A transient vanadium(III) neopentylidene complex. Redox chemistry and reactivity of the V=CHtBu functionality

Article abstract of DOI:10.1021/om800800g

The vanadium(III) bis(neopentyl) complex (PNP)V(CH₂ ^tBu)₂ (PNP = N[4-Me-2-(PⁱPr₂)C ₆H₃]₂^-), a complex readily prepared from alkylation of (PNP)VCl₂

Full text of DOI:10.1021/om800800g

Organometallics 2009, 28, 843–852
843
A Transient Vanadium(III) Neopentylidene Complex. Redox
Chemistry and Reactivity of the VdCHtBu Functionality
Uriah J. Kilgore, Caitlin A. Sengelaub,† Hongjun Fan, John Tomaszewski,
Jonathan A. Karty, Mu-Hyun Baik, and Daniel J. Mindiola*
Department of Chemistry, the Molecular Structure Center, and the School of Informatics, Indiana
UniVersity, Bloomington, Indiana 47405
ReceiVed August 18, 2008
t
-
The vanadium(III) bis(neopentyl) complex (PNP)V(CH2 Bu)2 (PNP ) N[4-Me-2-(PiPr2)C6H3]2 ), a
t
complex readily prepared from alkylation of (PNP)VCl2 with 2 equiv of LiCH2 Bu, serves as a precursor
to the transient vanadium(III) alkylidene complex “(PNP)VdCHtBu”. Two-electron oxidation of the
intermediate [(PNP)VdCHtBu] with chalcogen sources results in formation of the vanadium(V)
chalcogenide series (PNP)VdCHtBu(X) (X ) O, S, Se, Te). This family of chalcogenide-alkylidenes
has been studied via 51V NMR spectroscopy in combination with DFT computational methods. The redox
chemistry of [(PNP)VdCHtBu] and the reactivity of the alkylidene ligand are explored with the substrates
N3SiMe3, NtCtBu, N2, and azobenzene. It was discovered that NdN cleavage of the last substrate can
be achieved without oxidation of the metal.
vanadium(V) alkylidene complex, CpV(NAr)(CHPh)(PMe3) (Ar
) 2,6-(CHMe2)2C6H3), was reported.11 Unlike the heavier d0
alkylidene congeners (e.g., Ta(V)), which are typically prepared
by R-hydrogen abstraction involving high-valent metal precur-
sors, CpV(NAr)(CHPh)(PMe3) was generated via a two-electron-
redox process in which the precursor, CpV(NAr)(PMe3)2, was
oxidized by the Wittig reagent Ph3PdCHPh, resulting in an
alkylidene group-transfer reaction.11
Introduction
The utility of high-oxidation-state transition-metal alkylidenes
is well-known within the chemical community, as many of these
systems are capable of catalyzing important transformations such
as olefin metathesis,1 alkyne polymerization,2-4 and Wittig-type
reactions.1,5 More recently, high-oxidation-state transition-metal
alkylidenes have found a place in alkane metathesis catalytic
cycles, combining alkene metathesis and dehydrogenation/
hydrogenation catalysts.6-9 Although significant effort has been
dedicated to the development of nucleophilic “Schrock-type”
alkylidene complexes containing 4d and especially 5d transition
metals belonging to groups 5-7, the chemistry of 3d transition-
metal alkylidenes, especially for vanadium, is still in its infancy.
The first vanadium(III) alkylidene complex, CpVdCHtBu-
(dmpe) (dmpe ) bis(dimethylphosphanyl)ethane),10 was re-
ported by Teuben and co-workers in 1989. The synthesis of
this complex involved treatment of the V(III) bis(alkyl) precursor
with the chelating bis-phosphine ligand dmpe to promote
R-hydrogen abstraction, furnishing the terminal alkylidene unit.
It was not until 1994, however, that the first high-valent
High-oxidation-state alkylidene complexes of vanadium are
of particular interest to us, given their potential use in catalytic
group transfer chemistry due to the accessible V(V) T V(III)
redox shuffle. It has recently been established by Nomura et al.
that vanadium(V) alkylidenes can function as highly active ring-
opening metathesis polymerization (ROMP) catalysts.12-14 In
addition, silica-supported vanadium (IV) alkylidenes reported
in 1998 by Scott and co-workers were shown to be reactive
toward olefins, undergoing metathesis with styrene and cata-
lyzing its polymerization.15,16
In the context of group 5 metal chemistry, the PNP ligand
framework (PNP ) N[2-P(CHMe2)2-4-Me-C6H3]2-),17-19
a
* To whom correspondence should be addressed. E-mail: mindiola@
indiana.edu.
(11) Buijink, J.-K. F.; Teuben, J. H.; Kooijman, H.; Spek, A. L.
Organometallics 1994, 13, 2922.
(12) Yamada, J.; Fujiki, M.; Nomura, K. Organometallics 2005, 24,
2248.
† Present address: Department of Chemistry, Yale University, New
Haven, CT 06511.
(1) Schrock, R. R. Chem. ReV. 2002, 102, 145.
(2) Schrock, R. R.; Luo, S.; Lee, J. C.; Zanetti, N. C.; Davis, W. M
J. Am. Chem. Soc. 1996, 118, 3883.
(3) Buchmeiser, M. R. Chem. ReV. 2000, 100, 1565.
(4) Schlund, R.; Schrock, R. R.; Crowe, W. E. J. Am. Chem. Soc. 1989,
111, 8004.
(5) Mindiola, D. J.; Bailey, B. C.; Basuli, F. Eur. J. Inorg. Chem. 2006,
3135.
(6) Goldman, A. S.; Roy, A. H.; Huang, Z.; Ahuja, R.; Schinski, W.;
Brookhart, M. Science 2006, 312, 257.
(13) Nomura, K.; Atsumi, T.; Fujiki, M.; Yamada, J. J. Mol. Catal. A:
Chem. 2007, 275, 1.
(14) Nomura, K.; Onishi, Y.; Fujiki, M.; Yamada, J. Organometallics
2008, 27, 3818. For other examples of vanadium alkylidenes: (a) Zhang,
W.; Nomura, K. Organometallics 2008, 27, 6400. (b) Zhang, W.; Yamada,
J.; Nomura, K. Organometallics 2008, 27, 5353.
(15) Beaudoin, M. C.; Womiloju, O.; Fu, A.; Ajjou, J. A. N.; Rice, G. L.;
Scott, S. L. J. Mol. Catal. A: Chem. 2002, 190, 159.
(16) Ajjou, J. A. N.; Rice, G. L.; Scott, S. L. J. Am. Chem. Soc. 1998,
120, 13436.
(7) Ahuja, R.; Kundu, S.; Goldman, A. S.; Brookhart, M.; Vicente, B. C.;
Scott, S. L. Chem. Commun. 2008, 253.
(8) Blanc, F.; Coperet, C.; Thivolle-Cazat, J.; Basset, J.-M. Angew.
Chem., Int. Ed. 2006, 45, 6201.
(17) Winter, A. M.; Eichele, K.; Mack, H.-G.; Potuznik, S.; Mayer,
H. A.; Kaska, W. C. J. Organomet. Chem. 2003, 682, 149.
(18) Fan, L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2004, 23,
326.
(9) Coperet, C.; Maury, O.; Thivolle-Cazat, J.; Basset, J.-M. Angew.
Chem., Int. Ed. 2001, 40, 2331.
(10) Hessen, B.; Meetsma, A.; Teuben, J. H. J. Am. Chem. Soc. 1989,
111, 5977.
(19) Liang, L. C.; Lin, J. M.; Hung, C. H. Organometallics 2003, 22,
3007.
(20) Fryzuk, M. D.; MacNeil, P. A.; Rettig, S. J.; Secco, A. S.; Trotter,
J. Organometallics 1982, 1, 918.
10.1021/om800800g CCC: $40.75
2009 American Chemical Society
Publication on Web 12/18/2008

844 Organometallics, Vol. 28, No. 3, 2009
Kilgore et al.
Scheme 1. Synthesis of
1 and 2 from the Precursor
VCl3(THF)3
newer hybrid of the original Fryzuk ligand,20 has recently been
used as an ancillary support for transient Nb(III) fragments
capable of activating atmospheric nitrogen and cleaving the
NdN bond in azobenzene.21 Likewise, this scaffold stabilized
the first crystallographically characterized examples of terminal
niobium bis(alkylidenes)(bistrimethylsilylmethylidene and bis-
neopentylidene),22 as well as the first monomeric tantalum
bis(methylidene) complex, (PNP)Ta(CH2)2.23 In 1989, a related
monoanionic PNP ligand was applied to vanadium, and this
system was even used to construct some vanadium aryl and
alkyl complexes.24,25 More recently, we have shown the V(III)
Figure 1. Molecular structure of complex 1 depicting thermal
ellipsoids at the 50% probability level, with H atoms and iPr methyl
groups omitted for clarity. Distances are reported in Å and angles
in deg. Selected metrical parameters: V(1)-N(1), 1.950(3);
V(1)-Cl(1), 2.2687(7); V(1)-Cl(1A), 2.2687(7); V(1)-P(1),
2.4835(6); V(1)-P(1A), 2.4835(6); P(1)-V(1)-P(1A), 158.42(4);
N(1)-V(1)-Cl(1) 121.74(2); N(1)-V(1)-Cl(1A) 121.74(2);
Cl(1)-V(1)-Cl(1A), 116.53(5).
t
bis(neopentyl) complex (PNP)V(CH2 Bu)2 to be a ready precur-
sor to a series of terminal vanadium(V) complexes bearing
terminal alkylidene ligands when treated with various two-
electron oxidants such as N2CPh2, oxo sources such as OPPh3
(OdPMe3 or N2O can also be used), and sulfide sources such
as S8 and SdPPh3.26
and pyridine. Although 1 can be isolated and stored for later
use, we have found that this is generally unnecessary, as this
complex can generally prepared and used in situ en route to
the corresponding bis(alkyl) species by performing the subse-
quent alkylation step in a one-pot reaction (vide infra), in a
fashion similar to that described for preparation of the Ti(III)
Herein, we report further on the reactivity of the V(III) alkyl
t
species, (PNP)V(CH2 Bu)2, a molecule capable of undergoing
27
t
analogue (PNP)Ti(CH2 Bu)2.
two-electron-redox processes to generate terminal V(V) alky-
lidenes as well as other products derived from a transient
alkylidene fragment, [(PNP)VdCHtBu]. Some of the latter
chemistry includes metathesis, atmospheric nitrogen, activation,
and azobenzene NdN bond rupture. In addition to reactivity,
we also discuss and explore the 51V NMR spectroscopy of
chalcogenido alkylidene complexes derived from the two-
electron oxidation of [(PNP)VdCHtBu] through a combination
of experiment and DFT calculations.
Although 1 was typically used as a reactant in situ, we felt
compelled to confirm its connectivity by single-crystal X-ray
diffraction studies, as the close relatives “(PNP)TiCl2” and
“(PNP)NbCl2” eluded crystallographic analysis or isolation.21,27
Single crystals of 1 were grown by slow diffusion of hexane
into a dichloromethane solution at room temperature. The single-
crystal X-ray structure of 1 (Figure 1) displays a monomeric
(PNP)VCl2 species in a distorted-trigonal-bipyramidal geometry
with the N and two Cl atoms defining the base of the pyramid.
Half of the formula unit is crystallographically unique; V and
N are located on a 2-fold axis (3/4, y, 1/4). The observed
geometrical parameters are within the expected ranges. The
identity of 1 is further supported by mass spectral analyses (MS-
CI), further suggesting that the molecular ion is indeed the
monomeric and unsolvated (PNP)VCl2 fragment.
Results and Discussion
Incorporation of the Pincer Ligand and Subsequent
t
Synthesis of the Alkylidene Precursor (PNP)V(CH2 Bu)2. In
order to access the [(PNP)VdCHtBu] fragment (A), the PNP
framework must first be introduced on V(III), followed by
preparation of an appropriate vanadium alkyl precursor having
R-hydrogens, but ideally lacking ꢀ-hydrogens to prevent
competing side reactions such as ꢀ-hydrogen elimination.
Accordingly, the precursor (PNP)VCl2 (1) was conveniently
synthesized by treatment of VCl3(THF)3 in toluene/THF (3:1)
with 1 equiv of Li(PNP) at room temperature under an inert
atmosphere (Scheme 1). It should be noted that complex 1
appears to bind Lewis bases, given that solutions in solvents
such as toluene, benzene, and dichloromethane are red while a
blue solution is formed in Lewis basic solvents such as THF
Synthesis of the bis(neopentyl)vanadium complex (PNP)-
t
V(CH2 Bu)2 (2) was achieved by treatment of a -36 °C THF/
toluene solution of 1 (generated in situ) with 2 equiv of
t
neopentyllithium, LiCH2 Bu, in hexane (Scheme 1). Upon
addition of the lithium reagent, the red/purple suspension of 1
rapidly becomes green, and after workup of the reaction mixture,
complex 2 is collected in ∼78% yield as analytically pure green
crystals upon cooling a concentrated hexanes solution to -35
°C over 12 h. The complex is exceedingly air and moisture
sensitive. Over the course of several days, room-temperature
solutions of 2 under an atmosphere of nitrogen are converted
to [(PNP)VdCHtBu]2(µ2,η1:η1-N2) (3).26 The reactivity of 2
(21) Kilgore, U. J.; Yang, X.; Tomaszewski, J.; Huffman, J. C.; Mindiola,
D. J. Inorg. Chem. 2006, 45, 10712.
(22) Kilgore, U. J.; Tomaszewski, J.; Fan, H.; Huffman, J. C.; Mindiola,
D. J. Organometallics 2007, 26, 6132.
(23) Gerber, L. C. H.; Watson, L. A.; Parkin, S.; Weng, W.; Foxman,
B. M.; Ozerov, O. V. Organometallics 2007, 26, 4866.
(24) Danopoulos, A. A.; Edwards, P. G.; Parry, J. S.; Wills, A. R.
Polyhedron 1989, 8, 1767.
t
contrasts sharply with that of the putative “(PNP)Nb(CH2 Bu)2”,
a system which presumably undergoes a rapid series of
transformations such as R-hydrogen abstraction and dihydrogen
elimination steps, resulting in the high-valent bis(neopentyl-
idene) complex (PNP)Nb(CHtBu)2.22
(25) Danopoulos, A. A.; Edwards, P. G. Polyhedron 1989, 8, 1339.
(26) Kilgore, U. J.; Sengelaub, C. A.; Pink, M.; Fout, A. R.; Mindiola,
D. J. Angew. Chem., Int. Ed. 2008, 47, 3769.
(27) Bailey, B. C.; Huffman, J. C.; Mindiola, D. J.; Weng, W.; Ozerov,
O. V. Organometallics 2005, 24, 1390.

A Transient V(III) Neopentylidene Complex
Organometallics, Vol. 28, No. 3, 2009 845
Scheme 2. Synthesis of 4 and 5 via Treatment of 2 with [O]
Sources Such as R3PdO, N2O, TEMPO, R3NdO, and
Pyridine N-Oxide and [S] Sources Such as S8 and R3PdS,
Respectively
Figure 2. Molecular structure of complex 2 depicting thermal
ellipsoids at the 50% probability level, with H atoms and iPr methyl
groups omitted for clarity. Distances are reported in Å and angles
in deg. Selected metrical parameters: V(1)-N(10), 2.0230(11);
V(1)-C(36), 2.1085(14); V(1)-C(31), 2.1148(15); V(1)-P(2),
2.5882(4); V(1)-P(18), 2.5947(4); C(36)-V(1)-C(31), 121.46(6);
P(2)-V(1)-P(18), 151.750(13); C(32)-C(31)-V(1), 131.03(11);
C(37)-C(36)-V(1), 138.85(10).
Characterization of 2 was achieved through a combination
of solution magnetization measurements (Evans method), X-ray
diffraction data, and combustion analysis. The µeff value for 2
in C6D6 at 25 °C was found to be 2.74 ( 0.07 µΒ. This value
is in accord with the expected spin-only value of 2.83 µΒ for
the d2 configuration, thus indicating a high-spin vanadium(III)
system. Single crystals of 2 suitable for X-ray diffraction
analysis were slowly grown from diethyl ether at room tem-
perature. The crystal structure of 2 revealed the expected
monomeric bis(alkyl) species (Figure 2), with the vanadium
center confined in a highly distorted trigonal bipyramid when
the P-V-P linkage, 151.750(13)°, is treated as the primary
axis. Given the meridional constraint imposed by the aryl PNP
framework, the alkyl ligands are forced into a cis orientation
(121.46(6)°). The V-C distances of 2.1085(14) and 2.1148(15)
Å are within the range of other V-Calkyl distances of 2.062 and
2.061 Å reported for bis(alkyl) V(III) complexes originally
synthesized by Budzelaar as well a V(IV) bis(neopentyl) species
reported by Gambarotta and co-workers.28,29 The V-C-C bond
angles of 131.03(11) and 138.85(10)° in complex 2 are further
indicative of V-C alkyl bonds.
Reactions of 2 To Generate High-Valent and Terminal
Vanadium(V) Chalcogenido-Alkylidenes. Previously, we
synthesized the chalcogenido-alkylidene species (PNP)Vd
CHtBu(O) (4) (vide infra) and the analogue (PNP)VdCHtBu(S)
(5) via two-electron-redox processes stemming from the precur-
sor 2 and the corresponding chalcogen source (Scheme 2).26
The oxo-alkylidene species 4 has previously been synthesized
via treatment of 2 with Ph3PdO or N2O; we have also confirmed
that treatment of 2 with pyridine N-oxide or trimethylamine
N-oxide, with mild heating (50 °C), results in formation of 4
(Scheme 2). While these oxo-transfer type reactions were
expected, we have also made the curious discovery that 4 can
be generated in 49% yield by treating a hexane solution of 2
with the free radical 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO). After 2 days, the crude reaction mixture of TEMPO
and 2 is found by 1H NMR spectra and MS-CI to contain 4 and
as yet unidentified paramagnetic products. Complex 4 is
separated from the product mixture and isolated in pure form
by removing the volatiles in vacuo and extracting with hexam-
Scheme 3. Synthesis of Complexes 6 and 7 via Treatment of
2 with Se0 and Te0
ethyldisiloxane (TMS2O). Therefore, it appears that compound
4 is a highly favored thermodynamic product, since reagents
such as TEMPO are not typically sought as O atom sources.
Complex 5 can be similarly prepared from 2 and sulfide sources
such as S8, Ph3P)S, or Me3P)S (Scheme 2).
Unfortunately, the isolation of the heavier chalcogenide
analogues has proven somewhat more difficult. For instance,
the selenido alkylidene (PNP)VdCHtBu(Se) (6) can be prepared
by heating a dilute mixture of 2 and elemental Se to 60 °C in
benzene under an atmosphere of argon for 4 h to afford this
complex in 45% yield, as judged by 1H NMR spectroscopy using
an internal integration standard (1,4-dioxane) (Scheme 3). Crude
powders (judged by 1H NMR spectra)30 of 6 can be isolated by
storing concentrated hexane solutions at -36 °C for several
days. While attempts to isolate single crystals of 6 have failed,
spectroscopic data strongly support our assignment of the
complex in solution as that proposed. MS-CI reveals a molecular
ion, [M]+, at m/z 629.2007, and the isotopic distribution found
experimentally (most abundant ions: m/z 625.2005, 626.2046,
627.2021, 629.2007, 630.2033, and 631.2030) is in excellent
agreement with the calculated mass distribution (m/z 625.205,
626.206, 627.205, 629.202, 630.206, and 631.204).30 The 13C
NMR spectrum contains a broad resonance at 328.4 ppm, which
1
we assign to the alkylidene R-C while the H NMR spectrum
reveals a singlet at 9.24 ppm assigned to the corresponding
alkylidene R-H. These resonances are shown, via an HMQC
experiment, to correlate to one another. Due to the broadness
of the resonance at 328.4 ppm, NMR spectroscopic coupling
constants (JCH) could not be determined with certainty, even
(28) Desmangles, N.; Gambarotta, S.; Bensimon, C.; Davis, S.; Zahalka,
H. J. Organomet. Chem., 562, 53.
(29) Budzelaar, P. H.; van Oort, B. A.; Orpen, A. G. Eur. J. Inorg.
Chem. 1998, 1485.
(30) See the Supporting Information.

846 Organometallics, Vol. 28, No. 3, 2009
Kilgore et al.
when the solution was cooled to -50 °C. The 51V NMR
spectrum of 6 evinces one broad signal at 809 ppm (∆ν1/2
)
993 Hz, 25 °C, C6D6). As a result, the above data, with the
support of DFT NMR shift calculations (vide infra), strongly
support the identity of 6 as a monomer.
Likewise, attempts to crystallographically characterize
(PNP)VdCHtBu(Te) (7) have, thus far, been unsuccessful. To
prepare 7, complex 2 was treated with excess elemental Te
powder in THF, resulting in formation of a diamagnetic complex
which we propose to be the monomeric tellurido-alkylidene
1
on the basis of a combination of MS-CI, H/13C/31P NMR
studies, and 51V NMR spectroscopic data (1249 ppm, ν1/2
)
864 Hz). The isotopic distribution and intensities found in the
experimental MS-CI spectrum of 7 match those found in the
calculated spectrum quite closely, with the most intense ions
observed being m/z 674.1908, 675.1899, 677.1888, 679.1901
and the most intense ions calculated at m/z 674.191, 675.190,
677.193, and 679.192.30 Further, the 1H NMR spectrum of 7 is
consistent with our proposed formulation, while the 13C NMR
spectrum of 7 also corroborates our data, revealing an alkylidene
carbon resonance at 332.3 ppm (d8-toluene -28.0 °C). The
gHMQC NMR experiments conducted at -13 °C in d8-toluene
clearly reveal a correlation between the downfield carbon peak
at 332 ppm and a proton resonance at 6.7 ppm.
Figure 3. DFT optimized structures (B3LYP, 6-31G**) of
(PNP)VdCHtBu(dX) (complexes 4-7). Isopropyl methyls on
phosphorus and hydrogens are omitted for clarity.
Table 1. Selected Calculated (in Italics) and Experimental
Structural Data for Chalcogenido-Alkylidenesa
complex
VdX bond length
VdC bond length
VdC-C angle
4
5
6
7
1.595/1.599
2.0706/2.055
2.228
1.894/1.833
1.826/1.794
1.786
140.5/142.4
155.2/150.6
153.4
2.447
1.781
156.9
Solutions of pure 7 seem to be stable under an inert
atmosphere, but the presence of oxidants or traces of Te0
promote decomposition into as yet unidentified products.
Schrock and co-workers also observed a lack of stability in a
VdTe multiply bonded species in solution.31 In their case,
solutions of [N3N]VdTe (N3N3- ) (Me3SiNCH2CH2)3N) were
often found to decompose via deposition of a Te0 mirror. To
date, the only reported example of a complex bearing a terminal
VdTe ligand that is sufficiently stable for structural character-
ization via X-ray diffraction is [η4-Me8taa]VdTe ([η4-Me8taa]2-
) octamethyldibenzotetraaza[14]annulene), a species reported
by the group of Parkin in 1997.32
a Distances are reported in Å and angles in deg.
Scheme 4. Proposed Formation of Heavier Chalcogenide-
Alkylidenes from Se or Te Oxidation
As with compounds 4 and 5, we speculate that the heavier
alkylidene-chalcogenido derivatives are formed via activation
of Se or Te by the transient alkylidene A. We propose this
mechanism, given that N2 is activated by transient A to afford
3 when no other suitable substrates are present. At this point,
however, it is uncertain whether the chalcogen substrate (or N2)
promotes R-hydrogen abstraction in 2 to form the alkylidene
prior to oxidation of the metal center (Scheme 4).
51V NMR Spectra of the Chalcogenide-Alkylidenes. It is
well documented that the 51V NMR chemical shifts of high-
valent vanadium complexes exhibit inverse electronegativity
dependence; ligands bound to vanadium through oxygen or
nitrogen atoms induce high shielding, while ligands that are less
electronegative and more polarizable (Br-, I-, S2-, Se2-, Te2-)
result in deshielding of the 51V nucleus.33 The 51V NMR spectra
for this family of chalcogenide-alkylidene complexes,
(PNP)VdCHtBu(X) (X ) O, S, Se, Te), clearly illustrate an
inverse electronegativity dependence, as the observed 51V NMR
chemical shifts of complexes 4-7 follow a trend of increasing
chemical shift on proceeding down the chalcogen group. This
trend in 51V NMR spectroscopic shifts compares quite favorably
withthatofafamilyofvanadium(V)chalcogenide-chalcogenolates
prepared by the group of Arnold34 in 1996 and Gambarotta in
1997,35 as well as a series of V(V) chalcogenide complexes
reported by Schrock in 1994.31
(31) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Inorg. Chem. 1994,
33, 1448.
(32) Kisko, J. L.; Hascall, T.; Parkin, G. J. Am. Chem. Soc. 1997, 119,
7609. For other examples of transition metal complexes with the terminal
telluride: (a) Rabinovich, D.; Parkin, G. J. Am. Chem. Soc. 1991, 113, 9421.
(b) Christou, V.; Arnold, J. J. Am. Chem. Soc. 1992, 114, 6240. (c)
Siemeling, U.; Gibson, V. C. J. Chem. Soc., Chem. Commun. 1992, 1670.
(d) Rabinovich, D.; Parkin, G. J. Am. Chem. Soc. 1993, 115, 9822. (e)
Rabinovich, D.; Parkin, G. J. Am. Chem. Soc. 1995, 34, 6341. (f) Howard,
W. A.; Trnka, T. M.; Waters, M.; Parkin, G. J. Organomet. Chem. 1997,
528, 95. (g) Howard, W. A.; Parkin, G. J. Organomet. Chem. 1994, 472,
C1.
As additional support for our contention that 6 and 7 are
structurally analogous to complexes 4 and 5, DFT computational
methods were employed. A series of NMR-predictive DFT
(34) Gerlach, C. P.; Arnold, J. Inorg. Chem. 1996, 35, 5770.
(35) Ruppa, K. B. P.; Desmangles, N.; Gambarotta, S.; Yap, G.;
Rheingold, A. L. Inorg. Chem. 1997, 36, 1194.
(33) Rehder, D. Coord. Chem. ReV. 2008, 252, 2209.

A Transient V(III) Neopentylidene Complex
Organometallics, Vol. 28, No. 3, 2009 847
Table 2. Calculated (cc-pVTZ(-f)) and Experimental NMR
Chemical Shifts for Complexes 4-7
51V NMR shift (ppm)
complex
exptl
calcd
4
5
6
7
-58
570
809
-63
549
843
1249
1294
Figure 4. Molecular structure of complex 8 displaying thermal
ellipsoids at the 50% probability level with hexanes solvent. H
i
atoms and Pr methyl groups are excluded for clarity. Distances
are reported in Å and angles in deg. Selected metrical parameters:
V(1)-N(14),1.611(5);V(1)-C(14),1.928(6);V(1)-N(1),2.1084(15);
V(1)-P(1), 2.4890(3); N(14)-Si(15), 1.732(5); V(1)-N(14)-Si(15),
172.4(3); C(15)-C(14)-V(1), 145.8(5); N(14)-V(1)-C(14),
110.94(19); N(14)-V(1)-N(1), 130.34(18); C(14)-V(1)-N(1),
118.62(18).
Figure 5. Molecular structure of complex 9 displaying thermal
ellipsoids at the 50% probability level, with hexanes solvent and
H atoms excluded and the PNP ligand simplified to the first
coordination sphere for clarity. Distances are reported in Å and
angles in deg. Selected metrical parameters: V(1)-N(1), 2.081(2);
V(1)-N(2), 1.845(2); V1-N3, 1.717(2); V(1)-P(1), 2.4987(7);
V(1)-P(2), 2.4661(7); N(2)-N(2′), 1.222(4); N(3)-C(27), 1.383(3);
C(27)-C(28) 1.551(4); C(27)-C(32), 1.353(4); N(2′)-N(2)-V(1),
178.0(2); N(3)-V(1)-N(2), 122.75(10); N(3)-V(1)-N(1), 118.46(9);
N(2)-V(1)-N(1),118.72(8);P(2)-V(1)-P(1),152.19(3);C(27)-N(3)-V(1),
173.63(19); C(32)-C(27)-N(3), 127.4(3); C(32)-C(27)-C(28),
118.5(2); C27-C32-C33 135.0(3).
calculations of the 51V NMR chemical shifts for geometrically
optimized structures of monomeric (PNP)VdCHtBu(X) com-
plexes 4-7 (Figure 3 and Table 1) are in excellent agreement
with the experimental data (Table 2). We have also explored
the possibility of dimeric forms of 6 and 7, by either bridging
Te or Se or the alkylidene ligands. In both cases the dimeric
structures converged into two corresponding monomers during
geometry optimization. The optimized structures for complexes
4 and 5 seem to be consistent with experimentally determined
structures in that the differences between measured and calcu-
lated bond lengths and angles all fall under 5%. The calculated
NMR chemical shifts for all chalcogenide-alkylidenes (4-7)
agree within 10% of the experimental values. Calculated shifts
for 5-7 vary from experiment by less than 5%. We strongly
believe that these calculations, in addition to spectroscopic
evidence, lend support to our proposed connectivity of 6 and 7
in solution.
Reactivity of 2 with Trimethylsilyl Azide. Treatment of 2
withtrimethylsilylazideledtotheexpectedV(V)imido-alkylidene
species concurrent with N2 evolution. The diamagnetic complex
(PNP)VdCHtBu(NSiMe3) (8) was synthesized by treating a
thawing hexane solution of 2 with a thawing hexane solution
of trimethylsilyl azide and allowing the stirred solution to reach
room temperature. Complex 8 was characterized by a combina-
tion of X-ray diffraction data, multinuclear NMR spectroscopy,
and mass spectroscopy. Multinuclear NMR spectroscopic studies
of complex 8 are indicative of a V(V) alkylidene species: in
the 1H NMR a diagnostic alkylidene R-H resonance was located
at 12.51 ppm, and in the 13C NMR the R-C resonance was found
at 321.5 ppm. Mass spectroscopy (CI) yielded the [M]+ value
at 636.3340, which is in good agreement with the theoretical
value of [M]+, 636.3357.30 The reaction leading to formation
of 8 is not entirely clean, as side products are also observed by
1H and 13C NMR spectra; however, we were fortunate to isolate
small yellow, single crystals of 8 grown from concentrated
solutions pentane or hexanes at room temperature. Accordingly,
the X-ray diffraction analysis of single crystals of 8 reveals the
expected pseudo-trigonal-bipyramidal structure (Figure 4). A
cursory inspection of the structure of complex 8 indicates that
it is geometrically very similar to the molecular structures of 4
and 5. The vanadium-imide nitrogen bond of 1.611(5) Å is
somewhat shorter than those observed for the few crystallo-
graphically characterized V(V) imido-alkylidene species;11,36,37
this is, perhaps, an indication of a strong V-N pseudo triple
bond. The VdN-Si angle of 172.4(3)° is additionally supportive
of a vanadium-imide nitrogen pseudo triple bond. The
vanadium-alkylidene linkage, C(15)-C(14)-V(1), is consider-
ablymoredistortedthanotherexamplesofV(V)imido-alkylidenes
with a V-C-C angle of 145.8(5)°.
t
Reactivity of the Transient Alkylidene A with BuCtN.
Up to this point, the focus of this report has been on the synthesis
of five-coordinate V(V) alkylidenes, which we believe to result
from oxidation of the putative alkylidene fragment A; however,
we have not discussed reactivity at the alkylidene functionality
for such a species. It seemed likely that the alkylidene ligand
in putative A would display Wittig-like behavior in the presence
of unsaturated polarized substrates. Accordingly, treatment of
2 with tBuCtN under an atmosphere of N2 produced over 24 h
the compound [(PNP)VdNC(tBu))CHtBu]2(µ2,η1:η1-N2) (9)
(Figure 5). Formation of this complex is proposed to proceed
via a [2 + 2] cycloaddition reaction between the alkylidene
ligand in transient A and tBuCtN, resulting in a four-
cooordinate vinylimido species, [(PNP)VdNC(tBu))CHtBu],
which subsequently binds and reduces N2 to furnish the end-on
bridging dinitrogen as well as a terminal imide ligand (Scheme
5). A similar complex of vanadium was reported by Teuben et
al. in 1993; in this example, however, treating the V(III)
alkylidene CpVdCHtBu(dmpe) with tBuCtN results in forma-
(36) Yamada, J.; Fujiki, M.; Nomura, K. Organometallics 2005, 24,
2248.
(37) Basuli, F.; Bailey, B. C.; Brown, D.; Tomaszewski, J.; Huffman,
J. C.; Baik, M.-H.; Mindiola, D. J. J. Am. Chem. Soc. 2004, 126, 10506.

848 Organometallics, Vol. 28, No. 3, 2009
Kilgore et al.
N2 when coordination sites are available.39-41 However, one
cannot refute a different pathway to formation of compound 9.
For instance, intermediate A could activate N2 prior to Wittig-
like reactivity of the nitrile with the alkylidene ligand. This
pathway would also result in formation of a vanadium complex
having both N2 and alkylidene ligands.
Scheme 5. Proposed Mechanism to Formation of 9 via
Intermediate Aa
Previously, we reported the synthesis of the alkylidene-
dinitrogen complex 3, by mild heating of 2 under a bed of
atmospheric nitrogen over 12 h (vide supra). This prompted an
investigation of an alternative mechanism to formation of
compound 9 using complex 3. Accordingly, treatment of 3 with
excess tBuCtN at room temperature or under mild heating (40
1
°C) for 48 h did not result in a reaction according to both H
and 31P NMR spectra of the mixture (Scheme 5). The lack of
reactivity in 3 toward nitriles is not entirely unexpected, as this
molecule is nearly coordinatively saturated as well as sterically
hindered. However, this lack of reactivity of 3 implies that
Wittig-like chemistry most likely precedes N2 activation along
the formation of 9.
t
Characterization of 9 involved multinuclear NMR studies,
combustion analysis, and single-crystal X-ray diffraction studies.
Analogous to the structure of 3,26 half of the molecule is
crystallographically unique with an inversion on the N-N (-x,
a
Independently, treatment of 3 with BuCtN does not yield 9,
consistent with the proposed mechanism.
1
y, -z + /2) bond. The X-ray structure reveals a topologically
linear VdNdNdV bridging unit with a V-N-N angle of
178.0(2)°. The N(2)-N(2′) distance of 1.222(4) Å is indicative
of significant reduction in the NtN bond from that of free
dinitrogen (1.098 Å).42 Each NC(tBu)dCHtBu fragment is found
with the tBu groups trans to one another. Additionally, the imide
linkage, C(27)-N(3)-V(1), is virtually linear at 173.63(19)°,
thus implying the presence of a strong bond between the imide
nitrogen and vanadium. It should be noted that, prior to
crystallization, complex 9 appears as a mixture of two isomers
1
according to NMR studies. H and 13C NMR spectra of these
isomeric mixtures contain twice the number of resonances
expected in 9, with only a slight disparity in chemical shifts.
Once isolated as a crystalline material, complex 9 appears as
only one isomer, as evidenced by X-ray diffraction and solution
NMR studies.
NdN Cleavage of Azobenzene by Transient A. Previously
we have demonstrated that a transient and putative Nb(III)
species, “(PNP)NbCl2”, readily cleaves the NdN bond in
azobenzene by an overall 4e reduction, whereby a metal-ligand
cooperative process generates (PNPdNPh)NbCl2(NPh) (PNPd
NPh- ) N[2-P(CHMe2)2-4-Me-C6H3][2-PdNPh(CHMe2)2-4-
Me-C6H3]). In the case of V(III), compound 1 fails to react with
azobenzene at room temperature over several days. However,
treatment of 2 with 1 equiv of azobenzene results in formation
Figure 6. Molecular structure of complex 10 displaying thermal
ellipsoids at the 50% probability level. H atoms, iPr methyl groups,
and hexane solvent are excluded for clarity. Distances are reported
in Å and angles in deg. Selected metrical parameters: V(1)-N(3),
1.951(2); V(1)-N(2), 2.111(2); V(1)-C(27), 2.056(3); V(1)-N(1),
2.057(2); V(1)-P(2), 2.5405(8); V(1)-P(1), 2.5500(9); C(27)-N(2),
1.281(3);N(3)-V(1)-C(27),115.78(11);N(3)-V(1)-N(1),110.55(9);
C(27)-V(1)-N(1), 133.59(10); N(3)-V(1)-N(2), 151.47(10);
C(27)-V(1)-N(2),35.78(9);N(1)-V(1)-N(2),97.81(8);C(27)-N(2)-V(1),
69.79(16); N(2)-C(27)-V(1), 74.43(17); C(28)-C(27)-V(1),
153.7(2); N(2)-C(27)-C(28), 131.9(3); C(27)-N(2)-C(32),
136.5(3); C(32)-N(2)-V(1), 152.94(19); C(38)-N(3)-V(1),
139.56(19); C(28)-C(27)-N(2)-C(32), -5.8(5).
of
a
V(III) η2-iminoacyl-amide complex, [(PNP)V(η2-
PhN)CtBu)(NHPh)] (10), which can be isolated in 45% yield
as red crystals. Cleavage of the NdN bond in azobenzene by
vanadium(III) has precedent, but such a process is rare. The
group of Kawaguchi has reported that the vanadium(III)
tris(arylthiolato) complex V(SAr)3(THF) (Ar ) C6H3-2,6-
(Si(CH3)3)2) when treated with azobenzene results in formation
of a V(V) imide-sulfenamide complex, V(NPh)(PhNSAr)-
tion of [CpVdNC(tBu)dCHtBu](dmpe) without any evidence
of dinitrogen activation.38 In the formation of [CpVdNC-
(tBu)dCHtBu](dmpe), the starting complex, CpVdCHtBu-
(dmpe), is coordinatively saturated; thus, the probablility of N2
coordinating is diminished. However, in the case of compound
9, the putative intermediate, [(PNP)VdNC(tBu)dCHtBu], is a
low-valent and coordinatively unsaturated vinyl-imido frag-
ment, which seems primed for coordination and reduction of a
substrate such as N2. Vanadium(III) has been known to activate
(39) Song, J.-I.; Berno, P.; Gambarotta, S. J. Am. Chem. Soc. 1994,
116, 6927.
(40) Buijink, J. K. F.; Meetsma, A.; Teuben, J. H. Organometallics 1993,
12, 2004.
(41) Desmangles, N.; Jenkins, H.; Ruppa, K. B.; Gambarotta, S. Inorg.
Chim. Acta 1996, 250, 1.
(38) Hessen, B.; Buijink, J. K. F.; Meetsma, A.; Teuben, J. H.;
Helgesson, G.; Haakansson, M.; Jagner, S.; Spek, A. L. Organometallics
1993, 12, 2268.
(42) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular
Structure. IV. Constants of Diatomic Molecules; Van Nostrand Reinhold:
New York, 1979.

A Transient V(III) Neopentylidene Complex
Organometallics, Vol. 28, No. 3, 2009 849
Scheme 6. Proposed Mechanism of Formation of Compound 10 from NdN Bond Cleavage in Azobenzene
Table 3. Crystal Data and Experimental Parameters for Compounds 1, 2, and 8-10
1
2
8
9
10
empirical formula
formula wt
cryst color and shape,
size (mm3)
T (K)
C26H40Cl 2NP2V
550.37
red isometric block,
0.8 × 0.07 × 0.07
150(2)
C36H62NP2V
621.75
black cleaved block,
0.30 × 0.30 × 0.30
126(2)
C34H59N2P2SiV
636.80
yellow plate, 0.05
× 0.05 × 0.005
15(2)
C78H132N6P4V2
1379.66
black block, 0.22
× 0.18 × 0.12
150(2)
C46H67N3P2V
774.91
brown plate, 0.18
× 015 × 0.04
150(2)
λ (Å)
0.710 73
monoclinic, P2/n
0.710 73
orthorhombic, Pbca
0.442 80
monoclinic, C2/c
0.710 73
monoclinic, C2/c
0.710 73
triclinic, P1
j
cryst syst,
space group
unit cell dimens
a (Å)
11.5013(7)
9.2139(5)
12.9166(7)
18.6610(9)
19.7507(9)
19.8391(10)
11.6147(7)
31.6104(19)
11.6936(7)
28.613(2)
14.9540(11)
21.963(3)
13.1283(10)
13.6869(10)
14.0635(10)
66.438(2)
70.476(2)
82.018(2)
2183.1(3)
2
b (Å)
c (Å)
R (deg)
ꢀ (deg)
95.777(1)
95.694(1)
121.997(1)
γ (deg)
V (Å3)
1361.84(13)
2
1.342
0.693
2.21-26.75
7312.1(6)
8
1.130
0.382
2.05-30.02
4272.1(4)
4
0.990
0.187
0.80-19.93
7969.9(13)
4
1.150
0.358
1.60-26.42
Z
Fcalcd (Mg/m3)
1.179
0.334
1.62-25.36
µ (mm-1
)
θ range for
data collecn (deg)
no. of rflns collected
no. of unique rflns
no. of obsd rflns (I >
2σ(I))
15 994
2855 (Rint ) 0.0948)
2091
117 358
10 692 (Rint ) 0.0627)
7888
28 085
8025 (Rint ) 0.0447)
6045
48 069
8179 (Rint ) 0.0413)
6700
17 379
7943 (Rint ) 0.0439)
5231
max and min transmissn
no. of data/restraints/
params
0.9531 and 0.9466
2855/0/150
0.8964 and 0.7911
10 692/0/609
0.9991 and 0.9907
8025/20/220
0.9583 and 0.9254
8179/6/423
0.9868 and 0.9423
7943/3/483
a
goodness of fit on F2
1.023
0.993
1.107
1.062
0.996
final R indices
R1 ) 0.0359,
wR2 ) 0.0767
R1 ) 0.0631,
wR2 ) 0.0848
0.395 and -0.483
R1 ) 0.0337,
wR2 ) 0.0846
R1 ) 0.0543,
wR2 ) 0.0953
0.696 and -0.213
R1 ) 0.0518,
wR2 ) 0.1693
R1 ) 0.0651,
wR2 ) 0.1810
1.108 and -1.094
R1 ) 0.0491,
wR2 ) 0.1302
R1 ) 0.0625,
wR2 ) 0.1412
0.886 and -0.363
R1 ) 0.0447,
wR2 ) 0.1021
R1 ) 0.0885,
wR2 ) 0.1189
0.444 and -0.490
(I > 2σ(I))a
R indices
(all data)a
largest diff peak and
hole (e Å-3
)
a
2
2
2
Goodness of fit ) [∑[w(Fo - Fc2)2]/(Nobservns - Nparams)]1/2, all data. R1 ) ∑(|Fo| - |Fc|)/∑|Fo|. wR2 ) [∑[w(Fo - Fc2)2]/∑[w(Fo )2]]1/2
.
(SAr).43 In their studies, the NdN bond of azobenzene has been
cleaved in a metal-ligand 4e cooperative process. In our case,
we propose that complex 10 derives from a [2 + 2] cycload-
dition of azobenzene across the alkylidene moiety in transient
A to furnish a diazametallacyclobutane intermediate, in which
the proton originating from the alkylidene unit then undergoes
a 1,3-shift to promote cleavage of the weak N-N bond (Scheme
6). The magnetic moment of complex 10 (by the Evans method)
is consistent with an S ) 1 vanadium(III) ion (µeff ) 2.98 µB,
25 °C, C6D6). Mass spectroscopy (MS-CI) also corroborates the
identity of 10, yielding the ion [M - H]+, found at m/z 730.3608
(calculated [M - H]+ m/z 730.3618). As depicted in Figure 6,
the solid-state structure of 10 clearly exposes a three-membered
azametallacycle (η2-acylimine), V(1)-C(27)-N(2), as well as
a primary anilide ligand. This η2-acylimine feature is manifested
in the V(1)-C(27) distance of 2.056(3) Å, consistent with the
V-Calkyl as well as a dative bond to an iminoacyl nitrogen
(V(1)-N(2), 2.111(2) Å).44-48 The C(27)-N(2) bond length
of 1.281(3) Å is consistent with a CdN moiety, thus further
supporting our designation of 10 as having an η2-iminoacyl
fragment. Additionally, the torsion angle of the iminoacyl
fragment (C(28)-C(27)-N(2)--(32)) is -5.8(5)°, and C(28)
deviates 0.038 Å while C(32) is 0.133 Å from the plane defined
by the atoms V(1), C(27), and N(2), suggesting that a proton is
highly unlikely to be present on either C(27) or N(2). Overall,
the gross metrical parameters about the V-C-N ring system
in 10 are comparable to those of other known vanadium
(44) Ruiz, J.; Vivanco, M.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.
Organometallics 1993, 12, 1811.
(45) Vivanco, M.; Ruiz, J.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.
Organometallics 1993, 12, 1802.
(46) Castellano, B.; Solari, E.; Floriani, C.; Re, N.; Chiesi-Villa, A.;
Rizzoli, C. Organometallics 1998, 17, 2328.
(47) Vivanco, M.; Ruiz, J.; Floriani, C.; Chiesi-Villa, A.; Guastini, C.
Organometallics 1990, 9, 2185.
(48) Vivanco, M.; Ruiz, J.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.
Organometallics 1993, 12, 1794.
(49) An analysis of over 180 entries containing more than 425 V-N
amide linkages found in the CSD, via Conquest V1.10 on 10/10/08, suggests
an average V-N amide bond distance of approximately 2.0 Å.
(43) Komuro, T.; Matsuo, T.; Kawaguchi, H.; Tatsumi, K. Inorg. Chem.
2005, 44, 175.

850 Organometallics, Vol. 28, No. 3, 2009
Kilgore et al.
iminoacyl complexes reported in the literature.44-48 Finally, the
V(1)-N(3) bond length of 1.951(2) Å49 and the angle
C(38)-N(3)-V(1) of 139.56(19)° are indicative of a vanadium
primary anilide ligand. The formation of complex 10 contrasts
with all our other previous chemistry in that NdN rupture does
not occur by a reductive cleavage process, since the oxidation
state on vanadium is unchanged.
transferred to 4 Å molecular sieves. Celite, alumina, and 4 Å
molecular sieves were activated under vacuum overnight at 200
53
°C. Complexes 3-5,26 Li(PNP),51 LiCH2 Bu,52 and VCl3(THF)3
t
were prepared according to the literature. Complex 2 was prepared
with slight modifications to the procedure initially reported.26 CHN
analyses were performed by Midwest Microlabs, Indianapolis, IN.
1H, 51V, 31P, and 13C NMR spectra were recorded on Varian 500,
1
400, and 300 MHz NMR spectrometers. H and 13C NMR are
reported with reference to residual solvent resonances: for 1H NMR
residual C6H6 in C6D6 at 7.16 ppm and for 13C NMR C6D6 at 128
ppm. 31P NMR chemical shifts are reported with respect to external
H3PO4 (0.0 ppm). 51V NMR chemical shifts are reported with
respect to VOCl3 (0.0 ppm). Values of ν1/2 in 51V spectra are
reported with the line-broadening parameter set to 20. Solution
magnetic moments measurements were obtained by the method of
Evans.54,55 X-ray diffraction data were collected on a SMART6000
(Bruker) system or a Bruker APEX Kappa DUO using Mo radiation
under a stream of N2 (g) at low temperatures, except for compound
9 data, which were collected on a Bruker APEX II detector on a
D8 platform at ChemMatCARS, APS, Chicago. CI mass spectra
were collected with a MAT-95 XP mass spectrometer. Peak
matching was done with the CMASS and LIST subroutines of
XCalibur.
Preparation of (PNP)VCl2 (1). To a stirred suspension of
VCl3(THF)3 (86 mg, 0.23 mmol) in THF was added a solution of
Li(PNP) (100 mg, 0.230 mmol) in toluene. The red suspension
quickly gave way to a purple suspension, which was stirred
overnight to ensure complete reaction. Generally, when (PNP)VCl2
was used as a reactant, it was prepared in situ, assuming 100%
conversion. X-ray-diffraction-quality crystals were grown by slow
diffusion of hexanes into a saturated CH2Cl2 solution (∼1 mL) of
(PNP)VCl2 in an NMR tube.
Conclusions
The bis(neopentyl)vanadium(III) complex 2 has been shown
to be a versatile precursor to high-valent vanadium alkylidenes
as well as derivatives originating from the transformation of
the alkylidene functionality. Specifically, two-electron -redox
chemistry of the transient alkylidene complex A allows for the
isolation of novel high-valent vanadium alkylidene complexes
bearing terminal chalcogenides ranging from O2- to Te2-. While
the sulfide and oxo complexes are thermally stable, the selenide
and telluride derivatives undergo decomposition in solution to,
as yet, unidentified products. To assist in the characterization
of the heavier chalcogenide-alkylidenes, we have conducted a
combination of experimental and DFT-based computational
studies, which provide excellent agreement between our ex-
perimental and calculated 51V NMR spectroscopic data. Com-
puted structures corroborate well with the latter data and lend
strong support to our proposed connectivity of monomeric
(PNP)VdCHtBu)(X) (X ) O, S, Se, Te) complexes.
While the reactivity of the transient alkylidene fragment A
is seemingly dominated by such 2e redox chemistry with
chalcogen sources or oxidants such as N3SiMe3, the alkylidene
motif itself has been demonstrated to be a participant in [2 +
2] cycloaddition chemistry, as evidenced by the products of
1H NMR (25 °C, 300.06 MHz, CD2Cl2): paramagnetic δ 23.22
(∆ν1/2 ) 214 Hz), 11.08 (∆ν1/2 ) 211 Hz), 7.17 (∆ν1/2 ) 71 Hz),
3.42 (∆ν1/2 ) 39 Hz), 2.34 (∆ν1/2 ) 64 Hz), 1.27 (∆ν1/2 ) 25 Hz),
1.16 (∆ν1/2 ) 25 Hz), 0.88 (∆ν1/2 ) 75 Hz), -1.46 (∆ν1/2 ) 56
Hz), -2.57 (∆ν1/2 ) 174 Hz). Evans magnetic moment (25 °C,
399.97 MHz, CD2Cl2): µeff ≈ 2.4; we propose the low µeff is likely
due to very low solubility of 1 in most common organic solvents.
MS-CI: theoretical [M]+, m/z 549.1447; experimental [M]+, m/z
549.1451.
t
reactions with the substrates BuCtN and azobenzene. The
product resulting from treatment of 2 with tBuCtN derives from
cycloaddition with a transient alkylidene fragment as well as
reduction of dinitrogen, while azobenzene leads to a η2-
iminoacyl product thought to be derived from cycloaddition and
a 1,3-hydrogen shift resulting in N-N bond cleavage.
The ability of A to reduce N2 or other substrates, as well as
perform [2 + 2] cycloaddition chemistry, provides a unique
opportunity to study two important transformations taking place
at one metal center. Likewise, such a system might provide clues
on how to activate atmospheric nitrogen and subsequently
perform cycloaddition chemistry with this substrate. Other
current efforts in our laboratories include further exploration
of the redox chemistry and metathesis reactivity of the alkylidene
unit resulting from 2.
t
One-Pot Preparation of (PNP)V(CH2 Bu)2 (2). To a stirred
room-temperature THF/toluene solution of VCl3(THF)3 (1.73 g, 4.63
mmol) was slowly added Li(PNP) (2.02 g, 4.64 mmol) suspended
in toluene. The mixture was stirred overnight, resulting in a red-
purple suspension. The suspension was then cooled to -36 °C,
t
and to it was slowly added a solution of LiCH2 Bu (743 mg, 9.52
mmol) in hexanes at -36 °C. Rapidly the purple suspension gave
way to a green solution, which was stirred for 4 h at room
temperature. Solvent was removed in vacuo, and the remaining
green solid was extracted with hexane (∼75 mL). The resulting
green solution was filtered and concentrated. Small green crystals
were grown by cooling the hexane solution to -36 °C overnight
(76.1% yield in three crystal crops, 2.19 g, 3.52 mmol). The identity
Experimental Section
General Considerations. Unless otherwise stated, all operations
were performed in a M. Braun Laboratory Master double-drybox
under an atmosphere of purified nitrogen or using high-vacuum
standard Schlenk techniques under an argon atmosphere. Anhydrous
n-hexane, pentane, toluene, and benzene were purchased from
Aldrich in Sure-Seal reservoirs (18 L) and dried by passage through
two columns of activated alumina and a Q-5 column.50 Diethyl
ether and CH2Cl2 were dried by passage through two columns of
activated alumina.50 THF was distilled, under nitrogen, from purple
sodium benzophenone ketyl and stored under sodium metal (thin
cuts). Distilled THF was transferred under vacuum into glass bombs
before being brought into a drybox. C6D6 was purchased from
Cambridge Isotope Laboratory (CIL), degassed, and then vacuum-
1
of the compound was confirmed by comparison of its H NMR
spectrum with that of an authentic sample of 2 as well as magnetic
moment measurements (Evans method).
Synthesis of (PNP)VdCHtBu(O) (4) via Treatment of 2
with TEMPO. To a stirred room-temperature toluene solution of
2 (150 mg, 0.241 mmol) was added 1 equiv of TEMPO in toluene.
Stirring for 2 days led to formation of a brown solution which was
(51) Fan, L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2004, 23,
326.
(52) Schrock, R. R.; Fellmann, J. D. J. Am. Chem. Soc. 1978, 100, 3359.
(53) Gansauer, A.; Rinker, B. Tetrahedron 2002, 58, 7017.
(54) Evans, D. F. J. Chem. Soc. 1959, 2003.
(50) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(55) Sur, S. K. J. Magn. Reson. 1989, 82, 169.

A Transient V(III) Neopentylidene Complex
Organometallics, Vol. 28, No. 3, 2009 851
stripped of volatiles in vacuo. Complex 4 was extracted from the
crude mixture using hexamethyldisiloxane (TMS2O), and the extract
was filtered and dried under vacuum. The identity of the yellow
and stored at -36 °C. Storage overnight allows for isolation of
crude brown powders of 7 in ∼27% yield (85 mg, 0.126 mmol).
1H NMR (25 °C, 400.1 MHz, C6D6): δ 7.24 (m, 1H, Ar H),
7.07 (m, 1H, Ar H), 6.90 (m, 2H, Ar H), 6.80 (s, 1H, CHtBu), 6.79
(m, 1H, Ar H), 6.72 (m, 1H, Ar H), 2.35 (overlapped septets, 2H,
CH(CH3)2), 2.25 (m (obscured by Ar CH3 resonance), 2H,
CH(CH3)2), 2.17 (s, 3H, Ar CH3), 2.05 (s, 3H, Ar CH3), 1.65 (s,
9H, CHtBu), 1.62 (m, 3H, CH(CH3)2), 1.35 (doublet of doublets,
6H, CH(CH3)2), 1.25 (doublet of doublets, 3H, CH(CH3)2), 1.17
(doublet of doublets, 3H, CH(CH3)2), 1.10 (doublet of doublets,
3H, CH(CH3)2), 0.98 (m, 6H, CH(CH3)2). 13C{1H} NMR (25.0 °C,
125.7 MHz, C6D6): 332.3 (VdCHtBu), 160.4 (Ar), 160.2 (Ar),
132.7 (Ar), 132.3 (Ar), 132.2 (Ar), 127.0 (Ar), 125.5 (Ar), 120.9
(Ar), 119.4 (Ar), 117.9 (Ar), 116.6 (Ar), 116.0 (Ar), 45.3 (CtBu),
36.2 (CtBu), 28.1 (CMe2), 26.6 (CMe2), 21.8 (CMe2), 21.1 (ArMe),
20.9 (ArMe), 19.8 (CMe2), 19.2 (CMe2), 18.7 (CMe2), 18.2 (CMe2),
17.6 (CMe2), 17.4 (CMe2), 17.2 (CMe2), 16.6 (CMe2), 16.3 (CMe2);
one aryl resonance not located, likely obscured by solvent peak.
31P NMR (25 °C, 121.5 MHz, C6D6): δ 95 (doublet, broad). 51V
NMR (25 °C, 131.5 MHz, C6D6): δ 1249 ppm (ν1/2 ) 864 Hz).
gHMQC (-13 °C, C7H8): a strong correlation between peaks at
332 ppm (13C dimension) and 6.68 ppm (1H dimension) is observed.
MS-CI: theoretical [M]+, m/z 679.1915; found [M]+, m/z 679.1901.
Synthesis of (PNP)VdN(SiMe3)(CHtBu) (8). To a stirred -76
°C hexane solution of 2 (100 mg, 0.161 mmol) was slowly added
a -76 °C hexane solution of trimethylsilyl azide (19 mg, 0.16
mmol). The mixture was slowly warmed to room temperature and
stirred overnight, followed by removal of the volatiles in vacuo.
The red residue was taken up into hexane (∼5 mL) and filtered.
From the filtrate, yellow powder was obtained by concentration
and storage of this solution (∼1 mL). X-ray-quality crystals suitable
for diffraction studies can be manually separated from the powder.
The yellow powder and crystals are collected in approximately 24%
yield (25 mg, 0.039 mmol) as crude material.
1
product was confirmed by comparing the H NMR and 31P NMR
spectra and MS-CI spectrum with those of an authentic sample of
4. The yield was 49.0% based on 2 (66.7 mg, 0.118 mmol).
Synthesis of 4 via Treatment of 2 with Pyridine N-Oxide. To
a room-temperature C6D6 solution of 2 (28 mg, 0.045 mmol) in an
NMR tube with a J. Young screw top was added 1 equiv of pyridine
N-oxide (4.6 mg, 0.048 mmol). This mixture was then heated to
50 °C for 8 h, over which time the color of the solution became
significantly lighter. The identity of the product was confirmed to
1
be 4 by comparison of the H NMR and 31P NMR spectra and
MS-CI spectrum with those of an authentic sample.26 The yield of
4 as determined by use of a 5 µL 1,4-dioxane NMR internal
integration standard was 43%.
Synthesis of 4 via Treatment of 2 with Trimethylamine
N-Oxide. To a room-temperature C6D6 solution of 2 (29 mg, 0.047
mmol) NMR tube with a J. Young screw top was added 1 equiv of
trimethylamine N-oxide (3.6 mg, 0.048 mmol). This mixture was
then heated to 50 °C for 8 h. This was accompanied by a change
in color to yellow. The identity of the product was confirmed by
comparing the 1H NMR and 31P NMR spectra and MS-CI spectrum
with those of an authentic sample of (PNP)VdCHtBu(dO).26 The
yield of 4 as determined by use of a 5 µL 1,4-dioxane NMR internal
integration standard was 20%.
Synthesis of (PNP)VdCHtBu(Se) (6). To a room-temperature
C6D6 solution of 2 (75 mg, 0.12 mmol) in a NMR tube with a J.
Young screw top was added 2 equiv of Se (19 mg, 0.24 mmol).
The tube was sealed, and this mixture was then heated to 60 °C
for 5 h. The yield of 6 was 45%, as determined by as an internal
NMR integration standard (1,4-dioxane, 5 µL). The NMR solution
was then dried, extracted with pentane, filtered through Celite, and
concentrated in vacuo. Storage of this solution at -36 °C allowed
for isolation of green microcrystalline clumps, which were dried
and collected in 69% yield (52.2 mg, 0.083 mmol).
1H NMR (25 °C, 400.1 MHz, C6D6): δ 9.24 (s, 1H, dCHtBu),
7.20 (d of d, 1H, Ar H), 7.10 (d of d, 1H, Ar H), 6.91 (d, 1H, Ar
H), 6.83 (d, 2H, Ar H), 6.74 (d, 1H, Ar H), 2.37 (overlapped septets,
2H, CH(CH3)2), 2.24 (m (obscured), 1H, CH(CH3)2), 2.17 (m
(obscured), 1H, CH(CH3)2), 2.16 (s, 3H, Ar CH3), 2.09 (s, 3H, Ar
CH3), 1.58 (s, 9H, CHtBu), 1.55 (doublet of doublets, 3H,
CH(CH3)2), 1.40 (doublet of doublets, 3H, CH(CH3)2),; 1.34
(doublet of doublets, 3H, CH(CH3)2), 1.28 (doublet of doublets,
3H, CH(CH3)2), 1.13 (doublet of doublets, 3H, CH(CH3)2), 1.07
(doublet of doublets, 3H, CH(CH3)2), 0.99 (m, 6H, CH(CH3)2).
13C{1H} NMR (6.0 °C, 125.7 MHz, C6D6): δ 328.4 (VdCHtBu),
161.3 (Ar), 160.9 (Ar), 132.7 (Ar), 132.4 (Ar), 131.9 (Ar), 127.3
(Ar), 125.3 (Ar), 120.4 (Ar), 119.8 (Ar), 118.5 (Ar), 115.6 (Ar),
48.5 (CtBu), 35.2 (CtBu), 29.8 (CMe2), 26.4 (CMe2), 20.8 (Ar Me),
20.7 (Ar Me), 20.4 (CMe2), 19.5 (CMe2), 19.3 (CMe2), 18.8 (CMe2),
18.4 (CMe2), 18.0 (CMe2), 17.7 (CMe2), 17.6 (CMe2), 17.1 (CMe2),
16.1 (CMe2); one aryl resonance not located, likely obscured by
solvent resonances. 31P NMR (25 °C, 121.5 MHz, C6D6): δ 73.0
(broad). 51V NMR (25 °C, 131.5 MHz, C6D6): δ 809 (∆ν1/2 ) 993
Hz). MS-CI: theoretical [M]+, m/z 629.2018; found [M]+, m/z
629.2007.
1H NMR (25 °C, 400.1 MHz, C6D6): δ 12.51 (s, 1H, VdCHtBu),
7.26 (m, 2H, Ar H), 6.84 (m, 2H, Ar H), 6.78 (m, 2H, Ar H), 2.30
(overlapped septets, 2H, CH(CH3)2), 2.22 (septet, 1H, CH(CH3)2),
2.18 (s, 3H, Ar CH3), 2.13(s, 3H, Ar CH3), 2.08 (septet, 1H,
CH(CH3)2), 1.41 (doublet of doublets, 3H, CH(CH3)2), 1.27 (m,
12 H, CH(CH3)2), 1.05 (m, 6H, CH(CH3)2), 0.88 (m, 3H,
CH(CH3)2), 0.32 (s, 9H, SiMe3). 13C{1H} NMR (25 °C, 125.7 MHz,
C6D6): δ 322.5 (VdCHtBu), 162.1 (Ar), 161.5 (Ar), 132.4 (Ar),
132.1 (Ar), 131.7 (Ar), 125.5 (Ar), 124.6 (Ar), 120.4 (Ar), 118.1
(Ar), 117.7 (Ar), 117.5 (Ar), 116.4 (Ar), 45.7 (CtBu), 34.4 (CtBu),
26.6 (CMe2), 26.5 (CMe2), 20.8 (Ar Me), 20.7 (Ar Me), 20.3
(CMe2), 19.5 (CMe2), 19.4 (CMe2), 19.1 (CMe2), 18.9 (CMe2), 18.7
(CMe2), 18.6 (CMe2), 18.1 (CMe2), 16.3 (CMe2), 16.1 (CMe2), 4.3
(SiMe3). 31P NMR (25 °C, 121.5 MHz, C6D6): δ 52.4. 51V NMR
(25 °C, 131.5 MHz, C6D6): δ -123 (∆ν1/2 ) 1014 Hz). MS-CI:
theoretical [M]+, m/z 636.3357; found [M]+, m/z 636.3340.
Synthesis of [(PNP)VdNC(tBu))CHtBu]2(µ2,η1:η1-N2) (9). To
a stirred hexane solution of 2 (123 mg, 0.198 mmol) was added
NtCtBu at room temperature. The solution was stirred for 24 h,
and the volatiles were removed in vacuo. The red residue was taken
up into diethyl ether (∼5 mL) and the solution then filtered. X-ray-
diffraction-quality crystals were grown by concentration and storage
of a diethyl ether solution at room temperature. Storage of a
concentrated hexane solution at -36 °C allowed for isolation of
the complex as brown-red crystals in 30.7% yield (39.3 mg, 0.030
mmol). The crystalline material appeared to consist of only one
isomer. Over time in solution, complex 9 decomposed into an as
yet unidentified species.
Synthesis of (PNP)VdCHtBu(Te) (7). In a glovebox, to a room-
temperature THF solution of 2 (285 mg, 0.458 mmol) in a side-
arm reaction tube was added 7 equiv of Te powder (409 mg 3.21
mmol). The reaction tube was sealed, placed under vacuum, and
then removed from the glovebox. This mixture was heated to 50
°C with stirring in an oil bath for 2 days. The reaction mixture was
then dried in vacuo and then reintroduced into the glovebox under
vacuum. To the residue was added n-hexanes. The resulting brown
solution was filtered through a pad of Celite, concentrated in vacuo,
1H NMR (25 °C, 400.1 MHz, C6D6): δ 7.23 (m, 1H, Ar H),
7.19 (m, 1H, Ar H), 7.03 (m, 2H, Ar H), 6.83 (m, 2H, Ar H), 4.37
(s, 1H, N(tBu)CdCH(tBu)), 2.71 (septet, 1H, CH(CH3)2), 2.61
(septet, 2H, CH(CH3)2), 2.33 (septet, 2H, CH(CH3)2), 2.24 (s, 3H,
Ar CH3), 2.23 (s, 3H, Ar CH3), 1.54 (m, 3H, CH(CH3)2), 1.50 (s,

852 Organometallics, Vol. 28, No. 3, 2009
Kilgore et al.
9H, N (tBu)CdCH(tBu)), 1.46 (doublet of doublets, 3H, CH(CH3)2),
1.41 (doublet of doublets, 3H, CH(CH3)2), 1.33 (m, 3H, CH(CH3)2),
1.25 (m, 6H, CH(CH3)2), 1.12 (m, 3H, CH(CH3)2), 1.07 (s, 9H,
N(tBu)CdCH(tBu)), 0.98 (doublet of doublets, 3H, CH(CH3)2).
13C{1H} NMR (25 °C, 100.6 MHz, C6D6): δ 167.5 (N-C), 160.9
(Ar), 160.3 (Ar), 131.8 (Ar), 130.9 (Ar), 127.1 (Ar), 126.4 (Ar),
125.9 (Ar), 124.4 (Ar), 122.8 (Ar),119.8 (NC(tBu)dCH(tBu)), 118.9
(Ar), 115.4 (Ar), 40.1 (CtBu), 34.1 (CtBu), 32.4 (CtBu), 31.4 (CtBu),
28.6 (CMe2), 25.6 (CMe2), 24.4 (CMe2), 23.2 (CMe2), 21.0 (Ar
Me), 20.9 (Ar Me), 20.5 (CMe2), 20.1 (CMe2), 19.9 (CMe2), 19.8
(CMe2), 19.7 (CMe2), 18.5 (CMe2), 17.0 (CMe2); one methyl
resonance and aryl resonance were not located, aryl resonance likely
obscured by solvent. 31P NMR (25 °C, 121.5 MHz, C6D6): δ 51.8.
calculation was computed by additional single-point calculations
on each optimized geometry using Dunning’s correlation-consistent
triple- basis set cc-pVTZ(-f) that includes a double set of
polarization functions. For all transition metals, we used a modified
version of LACVP, designated as LACV3P, in which the exponents
were decontracted to match the effective core potential with the
triple- quality basis.
The models used in this study consist of up to ∼100 atoms,30
which represent the nontruncated molecules that were also used in
the related experimental work. These calculations challenge the
current state of computational capabilities, and the numerical
efficiency of the Jaguar program allows us to accomplish this task
in a bearable time frame.
Structural Studies. Crystal data and experimental parameters
for all structures are given in Table 3. Complete structural details
for each molecule are reported in the Supporting Information.
51V NMR (25 °C, 131.5 MHz, C6D ): δ 7 (∆ν1/2 ) 1480 Hz).
6
Anal. Calcd (C72H118N6P4V2): C, 66.85; H, 9.19; N, 6.5. Found: C,
67.19; H, 9.46; N, 6.46.
Synthesis of (PNP)V(C(tBu)dNPh)(NHPh) (10). To a stirred
hexane solution of 1 (50 mg, 0.080 mmol) was added 1 equiv of
azobenzene (14.6 mg, 0.080 mmol) at room temperature. The red
solution was stirred overnight, followed by removal of the volatiles
in vacuo. The residue was extracted with diethyl ether (∼5 mL)
and the solution filtered. From the filtrate, red crystals can be grown
by concentration (∼1 mL) and storage of this solution at room
temperature in 45% yield (26.5 mg in two crops, 0.036 mmol).
Magnetic moment (Evans method) (25 °C, 400.1 MHz, C6D6): µeff
) 2.98 ( 0.03 µB. 1H NMR (25 °C, 400.1 MHz, C6D6): δ 23.8
(ν1/2 ) 1300 Hz), 21.9 (ν1/2 ) 870 Hz), 20.4 (ν1/2 ) 1300 Hz),
17.6 (ν1/2 ) 290 Hz), 14.8 (ν1/2 ) 126 Hz), 3.7 (ν1/2 ) 113 Hz),
0.07 (ν1/2 ) 109 Hz), -0.95 (ν1/2 ) 84 Hz), -5.4 (ν1/2 ) 213 Hz),
-11.9 (ν1/2 ) 360 Hz), -17.6 (ν1/2 ) 210 Hz), -24.3 (ν1/2 ) 420
Hz). MS-CI: theoretical [M - H]+, m/z 730.3618; found [M -
H]+, m/z 730.3608.
Acknowledgment. We thank Indiana Universitys
Bloomington, the Camille and Henry Dreyfus Foundation,
the Alfred P. Sloan Foundation, and the U.S. National
Science Foundation for financial support of this research
(Grant No. CHE-0348941). U.J.K. acknowledges Baxter
Pharmaceutical Co. for financial support through a fellow-
ship. For X-ray diffraction studies requiring synchrotron
radiation, we wish to recognize ChemMatCARS Sector 15,
which is principally supported by the National Science
Foundation/Department of Energy under Grant No. CHE-
0535644. Use of the Advanced Photon Source was supported
by the U.S. Department of Energy, Office of Science, Office
of Basic Energy Sciences, under Contract No. DE-AC02-
06CH11357. CI mass spectra were recorded with the
assistance of Angela Hansen. Dr. John C. Huffman is also
thanked for collecting preliminary data on single crystals of
complex 2.
Computational Details. All calculations were carried out using
density functional theory as implemented in the Jaguar 6.0 suite56
of ab initio quantum chemistry programs. Geometry optimizations
were performed with the B3LYP57-60 and the 6-31G** basis set
with no symmetry restrictions. Transition metals were represented
using the Los Alamos LACVP basis.61-63 The NMR chemical shift
Supporting Information Available: Text, figures, tables, and
CIF files giving additional spectroscopic data and structural and
theoretical details. This material is available free of charge via the
Internet at http://pubs.acs.org.
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