Terminal amido and imido complexes of three-coordinate nickel

Full text of DOI:10.1021/ja010358a

J. Am. Chem. Soc. 2001, 123, 4623-4624
Scheme 1
4623
Terminal Amido and Imido Complexes of
Three-Coordinate Nickel
Daniel J. Mindiola and Gregory L. Hillhouse*
Searle Chemistry Laboratory, Department of Chemistry
The UniVersity of Chicago, Chicago, Illinois 60637
ReceiVed February 9, 2001
The chemistry of late-transition-metal complexes possessing
amido (NR2-) and imido (NR2-) ligands is much less developed
than that of the early- and mid-transition elements.1 This is
particularly true of the nickel triad, where the contrast between
these hard, nitrogen-donor ligands and the soft, electron-rich
metals is considered to be energetically destabilizing and is
exacerbated by the general lack of stabilizing π-donor interactions
involving the ligand lone pair(s) that are commonly found with
terminal amides and imides of the early- and mid-transition metals.
There has been increasing research activity in this area by us and
others,2-4 and it has been demonstrated that amido complexes of
Ni and Pd play key roles in a range of important reactions.5 Imido
complexes of the Ni triad are limited to a few bimetallic and
trimetallic examples possessing bridging (µ2 and µ3) NR2-
moieties.6,7 Stone’s 1970 report of (Ph2MeP)2MdNCF2CFHCF3
(M ) Pd, Pt) complexes incorporating an imido ligand with a
strongly electron-withdrawing fluoropropyl substituent provides
the only examples of terminal Group 10 imido derivatives,
although characterization was limited to 1H and 19F NMR
spectroscopic data, and the exact nature of these compounds is
unclear.8 To date, the only structurally characterized examples
of late-metal complexes possessing terminal imido ligands are
Bergman’s Ir derivatives of the general formulation Cp*IrtNR
(R ) aryl, alkyl, silyl).9
In the course of our studies of the reactions of aryl azides with
Ni(II) alkyls to give amide moieties,2 we have noted that azides
with bulky substituents (i.e., mesityl azide) yield products that
suggest involvement of nitrene (or imido) intermediates.10 More-
over, a recent report from Jones and Vicic presented compelling
evidence that a reactive terminal sulfido complex of Ni(II) was
generated on thermolysis of (PR2CH2CH2PR2)Ni(SH)(Ph), al-
though it was unstable with respect to dimerization via Ni-S-
Ni bridges.11 Herein we report a successful synthetic strategy for
the preparation of monomeric, three-coordinate nickel complexes
containing the chelating 1,2-bis(di-tert-butylphosphino)ethane
(dtbpe) ligand and an amido or imido nitrogen-donor ligand, along
with the structural characterization of the first terminal imido
complex of nickel, a species which features a nickel-nitrogen
multiple bond.
Our initial approach to preparing a terminal imido complex of
Ni(II) involved a modification of Jones’ method for the synthesis
of the transient “L2NidS”(described above).11 Specifically, we
wished to effect deprotonation or dehydrohalogenation of an
appropriate Ni(II) amide (i.e., LxNi(NHR)X). To inhibit imido
bridging, we used bulky substituents in both the ancillary phos-
phine ligand (dtbpe) as well as at nitrogen. Reaction of (dtbpe)-
NiCl2 (1)12 with 1 equiv of lithium 2,6-di-iso-propylphenylamide
(LiNH(2,6-(CHMe2)2C6H3)) gives a mixture of products, including
paramagnetic species. Reduction of 1 with KC8 in THF at -35
°C gives red crystals of the Ni(I) monochloride [(dtbpe)NiCl]2
(2) in 71% yield.13 Related Ni(I) chlorides of chelating diphos-
phine ligands have been reported.14 In contrast to 1, toluene
solutions of 2 react cleanly at -35 °C with Et2O solutions of
lithium 2,6-di-iso-propylphenylamide to afford the paramagnetic
arylamido complex (dtbpe)Ni{NH(2,6-(CHMe2)2C6H3)} (3) as
beet-red crystals in 92% yield (Scheme 1). This Ni(I) complex
(1) (a) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239. (b) Fryzuk, M.
D.; Montgomery, C. D. Coord. Chem. ReV. 1989, 95, 1. (c) Bryndza, H. E.;
Tam, W. Chem. ReV. 1988, 88, 1163. (d) Lappert, M. F.; Power, P. P.; Sanger,
A. R.; Srivastava, R. C. Metal and Metalloid Amides; Ellis Horwood:
Chichester; 1980. (e) Power, P. P. Comments Inorg. Chem. 1989, 8, 177.
(2) (a) Matsunaga, P. T.; Hess, C. R.; Hillhouse, G. L. J. Am. Chem. Soc.
1994, 116, 3665. (b) Koo, K.; Hillhouse, G. L. Organometallics 1995, 14,
4421. (c) Koo, K.; Hillhouse, G. L. Organometallics 1996, 15, 2669.
(3) Examples of Group 10 amides: (a) Bradley, D. C.; Hursthouse, M. B.;
Smallwood, R. J.; Welch, A. J. J. Chem. Soc., Chem. Commun. 1972, 872.
(b) Fryzuk, M. D.; MacNeil, P. A. J. Am. Chem. Soc. 1981, 103, 3592. (c)
Fryzuk, M. D.; MacNeil, P. A.; Rettig, S. J.; Secco, A. S.; Trotter, J.
Organometallics 1982, 1, 918. (d) Hope, H.; Olmstead, M. M.; Murray, B.
D.; Power, P. P. J. Am. Chem. Soc. 1985, 107, 712. (e) Bartlett, R. A.; Chen,
H.; Power, P. P. Angew. Chem., Int. Ed. Engl. 1989, 28, 316. (f) Villanueva,
L. A.; Abboud, K. A.; Boncella, J. M. Organometallics 1994, 13, 3921. (g)
VanderLende, D. D.; Abboud, K. A.; Boncella, J. M. Inorg. Chem. 1995, 34,
5319. (h) VanderLende, D. D.; Boncella, J. M.; Abboud, K. A. Acta
Crystallogr., Sect. C 1995, 51, 591. (i) Cowan, R. L.; Trogler, W. C. J. Am.
Chem. Soc. 1989, 111, 4750. (j) Seligson, A. L.; Cowan, R. L.; Trogler, W.
C. Inorg. Chem. 1995, 34, 5319. (k) Driver, M. S.; Hartwig, J. F. J. Am.
Chem. Soc. 1997, 119, 8232. (l) Bryndza, H. E.; Fultz, W. C.; Tam, W.
Organometallics 1985, 4, 939. (m) Klein, H.; Karsch, H. H. Chem. Ber. 1973,
106, 2438.
(4) (a) Holland, P. L.; Andersen, R. A.; Bergman, R. G.; Huang, J.; Nolan,
S. P. J. Am. Chem. Soc. 1997, 119, 12800 and references therein. (b) Holland,
P. L.; Smith, M. E.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc.
1997, 119, 12815. (c) Holland, P. L.; Andersen, R. A.; Bergman, R. G. J.
Am. Chem. Soc. 1996, 118, 1092.
(5) (a) Yang, B. H.; Buchwald, S. L. J. Organomet. Chem. 1999, 576, 125.
(b) Hartwig, J. F. Pure Appl. Chem. 1999, 71, 1417. (c) Wolfe, J. P.; Wagaw,
S.; Marcoux, J. F.; Buchwald, S. L. Acc. Chem. Res. 1998, 31, 805. (d)
Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852.
(6) Meij, R.; Stufkens, D. J.; Vrieze, K.; Brouwers, A. M. F.; Overbeek,
A. R. J. Organomet. Chem. 1978, 155, 123.
1
has been characterized by elemental analysis, IR and H NMR
spectroscopic methods,13 and by single-crystal X-ray diffraction.15
The molecular structure of 3 features a planar, three-coordinate
(10) (a) Lin, B.; Hillhouse, G. L. Abstracts of Papers, 213th ACS National
Meeting; San Francisco, CA, April, 1997; Abstract INOR 636. (b) Lin, B. L.
Ph.D. Thesis, The University of Chicago (Chicago, IL), December, 2000.
(11) (a) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1999, 121, 4070.
(b) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1999, 121, 7606.
(12) Bach, I.; Goddard, R.; Kopiske, C.; Seevogel, K.; Po¨rschke, K.-R.
Organometallics 1999, 18, 10.
(13) See the Supporting Information for complete spectroscopic and
analytical details for 2-5 and solution magnetic moments for 2 and 3.
(14) Scott, F.; Kru¨ger, C.; Betz, P. J. Organomet. Chem. 1990, 387, 113.
(15) Crystal data for 3: C30H58NNiP2, orthorhombic, P212121, a ) 10.8603-
(9) Å, b ) 16.6536(14) Å, c ) 17.3090(14) Å, Z ) 4, µ(Mo KR) ) 7.40
cm-1, T ) 100 K, V ) 3130.6(4) Å3, λ ) 0.71073 Å, Dc ) 1.174 mg/mm3.
Of 19258 data collected (red crystal, 1.70 e θ e 28.29) 7365 were independent
and observed with I > 2σ(I). Flack x parameter ) -0.004, 0.010 esd. All
non-hydrogen atoms were anisotropically refined, and hydrogen atoms were
idealized except for the H attached to N, which was located and refined
isotropically. R(F) ) 0.035 and R(wF2) ) 0.063.
(7) (a) Otsuka, S.; Nakamura, A.; Yoshida, T. Inorg. Chem. 1968, 7, 261.
(b) Muller, J.; Dorner, H.; Kohler, F. H. Chem. Ber. 1973, 106, 1122. (c)
Klein, H.-F.; Haller, S.; Ko¨nig, H.; Dartiguenave, Y.; Menu, M.-J. J. Am.
Chem. Soc. 1991, 113, 4673.
(8) McGlinchey, M. J.; Stone, F. G. A. J. Chem. Soc., Chem. Commun.
1970, 1265.
(9) (a) Gleuck, D. S.; Wu, J.; Hollander, F. J.; Bergman, R. G. J. Am.
Chem. Soc. 1991, 113, 2041. (b) Gleuck, D. S.; Hollander, F. J.; Bergman, R.
G. J. Am. Chem. Soc. 1989, 111, 2719.
10.1021/ja010358a CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/21/2001

4624 J. Am. Chem. Soc., Vol. 123, No. 19, 2001
Communications to the Editor
Figure 2. A perspective view of the molecular structure of 5 showing
the atom-labeling scheme. H-atoms have been omitted for clarity. Selected
metrical parameters: Ni-P(1) ) 2.1815(8), Ni-P(2) ) 2.1887(8), Ni-
N(1) ) 1.702(2), N(1)-C(71) ) 1.355(3) Å; P(1)-Ni-P(2) ) 90.94-
(3), P(1)-Ni-N(1) ) 132.50(7), P(2)-Ni-N(1) ) 136.46(7), Ni-N(1)-
C(71) ) 162.8(2)°.
Treatment of THF solutions of 4 with sodium bis(trimethyl-
silyl)amide at -35 °C results in deprotonation of 4 at nitrogen,
affording emerald-green crystals of the diamagnetic arylimido
complex (dtbpe)Ni{N(2,6-(CHMe2)2C6H3)} (5) in 65% isolated
yield. 5 was characterized by elemental analysis and IR and NMR
(1H, 13C, 31P) spectroscopy,13 and its monomeric structure was
confirmed by single-crystal X-ray diffraction.19
The molecular structure of 5 is shown in Figure 2. The most
salient structural features are a planar, three-coordinate nickel-
(II) center and a terminal imido ligand with a short Ni-N(1) bond
(1.702(2) Å). The Ni-N(1) bond length is comparable to the value
of 1.68 Å predicted by Pauling for a Ni-N bond order of two20
and is consistent with a symmetry-allowed π-bonding interaction
between a nitrogen lone pair and the empty in-plane Ni d-orbital
of π-symmetry. The imido ligand is significantly bent at nitrogen
(Ni-N(1)-C(71) ) 162.8(2)°) out of the Ni coordination plane
(as required by the in-plane π-bond). Steric interactions between
the imido’s aryl substituent and the phosphine’s tert-Bu groups
might be responsible for preventing this angle from being more
acute.
In summary, we have shown that a cationic Ni(II)-amido
complex can serve as a precursor to a Ni(II)-imido complex
containing a terminal imido moiety. It is noteworthy that while 5
is air-sensitive, it does not exhibit unusual thermal instability (in
fact, it is thermally robust), and it is somewhat surprising that
related three-coordinate complexes of Ni(II) with hard π-donor
ligands have not been previously prepared. We are currently
exploring the reaction chemistries of these three-coordinate nickel
complexes.
Figure 1. A perspective view of the molecular structure of 3 showing
the atom-labeling scheme. H-atoms (except H(1)) have been omitted
for clarity. Selected metrical parameters: Ni-P(1) ) 2.2094(8), Ni-
P(2) ) 2.2012(7), Ni-N(1) ) 1.881(2), N(1)-C(71) ) 1.373(3), N(1)-
H(1) ) 0.82(2) Å; P(1)-Ni-P(2) ) 91.91(3), P(1)-Ni-N(1) )
123.18(7), P(2)-Ni-N(1) ) 140.41(7), Ni-N(1)-C(71) ) 134.6(2), Ni-
N(1)-H(1) ) 111(2), C(71)-N(1)-H(1) ) 109(2)°.
nickel and a terminal, planar amido ligand (Ni-N(1) ) 1.881(2)
Å) (see Figure 1). The {P(1), P(2), Ni} and {N(1), C(71), H(1)}
planes are orthogonal with a 91° dihedral angle. The Ni-N
distance is comparable to those found for other terminal Ni
amides, such as the related three-coordinate Ni(I) complex (PPh3)2-
Ni{N(SiMe3)2} (Ni-N ) 1.88(1) Å)3a and several diverse Ni(II)
examples having (Ni-N)avg ≈ 1.88 Å (ranging from 1.93 to 1.82
Å).3c-e,h,4b
Recognizing that moving from the Ni(I) to Ni(II) manifold
should prove advantageous in preparing a terminal imido moiety
since a three-coordinate, d8 species could be stabilized by ligand-
to-metal π-donation, we sought to chemically oxidize 3 by one
electron. Common oxidants such as Cp2Fe+, Ag+, or I2 gave
myriad decomposition products, consistent with observations that
these oxidants are competent in triggering oxidatively induced
reductive elimination from alkyl Ni(II) amides and alkoxides via
Ni(III) intermediates.2,16 A cyclic voltammogram of a solution
of 3 (THF/TBAH) shows two closely spaced, quasireversible
oxidations at E1/2 ) -0.90 V and -0.52 V (relative to Fc/Fc+)
for the Ni(I)/Ni(II) and Ni(II)/Ni(III) couples, respectively. These
electrochemical data suggest that to effect the oxidation of 3 to
Ni(II), a very weak oxidant whose potential lies in this narrow
range is required to avoid over-oxidation to Ni(III). Inspection
of tabulations of oxidation potentials of reagents indicated a good
choice might be tropylium, having E0′ ) -0.65 V in acetonitrile.17
Accordingly, oxidation of THF solutions of 3 with tropylium
hexafluorophosphate at -35 °C gives the diamagnetic Ni(II)
amido salt [(dtbpe)Ni{NH(2,6-(CHMe2)2C6H3)}+][PF6-] (4) as
dark-green crystals in 82% isolated yield (Scheme 1). 4 was
characterized by elemental analysis, IR and NMR (1H, 13C, 19F,
31P) spectroscopy,13 and single-crystal X-ray diffraction. Although
disordered, the structure of 4 shows well-separated cations and
anions; the coordination geometry about Ni is similar to that
observed in 3, with a Ni-N bond length of 1.768(14) Å.18
Acknowledgment. We are grateful to the National Science Foundation
(CHE-9816341) for financial support of this research, to Dr. Ian Steele
for assistance with crystallography, to Daniel Haines for assistance with
electrochemical measurements, and to Cheslan Simpson and Ryan DaRe
for insightful discussions.
Supporting Information Available: Experimental, spectroscopic, and
analytical details; crystallographic details; atomic coordinates; bond angles
and distances; anisotropic thermal parameters; hydrogen atom coordinates;
least-squares planes; torsion angles (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
(16) (a) Matsunaga, P. T.; Hillhouse, G. L.; Rheingold, A. L. J. Am. Chem.
Soc. 1993, 115, 2075. (b) Matsunaga, P. T.; Mavropoulos, J. C.; Hillhouse,
G. L. Polyhedron 1995, 14, 175. (c) Koo, K.; Hillhouse, G. L.; Rheingold, A.
L. Organometallics 1995, 14, 456. (d) Han, R.; Hillhouse, G. L. J. Am. Chem.
Soc. 1997, 119, 8135. (e) Koo, K.; Hillhouse, G. L. Organometallics 1998,
17, 2924.
(17) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
(18) The structure of 4 suffered from conformational disorder of the tert-
Bu and CH2 groups of the dtbpe ligand, and the iso-Pr groups of the amido
ligand. A highly disordered THF of solvation was also confined in the
asymmetric unit, resulting in a poor quality structure. Details are included as
Supporting Information, but not reported here, except for selected metrical
parameters for the inner coordination sphere of Ni for comparison with 3 and
5.
JA010358A
(19) Crystal data for 5: C30H57NNiP2, triclinic, P1h, a ) 10.8639(10) Å,
b ) 11.1092(10) Å, c ) 14.5704(14) Å, R ) 67.700(2)°, â ) 72.419(2)°,
γ ) 75.351(2) °, Z ) 2, µ(Mo KR) ) 7.56 cm-1, T ) 100 K, V ) 1531.8(2)
Å3, λ ) 0.71073 Å, Dc ) 1.198 mg/mm3. Of 8973 data collected (emerald
green crystal, 1.55 e θ e 28.28) 6199 were independent and observed with
I > 2σ(I). All non-hydrogen atoms were anisotropically refined, and hydrogen
atoms were placed in idealized positions. R(F) ) 0.0475 and R(wF2) ) 0.0947.
(20) The Pauling and Schomaker-Stevenson covalent radii, with corrections
for electronegativity, predict bond lengths of 1.80 and 1.68 Å for Ni-N single
and double bonds, respectively. Pauling, L. The Nature of the Chemical Bond,
3rd ed.; Cornell University Press: Ithaca, NY, 1960.