Unusual transformations of a bipyridine ligand in attempts to trap a terminal chromium (III) alkylidene

Article abstract of DOI:10.1021/om0007421

Attempts to trap a terminal chromium alkylidene by reactions of dialkyl Cp*Cr(CH2SiMe3)2 (Cp= J/5-C5Me5) or metallacycle Cp*(THF)Cr(CH2)2SiMe2 with the bidentate ligand 1,2- bis(dimethyphosphino)ethane (dmpe) lead to dinuclear paramagnetic chromium alkyls [Cp*Cr(CH2SiMe3)2]2C-v1:'71-dmpe) (1) and lCp*Cr(CH2)2Si(Cli3)2l2(fi-i]1:ti1-dmpe) (2), re- ' spectively. On the other hand, the unusual mononuclear chromium complex (?/5-pentam-, . ethylcyclopentadienyl)(2-(2 )2 -dimethyl-2 -silapropyl)-l,2- dihydro[2)2']bipyridinerl,3''-diyljchromium(III) (3) was obtained by a reaction of Cp*(THF)Cr(CH2)2SiMe2 with 1 equiv of 2,2'-bipyridine (bipy). 3 is formed by carbon-carbon bond formation between one terminus , of the metallacyle and the 2-position of the bipy ligand, at the expense of the aromaticity of a pyridyl group. Single-electron oxidation of 3 with ferricenium hexafluorophosphate ([Cp2Fe]PFe) yielded 4, a dicationic dimer of 3 linked via a carbon-carbon bond between the bipy backbones. 1-4 were structurally characterized by X-ray.diffraction.

Full text of DOI:10.1021/om0007421

182
Organometallics 2001, 20, 182-187
Un u su a l Tr a n sfor m a tion s of a Bip yr id in e Liga n d in
Attem p ts to Tr a p a Ter m in a l Ch r om iu m (III) Alk ylid en e
Somying Leelasubcharoen, Kin-Chung Lam, Thomas E. Concolino,
Arnold L. Rheingold, and Klaus H. Theopold*
Department of Chemistry and Biochemistry, Center for Catalytic Science and Technology,
University of Delaware, Newark, Delaware 19716
Received August 24, 2000
Attempts to trap a terminal chromium alkylidene by reactions of dialkyl Cp*Cr(CH2SiMe3)2
(Cp* ) η5-C5Me5) or metallacycle Cp*(THF)Cr(CH2)2SiMe2 with the bidentate ligand 1,2-
bis(dimethyphosphino)ethane (dmpe) lead to dinuclear paramagnetic chromium alkyls
[Cp*Cr(CH2SiMe3)2]2(µ-η1:η1-dmpe) (1) and [Cp*Cr(CH2)2Si(CH3)2]2(µ-η1:η1-dmpe) (2), re-
spectively. On the other hand, the unusual mononuclear chromium complex (η5-pentam-
ethylcyclopentadienyl)(2-(2′′,2′′-dimethyl-2′′-silapropyl)-1,2-dihydro[2,2′]bipyridine-1,3′′-diyl)-
chromium(III) (3) was obtained by a reaction of Cp*(THF)Cr(CH2)2SiMe2 with 1 equiv of
2,2′-bipyridine (bipy). 3 is formed by carbon-carbon bond formation between one terminus
of the metallacyle and the 2-position of the bipy ligand, at the expense of the aromaticity of
a pyridyl group. Single-electron oxidation of 3 with ferricenium hexafluorophosphate ([Cp2-
Fe]PF6) yielded 4, a dicationic dimer of 3 linked via a carbon-carbon bond between the
bipy backbones. 1-4 were structurally characterized by X-ray diffraction.
In tr od u ction
(CH2SiMe3)2 have led us to propose a reaction mecha-
nism in which an alkylidene (A) and a metallacyclobu-
tane (B) intermediate coexist in rapid equilibrium.7 This
raised the question whether the alkylidene might be
trapped under suitable conditions. Specifically, since the
structural chemistry of Cp*CrIII is dominated by three-
legged piano stools,9 the use of bidentate chelating
ligands (L-L) might be expected to stabilize fragment
A in the form of Cp*(L-L)CrdCHSiMe3. Herein we
report on the dashing of that hope and on some
remarkable chemistry accompanying yet another escape
of the elusive CrIII alkylidene.
While chromium carbene complexes of the Fischer
type mark the historical beginning of metal carbon
multiple bonds and are abundant, simple alkylidenes
of chromium remain rare. Recent examples include
Gibson’s spectroscopically characterized (2,6-iPr2C6H3N)2-
(L)CrVIdCHCMe3 (L ) THF, PMe3),1 which surprisingly
do not react with alkenes, and Scott’s surface-grafted
(tSiO)2CrIVdCHEMe3 (E ) C, Si), which initiate the
polymerization of ethylene.2,3 Of particular interest in
the context of our work on organometallics of trivalent
chromium is Gambarotta’s proposed (TMEDA)CrIII
-
(dCHPh)(CH2Ph).4 Over the years we have tried on
occasion to isolate terminal Cp*CrIII alkylidenes by
various strategies, but to no avail. For example, neither
deprotonation of cationic CrIII alkyls5 nor steric crowding
of CrIII dialkyls yielded the desired species.6 While we
have obtained several dinuclear complexes featuring
bridging µ-CHR ligands (R ) H, alkyl),7,8 a terminal CrIII
alkylidene has so far eluded us.
Resu lts a n d Discu ssion
Reaction of Cp*Cr(CH2SiMe3)2 with monodentate
ligands merely produces adducts of the type Cp*(L)Cr-
(CH2SiMe3)2 (L ) pyridine, THF). Following precedent
from vanadium chemistry, where treatment of Cp-
(PMe3)V(CH2CMe3)2 with bis(dimethylphosphino)ethane
(dmpe) yielded the alkylidene Cp(dmpe)VdCHCMe3,10
we reacted the chromium dialkyl with the same biden-
tate phosphine. However, treatment of orange Cp*Cr-
(CH2SiMe3)2 with various amounts of dmpe yielded the
dinuclear adduct [Cp*Cr(CH2SiMe3)2]2(µ-η1:η1-dmpe) (1,
see Scheme 1).
However, the various products of the thermal decom-
position of the mononuclear 13-electron complex Cp*Cr-
(5) Noh, S.-K.; Heintz, R. A.; Theopold, K. H. Organometallics 1989,
8, 2071.
* Corresponding author. E-mail: theopold@udel.edu.
(1) Coles, M. P.; Gibson, V. C.; Clegg, W.; Elsegood, M. R. J .; Porrelli,
P. A. J . Chem. Soc., Chem. Commun. 1996, 1963.
(2) Scott, S. L.; Ajjou, J . A. N.; Paquet, V. J . Am. Chem. Soc. 1998,
120, 415.
(3) Ajjou, J . A. N.; Rice, G. L.; Scott, S. L. J . Am. Chem. Soc. 1998,
120, 13436.
(6) Bhandari, G.; Kim, Y.; McFarland, J . M.; Rheingold, A. L.;
Theopold, K. H. Organometallics 1995, 14, 738.
(7) Heintz, R. A.; Leelasubcharoen, S.; Liable-Sands, L. M.; Rhei-
ngold, A. L.; Theopold, K. H. Organometallics 1998, 17, 5477.
(8) Noh, S.-K.; Heintz, R. A.; J aniak, C.; Sendlinger, S. C.; Theopold,
K. H. Angew. Chem., Int. Ed. Engl. 1990, 29, 775.
(9) Theopold, K. H. Acc. Chem. Res. 1990, 23, 263.
(10) Hessen, B.; Meetsma, A.; Teuben, J . H. J . Am. Chem. Soc. 1989,
111, 5977.
(4) Hao, S.; Song, J .-I.; Berno, P.; Gambarotta, S. Organometallics
1994, 13, 1326.
10.1021/om0007421 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 11/30/2000

Unusual Transformations of a Bipyridine Ligand
Organometallics, Vol. 20, No. 1, 2001 183
Sch em e 1
F igu r e 2. Molecular structure of [Cp*Cr(CH2)2SiMe2]2(µ-
η1:η1-dmpe) (2, thermal ellipsoids are shown at 30% prob-
ability). Selected interatomic distances and angles are
listed in Table 2.
Ta ble 2. Selected In ter a tom ic Dista n ces a n d
An gles for [Cp *Cr (CH2)2SiMe2]2(µ-η1:η1-d m p e) (2)
Distances (Å)
Cr(1)-P(2)
Cr(1)-C(15)
Cr(1)-C(2)
Cr(1)-C(4)
C (14)-Si(1)
Cr(1)-centa
2.4292(16)
2.107(6)
2.271(5)
2.271(6)
1.860(6)
1.929(5)
Cr(1)-C(14)
Cr(1)-C(1)
Cr(1)-C(3)
Cr(1)-C(5)
C(15)-Si(2)
2.113(6)
2.281(5)
2.276(5)
2.279(5)
1.851(6)
F igu r e 1. Molecular structure of [Cp*Cr(CH2SiMe3)2]2(µ-
η1:η1-dmpe) (1, thermal ellipsoids are shown at 30% prob-
ability). Selected interatomic distances and angles are
listed in Table 1.
Angles (deg)
89.86(17) P(2)-Cr(1)-C (15)
83.6(2)
P(2)-Cr(1)-centa
P(2)-Cr(1)-C(14)
C(14)-Cr(1)-(15)
92.84(17)
126.9(15)
C(14)-Cr(1)-centa 125.6(15) C(15)-Cr(1)-centa 125.5(15)
Cr(1)-C(14)-Si(1)
87.8(2)
Cr(1)-C(15)-Si(1)
88.2(2)
Cr(1)-P(2)-C(13) 118.15(18) C(14)-Si(1)-C(15) 98.6(3)
Ta ble 1. Selected In ter a tom ic Dista n ces a n d
An gles for [Cp *Cr (CH2SiMe3)2]2(µ-η1:η1-d m p e) (1)
a
cent is the centroid of the Cp* ligand.
Distances (Å)
Cr(1)-P(1)
Cr(1)-C(15)
Cr(1)-C(2)
Cr(1)-C(4)
C (11)-Si(1)
Cr(1)-centa
2.4465(6)
2.144(2)
2.334(2)
2.268(2)
1.861(2)
1.955(2)
Cr(1)-C(11)
Cr(1)-C(1)
Cr(1)-C(3)
Cr(1)-C(5)
C(15)-Si(2)
2.123(2)
2.309(2)
2.296(2)
2.288(2)
1.869(2)
tane Cp*(THF)Cr(CH2)2SiMe2 in pentane had yielded
the bis(µ-alkylidene) complex [Cp*Cr(µ-CHSiMe3)]2,
among the erstwhile observations pointing toward an
interconversion between A and B. Consequently, addi-
tion of dmpe to Cp*(THF)Cr(CH2)2SiMe2 was tried. This
reaction yielded a green complex, to which we assign
the structure [Cp*Cr(CH2)2SiMe2)2]2(µ-η1:η1-dmpe) (2,
see Scheme 1). The molecular structure of 2 has been
determined by X-ray diffraction; the molecule is shown
in Figure 2, and selected interatomic distances and
angles are listed in Table 2. Once again, a dinuclear
complex held together by a bridging diphosphine was
produced. It is possible that the ligand substitution
reaction proceeds too fast to allow for rearrangement
of the coordinatively unsaturated metallacycle (B) to the
alkylidene species (A). However, another possible in-
terpretation posits that the greater stability of the
coordinatively saturated metallacycle is reflected in the
transition state of the trapping step, effectively draining
the equilibrating intermediate(s) into 2. The magnetic
behavior of 2 (µeff ) 5.6(1)µB, i.e., 3.9 µB per Cr) again
rules out any magnetic interaction between the metal
atoms. An alternative method of preparation of 2 is the
Angles (deg)
94.05(7) P(1)-Cr(1)-C (15)
P(1)-Cr(1)-C(11)
87.42(6)
124.7(9)
C(11)-Cr(1)-C(15) 95.57(9) P(1)-Cr(1)-centa
C(11)-Cr(1)-centa 125.0(9)
C(15)-Cr(1)-centa 120.7(9)
Cr(1)-C(11)-Si(1) 138.44(13) Cr(1)-C(15)-Si(2) 126.32(12)
Cr(1)-P(1)-C(21)
a
cent is the centroid of the Cp* ligand.
The molecular structure of 1 has been determined by
X-ray diffraction; the result is depicted in Figure 1, and
selected interatomic distances and angles are listed in
Table 1. The metric parameters are very similar to those
of analogous compounds and merit no particular discus-
sion. 1 finds close precedent in [CpCr(CH3)2]2(µ-η1:η1-
dmpe), which was prepared by Poli et al. in an unsuc-
cessful attempt to force CpCrIII to adopt a 17-electron
configuration (i.e., Cp(dmpe)CrMe2).11 The effective
magnetic moment of 1 at room temperature measured
µeff ) 5.6(1)µB (3.9 µB per Cr), consistent with the
absence of any magnetic interaction mediated by the
bridging diphosphine ligand.
1
thermal decomposition of 1; H NMR monitoring of a
solution of 1 heated to 60 °C showed the gradual
appearance of the resonances of 2 and SiMe4.
The structures of 1 and 2 suggested that dmpe, albeit
a potent bidentate ligand, is too flexible to serve our
purpose. In other words, we need a bidentate ligand that
by its very nature can coordinate to one metal only, thus
leaving only a single coordination site on chromium for
The formation of 1 suggested that alkane elimination
should precede the addition of the chelating ligand. In
that vein we note that the dissolution of metallacyclobu-
(11) Fettinger, J . C.; Mattamana, S. P.; Poli, R.; Rogers, R. D.
Organometallics 1996, 15, 4211.

184 Organometallics, Vol. 20, No. 1, 2001
Leelasubcharoen et al.
Ta ble 3. Selected In ter a tom ic Dista n ces a n d
Sch em e 2
An gles for Cp *Cr (η3-C14H18N2Si) (3)
Distances (Å)
Cr(1)-N(1)
Cr(1)-C(11)
Cr(1)-C(2)
Cr(1)-C(4)
Cr(1)-centa
Si(1)-C(13)
C(19)-C(20)
C(20)-C(21)
C(22)-C(23)
C(24)-N(2)
2.056(4)
2.098(5)
2.247(5)
2.295(5)
1.933(5)
1.903(5)
1.522(7)
1.500(7)
1.453(10)
1.358(7)
Cr(1)-N(2)
Cr(1)-C(1)
Cr(1)-C(3)
Cr(1)-C(5)
C(11)-Si(1)
C(13)-C(20)
N(2)-C(20)
C(21)-C(22)
C(23)-C(24)
1.981(4)
2.264(5)
2.290(5)
2.284(5)
1.828(6)
1.565(7)
1.485(7)
1.342(9)
1.369(8)
Angles (deg)
79.90(16) N(1)-Cr(1)-C(11)
N(1)-Cr(1)-N(2)
N(2)-Cr(1)-C(11)
92.87(18)
92.62(19) N(1)-Cr(1)-centa 129.4(19)
N(2)-Cr(1)-centa 129.5(19) C(11)-Cr(1)-centa 120.3(19)
Cr(1)-C(11)-Si(1) 117.5(2)
Si(1)-C(13)-C(20) 116.5(3)
Cr(1)-N(1)-C(19) 115.3(3)
Cr(1)-N(2)-C(20) 115.0(3)
C(20)-N(2)-C(24) 114.1(4)
C(11)-Si(1)-C(13) 111.1(2)
Cr(1)-N(1)-C(15) 125.8(3)
C(15)-N(1)-C(19) 118.8(4)
Cr(1)-N(2)-C(24) 128.6(4)
a
cent is the centroid of the Cp* ligand.
amido ligand. The coordination environment of the
chromium thus comprises one alkyl, one amide, and one
neutral pyridine donor ligand besides the Cp* ligand.
The formal oxidation state of chromium remains +III,
and the magnetic moment of 3 (µeff ) 3.6(1)µB) is
consistent with that assignment. It is notable that this
rearrangement breaks the aromaticity of one of the two
linked pyridine moieties. Indeed, careful inspection of
the bond distances in this newly formed nitrogen
heterocycle reveals pronounced bond length alternation.
Thus N(2)-C(20) (1.49 Å) and C(20)-C(21) (1.50 Å) are
clearly single bonds to the new quaternary carbon atom
(C(20)). The next four contiguous bonds in the ring (i.e.,
C(21)-C(22), 1.34 Å; C(22)-C(23), 1.45 Å; C(23)-C(24),
1.37 Å; and C(24)-N(2), 1.36 Å) show a pattern of bond
distances that is typical for 1,2-dihydropyridines, even
though the C(24)-N(2) distance may seem rather short
for a formal single bond. Molecular mechanics or
semiempirical quantum calculations (e.g., PM3) on the
free ligand give distances that are very close to the
observed ones;12 moreover, the relevant distances in the
crystal structure of an authentic 1,2-dihydropyridine
derivative are all but indistinguishable from those of
3.13 The Cr-N(2) distance of 1.98 Å is shorter than the
characteristic Cr-N distance for chromium bipyridine
complexes (2.08 Å) and supports the interpretation as
a Cr-amide bond. All these structural observations are
consistent with a reduction of the pyridine moiety to
the deprotonated form of a 1,2-dihydropyridine deriva-
tive. The metric parameters of the other pyridine ring
do not deviate from those expected of a bipyridine
ligand.
F igu r e 3. Molecular structure Cp*Cr(η3-C14H18N2Si) (3,
thermal ellipsoids are shown at 30% probability). Selected
interatomic distances and angles are listed in Table 3.
binding of the alkyl-derived moiety. Such a ligand is
2,2′-bipyridine (bipy). We remind the reader that reac-
tion of dialkyls Cp*(L)CrR2 with bipy resulted in ho-
molysis of a Cr-C bond and isolation of CrII alkyls of
the type Cp*(bipy)CrR.6 However, trapping of an A/B
mixture with bipy might reasonably be expected to favor
the alkylidene isomer A. With this notion in mind, we
investigated the reaction of Cp*(THF)Cr(CH2)2SiMe2
with bipyridine.
Addition of 1 equiv of bipy to a THF solution of Cp*-
(THF)Cr(CH2)2SiMe2 yielded a dark green solution.
Standard workup and recrystallization from toluene/
pentane yielded a new chromium complex (3, see
Scheme 2) that appeared to contain a bipy ligand as well
as a silicon-containing organic ligand. As the spectro-
scopic data were not sufficient to assign a structure to
the newly formed compound, a crystal structure deter-
mination was necessary to ascertain its identity. The
molecular structure of 3 is shown in Figure 3, and
selected interatomic distances and angles are listed in
Table 3. As anticipated, the product is a mononuclear
complex, but instead of the hoped for alkylidene, 3
contains a functionalized bipy ligand. Specifically, one
of the termini of the metallacycle has been transferred
from chromium to the 2-position of the bipyridine,
forming a new C-C bond and thereby creating a
quaternary carbon as well as transforming the adjacent
nitrogen from a neutral two-electron donor into an
The mechanism of formation of 3 is somewhat am-
biguous. Dissociative ligand substitution of THF by bipy
apparently yields the metallacycle Cp*(η2-bipy)Cr(CH2)2-
SiMe2. Such pseudo seven-coordinate Cp*CrIII species
are essentially unknown and likely to be highly desta-
bilized.9,14 One possible follow-up reaction involves a
migratory insertion of a N-C multiple bond of coordi-
(12) The calculations were performed with CS Chem3D Pro, Version
5.0; CambridgeSoft, 1999.
(13) Kozhushkov, S. I.; Brandl, M.; Yufit, D. S.; Machinek, R.;
Demeijere, A. Liebigs Ann.-Recl. 1997, 2197-2204.

Unusual Transformations of a Bipyridine Ligand
Organometallics, Vol. 20, No. 1, 2001 185
Ta ble 4. Selected In ter a tom ic Dista n ces a n d
An gles for [Cp *Cr (µ-η3:η3-C28H36N4Si2][P F 6]2
(4)‚2CH2Cl2
Distances (Å)
Cr(1)-N(1)
Cr(1)-C(11)
Cr(1)-C(2)
Cr(1)-C(4)
Cr(1)-centa
Si(1)-C(14)
C(19)-C(20)
C(15)-C(16)
C(17)-C(18)
C(19)-N(1)
2.031(5)
2.069(7)
2.257(7)
2.248(7)
1.894(6)
1.901(6)
1.511(9)
1.446(10)
1.499(9)
1.489(8)
Cr(1)-N(2)
Cr(1)-C(1)
Cr(1)-C(3)
Cr(1)-C(5)
C(11)-Si(1)
C(14)-C(19)
N(1)-C(15)
C(16)-C(17)
C(18)-C(19)
C(18)-C(18A)
2.070(5)
2.258(7)
2.226(7)
2.244(7)
1.848(7)
1.546(9)
1.278(9)
1.339(9)
1.559(8)
1.608(13)
F igu r e 4. Molecular structure of [Cp*Cr(µ-η3:η3-C28H36N4-
Si2][PF6]2 (4)‚2CH2Cl2; only the dicationic chromium com-
plex is shown (thermal ellipsoids are shown at 30%
probability). Selected interatomic distances and angles are
listed in Table 4.
Angles (deg)
N(1)-Cr(1)-N(2)
N(2)-Cr(1)-C(11)
N(2)-Cr(1)-centa
Cr(1)-C(11)-Si(1)
78.6(2) N(1)-Cr(1)-C(11)
96.7(3) N(1)-Cr(1)-centa
129.2(4) C(11)-Cr(1)-centa
116.9(4) C(11)-Si(1)-C(14)
89.3(2)
131.0(5)
119.6(5)
108.3(3)
126.4(5)
118.8(5)
125.2(5)
Si(1)-C(14)-C(19) 116.5(4) Cr(1)-N(1)-C(15)
Cr(1)-N(1)-C(19)
Cr(1)-N(2)-C(20)
C(20)-N(2)-C(24)
114.4(4) C(15)-N(1)-C(19)
116.5(4) Cr(1)-N(2)-C(24)
118.1(6)
nated bipy into either Cr-C bond of the metallacycle,
thereby forming 3 directly. The previously observed
insertion of nitriles into Cp*CrIII-alkyls provides some
precedent for this pathway. However, based on the
reactions of acyclic chromium dialkyls with bipy (see
above), we consider a homolytic cleavage of one Cr-C
bond, followed by attack of the tethered alkyl radical
on the bipy ligand, to be the more probable course of
reaction. Either way, we are not aware of any previous
report of this kind of attack on a coordinated bipyridine
ligand. The loss of aromaticity of a pyridine ring
(estimated at 130 kJ /mol) provides yet another measure
of the reluctance of CrIII to support a terminal alky-
lidene.
a
cent is the centroid of the Cp* ligand.
Sch em e 3
We believe that the apparent instability of the target
molecule is due to an unfavorable electronic interaction.
The filled p-orbital of the alkylidene dianion (here
CHSiMe32-) overlaps with a d-orbital of π-symmetry on
chromium; however, for CrIII (d3) the latter is actually
occupied by one electron; that is, this is a two-center/
three-electron interaction. The antibonding orbital of the
Cr-C π-bond bond is half-filled and the bond is weak.
It appears that the π-donation from the alkylidene is
not strong enough to enforce spin pairing to a doublet
state. In essence, the pairing energy of CrIII wins out
over the chance to form a chromium-carbon π-bond. If
this is true, removal of the offending electron, e.g., by
oxidation of the complex to CrIV (d2), might stabilize the
desired alkylidene. With this rationale in mind we have
investigated the products of the chemical oxidation of
3, in the hope that the rearrangement would prove
reversible.
Reaction of 3 with 1 equiv of [Cp2Fe]PF6 was fast and
yielded ferrocene and a new chromium complex, 4 (see
Scheme 2). J udging from its solubility characteristics
and IR spectrum, 4 was a hexafluorophosphate salt. Its
paramagnetism made a characterization by NMR spec-
troscopy impractical; hence the crystal structure of 4
was determined by X-ray diffraction. The result is
depicted in Figure 4, and selected interatomic distances
and angles are listed in Table 4. Obviously, 4 is not the
desired [Cp*(bipy)Cr(CHSiMe3)]PF6 but rather a dimer
of the one-electron oxidation product 3+. A 2-fold
rotational axis renders only half of the molecule unique.
The basic structure of 3 remains intact, but a new, long
carbon-carbon bond (1.61 Å) couples the original bipy
ligands of two such units at their 3-positions. This
further elaboration transforms the 1,2-dihydropyridine
anion of 3 into a neutral 2,3-dihydropyridine derivative.
The bond distances in this ring and the longer Cr-N
distance (Cr(1)-N(1), 2.03 Å) are consistent with this
picture. Despite the removal of one electron per metal
atom, the chromium retains its formal oxidation state
of +III; the ligand has been oxidized instead. The
magnetic moment of 4 (µeff ) 5.4(1)µB, i.e., 3.8 µB/Cr) is
consistent with magnetically dilute CrIII ions. In this
context, and as an explanation for the formation of 4, it
is instructive to consider the possible resonance forms
of the initial oxidation product 3+ (see Scheme 3).
Structure 3a + with its tetravalent chromium is probably
the most important contributor. However, delocalization
of the hole in the trio of SOMOs onto the unsaturated
ligand (or in other words charge transfer from the amide
to the metal) results in structures 3b+ and 3c+, with
radical character in the 3- or 5-position of the ring.
Coupling of two such radicals allows chromium to return
to its favored oxidation state of +III. The conjugated
double bonds of 3b+ probably render it slightly more
stable, hence the formation of the 3,3-coupled isomer 4
as the final product.
(14) For the ESR spectroscopic detection of a minute equilibrium
concentration of presumed [Cp*Cr(CN)4]2- see: Mattamana, S. P.; Poli,
R. Organometallics 1997, 16, 2427.

186 Organometallics, Vol. 20, No. 1, 2001
Leelasubcharoen et al.
UV/vis (pentane): 354 (ꢀ ) 10254 M-1 cm-1), 488 (ꢀ ) 1472
M-1 cm-1) nm. Mp: 131-133 °C (dec). µeff(295 K) ) 5.6(1)µB.
Anal. Calcd for C42H90Cr2Si4P2: C, 57.75; H, 10.39. Found: (1)
C, 51.86; H, 9.22; (2) C, 55.89; H, 9.39.18 MS, m/e: 649 (6.9%),
606 (35.7%), 553 (49.2%), 422 (13.3%), 322 (100%).
We note that the oxidative transformation of 3 into 4
does not necessarily invalidate our line of reasoning
concerning the stabilization of a chromium alkylidene.
The C-C bond formation leading to 3 may be irrevers-
ible, at least on the time scale of the oxidation and
subsequent dimerization (kdim ) 2 × 104 M-1 s-1) of the
latter.15
Syn th esis of [Cp *Cr ((CH2)2SiMe2)2] 2(µ-η1:η1-d m p e) (2).
A 880 mg (2.433 mmol) portion of Cp*Cr(CH2SiMe3)2 was
dissolved in 10 mL of THF, and the solution was heated to 50
°C for 18 h. To this dark green solution of Cp*(THF)Cr(CH2)2-
SiMe2 formed in situ was added 410 µL (2.458 mmol) of dmpe,
and the solution was allowed to stir for 1 h. The THF and
excess dmpe were removed by rotoevaporation to leave a green
solid. This solid was dissolved in toluene, layered with pentane,
and cooled to -30 °C to give 270 mg of 2 (32% yield). 1H NMR
(C6D6): -31.03 (br, 2H), -12.57 (br, 6H), 27.05 (br, 15H) ppm.
IR (KBr): 2933 (s), 2907 (s), 1481 (w), 1420 (s), 1375 (m), 1283
(m), 1231 (s), 1089 (m), 942 (s), 922 (s), 831 (s), 775 (m), 720
(m), 677 (s), 486 (s) cm-1. UV/vis (toluene): 408 (ꢀ ) 2797 M-1
cm-1), 625 (ꢀ ) 1805 M-1 cm-1) nm. µeff(295 K) ) 5.60µB. Anal.
Calcd for C34H66Cr2Si2P2: C, 58.59; H, 9.54. Found: (1) C,
54.95; H, 8.94; (2) C, 54.41; H, 10.00.18 MS, m/e: 680 (90.7%),
649 (53.5%), 606 (72.6%), 546 (100%), 510 (20.1%), 476 (17.4%),
376 (11.1%).
Syn th esis of (η5-P en tam eth ylcyclopen tadien yl)(2-(2′′,2′′-
d im eth yl-2′′-sila p r op yl)-1,2-d ih yd r o[2,2′]bip yr id in e-1,3′′-
d iyl)ch r om iu m (III), Cp *Cr (η3-C14H18N2Si) (3). A 1.54 mg
(4.258 mmol) sample of Cp*Cr(CH2Si(CH3)3)2 was dissolved in
10 mL of THF and then was heated at 50 °C for 18 h. To this
dark green solution was added 665 mg (2.458 mmol) of bpy,
and the solution was allowed to stir for 1 h. The THF was
removed by rotoevaporation, leaving a green-black solid. This
solid was dissolved in toluene and layered with pentane and
cooled at -30 °C to give 582 mg of 3 (32% yield). 1H NMR
(C6D6): -80.71, -71.86, -6.01, 2.87, 17.11, 42.97, 54.64 ppm.
IR (KBr): 2940 (s), 2855 (s), 1589 (m), 1486 (s), 1376 (m), 1256
(s), 1059 (w), 1024 (w), 976 (w), 895 (s), 833 (s), 790 (m), 726
(m), 676 (w) cm-1. UV/vis (toluene): 374 (ꢀ ) 8368 M-1 cm-1),
579 (ꢀ ) 1902 M-1 cm-1) nm. Mp: 158 °C (dec). µeff(296 K) )
3.64µB. Anal. Calcd for C24H33CrSiN2: C, 67.01; H, 7.74; N,
6.52. Found: C, 66.82; H, 8.01; N, 6.59. MS, m/e: 429 (16.0%),
343 (100%), 295 (22.4%), 208 (19.6%).
Syn t h esis of [Cp *Cr (µ-η3:η3-C28H 36N4Si2][P F 6]2 (4)‚
2CH2Cl2. A 250 mg (0.756 mmol) sample of [Cp2Fe]PF6 was
added to a stirred green solution of 325 mg (0.756 mmol) of 3
in 25 mL of THF. After a few minutes, the color of the solution
changed to brown-yellow. After the mixture was allowed to
stir for a half an hour the solvent was removed by rotoevapo-
ration, leaving a brown-black solid. This solid was washed with
pentane and filtered, giving a yellow filtrate and a red-black
solid. Pentane was evaporated from the filtrate to yield 130
mg of Cp2Fe as a yellow solid (92%). The red-black solid was
recrystallized from methylene chloride layered with pentane
at -30 °C, yielding 450 mg of 4‚2CH2Cl2 (90%). 1H NMR (CD2-
Cl2): -67.58, -19.69, 0.01, 7.17, 19.34, 23.27, 33.70, 44.03,
47.31 ppm. IR (KBr): 2918 (m), 1474 (m), 1431 (m), 842 (s),
737 (m), 557 (m). UV/vis (CH2Cl2): 378 (ꢀ ) 12613 M-1 cm-1),
463 (ꢀ ) 8570 M-1 cm-1) nm. Mp: >280 °C. µeff(297 K) ) 5.4-
(1)µB. Anal. Calcd for C50H70Cr2Si2N4P2F12Cl4: C, 45.53; H,
5.35; N, 4.25. Found: C, 45.74; H, 5.52; N, 4.22. MS, m/e (%):
343 (12%), 222 (5.3%), 170 (65.9%), 156 (64.8%), 119 (22.7%),
77 (100%).
Con clu sion s
Once again we have tried, and failed, to isolate a
terminal alkylidene bonded to the Cp*CrIII fragment.
The varied escape routes by which chromium avoids the
formation of such a species bespeak its fundamental
instability. In the case described herein it leads to the
loss of aromatic stabilization of half of a bipyridine
ligand. Higher oxidation states of chromium may be
needed to support a chromium-carbon double bond.
However, oxidation of rearrangement product 3 did not
reverse the functionalization of the bipyridine ligand.
Rather, further ligand coupling ensued. These results
reemphasize the notion that any terminal Cp*CrIII
alkylidene will be destabilized and therefore highly
reactive.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations of compounds
were performed using standard Schlenk, high-vacuum, and
inert atmosphere glovebox techniques. Pentane, diethyl ether,
tetrahydrofuran (THF), and toluene were distilled from purple
Na benzophenone/ketyl solutions. C6D6 was predried with Na
and stored under vacuum over Na/K alloy. CH2Cl2 and CD2-
Cl2 were predried with CaH2 and stored under vacuum over 4
Å molecular sieves. CrCl3(anhydrous) was purchased from
Strem Chemical Co. Trimethylsilylmethyllithium, dmpe, bipy,
and [Cp2Fe]PF6 were purchased from Aldrich Chemical Co.
CrCl3(THF)3,16 Cp*Li,17 Cp*Cr(CH2SiMe3)2,7 and Cp*(THF)-
7
Cr(CH2)2SiMe2 were synthesized by literature procedures.
NMR spectra were recorded on Bruker AM-250, WM-250,
or DRX-400 spectrometers and were referenced to the residual
protons of the solvent (C6D5H, 7.15 ppm; CDHCl2, 5.32 ppm).
FTIR spectra were recorded on a Mattson Alpha Centauri
spectrometer. UV-visible spectra were obtained with a Hewlett-
Packard HP845x spectrometer. Mass spectral analyses were
performed by the University of Delaware Mass Spectrometry
Facility. Elemental analyses were obtained from Schwarzkopf
Microanalytical Laboratory, Inc., Woodside, NY 11377. Room-
temperature magnetic susceptibilities were determined using
a J ohnson Matthey magnetic susceptibility balance, which
utilizes a modification of the Gouy method. Molar magnetic
susceptibilities were corrected for diamagnetism using Pascal
constants.
Syn th esis of [Cp *Cr (CH2SiMe3)2]2(µ-η1:η1-d m p e) (1). A
460 µL (2.757 mmol) portion of dmpe was slowly injected into
a stirred red solution of 1 g (2.765 mmol) of Cp*Cr(CH2SiMe3)2
in 10 mL of pentane. The color of the solution immediately
changed to blue-green. After the mixture was stirred for 1 h,
the solution was concentrated and crystallized at -30 °C to
yield 1.160 g of blue-purple solid 1 (96% yield). 1H NMR (C6D6):
-18.31 (br, 6H), -0.77 (br, 18H), 5.55 (br, 15H) ppm. IR
(KBr): 2940 (s), 2893 (s), 2827 (s), 1486 (w), 1421 (s), 1375
(m), 1390 (m), 1282 (m), 1233 (s), 1187 (m), 1175 (w), 927 (s),
(18) Despite repeated attempts, triply recrystallized samples of 1
and 2 did not give satisfactory elemental analyses. 1 is thermally labile,
and both 1 and 2 are very air-sensitive; we submit that decomposition
during shipping or handling is the cause of this failure. 1H NMR
spectra of these complexes are included in the Supporting Information;
however, the extreme broadening of the resonances due to the
paramagnetism of the complexes renders this way of demonstrating
purity ambiguous as well.
897 (s), 852 (s), 821 (s), 721 (s), 671 (s), 516 (w), 424 (w) cm-1
.
(15) Leelasubcharoen, S.; Lehmann, M. W.; Theopold, K. H.; Evans,
D. H. J . Electrochem. Soc., in press.
(16) Herwig, W.; Zeiss, H. H. J . Org. Chem. 1958, 23, 1404.
(17) Threlkel, R. S.; Bercaw, J . E.; Seidler, P. F.; Stryker, J . M.;
Bergman, R. G. Org. Synth. 1987, 65, 42.

Unusual Transformations of a Bipyridine Ligand
Organometallics, Vol. 20, No. 1, 2001 187
Ta ble 5. Cr ysta llogr a p h ic Da ta for [Cp *Cr (CH2SiMe3)2]2(µ-η1:η1-d m p e) (1),
[Cp *Cr (CH2)2SiMe2]2(µ-η1:η1-d m p e) (2), Cp *Cr (η3-C14H18N2Si) (3), a n d [Cp *Cr (µ-η3:η3-C28H36N4Si2)][P F 6]2
(4)‚2CH2Cl2
1
2
3
4
formula
fw
space group
a, Å
b, Å
c, Å
C
42H90Cr2P2Si4
C34H66Cr2P2Si2
696.99
P21/n
11.7706(2)
11.4745(2)
14.8858(3)
90.00
103.2229(12)
90.00
C24H33CrN2Si
429.61
P21/c
9.4575(2)
16.8771(2)
14.6778(2)
90.00
105.0810(10)
90.00
C50H70Cl4Cr2F12N4P2Si2
1319.02
P21212
14.5689(2)
18.6603(2)
10.86420(10)
90.00
90.00
90.00
2953.54(6)
2
873.44
P21/n
9.4135(2)
19.5070(2)
14.2857(2)
90.00
92.7572(6)
90.00
R, deg
â, deg
γ, deg
V, Å3
2620.24(4)
2
1957.20(6)
2
2262.11(6)
4
Z
cryst color, habit
D(calc), g cm-3
µ(Mo KR), cm-1
temp, K
diffractometer
radiation
R(F), %a
R(wF), %a
purple rod
1.107
5.92
green-blue plate
1.183
7.19
purple blade
1.261
5.70
brown plate
1.483
7.20
173(2)
173(2)
203(2)
173(2)
Siemens P4/CCD
Mo KR (λ ) 0.71073 Å)
4.42
14.68
6.65
6.93
16.20
7.26
21.27
18.13
Quantity minimized ) R(wF2) ) ∑[w(Fo - Fc2)2]/∑[(wFo2)2]1/2; R ) ∑∆/∑(Fo), ∆ ) |(Fo - Fc)|. w ) 1/[σ2(Fo2) + (aP)2 + bP], P ) [2Fc
a
2
2
+ Max(Fo, 0)]/3.
All software and sources of scattering factors are contained
in the SHELXTL (5.10) program library (G. Sheldrick, Siemens
XRD, Madison, WI).
Cr ysta llogr a p h ic Str u ctu r e Deter m in a tion s. Crystal,
data collection, and refinement parameters are given in Table
5. The systematic absences in the diffraction data of all
structures are uniquely consistent with the reported space
groups. The structures were solved using direct methods,
completed by subsequent difference Fourier syntheses and
refined by full-matrix least-squares procedures. The asym-
metric units of 1 and 2 contain half of the dimers, which both
lie on an inversion center. The asymmetric unit of 4 also
contains half of the dimer, which lies on a 2-fold axis, as well
as a PF6 counterion and a dichloromethane molecule. No
absorption correction was applied to the data of 1 and 3,
whereas SADABS absorption corrections were applied to the
data of 2 and 4. All non-hydrogen atoms were refined with
anisotropic displacement coefficients, and hydrogen atoms
were treated as idealized contributions.
Ack n ow led gm en t. This research was supported by
a grant from the National Science Foundation (CHE-
9876426). S.L. thanks the Ministry of Science, Technol-
ogy and Environment of Thailand for financial support.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra of
1 and 2, and tables of crystal data, structure solution and
refinement, atomic coordinates, interatomic distances and
angles, anisotropic thermal parameters, and H-atom coordi-
nates for 1, 2, 3, and 4. An X-ray crystallographic file, in CIF
format, is available through the Internet only. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM0007421