Terminal vanadium-neopentylidyne complexes and intramolecular cross-metathesis reactions to generate azametalacyclohexatrienes

Article abstract of DOI:10.1021/ja0472376

Four-coordinate vanadium complexes containing a terminal neopentylidyne functionality have been prepared by two consecutive α-hydrogen abstraction reactions both of which were induced by one-electron oxidations. Among these vanadium-alkylidyne complexes are the neutral and the cation (Nacnac)V_≡C^tBu(OTf) and [(Nacnac)V_≡C^tBu(THF)]⁺, respectively (Nacnac- = [Ar]NC(CH₃)CHC(CH₃)N[Ar], Ar = 2,6-(CHMe₂)₂C₆H₃). The vanadium-alkylidynes have been characterized by ¹H, ¹³C, ⁵¹V NMR spectroscopy and single-crystal X-ray diffraction and are consistent with a short VC bond. These alkylidynes were found to transform to azametalacyclohexatriene systems via an intramolecular cross-metathesis reaction. Kinetic studies of the transformation of (Nacnac)V_≡C^tBu(OTf) in C₇D₈ reveal the formation of the azametalacyclohexatriene to be independent of solvent (toluene vs THF) and the reaction to be first order in vanadium (k = 3.30(5) × 10^-5 s^-1 at 80 °C, with activation parameters ΔH_?= 25.4(3) kcal/mol, ΔS_? = -6(3) cal/molK). High-level DFT calculations on the full model suggest an intramolecular mechanism invoking only one transition state. The overall thermodynamic driving force for the reaction (ΔG) in solution phase was estimated to be -21.3 kcal/mol. Copyright

Full text of DOI:10.1021/ja0472376

Published on Web 08/05/2004
Terminal Vanadium-Neopentylidyne Complexes and Intramolecular
Cross-Metathesis Reactions to Generate Azametalacyclohexatrienes
Falguni Basuli, Brad C. Bailey, Douglas Brown, John Tomaszewski, John C. Huffman,
Mu-Hyun Baik,* and Daniel J. Mindiola*
Department of Chemistry, School of Informatics and Molecular Structure Center, Indiana UniVersity,
Bloomington, Indiana 47405
Received May 11, 2004; E-mail: mbaik@indiana.edu; mindiola@indiana.edu
High-oxidation state complexes containing alkylidyne function-
alities continue to attract much attention due to their involvement
in important reactions such as alkyne metathesis and CR group-
transfer.1 Prototypical among the immense family of group 5 and
6 early-transition metal alkylidynes is Schrock’s (tBuO)3WtC-
tBu,2 an active alkyne metathesis catalyst that is prepared by two
routes. One of these routes involves two consecutive R-hydrogen
abstraction reactions,1b,2a while the other is the direct rupture of
nitriles/alkynes with Chisholm’s (tBuO)3WtW(OtBu)3.1a,2b,c,3 Al-
though d0 early-transition metal complexes with the terminal
alkylidyne functionality are well-known, the vast majority of these
systems contain second and third row transition metals,1 and only a
handful of Fischer-type carbynes have been reported for V4 and Cr.1
Our interest in d0-metal complexes with metal-carbon multiple
bonds5 triggered the pursuit for the thus far unknown vanadium-
alkylidyne functionality. Herein, we report a new class of vana-
dium(V) complexes having a terminal neopentylidyne that were
prepared systematically by two consecutive R-hydrogen abstrac-
tions, each induced by one-electron oxidations. These complexes
can engage in intramolecular cross-metathesis reactions to afford
unusual azametalacyclohexatriene products. The energy profile of
this reaction has been examined in detail using DFT calculations.
Previously, we showed that one-electron oxidation of (Nacnac)V-
both reactions.6 The molecular structure for 2-OTf and [2-THF]-
[BPh4] reveals a highly distorted tetrahedral vanadium center with
a terminal alkylidyne ligand (Figure 1). In the crystal structures
for each compound the short V-C bond length is consistent with
a metal-ligand triple bond (2-OTf, 1.674(2) Å; [2-THF][BPh4],
1.696(3) Å). These values are clearly shorter than the average for
neutral and cationic four-coordinate vanadium-neopentylidene
complexes (VdC ≈ 1.79 Å, vide supra),5b or for the Fischer-
carbyne complex (CO)(dmpe)2VtCOSiPh3 (VtC, 1.754(8) Å).4
In addition, the sp-hybridization of the alkylidyne carbon is evident
from the linear V-CR-Câ angles (2-OTf, 177.6(9)°; [2-THF]-
[BPh4], 175.8(3)°). DFT calculations of 2-OTf indicate a Mayer-
bond order of 2.4, which lends additional support for the assignment
of the triple bond.6
Although stable as solids, complexes 2-OTf and [2-THF][BPh4]
transform slowly in solution to the vanadium-imido complex
supported by the chelating amido-vinyl ligand, (tBuCdC(Me)CHC-
(Me)N[Ar])VdNAr(OTf) (3)-OTf and [(tBuCdC(Me)CHC(Me)N-
[Ar])VdNAr(THF)][BPh4] [3-THF][BPh4], as evidenced by 1H and
13C NMR spectroscopy (85 and 81% isolated yield, respectively,
Figure 1).6 Complexes 3-OTf and [3-THF][BPh4] have been fully
characterized6 and are best described as an azametalacyclohexatriene
system resulting from a cross-metathesis transformation. The VNC4
metalacycle in the structure of 3-OTf is far from being planar (V
deviation from the NC4 plane is 1.21 Å).8 This feature also places
the metal center in contact with the â-carbons (C(3), 2.486(6) (Å);
C(5), 2.387(7) Å). Thus, the term azametalabenzene seems inap-
propriate for the vanadium systems reported here.
t
(CH2 Bu)2 (Nacnac-) [Ar]NC(CH3)CHC(CH3)N[Ar], Ar ) 2,6-
(CHMe2)2C6H3) with AgBPh4 followed by nucleophilic addition
of MgI2 promoted R-hydrogen abstraction to form the four-
coordinate neopentylidene complex (Nacnac)VdCHtBu(I) (Figure
1).5b Inspired by this observation and realizing that vanadium-
alkylidyne complexes are unknown, we alkylated (Nacnac)Vd
CHtBu(I) with LiCH2SiMe3 to form the neopentylidene-alkyl
species (Nacnac)VdCHtBu(CH2SiMe3) (1) in 72% yield after
recrystallization from pentane at -35 °C (Figure 1).6 At room
temperature, complex 1 exhibits a solution magnetic moment of
1.90 µΒ, and EPR spectra are in accordance with a V(IV) center.6
Single-crystal X-ray diffraction studies confirmed the proposed
connectivity and reveal a short VdC bond length (1.791(6) Å),
and an obtuse VdC-CtBu angle of 163.1(4)° signifying R-agostic
interaction of the VdCR-HR with the vanadium center (Figure 1).6
One-electron oxidation of 1 with AgOTf or AgBPh4 yields the
neutral (Nacnac)VtCtBu(OTf) (2)-OTf or cationic [(Nacnac)VtC-
tBu(THF)][BPh4] [2-THF][BPh4] four-coordinate alkylidyne com-
plexes in 59 and 65% yield, respectively (Figure 1). 1H NMR
spectra are consistent with (2)-OTf and [2-THF]+ retaining Cs
symmetry in solution, while the combination of 13C (δ:2-OTf, 375;
[2-THF][BPh4], 374)7 and 51V (δ: 2-OTf, -882; [2-THF][BPh4],
-956) NMR spectra suggests that both systems contain the terminal
vanadium(V)-neopentylidyne functionality.6
The reaction of 2-OTff3-OTf in C7D8 was determined to be
first order in vanadium with k ) 3.30(5) × 10-5 s-1 @ 74 °C.
Temperature dependence studies from 56 to 91 °C for the
2-OTff3-OTf transformation allowed for extraction of the activa-
tion parameters ∆Sq ) -6(3) cal/mol‚K-1, ∆Hq ) 25.4(3) kcal/
mol from the Eyring plot.6 In addition, the rate of formation of
3-OTf from 2-OTf was found to be independent of solvent (C7D8
vs THF-d8), suggesting no involvement of dissociative or associative
mechanisms.6
High-level DFT calculations of the 2-OTff3-OTf reaction also
support an intramolecular rearrangement invoking an azametala-
cyclobutene ring in the transition state 2-TS (Figure 2). The
computed activation parameters match the experimental results
q
q
(∆Scalc ) -10.3 cal/mol‚K-1, ∆Hcalc ) 28.8 kcal/mol) quite well.
In the reaction coordinate of 2-OTff3-OTf the cross-metathesis
reaction is thermodynamically downhill by 21.3 kcal/mol (∆G, 298
K). Calculations indicate no stable intermediate along the reaction
coordinate, suggesting a fast and smooth reaction to yield product
3-OTf once 2-TS is traversed (Figure 2).6 As illustrated in Figure
2, the triflate ligand adopts a bidentate coordination geometry in
To confirm the proposed connectivity for each vanadium-
alkylidyne we collected single-crystal X-ray diffraction data from
9
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J. AM. CHEM. SOC. 2004, 126, 10506-10507
10.1021/ja0472376 CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. Synthesis of neutral and cationic four-coordinate vanadium(IV) alkylidyne complexes as well as the thermolysis product of each. Only the core
structures of 1, 2-OTf, [2-THF]+, and 3-OTf, are depicted with 50% thermal ellipsoid plots. Hydrogen atoms with the exception of R-H, aryl groups with
the exception of ipso carbons, and SO2CF3 and carbon atoms on O(38) have been excluded for clarity.
continue to inspire organometallic chemists. We thank Indiana
University-Bloomington, the Camille and Henry Dreyfus Founda-
tion, and the National Science Foundation (0116050 to Indiana
University and CHE-0348941 to D.J.M.) for financial support.
D.J.M. thanks Dr. Allen Siedle for insightful discussions.
Supporting Information Available: Complete experimental prepa-
ration, computational, and crystallographic data for compounds 1-3.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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Figure 2. Calculated reaction energy profile for the conversion of 2-OTf
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to 3-OTf. Only the core structure is illustrated for clarity, all energies are
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activation parameters are given in brackets.
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transformation.
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(6) See Supporting Information for complete experimental, crystallographic,
and DFT calculation details.
In summary, we have demonstrated that the (Nacnac)V frame-
work is capable of stabilizing reactive vanadium-carbon double
and triple bonds. Interestingly, our strategy to prepare d0 metal
alkylidynes from vanadium-alkylidenes contrasts Schrock’s two-
electron reduction reactions of high-valent alkylidenes to prepare
TatC linkages (referred to as R-H elimination or 1,2-H migration).9
(7) Lowering the temperature of an NMR solution containing complexes 2,3
(-50 °C) increases the quadrupolar relaxation rate of the vanadium nucleus
7
(
51V, I ) /2, 99.6% abundant) and results in the effective decoupling of
the 51V scalar coupling from the 13C nucleus: Claridge, T. D. W. High-
Resolution NMR techniques in Organic Chemistry; Pergamon: Amster-
dam, 1999; pp 40-44.
t
In the presence of 1 equiv of LiCH2 Bu complex 2-OTf readily
(8) For an example of a planar metalapyridine system of group 5, see: Weller,
K. J.; Filippov, I.; Briggs, P. M.; Wigley, D. E. Organometallics 1998,
17, 322-329 and references therein.
polymerizes HCtCPh to afford Mn in excess of ∼7000. More
detailed studies of the polymerization reaction are currently in
progress in our laboratory.
Acknowledgment. This communication is dedicated to the
memory of Professor Ian P. Rothwell, whose work has and will
(9) Fellmann, J. D.; Tuner, H. W.; Schrock, R. R. J. Am. Chem. Soc. 1980,
102, 6608-6609.
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