[(TMEDA)Co(NO)2][BPh4]: A versatile synthetic entry point to four and five coordinate {Co(NO)2}10 complexes

Article abstract of DOI:10.1016/j.jorganchem.2011.04.038

[(TMEDA)Co(NO)₂][BPh₄] reacts with Group 1 salts of various monoanionic ligands to yield four and five coordinate {Co(NO) ₂}¹⁰ complexes. The synthesis of the four coordinate complex of the form [{LX}Co(NO)₂] via salt-metathesis reactions of [(TMEDA)Co(NO)₂][BPh₄] with [{ArNC(Me)CHC(Me)NAr} Li(OEt₂)] (Ar = 2,6-di-iso-propylphenyl) is reported. In addition [(TMEDA)Co(NO)₂][BPh₄] reacts with either KTp or a suite of cyclopentadienyllithium and cyclopentadienylsodium reagents, to generate the corresponding five coordinate [{L₂X}Co(NO)₂] complexes ({L₂X = C₅H₅, MeC₅H₄, Cp, ^tBuC₅H₄, Ph₂CHC₅H ₄, Me₃SiC₅H₄, ^tBuMe ₂SiC₅H₄, ⁱPr₃SiC ₅H₄, 1,3-(ⁱPr₃Si)₂C ₅H₃ and Tp). In support of existing precedent, the four coordinate complex is a thermally robust and readily isolable species while five coordinate complexes are thermally unstable transient intermediates that may either undergo dissociation of an NO ligand or be trapped by alkenes to form the corresponding metal dinitrosoalkane complexes. These reactions demonstrate that [(TMEDA)Co(NO)₂][BPh₄] provides a versatile synthetic entry point to cobalt dinitrosyl complexes and obviates the need for the repeated use of nitric oxide in the preparation of dinitrosoalkane complexes of cobalt.

Full text of DOI:10.1016/j.jorganchem.2011.04.038

Journal of Organometallic Chemistry 696 (2011) 3974e3981
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
[(TMEDA)Co(NO)2][BPh4]: A versatile synthetic entry point to four and five
coordinate {Co(NO)2}10 complexes
Mark R. Crimmin a, Lauren E. Rosebrugh a, Neil C. Tomson b, Thomas Weyhermüller b,
,
a
,
b,
**
Robert G. Bergman a , F. Dean Toste , Karl Wieghardt
*
*
a Department of Chemistry, University of California, Berkeley, CA 94720, United States
b Max-Planck-Institut für Bioanorganische Chemie, 34-36 Stiftstrasse, 45470 Mülheim an der Ruhr, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 March 2011
Accepted 29 April 2011
[(TMEDA)Co(NO)2][BPh4] reacts with Group 1 salts of various monoanionic ligands to yield four and five
coordinate {Co(NO)2}10 complexes. The synthesis of the four coordinate complex of the form [{LX}
Co(NO)2] via salt-metathesis reactions of [(TMEDA)Co(NO)2][BPh4] with [{ArNC(Me)CHC(Me)NAr}
Li(OEt2)] (Ar ¼ 2,6-di-iso-propylphenyl) is reported. In addition [(TMEDA)Co(NO)2][BPh4] reacts with
either KTp* or a suite of cyclopentadienyllithium and cyclopentadienylsodium reagents, to generate the
This paper is dedicated to Professor Kenneth
G. Caulton, for his fundamentally important
and useful contributions to inorganic and
organometallic chemistry, including the
work that facilitated the syntheses
described in this paper.
corresponding five coordinate [{L2X}Co(NO)2] complexes ({L2X
¼
C5H5, MeC5H4, Cp*, tBuC5H4,
Ph2CHC5H4, Me3SiC5H4, tBuMe2SiC5H4, iPr3SiC5H4, 1,3-(iPr3Si)2C5H3 and Tp*). In support of existing
precedent, the four coordinate complex is a thermally robust and readily isolable species while five
coordinate complexes are thermally unstable transient intermediates that may either undergo dissoci-
ation of an NO ligand or be trapped by alkenes to form the corresponding metal dinitrosoalkane
complexes. These reactions demonstrate that [(TMEDA)Co(NO)2][BPh4] provides a versatile synthetic
entry point to cobalt dinitrosyl complexes and obviates the need for the repeated use of nitric oxide in
the preparation of dinitrosoalkane complexes of cobalt.
Keywords:
Metal nitrosyl
Nitrosoalkane
Cobalt
Ligand-based reactivity
DNIC
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
distorted tetrahederal geometry at cobalt and are readily isolable,
five coordinate {Co(NO)2}10 complexes (IV) are transient reactive
The importance of nitric oxide (NO) to biological systems has
spurred recent research activity into the properties and reactivity of
1st row transition metal nitrosyl complexes [1]. While many
current studies are focused on dinitrosyl iron complexes (DNIC)
[1,2,3], due to their potential to mediate reactions involving NO
in vivo, far less attention has been paid to cobalt dinitrosyl
complexes despite the similarities between {Fe(NO)2}9 and
{Co(NO)2}10 complexes [4,5].
intermediates which, to date, have only be generated and charac-
terized in situ (Fig. 1). Although a number of dimeric and polymeric
{Co(NO)2}10 complexes are also known, including halide bridged
compounds of the form [Co(NO)2(
m
-X)]n (X ¼ Cl, Br, n ¼ 2; X ¼ I,
n ¼ N) [6], in the context of the current study, discussion is limited
to discrete mono-metallic complexes.
Four coordinate cationic species (I) supported by both non-
chelating and chelating amine, phosphine, phosphite and alkene
ligands have been synthesized and characterized [7]. Many have
been the subject of single crystal X-ray diffraction investigations [8].
A number of salts of the form I have been shown to exhibit
conductivity [7], and the electrochemistry of [{(EtO)3P}2Co(NO)2]
[BF4] has been studied [9]. In an example of applied reaction chem-
istry, [L2Co(NO)2]þXꢀ (L ¼ MeCN, PhCN, Me2CO; L2 ¼ COD, norbor-
nadiene; X ¼ PF6, BF4, ClO4) have been reported by Tkatchenko and
co-workers to be pre-catalysts for the oligomerization of butadiene,
norbornadiene, styrene, isoprene and phenylacetylene [10].
In the 1970s neutral four-coordinate complexes, [(acac)
Co(NO)2] (II, acac ¼ acetylacetonate) and [(sacsac)Co(NO)2] (II,
Existing monomeric {Co(NO)2}10 complexes can be organized
into four distinct classes. Where L is a neutral 2-electron ligand and
X is a mono-anionic ligand, these complexes can be described as:
mono-cationic four-coordinate, [L2Co(NO)2]þ (I), neutral four
coordinate, [LXCo(NO)2] (II), mono-anionic four coordinate,
[X2Co(NO)2]ꢀ (III) and neutral five-coordinate, [L2XCo(NO)2] (IV).
While the former four coordinate compounds (IeIII) demonstrate
* Corresponding authors. Tel.: þ1 510 642 1548; fax: þ1 510 642 7714.
** Corresponding author. Tel.: þ49 0 208 306 3609; fax: þ49 0 208 306 3952.
E-mail addresses: rbergman@berkeley.edu (R.G. Bergman), wieghardt@mpi-
muelheim.mpg.de (K. Wieghardt).
sacsac
¼
dithioacetylacetonate) were synthesized by salt-
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.04.038

M.R. Crimmin et al. / Journal of Organometallic Chemistry 696 (2011) 3974e3981
3975
Fig. 1. Monomeric dinitrosyl complexes of cobalt.
metathesis of [Co(NO)2Br]2 with lithium acetylacetonate and
2. Experimental
reaction of [(sacsac)2Co] with nitric oxide respectively [11,12]. More
recently Lippard and co-workers have reported not only a four
coordinate dinitrosyl) cobalt complex supported by an amino
(tropiminate) ligand in the context of nitric oxide sensing but also
2.1. General experimental
Unless otherwise indicated, operations were performed under
anhydrous conditions and inert atmosphere employing standard
Schlenk-line and glovebox techniques. Glassware was dried in an
oven at 160 ꢁC overnight or flame-dried prior to use. NMR spectra
were acquired using Bruker AV-300, AVQ-400, AVB-400 and AV-
500 spectrometers. Chemical shifts are reported as part per
the
b-diketiminate stabilized complex [{ArNC(Me)CHC(Me)NAr}
Co(NO)2] (Ar ¼ 2,6-di-iso-propylphenyl) [13,4c]. In addition to
these studies, a number of phosphine stabilized mono-halide
dinitrosyl complexes have been reported [14,15]. Thus Aresta,
Tkatchenko and co-workers have reported the synthesis of
[{PhP(OCH2CH2)2NH}Co(NO)2Cl] and its reversible reaction with
CO2 [14], while both [{(Ph2P]O)CH2CH2PPh2}Co(NO)2I] and
[(Ph3P)Co(NO)2I] have been studied by single crystal X-ray
diffraction [15].
Lippard and co-workers detailed the synthesis of an anionic
{Co(NO)2}10 complex of the form III in 2008 [4b]. As part of a wider
study on biologically relevant dinitrosyl iron complexes, the reac-
tion of [(PhS)4Co][Et4N]2 with NO was reported to yield
[(Ph2S)2Co(NO)2][Et4N]. While this work was pre-dated by a report
of the preparation of [(NC)2Co(NO)2][Na] in 1970 [16], character-
ization data on the latter complex did not include an Xeray
diffraction study.
million (ppm, d
) and 1H and 13C chemical shifts are referenced to
the corresponding residual protic solvent resonance. All mass
spectra (LR-MS and HR-MS) were recorded at the University of
California, Berkeley Microanalytical Facility with electrospray
ionization (ESI) or fast atom bombardment techniques (FAB) in
positive mode. Fast atom bombardment mass spectra were recor-
ded on a Micromass ZAB2-EQ magnetic sector instrument. Infrared
spectral data were recorded on a Thermo Scientific Nicolet iS10
spectrometer fitted with a Smart OMNI-transmission or Smart iTR
device as either KBR discs, neat solids or thin films. Elemental
analyses were recorded by the UC Berkeley micro-mass facility.
Thin layer chromatography was performed using 0.25 mm pre-
coated silica gel plates from Silicycle containing a fluorescent
indicator for visualization by UV light. Flash chromatography was
performed using MP Biomedicals SiliTech silica gel 32e63D, 60 Å.
Solvents were dried through a push-still system via passage
through activated alumina. Cobalt(II) chloride was dried prior to
use by heating under vacuum (120 ꢁC at 1 ꢂ 10ꢀ1 mbar) for 12 h.
Volatile alkenes were distilled from calcium hydride and freeze-
pump-thaw de-gassed prior to use. 5,6,7,8-Tetrafluoro-
1,4-dihydro-1,4-ethenonaphthalene [21] was synthesized by the
literature procedure. NO gas was purified by passage through
a ꢀ78 ꢁC trap before use. The cobalt complex 2 was synthesized by
a modification of the procedure reported by Caulton and co-
workers [7a]. The lithium salts [{ArNC(Me)CHC(Me)NAr}Li(OEt2)]
(Ar ¼ 2,6-di-iso-propylphenyl), [Li(tBuC5H4)], [Li{(Ph2CH)C5H4}],
[Li(C5H4SiMe3)], [Li{C5H4(SitBuMe2)}], [Li{(SiiPr3)C5H4}] and [Li{1,3-
(iPr3Si)2C5H3}] were prepared by literature procedures [22,23],
In an unusual example of ligand-based reactivity, Brunner and
Loskot reported that the reaction of [CpCo(m-NO)]2 with NO and
strained alkenes yielded the corresponding dinitrosoalkane
adducts (Scheme 1a) [17]. The scope was expanded and mechanism
of this reaction elucidated in subsequent studies from one of our
groups [18]. Consistent with the concerted addition of a five coor-
dinate reactive intermediate of the form IV to the alkene, binding of
the unsaturated substrate was shown to be reversible and stereo-
specific. Despite being air-sensitive and thermally unstable,
[CpCo(NO)2] (1) could be observed by NMR, UVevis and infrared
spectroscopy at low concentrations in solution. It is noteworthy
that 1 may act as a stoichiometric reagent for not only alkene dia-
mination, but also for the CeH functionalization of strained alkenes
through base-mediated reactions with Michael acceptors [19].
More recently, we have found that [Tp*Co(NO)] may be synthesized
via
a
salt-metathesis reaction between KTp* and [(TMEDA)
Co(NO)2][BPh4] (2) in THF solution [20]. Under a pressure of NO, the
former complex forms an equilibrium mixture with the corre-
sponding five-coordinate species [Tp*Co(NO)2] which in turn may
be trapped as the dinitrosoalkane adduct by addition to norbornene
(Scheme 1b).
We now report the synthesis and characterization of a number
of four and five-coordinate {Co(NO)2}10 complexes, by salt-
metathesis reactions of [(TMEDA)Co(NO)2][BPh4] with various
mono-anionic ligands, along with their reactions with alkenes. In
line with the expectations provided by existing studies (vide
supra), while a hydrocarbon soluble four coordinate system is
thermally robust and unreactive with alkenes and alkynes up to
100 ꢁC, five coordinate species are transient intermediates that
readily bind alkenes at temperatures as low as ꢀ78 ꢁC. In the
absence of an alkene trap the five coordinate species readily
dissociate NO.
Scheme 1. Representative alkene binding reactions of (a) [CpCo(NO)]2/NO and
(b) [Tp*Co(NO)]/NO.

3976
M.R. Crimmin et al. / Journal of Organometallic Chemistry 696 (2011) 3974e3981
Table 1
extraction of the crude reaction mixture in to hexanes. Removal
of the solvent and crystallization gave the product as a brown
solid (179 mg, 0.334 mmol, 60%). Xeray quality crystals were
obtained from slow evaporation of a concentrated pentane
solution. 1H NMR (d8-tol, 300 MHz, 298 K) 1.14 (d, 12H,
J ¼ 6.9 Hz), 1.21 (d, 12H, J ¼ 6.9 Hz), 1.73 (s, 6H), 3.11 (hept, 4H,
J ¼ 6.9 Hz), 5.13 (s, 1H), 6.98e7.10 (m, 6H); 13C NMR (d8-tol,
100 MHz, 298 K) 23.2, 24.6, 24.8, 27.6, 100.8, 124.0, 126.1, 142.6,
148.4, 164.2; Infrared (KBr disc, cmꢀ1) 2960, 2925, 2866, 1807,
1721, 1525, 1400; LR-MS (FAB, þve ion) 536 ([M]þ, 10%), 506
([MeNO]þ, 100%), 476 ([Me2NO]þ, 50%). Data match those
reported by Lippard and co-workers [4c].
X-ray experiment acquisition parameters and selected data on compounds 2 and 4j.
2
4j
Molecular formula
Formula weight (g molꢀ1
Crystal system
Space group
a (Å)
C30H36BCoN4O2
554.37
Orthorhombic
Pna21
21.083(2)
9.6623(9)
14.0373(13)
90
90
90
2859.5(5)
4
0.633
C23H34BCl2CoN8O2
595.22
Triclinic
Pꢀ1
)
10.7950(12)
11.2263(13)
12.6478(15)
102.699(2)
93.487(2)
110.050(2)
1389.0(3)
2
b (Å)
c (Å)
a
b
g
(deg)
(deg)
(deg)
V (Å3)
Z
m
r
q
(mmꢀ1
)
0.847
1.423
1.67e25.33
0.0362, 0.0909
0.0423, 0.0960
2.3.2. Representative procedure for the synthesis of cobalt
dinitrosoalkane complexes of norbornene (4aej)
(g cmꢀ3
)
1.288
range (o)
2.32e30.99
0.0272, 0.0686
0.0326, 0.0722
Under an inert atmosphere, a solution of the group 1 complex
(0.54 mmol, 1.2 equiv., see Table 3 for details) in THF (10 mL)
was added via cannula to a slurry of [(TMEDA)Co(NO)2][BPh4]
(250 mg, 0.45 mmol, 1 equiv.) and norbornene (4.5 mmol,10 equiv.)
in THF (10 mL) at ꢀ78 ꢁC. The reaction mixture was allowed to
warm to room temperature and stirred for several hours or over-
night (see Table 3 for details). The reaction could be monitored by
analysis of aliquots by thin layer chromatography on silica gel (3:1
hexanes/ethyl acetate eluent). Once the reaction was complete, the
THF was removed in vacuo and the crude product dissolved in
minimal methylene chloride and purified on silica gel using a 9:1
mixture of hexanes:ethyl acetate as the eluting solution. Products
isolated in this manner proved air- and moisture stable in the solid
state and could be handled on the bench-top. While full charac-
terization details are given in the supporting information, as an
example the data for 4f are the following: Black solid (0.149 g,
0.39 mmol, 86%). 1H NMR (CDCl3, 400 MHz, 298 K) 5.15 (s, 2H), 4.73
(s, 2H), 2.68e2.69 (m, 2H), 2.60e2.61 (m, 2H), 1.45e1.51 (m, 1H),
1.16e1.18 (m, 2H), 0.85e0.87 (m, 1H), 0.85 (s, 9H), 0.06 (s, 6H); 13C
NMR (CDCl3, 100 MHz, 298 K) 95.8, 94.2, 93.7, 92.9, 42.2, 30.9, 26.3,
26.2, 17.1, ꢀ5.9; Infrared (solid, cmꢀ1) 2953, 2926, 2855, 1403, 1357,
1352, 1265, 1251;; m/z (FAB, þve ion) 393 (100%, [M þ H]þ), 335
(55%, [MþHeC4H9]þ); HR-MS (ESI, þve) for C18H30O2N2CoSi
393.1403 found 393.1387.
R1 wR2b [I > 2
s
(I)]
a
R1 wR2b (all data)
a
Measured/independent reflections/Rint 81294/9063/0.0444 7783/4524/0.0204
a
R1 ¼ S jjFojꢀjFcjj/SjFoj.
b
2
wR2 ¼ {S[w(Fo ꢀFc2)2]/S[w(Fo2)2]}1/2
while [KTp*] was purchased from SigmaeAldrich and used without
further purification. All other reagents were obtained from
commercial suppliers (Aldrich, Strem) and used without further
purification. Full characterization data for compounds 4aeq are
presented in the supporting information.
2.2. Acquisition of X-ray data
Single crystal X-ray diffraction experiments for compound 4j
were conducted at UC Berkeley CheXray facility using a SMART
APEX diffractometer equipped with a fine-focus sealed tube, Mo
K/
a source, and Bruker APEX-I CCD detector. A multi-scan
absorption correction was applied, while structures were solved
with SIR-2004 and refined in SHELXL-97 [24]. Selected acquisition
data are presented in Table 1. CIF files for compounds 2 and 4j
were deposited in the CCDC database and are assigned as CCDC
815248 and 815249 respectively.
2.3. Synthetic procedures
2.3.3. Representative procedure for the synthesis of tris(pyrazolyl)
borate dinitrosoalkane complexes
2.3.1. Synthesis of [{ArNC(Me)CHC(Me)NAr}Co(NO)2] (Ar ¼ 2,6-di-
iso-propylphenyl) (3)
Under an inert atmosphere, a solution of [KTp*] (72.6 mg,
0.216 mmol, 1.2 equiv.) in THF (10 mL) was added via cannula to
a slurry of 2 (100 mg, 0.181 mmol, 1 equiv.) and alkene (1.81 mmol,
10 equiv.) in THF (10 mL) at ꢀ78 ꢁC. The reaction mixture was
allowed to warm to room temperature and stirred overnight. The
reaction could be monitored by analysis of aliquots by thin layer
chromatography on silica gel. Once complete the THF was removed
in vacuo and the crude product dissolved in minimal methylene
In a glovebox, [(TMEDA)Co(NO)2][BPh4] (306 mg, 0.552 mmol)
and [{ArNC(Me)CHC(Me)NAr}Li(OEt2)] (275 mg, 0.552 mmol)
were weighed out separately and transferred to a Schlenk vessel.
The vessel was sealed, removed from the box and attached to
a Schlenk line. After toluene (5 ml) was added the sealed vessel
was heated to 80 ꢁC for 4 days. Lithium tetraphenylborate may be
removed by filtration following evaporation of solvent and
Table 2
Selected bond angles (ꢁ) and bond lengths (Å) of selected four coordinate {Co(NO)2}10 complexes.
2
[(py)2Co(NO)2][BF4] [8c]
3a
[(PhS)2Co(NO)2] [Et4N] [4b]
CoeN
NeO
1.6630(12)
1.6636(12)
1.1475(16)
1.1582(15)
112.77(7)
168.53(13)
165.83(11)
2.0423(12)
2.0652(12)
1.654(6)
1.644(6)
1.130(8)
1.156(8)
115.6(3)
170.2(6)
170.1(6)
2.012(6)
2.012(6)
1.645(4), 1.649(4)
1.690(4), 1.693(4)
1.161(4), 1.154(4)
1.174(5), 1.169(5)
110.4(2), 110.9(2)
171.6(4), 174.3(4)
148.5(4), 150.3(4)
1.963(4), 1.970(4)
1.956(4), 1.961(4)
1.651(3)
1.684(3)
1.106(3)
1.126(3)
110.35(14)
174.9(2)
161.4(3)
2.2565(8)
2.2478(8)
NeCoeN
CoeNeO
CoeXb
[(py)2Co(NO)2]þBF4 and 3 X ¼ N and for [(PhS)2Co(NO)2]Et4N X ¼ S.
a
Data are within experimental error of those independently reported by Lippard and co-workers [4c].
X represents the heteroatoms of the neutral or mono-anionic supporting ligands, for 2.
b

M.R. Crimmin et al. / Journal of Organometallic Chemistry 696 (2011) 3974e3981
3977
2.3.4. Synthesis of [{tBuMe2SiC5H4}Co(
In a glovebox, [(TMEDA)Co(NO)2][BPh4] (200 mg, 0.362 mmol, 1
equiv.) and [Li{(tBuMe2Si)C5H4}] (67.2 mg, 0.362 mmol, 1 equiv.)
were weighed into separate vials. A slurry of the cobalt complex
meNO)]2
Table 3
Scope of the reaction of 2 with cyclopentadienyl and tris(pyrazolyl)borate anions in
the presence of norbornene.
was made in THF (4 mL) and
a solution of the cyclo-
pentadienyllithium reagent in THF (1 mL) added dropwise. The
reaction mixture was stirred for 0.5 h at room temperature and the
solvent removed under vacuum. The crude product was extracted
with 10 mL of benzene and flushed through a short silica pad (5 cm
depth ꢂ 4 cm diameter) eluting with benzene (40 mL). The resul-
[{L2X}M]
[NaCp]
Complex
Reaction time
Yielda
4a
4b
4c
4d
4e
4f
4g
4h
4i
4h
57%
66%
67%
74e94%
31%
75%
63e93%
63%
tant black-green solution was evaporated to dryness to give the
[Li{
h
5-MeC5H4}]
12h
12h
0.5h
4h
0.5h
0.5e12h
12h
12h
12h
t
product [{Me2 BuSiC5H4}Co(
m
eNO)]2 as a black solid (31.2 mg,
[LiCp*]
0.058 mmol, 32%). 1H NMR (CD2Cl2, 400 MHz, 298 K) 5.32 (m, 4H),
[Li{h5etBuC5H4}]
4.97 (m, 4H), 0.80 (s, 18H), 0.14 (s, 12H); 13C NMR (toluene-d8,
[Li{
[Li{
h
h
5-(Ph2CH)C5H4}]
5-Me3SiC5H4}]
100 MHz, 298 K) 94.7, 89.6, 26.1, 1.0, ꢀ5.9; Infrared (solid, cmꢀ1
)
[Li{h5etBuMe2SiC5H4}]
[Li{h5eiPr3SiC5H4}]
2924, 2852, 1588, 1533, 1468, 1247; LR-MS (EI, þve ion) 536 (60%,
[Mþ]).
[Li{
h
5-1,3-(iPr3Si)2C5H3}]
86%
[KTp*]
4j
65%b
a
3. Results and discussion
Reactions were conducted at 0.2 M in alkene, by addition of a solution of the
anion salt (1.2 equiv.) in THF to a solution of 2 and norbornene (10 equiv.) in THF
at ꢀ78 ꢁC. Following addition reaction mixtures were warmed to room temperature
and stirred for the specified time.
3.1. Four-coordinate {Co(NO)2}10 complexes
b
Alkene concentration was 0.1 M.
In accordance with the procedure reported by Caulton and co-
workers, [7aeb] the reaction of CoCl2, two equivalents of tetra-
methylethylenediamine (TMEDA) and NO in dry methanol followed
by addition of NaBPh4 allowed isolation of a mixture of [(TMEDA)
Co(NO)2][BPh4] (2) and [TMEDAꢃH][BPh4]. Crystallizing in the
orthorhombic space group Pna21, compound 2 is represented in
Fig. 2. Although insoluble in hydrocarbon solvents, compound 2 is
partially soluble in THF and has proven a versatile synthetic entry
point into dinitrosyl cobalt chemistry through salt-metathesis
reactions.
chloride and purified on silica gel using a 9:1 mixture of hex-
ane:ethyl acetate as the elutent. Products isolated in this manner
proved air and moisture stable in the solid state and could be
handled on the bench-top. While full characterization details are
given in the supporting information, as an example the data for 4k
are the following: Brown solid (62.8 mg, 62% yield). 1H NMR (CDCl3,
400 MHz, 298 K) 1.81 (d, 1H, J ¼ 9.2 Hz), 1.89 (s, 9H, Me), 2.29 (d, 1H,
J ¼ 9.2 Hz), 2.34 (m, 2H), 2.39 (s, 9H, Me), 3.36 (m, 2H, CHNO), 5.79
(s, 3H, ArH), 6.51 (s, 2H, C]CH); 13C NMR (CDCl3, 100 MHz, 298 K)
12.6, 13.7, 42.5, 42.7, 92.7, 107.5, 139.1, 145.2, 150.5; Infrared (KBr
disc, cmꢀ1) 3050, 2964, 2924, 2505, 1544, 1365, 1341; LR-MS
(ESI, þve ion) 509 (100%, [Mþ]), HR-MS (ESI, þve ion) Calc. for
C22H31O2N8BCo 509.1990 found 509.1982. Elemental analysis calc.
for C22H30O2BCoN8 C, 51.99; H, 5.95 N, 22.05 found C, 52.42; H,
6.04; N, 21.74.
Compound
2 readily reacts with [{ArNC(Me)CHC(Me)NAr}
LiꢃOEt2] (Ar ¼ 2,6-di-iso-propylphenyl) Scheme 2 in toluene solu-
tion at 80 ꢁC to yield [{ArNC(Me)CHC(Me)NAr}Co(NO)2] (3). The
reaction takes place with displacement of TMEDA from the coor-
dination sphere of 2 and elimination of LiBPh4. The black complex 3
isolated in this manner proved exceptionally soluble in hydro-
carbon solvents and must be handled under an anaerobic atmo-
sphere. Despite this, under nitrogen or argon, 3 is thermally robust
and may be heated at 100 ꢁC in toluene for up to 24 h without
notable decomposition. Lippard and co-workers have recently
reported the synthesis of 3 from a similar salt-metathesis reaction
employing [Co(NO)2(meCl)]2 in the place of 2 and provided single
crystal X-ray diffraction data on this complex [4c].
For comparison, the solid state data for the cationic complexes
[(py)2Co(NO)2][BF4] and 2 are presented alongside those of 3 and
the related anionic complex [(PhS)2Co(NO)2][Et4N] (Table 2)
[4b,8c]. To date all four-coordinate {Co(NO)2}10 complexes reported
display distorted tetrahedral geometry at the metal and compound
2 follows this trend. While comparison of cobalt-nitrogen bond
lengths to the supporting ligand in the cationic complexes 2 and
[(py)2Co(NO)2][BF4] (2.0423(12) and 2.012(6) Å respectively) to
those in 3 (1.956(4) and 1.961(4) Å) reveals a substantial shortening
of the bond distances from cobalt to the mono-anionic LX-type
Fig. 2. ORTEP representation of 2 thermal ellipsoids at 20% probability. Protons and the
tetraphenylborate anion have been omitted for clarity.
Scheme 2. Reaction of 2 with [{ArNC(Me)CHC(Me)NAr}Li(OEt2)] (Ar ¼ 2,6-di-iso-
propylphenyl) in toluene solution.

3978
M.R. Crimmin et al. / Journal of Organometallic Chemistry 696 (2011) 3974e3981
ligand relative to L2-type ligands, comparison of the bonding
parameters in the {Co(NO)2}10 fragment is more complicated.
Descriptions of the bonding subtleties of {Co(NO)2}10 complexes
have been the subject of a previous discussion [12]. While
cobalt-nitrogen bond lengths of the nitric oxide ligands lie in the
expected range of 1.61e1.73 Å [12], the lengthening of the nitrogen-
oxygen bond lengths in 3 (1.154(4)e1.174(5) Å) relative to those
found in 2 (1.1475(16)e1.1582(15) Å) occurs concomitantly with the
bending of the MeNeO angles from linearity (2, 165.83(11)e
168.53(13)ꢁ; 3, 148.5(4)e174.3(4)ꢁ) and a compression of the N(1)e
MeN(2) angle (2, 112.77(7); 3, 110.4(2) and 110.9(2)). The distortion
of the bond lengths and angles of the metal nitrosyl ligands in
3 relative not only to the cationic complexes 2 and [(py)2Co(NO)2]
[BF4], but also the anionic complex [(PhS)2Co(NO)2][Et4N], is
notable (Table 2) [4b,8c]. While interpretation of these solid-state
data should be approached cautiously, due to possible crystal
packing effects, the increased deviation from linear bonding modes
low solubility of 2 even in coordinating polar solvents, however,
prevented a detailed study of its chemistry with unsaturated
substrates. Thus, the reactions of 3 with cyclopentene, norbornene,
diphenylacteylene, isoprene or trans,trans-1,4-butadiene in either
d8-toluene or C6D6 solution were monitored by 1H NMR spectros-
copy. In all instances no reaction was observable up to tempera-
tures of 100 ꢁC over 12e48 h with starting materials remaining.
These results contrast dramatically to those found when generating
five coordinate {Co(NO)2}10 complexes from 2 by reaction with LiCp
or KTp* anions (vide infra), and while it may be argued that 3 posses
a sterically protected metal center, the ligand-based reaction was
expected to occur at the periphery of the coordination sphere.
Current data, however, cannot rule out the possibility that four
coordinate {Co(NO)2}10 complexes could coordinate alkenes to the
nitrosyl ligands.
3.2. Five-coordinate {Co(NO)2}10 complexes
of nitric oxide in 3 could be explained in terms of increased
p-back
donation to NO arising from increased electron density on the
The reaction of 2 with NaCp in THF (10 mM) at room tempera-
ture gave an immediate color change from brown to bright green
and the generation of [CpCo(NO)2] (1) could be monitored by react-
IR spectroscopy via the appearance of NO stretches at 1692 and
1611 cmꢀ1 [18]. Performing the reaction of 2 with a variety of
substituted cyclopentadienyl salts at ꢀ78 ꢁC in THF in the presence
of 10 equiv. of norbornene provided facile access to the corre-
sponding dinitrosoalkane complexes (4aei, Table 3). In these
reactions 2 acts as a convenient source of “[Co(NO)2]þ” obviating
the need to repeatedly use nitric oxide gas in the preparation of
cobalt dinitrosyl) complexes [17e19]. Furthermore, 2 may be
conveniently prepared on a multi-gram scale providing a useful
starting material for synthetic diversification. The reaction products
4aei proved air and moisture stable and were purified by flash
column chromatography on the bench-top.
While the reaction of 2 with a series of cyclopentadienyl salts
led to the generation and trapping of a number of [CpCo(NO)2]
derivatives, of more interest was the possibility to introduce new
ligand sets on cobalt through reaction with alternative metal salt
precursors. Tris(pyrazolyl)borate ligands have long been considered
as surrogates for cyclopentadienyl ligands [25e28], and the reac-
tion of 2 with potassium tris(3,5-dimethylpyrazolyl)borate in the
metal due to more efficient
p-donation from the b-diketiminate
ligand.
Examination of infrared spectroscopy data for four-coordinate
{Co(NO)2}10 complexes provides additional support for this
hypothesis. Compound 3 displays asymmetric and symmetric NeO
stretches at 1807 and 1721 cmꢀ1 respectively in the solid state, data
that compare well to those reported by Lippard and co-workers in
C6D6 solution (1801 and 1706 cmꢀ1) [4c] These absorptions lie
slightly outside the ranges established for compounds of the type II
(
nasym ¼ 1876e1820 cmꢀ1 and nsym ¼ 1798-1750 cmꢀ1) and are
consistent with a degree of back donation from the diketiminate
ligand increasing the electron density on the metal. The anionic
complex [(PhS)2Co(NO)2][Et4N] reported by Lippard and co-
workers displays v(NO) stretches at 1756 and 1678 cmꢀ1 in the
solid-state. Despite this complex possessing shorter NeO bond
lengths than 3 it displays only a minor perturbation of MeNeO
bond angles from idealized linear geometry in the reported solid-
state structure (Table 2) [4b].
Heating samples of 2 in the presence of 10 equiv. of norbornene
in THF in a sealed vessel over 14 h at 75 ꢁC provided no evidence for
new organometallic species derived from alkene binding to 2. The
Fig. 3. ORTEP representation of 4j, thermal ellipsoids at 20% probability H-atoms of omitted for clarity.

M.R. Crimmin et al. / Journal of Organometallic Chemistry 696 (2011) 3974e3981
3979
Table 4
Selected bond angles (o) and bond lengths (Å) for 4j, [CpCo{(m
-NO)2(C7H10)}], [TpiPr,MeCo(CO)2] and [Tp*Co(NO)].
[TpiPr,MeCo(CO)2] [28]
[Tp*CoNO] [20]
4j
[CpCo{(m-NO)2(C7H10)}] [17c]
CoeN(1)
CoeN(2)
e
1.625(5)
e
1.791(2)
1.793(2)
1.256(2)
1.248(3)
1.498(3)
1.504(3)
2.0140(19)
2.057(2)
2.0065(19)
85.45(9)
e
1.761(3)
1.764(3)
1.252(4)
1.247(4)
1.492(5)
1.487(5)
e
e
N(1)eO(1)
N(2)eO(2)
N(1)eC(1)
N(2)eC(2)
N(3)eCo
N(5)eCo
N(7)eCo
N(1)eCo-N(2)
CoeN(1)eO(1)
e
1.161(6)
e
e
e
e
e
e
1.985(8)
2.087(11)
2.002(12)
e
2.010(4)
2.009(3)
e
e
e
e
87.6(1)
e
e
173.5(6)
presence of 10 equiv. of norbornene in THF at ꢀ78 ꢁC followed by
warming to room temperature led to the dinitrosoalkane adduct 4j
in 65% yield (Table 3, entry 10). Attempts to observe the reaction
intermediate [Tp*Co(NO)2] formed from the reaction of 2 and KTp*
in THF at room temperature by react-IR spectroscopy failed and led
only to the observation and isolation of the previously reported
mono-nitrosyl complex [Tp*Co(NO)] [20].
Single crystals of 4j were grown by slow diffusion of pentane
into a concentrated dichloromethane solution. An ORTEP diagram
derived from a single crystal X-ray diffraction experiment is pre-
sented in Fig. 3 and selected bond angles and bond lengths are
presented in Table 4 along with those of related complexes. In the
solid state, the asymmetric unit contains a single molecule of 4j and
methylene chloride. The five-coordinate cobalt center of 4j possess
near perfect square-based pyramidal geometry (Tau factor ¼ 0.04)
[29]. Both CoeN and NeO bond lengths of the dinitrosoalkane
ligand reflect those reported for the cyclopentadienyl analogue as
does the N(1)eCoeN(2) bite angle (Table 4) [17c]. Consistent with
the square-based geometry at cobalt the equatorial arms of the tris-
(pyrazolyl)borate ligand demonstrate closer contacts to the metal
centre than the axial ligand (equatorial CoeN(3) 2.015(4) and
CoeN(7) 2.005(3) Å; axial CoeN(5) 2.059(4) Å). Similar observa-
tions have been made before in tris(pyrazolyl)borate chemistry
and [TpiPr,MeCo(CO)2], which also demonstrates a square-based
Table 5
Scope of the reaction of in situ generated [{L2X}Co(NO)2] complexes from 2 and [M{L2X}] with alkenes.
[M{L2X}]
Alkene
Product
Yield
[KTp*]
-
4k
4l
62%a
[Li{tBuMe2SiC5H4}]
86%b
[KTp*]
R2]R4] e(CH2CH]CH)e R1]R3]H
R2]R4] e(CH2CH]CH)e R1]R3]H
R1]R4]CO2Me R2]R3]H
4m
4n
4o
56%a
88%b
31%a
[Li{tBuMe2SiC5H4}]
[KTp*]
[Li{tBuMe2SiC5H4}]
4p
67%c
-
-
[NaCp]
endo-
4q D
exo-4q
77%d
þ
a
Reactions were conducted at 0.1 M in alkene, by addition of a solution of the anion salt (1.0e1.2 equiv.) in THF to a solution of 2 and alkene (10 equiv.) in THF at ꢀ78 ꢁC.
Following addition reactions were warmed to room temperature and stirred overnight.
b
Alkene concentration was 0.2 M.
The reaction was conducted at rt using 20 equiv. of 2,3-dimethylbut-2-ene (0.2 M), 1 h reaction time.
The reaction mixture was quenched at ꢀ78 ꢁC after 0.5 h, 2 equiv. of alkene (0.2 M), 1:1 mixture of endo and exo-isomers.
c
d

3980
M.R. Crimmin et al. / Journal of Organometallic Chemistry 696 (2011) 3974e3981
pyramidal geometry and possesses a cobalt center with an asym-
metrically bound tris(pyrazolyl)borate ligand (equatorial Co-N
1.985(8) and 2.002(2) Å; axial CoeN 2.087(11) Å) [28].
4. Summary and conclusions
The synthesis and characterization of a number of four and five-
coordinate {Co(NO)2}10 complexes by salt-metathesis reactions of
[(TMEDA)Co(NO)2][BPh4] with various mono-anionic ligands, along
with their reaction with alkenes, is reported. In support of existing
precedent, four coordinate complexes are thermally robust and
readily isolable species while five coordinate complexes are ther-
mally unstable transient intermediates that readily undergo
dissociation of an NO ligand. These latter complexes may be trap-
ped by alkenes to form the corresponding metal dinitrosoalkane
complexes. We are continuing to study the chemistry of dinitrosyl
complexes of the late transition metals.
Despite the apparent Cs-symmetry of complex 4j in the solid-
state, at 298 K in CDCl3, C6D6 and d8-toluene the metal bound
pyrazolyl groups are equivalent as observed by 1H NMR spec-
troscopy. Variable temperature experiments on 4j confirmed
that this observation was due to fast exchange of the pyrazolyl
ligands in the high temperature regime, and cooling of a d8-
toluene solution of 4j to 193 K revealed decoalescence of both
the methyl and methine proton resonances ascribed to the pyr-
azolyl moieties. Over the same temperature range resonances
attributed to the bicyclic hydrocarbon fragment showed no
decoalescence behaviour. While the methyl groups of Tp* split
into two pairs of resonances consistent with ‘freezing-out’ of the
Cs-symmetric structure observed in the solid state, perhaps most
informative was the decoalescence of the methine resonance;
this was present as a singlet at 5.60 ppm at 298 K but this signal
split into two singlet resonances at 5.70 and 5.77 ppm in a 1:2
ratio on cooling to 193 K. From the coalescence temperature
(Tc ¼ 216 ꢄ 3 K) and maximum peak separation (Dn ¼ 38 ꢄ 2 Hz)
the activation energy of the interconversion process may be
Acknowledgements
M.R.C. acknowledges the Royal Commission for the Exhibition of
1851 for provision of a research fellowship. M.R.C. is also indebted
to Antonio diPasquale for help with single crystal diffraction
experiments. N.C.T. thanks the Max-Planck-Gesellschaft for a
stipend. We are grateful for financial support from the NSF in the
form of a research grant to R.G.B. (CHE-0841786).
calculated as
D
Gy ¼ 10.6 ꢄ 0.2 kcal molꢀ1. Although the process
describes the interconversion of the axial and equatorial pyr-
azolyl ligands of the face-capping Tp* ligand, it remains possible
that this interconversion occurs via either (i) ligand dissociation,
bond rotation and ligand association or (ii) a Berry pseudo-
rotation. The current data cannot discriminate between these
mechanisms.
Appendix. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2011.04.038.
References
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