associated with the P center in 1 was detected at 451.6 ppm in
the 31P NMR spectrum, and effective C3-symmetry in C6D6
solution was evident through equivalent anilide residues by
1H and 13C NMR spectroscopy.16 X-ray crystallography sup-
ported the structural formulation for 1 (Fig. 1); expectedly, the
Ta–P bond length of 2.3052(11) Å is significantly shorter than
the respective distances in 5 and 3, and is similar to the distance
of 2.317(4) Å disclosed for the related four-coordinate phosphi-
nidene (tBu3SiO)3TavPPh.17a While the Ta–P–C “bend”
angle of 125.15(13)° is slightly more obtuse than that found
for (tBu3SiO)3TavPPh (110.2(4)°), it is in line with struc-
turally related trimethylsilyl- and stannyl-phosphinidenes of
niobium.8c,22
To demonstrate generality of this approach to phosphinidenes,
we targeted the cyclohexyl variant, 6 (Scheme 2). In this case,
the treatment of 4 with 1.5 equivalents of LiP(H)Cy in benzene
led to the isolation of 6 as a brown-red solid in 86% yield after
18 h. However, the supposed intermediate in this transformation,
phosphanide complex 7 (Scheme 2), was not observed when the
reaction mixture was followed by 1H and 31P NMR spectroscopy
(in C6D6). In attempts to spectroscopically identify 7, including
reactions conducted at low temperatures, only the starting
materials and the phosphinidene product 6 were detected in situ.
While an alternative sequence cannot be ruled out, we postulate
an analogous reaction to that which led to 3 (and ultimately 1)
takes place, but in this case intermediate 7 undergoes rearrange-
ment to the respective terminal phosphinidene much more
rapidly than phenyl counterpart 3. Characterization data obtained
for 6 revealed similar features to those found for 1. Notably, a
31P NMR resonance at 483.1 ppm was assigned to the P center
in 6. The crystallographically determined structure for 6 (Fig. 1)
showed a similar Ta–P bond length of 2.2888(8) Å and a slightly
less bent Ta–P–C angle (130.07(10)°) than that observed for 1, a
metric that likely arises from the increased steric demands of the
cyclohexyl substituent. Each phosphinidene crystal structure 2
and 6 reveals a rotated orientation of one of the three anilide
ligands perpendicular to the Ta–P vector, which is likely a geo-
metric consequence of maximizing π-electron donation to the
electrophilic Ta center.22
to produce other Ta-element multiple bonds via the installation
of main-group fragments atop the tantalaziridine platform used
herein, such as alkyl,23 silyl, or amido groups, each equipped
with a removable α-proton.
Acknowledgment is made to NSERC (Canada) for a post-
doctoral fellowship to M.A.R, NSF (CHE-071957 and
CHE-1111357) for support including materials and supplies, as
well as Nick Piro and Dan Tofan for assistance with
crystallography.
Notes and references
1 (a) S. Shah and J. D. Protasiewicz, Coord. Chem. Rev., 2000, 210, 181;
(b) F. Mathey, Angew. Chem., Int. Ed., 2003, 42, 1578; (c) L. Weber,
Eur. J. Inorg. Chem., 2007, 4095; (d) R. Waterman, Dalton Trans., 2009,
18.
2 For a recent review of nucleophilic phosphinidene complexes, see:
H. Aktaş, J. C. Slootweg and K. Lammertsma, Angew. Chem., Int. Ed.,
2010, 49, 2102.
3 R. R. Schrock, Chem. Rev., 2002, 102, 145.
4 (a) D. E. Wigley, Prog. Inorg. Chem., 1994, 42, 239; (b) A. P. Duncan
and R. G. Bergman, Chem. Rec., 2002, 2, 431; (c) N. Hazari and
P. Mountford, Acc. Chem. Res., 2005, 38, 839.
5 (a) J. D. Masuda, A. J. Hoskin, T. W. Graham, C. Beddie, M. C. Fermin,
N. Etkin and D. W. Stephan, Chem.–Eur. J., 2006, 12, 8696; (b) G. Zhao,
F. Basuli, U. J. Kilgore, H. Fan, H. Aneetha, J. C. Huffman, G. Wu and
D. J. Mindiola, J. Am. Chem. Soc., 2006, 128, 13575.
6 (a) A. H. Cowley, B. Pellerin, J. L. Atwood and S. G. Bott, J. Am.
Chem. Soc., 1990, 112, 6734; (b) U. J. Kilgore, H. Fan, M. Pink,
E. Urnezius, J. D. Protasiewicz and D. J. Mindiola, Chem. Commun.,
2009, 4521.
7 (a) B. F. Wicker, J. Scott, J. G. Andino, X. Gao, H. Park, M. Pink and
D. J. Mindiola, J. Am. Chem. Soc., 2010, 132, 3691; (b) R. Waterman
and T. D. Tilley, Chem. Sci., 2011, 2, 1320.
8 Salt metathesis reactions that produce phosphinidenes but lack the inter-
mediacy of M–P(H)R species have been reported: (a) P. B. Hitchcock,
M. F. Lappert and W.-P. Leung, J. Chem. Soc., Chem. Commun., 1987,
1282; (b) J. S. Freundlich, R. R. Schrock and W. M. Davis, J. Am. Chem.
Soc., 1996, 118, 3643; (c) J. S. Figueroa and C. C. Cummins, Angew.
Chem., Int. Ed., 2004, 43, 984; (d) E. B. Hulley, J. B. Bonanno,
P. T. Wolczanski, T. R. Cundari and E. B. Lobkovsky, Inorg. Chem.,
2010, 49, 8524.
9 (a) R. Melenkivitz, D. J. Mindiola and G. L. Hillhouse, J. Am. Chem.
Soc., 2002, 124, 3846; (b) H. Aktas, J. C. Slootweg, A. W. Ehlers,
M. Lutz, A. L. Spek and K. Lammertsma, Organometallics, 2009, 28,
5166, and references cited therein.
Given the precedent of related early transition-metal com-
plexes to engage in “PR” group transfer reactions,1,2,10 we anti-
cipated that the four-coordinate Ta phosphinidenes 2 and 6
would function as phospha-Wittig reagents. Consistent with
this regard, the separate treatments of 2 and 6 with pival-
dehyde (OvC(H)tBu) effected the production of the respective
phosphaalkenes, RPvC(H)tBu (R = Ph for 2, R = Cy for 6)
in reactions that were quantitative with respect to the conversion
of the aldehyde in situ, as assayed by NMR spectroscopy.16
In summary, we have reported herein the preparation of ter-
minal phosphinidenes 2 and 6 via apparent reaction sequences
involving the repositioning of a P-bound proton of tantalaziri-
dine phosphanide complexes (i.e. 3 and putative 7). We ascribe
at least some of the favorability of this base-promoted isomeriza-
tion to a confluence of ring-strain alleviation and basicity of the
alkyl fragment of the tantalaziridine functional group.18 Given
the considerable interest in phosphorus-element bond formation
reactions,1,2 we aim to explore further the reactivity of these
complexes beyond our preliminary experiments with pivalde-
hyde. Also, it remains to be seen if this strategy can be extended
10 D. W. Stephan, Angew. Chem., Int. Ed., 2000, 39, 314.
11 D. J. Mindiola, Acc. Chem. Res., 2006, 39, 813.
12 For a report of H-atom abstraction from a Ni–P(H)R precursor to generate
a NivPR complex, see: V. M. Iluc and G. L. Hillhouse, J. Am. Chem.
Soc., 2010, 132, 15148.
13 The preparation of
1 via an inter-metal phosphinidene transfer
reaction has been recently disclosed, see Ref. 7b. Our characterization
data for 1 differ somewhat from those reported and thus warrant inclusion
herein.
14 C. C. Cummins, R. R. Schrock and W. M. Davis, Angew. Chem., Int. Ed.
Engl., 1993, 32, 756.
15 M. A. Rankin and C. C. Cummins, J. Am. Chem. Soc., 2010, 132, 10021.
16 See ESI† for additional details.
17 (a) J. B. Bonanno, P. T. Wolczanski and E. B. Lobkovsky, J. Am. Chem.
Soc., 1994, 116, 11159; (b) N. Etkin, M. T. Benson, S. Courtenay,
M. J. McGlinchey, A. D. Bain and D. W. Stephan, Organometallics,
1997, 16, 3504; (c) M. P. Shaver and M. D. Fryzuk, Organometallics,
2005, 24, 1419; (d) For a conceptually related preparation of a Ta arsini-
dene from Me3SiAsH2, see Ref. 8b.
18 Reversible cyclometalation processes have figured importantly in the
reactivity of metallaziridine hydride complexes: (a) F. H. Stephens,
J. S. Figueroa, C. C. Cummins, O. P. Kryatova, S. V. Kryatov,
E. V. Rybak-Akimova, J. E. McDonough and C. D. Hoff, Organo-
metallics, 2004, 23, 3126; (b) J. S. Figueroa and C. C. Cummins, Dalton
Trans., 2006, 2161.
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