Mono-amidinate complexes stabilized by a new sterically-hindered amidine

Article abstract of DOI:10.1039/a905620c

A novel amidinate ligand incorporating a bulky terphenyl group is used to prepare unusual, low-coordinate lithium and yttrium mono-amidinate complexes.

Full text of DOI:10.1039/a905620c

Mono-amidinate complexes stabilized by a new sterically-hindered amidine†
Joseph A. R. Schmidt and John Arnold*
Department of Chemistry, University of California at Berkeley and Chemical Sciences Division, Lawrence Berkeley
National Laboratory, Berkeley, California 94720-1460, USA. E-mail: arnold@socs.berkeley.edu
Received (in Bloomington, IN, USA) 12th July 1999, Accepted 16th September 1999
A novel amidinate ligand incorporating a bulky terphenyl
group is used to prepare unusual, low-coordinate lithium
and yttrium mono-amidinate complexes.
The search for ancillary ligands capable of facilitating a wide
variety of catalytic processes is a challenging problem in
inorganic chemistry. Metal complexes incorporating amidinate
ligands have been actively studied over the past few years; a
broad range of chemistry has appeared and important applica-
tions, for example, olefin polymerization, have been de-
scribed.127 A major goal of work in this area is to prepare
ligands that can be easily tuned, sterically and electronically, in
order to promote the formation of complexes displaying unusual
structures and reactivities.
Scheme 1
We now report the synthesis of a novel amidinate ligand
incorporating a bulky terphenyl group that exerts steric control
in its metal derivatives. The synthesis of an unusual Li salt and
examples of Ln mono-amidinates are described. These are
noteworthy since bis-amidinates are generally formed in Ln
systems,8,9 a situation that parallels, to some extent, the related
cyclopentadienyl chemistry to which amidinates are frequently
compared.1,10
Previous efforts to prepare bulky amidinates relied on
adamantyl or C6H3Pri2-2,6 substitutions at the N atoms,11213
giving rise to ligands that are essentially bulky only in the
amidine plane [Fig. 1(a)]. In contrast, we envisaged use of a
terphenyl substituent on the amidine carbon atom to provide
steric hindrance not only in the plane of the ligand, but also
above and below this plane, resulting in a more ‘bowl’-shaped
environment [Fig. 1(b)]. Terphenyl groups are now well known
to support unusual coordination environments,14217 the closest
analogy to our work being the formation of hindered carbox-
ylate complexes reported recently.18,19
In contrast to simple non-bulky amidines examined previously,
the H NMR spectrum of Hdimb is complex at room temp.,
1
showing two independent sets of amidine resonances. TOCSY
data allowed deconvolution of the 1H NMR resonances into two
independent sets of peaks. As shown in Fig. 2, the four Pri
resonances are differentiated into two pairs: one pair from the
imine N and one pair from the amine N. A NOESY spectrum
showed chemical exchange between the two Pri groups in each
pair.22 We attribute this exchange to interconversion between
the Z- and E-syn isomers on the NMR timescale. Recently,
Boeré et al. reported similar structural effects in bulky N,NA-
bis(2,6-diisopropylphenyl)benzamidines.13
The sterically demanding amidine N,NA-diisopropyl(2,6-
dimesityl)benzamidine (Hdimb)20 was isolated in 94% yield by
addition of N,NA-diisopropylcarbodiimide to 2,6-dimesityl-
(lithiobenzene)21 followed by an aqueous work-up (Scheme 1).
Fig. 2 2D 1H NMR spectra and the indicated equilibrium.
Lithiation of the amidine with BunLi in hexanes, followed by
addition of N,N,NA,NA-tetramethylethylenediamine (tmeda)
gave the Li derivative [(dimb)Li(tmeda)] 20 shown in Scheme 1,
which was isolated in 64% yield as colorless crystals from Et2O.
1
The H NMR spectrum shows two different Pri resonances,
three inequivalent mesityl methyls, and two different mesityl
aromatic signals, indicating an unsymmetrical product with Cs
symmetry. For comparison, Li salts of simple amidines such as
[PhC(PhN)2]Li(tmeda) show spectra consistent with C2v sym-
metry in solution.23 The solid-state structure of [(dimb)Li-
(tmeda)]24 (Scheme 1) shows that the amidinate is coordinated
in a monodentate fashion to the Li which resides in a distorted
trigonal planar environment [sum of angles around Li =
358.4(3)°] made up of one amidinate N atom and both tmeda N
atoms. Some localization is evident in the N–C–N core, with
C(1)–N(1) 1.316(4) Å and C(1)–N(2) 1.337(4) Å. The amidi-
Fig. 1 Side (wire) and top (space-filling) views of (a) N,NA-bis(ada-
mantyl)neopentamidine (ref. 12) and (b) N,NA-diisopropyl(2,6-bismesi-
tyl)benzamidine.
† Electronic supplementary information (ESI) available: characterization
data.
Chem. Commun., 1999, 2149–2150
This journal is © The Royal Society of Chemistry 1999
2149

13 R. T. Boeré, V. Klassen, and G. Wolmershäuser, J. Chem. Soc., Dalton
Trans., 1998, 4147.
14 X. M. He, R. A. Bartlett, M. M. Olmstead, K. Ruhlandt-Senge, B. E.
Sturgeon and P. P. Power, Angew. Chem., Int. Ed. Engl., 1993, 32,
717.
15 B. Schiemenz and P. P. Power, Organometallics, 1996, 15, 958.
16 J. R. Su, X.-W. Li, R. C. Crittendon and G. H. Robinson, J. Am. Chem.
Soc., 1997, 119, 5471.
17 B. Twamley, C. D. Sofield, M. M. Olmstead and P. P. Power, J. Am.
Chem. Soc., 1999, 121, 3357.
nate N–Li bond [1.942(6) Å] is the shortest observed in a Li
amidinate complex and the N–C–N angle is substantially more
obtuse [126.5(3)°] than in related species.25 In contrast, the
N(tmeda)–Li bonds [2.115(6) and 2.126(6) Å] are typical.26 To
our knowledge, all reported complexes of the type (amidinate)-
Li(tmeda) display four-coordinate Li in solution and in the solid
state.23 We attribute the unusual Li coordination in our
compound to steric effects arising from the sterically bulky
amidinate ligand.
The Li salt is a useful reagent for the synthesis of metal
amidinates. For example, reaction of 1 equiv. of the Li
amidinate with YCl3(thf)3 proceeds as shown in Scheme 1, with
the colorless, crystalline product being isolated in moderate
yield from Et2O. The empirical formula, [(dimb)YCl4Li2-
(tmeda)2],20 follows straightforwardly from 1H NMR spectros-
copy and elemental analysis, and confirmation of the solid-state
structure is provided by X-ray diffraction.27,28 The six-
coordinate Y ion resides in a distorted octahedral coordination
environment with a single amidinate ligand coordinated in the
usual bidentate fashion. Examination of Y–Cl and Y–N bond
lengths (av. 2.66 and 2.32 Å, respectively) show no anomalous
values, although the N–C–N bond angle in the amidinate (ca.
111°) is nearly 5° less obtuse than in related Y com-
pounds.8,9,29231 Although mixed amidinate/cyclopentadienyl
and amidinate/cyclooctatetraenyl compounds have been pre-
viously characterized,31,32 studies involving Y compounds
having exclusively amidinate ligands as ancillary ligands have
only resulted in the formation of bis-amidinate compounds,8,9
making this the first example of a mono-amidinate yttrium
halide species.
18 D. Lee and S. J. Lippard, J. Am. Chem. Soc., 1998, 120, 12153.
19 J. R. Hagadorn, L. Que and W. B. Tolman, J. Am. Chem. Soc., 1998,
120, 13531.
20 NMR data (J/Hz): Hdimb (300 MHz) d (Z-syn) 7.180 (t, 1H, J 5), 6.967
(d, 2H, J 5), 6.864 (s, 4H), 3.439 (s, 1H), 3.420 (spt, 1H, J 6), 3.257 (spt,
1H, J 6), 2.279 (s, 12H), 2.208 (s, 6H), 0.872 (d, 6H, J 6), 0.483 (d, 6H,
J 6); (E-syn) 7.140 (t, 1H, J 5), 6.943 (d, 2H, J 5), 6.846 (s, 2H), 6.816
(s, 2H), 4.015 (spt, 1H, J 6), 3.484 (spt, 1H, J 6), 3.464 (s, 1H), 2.172 (s,
6H), 2.162 (s, 6H), 2.143 (s, 6H), 0.962 (d, 6H, J 6), 0.845 (d, 6H, J 6);
[(dimb)Li(tmeda)] (500 MHz) d 7.244 (t, 1H, J 7.5), 7.096 (d, 2H, J 7.5),
6.929 (s, 2H), 6.888 (s, 2H), 3.635 (spt, 1H, J 6), 2.978 (s, 6H), 2.798
(spt, 1H, J 6), 2.377 (s, 4H), 2.237 (s, 6H), 1.533 (s, 12H), 1.452 (s, 4H),
1.193 (d, 6H, J 6), 0.851 (d, 6H, J 6); [(dimb)YCl4Li2(tmeda)2] (500
MHz) d 7.124 (d, 2H, J 9), 7.082 (t, 1H, J 9), 7.009 (s, 4H), 3.453 (spt
d, 2H, JHH 6, JYH 2.5), 2.514 (s, 12H), 2.233 (s, 6H), 2.107 (s, 24H),
1.750 (s, 8H), 1.119 (d, 12H, J 6); [(dimb)Y{N(SiMe3)2}2] (500 MHz)
d 7.042 (t, 1H, J 7.5), 6.832 (d, 2H, J 7.5), 6.776 (s, 4H), 3.392 (spt d,
2H, JHH 6, JYH 2.5), 2.209 (s, 6H), 2.091 (s, 12H), 1.028 (d, 12H, J 6),
0.273 (s, 36H). Full characterization data are available in the
supplementary
1999/2149/).
information
(http://www.rsc.org/suppdata/cc/
21 C.-J. F. Du, H. Hart and K.-K. D. Ng, J. Org. Chem., 1986, 51, 3162.
22 J. K. M. Sanders and B. K. Hunter, Modern NMR Spectroscopy: A Guide
for Chemists, Oxford University Press, Oxford, 1993.
To test the robustness of the mono-amidinate moiety,
(dimb)Y, towards substitution chemistry, [(dimb)YCl4Li2-
(tmeda)2] was treated with KN(SiMe3)2 as shown in Scheme 1.
The metathesis proceeded smoothly to form [(dimb)Y{N-
(SiMe3)2}2]20 in excellent yield as colorless crystals from
pentane. The compound shows a simple 1H NMR spectrum and
the solid state structure again features a mono-amidinate
complex with the Y now four-coordinate in a distorted
tetrahedron.33 The Y–N(amidinate) bond lengths (av. 2.34 Å)
are nearly the same as those observed in the parent compound
and the N–C–N bond angle has opened slightly (113°), yet it
remains narrower than previously reported related com-
plexes.8,9,31 The Y–N(amide) bond lengths (av. 2.24 Å) are well
within the expected range.34
23 J. Barker, D. Barr, N. D. R. Barnett, W. Clegg, I. Cragg-Hine, M. G.
Davidson, R. P. Davies, S. M. Hodgson, J. A. K. Howard, M. Kilner,
C. W. Lehmann, I. Lopez-Solera, R. E. Mulvey, P. R. Raithby and R.
Snaith, J. Chem. Soc., Dalton Trans., 1997, 951.
24 Crystal data for C37H55N4Li: M = 562.81, orthorhombic, Pbca (no.
61), a = 17.7707(4), b = 19.7667(5), c = 20.3416(4) Å, V = 7145.4(2)
Å3, T = 160 K, Z = 8, m(Mo-Ka) = 0.061 mm21, 34508 reflections
measured, 7118 unique (Rint = 0.067), final R = 0.040, Rw = 0.043,
Rall = 0.056. CCDC 182/1423.
25 T. Gebauer, K. Dehnicke, H. Goesmann and D. Fenske, Z. Naturforsch.,
Teil B, 1994, 49, 1444 and references therein.
26 L. M. Engelhardt, W.-P. Leung, C. L. Raston, P. Twiss and A. H. White,
J. Chem. Soc, Dalton Trans., 1984, 321.
27 In the crystal structure, both tmeda units exhibit a degree of disorder.
The first tmeda unit, refined anisotropically, shows the usual twofold
disorder in the ethylene backbone, and only one of the two configura-
tions is shown. The region involving the second tmeda unit was found
to be occupied by either one tmeda or two Et2O units (ca. 50% each) in
the final crystallographic model and was refined isotropically. The
portion of the model with tmeda occupancy is shown here.
28 Crystal data for C44H73Cl4Li2N5OY: M = 932.69, monoclinic, P21/n
(no. 14), a = 10.9930(4), b = 16.4195(5), c = 29.0041(9) Å, b =
93.891(1)° V = 5223.2(3) Å3, T = 158 K, Z = 4, m(Mo-Ka) = 1.356
mm21, 25212 reflections measured, 9579 unique (Rint = 0.077), final R
= 0.054, Rw = 0.063, Rall = 0.137. CCDC 182/1423.
29 Q. Chen, Y. D. Chang and J. Zubieta, Inorg. Chim. Acta, 1997, 258, 257
and references therein.
30 H. Schumann, F. Erbstein, R. Weimann and J. Demtschuk,
J. Organomet. Chem., 1997, 536, 541 and references therein.
31 R. Duchateau, A. Meetsma and J. H. Teuben, Organometallics, 1996,
15, 1656.
The authors gratefully acknowledge the Department of
Defense Science and Engineering Graduate (NDSEG) Fellow-
ship Program for fellowship support (JARS), as well as Dr
Corey Liu for insightful discussions regarding the 2D NMR
data.
Notes and references
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33 Crystal data for C43H75N4Si4Y: M = 849.34, orthorhombic, Pbca (no.
61), a = 18.6326(6), b = 21.4487(5), c = 24.4451(7) Å, V = 9769.4(4)
Å3, T = 140 K, Z = 8, m(Mo-Ka) = 1.325 mm21, 47309 reflections
measured, 9772 unique (Rint = 0.085), final R = 0.030, Rw = 0.031,
Rall
= 0.105. CCDC 182/1423. For all structures see http://
www.rsc.org/suppdata/cc/1999/2149/ for crystallographic files in .cif
format.
34 H. Schumann, E. C. E. Rosenthal, G. Kociok-Kohn, G. A. Molander and
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Organometallics, 1998, 17, 4042.
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Chem. Commun., 1999, 2149–2150