A Two-Coordinate Nickel Imido Complex That Effects C-H Amination Page: 771
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JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
pubs.acs.org/JACS
A Two-Coordinate Nickel Imido Complex That Effects C-H Amination
Carl A. Laskowski,t Alexander J. M. Miller,' Gregory L. Hillhouse,*'t and Thomas R Cundari*'s
tGordon Center for Integrative Science, Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, United States
'Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, California 91125, United
States
BCenter for Advanced Scientific Computing and Modeling, Department of Chemistry, University of North Texas, Denton, Texas 76203,
United States
0 Supporting InformationABSTRACT: An exceptionally low coordinate nickel imido
complex, (IPr*)Ni=N(dmp) (2) (dmp = 2,6-dimesitylphenyl),
has been prepared by the elimination of N2 from a bulky aryl
azide in its reaction with (IPr*)Ni(r6-C7H8) (1). The solid-
state structure of 2 features two-coordinate nickel with a
linear C-Ni-N core and a short Ni-N distance, both
indicative of multiple-bond character. Computational stud-
ies using density functional theory showed a Ni=N bond
dominated by Ni(dr) -N(pr) interactions, resulting in two
nearly degenerate singly occupied molecular orbitals (SOMOs)
that are Ni-N jr* in character. Reaction of 2 with CO
resulted in nitrene-group transfer to form (dmp)NCO and
(IPr*)Ni(CO)3 (3). Net C-H insertion was observed in
the reaction of 2 with ethene, forming the vinylamine
(dmp)NH(CH=CH2) (5) via an azanickelacyclobutane
intermediate, (IPr*)Ni{N,C:K2-N(dmp)CH2CH2} (4).
T ransition-metal complexes containing multiple bonds with
main-group elements constitute an important class of orga-
nometallic reagents, exemplified by the active species in olefin
and alkane metathesis, aziridination, epoxidation, vicinal diami-
nation and oxyamination, and N2 reduction to ammonia.1-6
A prerequisite for transition-metal-element j-donor multiple
bonding is the availability of empty d orbitals on the metal center
that are of the correct symmetry and energetic disposition to
accept -electron density from the main-group element. Tradi-
tional synthetic strategies have employed high oxidation states
and/or coordinatively unsaturated complexes, both of which
lower the valence-electron count and increase the number of un-
occupied metal d orbitals. Extension of these principles suggests that
two-coordinate complexes might be well-suited for the formation of
multiple bonds, and because of their extreme coordinative unsatura-
tion, such complexes should show enhanced reactivity.
The accessibility of low-coordinate complexes supported by a
single neutral N-heterocyclic carbene (NHC) has accelerated
our research efforts directed toward two-coordinate late-metal
complexes.7 Utilizing the massive steric profile of IPr* [IPr* = 1,3-
bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene],"
we were able to prepare (IPr*)Ni(r6-C7H8) (1) via heteroge-
neous Mg reduction of (IPr*)(THF)NiC12 in the presence of
toluene (Scheme 1). Installation of an imide (RN2-) fragment is
often achieved through dinitrogen extrusion from organoazides
ACS Publications ( 2010 American Chemical Society(N3R).1'9'10 Protection of the Ni=N bond is crucial, and "cap-
ping" of the (IPr*)Ni subunit was accomplished with a large
terphenyl-substituted azide. Reaction of 1 with N3(dmp) (dmp =
2,6-dimesitylphenyl) resulted in vigorous N2 evolution and the
clean formation of (IPr*)Ni=N(dmp) (2), a 14-electron species,
as an olive-green solid (Scheme 1). 2 was characterized by X-ray
diffraction, superconducting quantum interference device
(SQUID) magnetometry, elemental analysis, and 1H NMR
spectroscopy. While 2 decomposes in CH2C12 solution, it is
stable in toluene or Et2O and does not change color or exhibit
signs of instability in THF.
X-ray studies of 2 showed a rigorously two-coordinate Ni
center whose very short Ni-N bond of 1.663(3) A is among the
shortest reported to date (Figure 1).11-13 Other than the two
coordination points, there are no short, stabilizing contacts
between Ni and the IPr* or (dmp)N2- ligands. The linear C-
Ni-N [174.24(13)] core is as expected for a coordination
number of 2, and the linear [171.6(3)] Ni-N-C unit reflects
Jr bonding between the imido ligand and nickel(II). A dihedral
angle of ~410, defined by the NHC core and the central C6H3
aromatic group of dmp, positions the dmp group in a twisted
conformation relative to IPr*.
While three-coordinate Ni(II) imido complexes have singlet
ground states,11,13 complex 2 exhibits a solid-state magnetic moment
of 2.77tB (SQUID, 60-300 K; Figure 2), indicative of the triplet
ground state of a high-spin d8 Ni center with a large zero-field
splitting (D = 24 cm-1). The high-spin configuration of 2 magnifies
the impact of the frontier orbitals' composition on bond order, as
electrons are promoted to antibonding orbitals, providing a formal
Ni=N double bond.
Orbital mixing accounts for the short Ni-N distance in 2, a
multiple bond in which r bonding dominates. An otherwise o*
3dz2 Ni orbital is greatly stabilized by symmetry-allowed mixing
with the 4s Ni orbital, increasing its nonbonding character.
Additionally, the short Ni-N distance allows for a strong Tr
interaction, which destabilizes the xr* molecular orbitals (Ni 3dzx
and 3dyz character) beyond the "nonbonding" 3dz2 orbital. The
resulting orbital ordering, in which the two nearly degenerate
singly occupied molecular orbitals (A = 0.03 eV) are 7r* in nature,
similar to Fe{N(tert-butyl)2}2,is was corroborated by density
functional theory (DFT) calculations at the B3LYP/6-311+G-
(d) level (Figure 2)16 and gives a formal Ni-N bond order of 2.
Received: November 10, 2010
Published: December 22, 2010dx.doi.org/10.1021/ja1101213 1J. Am. Chem. Soc. 2011, 133, 771-773
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Laskowski, Carl A.; Miller, Alexander J.M.; Hillhouse, Gregory L. & Cundari, Thomas R., 1964-. A Two-Coordinate Nickel Imido Complex That Effects C-H Amination, article, December 22, 2010; [Washington, D.C.]. (https://digital.library.unt.edu/ark:/67531/metadc107785/m1/1/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.