The Taming of Redox-Labile Phosphidotitanocene Cations

Article abstract of DOI:10.1002/chem.201805430

Tame d⁰ phosphidotitanocene cations stabilized with a pendant tertiary phosphane arm are reported. These compounds were obtained by one-electron oxidation of d¹ precursors with [Cp₂Fe][BPh₄]. The electronic structure of these compounds was studied experimentally (EPR, UV/Vis, and NMR spectroscopy, X-ray diffraction analysis) and through DFT calculations. The theoretical analysis of the bonding situation by using the electron localization function (ELF) shows the presence of π-interactions between the phosphido ligand and Ti in the d⁰ complexes, whereas dπ–pπ repulsion prevents such interactions in the d¹ complexes. In addition, CH–π interactions were observed in several complexes, both in solution and in the solid state, between the phosphido ligand and the phosphane arm. The d⁰ complexes were found to be light sensitive, and decompose through Ti?P bond homolysis to give Ti^III species. A naked d⁰ phosphidotitanocene cation has been trapped by reaction with diphenylacetylene, yielding a Ti/P frustrated Lewis pair (FLP), which was found to be less reactive than a previously reported Zr analog.

Full text of DOI:10.1002/chem.201805430

A Journal of
Accepted Article
Title: The Taming of Redox-Labile Phosphidotitanocene Cations
Authors: Adrien Thomas Normand, Quentin Bonnin, Stéphane
Brandès, Philippe Richard, Paul Fleurat-Lessard, Charles
Devillers, Cédric Balan, Pierre Le Gendre, Gerald Kehr, and
Gerhard Erker
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To be cited as: Chem. Eur. J. 10.1002/chem.201805430
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The Taming of Redox-Labile Phosphidotitanocene Cations
Adrien T. Normand,[a]* Quentin Bonnin,[a] Stéphane Brandès,[a] Philippe Richard,[a] Paul Fleurat-
Lessard,[a] Charles H. Devillers,[a] Cédric Balan,[a] Pierre Le Gendre,[a]* Gerald Kehr[b] and Gerhard
Erker[b]
Abstract: d0 phosphidotitanocene cations stabilized with a pendant
tertiary phosphane arm are reported. These compounds were
obtained by one-electron oxidation of d1 precursors with
[Cp2Fe][BPh4]. The electronic structure of these compounds was
studied experimentally (EPR, UV-vis and NMR spectroscopy, X-ray
diffraction analysis) and through DFT calculations. The theoretical
analysis of the bonding situation using the Electron Localization
Function (ELF) shows the presence of π interactions between the
phosphido ligand and Ti in the d0 complexes, whereas dπ-pπ
repulsion prevents such interactions in the d1 complexes. In addition,
CH/π interactions were observed in several complexes, both in
solution and in the solid state, between the phosphido ligand and the
phosphane arm. The d0 complexes were found to be light sensitive,
and decompose via Ti-P bond homolysis to give Ti(III) species. A
naked d0 phosphidotitanocene cation has been trapped by reaction
with diphenylacetylene, yielding a Ti / P frustrated Lewis pair (FLP)
which was found to be less reactive than a previously reported Zr
analogue.
We have recently reported on the chemistry of d0
phosphidozirconocene cations (C).[7] Compared to neutral
complexes, these species display enhanced reactivity due to the
combination of a coordinatively unsaturated Cp2Zr2+ fragment
with a nucleophilic PR2- ligand. This reactivity can be exploited in
i) the cooperative activation of CO2 and organic substrates; ii) the
catalytic hydrogenation of olefins; iii) the synthesis of Zr+ / P
frustrated Lewis pairs (FLPs) by cycloaddition with alkynes.
Unfortunately, we have thus far been unable to isolate these
species and conduct more detailed investigations into the nature
of the Zr-P bond.
Scheme 1. Group 4 metal complexes with anionic phosphorus ligands.
Introduction
In parallel to these studies, we have also been exploring the
possibility to generate phosphidotitanocene cations (C’).
Compared to the Zr analogues, these compounds require a
different synthetic approach. Indeed, the salt metathesis route
employed with Zr is unfeasible with Ti, since Ti(IV) compounds
are prone to one-electron reduction with concomitant production
of a phosphinyl radical.[9]
Group 4 metal complexes bearing anionic phosphorus ligands
have been investigated since the mid-1960s.[1] Numerous studies
focused on the reactivity and catalytic applications of Zr
phosphinidene (A) and Zr phosphido (B) complexes (Scheme 1,
R2P-: phosphido; RP2-: phosphinidene).[2],[3] By contrast, the
corresponding Ti complexes have been somewhat less explored;
initially, most reports dealt with the synthesis and characterization
of Ti phosphido complexes (B’) ─ including P-bridged bimetallic
compounds ─ with little emphasis on reactivity.[4] Later reports by
Harrod, Stephan and Le Gendre highlighted the importance of Ti
phosphido species in catalytic dehydrocoupling and
hydrophosphination reactions.[5] Finally, Ti phosphinidene
complexes (A’) have been studied by Mindiola.[6]
Scheme 2. Reaction of group 4 metals with alkali metal phosphides (M = Li, Na,
K).[10]
As a consequence, examples of isolable Ti(IV) phosphido
complexes are rather rare,[3a-b],[11] and to the best of our
knowledge, experimentally characterized examples of d0
phosphidotitanocene complexes are unknown. However, we
surmised that stable d0 phosphidotitanocene cations might be
obtained by oxidation of readily available neutral Ti(III) precursors
containing a pendant phosphine arm (Scheme 3).[12]
[a]
[b]
Dr. A. T. Normand, Dr Q. Bonnin, Dr S. Brandes, Dr P. Richard, Dr
C. H. Devillers, C. Balan, Prof P. Le Gendre
ICMUB, Université de Bourgogne Franche-Comté, UFR sciences et
techniques, 9 rue Alain Savary - BP 47870, 21078 Dijon Cedex
(France).
E-mail: adrien.normand@u-bourgogne.fr; pierre.le-gendre@u-
bourgogne.fr
Dr G. Kehr, Prof.G. Erker
Organisch-Chemisches Institut, Universität Münster
Corrensstrasse 40, 48149 Münster (Germany)
Supporting information for this article is given via a link at the end of
the document. CCDC 1871408-1871418 contain the supplementary
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/datarequest/cif.
Scheme 3. Synthesis of d0 phosphidotitanocene cations by one-electron
oxidation of neutral Ti(III) precursors.
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In this paper, we report the successful application of this
strategy and we address the question of the nature of the Ti-P
bond in C’ both experimentally and theoretically (DFT). Finally, we
compare the reactivity of a FLP derived from an unsubstituted
phosphidotitanocene cation with that of its previously reported Zr
counterpart.
Table 1. Relevant bond lengths [Å] and angles [°] for 2a-d.[a]
2a
2b
2c
2d
D[b]
Ti-CpP
Ti-Cp
2.0556(12)
2.0528(14)
2.6652(8)
2.6636(8)
137.40(5)
2.0713(8)
2.0527(9)
2.5990(6)
2.6641(5)
135.99(3)
2.0802(9)
2.0612(9)
2.6040(5)
2.6247(6)
135.24(4)
2.0804(11)
2.0617(12)
2.6499(8)
2.6600(8)
134.44(4)
2.088(5)[c]
2.074(4)
2.636(3)
2.658(3)
133.18(19)
Results and Discussion
Ti-P1[d]
Ti-P2[d]
The synthesis of the target phosphidotitanocene cations can
be effected in two steps from previously described [CpPCpTiCl2]
complexes 1.[13] Ti(III) precursors were first prepared on 1-2 mmol
scale by reacting 2 eq of lithium phosphide (PPh2Li or PCy2Li) with
1 (Scheme 4).[14]
Cp-Ti-
CpP
P1-Ti-
P2
93.40(2)
319.6(2)
85.33(2)
322.5 (1)
79.76(2)
333.8(1)
86.02(2)
325.3(2)
86.02(9)
324.5(5)
Σα(P2)
[a] Cp and CpP indicate the centroid of the Cp and CpP rings, respectively; [b]
literature values; [c] Ti-Cp distance; [d] P1: phosphane ligand, P2: phosphido
ligand.
The most salient structural feature of complexes 2a-d and D
is the pseudo tetrahedral geometry of the phosphido ligand. This
is reflected in the sum of angles around P1, with values ranging
from 319.6(2)° (2a) to 333.8(1)° (2c). Electronic repulsion
between the phosphorus lone pair and the singly-occupied 1a1
orbital of the d1 metal centre is most likely responsible for this
geometry,[2d],[15] which should entail a trigonal planar phosphido
ligand for the oxidized complexes (vide infra). The absence of π-
interactions between the phosphorus lone pair and the Cp2Ti
fragment is also evident from i) the orientation of the phosphido
ligand,[16] and ii) the relatively long Ti-P2 distances (from
2.6247(6), 2c, to 2.6641(5), 2b) which are close to the sum of
covalent radii of Ti and P (2.67±0.11Å).[17] These experimental
findings are corroborated by theoretical calculations conducted on
2b (vide infra).
Scheme 4. Synthesis of neutral Ti(III) precursors 2a-d.
Compounds 2a-d were obtained as green (2a, 2b, 2d) or
brown (2c) solids in moderate yield. They were characterized by
EPR spectroscopy, UV-vis spectroscopy, X-ray diffraction and
elemental analysis.
Unlike the TiCl2 precursors 1a-d, compounds 2a-d all show
an interaction between Ti and the pendant phosphine arm in the
solid state (Figure 2). Table 1 provides a summary of relevant
metric parameters; data for the analogous compound
[Cp2Ti(PPh2)(PMe3] (D) previously reported by Stephan are also
included for comparison.[4j]
It is interesting to compare the structures of 2b and 2c, since
-
the latter features the considerably more basic and bulkier PCy2
-
(compared to PPh2 ) phosphido ligand. First of all, the Ti-P2
distance in 2c is 0.039 Å shorter compared to 2b; the phosphido
ligand is also somewhat flatter in 2c, with a larger sum of angles
around P2 (+11.3 °). It is tempting to ascribe these observations
-
to increased π-interactions between Ti and PCy2 , but the
-
orientation of PCy2 is not adequate for this purpose. A closer
inspection of the structure of 2c suggests another explanation,
namely CH/π interactions.[18] Indeed, according to Nishio’s
formalism, 2c contains two Cy hydrogens in region 2 above one
of the Ph rings of the phosphane arm, and one in region 1 (see
the Supporting Information): therefore, these hydrogens satisfy
the criteria for CH/π interactions.[19] As a consequence, the Cy
ring is brought closer to the phosphane arm, with concomitant
flattening of the phosphido ligand, and shortening of the Ti-P2
distance. The narrower P1-Ti-P2 angle in 2c (79.76(2)°)
compared to 2b (85.33(2)°) is also likely a consequence of these
interactions. Noteworthy, 2b also features a hydrogen atom in
region 1 above one of the phosphane arm’s Ph ring, however this
is achieved simply by orienting both Ph rings in a perpendicular
fashion.
Figure 1. ORTEP drawings of the X-ray structures of 2a-d (ellipsoids drawn
at the 30% probability level, hydrogen atoms removed for clarity, except
where CH-π interactions are highlighted).
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The X-band EPR spectra of 2a-d were recorded in THF and
toluene at 295 K. Due to the extreme moisture sensitivity of these
compounds, hydrolysis products were generally observed in
increasing amounts over time. The EPR spectrum of 2b in toluene
shown in Figure 2 illustrates this phenomenon (see the supporting
information for a detailed discussion).
The absolute values of the coupling constant to the phosphido
ligand (A31P-2) range from 4.8 to 16.3.10-4 cm-1 in THF. These are
consistently smaller than the values of the coupling constant to
the phosphane arm (A31P-1), which span a much more restricted
range of 21.0 to 22.7.10-4 cm-1. These values are consistent with
those found by Stephan for D,[4j] and they are a consequence of
the higher s character of the Ti-phosphane bond. Indeed, NBO
calculations indicate ~40 % s character for the latter, vs ~20 % for
the Ti-phosphido bond (see the Supporting Information).
The UV-vis absorption spectra of 2a-d in THF are
characterized by local maxima (ε= 570-1410 cm-1.M-1) in the red
to near IR region (Figure 3).[20]
Figure 2. EPR spectrum of 2b in toluene showing the apparition of a hydrolysis
product over time.
Relevant parameters (isotropic g-factor, hyperfine and
superhyperfine coupling constants) are gathered in Table 2, along
with data for D previously reported by Stephan.[4j] Theoretical
values were obtained by DFT calculations (see the Supporting
Information for details).
Figure 3. UV-vis spectra of 2a-d in THF.
As TD-DFT is not reliable for open-shell systems,[21] we were
unable to confidently analyse these transitions theoretically, and
can therefore only speculate as to their exact nature.
Table 2. Relevant EPR parameters for 2a-d.[a]
We next turned our attention to the synthesis of the target
phosphidotitanocene cations 3, following the oxidation route
described above (Scheme 3). Thus, Ti(III) complexes 2a-d were
oxidized with [Cp2Fe][BPh4] in C6H5Br, following a modification of
a previously reported procedure.[13b]
2a
2b
2c
2d
D[b]
g
1.9885
1.9885
1.9865
1.9911
1.9912
1.9884
1.9922
1.9922
1.9896
1.9895
1.9896
1.9856
-
1.991
1.988
Calc.[c]
A47/49Ti
Calc.[c]
A31P-1
Calc.[c]
A31P-2
Calc.[c]
9.0
9.0
6.2
8.7
8.7
5.9
8.3
8.5
5.8
9.1
9.0
6.2
-
8.6
4.2
21.0
21.3
-20.9
23.3
23.3
-19.9
22.7
22.3
-19.1
22.4
22.3
-17.7
-
24.4
-29.8
6.3
8
9.9
11.0
12.0
15.4
16.3
17.3
15.7
4.8
5.3
10.5
-
2.1
-6.3
[a] spectra recorded in THF (first line) or toluene (second line) at 295 K; a
values in 10-4.cm-1 [b] literature values, spectrum recorded in toluene at 298
K.[c] Computed in toluene.
Remarkably, the values of g and A for each compound are
very similar in THF and toluene, which implies that phosphane
coordination is retained in THF. This behaviour is also observed
with the d0 phosphidotitanocene cations (vide infra).
Scheme 5. Synthesis of phosphidotitanocene cations 3b-d.
Complexes 3b-d were isolated in good to moderate yield after
workup (Scheme 5); however, complex 3a could not be isolated
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in pure form (the Cp region of the 1H NMR spectrum showed the
presence of impurities, and the mixture could not be purified).
We also attempted the synthesis of the unsubstituted
phosphidotitanocene complex [Cp2Ti(PCy2)][BPh4] (3e) following
the oxidation route described in Scheme 4. Addition of two
equivalents of PCy2Li to [Cp2TiCl2] in C6H5Br followed by oxidation
with [Cp2Fe][BPh4] led to a mixture of dia- and paramagnetic
species, possibly including the [Cp2Ti(PCy2)(PCy2H)]+ cation (see
the Supporting Information). When we repeated this experiment
in THF, the known compound [Cp2Ti(THF)2][BPh4] was isolated
as blue crystals in 23 % overall yield after recrystallization
(Scheme 6).[22] This complex was most probably formed by
homolysis of the Ti-P bond according to Scheme 2.
than the 2.67±0.11Å sum of covalent radii), a perfectly trigonal
planar phosphido ligand (Σα(P2) = 360.0(4)°) and an almost
perfect alignment of donor-acceptor orbitals (φ = -2.3(3) °). These
changes are especially remarkable if one considers that other
parameters (e.g. Ti-P1 distances), hardly change upon oxidation.
Scheme 6. Attempted syntheses of an unsubstituted d0 phosphidotitanocene
cation.
Figure 4. ORTEP drawings of the X-ray structures of 3b-g (ellipsoids drawn at
-
In light of the difficulties associated with the oxidation route,
we attempted a different approach involving the protonolysis of
the Cp2TiMe+ cation with PCy2H (Scheme 7). Interestingly, this
route did not afford 3e, but instead compound 3f, which contains
a coordinated PCy2H ligand. Attempts at preparing 3e by using
only 1 eq of PCy2H failed; rather, mixtures of 3f and Cp2TiMe+
were obtained. A PPh2 analogue (3g) could also be synthesized,
using PPh2H instead of PCy2H.
the 30% probability level, BPh4 anion and most hydrogen atoms removed for
clarity) and definition of the torsion angle φ.
Table 3. Relevant bond lengths [Å] and angles [°] for 3b, 3c and 3e.[a]
3b
3c
3e
Ti-CpP
Ti-Cp
2.0471(18)
2.040(3)
2.5883(12)
2.3599(14)
135.20(9)
90.72(4)
358.9 (3)
16.09(19)
2.0497(9)
2.0438(9)
2.6062(5)
2.3646(6)
133.77(4)
89.26(2)
359.98(12)
-15.85(8)
2.051(3)[c]
2.042(3)
2.5868(17)
2.3611(18)
134.89(12)
88.83(6)
360.0(4)
-2.3(3)
Ti-P1[d]
Ti-P2[d]
Cp-Ti-CpP
P1-Ti-P2
Σα(P2)
φ
Scheme 7. Synthesis of d0 phosphidotitanocene cations stabilized by
secondary phosphanes.
Altogether, these results suggest that mixed phosphane /
phosphidotitanocene cations are quite stable. Indeed, not only
could complexes 3b-f be isolated and studied spectroscopically
(NMR, EPR, UV-vis), but X-ray structures were also obtained for
3b, 3c and 3e (Figure 4).
The impact of oxidation is evident upon examination of the
solid-state structures of complexes 2b / 3b and 2c /3c (Tables 1
and 3). The Ti-P2 distance is 0.30 Å shorter in 3b vs 2b, and 0.26
Å shorter in 3c vs 2c. Additionally, the phosphido ligand in
complexes 3 is now much closer to a trigonal planar geometry
than in complexes 2, thus bringing the phosphorus lone pair
closer to Ti (Σα(P2) = 358.9(3) to 360.0(4)°). Moreover, the
orientation of the phosphido ligand (torsion angle φ) also brings
the lone pair into the same plane as the empty 1a1, b2 and 2a1
orbitals on Ti.[23] The unconstrained PPh2H complex 3e is a case
in point, with a Ti-P2 bond distance of 2.3611(18) Å (much shorter
[a] Cp and CpP indicate the centroid of the Cp and CpP rings, respectively; [b]
literature values; [c] Ti-Cp distance; [d] P1: phosphane ligand, P2: phosphido
ligand.
Therefore, there is considerable structural evidence to
support the hypothesis of π interactions between Ti and the
phosphido ligand in complexes 3. This is confirmed through the
theoretical analysis of the Ti-P interactions in 2b and 3b (see
Computational Details): in 2b, only one lone pair of the phosphido
ligand is involved in a Ti-P bond, while both are interacting with
the titanium atom in 3b (Figure 5) indicating π interactions in these
complexes.[24] This is in line with previous theoretical results
obtained on hypothetical d0 phosphidotitanocene complexes.[12]
A deeper analysis of the X-ray structures of 3b and 3c reveals
yet more interesting features. Firstly, as in the case of 2b and 2c,
CH/π interactions are observed between one of the Ph rings of
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the phosphane arm and CH bonds on the phosphido ligand
(Figure 4). Both hydrogens are located in region 1 above the Ph
rings (see the Supporting Information). Interestingly, in the case
of 3c, this does not seem to induce a narrower P1-Ti-P2 angle as
in 2c. Rather, since the Cy ring is engaged in only one CH/π
interaction with the Ph ring, both rings adopt a perpendicular
orientation, as observed with the Ph rings of 2b and 3b.
Figure 7. Aliphatic region of the 1H and 1H{31P} spectra of 3b in CD2Cl2 (500
MHz, 300 K) showing 3J coupling between P and H nuclei (t: residual toluene
signal).
Table 4. Values of θ (XRD) and 3JPH (NMR) for 3b and 3c .[a]
θ1 (°)
θ2 (°)
3JPH-1 (Hz)
44.9
3JPH-2 (Hz)
5.5
Figure 5. ELF basins in 2b and 3b. Bonding basin are shown in green whilst
lone pairs are in red. For clarity’s sake, only the basins between Ti and P are
shown, and the BPh4- anion is omitted.
3b
3c
172.1(3)
-174.66(12)
-69.5(4)
67.08(16)
43.5
6.1
[a] 1H spectra recorded at 500 MHz in CD2Cl2, 300 K.
Secondly, the chelate ring formed by the phosphane arm
causes one of the CH bonds of the CH2 group attached to the Cp
ring to become almost anti-periplanar to the P1-CH2 bond (Figure
6).
In the case of 3c, a shielded signal (0.00 ppm) is observed in
CD2Cl2 (500 MHz, 300 K) for one of the CH2 hydrogens of the Cy
ring. Interestingly, a shielded signal is also observed in d8-THF
(0.06 ppm) and C6D5Br (-0.17 ppm). The conformation of this
complex appears to be frozen in these solvents at 300 K, since
only one hydrogen experiences a shielding effect (probably
because of steric congestion due to the Cy ring).
Figure 6. Definition of torsion angles θ1 and θ2.
Examination of the 1H NMR spectrum of 3b and 3c indicates
that this conformation is retained in solution. Indeed, Figure 7
shows the aliphatic region of the 1H and 1H{31P} spectra of 3b: the
hydrogens of the Cp-CH2 group resonate at 2.87 and 2.04 ppm,
with 3JPH coupling constants of 44.9 and 5.5 Hz (Table 4).[25] Since
3J coupling between P and H nuclei follows a Karplus relationship,
with a maximum corresponding to anti-periplanar arrangement of
P-C and C-H bonds,[26] we conclude that the conformation of the
ethylene arm of 3b is similar in solution and in the solid state.[27]
The same conclusions can also be drawn for 3c.
The CH/π interactions observed in the X-ray structures of 3b
and 3c should entail considerably shielded 1H NMR signals for the
hydrogens involved, due to aromatic ring current effects. Indeed,
in the case of 3b, the ortho hydrogens of one of the Ph ring of the
phosphido ligand resonate as a triplet at 6.25 ppm in CD2Cl2 (500
MHz, 300 K), whereas the same hydrogens resonate above 7.28
ppm in 3d (Figure 8).[28] However, CH-π interactions are not
strong enough to prevent the free rotation of the Ph ring of the
phosphido ligand.
Figure 8. Conformational origin of shielded 1H NMR signals in 3b and 3c
(CD2Cl2, 500 MHz, 300 K). BPh4- anions omitted for clarity.
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In summary, there is considerable evidence to suggest that
the structures of 3b and 3c are the same in solution and in the
solid state. This correspondence indicates that the CpPCpTi
fragment (CpP = C5H5-CH2CH2PPh2) may be viewed as a pre-
organized host for CH/π interactions with incoming ligands or
substrates. Noteworthy, the neutral d1 analogues are also pre-
organized, since the X-ray structures of 2b and 2c show CH/π
interactions as well.
The 31P{1H} NMR spectra of 3b-g are also interesting in a
number of ways. First of all, the signals corresponding to the
-
phosphido ligand are very deshielded, around 410 ppm for PPh2
-
and 500 ppm for PCy2 (Table 5). For comparison, the
[Cp2ZrPCy2]+ cation (C1) gave a more shielded signal at 396.6
ppm (C6D5Br, 202 MHz, 299 K).[7a] In a recent paper, Eisenstein
noted that neutral d0 alkylidene complexes ─ which are valence
isoelectronic to cationic d0 phosphido complexes ─ display
considerably deshielded 13C NMR signals, similar to carbocations,
despite their nucleophilic carbene character.[29] The major
conclusion of that study was that the observed deshielding of the
carbene signal (compared to ethylene) was the result of more
efficient coupling between MC (ligand based) and π*MC (metal
based) orbitals, which are separated by a narrower gap than the
CC and π*CC orbitals of ethylene. Consequently, d0 alkylidene
complexes of 5d metals give less deshielded carbene signals than
isoelectronic 4d metals complexes, due to the higher energy of 5d
vs 4d empty orbitals. Although this would fall outside the scope of
the present paper, it would be interesting ─ in light of the
Eisenstein study ─ to investigate whether similar explanations
apply to i) the highly deshielded 31P NMR signal of
phosphidometallocene cations 3b-f and C1, and ii) the ~100 ppm
difference between Ti (3d) and Zr (4d) complexes.
Figure 9. UV-vis spectra of 3b-d in THF.
DFT calculations were conducted to investigate the nature of this
transition. Natural Transition Orbitals (Figure 10) indicate that it
corresponds to a LMCT band, mostly centred on the phosphido
ligand.[30]
Table 5. Relevant 31P{1H} NMR parameters for 3b-g.[a]
3b
3c
3d
3g
3f
δ-P1[b]
(ppm)
45.2
44.9
43.8
43.2
52.3
52.2
23.7
N.A.[c]
39.1
N.A.[c]
420.2[c]
N.A.[d]
500.4[c]
N.A.[d]
Figure 10. Natural Transition Orbitals for the highest energy UV-vis band of 3b.
δ-P2
(ppm)
414.1
408.8
506.4
496.7
401.2
395.1
2JPP
(Hz)
70.2
68.7
71.7
70.6
64.0
63.0
83.0
N.A.[c]
76.7
N.A.[c]
Since d0 phosphidotitanocene are usually unstable and
readily decompose to give Ti(III) and phosphinyl radicals
(Scheme 2), we were intrigued by the apparent stability of
complexes 3b-g. The presence of π interactions between the
Cp2Ti fragment and the phosphido ligand alone cannot explain
this stability, as illustrated by our failure to prepare complex 3e.
We reasoned that the additional phosphane ligand in 3b-g might
[a] spectra recorded in CD2Cl2 (first line) or d8-THF (second line) at 202
MHz, 300 K; [b] P1: phosphane ligand, P2: phosphido ligand; [c] spectra
recorded in C6D5Br; [d] 3e and 3f decomposed in THF.
Another striking feature of complexes 3b-f, visible in their
31P{1H} spectra, is the stability conferred by the chelating
phosphane arm in a coordinating solvent such as d8-THF. Indeed,
complexes 3b-d remain unaltered in this solvent, as evidenced by
the similar chemical shifts of P1 and P2 in CD2Cl2 vs d8-THF on
the one hand, and the 2JPP coupling constants on the other hand
(64.0 to 71.7 Hz in CD2Cl2 vs 63.0 to 70.6 Hz in d8-THF). In
contrast, dissolution of 3e or 3f in d8-THF releases free PPh2H or
PCy2H (see the Supporting Information).
We measured the UV-vis absorption spectra of 3b-d in THF
(Figure 9), and we observed intense bands (ε = 6920-7910 cm-
1.M-1) in the blue region (3b -> 437nm; 3c -> 419 nm; 3d -> 448
nm).
simply
provide
kinetic
stabilization
to
otherwise
thermodynamically unstable entities. To test this hypothesis, we
measured the EPR spectra of 3b-g and found that paramagnetic
Ti-P species were present in every case. In the case of 3f and 3g,
+
we could positively identify the Cp2Ti(THF)2 cation as the main
paramagnetic species (g = 1.9758, A47/49Ti = 11.4 x 10-4 cm-1). For
complexes 3b-d, the spectra revealed the presence of at least
three paramagnetic Ti-P species (see the Supporting Information).
Irradiation of 3g and 3g with a UV lamp for 20 minutes led to
mixtures in which only the Cp2Ti(THF)2+ cation was visible by EPR
spectroscopy (Figure 11).
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3
observed for compound E (Cp: 6.23 ppm, JPH = 0.7 Hz;
phosphane : -12.4 ppm).[32]
Interestingly, the high-resolution ESI mass spectrum of 4e
+
shows the expected presence of the Cp2Ti(C(Ph)C(Ph)PCy2
cation (with a distinctive cluster of peaks at 553.24758 Da), but
+
the elusive Cp2Ti(PCy)2 is also observed (375.17211 Da). This
suggests some degree of reversibility for the alkyne insertion
reaction, at least under the relatively forcing conditions of the
mass spectrometry experiment.
A PPh2 analogue of 4e was also prepared by reacting 3e with
a four-fold excess of diphenylacetylene. Although the insertion
reaction was much slower (72 h vs a few minutes), probably due
to the necessity to dissociate PPh2H first, compound 4e (not
shown) was isolated in 88 % yield. The preparation of 4e starting
from an isolated phosphidotitanocene cation indicates that alkyne
insertion probably occurs after oxidation by [Cp2Fe][BPh4] during
the synthesis of 4e.
Figure 11. EPR spectrum of 3g after 20 min irradiation with a UV lamp
Analysis of the reaction mixtures by 31P{1H} and 11B{1H} NMR
spectroscopy revealed i) the complete disappearance of 3f and
3g, ii) an intact B(C6F5)4 anion, iii) concomitant formation of
(PPh2)2 for 3g, and iv) the apparition of several P-containing
species in addition to PCy2H for 3f (some of which was present
before irradiation).[31]
Although 4e is stable enough to be isolated and characterized
by NMR spectroscopy, UV-vis spectroscopy, ESI-MS and X-ray
diffraction analysis (vide infra), it displays slightly broadened
signals by 1H NMR spectroscopy, suggesting the presence of
paramagnetic impurities. Indeed, the EPR spectrum of a solution
of 4e in d8-THF indicated the presence of small amounts of
[Cp2Ti(THF)2][BPh4] (see the Supporting Information), which
again points to the reversible insertion of diphenylacetylene into
the Ti-P bond of 3e.
-
For complexes 3b-d, the mixtures after UV irradiation were
-
again complex by EPR spectroscopy, but in each case the BPh4
anion was also intact, whilst the presence of (PPh2)2 (3b, 3d) or

(PCy2)2 (3c) strongly suggested that PR2 radicals had been
generated.
Irradiation of this solution with a UV lamp and monitoring the
reaction by EPR spectroscopy revealed that a new Ti-P product
Therefore, it appears that d0-phosphidotitanocene cations
decompose by Ti-P bond homolysis just like their neutral
counterparts. Compared to the unsubstituted complex 3e, the
additional stabilization provided by the phosphane ligand in 3b-g
enables the isolation of these otherwise fleeting species.
In light of these findings, we decided to investigate whether
complex 3e could be generated in situ and trapped with a suitable
reagent. We had previously shown that the Zr analogue of 3e (C1)
reacted with diphenylacetylene to give a Zr+ / P frustrated Lewis
pair (FLP),[7a] therefore we generated 3e in the presence of this
reagent, and indeed complex 4e was isolated in 27 % yield after
workup (Scheme 8).
was
being
formed
in
equimolar
quantities
with
[Cp2Ti(THF)2][BPh4]. Both 1H and 31P{1H} spectra showed the
complete disappearance of 4e, although the 11B{1H} NMR
spectrum did not evolve during photolysis. Borates are known to
decompose photolytically in the presence of electron
acceptors,[33] therefore we initially envisaged a non-innocent role
for the BPh4- anion. However, we could not find any evidence for
-
the oxidation of BPh4 , nor could we identify the diamagnetic by-
products of the reaction.
The reaction was repeated on preparative scale, and complex
4e’ was isolated in 40 % yield after workup (Scheme 9).
Scheme 9. Synthesis of 4e by photolysis of 4e.
Scheme 8. Synthesis of a Ti+ / P FLP by the trapping of a phosphidotitanocene
cation.
Complexes 4e and 4e’ were characterized by single crystal X-
ray diffraction analysis (Figure 12, Table 5). The metric
parameters of these complexes support their formulation as Ti-
alkenyl complexes with a chelating phosphane arm, rather than
Ti-carbene complexes bound to an alkylidene phosphonium
moiety.[34] The C11-C12 bond distances are consistent with a
double bond (4e: 1.344(2) Å; 4e’: 1.341(4) Å), whilst Ti-C11 and
C12-P distances indicate the presence of single bonds (4e:
2.178(2) Å and 1.813(2) Å; 4e’: 2.229(2) Å and 1.798(2) Å). The
Evidence for the formation of 4e initially came from the
1
comparison of its H and 31P{1H} NMR spectra with those of the
-
previously reported Zr analogue E (containing the MeB(C6F5)3
anion).[7a] For example, the Cp hydrogens in 4e resonate as a
doublet at 6.52 ppm (3JPH = 1.7 Hz) in CD2Cl2, indicative of
phosphane coordination to Ti, whilst the 31P{1H} spectrum shows
a signal at -7.6 ppm. Similar spectroscopic features were
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shorter Ti-P distance in 4e reflects the higher Lewis acidity of
Ti(IV) vs Ti(III) (4e: 2.6421(8) Å; 4e’: 2.7000(8) Å).
room temperature in CD2Cl2 did not yield the expected FLP
adducts: no reaction was observed in both cases. However, when
4e was reacted with ferrocene carboxaldehyde in CH2Cl2,
compound 5e was obtained in 83 % yield after workup (Scheme
10).
Figure 12. ORTEP drawings of the X-ray structures of 4e and 4e’ (ellipsoids
drawn at the 30% probability level, hydrogen atoms removed for clarity) Cp1
and Cp2 indicate the centroid of the Cp rings. Selected bond distances (Å) and
angles (°). 4e: Ti-Cp1 = 2.0347(9); Ti-Cp2 = 2.0379(9); Ti-P = 2.6421(8); Ti-C11
= 2.178(2); C11-C12 = 1.344(2); C12-P = 1.813(2); Cp1-Ti-Cp2 = 133.83(4); P-
Ti-C11 = 63.37(4); Σα(C11) = 359.8(2); Σα(C12) = 359.7(2). 4e’: Ti-Cp1 =
2.0562(13); Ti-Cp2 = 2.0623(13); Ti-P = 2.7000(8); Ti-C11 = 2.229(2); C11-C12
= 1.341(4); C12-P = 1.798(2); Cp1-Ti-Cp2 = 133.46(6); P-Ti-C11 = 61.22(6);
Σα(C11) = 359.6(3); Σα(C12) = 359.9(3).
Scheme 10. FLP reaction of 4e with ferrocene carboxaldehyde.
Compound 5e exhibits characteristic NMR features in CD2Cl2,
indicative of the activation of the CHO moiety by the Ti /P FLP.
Firstly, the 31P{1H} NMR spectrum shows a signal at 32.5 ppm (4e:
-7.6 ppm), consistent with the presence of a phosphonium moiety.
Secondly, the activated aldehydic proton resonates as a broad
signal at 6.12 ppm in the 1H spectrum, and couples both with the
carbons of the ferrocene Cp ring and the CH carbons of the Cy
rings in the HMBC spectrum. Finally, the activated aldehydic
carbon resonates as a doublet at 80.2 ppm (1JPC = 49.2 Hz) in the
13C{1H} spectrum.
The high resolution ESI spectrum of 5e displays a single
cluster of peaks at 767.25649 Da, which is again consistent with
FLP adduct formation.
Single crystals suitable for X-ray diffraction were obtained by
slow diffusion of pentane into a CH2Cl2 solution of 5e at -18 °C
(Figure 14).
In order to determine the redox properties of the 4e/4e’,
couple, voltammetric analyses were performed in THF, with 0.1 M
NaBPh4 as electrolyte. Under these conditions, 4e is reversibly
reduced at E1/2 = −0.420 V vs. SCE (Figure 13). The reduced
species was attributed to the Ti(III) complex 4e'. Complementary
cyclic and steady state voltammetry experiments conducted on a
mixture of 4e and 4e’ confirmed that these compounds form one
redox couple, and can be reversibly transformed into each other
(see the supporting information).
Epa = -0.385 V, Ipa = 3.45 µA
4
2
0
-2
-4
-6
E1/2 = -0.420 V
Ep = 70 mV
ipc / ipa = 1.03
Figure 14. ORTEP drawings of the X-ray structure of 5e (ellipsoids drawn at the
30% probability level, hydrogen atoms, BPh4 anion and CH2Cl2 solvate
molecules removed for clarity).
-
Epc = -0.455 V, Ipc = -5.16 µA
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
Compared to 4e, the Ti-C11 distance (2.231(3) Å) is
significantly elongated (+0.053(4) Å). The C11-C12 distance is
somewhat elongated (1.361(3) Å, +0.017(4) Å), whilst the C12-P
distance is slightly shortened (1.800(3) Å, -0.013(4) Å).
Noteworthy, the Fe-O distance (3.521(2) Å) is shorter than the
sum of Van der Waals radii (3.94 Å), a feature which was
previously observed in a related Zr complex.[35]
E / V vs. SCE
Figure 13. 4e in THF 0.1 M NaBPh4 ([4e] = 5 x 10-4 M, ν = 100 mV s-1, working
electrode: GC electrode Ø = 3 mm, counter electrode: Pt, Reference electrode:
SCE).
Finally, complex 4e’ failed to react with ferrocene
carboxaldehyde (toluene, room temperature), highlighting the
Finally, we investigated the FLP behaviour of 4e. By contrast
with the Zr analogue E, reaction with chalcone or 1 atm CO2 at
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greater Lewis acidity of the d0 titanocenium cation compared to its
neutral d1 sibling.
Elemental Analysis: a satisfactory elemental analysis could not be
obtained for this compound despite repeated attempts (C content was
always low by 5-10 %).
Compound 2c: In an Ar glovebox, a solution of PCy2Li (408 mg, 2.0
mmol) in THF (6 mL) was added to a solution of complex 1c (461 mg,
1.0 mmol) in THF (6 mL). The reaction mixture turned brown almost
instantly, volatiles were removed in vacuo outside the glovebox, and
toluene (30 mL) was added to the residue. The resulting suspension
was taken back inside the glovebox, filtered over diatomaceous earth
to remove LiCl, and the filtrate was evaporated outside the glovebox.
The residue was taken inside the glovebox and triturated in Et2O (20
mL), forming a brown precipitate. It was filtered over a sintered glass
frit, rinsed with 3x4 mL of a 1:1 mixture of Et2O/pentane, then dried on
the frit, yielding 2c as a brown powder (325 mg, 55 %). Single crystals
suitable for X-ray diffraction analysis were obtained by slow diffusion
of pentane into a toluene solution of 2c at -18°C. Elemental Analysis:
calcd for C36H45P2Ti: C, 73.59; H, 7.72. Found: C, 73.40; H, 7.60.
Conclusions
Although d0 phosphidotitanocene complexes are intrinsically
unstable due to their redox-labile nature, we have shown that they
can be isolated as phosphane-stabilized cations. This is
somewhat counter-intuitive, since d0 complexes are classified as
hard Lewis acids, whilst phosphorus-based ligands fall in the
category of soft Lewis bases. It appears that the phosphane-
stabilized phosphidotitanocene cations reported in this study are
unique in many ways: although they are stable enough to be
isolated and crystallographically characterized, they readily
decompose in the presence of light. Additionally, they have
strikingly similar structures in solution and in the solid state, a
consequence of the pre-organization of the phosphane-
Compound 2d: In an Ar glovebox, a solution of PCy2Li (408 mg, 2.0
mmol) in THF (6 mL) was added to a solution of complex 1c (461 mg,
1.0 mmol) in THF (6 mL). The reaction mixture turned green almost
instantly, volatiles were removed in vacuo outside the glovebox, and
toluene (30 mL) was added to the residue. The resulting suspension
was taken back inside the glovebox, filtered over diatomaceous earth
to remove LiCl, and the filtrate was evaporated outside the glovebox.
The residue was taken inside the glovebox and triturated in Et2O (20
mL), forming a green precipitate. It was filtered over a sintered glass
frit, rinsed with 3x4 mL of a 1:1 mixture of Et2O/pentane, then dried on
the frit, yielding 2d as a brown powder (307 mg, 52 %). Single crystals
suitable for X-ray diffraction analysis were obtained by slow diffusion
of pentane into a toluene solution of 2d at -18°C. Elemental Analysis:
calcd for C36H45P2Ti: C, 73.59; H, 7.72. Found: C, 73.34; H, 7.59.
functionalized metallocenium fragment in
a
favourable
conformation for CH/π interactions. Thus, in contrast to their
redox-labile behaviour, they show great structural resilience even
in coordinating solvents such as d8-THF. Finally, they give rise to
extremely high 31P NMR chemical shifts, in the 400-500 ppm
range, as a consequence of the double-bond character of the Ti-
P interaction.
Experimental Section
Compound 3b: In an Ar glovebox, 2b (574 mg, 1.0 mmol) and
[Cp2Fe][BPh4] (505 mg, 1.0 mmol) were mixed in C6H5Br (5 mL). The
reaction mixture gradually turned from green to red over 5 min;
residual particles of [Cp2Fe][BPh4] were observed. After decantation,
the supernatant was added to 50 mL of pentane under vigorous
agitation. A brick-red solid formed, it was filtered over a sintered glass
frit, suspended three times in pentane (5 mL) and dried on the frit. The
resulting powder was freed from residual C6H5Br and [Cp2Fe] by
dissolution in CH2Cl2 (4 mL), filtration over diatomaceous earth, and
precipitation as previously. Finally, the obtained product was stirred in
toluene (8 mL) for 3h, then filtered and rinsed with toluene and
pentane, yielding 3b as a brick-red powder containing 50 mol% of
toluene and 15 mol% of pentane (708 mg, 79 %). Single crystals
suitable for X-ray diffraction analysis were obtained by slow diffusion
of pentane into a CH2Cl2 solution of 3b at -18°C. Elemental Analysis:
calcd for C60H53BP2Ti(C7H8)0.5(C5H12)0.15: C, 81.10; H, 6.23. Found: C,
79.88; H, 6.04.
General
General information, detailed reaction procedures, analytical
details, and structural data of the new compounds are given in the
Supporting Information.
Synthesis
Compound 2a: In an Ar glovebox, a solution of PPh2Li (408 mg, 2.0
mmol) in THF (6 mL) was added to a suspension of complex 1a (487
mg, 1.0 mmol) in THF (8 mL). The reaction mixture turned green
almost instantly, and was stirred at room temperature for 30 min.
Volatiles were removed in vacuo outside the glovebox, and toluene
(15 mL) was added to the residue. The resulting suspension was
filtered over diatomaceous earth to remove LiCl, and the filtrate was
evaporated. The residue was taken inside the glovebox, and Et2O (15
mL) was added, forming a green precipitate. It was filtered over a
sintered glass frit, rinsed with 3x10 mL of Et2O, then 2x10 mL of
pentane, then dried on the frit, yielding 2a as a green microcrystalline
solid (374 mg, 62 %).
Compound 3c: In an Ar glovebox, 2c (196 mg, 0.33 mmol) and
[Cp2Fe][BPh4] (168 mg, 0 mmol) were mixed in C6H5Br (5 mL). The
reaction mixture gradually turned from green to red over 5 min;
residual particles of [Cp2Fe][BPh4] were observed, hence the mixture
was filtered over diatomaceous earth. The resulting solution was
added to 100 mL of pentane under vigorous agitation. An orange solid
formed, it was filtered over a sintered glass frit, suspended three times
in pentane (5 mL) and dried on the frit. The resulting powder was freed
from residual C6H5Br and Cp2Fe by stirring in toluene (20 mL). An oil
formed initially, which turned into a powder over 10 min. The solid was
filtered over a sintered glass frit, suspended three times in toluene (4
mL) then three times in pentane (4 mL) and dried on the fritt. Complex
3c was obtained as an orange powder containing 66 mol% of toluene
(178 mg, 59 %). Single crystals suitable for X-ray diffraction analysis
were obtained by slow diffusion of pentane into a CH2Cl2 solution of
3c at -18°C. Elemental Analysis: calcd for C60H64BP2Ti(C7H8)0.66: C,
80.30; H, 7.22. Found: C, 79.40; H, 7.17.
Single crystals suitable for X-ray diffraction analysis were obtained by
slow diffusion of pentane into a toluene solution of 2a at -18°C.
Elemental Analysis: calcd for C37H47P2Ti: C, 73.87; H, 7.87. Found: C,
73.17; H, 7.71.
Compound 2b: A solution of PPh2Li (769 mg, 4.0 mmol) in THF (40
mL) was added to a solution of complex 1b (920 mg, 2.0 mmol) in
THF (40 mL). The reaction mixture turned green almost instantly,
volatiles were removed in vacuo, and toluene (30 mL) was added to
the residue. The resulting suspension was filtered over diatomaceous
earth to remove LiCl, and the filtrate was evaporated. The residue was
taken inside the glovebox, and a mixture of Et2O (25 mL) and pentane
(25 mL) was added, forming a green precipitate. It was filtered over a
sintered glass frit, rinsed with 3x4 mL of a 1:1 mixture of Et2O/pentane,
then dried on the frit, yielding 2b as a green powder (770 mg, 67 %).
Single crystals suitable for X-ray diffraction analysis were obtained by
slow diffusion of pentane into a toluene solution of 2b at -18°C.
Compound 3d: In an Ar glovebox, 2d (218 mg, 0.37 mmol) and
[Cp2Fe][BPh4] (188 mg, 0.37 mmol) were mixed in C6H5Br (3 mL). The
reaction mixture gradually turned from green to red over 5 min;
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residual particles of [Cp2Fe][BPh4] were observed, hence the mixture
was filtered over diatomaceous earth. The resulting solution was
added to 100 mL of pentane under vigorous agitation. A brick-red solid
formed, it was filtered over a sintered glass frit, suspended three times
in pentane (10 mL) and dried on the frit. The resulting powder was
freed from residual C6H5Br and Cp2Fe by stirring in toluene (10 mL).
An oil formed, the supernatant was discarded, fresh toluene (10 mL)
was added and agitation was resumed. A powder formed over a few
hours. The solid was filtered over a sintered glass frit, suspended
three times in toluene (6 mL) then three times in pentane (10 mL) and
dried on the frit. Complex 3d (178 mg, 50 %) was obtained as an
orange powder.. Elemental Analysis: C60H64BP2Ti: C, 79.56; H, 7.12.
Found: C, 78.11; H, 7.32.
glovebox, and [Cp2Fe][BPh4] (485 mg, 0.9 mmol) was added. A blue
precipitate appeared gradually; the presence of (PCy2)2 was
ascertained by performing no lock 31P{1H} NMR experiment. After 1H
reaction, the reaction mixture was filtered over a sintered glass frit,
the precipitate was suspended twice in THF (1.5 mL) and twice in
pentane (3.0 mL) and dried on the frit. Complex [Cp2Ti(THF)2][BPh4]
was obtained as a blue-green powder (327 mg, 51 %). Material
suitable for elemental analysis was obtained by diffusion of pentane
into a THF solution (50 mL) of the crude product. Royal blue crystals
of [Cp2Ti(THF)2][BPh4] were obtained (152 mg, 45 %). Single crystals
suitable for X-ray diffraction analysis were obtained by cooling a
saturated solution of [Cp2Ti(THF)2][BPh4] in THF at -18°C. Elemental
Analysis: calcd for C36H42PTi: C, 78.64; H, 7.23. Found: C, 77.45; H,
7.11.
Attempted synthesis of 3e: In an Ar glovebox, Cp2TiCl2 (49.8 mg,
0.2 mmol) and PCy2Li (81.6 mg, 0.4 mmol) were mixed in C6H5Br (2
mL). The mixture turned chocolate brown over 5 min. It was filtered
over diatomaceous earth, then [Cp2Fe][BPh4] (101 mg, 0.2 mmol) was
added and the resulting mixture was stirred for 5 min, during which it
turned progressively red. It was filtered over diatomaceous earth in
order to remove a green-black insoluble solid. The homogeneous red
filtrate was added to 80 mL of pentane under vigorous agitation, to
precipitate an orange solid. The precipitate was filtered over a sintered
glass frit, suspended three times in pentane (10 mL) and dried on the
frit. An orange powder (30 mg) was isolated and analyzed by NMR
and EPR spectroscopy.
Compound 4e: In an Ar glovebox, Cp2TiCl2 (498 mg, 2.0 mmol) and
PCy2Li (816 mg, 4.0 mmol) were mixed in C6H5Br (8 mL). After 5 min,
the brown reaction mixture was filtered through diatomaceous earth
onto
a mixture of [Cp2Fe][BPh4] (1.01 mg, 2.0 mmol) and
diphenylacetylene (356 mg, 2.0 mmol). A green-brown solution was
obtained after 5 min, which was added to 120 mL of pentane under
vigorous agitation. A green solid formed, it was filtered over a sintered
glass frit, suspended three times in pentane (10 mL) and dried on the
frit. The resulting powder was freed from residual C6H5Br and Cp2Fe
by dissolution in CH2Cl2 (6 mL) and filtration over diatomaceous earth.
A blue-green solid was removed, and the resulting homogeneous
solution was evaporated to dryness. Toluene (100 mL) was added to
the residue, and a green solid formed. The suspension was stirred for
3 h, then filtered over a sintered glass frit. The solid was suspended
three times in toluene (15 mL) and three times in pentane (15 mL),
and dried on the frit. Complex 4e was obtained as an olive green
powder containing 40 mol% of toluene (483 mg, 27 %). Complex 4e
slowly degrades over time to give 4e’ (vide infra), which explains the
impossibility to obtain a satisfactory elemental analysis despite
repeated attempts on different batches. Single crystals suitable for X-
ray diffraction analysis were obtained by slow diffusion of pentane into
a CH2Cl2 solution of 4e at -18°C.
1
The H and 31P{1H} NMR spectrum suggested the presence of the
Cp2Ti(PCy2)(PCy2H)+ cation (compare with 3f) but impurities were
also observed. Moreover the 1H spectrum showed considerably
broadened signals, consistent with the presence of paramagnetic
impurities. The EPR spectrum revealed a mixture of species.
Compound 3f: In an Ar glovebox, Cp2TiMe2 (200 mg, 0.96 mmol) and
Ph3CB(C6F5)4 (888 mg, 0.96 mmol, 1 eq) were mixed for 5 min in 2
mL of C6H5Br at room temperature. A solution of PCy2H (401 mg, 2.02
mmol) in C6H5Br was added dropwise to the reaction mixture. The
resulting deep-red solution was stirred for 3h until a red precipitate
was formed. The suspension was added dropwise to 8 mL of pentane
under vigorous agitation to complete the precipitation of the desired
red compound. The supernatant was removed and the red precipitate
was washed twice with 5 mL of pentane. After removal of traces of
pentane in vacuo, the red precipitate was dissolved in CH2Cl2 and
precipitated from pentane as described above (N.B.: compound 3f
slowly decomposes in CH2Cl2, therefore this step must be performed
quickly). Removal of volatiles in vacuo yielded complex 3f (1.20 g,
83 %) as a red powder containing 50 mol% of pentane. Elemental
Analysis: calcd for C58H55BF20P2Ti(C5H12)0.5: C, 56.39; H, 4.77; Found:
C, 57.00; H, 5.00.
Compound 4e’: A solution of 4e (440 mg, 0.48 mmol) in THF (15 mL)
was irradiated for 4h30 with a 150 W UV lamp (Heraeus TQ 150).
Volatiles were removed in vacuo and the residue was stirred in 20 mL
of pentane inside an Ar glovebox for several hours. The resulting
suspension was filtered on a sintered glass frit, yielding a blue-green
solid and a brown solution. The solution was evaporated to dryness,
yielding 4e’ as a brown gum (105 mg, 40 %). Single crystals suitable
for X-ray diffraction analysis were obtained by cooling a saturated
solution of 4e’ in pentane at -18°C. Elemental Analysis: calcd for
C36H42PTi: C, 78.11; H, 7.65. Found: C, 77.27; H, 8.15.
Compound 3g: In an Ar glovebox, Cp2TiMe2 (200 mg, 0.96 mmol)
and [Ph3C][B(C6F5)4] (888 mg, 0.96 mmol, 1 eq) were mixed for 5 min
in 2 mL of C6H5Br at room temperature. A solution of PPh2H (376 mg,
2.02 mmol) in C6H5Br was added dropwise to the reaction mixture.
The resulting deep-red solution was stirred for 3h until a red
precipitate was formed. The suspension was added dropwise to 8 mL
of pentane under vigorous agitation to complete the precipitation of
the desired red compound. The supernatant was removed and the red
precipitate was washed twice with 5 mL of pentane. After removal of
traces of pentane in vacuo, the red precipitate was dissolved in CH2Cl2
and precipitated from pentane as described above (N.B.: compound
3g slowly decomposes in CH2Cl2, therefore this step must be
performed quickly). Removal of volatiles in vacuo yielded complex 3g
(980 mg, 83 %) as a red powder. Single crystals suitable for X-ray
diffraction analysis were obtained by slow diffusion of heptane in a
bromobenzene solution of the compound at -18°C. Elemental
Analysis: calcd for C58H31BF20P2Ti: C, 57.07; H, 2.45; Found: C, 56.71;
H, 2.54.
Compound 4g: In an Ar glovebox, compound 3g (400 mg, 0.32
mmol) was dissolved in C6H5Br (3 mL) with an excess of
diphenylacetylene (232 mg, 1.3 mmol) at room temperature and
stirred for 72 h. The evolution of the reaction was followed by 31P{1H}
no lock NMR experiments. The starting red solution became green.
The compound was precipitated by addition to 6 mL of pentane under
vigorous agitation. An oil formed, the supernatant was discarded and
the oil extracted twice with pentane. The black-green residue was
dissolved in 2 mL of dichloromethane and precipitated as described
above. Compound 4g was obtained as a light brown powder
containing 30 mol% of pentane after drying in vacuo (350 mg, 88 %).
Elemental Analysis: calcd for C60H30BF20PTi(C5H12)0.3: C, 59.47; H,
2.73; Found: C, 59.73; H, 2.44.
Compound 5e: In an Ar glovebox, compound 4e (170 mg, 0.19 mmol)
and ferrocene carboxaldehyde (42 mg, 0.20 mmol) were mixed in
CH2Cl2 (3 mL). After 5 min, the red reaction mixture was added to 50
mL of pentane under vigorous agitation. A red precipitate formed,
which was filtered over a sintered glass frit, rinsed with pentane and
dried under vacuum. Compoung 5e was obtained as a brown-red solid
(176 mg, 83 %) containing 75 mol% of pentane Single crystals
suitable for X-ray diffraction analysis were obtained by slow diffusion
of pentane into a CH2Cl2 solution of 5e at -18°C. Elemental Analysis:
Compound [Cp2Ti(THF)2][BPh4]: In an Ar glovebox, Cp2TiCl2 (249
mg, 1.0 mmol) and PCy2Li (408 mg, 2.0 mmol) were mixed in toluene
(5 mL). After 5 min, the brown reaction mixture was filtered through
diatomaceous earth and the filtrate was evaporated in vacuo outside
of the glovebox. The residue was dissolved in THF (10 mL) in the
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10.1002/chem.201805430
Chemistry - A European Journal
FULL PAPER
calcd for C71H72BFeOPTi(C5H12)0.75: C, 78.69; H, 7.16; Found: C,
77.88; H, 6.74.
Recherche Scientifique (CNRS), Agence Nationale de la
Recherche and Deutsche Forschungsgemeinschaft (MENOLEP
project) and the Conseil Regional de Bourgogne (PARI CDEA
program) is gratefully acknowledged.
Hydrolysis of Ti(III)-PPh2 complexes
In an Ar glovebox, the Ti(III)-PPh2 complexes (2a: 0.5 mmol; 2b: 0.19
mmol) were dissolved in 4 mL of THF and treated with 0.5 eq of
distilled water in THF. The solution turned blue-violet immediately.
The volatiles were removed in vacuo, the solid residue was triturated
with pentane, filtered and rinsed with pentane. Blue violet powders
were obtained in both cases after drying in vacuo (2a: 60 mg; 2b: 46
mg). EPR spectra of 2a and 2b were also recorded in “wet”,
unstabilized THF (degassed by free-pump-thaw) for comparison
purposes.
Keywords: transition metal phosphides • titanium • phosphorus
ligands • FLP • DFT
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The Ti(IV) complexes (3b, 4e: 0.05 mmol; 3c-g: 0.01 mmol) were
dissolved in THF or d8-THF and irradiated in sequences of 20 min with
a medium pressure Hg lamp (Heraeus TQ 150).
FLP reactions
Compound 4e (43.6 mg, 0.05 mmol) was dissolved in CD2Cl2.
Chalcone (10.4 mg, 0.05 mmol) was added and 1H and 31P{1H} NMR
spectra were recorded, showing only the presence of both reagents.
Compound 4e (43.6 mg, 0.05 mmol) was dissolved in CD2Cl2 and
placed in a Schlenk vessel. The vessel was quickly evacuated, and
refilled with 1 atm of CO2. The reaction mixture was stirred for 2 h, and
1H and 31P{1H} NMR spectra were recorded, showing only the
presence of 4e.
Compound 4e’ (55.4 mg, 0.1 mmol) was dissolved in toluene.
Ferrocene carboxaldehyde (21.4 mg, 0.1 mmol) was added and an
EPR spectrum was recorded, showing only the presence of 4e’.
Computational details
All DFT and TD-DFT calculations were carried out with the
Gaussian09 code,[36] tightening self-consistent field convergence
thresholds (10-10 a.u.). Geometries were optimized using the B3PW91
functional, with the def2-TZVP basis set for the titanium and the 6-
31+G(d,p) basis set for other atoms.[37] Vertical excitations were
computed with TD-DFT using the B3PW91 functional, with the def2-
TZVP basis set for the titanium and the 6-311++G(d,p) basis set for
other atoms. For each complex, 8 states were considered. The
solvent effects of THF were included according to the Polarizable
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Following the benchmark calculations of Hadt et al.[41] and our
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BPW91 functional with the aug-cc-pVTZ-J basis set for the Ti atom,[43]
and the IGLO-III basis set for other atoms.[44] The A parameters were
computed using the hybrid B3LYP functional with the same basis sets.
The aug-cc-pVTZ-J basis sets were taken from the EMSL Basis Set
Exchange Web site.[45] All parameters were computed in toluene
modelled as a PCM.
Acknowledgements
We thank Dr Jean-Claude Chambron and Dr Yves Canac for
stimulating discussions on CH/ interactions and record-breaking
31P NMR shifts in group 4 metal phosphides respectively.
We thank Ms Marie-José Penouilh-Suzette for measuring the
HRMS spectra. Financial support from the Centre National de la
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10.1002/chem.201805430
Chemistry - A European Journal
FULL PAPER
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