 reflections measured; 14374 independent
reflections (Rint =0.0372); final R values [I>2s(I)]: R1 =0.0338; wR2 =
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Heterometallacycles of Group 4 Metallocenes
FULL PAPER
0.0796; final R values (all data): R1 =0.0405; wR2 =0.0830; GOF on F2 =
1.018, 720 parameters.
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2843.
Crystal data for 4Hf: C34H30ClHfNP2·toluene; M=820.60 gmolÀ1; triclin-
¯
ic; space group P1; a=10.2413(2), b=11.6123(2), c=14.9773(3) ꢃ; a=
83.569(1), b=87.645(1), g =88.005(1)8; V=1767.65(6) ꢃ3; T=150(2) K;
Z=2; 1calcd =1.542 gcmÀ3; m=3.147 mmÀ1; absorption correction: multi-
scan (max. and min. transmission: 0.75 and 0.56); 59425 reflections mea-
sured; 8122 independent reflections (Rint =0.029); final R values [I>
2s(I)]: R1 =0.0215; wR2 =0.0546; final R values (all data): R1 =0.0235;
wR2 =0.0563; GOF on F2 =1.056, 389 parameters.
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Crystal data for 3Zr: C34H30NP2Zr·1.6 toluene; M=753.16 gmolÀ1; mono-
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U. Rosenthal, T. Beweries et al.
Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox,
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Inc., Wallingford CT, 2009.
[40] G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112–122.
Received: April 2, 2012
Published online: && &&, 0000
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Heterometallacycles of Group 4 Metallocenes
FULL PAPER
Metallacycles
M. Haehnel, S. Hansen,
A. Spannenberg, P. Arndt,
T. Beweries,* U. Rosenthal* &&&& — &&&&
Highly Strained Heterometallacycles
of Group 4 Metallocenes with Bis(di-
phenylphosphino)amide Ligands
All roads lead to Rome: Heterometal-
lacyclic complexes of the type
routes yielded the corresponding met-
allacycles in very high yields. Analysis
of the structure and bonding of these
complexes revealed that in-plane aro-
maticity plays an important role for
the stabilization of these species.
[Cp2M(k2-P,P-Ph2P-N-PPh2)] (Cp=h5-
cyclopentadienyl; M=Ti, Zr) can be
prepared by using four different syn-
thetic pathways (see scheme). All
All roads lead to Rome…
…and to the heterometallacycle
[Cp2M(k2-P,P-Ph2P-N-PPh2)]. In
their Communication on page &
& ff., U. Rosenthal, T. Beweries
et al. describe four different, yet
straightforward ways of synthe-
sising unusual four-membered
metallacycles of Group 4 metallo-
cenes based on a bis-
ACHTUNGTRENNUNG(diphenyl)amide ligand framework.
The cover picture shows the
drawing of the Colosseum as
a metaphor for the metallacyclic
product complex and roman roads
symbolising the synthetic pathways
leading to it.
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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