Four-coordinate phosphinidene complexes of titanium prepared by α-H-migration: Phospha-staudinger and phosphaalkene-insertion reactions

Article abstract of DOI:10.1021/ja036559r

α-Hydrogen migration in the phosphide (Nacnac)Ti=CH^tBu(PHR) (Nacnac- = [Ar]NC(Me)CHC(Me)N[Ar], Ar = 2,6^-iPr₂C₆H₃, R = C₆H₁₁, 2,4,6^-iPr₃C₆H_2<

Full text of DOI:10.1021/ja036559r

Published on Web 08/05/2003
Four-Coordinate Phosphinidene Complexes of Titanium Prepared by
r-H-Migration: Phospha-Staudinger and Phosphaalkene-Insertion Reactions
Falguni Basuli, John Tomaszewski, John C. Huffman, and Daniel J. Mindiola*
Department of Chemistry and Molecular Structure Center, Indiana UniVersity, Bloomington, Indiana 47405
Received June 7, 2003; E-mail: mindiola@indiana.edu
Chemistry of transition-metal complexes containing the terminal
phosphinidene1 functionality is exceedingly underdeveloped when
compared to the lighter nitrogen congener.2 In contrast to imide-
group transfer reactions, the concept of phosphinidene-group
transfer has received far less attention, presumably due to the lack
of stable systems having this functionality. Examples of early-
transition metal phosphinidenes include only a handful of complexes
composed of Zr3-5 and Ta.6,7 Other isolable phosphinidenes of the
early-metal series include some in group 6 (e.g., Mo and W).8-10
Akin to early-transition metals, the mid- to late-transition metal
phosphinidenes are also infrequent, yet of great interest, because
these systems can demonstrate a direct contrast between electron-
Figure 1. Molecular structures of 1b and 3c with thermal ellipsoids at the
50% probability level. Ethyl groups on O54 and Pr groups on the P-Trip
in 1b (C25, C27, C29) have been omitted. Aryl groups in 1b and 3c with
the exception of the ipso-carbons on the nitrogens have been also omitted
deficient and -rich complexes possessing this common functionality.
i
Rare examples of the latter family of elements include Fe,11 Ru,12,13
Os,13 Co,14 Rh,14 Ir,14,15 and Ni.16 In most of these compounds,
encumbering substituents on P are required, unless otherwise
stabilized electronically with electron-rich groups.1,17 Hitherto, the
only isolable group 4 terminal phosphinidenes reported to date are
the bis-cyclopentadienyl derivatives described by Stephan3 and
Protasiewicz.4
for clarity.
Scheme 1. Synthesis of 1a and 1b
Our recent report of the synthesis of four-coordinate titanium
alkylidene18 and imido18,19 complexes stimulated the pursuit of an
analogous titanium phosphinidene. Titanium phosphinidenes are
unknown,3d presumably due to hard-soft mismatch of these
elements. Herein, we report the first titanium complex containing
a terminal phosphinidene ligand. Judicious choice of substituent
on P can influence the kinetic stability of the reactive [TidPR]
fragment.
multiple bond.2 Both P2 and C3 are bound to Ti1 (2.572(3) and
2.123(8) Å, respectively), while the prior neopentylidene R-carbon
C19 has acquired an additional proton and is now attached to the
phosphorus as an alkyl group.20 Because of the low-coordination
environment on Ti, close interactions between the titanium center
and the olefinic carbons of the chelate ligand are also noted in the
structure of 1b (Ti1-C4, 2.420(8) Å; Ti1-C5, 2.456(9) Å).
Formation of 1 is proposed to occur via transmetalation of LiPHR
with (Nacnac)TidCHtBu(OTf) to furnish putative neopentylidene-
phosphide (Nacnac)TidCHtBu(PHR) (2) (R ) Cy (2a); R ) Trip
(2b)), which subsequently undergoes R-H-migration to give phos-
A strategy for preparing a low-coordinate and terminal titanium
phosphinidene complex involved treatment of the nucleophilic and
four-coordinate titanium alkylidene (Nacnac)TidCHtBu(OTf)18
(Nacnac- ) [Ar]NC(Me)CHC(Me)N[Ar], Ar ) 2,6-iPr2C6H3) with
the corresponding primary lithium phosphide. When 1 equiv of
LiPHR (R- ) Cy, Trip; Cy ) C6H11, Trip ) 2,4,6-iPr3C6H2) is
added to a cold ether solution of (Nacnac)TidCHtBu(OTf), the color
rapidly changes from red-brown to orange-brown. After workup,
t
t
complex ([Ar]NC(Me)CHC(Me)P[R][CH2 Bu])TidNAr(OEt2) (1)
phinidene (Nacnac)TidPR(CH2 Bu) (3) (R ) Cy (3a); R ) Trip
(R- ) Cy, 1a, 76% yield; R ) Trip, 1b, 89% yield) is isolated as
(3b)) (Scheme 2). Considered quite reactive, intermediate 3
rearranges through a “phospha-Staudinger” reaction to generate the
titanium-imide complex supported by the NP hybrid ligand
1
a dark-brown powder (Scheme 1).20 As an example, the H NMR
spectrum of 1b is consistent with these species having C1 symmetry,
while the 31P NMR shows only one diastereomer present (-43
ppm).20
t
([Ar]NC(Me)CHC(Me)P[R])TidNAr(CH2 Bu) (4) (R ) Cy (4a);
R ) Trip (4b)). Precedent for “Wittig or Staudinger-type” reactions
at the imine functionality of the Nacnac- ligand has been reported
previously by our group.18 Consequently, complex 1 would then
Large dark-orange blocks of 1 were grown from a saturated
pentane solution, and their connectivity was determined from single-
crystal X-ray diffraction methods.20 Both complexes are isostructural
and reveal a five-coordinate titanium complex in a highly congested
and distorted square pyramidal environment where the labile OEt2
ligand occupies an axial site.20 Upon inspection of the structure of
1b (Figure 1), it is evident that the [PTrip] fragment has undergone
migration to the â-carbon of the former Nacnac- ligand. The short
Ti1-N41 distance (1.732(6) Å) is clearly indicative of a Ti-Nimide
t
form from 4 by phosphaalkene insertion into the Ti-CH2 Bu bond
(Scheme 2). The latter reaction would be consistent with traditional
phosphaalkene chemistry because there is positive charge at the P
atom resulting from polarization in the σ bond.1d
Evidence for the intermediates generated during the formation
of 1b is substantiated by monitoring the reaction mixture at low
temperature using 31P NMR spectroscopy.20 Upon thawing of the
9
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J. AM. CHEM. SOC. 2003, 125, 10170-10171
10.1021/ja036559r CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Proposed Intermediates along the Reaction Pathway
To Generate 1a and 1b
Acknowledgment. We thank Indiana University-Bloomington,
the Camille and Henry Dreyfus Foundation, and the Ford Founda-
tion for financial support. D.J.M. is grateful to Dr. R. Waterman,
and Professors G. L. Hillhouse, C. C. Cummins, and K. G. Caulton
for insightful discussions. D.J.M. also thanks Prof. J. D. Prota-
siewicz for helpful suggestions and a generous gift of phosphine.
Supporting Information Available: Complete experimental prepa-
ration and crystallographic data for compounds 1-4 and the OEt2-free
adduct of 1b (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
References
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Ed. 2003, 42, 1578 and references therein.
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reagents in Et2O to -20 °C, a resonance assigned to 2b (19 ppm,
JP-H ) 556 Hz, purple solution)21 is observed. From -18 to -5
°C, the solution changes to green, and two resonances centered at
337 and 285 ppm are generated which vanish quickly to form a
resonance at 153 ppm. All three resonances lack P-H coupling,
thus consistent with formation of 3b and 4b, respectively. The two
downfield shift peaks for 3b are suggestive of two isomers being
present in solution (vide infra). Warming above -5 °C causes a
color change to red-brown and generates the resonance correspond-
ing to 1b (vide supra), concurrent with disappearance of the
downfield shifts attributed to 3b and 4b.20
(5) Other Zr phosphinidene complexes have been isolated by trapping
experiments. Mahieu, A.; Igau, A.; Majoral, J.-P. Phosphorus Sulfur 1995,
104, 235.
(6) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Angew. Chem., Int. Ed.
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In an effort to kinetically stabilize titanium-phosphinidene 3, a
likely intermediate generated from the reaction to form 1,
(Nacnac)TidCHtBu(OTf) was treated with the bulkier phosphide
LiPHMes* (Mes*- ) 2,4,6-tBu3C6H2) in pentane at -35 °C
(8) Hitchcock, P. B.; Lappert, M. F.; Leung, W. P. J. Chem. Soc., Chem.
Commun. 1987, 1282.
(9) Cowley, A. H.; Pellerin, B.; Atwood, J. L.; Bott, S. G. J. Am. Chem. Soc.
1990, 112, 6734.
(10) A uranium phoshinidene has also been prepared. Arney, D. S. J.; Schnabel,
R. C.; Scott, B. C.; Burns, C. J. J. Am. Chem. Soc. 1996, 118, 6780.
(11) (a) Mathey, F. Angew. Chem., Int. Ed. Engl. 1987, 26, 275. (b) Wit, J. B.
M.; van Eijkel, G. T.; de Kanter, F. J. J.; Schakel, M.; Ehlers, A. W.;
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38, 2596.
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20, 2657. (c) Sterenberg, B. T.; Udachin, K. A.; Carty, A. J. Organome-
tallics 2001, 20, 4463.
t
(Scheme 2). Accordingly, complex (Nacnac)TidPMes*(CH2 Bu)
(3c)20 is isolated in good yield (g78%), inasmuch as intermediate
(Nacnac)TidCHtBu(PHMes*) 2c is observed to form and decay
when monitoring the reaction mixture (31P NMR: Et2O, -10 °C,
31 ppm, JPH ) 555 Hz).20,21 Unlike putative 3a and 3b, phosphin-
idene 3c is relatively stable both in solution and in the solid state,
therefore suggesting that 3c is the kinetic product while 1a and 1b
are thermodynamic products. Large blocks of dark-green 3c were
grown from pentane, and the molecular structure is depicted in
Figure 1.20 The structure of 3c displays a four-coordinate titanium-
phosphinidene complex with a super short TidP bond (2.1831(4)
Å)22 and a linear TidP-Cipso angle (164.44(5)°). Examples of linear
MdP-R phosphinidene species are exceedingly rare and often arise
from formation of a pseudo-triple bond.1,6,9 31P NMR solution
spectra of 3c manifest two resonances at 242 and 216 ppm, also
consistent with two isomers present in solution. Formation of
these isomers could occur for steric reasons or by deviation of the
“TidPAr” fragment from the NCCCN ligand plane.23 Unfortunately
variable-temperature 31P NMR (25-40 °C) leads to decomposition
instead of interconversion between the two isomers.
(13) Termaten, A. T.; Nijbacker, T.; Schakel, M.; Lutz, M.; Spek, A. L.;
Lammertsma, K. Chem.-Eur. J. 2003, 9, 2200.
(14) Termaten, A. T.; Aktas, H.; Schakel, M.; Ehlers, A. W.; Lutz, M.; Spek,
A. L.; Lammertsma, K. Organometallics 2003, 22, 1827.
(15) Termaten, A. T.; Nijbacker, T.; Schakel, M.; Lutz, M.; Spek, A. L.;
Lammertsma, K. Organometallics 2002, 21, 3196.
(16) Melenkivitz, R.; Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2002,
124, 3846.
(17) Mathey, F.; Huy, N. H. T.; Marinetti, A. HelV. Chim. Acta 2001, 84, 2938.
(18) Basuli, F.; Bailey, B. C.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D.
J. J. Am. Chem. Soc. 2003, 125, 6052.
(19) Basuli, F.; Bailey, B. C.; Huffman, J. C.; Mindiola, D. J. Chem. Commun.
2003, 1554.
(20) See Supporting Information for complete experimental, spectral, and
crystallographic details.
(21) Significantly large P-H coupling constants have been reported: Brevard,
C.; Granger, P. Handbook of High-Resolution Multinuclear NMR; John
Wiley & Sons: New York, 1981.
(22) The TidP value is much shorter to Pauling’s predicted bond length of
2.288 Å for a double bond. Pauling, L. The Nature of the Chemical Bond,
3rd ed.; Cornell University Press: Ithaca, NY, 1960. A search in the CSD
reveals Ti-Phosphide bond lengths to be >2.5 Å.
(23) A similar fluxional process has been observed in four-coordinate scandium
species supported by the Nacnac- ligand used in the present study. Hayes,
P. G.; Piers, W. E.; Lee, L. W. M.; Knight, L. K.; Parvez, M.; Elsegood,
M. R. J.; Clegg, W. Organometallics 2001, 20, 2533.
In summary, we have shown that the four-coordinate titanium
neopentylidene can engage in R-deprotonation of phosphides to
yield reactive four-coordinate Ti(IV) phosphinidene species. The
titanium-phosphinidene complexes presented in this work are close
analogues of traditional phosphaalkenes, differing simply by the
presence of accessible d-orbitals.
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