Formation of C-C and C-N bonds in NiII ketimide complexes via transient NiIII aryl imides

Article abstract of DOI:10.1002/anie.200604547

Has a real nacnac for coupling: Ligand-to-metal electron transfer results from the reactions of the Ni^I synthon [{(nacnac)-Ni} ₂(μ-η³:η³-C₆H ₅Me)] (nacnac = CH-{CMeN(2,6-iPr₂C₆H ₃)}₂) with the azides 2,6-iPr₂C ₆H₃N₃ or 2,6-Me₂C₆H ₃N₃ to give Ni^II ketimides (see picture for example; C black, N light blue, Ni white). The reactions proceed via transient Ni^III imides, and the differing pathways are directed by the steric demands of the azide substituents. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200604547

Communications
DOI: 10.1002/anie.200604547
Ketimide Complexes
II
À
À
Formation of C C and C N Bonds in Ni Ketimide Complexes via
Transient NiIII Aryl Imides**
Guangcai Bai and Douglas W. Stephan*
À
À
À
À
The formation of C C and C heteroatom bonds is an
important general target of synthetic chemistry.[1–5] To these
ends, the chemistry of early-metal amides[6–11] has been
explored extensively. The corresponding chemistry of late-
transition-metal imido ligands has drawn much less atten-
tion.[12–14] In fact, while McGlinchey and Stone[15] reported in
transfer reactions affording the formation of newC C or C
N bonds, thus resulting in products that are described as NiII
ketimides.
We have previously reported the synthesis of [{(nacnac)-
Ni}2(m-h3:h3-C6H5Me)] (1).[37] This species, although best
formulated as a NiII dimer bridged by a reduced arene
fragment, behaves as a convenient NiI synthon. The reaction
of 1 with 2,6-iPr2C6H3N3 proceeded to generate a deep-green
solution. The subsequent work up and isolation of the dark
green solid gave 2 in 84% yield (Scheme 1). The molecule 2 is
=
1970 the generation of [(Ph2MeP)2M NCF2CFHCF3](M=Pd,
Pt) incorporating an imido ligand, the number of subsequent
reports was quite small until recently. In the late 1980s and
1990s Bergman and co-workers described the synthesis and
reactivity of Ir[16,17] and Os imides[18,19]
while Burrell and Steedman[20,21]
reported the complex [(h6-benzene)-
=
Ru NAr] (Ar= 2,6-iPr2C6H3). In 2001,
Mindiola and Hillhouse[22] showed the
deprotonation of the corresponding
amide to give the terminal NiII imide
=
[(tBu2PCH2CH2PtBu2)Ni NAr], which
undergoes reactions with CO and
benzyl isocyanide as well as ethylene to
give the corresponding carbodi-
imide[22,23] and aziridines,[24] respectively.
Low-valent metal centers have drawn
attention recently as reactions of CoI
and FeI have been shown to give the
corresponding double imido-bridged
dimers or trivalent imido com-
plexes.[25–33] Warren and co-workers[34]
have applied a similar strategy most
Scheme 1. Formation of 2 and 3 from 1. Ar=2,6-iPr2C6H3;Ar ’=2,6-iPr2C6H3 or 2,6-Me2C6H3.
recently to prepare both imido-bridged and terminal Ni
paramagnetic with a molecular magnetic moment of 3.6 mB in
C6D6 at 258C, consistent with the presence of unpaired spin.
The IR data provides a fingerprint for this species but was not
structurally informative. X-ray crystallography was per-
formed on single crystals of 2 grown from toluene.
=
imides of the form [{(nacnac)Ni}2(NAd)] and [(nacnac)Ni
NAd] (nacnac = CH{CMeN(2,6-iPr2C6H3)}2; Ad = 1-adaman-
tyl). The latter species react with phosphine to produce the
=
phosphinimide R3P NAd. In addition, Holland and co-work-
ers have described the ability of FeIII imides to abstract
H atoms from the ligand or external reagents,[35] while the
Warren group has described the reactivity of Cu aryl
imides.[36] Herein, we report the oxidation a NiI synthon by
aryl azides. In contrast to the alkyl azides, which give NiIII
imides, these reactions proceed to effect internal electron-
The X-ray crystallographic data[38] revealed that 2 is
correctly formulated as the bimetallic species [{(nacnac)-
Ni(NC6H2iPr2)}2] (Figure 1a). This molecule is a centrosym-
metric dimer; in each half the Ni center resides in an
approximately trigonally planar coordination sphere com-
posed of two N atoms of the nacnac ligand and a third N atom
À
derived from the azide fragment. The Ni N bond lengths to
the nacnac ligand were found to be 1.8631(19) Š and
1.8646(19) Š. The bite angle for the nacnac chelate in 2 is
94.41(8)8 while the metric parameters within the nacnac
[*] G. Bai, Prof. Dr. D. W. Stephan
Department of Chemistry and Biochemistry
University of Windsor
À
ligand are unexceptional. The Ni N(3) bond length is
Windsor, ON, N9B3P4 (Canada)
Fax: (+1)519-973-7098
1.711(2) Š, with a Ni-N-C angle of 166.1(2)8. This bond
=
length is markedly longer than that reported for [(nacnac)Ni
E-mail: stephan@uwindsor.ca
NAd] (1.662(2) Š),[34] similar to that reported for
[**] Financial support from NSERC of Canada is acknowledged.
[(CH2PtBu2)2Ni NC6H3iPr2] (1.702(2) Š),[39] and significantly
=
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
shorter than reported for [(CH2PtBu2)2NiNHC6H3iPr2]
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Chemie
1H NMR data for 3 are consistent with the presence of two
fragments derived from the azide reagent in addition to the
{(nacnac)Ni} fragment. Yields of 3 were increased to 74%
upon adjustment of the stoichiometry of Ni/azide to 1:2. The
X-ray crystallographic data[38] for 3 established the formula-
tion as the monometallic species [(nacnac)NiNC6H2Me2-
(NC6H3Me2)] (Figure 2). The geometry about the Ni center
as well as the metric parameters of the nacnac ligand are
similar to those observed in 2, and 3 is thus formulated in a
similar fashion as a NiII ketimide species. The Ni N bond
À
length for the ketimide ligand (1.699(3) Š) is slightly shorter
than that observed in 1, consistent with the presence of less
sterically demanding substituents. The resulting C(30)-N(3)-
Ni(1) angle is approximately linear at 175.0(3)8. The ketimide
À
N C bond length (1.289(4) Š) is consistent with double-bond
character. Similarly to 2, the bond lengths within the C6 ring
À
between N(3) and N(4) alternate in length. Thus, the C(31)
À
C(32) and C(34) C(35) bond lengths (1.339(5) Š) are shorter
À
than the remaining C C bonds, which average 1.459(6) Š.
The substituent on the C atom para to N(3) is N(4), which is
Figure 1. Pov-ray drawings of 2;(a) and (b) are orthogonal views.
Hydrogen atoms are omitted for clarity. In (b) the substituents on the
arene groups have been removed for clarity. C black, N light blue,
Ni white. Selected bond lengths [Š] and angles [8]: Ni(1)-N(3) 1.711(2),
Ni(1)-N(2) 1.8631(19), Ni(1)-N(1) 1.8646(19), N(3)-C(30) 1.269(3),
C(30)-C(31) 1.490(4), C(30)-C(35) 1.494(3), C(31)-C(32) 1.333(4),
C(32)-C(33) 1.494(4), C(33)-C(34) 1.490(4), C(33)-C(33)’ 1.569(6),
C(34)-C(35) 1.327(4);N(3)-Ni(1)-N(2) 132.36(9), N(3)-Ni(1)-N(1)
132.79(9), N(2)-Ni(1)-N(1) 94.41(8), C(30)-N(3)-Ni(1) 166.1(2), C(34)-
C(33)-C(32) 111.4(2), C(34)-C(33)-C(33) 110.4(3), C(32)-C(33)-C(33)’
111.2(3).
derived from addition of a second equivalent of 2,6-
À
Me2C6H3N3. The C(33) N(4) bond length is 1.299(5) Š. The
geometry at this remote N atom is bent with a C-N-C angle of
À
120.9(3)8 and an N C bond length to the terminal ring of
1.414(5) Š. These structural data support the description of
the {NC6H2Me2(NC6H3Me2)} unit as a para diketimine diene
fragment.
The formation of 2 and 3 is proposed to take place via
transient NiIII imide intermediates. Repeated attempts to
intercept a formal NiIII imide using other aryl azides such as
(1.881(2) Š).[39] The angles at the nickel center between
the nacnac N atoms and the third N atom [N(3)-Ni(1)-
N(2) and N(3)-Ni(1)-N(1)] average 132.6(2)8. Notably
À
the N(3) C(30) bond length is markedly short
À
(1.269(3) Š), as are the C(31) C(32) (1.333(4) Š) and
À
C(34) C(35) (1.327(4) Š) bonds in the ligand ring.
These lengths are indicative of double bonds, while the
remaining bonds within the ring are substantially longer
(av 1.495(5) Š). The two halves of the dimer are linked
through the C atom para to the N atom, C(33), with a
À
C(33) C(33)’ bond length of 1.569(6) Š. Saturation at
Scheme 2. Resonance forms of 3. L=nacnac.
the para C atom of this ring gives rise to a displacement
of the C atom from the plane of the C6 ring and results
in a gradual S curve of the dimerized ligand linkage
between the metal centers (Figure 1b). These data clearly
showthe reduction of the arene ring of the precursor azide,
resulting in the dimerization by coupling of the para carbon
atoms of two such fragments. Related back-to-back coupling
of imido ligands has been previously observed in the
oxidation of m-imido Rh dimers.[40] While the structural data
suggest a NiII ketimide formulation for 2, the observed
paramagnetism of 2 suggests that a NiIII imido radical
formulation is at least an accessible state (Scheme 2).
The analogous reaction of 1 with 2,6-Me2C6H3N3 was
performed. The liberation of N2 was evident immediately as
the solution became dark green. Subsequent work up of the
reaction afforded dark green crystals of 3 (Scheme 1). The
2,4,6-Me3C6H3N3 (MesN3) led only to unresolved, uncharac-
terized products. Nonetheless, cyclic voltammetric experi-
ments for 1 and the azides 2,6-Me2C6H3N3 and 2,6-iPr2C6H3N3
showed redox potentials consistent with the notion that the
NiI synthon is indeed a thermodynamically competent
reductant for the azides, which supports the notion of a
transient NiIII species.[41] In addition, literature precedent
supports this view; the groups of Holland[35] and Warren[34]
have shown that {(nacnac)FeI} and {(nacnac)NiI} are oxidized
by alkyl azides to give isolable MIII imides. In the case of 2 and
3, the electron-rich character of the aryl imide intermediates
prompt further reactivity. Formally, one can viewthe aryl
imide intermediates as NiII radicals which either dimerize or
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Communications
Technologies or Vacuum Atmospheres inert-atmosphere glove box.
Solvents were purified by employing a Grubbsꢀ column system.
1H NMR spectra were recorded on a Bruker Avance 300 and/or 500
spectrometer and referenced to SiMe4. Magnetic susceptibility was
determined by the Evanꢀs method. Combustion analyses were
performed in house. Compound 1 was prepared as previously
described in the literature.[37]
2: A solution of 1 (0.208 g, 0.2 mmol) in toluene (10 mL) was
added to a solution of 2,6-iPr2C6H3N3 (0.081 g, 0.4 mmol). Liberation
of N2 was immediately observed, and the solution became dark green.
The resulting mixture was stirred overnight. After concentration (to
3 mL) in vacuo, the resulting deep-green solution was kept at À358C
for one week to isolate dark green crystals of 2 (0.22 g, 84%). IR
(nujol): n˜ = 1924 (w), 1860 (w), 1704 (w), 1651 (w), 1623 (w), 1583 (w),
1548 (s), 1531 (s), 1494 (m), 1462 (s), 1396 (s), 1379 (s), 1319 (s), 1295
(m), 1250 (s), 1203 (m), 1179 (m), 1157 (m), 1121 (m), 1107 (m), 1054
(m), 1024 (m), 962 (m), 943 (m), 889 (m), 861 (m), 847 (m), 797 (s),
758 (m), 749 (m), 722 (m), 649 (w), 528 (m), 516 (w), 448 cmÀ1 (m);
magnetic susceptibility (c, C6D6, 258C): m = 3.61 mB. Elemental
analysis (%) calcd for C82H116N6Ni2 (1303.30): C 75.6, H 9.0, N 6.5;
found: C 75.9, H 8.7, N 6.4.
Figure 2. Pov-ray drawing of 3;(a) and (b) are orthogonal views.
3: 2,6-Me2C6H3N3 (0.118 g, 0.8 mmol) was added to a solution of 1
(0.208 g, 0.2 mmol) in toluene (10 mL). Liberation of N2 was
immediately observed, and the solution became dark green. The
resulting mixture was stirred overnight. Upon removal of toluene
in vacuo, hexane (5 mL) was added, and the resulting deep-green
solution was kept at À358C for two days to isolate dark green crystals
of 3 (0.12 g). After concentration of the filtrate to 3 mL, the solution
was kept at À358C for three days. An additional crop of 2 (0.09 g) was
formed. Yield: 0.21 g (74%). 1H NMR (C6D6): d = 7.01–6.88 (m, 9H,
Ar and 2,6-Me2C6H3), 6.20 (brs, 1H, MeCCH), 5.36 (brs, 1H,
MeCCH), 4.73 (s, 1H, g-CH), 4.10 (sept, 3JHH = 6.8 Hz, 4H, CHMe2),
1.90 (s, 2,6-Me2C6H3; and d, 3JHH = 6.8 Hz, CHMe2, 18H), 1.40 (s, 6H,
Hydrogen atoms are omitted for clarity. C black, N light blue, Ni white.
Selected bond lengths [Š] and angles [8]: Ni(1)-N(3) 1.699(3), Ni(1)-
N(1) 1.860(3), Ni(1)-N(2) 1.863(3), N(3)-C(30) 1.289(4), N(4)-C(33)
1.299(5);N(3)-Ni(1)-N(1) 133.46(13), N(3)-Ni(1)-N(2) 132.05(12),
N(1)-Ni(1)-N(2) 94.44(12), C(30)-N(3)-Ni(1) 175.0(3), C(33)-N(4)-
C(38) 120.9(3).
react further with azide to give 2 and 3 (Scheme 1). The
À
À
preferential formation of the C C bond in 2 and the C N
bond in 3 is attributed to the impact of steric effects. Large
ortho substituents on the azide preclude reaction of the
transient intermediate with excess azide, thus favoring back-
3
CMe), 1.25 (d, JHH = 6.9 Hz, 12H, CHMe2), 0.61 (s, 3H, MeCCH),
0.20 (s, 3H, MeCCH); 13C NMR (C6D6): 159.1, 159.0 (NC), 150.6
(CN(2,6-Me2C6H3)), 150.0, 142.9 (Ar), 140.9 (NC(Me)CH)), 131.6,
130.1, 126.9, 126.5, 124.1 (Ar and 2,6-Me2C6H3), 123.4 (MeCCH),
120.6 (MeCCH), 97.7 (g-C), 29.1 (CHMe2), 24.8, 23.8 (CHMe2), 21.9
(NCMe), 18.79 (2,6-Me2C6H3), 17.5, 17.4 ppm (MeCCH); IR (nujol):
n˜ = 2141(w), 2106 (w) 2051 (w), 1934 (w), 1901 (w), 1869 (w), 1836
(w), 1802 (w), 1706 (w), 1629 (m), 1587 (m), 1568 (m), 1548 (m), 1533
(s), 1513 (s), 1459 (s), 1438 (s), 1399 (s), 1320 (s), 1253 (s), 1218 (m),
1180 (m), 1162 (m), 1145 (w), 1101 (m), 1090 (m), 1056 (m), 1029 (s),
953 (m), 936 (m), 879 (m), 866 (w), 822 (m), 800 (m), 762 (s), 748 (m),
724 (m), 712 (m), 634 (w), 528 (m), 448 cmÀ1 (m). Elemental analysis
(%) calcd for C45H58N4Ni (713.70): C 75.7, H 8.2, N 7.9; found: C 75.5,
H 8.4, N 7.7.
À
to-back C C bond coupling (dimerization). In the case of 3,
the smaller ortho substitutents allowradical attack of azide
À
with loss of H2 and C N bond formation.
Preliminary DFT calculations using the models [CH-
(CMeNMe)2NiN(C6H6)] (4), derived from the crystallo-
graphic data for 2, were performed with the B3LYP-311
g(d,p) basis set. The HOMO of 4 is composed primarily of a
À
À
Ni N p bond while the LUMO has considerable N C p-bond
character. In contrast, analogous calculations for [CH-
(CMeNMe)2NiN(C6H4NMe)] (5) showed a more diffuse
HOMO with contributions from the p bonds of the nacnac
ORTEP figures of 2 and 3 can be found in the Supporting
Information. CCDC-626524 and 626525 contain the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
À
ligand, the C N bond, and a metal-based d orbital, whereas
the corresponding LUMO is clearly the p* orbital of the
NC6H4N fragment. These findings suggest that the extended
{NC6H4N} p system stabilizes the NiII ketimide nature of 3.
On the other hand, these results also suggest that facile access
of a triplet state of 2 accounts for the paramagnetism. Further,
more detailed computational studies of these features are
continuing.
Received: November 6, 2006
Published online: February 2, 2007
À
Keywords: C C coupling · density functional calculations ·
imido ligands · nickel · reduction
.
In summary, reduction of the transient NiIII imido species
À
À
results in the formation of a C C or C N bond. The utility of
such internal redox chemistry in the formation of extended
ring assemblies continues to be probed in our laboratories.
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Experimental Section
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b = 25.123(3),
c = 15.3140(19) Š,
b = 95.6460(10)8,
V=
3986.1(9) Š3, 7019 variables, 406 parameters, R = 0.0431, Rw =
¯
0.1098, GOF = 1.022. 3: triclinic, space group P1, a = 8.735(2),
b = 15.091(3), c = 15.875(4) Š, a = 89.290(2), b = 84.792(2), g =
83.995(2)8, V= 2072.7(8) Š3, 7271 variables, 451 parameters, R =
0.0572, Rw = 0.1382, GOF = 1.020.
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[NBu4][PF6] as the electrolyte.
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