Relative reactivity of the metal - Amido versus metal - Imido bond in linked Cp-amido and half-sandwich complexes of vanadium

Article abstract of DOI:10.1021/om8003306

Treatment of (η⁵-C₅H₄C ₂H₄NR)V(N-te-Bu)Me (R = Me, i-Pr) and C _pV(N-p-Tol)(N-/-Pr₂)Me (Cp = η⁵-C ₅H₅) with B(C₆F₅)³ or [Ph₃C][B(C₆F₅)₄] results in formation of the corresponding cations, [(η⁵-C₅H ₄C₂H₄NR)V(N-NBu)⁺ and [CpV(N-p-Tol)(N-i-Pr₂)]⁺. The latter could also be generated as its N,N-dimethylaniline adduct by treatment of the methyl complex with [PhNMe₂H][BAr₄] (Ar = Ph, C₆F ₅). Instead, the analogous reaction with the linked Cp-amido precursor results in protonation of the imidonitrogen atom. Sequential cyclometalation of the amide substituents gave cationic imine complexes [(η⁵- C₅H₄C₂H ₄NCR′₂)V(NH-i-Bu)]⁺ (R′ = H, Me) and methane. Reaction of cationic [(η₅-C₅H ₄C₂H₄NR)V(N-t-Bu)]⁺ with olefins affords the corresponding olefin adducts, whereas treatment with 1 or 2 equiv of 2-butyne results in insertion of the alkyne into the vanadium-nitrogen single bond, affording the monoand bis-insertion products [(η⁵-C ₅H₄C₂H₄N(i-Pr)C₂ Me ₂)V(N-t-Bu)]⁺)and [(η⁵-C₅H ₄C₂H₄N(i-Pr)C₄Me₄)V(N-t- Bu)]⁺. The same reaction with the half-sandwich compound [CpV(N-p-Tol)(N-i'-Pr₂)]⁺ results in a paramagnetic compound that, upon alcoholysis, affords sec-butylidene-p-tolylamine, suggesting an initial [2+2] cycloaddition reaction. The difference in reactivity between the V-N bond versus the V=N bond was further studied using computational methods. Results were compared to the isoelectronic titanium system CpTi(NH)(NH₂). These studies indicate that the kinetic product in each system is derived from a [2+2] cycloaddition reaction. For titanium, this was found as the thermodynamic product as well, whereas the insertion reaction was found to be thermodynamically more favorable in the case of vanadium.

Full text of DOI:10.1021/om8003306

Organometallics 2008, 27, 4071–4082
4071
Relative Reactivity of the Metal-Amido versus Metal-Imido Bond
in Linked Cp-Amido and Half-Sandwich Complexes of Vanadium
Marco W. Bouwkamp, Aurora A. Batinas, Peter T. Witte, Ton Hubregtse, Jeroen Dam,
Auke Meetsma, Jan H. Teuben, and Bart Hessen*
Molecular Inorganic Chemistry Department, Stratingh Institute for Chemistry, UniVersity of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
ReceiVed April 14, 2008
Treatment of (η5-C5H4C2H4NR)V(N-t-Bu)Me (R ) Me, i-Pr) and CpV(N-p-Tol)(N-i-Pr2)Me (Cp )
η5-C5H5) with B(C6F5)3 or [Ph3C][B(C6F5)4] results in formation of the corresponding cations, [(η5-
C5H4C2H4NR)V(N-t-Bu)]+ and [CpV(N-p-Tol)(N-i-Pr2)]+. The latter could also be generated as its N,N-
dimethylaniline adduct by treatment of the methyl complex with [PhNMe2H][BAr4] (Ar ) Ph, C6F5).
Instead, the analogous reaction with the linked Cp-amido precursor results in protonation of the imido-
nitrogen atom. Sequential cyclometalation of the amide substituents gave cationic imine complexes [(η5-
C5H4C2H4NCR′2)V(NH-t-Bu)]+ (R′ ) H, Me) and methane. Reaction of cationic [(η5-C5H4C2H4NR)V(N-
t-Bu)]+ with olefins affords the corresponding olefin adducts, whereas treatment with 1 or 2 equiv of
2-butyne results in insertion of the alkyne into the vanadium-nitrogen single bond, affording the mono-
and bis-insertion products [(η5-C5H4C2H4N(i-Pr)C2Me2)V(N-t-Bu)]+ and [(η5-C5H4C2H4N(i-Pr)C4Me4)V(N-
t-Bu)]+. The same reaction with the half-sandwich compound [CpV(N-p-Tol)(N-i-Pr2)]+ results in a
paramagnetic compound that, upon alcoholysis, affords sec-butylidene-p-tolylamine, suggesting an initial
[2+2] cycloaddition reaction. The difference in reactivity between the V-N bond versus the VdN bond
was further studied using computational methods. Results were compared to the isoelectronic titanium
system CpTi(NH)(NH2). These studies indicate that the kinetic product in each system is derived from
a [2+2] cycloaddition reaction. For titanium, this was found as the thermodynamic product as well,
whereas the insertion reaction was found to be thermodynamically more favorable in the case of vanadium.
reactions of ω-alkenylamines.5 Recently this mechanism has
also been postulated for the hydroamination reaction using the
group 4 metal catalyst {η5-C5Me4SiMe2N(t-Bu)}Zr(NMe2)Cl.5d
Our interest in the relative reactivity of the metal-amido and
metal-imido bond was kindled by our observations in the
chemistry of the linked Cp-amido vanadium imido cation [(η5-
C5H4C2H4N-i-Pr)V(N-t-Bu)]+ (2b).6 In our initial communica-
tion, which described the synthesis of the complex and its
reactivity toward olefins, we reported the first adduct of simple
olefins (ethylene and propylene) to a cationic d0 metal center.
However, no further reactivity with either the metal-amido or
the metal-imido bond was observed. Here we report the
reaction of cationic linked Cp-amido complexes and related half-
sandwich complex [CpV(N-p-Tol)(N-i-Pr2)(BrC6D5)][MeB-
(C6F5)3] (10) with alkynes, which gives evidence for both
insertion and [2+2] cycloaddition chemistry.
Introduction
Group 4 and 5 imido complexes have been studied extensively
as catalysts for the intermolecular hydroamination of alkynes
with amines.1,2 The most common mechanism proposed for this
transformation involves a [2+2] cycloaddition of the unsaturated
hydrocarbon with the metal-imido bond, followed by proto-
nation of the resulting metallacycle with a primary amine,
regenerating the metal imido catalyst.3 Likewise, a similar
mechanism in which the alkyne is reacting with the TidN bond
is proposed for the half-sandwich titanium catalyst CpTi(N-p-
Tol)(NH-p-Tol) (Cp ) η5-C5H5).4 However, for complexes of
this type, i.e., complexes that have both a metal amido and a
metal imido moiety, it is possible to invoke an alternative
mechanism in which the alkyne reacts with the Ti-N bond,
followed by protonation of the resulting aminoalkenyl ligand.
This is similar to the mechanism proposed for group 3 and
lanthanide-mediated intramolecular hydroamination/cyclization
Results and Discussion
Linked Cp-Amido Vanadium Imido Complexes. Vanadium
chloride complexes, (η5-C5H4C2H4NR)V(N-t-Bu)Cl (R ) Me,
i-Pr), were obtained from the reaction of t-BuNV(NMe2)2Cl6
with the ligand precursors C5H5C2H4NHR.7 Treatment of a
pentane solution of the chloride complexes with MeLi afforded
the corresponding methyl complexes (η5-C5H4C2H4NR)V(N-t-
* Corresponding author. E-mail: B.Hessen@rug.nl.
(1) For reviews on catalytic hydroamination reactions, see: (a) Mu¨ller,
T. E.; Beller, M. Chem. ReV. 1998, 98, 675. (b) Nobis, M.; Driessen-
Ho¨lscher, B. Angew. Chem., Int. Ed. 2001, 40, 3983. (c) Pohlki, F.; Doye,
S. Chem. Soc. ReV. 2003, 32, 104. (d) Bytschkov, I.; Doye, S. Eur. J. Org.
Chem. 2003, 935. (e) Alonso, F.; Beletskaya, I. P.; Yus, M. Chem. ReV.
2004, 104, 3079. (f) Doye, S. Synlett 2004, 1653. (g) Odom, A. L. J. Chem.
Soc., Dalton Trans. 2005, 225.
(2) For examples of group 5 hydroamination catalysts, see: (a) Anderson,
L. L.; Schmidt, J. A. R.; Arnold, J.; Bergman, R. G. Organometallics 2006,
25, 3394. (b) Anderson, L. L.; Arnold, J.; Bergman, R. G. Org. Lett. 2004,
6, 2519. (c) Lorber, C.; Choukroun, R.; Vendier, L. Organometallics 2004,
23, 1845.
(3) Pohlki, F.; Doye, S. Angew. Chem., Int. Ed. 2001, 40, 2305.
(4) Johnson, J. S.; Bergman, R. G. J. Am. Chem. Soc. 2001, 123, 2923.
(5) (a) Gagne´, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1992,
114, 275. (b) Gagne´, M. R.; Marks, T. J. J. Am. Chem. Soc. 1989, 111,
4108. (c) Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 9295. (d)
Stubbert, B. D.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 6149.
(6) Witte, P. T.; Meetsma, A.; Hessen, B.; Budzelaar, P. H. M. J. Am.
Chem. Soc. 1997, 119, 10561.
10.1021/om8003306 CCC: $40.75
2008 American Chemical Society
Publication on Web 07/24/2008

4072 Organometallics, Vol. 27, No. 16, 2008
Bouwkamp et al.
Scheme 1
Scheme 2
Table 1. Equilibrium Constants for Olefin Binding at 25 °C as
Determined by 1H NMR Spectroscopy
13C NMR shifts
Keq
Bu)Me (R ) Me, 1a; R ) i-Pr, 1b), which could be used as
convenient starting materials for cationic vanadium amido imido
complexes. Addition of solutions of compounds 1a and 1b to
B(C6F5)3 resulted in the clean formation of ionic species [(η5-
C5H4C2H4NR)V(N-t-Bu)][MeB(C6F5)3] (2a,b Scheme 1).6 De-
pending on the solvent, the compound was obtained predomi-
nantly as a contact ion-pair or solvent-separated ion-pair, which
was evident from the shifts observed in the 19F fluorine NMR
spectrum (amount of contact ion-pair observed for 2b: bro-
mobenzene-d5, <10%; CD2Cl2, 22%; chlorobenzene-d5, 33%;
C2D2Cl4, 66%; benzene-d6, >90%).8 Addition of a Lewis base,
such as THF-d8, results in clean formation of the corresponding
THF adduct. The analogous solvent-separated ion-pair with the
weakly coordinating anion [B(C6F5)4]- (2b′, Scheme 1) could
be prepared by treatment of compound 1b with [Ph3C]-
[B(C6F5)4]. The reaction of the methyl precursors with the borate
reagent [PhNMe2H][B(C6F5)4] in THF-d8 did not afford solvated
or N,N-dimethylaniline-stabilized [(η5-C5H4C2H4NR)V(N-t-Bu)]
olefin adduct
dCH2
101.3
103.3
92.7, 92.5
84.8
dCRR′
(25 °C)
4a
4b
5b
6b
99(10)
100(10)
44(4)
137.7, 135.4
177.5
23(2)
than tautomerization of the [(η5-C5H4C2H4NR)V(N-t-Bu)] cat-
ions. In a similar way, Nomura et al. reported the reaction of
vanadium methyl (2,6-Me2C6H3N)(t-Bu2CN)2VMe with aryl
alcohols, which afforded complexes derived from protonation
of an amide ligand, rather than of the methyl ligand, affording
complexes of the type (2,6-Me2C6H3N)(t-Bu2CN)(OAr)VMe (Ar
) aryl).10 Cyclometalation reactions similar to the one observed
here, involving alkyl-amide substituents, have been observed
previously in the thermolysis of the tantalum compound (η5-
C5Me5)Ta(NMe2)Me3.11
Reaction of Cationic Linked Cp-Amido Vanadium Imido
Complexes with Unsaturated Hydrocarbons. Addition of
olefins to bromobenzene-d5 solutions of cation 2b resulted in a
mixture of the starting material and the corresponding olefin
adducts [(η5-C5H4C2H4N-i-Pr)V(N-t-Bu)(CH2CRR′)][MeB-
(C6F5)3] (R ) R′ ) H, 4b; R ) Me, R′ ) H, 5b; R ) R′ )
Me, 6b; Scheme 2). In the case of propylene, two isomers were
observed in virtually equimolar amounts. In a similar way, the
ethylene adduct 4a could be generated by addition of ethylene
to a bromobenzene-d5 solution of 2a. As expected, equilibrium
constants for the binding of the olefins at 25 °C (listed in Table
1) decrease with increasing number of methyl substituents on
the olefin. However, there is virtually no effect of the substituent
on the amide nitrogen (99(10) vs 100(10) for 4a versus 4b).
Included in Table 1 are the 13C chemical shifts for the
coordinated olefins. From these data it is clear that the olefin is
highly polarized, with a strong contribution of the canonical
form in which the positive charge is located on the ꢀ-carbon
1
cations.9 Instead, H NMR spectroscopy indicated formation
of [(η5-C5H4C2H4NCR′2)V(NH-t-Bu)(THF-d8)][B(C6F5)4] (R′ )
H: 3a′; R′ ) Me: 3b′), free N,N-dimethylaniline, and methane
(Scheme 1). For 3a′, two doublets were observed for the
methylene protons of the aza-metallacyclopropane at δ 3.20 and
2.65 ppm (13C NMR: δ 64.4, JCH ) 163 Hz), whereas two
singlets corresponding to three protons each were observed at
δ 1.72 and 1.42 ppm for the methyl substituents in 3b′. In
addition, resonances at δ 3.6 and 3.7 ppm were observed for
the NH protons in 3a′ and 3b′, respectively. Complex 2b′ is
stable at room temperature in solution, suggesting that the
formation of the linked Cp-imine complexes results from initial
protonation of the tert-butylimido ligand, followed by methane
elimination involving a ligand Me-group in the case of the
formation of 3a′ and the i-Pr methine in the case of 3b′, rather
(7) (a) Sinnema, P. J.; Liekelema, K.; Staal, O. K. B.; Hessen, B.;
Teuben, J. H. J. Mol. Catal. A 1997, 128, 143. (b) Hughes, A. K.; Meetsma,
A.; Teuben, J. H. Organometallics 1993, 12, 1936.
(8) Horton, A. D.; De With, J.; Van der Linden, A. J.; Van de Weg, H.
Organometallics 1996, 15, 2672.
(9) An N,N-dimethylaniline adduct was prepared separately as a reference
compound by treating compound 1a with 1 equiv of N,N-dimethylaniline
(see Experimental Section for details).
(10) Yamada, J.; Fujiki, M.; Nomura, K. Organometallics 2007, 26,
2579.
(11) Mayer, J. M.; Curtis, C. J.; Bercaw, J. E. J. Am. Chem. Soc. 1983,
105, 2651.

ReactiVity of the Metal-amido Versus Metal-imido Bond
Organometallics, Vol. 27, No. 16, 2008 4073
Scheme 3
Scheme 4
(Scheme 2).12 In the case of the ethylene adducts 4a and 4b,
only one resonance is observed in the 13C NMR spectrum at δ
101.3 and 103.3 ppm, respectively, for the coordinated ethylene
molecule, suggesting fast rotation around the metal-ethylene
vector. Correspondingly, two resonances were observed for the
coordinated ethylene molecule in the 1H NMR spectrum of the
reaction mixtures at δ 4.71 and 4.29 ppm for 4a and δ 4.72
and 4.33 ppm for 4b. The degree of polarization of the olefins
increases with the number of methyl substituents on the distant
carbon, as expected from the relative stability of the corre-
+
sponding carbocations (RCMe2 > RCHMe+ > RCH2+) and
from the steric bulk on this carbon atom (Scheme 2). In addition
to the trend in 13C NMR chemical shifts, the polarization of
the olefin is manifested in a decrease in CH coupling constant
for the R-carbon of the olefins with increasing number of methyl
substituents on the olefin, suggesting increasing sp3 character
of the CH2 group (4b, JCH ) 164 Hz; 5b, JCH ) 157 and 159
Hz; 6b, JCH ) 156 Hz).
chloride, CpV(N-p-Tol)(N-i-Pr2)Cl, was prepared by addition
of NaCp to p-TolNV(N-i-Pr2)Cl2, which was obtained from the
addition of 2 equiv of di-isopropylamine to p-TolNVCl3
(Scheme 4). This is different from the route reported for the
tert-butylimido compound CpV(N-t-Bu)(NH-t-Bu)Cl, in which
t-BuNVCl3 was first converted to the half-sandwich compound
CpV(N-t-Bu)Cl2 and subsequently to CpV(N-t-Bu)(NH-t-Bu)Cl
by the addition of 2 equiv of tert-butylamine.13 However,
Treatment of [(η5-C5H4C2H4N-i-Pr)V(N-t-Bu)(BrC6D5)]-
[MeB(C6F5)3] (2b) with a small excess (1.2 equiv) of 2-butyne
did not result in the corresponding alkyne adduct. Instead, a
mixture of two new complexes was obtained. 1H NMR
spectroscopy indicated the formation of [(η5-C5H4C2H4N(i-
Pr)C2Me2)V(N-t-Bu)][MeB(C6F5)3] (7b, major) and [(η5-
C5H4C2H4N(i-Pr)C4Me4)V(N-t-Bu)][MeB(C6F5)3] (8b, minor),
resulting from an insertion reaction of the alkyne into the
metal-nitrogen single bond (Scheme 3). Compound 8b could
be obtained exclusively by addition of additional alkyne to the
14
reaction of CpV(N-p-Tol)Cl2 with di-isopropylamine results
in an intractable mixture of complexes, hence the choice of
introducing the amide ligand prior to the cyclopentadienyl
ligand. The vanadium methyl complex CpV(N-p-Tol)(N-i-
Pr2)Me (9, Scheme 4) was prepared by treatment of CpV(N-
p-Tol)(N-i-Pr2)Cl with methyllithium. The compound was
obtained as reddish-brown crystals from pentane at -10 °C,
which were suitable for a single-crystal X-ray analysis. An
ORTEP representation is depicted in Figure 1; see Table 2 for
pertinent bond lengths and angles. Included in Table 2 are the
data for the crystal structure of the linked Cp-amido compound
1b. The structure of 9 revealed a typical three-legged piano stool
type geometry with bond distances and angles similar to the
linked Cp-amide complex (η5-C5H4C2H4N-i-Pr)V(N-t-Bu)Me
(1b).
Addition of B(C6F5)3 to a solution of 9 in bromobenzene-d5
generated a reddish-brown solution of the solvated solvent-
separated ion-pair [CpV(N-p-Tol)(N-i-Pr2)(BrC6H5)][MeB-
(C6F5)3] (10) and a small amount (<10%, based on 19F NMR
spectroscopy) of the contact ion-pair. The same solvated cation
with the weakly coordinating [B(C6F5)4] anion could be prepared
by treatment of 9 with the borate reagent [Ph3C][B(C6F5)4] or
[PhNMe2H][B(C6F5)4] in C6D5Br (Scheme 5). In case of the
reaction with the anilinium reagent, the corresponding N,N-
dimethylaniline adduct [CpV(N-p-Tol)(N-i-Pr2)(PhNMe2)]-
[B(C6F5)4] (11′) was obtained cleanly. This is evident from the
1
reaction mixture. The H NMR spectra of 7b and 8b show
resonances for the i-Pr methine protons at 2.2 and 2.8 ppm,
respectively. In complexes where the i-Pr amide is directly
bound to vanadium and the i-Pr methine CH is directed toward
the metal center, the methine protons are typically found at 5-6
ppm.6 The upfield shifts observed for the mono- and bis-
insertion products, 7b and 8b, are therefore a clear indication
of an insertion reaction involving the V-N bond rather than a
[2+2] cycloaddition reaction with the VdN bond. Unfortu-
nately, the products could be isolated only as oils, despite
numerous crystallization attempts, impeding full characteriza-
tion.
Half-Sandwich Vanadium Imido Amido Complexes. As
mentioned in the Introduction, Bergman and co-workers have
reported the hydroamination reaction of alkynes with primary
amines catalyzed by CpTi(N-p-Tol)(NH-p-Tol),4 proposing a
mechanism that involves a [2+2] cycloaddition reaction of the
unsaturated hydrocarbon with the MdN bond and not an
insertion reaction with the M-N bond as we observe here.
Hence, we were interested in the reactivity of the nonlinked
cationic vanadium system [CpV(N-p-Tol)(N-i-Pr2)]+. Vanadium
(13) (a) Preuss, F.; Becker, H.; Hausler, H. J. Z. Naturforsch. 1987,
42b, 881. (b) Preuss, F.; Becker, H.; Wieland, T. Z. Naturforsch. 1990,
45b, 191.
(14) Buijink, J. K. F. Chemistry of vanadium-carbon single and double
bonds. Ph.D. Thesis, University of Groningen, The Netherlands, 1995.
(12) (a) Carpentier, J.-F.; Wu, Z.; Lee, C. W.; Stromberg, S.; Christopher,
J. N.; Jordan, R. F. J. Am. Chem. Soc. 2000, 122, 7750. (b) Vatamanu, M.;
Stojcevic, G.; Baird, M. C. J. Am. Chem. Soc. 2008, 130, 454.

4074 Organometallics, Vol. 27, No. 16, 2008
Bouwkamp et al.
Figure 1. ORTEP representation of 9 and 12′′ at the 30% probability level. Hydrogen atoms and the counterion of 12′′ are omitted for
clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) for 1b, 9,
Scheme 5
and 12′′
1b
9
12′′
V-Xa
2.103(3)
1.656(2)
1.854(2)
1.9835(15)
175.61(18)
86.10(11)
105.78(10)
96.87(12)
2.122(5)
1.666(2)
1.851(2)
2.006(2)
166.2(2)
96.68(16)
103.02(11)
98.54(15)
2.020(4)
1.665(4)
1.866(4)
1.988(3)
165.3(4)
98.96(18)
101.86(19)
96.60(16)
V-Nimido
V-Namido
V-Cpb
V-Nimido-Cimido
Namido-V-X
Namido-V-Nimido
Nimido-V-X
a X ) CMe for 1b and 9; X ) O1 for 12′′. b Centroid defined by the
carbon atoms of the Cp ligand.
1H NMR spectrum of the compound, which reveals, in addition
to the resonances expected for the C1-symmetric vanadium
cation, two singlets for the diastereotopic methyl groups on the
aniline nitrogen. It is remarkable that treatment of the Brønsted
acidic borate reagent [PhNMe2H][B(C6F5)4] with 9 and 1b
affords products from two different types of reactivity. This
might be explained by the increased basicity of the tert-butyl
imido ligand, relative to the p-Tol imido ligand, a result of the
electron-donating properties of the tert-butyl substituent. Nev-
ertheless, it cannot be excluded that in the case of both reactions
initial protonation occurs at the imido nitrogen, followed by
methane elimination through either 1,2-elimination or ꢀ-H
transfer to form a vanadium imido amido cation (11′) or a
vanadium amido imine cation (3a′,b′), respectively.
As for the linked Cp-amido complexes, also their half-
sandwich counterparts are obtained as oily substances. In
contrast, the THF adduct [CpV(N-p-Tol)(N-i-Pr2)(THF)][BPh4]
(12′′) could be obtained as an orange, crystalline material. This
compound was prepared from 9 and [PhNMe2H][BPh4] in THF
solution. Single crystals were obtained by cooling a saturated
THF solution. An ORTEP representation of the structure is
presented in Figure 1 (see Table 2 for pertinent bond distances
and angles). The metrical parameters of the compound are
similar to the vanadium methyl starting material, 9.
reaction (Scheme 6). Formation of such a species is also
supported by the observation that N-isopropylidene-isopropy-
lamine is released upon addition of PhSSPh to a reaction mixture
containing the paramagnetic species. Unfortunately the remain-
der of the reaction mixture is intractable. It is interesting to note
that the initially formed reaction mixture in which the imine is
still attached to the metal center is paramagnetic, whereas very
similar species, [(η5-C5H4C2H4NCR′2)V(NH-t-Bu)][B(C6F5)4]
(3a′,b′), were found to be diamagnetic. As yet, we have no good
explanation for these observations.
Despite the above-mentioned rearrangement to form a vana-
dium(III) species, [CpV(N-i-Pr2)(N-p-tol)]+ could be used as a
catalyst precursor in the hydroamination reaction of 2-butyne
with p-toluidine by pretreatment of the vanadium cation with
an excess of p-toluidine prior to the addition of the alkyne. This
way, the di-isopropylamide cation is converted into the p-
tolylamido cation [CpV(NH-p-Tol)(N-p-tol)(NH2-p-Tol)]+ (13′),
prohibiting the aforementioned ꢀ-hydrogen transfer from the
i-Pr amido group to afford cationic vanadium(III) complexes.
Nevertheless, the activity of the system is very low, especially
compared to the activities observed for the isoelectronic titanium
compound.4 Using the procedure described above, 2-butyne and
p-toluidine could be partially converted into N-isobutylidene-
p-tolylamine at 80 °C. After 5 days and 24% conversion
Hydroamination with Cationic Half-Sandwich Vanadium
Imido Amido Complexes. Treatment of cation 10 with 1 equiv
1
of 2-butyne resulted in a dark red reaction mixture. H NMR
spectroscopy revealed the formation of a paramagnetic com-
pound. Attempts to isolate the paramagnetic vanadium cation
were unsuccessful, but quenching the reaction mixture with
methanol-d4 and analysis by GC-MS revealed the formation of
partially deuterated sec-butylidene-p-tolylamine.15 This suggests
that the initially formed paramagnetic compound is the aza-
allyl complex [CpV{p-TolNC(Me)CHMe}(Me2CN-i-Pr)]+, the
result of a [2+2] cycloaddition reaction of the alkyne with the
VdN bond, followed by an intramolecular proton transfer
1
(determined by H NMR spectroscopy), the reaction mixture
had turned greenish-brown, indicating catalyst decomposition.
Indeed, no further reaction was observed upon prolonged heating
at 80 °C.
Computational Studies toward Alkyne Reactivity. The
preference for the linked Cp-amido vanadium cation for insertion
into the vanadium-amido bond differs remarkably from the
(15) sec-Butylidine-p-tolylamine-dn (n ) 1-4) was observed. As yet,
the reason for this distribution of isotopomers is not clear.

ReactiVity of the Metal-amido Versus Metal-imido Bond
Organometallics, Vol. 27, No. 16, 2008 4075
Scheme 6
Scheme 7
parallel to the VdN bond and one parallel to the V-N bond
(the first being slightly lower in energy by 0.47 kcal/mol). The
barrier for switching from one orientation of the alkyne ligand
to the other is expected to be minute. A similar result was
previouslyfoundfortheolefinadduct[(η5-C5H4C2H4NH)V(NH)(η2-
C2H4)]+.6 In the case of the corresponding titanium system, only
one minimum was found, in which the acetylene was bound
parallel to the TidN bond.18 The alkyne ligand in the calculated
adducts is bound in an asymmetric fashion, similar to what was
observed for the ethylene molecule in the calculated structure
of the cation [(η5-C5H4C2H4NH)V(NH)(η2-C2H4)].6 The Ct C
bonds in the alkyne ligands (M ) Ti: 1.213 Å; M ) V: 1.219
and 1.216 Å) are barely elongated upon coordinating to the metal
center, as expected for binding to a d0 metal center. The
calculated distance of free acetylene is 1.203 Å.
Whereas binding of acetylene is slightly uphill (∆G0 ) 1.35
kcal/mol) in the case of titanium, the binding energies in the
case of vanadium are -11.85 and -11.38 kcal/mol, depending
on the orientation of the alkyne. Thermal corrections have been
included in the calculations (see Experimental Section). The
increased binding energy when going from titanium to vanadium
can be explained by (a) the charge on the metal center19 and
(b) the small reorganization energy involved in the binding of
the alkyne in the case of vanadium. The “naked” vanadium
cation adopts a significantly pyramidalized structure (sum of
the angles around V: 342.3°), whereas the neutral titanium
analogue is virtually planar (sum of the angles around Ti:
360.0°). Accordingly, the Gibbs free energy associated with the
geometrical change necessary for accommodating the substrate
in the titanium case was calculated to be significantly higher
than vanadium (12.08 kcal/mol for M ) Ti versus 2.03 and
0.95 kcal/mol for M ) V).20
For both metal complexes the kinetic product is calculated
to be that originating from the [2+2] cycloaddition reaction.
The energies of activation are 1.66 (M ) Ti) and 2.82 (M )
V) kcal/mol for the cycloaddition reaction and 10.06 (M ) Ti)
and 6.34 (M ) V) kcal/mol for the insertion reaction. For
titanium, the cycloaddition product is found to be the thermo-
dynamic product as well, whereas the insertion product is the
energetically preferred product in the case of [CpV(NH)(NH2)]+.
proposed behavior of the isoelectronic Ti(IV) complexes
described by Bergman and co-workers.4 This prompted us to
study the insertion and [2+2] cycloaddition reaction by means
of computational methods (see Experimental Section for details).
The reaction coordinate for the insertion and the cycloaddition
reaction under investigation is summarized in Scheme 7. For
computational ease, truncated versions of the molecules were
studied in which the substituents on the imido and amido
nitrogen have been replaced by hydrogen atoms. Also the effect
of the counterion in the vanadium system has not been taken
into account. Figure 2 shows the calculated Gibbs free energy
diagram for both systems; see Table 3 for a summary of
thermodynamic and kinetic parameters. Bergman and co-
workers also studied the cycloaddition reaction of acetylene with
CpTi(NH)(NH2) using computational methods, affording similar
results.16,17
In addition to half-sandwich complexes of the type
[CpM(NH)(NH2)]n+
,
the linked system [(η5-C5H5C2-
H4NH)V(NH)]+ was studied (see Figure 3 and Table 3). Like
(17) The differences in energy can be explained by the use of different
software packages (Gaussian03 vs Jaguar 4.0 Quantum Chemistry program
package) and different basis sets for the metal center (LANL2DZ vs
LACV3P**). See Experimental Section for a detailed description of the
calculations used in this contribution.
(18) A geometry optimization starting with a structure in which the
alkyne is bound parallel to the Ti-N bond results in the same geometry as
found when starting with a structure in which the alkyne is bound parallel
to the TidN bond.
For vanadium, two different alkyne adducts were found with
virtually identical energies: one with the hydrocarbon bound
(19) Margl, P.; Deng, L.; Ziegler, T. Organometallics 1998, 17, 933.
(20) The distorted geometries of the [CpM(NH)(NH2)]n+ fragments are
no local minima but originate from a single-point calculation of the
fragments constructed from the corresponding alkyne adducts. A geometry
optimization of these fragments relaxes to the naked cations
(16) Straub, B. F.; Bergman, R. G. Angew. Chem., Int. Ed. 2001, 40,
4632.
[CpM(NH)(NH2)]n+
.

4076 Organometallics, Vol. 27, No. 16, 2008
Bouwkamp et al.
Figure 2. Calculated Gibbs free energy diagram for the insertion reaction of an alkyne into the M-N bond and the [2+2] cycloaddition
reaction with the MdN bond in [CpM(NH)(NH2)]n+ (red: M ) Ti, n ) 0; blue: M ) V, n ) 1). Ball-and-stick representations of the
stationary points for the vanadium reaction are shown.
Table 3. Pertinent Thermodynamic Parameters for Both the Insertion Reaction of an Alkyne into the M-N Bond and the [2+2] Cycloaddition
Reaction with the MdN Bond in [CpM(NH)(NH2)]n+ (M ) Ti, n ) 0; M ) V, n ) 1) and [(η5-C5H5C2H4NH)V(NR)]+ (R ) H, Me) (listed
energies are in kcal/mol at 298 K)
CpTi(NH)(NH2)
[CpV(NH)(NH2)]+
[(η5-C5H5C2H4NH)V(NH)]+
[(η5-C5H5C2H4NH)V(NMe)]+
∆G0(insertion)
-11.82
-19.88
10.06
1.66
21.87
21.55
-11.02
-7.19
6.36
2.82
17.36
10.01
-7.45
-9.66
8.70
2.16
16.15
11.82
-7.54
-4.09
9.53
3.99
17.07
8.08
∆G0(cycloaddition)
∆Gq(insertion)
∆Gq(cycloaddition)
∆Gq(-insertion)
∆Gq(-cycloaddition)
in the titanium system, no local minimum was found for the
adduct with the acetylene bound parallel to the metal-amido
bond. More importantly, the insertion reaction is no longer
thermodynamically favored in the linked Cp-amido system
Scheme 8
(∆G0
) -7.45 kcal/mol; ∆G0
) -9.66 kcal/
insertion
cycloaddition
mol). This can be explained by the increased steric bulk on the
amide nitrogen by introducing the ethylene bridge. Indeed, when
this effect is compensated for by introducing a methyl substituent
on the imido ligand, the [2+2] cycloaddition reaction is again
kinetically and the insertion reaction thermodynamically favored,
similar to the parent system [CpV(NH)(NH2)]+ (Figure 3).
These results illustrate the importance of the substituents on
the relative reactivity of the M-N and MdN bond.
For the sake of completeness, we also considered a third
potential reaction: [2+3] dipolar cycloaddition (Scheme 8). For
both titanium and vanadium systems, including the linked Cp-
amido vanadium cations, the product of such a [2+3] cycload-
dition reaction is thermodynamically favored over those from
the insertion and [2+2] cycloaddition reactions, especially when
considering the triplet state of the product. However, calculations
indicate that there is no single-step [2+3] dipolar cycloaddition
path. Instead, the system first undergoes [2+2] cycloaddition
as discussed above, and the addition product then rearranges to
the formal [2+3] cycloaddition product in a separate step with
higher activation energies. Thus, such a path does not seem to
be relevant to the easy rearrangements observed in the present
work (Table 4).
adduct. The binding energy for bromobenzene to the vanadium
cation CpV(NH)(NH2) was calculated to be -12.17 kcal/mol,
slightly higher than the calculated binding energy for acetylene.
The enthalpy contribution to the Gibbs free energy of binding,
-24.96 kcal/mol, is slightly higher than the binding enthalpy
calculated for the fluorobenzene adducts of d0 scandocene
cations21 and the chlorobenzene adduct of [(1,2-
C5H3Me2)2ZrMe]+.22 The V-Br-C angle of 109.2° found in
(21) Bouwkamp, M. W.; Budzelaar, P. H. M.; Gercama, J.; Del Hierro
Morales, I.; De Wolf, J.; Meetsma, A.; Troyanov, S. I.; Teuben, J. H.;
Hessen, B. J. Am. Chem. Soc. 2005, 127, 14310.
From experiment it is clear that in bromobenzene solution
the vanadium cation exists as the corresponding bromobenzene

ReactiVity of the Metal-amido Versus Metal-imido Bond
Organometallics, Vol. 27, No. 16, 2008 4077
Figure 3. Calculated Gibbs free energy diagram for both the insertion reaction of an alkyne into the V-N bond and the [2+2] cycloaddition
reaction with the VdN bond in [(η5-C5H4C2H4NH)V(NR)]+ (red: R ) H; blue: R ) Me). Ball-and-stick representations of the stationary
points for [(η5-C5H4C2H4NH)V(NMe)]+ are shown.
Table 4. Pertinent Thermodynamic Parameters for the [2+3] Cycloaddition Reaction of an Alkyne with [CpM(NH)(NH2)]n+ (M ) Ti, n ) 0; M
) V, n ) 1) and [(η5-C5H5C2H4NH)V(NR)]+ (R ) H, Me) (listed energies are in kcal/mol at 298 K)
CpTi(NH)(NH2)
[CpV(NH)(NH2)]+
[(η5-C5H5C2H4NH)V(NH)]+
[(η5-C5H5C2H4NH)V(NMe)]+
∆G0(2+3 singlet)
-2.49
-18.98
43.26
-23.29
-56.95
19.86
-23.27
-49.91
28.82
-14.62
-41.42
29.69
∆G0(2+3 triplet)
∆Gq(from metallacycle)
this study is very similar to the corresponding angle in the
chlorobenzene adduct Cp2Zr(ClC6D5)Me][B(C6F5)4] (115.0-
(1)°).23 The binding energy of the solvent molecule might
explain the poor productivity of the cationic vanadium catalyst
in the hydroamination reaction, as the bromobenzene adduct
might be regarded as a resting state of the catalyst, decreasing
the effective concentration of the active species.
The differences in activation energies and thermodynamic
preferences for the insertion and [2+2] cycloaddition reaction
are very small. As a result, the outcome of the reaction of these
types of metal imido amido complexes is likely to be highly
susceptible to changes in the substituents on both the amido
and imido ligands. Furthermore, the outcome of these reactions
is predicted to be highly dependent on subsequent reactivity,
such as the insertion of a second substrate molecule into the
newly formed metal-carbon bond (formation of compound 8b)
or the cyclometalation of ligand substituents (formation of
paramagnetic aza-allyl species).
reactions, whereas in the isoelectronic, cationic vanadium system
the insertion into the vanadium-nitrogen single bond is
thermodynamically favored. Nevertheless, the energy surface
of the two reactions is relatively flat; hence the outcome of the
reaction of metal amido imido complexes is most likely dictated
by the steric and electronic effects of the substitutents on the
two nitrogen-based ligands and by the reactions subsequent to
the initial insertion or cycloaddition reaction.
Experimental Section
General Considerations. All reactions and manipulations of air-
and moisture-sensitive compounds were performed under a nitrogen
atmosphere using standard Schlenk, vacuum line, and glovebox
techniques. Reagents were purchased from commercial suppliers
and used as received, unless stated otherwise. Deuterated solvents
were dried over Na/K alloy prior to use (C6D6, THF-d8) or degassed
and stored over molecular sieves (4 Å) (C6D5Br, C6D5Cl, CD2Cl2,
C2D2Cl4). The compounds C5H5C2H4NHR (R ) Me, i-Pr)7 and
t-BuNV(NMe2)2Cl6 were prepared using literature procedures. 1H,
19F, and 51V NMR spectra were recorded on a Varian Unity 500
Conclusions
1
or Varian Gemini 200 spectrometer. H and 13C NMR chemical
shifts are reported in ppm relative to tetramethylsilane using residual
solvent resonances as internal reference. 19F NMR chemical shifts
are reported in ppm relative to CFCl3 and 51V NNR chemical shifts
to VOCl3, which were both used as an external reference. Elemental
analyses were performed at the analysis department at the University
of Groningen. Reported values are the average of two independent
determinations.
Whereas a [2+2] cycloaddition-type mechanism is usually
proposed for the hydroamination reaction catalyzed by group 4
and 5 metal complexes, we here present an example where
insertion of an alkyne is observed into the V-N bond of a
cationic amido imido vanadium complex. Computational studies
have shown that half-sandwich amido imido complexes of
titanium have a clear preference for [2+2] cycloaddition
(η5-C5H4C2H4NMe)V(N-t-Bu)Cl. To a solution of 0.95 g (3.9
mmol) of t-BuNV(NMe2)2Cl in 20 mL of pentane was added 0.49 g
(4.0 mmol) of C5H5C2H4NHMe. The brown solution was refluxed
for 18 h, after which the color had changed to red. All volatiles
were removed in Vacuo, and the resulting solid was extracted twice
(22) Vanka, K.; Chan, M. S. W.; Pye, C. C.; Ziegler, T. Organometallics
2000, 19, 1841.
(23) Wu, F.; Dash, A. K.; Jordan, R. F. J. Am. Chem. Soc. 2004, 126,
15360.

4078 Organometallics, Vol. 27, No. 16, 2008
Bouwkamp et al.
with pentane (10 mL). The pentane solution was concentrated and
cooled to -20 °C, affording 0.80 g (2.9 mmol, 74%) of red crystals,
identified as (η5-C5H4C2H4NMe)V(N-t-Bu)Cl. 1H NMR (benzene-
d6, RT): δ 6.11 (m, 1H, Cp), 5.88 (m, 2H, Cp), 5.11 (m, 1H Cp),
4.60 (m, 1H, NCH2), 4.01 (s, 3H, NMe), 3.20 (m, 1H, NCH2), 2.46
(m, 1H, CpCH2), 1.98 (m, 1H, CpCH2), 1.19 (s, 9H, t-Bu). 13C{1H}
(benzene-d6, RT): δ 137.5 (Cipso of Cp), 116.2, 111.7, 100.6, 99.6
(4 H, Cp CH), 81.6 (NMe), 61.6 (NCH2), 30.9 (t-Bu Me), 28.3
(CpCH2). 51V NMR (benzene-d6, RT): δ -679 (∆ν1/2 ) 350). Anal.
Calcd for C12H20N2VCl: C, 51.72; H, 7.23; N, 10.05. Found: C,
51.28; H, 7.37; N, 9.99.
31.6 (q, 126 Hz, t-Bu Me), 29.9 (t, 129 Hz, CpCH2), 22.2 (q, 125
Hz, i-Pr Me), 21.1 (q, 125 Hz, i-Pr Me), 17 (v br, VMe). Cq of
t-Bu not observed. 51V NMR (benzene-d6, RT): δ -665 (∆ν1/2
)
320). IR: 656 (w), 667 (w), 689 (w), 814 (s), 851 (w), 868 (w),
957 (w), 990 (w), 1018 (w), 1036 (w), 1044 (w), 1071 (w), 1115
(w), 1148 (w), 1173 (w), 1213 (w), 1248 (s), 1333 (w), 1358 (s)
cm-1. Anal. Calcd for C15H27N2V: C, 62.92; H, 9.50; N, 9.78; V,
17.79. Found: C, 62.66; H, 9.49; N, 9.80; V, 17.68.
Generation of [(η5-C5H4C2H4NMe)V(N-t-Bu)][MeB(C6F5)3]
(2a). A solution of 17 mg (0.067 mmol) of 1a in 0.1 mL of C6D5Br
was added to a solution of 40 mg (0.078 mmol) of B(C5F5)3 in 0.4
(η5-C5H4C2H4N-i-Pr)V(N-t-Bu)Cl. To a solution of 3.3 g (13
mmol) of t-BuNV(NMe2)2Cl in 100 mL of pentane was added 2.0 g
(13 mmol) of C5H5C2H4NH-i-Pr. The brown solution was refluxed
for 18 h, after which the color had changed to red. All volatiles
were removed in Vacuo, and the resulting solid was extracted twice
with 50 mL of pentane. The pentane solution was concentrated and
cooled to -20 °C, yielding 3.3 g (11 mmol, 83%) of red crystals,
identified as (η5-C5H4C2H4N-i-Pr)V(N-t-Bu)Cl. 1H NMR (benzene-
d6, RT): δ 6.10 (m, 1H, Cp), 5.97 (m, 2H, Cp and CH of i-Pr),
5.87 (m, 1H, Cp), 5.13 (m, 1H, Cp), 4.66 (m, 1H, NCH2), 3.25
(dd, 6 and 13 Hz, 1H, NCH2), 2.47 (m, 1H, CpCH2), 1.76 (m,
1H,CpCH2), 1.19 (s, 9H, t-Bu), 1.01 (d, 7 Hz, 3H, i-Pr Me), 0.98
(d, 7 Hz, 3H, i-Pr Me). 13C{1H} NMR (benzene-d6, RT): δ 139.6
(Cipso of Cp), 115.1, 114.6, 100.5, 99.5 (4 CH of Cp), 72.3 (i-Pr
CH), 70.5 (NCH2), 30.0 (CpCH2), 31.2 (t-Bu Me), 21.2, 20.5 (2
i-Pr Me), Cq of t-Bu not observed. 51V NMR (benzene-d6, RT): δ
-674 (∆ν1/2 ) 360). IR: 652 (w), 810 (s), 837 (w), 876 (w), 1146
(w), 1169 (w), 1209 (s), 1225 (s), 1356 (s) cm-1. Anal. Calcd for
C14H24N2VCl: C, 54.82; H, 7.89; N, 9.13; V, 16.61; Cl, 11.56.
Found: C, 54.64; H, 7.92; N, 8.96; V, 16.45; Cl, 11.46.
1
mL of C6D5Br. H NMR spectroscopy showed full conversion to
compound 2a. The compound is mainly present as a solvent-
1
separated ion-pair. H NMR (bromobenzene-d5, -30 °C): δ 6.04
(br, 1H, Cp), 5.44 (br, 1H, Cp), 5.33 (br, 1H, Cp), 5.08 (br, 1H,
Cp), 4.44 (m, 1H, NCH2), 3.87 (s, 3H, NMe), 3.61 (m, 1H, NCH2),
2.48 (m, 1H, CpCH2), 2.28 (m, 1H, CpCH2), 0.92 (s, 9H, t-Bu).
13C{1H} (bromobenzene-d5, -30 °C): δ 143.0 (Cipso of Cp), 114.7,
108.1, 104.8, 104.1 (4 H of Cp), 84.9 (NMe), 79.2 (Cq of t-Bu),
64.5 (NCH2), 30.9 (t-Bu Me), 28.3 (CpCH2). 51V NMR (bromoben-
zene-d5, -30 °C): δ -565 (∆ν1/2 ) 7000).
[(η5-C5H4C2H4N-i-Pr)V(N-t-Bu)][MeB(C6F5)3] (2b). In 2 mL
of pentane 43 mg (0.15 mmol) of 1b was dissolved and slowly
added to a stirred solution of 100 mg (0.19 mmol) of B(C6F5)3 in
10 mL of pentane. The resulting suspension was stirred for an
additional 5 min. After 10 min an orange precipitate had settled
and the solution was decanted. The orange powder was washed
three times with 5 mL of pentane and dried in Vacuo. This yielded
97 mg (0.12 mmol, 81%) of compound 2b as an analytically pure,
orange powder. 1H NMR (benzene-d6, RT): of the contact ion-
pair: δ 5.86 (br, 1H, Cp), 5.80 (sept, 6 Hz, 1H, i-Pr CH), 5.59 (br,
1H, Cp), 5.46 (br, 1H, Cp), 4.74 (m, 1H, NC2), 4.52 (br, 1H, Cp),
2.98 (dd, 7 and 13 Hz, 1H, NCH2), 2.28 (dd, 7 and 13 Hz, 1H,
CpCH2), 1.44 (m, 1H, CpCH2), 0.84 (s, 9H, t-Bu), 0.63 (d, 6 Hz,
3H, i-Pr Me), 0.47 (d, 6 Hz, 3H, i-Pr Me), -0.20 (br, ∆ν1/2 ) 24
Hz, 3H, BMe). 13C{1H} NMR (benzene-d6, RT): δ 148.9 (d, JCF
) 242 Hz, C6F5), 142.9 (Cipso of Cp), 139.3 (d, JCF ) 240 Hz,
C6F5), 137.6 (d, JCF ) 245 Hz, C6F5), 112.6, 112.3, 102.1, 100.9
(4 CH of Cp), 75.5 (i-Pr CH), 72.8 (NCH2), 29.3 (CpCH2), 30.5
(t-Bu Me), 21.3, 20.2 (2 i-Pr Me), Cq of t-Bu and BMe not observed.
51V NMR (benzene-d6, RT): δ -514 (∆ν1/2 ) 1600). 19F NMR
(benzene-d6, RT): δ -133.6, -134.9* (o-F), -162.2*, -166.2 (p-
F), -166.8*, -168.7 (m-F). Resonances marked with an asterisk
(η5-C5H4C2H4NMe)V(N-t-Bu)Me (1a). To a solution of 1.2 g
(4.2 mmol) of (η5-C5H4C2H4NMe)V(N-t-Bu)Cl in 30 mL of Et2O
was added 2.8 mL of 1.53 M MeLi in Et2O (4.3 mmol). The
solution was stirred for 0.5 h, after which all volatiles were removed
in Vacuo. The resulting oil was stripped by addition of 2 × 5 mL
of pentane and removal in Vacuo. Extraction with 2 × 10 mL of
pentane and removal of the solvent afforded 0.89 g of the title
compound as a red oil. 1H NMR spectroscopy showed small
amounts of impuries in the region of 0-4 ppm. 1H NMR
(bromobenzene-d5, RT): δ 5.94 (br, 1H, Cp), 5.63 (br, 1H, Cp),
5.53 (br, 1H, Cp), 5.34 (br, 1H, Cp), 4.15 (m, 1H, NCH2), 3.79 (s,
3H, NMe), 3.47 (m, 1H, NCH2), 2.57 (m, 1H, CpCH2), 2.40 (m,
1H, CpCH2), 1.23 (s, 9H, t-Bu), 0.63 (br, ∆ν1/2 ) 12, 3H, VMe).
13C{1H} NMR (bromobenzene-d5, RT): δ 133.9 (Cipso of Cp), 114.7,
106.1, 102.3, 98.3 (4 CH of Cp), 78.6 (NMe), 58.9 (NCH2), 32.2
(t-Bu Me), 29.1 (CpCH2), Cq of p-Tol and VMe were not located.
51V NMR (bromobenzene-d5, RT): δ -679 (∆ν1/2 ) 700).
(η5-C5H4C2H4N-i-Pr)V(N-t-Bu)Me (1b). To a solution of 1.1 g
(3.7 mmol) of (η5-C5H4C2H4N-i-Pr)V(N-t-Bu)Cl in 20 mL of
pentane was added 4.5 mL of 0.88 M MeLi in Et2O (4.0 mmol).
The solution was stirred for 1 h, after which all volatile compounds
were removed in Vacuo. The resulting brown solid was extracted
twice with 30 mL of pentane and concentrated to ∼10 mL. Cooling
to -60 °C yielded 0.50 g (1.8 mmol, 49%) of the title compound
as red-brown, analytically pure crystals. 1H NMR (benzene-d6, RT):
δ 5.83 (m, 1H, Cp), 5.50 (m, 1H, Cp), 5.41 (m, 2H, Cp), 5.29
(sept, 7 Hz, 1H, i-Pr CH), 4.13 (m, 1H, NCH2), 3.30 (m, 1H, NCH2),
2.50 (ddd, 3, 7, and 13 Hz, 1H, CpCH2), 2.07 (m, 1H, CpCH2),
1.25 (s, 9H, t-Bu), 1.15 (d, 7 Hz, 3H, i-Pr Me), 0.95 (d, 7 Hz, 3H,
i-Pr Me), 0.69 (br, ∆ν1/2 ) 8 Hz, 3H, VMe). 13C{1H} NMR
(toluene-d8, -70 °C): δ 132.9 (Cipso of Cp), 112.7, 107.5, 100.6,
94.1 (4 CH of Cp), 70.4 (Cq of t-Bu), 67.1 (i-Pr CH), 66.5 (NCH2),
29.4 (CpCH2), 31.2 (t-Bu Me), 21.8, 20.7 (2 i-Pr Me), 17.7 (br,
∆ν1/2 ) 75, VMe). 13C NMR (benzene-d6, RT): δ 132.3 (s, Cq of
Cp), 113.0, 107.9, 100.9, 97.5 (d, 170, 172, 173, and 173 Hz, 4
CH of Cp), 67.5 (d, 142 Hz, i-Pr CH), 66.8 (t, 142 Hz, NCH2),
1
are from the contact ion-pair (>90%). H NMR (bromobenzene-
d5, RT): δ 6.06 (br, 1H, Cp), 5.73 (sept, 6 Hz, 1H, i-Pr CH), 5.52
(br, 1H, Cp), 5.37 (br, 1H, Cp), 5.13 (br, 1H, Cp), 4.70 (m, 1H,
NCH2), 3.56 (dd, 7 and 13 Hz, 1H, NCH2), 2.70 (dd, 6 and 13 Hz,
1H, CpCH2), 2.09 (m, 1H, CpCH2), 1.13 (br, ∆ν1/2 ) 25 Hz, 3H,
BMe), 1.01 (s, 9H, t-Bu), 0.99 (shoulder, i-Pr Me), 0.76 (d, 6 Hz,
3H, i-Pr Me). 13C{1H} NMR (bromobenzene-d5, RT): δ 148.9 (d,
JCF ) 239 Hz, C6F5), 143.2 (Cipso of Cp), 138.0 (d, JCF ) 241 Hz,
C6F5), 136.0 (d, JCF ) 248 Hz, C6F5), 112.8, 110.8, 103.4, 103.0
(4 CH of Cp), 75.8 (i-Pr CH), 73.9 (NCH2), 29.2 (CpCH2), 30.7
(t-Bu Me), 22.3, 20.7 (2 i-Pr Me), 11.5 (br, ∆ν1/2 ≈ 100 Hz, BMe),
Cq of t-Bu not observed. 51V NMR (bromobenzene-d5, RT): δ -544
(∆ν1/2 ) 1300 Hz). 19F NMR (bromobenzene-d5, RT): δ -133.4,
-134.5* (o-F), -162.0*, -165.7 (p-F), -166.3*, -168.1 (m-F).
Resonances marked with an asterisk are from the contact ion-pair
(<10%). 19F NMR (CD2Cl2, RT): δ -135.3*, -135.8 (o-F),
-163.3*, -165.7 (p-F), -167.5*, -168.5 (m-F). Resonances
marked with an asterisk are of the contact ion-pair (20%). 19F NMR
(chlorobenzene-d5, RT): δ -134.4 (overlap of solvent-separated
and contact ion-pair) (o-F), -161.9*, -164.9 (p-F), -166.2*,
-167.4 (m-F). Resonances marked with an asterisk are of the
contact ion-pair (33%). 19F NMR (C2D2Cl4, RT): δ -134.9,
-135.3* (o-F), -162.4*, -165.8 (p-F),-66.8*, -168.4 (m-F).
Resonances marked with an asterisk are of the contact ion-pair

ReactiVity of the Metal-amido Versus Metal-imido Bond
Organometallics, Vol. 27, No. 16, 2008 4079
(66%). Anal. Calcd for C33H27BF15N2V: C, 49.65; H, 3.41; N, 3.51;
V, 6.38. Found: C, 49.78; H, 3.28; N, 3.40; V, 6.31.
Generation of [(η5-C5H4C2H4NMe)V(N-t-Bu)(C2H4)][MeB-
(C6F5)3] (4a). An NMR tube equipped with a Teflon (Young) valve
was filled with 0.875 g of a 66 mM solution of 2a in C6D5Br. The
tube was connected to a vacuum line, frozen, and evacuated.
Subsequently a calibrated volume of ethylene was condensed into
the NMR tube, so that a pressure of approximately 1 bar was
obtained after thawing the NMR tube. The tube was closed and
thawed. After 1 h (to reach equilibrium) the sample was character-
Generation of [(η5-C5H4C2H4N-i-Pr)V(THF)(N-t-Bu)][MeB-
(C6F5)3]. A solution of 20 mg (0.07 mmol) of 1b in 0.1 mL of
C6D5Br was added to a solution of 10 mg (0.08 mmol) of B(C6F5)3
in 0.4 mL of C6D5Br, and 6 µL (0.07 mmol) of THF was added
subsequently by microsyringe. NMR showed clean conversion to
1
the title compound. H NMR (C6D5Br, RT): δ 6.12 (m, 1H, Cp),
1
ized by NMR spectroscopy. H NMR (bromobenzene-d5, RT): δ
5.89 (m, 1H, Cp), 5.64 (sept, 7 Hz, 1H, i-Pr CH), 5.37 (m, 1H,
Cp), 5.02 (m, 1H, Cp), 4.93 (m, 1H, NCH2), 3.48 (m, 1H, NCH2),
3.42 (m, 2H, R-H of THF), 3.32 (m, 2H, R-H of THF), 2.75 (m,
1H, CpCH2), 2.01 (m, 1H, CpCH2), 1.52 (m, 4H, ꢀ-H of THF),
1.02 (s, 9H, t-Bu), 0.93 (d, 7 Hz, 3H, i-Pr Me), 0.73 (d, 7 Hz, 3H,
i-Pr Me). 13C{1H} NMR (C6D5Br, RT): δ 143.2 (Cipso of Cp), 112.4,
111.8 (2 CH of Cp), 102.7 (br, 2 CH of Cp), 80.8 (R-C of THF),
75.0 (i-Pr CH), 73.2 (NCH2), 31.6 (t-Bu Me), 30.1 (CpCH2), 26.5
(ꢀ-C of THF), 22.2, 22.0 (2 i-Pr Me), Cq of t-Bu not observed.
51V NMR (C6D5Br, RT): δ -567 (∆ν1/2 ) 940).
Generation of [(η5-C5H4C2H4N-i-Pr)V(N-t-Bu)][B(C6F5)4]
(2b′). A solution of 10.5 mg (0.037 mmol) of 1b in 0.1 mL of
C6D5Br was added to a suspension of 39 mg (0.042 mmol) of
[Ph3C][B(C6F5)4] in 0.4 mL of C6D5Br. 1H NMR spectroscopy
revealed the formation of a species with an identical 1H NMR
spectrum to the solvent separated ion-pair, 2b, and Ph3CMe. 19F
NMR (bromobenzene-d5, RT): δ -133.5 (o-F), -163.9 (p-F), 167.7
(m-F).
5.74 (br, 1H, Cp), 5.52 (br, 1H, Cp), 5.22 (br, 1H, Cp), 4.96 (br,
1H, Cp) 4.71 (m, 2H, dCH2), 4.54 (m, 1H, NCH2), 4.29 (m, 2H,
dCH2), 3.79 (s, 3H, NMe), 3.63 (m, 1H, NCH2), 2.48 (m, 1H,
CpCH2), 2.25 (m, 1H, CpCH2), 0.91 (s, 9H, t-Bu). 13C{1H} NMR
(bromobenzene-d5, RT): δ 140.8 (Cipso of Cp), 108.0, 103.3, 101.5,
100.1 (4 CH of Cp) 101.3 (t, 165 Hz, dCH2), 84.9 (NMe), 64.4
(NCH2), 30.8 (t-Bu Me), 28.0 (CpCH2), Cq of t-Bu not observed.
51V NMR (bromobenzene-d5, -30 °C): δ -734 (∆ν1/2 ) 3500).
Generation of [(η5-C5H4C2H4N-i-Pr)V(N-t-Bu)(C2H4)][MeB-
(C6F5)3] (4b). An NMR tube equipped with a Teflon (Young) valve
was filled with 0.874 g of a 66 mM solution of 2b in C6D5Br. The
tube was connected to a high-vacuum line, frozen, and evacuated.
Subsequently, a calibrated volume of ethene was condensed into
the NMR tube, so that a pressure of approximately 1 bar was
reached after the NMR tube was closed and thawed out. After 1 h
(to reach equilibrium) the sample was characterized by NMR
spectroscopy. 1H NMR (bromobenzene-d5, RT): free olefin: δ 5.29
(s); [(η5-C5H4C2H4N-i-Pr)V(N-t-Bu)(C2H4)][MeB(C6F5)3]: δ 5.71
(br, 1H, Cp), 5.61 (br, 1H, Cp), 5.34 (sept, 6 Hz, 1H, i-Pr CH),
5.27 (br, 1H, Cp), 5.02 (br, 1H, Cp), 4.72 (m, 2H, dCH2), 4.61
(m, 1H, NCH2), 4.33 (m, 2H, dCH2), 3.26 (dd, 15 and 7 Hz, 1H,
NCH2), 2.70 (dd, 13 and 7 Hz, 1H, CpCH2), 1.91 (m, 1H, CpCH2),
0.94 (s, 9H, t-Bu), 0.82, 0.59 (d, 6 and 7 Hz, 2 i-Pr Me). 13C NMR
(bromobenzene-d5, -30 °C), free olefin: δ 123.9 (t, 160 Hz); [(η5-
C5H4C2H4N-i-Pr)V(N-t-Bu)(C2H4)][MeB(C6F5)3]: δ 141.9 (Cipso of
Cp), 109.6, 109.2, 103.1, 101.3 (d, 173, 173, 179 and 181 Hz, 4
CH of Cp), 103.2 (d, 164 Hz, dCH2), 76.3 (d, 143 Hz, i-Pr CH),
73.3 (t, 138 Hz, NCH2), 29.5 (t, partial overlap, CpCH2), 31.1 (q,
132 Hz, t-Bu Me), 22.6, 20.7 (q, 127 Hz, 2 i-Pr Me), Cq of t-Bu
not observed. 51V NMR (bromobenzene-d5, RT): δ -707 (∆ν1/2
) 750 Hz).
Generation of [(η5-C5H4C2H4N-i-Pr)V(N-t-Bu)(PhNMe2)]-
[MeB(C6F5)3]. The title compound was generated by the addition
of an excess (5 equiv) of N,N-dimethylaniline to a bromobenzene-
1
d5 solution of 2b. H NMR (bromobenzene-d5, RT): δ 7.22 (2H,
o-CH of Ph), 7.04 (t, 7 Hz, 2H, m-CH of Ph), 5.88 (overlapping,
2H, Cp and i-Pr CH), 4.96 (overlapping, 2H, Cp and NCH2), 4.65
(m, 1H, Cp), 3.93 (m, 1H, Cp), 3.38 (m, 1H, NCH2), 2.82 (s, 3H,
NMe), 2.59 (s, 3H, NMe), 2.50 (m, 1H, CpCH2), 1.84 (m, 1H,
CpCH2), 1.08 (s, 9H, t-Bu), 0.96 (d, 7 Hz, 3H, i-Pr Me), 0.87 (d,
7 Hz, 3H, i-Pr Me), p-CH of Ph not located. 51V NMR (C6D5Br,
RT): δ -551 (∆ν1/2 ) 830).
Generation of [(η5-C5H4C2H4NCH2)V(NH-t-Bu)(THF-d8)]-
[B(C6F5)4] (3a′). Compound 3a′ was prepared in the same way as
1
3b′ (see below). H NMR spectroscopy showed clean conversion
Generation of [(η5-C5H4C2H4N-i-Pr)V(N-t-Bu)(CH2CHMe)]-
[MeB(C6F5)3] (5b). The same procedure was used as for 4b. Only
now the propene pressure after thawing the NMR tube was
approximately 2 bar. Two isomers were formed (A and B) in a
to the title compound. 1H NMR (THF-d8, RT): δ 6.47 (br, 1H,
Cp), 6.29 (br, 1H, Cp), 6.18 (br, 1H, NH), 5.88 (br, 1H, Cp), 5.75
(br, 1H, Cp), 4.08 (m, 1H, NCH2), 3.62 (m, 1H, NCH2), 3.20 (d,
9H, dCH2), 2.65 (overlapping, CpCH2 and dCH2), 1.33 (s, 9H,
t-Bu). 13C NMR (THF-d8, -90 °C): δ 139.6 (Cipso Cp), 118.1,
106.8, 105.0, 98.7 (4H of Cp), 79.0 (br, Cq of t-Bu), 65.2 (t, 142
Hz, NCH2), 64.4 (t, 163 Hz, dCH2), 32.0 (t-Bu Me), 29.0 (CpCH2).
51V NMR (THF-d8, RT): δ -563 (∆ν1/2 ) 470).
1
∼4:5 ratio. H NMR (bromobenzene-d5, -30 °C), free olefin: δ
5.71 (m, 1H, dCHMe CH), 5.00 (d, 17 Hz, 1H, dCH2 cis-CH),
4.94 (d, 10 Hz, 1H, dCH2 trans-CH), 1.58 (d, 6 Hz, 3H, Me);
[(η5-C5H4C2H4N-i-Pr)V(N-t-Bu)(CH2CHMe)][MeB(C6F5)3]: δ 6.26
(m, dCHMeB), 6.01 (m, dCHMeA), 5.98, 5.93, 5.55, 5.43, 5.40,
5.34, 5.30, 5.08 (m, Cp), 5.66, 5.42 (i-Pr CH), 4.65, 4.39 (m, NCH2),
4.33 (d, 17 Hz, dCH2 cis-CHB), 4.01 (d, 17 Hz, dCH2 cis-CHA),
4.15 (d, 9 Hz, dCH2 trans-CHA), 3.65 (d, 8 Hz, dCH2 trans-CHB),
3.31, 3.16 (m, NCH2), 2.58, 2.54 (m, CpCH2), 1.99, 1.88 (m,
CpCH2), 1.32 (d, 5 Hz, dCHMeA), 1.28 (d, 5 Hz, dCHMeB), 0.98,
0.95 (s, t-Bu), 0.80, 0.72, 0.66, 0.56 (d, 6 Hz, i-Pr Me). 13C NMR
(bromobenzene-d5, -30 °C), free olefin: 134.4 (d, 155 Hz,
dCHMe), 116.8 (t, 153 Hz, dCH2), 20.5 (q, overlap, Me); [(η5-
C5H4C2H4N-i-Pr)V(N-t-Bu)(CH2CHMe)][MeB(C6F5)3]: δ 142.2,
141.8 (s, 2 Cipso of Cp), 137.7, 135.4 (d, 157 and 159 Hz, 2
dCHMe), 111.2, 110.3, 109.6, 109.2, 102.4, 102.3, 101.3, 101.2
(d, 182, 177, 175, 173, 174, 174, 177, and 177 respectively, 8 CH
of Cp), 92.7, 92.5 (t, 160 and 159 Hz, 2 dCH2), 77.0, 76.4 (t, 2
NCH2), 73.4, 72.7 (d, 2 i-Pr CH), 31.2, 30.9 (q, 127 and 128 Hz,
2 t-Bu Me), 29.7, 29.5 (2 CpCH2), 23.4, 22.9, 22.8, 22.6 (4 i-Pr
Me), 20.2, 20.1 (2 dCHMe), Cq of t-Bu not observed. 51V NMR
(bromobenzene-d5, RT): δ -646 and -650 (partial overlap).
Generation of [(η5-C5H4C2H4NCMe2)V(NH-t-Bu)(THF-d8)]-
[B(C6F5)4] (3b′). A solution of 30 mg (0.011 mmol) of 2b in 0.5
mL of THF-d8 was added to 84 mg (0.011 mmol) of
[PhNMe2H][B(C6F5)4]. Gas evolution was observed immediately
upon addition, and the color of the solution turned from brown to
reddish-brown while the anilinium reagent dissolved. 1H NMR
spectroscopy showed clean conversion to the title compound and
free N,N-dimethylaniline. 1H NMR (THF-d8, RT): δ 6.20 (m, 2H,
Cp), 5.88 (m, 1H, Cp), 5.69 (m, 1H, Cp), 5.50 (br, 1H, NH), 4.04
(m, 2H, NCH2), 2.78 (m, 1H, CpCH2), 2.69 (m, 1H, CpCH2), 1.99
(s, 3H, dCMe2), 1.91 (s, 3H, dCMe2), 1.38 (s, 9H, t-Bu). 13C{1H}
NMR (THF-d8, -50 °C): δ 140.0 (s, Cipso of Cp), 119.0 (d, 173
Hz, Cp CH), 107.6 (d, 176 Hz, Cp CH), 102.8 (d, 175 Hz, Cp
CH), 98.7 (d, 173 Hz, Cp CH), 79.1 (br, ∆ν1/2 ) 21 Hz, Cq of
t-Bu), 78.2 (s, dCMe2), 60.0 (t, 140 Hz, NCH2), 34.7 (q, 127 Hz,
dCMe2), 32.3 (q, 127 Hz, t-Bu Me), 31.4 (t, 129 Hz, CpCH2),
25.6 (q, 125 Hz, dCMe2). 51V NMR (THF-d8, RT): δ -354 (∆ν1/2
) 1900).

4080 Organometallics, Vol. 27, No. 16, 2008
Bouwkamp et al.
Generation of [(η5-C5H4C2H4N-i-Pr)V(N-t-Bu)(CH2CMe2)]-
[MeB(C6F5)3] (6b). This compound was prepared similarly to
compound 5b. The reaction mixture was characterized using NMR
Pentane (5 mL) was added twice and pumped off to remove residual
volatiles. Extraction of the red solid with 50 mL of pentane,
followed by concentrating and cooling the extract to -25 °C,
afforded 2.09 g (6.39 mmol, 68%) of p-TolNV(N-i-Pr2)Cl2 as a
red powder. 1H NMR (benzene-d6, RT): δ 7.26 (d, 8 Hz, 2H, p-Tol
CH), 6.58 (d, 9 Hz, 2H, p-Tol CH), 5.89 (sept, 6 Hz, 1H, i-Pr CH),
3.01 (br, 1H, i-Pr CH), 1.88 (s, 3H, p-Tol Me), 1.31 (d, 6 Hz, 6H,
i-Pr Me), 0.87 (d, 6 Hz, 6H, i-Pr Me). 13C NMR (benzene-d6, RT):
δ 138.6 (s, p-Tol Cipso), 129.2 (d, 160 Hz, p-Tol CH), 126.3 (d,
163,2 p-Tol CH), 61.2 (d, 138 Hz, i-Pr CH), 55.7 (d, 130 Hz, i-Pr
CH), 28.5 (q, 128 Hz, i-Pr Me), 21.2 (q, 127, p-Tol Me), 18.8 (q,
127 Hz, i-Pr Me), Cq of p-Tol was not observed. 51V NMR
(benzene-d6, RT): δ -67 (t, JV-N ) 96). Anal. Calcd for
C13H21N2VCl: C, 47.73; H, 6.47; N, 8.56. Found: C, 47.27; H, 6.42;
N, 8.24.
1
spectroscopy. H NMR (bromobenzene-d5, -30 °C): δ 5.79 (br,
1H, Cp), 5.60 (overlapping, 2H, Cp and i-Pr CH), 5.26 (br, 1H,
Cp), 4.99 (br, 1H, Cp), 4.54 (m, 1H, NCH2), 3.76 (s, 1H, dCH2),
3.69 (s, 1H, dCH2), 3.24 (m, 1H, NCH2), 2.60 (m, 1H, CpCH2),
1.82 (m, 1H, CpCH2), 1.72 (s, 3H, dCMe2), 0.96 (s, 3H, dCMe2),
0.94 (s, 9H, t-Bu), 0.87 (d, 7 Hz, 3H, i-Pr Me), 0.64 (d, 7 Hz, 3H,
i-Pr). 13C NMR (bromobenzene-d5, -30 °C): δ 177.5 (s, dCMe2),
142.3 (Cipso of Cp), 112.0, 110.7, 101.0, 101.4 (4 CH of Cp), 84.8
(t, 156 Hz, dCH2), 76.5 (i-Pr CH), 72.7 (NCH2), 30.9 (t-Bu Me),
29.9 (CpCH2), 29.1, 29.0, 23.9, 19.2 (i-Pr Me and dCMe2), Cq of
t-Bu not observed. 51V NMR (bromobenzene-d5, RT): δ -577
(∆ν1/2 ) 860).
Determination of Keq. Equilibrium constants (Keq) were deter-
mined from 1H NMR spectra of the reaction mixture of olefin
adduct and free olefin at 25 °C. The ratio olefin adduct:solvent-
separated ion-pair was determined by careful integration of well-
separated resonances of both complexes (mostly resonances cor-
responding to the Cp moiety or the C2H4 bridge). In order to have
reliable data, spectra were measured with a 25 s delay between
transients. The concentration of free olefin was also determined by
CpV(N-p-Tol)(N-i-Pr2)Cl. Toluene (30 mL) was condensed onto
a mixture of 0.536 g (1.64 mmol) of p-TolNV(N-i-Pr2)Cl2 and 0.146
g (1.66 mmol) of CpNa. The mixture was stirred for 3 h at -40 °C
and one night at room temperature, after which all volatiles were
removed in Vacuo. The red solid was extracted with pentane (2 ×
30 mL). Concentration of the red solution to 10 mL and slow
cooling to -25 °C afforded 0.47 g (1.26 mmol, 77%) of red crystals
identified as CpV(N-p-Tol)(N-i-Pr2)Cl. 1H NMR (benzene-d6, RT):
δ 7.19 (d, 8 Hz, 2H, p-Tol CH), 6.78 (d, 8 Hz, 2H, p-Tol CH),
5.83 (s, 5H, Cp), 4.96 (sept, 7 Hz, 1H, i-Pr CH), 3.33 (sept, 6 Hz,
1H, i-Pr CH), 2.03 (s, 3H, p-Tol Me), 1.85 (d, 7 Hz, 3H, i-Pr Me),
1.25 (d, 7 Hz, 3H, i-Pr Me), 1.04 (d, 7 Hz, 3H, i-Pr Me), 0.74 (d,
6 Hz, 3H, i-Pr Me). 13C{1H} NMR (benzene-d6, RT): δ 136.0 (p-
Tol Cipso), 129.2 (p-Tol CH), 125.1 (p-Tol CH), 108.6 (Cp), 65.0
(i-Pr CH), 58.5 (i-Pr CH), 31.1 (i-Pr Me), 27.4 (i-Pr Me), 21.2
(p-Tol Me), 19.4 (i-Pr Me), 17.6 (i-Pr Me), Cq of p-Tol was not
observed. 51V NMR (benzene-d6, RT): δ -591 (∆ν1/2 ) 400). Anal.
Calcd for C18H26N2VCl: C, 60.59; H, 7.35; N, 7.85; Cl, 9.94. Found:
C, 60.45; H, 7.44; N, 7.87; Cl, 10.14.
1
integration of its resonances in the H NMR spectrum. The value
for Keq was calculated using the formula Keq ) [olefin adduct] ×
[C6D5Br] × [ion-pair]-1 × [olefin]-1
.
Generation of [(η5-C5H4C2H4N(i-Pr)C2Me2)V(N-t-Bu)][MeB-
(C6F5)3] (7b). A solution of 10 mg (0.035 mmol) of (η5-
C5H4C2H4N-i-Pr)V(N-t-Bu)Me in 0.1 mL of C6D5Br was added to
a solution of 23 mg (0.045 mmol) of B(C6F5)3 in 0.4 mL of C6D5Br.
This solution was transferred into an NMR tube equipped with a
Teflon Young valve. The tube was connected to a high-vacuum
line, frozen in liquid nitrogen, and evacuated. Subsequently, 1.2
equiv of 2-butyne was condensed into the NMR tube, which was
1
then closed, thawed out, and kept at 0 °C for 10 min. H NMR
CpV(N-p-Tol)(N-i-Pr2)Me (9). A 1.6 M ether solution of MeLi
(2.2 mL, 1.6 M in ether, 3.5 mmol) was added to a cooled (-40
°C) solution of 1.18 g (3.31 mmol) of CpV(N-p-Tol)(N-i-Pr2)Cl in
50 mL of ether. The reaction mixture was stirred for 20 min at
-10 °C, resulting in a color change from red to orange, after which
the volatiles were removed in Vacuo. Pentane was added and
removed in Vacuo to remove residual ether. Extraction with 30 mL
of cold pentane and removal of the solvent in Vacuo yielded 0.81 g
(2.41 mmol, 73%) of yellow crystals of 9. During the entire workup
procedure, the compound was kept at -10 °C. 1H NMR (THF-d8,
RT): δ 7.00 (d, 9 Hz, 2H, p-Tol CH), 6.94 (d, 9 Hz, 2H, p-Tol
CH), 5.72 (s, 5H, Cp), 4.57 (sept, 6 Hz, 1H, i-Pr CH), 3.43 (br,
1H, i-Pr CH), 2.28 (s, 3H, p-Tol Me), 1.62 (d, 6 Hz, 3H, i-Pr Me),
1.40 (d, 7 Hz, 3H, i-Pr Me), 1.08 (d, 7 Hz, 3H, i-Pr Me), 0.92 (d,
7 Hz, 3H, i-Pr Me), 0.4 (s, VMe). 13C{1H} NMR (benzene-d6, RT):
δ 135.2 (Cipso of p-Tol), 130.5 (p-Tol CH), 126.2 (p-Tol CH), 107.6
(Cp), 63.4 (i-Pr CH), 56.4 (i-Pr CH), 33.3 (i-Pr Me), 28.2 (i-Pr
Me), 22.2 (p-Tol Me), 21.8 (i-Pr Me), 20.3 (i-Pr Me), V-Me not
observed. 51V NMR (benzene-d6, RT): δ -600 (∆ν1/2 ) 320). Anal.
Calcd for C19H29N2V: C, 67.84; H, 8.69; N, 8.33; V, 15.14. Found:
C, 67.65; H, 9.03; N, 8.27; V, 15.09.
Generation of [CpV(N-p-Tol)(N-i-Pr2)][MeB(C6F5)3] (10). To
a solution of 29 mg (0.057 mmol) of B(C6F5)3 in C6D5Br was added
slowly a solution of 14 mg (0.042 mmol) of 9 in C6D5Br. The
mixture was transferred to an NMR tube. 19F NMR indicated the
formation of a small amount of the contact ion-pair (<5%), the 1H
NMR data of which will not be described here. 1H NMR
(bromobenzene-d5, RT): δ 6.92 (overlapping doublets, 4H p-Tol
CH), 5.72 (s, 5H Cp), 5.11 (sept, 7 Hz, 1H, i-Pr CH), 3.50 (sept,
6 Hz, 1H, i-Pr CH), 2.19 (s, 3H, p-Tol Me), 1.51 (d, 6 Hz, 3H, i-Pr
Me), 1.15 (br s., 3H, B-Me), 1.12 (d, 6 Hz, 3H, i-Pr Me), 0.92 (d,
6 Hz, 3H, i-Pr Me), 0.84 (d, 7 Hz, 3H, i-Pr Me). 13C{1H} NMR
(bromobenzene-d5, RT): δ 148.7 (d, JCF ) 241 Hz, o-C6F5), 140.0
spectroscopy showed complete conversion to the title compound,
a small amount of 8b (see below), and traces of unidentified
impurities. 1H NMR (bromobenzene-d5, -30 °C): δ 6.53 (br, Cp),
5.71 (br, Cp), 5.27 (br, Cp), 4.99 (br, Cp), 2.86 (m, NCH2), 2.65
(m, NCH2), 2.37 (m, CpCH2), 2.21 (m, i-Pr CH), 2.00 (m, CpCH2),
1.84 (s, CMe), 1.16 (s, CMe), 1.03 (s, t-Bu Me), 0.63 (br, i-Pr
Me), 0.23 (br, i-Pr Me). 13C{1H} NMR (bromobenzene-d5, -30
°C): δ 132.9 (Cipso of Cp), 122.3 (CMe), 120.6, 110.2, 104.0, 95.1
(4 × Cp CH), 78.9 (Cq of t-Bu), 58.7, 56.6 (i-Pr CH and NCH2),
31.5 (t-Bu Me), 26.3, 24.2, 23.0, 21.3, 5.0 (2 × i-Pr Me, 2 × CMe
and CpCH2). 51V NMR (bromobenzene-d5, -30 °C): δ -181 (∆ν1/2
) 8900).
Generation of [(η5-C5H4C2H4N(i-Pr)C4Me4)V(N-t-Bu)][MeB-
(C6F5)3] (8b). Into the NMR tube used for the generation of 8a,
described above, an additional 2 equiv of 2-butyne was added. The
1
tube, after thawing, was kept at room temperature for 30 min. H
NMR spectroscopy showed complete conversion to the title
compound (with retention of the trace impurities). 1H NMR
(bromobenzene-d5, -30 °C): δ 5.56 (br, Cp), 5.1 (br, Cp), 5.27
(br, Cp), 5.09 (br, Cp), 2.83 (overlapping, i-Pr CH and NCH2),
2.37 (m, NCH2), 2.16 (m, CpCH2), 1.96 (m, CpCH2), 2.16 (s, CMe),
1.39 (s, CMe), 1.35 (s, CMe), 1.31 (s, CMe), 1.03 (s, t-Bu Me),
0.75 (br, i-Pr Me), 0.60 (br, i-Pr Me). 13C{1H} NMR (bromoben-
zene-d5, -30 °C): δ 135.1, 133.0, 129.2, 114.0 (Cipso of Cp and 3
× CMe), 113.8, 108.5, 105.4, 97.4 (4 × Cp CH), 77.5 (br, Cq of
t-Bu), 66.6 (i-Pr CH), 59.0 (NCH2), 31.1 (t-Bu Me), 26.0 (CpCH2),
30.6, 22.1, 20.8, 20.1, 18.9, 18.4 (2 × i-Pr Me, 4 × CMe). 51V
NMR (bromobenzene-d5, -30 °C): δ -436 (∆ν1/2 ) 8100).
p-TolNV(N-i-Pr2)Cl2. To a suspension of 2.46 g (9.37 mmol)
of p-TolNVCl3 in 50 mL of ether was added 2.85 mL (28.1 mmol)
of HN-i-Pr2 in 5 min. The suspension was stirred for 18 h at room
temperature, after which all volatiles were removed in Vacuo.

ReactiVity of the Metal-amido Versus Metal-imido Bond
Organometallics, Vol. 27, No. 16, 2008 4081
(Cipso p-Tol), 137.4 (d, JCF ) 249 Hz, p-C6F5), 136.6 (d, JCF ) 246
Hz, m-C6F5), 129.6 (p-Tol CH, overlap with solvent), 125.3 (p-
Tol CH), 110.1 (Cp), 70.2 (i-Pr CH), 62.2 (i-Pr CH), 32.2 (i-Pr
Me), 26.7 (i-Pr Me), 21.2 (i-Pr Me), 20.2 (p-Tol Me), 20.2 (i-Pr
Me). B-Me resonance was not observed. 19F NMR (bromobenzene-
d5, RT): δ -132.5 (d, 21 Hz, o-F), -132.8* (o-F), -160.3* (p-F),
-164.1 (t, 21 Hz, p-F), -164.7* (m-F), -166.6 (t, 20 Hz, m-F).
Resonances marked with an asterisk are from the contact ion-pair
(<5%). 51V NMR (bromobenzene-d5, RT): δ -397 (∆ν1/2 ) 2500).
Generation of [CpV(N-p-Tol)(N-i-Pr2)][B(C6F5)4] (10′). To a
solution of 11 mg (0.034 mmol) of 9 in 0.6 mL of C6D5Br was
added 31 mg (0.034 mmol) of [Ph3C][B(C6F5)4]. 1H NMR
spectroscopy indicated formation 10′ and Ph3CMe. 1H NMR
(bomobenzene-d5, RT): δ 7.16-6.92 (overlapping, 13H, p-Tol and
Ph3CMe), 5.72 (s, 5H, Cp), 5.11 (sept, 7 Hz, 1H, i-Pr CH), 3.50
(sept, 7 Hz, 1H, i-Pr CH), 2.20 (s, 3H, p-Tol Me), 2.04 (s, 3H,
Ph3CMe Me), 1.50 (d, 6 Hz, 3H, i-Pr Me), 1.12 (d, 7 Hz, 3H, i-Pr
Me), 0.91 (d, 6 Hz, 3H, i-Pr Me), 0.84 (d, 7 Hz, 3H, i-Pr Me).
13C{1H} NMR (bromobenzene-d5, RT): δ 149.2 (Cipso of p-Tol),
148.1 (d, JCF ) 238.78 Hz, B(C6F5)4), 140.0 (Cipso of p-Tol), 138.6
(d, JCF ) 239 Hz, B(C6F5)4), 136.1 (d, JCF ) 239 Hz, B(C6F5)4),
128.7 (p-Tol CH), 127.8 (p-Tol CH), 125.9 (Ph3CMe Ph), 125.3
(Ph3CMe Ph), 110.0 (Cp), 70.3 (i-Pr CH), 62.2 (i-Pr CH), 32.3
(i-Pr Me), 30.5 (i-Pr Me), 26.8 (i-Pr Me), 25.6 (Ph3CMe Me), 21.3
(p-Tol Me), 20.2 (i-Pr Me). 19F NMR (bromobenzene-d5, RT): δ
-132.2 (o-F), -162.4 (p-F), -166.3 (m-F).
Reaction of [CpV(N-i-Pr2)(N-p-Tol)][MeB(C6F5)3] with MeC-
CMe. Compound 10 was generated by addition of bromobenzene-
d5 (0.5 mL) to a mixture of 6.5 mg (0.019 mmol) of 9 and 9.9 mg
(0.019 mmol) of B(C6F5)3. The solution was attached to a vacuum
line and frozen in liquid nitrogen. Via a calibrated gas bulb, 4.2
mmHg (9.4 mL, 0.021 mmol) of 2-butyne was added. The tube
1
was thawed and the reaction mixture was analyzed by H NMR
spectroscopy, revealing formation of a paramagnetic compound.
1H NMR (bromobenzene-d5, RT): δ 73.3 (br, ∆ν1/2 ) 3500 Hz),
57.4 (br, ∆ν1/2 ) 4000 Hz), 30.4 (br, ∆ν1/2 ) 120 Hz), -15.5 (br,
∆ν1/2 ) 340 Hz). 19F NMR (bromobenzene-d5, 25 °C): δ -130.0
(br, o-C6F5), -163.0 (br, p-C6F5), -164.6 (br, m-C6F5). The
resulting solution was treated with methanol-d4 and analyzed by
GC-MS analysis, revealing the formation of sec-butylidene-p-
tolylamine-dn (n ) 1-4).
Generation of [CpV(N-p-Tol)(NH-p-Tol)(NH2-p-Tol)][B(C6-
F5)4] (13′). In an NMR tube, p-toluidine (0.08 g, 0.77 mmol) was
added to a freshly prepared solution of 10′ (0.08 mmol) in 0.6 mL
of C6D5Br. After 1 h at ambient temperature the solvent and
volatiles were removed in Vacuo and the remaining residue was
washed with pentane (2 × 0.5 mL). Removal of residual pentane
gave a red oil, which was identified as the title compound. 1H NMR
(bromobenzene-d5, RT): δ 12.77 (br s, 1H, NH), 7.14-7.06 (m,
15H Ph3CMe), 6.87 (d, 7 Hz, free p-toluidine), 6.86 (d, 6 Hz, 4H,
p-Tol), 6.76 (d, 6 Hz, 4H p-Tol), 6.41 (d, 6 Hz, free p-toluidine)
5.68 (s, 5H, Cp), 2.17 (br s, 6H, p-Tol Me), 2.13 (s, free p-toluidine),
2.04 (s, 3H CH3, Ph3CMe). 13C{1H} NMR (bromobenzene-d5, -25
Generation of [CpV(N-p-Tol)(N-i-Pr2)(PhNMe2)][B(C6F5)4]
(11′). [PhNHMe2][B(C6F5)4] (48 mg, 0.059 mmol) was added to a
solution of 9 (20 mg, 0.059 mmol) in C6D5Br (0.5 mL) at room
temperature. Gas evolution was observed. After 20 min the reaction
was complete and the solution turned from reddish-brown to intense
red. 1H NMR (bromobenzene-d5, RT): δ 7.2-7.0 (m, 6H, PhNMe2
CH and p-Tol CH, overlap with solvent), 6.89 (d, 8 Hz, 2H, p-Tol
CH), 5.34 (5H Cp), 4.88 (sept, 6 Hz, 1H, i-Pr CH), 3.52 (sept, 6
Hz 1H, i-Pr CH), 2.81 (s, 3H, NMe), 2.56 (s, 3H, NMe), 2.25 (s,
3H p-Tol Me), 1.78 (d, 7 Hz, 3H, i-Pr Me), 0.98 (d, 6 Hz, 6H, i-Pr
Me), 0.93 (d, 6 Hz, i-Pr Me). 13C{1H} NMR (bromobenzene-d5,
RT): δ 151.5 (Cipso of p-Tol), 148.7 (d, JCF ) 241 Hz, B(C6F5)4),
139.8 (Cipso of p-Tol), 135.7 (t, JCF ) 238 Hz, B(C6F5)4), 130.0,
127.0, 125.9 (p-Tol CH and Ar of PhNMe2), 119.5 (p-Tol CH),
110.0 (Cp), 68.7 (i-Pr CH), 61.5 (i-Pr CH), 58.5 and 50.0 (PhNMe2),
32.1 (i-Pr Me), 26.5 (i-Pr Me), 21.4 (p-Tol Me), 21.3 (i-Pr Me),
20.6 (i-Pr Me). Two PhNMe2 arene signals not observed. 19F NMR
(bromobenzene-d5, RT): δ -132.3 (o-F), -162.4 (t, 21 Hz, p-F),
-166.3 (m-F).
°C): δ 148.9 (Cipso Ph3CMe), 145.5 (Cipso p-Tol), 148.6 (d, JCF
)
241.19 Hz, o-C6F5), 143.8 (Cipso p-Tol), 143.6 (Cipso p-toluidine),
138.1 (overlapping doublets 243 and 234 Hz, p-C6F5 and m-C6F5),
131.2 (p-Tol CH, overlapping with the solvent), 129.7 (p-toluidine
CH), 129.6 (p-Tol CH, overlapping with solvent), 128.8 (p-Tol CH),
128.7 (Ph3CMe CH), 127.8 (Cipso p-toluidine), 127.7 (Ph3CMe CH),
127.2 (Cipso p-Tol), 126.6 (Cipso p-Tol), 126.5 (p-Tol CH, overlap-
ping with solvent), 125.9 (Ph3CMe CH), 125.1 (br, Cipso C6F5),
116.4 (p-Tol CH), 115.2 (p-toluidine CH), 110.3 (Cp), 52.5
(Ph3CMe), 30.5 (Ph3CMe), 21.6 (br, p-Tol Me), 20.5 (p-toluidine
Me), 20.4 (p-Tol Me). 19F NMR (bromobenzene-d5, RT): δ -133.4
(br, o-C6F5), -163.5 (t, 20 Hz, p-C6F5), -166.5 (t, 20 Hz, m-C6F5).
Hydroamination Experiment. Bromobenzene-d5 (0.4 mL) was
added to a mixture of 5.1 mg (0.016 mmol) of 9 and 12.0 mg (0.015
mmol) of [PhNMe2H][B(C6F5)4]. After 2 h the starting material
had disappeared and 1H NMR spectroscopy showed clean conver-
sion to 11′. To the reaction mixture was added 32.1 mg (0.300
mmol) of p-TolNH2, and the reaction mixture was warmed for 2.5 h
at 50 °C, resulting in disappearance of the di-isopropylamido cation
and formation of free N,N-dimethylaniline, di-isopropylamine, and
13′. Addition of 97 mmHg (56.5 mL, 0.295 mmol) of 2-butyne
and warming to 80 °C resulted in partial conversion of the two
substrates (24%, as determined by 1H NMR spectroscopy) and
formation of N-isobutylidene-p-tolylamine over a period of 5 days.
[CpV(N-p-Tol)(N-i-Pr2)(THF)][BPh4] (12′′). THF (5 mL) was
added to a mixture of 9 (0.20 g, 0.60 mmol) and [PhNMe2H][BPh4]
(0.40 g, 0.91 mmol), resulting in gas evolution. After 3 h the THF
was evaporated under reduced pressure, and the crude product was
washed with pentane (2 × 3 mL). The reaction mixture was
redisolved in 5 mL of THF and warmed to 80 °C. Cooling to room
temperature yielded 0.23 g (0.32 mmol, 54%) of orange crystals
1
of the title compound. H NMR (THF-d8, RT): δ 7.34-7.30 (br
(24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
ReVision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003.
m, 8H, o-CH BPh4), 7.22 (d, 8 Hz, 2H, p-Tol CH), 7.15 (d, 8 Hz,
2H, p-Tol CH), 6.89 (t, 7 Hz, 8H m-CH BPh4), 6.71 (t, 7 Hz, 4H
p-CH BPh4), 6.18 (s, 5H, Cp), 5.38 (sept, 6 Hz, 1H, i-Pr CH), 3.90
(sept, 6 Hz, 1H, i-Pr CH), 2.38 (s, 3H, p-Tol Me), 1.83 (d, 6 Hz,
3H, i-Pr Me), 1.34 (d, 7 Hz, 3H, i-Pr Me), 1.28 (d, 6 Hz, 3H, i-Pr
Me), 1.10 (d, 7 Hz, 3H, i-Pr Me). 13C{1H} NMR (benzene-d6, RT):
δ 166.4 (q, JCB ) 50 Hz, C, PhB), 140.9 (Cipso of p-Tol), 138.3
(CH, PhB), 131.4 (p-Tol CH), 127.5 (p-Tol CH), 126.7 (CH, PhB),
122.9 (CH, PhB), 118.1 (Cipso of p-Tol), 112.2 (Cp), 70.0 (i-Pr CH),
67.4 (THF), 61.0 (i-Pr CH), 31.2 (i-Pr Me), 27.4 (i-Pr Me), 25.6
(THF), 21.9 (i-Pr Me), 20.5 (p-Tol Me), 20.3 (i-Pr Me). Anal. Calcd
for C46H54N2VBO: C, 77.45; H, 7.52; N, 3.92. Found: C, 77.58;
H, 7.48; N, 3.77.

4082 Organometallics, Vol. 27, No. 16, 2008
Bouwkamp et al.
No further conversion of the reactants was observed upon prolonged
heating at 80 °C for 3 more days.
Acknowledgment. This investigation was supported by
The Netherlands Foundation for the Chemical Sciences (CW)
with financial aid from The Netherlands Organization for
Scientific Research (NWO). M.W.B. thanks The Netherlands
Organization for Scientific Research for a VENI grant. We
thank Peter H. M. Budzelaar (University of Manitoba) for
the useful discussions regarding the computational study.
Computational Method. All calculations were performed using
the Gaussian03 package.24 Geometries for all stationary points and
transition states were optimized using the B3LYP level of theory,
with the 6-311G basis set for C, H, and N and LANL2DZ for the
metal centers. Each calculation was followed by a single point
calculation using 6-311G** (C, H, and N) and LANL2DZ (Ti and
V) to obtain reliable energies, and a vibrational analysis using
6-311G (C, H, and N) and LANL2DZ (Ti and V) to extract thermal
corrections (enthalpy and entropy); free energies mentioned in the
text are for the gas phase, 298.15 K, 1 bar. No attempts were made
to correct for solvent effects or (in the case of V) for the presence
of a counterion.
Supporting Information Available: Crystallographic data for
compounds 9 and 12′′, coordinates for optimized structures, and
complete overview of the thermodynamic parameters. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM8003306