Benzyl anion abstraction from a (β-diiminato)Fe(II) benzyl complex

Article abstract of DOI:10.1039/b204454d

The 3-coordinate 12 VE iron(II) benzyl complex [(nacnac)- Fe(CH₂Ph)] reacts with B(C₆F₅)₃ to yield a paramagnetic contact ion pair with an η²-(o,m)-[PhCH₂B(C₆F₅) ₃] anion, which was characterised by X-ray diffraction.

Full text of DOI:10.1039/b204454d

Benzyl anion abstraction from a (b-diiminato)Fe(II) benzyl complex†
Timo J.J. Sciarone, Auke Meetsma, Bart Hesssen* and Jan H. Teuben
Dutch Polymer Institute/Center for Catalytic Olefin Polymerization, Stratingh Institute for Chemistry and
Chemical Engineering, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands.
E-mail: hessen@chem.rug.nl; Fax: (+31)50 363 4315; Tel: (+31)50 363 4322
Received (in Corvallis, OR, USA) 5th May 2002, Accepted 5th June 2002
First published as an Advance Article on the web 21st June 2002
The 3-coordinate 12 VE iron(II) benzyl complex [(nacnac)-
Fe(CH2Ph)] reacts with B(C6F5)3 to yield a paramagnetic
contact ion pair with an h2-(o,m)-[PhCH2B(C6F5)3] anion,
which was characterised by X-ray diffraction.
Iron(II) 2,6-diimine pyridine complexes have been found to be
highly efficient catalysts for the polymerisation of olefins when
activated with methyl alumoxane (MAO) cocatalyst.1 The
nature of the catalytically active species is still unproven,
though calculations have suggested it to be a 14 valence electron
(VE) cationic alkyl species.2 So far, well-defined dialkyl
derivatives of this system have not been reported, which
precludes generation of the proposed alkyl cations by alkyl
abstraction.‡ In fact, relatively little is known about the
reactivity of electron-deficient iron alkyl species.3 In order to
Fig. 1 Molecular structure of 1 (hydrogens not shown) with thermal
ellipsoids at 50% probability. Selected bond lengths (Å) and angles (°):
obtain information on alkyl abstraction behaviour and cation–
Fe(11)–N(11) 1.9831(11), Fe(11)–N(12) 1.9807(11), Fe(11)–C(130)
anion interactions in electron-deficient iron complexes, we have
2.0414(18), Fe(11)–C(130)–C(131) 113.18(11).
been studying ways to generate and study representative model
systems.
Reaction of 1 with the strong Lewis acid B(C6F5)3 in toluene,
One way to generate well-defined electron-deficient transi-
benzene or pentane results in selective abstraction of the benzyl
tion metal complexes is the use of sterically demanding
anion from the metal centre (Scheme 1). The ionic compound
ancillary ligands. b-Diketiminate (nacnac) ligands with 2,6-di-
[(nacnac)Fe][PhCH2B(C6F5)3] (2)∑, precipitates from solution,
substituted aryl groups on the nitrogens have been shown to
and single crystals were obtained by recrystallisation from
serve this purpose particularly well,4,5 and b-diketiminate
bromobenzene. The molecular structure of 2 (Fig. 2)§ reveals a
ligands have been used as ancillary ligand for olefin polymer-
2
contact ion pair in which the anion is h -coordinated to the iron
isation catalysts based on trivalent Ti, V, Cr and Sc.6
centre via the benzyl o- and m-carbons.
In order to study the stability of electron deficient alkyl
complexes of iron(II) and their reactivity towards commonly
used borane and borate activators, we have synthesised a 12 VE
iron(II) benzyl complex by alkylation of the ferrate [Li-
(THF)2][(nacnac)FeCl2] (nacnac = ArNCMeCHCMeNAr; Ar
= 2,6-diispropylphenyl) reported recently by Holland.5 Reac-
tion of this ferrate (generated in situ) with benzylmagnesium
bromide in THF cleanly affords the benzyl derivative, obtained
as paramagnetic air sensitive red crystals with composition
[(nacnac)Fe(CH2Ph)]·0.5C5H12 (1) after recrystallisation from
pentane.§
The distances of the iron centre to C(30) and C(31) are at least
0.25 Å shorter than the distances to the other ring carbons, but
Scheme 1
The molecular structure of 1 was determined by X-ray
diffraction (Fig. 1).¶ The unit cell contains two independent
molecules which do not differ significantly. The iron atom lies
in the plane defined by the three coordinating atoms, with the
sum of the angles around Fe equalling 360°. The same
coordination geometry was found for [(CH(C(tBu)NAr)2-
FeCH3] (Ar = 2,6-iPr-C6H3), which contains a sterically more
demanding b-diiminate ligand.7 Despite the formal electron
deficiency (12 VE) of the 3-coordinate iron atom, no secondary
interaction between the arene part of the benzyl group and the
iron atom is present: Fe(11)–C(130)–C(131) = 113.18(11)°.
This is in contrast to the usual observation in electron-deficient
2
group 4 metal benzyl complexes of h -benzyl bonding to the
2
metal centre.8 This absence of h -interaction in 1 is probably
related to the high-spin configuration of the metal centre in
(nacnac)FeR7 and the consequent lack of empty valence orbitals
on the metal centre.
Fig. 2 Molecular structure of 2 (hydrogens not shown) with thermal
ellipsoids at 50% probability. Selected bond lengths (Å) and angles (°): Fe-
N(1) 1.970(6), Fe–N(2) 1.973(6), Fe–C(30) 2.344(8), Fe–C(31) 2.340(8),
Fe–C(32) 2.592(7), Fe–C(33) 2.741(7), Fe–C(34) 2.716(8), Fe–C(35)
2.604(7), N(1)–Fe–N(2) 95.9(2), C(35)–C(36)–B 114.0(6).
† Electronic supplementary information (ESI) available: experimental data.
See http://www.rsc.org/suppdata/cc/b2/b204454d/
1580
CHEM. COMMUN., 2002, 1580–1581
This journal is © The Royal Society of Chemistry 2002

reduction to lower oxidation states yielding monoalkyl species, indicating
that, at least for Co, a Co(II) alkyl cation may not be the actual active
species.13,14
§ 1: Yield 71% based on diimine. Anal. C36H48N2Fe·0.5C5H12 requires C
76.98, H 9.06, N 4.66, Fe 9.30, found C 77.47, H 9.31, N 4.74, Fe 9.00%.
1
1
H NMR (300 MHz, THF-d8, rt) d(Dn ⁄ ) 36.8(70), 12.1(35), 219.7(897),
2
225.0(217), 229.0(290), 243.6(39), 250.4(40), 256.2(166) ppm (Hz).
¶
Crystal data for 1, C36H48N2Fe·0.5C5H12, M = 600.72, monoclinic, a
= 15.3319(9), b = 20.652(1), c = 22.408(1) Å, U = 7094.5(6) Å3, T =
125 K, space group P21/c, (no. 14), Z = 8, m = 4.52 cm21, l(Mo-Ka) =
0.71073 Å, 44924 reflections measured, 18149 unique (Rint = 0.0276),
wR(F2) = 0.1134 for 18149 reflections and 1181 parameters, R(F) =
0.0430 for 13810 reflections obeying Fo 4 4.0 s(Fo).
Fig. 3 19F NMR spectrum of 2 (470 MHz, C6D5Br, 253 K).
Crystal data for 2, C54H48N2FeBF15, M = 1076.62, monoclinic, a =
44.806(3), b = 13.349(1), c = 17.780(1) Å, b = 113.282(5)°, U =
9768.5(12) Å3, T = 110 K, space group C2/c, (no. 15), Z = 8, m = 4.06
cm21, l(Mo-Ka) = 0.71073 Å, 34522 reflections measured, 11209 unique
the arene C–C bond lengths do not show significant deviations
from aromaticity, ranging from 1.383(12) Å (C(30)–C(31)) to
1.42(1) Å (C(30)–C(35)). The plane through C(30), C(31) and
Fe makes an angle of 69.1(5)° with the diiminate coordination
plane and an angle of 37.2(6)° with the plane perpendicular to
the diiminate coordination plane and bisecting N(1)–Fe–N(2).
To the best of our knowledge, this coordination mode is
unprecedented for the PhCH2B(C6F5)3 anion, for which coor-
(Rint
= = 0.1767 for 11209 reflections and 851
0.3431), wR(F2)
parameters, R(F) = 0.0678 for 3022 reflections obeying Fo 4 4.0 s(Fo).
CCDC 185601 and 185603. See http://www.rsc.org/suppdata/cc/b2/
b204454d/ for crystallographic data in .cif or other electronic format.
∑
2: Yield 72% based on 1. Anal. C54H48N2FeBF15 requires C 60.24 H
4.49 N 2.60, found C 60.00, H 4.58 N 2.59%.
3
6
dination in h - and h -fashion has been reported.9 For the BPh4-
19F NMR (470 MHz, C6D5Br, 253.3 K): d 282.6 (o-F), 2128.4 (o-F),
2150.7 (m-F), 2152.8 (p-F), 2161.5 (p-F), 2163.4 (m-F). 19F NMR (188
MHz, THF-d8, rt): d 2131.8 (d, JFF = 21.45Hz, 6F, o-F), 2167.3 (t, JFF
= 19.66Hz, 3F, p-F), 169.5(m, 6F, m-F).
2
anion h -(m,p)-coordination to [CpAZrMe]+ has been proposed
2
based on 1H NMR data10 and h -(o,m)-coordination of [BPh4]2
to [(en)Cu(CO)]+ has been established by X-ray diffraction.11
In C6D5Br solution, low temperature 19F NMR spectroscopy
(Fig. 3) shows resonances for two [PhCH2B(C6F5)3] species:
one with chemical shifts normal for the free anion (71%) and
one in which the o-F and (to a lesser extent) m-/p-F resonances
are considerably downfield shifted and broadened due to
coordination to the paramagnetic Fe(II) ion (29%). Apparently
there is an equilibrium between the contact ion pair and a
species where the [nacnac]Fe-cation is solvated by bromo-
benzene.
1 G. J. P. Britovsek, V. C. Gibson, B. S. Kimberley, P. J. Maddox, S. J.
McTavish, G. A. Solan, A. J. P. White and D. J. Wiliams, Chem.
Commun., 1989, 849; B. L. Small, M. Brookhart and A. M. A. Bennett,
J. Am. Chem. Soc., 1998, 120, 4049.
2 D. V. Khoroshun, D. G. Musaev, T. Vreven and K. Morokuma,
Organometallics, 2001, 20, 2007; L. Deng, P. Margl and T. Ziegler, J.
Am. Chem. Soc., 1999, 121, 6479; E. A. H. Griffiths, G. J. P. Britovsek,
V. C. Gibson and I. R. Gould, Chem. Commun., 1999, 1333.
3 A. R. Hermes and G. S. Girolami, Organometallics, 1987, 6, 763; H. K.
Lee, B.-S. Luo, T. C. W. Mak and W.-P. J. Leung, J. Organomet. Chem.,
1995, 489, C71; J. L. Kisko, T. Hascall and G. Parkin, J. Am. Chem.
Soc., 1998, 120, 10561; N. Shirasawa, M. Akita, S. Hikichi and Y.
Moro-oka, J. Chem. Soc., Chem. Commun., 1999, 417; M. D. Fryzuk, D.
B. Leznoff, E. S. R. Ma, S. J. Rettig and V. G. Young Jr.,
Organometallics, 1988, 17, 2313.
Addition of THF to this solution results in a colour change
from red–brown to bright yellow. The resulting 19F NMR
spectrum of the THF adduct displays sharp resonances for the
free [PhCH2B(C6F5)3] anion, indicating that the Lewis base
THF has completely displaced the anion from the metal centre
(Scheme 2).
4 P. H. M. Budzelaar, N. N. P. Moonen, R. de Gelder, J. M. M. Smits and
A. W. Gal, Eur. J. Inorg. Chem., 2000, 753.
5 J. M. Smith, R. J. Lachicotte and P. L. Holland, Chem. Commun., 2001,
1542; P. L. Holland and W. B. Tolman, J. Am. Chem. Soc., 2000, 122,
6331.
6 W-K. Kim, M. J. Fevola, L. M. Liable-Sands, A. L. Rheingold and K.
H. Theopold, Organometallics, 1998, 17, 4541; V. C. Gibson, P. J.
Maddox, C. Newton, C. Redshaw, G. A. Solan, A. J. P. White and D. J.
Williams, Chem. Commun., 1998, 1651; P. G. Hayes, W. E. Piers and R.
McDonald, J. Am. Chem. Soc., 2002, 124, 2132; P. H. M. Budzelaar, A.
B. van Oort and A. G. Orpen, Eur. J. Inorg. Chem., 1988, 1485.
7 H. Andres, E. L. Bominaar, J. M. Smith, N. A. Eckert, P. L. Holland and
E. Münck, J. Am. Chem. Soc., 2002, 124, 3012.
8 M. Mena, M. A. Pellinghelli, P. Royo, R. Serrano and A. Tiripicchio, J.
Chem. Soc., Chem. Commun., 1986, 1118; G. R. Giesbrecht, A. Shafir
and J. Arnold, Chem. Commun., 2000, 2135; J. S. Rogers, R. J.
Lachicotte and G. C. Bazan, Organometallics, 1999, 18, 3976.
9 9. M. Bochmann, S. J. Lancaster, M. B. Hursthouse and K. M. A. Malik,
Organometallics, 1994, 13, 2235; X. Yang, C. L. Stern and T. J. J.
Marks, J. Am. Chem. Soc., 1994, 116, 10015; C. Pellecchia, A. Grassi
and A. Immirzi, J. Am. Chem. Soc., 1993, 115, 1160; L. W. M. Lee, W.
E. Piers, M. R. J. Elsegood, W. Clegg and M. Parvez, Organometallics,
1999, 18, 2947.
10 A. D. Horton and J. H. G. Frijns, Angew. Chem., Int. Ed. Engl., 1991, 30,
1152.
11 M. Pasquali, C. Floriani and A. Gaetani-Manfredotti, Inorg. Chem.,
1980, 19, 1191.
Scheme 2
In conclusion, the present study has shown that alkyl groups
from electron-deficient iron species can be conveniently
abstracted by a Lewis acid commonly used to activate early
transition-metal polymerisation catalyst precursors, and a new
coordination mode of the common [PhCH2B(C6F5)3]-counter-
ion was observed. Interaction of fluorinated borate anions with
paramagnetic transition-metal centres was seen to result in large
shifts in the 19F NMR spectrum, suggesting that paramagnetic
complexes may be used to probe even weaker cation–anion
interactions in solution. We are presently investigating this
possibility.
12 E. L. Dias, M. Brookhart and P. S. White, Organometallics, 2000, 19,
4995.
13 D. Reardon, G. Aharonian, S. Gambarotta and G. P. A. Yap,
Organometallics, 2002, 21, 786.
14 V. C. Gibson, M. J. Humphries, K. P. Tellmann, D. F. Wass, A. J. P.
White and D. J. Williams, Chem. Commun., 2001, 2252; T. M. Kooistra,
Q. Knijnenburg, J. M. M. Smits, A. D. Horton, P. H. M. Budzelaar and
A. W. Gal, Angew. Chem., Int. Ed., 2001, 40, 4719.
Notes and references
‡ Olefin adducts of the corresponding Ru(II) and Rh(III) alkyl cations have
been synthesised.12 For Mn(II) and Co(II), alkylation is complicated by
CHEM. COMMUN., 2002, 1580–1581
1581