Hydrogen-atom abstraction from Ni(I) phosphido and amido complexes gives phosphinidene and imide ligands

Article abstract of DOI:10.1021/ja107115q

Hydrogen-atom abstraction from M-E(H) to generate M=E-containing complexes (E = PR, NR) is not well studied because only a few complexes are known to undergo such reactions. Hydrogen-atom abstraction from nickel(I) phosphide and amide complexes led to the corresponding phosphinidene and imide compounds. These reactions are unparalleled in the organometallic chemistry of nickel and feature an unusual example of a transition-metal phosphinidene synthesized by hydrogen-atom abstraction.

Full text of DOI:10.1021/ja107115q

Published on Web 10/07/2010
Hydrogen-Atom Abstraction from Ni(I) Phosphido and Amido Complexes
Gives Phosphinidene and Imide Ligands
Vlad M. Iluc and Gregory L. Hillhouse*
Gordon Center for IntegratiVe Science, Department of Chemistry, The UniVersity of Chicago,
929 East 57th Street, Chicago, Illinois 60637
Received August 8, 2010; E-mail: g-hillhouse@uchicago.edu
targeted because it would parallel the phosphide/phosphinidene pair
Abstract: Hydrogen-atom abstraction from M-E(H) to generate
(dtbpe)Ni-PH(dmp) (1)/(dtbpe)NidP(dmp) (2) previously synthe-
MdE-containing complexes (E ) PR, NR) is not well studied
because only a few complexes are known to undergo such
reactions. Hydrogen-atom abstraction from nickel(I) phosphide
and amide complexes led to the corresponding phosphinidene
and imide compounds. These reactions are unparalleled in the
organometallic chemistry of nickel and feature an unusual
example of a transition-metal phosphinidene synthesized by
hydrogen-atom abstraction.
sized and characterized.5 Although related nickel amide and imide
complexes have been reported, (dtbpe)NidP(dmp) is the only nickel
phosphinidene to have been successfully isolated. Consequently,
the nickel amide (dtbpe)NisNH(dmp) (3) was synthesized in 80%
yield by the reaction of the Ni(I) chloride dimer [(dtbpe)NiCl]2 with
(dmp)NHLi, analogous to the synthesis of the Ni(I) amide
(dtbpe)NisNHAr (Ar ) 2,6-di-iso-propylphenyl).4 The new Ni(I)
complex 3, isolated as a bright-pink solid, is a one-electron
paramagnet with µeff ) 1.9 µB in CD2Cl2 solution measured by the
1
Evans method. As with most Ni(I) paramagnetic species, its H
NMR spectrum is broad and 3 is silent in both 13C and 31P NMR
Hydrogen-atom transfer reactions are important in areas promi-
nent in both chemistry and biology.1 Of these, reactions with
hydrogen-atom donors are common, especially when they involve
reactive metal-element (MdX, X ) O, N) multiple bonds.2 The
reverse process, hydrogen-atom abstraction to generate MdX-
containing complexes, is not as well studied because examples of
complexes known to undergo such reactions are rare.3 Herein we
report the successful conversion by hydrogen-atom abstraction of
nickel(I) phosphide and amide complexes to the corresponding
phosphinidene and imide derivatives. These reactions are unparal-
leled in the organometallic chemistry of nickel and include a unique
example of a transition-metal phosphinidene synthesized by this
hydrogen-atom abstraction route.
spectra. The N-H stretch in the IR spectrum (νNH ) 3276 cm-1
)
was found at lower energy than that for (dtbpe)Ni-NHAr (νNH
)
3345 cm-1),4 but it falls in the range of other reported amides.6
The solid-state structure of 3, determined by single-crystal X-ray
diffraction (Figure 1), shows a slight pyramidalization at nickel
(sum of angles ) 349.66°) with the Ni-N bond displaced from
the P(1)-Ni-P(2) plane by ∼29°. The amide hydrogen atom was
located in the electron-density map at 0.79(3) Å from the nitrogen
atom.
Scheme 1. Synthesis of (dtbpe)NidX(dmp) Complexes (X ) P, N)
Figure 1. Thermal-ellipsoid (50% probability) representation of (dtbpe)Nis
NH(dmp) (3, left) and (dtbpe)NidN(dmp) (4, right). H atoms, except on
N, were omitted for clarity. Selected metrical parameters for 3: Ni-N )
1.881(2), Ni-P(1) ) 2.2167(7), Ni-P(2) ) 2.2192(7), N-C(61) )
1.364(3), N-H ) 0.79(3) Å; Ni-N-C(61) ) 147.1(2), P(1) -Ni-P(2) )
92.27(3), P(1)-Ni-N ) 132.69(8), P(2)-Ni-N ) 124.70(8), Ni-N-H
) 104(2), C(61)-N-H ) 106(2)°. For 4: Ni-N ) 1.697(2), Ni-P(1) )
2.1717(6), Ni-P(2) ) 2.1397(6), N-C(31) ) 1.345(2) Å; Ni-N-C(31)
) 168.98(12), P(1)-Ni-P(2) ) 91.56(2), P(1)-Ni-N ) 142.46(5),
P(2)-Ni-N ) 125.29(5)°.
Our group has reported several three-coordinate nickel species,
which include amides, imides, phosphides, and phosphinidenes.4,5
The neutral amide/imide and phosphide/phosphinidene pairs differ
by the nickel oxidation state and one hydrogen atom. The
conversion from amide/phosphide to imide/phosphinidene was
achieved previously in two steps: oxidation of the neutral Ni(I)
center to cationic Ni(II), followed by deprotonation using a strong
base.4,5 We sought to determine whether an alternative route to
nickel-ligand multiple bonds, hydrogen-atom abstraction directly
from Ni(I) to Ni(II), is viable.
The bulky 2,4,6-tris-tert-butylphenoxyl radical (SMesO•)8 is
the simplest of stable phenoxyl radicals, and it was used by
Mayer9 for hydrogen-atom abstraction from N-H bonds in
transition-metal complexes and by Smith for the conversion of
A new amide/imide pair that contains 2,6-dimesitylphenyl (dmp;
mesityl ) 2,4,6-trimethylphenyl) as a nitrogen substituent was
9
15148 J. AM. CHEM. SOC. 2010, 132, 15148–15150
10.1021/ja107115q  2010 American Chemical Society

C O M M U N I C A T I O N S
[L]CosN(H)tBu to [L]CodNtBu (L ) phenyltris(1-tert-butyl-
imidazol-2-ylidene)borato).3a The reaction of 3 with SMesO•
generated the Ni(II) imide (dtbpe)NidN(dmp) (4, Scheme 1)
under mild conditions (ambient temperature, 12 h) and in high
yield (90%). The same reactivity was observed for (dtbpe)Nis
NHAr.10 1H, 31P, and 13C NMR spectra of 4 confirmed the presence
of a pseudo-C2V symmetric complex in solution, with magnetically
equivalent phosphorus nuclei (31P δ 119.23). The solid-state
structure of 4, determined by single-crystal X-ray diffraction (Figure
1), revealed a trigonal-planar geometry at the metal center with
the sum of angles around Ni at 359.31°. The Ni-N bond deviates
from the P(1)-Ni-P(2) bisector by ∼9°, and the Ni-N distance
of 1.697(2) Å is, within error, equal to the distance found for other
Ni(II) aryl imides.4,11 The Ni-N-C(31) angle at 168.98(12)°
deviates slightly from linearity, with the complex (dtbpe)NidNMes
being the only imide with collinear Ni, N, and C atoms.11 Steric
requirements of the dmp substituent force its central aryl ring to
lie perpendicular to the Ni coordination plane, in contrast to the
coplanar geometry found in other arylimides in this (dtbpe)Ni
system.4,11
Figure 3. Cyclic voltammogram for (dtbpe)Ni-NH(dmp) (3) at 100 mV/s,
10 mM in 1 M [nBu4N]PF6 in THF, Cp2Fe/Cp2Fe+ corrected.
Next, we probed the feasibility of pathway B. As mentioned, 4
can be independently synthesized from 3 by [Cp2Fe]+ oxidation to
give the cationic Ni(II) complex [(dtbpe)NisNH(dmp][B(ArF)4] (6),
followed by deprotonation using NaN(SiMe3)2. Based on the Ni(I)/
Ni(II) redox potential (E1/2 ) -0.55 V; Figure 3) and E(SMesOH/
SMesO•) ) -0.5 V,14 it is uncertain, however, whether SMesO• is
thermodynamically competent to oxidize 3. We also explored the
We also became interested in studying the analogous hydrogen-
abstraction process with the nickel phosphide complex 1 since no
such syntheses have been reported for phosphinidenes. The prepara-
tion of such complexes usually employs halogen abstrac-
tion,12a,b hydrogen-atom migration,12c-e methatesis,12f or dehy-
drohalogenation,13 in addition to the oxidation/deprotonation pro-
S
second step, deprotonation, by employing MesOK as a base to
1
convert 6 to 4. By monitoring this reaction by H NMR spectros-
copy, the reduction of 6 to 3 was observed initially and not its
deprotonation, a fact that seems to invalidate this mechanistic
pathway. Therefore, we propose that the transformations of 1/3 to
S
2/4 in the presence of MesO• occur by PCET (path C, Figure 2).
5
S
tocol detailed above. Reaction of MesO• with 1 led to 2 in 75%
isolated yield (Scheme 1). Interestingly, the phosphinidene (2)
and imide (4) complexes can be reconverted to the corresponding
Ni(I) phosphide (1) and amide (3) compounds in 87% and 75%
yield, respectively, by reaction with the H• donor nBu3SnH
(Scheme 1).
This proposal is in agreement with the PCET mechanism proposed
by Smith for the conversion of [L]CosN(H)tBu to [L]CodNtBu.3a
DFT calculations are consistent with the experimental data. DFT
calculations also support C as a viable, low-barrier mechanism since
the calculated reaction energies are -9.2/4.0 kcal/mol for the
conversion of 1/3 to 2/4 (Figure 2). The formation of the anionic
intermediate 5 and the cationic complex 6 are both energetically
uphill (103.0 and 22.3 kcal/mol, Figure 2), and the base and oxidant
energetics are included in the thermodynamic analysis.
In conclusion, we have described two examples of hydrogen-
atom abstraction from nickel phosphide and amide complexes to
form the corresponding phosphinidene and imide derivatives. These
processes were enabled by the synthesis of the dmp-substituted
imide and phosphinidene complexes, and their mechanism was
discussed on the basis of experimental and computational results.
In both cases, proton-coupled-electron transfer was found to be the
most viable reaction pathway. These transformations open an
avenue for the synthesis of late-transition-metal-element multiply
bonded species and contribute to the understanding of hydrogen-
atom abstraction processes in metal complexes.
Figure 2. Possible reaction pathways and calculated energies (kcal/mol;
solvent-corrected for benzene) for the conversion of 1 to 2 (X ) P) and 3
to 4 (X ) N); the organic reactants and products were omitted for clarity.
Acknowledgment. This work was supported by the National
Science Foundation through Grant CHE-0957816 to G.L.H.
Supporting Information Available: Crystallographic data for 3 and
4 (CIF). Synthetic and spectroscopic characterization of all complexes
and computational details (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
The conversion of 1/3 to 2/4 by SMesO• may follow three
pathways (Figure 2): (A) deprotonation (PT ) proton transfer)
followed by oxidation (ET ) electron transfer), (B) oxidation
followed by deprotonation, and (C) proton-coupled-electron transfer
(PCET). We disfavor path A since all our attempts to isolate the
anionic species [(dtbpe)NidN(dmp)-] (5) from the reduction of 4
led to decomposition, indicating that 5 is not stable. This observation
is supported by DFT calculations that show that the conversion of
2/4 to 7/5 is highly endothermic (101.0/99.0 kcal/mol, Figure 2).
References
(1) Mukhopadhyay, S.; Mandal, S. K.; Bhaduri, S.; Armstrong, W. H. Chem.
ReV. 2004, 104, 3981.
(2) (a) Kogut, E.; Wiencko, H. L.; Zhang, L.; Cordeau, D. E.; Warren, T. H.
J. Am. Chem. Soc. 2005, 127, 11248. (b) Eckert, N. A.; Vaddadi, S.; Stoian,
S.; Lachicotte, R. J.; Cundari, T. R.; Holland, P. L. Angew. Chem., Int. Ed.
2006, 45, 6868.
n
In addition, 3 is inert toward either BuLi or (Me3Si)2NNa, bases
stronger than SMesO• (for SMesOH•+: pKa ≈ 3),13 further supporting
that A is not a viable pathway in the transformation of 3 to 4
(3) (a) Cowley, R. E.; Bontchev, R. P.; Sorrell, J.; Sarracino, O.; Feng, Y.;
Wang, H.; Smith, J. M. J. Am. Chem. Soc. 2007, 129 (9), 2424. (b) Huynh,
M. H. V.; Meyer, T. J. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 13138.
(4) Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2001, 123, 4623.
S
mediated by MesO•.
9
J. AM. CHEM. SOC. VOL. 132, NO. 43, 2010 15149

C O M M U N I C A T I O N S
(5) Melenkivitz, R.; Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2002,
124, 3846.
(12) (a) Wolf, R.; Hey-Hawkins, E. Eur. J. Inorg. Chem. 2006, 6, 1348. (b)
Sa´nchez-Nieves, J.; Sterenberg, B. T.; Udachin, K. A.; Carty, A. J. J. Am.
Chem. Soc. 2003, 125, 2404. (c) Basuli, F.; Tomaszewski, J.; Huffman,
J. C.; Mindiola, D. J. J. Am. Chem. Soc. 2003, 125, 10170. (d) Basuli, F.;
Bailey, B. C.; Huffman, J. C.; Baik, M. H.; Mindiola, D. J. J. Am. Chem.
Soc. 2004, 126, 1924. (e) Graham, T. W.; Cariou, R. P. Y.; Sa´nchez-Nieves,
J.; Allen, A. E.; Udachin, K. A.; Regragui, R.; Carty, A. J. Organometallics
2005, 24, 2023. (f) Termaten, A. T.; Nijbacker, T.; Schakel, M.; Lutz, M.;
Spek, A. L.; Lammertsma, K. Organometallics 2002, 21, 3196. (g)
Termaten, A. T.; Aktas, H.; Schakel, M.; Ehlers, A. W.; Lutz, M.; Spek,
A. L.; Lammertsma, K. Organometallics 2003, 22, 1827.
(6) Gavenonis, J.; Tilley, T. D. J. Am. Chem. Soc. 2002, 124, 8536.
(7) (a) Bradley, D. C.; Hursthouse, M. B.; Smallwood, R. J.; Welch, A. J.
J. Chem. Soc., Chem. Commun. 1972, 15, 872. (b) Fryzuk, M. D.; MacNeil,
P. A.; Rettig, S. J.; Secco, A. S.; Trotter, J. Organometallics 1982, 1, 918.
(c) Hope, H.; Olmstead, M. M.; Murray, B. D.; Power, P. P. J. Am. Chem.
Soc. 1985, 107, 712. (d) Bartlett, R. A.; Chen, H.; Power, P. P. Angew.
Chem., Int. Ed. Engl. 1989, 28, 316.
(8) (a) Altwicker, E. R. Chem. ReV. 1967, 67, 475. (b) Hicks, R. G. Org. Biomol.
Chem. 2007, 5, 1321. (c) Manner, V. W.; Markle, T. F.; Freudenthal, J.;
Roth, J. P.; Mayer, J. M. Chem. Commun. 2008, 256.
(9) (a) Mader, E. A.; Manner, V. W.; Markle, T. F.; Wu, A.; Franz, J. A.;
Mayer, J. A. J. Am. Chem. Soc. 2009, 131, 4335. (b) Roth, J. P.; Yoder,
J. C.; Won, T.-J.; Mayer, J. M. Science 2001, 294, 2524.
(10) (dtbpe)NidNAr was isolated in 75% yield from the reaction of
(dtbpe)NisNHAr with SMesO• at room temperature in benzene.
(11) Waterman, R.; Hillhouse, G. L. J. Am. Chem. Soc. 2008, 130, 12629.
(13) Estimated value: Bordwell, F. G.; Cheng, J.-P. J. Am. Chem. Soc. 1991,
113, 1736.
(14) Steuber, F. W.; Dimroth, K. Chem. Ber. 1966, 99, 258.
JA107115Q
9
15150 J. AM. CHEM. SOC. VOL. 132, NO. 43, 2010