Coordination of ethene and propene to a cationic d0 vanadium center

Full text of DOI:10.1021/ja972219s

J. Am. Chem. Soc. 1997, 119, 10561-10562
10561
Coordination of Ethene and Propene to a Cationic
d0 Vanadium Center
Peter T. Witte, Auke Meetsma, and Bart Hessen*
Center for Catalytic Olefin Polymerization
Department of Chemistry, UniVersity of Groningen
Nijenborgh 4, 9747 AG Groningen, The Netherlands
Peter H. M. Budzelaar
Department of Inorganic Chemistry
UniVersity of Nijmegen
ToernooiVeld 1, 6525 ED Nijmegen, The Netherlands
Figure 1. Molecular structure of 2. Selected bond distances (Å) and
angles (deg): V-N(1) ) 1.854(2), V-N(2) ) 1.656(2), V-C(15) )
2.103(3), V-N(2)-C(11) ) 175.61(18), N(1)-V-C(15) ) 96.10(11),
N(1)-V-N(2) ) 105.78(10), N(2)-V-C(15) ) 96.87(12), V-N(1)-
C(7) ) 120.39(17), V-N(1)-C(8) ) 125.09(18).
ReceiVed July 7, 1997
The bonding of olefins to transition metals via the olefin
π-system is usually described in terms of (olefin)π-(M)d
σ-donation and (M)d-(olefin)π* π-backdonation.1 When the
metal center has a d0 configuration the π-backdonation contribu-
tion to the bonding is lacking, and the metal-olefin interaction
is expected to be relatively weak. Olefin adducts of d0-metal
centers are proposed as reaction intermediates in the catalytic
olefin polymerization by electrophilic cationic group 4 metal
alkyl species.2 Although circumstantial evidence on the interac-
tion of olefins with d0-metal centers has been present for some
time,3 actual observation of the olefin adducts has been limited
to one example of a cycloheptene adduct of a cationic W-
alkylidene,4 and two types of adducts (for Zr5 and Y6 ) in which
the olefin is held in proximity of the metal center by a covalent
tether. Here we report the generation and NMR-spectroscopic
characterization of adducts of simple olefins (ethene, propene)
with a cationic d0 vanadium(V) metal center, {[η5, η1-C5H4-
(CH2)2Ni-Pr]V(Nt-Bu)(η-olefin)}+, as well as quantumchemical
calculations on these new species.
Scheme 1
Scheme 2
The vanadium(V) complex [η5, η1-C5H4(CH2)2Ni-Pr]V(Nt-
Bu)Cl (1), with a linked Cp-amido ancillary ligand, was obtained
by reaction of C5H5(CH2)2NHi-Pr7 with the vanadium imido
complex (tBuN)V(NMe2)2Cl8 through dimethylamine elimina-
tion (Scheme 1). Reaction of 1 with MeLi yields the corre-
sponding methyl derivative [C5H4(CH2)2Ni-Pr]V(Nt-Bu)Me (2),
characterized by single crystal X-ray diffraction (Figure 1).
Reaction of the methyl complex 2 with the Lewis-acidic
borane B(C6F5)3 produces the ionic species [C5H4(CH2)2Ni-
Pr]V(Nt-Bu)[MeB(C6F5)3] (3), which was obtained analytically
pure. In C6D6 solvent 3 is present as a contact ion pair with
substantial interaction between the methyl group of the anion
and the cationic metal center.9 In C6D5Br solvent the anion in
3 is displaced to give a solvated cationic species. In this
solvated species and in Lewis-base adducts [C5H4(CH2)2Ni-
Pr]V(Nt-Bu)(L)+ (L ) THF, PMe3) the exchange between
coordinated base and excess free base is slow on the NMR
timescale. The electronic and steric properties of the system
may favor a dissociative rather than an associative displacement
mechanism.
Upon addition of ethene or propene to solutions of 3 in C6D5-
Br at ambient temperature an equilibrium between the solvated
cation of 3 and olefin adduct species is observed by NMR
spectroscopy (Scheme 2). The nature of the ethene adduct 4a
is apparent from its NMR spectroscopic features.11 The
chemical shifts and coupling constants of the coordinated ethene
(1H-NMR AA′BB′, consistent with rapid rotation around the
ethene-metal bond, δ 4.33, 4.72 ppm, 13C-NMR δ 103.2 ppm,
(1) Mingos, D. M. P. ComprehensiVe Organometallic Chemistry; Wilkin-
son, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol.
3, p 47 and references cited therein.
(2) For leading references on the role of cationic alkyl species in olefin
polymerization and metal-olefin π-interactions in these systems, see: (a)
Jordan, R. F. AdV. Organomet. Chem. 1991, 32, 325. (b) Marks, T. J. Acc.
Chem. Res. 1992, 25, 57. (c) Woo, T. K.; Fan, L.; Ziegler, T. Organome-
tallics 1994, 13, 2252. (d) Weiss, H.; Ehrig, M.; Ahlrichs, R. J. Am. Chem.
Soc. 1994, 116, 4919. (e) Yoshida, T.; Koga, N.; Morokuma, K. Organo-
metallics 1995, 14, 746.
(3) (a) Ballard, D. H. G.; Burnham, D. R.; Twose, D. L. J. Catal. 1976,
44, 116. (b) Nolan, S. P.; Marks, T. J. J. Am. Chem. Soc. 1989, 111, 8538.
(4) Kress, J.; Osborn, J. A. Angew. Chem., Int. Ed. Engl. 1992, 31, 1585.
(5) Wu, Z.; Jordan, R. F.; Petersen, J. L. J. Am. Chem. Soc. 1995, 117,
5867.
(6) Casey, C. P.; Hallenbeck, S. L.; Pollock, D. W.; Landis, C. R. J.
Am. Chem. Soc. 1995, 117, 9770.
(9) A good indication for interaction of the MeB(C6F5)3 anion with the
cationic metal center is found in the 19F NMR data, where a ∆δ(pF-mF)
value larger than 3 ppm is indicative of significant interaction (ref 10). For
3: (C6D6) ∆δ ) 4.6 ppm; (C6D5Br) ∆δ ) 2.4 ppm.
(10) Horton, A. D. Organometallics 1996, 15, 2675.
(11) For 4a: 1H NMR (500 MHz, C6D5Br, -30 °C) δ 5.71, 5.61, 5.27,
5.02 (m, Cp), 5.34 (sept, 6.4 Hz, i-Pr CH), 4.72, 4.33 (m, dCH2), 4.61,
3.26 (m, NCH2), 2.70, 1.91 (m, CpCH2), 1.19 (br, BMe), 0.94 (t-Bu), 0.82
(d, 6.4 Hz, i-Pr Me), 0.59 (d, 6.7 Hz, i-Pr Me); 13C NMR (125.7 MHz,
C6D5Br, -30 °C): δ 149.3 (d, JCF ) 241 Hz, o-CF), 141.9 (Cp C), 138.3
(d, JCF ) 244 Hz, p-CF), 137.3 (d, JCF ) 248 Hz, m-C6F5), 109.6, 109.2,
103.1, 101.3 (4 CH of Cp), 103.2 (d, JCH ) 164 Hz, dCH2), 76.3 (CH of
i-Pr), 73.3 (NCH2), 29.5 (CpCH2), 31.1 (CH3 of t-Bu), 22.6, 20.7 (2 CH3
of i-Pr), 11.8 (br, ∆ν1/2 ) 75 Hz, B-Me), Cquat of t-Bu not observed.
(7) (a) Sinnema, P. J.; Liekelema, K.; Staal, O. K. B.; Hessen, B.; Teuben,
J. H. J. Mol. Catal. 1997, in press. (b) Hughes, A. K.; Meetsma, A.; Teuben,
J. H. Organometallics 1993, 12, 1936.
(8) Prepared by a comproportionation reaction between Cl3V(N-t-Bu)
and (Me2N)3V(N-t-Bu).
S0002-7863(97)02219-1 CCC: $14.00 © 1997 American Chemical Society

10562 J. Am. Chem. Soc., Vol. 119, No. 43, 1997
Communications to the Editor
JCH ) 164.1 Hz) relative to free ethene (1H δ 5.29 ppm, 13C δ
123.9 ppm, JCH ) 159.6 Hz) distinguish 4a readily from a
possible aza-metallacyclic structure, like the one found for (t-
Bu3SiNH)(t-Bu3SiN)V[CH2CH2N(Si-t-Bu3)] with 1H δ 3.22
ppm, 13C δ 48.3 ppm, JCH ) 149 Hz.12 The propene adduct
4b is formed as two isomers in nearly equimolar amounts,
probably corresponding to two orientations of the methyl
substituent. Comparison of the NMR characteristics13 with those
of free propene shows that the -CH) carbon resonance is shifted
moderately downfield (+ 2.5 ppm), the )CH2 carbon signifi-
cantly upfield (-24 ppm), while the C-H and H-H coupling
constants are relatively little affected. These changes correspond
quite well with those observed for the structurally characterized
‘tethered’ d0-metal olefin adduct [Cp2Zr(OCMe2CH2CH2-
CH)CH2)]+, where a substantial amount of olefin polarization
was suggested.5
Figure 2. Optimized calculated geometry for [C5H4(CH2)2NH]V(NH)-
(C2H4)+. Selected distances (Å) and angles (deg): V-C(9) ) 2.43,
V-C(10) ) 2.54, C(9)-C(10) ) 1.36, V-N(1) ) 1.82, V-N(2) )
1.64, V-N(2)-H(11) ) 171.1, N(1)-V-N(2) ) 103.9, V-N(1)-
C(7) ) 125.6, V-N(1)-H(8) ) 120.3.
The olefin complexation is fully reversible, and 4 reverts to
solvated 3 upon pumping off the olefin. As observed with other
adducts in this system, the exchange between coordinated and
free olefin is remarkably slow. An increase in temperature shifts
metallic fragment needed to accomodate the ethene. A strong
pyramidality combined with a relatively strong olefin binding
energy was also found in DFT calculations on the ‘constrained
geometry’ olefin polymerisation catalyst model system, [C5H4-
1
the equilibrium more towards 3, but a broadening of the H-
NMR signals of the coordinated ethene is only seen at around
60 °C, suggesting a relatively strong bonding of the coordinated
olefin.
+
(SiH2)NH]ZrCH3 when compared to metallocene-type
systems.2c,15
Quantumchemical calculations (DFT/B3LYP)14 were per-
formed on the model system [C5H4(CH2)2NH]V(NH)(C2H4)+
of which the optimized geometry is shown in Figure 2. In the
lowest energy conformation the olefin is oriented parallel to
the metal-imido bond (although the orientation parallel to the
metal-amido bond is only 1.2 kcal mol-1 higher in energy). The
ethene molecule is bound asymmetrically, with V-C(ethene)
distances of 2.43 and 2.54 Å, d(C-C) ) 1.36 Å (free ethene
1.33 Å), and the closest C...N(imido) distance 2.63 Å. The
‘naked’ [C5H4(CH2)2NH]V(NH)+-fragment was found to be
nearly as pyramidal as in the ethene adduct, and the calculated
ethene bonding energy remarkably large (31 kcal mol-1). This
may be due to the small reorganisation energy of the organo-
The two remarkable features of the cationic (Cp-amido)V-
(imido)(olefin) system studied are a) the slow exchange between
coordinated and free olefin, and b) the reluctance to form a
vanada-aza-cyclobutene type product. Both features are likely
to stem from the ‘constrained geometry’ imposed by the linked
Cp-amido ancillary ligand. The strong bonding of the olefin
seems to be related to the preference of the (Cp-amide)V(imido)-
cation for a pyramidally distorted structure. The lack of
observation of the metalla-azacycle may have thermodynamic
rather than kinetic reasons. In the ‘unconstrained’ (imido)2V-
(amido) system12 the metallacycle is observed, but with rapid
reversibility even at low temperatures. Due to the Cp-amido
link in 4, the Cp(centroid)-V-N(amido) angle may not be able
to open up sufficiently to compensate for the expected small
C-V-N angle in the metalla-aza-cyclobutane, shifting the equi-
librium towards the olefin adduct.
(12) de With, J.; Horton, A. D. Organometallics 1993, 12, 1493. For
related systems, see: Walsh, P. J.; Hollander, F. J.; Bergman, R. G.
Organometallics 1993, 12, 3705 Bennett, J. L ; Wolczanski, P. T. J. Am.
Chem. Soc. 1994, 116, 2179.
(13) For 4b (C6D5Br, -30 °C) coordinated propene, two isomers 1H
Acknowledgment. This investigation was supported by the Neth-
erlands Foundation for Chemical Research (SON) with financial aid
from the Netherlands Organization for Scientific Research (NWO).
3
NMR δ 4.15, 4.01 and 3.65, 4.33 (CH2), 6.02 and 6.27 (CH), JHH ) 8.3
and 8.3 Hz (cis), 17.3 and 16.8 Hz (trans); 13C NMR δ 92.7 and 92.5 (CH2,
JCH ) 157.1 and 158.6 Hz), 137.7 and 135.4 (CH, JCH ) 158.5 and 158.6
Hz). For free propene (C6D5Br, -30 °C) 1H NMR δ 4.94, 5.00 (CH2),
Supporting Information Available: Experimental details and
spectroscopic and analytical data of the reported compounds; Details
of the structure determination of 2 including lists of positional and
thermal parameters and bond distances and angles; details of the
calculations and optimized Z-matrices for [C5H4(CH2)2NH]V(NH)+ and
its ethene adduct (2 conformations) (28 pages). See any current
masthead page for ordering and Internet access instructions.
3
5.71 (CH), JHH ) 9.9 Hz (cis), 17.0 Hz (trans); 13C NMR δ 116.8 (CH2,
JCH ) 155.8 Hz), 134.4 (CH, JCH ) 152.7 Hz).
(14) All calculations performed using the Gaussian 94 package (Gaussian
94, Revision E.1; Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M.
W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson,
G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Lahan, M. A.; Zakrewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Reploge, E. S.; Gomperts, R.; Martin, R. L.;
Fox, D. J.; Binkley, J. S.; DeFrees, D. J.; Baker, J.; Stewart, J. P.; Head-
Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian, Inc.: Pittsburgh, PA,
1995) using the B3LYP functional (Becke, A. D. J. Chem. Phys. 1993, 98,
5648).
JA972219S
(15) (a) Fan, L.; Harrison, D.; Woo, T. K.; Ziegler, T. Organometallics
1995, 14, 2018. (b) Woo, T. K.; Margl, P. M.; Lohrenz, J. C. W.; Blo¨chl,
P. E.; Ziegler, T. J. Am. Chem. Soc. 1996, 118, 13021.