Cleavage of Conjugated Alkenes by Cationic Osmium Nitrides: Scope of the Reaction and Dynamics of the Azaallenium Products

Article abstract of DOI:10.1021/om034404m

The cationic osmium(VI) nitride complex cis-[(terpy)OsNCl ₂]PF₆ (1; terpy = 2,2′6′,2″-terpyridine) reacts with a variety of aryl-substituted alkenes and acyclic and cyclic dienes to form the η²-azaallenium complexes cis-[(terpy)OsCl ₂(RR′C=N=CHR″)]PF₆ (2) by net nitrogen atom insertion into a C=C double bond. More electron-rich alkenes are more reactive, and electron-poor alkenes such as cinnamate esters do not react. Isomeric trans-[(terpy)OsNCl₂]PF₆ (3), as well as the tris(pyrazolyl)methane complex [(Tpm)OsNCl₂]PF₆ (4), give analogous products, although the scope of the reaction is restricted to dienes with these nitrides. The aryl-substituted azaallenium complexes cis-[(terpy)OsCl₂(η²(C,N)-R[Ar]C=N=CHR″)]PF ₆ display hindered rotation about the C-C_ipso bond, because π-stacking interactions favor a close approach of the aryl group to the terpyridine ligand. The osmium migrates slowly between the two C=N bonds of the azaallenium ligands, leading to both regio- and stereoisomerizations. These two processes can be observed separately in the case of cis-[(terpy)OsCl ₂([p-Me₂NC₆H₄]CH=N=CHCH ₃)]PF₆, with regioisomerization preceding stereoisomerization. The isomerizations appear to take place by an allene-roll mechanism, as judged by the similar rates observed for cyclic and acyclic azaallenium complexes.

Full text of DOI:10.1021/om034404m

1932
Organometallics 2004, 23, 1932-1946
Clea va ge of Con ju ga ted Alk en es by Ca tion ic Osm iu m
Nitr id es: Scop e of th e Rea ction a n d Dyn a m ics of th e
Aza a llen iu m P r od u cts
Aaron G. Maestri, Scheroi D. Taylor, Stephany M. Schuck, and Seth N. Brown*
Department of Chemistry and Biochemistry, 251 Nieuwland Science Hall,
University of Notre Dame, Notre Dame, Indiana 46556-5670
Received December 25, 2003
The cationic osmium(VI) nitride complex cis-[(terpy)OsNCl2]PF6 (1; terpy ) 2,2′:6′,2′′-
terpyridine) reacts with a variety of aryl-substituted alkenes and acyclic and cyclic dienes
to form the η2-azaallenium complexes cis-[(terpy)OsCl2(RR′CdNdCHR′′)]PF6 (2) by net
nitrogen atom insertion into a CdC double bond. More electron-rich alkenes are more
reactive, and electron-poor alkenes such as cinnamate esters do not react. Isomeric trans-
[(terpy)OsNCl2]PF6 (3), as well as the tris(pyrazolyl)methane complex [(Tpm)OsNCl2]PF6
(4), give analogous products, although the scope of the reaction is restricted to dienes with
these nitrides. The aryl-substituted azaallenium complexes cis-[(terpy)OsCl2(η2(C,N)-R[Ar]-
CdNdCHR′′)]PF6 display hindered rotation about the C-Cipso bond, because π-stacking
interactions favor a close approach of the aryl group to the terpyridine ligand. The osmium
migrates slowly between the two CdN bonds of the azaallenium ligands, leading to both
regio- and stereoisomerizations. These two processes can be observed separately in the case
of cis-[(terpy)OsCl2([p-Me2NC6H4]CHdNdCHCH3)]PF6, with regioisomerization preceding
stereoisomerization. The isomerizations appear to take place by an “allene-roll” mechanism,
as judged by the similar rates observed for cyclic and acyclic azaallenium complexes.
In tr od u ction
terpyridine) with aryl-substituted alkenes in which the
nitrogen atom inserts between the two carbons of the
alkene, cleaving the alkene to form η2-azaallenium
complexes (e.g., eq 1).9 While highly energetic reagents
Reactions of alkenes with metal-ligand multiple
bonds represent an important method of functionalizing
carbon-carbon double bonds. Prominent examples in-
clude reactions of metal oxo complexes with alkenes to
give epoxides1 or 1,2-diols2 and reactions of metal
carbene species to give cyclopropanes3 or the products
of olefin metathesis.4 Reactions of compounds with
metal-nitrogen multiple bonds are less common, though
imido complexes are clearly involved in manganese-
mediated aziridinations5 and probably involved in cop-
per-catalyzed aziridinations6 and metal nitrido com-
plexes have been reported to undergo [1,4]-additions
with dienes.7
such as ozone are known to cleave carbon-carbon
double bonds with concomitant formation of carbon-
heteroatom bonds, the ability of a stable metal nitride
to cleave such a strong, nonpolar bond is noteworthy,
and the formation of carbon-nitrogen bonds in this
manner has potential synthetic utility. Thus, we were
motivated to explore the scope and limitations of this
transformation. Here we report the extension of this
chemistry to a wide variety of conjugated alkenes,
including mono-, di-, and trisubstituted olefins, and to
cyclic and acyclic dienes as well as aryl-substituted
alkenes. Analogous reactions of the cationic osmium
nitride complexes trans-[(terpy)OsNCl2]PF68 and [(Tpm)-
OsNCl2]PF6 (Tpm ) tris(1-pyrazolyl)methane)10 with
dienes are also described, as are fluxional processes and
regio- and stereoisomerizations of the η2-azaallenium
complexes cis-[(terpy)OsCl2(RR′CdNdCHR′′)]PF6.
Several years ago, one of us reported a very unusual
reaction of cis-[(terpy)OsNCl2]PF6 (terpy ) 2,2′:6′,2′′-
8
* To whom correspondence should be addressed. E-mail:
Seth.N.Brown.114@nd.edu.
(1) (a) Strassner, T. Adv. Phys. Org. Chem. 2003, 38, 131-160. (b)
Katsuki, T. J . Mol. Catal. A 1996, 113, 87-107. (c) Barf, G. A.; Sheldon,
R. A. J . Mol. Catal. A 1995, 102, 23-39. (d) Ostovic, D.; Bruice, T. C.
Acc. Chem. Res. 1992, 25, 314-320.
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A.; Sharpless, K. B. In Catalytic Asymmetric Synthesis, 2nd ed.; Ojima,
I., Ed.; Wiley-VCH: New York, 2000; Chapter 6.
(3) Brookhart, M.; Studabaker, W. B. Chem. Rev. 1987, 87, 411-
432.
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(b) Armstrong, S. K. J . Chem. Soc., Perkin Trans. 1 1998, 371-388.
(c) Schuster, M.; Blechert, S. Angew. Chem., Int. Ed. Engl. 1997, 36,
2037-2056.
(5) (a) Groves, J . T.; Takahashi, T. J . Am. Chem. Soc. 1983, 105,
2073-2074. (b) Du Bois, J .; Tomooka, C. S.; Hong, J .; Carreira, E. M.
Acc. Chem. Res. 1997, 30, 364-372.
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34, 586-592.
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(10) Demadis, K. D.; El-Samanody, E.-S.; Meyer, T. J .; White, P. S.
Inorg. Chem. 1998, 37, 838-839.
(6) Brandt, P.; Sodergren, M. J .; Andersson, P. G.; Norrby, P.-O. J .
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10.1021/om034404m CCC: $27.50 © 2004 American Chemical Society
Publication on Web 03/17/2004

Cleavage of Alkenes by Cationic Osmium Nitrides
Organometallics, Vol. 23, No. 8, 2004 1933
Ta ble 1. Cr ysta llogr a p h ic Da ta for
cis-[(ter p y)OsCl2(η2(C,N)-(P h CH-
NdCHCHdCHCHdCHP h )]P F 6‚(CD3)2CO
(2h ‚(CD3)2CO)
Sch em e 1. Rea ction s of cis-[(ter p y)OsNCl2]P F 6 (1)
w ith Ar yla lk en es To Give th e Aza a llen iu m
Com p lexes
cis-[(ter p y)OsCl2(η2(C,N)-Ar R1CdNdCR2R3)]P F 6
(2a -p )
empirical formula
C36H27D6Cl2F6N4OOsP
fw
949.73
temp (K)
293
λ
0.710 73 Å (Mo KR)
space group
P1h
total no. of data collected
6341
no. of indep rflns
a (Å)
b (Å)
6341
11.7391(12)
13.2219(10)
14.1111(12)
94.154(6)
113.610(7)
111.576(7)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
1803.2(3)
Z
2
calcd F (g/cm3)
cryst size (mm)
1.749
0.22 × 0.10 × 0.05
µ (mm-1
)
3.796
R indices (I > 2σ(I))a
R indices (all data)a
goodness of fit on F2
R1 ) 0.0342, wR2 ) 0.0715
R1 ) 0.0490, wR2 ) 0.0774
1.045
R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) (∑[w(Fo - Fc2)2]/∑w(Fo2)2)1/2
.
a
2
Resu lts
R ea ct ion of cis-[(t er p y)OsNCl2]P F 6 w it h Ar yl-
a lk en es. The pale orange osmium(VI) nitrido complex
cis-[(terpy)OsNCl2]PF6 (1) reacts with a variety of aryl-
substituted alkenes in acetonitrile solution to give the
azaallenium complexes cis-[(terpy)OsCl2(η2(C,N)-ArR1-
CdNdCR2R3)]PF6 (2) (Scheme 1). The reactions are
carried out without precautions to exclude air or mois-
ture, and the blood red azaallenium complexes can be
isolated in good yield as analytically pure microcrystals
after recrystallization from acetonitrile-ether. Reac-
tions of â-methylstyrenes and even trisubstituted â,â-
or R,â-dimethylstyrenes proceed readily at room tem-
perature in the presence of a moderate excess of alkene,
while reactions of stilbenes and styrylalkenes (with the
exception of the very electron-rich p-hydroxy- and
p-(dimethylamino)stilbenes) are rather slow and require
mild heating to proceed to completion in a reasonable
time. The compounds are air-stable indefinitely as solids
and at least for several weeks in solution. The complexes
are very soluble in acetone and acetonitrile, moderately
soluble in methylene chloride, and insoluble in ether or
hydrocarbons.
The product of the reaction of 1 with 1,6-diphenyl-
1,3,5-hexatriene, 2h , has been characterized by X-ray
crystallography (Table 1). The geometry of the complex
(Figure 1, Table 2) is best viewed as octahedral, with
the initial cis orientation of the chlorides retained in
the product and with the sixth site occupied by a CdN
double bond of an azaallenium fragment. The terpyri-
dine ligand shows deviations from regular octahedral
geometry (the bond to the central nitrogen is shorter
F igu r e 1. Thermal ellipsoid plot (30% ellipsoids) of the
cation of cis-[(terpy)OsCl2(η2(C,N)-(PhCHNdCHCHdCH-
CHdCHPh)]PF6‚(CD3)2CO (2h ‚(CD3)2CO).
by 0.12 Å than the bonds to the outer nitrogens, and
the trans N11-Os-N31 angle of 153.4(2)° is rather
acute) that are typical of metal-terpyridine com-
plexes.11
The bonding in the azaallenium complex can be
regarded as a resonance hybrid between two canonical
structures, an Os(II) azaallenium complex and an
Os(IV) azametallacyclopropane. The metrical data of the
organic fragment (C1-N1 ) 1.374 Å, C1-N1-C2 )
137.5°) lie between these two extreme forms, with
substantial bending and lengthening of the bound
CdN bond compared to what would be expected for the
(11) Brown, S. N. Inorg. Chem. 2000, 39, 378-381.

1934 Organometallics, Vol. 23, No. 8, 2004
Maestri et al.
1
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for cis-[(ter p y)OsCl2(η2(C,N)-
(P h CHNdCHCHdCHCHdCHP h )]P F 6‚(CD3)2CO
(2h ‚(CD3)2CO)
to osmium and doubly bonded to nitrogen in the H (δ
6.62 for HR, δ 8.21 for the iminium hydrogen in 2a ) and
the 13C{1H} NMR spectra (δ 44.75 for CR, δ 163.12 for
the iminium carbon in 2a ) of the azaallenium complexes.
The fact that these resonances are sharp indicates that
these environments do not exchange on the NMR time
scale. As in free allenes,14 long-range coupling is ob-
Os-C1
2.182(5)
2.006(4)
2.082(4)
1.967(4)
2.087(4)
2.4197(14)
2.3715(14)
C1-N1
N1-C2
C2-C3
C3-C4
C4-C5
C5-C6
1.374(7)
1.293(7)
1.423(8)
1.334(9)
1.424(8)
1.339(9)
Os-N1
Os-N11
Os-N21
Os-N31
Os-Cl1
Os-Cl2
4
served, with J HH in Os(RCHNdCHR′) being about 2
Hz. In addition, the methyl hydrogens in complexes
2i-o derived from â-methylstyrenes consistently show
a detectable five-bond coupling to HR of 1.5-2 Hz as
well. The terpyridine ligand in each azaallenium com-
C1-Os-N1
38.0(2)
79.2(2)
78.5(2)
153.4(2)
177.49(13)
89.26(12)
88.34(5)
77.7(2)
C1-N1-C2
Os-N1-C1
Os-N1-C2
Os-C1-N1
Os-C1-C41
N1-C1-C41
N1-C2-C3
137.3(5)
77.9(3)
143.8(4)
64.0(3)
120.5(4)
120.6(5)
126.5(3)
N11-Os-N21
N21-Os-N31
N11-Os-N31
N21-Os-Cl1
N21-Os-Cl2
Cl1-Os-Cl2
C1-Os-N11
N1-Os-N31
1
plex 2 gives rise to 11 resonances in the H NMR and
15 in the 13C{1H} NMR spectra, consistent with an
unsymmetrical structure due to the asymmetry of CR.
Each of the compounds 2a -p has one aryl group
which shows hindered rotation about the aryl-Cipso
bond, giving rise to inequivalent ortho and meta reso-
81.9(2)
1
nances in the H NMR (δ 5.48, 6.72 (ortho) and 6.49,
free organic ligand. For comparison, 1,3-ditolyl-2-azaal-
lenium triflate has CdN bond lengths of 1.258 Å,12 close
to the 1.293(7) Å length of the uncoordinated CdN bond
in 2h . In contrast, the one previously structurally
characterized azaallenium complex, CpMo(CO)2(η2-
Tol2CdNdCTol2), clearly lies far toward the azametal-
lacyclopropane canonical form (C-N ) 1.43 Å, C-N-C
) 128.3°).13 This is consistent with stronger back-
bonding by Mo(0) than by Os(II). The twisting between
the two CdN planes characteristic of cumulenes is
clearly evident in the structure of 2h .
7.02 (meta) in 2a ). Both the observed barrier to rotation
and the upfield shifts strongly suggest that the orienta-
tion observed in the crystal of 2h , with the phenyl group
on C1 lying over the terpyridine ligand, is retained in
solution. The proximity to the terpyridine explains both
the barrier to rotation (due to steric difficulties in
rotating past the terpyridine) and the upfield shifts (due
to placement of protons in the shielding region of the
terpy). The pattern of aryl shifts, with one side of the
aryl group (both ortho and meta) more shifted than the
other side, and the para hydrogen not significantly
shifted, is consistent with a through-space effect, but
not with a through-bond effect that would be expected
if the upfield shift were due to electron donation by the
osmium-carbon bond.
Several features of the orientation of the azaallenium
ligand in 2h are noteworthy. First, the C1-N1 bond
eclipses the terpyridine N11-Os and N31-Os bonds.
Presumably this orientation is adopted for electronic
reasons, as it maximizes back-bonding by allowing
separate Os dπ orbitals to back-bond to the central
pyridine of the terpy ligand (which is the strongest
π-acceptor due to its short Os-N distance) and to the
π* orbital of the azaallenium ligand. Second, the phenyl
group bound to C1 lies over the terpyridine ring, with
the phenyl carbons C41-C46 only 3-4 Å from the mean
plane of the terpyridine. This cannot be a sterically
favorable position, and rotation of the azaallenium
fragment by 180° about the vector between the osmium
and the midpoint of the coordinated C-N bond would
give an electronically equivalent position that would
place the phenyl group between Cl1 and N31, a less
congested region of the molecule. Possibly the complex
adopts the observed orientation because the π-stacking
interaction between the electron-rich aryl group bound
to C1 and the electron-poor terpyridine is a favorable
one. Finally, the polyene chain in 2h adopts an s-cis
conformation between C2 and C3. However, this ap-
pears to be an artifact of crystal packing, since a large
vicinal coupling constant (J H2-H3 ) 10 Hz) is observed
in solution, indicative of an s-trans conformation. (The
double bonds between C3-C4 and C5-C6 are trans
both in the crystal and in solution.)
While rotation of the aryl group bound to CR is always
hindered, the barrier to rotation does vary, depending
on the steric and electronic features of the azaallenium
group. Sterically, larger substituents on the iminium
carbon impede rotation of the aryl group on CR. Thus,
while stilbene-derived compounds 2a-f, with aryl groups
on the iminium carbon, show sharp signals for the
R-aryl resonances (krot < 3 s-1 at room temperature),
complexes 2g,h , with alkenyl groups on the iminium
carbon, show broadening (krot ≈ 18 s-1) and the reso-
nances for methyl-substituted compound 2i are broader
still (krot ≈ 23 s-1). The complexes with gem-dimethyl
substitution on the iminium carbon (2m -o) show
stopped rotation on the NMR time scale at room
temperature, suggesting that the syn-methyl group
increases the steric profile of the anti-methyl group via
a buttressing effect. Electronically, more electron-donat-
ing substituents on the aryl group on CR result in slower
rotation. For example, the phenyl group in 2i shows
substantially broadened resonances, the p-tolyl group
in 2j shows moderate broadening, the p-anisyl group
in 2k shows only slight broadening, and no detectable
broadening is seen in the p-(dimethylamino)phenyl
group in the room-temperature NMR spectrum of 2l.
The presence of a slowly rotating aryl group with
Spectral data indicate that all of the compounds 2a -p
adopt structures in solution similar to that adopted by
2h in the solid state. In particular, very distinct
environments are apparent for the CH groups bound
1
upfield shifts in the H NMR can be used to assign the
regiochemistry of the azaallenium adducts 2. This
establishes, for example, that the products derived from
(12) Bo¨ttger, G.; Geisler, A.; Fro¨lich, R.; Wu¨rthwein, E.-U. J . Org.
Chem. 1997, 62, 6407-6411.
(13) Kilner, M. Adv. Organomet. Chem. 1972, 10, 115-198.
(14) Runge, W. In The Chemistry of the Allenes; Landor, S. R., Ed.;
Academic Press: New York, 1982; Vol. 3, pp 775-884.

Cleavage of Alkenes by Cationic Osmium Nitrides
Organometallics, Vol. 23, No. 8, 2004 1935
â-alkyl- or â-alkenylstyrenes (2g-p ) adopt structures
with the aryl group, rather than alkyl or vinyl groups,
on CR. Remarkably, this is true even for the R-substi-
tuted compound derived from 2-phenyl-2-butene, 2p ,
whose aryl resonances (δ 5.60, 6.65 (ortho), 6.73, 6.95
(meta), 7.25 (para)) clearly indicate that the aryl group
is bound to CR, even though this forms what is ef-
fectively a tertiary metal-carbon bond. The methyl
group on CR is shifted upfield (δ 0.94), with the CH3
group on the iminium carbon resonating at a chemical
shift (δ 2.40) typical of that seen in other products
derived from â-methylstyrenes (2i-l, δ 2.31-2.48).
The one exception to the ubiquity of aryl substitution
on CR occurs in the reaction of p-(dimethylamino)-â-
methylstyrene with 1 to form 2l as a 5:1 mixture of
isomers 2l-i and 2l-ii. The major isomer, 2l-i, does have
the p-(dimethylamino)phenyl group on CR (δ 5.29, 6.27
(ortho), 5.93, 6.37 (meta)), but the minor isomer does
not (δ 7.26 (2H, ortho), 6.67 (2H, meta)). This pattern,
coupled with the upfield shift of the CH3 group (δ 0.80),
indicates that 2l-ii is the regioisomer of 2l-i, with the
methyl group on CR and the aryl group on the iminium
carbon. Regioisomers are also observed in the reactions
of unsymmetrical stilbenes to give complexes 2b-d . In
each case the major regioisomer is the one with the
unsubstituted phenyl group bound to CR (2:1 for 2b, 4:1
for 2c, and 5:1 for 2d ; 2e exists as a single isomer by
NMR).
like the reaction of diphenylhexatriene to give 2h , is
completely regioselective, with only cleavage of the
terminal double bond. Presumably this site is preferred
for electronic rather than steric reasons, as this is the
less hindered site in methoxybutadiene but the more
1
hindered site in diphenylhexatriene. H and 13C NMR
spectra of 2q establish that the azaallenium group binds
with the CH2 group attached to osmium, with both the
equivalence of the two CH2 protons (δ 4.35, d, 2 Hz) and
the observation of equivalent 6,6′′-terpyridine protons
(δ 9.10) indicating that the molecule has a time-
averaged mirror plane. The methoxyvinyl group is
contained in that mirror plane and retains a trans
geometry and an s-trans conformation about the single
bond to the iminium carbon. Presumably the molecule
adopts the same binding orientation as seen in the
crystal of 2h , with the bound C-N bond eclipsing the
trans nitrogens of the terpyridine ligand, but rotation
of the group about the axis connecting osmium to the
midpoint of the C-N bond is rapid, as judged by the
retention of apparent Cs symmetry in acetone solution
even at -90 °C.
In contrast to the occasionally observed regioisomer-
ism, in no case do any of the isolated complexes show
evidence of stereoisomerism about the CdN bond. Thus,
all compounds with a single substituent on the iminium
carbon are assigned the E configuration about the
CdN bond, by analogy with crystallographically char-
acterized 2h and consistent with expectations from
steric effects. Complexes 2m -o (where no stereoisom-
erism is possible) show two methyl resonances in the
1H NMR, one at δ 2.40-2.43, assigned to the anti CH3
group by analogy to 2i-l, and another at δ 1.72-1.77,
assigned to the syn CH3 group. Both resonances show
long-range coupling to HR. All of the products derived
from trisubstituted alkenes also show one proton due
to the terpy 6,6′′-hydrogens that resonates downfield of
9.50 ppm; in contrast, all protons in 2a -l are upfield of
9.25 ppm. This downfield shift may be caused by a steric
interaction between the terpy and azaallenium groups.
Reaction with 2,5-dimethyl-2,4-hexadiene, which takes
place at a rate comparable to reaction with 1-methoxy-
1,3-butadiene, gives an azaallenium complex (2r ) of
opposite regioselectivity (eq 3). In this case, the presence
of the dimethylvinyl group on the carbon bound to
osmium is established by the lack of a mirror plane (e.g.,
6,6′′-H resonances at δ 9.28, 9.26) and by the inequiva-
lence of all four methyl groups (δ 2.47, 1.73, 1.40, 0.94
1
in the H NMR). The 13C NMR spectra of both 2q and
Unsubstituted styrenes such as styrene or 4-meth-
oxystyrene react with cis-[(terpy)OsNCl2]PF6 (1), but the
products formed are relatively unstable and no azaal-
lenium complex has been isolated from these reactions.
Similarly, while â-methylstyrenes with electron-donat-
ing substituents are excellent substrates for alkene
cleavage, more electron-poor p-cyano-, p-chloro-, or
p-fluoro-â-methylstyrenes react more slowly and do not
yield isolable products. The very electron-deficient
methyl cinnamate is inert to 1.
2r show the upfield shifts of the osmium-bound carbons
(δ 31.73 and 45.40, respectively) and downfield shifts
of the iminium-like carbons (δ 167.38, 183.21) charac-
teristic of the η2-azaallenium moiety.
The less electron-rich 2,4-hexadiene reacts rather
sluggishly with the nitride complex 1 to give a mixture
of three isomeric azaallenium complexes 2s, which are
isolated in 45% overall yield (eq 4). While the isomers
could not be separated from each other, they could be
identified by their distinctive coupling patterns in the
1H NMR. Ninety percent of the material consists of the
two possible regioisomers produced by cleavage of one
of the equivalent double bonds of the trans,trans-diene,
with the major isomer (2s-i) having the methyl group
bound to the iminium carbon (NdCHCH3, δ 7.35, qd,
5.5, 2.5 Hz) and the minor isomer (2s-ii) having the
methyl group bound to CR (OsCHCH3, δ 0.79, d, 5 Hz).
There is also 10% of an isomer (2s-iii) with the same
Clea va ge of 1,3-Dien es by cis-[(ter p y)OsNCl2]-
P F 6. Sufficiently electron-rich conjugated dienes also
undergo the alkene cleavage reaction exhibited by
stilbenes and substituted styrenes. Thus, while 1,3-
butadiene and 2,3-dimethyl-1,3-butadiene are rather
unreactive toward the osmium nitride complex 1, 1-meth-
oxy-1,3-butadiene reacts rapidly to give the insertion
product 2q in 88% isolated yield (eq 2). The reaction,

1936 Organometallics, Vol. 23, No. 8, 2004
Maestri et al.
regiochemistry as 2s-i, but with a cis-propenyl group
(3J HH ) 12 Hz in 2s-iii) rather than a trans-propenyl
group (3J HH ) 16 Hz in 2s-i) bound to CR. Presumably
2s-iii arises from attack on the cis,trans-diene present
in the commercial technical grade 2,4-hexadiene.
carbon directly bonded to osmium. Unlike the acyclic
compounds 2a -s, these cyclic azaallenium complexes
are somewhat unstable, decomposing in solution over
the course of a few days at room temperature. This may
reflect a modest amount of strain in these seven-
membered cyclic allenes. Free 1,2-cycloheptadiene is
estimated to be strained by about 20 kcal/mol,15 though
complexation to osmium is expected to substantially
reduce that strain (note that 1,2-cycloheptadiene is not
isolable, while a number of its metal complexes are16).
As previously described,7 cis-[(terpy)OsNCl2]PF6 (1)
reacts with the cyclic dienes 1,3-cyclohexadiene and
1-methoxy-1,3-cyclohexadiene by net [4 + 1] cycloaddi-
tion to give azanorbornene complexes of osmium(IV).
However, 1,4-dialkylcyclohexadienes do undergo alkene
cleavage to give cyclic azaallenium complexes in their
reactions with 1. For example, 1,4-dimethyl-1,3-cyclo-
hexadiene reacts rapidly with 1 to give a 2:1 mixture of
two regioisomeric azaallenium complexes 2t in overall
87% yield (eq 5). In this case, osmium is bound prefer-
R ea ct ion s of Ot h er Osm iu m (VI) Nit r id es w it h
1,3-Dien es. The cationic osmium(VI) nitrides trans-
[(terpy)OsNCl2]PF6 (3) and [(Tpm)OsNCl2]PF6 (4; Tpm
) tris(1-pyrazolyl)methane) have been reported to react
similarly to cis-[(terpy)OsNCl2]PF6 with a number of
reagents.17 However, 3 and 4 react much more slowly
with arylalkenes than does 1, and in no cases have
azaallenium complexes been isolated from these reac-
tions. For example, the tris(pyrazolyl)methane complex
4 reacts very slowly with trans-â-methylstyrene, with
only 57% consumption of 4 in 4 days at room temper-
ature (under conditions where 1 would be completely
consumed in 24 h). No azaallenium complex derived
from 4 could be identified by NMR in this reaction
mixture (maximum concentration <10% of total os-
mium), although the formation of significant amounts
of both acetaldehyde and benzaldehyde (31% and 22%
yield based on Os, respectively) on heating the reaction
mixture (in the presence of air and moisture) does
suggest that at least some alkene cleavage, probably
through an unstable azaallenium complex, is taking
place.
entially to the more substituted CdN bond of the cyclic
azaallenium ion, as indicated by the appearance of the
characteristically downfield iminium hydrogen in the
1H NMR of 2t-i (δ 7.72, d, 4 Hz), with the minor isomer
2t-ii displaying instead an upfield shift (δ 4.21) due to
HR. Note that the lack of a mirror plane in either isomer
excludes the possibility that this substrate has under-
gone [4 + 1] cycloaddition. R-Terpinene (1-isopropyl-4-
methyl-1,3-cyclohexadiene) also reacts with 1 to form a
mixture of isomeric azaallenium complexes 2u (eq 6).
Both of the inequivalent carbon-carbon double bonds
in R-terpinene are attacked, with a 3:1 preference for
cleavage of the less hindered methyl-substituted bond.
The major, methyl-substituted azaallenium ion again
forms two regioisomeric complexes differing in the site
of binding to osmium, though in this case the binding
selectivity is reversed, favoring the less substituted
CdN bond by a 2:1 ratio. Only the less substituted
CdN bond is observed to be bound to osmium in 2u -iii;
no complex is observed with the isopropyl group on the
The accessibility of alkene cleavage to the nitride
complexes 3 and 4 is demonstrated by their reactions
with certain 1,3-dienes, where stable azaallenium com-
plexes can be isolated. For example, both 3 and 4 react
with 1-methoxy-1,3-butadiene to give azaallenium com-
plexes 5q and 6q, respectively (eq 7). The resonances
(15) Early calculations15a estimated the strain energy in this allene
at 15 kcal/mol, while more recent calculations15b,c put the energy of
the cyclic cumulene 23 kcal/mol higher than that for unstrained 1,3-
cycloheptadiene:15d (a) Gasteiger, J .; Dammer, O. Tetrahedron 1978,
34, 2939-2945. (b) Angus, R. O., J r.; Schmidt, M. W.; J ohnson, R. P.
J . Am. Chem. Soc. 1985, 107, 532-537. (c) Bettinger, H. F.; Schleyer,
P. v. R.; Schreiner, P. R.; Schaefer, H. F., III. J . Org. Chem. 1997, 62,
9267-9275. (d) Roth, W. R.; Adamczak, O.; Breuckmann, R.; Lennartz,
H. W.; Boese, R. Chem. Ber. 1991, 124, 2499-2521.
(16) J ones, W. M.; Klosin, J . Adv. Organomet. Chem. 1998, 42, 147-
221.
(17) Meyer, T. J .; Huynh, M. H. V. Inorg. Chem. 2003, 42, 8140-
8160.

Cleavage of Alkenes by Cationic Osmium Nitrides
Organometallics, Vol. 23, No. 8, 2004 1937
Sch em e 2. Rea ction of tr a n s-[(ter p y)OsNCl2]P F 6
w ith 1-Meth oxy-1,3-cycloh exa d ien e To Give
Aza a llen iu m Com p lex 5v, w ith F u ll 1H NMR
Assign m en ts of th e Aza a llen iu m Moiety
due to the azaallenium moiety in these complexes are
very similar to those in the cis-terpy complex 2q, and
as in 2q, the complexes consist of a single regioisomer
with attack taking place exclusively on the less substi-
tuted bond of the diene and the osmium bound to the
less substituted bond of the azaallenium ligand. How-
ever, the trans complex 5q is spectroscopically distinct
from the cis isomer 2q. This is to be contrasted with
the reactions of the trans nitride 3 with slow-reacting
alkenes such as stilbenes, where the azaallenium prod-
ucts formed have exclusively the cis geometry around
osmium (i.e., 2a forms in the reaction of 3 with cis- or
trans-stilbene). The fact that azaallenium formation
with reactive substrates such as methoxybutadiene
takes place with retention of the stereochemistry around
osmium strongly suggests that formation of cis products
from 3 with stilbenes takes place by initial trans f cis
isomerization of the starting nitride, followed by ste-
reospecific (at Os) formation of the cis azaallenium
adducts 2. Stereospecific insertion is also seen in the
reaction of 3 with 2,5-dimethyl-2,4-hexadiene to give the
trans complex 5r , with no sign of the cis isomer 2r (eq
8). The regiochemistry of binding is the same as in the
[(terpy)OsNCl2]PF6 by [4 + 1] cycloaddition to give
azanorbornene complexes.7 However, a careful reanaly-
sis of the 1H NMR spectrum of the latter product
indicates that this formulation is incorrect and that
1-methoxy-1,3-cyclohexadiene in fact reacts with 3 at
room temperature to give the azaallenium complex 5v
(Scheme 2). An authentic [4 + 1] cycloadduct can be
observed at low temperature but readily dissociates
diene.18 Several key features of the spectrum of 5v are
incompatible with an azanorbornene structure but
consistent with an azaallenium structure. First, the 9
Hz coupling between Ha and Hg is inconsistent with Ha
occupying a bridgehead position. This coupling estab-
lishes that the Ha-C1-C7-Hg dihedral angle is about
140° (consistent with the ∼80° Ha-C1-C7-Hf dihedral
suggested by J af ) 2 Hz).19 Second, the coupling pattern
exhibited by the methylene hydrogens, with only one
large vicinal coupling (J ge ) 13 Hz, with J dg and J df both
about 4 Hz), is typical of a gauche conformation of the
CH2CH2 moiety, rather than the eclipsed conformation
imposed by the bicyclo[2.2.1] skeleton of the [4 + 1]
cycloadducts. The geminal coupling J de of 20 Hz is also
anomalously large for a norbornane skeleton and indi-
cates that the plane of the vinyl ether double bond
roughly bisects the allylic CH2 group.20 Finally, the
chemical shift of Hb (δ 8.22) is downfield of what would
be expected for an alkene (cf. δ 7.34 in the complex
trans-[(terpy)OsCl2(NC6H8)]PF67) but identical with that
shown by the acyclic azaallenium complex 5q.
cis isomer, as confirmed by the observation of four
separate methyl resonances (δ 3.06, 2.52, 2.22, 1.92),
but the terpyridine shows time-averaged symmetry in
1
the H NMR due to rapid rotation of the azaallenium
group about the vector connecting the Os to the mid-
point of the bound C-N bond. 2,4-Hexadiene also reacts
with 3, to give the trans complex 5s as a single regio-
and stereoisomer (eq 9). In this case, only binding of
The cyclic dienes 1,4-dimethyl-1,3-cyclohexadiene and
R-terpinene also react with trans-[(terpy)OsNCl2]PF6 to
give azaallenium complexes 5t and 5u , respectively, as
single regioisomers (eq 10). The spectra of the two
compounds are very similar, which suggests that it is
the osmium to the methyl-substituted carbon is ob-
served, as indicated by coupling of the iminium hydro-
3
gen (δ 8.33) to the alkene hydrogen at δ 7.17 with J )
10 Hz.
Both 1,3-cyclohexadiene and 1-methoxy-1,3-cyclohexa-
diene were previously reported to react with trans-

1938 Organometallics, Vol. 23, No. 8, 2004
Maestri et al.
the methyl-substituted double bond of R-terpinene that
is cleaved. The assignment of the isomer as the one with
the less substituted CdN bond bound to osmium is
based on the absence of a downfield-shifted iminium
1
resonance in the H NMR and on the 13C NMR, where
the iminium carbon resonances (δ 186.17 in 5t, 186.34
in 5u ) are quaternary and the CR resonances (δ 42.00,
42.23) are proton-coupled.
Regio- a n d Ster eoisom er iza tion s of Aza a llen iu m
Com p lexes. Isolated samples of the azaallenium com-
plexes 2 generally exist as single regioisomers, with the
few exceptions noted above. An important question
arises as to whether the observed distribution of isomers
is of thermodynamic or kinetic origin. While the obser-
vation of sharp, separate resonances for OsCH and
NdCH in cis-η2(C,N)-[(terpy)OsCl2(PhCHNdCHPh)]PF6
(2a ) indicates that regioisomerization is not occurring
on the NMR time scale, other studies indicate that
regioisomerization is reasonably facile on the chemical
time scale. For example, p-methoxystilbene reacts with
cis-[(terpy)OsNCl2]PF6 (1) to give a 4:1 mixture of
regioisomeric azaallenium complexes 2c, with the major
isomer containing the unsubstituted phenyl group on
the R-carbon. If this mixture is crystallized slowly, then
a pure sample of this major isomer can be obtained.
However, on dissolution of the pure major isomer in
acetonitrile, the compound isomerizes to the same 4:1
mixture of isomers initially observed with a half-life of
about 6 h at room temperature (eq 11).
F igu r e 2. Hammett plot of log K for the reaction cis-η2-
(C,N)-[(terpy)OsCl2([p-C6H4X]CHNdCHPh)]PF6 a cis-η2-
(C,N)-[(terpy)OsCl2(PhCHNdCH[p-C6H4X])]PF6 vs σX (CD3-
CN). Data for X ) NMe2 represent a lower limit on log K
and are not included in the correlation. Values for σX are
taken from ref 21.
Sch em e 3. In ter m ed ia tes in th e Rea ction of
tr a n s-p-Me2NC6H4CHdCHCH3 w ith
cis-[(ter p y)OsNCl2]P F 6 (1)
Thus, the observed distributions of regioisomers
formed from para-substituted stilbenes (2b-e) appear
to reflect the thermodynamic stability of the isomers.
The preference for the unsubstituted phenyl group for
occupying the site attached to the carbon bound to
osmium, rather than the iminium carbon, is general,
with the preference becoming more marked with the
increasing electron-donating ability of the substituent.
With the very strongly donating p-dimethylamino group,
this isomer is the only one observed by NMR (>20:1
preference). The effect can be quantified by a Hammett
analysis using the substituent σ values,21 which gives
F ) -2.0 ( 0.2 (Figure 2).
In a few other cases involving the most reactive
alkenes, nonequilibrium distributions of isomers can be
observed if reactions of the alkene are monitored by
NMR at low temperature. The reaction of p-(dimethyl-
amino)-â-methylstyrene with cis-[(terpy)OsNCl2]PF6 is
amenable to particularly detailed study, as three of the
four possible isomers are observed in kinetically well-
separated phases (Scheme 3). The initial exclusive
(>30:1) product when the reaction is allowed to proceed
at -40 °C, 2l-iii, has the aryl group on the carbon bound
1
to osmium (four aryl resonances in the H NMR), the
same regiochemistry as that of the major product
observed at equilibrium, 2l-i. 2l-iii differs from the
thermodynamic isomer in having the methyl group on
the iminium carbon syn to osmium, as judged by the
upfield chemical shift of the methyl group (δ 1.62) and
(18) Taylor, S. D.; Brown, S. N. Unpublished results.
(19) Bothner-By, A. A. Adv. Magn. Reson. 1965, 1, 195-316.
(20) Barfield, M.; Grant, D. M. J . Am. Chem. Soc. 1963, 85, 1899-
1904.
(21) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165-
195.

Cleavage of Alkenes by Cationic Osmium Nitrides
Organometallics, Vol. 23, No. 8, 2004 1939
downfield shift of one of the 6,6′′-terpy protons (δ 9.49),
both characteristic of compounds such as 2m -o, which
have syn methyl groups. At higher temperatures, the
kinetic isomer 2l-iii undergoes clean regioisomerization
to 2l-ii, which has the methyl group on CR (δ 0.80) and
the aryl group on the iminium carbon (two aryl reso-
nances) anti to osmium. Formation of 2l-ii is highly
selective at -10 °C, with further equilibration to the
equilibrium 5:1 mixture of 2l-i:2l-ii requiring ∼2 h at
room temperature. trans-Anethole behaves in a manner
qualitatively similar to p-(dimethylamino)-â-methyl-
styrene, forming a regioisomer analogous to 2l-iii at low
temperature. Isomerization of the kinetic isomer to the
thermodynamic isomer 2k is slower than that of 2l-iii
to 2l-ii but faster than that of 2l-ii to 2l-i, with a half-
life of about 4 h at -5 °C. No intermediate analogous
to 2l-ii is observed in the reaction of the trans-anethole.
Nonequilibrium distributions of isomers have also
been observed in low-temperature reactions of cis-
[(terpy)OsNCl2]PF6 (1) with the dienes 1-methoxybuta-
diene, 2,5-dimethyl-2,4-hexadiene, R-terpinene, and 1,4-
dimethyl-1,3-cyclohexadiene. The two acyclic dienes
form kinetic product mixtures in which both the isomer
present at equilibrium and its regioisomer are observed
early in the reaction (in a 1:2 ratio of 2q to its
regioisomer 2q-i for 1-methoxybutadiene and a 1:3 ratio
of 2r to 2r -i for 2,5-dimethyl-2,4-hexadiene). The cyclic
dienes react to form mixtures in which all the isomers
present at equilibrium are formed at low temperature,
but in a nonequilibrium ratio. Thus, 1 reacts with
R-terpinene to form 2u -i and 2u -ii in a 3:1 ratio,
equilibrating to 2:1, while dimethylcyclohexadiene forms
2t-i and 2t-ii in a roughly 1:1 ratio, equilibrating to 2:1.
(Intermediates are also observed prior to formation of
azaallenium complexes in both of the reactions with
cyclic dienes; the structure of these intermediates will
be the subject of a future report.) All of the isomeriza-
tions of diene-derived products take place rapidly below
room temperature. There is no sign of any isomerization
that changes which alkene has been cleaved; for ex-
ample, the ratio of products due to cleavage of the
methyl-substituted vs the isopropyl-substituted alkene
in R-terpinene (2u -i and 2u -ii vs 2u -iii) remains
unchanged even as the ratio of the regioisomers
2u -i:2u -ii evolves.
F igu r e 3. Time evolution of the reactions of cis-[(terpy)-
OsNCl2]PF6 with (a) 2,4-dimethyl-2,5-hexadiene and (b)
1,4-dimethyl-1,3-cyclohexadiene (CD3CN, -10 °C). Solid
lines represent the calculated fit to the kinetic schemes of
eqs 12 and 13 (a) and eqs 14-16 (b). In Figure 3b, I
represents an intermediate that precedes azaallenium
formation (see text). [Os] ) cis-[(terpy)OsCl2]PF6.
I is observed at equilibrium. Thus, the equilibration
between the regioisomers 2t-i and 2t-ii is most probably
a direct isomerization. The isomerization rate (k16 + k-16
) 1.21(7) × 10-4 s-1 at 263 K) for the cyclic compounds
2t is remarkably similar to the analogous isomerization
of the acyclic complexes 2r (k13 ) 0.86(7) × 10-4 s-1 at
263 K).
The isomerizations observed in the reactions of 1 with
excess 2,5-dimethyl-2,4-hexadiene and with excess 1,4-
dimethyl-1,3-cyclohexadiene have been monitored by 1H
NMR at -10 °C (Figure 3). The time evolution can be
fit well to a series of successive first-order (or pseudo-
first-order) reactions, as shown for the reaction with 2,5-
dimethyl-2,4-hexadiene (eqs 12 and 13) and for 1,4-
dimethyl-1,3-cyclohexadiene (eqs 14-16; all rate con-
stants measured in CD3CN at 263 K). Note that the
observed kinetics in the latter reaction rule out the
possibility that reversion of the products 2t-i and 2t-ii
to the intermediate I plays a significant role in their
interconversion, since the rate of appearance of the
products 2t is only moderately faster than that of their
interconversion. If the products reverted to I in order
to interconvert, then such similar rates would imply
that the forward and reverse rate constants for the
formation of 2t from I would have to be comparably
similar, contradicting the observation that no detectable

1940 Organometallics, Vol. 23, No. 8, 2004
Maestri et al.
Discu ssion
lectivity observed in all isolated complexes is thermo-
dynamic in origin. By noting the regioselectivity of the
products of reactions of 1 with 1,2-disubstituted alkenes,
one can construct a stability scale reflecting the prefer-
ence of the different substituents for location at CR
(relative to the anti position on the iminium carbon) in
complexes 2, with this preference decreasing in the
order Ph > H . p-Me2NC6H4 J CHdCR2 J CH3. The
only potential ambiguity here is in the comparison
between H and Ph, since no products have been isolated
from simple styrenes. The observation that cis-[(terpy)-
OsCl2(Ph[CH3]CdNdCHCH3)]PF6 (2p ) prefers to have
the phenyl-substituted carbon bound to osmium only
establishes such a preference conclusively relative to
location at the syn position on the iminium carbon.
However, a preference for phenyl over hydrogen at CR
seems likely, since it appears to outweigh an otherwise
noticeable reluctance to bind a tertiary center to osmium
(compare, for example, the exclusive location of the
C(CH3)2 group bonded to nitrogen and CHCHdCMe2
bonded to Os in 2r to the mere 2:1 preference for this
location of CH(CH3) vs CHCHdCHMe in 2s).
Scop e of Alk en e Clea va ge by Osm iu m Nitr id es.
The cationic terpyridine complex cis-[(terpy)OsNCl2]PF6
(1; terpy ) 2,2′:6′,2′′-terpyridine) reacts with a remark-
ably wide variety of conjugated alkenes to give aza-
allenium complexes, the products of net nitrogen atom
insertion into a CdC double bond. This nitride complex
can thus cleave extremely strong CdC double bonds
(BDE ≈ 155 kcal/mol) under remarkably mild conditions
(at or below room temperature in most cases). The
mechanism of this transformation will be discussed in
a forthcoming publication.22 Yields of the reaction are
generally high, and the reactions can be carried out in
the presence of air and moisture. Phenyl-substituted
alkenes or alkenes substituted with electron-rich aryl
groups react readily, with both 1,2-disubstituted and
trisubstituted alkenes giving stable products. A variety
of acyclic dienes react, as do 1,4-dialkylcyclohexadienes,
to give mildly strained complexes with seven-membered
azaallenium rings.
In contrast to normal patterns of organometallic
reactivity, the more substituted alkenes tend to react
more cleanly with 1. Thus, simple styrenes have so far
failed to yield isolable products with 1, although they
do induce decomposition of the osmium nitride. Less
substituted dienes, such as butadiene or isoprene, react
only very slowly with 1 and have not yielded isolable
products. There is a definite preference for electron-rich
alkenes, with electron-poor alkenes such as cinnamate
esters completely unreactive toward 1.
Other cationic nitrides, such as trans-[(terpy)OsNCl2]-
PF6 (3) and [(Tpm)OsNCl2]PF6 (4; Tpm ) tris(1-pyra-
zolyl)methane), also react with alkenes to give aza-
allenium complexes, although the scope of these reac-
tions is more limited. In particular, while a number of
dienes react with 3 and 4, we have not yet been able to
isolate any products from the reactions of arylalkenes
with these other nitrides. Arylalkenes such as â-meth-
ylstyrene do react with these nitrides, although the rate
is significantly slower than the rate of reaction with 1.
Indirect evidence suggests that azaallenium complexes
are formed in these reactions (for example, the observa-
tion of benzaldehyde and acetaldehyde when 4 is heated
with â-methylstyrene in the presence of air and mois-
ture). Thus, while the cis-terpyridine complex 1 appears
to be quantitavely best suited to this reaction, in terms
of both rates of reaction and stability of the products,
there is nothing that sets this complex qualitatively
apart from other cationic osmium nitrides. On the other
hand, less electrophilic neutral nitrides such as TpOsNCl2
(Tp ) hydridotris(1-pyrazolyl)borate)23 show little or no
reactivity toward alkenes that are readily cleaved by 1.
Th er m od yn a m ic Regioselectivity in Aza a llen i-
u m Bin d in g. Most of the azaallenium complexes cis-
[(terpy)OsCl2(RR′CdNdCHR′′)]PF6 (2) display substan-
tial regioselectivity with respect to which CdN double
bond is bound to osmium. In all cases where we have
been able to observe a nonequilibrium mixture of
regioisomers, isomerization takes place within hours at
room temperature; thus, we presume that the regiose-
On simple steric grounds, one would expect larger
substituents to avoid being located on the carbon bound
to osmium. However, apart from H, the reverse trend
is observed and is most marked for aryl substituents,
with phenyl groups strongly preferred in the R-position
over alkenyl or methyl, and probably even over hydro-
gen. A plausible explanation for the high predilection
for placing aryl (and possibly to a lesser extent, alkenyl)
groups on CR is that this allows their electron-rich π
systems to interact favorably by a π-stacking interaction
with the electron-poor π-system of the terpyridine
ligand. The importance of such interactions is suggested
by the observed orientation of the phenyl group in the
crystal structure of 2h , which appears to be maintained
in solution, judging from the anomalous chemical shifts
and hindered rotation observed in aryl-substituted
complexes 2a -p . Invoking π-stacking also explains the
qualitative trends observed in the aryl group rotation
rates of cis-η2(C,N)-[(terpy)OsCl2(ArCHdNdCHCH3)]-
PF6 (2i-l): the more electron-rich aryl groups should
interact more favorably with the terpyridine, and this
interaction will be lost in the transition state for aryl
group rotation, consistent with the observation that the
more electron-rich aryl groups rotate more slowly.
Finally, the trans azaallenium complexes 4, where
π-stacking is impossible, show only the expected steric
preferences for location at CR, with H . CH3 . CHd
CR2, and no examples of tertiary carbons bound to
osmium. While data are limited due to the paucity of
trans compounds (no aryl-substituted complexes are
available, for instance), the pattern is clearly distinct
from that observed with the cis complexes (compare, for
example, 2r -t with 5r -t).
Yet if the preference for aryl groups at CR is due to
π-stacking, it is surprising that the more electron-rich
aryl groups actually show a lower preference for locating
at this position. This trend is evident in the regioselec-
tivities of monosubstituted stilbenes (Figure 2) and in
the regioselectivity of azaallenium binding in cis-[(terpy)-
OsCl2(ArCHdNdCHCH3)]PF6 (2i-l), where only the
p-dimethylamino compound exhibits any of the R-CH3
regioisomer at equilibrium. However, electron-donating
(22) Maestri, A. G.; Braunecker, W. A.; Brown, S. N. Manuscript in
preparation.
(23) Crevier, T. J .; Mayer, J . M. J . Am. Chem. Soc. 1998, 120, 5595-
5596.

Cleavage of Alkenes by Cationic Osmium Nitrides
Organometallics, Vol. 23, No. 8, 2004 1941
Sch em e 4. P ossible Mech a n ism s for Regio- a n d
Ster eoisom er iza tion s of Aza a llen iu m Com p lexes
substituents are expected to interact favorably with the
electron-poor iminium carbon. For example, organic
reactions that involve deconjugation of carbon-hetero-
atom multiple bonds are typically disfavored by electron-
donating groups, such as in the hydration of benzalde-
hydes (F ) +1.7)24 or in the addition of methanol to
phenylimines of substituted benzaldehydes (F ) +1.1).25
Apparently, the stabilization of the iminium carbon is
more sensitive to the electron-donating ability of the aryl
group than is the strength of the π-stacking interaction,
which leads to a net preference for more electron-rich
aryl groups to be located on the iminium carbon.
Nevertheless, all of the data are consistent with a
significant interaction due to π-stacking that stabilizes
the R-aryl isomers relative to what would be expected
on the basis of steric considerations.
Mech a n ism of Regio- a n d Ster eoisom er iza tion s
of Osm iu m Aza a llen iu m Com p lexes. Nitrogen atom
insertion into CdC double bonds to form azaallenium
complexes 2, 5, and 6 appears to be irreversible. For
example, both cis- and trans-stilbene react with cis-
[(terpy)OsNCl2]PF6 (1) to form the same azaallenium
complex 2a , but the unreacted stilbene retains its
original geometry in both cases. This indicates that
dissociation of alkene and reinsertion is not occurring.
Even in cases where intramolecular migrations would
be possible, the site of nitrogen atom insertion does not
appear to be mobile. For example, the insertion products
derived from symmetrical butadienes (2g,r -t, 5r -t)
invariably show distinct resonances for the groups
attached to cleaved and uncleaved double bonds in the
NMR. Even on the chemical time scale, the invariance
with time of the ratio of products due to cleavage of the
methyl-substituted alkene in R-terpinene (2u -i and 2u -
ii) to the product due to cleavage of the isopropyl-
substituted alkene (2u -iii), even as the ratio of 2u -i to
2u -ii does change, strongly suggests that the carbon-
nitrogen bonds are set irreversibly in the alkene cleav-
age reaction.
In contrast, a number of azaallenium complexes 2
have been observed to undergo isomerizations affecting
the bonding of osmium to the CdNdC unit of the
azaallenium moiety. Both regioisomerizations (with the
osmium migrating between the two CdN bonds of the
azaallenium group in 2c,l,q,r ,t,u ) and stereoisomeriza-
tions (interchanging the syn and anti substituents on
the iminium group, in 2k ,l) have been seen. The most
illuminating example of this series of isomerizations is
provided by complex 2l, where regioisomerization and
stereoisomerization are both observed (Scheme 3). In
this case, the kinetically observed isomer cis,syn-η2-
(C,N)-[(terpy)OsCl2(ArCHdNdCHCH3)]PF6 (2l-iii, Ar )
p-Me2NC6H4) first undergoes regioisomerization to cis,an-
ti-η2(N,C)-[(terpy)OsCl2(ArCHdNdCHCH3)]PF6 (2l-ii),
which subsequently undergoes a second regioisomer-
ization to give the geometric isomer of 2l-iii, cis,anti-
η2(C,N)-[(terpy)OsCl2(ArCHdNdCHCH3)]PF6 (2l-i). No
direct stereoisomerization from 2l-iii to 2l-i is observed.
Although only stereoisomerization is observed in 2k , we
presume that regioisomerization precedes stereoisomer-
ization as for 2l but that the relative rate constants
differ enough that the intermediate regioisomer never
accumulates significantly.
Three plausible mechanisms for isomerization of the
azaallenium group are illustrated in Scheme 4. The
three pathways differ in the local symmetry of the
intermediates or transition states (neglecting details of
the substitution of the carbons): an η3-azaallyl inter-
mediate (path A) would have local Cs symmetry and an
η1-azaallyl intermediate (path B) would have C2v sym-
metry, while the transition state in path C would have
local C2 symmetry. The “allene-roll”26 mechanism of
path C, first suggested by Pettit27 and subsequently
investigated in detail by Vrieze28 and Rosenblum,29 is
well-known in metal-allene chemistry. The all-carbon
analogue of path B has also been suggested to explain
mutarotation of platinum allene complexes,30 and while
path A is unlikely in the all-carbon case because of the
need to develop a σ lone pair on the central carbon, it is
reasonable here with nitrogen in the central position.
Note that all three pathways predict that stereoisomer-
ization requires a two-step mechanism involving initial
regioisomerization followed by a regioreversion ac-
companied by stereochemical change. Formation of an
allyl intermediate would presumably proceed with
retention of stereochemistry at the iminium double
bond, so accessing the other stereoisomer requires prior
coordination to the iminium carbon (with the concomi-
tant twisting of the stereocenter). In the allene-roll
mechanism, access to the other face of the initially
coordinated CdN bond requires a transit through
coordination of the other CdN bond. Thus, while the
observation of stepwise isomerization in 2l is very
striking and does rule out gross dissociation of the
azaallenium ion from the osmium, it is of no use in
discriminating among the intramolecular pathways
A-C.
These pathways can be distinguished by contrasting
the behavior of the acyclic complex 2r , derived from 2,5-
(26) Gibson, V. C.; Parkin, G.; Bercaw, J . E. Organometallics 1991,
10, 220-231.
(27) Ben-Shoshan, R.; Pettit, R. J . Am. Chem. Soc. 1967, 89, 2231-
2232.
(28) Vrieze, K.; Volger, H. C.; Praat, A. P. J . Organomet. Chem.
1970, 21, 467-475.
(29) Foxman, B.; Marten, D.; Rosan, A.; Raghu, S.; Rosenblum, M.
J . Am. Chem. Soc. 1977, 99, 2160-2165.
(30) Cope, A. C.; Moore, W. R.; Bach, R. D.; Winkler, H. S. J . Am.
Chem. Soc. 1970, 92, 1243-1247.
(24) McClelland, R. A.; Coe, M. J . Am. Chem. Soc. 1983, 105, 2718-
2725.
(25) Calculated using substituent constants in ref 21 and equilib-
rium data from: Toullec, J .; Bennour, S. J . Org. Chem. 1994, 59, 2831-
2839.

1942 Organometallics, Vol. 23, No. 8, 2004
Maestri et al.
dimethyl-2,4-hexadiene, and its cyclic analogue 2t,
derived from 1,4-dimethyl-1,3-cyclohexadiene. Complex
2t contains a seven-membered ring, which would be
expected to be modestly strained because of the twist
of the azaallenium moiety. This strain should be re-
lieved on going to the planar allyl structures in path-
ways A and B, but not on going to the allene-like
transition state of pathway C. Thus, one would expect
that relief of strain would cause the cyclic complex 2t
to isomerize substantially faster than the acyclic com-
plex 2r if mechanism A or B operates. In fact, the two
complexes isomerize at very similar rates, with the
cyclic complex isomerizing less than 50% faster than the
acyclic complex at -10°. This is most consistent with
mechanism C.
This analysis closely parallels the studies of J ones and
co-workers on iron complexes of 1,2-cycloheptadiene,
[CpFe(CO)(L)(η2-C7H10)]+ (L ) CO (7a ), PPh3 (7b)).31
NMR experiments showed that allene proton exchange
takes place without interconversion of the diastereotopic
carbonyls (7a ) or diastereomeric complexes (7b), dem-
onstrating that the complexes isomerize exclusively by
the allene-roll mechanism, just as the acyclic analogues
do.28 However, the slow interconversion of the diaster-
eomers of the triphenylphosphine complex indicated
that the allyl cation intermediate was accessible, albeit
at ∼4-5 kcal/mol higher energy than the allene-roll
pathway. Apparently, the osmium azaallenium com-
plexes 2, like the iron allene complexes, prefer to
isomerize by an allene-roll mechanism, and the modest
strain in the seven-membered ring is insufficient to
divert them into a different mechanism in cyclic 2t or
7. One notable difference in the two cases is that, despite
following the same mechanism, the cyclic iron complex
7a does isomerize substantially faster than its acyclic
analogues (∆∆Gq > ∼2.4 kcal/mol),31a while the osmium
complexes 2r and 2t isomerize at very similar rates.
The limited data available on the electronic effects
on this isomerization are also consistent with mecha-
nism C. As the metal moves from binding to one C-N
bond to the other one, one would expect compensating
changes in the electron demand of the two carbons, with
CR becoming less electron-rich and the iminium carbon
becoming less electron-poor in the transition state.
Thus, electron-donating substituents on CR should ac-
celerate isomerization and electron-donating substitu-
ents on the iminium carbon should slow it down. Indeed,
the p-(dimethylamino)phenyl-substituted complex 2l
undergoes these reciprocal isomerizations at very dif-
ferent rates, with the isomerization that moves the aryl
group from CR to the iminium carbon (2l-iii f 2l-ii)
requiring ∼20 min at -10 °C and the isomerization that
moves the aryl group from the iminium carbon to CR
(2l-ii f 2l-i) requiring ∼2 h at 20 °C. The first
isomerization is significantly slower for the p-methoxy-
phenyl-substituted compound 2k (t1/2 ≈ 4 h at -5 °C),
and the second apparently significantly faster, since the
regioisomeric compound does not accumulate at -5 °C.
Note that one would expect mechanism A to be little
affected by the electronics at CR (since that carbon
remains bound to Os in the intermediate), while mech-
anism B should be relatively insensitive to substitution
at the iminium carbon (since that carbon remains
unbound).
Con clu sion s
The cationic nitride complex cis-[(terpy)OsNCl2]PF6
(1) reacts with a wide variety of conjugated alkenes by
net nitrogen atom insertion to give the η2-azaallenium
complexes 2. Reactions generally occur in high yield and
take place under mild conditions and in the presence of
air and moisture. trans-[(terpy)OsNCl2]PF6 (3) and
[(Tpm)OsNCl2]PF6 (4) react similarly, although the
scope of their reactions is more limited. The aryl-
substituted azaallenium complexes cis-[(terpy)OsCl2(η2-
(C,N)-ArR1CdNdCR2R3)]PF6 (2a-p) preferentially adopt
conformations in which the aryl group lies over the
terpyridine ring, both in the solid state and in solution,
apparently because of favorable π-stacking interactions.
The azaallenium groups in 2 undergo slow regioisomer-
ization, by an allene-roll type mechanism. Net stereo-
isomerization about the NdC bond is achieved by two
successive regioisomerizations, and both steps in this
sequence have been observed in one example.
Exp er im en ta l Section
Gen er a l P r oced u r es. cis-[(terpy)OsNCl2]PF6 (1) and trans-
[(terpy)OsNCl2]PF6 (3) were prepared by the method of Wil-
liams, Coia, and Meyer.8 Tpm32 and [(Tpm)OsNCl2]PF6 (4)33
was prepared according to literature procedures. All anhydrous
reactions were performed using oven-dried glassware under
nitrogen or argon. When needed during the organic syntheses,
anhydrous diethyl ether (Et2O) and tetrahydrofuran (THF)
were obtained by vacuum transfer of the respective solvent
from sodium benzophenone ketyl. All organometallic manipu-
lations were performed on the benchtop without precautions
to exclude air or moisture. 1H and 13C{1H} NMR spectra were
obtained on a General Electric GN-300 or a Varian Unity Plus
300 or 500 NMR spectrometer. Unless otherwise noted, all
NMR spectra were measured in CD3CN solution. IR spectra
were measured as evaporated films on a Perkin-Elmer PARA-
GON 1000 FT-IR spectrometer. Fast atom bombardment mass
spectra were obtained on
a J EOL J MS-AX505HA mass
spectrometer using 3-nitrobenzyl alcohol as a matrix. Peaks
reported are the mass number of the most intense peak of
isotopic envelopes; in all cases isotope patterns were in
agreement with values calculated from the molecular formulas.
Elemental analyses were performed by M-H-W Laboratories
(Phoenix, AZ).
Alkene substrates were commercially available or prepared
using literature procedures as noted, except for p-methyl-trans-
â-methylstyrene, p-(dimethylamino)-trans-â-methylstyrene,
p-(dimethylamino)-trans-â,â-dimethylstyrene, and (E)-2-phen-
yl-2-butene, whose preparations are described in the Support-
ing Information. Details of the preparation of certain key
osmium complexes are described in detail below, with other
compounds prepared by analogous procedures as noted. Full
1H and partial 13C{1H} NMR and IR data are given for selected
compounds. Full synthetic, spectroscopic (1H and 13C{1H}
NMR, IR, FABMS), and analytical details are provided for all
compounds in the Supporting Information.
cis-[(ter p y)OsCl2(1,2-η2-P h CHdNdCHP h )]P F6 (2a ). Into
a 50 mL pear-shaped round-bottom flask were weighed 169.8
(32) Grattan, T. C.; Brown, K. J .; Little, C. A.; J aydeep, J . S. L.;
Rheingold, A. L.; Reger, D. L. J . Organomet. Chem. 2000, 607, 120-
128.
(33) El-Samanody, E.-S.; Demadis, K. D.; Meyer, T. J .; White, P. S.
Inorg. Chem. 2001, 40, 3677-3686.
(31) (a) Manganiello, F. J .; Oon, S. M.; Radcliffe, M. D.; J ones, W.
M. Organometallics 1985, 4, 1069-1072. (b) Oon, S. M.; J ones, W. M.
Organometallics 1988, 7, 2172-2177.

Cleavage of Alkenes by Cationic Osmium Nitrides
Organometallics, Vol. 23, No. 8, 2004 1943
mg of cis-[(terpy)OsNCl2]PF6 (0.260 mmol) and 222.0 mg of
cis-stilbene (Aldrich; 1.23 mmol, 4.7 equiv). A 21 mL portion
of acetonitrile was added and the flask capped with a rubber
septum. The reaction mixture was heated in a 65 °C oil bath
for 17 h, over which time the solution turned dark red. The
acetonitrile was evaporated on a rotary evaporator, the residue
taken up in ∼10 mL of dichloromethane, and the solution set
aside. A considerable amount of undissolved material re-
mained in the flask. This was dissolved in CH3CN, the solution
stripped to dryness, and the residue extracted with a second
portion of CH2Cl2, which was combined with the first extract.
The combined CH2Cl2 extracts were allowed to stand over-
night, and the azaallenium complex was then isolated as a
dichloromethane solvate by suction filtration, washing with
10 mL of CH2Cl2 and 10 mL of Et2O, and then air-drying.
Yield: 157.1 mg (66%). 1H NMR: δ 5.48 (d, 8 Hz, 1H; ortho
OsCHPh); 6.49 (t, 7.5 Hz, 1H; meta OsCHPh), 6.62 (d, 2 Hz,
1H; OsCHPh), 6.72 (d, 7.5 Hz, 1H; ortho′ OsCHPh), 7.02 (t,
7.5 Hz, 1H; meta′ OsCHPh), 7.19 (tt, 7.5, 1 Hz, 1H; para
OsCHPh), 7.32 (t, 7.5 Hz, 2H; meta NdCHPh), 7.43 (m, 3H;
o, p-NdCHPh), 7.84 (ddd, 7, 5.5, 2 Hz, 1H; terpy H-5), 7.94 (t,
8 Hz, 1H; terpy H-4′), 7.96 (m, 1H; terpy H-5′′), 8.02 (dd, 8,
0.5 Hz, 1H; terpy H-3′), 8.09 (m, 2H; terpy H-3,4), 8.21 (d, 2
Hz, 1H; NdCHPh), 8.23 (dd, 8, 0.5 Hz, 1H; terpy H-5′), 8.26
(m, 2H; terpy H-3′′,4′′), 9.09 (dd, 5.5, 1 Hz, 1H; terpy H-6),
9.15 (dt, 5.5, 1 Hz, 1H; terpy H-6′′). 13C{1H} NMR: δ 163.12
(CHdN), 44.75 (OsCH). Anal. Calcd for C30H25Cl4F6N4OsP: C,
39.23; H, 2.74; N, 6.10. Found: C, 39.04; H, 2.26; N, 6.52.
cis-[(t er p y)OsCl2(η2(C,N)-P h CH dNdCH [C6H 4CH 3])]-
P F 6 (2b). A mixture of 86.0 mg of cis-[(terpy)OsNCl2]PF6
(0.1316 mmol), 128 mg of 4-methyl-trans-stilbene34 (0.6580
mmol, 5 equiv), a magnetic stirbar, and 9 mL of acetonitrile
was stirred in a 25 mL round-bottom flask for 42 h in a 65 °C
oil bath. The resulting dark red solution was reduced to half
its original volume on the rotary evaporator and then layered
with ether. The mixture was allowed to stand overnight in a
-20 °C freezer. The crystals were suction-filtered on a frit and
washed with 3 × 60 mL of Et2O to give 83.4 mg of dark red
2b, as a 2:1 mixture of isomers (total yield 75%). 1H NMR:
major isomer (Ph on CR), δ 9.16 (d, 5 Hz, 1H; terpy H-6′′), 9.08
(d, 5 Hz, 1H; terpy H-6), 8.50-7.85 (m, 9H; other terpyridine
protons), 8.28 (d, 2.5 Hz, 1H; NdCHAr), 7.32 (d, 8.5 Hz, 2H;
ortho NdCHAr), 7.26 (t, 8 Hz, 1H; para), 7.08 (d, 8 Hz, 2H;
meta NdCHAr), 7.00 (t, 8 Hz, 1H; meta′), 6.86 (d, 8.5 Hz, 1H;
ortho′), 6.61 (m, 1H; OsCHPh, partially obscured by minor
resonances), 6.48 (t, 7.5 Hz, 1H; meta), 5.64 (d, 8 Hz, 1H;
ortho), 2.19 (s, 3H; CH3); minor isomer (p-tolyl on CR, partial),
δ 9.14 (d, 5 Hz, 1H; terpy H-6′′), 9.06 (d, 5 Hz, 1H; terpy H-6),
8.50-7.85 (other terpyridine resonances, obscured), 8.42 (d,
2.5 Hz, 1H; NdCHPh), 7.45 (d, 8.5 Hz, 2H; ortho), 7.14 (t, 7.5
Hz, 1H; para), 6.83 (d, 8 Hz, 1H; meta Ar), 6.61 (m, 2H, OsCH
and meta′ Ar H, overlapped with major OsCHPh), 6.31 (d, 8
Hz, 1H; ortho′ or ortho Ar), 5.52 (d, 8.5 Hz, 1H; ortho′ or ortho
Ar), 2.04 (s, 3H; CH3). 13C{1H} NMR: δ 45.37 (OsCH, minor)
44.85 (OsCH, major), 21.66 (CH3, major), 21.4 (CH3, minor).
Anal. Calcd for C30H25Cl2F6N4OsP: C, 42.51; H, 2.97; N; 6.61.
Found: C, 42.32; H, 3.05; N, 6.43.
The acetonitrile was evaporated from the dark red solution
and the residue washed with ether. The red oil was taken up
in 1.5 mL of CH3CN, layered with 5 mL of Et2O, and allowed
to stand overnight. The red crystals were suction filtered on a
fritted-glass funnel, washed with 3 × 5 mL of Et2O, and air-
1
dried to yield 53.1 mg of 2i (81%). H NMR: δ 2.38 (dd, 5.4,
1.5 Hz, 3H; NdCHCH3), 5.56 (br d, 7 Hz, 1H; ortho), 6.29 (sl
br, 1H; OsCHPh), 6.67 (br m, 2H; meta, ortho′), 6.96 (br t, 7
Hz, 1H; meta′), 7.24 (tt, 7, 1 Hz, 1H; para), 7.40 (qd, 5.4, 2.5
Hz, 1H; NdCHCH3), 7.84 (ddd, 7.5, 5.5, 1.5 Hz, 1H; terpy H-5),
7.88 (t, 8 Hz, 1H; terpy H-4′), 7.95 (dd, 8, 1 Hz, 1H; terpy H-3′),
7.98 (ddd, 7, 5.5, 2 Hz, 1H; terpy H-5′′), 8.00 (ddd, 8, 1.5, 0.5
Hz, 1H; terpy H-3), 8.08 (td, 7, 1.5 Hz, 1H; terpy H-4), 8.25
(dd, 8, 1 Hz, 1H; terpy H-5′), 8.31 (m, 2H; terpy H-3′′,4′′), 9.10
(ddd, 5.5, 1.5, 0.5 Hz, 1H; terpy H-6), 9.13 (ddd, 5.5, 2, 1 Hz,
1H; terpy H-6′′). 13C{1H} NMR: δ 23.75 (CH3), 42.35 (Os-
CHPh), 168.57 (NdCHCH3). Anal. Calcd for C24H21Cl2F6N4-
OsP: C, 37.36, H, 2.74; N, 7.26. Found: C, 37.18; H, 2.80; N,
7.44.
Also prepared by this route was cis-[(terpy)OsCl2(1,2-η2-[4-
MeOC6H4]CHdNdCMe2)]PF6 (2n , 71%).
cis-[(t e r p y)OsCl2(η2(C,N )-P h CH dN dCH [C6H 4OH ])]-
P F 6 (2d ). A mixture of 95.4 mg of cis-[(terpy)OsNCl2]PF6
(0.1460 mmol), 143 mg of 4-hydroxy-trans-stilbene (Pfaltz and
Bauer, 0.730 mmol, 5.0 equiv), a magnetic stirbar, and 10 mL
of acetonitrile were stirred in a 25 mL round-bottom flask for
36 h at room temperature. The resulting dark solution was
reduced to half its original volume on the rotary evaporator
and then layered with ether. The mixture was allowed to stand
overnight in a -20 °C freezer. The crystals were suction-
filtered on a fritted funnel, washed with 3 × 50 mL of Et2O,
and air-dried to give 88.0 mg of 2d (5:1 mixture of isomers,
total yield 71%). 1H NMR: major isomer (Ph on CR), δ 9.20
(dt, 5.5, 1 Hz, 1H; terpy H-6′′), 9.13 (dd, 5.5, 1 Hz, 1H; terpy
H-6), 8.31-7.95 (m, 7 H), 7.91 (d, 2.1 Hz, 1H; NdCHAr), 7.87
(t, 8 Hz, 1H; terpy H-4′), 7.86 (ddd, 7, 5.5, 2 Hz, 1H; terpy
H-5), 7.31 (m, 2H; ortho NdCHAr), 7.21 (t, 7.5 Hz, 1H; para
OsCHPh), 7.02 (t, 7.5 Hz, 1H; meta OsCHPh), 6.72 (d, 8.5 Hz,
2H; meta NdCHAr), 6.52 (m, 2H; meta and ortho OsCHPh),
6.48 (d, 2.1 Hz, 1H; OsCHPh), 5.47 (d, 8 Hz, 1H; ortho
OsCHPh); minor isomer (p-HOC6H4 on CR, partial), δ 9.14 (dd,
5.5, 1 Hz, 1H; terpy H-6′′), 9.08 (dd, 5.5, 1 Hz, 1H; terpy H-6),
8.31-7.95 (8H, terpy, overlapped with major isomer), 7.82
(ddd, 7, 5.5, 1.5 Hz, 1H; terpy H-5), 7.44 (d, 7.5 Hz, 2H; ortho
NdCHPh), 7.33 (tt, 7.5, 1 Hz, 1H; para NdCHPh), 5.95 (dd,
8.5, 2.5 Hz, 1H; meta OsCHAr), 5.33 (dd, 8.5, 2.5 Hz, 1H; ortho
OsCHAr). 13C{1H} NMR: major isomer, δ 163.83 (NdCHAr),
44.98 (OsCHPh). IR (cm-1): 3300 (m, v br, νOH). Anal. Calcd
for C29H23Cl2F6N4OOsP: C, 41.00; H, 2.73; N, 6.59. Found: C,
40.82; H, 2.55; N, 6.43.
The following compounds were prepared analogously: cis-
[(terpy)OsCl2(η2(C,N)-PhCHdNdCH[C6H4NMe2])]PF6 (2e, 82%),
cis-[(terpy)OsCl2(η2(C,N)-[MeC6H4]CHdNdCHMe)]PF6 (2j, 78%),
cis-[(terpy)OsCl2(η2(C,N)-[MeOC6H4]CHdNdCHMe)]PF6 (2k ,
90%), cis-[(terpy)OsCl2(η2(C,N)-[Me2NC6H4]CHdNdCHMe)]-
PF6 (2l, 90%), cis-[(terpy)OsCl2(η2(C,N)-PhCHdNdCMe2)]PF6
(2m , 72%), cis-[(terpy)OsCl2(η2(C,N)-[Me2NC6H4]CHdNdCMe2)]-
PF6 (2o, 86%), cis-[(terpy)OsCl2(η2(C,N)-Ph(Me)CdNdCHMe)]-
PF6 (2p , 76%), cis-[(terpy)OsCl2(η2(C,N)-H2CdNdCHCHd
CHOMe)]PF6 (2q, 88%), and cis-[(terpy)OsCl2(η2(C,N)-[Me2Cd
CH]CHdNdCMe2)]PF6 (2r , 87%).
Spectroscopic data for 2l, as a 5:1 mixture of isomers 2l-i
(p-Me2NC6H4 on CR, NdCHCH3 anti to Os) and 2l-ii (CH3 on
CR), are as follows. 1H NMR: 2l-i, δ 9.12 (ddd, 5.5, 1.5, 0.5
Hz, 1H; terpy H-6′′), 9.07 (ddd, 5.5, 1.5, 0.5 Hz, 1H; terpy H-6),
8.48 (br t, 8 Hz, 1H; terpy H-5′′), 8.29 (ddd, 8, 1.5, 0.5 Hz, 1H;
terpy H-3′′), 8.26 (td, 8, 1.5 Hz, 1H; terpy H-4′′), 8.21 (dd, 8,
0.5 Hz, 1H; terpy H-5′), 8.07 (td, 8, 1.5 Hz, 1H; terpy H-4),
8.01 (ddd, 8, 1.5, 0.5 Hz, 1H; terpy H-3), 7.95 (dd, 8, 0.5 Hz,
1H; terpy H-3′), 7.85 (t, 8 Hz, 1H; terpy H-4′), 7.76 (ddd, 7.5,
5.5, 1.5 Hz, 1H; terpy H-5), 7.52 (qd, 5.5, 2.5 Hz, 1H;
Also prepared by this method were cis-[(terpy)OsCl2(η2(C,N)-
PhCHdNdCH[MeOC6H4])]PF6 (2c, 4:1 mixture of isomers,
71%), cis-[(terpy)OsCl2(1,2-η2-[4-MeOC6H4]CHdNdCH[4-Me-
OC6H4])]PF6 (2f, 85%), cis-[(terpy)OsCl2(1,2-η2-PhCHdNd
CHCHdCHPh)]PF6 (2g, 79%), and cis-[(terpy)OsCl2(1,2-η2-
PhCHdNdCHCHdCHCHdCHPh)]PF6 (2h , 61%).
cis-[(ter py)OsCl2(1,2-η2-P h CHdNdCHCH3)]P F6 (2i). Into
a screw-cap vial were placed 55.8 mg of cis-[(terpy)OsNCl2]-
PF6 (0.0854 mmol), 125.6 mg of trans-â-methylstyrene (Acros,
1.063 mmol, 12.4 equiv), and 5.5 mL of CH3CN. The vial was
capped and allowed to stand at room temperature for 24 h.
(34) Reetz, M. T.; Westermann, E.; Lohmer, R.; Lohmer, G. Tetra-
hedron Lett. 1998, 39, 8449-8452.

1944 Organometallics, Vol. 23, No. 8, 2004
Maestri et al.
NdCHCH3), 6.37 (dd, 9, 2.5 Hz, 1H; meta′), 6.27 (dd, 9, 2.5
Hz, 1H; ortho′), 6.08 (br, 1H; OsCHAr), 5.93 (dd, 9, 2.5 Hz,
1H; meta), 5.29 (dd, 9, 2 Hz, 1H; ortho), 2.88 (s, 6H; N(CH3)2),
2.31 (dd, 5.5, 2 Hz, 3H; NdCHCH3); 2l-ii, δ 9.20 (ddd, 5.5,
1.5, 0.5 Hz, 1H; terpy H-6′′), 9.11 (ddd, 5.5, 1.5, 0.5 Hz, 1H;
terpy H-6), 8.37 (td, 7.5, 1.5 Hz, 1H; terpy H-4′′), 8.37 (dd, 8,
1 Hz, 1H; terpy H-3′′), 8.33 (dd, 8, 1 Hz, 1H; terpy H-3), 8.31
(td, 7.5, 1.5 Hz, 1H; terpy H-4), 8.00 (obscured, 1H; terpy H-5′′),
7.97-7.73 (m, 3H; terpy H-3′,5′, 5), 7.95 (t, 8 Hz, 1H; terpy
H-4′), 7.26 (d, 9.5 Hz, 2H; ortho Ar), 7.24 (d, 2 Hz, 1H;
NdCHAr), 6.67 (d, 9.5 Hz, 2H; meta Ar), 5.08 (qd, 5.5, 2 Hz,
1H; OsCHCH3), 3.04 (s, 6H, N(CH3)2), 0.80 (d, 5.5 Hz, 3H;
OsCHCH3). 13C{1H} NMR ((CD3)2CO, 2l-i only): δ 168.05
(NdCHCH3), 56.00 ([CH3]2N), 43.69 (OsCHAr), 23.46
(NdCHCH3). Anal. Calcd for C26H26Cl2F6N5OsP: C, 38.34; H,
3.22; N, 8.60. Found: C, 38.35; H, 3.10; N, 8.50.
Spectroscopic data for 2m are as follows. 1H NMR: δ 9.51
(ddd, 5.5, 2, 1 Hz, 1H; terpy H-6′′), 9.22 (ddd, 5.5, 2, 1 Hz, 1H;
terpy H-6), 8.52 (d, 8.5 Hz, 1H; terpy H-3′′), 8.44 (td, 8, 1.5
Hz, 1H; terpy H-4′′), 8.31 (dd, 8, 0.5 Hz, 1H; terpy H-5′), 8.08
(td, 8, 1.5 Hz, 1H; terpy H-4), 8.03 (ddd, 7, 5.5, 2 Hz, 1H; terpy
H-5′′), 7.92 (d, 8.5 Hz, 1H; terpy H-3), 7.88 (ddd, 7, 5.5, 2 Hz,
1H; terpy H-5), 7.84 (dd, 8, 0.5 Hz, 1H; terpy H-3′), 7.76 (t, 8
Hz, 1H; terpy H-4′), 7.26 (tt, 7.5, 1 Hz, 1H; para), 6.96 (br td,
7.5, 1 Hz, 1H; meta′), 6.72 (br t, 7.5 Hz, 1H; meta), 6.64 (br d,
8 Hz, 1H; ortho′), 6.48 (br s, 1H; OsCHPh), 5.64 (br d, 8 Hz,
1H; ortho), 2.43 (d, 2 Hz, 3H; NdC[CH3][CH3] anti to Os), 1.72
(d, 2 Hz, 3H; NdC[CH3][CH3] syn to Os). 13C{1H} NMR: 184.37
(NdCMe2), 47.23 (OsCHPh), 31.95 (NdCH[CH3][CH3] anti to
Os), 21.35 (NdCH[CH3][CH3] syn to Os). Anal. Calcd for
C25H23Cl2F6N4OsP: C, 38.25; H, 2.95; N, 7.13. Found: C, 38.10;
H, 3.13; N, 6.98.
Spectroscopic data for 2p are as follows. 1H NMR: δ 9.43
(ddd, 5.5, 2, 1 Hz, 1H; terpy H-6′′), 9.11 (ddd, 5.5, 2, 1 Hz, 1H;
terpy H-6), 8.48 (d, 8.5 Hz, 1H; terpy H-3′′), 8.41 (td, 8, 1.5
Hz, 1H; terpy H-4′′), 8.25 (dd, 8, 0.5 Hz, 1H; terpy H-5′), 8.07
(td, 8, 1.5 Hz, 1H; terpy H-4), 8.02 (ddd, 7, 5.5, 1.5 Hz, 1H;
terpy H-5′′), 7.98 (d, 8.5 Hz, 1H; terpy H-3), 7.87 (ddd, 7, 5.5,
1.5 Hz, 1H; terpy H-5), 7.82 (dd, 8, 0.5 Hz, 1H; terpy H-3′),
7.76 (t, 8 Hz, 1H; terpy H-4′), 7.32 (q, 5 Hz, 1H; NdCHCH3),
7.25 (tt, 7.5, 1 Hz, 1H; para), 6.95 (td, 7.5, 1 Hz, 1H; meta′),
6.73 (br t, 7.5 Hz, 1H; meta), 6.65 (br d, 8 Hz, 1H; ortho′), 5.60
(br d, 8 Hz, 1H; ortho), 2.40 (d, 5 Hz, 3H; NdCHCH3 anti to
Os), 0.94 (s, 3H; OsCPhCH3). 13C{1H} NMR: 167.43
(NdCHCH3), 59.24 (OsCPhCH3), 24.89 (NdCHCH3), 18.52
(OsCPhCH3). Anal. Calcd for C25H23Cl2F6N4OsP: C, 38.25; H,
2.95; N, 7.13. Found: C, 38.08; H, 3.21; N, 7.04.
Spectroscopic data for 2q are as follows. 1H NMR: δ 9.10
(d, 5 Hz, 2H; terpy H-6,6′′), 8.46 (d, 8 Hz, 2H; terpy H-3′,5′),
8.33 (m, 4H; terpy H-4,4′′,5,5′′), 7.99 (t, 8 Hz, 1H; terpy H-4′),
7.94 (ddd, 8, 1.5, 0.5 Hz, 2H; terpy H-3,3′′), 7.22 (d, 12 Hz,
1H; NdCHCHdCHOMe), 6.97 (dt, 9.6, 1.8 Hz, 1H;
NdCHCHdCHOMe), 5.82 (dd, 12, 9.6 Hz, 1H; NdCHCHd
CHOMe), 4.35 (d, 1.8 Hz, 2H; OsCH2), 3.73 (s, 3H; OCH3). 13C-
{1H} NMR: δ 167.38 (NdCHR), 60.10 (OCH3), 31.73 (OsCH2).
Anal. Calcd for C20H19Cl2F6N4OOsP: C, 32.57; H, 2.60; N, 7.60.
Found: C, 33.00; H, 2.76; N, 7.53.
(CH3). Anal. Calcd for C23H25Cl2F6N4OsP: C, 36.19; H, 3.30;
N, 7.34. Found: C, 35.95; H, 3.48; N, 7.16.
cis-[(ter p y)OsCl2(η2(C,N)-[MeCHdCH]CHdNdCHMe)]-
P F 6 (2s). Into a 100 mL round-bottom flask were placed 102.0
mg of cis-[(terpy)OsNCl2]PF6 (0.1561 mmol), a magnetic stir
bar, and 10 mL of CH3CN. A 180 µL portion of 2,4-hexadiene
(Aldrich, 90% tech grade mixture of isomers, 1.56 mmol, 10
equiv) was then added via syringe, and the mixture was stirred
overnight. The acetonitrile solution was evaporated down to
∼3 mL under reduced pressure, and the dark red solution was
then passed down a flash column (silica) under argon, with
acetonitrile as eluent, and the volume was reduced again to 2
mL. The red solution was layered with Et2O, stored overnight
at -20 °C, and then filtered on a fritted glass funnel. The red
crystalline solid was washed with 3 × 30 mL of Et2O and air-
dried to yield 52.3 mg of 2s as a 6:3:1 mixture of isomers 2s-i,
2s-ii, and 2s-iii (0.0709 mmol, 45%). 1H NMR: 2s-i, δ 8.97
(dd, 5.5, 1.5 Hz, 1H; terpy H-6′′), 8.95 (dd, 5.5, 1.5 Hz, 1H;
terpy H-6), 8.42 (br d, 8 Hz, 2H; terpy H-3,3′′), 8.37 (br d, 8.5
Hz, 1H; terpy H-5′), 8.33 (br d, 7.5 Hz, 1H; terpy H-3′), 8.28
(td, 8, 1.5 Hz, 1H; terpy H-4′′), 8.19 (td, 8, 1.5 Hz, 1H; terpy
H-4), 8.03 (t, 8 Hz, 1H; terpy H-4′), 7.92 (ddd, 7.5, 5.5, 1 Hz,
1H; terpy H-5′′), 7.81 (ddd, 7, 5.5, 1 Hz, 1H; terpy H-5), 7.35
(qd, 5.5, 2.5 Hz, 1H; NdCHCH3), 5.52 (dq, 16, 7 Hz, 1H;
OsCHCHdCHCH3), 5.49 (br d, 10 Hz, 1H; OsCHCHdCHCH3),
4.23 (ddq, 16, 9, 2.5 Hz, 1H; OsCHCHdCHCH3), 2.40 (dd, 5.5,
1.5 Hz, 3H; NdCHCH3), 1.96 (dd, 2.5, 1.5 Hz, 3H; OsCHCHd
CHCH3); 2s-ii, δ 9.08 (dd, 5.5, 1.5 Hz, 1H; terpy H-6′′), 9.03
(dd, 5.5, 1 Hz, 1H; terpy H-6), 8.47 (d, 7 Hz, 1H; terpy H-3′′),
8.45 (d, 7 Hz, 1H; terpy H-3), 8.37 (br d, 8.5 Hz, 1H; terpy
H-5′), 8.34 (td, 7, 1.5 Hz, 1H; terpy H-4′′), 8.33 (br d, 7.5 Hz,
1H; terpy H-3′), 8.29 (td, 8, 1.5 Hz, 1H; terpy H-4), 7.97 (ddd,
7.5, 5.5, 1 Hz, 1H; terpy H-5′′), 7.90 (ddd, 7, 5.5, 1.5 Hz, 1H;
terpy H-5), 7.27 (dd, 9.5, 2.5 Hz, 1H; NdCHCHdCHCH3), 6.50
(dq, 15, 7 Hz, 1H; NdCHCHdCHCH3), 6.43 (dd, 15, 10 Hz,
1H; NdCHCHdCHCH3), 4.87 (br qd, 5, 2.5 Hz, 1H; Os-
CHCH3), 1.22 (dd, 7, 1.5 Hz, 3H; NdCHCHdCHCH3), 0.79 (d,
5 Hz, 3H; OsCHCH3) (terpy H-4′ was not found); 2s-iii, δ 9.02
(dd, 5, 1 Hz, 1H; terpy H-6′′), 8.89 (dd, 5.5, 1.5 Hz, 1H; terpy
H-6), 8.24 (td, 8, 1.5 Hz, 1H; terpy H-4′′), 8.21 (td, 8, 1.5 Hz,
1H; terpy H-4), 8.07 (t, 8.5 Hz, 1H; terpy H-4′), 7.86 (ddd, 7,
5.5, 1.5 Hz, 1H; terpy H-5′′), 7.80 (ddd, 7, 5.5, 1 Hz, 1H; terpy
H-5), 7.50 (qd, 5.5, 2.5 Hz, 1H; NdCHCH3), 6.13 (dq, 12, 7.5
Hz, 1H; OsCHCHdCHCH3), 5.64 (br d, 9.5 Hz, 1H; OsCHCHd
CHCH3), 4.16 (ddq, 12, 10, 1.5 Hz, 1H; OsCHCHdCHCH3),
2.43 (dd, 5.5, 2 Hz, 3H; NdCHCH3), 0.93 (dd, 7, 1.5 Hz, 3H;
OsCHCHdCHCH3). Anal. Calcd for C21H21Cl2F6N4OsP: C,
34.29; H, 2.88; N, 7.62. Found: C, 34.12; H, 3.02; N, 7.48.
Also prepared using analogous procedures were cis-[(terpy)-
OsCl2(η2-2-aza-1,5-dimethylcyclohepta-1,2,4-trienium)]PF6 (2t)
from 1 and 1,4-dimethyl-1,3-cyclohexadiene,35 isolated as a 2:1
mixture of isomers 2t-i (η2-1,2-isomer) and 2t-ii (η2-2,3-isomer)
in 87% overall yield, and the products of the reaction of 1 with
R-terpinene, cis-[(terpy)OsCl2(η2-[2,3]-2-aza-1-methyl-5-isopro-
pylcyclohepta-1,2,4-trienium)]PF6 (2u -i), cis-[(terpy)OsCl2(η2-
[1,2]-2-aza-1-methyl-5-isopropylcyclohepta-1,2,4-trienium)]-
PF6 (2u -ii), and cis-[(terpy)OsCl2(η2-[2,3]-2-aza-1-isopropyl-5-
methylcyclohepta-1,2,4-trienium)]PF6 (2u -iii), isolated as a
2:1:1 mixture in 75% yield. Spectroscopic data for 2t are as
follows. 1H NMR: 2t-i, δ 9.31 (dd, 5.5, 1.5 Hz, 1H; terpy H-6′′),
8.95 (dd, 5.5, 1.5 Hz, 1H; terpy H-6), 8.51 (d, 8 Hz, 1H; terpy
H-3′′), 8.46 (d, 8 Hz, 1H; terpy H-3), 8.40 (dd, 8, 0.5 Hz, 1H;
terpy H-5′), 8.36 (br td, 7.5, 2 Hz, 2H; terpy H-4,4′′), 8.35 (dd,
8, 1 Hz, 1H; terpy H-3′), 8.02 (ddd, 7, 5.5, 1 Hz, 1H; terpy H-5′′),
7.99 (t, 8 Hz, 1H; terpy H-4′), 7.94 (ddd, 7, 5.5, 1.5 Hz, 1H;
terpy H-5), 7.72 (d, 4.5 Hz, 1H; iminium H), 6.00 (br, 1H;
Spectroscopic data for 2r are as follows. 1H NMR: δ 9.28
(ddd, 5, 1.5, 0.6 Hz, 1H; terpy H-6′′), 9.26 (ddd, 5, 1.5, 0.6 Hz,
1H; terpy H-6), 8.45 (dd, 8, 2 Hz, 1H; terpy H-3 or 3′′), 8.41
(dd, 8, 2 Hz, 1H; terpy H-3 or 3′′), 8.37 (td, 9, 1.5 Hz, 1H; terpy
H-4′′), 8.33-8.29 (m, 2H; terpy H-3′,5′), 8.29 (ddd, 7, 5.5, 2
Hz, 1H; terpy H-5′′), 7.96 (td, 9, 1.5 Hz, 1H; terpy H-4), 7.92
(t, 8 Hz, 1H; terpy H-4′), 7.80 (ddd, 7, 5.5, 2 Hz, 1H; terpy
H-5), 5.63 (sl br d, 10 Hz, 1H; OsCHCHdCMe2), 3.92 (d of sp,
10, 1.2 Hz, 1H; OsCHCHdCMe2), 2.47 (d, 1.2 Hz, 3H;
NdC[CH3][CH3] anti to Os), 1.73 (d, 1.5 Hz, 3H; NdC[CH3]-
[CH3] syn to Os), 1.40 (d, 0.9 Hz, 3H; OsCHCHdC[CH3][CH3]),
0.94 (s, sl br, 3H; OsCHCHdC[CH3][CH3]). 13C{1H} NMR: δ
183.21 (Me2CdN), 45.40 (OsCHR), 30.51, 27.12, 21.71, 18.76
(35) 1,4-Dimethyl-1,3-cyclohexadiene was used as a 2:1 mixture with
1,4-dimethyl-1,4-cyclohexadiene; control experiments show that the
unconjugated diene is unreactive with 1 at room temperature. Prepa-
ration of the mixture of dienes: Brady, W. T.; Norton, S. J .; Ko, J .
Synthesis 1985, 704-705.

Cleavage of Alkenes by Cationic Osmium Nitrides
Organometallics, Vol. 23, No. 8, 2004 1945
alkene H), 2.46 (td, 14, 3.5 Hz, 1H; methylene), 1.93 (d, 1.5
Hz, 3H; CH3), 1.77 (td, 13, 5 Hz, 1H; methylene), 1.61 (ddd,
13, 7, 2.5 Hz, 1H; methylene), 1.58 (dtd, 13, 7, 2 Hz, 1H;
methylene), 0.69 (s, 3H; CH3); 2t-ii, δ 9.24 (dd, 5.5, 1.5 Hz,
1H; terpy H-6′′), 9.22 (dd, 5.5, 1.5 Hz, 1H; terpy H-6), 8.48 (d,
8 Hz, 1H; terpy H-3 or -3′′), 8.47 (d, 8 Hz, 1H; terpy H-3 or
-3′′), 8.33 (obscured, 1H; terpy H-4 or -4′′), 8.33 (br d, 7.5 Hz,
2H; terpy H-3′,5′), 8.31 (td, 8, 2 Hz, 1H; terpy H-4 or -4′′), 7.97
(ddd, 7, 5.5, 1.5 Hz, 1H; terpy H-5′′), 7.95 (t, 8 Hz, 1H; terpy
H-4′), 7.90 (ddd, 7, 5.5, 1.5 Hz, 1H; terpy H-5), 6.56 (br, 1H;
alkene H), 4.21 (br, 1H; HR), 2.99 (td, 13, 3.5 Hz, 1H;
methylene), 2.65 (m, 2H; methylene), 2.52 (dddd, 16.5, 7.5, 3,
1.5 Hz, 1H; methylene), 1.71 (d, 2 Hz, 3H; CH3), 1.32 (br s,
3H; CH3). Anal. Calcd for C23H23Cl2F6N4OsP: C, 36.28; H, 3.04;
N, 7.36. Found: C, 36.87; H, 3.34; N, 6.82.
acetonitrile were stirred in a screw-cap vial with a Teflon-lined
cap for 9 h. The resulting dark solution was reduced to half
its volume on the rotary evaporator and then layered with
ether. The mixture was allowed to stand overnight in a -20
°C freezer. The crystals were suction-filtered on a fritted
funnel, washed with 3 × 30 mL of Et2O, and air-dried to give
57.0 mg of 5r (90%). 1H NMR: δ 8.50 (d, 8 Hz, 2H; terpy
H-3′,5′), 8.47 (d, 8.5 Hz, 2H; terpy H-3,3′′), 8.44 (d, 6 Hz, 2H;
terpy H-6,6′′), 8.28 (t, 8 Hz, 1H; terpy H-4′), 8.05 (td, 8, 1.5
Hz, 2H; terpy H-4,4′′), 7.70 (ddd, 7.5, 5, 1.5 Hz, 2H; terpy
H-5,5′′), 6.54 (dsp, 10, 1.5 Hz, 1H; OsCHCHdCMe2), 5.14 (dsp,
10, 1.5 Hz, 1H; OsCHCHdCMe2), 3.06 (d, 1.5 Hz, 3H; OsNd
C[CH3][CH3]), 2.52 (d, 1.5 Hz, 3H; OsNdC[CH3][CH3]), 2.22
(d, 1.5 Hz, 3H; alkene CH3), 1.92 (d, 2 Hz, 3H; alkene CH3).
13C{1H} NMR: δ 185.31 (Me2CdN), 51.02 (OsCHR), 29.52,
26.64, 25.53, 19.86 (CH3). Anal. Calcd for C23H25Cl2F6N4OsP:
C, 36.19; H, 3.30; N, 7.34. Found: C, 36.25; H, 3.14; N, 7.32.
tr a n s-[(ter p y)OsCl2(1,2-η2-2-a za -5-m eth oxycycloh ep ta -
1,2,4-tr ien iu m )]P F 6 (5v). Into a 25 mL round-bottom flask
were placed 81.2 mg trans-[(terpy)OsNCl2]PF6 (3; 0.133 mmol),
79 µL of 1-methoxy-1,3-cyclohexadiene (Aldrich; 65% pure, 0.43
mmol, 3.2 equiv), a magnetic stirbar, and 5 mL of CH3CN. The
mixture was stirred at room temperature for 15 min and then
stored at -20 °C for 2 h. The dark solid that formed was
suction-filtered, washed three times with Et2O, and air-dried
tr a n s-[(ter py)OsCl2(η2(C,N)-(H2CdNdCHCHdCHOMe)]-
P F 6 (5q). Into a 50 mL round-bottom flask were added a
magnetic stirbar, trans-[(terpy)OsNCl2]PF6 (3; 82.1 mg, 0.126
mmol), and 8 mL of acetonitrile. A 130 µL portion of 1-meth-
oxy-1,3-butadiene (Aldrich, 1.28 mmol, 10 equiv) was injected
with a 100 µL syringe, and the flask was sealed with Parafilm.
The reaction mixture was stirred overnight for 15 h, over which
time the solution turned dark brown. The volume of acetoni-
trile was reduced by half on a rotary evaporator, and the
solution was flash-chromatographed on silica gel, with aceto-
nitrile as eluent, to yield a light brown solution. The brown
eluate was reduced to half its volume and then layered with
ether and stored at -20 °C overnight. The azaallenium
complex was then isolated as a brown solid by suction
filtration, washed with 50 mL of Et2O, and air-dried. Yield:
80.2 mg (0.109 mmol, 47%). 1H NMR: δ 8.74 (d, 5 Hz, 2H; terpy
H-6,6′′), 8.60 (d, 8 Hz, 2H; terpy H-3′,5′), 8.57 (dd, 9, 2.5 Hz,
2H; terpy H-3,3′′), 8.22 (dt, 10, 1.8 Hz, 1H; NdCHCHd
CHOMe), 8.16 (t, 8 Hz, 1H; terpy H-4′), 8.10 (td, 7.8, 1.5 Hz,
2H; terpy H-4,4′′), 7.78 (ddd, 7, 5.5, 1.5 Hz, 2H; terpy H-5,5′′),
7.72 (d, 12 Hz, 1H; NdCHCHdCHOMe), 6.62 (dd, 12, 10 Hz,
1H; NdCHCHdCHOMe), 5.45 (d, 1.8 Hz, 2H; OsCH2), 4.01
(s, 3H, OCH3). 13C{1H} NMR: δ 171.67, 171.36, 157.96, 155.31,
154.25, 141.82, 129.55, 126.26, 124.07, 117.96, 109.23, 60.86
(OCH3), 46.05 (OsCH2). Anal. Calcd for C20H19Cl2F6N4OOsP:
C, 32.57; H, 2.60; N, 7.60. Found: C, 32.69; H, 2.49; N, 7.64.
Also prepared by this method were trans-[(terpy)OsCl2(η2-
(C,N)-MeCHdNdCH[CHdCHMe])]PF6 (5s, 51%), trans-[(terpy)-
OsCl2(2,3-η2-2-aza-1,5-dimethylcyclohepta-1,2,4-trienium)]-
PF6 (5t, 72%), and trans-[(terpy)OsCl2(2,3-η2-2-aza-1-methyl-
5-isopropylcyclohepta-1,2,4-trienium)]PF6 (5u , 69%). Spectro-
scopic data for 5s are as follows. 1H NMR: δ 8.45 (d, 8 Hz,
2H; terpy H-3′,5′), 8.39 (d, 7.5 Hz, 2H; terpy H-3,3′′), 8.38 (t,
8 Hz, 1H; terpy H-4′), 8.33 (dd, 10, 2.5 Hz, 1H; iminium), 8.02
(dd, 5.5, 1 Hz, 2H; terpy H-6,6′′), 7.96 (td, 8, 1.5 Hz, 2H; terpy
H-4,4′′), 7.79 (ddd, 7.5, 5.5, 1 Hz, 2H; terpy H-5,5′′), 7.17 (ddq,
16, 10, 2.5 Hz, 1H; NdCHCHdCHCH3), 6.87 (dq, 16, 7 Hz,
1H; NdCHCHdCHCH3), 5.89 (br q, 7 Hz, 1H; OsCHCH3), 2.21
(dd, 7, 1 Hz, 3H; NdCHCHdCHCH3), 2.08 (d, 5.5 Hz, 3H;
OsCHCH3). 13C{1H} NMR: δ 161.54 (NdCHR), 43.49 (Os-
CHCH3), 19.13, 18.88 (CH3). Anal. Calcd for C21H21Cl2F6N4-
OsP: C, 34.29; H, 2.88; N, 7.62. Found: C, 34.79; H, 3.14; N,
7.19. Spectroscopic data for 5t are as follows. 1H NMR: δ 8.36-
8.31 (m, 5H; terpy H-3,3′,3′′,4′,5′), 7.92-7.86 (m, 4H; terpy
H-4,4′′,6,6′′), 7.54 (br t, 6.6 Hz, 2H; terpy H-5,5′′), 7.27 (br s,
1H; alkene H), 6.02 (br s, 1H; HR), 3.67 (td, 13, 4.5 Hz, 1H;
methylene), 3.02 (dt, 13.5, 4 Hz, 1H; methylene), 2.68 (d, 1.5
Hz, 3H; NdCCH3), 2.45 (m, 2H; methylene), 1.81 (s, 3H; alkene
CH3). 13C{1H} NMR: 186.17 (NdCR2), 42.00 (OsCHR), 39.74,
31.26 (CH2), 26.43, 22.92 (CH3). Anal. Calcd for C23H23Cl2F6N4-
OsP: C, 36.28; H, 3.04; N, 7.36. Found: C, 36.87; H, 2.95; N,
6.92.
1
to yield 62.3 mg of 5v as a black solid (65%). H NMR: δ 2.34
(tdd, 13, 9, 3 Hz, 1H; CHH′CH′′H′′′), 2.87 (dtd, 14, 4, 3 Hz,
1H; CHH′CH′′H′′′), 3.00 (dt, 20, 3 Hz, 1H; CHH′CH′′H′′′), 3.35
(dddd, 20, 13, 4, 1.5 Hz, 1H; CHH′CH′′H′′′), 3.89 (s, 3H; OCH3),
5.92 (dd, 6, 2 Hz, 1H, alkene), 6.53 (dt, 9, 2 Hz, 1H; HR), 7.80
(ddd, 7.5, 6, 1 Hz, 2H; terpy H-5,5′′), 8.10 (t, 8 Hz, 1H; terpy
H-4′), 8.11 (td, 8, 1 Hz, 2H, terpy H-4,4′′), 8.22 (dd, 6, 2 Hz,
1H; NdCH), 8.57 (dd, 8, 1 Hz, 2H; terpy H-3,3′′), 8.60 (d, 8
Hz, 2H; terpy H-3′,5′), 8.87 (d, 5.5 Hz, 2H; terpy H-6,6′′). Anal.
Calcd for C22H21Cl2F6N4OOsP: C, 35.35; H, 2.83; N, 7.49.
Found: C, 34.98; H, 3.04; N, 7.75.
[(Tp m )OsCl2(η2(C,N)-[H 2CdNdCH CH dCH OMe )]P F 6
(6q). A mixture of 87.2 mg of [(Tpm)OsNCl2]PF6 (4; 0.138
mmol) and 140 µL of 1-methoxy-1,3-butadiene (Aldrich, 1.38
mmol, 10 equiv) in 9 mL of CH3CN were stirred magnetically
in a screw-cap vial with a Teflon-lined cap for 3 h. The
resulting dark solution was reduced by half its volume on the
rotary evaporator and then layered with ether. After the
mixture stood overnight in a -20 °C freezer, the crystals were
suction-filtered on a fritted funnel, washed with 3 × 60 mL of
Et2O, and air-dried to give 91.0 mg of 6q (92%). 1H NMR: δ
8.93 (s, 1H; pz3CH), 8.44 (dt, 10, 1.6 Hz, 1H; NdCHCHd
CHOMe), 8.42 (obscured, 1H; pz 3,5-H), 8.36 (d, 2.7 Hz, 1H;
pz 3,5-H), 8.25 (d, 3.6 Hz, 2H; pz 3,5-H), 7.64 (d, 3.6 Hz, 2H;
pz 3,5-H), 7.62 (d, 12 Hz, 1H; NdCHCHdCHOMe), 6.68 (t,
2.7 Hz, 1H; pz 4-H trans to azaallenium), 6.58 (t, 2.7 Hz, 2H;
pz 4-H cis to azaallenium), 6.26 (dd, 12, 10 Hz, 1H; NdCHCHd
CHOMe), 4.86 (d, 1.8 Hz, 2H; OsCH2), 3.91 (s, 3H; OCH3). 13C-
{1H} NMR: δ 167.37 (NdCHR), 77.35 (pz3CH), 59.90 (OCH3),
32.54 (OsCH2). Anal. Calcd for C15H18Cl2F6N7OOsP: C, 26.08;
H, 2.52; N, 13.25. Found: C, 26.35; H, 2.72; N, 13.00.
Kin etics of Rea ction s of cis-[(ter p y)OsNCl2]P F 6 w ith
1,4-Dim eth yl-1,3-cycloh exa d ien e a n d 2,5-Dim eth yl-2,4-
h exa d ien e. Samples consisting of 7.7 mg of cis-[(terpy)OsNCl2]-
PF6 and ∼2 mg of terephthalaldehyde (as an internal standard)
dissolved in 0.5 mL of CD3CN were placed in 5 mm NMR tubes
with screw-cap tops fitted with Teflon-lined silicone rubber
septa. The tubes were cooled in the probe of a 500 MHz NMR
instrument to -10 °C. Reactions were initiated by ejecting the
sample, injecting 35 µL of the appropriate diene through the
septum, and reinserting into the precooled probe. 1H NMR
spectra were then acquired periodically over the course of
30 000 s. (In the reaction with 1,4-dimethyl-1,3-cyclohexadi-
ene,35 the tube was cooled to -40 °C while outside the probe;
5 min was allowed for reequilibration at -10 °C before data
collection was initiated.) Concentrations of the various species
tr a n s-[(ter p y)OsCl2(η2(C,N)-[Me2CdCH]CHdNdCMe2)]-
P F 6 (5r ). A mixture of 54.4 mg of trans-[(terpy)OsNCl2]PF6
(3; 0.0833 mmol), 65 µL of 2,5-dimethyl-2,4-hexadiene (Aldrich;
0.42 mmol, 5.0 equiv), a magnetic stirbar, and 5 mL of

1946 Organometallics, Vol. 23, No. 8, 2004
Maestri et al.
were obtained by integration relative to the intensity of the
terephthalaldehyde peak at δ 10.10, using the following
peaks: 1, δ 9.80 (2H); the intermediate I in the reaction with
1,4-dimethyl-1,3-cyclohexadiene, δ 11.22 (1H); 2t-i, δ 9.31 (1H);
2t-ii, δ 9.24 and 9.22 combined (2H). The thermodynamic
product in the reaction with 2,5-dimethyl-2,4-hexadiene (2r )
was measured by integration of its resonance at δ 7.80 (1H);
the kinetic product in the reaction was measured by difference,
using the overlapped terpy 6,6′′-H peaks at δ 9.35 to ascertain
the total product concentration. Good mass balance was
obtained in all cases, with constant total integrals measured
(about (10%) and no systematic variation of the overall
integral with time.
5, 1 Hz, 2H; terpy H-6,6′′), 8.66 (d, 10.5 Hz, 1H; iminium), 8.51
(d, 8 Hz, 2H; terpy H-3,3′′), 8.40 (m, 4H; terpy H-3′,4,4′′,5′),
8.03 (m, 3H; terpy H-4′,5,5′′), 5.65 (dt, 10.5, 1 Hz, 1H; alkene),
1.97 (sl br s, 3H, CHdC[CH3][CH3′]), 1.96 (sl br s, 3H, CHd
C[CH3][CH3′]), 1.05 (s, 6H; OsC[CH3]2).
X-r a y Cr ysta llogr a p h y of cis-[(ter p y)OsCl2(1,2-η2-P h -
C H dN dC H C H dC H C H dC H P h )]P F 6‚(C D 3)2C O
(2h ‚
(CD3)2CO). Dark red blocks of the complex were grown by slow
diffusion of ether into a solution of 2h in acetone-d6. A 0.22 ×
0.10 × 0.05 mm crystal was glued to the tip of a glass fiber in
the air and examined at 20 °C on an Enraf-Nonius CAD4
diffractometer using Mo KR radiation with a graphite mono-
chromator (λ ) 0.710 73 Å). The crystal was triclinic (space
group P1h). The unit cell was determined on the basis of 25
reflections with 12.0° < θ < 13.0°. A total of 6341 reflections
with 2θ < 50° were collected. Crystal quality was monitored
by recording 3 standard reflections approximately every 170
reflections measured, and a linear correction was applied to
correct for the slight (2%) decay that was observed. An
The time courses of the reactions were simulated by
nonlinear least-squares fitting to the appropriate analytical
expressions for successive first-order (or pseudo-first-order)
equations36 using Microsoft Excel,37 with uncertainties in the
calculated parameters calculated as described in the litera-
ture.38
empirical absorption correction was applied (µ ) 3.796 mm-1
,
Gen er a tion a n d 1H NMR Ch a r a cter iza tion of Un sta ble
Aza a llen iu m Isom er s. The following isomers are thermo-
dynamically unstable and were characterized in situ when
generated at low temperature from 1 and an excess of the
appropriate alkene. Some resonances are obscured by isomeric
products and resonances from excess alkene. 1H NMR data
for cis,syn-[(terpy)OsCl2(η2(C,N)-[Me2NC6H4]CHdNdCHMe)]-
PF6 (2l-iii) at -40 °C: δ 9.49 (d, 5.5 Hz, 1H; terpy H-6′′), 9.16
(d, 5.5 Hz, 1H; terpy H-6), 8.49 (d, 8.5 Hz, 1H; terpy H-3′′),
8.42 (td, 7.5, 1.5 Hz, 1H; terpy H-4′′), 8.25 (d, 8 Hz, 1H; terpy
H-3), 8.24 (qd, 6, 2 Hz, 1H; iminium), 8.03 (m, 2 H; terpy
H-5,5′′), 7.80 (m, 4H; terpy H-3′,4,5′,4′), 6.42 (quin, 2 Hz, 1H;
HR), 6.36 (d, 8.5 Hz, 1H; Ar H), 6.21 (d, 8.5 Hz, 1H; Ar H),
5.94 (d, 8.5 Hz, 1H; Ar H), 5.29 (d, 8.5 Hz, 1H; Ar H), 1.62 (dd,
6, 2 Hz, 3H; NdCHCH3). 1H NMR at -5 °C for cis,syn-[(terpy)-
OsCl2(η2(C,N)-[MeOC6H4]CHdNdCHMe)]PF6 (2k -iii): δ 9.50
(d, 5.5 Hz, 1H; terpy H-6′′), 9.20 (d, 5.5 Hz, 1H; terpy H-6),
8.47 (d, 8.5 Hz, 1H; terpy H-3′′), 8.42 (td, 7.5, 1.5 Hz, 1H; terpy
H-4′′), 8.25 (d, 8.5 Hz, 1H; terpy H-5′), 8.22 (qd, 6, 2.5 Hz, 1H;
iminium), 8.04 (m, 2 H; terpy H-5,5′′), 7.90 (m, 1H; terpy H-3),
7.83 (m, 2H; terpy H-3′,4), 7.75 (t, 8 Hz, 1H; terpy H-4′), 6.64
(quin, 2 Hz, 1H; HR), 5.48 (br, 1H; Ar H), 1.62 (dd, 5, 2 Hz,
3H; NdCHCH3). 1H NMR of cis-[(terpy)OsCl2(η2(N,C)-CH2d
NdCH(CHdCHOMe)]PF6 (2q-i) at -20 °C: δ 8.92 (d, 5 Hz,
1H; terpy H-6′′), 8.26 (td, 8, 1 Hz, 1H; terpy H-4 or 4′′), 7.91
(ddd, 7.5, 5.5, 1 Hz, 1H; terpy H-5′′), 7.83 (dd, 9, 2.5 Hz, 1H;
iminium), 7.80 (ddd, 7.5, 5.5, 1 Hz, 1H; terpy H-5), 7.45 (dd,
9, 2.5 Hz, 1H; iminium). 1H NMR of cis-[(terpy)OsCl2(η2(C,N)-
Me2CHdNdCH(CHdCMe2)]PF6 (2r -i) at -20 °C: δ 9.34 (dd,
transmission factors 0.7049-0.8369). The osmium atom was
located on a Patterson map. The remaining non-hydrogen
atoms were found on difference Fourier syntheses. Hydrogens
in the complex were placed in calculated positions, except for
the two bonded to C1 and C2, which were located on the
difference Fourier maps and refined isotropically. Final full-
matrix least-squares refinement on F2 converged at R ) 0.0342
for 5324 reflections with Fo > 4σ(Fo) and R ) 0.0490 for all
data (wR2 ) 0.0715, 0.0774, respectively). All calculations used
SHELXTL (Bruker Analytical X-ray Systems), with scattering
factors and anomalous dispersion terms taken from the
literature.39
Ack n ow led gm en t. We thank Dr. Maoyu Shang for
assistance with the X-ray crystallography. Major finan-
cial support of this work by the National Science
Foundation (Grant No. CHE-97-33321-CAREER) is
gratefully acknowledged. S.N.B. also thanks the Camille
and Henry Dreyfus Foundation (New Professor Award),
DuPont (Young Professor Award), and the Dow Chemi-
cal Co. (Innovation Recognition Program) for support.
A.G.M. acknowledges support by a J . Peter Grace
Fellowship.
Su p p or tin g In for m a tion Ava ila ble: Text giving full
synthetic details and spectroscopic data on all compounds and
tables giving and crystallographic data; crystallographic data
are also available as CIF files. This material is available free
of charge via the Internet at http://pubs.acs.org.
(36) Espenson. J . H. Chemical Kinetics and Reaction Mechanisms;
McGraw-Hill: New York, 1995.
(37) Harris, D. C. J . Chem. Educ. 1998, 75, 119-121.
(38) de Levie, R. J . Chem. Educ. 1999, 76, 1594-1598.
OM034404M
(39) International Tables of Crystallography; Kluwer Academic
Publishers: Dordrecht, The Netherlands, 1992; Vol. C.