Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9291
would be very unstable, and it would partially decom-
pose, thus liberating CO that rapidly would react with
undecomposed molecules to give the electron-precise and
very stable tricarbonyl [Mo2Cp2(μ-PPh2)(CO)3(NO)].
Decomposition of D is then expectedly avoided in the
presence of isocyanide, since any ligand being present (L)
would rapidly react with D to give an electron precise
intermediate E, perhaps still retaining the nitrosyl bridge,
if we use the results of the carbonylation reactions of the
isoelectronic complexes [Mo2Cp2(μ-PCy2)(μ-X)(CO)2]
(X = alkenyl, COMe) as a model.26 The intermediate E
would easily rearrange in different ways eventually open-
ing the nitrosyl bridge, then leading to the dicarbonyl 5 or,
when L = CO, to the tricarbonyl [Mo2Cp2(μ-PPh2)-
(CO)3(NO)] as noted above. Alternatively, when L =
CNtBu the intermediate E might evolve through decar-
bonylation, perhaps induced by the presence on the same
metal atom of the relatively bulky CNtBu ligand, to give a
new unsaturated intermediate F, which then would reach
the electronic saturation by a rearrangement of the iso-
cyanide ligand, from terminal to the μ-η1:η2 bridging
mode, this being accompanied by a displacement of the
bridging nitrosyl to a terminal position, thus explaining
the formation of the isocyanide-bridged complex 6.
Concluding Remarks. The formation of the nitride-
bridged complexes 2 (which in turn are the precursors
of the nitride-bridged compounds 3 and 4) can be under-
stood as derived initially from an orbitally controlled
nucleophilic attack of a metal atom of the anion 1 to the N
atom of the nitrosyl ligand in the cations [MCp0-
(NO)(CO)2]þ. Because of the electronic and coordinative
unsaturation of the resulting trimetallic intermediate, the
system does not evolve through a CO2 elimination path-
way, as previously observed by us in the reactions of the
electron-precise anions of type [MCp(CO)2L]-, and this
gives further support to our previous hypothesis that a
close approach of the carbonyl ligands to the bridging
nitrosyl ligand in the intermediates initially formed is
essential for this particular reaction pathway to be op-
erative. Instead, we trust that the unsaturated dimolyb-
denum center derived from 1 allows and induces the
simultaneous coordination of both the N and O atoms
of the nitrosyl ligand to partially relieve this deficiency,
thus initiating a progressive weakening of the N-O bond
in the nitrosyl ligand eventually leading to its full clea-
vage, to yield metal-bound nitride and oxo ligands and
thus achieving the complete electronic saturation of the
resulting molecule. The presence of a metal initially
bound to the nitrosyl ligand is essential to the overall
process, since otherwise this ligand binds the unsaturated
dimolybdenum center in non-activated (with respect to
the N-O bond cleavage) coordination modes involving
only the nitrogen atom, as observed in the nitrosylation
reactions of the anion 1. Further work will be needed to
prove the use of the above ideas for a better understand-
ing of the ways in which transition-metal complexes can
cleave the strong bond of the NO molecule, and for the
design of polymetallic substrates able to carry out this
process in an efficient and even catalytic way.
atmosphere using standard Schlenk techniques. Solvents were
purified according to literature procedures and distilled prior to
their use.39 Petroleum ether refers to that fraction distilling in
the range 338-343 K. Compounds [MCp0(CO)2(NO)]BF4
[M = Mn (1),40 Re (2)41] and THF solutions of Na[Mo2Cp2-
(μ-PPh2)(μ-CO)2] (1)13 were prepared as described previously.
Chromatographic separations were carried out on alumina
using jacketed columns cooled by tap water (ca. 285 K) or by
a closed 2-propanol circuit, kept at the desired temperature with
a cryostat. For this purpose, commercial aluminum oxide
(Aldrich, activity I, 150 mesh) was degassed under vacuum prior
to use. The latter was mixed under nitrogen with the appropriate
amount of water to reach the activity desired. All other reagents
were obtained from the usual commercial suppliers and used as
received. IR stretching frequencies were measured in solution or
KBr discs. Nuclear Magnetic Resonance (NMR) spectra were
routinely recorded at 300.13 (1H), 121.50 (31P{1H}), or 75.47
MHz (13C{1H}) at 290 K in CD2Cl2 solutions unless otherwise is
stated. Chemical shifts (δ) are given in ppm, relative to internal
tetramethylsilane (1H, 13C) or external 85% aqueous H3PO4
(31P). Coupling constants (J) are given in hertz.
Preparation of [Mo2ReCp2Cp0(μ-N)(μ-O)(μ-PPh2)(CO)4]
(2a). Solid [ReCp0(CO)2(NO)]BF4 (0.075 g, 0.17 mmol) was
added to a freshly prepared THF solution (10 mL) containing
about 0.17 mmol of compound 1, cooled at 193 K. The mixture
turned into black-green instantaneously and was further stirred
for 30 min while allowing it to warm up to 243 K, to yield an
orange mixture. The solvent was then removed under vacuum,
the residue was extracted with a minimum CH2Cl2, and the
extract was chromatographed on alumina (activity 3.5) at 243 K.
Elution with dichloromethane-petroleum ether (1:1) gave a
minor orange fraction containing some [Mo2(μ-PPh2)(CO)3-
(NO)]. Elution with THF-petroleum ether (1:1) gave a major
orange fraction which yielded, after removal of solvents under
vacuum at 243 K, compound 2a as a brown-orange powder
(0.095 g, 62%). This compound is thermally unstable, and it
transforms into 3a above 253 K in dichloromethane solution.
Thus satisfactory elemental analysis for this compound could
not be obtained. 1H NMR (400.13 MHz, 253 K): δ 8.3-6.7 (m,
Ph, 10H), 5.50, 5.24 (2s, 2 ꢁ 5H, Cp), 5.11, 4.99, 4.94, 4.89 (4m, 4
ꢁ 1H, C5H4), 2.11 (s, 3H, Me). 13C{1H} NMR (100.63 MHz, 253
K): δ 239.7 (d, JCP = 27, MoCO), 233.7 (s, MoCO), 206.5, 206.1
(2s, ReCO), 141.5 [d, JPC= 41, C1(Ph)], 136.5 [d, JPC = 49,
C1(Ph)], 135.5-128.7 (m, Ph), 110.5 [s, C1(C5H4)], 105.2, 97.6
(2s, Cp), 83.5, 83.4, 83.1, 82.6 [4s, C2,3(C5H4)], 13.9 (s, Me).
Preparation of [Mo2ReCp2Cp0(μ-N)(μ-O)(μ-PPh2)(CO)3]
(3a). A solution of compound 2a (0.046 g, 0.05 mmol) in
dichloromethane (20 mL) was stirred at room temperature for
2 h to give a dark green solution which was filtered through
alumina (3 ꢁ 2.5 cm, activity 3.5) at 285 K. Addition of
petroleum ether to the filtrate and removal of solvents under
vacuum gave compound 3a as an emerald-green, air-sensitive
powder (0.039 g, 88%). Anal. Calcd for C31H27Mo2NO4PRe: C,
1
42.00; H, 3.07; N, 1.58. Found: C, 41.60; H, 3.25, N, 1.70. H
NMR (200.13 MHz): δ 8.2-6.5 (m, Ph, 10H), 6.05 (s, 5H, Cp),
5.59 (d, JPH = 1, 5H, Cp), 4.96, 4.68 (2m, 2 ꢁ 1H, C5H4), 4.75
(m, 2H, C5H4), 1.71 (s, 3H, Me). 13C{1H} NMR (100.63 MHz,
253 K): δ 240.0 (d, JCP = 18, MoCO), 207.3, 206.5 (2s, ReCO),
144.1 [d, JPC= 28, C1(Ph)], 138.1 [d, JPC = 52, C1(Ph)],
135.8-128.5 (m, Ph), 110.3 [s, C1(C5H4)], 104.1, 99.4 (2s, Cp),
84.2, 84.1, 83.9, 81.6 [4s, C2,3(C5H4)], 14.0 (s, Me).
Preparation of [Mo2MnCp2Cp0(μ-N)(μ-O)(μ-PPh2)(CO)3]
(3b). Solid [MnCp0(CO)2(NO)]BF4 (0.052 g, 0.17 mmol) was
added to a freshly prepared THF solution (10 mL) containing
(39) Armarego, W. L. F.; Chai, C. Purification of Laboratory Chemicals,
5th ed.; Butterworth-Heinemann: Oxford, U.K., 2003.
Experimental Section
(40) Connelly, N. G. Inorg. Synth. 1974, 15, 91.
General Procedures and Starting Materials. All manipula-
tions and reactions were carried out under a nitrogen (99.995%)
(41) Tam, W.; Liu, G. Y.; Wang, K. W.; Kiel, W. A.; Wong, V. K.;
Gladysz, J. A. J. Am. Chem. Soc. 1982, 104, 141.