Isocyanate and carbodiimide synthesis by nitrene-group-transfer from a nickel(II) imido complex

Article abstract of DOI:10.1039/b204846a

The imido complex (dtbpe)Ni{N(2,6-(CHMe₂)₂C₆H₃)} reacts with CO and CNCH₂Ph with addition at the Ni-N bond to give (dtbpe)Ni{C,N:η²-C(O)N(2,6-(CHMe₂)₂C ₆H₃)} and (dtbpe)Ni{C,N:η²-C(NCH₂Ph)N(2,6-(CHMe₂) ₂C₆H₃)}; both) complexes react further with CO to liberate the isocyanate and carbodiimide ligands with formation of (dtbpe)-(Ni(CO)₂.

Full text of DOI:10.1039/b204846a

Isocyanate and carbodiimide synthesis by nitrene-group-transfer from a
nickel(II) imido complex†
Daniel J. Mindiola and Gregory L. Hillhouse*
Searle Chemistry Laboratory, Department of Chemistry, The University of Chicago, Chicago, Illinois 60637,
USA. E-mail: g-hillhouse@uchicago.edu
Received (in Purdue, IN, USA) 19th May 2002, Accepted 9th July 2002
First published as an Advance Article on the web 23rd July 2002
The imido complex (dtbpe)Ni{N(2,6-(CHMe2)2C6H3)} reacts
with CO and CNCH2Ph with addition at the Ni–N bond to
give (dtbpe)Ni{C,N:h2-C(O)N(2,6-(CHMe2)2C6H3)} and
(dtbpe)Ni{C,N:h2-C(NCH2Ph)N(2,6-(CHMe2)2C6H3)}; both
complexes react further with CO to liberate the isocyanate
and carbodiimide ligands with formation of (dtbpe)-
Ni(CO)2.
examples for Group 10 complexes,3 including [(h-C5Me5)-
Ni(m2-NHTol)]2 which reacts with CO to give (h-C5Me5)-
Ni{C(O)NHTol}(CO).7
Treatment of a THF solution of 3a with NaN(SiMe3)2 effects
deprotonation of 3a at nitrogen to give the isocyanate complex
2
(dtbpe)Ni{C,N-h -C(O)N(2,6-(CHMe2)2C6H3)} (4) as yellow
crystals in 71% isolated yield Scheme 1).† The IR spectrum of
4 shows no N–H stretch and a strong nCO at 1719 cm21. The
higher carbonyl stretching vibration in neutral 4 relative to
cationic 3a (Dn = 142 cm21) suggests that the isocyanate
The chemistry of transition-metal complexes containing imido
ligands (RN22) is extensive, with these hard, nitrogen-donor
ligands exhibiting diverse reactivities ranging from inert
“spectator” ligands to very ractive moieties capable of activat-
ing hydrocarbon C–H bonds.1,2 Terminal imido ligands are far
more prevalent in complexes of high-valent, early- and mid-
transition metals than in complexes of the later-transition
elements. The contrast between hard, nitrogen-donor imides
and soft, electron-rich late-transition metals has been argued to
destabilize the strong p-donor interactions which characterize
the bonding of imides in higher-valent compounds.3 The d6
2
ligand is coordinated to Ni in an h :C,N fashion in 4. An X-ray
diffraction study on crystals of 4, although disordered, also
confirms this connectivity.8† Elemental analysis and multi-
nuclear NMR spectroscopy (1H, 13C, 31P) support this formula-
tion. When the deprotonation reaction is monitored by NMR
5
iridium imido complexes (h -C5Me5)Ir·NR (R = aryl, alkyl,
6
silyl) and the d6 ruthenium complex (h -p-cymene)-
Ru·N{2,4,6-(CMe3)3C6H2} stood for some time as the only
examples of terminal imido complexes possessing a d-electron
count greater than four.4,5 We recently reported the synthesis of
a d8 nickel imido complex, (dtbpe)Ni{N(2,6-(CHMe2)2C6H3)}
(1; dtbpe = 1,2-bis(di-tert-butylphosphino)ethane), by deproto-
nation of the amido ligand of [(dtbpe)Ni{NH(2,6-
(CHMe2)2C6H3}][PF6] (2a).6 Herein we describe the reactions
of carbon monoxide and benzyl isocyanide with the imido
ligand of 1 that result in the complete formal nitrene-transfer to
CO and C·NCH2Ph with elimination of the corresponding
isocyanate and carbodiimide products.
Reaction of THF solutions of the cationic amido complex 2a
with an excess of carbon monoxide results in insertion of one
equivalent of CO into the Ni–N bond of 2a to give the
Scheme 1
2
carboxyanilide complex salt [(dtbpe)Ni{C,O:h -OCNH(2,6-
(CHMe2)2C6H3)}][PF6] (3a) as pale yellow microcrystals in
79% isolated yield (Scheme 1).† 3a was characterized by
elemental analysis and IR (nNH = 3351, nCO = 1577 cm21) and
1
multinuclear NMR (1H, 13C, 19F, 31P) spectroscopy. The H
NMR resonance for the NH proton appears at d 8.14 (bs s), and
inequivalent 2CH2P(t-Bu)2 arms of the dtbpe ligand are
1
indicated in both the H and 31P NMR spectra. The 13C{1H}
NMR spectrum of 3-13C (prepared by reaction of 2a with 13CO)
shows a well-resolved doublet-of-doublets for the carbonyl
moiety coupled to two inequivalent phosphorus nuclei (d 183.2,
2JCP = 10.6, 65.3 Hz), and the IR spectrum of 3-13C shows the
13CO
typical isotopic shift for the carbonyl stretch (n
= 1548
cm21, Dn = 29 cm21). Crystallographic analysis of an
2
analogously prepared tetraarylborate salt, [(dtbpe)Ni{C,O:h -
OCNH(2,6-(CHMe2)2C6H3)}][B(3,5-(CF3)2C6H3)4]
(3b),‡
Fig. 1 A perspective view of the complex cation of 3b. H-atoms, except that
attached to N, have been omitted for clarity. Selected metrical parameters:
Ni–O 1.910(3), Ni–C(70) 1.823(4), Ni–P(1) 2.2125(11), Ni–P(2)
2.1530(11), O–C(70) 1.270(4), N–C(70) 1.314(4) Å; P(1)–Ni–P(2)
92.08(4), P(1)–Ni–O 110.97(8), P(1)–Ni–C(70) 150.48(12), P(2)–Ni–C(70)
117.04(12), P(2)–Ni–O 156.65(8), O–Ni–C(70) 39.69(13), Ni–C(70)–O
73.8(2), Ni–O–C(70) 66.5(2), Ni–C(70)–N 160.3(3), O–C(70)–N 125.7(3),
C(70)–N–C(71) 123.4(3)°.
confirmed that the carboxyanilide ligand is O,C-bound to Ni
(see Fig. 1). Carbon monoxide insertion into late-transition
metal–amide bonds has been observed, and there are several
† Electronic supplementary information (ESI) available: complete experi-
mental, spectroscopic, analytical, and crystallographic details. See http://
www.rsc.org/suppdata/cc/b2/b204846a/
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CHEM. COMMUN., 2002, 1840–1841
This journal is © The Royal Society of Chemistry 2002

spectroscopy (1H, 13C, 31P), an intermediate whose spectra are
50% yield with substantial decomposition of 7 to give free
benzyl isocyanide also occurring.
2
consistent with the C,O:h -isocyanate isomer is observed and it
cleanly converts to 4 over a period of several hours.
It is notable that for low-valent early-metals (e.g.,
WCl2(PMePh2)4), thermodynamics favour interactions of iso-
cyanates and carbodiimides in the reverse sense of the reactions
reported herein, that is they oxidatively add to give stable
imido–carbonyl and imido–isocyanide complexes such as
W(NR)Cl2(CO)(PMePh2)2 and W(NR)Cl2(CNR)(PMePh2)2.10
Although late-metal terminal imido complexes are quite rare,
Treatment of a Et2O solution of the neutral Ni(II) imido
complex 1 with carbon monoxide (1 atm, 22 °C) results in a
color change from emerald-green to pale yellow over a 2 h
period with formation of the free arylisocyanate ONCNN(2,6-
(CHMe2)2C6H3) (5) along with the Ni(0) dicarbonyl complex
(dtbpe)Ni(CO)2 (6),9 depicted in Scheme 1.† The carbonylation
1
5
of 1 gives 5 and 6 quantitatively, as determined by H NMR
(h -C5Me5)Ir·NR has been shown to react with CO and C·NR
integration against an internal standard (6 was isolated from the
reaction mixture as tan crystals in 60% yield). Identification of
5 as o-diisopropylphenyl isocyanate was supported by GC/MS
analysis of the reaction mixture (m/z = 203, M+) as well as by
spectroscopic comparison with an authentic sample. Reaction of
1 with 1 equivalent of CO gives a mixture containing unreacted
1 along with 4, 5 and 6 (as determined by IR and multinuclear
NMR spectroscopy), strongly implicating 4 as an intermediate
in the carbonylation reaction of 1 that gives 5 and 6; the
subsequent reaction of 4 with CO is fast with respect to the
initial formation of 4 (Scheme 1). Consistent with this
mechanism, addition of CO to a solution of pure 4 gives 5 and
6 rapidly and quantitatively.
via addition accross the Ir–N bond to give 18-electron species
5
5
(h -C5Me5)Ir(CO)(RNCO) and (h -C5Me5)Ir(CNR)(RNCNR).
These compounds, however, are apparently stable with respect
to complete nitrene-group transfer with reductive elimination to
give the metal-free heterocumulenes,4 although free carbodii-
mide formation has been observed in the reaction of an
isocyanide with a tantalum imido complex.11
This research was supported by a grant from the National
Science Foundation to G. L. H. and by fellowships from the
Ford Foundation and the National Institutes of Health to D. J.
M.
Notes and references
‡ Crystal data for 3b·2Et2O, C69H80BF24NNiO2.5P2: triclinic, P1, a =
The coordination chemistry of isocyanides often parallels
that of the isolobal CO molecule, and in a reaction related to that
observed with carbon monoxide, treatment of cold (235 °C)
diethyl ether solutions of 1 with benzyl isocyanide results in
addition of C·NCH2Ph across the Ni–N double bond to give the
¯
12.495(3), b = 16.167(4), c = 18.487(4) Å, a = 98.050(2), b = 91.659(4),
g = 91.399(4), Z = 2, m(Mo-Ka) = 0.410 mm21. Of 17173 total reflections
(yellow block, 1.27 @ q @ 24.00°), 11460 were independent and observed
(Rint = 4.09%) with I > 2s(I). A semi-empirical absorption correction was
performed using psi-scans. Direct methods were used to locate Ni, P, O, N,
F, B, and C atoms from the E-map. No anomalous bond lengths or thermal
parameters were noted except for one disordered diethyl ether molecule of
solvation which resided at an inversion centre (O2S). One peripherical CF3
group of the counter-anion suffered from disorder but converged normally
during refinement. All non-hydrogen atoms were refined anisotropically,
and hydrogen atoms were refined isotropically and fit to idealised positions.
GoF on F2 = 1.069; R(F) = 5.98% and R(wF) = 15.03%. Crystal data for
2
unsymmetrical carbodiimide adduct (dtbpe)Ni{C,N:h -
C(NCH2Ph)N(2,6-(CHMe2)2C6H3)} (7) as pale-pink crystals in
50% yield (Scheme 1).† NMR data (1H, 13C, 31P) are typical for
square-planar Ni(II) and a strong n
is observed in the IR
CN
spectrum at 1684 cm21. The solid-state structure of 7 was
crystallographically determined, and a view of the molecule is
shown in Fig. 2.‡ We see no evidence in solution for isomers,
i.e., binding of the carbodiimide ligand through the benzylic-
CN bond. Unlike the reaction of 1 with CO, excess benzyl
isocyanide does not displace the carbodiimide ligand. However,
exposure of 7 to an excess of CO results in elimination of the
carbodiimide PhCH2NNCNN{2,6-(CHMe2)2C6H3} (8; GC/MS,
m/z = 292, M+) and formation of 6 (Scheme 1). It is noteworthy
that the reaction of 7 with CO is not as clean as that of 4 with CO
(to give 5). 1H NMR and IR analyses indicates 8 is formed in ~
7, C38H64F6N2NiP2: monoclinic, P21/n,
a = 10.8143(7) Å, b =
17.9376(12) Å, c = 19.3299(14) Å, b = 100.1160(10)°, Z = 4, m(Mo Ka)
= 0.640 mm21. Of 22171 total reflections (pink prism, 2.02 = q = 28.27°),
8619 were independent and observed (Rint = 4.81%)with I > 2s(I). A
semi-empirical absorption correction was performed using psi-scans. Direct
methods were used to locate the Ni, P, N, and C atoms from the E-map. All
non-hydrogen atoms were refined anisotropically, and hydrogen atoms
were refined isotropically and fit to idealised positions. No anomalous bond
lengths or thermal parameters were noted. GoF on F2 = 1.114; R(F) =
5.02% and R(wF) = 12.02%. CCDC reference numbers 186268–186270.
See http://www.rsc.org/suppdata/cc/b2/b204846a/ for crystallographic data
in CIF or other electronic format.
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Fig. 2 A perspective view of the molecular structure of 7. H-atoms have
been omitted for clarity. Selected metrical parameters: Ni–N(1) 1.879(2),
Ni–C(10) 1.857(2), Ni–P(1) 2.2360(6), Ni–P(2) 2.1517(6), N(1)–C(10)
1.318(3), N(1)–C(71) 1.411(3), N(2)–C(10) 1.275(3), N(2)–C(101)
1.474(3) Å; P(1)–Ni–P(2) 92.93(2), P(1)–Ni–N(1) 121.07(6), P(1)–Ni–
C(10) 162.35(8), P(2)–Ni–C(10) 104.63(8), P(2)–Ni–N(1) 144.97(6), N(1)–
Ni–C(10) 41.32(9), Ni–C(10)–N(1) 70.22(13), Ni–N(1)–C(10) 68.46(13),
Ni–C(10)–N(2) 144.3(2), N(1)–C(10)–N(2) 145.5(2), C(10)–N(2)–C(101)
119.1(2), C(10)–N(1)–C(71) 134.5(2)°.
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2
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