N-N bond cleavage in a nitrous oxide-NHC adduct promoted by a PNP pincer cobalt(I) complex

Article abstract of DOI:10.1016/j.poly.2015.09.001

The reactivity of a PNP cobalt(I) complex towards the nitrous oxide N-heterocyclic carbene adduct (NHC-N₂O) was explored. Several products were isolated including the cobalt nitrosyl complex (HPNP)Co(NO) (1) and the NHC-derived imine NHC=NH (2)

Full text of DOI:10.1016/j.poly.2015.09.001

Accepted Manuscript
N-N Bond Cleavage in a Nitrous Oxide-NHC Adduct Promoted by a PNP Pincer
Cobalt(I) Complex
Clément Camp, Lara C.E. Naested, Kay Severin, John Arnold
PII:
S0277-5387(15)00493-3
DOI:
http://dx.doi.org/10.1016/j.poly.2015.09.001
POLY 11515
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
14 August 2015
1 September 2015
Please cite this article as: C. Camp, L.C.E. Naested, K. Severin, J. Arnold, N-N Bond Cleavage in a Nitrous Oxide-
NHC Adduct Promoted by a PNP Pincer Cobalt(I) Complex, Polyhedron (2015), doi: http://dx.doi.org/10.1016/
j.poly.2015.09.001
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N-N Bond Cleavage in a Nitrous Oxide-NHC Adduct
Promoted by a PNP Pincer Cobalt(I) Complex.
Clément Camp,a Lara C. E. Naested,a Kay Severinb and John Arnold*,a
[a] Department of Chemistry, University of California, Berkeley, California 94720, United States.
[b] Ecole Polytechnique Fédérale de Lausanne (EPFL), Institut des Sciences et Ingénierie
Chimiques, CH-1015 Lausanne, Switzerland.
KEYWORDS. Cobalt; pincer ligand; nitrous oxide; N-heterocyclic carbene; NHC; nitrosyl
ABSTRACT
The reactivity of a PNP cobalt(I) complex towards the nitrous oxide N-heterocyclic carbene
adduct (NHC-N2O) was explored. Several products were isolated including the cobalt nitrosyl
complex (HPNP)Co(NO) (1) and the NHC-derived imine NHC=NH (2). This study provides a rare
example of metal-mediated rupture of the N-N bond in a nitrous oxide derivative, a route that
could be used to generate late-metal nitrosyls.
INTRODUCTION
From a thermodynamic point of view, nitrous oxide (N2O) is a strong oxidant.[1] In the presence
of reducing agents, N2O typically liberates molecular nitrogen and an oxygen atom.[2–18] N2O
has thus been proposed as an environment friendly oxygen-atom transfer reagent.[8,11–13] Yet
transforming N2O into a useful oxidant is a scientifically challenging problem since its utilization
is hampered by its kinetic inertness.[2] Additionally, N2O is one of the most important currently
emitted ozone-depleting substances[19,20] and is an important greenhouse gas, with a global
warming potential 300 times greater than CO2 on a per-molecule basis.[21] The discovery of

new technologies able to use N2O as an oxidant or a commodity feedstock reagent or
decompose N2O into environmentally harmless molecules at low temperature is therefore an
attractive objective for catalysis research.[2] For the above reasons, the design of well-defined
organometallic species able to activate N2O is a desirable objective.
Metal-mediated N-N bond cleavage of N2O could provide new opportunities to the field, but is a
rarely observed phenomenon, N-O bond cleavage being most of the time thermodynamically
favored. To the best of our knowledge, direct N-N bond reductive cleavage of N2O derivatives
promoted by well-defined organometallic species is limited to the examples drawn in Figure 1.
Seminal work by Cummins and co-workers demonstrated the ability of tris-amido Mo(III)
complexes to selectively promote N-N bond rupture of N2O, yielding molybdenum nitride and
nitrosyl species rather than molybdenum oxo and N2.[22] Detailed thermodynamic and
mechanistic investigations showed that the N-N vs N-O bond cleavage selectivity was kinetically-
driven.[23] Sita and co-workers reported facile nitrous oxide N-N bond rupture at a low-
coordinate Mo(II) carbonyl center to produce a nitrosyl isocyanate complex (Figure 1).[24] Yet
this process is not selective, and competitive N-O bond cleavage of N2O is observed under the
same experimental conditions. Recently, one of our groups observed that N-heterocyclic
carbenes (NHCs) were able to form stable adducts with nitrous oxide (NHC-N2O) under ambient
conditions.[25,26] Unlike free N2O, which is a poor ligand, these adducts exhibit a rich and
diverse coordination chemistry[27,28] therefore opening new avenues for metal-mediated
transformation of N2O. One particular feature is the substantial weakening of the N-N bond in
the NHC-bound N2O group, promoting facile insertion of Ni(0) into this bond to yield a dinuclear
iminato nitrosyl complex.[27]
2

Figure 1. Reported examples of N-N bond scission in N2O and NHC-activated N2O promoted by
organometallic derivatives.
We therefore became interested in further exploring the reactivity of NHC-N2O with late
transition metal centers in more crowded steric environments to avoid dimerization. To this
end, we identified low-valent cobalt complexes supported by tridentate pincer ligands as
particular attractive prospects. The steric crowding imposed by the bulky substituents at the
donor atoms combined with the chelate-enforced nature of the tridentate pincer skeleton
allows isolation of highly reactive cobalt species that have demonstrated attractive potential for
small molecules activation.[29–37] In particular, the Arnold group recently reported that
reduced cobalt complexes supported by a PNP pincer ligand can promote the unusual activation
of H2.[38] We herein report the reaction of the cobalt(I) complex (HPNP)CoCl[33] (HPNP = (iPr2P-
CH2-CH2)2NH) with the N-heterocyclic carbene-activated nitrous oxide adduct NHC-N2O (NHC =
1,3-Dimesitylimidazolin-2-ylidene) leading to a diverse array of products including the cobalt
nitrosyl complex (HPNP)Co(NO) (1) and the NHC-derived imine NHC=NH (2) resulting from the
N-N bond rupture of the nitrous oxide moiety in NHC-N2O.
3

EXPERIMENTAL SECTION
General Considerations: The syntheses and manipulations described below were performed
either using standard Schlenk line techniques or in an MBraun inert atmosphere drybox under
an atmosphere of purified nitrogen (<1 ppm O2/H2O). Glassware and cannulae were stored in an
oven at ∼160 °C for at least 12 h prior to use. Hexanes, DCM and toluene were purified by
passage through a column of activated alumina, stored over 3 or 4 Å molecular sieves, and
degassed prior to use. C6D6 was dried over sodium/benzophenone and then vacuum-transferred
to a storage flask and freeze-pump-thaw degassed before being stored over activated molecular
sieves in the drybox. NHC-N2O[25,26] and (HPNP)CoCl[33] were prepared using literature
procedures. All other reagents were acquired from commercial sources and used as received.
NMR spectra were recorded on Bruker AVQ-400, DRX-500 and AV-600 spectrometers. Chemical
shifts were measured relative to residual solvent peaks, which were assigned relative to an
external TMS standard set at 0.00 ppm. 31P chemical shifts were referenced to an external
1
standard (Ph3PO for 31P set at 23 ppm). H and 13C NMR assignments were routinely confirmed
1
1
by H−1H COSY and H−13C HSQC experiments. Infrared (IR) spectra were recorded with a
Thermo Scientific Nicolet iS10 series FTIR spectrophotometer as a Nujol mull between KBr
plates. The uncorrected melting points were determined using sealed capillaries prepared under
nitrogen on an Optmelt SRS. Elemental analyses were performed at the School of Human
Sciences, Science Center, London Metropolitan University. The X-ray structural determinations
were performed at CHEXRAY, University of California, Berkeley on a Bruker SMART APEX II
QUAZAR diffractometer.
Reaction of (HPNP)CoCl with NHC-N2O:
A yellow suspension of NHC-N2O (87 mg, 0.25 mmol, 1 eq.) in 13 mL of toluene was added to a
bright blue toluene (6 mL) solution of (HPNP)CoCl[33] (300 mg, 0.75 mmol, 3 eq.). The resulting
mixture gradually turned dark red and was stirred for 16 h at room temperature. The volatiles
were removed under vacuum. The dark red solid residue was extracted with 20 mL hexanes and
filtered to give a dark brown hexane filtrate (A) and red-pink solids. The remaining red-pink
solids were further extracted with 26 mL hexanes and filtered, affording a red hexanes filtrate
4

(B) and purple-pink solids. The latter solid was extracted with DCM; concentration and cooling
to -40 °C afforded pink needles of (HPNP)CoCl2 (3) (60 mg, 55 %).
Compounds 1, 2 and 4 were separated by fractional crystallization from hexanes solutions A and
B. The dark-brown hexane filtrate A was concentrated to 10 mL and stored at -40°C in a freezer
for 24 h. This produced a crystalline material containing a mixture of crystals of (1) and (2) and a
brown filtrate which was further concentrated to 6 mL and stored at -40 °C for another 24 h.
This yielded large dark brown block-shaped crystals of (PNP)CoCl (4) that were recovered and
dried under vacuum (43 mg, 43 %). The red hexane filtrate B was concentrated to 15 mL and
stored at -40°C in a freezer for 24h. This afforded red block-shaped crystals of complex
(HPNP)Co(NO) (1) that were recovered and dried under vacuum (22 mg, 22 %). The remaining
pale brown hexane filtrate was concentrated and stored at -40 °C for another 24 h to yield
compound NHC=NH (2) as light brown needle-shaped crystals (9 mg, 11 %).
Single crystals of 1 and 2 suitable for X-ray diffraction were obtained upon cooling to -40°C
concentrated solutions of the corresponding compounds. The identities of (HPNP)CoCl2[39] (3)
1
and (PNP)CoCl[33] (4) were confirmed by H and 31P NMR spectroscopy as well as X-ray
diffraction checking of the crystallographic unit-cell.
(HPNP)Co(NO) (1): 1H NMR (400 MHz, C6D6, 293 K) δ = 2.9 (br, 1H, NH), 2.6 (m, 2H, PCH(CH3)2),
2.1 (m, 2H, CH2), 1.5 (m, 12H, PCH(CH3)2), 1.4 (m, 2H, PCH(CH3)2), 1.3 (m, 2H, NCH2), 1.1 (m, 12H,
PCH(CH3)2), 0.9 (m, 2H, NCH2), 0.7 (m, 2H, CH2). 31P{1H} NMR (243 MHz, C6D6, 293 K) δ = 80.6 (br,
P(iPr)2). 13C{1H} NMR (125.8 MHz, C6D6, 293 K) δ = 49.8 (NCH2), 28.5 (PCH(CH3)2), 26.8
(PCH(CH3)2), 25.4 (PCH2), 20.8 (PCH(CH3)2), 20.7 (PCH(CH3)2), 19.8 (PCH(CH3)2), 18.9 (PCH(CH3)2).
FT-IR (cm-1) 3265 (s, N-H), 2953 (s, C-H), 2923 (s, C-H), 2853 (s, C-H), 1595 (s, N=O), 1467 (s),
1378 (s), 1260 (s), 1239 (m), 1176 (m), 1093 (s), 1074 (s), 1046 (s), 1021 (s), 886 (s), 813 (s), 768
(m), 677 (s), 612 (s). M.p. 180 °C. Anal. Calcd for C16H37N2OP2Co: C, 48.73; H, 9.46; N, 7.10.
Found: C, 48.55; H, 9.37; N, 6.99.
1
1,3-Dimesitylimidazolin-2-imine NHC=NH (2): H NMR (400 MHz, C6D6, 293 K) δ = 6.8 (s, 4H,
CHAr), 5.8 (s, 2 H, CH=CH), 2.2 (s, 12H, o-CH3), 2.1 (s, 6H, p-CH3). 13C{1H} NMR (125.8 MHz, C6D6,
5

293 K) δ = 150.8 (C=NH), 138.1 (CAr), 136.8 (CAr), 133.7 (CAr), 129.3 (CHAr), 111.9 (CH=CH), 21.0 (o-
CH3), 18.1 (p-CH3). FT-IR (cm-1) 3375 (m, br, N-H), 3126 (m, =C-H), 2955 (s, C-H), 2922 (s, C-H),
2853 (s, C-H), 1697 (s, C=N), 1609 (w), 1491 (s), 1463 (s), 1413 (s), 1377 (s), 1260 (s), 1237 (m),
1096 (s), 1016 (s), 909 (m), 855 (w), 798 (m), 724 (w), 676 (m), 568 (m). M.p. 176 °C.
X-ray Crystallography. X-ray structural determinations were performed on a Bruker SMART
QUAZAR diffractometer which is a 3-circle diffractometer that couple a CCD detector with a
sealed-tube source of monochromated Mo Kα radiation (λ = 0.71073 Å). A crystal of appropriate
size was coated in Paratone-N oil and mounted on a Kapton® loop. The loop was transferred to
the diffractometer, centered in the beam, and cooled by a nitrogen flow low-temperature
apparatus that had been previously calibrated by a thermocouple placed at the same position as
the crystal. Preliminary orientation matrices and cell constants were determined by collection of
60 10 s frames, followed by spot integration and least-squares refinement. The reported cell
dimensions were calculated from all reflections with I > 10 σ. The data were corrected for
Lorentz and polarization effects; no correction for crystal decay was applied. An empirical
absorption correction based on comparison of redundant and equivalent reflections was
applied using SADABS.[40] All software used for diffraction data processing and crystal-structure
solution and refinement are contained in the APEX2 program suite (Bruker AXS, Madison, WI).
Thermal parameters for all non-hydrogen atoms were refined anisotropically. For all structures,
2
2 2
2 2
R1 = Σ(|Fo| - |Fc|)/Σ(|Fo|); wR2 = [Σ{w(Fo - Fc ) }/Σ{w(Fo ) }]1/2. Thermal ellipsoid plots were
created using Mercury supplied with Cambridge Structural Database (CCDC: Cambridge, U.K.,
2004-2009).
6

Table 1: X-ray crystallographic parameters for complexes compounds 1 and 2.
(HPNP)Co(NO) (1)
C16H37N2OP2Co
394.34
NHC=NH (2)
empirical formula
formula weight
temperature (K)
crystal system
space group
C21H25N3
319.44
100(2)
100(2)
monoclinic
P 21/c
triclinic
P -1
a (Å)
10.1004(7)
14.1307(11)
15.0820(11)
90
6.8922(6)
8.5144(8)
15.9322(16)
101.414(4)
90.344(6)
102.292(4)
894.32(15)
2
b (Å)
c (Å)
α (deg)
106.623(3)
90
β (deg)
γ (deg)
volume (Å3)
2062.6(3)
4
Z
density (g.cm-3)
absorption coefficient (mm-1)
F(000)
1.270
1.186
0.991
0.071
848
344
total no. reflections
unique reflections [R (int)]
R1a (I > 2σ(I))
wR2a (all data)
largest diff. peak and hole (e.A-3)
goodness of Fit
29022
7825
3772 [0.036]
0.0286
3220 [0.037]
0.0479
0.0709
0.1291
0.450 and -0.248
0.973
0.184 and -0.209
1.013
7

RESULTS AND DISCUSSION
Reaction of the cobalt(I) complex (HPNP)CoCl[33] with the N-heterocyclic carbene activated
nitrous oxide adduct NHC-N2O proceeds in 16 hours at room temperature. The reaction was
performed using a 3:1 Co(I):NHC-N2O ratio according to Scheme 1.
Scheme 1. Reaction of (HPNP)CoCl with NHC-N2O.
Following workup, four reaction products were isolated (Scheme 1). The four-coordinate cobalt
nitrosyl species (HPNP)Co(NO) (1) was isolated as bright red crystals in 22% yield. Contrary to
the reaction of Ni(COD)2 with NHC-N2O that yields a dinuclear nickel nitrosyl complex with
imidazolin-2-iminato ligands bridging two nickel centers (see Figure 1),[27] the iminato moiety is
not bound to cobalt in the final product but is isolated as the free imine organic product
1,3-Dimesitylimidazolin-2-imine NHC=NH (2). This is likely the result of the sterically demanding
HPNP ligand environment, together with the presence of a reactive amine moiety at the ligand
(see below). The 13C NMR spectrum of 2 exhibits a characteristic resonance at δ = 150.8 ppm
(Figure S8) and a sharp νCN at 1697 cm-1 (Figure S10) indicative of an imine moiety. The solid-
state structure of 2, as determined by single crystal X-ray diffraction (Figure 2), further confirms
the molecular identity of 2 and compares well with previously reported imidazolin-imine
compounds.[41–43] No H-bonding intermolecular interactions are observed in the solid-state.
8

Figure 2. Solid-state molecular structure of compound NHC=NH (2) determined by single-crystal
X-ray diffraction. Hydrogens are removed for clarity. Carbon (grey) and nitrogen (blue) atoms
are represented with 50 % probability ellipsoids.
Table 2. Selected bond distances (Å) for compound NHC=NH (2).
N(3)-C(1)
C(1)-N(1)
N(1)-C(2)
1.284(2)
1.389(2)
1.400(2)
C(2)-C(3)
C(3)-N(2)
N(2)-C(1)
1.338(3)
1.401(2)
1.384(2)
Two cobalt(II) chloride species are also formed during the course of the reaction: (HPNP)CoCl2
(3) and (PNP)CoCl (4), the latter featuring a deprotonated PNP ligand acting as an amido-
diphosphine donor. These species were isolated previously[33,39] and, in the present case,
were identified both by NMR spectroscopy and by inspection of the unit cell parameters of
crystalline samples. Given the relative complexity of the reaction mixture and the difficulty of
separation of the multiple reaction products, isolated yields for 1-4 are moderate.
Performing the reaction with a 1:1 (HPNP)CoCl:NHC-N2O ratio resulted in the full conversion of
the cobalt(I) complex (HPNP)CoCl to the same products (1-4) and unreacted NHC-N2O was
recovered, in agreement with the stoichiometry proposed in Scheme 1. Because of the slow
nature of this heterogeneous reaction and the presence of several paramagnetic species in the
9

reaction mixture, it was not possible to use low-temperature NMR spectroscopy to probe for
any reaction intermediates.
1
Complex 1 was fully characterized by FT-IR, H and 31P NMR spectroscopies, single-crystal X-ray
1
diffraction and elemental analysis. The H NMR spectrum for 1, recorded at 23°C in C6D6 (Figure
S1), displays nine resonances in the diamagnetic region consistent with the presence of a Cs-
symmetric solution species with one signal for the N-H proton, four signals for the methylenic
bridges protons and four signals for the isopropyl substituents. A single resonance at δ = 80.6
ppm was observed in the 31P{1H} NMR spectrum of 1 (Figure S2) for the phosphine donors of the
PNP ligand, in accord with a S = 0 species. This lies in stark contrast with the S = 1 Co(I) complex
(HPNP)CoCl whose 1H NMR spectrum features broad paramagnetic resonances in the -14 to +65
ppm region.[33]
The IR spectrum for 1 features a characteristic absorption band at 3265 cm-1 corresponding to
the N-H vibration of the (HPNP) ligand and a strong absorption band at 1595 cm-1 assigned to
the N-O stretch (Figure S6). While in the wide range of frequencies observed in nitrosyl
complexes,[44,45] this feature appears at relatively low energy for a terminal linear nitrosyl
ligand and is consistent with a reduced NO bond order.[46] Similar low energy NO stretches are
observed in related {CoNO}10 complexes[47,48] (see Table 4 below).
The molecular structure of 1 was further confirmed by X-ray diffraction analysis. In the crystal
structure (Figure 3) the cobalt center adopts a distorted trigonal-pyramidal arrangement with
two phosphorus donors from the phosphines of the HPNP pincer ligand and the nitrogen from
the NO donor in the trigonal plane, and the nitrogen from the amine (τ4 = 0.79) in the apical
position.[49] A similar geometric environment is found in the related (HPNP)CoCl cobalt(I)
complex.[33]
The P1−Co1−P2, N2−Co1−P1, and N2−Co1−P2 angles are 120.63(2)°, 114.16(6)°, and 117.20(6)°,
respectively, and the Co atom is displaced by 0.326 Å above the N2−P1−P2 plane. The P1-Co1-P2
and N1-Co1-N2 planes are perpendicular (dihedral angle of 89.6°). The NO group is found to be
10

nearly linearly bound to Co with a Co-N(2)-O(1) angle of 172.4(2)°, a common feature of metal
nitrosyl complexes featuring strong π-backbonding. The cobalt-nitrosyl Co-N(2) bond (1.632(2)
Å) is much shorter than that of the cobalt-amine Co-N(2) (2.109(2) Å) and suggests multiple
bonding between Co and NO. The nitrosyl N-O bond distance of 1.213(2) Å falls in between that
of free (NO)● and that of (NO)− (resp. 1.154 Å and 1.26 Å).[44] These structural parameters
compare well with those of pseudotetrahedral {CoNO}10 complexes (see below) (triphos)Co(NO)
(triphos = MeC(CH2CH2PPh2)3)[47] (Co-N-O = 174.6(6)°, Co-N = 1.620(5) Å; N-O = 1.211(6) Å) and
((R)-prophos)(tBuNC)Co(NO) (prophos = Ph2PCH2CHMePPh2)[50] (Co-N-O = 178.5(8)°, Co-N =
1.632(8) Å; N-O = 1.21(1) Å). Finally the Co-P bond lengths are alike that of related
compounds.[33,51]
N1
P1
P2
Co1
N2
O1
O1
N2
N1
P2
P1
Co1
Figure 3. Top and side views of the solid-state molecular structure of compound (HPNP)Co(NO)
(1) determined by single-crystal X-ray diffraction. Hydrogens are removed for clarity. Carbon
11

(grey), cobalt (purple), nitrogen (blue), oxygen (red) and phosphorus (orange) atoms are
represented with 50 % probability ellipsoids.
Table 3: Selected bond distances (Å) and angles (°) for compound (HPNP)Co(NO) (1).
Co-N(2)
Co-N(1)
Co-P(1)
Co-P(2)
N(2)-O(1)
1.632(2)
2.109(2)
2.1991(6)
2.1997(6)
1.213(2)
P(1)-Co-N(1)
N(1)-Co-P(2)
P(1)-Co-N(2)
N(2)-Co-P(2)
Co-P(1)-C(7)
Co-P(2)-C(10)
Co-N(1)-C(8)
Co-N(1)-C(9)
C(8)-N(1)-C(9)
85.63(5)
85.74(5)
114.16(6)
117.20(6)
103.23(6)
102.39(7)
112.12(12)
112.64(12)
111.31(16)
Co-N(2)-O(1)
P(1)-Co-P(2)
N(1)-Co-N(2)
172.4(2)
120.63(2)
127.68(8)
Complex 1 can formally be viewed as Co-I-NO+ species: this designation helps rationalize the
redox chemistry occurring in the course of the reaction, the monovalent starting material
(HPNP)CoCl leading to two divalent cobalt by-products and a Co(-I) main product. However,
description of the bonding in metal nitrosyl complexes often presents a challenge. Co-NO bonds
are highly covalent with close relative energy of the metal-d and NO-π* orbitals.[44] The nitrosyl
ligand is thus non-innocent and can be formally treated as NO2−, NO−, NO● or NO+. Several
limiting valence bond descriptions, presented on Figure 4, can thus be proposed for 1. Note that
oxidation states assignments are a formalism and not a rigorous description of the actual
electron distribution in the complex. Furthermore, the bonding picture is often very complex
and can involve strong antiferromagnetic interactions and multi-configurational ground-states
which challenges usual descriptions of bonding.[44,52,53] For these reasons, metal-nitrosyl
derivatives are often discussed using the Feltham-Enemark notation,[54] {MNO}x, where x is the
total number of electrons in the metal-d + NO-π* orbitals, which purposefully makes no
assumption on the formal charge on the metal or on the NO ligand.
12

Figure 4. Limiting valence bond descriptions of S = 0 {CoNO}10 complexes.
Table 4 lists some physicochemical data for 4-coordinate terminal mono-nitrosyl cobalt
complexes which fall into two categories: {CoNO}10 derivatives featuring 18 valence electrons
(VE) and {CoNO}9 derivatives featuring 17 VE. From this literature survey, it appears that 4-
coordinate {CoNO}10 species share common characteristics of which compound 1 is a typical
example: red-brown color, pseudo-tetrahedral or trigonal-planar geometries, linearly-bound
nitrosyl and S = 0 spin state. Additionally, these species feature substantially low NO vibrational
stretches (1655-1595 cm-1) and long N-O bond lengths (around 1.21 Å) compared to {CoNO}9
species which both suggest stronger back-donation with higher electron density located on the
nitrosyl moiety.
A different situation is found for the square-planar (iPrPDI)Co(NO) complex[53] (iPrPDI = 2,6-(2,6-
iPr2−C6H3−N=CMe)2C5H3N) which adopts a {CoNO}9 rather than the expected 18 VE {CoNO}10
configuration. This compound is best described as (iPrPDI●−)CoI(NO●) with a closed-shell square
planar d8 Co(I) center. This unusual biradical electronic configuration arises from the presence
of the iminopyridine redox-active ligand. Lastly, Wieghardt, DeBeer, Toste, Bergman and
coworkers have used detailed spectroscopic methods in conjunction with advanced
computational studies to elucidate the electronic structure of (Tp*)Co(NO).[52] They concluded
that the major component of the ground state of this {CoNO}9 complex is a S = 3/2 Co(II) metal
center anti-ferromagnetically coupled to a triplet NO−ꢀanion. However, multi-reference
configuration is an important characteristic of this system with significant contribution from the
CoI(S = 1)-NO●(S = ½) configuration to the final wave function. Similar electronic structure is
13

probably found in [(N(CH2CH2PR2)3)Co(NO)][BPh4][55] which share common characteristics (S =
½, green color, tetrahedral geometry) with (Tp*)Co(NO).
In view of literature precedents for the oxidation chemistry of PNP-supported cobalt complexes,
the N-N bond cleavage vs oxygen-atom transfer selectivity observed herein is remarkable.
Indeed, Caulton and coworkers observed recurrent ligand non-innocence in cobalt complexes
supported by the tBuPNP ligand set (tBuPNP = (tBu2PCH2SiMe2)2N-)[56–58] due to the presence of
vulnerable phosphine functionalities. Notably, when exposed to oxygen-atom transfer reagents
(O2, PhI=O), the three-coordinate Co(I) complex (tBuPNP)Co is oxidized at the phosphine moieties
to give phosphine-oxide derivatives.[57] Particularly relevant to our work, the same group
reported the reaction of (tBuPNP)Co with 3 equiv. of N2O which led to complete consumption of
(tBuPNP)Co to yield a new paramagnetic species of Cs symmetry. Although full characterization
of this reaction product was not successful, the authors concluded that oxidation most likely
occurs at the phosphine moieties since the same species was formed when 3 equiv. of Me3NO
were used in place of N2O.[57]
The reactivity of (HPNP)CoCl with NHC-N2O observed herein is therefore in stark contrast with
that previously observed for (tBuPNP)Co with N2O since no oxygen-atom transfer to the
phosphine moieties is observed. Although O-atom transfer reaction from NHC-N2O has been
observed with oxophilic early transition metals,[28] the N-N bond cleavage is likely driven by the
weakening of the N2O N-N bond upon NHC coordination.
A tentative general reaction scheme is proposed in Scheme 2 to account for the formation of
compounds 1-4. The first step most likely involves a disproportionation process of the Co(I)
center in (HPNP)CoCl to generate (HPNP)CoCl2 (3) and (HPNP)Co(NO) (1). Two possible
mechanisms can be envisioned for the N-N bond rupture in NHC-N2O: either a homolytic bond
cleavage, generating a imidazolyn-2-iminato radical NHC=N● or a heterolytic bond cleavage
generating a imidazolyn-2-iminato anion NHC=N−. Subsequently, this nucleophilic reaction
intermediate abstracts a proton from the amine group of the HPNP ligand in 1, yielding NHC=NH
14

(2) and (PNP)CoCl (4), observed experimentally. The overall reaction stoichiometry is therefore
in agreement with the experiment.
Scheme 2. Proposed mechanistic schemes to account for the formation of compounds 1-4.
Homolytic bond-cleavage:
Heterolytic bond-cleavage:
Preliminary reactivity studies involving the Co(I) derivative (PNP)Co(N2)[33] towards NHC-N2O
were attempted. Invariably of the reaction conditions used, intractable mixtures of compounds
were produced, among which 1 could be identified. This suggests that the formation of late
metal-nitrosyls from NHC-N2O might be a more general process.
15

CONCLUSIONS
In summary we have discovered a rare example of cobalt(I)-based cleavage of the N-N bond in a
nitrous-oxide NHC adduct. This reactivity is an unusual route to the formation of an electron-
rich cobalt-nitrosyl complex (HPNP)Co(NO). Several co-products are produced in the course of
the reaction, including the free 1,3-Dimesitylimidazolin-2-imine NHC=NH resulting from the
cleaved nitrous oxide moiety in NHC-N2O. The relative complexity of this process can be
attributed to the lability of the chloride anion together with the presence of a reactive amino
group at the HPNP pincer ligand. This work highlights the potential of low-valent metal centers
to promote the transformation of NHC-activated N2O that may ultimately be exploited in
catalysis. Future studies will be directed in order to understand the mechanism involved in this
transformation as well as the impact of the nature of the Co(I) reagent and the scope of these
reactions.
ACKNOWLEDGMENTS
The authors gratefully acknowledge funding of the synthetic and characterization studies at
Berkeley by the NSF (CHE-1465188). We acknowledge Benjamin Kriegel for helpful discussions
and Euro Solari from the EPFL, Switzerland, for generously providing NHC-N2O. We thank Dr A.
DiPasquale for experimental assistance with X-ray diffraction studies.
Appendix A. Supplementary data
CCDC 1418266 and 1418267 contain the supplementary crystallographic data for compounds 1
and
2.
These
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-
mail:deposit@ccdc.cam.ac.uk. Supplementary data (1H, 13C and 31P NMR and IR spectra for
compounds 1 and 2) can be found in the online version at doi:<include_doi_here>.
AUTHOR INFORMATION
*phone: +1 510 643 5181; e-mail: arnold@berkeley.edu
16

Notes
The authors declare no competing financial interest
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20

Table 4. Selected data for 4-coordinate terminal mono-nitrosyl cobalt complexes.
Spin
state
F.-E.
notation
Compound
Geometry
VE
Color
νNO (cm-1)
Co-N (Å)
N-O (Å)
Co-N-O (°)
Reference
pseudo trigonal
pyramidal
(HPNP)Co(NO) (1)
S = 0
18
{Co(NO)}10
red
1595
1.632(2)
1.213(1)
172.4(2)
this work
(triphos)Co(NO)
(prophos)(tBuNC)Co(NO)
(np3)Co(NO)
pseudo tetrahedral S = 0
pseudo tetrahedral S = 0
pseudo tetrahedral S = 0
18
18
18
{Co(NO)}10
{Co(NO)}10
{Co(NO)}10
red
1630
1655
1620
1.620(5)
1.632(8)
/
1.211(6)
1.21(1)
/
174.6(6)
178.5(8)
/
[47,48]
[50]
red
brown
[55]
1.643(14)
and 1.737(6)
1.101(7) and
1.167(16)
166.4(5) and
169.1(10)
(iPrPDI)Co(NO)
square planar
S = 0
17
{Co(NO)}9
pink
1712
[53]
1.628(5) and
1.625(5)
1.161(6) and
1.167(6)
173.5(6) and
175.5(6).
(Tp*)Co(NO)
tetrahedral
tetrahedral
S = 1/2 17
S = 1/2 17
{Co(NO)}9
{Co(NO)}9
green
green
1732
1680
[52,59]
[55]
[(np3)Co(NO)][BPh4]
1.83(6)
1.14(10)
165(7)
Nitrosyl carbonyl cobalt derivatives were omitted from this analysis. VE = valence electrons, F.-E. = Feltham and Enemark, triphos = MeC(CH2−CH2−PPh2)3),
np3 = tris(2-diphenylphosphinoethyl)amine, prophos = (R)-Ph2P−CH2−CHMe−PPh2, iPrPDI = 2,6-(2,6-iPr2−C6H3−N=CMe)2C5H3N, Tp* = hydro-tris(3,5-Me2-
pyrazolyl)borate
21

Graphical abstract
22

A PNP cobalt(I) complex was allowed to react with the nitrous oxide N-heterocyclic carbene adduct
(NHC-N2O). Several products were isolated and two were characterized including the cobalt nitrosyl
complex (PNP)CoNO and the 1,3-dimesitylimidazolin-2-imine NHC=NH. Heterolytic or homolytic bond
cleavage of the N-N bond from NHC-N2O could be used to generate late-metal nitrosyls.
Synopsis
23