Redox and lewis acid reactivity of unsaturated OsII

Article abstract of DOI:10.1002/ejic.201000503

A synthesis of [(PNP)OsI] {PNP = (tBu₂PCH₂SiMe ₂)₂N} permits evaluation of its reactivity, both Lewis acidity and reducing power (i.e., ability to be oxidized). It binds two molecules of PhCN, into trans sites, but only one of ethylene, and, upon binding of one N₂, there is heterolytic splitting of one tBu C-H bond to put the proton on amide N and the carbon on Os, leaving divalent metal in [{PN(H)P*}Os(N₂)(I)]. Two moles of H₂ add, forming [{PN(H)P}OsH(H₂)I], via H-H bond heterolysis. Thermolysis of [(PNP)OsI] gives the product of adding a tBu methyl C-H bond across the Os/N bond, and also net dehydrogenation of this intermediate, forming a carbene complex; the released H₂ forms [(PNP)OsH₂I], and the chemistry of [(PNP)Os] hydridohalides is described. Reaction with O₂ occurs with no detectable intermediate, to completely split the O=O bond, and form trans-[(PNP)Os(O)₂I], a product of four electron redox change. Attempted two electron oxidation by oxygen atom transfer with pyridine N-oxide or Me₃NO or N₂O surprisingly effect transposition of N from its silyl substituents onto the metal, and replace N by O, forming a nitride complex of a bis(silyl ether, phosphane) chelate whose oxygen fails to bind to Os. The product is thus four-coordinate, tetrahedral [(POP)Os(N)I], with an Os/N triple bond. Synthesis of [(PNP)OsI] {PNP = (tBu₂PCH ₂SiMe₂)₂N} shows this paramagnetic 14-valence electron species to rapidly add H₂ or ethylene, and to split O ₂ rapidly at -78 °C to give the hexavalent species illustrated, [(PNP)Os(O)₂I]; reaction of [(PNP)OsI] with O-atom transfer reagents gives a product of transposition of amide N with O, [(POP)Os(N)I].

Full text of DOI:10.1002/ejic.201000503

FULL PAPER
DOI: 10.1002/ejic.201000503
Redox and Lewis Acid Reactivity of Unsaturated OsII
Nikolay Tsvetkov,[a] Maren Pink,[a] Hongjun Fan,[a] Joo-Ho Lee,[a] and
Kenneth G. Caulton*[a]
Keywords: Osmium / Pincer / Oxygen reduction / Hydrogen / C-H bond cleavage
A synthesis of [(PNP)OsI] {PNP = (tBu2PCH2SiMe2)2N} per- istry of [(PNP)Os] hydridohalides is described. Reaction with
mits evaluation of its reactivity, both Lewis acidity and reduc-
ing power (i.e., ability to be oxidized). It binds two molecules
of PhCN, into trans sites, but only one of ethylene, and, upon
binding of one N2, there is heterolytic splitting of one tBu C–
H bond to put the proton on amide N and the carbon on Os,
leaving divalent metal in [{PN(H)P*}Os(N2)(I)]. Two moles of
H2 add, forming [{PN(H)P}OsH(H2)I], via H–H bond heteroly-
sis. Thermolysis of [(PNP)OsI] gives the product of adding a
tBu methyl C–H bond across the Os/N bond, and also net
dehydrogenation of this intermediate, forming a carbene
complex; the released H2 forms [(PNP)OsH2I], and the chem-
O2 occurs with no detectable intermediate, to completely
split the O=O bond, and form trans-[(PNP)Os(O)2I], a product
of four electron redox change. Attempted two electron oxi-
dation by oxygen atom transfer with pyridine N-oxide or
Me3NO or N2O surprisingly effect transposition of N from its
silyl substituents onto the metal, and replace N by O, forming
a nitride complex of a bis(silyl ether, phosphane) chelate
whose oxygen fails to bind to Os. The product is thus four-
coordinate, tetrahedral [(POP)Os(N)I], with an Os/N triple
bond.
(PNP)MCl species offer are to probe the extent to which a
spin change influences reaction rate and character, as well
as the functionality of the amide center. In addition, if os-
mium here is truly a potent reductant, then there is the pos-
sibility that it can function to create a π acid ligand, even
by H–C(sp3) bond cleavage to yield a carbene.
Introduction
Four-coordinate d6 species are rare,[1] due to the special
stability of more saturated square pyramidal five-coordi-
nate, or octahedral alternatives. Synthetic approaches must
be carried out in the absence of potential donors, including
N2, but also with massive steric shielding to avoid halide
bridging which pervades divalent Ru and Os chemistry. This
led us to the d6 RuII compounds (PNP)RuX {PNP =
(tBu2PCH2SiMe2)2N; X = Cl, F, O3SCF3}[2,3] The unusual
triplet ground state for these molecules requires that two d
orbitals have an energy separation comparable to spin pair-
ing energy, and this separation is influenced by the degree
of amideǞRu π-donation. We were interested in how
things change on going to the osmium analogs of (PNP)-
RuX, where d orbital splitting is generally larger than for 4d
metals, but where amide/dπ overlap should be larger. Direct
comparison of any osmium analog of the unusual four-co-
ordinate, 14 valence electron, planar, spin triplet d6 mole-
cule (PNP)RuX requires avoiding a synthetic precursor,
In this report, we furnish a synthesis of [(PNP)OsI]
which permits evaluation of its reactivity, both Lewis acid-
ity and reducing power (ability to change to higher formal
oxidation state). Being so very unsaturated, it binds many
donors within time of mixing, but it also rapidly consumes
two molecules of H2, heterolytically splitting[5–9] one and
coordinating the other intact. In the absence of any suitable
reaction partner, a higher activation barrier process for
[(PNP)OsI] gives the product of net dehydrogenation of a
methyl group, a carbene complex; the released H2 forms
[(PNP)OsH2I], and the chemistry of unusual “daughter”
radical (PNP)OsIII hydridohalides is described. In contrast
to some of these nonredox heterolyses, reaction with O2 oc-
curs with no detectable intermediate, to completely split the
O=O bond, and form trans-[(PNP)Os(O)2I], a product of
four electron redox change; the mechanism of this transfor-
mation is deduced, from experimental and DFT studies. At-
tempted two electron oxidation, by oxygen atom transfer
with pyridine N-oxide or Me3NO or N2O, surprisingly ef-
fect transposition of N from its silyl substituents onto the
metal, and replace N by O, forming tetrahedral [(POP)-
Os(N)I] with a bis(silyl ether, phosphane) chelate and an
Os/N triple bond.
[areneOsX2]2,
bearing
benzylic
hydrogens,
since
[(CH3C6H4iPr)OsCl2]2 has been shown[4] to react with
Li(PNP) with triple intramolecular dehydrogenation of the
methyl group to form the carbyne complex [(PNP)-
Os(ϵCC6H4iPr)(H)2]. The unique opportunities which d6
[a] Department of Chemistry, Indiana University,
Bloomington, IN, USA
Fax: +1-812-855-8300
E-mail: caulton@indiana.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000503.
View this journal online at
wileyonlinelibrary.com
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Eur. J. Inorg. Chem. 2010, 4790–4800

Redox and Lewis Acid Reactivity of Unsaturated OsII
1
characterized by its H NMR intensities, including that of
Results and Discussion
coordinated ethylene, and also the presence of two chemical
shifts each for tBu, for SiMe, and for SiCH2 protons. This
unexpected 1:1 stoichiometry may be associated with the
fact that one ethylene binds in a way that folds iodide
downwards, crowding the sixth coordination site.
Synthesis of (PNP)OsI (1)
Reaction of [(C6H6)OsI2]2 with (PNP)MgCl in THF for
48 h at 22 °C forms pentane soluble green [(PNP)OsI] in
40% yield. Judging by the absence of a 31P NMR signal
(due to extremely rapid relaxation) and three 1H NMR
chemical shifts in the range +4 to –5.5 ppm, the molecule
is paramagnetic and of C2v symmetry. The 1H NMR signals
are so sharp that the tBu resonance can be resolved into a
virtual triplet (6 Hz P/H spin coupling). A DFT(B3LYP)
geometry optimization of this species[10] shows that the trip-
let is the ground state, but the singlet is only 1.7 kcal/mol
higher, whereas the singlet for the Ru analog lies 12.8 kcal/
mol higher. The spin density is 1.93 e on Os, indicating little
delocalization onto the ligands. We describe now
(Scheme 1) how the molecule shows its unsaturation by
binding of PhCN, H2, or C2H4, each in time of mixing[11]
at 25 °C.
c. Dihydrogen
This reagent gives both an η2-H2 adduct and H2 addition
across the Os–N bond, thus no redox change at osmium.[12]
This reaction also shows the unsaturation of [(PNP)OsI],
but with the additional feature of heterolytic splitting at the
amide nitrogen. Reaction of [(PNP)OsI] with 2 equiv. (or
1 atm) H2 in benzene at 25 °C forms (Scheme 1) only Cs-
symmetric diamagnetic [{PN(H)P}OsH(H2)I], 3, in time of
mixing. This shows an NH proton at 2.3 ppm and intensity
2:1 hydride signals at –13.9 and –12.7 ppm. The T1(mini-
mum) value of this intensity 2 signal, 92 ms at 500 MHz,
indicates it to be a molecular dihydrogen ligand, but with a
highly elongated H···H distance.[13]
Reaction of [(PNP)OsI] with only equimolar H2 in ben-
zene forms diamagnetic red [(PNP)OsH2I] (see below for
fuller characterization). Addition of 1 atm of H2 to a solu-
tion of [(PNP)OsH2I] causes complete conversion in less
than 5 min to yellow [{PN(H)P}OsH(H2)I]. With regard to
possible H2 transfer reactions, we observed that combining
equimolar [(PNP)OsI] and [{PN(H)P}OsH(H2)I] in ben-
zene shows NMR signals of only the individual species, so
this is not a viable synthesis of [(PNP)OsH2I], and there is
no H or 31P NMR coalescence of these two species.
1
Scheme 1.
Lewis Base Adducts
Independent Syntheses and Characterization of the
Halohydrides [(PNP)OsHx(halide)]
a. Benzonitrile
While the above reactivity with H2 shows a reluctance
to reach OsIV, we find a significant preference for trivalent
osmium in paramagnetic monomeric species.
Addition of PhCN (2 equiv.) to [(PNP)OsI] in benzene
forms (Scheme 1) diamagnetic trans-[(PNP)OsI(PhCN)2], 2,
identifiable by 1H and 31P NMR spectra consistent with C2v
symmetry. Binding two benzonitriles in trans positions is
also characteristic of (PNP)RuCl.
[(PNP)OsH2Cl]
Reaction of [(PNP)Os(H)3][4] with N-chlorosuccinimide
(NCS) in benzene occurs within minutes (in spite of the
poor solubility of the NCS), with complete consumption of
Os reagent to give mainly diamagnetic [(PNP)OsH2Cl] in
addition to small amounts of [(PNP)OsHCl] (see below).
The major conversion here is the H/Cl replacement reaction
typical of radical Cl sources. While [(PNP)OsH2Cl] shows
only one hydride NMR signal, a triplet, there are two tBu
and two SiMe chemical shifts, indicating Cs symmetry. The
hydrides are therefore either a dihydrogen molecule, or flux-
ional and coalesced, in a cis-dihydride of OsIV.
b. Ethylene
Green [(PNP)OsI] reacts in time of mixing[11] with ethyl-
ene (3 equiv.; 1 atm) to give (Scheme 1 and Scheme 2) a
dark red diamagnetic 1:1 adduct 5a, [(PNP)Os(C2H4)I],
Similarly, the reaction of orange [(PNP)Os(H)3] with
equimolar N-iodosuccinimide (NIS) in benzene or THF
gives complete conversion within less than 10 min to the
metastable (see below) Cs-symmetric [(PNP)OsH2I].
[(PNP)OsHCl]
The reaction of [(PNP)OsH(N2)][14] with equimolar (or
Scheme 2.
excess) N-chlorosuccinimide (NCS) in benzene or toluene
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FULL PAPER
(for low-temperature studies) gives rapid reaction and com- Ru). This can be explained by the Singly Occupied Molecu-
plete conversion of the reagent complex to form [(PNP)- lar Orbital of the trivalent metals being π*(M/N) in charac-
OsHCl], identifiable by its unusual (i.e., paramagnetically ter, so going from a d5 to a d6 configuration further popu-
shifted) proton chemical shifts from +12 to –19 ppm (hy- lates an M/N anti-bonding orbital, thus lengthening the
dride not detected). There are two tBu chemical shifts and bond. This of course supports the idea that the amide N is
two SiMe chemical shifts, which prove Cs, not C2v sym- a π donor to the metal.
metry, showing that this molecule is stereochemically rigid
and that it cannot be planar [(PNP)OsCl] (see Exp. Sec-
tion), whose Ru analog[2] is paramagnetic (a ground-state
spin-triplet in spite of the d6 configuration). The molecule
shows no 31P NMR signal, consistent with rapid relaxation
broadening the phosphorus NMR signal to invisibility. The
line widths of the proton NMR signals are typically 115 Hz.
An Evans method magnetic susceptibility determination in
benzene at 22 °C gave µ = 1.73 B.M., consistent with S =
1/2 and thus oxidation state OsIII. The selectivity of this
NCS reaction is remarkable, since it might have been ex-
pected to replace hydride by Cl {as it does with [(PNP)-
Os(H)3], above}, not add Cl. The loss of N2 ligand here is
wholly consistent with the reaction being an oxidation at
Os (less back donation in the product), so apparently the
Figure 1. ORTEP drawing (50% probability ellipsoids) of [(PNP)-
radical Cl source NCS simply encounters a reducing agent,
(PNP)OsII, not an H atom donor towards the imidyl radi-
cal, due to the stability characteristic of trivalent osmium.
OsHCl], with hydrogen atoms deleted. Unlabelled atoms are car-
bons, tBu methyls have been deleted, and the hydride was not evi-
dent in the X-ray determination. See also Table 1.
Table 1. Selected bond lengths [Å] and angles [°] for (PNP)M(H)-
Cl*.
Reactivity of [(PNP)OsH2Cl]
M = Os
M = Ru
Over a period of 5 days at 25 °C in benzene, [(PNP)-
OsH2Cl] is observed to convert to [(PNP)OsHCl], which is
apparently a thermodynamic sink in this class of molecules.
This reaction is slow, apparently due to the need to transfer
only one H.
M–N1
M–P1
M–C11
N1–M–P1
P1*1–M–P1[a]
P1*1–M–C11
N1–M–C11
P1–M–C11
2.020(3)
2.029(4)
2.3733(8)
2.4260(16)
89.22(2)
178.43(4)
90.18(4)
161.51(6)
91.31(4)
2.3679(7)
2.3951(14)
89.074(15)
178.15(3)
90.80(4)
163.29(5)
90.80(4)
Over the course of hours to days at 25 °C, the iodo ana-
log [(PNP)OsH2I] also converts to paramagnetic [(PNP)-
OsHI], and the released H2 combines with [(PNP)OsH2I] to
form [{PN(H)P}OsH(H2)I]. As a result, the reaction
(above) of [(PNP)Os(H)3] with NIS already produces some
amounts of [(PNP)OsHI] and [{PN(H)P}OsH(H2)I] even
within 2 h of mixing. The overall balanced reaction would
convert 3 [(PNP)OsH2I] into a 2:1 ratio of [(PNP)OsHI]
and [{PN(H)P}OsH(H2)I]. In this reaction, there are no sig-
nificant rate differences between these chloride and iodide
analogs. In an independent experiment, reaction of the radi-
cal [(PNP)OsHI] with 1 atm H2 also causes conversion to
[{PN(H)P}OsH(H2)I], but only first detectable after 2 h.
The slow rate of this reaction is suggested to be due to the
fact that the first step, delivering one H from H2, encoun-
ters some significant barrier.
[a] *Cell constants, Ru[Os]: a = 20.682(2) Å [20.714(2)]; b =
9.9430(8) [9.9650(10)];
110.993(4)° [110.728(2)].
c = 15.5568(12) [15.6128(16)]; β =
Independent synthetic work attempting to install an alkyl
group on the (PNP)Ru fragment led instead to crystalli-
zation of the molecule (PNP)RuHCl, identified only by its
crystal structure (Table 1). The synthetic method was reac-
tion of (PNP)RuCl with LiCH2SiMe3 in THF at –70 °C,
and subsequent crystal growth by slow evaporation of sol-
vent; the product showed no 31P NMR signal, consistent
with paramagnetism. For this Ru analog, the hydride was
visible in the difference Fourier maps, as well as by its in-
ferred influence on bending the angle N–Ru–Cl from 180°.
In fact the Ru analog is crystallographically isomorphous
with the Os compound, so they pack identically. Table 1
compares the structural parameters of the two analogs,
showing both to be square pyramidal.
The single-crystal X-ray structure of [(PNP)OsHCl] de-
termined on crystals grown from benzene (Figure 1 and
Table 1) shows a square pyramidal geometry with basal
chloride. This is consistent with a formula [(PNP)OsHCl],
where the hydride (not detected in the X-ray determination)
is deduced from the bending of the chloride ligand out of
the OsP2N plane. The Os–N(amide) distance, 2.020(3) Å, is
significantly shorter than comparable distances for d6 com-
Thermolysis of Olefin Adducts of [(PNP)OsI]
The general tendency[15] of the 5d metals to transform
pounds of five-coordinate (PNP)MII structures (M = Os or hydrocarbyl moieties into π-acid ligands is evident in the
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Redox and Lewis Acid Reactivity of Unsaturated OsII
reaction of [(PNP)OsI]. Heating [(PNP)OsI(C2H4)] in ben- heterolysis to give {[PN(H)P*]RuN2}OTf, where the triflate
zene for 1 h at 90 °C completely consumes this reagent hydrogen bonds to the NH proton, instead of binding to
(Scheme 2), forming primarily carbene 8, “(PNP=)OsI,” the metal (cf. iodide on Os here in 8);[16] the difference in
and a small amount of [(PNP)OsH2I], due its characteristic products here is that Ru is five coordinate, while Os is six
conversion (see above) to the also observed paramagnetic coordinate.
[(PNP)OsHI]. Carbene [(PNP=)OsI] is identified by the
DFT(B3LYP) calculation shows that N2 binds, to form
characteristic M = CH(R) 1H NMR signal at δ = 16 ppm as a simple adduct with [(PNP)OsI] with a ∆E of –13.2 kcal/
a doublet of doublets (carbene H coupling to inequivalent mol (for Ru this is changed to + 1.9 kcal/mol), consistent
phosphorus), and also three tBu and four SiMe signals. with observation. However, calculated to be still 15.7 kcal/
Free ethylene is observed, indicating that it is not a fully mol more stable is the observed product, 4, of heterolytic
effective scavenger of the released hydrogen. Since this reac- splitting of one of the tBu C–H bonds across the Os/NSi2
tion is faster than thermolysis of [(PNP)OsI] itself (see Ex- bond; in contrast to this is the corresponding energy for C–
perimental), it does not involve that species, in spite of H splitting involving ruthenium, which is only –1.4 kcal/
forming some of the same products.
mol.
Reaction of [(PNP)OsI] with tert-butyl ethylene (1:2 mol
ratio) in C6D6 completely consumes the Os reagent in 4 h
at 90 °C to form [(PNP=)OsI], together with some amount
of paramagnetic [(PNP)OsHI] and a 50% yield of a dia-
magnetic hydride complex assigned as [(PNP)Os(H)(I)-
(CH=CHtBu)] due to resonances consistent with Cs sym-
metry. This is the product of vinylic C–H oxidative addition
to [(PNP)OsI]. Again this disappearance of [(PNP)OsI] is
faster than that of pure [(PNP)OsI] under the same condi-
tions, which shows the active participation of the olefin.
Monocarbonyl Adduct [(PNP)Os(CO)I]
It is thus unusual that it is not the hydrogen acceptor
ligand L = C2H4 in [(PNP)OsI(L)] that readily (25 °C) gives
the carbene, but rather L = N2. To test whether π-acidity is
the causal factor, we considered the example where L = CO.
It is possible to synthesize the five coordinate analog
[(PNP)Os(CO)I] by careful addition of equimolar CO to a
frozen solution of [(PNP)OsI] in C6D6, with vigorous mix-
ing during melting. Even here, some [(PNP)Os(CO)2I] is
also formed, and fails to conproportionate with unreacted
[(PNP)OsI] to form [(PNP)Os(CO)I], due apparently to ste-
ric bulk of these two molecules. Addition of more CO con-
verts the mixture completely to [(PNP)Os(CO)2I], identified
as the trans isomer by its NMR indicating C2v symmetry.
Synthesis and Reactivity of [(PNP)Os(N2)I]
Adding 1 atm N2 at –70 °C to [(PNP)OsI] in THF forms
(Scheme 2) a diamagnetic 1:1 adduct (characterized by
NMR spectra at low temperature to get higher conversion
to adduct) consistent with Cs symmetry. Low-temperature
infrared spectra in pentane show the adduct has an NN
stretching frequency of 2112 cm–1 and this stretch loses in-
tensity as the sample warms, consistent with this same evol-
ution evident in the following variable temperature NMR
spectra. A sample mixed and conserved at low temperature
evolves, beginning already at 0 °C and completely within
0.5 h at 25 °C, to form one product. One contribution to
this slow rate is that the N2 adduct formation is less at
higher temperatures. The lack of symmetry, via the resolu-
tion of four SiMe and three tBu chemical shifts together
with two intensity three methyl signals are all consistent
The monocarbonyl shows Cs symmetry by its 31P and H
1
NMR spectra. The observed CO stretching frequency of
trans-[(PNP)Os(CO)2I] is 1941 cm–1, while that of the
monocarbonyl is lower, 1892 cm–1, as expected. The mono-
carbonyl shows no sign of metallation of its C–H bond af-
ter 24 h at 25 °C. It is thus not simply the π-acidity of L
that promotes C–H bond heterolysis in unsaturated
[(PNP)Os(L)I].
Reaction of [(PNP)OsI] with O2
Oxygen provides an opportunity to probe the multielec-
with a product where a C–H bond of one tBu methyl group tron reducing potential of OsII in the PNP ligand environ-
has been cleaved to give [{PN(H)P*}OsI(N2)] (4). Detection ment. Reaction (1:1 O2/Os) of these reagents occurs in time
of one NH proton establishes that this is an example of H– of mixing at 25 °C with a color change from green to yellow
C(sp3) bond heterolysis. The osmium oxidation state is thus to give complete conversion (Scheme 1) to a benzene-solu-
unaltered. The NN stretching frequency of [{PN(H)P*}- ble diamagnetic product 6 of C2v symmetry (1H and 31P
OsI(N2)] is 2038 cm–1, indicating only modest back do- NMR evidence). The CI (methane) mass spectrum of the
nation to the relatively poor π-acid N2. This C–H bond isolated product shows [(PNP)OsO2]+ as the highest mass,
heterolysis contrasts starkly with the lack of such reaction supporting a formula (PNP)OsO2I for 6, which, together
at 25 °C with an ethylene ligand, giving instead the simple with the C2v symmetry, would require full scission of the
adduct [(PNP)Os(C2H4)I], and suggests that these two li- O/O bond (i.e., a rare 4-electron redox change)[17–19] or, if
gands elicit very different selectivities: apparently the π- iodide were not coordinated, then a salt of the cation
acidity of N2 leaves Os sufficiently electrophilic that it can (PNP)OsO2+. Crystals grown by cooling from toluene were
bind, then rapidly heterolytically split an alkyl C–H bond. determined by X-ray diffraction (Figure 2) to be the molec-
This reaction bears some similarity to that of N2 with ular species [(PNP)Os(O)2I] with the two oxido ligands mu-
(PNP)Ru(OTf), OTf = O3SCF3, which triggers H–C bond tually trans [O/Os/O = 174.90(13)°]. The geometry has no
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rigorous symmetry, but is in fact very close to C2-symmet- large ЄO–Os–O (121.5°) and yet the heterolytic splitting of
ric. The Os/O distances, 1.749(3) and 1.751(3) Å, are iden- this bond is not unusually easy (∆E = 28.6 kcal/mol in eq.
tical to those in trans-[Os(O)2(PiPr3)2].[20] The Os/I distance, d). The special stability of the doublet [(PNP)Os(O)2] radi-
2.8140(3) Å is unexceptional. The NSi2 group is twisted to cal VB is perhaps reflected in the fact that 25% of the spin
a large degree (dihedral angles 31 and 35°), as if to avoid density is located on each oxido ligand; the formal metal
overlap of the amide lone pair (N is planar) with the one oxidation state is thus less than +5 since spin density on the
filled dπ orbital. When the reaction is observed (low-tem- oxido ligand means it is less negative than 2–. Note also
1
perature H and 31P NMR) following mixing at –78 °C in that ЄO–Os–O (127.8°) in [(PNP)Os(O)2] is nearly un-
toluene, no intermediate is detected; only reactant disap- changed from that in cis-[(PNP)Os(O)2I].
pearance and growth of the above product are seen, in spite
of Cs-symmetric [(PNP)Os(η1 or η2-O2)I] being anticipated.
The barriers for two reaction steps (the 4 electron redox
change and the isomerization of the two O to being mutu-
ally trans) are thus both low. Likewise, when these reagents
are combined at 25 °C in an Os/O2 mol ratio of 1:0.5, only
50% yield of [(PNP)Os(O)2I] together with 50% unreacted
[(PNP)OsI] are observed, so no mono-oxido species or in-
termediate is seen in this way.
Scheme 3. Relative energies [∆E(SCF) + ∆G(solv) in kcal/mol] for
reactions of O2 with [(PNP)Osl].
If indeed the mechanism involves Os–I bond homolysis,
this step should be much more endothermic if the halogen
is fluorine. The energy of forming neutral F· will be very
unfavorable, hence halting the reaction at cis-[(PNP)Os-
(O)2F]. Indeed, reaction of [(PNP)OsF] with equimolar O2
Figure 2. ORTEP drawing (50% probabilities) of the non-hydrogen
at 22 °C in benzene immediately yields a Cs-symmetric
atoms of [(PNP)Os(O)2I], showing selected atom labeling. Unlab-
product showing a JPF coupling (48 Hz) in both 31P and
elled atoms are carbon. Selected structural parameters: Os–O1,
1.749(3) Å; Os–O2, 1.751(3); Os–N, 2.082(3); Os–P1, 2.5212(10);
1
19F NMR spectra; the H NMR spectrum confirms these
conclusions, thus supporting the homolysis mechanism for
the conversion of cis to trans dioxido product. This cis spe-
cies is sufficiently kinetically trapped regarding cisǞtrans
conversion that this isomerization is never observed. In-
stead, with a half-life of ca. 10 min, it deposits a black solid
(metal oxide).
Os–P2, 2.5024(10); Os–I, 2.8140(3); N–Os–I, 179.18(9)°; O1–Os–
O2, 174.90(13); P2–Os–P1, 172.08(3);O1–Os–N, 92.55(13); O2–Os–
N, 92.55(13).
Scheme 3 (see also Supporting Information) shows rela-
tive DFT(B3LYP) electronic energies (corrected for sol-
vation using a self-consistent reaction field based on numer-
ical solutions of the Poisson–Boltzmann equation) of a
variety of candidate intermediates; bold arrows map the
lowest energy intermediates for what is seen to be a highly
exothermic process. Ionic mechanisms (a, c, and d) to
Single O-Atom Transfer Product, [“(PNP)Os(O)I].”
We also sought a monoxide complex, [(PNP)Os(O)I].
change the oxido stereochemistry to trans involve large en- Equimolar [(PNP)OsI] and pyridine N-oxide react in time
ergy penalties, but homolysis of the Os–I bond in hexaval- of mixing in THF, with color change from green to dark
ent cis-[(PNP)Os(O)2I] is remarkably easy (BDE is 4.2 kcal/ red, to give a single product 7. Removal of all volatiles and
mol), and is repaid by a much larger BDE (32.0 kcal/mol) recrystallization from pentane at –40 °C gives a single prod-
of forming trans-[(PNP)Os(O)2I]. Since this 4.2 kcal/mol uct, exhibiting one 31P chemical shift, two tBu (triplets),
BDE is less than T∆S at our operating temperatures, Os–I two SiMe and two CH2 proton chemical shifts, all, consis-
bond homolysis from IVB will be spontaneous, and isomer- tent with Cs symmetry and diamagnetism. The methane CI
ization will probably occur in a geminate radical pair, thus mass spectrum shows a molecular ion for mass [“(PNP)-
preventing the cis intermediate from achieving detectable OsOI].” Equimolar [(PNP)OsI] and Me3NO also react
concentration, in agreement with experiment. The weak completely in time of mixing at 25 °C to give this same
bond in cis-[(PNP)Os(O)2I] is reflected in a very long Os–I product. While metallocentric thinking, and the idea that
distance (3.119 Å vs. 2.971 in the trans isomer), and a this OsII center is a reducing agent, would assign this prod-
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Redox and Lewis Acid Reactivity of Unsaturated OsII
uct as [(PNP)Os(O)I], in fact the product has undergone a undetected [(PNP)OsIVOI] is evidently sufficiently nucleo-
surprising transposition of the atoms N and O compared philic that it is thermodynamically preferred to have two Si
to this formula. A single-crystal X-ray diffraction structure migrate there, leaving a nitride ligand instead of oxido.
determination (Figure 3) shows the molecule to have the Cs
symmetry deduced from NMR spectra, but the monatomic
ligand attached to Os has bond length too short for Os/O
Conclusions
but appropriate for an Os/N triple bond.[21] The heteroatom
This work shows the low barrier for [(PNP)OsI] to ad-
duct formation with Lewis bases, as well as the reaction
with H2. When no such reagent (or solvent, e.g. benzene) is
available, then, with a higher barrier, attack on its own tBu
group can occur twice, forming 8. Note however that
[(PNP)OsI] is protected by a high activation energy from
attacking its own ligand.
In the thermolysis and hydrocarbon reactivity work re-
ported here, only [(PNP)OsH2(halide)] and [(PNP)OsHI-
(CH=CHtBu)] are OsIV. The remainder of the halide species
stays at the divalent oxidation state of the reagent. Even
when this involves cleavage of an H–C(sp3) bond, the fa-
vored oxidation state is OsII, and this is possible by formal
addition of the C–H bond across the Os–amide bond, a
process best described as C–H bond heterolysis. This is not
the favored reaction for pincer ligands with sp2 or sp3 C
atom at the central anionic site,[23–27] but it is the reaction
between Ru–NR2 and the H2 σ bond, in the Noyori hydro-
genation catalyst, forming H–Ru–NHR2.[5–9] This differ-
ence is attributed to the lone pair on the central pincer do-
nor for anionic PNP pincers.
in the pincer ligand does not coordinate to Os, but is 3.14 Å
distant from it, consistent with a poorly nucleophilic bis(si-
lyl ether) O atom. The molecule is thus [(POP)Os(N)I], con-
taining four coordinate osmium, with a flattened tetrahe-
dral coordination geometry. The uncharged POP ligand
thus spans a wide edge of the tetrahedron, with a P1–Os1–
P2 angle of 138.35(3)°, but the other angles around Os
range from 96 to 117°. The Os···N distance, 1.638(4) Å, is
identical to the Ru···N distance in (PNP)RuN.[22] Does the
oxido group ever reside on Os in intermediates in any of
these amine oxide reactions, or are these all examples of
oxygen attack directly on silicon? One approach to this is
the observation that addition of non-nucleophilic N2O to
[(PNP)OsI] at –78 °C, followed by mixing with minimal
heating of the solution, shows that the reaction has gone
to completion to form the product characterized above; no
intermediate is detected. We believe that N2O lacks the nu-
cleophilicity to directly attack silicon. This suggests that the
unobserved primary product [(PNP)Os(O)I], when formed
from N2O, has a low barrier to double SiǞO migration.
The rapid rate of this migration also excludes [(PNP)Os-
(O)I] as an intermediate in the reaction of [(PNP)OsI] with
O2.
The heterolytic character of the C–H cleavage offers a
special advantage when, as is often the case here, a halide
ligand is present [Equation (1)]. If a base B can be found
to subsequently do dehydrohalogenation, then a metal/alkyl
functionality has been formed from an alkyl group without
need of alkali metal or Grignard reagents and without oxid-
izing the metal. Success in such dehydrohalogenation de-
pends on finding a base stronger than the amide nitrogen.
(1)
Figure 3. ORTEP drawing of the non-hydrogen atoms of [(POP)-
Os(N)I] (50% probability) showing selected atom labeling. Unlab-
elled atoms are carbon. Selected structural parameters: Os1–N1,
1.638(4) Å; Os1–P1, 2.3928(9); Os1–P2, 2.3938(10); Os1–I1,
2.7486(3); Si1–O1, 1.643(3); Si2–O1, 1.639(3); N1–Os1–P1,
101.64(11)°; N1–Os1–P2, 100.48(12); P1–Os1–P2, 138.35(3); N1–
Os1–I1, 117.33(14); P1–Os1–I1, 96.24(2); P2–Os1–I1, 104.21(2);
Si2–O1–Si1, 140.24(18). Nonbonded Os–O1 = 3.14 Å.
Why does N2 trigger C–H bond heterolysis? This could
be due to the fact that product 4 is an 18 electron species,
which is less true of [(PNP)OsI(N2)] due to Si2NǞOs π-
donation. But this five-coordinate adduct also apparently
DFT(B3LYP) geometry optimization calculations on the has a more electron rich amide nitrogen than does [(PNP)-
two isomeric structures[10] gave five coordinate [(PNP)- OsI], thus preparing it for C–H bond heterolysis. Finally
Os(O)I] as well as the observed four coordinate POP struc- product 4 annihilates any 4 electron repulsion between the
ture, with the latter in very good agreement with experiment amide lone pair and an occupied d orbital in [(PNP)-
with regard to bond lengths and angles. Consistent with OsI(N2)].
the logic above, the calculated Os···O distance, 1.81 Å, is
Although halogen effects are often not very dramatic, the
distinctly longer than the Os···N distance, 1.65 Å. The ni- F/I comparison used here offers special mechanistic poten-
tride isomer is more stable than the terminal oxido isomer tial. In addition, the DFT calculations here reveal that M–
by 20.9 kcal/mol, something that probably could not be pre- I bond homolysis should be considered more generally, es-
dicted since the two isomers have the same metal oxidation pecially where the metal oxidation state is high. Regarding
state and valence electron count. In summary, the oxido in a synthetic application, one can disfavor some mechanisms
Eur. J. Inorg. Chem. 2010, 4790–4800
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FULL PAPER
in vacuo, 40 mL of pentane was added and stirred for 24 h at 22 °C,
then filtered. The pentane solution was concentrated to 5 mL and
kept overnight at –40 °C. Green powder was filtered and dried;
by choosing the metal fluoride variant, allowing kinetic to
dominate thermodynamic product control.
The exceptions to the above tendency to retain OsII are
the more potent reagents amine oxide and O2, where OsIV
and OsVI, respectively, are achieved. In these products, the
strength of the multiple bonds to O and N dominate the
thermodynamics, and NǞOs π-donation in [(PNP)Os-
(O)2I] diminishes the amide basicity to the extent that C–H
heterolysis does not occur there.
1
yield 0.94 g (40%). H NMR of species 1 (C6D6, 25 °C): δ = –5.42
(t, J = 4.6 Hz, 4 H, CH2), –0.62 (t, J = 6.1 Hz, 36 H, tBu), 4.62 [s,
12 H, Si(CH3)2] ppm. CI-MS (m/z): 765.1687; calcd. for C22H50-
INOsP2Si2: [M – 2H]+ 765.1616.
Reaction of [(PNP)OsI] (1) with PhCN:
These results show both reductive and Brønsted basic
reactivity of the Os-amide bond in [(PNP)OsI], as well as
kinetic facility of accessing higher oxidation states, leading
to rapid formation of the thermodynamic isomer of the
OsVI dioxido species. The ability of the PNP ligand to sup-
port metal oxidation states from +2 to +6 may be one of
its special strengths.
Experimental Section
General Considerations: All manipulations were performed using
standard Schlenk techniques or in an argon-filled glovebox unless
otherwise noted. Pentane and THF were purified using an Innov-
ative Technologies solvent purification system, Pure Solv 400-6-
MD. [D8]THF and [D6]benzene were dried under Ph2CO/Na, vac-
uum transferred and stored in the glovebox under argon. N2O, H2,
N2, C2H4 and O2 were used as received from commercial vendors.
OsCl3·3H2O was purchased from Colonial Metals, Inc. 1,3-Cyclo-
hexadiene was distilled. PhCN was distilled from P2O5 and de-
gassed. C5H5NO was distilled in vacuo (1 mm at 130 °C, 170 °C oil
bath) to give a colorless liquid, which crystallized in 15 min into a
white solid. [(PNP)Os(H)3] was synthesized according to the litera-
ture.[4] NMR chemical shifts are reported in ppm relative to protio
impurities in the deuterio solvents; “ps.-t” stands for pseudo-triplet.
Coupling constants are given in Hz. 31P NMR spectra are refer-
enced to external standards of H3PO4. NMR spectra were recorded
with a Varian Unity INOVA instrument (400 MHz 1H; 162 MHz
31P; 101 MHz 13C). Infrared spectra were recorded on a Nicolet
510P FT-IR spectrometer. Mass spectra were acquired on a PE-
Sciex API III triple quadrupole spectrometer. “PNP” is N(Si-
Me2CH2PtBu2)2.
10 mg of [(PNP)OsI] (0.013 mmol) in a J-Young NMR tube was
dissolved in 0.5 mL of C6D6. 2.7 mg of PhCN (0.026 mmol,
2.6·10–3 mL) was added to the solution via syringe. The green reac-
tion mixture turned to purple in time of mixing with complete con-
version to [(PNP)Os(PhCN)2I] (2). 1H NMR (C6D6, 25 °C): δ =
0.59 (s, 12 H, SiMe), 1.24 (t, J = 4.6 Hz, 4 H, CH2), 1.57 (br.s, 36
H, tBu), 6.8–6.9 (m, 6 H, Ar), 7.51 (d, J = 7.6 Hz, 4 H, o-Ar) ppm.
31P{1H} NMR (C6D6, 25 °C): δ = 4.8 (s) ppm. In benzene, excess
PhCN slowly displaces the iodide to precipitate a red solid, shown
by 1H NMR integration (in CD2Cl2) and mass spectrometry to
giveacationcontainingthreePhCN;theproductisthusthesalt[(PNP)-
Os(NCPh)3]I. 1H NMR (CD2Cl2, 25 °C): δ = 0.30 (s, 12 H, SiMe),
1.30 (t, J = 4.4 Hz, 4 H, CH2), 1.46 (t, J = 6.4 Hz, 36 H, tBu),
7.55–7.70 (m, 15 H, Ar) ppm. 31P{1H} NMR (CD2Cl2, 25 °C): δ =
20.9 (s) ppm. ESI-MS (THF): (m/z) 949.3994; calcd. for
C43H67N4OsP2Si2: [M]+ 949.3994.
Reaction of [(PNP)OsI] (1) with C2H4:
[(C6H6)OsI2]2:[28] 1.00 g of OsCl3·3H2O, 2.0 mL of cyclohexa-1,3-
diene and 1.2 g of I2 was dissolved in the 50 mL of EtOH. The
reaction mixture was refluxed with vigorous stirring for 12 h, co-
oled and a brown powder was filtered, washed with EtOH (10 mL)
and Et2O (10 mL). 1.48 g (99%) of product was obtained after dry-
ing in vacuo. This reagent was used without further purification.
Its solubility in benzene is too low to produce a proton NMR sig-
1
nal, but it dissolves, with adduct formation, in MeCN. H NMR
(CD3CN, 25 °C): δ = 6.07 (s, C6H6) ppm.
[(PNP)OsI] (1):
20 mg of [(PNP)OsI] (0.026 mmol) in a J-Young NMR tube was
dissolved in 0.5 mL of C6D6 and was degassed through three
freeze-pump-thaw cycles using liquid N2. 760 mm of C2H4
(3.5 equiv.) was added to the evacuated head space of the frozen
solution in the tube. The green reaction mixture turned to dark red
in time of mixing upon thawing, with complete conversion to
[(PNP)Os(C2H4)I] (5). 1H NMR (C6D6, 25 °C): δ = 0.45, 0.47 (both
s, 6 H each, SiMe), 0.73 (dt, J = 12 and 4.6 Hz, 2 H, CH2), 1.12
(br.s, 18 H, tBu), 1.23 (dt, J = 12 and 4.6 Hz, 2 H, CH2), 1.43 (t,
J = 6.1 Hz, 18 H, tBu), 4.11 (br.s, 4 H, C2H4) ppm. 31P{1H} NMR
(C6D6, 25 °C): δ = –10.5 ppm.
1.83 g of (PNP)MgCl[29] (0.0030 mol) was added to a suspension
of 1.60 g of dimer [C6H6OsI2]2 (0.0015 mol) in 40 mL of THF. The
reaction was stirred for 48 h at 22 °C. All volatiles were removed
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Redox and Lewis Acid Reactivity of Unsaturated OsII
Reaction of [(PNP)OsI] (1) with H2:
[(PNP)OsH2I]: 1H NMR ([D8]toluene, 25 °C): δ = 0.27 and 0.31
(both s, 6 H each, SiMe), 1.16 and 1.55 (both t, 18 h each, PtBu)
ppm. 31P{1H} NMR ([D8]toluene, 25 °C): δ = 45.0 (s) ppm. For a
sample of [(PNP)OsH2I] generated from [(PNP)OsI] and 1 equiv.
1
of H2, H NMR ([D8]toluene, –50 °C): δ = –12.2 and –25.2 (both
broad, equal intensity, OsH2) ppm; these signals vanish due to
broadening (fluxionality) upon raising the temperature. 31P NMR
was unchanged at this temperature.
Reaction of [(PNP)OsHI] with H2: A sample of [(PNP)OsHI] (solu-
tion in C6D6) was degassed through three freeze-pump-thaw cycles.
760 mm of H2 (ca. 3.6 equiv.) was added to the evacuated head
space of the frozen solution. The reaction mixture was allowed to
melt and was stirred at 22 °C. NMR was recorded periodically,
showing slow (completed in 24 h) formation of known [{PN(H)P}-
Os(H2)(H)(I)].
20 mg of [(PNP)OsI] (0.026 mmol) in a J-Young NMR tube was
dissolved in 0.5 mL of C6D6 and was degassed through three
freeze-pump-thaw cycles using liquid N2. 760 mm of H2
(ca. 1.7 equiv.) was added to the evacuated head space of the frozen
solution. The green reaction mixture turned to yellow in time of
1
mixing with complete conversion to [{PN(H)P}Os(H)(H2)I], 3. H
NMR (C6D6, 25 °C): δ = –13.92 [t, J = 11.9 Hz, 2 H, Os(H2)],
–12.67 (t, J = 15.6 Hz, 1 H, OsH), 0.17, 0.41 [both s, 6 H each,
Si(CH3)2], 1.21 (t, J = 5.8 Hz, 18 H, tBu), 1.49 (t, J = 6.3 Hz, 18
H, tBu), 2.32 (br.s, 1 H, NH) ppm. 31P{1H} NMR (C6D6, 25 °C):
δ = 42.6 (s) ppm. T1 values of a solution in THF were measured
at 500 MHz in the range +25 to –55 °C. The signal for the intensity
two hydrogens on Os reaches a broad minimum from about –45 to
–55 °C, at a value of 92 ms. At this same temperature, the T1 value
for the intensity one hydride is always larger, and is 174 ms at
–55 °C.[10]
Reaction of [(PNP)OsH(N2)] with N-Chlorosuccinimide (NCS),
Forming [(PNP)OsHCl]: [(PNP)OsH(N2)] (10 µmol) solution in
0.5 mL of C6D6 was prepared in a J-Young tube. 1.3 mg of NCS
was added to the reaction tube. After 30 min, the volatiles were
removed in vacuo. The mixture was extracted into pentane and
undissolved solid was removed by filtration. The filtrate was dried
in vacuo to yield 5.8 mg (86%) of [(PNP)OsHCl]. 1H NMR (C6D6,
25 °C): δ = –19.72 (br. s, 4 H, SiCH2), –5.16 (br. s, 18 H, tBu),
–3.66 (br. s, 18 H, tBu), 10.29 (br. s, 6 H, SiMe2), 12.01 (br. s, 6 H,
SiMe2); Os-H signal was not observed due to paramagnetic broad-
ening of the signal ppm.
Reaction of [(PNP)Os(H)3] with NCS to [(PNP)OsH2Cl]: 6 mg of
[(PNP)Os(H)3] (10 µmol) in a J-Young tube was dissolved in
0.5 mL of C6D6. 1.5 mg of NCS was added to the tube. In 20 min,
complete consumption of [(PNP)Os(H)3] was observed by NMR
spectroscopy. Formation of [(PNP)OsH2Cl] was confirmed by
NMR spectra with a small amount of [(PNP)OsHCl] also evident.
1H NMR of [(PNP)OsH2Cl] (C6D6, 25 °C): δ = 0.26 (s, 6 H, SiMe),
0.35 (s, 6 H, SiMe), 1.18 (ps.-t, J = 6 Hz, 18 H, PtBu), 1.48 (ps.-t,
J = 6 Hz, 18 H, PtBu) ppm. OsH2 and SiCH2 signals were not
observed, the latter due to overlapping with other resonances.
31P{1H} NMR (C6D6, 25 °C): δ = 42.4 (s) ppm. Over 5 days, the
signals from [(PNP)OsH2Cl] decreased while the NMR signals for
new diamagnetic product and paramagnetic [(PNP)OsHCl] in-
creased. We assigned the new diamagnetic product to [{PN(H)-
P}Os(H)2Cl(succinimide)].[30] 1H NMR (C6D6, 25 °C): δ = –7.10 (t,
JHP = 10.8 Hz, 2 H, OsH2), 0.02 (s, 6 H, SiMe), 0.29 (s, 6 H, SiMe),
1.31 (ps.-t, J = 6 Hz, 18 H, PtBu), 1.56 (ps.-t, J = 6 Hz, 18 H, PtBu)
ppm. SiCH2 signals were not observed due to overlapping with
other resonances. 31P{1H} NMR (C6D6, 25 °C): δ = 15.4 (s), This
declines in intensity, with growth of [(PNP)OsH2Cl] and [(PNP)-
OsHCl] as described above, all due to reaction with available H2.
Finally this reaction comes to equilibrium with some [{PN(H)-
P}Os(H)2Cl(succinimide)] persisting, due to the limited amount of
available H2 ppm.
Thermolysis of [(PNP)OsI] in Inert Solvent: Studies where 20 mg
[(PNP)OsI] is heated (1 to 18 h) in [D12]cyclohexane in the the
range 60–90 °C and periodically monitored by 1H and 31P NMR
spectroscopy show formation first of a species with an AX 31P{1H}
NMR pattern, and more slowly the formation of the dehydrogena-
tion product (PNP=)OsI, 8 (see below for characteristic NMR sig-
nature), together with [(PNP)OsH2I] (see above). We have been un-
able to fully identify the AX product (designated here as X), but
believe it to involve reaction of one backbone silicon with the io-
dide; its 1H NMR spectrum shows two 1H NMR tBu doublets but
apparently three silyl methyl chemical shifts. The amounts of
(PNP=)OsI and [(PNP)OsH2I] are approximately equal, in agree-
ment with the stoichiometry shown in Equation (2).
(2)
Reaction of [(PNP)Os(H)3] with NIS: 15.8 mg of [(PNP)Os(H)3]
(0.0246 mmol) was dissolved in 10 mL of THF. 5.8 mg
(0.0258 mmol) of N-iodosuccinimide was added at one time, with
vigorous stirring. The brown reaction mixture turned dark red in
time of mixing. After 2 h of stirring all volatiles were removed in
vacuo, the residue was extracted with pentane (2ϫ20 mL), filtered
through Celite, and the pentane extracts concentrated to dryness
Formation of 8 is accelerated by the presence of equimolar
PhN=NPh, which scavenges hydrogen (new aryl and NH signals
appear during this conversion). At no time during the thermolysis
of [(PNP)OsI] is any hydride signal detected by 1H NMR except
for that of [(PNP)OsH2I].
Unidentified Product X: 1H NMR (C6D12, 25 °C): δ = 0.17, 0.22,
0.37 (all s, 3 H each, SiMe), 1.04 and 1.25 (both d, PtBu) ppm.
31P{1H} NMR (C6D12, 25 °C): δ = –8.1 (d, J = 302 Hz), 51.1 (d, J
= 302 Hz) ppm.
1
in vacuo. Both H and 31P NMR (C6D6) showed the formation of
the mixture of paramagnetic [(PNP)OsHI] and diamagnetic
[(PNP)OsH2I] (see spectra below) and [{PN(H)P}Os(H2)(H)(I)] in
1
ratio 1:0.1:0.23. H NMR (C6D6, 25 °C) of paramagnetic [(PNP)-
OsHI]: δ = –19.2 and –18.7 (two br. s, 2 H each, CH2); –5.74 and
–4.11 (two br. s, 18 H each, tBu); 11.26 and 11.48 (two br. s, 6 H
each, SiMe2) ppm.
[(PNP=)OsI] (8): 1H NMR (C6D6, 25 °C): δ = –0.11, 0.20, 0.46,
0.46 (all s, 3 H each, SiMe), 0.70 and 1.28 (both d, 3 H each,
CMe2), 1.40, 1.48, 1.51 (all d, 9 H each, PtBu), 15.98 (d.d, J = 10.6
Eur. J. Inorg. Chem. 2010, 4790–4800
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FULL PAPER
and 34.7 Hz, Os-CH=) ppm. 31P{1H} NMR (C6D12, 25 °C): δ =
0.71 (d, J = 349 Hz), 24.51 (d, J = 349 Hz) ppm.
tane and was degassed through three freeze-pump-thaw cycles
using liquid N2. 760 mm (14 equiv.) of N2 was added to the evacu-
ated head space of the frozen solution. This solution was put far
into the Dewar (to cool the upper glass walls) with acetone at
–80 °C, allowed to melt, then the tube was shaken fast to minimize
heating at the walls. The cold tube was transferred into the glove
box where the solution was transferred into the pre-cooled (–40 °C)
IR cell. The IR spectrum was recorded as quickly as possible to
prevent sample warming. IR (pentane, approximately –40 °C):
ν(N2) 2112 cm–1. IR spectra observed upon progressive warming
shows decay of this adduct, [(PNP)Os(N2)I], with formation of 4.
Thermolysis of [(PNP)OsI(C2H4)]: A solution of [(PNP)OsI(C2H4)]
was heated in benzene for 1 h at 90 °C. 31P NMR showed complete
conversion of [(PNP)OsI(C2H4)] to a mixture of [(PNP=)OsI]
1
(54%), X (15%), and [(PNP)OsH2I] (4%). H NMR showed para-
magnetic peaks for [(PNP)OsHI].
Thermolysis of [(PNP)OsI] in the Presence of (tBu)HC=CH2: 20 mg
of [(PNP)OsI] (0.026 mmol) and 2.2 mg of tert-butyl ethylene
(0.026 mmol) were dissolved in 0.5 mL of C6D6. Heating at 90 °C
for 3.5 h showed (1H and 31P NMR) complete consumption of
[(PNP)OsI] and formation of a mixture of [(PNP=)OsI], together
with some amount of paramagnetic [(PNP)OsHI], including 50%
Reaction of [(PNP)OsI] (1) with CO: 20 mg of [(PNP)OsI]
(0.026 mmol) in a J-Young NMR tube was dissolved in 0.5 mL of
C6D6 and was degassed through three freeze-pump-thaw cycles
using liquid N2. 220 mm of CO (1 equiv.) was added to the evacu-
ated head space of the frozen solution in the tube. The green reac-
tion mixture turned to deep green in time of mixing with 90%
conversion to [(PNP)Os(CO)I]. Also present in the solution is
1
[(PNP)Os(H)(I)(CH=CHtBu)]: H NMR (C6D6): δ = –10.3 (t, J =
18.7 Hz, 1 H, Os-H), 0.09, 0.34 (both s, 6 H each, SiMe), 1.36, 1.62
(both t, J = 6.1 and 6.3 Hz, 18 H each respectively, tBu) ppm; other
signals were not seen due to complexity of the mixture. 31P{1H}
NMR (C6D6): δ = 22.9 (s) ppm.
1
[(PNP)Os(CO)2I]. For 5b, H NMR (C6D6, 25 °C): δ = 0.31, 0.36
Reaction of [(PNP)OsI] (1) with N2:
(both s, 6 H each, SiMe), 0.74–0.80 (m, 2 H, CH2), 1.33, 1.41 (both
t, J = 6.4 Hz, 18 H each, tBu) ppm; the other Si-CH2 was not
observed due to overlapping with other stronger resonances.
31P{1H} NMR (C D , 25 °C): δ = 41.1 ppm. IR (C D ): ν = 1892
˜
6
6
6
6
cm–1. NMR observation after 12 h did not reveal any changes in
1H and 31P NMR spectroscopy. Another 220 mm of CO was then
added to the evacuated head space of the frozen solution. The
green reaction mixture turned to yellow in time of mixing with
complete conversion to [(PNP)Os(CO)2I]. 1H NMR (C6D6, 25 °C):
δ = 0.32 (s, 12 H, SiMe), 1.10–1.12 (m, 4 H, CH2), 1.38 (br.s., 36
H, tBu) ppm. 31P{1H} NMR (C6D6, 25 °C): δ = 24.6 ppm. IR
10 mg of [(PNP)OsI] (0.013 mmol) in a J-Young NMR tube was
dissolved in 0.5 mL of C6D6 and was degassed through three
freeze-pump-thaw cycles using liquid N2. 760 mm of N2 (7 equiv.)
was added to the evacuated head space of the frozen solution. The
green reaction mixture turned to yellow in two hours with 92%
conversion to [{PN(H)P*}Os(N2)(I)], 4. 1H NMR (C6D6, 25 °C): δ
= –0.01, 0.01, 0.72, 0.79 (all s, 3 H each, SiMe), 1.01 (d, J =
11.5 Hz, 9 H, tBu), 1.35 (d, J = 13.2 Hz, 9 H, tBu), 1.56 (d, J =
12.5 Hz, 9 H, tBu), 1.73 (d, J = 13.5 Hz, 3 H, CH3C), 2.71 (ddd,
J = 27.6, J = 10.5, J = 2.4 Hz, 1 H, Os-CH2) ppm. 0.5–0.2 (SiCH2);
other CH2 signals and the other CH3 signal are hidden by the tBu
signals. 31P{1H} NMR (C6D6, 25 °C): δ = –32.9 (d, J = 286 Hz),
(C D ): ν = 1941 cm–1.
˜
6
6
Reaction of [(PNP)OsI] (1) with O2
Variable-Temperature Monitoring of Reaction of [(PNP)OsI] with
O2:
20.8 (d, J = 285 Hz) ppm. IR (C D solution): ν(N ) = 2038 cm–1.
˜
6
6
2
Variable-Temperature NMR Monitoring of Reaction of [(PNP)OsI]
with N2: 20 mg of [(PNP)OsI] (0.026 mmol) in a J-Young NMR
tube was dissolved in 0.5 mL of [D8]THF and was degassed
through three freeze-pump-thaw cycles using liquid N2. 760 mm
(3.4 equiv.) of N2 was added to the evacuated head space of the
frozen solution in the tube. This solution was put far into the De-
war (to cool the upper glass walls) with acetone at –80 °C, allowed
to melt, then the tube was shaken fast to minimize heating at the
walls and NMR spectra were recorded beginning from –60 °C. The
green reaction mixture turned to dark green in time of mixing with
more than 90% conversion to [(PNP)Os(N2)I]. The reaction mix-
20 mg of [(PNP)OsI] (0.026 mmol) in a J-Young NMR tube was
dissolved in 0.5 mL of [D8]THF and was degassed through three
freeze-pump-thaw cycles using liquid N2. 220 mm (1 equiv.) of O2
was added to the evacuated head space of the frozen solution in
the tube. This solution was put far into a Dewar (to cool the upper
glass walls) with acetone at –80 °C, allowed to melt, then the tube
was shaken fast to minimize heating at the walls and NMR spectra
were recorded beginning from –60 °C. The green reaction mixture
turned to yellow in time of mixing with complete conversion to
[(PNP)Os(O)2I] (6). Observation of 1H and 31P NMR at –60 °C
revealed the absence of the starting complex and formation of 6.
The reaction mixture was warmed to 25 °C, where 1H and 31P
1
ture was warmed to 25 °C, with recording H and 31P NMR every
1
10°. For [(PNP)Os(N2)I], H NMR (THF, –60 °C): δ = 0.19, 0.35 NMR showed no changes.
(both s, 6 H each, SiMe), 1.42, 1.55 (br.s, 18 H each, tBu) ppm;
NMR Assay with Different Os/O2 Ratios: 20 mg of [(PNP)OsI]
CH2 were not assigned with confidence. 31P{1H} NMR (THF,
–60 °C): δ = 23.40 ppm. NMR observations at –50, –40, –30, –20,
and –10 show decay of the adduct with formation of starting
[(PNP)OsI]. Initial formation of final product 4 was seen at 0 °C
(this is the only diamagnetic product at 25 °C in 40 min).
(0.026 mmol) in a J-Young NMR tube was dissolved in 0.5 mL of
deuterated benzene and was degassed through three freeze-pump-
thaw cycles using liquid N2. 110 mm (0.5 equiv.) of O2 was added
to the evacuated head space of the frozen solution in the tube.
NMR observation after 10 min at 22 °C revealed the formation of
the mixture of the product and starting material in ratio 1:1. Ad-
dition of another 0.5 of equivalent of O2 showed complete conver-
Low-Temperature IR Study of the Reaction of [(PNP)OsI] with N2:
5 mg of [(PNP)OsI] (0.007 mmol) was dissolved in 0.5 mL of pen-
4798
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Eur. J. Inorg. Chem. 2010, 4790–4800

Redox and Lewis Acid Reactivity of Unsaturated OsII
sion to the product. Addition of 1 more equivalent of O2 showed
no changes, even after 24 h at 22 °C. Crystals of [(PNP)OsO2I] suit-
able for X-ray determination analysis were grown by cooling of
CI-MS (m/z): 783.1725; calcd. for C22H52INOOsP2Si2: [M]+
783.1717.
CCDC-778419 [for (PNP)RuHCl], -778420 [for (PNP)OsHCl],
-778421 [for (PNP)Os(O)2I], and -778422 [for (PNP)Os(N)I] con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
1
saturated toluene solution at –40 °C. H NMR (C6D6, 25 °C): δ =
0.38 [s, 12 H, Si(CH3)2], 1.03 (t, J = 6.3 Hz, 4 H, CH2), 1.44 (t, J
= 6.4 Hz, 36 H, tBu) ppm. 31P{1H} NMR (C6D6, 25 °C): δ = 21.1
(s) ppm. IR (pentane solution): ν(Os=O) 836 cm–1, assigned by
˜
comparison to the solution IR of [(PNP)OsI]. CI-MS (m/z):
672.2596; calcd. for C22H52NO2OsP2Si2: [M – I]+ 672.2621.
Supporting Information (see footnote on the first page of this arti-
cle): T1 measurement for [{PN(H)P}Os(H)(H2)I], details of X-ray
structure determination, computational details, and NMR spectra.
[(PNP)OsCl]: 105 mg of [(PNP)OsI] (0.137 mmol) and 94 mg of
Bu4NCl (0.411 mmol, 3 equiv.) in 50 mL of Et2O were vigorously
stirred for 24 h at 22 °C. Following filtration, the filtrate was con-
centrated and dried to give 86 mg of [(PNP)OsCl] (93%). 1H NMR
(25 °C, benzene): δ = –2.51 (t, J = 4.5 Hz, 4 H, CH2), 0.79 (t, J =
7.1 Hz, 36 H, tBu), 2.27 [s, 12 H, Si(CH3)2] ppm. These chemical
shifts indicate paramagnetism.
Acknowledgments
This work was supported by the National Science Foundation
(NSF) (NSF-CHE-0544829). We thank Dr. Michael Ingleson for
the crystalline sample of (PNP)RuHCl.
[(PNP)OsF]: 18 mg of [(PNP)OsCl] (0.027 mmol), 2.5 mg of
NMe4F (0.027 mmol) and 12.3 mg (0.081 mmol) of CsF in 0.6 mL
of [D8]THF were stirred for 2 h at 22 °C. All volatiles was removed
in vacuo, 10 mL of pentane was added and stirred for 10 min at
22 °C, then filtered. The pentane solution was concentrated and
dried in vacuo to give 16 mg (90%) of [(PNP)OsF]. 1H NMR
(25 °C, benzene): δ = 0.28 (t, J = 4.6 Hz, 4 H, CH2), 0.46 [s, 12 H,
Si(CH3)2], 1.68 (t, J = 6.1 Hz, 36 H, tBu) ppm. No 31P NMR signal
could be detected. Direct synthesis of [(PNP)OsF] from [(PNP)OsI]
was not successful due to high base sensitivity of the latter to F–,
evident by a product with an AB 31P{1H}NMR spectrum due to
attack on a tBu CH bond.
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Reaction of [(PNP)OsF] with O2: 16 mg of [(PNP)OsI]
(0.026 mmol) in a J-Young NMR tube was dissolved in 0.5 mL of
deuterated benzene and was degassed through three freeze-pump-
thaw cycles using liquid N2. One equivalent of O2 was added to
the evacuated head space of the frozen solution in the tube. NMR
observation after 10 min at 22 °C revealed the formation cis-
1
[(PNP)Os(O)2F]. H NMR (C6D6, 25 °C): δ = 0.43, 0.52 [both s, 6
H each, Si(CH3)2], 1.37, 1.42 (both t, J = 7.2 Hz, 18 H each, tBu),
CH2 protons were not seen due to overlap with other signals ppm.
31P{1H} NMR (C6D6, 25 °C): δ = 49.1 (d, J = 48 Hz) ppm. 19F
NMR (C6D6, 25 °C): δ = –201.5 (t, J = 48) ppm. Signals of this
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product 7. The same product is obtained analogously using
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Received: May 4, 2010
Published Online: September 7, 2010
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Eur. J. Inorg. Chem. 2010, 4790–4800