Phosphaazaallene and phosphinylimide complexes stemming from a terminal and four-coordinate titanium phosphinidene

Article abstract of DOI:10.1039/b311748k

Treatment of the terminal titanium-phosphinidene (Nacnac)Ti=PMes*(CH₂^tBu) (Nacnac^- = [2,6-ⁱPr₂C₆H₃]- NC(CH₃)CHC(CH₃)N[2,6-ⁱPr₂ C₆H₃], Mes*^- = 2,4,6-^tBu₃-C₆H₂) with CN^tBu and N₂CPh₂, in pentane at -35°C, affords η²-(N,C)-phosphaazaallene and phosphinylimide complexes, respectively.

Full text of DOI:10.1039/b311748k

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Phosphaazaallene and phosphinylimide complexes stemming from a
terminal and four-coordinate titanium phosphinidene†
Falguni Basuli, Lori A. Watson, John C. Huffman and Daniel J. Mindiola*
Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington,
Indiana 47405, USA. E-mail: mindiola@indiana.edu
Received 23rd September 2003, Accepted 2nd October 2003
First published as an Advance Article on the web 14th October 2003
Treatment of the terminal titanium–phosphinidene
ArNCHCHCHNAr, Ar = 2,6-Me2C6H3, Mes = 2,4,6-Me3C6H2)
(Nacnac)Ti᎐PMes*(CH tBu) (Nacnac؊ ؍ [2,6-iPr2C6H3]–
using density funtional theory (DFT).† The optimized
geometry of the model complex is in close agreement with
the experimentally observed X-ray structural parameters of
᎐
2
NC(CH3)CHC(CH3)N[2,6-iPr2C6H3], Mes*؊ ؍ 2,4,6-tBu3-
C6H2) with CNtBu and N2CPh2, in pentane at ؊35 ЊC,
affords ꢀ2-(N,C)-phosphaazaallene and phosphinylimide
complexes, respectively.
1.† When inspecting the frontier orbitals in (Nacnac*)Ti᎐
᎐
PMes(Me), it is observed that the HOMO is the π-bond of the
Ti–P double linkage (Fig. 1), while the LUMO† is composed
predominantly of π* character localized about the N–C bonds
of the Nacnac* backbone and a node at the γ-C. More import-
ant, the stability of the Ti–P bond and the linear Ti–P–Cipso
angle is attributed to the HOMO-1 orbital. The HOMO-1,
which is orthogonal to the HOMO, indicates that there is sig-
nificant π-donation of the lone pair on P to an empty d-orbital
on titanium (Fig. 1). Theoretical results suggest tantalizingly
that complex 1 has a reactive phosphinidene functionality and
Titanium phosphinidene complexes are rare, presumably due to
the mismatch between the hard titanium atom and the soft
P-containing ligand.1,2 To date, only a handful of group 43 and
54 terminal phosphinidene complexes have been described in
the literature. The pursuit of systems having such a reactive
functionality stems from their ability to engage in “phospha-
Staudinger or Wittig”-type transformations.3,4 We recently
reported the synthesis of a four-coordinate titanium phos-
t
phinidene complex (Nacnac)Ti᎐PMes*(CH Bu) 1 (Scheme 1)
that the bonding in Ti–P should be better portrayed as a
᎐
2
by α-hydrogen migration.5 The preparation of 1 was remark-
able since organotransition-metal complexes with low-
coordination numbers are inherently reactive and consequently
provide useful templates to study fundamental processes such
as small-molecule activation and group transfer.3,4a,5,6 The
present study establishes that a coordinatively unsaturated and
terminal titanium–phosphinidene complex4 supported by a
sterically demanding β-diketiminate ligand, denoted “Nacnac”
(NacnacϪ = ArNC(Me)CHC(Me)NAr, Ar = 2,6-iPr2C6H3), can
react with CNtBu and N2CPh2 to afford novel complexes having
bound η2-(N,C)-phosphaazaallene and phosphinylimide
ligands, respectively.
᎐
pseudo-triple bond (Ti᎐P).
᎐
᎐
Fig. 1 A perspective view showing the Ti᎐P π-orbitals (HOMO and
᎐
HOMO-1) for the model complex of 1, namely (Nacnac*)Ti᎐
᎐
PMes(Me).
᎐
In order to demonstrate the reactivity of the Ti᎐P functional-
᎐
ity we carried out reactions of complex 1 with CNtBu and
N2CPh2. When a pentane solution of 1 is treated with 3 equiv of
CNtBu, the color rapidly changes from green to dark brown.
After recrystallization from pentane, the η2-(N,C)-phospha-
azaallene titanium complex (η1-Nacnac)Ti(CNtBu)(η2-(N,C)–
Scheme 1 Synthesis of titanium phosphinidene 1 and subsequent
reactions to prepare complexes 2 and 3.
t
tBuN᎐CCH Bu)(η2-(N,C)–tBuN᎐C᎐PMes*) (2) is isolated in
᎐
᎐ ᎐
2
60% yield as dark plates. Diagnostic features associated with 2
include the observation of stretches at 2210 and 2187 cmϪ1, and
1551 and 1528 cmϪ1, which may be assigned to νCN and νCP
stretches, respectively. There are two 31P NMR resonances
centered at Ϫ8.53 and Ϫ17.58 ppm, which we attribute to the
presence of two isomers in solution. In addition, the 13C NMR
resonance of the central allene carbon of the coordinated
phosphaazaallene is consistent with η2-(N,C) binding (δ 217.6
and 213.8 ppm, JCP = 166 and 172 Hz, respectively)† for the
two isomers.7 Phosphaazaallenes are reactive molecules, and
examples of early-transition metal complexes having this type
of ligand are scarce.7,8
Isolation of complex 1 was achieved by kinetic stabilization
using a sterically bulky substituent on P (Mes*). Otherwise,
complex 1 transforms by a “phospha-Staudinger” reaction.5
However, the low-coordination environment coupled with the
kinetic stability of the Ti᎐P bond, the unusually short
᎐
Ti–P bond (2.1831(4) Å)5 and the obtuse Ti–P–Cipso angle
(164.44(5)Њ)5 raised fundamental questions regarding the
nature of bonding and structure in complex 1. Theoretical cal-
culations were thus performed on the titanium phosphinidene
model complex (Nacnac*)Ti᎐PMes(Me) (Nacnac*Ϫ
=
᎐
To further support the connectivity in 2, single crystals were
grown from a saturated pentane solution cooled to Ϫ35 ЊC, and
the molecular structure was determined by X-ray diffraction
studies (Fig. 2).‡ The structure of 2 displays one of the two
possible isomers and reveals a coordinatively saturated titanium
† Electronic supplementary information (ESI) available: Complete
experimental, spectroscopic, analytical, and crystallographic details for
2 and 3, as well as DFT computational details for the model complex of
1, namely (Nacnac*)Ti᎐PMes(Me). See http://www.rsc.org/suppdata/
᎐
dt/b3/b311748k/
D a l t o n T r a n s . , 2 0 0 3 , 4 2 2 8 – 4 2 2 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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cleave the P᎐C bond of Mes*P᎐C᎐NPh to afford the phos-
᎐
᎐ ᎐
phinidene–isocyanide complex trans-WCl2(CNPh)(PMes*).11
The microscopic reverse of the bond breaking reaction of
phosphaazaallenes support the notion that Ti᎐P bonds are
᎐
inherently reactive scaffolds and powerful phosphinidene
group-transfer reagents.
For financial support of this research, we thank Indiana
University-Bloomington, the Camille and Henry Dreyfus
Foundation, and the Ford Foundation. D.J.M. thanks Prof.
A. L. Odom and Prof. D. Lee for insightful comments and the
Indiana University Information Technology Research and
Academic Computing Services for access to the high perform-
ance computing system. L.A.W. acknowledges the National
Science Foundation for a pre-doctoral fellowship.
Fig. 2 Molecular structure of 2 with thermal ellipsoids at the 50%
probability level. All H-atoms and aryl groups on the nitrogens with the
exception of the ipso-carbon atoms C(27) and C(13) have been omitted
Notes and references
t
for clarity. Methyl groups on the Bu fragments (C(4), C(47), C(43),
C(68), C(64), C(72), and C(93)) have been also removed for clarity.
¯
‡ Crystal data for 2, C77H132N5PTi: triclinic, P1, a = 12.297(2), b =
13.182(3), c = 24.843(5) Å, α = 103.400(5), β = 92.806(6), γ = 91.863(6)Њ,
Z = 2, µ(Mo–Kα) = 0.171 mmϪ1, V = 3908.8(13) Å3, Dc = 1.025 mg
mmϪ3, GoF on F 2 = 1.022. Out of a total of 47023 reflections collected,
13721 were independent and 9219 were observed (Rint = 0.116) with
I > 2σ(I ) (dark plate, 0.35 × 0.30 × 0.30 mm, 25.01Њ ≥ Θ ≥ 1.97Њ).
All non-hydrogen atoms were refined with anisotropic displacement
parameters. The final full matrix least squares refinement converged
to R1 = 0.0869 (F) and wR2 = 0.2567 (F 2). Two disordered pentane
molecules were confined in the asymmetric unit.
center with an η1-bound NacnacϪ ligand, one isonitrile
occupying the axial position, as well as two additional iso-
nitriles inserting into the Ti–C and Ti᎐P bonds. Both the
᎐
iminato and phosphaazaallene ligands are η2-(N,C) bound to
the titanium center.† There is substantial reduction of the N᎐C
᎐
bond (N(51)–C(52), 1.307(5) Å) of the phosphaazaallene
ligand relative to that for the metal-free form.7–9
¯
§ Crystal data for 3, C65H91N4PTi: triclinic, P1, a = 11.696(2), b =
When diphenyldiazomethane is added slowly to 1 in pentane,
13.078(4), c = 20.218(4) Å, α = 81.562(6), β = 78.932(5), γ = 80.736(5)Њ,
Z = 2, µ(Mo–Kα) = 0.212 mmϪ1, V = 2974.1(11) Å3, Dc = 1.125 mg
mmϪ3, GoF on F 2 = 0.915. Out of a total of 12447 reflections collected,
11628 were unique and 6497 were observed (Rint = 0.053) with I > 2σ(I )
(dark brown block, 0.25 × 0.25 × 0.25 mm, 27.54Њ ≥ Θ ≥ 2.22Њ). There is
an occupational disorder of the phosphorus atom with an approximate
ratio of 84 : 16. In addition, there is a rotational disorder about two
methyl groups of the iPr ligand having alternate positions. The final full
matrix least squares refinement converged to R1 = 0.0624 (F) and wR2 =
0.1530 (F 2). CCDC reference numbers 218483 and 218484. See http://
www.rsc.org/suppdata/dt/b3/b311748k/ for crystallographic data in
CIF or other electronic format.
t
complex (Nacnac)Ti᎐N[P(CH Bu)(Mes*)](N᎐CPh ) (3) is
᎐
᎐
2
2
formed cleanly in 68% yield subsequent to recrystallization
from pentane at Ϫ35 ЊC. † Phosphinylimide complex 3 displays
one 31P NMR resonance at 99.7 ppm and represents the first
example of an sp2-phosphorus attached to a titanium–imide
motif. Single and dark-brown crystals of 3§ also support the
proposed connectivity revealing a four-coordinate titanium
complex containing a short Ti᎐Nimide bond length of 1.727(3) Å
᎐
(Ti(1)–N(47)) and Ti–N–P angle of 157.4(7)Њ with a substanti-
ally pyramidalized phosphino-P (Fig. 3). Other salient features
in the structure of 3 include a linear ketiminate ligand (Ti(1)–
N(33)–C(34), 174.5(3)Њ) having a short Ti(1)–N(33) bond of
1.918(3) Å, both of which indicate considerable π-bonding
character in the titanium–N{ketiminate} linkage. Complex 3 forms
1 (a) A. H. Cowley, Acc. Chem. Res., 1997, 30, 445; (b) A. H. Cowley
and A. R. Barron, Acc. Chem. Res., 1988, 21, 81; (c) F. Mathey,
Angew. Chem., Int. Ed., 2003, 42, 1578.
2 R. G. Pearson, Inorg. Chem., 1988, 27, 734.
likely from insertion of N CPh into the Ti᎐P bond, and sub-
3 (a) T. L. Breen and D. W. Stephan, J. Am. Chem. Soc., 1995, 117,
11914 and refs. therein; (b) T. Graham and D. W. Stephan, private
communication; (c) E. Urnezius, K.-C. Lam, A. L. Rheingold and
J. D. Protasiewicz, J. Organomet. Chem., 2001, 630, 193.
᎐
2
2
sequent alkyl insertion as well as unusual N–N bond cleavage
of the putative phosphaazine intermediate. Phosphinylimide 3
could be best described as a η1-phosphahydrazido complex of
titanium. Analogous Ti and Zr η1-hydrazido complexes display
rich chemistry differing significantly from monomeric alkyl and
aryl imido complexes.10 Compounds 2 and 3 are exceedingly
reactive and decompose over several hours in solution or in the
solid state.
4 (a) C. C. Cummins, R. R. Schrock and W. M. Davis, Angew. Chem.,
Int. Ed. Engl., 1993, 32, 756; (b) J. B. Bonanno, P. T. Wolczanski and
E. B. Lobkovsky, J. Am. Chem. Soc., 1994, 116, 11159.
5 F. Basuli, J. Tomaszewski, J. C. Huffman and D. J. Mindiola, J. Am.
Chem. Soc., 2003, 125, 10171.
6 (a) C. C. Cummins, C. P. Schaller, G. D. Van Duyne, P. T. Wolczan-
ski, A. W. Chan and R. Hoffmann, J. Am. Chem. Soc., 1991, 113,
2985; (b) P. J. Walsh, F. J. Hollander and R. G. Bergman, J. Am.
Chem. Soc., 1988, 110, 8729; (c) R. Melenkivitz, D. J. Mindiola and
G. L. Hillhouse, J. Am. Chem. Soc., 2002, 124, 3846; (d ) D. J.
Mindiola and G. L. Hillhouse, J. Am. Chem. Soc., 2001, 123, 4623;
(e) D. J. Mindiola and G. L. Hillhouse, J. Am. Chem. Soc., 2002, 124,
9976; ( f ) D. J. Mindiola and G. L. Hillhouse, Chem. Commun.,
2002, 1840; (g) S. Thyagarajan, D. T. Shay, C. D. Incarvito, A. L.
Rheingold and K. H. Theopold, J. Am. Chem. Soc., 2003, 125, 4440.
7 J. B. Alexander, D. S. Glueck, G. P. A. Yap and A. L. Rheingold,
Organometallics, 1995, 14, 3603.
Our work in the synthesis of a titanium phosphaazaallene
complex contrasts with previous experiments by Cowley and
co-workers where it was shown that WL Cl (L᎐PMePh ) can
᎐
4
2
2
8 R. Appel and F. Knoll, Adv. Inorg. Chem., 1989, 33, 259.
9 X.-G. Zhou, L.-B. Zhang, R.-F. Cai, Q.-J. Wu, L.-H. Weng and
Z.-E. Huang, J. Organomet. Chem., 2000, 604, 260.
10 (a) N. Wiberg, H.-W. Häring, G. Huttner and P. Friedrich,
Chem. Ber., 1978, 111, 2708; (b) P. J. Walsh, M. J. Carney and R. G.
Bergman, J. Am. Chem. Soc., 1991, 113, 6343; (c) A. J. Blake, J. M.
McInnes, P. Mountford, G. I. Nikonov, D. Swallow and D. J.
Watkin, J. Chem. Soc., Dalton Trans., 1999, 379; (d ) J. L. Thorman
and L. K. Woo, Inorg. Chem., 2000, 39, 1301; (e) Y. Li, Y. Shi and
A. L. Odom, private communication.
Fig. 3 Molecular structure of 3 with thermal ellipsoids at the 50%
probability level. All H-atoms and aryl groups on the nitrogens with the
exception of the ipso-carbon atoms (C(7) and C(21)) have been omitted
for clarity.
11 A. H. Cowley, B. Pellerin, J. L. Atwood and S. G. Bott, J. Am. Chem.
Soc., 1990, 112, 6734.
D a l t o n T r a n s . , 2 0 0 3 , 4 2 2 8 – 4 2 2 9
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