Novel μ-methyl complexes of vanadium and their relevance to bimolecular deactivation of homogeneous imidovanadium polymerisation catalysts

Article abstract of DOI:10.1039/a705256a

Attempted dimethylation of [CpV(NC₆H₃Prⁱ₂-2,6)]Cl₂ using Grignard reagents affords novel μ-methyl complexes arising by reductive dimerisation; the structure of one of these products, [CpV(NC₆

Full text of DOI:10.1039/a705256a

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Novel m-methyl complexes of vanadium and their relevance to bimolecular
deactivation of homogeneous imidovanadium polymerisation catalysts
Michael C. W. Chan,a Jacqueline M. Cole,b Vernon C. Gibson*a and Judith A. K. Howardb
a Department of Chemistry, Imperial College, South Kensington, London, UK SW7 2AY
b Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE
Attempted dimethylation of [CpV(NC6H3Pri2-2,6)]Cl2 using
Grignard reagents affords novel m-methyl complexes arising
by reductive dimerisation; the structure of one of these
products, [CpV(NC6H3Pri2-2,6)(m-Me)2]2(m-Mg) 2, reveals
V–(m-Me)–Mg, V–Mg, N–Mg and C–H···Mg interactions.
The paramagnetic magnesium-containing divanadium com-
plex 2 was isolated from the reaction of 1 with excess MeMgBr
in diethyl ether (Scheme 1). The molecular structure is shown in
Fig. 1.‡ Complex 2 crystallises as a trimetallic V2Mg cluster in
the centrosymmetric space group P21/c, with the magnesium
atom located at the centre of inversion. The structure consists of
two methyl groups bridging between the vanadium atom of the
[CpV(NAr)] fragment and the magnesium core. The V–Mg
distance of 2.594(1) Å is similar to other proposed metal–Mg
bonds11,12 and therefore may be viewed as a direct V–Mg
interaction. The N–Mg distance of 2.366(6) Å is within the
range for a genuine bonding interaction,13 although the V–N
distance of 1.724(6) Å (1.60–1.68 Å in terminal imides,14
1.84–1.85 Å in bridging imides3a,8,15) and the V–N–Cipso angle
of 160.6(5)° suggest that this interaction is weak and that the
imido unit behaves more as a terminal ligand to the vanadium
centre than a bridging group between V and Mg. The carbon
atoms of the bridging methyl groups are located slightly closer
to the vanadium [V–Cmethyl 2.200(7), 2.227(7) Å; Mg–Cmethyl
2.382(7), 2.408(7) Å] and the V–Cmethyl distances are longer
than those reported for vanadium complexes with terminal alkyl
groups (2.04–2.08 Å).3,4,8,16 The two V–Cmethyl–Mg angles are
identical [68.4(2)°] and are typical for methyl groups bridging
two metals.11,16 The two highly distorted V–C–H angles (55,
77°) and the particularly short Mg–H separations (2.04, 2.35 Å)
for H(18c) and H(19b) respectively,‡ indicate the presence of
C–H···Mg agostic interactions [Fig. 1(b)]. The presence of the
There is presently much interest in the development of new-
generation ‘non-metallocene’ catalysts for the polymerisation
of a-alkenes. These include, for example, alkoxide,1 thiolate,2
amide,3 imide,4 tetradentate Schiff-base5 and macrocyclic
procatalysts.6 In recent work we have been investigating the
isolobal relationship between group 4 metallocenes and group 5
half-sandwich imido species7 and have found that vanadium
complexes of the type [CpV(NR)Cl2] are particularly active
amongst these group 5 metal pro-catalysts.4a Typically, they
give high initial polymerisation activities but die over the course
of a few minutes, a kinetic profile that is not uncommon in many
homogeneous Ziegler-type polymerisation systems. In order to
gain more insight into the deactivation process and to be able to
control this behaviour, we set out to synthesise well defined
cationic alkyl versions of these catalysts. Methyl derivatives
were targeted since such species have proved to be suitable
precursors in metallocene systems.8 Here, we describe two
highly unusual products arising from reactions of
[CpV(NAr)Cl2] (Ar = C6H3Me2-2,6) 1 with methylmagnesium
halides under differing conditions of solvent and Grignard
reagent. The products are structurally exceptional and provide
insight into the bimolecular deactivation process occurring in
[CpV(NR)] polymerisation systems.†
(a)
The synthesis of the starting complex 1 has been described
previously by Teuben and coworkers.9 During the course of our
studies, we have found that 1 and related imido complexes can
also be prepared by an imido exchange route according to
Scheme 1, a methodology that appears to be quite general.10
Thus treatment of [CpV(NBut)Cl2] with 2,6-diisopropylaniline
in hot 1,2-dichloroethane affords [CpV(NAr)Cl2] 1 in high
yield.
C(19)
V(1)
N(1)
C(18)
C(1)
C(1a)
C(18a)
Mg
N(1a)
V(1a)
C(19a)
i,
(b)
H(18a)
H(18b)
V(1)
Cl
Cl
Cl
Cl
C(18)
V
V
NH2
NH2
H(19c)
H(18c)
N
N
C(19)
H(19b)
–
N(1)
H(19d)
C(19a)
1
Mg
ii
iii
N(1a)
Me
Me
H(19e)
V(1a)
H(18d)
V
N
V
Mg
Me
H(18f)
H(18e)
C(18a)
N
Me
N
N
Fig. 1 (a) The molecular structure of 2 with key atoms labelled. H atoms are
omitted for clarity. Selected dimensions (Å and °): V(1)–N(1) 1.724(6),
V(1)–C(18) 2.200(7), V(1)–C(19) 2.227(7), V(1)–Mg 2.594(1), Mg–N(1)
2.366(6), Mg–C(18) 2.408(7), Mg–C(19) 2.382(7); N(1)–V–C(18) 100.5,
N(1)–V–C(19) 101.3(3), C(18)–V–V(19) 91.0(3), N(1)–V–Mg 62.7(2),
C(18)–V–Mg 59.6(2), C(19)–V–Mg 58.6(2), V(1)–N(1)–C(1) 160.6(5),
V(1)–C(18)–Mg 68.4(2), V(1)–C(19)–Mg 68.4(2); (b) view of the magne-
sium core showing the agostic C–H···Mg interactions.
Me
V
V
2
3
Scheme 1 Reagents and conditions: i, (CH2Cl)2, 75 °C, 10 d; ii, 2.5
MeMgBr, Et2O, 12 h; iii, 3 MeMgCl in thf–Et2O, 12 h
Chem. Commun., 1997
2345

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monochromated Mo-Ka X-radiation (l = 0.710 73 Å); no absorption
correction was made. Structure solved (by direct methods) and refined using
SHELXL-93. 4857 reflections were used in the full-matrix least-squares
refinement. Refinement of positional and anisotropic thermal parameters
for all non-H atoms (364 parameters) converged to R1[I > 2s(I)] = 0.0531
and wR2[I > 2s(I)] = 0.1476 [w = 1/(s2Fo2 + 0.0650P2 + 5.1588P)] with
S = 1.208. Hexane solvent H atoms were not modelled. CCDC 182/609.
C(35)
N(2)
C(23)
V(2)
N(1)
V(1)
C(11)
1 A. van den Linden, C. J. Schaverien, N. Meijboom, C. Ganter and
A. G. Orpen, J. Am. Chem. Soc., 1995, 117, 3008.
2 T. Miyatake, K. Mizunuma and M. Kakugo, Makromol. Chem.
Macromol. Symp., 1993, 66, 203.
3 For recent examples, see: (a) J. D. Scollard, D. H. McConville and
S. J. Rettig, Organometallics, 1997, 16, 1810; (b) F. G. N. Cloke,
T. J. Geldbach, P. B. Hitchcock and J. B. Love, J. Organomet. Chem.,
1996, 506, 343; (c) R. Baumann, W. M. Davis and R. R. Schrock, J. Am.
Chem. Soc., 1997, 119, 3830; (d) S. Tinkler, R. J. Deeth, D. J. Duncalf
and A. McCamley, Chem. Commun., 1996, 2623; (e) F. Ja¨ger,
H. W. Roesky, H. Dorn, S. Shah, M. Noltemeyer and H.-G. Schmidt,
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Chem. Ber., 1996, 129, 1087; (g) H. Mack and M. S. Eisen,
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4 (a) M. P. Coles and V. C. Gibson, Polym. Bull., 1994, 33, 529; (b)
M. P. Coles, C. I. Dalby, V. C. Gibson, W. Clegg and M. R. J. Elsegood,
J. Chem. Soc., Chem. Commun., 1995, 1709.
Fig. 2 The molecular structure of 3 with key atoms labelled. H atoms are
omitted for clarity. Selected dimensions (Å and °): V(1)–N(1) 1.867(3),
V(1)–N(2) 1.882(3), V(2)–N(1) 1.896(3), V(2)–N(2) 1.894(3), V(1)–C(35)
2.215(4), V(2)–C(35) 2.310(4), V(1)–V(2) 2.324(1); V(1)–C(35)–V(2)
61.8(1), V(1)–N(1)–V(2) 76.3(1), V(1)–N(2)–V(2) 76.9(1), V(1)–N(1)–
C(11) 143.8(2), V(2)–N(1)–C(11) 139.9(2), V(1)–N(2)–C(23) 142.4(2),
V(2)–N(2)–C(23) 141.6(2).
magnesium core with its attendant weak ligand interactions
(bonding distances 2.366–2.594 Å) appears to lend significant
stability to this unusual product.
By contrast, in the presence of diethyl ether and tetrahydro-
furan, 1 reacts with an excess of MeMgCl to afford the
paramagnetic mono m-methyl complex 3 (Scheme 1, Fig. 2).
The crystal structure of 3‡ consists of a single methyl and two
arylimido groups bridging between two [CpV] units. The
distances V(1)–C(35) [2.310(4) Å], V(2)–C(35) [2.215(4) Å]
and the V(1)–C(35)–V(2) angle [61.8(1)°] are comparable with
2. In this case, however, there are no close interactions of the
bridging methyl C–H bonds with the adjacent metal centres.
The V–N distances [av. 1.885(4) Å] are longer than for 2 owing
to the decreased bond order resulting from the fact that the
imido substituents are bridging. The V(1)–V(2) distance of
2.324(1) Å is within the range for a bonding interaction,17 and
the V–Cp(centroid) vectors are almost coaxial with the V–V
bond. Finally, the bridging methyl and imides are arranged
around the V–V axis such that C(35), N(1) and N(2) are
approximately 120° to each other. We note that Teuben and
coworkers have reported a related tolylimido complex, [CpV(m-
NC6H4Me-p)Me]2, which contains bridging imido ligands and
terminal methyl groups.18
5 E. B. Tjaden, D. C. Swenson and R. F. Jordan, Organometallics, 1995,
14, 371.
6 H. Brand, J. A. Capriotti and J. Arnold, Organometallics, 1994, 13,
4469; R. Uhrhammer, D. G. Black, T. G. Gardner, J. D. Olsen and
R. F. Jordan, J. Am. Chem. Soc., 1993, 115, 8493.
7 D. N. Williams, J. P. Mitchell, A. D. Poole, U. Siemeling, W. Clegg,
D. C. R. Hockless, P. A. O’Neil and V. C. Gibson, J. Chem. Soc., Dalton
Trans., 1992, 739; J. K. Cockcroft, V. C. Gibson, J. A. K. Howard,
A. D. Poole, U. Siemeling and C. Wilson, J. Chem. Soc., Chem.
Commun., 1992, 1668; U. Siemeling and V. C. Gibson, J. Organomet.
Chem., 1992, 426, C25; P. W. Dyer, V. C. Gibson, J. A. K. Howard,
B. Whittle and C. Wilson, J. Chem. Soc., Chem. Commun., 1992,
1666.
8 See for example: R. F. Jordan, C. S. Bajgur, R. Willet and B. Scott,
J. Am. Chem. Soc., 1986, 108, 7410; G. C. Hlatky, H. W. Turner and
R. R. Eckman, J. Am. Chem. Soc., 1989, 111, 2728, J. A. Ewen and
M. J. Elder, Macromol. Chem., Macromol. Symp., 1993, 66, 179;
J. C. W. Chien, W. M. Tsai and M. D. Rausch, J. Am. Chem. Soc., 1991,
113, 8570; X. Yang, C. L. Stern and T. J. Marks, J. Am. Chem. Soc.,
1991, 113, 3623; 1994, 116, 10015.
9 J.-K. F. Buijink, A. Meetsma, J. H. Teuben, H. Kooijman and
A. L. Spek, J. Organomet. Chem., 1995, 497, 161.
10 For Ir: D. S. Glueck, J. Wu, F. J. Hollander and R. G. Bergman, J. Am.
Chem. Soc., 1991, 113, 2041. For Os: R. I. Michelman, R. G. Bergman
and R. A. Anderson, Organometallics, 1993, 12, 2741. For Mo:
P. W. Dyer, V. C. Gibson, E. L. Marshall, W. Clegg, M. J. Elsegood,
A. Bell, J. Chem. Soc., Chem. Commun., 1994, 2247; 2547. For Cr:
M. P. Coles, C. I. Dalby, V. C. Gibson, W. Clegg and M. R. J. Elsegood,
Polyhedron, 1995, 14, 2455. For Ti: P. E. Collier, S. C. Dunn,
P. Mountford, O. V. Shishkin and D. Swallow, J. Chem. Soc., Dalton
Trans., 1995, 3743.
Since dialkylation of a dihalide precursor is the prevalent
route to a well defined cationic alkyl catalyst species, these
observations provide an explanation for the rapid deactivation
of [CpV(NR)] procatalysts and are likely to be of general
relevance to other rapidly deactivating catalyst systems.
The EPSRC and the ILL are thanked for studentships to
M. C. W. C. and J. M. C. respectively and BP Chemicals Ltd. is
thanked for a CASE studnetship (to M. C. W. C.).
11 Ni–Mg 2.615 Å: W. Kaschube, K. R. Po¨rschke, K. Angermund,
C. Kru¨ger and G. Wilke, Chem. Ber., 1988, 121, 1921.
12 Co–Mg 2.565, 2.480 Å: K. Jonas, G. Koepe and C. Kru¨ger, Angew.
Chem., Int. Ed. Engl., 1986, 25, 9; Mo–Mg 2.853, 2.737 Å:
M. L. H. Green, G. A. Moser, I. Packer, F. Petit, R. A. Forder and K.
Prout, J. Chem. Soc., Chem. Commun., 1974, 839.
13 For example: H. Viebrock and E. Weiss, J. Org. Chem., 1994, 464,
121.
14 W. A. Nugent and J. M. Mayer, Metal–Ligand Multiple Bonds, Wiley
Interscience, New York, 1988.
15 G. A. Solan, P. G. Cozzi, C. Floriani, A. Chiesi-Villa and C. Rizzoli,
Organometallics, 1994, 13, 2572.
16 S. K. Noh, S. C. Sendlinger, C. Janiak and K. H. Theopold, J. Am. Chem.
Soc., 1989, 111, 9127; S. K. Noh, R. A. Heintz, S. C. Sendlinger,
C. Janiak and K. H. Theopold, Angew. Chem., Int. Ed. Engl., 1990, 29,
775.
17 F. A. Cotton and M. J. Millar, J. Am. Chem. Soc., 1977, 99, 7886;
H. Vahrenkamp, Chem. Ber., 1978, 111, 3472.
18 J.-F. K. Buijink, J. H. Teuben, H. Kooijman and A. L. Spek,
Organometallics, 1994, 13, 2922; J.-K. F. Buijink, A. Meetsma,
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497, 161.
Footnotes and References
* E-mail: v.gibson@ic.ac.uk
† Satisfactory elemental analyses have been obtained.
‡ Crystal data: 2: C19H28Mg0.5NV, Mr = 333.52, orange-red irregular
plate-shaped crystal (0.40 3 0.36 3 0.15 mm), monoclinic, space group,
P21/c, a = 10.220(2), b = 9.909(2), c = 18.658(3) Å, b = 104.94(1)°,
U
= = 4, T = 150(2) K, Dc = ,
1825.6(6) Å3, Z 1.213 g cm23
F(000) = 712, m = 0.556 mm21. 2184 unique reflections (2q < 45°) were
measured on a Siemens P4 diffractometer with graphite monochromated
Mo-Ka X-radiation (l = 0.710 73 Å); no absorption correction was made.
Structure solved (by Patterson methods) and refined using SHELXL-93.
2179 reflections were used in the full-matrix least-squares refinement.
Refinement of positional parameters for all non-H atoms and anisotropic
thermal parameters for V (91 parameters) converged to R1[I > 2s(I)]
= 0.0602 and wR2[I > 2s(I)] = 0.0912 [w = 1/(s2F02 + 0.0003P2) where
P = (Fo2 + 2Fc )/3] with S = 0.729. All bridging H atoms were found in
2
the difference map but not refined. 3: C35H47N2V2·0.5C6H12, Mr = 597.63,
deep red diamond-shaped crystal (0.80 3 0.62 3 0.40 mm), monoclinic,
space group, P21/n, a = 9.924(2), b = 21.653(4), c = 16.235(3) Å,
b = 98.73°, U = 3448(1) Å3, Z = 4, T = 150(2) K, Dc = 1.151 g cm23
,
F(000) = 1268, m = 0.565 mm21. 4865 unique reflections (2q < 46.5°)
were measured on a Siemens SMART-CCD diffractometer with graphite
Received in Basel, Switzerland, 22nd July 1997; 7/05256A
2346
Chem. Commun., 1997