Synthesis, structure and reactivity of the hexaphospha-titanocene, bis-(η5-3,5-di-tert-butyl-1,2,4-triphosphacyclopentadienyl)titanium(II)

Article abstract of DOI:10.1039/a905469c

The novel, fully structurally characterised, 14e-titanocene [Ti(η⁵-(P3C2tBu2)2] made by (i) cocondensation of titanium vapour with the phosphaalkyne tBuCP (ii) reaction of tBuCP with [Ti(η⁶-C6H5Me)] at -78 deg C or (iii) by heating T

Full text of DOI:10.1039/a905469c

Synthesis, structure and reactivity of the hexaphospha-titanocene,
5
bis-(h -3,5-di-tert-butyl-1,2,4-triphosphacyclopentadienyl)titanium(ii)
F. Geoffrey N. Cloke,* John R. Hanks, Peter B. Hitchcock and John F. Nixon*
School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton, UK BN1 9QJ. E-mail:
J.Nixon@sussex.ac.uk
Received (in Cambridge, UK) 7th July 1999, Accepted 26th July 1999
The novel, fully structurally characterised, 14e-titanocene
compounds are known, the only other reported hexaphospha-
metallocene structures are for Cr, Fe and Ru13–15 although there
also exists an unpublished Mn structure.16 As expected, the
metal to ring-centroid distances steadily decrease in the series
5
[Ti(h -(P3C2But2)2] made by (i) cocondensation of titanium
vapour with the phosphaalkyne ButCP (ii) reaction of ButCP
6
°
with [Ti(h -C6H5Me)2] at 278 C or (iii) by heating TiCln
with n(KP3C2But2) (n = 2, 3) in toluene at 110 °C, undergoes
an unusual [2+2] cycloaddition reaction with ButCP to give
5
[M(h -P3C2But2)2] (M = Ti, Cr, Mn, Fe), as the size of the metal
decreases ( Ti 1.942, Cr 1.815, Mn 1.710, Fe 1.687 Å). A similar
trend in metal–ring distances has been observed and rationalised
[Ti(h -P3C2But2)(h -h -P4C3But3)].
5
3
2
5
in the analogous metallocenes [M(h -C5H5)2] (M = V, Cr,
5
Despite several attempts by different workers using a variety of
routes, isolation and structural characterisation of a monomeric,
(non C–H activated), titanocene has failed owing to the high
Fe).17–20 The [M(h -P3C2But2)2] complexes all have slightly
bent sandwich structures with centroid–metal–centroid angles
of 173.5° (Cr), 173.2° (Fe), 171.0° (Mn). Since the 18e Fe and
Ru compounds and the 16e Cr compound would be expected to
have parallel rings, the slight distortion has been attributed to
the steric effect of the tert-butyl groups being unable to fully
5
reactivity of the carbene-equivalent [Ti(h -C5H5)2] unit. The
addition of neutral ligands such as CO, N2, PF3, PMe3, C2H4
5
and bulky ethynes facilitated isolation of derivatives of [Ti(h -
5
5
C5H5)2] and [Ti(h -C5Me5)2],1–7 however, the use of bulky
intermesh. As expected, the pentaphosphametallocenes, [M(h -
5
substitutents on the cyclopentadienyl rings proved less success-
P3C2But2)(h -P2C3But3)] (M = V, Fe)10,14 have essentially
5
ful until the very recent report of [Ti(h -Cps)2] [Cps
=
parallel rings as here the tert-butyl groups can fully inter-
mesh.
C5Me4(SiMe2But)] by Lawless and coworkers8
Previously9,10 we showed that cocondensation of the phos-
phaalkyne ButCP with transition metal vapours affords a wide
variety of organometallic compounds of which those containing
the di- and tri-phosphorus substituted cyclopentadienyl rings
are by far the most common. We now report that cocondensa-
tion of titanium atoms with ButCP yields the first example of a
phosphorus substituted titanocene which has been fully charac-
terised and its molecular structure established by a single crystal
X-ray diffraction study.
Only limited structural comparisons can be made between 1
and other titanocenes because the parent titanocene is not
monomeric but exists instead as various dimers.1 Likewise,
5
although decamethyltitanocene [Ti(h -C5Me5)2] is monomeric,
(in equilibrium with a C–H activated species), no structural data
are available.1 The X-ray structure of the very recently
5
synthesised [Ti(h -Cps)2] shows that the two rings are planar,
staggered and exactly parallel, with the SiMe2But substituents
rotated by 180° with respect to each other.8 The metal–centroid
distance [2.014(8)Å] is larger than that determined for 1
[1.942(2) Å], presumably either because of the more sterically
demanding Cps ligand and/or the stronger bonding of the
phosphorus substituted cyclopentadienyl rings.9 The exactly
parallel nature of the two Cps rings is also presumably due to
steric factors. Although 1 has a 3d2 electronic configuration, it
is found to be diamagnetic. At room temperature the 1H,
Cocondensation of electron beam generated11 titanium atoms
with an excess of ButCP at 77 K (i) affords the hexaphospha-
5
titanocene complex [Ti(h -P3C2But2)2] 1 (ca. 2.5% after work
up, based on titanium) (Scheme 1). 1, which was obtained as
dark-red air and moisture-sensitive crystals by sublimation (180
°C, 1 3 1025 mbar) followed by recrystallisation from pentane
(220 °C)† can also be prepared, in slightly better yield (ii) by
6
adding an excess of ButCP to a pentane solution of [Ti(h -
C6H5Me)2] at 278 °C or in moderate (ca. 30%) yield (iii) by
heating TiCln with n(KP3C2But2)12 (n = 2, 3) in toluene at
110 °C.
A single crystal X-ray diffraction study‡ (Fig. 1) confirms the
formulation of 1 and shows it to be a slightly bent, monomeric
sandwich compound (angle between the planes of the two
P3C2But2 rings
= 16.05°; centroid–Ti–centroid angle =
173.9°). The rings are approximately planar, with the tert-butyl
groups staggered so as to minimize inter-ring interactions and
the phosphorus atoms lying slightly above the plane containing
the carbons. Although several phosphorus-containing sandwich
5
Fig. 1 Molecular structure of [Ti(h -P3C2But2)2], 1. Selected distances (Å)
and angles(°): Ti–M(1) 1.942(2), Ti–C(1) 2.354(2), Ti–C(2) 2.470(2), Ti–
P(1) 2.477(1), Ti–P(2) 2.535(1), Ti–P(3) 2.637(1), P(1)–C(1) 1.790(2),
P(1)–P(2) 2.167(1), P(2)–C(2) 1.784(2), P(3)–C(2) 1.739(2), P(3)–C(1)
1.764(2), C(1)–P(1)–P(2) 97.84(7), C(2)–P(2)–P(1) 98.72(7), C(2)–P(3)–
C(1) 99.92(10), P(3)–C(1)–P(1) 121.47(12), P(3)–C(2)–P(2) 121.74(12),
M(1)–Ti–M(1)A 173.9(1), angle between the least-squares planes of the two
rings 16.05(0.07). M(1) and M(1)A refer to the centroids of the two rings.
Scheme 1
Chem. Commun., 1999, 1731–1732
1731

‡ Crystal data: for 1: C20H36P6Ti, M = 510.2, orthorhombic, space group
C2221 (no. 20), a = 11.632(6), b = 16.114(5), c = 13.920(4) Å, U =
2609(2) Å3; Z = 4, Dc = 1.30 Mg m23, T = 173(2) K. Data were obtained
with an Enraf-Nonius CAD4 diffractometer using Mo-Ka radiation, l =
0.71073 Å, (m = 0.70 mm21), on a crystal of dimensions 0.4 3 0.4 3 0.2
mm; 2112 unique reflections were measured for 2 < 2q < 30° and 2035
reflections with I > 2s(I) were used in the refinement. The final residuals
were R = 0.025, wR2 = 0.062 for I > 2s(I). Hydrogen atoms were refined
isotropically. The molecule lies on a crystallographic two fold rotation
axis.
¯
For 2: C25H45P7Ti, M = 610.3, triclinic, space group P1(no.2), a =
10.299(5), b = 11.910(6), c = 14.230(10) Å, a = 86.48(5), b = 71.00(5),
g = 68.75(4)°, U = 1535(2)Å3; Z = 2, Dc = 1.32 Mg m23, T = 173(2) K.
Data were obtained with an Enraf-Nonius CAD4 diffractometer using Mo-
Ka radiation, l = 0.71073 Å, (m = 0.66 mm21), on a crystal of dimensions
0.4 3 0.3 3 0.05 mm; 5362 unique reflections were measured for 2 < 2q
< 25° and 4021 reflections with I > 2s(I) were used in the refinement. The
final residuals were R = 0.077, wR2 = 0.196 for I > 2s(I).
CCDC 182/1348. See http://www.rsc.org/suppdata/cc/1999/1731/ for
crystallographic files in .cif format.
5
3
2
t
Fig. 2 Molecular structure of [Ti(h -P3C2But2)(h -h -P4C3 Bu3)] 2. Se-
lected distances (Å): Ti–M(1) 2.120(6), Ti–C(2) 2.503(6), Ti–C(11)
2.210(6), Ti–C(16) 2.465(5), Ti–C(17) 2.663(6), Ti–P(2) 2.472(2), Ti–P(3)
2.662(2), Ti–P(4) 2.577(2), Ti–P(5) 2.609(2), Ti–P(6) 2.673(3), Ti–P(7)
2.709(2), C(1)–P(1) 1.876(5), P(1)–P(2) 2.216(2), P(2)–C(2) 1.790(5),
C(2)–P(3) 1.752(5), P(3)–C(1) 1.831(6), P(1)–C(11) 1.858(5), C(11)–P(4)
1.828(5), P(4)–C(1) 1.909(5). M(1) refers to the centroid of the ring
P(5)C(16)P(7)C(17)P(6).
§ NMR for 1 (d8-toluene, 298 K). 1H (300.13 MHz): d 0.18 (s, 36 H,
2
Ti{P3C2[C(CH3)3]2}2). 13C{1H} (75.47 MHz): d 35.2 (d, JCP 7 Hz,
2
Ti{P3C2[C(CH3)3]2}2), 42.6 (d, JCP 18 Hz, Ti{P3C2[C(CH3]3]2}2).
31P{1H} (121.49 MHz): d 342.5 (t, 1P, 2JPP 44.2 Hz, PPCPC), 386.0 (d, 2P,
2JPP = 46.7 Hz, PPCPC).
NMR for 2 (d6-benzene, 298 K). 1H (300.13 MHz): d 1.05 (s, 9 H) 1.29
(s, 9 H), 1.38, 1.40 [2s (overlapping) 18 H], 1.67 (s, 9 H). 31P{1H} (121.49
MHz): dP(1) 260.3 (ddd, 1JP(1)P(2) 290, 2JP(1)P(3) 12, 2JP(1)P(4) 12 Hz), dP(2)
1
2
2
309 (dd, JP(1)P(2) 290, JP(2)P(3) 40 Hz), dP(3) 98.7 (dddd, JP(2)P(3) 40,
2JP(3)P(4) 32, 2JP(3)P(1) 12, 2JP(3)P(5) = 5 Hz), dP(4) 283.9 (dddd of m, 2JP(4)P(6)
70, 2JP(4)P(3) 32, 2JP(4)P(5) or P(1) 14, 2JP(4)P(1) or P(5) = 12, plus other ca. 5 Hz
inter-ring couplings), dP(5) 260.8 [dd, 1JP(5)P(6) 430, 2JP(5)P(7) 50, 2JP(5)P(4) 12
Hz (inter-ring)], dP(6) 301.3 [dd, 1JP(6)P(5) 430, 2JP(6)P(7) 52, 2JP(6)P(4) 70 Hz
(inter-ring)], dP(7) 225.2 (dd, 2JP(7)P(6) = 50, 2JP(7)P(5) 50 Hz).
13C{1H} and 31P{1H} NMR spectra§ are all sharp (and
temperature invariant over the range 185–385 K) and indicate
that in solution the two P3C2But2 rings are equivalent and that
there is no inter-ring coupling. This is in contrast to the iron and
ruthenium analogues which exhibit fluxional behaviour and
inter-ring coupling constants of 53 Hz (Fe) and 37.4 Hz
(Ru).13,14
1 G. P. Pez and J. N. Armor, Adv. Organomet. Chem., 1981, 19, 1.
2 R. D. Sanner, D. M. Duggan, T. C. McKenzie, R. E. Marsh and J. E.
Bercaw, J. Am. Chem. Soc., 1976, 98, 8358.
3 B. H. Edwards, R. D Rogers, D. J. Sikora, J. L. Atwood and M. D.
Rausch, J. Am. Chem. Soc., 1983, 105, 416.
4 L. B. Kool, M. D. Rausch, H. G. Alt, M. Herberhold, U. Thewalt and B.
Wolf, Angew. Chem., Int. Ed. Engl., 1985, 24, 394.
5 S. A. Cohen, P. R. Auburn and J. E. Bercaw, J. Am. Chem. Soc., 1983,
105, 1136.
6 V. Varga, K. Mach, M. Polásek, P. Sedmera, J. Hiller, U. Thewalt and
S. I. Troyanov, J. Organomet. Chem., 1996, 506, 241.
7 V. V. Burlakov, A. V. Polyakov, A. I Yanovsky, Y. T. Struchkov, V. B.
Shur, M. E. Vol’pin, U. Rosenthal and H. Görls, J. Organomet. Chem.,
1994, 476, 197.
8 P. B. Hitchcock, F. M. Kerton and G. A. Lawless, J. Am. Chem. Soc.,
1998, 120, 10264.
9 K. B. Dillon, F. Mathey and J. F. Nixon, Phosphorus: The Carbon Copy,
John Wiley and Sons, Chichester 1998 and references therein.
10 F. G. N. Cloke, K. R. Flower, P. B. Hitchcock and J. F. Nixon, J. Chem.
Soc., Chem. Commun., 1995, 1659.
11 F. G. N. Cloke and M. L. H. Green, J. Chem. Soc., Dalton Trans., 1981,
1938.
12 C. Callaghan, G. K. B. Clentsmith, F. G. N. Cloke, P. B. Hitchcock,
J. F. Nixon and D. M. Vickers, Organometallics, 1999, 18, 793.
13 P. B. Hitchcock, J. F. Nixon and R. M. Matos, J. Organomet. Chem.,
1995, 490, 155.
Decamethyltitanocene forms dinitrogen complexes (some
reversibly), and although 1 does not react with dinitrogen, it
readily undergoes addition reactions with CO or tBuNC to give
the 16e and 18e complexes [Ti(h -P3C2But2)2L] (L = CO) and
5
[Ti(h -P3C2But2)2L2] (L = CO, ButNC) respectively, which will
5
be reported in detail elsewhere.21 However, with an excess of
ButCP the novel red–brown 14e-complex [Ti(h -
5
P3C2But2)(h +h -P4C3But3)] 2 is formed as a result of an
3
2
5
unusual [2+2] cycloaddition with a PNC bond of one of the h -
P3C2But2 rings of 1 (Scheme 1). No reaction is observed in an
analagous experiment with ButCN. The molecular structure of
2, determined by a single crystal X-ray diffraction study,‡ is
5
shown in Fig. 2. The titanium is attached in an h -fashion to the
one remaining P3C2But2 ring and h +h - ligated to the new
P4C3But3 moiety. The 31P{1H} NMR spectrum of 2 exhibits the
expected seven different resonances and the 1H NMR spectrum
contains the expected five singlets.§
3
2
The mechanism of formation of 2 from 1 might involve the
intermediacy of an h - (or less likely h -ligated) ButCP complex
1
2
5
3
and the subsequent h - to h -ring slippage of one of the
P3C2But2 facilitating a [2+2] cycloaddition with the resulting
uncoordinated PNC bond leading to the product 2. Interestingly,
although 2 can be sublimed (170 °C, 1 3 1025 mbar) with only
slight decomposition to 1, it was not isolated from the original
cocondensation experiment.
14 R. Bartsch, P. B. Hitchcock and J. F. Nixon,. J. Chem. Soc., Chem.
Commun., 1987, 1146.
15 R. Bartsch, P. B. Hitchcock and J. F. Nixon,. J. Organomet. Chem.,
1988, 356, C1.
16 U. Zenneck, personal communication.
17 R. D Rogers, J. L. Atwood, D. Foust and M. D. Rausch, J. Cryst. Mol.
Struct., 1981, 11, 183.
To our knowledge, the only related metallocene ring addition
reactions previously reported concern nickelocene and fluoro-
alkenes or an activated norbornadiene however no structural
data are available for comparison.22,23
18 K. R Flower and P. B. Hitchcock, J. Organomet. Chem., 1996, 507,
275.
We thank Dr A. Abdul-Sada for recording the mass spectrum
of 1 and the EPSRC for financial support (for J. R. H.).
19 P. Seiler and J. Dunitz, Acta Crystallogr., Sect. B., 1979, 35, 1069.
20 A. Haaland, Acc. Chem. Res., 1978, 12, 415.
21 F. G. N. Cloke, J. R. Hanks, P. B. Hitchcock and J. F. Nixon, papers in
preparation.
Notes and references
† C20H36P6Ti: MS (EI, 70 eV), m/z 510 (100%) (M+), 57 (8%) (But+) with
the expected isotope pattern. Elemental analysis: C, 47.08 (46.57), H, 7.11
(7.15%).
22 M. Dubeck, J. Am. Chem. Soc., 1960, 82, 6193.
23 D. W. McBride, R. L. Pruett, E. Pitcher and F. G. A. Stone J. Am. Chem.
Soc., 1962, 84, 497.
Communication 9/05469C
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Chem. Commun., 1999, 1731–1732