Zwitterionic and cationic titanium and vanadium complexes having terminal M-C multiple bonds. The Role of the β-diketiminate ligand in formation of charge-separated species

Article abstract of DOI:10.1021/om900115v

Treatment of the neopentylidene complex ([ArNC(Me)]₂CH)Ti= CH¹Bu(OTf) (1) with a strong base such as an alkyl reagent (lithium or potassium salt) results in deprotonation of the βdiketiminate β-methyl group to form complex (ArN(Me)CC

Full text of DOI:10.1021/om900115v

Organometallics 2009, 28, 4115–4125 4115
DOI: 10.1021/om900115v
Zwitterionic and Cationic Titanium and Vanadium Complexes Having
Terminal M-C Multiple Bonds. The Role of the β-Diketiminate Ligand in
Formation of Charge-Separated Species
Debashis Adhikari,§,† Falguni Basuli,§,‡ Justin H. Orlando,† Xinfeng Gao,†
John C. Huffman,† Maren Pink,† and Daniel J. Mindiola*,†
†Molecular Structure Center and the Department of Chemistry, Indiana University, Bloomington, Indiana
47405. § First and second author contributed equally to this work. ‡ Present address: Nuclear Medicine,
Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts 02115
Received February 12, 2009
Treatment of the neopentylidene complex ([ArNC(Me)]2CH)TidCHtBu(OTf) (1) with a strong
base such as an alkyl reagent (lithium or potassium salt) results in deprotonation of the β-
diketiminate β-methyl group to form complex (ArN(Me)CCHC(CH2)NAr)TidCHtBu(THF) (2)
along with liberation of the alkane. Likewise, ligand deprotonation of ([ArNC(Me)]2CH)VtCtBu-
(OTf) (3), in THF or Et2O, results in formation of the alkylidyne-solvent adduct (ArNC(Me)CHC-
(CH2)NAr)VtCtBu(L) (L = THF, 4-THF; L = Et2O, 4-OEt2), concomitant with alkane formation.
The connectivity in compound 4-Et2O has been established by single-crystal X-ray diffraction
studies. Compounds 2 and 4-THF react with B(C6F5)3 to produce the first examples of terminal
titanium alkylidene and vanadium alkylidyne zwitterions, namely, the borane adducts (ArNC(Me)-
CHC(CH2B(C6F5)3)NAr)TidCHtBu(THF) (5) and (ArNC(Me)CHC(CH2B(C6F5)3)NAr)VtCtBu-
(THF) (6), respectively. The solid state structure for each zwitterion was also obtained. Compounds 2
and 4-THF react readily with [HNMe2Ph][B(C6F5)4] to produce the discrete salts [([ArNC(Me)]2CH)-
TidCHtBu(THF)][B(C6F5)4] (7) and [([ArNC(Me)]2CH)VtCtBu(THF)][B(C6F5)4] (8), respectively,
via protonation of the methylene group in the N,N0-chelating ligand ArNC(Me)CHC(CH2)NAr2- to
re-form the β-diketiminate scaffold. The reactivity of 7 allows for spectroscopic elucidation only at
low temperatures, while complex 8 has been fully characterized including a single-crystal X-ray
structure.
Introduction
metric Wittig-like reagent for the methylenation of carbonyl-
based functional groups.3,4 The polarized nature of the
masked TidC multiple bond renders this system more
reactive than prototypical phospha-Wittig reactants, and
as a result, complex Cp2Ti(CH2AlCl(Me)2) works particu-
larly well for sterically encumbered carbonyls present in
reagents such as aldehydes, esters, lactones, and amides.5
Likewise, the AlMe2Cl moiety in Tebbe’s complex reduces
nucleophilic character at the methylidene carbon, conse-
quently making this highly polarized group less basic than
the more ubiquitous phospha-Wittig compounds.5 Analo-
gous to Cp2Ti(CH2AlCl(Me)2), treatment of Cp2Ti(Me)2
with AlMe3 does produce chloride-free Cp2Ti(CH2Al-
(Me)3), but such a complex was never isolated in pure form
due to the presence of unreacted dimethyl precursor.1 Fol-
lowing Tebbe’s work, few examples of aluminum-stabilized
titanium analogues have been reported. For example,
Tebbe’s reagent, Cp2Ti(CH2AlCl(Me)2),1 is a Lewis-acid-
stabilized methylidene complex generated from the addition
of 2 equiv of AlMe3 to Cp2TiCl2 (eq 1). This complex was
perhaps one of the first d0 transition metal systems to per-
form olefin metathesis in a catalytic manner and represents a
seminal discovery toward the application of high-valent
metal-carbon multiple bonds in the realm of organic meth-
odology.2 In general, Tebbe’s complex is a classic stoichio-
*Corresponding author. E-mail: mindiola@indiana.edu.
(1) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. J. Am. Chem. Soc.
1978, 100, 3611.
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1979, 101, 5074.
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Petasis, N. A.; Hu, Y.-H. J. Org. Chem. 1997, 62, 782. (c) Petasis, N. A.;
Lu, S.-P.; Bzowej, E. I.; Fu, D.-K.; Staszewski, J. P.; Akritopoulou-
Zanze, I.; Patane, M. A.; Hu, Y.-H. Pure Appl. Chem. 1996, 68, 667. (d)
Elschenbroich, C. In Organometallics, 3rd ed.; Wiley-VCH: Weinheim,
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€
Kruger and co-workers published a rare example of a
titanium alkylidene system stabilized by an organoalumi-
num moiety (eq 2).6
(4) In the absence of Al(CH3)3, complex Cp2Ti(CH3)2, which is fairly
tolerant to air and moisture, can extrude CH4 thermolytically: (a)
Petasis, N. A.; Bzowej, E. I. J. Am. Chem. Soc. 1990, 112, 6392. (b)
Petasis, N. A.; Bzowej, E. I. J. Org. Chem. 1992, 57, 1327. (c) Petasis, N.
A.; Akritopoulou, I. Synlett 1992, 665. (d) Petasis, N. A.; Lu, S.-P.
Tetrahedron Lett. 1995, 36, 2393.
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Principles and Applications of Organotransition Metal Chemistry; Uni-
versity Science Books: Mill Valley, 1987; pp 813- 815. (b) Pine, S. H.;
Zahler, R.; Evans, D. A.; Grubbs, R. H. J. Am. Chem. Soc. 1980, 102, 3270.
(6) Kruger, C.; Mynott, R.; Siedenbiedel, C.; Stehling, L.; Wilke, G.
Angew. Chem., Int. Ed. 1991, 30, 1668.
€
r
2009 American Chemical Society
Published on Web 06/18/2009
pubs.acs.org/Organometallics

4116 Organometallics, Vol. 28, No. 14, 2009
Adhikari et al.
Recently, our group discovered an alkylidyne analogue to
Tebbe’s complex, namely, the Lewis-acid-stabilized
group 4 species (PNP)Ti[C(tBu)Al(Me)3] (PNP-= N[ 2-P-
(CHMe2)2-4-MeC6H3]2) (eq 3).7 Parallel to this contribu-
tion, Roesky and co-workers reported an unprecedented
“Ti-CH-” linkage stabilized by a Al2(Me)+5 fragment
(eq 4).8 If one excludes the titanium methyl ligand as well
as Al2(Me)+5 , compound ([ArNC(Me)]2CH)Ti(Me)dCH-
(Al2Me5) (Ar=2,6-iPrC6H3) could be best diagnosed
as a parent methylidyne source.8 Unfortunately, the
anionic nature of the TiCH group, coupled with the re-
active Ti-CH3 linkage and fragile imine group present
within the β-diketiminate ligand, precluded clean methyli-
dyne “CH” group transfer chemistry.8 Roesky and co-
workers also complemented work on the heavier con
N,N0-chelating, bis-anilide ligand (ArNC(Me)CHC(CH2)-
NAr) .
2- 14,15 Due to the nucleophilic nature of the methylene
motif in the scaffold (ArNC(Me)CHC(CH2)NAr)2-, this
ligand can be readily protonated, with reagents such
[NHMe2Ph][B(C6F5)4], to reassemble the original β-diketi-
minate framework on the metal, but as cationic species
due to the nature of the weakly coordinating anion.
Likewise, Lewis acid coordination to the methylene moiety
of (ArNC(Me)CHC(CH2)NAr)2- can yield charge-sepa-
rated species retaining the terminal M-C multiply bonded
ligand.
geners of group
4 by preparing [(Cp*M)3Al6Me8-
(μ3-CH2)2(μ4-CH)4(μ3-CH)] clusters (M = Zr and Hf).9
A similar “TiCH”-trapped product was reported by
Stephan et al., namely, the complex Cp*Ti(μ-Me)-
(μ-NPiPr3)(μ3-CH)(AlMe2)2.10 If one takes into considera-
tion other groups outside AlMe+2 , then another Lewis-acid-
stabilized TiCH group that merits mentioning is Mena’s
[CpTiO]3(μ3-CH) cluster as well as its subsequent deriva-
tives.11
Although the discovery of Tebbe’s complex and its role
as a methylidene (e.g., CH2) group transfer reagent was
reported over 30 years ago, examples of zwitterionic titanium
complexes retaining a terminal alkylidene ligand, to our
knowledge, have not been reported. In fact, cationic titan-
ium complexes bearing the terminal alkylidene have also
remained elusive, presumably due to the inherent reacti-
vity of the TidC bond. This characteristic also applies
to vanadium alkylidene12 and alkylidyne13 cations, which
have been isolated and fully characterized by us only
recently.
Results and Discussion
Herein we report in this paper a unique approach to
preparing zwitterionic and cationic titanium neopentyli-
denes as well as zwitterionic and cationic vanadium neopen-
tylidynes. The ability of the β-diketiminate’s β-methyl
hydrogens to tautomerize in [ArNC(Me)]2CH- (Ar =
2,6-iPr2C6H3) allows for facile deprotonation to form the
Deprotonation of the β-Diketiminate Ligand [ArNC-
(Me)]2CH- (Ar = 2,6-iPr2C6H3) on Titanium(IV) and Va-
nadium(V) Complexes. It has been well established that the β-
diketiminate ancillary framework has vulnerable sites, espe-
cially in the context of early transition metal or main group
organometallic chemistry.16 Of the many different modes of
transformation involving the β-diketiminate ligand, perhaps
the most common pathway is intramolecular C-H bond
activation of peripheral groups stemming from the R-nitro-
gen atoms.15b,16-22 Other reactions of this ligand type in-
clude electrophilic activation at the γ-carbon due to
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Article
Organometallics, Vol. 28, No. 14, 2009 4117
delocalization of charge about the NCCCN ring,16,23-27 as
well as deprotonation of the β-methyl group composing the
NCCCN ring.14,15 Very recently, our group has determined
that the iminefunctionality belonging tothe NCCCNring can
undergo smooth cross-metathesis, intramolecularly, with
complexes having a titanium21,28 and vanadium neopentyli-
dene12 ligand. Other similar groups such as phosphinidenes
have been also reported to undergo Wittig/Staudinger-like
exchange reaction with the imine group of the β-diketiminate
ligand.29 Although the latter type of intramolecular reaction
reveals the parallel reactivity the M-C or M-P multiple
bond, such a transformation about the β-diketiminate ligand
resultsininactivation, bymeansofa grouptransfer process, of
the functionality that is of interest to us.
Scheme 1
Even though the β-diketiminate ligand can readily under-
go deprotonation of the β-methyl group, such a transforma-
tion can be avoided by incorporating a tBu group.15b,21,30,31
Despite this, we are interested in such a deprotonation
reaction taking place in compounds such as ([ArNC-
(Me)]2CH)TidNAr(OTf)15a and ([ArNC(Me)]2CH)Tid
CHtBu(OTf) (1)15b,21 given that a new, perhaps even more
robust ligand framework, such as (ArNC(Me)CHC(CH2)-
NAr)2- was readily assembled without demetalation
(Scheme 1). Intuitively, the nature of the (ArNC(Me)CHC-
(CH2)NAr)TidX(THF) (X = CHtBu, NAr) scaffold sug-
gested that an intermolecular cross-metathesis reaction was
no longer viable given that the vulnerable imine motif was
absent. Likewise, the coordination environment at the metal
center was relatively unperturbed, concurrent with retention
of the functional group of interest: TidCHtBu or TidNAr
(Scheme 1).15 In addition, the dianionic nature of the newly
formed ligand implied that a more electron rich metal system
was accessible, but also having potentially labile ligands such
as THF or Et2O. Although Lappert et al. were the first to
report deprotonation and protonation reactions of β-diketi-
minates,32 only recently have other research groups explored
(ArNC(Me)CHC(CH2)NAr)2- as ancillary ligands.33
We were particularly intrigued with the preparation of
complex (ArNC(Me)CHC(CH2)NAr)TidCHtBu(THF) (2),
shown in Scheme 1a,15b since this system could offer access to
a low-coordinate, electron-rich titanium complex having a
terminal alkylidene moiety. Preparation of complex 2 is
relatively simple: invoking the use of a strong base,
KCH2Ph34 in THF,15b with the alkylidene precursor 1
(Scheme 1a). A similar deprotonation of the ligand back-
bone has been documented earlier by our group for the
titanium imido system ([ArNC(Me)]2CH)TidNAr(OTf)
(Scheme 1b).15a The yield of 2 (64% isolated yield) was
relatively unperturbed by using an alternative base such
as LitBu in THF at -35 °C. The use of a coordinating
solvent such as THF was critical to formation of 2 since
alkylation of 1 with the same reagents, in pentane
or toluene solution at -35 °C, yielded intractable pro-
ducts. Only in the case of LiCH2SiMe3, in pentane, was
alkylation successful enough in 1 to allow for isolation of
the titanium alkylidene-alkyl ([ArNC(Me)]2CH)TidCHtBu-
(CH2SiMe3).15b Unfortunately, treating ([ArNC(Me)]2CH)-
TidCHtBu(CH2SiMe3) with THF results in complicated
mixtures, from which we were unable to identify 2
(Scheme 2). Therefore, we propose that formation of 2 from
1 most likely involves either a tautomerization process
promoted by THF or an ionization of the base by THF,
which then deprotonates the methyl group of the ligand,
concertedly. On the basis of our independent synthetic
work, the alkyl complex ([ArNC(Me)]2CH)TidCHtBu-
(CH2SiMe3) does not appear to be an intermediate to for-
mation of 2. THF binding to precursor ([ArNC(Me)]2CH)-
TidCHtBu(OTf) is not unreasonable to propose given that
the trimethylsilymethylidene derivative ([ArNC(Me)]2CH)-
TidCHSiMe3(OTf)(THF) readily coordinates THF.35 We
cannot, however, discard the possibility of THF displacing
the OTf- in a precursor such as 1 or the ionization of the
alkyl reagent by THF to generate a more powerful base.
Therefore, it is evident that THF blocks other, slower, but
competitive processes such as double C-H activation of the
isopropyl methines as well as intramolecular cross-metathe-
sis with the β-diketiminate imine group to form the products
shown in Scheme 2.15b The latter two products are observed
when ([ArNC(Me)]2CH)TidCHtBu(CH2SiMe3) is allowed
(23) (a) Kuhn, N.; Lanfermann, H.; Schmitz, P. Liebigs Ann. Chem.
1987, 727. (b) Kuhn, N.; Kuhn, A.; Boese, R.; Augart, N. J. Chem.
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B. A.; Duboc-Toia, C.; Le Pape, L. L.; Yokota, S.; Tachi, Y.; Itoh, S.;
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1998, 120, 9384.
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M. F; Nycz, J. E. Chem. Commun. 2003, 1142. (b) Ragogna, P. J.;
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Mindiola, D. J. Chem. Commun. 2005, 2250.
(31) Budzelaar, P. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J. Inorg.
Chem. 1998, 1485.
(32) Hitchcock, P. B.; Lappert, M. F.; Protchenko, A V. Chem.
Commun. 2005, 951.
(33) (a) Jana, A.; Schulzke, C.; Roesky, H. W. J. Am. Chem. Soc.
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4118 Organometallics, Vol. 28, No. 14, 2009
Adhikari et al.
Scheme 2
to stir in hexane or pentane for 10 days at room temperature
and in the absence of donor solvents (Scheme 2).15b
(JC-H = 158 Hz).38 Given the breakdown of symmetry in the
dianionic ligand, complex 4-THF belongs to point group C1,
which explains the spectroscopic observation of four inequi-
valent isopropyl groups on the supporting N,N0-chelating
ligand as well as two distinctly different β-carbons compos-
ing the NCCC0N ring (13C NMR: 152.6 and 141.1 ppm).
Chemically shifted resonances, from that of free THF, are
The transformation of 1 to 2 is signified by a drastic upfield
chemical shift of the alkylidene C-H group, when judged by
both the 13C (from 271 to 254 ppm) and 1H (from 5.69 to 5.05
ppm) NMR spectroscopic resonances. The similarity in the
JC-H values for the alkylidene carbons (([ArNC(Me)]2CH)-
TidCHtBu(CH2SiMe3), 99 Hz; 2, 93 Hz) suggests that only a
fraction of distortion about the alkylidene moiety group has
taken place when the β-diketiminate ligand is deprotonated
(Table 1).
1
also observed in the H NMR spectrum of 4-THF. Exam-
ination of the reaction mixture after separation of 4-THF
suggests a myriad of other products formed from this reac-
tion. In fact, allowing the reaction to stir for longer than 10-
20 min results in very low yields of 4-THF being isolated.
Unfortunately, multiple attempts to grow single crystals of 4-
THF suitable for X-ray diffraction studies were unsuccessful.
For this reason we turned our attention to preparing a close
derivative of 4-THF by replacing the THF ligand with Et2O.
Accordingly, treatment of a cold pentane solution of 3
(having a few drops of Et2O) with 1 equiv of LitBu (keeping
the solution at -35 °C during the addition and stirring)
resulted also in deprotonation of the CH3 group in the β-
carbon, to form 4-OEt2 in 58% (isolated) yield after rapid
workup of the reaction mixture. Complex 4-OEt2 displays
virtually identical spectroscopic signatures to 4-THF, but the
complex can be isolated as single crystals from a hexane
solution cooled to -35 °C (see Experimental Section).
Table 1 compiles salient NMR spectroscopic data for com-
plexes 4-THF and 4-OEt2.
The structure of crystallized 4-OEt2 reveals a rare example
of a low-coordinate vanadium alkylidyne confined in a
pseudo-tetrahedral environment and displaying C1 symme-
try (Figure 1). The space group, P2(1), also corroborates the
chirality present in the system in the solid state. A short
VtC bond length of 1.683(4) A is consistent with a terminal
alkylidyne moiety,13,36 while the vanadium alkylidyne group
is essentially linear (171.0(3)o), where the plane defined by
the atoms VtC-C bisects the N-V-N plane. Although the
molecular structure of 4-OEt2 suffered from a highly dis-
ordered Et2O ligand (V-O, 2.022(3) A), connectivity was
unambiguously confirmed and was consistent with our
NMR spectral data (vide supra). The dianionic nature of
the N,N0-chelating ligand in 4-OEt2 is supported by the short
V-N bond lengths (1.894(2) and 1.886(3) A, Table 2), which
deviate from V(V)-Nnacnac distances observed in the two
examples of four-coordinate vanadium alkylidynes known
The isolobal relationship between ([ArNC(Me)]2CH)-
TidNAr(OTf) and the vanadium alkylidyne complex
([ArNC(Me)]2CH)VtCtBu(OTf) (3) prompted us to inves-
tigate how an alkyl group would react with the latter. Given
that vanadium alkylidynes, until recently,13 were an un-
known class of molecules,36 we inquired if the alkylidyne
functionality would be preserved while transforming the β-
diketiminate ligand. Accordingly, treatment of 3 with 1 equiv
of KCH2Ph in THF at -35 °C promotes a rapid color change
from brown to red. Rapid workup of the reaction after 5-10
min allows for isolation of the alkylidyne-THF complex
(ArNC(Me)CHC(CH2)NAr)VtCtBu(THF) (4-THF) in
46% yield subsequent to recrystallization overnight from
pentane at -35 °C (Scheme 3). Complex 4-THF is sparingly
soluble in pentane or hexane, thus allowing for clean crystal-
lization from the reaction mixture. Allowing the reaction to
stir longer than 10 min results in lower yields of 4-THF.
Compound 4-THF reveals a diagnostic alkylidyne resonance
in the 13C NMR spectrum centered at 346.7 ppm. The
alkylidyne resonance was broad and only observable upon
cooling of the NMR probe to -50 °C. Very likely, the
lowering of the temperature of an NMR solution containing
complex 4-THF increases the quadrupolar relaxation rate of
the vanadium nucleus (51V, I = 7/2, 99.6% abundant), thus
resulting in the effective decoupling of the 51V scalar cou-
pling from the 13C nucleus.37 Other diagnostic spectroscopic
features for complex 4-THF include the observation of two
inequivalent protons for the methylene group composing the
dianionic ligand, which are centered at 3.52 and 3.29 ppm.
The two resonances were clearly correlated to the sp2-
hydridized carbon at 79.78 ppm via an HMQC experiment
(36) An example of a Fischer-type carbyne complex of vanadium has
been reported. Protasiewicz, J. D.; Lippard, S. J. J. Am. Chem. Soc.
1991, 113, 6564.
(37) Claridge, T. D. W. In High-Resolution NMR Techniques in
Organic Chemistry; Pergamon: Amsterdam, 1999; pp 40-44.
(38) Summers, M. F.; Marzilli, L. G.; Bax, A. J. Am. Chem. Soc. 1986,
108, 4285.

Article
Organometallics, Vol. 28, No. 14, 2009 4119
Table 1. Selected Multinuclear NMR Spectroscopic Features for Complexes 1-8a
1
2
3
4-THF
4-OEt2
5
6
7
8-THF
1H NMR: MdCH
1H NMR: CH3 of β carbon
in chelate ligand
5.23
1.47
5.05
1.65
3.13
1.53
3.33
1.67
1.49
4.34
1.59
1.43
1.63
5.12
1.84
1H NMR: γ-CH of chelate
ligand
4.79
4.92
4.38
4.33
5.66
4.99
4.82
1H NMR: β-CH2 of chelate
ligand
3.64, 3.25
3.52, 3.29
3.49, 3.24
2.96, 2.43
2.56, 2.17
13C NMR: MC (JC-H
)
271.5 (95)
168.2
254.0 (93)
153.2
375.1
167.2
346.7
152.6
343.6
153.1
260.0 (na)
168.0
365.8
161.8
269.3 (na)
168.4
374.7
167.5
13C NMR: β-carbon of
chelate ligand
13C NMR: γ-carbon of
95.5 (162)
99.2 (154)
93.8 (na)
94.59 (160)
94.93 (156)
91.05 (na)
89.83 (na)
91.8 (na)
90.47 (na)
chelate ligand (JC-H
13C NMR: β-C, and
adjacent CH2 of
)
141.9,
83.0 (162)
141.1, 79.78
(158)
141.8, 80.23
(158)
181.0,
31.90 (na)
180.2,
30.68 (na)
chelate ligand (JC-H
11B NMR: B(C6F5)3
19F NMR
)
-14.5
-15.7
-15.6
-15.6
-77.7
-78.2
-131.4,
-160.9,
-165.3
-131.2,
-161.7,
-165.9
489(2671)
-132.4,
-162.5,
-166.4
-133.0,
-163.4,
-167.3
548(1966)
51V NMR: (Δν1/2
)
618(1446)b 286(680)
310(741)
a Chemical shifts are reported in ppm. Coupling constants and linewidths are reported in Hz. b The original value reported at -882 ppm was
incorrectly referenced.13
Scheme 3
(1.92-1.96 A).13 In regard to the N,N0-chelating scaffold in
4-OEt2, the N-Cβ distances are considerably longer than
those reported for the parent β-diketiminate ligand in pre-
cursor 3 (Table 2).13 Due to the conjugated nature of the diene
motif in the N,N0-chelating framework, the methylene hydro-
gens, which were located and refined isotropically, are in the
plane defined by the atoms NCCC0N. Table 2 lists selected
metrical parameters for the single-crystal structure of 4-OEt2.
As noted earlier, the use of a polar solvent such as THF in
conjunction with a strong base resulted in deprotonation of
the β-diketiminate ligand in 1. Unfortunately, multiple
Figure 1. Perspective view of the solid state structure of 4-OEt2
with thermal ellipsoids at the 50% probability level (for artistic
purpose, the ethyl groups on O38 are displayed with isotropic
attempts to alkylate 3 with LiCH2SiMe3 or KCH2Ph in
nonpolar media (toluene, pentane, hexane) resulted in de-
composition mixtures, from which the hypothetical species
“([ArNC(Me)]2CH)VtCtBu(R)” was never isolated.
parameters due to disorder). Hydrogens, with the exception of
the methylene group, have been omitted for clarity. Isopropyl
groups on the bis-anilide aryls have also been omitted for clarity.
Table 2 lists pertinent metrical parameters, and crystallographic
parameters are listed in Table III (see Supporting Information).
Synthesis and Characterization of a Zwitterionic Titanium
(IV) Neopentylidene and Vanadium(V) Neopentylidyne. The
possibility to transform the β-diketiminate framework,
[ArNC(Me)]2CH-, into a more robust N,N0-chelating li-
gand, ArNC(Me)CHC(CH2)NAr2-, provided the impetus
to explore the possibility of abstracting the coordinated
Lewis base (THF) or exploiting the chemistry at the TidC
or VtC linkage. Accordingly, when a toluene solution of 2
was treated with solid B(C6F5)3 and in the absence of
coordinating solvents, an oily residue was formed immedi-
ately without a significant color change of the solution.
Crystallization of this residue from toluene/pentane at
-35 °C allowed for isolation of pure (ArNC(Me)CHC-
(CH2B(C6F5)3)NAr)TidCHtBu(THF) (5) in 86% yield as
1
a brown-red material (Scheme 4). On the basis of H, 13C,
19F, and 11B NMR spectroscopy, the borane has coordinated
to the terminal methylene group to yield a zwitterionic
titanium alkylidene species. Specifically, the 1H NMR spec-
trum of 5 revealed a relatively unperturbed alkylidene R-
hydrogen (3.02 ppm) in comparison to 2, while the 13C NMR
spectrum demonstrated that a terminal neopentylidene func-
tionality (260 ppm, JC-H = 85 Hz) and coordinated THF

4120 Organometallics, Vol. 28, No. 14, 2009
Table 2. Selected Metrical Parameters for the Solid State Structures of Complexes 4, 5, 6, and 8-OEt2
Adhikari et al.
a
M = Ti or V
1
2b
3
4-OEt2
5 3C7H8
6 2.25C7H8 0.25C6H14
8-OEt2
3
3
3
M-C
1.830(3)
2.012(3)
2.025(3)
1.344(4)
1.343(4)
1.410(4)
1.403(4)
1.957(2)
1.86(1)
1.993(8)
1.942(9)
1.42(2)
1.48(2)
1.51(3)
1.39(3)
2.130(8)
1.674(2)
1.924(2)
1.9364(18)
1.347(3)
1.337(3)
1.404(3)
1.398(3)
1.9887(19)
1.683(4)
1.894(2)
1.886(3)
1.394(4)
1.410(4)
1.420(5)
1.422(5)
2.022(3)
1.830(4)
1.979(3)
2.007(3)
1.354(4)
1.337(4)
1.429(4)
1.398(4)
2.071(2)
1.657(5)
1.502(4)
168.3(3)
97.37(11)
113.36(14)
106.77(14)
112.08(11)
121.91(10)
105.45(13)
123.4(3)
119.4(3)
131.9(3)
1.147
1.686(3)
1.932(3)
1.941(2)
1.345(4)
1.352(4)
1.406(4)
1.412(4)
2.002(2)
1.647(5)
1.514(4)
175.6(3)
97.67(10)
107.36(13)
105.11(13)
122.54(10)
120.28(10)
102.46(13)
123.1(3)
119.2(3)
130.5(3)
1.081
1.6809(15)
1.9509(12)
1.9359(12)
1.3379(19)
1.3547(18)
1.407(2)
M-NR
M-NR
NR-Cβ
NR-Cβ
Cβ-Cγ
Cβ-Cγ
1.419(2)
2.0110(11)
M-O
B-CH2
H2CdCβ
M-C-C
NR-M-NR
NR-M-C
NR-M-C
NR-M-O
NR-M-O
O-M-C
NR-Cβ-Cγ
NR-Cβ-Cγ
Cβ-Cγ-Cβ
M deviation
1.413(14)
161.7(8)
101.5(4)
115.7(4)
114.4(4)
107.2(3)
113.3(3)
104.9(4)
125.8(9)
113.7(9)
134(1)
1.418(5)
171.0(3)
163.9(3)
177.61(19)
96.83(8)
174.36(12)
97.76(5)
94.18(10)
109.83(13)
111.15(13)
115.62(10)
112.26(10)
112.48(13)
123.5(3)
122.8(3)
129.9(3)
1.129
104.45(13)
107.03(15)
106.77(16)
116.91(11)
115.51(12)
105.52(14)
120.6(3)
119.9(3)
133.8(3)
0.990
105.58(10)
106.65(10)
118.08(9)
119.64(9)
108.60(10)
121.8(2)
121.5(2)
130.1(2)
1.116
105.46(6)
105.46(6)
121.97(5)
120.63(5)
103.90(6)
121.15(13)
121.85(13)
130.11(14)
1.112
0.851
a Metrical parameters for complexes 1-3 are included for comparison.13,15b,21,28a Distances are reported in angstroms and angles in degrees. NR, Cβ,
and Cγ represent the atoms of the NCCCN ring. M deviation represents deviation of the metal ion above or below the plane defined by the NCCC0N or
NCCCN ring composing the chelating ligand. b The solid state structure was of poor quality due to badly split crystals.15b
(resonance at 3.94 and a broad resonance at 1.84 ppm) were
Scheme 4
retained in 5. The much lower JC-H versus 1 or 2 suggest a
stronger R-hydrogen agostic interaction taking place with
the more electron deficient titanium(IV) center. Rehybridi-
zation at the former borane group is also evident from the
broad 11B NMR chemical shift at -14.5 ppm (relative to
Et2OfBH3 referenced at 0 ppm), a value upfield shifted
from that of free borane (75 ppm). The 11B NMR spectro-
scopic chemical shift experienced in 5 is analogous to that for
methide-abstracted borane zwitterions.39 Binding of borane
to the CH2 group caused a dramatic upfield shift in the 13
C
NMR and 1H resonances for both the methylene carbon and
hydrogens (carbon at 31.33 ppm, and diastereotopic hydro-
gens at ∼2.7 and 2.31 ppm, respectively, and having a 2JH-H
value of 20 Hz). Other than these diagnostic features, both
1
the H and 13C NMR spectra are consistent with an asym-
metrical NCCC0N chelating framework analogous to 2 (two
inequivalent β-carbon resonances from the NCCC0N ring as
well as inequivalent isopropyl methine and aryl groups, vide
supra). Abstraction of THF or chemistry related to the
alkylidene moiety was not observed with B(C6F5)3, thus
suggesting that the most accessible and nucleophilic site for
Lewis acid coordination was the “CH2” group in the new
ligand scaffold (ArNC(Me)CHC(CH2)NAr)Ti2+. Forma-
tion of 5 can be rationalized on the basis of a charge
delocalizing about the butadieneamine “NRCCγCβCH2”
motif of 2, to form a zwitterion 2a shown in Scheme 4.
Charge separation in complex 5 can resonate in several
forms (5a-d), as suggested in Scheme 4. To unambiguously
confirm the coordination of borane to 2, we resorted to
single-crystal X-ray analysis (Figure 2). Table 2 lists selected
metrical parameters for the single-crystal structure of 5.
While coordination of borane to the methylene group results
in an overall fixed negative change at boron, positive charge
delocalization can be proposed to be distributed about
several atoms in the NCCC0NTi ring, including the Ti center.
The metrical parameters observed in the solid state structure
of 5 suggest that all resonances, 5a-d, should be considered,
given the resemblance for the pair of Ti-NR and NR-Cβ
distances (1.979(3) and 2.007(3) A; 1.354(4) and 1.337(4) A,
respectively). Although canonical forms 5a and 5c seem
intuitive based on electropositive character at the metal
center, resonances 5b and 5d seem the most logical to
propose in terms of carbocation character at the β-position
being stabilized by NR-lone pair donation. In fact, 13C NMR
spectroscopic data revealed that both β-carbons composing
the NCCC0N plane had significantly shifted downfield
(181.0 and 168.1 ppm, respectively, with respect to 153.2
and 141.9 ppm in 2), in accord with these atoms having more
carbocation-like character. A compiled list of salient spec-
troscopic data is shown in Table 1.
Due to the carbocation character of the β-carbon in
complex 5, the N,N0-chelating scaffold in 2 should, in
principle, be a better donor ligand than the chelating ligand
in the zwitterion. Judging from the crystallographic data,
(39) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113,
3623.

Article
Organometallics, Vol. 28, No. 14, 2009 4121
Figure 3. Perspective view of the solid state structure of 6 with
thermal ellipsoids at the 50% probability level. Hydrogens, with
the exception of the methylene groups (C5), have been omitted
for clarity. Isopropyl aryl groups on the bis-anilide ligand,
disordered solvent, and the perfluorophenyl groups on boron
(with the exception of the ipso carbons, C35, C41, and C47) have
also been omitted for clarity. Table 2 lists pertinent metrical
parameters, and crystallographic parameters are listed in Table
III (see Supporting Information).
Figure 2. Perspective view of the solid state structure of 5 with
thermal ellipsoids at the 50% probability level. Hydrogens, with
the exception of alkylidene and methylene groups (C66 and
C20), have been omitted for clarity. Isopropyl aryl groups on the
bis-anilide ligand, disordered solvent, and the perfluorophenyl
groups on boron (with the exception of the ipso carbons, C34,
C45, and C56) have also been omitted for clarity. Table 2 lists
pertinent metrical parameters, and crystallographic parameters
are listed in Table 3 (see Supporting Information).
complex.40 Most notably, the resulting complex shows fea-
tures of a C1 symmetric molecule that results from B(C6F5)3
binding to the methylene group in the backbone. The hydro-
gens on the methylene group are also diastereotopic, showing
Scheme 5
2
identical JH-H values to that for the titanium analogue 5
(vide supra). The proton and carbon chemical shifts vary
remarkably due to a change in hybridization of the center of B
(C6F5)3 binding (1H NMR: from 3.52 and 3.29 in 4-THF to
2.56 and 2.17 ppm in 6; 13C NMR: from 79.78 in 4-THF to
31.84 ppm in 6). In addition, the 19F NMR spectrum reveals
the difference between meta and para positions (Δ =
4 ppm), which is indicative of a four-coordinate borate
group.41 On the basis of our data, we confidently propose
this new material to be the zwitterionic vanadium alkylidyne
depicted in Scheme 5. In fact, addition of donor solvents such
as Et2O or THF does not appear to transform 6 back to 4-
THF or 4-OEt2 concurrent with (F5C6)3BrTHF ejection.
A solid state structure of 6 confirms the proposed connec-
tivity (Figure 3) and displays structural features analogous to
the titanium derivative 5. As anticipated, borane coordination
to C5 results in rehybridization of carbon and boron to a
tetrahedral geometry concurrent with elongation of the C5-
C4 bond (1.514(4) A) from that of its ether analogue of the
neutral derivative, namely, 4-OEt2 (Table 2, 1.418(5) A).13 The
diastereotopic nature of the hydrogen on C5 is clearly visible
from the structure. Preservation of the terminal VtC group is
evident from the short bond length of 1.686(3) A and linear
angle of 175.6(3)o. The vanadium-THF distance in 6 (V1-
O1, 2.002(2) A) is comparable to that of 5, but shorter than
that of the borane-free complex 4-THF (2.022(3) A, vide
supra), again implying a more electron deficient metal center.
Synthesis and Characterization of a Cationic Titanium(IV)
Neopentylidene and a Vanadium(V) Neopentylidyne. Given
the nucleophilic nature of the terminal methylene moiety in
borane coordination to C20 results not only in rehybridiza-
tion of carbonand boron to sp3 but elongation of the C20-C5
(1.502(4) A) from that of its neutral species 2 (Table 2, 1.413
(14) A).15 Likewise, the terminal TidC linkage is short (1.830
(4) A) and the angle is distorted significantly from a proto-
typical sp2-hybridized carbon, thus suggesting an R-hydrogen
agostic interaction taking place with the Ti(IV) center. In
addition, the alkylidene hydrogen is oriented perpendicular to
the THF, with the metal center deviating 1.147 A above the
NCCC0N plane. The titanium-THF distance in 5 (Ti1-O71,
2.071(2) A) is significantly shorter than in 2 (2.130(8) A), thus
implying a more electron deficient Ti center.
When a toluene solution containing 4 was added to
borane, a similar reaction to that for the formation of 5
was observed. The resulting complex (ArNC(Me)CHC-
(CH2B(C6F5)3)NAr)VtCtBu(THF) (6) was isolated in pure
form and good yield (87%) as a red solid after crystallization
from a toluene/hexane-layered solution (Scheme 5). Multi-
nuclear NMR spectroscopy and X-ray crystallographic data
proved the identity of the complex unambiguously. Several
salient spectroscopic features of this new complex are the 13C
NMR spectrum displaying a broad resonance for the alky-
lidyne carbon at 366 ppm (-50 °C) as well as a broad 51V
NMR resonance at 489 ppm (Δν1/2 = 2671 Hz). Both of the
13C and 51V NMR spectroscopic features are consistent
with a terminal and diamagnetic neopentylidyne vanadium
(41) (a) Blackwell, J. M.; Piers, W. E.; Parvez, M. Org. Lett. 2002, 2,
695. (b) Horton, A. D.; With, J.-D. Organometallics 1997, 16, 5424.
(40) Rehder, D. Coord. Chem. Rev. 2008, 252, 2209.

4122 Organometallics, Vol. 28, No. 14, 2009
Adhikari et al.
Scheme 6
Scheme 7
compounds 2 and 4, and the fact that protonation studies
occur at this site,32 we suspected that a similar transforma-
tion would happen in our system, to re-form the β-diketimi-
nate ancillary. Accordingly, treatment of 2 in toluene with
solid [HNMe2Ph][B(C6F5)4] at 25 °C rapidly caused an oily
product to separate from the medium but without a signifi-
cant color change in the solution. Examination of the reac-
([ArNC(Me)]2CH)TidCHtBu(CH2SiMe3)15b with a reagent
such as [HNMe2Ph][B(C6F5)4] has not been successful.
Therefore, our strategy to reconstruct the β-diketiminate
ligand from the chelate scaffold in complex 2 appears to be
selective and relatively clean, despite the reactive nature of
the product. To our knowledge, complex 7 represents the
only documented example of a discrete cationic titanium (or
group 4) complex bearing the terminal alkylidene group.
Because we could not obtain solid state structural infor-
mation for 7, we decided to generate a cationic alkylidyne
analogue of vanadium. The stability of such a scaffold,
[([ArNC(Me)]2CH)VtCtBu]+, should in principle be great-
er given the additional M-C π-bond involved, thence pro-
viding a higher formal electron count on the V(V) ion as
opposed to the Ti(IV) analogue. This hypothesis was further
corroborated by the fact that a terminal cationic vanadium
alkylidyne complex, [([ArNC(Me)]2CH)VtCtBu(THF)]-
[BPh4],13 has been isolated. This complex was remarkably
stable (decomposition occurred only after 20 days at 65 °C)
to an intramolecular cross-metathesis transformation of the
neopentylidyne with the imine group composing the β-
diketiminate NCCCN template.21,28 In view of that, when
a toluene solution of 4-THF was added to solid [HNMe2Ph]-
[B(C6F5)4], an immediate product was observed to form
upon the dissolution of solid [HNMe2Ph][B(C6F5)4]. Con-
centrating the solution resulted in gradual precipitation of
the salt [([ArNC(Me)]2CH)VtCtBu(THF)][B(C6F5)4] (8-
THF) in 81% yield (Scheme 7). The connectivity in complex
1
tion mixture by H NMR spectroscopy of the oily residue
revealed formation of the salt that we speculate to be
[([ArNC(Me)]2CH)TidCHtBu(THF)][B(C6F5)4] (7) along
with some other byproducts (Scheme 6). For instance, the
1H NMR spectrum revealed formation of a C2 symmetric
ligand scaffold, namely, re-formation of the β-diketiminate
ligand [ArNC(Me)]2CH-. Unfortunately, longer time
frames (from minutes to hours) resulted in gradual decom-
position of 7 to a myriad of products, especially if the
solution was kept at room temperature or if an attempt
was made to isolate a solid. The thermal instability of 7 is not
surprising, since 1 itself has a t1/2 of 45 min at 57 °C.21 In fact,
the nature of the pseudo halide or halide affects the rate for
the decomposition of the [([ArNC(Me)]2CH)TidCHtBu]+
scaffold to the point where weaker π-donors (Br- and I-)
result in enhanced rate of decomposition.21 In the case of 7,
the presence of a more labile ligand such as THF presumably
facilitates the rate for decomposition of the latent low-
coordinate titanium alkylidene via intramolecular C-H
activation as well cross-metathesis pathways among many
other possibilities. Our inability to obtain suitable single
crystals for diffraction studies as well as the poor character-
ization of 7 prompted us to conduct an in situ, low-tempera-
ture study of this reactive species in bromobenzene-d5. Hence
the 1H NMR spectrum of 7, generated in situ at lower
temperatures, was greatly improved and confirmed forma-
tion of a Cs symmetric system by the presence of diagnostic
alkylidene R-hydrogen (3.17 ppm) as well as the methine γ-
CH resonance for the β-diketiminate backbone (4.82 ppm)
and equivalent β-methyl groups (168.4 ppm). Maintaining
the sample cooled to -15 °C allowed us to collect relatively
clean 13C, 19F, and 11B NMR spectroscopic data for 7, for the
first terminal alkylidene salt of a group 4 metal (see Experi-
mental Section for all details and Supporting Information
for spectra). The 13C NMR spectroscopic data confirmed the
presence of a Cs symmetric terminal alkylidene resonance at
269.3 ppm (we were unable, however, to collect reliable JC-H
data due to the poor solubility of 7 in bromobenzene-d5).
However, we anticipate a low JC-H value, if not comparable,
to that of 5, most likely due to the unsaturated and thus
electron-deficient nature of the Ti(IV) center. Overall, the
spectral features for 7 are analogous to 1 and other Cs
symmetric neopentylidene complexes supported with the β-
diketiminate ligand [ArNC(Me)]2CH-.22 Likewise, the simi-
larity of the 1H NMR spectra of 1 and 7 apparently suggests
the R-hydrogen in 7 to be oriented anti with respect to the
THF ligand.21,28a In contrast, treating 1 with Li[B(C6F5)4]-
(OEt2) results in complicated mixtures, even when the reac-
tion was performed at low temperatures in bromobenzene-
d5. Likewise, attempts to protonate the alkyl ligand in
8-THF is assigned on the basis of 1H, 13C, 19F, 11B, and 51
V
NMR spectra (Table 1). While the 1H NMR spectrum con-
firmed the formation of a C2 symmetric β-diketiminate ligand
by protonation of the methylene moiety in 4-THF, the 13C
NMR spectrum unambiguously established retention of the
alkylidyne unit (374.7 ppm, -55 °C). The combination of 19F,
11B, and 51V (548 ppm, Δν1/2 = 1966 Hz) NMR spectral data
also endorsed the formation of a discrete diamagnetic vana-
dium salt having the weakly coordinating anion B(C6F5)4.
Therefore, the solution NMR spectroscopic data for com-
pound 8-THF suggests a symmetrically identical species to
the BPh4 salt derivative reported earlier by us.13 Formation of
8-THF was also achieved by treating 3 with Li[B(C6F5)4]-
(OEt2) in THF at -35 °C (Scheme 7). Unlike complex 7, which
gradually decayed in solution at room temperature, complex 8-
THF appears to be relatively stable in solutions of THF,
bromobenzene, and halogenated solvents such as CH2Cl2.
Although we obtained complete NMR spectroscopic data
for 8-THF, single crystals suitable for diffraction analysis
were obtained with 8-OEt2 (multinuclear NMR spectro-
scopic data is virtually identical to 8-THF). Single crystals
were grown from a toluene/hexane-layered solution cooled
to -35 °C over 3 days, and the solid state structure was
determined by X-ray diffraction analysis. Figure 4 depicts a
four-coordinate vanadium(V) center having overall Cs sym-
metry consistent with the NMR spectral data (vide supra).
The near-linear (174.36(12)o) arrangement of V1-C38-C39

Article
Organometallics, Vol. 28, No. 14, 2009 4123
zwitterions such as compounds 5 and 6, given that these
systems could be studied as catalysts42 or as potential sources
ofzwitterionic, low-coordinatetitanium alkylidenesandvana-
dium alkylidynes by displacement of the neutral ligand THF.
Experimental Details
General Considerations. Unless otherwise stated, all opera-
tions were performed in an M. Braun Lab Master double drybox
under an atmosphere of purified nitrogen or using high-vacuum
standard Schlenk techniques under an argon atmosphere.43
Anhydrous n-hexane, pentane, toluene, and benzene were pur-
chased from Aldrich in Sure Seal reservoirs (18 L) and dried by
passage through two columns of activated alumina and a Q-5
column. Diethyl ether was dried by passage through a column of
activated alumina. THF was distilled, under nitrogen, from
purple sodium benzophenone ketyl and stored over sodium
metal. Distilled THF was transferred under vacuum into collec-
tion vessels before being carried into a drybox. C6D6 was
purchased from Cambridge Isotope Laboratory (CIL), de-
gassed, and vacuum transferred to 4 A molecular sieves. BrC6D5
was purchased from CIL and degassed three times. After
transferring into the glovebox it was passed through alumina
in a pipet column. Celite, alumina, and 4 A molecular sieves were
activated under vacuum overnight at 200 °C. All the starting
materials such as 1, 2, and 3 were prepared following the
literature procedure.13,15b B(C6F5)3 and [HNMe2Ph][B(C6F5)4]
were purchased from Boulder Scientific. [HNMe2Ph][B(C6F5)4]
was dried at 60 °C for 2 days under reduced pressure to remove
traces of CH2Cl2. KCH2Ph was prepared according to the
literature procedure.44 CHN analyses were performed by Mid-
west Microlab, Indianapolis, IN. 1H, 13C, and 19F NMR spectra
were recorded on Varian 400 or 300 MHz NMR spectrometers.
1H NMR spectra are reported with reference to solvent reso-
nances (residual C6D5H in C6D6, 7.16 ppm). 11B and 51V NMR
spectra were collected using a Varian 500 MHz NMR spectro-
Figure 4. Perspective view of the solid state structure of 8-OEt2
with thermal ellipsoids at the 50% probability level. Hydrogens
have been omitted for clarity. Isopropyl groups on the β-
diketiminate aryls, disordered solvent, and the borate anion B
(C6F5)4 have also been omitted for clarity. Table 2 lists pertinent
metrical parameters, and crystallographic parameters are listed
in Table 3 (see Supporting Information).
and short V1-C38 distance (1.6809(15) A) in complex 8-
OEt2 are clearly indicative of a vanadium alkylidyne as in the
BPh4 salt analogue (1.696(3) A).13 The vanadium center is
displaced 1.112 A above the β-diketiminate plane (Table 2
provides salient metrical parameters for the solid state
structure of 8-OEt2). The solid state structure is consistent
with the protonation taking place at the formal methylene
group since the V-N distances fall within the expected
values for a [ArNC(Me)]2CH]V scaffold (∼1.943 A).15
meter. BF3 OEt2 (set at 0 ppm) and VOCl3 (set as 0 ppm) were
3
used as external standards for 11B and 51V NMR, respectively.
X-ray diffraction data were collected on a SMART6000 (Bru-
ker) system under a stream of N2 (g) at low temperatures.45,46
Synthesis of the Complex (ArNC(Me)CHC(CH2)NAr)
VtCtBu(OEt2) (4-OEt2). In a vial was dissolved 3 (80 mg,
0.12 mmol) in pentane (10 mL), and the solution was cooled
to -35 °C. To the cold solution was added a cold (-35 °C)
pentane (5 mL) solution of tBuLi [7.46 mg, 0.12 mmol], and the
color of the solution changed to brown-red. The solution was
stirred for 5 min, and 4 drops of ether were added. The solution
was stirred for a further 1 h and dried under vacuum. The residue
was extracted with hexane and filtered. The filtrate was reduced
in volume in vacuo and cooled to -35 °C to afford 4-OEt2 as
dark red crystals [43 mg, 0.07 mmol, 58% yield].
Conclusions
We have demonstrated that deprotonating the β-methyl
group of the (ArNC(Me)]2CH)M (M = Ti(IV) and V(V))
scaffold provides the dianionic N,N0-chelating (ArNC(Me)-
CHC(CH2)NAr) ligand. The nucleophilic character at the
terminal methylene motif of this ligand has allowed us to
convert such an ancillary support to a zwitterion by addition
of B(C6F5)3 or to a discrete salt by protonation with
[HNMe2Ph][B(C6F5)4]. Therefore, the sometimes vulnerable
nature of the β-diketiminate ligand, [ArNC(Me)]2CH-, can
play an important role in assembling the first examples of
charge-separated species of Ti and V compounds, specifi-
cally compounds having terminal metal-carbon multiple
bonded ligands. We are currently exploring the reactive
nature of the M-C multiple bond in compounds such as 2
and 4 since these systems appear to be more kinetically stable
than compounds supported by the original β-diketiminate
ligand. This feature is likely due to the fact that the vulner-
able imine group is no longer available for the intramolecular
Wittig-like rearrangement. We are also beginning to uncover
the chemistry offered by unusual alkylidene and alkylidyne
For 4-OEt2: 1H NMR (23 °C, 399.8 MHz, C6D6): δ 7.49-7.25
(m, 6H, C6H3), 4.33 (s, 1H, ArNC(Me)CHC(CH2)NAr), 4.23
(septet, 1H, CHMe2), 4.15 (septet, 1H, CHMe2), 4.03-3.92 (m,
5H, CHMe2, O(CH2CH3)2), 3.86 (septet, 1H, CHMe2), 3.49 (s,
1H, ArNC(Me)CHC(CH2)NAr), 3.24 (s, 1H, ArNC(Me)-
CHC(CH2)NAr), 1.67 (d, 3H, CHMe2), 1.63-1.58 (over-
lapping doublets, 9H, CHMe2), 1.54 (d, 3H, CHMe2), 1.48
(d, 3H, CHMe2), 1.43 (s, 3H, ArNC(Me)CHC(CH2)NAr), 1.37
(43) For a general description of the equipment and techniques used
in carrying out this chemistry see: Burger, B. J.; Bercaw, J. E. In
Experimental Organometallic Chemistry; Wayda, A. L., Darensbourg,
M. Y., Eds.; ACS Symposium Series 357; American Chemical Society:
Washington D.C., 1987; p 79.
(42) Borane coordination to remote sites of ancillary ligands has been
used to generate efficient catalysts. (a) Azoulay, J. D.; Rojas, R. S.;
Serrano, A. V.; Ohtaki, H.; Galland, G. B.; Wu, G.; Bazan, G. C. Angew.
Chem., Int. Ed. 2009, 48, 1089. (b) Chen, Y.; Wu, G.; Bazan, G. C.
Angew. Chem., Int. Ed. 2005, 44, 1108. (c) Lee, B. Y.; Bu, X.; Bazan, G.
C. Organometallics 2001, 20, 5425. (d) Wasilke, J.-C.; Komon, Z. J. A.;
Bu, X.; Bazan, G. C. Organometallics 2004, 23, 4174. (e) Kim, Y. H.;
Kim, T. H.; Yeoul, B.; Woodmansee, L. D.; Bu, X.; Bazan, G. C.
Organometallics 2002, 21, 3082.
(44) Bailey, P. J.; Coxall, R. A.; Dick, C. M.; Fabre, S.; Henderson, L.
~
ꢀ
C.; Herber, C.; Liddle, S. T.; Lorono-Gonzalez, D.; Parkin, A.; Parsons,
S. Chem.;Eur. J. 2003, 9, 4820.
(45) SAINT 6.1; Bruker Analytical X-Ray Systems: Madison, WI.
(46) SHELXTL-Plus V5.10; Bruker Analytical X-Ray Systems:
Madison, WI.

4124 Organometallics, Vol. 28, No. 14, 2009
Adhikari et al.
(d, 3H, CHMe2), 1.30 (d, 3H, CHMe2), 0.98 (t, 4H, O(CH2-
CH3)2), 0.46 (s, 9H, VtCCMe3). 13C NMR (-50 °C, 100.6
MHz, C6D5CD3): δ 343.6 (VtCCMe3). 13C NMR (23 °C, 100.6
MHz, C6D6): δ 153.6 (aryl), 153.1 (ArNC(Me)CHC(CH2)NAr),
151.6 (aryl), 144.0 (aryl), 143.9 (aryl), 143.5 (aryl), 143.4 (aryl),
141.8 (ArNC(Me)CHC(CH2)NAr), 125.3 (aryl), 125.2 (aryl),
124.6 (aryl), 123.8 (aryl), 123.3 (aryl), 122.9 (aryl), 94.93 (d, JC-H
=156 Hz, ArNC(Me)CHC(CH2)NAr), 80.23 (t, JC-H=158 Hz,
ArNC(Me)CHC(CH2)NAr), 74.41 (OEt2), 56.26 (VtCCMe3),
29.30 (VtCCMe3), 29.10 (CHMe2), 28.60 (CHMe2), 28.56
(CHMe2), 28.37 (CHMe2), 26.34 (Me), 25.89 (Me), 25.85
(Me), 25.74 (Me), 25.41 (Me), 25.07 (Me), 24.87 (Me), 24.35
(Me), 23.13 (Me), 13.79 (OEt2). 51V NMR (23 °C, 100.6 MHz,
C6D6): δ 310 (Δν1/2=741 Hz). Satisfactory elemental analysis
was not obtained due to the thermal instability of the complex.
Synthesis of the Complex (ArNC(Me)CHC(CH2)NAr)-
VtCtBu(THF) (4-THF). In a round-bottom flask was dissolved
3 (160 mg, 0.24 mmol) in THF (20 mL), and the solution was
cooled to -35 °C. To the cold solution was added a cold (-35 °C)
THF (2 mL) solution of KCH2Ph (30.2 mg, 0.24 mmol), and the
color of the solution changed to brown-red immediately after the
addition. The solution was stirred for 5 min and dried in vacuo.
The dried mass was extracted with 20 mL of pentane and filtered.
The filtrate was reduced in volume to 5 mL and stored at
-35 °C to yield bright red crystals of 4-THF within 24 h (65 mg,
0.11 mmol, 46% yield).
(TidCHCMe3), 180.96 (C(Me)CHC(CH2B), 168.04 (C(Me)-
CHC(CH2B), 163.11 (aryl), 149.20 ((C6F5)3B), 146.83
((C6F5)3B), 145.51 (aryl), 144.82 (aryl), 142.29 (aryl), 141.10
(aryl), 140.35 (aryl), 140.14 (aryl), 139.75 (aryl), 139.54 (aryl),
138.42 ((C6F5)3B), 135.97 ((C6F5)3B), 128.38 (aryl), two aryl
peaks are buried under solvent BrC6D5 resonances. 91.05 (C-
(Me)CHC(CH2B)), 78.52 (THF), 51.01 (TidCHCMe3) 34.37
(CHMe2), 31.90 (C(Me)CHC(CH2B) 30.46 (CHMe2), 29.84
(CHMe2), 29.60 (CHMe2), 28.69 (Me), 28.32 (Me), 27.75
(Me), 25.92 (THF), 25.66 (Me), 24.49 (Me), 24.20 (Me), 23.71
(Me), 23.26 (Me), 23.04 (Me), 22.74 (Me). 19F NMR (23 °C,
282.3 MHz, BrC6D5): δ -131.35 (B(C6F5)3), -160.94 (B-
(C6F5)3), -165.27 (B(C6F5)3). 11B NMR (24 °C, 160.61 MHz,
BrC6D5): δ -14.45. Satisfactory elemental analysis was not
obtained due to the thermal instability of the complex.
Synthesis of the Complex (ArNC(Me)CHC(CH2B(C6F5)3)-
NAr)VtCtBu(THF) (6). In a vial was taken 4-THF (20 mg,
0.033 mmol) dissolved in 2 mL of toluene. The toluene solution
was added to an another vial containing solid B(C6F5)3 (16.9
mg, 0.033 mmol). No color change was observed immediately
after the addition. The mixture was stirred for 30 min, after
which it was dried in vacuo. The dried mass was washed with 3
mL of pentane and dried again. The dried mass was dissolved in
toluene/hexane and stored at -45 °C for 12 h to yield red crystals
of 6 (32 mg, 0.029 mmol, 87% yield). Multinuclear NMR of the
dried mass proved the identity of the desired complex.
For 4-THF: 1H NMR (23 °C, 399.8 MHz, C6D6): δ 7.51-7.26
(m, 6H, C6H3), 4.38 (s, 1H, ArNC(Me)CHC(CH2)NAr), 4.23
(septet, 1H, CHMe2), 4.15 (br, 4H, THF), 4.00 (overlapping
septets, 2H, CHMe2), 3.55 (septet, 1H, CHMe2), 3.52 (s, 1H,
ArNC(Me)CHC(CH2)NAr), 3.29 (s, 1H, ArNC(Me)CHC-
(CH2)NAr), 1.70 (d, 3H, CHMe2), 1.64 (d, 3H, CHMe2),
1.61-1.55 (m, 9H, overlapping singlet of ArNC(Me)CHC-
(CH2)NAr and doublets of CHMe2), 1.49 (d, 3H, CHMe2),
1.44 (br, 4H, THF), 1.38 (overlapping doublets, 6H, CHMe2),
1.27 (d, 3H, CHMe2), 0.49 (s, 9H, VtCCMe3). 13C NMR
(-50 °C, 100.6 MHz, C6D5CD3): δ 346.7 (VtCCMe3). 13C
NMR (23 °C, 100.6 MHz, C6D6): δ 153.1 (aryl), 152.6 (ArNC-
(Me)CHC(CH2)NAr), 151.6 (aryl), 144.3 (aryl), 143.9 (aryl),
143.5 (aryl), 143.4 (aryl), 141.1 (ArNC(Me)CHC(CH2)NAr),
125.5 (aryl), 125.4 (aryl), 124.7 (aryl), 123.9 (aryl), 123.2 (aryl),
123.0 (aryl), 94.59 (d, JC-H = 160 Hz, ArNC(Me)CHC(CH2)-
NAr), 79.78 (t, JC-H = 158 Hz), ArNC(Me)CHC(CH2)NAr),
78.67 (THF), 56.24 (VtCCMe3), 29.85 (VtCCMe3), 29.52
(CHMe2), 28.60 (CHMe2), 28.80 (CHMe2), One CHMe2 is
buried. 26.55 (Me), 26.46 (Me), 26.31 (Me), 26.15 (Me), 25.64
(THF), 25.14 (Me), 24.96 (Me), 24.80 (Me), 24.46 (Me),
1H NMR (23 °C, 300.07 MHz, CDCl3): δ 7.29-7.19 (m, 6H,
C6H3), 5.12 (s, 1H, ArNC(Me)CHC(CH2B)NAr, 3.10-2.88
(overlapped septets, 2H, CHMe2), 2.71-2.56 (overlapped sep-
tets, 2H, CHMe2), 2.56 (d, 1H, ArNC(Me)CHC(CH2B)NAr,
2JH-H = 20 Hz), 2.30 (br, 4H, THF), 2.17 (d, 1H, ArNC(Me)-
2
CHC(CH2B)NAr), JH-H = 20 Hz), 1.63 (s, 3H, ArNC(Me)-
CHC(CH2B)NAr), 1.59-1.44 (overlapping doublets, 9H,
CHMe2), 1.25-1.16 (m, 13H, overlapped resonance of CHMe2
and THF), 1.02 (d, 3H, CHMe2), 0.84 (d, 3H, CHMe2), 0.10 (s,
9H, VtCCMe3). 13C NMR (-50 °C, 100.6 MHz, CDCl3): δ 366
(VtCCMe3). 13C NMR (23 °C, 125.89 MHz, CDCl3): δ 180.21
(ArNC(Me)CHC(CH2B)NAr, 161.83 (ArNC(Me)CHC(CH2B)-
NAr), 149.24 ((C6F5)3B), 148.29 (aryl), 147.38 (aryl), 147.08
((C6F5)3B), 141.12 (aryl), 140.61 (aryl), 137.72 ((C6F5)3B),
135.19 ((C6F5)3B), 127.70 (aryl), 127.50 (aryl), 124.89 (aryl),
124.65 (aryl), 123.71 (aryl), 123.60 (aryl), 89.83 (ArNC(Me)-
CHC(CH2B)NAr), 65.90 (THF), 60.04 (VtCCMe3), 31.84
(ArNC(Me)CHC(CH2B)NAr),
30.54
(CHMe2),
28.64
(CHMe2), 28.41 (CHMe2), 28.13 (VtCCMe3), 27.73 (CHMe2),
26.34 (THF), 25.84 (Me), 25.04 (Me), 24.84 (Me), 24.50 (Me),
23.88 (Me), 23.52 (Me), 23.27 (two Me overlapped). 13C NMR
(-50 °C, 100.6 MHz, CDCl3): δ 365.75 (VtCCMe3). 19F NMR
(25 °C, 282.32 MHz, CDCl3): δ -131.22 (B(C6F5)3), -161.65 (B-
(C6F5)3), -165.89 (B(C6F5)3). 51V NMR (24 °C, 131.58 MHz,
CDCl3): δ 489 (Δν1/2 = 2671 Hz). 11B NMR (24 °C, 160.61 MHz,
CDCl3): δ -14.47. Satisfactory elemental analysis was not
obtained due to the thermal instability of the complex.
23.31 (Me). 51V NMR (23 °C, 100.6 MHz, C6D6): δ 286 (Δν1/2
=
680 Hz). Anal. Calcd for C38H57N2OV: C, 74.97; H, 9.44; N,
4.60. Found: C, 75.22; H, 9.48; N, 4.83.
Synthesis of the Complex (ArNC(Me)CHC(CH2B(C6F5)3)-
NAr)TidCHtBu(THF) (5). In a vial 2 (30 mg, 0.50 mmol) was
taken in 2 mL of toluene and added to solid B(C6F5)3 (26 mg,
0.50 mmol) along with stirring. No significant color change was
observed after the mixing. The solution mixture was stirred for
30 min and then dried in vacuo. Multinuclear NMR proved the
identity of the complex.
Synthesis of the Complex [([ArNC(CH3)]2CH)TidCHtBu-
(THF)][B(C6F5)4] (7). In a vial 2 was taken (15 mg, 0.025 mmol)
in 2 mL of toluene and added to the solid [PhNMe2H][B(C6F5)4]
(21.1 mg, 0.025 mmol). After the addition the color of the
mixture remained brown without any change. The mixture
was stirred for 30 min, after which it was dried in vacuo. The
dried mass was washed with hexane (3 times taking 2 mL each)
and dried again. Multinuclear NMR spectroscopy proved the
identity of the synthesized complex.
1H NMR (23 °C, 399.8 MHz, BrC6D5): δ 7.22-6.81 (m, 6H,
C6H3), 5.52 (s, 1H, ArNC(Me)CHC(CH2B)NAr), 4.03-3.81
(m, 4H, THF), 3.02 (s, 1H, TidCHtBu, based on HMQC),
2.83-2.72 (m, 2H, CHMe2 and ArNC(Me)CHC(CH2B(C6F5)4)-
2
NAr resonances overlapped with JH-H not resolved), 2.60
(septet, 1H, CHMe2), 2.51 (septet, 1H, CHMe2), 2.42 (septet,
1H, CHMe2), 2.31 (d, 1H, ArNC(Me)CHC(CH2B(C6F5)4)NAr,
2JH-H = 20 Hz), 1.84 (br, 4H, THF), 1.51 (d, 3H, CHMe2),
1.42-1.33 (m, 6H, ArNC(Me)CHC(CH2B(C6F5)4)NAr and
CHMe2 resonances overlapped), 1.30 (d, 3H, CHMe2), 1.13
(overlapping doublets, 4H, CHMe2), 0.97 (d, 3H, CHMe2), 0.76
(d, 3H, CHMe2), 0.69 (d, 3H, CHMe2), 0.23 (s, 9H, TidCHtBu).
1H NMR (23 °C, 399.8 MHz, BrC6D5): δ 7.21-7.00 (m, 6H,
C6H3), 4.82 (s, 1H, ArNC(Me)CHC(Me)NAr), 3.84 (br, 4H,
THF), 3.17 (s, 1H, TidCHtBu, based on HMQC), 2.69 (septet,
2H, CHMe2), 2.18 (septet, 2H, CHMe2), 1.68 (br, 4H, THF),
1.49 (s, 1H, ArNC(Me)CHC(Me)NAr), 1.23 (d, 6H, CHMe2),
1.08 (d, 6H, CHMe2), 0.97 (d, 6H, CHMe2), 0.83 (d,
6H, CHMe2), 0.18 (s, 9H, TidCHtBu). 13C NMR (-15 °C,
13C NMR (-5 °C, 100.6 MHz, BrC6D5):
δ
260.01
100.6 MHz, BrC6D5): δ 269.31 (TidCHCMe3), 168.43

Article
Organometallics, Vol. 28, No. 14, 2009 4125
(C(Me)CHC(Me)), 149.75 ((C6F5)3B), 147.37 ((C6F5)3B),
143.90 (aryl), 140.54 (aryl), 139.27 (aryl), 137.79 ((C6F5)3B),
135.36 ((C6F5)3B), 128.51 (aryl), 125.27 (aryl), 124.02 (aryl),
91.80 (C(Me)CHC(Me), 78.21 (THF), 52.51 (TidCH-
CMe3), 31.58 (CHMe2), 30.17 (CHMe2), 28.62 (Me), 25.85
(Me), 25.68 (THF), 23.95 (Me), 23.82 (Me), 24.18 (Me), 22.91
(Me). 19F NMR (23 °C, 282.3 MHz, BrC6D5): δ -132.41 (B-
(C6F5)4), -162.51 (B(C6F5)4), -166.40 (B(C6F5)4). 11B NMR
(24 °C, 160.61 MHz, BrC6D5): δ -15.65. Elemental analysis was
not obtained due to the thermal instability of the complex.
Synthesis of the Complex [([ArNC(CH3)]2CH)VtCtBu-
(THF)][B(C6F5)4] (8-THF). In a vial was taken 4-THF (15 mg,
0.025 mmol), which was dissolved in 3 mL of toluene. The
toluene solution was added to an another vial containing
solid [NHMe2Ph][B(C6F5)4] (20.6 mg, 0.025 mmol). No color
change was observed immediately after the addition. The
mixture was stirred for 40 min, after which it was dried in vacuo.
The dried mass was washed with 3 mL of pentane and
dried again. Multinuclear NMR of the dried mass proved the
identity of the desired complex (25.6 mg, 0.020 mmol, 81%
yield).
100.6 MHz, CDCl3): δ 374.7 (VtCCMe3). 13C NMR (23 °C,
125.89 MHz, CDCl3): δ 167.53 (ArNC(Me)CHC(Me)NAr),
149.30 ((C6F5)4B), 147.23 (aryl), 146.92 ((C6F5)4B), 140.19
(aryl), 139.50 (aryl), 137.45 ((C6F5)4B), 135.02 ((C6F5)4B),
128.43 (aryl), 125.12 (aryl), 123.98 (aryl), 90.47 (ArNC(Me)-
CHC(Me)NAr), 65.86 (THF), 61.80 (VtCCMe3), 31.49
(CHMe2), 28.79 (CHMe2), 27.96 (Me), 26.48 (THF), 25.71
(Me), 24.55 (Me), 24.21 (Me), 24.11 (Me). One -Me resonance
is buried under other resonances. 19F NMR (25 °C, 282.32 MHz,
CDCl3): -133.03 (B(C6F5)4), -163.43 (B(C6F5)4), -167.31 (B-
(C6F5)4). 51V NMR (24 °C, 131.58 MHz, CDCl3): δ 548 (Δν1/2
= 1966 Hz). 11B NMR (24 °C, 160.61 MHz, CDCl3): δ -15.63.
Multiple attempts to obtain satisfactory elemental analysis
failed. The ether analogue, 8-OEt2, was prepared similarly to
8-THF, except substituting THF for Et2O.
Acknowledgment. We thank Indiana University-
Bloomington, the Dreyfus Foundation, the Sloan Foun-
dation, and the NSF (CHE-0348941) for financial sup-
port of this research.
1H NMR (23 °C, 400.11 MHz, CDCl3): δ 7.27-7.24 (m, 6H,
C6H3), 4.82 (s, 1H, ArNC(Me)CHC(Me)NAr), 2.97 (septet, 2H,
CHMe2), 2.59 (septet, 2H, CHMe2), 2.21 (br, 4H, THF), 1.84 (s,
6H, ArNC(Me)CHC(Me)NAr), 1.49 (d, 6H, CHMe2), 1.24 (d,
6H, CHMe2), 1.13 (d, 6H, CHMe2), 1.04 (d, 6H, CHMe2),0.73
(br, 4H, THF), 0.05 (s, 9H, VtCCMe3). 13C NMR (-55 °C,
Supporting Information Available: Complete crystallographic
data (4 OEt2, 5, 6, and 8-OEt2) and synthetic and spectroscopic
details (including multinuclear NMR spectra for complexes 4-
8) for new compounds are available free of charge via the
Internet at http://pubs.acs.org.
3