Well-defined imidotitanium alkyl cations: Agostic interactions, migratory insertion vs. [2+2] cycloaddition, and the first structurally authenticated AlMe3 adduct of any transition metal alkyl cation

Article abstract of DOI:10.1039/b504567c

The imidotitanium alkyl cations [Ti(N^tBu)(Me₃[9] aneN₃)R]⁺ (R = Me (3⁺) or CH ₂SiMe₃ (4⁺)) possess either a very weak α-agostic or β-Si-C agostic interactions, respectively, according to ¹³C and ²⁹Si NMR and DFT studies; reaction of 4 ⁺ with ⁱPrNCNⁱPr gives totally selective insertion into the Ti-alkyl bond; reaction of 3⁺ with AlMe ₃ gives the first structurally characterised AlMe₃ adduct of a transition metal alkyl cation (Me₃[9]aneN₃ = 1,4,7-trimethyltriazacyclononane). The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b504567c

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Well-defined imidotitanium alkyl cations: agostic interactions,
migratory insertion vs. [2+2] cycloaddition, and the first structurally
authenticated AlMe3 adduct of any transition metal alkyl cation{
Paul D. Bolton,a Eric Clot,*b Andrew R. Cowleya and Philip Mountford*a
Received (in Cambridge, UK) 1st April 2005, Accepted 5th May 2005
First published as an Advance Article on the web 1st June 2005
DOI: 10.1039/b504567c
Reaction of [Ti(NtBu)(Me3[9]aneN3)R2] (R 5 Me (1) or
CH2SiMe3 (2)) with TB in C6D5Br gave the cations
[Ti(NtBu)(Me3[9]aneN3)R]+ (R 5 Me (3+) or CH2SiMe3 (4+))
whose NMR data suggest they exist as solvent separated ion
pairs.7{ The corresponding reaction of 1 with TB in CD2Cl2 led to
decomposition, but for 2 the formally 14 valence electron cation 4+
was again quantitatively formed and, remarkably, is stable for days
at room temperature in CD2Cl2. The reaction between 2 and TB
followed by Ph3PO gave the fully characterised adduct
[Ti(NtBu)(Me3[9]aneN3)(CH2SiMe3)(Ph3PO)][BArF4] (5-BArF4) in
66% isolated yield (Scheme 1).{
The imidotitanium alkyl cations [Ti(NtBu)(Me3[9]aneN3)R]+
(R 5 Me (3+) or CH2SiMe3 (4+)) possess either a very weak
a-agostic or b-Si–C agostic interactions, respectively, according
to 13C and 29Si NMR and DFT studies; reaction of 4+ with
iPrNCNiPr gives totally selective insertion into the Ti–alkyl
bond; reaction of 3+ with AlMe3 gives the first structurally
characterised AlMe3 adduct of a transition metal alkyl cation
(Me3[9]aneN3 5 1,4,7-trimethyltriazacyclononane).
Substantial effort has been made over the past 10–15 years to
develop early transition metal ‘post-metallocene’ Ziegler type olefin
polymerisation catalysts,1 and imido compounds (LnMLNR)2
have also been extensively studied in this regard.3 We recently
reported that certain macrocycle-supported imidotitanium dichlor-
ides [Ti(NR)(Me3[9]aneN3)Cl2] (I–R, Chart 1), isolobal with
Group 4 metallocene dichlorides II, form (for bulky alkyl groups
R) extremely active ethylene polymerisation catalysts with
methyl aluminoxane (MAO).4 Activation of the dialkyls
[Ti(NtBu)(Me3[9]aneN3)R2] (R 5 Me (1) or CH2SiMe3 (2)) with
[CPh3][BArF4] (TB, ArF 5 C6F5) also afforded very active species.
The catalyst systems I–tBu/MAO and 1/TB are the most active
imido-supported Ziegler catalysts described to date.
The formally 14 valence electron 3+ and 4+ are isolobal8
analogues of Group 4 metallocenium cations [Cp2MR]+, active
species in Ziegler–Natta olefin polymerisation.1,5 The electron-
deficient nature of 3+ and 4+ raises the possibility of agostic
interactions9 and so further NMR studies were carried out,
together with DFT calculations{ on the model dialkyl compounds
[Ti(NMe)(H3[9]aneN3)R2] (R 5 Me (1a) or CH2SiMe3 (2a)) and
cations [Ti(NMe)(H3[9]aneN3)R]+ (R 5 Me (3a+) or CH2SiMe3
(4a+)). Calculations on 3a+ found a very weak a-C–H agostic
interaction with one Ti–C–H angle of 105.9u and two of 115.7u and
113.8u (similar interactions have been found in metallocenium
methyl cations10) which resulted in a 5.5 Hz increase in the
calculated average 1JCH for the Ti–Me ligand (107.5 Hz) compared
to that in model dimethyl 1a (102 Hz). Constraining the Ti–Me
moiety in 3a+ to be non-agostic (only ca. 2 kJ mol21 less stable)
with equivalent Ti–C–H angles of 109.5u gave an average 1JCH that
was 11 Hz greater than that in 1a.11a The observed (C6D5Br) 1JCH
for 1 and 3+ of 111 and 116 Hz (D1JCH 5 5 Hz) are consistent with
the agostic structure computed by DFT.7 However, given the weak
nature of the a-agostic interaction this conclusion is only tentative
and calculations on extended systems are in progress. There are
few reports of calculations of spin–spin coupling constants within
ligands of transition metal complexes.11b The experience gained on
organic systems indicates that hybrid functionals (B3PW91 in the
calculations reported herein) are particularly efficient. Accuracies
Remarkably, despite the many detailed studies of metallocenium
and non-metallocenium alkyl cations (deemed to be the active
species in Ziegler polymerisation),1,5 very little is known about
catalytically-active imido-supported alkyl cations (imido com-
pounds themselves have been of enormous interest for over
20 years2). The only report to date is from Gibson who described
NMR evidence for the bis(imido)chromium benzyl cation
[Cr(NtBu)2(CH2Ph)]+ (III) and its PMe3 adduct.6 In this contribu-
tion we report our initial studies of well-defined imidotitanium
alkyl cations.
Chart 1
{ Electronic supplementary information (ESI) available: characterising
Scheme 1 Reactions of [Ti(NtBu)(Me3[9]aneN3)(CH2SiMe3)]+ (4+).
data and details of the DFT calculations; molecular structure of 6-BArF
See http://www.rsc.org/suppdata/cc/b5/b504567c/
.
4
2
[BArF
] anion is omitted. All reactions in CH2Cl2.
4
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of better than 10% (with respect to experimental values) are
considered to be excellent.11c,d
model substrate. The bulkier alkyl cation 4+ was chosen so that the
competing TiLNCMe3 and Ti–CH2SiMe3 reactive sites would be
as sterically equivalent as possible.
The observed 29Si NMR shifts (CD2Cl2) of d 21.8 and
20.7 ppm in the six-coordinate complexes 2 and 5+ are within
normal ranges but the high-field value of 215.9 ppm in 4+ is very
unusual. The DFT calculated structure of 4a+ (Fig. 1) shows a
Cation 4+ reacts quantitatively on an NMR tube scale to form
the migratory insertion product [Ti(NtBu)(Me3[9]aneN3)-
{Me3SiCH2C(NiPr)2}]+ (6+) which was isolated as 6-BArF in
4
31 % yield, Scheme 1).{ The molecular structure{{ of 6+
unambiguously confirms the site of attack of the carbodiimide
substrate and the formation of a trimethylsilylmethyl-substituted
diisopropylformamidinate moiety.
clear b-Si–C Ti interaction, and the calculated 29Si NMR shift of
…
d 217.6 ppm for 4a+ agrees well with that for the experimental
system, as does that for the neutral dialkyl 2a (calcd. 21.2 ppm vs.
21.8 ppm found for 2). Horton has observed upfield shifted 29Si
resonances in the isolated cationic zirconocene vinyl complex
Cationic alkyl complexes ‘‘[LnM–R]+’’ are generally accepted as
being the active species in Ziegler-type olefin polymerisation
catalysis. However, in systems activated by MAO (typically
containing up to 15 wt% ‘‘AlMe3’’) the catalyst resting state is
[Cp2Zr{C(LCMe2)SiMe3}]+ which features a b-Si–C Zr con-
…
tact,12a and b-Si–C interactions are well-known in neutral Group 3
complexes containing CH(SiMe3)2 and N(SiMe3)2 ligands.9c,12b
Imido-supported transition metal alkyl cations have at least two
potential sites for reaction with unsaturated substrates. Jensen and
Børve have suggested that ethylene addition to one of the CrLNR
bonds in catalytically relevant species akin to III can be preferred
over insertion into the Cr–alkyl bond,13 raising fundamental
questions of site selectivity and mechanism in imido-based Ziegler
catalysts (note that MLNR bonds for Ti and Zr in particular
undergo a wide range of cycloaddition reactions2). Legzdins et al.
have reported that neutral bis(imido)tungsten alkyls also react
preferentially at a WLNR bond.14 In an attempt to address these
issues we very recently undertook detailed experimental and DFT
studies of the reactions of the well-defined imidotungsten methyl
cation [W(N2Npy)(NPh)Me]+ IV (Chart 1) with unsaturated
probably
a
cationic bimetallic species of the type
[LnM(m-R)2AlR2]+.16 Such species are also important in chain
transfer (to aluminium) and catalyst deactivation. Although
cationic Group 4 (or later) bimetallic cations of this type have
been isolated and studied spectroscopically and computationally,
none has been structurally authenticated to date.17 We showed
previously4 that the presence of ‘‘AlMe3’’ is probably important in
chain transfer and molecular weight distributions in the catalyst
system I/MAO, and Gibson has correlated chain transfer to
aluminium with the presence of bimetallic complexes
[LnM(m-R)2AlR2]n+ as catalyst resting states.5c
substrates
(N2Npy
5
(2-NC5H4)C(Me)(CH2NSiMe3)2).15
(1)
Unfortunately, experimental tests of the DFT-predicted preference
for insertion into W–Me instead of cycloaddition to WLNPh were
frustrated by the reactivity of the Namide lone pairs of the
supporting ligand which offered an alternative, kinetically
favoured pathway.
The well-defined, metallocenium-like cations 3+ and 4+ provide a
unique opportunity for addressing experimentally the fundamental
question of site-selectivity in imido-supported Ziegler catalysts.
Since both Group 4 imido compounds [LnMLNR] and metal–
alkyl cations [LnM–R]+ are known to react with carbodiimides
RNCNR to form [2+2] cycloaddition2b,c or alkyl migratory
insertion products respectively, iPrNCNiPr was selected as a
Reaction of [Ti(NtBu)(Me3[9]aneN3)Me]+ (3+) with Al2Me6
(0.5 equiv., C6D5Br) quantitatively formed the heterobimetallic
cation [Ti(NtBu)(Me3[9]aneN3)(m-Me)2AlMe2]+ (7+); addition of
TB to a solution of 1 and Al2Me6 in CH2Cl2 afforded 7-BArF4 in
53 % isolated yield (eqn. 1). The molecular structure{{ (Fig. 2)
Fig. 2 Molecular structure of [Ti(NtBu)(Me3[9]aneN3)(m-Me)2AlMe2]+
Fig. 1 DFT(B3PW91) calculated structure of [Ti(NMe)(H3[9]-
aneN3)(CH2SiMe3)]+ 4a+. Ti–CH2 2.100; Ti C1 2.534; Si–C1 1.973;
(7+). Selected H atoms and [BArF 2 anion are omitted. H atoms shown as
]
4
…
spheres of arbitrary radius. Ti(1)–C(1) 2.344(2); Ti(1)–C(2) 2.335(2); Ti(1)–
…
N(1) 1.6978(16); Ti(1) H(3) 2.17(3), Ti(1) H(6) 2.17(3), Al(1)–C(1)
˚
2.081(2); Al(1)–C(2) 2.075(2); Al(1)–C(3) 1.963(3); Al(1)–C(4) 1.981(3) A.
…
˚
Si–C2 1.893; Si–C3 1.883 A; Ti–CH2–Si 90.7u, CH2–Si–C1 106.4u, CH2–
Si–C2 112.9u, CH2–Si–C3 114.6u.
3314 | Chem. Commun., 2005, 3313–3315
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Mw 5 1141.40, monoclinic, P 21/n, a 5 13.7837(2), b 5 19.5766(3),
shows the presence of pentacoordinate bridging methyl groups
3
˚
˚
c 5 18.6673(2) A, a 5 90, b 5 109.7558(7), c 5 90u, U 5 4740.67(11) A ,
Z 5 4, T 5 150 K, m 5 0.428 cm21, 7266 reflections I . 3s(I), Rint 5 0.047,
R 5 0.0359, Rw 5 0.0371. CCDC 268643 and 268644. See http://
www.rsc.org/suppdata/cc/b5/b504567c/ for crystallographic data in CIF or
other electronic format.
Ti(m-Me)Al and terminal Al–Me groups which are also clearly
1
distinguished in the H and 13C NMR spectra.{
The cation 7+ possesses an approximately tetrahedral Al centre
and an octahedral Ti (ignoring any bridging H atoms). The Ti–N
and terminal Al–C distances are within usual ranges and we will
focus our discussion on the central Ti(m-Me)2Al moiety, the point
1 V. C. Gibson and S. K. Spitzmesser, Chem. Rev., 2003, 103, 283.
2 Selected reviews: (a) D. E. Wigley, Prog. Inorg. Chem., 1994, 42, 239; (b)
L. H. Gade and P. Mountford, Coord. Chem. Rev., 2001, 216–217, 65;
(c) A. P. Duncan and R. G. Bergman, Chem. Rec., 2002, 2, 431.
3 P. D. Bolton and P. Mountford, Adv. Synth. Catal., 2005, 347, 355
(review).
4 N. Adams, H. J. Arts, P. D. Bolton, D. Cowell, S. R. Dubberley,
N. Friederichs, C. Grant, M. Kranenburg, A. J. Sealey, B. Wang,
P. J. Wilson, A. R. Cowley, P. Mountford and M. Schro¨der, Chem.
Commun., 2004, 434.
5 Metallocene systems: (a) H. H. Brintzinger, D. Fischer, R. Mu¨lhaupt,
B. Rieger and R. M. Waymouth, Angew. Chem., Int. Ed., 1995, 34,
1143; (b) M. Bochmann, J. Organomet. Chem., 2004, 689, 3982; (c) For
a recent overview of mechanism and leading references: G. J.
P. Britovsek, S. A. Cohen, V. C. Gibson and M. van Meurs, J. Am.
Chem. Soc., 2004, 126, 10701.
6 M. P. Coles, C. I. Dalby, V. C. Gibson, W. Clegg and M. R. J. Elsegood,
J. Chem. Soc., Chem. Commun., 1995, 1709.
7 Weak solvation by C6D5Br is possible. See the following and references
therein: F. Wu, A. K. Dash and R. F. Jordan, J. Am. Chem. Soc., 2004,
126, 15360.
8 D. S. Williams, M. H. Schofield and R. R. Schrock, Organometallics,
1993, 12, 4560; T. A. Albright, J. K. Burdett and M.-H. Whangbo,
Orbital Interactions in Chemistry; Wiley-Interscience, New York, 1985.
9 Recent reviews: (a) W. Scherer and G. S. McGrady, Angew. Chem., Int.
Ed., 2004, 43, 1782; (b) E. Clot and O. Eisenstein, Struct. Bonding
(Berlin), 2004, 113, 1; (c) G. I. Nikonov, J. Organomet. Chem., 2001,
635, 24.
˚
of most interest. As expected, the Ti–C distances (av. 2.339 A) are
significantly lengthened in comparison with those for the dimethyl
4
precursor 1 (av. 2.213 A). The bridging Al–Me distances are also
˚
˚
˚
longer (av. 2.078 A) than the terminal ones (av. 1.972 A). The H
atoms for the m-Me and Al–Me groups were located from a
Fourier difference map and refined positionally and isotropically.
Notwithstanding the inherent imprecisions concerning H atom
location using X-ray diffraction, the geometry at the two m-Me
ligands is much better described as the approximately square based
bipyramidal geometry found by neutron diffraction for the m-Me
ligands in Al2Me618 rather than the trigonal bipyramidal geometry
found for the Nd(m-Me)2Al groups in [Nd(AlMe4)3]17a (again by
neutron diffraction). There is no statistically significant lengthening
of individual C–H bonds of the bridging methyl groups, nor is the
average C–H distance significantly different between the terminal
1
and bridging ligands (in line with the measured JCH values
mentioned above). Furthermore, only one H atom per m-Me ligand
…
forms
a
close contact to titanium (Ti(1) H(3)
˚
5
Ti(1) H(6) 5 2.17(3) A) in contrast to the structures found for
…
neutral yttrium or lanthanide compounds with M(m-Me)2AlMe2
…
or related units which have two close M H contacts per m-Me
10 T. K. Woo, L. Fan and T. Ziegler, Organometallics, 1994, 13, 2252;
J. C. Green and C. N. Jardine, J. Chem. Soc., Dalton Trans., 2001, 274.
11 (a) The 1JCH for Ti–Me would be expected to increase with decreasing
electron density at Ti, regardless of the presence or otherwise of
a-agostic interactions. W. C. Finch, E. V. Anslyn and R. H. Grubbs,
J. Am. Chem. Soc., 1988, 110, 2406; (b) J. Autschbach, Struct. Bonding
(Berlin), 2004, 112, 1; (c) J. E. Peralta, G. E. Scuseria, J. R. Cheeseman
and M. J. Frisch, Chem. Phys. Lett., 2003, 375, 452; (d) O. B. Lutnaes,
T. A. Ruden and T. Helgaker, Magn. Reson. Chem., 2004, 42, S117.
12 (a) A. D. Horton and A. G. Orpen, Organometallics, 1991, 10, 3910; (b)
For leading references see: W. T. Klooster, L. Brammer, C. J. Schaverien
and P. H. M. Budzelaar, J. Am. Chem. Soc., 1999, 121, 1381; (c)
D. L. Clark, J. C. Gordon, P. J. Hay, R. L. Martin and R. Poli,
Organometallics, 2002, 21, 5000; (d) L. Perrin, L. Maron, O. Eisenstein
and M. F. Lappert, New J. Chem., 2003, 27, 121.
group (note again the neutron diffraction study of
[Nd(AlMe4)3]17a). DFT calculations on a model of 7+ (namely
[Ti(NMe)(H3[9]aneN3)(m-Me)2AlMe2]+) reproduced the experi-
mental structure very well, including the m-Me group H atom
positions and the geometry at these carbon atoms; an alternative
…
structure with two close M H contacts for one m-Me group was
found to be 10.7 kJ mol21 higher in energy and corresponds to the
transition state for H exchange within this bridging Me group. It
appears that the modeling of transition metal cations
[LnM(m-Me)2AlMe2]+ by neutral rare earth analogues is appro-
priate only to a first approximation, and that the geometry and
orientation of the m-methyl ligands can differ.19
13 V. R. Jensen and K. J. Børve, Chem. Commun., 2002, 543; V. R. Jensen
and K. J. Børve, Organometallics, 2001, 20, 616.
14 P. Legzdins, E. C. Phillips, S. J. Rettig, J. Trotter, J. E. Veltheer and
V. C. Yee, Organometallics, 1992, 11, 3104.
15 B. D. Ward, G. Orde, E. Clot, A. R. Cowley, L. H. Gade and
P. Mountford, Organometallics, 2005, 24, 2368.
We thank EPSRC, DSM Research BV, CNRS and British
Council for support. We acknowledge the use of the EPSRC
National Service for Computational Chemistry Software and the
UK Computational Chemistry Facilty.
16 (a) M. Bochmann and S. J. Lancaster, Angew. Chem., Int. Ed., 1994, 33,
1634; (b) For other examples and leading references see ref. 5 and
R. A. Petros and J. R. Norton, Organometallics, 2004, 23, 5105.
17 A number of rare earth alkyl AlR3 adducts have been structurally
characterised, but in most cases H atoms for the M(m-alkyl)2Al moiety
were not refined. For examples where H atom location is reported see:
(a) W. T. Klooster, R. S. Lu, R. Anwander, W. J. Evans, T. F. Koetzle
and R. Bau, Angew. Chem., Int. Ed., 1998, 37, 1268; (b) M. G. Klimpel,
R. Anwander, M. Tafipolsky and W. Scherer, Organometallics, 2001,
20, 3983.
Paul D. Bolton,a Eric Clot,*b Andrew R. Cowleya and
Philip Mountford*a
aChemistry Research Laboratory, University of Oxford, Mansfield
Road, Oxford, UK OX1 3TA
bLSDSMS (UMR 5636 CNRS), cc 14, Universite´ Montpellier 2, 34095
Montpellier cedex 5, France
Notes and references
{ Crystal data for 6-BArF4: C48H55BF20N6SiTi, Mw 5 1182.76, triclinic,
18 O. Yamamoto, J. Chem. Phys., 1975, 63, 2988.
+
…
¯
˚
P1, a 5 11.9619(2), b 5 12.7619(2), c 5 17.9840(2) A, a 5 75.5723(5),
19 A recent DFT study of [Cp2Zr(m-Me)2AlMe2] predicted one Zr H–C
unit per m-Me group, consistent with the metallocenium-like cation 7+
reported here: I. I. Zakharov and V. A. Zakharov, Macromol. Theory
Simul., 2002, 11, 352.
3
˚
b 5 80.1952(5), c 5 86.9238(5)u, U 5 2619.78(7) A , Z 5 2, T 5 150 K,
m 5 0.299 cm21, 7976 reflections I . 3s(I), Rint 5 0.049 R 5 0.0402,
Rw 5 0.0476. Data for 7-BArF4?CH2Cl2: C41H42AlBF20N4Ti?CH2Cl2,
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Chem. Commun., 2005, 3313–3315 | 3315