Steric engineering of C-F activation with lanthanoid formamidinates

Article abstract of DOI:10.1039/b419047e

Reaction of lanthanum with Hg(C₆F₅)₂ and bulky N,N′-bis(2,6-diisopropylphenyl)formamidine (HDippForm) in tetrahydrofuran gives [LaF{DippForm}₂(THF)] with a rare terminal Ln-F bond, and a high yield of a novel fu

Full text of DOI:10.1039/b419047e

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Steric engineering of C–F activation with lanthanoid formamidinates
Marcus L. Cole,{ Glen B. Deacon, Peter C. Junk* and Kristina Konstas
Received (in Cambridge, UK) 21st December 2004, Accepted 31st January 2005
First published as an Advance Article on the web 15th February 2005
DOI: 10.1039/b419047e
[LaF(DippForm)2(THF)]?C7H8 (1?C7H8) after crystallisation from
toluene (Scheme 1, Ln 5 La).{ By contrast, reaction of La metal
with Hg(C6F5)2 and the smaller N,N9-bis(2,6-dimethylphenyl)-
Reaction of lanthanum with Hg(C6F5)2 and bulky
N,N9-bis(2,6-diisopropylphenyl)formamidine (HDippForm) in
tetrahydrofuran gives [LaF{DippForm}2(THF)] with a rare
terminal Ln–F bond, and a high yield of a novel functionalised
formamidine, DippForm((CH2)4OC6F4H-o).
formamidine
yields
a
lanthanum
tris(formamidinate)
1
[La(XylForm)3(THF)]?2THF.7 1?C7H8 was characterised{ by H,
13C{1H} and IR spectroscopy, elemental analyses and location of
19F NMR resonance at +150.2 ppm (La–F).8 Scheme 1 appears
The activation of unsaturated and saturated fluorocarbon C–F
bonds by divalent lanthanoid species is the main source of the few
examples of heteroleptic lanthanoid fluorides [Ln(L)2F]
(L 5 anionic ligand).1–3 As an alternative approach with general
applicability, redox transmetalation/protolytic ligand exchange
syntheses using bis(pentafluorophenyl)mercury3 could be
employed if the proposed [Ln(C6F5)L2] intermediate were to
undergo C–F activation to yield [Ln(F)L2] and tetrafluoroben-
zyne3 before protolysis (to give LnL3) could occur. This demands
steric inhibition of the final exchange step and obviates the need
for an isolable divalent precursor. The ready accessibility4 of
N,N9-diarylformamidinates (potential cyclopentadienide mimics5)
makes them attractive candidates for steric tuning, but no
lanthanoid complexes have yet been published. We now report
that with a suitable choice of ligand, sterically engineered C–F
activation can be achieved leading to the first lanthanoid
organoamide fluorides,6 which have rare terminal Ln–F bonding.
Unexpectedly, chemoselective trapping of the expelled tetrafluoro-
benzyne also occurs giving a novel functionalised formamidine.
Elemental lanthanum readily reacts with bis(pentafluoro-
phenyl)mercury and N,N9-bis(2,6-diisopropylphenyl)formamidine
(HDippForm) in THF in a 1 : 1.5 : 3 stoichiometry to form
a
general across a range of lanthanoid sizes, having also been
observed for Ln 5 Nd and Tm.
An X-ray crystal structure determination§ (Fig. 1) shows the
lanthanum centre is six-coordinate with cisoid DippForm ligands.
Its geometry may be described using the formamidinate ligands as
point charges located at the NCN carbon atom. This generates a
distorted tetrahedral geometry that boasts not only a rare terminal
fluoride2c,9 but also the first lanthanoid fluoride supported by
organoamide ligands (F(1)–La(1)–C(25) 105.1(1)u, O(1)–La(1)–
˚
C(50) 104.0(1)u). The La–F bond length in 1 (2.136(2) A) is short
relative to known La–F interactions in metal organic com-
pounds.6a,10 These are all bridging in nature e.g. [{La(1,3-
6a
(SiMe3)2C5H3)2F}2]; 2.349(7) A and 2.324(9) A. However, this
˚
˚
bond length is comparable with terminal Ln–F bond lengths found
˚
in [Yb(F)(C5Me5)2(THF)] (2.026(2) A), [Yb(F)(C5Me5)2(Et2O)]
2c
(2.015(4) A) and [Sm(F)(Tp*)2] (Tp* 5 hydrotris(3,5-dimethyl-
˚
9
pyrazolyl)borate, Sm–F 2.090(7) A) when allowance is made for
˚
˚
differences in ionic radii (six-coordinate lanthanum; 1.032 A, eight
˚
coordinate ytterbium; 0.985 A and seven coordinate samarium;
Fig. 1 Molecular structure of 1, POV-RAY illustration, 40% thermal
ellipsoids, all hydrogen atoms and lattice solvent omitted for clarity.
˚
Selected bond lengths (A) and angles (u): La(1)–F(1) 2.136(2), La(1)–O(1)
Scheme 1
2.539(3), La(1)–N(1) 2.538(3), La(1)–N(2) 2.578(3), La(1)–N(3) 2.543(3),
La(1)–N(4) 2.560(3), O(1)–La(1)–F(1) 85.1(1), N(1)–La(1)–N(2) 53.0(1),
N(3)–La(1)–N(4) 53.0(1), O(1)–La(1)–N(1) 132.1(1), O(1)–La(1)–N(2)
79.4(1), F(1)–La(1)–O(1) 85.1(1), F(1)–La(1)–N(1) 109.2(1), F(1)–La(1)–
N(2) 104.0(1), F(1)–La(1)–N(3) 127.6(1), F(1)–La(1)–N(4) 97.7(1).
{ Current address: Department of Chemistry, University of Adelaide,
South Australia 5005, Australia.
*peter.junk@sci.monash.edu.au
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Notes and references
{ General procedure: Lanthanum metal filings (0.10 g, 0.72 mmol),
bis(pentafluorophenyl)mercury (0.37 g, 0.69 mmol) and N,N9-bis(2,6-
diisopropylphenyl)formamidine (0.50 g, 1.4 mmol) were stirred in
tetrahydrofuran (20 cm3) under purified nitrogen at room temperature
for 48 h. Filtration followed by drying under vacuum yielded a colourless
powdered material that was extracted into toluene (10 cm3). Cooling at
–30 uC yielded 1?C7H8 as colourless elongated hexagonal plates in three
batches over several days (total 0.37 g, 77% by [Hg(C6F5)2]), mp 284 uC
(dec.). Found: C 67.86; H 7.66; La 14.65; N 5.13 C54H78FLaN4O; 1 (with
loss of C7H8) requires C 67.76; H 8.21; La 14.51; N 5.85%. IR (Nujol):
1937 w, 1858 w, 1799 w, 1603 s, 1583 s, 1311 m, 1267 m, 1183 s, 1108 s,
1050 m, 1028 m, 1019 m, 800 m, 756 s, 730 s, 697 s, 673 w cm21. 1H NMR
(300.1 MHz, C6D6, 300 K): d 5 8.23 (s, 2H; NC(H)N), 7.27–7.06 (m, 17H;
Ar–H), 3.58 (h, 3J(H,H) 5 6.8 Hz, 8 H; CH, iPr), 3.47 (m, 4H; OCH2,
THF), 2.18 (s, 3H; CH3, toluene), 1.31 (m, 4H; CH2, THF), 1.27 (d,
3J(H,H) 5 6.8 Hz, 48 H; CH3, iPr). 13C{1H} NMR (75.5 MHz, C6D6,
300 K): d 5 166.7 (NC(H)N), 145.2, 141.8, 137.9 (Ar–C), 129.2, 128.3,
125.5, 123.4, 122.2 (Ar–CH), 66.7 (OCH2, THF), 27.5 (CH, iPr), 24.9
(CH2, THF), 22.5 (CH3, iPr), 20.1 (CH3, toluene). 19F NMR (282.4 MHz,
C6D6, 300 K, CCl3F): d 5 150.2 (s; La–F). 2: Concentration of the mother
liquor from 1?C7H8 (,1 cm3) and cooling at –30 uC overnight yielded
compound 2 as colourless rods (0.10 g, 81% by [Hg(C6F5)2]), mp 160 uC.
High resolution ESMS: m/z 585.3462, C35H45F4N2O [M + H]+ requires
585.3390. IR (Nujol): 1665 s, 1636 s, 1528 m, 1523 s, 1386 m, 1364 m,
1289 m, 1233 m, 1150 w, 1097 m, 950 m, 923 m, 821 w, 756 s, 729 s, 687 s,
673 w cm21. 1H NMR (300.1 MHz, C6D6, 300 K): d 5 7.31–7.14 (m, 7H;
Ar–H and NC(H)N, DippForm), 5.96 (dddd, 3J(o-H,F) 5 11.3 Hz,
4J(m-H,F) 5 8.5 Hz, 4J(m-H,F) 5 5.7 Hz, 5J(p-H,F) 5 2.8 Hz, 1H; Ar–5-
H), 3.85 (m, 2H; OCH2), 3.48 (h, 3J(H,H) 5 6.8 Hz, 2H; CH, iPr), 3.36 (m,
4H; NCH2 and CH, iPr), 1.96 (m, 2H; CH2), 1.52 (m, 2H; CH2), 1.31 (d,
3J(H,H) 5 6.9 Hz, 6H; CH3, iPr), 1.19 (d, 3J(H,H) 5 6.8 Hz, 12H; CH3,
iPr), 1.15 (d, 3J(H,H) 5 6.9 Hz, 6H; CH3, iPr). 13C{1H} NMR (75.5 MHz,
C6D6, 300 K): d 5 147.0 (s; NC(H)N), 138.5, 138.1, 128.4, 128.0, 125.5,
124.4, 122.1, 121.9 (s; Ar–C), 96.6 (dd, J(o-C,F) 5 22.5 Hz, J(p-C,F) 5
2.9 Hz; ArF–CH), 68.5 (s; NCH2), 48.9 (s; OCH2), 27.5, 27.4, 27.1 (s; CH,
iPr), 25.8 (s; CH2), 24.1 (s; CH3, iPr), 23.1 (s; CH2), 22.9, 22.5 (s; CH3, iPr)
(carbon resonances for the 2,3,4,5-tetrafluorophenyl group, excepting the
CH resonance, not observed). 19F NMR (282.4 MHz, C6D6, 300 K,
CCl3F): d 5 2140.0 (ddd, 3J(o-F,F) 5 22.6 Hz, 4J(o-F,H) 5 11.3 Hz,
5J(p-F,F) 5 8.5 Hz, 1F; Ar–4-F), 2155.8 (ddd, 3J(o-F,F) 5 21.3 Hz,
3J(o-F,F) 5 19.8 Hz, 5J(p-F,H) 5 2.8 Hz, 1F; Ar–2-F), 2160.0 (dddd,
3J(o-F,F) 5 19.8 Hz, 5J(p-F,F) 5 8.5 Hz, 4J(m-F,H) 5 8.5 Hz,
4J(m-F,F) 5 2.8 Hz, 1F; Ar–1-F), 2167.5 (dddd, 3J(o-F,F) 5 22.6 Hz,
3J(o-F,F) 5 21.3 Hz, 4J(m-F,H) 5 5.7 Hz, 4J(m-F,F) 5 2.8 Hz, 1F; Ar–3-
F). Fluorines numbered as in Fig. 1. 19F NMR monitoring of reaction
mixtures revealed Hg(C6F5)2, C6F5H and 2 as the sole fluorocarbon
species.
Fig. 2 Molecular structure of 2, POV-RAY illustration, 40% thermal
ellipsoids, all hydrogen atoms except H(35) omitted for clarity. Selected
˚
bond lengths (A) and angles (u): N(1)–C(25) 1.358(2), N(2)–C(25) 1.277(2),
N(1)–C(26) 1.472(2), C(29)–O(1) 1.444(2), N(1)–C(25)–N(2) 123.4(2).
1.020 A).11 The aryl groups of 1 are near-orthogonal to the NCN
˚
backbone as is typical of the DippForm ligand (range of aryl :
NCN torsion angles; 74.1(3)u to 88.2(3)u).4,12
It is likely that compound 1 results from C–F activation in
the pentafluorophenyllanthanum intermediate [La(C6F5)-
(DippForm)2] (cf. isolable [Ln(C6F5)(L)2] compounds, L 5 C5H5
or C5Me5),2b,6c,13 wherein steric bulk prohibits protonation of the
fluoroaryl group giving [La(DippForm)3] by the remaining
equivalent of HDippForm (cf. formation of [La(XylForm)3-
(THF)], above7). Elimination of tetrafluorobenzyne from MC6F5
complexes (the archetypal path of Ln–F formation) normally
gives complex fluoroaromatics, in particular 2-nonafluorobiphe-
nyls, from insertion of the benzyne into M–C6F5 bonds.1a,14
However, further work-up of the La–HDippForm–[Hg(C6F5)2]
reaction mixture enables high yield isolation (81%) of a single
fluorocarbon co-product; viz. the functionalised formamidine 2{,
identified by spectroscopic analysis and X-ray crystallography
(Fig. 2).§
3
5
The structure exhibits a DippForm with discrete single (N(1)–
˚
˚
C(25) 1.358(2) A) and double (N(2)–C(25) 1.277(2) A) C–N bonds
tethered to a 2,3,4,5-tetrafluorophenyl group by a ring opened
THF.15 The formation of 2 can be attributed to protonation of
tetrafluorobenzyne by HDippForm to give the formamidinate ion
§ Crystalline samples of compounds 1?C7H8 and 2 were mounted upon
glass fibres in silicone grease at 123(2) K (2) or 173(2) K (1?C7H8). Enraf-
Nonius Kappa CCD diffractometer, graphite monochromated Mo Ka
˚
X-ray radiation (l 5 0.71073 A). Data were corrected for absorption by the
+
DENZO-SMN package. Lorentz polarisation and absorption corrections
were applied. Structural solution and refinement was carried out using the
SHELX suite of programs with the graphical interface X-Seed. Crystal
and HC6F4 , the latter being trapped by THF. Nucleophilic attack
+
of the amidinate on the resulting oxonium ion, HC6F4 OC4H8,
which is activated at the THF a-carbon, yields 2.15
¯
data for 1?C7H8: C61H86F1N4O1La1, M 5 1049.25, triclinic, P1 (no. 2),
˚
Thus, C–F activation for elements without a readily stable LnII
state16 can be achieved by steric engineering giving both the first
organoamide supported lanthanoid fluorides and novel functio-
nalised fluoroarenes/formamidines. Continuing studies suggest
C–F activation of perfluoroaryls with a single ortho-fluorine
substituent is also possible. Accordingly La metal, Hg(o-HC6F4)2
a 5 12.6721(9), b 5 13.6678(12), c 5 17.264(3) A, a 5 92.904(4),
3
˚
b 5 104.552(6), c 5 91.186(4)u, V 5 2888.8(6) A , Z 5 2, Dc
5
1.206 g cm23, F000 5 1108, m 5 0.784 mm21, 2hmax 5 55.7u, 42471
reflections collected, 13485 unique (Rint 5 0.1285). Final GooF 5 1.012,
R1 5 0.0576, wR2 5 0.1013, R indices based on 9041 reflections with I .
2s(I) (refinement on F2), 694 parameters, 0 restraints. Crystal data for 2:
C35H44F4N2O, M 5 584.72, monoclinic, P21/n (no. 14), a 5 10.6132(3),
˚
3
˚
b 5 20.4222(7), c 5 14.8655(6) A, b 5 98.5310(10)u, V 5 3186.37(19) A ,
Z 5 4, Dc 5 1.219 g cm23, F000 5 1248, m 5 0.089 mm21, 2hmax 5 56.3u,
25587 reflections collected, 7680 unique (Rint 5 0.1111). Final
and bis(2,6-diisopropylphenyl)formamidine yield
DippForm{(CH2)4OC6F3H2-2,6}.7
1
and
GooF 5 0.938, R1 5 0.0568, wR2 5 0.1118, R indices based on 3758
reflections with I . 2s(I) (refinement on F2), 387 parameters, 0 restraints.
CCDC 253456 for compound 1 and CCDC 253457 for compound 2. See
http://www.rsc.org/suppdata/cc/b4/b419047e/ for crystallographic data in
.cif or other electronic format.
We thank the Australian Research Council for continued
support.
Marcus L. Cole,{ Glen B. Deacon, Peter C. Junk* and Kristina Konstas
School of Chemistry, Monash University, P.O. Box 23, Victoria 3800,
Australia. E-mail: peter.junk@sci.monash.edu.au;
1 (a) J. L. Kiplinger, T. G. Richmond and C. E. Osterberg, Chem. Rev.,
1994, 94, 373; (b) J. Burdenuic, B. Jedlicka and R. H. Crabtree, Chem.
Fax: +61 (0)3 9905 4597; Tel: +61 (0)3 9905 4570
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Ber./Recl., 1997, 130, 145; (c) H. Dorn, E. F. Murphy, S. A. A. Shah
and H. W. Roesky, J. Fluorine Chem., 1997, 86, 121.
relative to 1 presumably resulting from a Sm(III) induced paramagnetic
shift.
9 A. C. Hillier, X. W. Zhang, G. H. Maunder, S. Y. Liu,
2 For examples see: (a) C. J. Burns, D. J. Berg and R. A. Andersen,
J. Chem. Soc., Chem. Commun., 1987, 272; (b) C. J. Burns and
R. A. Andersen, J. Chem. Soc., Chem. Commun., 1989, 136; (c)
P. L. Watson, T. H. Tulip and I. Williams, Organometallics, 1990, 9,
1999; (d) G. B. Deacon, S. C. Harris, G. Meyer, D. Stellfeldt,
D. L. Wilkinson and G. Zelesny, J. Organomet. Chem., 1998, 552, 165;
(e) G. B. Deacon, G. Meyer and D. Stellfeldt, Eur. J. Inorg. Chem.,
2000, 1061.
ˆ
T. A. Eberspacher, M. V. Metz, R. McDonald, A. Domingos,
M. Marques, V. W. Day, A. Sella and J. Takats, Inorg. Chem., 2001,
40, 5106.
ˇ
10 (a) F. Perdih, A. Demˇsar, A. Pevec, S. Patricek, I. Leban, G. Giester,
J. Sieler and H. W. Roesky, Polyhedron, 2001, 20, 1967; (b)
H. Takemura, S. Nakashima, N. Kon, M. Yasutake, T. Shinmyozu
and T. Inazu, J. Am. Chem. Soc., 2001, 125, 9293.
3 G. B. Deacon, C. M. Forsyth and S. Nickel, J. Organomet. Chem., 2002,
647, 50.
4 M. L. Cole, A. J. Davies, C. Jones and P. C. Junk, J. Organomet.
Chem., 2004, 689, 3093.
11 R. D. Shannon, Acta Crystallogr., Sect. A, 1976, 32, 751.
12 M. L. Cole and P. C. Junk, J. Organomet. Chem., 2003, 666, 55.
13 (a) G. B. Deacon and D. L. Wilkinson, Inorg. Chim. Acta, 1988, 142,
155; (b) I. Castillo and T. D. Tilley, J. Am. Chem. Soc., 2001, 123, 10526.
14 (a) S. C. Cohen and A. G. Massey, Adv. Fluorine Chem., 1970, 6, 83; (b)
G. B. Deacon, A. J. Koplick, W. D. Raverty and D. G. Vince,
J. Organomet. Chem., 1979, 182, 171.
15 For examples of f-block induced THF ring-opening see: (a) W. J. Evans,
T. A. Ulibarri, L. R. Chamberlain, J. W. Ziller and D. Alvarez,
Organometallics, 1990, 9, 2124; (b) H. Schumann, M. Glanz, H. Hemling
and F. H. Go¨rlitz, J. Organomet. Chem., 1993, 462, 155; (c) L. R. Avens,
D. M. Barnhart, C. J. Burns and S. D. McKee, Inorg. Chem., 1996, 35,
537; (d) C. Boisson, J. C. Berthet, M. Lance, M. Nierlich and
M. Ephritikhine, Chem. Commun., 1996, 2129.
5 (a) R. Kempe, Angew. Chem., 2000, 114, 478, Angew. Chem., Int. Ed.,
2000, 39, 468; (b) P. W. Roesky, Chem. Soc. Rev., 2000, 29, 335; (c)
W. E. Piers and D. J. H. Emslie, Coord. Chem. Rev., 2002, 233, 131; (d)
F. T. Edelmann, D. M. M. Freckmann and H. Schumann, Chem. Rev.,
2002, 102, 1851.
6 For recent examples where a trivalent precursor is used to generate a
heteroleptic lanthanoid fluoride please see: (a) Z. Xie, K. Chui, Q. Yang,
T. C. W. Mak and J. Sun, Organometallics, 1998, 17, 3937; (b)
W. J. Evans, D. G. Giarikos, M. A. Johnston, M. A. Greci and
J. W. Ziller, J. Chem. Soc., Dalton Trans., 2002, 502; (c) L. Maron,
E. L. Werkema, L. Perrin, O. Eisenstein and R. A. Andersen, J. Am.
Chem. Soc., 2005, 127, 279.
16 At least one example of a LaII, NdII and a TmII molecular complex is
known despite considerable preparative and handling difficulties:
M. C. Cassani, D. J. Duncalf and M. F. Lappert, J. Am. Chem. Soc.,
1998, 120, 12958; M. N. Bochkarev, I. L. Fedushkin, S. Dechert,
A. A. Fagin and H. Schumann, Angew. Chem., Int. Ed., 2001, 40, 3176;
W. J. Evans, N. T. Allen and J. W. Ziller, Angew. Chem., Int. Ed., 2002,
41, 359.
7 M. L. Cole, G. B. Deacon, P. C. Junk and K. Konstas, unpublished
results.
8 No relevant examples of lanthanum fluoride 19F NMR data could be
found, however, the terminal samarium fluoride species [Sm(F)(Tp*)2]
listed in ref. 8 exhibits a 19F NMR signal at –172.3 ppm, the large shift
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Chem. Commun., 2005, 1581–1583 | 1583