A terminal Ni(III)-imide with diverse reactivity pathways

Article abstract of DOI:10.1021/ja0533186

The synthesis and structure of the β-diketiminato Ni(I) lutidine adducts [Me_xNN]Ni(2,4-lutidine) (x = 2 (2); x = 3 (3)) are described which serve as synthons to the "naked" 13-electron [Me_xNN]Ni fragments in reactions with N₃Ad to give Ni-imido complexes. The singly bridged imide {[Me₂NN]Ni}₂(μ-NAd) (4) possesses short Ni-Ni (2.506(1) A) and Ni-N(imido) distances (1.732(4)-1.752(4) A). Steric modification of the β-diketiminate ligand to include an additional methyl group in the N-aryl 4-position affords the Ni(III) terminal imide [Me₃NN]Ni=NAd (8) isolated in 52% yield. The X-ray structure of terminal imide 8 reveals a contracted Ni-N(imido) bond distance (1.662(2) A) and an only somewhat bent imido linkage (Ni-N-C = 164.5(2)°) consistent with a significant degree of multiple bond character. Frozen glass EPR studies of 5 indicate a rhombic environment in which one of the signals exhibits strong hyperfine coupling (A = 22 G) to the imido ¹⁴N (I = 1) nucleus. The terminal imide 5 undergoes complete imido group transfer to CO and CNBu^t to give AdNCO and AdNCNBu^t, respectively, as well as with PMe3 to afford AdN=PMe₃. Exemplifying the radical character at the imido N atom, 5 adds to cobaltocene and abstracts a H atom from 1,4-cyclohexadiene to give the Ni(II)-amides [Me₃NN]Ni-NAd(η⁴-C₅H₅)CoCp (7) and [Me₃NN]Ni-NHAd(8). Copyright

Full text of DOI:10.1021/ja0533186

Published on Web 07/20/2005
A Terminal Ni(III)-Imide with Diverse Reactivity Pathways
Elzbieta Kogut, Heather L. Wiencko, Libei Zhang, Douglas E. Cordeau, and Timothy H. Warren*
Department of Chemistry, Georgetown UniVersity, Box 571227, Washington, D.C. 20057-1227
Received May 20, 2005; E-mail: thw@georgetown.edu
Scheme 1. Synthesis and Reactivity of [MexNN]Ni(2,4-lutidine)
Terminal imido species of the later transition metals have been
suggested as key intermediates in catalytic alkene aziridination.1,2
While iridium and ruthenium imides, such as Cp*IrtNtBu3 and
(η6-benzene)RudNAr4 (Ar ) 2,6-iPr2C6H3), remained for some time
as isolated examples of late metal terminal imides, the anticipated
lower metal-ligand bond strengths of first row analogues identify
them as attractive synthetic targets to explore the chemistry of the
imido functionality. Hillhouse and Mindiola prepared (tBu2PCH2-
CH2PBut2)NidNAr by deprotonation of the corresponding cationic
Ni(II)-anilide,5 which undergoes facile imide group transfer to CO
and benzyl isocyanide6 as well as with ethylene under prolonged
heating.7 Peters,8 Theopold,9 and Meyer10 have explored trigonal
Co(I) and Fe(I) platforms with organoazides to give trivalent imido
complexes. We have recently shown that the 18-electron Co(I)
â-diketiminate [Me2NN]Co(η6-toluene) reacts with organoazides
to give the doubly bridged imide {[Me2NN]Co}2(µ-NAr′)2 (Ar′ )
3,5-Me2C6H3) or a three-coordinate terminal species [Me2NN]Cot
NAd (1) (Ad ) 1-adamantyl).11 Herein, we describe the synthesis
of related three-coordinate d9 Ni(I) adducts, which react with AdN3
to form nickel-imido species, including a terminal Ni(III)-imide
with diverse modes of reactivity.
Treatment of the square planar Ni(II)-alkyl [Me2NN]Ni-
(CH2CH3)(2,4-lutidine)12 with 80 psi H2 in ether results in the rapid
formation of the monovalent [Me2NN]Ni(2,4-lutidine)13 (2) that may
be isolated as red crystals in 64% yield. More conveniently, addition
of Tl[Me2NN] to NiCl2(2,4-lutidine)2 in THF followed by reduction
with 0.5% Na/Hg after filtration of initially formed TlCl provides
2 in 81% isolated yield. At room temperature and in the presence
of excess lutidine, this complex exhibits a magnetic moment of
1.8 µB in benzene-d6 (Evans method) consistent with the presence
of one unpaired electron, and glass EPR spectra at 77 K in toluene
indicate a rhombic environment (g1 ) 2.435, g2 ) 2.133, g3 )
2.068). The X-ray structure of 2 (Scheme 1) shows it to be “bent”
trigonal planar as the N1-Ni-N3 angle of 153.58(9)° is consider-
ably more obtuse than the N2-Ni-N3 angle of 108.78(9)°, similar
in structure to the d10 [Me2NN]Cu(2,4-lutidine).14
coordinate [Me3NN]NidNAd (5) in 52% yield (Scheme 1). The
X-ray structure of 5 (Figure 2) is very similar to that of [Me2NN]-
CotNAd (1) with a slightly longer Ni-N(imido) distance of
1.662(2) Å (1.624(4) Å in 1)11 and a Ni-N-C angle of 164.5(2)°.
The glass EPR spectrum of 5 in toluene at 77 K indicates a rhombic
environment (g1 ) 2.162, g2 ) 2.038, g3 ) 1.937). Significantly,
the signal at g ) 2.038 is a 1:1:1 triplet with A ) 22 G, suggesting
substantial involvement of the imido nitrogen (14N; I ) 1) in the
singly occupied molecular orbital of this low-spin species. This
orbital picture is corroborated by DFT calculations, which indicate
significant spin density on the imido N atom (57%) as a result of
the two-center, three-electron interaction involving the Ni dxy and
imido N px orbitals (Figure 1).16
Initial reactivity studies of the terminal imide 5 demonstrate
complete imido group transfer to select electrophiles (Scheme 2).
Reaction of 5 with CO and CNBut results in the rapid formation
of AdNCO and AdNCNBut in 76 and 84% yields (GC). In the
reaction with CO, the [Me3NN]Ni fragment is trapped as the
diamagnetic {[Me3NN]Ni}2(µ-CO)2 (6) isolated in 85% yield. In
light of this apparent nucleophilic behavior for 5, we were surprised
to see that complete group transfer to PMe3 readily occurs to give
Me3PdNAd in 91% yield (31P NMR).
Reaction of 2 with 1 or 0.5 equiv of N3Ad results in the formation
of the paramagnetic, singly bridged dinickel imide {[Me2NN]Ni}2-
(µ-NAd) (4), which may be isolated in 34% yield (Scheme 1). The
X-ray structure of 4 (Figure 2) shows the [Me2NN]Ni fragments
to be roughly orthogonal (87.9° twist angle). This singly bridged
species exhibits considerably shortened Ni-N(imido) bond dis-
tances (1.732(4) and 1.752(4) Å) as well as a contracted Ni-Ni
distance (2.506(1) Å) relative to corresponding distances in the
doubly bridged {[Me2NN]Co}2(µ-NAr′)2.11 The low solution mag-
netic moment of 4 (µeff ) 0.95 µB in benzene-d6 at RT) is
corroborated by solid-state SQUID measurements which suggest
the possibility of weak antiferromagnetic coupling.
A separate manifold of one-electron reactivity results from the
radical character at the imido N atom. As cobaltocene is well-known
to add to organic radicals R• to give (η4-C5H5R)CoICp,17 the reaction
A slight peripheral steric modification to the â-diketiminate
ligand allows the isolation of a terminal d7 imide.15 Reaction of
N3Ad with [Me3NN]Ni(2,4-lutidine) (3) that possesses an additional
methyl group in the 4-position of each N-aryl ring affords the three-
Figure 1. Orbital interactions in â-diketiminato terminal imides.
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C O M M U N I C A T I O N S
Figure 2. X-ray structures of {[Me2NN]Ni}2(µ-NAd) (4), [Me3NN]NidNAd (5), [Me3NN]Ni-NAd(η4-C5H5)CoCp (7), and [Me3NN]Ni-NHAd (8).
Scheme 2. Reactivity of [Me3NN]NidNAd (5)
(PDF); crystallographic details for 2-5, 7, and 8 (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
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Both 2 and 3 serve as synthons to the 13-electron â-diketiminato
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(16) See Supporting Information for details.
Acknowledgment. T.H.W. thanks Georgetown University, the
ACS Petroleum Research Fund (Type-G), and the NSF CAREER
program for financial support. We also thank Prof. Jeffrey Petersen
for obtaining EPR spectra of 2, 3, and 5, and Prof. Karsten Meyer
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Supporting Information Available: Detailed experimental and
DFT calculation procedures with spectroscopic and analytical data
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