η2-organoazide complexes of nickel and their conversion to terminal imido complexes via dinitrogen extrusion

Article abstract of DOI:10.1021/ja805530z

1-Adamantyl- and mesitylazide react with [(dtbpe)Ni]₂(η²-μ-C₆H₆) to give the η² organic azide adducts (dtbpe)Ni(η²-N₃R) (R = Ad, 3a; Mes, 3b) that have been isolated in good yiel

Full text of DOI:10.1021/ja805530z

Published on Web 08/26/2008
η2-Organoazide Complexes of Nickel and Their Conversion to Terminal Imido
Complexes via Dinitrogen Extrusion
Rory Waterman† and Gregory L. Hillhouse*
Gordon Center for IntegratiVe Science, Department of Chemistry, The UniVersity of Chicago,
929 East 57th Street, Chicago, Illinois 60637
Received July 16, 2008; E-mail: g-hillhouse@uchicago.edu
Our reports of the syntheses and structures of the three-coordinate
nickel imido, carbene, and phosphinidene complexes (dtbpe)Nid
N(2,6-iPr2C6H3) (1; dtbpe ) 1,2-bis(di-tert-butylphosphino)ethane),1
(dtbpe)NidCPh2,2 and (dtbpe)NidP(2,6-Mes2C6H3)3 initiated ongo-
ing studies of rich group-transfer reaction chemistries of these NidX
multiply bonded species.2,4 Imido complex 1 is best prepared by
1-e- oxidation of the Ni(I) amido complex (dtbpe)Ni{NH(2,6-
iPr2C6H3)} followed by deprotonation of the resulting cationic Ni(II)
amide (dtbpe)Ni{NH(2,6-iPr2C6H3)}+.1 In search of a simplified
and general preparation of (dtbpe)NidNR complexes, we have
Figure 1. Structures of 3a (l) and 3b (r) with thermal ellipsoids at 35%
investigated the reactions of organoazides with Po¨rschke’s labile
Ni(0) benzene complex {(dtbpe)Ni}2(η2-µ-C6H6) (2).5 Organoazides
have long been recognized as useful synthons for early transition
metal imido moieties.6 For late metals, Warren has reported the
syntheses of Ni(III) imido complexes from the reaction of RN3 with
[Me2C3H(NMes)2]Ni(2,4-lutidine),7 and Peters and Theopold have
applied this strategy to Co systems.8,9 This approach seemed
particularly attractive in our system in light of the observation that
the η2-diphenyldiazomethane adduct (dtbpe)Ni(η2-N2CPh2), pre-
pared from N2CPh2 and a suitable Ni(0) precursor, cleanly converts
to (dtbpe)NidCPh2 upon thermolysis in the presence of catalytic
Sm(OTf)3.2
probability and H-atoms omitted. Select bond lengths (Å) and angles (deg)
for 3a: Ni-N(1) ) 1.910(2), Ni-N(2) ) 1.810(2), N(1)-N(2) ) 1.244(2),
N(2)-N(3) ) 1.298(3), N(3)-C(31) ) 1.484(3). P(1)-Ni-P(2) ) 93.75(2),
P(1)-Ni-N(2) ) 114.60(6), P(2)-Ni-N(1) ) 112.72(6), N(1)-Ni-N(2)
) 38.97(8), N(1)-N(2)-N(3) ) 142.7(2), N(2)-N(3)-C(31) ) 113.6(2).
For 3b: Ni-N(1) ) 1.906(2), Ni-N(2) ) 1.811(2), N(1)-N(2) ) 1.239(3),
N(2)-N(3) ) 1.308(3), N(3)-C(31) ) 1.438(3). P(1)-Ni-P(2) ) 92.90(3),
P(2)-Ni-N(2) ) 115.61(7), P(1)-Ni-N(1) ) 112.84(7), N(1)-Ni-N(2)
) 38.85(9), N(1)-N(2)-N(3) ) 138.7(2), N(2)-N(3)-C(31) ) 112.8(2).
Scheme 1
Reaction of hexane solutions of 2 with 1-azidoadamantane
affords the unusual 1-adamantylazide adduct (dtbpe)Ni(η2-N3Ad)
(3a, Ad ) 1-adamantyl) as analytically pure, golden crystals in
76% isolated yield (Scheme 1). Mesitylazide reacts in an analogous
fashion with 2 in a dilute toluene solution to give (dtbpe)Ni(η2-
N3Mes) (3b) as yellow crystals in 64% isolated yield, but this
complex is somewhat thermally unstable, slowly decomposing at
ambient temperature even in the solid state. 1H and 31P NMR
spectra of 3a and 3b demonstrate the Cs-symmetry of the complexes
(inequivalent 31P nuclei with pairwise-inequivalent tert-butyl
groups). Addition of MesN3 to C6D6 solutions of 3a results in
formation of 3b and AdN3, suggesting the RN3 ligand is labile in
that solvent (Scheme 1).
The η2-coordination of the N3R ligands in 3a and 3b was
confirmed by X-ray crystallography. The solid-state structures are
shown in Figure 1 and feature planar geometry at nickel and
delocalized π-bonding in the N3 framework (for 3a, N(1)-N(2) )
1.244(2), N(2)-N(3) ) 1.298(3) Å). This is a unique coordination
mode for organoazide ligands and is similar to the η2-coordination
observed for diazoalkane and diazonium complexes of the (dtbpe)Ni
fragment.2,10 Cummins has reported a structurally related η2-P2NR
complex of niobium.11 In the few instances where a metal complex
bears an organic azide, the ligand is bound as a diazenylimido
Chart 1
through the aromatic ring is known for [(η6-ArN3)Mn(CO)3+][PF6
]
-
2-
(RN3 as A, B, C; Chart 1)12-15 or η1 as neutral RN3 through
(Ar ) Ph, p-Tol).19
either the R or γ N-atoms (D, E).16-18 η6-Aryl azide coordination
Thermolysis of benzene solutions of either pure 3a or 3b yields
myriad products. However, heating benzene solutions of pure 3a
with a substoichiometric amount of 2 (∼5 mol %) gives analytically
† Current address: Department of Chemistry, University of Vermont, Burlington,
VT 05405.
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10.1021/ja805530z CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
olefins (aziridination) with RN3 using 2 as a catalyst precursor. Such
group-transfer reactions are by nature restricted to being stoichio-
metric ones when a multistep synthetic protocol involving Ni(I) is
required to access the key L2NidNR complexes.4a,22 Two features
of this system, however, appear to obviate its development into a
functional catalytic process. First, the success of the conversion of
2 to 4 relies on having a sufficiently labile ligand, like η2-C6H6, in
the Ni(0) precursor complex. Common olefins (e.g., ethylene,
1-hexene, 1,5-cyclooctadiene) bind too strongly to the (dtbpe)Ni-
fragment and are not displaced by RN3 to give η2-N3R complexes.
Second, the reaction of RN3 with 4 to give RNdNR is faster at
reasonable N3R concentrations than is the aziridination of olefins
by 1.4a,20 It is noteworthy that these limiting factors can be
overcome in the related cyclopropanation of ethylene by N2CPh2,
where modest catalytic turnover is observed using (dtbpe)Nid
CPh2.4a
Figure 2. Perspective views of 4a (l) and 4b (r) with thermal ellipsoids
drawn at 35% probability and H-atoms omitted. Select bond lengths (Å)
and angles (deg) for 4a: Ni-N ) 1.673(2), N-C(31) ) 1.417(3), Ni-P(1)
) 2.1371(8), Ni-P(2) ) 2.1405(8), Ni-N-C(31) ) 163.0(2), P(1)-Ni-P(2)
) 91.69(3). For 4b: Ni-N(1) ) 1.703(4), N-C(31) ) 1.347(7), Ni-P(2)
) 2.179(1), Ni-N-C(31) ) 180, P(1)-Ni-P(2) ) 91.12(5).
pure dark red crystals of (dtbpe)NidNAd (4a) in 81% yield
(Scheme 1). The beneficial role of added 2 appears to be to keep
the equilibrium concentration of free AdN3 low (favoring 3a in
the equilibrium shown in Scheme 1). We have shown that AdN3
undergoes a side reaction with 4a to effect its decomposition
(forming azoadamantane, AdNdNAd).20 The terminal mesityl
imido complex (dtbpe)NidNMes (4b) can be similarly prepared,
but the best yields are obtained by treatment of concentrated
petroleum ether solutions of 2 with MesN3 to afford 4b in 84%
yield as analytically pure turquoise crystals. 1H and 31P NMR
spectra of complexes 4a and 4b are consistent with pseudo C2V-
symmetry and present features typical of a terminal imido complex
of nickel(II).1 Both imido complexes 4a and 4b were crystallo-
graphically characterized (Figure 2). These complexes and previ-
ously reported imido 1 highlight interesting structural features
associated with the nitrogen substituent. The metrical parameters
of 4b and 1 are nearly identical except that the NisNsC angle is
rigorously linear for 4b while it is slightly bent at 162.8(2)° in 1.
The alkylimido 4a has a NisNsC angle (163.0(2)°) similar to that
for 1 with a slightly shorter NisN bond (∆ ∼0.03 Å).1
Acknowledgment. This work was supported by the National
Science Foundation through Grant CHE-0615274 (to G.L.H.) and
a predoctoral GAANN Fellowship from the Department of Educa-
tion (to R.W.).
Supporting Information Available: Experimental procedures with
characterization data and kinetic data (PDF) as well as crystallographic
information for 3a, 3b, 4a, and 4b (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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