Conversion of (η5-C5H5)Co(PPh 3)2 and Nitro Compounds to Mononuclear η 1(N)-Nitrosoalkyl and Dinuclear μ-η1(N):η 2(N,O)-Nitrosoaryl Complexes

Article abstract of DOI:10.1021/om034083e

The room-temperature reaction of (η⁵-C₅H ₅)Co(PPh₃)₂ (5) with aliphatic nitro compounds, RNO₂, where R = Me, Et, iPr, leads to formation of the corresponding η¹-nitroso derivativ

Full text of DOI:10.1021/om034083e

5
268
Organometallics 2003, 22, 5268-5273
5
Con ver sion of (η -C H )Co(P P h ) a n d Nitr o Com p ou n d s
5
5
3 2
1
to Mon on u clea r η (N)-Nitr osoa lk yl a n d Din u clea r
1
2
µ-η (N):η (N,O)-Nitr osoa r yl Com p lexes
J oseph M. O’Connor* and Kevin D. Bunker
Department of Chemistry and Biochemistry, 9500 Gilman Drive, University of California at
San Diego, La J olla, California 92093-0358
Received August 2, 2003
5
The room-temperature reaction of (η -C
5
H
5
)Co(PPh
3
)
2
(5) with aliphatic nitro compounds,
1
RNO
(
2
, where R ) Me, Et, iPr, leads to formation of the corresponding η -nitroso derivatives
5
1
η -C
5
H
5
)Co(PPh
3
)[η (N)-N(O)R] (6-Me, R ) Me; 6-Et, R ) Et; 6-Pr, R ) iPr). Aromatic nitro
, where Ar ) -C , -p-C CtCTMS, undergo reaction with 5 to give
compounds, ArNO
2
6
H
H
5
6 4
H
1
2
5
1
2
the µ-η :η -nitroso derivatives, [(η -C
R ) -p-C CtCTMS). Complexes 6-Me and 7-Ar were characterized by X-ray crystal-
lographic analyses.
5
5
)Co]
2
[µ-η (N):η (N,O)-N(O)R]
2
(7-Ph, R ) -C
6 5
H ; 7-Ar,
6
H
4
In tr od u ction
Nitroso-alkyl and -aryl complexes are of interest as
Sch em e 1
intermediates in the metal-catalyzed carbonylation of
nitroaromatics, as inhibitors of cytochrome catalytic
1
function, and as models for metal-dioxygen complexes.
Among the useful synthetic routes toward metal-
nitroso complexes is the deoxygenation of organic nitro
compounds by metal-carbonyl and metal-phosphine
2
-5
complexes.
In previous reactions of this type, the
mononuclear metal-nitroso products contain a side-
2
bound η (N,O)-N(O)R nitroso ligand. For example, Ber-
man and Kochi reported the reaction of Ni(PPh3)4 (1)
nitroaryls undergo reaction with 5 to give the dinuclear
2
1
2
5
1
2
with 2-nitro-2-methylpropane to give the η (N,O)-nitroso
µ-η :η -nitroso derivatives [(η -C H )Co] [µ-η (N):η -
5
5
2
2
complex 2 and triphenylphosphine oxide (Scheme 1).
(N,O)-N(O)R] (7-Ph, R ) -C H ; 7-Ar, R ) -p-C H -
2
6
5
6
4
In a similar fashion, the carbonyl complex Ru(dppe)-
CtCTMS). To our knowledge, this is the first example
(
CO)3 (3) undergoes reaction with nitrobenzene to give
in which a metal complex undergoes reaction with a
2
3
1
5,6
the η (N,O)-nitroso complex 4 and carbon dioxide.
nitro compound to give an η (N)-nitroso complex, and
the first observation wherein the nature of the nitro
organic group (alkyl or aryl) determines the binding
2
Detailed mechanistic studies, by Kochi and by Glad-
felter,3 indicated that both reactions proceed via an
oxygen atom transfer induced by a prior electron
transfer from the metal to the nitro substrate.
1
1
2
mode (η (N) or µ-η (N):η (N,O)) in products formed from
the same starting complex.
Herein we report that nitroalkanes undergo reaction
5
4
1
with (η -C5H5)Co(PPh3)2 (5) to give the known η -
Resu lts a n d Discu ssion
5
1
nitroso derivatives (η -C5H5)Co(PPh3)[η (N)-N(O)R] (6-
Me, R ) Me; 6-Et, R ) Et; 6-Pr, R ) iPr). In contrast,
When nitromethane (0.074 mmol) is added to a
benzene-d6 solution (0.32 mL) of the cobalt bis(phos-
7
1
phine) complex 5 (0.018 mmol), the η (N)-nitrosomethane
complex 6-Me is formed within 90 min at room temper-
*
To whom correspondence should be addressed. E-mail:
jmoconnor@ucsd.edu.
1) (a) Lee, J .; Chen, L.; West, A. H.; Richter-Addo, G. B. Chem. Rev.
(
1
ature (Scheme 2). Integration of the H NMR resonances
2
002, 102, 1019-1065. (b) Richter-Addo, G. B. Acc. Chem. Res. 1999,
for 6-Me at δ 4.42 (s, 5H, C5H5) and 2.58 (d, J PH ) 2.5
Hz, 3H, CoNOCH3), relative to the methyl hydrogen
resonance of the toluene internal standard, indicated a
56% yield of 6-Me. An infrared spectrum (NaCl) of the
crude reaction mixture exhibited a band attributed to
3
2, 529-536.
(
2) Berman, R. S.; Kochi, J . K. Inorg. Chem. 1980, 19, 248-254.
3) (a) Skoog, S. J .; Gladfelter, W. L. J . Am. Chem. Soc. 1997, 119,
(
1
1049-11060 and references therein. (b) Skoog, S. J .; Cambell, J . P.;
Gladfelter, W. L. Organometallics 1994, 13, 4137-4139.
4) (a) Weiner, W. P.; Bergman, R. G. J . Am. Chem. Soc. 1983, 105,
922-3929. (b) Weiner, W. P.; White, M. A.; Bergman, R. G. J . Am.
Chem. Soc. 1981, 103, 3612-3614.
5) The conversion of nitro compounds to η -nitroso porphyrin
(
3
-
1
the ν(NO) stretch at 1301 (s) cm and strong bands
1
(
2
complexes in the presence of sodium dithionite involves a nitro to
nitroso reduction, followed by metal binding of RNO: (a) Ricoux, R.;
Boucher, J .-L.; Mansuy, D.; Mahy, J .-P. Biochem. Biophys. Res.
Commun. 2000, 278, 217-223. (b) Renodon, A.; Boucher, J .-L.; Wu,
C.; Gachhui, R.; Sari, M.-A.; Mansuy, D.; Stuehr, D. Biochemistry 1998,
(6) The reaction of Co(PMe
3
)
4
and 2-nitrophenol gives an η -2-
nitrosophenolato complex: Klein, H.-F.; Schmidt, A.; Fl o¨ rke, U.; Haupt,
H.-J . Inorg. Chim. Acta 2003, 342, 171-178.
(7) (a) Yamazaki, H.; Hagihara, N. J . Organomet. Chem. 1977, 139,
157. (b) Wakatsuki, Y.; Yamazaki, H. Inorg. Synth. 1989, 26, 189-
200.
3
7, 6367.
1
0.1021/om034083e CCC: $25.00 © 2003 American Chemical Society
Publication on Web 11/11/2003

Nitroso-Alkyl and -Aryl Complexes
Organometallics, Vol. 22, No. 25, 2003 5269
Sch em e 2
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
for 6-Me a n d 7-Ar
6
-Me
7-Ar
mol formula
mol wt
cryst syst
space group
temp, K
a, Å
C24H23CoNOP
431.33
triclinic
P1h
100(2)
10.0499(5)
10.8886(6)
20.1659(11)
80.6070(10)
82.4910(10)
66.8450(10)
1996.39(18)
4
C32H36Co2N2O2Si2
654.67
monoclinic
P21/c
100(2)
14.722(4)
8.600(3)
12.232(4)
90
92.035(4)
90
1547.6(8)
2
1.405
b, Å
c, Å
1
F igu r e 1. ORTEP view of the η -nitroso complex 6-Me
R, deg
â, deg
with the atom-numbering scheme (50% probability thermal
ellipsoids). Hydrogen atoms are omitted for clarity.
γ, deg
3
V, Å
Z
Ta ble 2. Selected Bon d Len gth s a n d An gles for
3
Dcalcd, Mg/m
abs coeff, mm
cryst size, mm
wavelength, Å
no. of rflns collected
no. of indep rflns
1.435
0.954
6-Me
-
1
1.180
3
Bond Lengths (Å)
0.25 × 0.25 × 0.10 0.38 × 0.13 × 0.09
0.710 73
0.710 73
4007
4007 (R(int) )
0.0000)
A
B
dist
A
B
dist
12561
8757 (R(int) )
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)]
N(1)
P(1)
C(20)
C(21)
C(22)
C(23)
1.7822(16)
2.1835(5)
2.1099(19)
2.0901(18)
2.1006(18)
2.1148(19)
P(1)
Co(1)
N(1)
N(1)
P(1)
P(1)
C(14)
C(24)
O(1)
C(1)
C(2)
C(8)
1.8409(18)
2.0963(19)
1.2846(19)
1.480(2)
1.8359(17)
1.8354(18)
0
.0159)
full-matrix least squares on F2
refinement method
no. of data/params
goodness of fit on F
8757/553
0.978
4007/203
1.016
2
final R indices
R1 ) 0.0324,
wR2 ) 0.0814
R1 ) 0.0391,
wR2 ) 0.0835
R1 ) 0.0525,
wR2 ) 0.1313
R1 ) 0.0631,
wR2 ) 0.1358
1.603 and -0.640
Bond Angles (deg)
R indices (all data)
A
B
C
angle
A
B
C
angle
largest diff peak and 0.530 and -0.233
N(1) Co(1) P(1)
Co(1) N(1) O(1) 126.33(12) Co(1) P(1) C(8)
90.92(5)
Co(1) P(1) C(2)
109.06(6)
119.23(6)
-
3
hole, e Å
Co(1) N(1) C(1) 123.46(13) Co(1) P(1) C(14) 118.57(6)
attributed to triphenylphosphine oxide at 1196 and 1118
-
1
cm . The NO stretch in the IR spectrum of CH3NO (8)
distance in 9. This lengthening of the N-O bond relative
to the free nitroso compounds is a reflection of signifi-
cant back-bonding from cobalt(I) to the π* orbital of the
nitroso ligand. The geometry about nitrogen is trigonal
planar, as determined by only slight deviations of the
Co (+0.004 Å), C (+0.004 Å), O (+0.004 Å), and N
(-0.012 Å) atoms from the mean plane defined by those
atoms. The C(1)-N(1)-O(1) angle of 110.18(15)° in 6-Me
is 3° smaller than in nitrosomethane and ca. 15° less
than in 9. DFT-B3LYP calculations by Niu and Hall on
-1
-1
is observed at 263 cm higher wavenumber (1564 cm
gas-phase value) than that observed for 6-Me. The E
dimer, [CH3N(O)dN(O)CH3] (9), which has significant
NO single-bond character, exhibits ν(NO) at 1282 cm
The related reactions of 5 (0.063 M) with either nitro-
ethane (3 equiv) or 2-nitropropane (3 equiv) lead to
formation of the known η -nitroso complexes 6-Et (53%
yield) and 6-Pr (38% yield), respectively. In addition to
a lower yield, the 2-nitropropane reaction takes 43 h
longer to reach completion than does reaction of 5 with
either nitromethane or nitroethane.
,
8
1
a
-
1 9
.
1
5
1
(η -C5H5)Co(PH3)[η (N)-N(O)Me] produced the same
1
2
110° C-N-O angle; thus, the relatively small 110°
angle in 6-Me does not appear to be due to steric
congestion between the nitroso ligand and the PPh3
ligand.
An X-ray crystallographic analysis of 6-Me permits a
comparison of bond distance and angle data with those
1
0
determined for both CH3NO (8) and the E dimer
CH3N(O)dN(O)CH3] (9)11 (Figure 1, Tables 1 and 2).
The spectroscopic and structural properties of the
nitroso complexes were similar to those previously
reported by Bergman for the same complexes prepared
[
For 6-Me, the N(1)-O(1) distance of 1.2845(19) Å is 0.07
Å longer than the N-O distance in nitrosomethane
1.211 Å) and slightly longer than the 1.25(2) Å N-O
5
from (η -C5H5)Co(CO)2, by the route shown in Scheme
(
4
5
3
. The key steps are alkylation of Na[(η -C5H5)Co(NO)],
migratory insertion of the alkyl ligand into the cobalt-
(
(
(
8) Luttke, W. Z. Electrochem. 1957, 61, 976.
9) Gowenlock, B. G.; Trotman, J . J . Chem. Soc. 1955, 4190-4196.
10) Turner, P. H.; Cox, A. P. J . Chem. Soc., Faraday Trans. 1978,
(12) (a) Niu, S.; Hall, M. B. J . Am. Chem. Soc. 1997, 119, 3077-
3086. (b) Niu, S.; Hall, M. B. J . Phys. Chem. 1997, 101, 1360-1365.
(c) Hofmann, P.; Padmanabhan, M. Organometallics 1983, 2, 1273-
1284. (d) Poli, R.; Smith, K. M. Eur. J . Inorg. Chem. 1999, 877-880.
2
6
, 533-559.
11) Van Meerssche, M.; Germain, G. Bull. Soc. Chim. Belg. 1959,
8, 244-257.
(

5
270 Organometallics, Vol. 22, No. 25, 2003
O’Connor and Bunker
Sch em e 3
Sch em e 4
nitrosyl bond, and trapping of the unsaturated cobalt-
benzene-d6 to give 7-Ph in 85% yield. Although 7-Ph was
nitroso intermediate by added triphenylphosphine. The
not previously characterized by NMR spectroscopy, it
5
13
yields based on (η -C5H5)Co(CO)2 ranged from 9 to 32%
was structurally characterized. In addition to the
for the four-step sequence.
relatively high field C5H5 hydrogen chemical shift in the
H NMR spectrum of 7-Ph, the phenyl carbon reso-
1
In addition to being a relatively lengthy route toward
complexes 6, the alkyl halide route shown in Scheme 3
is presumably not applicable to the synthesis of ni-
trosoaryl complexes from aryl halides. The reaction of
nances are observed at 164.5 (ipso), 129.0, 124.6, and
1
3
1
122.3 ppm in the C{ H} NMR spectrum (C6D6). These
values are very similar to those for nitrosobenzene
1
4
5
with nitroaryls should in principle provide a conve-
(166.1, 135.5, 129.4, and 120.9 ppm).
nient route toward nitrosoaryl analogues of 6. Despite
the employment of reaction conditions and concentra-
tions similar to those utilized for the synthesis of 6, the
room-temperature reaction of 5 (0.031 mmol) with
nitrobenzene (0.092 mmol) led to formation of the
An interesting test of chemoselectivity in the nitro to
nitroso conversion is found in the reaction of 5 with 1-(4-
nitrophenyl)-2-(trimethylsilyl)acetylene. Alkynes are
known to undergo facile reaction with 5 to give high
5
2
1
yields of cobalt-alkyne complexes, (η -C5H5)Co(η -R Ct
1
2
5
1
2
dinuclear µ-η :η -nitroso complex [(η -C5H5)Co]2[µ-η (N):
CR )(PPh3), as well as cobaltacyclopentadienes and
2
15
η (N,O)-N(O)C6H5]2 (7-Ph, Scheme 4) in 21% yield. It
arenes derived therefrom. In this work, 1-(4-nitro-
1
was readily apparent from the high-field H NMR
phenyl)-2-(trimethylsilyl)acetylene (0.013 mmol, 0.039
(
benzene-d6) chemical shift of the cyclopentadienyl
M) underwent reaction with 5 (0.010 mmol) in benzene-
5
hydrogen resonance at δ 4.04 that 7-Ph was not a simple
analogue of 6, for which the average cyclopentadienyl
hydrogen resonance was observed at δ 4.43. The dimer
nature of the complex was revealed from the mass
spectrometry analysis, and a literature search revealed
that 7-Ph had been previously prepared from nitrosoben-
d6 at room temperature (24 h) to give the dimer [(η -
1
2
C5H5)Co]2[µ-η (N):η (N,O)-(TMSCtC)C6H4(NO)}2 (7-Ar)
in 34% yield.
An X-ray crystallographic analysis of 7-Ar confirmed
the centrosymmetric µ-nitroso formulation, with non-
bonded Co-Co and Co(1)-O(1A) distances of 2.9908(9)
and 2.715 Å, respectively (Figures 2 and 3, Tables 1 and
3). The Co(1)-N(1) distance of 1.920(2) Å in 7-Ar is
slightly shorter than the Co(1)-N(1A) distance of 1.938-
5
2
13
zene and (η -C5H5)Co(η -CH2dCH2)2 by Floriani.
A
more convenient starting material than the bis(ethyl-
ene) complex is the bis(phosphine) 5, which we have
determined to also undergo reaction with nitrosoben-
zene over the course of 90 min at room temperature in
(
14) Boyd, A. S. F.; Browne, G.; Gowenlock, B. G.; McKenna, P. J .
Organomet. Chem. 1988, 345, 217-220.
15) For leading references see: (a) O’Connor, J . M.; Bunker, K. D.
(
(
13) Stella, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J . Chem.
J . Organomet. Chem. 2003, 671, 1-7. (b) Vollhardt, K. P. C. Angew.
Chem., Int. Ed. Engl. 1984, 23, 539.
Soc., Dalton Trans. 1988, 545-547.

Nitroso-Alkyl and -Aryl Complexes
Organometallics, Vol. 22, No. 25, 2003 5271
that appear to stem from nonbonding interactions
between the trimethylsilyl groups on neighboring dimers
within the lattice (Figure 4, in the Supporting Informa-
tion).
While the modest yields for the formation of 6 and 7
preclude mechanistic studies, it is reasonable that the
reaction mechanism may involve an oxygen atom trans-
fer induced by a prior electron-transfer process, as was
2
3
established by Kochi and by Gladfelter for the forma-
tion of complexes 2 and 4 (Scheme 1). Scheme 5 depicts
a possible mechanism for the 5 to 6 conversion, by
analogy to the Kochi mechanism for the conversion of
1
2
F igu r e 2. ORTEP view of the µ-η (N):η (N,O)-nitroso
complex 7-Ar, with the atom-numbering scheme (50%
probability thermal ellipsoids). Hydrogen atoms are omit-
ted for clarity.
5
1 to 2. Loss of triphenylphosphine from 5 to give [(η -
C5H5)Co(PPh3)] (10) is well established for substitution
1
7,18
reactions of 5.
Single-electron transfer (possibly
inner-sphere) from 10 to RNO2 leads to the ion pair 11,
involving a cobalt radical cation and nitro radical anion.
Kochi has demonstrated that nitroaryl radical anions
generated by electrochemical techniques in the presence
2
of phosphine do not give phosphine oxide. Collapse of
F igu r e 3. Side view of 7-Ar showing deviations of atoms
from the O(1)-N(1)-O(1A)-N(1A) plane.
the ion pair 11 would give intermediate 12, followed by
cyclization to 13. Subsequent fragmentation of 13 to
triphenylphosphine oxide and trapping of the resultant
Ta ble 3. Selected Bon d Len gth s a n d An gles for
2
7
-Ar
η -nitroso intermediate 14 with triphenylphosphine
would then give the nitroso species 6. Computational
Bond Lengths (Å)
studies by Niu and Hall indicate that intermediate 14
A
B
dist
A
B
dist
12
(
R ) Me) would have a weak Co-O interaction, with
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
Co(1)
O(1)
N(1)
1.9032(19)
1.920(2)
1.938(2)
2.066(3)
2.072(3)
2.044(3)
2.078(3)
Co(1)
N(1)
N(1)
C(4)
C(7)
C(8)
C(16)
O(1)
C(1)
C(7)
C(8)
Si(1)
2.073(3)
1.374(3)
1.411(3)
1.437(4)
1.203(4)
1.848(3)
a 2.09 Å Co-O distance and a Co-N-C angle of 156.8°.
The calculations further indicated that 14 (R ) Me)
associates phosphine to give the PH3 analogue of 6 with
essentially no barrier. Alternative mechanisms would
include one involving an outer-sphere single electron
transfer from 5 to RNO2, followed by recombination to
N(1A)
C(12)
C(13)
C(14)
C(15)
1
9
1
8-electron bis(phosphine) analogues of 12 and 13, with
Bond Angles (deg)
angle
loss of OPPh3 from 13 directly giving 6. A mechanism
of this type is disfavored by our observation that the
rate of reaction for 5 and nitromethane is retarded by
A
B
C
A
B
C
angle
O(1) Co(1) N(1)
42.13(8) O(1) N(1) Co(1A) 108.92(15)
O(1) Co(1) N(1A) 91.73(9) C(1) N(1) Co(1A) 127.52(18)
N(1) Co(1) N(1A) 78.34(10) Co(1) N(1) Co(1A) 101.66(10)
2
0
added triphenylphosphine.
N(1) O(1) Co(1)
O(1) N(1) C(1)
O(1) N(1) Co(1)
69.58(12) C(2) C(1) N(1)
115.4(2) C(6) C(1) N(1)
68.29(13) C(4) C(7) C(8)
123.3(2)
116.9(2)
177.0(3)
175.0(3)
In principle, intermediate 14 could lead to both
mononuclear η -nitroso products 6 and dinuclear µ-η :
η -nitroso products 7. Floriani speculated that the
1
1
2
C(1) N(1) Co(1) 119.58(17) C(7) C(8) Si(1)
structural features of 7-Ph suggest that its formation
5
2
from (η -C5H5)Co(η -CH2dCH2)2 and nitrosobenzene
(2) Å, and both Co-N distances are much longer than
may actually occur via reaction of the nitrosobenzene
1
the 1.7822(16) Å Co-N distance observed in the η -
nitroso complex 6-Me. The N(1)-O(1) distance of 1.374-
13
dimer with cobalt. If a mechanism similar to that
shown in Scheme 5 is operative, then the nitrosoaryl
(3) Å in 7-Ar is also significantly longer than the
1
.2845(19) Å N-O distance in 6-Me and is consistent
(
16) Baldridge, K. K.; Donovan-Merkert, B. T.; O’Connor, J . M.; Lee,
2
with a Co(III) formal oxidation state in 7-Ar. The η -
nitroso group exhibits a Co(1)-N(1)-O(1) angle of
6
L. I.; Closson, A.; Fandrick, D.; Tran, T.; Bunker, K.; Fouzi, M.; Gantzel,
P. Org. Biomol. Chem. 2003, 1, 763-766 and references therein.
(
17) Complex 5 undergoes rapid reversible phosphine dissociation
1
4.29(13)° in 7-Ar which compares with the η -nitroso
in toluene: J anowicz, A. H.; Bryndza, H. E.; Bergman, R. G. J . Am.
Chem. Soc. 1981, 103, 1516-1518.
Co-N-O angle in 6-Me of 126.33(12)° and the Co(1A)-
N(1)-C(1) angle of 127.52(18)° in 7-Ar. The O(1)-N(1)-
O(1A)-N(1A) plane bisects the plane of the cyclopen-
tadienyl ligands at an angle of 8.1°, and the mean plane
of the phenyl ring bisects the O(1)-N(1)-O(1A)-N(1A)
plane by 17.3° (Figure 3). The C(4)-C(7)-C(8) angle and
C(7)-C(8)-Si angle are 177.0(3) and 175.0(3)°, respec-
tively, and there is a pronounced bending of the alkyne
sp carbons and the silicon atom away from the O(1)-
N(1)-O(1A)-N(1A) plane. The deviations C(7), C(8),
and Si(1) from that plane are 1.72, 2.19, and 3.06 Å,
respectively. The bending at C(7) and C(8) is attributed
to the well-established ease of bending about sp-hybrid-
ized carbon centers16 and crystal lattice packing forces
(18) Triphenylphosphine does not react with nitro compounds under
the mild conditions employed for conversion of 5 to 6 and 7: Boyer, J .
H.; Ellzey, S. E., J r. J . Org. Chem. 1961, 26, 4684-4685.
(
to give an analogue of 6, the calculations also indicate that 14 is
probably not an intermediate in the NO insertion shown in Scheme 3
(see Figure 3 in ref 12a).
19) Although calculations indicate that 14 readily associates PH
3
(
20) It proved difficult to monitor the reaction of 5 with nitromethane
1
1
by H NMR spectroscopy due to severe line broadening in the H NMR
spectrum. However, after 90 min at room temperature, the signals
became sharp and it was possible to establish
integration relative to toluene as internal standard (see the Experi-
mental Section). A H NMR spectrum of a benzene-d
solution of 5 (11.9 mg, 0.018 mmol, 0.027 M), triphenylphosphine (48.4
mg, 0.184 mmol, 0.272 M), nitromethane (3.0 µL, 0.055 mmol), and
toluene (internal standard) exhibited sharp resonances and essentially
no reaction after 90 min. After 7 h a H NMR spectrum indicated an
11% yield of 6-Me and after 24 h only a 28% yield.
a 56% yield by
1
6
(0.68 mL)
1

5
272 Organometallics, Vol. 22, No. 25, 2003
O’Connor and Bunker
Sch em e 5
Sch em e 6
readily available nitroalkanes by metal-phosphine
complexes. This report represents a second example of
such a nitroalkane to metal-nitrosoalkane transforma-
tion and the first reduction of nitroalkanes by an
1
organometallic complex to give η (N)-nitrosoalkane
products. Although the yields are modest, this synthetic
route is currently the most direct approach to the cobalt
1
η -nitrosoalkanes described herein. In addition, the
reactions of [(η -C5H5)Co(PPh3)2] (5) with nitro com-
ligand would be generated within the coordination
sphere of cobalt and therefore would be unlikely to
involve in situ formation of a nitroso dimer. On the basis
of the barrierless association of PH3 to 14, it is unlikely
that 14 would undergo dimerization to give 7 at a rate
competitive with trapping by triphenylphosphine. The
5
pounds are unusual in that nitroalkanes give mono-
1
nuclear η (N)-nitroso products, whereas nitroaryls give
1
2
µ-η (N):η (N,O)-nitroso products.
Exp er im en ta l Section
1
1
2
preference for η binding with nitrosoalkyls and µ-η :η
binding for nitrosoaryls may be due to the better
π-acceptor ability of the nitrosoaryl ligands relative to
the nitrosoalkyl ligands. Computational studies indicate
η5-C
1
(
5 5 3
H )Co(P P h )[η (N)-N(O)Me] (6-Me). Under a nitro-
gen atmosphere, nitromethane (4.0 µL, 0.074 mmol) was
syringed into an NMR tube containing a benzene-d (0.32 mL)
6
2
2
5
that the η (N,O)-nitroso complex (PH3)2Pt[η (N,O)-N(O)-
5 5 3 2
solution of (η -C H )Co(PPh ) (5; 11.7 mg, 0.018 mmol, 0.056
2
CF3] is more stable than (PH3)2Pt[η (N,O)-N(O)CH3],
consistent with the better π-acceptor ability of CF3NO.
The observation that (PMe3)2Co(Me)2(NO) undergoes
M) and toluene (internal standard). The tube was sealed under
a nitrogen atmosphere and maintained at room temperature
2
1
1
for 90 min, at which time a H NMR spectrum of the sample
1
2
indicated a 56% yield of 6-Me.
rearrangement to the µ-η :η -nitrosomethane complex
2
2
In a preparative-scale experiment, nitromethane (245 µL,
.52 mmol) was syringed into a benzene solution (15 mL) of 5
1
5
provides an interesting counterpoint to the forma-
tion of the cobalt-nitrosomethane compound 6-Me
Scheme 6). The differing binding modes for nitroso-
4
(0.980 g, 1.51 mmol) under a nitrogen atmosphere. After the
(
mixture was stirred at room temperature for 20 h, a portion
of the sample was removed and examined by IR spectroscopy
methane in 6 and 15 may be the result of a more
electron-rich metal center in 15, which would favor a
cobalt(III) product.
(see below). The solvent was removed from the remainder of
the sample and the residue washed with hexanes (3 mL). The
crude product was then dissolved in benzene (∼0.25 mL), and
hexanes (∼1.5 mL) were added to precipitate a mixture of
triphenylphosphine oxide (TPPO) and black crystals of 6-Me.
Several X-ray-quality crystals of 6-Me were selected, and a
crystal structure determination was carried out. Data for 6-Me
are as follows. IR (neat, NaCl; crude mixture containing 6-Me
and TPPO): 1589 (w, TPPO), 1481, 1436, 1301, 1195 (s,
Con clu sion s
Ligand substitution by nitrosoalkanes provides the
1
most direct route to η (N)-nitrosoalkane complexes;
however, this approach has been primarily limited to
t
nitroso compounds with bulky alkyl groups ( BuNO and
-
1 1
TPPO), 1118 (s, TPPO) cm . H NMR (C
6
D
6
): δ 2.58 (d, 3H,
), 7.05 (m, 9H, C ),
6 5 6 6
H ). C{ H} NMR (C D ): δ 66.1, 81.4, 129.4,
i
PrNO), due in part to the favorable formation of nitroso
J
PH ) 2.5 Hz, CH
3
), 4.42 (s, 5H, C
.75 (m, 6H, C
34.4 (d, J PC ) 11.4 Hz), 135.7, 160.5.
5
H
5
6 5
H
dimers and tautomers in the less bulky nitroso com-
13
1
7
1
5
,23
pounds.
A promising route toward less sterically
encumbered nitrosoalkyl complexes is the reduction of
(
23) (a) Mansuy, D.; Battioni, P.; Chottard, J .-C.; Riche, C.; Chiaroni,
(
21) Fantucci, P.; Pizzotti, M.; Porta, F. Inorg. Chem. 1991, 30,
A. J . Am. Chem. Soc. 1983, 105, 455-463. (b) Pilato, R. S.; McGettigan,
C.; Geoffroy, G. L. Organometallics 1990, 9, 312-317. (c) Mansuy, D.;
Dreme, M.; Chottard, J . C. J . Organomet. Chem. 1978, 161, 207-220.
(d) Mansuy, D.; Dreme, M.; Chottard, J .-C.; Girault, J .-P.; Guilhem,
J . J . Am. Chem. Soc. 1980, 102, 844-845.
2
277-2282.
(
22) (a) Klein, H.-F.; Karsch, H. H. Chem. Ber. 1976, 109, 1453-
1
1
464. (b) Middleton, A. R.; Wilkinson, G. J . Chem. Soc., Dalton Trans.
981, 1898-1905.

Nitroso-Alkyl and -Aryl Complexes
Organometallics, Vol. 22, No. 25, 2003 5273
η5-C
H
)Co(P P h
)[η (N)-N(O)Et] (6-Et). In a fashion
1
0-5% ethyl acetate/hexanes gradient) to give 7-Ar (12.1 mg,
16% yield) as an air-sensitive black crystalline powder, mp
<130 °C dec. IR (NaCl, neat): 3072 (m), 2953 (vs), 2898 (m),
2153 (s, CtC), 1725 (m), 1589 (m), 1531 (w), 1483 (s), 1409
(s), 1247 (s), 1233 (w), 1155 (w), 1104 (m), 1021 (m), 838 (s),
(
5
5
3
similar to the reaction above, a dry benzene-d
solution of 5 (18.6 mg, 0.029 mmol, 0.063 M) and nitroethane
6.2 µL, 0.086 mmol) gave a 53% yield of 6-Et after 4 h at room
temperature. IR (neat, NaCl; crude reaction mixture contain-
ing 6-Et and TPPO): 1585, 1479, 1434, 1335, 1305, 1198 (s,
6
(0.46 mL)
(
-
1
1
801 (s), 723 (s) cm . H NMR (C
6
D
6
): δ 0.28 (s, 18H, TMS),
), 7.45 (d, 4H, J ) 8.0 Hz, C ), 7.60-8.20
): δ 0.26 (s, 18H, TMS), 4.19
), 7.85-8.20 (br
): δ 0.4, 81.2, 94.6, 106.9,
-
1 1
TPPO), 1118 (s, TPPO) cm . H NMR (C
6
D
6
) of 6-Et: δ 1.16
), 3.23 (q, 2H, J ) 7.0 Hz, CH ), 4.43
), 7.03 (m, 9H, C ), 7.72 (m, 6H, C ).
)[η (N)-N(O)iP r ] (6-P r ). In a fashion
(0.46 mL)
3.92 (s, 10H, C
(br m, 4H, C
(s, 10H, C
m, 4H, C
118 (v br), 119.4, 133.0, 164.6. C{ H} NMR (CDCl
81.0, 94.4, 105.6, 119 (v br), 118.8, 132.8, 163.7. HRMS
(FAB): calcd for C32 Si , 654.0979; found, 654.0975.
5
H
5
1
6 4
H
(
(
t, 3H, J ) 7.0 Hz, CH
s, 5H, C
(
3
2
6
H
4
). H NMR (CDCl
3
5
H
5
6
H
5
6
H
5
5
H
5
6 4
), 7.37 (d, 4H, J ) 8.8 Hz, C H
5
1
13
1
η -C
5
H
5
)Co(P P h
3
6
H
4
). C{ H} NMR (C
6
D
6
1
1
3
similar to the reaction above, a dry benzene-d
6
3
): δ -0.4,
solution of 5 (18.6 mg, 0.029 mmol, 0.063 M) and 2-nitropro-
pane (7.7 µL, 0.086 mmol) gave a 38% yield of 6-Pr after 47 h
at room temperature. IR (neat, NaCl; crude mixture of 6-Pr
and TPPO): 1586, 1480, 1435, 1378, 1357, 1309, 1235, 1198
H
36Co
2
N
2
O
2
2
X-r a y Str u ctu r e Deter m in a tion of 6-Me. In the drybox,
3
a single crystal with dimensions 0.25 × 0.25 × 0.10 mm was
-
1
1
(
3
6
(
4
6
s, TPPO), 1118 (s, TPPO) cm . H NMR (C
6
D
6
): δ 1.03 (t,
immersed in Paratone and placed on a glass fiber. Data were
collected on a Bruker SMART (APEX) CCD diffractometer by
using a graphite monochromator with Mo KR radiation (λ )
0.710 73 Å) at 100 K. The structure was solved by direct
H, J ) 6.0 Hz, CH
.5 Hz, CH), 7.00 (m, 9H, C
): δ 0.90 (t, 3H, J ) 6.0 Hz, CH
3
), 4.45 (s, 5H, C
), 7.72 (m, 6H, C
), 4.49 (s, 5H, C
), 7.61 (m,
5 5
H ), 4.77 (quint, 1H, J )
1
6
H
5
6
H
5
). H NMR
),
THF-d
8
3
5 5
H
2
.94 (quint, 1H, J ) 6.5 Hz, CH), 7.30 (m, 9H, C
6
H
5
methods and refined by full-matrix least squares on F . All
H, C
[
6
5
H
(η -C
5
).
hydrogen atoms were geometrically fixed on their attached
1
2
24
5
H
5
)Co]
2
[µ-η (N):η -(N,O)-N(O)C
6
H
5
]
2
(7-P h ). In the
atoms during the refinement. Table 1 summarizes the crystal
drybox, complex 5 (20 mg, 0.031 mmol, 0.054 M) and toluene
internal standard) were dissolved in dry benzene-d (0.57 mL).
Nitrobenzene (9.5 µL, 0.092 mmol, 0.16 M) was added by
data collection and refinement parameters. The ORTEP draw-
ing in Figure 1 illustrates one of the two crystallographically
distinct molecules in the unit cell. Data for both molecules in
the unit cell are given in the Supporting Information.
X-r a y Str u ctu r e Deter m in a tion of 7-Ar . A single crystal
(
6
syringe, and the tube was sealed under a nitrogen atmosphere.
1
After 22 h at room temperature, a H NMR spectrum of the
3
sample indicated a 21% yield of 7-Ph.
In a preparative-scale experiment, nitrobenzene (0.22 mL,
with dimensions 0.38 × 0.13 × 0.09 mm was immersed in
Paratone and placed on a glass fiber. Data were collected on
a Bruker SMART (APEX) CCD diffractometer by using a
graphite monochromator with Mo KR radiation (λ ) 0.710 73
Å) at 150 K. The crystal was a twin, and the reflections were
resolved using GEMINI (v. 1.02) and then integrated using
SAINT (v6.35A). The data were massaged by SADABS (v2.04)
and TWINABS (v1.02). The structure was then refined by full-
2
.13 mmol) was added by syringe to a benzene (15 mL) solution
of 5 (0.455 g, 0.702 mmol). After the mixture was stirred for
3 h at room temperature, the volatiles were evaporated and
2
the residue chromatographed (alumina, 0-5% ethyl acetate/
hexanes gradient) to give 7-Ph (24.0 mg, 7.4% yield) as an air-
sensitive black crystalline powder. IR (NaCl, neat): 3053 (m),
2
24
2
923 (vs), 2853 (s), 1725 (m), 1589 (m), 1525 (w), 1480 (s), 1435
matrix least squares on F . All hydrogen atoms were geo-
metrically fixed on their attached atoms during the refinement.
Table 1 summarizes the crystal data collection and refinement
parameters.
(
s), 1345 (m), 1276 (m), 1198 (m), 1118 (s), 1071 (w), 1042 (vw),
1
1
4
C
026 (w), 744 (s), 720 (s), 695 (vs) cm-1. H NMR (C
.04 (s, 10H, C ), 7.02 (m, 6H, C ), 7.95-8.43 (br m, 4H,
). C{ H} NMR (C ): δ 81.3, 122.3, 124.6, 129.0, 164.5
of C ). HRMS (FAB): calcd for C22 , 462.0189;
found, 462.0187.
6 6
D ): δ
5
H
5
6 5
H
1
3
1
6
H
5
6 6
D
(C
i
6
H
5
2 2 2
H20Co N O
Ack n ow led gm en t. The support of the National
Science Foundation (Grant No. CHE-9975939) is grate-
fully acknowledged. Acknowledgment is made to the
donors of the Petroleum Research Fund, administered
by the American Chemical Society, for partial support
of this research.
5
1
2
[
(η -C
5
H
5
)Co]
2
[µ-η (N):η (N,O)-N(O)C
6
H
4
C≡CTMS]
2
(7-
Ar ). In the drybox, complex 5 (6.3 mg, 0.010 mmol, 0.029 M)
and toluene (internal standard) were dissolved in dry benzene-
d (0.34 mL). 1-(4-Nitrophenyl)-2-(trimethylsilyl)acetylene (2.9
6
mg, 0.013 mmol, 0.039 M) was added, and the tube was sealed
under a nitrogen atmosphere. After 24 h at room temperature,
1
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
data, as CIF files, for 6-Me and 7-Ar. This material is available
free of charge via the Internet at http://pubs.acs.org.
a
H NMR spectrum of the sample indicated a 34% yield of
7
-Ar.
In a preparative-scale experiment, 1-(4-nitrophenyl)-2-(tri-
methylsilyl)acetylene (50.6 mg, 0.231 mmol) was added to a
benzene (15 mL) solution of 5 (0.151 g, 0.233 mmol). After the
mixture was stirred for 23 h at room temperature, the volatiles
were evaporated and the residue chromatographed (alumina,
OM034083E
(
24) Sheldrick, G. M. SHELXTL, version 6.1, Software Reference
Manual; Bruker AXS: Madison, WI, 1997.